Origin of contemporaneous tholeiitic and K-rich alkalic lavas: a case study from the northern Deccan Plateau, India

Mahoney, J. J. ; Macdougall, J. D. ; Lugmair, G. W. ; Gopalan, K. ; Krishnamurthy, P. (1985) Origin of contemporaneous tholeiitic and K-rich alkalic lavas: a case study from the northern Deccan Plateau, India Earth and Planetary Science Letters, 72 (1). pp. 39-53. ISSN 0012-821X

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Official URL: http://linkinghub.elsevier.com/retrieve/pii/001282...

Related URL: http://dx.doi.org/10.1016/0012-821X(85)90115-3

Abstract

Concurrently erupted, alternating Deccan Trap flows of tholeiitic and potassic alkalic basalt outcrop along both banks of the Narmada River near Navgam. Nearby, around Rajpipla, early tholeiites are overlain by K-rich alkalic flows and intrusives, which are themselves cut by later tholeiitic dikes. Nd and Sr isotopic ratios of a wide variety of rocks from both areas form a single correlated array, which reflects mixing between positive εJUV and negative εJUV endmembers. There is an almost complete overlap between values for tholeiitic and alkalic samples; thus, both alkalic and tholeiitic primary magmas must have been produced that were isotopically much alike. A Rajpipla rhyolite also falls on the array, near the midpoint. For positive values of εJUV(T) the array is indistinguishable from that defined by the lower group of tholeiites at Mahabaleshwar, some 450 km to the south, implying a similar or identical high εJUV mantle source. The negative εJUV component, apparently different from either of the two at Mahabaleshwar, may have been continental crust or enriched mantle. Both alkalic and tholeiitic groups display wide overlapping ranges in incompatible elements other than K, Rb, and Ba-particularly in Sr and Nb. This partial decoupling of incompatible elements, in conjunction with the isotopic similarity between the two classes of rocks, is strongly suggestive of an enrichment event in portions of the mantle source shortly before magmatism.

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