105 / 1971-01-01 00:00:00

Late Permian Back-arc Basin Development In The Western Jinshajiang Suture Zone, Central Qinghai-tibe

The Central Qinghai-Tibetan Plateau (CQTP) 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-Tibetan Plateau and even the whole world. The Jinshajiang Suture Zone (JSZ) is generally considered as an extremely important suture zone in the CQTP. However, the nature of JSZ and the way which JSZ extends westward remains hotly debated (e.g. Wang et al., 2000; Reid et al., 2005; Metcalfe, 2006; Pullen et al., 2008; Fan et al., 2010; Yang et al.,2012). In this paper, we provide new geochronological, geochemical and isotopic data of Yushu mafic-ultramafic rocks from the western JSZ that help to constrain the Palaeo-Tethyan tectonic evolution of the CQTP.
The Yushu mafic-ultramafic rocks have more uniform zircon U-Pb ages, which range from 258±2Ma to 257±1Ma. However, they display more complex chemical compositions. The Geriliha 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 also characterized by significant high FeOt and TiO2, similar to the typical Fe-Ti rich mafic rocks from the Panzhihua, Eastern Greenland and Northern Somalia (e.g. Zhou et al., 2005). 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 contents. 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 and olivine pyroxenites are distributed in the Duocai-Longbao ophiolitic mélange as slices. 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 (Leat et al.,2002). The two rock types also have relatively low εNd (t) (-0.6-0 and +0.7-+1.7, respectively) and high Isr (0.709-0.712 and 0.709, respectively). The Zhiduo mafic rocks are exposed in the Duocai-Longbao ophiolitic mélange as slices or massifs and they were usually regarded as components of ophiolite. The Zhiduo layered gabbros and dolerites have the same chemical compositions as the Geriliha 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 TiO2 and OIB-like compositions. The two rock types have relatively low εNd (t) (+0.7 and +0.3-+0.5, respectively) and high Isr (0.707 and 0.707, respectively). However, the Zhiduo isotropic gabbros have moderate FeOt and TiO2 and 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 Geriliha gabbros, Zhiduo layered gabbros and dolerites have similar genetic mechanisms related to partial melting of a 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 fO2 resulted 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 JSZ could not extend westward to the internal Yushu-North Qiantang area and the Yushu mafic-ultramafic 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.