Shear Zone-hosted Migmatites (Eastern India): the Role of Dynamic Melting in the Generation of REE-depleted Felsic Melts, and Implications for Disequilibrium Melting |
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Authors: | Bhadra Subhadip; Das Suman; Bhattacharya A |
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Affiliation: | 1Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India
2Department of Earth Sciences, Pondicherry University, Kalapet, Pondicherry 605014, India |
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Abstract: | In the Ranmal migmatite complex, non-anatectic foliated graniteprotoliths can be traced to polyphase migmatites. Structuralmicrotexturalrelations and thermobarometry indicate that syn-deformationalsegregationcrystallization of in situ stromatic and diatexiteleucosomes 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 compositionis straddled by element abundances in the leucosome and themesosome. 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 andthe mesosome, whereas Na2O and Al2O3 abundances are similar.The protolith and the mesosome have negative Eu anomalies, butprotolith-normalized abundances of REE-depleted leucosomes showpositive Eu anomalies. The congruent melting reaction for leucosomeproduction is inferred to be 0·325 quartz+0·288K-feldspar+0·32 plagioclase+0·05 biotite+0·014hornblende+0·001 apatite+0·001 zircon+0·002sphene=melt. Based on the reaction, large ion lithophile element,REE and Zr abundances in model melts computed using dynamicmelting approached the measured element abundances in leucosomesfor >0·5 mass fraction of unsegregated melts withinthe mesosome. Disequilibrium-accommodated dynamic melting andequilibrium crystallization of melts led to uniform plagioclasecomposition in migmatites and REE depletion in leucosome. KEY WORDS: migmatite; REE; trace element; partial melting; PT conditions |
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Keywords: | : migmatite REE trace element partial melting P T conditions |
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