Petrological characterization and tectonic significance of retrogressed garnet peridotites, Raobazhai area, North Dabie Complex, east‐central China
Petrological characterization and tectonic significance of retrogressed garnet peridotites, Raobazhai area, North Dabie Complex, east‐central China
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DOI：
10.1046/j.1525-1314.2000.00237.x
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发表时间：
2000-03
影响因子：
3.4
通讯作者：
Tsai;Liou;Ernst
中科院分区：
文献类型：
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作者：
Tsai;Liou;Ernst
北大别杂岩的饶八寨超镁铁质岩体被重新解释为一个幔源橄榄岩片，被包围在周围的麻粒岩相-角闪岩相片麻岩中，并与周围的麻粒岩相-角闪岩相片麻岩等面变质。超镁铁质岩片主要由变辉橄榄岩组成，但也包括变凝灰岩和糜棱二辉橄榄岩亚单元。岩石中含有尖晶石，但不含石榴石和斜长石。然而，在糜棱岩二辉橄榄岩中，尖晶石、斜方辉石和单斜辉石的细粒共生体在二维上勾勒出类似石榴石习性的区域;宽束微探针分析表明，在火成石榴石前体之后存在假晶。变质杂砾岩和变质斜方辉石的矿物组合为：橄榄石（Fo 92）+斜方辉石（En 92）+透闪石-镁角闪石+Mg-Al-铬铁矿，表明角闪岩相重结晶。糜棱岩二辉橄榄岩的矿物组合为：橄榄石（Fo 90）+斜方辉石（En 90）+单斜辉石+含铬尖晶石+闪长角闪岩，指示麻粒岩相-角闪岩相变质作用。相平衡和地质温度估计表明，饶八寨Meta超镁铁岩至少经历了三个阶段的重结晶：（I）950-990 °C，（II）750-860 °C，和（III）670-720 °C，假设平衡在尖晶石橄榄岩稳定场（c. 6-15根据推测的石榴石赝象，早期高压阶段（≥18 kbar）是可能的。岩石化学和地质温压数据表明，超镁铁岩片代表地幔楔的一个片段，在构造上并入俯冲大陆地壳，并在麻粒岩相至角闪岩相条件下重新平衡，同时被折返至浅层。
The Raobazhai ultramafic body of the North Dabie Complex is re‐interpreted as a mantle‐derived peridotitic slice enclosed in, and isofacially metamorphosed with, surrounding granulite‐to‐amphibolite facies gneisses. The ultramafic sheet consists mainly of metaharzburgite, but includes subunits of metadunite and mylonitic lherzolite. The rocks contain spinel but neither garnet nor plagioclase. However, in the mylonitic lherzolite, fine‐grained intergrowths of spinel, orthopyroxene and clinopyroxene outline domains resembling the habit of garnet in two dimensions; broad‐beam microprobe analyses imply pseudomorphs after a pyropic garnet precursor. The mineral assemblage of the metadunite and metaharzburgite is: olivine (Fo92)+orthopyroxene (En92)+tremolitic‐to‐magnesiohornblende+Mg–Al‐chromite, indicating amphibolite facies recrystallization. The mineral assemblage of the mylonitic lherzolite is: olivine (Fo90)+orthopyroxene (En90)+clinopyroxene+Cr‐bearing spinel+pargasitic amphibole, indicative of granulite‐to‐amphibolite facies metamorphism. Phase equilibria and geothermometric estimations show that the Raobazhai meta‐ultramafics have undergone at least three stages of recrystallization: (I) 950–990 °C, (II) 750–860 °C, and (III) 670–720 °C, assuming equilibrium in the spinel peridotite stability field (c. 6–15 kbar), although an early, high‐pressure stage (≥18 kbar) is probable, based on the inferred garnet pseudomorphs. Petrochemical and geothermobarometric data suggest that the ultramafic slice represents a fragment of the mantle wedge, tectonically incorporated into subducted continental crust and re‐equilibrated at granulite‐to‐amphibolite facies conditions while being exhumed to shallow levels.