Timing Of High-pressure Metamorphism Research Articles

Constraints on the thermal evolution of metamorphic core complexes from the timing of high-pressure metamorphism on Naxos, Greece

Metamorphic core complexes are classically interpreted to have formed during crustal extension, although many also occur in compressional environments. New U−(Th)−Pb allanite and xenotime geochronologic data from the structurally highest Zas Unit (Cycladic Blueschist Unit) of the Naxos metamorphic core complex, Greece, integrated with pressure−temperature−time (P−T−t) histories, are incorporated into a thermal model to test the role of crustal thickening and extension in forming metamorphic core complexes. Metamorphism on Naxos is diachronous, with peak metamorphic conditions propagating down structural section over a ∼30−35 m.y. period, from ca. 50 Ma to 15 Ma. At the highest structural level, the Zas Unit records blueschist-facies metamorphism (∼14.5−19 kbar, 470−570 °C) at ca. 50 Ma, during northeast-directed subduction of the Adriatic continental margin. The Zas Unit was subsequently extruded toward the SW and thrust over more proximal continental margin and basement rocks (Koronos and Core units). This contractional episode resulted in crustal thickening and Barrovian metamorphism from ca. 40 Ma and reached peak kyanite-sillimanite−grade conditions of ∼10−5 kbar and 600−730 °C at 20−15 Ma. Model P−T−t paths, assuming conductive relaxation of isotherms following overthrusting, are consistent with the clockwise P−T−t evolution. In contrast, extension results in exhumation and cooling of the crust, which is inconsistent with key components of the thermal evolution. Barrovian metamorphism on Naxos is therefore interpreted to have resulted from crustal thickening over a ∼30−35 m.y. time period prior to extension, normal faulting, and rapid exhumation after a thermal climax at ca. 15 Ma.

Geochemistry, Lu–Hf garnet ages, and P–T conditions of blueschists from the Meliatic and Fatric nappes, Western Carpathians: Indicators of Neotethyan subduction

Basaltic rocks from ophiolitic mélanges provide information on geodynamic setting, origin, and later tectonometamorphic conditions. This paper resolves the P–T conditions and timing of high-pressure metamorphism in an accretionary wedge that formed during the Late Jurassic closure of the Neotethyan Meliata Basin. Blueschist-facies metabasites of the Meliatic Bôrka Nappe and the Albian conglomerate pebbles of the Fatric Klape Nappe contain rare assemblages of garnet in association with glaucophane, phengite, rutile, and epidote (±albite). Here, we compare a Lu–Hf garnet age from the Meliatic Bôrka Nappe in the southern margin of the Inner Western Carpathians (IWC) with a garnet age from inferred Meliatic blueschists of the Klape Nappe overlying the IWC northern margin. The Hačava type locality of the Bôrka Nappe hosts calc-alkaline type metabasite (∼VAB-C; εNd(245) = +0.9) embedded within Middle to Upper Triassic marbles of the Neotethyan Meliata Basin northern continental margin. The investigated Klape Nappe island arc tholeiite (∼VAB-T; εNd(240) = +5.9) blueschist pebble, and associated deepwater metasedimentary rock pebbles were found in a conglomerate layer of unmetamorphosed Albian flysch. The garnet ages of 153.95±0.69 Ma and 152.1±1.5 Ma correspond to closure of the Meliata Basin during southward intraoceanic and continental margin subduction. This was followed by the exhumation of HP blocks in serpentinite mélange and the formation of an accretionary wedge with included anchimetamorphosed Jurassic flysch. The P–T conditions of the blueschists were constrained by Perple_X modelling to be 520°C and 1.55 GPa for the Bôrka Nappe, and 490–510°C and 1.68–1.72 GPa for a pebble from the Klape Nappe conglomerate. The similarity of garnet dates and metamorphic conditions between the two samples suggest that the blueschists formed during the Late Jurassic Meliatic subduction. The north-vergent Meliatic nappes are the inferred source of the Albian flysch conglomerates deposited in the foreland Fatric Basin. This material was subsequently transported by the Fatric Klape Nappe to the IWC orogenic front during the Turonian, following the closure of the Fatric Basin.

