The Paleozoic Jinshajiang ophiolitic mélange in southwest China marks an important branch ocean (i.e., the Jinshajiang Ocean) of the Paleo-Tethys. Basic-intermediate rocks are widespread features in the mélange; their formation age is well known, but the petrogenesis has not been well studied, which means that the evolutionary history of the Jinshajiang Ocean is not well constrained. To understand the nature of the mélange and the ocean, we present a set of elemental and isotopic data from two typical crustal sequences in two areas of the Jinshajiang ophiolitic mélange, Zhiyong and Baimaxueshan. The basalts in the ca. 343 Ma Zhiyong crustal sequence show mid-ocean-ridge basalt–like geochemical compositions with Nb/La ratios of 0.98–1.15 and εNd(t) values of +6.5 to +7.7, indicating that the basalts formed in the spreading ridge of the ocean. In contrast, the 283 Ma Baimaxueshan crustal sequence consists of gabbros and basaltic-andesitic lavas, which have an arc affinity with Nb/La ratios of 0.54–0.67 and εNd(t) values of +5.1 to +6.5. The geochemical differences were not caused by crustal assimilation but reflect mantle metasomatism by fluids dehydrated from the subducting slab. Therefore, we propose that the Zhiyong and Baimaxueshan crustal sequences formed in seafloor spreading and subduction settings, which were related to the opening and closure of the ocean, respectively.
Ophiolites are fragments of ancient oceanic lithosphere, including oceanic crust and the underlying upper mantle, which were generated in a variety of tectonic settings and then emplaced onto a continental margin. Specifically, ophiolites can be generated at almost every stage of the whole life cycle of oceanic basins, from continental rifting to seafloor spreading, ocean closure, and final continental collision (Dilek and Furnes, 2011, 2014; Pearce, 2014). Therefore, identification of the formation environment of ophiolites is crucial to reconstruct the plate history. Oceanic crust in ophiolites, including lavas and plutonic rocks, is generated by a combination of multiple processes, from mantle metasomatism to partial melting to magma fractionation; therefore, the crustal sequence carries an important record about the igneous processes that occurred during the formation of the oceanic lithosphere, which in turn can help to reveal the tectonic origin of ophiolites and provide insights into the evolution of oceanic basins (e.g., Miyashiro, 1973; Pearce and Robinson, 2010; Pearce, 2014).
The Paleo-Tethys Ocean was a Paleozoic ocean separating the Gondwana and Laurasia supercontinents; it started to open in the Devonian and closed in the Late Triassic (Şengör, 1987; Stampfli and Borel, 2002; Metcalfe, 2013). Paleo-Tethys–related suture zones containing ophiolitic mélanges are now preserved in the Sanjiang area, southwest China (Fig. 1; Metcalfe, 1996, 2006, 2011, 2013; Mo et al., 1998; Zhong, 1998; Deng et al., 2014; Wang et al., 2018). The Changning-Menglian suture marks the collisional closure of the main Paleo-Tethys Ocean, which separated the Baoshan-Sibumasu block to the west from the Qamdo-Simao block to the east (e.g., Metcalfe, 1996; Mo et al., 1998; Zhong, 1998; Jian et al., 2008, 2009a, 2009b). The Ailaoshan and the Jinshajiang sutures resulted from closure of a branch ocean or a back-arc basin (Zhong, 1998; Wang et al., 2000a). A typical ophiolite in the Ailaoshan suture zone, the Shuanggou ophiolite is analogous to those formed at slow-spreading ridges or at continental margins, with crustal sequences similar to normal mid-ocean-ridge-basalts (N-MORB; Yumul et al., 2008; Hu et al., 2015; Lai et al., 2014a). The Jinshajiang suture has been proposed to be connected with the Ailaoshan suture. However, previous studies on the ophiolite in the Jinshajiang suture mainly focused on geochronology (e.g., Jian et al., 2008, 2009b) but paid less attention to petrogenesis and geochemistry, especially to the basic-intermediate crustal sequence. As a consequence, we have limited knowledge of the Jinshajiang ophiolite and the lithosphere of the Jinshajiang Ocean, for example, whether the Jinshajiang Ocean developed from a back-arc basin due to the subduction of the main Paleo-Tethys Ocean plate (Metcalfe, 1996, 2006, 2011, 2013; Mo et al., 1998; Zhong, 1998; Jian et al., 2009a, 2009b; Fan et al., 2010). The relation between the Jinshajiang suture and the Ailaoshan suture also needs to be checked regarding the geochemical difference between crustal sequences in the Ailaoshan and Jinshajiang ophiolites (e.g., Jian et al., 2009a, 2009b; Hu et al., 2015; Lai et al., 2014a). To help resolve some of these issues, we carried out detailed elemental and isotopic studies of the crustal sequences (e.g., gabbros and basalts) of the Jinshajiang ophiolite and integrated the results with previous geochronological studies (e.g., Jian et al., 2008, 2009b). Our aims were (1) to constrain the mantle source and genesis of these rocks and (2) to clarify the tectonic history of the ocean.
