胶北地区早前寒武纪孔兹岩系的变质演化
Metamorphic evolution of Early Precambrian khondalite series in North Shandong Province
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摘要: 在胶北地区出露大面积早前寒武纪孔兹岩,主体岩石为富铝片岩-片麻岩,夹有大理岩等变沉积岩。岩相学、成因矿物学和变质反应性质的研究以及温压条件估算结果表明,研究区孔兹岩系经历了四个阶段的变质演化。早期进变质阶段 (M1) 的矿物组合为石榴石及其内部的黑云母+斜长石+石英;峰期中-高压麻粒岩相变质阶段 (M2) 的矿物组合以含蓝晶石的麻粒岩相矿物组合为标志,该阶段最高的温压条件可达760℃、10.0kb;峰后近等温减压麻粒岩相退变质阶段 (M3) 发生一系列典型减压反应,以新生堇青石的出现为标志,形成了含堇青石的中-低压麻粒岩相矿物组合,稳定的温压条件为750~800℃、4.0~5.4kb;晚期降温退变质阶段 (M4) 以石榴石转变形成细小黑云母等退变矿物为特征,该阶段的温压条件为550~650℃、4.0kb。孔兹岩系变质演化P-T轨迹具有顺时针型式,先后经历了近等温减压 (ITD) 和近等压冷却 (IBC) 的P-T演化,标志着胶北地区早前寒武纪孔兹岩系曾经历了地壳加厚-构造隆升的动力学过程。Abstract: Early Precambrian khondalite series, which mainly consist of Al-rich schists and gneissic rocks with thin layers of marble, are widespread in north Shandong Province, SE Sino-Korean craton. Four stages of the metamorphic history have been recognized in mineral assemblages based on petrographic features, genetic mineralogy, metamorphic reaction integrated with garnet, biotite and cordierite zonation patterns. The P-T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. Garent along with fine-grained inclusion minerals, defines a prograde assemblage (M1) garnet+biotite+plagioclase+quartz. The peak of metamorphism (M2) at about 760℃ and 10.0kb is defined by kyanite-bearing granulite-facies mineral assemblage. Symplectite or coronas of cordierite surrounding relict garnet indicate garnet occurring breakdown reaction at conditions around 750~800℃ and 4.0~5.4kb. Kelyphites of fine-grained biotite ±quartz around garnet formed during the cooling process (M4) at about 550~650℃ and ~4.0kb. These results define a sequential P-T path containing near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P-T paths of the Early Precambrian khondalite series in North Shandong Province imply a model of thickening followed by rapid uplifting in a collisional orogen.
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Key words:
- Genetic mineralogy /
- Metamorphic reaction /
- P-T path /
- Khondalite series /
- North Shandong Province
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