Prograde Regional Metamorphism Research Articles

Lithostratigraphy and metamorphism of the Khudi-Tal area, west central Nepal

The study is mainly focused on lithostratigraphy, petrography and geological structures to decipher the metamorphic facies and metamorphic history of the Khudi-Tal area along the Marsyangdi Valley, west central Nepal. Geological mapping was carried out covering 142 sq.km, on the scale of 1:50000. The Main Central Thrust (MCT) separates lithological succession of the Lesser Himalaya and Higher Himalaya around Bahundanda, Probhi and Naiche of the Khudi-Tal area. Lithological succession is dominated by metapelitic to psammatic schist, siliceous to dolomitic marble, quartzites, graphitic to garnet schist, kyanite garnet gneiss and banded gneiss from south to north. The kyanite isograd follow the MCT separating Kyanite zone with mineral assemblages Ky+Grt+Qtz+Ms+Bt+Pl of the Higher Himalaya and Garnet zone as Grt+Bt+Ms+Chl+K-Fel+Pl+Qtz in pelitic rocks, Bt+Ms+Chl+Plag+Qtz in Psammatic rocks and Cal/ Dol+Qtz+Pl+Bt+Ms in calcareous rock of the Lesser Himalaya. Based on the mineral assemblages, the Higher Himalaya belongs to Amphibolite Facies whereas the Lesser Himalaya belongs to Epidote-Amphibolite Facies. The lithological succession of the Higher and the Lesser Himalaya has undergone poly-phase metamorphism. In the Lesser Himalayan zone rocks, the regional burial metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type garnet grade inverted metamorphism M2 and later post-MCT retrogressive metamorphism M3. In the Higher Himalaya rocks, the early phase kyanite bearing medium temperature/ high pressure regional prograde metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type inverted metamorphism M3 (syn-MCT/ neo-Himalayan) and over printed by later phase retrogressive metamorphism M3 (post-MCT).

El Álamo district (Baja California, México): A hint of a new Cordilleran orogenic gold belt?

The El Álamo is one of the most important historic gold mining districts in the Peninsular Ranges batholith. Gold mineralizations at El Álamo consist in fault-fill laminated quartz veins with small amounts of sulfides and oxides, and lesser amounts of gold. Veins are mainly emplaced along compressional brittle-plastic shear zones within the El Álamo quartz diorite (146.6 ± 1.6 Ma, U–Pb zircon), which is intruded by the San Marcos dike swarm (126.8 ± 1.7 Ma, U–Pb zircon). Several lines of evidence indicate that the dike swarm is older than quartz veins, and the formation of the later is related to the Peninsular Ranges orogeny (∼110−98 Ma). Gold bearing quartz veins formed during regional prograde metamorphism (greenschist facies), and the veins subsequently underwent plastic deformation, dynamic recrystallization and metamorphism. Gold remobilization and concentration seems to have been largely enhanced by metamorphism, probably accounting for very high grade, albeit sporadic, bonanza ores, typical of the district. 40Ar/39Ar dating of hydrothermal white micas from the auriferous veins yielded ages of 97.86 ± 0.17 Ma at the Ulises mine and 97.46 ± 0.17 Ma at the La Americana mine. These ages are interpreted as the minimum age of the regional metamorphism in the area. Based on their primary characteristics, the gold deposits from El Álamo district can be ascribed to the orogenic gold type. These deposits are similar to other known deposits in the same orogenic context (e.g., Julian district; southern California), and they probably conform a new Cordilleran orogenic gold belt that could extend over much of the exhumed axis of the Peninsular Ranges batholith.

