Blueschist-facies Rocks Research Articles

Eclogite-, amphibolite- and blueschist-facies rocks from Diego de Almagro Island (Patagonia): Episodic accretion and thermal evolution of the Chilean subduction interface during the Cretaceous

Few localities in the Patagonian Andes expose remnants of the Mesozoic Chilean paleo-accretionary complex. We focus on the Diego de Almagro Island high-pressure/low-temperature (HP/LT) Complex, a pluri-kilometer thick sequence comprising metavolcanic rocks with oceanic affinities and metasedimentary rocks. In this study, the deepest segments of the Chilean subduction interface in Patagonia are characterized for the first time. Despite its apparent homogeneity, the complex is actually composed of two tectonic units with distinct ages of metamorphism and thermal evolution: the garnet amphibolite (GA) and the underlying blueschist (BS) units.The GA unit mafic rocks exhibit epidote, phengite, titanite, rutile, chloritoid and paragonite inclusions in prograde garnet I, diopside + albite intergrows replacing omphacite inclusions in garnet II, and relict omphacite (XJd45) included in edenitic-pargasitic amphiboles. Thermobarometric results show that these rocks were buried along a relatively cold prograde path (c. 11°C/km) and reached eclogite-facies near peak pressure conditions (c. 550–600°C, 1.6 GPa). The GA unit underwent a pervasive stage of amphibolitization during decompression at c. 1.3 GPa. Field and petrological observations, together with multi-mineral Rb-Sr dating, indicate that amphibolitization of the GA unit took place along the subduction interface at c. 120Ma in a slightly warmer subduction regime (c. 13–14°C/km), in agreeement with the formation of coetanoeus amphibolites at c. 35km. The underlying BS unit (i) yields four consistent Rb-Sr deformation ages of c. 80Ma, i.e. 40Ma younger than the overlying rocks from the GA unit; (ii) exhibits slightly cooler peak metamorphic conditions (c. 520–550°C, 1.6 GPa) indicating burial along a prograde path of c. 10°C/km (iii) does not show amphibolite-facies overprint as seen in the GA unit. After a long residence time under amphibolite-facies conditions, the amphibolitized rocks of the GA unit partly recrystallized during blueschist-facies deformation at around 80Ma, as shown by silica-richer rims around phengite and glaucophane overgrowths around hornblende.These new results: (i) shed light on the dynamics of the deep Chilean paleo-accretionary wedge, (ii) reveal that the GA unit underwent two stages of exhumation, likely juxtaposed with the BS unit near 40km depth, and (iii) suggest that the subduction thermal gradient on Diego de Almagro Island fluctuated between c. 10–11°C/km and 13–14°C/km during Cretaceous times.

Internal geometry of the central Sesia Zone (Aosta Valley, Italy): HP tectonic assembly of continental slices

Detailed field mapping reveals that the Sesia Zone is subdivided into two complexes with the Barmet Shear Zone (BSZ) outlining the tectonic contact between them. This greenschist-facies contact reflects a metamorphic gap between the Internal Complex (eclogite facies, eclogitic micaschists dominant) and the External Complex (epidote blueschist facies). The BSZ comprises a wedge shape area in which fragments and slices of orthogneiss and paragneiss are wrapped by siliceous dolomite marbles displaying a mylonitic foliation. Conspicuous cornieules and high pressure breccias occur along this contact. We propose that the eclogite facies Internal Complex is subdivided into three basement units, called sheets, delimited by discontinuous metasedimentary trails of probable Mesozoic age. Thin monocyclic bands thus separate kilometre scale polycyclic sheets. The External Complex comprises three epidote blueschist facies sheets of comparable size, which are separated by lenses retaining a pre-Alpine high temperature imprint. These weakly overprinted fragments (parts of the classically termed 2DK zone) are aligned along greenschist facies shear zones that separate the gneissic sheets. The BSZ, with a wedge rich in meta-sediments, chiefly siliceous dolomite marbles, is a key element in which fragmentation and reworking of materials from the internal and external complexes are evident. A carbonate breccia occurs in this shear zone, with clasts displaying a HP foliation randomly oriented in a ductile carbonate matrix. Siliceous dolomite marbles appear to have acted as lubricants to accommodate deformation related to the juxtaposition of the two basement complexes during exhumation. We propose a model of the Sesia Zone, with the BSZ as the thrust responsible for the juxtaposition of eclogite facies rocks of the Internal Complex on top of epidote blueschist facies rocks of the External Complex. The two complexes were already assembled when this shear zone became active. The entire stack was finally rotated (40–60°) during the Vanzone Phase.

