Deformation, petrogenesis and tectonic implications of late Permian alkaline mafic rocks in the northern Yidun terrane

The Yidun terrane which is located at the north of the Tethyan tectonic domain of Eastern Tibet, is an essential place for understanding the evolution of the Garze-Litang ocean and the eastern Tethys. In this study, we focus on the northern Yidun terrane, where a set of igneous-sedimentary rock assemblage is exposed, 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. One gabbro yields a weighted U-Pb zircon age of 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 gabbros and basalts show alkaline affinities and have variably low contents of SiO2 (41.08-52.45 wt%) and variably high contents of MgO (3.17-14.62 wt%), Fe2O3 (7.17-16.57 wt%), TiO2 (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. It is suggested that their genesis was probably related to the Emeishan igneous activity during the late Permian. 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.