Metamorphic evolution of migmatites from the deep Famatinian arc crust exposed in Sierras Valle Fértil–La Huerta, San Juan, Argentina

Abstract

Meta-supracrustal migmatitic packages in the Sierras Valle Fértil–La Huerta of northwestern Argentina occur as wedge- or strip-shaped septa interlayered among mafic to intermediate igneous plutonic rocks. Meta-supracrustal rocks were metamorphosed under granulite-facies conditions during the development of the Famatinian magmatic arc, and are among the structurally deepest rocks exposed within the belt dominated by Ordovician plutonism. Petrographic analysis, mineral chemistry and whole rock geochemistry of granulite-facies migmatites are used to argue that the meta-supracrustal packages comprise a sequence of pelitic to quartzo-feldspathic sedimentary rocks that achieved peak metamorphic P– T conditions of 5.2–7.1 kbar and 770–840 °C. There are no resolvable differences in peak P– T conditions for migmatites separated 70 km along strike of the Sierras Valle Fértil and La Huerta, suggesting that similar levels of the Famatinian paleo-arc crust are currently exposed in these ranges. Idioblastic poikilitic garnets displaying weak to absent chemical zoning profiles developed at or close to the peak metamorphic stage are used in conjunction with petrogenetic grid constraints to interpret the prograde evolution. At the time the supracrustal rocks experienced maximum thermal conditions, they underwent dehydration partial melting. Microtextural features show that felsic melt (leucosome) back reacted with the adjacent coexisting mineral assemblage (mesosome). These observations are interpreted as evidence that the migmatites evolved through a continuous heating–cooling cycle with minor pressure change. This is consistent with the general lack of reaction textures denoting decompression at high temperatures, and with the possibility that in some migmatites retrograde reactions formed staurolite. Collectively, these features indicate that when the migmatites attained peak thermal conditions, the deepest exposed arc crust was about 20–25 km beneath the Ordovician surface. Comparing these results with metamorphic studies elsewhere in the Famatinian arc between 31° and 32°S. indicates that much of the main-arc records primarily prograde P– T trajectories associated with a regional contact metamorphism, but that specific locations in the back-arc, main-arc and accretionary wedge also record post-peak retrogression during crustal exhumation. These differences are attributed to the fact that a collisional orogeny closely followed the cessation of arc magmatism, a collision we infer to be associated with the accretion of a Laurentian terrane to the Gondwana margin. The results of this study therefore provide important insights into the geodynamic context of the formation and closure of the central segment of the Famatinian magmatic arc.