Abstract

The Kalatage inlier in the Dananhu-Haerlik arc is one of the most important arcs in the Eastern Tianshan, southern Altaids (or Central Asian orogenic belt). Based on outcrop maps and core logs, we report 16 new U-Pb dates in order to reconstruct the stratigraphic framework of the Dananhu-Haerlik arc. The new U-Pb ages reveal that the volcanic and intrusive rocks formed in the interval from the Ordovician to early Permian (445–299 Ma), with the oldest diorite dike at 445 ± 3 Ma and the youngest rhyolite at 299 ± 2 Ma. These results constrain the ages of the oldest basaltic and volcaniclastic rocks of the Ordovician Huangchaopo Group, which were intruded by granite-granodiorite-diorite plutons in the Late Ordovician to middle Silurian (445–426 Ma). The second oldest components are intermediate volcanic and volcaniclastic rocks of the early Silurian Hongliuxia Formation (S1h), which lies unconformably on the Huangchaopo Group and is unconformably overlain by Early Devonian volcanic rocks (416 Ma). From the mid- to late Silurian (S2-3), all the rocks were exhumed, eroded, and overlain by polymictic pyroclastic deposits. Following subaerial to shallow subaqueous burial at 416–300 Ma by intermediate to felsic volcanic and volcaniclastics rocks, the succession was intruded by diorites, granodiorites, and granites (390–314 Ma). The arc volcanic and intrusive rocks are characterized by potassium enrichment, when they evolved from mafic to felsic and from tholeiitic via transitional and calc-alkaline to final high-K calc-alkaline compositions with relatively low initial Sr values, (87Sr/86Sr)i = 0.70391–0.70567, and positive εNd(t) values, +4.1 to +9.2. These new data suggest that the Dananhu-Haerlik arc is a long-lived arc that consequently requires a new evolutionary model. It began as a nascent (immature) intra-oceanic arc in the Ordovician to early Silurian, and it evolved into a mature island arc in the middle Silurian to early Permian. The results suggest that the construction of a juvenile-to-mature arc, in combination with its lateral attachment to an incoming arc or continent, was an important crustal growth mechanism in the southern Altaids.

INTRODUCTION

The southern Altaids, or Central Asian orogenic belt, is one of the most important sites of Neoproterozoic–Phanerozoic juvenile crustal growth in the world (Fig. 1A; Şengör et al., 1993; Şengör and Natal’in, 1996; Windley et al., 2007; Wilhem et al., 2012; Xiao et al., 2015). The Altaids formed by the successive accretion of oceanic and continental arcs and accretionary complexes (Coleman, 1989; Dobretsov et al., 1995; Gao et al., 1998; Buchan et al., 2002; Bazhenov et al., 2003; Xiao et al., 2003, 2004, 2008, 2010; Li, 2004; Li et al., 2006a; Ma et al., 1997; Shi et al., 2010; Wainwright et al., 2011a, 2011b). This accretion involved major vertical crustal growth through arc magmatism in convergent and/or extensional settings during the late Paleozoic and early Mesozoic (Han et al., 1997, 1998, 2004; Chen et al., 2000, 2006; Jahn et al., 2000; Wu et al., 2000, 2002; Jahn, 2004; Yuan et al., 2007; Mao et al., 2014b).

The Eastern Tianshan (Fig. 1), which is situated in the South Tianshan–Solonker suture zone of the southern Altaids (Xiao et al., 2004, 2010; Mao et al., 2014b, 2018), involved episodic amalgamation and accretion of continental margin arcs, island arcs, ophiolites, and accretionary wedges (Ma et al., 1997; Li, 2004; Xiao et al., 2004; Wang et al., 2006; Windley et al., 2007). Consequently, diverse models have been applied to better understand the geological evolution of the Chinese Eastern Tianshan in the Paleozoic, and many controversies remain, especially regarding the stratigraphic framework, relationships among formations, the concept and nature of the Dananhu-Haerlik arc (Xiao et al., 1992; Ma et al., 1997; Xiao et al., 2004; Wang et al., 2006; Muhetaer et al., 2010; Li et al., 2016; Zhang et al., 2016), and the detailed geological evolution of the associated tectonic belts (Xiao et al., 1992; Ma et al., 1997; Feng et al., 2002; Li, 2004; Xiao et al., 2004; Wang et al., 2006; Wang et al., 2014; Li et al., 2016).

In order to better understand the tectonic evolution of the Eastern Tianshan, we need a detailed temporal-tectonostratigraphic framework for the highly thrusted sequences. In this paper, we focus on one portion of the Dananhu-Haerlik arc: the Kalatage geological inlier, which is mainly composed of weakly metamorphosed Ordovician to Permian volcanic, volcaniclastic, and granitic rocks, and minor marbles together with several types of mineral deposits (see Figs. 2 and 3; Mao et al., 2019), such as volcanogenic massive sulfide (VMS)–type Cu-Zn-(Au-Ag) deposits, porphyry-type Cu (Au) deposits, subvolcanic hydrothermal vein–type Cu (Au) deposits, and magmatic Cu-Ni sulfide deposits. We reconstructed the Paleozoic stratigraphic framework of the volcanic and sedimentary rocks at Kalatage mainly from our outcrop mapping and core logging combined with 16 new U-Pb ages and geochemical data to better understand the history of magmatic activity in the arc. These results improve our knowledge of the geological evolution of the Dananhu-Haerlik arc and place new constraints on the tectonic evolution of the southern Altaids.

GEOLOGICAL SETTING

Tectonic Background of the Eastern Tianshan

The Chinese Eastern Tianshan is a 300-km-wide, 1000-km-long orogenic collage that occupies a key position in the southern Altaids between the South Tianshan and Beishan (Fig. 1A; Xiao et al., 2004). This area consists of the South Tianshan zone, the Central Tianshan block, the Yamansu arc, and the Dananhu-Haerlik arc (Fig. 1B).

The South Tianshan zone is an accretionary complex located between the Central Tianshan block and the Tarim craton and is bounded to the north by the Kawabulake-Xingxingxia fault (Fig. 1B). This zone includes Silurian–Carboniferous turbidites, Silurian–late Carboniferous ophiolites, cherts, volcaniclastic rocks, and mélanges, and Devonian–early Carboniferous eclogites and blueschists (Gao et al., 1998; Xiao et al., 2004; Du et al., 2019).

The Central Tianshan block is located between the Kawabulake-Xingxingxia and Aqikekuduke-Shaquanzi faults (Fig. 1B), and it contains Precambrian amphibolite-facies migmatitic gneisses, quartz schists, and marbles with U-Pb ages ranging from 1900 Ma to 900 Ma (Hu et al., 2000; Xiu et al., 2002; Liu et al., 2004; Li et al., 2009a; Shi et al., 2010; Huang et al., 2015), as well as Paleozoic arc volcanic and volcaniclastic rocks and intrusions with Ordovician to Triassic ages (Hu et al., 2007; Li et al., 2001; Zhang et al., 2004b, 2005a, 2005b; Mao et al., 2006; Sun et al., 2006).

The Devonian to Permian Yamansu arc is separated from the Central Tianshan block by the Aqikekuduke-Shaquanzi fault (Windley et al., 1990; Shu et al., 1999; Xiao et al., 2004a, 2006, 2008a; Wu et al., 2005b). The Yamansu arc consists of Devonian–Permian calc-alkaline andesites, basalts, rhyolites, tuffs, and volcaniclastic rocks interbedded with weakly metamorphosed, fine-grained clastic and carbonate sediments, and later granitic intrusions (Ji et al., 1994; Ma et al., 1997; Gu et al., 1999; Xiao et al., 2004; Yang et al., 1996, 1998; Du et al., 2018b; Zhao et al., 2018; Han et al., 2019; Long et al., 2020).

The Paleozoic Dananhu-Haerlik arc, which is located between the Kangguer and the Kalameili faults (Ma et al., 1997; Xiao et al., 2004; Li, 2004), contains the Dananhu-Haerlik arc, the late Paleozoic Bogda intra-arc basin, and the late Paleozoic Kangguer forearc. The Ordovician to Permian Dananhu-Haerlik arc consists of Ordovician to Permian calc-alkaline mafic-felsic lavas, volcaniclastic tuffs, and flysch (Ma et al., 1997; Li, 2004; Xiao et al., 2004; Hou et al., 2005; Tang et al., 2006; Mao et al., 2014a). Ordovician–Permian arc-related granitic intrusions are abundant in the arc (Song et al., 2002a; Li et al., 2004; Chen et al., 2005; Hou et al., 2005; Sun et al., 2005, 2007; Wu et al., 2005a; Cao et al., 2006; Guo et al., 2006; Du et al., 2018a; Zhang et al., 2018). The Bogda intra-arc basin, which formed within the Dananhu-Haerlik arc during the Carboniferous to early Permian, is dominated by basalts, dacites, rhyolites, mafic to felsic volcaniclastic rocks, and rare interbedded andesites and sediments; the intrusions are mainly early Permian in age (Gu et al., 2001a, 2001b; Xiao et al., 2004; Li, 2004; Li et al., 2006b; Wang et al., 2010; Chen et al., 2011, 2013; Shu et al., 2011; Gao et al., 2014). The Kangguer forearc, which is located at the southern margin of the Dananhu-Haerlik arc, contains strongly metamorphosed and deformed mélanges that include lenses of Devonian–Carboniferous meta-basalt, tuff, chert, limestone, sandstone, deep-sea turbidites, and ophiolites (Ji et al., 1994; Yang et al., 1996; Ma et al., 1997; Zhou et al., 2001; Xiao et al., 2004; Li et al., 2005, 2008). Also, many early Permian mafic-ultramafic zoned complexes (such as the Huangshan complex) occur along several hundred kilometers of the forearc (Ma et al., 1997; Mao et al., 2002; Han et al., 2004; Xiao et al., 2004; Zhou et al., 2004; Qin et al., 2011).

To date, because of the absence of fossils and geochronological data with which to constrain the tectono-stratigraphic framework and geochemical data to constrain the volcanic and intrusive evolution, many diverse geological models have been proposed for the evolution of the Dananhu-Haerlik arc (Ma et al., 1997; Yang et al., 1996; Feng et al., 2002; Li, 2004; Xiao et al., 2004, 2010, 2015, 2018; Wang et al., 2006).

Geology of the Kalatage Inlier

The Kalatage geological inlier (40 × 10 km; Figs. 1B and 2), which is the focus of this paper, is located in the center of the Paleozoic Dananhu-Haerlik arc within the Turpan Basin (Mao et al., 2010a; Mao, 2014). The geological character and evolution of the inlier are complex. The inlier is an anticline with NW-, NNW- and ENE-trending faults. The inlier, which is surrounded by Cretaceous and younger sedimentary rocks, consists predominantly of low-grade metamorphic Ordovician to Permian mafic to felsic volcanic strata and mafic to felsic intrusions. The core of the geological inlier is mainly composed of Ordovician to Permian mafic-felsic volcanic rocks, volcaniclastic rocks, minor sedimentary rocks, and mafic to felsic granitic intrusions (Mao et al., 2010a; Tang et al., 2006). Devonian to Permian volcanic and sedimentary strata unconformably overlie and surround the volcanic strata of the core (Fig. 2; Table 1). The extensive Ordovician–Carboniferous arc intrusions are composed of diorite, granitic diorite, and granite with minor gabbro. The largest Kalatage intrusion is an early Paleozoic calc-alkaline arc-generated granite with a zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb age of 429 ± 3 Ma (see Fig. 2; Li et al., 2006c; Mao, 2014). There are four types of mineral deposits hosted by the volcanic strata and intrusions in the inlier core (see Fig. 3): (1) early Paleozoic volcanogenic massive sulfide (VMS) deposits such as the Honghai Cu-Zn-(Au-Ag) deposit, which contains 0.27 Mt of Cu and 0.27 Mt of Zn, accompanied by Au, Ag, Ga, In, Tl, Cd, and Ga elements (Mao et al., 2010; Yang et al., 2018; Mao et al., 2019); (2) subvolcanic hydrothermal vein deposits such as the Meiling, Hongshi, and Hongshan Cu (Au) deposits (Fang et al., 2002; Yu et al., 2019); (3) porphyry-skarn deposits like the Yudai and Xierqu Cu (Au) deposits (Mao et al., 2018); and (4) Cu-Ni sulfide deposits related to mafic-ultramafic complexes (e.g., the Yueyawan Cu-Ni deposit; Sun et al., 2019).