Vein topology, structures, and distribution during the prograde formation of an Archean gold stockwork

Abstract The Archean Cheechoo stockwork gold deposit is hosted by a felsic intrusion of tonalitic-granodioritic composition and crosscutting pegmatite dikes in the Eeyou Istchee James Bay area of Quebec, Canada (Archean Superior craton). The evolution of the stockwork is characterized herein using field relationships, vein density, and connectivity measurements on drill core and outcrop zones. The statistical distribution of gold is used to highlight mechanisms of stockwork emplacement and gold mineralization and remobilization. Two statistical populations of gold concentration are present. Population A is represented by gold grades below 1 g/t with a lognormal cumulative frequency. It is widespread in the hydrothermally altered (albite and quartz) and mineralized facies of the pluton. It is controlled by the development of quartz-feldspar-diopside veins as shown by the similar lognormal distribution of grades and vein density and by the correspondence of grades with network connectivity. Diopside and actinolite porphyroblasts in deformed veins within sodic and calcsilicate alteration zones are evidence for auriferous vein emplacement prior to the amphibolite facies peak of metamorphism. Population B (>1 g/t) is erratic and exhibits a strong nugget effect. It is present throughout the mineralized portion of the pluton and in pegmatites. This population is interpreted as the result of gold remobilization during prograde metamorphism and pegmatite emplacement following the metamorphic peak. The pegmatites are interpreted to have scavenged gold emplaced prior to peak metamorphism. These results show the isotropic behavior of the investigated stockwork during regional deformation and its development during the early stages of regional prograde metamorphism.

Precise Location and Mapping of the Main Central Thrust Zone in Reference to Micro-Structures and Deformation along Khudi-Tal Area of Marsyangdi Valley

Geological mapping was carried out along Marsyangdi valley in the Khudi - Dahare -Tal area on a scale of 1: 50,000 covering about 142 square kilometers. Recent study aims to locate the Main Central Thrust (MCT) precisely based on lithostratigraphy, micro-structures, deformation, and metamorphism. Several thin sections were observed to study the metamorphism, deformation, and micro-structures developed in the rocks. The rocks sequences in both the Higher Himalaya and the Lesser Himalaya have undergone polyphase metamorphism and deformation. The Lesser Himalaya experienced first burial metamorphism (M1) followed by garnet grade inverted metamorphism related to the MCT activity (M2) followed by retrograde metamorphism (M3) whereas the Higher Himalaya has undergone regional high-pressure/ high-temperature kyanite/ sillimanite- grade prograde regional metamorphism (M1) followed by the (M2) related to ductile sharing which in turn is overprinted by the later post-tectonic retrograde garnet to chlorite grade metamorphism during exhumation. The polyphase deformation is indicated by the cross-cutting foliation and many other features. The deformation phase D1 is associated with the development of the bedding parallel foliation due to burial in both the Higher Himalaya and the Lesser Himalaya. Isoclinal folds and crenulation cleavage were developed before the collision is categorized as D2. Development of nearly N- S trending mineral and stretching lineation, south vergent drag folds, folded S2 cleavage and microscopic shear sense indicators, rotated syn- tectonic garnet grains, etc. were developed during the deformation D3 related to the ductile shearing through the MCT. Various brittle faults and shear zones cross-cutting all earlier features were developed during D4 during the upheaval. The rocks in the MCT zone are affected by intense sharing and mylonitization as indicated by the presence of many mylonitic structures in the thin sections throughout the Lesser Himalaya in the area. Features like polygonization and ribbon quartz with evidence of sub-grain rotation, mica fish, syn-tectonic rotated garnet grains indicate the ductile shearing in the MCT area suggesting the dynamic recrystallization in the MCT zone whereas rocks of the Higher Himalaya show the evidence of recrystallization under static condition. The MCT zone was mapped precisely based on the microstructures and deformation.

Metamorphic fingerprints of Fe-rich chromitites from the Eastern Pampean Ranges, Argentina

