Metamorphic Facies Series Research Articles

Early Paleozoic metamorphic P–T–t evolution of the northern Cathaysia Block (SE China): Implications for thermal history and geodynamic setting

The tectono-thermal evolution of the Early Paleozoic orogeny in South China has been debated. In this contribution, detailed studies of petrography, mineral chemistry, phase equilibria modeling, conventional geothermobarometry, and zircon–monazite geochronology were conducted on metamorphic rock samples from the Zhoutan, Longyou, and Chencai groups in the northern Cathaysia Block, South China. All rocks record clockwise P–T paths, including the Pmax (pre-Tmax), Tmax, and post-peak retrograde stages. Sillimanite-garnet-mica gneiss from the Zhoutan Group is characterized by decompression and heating from the Pmax (0.97 GPa/710 °C) to the Tmax stage (0.70–0.85 GPa/795–815 °C). Garnet-amphibole gneiss from the Longyou Group and sillimanite-garnet-biotite gneiss from the Chencai Group experienced Tmax metamorphism at 0.90–1.00 GPa/865–880 °C and 0.90–1.00 GPa/805–820 °C, respectively. LA-ICP-MS zircon–monazite UPb dating yields 453–446 Ma for the retrograde metamorphism of the Zhoutan Group, ∼457 Ma and ∼432 Ma for the peak and retrograde metamorphism, respectively, of the Longyou Group, and 441–436 Ma for the peak metamorphism of the Chencai Group. This indicates that the Early Paleozoic crustal thickening event may have occurred 10–20 Ma earlier in the western region than in the eastern region of the northern Cathaysia Block. Integrated Early Paleozoic P–T–t data from the Cathaysia Block provide medium-pressure facies series conditions without the products of high-pressure facies series metamorphism, which are common in subduction-to-collision orogens. Based on the current metamorphic characteristics and other geological data for Early Paleozoic orogeny in South China, we infer that the current tectonic domains developing metamorphism–magmatism–deformation may not be the “first scene” of subduction-to-collision. The stress that caused the Early Paleozoic orogeny likely originated from the ancient plate boundaries of South China.

Geochemical variations of anatectic melts in response to changes of P–T–H2O conditions: Implication for the relationship between dehydration and hydration melting in the Himalayan orogen

The anatectic mechanism is the most fundamental criterion in determining the petrogenetic relationship between migmatite, granulite and granite at convergent plate margins. Although two main anatectic mechanisms, namely water-absent (dehydration) and water-fluxed (hydration) melting, have been identified in many collisional orogens, their spatiotemporal relationship and genetic connection remain obscure. To address this issue, we conduct an integrated study of forward phase equilibrium and trace element modelling for partial melting of metapelite in the Himalayan orogen. The results are used to decipher changes of phase relation and melt composition in response to changes of temperature, pressure and water content. Pressure and water content mainly affect the solidus and evolving residual mineral assemblages, whereas temperature controls the anatectic reaction. Hydration melting at lower pressure and temperature would produce anatectic melts with relatively high SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios, but low Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios. In contrast, dehydration melting at higher pressure and temperature would generate anatectic melts with relatively high Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios, but low SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios. A comparison of geochemical compositions between the modelled melts and the two groups of well-characterized leucogranites in the Himalayan orogen indicates that the coeval dehydration and hydration melting would likely occur at the deeper and shallower crustal levels, respectively. In combination with the secular evolution of metamorphic facies series in the Himalayan orogen, we propose a spatiotemporally coupled dehydration-hydration melting mechanism. The early metamorphism at lower geothermal gradients would result in metamorphic dehydration in the lower crust and hydration in the upper crust during the syn-collisional period in the Early Cenozoic. As a result, the later metamorphism at higher geothermal gradients would lead to dehydration melting in the lower crust and hydration melting in the upper crust during the post-collisional period in the Late Cenozoic. This study highlights the applicability of phase equilibrium and trace element modelling to reveal the geochemical variations of anatectic melts in response to changes of P–T–H2O conditions. This provides new insights into the relationship between dehydration and hydration melting in collisional orogens.

