Origin of mafic and ultramafic cumulates from the Ditrău Alkaline Massif, Romania

Abstract

Mafic-ultramafic cumulates enclosed in gabbroic-dioritic rocks form part of the Mesozoic Ditrău Alkaline Massif in the Eastern Carpathians, Romania. The poikilitic olivine-and pyroxene-rich and nearly mono mineralic hornblendite rocks display typical cumulate textures with early crystallised olivine ), diopside and augite. In the early stages of their genesis the amphibole was intercumulus while in later stages it acquired cumulus status as the fractionating magma evolved. Using major and trace element compositions of minerals and whole-rock samples the origin of these cumulates is determined and the parental magma composition and depth of emplacement are calculated.Cumulus clinopyroxene has more primitive composition than intercumulus amphibole suggesting closed system fractionation for the evolution of poikilitic olivine-and pyroxenerich cumulates. The evolution of the amphibole-rich mesocumulates is more clearly the result of closed system crystallisation dominated by the precipitation of clinopyroxene and amphibole cumulus crystals.Lamprophyre dykes of the Ditrău Alkaline Massif are proposed to reflect multiple basanitic parental magma batches from which the cumulus olivine and clinopyroxene crystallised. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT2 Relative to these dykes the calculated equilibrium melts for intercumulus amphibole in the cumulates was more primitive whilst that for the cumulus amphibole was more evolved. The calculated crystallisation temperature and pressure of ~1000-1050 °C and ~0.7 GPa, based on the composition of the amphiboles, indicate crystallisation at lower crustal depths. Rare earth element compositions are consistent with an intra-plate tectonic setting.

Insights into the evolution of an alkaline magmatic system: An in situ trace element study of clinopyroxenes from the Ditrău Alkaline Massif, Romania

“…10a) by Batki et al (2014) and as cumulate micro-and A C C E P T E D M A N U S C R I P T macrocrysts in the Ditrău hornblendites (Fig. 10b, c) by Pál-Molnár et al (2015b). The term 'phenocryst' is used to phases which are in equilibrium with the host melt (e.g., Cox et al, 1979;Streck, 2008), whilst 'antecrysts' are defined as crystals did not crystallise directly from the magma in which they are now hosted but represent crystals recycled by other magmas related to the same magmatic system (e.g., Charlier et al, 2005;Davidson et al, 2007;Streck, 2008;Francalanci et al, 2012;Ubide et al, 2012Ubide et al, , 2014a.…”

“…Hornblendite cumulate, nepheline syenite, camptonite, tinguaite and some of the ijolite samples have formed part of previous petrological and mineralogical studies (Fall et al, 2007;Batki et al, 2004;2012a,b, 2014Pál-Molnár, 2000;Pál-Molnár, 2010b, Batki andPál-Molnár, 2011;Pál-Molnár et al, 2015b).…”

“…The cumulates span a wide range of compositions but predominantly consist of olivine and/or clinopyroxene to essentially mono-mineralic hornblendite. A complete igneous rock series from hornblendite to gabbro, diorite, monzodiorite, monzonite, quartz monzonite, syenite, quartz syenite and A C C E P T E D M A N U S C R I P T granite can be observed from the north-west to the north-east part of the massif (Pál-Molnár et al, 2015b) (Fig. 1c).…”

“…The petrogenesis of the Ditrău Alkaline Massif has been in the focus of many studies in the last 150 years (e.g., Streckeisen, 1954Streckeisen, , 1960Codarcea et al, 1957;Streckeisen and Hunziker, 1974;Pál-Molnár and Árva-Sós, 1995;Dallmeyer et al, 1997;Kräutner and Bindea, 1998;Morogan et al, 2000;Pál-Molnár, 2000, 2010bFall et al, 2007;Batki et al 2014;Pál-Molnár et al, 2015b). However, because of the wide range of lithologies and complex field relationships, contrasting models for the origin and magmatic evolution of the massif have been proposed.…”

“…They attributed an important role to assimilation and fractionation, in conjunction with the mixing of felsic and basanitic melts. The hornblendites are interpreted as gravitational cumulates on a magma chamber floor (Pál-Molnár, 2000, 2010bPál-Molnár et al, 2015b) or as disrupted bodies of former sidewall cumulates (Morogan et al, 2000). Modelling suggests that camptonite dykes have been generated by 1-4% partial melting of an enriched, amphibole-bearing garnet lherzolite mantle source.…”