U-Pb dating of zircons from quartz diorite and its enclaves at Tongguanshan in Anhui and its petrogenetic implication

The genesis of intermediate-silicic intrusive rocks and their enclaves in Tongling are closely related to Fe, Cu mineralization, which may provide petrologic information on lithospheric dynamics in this region. Tongguanshan quartz diorite and its enclaves, representatives of the Mesozoic intermediate-silicic intrusive rocks in the Tongling area, were studied using approaches of petrography, electron microprobe mineral chemistry and the LA-ICPMS zircon U-Pb dating. The chemical compositions of pyroxene and amphibole in pyroxene-amphibole cumulate, quartz-diorite host rock from Tongguanshan show correlative variations. The Al contents of the minerals in pyroxene-amphibole cumulate are higher than those in the host rocks, indicating that the cumulate crystals were crystallized before the emplacement of host magma. The consistent mineral chemical compositions between the Tongguanshan quartz diorite and the microgranular diorite enclave indicate that the enclave and host rock were crystallized under similar conditions and therefore are closely genetically related. The LA-ICPMS zircon U-Pb dating suggests that the crystallization age of Tongguanshan quartz diorite is 137.5±1.1 Ma, consistent with the previous dates by different methods. However, the present U-Pb dating also revealed the presence of late-Archean relict cores, demonstrating that older lower crustal materials were involved in the generation of Mesozoic magmatic rocks in this area. Furthermore, the dating results show that the crystallization age of the microgranular diorite enclave is 137.5 ± 2.4 Ma, the same as the crystallization age of the host magma. By integrating the previous Sr-Nd-Pb isotope data, it is also discussed the petrogenesis of quartz diorite and its enclaves in Tongguanshan.