Increase In Metamorphic Grade Research Articles

Garnet chemical zoning: a clue for the tectono-metamorphic history of the Proterozoic Mayombe chain (West Congo Belt), Congo-Brazzaville

Abstract The Mayombe chain of Congo is part of the West Congo Belt, which belonged to the western Gondwana supercontinent. It consists of Paleoproterozoic gneisses and schists that are tectonically stacked and overthrust Neoproterozoic low-grade metamorphic rocks. Although Neoproterozoic context of the chain is relatively well established, the tectono-metamorphic evolution of its Paleoproterozoic basement still under discussion. Petrography, garnet chemistry and phase equilibria modelling were used to constrain tectono-metamorphic evolution of meta-plutonic and meta-sedimentary rocks from the Western Domain of the Mayombe chain. Microprobe analysis reveals three garnet types: (i) 2-stage garnets with distinct cores (Grt1) and rims (Grt2), (ii) unzoned garnet showing narrow diffusion zones along cracks and rims and (iii) syn-kinematic garnet with normal growth zoning. These complex and simple features of garnet growth are, respectively, related to a polycyclic evolution linked in this area to: (i) the superposition of Eburnean (c. 2000 Ma) and Pan-African (c. 600 Ma) orogenies and (ii) a monocyclic evolution related to a single Pan-African event taking into account ages of the protoliths. The oldest metamorphic assemblage (Eburnean) is preserved in amphibolite facies conditions marked by the first generation of garnet, whereas the younger (Pan-African) event varies from amphibolite facies in the southwest (4–6 kbar, 550°C–600°C) to greenschist facies in the northeast (4–6 kbar at 450°C–550°C) confirming the westward increase in metamorphic grade during the Pan-African event. Mineral equilibria modelling shows also a relatively HP episode culminated at 11.5–12.5 kbar and 525°C–550°C which tectonic environment stills less understood.

Boron isotope behavior during metamorphic dehydration and partial melting of continental crust: Constraints from tourmaline in metapelites and leucocratic dikes, Himalayan orogen

Crustal rocks in collisional orogens often underwent the metamorphic transition from dehydration to anatexis in response to an increase of geothermal gradients, but it remains obscure how the two types of metamorphism influenced each other. This issue can potentially be addressed by an integrated study of mineralogy and geochemistry for tourmaline in protoliths and their anatectic products. In this paper, we present new results from comprehensive analyses of major-trace elements and B isotopes in tourmaline together with whole-rock Sm–Nd isotopes in well-characterized metapelites and leucocratic dikes from the Himalayan orogen. The leucocratic dikes show whole-rock εNd(t) values of −15.7 to −15.4, falling within an εNd(t) range of −17.7 to −13.8 for the metapelites. In the metapelites, metamorphic tourmaline (Tur-M) is dravitic and has relatively low δ11B values of −12.0 to −9.0‰. As the metamorphic grade increases, the δ11B values of Tur-M decrease, which is linked to progressive metamorphic dehydration. In the leucocratic dikes, three types of tourmaline are identified by different petrological and geochemical criteria. The tourmaline cores (Tur-LC) are schorlitic and have relatively high δ11B values of −8.7 to −7.7‰, and this type is ascribed to an anatectic origin. The tourmaline rims (Tur-LR) corroding the Tur-LC are dravitic and have relatively high δ11B values of −9.6 to −7.6‰, and this type is attributed to a fluid origin. Another type of anatectic tourmaline (Tur-L) occurs as grains with homogeneous oscillatory zoning that are dravitic and have slightly higher δ11B values of −10.4 to −5.9‰. The anatectic Tur-LC in the leucocratic dikes has higher δ11B values than the metamorphic Tur-M in the metapelites. We attribute this difference to the addition of external fluids with high δ11B values to the metapelite source at the overlying crustal levels. The influxed fluids most likely originated from metamorphic dehydration of lithologically similar metapelites at deeper crustal levels, as evidenced by the Mg-rich characteristics of Tur-L and Tur-LR. The metamorphic fluids would ascend into shallow crustal levels through faults and shear zones, which could not only trigger fluid-fluxed (hydration) melting of the metapelites but also result in fluid activity in later stages. This is further verified by phase equilibrium modelling and model calculation of B geochemical behavior during both metamorphic dehydration and partial melting. On this basis, a spatiotemporally coupled dehydration-hydration melting mechanism is proposed to link the early dehydration metamorphism to the later anatectic metamorphism in the collisional orogen.

