安徽大别山北部石榴辉石麻粒岩岩石成因及演化

Petrogenesis and the evolution of garnet-pyroxene granulite from the northern Dabie Mountain, Anhui

石榴辉石麻粒岩主要产在北大别 ,以团块状 ,沿铁镁质超基性岩块走势分布。它们的主要矿物成分由石榴石、透辉石、紫苏辉石、斜长石等组成 ,在石榴石边部分别形成角闪石 +斜长石 ,紫苏辉石 +斜长石或透辉石 +斜长石后成合晶结构。通过矿物温度压力计计算 ,高峰期变质温度 740℃～ 890℃ ,压力 0 .8～ 1.5 2 GPa ;退变质期温度 70 0℃～ 86 2℃ ,压力 0 .6 9～0 .79GPa,表明石榴辉石麻粒岩 PT轨迹为等温降压顺时针轨迹。通过主量元素 D(X)函数及有关图解判别 ,表明原岩相当基性火成岩或火山岩。稀土元素 ,微量元素地球化学特征显示上述岩石具有大洋中脊拉斑玄武岩及岛弧、火山弧玄武岩元素分布特征。将 REE数据投影到 (L a/ Sm) - L a关系图中 ,投影点沿部分熔融斜线分布 ,提示原岩岩浆是地幔岩部分熔融产物。本区石榴辉石麻粒岩 ,位于大别山北部 ,华北 -扬子板块俯冲 -碰撞结合部 ,根据上述岩石形成的温压条件 ,表明是在板块碰撞过程产生挤压 ,地壳增厚 。

Garnet-pyroxene granulites occur in the Northern Dabie mountain and are distributed as blocks and lens along the margin of mafic-ultrabasic rocks. They are composed mainly of garnet + diopsit + hypersthene + plagioclase. There are symplektites of Am + Pl, Cpx + Pl or Hy + Pl in different granulites, respectively. By use of geobarometers and geothermometers, the peak metamorphic P-T is about 740℃~890℃, 0.8~1.52GPa; and the retrograde P-T; 700℃~862℃, 0.69~0.79GPa. It is shown that the garnet-pyroxene granulites have a clockwise P-T paths by isothermal decompression process.The rock chemistry, REE and trace element geochemistry of them are researched in this paper. The results indicate that their original rocks, may be a basic pyrogenic or volcanic rocks and have the trace element distribution feature of oceanic basalt. By means of distribution in figure of (La/Sm)-La for the REE data, the original rocks of the garnet-pyroxene granulites would be derived from a mantle partial melting. The garnet-pyroxene granulites in the area are located at the suture of subduction-collision orogeney between the North China and Yangtze plaleo-continents in the Northern Dabie. According to the P-T condition, the garnet-pyroxene granulites were formed under the high pressure granulite facies produced by the plate conllision leaded to the compression and crustal thickening.