Metamorphic Types Research Articles

Tectonic evolution of low-grade metamorphosed rocks of the Cretaceous Shimanto accretionary complex, Central Japan

We reconstructed the tectono-metamorphic evolution of the low-grade metamorphosed Cretaceous Shimanto accretionary complex in the Kanto Mountains, Central Japan, based on radiolarian fossils, metamorphic temperatures derived from illite crystallinity analysis, and timing of metamorphism based on illite K–Ar dating. The accretionary age of the Kobotoke Group is Turonian to Maastrichtian (66–94 Ma), based on radiolarian fossils. Illite crystallinity data indicate metamorphic temperatures of approximately 300 °C. The illite K–Ar ages constrain the timing of metamorphism to the Middle Eocene around 40 Ma. Combining our results and previous study, we defined two types of low-grade metamorphism within Cretaceous Shimanto accretionary complex of the Kanto Mountains. The early metamorphism, in excess of 300 °C, was related to the uplift of the Sambagawa metamorphic rocks, in turn associated with the subduction of the Kula–Pacific ridge during the Late Cretaceous (65–75 Ma). This metamorphism is recorded in the Otaki Group within the northernmost part of the complex in the Kanto Mountains. Subsequent to the subduction of the Kula–Pacific ridge, a later period of metamorphism, recorded in the Kobotoke Group, is characterized by the thermal effects of the subduction of the young, hot Pacific Plate during the Middle Eocene. The effect of the early metamorphism occurred synchronously 500 km along the trench from Southwest to Central Japan. The later metamorphism occurred at 50 Ma in Kyushu and Shikoku of Southwest Japan, and at 40 Ma in the Kanto Mountains of Central Japan. This difference in the timing of metamorphism between Southwest and Central Japan is explained by the northward migration of the young, hot Pacific Plate.

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.

P-T paths derived from garnet growth zoning in Danba domal metamorphic terrain, Sichuan Province, West China

Danba (丹巴) domal metamorphic terrain belongs to Songpan (松潘)-Ganze (甘孜) orogenic belt, where typical Barrovian and Buchan metamorphic zones are preserved. The former included chlorite, biotite, garnet, staurolite, kyanite and sillimanite zones, while the latter only developed silimanite+muscovite and sillimanite+K-feldspar zones. Integrated study has been carried on metamorphic reactions of garnet production and consumption, P-T paths and P-T-X-M phase relation and thermal tectonic model for Danba metamorphic zones. Petrological textures in thin sections show that garnet production and consumption in kyanite-sillimanite zone is mainly attributed to Chl+Ms+Pl+Q= Grt+Bt+H2O and kyanite=sillimanite respectively. Based on mineral compositions, the geothermobarometry gives an average P, T condition of (4.9±0.3)×108 Pa, 543±30 °C for the first growth stage of the garnet and (5.8±0.3)×108 Pa, 534±29 °C for the second stage of garnet growth respectively. Anti-counter clockwise P-T paths were drawn using Gibbs method by NCMnKFMASH system for sample G98686 in the kyanite zone. The P-T-X-M modeling for the first mineral assemblages shows that the prediction is similar to the measured values in gossular, almandine and spessartine but mole fraction of pyrope and Fe/(Fe+Mg) deviated far from the contours; while that for the second mineral assemblages exhibits that the prediction is consistent with the measured value of pyrope, grossular content and Fe/(Fe+Mg) of garnet. A thermal tectonic model that there are at least three structure levels across the thrust-decollement zones is presented according to the P-T paths, metamorphic grades and deformation styles for the staurolite-kyanite zone of the Barrovian type metamorphism, which will provide some constraints for the evolution of the nappe complex.

40Ar/39Ar ages of the high-P/T metamorphic rocks of the Heilongjiang Complex in the Jiamusi Massif, northeastern China

The Heilongjiang Complex in northeastern China is composed mainly of alternating blueschists and pelitic schists which underwent high-P/T type metamorphism of the epidote blueschist facies. Phengites in the pelitic schists from two areas in the Heilongjiang Complex yielded 40Ar/39Ar plateau ages of 165 ± 0.8 Ma (Mudanjiang area) and 171 ± 0.7 Ma (Yilan area). Phengites in blueschists from the Yilan area give 40Ar/39Ar isochron ages of 145 ± 0.9 Ma and 146 ± 1.3 Ma. The previously reported peak metamorphic temperature of 350-450 °C for the Heilongjiang Complex is close to the Ar retention temperatures of white mica, suggesting that the ages reported here closely date the metamorphic culmination in this complex. These ages indicate that a widespread high-P/T metamorphism occurred in the Heilongjiang Complex at 165-171 Ma. Subsequent to this period, a similar type of metamorphism continued at least until 145-146 Ma in the Yilian area.

