Evolution of the Palaeozoic basement of the Northpatagonian Andes in the San Martín de los Andes area (Neuquén, Argentina)Petrology, age and correlations.

Résumé

In the surroundings of San Martín de los Andes (Argentinian Patagonia), the Palaeozoic basement consists of metamorphic and plutonic rocks. Metamorphic rocks are schists, gneisses and migmatites that are typically strongly deformed. Considering the geochemical and petrographic characteristics of these metamorphic rocks, their protolith could be considered as a sequence of pelites and greywackes. Detrital zircon analysis (U-Pb Q-ICP-MS) on these rocks has yielded a maximum depositional age of 501.2 ± 9.3 Ma. (Series 3 Cambrian) for the sedimentary protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since some relics of a former foliation seem to be also present in some samples. The regional foliation is, in turn, locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphism includes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut it in presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a Concordia age of 434.1 ±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatectic age constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lower Silurian). The igneous rocks of the basement are granodiorites and tonalities with minor gabbros that cut the metamorphic fabrics and contain xenoliths and roof pendants of the metamorphic rocks. These plutonic rocks are affected by low-grade metamorphic conditions, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age of these igneous rocks has been constrained by several authors by U-Pb zircons between ca. 370-400 Ma (Devonian). The age of sedimentation of the protolith and its metamorphic evolution seems to be related to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonian igneous rocks of San Martín de los Andes would represent a Devonian magmatic arc, related to asubduction developed at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.