Abstract
The Jacupiranga Complex (Cajati, SP) is one of several Meso-Cenozoic alkaline units intrusive along the Parana Basin margins, in the Central-Southeastern part of the South American Platform. This unit comprises a large variety of lithotypes: dunites, whehrlites, clinopyroxenites, rocks from the ijolite series, diorites, syenites, manzonites, phonolites, lamprophyres, and carbonatites. While carbonatites have been extensively investigated over the last decades, as they host an important phosphate ore deposit, little attention has been paid to the silicate rocks. The current study presents new geochronological, mineralogical, geochemical, and isotopic data on the Jacupiranga Complex, in order to better understand the origin and evolution of the unit. 40Ar/39Ar ages for different lithotypes range from 133.7±0.5 Ma to 131.9±0.5 Ma, while monzonite zircon analyzed by SHRIMP yields a U-Pb concordia age of 134.9±0.65 Ma, indicating that the Jacupiranga emplacement was contemporaneous with the extrusion of the tholeiites of the Paraná Magmatic Province. There seems to be no obvious age progression for clinopyroxenites, diorites, or lamprophyres, although the monzonite yield both 40Ar/39Ar and U-Pb ages older than those of the other rocks. Geochemical compositions of the silicate rocks are used to evaluate two main magma-evolution trends for that unit: (1) a strongly silica-undersaturated series, probably related to nephelinite melts and (2) a mildly silica-undersaturated series related to basanite melts. Lamprophyre dikes within the complex are considered as good representatives of the basanite parental magma. Compositions of the calculated melts in equilibrium with diopside cores from clinopyroxenites are quite similar to those of the lamprophyres, suggesting that at least a part of the clinopyroxenites is related to the basanite series. Meladiorite and monzonite show petrographic features and geochemical and isotope compositions (87Sr/86Sri: 0.705979-0.706086 and 143Nd/144Ndi: 0.511945-0.512089) suggestive of crustal assimilation, although it may be relegated to a local process and to some basanite batches. Carbonatites yield isotopic ratios (Nd and Pb) and trace elements composition (e.g. Ba/La, Nb/Ta) that preclude a link by liquid immiscibility with the silicate rocks. Two scenarios are envisaged: a primary magma of carbonatite composition originated by direct partial melting of the mantle or an origin by immiscibility from a hypothetical silicate magma currently unknown in the complex. Nd-Sr-Pb-Hf isotopic data indicate an important contribution of the subcontinental lithospheric mantle (SCLM) in the genesis of those rocks. Lamprophyres and calculated melts in equilibrium with clinopyroxene show relatively high CaO/Al2O3 and La/Zr ratios and low Ti/Eu, indicating a lithospheric mantle metasomatized by CO2-rich fluids and vein-plus-wall-rock melting mechanisms. Compositional variations among those liquids are attributed to the mixing between metasomatic veins partial melt and peridotite partial melt, besides as well as to the differences in the clinopyroxene/garnet ratios on in the mantle. ΔεHf variations suggest a slightly higher role of the wall-rock peridotite as a source component for the nephelinites, whereas the basanite parental magma is mainly related to the wehrlite veins, although both are enriched magmas. Depletion in Nb-Yb, enrichment of LREE relative to HREE, and enrichment in Cs, Rb and Sr in the lamprophyres suggest that the metasomatic reactions in the mantle source were caused by slab-derived fluids. TDM model ages indicate the heterogeneous nature of the mantle source and are coherent with the values generally obtained for the alkaline occurrences from the Central-Southeastern part of the South-American Platform.