GEOCHEMICAL CONSTRAINTS FOR THE PERMIAN TUNNEL TONALITE: IMPLICATIONS FOR THE EVOLUTION OF THE SW MARGIN OF THE NORTH PATAGONIAN MASSIF

LÓPEZ DE LUCHI, MÓNICA G.; MARTÍNEZ DOPICO, CARMEN; GRILLO VIDAL, CAROLINA

Congreso; XXI Congreso Geológico Argentino; 2022

composed of mesocratic biotite-amphibole tonalites and minor diorites in sharp contact with the CushamenFormation (Fig 1A). The more continuous outcrops appear at both sides of the railway tunnel, as a 0.5 km wide and 5 km long belt along the Río Chico River, 0.5 km north from the town. Scattered outcropsare seen to the east of the belt up to 1.5 km NE of the Cerro Colorado. Cerredo and López de Luchi (1997)and Cerredo et al. (2000) described the petrography, microstructures, and relationship with the regionaldeformation. The crystallization of magmatic epidote+allanite after plagioclase and before biotite suggestspressures ca. 8.5-9.5 kbar which would correspond to the intrusion depth. The high modal (> 10%) amountof epidote indicates that it was formed near the solidus (Cerredo and López de Luchi 1997). SHRIMP zirconage is 295±2 Ma (CUS 131, Pankhurst et al. 2006) whereas a zircon age on magnetic fractions is 286± 13 Ma(AB 70, Varela et al. 2005).Di erent tectonic models were proposed for the 330 to 270 Ma evolution of the west-southwest sectorof the North Patagonian Massif (NPM). Pankhurst et al. (2006) proposed ~335-325 Ma subduction relatedI- type magmatism, ~320-310 Ma S-type granitoids due to the collision of the Deseado Massif and ~310-240Ma metaluminous and peraluminous magmatism controlled by compressive deformation followed by slabbreak-o . In the Sierra de Mamil Choique. López de Luchi and Cerredo (2008) considered that the emplacementof most of the 283 to 254 Ma Sierra de Mamil Choique granitoids was controlled by a regional stress eld that also a ected the host Cushamen Formation. Marcos et al. (2020) proposed metamorphism andmagmatism from 330 to 300 Ma, an episode of partial melting and migmatitization at 290 Ma and basementuplift starting at 265 Ma. Renda et al. (2021) suggested a ca. 330-320 Ma regional metamorphism, coevalwith syntectonic (~330-300 Ma) magmatism and proposed that the syntectonic emplacement of the 314.1± 2.2 Ma tonalite in the Paso del Sapo Complex would indicate crustal thickening and ~NE-SW contractionwith a top-to-the-SW tectonic transport direction which would be active up to ca.303 Ma. In consequenceconstraining the depth of the source for the 314 to ca. 290 Ma magmatism is critical to address the inferredthickening of the crust.We analyze the geochemical signature of the 295 Ma Tunnel Tonalite based on six new chemical datatogether with three more taken from Varela et al. (2015) and Pankhurst et al. (2006) and we compare themwith the available chemical data for basic to mesosilicic rock that cover the time span from 325 to 290 Maand crop out from Bariloche area to Laguna del Toro, Paso del Sapo and Sierra de Pichiñanes (Dalla Salda etal. 1991, Pankhurst et al. 2006, Varela et al. 2015) as well as with our unpublished data for the amphibolitesof the Cushamen Formation cropping out along Cañadón La Angostura.TT show SiO2 between 60.3-63.70% and de ne a subalkaline, metaluminous (A/CNK. 0.85) to mildlyperaluminous (A/CNK:1.07) medium-K calc-alkaline series (Fig.1B). In binary variation diagrams (not shown)tonalites exhibit well de ned trends with negative slopes against SiO2 for Al2O3, Fe2O3, MgO, CaO, MnO andTiO2 whereas the alkalis and P2O5 de ne slightly positive trends. Ni(15), Cr(31-23), Co(16-12) and Sc arelow whereas LILE, Sr and LREE contents are relatively high. Sc, Co, Ni and Cr low values would exclude aprimitive source. Clinoamphibole and plagioclase as fractionating phases are suggested by Y and Sr decreaseagainst SiO2. Relative to normal mid-ocean ridge basalts (MORB) samples show selective enrichments in largeion lithophile elements (LILE: e.g., Sr, K, Rb, Ba, Th) and to a lesser extent in La and Ce, but depletions insome high  eld-strength elements (HFSE) such as Ta, Nb, Zr, and Hf. Nb-Ti troughs are typical for arc-relatedrocks. REE patterns indicate both a marked increase in LREE and a decrease in HREE. Eu/Eu* varies from0.85 to 0.94. Tectonic diagrams based on major and trace elements (Fig. 1B) indicate a pre-plate collisionsetting or arc magmatism (Fig. 1C). Dy/Dy* values, which are a measure of the concavity of a REE pattern(Davidson et al. 2013) vary from 0.5 to 0.7 and decrease with SiO2 whereas DyN/YbN increases which mightalso result from amphibole fractionation. LaN/YbN varies from 18 to 27 and Sr/Y from 27-43 (Fig. 1D y 1E).Profeta et al. (2015) proposed that these ratios could provide information on episodes of thin versus thickcrust in the geologic record of subduction-related arc. Values of these ratios would suggest an averagecrustal thickness of 50-60 km. which could suggest that TT might represent an arc magmatism developedin a relatively thick crust. I type magmatism from 330-320 Ma (Pankhurst et al. 2006), i.e. orthoamphibolites of Cañadón de LaMosca, Cordón del Serrucho and Cushamen Formation would indicate a relatively thin crust based on lowLaN/YbN (2 to 7) and Sr/Y (12-18); moderate Sr/Y (28-35) in two samples of the Guillelmo lake might berelated to amphibole fractionation (Fig. 1D y 1E). For the interval 320-310 Ma higher LaN/YbN (15-21) andmoderate Sr/Y (17-34) would indicate a thicker crust.In synthesis TT as well as the 294±3 Ma Laguna del Toro only available chemical data would supporta thick crust at ca. 295 Ma. The  rst evidence of a thick crust is indicated by the 314 Ma tonalite of Pasodel Sapo as proposed Renda et al. (2021) and perhaps by the 318 Ma biotite-garnet 62% SiO2 granitoid ofSierra de Pichiñanes (Pankhurst et al. 2006). Crustal thickness might remain stable or slightly increase from314 to 295 Ma. If this scenario corresponds to a collision or to a change on the convergence rate along anactive margin remains to be solved.