Detrital zircon ages indicate an Early Cretaceous episode of blueschist-facies metamorphism in southern Alaska: Implications for the Mesozoic paleogeography of the northern Cordillera

Detrital zircon U-Pb ages are presented from the Liberty Creek schist in the central Chugach Mountains that indicate two distinct periods of preservation of blueschist-facies metamorphism along the southern Alaskan margin. A maximum depositional age (MDA) of 136 Ma demonstrates that the Liberty Creek schist was deposited long after the Early Jurassic cooling ages (196–185 Ma) recorded in other western Alaskan schist bodies containing blueschist-facies rocks, thus revealing two distinct blueschist-facies preservation events: an Early Jurassic event and a post–Early Cretaceous event. This Early Cretaceous depositional age also indicates that there have been major reorganizations within this subduction complex because the Potter Creek assemblage (MDA of 169–156 Ma), directly south of the Liberty Creek schist, is an older but more shallowly exhumed assemblage. Strike-slip faulting has rearranged the accretionary complex by carrying the Potter Creek assemblage outboard and south of the Liberty Creek schist. The predominance of 140–130 Ma zircons in the Liberty Creek schist sample and a population of detrital zircons that is distinct from nearby terranes suggest a sedimentary source different from other related accretionary assemblages. Three suggested Cordilleran source terranes are the Chitina Valley batholith immediately to the east; the Firvale suite of the Coast plutonic complex, ∼1500 km to the southeast near Vancouver, British Columbia; or our preferred source, the southern Mexican Guerrero terrane, ∼3000 km to the southeast. The detrital zircon signature of the Liberty Creek schist and these distances to potential sources support models suggesting thousands of kilometers of strike-slip movement along the western Cordillera since Cretaceous time, consistent with the Baja–British Columbia hypothesis.

Proto-Japan and tectonic erosion: Evidence from zircon geochronology of blueschist and serpentinite

Blueschist facies rocks in the Shikoku island of southwest Japan were extruded from depth as blocks in the Kurosegawa serpentinite melange, where they occur in association with other tectonic blocks of tonalite-trondhjemite-granodiorite (TTG) rocks and calc-alkaline volcanics. Here we report magmatic zircons in the blueschist that yield 206Pb/238U mean ages in the range 505 ± 3 to 503 ± 3 Ma. These zircons crystallized at a mid-ocean ridge within basaltic rocks, and migrated to the continental margin of paleo-Asia. Subduction of the oceanic lithosphere into the mantle at a depth of ∼40 km resulted in metamorphism under jadeite-glaucophane facies conditions at 250 Ma. The blueschist facies rocks were then exhumed to the surface along a splay fault connected with the forearc region through tectonic extrusion. Zircons separated from serpentinite yielded a 206Pb/238U mean age of 152 ± 3 Ma, marking the timing of serpentine protrusion to the surface, consistent with geologic constraints. The serpentinites also contain minor ca. 500 Ma zircons, similar to those in the blueschist. The predominance of detrital ca. 150 Ma zircons in the serpentinite suggests extensive tectonic mixing of Jurassic trench turbidites with the mantle wedge. The Kurosegawa serpentinite melange belt extends along the strike of the orogenic belt for more than 800 km, parallel to the active trench. We propose that this belt marks the location of extensive tectonic erosion and provides an excellent case study for the destruction of continental crust along a convergent plate boundary.

Tectonometamorphic evolution of blueschist-facies rocks in the Phyllite-Quartzite Unit of the External Hellenides (Mani, Greece)

Tectonometamorphic evolution of blueschist-facies rocks in the Phyllite-Quartzite Unit of the External Hellenides (Mani, Greece)

Petrofabrics of high-pressure rocks exhumed at the slab-mantle interface from the “point of no return” in a subduction zone (Sivrihisar, Turkey)

