Petrogenesis And Geodynamic Setting Of Permian To Triassic Mafic Rocks In The Yushu Area, Central Qinghai-tibetan Plateau

The Qinghai-Tibet Plateau is the highest, largest and youngest plateau in the world. It has experienced a complex Tethyan tectonic evolution (Proto-Tethys, Palaeo-Tethys and Neo-Tethys included) and is generally recognized as the best natural laboratory for studying continental geodynamics and plate-tectonics on a global scale. The central Qinghai-Tibet Plateau is mainly characterized by the development of the Palaeo-Tethyan tectonic-magmatic system, which involves the complex evolutionary processes of multiple terranes and suture zones, and provides an important window into understanding Palaeo-Tethyan tectonic evolution and mineralization of Qinghai-Tibet Plateau and even the whole world. In the past few decades, most scholars at home and abroad have focused on the study of Palaeo-Tethyan tectonic evolution of the central-southern Qinghai-Tibet Plateau, whereas the study on the central-western Qinghai-Tibet Plateau is seldom. Whether the Jinsha and Garze-Litang oceans extend to the central-western area and the area has experienced the same Permian to Triassic tectonic evolution as the central-southern area are still unknown.The Yushu area is located in the joint region of the western Jinshajiang and Ganze-Litang suture zones. With the subduction of Palaeo-Tethyan oceans and the final closure, Permian to Triassic magmatic rocks are widespread in the area and the mafic rocks have a broader age range than that of the felsic rocks. Therefore, the author chose the Permian to Triassic mafic rocks as the research object, and detailed researches on their zircon U-Pb geochronology, petrography, mineralogy, geochemistry and Sr-Nd isotopic composition were carried out. What’s more, the previous research results on this area were combined to deeply analyze the genetic mechanism and tectonic setting, and the relevant scientific issues are discussed on the basis of existing knowledge. At last, a new model for the Palaeo-Tethyan tectonic evolution of the central-western Qinghai-Tibet Plateau was built. The main results are obtained as follows: 1. The ages of the mafic rocks from the Yushu area range from Early Permian to Late Triassic (Zircon U-Pb ages:284-213Ma). And they are largely concentrated in four periods, which includes Early Permian (-284Ma), Late Permian (258-257Ma), Middle Triassic (236-232Ma) and Late Triassic (-213Ma). Zircon LA-ICP-MS U-Pb results yielded nine206pb/238U ages of284±2Ma,258±2Ma,258±Ma,258±2Ma,257±Ma,232±Ma,234±3Ma,236±2Ma and213±2Ma, which could be interpreted as the crystallization ages of Zaduo gabbro porphyrites, Yushu gabbros, Haxiu gabbros, Zhiduo dolerites, Zhiduo isotropic gabbros, Gequ gabbros, Chayong pillow basalts, Xiewu gabbros and Longbao lamprophyric dikes, respectively.2. Early Permian mafic rocks are rarely exposed in the Yushu area and their occurrences are generally associated with ultramafic rocks. The Zaduo mafic-ultramafic rocks are composed of gabbros, gabbro porphyrites and olivine pyroxenites and they intruded the Carboniferous Zaduo group of North Qiangtang block as dikes. Olivines from the olivine pyroxenites have relatively high Fo values (78.47-81.75) and they belong to peridot in composition. Pyroxenes from the gabbros are characterized by relatively high Ca, Ti and Mg, which are similar to pyroxenes from the Panzhihua mafic-ultramafic rocks. And they belong to diopside in composition. The Zaduo mafic-ultramafic rocks belong to the alkaline basalt series in composition. In the MORB-normalized trace element spider diagrams, all the samples show strong enrichment of LREE, Th, Nb, Ta and Ti, slight depletion of Zr and Hf, generally comparable to OIB and Emeishan basalts. Relatively high εNd(t) values (+3.7-+3.9,+2.2and+3.8-+4.2, respectively) and low Isr values (0.706-0.712,0.704and0.707-0.708, respectively) suggests a depleted mantle. Based on mineralogical, geochemical and Sr-Nd isotopic compositions, we suggest that the Zaduo mafic-ultramafic rocks were derived from an OIB-like asthenosphere mantle, and generated by partial melting of garnet+minor spinel Iherzolite. In addition, the olivine pyroxenite represents mixtures of70-80vol%with20-30vol%cumulus silicate minerals (predominantly olivine) and the gabbro hadn’t undergone the accumulation of pyroxene. Combining with regional data, we proposed that the Zaduo mafic-ultramafic rocks could have formed in a continent rift, which was related to Early Permian spreading of the Longmu Co-shuanghu ocean basin.3. Late Permian mafic rocks are widely distributed in the Yushu area relative to the Early Permian mafic rocks and they also have more complex chemical compositions. The Yushu gabbros intruded into the Yushu-Zhongdian block as dikes. Note that the block might be a part of northern North Qiangtang block. Pyroxenes from the gabbros display relatively high Ti and Fe. And they belong to augite in composition and follow the rift-cumulate trend. The gabbros are characterized by significant high FeOt and TiO2, similar to the typical Fe-Ti rich mafic rocks from the