Prograde Evolution Research Articles

40Ar–39Ar laser dating of ductile shear zones from central Corsica (France): Evidence of Alpine (middle to late Eocene) syn-burial shearing in Variscan granitoids

The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. The most debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40Ar–39Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P–T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike ~NNE–SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P–T conditions attained during shearing at ~300°C and minimum pressures of ~0.6GPa. In-situ 40Ar–39Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, ~45–36Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in-situ ages at ~34Ma, but a hump-shaped age spectrum, with maximum ages of ~35Ma and intermediate- to high-temperature steps as young as ~33–32Ma. Results indicate that the studied samples consist of an earlier celadonite-poor white mica with a minimum age of ~46Ma, overgrown by a synshear high-celadonite white mica, developed at greater depth between ~37 and 35Ma; faint late increments in shearing occurred at ≤33–32Ma, when white mica incipiently re-equilibrated during exhumation. Results suggest that ductile shearing with a dominant strike-slip component pervasively deformed the Corsican basement complex during the emplacement and progressive thickening of the Alpine orogenic wedge and broaden the extent of the domain affected by the Alpine tectonometamorphic events. Integration of petrological modeling and geochronological data shows that the Variscan basement of central Corsica, close to the Alpine nappes, was buried during the late Eocene by ≥18km of Alpine orogenic wedge and foreland deposits. Our results, combined with previously published apatite fission-track data, imply an overburden removal ≥15km from the late Eocene (Priabonian) to the early Miocene (Aquitanian), pointing to a minimum average exhumation rate of 1.3–1.5mm/a.

Prograde evolution of Sulu UHP metamorphic rock in Yangzhuang, Junan region, deduced by combined Raman and petrological studies

The prograde metamorphic history of the Sulu ultrahigh-pressure metamorphic terrane has been revealed using Raman-based barometry of the SiO2 phases and other mineral inclusions in garnet porphyroblasts of a coesite eclogite from Yangzhuang, Junan region, eastern China. Garnet porphyroblasts have inner and outer segments with the boundary being marked by discontinuous changes in the grossular content. In the inner segment, the SiO2 phase inclusions are α-quartz with no coesite or relict features such as radial cracks. The residual pressures retained by the quartz inclusions systematically increase from the crystal centre to the margin of the inner segment. The metamorphic conditions estimated by calculation from the residual pressure and conventional thermodynamic calculation range from 500 to 630 °C and 1.3 to 2.3 GPa for the stage of the inner segment. Coesite and its pseudomorph occur as inclusions in the outer segment of the garnet and matrix omphacite. This occurrence of coesite is consistent with the pressure and temperature conditions of 660–725 °C and 3.1 GPa estimated by conventional geothermobarometry. Our results suggest that the quartz inclusions in the inner segment were trapped by garnet under α-quartz-stable conditions and survived phase transition to coesite at the peak metamorphic stage. The SiO2 phases and other inclusions in the garnet have retained evidence of the pre-eclogite prograde stage even during exhumation stage. The combined Raman spectroscopic and petrological approaches used here offers a powerful means for obtaining more robust constraints prograde stages involving garnet growth where different SiO2 phases are present as inclusions.

Multi-stage barite crystallization in partially melted UHP eclogite from the Sulu belt, China