Chromitites hosted in the serpentinized harzburgite bodies from Los Congos and Los Guanacos (Eastern Pampean Ranges, north Argentina) record a complex metamorphic evolution. The hydration of chromitites during the retrograde metamorphism, their subsequent dehydration during the prograde metamorphism and the later-stage cooling, have resulted in a threefold alteration of chromite: i) Type I is characterized by homogeneous Fe3+- and Cr-rich chromite; ii) Type II chromite contains exsolved textures that consist in blebs and fine lamellae of a magnetite-rich phase hosted in a spinel-rich phase; iii) Type III chromite is formed by variable proportions of magnetite-rich and spinel-rich phases with symplectitic texture. Type I chromite shows lower Ga and higher Co, Zn and Mn than magmatic chromites from chromitites in suprasubduction zone ophiolites as a consequence of the redistribution of these elements between Fe3+-rich non-porous chromite and silicates during the prograde metamorphism. Whereas, the spinel-rich phase in Type III chromite is enriched in Co, Zn, Sc, and Ga, but depleted in Mn, Ni, V and Ti with respect to the magnetite-rich phase, due to the metamorphic cooling from high-temperature conditions. The pseudosection calculated in the fluid-saturated FCrMACaSH system, and contoured for Cr# and Mg#, allows us to constrain the temperature of formation of Fe3+-rich non-porous chromite by the diffusion of magnetite in Fe2+-rich porous chromite at <500 ºC and 20 kbar. The subsequent dehydration of Fe3+-rich non-porous chromite by reaction with antigorite and chlorite formed Type I chromite and Mg-rich olivine and pyroxene at >800 ºC and 10 kbar. The ultimate hydration of silicates in Type I chromite and the exsolution of Type II and Type III chromites would have started at ~600 ºC. These temperatures are in the range of those estimated for ocean floor serpentinization (<300 ºC and <4 kbar), the regional prograde metamorphism in the granulite facies (800 ºC and <10 kbar), and subsequent retrogression to the amphibolite facies (600 ºC and 4-6.2 kbar) in the host ultramafic rocks at Los Congos and Los Guanacos. A continuous and slow cooling from granulite to amphibolite facies produced the exsolution of spinel-rich and magnetite-rich phases, developing symplectitic textures in Type III chromite. However, the discontinuous and relatively fast cooling produced the exsolution of magnetite-rich phase blebs and lamellae within Type II chromite. The P-T conditions calculated in FCrMACaSH system and the complex textural and geochemical fingerprints showed by Type I, Type II and Type III chromites leads us to suggest that continent-continent collisional orogeny better records the fingerprints of prograde metamorphism in ophiolitic chromitites.

Tectonic evolution of the hinterland of the Zagros Orogen revealed from the deformation of the Golpaygan Metamorphic Complex, Iran

In the southwestern part of the Central Iran zone, the Golpaygan Metamorphic Complex (GMC) was evolved during the Zagros orogeny. This complex is composed of different metamorphic rocks including schist, marble, slate, gneiss and amphibolite. The metamorphic complex is overlain by a cover sequence including Permian carbonates, Jurassic clastic rocks, Cretaceous carbonates and Eocene clastic rocks. The internal structure of the GMC is characterized by four deformational stages. The first stage (D1) developed contemporaneously with the prograde regional metamorphism and resulted in formation of the first generation folds and prevailing axial plane foliation. The second stage of deformation (D2) was accompanied by deformation partitioning and formation of contractional shear zones. These structures were overprinted by different scale folds of the third stage (D3). The fourth stage of deformation (D4) is characterized by generation of the extensional shear zones which gradually transformed into normal faults. D1 and accompanied metamorphism may have resulted from steep subduction of the Neotethys Ocean beneath the active margin of the Central Iran zone during Jurassic times. D2–D3 structures and retrograde metamorphism may have been formed by a regional shortening and transpression deformation event which resulted in exposure of the metamorphic rocks in shallower levels during Cretaceous flat slab subduction. This condition lasted until Paleocene time when the slab roll-back caused extension and thinning of the overriding Central Iran plate. The subsequent decompression and regional extension resulted in intrusion of magmatic rocks and generation of D4 which formed the GMC and related magmatism in Paleocene-Eocene time.

A case of regional metamorphism of Buchan type (andalusite-cordierite) in the Nortern Santander Massif, Eastern Cordillera (Colombia)

The Lower Paleozoic Silgara Formation in the northern Santander Massif was affected by Caledonian prograde regional metamorphism, which varies from low to medium grade and is represented by greenschist, epidote-amphibolite and amphibolite facies. The Silgara Formation shows evidence of a regional metamorphism of Buchan type (andalusite-cordierite) attributed to a crust that was affected by a considerable addition of heat due to magmas which was overimposed on the Barrovian metamorphism that characterize this massif. An anticlockwise PT path reveals that the initial conditions are represented by the mineral assemblage of chloritoid+pryrophyllite+chlorite (all assemblages with quartz+muscovite) in greenschist facies and the final conditions correspond to the mineral assemblage of sillimanite+biotite+cordierite (+muscovite+quartz+garnet if sufficient MnO and CaO are present) in amphibolite facies due to the reaction andalusite = sillimanite occurred at 600 °C and 2.4 kbar. Additionally, in these rocks several deformation events and evidences of shearing and intracrystalline deformation were found.

