山东半岛基性高压麻粒岩的成因矿物学及变质演化
Genetic mineralogy and metamorphic evolution of mafic high-pressure (HP) granulites from the Shandong Peninsula, China
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摘要: 在山东半岛的平度-莱西-莱阳-烟台一带,广泛出露基性高压麻粒岩,它们呈大小不等的透镜体或不规则脉状体产于TTG片麻岩中,且与超镁铁质岩石密切“伴生”。根据岩相学、矿物相转变、变质反应以及温压条件的系统研究结果,确定山东半岛基性高压麻粒岩经历了四个阶段的变质演化。早期进变质阶段 (M1) 的标志性矿物组合以石榴石及其内部的细粒单斜辉石+斜长石±石英为特征,记录的温压条件为T=740~770℃、P=0.9~1.0GPa;峰期高压麻粒岩相变质阶段 (M2) 的矿物组合以基质中石榴石(富Ca核部)+单斜辉石(富Al核部)+斜长石(富Na核部)±石英为特征,记录的温压条件为T=850~880℃、P=1.45~1.65GPa;峰后近等温减压 (中压) 麻粒岩相退变质阶段 (M3),以发生一系列典型减压反应和斜方辉石的大量出现为标志,典型的矿物组合为斜方辉石+单斜辉石+斜长石+磁铁矿±角闪石,记录的温压条件为T=780~830℃、P=0.65~0.85GPa;晚期角闪岩相退变质阶段 (M4) 以石榴石发生降温反应、形成角闪石+斜长石+磁铁矿±石英等退变矿物组合为特征,记录的温压条件为T=590~650℃、P=0.62~0.82GPa。高压基性麻粒岩的变质演化的P-T轨迹具有顺时针型式,先后经历了近等温减压过程 (ITD) 和近等压降温过程 (IBC) 变质演化过程,指示研究区基性高压麻粒岩是古老陆块之间在俯冲-碰撞造山过程中加厚下地壳折返地表的产物。该项成果对于重新认识华北克拉通在早前寒武纪古老陆块的碰撞-拼贴及其演化的动力学过程具有重要科学意义。Abstract: Mafic high-pressure (HP) granulites as enclaves and irregular vein-like blocks are widespread distributed in TTG gneiss in the Pingdu-Laixi-Laiyang-Yantai area of Shandong Peninsula, southeastern Sino-Korean craton. On the basis of petrography, mineral transformation, metamorphic reaction integrated with garnet, clinopyroxene and plagioclase zonation patterns, four stages of the metamorphic evolution have been identified in mineral assemblages, including early prograde metamorphic stage (M1), peak HP granulite-facies metamorphic stage (M2), near isothermal decompression granulite-facies retrograde stage (M3), and late amphibolite-facies retrograde stage (M4). The P-T conditions for each metamorphic stage were estimated using thermodynamical and experimental geothermometers and geobarometers. The core of garnet, together with mineral inclusions, is regarded as a prograde assemblage (M1) of garnet+clinopyroxene+plagioclase±quartz, which formed at 740~770℃ and 0.9~1.0GPa. The peak HP granulite-facies metamorphism (M2) is defined by high-Ca cores in garnet, high-Al cores in clinopyroxene and high-Na cores in plagioclase ±quartz as matrix minerals, yielding 850~880℃ and 1.45~1.65GPa. Symplectites or coronas of orthopyroxene+clinopyroxene+plagioclase+magnetite±amphibole around garnet porphyroblasts indicate garnet decompressional reactions (M3) occurring at P-T conditions of 780~830℃ and 0.65~0.85GPa. Symplectites of amphibole+plagioclase+magnetite±quartz surrounding garnet formed at a cooling retrograde stage (M4) with P-T conditions of 590~650℃ and 0.62~0.82GPa. These data indicate that the mafic HP granulites record an integrated P-T path including prograde, near-isothermal decompression (ITD) and near-isobaric cooling (IBC) evolutional process. The clockwise ITD and IBC P-T paths of the mafic HP granulites in the Shandong Peninsula support a geodynamic model of crust thickening followed by extension in a continent-continent collisional orogen.
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Key words:
- Mafic high-pressure granulites /
- Genetic mineralogy /
- P-T path /
- Shandong Peninsula
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