Metasediments Covering Ophiolites in the HP Internal Belt of the Western Alps: Review of Tectono-Stratigraphic Successions and Constraints for the Alpine Evolution

Ophiolites of the Alpine belt derive from the closure of the Mesozoic Tethys Ocean that was interposed between the palaeo-Europe and palaeo-Adria continental plates. The Alpine orogeny has intensely reworked the oceanic rocks into metaophiolites with various metamorphic imprints. In the Western Alps, metaophiolites and continental-derived units are distributed within two paired bands: An inner band where Alpine subduction-related high-pressure (HP) metamorphism is preserved, and an outer band where blueschist to greenschist facies recrystallisation due to the decompression path prevails. The metaophiolites of the inner band are hugely important not just because they provide records of the prograde tectonic and metamorphic evolution of the Western Alps, but also because they retain the signature of the intra-oceanic tectono-sedimentary evolution. Lithostratigraphic and petrographic criteria applied to metasediments associated with HP metaophiolites reveal the occurrence of distinct tectono-stratigraphic successions including quartzites with marbles, chaotic rock units, and layered calc schists. These successions, although sliced, deformed, and superposed in complex ways during the orogenic stage, preserve remnants of their primary depositional setting constraining the pre-orogenic evolution of the Jurassic Tethys Ocean.

Metamorphic gabbro and basalt in ophiolitic and continental nappes of the Zermatt region (Western Alps)

The composition of meta-gabbro and meta-basalt occurring abundant and widespread in all nappes of the nappe stack exposed in the Zermatt region of the Western Alp shows distinct patterns related to the geodynamic origin of metamorphic basic rocks. Eclogitic meta-basalts of the ophiolitic Zermatt-Saas Unit (ZSU) show enriched MORB signatures. The meta-basalts (eclogites) of the continental fragment of the Theodul Glacier Unit (TGU) derive from pre-Alpine metamorphic continental intraplate basalts. Meta-basalts (eclogites) from the continental basement of the Siviez-Mischabel nappe (SMN) derive from MORB thus a genetic relation to the TGU eclogites can be excluded. All basic igneous rocks experienced post-magmatic alteration by fluid-rock interaction ranging from processes at the seafloor, in the shallow crust, during subduction zone hydration, in the exhumation channel and late Alpine regional metamorphisms. The consequences of these alteration processes can be identified at various levels in the rock composition data. It was found that the REE data are little affected by fluid-rock alteration. Some trace elements, notably Cs, Rb, and Ba are typically massively altered relative to igneous compositions in all three groups of meta-basalts. Generally, meta-basalts from the TGU and the SMN preserved the features of the original composition whilst the ZSU meta-volcanic rocks experienced massive alteration. For the ZSU meta-volcanic rocks it is evident that Zr was gained and Y lost during high-pressure fluid-rock interaction indicating a mobile behavior of the two elements during HP-metamorphism in contrast to their behavior in hydrothermal near-surface fluid-rock interaction.

Tectonostratigraphy of the Southernmost Scandinavian Caledonides: testing the Shetland correlation and the Laurentian/Renlandian link

