大别山东段超高压变质带中变质花岗岩的矿物化学和地球化学特征

大别山东段超高压变质带中变质花岗岩富硅、贫钙、贫铝，属偏碱性花岗岩，围岩为含榴辉岩包体的超高压副片麻岩。变质花岗岩稀土元素总量多在（100－200）×10^-6，具有较大的负铕异常，其原岩应为壳源型花岗岩。元素地球化学特征表明变质花岗岩原岩与古造山作用有关。变质花岗岩中存在大量由岩浆型内核和变生型边缘构成的变质增生锆石。结合锆石U－Pb年龄资料认为，变质花岗岩应由古老花岗岩变质形成，而不是超高压变质作用之后部分地壳岩石重熔的产物。岩石中有富锰和贫锰两种石榴子石，通过富锰石榴子石－黑云母、贫锰石榴子石－多硅白云母等矿物对的温压计算可知变质花岗岩在400－500℃、0.6-0.8GPa条件下经历过变质作用。但几种间接证据反映出变质花岗岩可能经历过超高压变质作用。

Mineral Chemistry and Geochemical Characteristics of Metagranitoids from the Ultrahigh Pressure Metamorphic Belt,East Dabie Mountains and Their Geological Significance

The studied metagranitoids are located in the ultrahigh pressure metamorphic belt in the Dabie Mountains and belong to sub-alkaline granites. They are chemically rich in SIO2 and poor in CaO and AL2O3 The surrounding rocks are ultrahigh-pressure metamorphic gneisses with eclogite enclaves．The total REE contents of the most samples are(100～200)x10-6 ．All samples have obviously negative Eu anomalies ．Their protolith was originated from Neoproterozoic ganites．Combined with geochemical characters of the metagranitoids，they are thought to be relatd to paleo-orogeny．There are numbers of metamorphic overgrowth zircons in the granitoids， which are composed of mixing magmatic core and metabolic overgrowth rim. Combination of mineral chemistry and U-Pb ages of zircons indicates that the metagranitoids were or inated and metamorphosed from paleo-granites，but not Mesozoic remelting granites formed later than ultrahigh pressure metamorphism．There are two kinds of garnet in the rocks,one is poor in Mn，another is rich in Mn．By using geothermobarometry of garnet-botite and garnet-phengite mineral pairs，it can be proved that metagranitoids underwent metamorphism at pressure 0．6～0．8GPa and temperature 420～ 500℃. Some indirect evidence shows that the metagranitoids might have experienced a Mesozoic ultrahigh pressure metamorphism ．