Petrogenesis Of Early Mesozoic Granites In Western Kunlun Orogen And Its Implications For Paleo-Tethys Tectonic Evolution

The western Kunlun orogen is located in the northwestern Tibet plateau, occupying a key tectonic position at the junction between the Pan-Asia and the Tethyan tectonic domains. The orogen adjoins the Tarim Basin in the north and the Tianshuihai Terrane in the south, bounded by the Oytag-Kudi suture and Mazha-Kangxiwa suture, respectively. At present, the acceptalbe viewpoint is that the western Kunlun orogen has undergone multi-period orogenic events. However, the tectonic evolutionary process of the western Kunlun orogen remains controversial. In addition, its orogenic history is closely associated with the final collage between the Eurasia and Qiangtang Block. There are two episode of granitoid magmatism in western Kunlun area, including the northern (Paleozoic) and southern (Mesozoic) magmatic belts. In this study, we presented the results of an integrated petrographic, mineralogical, geochronological, and geochemical study performed on six granitic plutons and their dark enclaves from the southern magmatic belt. The aim was finally to shed light on the long-debated tectonic evolutionary model of the Paleo-Tethys.In this work, we studied six granitic plutons (Yuqikapa, Muztaga, Taer, South Kudi, Arkarz and Mazha) and associated dark micrograular enclaves collected from the southern magmatic belt in the western Kunlun area. Most plutons are located in the Western Kunlun terrane, except for the Mazha pluton. Detailed SHRIMP zircon U-Pb dating indicates that these plutons and associated dark microgranular enclaves were emplaced in the Triassic (ca.243 to 209 Ma).The Yuqikapa pluton are emplaced in the Middle Triassic Anisian (～243 Ma), composed of two-mica granites (monzogranite-syenogranite-alkali-felspar granite), with medium granitic texture and gneissic structure. Main rock-forming minerals include K-feldspar, quartz, plaioglase, biotite and muscovite. These granites are high-K calc-alkaline and peraluminous. They show initial 87Sr/Sr ratios of 0.7071-0.7077, εNd(T) of -4.4 to -6.3 and εHf(T) (in-situ zircon) of -3.3. Elemental and isotopic data suggest that they were formed by "wet" anatexis of the Precambrian metasedimentary and metaigenous rocks in the continental collision (overthrust) zone.The Muztaga and Taer plutons were emplaced in the Late Triassic Carnian (-234-227 Ma), consisting of granodiorite and monzogranite. Minerals assemble includes plaioglase, K-feldspar, quartz, amphibole and biotite. These granites have medium granitic texure and massive structure, locally being gneissic. The samples are high-K calc-alkaline and most of them are metaluminous, but some samples are weakly peraluminous. The two plutons contain abundant dark microgranular enclaves and have SHRIMP zircon U-Pb ages of 233 to 226 Ma, consistent with them of their granitic host rocks. The Muztaga and Taer host rocks show initial 87Sr/86Sr ratios of 0.7052-0.7104, εNd(T) of -3.6 to -7.3 and sHf(T) (in-situ zircon) of -3.1 to-1.0. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metasedimentary and metaigenous rocks in the normal lower-crust (< 40 km) for the Muztaga pluton (close to the Mazha-Kangxiwa suture) and in the thickened lower-crust (-40-50 km) for the Taer pluton (away from the suture), triggered by underplating of coeval (233-226 Ma) mantle-derived enclave-forming magmas. The enclaves are basic (SiO2 248.0-51.6 wt.%) to intermediate (SiO2 53.0-54.6 wt.%) with high K2O (1.4-4.2 wt.%). Compared to their respective host granitoids, they generally have similar εNd(T) (-3.4 to -8.2), low initial Sr/Sr ratios (0.7034-0.7084) and high initial εHf(T) (-1.1 to+1.2). The enclave magams are considered to have been derived from the partial melting of subduction-modified mantle in the spinel stability field (<60 km) (enclaves in the Muztaga pluton) and the spinel-garnet transition zone (-60-80 km) (enclaves in the Taer pluton).Petrogenesis of the Middle Triassic Anisian and Late Triassic Carnian granites and associated mafic enclaves suggests that final closure of the Paleo-Tethys between the western Kunlun terrane and Tianshuihai terrane occurred in the Middle Triassic Anisian (-243 Ma). In the Late Triassic Carnian (234-227 Ma), the Carnian granites and associated dark microgranular enclaves were emplaced in a post-collisional extensional regime, casused by the break-off of Paleo-Tethyan oceanic slab.The South Kudi, Arkarz and Mazha plutons were emplcaed in the Late Triassic Norian (215-209 Ma) and show a southward-younging trend. The South Kudi pluton (215 Ma) is composed of high-K calc-alkaline granodiorite and monzogranite, with initial 87Sr/86Sr ratios of 0.7093-0.7099, εNd(T) of -4.9 to -5.4 and εHf(T) (in-situ zircon) of 0.3. Elemental and isotopic data suggest that the granitoids were generated by partial melting of the Precambrian metasedimentary-igneous basement in the normal lower crust (< 40 km) of the western Kunlun terrane triggered by underplating of basaltic magma. The Arkarz pluton (213 Ma) consists of high-K calc-alkaline monzogranite, syenogranite and alkali-feldspar granite and contains abundant microgranular envlaves, located in the south of South Kudi pluton. The host granites have initial 87Sr/86Sr ratios of 0.7071-0.7085, εNd(T) of -3.7 to -4.8 and εHf(T) (in-situ zircon) of -0.7, and were also generated by partial melting of the Precambrian metasedimentary-igneous basement in the normal lower-crust (< 40 km) of the western Kunlun terrane triggered by underplating of enclave-forming potassic magma. The enclaves are mainly basic (SiO2 48.0-54.9 wt.%) and potassic, showing similar geochemical characteristics to those of potassic magmas formed in the continental arc setting. They have initial 87Sr/86Sr ratios of 0.7071-0.7080 and εNd(T) o -2.6 to -4.5. Elemental and isotopic data suggest that the enclave magmas have been derived by partial melting of the metasomatized mantle wedge in the spinel-garnet transition zone (-60-80 km). The Mazha pluton (209 Ma) is located in the Tianshuihai terrane, composed of high-K calc-alkaline tonalite and granodiorite. It has initial 87Sr/86Sr ratios of 0.7087-0.7097, εNd(T) of -6.3 to -6.5 and εHf(T) of -2.8. Elemental and isotopic data suggest that the granitoids were generated by mixing of mantle-derived basaltic magma with crust-derived felsic magma.Petrogenesis of the Late Triassic Norian granites and assoicated dark microgranular enclaves suggests the western Kunlun-Tianshuihai terrane is an active continental margin, in response to the northward subduction of the Paleo-Tethys between the Tianshuihai and Karakorum terranes. Thus, the closure of the oceanic basin between the two terranes is not earlier than the age of 209 Ma.The collision of the Karakorum terrane with the Tianshuihai terrane most likely occurred during the Early Jurassic considering the onset of terrestrial red molasse deposition in the Tianshuihai terrane since the Early Jurassic.