High-pressure granulite-facies metamorphism in the junction between the Siberian and Kazakhstan-Junggar continents and implications for the assembly of Pangea

The assembly of Pangea is usually thought to be finally completed by the accretion of the Siberian continent to Laurussia, but the exact timing of the accretion is poorly constrained in global paleogeographic reconstructions, which lagged behind the Kazakhstan-Laurussian amalgamation in the latest Mississippian and was possibly coeval with, or after, the amalgamation of the Tarim and Kazakhstan-Junggar continents in the Pennsylvanian. In this respect, the geological features, such as the medium-pressure facies series metamorphism in the Irtysh Complex, NW China, from the junction between the Siberian and Kazakhstan-Junggar continents are expected to provide crucial constraints on the timing of the accretion. The metamorphic rocks in the Irtysh Complex record the high-pressure granulite-facies metamorphism with the assemblage of kyanite + K-feldspar + garnet and show a complete clockwise metamorphic P-T-t path, involving rapid burial to the peak P-T conditions of ∼11 Kbar and 800 °C at ∼300 Ma and complicated exhumation to the surface during the Permian. The Pennsylvanian (∼322–300 Ma) rapid burial was coeval with the collision of Siberian and Kazakhstan-Junggar continents, whereas the Permian exhumation was triggered by post-collisional extension and sinistral shearing and can be divided into three substages: isothermal decompression at ∼300 Ma, near-isobaric cooling (∼300–268 Ma), and decompression-cooling to the surface prior to ∼253 Ma. The high-pressure granulite-facies metamorphism suggests that the Siberian continent was amalgamated with the Kazakhstan-Junggar continent in the Pennsylvanian, almost coeval with the amalgamation of the Tarim and Kazakhstan-Junggar continents. Therefore, the assembly of Pangea was completed in the latest Carboniferous at least.

Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

AbstractCrustal metamorphism under extreme pressure-temperature conditions produces characteristic ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) mineral assemblages at convergent plate boundaries. The formation and evolution of these assemblages have important implications, not only for the generation and differentiation of continental crust through the operation of plate tectonics, but also for mountain building along both converging and converged plate boundaries. In principle, extreme metamorphic products can be linked to their lower-grade counterparts in the same metamorphic facies series. They range from UHP through high-pressure (HP) eclogite facies to blueschist facies at low thermal gradients and from UHT through high-temperature (HT) granulite facies to amphibolite facies at high thermal gradients. The former is produced by low-temperature/pressure (T/P) Alpine-type metamorphism during compressional heating in active subduction zones, whereas the latter is generated by high-T/P Buchan-type metamorphism during extensional heating in rifting zones. The thermal gradient of crustal metamorphism at convergent plate boundaries changes in both time and space, with low-T/P ratios in the compressional regime during subduction but high-T/P ratios in the extensional regime during rifting. In particular, bimodal metamorphism, one colder and the other hotter, would develop one after the other at convergent plate boundaries. The first is caused by lithospheric subduction at lower thermal gradients and thus proceeds in the compressional stage of convergent plate boundaries; the second is caused by lithospheric rifting at higher thermal gradients and thus proceeds in the extensional stage of convergent plate boundaries. In this regard, bimodal metamorphism is primarily dictated by changes in both the thermal state and the dynamic regime along plate boundaries. As a consequence, supercontinent assembly is associated with compressional metamorphism during continental collision, whereas supercontinent breakup is associated with extensional metamorphism during active rifting. Nevertheless, aborted rifts are common at convergent plate boundaries, indicating thinning of the previously thickened lithosphere during the attempted breakup of supercontinents in the history of Earth. Therefore, extreme metamorphism has great bearing not only on reworking of accretionary and collisional orogens for mountain building in continental interiors, but also on supercontinent dynamics in the Wilson cycle.

