HP Rocks Research Articles

Multi-Stage Metamorphism of the UHP Pelitic Gneiss from the Southern Altyn Tagh HP/UHP Belt, Western China: Petrological and Geochronological Evidence

The kyanite-bearing garnet pelitic gneiss from the Jianggalesayi area in southern Altyn Tagh high pressure/ultra-high pressure belt was proved to have been experienced UHP metamorphism (>12 GPa) by the discovery of kyanite and spinel exsolution microstructure in quartz (precursor stishovite). In this study, three stages of retrograded metamorphism (M2–M4) after the UHP metamorphism (M1) were identified for the UHP pelitic gneiss. The HP granulite-facies stage (M2) was characterized by the mineral assemblage of garnet+kyanite+K-feldspar+rutile+quartz±ilmenite, recording the P-T condition of >1.12 GPa and ≈850–930 oC. The granulite-facies stage (M3) was represented by the mineral assemblage of garnet rim+K-feldspar+sillimanite (Sill1)+biotite (Bt1)+plagioclase (Pl1)+ilmenite+quartz, and confined under P-T conditions of 0.5–0.8 GPa and ≈770–795 oC. The late cooling stage M4 was accompanied by the appearance of fine-grained Pl2, Sill2 and Bt2 in the matrix, and the P-T conditions were 0.4–0.6 GPa and 350 km) to the lower crust depth (≈40–20 km), resulting in an average exhumation rate of 9.11–9.70 mm/yr. In the southern Altyn Tagh region, the HP and UHP rocks from different areas had identical peak metamorphic ages. Therefore, contemporary UHP and HP rocks with different metamorphic evolutions were recognized coexisting in the same orogenic belt, which can be interpreted by the model of subduction channel. The continental crustal were subducted to different depths along the direction of the subduction channels at ≈500 Ma, suffered different grade metamorphism, and then returned to the surface along the subduction channel.

Channel flow, tectonic overpressure, and exhumation of high-pressure rocks in the Greater Himalayas

Abstract. The Himalayas are the archetype of continental collision, where a number of long-standing fundamental problems persist in the Greater Himalayan Sequence (GHS): (1) contemporaneous reverse and normal faulting, (2) inversion of metamorphic grade, (3) origin of high- (HP) and ultrahigh-pressure (UHP) rocks, (4) mode of ductile extrusion and exhumation of HP and UHP rocks close to the GHS hanging wall, (5) flow kinematics in the subduction channel, and (6) tectonic overpressure, here defined as TOP = P∕PL where P is total (dynamic) pressure and PL is lithostatic pressure. In this study we couple Himalayan geodynamics to numerical simulations to show how one single model, upward-tapering channel (UTC) flow, can be used to find a unified explanation for the evidence. The UTC simulates a flat-ramp geometry of the main underthrust faults, as proposed for many sections across the Himalayan continental subduction. Based on the current knowledge of the Himalayan subduction channel geometry and geological/geophysical data, the simulations predict that a UTC can be responsible for high TOP ( > 2). TOP increases exponentially with a decrease in UTC mouth width, and with an increase in underthrusting velocity and channel viscosity. The highest overpressure occurs at depths < −60 km, which, combined with the flow configuration in the UTC, forces HP and UHP rocks to exhume along the channel's hanging wall, as in the Himalayas. By matching the computed velocities and pressures with geological data, we constrain the GHS viscosity to be ≤ 1021 Pa s, and the effective convergence (transpression) to a value ≤ 10 %. Variations in channel dip over time may promote or inhibit exhumation (> or < 15°, respectively). Viscous deformable walls do not affect overpressure significantly enough for a viscosity contrast (viscosity walls to viscosity channel) of the order of 1000 or 100. TOP in a UTC, however, is only possible if the condition at the bottom boundary is no-outlet pressure; otherwise it behaves as a leaking boundary that cannot retain dynamic pressure. However, the cold, thick, and strong lithospheres forming the Indian and Eurasian plates are a good argument against a leaking bottom boundary in a flat-ramp geometry, and therefore it is possible for overpressure to reach high values in the GHS.

Phase Equilibrium Modeling of Retrograded Eclogite at the Kekesu Valley, Eastern Segment of SW Tianshan Orogen and Tectonic Implications

