Hydrous melting of metasomatized mantle wedge and crustal growth in the post-collisional stage: Evidence from Late Triassic monzodiorite and its mafic enclaves in the south Qinling (central China)

Abstract

Triassic collision between the Yangtze and North China blocks is a key aspect of the evolution of the Paleo-Tethys in East Asia. This paper reports age and geochemistry for Late Triassic monzodiorite and its mafic enclaves from the south Qinling (central China). The host monzodiorite and mafic enclaves have identical zircon U-Pb ages of 227 ± 3 Ma and 221 ± 3 Ma, respectively. The host monzodiorite displays moderate SiO 2 (62.44-63.85 wt%) and MgO (3.44-4.47 wt%) contents and high Cr (157-244 ppm) and Ni (78-108 ppm) contents. It has evolved whole-rock Sr-Nd isotopic compositions (ε Nd(t) = -6.2 to -5.0) and slightly positive zircon ε Hf(t) values (up to +6.4). Given these characteristics in combination with high Th/Nb and Nb/Yb ratios, the host monzodiorite is considered to have been derived from hydrous melting of metasomatized mantle lithosphere. Its moderate Sr/Y and low Yb/Lu ratios indicate the fractional crystallization of hornblende. The mafic enclaves have lower SiO 2 (52.85-58.53 wt%) and higher MgO (8.26-9.45 wt%) contents. Most zircons in the mafic enclaves display positive εHf(t) values of +0.9 to +16.5. These features indicate that the pristine mafic melt was derived from depleted mantle lithosphere. Minor grains in the mafic enclaves display lower εHf(t) values (-4 to 0) than the zircons in the host monzodiorite, suggesting that the mafic melt had incorporated some evolved crustal component before it intruded into the host monzodiorite chamber. In summary, in the circumstance of slab break-off and asthenosphere upwelling, hydrous melting of mantle lithosphere has contributed greatly to crustal-derived granites in the Qinling orogenic belts. These results have the following implications for the Triassic granites in the Qinling orogenic belts: (1) hydrous melting of metasomatized mantle wedge greatly contributes to crustal growth in orogenic processes; and (2) mantle-derived hydrous mafic melts induced the extensive melting of crust and led to the voluminous Triassic granites in the Qinling orogen.