Barite inclusions in rock-forming and accessory minerals and in multiphase solid inclusions (MSI) in ultrahigh pressure (UHP) eclogites have been controversially interpreted to record the presence of high-salinity oxidizing fluids1 close to the peak of metamorphism. A detailed petrographic and microstructural study of barite in partially melted UHP eclogite from Yangkou, in the central Sulu belt of China demonstrates that barite precipitation is a multi-stage process. The different generations of barite provide insight into the evolution of fluid systems as P-T conditions evolved through the late prograde stage of subduction to the peak of UHP metamorphism and subsequently during exhumation. Five microstructural types of barite are recognized in this study. Type I barite (variable high to low Sr/Ba) occurs as equant primary inclusions with rutile in garnet and omphacite within coesite-bearing eclogite. Zr-in-rutile thermometry on the primary inclusions of rutile yields T = 658–699 °C at P = 2.5–4.5 GPa. Thus, barite inclusions were likely precipitated from an internally buffered fluid during the late prograde evolution. Type II barite (low Sr/Ba) occurs in MSI located toward the rims of garnet and omphacite. Since peak pressure was above the second critical endpoint for basaltic compositions, the MSI are inferred to represent aliquots of a silica-rich supercritical fluid trapped during the late prograde evolution close to the metamorphic peak. Type III barite (low Sr/Ba) occurs in pseudomorphs after phengite inclusions in garnet. The replacement phase assemblage formed during exhumation, by in situ melting of phengite. Type IV barite (low Sr/Ba) forms coarse-grained patches associated with sub-solidus replacement of omphacite by hornblende and albite symplectites along grain boundaries. Type V barite (low Sr/Ba) occurs as grain-boundary veinlets, intergranular grains, and closed rings around pyrite that is partly replaced by hematite and goethite. We interpret the types IV and V barite to have precipitated from an internally generated grain-boundary aqueous fluid, which is expected to be a response to H 2 O exsolving from garnet and omphacite during low-pressure amphibolite facies conditions. Therefore, barite precipitated during the late prograde subduction and the retrograde exhumation of UHP eclogite yields information about the mobility of Ba, Sr, and S during the metamorphic evolution.

Mesoproterozoic UHT metamorphism in the Southern Irumide Belt, Chipata, Zambia: Petrology and in situ monazite dating

The Irumide Belt sensu lato of eastern Zambia situated between the Bangweulu Block and the Zimbabwe Craton is one of the Mesoproterozoic belts in southern Africa, which developed during the formation of the supercontinent Rodinia. Though in several of these belts the orogenic evolution seems to be connected to a long lasting high temperature history (∼200Ma), the duration and geodynamic causes of the metamorphism in this particular belt have not been studied.Here we report the results of an integrated petrological study and texturally controlled in situ monazite dating of spinel–quartz and orthopyroxene–cordierite granulites and associated charnockites and mafic rocks from the Chipata Terrane of the Southern Irumide Belt (SIB), which revealed a single stage high to ultra-high temperature (HT, UHT) metamorphism during the Mesoproterozoic Irumide orogeny. The earliest textural relicts of the prograde evolution are patchy sillimanite aggregates interpreted to represent pseudomorphs after andalusite. Sillimanite inclusions in garnet indicate garnet growth in the sillimanite stability field. During peak metamorphism garnet+sillimanite reacted to form spinel+quartz. In associated garnet–orthopyroxene–cordierite granulites, peak orthopyroxene has high Al2O3 contents of up to 7.9wt%. These peak metamorphic features point to UHT conditions of 900–1000°C (UHT) at ∼5–6kbar, in agreement with the occurrence of inverted pigeonite in associated charnockites. Orthopyroxene+cordierite coronas around prograde garnet in the second assemblage indicates decompression at UHT conditions after peak metamorphism. Subsequent isobaric cooling (IBC) led to reversal of mineral reactions and thus to replacement of spinel+quartz by garnet 2+sillimanite 2+cordierite. The IBC at still high pressures is also indicated by the formation of garnet and clinopyroxene coronas between orthopyroxene and plagioclase in associated mafic rocks. During late-stage retrogression the formation of kyanite+chlorite+quartz±staurolite±magnesite at the expense of cordierite and garnet in garnet–cordierite–sillimanite gneiss indicates prevailing high pressures (>4–5kbar) during cooling below temperatures of ∼550°C.Monazite grains occurring in different textural settings (inclusions in garnet, and in matrix) revealed ages between 1031±4 and 1013±4Ma for the peak HT to UHT metamorphism of the Chipata terrane of the SIB. The metamorphism is almost synchronous with the age of intracrustal melting in the northern Irumide Belt (IB; 1020–950Ma) and slightly younger than the age of continental margin arc magmatism in the SIB (∼1090–1040Ma). The metamorphism therefore most likely occurred as a consequence of subduction related magmatism and the collision between the IB (margin of the Bangweulu block) and the SIB (margin of an unknown southern craton) during the Mesoproterozoic Irumide orogeny.