In this study, we report newly found strongly retrograded eclogite blocks in the Kekesu Valley, eastern segment of southwestern Tianshan, northwestern China. Based on detailed petrographical studies and mineral chemistry analyses, we constrain P-T evolution of two representative samples via phase equilibrium modeling method using THERMOCALC software. The peak stage is evidenced by the primary inclusions of omphacite+glaucophane+lawsonite (inferred)+chlorite+white mica in garnet. Using grossular and pyrope isopleth thermobarometry in garnet, the P peak conditions for this stage were constrained to be ~23 kbar, 480–500 °C and the T peak conditions were constrained to be 19–20 kbar, 500–520 °C. Subsequent exhumation led to the transformation of lawsonite to epidote. Using Si-inphengite barometry, the P was constrained to be 16–20 kbar at T of 500–520 °C for this stage. Further retrograde stage was evidenced by the mineral assemblages of glaucophane+garnet+epidote+paragonite+ chlorite+white mica+quartz. Later stage retrogression is evidenced by glaucophane+epidote+chlorite+ quartz+white mica+albite+actinolite with P of 5–9 kbar when T<490 °C, indicating the eclogite may have been exhumed to the middle crust level and overprinted by blueschist-greenschist facies metamorphism. The results show that eclogite at the eastern segment of SW Tianshan shares similar P-T path to the HP rocks from the western segment, implying a similar P-T evolution for these rocks.

LATE-AND POST-MIOCENE GEODYNAMIC EVOLUTION OF THE MESOGEA BASIN (EAST ATTICA, GREECE): CONSTRAINTS FROM SEDIMENT PETROGRAPHY AND STRUCTURES

In Attica, from the Miocene through the Quaternary, successive generations of detachment faults caused exhumation and denudation of Alpine HP rocks and – later on -formation of sedimentary basins. The Mesogea low angle detachment fault separates the HP rocks exposed at the southern flank of the Penteli Mtfrom the Late - post-Late Miocene Mesogea basin. Combined sedimentary-petrologic and structural analyses reveal the following: (i) Late Miocene sediments include material from unmetamorphosed source areas suggesting that, until then, parts of the HP rocks were buried under the (largely unmetamorphosed) Pelagonian nappe unit, (ii) Post-Late Miocene sediments exclusively contain clasts from high-P source areas and show downward bending of the layering that accommodates slip along a lis trie fault surface. Close to the Penteli Mt, within the post-Late-Miocene sediments gravity sliding-blocL· of metamorphic rocks occur. All this indicates post-Late Miocene activity along this detachment fault controlled rapid surface uplift/relief formation, denudation and fast erosion of HP rocks in the Penteli Mt.

The coherent ultrahigh-pressure terrane of the Tianshan meta - ophiolite belt, NW China

Blocks of basic rocks metamorphosed at high- and ultrahigh-pressures (HP-UHP) are often embedded in metasedimentary or granitic rocks that do not show evidence for HP metamorphism. The contrasting maximum depth reached by these isolated rock fragments, normally eclogites, and the enveloping rocks is commonly seen as evidence for a form of tectonic mélange. In the Tianshan meta-ophiolite belt of NW China, some reported eclogites and pelitic-felsic schists adjacent to coesite-bearing rocks apparently lack evidence of UHP conditions. Their deduced maximum pressures are 1.5–2.5 GPa. These HP blocks have been regarded as dismembered ophiolite fragments each with its own metamorphic history generated by a forced return flow in a subduction channel. Here we reappraise the P-T estimates, formerly derived by various P-T tools, of these reported HP rocks, including pelitic schists, mafic eclogites and eclogitic rocks of volcaniclastic origin, from the Akeyazi area. P-T pseudosections in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O contoured for garnet core or mantle composition (thermocalc version 3.33) have been used for deducing peak P-T conditions. It was found that all investigated samples were subject to UHP metamorphism (2.8–3.5 GPa at 520–580 °C). The used thermodynamic dataset for these P-T estimates has also been testified valid for typical UHP-LT rocks worldwide. We suggest that garnet isopleth thermobarometry-derived P-T estimates are potentially robust in accordance with UHP mineralogical evidence such as coesite, but are sensitive to garnet solution models and effective bulk compositions. Inconsistent P-T results and related tools involving sodic clinopyroxene and phengite should be dismissed in many cases, if not all. The consequences of the reevaluated P-T data are that eclogites, both interlayered and blocky, and the associated host rocks comprise a coherent Tianshan UHP terrane, not a highly disrupted mélange.

Divergent plate motion drives rapid exhumation of (ultra)high pressure rocks

Exhumation of (ultra)high pressure [(U)HP] rocks by upper-plate divergent motion above an unbroken slab, first proposed in the Western Alps, has never been tested by numerical methods. We present 2D thermo-mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate, followed by upper-plate divergent motion away from the lower plate. Results demonstrate how divergent plate motion may trigger rapid exhumation of large volumes of (U)HP rocks directly to the Earth's surface, without the need for significant overburden removal by erosion. Model exhumation paths are fully consistent with natural examples for a wide range of upper-plate divergence rates. Exhumation rates are systematically higher than the divergent rate imposed to the upper plate, and the modeled size of exhumed (U)HP domes is invariant for different rates of upper-plate divergence. Major variations are instead predicted at depth for differing model scenarios, as larger amounts of divergent motion may allow mantle-wedge exhumation to shallow depth under the exhuming domes. The transient temperature increase, due to ascent of mantle-wedge material in the subduction channel, has a limited effect on exhumed continental (U)HP rocks already at the surface. We test two examples, the Cenozoic (U)HP terranes of the Western Alps (continental upper plate) and eastern Papua New Guinea (oceanic upper plate). The good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre-Cenozoic (U)HP terranes where the geologic record of exhumation is only partly preserved.

