Tectonometamorphic Unit Research Articles

Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps

In the Southern Steep Belt, Italian Central Alps, relicts of the pre-Alpine continental crust are preserved. Between Valtellina and Val Camonica, a poly-metamorphic rock association occurs, which belongs to the Austroalpine units and includes two classically subdivided units: the Languard-Campo nappe (LCN) and the Tonale Series (TS). The outcropping rocks are low to medium grade muscovite, biotite and minor staurolite-bearing gneisses and micaschists, which include interlayered garnet- and biotite-bearing amphibolites, marbles, quartzites and pegmatites, as well as sillimanite-bearing gneisses and micaschists. Permian intrusives (granitoids, diorites and minor gabbros) emplaced in the metamorphic rocks. We performed a detailed structural, petrological and geochronological analysis focusing on the two main lithotypes, namely, staurolite-bearing micaschists and sillimanite-bearing paragneisses, to reconstruct the Variscan and Permian-Triassic history of this crustal section. The reconstruction of the tectono-metamorphic evolution allows for the distinction between two different tectono-metamorphic units during the early pre-Alpine evolution (D1) and predates the Permian intrusives, which comprise rocks from both TS and LCN. In the staurolite-bearing micaschists, D1 developed under amphibolite facies conditions (P = 0.7–1.1 GPa, T = 580–660 °C), while in the sillimanite-bearing paragneisses formed under granulite facies conditions (P = 0.6–1.0 GPa, T> 780 °C). The two tectono-metamorphic units coupled together during the second pre-Alpine stage (D2) under granulite-amphibolite facies conditions at a lower pressure (P = 0.4–0.6 GPa, T = 620–750 °C) forming a single tectono-metamorphic unit (Languard-Tonale Tectono-Metamorphic Unit), which comprised the previously distinguished LCN and TS. Geochronological analyses on zircon rims indicate ages ranging between 250 and 275 Ma for D2, contemporaneous with the emplacement of Permian intrusives. This event developed under a high thermal state, which is compatible with an extensional tectonic setting that occurred during the exhumation of the Languard-Tonale Tectono-Metamorphic Unit. The extensional regime is interpreted as being responsible for the thinning of the Adriatic continental lithosphere during the Permian, which may be related to an early rifting phase of Pangea.

Geologic evolution of a Cretaceous tectonometamorphic unit in the Franciscan Complex, western California

ABSTRACTThe Franciscan Yolla Bolly terrane of the NE California Coast Ranges consists mainly of quartzose metagreywackes containing sparse high-pressure/low-temperature (HP/LT) neoblastic minerals, including ubiquitous lawsonite. Some Yolla Bolly rocks also contain one or more of the newly grown phases, pumpellyite, aragonite, glaucophane, and/or jadeitic pyroxene. These blueschist-facies metasandstones recrystallized under physical conditions of ~200–300°C and ~8 kbar at subduction-zone depths approaching 30 km. Petrologically similar Franciscan metaclastic-rich map units – Yolla Bolly terrane-like rocks, here designated the ‘YB’ unit – crop out in the central and southern California Coast Ranges. Recently published detrital zircon U‒Pb SIMS and LA-ICPMS data for 19 ‘YB’ metagreywackes indicate maximum ages of formation as follows: ~110–115 Ma (8) in the NE California Coast Ranges; ~95–107 Ma (7) in the San Francisco Bay area + Diablo Range; and ~85–92 Ma (4) in the dextrally offset Nacimiento Block. These fault-bounded ‘YB’ strata do not constitute coeval parts of a single tectonostratigraphic unit. Instead the term tectonometamorphic is proposed for such time-transgressive map units. Based on the current and likely Cretaceous 30° angular divergence between NS-palaeomagnetic stripes of the Farallon oceanic plate and the NNW-trending California convergent margin, I infer that arrival at the arc margin and underflow of a relatively thick segment of oceanic crust and its largely clastic sedimentary blanket may have resulted in progressive southeastward migration of an accreted, subducted, then exhumed HP/LT metagreywacke section. During the ~30 million year interval, ~115–85 Ma, the locus of ‘YB’ accretion, underflow, and tectonic regurgitation evidently moved SE along an ~1000 km stretch of the accretionary margin of western California.