The Abitibi-Opatica transition, Superior Province, Quebec, Canada: Structural analysis, 40Ar/39Ar thermochronology and implications for Archean tectonics

We present a structural and metamorphic study of the Abitibi greenstone belt (AGB) and Opatica Plutonic Belt (OPB) of the Archean Superior Province. The AGB-OPB contact is considered as an archetype example of Archean subduction, based on a LITHOPROBE seismic profile showing a North-dipping lithospheric-scale reflector interpreted as the vestige of subduction. Our mapping indicates that the AGB overlies the OPB, and that the AGB-OPB contact does not show evidence of significant shear deformation, as expected for a major upper plate-lower plate boundary. There is no metamorphic break at the AGB-OPB transition but rather a progressive increase of metamorphic grade toward the OPB. Therefore, we think that the OPB simply exposes the deepest part of a composite AGB-OPB sequence. 40Ar/39Ar ages suggest that the AGB and OPB rocks were exhumed from amphibolite-facies conditions at ∼ 2678 Ma and ∼ 2668 Ma, respectively, and then that both terranes share the same succession of thermal disturbances at c. 2650 Ma and 2600 Ma, the latter corresponding to shearing along the Nottaway River Shear Zone. We suggest that progressive cooling of both assemblages was accompanied by strain localisation along strike-slip shear zones and occurred when lateral flow of the lower crust became predominant over the convective overturn and vertical crustal material transfer. Comparison with adjacent areas suggests that regional metamorphism has been coeval over a large region, which is consistent with pervasive deformation and slow cooling as expected for vertical tectonic models and diapiric magmagenesis and ascent during the Archean.

P–T–t Path of Unusual Garnet–Kyanite–Staurolite– Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology

Abstract Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.

Subducted fragments of the Liguro-Piemont ocean, Western Alps: Spatial correlations and offscraping mechanisms during subduction

Fragments of subducted slow-spreading oceanic lithosphere are exposed continuously in the Liguro-Piemont domain of the Western Alps. By combining new and literature petrological data, interpolated maps of maximum temperatures, maximum Si contents of phengite as a proxy for peak pressure and thermodynamic modelling, we provide a detailed framework of the peak metamorphic conditions experienced by the distinct subduction slices. High-resolution mapping confirms the marked eastward increase in metamorphic grade throughout the domain, as well as within some slices. The compilation of lithostratigraphic, structural and radiochronological data and the estimation of sediment/mafic-ultramafic ratio for each slice allow refining the origin of these tectonometamorphic units within the former oceanic domain. The refined structural sketchmap allows to restore the geometries of the Alpine subduction at peak burial conditions. Results point to a trimodal distribution of the units with an increase in metamorphic conditions from the Upper (LPU; 320-400°C- 1.2-1.9 GPa) to the Middle (LPM; 415-475°C- 1.7-2.2 GPa) and to the Lower units (LPL; 500-580°C- 2.2-2.8 GPa). The blueschist-facies LPU and LPM units are dominated by sediments (>90%), whereas the eclogitic LPL units are far richer in mafic-ultramafic rocks (>40%). These characteristics, along with lithostratigraphic differences, reflect major differences in their initial paleogeography and/or in the mechanisms responsible for material offscraping from the downgoing slab. The peak burial depths of the LPU, LPM and LPL units are similar to those inferred for slicing and underplating in both modern and fossil subduction zones. Petrological and lithostratigraphic data suggest that the offscraping of the LPU and LPM units was mostly controlled by lithological contrasts, within pelagic shales or along contacts within the uppermost serpentinized mantle. In contrast, major dehydration reactions (such as lawsonite breakdown in sediments) likely controlled the offscraping of the LPL units at eclogite-facies conditions, possibly through high fluid pressure conditions and rocks embrittlement.

