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

RAPALINI, AUGUSTO E.; LÓPEZ DE LUCHI, MÓNICA G.; MARTÍNEZ DOPICO, CARMEN I.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2021

0895-9811

We present the first geochronologically constrained igneous stratigraphy of the Permian to Early Triassic magmatismof the Nahuel Niyeu and Yaminu´e areas, northern Patagonia. This magmatism was separated into theEarly Permian Granitoids (>270 Ma, EPG; Navarrete Plutonic Complex) and the Late Permian-Early TriassicGranitoids (260-245 Ma, LPETG) that are made up of the Madsen Tonalite (2̴ 52 Ma, MT), the Yaminue´ MetaigneousComplex (2̴ 61-249 Ma, YMIC), and the Ramos Mexía Igneous Complex (2̴ 45 Ma, RMIC). Structuralobservations 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 Ordovicianmetamorphic basement i.e. RMIC or they are synkinematic sheeted intrusion (YMIC) concordantly emplaced inDevonian 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 pressureconstrains their emplacement at 2.9±0.5 kbar. The LPETG have a wide range of compositions. From sheeted6̴ 2?75% SiO2 tonalites, to leucogranites (YMIC) to undeformed ̴60?72% tonalites to monzogranites (MT andRMIC). They show low to moderate Ba?Sr, 0.5< K2O/Na2O< 2.8 and Sr/Y< 50. Mafic to intermediate crustalsources are inferred for the LPETG. In contrast with the EPG, the YMIC was emplaced at 5.2±0.3 Kbar. Thecalculated average pressure of RMIC is 2.8±0.25 kbar. Since RMIC intrudes YMIC, we conclude that there was anuplift of ca. 9 km between ̴249 and 245 Ma. The major compressional event (Dn+1) that controlled the synkinematicemplacement of the YMIC is bracketed between 261 and 249 Ma. Tectonic processes associated with theuplift of YMIC from 20 to 11 km in depth and the coeval emplacement of RMIC at 245±1 Ma could correspond tothe collapse of a thickened crust.