Late-Neoproterozoic ultrahigh-temperature metamorphism in the Highland Complex, Sri Lanka

The Sri Lankan crustal blocks preserve the records of Neoproterozoic tectonothermal events associated with the final assembly of the Gondwana supercontinent. Here we present petrological and U–Pb zircon geochronological data on a suite of ultrahigh-temperature (UHT) metapelites and mafic granulite from the Highland Complex in central Sri Lanka which provide convincing evidence for Late Neoproterozoic UHT metamorphism. Three sapphirine granulites, two corundum bearing metapelites and one mafic granulite were studied for petrology and zircon U–Pb geochronology. The petrographic observations coupled with conventional thermobarometric calculations indicate that the pelitic granulites attained P=11–12kbar and T=850°C during prograde evolution. Subsequently the rocks have undergone a prograde decompression stage with increasing temperature up to UHT conditions. The peak metamorphism was attained at P–T conditions around 900–975°C and 9–10kbar, followed by near isobaric cooling and rapid decompression.LA-ICPMS zircon U–Pb analyses of magmatic cores in detrital zircons from the sapphirine granulites show 206Pb/238U ages ranging from 834±12 to 722±14Ma, reflecting the timing of crystallization of these grains in their protoliths. Metamorphic zircons in these samples yield ages ranging 535±5 and 567±4Ma representing the age of the Neoproterozoic UHT metamorphism. The metamorphic zircons from corundum-bearing metapelites yield ages from 530±4 to 578±4Ma. An older age population between 1676Ma and 2365Ma from these metasediments suggests the timing of emplacement of the Paleoproterozoic sources. Our data suggest variable degree of input from multiple sources into the protolith sediments of the Highland Complex prior to the latest Neoproterozoic UHT metamorphism at ∼530–580Ma which is broadly coeval with the collisional assembly of the Gondwana supercontinent.

THE OCCURRENCE OF HIGH-TEMPERATURE SKARNS FROM ORAVIŢA (BANAT, ROMANIA): A MINERALOGICAL OVERVIEW

A shallow-level dioritic pluton of Upper Cretaceous age caused extensive contact metamorphism of a Mesozoic calcareous sequence at Oraviţa-Ciclova in Romania. The peak metamorphic assemblage (stage I) includes wollastonite, akermanite-rich melilite, Ti-rich calcic garnet, and pyrrhotite. A second, subsequent paragenesis (stage II) is due to local decreases in fluid pressure and includes gehlenite, Ti-poor calcic garnet, monticellite, Al-rich diopside, magnetite, perovskite, and probably a Ca-Si mineral (kilchoanite?). An early hydrothermal stage (stage III) produced retrogression of the first two stages and a paragenesis including vesuvianite, clintonite, fukalite, katoite, and hydroxylellestadite. A second, late hydrothermal event (stage IV) resulted in the formation of clinochlore, talc, epidote, zeolites (scolecite and thomsonite-Ca), thaumasite, and tobermorite. A last, weathering, paragenesis corresponding to stage V includes allophane (C-S-H gels) and probably portlandite, unpreserved because of its transformation to aragonite, then calcite. The peak metamorphic conditions during the prograde evolution are estimated to have been 750–800 °C for a fluid pressure of up to 0.5 kbar. The main constants determined from chemical, optical, and X-ray powder analyses of these mineral species are reported here. This work documents the first occurrence of katoite and the second occurrences of perovskite, fukalite, and hydroxylellestadite in the Romanian skarns.