Panzhihua, Eastern Greenland and Northern Somalia. The rocks are plotted in the area between the sub-alkaline and alkaline basalt series in the Zr/TiO2-Nb/Y classification diagram and they have relatively high REE. In the MORB-normalized trace element spider diagrams, they display strong enrichment of LREE and Th, depletion of P and weak depletion of Nb, Ta and Ti, similar to OIB. The rocks have relatively low εNd(t)(0-+0.7) and high Isr(0.706-0.709). The Haxiu gabbros are distributed in the Duocai-Longbao ophiolitic melange as slices and some olivine pyroxenites xenoliths are well preserved in the gabbros. Olivines from the olivine pyroxenites have relatively high Fo values (78.29-83.80) and they belong to peridot in composition. Pyroxenes from the gabbros are characterized by relatively high Mg and Cr and low Ti. And they belong to diopside in composition and follow the arc-cumulate trend. The Haixu gabbros and olivine pyroxenites have similar chemical composition to the primitive magmas and they belong to the alkaline basalt series in composition. The two rock types have very low REE and weak LREE enrichment. In the MORB-normalized trace element spider diagrams, they show enrichment of LREE and Th, obvious depletion of Nb, Ta, P and Ti, generally comparable to the IAT from intra-oceanic arcs and the primitive arc magmas which were derived from a subduction-modified asthenosphere mantle underneath the continental arc, Antarctic Peninsula. The two rock types also have relatively low εNd(t)(-0.6-0and+0.7-+1.7, respectively) and high Ir (0.709-0.712and0.709, respectively). The Zhiduo mafic rocks are exposed in the Duocai-Longbao ophiolitic melange as slices and massifs and they were usually regarded as components of ophiolite. The Zhiduo layered gabbros and dolerites have the same chemical compositions as the Yusu gabbros. They are also plotted in the area between the sub-alkaline and alkaline basalt series in the Zr/TiO2-Nb/Y classification diagram and they have significant high FeOt and TiO2and OIB-like compositions. The two rock types have relatively low eNd (t)(+0.7and+0.3-+0.5, respectively) and high Isr (0.707and0.707, respectively). However, the Zhiduo isotropic gabbros have moderate FeOt and TiO2and belong to the sub-alkaline basalt series in composition. In the MORB-normalized trace element spider diagrams, they are characterized by enrichment of LREE and Th, obvious depletion of Nb, Ta, P and Ti, similar to the mafic rocks derived from a subduction-modified lithospheric mantle. And they have relatively low εNd(t)(-1.2-1.0) and high ISr (0.708-0.711). Based on mineralogical, geochemical and Sr-Nd isotopic compositions, we suggest that the Yushu gabbros, Zhiduo layered gabbros and dolerites have similar genetic mechanisms related to partial melting of spinel-garnet Iherzolite mantle source metasomatized by subducted, slab-derived melts and aqueous fluids. And high degree of fractional crystallization dominated by olivine and clinopyroxene coupled with low fO2resulted in their Fe-Ti enrichment. The Haxiu gabbros could be derived from an asthenosphere mantle metasomatized by aqueous fluids and related to partial melting of a spinel Iherzolite mantle source. The Haxiu olivine pyroxenites were generated by relatively high degree of partial melting of a spinel Iherzolite mantle source. The Zhiduo isotropic gabbros might be derived from an lithospheric mantle metasomatized by aqueous fluids and sediments and related to partial melting of a spinel Iherzolite mantle source. Combining with regional data, we proposed that the Late Permian mafic rocks could have formed in a back-arc extension environment, which might be related with Late Permian rollback of the Longmu Co-shuanghu oceanic slab.4. The Middle Triassic Gequ mafic rocks are mainly composed of gabbros and intrude the Triassic strata in the North Qiangtang block as dikes. Note that the Triassic strata conglomerates unconformably overlying Triassic to Permian marine carbonates. Pyroxenes from the gabbros display relatively high Ca, Ti and Fe. And they belong to diopside in composition and follow the rift-cumulate trend. The rocks belong to the tholeiitic basalt series in composition and they show enrichment of LREE and Th and depletion of Nb, Ta and Ti relative to MORB. All the rocks have relatively lowd (t)(+0.5-+2.0) and high Ir (0.706-0.708). Based on mineralogical, geochemical and Sr-Nd isotopic compositions, we suggest that the Gequ mafic rocks could be derived from a lithospheric mantle metasomatized by5-10vol%crustal materiels, and generated by partial melting of a spinel+minor garnet Iherzolite mantle source. And fractional crystallization of olivine, clinopyroxene and plagioclase is also suggested. Combining with regional data, we proposed that the Gequ mafic rocks could have formed in a post-collisional extension environment, which might be related with Middle Triassic break-off of Longmu Co-shuanghu oceanic slab.5. The Middle Triassic Chayong and Xiewu mafic rocks were generally regarded as the significant components of the western Garze-Litang ophiolite. Pyroxenes from the Chayong gabbros have relatively