Geology of the Eastern Ligurian Alps: a review of the tectonic units

The Alpine and Apennine belts come into contact in Central Ligu ria, in an extremely complex structural arrangement; this junction is usually referred to as the Ligurian Knot. The geological mapping for the 213-230 Genova and 212 Spigno Monferrato 1:50,000 quadrangles necessitated the updating of the stratigraphic, structural and metamorphic descriptions of this area, and a redefinition of some of the units and geological ensembles already established in Central Liguria, i.e. the Voltri Group, the Sestri-Voltaggio Zone and the Val Polcevera Unit. We propose a new definition of tectonic and tectono-metamorphic units, grouped in three main associations, as follows: a) units derived from oceanic crust and mantle; b) units derived from a continental margin; c) Flysch units derived from the sedimentary cover of an oceanic basement. In group a), we propose to keep the already established Palmaro-Caffarella, Cravasco-Voltaggio, Varazze and Figogna units and to establish a new Voltri tectonometamorphic Unit, including all the rocks showing a metamorphic climax in eclogite facies conditions, with a variably developed greenschist facies overprint. In group b) we propose to keep the already established Gazzo-Isoverde Unit and to define a new Angassino- Terma tectono-metamorphic Unit, encompassing limited outcrops of quartzite and dolomitic limestone associated with the Voltri rocks. In group c) we propose to replace the former Val Polcevera Unit with new Ronco, Montanesi and Mignanego tectonic units. We propose to abandon the term Voltri Group, for the term Voltri Massif that can be used to define a set of units, including the Voltri, Varazze, Palmaro-Caffarella and Angassino-Terma units, irrespective of their different lithology and/or paleogeographic derivation and/or metamorphism. For the Sestri-Voltaggio Zone, we propose to use this term only for the Cravasco-Voltaggio and Gazzo-Isoverde units (and not the Figogna Unit), to emphasize their common tectonometamorphic evolution.

Contrasting P– T– t paths from the basement of the Tisia Unit (Slavonian Mts., NE Croatia): Application of quantitative phase diagrams and monazite age dating

Medium-grade mica schists and intercalated paragneisses and amphibolites from the basement of the Tisia Unit, Slavonian Mountains., northeastern Croatia, contain complexly zoned garnets. At the Kutjevo locality, mica schists are characterised by garnets with Mn-rich cores and Ca-rich rims. Mn decreases steadily from core to rim and Ca increases abruptly. This is in contrast to the paragneisses and amphibolites which contain garnets with smoothly decreasing Ca from core to rim. Quantitative phase diagrams and garnet composition isopleths calculated from bulk rock analyses reveal that the Ca-poor garnet cores in the mica schists formed during an earlier event at 584–592 °C and 6.4–7.8 kbar. Ca-rich rims formed at conditions of 600–660 °C and 11–12 kbar — calculated using garnet isopleths and mineral thermobarometry. The paragneiss and amphibolite provide similar P– T information for the later peak event (ca. 650 °C, 10–12 kbar) but do not preserve a record of the earlier, lower P– T event and modelling shows that garnet was not stable at these conditions. Contrary to previous studies on this outcrop and rock type, no staurolite was observed and quantitative phase diagrams contoured for H 2O mode isopleths indicate that the rock did not cross staurolite-bearing fields during the retrograde P– T path. Mica schists from the Krndija locality contain zoned polyphase garnets. Phase diagram calculations reveal that Ca-rich garnet cores formed between 520 and 630 °C and 7–8 kbar. Rims have a lower Ca content and formed at considerably reduced pressures together with andalusite and staurolite at ca 530–570 °C and 3–4 kbar. Since both localities were traditionally considered to be part of the same tectono-metamorphic unit, evidence presented here clearly shows that this cannot be the case. EMP monazite ages are Variscan (350 Ma) in the Krndija mica schists and around pre- or early Variscan (440 Ma) in the Kutjevo mica schists. We therefore propose a more complex internal structure and metamorphic history for this area than previously recognised suggesting a metamorphic evolution of the Slavonian Mountains that includes a pre-Variscan and a Variscan cycle.

