Abstract
We report the first findings of several occurrences of lawsonite and metamorphic aragonite in the meta-sediments from the Combin Zone (Piemonte–Liguria ocean, Western Alps), where the early blueschist-facies episode is poorly documented. New field and metamorphic data (thermodynamic modelling and Raman spectroscopy on carbonaceous material) are used to elucidate the P–T evolution and fluid composition of the Combin Zone and investigate the lawsonite growth and breakdown reactions. Two tectonometamorphic units have been identified within the Combin Zone with distinct geometry, lithological content and P–T conditions. In the higher grade unit, metamorphic aragonite occurs as inclusions in titanite. Lawsonite and garnet were stable at peak P–T conditions (~ 16–17 kbar and 460–480 °C) at very low X(CO2) values. Lawsonite is systematically pseudomorphed, but preserves hourglass zoning or internal fabric associated with the prograde ductile deformation. The lower grade unit (~ 8 ± 1 kbar ~ 370–400 °C) is discontinuously exposed along the western base of the continental Dent Blanche nappe and records P–T conditions similar to those recorded by the Dent Blanche nappe. A metamorphic discontinuity is, therefore, documented between the largest part of the Combin Zone on the one hand, and the Dent Blanche nappe on the other hand. The discovery of lawsonite and metamorphic aragonite allows a better understanding of the large-scale metamorphic structure of the Western Alps.
Highlights
IntroductionLawsonite CaAl2Si2O7(OH)2⋅H2O and aragonite (the orthorhombic polymorph of CaCO3) are diagnostic minerals of low-temperature–high-pressure conditions, as experimentally established since the 1960s (lawsonite: Newton and Kennedy 1963; Nitsch 1972; Schmidt and Poli 1994; Communicated by Hans Keppler.Auvergne, CNRS, IRD, OPGC, 63000 Clermont‐Ferrand, FranceSchmidt 1995; aragonite: Johannes and Puhan 1971; Hacker et al 2005).Lawsonite has been recognized as an essential carrier of H2O in subduction zones (Poli and Schmidt 1997; Schmidt and Poli 1998; Tsujimori and Ernst 2014; Whitney et al 2020), and its potential for geochronology shown in several localities (e.g. Mulcahy et al 2009; Vitale Brovarone and Herwartz 2013)
Lawsonite CaAl2Si2O7(OH)2⋅H2O and aragonite are diagnostic minerals of low-temperature–high-pressure conditions, as experimentally established since the 1960s.Lawsonite has been recognized as an essential carrier of H2O in subduction zones (Poli and Schmidt 1997; Schmidt and Poli 1998; Tsujimori and Ernst 2014; Whitney et al 2020), and its potential for geochronology shown in several localities (e.g. Mulcahy et al 2009; Vitale Brovarone and Herwartz 2013)
Due to its large H 2O content, the presence of lawsonite in metabasic rocks requires a significant addition of water before the subduction metamorphism, typically by hydrothermal metamorphism following the emplacement of these magmatic rocks (Poli and Schmidt 2002; Staudigel 2014)
Summary
Lawsonite CaAl2Si2O7(OH)2⋅H2O and aragonite (the orthorhombic polymorph of CaCO3) are diagnostic minerals of low-temperature–high-pressure conditions, as experimentally established since the 1960s (lawsonite: Newton and Kennedy 1963; Nitsch 1972; Schmidt and Poli 1994; Communicated by Hans Keppler.Auvergne, CNRS, IRD, OPGC, 63000 Clermont‐Ferrand, FranceSchmidt 1995; aragonite: Johannes and Puhan 1971; Hacker et al 2005).Lawsonite has been recognized as an essential carrier of H2O in subduction zones (Poli and Schmidt 1997; Schmidt and Poli 1998; Tsujimori and Ernst 2014; Whitney et al 2020), and its potential for geochronology shown in several localities (e.g. Mulcahy et al 2009; Vitale Brovarone and Herwartz 2013). Lawsonite CaAl2Si2O7(OH)2⋅H2O and aragonite (the orthorhombic polymorph of CaCO3) are diagnostic minerals of low-temperature–high-pressure conditions, as experimentally established since the 1960s Schmidt 1995; aragonite: Johannes and Puhan 1971; Hacker et al 2005). In addition to specific P–T conditions, lawsonite occurrence is restricted to rocks containing enough CaO and A l2O3, typically mafic rocks (meta-basalt and meta-gabbro), and Ca-rich sedimentary rocks like metagreywackes, originally containing a significant proportion of volcanic clasts, and meta-marls 12 wt%), the presence of lawsonite in metabasic rocks requires a significant addition of water before the subduction metamorphism, typically by hydrothermal metamorphism following the emplacement of these magmatic rocks (Poli and Schmidt 2002; Staudigel 2014). Meta-marls are initially always H 2O-saturated since their deposition and diagenesis on top of the oceanic crust
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