Prograde evolution of the Scottish Caledonides and tectonic implications

Recent thermometric analyses of samples collected in thrust-parallel structural transects across the Scandian (435–415Ma) orogenic wedge in northwest Scotland provide a comprehensive characterization of the synorogenic retro-wedge thermal architecture. However, the paucity of petrologically-important metamorphic mineral phases (e.g., staurolite, Al-silicates) has limited investigation of pressure–temperature (P–T) histories, which hinders our ability to examine the nature of orogen-scale kinematic and thermal coupling. New data collected along a foreland-to-hinterland transect from the Moine to the Naver thrust sheets provides additional constraints for characterizing the prograde metamorphic evolution. In addition, we characterized Ti diffusion profiles in quartz inclusions in garnet to constrain duration of metamorphic heating. These results are used to develop coupled kinematic-thermal models of Scandian orogenic evolution.Early garnet core growth conditions are constrained by isopleth intersections, with peak P–T estimates determined by conventional exchange and net transfer thermobarometry and thermodynamic calculations. Most samples follow normal prograde heating and burial profiles, with peak conditions of 450°C and 5.0kbar in the immediate hanging wall to the Moine thrust, increasing in temperature and pressure to 733°C and 9.5kbar in the immediate hanging wall to the Naver thrust. These normal prograde pressure trajectories are interpreted to reflect burial of incipient thrust sheets beneath the overriding wedge at the leading edge of the orogen. Prograde heating coeval with burial is interpreted to result from surface-directed isotherm perturbation due to thrust-related advection in the overriding wedge. One exception to this is a sample from the top of the Moine thrust sheet, where prograde heating occurs during decompression (540°C and 8.1kbar to 590°C and 7.0kbar). In this case, the short lag times between motion on the Moine and Ben Hope thrusts may have limited advectionary heating until after exhumation associated with motion on the underlying Moine thrust was underway. Ti diffusion profiles in quartz inclusions in garnet suggest the near-peak thermal evolution of these rocks occurred over very short time scales (<200,000years). While most of the garnets are inferred to be Scandian in age, we document evidence for pre-Scandian garnet cores in structurally higher (more hinterland positioned) samples that must have grown under higher temperatures. In the hanging wall of the Moine thrust, high grossular garnets with estimated formation conditions >9kbar are probably of detrital origin.

UHP kyanite eclogite associated with garnet peridotite and diamond-bearing granulite, northern Bohemian Massif

Kyanite eclogites enclosed in garnet peridotites may provide important information on P-T evolution of orogenic peridotites in deep subduction and collision zones. Kyanite eclogite interlayered with garnet peridotite occurs in the borehole T-7, in the Saxothuringian basement of the northern part of the Bohemian Massif. This orogenic peridotite of mantle origin is associated with felsic granulites, which contain diamond as a consequence of deep subduction of the continental crust. Here, we report on the metamorphic evolution of kyanite eclogite, which shows a well-preserved peak-pressure mineral assemblage of garnet, omphacite, kyanite and phengite. Conventional geothermobarometry, average PT method and thermodynamic modelling constrain the metamorphic conditions of this assemblage up to 3.5–4.5GPa at 900–1050°C. Two compositional types of garnet, i.e., Mg-rich and Ca-rich, have been recognised. Thermodynamic modelling shows that the composition of Ca-rich garnet with XCa (0.35–0.37) in the core corresponds to stability of garnet at 3.5–4.5GPa. Amphibole and zoisite are preserved as inclusions in garnet cores, and they are stable below 2.5GPa, indicating that garnet grew at the expense of these phases at increasing P-T conditions during the prograde evolution of the rock. A post-peak metamorphism decompression and cooling are recorded by decrease of Ca-Eskola end-member in omphacite, drop in XMg and XGrs at garnet rim and a very restricted formation of pargasitic amphibole in the matrix. The absence of symplectites after omphacite in the investigated eclogite may be due to a very low content of quartz and possibly also fluid in the rock. Our study suggests that kyanite-bearing eclogite underwent UHP metamorphism as a consequence of subduction, together with interlayered garnet peridotite. Both rocks were incorporated into the subducted continental crust (diamond-bearing granulites) during the Variscan orogeny.

