Publication: Magmatic evolution of the Peñón Rosado granite: Petrogenesis of
garnet-bearing granitoids
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Publication Date
1995
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Elsevier Science B.V., Amsterdam
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Abstract
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 surrounding
metasedimentary rocks and subsequent differentiation by fractional crystallization during emplacement, with garnet occurring
throughout 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 and
the proportion of garnet crystallizing are roughly controlled by the evolving composition of the different granitic facies.Geothermobaric
calculations reveal an initial crystallization temperature of 764°–792 °Cand a pressure of 5.9–6.0 kb, indicating that the parental magma
was emplaced atmiddle crustal depths (∼1920
km) in moderate–low magmatic temperature conditions.Major (CaO, Na2O,K2O) and
trace element (Rb, Sr, Ba) contents in the Peñón Rosado granite strongly suggest anatexis was the outcome of H2O-fluxed melting of
metagreywacke, with heat input from a major metaluminous suite. Our studies reveal that garnet formed by direct crystallization from
peraluminous magma in equilibrium with solid phases such as biotite and white mica.We confirm previous suggestions that zoning in
garnet is strongly temperature-dependent. Thus, magmatic garnets in granitic rocks crystallized above ∼ 700 °C have “spessartine
inverse bell-shaped profile” or are unzoned, whereas garnet exhibiting “spessartine bell-shaped profile” must be considered of
metamorphic origin (i.e., xenocrystic) or formed in very felsic magmas (SiO2=73–76%) crystallizing below ∼ 700 °C.
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