Ultrametamorphism in Precambrian granulite terranes: Evidence from Mg‐AI granulites and calc‐silicate granulites of the Eastern Ghats, India
作者:
Somnath Dasgupta,
Pulak Sengupta,
期刊:
Geological Journal
(WILEY Available online 1995)
卷期:
Volume 30,
issue 3‐4
页码: 307-318
ISSN:0072-1050
年代: 1995
DOI:10.1002/gj.3350300310
出版商: John Wiley&Sons Ltd
关键词: ultrametamorphism;Eastern Ghats;India;Mg‐Al granulites;calc‐silicate granulites
数据来源: WILEY
摘要:
AbstractHigh Mg‐Al granulites and calc‐silicate granulites provide evidence for ultra‐high temperatures of metamorphism (ca. 1000°C) at moderate pressures (9–10 kbar) in the Eastern Ghats Belt, India. Lack of proper geochronological data prevents the dating of this extreme metamorphism. High Mg‐Al granulites contain different subsets of mineral assemblages involving spinel, quartz, sapphirine, cordierite, orthopyroxene, garnet and sillimanite coexisting with either rutile‐ilmenite or titanohaematite‐ferrianilmenite. These high Mg‐AI rocks are poor in Zn and Cr, as reflected primarily in the composition of spinel. Evidence of ultra‐high temperature metamorphism comes from (i) textural interpretation of the former coexistence of spinel‐cordierite‐quartz and sapphirine‐quartz and stabilization of the assemblages orthopyroxene‐sillimanite‐cordierite and spinel‐quartz‐sapphirine‐garnet and (ii) the high Al2O3 content of orthopyroxene coexisting with garnet and/or cordierite. Consideration of the sequence of deduced mineral reactions in petrogenetic grids in the system FMAS attests to an anticlockwise P‐T path of evolution for the granulites. In calc‐silicate granulites stabilization of nearly pure meionite and of the wollastonite‐plagioclase‐andradite‐rich garnet, wollastonite‐scapolite‐grandite garnet‐calcite association corroborate high temperatures of metamorphism. Conventional mineralogical geothermobarometry in all the rocks record lower temperatures (maximum 950°C) at 9–10 kbar pressures, attributed to resetting of the mineral compositions during cooling. Following peak metamorphism, the rocks firstly experienced near‐isobaric cooling followed by near‐isothermal decompression. On the basis of the available evidence it appears that non‐extensional lithospheric thinning and/or heat input from basic/enderbitic magma are the causes of such ultra‐high temperature me
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