Plutons within continental strike-slip shear zones bear important geological processes on late-stage plate transpression and continent-continent collision and associated lateral block extrusion. Where, when, and how intrusions and shearing along transpressional strike-slip shear zones respond to plate interactions, however, are often debated. In this study, we investigated migmatite associated leucogranite and pegmatite from the exhumed >1000-km-long Ailao Shan-Red River left-lateral strike-slip shear zone in Southeast Asia that was active during India-Eurasia plate convergence. Most zircons from the migmatites and leucogranitic intrusions present inherited core-rim structure. The depletion of rare earth element patterns and positive Eu anomalies suggest that leucosomes and leucogranites are the result of crustal anatexis. Zircon rims from the foliated migmatites and leucogranites record U-Pb ages of 41–28 Ma, revealing the timing of the Cenozoic crustal anatexis event along this strike-slip shear zone. Ages of the magmatic zircons from the unfoliated pegmatites provide the timing of the termination of a high-temperature tectono-thermal event and ductile left-lateral shearing at 26–23 Ma. The Cenozoic crustal anatexis along the Ailao Shan-Red River strike-slip shear zone indicates that thickened crust underneath the shear zone involved previously subducted crust. We propose that the Cenozoic thermal state has an important effect on the crustal anatexis and thus on the rheological behavior of the lithosphere by thermal weakening, which plays an essential role in localizing the initiation of the deep-seated lower-crustal shear zone.
Crustal anatexis generally occurs at different geological settings including environments of crustal thickening or post-collisional collapse (e.g., Brown, 2001; Cao et al., 2019; Shakerardakani et al., 2020). Melt segregation and extraction are evidenced by the presence of leucosomes within migmatites, which record the melt flow network through the crust (e.g., Brown, 2001; Searle et al., 2010; Vanderhaeghe, 2009; Yu et al., 2019). Highly evolved peraluminous leucogranites are widespread throughout orogenic belts, especially the leucogranites associated with continental collision formed by partial melting of aluminum-rich metasedimentary rocks buried at lower crustal depth (e.g., Patiño-Douce and Johnston, 1991; Scaillet et al., 1995; Brown, 2007, 2013; Shakerardakani et al., 2020). Exhumation of migmatites and leucogranites within the crust is generally favored by post-collisional collapse or exhumation of metamorphic core complexes along crustal-scale strike-slip shear zones (e.g., Hutton, 1988; Searle, 2006; Searle et al., 2010; Cao et al., 2011a; Liu et al., 2015b, 2020). The presence of melt weakens rock strength and promotes faster deformation, ultimately influencing how orogenic belts form (e.g., Rosenberg and Handy, 2005; Vanderhaeghe, 2009; Alina et al., 2019). Therefore, accurate dating of the timing of melting, determining the protoliths, and characterizing the structure and pressure-temperature (P-T) conditions of partial melting are important for understanding the evolutionary processes and dynamics of the late stages of orogenesis (e.g., Labrousse et al., 2011).
The Sanjiang (Jinshajiang, Lancangjiang, and Nujiang) region in Southeast Asia, and its lithospheric structure prior to Cenozoic times was dominantly characterized by sub-parallel sutures, subduction-modified mantle and crust, Mesozoic basins between sutures, and primary polymetallic ore accumulations (Fig. 1) (e.g., Deng et al., 2014). During Cenozoic times, the Sanjiang region has undergone significant crustal thickening, tectonic extrusion, and reorientation of structures triggered by the India-Eurasia collision. Several continental-scale exhumed strike-slip fault zones, such as the Ailao Shan-Red River shear zone (ASRR-SZ), Chongshan shear zone, and Gaoligong shear zone developed in the Sanjiang region (Leloup et al., 2001; Searle et al., 2010; Cao et al., 2011a, 2011b, 2017; Chen et al., 2019; Liu et al., 2020) (Figs. 1 and 2). These Cenozoic strike-slip shear zones have been one of the best places to study the relationships between tectonics, magmatism, thermal evolution, and dynamics of intracontinental deformation in Southeast Asia (Tapponnier et al., 1982, 2001; Yin and Harrison, 2000; Royden et al., 2008; Xu et al., 2015; Liu et al., 2020) (Fig. 1). Several popular models have been proposed to describe the tectonic deformation, including (1) rigid block extrusion, where blocks (e.g., Baoshan block, Simao block, Indochina block) are segmented by the major transpressional strike-slip shear zones that record the deformation and metamorphic history of the region (Tapponnier et al., 1982), (2) continuous deformation as a viscous sheet around the southeast Tibet Plateau (England and Houseman, 1986), and (3) crustal channel flow, where Tibet Plateau materials were transported to the southeastern margin by the weak zones of mid-lower crust (Royden et al., 1997; Clark and Royden, 2000). Recent high-resolution crustal velocity models also have proved the existence of weak zones in the mid-lower crust and some spatial relationships between the crustal weak zones and the major shear zones (Liu et al., 2014; Qiao et al., 2018).
Previous studies have revealed that these large-scale transpressional and/or transtensional shear zones appear as highly localized ductile deformation zones along the high-grade metamorphic and mylonitic belts, which are also the sites of former suture zones later reactivated as strike-slip shear zones (Metcalfe, 2006; Cao et al., 2011a; Xu et al., 2015; Dong et al., 2019; Liu et al., 2020). The deep-seated mylonitic shear zones have accommodated large displacement, magmatism, and intracontinental deformation related to the southeastward lateral extrusion of continental fragments from the Tibetan Plateau in front of the India-Eurasia collision zone (Tapponnier et al., 1986, 2001). Along the strike-slip ASRR-SZ, widespread outcrops of migmatites, leucogranites, and granitic pegmatites indicate the occurrence of intense crustal anatexis (e.g., Cao et al., 2011a; Liu et al., 2015b, 2020; Wang et al., 2019) (Fig. 2). These granitic intrusions together with metamorphic rocks exhumed along the strike-slip shear zone are strongly influenced by the tectonic evolution of the Sanjiang region. These granitic intrusions provide an excellent opportunity to investigate not only the timing of shearing, but also the formation and evolution of anatectic melts, and the relationship between crustal anatexis and the formation of shear zones.
Many of the critical interpretations focus on whether metamorphism is pre-kinematic or syn-kinematic with respect to left-lateral shearing and whether the leucogranite dikes are pre-, syn-, or post-kinematic with respect to left-lateral shearing (Leloup et al., 1995; Liang et al., 2007; Searle et al., 2010; Cao et al., 2009, 2011a, 2011b, 2011c; Liu et al., 2020). The metamorphic and igneous rocks were previously interpreted as having formed because of shear heating during left-lateral strike-slip shearing (Tapponnier et al., 1982, 1986, 1990; Leloup and Kienast, 1993; Leloup et al., 1995). Searle et al. (2010) suggested that ductile left-lateral strike-slip shear fabrics were superimposed on all lithologies at high temperatures (∼500–550 °C) after peak metamorphism and after granitic intrusion (Jolivet et al., 2001). The earlier deformed leucogranites occurred at 32–24 Ma, thus constraining the age of ASRR left-lateral ductile shearing (Cao et al., 2009, 2011a, 2011b, 2011c). Jolivet et al. (2001) suggested that Oligo-Miocene deformation in north Vietnam along the ASRR left-lateral strike-slip shear was restricted to the upper and middle crust above a horizontal shear zone. Besides, the temporal relations between the shear zone structures and magmatic intrusions remain a matter of debate, especially contested is whether crustal anatexis was initiated prior to, or coeval with, the left-lateral shearing (e.g., Leloup and Kienast, 1993; Leloup et al., 1995, 1999, 2007; Zhang and Schärer, 1999; Jolivet et al., 2001; Gilley et al., 2003; Searle, 2006; Cao et al., 2009, 2011a, 2011b, 2017; Liu et al., 2020).