Magmatic and metamorphic history of the Proterozoic Lesser Himalayan Crystallines from Bomdila area, Arunachal Pradesh, NE Lesser Himalaya, India: Constraints from whole rock and mineral chemistry

The excellent exposures of plentiful Proterozoic granitic bodies in the north‐east Himalaya offer a chance to explore the evolution of continental crust in the Early Proterozoic period. The present comprehensive study from Bomdila area, western Arunachal Pradesh, NE India documents petrogenesis of Palaeoproterozoic Bomdila granite gneiss and metamorphic history of the region using the whole rock and mineral chemistry, and suitable mineral thermobarometry. The gneisses, with quartz, k‐feldspar, plagioclase, and mica mineralogy, are characterized by enrichment in rare earth elements (total REE up to 290 ppm), Rb, Ba, Th, and depletion in mafic elements (Ni, Cr, V, Sc). The gneisses are subalkaline and peraluminous with A/CNK (Al2O3/CaO + Na2O + K2O) > 1.1, indicating S‐type nature of the granites. The gneisses show fractionated REE patterns (LaN/YbN = up to 22) with LREE enrichment and HREE depletion. Geochemical characteristics such as enriched SiO2, Al2O3, alkalis, LREE, high incompatible element ratios (Rb/Sr and Ba/Sr), and depleted mafic contents with noticeable negative Eu anomalies (Eu/Eu* = 0.2 – 0.6) suggest an origin of gneisses through high‐temperature (>800°C) shallow crustal melting of a pelitic source similar to the metasedimentary enclaves within the gneisses. Mineral thermobarometric calculations suggest a tightly constrained P–T condition of 7.5–8.6 kbar and 560–590°C for the gneiss and schist from the study area. Almost identical P–T conditions of metamorphism from different lithologies attests that the Bomdila area may have attained an upper amphibolitic facies condition, possibly concomitant to the Himalayan Orogeny. Though rudimentary, temperature estimates from Ti‐in biotite thermometer suggest an increase in metamorphic grade towards the Main Central Thrust.

Lateral Termination of a Cycladic‐Style Detachment System (Hymittos, Greece)

AbstractThe bedrock of Hymittos, Attic peninsula, Greece, exposes a pair of low‐angle crustal‐scale ductile‐then‐brittle detachment faults. The uppermost detachment fault separates sub‐greenschist facies phyllite and marble of a Pelagonian Zone hanging wall, from greenschist facies metasedimentary schist, calc‐schist, and marble correlated to the Cycladic Blueschist Unit. A second, structurally lower detachment fault subdivides the metamorphic rocks of the Cycladic blueschist unit footwall into middle and lower units. There is a marked step in metamorphic grade between the sub‐greenschist facies uppermost package, and the middle‐to‐upper greenschist facies middle and lower packages. A suite of new white mica40Ar/39Ar and zircon (U‐Th)/He dates indicates accommodation of deformation along these faults occurred from the late Oligocene to the late Miocene with both faults active during the middle Miocene. The structures have clear top‐S/SSW kinematics determined from flanking folds, sigmoids, shear bands, stair‐stepping of strain shadows on porphyroclasts, and SCC' fabrics. The ductile‐to‐brittle deformation of the structures, morphology of the massif, and the increase in metamorphic grade suggest these low‐angle structures are part of a major, crustal‐scale extensional complex, located at the northwest end of the West Cycladic Detachment System, that accommodated Miocene bivergent exhumation of Attic‐Cycladic metamorphic core complexes in the central Aegean. Taken together, the above data suggest that multiple coeval detachment branches may form in areas with high strain gradients to accommodate the mechanically necessary termination of Cycladic‐style detachment systems.