Mantle Lithosphere Rheology, Vertical Tectonics, and the Exhumation of (U)HP Rocks

AbstractNumerical modeling results indicate that mantle lithosphere rheology can influence the pressure‐temperature‐time (P‐T‐t) trajectories of continental crust subducted and exhumed during the onset of continental collision. Exhumation of ultrahigh‐pressure (~35 kbar)/high‐temperature (~750°C) metamorphic rocks is more prevalent in models with stronger continental mantle lithosphere (e.g., dry), whereas high‐pressure (~9–22 kbar)/low‐temperature (350°C–630°C) metamorphic rocks occur in models with weaker rheology (e.g., hydrated) for the same layer. In the latter case, the buried crustal rocks can remain encased in ablatively subducting mantle lithosphere, reach only moderate temperatures, and exhume by dripping/detachment of the lithospheric root. In this transition from subduction to a dripping style of “vertical tectonics,” burial and exhumation of crustal rocks are driven without imposed far‐field plate convergence. The model results are compared against thermobarometric P‐T estimates from major (ultra)high‐pressure metamorphic terranes. We propose that the exhumation of high‐pressure/low‐temperature metamorphic rocks in Tavşanlı and Afyon zones in western Anatolia may be caused by viscous dripping of mantle lithosphere suggesting a weaker continental mantle lithosphere, whereas (ultra)high‐pressure exhumation (e.g., Dabie Shan‐eastern China and Dora Maira‐western Alps) may be associated with plate‐like subduction. In the latter case, the slab is much stronger and deformation is localized to the subduction interface along which rocks are buried to &gt;100 km depth before they are exhumed to the near surface.

Mantle wedge exhumation beneath the Dora-Maira (U)HP dome unravelled by local earthquake tomography (Western Alps)

In continental subduction zones, the behaviour of the mantle wedge during exhumation of (ultra)high-pressure [(U)HP] rocks provides a key to distinguish among competing exhumation mechanisms. However, in spite of the relevant implications for understanding orogenic evolution, a high-resolution image of the mantle wedge beneath the Western Alps is still lacking. In order to fill this gap, we perform a detailed analysis of the velocity structure of the Alpine belt beneath the Dora-Maira (U)HP dome, based on local earthquake tomography independently validated by receiver function analysis. Our results point to a composite structure of the mantle wedge above the subducted European lithosphere. We found that the Dora-Maira (U)HP dome lays directly above partly serpentinized peridotites (Vp ~7.5km/s; Vp/Vs=1.70–1.72), documented from ~10km depth down to the top of the eclogitized lower crust of the European plate. These serpentinized peridotites, possibly formed by fluid release from the subducting European slab to the Alpine mantle wedge, are juxtaposed against dry mantle peridotites of the Adriatic upper plate along an active fault rooted in the lithospheric mantle. We propose that serpentinized mantle-wedge peridotites were exhumed at shallow crustal levels during late Eocene transtensional tectonics, also triggering the rapid exhumation of (U)HP rocks, and were subsequently indented under the Alpine metamorphic wedge in the early Oligocene. Our findings suggest that mantle-wedge exhumation may represent a major feature of the deep structure of exhumed continental subduction zones. The deep orogenic levels here imaged by seismic tomography may be exposed today in older (U)HP belts, where mantle-wedge serpentinites are commonly associated with coesite-bearing continental metamorphic rocks.

Investigation of Ge and Ga exchange behaviour and Ge isotopic fractionation during subduction zone metamorphism

