Eclogitic metatrondhjemites from metaophiolites of the Western Alps

In the Urtier valley (southern Aosta Valley, Italy), the Piemonte metaophiolites mainly consist of serpentinized peridotites including pods and boudinaged layers of Fe-metagabbro and trondhjemite transposed in the main eclogitic foliation. The contact between serpentinized peridotites and Fe-metagabbro/trondhjemite is locally lined by chloriteschist and rodingite. The high pressure parageneses in the Fe-metagabbro are omphacite-garnet-rutile-glaucophane-phengite, and in the metatrondhjemite plagioclase-quartz-phengite-clinozoisite-epidote-garnet, respectively. Bulk-rock major and trace elements in addition to O isotope analyses were performed in both rock types. Fe-metagabbros are characterized by MgO wt% ranging between 6.11 and 9.63%, EREE= 20-101 ppm, (La/Yb)N = 0.22-0.91; trondhjemites have SiO2 43%, Al2O3 ranging between 21 and 24%, CaO ranging between 17 and 20%, EREE = 172 - 272 ppm, (La/Yb)N ranging between 7.78 and 13.70. The 18O is 5.9 per-mil in a Fe-metagabbro sample and 7.4 per-mil in a trondhjemite sample, suggesting that these rocks have been affected by a weak oceanic low temperature alteration. The high CaO content may indicate a metasomatic process which could have occurred during the oceanic stage or at high pressure conditions

Record of Jurassic mass transport processes through the orogenic cycle: Understanding chaotic rock units in the high-pressure Zermatt-Saas ophiolite (Western Alps)

The eclogite facies Zermatt-Saas ophiolite in the Western Alps includes a composite chaotic unit exposed in the Lake Miserin area, in the southern Aosta Valley region. The chaotic unit is characterized by a block-in-matrix texture consisting of ultramafic clasts and blocks embedded within a carbonate matrix. This unit overlies massive serpentinite and ophicarbonate rocks and is unconformably overlain by layered calcschist. Despite the effects of subduction and collision-related deformation and metamorphism, the internal stratigraphy and architecture of the chaotic unit are recognizable and are attributed to different types of mass transport processes in the Jurassic Ligurian-Piedmont Ocean. This finding represents an exceptional record of the preorogenic history of the Alpine ophiolites, marked by different pulses of extensional tectonics responsible for the rough seafloor topography characterized by structural highs exposed to submarine erosion. The Jurassic tectonostratigraphic setting envisioned is comparable to that observed in present-day magma-poor slow- and ultraslow-spreading ridges, characterized by mantle exposure along fault scarps that trigger mass transport deposits and turbiditic sedimentation. Our preorogenic reconstruction is significant in an eclogitized collisional orogenic belt in which chaotic rock units may be confused with the exclusive product of subduction-related tectonics, thus obscuring the record of an important preorogenic history. \ua9 2017 Geological Society of America

Permeability Structure of the Lava-Dike Transition of 15 My Old Oceanic Crust Formed at the East Pacific Rise

The permeability structure of oceanic crust controls both the spatial and temporal extent of hydrothermal circulation, but the detailed geometry of fractures in seafloor rocks is not well known. We apply an equivalent channel model to veins, joints, faults, and breccias preserved in recovered cores from ODP-IODP Hole 1256D to calculate paleo-permeability. In the ~250 m transition between dikes and lavas, paleo-permeability is 10-13~10-14 m2 with narrow zones of >10-9 m2 that presumably act as conduits for the largest volume of fluids. Most of these high permeability zones are oriented vertically as a result of diking events into a significant thickness of lavas outside of the neovolcanic zone. After an increase in permeability due to off-axis diking events, fluid temperatures drop, pathways are sealed, and the permeability of the upper oceanic crust drops significantly

Factors Controlling Hydrothermal Nickel and Cobalt Mineralization—Some Suggestions from Historical Ore Deposits in Italy