P-T and Ar–Ar age constraints on low- to high-grade metabasites from the Buhwa Greenstone Belt, southern Africa: Implications for Neoarchean to Paleoproterozoic thermal evolution along the Limpopo Complex–Zimbabwe Craton boundary

This study addresses the Neoarchean to Paleoproterozoic thermal evolution from the Buhwa Greenstone Belt (BGB) along the contact zone between the Archean Zimbabwe Craton (ZC) and a granulite terrane of the Northern Marginal Zone (NMZ) of the Limpopo Complex. Available tectonic models suggest a thrusting of the hot NMZ over and onto the cold ZC, which possibly gave rise to local heating of the margin of the craton by a “hot-iron effect”. Petrographic, pressure–temperature data, along with hornblende Ar–Ar ages for metabasites from the BGB were coupled with a discussion of the tectonics of this region. The metabasites typically occur interlayered with metasediments. Four types of metabasites occur, namely, calcic amphibole–epidote schist, amphibolite (garnet-bearing and garnet-free), calcic amphibole–clinopyroxene schist, and mafic granulite, indicating a varying metamorphic grade from epidote–amphibolite facies to granulite facies. The lower-grade calcic amphibole–epidote schists occur in the northwestern margin of the belt, whereas the higher-grade amphibolites and mafic granulites are present along the southeastern margin of the BGB adjacent to the granulite terrane. The increase in metamorphic grade towards the NMZ of the Limpopo Complex is supported by temperature estimates of <500 °C to >700 °C from northwest to south east. The results of our study are consistent with the pervailing tectonic model which considers the NMZ–ZC boundary as a major thrust-sense shear zone dipping southward, and the hot NMZ thrusted over the cold ZC. New hornblende Ar–Ar ages indicated two discrete thermal events recorded in the BGB rocks: 2525 ± 19 Ma from mafic granulite and 2020 ± 8 Ma to 1968 ± 10 Ma from amphibolites. The Neoarchean age probably corresponds to a cooling age after the peak metamorphism, indicating that the NMZ thrusting onto the ZC took place at >2525 Ma. The age of the thrusting of the NMZ is younger than that of the Southern Marginal Zone (SMZ) of the Limpopo Complex onto the Kaapvaal Craton (KC) (ca. 2.72–2.69 Ga). Therefore, the formation of the Limpopo Complex is not because of a simple collision of the KC and the ZC, but the SMZ-KC collision is more than 100 Myr earlier than the NMZ–ZC collision, suggesting multiple collision for the formation of the Limpopo Complex. In contrast, the younger Paleoproterozoic thermal event coincidental with the last high-grade metamorphic event within the Limpopo Complex might correspond to a thermal reactivation along the NMZ–ZC boundary possibly caused by the global ca. 2.0 Ga orogenic events.

Petrology and phase equilibrium modeling of granulites from Obudu in the Benin-Nigerian Shield, Southeastern Nigeria: implications for clockwise P-T evolution in a collisional orogen

The Eastern Nigerian terrane forms the eastern flank of the Trans-Saharan Orogenic Belt within the Benin-Nigerian Shield. We report new petrological and P–T data of metamorphic rocks (migmatitic gneisses with subordinate granite gneiss, meta-quartz diorite, and meta-ultramafic rock) from Obudu in the southeastern flank of the Benin-Nigerian Shield for unraveling the tectono-metamorphic signatures of the eastern margin of the Trans-Saharan Orogenic Belt. The migmatized pelitic gneiss contains a peak mineral assemblage of plagioclase + rutile + garnet + biotite + K-feldspar + quartz ± liquid, based on which the peak metamorphic conditions are constrained as 850–890 °C and 9–10 kbar using phase equilibria modeling, optimal thermobarometry, and garnet-biotite geothermometry. Retrograde conditions were also estimated for a plagioclase + quartz + garnet + biotite + cordierite + rutile + sillimanite assemblage as 3.8 kbar/590 °C to 6.8 kbar/780 °C. The peak condition under granulite facies confirmed a marked increase of metamorphic grade from the greenschist facies in the western Benin-Nigerian Shield through the middle-upper amphibolite facies in the north-central Nigerian terranes to the granulite facies in the eastern part of the Benin-Nigerian Block. The results presented in this study are the first evidence of granulite-facies metamorphism from the easternmost parts of the Benin-Nigerian Shield. The clockwise path obtained in this study might suggest a continent-continent collisional setting for the evolution of this region related to the closure of the Goias-Pharusian Ocean and the formation of the Trans-Saharan Orogenic Belt.