Germanium (Ge) and gallium (Ga) are slightly to moderately incompatible trace elements during magmatic processes and may be indicators of the nature of the mantle source. Because of their solubility in fluids, Ge and Ga are also geochemical tracers of various hydrothermal processes. Understanding high-pressure/low-temperature (HP–LT) recycling processes of Ge and Ga through fluid-mediated mass transfers in subduction zones can then allow discussing whether they can be source of geochemical heterogeneities in the mantle wedge. We have analysed Ge and Ga abundances and Ge isotope composition of a series of well-characterised HP–LT metabasites of mid-ocean ridge basalt (MORB) affinity and pelitic micaschists from the Ile de Groix (France), to determine Ge and Ga behaviour and Ge isotope fractionation during subduction zone metamorphism (SZM). Metabasites have Ge and Ga contents of 1.2–2.1ppm and 17–22ppm, respectively, typical of tholeiitic basalts. Garnet-bearing blueschists and eclogites have δ74Ge values (+0.42 to +0.65‰) similar within error to that of tholeiitic basalts (+0.55 to +0.57‰; Luais, 2012).Our study shows that during the prograde metamorphism, Ge and Ga abundances and δ74Ge values do not vary from blueschists to eclogites, owing to the large stability field of Ge- and Ga-hosting minerals (epidote, garnet, titanite, amphiboles and omphacite) under varying P–T conditions. During the retrograde metamorphism, the slight decrease of the Ga content of c. 8% in greenschists (19±1ppm; 2σ) compared to HP rocks (21±1ppm; 2σ) suggests Ga loss during retrogression, whereas Ge abundances remain within the same range as blueschists and eclogites. In the garnet-bearing greenschists (early stages of retrogression), the δ74Ge values (+0.38 to +0.49‰) remain similar within error to HP rocks. However, albite- and calcite-bearing greenschists have higher δ74Ge values (+0.84 to +0.98‰), showing evidence of Ge fractionation under intensive fluid-rock interactions (late stage of retrogression). Micaschists have Ge and Ga abundances within the same range as continental crust and schists. Their variations in δ74Ge (+0.29 to +0.70‰) reflect variations in the sedimentary source and protolith composition.The relatively small variation of δ74Ge values in metabasites compared to mantle rocks implies that the subducted oceanic crust cannot trigger Ge isotopic heterogeneities in the mantle through fluid-induced metasomatism or recycling in the deep mantle. However, during the late stages of retrogression, migration of oxidising fluids along the crust-mantle wedge may be responsible for a resolvable increase of the δ74Ge value of the mantle wedge along the subduction channel. Besides, the larger range of Ge concentrations and δ74Ge values in the micaschists suggests that recycling of subducted sediments may generate mantle Ge elemental and isotopic heterogeneities.

Protracted (~ 30 Ma) eclogite-facies metamorphism in northern Victoria Land (Antarctica): Implications for the geodynamics of the Ross/Delamerian Orogen

Northern Victoria Land, Antarctica, plays a key role in any geodynamic reconstructions of the paleo-Pacific margin of Gondwana because it represents the along-strike continuation of Australia in Antarctica and hosts the only Paleozoic eclogites known along the Transantarctic Mountains. The architecture and evolution of the early Cambrian to Early Ordovician Ross–Delamerian Orogen in northern Victoria Land, however, remain controversial. Accurate geodynamic reconstructions have been hindered by a dearth of temporal metamorphic constraints. This study reports laser-ablation split-stream ICP-MS data on zircon in newly recognized eclogite, that preserves a memory of the prograde evolution, and on zircon from its country rock from the Lanterman Range of northern Victoria Land. Geochronological data are supplemented for the eclogite by whole-rock geochemical and petrological constraints, that indicate an E-MORB protolith derived from a depleted mantle source, and P–T conditions for the eclogite-facies stage of ~700°C and 1.7–2.4GPa. U–Pb ages, trace-element contents, cathodoluminescence imaging and textural/inclusion relationships testify to protracted zircon growth/recrystallization history under eclogite-facies conditions, that also included (re)crystallization during part of the prograde path. Results extend the onset of eclogite-facies metamorphism in northern Victoria Land back to ~530Ma and, together with geochronological evidence for the emplacement of calc-alkaline granitoids at ~530–520Ma, suggest that UHP–HP rocks formed by accretion of material beneath an active continental margin. Regionally, this interpretation implies that burial of mafic rocks and associated siliciclastic sequences to mantle depths commenced significantly earlier than previously believed (~500Ma), thereby contrasting with the common belief that high-pressure metamorphism was linked to arc–continent collision. New and old results reveal the coexistence along the same ridge of two eclogite types: (1) medium-grained, “colder” and undeformed eclogites, that recorded both prograde and peak conditions, and (2) finer-grained, “hotter” and slightly deformed eclogites, that recorded only metamorphic peak conditions with hints of the subsequent retrogression. We propose that “prograde/peak” eclogites formed during an active continental margin stage and “peak” eclogites during the subsequent island arc–continental arc collision.

Na-P concentrations in high-pressure garnets: A potentially rich, but riskyP-Trepository

Establishing the history of HP and UHP metamorphic rocks is important for quantifying Earth dynamics. The history of these rocks defines, among other things, paleo-subduction rates, P-T paths, and the kinematics of continent-continent collision. Although the appearance of certain minerals, such as coesite, stishovite, or diamond, provides unequivocal evidence of an HP or UHP component to the history of a rock, they cannot provide details of the P-T-t path a rock has experienced. However, complex solid solutions can. This reflects the fact that solid solutions have the potential to provide a continuous thermodynamically controlled response to evolving P-T conditions. To the extent that such solid-solution characteristics are preserved unmodified in a mineral throughout its history, a detailed description of the trajectory of the rock during burial …

Tracking the timing of subduction and exhumation using 40Ar/39Ar phengite ages in blueschist- and eclogite-facies rocks (Sivrihisar, Turkey)

