Petrogenetic study of the Lonco Trapial volcanism and its comparison with the Early-Middle Jurassic magmatic units from northern Patagonia

Abstract

One of the largest silicic igneous provinces of the world, the Chon Aike Volcanic Province, is related to an extensional regime developed in Patagonia in Early to Middle Jurassic times. This work presents new geochemical analyses for major and trace elements from different units in the central domain of northern Patagonia, units known as Lonco Trapial and Garamilla formations. Additionally, some samples were analyzed from the Piltriquitrón and Lago La Plata formations in the western domain of northern Patagonia. One new U–Pb age of a rhyolitic dike (177.26 ± 0.96 Ma) and new Rb–Sr and Sm–Nd determinations from the Lonco Trapial Formation are also presented in this work. Results presented here fill a gap in geochemical data for the area with respect to the temporal and spatial coverage of the Chon Aike Volcanic Province in northern Patagonia. Our comparative study shows that all Early-Middle Jurassic igneous rocks from this area have mixed characteristics between slab-derived subduction-related settings and intraplate magmas. In the central domain melting must have involved both mantle and lower crust, so that a mixed source is envisaged. Ascent to shallow crustal levels with subsequent low-pressure differentiation followed by assimilation could explain the geochemical features of Lonco Trapial magmas from andesites to subordinated rhyolites. The arc signature of Lonco Trapial magmas located far from the volcanic arc is considered as the result of decompression melting of mantle enriched by previous subduction events rather than by contemporaneous Early Jurassic subduction involving additional contributions of crustal materials. Upper crustal contamination was facilitated by eastwards increasing crustal thicknesses, so Lonco Trapial magmas had to traverse a thicker crust on their way to the surface.