High-pressure Ultrahigh-pressure Research Articles

Fluid–rock interaction and element mobilization in UHP metabasalt: Constraints from an omphacite–epidote vein and host eclogites in the Dabie orogen

A petrological study is carried out for a high-pressure (HP) omphacite–epidote vein and its host ultrahigh-pressure (UHP) eclogites at Ganghe in the Dabie orogen. The UHP eclogite shows systematic changes toward the HP vein in mineral assemblage, mineral proportion and bulk-rock composition. The eclogite far away from the vein (>20cm) contains abundant lawsonite pseudomorphs indicated by the aggregates of epidote, kyanite and quartz porphyroblasts. The abundance and grain size of these porphyroblasts decrease gradually toward the vein in the nearby eclogite (<20cm), and they are almost absent in the eclogite in direct contact with the vein (<2.5cm). The omphacite–epidote vein contains high amounts of the components hosted in the minerals that were lost in the host eclogite, suggesting significant fluid–rock interaction and mass transfer in the eclogite–vein system during the fluid flow and vein formation. A detailed UHP fluid–eclogite interaction with various element mobilities is inferred from a combined study of metamorphic petrology, geochemistry and mass-balance calculations. The HP vein was formed by fluid liberation from the lawsonite breakdown in the host UHP eclogite at 2.8–3.0GPa and 650–680°C. The released UHP aqueous fluid reacted with the eclogite and dissolved kyanite, coesite, epidote, omphacite, rutile, zircon and apatite. This resulted in significant transfers of SiO2, Al2O3, CaO, Fe2O3, Na2O, TiO2, P2O5, LILE (Sr and Pb), REE and HFSE (Nb, Ta, Zr, Hf, U and Th) into the UHP vein-forming fluid. The solubilities of these minerals and relevant element mobilities gradually increase toward the fluid due to the increase of fluid/eclogite ratios. Fluid-mobile elements (e.g., Cs, Rb, K and Ba) were gained in the eclogite by the formation of late-stage phengite and amphibole, which consumed the interstitial fluid after the precipitation of vein minerals.Therefore, it is inferred that the UHP fluid is an efficient medium to transport not only fluid-mobile elements such as LILE and LREE but also nominally fluid-immobile elements such as HFSE and HREE. Local fluid–rock ratios and solubilities of the host minerals have largely controlled the mobilities of elements. In particular, the dissolution of minerals is a key mechanism to transfer the elements into the HP–UHP fluids released from the cold subducting slab.

Eclogite origin and timings in the North Qinling terrane, and their bearing on the amalgamation of the South and North China Blocks

The amalgamation of South (SCB) and North China Blocks (NCB) along the Qinling-Dabie orogenic belt involved several stages of high pressure (HP)-ultra high pressure (UHP) metamorphism. The new discovery of UHP metamorphic rocks in the North Qinling (NQ) terrane can provide valuable information on this process. However, no precise age for the UHP metamorphism in the NQ terrane has been documented yet, and thus hinders deciphering of the evolution of the whole Qinling-Dabie-Sulu orogenic belt. This article reports an integrated study of U–Pb age, trace element, mineral inclusion and Hf isotope composition of zircon from an eclogite, a quartz vein and a schist in the NQ terrane. The zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning, high Th/U and 176Lu/177Hf ratios, pronounced Eu anomalies and steep heavy rare earth element (HREE) patterns. The zircon cores yield an age of 796 ± 13 Ma, which is taken as the protolith formation age of the eclogite, and implies that the NQ terrane may belong to the SCB before it collided with the NCB. The ɛHf(t) values vary from −11.3 to 3.2 and corresponding two-stage Hf model ages are 2402 to 1495 Ma, suggesting the protolith was derived from an enriched mantle. In contrast, the metamorphic zircon rims show no zoning or weak zoning, very low Th/U and 176Lu/177Hf ratios, insignificant Eu anomalies and flat HREE patterns. They contain inclusions of garnet, omphacite and phengite, suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions, and their weighted mean 206Pb/238U age of 485.9 ± 3.8 Ma was interpreted to date the timing of the eclogite facies metamorphism. Zircon in the quartz vein is characterized by perfect euhedral habit, some oscillatory zoning, low Th/U ratios and variable HREE contents. It yields a weighted mean U–Pb age of 480.5 ± 2.5 Ma, which registers the age of fluid activity during exhumation. Zircon in the schist is mostly detrital and U–Pb age peaks at c. 1950 to 1850, 1800 to 1600, 1560 to 1460 and 1400 to 1260 Ma with an oldest grain of 2517 Ma, also suggesting that the NQ terrane may have an affinity to the SCB. Accordingly, the amalgamation between the SCB and the NCB is a multistage process that spans c. 300 Myr, which includes: the formation of the Erlangping intra-oceanic arc zone onto the NCB before c. 490 Ma, the c. 485 Ma crustal subduction and UHP metamorphism of the NQ terrane, the c. 430 Ma arc-continent collision and granulite facies metamorphism, the 420 to 400 Ma extension and rifting in relation to the opening of the Palaeo-Tethyan ocean, the c. 310 Ma HP eclogite facies metamorphism of oceanic crust and associated continental basement, and the final 250 to 220 Ma continental subduction and HP–UHP metamorphism.