Early Paleozoic high-temperature metamorphism of garnet amphibolite in the Longyou area, Cathaysia Block of South China: P–T path and tectonic implications

Garnet amphibolite can provide crucial information on tectonic processes. Recent progresses on garnet amphibolite in the Longyou area of the Cathaysia Block, South China have attracted geologists’ wide concerns and discussions about the regional metamorphic history and geodynamic setting. The Longyou garnet amphibolite shows a prominent porphyroblastic texture involving garnet (~40 vol%), hornblende, clinopyroxene, quartz and ilmenite, with minor plagioclase (<5 vol%), sphene, rutile, pyrite and apatite. Garnet presents as aggregations with double-/multi-nuclear structure and has embayed rims surrounded by quartz and plagioclase. Clinopyroxene is partially replaced by late hornblende and plagioclase. A clockwise high-temperature P–T evolution involving four stages has been determined. The generalized prograde stage shows increases of P–T conditions. The Pmax and Tmax stages lie in P–T conditions of 13 ± 0.6 kbar/808 ± 13 °C and 10 ± 0.9 kbar/840 ± 10 °C, reflecting apparent thermal gradients of ~17 °C/km and 22–23 °C/km, respectively. Both stages match medium-pressure facies series metamorphism. The late retrograde stage shows decompression and cooling. A P–XMg [=Mg/(Mg + Fe) in mole] pseudosection drawn at T = 840 °C suggests that low bulk-rock XMg contents (=0.29–0.33) in the Longyou garnet amphibolite are responsible for the high content of garnet in the rock. A P–M(Na2O) pseudosection at T = 840 °C reveals that the low content of plagioclase in the rock is caused by low bulk-rock Na2O contents (=0.44–1.18 mol.%). The integrated P–T–t data from the Cathaysia Block indicate that the early Paleozoic metamorphism can be connected to an intraplate orogenic setting.

Ultra-High Pressure Metamorphism and Geochronology of Garnet Clinopyroxenite in the Paleozoic Dunhuang Orogenic Belt, Northwestern China

Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure–temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790–920 °C/28–41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389–370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried.

A review of the chondrite–achondrite transition, and a metamorphic facies series for equilibrated primitive stony meteorites

AbstractHere, the petrological features of numerous primitive achondrites and highly equilibrated chondrites are evaluated to review and expand upon our knowledge of the chondrite–achondrite transition, and primitive achondrites in general. A thermodynamic model for the initial silicate melting temperature and progressive melting for nearly the entire known range of oxidation states is provided, which can be expressed as Tm = 0.035Fa2−3.51Fa + 1109 (in °C, where Fa is the proportion of fayalite in olivine). This model is then used to frame a discussion of textural and mineralogical evolution of stony meteorites with increasing temperature. We suggest that the metamorphic petrology of these meteorites should be based on diffusive equilibration among the silicate minerals, and as such, the chondrite–achondrite transition should be defined by the initial point of silicate melting, not by metal–troilite melting. Evidence of silicate melting is preserved by a distinctive texture of interconnected interstitial plagioclase ± pyroxene networks among rounded olivine and/or pyroxene (depending on ƒO2), which pseudomorph the former silicate melt network. Indirectly, the presence of exsolution lamellae in augite in slowly cooled achondrites also implies that silicate melting occurred because of the high temperatures required, and because silicate melt enhances diffusion. A metamorphic facies series is defined: the Plagioclase Facies is equivalent to petrologic types 5 and 6, the Sub‐calcic Augite Facies is bounded at lower temperatures by the initiation of silicate melting and at higher temperatures by the appearance of pigeonite, which marks the transition to the Pigeonite Facies.

Metamorphic P–T evolution of the Gotsu blueschists from the Suo metamorphic belt in SW Japan: Implications for tectonic correlation with the Heilongjiang Complex, NE China

In the Gotsu area of the c. 200 Ma high-P/T Suo metamorphic belt in the Inner Zone of southwest Japan, blueschists occur as lenses or layers within pelitic schists. Prograde, peak, and retrograde stages are distinguished in the blueschists, and the prograde and the peak metamorphic conditions are determined using pseudosection modelling in the NCKFMASHO system. The prograde metamorphic stage is defined by inclusions in porphyroblastic epidote and glaucophane, such as phengite, chlorite, albite, epidote and glaucophane/winchite, and the estimated metamorphic conditions are <325 °C and < 4–5 kbar at the boundary between the glaucophane schist facies and the greenschist facies. The peak metamorphic stage is well-defined by the schistosity-forming minerals, i.e. epidote, glaucophanic amphibole, phengite, and chlorite, suggesting the glaucophane schist facies conditions of 475–500 °C and 14–16 kbar. Actinolite/magnesiohornblende, chlorite, and albite replacing the peak stage minerals suggest the retrograde metamorphism into the greenschist facies. The metamorphic facies series of the Suo belt is defined by pumpellyite–actinolite facies to epidote–blueschist facies, and it has a relatively lower-P/T compared with the c. 300 Ma Renge belt in the Inner Zone of southwest Japan, which is defined by a sequence of lawsonite–blueschist facies to glaucophane–eclogite facies. The P– $$ {M}_{{\mathrm{H}}_2\mathrm{O}} $$ pseudosection and water isopleth show that the rocks were dehydrated during the initial stage of the exhumation and remained in water-saturated conditions. Similarities of the detrital zircon and peak metamorphic ages of the blueschists from the Suo metamorphic belt in southwest Japan and the Heilongjiang Complex in northeast China suggest that both metamorphic belts were probably formed in the same Paleo-Pacific subduction system in the Late Triassic to Jurassic period.