In this contribution, we present here a combined study of zircon internal structure, zircon U-Pb ages, whole-rock geochemical composition, and Ti-in-zircon thermometry of the migmatites and leucogranites within the ASRR-SZ. We attempt to define the deformation and timing of migmatite formation and leucogranitic magmatism within the ASRR-SZ.
THE AILAO SHAN-RED RIVER SHEAR ZONE AND STRUCTURE OF THE AILAOSHAN MASSIF
The Exhumed Ailao Shan-Red River Shear Zone and Metamorphic Massifs
The ASRR-SZ is a >1000-km-long continental-scale shear belt geographically located in southeastern Tibet. The area formed at the closure of one of the representative branches of the Paleotethys ocean (e.g., Wang et al., 2019), and subsequent amalgamation of Gondwana-derived micro-continental blocks and Paleozoic arc terranes (Metcalfe, 2013; Deng et al., 2014). The Paleotethyan suture is termed the Jinshajiang-Ailaoshan suture zone, and the ASSR-SZ is superimposed on it. The strike-slip activity along the ASRR-SZ is assumed to have led to over 500 km of southeastward displacement of the Indochina block relative to the Yangtze-South China block and is accompanied by the opening of the South China Sea (Fig. 1) (e.g., Tapponnier et al., 1986, 1990; Peltzer and Tapponnier, 1988; Leloup et al., 1995; Chung et al., 1997; Searle, 2006).
Along the ASRR-SZ, four narrow NW-SE oriented high-grade metamorphic massifs are developed including the Xuelongshan (XLS), Diancangshan (DCS), and Ailaoshan (ALS) massifs in China and the Day Nui Con Voi (DNCV) massif in Vietnam (Fig. 1). The ALS massif, the largest of the four massifs along the shear zone, is cored by high-grade metamorphic rocks previously considered to be a Mesoproterozoic unit, i.e., the ALS Group (Yunnan BGMR, 1983) (Fig. 2). The high-grade core is bounded by the Red River fault to the northeast and the ALS fault to the southwest, separating the high-grade core of the Yangtze plate from the low-grade metamorphic Paleozoic–Mesozoic unit of the Indochina block. Faulting along these faults contributed to the late Cenozoic exhumation of the massifs (Leloup et al., 1995; Cao et al., 2011a). The high-grade ALS metamorphic core consists of garnet-sillimanite-biotite paragneisses, augen/banded migmatites, amphibolites, schists, gneisses, calc-silicates, and marbles. The peak metamorphic grades are upper amphibolite to granulite facies (780–840 °C and ∼9.5 kbar) (Leloup and Kienast, 1993; Wang et al., 2019) and migmatization is widely developed in the high-grade metamorphic core (Wang et al., 2017). Amphibolites, gneiss, and migmatite yield zircon ages at 815–800 Ma, 770–750 Ma, ca. 240 Ma, and ca. 30 Ma (Lin et al., 2012; Cai et al., 2014). The Paleozoic–Mesozoic low-grade metamorphic belt consists of upper to lower greenschist-facies metamorphic rocks, e.g., micaschists, garnet-micaschists, and phyllites (Yunnan BGMR, 1983; Leloup et al., 1995).
Geochronology Data of the Ailao Shan-Red River Shear Zone
Structural and thermochronological studies conducted on the massifs have focused on the tectono-magmatic evolution with the aim of constraining the timing and amount of displacement, the rate of movement along the ASRR-SZ, and the exhumation processes of the metamorphic massifs (e.g., Schärer et al., 1990, 1994; Harrison et al., 1992; Leloup and Kienast, 1993; Gilley et al., 2003; Cao et al., 2011a, 2011b, 2017; Liu et al., 2015b, 2020; Chen et al., 2015; Cheng et al., 2018). Controversy still exists over the age of initiation and the duration of strike-slip shearing and the processes and mechanisms of exhumation along the ASRR-SZ.
Early metamorphic, structural, and thermochronological analysis (e.g., zircon, Ar/Ar, and fission track) revealed that the left-lateral ductile shearing along the ASRR-SZ occurred at the period of ca. 31–17 Ma (e.g., Schärer et al., 1990, 1994; Harrison et al., 1992; Leloup and Kienast, 1993; Leloup et al., 1995, 2001; Harrison et al., 1996; Zhang and Schärer, 1999; Wang et al., 2000, 2001; Jolivet et al., 2001; Gilley et al., 2003; Cao et al., 2009, 2011a; Liu et al., 2012, 2020; Tang et al., 2013; Chen et al., 2015, 2017a). Searle (2006) interpreted the metamorphism of the DNCV metamorphic rocks in Vietnam (ca. 220–44 Ma) to have predated shearing and is, therefore, unrelated to shearing along the ASRR-SZ (Gilley et al., 2003). He argued that the shear fabrics associated with the left-lateral slip postdated peak metamorphic conditions at relatively low temperatures. He suggested that the left-lateral shear along the ASRR-SZ initiated at 21 Ma, instead of 35 Ma. Liang et al. (2007) presented high potassic alkaline intrusions with a genetic relationship to the ASRR-SZ and suggested that the onset of left-lateral movements along the ASRR-SZ began before 36 Ma. However, some researchers argued for a sequence of calc-alkaline and alkaline magmatic rocks (e.g., granodiorite, syenite, monzogranite, trachyte, and lamprophyre), and that most leucogranites exposed along the ASRR-SZ are pre-kinematic and there is no spatial or temporal link between intrusions and ASRR-SZ strike-slip shearing (Searle, 2006; Chung et al., 2008). Based on combined structural, microstructural analysis, and U-Pb zircon dating, Cao et al. (2009, 2011a, 2011b) reported that the left-lateral shearing of the Diancangshan massif along the ASRR-SZ initiated at ca. 31 Ma, culminated between ca. 27 and 21 Ma under high-temperature metamorphic and deformation conditions, and slowed down at ca. 20 Ma to low-temperature shearing (Cao et al., 2011b; Chen et al., 2015; Liu et al., 2020).
The Ailaoshan (ALS) Massif and Granitic Intrusions
In the ALS massif along the ASRR-SZ, the Oligo-Miocene left-lateral shearing is characterized by strong mylonitization generating a sub-vertical foliation, a subhorizontal mineral and/or stretching lineation in the metamorphic rocks and magmatic intrusions (Fig. 2). Lineation predominates over foliation in the fabric components of the mylonites, which show typical L>>S fabrics. Locally there are L-S tectonites defined by a prominent stretching lineation and an equally developed foliation (Cao et al., 2011a, 2011b; Chen et al., 2015). The strong stretching mineral lineation L is subhorizontal or gently plunging to the NW/NNW or SE/SSE with a plunge angle of less than 30° (Fig. 2). When present, the mylonitic foliation is commonly parallel to the compositional layering, and strikes approximately NNW-SSE, with variable dip angles. The mylonitic foliation dips steeply west along the western side of the zone and steepens to nearly vertical in the narrow central zone. Along the eastern and southeastern sides, the foliation in the gneiss and mica-schist dips gently to the east and northeast. Close to the northern termination, the dip angles of the foliation are steeper than at the southern end, and the foliation is often near to a subvertical position (Fig. 2).