high Ca, Al, Ti and Mg. And they belong to diopside in composition and follow the rift-cumulate trend. The Chayong mafic rocks belong to the tholeiitic basalt series in composition. In the MORB-normalized trace element spider diagrams, they are characterized by enrichment of Th and LREE, weak depletion of Nb, Ta, Zr, Hf and Ti, comparable to E-MORB. All the rock types (pillow basalts, gabbros and dolerites included) have relatively high εNd (t)(+2.7-+3.6,+3.1-+3.4and+1.7, respectively) and low Isr (0.705-0.708,0.705and0.706, respectively). The Xiewu pillow basalts belong to the tholeiitic basalt series in composition and they have low REE. In the MORB-normalized trace element spider diagrams, they display enrichment of LREE, Th, Nb and Ta, weak depletion of P, Zr and Hf, similar to E-MORB. However, the Xiewu gabbros belong to alkaline basalt series in composition and they have OIB-like composition, which display enrichment of LREE, Th, Nb, Ta and Ti and depletion of P relative to MORB. All the two rock types have relatively low εNd(t)(+0.9-+1.0and+0.2-+0.6, respectively) and high Isr (0.710-0.711and0.709, respectively). Based on mineralogical, geochemical and Sr-Nd isotopic compositions, we suggest that the Chayong and Xiewu mafic rocks could be derived from two different depleted mantles metasomatized by crustal materiels and enriched components of an asthenosphere mantle (e.g., OIB), respectively. In addition, the Chayong mafic rocks were generated by partial melting of a spinel-garnet Iherzolite mantle source and fractional crystallization of olivine, clinopyroxene and plagioclase might have occurred during the magma evolution. The Xiewu pillow basalt were formed by partial melting of a spinel Iherzolite mantle source and only some fractional crystallization of olivine were occurred, whereas the Xiewu gabbros were generated by partial melting of a spinel-garnet Iherzolite mantle source and it had undergone fractional crystallization of olivine, clinopyroxene and plagioclase. Combining with regional data, we proposed that the Chayong and Xiewu mafic rocks could have formed in a continent rift environment, which might be related to Middle Triassic spreading of the western Garze-Litang Ocean basin.6. The Late Triassic mafic rocks are rarely exposed in the Yushu area, relative to the Middle Triassic rocks. The Longbao mafic rocks are mainly composed of calc-alkaline lamprophyric dikes and they intruded the Triassic arc magmatic rocks. The rocks have moderate SiO2, relatively low TiO2and FeOt/MgO and high Al2O3, MgO, Cr and Mg#, comparable to primitive magmas. They belong to the sub-alkaline basalt series and have relatively high REE. In the MORB-normalized trace element spider diagrams, they are characterized by enrichment of LREE and Th, strong depletion of Nb, Ta and Ti, similar to the mafic rocks which are from continental arcs or derived from a subduction-modified lithosphere mantle. All the rocks have enriched Sr and Nd compositions and display relatively low εNd(t)(-2.3-1.5) and high Isr(0.705). Based on geochemical and Sr-Nd isotopic compositions, we suggest that the Longbao mafic rocks could be derived from a lithospheric mantle metasomatized by aqueous fluids and generated by partial melting of a phlogopite-bearing spinel Iherzolite mantle source. Fractional crystallization of olivine might also have occurred during the magma evolution. Combining with regional data, we proposed that the Longbao lamprophyric dikes could have formed in a continent arc environment, which might be related to Late Triassic S-directed subduction of the western Garze-Litang Ocean basin.7. Based on our results and systematic summaries of the previous published data, we proposed that the Jinshajiang suture zone could not extend westward to the internal Yushu-North Qiantang area and the previous defined western Jinshajiang suture zone might be the important component of the Garze-Litang suture zone. The central-western Qinghai-Tibet Plateau could have experienced a different Palaeo-Tethyan tectonic evolution from the central-southern area and the tectonic evolutionary history are described as follows:(1) From Early Carboniferous to the middle stage of Early Permian, the Longmu Co-Shuanghu ocean basin had continued to expand and the western Garze-Litang Ocean basin could had been opened.(2) From the late stage of Early Permian to Early Triassic, the Longmu Co-Shuanghu ocean basin had continued to subduct underneath the North Qiangtang block.(3) At the early stage of Middle Triassic, the Longmu Co ocean basin could be finally closed, while the western Garze-Litang Ocean basin still expanded.(4) From the late stage of Middle Triassic to Late Triassic, the break-off of the Longmu Co-Shuanghu oceanic slab resulted in a large number of bimodal volcanisms in the south area of the North Qiangtang block, while the S-directed subduction of the western Garze-Litang Ocean basin led to a lot of low potassic, calc-alkaline diorites/andesites in the north area of the North Qiangtang block.(5) From the late stage of Late Triassic to Early Jurassic, the western Garze-Litang Ocean basin could be finally closed.