The nomenclature of rocks within the Kalatage inlier core has traditionally been based on lithological similarities to established rocks elsewhere, but even these have controversial biostratigraphic ages. For example, the Lower Devonian Kalatage Group (Fig. 2) was assigned an age of early Devonian (D1k) because it underlies the Dananhu Formation (D1d), which has an early Devonian biostratigraphic age (Qin et al., 2001; Tang et al., 2006). The Middle Ordovician Daliugou Formation (O2d) belongs to the Middle Ordovician Huangchaopo (OH) Group and consists of basalt, andesite, dacite, rhyolite, and volcaniclastic rocks. Another adjacent group of basalts, andesites, dacites, rhyolites, and pyroclastic and sedimentary rocks was assigned to the Middle–Upper Silurian Hongliuxia Formation (S2–3h) because it is surrounded by the Dananhu Formation and because the rock assemblage is similar to that of the Daliugou and Hongliuxia Formations in the Haerlik Mountain area in Xinjiang (I.R.G.S., 2003; Xiao et al., 2004; Li et al., 2006c). Mao et al. (2010a) reported a zircon secondary ion mass spectrometry (SIMS) U-Pb age of 416 ± 7 Ma for a dacite, indicating that the volcanic group erupted during the Ordovician to Devonian. In this study, we will focus on the volcanic-sedimentary rocks in the core of the Kalatage inlier in order to reconstruct the stratigraphic framework, which we constrained using the geochemical characteristics of the volcanic and intrusive rocks. In so doing, we aimed to gain a better understanding of the tectonic and evolutionary processes of the Dananhu-Haerlik arc, which have major implications for the tectonic evolution and mineralization of the southern Altaids.

LITHOSTRATIGRAPHY AND SUBDIVISIONS

The lithostratigraphy is based on observations from outcrops and logged drill cores. Three Paleozoic stratigraphic units are defined (Fig. 2; Table 1). The oldest, unit 1, is composed of Ordovician and/or older basaltic and pyroclastic rocks. The middle unit, unit 2, is the overlying Silurian Hongliuxia Group, which is composed of andesitic-felsic volcanic flows, tuffs, and volcaniclastic rocks. The youngest, unit 3, is a Devonian to Carboniferous group that unconformably overlies the Ordovician–Silurian groups and consists of dacitic to rhyolitic flows, tuffs, and volcaniclastic rocks, as well as conglomerates, siltstones, and sandstones.

Unit 1—Basaltic Rocks

A group of basaltic to basaltic-andesitic volcanic and fragmental volcaniclastic rocks is exposed over an area several kilometers wide and tens of kilometers long around the Kalatage granite in the northern Kalatage inlier. In the Hongshan area, where the exposed rocks are 300–500 m wide, only one 500-m-deep drill core has penetrated the entire volcanic pile and reached the underlying Kalatage granite. Other cores in the Honghai and Meiling areas passed through several hundred meters of basaltic rocks without reaching the basement (Fig. 3).

The basalts are composed of olivine, augite, and plagioclase phenocrysts (1%–2%), volcanic glass (>90%), and minor magnetite and ilmenite (Fig. 4A), but some have been highly altered by silicification, sericitization (Fig. 4B), and pyrite enrichment along faults that have undergone hydrothermal activity (e.g., Honghai). Some basalts are weakly altered by chloritization and epidotization and have siliceous and calcareous amygdules filled with pyrite and chalcopyrite.

Unit 2—Andesitic Rocks

This unit consists of mineralized volcanic and fragmental rocks, which are unconformably overlain by dacites and rhyolites; the rocks are only exposed east of Hongshan and north of Hongshi (Fig. 2). The rocks in unit 2 are characterized by andesitic lavas, volcaniclastic breccias, lapilli tuffs and ash, and andesitic fragmental rocks, which have undergone chloritization, epidotization, and pyrite-malachite mineralization.

Unit 3—Andesite-Dacite-Rhyolite

At the bottom of this unit, slightly epidotized and structureless polylithic breccias up 20 m thick unconformably overlie Silurian–Ordovician mineralized rocks (Fig. 3); i.e., they unconformably cover the massive sulfide ore of the Honghai VMS deposit (Fig. 4E). Typically, the clastic rocks are matrix-supported and composed of heterolithic clasts, which mainly consist of fine-grained dacitic crystal clasts, crystal fragments, and mineralized tuffaceous sedimentary fragments of disseminated pyrite, together with massive Cu-Zn breccias, siliceous rocks, and granite (Fig. 4G). The mineralized clasts are common in the lower part of the breccias, and the abundance of these clasts decreases upward. The compositions of the clasts are similar to those of polylithic breccias below highly fossiliferous, early Devonian rocks along the margin of the Kalatage inlier (Fig. 4F). Locally, the matrix has subordinate welded textures, armored lapilli, and plagioclase crystals.

This 300-m-thick intermediate-acidic volcanic pile rests unconformably on different volcanic rocks, e.g., unit 1 basalts, unit 2 polylithic breccias, a massive Cu (Zn) sulfide orebody (Fig. 4E), and volcanic-sedimentary rocks (Fig. 3). The intermediate-acidic volcanic rocks consist of tuffaceous sediments interbedded with intermediate-felsic, coherent and fragmental rocks that include dacitic to rhyolitic lavas, ignimbrites, agglomerates, lapilli tuffs, and volcaniclastic tuffs and breccias, which are interbedded with minor conglomerates, siltstones, sandstones, andesite lavas, and coherent volcaniclastic rocks.

Some dacites also contain multiple welded tuffs, which are separated by unwelded beds of lapilli tuff and interbedded with dacitic lavas (Figs. 4H–4L). The volumetrically dominant lithic clasts and dacites are typically brown, but some are purple-red. The dominant welded tuffs contain fine-grained porphyritic clasts and plagioclase and minor quartz phenocrysts in a fine-grained feldspathic and/or cryptocrystalline glass groundmass (Fig. 4H–4J). The tuffs are composed of volcanic clasts, plagioclase, and subordinate quartz phenocrysts in an ash matrix (Fig. 4K), and they have rhyolitic textures with juvenile, amoeboid-shaped clasts. The welded and delicate volcanic textures indicate deposition in a subaerial to possibly shallow subaqueous environment.

The rhyolites and coherent welded tuffs, located in the Meiling and Hongshan districts, are composed of small lava grains, and plagioclase and quartz phenocrysts in a fine-grained feldspathic and quartzose groundmass.

In the Honghai, Hongshi, and Meiling areas, several thin, coherent clastic sedimentary beds up to 20 m thick overlie dacites in a >100 m2 area (Fig. 5). In some drill cores from the Honghai, Hongshi, Meiling and Yudai areas, thin, coherent tuffaceous sediments are interbedded with dacites; the sediments consist of tuffaceous sandstone and mudstone, bedded tuff, and lapilli tuff. In some drill cores, thin (0.1–10 m) sedimentary beds are typified by large, discontinuous thickness variations, rapid compositional changes, and small distribution areas (the largest is no greater than 0.5 km2; Figs. 5 and 6).

Most of these rocks have been extremely altered by hydrothermal fluids, with the result that high degrees of alteration have masked protolith textures and modified original rock compositions; however, primary protolith textures are locally preserved in layers such as fine-grained breccias. In the center of the hydrothermal system, alterations include clay formation, sericitization, chloritization, and silicification, whereas in the outer ring, alteration includes chloritization, epidotization, and silicification. All the rocks are rich in sulfide minerals, especially disseminated pyrite (1%–20%), and they are transected by copper-quartz veins (composed of quartz, pyrite, chalcopyrite, and sphalerite) and alteration breccias.

Intrusions

In the core of the Kalatage inlier, several large intrusions of dikes and plutons range in composition from gabbro to granite and in age from Ordovician to Carboniferous. Early Paleozoic (Ordovician to Silurian) intrusions composed of gabbro, diorite, granodiorite, and granite have intruded the Ordovician mafic volcanic rocks. Late Paleozoic intrusions that occur within the caldera and along syngenetic faults are mainly intermediate to felsic plugs and dikes consisting of diorite, granodiorite, and granite (Fig. 7). The petrologic characteristics of the intrusions are presented in Table 2 and Figure 7.

U-Pb GEOCHRONOLOGY

Prior to this study, isotopic ages from the Kalatage inlier were limited, and the ages of the supracrustal volcanic rocks were not well defined. In this study, the ages of 16 samples dated by SIMS, LA-ICP-MS, and sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronological methods are presented in Supplemental Tables S1, S2, and S31 and summarized in Table 3. Combined with other age data (Table 3; Mao et al., 2010a, 2014b; Mao, 2014; Deng et al., 2016; Li et al., 2016; Long et al., 2016), we aimed to establish the chronological framework of the magmatism in the Kalatage inlier.

Pre-Devonian Magmatic Activity

Several pre-Devonian intermediate and felsic magmatic rocks have been dated in the Kalatage inlier and Haerlik Mountain area in the Dananhu-Haerlik arc. For example, Long et al. (2016) reported that two diorites, which intruded unit 1 rocks in the Dongerqu district, have zircon SHRIMP U-Pb ages of 442.6 ± 5.3 Ma and 437.1 ± 3.8 Ma (Fig. 2). Li et al. (2016) reported an andesite with an LA-ICP-MS zircon U-Pb age of 438.4 ± 4.9 Ma in the eastern Kalatage inlier. Three zircon SHRIMP U-Pb ages of 462 ± 9 Ma for a granite, 447 ± 11 Ma for a granodiorite, and 448 ± 7 Ma for a quartz diorite (Chao et al., 2006) and a zircon SHRIMP U-Pb age of 429.6 ± 6.2 Ma for a quartz diorite (Guo et al., 2006) were reported in the Haerlik Mountain area. Zhang et al. (2016) and Lin et al. (2017) dated mineralized diorite-granodiorite units in the Yuhai and Sanchakou Cu deposits in the Sanchakou area and reported a zircon SHRIMP U-Pb age of 422 ± 4 Ma and a zircon LA-ICP-MS U-Pb age of 425 ± 4 Ma, respectively.

For this study, we obtained two zircon SIMS U-Pb ages from a diorite and a granodiorite dike that intrude basaltic rocks. The zircons in the diorite (08KL03–5) were fine grained, mostly colorless, transparent, and well crystallized, with grain sizes of (40–50) × (50–100) μm, and cathodoluminescence (CL) images revealed that the zircons had a single composition and typical rhythmic magmatic zones. The samples concentrated in a small area along the concordia line, and 14 analyzed zircon grains gave a 206Pb/238U weighted average age of 445 ± 3 Ma (mean square of weighted deviates [MSWD] = 0.59; Fig. 8A). The zircons of the granodiorite (08KL08–2) were also well crystallized, with grain sizes of (30–80) × (50–100) μm, and CL images showed that all the zircons had typical rhythmic magmatic zones. The samples concentrated in a small area along the concordia line, and 13 analyzed zircon grains gave a 206Pb/238U weighted average age of 439 ± 5 Ma (MSWD = 2.0; Fig. 8B).

