Constraints on the evolutionary history of aluminous mafic rocks in the Ronda peridotite massif (Spain) from trace-element compositions of clinopyroxene and garnet

Abstract

Multiple trace-element compositions of clinopyroxene and garnet in aluminous mafic layers (±corundum and/or sapphirine) alternating with peridotites from the Ronda massif, Spain, were determined by an ArF 193 nm excimer laser system coupled with a quadrupole ICP-MS. The studied aluminous mafic rock is divided into several sublayers based on the modal mineral variations. Trace-element compositions of minerals vary between sublayers. Primitive mantle-normalized trace-element distribution patterns have more significant positive Eu anomaly for both garnet and clinopyroxene in the corundum-bearing sublayers than for those in the other sublayers whereas corundum-bearing sublayers contain more abundant secondary plagioclase than corundum-free sublayers. These geochemical signatures of the corundum-bearing sublayers intercalated with corundum-free sublayers were acquired as plagioclase-rich portions of gabbroic layered sequence originated at the lower crustal to the uppermost mantle sections rather than selective accumulation of metamorphic plagioclase due to deformation during the late stage of exhumation of the massif. Partition coefficients of trace elements between clinopyroxene and garnet show systematic variations with respect to major element composition, indicating local attainment of chemical equilibrium. The trace element compositions of clinopyroxene and garnet were equilibrated with secondary metamorphic plagioclase, i.e., at granulite-facies conditions, because plagioclase is required for the geochemical balance in Sr and Eu contents of the whole-rock trace element compositions. Positive Eu anomaly of clinopyroxene (±garnet) is also found in mantle-derived aluminous eclogitic xenoliths and eclogitic mineral inclusions in diamond captured in kimberlites, and is thus a useful indicator to explore recycled crustal components in the mantle.

Orogenic, Ophiolitic, and Abyssal Peridotites

“…This model is based on the extreme diversity of isotopic compositions in mafic rocks and on their isotopic differences with peridotites (Blichert-Toft et al, 1999;Hamelin and Allègre, 1988;Polvé and Allègre, 1980;Roy-Barman et al, 1996). According to Kornprobst et al (1990), the recycling hypothesis is further supported by the existence of positive europium anomalies in garnet pyroxenites from Ronda and Beni Bousera ( Figure 28; Kornprobst et al, 1990;Morishita et al, 2003aMorishita et al, , 2009aPearson et al, 1989). These anomalies are considered as evidence for a prograde evolution of the pyroxenites (see also, Morishita et al, 2004).…”

“…In the Ronda, Beni Bousera, and Horoman orogenic lherzolite bodies, these conditions are constrained by the presence of corundum (AEsapphirine) and/or graphitized diamond pseudomorphs Kornprobst et al, 1990;Morishita and Arai, 2001;Morishita et al, 2003aMorishita et al, , 2009aPearson et al, 1989Pearson et al, , 1993Pearson et al, , 1995. When these rocks are included, the deformed pyroxenites show a broad range of major-element compositions on the SiO 2 / MgO versus Al 2 O 3 diagram ( Figure 27).…”

“…This would imply that a number of garnet pyroxenites crystallized primarily as LP, plagioclase-bearing cumulates and then followed a prograde metamorphic evolution to higher-pressure mantle conditions. This second alternative would account for the positive europium anomalies commonly observed in these pyroxenites (Pearson et al, 1989;Kornprobst et al, 1990;Morishita et al, 2003aMorishita et al, , 2009a; Figure 28). …”

“…Several studies were also performed on the Beni Bousera peridotite (Morocco), including pioneering and advanced geochemical studies of mafic layers (Blichert-Toft et al, 1999;Gueddari et al, 1996;Kumar et al, 1996;Kornprobst et al, 1990;Allègre, 1979, 1982;Pearson and Nowell, 2004;Pearson et al, 1989Pearson et al, , 1991aPolvé and Allègre, 1980). Recent studies on Ronda and Beni Bousera were mostly focused on deciphering the significance of the different types of pyroxenites and further constraining the mechanisms of lithospheric melting and related melt ingress and refertilization Crespo et al, 2006;El Atrassi et al, 2011;Gysi et al, 2011;Marchesi et al, 2010Marchesi et al, , 2012Morishita et al, 2009a;Soustelle et al, 2009). …”

Depleted subcontinental lithospheric mantle and its tholeiitic melt metasomatism beneath NE termination of the Eger Rift (Europe): the case study of the Steinberg (Upper Lusatia, SE Germany) xenoliths

“…Their origin is related to formation of symplectitic clinopyroxene. Thus, the origin of this pattern is different from those with Eu-positive anomaly which occur seldom in clinopyroxene coexisting with plagioclase in mafic rocks subjected to metamorphic recrystallization (e. g., Seifert and Chadima 1989;Morishita et al 2009). …”