Fractional Crystallization and Liquid Immiscibility Processes in the Alkaline-Carbonatite Complex of Juqui (So Paulo, Brazil) |
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Authors: | BECCALUVA, L. BARBIERI, M. BORN, H. BROTZU, P. COLTORTI, M. CONTE, A. GARBARINO, C. G., C.B. GOMES M., ACCIOTTA MORBIDELLI, L. RUBERTI, E. SIENA, F. TRAVERSA, G. |
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Affiliation: | 1Istituto di Mineralogia Universita di Ferrara, C.so Ercole I'D'Este 32, 44100 Ferrara, Italy 2Dipartimento di Scienze della Terra, Universita di Roma, Roma, Italy 3Escola Politcnica, Uiversidade de So paulo, Brazil 4Departmento di Scienze della Terra, Universit di Cagliari, Cagliari, Italy 5Istituto di Giacimenti Minerari, Universit di Cagliari, Cagliari, Italy 6Instituto de Geocincia, Universidade de s o Paulo, Brazil 7Dipartimento di Scienze della Terra, Universit di Cagliari, Cagliari, Italy 8Dipartimento di Scienze della Terra, Universit di Perugia, Perugia, Italy |
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Abstract: | The Juqui circular intrusion, which is Cretaceous in age (130135Ma), crops out in the Precambrian gneissic basement in Brazilover an area of 14 km2. It consists of olivine clinopyroxen-itecumulates (with minor olivine gabbros) in the northeastern sector(74 vol.%), whereas ijolites-melteigites-urtites (4%) and nephelinesyenites with minor essexites and syenodiorites (21%) outlinesubannular concentric patterns with an Mg-carbonatite core (1%), in the southwestern part of the complex. Petrographical, bulk rock, and mineral compositional trendsindicate that the origin of the complex can be largely accountedfor by shallow-level fractional crystallization of a carbonatedbasanitic parental magma. Such a magma was generated deep inthe subcontinental lithosphere by low-degree partial meltingof a garnet-phlogopite peridotite source. Mass-balance calculations in agreement with field volume estimatespermit definition of several fractionation stages of the magmaticevolution under nearly closed-system conditions, with inwarddevelopment of zonally arranged side-wall cumulates. These stagesinvolved: (1) fractionation from basanite to essexite magma(liquid fraction F = 335%) by crystallization of olivineclinopyroxenite plus minor olivine alkali gabbro cumulates;(2) derivation of the least differentiated mafic nepheline syenite(F = 55 %) from essexitic magma by subtraction of a syenodioriteassemblage; (3) exsolution of a carbonatite liquid ( 5%) froma CO2-enriched mafic nepheline syenite magma, which also underwentcontinuous fractionation giving rise to ijolite-melteigite-urtitecumulates. The proportion of cumulus clinopyroxene and biotiteand intercumulus nepheline and alkali feldspar in these lastrocks, as well as the absence of alkalis in carbonatite, maybe attributed, at least in part, to loss of alkali-rich hydrousfluids released during and after the unmixing formation of thetwo conjugate liquids. The KD values determined for Mg-carbonatite/nepheline syeniteare lower (1429) for light rare earth elements(LREE) than for REE from Eu to Yb (4678),in contrast to recent experimental results (Hamilton et al.,1989). A possible explanation is that Juquia Mg-carbonatiterepresents an alreadydifferentiated magma, which underwent extensivefractionation of LREE-enriched calcite. In this way, the highvariability of K0 REE patterns observed in several alkaline-carbonatitecomplexes can also be accounted for. The remarkably constant initial 87Sr/86Sr ratios (mostly between07052 and 07057) support the interpretation ofthe intrusion as having been generated by fractrional crystallizationand liquid immiscibility from a common parental magma. Iligherisotopic ratios (0706007078), found mainlyin dykes and in the border facies of the intrusion, may be dueto contamination by the gecissic basement. |
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