Two granites, a granodiorite, and a diorite of the Kalatage granitic pluton gave four new zircon LA-ICP-MS U-Pb ages. The zircons of granite sample (10KL04) were mostly colorless, transparent, and well crystallized, with grain sizes of (20–60) × (80–150) μm, and CL images showed that the zircons had a single composition and typical rhythmic magmatic zones. The samples concentrated on the concordia line, and 16 analyzed zircon grains yielded a 206Pb/238U weighted average age of 429 ± 3 Ma (MSWD = 1.2, n = 16; Fig. 8C). The zircons from granite sample (10KL05) were well crystallized, with grain sizes of (50–100) × (50–200) μm, and CL images revealed that they were relatively dark, with a single composition and typical rhythmic magmatic zones. The samples lie on the concordia line, and 16 analyzed zircon grains that concentrated in a small area along the concordia line gave a 206Pb/238U weighted average age of 429 ± 3 Ma (MSWD = 0.86, n = 16; Fig. 8D). Four zircon grains plotted below the concordia line and indicate minor radiogenic Pb loss (Fig. 8D). Two xenocrystic zircons with 206Pb/238U ages of 491 ± 5 Ma and 500 ± 4 Ma plotted below the concordia line and indicate radiogenic Pb loss (Fig. 8D). The zircons in the granodiorite sample (10KL07) were well crystallized, with grain sizes of (30–120) × (30–200) μm, and CL images showed they were relatively dark, with a single composition and typical rhythmic magmatic zones. The 17 analyzed zircon grains concentrated in a small area along the concordia line and gave a 206Pb/238U weighted average age of 427 ± 2 Ma (MSWD = 1.12, n = 17; Fig. 8E). The zircons from the diorite sample (10KL06) were fine grained with sizes of (30–80) × (50–200) μm, and CL images revealed they were relatively dark, with a single composition and typical rhythmic magmatic zones. In total, 15 analyzed zircon grains concentrated in a small area along the concordia line and yielded a 206Pb/238U weighted average age of 426 ± 2 Ma (MSWD = 1.13, n = 15; Fig. 8F).

The zircons separated from a tuffaceous sediment of unit 2 were mostly colorless, transparent, and well formed, with grain sizes of (40–60) × (40–120) μm. CL images showed that the zircons had rhythmic zones typical of mafic magmatic rocks (Fig. 9A). The zircon SHRIMP U-Pb analytical results from 10 grains are presented in Figure 9A. The 206Pb/238U ages ranged from 425.3 ± 6.1 Ma to 453.3 ± 7.5 Ma, concentrated in a small area on the concordia line, of which 27 analyzed zircons gave a 206Pb/238U weighted average age of 438 ± 4 Ma (MSWD = 1.8, n = 10; Fig. 9A).

In summary, these geochronological data indicate that magmatism was active in the early Paleozoic (462–422 Ma).

Devonian to Carboniferous–Permian Magmatic Activity

Many Devonian to early Permian isotopic ages have been published for volcanic and intrusive rocks in the Dananhu-Haerlik arc (Table 3). For example, in the Tuwu porphyry Cu deposit area, the ages of ore-bearing, arc-related volcanic rocks range from Devonian to Carboniferous (416 Ma to 322 Ma; Rui et al., 2002; Chen et al., 2005; Hou et al., 2005; Wu et al., 2005a; Li et al., 2011). In the Dananhu and Kezier areas, granites have yielded zircon U-Pb ages of 383 Ma to 357 Ma (Song et al., 2002a), and several late Paleozoic (316–298 Ma) intermediate and felsic intrusions have been reported in the Haerlik area (Sun et al., 2005, 2007; Wang et al., 2009). On the southern margin of the Dananhu-Haerlik arc along the Kanggurtag fault, a belt of ultramafic-mafic complexes several hundred kilometers long has ages ranging from 285 Ma to 269 Ma (Mao et al., 2002; Han et al., 2004; Zhou et al., 2004; Li et al., 2006b; Han et al., 2010; Qin et al., 2011), and an early Permian adakitic granite in the Shanchakou porphyry Cu deposit has an age of 278 ± 4 Ma (Li et al., 2004). In the Turpan Basin and Bogda Mountains, alkali volcanic rocks and an intrusion in a fault have ages ranging from 285 Ma to 269 Ma (Chen et al., 2011).

Devonian to Carboniferous magmatic rocks are abundant in different stratigraphic units and multiple intrusions. In the margins of the Kalatage inlier, Devonian to Permian magmatic and fossiliferous sedimentary rocks are extensive (XBGMR, 1993; I.R.G.S., 2003). Yu (2016) reported two SHRIMP zircon U-Pb ages of 318 ± 4 Ma and 313 ± 7 Ma from purple dacites from a deep core in the Meiling Cu (Au) deposit, and Mao et al. (2010a) reported a U-Pb age of 416 ± 6.7 Ma from dacitic rocks at the bottom of unit 3 in the Honghai deposit. We dated five extrusive and four granitic intrusive rocks by LA-ICP-MS (Figs. 8 and 9), and the results indicate that these rocks have ages that range from Devonian to Carboniferous (416–300 Ma), which are consistent with the ages of Devonian–Permian magmatic and fossiliferous sedimentary rocks that are extensive in the margin of the Kalatage inlier (I.R.G.S., 2003; XBGMR, 1993).

The zircons of dacite sample (09HSB01) from the Hongshan area are mostly colorless, transparent, and well formed, with grain sizes of (40–90) × (60–150) μm, and CL images showed that they had a relatively singular composition and typical rhythmic magmatic zones. The zircon SIMS U-Pb analytical results from 16 grains are presented in Figure 9B. They concentrated on the concordia line and yielded a 206Pb/238U weighted average age of 351.1 ± 4.4 Ma (MSWD = 2.5, n = 16; Fig. 9B). In the Honghai area, a dacite (08KL03–10) from the top of a drill core, which was covered by volcanic and sedimentary rocks, contained zircons that were mostly colorless, transparent, and well formed, with grain sizes of (60–100) × (80–120) μm, and CL images showed that the zircons had a relatively singular composition and typical rhythmic magmatic zones. The zircon SIMS U-Pb analytical results from 13 grains are presented in Figure 9B. They concentrated on the concordia line and gave a weighted age of 353.6 ± 2.8 Ma (MSWD = 0.38, n = 13; Fig. 9C).

A purple dacitic ignimbrite (12KL20) from the top of a volcanic and sedimentary sequence in the Meiling area (Fig. 5) was dated by the LA-ICP-MS U-Pb method. The zircons had grain sizes of (30–100) × (50–150) μm, and CL images showed that some had core-rim and solution structures (Fig. 9D). Most of the zircons plotted below the concordia line, which reflects radiogenic Pb loss (Fig. 9D). The 15 youngest zircon grains concentrated in a small area and plotted a little below the concordia line, yielding a weighted age of 301 ± 1 Ma (MSWD = 0.77, n = 15; Fig. 9D). Two groups of xenocrystic ages based on three analyses yielded 206Pb/238U weighted ages of 334 ± 2 Ma and 398 ± 2 Ma. Two spot ages that clearly reflected radiogenic Pb loss plotted far below the concordia line (206Pb/238U ages of 300 ± 2 Ma and 292 ± 2 Ma). These data are consistent with zircon grains that have core-rim and solution structures, as indicated by their CL images (Fig. 9D).

Two rhyolitic lava samples, collected from the top of volcanic and sedimentary sequences in the Hongshan and Meiling areas, were from subvolcanic hydrothermal vein–type Cu (Au) deposits, which were dated by the LA-ICP-MS U-Pb method. The zircons from the Hongshan sample (12KL18) were well crystallized, with grain sizes of (20–100) × (20–200) μm, and CL images showed typical rhythmic magmatic zones, and some zircons indicated solution structures (Fig. 9E). The nine youngest zircon grains concentrated in a small area and plotted along the concordia line, and these gave a weighted age of 301 ± 2 Ma (MSWD = 1.8, n = 9; Fig. 9E). Four grains were xenocrystic (206Pb/238U ages of 338 ± 2 Ma, 357 ± 2 Ma, 420 ± 3 Ma, and 422 ± 2 Ma) and have undergone minor radiogenic Pb loss (Fig. 9E). The zircons from the Meiling sample (12KL19) were well crystallized, with grain sizes of (20–100) × (20–150) μm, and CL images showed typical rhythmic magmatic zones; some zircon grains had solution structures (Fig. 9F). The five youngest zircon grains concentrated in a small area and plotted along the concordia line, and these yielded a weighted age of 299 ± 2 Ma (MSWD = 0.68, n = 5; Figs. 4 and 9F). Three grains (206Pb/238U ages of 324 ± 2 Ma, 465 ± 3 Ma, and 480 ± 2 Ma) and a group of grains (with a weighted age of 436 ± 2 Ma, MSWD = 0.37, n = 6) were xenocrystic and indicated minor radiogenic Pb loss (Fig. 9F).

Zircons from four granites that intruded the dacitic volcanic-sedimentary sequence from the Meiling area were dated by LA-ICP-MS U-Pb methods, and results are presented in Figure 8. A granodiorite dike sample (13KL02) from north of the Meiling Cu (Au) deposit had well-crystallized zircons with grain sizes of (50–100) × (80–150) μm, and CL images revealed that they have a singular composition and typical rhythmic magmatic zones. The 14 analyzed zircon grains concentrated in a small area along the concordia line and yielded a 206Pb/238U weighted average age of 334 ± 4.7 Ma (MSWD = 0.35, n = 14; Fig. 8G). The zircons of granite sample (13KL10) from north of the Meiling Cu (Au) deposit were well crystallized, with grain sizes of (60–100) × (100–160) μm, and CL images revealed that they had a singular composition and typical rhythmic magmatic zones. The 14 analyzed zircon grains concentrated in a small area along the concordia line and gave a 206Pb/238U weighted average age of 322.8 ± 3.6 Ma (MSWD = 0.68, n = 14; Fig. 8H).

The zircons from a mineralized granite porphyry (13KL05) from the Meiling Cu (Au) deposit were well crystallized, with grain sizes of (40–80) × (80–200) μm, and CL images showed they had a single composition and typical rhythmic magmatic zones. The 16 analyzed zircon grains concentrated in a small area along the concordia line and gave a 206Pb/238U weighted average age of 323.4 ± 4.2 Ma (MSWD = 0.51, n = 16; Fig. 8I). A granitic dike (10ZB46), which intruded basaltic and dacitic lavas, lapilli tuffs, ignimbrites, and volcaniclastic sedimentary rocks, contained zircons that were well crystallized, with grain sizes of (30–50) × (70–120) μm, and CL images showed they had a singular composition and typical rhythmic magmatic zones. The 17 analyzed zircon grains concentrated in a small area along the concordia line and yielded a 206Pb/238U weighted average age of 314 ± 2 Ma (MSWD = 0.32, n = 17; Fig. 8J). Mao et al. (2018) reported two U-Pb ages of 389.7 ± 2.5 Ma and 382.1 ± 2.4 Ma for diorites from the Yudai and Xierqu districts, respectively.

In summary, these zircon U-Pb isotopic ages of rocks and mineralization (e.g., sulfide Re-Os isotopic ages) indicate that magmatism was active from the Devonian to early Permian (416–269 Ma) in the Dananhu-Haerlik arc.

Stratigraphic Divisions and Framework

Several isotopic ages recently reported from volcanic and intrusive rocks in the Kalatage inlier indicate that the volcanic rocks range in age from the Ordovician to Carboniferous (Table 3). In this study, we investigated the field relations, regional stratigraphy, and geochronology in order to reconstruct the following stratigraphic framework for the Kalatage inlier (Fig. 10; Table 1).

Ordovician Volcanic Sequence

Pre-Devonian volcanic rocks of the Kalatage inlier mainly consist of basalt, andesitic basalt, and volcaniclastic rocks. Due to the lack of geochronological data, their ages are best constrained by crosscutting intrusions and by the ages of cover rocks. The 445–426 Ma granites, granodiorites, and diorites that intrude unit 1 rocks provide a minimum age for the basalts and andesitic basalts (Fig. 8; Table 1). The oldest granodioritic intrusion has a zircon U-Pb age of 445 ± 3 Ma, and thus unit 1 must be Ordovician or older. Additionally, the cover sedimentary tuffs and massive sulfide ores have ages of 438–434 Ma (Figs. 3 and 10; Mao, 2014; Deng et al., 2016), which suggests that the minimum age of these mafic volcanic rocks is older than 445 Ma.