The highest pressure recorded by metamorphic rocks exhumed from oceanic subduction zones is ~2.5 GPa, corresponding to the maximum decoupling depth (MDD) (80 ± 10 km) identified in active subduction zones; beyond the MDD (the “point of no return”) exhumation is unlikely. The Sivrihisar massif (Turkey) is a coherent terrane of lawsonite eclogite and blueschist facies rocks in which assemblages and fabrics record P-T-fluid-deformation conditions during exhumation from ~80 to 45 km. Crystallographic fabrics and other features of high-pressure metasedimentary and metabasaltic rocks record transitions during exhumation. In quartzite, microstructures and crystallographic fabrics record deformation in the dislocation creep regime, including dynamic recrystallization during decompression, and a transition from prism slip to activation of rhomb and basal slip that may be related to a decrease in water fugacity during decompression (~2.5 to ~1.5 GPa). Phengite, lawsonite, and omphacite or glaucophane in quartzite and metabasalt remained stable during deformation, and omphacite developed an L-type crystallographic fabric. In marble, aragonite developed columnar textures with strong crystallographic fabrics that persisted during partial to complete dynamic recrystallization that was likely achieved in the stability field of aragonite (P > ~1.2 GPa). Results of kinematic vorticity analysis based on lawsonite shape fabrics are consistent with shear criteria in quartzite and metabasalt and indicate a large component of coaxial deformation in the exhuming channel beneath a simple shear dominated interface. This large coaxial component may have multiplied the exhuming power of the subduction channel and forced deeply subducted rocks to flow back from the point of no return.

Late Cretaceous eclogitic high-pressure relics in the Bitlis Massif

A new occurrence of eclogites was found in the Kesandere valley in the eastern most part of the Bitlis complex, SE Anatolia. These high-pressure (HP) relics were preserved in calc-arenitic metasediments within the high-grade metamorphic basement of the Bitlis complex. The eclogitic parageneses were strongly overprinted during decompression and heating. These new eclogites locality complements the evidence of blueschist-facies metamorphism documented recently in the meta-sedimentary cover sequence of this part of the Bitlis complex. Thermodynamic calculations suggest peak conditions of ca. 480–540 °C/1.9–2.4 GPa. New U/Pb dates of 84.4 ± .9 and 82.4 ± .9 Ma were obtained on zircons from two Kesandere eclogite samples. On the basis of geochemical criteria, these dates are interpreted to represent zircon crystallization during the eclogitic peak stage. Kesandere eclogites differ from those previously described in the western Bitlis complex (Mt. Gablor locality) in terms of lithologic association, protolithic origin, and peak P–T conditions (600–650 °C/1.0–2.0 GPa, respectively). On the other hand, eclogitic metamorphism of Kesandere metasediments occurred shortly before blueschist-facies metamorphism of the sedimentary cover (79–74 Ma 40Ar/39Ar white mica). Therefore, the exhumation of Kesandere eclogites started between ca. 82 and 79 Ma, while the meta-sedimentary cover was being buried. During this short time span, Kesandere eclogite were likely uplifted from ~65 to 35 km depth, indicating a syn-subduction exhumation rate of ~4.3 mm/a. Subsequently, eclogite- and blueschist-facies rocks were likely retrogressed contemporarily during collision-type metamorphism (around 72–69 Ma). The Bitlis HP rocks thus sample a subduction zone that separated the Bitlis–Pütürge (Bistun?) block from the South-Armenian block, further north. To the south, Eocene metasediments of the Urse formation are imbricated below the Bitlis complex. They contain (post Eocene) blueschists, testifying separation from the Arabian plate and southward migration of the subduction zone. The HT overprint of Kesandere eclogites can be related to the asthenospheric flow provoked by subducting slab retreat or break off.

New evidence of blueschist facies rocks and their geotectonic implication for Variscan suture(s) in the Bohemian Massif

AbstractBlueschist facies rocks, exposed within consolidated continental blocks, provide some of the best evidence for the existence of previous suture(s). They usually occur as lenses or layers embedded within greenschist or amphibolite facies rocks and indicate reequilibration at medium‐ to low‐pressure conditions. In the Bohemian Massif, a few occurrences of blueschists have been reported, and here, new evidence of high‐pressure (HP) metamorphism in various lithologies is presented that suggests a larger extent of blueschist facies rocks along the northern border of this Massif. An earlier blueschist facies metamorphism is documented by inclusions of glaucophane in garnet, epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses. The estimated P–T conditions, obtained using pseudosections and mineral isopleths, correspond to blueschist and low‐temperature eclogite facies conditions (1.1–2.0 GPa at 350–550 °C). Together with medium‐temperature eclogites from different units in the Bohemian Massif they indicate a geothermal gradient of 8–10 °C km−1, which is typical for cool subduction. Radiometric dating on phengite from metapelites confirms an early Palaeozoic cooling age of c. 360 Ma for this HP metamorphic event. The presence of blueschist facies rocks, their P–T relations and age constraint together with those from eclogite facies rocks allows us to locate the Variscan suture, which straddles the SE margin of Saxothuringian Zone from Erzgebirge to Sudetes, and its possible continuation to the Moldanubian Zone, where eclogite facies and UHPM rocks are abundant.

