Evolution of the granulites and sub-division of the granulite facies in Ceylon

Abstract

Three subfacies of the granulite facies are recognized: garnet-diopsidequartz subfacies; pyroxene-granulite subfacies; and hornblende-granulite subfacies The erection of a garnet-diopside-quartz subfacies follows De Waard's suggestion (1965) of a garnet-clinopyroxene subfacies. Two major divisions of the hornblendegranulite subfacies are now recognized, (i) garnet-biotite division, (ii) cordierite division. Three sub-divisions of the former are suggested on the basis of critical basic assemblages. Since wollastonite has been found to have a limited areal occurrence within the latter, a two-fold sub-division of the cordierite division is suggested on the basis of the assemblages, (a) calcite-quartz, (b) wollastonite. A seven-fold sub-division of the granulite facies is thus proposed for Ceylon rocks. An attempt is made to trace briefly the major events in the metamorphic history of the rocks supported by available geochronological evidence.

Proterozoic crustal evolution of southernmost India and Sri Lanka

“…The presence of a metamorphic field gradient was questioned by Perera (1984) but has been verified by later workers. Notably, the restriction of the high-P assemblage garnet + clinopyroxene + quartz to the eastern half of the HC (Hapuarachchi 1972) has been confirmed (Schenk et al 1991).…”

“…Alternatively, Cooray (1962Cooray ( , 1978, Crawford & Oliver (1969), Berger (1973), Hapuarachchi (1972) and Newton & Hansen (1986) considered the VC as a retrogressed equivalent of the HC. However, isotopic data (Milisenda et al 1988;H61zl et al 1991Liew et al 1991a,b) have now firmly established that the VC is younger than the HC and is neither a basement to the HC metasediments nor its retrogressed equivalent.…”

“…Instead, the contact between these units represents a major thrust (Hatherton et al 1975;Kr6ner 1986;Kriegsman 1991Kriegsman , 1993Kleinschrodt 1994;Btichel 1991Btichel , 1994. The presence of serpentinized ultramafic bodies (Dissanayake & van Riel 1978), copper-iron mineralizations (Munasinghe & Dissanayake 1982) and retrogressed chamockites (Cooray 1962;Hapuarachchi 1972;Btichel 1994) testifies to the importance of this contact.…”

“…For example, Almond's (1994) D4 and D5 are interpreted as late stages of D3; D3 of Kehelpannala (1997) and Kehelpannala & Ratnayake (2001) is grouped into D2, and their D4-D6 are all grouped into D3 in this paper, because they are interpreted as manifestations of the same bulk stress field. Hapuarachchi (1972), in the first comprehensive review of the metamorphism of Sri Lankan granulites, recognized several distinct metamorphic domains and postulated a general westward decrease of maximum recorded pressures. Newton & Hansen (1986) also advanced arguments for westward decreasing P and T across the Sri Lankan granulite terrain and proposed the presence of a tilted crustal section.…”

“…) suggests that metamorphic conditions at that stage were similar for all domains. Hapuarachchi (1972) mentioned a metamorphic evolution defined by an early high-pressure stage and the subsequent appearance of low-pressure assemblages, including andalusite. Similarly, Perera (1984) and Sandiford et al (1988) described a peak P-T stage, followed by growth of typical decompression textures in metabasites.…”