Discrimination between ophiolitic metabasalts, north D'Urville Island, New Zealand

Abstract

Whole-rock minor and trace element abundances, relict clinopyroxene compositions and petrographic features discriminate between 2 metabasalt units within the Patuki Volcanic complex, north D'Urville Island, New Zealand. The majority of the Patuki metabasalts show P, Zr, Nb, and light rare earth element (LREE) depletion and exhibit tholeiitic differentiation characteristics. They show close similarities to many abyssal tholeiites produced at modern spreading ridges (Ntype ocean ridge basalts). Pristine relict clinopyroxene preserves Ti-poor endiopside and endiopsidic augite compositions somewhat enriched in Cr, and shows systematic iron enrichment. A smaller proportion of the metabasalts are enriched in Nb, LREE, and related hygromagmatophile elements, but retain phenocryst assemblages and some trace element abundances similar to ocean-floor basalts. They contain Ca- and AI-rich titaniferous salites. These metabasalts are atypical of ophiolitic metabasalts and of metabasalts formed at normal ridge segments and show closest similarity to largeion- lithophile element enriched, ocean-floor tholeiites (E-type mid-ocean ridge basalts). At north D’Urville Island, the N-type metabasalts form an upper unit composed of massive and more rarely pillowed flows, and the group of E-type metabasalts form a lower unit composed of pillow lava and rare tabular bodies of ophitic spilite, associated with volcanic breccia. Interbedded marine sediments include sandstone, argillite, mudstone and minor tuffs, and ophiolitic breccia The trace element data indicate that the 2 suites of Patuki metabasalts at north D’Urville Island are derived from geochemically distinct mantle sources, similar to sources from which N- and E-type midocean ridge basalts originate. The data imply mantle heterogeneity beneath a poorly evolved Middle Permian seafloor.