GEOLOGICAL BACKGROUND AND SAMPLE PETROGRAPHY
The N-S–striking Jinshajiang ophiolite-bearing orogenic belt is located in the eastern part of the Sanjiang area, southwest China (Fig. 1; Mo et al., 1998; Zhong, 1998; Wang et al., 2000a; Jian et al., 2009a, 2009b; Wang et al., 2018). It separates the Qamdo-Simao block to the west and the Zhongza block to the east. The basement of the Qamdo-Simao block consists of the high-grade Damenglong and Chongshan complexes (Zhong, 1998; Wang et al., 2000a, 2000b, 2006). The Damenglong complex contains greenschists, metavolcanics, graywacke, iron formation, and amphibolite with granitoid intrusions (the Lincang batholith; Yang et al., 1994), and the Chongshan complex is composed of gneiss, amphibolite, and graywacke intercalated with phyllitic basic-intermediate metavolcanics and medium- to thick-bedded, fine- to medium-grained marble (Wei et al., 1984; Wu et al., 1984). Metasediments of the Lower Ordovician section, as the oldest exposed sedimentary rocks in the Qamdo-Simao block, are characterized by slates, phyllites, quartzites, and meta-limestones that are analogous to those in the Lower Ordovician of the Yangtze block. The Middle Devonian section unconformably overlies the Lower Ordovician and is characterized by basal conglomerates and shallow-marine sediments. Moreover, the Carboniferous and the Permian strata are mainly composed of shallow-marine, paralic, and continental sediments with coal measures in places, while the Lower–Middle Triassic strata consist of shallow-marine clastics and carbonates (Bureau of Geology and Mineral Resources of Yunnan Province [BGMRYP], 1990; Metcalfe, 2006). The variation in sedimentary facies indicates that the Qamdo-Simao block had become part of the Yangtze block before Middle Devonian time (Wang et al., 2000a, 2006; Zhong, 1998) but was later separated from the Yangtze block, thereby leading to the birth of the Jinshajiang Ocean.
In the Zhongza block, a massive basaltic flow with compositions comparable to the Emeishan flood basalt disconformably overlies the Lower Permian section, and it is overlain by the Upper Permian sedimentary rocks (Bureau of Geology and Mineral Resources of Sichuan Province [BGMRS], 1991). The Lower Permian section contains massive limestone-bearing purple pelitic bands and fossils such as Sumatrina longisssima, Neoschwagerina cheni, Chusenlla schwagerinae formis, Pseudodoliolina tubaensiss, and coral. The Lower Permian section in the Zhongza block is comparable with that in the Yangtze block. However, the Upper Permian section in the Zhongza block is characterized by more submarine sedimentary deposits, including sandstone, siltstone, and limestone and black shale interbedded with coal, relative to that in the Yangtze block. This feature indicates that the Zhongza-Zhongdian block was a part of the Yangtze block but was then separated from it in the Permian in response to the opening of the Garze-Litang Ocean in the late Paleozoic (Li et al., 2010).
The Jinshajiang orogenic belt has four parts, from east to west, including a foreland fold-and-thrust belt, an ophiolite mélange zone, a collision-related volcano-sedimentary zone, and a continental margin arc zone (Fig. 2; Zi et al., 2013). The foreland thrust zone is made up of a series of west-verging structural slices, including Devonian–Permian turbidites, carbonates, and basalts. These rocks were developed on the passive margin of the Yangtze block (Leloup et al., 1995; Li et al., 2002). The ophiolite mélange zone marks the ancient lithosphere of the Jinshajiang Ocean and is thought to connect with the Ailaoshan ophiolite mélange zone to the south (e.g., Wang et al., 2000a). The collisional volcano-sedimentary zone was built along the margin of the Qamdo-Simao block, and it is characterized by an association of bimodal (felsic and mafic) volcanic rocks with the Triassic turbidites and clastic sequences, suggesting an abyssal or bathyal environment (Mou and Wang, 2000; Yu et al., 2000; Tan, 2002). The continental margin arc is located at the eastern margin of the Qamdo-Simao block. It was generated by westward subduction of the Jinshajiang oceanic plate in the Permian (Zi et al., 2012a, 2012b, 2012c, 2013).