Petrology and age of Precambrian Aksu blueschist, NW China

The first occurrence of blueschist in the Neoproterozoic signifies a transition in thermal regime from a warmer Archean–Proterozoic Earth to modern colder subduction characterized by blueschist facies or ultrahigh pressure metamorphism. To evaluate the thermal regime of the transition, we apply phase equilibrium modeling to the Aksu blueschist in northwestern China to constrain the pressure (P) and temperature (T) conditions of the peak mineral assemblages and to calculate the thermal regime recorded by the blueschist. Phase equilibrium modeling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–O system indicates that the peak mineral assemblage (glaucophane/riebeckite + epidote + phengite + albite + quartz + chlorite) was developed at P–T conditions of 6.8–8.7 kbar and 320–410 °C for a stilpnomelane-bearing epidote blueschist (AK04) and of 6.5–8.0 kbar and 310–380 °C for an epidote blueschist (AK06). These P–T data define an apparent thermal gradient of ∼470 °C/GPa for the peak stage which is typical of the high-pressure intermediate type metamorphic facies series. In addition, we present new LA–ICP–MS U–Pb ages from detrital zircons from two blueschists (AK04 and AK06) and SHRIMP U–Pb ages from zircon from a cross-cutting post-metamorphic mafic dyke (AK134) to constrain the age of the metamorphism. The youngest concordant detrital zircon ages give a maximum depositional age for the blueschist protoliths of 806 ± 10 (AK04) and 803 ± 10 Ma (AK06), respectively, and the SHRIMP U–Pb ages of zircons from the mafic dyke yields a weighted mean age of 769 ± 19 Ma interpreted to date crystallization. These results constrain the age of Aksu blueschist facies metamorphism to between 769 ± 19 and c. 805 Ma.

On the significance of short-duration regional metamorphism

Short-duration regional metamorphism is a recently observed and poorly understood phenomenon in metamorphic geology. In this review, it is defined as metamorphism on time scales that limit length scales (of the associated thermal anomaly) to significantly less than the thickness of the orogenic crust (&lt;10 myr) or subducted oceanic lithosphere (&lt;5 myr). Without appealing to exceptional heat sources, thermal models have been unable to account for peak metamorphic temperature during collisional orogenesis and subduction. This observation, combined with restricted time scales for regional metamorphism, suggests that metamorphic facies series can record atypical and transient thermal conditions (related to punctuated and localized heat advection and/or production), rather than normal, ambient conditions for the tectonic setting to which they are allied. High-precision geochronology can resolve short-duration metamorphic estimates of 1 – 10 myr. However, diffusion geospeedometry typically yields extremely short metamorphic durations (&lt;1 myr); tools in metamorphic geology may have matured to the point that the discipline is beginning to recognize episodicity and criticality in deep processes. New, very high-precision petrochronology techniques offer great potential to probe the veracity of extremely short metamorphic durations being obtained from diffusion geospeedometry. Benchmarking of these new very high-precision petrochronology techniques must become a priority for metamorphic geology.