Three-dimensional evaluation of fabric evolution and metamorphic reaction progress in polycyclic and polymetamorphic terrains: a case from the Central Italian Alps

Abstract The 3D reconstruction of geological bodies is an excellent tool for the representation of crustal structures and is applied here to understand related heterogeneities in the grain-scale fabrics; the western portion of the Languard–Tonale Alpine tectono-metamorphic unit (Austroalpine domain, Central Alps) allows evaluation of the per cent volume of textural reworking during polyphase pre-Alpine and Alpine deformations. The structural and metamorphic overprinting during the last deformation imprint involved less than 50% of rock volume; this estimate is obtained by discriminating domains that homogeneously recorded structural and metamorphic re-equilibration during crenulation–decrenulation cycles. These domains are reconstructed using a geograhpical information system (GIS) to manipulate field data and interpretative cross-sections as a means to constrain their 3D volumes. The degree of fabric evolution is integrated at the microscale with the estimate of the reactants/products ratio to infer the progress of metamorphic transformation related to advancing degree of mechanical reactivation. The correlation between degree of fabric evolution and progress of synkinematic metamorphic reactions shows that differences between pristine mineral assemblages v. pre-existing fabrics influence the rate of reaction accomplishment. Fabric evolution and degree of metamorphic transformation increase proportionally once above the threshold value of 60% of volume affected by fabric rejuvenation; metamorphic degree also influences the progress of metamorphic reactions.

Vestiges of lost tectonic units in conglomerate pebbles? A test in Permian sequences of the Southalpine Orobic Alps

AbstractMicrostructural analysis andP–Testimates of metamorphic pebbles in Permian conglomerates of the Central Southern Alps, representing the erosion product of the collapsing Variscan chain, are the discriminating tools for determining the metamorphic sequences representing potential sources of the conglomerates. In the selected case, basement units are precisely outlined on the basis of qualityP–T–d–tpaths that allow reconstruction of their metamorphic evolutions (tectonometamorphic units); this facilitates individuation of the basement sources with much better confidence. The lower Permian volcaniclastic sequence of the Eastern Orobic Basin, which overlies the Variscan Val Vedello basement, comprises the Aga and Vedello conglomerates, which are the oldest deposits containing a considerable amount of up to metre-sized metamorphic pebbles. Microstructural and mineral chemical data on metamorphic pebbles of the Aga and Vedello conglomerates were used to infer quantitative pre-PermianP–Tevolutions, which are compared with those of the tectonometamorphic units constituting the surrounding Southalpine metamorphic basement. Two types ofP–Tevolution are recorded in the metamorphic pebbles of Aga and Vedello conglomerates: Type 1 is characterized by an amphibolite-facies imprint, followed by greenschist retrogression; Type 2 is characterized by three successive greenschist-facies re-equilibrations. The Type 1P–Tevolution of metamorphic pebbles matches with that of the adjacent tectonometamorphic unit of the Val Vedello basement. Type 2 is similar to those recorded in units B and C of the North East Orobic basement, and it differs from that of the adjacent Val Vedello basement. This suggests that the Aga and Vedello conglomerates were fed by two different basement sources: one consisting of the present day Val Vedello basement, and the other compatible with units B and C of the North East Orobic basement. According to theP/Tratios of theTmax–PTmaximprints, both basement sources recorded the Variscan collision but at a different crustal level. The age (c.278 Ma) of the Aga and Vedello conglomerates constrains the minimum exhumation age for their basement sources.

和歌山県清水－美里地域の毛原層のローソン石を含む泥質片岩

This is the first report of lawsonite in pelitic schists from the Kebara Formation, a tectonometamorphic unit exposed between the Sambagawa metamorphic belt and the Chichibu belt in western Kii Peninsula. Lawsonite and pumpellyite have previously been reported from basic schists in the Kebara Formation, suggesting high-P/T metamorphic conditions. The pelitic schists consist mainly of quartz, albite and chlorite along with minor carbonaceous matter and rare phengite, lawsonite, calcite and titanite. The mineral assemblage of the lawsonite-bearing pelitic schists is chlorite+phengite +lawsonite+albite+quartz. This assemblage is basically the same as that reported for lawsonite-bearing pelitic schists from the chlorite zone in the Sambagawa metamorphic belt, i.e. in the Ise area in eastern Kii Peninsula and in the Besshi nappe complex exposed along the Asemigawa River in central Shikoku. These various lawsonite-bearing pelitic schists probably experienced similar high-P/T metamorphic conditions suggesting that the entire Kebara Formation experienced similar high-P/T metamorphism to that of the chlorite zone in the Sambagawa belt.