The Jinshajiang ophiolites consist of a number of blocks that occur within a N-S–striking tectonic mélange zone along the Jinsha River over a distance of hundreds of kilometers (Fig. 1). The ophiolite is composed of dismembered peridotite, cumulate rocks, and volcanic rocks with intercalated limestone and radiolarian cherts (Mo et al., 1998; Wang et al., 2000a). These rocks are blocks or olistoliths enclosed in a turbiditic sequence of Middle Triassic age unconformably covered by Late Triassic continental red sandstone (Faure et al., 2016). Mantle peridotites are dominated by harzburgites with minor dunites, which have been largely serpentinized. Large-size (1 km across) peridotites are exposed as irregular veins or strip bodies, while small-size (<100 m across) peridotites occur as dismembered lenticular bodies. Cumulate ultramafic rocks including harzburgites, dunites, and pyroxenites only occur in the Xumai area; these rocks are separated from the mantle rocks by faults (Mo et al., 1998). The typical rock assemblage in the cumulate rocks is represented by an association of layered gabbros and anorthites (Wang, 1985; Mo et al., 1998; Jian et al., 2008). Pillow lavas range from basalt to andesite in composition and are widely distributed in the mélange; the pillow structure can be up to 3 m in width with developed chilled rims, as well as radiated fractures, indicating they were produced by submarine eruption processes (Wang, 1985). This study carried out sampling in two typical areas of the Jinshajiang ophiolitic mélange, i.e., the Baimaxueshan area and the Zhiyong area. In the Baimaxueshan area, serpentinized harzburgites are scattered within sheared sandy silt, while the crustal sequence is mainly composed of layered gabbros and pillow lavas. The age of this crustal sequence has been constrained as 285–282 Ma based on zircon U-Pb dating from the gabbros (Jian et al., 2008). Gabbro samples exhibit a typical gabbroic/ophiolitic texture and consist of subhedral-euhedral plagioclase (∼65 modal%) and xenomorphic-subhedral granular clinopyroxene (∼30 modal%; Fig. 3A). The lavas are basalts and andesites with a typical porphyritic texture. Phenocrysts of plagioclase outnumber those of clinopyroxene (Fig. 3B). The groundmass of the andesites consists of small microlites of feldspar embedded in glass (Fig. 3C). In the Zhiyong area, the crustal sequence is composed of pillow lava and layered amphibole gabbros with ages of 343.5 ± 2.7 Ma (Jian et al., 2008, 2009b). The Zhiyong lavas are typical amygdaloidal basalts with porphyritic textures consisting of plagioclase phenocrysts in an intergranular groundmass (Fig. 3D).
Whole-rock major oxides were measured with a ME-XRF 06 spectrometer with <5% relative standard deviation at the ALS Laboratory, Guangzhou, China. Trace elements were analyzed with an inductively coupled plasma–mass spectrometer (ICP-MS) with an analytical precision better than 5% at the State Key Laboratory of Ore Deposit Geochemistry, Chinese Academy of Sciences (SKLODG), Guiyang, China. The detailed procedure for basic-intermediate rocks was described by Qi et al. (2000).
Whole rock Sr-Nd isotopes were analyzed on a Thermo Fisher Triton thermal ionization mass spectrometer (TIMS) at SKLODG. The analytical procedure was as follows: 0.11 g of rock powder was added in Teflon capsules with mixed HF + HNO3 + HClO4. After complete dissolution of the sample, the solution was dried, and 6 mol/L HCl was then added and evaporated to dryness (twice). The residue was dissolved with 1.5 mL HCl at 2.5 mol/L. After centrifugal separation, 1 mL of the supernatant was passed through a cation exchange resin (AG50W × 12), and 5 mol/L HCl was used to elute Sr. Then, 6 mol/L HCl was used to elute rare earth elements (REEs), and the solution was dried. Next, 0.1 mol/L HCl was passed through an anion exchange resin (P507), and 0.2 mol/L HCl was added to elute Nd. Mass fractionation corrections for Sr and Nd isotopic ratios were based on values of 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. Measured values were 87Sr/86Sr = 0.710250 ± 7 (2σ) for NBS987, and 143Nd/144Nd = 0.512612 ± 8 (2σ) and 143Nd/144Nd = 0.512104 ± 5 (2σ) for the BCR-2 and JNdi-1 standards, respectively.
Major-oxide and trace-element data are listed in Table 1. The Zhiyong lavas are basaltic in composition, with SiO2 from 48.48% to 51.42% and MgO from 6.28% to 7.58% (Fig. 4A). With decreasing MgO, Al2O3 decreases from 16.83% to 13.95% (Fig. 4B), while Fe2O3 increases from 7.18% to 10.76% (Fig. 4C). In the Baimaxueshan crustal sequence, the gabbros have SiO2 from 47.10% to 53.08% and MgO from 4.37% to 8.90%, while the lavas have SiO2 contents ranging from 46.56% to 58.48% and lower MgO contents from 2.74% to 4.91%, which show positive correlations with Al2O3 (12.19%–14.62%) and Fe2O3 (9.22%–11.93%; Figs. 4B and 4C).
