Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements, REE geochemistry and Sr—Nd—O isotope systematics of ultramafic dykes

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, Sr i ∼ 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 ε Nd (t) values vary from +1.23 to −3.27 whereas δ 18 O 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 ε Nd , low Sr i (0.702) and low δ 18 O 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.

Precambrian mafic magmatism in the Singhbhum craton, eastern India

“…The thick intrusion of Keshargaria studied in detail, is constituted largely of poikilitic harzburgite and the intrusion is immediately flanked on either sides by thin dolerite dykes (Saha et al 1972). There is no 'contaminated' petrographic type corresponding to lherzolite in which olivine to coexists with quartz and hornblende, as reported by Roy et al (2004). However, these two minerals (viz.…”

“…The more common type shows coarse tabular to elongate enstatite crystal cumulate coexisting with oval olivine in association with very minor late intercumulus clinopyroxene. But the proportion of clinopyroxene is never so high to designate them as lherzolite (Roy et al 2004). Mineralogy, majorand trace element characters of the other petrographic type viz.…”

“…Poikilitic harzburgite occurs as a major component of Keshargaria dyke intrusion (Saha et al 1972). From trace element and isotopic compositions based on inadequate field and pertrographic characters of Keshargaria body, a late Archaean enrichment of mantle has been proposed (Roy et al 2004). Such an antiquity may be ingored because poikilitic harzburgite occurs as diapiric intrusion within Proterozoic Singhbhum Basin (Bose and Goles, 1971;Bose et al 1989).…”

“…There is no petrographic variant of the ultramafic unit with dominant clinopyroxene (Saha et al 1972) though the immediately adjoining dolerites reveal fresh clinopyroxene. Based on inadequate field and petrographic study of Keshargaria body and having no cognition of the ultradepleted poikilitic harzburgite member, Roy et al (2004) derived a late Archaean Rb-Sr isochron age (2613±177 Ma) for the Keshargaria intrusion with 'lherzolite' variant. So far as the of age of the ultramafics is concerned, it is very significant to observe that thick diapiric ultramafic body with ultradepleted poikilitic harzburgite member (as at Keshargaria) was emplaced (at Roro-Jojohatu area near Chaibasa) within cover sediments of the PSB (Bose and Goles, 1971), and has been reported as "marginal basin ophiolite" (Bose and Chakraborti, 1981).…”

جيوكيمياء القواطع المافية في أجزاء من معقدة سينغابوم الجرانيتية: الأصل الصخري والوضع التكتونى

“…The rocks of the BIF Group overly unconformably the Singhbhum granitoid complex. The stabilization of this craton is attested by the intrusion of spectacular set of reticulating dyke swarms know as Newer dolerite dykes (Dunn and Dey 1942;Roy et al 2004) mainly concentrated in the southern part of Singhbhum shear zone. Published K-Ar dates of these dykes give a long range from 2,144 to 950 Ma (Sarkar and Saha 1977;Mallick and Sarkar 1994) indicating them to be of definite Proterozoic age.…”