Fluid regime of initial granite-forming processes in metamorphic complexes of various pressure: A case study of the western Baikal metamorphic belt

Syncollisional granite generated in zonal metamorphic complexes at 500–700°C and various pressure and in the presence of fluid of various composition are proved to differ in proportions of granite material generated by metamorphic, metasomatic, and magmatic processes. Monometamorphic low-pressure complexes devoid of the influx of foreign solutions are characterized by the most intense metamorphic differentiation: the dissolution and redeposition of quartz and plagioclase into lower pressure zones and the development of plagiomigmatites. The process starts with deformations of the schists at 500–550°C. The main factors (at a constant total composition of the rock) is an uneven pressure distribution and metamorphic solutions. This stage is followed by metasomatic granitization, which is initiated if shear motions and decompression are associated with influx of silicic alkaline solutions into the rocks and their transformation into migmatite (up to granite-gneiss). Granitization during this stage is controlled by an increase in the permeability of the rocks and the introduction of perfectly mobile Si, Na, and K and removal of Mg, Ca, and Fe, which modifies the composition of the rocks and makes them similar to granite. Metasomatic granitization may take place in low-pressure polymetamorphic complexes. Under a high lithostatic pressure and in the presence of water-carbon dioxide fluids (kyanite-sillimanite facies series of metamorphism), the magmatic process begins at higher temperatures. The early stages are dominated by metamorphic differentiation and metasomatic sodic migmatization. The rocks modified by these processes (their salic and mafic minerals are segregated) are affected by anatexis at 650–700°C (partial melting of the granite eutectic) and, at higher temperatures, by diatexis, which generates granite melts able to migrate. These processes are variably pronounced in all metamorphic complexes.

The tectono‐metamorphic evolution of the very low‐grade hangingwall constrains two‐stage gneiss dome formation in the Montagne Noire (Southern France)

AbstractThe Montagne Noire in the southernmost French Massif Central is made of an ENE‐elongated gneiss dome flanked by Palaeozoic sedimentary rocks. The tectonic evolution of the gneiss dome has generated controversy for more than half a century. As a result, a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms. Most of these models are based on data from the gneiss dome itself. Here, new constraints on the dome evolution are provided based on a combination of very low‐grade petrology, K–Ar geochronology, field mapping and structural analysis of the Palaeozoic western Mont Peyroux and Faugères units, which constitute part of the southern hangingwall of the dome. It is shown that southward‐directed Variscan nappe‐thrusting (D1) and a related medium‐P metamorphism (M1) are only preserved in the area furthest away from the gneiss dome. The regionally dominant pervasive tectono‐metamorphic event D2/M2 largely transposes D1 structures, comprises a higher metamorphic thermal gradient than M1 (transition low‐P and medium‐P metamorphic facies series) and affected the rocks between c. 309 and 300 Ma, post‐dating D1/M1 by more than 20 Ma. D2‐related fabrics are refolded by D3, which in its turn, is followed by dextral‐normal shearing along the basal shear zone of both units at c. 297 Ma. In the western Mont Peyroux and Faugères units, D2/M2 is largely synchronous with shearing along the southern dome margin between c. 311 and 303 Ma, facilitating the emplacement of the gneiss dome into the upper crust. D2/M2 also overlaps in time with granitic magmatism and migmatization in the Zone Axiale between c. 314 and 306 Ma, and a related low‐P/high‐T metamorphism at c. 308 Ma. The shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome, and also with the peak of metamorphism in the hangingwall. Both, the intensity of D2 fabrics and the M2 metamorphic grade within the hangingwall, decrease away from the gneiss dome, with grades ranging from the anchizone–epizone boundary to the diagenetic zone. The related zonation of the pre‐D3 metamorphic field gradients paralleled the dome. These observations indicate that D2/M2 is controlled by the exhumation of the Zone Axiale, and suggest a coherent kinematic between the different crustal levels at some time during D2/M2. Based on integration of these findings with regional geological constraints, a two‐stage exhumation of the gneiss dome is proposed: during a first stage between c. 316 and 300 Ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull‐apart‐like geometry. The second stage from 300 Ma onwards was characterized by northeast to northward extension, with exhumation accommodated by north‐dipping detachments and hangingwall basin formation along the northeastern dome margin.

