The Cretaceous Banhadão alkaline complex in southeastern Brazil presents two potassic SiO 2-undersaturated series. The high-Ca magmatic series consist of initially fractionated olivine (Fo 92-91) + diopside (Wo 48-43En 49-35Ae 0-7), as evidenced by the presence of xenocrysts and xenoliths. In that sequence, diopside (Wo 47-38En 46-37Ae 0-8) + phlogopite + apatite + perovskite (Prv >92) crystallized to form the phlogopite melteigite and led to the Ca enrichment of the magma. Diopside (Wo 47-41En 32-24 Ae 3-14) continued to crystallize as an early mafic mineral, followed by nepheline (Ne 74.8-70.1Ks 26.3-21.2Qz 7.6-0.9) and leucite (Lc 65-56) and subsequently by melanite and potassic feldspar (Or 85-99Ab 1-7) to form melanite ijolites, wollastonite-melanite urtites and melanite-nepheline syenites. Melanite-pseudoleucite-nepheline syenites are interpreted to be a leucite accumulation. Melanite nephelinite dykes are believed to represent some of the magmatic differentiation steps. The low-Ca magmatic series is representative of a typical fractionation of aegirine-augite (Wo 36-29En 25-4Ae 39-18) + alkali feldspar (Or 57-96Ab 3-43) + nepheline (Ne 76.5-69.0Ks 19.9-14.4Qz 15.1-7.7) + titanite from phonolite magma. The evolution of this series from potassic nepheline syenites to sodic sodalite syenites and sodalitolites is attributed to an extensive fractionation of potassic feldspar, which led to an increase of the NaCl activity in the melt during the final stages forming sodalite-rich rocks. Phonolite dykes followed a similar evolutionary process and also registered some crustal assimilation. The mesocratic nepheline syenites showed interactions with phlogopite melteigites, such as compatible trace element enrichments and the presence of diopside xenocrysts, which were interpreted to be due to a mixing/mingling process of phonolite and nephelinite magmas. The geochemical data show higher TiO 2 and P 2O 5 contents and lower SiO 2 contents for the high-Ca series and different LILE evolution trends and REE chondrite-normalized patterns as compared to the low-Ca series. The 87Sr/ 86Sr, 143Nd/ 144Nd, 206Pb/ 204Pb and 208Pb/ 204Pb initial ratios for the high-Ca series (0.70407-0.70526, 0.51242-0.51251, 17.782-19.266 and 38.051-39.521, respectively) were slightly different from those of the low-Ca series (0.70542-0.70583, 0.51232-0.51240, 17.758-17.772 and 38.021-38.061, respectively). For both series, a CO 2-rich potassic metasomatized lithospheric mantle enriched the source with rutile-bearing phlogopite clinopyroxenite veins. Kamafugite-like parental magma is attributed to the high-Ca series with major contributions from the melting of the veins. Potassic nephelinite-like parental magma is assigned to the low-Ca series, where the metasomatized wall-rock played a more significant role in the melting process.
The Banhadao alkaline complex, south eastern Brazil: source and evolution of potassic SiO2-undersaturated hich-Ca and low Ca magmatic series.
DE MIN, ANGELO;
2012-01-01
Abstract
The Cretaceous Banhadão alkaline complex in southeastern Brazil presents two potassic SiO 2-undersaturated series. The high-Ca magmatic series consist of initially fractionated olivine (Fo 92-91) + diopside (Wo 48-43En 49-35Ae 0-7), as evidenced by the presence of xenocrysts and xenoliths. In that sequence, diopside (Wo 47-38En 46-37Ae 0-8) + phlogopite + apatite + perovskite (Prv >92) crystallized to form the phlogopite melteigite and led to the Ca enrichment of the magma. Diopside (Wo 47-41En 32-24 Ae 3-14) continued to crystallize as an early mafic mineral, followed by nepheline (Ne 74.8-70.1Ks 26.3-21.2Qz 7.6-0.9) and leucite (Lc 65-56) and subsequently by melanite and potassic feldspar (Or 85-99Ab 1-7) to form melanite ijolites, wollastonite-melanite urtites and melanite-nepheline syenites. Melanite-pseudoleucite-nepheline syenites are interpreted to be a leucite accumulation. Melanite nephelinite dykes are believed to represent some of the magmatic differentiation steps. The low-Ca magmatic series is representative of a typical fractionation of aegirine-augite (Wo 36-29En 25-4Ae 39-18) + alkali feldspar (Or 57-96Ab 3-43) + nepheline (Ne 76.5-69.0Ks 19.9-14.4Qz 15.1-7.7) + titanite from phonolite magma. The evolution of this series from potassic nepheline syenites to sodic sodalite syenites and sodalitolites is attributed to an extensive fractionation of potassic feldspar, which led to an increase of the NaCl activity in the melt during the final stages forming sodalite-rich rocks. Phonolite dykes followed a similar evolutionary process and also registered some crustal assimilation. The mesocratic nepheline syenites showed interactions with phlogopite melteigites, such as compatible trace element enrichments and the presence of diopside xenocrysts, which were interpreted to be due to a mixing/mingling process of phonolite and nephelinite magmas. The geochemical data show higher TiO 2 and P 2O 5 contents and lower SiO 2 contents for the high-Ca series and different LILE evolution trends and REE chondrite-normalized patterns as compared to the low-Ca series. The 87Sr/ 86Sr, 143Nd/ 144Nd, 206Pb/ 204Pb and 208Pb/ 204Pb initial ratios for the high-Ca series (0.70407-0.70526, 0.51242-0.51251, 17.782-19.266 and 38.051-39.521, respectively) were slightly different from those of the low-Ca series (0.70542-0.70583, 0.51232-0.51240, 17.758-17.772 and 38.021-38.061, respectively). For both series, a CO 2-rich potassic metasomatized lithospheric mantle enriched the source with rutile-bearing phlogopite clinopyroxenite veins. Kamafugite-like parental magma is attributed to the high-Ca series with major contributions from the melting of the veins. Potassic nephelinite-like parental magma is assigned to the low-Ca series, where the metasomatized wall-rock played a more significant role in the melting process.Pubblicazioni consigliate
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