There is a consensus that the unit 1 basalt rocks are older than 445 Ma. Therefore, combined with regional stratigraphic divisions, we assign these volcanic rocks to the Ordovician Huangchaopo (OH) Group. Several Late Ordovician (462–447 Ma) intermediate and felsic intrusions in the Haerlik Mountain area, which is 200 km east of the Kalatage inlier, were reported by Cao et al. (2006).

Early Silurian Volcanic Sequence

The unit 2 volcanic rocks are stratigraphically above unit 1. Our new SHRIMP U-Pb dates demonstrate that tuffaceous sediments have an age of 437.8 ± 3.9 Ma (Figs. 3 and 10). The VMS deposit at the top of the sequence (Figs. 3 and 10) has a chalcopyrite Re-Os isochronal age of 434 ± 4 Ma (Deng et al., 2016), and a mineralized fragmental sequence has a sericite age of 424 ± 7 Ma (Mao et al., 2010a). Li et al. (2016) reported that an andesite in the eastern Kalatage inlier has a LA-ICP-MS zircon U-Pb age of 438.4 ± 4.9 Ma.

In summary, there was major intermediate volcanism and mineralization in the early Silurian (445–434 Ma). Combined with regional and previous stratigraphic divisions in the Kalatage inlier, we assign these volcanic rocks to the Hongliugou Formation (S1h).

Middle to Late Silurian Sedimentary Sequence

A several-meter-thick polymictic conglomerate contains clasts of granite, volcanic and volcaniclastic rocks, chert, tuff, and massive sulfides in a matrix of pyroclasts and ash. The sequence is stratigraphically unconformable on unit 2 and is covered by Devonian (416 ± 7 Ma) dacitic volcano-sedimentary rocks at the bottom of unit 3 (Figs. 3 and 10; Table 1; Mao et al., 2010a), suggesting that these rocks are middle to late Silurian in age (434–416 Ma).

Devonian to Carboniferous–Permian Volcano-Sedimentary Sequence

In the core of the Kalatage inlier, unit 3 Devonian to Carboniferous (416–299 Ma) volcano-sedimentary rocks unconformably overlie mid- to late Silurian polymictic conglomerates and/or an early Silurian and/or Ordovician volcano-sedimentary sequence (Figs. 3 and 10; Table 1). In the Honghai area, dacitic rocks (416 ± 7 Ma) that overlie the Honghai massive orebody are 8–18 m.y. younger than dated sulfides that were collected from a lensoidal massive orebody at the top of the mineralized fragmental sequence, which indicates that a significant unconformity is present at the base of the Dananhu Formation. The upper age of the volcanic sequence is directly constrained by the age of overlying dacitic rocks (354 ± 3 Ma) in the Honghai area (Figs. 3 and 10) and by the age of red dacitic rocks (351 ± 4 Ma) that overlie the Dananhu Formation north of the Hongshan area. In the Honghai-Meiling area, brown, red, and purple dacites and rhyolites at the top of unit 3 have U-Pb crystallization ages of 301 ± 1 Ma, 301 ± 2 Ma, and 299 ± 2 Ma (Figs. 9D, 9E, and 9F). Notably, Yu (2016) reported that two purple dacites yielded zircon SHRIMP U-Pb ages of 313 ± 7 Ma and 318 ± 4 Ma, respectively, from a 120–150-m-deep drill core in the Meiling district. Therefore, these geochronological data indicate that the upper part of the volcanic sequence formed during the Devonian to Carboniferous (416–299 Ma). These isotopically defined divisions are consistent with the ages of Devonian to Carboniferous highly fossiliferous sediments as listed in Table 1, and they also are similar to sediments reported elsewhere in the Dananhu-Haerlik arc (Zuo et al., 1990, 1991; XBGMR, 1993; Ma et al., 1997; Song et al., 2004; Xiao et al., 2004).

GEOCHEMISTRY

We systematically investigated the geochemistry of rocks in the Kalatage inlier to improve the framework and reconstruction of the Paleozoic Dananhu-Haerlik volcanic arc. The major- and trace-element data are listed in Supplemental Tables S4 and S5, respectively (see footnote 1). The Rb-Sr and Sm-Nd isotopic data of the gabbroic, granitic, and rhyolitic samples are given in Table 4.

Major- and Trace-Element Geochemistry

Ordovician Mafic Volcanic Rocks

In the Ordovician, a fissure volcanic eruption gave rise to amygdaloidal basaltic, gabbroic, and volcaniclastic rocks on the oceanic floor from Kalatage to Haerlik in the Dananhu-Haerlik arc (XBGMR, 1993). During mineralization, these rocks underwent different degrees of epidotization, chloritization, and carbonization, but we collected fresh samples for our analyses.

Fresh basaltic rocks have pyroxene and plagioclase phenocrysts, and a few samples exhibited weak chloritized alteration. The rocks are characterized by relatively low TiO2 (0.52–0.93 wt%) contents, high Mg# ratios (51–63), and Na2O > K2O. In a Zr/Ti versus Nb/Y diagram (Winchester and Floyd, 1977), all samples plot within basalt to andesite-basalt fields (Fig. 11A), and in a Zr versus Y diagram, the rocks plot in the tholeiitic and transitional series areas. These basaltic rocks are enriched in rare earth elements (REEs: La/YbN = 1.45–3.39; Fig. 12A) and feature symmetrical light REE (LREE) and heavy REE (HREE) enrichments (La/SmN = 1.18–2.16, Gb/YbN = 1.06–1.36). Most samples have a weak positive Eu anomaly (Eu/Eu* = 0.92–1.13), which indicates cumulus crystallization of plagioclase during formation. A clear symmetrical negative Ce anomaly (Ce/Ce* = 0.89–0.93) suggests input of subduction-related material into the parent magma and/or interaction with seawater (Hole et al., 1984). In a primitive mantle–normalized trace-element diagram (Fig. 12B), these basaltic rocks display depletions in Nb, Ta, and Ti relative to large ion lithophile elements (LILEs) and REEs, and enrichments in LILEs (Rb, Ba, K, U, and Sr), Pb, and Th.

Most samples of highly epidotized and chloritized basalts and andesitic basalts were collected from below the massive orebody in the Honghai area; however, sample 08KL01–3, from north of the Hongshi district, is also highly epidotized (Fig. 4B). The major-element compositions of the strongly altered basaltic rocks (silicified, sericitized, chloritized, and pyritized) are notably different from the unaltered rocks, being characterized by high SiO2 contents (58.57–76.07 wt%), loss on ignition (LOI) values (3.2–6.7 wt%), and K2O contents (1.78–3.89 wt%); relatively low TiO2 (0.32–0.63 wt%) contents; and K2O > Na2O resulting from the sericitization.

Early Silurian Intermediate Volcanic Rocks

Nineteen volcanic breccia samples from the core of the Honghai Cu-Zn deposit were chemically analyzed. Their major-element compositions were notably changed by the alteration, with large ranges of SiO2 contents (59.45–78.60 wt%), TiO2 contents (0.3–0.6 wt%), and Mg# values (14–58). These rocks have a geochemical pattern that is transitional between the tholeiitic and calc-alkaline series (Fig. 11B). In a Zr/Ti versus Nb/Y diagram (Winchester and Floyd, 1977), most samples plot within the andesite field, and some plot within the dacite field (Fig. 11A). Most rocks belong to the transitional magmatic series, but a few plot in the tholeiitic and calc-alkaline fields on a Zr versus Y diagram (Fig. 11B). These rocks are enriched in REEs (La/YbN = 1.37–5.53; Fig. 12C) and have symmetrical LREE-enriched patterns (La/SmN = 1.53–3.33). Most of the rocks have a weak negative Eu anomaly (Eu/Eu* = 0.48–1.27), and all display depletions in Nb, Ta, and Ti relative to LILEs and REEs, and enrichments in LILEs (Rb, Ba, K, U, and Sr), Pb, and Th (Fig. 12D).

Devonian to Carboniferous Intermediate to Felsic Volcanic Rocks

The Devonian–Carboniferous intermediate to felsic volcanic rocks are almost unaltered and range in composition from andesite to rhyolite (Fig. 13A). Most samples are characterized by Na2O > K2O, except for the rhyolites (Na2O < K2O). The majority of the intermediate to felsic rocks plot in the low-K to high-K calc-alkaline field in a SiO2 versus K2O diagram (Fig. 13B). All samples are characterized by REE enrichment patterns (La/YbN = 1.48–5.66; Fig. 12E) with negative Eu anomalies (Eu/Eu* = 0.33–0.95). The samples are enriched in LILEs (Rb, Ba, U, and K), Th, Pb, Zr, and Hf and depleted in Nb, Ta (Nb/La = 0.19–0.55 and Zr/Nb = 25–82), Sr, P, and Ti in primitive mantle–normalized diagrams (Fig. 12F). The rhyolite rocks have relatively stronger negative Eu anomalies (Eu/Eu* = 0.33–0.52; Fig. 12G) than the other rocks. These samples are also enriched in Rb, K, Pb, Zr, and Hf and, compared to the other rocks, exhibit stronger Nb depletions (Nb/La = 0.21–0.31 and Zr/Nb = 55–67) and P, Sr, and Eu anomalies.

Late Ordovician Granodiorite Dikes

Late Ordovician granodiorite dike samples have low K2O contents (0.3–0.8 wt%) and Na2O > K2O (Fig. 13A), and they plot as dacite in a SiO2 versus (K2O + Na2O) diagram (Fig. 13A) and in the low-K calc-alkaline field in a SiO2 versus K2O diagram (Fig. 13B). These rocks are characterized by a weak enrichment in REEs (La/YbN = 2.51–2.91; Fig. 14A), have a weak negative Eu anomaly (Eu/Eu* = 0.69–0.79), and have a symmetrical negative Ce anomaly (Ce/Ce* = 0.92–0.93). The rocks are depleted in Nb, Ta, and Ti relative to LILEs and REEs and are enriched LILEs (Rb, Ba, K, and U) and Pb, Th, Zr, and Hf in a primitive mantle–normalized trace-element diagram (Fig. 14B).

Early Silurian Diorite

An early Silurian diorite dike (with an age of 439 ± 5 Ma) has a narrow range of SiO2 contents (60.3–60.8 wt%) and Na2O > K2O (Fig. 13). The samples plot in the andesite field in a SiO2 versus (K2O + Na2O) diagram (Fig. 13A) and in the medium-K calc-alkaline field in a SiO2 versus K2O diagram (Fig. 13B), and they are enriched in LREEs (La/YbN = 4.86–5.28; Fig. 14C) and have a symmetrical negative Eu anomaly (Eu/Eu* = 0.81–0.87) and a Ce anomaly (Ce/Ce* = 0.92–0.94). Furthermore, they are depleted in Nb, Ta, and Ti relative to LILEs, and REEs, and they are enriched in LILEs (Rb, Ba, K, U, and Sr), Pb, and Th in a primitive mantle–normalized trace-element diagram (Fig. 14D).

Middle to Late Silurian Kalatage Pluton

This mid- to late Silurian pluton, which is the largest intrusion in the Kalatage complex, is characterized by a wide range of SiO2 (58.5–79.3 wt%) and K2O (0.2–2.2 wt%) contents, and Na2O > K2O. The pluton samples plot in the andesite, dacite, and rhyolite fields in a SiO2 versus (K2O + Na2O) diagram (Fig. 13A) and in the low-K calc-alkaline to medium-K calc-alkaline field in a SiO2 versus K2O diagram (Fig. 13B). The rocks are enriched in REEs (La/YbN = 1.04–4.21; Fig. 14E), and the granites and diorites have negative or no Eu anomalies (Eu/Eu* = 0.58–1.03), indicating plagioclase fractionation. The rocks are depleted in Nb, Ta, Ti, and Sr relative to LILEs and REEs, and they are enriched in LILEs (Rb, Ba, K, and U), Pb, and Th in a primitive mantle–normalized trace-element diagram (Fig. 14F).