造山帯の分類とその意義：古造構場復元の束縛条件

The geotectonic division of the Japanese islands has been proposed by Isozaki and Maruyama (1991) such that the eastern extension of the Dabie-Sulu 240-220 Ma collisional UHP-HP belt (DSB) passes through the Korean peninsula to Japan. The corresponding belt in Japan is the Higo-Unazuki-Hitachi-Takanuki belt (HUHTB), because the protoliths are a unique A-type (platform sediments) and the metamorphic facies series belongs to the intermediate-pressure type, in addition to 240-220 Ma of metamorphic ages. Ishiwatari and Tsujimori (2012) claim that Maruyama et al. (2011) did not evaluate a new proposal by Ishiwatri and Tsujimori (2003), who proposed that the DSB extends not to the HUHTB but to the Sangun belt in a complex manner because of the promontory nature of the continental margin. This is a Q-A report requested by Ishiwatari and Tsujimori (2012). In this paper the author first introduces a classification of orogenic belts based on protoliths and its great significance for understanding the history of complex orogenic belts, following the original article by Maruyama et al. (1996), in addition to current topics on the role of tectonic erosion. Preceding the final formation of the collisional orogen, the Pacific-type orogen must have been present structurally above the collisional orogen against the hanging wall of the continent or arc. If not, it suggests the presence of tectonic erosion. Moreover, the size of any of regional metamorphic belt, arc, and TTG belt could be an excellent indicator of the scale of tectonic erosion. The Triassic DSB continues not to Sangun belt, but to the HUHTB, for four reasons: first, the protolith of those belts, second; the nature of regional metamorphism along intermediate-pressure type; third, structural units above and below the HUHTB; and finally, paleogeographic reconstruction of the Triassic North and South China cratons. The tectonic juxtapositions of the four so-called Sangun BS belts —450 Ma, 340 Ma, 250-210 Ma, and 170-180 Ma— against the HUHTB in a narrow zone as klippes can best be interpreted by extensive tectonic erosion.

A new perspective on metamorphism and metamorphic belts

The discovery of ultrahigh-pressure rocks from collision-type orogenic belts has revolutionized the classic interpretation of (1) progressive and retrogressive metamorphism recorded on surface exposures of regional metamorphic belts, (2) geochronology of the various stages of metamorphism, (3) origin of metamorphic textures, (4) P– T– t path, (5) metamorphic facies series, (6) exhumation model, and (7) role of fluids during regional metamorphism. Based mainly on our recent studies of the Kokchetav, Dabie Shan, Indonesia, Franciscan and Sanbagawa belts, we suggest the following revolutionary paradigm shifts. The so-called mineral isograds defined on the maps of regional metamorphic belts were a misunderstanding of the progressive dehydration reaction during subduction because extensive late-stage hydration has mostly obliterated the progressive minerals in pelitic–psammitic and metabasic rocks. Progressively zoned garnet has survived as the sole progressive mineral that was unstable with the majority of matrix-forming minerals. The classic Barrovian isograds should therefore be carefully re-examined. The well-documented SHRIMP chronology of spot-dating zoned zircons with index minerals from low- P in the core, through HP–UHP in the mantle to low- P on the rim clearly shows that the slow exhumation speed of 23–40 My from mantle depth to mid-crustal level was followed by mountain building with doming at latest stage. Extensive hydration of the UHP–HP unit occurred due to fluid infiltration underneath, when the UHP–HP unit intruded the low-grade to unmetamorphosed unit at a mid-crustal level. Most deformation textures such as mineral lineations, porphyroblasts, pull-apart, or boudinaged amphiboles, formed during extensive hydration at the late stage do not constrain the progressive stress regime. The P– T–time path determined by thermobarometry using mineral inclusions in garnet and forward modeling of garnet zoning, independent of the matrix minerals, indicates an anticlockwise trend in the P– T space, and follows an independent P– T change in the metamorphic facies series. This is consistent with the numerically calculated geotherm along the Wadati–Benioff plane. Collision-type orogenic belts have long been regarded as being characterized by the intermediate-pressure type metamorphic facies series. The kyanite–sillimanite is an apparent type facies series formed by the late-stage extensive hydration. In contrast, the original high- P to ultrahigh- P type facies series with an anticlockwise kink-point at around 10 kb is a progressive type. A collision-type regional metamorphic belt crops out as a very thin unit sandwiched between overlying and underlying low- P or weakly metamorphosed units. The metamorphic belt has an aspect ratio (thickness vs width) of 1:100, and it extends for several hundreds to a thousand km. It resembles a thin mylonitic intrusion from the mantle extending from 100 to 200 km depth into the crustal rock unit. The underlying unit is thermally metamorphosed in the andalusite–sillimanite type facies series. The major reason for the misunderstanding of the progressive metamorphism in collision-type orogenic belts is the underestimation of the role of fluids derived from the underlying low-grade metamorphic unit, when juxtaposed at a mid-crustal level. The circulation of fluids along the plate boundaries is more important than a P– T change.

