Nature and origin of Mesozoic granitoids and associated mineralization in the Sanjiang Tethys Orogeny, SW China: the Xiuwacu complex example

摘要

The magmatic generation for the Late Triassic-Early Jurassic (similar to 215-200 Ma) and Early Cretaceous-Late Cretaceous (similar to 108-79 Ma) post-collisional granites in the Sanjiang Tethys orogeny remain enigmatic. The Xiuwacu complex, located in the southern Yidun Terrane, consists of biotite granite with a weight mean Pb-206/U-238 age of 199.8 +/- 2.5 Ma, aplite granite of 108.2 +/- 2.3 Ma, monzogranite porphyry of 80.8 +/- 1.0 Ma, and diorite enclaves of 79.2 +/- 0.9 Ma and 77.9 +/- 0.8 Ma. The Late Triassic biotite granites show I-type granite affinities, with high SiO2 contents, high Mg-# values, high zircon delta O-18 values, and negative whole-rock & Nd(t) values, indicating a predominant ancient crustal source with the input of juvenile materials. Their fractionated REE patterns and concave-upward middle-to-heavy REE patterns require garnet-bearing amphibolite as the melt source. The Cretaceous highly fractionated aplite granites and monzogranite porphyries have relatively high SiO2 contents, high (Na2O + K2O)/CaO ratios, high zircon delta O-18 values, and enriched whole-rock Sr-Nd isotopic signatures, suggesting that their parent magmas were likely originated from the ancient middle- to lower crust. Their significant negative Eu anomalies and obvious depletions in Nb, Sr, and Ti demonstrate that the Cretaceous granitic magmas had experienced more fractionation than the Late Triassic felsic magmas. The Late Cretaceous diorite enclaves show low SiO2 contents, high Mg-# values, and high zircon delta O-18 values, suggesting that they were probably derived from the partial melting of subcontinental lithospheric mantle enriched by the Late Triassic subduction. The Late Triassic-Early Jurassic and Early Cretaceous-Late Cretaceous magmatism witnessed the post-collisional setting and intraplate extensional setting in response to the slab break-off and lithospheric-scale transtensional faulting, respectively. The partial melting of subduction-modified lithospheric mantle or/and residual sulphide cumulates within the lower crust during the origination of Late Cretaceous magmas could have provided metals for the formation of Xiuwacu deposit.