The Permian to early Triassic granitoids of the Nahuel Niyeu - Yaminué area, northern Patagonia: Igneous stratigraphy, geochemistry and emplacement conditions

Abstract

We present the first geochronologically constrained igneous stratigraphy of the Permian to Early Triassic magmatism of the Nahuel Niyeu and Yaminué areas, northern Patagonia. This magmatism was separated into the Early Permian Granitoids (>270 Ma, EPG; Navarrete Plutonic Complex) and the Late Permian-Early Triassic Granitoids (260-245 Ma, LPETG) that are made up of the Madsen Tonalite (̴252 Ma, MT), the Yaminué Metaigneous Complex (̴261-249 Ma, YMIC), and the Ramos Mexía Igneous Complex (̴245 Ma, RMIC). Structural observations indicate that EPG are undeformed and intrude a low-grade Ordovician metamorphic basement. LPETG are either mostly undeformed granitoids intruding Cambrian mylonites, i.e. MT or a low-grade Ordovician metamorphic basement i.e. RMIC or they are synkinematic sheeted intrusion (YMIC) concordantly emplaced in Devonian medium grade metaclastic rocks. The EPG are high Ba–Sr granitoids with 66–73% SiO2, K2O/Na2O< 1, and Sr/Y> 34. Major element chemistry indicates mafic sources and volcanic-arc affinities. Average pressure constrains their emplacement at 2.9±0.5 kbar. The LPETG have a wide range of compositions. From sheeted ̴62–75% SiO2 tonalites, to leucogranites (YMIC) to undeformed ̴60–72% tonalites to monzogranites (MT and RMIC). They show low to moderate Ba–Sr, 0.5< K2O/Na2O< 2.8 and Sr/Y< 50. Mafic to intermediate crustal sources are inferred for the LPETG. In contrast with the EPG, the YMIC was emplaced at 5.2±0.3 Kbar. The calculated average pressure of RMIC is 2.8±0.25 kbar. Since RMIC intrudes YMIC, we conclude that there was an uplift of ca. 9 km between ̴249 and 245 Ma. The major compressional event (Dn+1) that controlled the synkinematic emplacement of the YMIC is bracketed between 261 and 249 Ma. Tectonic processes associated with the uplift of YMIC from 20 to 11 km in depth and the coeval emplacement of RMIC at 245±1 Ma could correspond to the collapse of a thickened crust.