The early Paleozoic evolution of the Paleo-Pacific Gondwana margin: A structural, petrographic, and geochemical study in the Puncoviscana Formation.

摘要

The Pampean Orogen is one of the few turbidite-dominated, chevron-folded orogens in the world that preserve evidences of an early-Paleozoic convergent orogenic system that extended along the western margin of Gondwana. A better understanding of the evolution of this margin during the late Proterozoic-early Paleozoic is critical to comprehend the crustal development of western Gondwana and global plate reconstructions during the remainder of the Paleozoic. This study focuses on Pampean Vendian-aged turbidites of the Puncoviscana Formation and higher-grade equivalent rocks to the south, where evidence of the lower Paleozoic convergence is preserved. A series of transects through the Puncoviscana Formation and metasedimentary equivalent rocks from deeper structural levels demonstrates that an early stage compaction cleavage is folded into chevron structures with an associated tectonic cleavage, prior to erosion and deposition of the overlying Middle to Late Cambrian Meson Group. Changes in structural style are gradational, from the chevron-folded Puncoviscana Formation southward into progressively higher grade, ductily-folded metasedimentary rocks. Consequently, all mesoscopic ductile fold structures in the region can be explained with one major deformation episode. Pre-chevron folding compaction-related spaced cleavage in Puncoviscana Formation banded sandstones gradually transitions into compositionally banded schists and gneisses to the south due to pressure solution and mica growth becoming increasingly more important with increasing structural depth and metamorphic grade. As mica content increases, dissolution of quartz and feldspar also increase, favoring the passive concentration of mica. Thus, the formation of a quasi-gneissic compositional banding does not require tectonic transposition of an older planar fabric. Major and five trace-element concentrations in Puncoviscana Formation rocks and higher-grade equivalents are similar, revealing the relative homogeneity of their source. Puncoviscana Formation Pampean structures are very similar in geometry and orientation to Pre-Ordovician structures documented in Sierra de Quilmes metasediments, suggesting that these rocks underwent a common tectono-metamorphic history. The homogeneity in structural style and orientation, metamorphic grade variation and geochemical composition in the basement rocks of northern and eastern Sierras Pampeanas argue against previously published tectonic models of allochthonous terrane collision during the Pampean Orogeny.