Effect of oil exploration and production on the salinity of a marginally permeable aquifer system in the Thar Jath-, Mala- and Unity Oilfields, Southern Sudan

The study area, located in the Subtropics west of the East African Rift, is characterized - on the basis of its hydrology - by an alluvial aquifer system that is made up of predominantly fi ne-grained early Tertiary and Quaternary sediments. These sediments were deposited by the River Nile in large lakes. Nowadays, there is little natural new water recharge from seeping precipitation, due to marginal permeability and the formation of extensive wetlands. In order to search for possible links between an increasingly high salinity in drinking water and the exploration and production of oil in Unity State, South Sudan, a total of 90 water samples from 76 different sampling sites were take n, 74 have been analysed in detail. The samples were taken from the surface water of swamps, from boreholes in various settlements in the area, from produced water storage basins, from production facilities of the oil industry and from mud pits at abandoned as well as from still active exploration drilling sites. Results of these analyses facilitated the identifi cation of six water types that were hydro- chemically distinguishable, and that were characterized by signifi cantly variable ionic relationships and salt contents. Waters unaffected by human activity are characterized by prograde quality development, controlled by water-rock reactions, and they range from a hydrogen carbonate type to a sulphate type. The total dissolved content increases only gradually with the geogenic, prograde evolution of groundwater close to the surface, and ranges in terms of electrical conductivity from 6,000 μS/cm) with simultaneous dominance of chloride and sulphate contents. This anion dominance occurs also partially in produced water storage basins of oil production facilities, especially in mud pits of exploration operations. Due to the direction of groundwater fl ow, the spatial distribution of salt contamination and the hydro-geological boundary conditions found in low water permeable alluvial deposits, the cause of high conductivities in drinking water wells can only be attributed to selective seepage of salt-containing water from the basins and mud pits of those three oilfi elds operating in the study area. Other sources of salt contamination, e.g. deepwater rises or groundwater affected by evaporation, were not identifi ed. In order to guarantee the supply of drinking water for the population in the region, initially two deep wells were sunk (down to 280 m and 300 m depth respectively), and drinking water was pumped from the deepest aquifer, which had not yet been explored and developed. The latter has sulphate-dominated water of low to medium conductivity, which circulates in thin lenses of sand as well as in predominantly silty, fi ne sand layers. A clay layer, approximately 50 m thick, separates the fi rst partly salinized aquifer from the underlying newly explored groundwater horizon. This horizon is supplied only by very slow processes of new accumulation (re-charge) processes, extending over decades. With Open Access Article

Multistage growth of Fe–Mg–carpholite and Fe–Mg–chloritoid, from field evidence to thermodynamic modelling