Metamorphic patterns in orogenic systems and the geological record

Abstract Regional metamorphism occurs in plate boundary zones. Accretionary orogenic systems form at subduction boundaries in the absence of continent collision, whereas collisional orogenic systems form where ocean basins close and subduction steps back and flips (arc collisions), simply steps back and continues with the same polarity (block and terrane collisions) or ultimately ceases (continental collisions). As a result, collisional orogenic systems may be superimposed on accretionary orogenic systems. Metamorphism associated with orogenesis provides a mineral record that may be inverted to yield apparent thermal gradients for different metamorphic belts, which in turn may be used to infer tectonic setting. Potentially, peak mineral assemblages are robust recorders of metamorphic P and T , particularly at high P – T conditions, because prograde dehydration and melting with melt loss produce nominally anhydrous mineral assemblages that are difficult to retrogress or overprint without fluid influx. Currently on Earth, lower thermal gradients are associated with subduction (and early stages of collision) whereas higher thermal gradients are characteristic of back-arcs and orogenic hinterlands. This duality of thermal regimes is the hallmark of asymmetric or one-sided subduction and plate tectonics on modern Earth, and a duality of metamorphic belts will be the characteristic imprint of asymmetric or one-sided subduction in the geological record. Accretionary orogenic systems may exhibit retreating trench–advancing trench cycles, associated with high (>750 °C GPa −1 ) thermal gradient type of metamorphism, or advancing trench–retreating trench cycles, associated with low (<350 °C GPa −1 ) to intermediate (350–750 °C GPa −1 ) thermal gradient types of metamorphism. Whether the subducting boundary advances or retreats determines the mode of evolution. Accretionary orogenic systems may involve accretion of allochthonous and/or para-autochthonous elements to continental margins at subduction boundaries. Paired metamorphic belts, sensu Miyashiro, comprising a low thermal gradient metamorphic belt outboard and a high thermal gradient metamorphic belt inboard, are characteristic and may record orogen-parallel terrane migration and juxtaposition by accretion of contemporary belts of contrasting type. A wider definition of ‘paired’ metamorphism is proposed to incorporate all types of dual metamorphic belts. An additional feature is ridge subduction, which may be reflected in the pattern of high d T /d P metamorphism and associated magmatism. Apparent thermal gradients derived from inversion of age-constrained metamorphic P – T data are used to identify tectonic settings of ancient metamorphism, to evaluate the age distribution of metamorphism in the rock record from the Neoarchaean Era to the Cenozoic Era, and to consider how this relates to the supercontinent cycle and the process of terrane export and accretion. In addition, I speculate about metamorphism and tectonics before the Mesoarchaean Era.

The geological characteristics of oceanic-type UHP metamorphic belts and their tectonic implications: Case studies from Southwest Tianshan and North Qaidam in NW China

The geological characteristics of ultrahigh-pressure (UHP) metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolithic assemblage of typical oceanic crust, the peak metamorphic temperature <600°, P-T path undergoing blueschist facies during prograde and retrograde metamorphic evolution, respectively, with low geothermal gradient of cold subduction. The further study of oceanic-type UHP metamorphic belt is very significant for constructing metamorphic reaction series of cold subduction zone, for understanding how aqueous fluids were transported into deep mantle and for classifying the types of UHP metamorphism in cold subduction zone. The uplift and exhumation mechanism of oceanic UHP metamorphic rocks is one of the most challenging problems in the study of UHP metamorphism, which is very important for understanding the geodynamic mechanism of solid Earth. As a traveler subducted into the mantle depth and then uplifted to the surface, oceanic-type UHP metamorphic belts witness the bulk process from the subduction to exhumation and is an ideal target to study the geochemical behavior and cycling of elements in subduction zones. The tectonic evolution of one convergent orogenic belt can be usually divided into two stages of oceanic subduction and followed continental subduction and collision, and the two best-established examples of orogenic belts are Alps and Himalaya. Therefore, the study of oceanic-type UHP metamorphic belt is the frontier of the current plate tectonic theory. As two case studies, the current status and existing problems of oceanic-type UHP metamorphic belts in Southwest Tianshan and North Qaidam, NW China, are reviewed in this paper.

