FRACTIONAL CRYSTALLIZATION AND LIQUID IMMISCIBILITY PROCESSES IN THE ALKALINE-CARBONATITE COMPLEX OF JUQUIA (SAO-PAULO, BRAZIL)

The Juquia circular intrusion, which is Cretaceous in age (130-135 Ma), crops out in the Precambrian gneissic basement in Brazil over an area of approximately 14 km2. It consists of olivine clinopyroxenite cumulates (with minor olivine gabbros) in the northeastern sector (74 vol.%), whereas ijolites-melteigites-urtites (4%) and nepheline syenites with minor essexites and syenodiorites (21%) outline subannular concentric patterns with an Mg-carbonatite core (1%), in the southwestern part of the complex. Petrographical, bulk rock, and mineral compositional trends indicate that the origin of the complex can be largely accounted for by shallow-level fractional crystallization of a carbonated basanitic parental magma. Such a magma was generated deep in the subcontinental lithosphere by low-degree partial melting of a garnet-phlogopite peridotite source. Mass-balance calculations in agreement with field volume estimates permit definition of several fractionation stages of the magmatic evolution under nearly closed-system conditions, with inward development of zonally arranged side-wall cumulates. These stages involved: (1) fractionation from basanite to essexite magma (liquid fraction F = 33.5%) by crystallization of olivine clinopyroxenite plus minor olivine alkali gabbro cumulates; (2) derivation of the least differentiated mafic nepheline syenite (F = 5.5%) from essexitic magma by subtraction of a syenodiorite assemblage; (3) exsolution of a carbonatite liquid (approximately 5 %) from a CO2-enriched mafic nepheline syenite magma, which also underwent continuous fractionation giving rise to ijolite-melteigite-urtite cumulates. The proportion of cumulus clinopyroxene and biotite and intercumulus nepheline and alkali feldspar in these last rocks, as well as the absence of alkalis in carbonatite, may be attributed, at least in part, to loss of alkali-rich hydrous fluids released during and after the unmixing formation of the two conjugate liquids. The K(D) values determined for Mg-carbonatite/nepheline syenite are lower (1.4-2.9) for light rare earth elements (LREE) than for REE from Eu to Yb (4.6-7.8), in contrast to recent experimental results (Hamilton et al., 1989). A possible explanation is that Juquia Mg-carbonatite represents an already differentiated magma, which underwent extensive fractionation of LREE-enriched calcite. In this way, the high variability of K(D) REE patterns observed in several alkaline-carbonatite complexes can also be accounted for. The remarkably constant initial Sr-87/Sr-86 ratios (mostly between 0.7052 and 0.7057) support the interpretation of the intrusion as having been generated by fractional crystallization and liquid immiscibility from a common parental magma. Higher isotopic ratios (0.7060-0.7078), found mainly in dykes and in the border facies of the intrusion, may be due to contamination by the gneissic basement.