The Kerala Khondalite Belt, close to the Achankovil shear zone in the north, exposes an association of predominantly metasedimentary rocks (leptynitic garnet-biotite gneisses, khondalites and cordierite-garnet-sillimanite gneisses, quartzites) which has been subjected to polyphase deformation and high-grade metamorphism (T > 750°C; P > 5kbar) during the Pan-African orogeny (550 m.a.) (Braun et al., 1995). Rare, strongly deformed bands of calc-silicate rocks and humite-bearing marbles occur in the region of Ambasamudram (Krishnanath, 1981). The setting and paragenetic evolution of these rocks is dealt with in this contribution.
The marbles are characterized by a coarsely recristallized calcite matrix with granoblastic and sutured textures. The silicates, spinel and dolomite occur in different modal amount and distribution, mainly as intergranular phases. Within the studied marble band three mineral associations were observed: I. Chondrodite-humite-calcite-dolomite-amphibole-phlogopite-spinel-diopside. II. Clinohumite-calcite-dolomite-forsterite-amphibole-phlogopite.
Silicates and spinel are nearly pure Mg-endmembers (XFe < 10 mol %) and exhibit no compositional zoning. The humite group phases, amphibole and phlogopite in assemblages I and II, which occur exclusively in the northern sector, compared to assemblage III show higher fluorine contents. Ti contents of the humite minerals are low (Ti < 0.1 c.f.u).
Microtextures document an early coronitic replacement of forsterite by diopside, tremolite, clinohumite and carbonates, followed by the replacement of clinohumite and chondrodite by coronitic assemblages of diopside, tremolite and carbonates through a series of retrograde reactions:
2 Fo + 4 Cal + 2CO2 Æ Di + 3 Dol
8 Fo + 13 Cal + 9CO2 + H2O + HF Æ Tr + 11 Dol
4 Fo + Dol + H2O + HF Æ Chu + Cal + CO2
Chu + 9 Cal + 5 CO2 Æ 2 Di + 7 Dol + H2O + HF
2 Chu + 15 Cal + 11 CO2 Æ Tr + 13 Dol + H2O + HF
Chu + 26 Di + 6 H2O + 12CO2 Æ 7 Tr + 12 Cal + HF
4 Chn + 17 Cal + 13 CO2 Æ Tr + 15 Dol + 3 H2O + HF
Humite was formed by late-stage replacement of chondrodite in accordance with earlier studies (Tilley, 1951; Rosen & Raith, 1995), which suggests that humite may not be stable at high temperature (cf. Rice, 1980).
To evaluate the reaction histories of the rocks, P-T-Xfluid grids were calculated for the CaO-MgO-SiO2-H2O-CO2-HF model system with the GEOCALC-software (Berman et al., 1991), implementing the thermodynamic data of clinohumite (Duffy et al., 1979). The chemographic relations for the inferred P,T conditions show that the clinohumite- and chondrodite-bearing coronitic assemblages must have formed in the presence of extremely CO2-poor fluids (XCO2<0,2). As the peak-metamorphic Fo+Di+Spl assemblages in the marbles equilibrated at moderate to high XCO2, the retrograde reactions were triggered by the influx of HF-bearing, aqueous fluids from external sources. To further characterize the fluid regime, a stable isotope study is in progress.
Berman, R.G., Canad. Mineralogist 29, 857-887 (1991).
Braun, I., Raith, M. & Ravindra Kumar, R.G., J. metam. Geology, submitted 1995.
Duffy, C.J. & Greenwood, H.J., Am.Min. 64, 1156-1174 (1979).
Krishnanath, R., Geol. Society of India, 22, 235-242 (1981).
Rice, J.N., Contrib. Mineral. Petrol. 71, 219-235 (1980).
Rosen, K. & Raith, M., Terra abst. l.7, 316 (1995).
Tilley, C.E., Min.Mag., 29, 621-666 (1951).