The Permian-Triassic Intracratonic Magmatic Corridor of La Pampa province, Argentina: new U-Pb SHRIMP ages, Hf isotopic composition, and geodynamic implications

Abstract

A study and review of the magmatic units that make up the Permian-Triassic Intracratonic Magmatic Corridor of La Pampa (CMPT-LP), south-central Argentina, has been carried out. New U-Pb SHRIMP ages, Hf isotopic composition, and chemical composition of some units of the CMPT-LP have been obtained, i.e. 1) Chacharramendi Syenogranite, age 254.7 ± 1.3 Ma (Late Permian, Lopingian), Hf model age of 1640 Ma

with ε_Hf average of -5.68; 2) Lihue Calel Rhyolite, 239.3 ± 1.5 Ma (Middle Triassic), Hf model age of 1640 Ma with ε_Hf average -5.56. This indicates that the two dated units were crystallized from a mantle-derived melt that incorporated crust of late Statherian up to Calymmian age, taking also into account a Nd model age of 1483 Ma for the more distal (southeasternmost) sector (López Lecube Granite) of the CMPT-LP obtained by other authors.

New geochemical data reported here, analyzed together with previous geochemical data, denote the anorogenic signature –intraplate environment– of the CMPT-LP magmatic corridor, highlighting its consistency with A₁-type granites due to the similarity of their sources with those from which the oceanic island basalts (OIB) derive, as evidenced, e.g., by Y/Nb ratios <1.2. The classification of the rocks of the CMPT-LP mainly within the high potassium type is also consistent with its occurrence in an intraplate tectonic environment. The magmatism in the CMPT-LP has been considered as part of the Choiyoi Group, which is also related to late Permian to Early Triassic extension. Nevertheless, there is a marked contrast between the anorogenic environment of the CMPT-LP and the orogenic context of the Choiyoi Magmatic Arc. For this reason, the convenience of excluding the CMPT-LP from the Choiyoi Group is analyzed. In addition, the tectonic frames of the orogenic belts adjoining the CMPT-LP –San Rafael (or Sanrafaelic, or Choiyoi Arc-retroarc) and Northpatagonian Gondwanide belts– are compared briefly.