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.

Tectonic evolution of the southeastern part of the Pohorje Mountains (Eastern Alps, Slovenia)

Tectonic evolution of the southeastern part of the Pohorje Mountains (Eastern Alps, Slovenia)Field relations and deformation structures in the southeastern part of the Pohorje Mountains constrain the tectonic evolution of the Austroalpine high-pressure/ultrahigh pressure (HP/UHP) terrane. The Slovenska Bistrica Ultramafic Complex (SBUC) forms a large (ca. 8 × 1 km size) body of serpentinized harzburgite and dunite including minor garnet peridotite and is associated with partly amphibolitized eclogite bodies. The SBUC occurs in the core of an isoclinal, recumbent, northward closing antiform and is mantled by metasedimentary rocks, mostly gneisses and a few marbles, including isolated eclogite/amphibolite lenses. Before this folding, the SBUC formed the deepest part of the exposed terrane. We interpret the SBUC to be derived from near-MOHO, uppermost mantle which was intruded by gabbros in the subsurface of a Permian rift zone. During Cretaceous intracontinental subduction, the SBUC was most likely part of the footwall plate which experienced HP to UHP metamorphism and was folded during exhumation. In the Miocene, the Pohorje Pluton intruded and, subsequently, the metamorphic rocks together with the pluton were deformed probably due to east-west extension and contemporaneous north-south shortening, thus forming an antiformal metamorphic core complex.

Subduction zone metamorphism – pioneering contributions from the Alps

Circum-Pacific-type high-pressure (HP) metamorphic belts occupy contractional sutures marked by intensely deformed, imbricated nappes that verge seaward. Mineral assemblages reflect recovered depths of ∼25–50 km for mafic and felsic crustal units carried down on an underlying oceanic plate; such units recrystallized prior to disengagement and exhumation of slices of chiefly low-density rock. Low-grade parts of Alpine-type subduction complexes are similar to regurgitated circum-Pacific-type HP belts, but coesite ± microdiamond inclusions in tough, unreactive host minerals show that lithospheric underflow and continental collision involve partial recovery of ultra-high-pressure (UHP) rocks from depths of 100–140 km. Associated garnet peridotites from the Central Alps and several other collision sutures display exsolution lamellae and intergrowths suggesting the former existence of majoritic garnet and additional phases requiring formation at >300 km, attesting to earlier, unrelated mantle upwelling, or to the resurrection of profoundly subducted and back-reacted complexes. HP and UHP terranes return to mid-crustal levels mainly driven by buoyancy. Times of deep-seated storage and rates of exhumation are short; mineral parageneses, intervals of HP–UHP recrystallization and nappe structures reflect the two-way, relatively rapid transport of now-exposed subduction complexes. More fully than any other areas, integrated mineralogic-geochemical-structural studies of the Alps have illuminated relationships between metamorphic petrology and convergent plate tectonics. HP–UHP transformations of mafic and quartzofeldspathic rocks initially formed at, or near, the Earth's surface, imbricate structures of subduction complexes including their prograde metamorphic zonations + retrograde parageneses, association with deep-seated UHP mantle rocks and speedy ascent were first documented in the Alps.