The Zhiyong basalts display flat or slightly light (L) REE–enriched REE patterns with low (La/Yb)N (1.10–1.95) and (Sm/Yb)N ratios (1.23–1.36) and variable REE concentrations (35.9–91.5 ppm; Fig. 5A). In the primitive mantle–normalized trace-element diagram, the Zhiyong basalts do not exhibit any depletions in high field strength elements (HFSEs; e.g., Nb and Ta; Fig. 5B). In comparison, the Baimaxueshan lavas are characterized by flat or slightly LREE-enriched patterns (Fig. 5C), with low (La/Yb)N (1.00–1.78) and (Sm/Yb)N ratios (1.01–1.27) and relatively constant REE concentrations (50.46–63.22 ppm), but in a primitive mantle–normalized spider diagram, they show considerable depletion in HFSEs (e.g., Nb, Ta) and variable enrichment in Pb (Fig. 5D). The gabbros exhibit trace-element features (Figs. 5E and 5F) similar to the Baimaxueshan lavas. In particular, the lavas show variable depletions of Sr, while the gabbros exhibit positive Sr anomalies, indicating that Sr concentrations were mainly controlled by the crystallization of plagioclase.
Isotope data are listed in Table 2. The initial isotope compositions were calculated for ages of 343 Ma and 283 Ma for the Zhiyong and Baimaxueshan rocks, respectively (Jian et al., 2008, 2009b). The Zhiyong lavas without any Nb depletion tend to have slightly higher εNd(t) values (6.5–7.1) but lower (87Sr/86Sr)i ratios (0.704052–0.705452) than those of the Baimaxueshan lavas and gabbros, which have εNd(t) values from +5.1 to +6.5 and (87Sr/86Sr)i values from 0.704531 to 0.707347 (Fig. 6).
MORB Preserved in the 343 Ma Zhiyong Crustal Sequence
The Zhiyong crustal sequence contains trace-element patterns (e.g., flat REE chondrite-normalized patterns; Fig. 5) and isotope compositions (εNd[t] from +6.5 to +7.1; Fig. 6) that have strong affinities to enriched (E) MORB. The negative correlations between Fe2O3 and MgO suggest that the fractionation of clinopyroxene was limited (Fig. 4C), whereas the overall positive correlations between Al2O3 and MgO reflect the fractionation of plagioclase and olivine (Fig. 4B). Scandium (Sc) is a useful indicator for the fractionation of clinopyroxene because it is highly compatible with clinopyroxene but incompatible with plagioclase and olivine (Niu et al., 2002). Similar to the correlation between Fe2O3 and MgO, Sc steadily increases with decreasing MgO (Fig. 4D), implying the absence of clinopyroxene during fractionation. Thus, according to the correlation between the major elements, the fractionation crystallization sequence was dominated by olivine and plagioclase with negligible clinopyroxene fractionation. The fractionation sequence is similar to that of MORB, which takes place in a low-pressure anhydrous environment, because petrological experiments illustrate that decompression can expand the liquidus crystallization fields of plagioclase and olivine and suppress the field of clinopyroxene (O’Hara, 1968; Bender et al., 1978; Presnall et al., 1978, 1979; Grove et al., 1982; Fujii and Bougault, 1983), and a high magmatic water content, which is associated with subduction, can expand the stability field of clinopyroxene at the expense of plagioclase (Gaetani et al., 1993).
All these features support the interpretation that the Zhiyong basalts formed by upwelling of the asthenosphere beneath a spreading ridge. Compared with the diabases from the Ailaoshan ophiolite (Xu and Castillo, 2004; Hu et al., 2015), which represent the oceanic crust of the Ailaoshan Ocean, the Zhiyong basalts show not only a relatively enrichment in LREEs, but also slightly lower Nd isotope ratios (Fig. 6). Because there is no evidence of involvement of a slab component or crustal assimilation (e.g., Nb-Ta depletion), the data illustrate that the asthenosphere beneath the ocean was similar to that of enriched depleted MORB mantle (E-DMM) as proposed by Workman and Hart (2005), providing evidence of heterogeneity in the upper mantle of the Paleo-Tethys. The melting of the source was in the low-pressure spinel field, as reflected by the flat REE pattern of the lavas. Following the batch melting equation of Shaw (1970), the Zhiyong lavas were generated by moderate-degree (∼10%) partial melting (Fig. 7).
