The Pariquera Acu K-alkaline complex and southern Brazil lithospheric mantle source characteristics

Four of the most important K-alkaline magmatic complexes of southern Brazil are associated with two main NW-trending structures. The Lages occurrence is linked to the uplift of a large crustal block, the Lages Dome, tectonica lly controlled by old NW-trending fault zones. The Jacupiranga, Juquia and Pariquera Acu (PGAM) occurrences are related to the Ponta Grossa Arch, a NW -trending uplift structure that produced tensional tectonic lineaments with the same direction and influenced the emplacement of both alkaline rocks a nd dyke swarms associated with Parana flood basalt activity. The Pariquera Acu complex, petrographically and geochemically described here for the firs t time, comprises coarse- to fine-grained rocks representing two main suite s with different degrees of alkalinity: theralite to essexite and syenogabb ro to syenodiorite. In both series highly evolved products are lacking. The presence of carbonatites is inferred. In the Jacupiranga complex (ca. 130 Ma), clinopyroxenite (jacupirangite) cumulates with minor dunites are assoc iated with ankaratrites, alkali gabbros and leucocratic rocks such as syeni tes, nepheline syenites and peralkaline phonolites. Many phases of both cal cic and magnesian carbonatite intrusions, to which relevant fenitization pr ocesses of jacupirangite are related, characterize the district. The Juquia complex (130-135 Ma) consists of prevalent olivine clinopyroxenite cumulat es and nepheline syenites, the latter arranged in subanular concentric patt erns together with subordinate ijolites-melteigites-urtites, essexites and syenodiorites. The central part of the intrusion is occupied by a small Mg- carbonatite body. The Lages complex (78-63 Ma) is made up of mafic-ultramaf ic rock-types (olivine melilitites, olivine nephelinites, basanites) and pr evalent leucocratic bodies mainly consisting of peralkaline phonolites. Kim berlitic breccias and carbonatites are also present. Petrography, mineral a nd bulk-rock chemistry and mass balance calculations show that shallow-leve l fractional crystallization processes controlled the evolution from mafic to leucocratic rocks in all four complexes. Starting from a mantle source c omposition as suggested for the eastern Paraguay alkaline (ASU) magmatism, mass balance calculations indicate that primary magmas with high LILE conte nts and high La/Yb ratios are consistent with a garnet peridotite or phlogo pite-bearing garnet peridotite solid residuum and less than 6% melting. The mantle sources are inferred to have been derived by different incompatible element enrichment probably associated with two late Proterozoic metasomat ic processes: ASU (1.1 Ga) and Alto Paranaiba Igneous Province (APIP; 0.5 G a). Mainly on the basis of geochemical evidence (e.g. Nb-Ta anomaly), mantl e sources for PGAM and Lages Ol-melilitites and Ol-nephelinites seem to hav e been affected by an APIP-like event. On the other hand, an ASU-like metas omatic process is suggested for the Lages basanite mantle source, which may be indicative of vertical mantle heterogeneity. Geochemical data also poin t to a large-scale northward increase in concentration of incompatible elem ents (e.g. Lages Ol-melilitites and Ol-nephelinites vs APIP kamafugites). A n attempt to correlate the chemical composition of silicate rocks and carbo nate metasomatism shows that the mantle sources for PGAM and Lages occurren ces plot on a mixing curve linking depleted peridotites and the average Ca- carbonatite. This fact supports the view that carbonatitic fluids/melts hav e been involved in the metasomatic processes responsible for variable incom patible element enrichment of the lithospheric mantle of SE Brazil. (C) 200 0 Elsevier Science Ltd. All rights reserved.