UHP Prp-whiteschists from the Dora-Maria Massif represent metasomatic rocks originated at the expense of post-Variscan granitoids by the influx of fluids along shear zones. 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. Three generations of Prp, with different composition and mineral inclusions, have been distinguished: i) prograde PrpI, as, large cores of megablasts and small cores of porphyroblasts; ii) peak PrpII, as inner rims of megablasts and porphyroblasts and cores of small neoblasts; iii) early retrograde PrpIII, locally constituting an outer rim. Two generations of fluid have been recognised: i) a prograde MgCl2-rich brine (6-28 wt% NaCl eq., with Si and Al) trapped during growth of Prp I; ii) a peak alumino-silicate supercritical liquid containing Mg and alkalis, and subordinate P, Cl, S, CO32-, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), Th and U. We propose a model, that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists. During Alpine metamorphism, the post-Variscan metagranite experienced a prograde metamorphism at HP conditions (stage A: P ~ 1.6 GPa and T ≤ 600°C), as testified by the growth of an Alm-rich garnet in some xenoliths. At stage B (1.7-2.1 GPa and 560-590°C), the influx of external fluids originated from Atg breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies in the orthogneiss towards a whiteschist composition. During successive stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), metasomatic fluid influx, coupled with internal dehydration reactions involving Mg-chlorite, promoted the growth of Prp I in the presence of MgCl2-brine. At the metamorphic peak (stage D: 4.0-4.3 GPa and 730°C), Prp II growth occurred in the presence of a supercritical liquid, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligeable.
LILE-enriched fluids generated at UHP conditions in Dora-Maira whiteschists (western Alps)
FERRANDO, Simona;COMPAGNONI, Roberto
2008-01-01
Abstract
UHP Prp-whiteschists from the Dora-Maria Massif represent metasomatic rocks originated at the expense of post-Variscan granitoids by the influx of fluids along shear zones. 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. Three generations of Prp, with different composition and mineral inclusions, have been distinguished: i) prograde PrpI, as, large cores of megablasts and small cores of porphyroblasts; ii) peak PrpII, as inner rims of megablasts and porphyroblasts and cores of small neoblasts; iii) early retrograde PrpIII, locally constituting an outer rim. Two generations of fluid have been recognised: i) a prograde MgCl2-rich brine (6-28 wt% NaCl eq., with Si and Al) trapped during growth of Prp I; ii) a peak alumino-silicate supercritical liquid containing Mg and alkalis, and subordinate P, Cl, S, CO32-, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), Th and U. We propose a model, that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists. During Alpine metamorphism, the post-Variscan metagranite experienced a prograde metamorphism at HP conditions (stage A: P ~ 1.6 GPa and T ≤ 600°C), as testified by the growth of an Alm-rich garnet in some xenoliths. At stage B (1.7-2.1 GPa and 560-590°C), the influx of external fluids originated from Atg breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies in the orthogneiss towards a whiteschist composition. During successive stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), metasomatic fluid influx, coupled with internal dehydration reactions involving Mg-chlorite, promoted the growth of Prp I in the presence of MgCl2-brine. At the metamorphic peak (stage D: 4.0-4.3 GPa and 730°C), Prp II growth occurred in the presence of a supercritical liquid, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligeable.
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