Zn and Pb mobility during metamorphism of sedimentary rocks and potential implications for some base metal deposits

Comprehension of the genesis of Pb-Zn ore systems is currently limited by a poor understanding of where these metals are sourced from. Our study of metal mobility during regional metamorphism in the Mt. Lofty Ranges, South Australia, demonstrates that in staurolite-absent siliciclastic metasedimentary rocks, biotite contains >80 % of the bulk rock Zn, as well as a considerable proportion of the total Pb. Fluid flow through these metasedimentary rocks led to a continuous depletion of Pb and Zn on a mineral and bulk rock scale during prograde regional metamorphism. We calculate that ∼80 % of the bulk rock Zn and ∼50 % of the bulk rock Pb were mobilised, mainly through reactions involving biotite. These reactions led to a calculated Pb and Zn “loss” of ∼2.7 and 27 Mt, respectively, in the high-grade metamorphic zone. Halogen contents of apatite and biotite and bulk rock Zn isotope data provide evidence that Cl-rich metamorphic fluids were important for metal transport. Hence, fluid flow accompanying prograde metamorphism of typical sedimentary rocks can mobilise base metals to the degree required to potentially supply significant Pb-Zn ore systems.

Phase equilibria constraints on the melt fertility of crustal rocks: the effect of subsolidus water loss

AbstractDuring regional prograde metamorphism, H2O generated by ongoing dehydration reactions is likely to be continuously lost from a rock by compaction. Classical melting experiments cannot easily simulate this phenomenon, because ideally, all run products are conserved within the experimental charge, although significant equilibration and H2O generation may occur during heating. Phase equilibria modelling is used to consider the effect of subsolidus water loss (SWL) on subsequent melting relationships of felsic lithologies (including metapelite, metagreywacke and metatonalite) in the suprasolidus. SWL drives the bulk composition towards the minimum saturation point on the boundary of the wet‐melting field and results in significantly reduced subsequent melt generation when compared to melting experiments involving conservation of subsolidus H2O. This effect is most significant at P–T conditions just above the solidus. For initially hydrated rocks, the reduction in melt production causes rheologically critical thresholds (e.g. melt connectivity threshold, melt escape threshold and the solid‐to‐liquid transition) to be intersected at temperatures generally more than 100 °C, higher than predicted by idealized experimental melting curves.

Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry

Determining early orogenic processes within the Pamir-Tibet orogen represents a critical step toward constructing a comprehensive model on the tectonic evolution of the region. Here we investigate the timing and cause of prograde metamorphism of Cenozoic metamorphic rocks from the Pamir plateau through Lu-Hf geochronology, U-Pb rutile thermochronology, and garnet thermometry. Regional prograde metamorphism and heating to 750–830 °C, as constrained by thermometry, occurred between 37 and 27 Ma. Prograde growth of garnet first occurred in the South Pamir and spread to the Central Pamir during the following 10 m.y. The early metamorphism is attributed to high mantle heat flow following the ca. 45 Ma break-off of the Indian slab south of the Pamir. Our investigation confirms a long-lived thermal history of the Pamir deep crust before the Miocene, and provides a causal link between break-off, enhanced mantle heat flow, and prograde heating of the subduction hanging wall.