<p>Geological mapping, zircon U–Pb dating of 28 samples, and mica <sup>40</sup>Ar–<sup>39</sup>Ar dating of 7 samples in the Stavanger–Ryfylke region (Stavanger, Suldal, Nedstrand, Randøy) characterizes the tectonostratigraphy of the southernmost nappes in the Scandinavian Caledonides. Four main tectonostratigraphic levels are described. (1) The lowest phyllite/mica schist nappes –Buadalen, Holmasjø, Lower Finse, Synnfjell– represent the Cambro–Ordovician sediment cover of the Baltic margin. (2) The overlying nappes –Madla, Storheia, Dyrskard, Hallingskarvet– consist of felsic metaigneous rocks with a consistent age between c.1525 and 1493 Ma. They host c.1040 Ma intrusives and c.1025 Ma Sveconorwegian metamorphism. They likely represent transported Baltican (Sveconorwegian) basement, widely exposed in S Norway. (3) The overlying nappes –Sola, Boknafjord, Kvitenut, Revseggi– are more diverse and lack counterparts in the exposed Baltican crust. The Sola nappe, near Stavanger, comprises a marine succession –Kolnes succession– of mica schist, metasandstone, marble, amphibolite and felsic metavolcanic rocks. The metavolcanic rocks –Snøda metadacite–rhyolite– are fine-grained mica gneisses, with calc-alkaline composition. Their extrusion age of c.941–934 Ma date deposition of the sequence. Detrital zircons in a metasandstone sample (n=138) yield main age modes at c.1040, 1150 and 1395 Ma, as well as significant Paleoproterozoic and Archaean modes. The Kolnes succession was affected by Taconian/Grampian metamorphism peaking in eclogite-facies conditions between c.471 and 458 Ma (Smit et al., 2010), followed by regional cooling around 445–435 Ma. Leucogranite bodies (c.429 Ma) cut the Grampian fabric. Several <sup>40</sup>Ar–<sup>39</sup>Ar white mica and biotite plateau ages constrain the timing of Scandian top-to-the SE nappe stacking at c.420 Ma. The Boknafjord nappe in Nedstrand comprises a c.932 Ma augen gneiss, overlain successively by amphibolite and mica schist units. Preliminary detrital zircon data (n=11) imply an Ordovician (<459 Ma) deposition for the mica schist. (4) The highest nappes –Karmsund and Hardangerfjord– host the Karmøy and Bømlo ophiolite complexes. These complexes comprise a c.493 Ma supra subduction zone ophiolite, intruded by c.485–466 Ma volcanic arc plutonic rocks, and unconformably overlain by fossiliferous upper Ordovician (<c.445 Ma) clastic sediments (Pedersen and Dunning, 1997).</p><p>We propose that the Iapetan Karmøy–Bømlo ophiolite complexes were accreted onto the Kolnes succession on the Laurentian side of the Iapetus realm, during the Grampian orogeny, before integration of both in the Scandian nappe pile. The age of HP metamorphism in the Kolnes succession (471–458 Ma) matches the inferred timing for obduction of the Karmøy–Bømlo complexes (485–448 Ma). The evidence for a Laurentian margin obduction stems from a conspicuous similarity with Shetland. On Shetland, the c.492 Ma Unst–Fetlar ophiolite complex was obducted during the Grampian orogeny onto Neoproterozoic Laurentian marine sequences (psammite-marble-mica gneiss) of the Westing, Yell Sound and East Mainland successions. The Westing and Yell Sound successions are characterized by a c. 944–925 Ma, Renlandian, high-grade metamorphism, a dominant detrital zircon mode at 1030 Ma, and common Archean detrital zircons. They correlate well with the Kolnes succession and suggest an ancestry along the Neoproterozoic Renlandian active margin of Laurentia and Rodinia, before opening of Iapetus. </p>

Petrologic constraints on subduction zone metamorphism from a coesite-bearing metapelite in the Northern Appalachian Orogen