A critical assessment of the tectono-thermal memory of rocks and definition of tectono-metamorphic units: evidence from fabric and degree of metamorphic transformations

Abstract A correlation procedure of scattered tectonic and metamorphic imprints in the reactivated crust is elaborated from recent analytical work in three Alpine metamorphic complexes. It consists of: interpretation of the time-sequence of tectonic fabrics and test of their kinematic coherence; determination of paragenetic compatibility among the mineralogical support of mesoscopic fabrics; cross-validation of mineral transformation over-prints; construction of P-T-d-t paths using a time-sequence of parageneses. The representation of structural and metamorphic information conveys the full tectono-metamorphic history on maps displaying combined tectonic and metamorphic effects. Shape and size definition of metamorphic units, now individuated mainly using their lithological homogeneity and dominant metamorphic imprint, is improved. The analysis of interaction between fabric and metamorphic imprint distributions, proposed in three Alpine examples, shows that the dominant metamorphic imprint does not coincide with T max - P T max of each inferred P-T-d-t loop; the dominant metamorphic imprint is that given by the mineralogical support of the most pervasive fabric. Different metamorphic imprints may dominate in adjacent areas of a single tectono-metamorphic unit (TMU), or equivalent metamorphic imprints may dominate in different TMUs. Therefore, lithostratigraphic setting and dominant metamorphic imprint are inefficient to contour TMUs in terrains with polyphase deformation and metamorphism, without considering multiscale heterogeneity of superposed synmetamorphic fabrics.

Low-grade metamorphic rocks from the Pulur complex, NE Turkey: implications for the pre-Liassic evolution of the Eastern Pontides

In the Pulur complex (Sakarya Zone, Eastern Pontides, Turkey) a low-grade tectonometamorphic unit (Dogankavak) is exposed in three tectonic windows beneath a complex medium-pressure high-temperature metamorphic unit of late Carboniferous age. The thrust plane between both units is transgressively covered by Liassic conglomerates. The Dogankavak unit comprises a sequence of metabasites with MORB-type chemical compositions and phyllites, with subordinate calcareous phyllites, marbles, quarzofeldspathic schists and metacherts. This sequence is interpreted as a former accretionary complex related to the consumption of the Palaeotethys. Mineral parageneses in the metabasites allow for the distinction of two domains with slightly different peak metamorphic conditions, i.e. 375–425 °C/0.5–0.8 GPa (greenschist facies) and 400–470 °C/0.6–1.1 GPa (albite-epidote amphibolite facies). The age of metamorphism is constrained at ~ 260 Ma (early Late Permian) by two Rb-Sr mineral-whole rock ages (hornblende, phengite) and one 40Ar/39Ar single step total fusion age (phengite). In conjunction with previous data on other accretionary complexes in the Sakarya zone in Northern Turkey, the data presented in this study suggest a continuous subduction of the Palaeotethys at least from Early/Late Permian to Late Triassic and a discontinuous preservation of accretion complexes in both space and time.

Ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the European Variscan Belt

The content and isotopic composition of ammonium ( NH 4 +) were measured by using Kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the European Variscides, i.e., the Erzgebirge and the Zone of Erbendorf-Vohenstrauss in Germany. The studied Erzgebirge schists represent an early Paleozoic passive margin sequence that reappears in different deep subducted metamorphic units. This allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism. Ammonium is found to be progressively depleted, accompanied by a shift in δ 15N from the Low-grade Units (≈2 kbar/300°C) with 638 ± 124 ppm and δ 15N = +2.2 ± 0.6‰, to the Garnet-Phyllite Unit (≈9 kbar/470°C) with 621 ± 190 ppm and δ 15N = +3.5 ± 0.9‰, the Mica Schist/Eclogite Unit (≈12 kbar/550°C) with 394 ± 113 ppm and δ 15N = +3.9 ± 0.8‰ and to the Gneiss/Eclogite Unit (>12 kbar/730°C) with 99 ± 32 ppm and δ 15N = +7.7 ± 2.0‰. Using equilibrium models for Rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release. Only for the mica schists, this isotope fractionation can be explained by the loss of molecular nitrogen. In comparison with the early Paleozoic schists, the Proterozoic ortho- and paragneisses of the Erzgebirge Gneiss Unit (≈6–8 kbar/650°C) contain significantly lower amounts of ammonium (≈70 ppm) and reach relatively low δ 15N values (+2.5‰ to +3.6‰). The Paragneisses from the Zone of Erbendorf-Vohenstrauss (ZEV) sampled from the KTB pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies (7 kbar/650–700°C). The metagraywackes have NH 4 + contents of ≈80 ppm and δ 15N of approximately +6‰. Gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in NH 4 + (>250 ppm) and δ 15N up to +12‰, which suggest late fluid-rock interactions with a nitrogen- and 15N-enriched fluid.