Mica Inclusions inside Host Mica Grains from the Sutlej Section of the Higher Himalayan Crystallines, India-Morphology and Constrains in Genesis

Biotite and muscovite inclusions inside mica host minerals from the Sutlej section of the Higher Himalayan Crystalline were studied under an optical microscope. These inclusions formed possibly by local recrystallization of mica grains during regional prograde metamorphism, with some affected by top-to-SW shear leading to parallelogram shapes. Recrystallization may have been assisted by solution transfer along the cleavage planes of the host grains. The relative competency of deformed phyllosilicate inclusions with the same or different composition to the host depends on the size and orientation of (001) cleavage planes of the inclusions relative to the host. Shearing of mica inclusions led to their parallelogram geometries within the contained mica inclusions. Some of the sheared inclusions deflect cleavage planes in the host minerals and define flanking microstructures. Trapezoid-shaped inclusions are a new finding that deserves more attention for their genesis. These structurally anisotropic inclusions did not originate from sub-grains, secondary infillings or retrogression. These inclusions are also not related to pseudomorphism, isomorphism, folding of the bulk rock etc. Some of the inclusions formed by recrystallization of the host mineral during top-to-SW ductile shear.

The “Permian event” in the Eastern European Alps: Sm–Nd and P–T data recorded by multi-stage garnet from the Plankogel unit

Kyanite–staurolite–garnet micaschists from the Plankogel unit, Saualpe–Koralpe region, Eastern European Alps, document a poly-metamorphic evolution. Textures indicate two stages of garnet growth, interpreted as (i) relics of an early, pre-Alpine metamorphic event and (ii) newly-grown garnet of Alpine age. The age of the first crystallisation event during prograde Permian metamorphim is constrained by Sm–Nd dating of the almandine-rich garnet core domains. Mineral–wr ages of nine high-Sm/Nd garnet single crystal core fractions from three distant localities (Laaken, SE Koralpe; Plankogel, NW Saualpe; Friesach, E Gurktal Alps) range between 275.0 ± 2.7 and 264.1 ± 3.6 Ma. The pooled mean regression age of just the garnet fractions is 269.6 ± 6.2 Ma (MSWD = 4.3; n = 9), with an initial Nd isotope composition of εNd(270) = − 8.6. Leaching experiments and whole rock analyses revealed slight Nd isotope disequilibrium between garnet porphyroblasts, REE-rich inclusions, and the matrix of the polyphase mineral assemblage. Kyanite inclusions in the inner and sillimanite in the outer parts of the garnet cores, decreasing grossular and increasing pyrope contents indicate variable pressure/temperature conditions during Permian garnet growth. Modelling of mineral assemblages in pressure–temperature pseudosections suggests that the first-generation, Permian garnet crystallized at about 0.46 GPa and 540 °C, whereas the eo-Alpine garnet documents distinctly higher pressures of 1.0 GPa and 600 °C. The data document regional prograde metamorphism in metapelites, at low or moderate pressure conditions, during the Middle Permian. Crustal-scale extensional processes and heat input from the mantle, induced by ongoing Paleotethys subduction and Neotethys opening in the course of the separation of Europe and Africa-related Adria were responsible for this low-P tectonothermal event.

Monazite begets monazite: evidence for dissolution of detrital monazite and reprecipitation of syntectonic monazite during low-grade regional metamorphism

Back-scattered electron (BSE) imaging and X-ray element mapping of monazite in low-grade metasedimentary rocks from the Paleoproterozoic Stirling Range Formation, southwestern Australia, reveal the presence of distinct, high-Th cores surrounded by low-Th, inclusion-rich rims. Previous geochronology has shown that the monazite cores are older than 1.9 Ga and overlap with the ages of detrital zircon grains (∼3.5–2.0 Ga), consistent with a detrital origin. Many cores have scalloped and embayed surfaces indicating partial dissolution of former detrital grains. Textural evidence links the growth of the monazite rims (∼1.2 Ga) to deformation and regional metamorphism during the Mesoproterozoic Albany-Fraser orogeny. These results indicate that high-Th detrital monazite is unstable under low-grade metamorphic conditions (<400°C) and was partially or completely dissolved. Dissolution was followed by near-instantaneous reprecipitation and the formation of low-Th monazite and ThSiO4. This reaction is likely to operate in other low-grade metasedimentary rocks, resulting in the progressive replacement of detrital monazite by metamorphic monazite during regional prograde metamorphism.

