Field, petrological and geochemical data is reported on the Jacupiranga alkaline-carbonatite complex (133-131 Ma) which, together with other alkaline complexes of southern Brazil and south-western Africa, occurs in the central part of - and is coeval with - the Paranà-Etendeka CFB province. It consists of a shallow intrusion in the Precambrian crystalline basement, and can be subdivided in two main diachronous plutonic bodies: an older dunite-gabbro-syenite in the NW and a younger clinopyroxenite – ijolite (s.l.) in the SE, later injected by a carbonatitic core (< 1% volume). A petrogenetic model based on bulk rock major and trace element, mineral chemistry and Sr-Nd-Pb-C isotopes indicate that the two silicate intrusions generated from different parental magmas that rose from distinct mantle sources and evolved at shallow level in two zoned cup-shaped plutonic bodies. The first intrusion was generated by OIB-like alkaline to mildly alkaline parental basalts that initially led to the formation of a dunitic adcumulate core, surrounded by gabbroic cumulates, in turn injected by subanular syenite intrusive and phonolite dikes. Mela-nephelinitic (± melilite) melts likely generated deep (≥ 3 GPa) in the lithosphere were the parental magmas of the second intrusion and gave rise to large coarse-grained clinopyroxenite ad- to meso-cumulates, in turn surrounded and partially cut, by semi-annular fine-layered melteigite-ijolite-urtite ortho-cumulates. Isotopically, carbonatites do not evidence genetic links with the associated silicate intrusions, thus suggesting either a direct mantle origin or shallow liquid immiscibility from hypothetical silicate magmas currently not observed in the complex. An important result arising from the new Sr-Nd-Pb isotopes on Jacupiranga rocks is the clear correspondence of the clinopyroxenite-ijolite (s.l.) intrusion with the “Gough component” recently identified as the initial plume signature that characterizes the magmatic activity since 132 Ma, encompassing the oldest part of the Walvis Ridge volcanism and the Etendeka picrite-basalt association. Therefore, a model is proposed to account for the Early Cretaceous tectonomagmatic evolution of western Gondwana, where the impinging proto-Tristan (Walvis) mantle plume caused lithospheric arching, extension and radial fracturing of the south American-African plate triggering widespread small volume alkaline-carbonatite episodes mostly coeval with the eruption of the majority of Paranà-Etendeka CFB; the model could explain the coexistence of 1) small degree alkaline melts mostly generated from lithospheric mantle sources that, for the deepest magmas such as Jacupiranga parental melanephelinites, may also record the signature of sublithospheric plume-related geochemical components; and 2) higher degree melting CFB picrites generated from the hottest and deepest sublithospheric mantle sources at the core of the plume head.
The alkaline-carbonatite complex of Jacupiranga (Brazil) revisited: magma genesis, mode of emplacement and tectono-magmatic significance.
BECCALUVA, Luigi;BIANCHINI, Gianluca;NATALI, Claudio;SIENA, Franca
2016
Abstract
Field, petrological and geochemical data is reported on the Jacupiranga alkaline-carbonatite complex (133-131 Ma) which, together with other alkaline complexes of southern Brazil and south-western Africa, occurs in the central part of - and is coeval with - the Paranà-Etendeka CFB province. It consists of a shallow intrusion in the Precambrian crystalline basement, and can be subdivided in two main diachronous plutonic bodies: an older dunite-gabbro-syenite in the NW and a younger clinopyroxenite – ijolite (s.l.) in the SE, later injected by a carbonatitic core (< 1% volume). A petrogenetic model based on bulk rock major and trace element, mineral chemistry and Sr-Nd-Pb-C isotopes indicate that the two silicate intrusions generated from different parental magmas that rose from distinct mantle sources and evolved at shallow level in two zoned cup-shaped plutonic bodies. The first intrusion was generated by OIB-like alkaline to mildly alkaline parental basalts that initially led to the formation of a dunitic adcumulate core, surrounded by gabbroic cumulates, in turn injected by subanular syenite intrusive and phonolite dikes. Mela-nephelinitic (± melilite) melts likely generated deep (≥ 3 GPa) in the lithosphere were the parental magmas of the second intrusion and gave rise to large coarse-grained clinopyroxenite ad- to meso-cumulates, in turn surrounded and partially cut, by semi-annular fine-layered melteigite-ijolite-urtite ortho-cumulates. Isotopically, carbonatites do not evidence genetic links with the associated silicate intrusions, thus suggesting either a direct mantle origin or shallow liquid immiscibility from hypothetical silicate magmas currently not observed in the complex. An important result arising from the new Sr-Nd-Pb isotopes on Jacupiranga rocks is the clear correspondence of the clinopyroxenite-ijolite (s.l.) intrusion with the “Gough component” recently identified as the initial plume signature that characterizes the magmatic activity since 132 Ma, encompassing the oldest part of the Walvis Ridge volcanism and the Etendeka picrite-basalt association. Therefore, a model is proposed to account for the Early Cretaceous tectonomagmatic evolution of western Gondwana, where the impinging proto-Tristan (Walvis) mantle plume caused lithospheric arching, extension and radial fracturing of the south American-African plate triggering widespread small volume alkaline-carbonatite episodes mostly coeval with the eruption of the majority of Paranà-Etendeka CFB; the model could explain the coexistence of 1) small degree alkaline melts mostly generated from lithospheric mantle sources that, for the deepest magmas such as Jacupiranga parental melanephelinites, may also record the signature of sublithospheric plume-related geochemical components; and 2) higher degree melting CFB picrites generated from the hottest and deepest sublithospheric mantle sources at the core of the plume head.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
