Late Cretaceous–Eocene magmatism induced by slab rollback and breakoff in the Tengchong terrane, SW China

Abstract

ABSTRACT Late Cretaceous–Eocene granites are widespread in the Tengchong terrane and related to subduction and closure of the Neo–Tethyan Ocean. However, the reason of their compositional and spatial variations is still under explored. This study reports zircon U–Pb geochronological, whole–rock and Nd isotopic data for Guyong, Houqiao and Menglong granites in the Tengchong terrane. Our data show that the three plutons crystallized at 70.4 ± 0.3Ma, 64.4 ± 0.3Ma and 47.6 ± 0.2Ma. Guyong monzogranites show an I–type affinity and may have experienced fractional crystallization of plagioclase, biotite and sphene. Negative εNd(t) values (−9.19 to −11.01) with two-stage model ages (T2DM = 1.59–1.78 Ga) indicate that Guyong monzogranites are derived from a predominantly Paleoproterozoic crustal source with little involvement of mantle–derived materials in an extensional setting. Lower εNd(t) (−9.33 to −11.63) and negative correlations of HREE, LREE, Th, and Y vs. SiO2 indicate that Houqiao plutons are S–type granites derived from a pure crustal source during crustal thickening. Low Zr/Hf and Nb/Ta ratios, zircon saturation temperature (TZr = 750°C–786°C) and negative δEu (0.11–0.40) of Menglong biotite granites indicate that they have experienced fractional crystallization of plagioclase, biotite and sphene. Relatively high εNd(t) (−6.81 to −7.07) values indicate that they are products of felsic magma mixed with mafic magma, which is also supported by numerous coeval mafic–intermediate rocks (Eocene) such as metagabbros, diorites and MMEs (mafic microgranular enclaves) in the western Tengchong terrane. A process of slab rollback and slab breakoff may have triggered the late Cretaceous–Eocene magmatism and the compositional variation of associated granitoids in the Tengchong terrane.