Evolution of the Palaeozoic basement of the North Patagonian Andes in the San Martín de los Andes area (Neuquén, Argentina): petrology, age and correlations

Abstract

In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphic and plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemical and petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes. Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian) for this sedimentary protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regional foliation is 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 U-Pb 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, tonalities, and some gabbros that cut the metamorphic basement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grade metamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age of these igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370 and 400 Ma (Devonian). The maximum sedimentation age for 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 could represent a Devonian magmatic arc, related to subduction processes developed at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.

The Late Paleozoic Tectonometamorphic Evolution of Patagonia Revisited: Insights From the Pressure‐Temperature‐Deformation‐Time (P‐T‐D‐t) Path of the Gondwanide Basement of the North Patagonian Cordillera (Argentina)

“…This is consistent with the crustal thickening event suggested by Cerredo and López de Luchi () for this Late Paleozoic magmatism. P‐T‐t data of the basement show a similar trend, with younger ages of metamorphism for higher‐pressure peak metamorphic conditions (sections and , Martínez et al, ; Serra‐Varela et al, , ), although this trend could alternatively be explained by differential exhumation. Nevertheless, a progressive crustal thickening is further supported by the prograde P‐T path of the Challhuaco sample (Figure ).…”

“…On the other hand, a correct understanding of the coupling between deformation and metamorphism is critical for the reconstruction of P-T-D-t paths of deformed metamorphic rocks (Williams & Jercinovic, Cingolani et al, 2011;Hervé et al, 2013;Serra-Varela et al, 2018). Inset shows area of the map in South America, whereas the yellow rectangle indicates location of the map of Figure 2.…”

“…Sketch map showing main exposures of Paleozoic basement rocks in northwestern Patagonia (modified after Cingolani et al, ; Hervé et al, ; Serra‐Varela et al, ). Inset shows area of the map in South America, whereas the yellow rectangle indicates location of the map of Figure .…”

Southward-Directed Subduction of the Farallon–Aluk Spreading Ridge and Its Impact on Subduction Mechanics and Andean Arc Magmatism: Insights From Geochemical and Seismic Tomographic Data

“…The oldest rocks of the Paleozoic basement correspond to Late Paleozoic metamorphic complex with U/Pb zircon ages between ∼420 and 380 Ma and biotite K/Ar ages between ∼375 and 310 Ma (Figures 1, 3; Basei et al, 1999;Lucassen et al, 2004;Varela et al, 2005;Serra-Varela et al, 2019), whose evolution is related to the Patagonian Famatinian orogeny (Serra-Varela et al, 2019). Granites with ages between ∼350 and 270 Ma intruded the metamorphic basement (Figure 3; FIGURE 2 | Detailed map showing the main geological units recognized in the Southern Central Andes between 35 and 36 • S. The black box shows the studied area where the Los Ángeles Unit crops out.…”

“…Based on Fennell et al (2019) and Nullo et al (2005). Varela et al, 1994;Basei et al, 1999;Lucassen et al, 2004) during the onset of subduction linked to the initiation of the Gondwanian orogenic cycle (Serra-Varela et al, 2019).…”