Blueschist-facies Units Research Articles

Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps)

ABSTRACT In the Valmala sector of the southern Dora Maira Massif (Western Alps), two different eclogite- and blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to point out that the tectonic slices within the Valmala Tectonic Unit consist of ‘native’ rock slices wrenched from the overlying Dronero Unit, and ‘exotic’ rocks likely sourced from other units of the Dora Maira and from a continental margin and an oceanic basin. On the contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit.

Fingerprints of forearc element mobility in blueschist-facies metaconglomerates, Catalina Schist, California

ABSTRACTMetaconglomerates in the lawsonite–blueschist facies unit of the Catalina Schist (California) contain gabbroic and dioritic clasts exhibiting evidence for extensive metasomatism during high-P/T metamorphism. We performed whole-rock and in situ analyses of these metaconglomerate clasts to better constrain the composition of infiltrating fluids and to elucidate the history of chemical alteration. Petrographic evidence for this alteration includes replacements of plagioclase by phengite and sodic amphibole rims developed on igneous hornblende. These observations regarding mineral replacement are reinforced by corresponding shifts in chemical compositions. Relative to compositions of presumed protoliths, whole-rock compositions of the metaconglomerate clasts show enrichments in elements that are relatively mobile in aqueous fluids (LILE: K, Rb, Cs, and Ba; Li, B, N), and elevated δ15N, and show depletions in Ca and Sr. Electron and ion microprobe data, and analyses of mineral separates, show that phengite and sodic amphibole are enriched in LILE and Li and B, respectively, relative to the igneous phases they have replaced. Oxygen and C isotope compositions of finely disseminated calcite in the clasts, and of calcite in veins cross-cutting or mantling the clasts, are consistent with crystallization from fluids previously equilibrated with metasedimentary rocks within the same unit. The same fluids are implicated as the source for the Li, B, N, and LILE enrichments. These metaconglomerate clasts provide unique records of forearc metasomatism due to the presumed extremely low and well-constrained concentrations of fluid-mobile elements in their protoliths and the previously published, larger-scale fluid–rock context into which the observed metasomatic changes can be placed.

Cold subduction of the Neotethys: the metamorphic record from finely banded lawsonite and epidote blueschists and associated metabasalts of the Nagaland Ophiolite Complex, India

AbstractIn this study, we have deduced the thermal history of the subducting Neotethys from its eastern margin, using a suite of partially hydrated metabasalts from a segment of the Nagaland Ophiolite Complex (NOC), India. Located along the eastern extension of the Indus‐Tsangpo suture zone (ITSZ), the N–S‐trending NOC lies between the Indian and Burmese plates. The metabasalts, encased within a serpentinitic mélange, preserve a tectonically disturbed metamorphic sequence, which from west to east is greenschist (GS), pumpellyite–diopside (PD) and blueschist (BS) facies. Metabasalts in all the three metamorphic facies record prograde metamorphic overprints directly on primary igneous textures and igneous augite. In the BS facies unit, the metabasalts interbedded with marble show centimetre‐ to metre‐scale interlayering of lawsonite blueschist (LBS) and epidote blueschist (EBS). Prograde HP/LT metamorphism stabilized lawsonite + omphacite (XJd = 0.50–0.56 to 0.26–0.37) + jadeite (XJd = 0.67–0.79) + augite + ferroglaucophane + high‐Si phengite (Si = 3.6–3.65 atoms per formula unit, a.p.f.u.) + chlorite + titanite + quartz in LBS and lawsonite + glaucophane/ferroglaucophane ± epidote ± omphacite (XJd = 0.34) + chlorite + phengite (Si = 3.5 a.p.f.u.) + titanite + quartz in EBS at the metamorphic peak. Retrograde alteration, which was pervasive in the EBS, produced a sequence of mineral assemblages from omphacite and lawsonite‐absent, epidote + glaucophane/ferroglaucophane + chlorite + phengite + titanite + quartz through albite + chlorite + glaucophane to lawsonite + albite + high‐Si phengite (Si = 3.6–3.7 a.p.f.u.) + glaucophane + epidote + quartz. In the PD facies metabasalts, the peak mineral assemblage, pumpellyite + chlorite + titanite + phengitic white mica (Si = 3.4–3.5 a.p.f.u.) + diopside appeared in the basaltic groundmass from reacting titaniferous augite and low‐Si phengite, with prehnite additionally producing pumpellyite in early vein domains. In the GS facies metabasalts, incomplete hydration of augite produced albite + epidote + actinolite + chlorite + titanite + phengite + augite mineral assemblage. Based on calculated T–M(H2O), T–M(O2) (where M represents oxide mol.%) and P–T pseudosections, peak P–T conditions of LBS are estimated at ~11.5 kbar and ~340 °C, EBS at ~10 kbar, 325 °C and PD facies at ~6 kbar, 335 °C. Reconstructed metamorphic reaction pathways integrated with the results of P–T pseudosection modelling define a near‐complete, hairpin, clockwise P–T loop for the BS and a prograde P–T path with a steep dP/dT for the PD facies rocks. Apparent low thermal gradient of 8 °C km−1 corresponding to a maximum burial depth of 40 km and the hairpin P–T trajectory together suggest a cold and mature stage of an intra‐oceanic subduction zone setting for the Nagaland blueschists. The metamorphic constraints established above when combined with petrological findings from the ophiolitic massifs along the whole ITSZ suggest that intra‐oceanic subduction systems within the Neotethys between India and the Lhasa terrane/the Karakoram microcontinent were also active towards east between Indian and Burmese plates.