Structure, tectonics and metamorphic development of the Sancti Spiritus Dome (eastern Escambray massif, Central Cuba)

The Sancti Spiritus Dome of the eastern Escambray (Cuba) represents a metamorphic fold and thrust structure which was part of the Cretaceous subduction-accretion complex of the Greater Antillean Arc. On the basis of structural data and pressure-temperature-time evolution the metamorphic complex can be subdivided into four units interpretable as nappes: a high-grade greenschist-facies unit (Pitajones unit), a high-pressure tectonic melange (Gavilanes unit), high-pressure amphibolites (Yayabo unit) and – tectonically overlying - low-pressure metagabbros of the Greater Antillean Arc (Mabujina unit). The oldest rock fabrics are preserved in eclogite- and blueschist-facies rocks of the Gavilanes unit, indicating arc-parallel extension. Maximum metamorphic conditions are recorded in eclogites (16-20 kbar, 580-630 °C) and garnet-mica schists (16-23 kbar, 530-610 °C) of the Gavilanes unit. Field observations and fabric studies show that greenschist-facies dynamic indicators are dominated by top-to-NE tectonic transport in the lowermost nappes. The greenschist-facies shear zone between the Yayabo unit and the Mabujina unit is viewed as the main detachment zone between the subduction complex and the overlying arc complex. Active subduction ceased at about 70 Ma, followed by rapid uplift, exhumation and thrusting to the north.

Fluid migration above a subducted slab — Thermodynamic and trace element modelling of fluid–rock interaction in partially overprinted eclogite-facies rocks (Sesia Zone, Western Alps)

The amount and composition of subduction zone fluids and the effect of fluid–rock interaction at a slab–mantle interface have been constrained by thermodynamic and trace element modelling of partially overprinted blueschist-facies rocks from the Sesia Zone (Western Alps). Deformation-induced differences in fluid flux led to a partial preservation of pristine mineral cores in weakly deformed samples that were used to quantify Li, B, Sr and Pb distribution during mineral growth, -breakdown and modification induced by fluid–rock interaction. Our results show that Li and B budgets are fluid-controlled, thus acting as tracers for fluid–rock interaction processes, whereas Sr and Pb budgets are mainly controlled by the fluid-induced formation of epidote. Our calculations show that fluid–rock interaction caused significant Li and B depletion in the affected rocks due to leaching effects, which in turn can lead to a drastic enrichment of these elements in the percolating fluid. Depending on available fluid-mineral trace element distribution coefficients modelled fluid rock ratios were up to 0.06 in weakly deformed samples and at least 0.5 to 4 in shear zone mylonites. These amounts lead to time integrated fluid fluxes of up to 1.4 ∙ 10 2 m 3 m − 2 in the weakly deformed rocks and 1–8 ∙ 10 3 m 3 m − 2 in the mylonites. Combined thermodynamic and trace element models can be used to quantify metamorphic fluid fluxes and the associated element transfer in complex, reacting rock systems and help to better understand commonly observed fluid-induced trace element trends in rocks and minerals from different geodynamic environments.

Textural and age relations of polymetamorphic rocks in the HP Meliata Unit (Western Carpathians)

Slices and tectonic blocks of amphibolite facies crustal rocks overprinted by blueschist facies metamorphism occur in the Middle-Jurassic Meliata high-pressure unit of the Western Carpathians. The Early Paleozoic age for relic muscovite is known from some micaschists with blueschist facies overpint in the eastern sector of the Meliata unit. Based on lithology, some amphibolite facies rocks are comparable with the Early Paleozoic basement rocks in the Gemeric unit. In this work, we present results of petrology and Ar–Ar and Rb–Sr geochronology from blueschist facies rocks with relicts of hornblende, muscovite, garnet, and rutile. The older amphibolite facies metamorphism was followed by retrogression prior to the blueschist facies overprint. The blueschist facies metamorphic minerals are Ca-rich garnet, glaucophane, phengite, and albite. The coarse-grained relict muscovite indicates an Early Paleozoic age using the Ar–Ar method and a Middle-Jurassic age using the Rb–Sr technique. The presence of amphibolite facies rocks within the Meliata unit is interpreted as involvment of Variscan/Pre-Variscan basement-unit rocks in the Middle-Jurassic subduction of the Meliata-Hallstatt oceanic basin.