Asymmetric sheath folds are common both in sheared gneisses (such as sillimanite- or amphibolitic gneiss) and granitic intrusions, which suggest intense ductile shearing in the entire shear zone. In the low-strain zones, mineral grains from the protoliths are flattened and elongated and form a foliation and stretching lineation in the mylonitic rocks. Evidence of dynamic recrystallization by grain boundary migration or subgrain rotation recrystallization of quartz grains and bulging recrystallization of feldspar is preserved implying deformation at high-temperature conditions (Liu et al., 2015a). High-temperature deformation conditions are also indicated by the linear alignment of syn-shearing mineral grains, such as sillimanite, in the gneisses. Outcrop and microscopic observations of sheared rocks from the massifs reveal abundant widespread left-lateral shear sense indicators, although right-lateral shear indicators are also observed in many localities.
Along the entire ALS massif, widespread outcrops of migmatites, leucogranites, and pegmatites indicate the occurrence of crustal partial melting in the high-grade metamorphic core (Fig. 3). Different types of intrusions show variable degrees of deformation ranging from strongly sheared layered granites/migmatites and leucogranites with a penetrative foliation and lineation to undeformed pegmatites. Deformation and recrystallization that accompanies anatectic melting can unequivocally be ascribed to left-lateral ductile shearing under high-temperature metamorphic conditions along the ASRR-SZ. Field and microscopic observations reveal that the pre- and syn-kinematic intrusions are coeval with high-temperature mylonitization. Widespread shear sense indicators in the intrusions such as sheared veins, S-C fabrics, shear bands, mica-fish arrays, asymmetric fabrics (e.g., sigma- and delta-type feldspar and hornblende porphyroclasts) (Figs. 3 and 4), and asymmetric pressure shadows suggest that the high-grade metamorphic rocks and most leucocratic intrusions experienced progressive left-lateral shearing. However, post-kinematic granitic pegmatite dikes cross-cutting the mylonitic foliation and stretching lineations of host rocks (Fig. 3G) record the latest strain increment.
SAMPLES AND STRUCTURAL CHARACTERISTICS
Several generations of migmatites and leucogranites are exposed in the highly sheared metamorphic rocks in the ALS massif. Field and microscopic observations reveal the existence of pre-shearing, syn-kinematic, and post-kinematic leucocratic intrusions, so that the combined ages of crystallization of the granitic rocks can be suitably applied to constrain the timing of ductile shearing along the ASRR shear zone. Ten samples in the ALS massif have been selected including two foliated migmatites (samples AL082-3 and AL082-1), four foliated leucogranites (AL089-3, AL089-4, AL076-1, and AL007-3) and four weakly or unfoliated pegmatites (AL069-6, AL069-9, AL104-1, and AL116-1). Sample locations are given in Figure 2 and specific sample descriptions are shown in Table S11. The structural relationships between granitic rocks and their wall rocks, and the deformation microstructures in the intrusions are presented in Figures 3 and 4, respectively.
The foliated stromatitic leucogranites from the ALS massif are either deformed granitic intrusions or metapelites (Figs. 3A–3C), which display strong migmatization defined by intercalation of compositional layers of melanosome and plagioclase (Pl)-rich leucosome (Figs. 3A–3C). The migmatitic gneisses are composed predominantly of K-feldspar, plagioclase, quartz, hornblende, and biotite with the accessory minerals including apatite, zircon, monazite, and xenotime (Figs. 4A–4C). The melanosome layers are enriched in dark restite minerals, such as garnet, hornblende, and biotite, with minor and variable amounts of plagioclase, K-feldspar, titanite, apatite, and zircon. The mesosome (AL082-1, including hornblende, plagioclase, K-feldspar, quartz, and titanite) and the melanosome (AL082-3, including hornblende, biotite, plagioclase, K-feldspar, and quartz) represents the mafic residuum (Figs. 3A–3C). Within the migmatitic gneiss, the leucogranitic sills or dikes are generally subparallel to the gneissosity/foliation of the host gneisses (Figs. 3A–3C). These leucogranites occur mainly in dikes of varying sizes (Figs. 3C–3G). Deformed leucogranitic dikes, such as nodular, lenticular, lanceolate, and pinch-and-swell structures, are found in the metamorphic rocks (Fig. 3B). In some outcrops, the leucogranite is cut and crossed by pegmatite dikes.
In the leucogranites, the main minerals include plagioclase, K-feldspar, quartz, muscovite, and biotite; the accessory minerals are dominated by garnet, apatite, zircon, and monazite (Figs. 4D–4I, Supplementary Figs. S1A–S1E; see footnote 1). Most leucogranites show various shear deformation structures (Fig. 4A). The rotated K-feldspar porphyroclasts and fine-grained matrix are generally well developed. The K-feldspar grains occur as σ-, δ-, and ϕ-type porphyroclasts, indicating a left-lateral shear sense (Figs. 4D–4I). The K-feldspar porphyroclasts are characterized by crystal-plastic deformation and strong dynamic recrystallization, surrounded by polycrystalline aggregate bands of quartz (Fig. 4D). High-temperature quartz ribbons and aggregates also developed by linking multiple rectangular-shaped (or similar) quartz grains. The feldspar fish fabrics occur in the quartz ribbons, indicating the left-lateral shear sense (Fig. 4I). The polycrystalline quartz bands and fibrous sillimanite aggregates indicate a high-temperature metamorphic condition. (Fig. 4G). Garnet is present as idiomorphic grains and is free from inclusions, only a few of the grains exceed 50 μm in size. The matrix is composed of fine-grained plagioclase, and quartz and biotite aggregates (Fig. 4H). The plastic flow structure is consistent with the orientation of foliation and lineation of the mylonites in the ALS massif (Fig. 2).
The pegmatites are composed of variable proportions of K-feldspar, plagioclase, quartz, biotite, and muscovite, with accessory magnetite, apatite, and zircon. In individual outcrops, these weakly to undeformed dikes have variable widths (a few centimeters to meters) (Supplementary Figs. S1F–S1H, S3G, S3H, and S4J–S4L; see footnote 1). Most of these pegmatite intrusions cut the high temperature mylonitic foliation and lineation of their country rocks at small or high angles indicating the pegmatite dikes are post-shearing intrusions. Some of the pegmatites show only very weak ductile solid-state and brittle deformation microstructures. In contrast to the foliated leucogranites, the microstructures of post-shearing pegmatites include mainly K-feldspar grains of subhedral crystal shape. Some feldspars exhibit intragranular reactivation of the cleavage planes of their crystals. The quartz grains are slightly elliptical and exhibit undulose extinction. The fine-grained recrystallization of quartz grains occurs locally at the boundaries of coarse older grains.
ZIRCON U-Pb GEOCHRONOLOGY AND TRACE-ELEMENT CHEMISTRY
Ten samples in the ALS massif have been selected including two foliated migmatites (samples AL082-3 and AL082-1), four foliated leucogranites (AL089-3, AL089-4, AL076-1, and AL007-3), and four weakly or unfoliated pegmatites (AL069-6, AL069-9, AL104-1, and AL116-1). Zircon U-Pb age and trace element data analyzed by laser ablation–inductively coupled plasma–mass spectrometry are summarized in Supplementary Tables S2 and S3, respectively (see footnote 1). Data with excessive concentrations of U and Th that could be unsuitable for common Pb correction were excluded from the interpretation.
Zircons from all these intrusions are mostly subhedral to ovoid in morphology with an average crystal length of over 150 μm and a general width-to-length ratio of 1:2.5 (Supplementary Fig. S2). Based on their internal structure, two types of zircon grains are recognized. Most zircons bear inherited cores showing a core-rim texture, whereas some grains lack inherited cores and display an obvious oscillatory zonation (Supplementary Fig. S2). The detailed results are described as follows (Figs. 5 and 6).