Study on test method of heat release intensity and thermophysical parameters of loose coal

Accurate determination of basic parameters of coal spontaneous combustion process can not only directly or indirectly characterize the spontaneous combustion of coal, but also provide a reliable theoretical basis for the accurate prediction of coal spontaneous combustion period and the proposition of prevention measures of spontaneous combustion. In this paper, based on the theory of porous medium heat transfer, the mathematical models of parameters such as oxidative heat release intensity, thermal conductivity, heat capacity and activation energy of loose coal were established by using element balance method. Then, an integrated experiment system for testing characteristic parameters of coal spontaneous combustion was designed and set up, and the characteristic parameters of loose coal spontaneous combustion were accurately tested through the spontaneous combustion heating experiment on large-volume coal. The results show that the thermal conductivities of loose coal samples all decrease with the increase of voidage and rise with the increase of metamorphic grade. The heat capacity grows with the increase of volatile content. The oxidative heat release intensities of samples were increased exponentially with the rising temperature, which first rises slowly in the temperature range of 60–70 °C and then increases rapidly after 70 °C. The sample with a lower metamorphic grade and a stronger spontaneous combustion tendency owns greater oxidative heat release intensity, which grows with the increase of dynamic absorbed oxygen. At the same temperature, the large-size sample has smaller oxidative heat release intensity. The activation energy of coal samples increases with the rise of temperature and metamorphic grade. The experimental results indicate that the test system and method are reliable and can achieve the goal of rapid test.

Non-isothermal study of gasification process of coal char and biomass char in CO2 condition

AbstractNon-isothermal method was used to study gasification characteristics of three coal chars and one biomass char. Four chars were made from anthracite coal (A), bituminous coal (B), lignite coal (L), and wood refuse (W), respectively. The gasification process was studied by random pore model (RPM), unreacted core model (URCM) and volumetric model (VM). With an increase in metamorphic grade, the gasification reactivity of coal char decreased, and the gasification reactivity of biomass char was close to that of low metamorphic coal char. With an increase in heating rate, the gasification of all samples moved towards high temperature zone, and the whole gasification time decreased. It was concluded from kinetics analysis that the above-mentioned three models could be used to describe the gasification process of coal char, and the RPM fitted the best among the three models. In the RPM, the activation energies of gasification were 193. 9, 225. 3 and 202. 8 kJ/mol for anthracite coal char, bituminous coal char and lignite coal char, respectively. The gasification process of biomass char could be described by the URCM and VM, while the URCM performed better. The activation energy of gasification of wood refuse char calculated by the URCM was 282. 0 kJ/mol.

Chemical zoning of Ca-amphiboles in amphibolites, from the Hamedan area, West Iran

Amphibolite layers of the Hamedan area (Sanandaj-Sirjan zone, west Iran) contain amphibole crystals with strong optical zoning. These amphibolites occur as interlayers in middle Jurassic, Buchan-type andalusite-garnet-staurolite and sillimanite-garnet-andalusite schist of the area that were intruded by late Jurassic magmatic bodies of the Alvand plutonic complex. Electron microprobe analyses results show that the zoned amphiboles have ferrohornblende cores that change to ferroedenite toward the rims. The core-to rim increase of Al, Fe, Na, and K and decrease of Si and Mg along with edenitic substitution in amphiboles is consistent with increase in metamorphic grade. Thermobarometry calculations based on amphibole composition and hornblende-plagioclase thermometry provided 492 to 508 o C and 4.3 to 4.9 kbar for the inner cores, 495 to 514 o C and 4.5 to 5.1 kbar for the outer cores, 538 to 564 o C and 5.3 to 5.8 kbar for the inner rims and 552 to 573 o C and 5.5 to 5.9 kbar for the outer rims, respectively. These results point to a nearly isobaric prograde P-T path for the Hamedan area amphibolites, compatible with a metamorphic evolution dominated by the thermal perturbation associated with the late Jurassic magmatism of the northern Sanandaj-Sirjan zone.