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.

Metamorphic evolution of Neoproterozoic metapelites and gneisses in the Sinai, Egypt: Insights from petrology, mineral chemistry and K–Ar age dating

Petrology, mineral chemistry, and age dating of the metapelitic schists and gneisses in three metamorphic complexes of Sinai have been studied and reported with the aims of characterizing the nature, type, and age of metamorphism as well as the metamorphic evolution of the Sinai metamorphic complexes. Nine representative samples of metapelitic schists, migmatites, and gneisses from the Taba Metamorphic Complex (TMC), the Kid Metamorphic Complex (KMC) and the Feiran-Solaf Metamorphic Complex (FSC) in southern Sinai Peninsula have been analyzed by electron microprobe for their mineral assemblages and K–Ar geochronological methods for biotite separates. Garnet from the metapelites and migmatites are almandine-rich and usually show zoning with variable X Fe values. The estimated P–T metamorphic conditions of the TMC range from 560–578 °C/3–4.5 kbar for the metapelitic schists, to 685 °C/5.3 kbar for the migmatites, while those for the metapelitic schists in the KMC are 590 °C/4–5.9 kbar in the Umm Zariq Formation, and 636 °C/4.7 kbar in the northern part of the Malhaq Formation. The P–T metamorphic conditions of the hornblende–biotite gneisses from the SZ of the FSC are 634–641 °C/4–5 kbar. These P–T conditions and mineral assemblages indicate amphibolite facies conditions of the LP/HT-type of metamorphism. The K–Ar biotite cooling ages range from 594 to 617 Ma for the TMC, from 593 to 609 Ma for the KMC, and from 589 to 602 Ma for the FSC. It could be suggested that the cooling metamorphic ages of all complexes lie at approximately 600 Ma, and thus point to a uniform and single metamorphic event for the whole complexes in the southern Sinai Peninsula. The estimated geothermal gradient is in the range of 30–50 °C/km. Such a steep gradient and the LP/HT mineral assemblages suggest that they were formed in an extensional setting and a heat flow was transferred from nearby granite intrusions.

Metamorphism of metasediments at the scale of an orogen: a key to the Tertiary geodynamic evolution of the Alps

Abstract Major discoveries in metamorphic petrology, as well as other geological disciplines, have been made in the Alps. The regional distribution of Late Cretaceous–Tertiary metamorphic conditions, documented in post-Hercynian metasediments across the entire Alpine belt from Corsica–Tuscany in the west to Vienna in the east, is presented in this paper. In view of the uneven distribution of information, we concentrate on type and grade of metamorphism; and we elected to distinguish between metamorphic paths where either pressure and temperature peaked simultaneously, or where the maximum temperature was reached at lower pressures, after a significant temperature increase on the decompression path. The results show which types of process caused the main metamorphic imprint: a subduction process in the western Alps, a collision process in the central Alps, and complex metamorphic structures in the eastern Alps, owing to a complex geodynamic and metamorphic history involving the succession of the two types of process. The western Alps clearly show a relatively simple picture, with an internal (high-pressure dominated) part thrust over an external greenschist to low-grade domain, although both metamorphic domains are structurally very complex. Such a metamorphic pattern is generally produced by subduction followed by exhumation along a cool decompression path. In contrast, the central Alps document conditions typical of subduction (and partial accretion), followed by an intensely evolved collision process, often resulting in a heating event during the decompression path of the early-subducted units. Subduction-related relics and (collisional/decompressional) heating phenomena in different tectonic edifices characterize the Tertiary evolution of the Eastern Alps. The Tuscan and Corsica terrains show two different kinds of evolution, with Corsica resembling the western Alps, whereas the metamorphic history in the Tuscan domain is complex owing to the late evolution of the Apennines. This study confirms that careful analysis of the metamorphic evolution of metasediments at the scale of an entire orogen may change the geodynamic interpretation of mountain belts.

