Tertiary nephelinitic magmatism in Eastern Paraguay: Petrology, Sr-Nd isotopes and genetic relationships with associated spinel-peridotite xenoliths
Comin-Chiaramontk, Piero; Civetta, Lucia; Petrini, Riccardο; Piccirillo, Enzο Michele; Bellieni, Giulianο; Censi, Paolo; Bitschene, Peter; Demarche, Gabriella; De Min, Angelo; Gomes, Celso De Barro; Castillo, Ana Maria; Velazquez, Juan Carlos
European Journal of Mineralogy Volume 3 Number 3 (1991), p. 507 - 526
published: Jun 14, 1991
manuscript accepted: Jan 11, 1991
manuscript received: Jun 7, 1990
ArtNo. ESP147050303004, Price: 29.00 €
Abstract Plugs, flows and dykes of nephelinitic (and ankaramitic) rocks occur near Asuncion (Eastern Paraguay ; 61-39 Ma). This magmatism occurs at the western boundary of the Parana basin and is associated with NW-SE-trending rift and antiform structures. Nephelinites contain variable amounts of crustal (sedimentary, metamorphic and volcanic) and mantle (spinel-peridotite) xenoliths. The genesis of the Asuncion nephelinites requires a mantle source geochemically and isotopically distinct from that represented by the spinel-peridotite nodules included in the lavas. The most likely mantle source is a garnet peridotite characterized by small-scale heterogeneities, at least in terms of Sr and Nd isotopes. This mantle source was subjected to (1) low degrees of partial melting (e.g. 3 - 4 %), (2) enrichment in Nb, Ba, La, Ce, Sr and P relative to a primordial mantle. It is suggested that the enrichment in incompatible elements may be due to metasomatic processes related to a thermal anomaly responsible for the generation of the Lower Cretaceous Parana flood lavas. The initial Sr isotope ratios of the Asuncion nephelinites (R0 = 0.7036-0.7039) indicate a mantle source distinct from that of the Lower Cretaceous magmatism (Central-Eastern Paraguay: alkaline magmatism, R0 = 0.7073 ; tholeiitic flood basalts, R0 = 0.7059). On the whole, the Cretaceous-Tertiary alkaline and ultra-alkaline magmatism of Central-Eastern Paraguay appears closely linked to rift structures trending NW-SE and WNW-ESE. This may be an indication of a tectono-magmatic event(s) associated with stress-regime changes during the South America-Africa continental separation.