Microporosity linked fluid focusing and monazite instability in greenschist facies para-conglomerates, western India

Abstract

In a deformed para-conglomerate (western India), monazites occur as discrete grains (50-250 mu m long axis) in tonalite clasts, in the warping chlorite-biotite-calcite schist layers, and as disseminated grains (less than 5 mu m) along with xenotime and huttonite forming veins in schist layers. The veins are absent in clasts. Chlorite and biotite defining schistosity in schist layers and in synchronously-formed extensional veins/fractures in clasts are chemically identical. delta 18O (VSMOW) and delta 13C (VPDB) in calcite in schistosity, in clasts, and in "beards" in pressure shadow zones around clasts are tightly bracketed, 11.4-12.2ppt and -3.1 to -3.6ppt respectively. Margins of clast-hosted monazites are partly altered to fluorapatite - huttonite plus or minus allanite plus or minus Y-xenotime. By contrast, monazite alteration in schist layers is extensive. In schist layers, rafts of embayed monazite occur in fluorapatite-dominated corona studded with idiomorphic huttonite, Y-xenotime plus or minus allanite. Mass balance calculation in a well-defined alteration zone around monazite in schist layer with P conserved indicate large gain in mass fraction of lower-abundance, less soluble HREEs, Y and Th in the alteration zone was compensated by minor loss in mass fraction of higher abundance elements (LREEs, MREEs) resulting in negligible net mass change. Monazite decomposition was associated with approx 50 percent dilation strain. Yb, Th and U abundances in alteration zones around decomposed monazites in schist layer are computed to be manifold higher than average schists. The contemporaneous formation of pore microstructures, the chemical homogeneity among ferromagnesian minerals and C-O isotope data in calcite in different textural domains, and results of mass balance calculations are compatible with deformation-aided porosity generation, synchronous fluid-mediated advection transport of elements, and dissolution-re-precipitation of mica-calcite aggregates at upper crustal conditions, approx 500 degrees C, approx. 4 kbar. Monazite instability was induced by strain-focused influx of HREE, Y, Th and U rich fluids from external sources. The better preservation of within-clast monazite relative to the ones in weak-to-shear schist layers is attributed to limited fluid access through fewer porosity networks in actively deforming clasts.