Devonian to Carboniferous Arc Intrusions

Devonian to Carboniferous intrusions have similar geochemical characteristics as equivalent volcanic rocks of the same age, which are characterized by a wide range of SiO2 (59.5–78.2 wt%) and K2O (0.3–5.1 wt%) contents. Most samples have a Na2O > K2O pattern, except for late Carboniferous intrusions and dikes (with Na2O < K2O) located in the center of the calderas (e.g., the granitic dikes in the Meiling area and an intrusion in the Honghai area). The samples plot in the andesite-dacite field in a SiO2 versus (K2O + Na2O) diagram (Fig. 13A) and in the low-K calc-alkaline to high-K calc-alkaline field in a SiO2 versus K2O diagram (Fig. 13B). The late-stage intrusions and dikes (with Na2O < K2O) plot as medium-K to high-K calc-alkaline magmas.

These rocks are enriched in REEs (La/YbN = 1.75–4.63; Fig. 14G) and have a negative to weak positive Eu anomaly (Eu/Eu* = 0.38–1.08), which is indicative of plagioclase fractionation. These rocks display depletions in Nb, Ta, and Ti compared to LILEs and REEs and are enriched in LILEs (Rb, Ba, K, U, and Sr), Pb, and Th (Fig. 14H). Compared to the other rocks, the late-phase intrusions and dikes (late Carboniferous) have stronger Eu negative anomalies (Eu/Eu* = 0.47–0.92) and are enriched in Rb, K, Pb, Zr, and Hf with stronger Nb depletion and P, Sr, and Eu anomalies (Figs. 14I and 14J).

Isotopic Geochemistry

The arc volcanic and intrusive rocks from the Kalatage inlier are characterized by relatively low initial Sr values, (87Sr/86Sr)i = 0.70391–0.70567, and positive εNd(t) values, +4.1 to +9.2, and trend toward decreasing εNd(t) with time (Fig. 15; Table 4).

The Ordovician mafic volcanic rocks have relatively low (87Sr/86Sr)i values (0.70459–0.70541) and high εNd(t) values (+7.5–+8) (Fig. 15A), and the two-stage depleted mantle model ages are young (T2DM = 564–524 Ma; Table 4). Devonian–Carboniferous intermediate to felsic volcanic rocks have relatively low (87Sr/86Sr)i values (0.70505–0.70523) and high εNd(t) values (+6.4–+8.8) (Figs. 15A and 15B) and young T2DM (587–346 Ma; Table 4).

The Ordovician granodiorite dikes are characterized by relatively low (87Sr/86Sr)i values (0.70519), high εNd(t) values (+7.2), and young T2DM (598 Ma; Fig. 15). An early Silurian diorite has relatively low (87Sr/86Sr)i values (0.70412), high εNd(t) values (+7.5), and young T2DM (570 Ma; Fig. 15). A mid–late Silurian pluton has relatively low (87Sr/86Sr)i values (0.70391–0.70405) and the highest εNd(t) values (+7.8–+9.2) of the analyzed magmatic rocks in the Kalatage inlier (Fig. 15). Only one Devonian–Carboniferous granite sample (08KL11) collected from the Honghai caldera was analyzed for Rb-Sr and Sm-Nd isotopes. This high-K calc-alkaline granite had the highest (87Sr/86Sr)i value (0.70567) and the lowest εNd(t) value (+4.1) of the analyzed magmatic rocks in the Kalatage inlier (Fig. 15).

DISCUSSION

Our new geologic, stratigraphic, U-Pb (zircon), and geochemical results enable us to place robust constraints on the age, origin, and evolution of the calderas and volcanos in the Kalatage inlier.

Setting of Volcanic Activity

The occurrence of coherent pillow basalts, amygdaloidal basalts, gabbros, volcaniclastic rocks, and a few clastic rocks suggests that the volcanic Huangchaopo Group formed in a completely subaqueous environment in the Ordovician. On a regional scale, the basaltic rocks are discontinuously exposed for more than 200 km from Kalatage to the Dananhu area and farther to the Haerlik Mountain area. This pattern precludes large-scale basaltic eruptions in the Dananhu island arc, and thus, the basaltic rocks were probably extruded in a volcano fissure eruption.

The early Silurian Hongliuxia Formation contains coherent, amygdaloidal basaltic andesite, volcaniclastics, sedimented tuffs, and massive sulfide deposits, which lie conformably on Ordovician mafic rocks. The presence of laminated tuffaceous sediments intercalated with laminated sulfides and a massive Cu-Zn sulfide lens at the top of the volcano-sedimentary pile suggest that the rocks were exposed in a deep ocean floor, comparable to modern seafloor sulfide deposits. The presence of localized tuffaceous volcanic breccias and sedimentary rocks in the Honghai area indicates formation in a vent-proximal setting, such as the upper flank of an oceanic arc volcano.

From the middle to late Silurian (for ∼10 m.y.; Fig. 10), the variably thick polymictic conglomerate (0–20 m) between the early Silurian Hongliuxia Formation and Devonian–Carboniferous shallow-marine volcano-sedimentary rocks was deposited during transition of the arc from a deep oceanic to subaerial setting and associated exhumation of basement rocks. The polymictic conglomerate that contains heterolithic angular clasts represents an immature sedimentary and/or volcaniclastic deposit that formed around the Kalatage inlier. Additionally, the Honghai area features an immature polylithic breccia containing clasts of silica breccia, massive sulfides, granitic breccia, and volcaniclastic tuff. The presence of conglomerates with clasts of weathered granite, silicalite (composed of silica in form of jasper or diatomaceous earth), massive sulfides, dacite, and volcaniclastic tuff suggests that a period of erosion and crustal uplift lasted for up to ∼10 m.y. from 434 Ma to 416 Ma, and it indicates a change in the site of deposition and eruption from deep ocean to subaerial or shallow subaqueous.

During the Devonian to Carboniferous, the core of the Kalatage inlier was an oceanic island arc. In the Early Devonian, debris aprons were composed of volcanic detritus–rich turbidites and volcaniclastic sediments, although several Devonian to Carboniferous polylithic intermediate to felsic volcanic breccias may be caldera collapse breccias that formed from the slumping of oversteepened walls produced by caldera subsidence or from erosion associated with surface uplift. Following the deposition of the polylithic breccias, the island arc was buried under shallow-marine sediments. The calcareous debris limestone aprons can be interpreted as evidence that the island arc was located in a low latitude. The fact that the early Devonian marine limestones are repeatedly intercalated with volcaniclastic rocks precludes the possibility that the volcanism was only located in the center of the island arc. Above these limestones, there are red, white, and brown clastic sediments interbedded with volcaniclastic rocks and dacitic lavas, which suggests that deposition occurred under shallow-marine or subaerial conditions. The presence of several polylithic breccias indicates that the island arc underwent multiple episodes of rifting and subsidence, and the presence of calcareous debris limestone lenses may indicate that the Kalatage volcano was a low-latitude atoll island that erupted during the Early Devonian. The alignment of the Kalatage, Dacaotan, and Dananhu volcanos likely constituted an island-arc chain.

The Devonian to Carboniferous rocks in the Kalatage inlier core mainly consist of red and brown volcaniclastic, intermediate-felsic, and volcano-sedimentary rocks. Fluvial to shallow-marine sediments are intercalated with volumetrically minor but widespread intermediate-felsic volcanic and volcaniclastic rocks. The volcanic welding textures in the volcanic rocks suggest subaerial deposition in hot, pyroclastic flows; welding textures form in modern volcanic deposits due to the heat and pressure of overlying flows (Lipman, 1984; Geshi et al., 2002; Cole et al., 2005; Acocella, 2007). The Kalatage inlier contains several caldera centers at Yudai, Hongshan, AP7, Meiling, Hongshi, and Honghai, which are aligned roughly NW-SE, and that is parallel to the general volcano orientation; similar aligned caldera chains are typical of modern volcanic island arcs (Francis, 1983; Lipman, 1984; Geshi et al., 2002).

The distribution of these subaerial, volcanic calderas is discontinuous; they were individual caldera centers within the volcano. Outwards from the volcano’s center, the lithologies change from proximal to distal as follows: volcanic rocks (dacite and/or rhyolite lava ± breccia lava) + agglomerates ± volcaniclastic/tuff sediments ± maar lacustrine sediments, volcaniclastic material (volcano breccia, tuff breccia, and breccia tuff) + ignimbrites ± volcanic rocks to volcaniclastic flows + ignimbrites (Figs. 4 and 5). These lithologies are consistent with those observed around a modern, explosive volcano in an island-arc setting (Lipman, 1984; Acocella et al., 2012). The Devonian and Carboniferous sedimentary rocks are indicative of deposition in a shallow-marine and/or subaerial environment.

In summary, Dananhu-Haerlik arc was a typical oceanic island arc from the Devonian to Carboniferous, which is consistent with recent regional studies (Xiao et al., 2004; Han et al., 2006; Li et al., 2008; Chen et al., 2011; Zhang et al., 2016; Li et al., 2016). During the Early Devonian, the arc was situated in a low latitude, but from the Middle Devonian to late Carboniferous, the arc was in the Northern Hemisphere (Xiao et al., 2004, 2008a, 2008b).

Tectonic Setting of Magmatism

Ordovician Magmatism

Geochemical analyses reveal that the Ordovician basaltic rocks formed from a subduction-related magma with a relatively low TiO2 content; an enrichment in LREEs, LILEs (Rb, Ba, U, K), and Pb, Th, Zr, and Hf; a depletion in Nb, Ta, Eu, P, and Ti (Figs. 12A and 12B); relatively low (87Sr/86Sr)i values (0.70459–0.70541); and high εNd(t) values (+7.5–+8) (Fig. 15). Most samples have Ba/La ratios that exceed 20 (except for two that are lower than 20), which is consistent with an arc environment (Kay et al., 1994). All mafic rocks plot as tholeiitic to calc-alkaline arc basalts on a Ti-Zr-Y ternary diagram (Fig. 11C), and all rocks plot in the arc tholeiitic field, except for a few that have arc calc-alkaline characteristics, in a Ti-V diagram (Fig. 11D). A Y versus Cr plot suggests that these Ordovician basalts have volcanic arc signatures, and they plot in the same field as the Tofua and Mariana Island arcs (Fig. 11E). In a Th-Hf-Ta ternary diagram, the samples plot as volcanic arc basalts (Fig. 11F). These mafic rocks have relatively low (87Sr/86Sr)i and high εNd(t) values (Table 4; Fig. 15), a composition that indicates a largely juvenile-depleted, mantle-derived magmatic source without assimilation of Precambrian continental material. The negative Ce anomalies, which have relatively low Ce/Pb (1.1–6.7) and Nb/U (2.2–11.0) ratios, indicate the presence of subduction-related material in the parent magma (Hole et al., 1984; Klein and Karsten, 1995). The regional geology in the Haerlik Mountain area demonstrates that the Ordovician basaltic rocks belong to an Ordovician immature island arc (Ma et al., 1997; Xiao et al., 2004).

The Late Ordovician granodiorites have a composition typical of low-K calc-alkaline arc magmatism characterized by enrichment in LREEs, LILEs (Rb, Ba, K, and U), Pb, Th, Zr, and Hf; depletion in Nb, Ta, and Ti (Wilson, 2001); low (87Sr/86Sr)i values (0.70519), and high εNd(t) values (+7.2). All rocks plot as volcanic arc granitoids on Y versus Nb and (Yb + Nb) versus Rb diagrams (Figs. 13C and 13D). Several Late Ordovician subduction-related intermediate and felsic intrusions have been reported in the Haerlik Mountain area 200 km east of the Kalatage inlier. For example, a K-feldspar granite, a granodiorite, and a quartz diorite in the Haerlik Mountain area have SHRIMP zircon ages of 462 ± 9 Ma, 447 ± 11 Ma, and 448 ± 7 Ma, respectively (Cao et al., 2006).