The role of structural and metamorphic memory in the distinction of tectono-metamorphic units: the basement of the Como lake in the Southern Alps

The concept of ‘metamorphic field gradient’ applied to the lithologically homogeneous deep crust of the Southern Alps suggests the existence of two metamorphic units. The comparison of P-T-d-t paths, derived in adjacent portions of this basement and supported by a continuous foliation trajectory map helps to distinguish three tectono-metamorphic units, corresponding to the Domaso–Cortafò, Dervio–Olgiasca and Monte Muggio zones. The degree of granular scale reorganisation of the prevailing planar fabric is considered in the three zones together with the relative chronology of the structural imprints and the metamorphic environments in which they developed. This approach emphasizes that the dominant metamorphic imprint of each unit coincides with that of the most pervasive fabric at the regional scale when the degree of fabric evolution is sufficiently high, and not with the T max– P Tmax recorded in each tectono-metamorphic unit. In terrains that underwent polyphase deformation and metamorphism the ‘metamorphic field gradient’ concept cannot therefore be utilised to discriminate tectono-metamorphic units.

Plutonism in the Variscan Odenwald (Germany): from subduction to collision

Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from a subduction to a collisional setting. For ∼362 Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial isotopic compositions (eNd=+3.4 to +3.8;87Sr/86Sr =0.7035–0.7053;δ18O=6.8–8.0‰) and chemical signatures (e.g., low Nb/Th, Nb/U, Ce/Pb, Th/U, Rb/Cs) indicate a subduction-related origin by partial melting of a shallow depleted mantle source metasomatized by water-rich, large ion lithophile element-loaded fluids. In the central (unit II) and southern (unit III) Odenwald, syncollisional mafic to felsic granitoids were emplaced in a transtensional setting at approximately 340–335 Ma B.P. Unit II comprises a mafic and a felsic suite that are genetically unrelated. Both suites are intermediate between the medium-K and high-K series and have similar initial Nd and Sr signatures (eNd=0.0 to –2.5;87Sr/86Sr=0.7044–0.7056) but different oxygen isotopic compositions (δ18O=7.3–8.7‰ in mafic vs 9.3–9.5‰ in felsic rocks). These characteristics, in conjunction with the chemical signatures, suggest an enriched mantle source for the mafic magmas and a shallow metaluminous crustal source for the felsic magmas. Younger intrusives of unit II have higher Sr/Y, Zr/Y, and Tb/Yb ratios suggesting magma segregation at greater depths. Mafic high-K to shoshonitic intrusives of the southern unit III have initial isotopic compositions (eNd=–1.1 to –1.8;87Sr/86Sr =0.7054–0.7062;δ18O=7.2–7.6‰) and chemical characteristics (e.g., high Sr/Y, Zr/Y, Tb/Yb) that are strongly indicative of a deep-seated enriched mantle source. Spatially associated felsic high-K to shoshonitic rocks of unit III may be derived by dehydration melting of garnet-rich metaluminous crustal source rocks or may represent hybrid magmas.

Pressure-Temperature and Fluid Evolution of Quartzo-Feldspathic Metamorphic Rocks with a Relic High-Pressure, Granulite-Facies History from the Central Erzgebirge (Saxony, Germany)

