Precambrian mafic magmatism in the Singhbhum craton,eastern India |
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| Authors: | Mihir K Bose |
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| Institution: | (1) Department of Geology, Presidency College, 86/1 College Street, Kolkata, 700 073, India |
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| Abstract: | The Singhbhum craton has a chequred history of mafic magmatism spanning from early Archaean to Proterozoic. However, lack
of adequate isotopic age data put constraints on accurately establishing the history of spatial growth of the craton in which
mafic magmatism played a very significant role. Mafic magmatism in the craton spreads from ca.3.3 Ga (oldest “enclaves” of
orthoamphibolites) to about 0.1 Ga (‘Newer dolerite’ dyke swarms). Nearly contemporaneous amphibolite and intimately associated
tonalitic orthogneiss may represent Archaean bimodal magmatism. The metabasic enclaves are appreciably enriched and do not
fulfill the geochemical characteristics of worldwide known early Archaean (>3.0 Ga) mafic magmatism. The enclaves reveal compositional
spectrum from siliceous high-magnesian basalt (SHMB) to andesite. However, the occurrence of minor depleted boninitic type
within the assemblage has so far been overlooked. High magnesian basalt with boninitic character of Mesoarchaean age is also
reported in association with supracrustals from southern fringe of the granitoid cratonic nucleus. The subcontinental lithospheric
mantle (SCLM) below the craton is conjectured to have initiated during the early Archaean. Significantly, recurrence of depleted
magma types in the craton is observed during the whole span of mafic igneous activity which has been vaguely related to “mantle
heterogeneity”, although the alternative model of sequential mantle melting is also being explored.
The Singhbhum craton includes the Banded Iron Formation (BIF) associated mafic lavas, MORB-like basic and komatiitic ultrabasic
bimodal volcanism — documented as Dalma volcanics, Dhanjori lavas, and the Proterozoic Newer dolerite dykes. Three different
types of REE fractionation patterns are observed in the BIF-associated mafic lavas. These are the REE unfractionated type
is more depleted than N-MORB and some lavas with boninitic type of REE distribution. MORB-like basic and komatiitic ultrabasic
(Dalma volcanics) are emplaced within the Proterozoic Singhbhum Basin (PSB). The vista of magmatism in the basin was controlled
by a miniature spreading centre represented by the mid-basinal Dalma volcanic ridge. The volcano-sedimentary basinal domain
of Dhanjori emerged at the interface of two subprovinces (viz. the mobile volcano-sedimentary belt of PSB and rigid granite
platform) under unique stress environment related to extensional tectonic regime. Trace element distribution in Dhanjori lavas
is remarkably similar to that in PSB minor intrusions and lavas (except a Ta spike in the latter). The Proterozoic Newer dolerite
dykes within Singhbhum nucleus manifest an unusually wide spam of intrusive activity (ca 2100 Ma to 1100 Ma) and unexpectedly
uniform mantle melting behaviour. |
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| Keywords: | Boninitic magma Siliceous high magnesian basalt (SHMB) Komatiite N-MORB Mantle characteristics Singhbhum craton |
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