A Vestige of Earth's Oldest Ophiolite

A sheeted-dike complex within the approximately 3.8-billion-year-old Isua supracrustal belt (ISB) in southwest Greenland provides the oldest evidence of oceanic crustal accretion by spreading. The geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid-ocean ridge-like setting, and their oxygen isotopes suggest a hydrothermal ocean-floor-type metamorphism. The pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association: the Paleoarchean Isua ophiolite complex. These sheeted dikes offer evidence for remnants of oceanic crust formed by sea-floor spreading of the earliest intact rocks on Earth.

The thermal history of equilibrated ordinary chondrites and the relationship between textural maturity and temperature

The compositions and textures of phases in eleven equilibrated ordinary chondrites from the H, L, and LL groups spanning petrographic types 4–6 were studied and used to constrain the thermal histories of their parent bodies. Based on Fe–Mg exchange between olivine and spinel, average equilibration temperatures for type 4–6 chondrites encompass a small range, 586–777 °C, relative to what is commonly assumed for peak temperatures (600–950 °C). The maximum temperatures recorded by individual chondrites, which are minima relative to peak metamorphic temperatures, increase subtly but systematically with metamorphic type and are tightly clustered for H4-6 (733–754 °C) and LL4-6 (670–777 °C). For the Ls, Ausson (L5) records a higher maximum olivine–spinel temperature (761 °C) than does the L4 chondrite Saratov (673 °C) or the L6 chondrite Glatton (712 °C). Our data combined with olivine–spinel equilibration temperatures calculated for other equilibrated ordinary chondrites using mineral compositions from the literature demonstrate that, in general, type 4 chondrites within each chemical group record temperatures lower than or equal to those of types 5–6 chondrites. For H chondrites, the olivine–spinel closure temperature is a function of spinel grain size, such that larger grains, abundant in types 5–6 chondrites, record temperatures of ∼740 °C or more while smaller grains, rare in types 5–6 but abundant in type 4 chondrites, record lower temperatures. Olivine–spinel temperatures in the type 6 chondrites Guareña and Glatton are consistent with rapid (50–100 °C/Myr) cooling from high temperatures in the ordinary chondrite parent bodies. With one exception (∼500 °C/Myr), olivine–spinel data for St.-Séverin (LL6) are consistent with similar cooling rates. Cooling rates of order 100 °C/Myr at ∼750 °C for type 6 chondrites are considerably higher than previously determined cooling rates for lower temperatures (⩽550 °C) based on metallography, fission tracks, and geochronology. For H chondrites, current thermal models of an “onion shell” parent body are inconsistent with a small range of peak temperatures based on olivine–spinel and two pyroxene thermometry combined with a wide dispersion of cooling rates at low temperatures. Equilibrated chondrites may have sampled regions near a major transition in physical properties such as near the base of a regolith pile.

Numerical modeling of thermal structure, circulation of H 2O, and magmatism–metamorphism in subduction zones: Implications for evolution of arcs

Numerical models on thermal structure, convective flow of solid, generation and transportation of H 2O-rich fluid in subduction zones are consolidated to have a comprehensive view of the subduction zone processes: heat balance, circulation of H 2O magmatism–metamorphism, growth of arcs and continental margins. A large scale convection model with steady subduction of a cold old slab (130 Myr old) predicts rapid (∼ 100 Myr) cooling of subduction zones, resulting in cessation of magmatism. The model also predicts that the mantle temperature beneath arcs and continental margins is greatly affected by the effective temperature of the subducting slab, i.e., the age of the subducting slab. If subduction of a young hot slab, including ridge subduction, occurs every 60 to 120 Myr as is suggested for eastern Asia, the average temperature beneath arcs is increased by about 300 °C, which may explain the long-lasting magmatism in eastern Asia. Associated with subduction of young slabs and ridges, thermal structure and circulation of H 2O are greatly modified to cause a transition from (1) normal arc magmatism, (2) forearc mantle melting, to (3) slab melting to produce a significant amount (100 km 3) of granitic melts, associated with both high- P/ T and low- P/ T type metamorphism. The last stage of (3) can result in formation of a granitic batholith belt and a paired metamorphic belts. Synthesis of the numerical models and observations suggest that episodic subduction of young slabs and ridges can explain heat source for generating a large amount of granitic magmas of batholiths, synchronous formation of batholith and regional metamorphic belts, and P– T conditions of the paired metamorphism. Even the high- P/ T metamorphism requires an elevated geothermal structure in the forearc region, associated with ridge subduction. Although the emplacement of the batholiths and the regional metamorphic belts, and the mass balance in subduction zones are not well constrained at present, the episodic event associated with ridge subduction is thought to be essential for net growth of arcs and continental margins, as well as for the long-term heat balance in subduction zones.

