Abstract
Fine spherulitic intergrowths of quartz and feldspar in felsite inclusions (FIs) in garnet in five representative granulite-facies rocks from northern, central and southeastern parts of the Highland Complex (HC), Sri Lanka, are presented together with an experimentally reproduced spherulitic intergrowth of similar dimensions. FIs show characteristics indicative of unexpectedly rapid cooling after far-from-equilibrium crystallization of trapped partial melts formed during high-grade metamorphism, because granulite-facies rocks in continental collision orogens generally have been presumed to be cooled and exhumed slowly from the depths for more than millions of years. Sri Lanka is the key place to unravel the mystery of rapid cooling of huge granulite masses and relevant geological processes because of the most widespread but apparently restricted occurrence of FIs along the marginal part of the HC in addition to accumulated geological, geochemical, and geochronological data.
Highlights
Sri Lanka extensively exposes high-grade regional metamorphic rocks, which show various notable features; (1) arrested charnockitization (e.g., Hansen et al, 1987; Hiroi et al, 1990; Yoshida and Santosh, 1994; Kehelpannala, 1999; Perchuk et al, 2000, and references therein), (2) high purity vein graphite deposits (e.g., Erdosh, 1970; Hapuarachchi, 1977; Dissanayake, 1986; Hewathilake et al, 2018 ; Touret et al, 2019, and references therein), (3) occurrence of ultrahigh-temperature (UHT) mineral assemblages such as sapphirine + quartz and orthopyroxene + sillimanite + quartz (e.g., Osanai, 1989; Hiroi et al, 1994, Kriegsman and Schumacher, 1999; Sajeev and Osanai, 2004; Dharmapriya et al, 2015; Osanai et al, 2016a, and references therein), and so on
felsite-nanogranite inclusions (FNIs) in this sample are composed of spherulitic intergrowths of sodic plagioclase + quartz, which are accompanied by more coarse-grained biotite, apatite, and monazite
A schematic model of FNI-forming and preserving processes is shown in Figure 8 after Hiroi et al (2014, 2019)
Summary
Sri Lanka extensively exposes high-grade regional metamorphic rocks, which show various notable features; (1) arrested charnockitization (e.g., Hansen et al, 1987; Hiroi et al, 1990; Yoshida and Santosh, 1994; Kehelpannala, 1999; Perchuk et al, 2000, and references therein), (2) high purity vein graphite deposits (e.g., Erdosh, 1970; Hapuarachchi, 1977; Dissanayake, 1986; Hewathilake et al, 2018 ; Touret et al, 2019, and references therein), (3) occurrence of ultrahigh-temperature (UHT) mineral assemblages such as sapphirine + quartz and orthopyroxene + sillimanite + quartz (e.g., Osanai, 1989; Hiroi et al, 1994, Kriegsman and Schumacher, 1999; Sajeev and Osanai, 2004; Dharmapriya et al, 2015; Osanai et al, 2016a, and references therein), and so on. Nanogranites exhibit similar features to those of FIs when they are inequigranular and contain granophyric to micrographic intergrowths of quartz and feldspar (Cesare et al, 2011) Because of their similarities, both FIs and nanogranites will be collectively referred to as felsite-nanogranite inclusions (FNIs). Poikiloblastic garnet contains sporadic inclusions of sillimanite, plagioclase, alkali feldspar, quartz, biotite, ilmenite, and apatite in addition to FNIs (Fig. 2). FNIs in this sample are composed of spherulitic intergrowths of sodic plagioclase + quartz, which are accompanied by more coarse-grained biotite, apatite, and monazite. Poikiloblastic garnet contains numerous inclusions of sillimanite, plagioclase, alkali feldspar, quartz, biotite, spinel, rutile, ilmenite, monazite, and apatite in addition to FNIs. FNIs contain relatively coarse-grained biotite, sodic plagioclase and quartz surrounding fine spherulitic intergrowth of quartz + alkali feldspar (Fig. 3). No Sample number Sil Opx Grt Bt Pl Afs Qz Rt Ilm Zrn Mnz Ap St* Hc* Crn* Ky* And♦
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