Short-lived plutonism coeval with sediment underplating and high-grade metamorphism in the inner Famatinian magmatic arc of the Sierras Pampeanas, Argentina

CASQUET, C.; PANKHURST, R.J; RAPELA, C.W.; DAHLQUIST, J.A.; BALDO, E.G; GALINDO, C.; FANNING, C.M

Avila

Simposio; 7th Hutton Symposium on Granites and Related Rocks; 2011

Universidad de Granada

A wide Early-to-Middle Ordovician magmatic arc represented by I- and S-type belts of plutons and volcanic rocks has long been recognized in the Sierras Pampeanas and has been unanimously related to westward subduction of the Iapetus Ocean along the western margin of between ca. 495 and 465 Ma as part of the Famatinian orogeny. The inner belt of the Famatinian arc stretches from the Sierra de Valle Fertil in the south to the Sierra de Asperecitos near the township of Villa Union. It consists mainly of elongated bodies of gabbro, tonalite and granodiorite largely concordant with foliation of the host rocks. The gabbros are the oldest intrusive rocks; tonalites and granodiorites are slightly younger on geological grounds. The host rocks are middle-crust upper amphibolite to granulite facies (6.8 kbar, 760ºC) metasedimentary rocks, mostly metapelitic migmatites. Discordant granite stocks and pegmatite sheets are common. A screen of lower-crust (12.1 ± 1 kbar, 770 ± 20ºC) mylonitic metasedimentary rocks is tectonically juxtaposed outboard of the arc (Loma de las Chacras, Valle Fértil). This section of the Famatinian arc and the lower crust were subsequently steeply tilted, exposing deep levels of the belt. The meta-sedimentary rocks that host the plutons were part of a post-Pampean (i.e., younger than 510 Ma) sedimentary basin. Their metamorphism is dated at 470 ± 3 Ma using zircon overgrowths on detrital grains. The plutonic rocks – both early and late relative to foliation development – yield ages that are within error of the age of metamorphism, suggesting that they were coeval and that magmatism was generated very rapidly. Granulites outboard of the arc, also with a metamorphic age of ~470 Ma, contain detrital zircons derived from the magmatic arc (some volcanic in origin) with an age of 469 ± 6 Ma). These rocks are interpreted as part of an accretionary wedge dragged down the subduction zone and were underplated about 40 km beneath the magmatic arc. Thus sedimentation, subduction and granulite facies metamorphism and deformation took place in a very short interval (probably < 5Ma) coeval with magmatism. The short tectonothermal event at ca. 470 Ma in the SVF-Asperecitos belt of the Famatinian arc was largely coincident with nappe imbrication and relatively high P/T metamorphism (13 ± 1 kbar, 600 ± 50ºC) in the allochthonous Precordillera terrane. The latter_accreted to the arc along the proto- Bermejo major fault, which led to interruption of subduction and arc magmatism.