Genesis of post-collisional granitoids and basement nature of the Junggar Terrane, NW China: Nd–Sr isotope and trace element evidence

Abstract

The post-collisional magmatism of the Junggar Terrane is characterized by intrusion of large amounts of granitoids and minor basic/ultrabasic rocks. The granitoids comprise two magmatic suites: calc-alkaline and alkaline, which were emplaced contemporaneously at about 294 Ma. The calc-alkaline rocks are typically sodium-rich (Na 2O/K 2O=1.1–3.5) and metaluminous (A/NK >1.0, A/CNK=0.78–1.04). They show mildly fractionated REE patterns ((La/Yb) N<15) and spidergrams with strong depletion of Nb, Ti and, to a lesser extent, Sr. The alkaline granites have high contents of SiO 2 (75–78%), alkalis, Nb, HREE, Y, Sn, F and high FeO/MgO ratios and huge Sr and Eu depletion in the spidergrams. Moreover, they display characteristic tetrad REE patterns and non-CHARAC trace element behaviour. The two rock suites have similar initial Nd and Sr isotopic compositions, with ε Nd( T) in the range +5.2 to +7.1 and I Sr mostly in the range 0.7031–0.7041. This points to a predominance of juvenile components in their sources. The calc-alkaline rocks are most probably derived by dehydration-melting of a basic lower crust leaving behind a granulite residue. The process was probably triggered by underplating of mantle-derived basic magmas in an extensional regime. The alkaline granites are considered to have formed by differentiation of the calc-alkaline granitoids. Our study argues for a juvenile continental crust for the basement of the Junggar terrane, which is likely dominated by early Paleozoic oceanic crust and arc complex that were deeply buried during the late Paleozoic subduction and accretion.