Changshagou adakite, an outcrop in the middle segment of the South Altyn Tagh ultra-high pressure metamorphism （UHPM） terrane, contains medium-K cal-alkaline and weakly peraluminous compositions （SiO2 = 66.79% to 68.65%, Al2O3 = 17.48% to 18.31%, K20 ＋ Na20 = 6.32% to 6.88%, K2O/Na2O = 0.25 to 0.33, A/CNK = 1.01 to 1.06）. This outcrop is also enriched with large ion lithophile elements but with depleted high-field strength elements （HFSE） showing clearly negative Nb, Ta, and Ti anomalies. REE distribution patterns show a positive anomaly of Eu （6Eu = 1.15 to 1.31） and weakly enriched with LREE compared with HREE （LREE/HREE = 1.02 to 4.20）. Experimental results and several characteristics, including relatively low Nb/Ta ratios （6.03 to 8.45） and high Sr, Sr/Y, （La/Yb）N and low Y and Yb, which indicate the presence of residual garnet and the absence of plagioclase in the source region, show that adakite may form at a pressure ranging from 1.2 GPa to 1.5 GPa and at a temperature of approximately 900~C. Low Cr, Ni, and Mg# values, trace element patterns, and SiO2- Mg# and SiO2-MgO diagrams indicate that rocks are formed by the partial melting of a thickened lower continental crust. LA-ICP-MS in situ U-Pb dating yields two group ages： 503.1±1.7 Ma （core） and 453.1±3.0 Ma （rim）. The Th/U ratios of the core and the rim are 0.11 to 0.40 and 0.03 to 0.07, respectively. Considering the zircon CL image characteristics, Th/U ratios, and previous studies on regional UHPM rocks, adakite formed at 503.1 ± 1.7 Ma and underwent a tectothermal event as a result of the break-off of the Altyn deep subducted continental crust at 453.1 ± 3.0 Ma.
KANG LeiLIU LiangWANG ChaoCAO YutingYANG WenqiangWANG YaweiLIAO Xiaoying
