U-Pb zircon age, geochemical, and Sr-Nd isotopic constraints on the age and origin of granodiorite porphyries in the Guilong area, southern Yunnan Province, South China

摘要

Associated with several important polymetallic mineral deposits, the origin of the investigated Guilong area granodiorite porphyries (southern Yunnan Province) are presently poorly understood. For this reason we have undertaken a geochronological and geochemical study of these felsic intrusive rocks that includes: U-Pb zircon age-dating, whole-rock geochemical, and Sr-Nd isotopic data, and that are presented and discussed in this paper. Laser ablation inductively coupled plasma mass spectrometry, U-Pb zircon analyses yield an age of 252.5 ± 1.0 Ma for one sample of the felsic rocks. The Guilong area granodiorite porphyries are characterized by variable and high (~(87)Sr/~(86)Sr)_i, ranging from 0.7103 to 0.7143, and relatively low ε_Nd(t) values from-13.1 to-11.6. They also exhibit negative Nb, Ta, Ti and Sr anomalies on a primitive mantle-normalized multi-element diagram. Based upon their petrological and geochemical characteristics, we suggest that these rocks derive through the partial melting of crustal rocks at a shallow level. The parental magmas to the Guilong granodiorite porphyries underwent fractional crystallization of K feldspar, plagioclase, and ilmenite or rutile prior to emplacement. Magma generation at around 250 Ma (during Permian times) occurred in response to tectonic interactions between the Yangtze Platform and other crustal blocks, that provided the heat source and elevated geotherms required for partial melting of crust.