Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements,REE geochemistry and Sr—Nd—O isotope systematics of ultramafic dykes |
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Authors: | A Roy A Sarkar S Jeyakumar S K Aggrawal M Ebihara H Satoh |
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Institution: | (1) Coal Wing, Geological Survey of India, DK-6, Sector-II, Salt Lake, 700 091 Kolkata, India;(2) Department of Geology and Geophysics, Indian Institute of Technology, 721 302 Kharagpur, West Bengal, India;(3) Fuel Chemistry Division, Bhaba Atomic Research Centre, 400 085 Mumbai, India;(4) Faculty of Science, Tokyo Metropolitan University, 192-03 Tokyo, Japan;(5) Geological Survey of Japan, 305-8567 Tsukuba, Japan |
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Abstract: | Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and
lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered
to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study
indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma,
Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest
stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile
elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots
exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)N ∼ 2–3 times chondrite, (Gd/Yb)N ∼ 1). The εNd(t) values vary from +1.23 to -3.27 whereas δ18O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace
and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised
by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted,
variable εNd, low Sri(0.702) and low δ18O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the
granitic crust which possibly originated due to the same thermal pulse. |
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Keywords: | Ultramafic dykes Rb-Sr & Sm-Nd source enrichment eastern Indian craton |
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