Oceanic Subduction Recorded by the 283 Ma Baimaxueshan Crustal Sequence
In comparison with the Zhiyong lavas, the lavas and gabbros in the 283 Ma Baimaxueshan crustal sequence are characterized by significant depletions in HFSEs (e.g., Nb) with relatively lower εNd(t) values and higher Sr isotope values (Figs. 5 and 6). These geochemical features clearly indicate the involvement of sediments in the genesis of the Baimaxueshan crustal sequence, which was either related to metasomatism in the mantle source or crustal contamination during magma fractionation (Jahn et al., 1999; Fan et al., 2010; Bezard et al., 2014). To discriminate between these two processes, the relations between indices of magma fractionation (e.g., SiO2) and whole-rock trace elements are often used (e.g., Fan et al., 2010; Bezard et al., 2014). As shown in Figure 8, the subchondritic Nb/La ratios and εNd(t) values remain constant irrespective of the variation of the SiO2 contents, implying that these values were not controlled by magma fractionation. In other words, these compositional features of the Baimaxueshan crustal sequence were unrelated to crustal contamination processes, so they were more likely a result of mantle metasomatism.
Located in the foreland thrust zone, the Xiaruo-Tuoding area contains a complete stratigraphic sequence from the Devonian to Permian, which developed on the volcanic-rifted passive margin of the Yangtze block (Feng et al., 1999; Sun and Jian, 2004; Xiao et al., 2008). The sequence records detailed information about the evolution of the ocean. In the recovered stratigraphic sequence, the Upper Devonian sequence is composed of gray marlite, crystalline limestone, and black mudstone, indicating a neritic-bathyal setting. Moreover, the Lower Carboniferous consists of gray phyllite and quartz schist intercalated with bathyal fossiliferous crystalline limestone (Feng et al., 1999). Basalts in the Lower Carboniferous show affinities with the oceanic-island basalt (OIB; Zhang et al., 1996). The Upper Carboniferous consists of carbonate breccia and turbidite limestone with synsedimentary slump and scour structures. The Upper Carboniferous also is composed of basic volcanic breccia with OIB-like geochemical compositions (Fig. 5; Xiao et al., 2008). The lithology of the Lower Permian section is complex, including turbidite sandstone, pebbled sand shale, conglomerate, carbonate-slipping rock, breccia, turbidite limestone, crystalline limestone, braided limestone, and basalt. Furthermore, the characteristics of the Upper Permian sedimentary combination indicate that the area was located in a continental slope to shallow sea environment. Based on the characteristics of the stratigraphic sequence, Sun and Jian (2004) proposed that the Jinshajiang Ocean was a mature ocean in the Carboniferous after continental breakup in the Late Devonian, and it started to close in the Permian.
Therefore, the previous discussion indicates that the Baimaxueshan crustal sequence likely formed in an arc system during the closure stage of the Jinshajiang Ocean, when the mantle source had been metasomatized by fluids or melts released from the slab. We further propose that this crustal sequence formed in an oceanic arc rather than a continental arc, where crustal assimilation is very common. The oceanic arc setting is also consistent with previous studies on the 283 Ma tonalite in the Jiyudu area of the Jinshajiang mélange (Zi et al., 2012b). In general, the development of oceanic subduction can be separated into three stages (Ishizuka et al., 2011; Reagan et al., 2010; Whattam and Stern, 2011): (1) When oceanic lithosphere starts to sink, the asthenosphere will upwell into the overlying void and further melt due to decompression, leading to the formation of forearc basalts, which are chemically similar to MORB (Reagan et al., 2010). (2) As subduction continues, the depleted mantle will further melt due to the slab input. Magma in this stage is generally boninitic, with high SiO2 and MgO contents, enriched in the fluid mobile trace elements (e.g., Rb, Ba, K, LREEs) but depleted in fluid immobile elements (e.g., heavy [H] REEs and HFSEs). The REE patterns of boninites are usually U-shaped, reflecting middle (M) REE depletion relative to LREEs and HREEs (Crawford et al., 1989; Tatsumi and Maruyama, 1989; Taylor et al., 1994; Falloon and Danyushevsky, 2000). (3) The “normal” arc lavas then form as the subduction zone steps into the mature stage with a change to flux melting in counterflowing mantle. The magmas in different stages of subduction form different series. On the one hand, boninite is a special rock type in a subduction zone, with >53 wt% SiO2, TiO2 < 0.5 wt%, and Mg# > 0.65 (Crawford et al., 1989). Boninites may be the primary magma of many high-magnesium andesites (HMA) in an arc system. On the other hand, normal arc lavas range from basaltic andesites (BA) to andesites (A), dacites (D), and rhyolites (R) (the BADR series). From Figure 4A, it is clear that the Baimaxueshan lavas are clearly distinguished from the HMA series, but they vary within the range of the BADR series. Therefore, the 283 Ma Baimaxueshan crustal sequence was not related to boninites, because it has an affinity with normal arc lavas. The available evidence suggests that the Baimaxueshan crustal sequence marks a mature oceanic arc in the Jinshajiang Ocean.