The low (87Sr/86Sr)i values, high εNd(t) values, and T2DM model ages of the Ordovician magmatic rocks suggest the arc had no Precambrian continental basement. The T2DM model ages are older than the rock crystallization age, and they may represent interaction between the magma and older arc crustal rocks at depth; that would be consistent with our two zircon xenocrysts dated at ca. 500 Ma. No Precambrian zircon xenocrysts were found, which suggests that the arc did not have a Precambrian continental basement, and this conclusion is consistent with the regional geology (Xiao et al., 2004; Han et al., 2006; Li et al., 2008; Chen et al., 2011; Zhang et al., 2016; Li et al., 2016). Accordingly, the data enable us to conclude that these Ordovician magmatic rocks came from a juvenile source without any assimilation of Precambrian continental material, and we infer that the Ordovician Dananhu-Haerlik arc was a nascent (immature) island arc that developed on oceanic crust.

Early Silurian Magmatism

The geochemical data from the early Silurian andesitic rocks indicate a subduction-generated evolution. These transitional magmatic series volcanic rocks are characterized by depletions in Nb, Ta, and Ti, and enrichments in LILEs (Rb, Ba, K, U, and Sr), Pb, and Th (Wilson, 2001). Most samples have Ba/La ratios that exceed 20, which is consistent with an arc environment (Kay et al., 1994). All samples plot as calc-alkaline arcs on a Ti-Zr-Y ternary diagram (Fig. 11C). The Y versus Cr relationship suggests that the Ordovician basalts, which plot in the same field as the Tofua and Mariana Island arcs and as volcanic arc basalts in a Th-Hf-Ta ternary diagram (Fig. 11F), formed in a volcanic arc (Fig. 11E). Li et al. (2016) reported a subduction-related high-Mg andesite with an LA-ICP-MS zircon U-Pb age of 438.4 ± 4.9 Ma in the eastern Kalatage inlier.

The early Silurian granitic intrusive rocks are medium-K and calc-alkaline rocks. The rocks are typical of arc magmatism being characterized by depletions in Nb, Ta, and Ti, enrichments in LILEs (Rb, Ba, K, and U), Pb, and Th (Fig. 14; Wilson, 2001), relatively low (87Sr/86Sr)i values (0.70412), and high εNd(t) values (+7.5) (Fig. 15). All samples plot as volcanic arc granitoids on Y versus Nb and (Yb + Nb) versus Rb diagrams (Figs. 13C and 13D). Notably, early Silurian arc-related, slab melt–derived adakitic diorite intrusions were reported by Chen et al. (2017) to have a zircon U-Pb age of 432 ± 3 Ma. Long et al. (2016) demonstrated that diorites from the Dongerqu area in the northern Kalatage inlier (Fig. 2) have zircon SHRIMP U-Pb ages of 442.6 ± 5.3 Ma and 437.1 ± 3.8 Ma. All geochronological studies indicate that these diorites formed in the Late Ordovician to early Silurian, and that the formation of the younger diorites was coeval with that of the Honghai VMS deposit. Chalcopyrite and pyrite from the Honghai VMS Cu-Zn deposit have Re-Os isochron ages of 434 ± 4 Ma and 436 ± 2 Ma, respectively (Deng et al., 2016; Mao, 2014; Mao et al., 2019).

Middle to Late Silurian Magmatism

The mid- to late Silurian granitic intrusive rocks are low-K, and calc-alkaline to medium-K calc-alkaline in composition. They are typical of arc magmatism, being characterized by depletions in Nb, Ta, and Ti; enrichments in LILEs (Rb, Ba, K, and U), Pb, and Th (Fig. 14; Wilson, 2001); relatively low (87Sr/86Sr)i values (0.70391–0.70405); and high εNd(t) values (+7.8–+9.2) (Fig. 15; Li et al., 2006c). All the samples plot as volcanic arc granitoids on Y versus Nb and (Yb + Nb) versus Rb diagrams (Figs. 13C and 13D). The mid- to late Silurian subduction-related intermediate intrusions are common in the Dananhu-Haerlik arc and include an arc-related quartz diorite (429.6 ± 6.2 Ma) in the Haerlik Mountain area (Guo et al., 2006) and a subduction-related adakitic diorite-granodiorite (425–422 Ma) in the Sanchakou area (Zhang et al., 2016; Lin et al., 2017). In summary, the Kalatage inlier was an oceanic arc that formed in the middle to late Silurian.

Devonian to Early Permian Magmatism

All the geochemical data from Devonian to Carboniferous volcanic rocks suggest that these intermediate to felsic volcanic rocks were generated by subduction. Low-K to high-K calc-alkaline andesite, dacite, and rhyolite are characterized by low TiO2 contents, depletions in Nb, Ta, and Ti, and enrichments in LILEs (Rb, Ba, K, U, and Sr), Pb, and Th. The negative Ti anomalies of the youngest rhyolite are related to titanite/ilmenite fractionation (Fig. 12J). All trace-element characteristics of the rhyolite suggest intense interaction between a highly fractionated magma and aqueous hydrothermal fluids during an advanced stage of volcanic activity (Jahn et al., 2001; Zhao et al., 1992, 2002). Most samples also have Ba/La ratios that exceed 20, which is consistent with an arc environment (Kay et al., 1994). The rocks have relatively high Pb (1.7–66.3 × 10−6) and Th (0.8–6.1 × 10−6) contents and relatively low Ce/Pb (0.7–3.4) and Nb/U (1.0–6.7) ratios, which are indicative of incorporation of subduction-related material into the parent magma (Dilek et al., 2008; Klein and Karsten, 1995). All the samples plot as volcanic arc granites and syncollisional granites in a Nb versus Y diagram (Fig. 13C) and as volcanic arc granitoids in a (Yb + Nb) versus Rb diagram (Fig. 13D).

These Devonian to Carboniferous intermediate to felsic intrusions, which are composed of low-K to high-K calc-alkaline series rocks, are typical subduction-related arc magmatic rocks. They are typified by low TiO2 contents; depletions in Nb, Ta, and Ti; and enrichments in LILEs (Rb, Ba, K, U, and Sr), Pb, and Th; and they have the highest (87Sr/86Sr)i value (0.70567) and lowest εNd(t) value (+4.1) relative to the older magmatic rocks in the Kalatage inlier.

Several Middle to Late Devonian (390–382 Ma) intrusions, which are products of adakitic magmatism, are located in the Yudai and Xierqu ore districts (Mao et al., 2018), and they have high Sr contents (310–1020 ppm), strong depletion of HREEs (e.g., Yb = 0.80–1.44 ppm), and high Sr/Y ratios (36–90). They plot as diorites and dacites in a total alkali–silica (TAS) diagram and in the low-K to medium-K calc-alkaline composition series (Mao et al., 2018). All the major- and trace-element geochemical features of these rocks indicate an affinity with adakitic rocks (Defant and Drummond, 1990; Drummond et al., 1996; Martin, 1999; Martin et al., 2005; Defant et al., 2002; Moyen, 2009; Moyen and Martin, 2012). Porphyritic rocks plot as adakites in a Sr/Y versus Y discrimination diagram, and they have relatively low (87Sr/86Sr)i values (0.70412–0.70462) and high εNd(t) values (+6.1–+7.0), which are similar to most typical adakites, with Sr-Nd values close to those of mid-ocean-ridge basalt (MORB; Defant and Drummond, 1990; Martin, 1999; Martin et al., 2005; Defant et al., 2002). These adakitic rocks have relatively high MgO (Mg#), Cr, Ni, Co, and V contents; low TiO2 contents (<0.87 wt%); and high Cr/Ni ratios (0.75–1.14), all of which are consistent with a slab melt source that had been modified by mantle wedge interactions (Defant and Drummond, 1990; Sen and Dunn, 1994; Yogodzinski et al., 1995; Martin, 1999; Rapp et al., 1999; Defant et al., 2002; Xu and Ma, 2003; Wang et al., 2007). All these chemical constraints suggest that the adakitic intrusions were derived from a basaltic, subducted oceanic slab source.

Many Devonian to early Permian arc-related volcanic rock ages have been published from the Dananhu-Haerlik arc (Table 3). Seventy-five kilometers southeast of Kalatage, ore-bearing, arc-related andesitic to basaltic volcanic rocks from the Tuwu porphyry Cu deposit range in age from Devonian to Carboniferous with Sm-Nd and zircon U-Pb ages from 416 Ma to 360 Ma (Rui et al., 2002). Additionally, an andesite has a zircon SHRIMP U-Pb age of 336.5 ± 6.6 Ma, and a basalt has an Sm-Nd isochron age of 334 ± 36 Ma (Hou et al., 2005). Also, Li et al. (2011) reported a zircon LA-ICP-MS U-Pb age of 322.9 ± 6.4 Ma for arc-related volcanic rocks in the Kezier area, which is 30 km northwest of the Tuwu deposit.

Several Devonian to Permian arc-related intrusions have been reported in the Dananhu-Haerlik arc. For example, an adakitic granite porphyry in the Chihu porphyry Cu-Mo deposit east of the Tuwu deposit has a SHRIMP U-Pb age of 322 ± 10 Ma (Wu et al., 2005a). Adakitic plagiogranite porphyries in the Tuwu and Yandong porphyry Cu deposits have zircon SHRIMP U-Pb ages of 334 ± 3 Ma and 333 ± 4 Ma, respectively (Chen et al., 2005). Two arc-related granitic batholiths in the Dananhu and Kezier areas have zircon SHRIMP U-Pb ages of 383 ± 9 Ma and 357.3 ± 6.2 Ma, respectively (Song et al., 2002a). Some late Paleozoic arc-related intermediate and felsic intrusions in the Haerlik Mountain area include a diorite and a granite with zircon SHRIMP U-Pb ages of 316 ± 3 Ma and 311 ± 9 Ma, respectively (Sun et al., 2005, 2007), and a granite with a zircon LA-ICP-MS U-Pb age of 298 ± 2 Ma (Wang et al., 2009). A subduction-related, Alaska-type ultramafic-mafic complex belt that is several hundred kilometers long formed along the Kanggurtag fault from 285 Ma to 269 Ma on the southern margin of the Dananhu-Haerlik arc (Xiao et al., 2004). There is also an early Permian subduction-related adakitic granite in the Shanchakou porphyry Cu deposit that has an age of 278 ± 4 Ma (Li et al., 2004).

In summary, these data indicate that the Dananhu-Haerlik arc was active from the Devonian to the early Permian (416–269 Ma).

