%0 Journal Article
%A Dahlquist, Juan A.
%A Galindo Francisco, María Del Carmen
%A Pankhurst, R.J.
%A Rapela, Carlos W.
%A Alasino, Pablo H.
%A Saavedra, Julio
%A Fanning, C.M.
%T Magmatic evolution of the Peñón Rosado granite: Petrogenesis ofgarnet-bearing granitoids
%D 1995
%@ 0024-4937
%U https://hdl.handle.net/20.500.14352/56987
%X Garnet is an uncommon accessory mineral in igneous rocks but is petrologically significant. The Peñón Rosado granite (469± 4Ma)at Cerro Asperecito is an S-type granite that contains an unusual amount of magmatic garnet. Combined petrology, chemistry/mineralogy and whole-rock geochemistry indicates that the magma was produced by partial melting of the surroundingmetasedimentary rocks and subsequent differentiation by fractional crystallization during emplacement, with garnet occurringthroughout the crystallization sequence. Three facies are recognised: PRG1 (SiO2=65.70%) represents cumulates, PRG2(SiO2=70.88%) represents a differentiated melt, and PRG3 (SiO2=74.59%) a residual melt. The fractionation of Mn in garnet andthe proportion of garnet crystallizing are roughly controlled by the evolving composition of the different granitic facies.Geothermobariccalculations reveal an initial crystallization temperature of 764°–792 °Cand a pressure of 5.9–6.0 kb, indicating that the parental magmawas emplaced atmiddle crustal depths (∼1920km) in moderate–low magmatic temperature conditions.Major (CaO, Na2O,K2O) andtrace element (Rb, Sr, Ba) contents in the Peñón Rosado granite strongly suggest anatexis was the outcome of H2O-fluxed melting ofmetagreywacke, with heat input from a major metaluminous suite. Our studies reveal that garnet formed by direct crystallization fromperaluminous magma in equilibrium with solid phases such as biotite and white mica.We confirm previous suggestions that zoning ingarnet is strongly temperature-dependent. Thus, magmatic garnets in granitic rocks crystallized above ∼ 700 °C have “spessartineinverse bell-shaped profile” or are unzoned, whereas garnet exhibiting “spessartine bell-shaped profile” must be considered ofmetamorphic origin (i.e., xenocrystic) or formed in very felsic magmas (SiO2=73–76%) crystallizing below ∼ 700 °C.© 2006 Elsevier B.V. All rights reserved.
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