Chemical heterogeneity in an enderbite-hosted pseudotachylite,eastern India: evidence for syn-deformation multi-reaction melting in pseudotachylite |
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| Authors: | Radhika?Patro S?N?Mahapatro Email author" target="_blank">A?BhattacharyaEmail author N?C?Pant J?K?Nanda A?Dey A?K?Tripathy |
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| Institution: | (1) Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, 721 302, India;(2) Geological Survey of India, Bhubaneswar, 751 012, India;(3) Department of Geology, Delhi University, Delhi, 110 007, India;(4) Geological Survey of India, S-4/113 Niladrivihar, Bhubaneswar, 751021, India; |
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| Abstract: | The origin of chemical and mineralogical heterogeneity in tens-of-microns wide layers and domains in enderbite-hosted couple-of-centimeters
wide pseudotachylite vein is examined based on the results of BSE and X-ray element imaging, and electron probe microanalyses
of major elements of host-rock minerals, clasts, microphenocrysts, and pseudotachylite matrix. The pseudotachylite layers
and domains containing variable proportions of orthopyroxene and magnetite microphenocrysts continue as mantles around quartz,
K-feldspar, plagioclase and garnet clasts. The clasts are chemically modified along margins and intra-clast pseudotachylite
injections. The chemical modifications are extensive in smaller clasts <5 μm diameter. At least three chemically distinct
layers and domains in the pseudotachylite, and their fine-grained matrices, plot in sharply defined, well-segregated and non-overlapping
fields in FeO + MgO−Al2O3–SiO2, FeO–CaO–MgO and CaO–Na2O–K2O and FeO vs. FeO/FeO + MgO diagrams. The compositions of the layers and domains—smeared between a feldspar + quartz component
and a ferromagnesian component of garnet + Fe–Ti oxides (±orthopyroxene)—possibly correspond to fractionated quenched melts,
or admixtures of microphenocrysts that cannot be resolved by the microbeam techniques employed. The compositional variations
are incompatible with deformation-driven crystal fractionation in melt. Instead the layers and domains possibly are crystal-melt
mushes produced by syn-deformation ultra-high temperature (1,250–1,375°C) melting reactions involving variable proportions
of host-rock minerals determined by time-transient local phase aggregates experiencing strain. The similar element variation
trends in pseudotachylite examined here and those reported from anorthosite, metapelite and charnockite elsewhere suggests
local phase aggregate controlled multi-reaction melting is a phenomenon commoner than hitherto realized in pseudotachylites. |
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