Migmatite and Foliated Leucogranites
Samples AL082-3 and AL082-1 are collected from the same outcrop (Fig. 3B). Zircons from the melanosome sample (foliated migmatite, AL082-3) are mostly subhedral with crystal lengths of 100 to 200 μm and a general width-to-length ratio of 1:2–1:3. Zircons from the mesosome (foliated migmatite, AL082-1) are generally subhedral with an average crystal length of 300 μm and a long-prismatic crystal habit (width to length = 1:3). In the cathodoluminescence (CL) images, zircons present a zoned core-rim structure with puzzled cores (Supplementary Figs. S2A and S2B). In the melanosome sample AL082-3, ten inherited zircon cores yield apparent 206Pb/238U ages ranging from 414 Ma to 1160 Ma (Th/U = 0.3–0.9) (Figs. 5A). Ten zircon rims yield 206Pb/238U ages between 30.8 ± 0.6 and 41.7 ± 1.8 Ma, with a weighted mean of 34.1 ± 0.7 Ma calculated from six analyses (Th/U = 0.002–0.02) (Fig. 5B). In the mesosome sample (AL082-1), one inherited zircon core yields an apparent 206Pb/238U age of 549.3 ± 5.6 Ma (Th/U = 0.99). Seven zircon rims yield 206Pb/238U ages between 34.6 ± 0.5 and 37.0 ± 0.8 Ma, with a weighted mean of 35.7 ± 0.9 Ma (Th/U = 0.1–0.9) (Figs. 5E and 7C). The zircon rims of the sample AL082-3 have present left-dipping rare earth element (REE) patterns with strong enrichment of heavy (H)REE, positive Ce (Ce/Ce* = 1.17–110.45) and negative Eu anomalies (Eu/Eu* = 0.02–0.47) (Fig. 5C). Compared with the inherited rims, the cores have higher REE contents (Fig. 5C). In sample AL082-1, the zircon rims have left-dipping REE patterns with strong enrichment of HREE, positive Ce (Ce/Ce* = 1.48–193.1) and negative Eu anomalies (Eu/Eu* = 0.30–0.83). The rims with higher Th/U ratios have higher REE contents (Fig. 5F). In addition, the distinctly high U and relatively low Th contents with unusually low Th/U ratios of rims are common features of anatectic zircons (Rubatto, 2017).
Samples AL089-3 and AL089-4 are foliated leucogranites collected from the same outcrop (Fig. 3D). Sample AL089-3 contains sillimanite and garnet, and sample AL089-4 contains garnet, these characteristics may reflect muscovite dehydration-melting reaction in the source region. Zircons from the two samples are mostly subhedral with crystal lengths ranging from 150 to 200 μm and a general width-to-length ratio of 1:2–1:3.5. In the CL images, zircons present a core-rim-zoned and oscillatory-zoned structure in the rim. Some zircon cores display an oscillatory zonation (Supplementary Figs. S2C and S2D). The inherited zircon cores from the leucogranite (AL089-4) yield apparent 206Pb/238U ages of 80.3 ± 2.5 Ma and 154.1 ± 2.1 Ma. Twenty analyses on the zircon rims yield 206Pb/238U ages between 28.3 ± 1.1 and 36.0 ± 0.8 Ma, with a weighted mean age of 29.7 ± 0.4 Ma calculated from seventeen analyses (Th/U = 0.30–0.29) (Figs. 5G,5H, and 7G). Two inherited zircon cores from leucogranite (AL089-3) yield apparent 206Pb/238U ages of 227.7 ± 2.1 Ma and 255.8 ± 6.9 Ma. Four analyses on the zircon rims yield 206Pb/238U ages between 119.6 ± 7.5 and 76.6 ± 2.8 Ma. Thirteen analyses on the zircon rims and single zircons yield 206Pb/238U ages between 27.1 ± 0.5 Ma and 36.9 ± 0.9 Ma, with a weighted mean of 36.0 ± 1.0 Ma calculated from 11 analyses (Th/U = 0.04–0.35) (Figs. 5J, 5K, and 7E). The zircon rims of samples AL089-3 and AL089-4 have similar left-dipping REE patterns with strong enrichment of HREEs, positive Ce (Ce/Ce* = 10.2–192.2) and negative Eu anomalies (Eu/Eu* = 0.03–0.40). Compared with the rims, the inherited cores have lower Eu/Eu* ratios (0.12–0.47) and HREE contents (Figs. 5I and 5L).
Sample AL007-3 is a mylonitic leucogranite. Zircon grains are generally subhedral with average crystal length of 250 μm and have a long prismatic crystal habit (width to length ratio = 1:3). In CL images, zircons present a core-rim zone-structure with puzzled cores (Supplementary Fig. S2E). Two inherited zircon cores from the mylonitic leucogranite (AL007-3) yield apparent 206Pb/238U ages ranging from 281.5 to 1123 Ma, one spot age is 1123 ± 20.4 Ma, three spot ages are at 732.2 ± 8.8 Ma, 690 ± 6.1 Ma, and 558.2 ± 12.6 Ma (Figs. 5M and 7B). Two spot ages are 281.5 ± 4.1 Ma and 398.1 ± 6.8 Ma, respectively. Thirteen analyses on the zircon rims and unzoned zircons yield 206Pb/238U ages between 27.1 ± 0.5 Ma and 36.9 ± 0.9 Ma, with a weighted mean of 34.0 ± 0.3 Ma calculated from seven analyses (Th/U = 0.04–0.35) (Fig. 5N). The zircon rims of the sample AL007-3 have left-dipping REE patterns with a strong enrichment of HREEs, positive Ce (Ce/Ce* = 4.9–149.4) and negative Eu anomalies (Eu/Eu* = 0.42–0.90). Compared with the inherited rims, the cores have lower Eu/Eu* ratios (0.09–0.45) and HREE contents (Fig. 5O).
Sample AL076-1 is a deformed leucogranite. Zircons from the sample are mostly subhedral with crystal lengths ranging from 150 to 300 μm and a general width-to-length ratio of 1:2–1:4. In the CL images, zircons present the core rim-zoned structure with puzzled cores (Supplementary Fig. S2F). Two inherited zircon cores from the leucogranite (AL076-1) yield apparent 206Pb/238U ages ranging from 629.9 Ma to 790.2 Ma. Two spot ages are at 629.9 ± 4.9 Ma and 674.6 ± 5.7 Ma, seven spot ages are between 732.1 ± 7 and 790.2 ± 8.1 Ma. The main Th/U ratios of these zircon rims are between 0.66 and 1.24. Eleven analyses on the zircon rims yield 206Pb/238U ages between 32.3 ± 0.5 and 36.1 ± 0.6 Ma, with a weighted mean of 32.8 ± 0.32 Ma calculated for seven analyses (Th/U < 0.01) (Figs. 5P,5Q, and 7I). The zircon rims of the sample AL076-1 have left-dipping REE patterns with strong enrichment of HREE, positive Ce (Ce/Ce* = 3.2–54.3) and negative Eu anomalies (Eu/Eu* = 0.11–0.75). Compared with the rims, the inherited cores have Eu/Eu* ratios of 0.13–0.36 and higher REE contents (Fig. 5R).
