Age,Geochemistry And Petrogenesis Of Late Triassic Laocheng Plutons In South Qinling,Central China

The Laocheng granite exposed the Ningshan area,together with the Wulong granite pluton,consists of an important part of the Early Mesozoic granitoid magmatism in central South Qinling.Abundant mafic enclaves can be found in different units of the Laocheng granite.Analytic results of zircon U-Pb dating show that the Laocheng granite was emplaced in 219-213 Ma and its mafic enclaves crystallized in ca.211 Ma,diorite crystallized in ca.214 Ma,more or less simultaneous with the formation of the Wulong granite.Geochemical data shows that the Laocheng granites are mainly granodiorite and quartz monzonite in composition with characteristics of low silica contents,alkali-rich,weak peraluminous,and high Mg#(43?67).It belongs to the high-K calc-alkaline I-type series.Mafic enclaves hosting in the granite are mainly dioritic,having low contents in SiO2(52.6%?62.4%),alkali-rich and high Mg#(63?72),belonging to the shoshonite series.The diorites also have low contents in SiO2(52.6%?55.26%),alkali-rich and high Mg#(54?63),belonging to the shoshonite series.Compared with the host granite,the enclaves and the diorites have lower initial 87Sr/86Sr ratios ranging from 0.7043 to 0.7058,relatively high initial ?Nd values from-0.7 to-4.3 and higher Mg#,suggesting the primary magma of the enclaves and the diorites might have originated from a basaltic magma.Whole-rock geochemistry and Sr-Nd isotopic composition suggest that the composition of two magmas have exchanged prior to the formation of the Laocheng granite and an equilibrium had been reached.Compared with Sr-Nd isotopic composition of basement rocks exposed in South Qinling,it is suggested that the magma of the Laocheng granite originated from Neoproterozoic mafic crustal material.Relationship between the enclaves and the host granite indicates a magma mixing between the crystallizing acid magma and the basic magma slightly later injected into the magma chamber so that the enclaves and the granites formed.These two magmas were not completely miscible during the mixing process.