Eclogite Facies Regional Metamorphism of Hydrous Mafic Rocks in the Central Alpine Adula Nappe

Abstract

The Adula Nappe is a slice of Pre-Mesozoic continental basement affected by Early Alpine (Mesozoic or Lower Tertiary) high-pressure metamorphism. Mineral compositions in mafic rocks containing omphacite -I-garnet + quartz record a continuous regional trend of increasing recrystallization temperatures and pressures that can be ascribed to this regional high-pressure metamorphic event. P-T estimates derived from mineral compositions grade from about 12 kb and 500 °C or less in the north of the nappe to more than 20 kb/800 °C in the south.The regional P-T trend is associated with a mineralogical transition from assemblages containing additional albite and abundant amphiboles, epidote minerals, and white micas in the north (omphacite-garnet amphibolites) to kyanite eclogites containing smaller amounts of hornblende and zoisite in the south. Textures and mineral compositional data show that these hydrous and anhydrous silicates associated with omphacite + garnet-I-quartz are primary parts of the high-pressure assemblages. Observed phase relations between these primary silicates, theoretical Schreinemakers analysis, and the thermobarometric results, together indicate that the regional transition from omphacite amphibolites to kyanite eclogites can be described by two simplified reactions: alb + epi + hbl = omp + kya -I-qtz + par (H 2 O-conserving)par + epi + hbl + qtz = omp + kya + H 2 O (dehydration)which have the character of isograd reactions. Local variations of water activity (a H2<3 ) as indicated by isofacial mineral assemblages, and the H 2 O-conserving character of the reaction (15), are interpreted to reflect largely H 2 O-undersaturated and predominantly fluid-absent high-pressure metamorphism within the northern part of the nappe. The omphacite amphibolites and paragonite eclogites in this area are thought to have formed by H 2 O-conserving reactions from Pre-Mesozoic high-grade amphibolites, i.e. from protoliths of similar bulk H 2 O-content.The second 'isograd' (12) is interpreted to mark the regional transition from largely fluid-absent metamorphism in the north to fluid-present metamorphism in the south, where metamorphic pressures and temperatures in excess of 12-15 kb and 500-600 °C were sufficient for prograde in-situ dehydration of similar hydrous protoliths to kyanite eclogites. The observation of abundant veins, filled with quartz + kyanite + omphacite, suggests that a free fluid coexisted locally with the kyanite eclogites of the southern Adula Nappe at some time during progressive dehydration.

Petrology of the nyböite-bearing eclogite in the Donghai area, Jiangsu Province, eastern China

“…5) are higher than those from the pair for normal omphacite (0.35 < Xjd < 0.6). A similar tendency has been reported in an Alpine eclogite by Koons (1984) and Heinrich (1986). They ascribed this fact to the preferential incorporation of Fe 2+ into M2 sites in jadeitic pyroxene.…”

EBSD-measured lattice-preferred orientations and seismic properties of eclogites

“…This lens-shaped peridotite massif, 1-km long, is surrounded by eclogite boudins partly converted to an amphibolite assemblage (Möckel, 1969;Ernst, 1977). Thermobarometric investigations (Heinrich, 1986) suggest P -T metamorphic conditions in the range 1.8 -3.5 GPa and 750 -900°C.…”

New stratigraphic data from the Lower Penninic between the Adula nappe and the Gotthard massif and consequences for the tectonics and the paleogeography of the Central Alps

“…This barrowian metamorphism is preceded by a subduction related high-pressure event, ranging from blueschist to eclogite conditions (e.g. Heinrich, 1982Heinrich, , 1986Nagel, 2008;Wiederkehr et al, 2008). The metamorphic evolution of the metasediments in the northern part of the Lepontine dome has been recently reviewed and improved by Wiederkehr et al (2008Wiederkehr et al ( , 2009Wiederkehr et al ( , 2011.…”