Petrogenesis of the Maowu pyroxenite-eclogite body from the UHP metamorphic terrane of Dabieshan:chemical and isotopic constraints

摘要

The Maowu eclogite-pyroxenite body is a small (250 X 50m) oayered intrusion that occurs in the ultra-high-pressure(UHP) metamorphic terrane of Dabieshan,Chinal.Like the adjacent Bixiling complex,the Maowu intrusionwas initially emplaced at a crustal level,then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic.This paper presents the results of a geochemical and isotopic collision of the metamorphosed Maowu body.The maowu intrusion has undergone open system chemical and sotopic behavior three times.Eqarly crustal contamination during magmatic differentiation is manifested by high initial ~87Sr/~86Sr ratios (0.707-0.708) and inhomogeneous negative epsilon_Nd(T) values of -3 to -10 at 500 Ma(Probable protolith age).Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites,lack of wholerock(WR)Sm-Nd isochronal relationship,low delta~18O values and an extreme enrichment of Th and REE in a clinopyroxenite.Finally,K nd Rb depletion during UHP metamorphism is deduced from the high initial ~87Sr/~86Sr ratios unsupported by in situ Rb/Sr ratios.Laser ICP-MS spot analyses on mineral grains show that(1) Grt and Cpx attained chemica equilibrium during UHP metamorphism,(2)Cpx/Grt partition coefficients for REE correlate with Ca,and (3)LREE aboundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx),implying that the presence of LREE-rich accessory phases,such as monazite and apatite,is required to account for the Ree budget.Sm-Nd isotope analyses of minerals yielded three internal isochrons with ages of 221+-5 Ma and epsilon(T)=-6.9 for a garnet clinopyroxenite.The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes menttioned above must have occurred prior to the UHP metamorphism.These Sm-Nd ages agree with published zircon and monazite U-Pb ages and constrain the time of UHP metamorphism to 220-236 Ma.The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses,but the two bodies are distinguished by their magma-chamber processes.The Bixiling magmas were contaminated by the lower crust,whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation.The two complexes represent two distinct suites of magmatic rocks,which have resided in the continental crust for about 300-400 Ma before their ultimate suduction to mantle depths,UHP metamorphism and return to the curstal level.