Geochronologic studies of high-pressure/low-temperature rocks can be used to determine the timing and rates of burial and exhumation in subduction zones by dating different stages of the pressure–temperature history. In this study, we present new in situ UV laser ablation 40Ar/39Ar phengite ages from a suite of lawsonite blueschist- and eclogite-facies rocks representing different protoliths (metabasalt, metasediment), different structural levels (within and outside of a high-strain zone), and different textural positions (eclogite pod core vs. margin) to understand the timing of these events in an exhumed Neo-Tethyan subduction zone (Sivrihisar Massif, Tavsanli Zone, Turkey). Weighted mean in situ 40Ar/39Ar ages of phengite from the cores of lawsonite eclogite pods (90–93 Ma) are distinctly older than phengite from retrogressed, epidote eclogite (82 ± 2 Ma). These ages are interpreted as the age of peak and retrograde metamorphism, respectively. Eclogite records the narrowest range of ages (10–14 m.y.) of any rock type analyzed. Transitional eclogite- and blueschist-facies assemblages and glaucophane-rimmed lawsonite + garnet + phengite veins from eclogite pod margins record a much wider age range of 40Ar/39Ar ages (~20 m.y.) with weighted mean ages of ~91 Ma. Blueschists and quartzites record more variable 40Ar/39Ar ages that may in part be related to structural position: samples within a high-strain zone at the tectonic contact of the HP rocks with a meta-ultramafic unit have in situ UV laser ablation 40Ar/39Ar ages of 84.0 ± 1.3–103.7 ± 3.1 Ma, whereas samples outside this zone range to older ages (84.6 ± 2.4–116.7 ± 2.7 Ma) and record a greater age range (22–38 m.y.). The phengite ages can be correlated with the preservation of HP mineral assemblages and fabrics as well as the effects of deformation. Collectively, these results show that high-spatial resolution UV laser ablation 40Ar/39Ar phengite data, when considered in a petrologic and structural context, may document prograde (burial) and retrograde (exhumation) stages of subduction metamorphism.

Re–Os and Lu–Hf isotopic constraints on the formation and age of mantle pyroxenites from the Bohemian Massif

We report on the Lu–Hf and Re–Os isotope systematics of a well-characterized suite of spinel and garnet pyroxenites from the Gföhl Unit of the Bohemian Massif (Czech Republic, Austria). Lu–Hf mineral isochrons of three pyroxenites yield undistinguishable values in the range of 336–338Ma. Similarly, the slope of Re–Os regression for most samples yields an age of 327±31Ma. These values overlap previously reported Sm–Nd ages on pyroxenites, eclogites and associated peridotites from the Gföhl Unit, suggesting contemporaneous evolution of all these HT–HP rocks. The whole-rock Hf isotopic compositions are highly variable with initial εHf values ranging from −6.4 to +66. Most samples show a negative correlation between bulk rock Sm/Hf and εHf and, when taking into account other characteristics (e.g., high 87Sr/86Sr), this may be explained by the presence of recycled oceanic sediments in the source of the pyroxenite parental melts. A pyroxenite from Horní Kounice has decoupled Hf–Nd systematics with highly radiogenic initial εHf of +66 for a given εNd of +7.8. This decoupling is consistent with the presence of a melt derived from a depleted mantle component with high Lu/Hf. Finally, one sample from Bečváry plots close to the MORB field in Hf–Nd isotope space consistent with its previously proposed origin as metamorphosed oceanic gabbro. Some of the websterites and thin-layered pyroxenites have variable, but high Os concentrations paralleled by low initial γOs. This reflects the interaction of the parental pyroxenitic melts with a depleted peridotite wall rock. In turn, the radiogenic Os isotope compositions observed in most pyroxenite samples is best explained by mixing between unradiogenic Os derived from peridotites and a low-Os sedimentary precursor with highly radiogenic 187Os/188Os. Steep increase of 187Os/188Os at nearly uniform 187Re/188Os found in a few pyroxenites may be connected with the absence of primary sulfides, but the presence of minor late stage sulfide-bearing veinlets likely associated with HT–HP metamorphism at crustal conditions.

Kinematics of syn- and post-exhumational shear zones at Lago di Cignana (Western Alps, Italy): constraints on the exhumation of Zermatt–Saas (ultra)high-pressure rocks and deformation along the Combin Fault and Dent Blanche Basal Thrust