South Variscan terrane accretion: Sardinian constraints on the intra-Alpine Variscides

Structural data are presented that show that the Sardinian Variscides form the boundary of the Variscan intra-Alpine terrane, thrust onto a Gondwana foreland. This model replaces the currently held hypothesis that the south Armorican suture zone continues into the Sardinian Variscides. The orogenic activity propagated toward the foreland in two distinct waves: (1) thrusting, burial and Barrovian type metamorphism that reached the internal orogen in Late Devonian times and lasted in the external orogen until Early Carboniferous times. Transpressional strain partitioning is suggested by frontal thrusting in the external orogen and lateral displacement in the internal orogen. (2) Normal faulting, exhumation and local Buchan type metamorphism were superimposed on the internal orogen probably since Early Carboniferous times and reached the external orogen contemporaneously with the Late Carboniferous emplacement of voluminous granitic intrusions. Extensional doming and lateral displacement indicate a transtensional regime. The sense of orogen-parallel displacement during both stages was left-lateral on inward dipping movement planes and right-lateral on outward dipping movement planes. The resulting left-lateral displacement between the intra-Alpine terrane and the Gondwana foreland is due to the bending of the Variscan belt, during which the Sardinian segment underwent a bookshelf type movement within an overall right-lateral displacement regime.

The b0 lattice parameter and chemistry of phengites from HP/LT metapelites

The statistical b 0 method is based on the fact that the cumulative b 0 values of white K-micas (WKM) from metapelites of a specific bulk composition can be used, in the lowermost T range of metamorphism, as a semiquantitative P indicator: mean b 0 values (and related celadonite content) in WKM increase with increasing P, thus characterizing the baric type of metamorphism. The solid petrological framework of this method includes specific constraints on rock bulk composition and need of other conditions. In this way, a reference b 0 scale for LT metapelites has become available in the literature, covering the whole range of metamorphic P. However, after many publications based on it, which made a large amount of data available, revision of this b 0 scale is now opportune, and the primary need is a better definition of the reference b 0 values and WKM chemistry for HP/LT metapelites. Such a better definition is the aim of this paper. The considered HP/LT metapelites come from various terrains: Schistes Lustres of W Alps, Diablo Range (Franciscan Complex of California), Phyllite-Quartzite Unit of Crete and Peloponnese, UHP gneiss from Dora Maira, Attic-Cycladic Complex of Sifnos, Tavsanli Zone and Nilufer area of NW Turkey. In addition, WKM chemical and b 0 data were used from various sources. The new chemical and b 0 data (EMPA and XRPD) allowed us: ( i ) to establish the reference b 0 and chemistry of WKM from HP/LT metapelites: b 0 turns out to be around 9.040 A; ( ii ) to confirm and extend to HP/LT metapelites the petrological basis of the b 0 method, particularly the control of mineral assemblages on b 0 values; ( iii ) to glimpse more complex relationships between b 0 and Si than those currently known, and particularly the importance of the Fe 2+ /(Mg + Fe 2+ ) ratio; ( iv ) to ascertain the validity of the b 0 method for detecting generic HP/LT conditions; (ν) to ascertain the inability of the b 0 method to discriminate, within the above HP field, differences between various P/T regimes, mainly due to still poorly known crystal-chemical problems, and the fact that comparative b 0 interpretations of various HP/LT sample populations from different belts are impeded by poor understanding of the extent of reequilibration of WKM at higher T and lower P during exhumation.