We compare three poorly known, historical Ni–Co-bearing hydrothermal deposits in dierent geological settings in Italy: The Ni–Co–As–Sb–Au-bearing Arburese vein system (SW Sardinia), the Co–Ni–As-rich Usseglio vein system (Piedmont), and the small Cu–Ag–Co–Ni–Pb–Te–Se stockwork at Piazza (Liguria). These deposits share various (mineralogical, chemical, thermal, and stable isotopic) similarities to the Five Element Vein-type ores but only the first two were economic for Co–Ni. The Sardinian Ni-rich veins occur in Paleozoic basement near two Variscan plutons. Like the Co-rich Usseglio vein system, the uneconomic Piazza deposit is hosted in an ophiolite setting anomalous for Co. The Sardinian and Usseglio deposits share a polyphasic assemblage with Ni–Co–As–Sb–Bi followed by Ag-base metal sulfides, in siderite-rich gangue, whereas Piazza shows As-free, Ag–Pb–Te–Se-bearing Co–Ni–Cu sulfides, in prehnite–chlorite gangue. Fluid inclusions indicated Co–Ni arsenide precipitation at 170 C for Usseglio, whereas for the Sardinian system late sulfide deposition occurred within the 52–126 C range. Ore fluids in both systems are NaCl-CaCl2-bearing basinal brines. The chlorite geothermometer at Piazza provides the range of 200–280 C for ore deposition from CO2-poor fluids. Enrichments in Se and negative 13C in carbonates suggest interaction with carbonaceous shales. These deposits involve issues about source rocks, controls on Co/Ni and possible role of arsenic and carbonate components towards economic mineralization

Tables S1_Tartarotti et al. submitted to TECTONICS

Table S1. Sample list with geographic location and mineral parageneses (Mineral Abbreviation according to Siivola & Schmid, 2007). Omp(c): coarse-grained omphacite; Omp(f): fine-grained omphacite. Mineral abbrevtiation after Siivola and Schmid (2007)

The Antrona nappe : lithostratigraphy and metamorphic evolution of ophiolites in the Antrona valley (Pennine Alps)

The Antrona ophiolite (western Central Alps) represents a tectonic fragment of the oceanic lithosphere of the Upper Jurassic - Lower Cretaceous Ligurian- Piedmont basin, a section of the Western Alpine Tethyan Ocean. The Antrona ophiolite occurs at lower structural levels in the Alpine nappe stack and is sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The Monte Rosa Nappe is overlain by the Zermatt Saas ophiolitic Unit. Despite the tectono-metamorphic reworking, the Antrona ophiolite exhibits all typical lithologies of oceanic lithosphere: ultramafic and mafic plutonic rocks, mafic volcanic rocks and deep-sea sediments can be recognized. Several rock types were distinguished among mafic rocks. New findings and inferences are: i) the occurrence of probable relics of magmatic structures in some amphibolites, inferred to be pillow lavas or pillow breccia; ii) the occurrence of lawsonite pseudomorphs-bearing amphibolites, not described so far in the study area as precursor of the origin of epidote-amphibolites the Antrona Valley. A qualitative P-T diagram deduced from the stability conditions of mineral parageneses is presented and compared with published P-T paths for the Antrona and Zermatt-Saas ophiolites. The metamorphic evolution of the studied rocks is characterized by blueschist prograde path followed by high pressure (eclogitic) metamorphic peak. P-T estimates for the metamorphic peak were calculated by the Na-clinopyroxene garnet equilibria and the jadeite content in omphacite. T = 372\ub0C for a nominal pressure of P = 1 GPa and T = 386\ub0C for a nominal pressure of P = 1.5 Gpa were obtained. Jd30 as maximum Jadeite content suggests P > 1 Gpa. Retrograde path, although not well constrained, is dominated by epidote-amphibolite/ amphibolite facies conditions, in accord with published data, differing from those inferred so far for the overlying Zermatt-Saas ophiolite

The St. Marcel Valley, Western Alps: metaophiolites, metasedimentary sequence and tectonic setting

In the Southern Aosta Valley, the St. Marcel Valley metaophiolites consist of mainly metavolcanics and their sedimentary cover metamorphosed under HP subduction-related metamorphism. A detailed geological map carried out in the St. Marcel Valley reveals that the metasedimentary cover, although transposed by the Alpine tectonics, is essentially made of three main terms, that are Mn-rich metaquartzites, marble, and calcschists. The metasedimentary sequence is quite comparable with the unmetamorphosed sequence made of radiolarian cherts, Calpionella limestones and Palombini shales covering the Ligurian ophiolites. The St. Marcel Valley metaophiolites represent the upper crustal section of the Mesozoic Tethyan ocean and its pelagic sedimentary cover, overthrusting the serpentinite unit of the Mount Avic, to the East. Within the Piemonte nappe stack, the St.Marcel metaophiolites are located at the higher structural level