Timing and conditions of high-pressure metamorphism in the western Grenville Province: Constraints from accessory mineral composition and phase equilibrium modeling

Previous geochronological analyses of high pressure (HP) metamorphic rocks in the western Grenville Province, Ontario, Canada have yielded precise U–Pb zircon ages; however, uncertainty has remained as to whether these ages represent the timing of HP metamorphism or the granulite/amphibolite facies overprint accompanying exhumation to a hot middle orogenic crust. Detailed study of these HP rocks, involving garnet, rutile, and zircon trace element analysis, phase equilibrium modeling, and zircon U–Pb geochronology, has yielded much improved constraints on the timing and conditions of HP metamorphism. Zircon from five of the six HP samples yield anchored discordia upper intercept and 207Pb/206Pb weighted average ages between 1097 and 1085 Ma, and typically have trace element compositions consistent with growth in a garnet-rich, plagioclase-poor eclogite-type assemblage (i.e. no negative Eu anomaly and flat HREE trends). Titanium-in-zircon and Zr-in-rutile thermometry indicates that the range of zircon crystallization temperatures for most samples (643–767 °C) is close to that of rutile inclusions in garnet (668–753 °C) and matrix rutile (690–772 °C). Phase relations in a pseudosection calculated for the sample that best preserves the HP assemblage indicate that: (1) the stability field for the inclusions observed in garnet and kyanite is between 11.5 < P < 14 kbar and 600 < T < 700 °C, and (2) zircon and rutile crystallization temperatures intersect the inferred HP assemblage field (Grt + Cpx + Ky + Rt + Hbl + Qtz) and garnet and kyanite modal isopleths at P > ~ 15 kbar, indicating that the ca. 1090 Ma zircon ages date metamorphism at eclogite facies conditions. Thus, the deep burial of mafic lower crust that resulted in HP metamorphism in the western CGB occurred just prior to the main “Ottawan” phase of continental collision in the western Grenville Province (ca. 1080–1040 Ma).

A major synmetamorphic Early Devonian thrust and extensional fault system in the Mid Norway Caledonides: relevance to exhumation of HP and UHP rocks

Abstract The study of complex orogenic belts commonly begins in the frontal regions with well-defined tectonostratigraphy, and relatively simple structure and metamorphism, and proceeds into the progressively more complex hinterland, which nevertheless may contain the best geochronological record of the most intense orogenic events. The northern part of the Western Gneiss Region in the hinterland of the Scandian orogen contains a robust U–Pb zircon geochronological framework on rocks subjected to high-pressure (HP) and ultra-high-pressure (UHP) metamorphism that has implications for tectonic development both there and towards the foreland. HP and UHP eclogite crystallization occurred at 415–410 Ma (Early Devonian, Lochkovian to Pragian), followed by pegmatite crystallization at c. 395 Ma (Late Emsian) during exhumation and return to amphibolite-facies conditions, thus limiting the process to 15–20 myr. The nature and sequence of events are much more complex than in the foreland, causing difficulty in correlation, yet the combined tectonics in the two regions provides the necessary context to explain, for example, how rocks were subjected to deep-seated, high-temperature metamorphism and then exhumed to shallower levels. Here, we suggest how a recently recognized extensional detachment fault and a recently recognized out-of-sequence thrust might be linked to the timing of HP metamorphism and later exhumation. The postulated Agdenes extensional detachment in its footwall has basement gneisses containing Mesoproterozoic igneous titanite fully reset at 395 Ma, as well as Devonian pegmatites, and in the hanging wall Ordovician to Early Silurian granitoids of the Støren Nappe containing igneous titanite barely influenced by Devonian recrystallization and no evidence of post-Ordovician melts. This implies removal of a significant crustal section on a large-scale detachment. Rocks both above and below are overprinted by the same late, subhorizontal, sinistral ductile extensional fabric, obscuring any fabrics produced during development of the detachment itself. Eastern Trollheimen escaped the late, strong, subhorizontal overprint, and shows: (1) early emplacement of thrust nappes of Lower and Middle Allochthons over Baltican basement and its Late Neoproterozoic quartzite cover; (2) major, SE-directed, recumbent folding of the entire thrust-imbricated sequence; and (3) major, out-of-sequence, SE-directed thrusting (Storli Thrust), for an 80 km minimum transport across-strike, of the recumbent-folded sequence over deeper, less deformed, lower basement gneisses and unconformable Neoproterozoic quartzite cover. The upper basement contains boudins of eclogite and garnet-corona gabbro lacking in the lower basement. Similar basement imbrications occurred in the Tømmerås window, the Skarddøra Antiform, the Mullfjället Antiform and the Grong–Olden Culmination, up to 240 km NE of Trollheimen, as well as in the Reksdalshesten antiform 100 km west, all within the postulated minimum 400×180 km area of the Agdenes detachment.

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.

