Trace element composition of parental magmas from mafic-ultramafic cumulates determined by in situ mineral analyses: The Juqui¨¢ mafic-ultramafic alkaline-carbonatite massif, SE Brazil

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• 作者：Rogé ; rio Guitarrari Azzone ; ^{rgazzone@usp.br} ; Gaston E.R. Enrich ; Celso de Barros Gomes ; Excelso Ruberti
• 关键词：Clinopyroxene ; Olivine ; Laser ablation ; Mafic&ndash ; ultramafic cumulates ; Parental magma ; Alkaline magmatism ; Ponta Grossa Arch
• 刊名：Journal of South American Earth Sciences
• 出版年：2013
• 出版时间：January, 2013
• 年：2013
• 卷：41
• 期：Complete
• 页码：5-21
• 全文大小：1538 K

文摘

Major and trace element analyses of relict cores of cumulus minerals (olivine and clinopyroxene) from primitive rocks of the Juqui¨¢ mafic-ultramafic alkaline-carbonatite complex (Early Cretaceous) in the Ponta Grossa Arch Alkaline Province, southeastern Brazil, were used to calculate the equilibrium melt compositions. Olivine relict cores are compositionally restricted to the Fo_83-74 interval, and they exhibit significant concentrations of Mn (2220-3001?ppm), Ni (1188-2327?ppm), Ca (175-649?ppm), Co (169-216?ppm), Zn (115-215?ppm), Ti (55-305?ppm), Cr (4-320?ppm) and P (31-154?ppm). Clinopyroxene relict cores are essentially diopside, with minor amounts of hedenbergite and tschermakite components. The trace element contents found in clinopyroxene cores are Ni (196-339?ppm), V (99-318?ppm), Sr (110-260?ppm), Zr (12-163?ppm), Sc (50-78?ppm), P (16-118?ppm), Zn (16-48?ppm) and Co (25-43?ppm). The rare earth element (REE) concentrations are variable (28-240?ppm), with the lighter elements present in the greatest abundance, particularly Ce (up to 83?ppm).

The melt compositions in equilibrium with olivine and clinopyroxene cores from different samples suggest that distinct basanite magma batches have played a role in the formation of the Juqui¨¢ cumulate rocks. These calculated liquids have compositions that are quite similar to those of the nearby lamprophyre dikes. The CaO/Al₂O₃ values (>0.8) from the lamprophyre dikes and the high La/Zr and low Ti/Eu values from the calculated liquids point to a previous carbonatite metasomatic event in the magmatic source that lead to the formation of wehrlite veins in a peridotitic mantle. The variation in the Hf/Y ratio implies differences in cpx/gt modal proportions in the source. Non-modal mantle batch-melting models indicate that a homogeneous source with little variation in the degree of partial melting cannot explain the trace element differences among the calculated liquids. The models suggest that the geochemical differences represent (1) mixing between metasomatic vein partial melt and garnet lherzolite partial melt and (2) clinopyroxene/garnet modal ratios of the residual mantle.