Serpentine polymorphs andP-Tevolution of metaperidotites and serpentinites in the Takab area, NW Iran

AbstractThe Takab meta-ultramafic rocks of northwestern Iran crop out in association with a variety of metamorphic rocks and they can be divided into three types on the basis of dominant mineral assemblage and degree of serpentinization: metaperidotites, talc-serpentine rocks and serpentinites. The peridotite protoliths were mainly spinel-harzburgites and -dunites. The talc-serpentine rocks formed under low-grade conditions and their dominant mineral assemblage is lizardite/chrysotile + talc + calcite/dolomite. Pseudomorphic textures after olivine and pyroxene are typical. Later, lower amphibolite-facies metamorphism produced antigorite, clinochlore and tremolite. Late alteration, assisted by local shearing, produced more antigorite and late lizardite in the metaperidotites. The mineral chemistry of the serpentine polymorphs is variable. Compositions of coexisting spinel and olivine point to an ophiolitic origin. Thermometric estimates from co-existing relict olivine, pyroxenes and spinel are in the range 1000–1200ºC; clinopyroxene barometry yields a pressure of 24± 2.7 kbar for the mantle conditions. Exhumation and hydration followed, and the stability of lizardite/chrysotile constrains the T and XCO2to &lt;280ºC and &lt;0.002, respectively. Subsequent prograde regional metamorphism took place in an orogenic setting, reaching peak temperatures in the range 410–540ºC at XCO2&gt;0.1. A Neoproterozoic–Early Cambrian age for the Takab protoliths is suggested on the basis of correlations with the Central Iran Zone. The strips of ultramafic rock in the Takab complex can be attributed to lithospheric remnants of the Proto-Tethyan ocean which closed during the Pan-African orogeny.

A shear zone related greenstone belt hosted gold mineralization in the Archean of West Greenland. A petrographic and combined Pb–Pb and Rb–Sr geochronological study

Widespread gold showings have been found in an Archean greenstone belt on Storø in Godthåbsfjord, West Greenland. The Storø Greenstone Belt (SGB) comprises metasedimentary and metavolcanic rocks with minor magnetite-rich bands. Gold mineralization is hosted by amphibolites, mica schists and garnetite rocks. Gold occurs in various mineral associations, but its presence in löllingite–arsenopyrite–pyrrhotite parageneses is prevalent. To a lesser extent, native gold occurs within seriticized plagioclase, in sheeted quartz veins, and as inclusions in garnet. Based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold-bearing löllingite–arsenopyrite–pyrrhotite assemblage on Storø: In the first stage, arsenopyrite likely formed with gold in solid solution. In the second stage, gold-bearing arsenopyrite was replaced by löllingite and pyrrhotite. This took place during regional prograde metamorphism. Gold accumulated within löllingite as discrete grains. During subsequent retrograde metamorphic conditions, löllingite and pyrrhotite reacted and formed arsenopyrite, and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite–löllingite grain boundaries. During a later event a garnet overgrowth partly affected the gold-bearing löllingite–arsenopyrite–pyrrhotite assemblage. The isotopic composition of Pb released from these garnets by Pb stepwise leaching experiments (PbSL) and the bulk Pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of Pb in the fluid from which first the sulfides and subsequently garnet were formed. Bulk Pb analyses and PbSL experiments of arsenopyrite and PbSL experiments of garnet yield ages of 2863 ± 24 Ma (2 σ; MSWD = 0.2) for arsenopyrite and 2748 ± 62 Ma (2 σ; MSWD = 220) for garnet. The 2863 ± 24 Ma age is thus a minimum age for the mineralization. Rb–Sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal Middle Proterozoic (ca. 1.65 Ga) mineral ages, well in accordance with a regional reheating event known to have affected the area, and are thus interpreted as cooling ages thereof. This regional reheating event has apparently not affected the Pb–Pb isotopic system of arsenopyrite, implying that the blocking temperature of the U–Pb system in arsenopyrite is likely to be above ∼ 300 °C. The Late Archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time. The ore textures preserved today are interpreted as those formed under this regional metamorphic event.

