The UHP pyrope whiteschists from the Brossasco-Isasca Unit of the Southern Dora-Maria Massif (Italian Western Alps) are metasomatic rocks originated at the expense of post-Variscan granitoids by the influx of fluids along shear zones at high pressure. Geochemical analyses reveal that whiteschists are strongly enriched in Mg and depleted in Na, K, Ca, Ba, LILE (Cs, Pb, Rb, K, Sr, Ba), and Eu with respect to the average continental crust. Minero-petrographical investigations reveal the presence of three generations of pyrope, with different composition and mineral inclusions: (1) a prograde Prp I, which constitutes the large core of megablasts and the small core of porphyroblasts; (2) a peak Prp II, which constitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (3) a early retrograde Prp III, which locally constitutes an outer rim. Fluid inclusion study allows to distinguish two generations of fluid inclusions: (1) primary fluid inclusions in prograde kyanite that represent a MgCl2-rich brine (6–28 wt% NaCleq with Si and Al as other dissolved cations) trapped during growth of Prp I (type-I fluid); (2) primary multiphase-solid inclusions in Prp II that are remnants of a peak alumino-silicate supercritical liquid containing Mg and alkalis, as well as subordinate P, Cl, S, CO32-, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), Th, and U (type-II fluid). We propose a model that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists and that could also explain the genesis of other lithologies of the Alps, characterized by very-high Mg content and low Ca and Na contents. During Alpine metamorphism, the post-Variscan metagranite of the Brossasco-Isasca Unit experienced a prograde metamorphism at HP conditions (stage A: P ~ 1.6 GPa and T ≤ 600 °C), as indicated by the growth of an almandine-rich garnet in some former xenoliths. At stage B (1.7–2.1 GPa and 560–590 °C), the influx of external fluids, originated from antigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies in the orthogneiss toward a whiteschist composition. During stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), the metasomatic fluid influx, coupled with internal dehydration reactions involving Mg-chlorite, promoted the growth of Prp I in the presence of the type-I MgCl2-brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730 °C), the Prp II growth occurred in the presence of a type–II supercritical liquid, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligible.
Metasomatism of continental crust during subduction: the Dora-Maira UHP whiteschists (Italian Western Alps)
FERRANDO, Simona;COMPAGNONI, Roberto
2009-01-01
Abstract
The UHP pyrope whiteschists from the Brossasco-Isasca Unit of the Southern Dora-Maria Massif (Italian Western Alps) are metasomatic rocks originated at the expense of post-Variscan granitoids by the influx of fluids along shear zones at high pressure. Geochemical analyses reveal that whiteschists are strongly enriched in Mg and depleted in Na, K, Ca, Ba, LILE (Cs, Pb, Rb, K, Sr, Ba), and Eu with respect to the average continental crust. Minero-petrographical investigations reveal the presence of three generations of pyrope, with different composition and mineral inclusions: (1) a prograde Prp I, which constitutes the large core of megablasts and the small core of porphyroblasts; (2) a peak Prp II, which constitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (3) a early retrograde Prp III, which locally constitutes an outer rim. Fluid inclusion study allows to distinguish two generations of fluid inclusions: (1) primary fluid inclusions in prograde kyanite that represent a MgCl2-rich brine (6–28 wt% NaCleq with Si and Al as other dissolved cations) trapped during growth of Prp I (type-I fluid); (2) primary multiphase-solid inclusions in Prp II that are remnants of a peak alumino-silicate supercritical liquid containing Mg and alkalis, as well as subordinate P, Cl, S, CO32-, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), Th, and U (type-II fluid). We propose a model that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists and that could also explain the genesis of other lithologies of the Alps, characterized by very-high Mg content and low Ca and Na contents. During Alpine metamorphism, the post-Variscan metagranite of the Brossasco-Isasca Unit experienced a prograde metamorphism at HP conditions (stage A: P ~ 1.6 GPa and T ≤ 600 °C), as indicated by the growth of an almandine-rich garnet in some former xenoliths. At stage B (1.7–2.1 GPa and 560–590 °C), the influx of external fluids, originated from antigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies in the orthogneiss toward a whiteschist composition. During stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), the metasomatic fluid influx, coupled with internal dehydration reactions involving Mg-chlorite, promoted the growth of Prp I in the presence of the type-I MgCl2-brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730 °C), the Prp II growth occurred in the presence of a type–II supercritical liquid, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligible.
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