The Carboniferous to Jurassic evolution of the pre-Alpine basement of Crete: constraints from U-Pb and U-(Th)-Pb dating of orthogneiss, fission-track dating of zircon, structural and petrological data

Abstract The pre-Alpine evolution of the external Hellenides is poorly constrained because of the Alpine impact which largely erased the older orogenic imprints. Only a few outcrops with pre-Alpine basement exist, one of which is located in eastern Crete. The pre-Alpine basement, part of the Phyllite-Quartzite Unit, is composed of four sub-complexes, which are different in protolith age, type and age of metamorphism, and postmetamorphic cooling history. The lowermost, Kalavros crystalline complex (KCC) underwent Permian amphibolite-facies metamorphism related to top-to-the-NE shearing. The KCC exhibits a four-stage garnet zonation and a late, high-temperature event associated with the growth of K-feldspar. The KCC is overlain by the Myrsini crystalline complex (MCC), which underwent Carboniferous amphibolite facies metamorphism associated with top-to-the-north shearing. Late cooling of the MCC is documented by Jurassic fission track ages of zircon. The Chamezí crystalline complex underwent upper greenschist-facies metamorphism related to top-to-the-north shearing. In addition, the Vaí crystalline complex, in an uncertain structural position, is characterized by Triassic emplacement of granite, followed by amphibolite-facies top-to-the-NW shearing and cooling, as is indicated by Jurassic fission-track ages of zircon. A preliminary tectonic model is presented, which invokes south-directed subduction, collision and accretion of the crystalline complexes to the northern margin of Gondwana.

Timing of Uralian orogenic gold mineralization at Kochkar in the evolution of the East Uralian granite-gneiss terrane

Gold mineralization at Kochkar (Urals, Russia) is hosted mainly by quartz lodes, which developed at lithological contacts between mafic dikes and granitoids of the Plast massif during late Carboniferous to early Permian, regional E–W compression in the East Uralian Zone (EUZ). The alteration mineralogy in mafic dikes comprises biotite, actinolite, albite, K-feldspar, quartz, epidote, tourmaline, sericite, pyrite, arsenopyrite, chalcopyrite, sphalerite, fahlores, galena, bismuthinite, and gold, and in Plast granitoids quartz, sericite, calcite, epidote, and ore minerals. Geochemically, an enrichment of Si, K, Rb, Ba, S, base metals, W, and Au can be observed. The ore fluid had δ18O values between 8.2‰ and 9.5‰ typical for metamorphic or deep magmatic fluids. The tectonometamorphic evolution of the EUZ is marked by peak metamorphic conditions at 635±40°C and 5–6 kbar through 500±20°C during gold mineralization, and 300–350°C and 2–3 kbar. The last event was dated on a late, barren quartz vein formed during greenschist facies metamorphism at 265±3 Ma by the Rb–Sr method. Fluids related to this overprint had a δ18O value of 5.2‰ and an initial 87Sr/86Sr ratio of 0.70685 indicating that they are largely equilibrated with metamorphic lithologies of the EUZ. The Plast granitoids and the adjacent Borisov granite, which was dated at 358±23 Ma (U–Pb zircon age), have an adakitic character. This, together with the arc-signature of the mafic dikes, supports the setting of the EUZ within the Valerianovsky continental arc. Eastward subduction of the Uralian Ocean below this arc began during the late Devonian to early Carboniferous. Between 320 and 265 Ma, the oblique closure of the ocean resulted in doming of granitoid massifs in a sinistral transpressional regime, subsequent retrograde gold mineralization during E–W compression and a later greenschist facies overprint. This long-lasting retrograde evolution of the EUZ was caused by the lack of postcollisional collapse. Heat for a “deep-later" type of metamorphism and triggering the auriferous fluid system was supplied by radiogenic heating of an overthickened crust. The greenschist facies overprint at Kochkar and coeval crustal melting in the EUZ was additionally initiated by local external heating of the terrane. This could have been caused by syn- to postcollisional slab rollback or delamination resulting in magmatic underplating of the EUZ, which postdates orogenic gold mineralization at Kochkar. The tectonic interpretation of the EUZ indicates that gold mineralization at Kochkar formed in a mid-crustal environment of a continental magmatic arc at the cessation of active subduction predating post orogenic plutonism.