Unfoliated Leucogranites and Granitic Pegmatites
Zircons from the pegmatite sample AL104-1 are mostly euhedral with crystal lengths ranging from 200 to 350 μm and a width-to-length ratio of 1:2–1:3. In CL images, zircons display an oscillatory zonation (Supplementary Fig. S2J). Sixteen spot ages of zircon rims are between 23.9 ± 0.4 and 25.9 ± 0.5 Ma, and the weighted mean age of the sixteen analyses is 25.0 ± 0.43 Ma (Th/U = 0.35–0.92) (Figs. 6A,6B, and 7H). The zircon rims of sample AL104-1 have left-dipping REE patterns with strong enrichment of HREE, positive Ce (Ce/Ce* = 3.2–54.3) and negative Eu anomalies (Eu/Eu* = 1.86–50.29) (Fig. 6C).
Samples AL069-6 and AL069-9 are an unfoliated leucogranite and a granitic pegmatite, respectively, collected from the same outcrop. Zircons from the two samples are mostly subhedral with crystal lengths from 120 to 200 μm and a width-to-length ratio of 1:2–1:3.5. In CL images, zircons present a zoned core-rim structure with puzzled cores (Supplementary Figs. S2G and S2I). Two inherited zircon cores from the leucogranite (AL069-6) yield apparent 206Pb/238U ages of 650 ± 11.5 Ma and 826 ± 8.4 Ma. Twenty analyses on the zircon rims of fourteen zircons yield 206Pb/238U ages between 22.0 ± 0.4 Ma and 25.9 ± 0.5 Ma, with a weighted mean age of 24.1 ± 0.28 Ma (Th/U = 0.59–1.02) (Figs. 6D,6E, and 7D). Ten analyses on the zircon rims from the granitic pegmatite (AL069-9) yield 206Pb/238U ages between 24.2 ± 0.3 and 26.2 ± 0.4 Ma, with a weighted mean age of 25.44 ± 0.50 Ma calculated from nine analyses (Th/U = 0.004–0.009) (Figs. 6G,6H, and 7F). The zircon rims of samples AL069-6 and AL069-9 have similar left-dipping REE patterns with strong enrichment of HREEs, positive Ce (Ce/Ce* = 1.42–58.33) and negative Eu anomalies (Eu/Eu* = 0.05–0.21) (Figs. 6F and 6I).
Zircons from the unfoliated granitic pegmatite (Sample AL116-1) are mostly subhedral with crystal lengths of 80–120 μm and a width-to-length ratio of 1:1–1:2. In the CL images, most zircon grains display well developed oscillatory zonation, some zircons present a core-rim structure (Supplementary Fig. S2H). Two spot ages are at 587.8 ± 10.8 Ma and 817.4 ± 9.7 Ma. Sixteen analyses on the zircon rims yielded 206Pb/238U ages between 22.4 ± 0.3 and 30.1 ± 0.5 Ma, with a weighted mean age of 23.0 ± 0.4 Ma calculated from the sixteen analyses (Th/U = 0.11–0.53) (Figs. 6J, 6K, and 7J). The zircon rims of sample AL116-1 have left-dipping REE patterns with strong enrichment of HREE, positive Ce (Ce/Ce* = 21.56–753.20) and negative Eu anomalies (Eu/Eu* = 0.12–0.36) (Fig. 6L).
The zircons from unfoliated leucogranites and granitic pegmatites (excluding AL069-9) display clear oscillatory zones. In addition, with characteristics of HREE enrichment, positive Ce anomalies and negative Eu anomalies argue for magmatic zircons. The zircons from sample AL069-9 have obvious Ce positive anomalies and Eu negative anomalies, which are similar to magmatic zircons. However, the zircons with extremely low Th/U ratios are obviously different from magmatic zircons, indicating that zircons were formed in the transitional stage between magma and hydrothermal fluid (Fig. S2) (Hoskin and Black, 2000).
WHOLE-ROCK MAJOR AND TRACE ELEMENT ANALYSES
Major and trace element compositions of the selected samples of the melanosome layer, plagioclase (Pl)-rich leucosome, and K-feldspar (Kfs)-rich pegmatite from the ALS massif are listed in Table S4 and Table S5 (see footnote 1). The melanosome and mesosome layers have obviously higher Fe2O3T (1.78–5.4 wt%), MgO (0.54–3.64 wt%), CaO (1.97–6.08 wt%), TiO2 (0.3–0.58 wt%), and P2O5 (0.19–0.2 wt%) contents, and relatively lower SiO2 (60.12–69.04 wt%) contents than those in the Pl-rich leucosome layers and the Kfs-rich pegmatite dikes. The SiO2 contents of the Pl-rich leucosome layer ranges from 65.63 to 72.6 wt%. The contents of Al2O3 and Na2O of the Pl-rich leucosome layer are similar to those of the melanosome layer. The Pl-rich leucosomes has obviously lower Fe2O3T (0.13 wt%), MgO (0.02–0.07 wt%), CaO (1.27–1.41 wt%), TiO2 (0.01–0.02 wt%), and P2O5 (0.01–0.10 wt%) contents, and relatively higher K2O (7.27–7.91 wt%) than the melanosome and mesosome layers. The Kfs-rich pegmatite has relatively high SiO2 (72.69–76.18 wt%) and K2O (2.6–7.41 wt%) contents, low Fe2O3T (0.84–1.81 wt%), MgO (0.1–02 wt%), CaO (0.94–1.22 wt%), Na2O (2.41–4.93 wt%), TiO2 (0.057–0.11 wt%), and P2O5 (0.012–0.092 wt%) contents, and a moderate Al2O3 (13.81–14.69 wt%) content (Supplementary Table S4). In the total alkali versus silica diagram (Fig. 8A), the pegmatite into the field of granite. The A/NK ratios and A/CNK ratios range from 1.13 to 1.84 and 0.82–1.10, respectively, indicating the leucosome and pegmatite are peraluminous, and the melanosome layer belong to a metaluminous composition (Fig. 8B).
The melanosome and mesosome layers, Pl-rich leucosomes and Kfs-rich pegmatites have also distinct trace-element compositions. Figure 8A shows that the melanosome layer has much higher REE contents than the other rock types. The chondrite-normalized REE patterns of the leucosomes and pegmatites are from 5.28 to 160.51, which are lower than that in the melanosome (Fig. 8C) (chondrite values from Sun and McDonough, 1989). The Eu anomalies of melanosomes, leucosomes, and leucogranites are from 0.47 to 0.85, the Eu anomalies of leucosomes are from 3.36 to 22.26. The melanosome layers and pegmatites have negative Eu anomalies, the Pl-rich leucosomes have pronounced positive Eu anomalies (Fig. 8C). These characteristics are typical for Pl-rich leucosomes (e.g., Yang et al., 2019a), the Pl-rich leucosomes represent a type of felsic refractory residuum (Taylor et al., 2014; Nicoli et al., 2017). The melanosome, leucosomes, and pegmatites also have trace-element compositions in the primitive mantle-normalized spidergram. The Pl-rich leucosomes and the Kfs-rich pegmatites are enriched in large-ion lithophile elements (LILEs, e.g., Rb, Ba, K, Sr, Pb, and U). The melanosome and mesosome layer are enrichment in light (L)REEs (Fig. 8C, Rb, Sr, Th, La, and Pb are enrichment in the primitive mantle-normalized spidergram (Fig. 8D). We conclude that MgO, Fe2O3T, P2O5, and REEs are expected to increase in the residual melanosome and mesosome layers after partial melting. Meanwhile, the Pl-rich leucosomes and the Kfs-rich pegmatites became enriched in LILEs and low in LREE concentrations, which are also due to the fact that the main phases are cumulate quartz, plagioclase, and K-feldspar.