In subduction zones, sediments and altered oceanic crust (AOC) of the subducting slab contribute to the overlying mantle wedge, which eventually leads to chemical variations of the lavas (Nielsen and Marschall, 2017). These crustal materials may be transported though melts and fluids that are released from the sediments and AOC of the slab (Brenan et al., 1995; Kessel et al., 2005; Plank, 2005). The effects of melt and fluid metasomatism are different. Fluids from dehydration of a slab are enriched in the fluid mobile elements and depleted in fluid immobile elements, while melts from melting of a slab are enriched in both fluid mobile and fluid immobile elements. For example, a slab fluid has lower Nd concentrations and in turn has minor effects on the Nd isotopic compositions of the mantle source overlying the subducted slab. In contrast, slab melts have higher Nd concentrations with radiogenic Nd isotopic compositions, and, consequently, metasomatism by slab melts can significantly change the Nd isotopic composition of the upper mantle (Peng et al., 2008). Therefore, still-unradiogenic Nd isotope ratios in the Baimaxueshan crustal sequence favor a slab-derived fluid modification process. Both Th and Nd are not mobile during slab dehydration (Brenan et al., 1995); therefore, the input of a slab fluid may not change the Th/Nd ratio of the source and of the lavas. In contrast, sediments usually have higher Th/Nd ratios than the mantle, and it has been experimentally determined that a sediment melt has much higher Th/Nd ratios (Johnson and Plank, 2000); therefore, the addition of a high-Th/Nd slab melt will greatly change the composition of the source and the lavas. As to the Baimaxueshan lavas, in comparison to the variation of Sr/Nd ratios and Nd isotope ratios, the Th/Nd ratios are very low with limited variations (Fig. 9). This suggests that the predominate metasomatizing agent was a fluid dehydrated from a slab rather than a slab melt. We used two end-member mixing to estimate the nature and the proportion of involved fluid in the mantle source (Li et al., 2012). Our quantitative calculations indicate that the source of the Baimaxueshan crustal sequences had been modified by addition of <5% sediment-derived fluid (Fig. 10).
Comparison between the Jinshajiang and Ailaoshan Sutures
The Ailaoshan suture zone is located to the southeast of the Jinshajiang suture zone (Fig. 1), and it has lithological assemblages and deformation-metamorphic history comparable to the Jinshajiang suture zone. Therefore, it has been widely accepted that the two sutures are contiguous and represent the same ocean basin, i.e., the Jinshajiang-Ailaoshan Ocean (Wang et al., 2000a). This point is important for reconstructing the paleogeography of the Paleo-Tethys. Nevertheless, by integrating our data with relevant information in the literature, several differences between the two sutures should be taken into consideration.
First of all, the Ailaoshan and Jinshajiang sutures are ∼300 km apart (Fig. 1). The ophiolite in the Ailaoshan suture zone is characterized by the dominance of lherzolite in the mantle sequence, minor portions of the mafic rocks, and absence of sheeted dikes. These features are similar to those of lithosphere at slow-spreading ridges or at continental margins (Pamić et al., 2002; Schaltegger et al., 2002; Montanini et al., 2008). Jian et al. (2009b) reported Late Devonian ages for a diabase (382.9 ± 3.9 Ma) and a plagiogranite sample (375.9 ± 4.2 Ma), indicating that the oceanic crust preserved in the Ailaoshan ophiolite is older than the oceanic crust preserved in the Jinshajiang ophiolite. These temporal and spatial differences thus indicate diachronous opening of the Jinshajiang-Ailaoshan Ocean (Zhong, 1998; Wang et al., 2000a).
Second, high-grade metamorphic rock assemblages in the Ailaoshan suture are considered to be representative of the continent-ocean transition zone, and they are in fault contact with the Yangtze block to the east and an olistostrome to the west (Jian et al., 2009a, 2009b). These geological features are characteristic of a nonvolcanic rifted margin (Froitzheim and Manatschal, 1996; Hopper et al., 2004), and on these grounds, Jian et al. (2009b) proposed a detached-rifting system, which caused the thinning of the lithosphere and the seafloor spreading of the Ailaoshan segment. In contrast, the Jinshajiang ophiolitic mélange is associated with a volcanic-rifted margin, as indicated by the stratigraphic sequence in the Xiaruo-Tuoding area. Mafic volcanic rocks interbedded in Carboniferous and Permian sedimentary rocks exhibit OIB-like geochemical compositions with high LREE and HFSE contents (Fig. 5; Zhang et al., 1996; Xiao et al., 2008) and Nd isotope compositions (εNd[t] = −1.43 to +1.9) close to primitive mantle (Fig. 6; Xiao et al., 2008). It should be noted that these geochemical and isotopic signatures are very similar to those of the Emeishan flood basalts, which are located to the east of the Jinshajiang suture zone (Fig. 1; Xiao et al., 2008).