Paleocene–Eocene high-grade metamorphism, anatexis, and deformation in the Thor–Odin dome, Monashee complex, southeastern British Columbia

The Thor–Odin dome of the Monashee complex, in the southeastern Canadian Cordillera, comprises Paleoproterozoic basement gneiss with infolds of unconformably overlying rocks of a supracrustal cover sequence and is the deepest exposed structural level in the Omineca belt. Orthogneiss and paragneiss of the basement are migmatitic and contain ubiquitous stromatic leucosome and discrete phenocrystic and pegmatitic vein-type leucosome, which are all interpreted to have formed as a result of in situ melting. The stromatic leucosome is infolded with the country rock (F2), contains a weakly developed foliation, and has a biotite-rich melanosome. The phenocrystic and pegmatitic vein-type leucosome crosscut the stromatic leucosome and the transposition foliation (S2). Evidence to support an igneous and anatectic source for the leucosome includes (i) petrography, (ii) major and trace element chemistry, (iii) zircon morphology, and (iv) peak pressure–temperature (P–T) conditions. Sensitive high-resolution ion microprobe (SHRIMP) 206Pb/238U zircon dates range from ca. 56 to 54 Ma and are interpreted to represent the age of leucosome crystallization. Zircon commonly contains discrete ca. 2.6-1.8 Ga cores that are interpreted as detrital grains inherited from the host paragneiss. Anatexis was ongoing by ca. 56 Ma, as a result of regional prograde metamorphism, and was coincident, at least in part, with the formation of the penetrative S2 transposition foliation and large recumbent F2 tight to isoclinal folds. Anatexis continued during F3 and F4 folding. Melting may have continued until ca. 51 Ma, driven by decompression reactions, and was concomitant with the D5 extensional deformation.

Tectonic evolution and significance of Silurian – Early Devonian hinterland-directed deformation in the internal Humber zone of the southern Quebec Appalachians

In the southern Quebec Appalachians, the early tectonic history of the Laurentian margin (Humber zone) comprises foreland-propagating, northwest-directed thrust faulting, nappe emplacement, and regional prograde metamorphism in response to the obduction of large ophiolitic nappes during the Taconian orogeny. In the internal Humber zone, this event is dated at 462 ± 3 Ma (late Middle Ordovician), which is interpreted to represent the timing of near-peak Taconian metamorphism. Superimposed hinterland-directed structures are accompanied by retrograde metamorphism and consist of back thrusts and normal faults, which respectively delimit the northwestern and southeastern limbs of the Sutton and Notre-Dame mountains anticlinoria, both salient structures of the internal Humber zone of southern Quebec. Geochronologic data on the timing of hinterland-directed deformation vary from 431 to 411 Ma. Two tectonic models are presented and discussed, which may account for the Silurian – Early Devonian evolution of the Laurentian margin: (1) back thrusting and syn- to post-compressional crustal extension in response to the tectonic wedging of basement-cored duplexes inducing delamination of supracrustal rocks; (2) tectonic exhumation of the internal Humber zone by extensional collapse. Evidence for Silurian – Early Devonian extensional tectonism in the Humber zone provides the basement infrastructures necessary for the creation and the onset of sedimentation in the Gaspé Belt basins (e.g., Connecticut Valley – Gaspé synclinorium). Several structural, metamorphic features in the internal Humber zone of the northwestern New England Appalachians yield analogous characteristics with those of southern Quebec and may have shared a similar Silurian – Early Devonian tectonic evolution.

Very low-grade metamorphism of sedimentary rocks of the Meliata unit, Western Carpathians, Slovakia: implications of phyllosilicate characteristics