<p>The Caledonian orogen formed following Paleozoic subduction of the Iapetus Ocean and preserves evidence of ultrahigh-pressure (UHP) metamorphism and exhumation of crustal rocks from mantle depths. The Appalachian orogen similarly formed in the Paleozoic following subduction of Iapetus Ocean crust, but evidence for (U)HP metamorphism in exhumed Appalachian rocks has been challenging to identify. We present results from a metapelite from high-pressure rocks of the Tillotson Peak Complex in the northern Appalachians, which formed during the middle-Ordovician Taconic orogeny. This sample contained mm-cm scale garnet porphyroblasts that host abundant mineral inclusions. Confocal Raman microspectroscopy of inclusions in the rims of a garnet porphyroblast identified relic coesite, preserved as a bi-mineralic inclusion composed of coesite in α-quartz. Raman depth profiling and 2-dimensional mapping indicate the relic coesite is ~10 μm<sup>3</sup>, suggesting that mineralogical evidence of UHP metamorphism in the Appalachians may be preserved only as μm-scale inclusions contained in polymetamorphosed rocks. We applied quantitative WDS X-ray maps acquired with electron microprobe, quartz-in-garnet elastic thermobarometry, and Zr-in-rutile trace element thermometry to further constrain the metamorphic history of the coesite-bearing metapelite. Garnet zoning patterns in conjunction with elastic and trace element thermobarometry applied to co-entrapped mineral inclusions suggest that garnet nucleated at 14-15.5 kbar and 420-520 °C, and continuously crystallized to 15-19.5 kbar and 470-560 °C during subduction zone metamorphism. Peak metamorphic conditions based on the stability field of coesite and on Zr-in-rutile thermometry from inclusions in the garnet rims suggest UHP metamorphism at >28 kbar and 530 °C. UHP metamorphism of pelitic sediments within the Taconic paleo-subduction zone invite comparisons with similar UHP rocks in the Caledonian orogeny. Future studies of UHP metamorphism in the Appalachian orogen will focus on constraining: 1) the spatial and temporal scales of UHP metamorphism, 2) the retrograde/exhumation P–T path of the coesite-bearing metapelite, and 3) the P–T history of other nearby metamorphic units, such as the Tillotson peak metabasites, to evaluate if these units shared a similar metamorphic history.</p>

Large meta-eclogite massifs within the Western Gneiss Region, Scandinavian Caledonides: Subducted ocean-continent transition?

<p>The northern part of the Western Gneiss Region (WGR) has distinctive belts of allochthonous metasediments and mafic rocks lying within tight infolds into the Baltica basement. They outcrop from the Grong-Olden Window to the Norwegian coast, possibly as far SW as Sørøyane, predominantly comprising metapelite and amphibolite with psammite, marble, calc-silicate, local large eclogite (>4km) lenses and ultramafites. These supracrustal lithotectonic units are attributed to the Blåhø Nappe, correlated with the Seve Nappe Complex (SNC) in its main outcrop in Sweden, which is considered to represent the pre-Caledonian continent-ocean transition (COT) of Baltica. They closely resemble the Lower Seve Nappe in northern Sweden where large amphibolite massifs with marbles are common, along with local eclogites. At least some have geochemical characteristics of spilitised extrusive MORB basalt in contrast to the better known, Neoproterozoic Baltoscandian Dyke Swarm in the SNC.</p><p>In the WGR near Molde a >10km long massif of such “amphibolite” at Tverrfjella commonly exhibits a relict high-P granulite precursor that has, in turn, overprinted eclogite. It encloses marble, scapolite-bearing calc-silicate, garnet peridotite (harzburgite) and Cu ores. Marble and meta-eclogite are intermixed which, along with its high Na spilitic character, suggests that the protolith was extrusive. Limited geochemical data suggest MORB composition. P-T estimates for eclogites in adjacent belts suggest UHP, possibly diamond-stable, conditions; in Sørøyane the well-known Ulsteinvik eclogite contains coesite. In the Molde area some of the mafic rocks and metasediments have partially melted. Eclogite metamorphism was Scandian in the Tverrfjell massif at 418 ± 11 Ma, with similar ages but tighter errors for adjacent belts and Ulsteinvik. These are significantly younger than ages for (U)HP metamorphism in the main SNC outcrop in Sweden, where early Ordovician subduction with a latest Ordovician granulite overprint is recorded. However, metapelites in other Blåhø-like supracrustal belts in the WGR do seem to record this earlier history as does one eclogite, consistent with the “double-dunk” hypothesis in this hinterland region. The protolith age of the metabasalts is unknown; analogy with the BDS suggests Neoproterozoic, but some zircon data from the WGR may suggest magmatic crystallisation during the Ordovician.<sup></sup>O-isotopes indicate that the marbles were Palaeozoic, rather than Proterozoic, carbonates. Overall, the available literature data show that some large mafic massifs in the WGR, with associated metasediments and peridotites, are allochthonous with respect to Baltica basement; they represent major additions of extrusive basalt to a far-distal COT or fully oceanic basin that have been subducted at least once during the Caledonian Wilson cycle. Isotopic data hint that at least some of their protoliths are unusually young. These supracrustal belts certainly merit closer attention.</p>

