Patagonia-Antarctica Early Paleozoic conjugate margins: Cambrian synsedimentary silicic magmatism, U-Pb dating of K-bentonites, and related volcanogenic rocks

摘要

This study describes the stratigraphic features, petrology and geochemistry and geochronology constraints of K-bentonites, ignimbrites and related volcanogenic rocks interbedded in the marine sedimentary sequence of the El Jagüelito Formation from northern Patagonia basement, southwestern Gondwana margin (41°33′S-65°15′W, South America). Six SHRIMP and ICP-MS U-Pb zircon ages from pyroclastic and volcanogenic sedimentary rocks indicate two stages of synsedimentary volcanism at c. 530 and c. 515?Ma which constraint the whole volcano-sedimentary pile to Early-Middle Cambrian. Distal volcanic ash fall deposits of K-bentonites and proximal pyroclastic flows of ignimbrites were erupted from three subaerial to subaqueous eruptive centers and are associated with volcanogenic facies and sub-volcanic intrusive equivalents. The volcanic rocks were deposited synchronously with on-going dynamic siliciclastic sedimentation into an actively subsiding basin.Dacitic K-bentonites and high-silica rhyolitic ignimbrites belong to the same high-K calc-alkaline/shoshonitic magma series and have a similar peraluminous signature. Their HFSE patterns with relative depletion in Nb, Sr, P, and Ti and the LREE-enriched patterns with a negative Eu anomaly indicate characteristics of subduction-related magmas associated with active continental arc magmatism. The geochemical discrimination diagrams together with geological features suggest an extensional tectonic setting for K-rich magmas within the overall convergent-margin system. Silicic, more potassic magmas with the inherited subduction-related character of the El Jagüelito Formation erupted on a back-arc basin. Their volcanic rocks are products of a mixture of fractionated mafic magmas and partial crustal melts. The integration of the Cambrian synsedimentary explosive volcanism of the El Jagüelito Formation with coeval magmatism of northern Patagonia led the interpretation to the continental scale of the paleo-Pacific margin of Gondwana by Early Paleozoic. The El Jagüelito Formation has provided consistent lithological, stratigraphic, geochemical and geochronological arguments to assess continental comparisons and a paleogeographic reconstruction between the eastern North Patagonian Massif and East Antarctica that fit in well the hypothesis that the northern Patagonia basement once occupied a position adjacent to East Antarctica. A series of geodynamic stages is proposed that allow for providing a Cambrian framework for understanding the magmatic arc-back arc system modified by episodic pulses of extension associated with slab-roll back which terminated in detachment of northern Patagonia from the East Antarctica continental margin by Late Cambrian. The several lines of geological evidence discussed in this paper point to a parautochthonous origin of the eastern North Patagonian Massif as an outboard assemblage that represents the conjugate margin of the Pensacola-Queen Maud-Ellsworth-Whitmore Mountains of Antarctica.