METAMORPHIC AND DEFORMATION CONDITIONS
The activity of SiO2 and TiO2 varies in different geological settings for rocks and out-of-context zircons with undefined αTiO2 and αSiO2. Watson and Harrison (2005) suggested that most natural melts are capable of crystallizing zircons (i.e., saturated in ZrSiO4) and have TiO2 activities exceeding ∼0.5. The activity of TiO2 = 1 reflects rutile saturation, while the activity of TiO2 = 0.7 reflects titanite and titanomagnetite saturation. Most silicic melts have an activity of SiO2, αSiO2> = 0.3, and the activity of αSiO2 = 1 when contain quartz. In this study, where all samples contain quartz, we use the activity of αSiO2 = 1. Only AL082-1 contains titanite, so the activity of αTiO2 in this sample is 0.7, and the αTiO2 activity of other samples is 0.5. Formula (2) is used to estimate the temperature of crystallization during metamorphism. The calculated temperatures of Ti-in-zircon thermometer in migmatites, leucogranites, and pegmatites are as given in Figure 8 and Supplementary Table S1. The calculated temperatures of Ti-in-zircon thermometer in migmatite, leucogranites, and leucopegmatites versus the ages, Th/U ratio are presented in Figure 8. The average of Ti-in-zircon temperatures in inherited zircon rims of foliated melanosome/mesosome (samples AL082-1 and AL082-3) is at 724 °C, and the average of Ti-in-zircon temperatures in zircon cores of sample AL082-1 is at 862 °C. The Ti-in-zircon temperatures in inherited zircon cores of mylonitic leucogranites (samples AL007-3, AL076-1, and AL089-3) have averages of 816 °C, 751 °C, 810 °C, respectively. The range of Ti-in-zircon temperatures in zircon rims from mylonitic leucogranites (samples AL089-3, AL089-4, AL076-1, and AL007-3) are ∼665–781 °C. The main range of Ti-in-zircon temperatures on zircon rims from unfoliated pegmatites (samples AL104-1, AL116-1, AL069-6, and AL069-9) are ∼668–721 °C.
Hornblende-Plagioclase Thermobarometry of Migmatites
Pressure and temperature estimates of these rocks are based on geothermobarometry of hornblende and plagioclase, which are based on hornblende solid-solution models and are well constrained by natural and experimental systems. Two hornblende-plagioclase geothermometers were proposed by Holland and Blundy (1994), based on the edenite-tremolite reaction (edenite + 4 quartz = tremolite + albite) (thermometer A) and the edenite-richterite reaction (edenite + albite = richterite + anorthite) (thermometer B). Thermometer B is preferable, based on comparison to other thermometers as suggested by Anderson (1996). Therefore, we chose thermometer B to calculate the temperatures of migmatites. Anderson and Smith (1995) proposed a temperature-corrected Al-in-hornblende barometer by calibrating the methods of Johnson and Rutherford (1989) and Schmidt (1992) and comparing them with results obtained from the Anderson and Smith (1995) calibration. We use the calibrated formulas to calculate the pressure of migmatites. Electron microprobe analyses are listed in Supplementary Table S6 (see footnote 1). The calculation results show that the temperature range of sample AL082-1 is 735–860 °C, and its pressure range is 0.09–0.36 Gpa (Fig. 9E); the temperature range of sample AL082-3 is 710–820 °C, and its pressure range is 0.35–0.62 Gpa (Fig. 9F).
Deformed Quartz Electron Backscatter Diffraction (EBSD) Crystallographic Preferred Orientation (CPO) Results
To further constrain the deformation conditions of the granitic rocks after its intrusions, four representative samples of mylonitic leucogranites were selected for CPO EBSD measurements from the representative dated samples of the ALS massif. Under the microscope, the feldspar shows plastic deformation and mechanical twins, and myrmekites developed around K-feldspar porphyroclasts. The quartz forms mainly elongated polycrystalline aggregates, showing evidence of grain boundary migration recrystallization in the form of irregular serrated grain boundaries. All CPOs of quartz were measured in the polycrystalline quartz ribbons and aggregates as shown in Figure 10. All the EBSD pole figures from three samples show quartz c-<0001= maximum near the X-axis direction. The m (10–10) and a (11–20) form a small girdle in the YZ plane, and r (10–11) exhibits a weak maximum. The dominant slip system is prism slip. The quartz aggregates from sample AL007-3 shows c-<0001= maximum near the Y-axis. The m (10–10) and a (11–20) form great circle girdles in the XZ plane, and r (10–11) shows a weak maximum in the XY plane. The dominant slip system is prism <c= and <a= slip, indicating high-temperature deformation conditions (Stipp et al., 2002).
Cenozoic Crustal Anatexis along the ASRR-SZ
The migmatites and leucogranites are important rocks within the ASRR-SZ and record a crustal anatexis event. Thus, understanding the petrogenesis and geodynamics of the Cenozoic migmatites and leucogranites provide important constraints on the formation and evolutionary history of these mechanically weak zones and its surrounding regions.
Our new zircon U-Pb ages of 41–28 Ma from mylonitic leucogranites with metamorphic and deformation fabrics in the ASRR-SZ reveal overprinted Oligocene and Miocene metamorphism, intrusion, and deformation. Studies have addressed that using Th/U ratios as a criterion for distinguishing magmatic zircon and metamorphic zircons (e.g., Rubatto, 2002, 2017; Hoskin and Schaltegger, 2003). The Th/U ratios are also used to distinguish magmatic (mainly Th/U> 0.2) and metamorphic (mainly Th/U < 0.1) zircons in this study. The average age ranges of the deformed migmatites and leucogranites are from 30 to 35 Ma (mainly Th/U < 0.1), the inherited cores have higher REE contents than rims. The rims are related to the degree of crustal anatexis and the equilibrium between the melt and the residual phase (Figs. 5C,5F, 5I, 5L, and 5R). Recently, Wang et al. (2019) reported that the relic mineral assemblage preserved as inclusions in garnet porphyroblasts belongs to prograde stage and records P-T conditions of lower amphibolite facies and dehydration reactions (e.g., dehydration of muscvite and/or biotite) and mineral assemblages (sillimanite, K-feldspar) formed at the peak granulite facies metamorphic stage that yielded P-T conditions of 780–840 °C, of which ∼0.95 GPa occurred in Cenozoic times at ca. 32–35 Ma (Palin et al., 2013; Wang et al., 2019). The post-peak near isothermal decompression stage is characterized by garnet developed as rotated porphyroclasts, which have visible trailing and reaction edges of ″white eye structures,″ which show P-T conditions of lower amphibolite facies (Leloup et al., 2001; Cao et al., 2011a). The metamorphic zircon rims formed in the process of crustal anatexis show complex core-rim characteristics and record the time of corresponding crustal anatexis (Foster et al., 2001; Keay et al., 2001; Imayama et al., 2012). The Ti-in-zircon temperatures on zircon rims from these leucogranites and migmatites represent the temperature of crustal anatexis. The Ti-in-zircon temperature on zircon rims from foliated migmatites and leucogranites range mainly from 749 to 781 °C (Figs. 9C and 9D), which is consistent with the studied metamorphic and crustal anatexis conditions. It reveals the significance of zircon rim ages from the foliated migmatites and leucogranites, which give U-Pb ages of 41–28 Ma with low Th/U ratios, recording the timing of Cenozoic crustal anatexis and peak metamorphism along the ASRR-SZ and the depth of anataxis which formed at ∼30 km (Fig. 11B).