Subduction-zone metamorphism, calc-alkaline magmatism, and convergent-margin crustal evolution

At the dawn of the age of plate tectonics, Akiho Miyashiro published a seminal paper in which he drew attention to significant differences in regional metamorphic phase assemblages, described contrasting geologic occurrences, and inferred their characteristic ranges of physical conditions. He advanced the paired metamorphic belt concept, involving an oceanward, narrow, high-P/low-T blueschist zone intimately intermixed with ophiolites, and a landward, broad, low-P/high-T realm associated with arc volcanic–plutonic rocks. More recent studies have illuminated the following: (1) Two main types of convergent plate junction can be distinguished, Pacific underflow of thousands of km of oceanic lithosphere, and Alpine closure of an intervening oceanic basin leading to continental collision. (2) Plate descent carries mafic + felsic lithologies to depths of ~ 35–120 km or more at relatively low temperatures, producing high-pressure (HP) Pacific-type and ultrahigh-pressure (UHP) Alpine-type metamorphic terranes, respectively. (3) Exhumed HP–UHP complexes display low-aggregate bulk densities, reflecting chiefly buoyancy-driven ascent of quartzofeldspathic allochthons. (4) At a depth of ~ 35 km in a warm-lithosphere subduction zone, sialic crust should melt if an aqueous fluid were abundant; it does not, hence the activity of H 2O must be low at such depths. (5) Global observations indicate that volcanic–plutonic arcs are sited 100 ± 20 km above subducting oceanic plates; devolatilization of sinking, hydrated oceanic crust ± mantle promotes the generation of calc-alkaline arc melts in this magmagenic zone. (6) Pacific-type underflow of basaltic crust-capped plates produces new and recycled continental crust, whereas Alpine-type convergence reshuffles collided terranes but does not generate juvenile sialic crust.

Wellbore breakouts of the main borehole of Chinese Continental Scientific Drilling (CCSD) and determination of the present tectonic stress state

The Sulu–Dabie high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt as the product of subduction-collision between the northern China plate and Yangtze plate underwent a process of formation and evolution from deep subduction→exhumation→extension→slow uplift. The study of its modern tectonic stress field has great significance for a complete understanding of the process of formation and evolution of the HP–UHP metamorphic belt, especially the exhumation and uplift of the belt. Wellbore breakouts are the most visual tectonic phenomenon which can characterize the modern stress action in the main borehole of Chinese Continental Scientific Drilling (CCSD). Ultrasonic borehole televiewer reflection wave data show that wellbore breakouts began to occur at 1216 m depth of the main borehole. A total of 143 borehole televiewer images were collected from 1216 to 5118 m depth (hole completion depth). After data processing and statistics, the average azimuth of the long dimension of the wellbore breakout obtained was 319.5° ± 3.5°, indicating that the average azimuth of the maximum horizontal principal stress causing wellbore breakout initiation was 49.5° ± 3.5°. The maximum and minimum horizontal principal stress values at 52 depths in the interval of 1269 to 5047 m were estimated using the elements of wellbore shapes (wellbore depth and width), combined with the cohesive strength and internal frictional angle of the rock obtained by rock mechanical tests on samples, and the static load stresses at corresponding depths were calculated according to the rock density logging data. The results indicate that: the maximum and minimum horizontal principal stresses are 41.4 and 25.3 MPa at 1269 m depth respectively and 164.7 and 122 MPa at 5047 m depth respectively; the maximum vertical stress is 141.3 MPa at 5047 m depth with a density of 2.8 g/cm 3; the in-situ stresses increase nearly linearly with depth. The magnitudes and directions of the three principal stresses reflect that the regional stress field around the CCSD main borehole is mainly in a strike-slip state, which is consistent with the basic features of the regional stress field determined using other methods.

Petrology and geochronology of HP–UHP rocks from the South Altyn Tagh, northwestern China