Timing of HP metamorphism in the Schistes Lustrés of Alpine Corsica: New Lu–Hf garnet and lawsonite ages

Alpine Corsica is crucial in understanding Western Mediterranean geodynamics. Available paleotectonic models attribute the high-pressure terrains of Corsica to the Western Alps (east dipping subduction), or to the Apennines (west-dipping subduction) or to a flip from one to the other subduction zone. Constraints on the timing of high-pressure metamorphism in Alpine Corsica are sparse and ages range from Late Cretaceous to Late Eocene. This represents a major point of debate. Here we present high-precision garnet Lu–Hf ages for three samples from the highest grade, Lawsonite-eclogite unit yielding ages of 34.23±1.6Ma; 34.11±0.89Ma, 34.65±0.55Ma. In addition to the well-established garnet dating, we employed lawsonite Lu–Hf analyses on a lawsonite-bearing (garnet-free) metagabbro from the neighboring lawsonite-blueschist unit that yielded an age of 37.5±1.3Ma. Because metamorphic temperatures of the eclogite-facies samples (ca. 500–550°C) are below the Lu–Hf closure temperature in garnet and since the eclogitic garnet displays a core-to-rim increase in Mn concentrations typical of prograde zoning, we attribute the above Lu–Hf ages to prograde subduction metamorphism. Similarly, the age of lawsonite in the blueschist-facies unit can be safely referred to prograde-peak conditions based on the available clockwise metamorphic path. The slightly older age of the lawsonite-blueschist sample suggests a diachronous timing in the achievement of high-pressure conditions in the eclogite and in the blueschist facies units. Since both units are thought to derive from a former ocean–continent transition zone, our geochronological data provide precise time constraints to the subduction history of Alpine Corsica in the frame of the Western Mediterranean geodynamics. Our data provide Late Eocene ages for the subduction event in Corsica, which are significantly younger than previously published Cretaceous ages. This calls for partial revision of the current evolutionary models on the Corsican segment of the Alpine belt.

Thermochronological constraints on two-stage extrusion of HP/UHP terranes in the Dabie–Sulu orogen, east-central China