The differential exhumation processes along the strike-slip shear zone occurred after the 28 Ma sinistral shearing terminated asynchronously (Fig. 11). Our EBSD analysis of the quartz ribbons or aggregates from the leucogranites presents a maximum c-<0001= in the X- and Y-axis directions, indicating deformation dominated by the high-temperature prism <c= and <a= slip system (550–660 °C) (Stipp et al., 2002). The results of deformation conditions suggest that the post-peak near-isothermal decompression stage is consistent with the left-lateral ductile shearing (Leloup et al., 2001; Liu et al., 2013; Wang et al., 2019). The distribution of pegmatites is limited to the high-grade rocks, which implies intense magmatism at depth related to late ductile shearing along the shear zone. The unfoliated leucogranites and pegmatites present the post-shearing intrusions. Their U-Pb isotope ages constrain the age of ductile shearing along the shear zone from ca. 28 to 23 Ma (Fig. 12) (or possibly extending to 17 Ma; Leloup et al., 2001 and references therein). This conclusion is compatible with previous Ar-Ar dating results of potassium minerals (e.g., white mica) from the three major massifs of the shear zone (Fig. 11A) (Day Nui Con Voi—Wang et al., 1998, 2000; Ailaoshan, Diancangshan—Chen et al., 2015; Diancang Shan—Cao et al., 2011b).
Middle-Lower Crustal Weak Zone and Initiation of Deep-Seated Shearing
When and how a major strike-slip shear zone is initiated and shearing responds to plate interactions is extensively debated (e.g., Tapponnier and Molnar, 1976; Cao and Neubauer, 2016 and references therein). Recently, it is believed that rheological weakening mechanisms play an important role in fault localization as faults generally nucleate in the weakest zone within the crust and/or lithosphere (Handy et al., 2005; Dayem et al., 2009a; Molnar and Dayem, 2010; Cao and Neubauer, 2016 and references therein). Fossen and Rotevant (2016) suggests that the faults initiated as individual segments, whose fault tips at some point interact at the locations of the salient, form curved fault segments. Here, we propose that the initiation of the ASRR-SZ is at depth where temperature caused rheological weakening and localization along hot-to-cold contacts deep within the crustal part of the lithosphere. These processes resulted in the specific thermal and structural architecture of the ASRR-SZ.
For the ASRR-SZ, petrographic, tectonic, and metamorphic studies indicated that the four narrow NW-SE oriented high-grade metamorphic massifs experienced strong left-lateral ductile deformation resulting in its formation (e.g., Leloup et al., 2007; Liang et al., 2007; Cao et al., 2011a, 2011b; Lu et al., 2012; Lin et al., 2012; Tang et al., 2013; Liu et al., 2015b). Mylonitic fabrics formed by left-lateral strike-slip shearing after peak conditions of metamorphism and after granite intrusions (Chung et al., 1997, 2008; Jolivet et al., 2001; Searle, 2006; Anczkiewicz et al., 2007). Recently, crustal low-velocity zones, revealed by tomography and magnetotelluric imaging results, imply a mid-lower crustal weak zone (at a depth of 20–40 km) (Bai et al., 2010; Liu et al., 2014; Qiao et al., 2018). These weak zones, defined by various geophysical imaging methods, have shown a strong correlation with the main strike-slip shear zone systems in this area of the Sanjiang region, which emphasizes the important role played by the main fault systems in the tectonic evolution of SE Tibet. Especially, it is noteworthy that the high heat flow value in Southeast Asia also presents convincing proof of crustal weak zones where shear heating or even partial melting exists (Clark and Royden, 2000). Studies of exhumed mylonitic strike-slip shear zones have described the presence of surface rocks which originated from great depths. Close spatial-temporal relationship and compatible rates of processes between the pre-kinematic or contemporaneous granitic intrusions and continent-scale strike-slip faulting have been documented worldwide in several cases (e.g., Hutton et al., 1990; McCaffrey, 1992; Tikoff and Teyssier, 1994; Tommasi et al., 1994; Neves et al., 1996; Brown and Solar, 1998; Rosenberg, 2004; Rosenberg et al., 2007; Cao et al., 2011a; Brown, 2001, Cao and Neubauer, 2016; Liu et al., 2020).
Along the exhumed continental-scale ASRR-SZ, widespread outcrops of migmatites and granitic intrusions occur. Large-scale high-potassium intrusions (ca. 40–32 Ma) are also distributed along and outside the shear zone (Leloup et al., 1995; Searle, 2006; Liang et al., 2007; Cao et al., 2011a; Flower et al., 2013; Chen et al., 2015) (Figs. 3 and 4). The widespread exposures of leucogranitic intrusions in the four high-grade massifs (Fig. 12) imply that intense crustal anatexis occurred along the ASRR-SZ (Fig. 12). Undeformed mafic or lamprophyre dikes (intruded at ca. 36 Ma) are also common outside the shear zone (e.g., Huang et al., 2002; Tran et al., 2010; Chen et al., 2014; Lu et al., 2015) and do not show any evidence of ductile deformation related to the shear zone. Their counterparts within the shear zone, however, were highly sheared into mylonitic rocks similar to their wall rocks. Such rocks possess complex structural, microstructural, and EBSD textures, reflecting a long history of superposition by progressive high-temperature ductile shearing. These characteristics are compatible with those of previously described pre- and syn-shearing intrusions (see also Searle et al., 2010; Cao et al., 2011a; Tang et al., 2013; Liu et al., 2015b, 2020, and references therein). Pre- and syntectonic plutons are emplaced preferentially within or at the margins of wide, regional-scale strike-slip shear zones, which can intersect major lithological boundaries (Weinberg et al., 2004) and indicate rheological weakening and localizing along hot-to-cold contacts deep within the crust and mantle lithosphere (Cao and Neubauer, 2016; van der Werf et al., 2017).
Magmatic weakening of the lithosphere has received peculiar attention in the context of continental faulting (e.g., Brown and Solar, 1998; Rosenberg, 2004; Cao and Neubauer, 2016 and references therein). In the present study, together with previous studies (e.g., Cao et al., 2011a, 2011b; Tang et al., 2013; Chen et al., 2015), the foliated leucogranitic intrusions along the ASRR-SZ are bracketed in the age range of regional high-potassium intrusions from ca. 41 to 28 Ma (Fig. 12). It reveals the significance of metamorphic zircon rim ages related to multiple crustal anatexis and peak temperature conditions of metamorphism. Due to the ascension and emplacement of intrusions, the instability localizing the faulting comes from rheologically deep-seated levels of the wrench zone, which is a potential mechanism for the inception of major strike-slip faults. During further development, the instability of uprising magma focuses fault motion in the thermally weakened lower crust. Based on the rheological contrasts, melt-induced strain localization and lateral flow of anatectic crust can be postulated to localizing major strike-slip faults confining laterally extruding blocks.
Multiple Tectonics-Related Magmatic Events and Model of South China Block Subduction during Eocene
In the southeastern Tibet Plateau, there are two major tectonic domains of the South China plate to the east and the Sanjiang region in the Southeast Asia Tethys domain to the west separated by the ASRR-SZ (Fig. 1). The South China plate is mainly composed of three parts: the Yangtze block to the north, the Cathaysia block to the south, and the Qin-Hang structural belt between them (Chen et al., 2019). Neoproterozoic magmatic rocks, including contemporaneous granitic and mafic rocks, are widely developed along the margins of the South China plate, especially along its northern and western margins (e.g., Qi et al., 2012; Cai et al., 2014, 2015; Wang et al., 2016; Li et al., 2018a, 2018b; Chen et al., 2019, Zhou et al., 2020) (Figs. 12 and 13).
