Geochronology and geochemical constraints on petrogenesis of Early Paleozoic granites from the Laojunshan district in Yunnan Province of South China

Abstract

The Early Paleozoic Orogen in South China has been a topic of hot debate; in particular with regard to the relative contributions of crustal and mantle sources to the generation of granites. To address this issue, we report geochronological and geochemical data for three related suites of granites (Tuantian, Suite 1, Nanlao, Suite 2, and Laochengpo, Suite 3) from the Laojunshan district in the southeast Yunnan Province, western South China Block (SCB). LA-ICP-MS U-Pb dating of zircons indicates magmatic crystallization ages of ~436Ma, ~430Ma and ~427 Ma for Suites 1, 2, and 3 granites, respectively. The εHf(t) values of magmatic zircons in all three suites are heterogeneous, ranging between +3 and −14. All three suites also contain inherited zircon cores with Proterozoic U-Pb and Hf model ages. Bulk compositions form well-defined mixing arrays, with Suites 1 and 2 being strongly peraluminous (A/CNK >1.1), whereas Suite 3 is weakly peraluminous (A/CNK=1.0–1.1). Suites 1 and 2 have lower TFe2O3, Al2O3, MnO, MgO, CaO, TiO2, Na2O, Mg# and Nb/Ta but higher K2O, Rb/Sr, Rb/Ba and εNd(t) than those of Suite 3. These results suggest that Suite 1 and Suite 2 were formed by partial melting of Proterozoic metasedimentary rocks with little or no input of mantle-derived materials, but Suite 3 was derived from a mixture of a crustal melt with mantle derived mafic magma. The apparent paradox that the mantle-derived component has lower εNd(t) values than the crustal components is consistent with the very low εNd(t) values found in mantle-derived basalts in the SCB. The Wuyi-Yunkai orogenic collapse may have caused lower crustal melting of the Cathaysia Block. The early Paleozoic granitoid magmatism with mafic magma input indicates an extensional environment in the western SCB, in response to post-collisional orogenic collapse. Asthenospheric upwelling and basaltic underplating probably contributed heat and melts triggering felsic magmatism in the western SCB.