Stratigraphy-related, low-pressure metamorphism in the Hardey Syncline, Hamersley Province, Western Australia

The late Archean Fortescue Group and Paleoproterozoic Hamersley Group are exposed in the Hardey Syncline located at the southwestern edge of the Hamersley Basin, Pilbara Craton, Western Australia. The secondary mineral assemblages and compositions of the basaltic rocks of the Fortescue and Hamersley Groups reveal the metamorphic conditions of the study area. The estimated metamorphic grade ranges from prehnite–actinolite facies (Hamersley Group), through greenschist facies (Fortescue Group), to a transition between greenschist facies and actinolite–calcic plagioclase facies (Fortescue Group), indicating a low-pressure type metamorphic facies series. The metamorphic grade increases northward, which is opposite to the general southward increase of the regional metamorphic grade. Furthermore, the change of metamorphic grade strongly correlates with stratigraphy, and the metamorphic temperature increases with stratigraphic depth. These observations suggest that the metamorphism of the study area was caused by a thermal event before the folding due to the Ophthalmian orogeny that affected most of the Hamersley Province. Considering the presence of 2.2 Ga dolerite sills in the Hardey Syncline and the low-pressure metamorphic condition, it is suggested that the metamorphism of the study area was caused by the 2.2 Ga continental rifting, which is consistent with the reported metamorphic age.

Paired metamorphic belts revisited

The modern plate tectonics regime is characterized by a duality of thermal environments, one representing the subduction zone and the other representing the arc–backarc or orogenic hinterland. This duality is the hallmark of one-sided (asymmetric) subduction, and the characteristic imprint of one-sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt, one of high d T/d P type and the other low d T/d P type. The broadly contemporaneous occurrence of granulite and ultrahigh-temperature metamorphism with eclogite–high-pressure granulite metamorphism in the geological record since the Neoarchean Era is evidence of dual thermal environments and indicates that subduction has operated on Earth since that time. Classic ‘paired’ metamorphic belts in which an inboard high d T/d P metamorphic belt is juxtaposed against an outboard low d T/d P metamorphic belt along a tectonic contact—such as the Ryoke and Sanbagawa belts in Japan—are found in Phanerozoic accretionary orogens of the circum-Pacific. Generally, they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far-traveled. This is a consequence of the difference between globally-continuous subduction, generating a low-to-intermediate d T/d P environment in the subduction zone and a high d T/d P environment in the arc–backarc system, and metamorphic imprints in the geological record that represent discrete ‘events’ due to changes in plate kinematics or subduction boundary dynamics, or as a result of collision of ridges, arcs or continents with the upper plate at the trench. The concept of ‘paired’ metamorphic belts may be generalized and extended more widely than in the original proposition to subduction-to-collision orogenic systems in addition to accretionary orogenic systems. In this wider application, the term “paired metamorphic belts” may be used for “penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients, one warmer and the other colder, juxtaposed by plate tectonics processes” (Brown, 2009). This extends the original concept of Miyashiro (1961) beyond the simple pairing of high d T/d P and low d T/d P metamorphic belts in circum-Pacific accretionary orogens, and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings. This is particularly useful in subduction-to-collision orogenic systems, where the suture and lower plate materials will register the imprint of low-to-intermediate d T/d P and the upper plate will register penecontemporaneous high d T/d P metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record.

