Petrogenesis of the Late Jurassic Laomengshan rhyodacite (Southeast China): constraints from zircon U-Pb dating, geochemistry and Sr-Nd-Pb-Hf isotopes

摘要

Extensive magmatism occurred in southeast China during Late Jurassic time, forming large-scale granitic and volcanic rocks associated with non-ferrous, rare earth and rare, radioactive metal deposits. The Shuikoushan Pb-Zn-Au orefield is a typical example located in Hunan Province. This study reports LA-ICP-MS zircon U-Pb ages, whole-rock chemistry, and Sr-Nd-Pb isotopic compositions, and in situ Hf isotopic geochemistry of zircons from the Laomengshan rhyodacite in the Shuikoushan Pb-Zn-Au orefield. Zircon U-Pb dating yields a weighted average age of 156.7 +/- 1.6Ma for the intrusion of the rhyodacite. The rhyodacite samples are mainly shoshonitic series, having metaluminous to weakly peraluminous A/CNK values ranging from 0.96 to 1.09, with moderately high magnesium content (Mg#=42.4-47.5). Samples display high (Sr-87/Sr-86)(i) values (0.71165-0.71176), low epsilon(Nd)(T) values (-10.7 to -10.3), old Nd model ages (T-DM=1.73-1.86Ga), and relatively homogeneous Pb isotopic compositions [(Pb-206/Pb-204)(i)=18.365-18.412, (Pb-207/Pb-204)(i)=15.663-15.680, and (Pb-208/Pb-204)(i)=38.625-38.666]. The zircons exhibit enriched epsilon(Hf)(T) values (-16.22 to -9.86) and old two-stage Hf model ages (T-DM2=1.82-2.22Ga). All the above data indicate that the Laomengshan rhyodacite originated from melting Palaeoproterozoic basement, perhaps contaminated by subordinate mantle melts. Intense extension and thinning of the continental lithosphere during Late Jurassic time resulted in melting of upwelling asthenosphere, and mafic mantle melts interacted with and melted Palaeoproterozoic lower crust, thus forming the Laomengshan rhyodacite.