Last, the diabases in the Ailaoshan ophiolite, which represent ancient oceanic crust, have N-MORB–like trace-element characteristics with LREE-depleted patterns and positive εNd(t) values (+9.7 to +11.6; Figs. 5 and 6; Hu et al., 2015). In contrast, the Zhiyong crustal sequence, which represents oceanic crust in the spreading ridge of the Jinshajiang Ocean, is characterized by E-MORB–like compositions with higher LREE/HREE ratios and lower εNd(t) values. The presence of E-MORB at mid-ocean ridges has long been considered as evidence for plume-ridge interactions (Schilling, 1973; Schilling et al., 1983; Le Roux et al., 2002; Cushman et al., 2004). Together with the presence of the volcanic-rift margin containing OIB-like basalts, we propose that, unlike the Ailaoshan segment, the genesis of the oceanic crust in the Jinshajiang segment of the Ailaoshan-Jinshajiang Ocean was affected by a long-lived Paleo-Tethys mantle plume, since the stage of continent rifting. The Paleo-Tethys mantle plume might have subsequently produced the ca. 260 Ma Emeishan flood basalt (Xiao et al., 2008).
Evolution of the Jinshajiang-Ailaoshan Ocean
The tectonic environment of the Jinshajiang-Ailaoshan Ocean has been hotly debated (Fan et al., 2010; Jian et al., 2009a, 2009b). To date, two different tectonic settings have been proposed: (1) a subduction-related back-arc basin (Metcalfe, 1996, 2006; Wang et al., 2000a) and (2) a subduction-unrelated branch ocean with a slow spreading velocity (Mo et al., 1998; Zhong, 1998). In this contribution, we propose that the first model of subduction-related back-arc basin is inappropriate. First, in terms of regional geology, the requisite associated arc related to subduction of the main Paleo-Tethys Ocean and the opening of the Jinshajiang-Ailaoshan Ocean has not been located or reported. Second, as proposed in this study, the 343 Ma Zhiyong crustal sequence represents the oceanic crust of the Jinshajiang Ocean, while the oceanic crust in the Ailaoshan ophiolite is ∼383 m.y. old. Additionally, the oldest igneous rocks in the Jinshajiang mélange are the low-Ti continental flood basalt (CFB) xenoliths (ca. 443–401 Ma), which show enriched LREE and high Ba, Sr, Nb, and Ta concentrations, indicating the onset of crustal extension (Jian et al., 2009b). These ages are much older than the age of subduction of the main Tethys Ocean, which was in the Early Permian, as suggested by the geochronological data of Jian et al. (2009b). For example, Alaskan-type mafic-ultramafic complexes and arc rocks formed by subduction of the main Paleo-Tethys plate yield zircon ages of 306.2 ± 4.6 Ma for a tonalite, 301.0 ± 2.9 Ma for a microgabbro, 297.1 ± 2.0 Ma for a plagioclase hornblendite, and 281.3 ± 1.7 Ma for a diabase (Jian et al., 2009b). Third, the oceanic crust of the Jinshajiang-Ailaoshan Ocean, represented by the 343 Ma Zhiyong crustal sequence and the 383 Ma Ailaoshan diabases, lacks evidence of involvement of slab components during its genesis. Based on this evidence, we therefore propose that the Jinshajiang-Ailaoshan Ocean represents a branch ocean rather than a back-arc basin in the Paleo-Tethys. The evolution of the Jinshajiang-Ailaoshan Ocean is presented in Figure 11.
In the Early Devonian, the Qamdo-Simao block started to rift from the Yangtze block (Fig. 11A). The rifted western Yangtze margin consisted of two segments, the Ailaoshan segment in the south and the Jinshajiang segment in the north (Wang et al., 2000a). The Ailaoshan segment was a nonvolcanic rift margin (Jian et al., 2009a, 2009b). Juxtaposition of lower-crustal and mantle rocks in the high-grade Ailaoshan metamorphic complex indicates that detachment faulting caused the removal of crust and consequent exhumation of mantle (Jian et al., 2009a, 2009b; Hu et al., 2015). Crustal extension of the Jinshajiang segment might have taken place at 443–401 Ma, as recorded by the low-Ti CFB xenoliths in the Jinshajiang ophiolite (Jian et al., 2009a, 2009b). OIB-like basalts preserved in the Carboniferous–Permian strata postdate the rifting of the Jinshajiang segment, indicating that the rift margin was distal to the mantle plume in the Early Devonian.