Metamorphic and tectonic evolution of a structurally continuous blueschist-to-Barrovian terrane, Sivrihisar Massif, Turkey

Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn- to post-subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist-to-Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod-shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have 40Ar/39Ar white mica ages that are >20 Myr younger than those of the blueschists, are also dominated by marble, but rod-shaped calcite has been progressively recrystallized into massive marble within a ∼200-m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ∼1 km present-day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite-rich quartzite; in one location, all three are in the same rock. Andalusite pre-dates Barrovian metamorphism, kyanite is both pre- and syn-Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite-in isograd are evidence for pre-Barrovian subduction metamorphism preserved at the low-T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn-kinematic high-T minerals defining the fabric of L-tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation.

Glaucophane schists and associated rocks from Sifnos (Cyclades, Greece): New constraints on the P–T evolution from oxidized systems

The island of Sifnos (Cyclades, Greece) is famous for its spectacular blueschist-facies rocks and eclogites. Their very well preserved high-pressure assemblages are characterized by the occurrence of Fe + 3 -rich minerals such as aegirine-rich pyroxene, riebeckite-rich amphibole, magnetite and deerite. Therefore, the common model system NCFMASH (Na 2O–CaO–FeO–MgO–Al 2O 3–SiO 2–H 2O), in which all iron is assumed to be ferrous, is not sufficient to completely model these highly oxidized assemblages. We propose new constraints on the P–T evolution of the glaucophane schists and associated rocks from Vroulidia Bay (northern Sifnos) using P–T isochemical diagrams (pseudosections) modelled for the oxidized N(K)CFMASHO system [Na 2O–(K 2O)–CaO–FeO–Fe 2O 3–MgO–Al 2O 3–SiO 2–H 2O], and calculated using solid solution models involving Fe + 3 -end-members. The resulting P–T conditions could be fitted by a simple trajectory consisting of a smooth clockwise P–T loop, with two distinct high-pressure events at T = 450–500 °C, P > 2.0 GPa (assemblage A) and T = 525–565 °C, P > 2.1 GPa (assemblage B), respectively, followed by cooling and decompression. Geospeedometry based on 40Ar/ 39Ar data, however, allows constraint as to the duration of individual thermal excursions associated with these events. Should these limits be taken into account, the P–T envelope derived in this paper encompasses a more complex P–T history, with individual excursions that potentially involve relatively short periods of temperature increase ± pressure variation.

Triassic continental subduction in central Tibet and Mediterranean-style closure of the Paleo-Tethys Ocean

The Qiangtang metamorphic belt (QMB) in central Tibet is one of the largest and most recently documented high-pressure (HP) to near-ultrahigh-pressure (near-UHP) belts on Earth. Lu-Hf ages of eclogite- and blueschist-facies rocks within the QMB are 244–223 Ma, indistinguishable from the age of UHP metamorphism in the Qinling-Dabie orogen. Results of a U-Pb detrital zircon study suggest that protoliths of the QMB include upper Paleozoic Qiangtang continental margin strata and sandstones that were derived from a Paleozoic arc terrane that developed within the Paleo-Tethys Ocean to the north. We attribute QMB HP metamorphism to continental collision between the Qiangtang terrane and a Paleo-Tethys arc terrane. This collision, and the coeval South China–North China collision, may have slowed convergence between Laurasia and Gondwana-derived terranes and initiated Mediterranean-style rollback and backarc basin development within much of the remnant Paleo-Tethys Ocean realm.

Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California

Research Article| November 01, 2007 Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California Jeffrey R. Unruh; Jeffrey R. Unruh 1William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Search for other works by this author on: GSW Google Scholar Trevor A. Dumitru; Trevor A. Dumitru 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Thomas L. Sawyer Thomas L. Sawyer 3Piedmont Geosciences, 10235 Blackhawk Drive, Reno, Nevada 89506, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Jeffrey R. Unruh 1William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Trevor A. Dumitru 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Thomas L. Sawyer 3Piedmont Geosciences, 10235 Blackhawk Drive, Reno, Nevada 89506, USA Publisher: Geological Society of America Received: 13 Jun 2006 Revision Received: 15 Jan 2007 Accepted: 24 Jan 2007 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 The Geological Society of America, Inc. GSA Bulletin (2007) 119 (11-12): 1347–1367. https://doi.org/10.1130/B26057.1 Article history Received: 13 Jun 2006 Revision Received: 15 Jan 2007 Accepted: 24 Jan 2007 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeffrey R. Unruh, Trevor A. Dumitru, Thomas L. Sawyer; Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California. GSA Bulletin 2007;; 119 (11-12): 1347–1367. doi: https://doi.org/10.1130/B26057.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In the California Coast Ranges east of San Francisco, post–3.5 Ma folding and erosion of the Mount Diablo anticline have created unusual three-dimensional exposures of late Mesozoic–Cenozoic stratigraphic and structural relations among rocks of the Franciscan subduction complex, Coast Range ophiolite, and Great Valley forearc basin. These relations offer new insights into the kinematics of Late Cretaceous–early Tertiary synsubduction extension, attenuation, and blueschist exhumation within the ancestral California forearc region. Map relations and subsurface data reveal that marine strata of the Great Valley forearc basin northeast of Mount Diablo accumulated in an ∼100-km-long, north-south–trending, extensional graben system. Normal faults in the graben system cut steeply downsection through Late Cretaceous and early Tertiary growth strata and terminate against the Clayton–Marsh Creek fault, which is at a low angle to bedding and is interpreted to be the basal detachment for part of the graben system. The Clayton–Marsh Creek fault in turn is linked to a system of faults that juxtapose blueschist facies rocks of the underlying ancestral Franciscan accretionary prism against attenuated remnants of forearc crust that were never deeply buried, including the Coast Range ophiolite and basal Great Valley strata. These faults are the local expression of the Coast Range fault in this area. Franciscan rocks exposed beneath the faults were metamorphosed at ≥20 km depths after 108 Ma, based on jadeitic pyroxene and 108 Ma detrital zircons (U-Pb) in metagraywackes. Apatite fission-track data indicate that these blueschist facies rocks were subsequently uplifted and exhumed from ∼9 km to ∼3 km depths in latest Cretaceous–early Tertiary time, coeval with the subsidence and extension in the structurally overlying forearc graben system. Much of their earlier rise from ≥20 km to 9 km depth presumably also occurred coeval with graben development, although our data cannot determine this directly. These relations demonstrate that the California forearc around Mount Diablo was in an extensional regime undergoing active sedimentation during major Franciscan exhumation, and thus support models for exhumation of Franciscan blueschist facies rocks via synsubduction extensional processes, rather than via erosional processes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Variscan amphibolite-facies rocks from the Kurtoğlu metamorphic complex (Gümüşhane area, Eastern Pontides, Turkey)

The Kurtoglu metamorphic complex, that forms part of the pre-Liassic basement of the Sakarya zone in northern Turkey, consists of at least two tectonic units. Blueschist-facies rocks of unknown metamorphic age in the southern part of the complex are tectonically overlain by Variscan low-pressure high-temperature metamorphic rocks. The latter comprise mica schists and fine-grained gneisses, cut by metaleucogranitic dikes, as well as migmatitic biotite gneisses and subordinate amphibolite intercalations. Structural data indicate that metamorphism and penetrative deformation occurred after dyke intrusion. Peak metamorphic conditions of the mica schists, fine-grained gneisses and metaleucogranites are estimated to ∼650°C and ∼0.4 GPa, based on phase relationships in the system NCKFMASH, Fe–Mg partitioning between garnet and biotite as well as garnet-aluminosilicate-quartz-plagioclase (GASP) and garnet-plagioclase-biotite-quartz (GBPQ) barometry. Peak temperatures of the migmatitic biotite gneisses and amphibolite intercalations are not well constrained but might have been significantly higher (690–740°C), as suggested from hornblende-plagioclase thermometry. 40Ar–39Ar incremental dating on muscovite and biotite fractions from the mica schists and fine-grained gneisses yielded plateau ages of ∼323 Ma. Significantly older model ages of ∼329 and ∼337 Ma were obtained on muscovite fractions from two metaleucogranite samples. These fractions contain both relict igneous and newly formed metamorphic muscovite.

