北秦岭超高压地体折返过程中的多期次部分熔融

本文对丹凤地区秦岭岩群含柯石英超高压变质地体长英质片麻岩中的混合岩化长英质浅色体和含石榴子石暗色包体的花岗质脉体进行了详细的矿物学、地球化学和锆石U-Pb年代学以及Lu-Hf同位素研究。其中,长英质浅色体显示了近原位熔融的高硅、富钾的过铝质花岗岩地球化学特征;锆石的CL图像呈灰黑色,均匀无结构或云雾状内部结构,Th/U比值0. 008,并含有钾长石、斜长石、石英和磷灰石等包裹体,显示深熔锆石的特征;花岗质脉体暗色包体中的石榴子石显示核-边成分环带,其中核部成分与秦岭岩群长英质片麻岩中石榴子石成分一致,边部Sps含量升高,显示熔体改造或退变质扩散特征,寄主花岗质脉体显示重稀土强烈亏损的与石榴子石平衡的熔体特征,指示它们是秦岭岩群含石榴子石长英质片麻岩部分熔融的产物。锆石LA-ICP-MS定年得到长英质浅色体和花岗质脉体的结晶年龄分别为445±4Ma和420±1Ma,明显晚于本区的超高压变质时代,而与折返过程中麻粒岩相和角闪岩相退变质叠加的时代基本一致。结合区域地质和前人的研究成果,提出秦岭岩群在深俯冲板块的折返过程中,分别在445Ma和420Ma发生了两期部分熔融作用。

Multi-stages partial melting of North Qinling ultrahigh pressure metamorphic terrane during exhumation

This paper presents an integrated study of petrography, geochemistry, zircon U-Pb dating and Lu-Hf isotopic composition on felsic leucosome and granite vein with garnet-bearing enclave from Dasigou coesite-bearing ultrahigh pressure metamorphic terrane in Qinling Group. The felsic leucosome occurs as migmatitic veins in felsic gneisses from Qinling Group, indicating they were crystallized from in-situ anatectic melt. Geochemical data show that felsic leucosome is peraluminous granite with high K₂O and SiO₂ contents. Zircons from felsic leucosome show no zoning or weak zoning, very low Th/U ratios (<0.008) and contain micro-inclusions of plagioclase, quartz, K-feldspar and apatite, indicating that they are anatectic origin. Garnets from the granite vein with garnet-bearing enclave show obvious core-rim structure. The garnet cores are similar with garnets from HP-UHP garnet-bearing paragneisses in Qinling Group in composition and the garnet rims have higher Sps contents than cores, indicating that they were modified by anatectic melt or retrograde diffusion. The granite vein with garnet-bearing enclave shows strong LREE enrichment and HREE depletion patterns, suggesting the magma was in equilibrium with garnet, and was a partial melting product of garnet-bearing paragneisses in Qinling Group. Zircon LA-ICP-MS U-Pb dating yielded crystallization ages of 445±4Ma and 420±1Ma for the felsic leucosome and granite vein with garnet-bearing enclave, respectively. These ages are obviously later than the UHP metamorphic age but similar with the granulite facies overprint and amphibolite facies retrograde ages of the UHP eclogites and felsic gneisses in North Qinling respectively. Combined with the previous results and the geological background, we propose that Qinling Group experienced two stages of partial melting at around 445Ma and 420Ma during exhumation.