Along the Qinling–Dabie–Sulu orogenic belt in China crops out the world's largest terrane composed of ultrahigh-pressure (UHP) metamorphic rocks. Differences in the timing and mechanisms of oceanic and continental subductions are assumed to be responsible for different ages of high-pressure (HP) and UHP slices in different parts of the belt. The western part of the Dabie orogen (western Dabie terrane) holds a key to understanding of the transition from oceanic to continental subduction. This paper reports geochronological results to test a two-stage tectonic model for the exhumation of HP/UHP rocks in western Dabie. This model involves two different stages and types of extrusion for exhumation of the HP/UHP rocks in east-central China. Mica Ar/Ar ages, ranging from 241 to 231 Ma, indicate a general middle Triassic cooling probably driven by early upward extrusion during the collision between the North and South China Blocks. Late Triassic–Early Jurassic cooling was associated with later eastward extrusion, ranging from 200 to 184 Ma. The second event is recorded also in mica in the region that was not affected by later deformation and magmatism. The lateral movement along lithosphere-scale faults resulted in the eastward extrusion of the HP–UHP metamorphic terrane, which was followed, in the Late Triassic–Early Jurassic time, by a major compressive event. These two extrusion events are correlative with the two stages of Triassic exhumation of the western Dabie HP–UHP rocks, respectively. Wintin the framework of the Qinling–Dabie–Sulu orogenic belt, it is suggested for western Dabie that the subduction/exhumation of blueschist-facies unit is related to the Mianlue suture, whereas the subduction/exhumation of HP/UHP eclogite-facies units is related to the Shangdan suture.

Rb–Sr and 40Ar/ 39Ar ages in blueschists from the Ile de Groix (Armorican Massif, France): Implications for closure mechanisms in isotopic systems

Rb–Sr and 40Ar/ 39Ar data mainly ranging from 345 ± 5 to 361 ± 5 Ma and from 348.4 ± 1.4 to 364.6 ± 1.4 Ma, respectively, have been obtained on phengite from the HP/LT rocks from the Ile de Groix (Armorican Massif, France), which represents one of the few occurrences of Variscan blueschists. Petrological data show two blueschist-facies units, recording distinct peak P– T conditions (about 16–20 kbar, 500 °C and 12–16 kbar, 450 °C in the upper and lower units, respectively) followed by a nearly isothermal decompression in the greenschist facies. Rb–Sr and 40Ar/ 39Ar results, defining two groups of ages mainly around 345–353 Ma and 358–366 Ma, are consistent in well-preserved blueschist-facies rocks as well as in strongly overprinted samples from both units. One sample excepted, there is no evidence of inherited or excess argon and resetting of the isotopic systems. Oldest Rb–Sr and 40Ar/ 39Ar ages are found in rocks with well-preserved blueschist-facies parageneses, while younger ages are obtained on samples with a strong greenschist-facies overprint. This suggests that the closure of the K–Ar system is diachronous in samples, which record the same P– T evolution. A closure of the K–Ar clock at around 450–500 °C in phengite from rocks left nearly undeformed after the blueschist-facies metamorphism is in agreement with petrological, geochemical and structural data. Thus, in these samples, the oldest Rb–Sr and 40Ar/ 39Ar ages (around 360–370 Ma) represent the age of the HP metamorphism. In the overprinted samples, later closure of the K–Ar and Rb–Sr clocks is the result of retrogression and continuing ductile deformation. The small age difference between the oldest and the youngest Rb–Sr and 40Ar/ 39Ar ages (around 10–20 Ma) argues in favour of a relatively fast cooling.

Recent advances in Alpine studies: tracking the Caledonian–Variscan belt in the internal western Alps

Recent advances in western Alps knowledge haveconcerned several key geodynamic issues. The deepstructure of the belt, and then the implication ofthe lower crust and lithospheric mantle in the colli-sional deformation were recognized through seismic–reflection and tomographic studies. The origin ofthe Piedmont–Ligurian Ocean and adjacent palaeo-margins was thoroughly elaborated through compar-ison with central Atlantic and the Galicia margin(it is worth to point here another striking similarity,i.e. with the oceanic margin of southwestern Aus-tralia [4]). A wealth of field and laboratory stud-ies have been devoted to the Alpine synkinematicmetamorphism, and particularly to the early, high-pressure–low-temperature (HP–LT) metamorphism,which increases from the Fe–Mg carpholite-bearing,low-grade blueschist-facies units of internal Briancon-nais, to the eclogite and ultra-high pressure (UHP)units of the internal massifs (IM) and overthrust-ing ophiolites [9,22 (and references therein)]. Thepeak pressure mineral assemblages from the ophio-lites and IM are now clearly dated from the Earlyto Late Eocene. Kinematic and thermal models havebeen coined taking into account the chemical compo-sition of the crust [14]. The HP–UHP metamorphism