The Gneiss-Eclogite Unit is a composite tectonometamorphic unit within the Variscan Erzgebirge mega-antiform. It comprises migmatitic para- and orthogneisses, high-temperature (HT) mylonites, kyanite-bearing granulites, eclogites and garnet peridotites. Four different quartzo-feldspathic assemblages are recognized, in which maximum conditions of up to 830°C and 21 kbar were determined. The assemblages are characterized by the nearly complete prograde breakdown of biotite, by high grossular content (23–47 mol %) of garnet in the presence of albite, and high Si contents of phengite [3.3–3.4 per formula unit (p.f.u.)]. Water activities at this stage are variable and range from <0.15 to >0.4. The maximum pressures indicated for individual rock volumes may vary considerably between 12 and 24 kbar at 700–800°C, so that non-coherency of the entire Gneiss-Eclogite Unit appears likely during the high-pressure event itself. After decompression, concomitant with penetrative HT myulonitization, hydration led to overprinting of the rocks to variable degrees, owing to channelized fluid influx. Partial equilibration at medium-pressure conditions of about 7–10 kbar and 600–700°C occurred, involving abundant retrograde migmatization. The water activity increased to 0.5–1.0. During later exhumation, deformation and re-equilibration at 2–3 kbar and 400–500°C were concentrated in local, discrete, ductile normal fault zones. The kinked geometry of the PT path is thus characterized by (1) high-pressure (HP) equilibration, followed by near-isothermal decompression at high temperatures, during which rocks from different depths were amalgamated, and (2) extensive hydration and re-equilibration at medium pressures, followed by rapid cooling during continued uplift, when the entire unit came into contact with cooler, now over- and underlying units. This scenario is attributed to continent collision, orogenic collapse and disintegration of the HP unit during continuing collision, crustal stacking and uplift controlled by extension.

Metamorphic evolution of ultrahigh-pressure garnet peridotites from the Variscan Vosges Mts. (France)

In the Central Vosges Mts. (France) of the Variscan belt, MgCr garnet peridotite bodies occur within the uppermost tectonometamorphic unit (Leptynitic granulites) as lenses in low-pressure/high-temperature metamorphic rocks. Neglecting late-stage serpentinization, the metamorphic evolution of these rocks was characterized by four stages. During stage I, the rocks were equilibrated at high pressures and temperatures (> 4.9 GPa/ > 950°C, in most cases > 1100°C), either within or near to the stability field of diamond. Stage II is documented by the formation of coronas around relict garnet with the assemblage orthopyroxene ± clinopyroxene ± amphibole + spinel ± plagioclase. During stage III, the remaining garnet was transformed to very fine-grained kelyphite consisting of orthopyroxene + amphibole + spinel ± plagioclase. Small relict garnet grains are preserved in one peridotite only. Stage IV corresponds to the late formation of tremolitic hornblende and chlorite which partially replaced the pseudomorphs after garnet or occur along cracks in the matrix of some rocks. Compositional zoning patterns of pyroxene porphyroclasts suggest that initial decompression was either accompanied by a moderate increase in temperature or nearly isothermal. Garnet breakdown textures and compositions of minerals grown during stages II and III also suggest rapid decompression at still elevated temperatures (1000-720°C).

Relict granulites in the Ross orogen of northern Victoria Land (Antarctica), I. Field occurrence, petrography and metamorphic evolution

Granulite-facies rocks occur as mappable relics in the Wilson Terrane (northern Victoria Land), the lithotectonic unit of the Cambro—Ordovician Ross orogen closest to the East Antarctic craton. Despite the widespread amphibolite-facies overprinting of Ross age, large-scale low-strain tectonic lozenges preserve a layered unit of felsic and quartz-poor garnet + orthopyroxene ± cordierite metasedimentary granulite, with minor, metre-thick, layers of mafic two-pyroxene granulite and rare lenses of marble. Large bodies of massive enderbite are also present and locally show discordant, intrusive contacts with respect to the layered metasedimentary sequence. Mineral assemblages, reaction textures and geothermobarometric estimates in granulite rocks point to a pre-Ross decompressional evolution from higher-pressure (P = 7.7 ± 0.7 kbar at T = 820 ± 100°C) to lower-pressure (6.3 ± 0.4 kbar and 830 ± 50°C) granulite-facies conditions. Geological and petrological data suggest that the granulite-facies rocks of the Wilson Terrane form a distinct tectonometamorphic unit very similar to other Neoproterozoic granulite-facies terrains of the East Antarctic craton. In this aspect, the occurrence of granulite-rocks within the Transantarctic Mountains strongly suggests the reactivation of the palaeo-Pacific margin of East Antarctica during the Ross orogeny.

