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Lal, R. K.
- Mineralogy of Chromiferous Quartzites from South India
Authors
1 Meneralogisch-Petrographisches Institut, der Universitat Olshausenstr, 40-60, D 2300 Kiel, DE
2 Mineralogisch-Petrologisches Institut, der Universitat Bonn, Poppelsdorfer Schloss, D 5300 Bonn, DE
3 Geological Survey of India, Southern Region, Hyderabad 500001, IN
4 Department of Geology, Banaras Hindu University, Varanasi 221005, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 24, No 10 (1983), Pagination: 502-521Abstract
The mineralogy of chromiferous quartzites from Archaean greenstone belts of southern Peninsular India has been studied. The majority of the quartzites are metamorphosed clastic sediments derived from a sialic source with enclaves of ultramafic chromite-bearing rocks. For the quartzites from the Ghatti Hosahalli belt, however, a volcanic-exhalative origin is most likely. The chromium in the sediments was fixed in the detrital chromite and/or the clay minerals.
The typical mineral assemblages developed in a wide range of metamorphic conditions from greenschist to amphibolite facies comprise fuchsite, quartz, kyanite and accessory rutile, tourmaline and Cr-spinel. Microprobe analyses show high chromium contents for fuchsite (2-4wt. % Cr2O3), kyanite (up to 6wt. % Cr2O3), rutile (1.0-2-6wt. % Cr2O3) and tourmaline (2.5-7.6 wt. % Cr2O3). Cr-spinels are essentially solid solutions between hercynite and chromite (Mg/(Mg+Fe2+)=0.060.15; Cr/(Cr+Al)=0.42-0.75). Comparison with igneous chromites indicates that compositional reequilibration during metamorphism was characterised by replacement of Mg by Fe2+ and, to a lesser extent, of Cr3+ by Al. The micas of chromiferous quartzites from the Chitradurga area have high Cr and Ba contents (1.4-3.9wt. % Cr2O3; 0.5-8.5 wt. % BaO) and represent solid solutions extending from muscovite to Cr-oellacherite, A baryte-bearing quartzite from the Ghatti Hosahalli belt contains the rare assemblage quartz, baryte, celsian (Cs85Or11Ab4), fuchsite (15 wt. % Cr2O3; 8.5 wt. % BaO), uvarovite (Uv71Gro28) and tremolite.
The regular distribution of chromium between mica and associated Cr-spinel, rutile and tourmaline indicates equilibration during metamorphism.
- Internally Consistent Geothermobarometers in the System FeO-MgO-Al2O3-SiO2-H2O Involving Garnet, Cordierite, Aluminosilicate and Quartz and their Application to Metapeiites
Authors
1 Department of Geology, Banaras Hindu University, Varanasi-221 005, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 49, No 6 (1997), Pagination: 647-660Abstract
Reactions involving garnet, cordierite, aluminosilicate and quartz in FMASH system are calibrated as geothermobarometers, namely 1/2 Fe-crd + 1/3 Pyrope = 1/2 Mg-crd + 1/3 Almandine (1); 2 Almandine + 4 sillimanite/andalusite + 5 quartz = 3 Fe-crd (2 and 4); and 2 Pyrope + 4 sillimanite/andalusite + 5 quartz = 3 Mg-crd (3 and 5).
T = [6170+O.031(P-1)-400(XFe-XMg)Crd-A]/(RlnKD+2.812+B) (1)
A= 166 X2Mg-506 X2Fe+680 XFeXMg+336 (XCa+XMa) (XMg-XFe)-
XCa - 358 XMn and B = 1.5 XCa
For Fe and Mg end-member reactions involving sillimanite, the following geobarometric formulation is obtained:
PFe=1+[{(24.02-6RlnKFe)T-17369}/4.003] (2)
PMg=1+[{(7.238-6RlnKMg)T+19640}/3.82] (3)
Similarly for reactions involving and alusite, PFe and PMg expressions are : PFe=1+[{(26.04-6RlnKFe)T-19471}/3.86] (4)
PMg = 1+[{(9.25-6RlnKMg)T+17536}/3.68) (5)