Sr, Nd, Pb and O Isotopes of Minettes from Schirmacher Oasis, East Antarctica: a Case of Mantle Metasomatism involving Subducted Continental Material

Abstract

Numerous minette dykes intersect the Precambrian crystalline baseINTRODUCTION ment of Schirmacher Oasis, East Antarctica. This study presents Minette magmas are generally thought to represent smallnew Sr, Nd, Pb and O isotope data for 11 minette samples from degree melts that form at depths of >50–150 km within four different dykes. The samples are characterized by relatively the lithospheric mantle. The deep origin of such lamhigh Sr/Sr (0·7077–0·7134), Pb/Pb (15·45–15·55) prophyric magmas is indicated by entrained mantle xenoand Pb/Pb (37·8–39·8), combined with low Nd/Nd liths of spinel peridotite or garnet peridotite composition, ( Nd =−6·5 to−25·1) and variable Pb/Pb (16·8–18·1). and the geochemistry of the magmas (e.g. Rogers et al., The O values are high, ranging from +6·5 to +9·5‰ 1982; Stille et al., 1989; Wyman & Kerrich, 1993; Carlson SMOW. Rb/Sr whole-rock–biotite isochrons suggest an age of & Irving, 1994; Carmichael et al., 1996). Minettes typically >455 Ma for emplacement of the minette dykes. The major and display intermediate to basic compositions with high compatible element geochemistry of the minettes indicates derivation contents of MgO, Cr and Ni, and high mg-number. of the magmas from a mantle source. The enriched isotopic and Additionally, however, minettes are also characterized trace element signatures of the dykes cannot be due to contamination by high concentrations of large ion lithophile elements of the ascending magmas by continental crust. Rather, the geochemical (LILE), particularly Ba, Sr and Rb, and they are enriched characteristics of the minettes are most reasonably explained by in light rare earth elements (LREE). Thus minettes display partial melting of a lithospheric mantle source that was enriched high abundances of both compatible and highly inby metasomatic fluids derived from recycled continental crust. If compatible trace elements. This requires the involvement mantle enrichment took place just before dyke emplacement, the of at least two distinct source components for the genisotopic systematics of the minettes must be inherited directly from eration of minette magmas: (1) a peridotitic mantle resthe metasomatic agents, and this would indicate derivation of the ervoir and (2) a component enriched in LILE and LREE. fluids from recycled lower continental crust. It is conceivable that the enrichment of incompatible trace elements results from the contamination of the magmas with crustal materials during dyke emplacement.