The evolution of the Sesia Zone (Western Alps) from Carboniferous to Cretaceous: insights from zircon and allanite geochronology

Microscale dating of distinct domains in minerals that contain relics of multiple metamorphic events is a key tool to characterize the polyphase evolution of complex metamorphic terranes. Zircon and allanite from five metasediments and five metaintrusive high-pressure (HP) rocks from the Eclogite Micaschist Complex of the Sesia Zone were dated by SIMS and LA-ICP-MS. In the metasediments, zircon systematically preserves detrital cores and one or two metamorphic overgrowths. An early Permian age is obtained for the first zircon rim in metasediments from the localities of Malone Valley, Chiusella Valley and Monte Mucrone (292 ± 11, 278.8 ± 3.6 and 285.9 ± 2.9 Ma, respectively). In the Malone Valley and Monte Mucrone samples, the early Permian ages are attributed to high-temperature metamorphism and coincide with the crystallization ages of associated mafic and felsic intrusions. This implies that magmatism and metamorphism were coeval and associated to the same tectono-metamorphic extensional event. In the Malone Valley, allanite from a metasediment is dated at 241.1 ± 6.1 Ma and this age is tentatively attributed to a metasomatic/metamorphic event during Permo-Triassic extension. Outer zircon rims with a late Cretaceous age (67.4 ± 1.9 Ma) are found only in the micaschist from Monte Mucrone. In metagabbro of the Ivozio Complex, zircon cores yield an intrusive age for the protolith of 340.7 ± 6.8 Ma, whereas Alpine allanite are dated at 62.9 ± 4.2 and 55.3 ± 7.3 Ma. The Cretaceous ages constrain the timing of the HP metamorphic stage. The presence of zircon overgrowth only in the central area of the Eclogite Micaschist Complex is attributed to local factors such as (1) multiple fluid pulses at HP that locally enhanced zircon dissolution and recrystallization, and (2) slightly higher temperatures reached in this area during HP metamorphism.

Subduction of a rifted passive continental margin: the Pohorje case of Eastern Alps–constraints from geochronology and geochemistry

This study presents geochronological and geochemical data from newly dated Permian granitic orthogneisses associated with the Eclogite-Gneiss unit (EGU) from the southernmost part of the Austroalpine nappe stack, exposed within the Pohorje Mountains (Slovenia). LA-ICP-MS zircon U–Pb ages of two samples of the augen-gneisses are 255 ± 2.2 Ma and 260 ± 0.81 Ma, which are interpreted as the age of magmatic crystallization of zircon. In contrast, all round zircons from leucogneisses give Cretaceous ages (89.3 ± 0.7 Ma and 90.8 ± 1.2 Ma), considered as the age of UHP/HP metamorphism. The round zircons overgrew older euhedral zircons of Permian and rare older ages tentatively indicating that these rocks are of latest Permian age, too. Zircon εHf(t) values of the four orthogneiss samples are between − 13.7 and − 1.7 with an initial 176Hf/177Hf ratio ranging from 0.282201 to 0.282562; T DM C is Proterozoic. The augen-gneisses show geochemical features, e.g. high (La/Lu)N ratios and strong negative Eu anomalies, of an evolved granitic magma derived from continental crust. The leucogneisses are more heterogeneously composed and are granitic to granodioritic in composition and associated with eclogites and ultramafic cumulates of oceanic affinity. We argue that the Permian granitic orthogneisses might be derived from partial melting of lower crust in a rift zone. We consider, therefore, that segment of the EGU is part of the distal Late Permian rift zone, which finally led to the opening of the Meliata Ocean during Middle Triassic times. If true, the new data also imply that the Permian stretched continental crust was potentially not much wider than ca. 100 km, was subducted and then rapidly exhumed during early Late Cretaceous times.

