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

Abstract

Felsite inclusions (FIs) in granulite garnets from continental collision orogens are supercooled felsic melt inclusions. Not only do these represent non-equilibrium textures, such as dendritic and spherulitic crystals of quartz and other minerals, but the porphyritic texture is that of hypabyssal and volcanic rocks. This paper presents representative zoned quartz phenocryst-bearing FIs in various granulites from the Highland Complex in central Sri Lanka, the Lützow-Holm Complex in East Antarctica, the Grenville Province in southeastern Canada, and the Kerala Khondalite Belt in southern India. Quartz phenocrysts show simple and definite cathodoluminescence (CL) zoning with euhedral bright cores and dark rims and overgrowths. Euhedral quartz never develops in subsolidus metamorphic rocks except for vein quartz, and is characteristic of growth in a melt phase. Groundmass quartz surrounding quartz phenocrysts within the FIs is always dark in CL, whereas matrix quartz outside garnet and single-grain inclusion quartz in garnet are usually brighter than the cores of quartz phenocrysts. The close relationship between violet CL emission intensity and Ti content of quartz indicates different growth temperatures in harmony with the inferred crystallization sequence. The preservation of CL growth zoning in quartz phenocrysts and fine non-equilibrium textures imply that the cooling rates of some granulites are 1 to 2 orders of magnitude faster than so far presumed.