The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps

AbstractThermal models for Barrovian metamorphism driven by doubling the thickness of the radiogenic crust typically meet difficulty in accounting for the observed peak metamorphic temperature conditions. This difficulty suggests that there is an additional component in the thermal budget of many collisional orogens. Theoretical and geological considerations suggest that viscous heating is a cumulative process that may explain the heat deficit in collision orogens. The results of 2D numerical modelling of continental collision involving subduction of the lithospheric mantle demonstrate that geologically plausible stresses and strain rates may result in orogen‐scale viscous heat production of 0.1 to &gt;1 μW m−3, which is comparable to or even exceeds bulk radiogenic heat production within the crust. Thermally induced buoyancy is responsible for crustal upwelling in large domes with metamorphic temperatures up to 200 °C higher than regional background temperatures. Heat is mostly generated within the uppermost mantle, because of large stresses in the highly viscous rocks deforming there. This thermal energy may be transferred to the overlying crust either in the form of enhanced heat flow, or through magmatism that brings heat into the crust advectively. The amplitude of orogenic heating varies with time, with both the amplitude and time‐span depending strongly on the coupling between heat production, viscosity and collision strain rate. It is argued that geologically relevant figures are applicable to metamorphic domes such as the Lepontine Dome in the Central Alps. We conclude that deformation‐generated viscous dissipation is an important heat source during collisional orogeny and that high metamorphic temperatures as in Barrovian type metamorphism are inherent to deforming crustal regions.

Pseudosection analysis for the Sanbagawa pelitic schist and its implication to the thermal structure of high-pressure intermediate type of metamorphism

Phase equilibrium relationships for the pelitic schists were examined by pseudosection method using the bulk rock composition of Sanbagawa pelitic schist in the garnet zone of central Shikoku, Japan. The obtained P-T pseudosections reproduce the observed sequence of mineral zones in the Sanbagawa pelitic schist: the chlorite, garnet and biotite zones with an increasing order of metamorphic grade. The effect of MnO content on the stability of garnet was examined on P-T pseudosections by changing the MnO content of the bulk rock composition from 0.0 to 0.2 wt%. It follows that the characteristic sequence of the mineral zones observed in the Sanbagawa pelitic schist owes to the higher pressure of metamorphism along with the MnO content. (Communicated by Nobuo MORIMOTO, M.J.A.)

Barrovian and Buchan type metamorphism in the Pan-African Kaoko belt, Namibia: implications for its geotectonic position within the framework of Western Gondwana

The polyphase deformed Pan-African Kaoko belt, Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, consists of pre-Pan-African basement gneisses, Pan-African volcano-sedimentary cover sequences and large granitoid intrusions. The eastern part of the Kaoko belt comprises a nearly complete Barrovian sequence of medium to high temperature/medium pressure metapelitic rocks, which range in grade from greenschist- to upper amphibolite-facies conditions. From east to west, north-northwest to northwest trending subparallel garnet-, staurolite-, kyanite-, and kyanite-sillimanite-muscovite metamorphic zones can be distinguished, whereas the western Kaoko belt is dominated by a high temperature/low pressure Buchan-type metamorphism with sillimanite-K-feldspar- and garnet-cordierite-sillimanite-K-feldspar metamorphic zones. The two metamorphic terranes are tectonically separated by the Puros shear zone. The pressure-temperature path for each metamorphic zone was inferred by using observed mineral assemblages and reaction textures, a combination of various geothermobarometers and, in particular, pseudosections. Peak metamorphic conditions of the Barrovian and Buchan zones indicate peak pressure-temperature ratios of ~20°C/km and 50°C/km, respectively. The Barrovian-type terrane is probably linked to a fold-and-thrust belt, the Buchan-type terrane may be linked to a magmatic arc setting. Similarities between the Kaoko belt in northwest Namibia and the central Ribeira belt in South America might point to a common geological history.

BUCHER, K. &amp; FREY, M. 2002. Petrogenesis of Metamorphic Rocks, 7th ed. xvi + 341 pp. Berlin, Heidelberg, New York: Springer-Verlag. Price Euros 49.95 (+VAT at local rate), SFr 86.00, £35.00, US $59.95 (hard covers). ISBN 3 540 43130 6

BUCHER, K. &amp; FREY, M. 2002. Petrogenesis of Metamorphic Rocks, 7th ed. xvi + 341 pp. Berlin, Heidelberg, New York: Springer-Verlag. Price Euros 49.95 (+VAT at local rate), SFr 86.00, £35.00, US $59.95 (hard covers). ISBN 3 540 43130 6 - Volume 140 Issue 5