Transient, synobduction exhumation of Zagros blueschists inferred from P‐T, deformation, time, and kinematic constraints: Implications for Neotethyan wedge dynamics

We present the first P‐T, deformation time, and kinematic constraints on the only known blueschist facies rocks (BS) present in the Zagros (Hajiabad area). The BS were underplated below the Sanandaj‐Sirjan zone and crop out as kilometer‐scale bodies within extensive colored melange units marking discontinuously the Neotethyan suture zone. P‐T estimates point to high‐pressure/low‐temperature (HP‐LT) conditions around 11 kbar and 520–530°C for the majority of BS, along a ∼15°C km−1 gradient. Some exotic blocks in matrix serpentinite reached 17–18 kbar at ∼500°C. In situ laser probe 40Ar‐39Ar radiometric age constraints on phengite cluster between 85 and 95 Ma and suggest that (1) synconvergence exhumation of Zagros BS from 35–50 km to depths <15–20 km was accomplished before 80 Ma, (2) BS exhumation corresponded to a transient process with respect to the long‐lived subduction beneath Iran (∼150–35 Ma), and (3) age constraints for Zagros BS are 5–10 Myr older than for the nearby Oman HP‐LT rocks and broadly coincide with obduction processes in the region (circa 95–70 Ma). We propose that the mechanical coupling across the Neotethyan subduction zone (NSZ) beneath Iran was modified by the large‐scale plate rearrangement accompanying obduction, allowing for a short‐lived exhumation of Zagros BS. Exhumation ceased at the end of obduction, when subduction of the Arabian continental margin stopped. Kinematic calculations suggest that convergence velocities across the NSZ likely doubled (to ∼5–6 cm yr−1) during the period 118–85 Ma, so that BS exhumation may have been promoted by a combination of obduction movements and increased convergence velocities.

Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean

In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time.

Contrasting structural and P‐T evolution of tectonic units in the southeastern Betics: Key for understanding the exhumation of the Alboran Domain HP/LT crustal rocks (western Mediterranean)

Thermobarometry using mica‐chlorite local equilibria and structural analysis of three tectonic units of the Alpujarride complex in the southeastern Betics indicate that rocks that underwent low‐pressure/low‐temperature metamorphism are found below higher‐grade blueschist facies rocks. Moreover, the high‐pressure/low‐temperature (HP/LT) units underwent contrasting (P‐T) and structural evolutions. The structurally highest unit was exhumed in a typical postorogenic high‐temperature geothermal context in the andalusite stability field, while the intermediate unit was exhumed following a cooling and decompression P‐T path within the kyanite stability field. The superposition of the three units occurred during the lower Miocene related with north directed brittle‐ductile shear zones and associated north vergent overturned folds, after postorogenic exhumation of the top unit. The coexistence of thrusting with lower and middle Miocene brittle extensional systems active in shallower levels of the Alboran orogenic wedge implies that extension was synorogenic. These data strengthen the hypothesis that rocks thinned by postorogenic extension during the latest Oligocene and lower Miocene were entrained in the lower to middle Miocene Alboran orogenic wedge.

Discovery of Paleozoic Fe-Mg carpholite in Motalafjella, Svalbard Caledonides: A milestone for subduction-zone gradients

Paleozoic blueschist facies rocks are relatively scarce on Earth due to warmer geother- mal gradients at that time and/or later reequilibration. Ferro-magnesiocarpholite (Fe-Mg carpholite), the typical low-temperature blueschist facies index mineral in metapelites, was discovered 30 yr ago and is known only in Tethyan belts metamorphosed ,80 m.y. ago. Herein we report the discovery of Paleozoic Fe-Mg carpholite in the ca. 470 Ma blueschists of Motalafjella, Svalbard Caledonides, the oldest known occurrence on Earth. The car- pholite-bearing rocks reached pressure-temperature (P-T) conditions of 15-16 kbar and 380-400 8C and followed a nearly isothermal exhumation path. In the cooling Earth per- spective, these P-T estimates for Motalafjella blueschists demonstrate the existence of cold subduction-zone gradients (;7 8C/km) from the middle Paleozoic onward.