Garnet-Rich Veins in an Ultrabasic Amphibolite from NE Sardinia, Italy: An Example of Vein Mineralogical Re-Equilibration during the Exhumation of a Granulite Terrane

A complex system of mono- and polymineralic centimeter-thick veins occurs within the ultrabasic amphibolites of Montigiu Nieddu hill in northeastern Sardinia, and they are filled with garnet, amphibole, chlorite, and epidote. Some garnet-rich veins are margined by an amphibole layer at the interface with the host rock and/or show replacement of epidote concentrated in the vein core. Together with homogeneous matrix garnet (Grt1), millimetric, euhedral, and strongly zoned garnet porphyroblasts occur within these veins. The estimated pressure–temperature conditions (P = 1.0–1.7 GPa, T = 650–750 °C) for the formation of Grt1 match the metamorphic peak and early exhumation derived previously for the host rocks and confirm that the garnet veins also formed under high-pressure (HP) conditions. The igneous protolith of the host rocks experienced HP metamorphism in a subduction zone and underwent exhumation in an exhumation channel. The vein system in the ultrabasic amphibolites formed by cyclic hydrofracturing as rapid and transient events such as crack-seal veining. The growth of multiple vein-filling mineral assemblages indicates the formation of separate vein-producing cycles.

Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India

Growth of corundum in metamorphosed anorthosites and related basic-ultra-basic rocks is an exceptional feature, and its origin remains elusive. We describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from two localities, Manavadi (MvAm) and Ayyarmalai (AyAm), contain anorthite lenses (An90–99) with euhedral to elliptical outline set in a finer-grained matrix of calcic plagioclase (An85–90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic megacrysts, and the matrix are both recrystallized under static condition presumably during the regional high pressure (HP) metamorphism (~800 °C, 8–11 kbar) at ~2.45 Ga. Corundum occurs in the core of some of the recrystallized anorthite lenses (An95–99) in two modes: (1) Dominantly, it forms aggregates with magnetite (with rare inclusion of hercynite; in MvAm) or spinel (and occasionally hematite-ilmenite; in AyAm). The aggregates cut across the polygonal grain boundaries of the anorthite and contain inclusions of anorthite. (2) Corundum also occurs along the grain boundaries or at the triple junctions of the polygonal anorthite grains, where it forms euhedral tabular grains, sieved with inclusions of anorthite or forms skeletal rims around the recrystallized anorthite, such that it seems to be intergrown with anorthite. Combined petrological data and computed phase relations are consistent with growth of corundum in an open system during regional metamorphism in the presence of intergranular fluids. Two mechanisms are proposed to explain the formation of the corundum in the amphibolites: (1) corundum + magnetite/spinel aggregates formed dominantly by oxy-exsolution of pre-existing Al-Fe-Mg-(Ti)-spinel. This pre-existing spinel may be primary magmatic inclusions within the anorthite phenocrysts or could have formed due to reaction of primary magmatic inclusions of olivine with the host anorthite. Pseudosections of fO2-nH2O-T-P in the CaO–FeO–MgO–Al2O3–SiO2–H2O (CFMASH) system indicate that fO2 and H2O strongly influence the formation of corundum + amphibole from the initial magmatic assemblage of anorthite (phenocrysts) + spinel ± olivine (inclusions). (2) The corundum with anorthite presumably formed through desilification and decalcification of anorthite, as is indicated by computed phase relations in isobaric-isothermal chemical potential diagrams (µSiO2-µCaO) in parts of the CASH system. Growth of corundum in this mode is augmented by high activity of anorthite in plagioclase, high pressure, and low-to-medium temperature of metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultra-basic rocks in general, which should provide new insight into lower crustal processes like high-pressure metamorphism.