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

VERDECCHIA, SEBASTIÁN O.; CASQUET, CÉSAR; BALDO, EDGARDO G.; LARROVERE, MARIANO A.; LEMBO WUEST, IVÁN; BENITEZ, MANUELA; RAMACCIOTTI, C.; MURRA, J.; PANKHURST, R.J.

GEOLOGICAL MAGAZINE

CAMBRIDGE UNIV PRESS

Lugar: Cambridge; Año: 2023

0016-7568

Subduction along the SW margin of Gondwana began in the late Cambrian, leading to the Famatinian orogeny which continued until the early Devonian. This paper focuses on one discrete orogenic belt that formed during the Rinconada phase of the Famatinian orogeny (444–410 Ma), which is well exposed at Sierra de Ramaditas and neighboring sierras in western Argentina. The Ramaditas Complex is formed by high-grade metasedimentary and meta-ultrabasic rocks and amphibolites. This Complex forms the upper nappe of a thrust stack resulting from westward thrusting. Deformation mainly consists of an early high temperature S1 foliation (stromatic migmatites), coeval with thrusting and metamorphism. Metamorphism attained peak P–T conditions of 6.0 – 6.9 kbar and 795 – 810º C, at ca. 440 Ma, i.e., coincident with the Rinconada orogenic phase. The intermediate nappes and the lower unit crop out in the nearby sierras of Maz and Espinal and underwent medium to low-grade Silurian metamorphism respectively coeval with the upper nappe, together defining an inverted Barrovian-type metamorphism with T decreasing and P increasing downwards across the thrust stack. We argue that the Rinconada orogenic phase developed near the continental margin of SW Gondwana, during a magmatic lull following accretion of the Precordillera terrane to the continental margin at ca. 470 Ma. The active margin jumped westward after accretion, and flat-slab subduction resumed in the early Silurian provoking thrusting and imbrication of nappe stack under the still hot root (800 – 900º C) of the older ca. 470 Ma Famatinian magmatic arc. This “hot-iron” process explains both the inverted Barrovian-type metamorphism and the missing overburden of 21 to 24 km implied by the P estimate.