The evolution of a mid-crustal thermal aureole at Cerro Toro, Sierra de Famatina, NW Argentina

Abstract

A sheeted tonalite complex at Cerro Toro in western Sierra de Famatina, Argentina, was emplaced in the middle crust during an early phase of the Ordovician Famatinian orogeny. The intrusion produced a large-scale thermal aureole that provoked significant melting of the host rocks and hybridization of the anatectic melts with the tonalitic magmas. Three zones were distinguished in a cross-section: (1) an external zone consisting of metatexitic gneisses, amphibolites and minor tonalites, (2) an intermediate zone formed by screens of highly melted gneisses and amphibolites lying between the tonalite sheets: stoped blocks of gneisses within the tonalite sheets, small bodies of metagabbro and hybrid rocks are common in this zone, (3) an internal zone formed almost exclusively of massive tonalite. Incongruent melting of biotite in gneisses of the intermediate zone produced abundant cordierite and garnet; anatectic granitoids developed in the innermost part at 750-800ºC and ca. 5kbar. High water activities within this zone eventually promoted significant melting of plagioclase + quartz in the gneisses. Pooling of anatectic magmas resulted in allochthonous bodies of variable size either concordant or discordant to the regional structural grain, and part were retained in or close to the residuum. Allochthonous granitoids did not reach isotopic equilibrium with the source protolith probably due to fast extraction of the melts. The Cerro Toro contact aureole shows that assimilation of metasedimentary rocks through partial melting can be an important mechanism during emplacement of tonalitic magmas at middle crust levels. Thus the chemistry of cordilleran magma can be modified as they rise through the crust. AbstractA sheeted tonalite complex at Cerro Toro in western Sierra de Famatina, Argentina, was emplaced in the middle crust during an early phase of the Ordovician Famatinian orogeny. The intrusion produced a large-scale thermal aureole that provoked significant melting of the host rocks and hybridization of the anatectic melts with the tonalitic magmas. Three zones were distinguished in a cross-section: (1) an external zone consisting of metatexitic gneisses, amphibolites and minor tonalites, (2) an intermediate zone formed by screens of highly melted gneisses and amphibolites lying between the tonalite sheets: stoped blocks of gneisses within the tonalite sheets, small bodies of metagabbro and hybrid rocks are common in this zone, (3) an internal zone formed almost exclusively of massive tonalite. Incongruent melting of biotite in gneisses of the intermediate zone produced abundant cordierite and garnet; anatectic granitoids developed in the innermost part at 750-800ºC and ca. 5kbar. High water activities within this zone eventually promoted significant melting of plagioclase + quartz in the gneisses. Pooling of anatectic magmas resulted in allochthonous bodies of variable size either concordant or discordant to the regional structural grain, and part were retained in or close to the residuum. Allochthonou...

O–H–Sr–Nd isotope constraints on the origin of the Famatinian magmatic arc, NW Argentina

“…1; e.g. Saavedra et al 1992;Alasino et al 2014;Bellos et al 2015). At these localities, there are steeply dipping N-S-trending sheets of tonalite and less abundant granodiorite with scarce gabbro and mafic-ultramafic cumulates.…”

“…At these localities, there are steeply dipping N-S-trending sheets of tonalite and less abundant granodiorite with scarce gabbro and mafic-ultramafic cumulates. Host rocks are high-grade metasedimentary rocks (c. 4-5 kbar; Rossi & Toselli, 2005;Alasino et al 2014). Water-fluxed melting was invoked by Alasino et al (2014) to explain the high degree of melting in the host rocks in the regional thermal aureoles.…”

“…Host rocks are high-grade metasedimentary rocks (c. 4-5 kbar; Rossi & Toselli, 2005;Alasino et al 2014). Water-fluxed melting was invoked by Alasino et al (2014) to explain the high degree of melting in the host rocks in the regional thermal aureoles. Hybrid granitoids were formed by variable mixing of anatectic leucogranite with tonalite at magma chamber margins (Alasino et al 2014).…”

“…These samples were analysed for whole-rock major oxides and trace elements using an inductively coupled plasma mass spectrometer (ICP-MS) at Activation Laboratories, Ancaster, Ontario, Canada, under the '4LithoResearch' package, following the procedure described at http://www.actlabs.com (online Supplementary Table S1, available at http://journals.cambridge.org/geo). Additionally, we have taken into consideration the Sr and Nd isotope analyses reported by Dahlquist et al (2012), Alasino et al (2014Alasino et al ( , 2016 and Rapela et al (2018) and for the remaining samples in Table 2 (all recalculated to 470 Ma) ( Table 3).…”

Metamorfismo de alto gradiente P/T en la Sierra de Pie de Palo (Sierras Pampeanas, Argentina): modelado de equilibrio de fases minerales e implicancias geodinámicas en el antearco famatiniano

“…La región del arco magmático y el trasarco se caracterizan por un metamorfismo de bajo gradiente P/T y trayectorias antihorarias con presiones menores a 8 kbar y temperaturas de hasta ca. 900 ºC, encontrándose la zona de mayor profundidad en las Sierras de Cerro Toro y Valle Fértil, y la zona más somera en las sierras de Famatina, Chepes y Velasco (Dahlquist y Baldo, 1996;Hauzenberger et al, 2001;Büttner et al, 2005;Dahlquist et al, 2005;Murra y Baldo, 2006;Steenken et al, 2006;Delpino et al, 2007;Castro de Machuca et al, 2008;Otamendi et al, 2008;Gallien et al, 2010Gallien et al, , 2012Larrovere et al, 2011, Alasino et al, 2014. En el dominio al oeste del arco magmático famatiniano (Sierra de Pie de Palo, Loma de Las Chacras) es donde se desarrolla un metamorfismo de mayor presión alcanzando valores de hasta 13 kbar (Baldo et al, 1998;Casquet et al, 2001;van Staal et al, 2011;Mulcahy et al, 2011Mulcahy et al, , 2014.…”