Detailed field and petrological studies on the South Altyn Tagh in the last decade show that high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks are mainly exposed at Jianggalesayi and Qingshuiquan in the west, and at Yinggelisayi in the middle of the terrane. The South Altyn Tagh UHP rocks occur as lensoid blocks in granitic gneiss, paragneiss and marble and include eclogite, kyanite-garnet-bearing pelitic gneiss, magnesite-bearing garnet lherzolite, K-feldspar-bearing garnet clinopyroxenite and garnet-bearing granitic gneiss. Zircon U–Pb SHRIMP and LA-ICP-MS dating, combined with CL and trace element analysis yield geochronological data for different South Altyn Tagh HP/UHP rocks as follows: metamorphic ages of 493 ± 4 and 499 ± 27 Ma for the Jianggalesayi eclogite and its country rock respectively, 509 ± 12, 489 ± 8 and 487 ± 10 Ma for Yinggelisayi magnesite-bearing garnet lherzolite, K-feldspar-bearing garnet clinopyroxenite and garnet-bearing granitic gneiss, and 475 ± 4 and 486 ± 5 Ma for Qingshuiquan garnet clinopyroxenite and kyanite-bearing garnet-biotite gneiss. Magmatic protolith ages of eclogite, garnet lherzolite, garnet clinopyroxenite and garnet-bearing granitic gneiss are 754 ± 9, 844 ± 9, 821 ± 27 and 842 ± 18 Ma, respectively. Metamorphic ages of these rocks are consistent within errors, indicating that the South Altyn Tagh HP/UHP rocks were formed during 475–509 Ma; these are 20–80 Ma older than dated North Qaidam HP/UHP rocks (420–457 Ma). The occurrence of possible stishovite in pelitic gneiss and the older HP/UHP metamorphic ages suggest that the South Altyn Tagh belt may not necessarily be a western extension of the North Qaidam HP/UHP metamorphic belt, offset along the Altyn Tagh strike-slip fault.

Alpine and Pacific styles of Phanerozoic mountain building: subduction‐zone petrogenesis of continental crust

AbstractA broad continuum exists between two distinct end‐member types of mountain building. Alpine‐type orogenic belts develop during subduction of an ocean basin between two continental blocks, resulting in collision. They are characterized by an imbricate sequence of oceanward verging nappes; some Alpine belts exhibit superimposed late‐stage backthrusting. Sediments are chiefly platform carbonates and siliciclastics, in some cases associated with minor amounts of bimodal volcanics; pre‐existing granitic gneisses and related continental rocks constitute an autochthonous–parautochthonous basement. Metamorphism of deeply subducted portions of the orogen ranges from relatively high‐pressure (HP) to ultrahigh‐pressure (UHP). Calcalkaline volcanic–plutonic rocks are rare, and have peraluminous, S‐type bulk compositions. In contrast, Pacific‐type orogens develop within and landward from long‐sustained oceanic subduction zones. They consist of an outboard oceanic trench–accretionary prism, and an inboard continental margin–island arc. The oceanic assemblage consists of first‐cycle, in‐part mélanged volcaniclastics, and minor but widespread cherts ± deep‐water carbonates, intimately mixed with disaggregated ophiolites. The section recrystallized under HP conditions. Recumbent fold vergence is oceanward. A massive, slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable, non‐subducted plate. It consists of abundant, dominantly intermediate, metaluminous, I‐type volcanics resting on old crust; both assemblages are thrown into open folds, intruded by comagmatic I‐type granitoids, and metamorphosed locally to regionally under high‐T, low‐P conditions. In the subduction channel of collisional and outboard Circumpacific terranes, combined extension above and subduction below allows buoyancy‐driven ascent of ductile, thin‐aspect ratio slices of HP–UHP complexes to midcrustal levels, where most closely approached neutral buoyancy; exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume. At massive sialic buildups associated with convergent plate cuSPS (syntaxes), tectonic aneurysms may help transport HP–UHP complexes from mid‐ to upper‐crustal levels. The closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts, so volcanic–plutonic belts are poorly developed in Alpine orogens compared with Circumpacific convergent plate junctions. Generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation. Phase equilibrium studies show that, under typical subduction‐zone P–T trajectories, clinoamphibole ± Ca–Al hydrous silicates constitute the major hydroxyl‐bearing phases in deep‐seated metamorphic rocks of MORB composition; other hydrous minerals are of minor abundance. Ca and Na clinoamphiboles dehydrate at pressures of above approximately 2 GPa, but low‐temperature devolatilization may be delayed by pressure overstepping; thus metabasaltic blueschists and amphibolites expel H2O at melt‐generation depths, and commonly achieve stable eclogitic assemblages. Partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions. For reasonable subduction‐zone geothermal gradients however, white micas ± biotites remain stable to pressures &gt;3 GPa. Accordingly, attending descent to depths of &gt;100 km, mica‐rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of H2O, and transform incompletely to stable eclogite‐facies assemblages. Underflow of amphibolitized oceanic lithosphere thus generates most of the deep‐seated volatile flux, and the consequent partial melting to produce the calcalkaline suite, along and above a subduction zone; where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths, volatile flux severely diminishes. Thus, continental collision in general does not produce a volcanic–plutonic arc whereas in contrast, the long‐continued contemporaneous underflow of oceanic lithosphere does.

