Silurian inverted Barrovian-type metamorphism in the Western Sierras Pampeanas (Argentina). A case of top to bottom heating?

VERDECCHIA, SEBASTIÁN O.; CASQUET, C.; BALDO, E.G.; LARROVERE, M.A.; LEMBO WEST C.I.; BENITEZ, M.E.; RAMACCIOTTI, C.D.; MURRA, J.A.; PANKHURST, R. J.

GEOLOGICAL MAGAZINE

CAMBRIDGE UNIV PRESS

Lugar: Cambridge; Año: 2023

0016-7568

This paper focuses on one orogenic belt that formed during theRinconada phase on the final stage of the Famatinian orogeny, between445 and 410 Ma which is well exposed at Sierra de Ramaditas andneighboring ranges in western Argentina. The Ramaditas Complex isformed by metasedimentary and meta-ultrabasic rocks and amphibolites.This complex forms the upper nappe of a thrust stack resulting fromwestward thrusting. Deformation consists of an early high temperatureS1 foliation (stromatic migmatites), coeval with thrusting andmetamorphism. Metamorphism attained peak P–T conditions of 6.0 – 6.9kbar and 795 – 810º C, at ca. 440 Ma, i.e., coincident with theRinconada orogenic phase. The lower unit and intermediate nappes cropout in the nearby sierras of Maz and Espinal and underwent low- tomedium-grade Silurian metamorphism respectively together with theupper nappe, defining an inverted Barrovian-type metamorphism with Tdecreasing and P increasing downwards across the thrust stack (i.e.westward). We argue that the Rinconada orogenic phase developed nearthe continental margin of SW Gondwana, during a magmatic lullfollowing accretion of the Precordillera terrane to the continental marginat ca. 470 Ma. The active margin jumped to the west after accretion, andflat-slab subduction resumed in the early Silurian, provoking thrustingand imbrication of nappe stack under the still hot root (800 – 900º C) ofthe older Famatinian magmatic arc. This “hot-iron” process explains boththe inverted Barrovian-type metamorphism and the missing overburdenof 21 to 24 km implied by the P estimate.