The ASRR-SZ is an important tectonic belt located on the western margin of the South China plate and the southeastern margin of the Tibet Plateau, which has been widely advocated to be the result of the long-term interaction between the Indochina and South China blocks. In our study, according to the age statistics of all zircons from migmatites, leucogranites, and pegmatites, the protoliths of the granitic rocks in the ASRR-SZ have zircons with a wide age range from 1160 to 23 Ma. Most zircons exhibit inherited cores, showing a core-rim structure, whereas some, the Cenozoic ones, lack inherited cores and display an obvious oscillatory zonation. The zircon rims from the foliated migmatites and leucogranites give U-Pb ages of 41–28 Ma with low Th/U ratios (mainly <0.1), and the magmatic zircons from the unfoliated leucogranites present the ages of 26–23 Ma with high Th/U ratios (>0.3). The results of our U-Pb zircon dating, together with previous studies, demonstrate that several tectono-magmatic events occurred in the ASRR-SZ. The 1160–1105 Ma inherited cores may be related to Precambrian basement of the Indochina block, which formed in mid-Proterozoic times (Wang et al., 2020). Mid-Neoproterozoic magmatism during 810–751 Ma and 732–739 Ma has been recognized within the Ailaoshan massif (Figs. 12 and 13), the 630–790 Ma inherited cores have been proved to be related to the Neoproterozoic subduction-related magmatism in the Ailaoshan massif, which records the Neoproterozoic convergence around the Yangtze block and the welding of the Yangtze and Cathaysia blocks along the mid-Neoproterozoic Jiangnan orogen (Cai et al., 2014, 2015; Wang et al., 2016) (Fig. 13). A large number of tectono-magmatic and thermochronological data indicate that the ASRR tectonic zone records multi-episodic tectono-magmatic events at ca. 830–770 Ma, 420–470 Ma, 380–360 Ma, 240–250 Ma, and 20–35 Ma (Wang et al., 2010; Deng et al., 2014, Leloup and Kienast, 1993, Leloup et al., 1995; Searle, 2006; Cao et al., 2011a). Sm-Nd TDM model ages of high-grade metamorphic rocks in the ASRR-SZ range from 2.3 to 1.0 Ga (Zhai et al., 1990; Liu et al., 2020). The South China block is separated from the Simao block in the west by the Ailao Shan suture, which formed ca. 360–380 Ma (e.g., Wang et al., 2000; Jian et al., 2009a).
The distribution and assembly of the various blocks and fragments that constitute East and Southeast Asia records their origin along the northern margin of Gondwana, the opening and subsequent consumption of Tethyan Oceans, and final accretion to the growing Asian continent (e.g., Sone and Metcalfe, 2008; Metcalfe, 1996, 2006, 2013; Wang et al., 2016). Numerous igneous rocks relate to the Paleotethyan evolution of the Southeast Asia region. Wang et al. (2019) has suggested that a switch from subduction of the main East Paleotethyan ocean to the collision of the Sibumasu with Simao/Indochina blocks occurred at ca. 237 Ma. The timing of initial, syn-, and post-collision events along the Jinshajiang-Ailaoshan-Song Ma suture zone, with its record of back-arc basin closure, is at ca. 247 Ma, 247–237 Ma, and 237–200 Ma. Although, the temporal evolution of the East Paleotethyan system from subduction to collision is debated, in the magmatic inherited cores, the ages of 245–280 Ma, record the history of consumption of the Paleotethyan ocean, indicate the presence of Permo-Triassic magmatic and tectonothermal events during collision (Carter et al., 2001; Searle et al., 2010; Cao et al., 2015; Wang et al., 2019).
Most of Th/U ratios show that inherited cores bear a high (>0.3) ratio indicating magmatic crystallization. The ages of inherited cores concentrated in a few intervals, 1160–1105 Ma, 630–790 Ma, 549–414 Ma, 280–228 Ma, and 154–76 Ma (Fig. 7 K). The zircon rim ages from plutonic rocks give U-Pb ages of 41–28 Ma with low Th/U ratios, recording the timing of Cenozoic crustal anatexis and peak metamorphism along the ASRR-SZ. It is noteworthy that the Neoproterozoic inherited zircons in the plutonic rocks from this study together with studies previously reported in the ASRR-SZ are dominated by ages of 740–800 Ma (e.g., Cai et al., 2014, 2015; Liu et al., 2013, 2015a; Wang et al., 2016; Li et al., 2018a, 2018b; Wang et al., 2019). Studies also present two tectonic belts of mid-Neoproterozoic granitoids that occur in the South China block (summarized in Fig. 13), one is in the Jiangnan orogenic belt, which is a welding of the Cathaysia block with the Yangtze block at 850–800 Ma (Fig. 13B), and the other one is along the western margin of the Yangtze block, where granitoid ages are between 750 and 800 Ma (Fig. 13A). The inherited zircons along the ASRR-SZ with these age populations can give important constraints on source and type of crust, from which magma was extracted by anatexis. This could simply indicate thickened crust underneath the ASRR-SZ or from a deeper level, e.g., subducted crust. Such subducted crust of the South China block was reported by seismic surveys (e.g., Li et al., 2008; Zhang et al., 2012; Flower et al., 2013) and postulated by the genesis of K-rich magmas, which formed during the Eocene across the ASRR-SZ.
Tectono-magmatic analysis along the ASRR-SZ reveals a sequential tectonic process that occurred within the framework of India-Eurasia plate convergence. The new zircon U-Pb age data, coupled with our meso- and microstructural observations and textures, not only allow us to establish the temporal framework of Cenozoic crustal anatexis event along the ASRR-SZ, but also provide an alternative hypothesis correlating crustal anataxis and the thermal state related to formation of the ASRR-SZ. This new data contributes to the improvement of current geodynamic models for the post-collisional stage of the India-Eurasia plate interaction in Southeast Asia.
(1) The tectono-magmatic and thermochronological data indicate that the ASRR tectonic zone records inherited multi-episodic tectonothermal events. The ages of inherited zircon cores concentrate in a few intervals, 1160–1105 Ma, 630–790 Ma, 549–414 Ma, 280–228 Ma, and 154–76 Ma. The ages of zircon rims from the foliated migmatites and leucogranites give U-Pb ages of 41–28 Ma associated with low Th/U ratios, and the magmatic zircons from the unfoliated leucogranites present ages of 26–23 Ma with high Th/U ratios.
(2) The zircon rim ages from the regional migmatites and leucogranites record the timing of Cenozoic crustal anataxis and a peak metamorphic event along the ASRR-SZ that occurred between 41 and 28 Ma. The Ti-in-zircon temperature on zircon rims from migmatites and leucogranites range mainly from 749 to 781 °C, which is consistent with the studied metamorphic and crustal anatexis conditions. The termination of ductile left-lateral shearing of the ASRR-SZ is 28–23 Ma, possibly extending to 17 Ma, which is constrained by U-Pb isotope ages from the unfoliated leucogranites and pegmatites representing post-shearing intrusions.
(3) We propose that the initiation of the continental exhumed shear zone was at a depth where a pluton-controlled tectonic setting resulted in rheological weakening and localizing along hot-to-cold contacts deep within the crust lithosphere. Due to the ascension and emplacement of granite intrusions, the instability localizing the shear zone comes from rheological weakening of deep-seated levels of the wrench zone, which is a potential mechanism for the inception of major strike-slip shear zone development.
We gratefully acknowledge careful reviews and constructive suggestions by two anonymous reviewers. This work was financially supported by the National Key Research and Development Program (grant no. 2017YFC0602401), the National Natural Science Foundations of China (grant no. 41972220, 4188810), and the Excellent Youth Fund of the National Natural Science Foundation of China (Grant no. 41722207).