Pre-collisional ductile extension in the internal western Alps (Sesia zone, Italy)

Early Alpine eclogites in the western Alps were formed in the Cretaceous prior to the main stage of continental collision in the Eocene. These early Alpine eclogites were exhumed to shallow crustal levels by the end of the Cretaceous, but Mesozoic (pre-collisional) extensional structures, required to unroof these rocks, are not common. Most of the extensional structures described in the western Alps are younger, they postdate Eocene continental collision or are coeval with it and may have helped the unroofing of the usually lower grade, collision-related, blueschist facies rocks. The Sesia zone, at the top of the western Alps structural pile, is a slice of a polymetamorphosed continental basement partly dominated by Eoalpine (130-110 Ma) eclogite facies mineral assemblages. The radiometric data show that these eclogites cooled below ca. 350°C at 60 Ma, so that the major part of the orogenic cycle, including burial, high-pressure metamorphism, cooling and decompression of this unit, was completed prior to continental collision. The present study analyses a NW-dipping, 2 km thick zone of medium- P ductile deformation that divides the Sesia zone into a lower, eclogite-bearing (EMS) unit and an upper blueschist facies unit (II DK). Ductile deformation was mainly confined to the footwall, EMS, whereas the hangingwall, II DK, preserves pre-Alpine structures and mineralogy and remained rigid. Rare eclogite facies nodules occur in this zone of ductile deformation, but medium- P dynamic recrystallization in the stability field of albite is predominant. Mineral chemistry and geothermobarometry of a metaquartz diorite, that occupies a significant part of this deformation zone, show that the ductile fabric in this rock formed at estimated P-T conditions of 8–10 kbar and 500°C. ‘S C’ fabric in the vicinity of the EMS-II DK contact indicates a top-to-the-WNW sense of motion, which is down-dip on the NW-dipping foliation in the region, and extensional with respect to the orientation of the NW-dipping tectonic contact. The interpretation of the medium- P fabric in the EMS as extensional is supported also by the fact that the hangingwall II DK has lower (Alpine) P-T. A WNW-directed Cretaceous extension of the type described in the present work may have dispersed the overthickened Adriatic margin towards the still open Piemont ocean. The later was then sufficiently large to provide the space which is required if the exhumation of deeply buried rocks is a result of horizontal spreading of orogenic wedges. However, the scale of the extensional deformation deduced from the metamorphic discontinuity across the II DK-EMS contact (ca. 2 kbar,) is certainly insufficent to account for the entire vertical motion implied by the emergence of the Sesia eclogites.

Compression versus extension in the exhumation of the Dora-Maira coesite-bearing unit, Western Alps, Italy

Several recent papers suggest that uplift and exhumation of high-pressure, low-temperature (HP-LT) metamorphic rocks are due to crustal extension, coeval with and/or post-dating the contraction responsible for the metamorphism. A detailed survey of the coesite-bearing unit of the southern Dora-Maira massif offers an opportunity to discuss the relative importance of extension and compression in the exhumation tectonics of ultra-high-pressure (UHP) rocks. This UHP unit consists of pre-Alpine basement rocks, which equilibrated at a depth of 100 km, probably 120-100 Ma ago. It is overlain by two units with a similar lithology and Eoalpine metamorphic age and high P/T gradients, which equilibrated at 60 and 50 km, respectively. This Eoalpine basement complex is intercalated between blueschist facies units. The whole pile was affected by a Mesoalpine greenschist facies mylonitic deformation, during which the early contacts were reactivated as low-angle shallow-dipping shear zones. Extension is documented only during the Mesoalpine collisional period, and is referred to the gravity collapse of a thickened orogenic wedge, with an exhumation rate of about 1 mm/yr. The most important part of the exhumation of the UHP rocks occurred earlier, at a higher rate, close to 4 mm/yr. A forced return flow, or extrusion tectonics of imbricate slices, in an on-going subduction setting seems appropriate to account for this early stage. The fact that the omission of intermediate pressure rocks between juxtaposed units is an unreliable criterion for demonstrating extension is discussed.