Ductility contrast induced by silicification in pelitic schist of the Ryoke metamorphic belt, Japan

Contrasting ductility is recognized in the rocks of Cretaceous Ryoke metamorphic belt in Iwakuni area, southwest Japan. Pelitic schist is ubiquitous in the region and differences in mineral assemblages mark increase in metamorphic grade. The area has been graded as chlorite-biotite zone in the north progressing into biotite- and muscovite-cordierite zones in the south. Pelitic schist near the boundary between the biotite- and muscovite-cordierite zones has undergone partial silicification to form whitish silicified schist layers which contain two types of quartz veins: those parallel to foliation in the host rock are called schistosity-concordant veins, and those inclined to host rock foliation, schistosity-discordant veins. In this study we examined the quartz structure in the silicified schist and in both types of veins to understand the ductility contrast induced by the silicification process. Crystallographic orientations of quartz in the veins and silicified schist rocks were studied using the Scanning Electron Microscopy (SEM) based Electron Back Scatter Diffraction (EBSD) technique. Quartz c-axis orientations in the silicified schist are nearly random, demonstrating an absence of post-silicification ductile deformation. Quartz grains in the schistosity-concordant veins have preferred c-axis orientations perpendicular to the schistosity indicating ductile shortening. In contrast, schistosity-discordant veins display distinct quartz c-axis fabric than that found in the schistosity-concordant veins. This is because the two types of host rocks exhibit a difference in ductility during deformation. The presence of deformed quartz veins in the undeformed silicified schist indicates transformation of the ductile pelitic schist into the brittle silicified schist at mid-crustal levels where these rocks originate, hence forming contrasting rock layers. Schistosity-concordant veins in the biotite-rich pelitic schist deformed with its host rock in a ductile manner while the schistosity-discordant veins in the neighboring silicified schist were left intact. Silicification of the pelitic schist may have been caused by the silica-rich geofluids produced by subsurface processes. Geofluids responsible for the occurrence of such mechanically contrasting layers mark an increase in seismic reflectivity at mid-crustal depths and may be potential reflectors of seismic waves giving rise to the so-called “bright spots”.

Influence of shear heating on microstructurally defined plate boundary shear zones

In plate-boundary scale ductile shear zones defined by microstructural weakening, shear heating may lead to a temperature increase over 5 m.y. of up to 80 °C just below the brittle ductile transition, up to 120 °C just below the Moho, and to thermal boundary zones tens of km wide on either side of the shear zone. Where rock strength is highest, shear zones are narrow (∼1 km), and thermal gradients within the shear zone itself are low, so there is no tendency for increased localization. Heating results in thermal weakening, but this is partly offset by grain growth related to the decrease in stress. In shear zones of the order of 10 km width, shear stress, strain rate, and hence heat generation are lower, and thermal gradients are insufficient to cause additional strain localization. Temperature increases in the mid-crust are of the order of 10 °C, insufficient to cause partial melting or an increase in metamorphic grade. In the upper mantle, shear zones may be 50 km or more wide, and the temperature increase is less than 20 °C in 5 m.y., but temperature differences between center and margin may enhance the strain rate at the center by up to 18%.