We provide new insights into the prograde evolution of HP/LT metasedimentary rocks on the basis of detailed petrologic examination, element-partitioning analysis, and thermodynamic modelling of well-preserved Fe–Mg–carpholite- and Fe–Mg–chloritoid-bearing rocks from the Afyon Zone (Anatolia). We document continuous and discontinuous compositional (ferromagnesian substitution) zoning of carpholite (overall X Mg = 0.27–0.73) and chloritoid (overall X Mg = 0.07–0.30), as well as clear equilibrium and disequilibrium (i.e., reaction-related) textures involving carpholite and chloritoid, which consistently account for the consistent enrichment in Mg of both minerals through time, and the progressive replacement of carpholite by chloritoid. Mg/Fe distribution coefficients calculated between carpholite and chloritoid vary widely within samples (2.2–20.0). Among this range, only values of 7–11 correlate with equilibrium textures, in agreement with data from the literature. Equilibrium phase diagrams for metapelitic compositions are calculated using a newly modified thermodynamic dataset, including most recent data for carpholite, chloritoid, chlorite, and white mica, as well as further refinements for Fe–carpholite, and both chloritoid end-members, as required to reproduce accurately petrologic observations (phase relations, experimental constraints, Mg/Fe partitioning). Modelling reveals that Mg/Fe partitioning between carpholite and chloritoid is greatly sensitive to temperature and calls for a future evaluation of possible use as a thermometer. In addition, calculations show significant effective bulk composition changes during prograde metamorphism due to the fractionation of chloritoid formed at the expense of carpholite. We retrieve P–T conditions for several carpholite and chloritoid growth stages (1) during prograde stages using unfractionated, bulk-rock XRF analyses, and (2) at peak conditions using compositions fractionated for chloritoid. The P–T paths reconstructed for the Kutahya and Afyon areas shed light on contrasting temperature conditions for these areas during prograde and peak stages.

PT-path reconstruction via unraveling of peculiar zoning pattern in atoll shaped garnets via image assisted analysis: an example from the Santa Lucia del Mela garnet micaschists (northeastern Sicily-Italy)

Garnet micaschists nearby Santa Lucia del Mela - Peloritani Mountains (North-eastern Sicily, Italy), were studied by integrating information obtained from petrographic and image-assisted analyses of mineral phase compositions by using X-ray maps and thermodynamic modeling. These rocks are characterized by the presence of atoll garnets, and preserve a continuous array of textures from pristine garnets, through peninsula or island textures, to atoll textures. Completely substituted garnets represent the final stage of the process. These textures result from the metamorphic evolution of the rocks, consisting of two main stages of recrystallization-deformation events. The sequence of mineral parageneses associated to successive deformation events is as follows: early D1 is characterized by the assemblage wm1 + chl1 + bt1 + pl1 + grt1 + qtz + ilm + ap; late D1 by grt2 + wm2 + chl2+ bt2 + pl2 + qtz + ilm + ap + hem; D2 (associated to garnet core transformation) by grt3 + bt3 + pl3 + wm3 + qtz + chl3 + ilm. On the basis of textural observations and mineral parageneses, the atoll formation is related to the substitution of garnet cores during the D2 stage, with the consumption of garnet cores (grt1) and production of new garnet rim (grt3) with wm, bi, qtz and pl. The recognition of the different stages of garnet growth and associated parageneses has been obtained using image analysis coupled with compositional maps, performed via multivariate statistical analysis of four selected X-Ray map arrays. Following the textural and mineral-chemical analyses, PT conditions associated to the three recrystallisation stages have been determined via pseudosection computations. Results indicate that the analyzed rock-types have recorded a clock-wise PT trajectory during the Variscan orogeny, defined by a prograde evolution, in amphibolite facies conditions, preserved in relic garnet cores, followed by an initial decompressional evolution toward lower amphibolite facies conditions associated to a partial heating. During this last stage the multi stadial atollization process took place, retrogradely evolving toward a green-schist facies conditions, constrained through the compositions of the pseodomorphyc mineral aggregates gradually substituting previously formed garnets.

Retrogressed lawsonite blueschists from the NW Iberian Massif: P–T–t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

