Early Permian–Late Triassic Magmatism in the Tuotuohe Region of the Qinghai–Tibet Plateau: Constraints on the Tectonic evolution of the Western Segment of the Jinshajiang Suture

In this paper we present new zircon U-Pb ages,whole-rock major and trace element analyses,and zircon Hf isotopic data for magmatic rocks in the Tuotuohe region of the western segment of the Jinshajiang suture.Our aim is to constrain the Early Permian-Late Triassic tectonic evolution of the region.Zircons from the magmatic rocks of the Tuotuohe region are euhedralsubhedral in shape and display fine-scale oscillatory zoning as well as high Th/U ratios （0.4-4.6）,indicating a magmatic origin.The zircon U-Pb ages obtained using LA-ICP-MS are 281 ± 1 Ma,258 ± 1 Ma,244 ± 1 Ma,and 216 ± 1 Ma,which indicate magmatism in the Early Permian-Late Triassic.A diorite from Bashihubei （BSHN） has SiO2 =57.18-59.97 wt％,Al2O3=15.70-16.53 wt％,and total alkalis （Na2O ＋ K2O） =4.46-6.34 wt％,typical of calc-alkaline and metaluminous series.A gabbro from Bashibadaoban （BSBDB） belongs to the alkaline series,and is poor in SiO2 （45.46-54.03 wt％） but rich in Al2O3 （16.19-17.39 wt％） and total alkalis （Na2O ＋ K2O =5.48-6.26 wt％）.The BSHN diorite and the BSBDB gabbro both display an enrichment of LREEs and LILEs and depletion of HFSEs,and they have no obvious Eu anomaly; they have relatively low MgO contents （2.54-4.93 wt％）,Mg# values of 43 to 52,and low Cr and Ni contents （8.07-33.6 ppm and 4.41-14.2 ppm,respectively）,indicating they differentiated from primitive mantle magmas.They have low Nb/U,Ta/ U,and Ce/Pb ratios （1.3-9.6,0.2-0.8,and 0.1-18.1,respectively）,and their initial Hf isotopic ratios range from ＋9.6 to ＋16.9 （BSHN diorite） and ＋6.5 to ＋12.6 （BSBDB gabbro）,suggesting their primary magmas were derived mainly from the partial melting of a mantle wedge that had been metasomatized by subduction fluids.Taking all the new data together,we conclude that the western and eastern segment of the Jinshajiang suture regions underwent identical processes of evolution in the Early Permian-Late Triassic：oceanic crust subduction before the Early Permian,continental collision during the Early-Middle Triassic,and post-collisional extension from the Late Triassic.

Timing of formation of the Western Jinshajiang and Ganze–Litang sutures:evidence from the Duocai Granite in the Zhiduo region,West China

The timing of the closure of the Paleo-Tethys Ocean in West China remains debated.To investigate this problem,we examined the geochemical characteristics,zircon U-Pb chronology,and Hf isotopes of monzogranites and quartz diorites from the Duocai granite in the Zhiduo region,at the intersection of the Jinshajiang and GanzeLitangsutures.The monzogranites have the chemical characteristics of calc-alkalineⅠ-type granites and yield a weighted mean zircon U-Pb age of 234.6±0.9 Ma[mean square weighted deviation(MSWD)=0.36].InitialεHf(-t)values are high and positive,ranging from+7.9 to+13.6 with a mean of+10.7,corresponding to two-stage Hf isotope model ages(TDM2)of 762-395 Ma.Geochemical and isotopic data indicate that the source magma of the monzogranites formed from mantle-derived magmas mixed with the overlying crustal materials.The quartz diorites,which also have compositional characteristics of calcalkaline I-type granites,yield a weighted mean zircon UPb age of 209.1±0.7 Ma(MSWD=0.29).InitialεHf(-t)values range from-2.5 to+0.6 with a mean of-1.5,with the corresponding TDM2 of 1402-1210 Ma.Geochemical and isotopic data indicate that the primary magma of the quartz diorites was derived mainly from partial melting of the mafic lower crust and small amount mantle-derived magma involved.Combining these results with regional data,the studied granites are inferred to have formed as a result of continuous subduction of plates underlying the Western Jinshajiang Ocean-Ganze-Litang Ocean from 234 to 209 Ma,and were unrelated to subduction of the South Jinshajiang oceanic plate.We suggest that the Western Jinshajiang Ocean-Ganze-Litang Ocean closed by the end of Late Triassic.