Arenigian tholeiitic basalts in the Famatina Ordovician basin, northwestern Argentina: emplacement conditions and their tectonic significance.

Abstract

ABSTRACT. This study is focused on the analysis of volcanic deposits that crop out at the middle portion of the Las Planchadas range, northern part of the Famatina System in Argentina. These volcanic rocks are records of an Ordovician effusive basaltic volcanism that took place under subaqueous marine conditions. Along the study area crop out an Arenigian volcanic and volcaniclastic rocks succession with massive and autoclastic lavas, hyaloclastites of basaltic composition and volcaniclastic sandstones and mudstones. Large volumes of the volcanic deposits were strongly affected by fragmentation processes during their subaqueous emplacement and in situ accumulated as basaltic breccias. The same volcanic-volcaniclastic association crops out to the south of the Las Planchadas range, forming a basaltic belt with similar characteristics. The geochemical features of the basalts are compatible with depleted mid-ocean ridge basalt (MORB)-like source for the magma, with contribution of subducted related components such as water rich marine hemipelagic sediments, compatible with a back arc geotectonic setting developed along the northern part of the Famatina System during the Arenigian.Keywords: Tholeiitic basalts, Syn-eruptive Hyaloclastic deposits, Ordovician, Famatina belt. RESUMEN. Basaltos toleíticos arenigianos en la cuenca ordovícica famatiniana, noroeste de Argentina: condiciones de emplazamiento y su implicancia tectónica. El presente trabajo se enfoca en el análisis de los depósitos volcánicos que af loran en el tramo medio de la sierra de Las Planchadas, norte del Sistema de Famatina, Argentina. Estas rocas representan un registro completo del volcanismo ordovícico, correspondiente a efusiones basálticas emplazadas en un ambiente submarino. En la zona estudiada af lora una sucesión volcánica-volcanoclástica representada por lavas macizas y autoclásticas de composición basáltica, hialoclastitas basálticas, psamitas y pelitas volcanoclástica. Extensos volúmenes de estos depósitos fueron afectados fuertemente por procesos de fragmentación durante su emplazamiento subácueo, acumulándose in situ como brechas basálticas. Asociaciones volcánicas-volcanoclástica con idénticas características también af loran hacia el sur, en la misma sierra de Las Planchadas, y forman un cinturón volcánico de basaltos. Las características geoquímicas de estas volcanitas son comparables con basaltos empobrecidos, originados en zonas de dorsales oceánicas (MORB) y con componentes propios de subducción, como la participación de sedimentos hemipelágicos ricos en agua. Estas características son indicativas de un ambiente tectónico de retroarco ligado a la evolución del tramo norte del Sistema de Famatina durante el Arenigiano. Ordovícico, Cinturón famatiniano. 124 Arenigian tholeiitic basalts in the Famatina Ordovician basin, northwestern Argentina... Palabras clave: Basaltos toleíticos, Depósitos hialoclásticos

Early Paleozoic accretionary orogenies in NW Argentina: Growth of West Gondwana

“…This in turn shows that the arc/back-arc boundary migrated in time and space. This point is reinforced by the findings of Cisterna et al (2017), who described Arenigian MORB indicative of a back-arc, that are contemporaneous with and north of the intra-arc or inter-arc volcaniclastic sedimentary in the same mountain range described by Astini and Dávila (2004).…”

“…Above the Puncoviscana sequence, and separated by an angular unconformity, there are a number of deformed and metamorphosed younger Cambrian and Ordovician shallow marine basins (e.g. Astini, 2008;Bahlburg, 1998;Cisterna et al, 2017;Zimmermann and Bahlburg, 2003).…”

“…She interpreted these peaks to indicate several melting pulses and discussed the possibility that they reflect cyclic heating and/ or cyclic water influx into a hot region triggering melting events. This requires a long-lasting heat source to maintain the rocks close to their solidus for a long time, and could be a combination of back-arc extension and asthenospheric melting (Cisterna et al, 2017;Coira et al, 2009a), followed by the thickening of a crust comprising turbidites rich in HPEs. Wolfram (2017) argued that cyclic melting in the back-arc could signal pulsation in mantle heat transfer to the supra-subduction crust.…”

“…The picture that emerges from the review of the sedimentary record is that marine sedimentation dominated by turbidites defines an extensional period that started early in the Famatinian cycle at 490 Ma and continued until 460 Ma (early Caradocian) (Table 1) (e.g. Astini, 2008;Cisterna et al, 2017;Hauser et al, 2011) suggesting that the region was below sea level over this ~30 Ma period. Sedimentation was accompanied by basaltic magmatism, which ended at 470 Ma, just before peak magmatism in the arc itself and maximum basin subsidence between 470 and 460 Ma.…”