Superzoned garnets in the coesite-bearing Brossasco-Isasca Unit, Dora-Maira massif, Western Alps, and the origin of the whiteschists

Rare centimeter-sized superzoned garnets (SZGs) were discovered in two coesite-bearing whiteschists of the Brossasco-Isasca Unit (BIU), southern Dora-Maira massif (DMM), Western Alps. The superzoned garnet consists of a reddish-brown almandine core crowded with inclusions of staurolite, chloritoid, kyanite, chlorite and paragonite, and of a pinkish pyrope rim with sporadic inclusions of kyanite, and magnesian chlorite. The core–rim contact is relatively sharp and marks the termination of the inclusion-rich portion. The core composition of the superzoned garnet is almost identical to, or slightly richer in Mg, than that of the rim of porphyroblastic garnet in metapelites from the same unit. In the rim of the superzoned garnet, Mg–Fe ratio increases abruptly towards the outermost rim, whose composition is identical to that of the common pyrope in the whiteschist. At the core–rim boundary, there is no chemical gap. Chloritoid and staurolite are common inclusions in the core of the superzoned garnet in the whiteschist and in the porphyroblastic garnet in the metapelite. The staurolite composition (Si=2.00 and total R 2+<2.0 for O=23 basis) and its reverse Fe–Mg distribution with respect to garnet suggest a HP origin. The Fe–Mg distribution between chloritoid and garnet is reverse in the superzoned garnet, but normal in the garnet of metapelite. Because normal Fe–Mg distribution was reported from other eclogite-facies metapelites, a model petrogenetic grid was constructed in the FMASH model system considering St, Cld, Ky, Chl, Grt, and assuming the following Fe–Mg partitioning of St>Grt>Cld>Chl. The resulting petrogenetic grid suggests that the core of the superzoned garnet contains incompatible assemblages, such as St–Cld–Chl vs. Cld–Chl–Ky. New and literature data and results of experiments in the KFASH system suggest that: (1) the superzoned garnet was formed under a single prograde high-pressure/ultra high-pressure (HP/UHP) Alpine metamorphism, (2) the almandine inclusion-rich core of the superzoned garnet crystallized at disequilibrium in a pelitic composition system at around 600°C and less than 16 kbar, probably from a former metapelite xenolith included in a Variscan granitoid, and (3) the chemical environment of the host rock suddenly changed from the normal pelite to the whiteschist composition by a metasomatic process during the rim growth, i.e., at a stage close to the UHP climax.

Compositions of Jurassic detrital garnets in Hefei Basin and its implication to provenance reconstruction and stratigraphic correlation

The compositions of Jurassic detrital garnets in Hefei Basin are complicated. Contents of the end member are from zero to 43% for pyrope, from less than 1% to 50% for grossular, from 2% to 92% for almandine, and from zero to 88% for spessartine. Part of relatively pyrope-rich detrital garnets might be originated from high pressure-ultrahigh pressure metamorphic rocks. Contents of spessartine in garnets from the present metamorphic rocks of the Dabie Mountains, including the greenschists of Foziling Group, are lower than 30%. Therefore, they would not be the source of the spessartine-rich detrital garnets in the Jurassic sedimentary rocks of the Hefei Basin. Chemical compositions of the Jurassic detrital garnets in the Hefei Basin have some characteristics in the distribution with strata, which can be applied to study of the sedimentary filling sequence and stratigraphic correlation.

Two contrasting petrotectonic domains in the Kokchetav megamélange (north Kazakhstan): Difference in exhumation mechanisms of ultrahigh‐pressure crustal rocks, or a result of subsequent deformation?