The Meliata unit represents a melange-like accretionary wedge, containing blueschist facies tectonic blocks and slices in a Triassic and Jurassic sedimentary matrix. The blueschist facies rocks are tectonic remnants of the subducted parts of the Meliata-Hallstatt branch of the Tethys. The phyllosilicate assemblages in very low-grade metapelites represent metastable disequilibrium stages which the assemblages have reached during reaction progress. Therefore, temperature and pressure values of low-T metamorphism of the sedimentary series and the late stages of decompressional cooling of blueschist facies rocks, obtained by phyllosilicate "crystallinity", chlorite thermometric and white K-mica geobarometric methods, can be regarded as semiquantitative estimates. However, results of chlorite–white mica thermobarometry suggest that local equilibrium was approached at a microscopic scale. For deciphering the age relations of prograde and retrograde events, K–Ar isotope geochronological methods were applied. The sedimentary series and related basalts of the Meliata unit experienced high-T anchizonal prograde regional metamorphism, the temperature and pressure of which can vary between ca. 280 and 350 °C and ca. 2.5 and 5 kbar. White K-mica b geobarometry suggests possible minimal pressures of ca. 1.5 to 3 kbar. The mylonitic retrogression of blueschist facies phyllites is characterised by 340 °C and 4 kbar (minimal P). The low-T prograde metamorphism was synchronous with the retrograde metamorphism of the blueschists. The ages of these two events may be between ca. 150 and 120 Ma, culminating most probably at around 140–145 Ma. Thus, the Upper Jurassic (lowermost Cretaceous) very low-grade metamorphism of the Meliata unit is younger than the subduction-related, 160–155 Ma blueschist facies event, and definitely older than the Cretaceous (100–90 Ma) metamorphism of the footwall Gemer Palaeozoic.

Dehydration of diasporite to corundite in nature and experiment

The diasporite-corundite rock transformation, which releases 6–8 wt% H 2 O in an average metabauxite, was studied experimentally. The results are compared with petrological observations on the island of Naxos (Greece), where the transformation occurred in metakarst bauxites during prograde regional metamorphism. Dehydration experiments were started with fine-grained natural diasporite embedded in marble. The samples were first annealed in the diaspore stability field, then slowly brought to the final pressure-temperature ( P - T ) conditions in the corundum field and kept there five to seven days. Overstepping the diaspore-corundum equilibrium by ∼30 °C at 8 kbar resulted in partial dehydration of diaspore. As with the corundum-in isograd on Naxos, the corundum grew preferentially along the bauxite-marble contact. Experiments at 17–40 kbar with T oversteps of 40–150 °C resulted in complete diaspore breakdown. A high-porosity zone containing corundum and silicates developed along the bauxite-marble boundary, resulting from the solid volume decreases associated with the diaspore-corundum and decarbonation reactions. In nature, the marble similarly behaved as a barrier for liberated fluid, as indicated by coarse corundum- chloritoid segregations along metabauxite rims. In the 30–40 kbar experiments, the porous contact zone acted as fluid pathway, allowing partial dissolution of metabauxite. This demonstrates pronounced Al, Fe, and Ti mobility at high P and T of 600–800 °C.

Release of CO2 from carbonate rocks during regional metamorphism of lithologically heterogeneous crust

Prograde regional metamorphism drives CO 2 from carbonate rock to crustal fluids that ascend and ultimately interact with the atmosphere and oceans. The observed loss of CO 2 from metamorphic belts remains problematic, however, because the cooling that accompanies fluid ascent favors reactions that add CO 2 to metacarbonate rock by removing CO 2 from fluids. A new two-dimensional model of coupled mass transfer, chemical reaction, and heat transport was developed to assess how rock devolatilization proceeds along the upward escape paths of crustal fluids during prograde metamorphism. The model is based on upper greenschist to lower amphibolite facies growth of amphibole in metacarbonate layers and garnet and biotite in intercalated metapelite layers of the Wepawaug Schist, Connecticut (Acadian orogeny). The modeling indicates that during heating, CO 2 concentrations were larger in metacarbonate layers than in adjacent metapelite layers because amphibole growth in metacarbonates produced CO 2 , whereas garnet and biotite growth in metapelites produced H 2 O. The resulting cross-layer concentration gradients drove H 2 O into the metacarbonate layers and CO 2 out by diffusion and the transverse component of mechanical dispersion. Such cross-layer mass transfer can continually force rock decarbonation while fluids ascend, dominating the effects of cooling, unless fluid fluxes are large and prograde heating rates are small. Consequently, prograde metamorphism of carbonate-bearing sedimentary sequences containing significant amounts of pelitic rock will release CO 2 to regionally migrating fluids in a wide range of orogenic settings, regardless of whether flow is in a direction of increasing or decreasing temperature. Regional CO 2 release can be driven by outcrop-scale processes of volatile exchange between contrasting lithologies.