Kinematic analyses of shear zones at Lago di Cignana in the Italian Western Alps were used to constrain the structural evolution of units from the Piemont–Ligurian oceanic realm (Zermatt–Saas and Combin zones) and the Adriatic continental margin (Dent Blanche nappe) during Palaeogene syn- and post-exhumational deformation. Exhumation of Zermatt–Saas (U)HP rocks to approximately lower crustal levels at ca. 39 Ma occurred during normal-sense top-(S)E shearing under epidote–amphibolite-facies conditions. Juxtaposition with the overlying Combin zone along the Combin Fault at mid-crustal levels occurred during greenschist-facies normal-sense top-SE shearing at ca. 38 Ma. The scarcity of top-SE kinematic indicators in the hanging wall of the Combin Fault probably resulted from strain localization along the uppermost Zermatt–Saas zone and obliteration by subsequent deformation. A phase of dominant pure shear deformation around 35 Ma affected units in the direct footwall and hanging wall of the Combin Fault. It is interpreted to reflect NW–SE crustal elongation during updoming of the nappe stack as a result of underthrusting of European continental margin units and the onset of continental collision. This phase was partly accompanied and followed by ductile bulk top-NW shearing, especially at higher structural levels, which transitioned into semi-ductile to brittle normal-sense top-NW deformation due to Vanzone phase folding from ca. 32 Ma onwards. Our structural observations suggest that syn-exhumational deformation is partly preserved within units and shear zones exposed at Lago di Cignana but also that the Combin Fault and Dent Blanche Basal Thrust experienced significant post-exhumational deformation reworking and overprinting earlier structures.

Deciphering protoliths of the (U)HP rocks in the Makbal metamorphic complex, Kyrgyzstan: geochemistry and SHRIMP zircon geochronology

The (ultra)high-pressure ( U ) HP Makbal metamorphic complex (MMC) belongs to a chain of early Paleozoic ( U ) HP terranes to the south of the Paleo-Kazakhstan continent, a major component of the Central Asian Orogenic Belt. We present geochemical characteristics, reconstructions of geodynamic settings and constraints on the emplacement and depositional ages of metamorphosed intrusive and sedimentary rocks of the MMC. The MMC comprises variably metamorphosed juxtaposed packages of mica schists, hosting boudins of metamorphosed mafic rocks, metaquartzites and metacarbonates in a tectonic melange. The rocks, metamorphosed under ( U ) HP conditions, are situated in the central part of the terrane. The UHP garnet–talc–chloritoid schists are believed to have formed from immature pelitic sediments with maximum depositional ages of 700–650 Ma. The ( U ) HP metaquartzites comprise mature sediments derived from a Paleoproterozoic continental source. They were formed in a passive-margin environment and have a maximum depositional age of ca . 1600 Ma. The chemical composition of the MMC metabasic rocks is compatible with subalkaline tholeiitic basalts with E-MORB compositions; however, some samples have N-MORB and OIB affinities. The ages of zircon xenocrysts from the metabasic rocks show that the mafic intrusions are probably younger than ca . 700 Ma. The metabasic rocks represent dikes and sills emplaced into immature pelitic sediments in a rift-related or continental-slope environment at the final stages of the Rodinia break up (700–600 Ma) when shallow mantle melting produced relatively primitive tholeiitic compositions. The whole sequence, including the quartzites, was later juxtaposed with other passive-margin sediments and metamorphosed under ( U ) HP conditions in a Caledonian subduction zone between 500 and 475 Ma. The ages of detrital zircon grains from the MMC rocks indicate derivation from a Paleoproterozoic continental source and match the age pattern characteristic for the Northern Tien Shan, with major peaks at 1.95–1.75 Ga and smaller peaks between 2.7 and 2.5 Ga and at ca . 1.0 Ga. The ages between 1.65 and 1.4 Ga are uncharacteristic for the Kyrgyz Northern Tien Shan, instead may be correlated with recently documented voluminous early Mesoproterozoic arc magmatism in the Central Tien Shan Arc Terrane in China. The resetting of the U–Th–Pb system and the growth of new zircon in the MMC rocks were probably defined by the permeability to aqueous fluids. An age of 456 ± 4 Ma, obtained for the rims of zircon xenocrysts from a small amphibolite boudin in the southern part of the MMC, corresponds with the age of an adjacent Ordovician granite and indicates that Paleozoic intrusions could have provided sufficient heat to cause resetting and/or growth of new zircon in the MMC rocks.

High-pressure granulites of the Podolsko complex, Bohemian Massif: An example of crustal rocks that were subducted to mantle depths and survived a pervasive mid-crustal high-temperature overprint

