Geochemical constraints on the petrogenesis of Triassic alkaline basalts of Sierra de Valle Fertil, Western Sierras Pampeanas, Argentina: implications for their origin, evolution and tectonic setting

摘要

The first geochemical and Sr-Nd isotope data of a series of Triassic basaltic rocks from Sierra de Valle Fertil, Western Sierras Pampeanas, Argentina, are herein presented. The mafic rocks are the result of continental intraplate magmatism related to an extensional tectonic setting after the climax of the Gondwanide orogeny (Upper Carboniferous-Permian). Major minerals phases of these rocks are olivine, clinopyroxene, plagioclase and Fe-Ti oxides. Olivine and clinopyroxene occur both as phenocrysts and as groundmass, while feldspar is mostly restricted to the groundmass. The rocks belong to a typical alkaline series displaying SiO2 contents of 43.9-51.6 wt%, Na2O between 3.1 and 5.4 wt%, K2O between 0.9 and 2.9 wt% and high TiO2 2.2-3.6 wt% and P2O5 0.5-1.2 wt% contents. Nepheline on its CIPW normative composition indicates that it crystallized from silica-undersaturated magmas. Samples have moderate to low Mg# (58.1-15.9) values and Cr and Co contents that indicate they are not primary magmas but have undergone fractional crystallization with earlier fractionation of olivine and clinopyroxene. Geochemical patterns are similar to those found in the ocean island basalts (OIBs), with enrichment of alkalis and incompatible elements: LILE, HFSE and LREE, strong fractionation of HREE relative to LREE [(La/Yb)(N) = 9.6-22.4), no depletion of Nb and Ta and absence of negative Eu anomaly. Observed LILE-enrichment is indicated by the alkaline nature of the basaltic rocks and suggests melting of an enriched mantle. The Eu and Sr positive anomalies, not related to plagioclase accumulation, argue against significant plagioclase fractionation and are original geochemical characteristics of the primary magma. The chemical features, together with high incompatible-element ratios, are consistent with low degrees of partial melting of a dominantly garnet-pyroxenite mantle source. The trace-element patterns and isotopic data with Nd-143/Nd-144((i)) (0.51257-0.51267), positive epsilon Nd-(i) and low Sr-87/Sr-86((1)) (0.70292-0.70339) values, suggest a mantle source influenced by an enriched-component. Active subduction-related mechanisms promoted the conditions for decompression melting in the asthenospheric mantle. Primary basaltic magma experienced subsequent compositional modifications by fractional crystallization in shallow-level low-pressure magma storage reservoirs. There is no evidence of crustal contamination during magma ascent.