Eclogitization and exhumation of Caledonian continental basement in Lofoten, North Norway

U-Pb and 40 Ar/ 39 Ar isotopic data combine with structural and petrological information to allow insights into the timing of Caledonian tectonic burial and exhumation of lower crustal rocks now exposed in the Lofoten Islands of North Norway (latitude 68° N). Severely retrogressed eclogites occur in rare lenses or, even more rarely, hydrated shear zones within Archaean and Proterozoic granulite-facies Baltic basement gneisses. The timing of high-pressure metamorphism in Lofoten has been difficult to determine because retrogression has disturbed mineral isotopic systems and zircon apparently was not generated, leaving its tectonic significance uncertain. Recently discovered pre- and post-kinematic felsic injections provide the opportunity to bracket the age of eclogitization. U/Pb analyses of zircon and xenotime from a prekinematic syenogranite dyke that cuts the mafic host to one of the retro-eclogite lenses intruded at 1800 ± 5 Ma, and we interpret the strong disturbance of its U-Pb system at 478 ± 41 Ma to approximate the age of eclogitization. Omphacite breakdown textures imply rapid isothermal decompression during initial uplift, followed by slow uplift. Syn-upper-amphibolite-facies thrust emplacement of the overlying Leknes Group at ca. 464 Ma implies that the Lofoten basement resided in the lower crust for ca. 14 m.y. 40 Ar/ 39 Ar cooling dates from the retro-eclogites trace their exhumation path into the middle crust (hornblende ca. 433 Ma) where they resided before being slowly elevated to levels at the base of the ductile-brittle transition in the early Carboniferous (muscovite ca. 343 Ma). The Middle Ordovician Lofoten eclogites likely formed at a time similar to the “early” group of Eocaledonian eclogites (ca. 505–450 Ma) found in the Tromso, Seve, and Bergen Arcs allochthons that were later thrust onto Baltica during the main Scandian (Siluro–Devonian) collision. Lofoten eclogites appear to be ca. 50 m.y. older than the “late” group of autochthonous, Scandian (ca. 425–400 Ma) high-pressure and ultrahigh-pressure (UHP) eclogites of the Western Gneiss Region (WGR), and the former preserve a much longer (ca. 100 m.y.) exhumation history. To date, there is no evidence to indicate either UHP or Scandian eclogite-facies metamorphism in Baltic basement along coastal Norway north of latitude 64° N. Middle Ordovician eclogites of Scandinavia are contemporaneous with Taconic eclogites found in thrust nappes of the Appalachian Orogen. If the Lofoten eclogites correlate to those in the Tromso Nappe Complex (Uppermost Allochthon), then both terranes might represent Laurentian relics emplaced during the Scandian and left orphaned on the conjugate side of the orogen when the North Atlantic began to open in the Eocene. Lithologic, petrologic, kinematic, provenance, and palinspastic information favor, however, correlation with eclogites in transitional Baltic-Iapetan crust of the Seve Nappe Complex (Upper Allochthon), which provides a piercing point linking eclogites in autochthonous Baltic crust now exposed in the most internal parts of the orogen with those transported and preserved in the Swedish foreland. We suggest a broad twofold subdivision for the Eocaledonian eclogite provinces into those that are (1) Baltic derived, and (2) exotic with respect to Baltica. Middle Ordovician eclogites falling under category (1) can further be subdivided into those occurring in autochthonous Baltic basement (i.e., Lofoten), and those in thrust translated terranes. Apparently, continental crust in a collisional setting can be subducted to mantle depths and show only very sparse evidence of this tectonic history.

The occurrence and timing of high-pressure metamorphism on Margarita Island, Venezuela: a constraint on Caribbean-South America interaction

Abstract The metamorphic rock sequences exposed on the Island of Margarita, Venezuela, located in the southeastern corner of the Caribbean Plate margin, are composed of a high-pressure/low-temperature (HP/LT) nucleus subducted to at least 50 km depth, now structurally overlain by lower-grade greenschist-facies units lacking any sign of high-pressure subduction-zone metamorphism. The HP/LT nucleus involves protoliths of both oceanic (metabasalts and intimately associated carbonaceous schists of the La Rinconada unit; peridotite massifs) and continental affinity (metapelites, marbles and gneisses of the Juan Griego unit). All HP/LT units were joined together prior to the peak of high-pressure metamorphism, as shown by their matching metamorphic pressure–temperature evolution. The metamorphic grade attained produced barroisite as the regional amphibole. Glaucophane is not known from Margarita. Contrary to a widely propagated assumption, there are no major nappe structures post-dating HP/LT metamorphism anywhere within the high-pressure nucleus of Margarita Island. U–Pb zircon dating of key tonalitic to granitic intrusive rocks provides the following constraints: (1) the Juan Griego unit is heterogeneous and contains Palaeozoic as well as probable Mesozoic protolith; (2) the peak of HP/LT metamorphism, that is maximum subduction, is younger than 116–106 Ma and older than 85 Ma, most probably c. 100–90 Ma, a time span during which the southeastern Caribbean/South American border was clearly a passive margin. The assembly of Margaritan protoliths and their HP/LT overprint occurred far to the west in northwestern South America, a scenario completely in accord with the details of the Pacific-origin model outlined by Pindell &amp; Kennan. Juxtaposition of the greenschist-facies units occurred after exhumation into mid-crustal levels after c. 80 Ma.

Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps

AbstractThe Schistes Lustrés (SL) suture zone occupies a key position in the Alpine chain between the high‐pressure (HP) Brianconnais domain and the ultrahigh‐pressure (UHP) Dora Maira massif, and reached subduction depths ranging from c. 40–65 km (Cottian Alps). In order to constrain the timing of HP metamorphism and subsequent exhumation, several phengite generations were differentiated, on the basis of habit, texture, paragenesis and chemistry, as belonging to the first or second exhumation episode, respectively, D2 or D3, or to earlier stages of the tectono‐metamorphic evolution. Ten carefully selected samples showing D2, D3 (D2 + D3), or earlier (mostly peak temperature) phengite population(s) were subjected to laser probe 40Ar/39Ar analysis. The data support the results of the petrostructural study with two distinct age groups (crystallization ages) for D2 and D3 phengite, at 51–45 and 38–35 Ma, respectively. The data also reveal a coherent age cluster, at 62–55 Ma, for peak temperature phengite associated with chloritoid which were preserved in low strain domains. The age of the D3 event in the SL complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs. Exhumation rates of the order of 1–2 mm yr−1 are obtained for the SL complex, which are compatible with velocities documented for accretionary wedge settings. Similarly, cooling velocities are only moderate (c.5 °C Myr−1), which is at variance with recent estimates in the nearby UHP massifs.

Early Tertiary eclogite facies metamorphism in the Monviso Ophiolite

Although crucial for the construction of tectonic models of the Alps the timing of high pressure metamorphism is still poorly determined and controversial. It is likely to vary from one tectonic unit to another depending on when each became involved in subduction. This in turn relates to palaeogeographic position with respect to the active ocean basin. Well defined, reliable geochronological data are too few to test this hypothesis. This paper extends the database by determining Sm–Nd mineral isochrons on two samples from the Monviso Ophiolite in the Piemonte Zone, carefully selected to minimize the problems of Sm–Nd dating of eclogites encountered elsewhere in the Alps. The dated samples have eclogite facies mineral assemblages typical of the Lago Superiore unit of the ophiolite; mineral compositions are similar to previously reported samples and indicate pressures of around 2 GPa and temperatures of 400–500 °C. Sm–Nd isochron ages of 60±12 and 62±9 Ma are defined by garnet and clinopyroxene, while the Rb–Sr age on phengite which is part of the high‐P assemblage is 40±1 Ma. The new data fit an emerging pattern of ages in which high‐P metamorphism in the oceanic realm is Early Tertiary, with slightly older ages in the overlying Sesia Zone and younger, Oligocene ages in the underlying internal basement massifs which only became involved in subduction when closure of the Piemont ocean was complete.

Timing of high-pressure metamorphism in the Yukon – Tanana terrane, Canadian Cordillera: constraints from U – Pb zircon dating of eclogite from the Teslin tectonic zone

Zircon from eclogite near Last Peak in the Teslin tectonic zone yielded a U–Pb isotopic age of 269 + 2 Ma (2σ), the first precise age for such a rock in the Yukon –Tanana terrane of the Canadian Cordillera. Both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin, and the U – Pb age therefore constrains the timing of peak high-pressure metamorphism in this rock. The U – Pb age demonstrates for the first time that an Early Permian high-pressure metamorphic event occurred in rocks now making up the Teslin tectonic zone, and possibly elsewhere in the Yukon – Tanana terrane. This U – Pb age provides a new geochronologic "pin" in the evolution of the Yukon – Tanana terrane prior to its Mesozoic accretion to the North American continental margin and, combined with recent 40Ar/39Ar muscovite data, indicates that high-pressure metamorphism at this time was a relatively short-lived event.