The Brahmaputra tale of tectonics and erosion: Early Miocene river capture in the Eastern Himalaya

The Himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion. Within the broader orogen, fluvial drainages partly reflect surface uplift, different climatic zones and a response to crustal deformation. In the eastern Himalaya, the unusual drainage configuration of the Yarlung Tsangpo–Brahmaputra River has been interpreted either as antecedent drainage distorted by the India–Asia collision (and as such applied as a passive strain marker of lateral extrusion), latest Neogene tectonically-induced river capture, or glacial damming-induced river diversion events.Here we apply a multi-technique approach to the Neogene paleo-Brahmaputra deposits of the Surma Basin (Bengal Basin, Bangladesh) to test the long-debated occurrence and timing of river capture of the Yarlung Tsangpo by the Brahmaputra River. We provide U–Pb detrital zircon and rutile, isotopic (Sr–Nd and Hf) and petrographic evidence consistent with river capture of the Yarlung Tsangpo by the Brahmaputra River in the Early Miocene. We document influx of Cretaceous–Paleogene zircons in Early Miocene sediments of the paleo-Brahmaputra River that we interpret as first influx of material from the Asian plate (Transhimalayan arc) indicative of Yarlung Tsangpo contribution. Prior to capture, the predominantly Precambrian–Paleozoic zircons indicate that only the Indian plate was drained. Contemporaneous with Transhimalayan influx reflecting the river capture, we record arrival of detrital material affected by Cenozoic metamorphism, as indicated by rutiles and zircons with Cenozoic U–Pb ages and an increase in metamorphic grade of detritus as recorded by petrography. We interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen. Whole rock Sr–Nd isotopic data from the same samples provide further support to this interpretation. River capture may have been caused by a change in relative base level due to uplift of the Tibetan plateau. Assuming such river capture occurred via the Siang River in the Early Miocene, we refute the “tectonic aneurysm” model of tectonic–erosion coupling between river capture and rapid exhumation of the eastern syntaxis, since a time interval of at least 10 Ma between these two events is now demonstrated. This work is also the first to highlight U–Pb dating on detrital rutile as a powerful approach in provenance studies in the Himalaya in combination with zircon U–Pb chronology.

Regional metamorphism of the Early Palaeozoic Greenland Group, South Westland, New Zealand

Early Palaeozoic basement rocks in South Westland consist of regionally metamorphosed Greenland Group of probable Ordovician depositional age. Correlative and formerly contiguous Gondwana margin rocks are also found in Antarctica and eastern Australia. Major protoliths are variably foliated sandstone and mudstone, with rare scattered calcareous lenses. Field, petrographic and electron microprobe data reveal a progressive, regional, southeast-wards increase in metamorphic grade in a 2600 km2 area between Martins Bay and Fox Glacier. We define new chlorite, biotite–albite, biotite–calcic plagioclase and sillimanite–microcline zones. Minerals such as cordierite and andalusite, along with semi-schistose textures, indicate a low P/T (pressure/temperature) Buchan style of regional metamorphism spanning the lower greenschist to upper amphibolite facies. This contrasts with medium to high P/T metamorphism in the Haast Schist east of the Alpine Fault. In South Westland, peak metamorphism is loosely constrained as Devonian to Carboniferous (330–375 Ma) in age; the present-day isograd pattern was mostly established by the Late Cretaceous and has not been strongly influenced by Alpine Fault deformation.

Folding of granite and Cretaceous exhumation associated with regional-scale flexural slip folding and ridge subduction, Kitakami zone, northeast Japan

The early Cretaceous granitic plutons intrude the Kitakami zone, northeast Japan, whose southern and northern regions consist of forearc basin and accretionary complex rocks, respectively. All country rock of the Kitakami zone exhibit prominent pressure-solution cleavage and associated folds formed during shortening with a small component of sinistral shear, whereas most plutons show only igneous textures. The Kesengawa granite and some other plutons, however, have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks. We document a tectonic fold with axial planar foliation in part of the Kesengawa granite. The metamorphic minerals associated with the contact aureole and country rocks of the Kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled. The Kesengawa pluton and country rocks of the southern Kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern Kitakami zone are overturned and verge to the east. The Kitakami basement is not bounded by normal or reverse faults, so the style of regional exhumation does not resemble the upright or inclined extrusion noted in other regions, nor is the exhumation associated with extensional doming. Instead, the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines. The regional shortening of accretionary prism, forearc basin, and older forearc basement was associated with intrusion of adakitic plutons that thermally weakened the forearc basin and enhanced the deformation and exhumation. The adakitic magmatism and forearc shortening resulted from subduction of the buoyant Izanagi–Kula ridge, a regional event known as the Oshima orogeny.

