The effects of source composition and melting conditions on the composition of syn-exhumation granites in collisional orogen

Abstract

Partial melting of the deeply subducted continental crust during exhumation has been widely recognized recently in collisional orogens. However, it is little known about the effects of source composition and melting conditions on the composition of syn -exhumation granitic rocks. This is investigated by studying zircon and titanite U Pb geochronology and whole-rock geochemistry for syn -exhumation granites from the Weihai area in the Sulu orogen. The U Pb dating of relict zircon cores yields concordant Neoproterozoic and Late Triassic ages, respectively, for the meta-igneous protolith and granulite-facies metamorphism. Magmatic zircon rims give U Pb ages of 207 ± 1 to 214 ± 2 Ma for the formation of Weihai granites, in agreement with magmatic titanite ages of 209 ± 6 to 217 ± 5 Ma in the Late Triassic. The compositional zonation of titanite shows decreases of Nb/Ta and (La/Yb) N from core to rim, indicating the geochemical changes during granitic magmatism. In general, these granites show arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions with an affinity to deeply subducted crustal slices from the South China Block. Compared with previously reported syn -exhumation granites from Lanshan in the southern Sulu orogen, the Weihai granites in the northeastern Sulu orogen exhibit different Sr-Nd-O isotope compositions. The Weihai granites have relatively low SiO 2 and Na 2 O contents but high Al 2 O 3 and K 2 O contents with high A/CNK and K/Na ratios. They contain high Ba and Sr but low Y with high Sr/Y and (La/Yb) N ratios. Both zircon and whole-rock of the Weihai granites exhibit no significant negative Eu anomalies. Based on the differences in geochemical features and the spatial-temporal distributions, it is inferred that the syn -exhumation granites in the Weihai area are derived from decompressional melting of the lower crustal slice at greater depth, whereas those in the Lanshan area are derived from decompressional melting of the upper crustal slice at relatively shallower depth. These differences indicate that both source compositions and melting conditions have exerted significant influences on the geochemical composition of syn -exhumation granites in the collisional orogen. This has bearing on the crust-mantle interaction in continental subduction channel through metasomatic reaction of the mantle wedge with these granitic melts produced during the exhumation. • The Weihai granites in the northern Sulu orogen indicate syn -exhumation magmatism. • The Weihai granites are characterized by relatively high Sr/Y and (La/Yb) N ratios. • Syn-exhumation granites in the Sulu orogen are distinct from south to north. • Differences in source compositions and melting conditions caused the compositional diversity of granites.