POTASSIC AND SODIC IGNEOUS ROCKS FROM EASTERN PARAGUAY - THEIR ORIGINFROM THE LITHOSPHERIC MANTLE AND GENETIC-RELATIONSHIPS WITH THE ASSOCIATED PARANA FLOOD THOLEIITES

Eastern Paraguay represents the westernmost fringe of Early Cretaceous Parana flood tholeiites (Serra Geral Formation, SGF). Besides the SGF eastern Paraguay has been the site of alkaline magmatism since Mesozo ic times: (1) Late Permian-Early Triassic sodic intrusions and lavas; (2) Early Cretaceous potassic igneous rocks and very scarce sodic lava s; (3) Late Cretaceous-Oligocene sodic lavas. Two distinct magmatic ev ents are dominant in the Asuncion-Sapucai graben (ASU) of eastern Para guay: (1) wide-spread potassic magmatism and SGF tholeiites (Early Cre taceous); (2) Asuncion sodic magmatism (Late Cretaceous-Oligocene). Th e potassic rocks from a compositional continuum from moderately to str ongly potassic. Two potassic suites are proposed, i.e. basanite to pho nolite and alkali basalt to trachyte and their intrusive analogues. Th e sodic rocks include ankaratrites, nephelinites and phonolites. Two s imilar but distinct parental magmas have been inferred for the potassi c suites, both characterized by strongly fractionated REE and negative 'Ta-Nb-Ti'. anomalies: Slight positive Ta and Nb anomalies distinguis h the sodic rocks. Sr-Nd isotope data confirm the distinction of the p otassic locks, enriched zn radiogenic Sr and late, in radiogenic Nd, f rom the sodic rocks, close to bulk Earth. Crustal contamination does n ot appear to have been significant in the generation of the investigat ed rocks, supported by delta(18)O data. The source of potassic rocks i s constrained by high LILE, LREE, Th, U and K, relative to a primitive mantle composition and a garnet peridotite is favoured as a possible mantle source for the investigated rocks. The close association of pot assic and sodic rock suites in the ASU demands that their parental mag mas derived from a heterogeneous subcontinental mantle, variously enri ched in incompatible elements. Significant H2O, CO2 and F are expected in the mantle source(s) for the occurrence of coeval carbonatites. An y genetic hypothesis based an a 'mantle plume' system is constrained b y strong lithospheric mantle characteristics. This does not preclude t hat thermal perturbations from the asthenosphere may have triggered ma gmatic activity in the lithospheric mantle. Model ages indicate that t wo distinct metasomatic events may have occurred during Late and Early -Middle Proterozoic as precursor to the genesis of tholeiitic and alka line magmatism in the Parana Basin. These metasomatic processes were c hemically distinct, indicated by the strong differences in Ti, LILE an d HFSE concentrations found in both alkaline provinces (e.g. potassic rock from ASU vs Alto Paranaiba Igneous Province) and Parana tholeiite s (low vs high Ti). In general, the relationships between the alkaline rocks from southeastern Brasil, ie. Alto Paranaiba, Ponta Grossa Arch , Serra do Mar, Lages and the flood basalts of the Parana Basin, suppo rt a common origin in the lithospheric mantle. Sr-Nd isotope and other geochemical data indicate that a significant role war played by a man tle component depleted in incompatible elements and with high Sm/Nd ra tio. This component (N-MORB type) would be represented by the depleted portions of a mantle which was variously metasomatized during Protero zoic times. The isotopic and geochemical features of the modern Trista n da Cunha plume are distinctly different from the component depleted in incompatible elements, and its contribution is not apparent in the compositions of the Parana tholeiites.