Petrogenesis of the Arc Magmatism

Our geochemical studies suggest that the arc-related volcanic rocks in the Kalatage inlier became more felsic over time, changing from Ordovician basalts to early Silurian andesites to Devonian–Carboniferous dacites and finally to late Carboniferous rhyolites. These lithological changes correspond to geochemical changes in composition from tholeiitic to transitional to low-K and medium-K calc-alkaline to high-K and medium-K calc-alkaline (Figs. 11 and 13), with progressive enrichments in K (Fig. 16A). Overall, the arc volcanic rocks generally have Na2O > K2O patterns, except for the last phase of rhyolites and some hydrothermally altered samples, which have Na2O < K2O patterns. Except for the altered samples, the rocks became enriched in K2O, and their TiO2 contents and Na2O/K2O ratios decreased over time (Figs. 16B and 16C). In addition, characteristic trace elements show distinctive changes over time. The total REE (ΣREE) contents and REE fractionation increase (Figs. 16D and 16E), and negative Eu anomalies become stronger over time (Fig. 16F). The Pb and Yb contents and Th/Yd ratios increase with time, except for some of the final rhyolites, in which these parameters notably decrease (Figs. 16G–16I). The rocks contain increasing enrichments in LILEs (Rb, Ba, K, Sr, U) and Pb over time and later depletions in high field strength elements (HFSEs; Wilson, 2001). All the volcanic rocks plot in the volcanic arc fields in Figures 11 and 13. The arc-related intrusions in the Kalatage district consist of diorites, granodiorites, and granites, and the Devonian and Carboniferous intrusions have similar geochemical characteristics as their volcanic counterparts. Overall, these rocks become more felsic with time, although there is not a uniform change in SiO2 content from the Ordovician to the late Carboniferous. The arc intrusions became enriched in K (Fig. 17A) and changed in composition from Ordovician low-K calc-alkaline compositions to early Silurian medium-K calc-alkaline compositions, to mid-Silurian, Devonian, and Carboniferous low-K and medium-K calc-alkaline compositions, and finally to late Carboniferous high-K and medium-K calc-alkaline compositions. In these rocks, the Al2O3/(CaO + Na2O + K2O) (A/CNK) ratios increased and the Na2O/K2O ratios decreased with time (Figs. 17B and 17C). The arc intrusions show a large range in characteristic trace elements at different times, but there were no obvious, regular changes over time (Figs. 17D–17F). The rocks are characterized by weakly to moderately fractionated REEs, weak to moderate negative Eu anomalies, enrichments in LILEs (Rb, Ba, K, and U) and Pb, and depletions in HFSEs, all of which are indicative of a volcanic arc (Wilson, 2001). All the intrusive rocks plot in the typical arc field in Figures 13C and 13D. In addition, several dioritic porphyritic intrusions with high Sr/Y ratios were mixed with normal arc intrusive rocks in the Middle and Late Devonian, and these intrusions plot within the field of adakitic rocks (Mao et al., 2018). These intrusions still have relatively high Na2O/K2O ratios and Sr contents, low Y and Yb contents, and relatively low (87Sr/86Sr)i values and high εNd(t) values, which are similar to those of modern MORB (Mao et al., 2018). Accordingly, we conclude that the magma was likely generated by the melting of subducted basaltic oceanic slabs.

The arc magmatism evolved over time from tholeiitic to calc-alkaline, which reflects the maturation of the island arc. The Ordovician tholeiitic basalts and low-K calc-alkaline intrusions are evidence of a nascent arc. The early Silurian transitional-intermediate volcanic rocks and low-K to medium-K calc-alkaline intrusions demonstrate that the arc was in a transitional stage during the evolutionary trend toward a more mature arc. Finally, the large volume of middle Silurian calc-alkaline intrusions suggests a significant transition to a mature stage. The Devonian to Carboniferous arc magmatism evolved from low- and medium-K calc-alkaline to high-K calc-alkaline compositions, reflecting a relatively thick and mature arc. During the Middle to Late Devonian, a significant magmatic change led to a porphyry mineralization event at 390–382 Ma (Mao et al., 2018).

Nature of the Dananhu-Haerlik Arc

In recent decades, no consensus was reached on the nature of the Junggar terrane basement. For example, one model suggested that the Junggar terrane has a Precambrian basement (Xiao et al., 1992; He et al., 1994; Ma et al., 1997; Li, 2004; Charvet et al., 2007; Bazhenov et al., 2003, 2012; Zhang et al., 2013; Xu et al., 2015), but another model envisaged a basement of Paleozoic oceanic crust (Jiang, 1984; Hsü, 1988; Li, 1989; Carroll et al., 1990; Dobretsov et al., 1995; Windley et al., 2007), and a third model proposed a suprasubduction, oceanic-island-arc basement (Coleman, 1989; Chen and Jahn, 2004; Şengör et al., 1993; Jahn et al., 2000; Şengör and Natal’in, 1996; Xiao et al., 2004, 2006, 2008a, 2008b, 2009, 2010, 2013, 2015). In parallel with this controversy, the nature of the Dananhu-Haerlik arc has been debated for several decades. Several authors have argued that the arc formed in a continental active margin on the Turpan-Junggar plate (Xiao et al., 1992; Ma et al., 1997; Feng et al., 2002; Li, 2004; Li et al., 2006a; Muhetaer et al., 2010; Wang et al., 2006), whereas more recent studies demonstrate that the arc is an intra-oceanic island arc (Xiao et al., 2004; Han et al., 2006; Li et al., 2008; Chen et al., 2011; Li et al., 2016; Zhang et al., 2016).

In this study, the volcanic and intrusive rocks have relatively low initial Sr values, (87Sr/86Sr)i = 0.70391–0.70567, and high εNd(t) values (+4.1−+9.2), close to the mantle array (Table 4; Fig. 15A), which is indicative of a largely juvenile depleted mantle–derived magmatic source. Overall, there is a general trend toward decreasing εNd(t) with time (Fig. 15B), which reflects the maturing of the island arc. The T2DM model ages for the volcanic and intrusive rocks in the Kalatage inlier range from 727 to 346 Ma. Most of the T2DM model ages older than the crystallization age of the rocks may represent interaction between the magma and older arc crustal rocks at depth, which is consistent with our two zircon xenocrysts dated at ca. 500 Ma. No Precambrian zircon xenocrysts were found, indicating that the arc was an intra-oceanic arc, which agrees with previous regional studies (Xiao et al., 2004; Han et al., 2006; Li et al., 2008; Chen et al., 2011; Zhang et al., 2016; Li et al., 2016). Therefore, we conclude that the arc was far from any continent and that it matured from the Ordovician to early Permian.

Geodynamic Evolution of the Paleozoic Dananhu-Haerlik Arc

The concept, nature, and geological evolution of the Dananhu-Haerlik arc have long been controversial, and several models have been proposed to explain its genesis. One model suggested that the arc formed by south-dipping subduction of the Kalameili Ocean from the Ordovician to Carboniferous (Xiao et al., 1992; Ma et al., 1997; Feng et al., 2002; Zhang et al., 2004a, 2006; Mao et al., 2019); another model proposed that the arc formed by north-dipping subduction of the Kangguer Ocean from the Ordovician to Carboniferous (Li et al., 2005, 2006c, 2008; Wang et al., 2006; Muhetaer et al., 2010; Wang et al., 2014; Li et al., 2016; Zhang et al., 2016). According to a third model, south-dipping subduction of the Kalameili Ocean formed the Haerlik arc, and north-dipping subduction of the Kangguer Ocean formed the Dananhu arc at different times (Xiao et al., 2004; Li, 2004).

The Kalatage inlier, which is located in the Turpan Basin, is one of the most important parts of the Dananhu-Haerlik arc in the Eastern Tianshan because it contains the best example of a volcanic caldera so far recorded in the entire Altaids. The Dananhu-Haerlik arc underwent a complex sequential evolution (Windley et al., 1990; Ma et al., 1997; Li, 2004; Xiao et al., 2004; Wang et al., 2006). Our geological, stratigraphic, geochronologic, and geochemical studies of the Kalatage inlier enabled us to reconstruct the temporal-stratigraphic-structural sequence of the volcanic and sedimentary rocks, which in turn reflects the magmatic evolution. We consider that the stratigraphic framework and geological evolution of the Haerlik and Dananhu arcs was similar; the Dananhu-Haerlik arc was a nascent (immature) intra-oceanic island arc created by north-dipping subduction during the Late Ordovician–Silurian, and southward accretion formed the Devonian–Carboniferous Tuwu arc belt (Ma et al., 1997; Xiao et al., 2004; Han et al., 2006; Wang et al., 2006; Li et al., 2016). From the Devonian to Carboniferous, the arc increased in maturity, but it also underwent complex tectonic deformation as it grew by north-dipping subduction during the Devonian to early Carboniferous and by back-arc extension and southward magmatic front migration in the early to mid-Carboniferous (Ma et al., 1997; Xiao et al., 2004; Wang et al., 2006; Li, 2004; Muhetaer et al., 2010). Adakitic rocks and the Tuwu porphyry Cu deposit developed during the early Carboniferous (334–333 Ma; Xiao et al., 2004; Chen et al., 2005; Wang et al., 2006, 2007; Zhang et al., 2004a). By the late Carboniferous, as the Kalameili Ocean closed, the Dananhu-Haerlik arc became attached to the Angara margin to the north (Ma et al., 1997; Li, 2004; Xiao et al., 2004).

Because our results have been integrated with published geological data (Ma et al., 1997; Li, 2004; Xiao et al., 2004; Wang et al., 2006; Li et al., 2008; Zhang et al., 2016; Li et al., 2016; Mao et al., 2018; data in Table 1 herein), we modified previous tectonic models to produce a new synthesis and geodynamic history of the Dananhu-Haerlik arc from the Ordovician to the Carboniferous, as follows (Fig. 18):

During the Ordovician, a nascent arc grew by incipient subduction in the Paleo-Asian Ocean, which is evidenced by the large volume of tholeiitic basaltic rocks and a few low-K calc-alkaline felsic intrusions in the Kalatage-Dananhu-Haerlik region (XBGMR, 1993; Ma et al., 1997; I.R.G.S., 2003; Xiao et al., 2004; Cao et al., 2006). During the early Silurian, oceanic ridge subduction caused tholeiitic basaltic arc volcanism to shut down, and the increasing maturity of the arc produced several, new, transitional series of chemically intermediate rocks, including high-Mg andesites (Li et al., 2016), a few low-K calc-alkaline intrusions, adakitic diorites (Long et al., 2016; Chen et al., 2017; Mao et al., 2019), and a VMS-type Cu-Zn-(Au-Ag) deposit (Mao et al., 2010, 2019; Mao, 2014; Deng et al., 2016). In the mid-late Silurian (S2–3), the arc reached peak maturity. The continuous oceanic ridge subduction generated a large volume of calc-alkaline intermediate-felsic rocks from 429 to 426 Ma, together with the Yuhai adakite-related porphyry-type Cu deposits north of Huangshan (Fig. 1B; Guo et al., 2006; Zhang et al., 2016), and it caused contraction in the forearc and uplift of the arc. The uplifted mature arc became emergent in the Early Devonian, when a low-latitude shallow-marine to subaerial atoll formed around the arc.

The Devonian to Carboniferous arc produced subaerial to shallow-marine calc-alkaline intermediate-felsic volcanic and intrusive rocks, which are indicative of a mature arc. From the Devonian to Carboniferous, the magmatic front of the Dananhu-Haerlik arc migrated southward (Xiao et al., 2004; Li, 2004; Mao et al., 2018; see also data in Table 3), and the magmatic products, with their pronounced potassium content, exhibited a progressive evolution from low- and medium-K to high-K calc-alkaline compositions (Rui et al., 2002; Wu et al., 2002; Zhang et al., 2004a, 2006; Sun et al., 2005, 2007; Li et al., 2004, 2005, 2011; Wang et al., 2009; Zhou et al., 2010; Mao et al., 2018; this study). During the mid-Devonian (390–382 Ma), adakitic rocks began to form in response to oceanic slab melting, which led to formation of the Yudai porphyry-type Cu-Au deposit (Mao et al., 2018). In the early Carboniferous, further adakitic activity gave rise to the Tuwu porphyry Cu deposit belt at 334–333 Ma (see Fig. 18; Rui et al., 2002; Wu et al., 2002; Xiao et al., 2004; Chen et al., 2005; Wang et al., 2006, 2007; Zhang et al., 2006; Mao et al., 2018). Back-arc extension led to formation of the Xiaopu interarc basin (Gu et al., 2001a, 2001b; Xiao et al., 2004; Li, 2004; Wang et al., 2010; Chen et al., 2013; Shu et al., 2011; Gao et al., 2014). By the late Carboniferous, the mature Dananhu-Haerlik arc contained high-K calc-alkaline volcanic and subvolcanic intrusions (Wang et al., 2009; Zhou et al., 2010; Zhang et al., 2017), which drove hydrothermal convection that formed the vein-type Cu (Au) deposits in a shallow crust, e.g., the Meiling, Hongshan, and Hongshi vein-type Cu (Au) deposits in the Kalatage inlier (Mao, 2014; Mao et al., 2014a; Yu, 2016; Yu et al., 2016) and the Xiaoshitouquan vein-type Ag-Cu-(Pb-Zn) deposits in the southern Haerlik Mountains area (Zhang et al., 2017). The mature Dananhu-Haerlik (Kalatage) arc finally docked with the Angaran active margin to the north when the Kalameili Ocean closed, resulting in lateral enlargement of the Angaran craton in the late Carboniferous (Ma et al., 1997; Li, 2004; Xiao et al., 2004, 2006, 2008a, 2015). Northward subduction resulted in formation of the Sanchakou adakitic intrusions, felsic intrusions, volcanic rocks, and Alaskan-type mafic-ultramafic complexes in the arc, and magmatic Cu-Ni sulfide deposits during the early Permian (Li et al., 2004; Xiao et al., 2004; Wang et al., 2009; Zhou et al., 2010; Qin et al., 2011; Chen et al., 2011; Mao et al., 2014b).

