Geochemistry and Zircon U-Pb Dating of Changshagou Adakite from the South Altyn UHPM Terrane: Evidence of the Partial Melting of the Lower Crust

Abstract

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%, K2O + Na2O = 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 (δEu = 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.