Seafloor spreading in the Ailaoshan segment occurred at ca. 380 Ma (Fig. 11B), leading to the generation of the Ailaoshan ophiolite (Jian et al., 2009a, 2009b). The lithological characteristics of the Ailaoshan ophiolite, including the dominance of lherzolite in the mantle sequence, minor portions of mafic rocks, and absence of the sheeted dikes, is similar to those formed at slow spreading ridges (Pamić et al., 2002; Schaltegger et al., 2002; Montanini et al., 2008). Oceanic crust in the Ailaoshan segment is similar to N-MORB (Hu et al., 2015). As a result, the Ailaoshan segment has been interpreted as a slow-spreading oceanic basin (Zhang et al., 1995; Zhong, 1998; Jian 2009a, 2009b). In contrast, the Jinshajiang segment was still at the rifting stage at that time. At 340 Ma, deep melt from the mantle plume–established rift systems transformed the established Jinshajiang rift system into a volcanic rift margin and formed the OIB-like basalts preserved in the volcanic rift margin. This led to the seafloor spreading in the Jinshajiang segment (Fig. 11C). Meanwhile, the asthenosphere in the Jinshajiang segment was influenced by the plume, and therefore could produce oceanic crust comparable to E-MORB.
After reaching its maximum expansion in the late Carboniferous, the Jinshajiang–Ailaoshan Ocean started to close by westward subduction under the Qamdo-Simao block in the Early Permian. Oceanic subduction took place almost simultaneously in the Jinshajiang and Ailaoshan segments (Fig. 11D), as documented by the 280 Ma Baimaxueshan crustal sequence in the Jinshajiang ophiolitic mélange and the 287 Ma Wusu basalt in the Western Ailaoshan volcanic belts (Fan et al., 2010). Subduction of the Jinshajiang oceanic lithosphere promoted the Zhongza block to separate from the Yangtze block, resulting in opening of the Garze-Litang Ocean along the western Yangtze during Early Permian time (Li et al., 2010). The closure of the ocean also caused the change in the relative position between the mantle plume and the Yangtze block, which promoted formation of the ca. 260 Ma Emeishan flood basalt to the east of the Jinshajiang Ocean (Fig. 11D; Xiao et al., 2008). The consumption of the ocean transited to collision between the Qamdo-Simao block and the Yangtze block in the Late Permian–Early Triassic, with formation of associated syncollisional granitoid plutons close to the Baimaxueshan crustal sequence (Zi et al., 2012a) and within the Western Ailaoshan volcanic belts (Fig. 11E; Lai et al., 2014b). Meanwhile, crust formed either in the spreading ridge or in the oceanic arc was accreted to the front of the arc. These rocks acted as olistostromes enclosed in a Middle Triassic turbiditic sequence in the accretionary prism (Wang et al., 2000a; Faure et al., 2016). The collision then resulted in the emplacement of the ophiolitic mélange and ended in the Triassic, as indicated by regional Permian to Triassic disconformities (BGMRYP, 1990).
In this study, we focused on two different crustal sequences of the Jinshajiang ophiolite. Based on previous geochronological studies, it is proposed that these two crustal sequences record different stages in development of the Jinshajiang Ocean. The 343 Ma Zhiyong lavas show geochemical affinities to E-MORB, suggesting that they formed in the spreading ridge of the Jinshajiang Ocean. In contrast, the 283 Ma Baimaxueshan lavas and gabbros have volcanic arc–like compositions with depletions in HFSEs and lower Nd isotope ratios, indicating that the Baimaxueshan crustal sequence formed in an oceanic subduction zone. Comparison with the Ailaoshan ophiolite highlights the influence of a mantle plume on the evolution of the Jinshajiang suture zone. Integrating our data with data in the literature, we proposed an evolutionary model of the Jinshajiang-Ailaoshan Ocean from its rifting in the Devonian to final collision between the Qamdo-Simao block and the Yangtze block in the Late Permian–Early Triassic.
We appreciate the assistance of Jing Hu, Yifan Yin, and Xiaobiao Li with trace-element and Sr-Nd isotope measurements at the State Key Laboratory of Ore Deposit Geochemistry, Chinese Academy of Sciences. This study was jointly supported by the National Natural Science Foundation of China (41425011) and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18000000). We are grateful to Xiao Wenjiao, Michel Faure, and an anonymous reviewer, and the editor, Damian Nance, who kindly provided constructive comments that greatly improved the quality of the manuscript. We would also like to thank Xiaohu He, Chang Zhang, and Qiwei Li for their suggestions and discussions during the revision of this paper.