Phanerozoic crustal growth: U–Pb and Sr–Nd isotopic evidence from the granites in northeastern China

Abstract

Northeastern China (NE China) is an eastern part of the Central Asian Orogenic Belt (CAOB). Its geology is distinguished by extensive distribution of Phanerozoic granitic and acid volcanic rocks. The granitoids comprise mainly I- (especially the highly fractionated felsic I-) and A-type granites; S-type granites are conspicuously absent. New zircon U–Pb and Rb–Sr mineral isochron ages, together with previously published results, indicate that these granites were emplaced in Late Paleozoic to Late Mesozoic times. Most of the granites are characterized by low initial 87Sr/ 86Sr ratios (0.705±0.001), positive ε Nd( T) values (+4 to 0) and fairly young T DM model ages (500–1300 Ma), suggesting that a significant proportion of the granites is of juvenile origin. In contrast, some granites emplaced within Precambrian terranes (or microcontinents) in this area show negative ε Nd( T) values of 0 to −8, and higher T DM ages of 1500–2000 Ma. This suggests that the Precambrian basement rocks have exerted a significant role in the generation of these granites. Sr–Nd isotopic modeling shows that these granites are likely to have been produced by partial melting of mixed sources, mostly probably of Phanerozoic underplated basaltic and lower crustal rocks. The generation of the granites with negative ε Nd( T) values involved a smaller proportion of basaltic component (≈40%), whereas the dominant granites with positive ε Nd( T) values require a much greater amount of basaltic material (55–85%). The emplacement of granitic rocks with such a large proportion of mantle-derived component indicates a massive addition of juvenile continental crust in NE China. We propose that the granites were formed in an anorogenic tectonic setting and that underplating of basaltic liquid as a result of extension is an important mechanism for the continental growth.