Relict granulites in the Ross orogen of northern Victoria Land (Antarctica), I. Field occurrence, petrography and metamorphic evolution

Granulite-facies rocks occur as mappable relics in the Wilson Terrane (northern Victoria Land), the lithotectonic unit of the Cambro—Ordovician Ross orogen closest to the East Antarctic craton. Despite the widespread amphibolite-facies overprinting of Ross age, large-scale low-strain tectonic lozenges preserve a layered unit of felsic and quartz-poor garnet + orthopyroxene ± cordierite metasedimentary granulite, with minor, metre-thick, layers of mafic two-pyroxene granulite and rare lenses of marble. Large bodies of massive enderbite are also present and locally show discordant, intrusive contacts with respect to the layered metasedimentary sequence. Mineral assemblages, reaction textures and geothermobarometric estimates in granulite rocks point to a pre-Ross decompressional evolution from higher-pressure ( P = 7.7 ± 0.7 kbar at T = 820 ± 100°C) to lower-pressure (6.3 ± 0.4 kbar and 830 ± 50°C) granulite-facies conditions. Geological and petrological data suggest that the granulite-facies rocks of the Wilson Terrane form a distinct tectonometamorphic unit very similar to other Neoproterozoic granulite-facies terrains of the East Antarctic craton. In this aspect, the occurrence of granulite-rocks within the Transantarctic Mountains strongly suggests the reactivation of the palaeo-Pacific margin of East Antarctica during the Ross orogeny.

Extensional ductile tectonics of the Sioule metamorphic series (Variscan French Massif Central)

In the Northern part of the Variscan French Massif Central, the Sioule series, from top to bottom, consists of a pre-Visean granite, migmatite, gneiss and mica schist. Two ductile deformations have been recognized. The earlier phase is characterized by a north-east-south-west trending stretching lineation; the second phase, characterized by a north-west-south-east trending mineral, stretching and crenulation lineation, is better marked in the lower mica schist part than in the upper granito-gneissic part. This second phase occurred during retrogression of the metamorphic rocks; related shear criteria indicate a top to the south-west shear. The Namurian-Westphalian magmatic bodies such as the Echassieres leucogranite, Pouzol-Servant microgranite and numerous north-east -south-west trending microgranite dykes are emplaced in extensional fractures related to the same north-west-south-east maximum stretching direction. The asymmetrical shapes of the two granitic massifs indicate that they intruded towards the south-east. The synkinematic retrogression of the metamorphic rocks, the shape of the magmatic bodies and a re-examination of the numerous available data support the interpretation that the deformation is due to the extensional tectonic regime related to the Variscan crustal re-quilibration. This interpretation is in agreement with the correlation of the Sioule series with the Chavanon series. The two series belong to a unique tectono-metamorphic unit left-laterally offset by the Stephanian motion of the Sillon Houiller fault. This study also shows that the Sillon Houiller did not play a significant part during the Namurian-Westphalian extensional tectonics of the Massif Central.

Pretectonic tholeiitic volcanism and related transitional plutonism in the Kidal assemblage (Iforas Pan-African Belt, Mali)

The Kidal assemblage corresponds to a high-grade tectonometamorphic unit situated beneath the IGU granulitic nappe. Pretectonic magmatic rocks of intermediate to ultrabasic compositions form more than 60% of the volume and were intensely foliated during the D1 event. Four groups have been defined: (i) alkaline leptynites associated with aluminous quartzites and dated at 1837 + 17/−9 Ma (Caby and Andreopoulous-Renaud 1983, J. Afr. Earth Sci. 1, 193–197); (ii) metadiorites-metatonalite plutons, far the most abundant; (iii) amphibolite bands, locally associated with metaconglomerates (O. Bourhessa), lying directly upon the quartzites and marbles attributed to the Upper Proterozoic; (iv) narrow lenes and large bodies of metagabbros associated with metaperidotites and some meta-anorthosites. The amphibolites show T. MORB affinities (low alkali and incompatible element contents and almost flat REE patterns), and this suggests a spreading environment. The gabbroic suite magmas, with various cumulates, are distributed between three end-members (olivine peridotites, pyroxenites, anorthosites). The fine grained gabbros show a close chemical similarity with the amphibolites and this suggests a direct genetic relationship between the two suites. The chemical characters of the metadiorite-metatonalite suite conform to typical diorite-tonalite-trondhjemite continental suites in a spreading environment. The magmatic units of the Kidal assemblage show significant lithological and geochemical similarities with the supracrustal basic volcanics and intrusive complexes of Archean high-grade belts. The postulated extensional behaviour of the Kidal assemblage may be considered to have indirectly induced the high-strain and dominantly rotational tectonic regime evidenced in this domain.