Amphibolite and blueschist–greenschist facies metamorphism, Blue Mountain inlier, eastern Jamaica

AbstractCretaceous (possibly older) metamorphic rock occurs mainly in the Blue Mountain inlier in eastern Jamaica. Fault‐bounded blocks reveal two styles of metamorphism, Westphalia Schist (upper amphibolite facies) and Mt. Hibernia Schist (blueschist (BS)–greenschist (GS) facies). Both Westphalia Schist and Mt. Hibernia Schist preserve detailed records of retrograde P–T paths. The paths are independent, but consistent with different parts of the type‐Sanbagawa metamorphic facies series in Japan. For each path, phase relationships and estimated P–T conditions support a two‐stage P–T history involving residence at depth, followed by rapid uplift and cooling. Conditions of residence vary depending on the level in a tectonic block. For the critical mineral reaction (isograd) in Westphalia Schist, conditions were P ∼7.5 kbars, T ∼600°C (upper amphibolite facies). Retrograde conditions in Hibernia Schist were P = 2.6–3.0 kbars, T = 219–237°C for a(H2O) = 0.8–1.0 (GS facies). Mt. Hibernia Schist may represent a volume of rock that was separated and uplifted at an early time from an otherwise protracted P–T path of the sort that produced the Westphalia Schist. Reset K–Ar ages for hornblende and biotite indicate only that retrograde metamorphism of Westphalia Schist took place prior to 76.5 Ma (pre‐Campanian). Uplift may have commenced with an Albian–Aptian (∼112 Ma) orogenic event. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Large P–T gap between Ballantrae blueschist/garnet pyroxenite and surrounding ophiolite, southern Scotland, UK: Diapiric exhumation of a Caledonian serpentinite mélange

The Ballantrae ophiolite in southern Scotland includes a NEE–SWW-trending serpentinite mélange that contains blocks of mafic blueschist and high-pressure, granulite facies, metapyroxenite (Sm–Nd metamorphic age: 576 ± 32 and 505 ± 11 Ma). Tectonic blocks of mafic schist are less than 3 × 3 m in size, and have greenschist, blueschist or epidote amphibolite facies assemblages corresponding to the high-pressure intermediate-type metamorphic facies series. Adjacent rocks of the serpentinite mélange are hydrothermally-altered MORB-like ophiolitic basalt (prehnite–pumpellyite facies), dolerite (actinolite–oligoclase sub-facies) and gabbro (amphibolite facies), all with assemblages that are diagnostic of the low-pressure metamorphic facies series. The difference in metamorphic facies series and parageneses of minerals between the high-pressure mafic blocks and the adjacent, low-pressure ophiolitic meta-basic rocks suggests that the former were exhumed from > 25 km depth within a cold subducted slab, and were juxtaposed with the latter, the bottom of a MORB-like ophiolite in the hanging wall of a trench. An ENE–WSW-trending, 501 ± 12 Ma volcanic arc belt extends for 3 km south of the serpentinite mélange. We suggest that ridge subduction associated with a slab window created arc-related gabbro (483 ± 4 Ma) at Byne Hill and within-plate gabbro (487 ± 8 Ma) at Millenderdale. Final continental collision created the duplex structure of the Ballantrae complex that includes the HP blocks and serpentinite mélange. These relations define diapiric exhumation in the Caledonian orogen of SW Scotland.

Re-interpretation of progressive metamorphism, facies series, P-T-t path and exhumation model for the collisional orogenic belts

Recent discovery of ultra-high pressure (UHP) mineralogy, and descriptions of geology, petrology, geochronology and geochemistry of the Phanerozoic collisional orogens have changed the basic concepts of metamorphic facies series, P-Ttime path, progressive metamorphism, Barrovian-type metamorphism and tectonic model. The Himalayan orogen has served as a world standard for the collisional orogenic belt for a long time, characterized by Barrovian-type metamorphism (intermediate-P type metamorphism within the stability field of plagioclase), tectonic overlapping of double-continental crust, and thermal relaxation with buoyant unroofing (England and Thompson 1984). Recent discovery of coesite-bearing eclogite in the western Himalaya and eclogites from the central to eastern Himalaya can not be explained by the previous models. In the followings, summarizing the new constraints on orogenic process, I try to explain the collision orogeny including Himalaya.

Progressive metamorphism of the Taitao ophiolite; evidence for axial and off-axis hydrothermal alterations

We estimated metamorphic conditions for the ∼ 6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge. Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from ∼ 230 °C in the volcanic unit to ∼ 380 °C at the bottom of the gabbro unit, systematically ∼ 200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration. Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142).