Abstract In northern Kazakhstan the WNW striking Kokchetav megamélange includes different crustal sequences with high‐pressure/ultrahigh‐pressure (HP/UHP) remnants of their 540–520 Ma subduction metamorphism. Two domains separated by the north‐east trending Chaglinka fault are distinguished. The western domain exhibits NE–SW structures within a single Kumdy–Kol megaunit of diamond‐bearing UHP metasediments and high‐temperature (HT) eclogites. The eastern domain consists of the composite Kulet megaunit with the Kulet UHP unit (coesite‐bearing metasediments, whiteschists and eclogites), the Enbek–Berlyk medium‐pressure (MP) unit (kyanite‐bearing, high‐alumina rocks with interleaved coronitic metagabbro), and ortho‐ and paragneisses with eclogites and amphibolites included. All eclogites in the eastern domain are of the relatively low temperature (LT) type. Sillimanite is common and appears after kyanite in the sheared MP unit. A regional and moderately ESE plunging linear fabric coincides with the fold‐axis of the foliation poles from the eastern domain. Whether this also reflects a regional top to the WNW transport, as inferred from the dextral strike‐slip on steeply to SSW dipping foliation, needs further study. Top to the WNW shear is shown by weakly inclined low pressure (LP) cordierite rocks that flank the eastern domain in the south. Some new 39Ar/40Ar mica cooling ages (519, 521 Ma) from the Kulet UHP micaschists reflect the same early stage evolutionary event as was previously shown for the Kumdy–Kol UHP rocks (515, 517 Ma) in the west. Similar 39Ar/40Ar ages (500, 517 Ma) are recorded by micas and amphibole that outline a top to NNW shear fabric in the non‐subducted Proterozoic basement, north of the megamélange. A 447 Ma overprint of the MP sequences is considered to reflect the strike‐slip deformation with sillimanite and the reworking of an early kyanite‐bearing tectonite. Biotites from the LP cordierite rocks yielded approximately 400 Ma 39Ar/40Ar ages. In case they reflect the WNW shear deformation, the latter is considered to be associated with a regional granite magmatism (420–460 Ma) extending south of the eastern domain. In their present different structural domains the Kulet and Kumdy–Kol UHP units display a similar early stage event. Subsequent LP deformation, which is likely to be associated with regional granite magmatism (420–460 Ma), is assumed to have obliterated any common or uniform early exhumation structure for the whole megamélange. The north‐east structured Kumdy–Kol domain is assumed to have preserved the most information about the early stage exhumation. This domain is at an angle to the regional WNW strike of the megamélange.

High-pressure blueschists and eclogites in Hong'an: a framework for addressing the evolution of high- and ultrahigh-pressure rocks in central China

The Hong'an region in the Qinling–Dabie collisional zone in eastern China hosts a series of metamorphic rocks exposing a south-to-north distribution from blueschist/blueschist–greenschist, amphibolite, eclogite (kyanite free) and kyanite–eclogite to coesite–eclogite facies rocks that represent progressively deeper levels of the Mesozoic subduction–collision complex. The Hong'an area is interesting for three reasons: (1) it escaped the thermal and structural overprint imparted on much of the Dabie Mountains during Early Cretaceous intrusion of voluminous granites and granodiorites; (2) the high-pressure (HP) Hong'an eclogites are widely distributed, often preserve prograde crystallization histories and can be directly linked in time and space to the blueschist/blueschist–greenschist rocks exposed to the south; (3) the blueschist/blueschist–greenschist facies rocks are generally better exposed than their equivalents in the southeastern Dabie Mountains and offer some opportunity for simultaneous structural and metamorphic analysis. The Hong'an area HP rocks offer perhaps the closest approximation to a preserved snapshot of Mesozoic pressure–temperature ( P– T) conditions attending early subduction–exhumation in the region, and are thus essential to generating a coherent picture of the dynamics attending both metamorphism and exposure of the coeval ultrahigh-pressure (UHP) rocks. The purpose of this contribution is twofold: (1) to document previously unpublished metamorphic and structural data characterizing these HP sequences and their relative continuity in Hong'an; (2) to incorporate these data with recent geochronologic, structural and paleomagnetic information in the context of protracted, late Paleozoic through Mesozoic subduction, collision and exhumation. Metamorphism and exhumation of some of the southern Hong'an HP sequences appear to have occurred concomitant with oceanic subduction immediately to the west, and thus may have preceded widespread continental subduction/collision. Moreover, all of the HP–UHP sequences in the region were exhumed before the end of collision between the Sino-Korean and Yangtze cratons at ca. 160 Ma. Exhumation of HP–UHP rocks both before and during continental plate collision is neither novel for central China nor for other HP–UHP zones, but is important to take into account when reconstructing the evolution of such orogens.