The Podolsko complex, Bohemian Massif, is a mid-crustal migmatite–granite dome exposed along a tectonic boundary separating the upper crust from the deeply eroded interior of the Variscan orogen, referred to as the Moldanubian Zone. This study examines metamorphic history of mafic and felsic granulites that occur in this complex as minor lenses or layers hosted in pervasively anatectic rocks. The mafic granulite contains garnet with preserved high-Ca cores, which based on pseudosection modelling indicates pressure conditions near the coesite stability field at temperatures of ca. 550–600°C. The relicts of an earlier eclogite-facies stage have been overprinted by a later granulite-facies assemblage consisting of ternary feldspar, orthopyroxene, and spinel in the mafic granulite and sillimanite and spinel in the felsic granulite. Composition of younger garnet (in rims and as smaller grains) in both granulites suggests that a near isothermal decompression of these rocks was followed by heating that reached temperature of ca. 900°C at pressure of ca. 0.5GPa. It is thus concluded that the granulites underwent at least two temporally separate tectonometamorphic events: they were first subducted to mantle depths and exhumed rapidly at relatively low temperatures and then near isobarically heated at mid-crustal levels. The preservation of earlier eclogite-facies garnet in the mafic granulite indicates that the latter event was short-lived and was followed by near isobaric cooling. The geologically brief granulite-facies metamorphism was previously explained as a result of slab break-off and mantle upwelling after the main phase of microplate convergence in the Bohemian Massif. To put the Podolsko complex into a broader tectonic context, we synthesize the available petrologic and structural data from the correlative (U)HP assemblages of the Moldanubian Zone to suggest that they typically do not preserve structural record of the subduction stage, only rarely preserve an early exhumation phase to the middle crust, and most, if not all, have been extensively overprinted by late-stage processes at shallower crustal levels. This synthesis also reveals that kinematics and thus tectonic processes driving the final exhumation of the (U)HP rocks were diverse and may vary from one location to another. These local settings include exhumation in the footwall of major extensional detachments, due to folding and subsequent lateral extrusion, and along crustal-scale strike-slip shear zones. The extensive late-stage overprint masking the earlier (pre-340Ma) subduction history is greatly exemplified in the Podolsko complex where the granulites are only small relics in pervasively anatectic rocks melted at mid-crustal conditions and then rapidly exhumed along extensional detachments to upper-crustal levels.

Late Palaeozoic 40Ar/39Ar ages of the HP-LT metamorphic rocks from the Kekesu Valley, Chinese southwestern Tianshan: new constraints on exhumation tectonics

abstractAlthough numerous ages have been obtained for the Chinese southwestern Tianshan high pressure/ultrahigh pressure-low temperature (HP/UHP-LT) metamorphic belt in the past two decades, its exhumation history is still controversial. The poor age constraint was related to the appealing low metamorphic temperatures and excess Ar commonly present under HP/UHP conditions. This study aims to provide new age constraints on the orogen’s exhumation by obtaining 40Ar/39Ar mica ages using the conventional step-heating technique, with emphasis on the avoidance of excess Ar contamination. From a cross section along the Kekesu Valley, four samples, three from the HP-LT metamorphic belt (TK050, TK051, and TK081) and one from the southern margin of the low pressure metamorphic belt (TK097), were selected for 40Ar/39Ar dating. Phengites from garnet glaucophane schist TK050 and the surrounding rock garnet phengite schist TK051 yield comparable plateau ages of 321.4 ± 1.6 and 318.6 ± 1.6 Ma, respectively, while epidote mica schist TK081 gives a younger plateau age of 293.3 ± 1.5 Ma. Considering the chemical compositions of phengites, mineral assemblages, and microstructures in the thin slices, we suppose that the former represents the time the HP rocks retrograded from the peak stage (eclogite facies) to the (epidote)-blueschist facies, whereas the latter reflects greenschist facies overprinting. Biotite and muscovite from two-mica quartzite TK097 give similar plateau ages of 253.0 ± 1.3 and 247.1 ± 1.2 Ma, interpreted to date movement on the post collisional transcrustal South Nalati ductile shear zone. By combining our new ages with published data, a two-stage exhumation model is suggested for the Chinese southwestern Tianshan HP/UHP-LT metamorphic belt: initial fast exhumation to a depth of about 30–35 km by ~320 Ma was followed by relatively slow (~1 mm year–1) uplift to ~10 km by ~293 Ma.

Current challenges for explaining (ultra)high‐pressure tectonism in the Pennine domain of the Central and Western Alps

