Paragonite eclogites from Dabie Shan, China: re-equilibration during exhumation?
Paragonite eclogites from Dabie Shan, China: re-equilibration during exhumation?
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中国大别山的钠辉石榴辉岩：挖掘过程中的重新平衡？
DOI：
10.1111/j.1525-1314.1995.tb00233.x
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发表时间：
1995
影响因子：
3.4
通讯作者：
A. Okay
中科院分区：
文献类型：
--
作者：
A. Okay
大别山超高压变质带的榴辉岩中发现了与石榴子石、绿辉石和蓝晶石结构平衡的钠长岩。钠云母、绿辉石和蓝晶石之间的平衡反应表明在c. 700摄氏度然而，其中一个钠长榴辉岩也包含明确的石英假像柯石英后，作为石榴石中的包裹体，这表明在相同的温度下的最低压力为27千巴。来自同一岩石的不同压力估计表明，超高压榴辉岩中的基质矿物在较低的压力下重结晶，并不代表超高压组合的峰值。通过校准石榴石+锆石/斜锆石+蓝晶石+石英/柯石英地质压力计，并将其应用于适当的榴辉岩相岩石的超高压和高压岩石来测试这一假设。这四种矿物在10 ~ 60 kbar的压力范围内共存，在这个宽的压力范围内，石榴石的钙铝榴石含量反映了平衡压力，其基础是反应锆石/斜锆石=钙铝榴石+蓝晶石+石英/柯石英+ H，O。地质压力计的结果与其他造山带榴辉岩的独立压力估计值吻合得很好。对于大别山的钠长榴辉岩，地质压力计指示的压力在石英稳定场中，证实了以前含柯石英的钠长榴辉岩在较低的压力下重新平衡。另一方面，石榴石从其他柯石英轴承，但不含钠云母蓝晶石-zoojiang榴辉岩显示了非常广泛的钙铝榴石含量的变化，对应的压力变化从柯石英到石英领域。这种广泛的变化，部分原因是由于在石榴子石中钙铝榴石成分的边缘减少，是由部分平衡的矿物组合在折返过程中。
Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh-pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c. 700" C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh-pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh-pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh- and high-pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H,O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite-bearing paragonite-eclogite has re-equilibrated at lower pressures. On the other hand, garnets from other coesite-bearing but paragonite-free kyanite- zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.