Journal Article

Whole-rock, Phosphate, and Silicate Compositional Trends across an Amphibolite- to Granulite-facies Transition, Tamil Nadu, India

Edward C. Hansen and Daniel E. Harlov

in Journal of Petrology

Volume 48, issue 9, pages 1641-1680
Published in print September 2007 | ISSN: 0022-3530
Published online August 2007 | e-ISSN: 1460-2415 | DOI: http://dx.doi.org/10.1093/petrology/egm031
Whole-rock, Phosphate, and Silicate Compositional Trends across an Amphibolite- to Granulite-facies Transition, Tamil Nadu, India

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Chemical trends from north (amphibolite facies) to south (granulite facies) along a 95 km traverse in Tamil Nadu, Southern India, include: whole-rock depletion of Rb, Cs, Th and U, enrichment in Ti and F, and depletion in Fe and Mn in biotite and amphibole; increases in Al and decreases in Mn in orthopyroxene; enrichment of fluorapatite in F. K-feldspar blebs are widespread along quartz–plagioclase grain boundaries, and could indicate either partial melting or metasomatism. In the northernmost portion of the traverse the principal rare earth element (REE)-bearing minerals are allanite and titanite. South of a clinopyroxene isograd, monazite grains independent of fluorapatite are the major REE- and Th-bearing phase. Further south independent monazite is rare but Th-free monazite inclusions are common in fluorapatite. During prograde metamorphism, independent monazite was replaced by REE-rich fluorapatite in which the monazite inclusions later formed. The loss of independent monazite was accompanied by a loss of whole-rock Th and possibly a small depletion in light REE. Most mineralogical features along the traverse can be accounted for by progressive dehydration and oxidation reactions. Trace-element depletion is best explained by the action of an externally derived low H2O activity brine migrating from a source at greater depth, possibly preceded or accompanied by partial melting.

Keywords: granulite facies; charnockite; metasomatism; Archean; Tamil Nadu, India; fluorapatite; monazite; allanite; titanite; biotite

Journal Article.  21273 words.  Illustrated.

Subjects: Petrology

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