Mg-metasomatic rocks (e.g. whiteschists, leucophyllites) derived from post-Variscan granitoids are widespread in the Alps and the presence of features in common indicates that the genetic process is likely similar in the entire range of the Alps. This process assumes the presence of highly channelized fluids, derived from ultramafic rocks previously interacting with seawater, that infiltrate the continental crust along shear zones. The infiltration can occur during rifting, continental subduction and/or continent–continent collision (Ferrando, 2012). Syn-subduction Mg-metasomatism produced by the influx of external fluids generated by antigorite breakdown from subducting oceanic serpentinites is well recorded in two UHP crustal lithologies from the Western Alps. The first example is represented by the typical UHP whiteschists of the Brossasco-Isasca Unit (Dora-Maira Massif). Some whiteschists, derived from xenoliths of pelitic composition within metagranites, still preserve relics of the former pre-metasomatic mineral assemblage equilibrated during prograde Alpine metamorphism (ca. 600°C and 1.6 GPa; Compagnoni & Hirajima, 2001). Microstructural evidence indicates that the Mg-metasomatism occurred at 1.7–2.1 GPa and 560–590 °C and possibly continued up to 2.8 GPa and 650°C in the presence of MgCl2-brines (Ferrando et al., 2009). The second example is from coesite + chloritoid + garnet ± glaucophane talcschists used to produce four quernstones unearthed in the ruins of a villa rustica belonging to the Roman imperial period (Costigliole Saluzzo, Western Alps; Groppo et al., 2016). These Mg-metasomatic rocks, recording peak conditions of 480–510°C and 2.7–3.1 GPa, belong to a still unmapped UHP Unit of the Western Alps. The former protolith is likely a Fe-rich metapelite, and relics of chloritoid still rich in Fe within garnet indicate that Mg-metasomatism occurred during prograde Alpine metamorphism (P<2.5 GPa).
Mg-metasomatism during subduction: two examples from UHP crustal lithologies of the Western Alps.
FERRANDO, Simona
2016-01-01
Abstract
Mg-metasomatic rocks (e.g. whiteschists, leucophyllites) derived from post-Variscan granitoids are widespread in the Alps and the presence of features in common indicates that the genetic process is likely similar in the entire range of the Alps. This process assumes the presence of highly channelized fluids, derived from ultramafic rocks previously interacting with seawater, that infiltrate the continental crust along shear zones. The infiltration can occur during rifting, continental subduction and/or continent–continent collision (Ferrando, 2012). Syn-subduction Mg-metasomatism produced by the influx of external fluids generated by antigorite breakdown from subducting oceanic serpentinites is well recorded in two UHP crustal lithologies from the Western Alps. The first example is represented by the typical UHP whiteschists of the Brossasco-Isasca Unit (Dora-Maira Massif). Some whiteschists, derived from xenoliths of pelitic composition within metagranites, still preserve relics of the former pre-metasomatic mineral assemblage equilibrated during prograde Alpine metamorphism (ca. 600°C and 1.6 GPa; Compagnoni & Hirajima, 2001). Microstructural evidence indicates that the Mg-metasomatism occurred at 1.7–2.1 GPa and 560–590 °C and possibly continued up to 2.8 GPa and 650°C in the presence of MgCl2-brines (Ferrando et al., 2009). The second example is from coesite + chloritoid + garnet ± glaucophane talcschists used to produce four quernstones unearthed in the ruins of a villa rustica belonging to the Roman imperial period (Costigliole Saluzzo, Western Alps; Groppo et al., 2016). These Mg-metasomatic rocks, recording peak conditions of 480–510°C and 2.7–3.1 GPa, belong to a still unmapped UHP Unit of the Western Alps. The former protolith is likely a Fe-rich metapelite, and relics of chloritoid still rich in Fe within garnet indicate that Mg-metasomatism occurred during prograde Alpine metamorphism (P<2.5 GPa).
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