Trace Element Variations in Minerals from Tholeiitic Gabbros (Northern Apennine Ophiolites, Italy)

R. Tribuzio Dipartimento di Scienze della Terra, Università di Pavia, Via Abbiategrasso 209, I-27100 Pavia, Italy

R. Vannucci Dipartimento di Scienze della Terra, Università di Pavia, Via Abbiategrasso 209, I-27100 Pavia, Italy

M. Tiepolo Dipartimento di Scienze della Terra, Università di Pavia, Via Abbiategrasso 209, I-27100 Pavia, Italy

L. Ottolini C.N.R., Centro di Studio per la Cristallochimica e la Cristallografia, Via Abbiategrasso 209,

I-27100 Pavia, Italy

The ophiolite sequence from northern Apennine (Italy) is the base of the sedimentary cover which include pelagic, trench and lower slope deposits ranging in age from upper Jurassic to early Paleocene. The gabbroic rocks outcrop as bodies of variable size within tectonitic lherzolites. Mg-gabbroids (olivine-cumulates, troctolites, olivine-gabbros and gabbros) locally show cumulate layered textures, characterized by modal and/or grain size variations. Fe-gabbroids (gabbronorites and Fe-Ti-oxides-diorites) mainly outcrop as clasts in ophiolitic breccias.

Within a same rock sample, minerals are generally unzoned and uniform. The anorthite content of plagioclase varies from 70 mol % in troctolites to 30 mol % in Fe-Ti-diorites and gradually decreases with decreasing Mg/Mg+Fe ratio (Mg#) of both olivine and clinopyroxene. Clinopyroxene has Al and Cr contents (up to 0.19 apfu and 0.04 apfu, respectively) which decrease with decreasing Mg# (0.9 to 0.6). Olivine has forsterite ranging from 88 to 68 mol %. Common accessory minerals, modally increasing from Mg- to Fe-gabbroids, are apatite, Fe-Ti-oxides and Ti-pargasite.

Rare Earth and trace elements in minerals have been analyzed by secondary ion mass spectrometry with a Cameca IMS 4F ion microprobe. The REE pattern of clinopyroxene is characterized by LREE-depletion and slight negative Eu anomaly. Clinopyroxene shows a large increase in Zr, Y, REE (by about one order of magnitude) with decreasing Mg#. Sr, Sc, V and Ti display a restricted range of variations, whereas Cr shows an abrupt negative correlation with Mg#. The REE pattern of plagioclase is characterized by a steady decrease (up two orders of magnitude) from La to Yb, and a strong positive Eu anomaly. Plagioclase has constant, relatively high contents of Sr and Ti. In both clinopyroxene and plagioclase, the LaN/SmN ratio gradually increases with total REE contents.

Olivine, plagioclase, clinopyroxene and Ti-pargasite do not account for the whole rock inventory of incompatible trace elements. In particular, a large proportion of REE and Y budget of the whole rocks (up to 70 % in Fe-Ti-diorites) is incorporated into accessory apatite. Mass balance calculations moreover indicate that trace amounts of Zr-rich minerals must also be present.

The theoretical trace element composition of the parental liquids of the Mg-richest clinopyroxenes closely resembles that of present-day, primitive N-MORB. As a whole, the trace element variations of liquids in equilibrium with clinopyroxene and plagioclase are consistent with fractional crystallization controlled by olivine, plagioclase, pyroxene and Fe-Ti-oxides. However, REE compositions of more evolved melts (i.e., in equilibrium with the Mg-lowest clinopyroxenes) are more enriched than differentiated N-MORB. In other words, clinopyroxene and plagioclase from both Mg-richest and Mg-lowest gabbroids cannot be related to realistic N-MORB compositions through the same sets of DREEMin/Liq. Despite large variations in LREE compositions of clinopyroxenes from mafic cumulates are commonly used to argue against a process of low pressure fractionational crystallization (Ross and Elthon, 1993), it seems plausible that the observed REE increase in clinopyroxene and plagioclase from late cumulates is partly due to variations in DREEMin/Liq during igneous differentiation

The values of DREECpx/Liq are inferred to gradually increase due to decreasing temperature conditions, changes in melt and clinopyroxene compositions, in accordance with the experimental results on the partitioning between clinopyroxene and melt (Sisson, 1991; Gallahan and Nielsen, 1992). The variations in total REE contents of plagioclases are less marked than those of clinopyroxenes. However, the variations in DREEPl/Liq are probably strongly controlled by the increasing Na/Ca ratio of plagioclases. In fact, due to charge balance requirements, increasing Na/Ca is inferred to disfavour the incorporation of trivalent REE into plagioclase.


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