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Journal of Petrology Advance Access originally published online on November 30, 2006
Journal of Petrology 2007 48(3):435-457; doi:10.1093/petrology/egl066
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Shear Zone-hosted Migmatites (Eastern India): the Role of Dynamic Melting in the Generation of REE-depleted Felsic Melts, and Implications for Disequilibrium Melting

Subhadip Bhadra1,2, Suman Das1 and A. Bhattacharya1,*

1Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India
2Department of Earth Sciences, Pondicherry University, Kalapet, Pondicherry 605014, India

RECEIVED JULY 11, 2005; ACCEPTED OCTOBER 24, 2006


  Abstract

In the Ranmal migmatite complex, non-anatectic foliated granite protoliths can be traced to polyphase migmatites. Structural–microtextural relations and thermobarometry indicate that syn-deformational segregation–crystallization of in situ stromatic and diatexite leucosomes occurred at 800°C and 8 kbar. The protolith, the neosome, and the mesosome comprise quartz, K-feldspar, plagioclase, hornblende, biotite, sphene, apatite, zircon, and ilmenite, but the modal mineralogy differs widely. The protolith composition is straddled by element abundances in the leucosome and the mesosome. The leucosomes are characterized by lower CaO, FeO+MgO, mg-number, TiO2 , P2O5 , Rb, Zr and total rare earth elements (REE), and higher SiO2 , K2O, Ba and Sr than the protolith and the mesosome, whereas Na2O and Al2O3 abundances are similar. The protolith and the mesosome have negative Eu anomalies, but protolith-normalized abundances of REE-depleted leucosomes show positive Eu anomalies. The congruent melting reaction for leucosome production is inferred to be 0·325 quartz+0·288 K-feldspar+0·32 plagioclase+0·05 biotite+0·014 hornblende+0·001 apatite+0·001 zircon+0·002 sphene=melt. Based on the reaction, large ion lithophile element, REE and Zr abundances in model melts computed using dynamic melting approached the measured element abundances in leucosomes for >0·5 mass fraction of unsegregated melts within the mesosome. Disequilibrium-accommodated dynamic melting and equilibrium crystallization of melts led to uniform plagioclase composition in migmatites and REE depletion in leucosome.

KEY WORDS: migmatite; REE; trace element; partial melting; P–T conditions

*Corresponding author. Present address: Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India. Telephone: (+91)-3222-283350. Fax: (+91)-3222-255303/282268. E-mail: abbhat55{at}yahoo.com


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