Plagiogranites in Suture Zones: the Case of the Cuyano Proto-Ocean in the Lower Paleozoic Southwestern Gondwana Margin, Argentina

The Western Argentine Precordillera comprises mafic-ultramafic rocks for more than 400 km in length indicating a suture zone between previously accreted terranes. This belt is discontinuously exposed in the Central Andes between 29° and 33°S and related to the collision of the (western) Chilenia terrane against the (eastern) Cuyania terrane in the Middle-Upper Devonian. Mafic and ultramafic units such as basalts, gabbros, layered gabbros and wehrlites of Late Neoproterozoic to Late Silurian ages are the main lithology. Small bodies (25 m2) of intrusive felsic rocks, however, not mentioned in previous literature were identified in the northern section of this belt. Leucocratic rocks lie as felsic layers in cumulate gabbros or as irregular intrusive bodies within gabbroic sills. Thin section analyses exhibit fine to medium-grained plagiogranites with granophyric textures. They consist of tonalities and diorites that comprise 15 to 35% of quartz, 60 to 35% of plagioclase, 25 to 30% of hornblende, with subordinate clinopyroxene and biotite. They are moderately altered to chlorite, epidote, secondary quartz and sericite. Whole-rock geochemical analyses agree with E-MORB-type tholeiitic affinities with minor crustal contamination. Our observations support a model involving an oceanic "Cuyano" basin emplaced in an asymmetrically stretched continental margin likely generated by a large scale detachment fault. In this context, the presence of plagiogranite units related with gabbroic sills indicates that at least a small volume of gabbroic magma has been differentiated. Field, petrographic and geochemical evidences support a low-pressure differentiation of tholeiitic basaltic magmas for the plagiogranites origin. This would have occurred in the southwestern margin of Gondwana (the Cuyania terrane) during Early Paleozoic times.