Blueschist facies terranes in the Variscan Ibero-Armorican Arc are restricted to scarce and relatively small areas. One of these examples is the Ceán Unit, which is the westernmost exposure of the middle allochthonous sheet of the Variscan belt in the Malpica–Tui Complex (NW Iberian Massif). The Ceán Unit is a highly condensed metamorphic succession with a lower part in the blueschist facies and an upper part without HP relicts. It comprises variable proportions of glaucophane–chloritoid-bearing metapelites and mafic rocks with abundant well-preserved pseudomorphs after euhedral lawsonite. Both lithologies show systematic changes in texture and mineral composition that are spatially related depending on deformation. The metamorphic evolution of the metabasic rocks has been constrained in the P–T space through pseudosection approach and is characterised by H2O-undersaturated prograde evolution induced by the crystallisation of lawsonite. Peak conditions in the blueschist/LT-eclogite facies have been constrained at ca. 2.2 GPa and 560 °C. Exhumation-related metamorphism is characterised by a nearly isothermal decompression from the lawsonite-bearing fields to fields with stable albite at P ≈ 1 GPa. This lead to the pseudomorphism of lawsonite in the early-decompression stages, and a subsequent amphibolite–greenschist facies overprint at P < 0.8 GPa and T ≈ 440–480 °C. The preservation of the lawsonite crystal shape despite complete retrogression indicates that pseudomorphism occurred as a static process and that particular levels of the blueschist host rock were not affected by penetrative deformation during exhumation. 40Ar/39Ar step heating of phengitic muscovite from the pelitic schists interbedded with the lawsonite pseudomorph-bearing metabasic rocks yield plateau ages of ca. 363 ± 2 and 354 ± 1 Ma. The older age is interpreted as the age of the peak blueschist facies metamorphism. The age of 355 Ma is interpreted as a cooling age and is inferred to represent some point relatively close to peak conditions at the onset of the isothermal decompression. 40Ar/39Ar dating of muscovite from the quartzo-feldspathic mylonites of the Bembibre–Ceán detachment, at the base of the Ceán Unit, yields an age of ca. 337 ± 3 Ma, interpreted as the age of the post-nappe extensional tectonics. Similar data obtained from the blueschists of Ile de Groix (Armorican Massif; Bosse et al. in Chem Geol 220:21–45, 2005) support the equivalence of the Ceán Unit and the Upper Unit of Ile de Groix along the Ibero-Armorican Arc and suggest that these units share a blueschist facies event constrained at ca. 360–370 Ma, that is inferred to represent the Late Devonian–Early Carboniferous subduction of the northern margin of Gonwana beneath Laurussia.

Osumilite–melt interactions in ultrahigh temperature granulites: phase equilibria modelling and implications for the P–T–t evolution of the Eastern Ghats Province, India

AbstractThe exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine‐grained mineral intergrowths comprising cordierite + K‐feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close‐to‐isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post‐peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter‐clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar−1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step‐wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post‐peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close‐to‐isobaric cooling.

Contrasting exhumation P–T paths followed by high-P rocks in the northern Caribbean subduction–accretionary complex: Insights from the structural geology, microtextures and equilibrium assemblage diagrams

The Río San Juan metamorphic complex exposes a segment of a high-pressure subduction–accretionary complex built during convergence between the Caribbean island arc and the North America continental margin. It is composed of accreted arc- and oceanic-derived metaigneous rocks, serpentinized peridotites and minor metasediments forming a structural pile. Combined structural geology, microtextural relations, multi-equilibrium calculations and thermodynamical modelling, together with published isotopic ages, allow reconstructing the metamorphic P–T–t paths of each nappe/unit and their links to the structural evolution. In all units of the complex, three major stages (M1 to M3) in the tectonothermal evolution have been distinguished. The M1 stage corresponds to the prograde evolution towards the pressure-peak of metamorphism under blueschist or eclogitic-facies conditions. The M2 stage is related to the main retrogressive event and is characterized by the S2–L2 fabric development in all lithologies and at all scales. The M3 stage represents continuous exhumation from ductile to ductile–brittle deformation regimes. However, the shape of the retrograde P–T path, the age of the exhumation-related D2 structures and the tectonic significance of D2 deformation are different in each structural unit.In the upper structural levels of the Jagua Clara serpentinitic-matrix mélange, the counter-clockwise P–T path of the eclogite blocks is typical of rocks exhumed in the early stages of intra-oceanic subduction zones. The clockwise P–T path obtained for the lower Cuaba unit is characterized by a strong isothermal decompression from the garnet-epidote amphibolite and eclogite-facies pressure-peak. This P–T evolution can be explained by rapid exhumation caused by extensional tectonics, in relation to a major modification of convergence conditions across the subduction zone. The P–T path also explains local syn-M2 partial melting processes, because it crosses the wet solidus for IAT mafic compositions. The P–T path obtained for the high-P Guineal Schists, with exhumation trajectory following the burial trajectory, can be related to exhumation during active subduction. This exhumation was most likely driven by a combination of underthrusting of tectonic units and erosion processes.Available geochronological data and T–t/P–t estimates reveal a Late Campanian to Maastrichtian retrograde M2 metamorphism in the lower structural units of the complex during a consistent D2 top-to-the-NE/ENE tectonic transport. A similar tectonic transport has also been recognized in the metasedimentary nappes of the Samaná complex during Eocene to earliest Miocene. These relations indicate a northeastward progradation of deformation during the successive tectonic incorporation of arc, oceanic and continental-derived terrains to the developing Caribbean subduction–accretionary complex.

