Detrital white mica single\|crystal Ar\|Ar ages from the Himalayan foreland sedimentary record are potentially a powerful tool in constraining source regions, tectonic evolution, and the timing and extent of exhumatio...Detrital white mica single\|crystal Ar\|Ar ages from the Himalayan foreland sedimentary record are potentially a powerful tool in constraining source regions, tectonic evolution, and the timing and extent of exhumation (Copeland 1990, Najman 1997, White 1999). However such studies require large numbers of analyses, and Ar/Ar total\|fusion of white mica grains is a very rapid and cost\|effective technique.Such analyses, however, do result in the sacrifice of internal consistency checks inherent in, for example, Ar/Ar step\|heating analysis.展开更多
The Lesser Himalaya in central Nepal consists of greenschist\|facies metasedimentary rocks, which are overthrust by the amphibolite\|facies rocks of the Higher Himalaya along the Upper Main Central Thrust (UMCT). Desp...The Lesser Himalaya in central Nepal consists of greenschist\|facies metasedimentary rocks, which are overthrust by the amphibolite\|facies rocks of the Higher Himalaya along the Upper Main Central Thrust (UMCT). Despite a number of works on metamorphism of the MCT zone and the Higher Himalaya, the low\|grade metamorphic rocks to the south of the MCT zone have not been studied yet.In the present study, an attempt was made to constrain the metamorphic events and thermal structure of the whole Lesser Himalaya by means of white mica (Ms) compositions. About 600Ms grains in 48 metapelitic samples from the Lesser Himalaya were analysed by the EPMA. Compositional zoning in individual Ms grains was checked by means of X\|ray compositional mapping. There exist wide intersample, intrasample and within grain compositional variations in Ms in the samples from the Lesser Himalaya. Sheared Ms phenocrysts in pegmatites and gneisses have compositions very close to that of the ideal muscovite. Detrital Ms show wide variation in phengite content, most of which are poor in phengite content, and are most probably derived from older higher\|grade metamorphic rocks.展开更多
文摘Detrital white mica single\|crystal Ar\|Ar ages from the Himalayan foreland sedimentary record are potentially a powerful tool in constraining source regions, tectonic evolution, and the timing and extent of exhumation (Copeland 1990, Najman 1997, White 1999). However such studies require large numbers of analyses, and Ar/Ar total\|fusion of white mica grains is a very rapid and cost\|effective technique.Such analyses, however, do result in the sacrifice of internal consistency checks inherent in, for example, Ar/Ar step\|heating analysis.
文摘The Lesser Himalaya in central Nepal consists of greenschist\|facies metasedimentary rocks, which are overthrust by the amphibolite\|facies rocks of the Higher Himalaya along the Upper Main Central Thrust (UMCT). Despite a number of works on metamorphism of the MCT zone and the Higher Himalaya, the low\|grade metamorphic rocks to the south of the MCT zone have not been studied yet.In the present study, an attempt was made to constrain the metamorphic events and thermal structure of the whole Lesser Himalaya by means of white mica (Ms) compositions. About 600Ms grains in 48 metapelitic samples from the Lesser Himalaya were analysed by the EPMA. Compositional zoning in individual Ms grains was checked by means of X\|ray compositional mapping. There exist wide intersample, intrasample and within grain compositional variations in Ms in the samples from the Lesser Himalaya. Sheared Ms phenocrysts in pegmatites and gneisses have compositions very close to that of the ideal muscovite. Detrital Ms show wide variation in phengite content, most of which are poor in phengite content, and are most probably derived from older higher\|grade metamorphic rocks.