Implications for Crustal Growth of the Altaids

The Altaid orogen was an important site of juvenile crustal growth during the Neoproterozoic–Phanerozoic (Şengör et al., 1993; Şengör and Natal’in, 1996; Jahn et al., 2000; Jahn, 2004; Windley et al., 2007; Xiao et al., 2010; Wilhem et al., 2012).

After decades of study, a consensus has emerged that the Altaid orogen formed by the successive lateral accretion of small continental blocks, arcs, and accretionary complexes (Coleman, 1989; Windley et al., 1990, 2007; Şengör et al., 1993; Dobretsov et al., 1995; Şengör and Natal’in, 1996; Gao et al., 1998; Buchan et al., 2002; Bazhenov et al., 2003; Xiao et al., 2003, 2004a, 2006, 2008a, 2008b, 2010; Li, 2004; Li et al., 2006a; Shi et al., 2010). The volcanic island arcs and continental arcs are some of the most important accretionary components. Şengör et al. (1993; see also Şengör and Natal’in, 1996) proposed that a single, giant subduction-accretion complex was contorted in an oroclinal bend to form the Altaids. The large amount of crustal growth was facilitated by the emplacement of enormous volumes of volcanic, pyroclastic, and intrusive rocks in and around major volcanic arcs in the late Paleozoic (Han et al., 1997, 1998, 2004; Chen et al., 2000; Jahn et al., 2000, 2001; Chen and Jahn, 2004; Jahn, 2004; Chen and Arakawa, 2005; Mao et al., 2006, 2008, 2010, 2014a, 2018; Wang et al., 2007; Wu et al., 2000, 2002; Yuan et al., 2007). In the Dananhu-Haerlik arc, all the magmatic rocks have low (87Sr/86S) values and high εNd(t) values (+4.1–+9.2), and no old xenocrystic or inherited zircons have ever been reported, suggesting generation from a juvenile depleted mantle with no assimilation of older continental material. This conclusion is consistent with the stratigraphic evolution of the arc, because the arc mainly consists of pyroclastic and volcanic rocks and lacks mature epicontinental clastic sediments (Ma et al., 1997; Xiao et al., 2004). These relations suggest that the Dananhu-Haerlik arc developed offshore and far from any ancient craton or continent, which is consistent with the regional geology (Xiao et al., 2004, 2015, 2018; Han et al., 2006; Li et al., 2008; Chen et al., 2011; Li et al., 2016; Zhang et al., 2016). All these data indicate that the whole Dananhu-Haerlik arc represents a major new growth of continental crust in the Paleozoic. As shown in Figure 1B, the Dananhu-Haerlik arc accounts for at least one third of the total Eastern Tianshan. Therefore, these factual relations demonstrate that this island arc was an important means of crustal growth in the Eastern Tianshan.

In summary, island-arc crustal growth in the Eastern Tianshan was accomplished by successive emplacements of immense volumes of juvenile magma in a long and protracted subduction-generated process, which together with terminal lateral accretion, was a key continental growth mechanism in the southern Altaids.

CONCLUSIONS

Our stratigraphic and geochronologic studies of the Kalatage inlier enabled us to reconstruct the stratigraphic framework of the Kalatage inlier and reveal that the volcanic and intrusive rocks formed in the Ordovician to early Permian; the oldest diorite dike is 445 ± 3 Ma, and the youngest rhyolite is 299 ± 2 Ma.

The arc began as a nascent (immature) intra-oceanic arc during the Ordovician to early Silurian and evolved into a mature island arc in the middle Silurian to early Permian. The environment evolved from deep ocean (438–434 Ma) to subaerial to shallow subaqueous burial (416–300 Ma) The arc volcanic and intrusive rocks are characterized by potassium enrichment, low initial Sr values, (87Sr/86Sr)i = 0.70391–0.70567, positive εNd(t) values, +4.1 to +9.2, and Sr-Nd isotope enrichment.

The results suggest that the long-lived arc was an important crustal growth mechanism in the southern Altaids.

APPENDIX

Zircon U-Pb Dating

Zircon Separation and Mounting

Zircon crystals were separated from crushed rocks using a combination of heavy liquid and magnetic techniques. The grains were handpicked in alcohol under a binocular microscope and mounted in epoxy resin. The zircon mount was polished with a diamond compound to reveal the zircon midpoints. To identify the internal features of the zircons (zoning, structures, alteration, fractures, etc.), cathodoluminescent images were captured with a Cameca electron microprobe at the Institute of Geology and Geophysics (IGG), Chinese Academy of Sciences, Beijing.

SIMS Zircon U-Pb Dating

Analyses were carried out at the IGG, Chinese Academy of Sciences, Beijing. Measurements of U, Th, and Pb were obtained with a Cameca IMS-1280 SIMS instrument following the procedures of Q.L. Li. et al. (2010). The U-Th-Pb ratios and absolute abundances were determined relative to the standard zircon 91500, analyses of which were interspersed with those of unknown grains following operating and data processing procedures similar to those described by Li et al. (2009b). The mass resolution used to measure the Pb/Pb and Pb/U isotopic ratios was 5400. A long-term uncertainty of 1.5% (1 RSD) for standard zircon 206Pb/238U measurements was propagated to the unknowns. Measured compositions were corrected for common Pb using nonradiogenic 204Pb; the corrections are sufficiently small to be insensitive to the chosen common Pb composition. An average of present-day crustal compositions (Stacey and Kramers, 1975) was used for the common Pb, on the assumption that common Pb is largely surface contamination introduced during sample preparation. Uncertainties in the individual analyses in the data tables are reported at the 1σ level, and mean ages for pooled U/Pb (and Pb/Pb) analyses are cited with a 95% confidence interval. Data reduction was conducted using the ISOPLOT/Ex v. 2.49 program (Ludwig, 2001).

LA-ICP-MS Zircon U-Pb Dating

Measurements were carried out at the multicollector ICP-MS (MC-ICP-MS) laboratory at the Tianjin Institute of Geology and Mineral Resources, Tianjin, China. A UP193-FX laser-ablation system with a 193 nm ArF excimer laser was coupled with a Thermo Fisher ICP-MS instrument. Helium was used as the carrier gas to enhance the transport efficiency of the ablated material. The analyses were conducted with a beam diameter of 50 µm, a typical ablation time of ∼30 s for 200 cycles for each measurement, a repetition rate of 10 Hz, and a laser power of 100 mJ/pulse (Wu et al., 2006). U, Th, and Pb concentrations were calibrated using 29Si as an internal standard and NIST SRM 610 as the external standard. The 207Pb/206Pb and 206Pb/238U ratios were calculated using GLITTER 4.0 (Johnson et al., 2008) and were then corrected using the Harvard zircon 91500 as an external standard. The 207Pb/235U ratios were calculated from the values of 207Pb/206Pb and 206Pb/238U. The common Pb correction was applied according to the method proposed by Andersen (2002). The weighted mean U-Pb ages and concordia plots were processed using ISOPLOT 3.0, and the detailed procedure can be found in Xie et al. (2008).

SHRIMP Zircon U-Pb Dating

Zircon grains from sample 14KLHK01 were separated using conventional heavy liquid and magnetic techniques. Representative zircon grains were handpicked under a binocular microscope and mounted in an epoxy resin disc, and the grains were then polished and coated with a gold film. The analyses were conducted at the Chinese Academy of Geological Sciences, Beijing. The zircons were documented with transmitted and reflected light micrographs and cathodoluminescence images to reveal their external and internal structures. The U-Th-Pb isotopic analyses were performed using a sensitive high-resolution ion microprobe (SHRIMP-II). Details of the SHRIMP analytical procedures for zircon were described by Song et al. (2002b). Interelement fractionation in the ion emissions from the zircon grains was corrected relative to the Research School of Earth Sciences (RSES) reference TEMORA 1 (417 Ma; Black et al., 2003). Data reduction was carried out using the ISOPLOT/Ex v. 2.49 program (Ludwig, 2001).

Major- and Trace-Element Analyses

The measurements were carried out at the analytical laboratory of the Beijing Research Institute of Uranium Geology. The major oxides were determined by X-ray fluorescence spectrometry (XRF) with an analytical error of less than 5%. Loss on ignition (LOI) values were determined by heating the sample powder to 1000 °C for 1 h. The trace elements, which included rare earth elements, were determined by ICP-MS analysis. The whole-rock powders (40 mg) were dissolved in screw-top Teflon beakers using an HF + HNO3 mixture for 5 d at 200 °C, and then they were dried and digested with HNO3 at 150 °C for 1 d (twice). Finally, the dissolved samples were diluted to 50 mL with 1% HNO3 before the analyses were conducted. An internal standard solution containing the single element In was used to monitor drift in the mass response during counting, and the precision was generally better than 2%–5%.

Sr-Nd Isotopic Analyses

Analyses were conducted at the IGG, Chinese Academy of Sciences, Beijing. The Rb-Sr and Sm-Nd isotopic ratios were measured with a Finnigan MAT262 thermal ionization mass spectrometer (TIMS), which is housed at the Laboratory for Radiogenic Isotope Geochemistry. The measurements were obtained following the isotope dilution procedures of Zhou et al. (2002) and Chen et al. (2000). A static multicollection mode was used during the measurements, and a traditional cation exchange technique was adopted for chemical separations. The mass fractionation corrections for Sr and Nd isotopic ratios were based on 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Repeated measurements of the La Jolla Nd standard and NBS987 during the measurement period yielded ratios of 143Nd/144Nd = 0.511861 ± 9 (2σ) and 87Sr/86Sr = 0.710254 ± 10 (2σ), respectively. The total procedural blanks for Sr and Nd were ∼10−9 g and ∼10−11 g, respectively.

ACKNOWLEDGMENTS

We thank Lingli Long, Xiaohua Deng, Huayong Chen, Ji’en Zhang, Zhiyoung Zhang, Dongfang Song, and Bo Wan for their helpful discussions. We appreciate editors and formal reviewers for their constructive reviews. This study was financially supported by funds from the National Key R&D Program of China (2017YFC0601201), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18020203), the Chinese National Basic Research 973 Program (2014CB440803), the Chinese Ministry of Land and Resources for Public Welfare Industry Research (201411026–1), the Chinese Geological Survey Project (DD20160071), and the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (QYZDJ-SSW-SYS012). This is a contribution to International Geoscience Programme (IGCP) Project 662, “Orogenic Architecture and Crustal Growth from Accretion to Collision.”

1Supplemental Material. Zircon SIMS, LA-ICP-MS, and SHRIMP U-Pb geochronological dating, major, and trace element data (listed in Supplemental Tables S1 to S5, respectively). Please visit https://doi.org/10.1130/GEOS.S.13232570 to access the supplemental material, and contact editing@geosociety.org with any questions.
Science Editor: Shanaka de Silva
Associate Editor: Alan Whittington
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