Orthopyroxene-enrichment in the lherzolite-websterite xenolith suite from Paleogene alkali basalts of the Poiana Ruscă Mountains (Romania)

Abstract

Abstract:In this paper we present the petrography and geochemistry of a recently collected lherzolite-websterite xenolith series and of clinopyroxene xenocrysts, hosted in Upper Cretaceous-Paleogene basanites of Poiana Ruscă (Romania), whose xenoliths show notable orthopyroxene-enrichment. In the series a slightly deformed porphyroclastic-equigranular textured series could represent the early mantle characteristics, and in many cases notable orthopyroxene growth and poikilitic texture formation was observed. The most abundant mantle lithology, Type A xenoliths have high Al-and Na-contents but low mg# of the pyroxenes and low cr# of spinel suggesting a low degree ( < 10 %) of mafic melt removal. They are also generally poor in overall REE-s (rare earth elements) and have flat REY (rare earth elements + Y) patterns with slight LREE-depletion. The geochemistry of the Type A xenoliths and calculated melt composition in equilibrium with the xenolith clinopyroxenes suggests that the percolating melt causing the poikilitization can be linked to a mafic, Al-Na-rich, volatile-poor melt and show similarity with the Late Cretaceous-Paleogene (66-72 Ma) subduction-related andesitic magmatism of Poiana Ruscă. Type B xenoliths, with their slightly different chemistry, suggest that, after the ancient depletion, the mantle went through a slight metasomatic event. A subsequent passage of mafic melts in the mantle, with similar compositions to the older andesitic magmatism of Poiana Ruscă, is recorded in the pyroxenites (Fe-rich xenoliths), whereas the megacrysts seem to be cogenetic with the host basanite. The Poiana Ruscă xenoliths differ from the orthopyroxene-enriched mantle xenoliths described previously from the Carpathian-Pannonian Region and from the Dacia block.

Petrologic characteristics of the upper mantle beneath island arcs

“…Ternary plots showing the modal amounts of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) in peridotites and related xenoliths from arcs and arc-related settings. Data sources: Ishimaru (2004), Bryant et al (2007), Ishimaru et al (2007), and Ionov (2010) for Kamchatka; Kida (1988) and for the Philippines; Abe (1997), Abe et al (1992), Arai et al (2000Arai et al ( , 2001, and Hirai (1986) for Japan; Barsdell and Smith (1989), Franz et al (2002), andMcInnes et al (2001) for the SW Pacific; Brandon and Draper (1996), Francis (1987), Harder and Russell (2006), and Peslier et al (2002) for Alaska and Canada; Downes et al (2004), Smith and Riter (1997), and Smith et al (1999) for the USA; Blatter and Carmichael (1998) and Luhr and Aranda-Gómez (1997) for Mexico; Parkinson et al (2003) for the Lesser Antilles; Faccini et al (2013), Rivalenti et al (2004a, b), and Schilling et al (2005) Downes et al (1992), Downes et al (1995), Nédli et al (2015), Szabó and Taylor (1994), Szabó et al (1995), and Vaselli et al (1995) for the Pannonian Basin. , 2003a, 2004Ishimaru et al, 2007;Bryant et al, 2007Takahashi, 1986Ichiyama et al, 2013Ishimaru et al, 2007 ;…”