AbstractThe Pennine domain of the Central and Western (PCW) Alps, including the Dora‐Maira, Monte Rosa, Gran Paradiso, Adula/Cima Lunga nappes and the Zermatt‐Saas zone underwent ultrahigh‐ or high‐pressure [(U)HP &gt;1.5 GPa] metamorphism during the Alpine orogeny. We review structural, petrological and geochronological data for the (U)HP units in the PCW Alps (i) to clarify the relationship between (U)HP metamorphism and deformation, (ii) to confront published exhumation models for the (U)HP units with the reviewed data and (iii) to evaluate consequences of different pre‐Alpine paleogeographic settings (Penrose‐type ocean v. hyperextended margins) on the Alpine orogeny. The review indicates that (i) peak pressures are recorded only in minor volumes of the corresponding tectonic nappes; (ii) (U)HP rocks occur within coherent and imbricate thrust sheets which show substantial pressure jumps; (iii) peak pressures are mostly associated with a top‐to‐the‐foreland kinematics; (iv) decompression from (U)HP (4 to &gt;1.5 GPa) to greenschist or amphibolite facies (~1 GPa) metamorphic conditions was fast (&lt; c. 2 Ma); and (v) accreted fragments of the Alpine–Tethyan basins reflect Mesozoic hyperextended margins rather than mature Penrose‐type oceans. The comparison of these characteristics with numerical exhumation models suggests that exhumation of (U)HP rocks by buoyancy‐driven return flow within a subduction channel under near‐lithostatic pressure is unlikely because (i) models predict volumes of (U)HP rocks that are more than an order of magnitude larger as observed in the PCW Alps and (ii) the required top‐to‐the‐hinterland (U)HP kinematics in the hangingwall of (U)HP units has not been observed until now. Also, the buoyancy‐driven exhumation of the individual crustal (U)HP units within weak (≤1019 Pa s) and denser (&gt; ~3180 kg m−3) rocks is questionable, because there is no evidence of such rocks around the (U)HP units. An alternative model, which could explain the main characteristics of the (U)HP units in the PCW Alps, is an orogenic wedge model that (i) involves dynamic stresses deviating from lithostatic pressure and (ii) is formed during the convergence of hyperextended margins. Deviations of dynamic stresses from the lithostatic pressure and local pressure variations cannot be excluded during the Alpine orogeny, but these deviations and variations have not been clearly identified until now.

Tectonic overpressure and underpressure in lithospheric tectonics and metamorphism

AbstractThe lithostatic pressure concept is most commonly applied on a geological scale for lithospheric processes and related evolution of metamorphic rock complexes. Here, various aspects of non‐lithostatic overpressure and underpressure phenomena in lithospheric tectonics and metamorphism are reviewed on the basis of recently published literature. The main conclusion from this short review is that these phenomena certainly exist in nature on all time and space scales including geological ones. They are, in particular, responsible for some geological processes, which are otherwise difficult to explain, such as downward water suction into the interior of subducting slabs. Magnitudes of overpressure and underpressure are strongly variable and may potentially reach up to ±100% of the lithostatic pressure and up to a GPa‐level. These magnitudes depend mainly on the rheology of deforming rocks and on the nature of related tectonic process. Rheological heterogeneity of deforming rock units, which is common in nature, has a tendency to enhance overpressure and underpressures. Large overpressure can typically be expected in rheologically strong (dry) bending rock units, in particular in the mantle lithosphere. However, rheological weakness of rocks and small local deviatoric stresses do not guarantee the absence of large overpressures in these rocks. Therefore, the influence of significant tectonic overpressure and/or underpressure cannot be excluded for any metamorphic complex a priori but should be instead tested by exploring realistic thermomechanical models for envisaged tectono‐metamorphic scenarios. Many lithospheric rocks subjected to large overpressures and underpressures cannot be studied as they do not exhume to the surface. Some controversy exists concerning overpressure magnitudes for the ultrahigh‐pressure (UHP) rocks and several conflicting hypotheses are proposed, which need to be thoroughly tested in the future. In this respect, the Alpine region may offer a unique opportunity for the testing of geological‐scale overpressures in (U)HP rocks by combining structural‐geological and petrological data with realistic lithospheric‐scale numerical modelling.

Slab rupture and delamination under the Betics and Rif constrained from receiver functions

We map the lithospheric structure under the westernmost Mediterranean convergent setting interpreting P-receiver functions obtained from a dense seismic network. No orogenic root occurs under the eastern and great part of the central Betics. However, the subducted South Iberian continental lithosphere is found beneath the western Betics where the Iberian Moho reaches depths of approximately 65km, dipping gently towards the SE. Meanwhile, at the Rif, strong crustal and lithospheric thickness contrasts occur across the Nekor NW-SE sinistral fault that overlies the region of present slab tearing. East of the Nekor fault there is no orogenic root and the crust has been thinned to approximately 22km, whilst to the west the crust reaches 55km thickness and the Maghrebian continental lithosphere is attached to the lithospheric slab imaged by tomography under the Alboran basin. These data suggest that subduction rollback under the Alboran and Algerian basins, together with continental slab tearing or detachment producing edge delamination under the Betics and Rif have been the main tectonic mechanisms driving extension, magmatism and regional uplift in the westernmost Mediterranean since the Late Miocene until present. The surface expression of edge-delamination and slab tearing is marked by regional uplift, denudation of HP rocks in elongated core-complex type domes, late Miocene volcanism in the Eastern Betics and Rif, and by large NE-SW strike-slip transfer faults like the Alpujarras, Crevillente, Torcal or Nekor faults that accommodate strong gradients in crustal displacements. The Iberian slab is still attached to the oceanic slab imaged under the Alboran basin at the western Betics where intermediate depth seismicity, recent dextral strike-slip faulting and folding, could reflect slab tearing. Meanwhile, active faulting and differential GPS-measured displacements would mark slab tearing beneath the Rif coinciding with the trace of the sinistral Nekor fault.