Geochemical Characteristics and Tectonic Significance of Triassic Granite from Taer Region, the Northern Margin of West Kunlun

Late Hercynian-early Indosinian （Triassic） granite is widely distributed around the Taer region of the northern margin of West Kunlun. The rock mass is mainly composed of calc-alkaline porphyroid biotite adamellite and characterized by SiO2-rich, high-Ca, moderate-alkaline, and strongly peraluminous attributes, and relatively low ~REE with LREE enrichment and a moderate Eu anomaly. As shown in the trace element spider web diagram, distinct peaks appear for Th, La, Nd, and Zr and clearly low values appear for Ba, Nb, Sr, P, and Ti. Further, compared with the primitive mantle, Rb/Sr and Rb/Ba are considerably higher and Nd/Th and Nb/Ta are relative low, all falling into the scope of the crust-origin rocks, indicating the characteristics of the crust-origin S-type granite. The rock mass＇s zircon U-Pb isotopic age is determined to be 235.7 -~ 3.9 Ma. On the basis of the age data, spatio-temporal location, lithology, and geochemistry of the rock mass, we conclude that the formation of the rock mass is closely related to the strong compressional orogenic movement （240 Ma） of the Tianshuihai terrane and the South Kunlun terrane. The rock mass is the product of the collision orogenic movement. However, distinct differences are observed between the studied rock mass and the synorogenic Bulunkou rock mass, which may be caused by the different collision strength and different positions with respect to the collision zone.

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.

Geochemistry, Geochronology and Lu-Hf Isotopes of Peraluminous Granitic Porphyry from the Walegen Au Deposit, West Qinling Terrane

Walegen Au deposit is closely correlated with granitic intrusions of Triassic age, which are composed of granite and quartz porphyries. Both granite porphyry and quartz porphyry consist of quartz, feldspar and muscovite as primary minerals. WeaMy peraluminous granite porphyry （A/ CNK=1.10-1.15） is enriched in LREE, depleted in HREE with Nb-Ta-Ti anomalies, and displays subduction-related geochemistry. Quartz porphyry is strongly peraluminous （A/CNK=1.64-2.81） with highly evolved components, characterized by lower TiO2, REE contents, Mg#, K/Rb, Nb/Ta, Zr/Hf ratios and higher Rb/Sr ratios than the granite porphyry. REE patterns of quartz porphyry exhibit lanthanide tetrad effect, resulting from mineral fractionation or participation of fluids with enriched F and CI. LA- ICP-MS zircon U-Pb dating indicates quartz porphyry formed at 233±3 Ma. The ages of relict zircons from Triassic magmatic rocks match well with the detrital zircons from regional area. In addition, εHf（t） values of Triassic magmatic zircons from the granite and quartz porphyries are -14.2 to -9.1 （with an exception of ＋4.1） and -10.8 to -8.6 respectively, indicating a crustal-dominant source. Regionally, numerous Middle Triassic granitoids were previously reported to be formed under the consumption of Paleotethyan Ocean. These facts indicate that the granitic porphyries from Walegen Au deposit may have been formed in the processes of the closing of Paleotethyan Ocean, which could correlate with the arc-related magmatism in the Kunlun orogen to the west and the Qinling orogen to the east.