Anatomy of garnets in a Jurassic granite from the south-eastern margin of the North China Craton: Magma sources and tectonic implications

摘要

The Late Jurassic Jingshan granite located at the south-eastern margin of the North China Craton contains abundant garnets which can be subdivided into three types based on texture and composition: (ⅰ) euhe-dral garnet in mafic biotite and garnet rich enclave (Grt Ⅰ), (ⅱ) coarse-grained garnet (Grt Ⅱ) in the host granite, and (ⅲ) small euhedral garnet in aplite (Grt Ⅲ). In general, Grt I has higher FeO, CaO and lower MnO contents than Grt Ⅱ. Grt Ⅲ has higher Mn, but lower Ca contents than others. Grt I has lower MREE and HREE contents than Grt Ⅱ. Grt Ⅲ has prominent and distinctly negative Eu anomaly as well as higher MREE composition compared to the others. Systematic variations in oxygen isotope compositions are observed among the three garnet types, with δ~(18)O values of ＜3.8‰ in most of Grt I, 3.8-4.7‰, in most Grt Ⅱ (for inclusion-free garnets), and typically ＞4.7‰ in Grt Ⅲ. Some of the Grt Ⅱ and Grt Ⅲ display two distinct zonings with cores having similar major and trace element compositions to Grt I. Cathodoluminescence (CL) images revealed that the zircons from different garnet-bearing samples possess fine-scale oscillatory zoned magmatic rims with inherited cores. In situ zircon U-Pb dating and trace element analyses show that the dark-luminescent magmatic rims all have Jurassic concordia ages (~160 Ma) and similar trace element patterns. Most of the inherited cores also display similar Triassic ages of 210-236 Ma, which is similar to the ages of ultrahigh pressure (UHP) metamorphic rocks of the Dabie-Sulu orogen (230 Ma). In addition, Jurassic concordia ages were also found in a zircon inclusion in Grt I, implying that the Grt I was formed shortly before the main magmatic event. The age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events. Based on the zircon U-Pb ages from different garnet-bearing samples, the major element, trace element, oxygen isotope, and zoning textures of the three kinds of garnet we suggest that Grt I may be peri-tectic garnet, whereas Grt Ⅱ and Ⅲ are probably the results of magmatic dissolution-precipitation processes and re-equilibration of garnets with changing magmatic conditions during melting, differentiation, crystallization, and cooling within the granite. We conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the Jingshan granites are from Dabie-Sulu orogen representing the South China Craton.