Variations in the Illite to Muscovite Transition Related to Metamorphic Conditions and Detrital Muscovite Content: Insight from the Paleozoic Passive Margin of the Southwestern United States

X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been widely used in regional studies of low-grade metamorphism. Variations in detrital muscovite content of sediments are also measurable with XRD and may, in fact, be difficult to distinguish from metamorphic gradients. We utilize field transects to isolate detrital muscovite versus metamorphic reaction components of illite-muscovite variations in Paleozoic rocks of the western United States. One transect focuses on a single stratigraphic interval (Middle Cambrian pelites) and extends from Death Valley in the west to the Grand Canyon in the east. Detrital muscovite content is roughly constant along this 500-km-long transect, so illite-muscovite variations are ascribed to differences in low-grade metamorphism. In terms of both illite crystallinity and polytype composition, there is an overall increase in metamorphic grade to the west along the transect, from the diagenetic metapelitic zone at the Grand Canyon to epizone-low anchizone conditions near Death Valley. Most of the regional variation in illite parameters occurs between Frenchman Mountain and the southern Nopah Range, corresponding to the transition from cratonic to miogeoclinal facies and also to the leading edge of the Sevier thrust belt. In contrast to this horizontal transect that highlights metamorphic reactions, a vertical transect through the Paleozoic stratigraphy of the Grand Canyon targets temporal variations in the flux of detrital muscovite. In the Grand Canyon, illite crystallinity reaches a maximum at the base of the Pennsylvanian Supai Group before decreasing upsection. Deposition of the Supai Group was coeval with formation of basementcored uplifts during the Ancestral Rocky Mountains orogeny, and we suggest that the upsection change in illite composition at the Grand Canyon reflects input of detrital muscovite eroded from these uplifts

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.

Counterclockwise P– T evolution of the Aghil Range: Metamorphic record of an accretionary melange between Kunlun and Karakorum (SW Sinkiang, China)

This paper describes the metamorphic evolution and the tectonic significance of the Aghil Range, a poorly known terrane located between Kunlun and Karakorum north of K2 in the framework of western Tibet. The Aghil Range consists of different units separated by syn- to late-metamorphic thrusts and post-metamorphic faults of similar attitude; among other units, the Surukwat Complex is a composite sequence of thrust sheets trending WNW–ESE and steeply dipping SSW showing a general increase of metamorphic grade from lower to higher structural levels. P– T pseudosections and conventional thermobarometry of metapelites at the top of the Surukwat Complex tightly constrain the P– T path of this highest-grade metamorphic portion of the Aghil Range. The prograde path is characterized by an early increase in both P and T and by a later, nearly isothermal, increase in P, from 500 < T < 530 °C and 0.25 < P < 0.40 GPa to 580 < T < 600 °C and 0.80 < P < 0.90 GPa. The peak metamorphic event is constrained at 550 < T < 590 °C and 0.77 < P < 0.91 GPa. The retrograde path is characterized by decompression associated to a slight cooling to T ~ 500 °C and P < 0.5 GPa. Altogether, the petrology of the studied rocks suggests a P– T path with a narrow counterclockwise shape. The studied sequence could represent the result of the early subduction of an accretionary complex, interpreted further eastward according to a melange underthrusting model. As concerning the tectonic setting of this terrane, a number of geologic and petrologic similarities link the Aghil Range and the central Qiangtang metamorphic belt, suggesting that the Aghil Range is the possible NW extension of the Qiangtang microplate separating Kunlun from Karakorum.