Deformation,Petrogenesis And Tectonic Implications Of Late Permian Alkaline Mafic Rocks In The Northern Yidun Terrane

The Yidun terrane which is located in the Tethys tectonic domain in eastern Tibet,is an essential geological structural unit of the Sanjiang orogenic belt and an important place for understanding the tectonic evolution of the Ganzi-Litang Ocean.The Yidun terrane is composed of a Zhongza block in the west and an Yidun island arc in the east.The Zhongza block was regarded to split from the Yangtze block with the opening of the Garze-Litang ocean in the late Paleozoic,and the Yidun island arc was a continental volcanic arc formed by the westward subduction of the Garze-Litang oceanic lithosphere in the late Triassic.However,how the Garze-Litang ocean basin formed remains controversial.One group suggests that the Garze-Litang ocean was a branch ocean basin of the Paleo-Tethys and opened by the Emeishan mantle plume upwelling.In contrast,many researchers propose that the Garze-Litang ocean was a back-arc ocean basin formed by the eastward subduction and roll-back of the Paleo-Tethys oceanic lithosphere.The Upper Permian Gangdagai formation volcanic rocks are widely developed in the Yidun terrane,and their source is considered to be directly related to the Emeishan mantle plume.However,studies of the Gangdagai formation volcanic rocks are mostly concentrated in the southern part of Yidun terrane.In this study,we focus on the northern Yidun terrane,where a set of igneous-sedimentary rock assemblage is exposed and mainly consists of phyllites,siliceous rocks,limestones and mafic rocks composed of meta-gabbros and meta-basalts.Three stages of structural deformation were widely developed in this area.D1 is represented by thrust faults along NW trending,NW/NNW-dipping foliation,rootless isoclinal folds with NW-trending axial plane and interlayer cleavages,all of which correspond to those in the Garze-Litang suture zone to the west.D2 and D3 is represented by N-S trending normal faults and E-W trending strike-slip faults plus NE-trending reverse faults,respectively.Gabbro shows residual gabbroic texture and is mainly composed of clinopyroxene（30–35 wt.%）,altered plagioclase（30–35 wt.%）,chlorite（20–30 wt.%）and minor accessory minerals（apatite,calcite,magnetite etc.）.Basalt is characterized by cryptocrystalline texture and mainly composed of fine-grained clinopyroxene（25–35wt.%）,plagioclase（30–35 wt.%）,glassy minerals and minor accessory minerals（chlorite,magnetite etc.）.Zircon U-Pb dating of gabbros constrains the formation age of mafic rocks as 256±6.2 Ma,which is later than the initial expanding timing of the Garze-Litang oceanic basin but corresponds to the volcanic activity timing（ca.260 Ma）of the Emeishan igneous province to the east.The dated zircons have a wide range of ¹⁷⁶Hf/¹⁷⁷Hf ratios（0.282652–0.282756）,and yield positiveεHf（t=256 Ma）values of+0.88to+3.47,indicating a long-term depleted mantle source for the studied mafic assemblage.The gabbros and basalts show alkaline affinities and have variably low contents of Si O₂（41.08-52.45 wt%）and variably high contents of Mg O（3.17-14.62 wt%）,Fe₂O₃（7.17-16.57 wt%）,Ti O₂（1.12-3.93 wt%）,Na₂O（0.58-4.39 wt%）and K₂O（0.07-2.08 wt%）.They are characterized by enrichment of light rare earth elements（REEs）,positive Ba anomalies,negative Zr and Hf anomalies with no obvious negative Nb and Ta anomalies,which are geochemically similar to those of typical ocean island basalt（OIB）and also show great similarity with the Emeishan flood basalts.On the Th/Yb versus Nb/Yb diagram,the studied gabbros and basalts are characterized by consistent and high Th/Yb ratios with most of them plotting in the ocean island basalts（OIB）field.On the Zr/Y versus Zr diagram,all of them plot in the within-plate basalt（WPB）field because of their high Zr/Y ratios,indicating that the area was in an extensional tectonic background.La/Sm-Sm/Yb melt modelling shows that the magma source of the gabbros is concentrated in the garnet lherzolite area with deep melting,whereas the magma source of the basalts plots within the spinel lherzolite-spinel+garnet lherzolite array with shallow melting,indicating their variable mantle melting depth.The geochemical characteristics of mafic rock are highly consistent with those of the Emeishan basalts and their formation timing also corresponds to the volcanic activity timing（ca.260 Ma）of the Emeishan igneous province,implying their genesis was probably related to the Emeishan igneous activity during the late Permian.The primary relationship of intrusive contact between the Emeishan-affinity mafic rocks and marine sedimentary strata implies that the opening time of the ocean basin was earlier than the formation age of mafic rocks,that is,256 Ma.Our study indicates that the opening of the Garze-Litang ocean could not have been directly related to the Emeishan mantle plume,although,it has widely affected the eastern Paleo-Tethys tectonic domain during late Permian.Combined with the evolution history of the Ganzi Litang Ocean,we propose that the Emeishan mantle plume may have further promoted the expansion of the Garze-Litang Ocean in the late Permian.