Remarkable petrographic features of Sri Lankan granulites; with special regard to unexpected occurrence of “felsite-nanogranite inclusions” in garnet

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-fromequilibrium 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.

Zoned quartz phenocrysts in supercooled melt inclusions in granulites from continental collision orogens

“…Grt+ Qz+V Opx+ Crd trapping of hydrous partial melts in garnet cooling to form phenocrysts in and from melts cracking of garnet by deformation and overpressure, resulting in decompression and degassing of trapped hydrous melts non-equilibrium crystallization rapid cooling to preserve fine textures and quartz zoning F I G U R E 9 P-T diagram showing the schematic paths followed by melt inclusions to be FNIs and host granulites, modified after fig. 8 of Hiroi (2020). The P-T paths are decoupled because of the substantial overpressure caused by the large difference in compressibility between melt inclusion and host garnet (Angel, Nimis, Mazzucchelli, Alvaro, & Nestola, 2015;Ferrero et al, 2016).…”

“…The bright areas of quartz SEM-CL images almost always correspond to blue-violet colours (~400 nm) while dark areas red-brown colours (> 600 nm). 3.1 | Garnet-biotite gneiss from the Highland Complex (sample 88111503A) Sample location is shown in Table 1 and Hiroi (2020). Besides being crowded with many mineral inclusions such as biotite, quartz, rutile, ilmenite, sulfide and carbonate minerals, porphyroblastic garnet also contains FNIs (Figure 1a).…”

“…Nanogranites consist of cryptocrystalline multiphase aggregates with an equigranular, hypidiomorphic to allotriomorphic texture. Cesare et al (2009) Hiroi et al (2014Hiroi et al ( , 2019 and Hiroi (2020) additionally argued that felsite inclusions are significant indicators of an unexpectedly rapid cooling history of the host granulites during exhumation. Cesare, Acosta-Vigil, Ferrero, and Bartoli (2011) and Ferrero, Ziemann, Angel, O'Brien, and Wunder (2016) showed nanogranite inclusions exhibit similar features to those of felsite inclusions when they are inequigranular and contain granophyric to micrographic intergrowths of quartz and feldspar.…”

“…Hiroi et al (2014) reported similar, but distinct melt inclusions in garnet in granulites from various geologic ages ranging from Early Proterozoic to Middle Paleozoic and wide global distribution (the Limpopo Belt, the Grenville Province, the Lützow‐Holm Complex of East Antarctica, the Highland Complex of Sri Lanka, and the southern Bohemian Massif), and named them ‘felsite inclusions’ to emphasize their characteristic textures similar to hypabyssal and volcanic rocks. Hiroi et al (2014, 2019) and Hiroi (2020) additionally argued that felsite inclusions are significant indicators of an unexpectedly rapid cooling history of the host granulites during exhumation. Cesare, Acosta‐Vigil, Ferrero, and Bartoli (2011) and Ferrero, Ziemann, Angel, O'Brien, and Wunder (2016) showed nanogranite inclusions exhibit similar features to those of felsite inclusions when they are inequigranular and contain granophyric to micrographic intergrowths of quartz and feldspar.…”

“…5) described a porphyritic FNI with biotite phenocryst surrounded by dendritic quartz and K‐feldspar. Hiroi (2020) presented some examples of spherulitic intergrowths of quartz and feldspar in FNIs from Sri Lanka in some detail. The purpose of the present paper is to document and demonstrate the significance of FNIs containing euhedrally zoned quartz phenocrysts, as revealed by scanning electron microscopy cathodoluminescence imaging of Ti concentrations (Wark & Watson, 2006; Watt, Wright, Galloway, & McLean, 1997).…”