Search

A metasomatic horizon (MH) occurs between the metaophiolite (serpentinite and metaophicarbonates) basement and metasedimentary sequence (chaotic rocks and calcschists) of the Lake Miserin Ophiolite, in the high pressure Zermatt-Saas Zone of the Northwestern Alps. Macro- and microstructural analyses combined with petrological and geochemical investigations of the MH and surrounding lithologies unravelled a polyphase blastesis-deformation history, which led to the formation of a complex fabric and minero-chemical alteration of the serpentinite basement-metasediments interface. Dehydration, decarbonation and carbonation interplayed from early Alpine subduction up to HP-LT metamorphic peak (T = 550-630°C, P = 1.8-2.5 GPa), to produce a distinctive, pervasive amphibole (tremolite/actinolite) replacement both in carbonate-rich and serpentinite-rich domains pertaining to the MH protoliths, i.e., serpentinite and carbonatebearing metabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average d18OVSMOW and d13CVPDB values of dolomite within the MH (+14.4‰ and +0.7‰ respectively) lie between those of the metaophicarbonates and of calcschist. These results suggest that Mg-H2O-rich fluids from the dehydrating slab, CO2 released by decarbonation and SiO2-rich fluids evolved in calcschists mixed together and circulated mostly along the metaophiolite basement/metasediments interface, where the MH developed and recorded a preferential channel for mixed metamorphic fluid flow. These findings highlight and confirm that the study of metasomatic rocks in convergent systems is crucial to comprehend the behaviour of different fluids circulating, mixing and interacting with lithologies along slab-parallel discontinuities, which act as major fluid conduits for deep volatile recycling. [ABSTRACT FROM AUTHOR]

The eclogite-facies Monviso meta-ophiolite Complex in the Western Alps represents a well-preserved fragment of oceanic lithosphere and related Upper Jurassic – Lower Cretaceous sedimentary covers. This meta-ophiolite sequence records the evolution of an oceanic core complex formed by mantle exhumation along an intra-oceanic detachment fault (the Baracun Shear Zone), related to the opening of the Ligurian–Piedmont oceanic basin (Alpine Tethys). On the basis of detailed geological mapping, and structural, stratigraphic and petrological observations, we propose a new interpretation for the tectonostratigraphic architecture of the Monviso meta-ophiolite Complex, and discuss the role played by structural inheritance in its formation. We document that subduction- and exhumation-related Alpine tectonics were strongly influenced by the inherited Jurassic intra-oceanic tectonosedimentary physiography. The latter, although strongly deformed during a major Alpine stage of non-cylindrical W-verging folding and faulting along exhumation-related Alpine shear zones (i.e. the Granero–Casteldelfino and Villanova–Armoine shear zones), was not completely dismembered into different tectonic units or subduction-related mélanges as suggested in previous interpretations. The present-day architecture of the Monviso meta-ophiolite Complex results from nappe-scale folding with a significant component of shearing, and strain partitioning of the Alpine deformation, which were controlled by the inherited occurrence of (i) lateral and vertical variations of facies and thickness of sediments, (ii) an intra-oceanic fault-rock assemblage, which acted as weak horizons in concentrating deformation, and (iii) remnants of a volcanic ridge, which consists of massive metabasalt. Thus, the recognition of pre-collisional, intra-oceanic, tectonostratigraphic inheritance represents an important step in reconstructing the tectonic evolution of meta-ophiolite units in orogenic belts. [ABSTRACT FROM AUTHOR]

In the Inner Western Alps, three different types of block-in-matrix structures (BIMs) formed sequentially through time at a convergent plate margin. These show the superposition of progressive deformation from (i) subduction to eclogite-facies depths, (ii) collision, accretion, and exhumation of oceanic crust, represented by the Monviso Meta-ophiolite Complex, to (iii) collision, accretion, and exhumation of the continental Dora Maira units. The Type 1 occurs in the metasedimentary cover of the Dora Maira Unit and consists of a map-scale broken formation with boudinaged ‘native’ blocks of marble (Early Jurassic) in a calcschist matrix. It results from the tectonic overprinting of exhumation-related folding (D2-stage) on an earlier subduction-related dismembered succession (D1-stage). Type 1 also includes ‘non-mappable’ BIMs with ‘exotic’ blocks, resulting from the gravitational collapse of the Triassic carbonate platform of European Continental Margin, triggered by the Early Jurassic rifting. In the Monviso Meta-ophiolite Complex, Types 2 and 3 represent tectonically induced broken and dismembered formations, respectively. They differ from each other in the degree of stratal disruption of primary interbedded horizons of mafic metabreccia (Type 3) and mafic metasandstone (Types 2 and 3) sourced by the Late Jurassic–Early Cretaceous denudation of an oceanic core complex. Dismembered interbeds (Type 2) and isolated blocks were mixed together (Type 3) by the overlap of D2 tectonics and late- to post-exhumation extensional shearing (D3-stage). Development of these types of BIMs may be common in many exhumed convergent plate margins, where severe tectonics and metamorphic recrystallization under high-pressure conditions normally prevent the reconstruction of BIMs or mélange-forming processes. Our findings show that documenting the mode and time of the processes forming BIMs is highly relevant in order to reconstruct the oceanic seafloor morphology and composition of associated stratigraphic successions, and their control in the evolution of those convergent plate margins. [ABSTRACT FROM AUTHOR]

The Mount Avic massif consists of serpentinized peridotite exposed in the southern Aosta valley (Northwestern Alps), covering an area of ca. 180 km2. The 1:10,000 scale geological map is located in the southern portion of the massif, where serpentinite is in contact with ophiolitic rocks pertaining to the Piemonte Zone, which represents the fossil Mesozoic Tethyan ocean. Southwards, ophiolites are overthrusted by the continental-derived Austroalpine Mont Glacier unit. Serpentinite consists of antigorite, magnetite, and coarse grained Ti-clinohumite, olivine, and diopside, which are reminiscent of the original mantle texture. Rodingitic mafic dykes are intruded within serpentinite; other mafic rocks, consisting of (not rodingitized) metagabbro and metabasalt with relict eclogitic minerals, occur as tectonic slices associated with serpentinite, calcschist and sulphide-rich epidosite. The map gives detailed and updated information on the structure and lithostratigraphy of the Mount Avic ophiolites, providing an insight to the mantle-crust transition of the Tethyan oceanic lithosphere. [ABSTRACT FROM PUBLISHER]

Holes BA1B and BA3A were drilled into the Wadi Tayin Massif, southern ophiolite complex of Oman, a fragment of the Tethyan oceanic lithosphere obducted onto the Arabian continent. Within the sequence, we have studied a portion of the shallow mantle, composed mainly of strongly serpentinised harzburgite that embeds dunitic levels, the biggest being over 150 m thick. The formation of thick dunitic channels, already approached via published structural and mathematical models, is here investigated with a mineral chemistry approach. We focused on Cr-spinel, the only widespread phase preserved during serpentinization, whose TiO2 content displays a wide variability from low in harzburgite, (TiO2 < 0.25 wt. %), typical of non-metasomatised ophiolite mantle, to moderately high in dunite (TiO2 < 1.10 wt. %) characterizing a rock/melt interactions. The high variability of TiO2, accompanied by similar patterns of Cr# and Mg# is observed, in a fractal pattern, at all scales of investigation, from the whole channel scale to the single thin section, where it affects even single grain zonings. Our results suggest that the over 150 m thick dunite channel here investigated was formed by coalescence of different scale melt channels and reaction zones with different sizes, confirming the published structural model. [ABSTRACT FROM AUTHOR]