Blueschist‐facies metapelites from the Malpica–Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high‐pressure assemblages

AbstractThe Malpica–Tui Unit (Galicia, NW Spain) records eclogite‐ and blueschist‐facies metamorphism during the onset of the Variscan orogeny in Europe. Petrological analysis involving pseudosections calculated using thermocalc shows that the Upper Sheet of this unit, the Ceán Schists, recorded a three‐stage metamorphic evolution involving (i) Early subduction‐related medium‐pressure/low‐temperature metamorphism (M1) constrained at ∼350–380 °C, 12–14 kbar, which is only recorded in the basal part (lower metapelites, LM) of the Ceán Schists. (ii) Subduction‐related blueschist facies prograde metamorphism (M2) going from ∼19 kbar, 420 °C to 21 kbar, 460 °C in the LM, and from 16 kbar 430 °C to 21–22 kbar, 520 °C in the structurally upper metapelites (UM). (iii) Exhumation‐related metamorphism (M3) is characterized by a decompression to 8–10 kbar, 470–490 °C in the LM. This decompression is also recorded in the UM, but it was not possible to estimate precise P–T conditions. The calculations indicate that (i) the prograde evolution in subduction zones may occur in fluid‐undersaturated conditions due to the crystallization of lawsonite, even in metapelitic rocks. This significantly influences phase equilibria and hence the P–T estimates. (ii) The proportion of ferric iron also has a strong influence on phase equilibria, even in metapelites. However, the analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. The use of P–T–X(Fe2O3) pseudosections together with petrographic observations is then necessary to estimate the real oxidation state of the rocks and correctly evaluate the P–T conditions.

Retrograde alterations of phyllosilicates in low-grade metapelite: a case study from the Szendrő Paleozoic, NE-Hungary

Frequently, at temperatures lower than the metamorphic peak conditions, slates undergo mineral transformations, usually mediated by fluids. We have studied core material of an epizonal slate series (Szendrő Mountains, NE-Hungary) to reconstruct the post-metamorphic lower-T hydrothermal alterations using petrographic, X-ray diffraction, electron microprobe methods, and transmission electron microscopy. The borehole crosscuts an upper part of the ca. 600–800 m Lower-Carboniferous flysch-like Szendrő Phyllite Formation. The samples were metamorphosed reaching epizonal conditions with a mineral assemblage characterized by quartz, muscovite, chlorite and albite. Even in the freshest samples, break-up and loosening of the regional metamorphic structure was observed, with cracks parallel to or crossing the cleavage. In the upper part, chlorite and albite are almost absent, while the presence of paragonite, mixed Na–K mica, and mixed-layered minerals with smectitic component are characteristic. Goethite, halloysite, and subordinate kaolinite are present in the most altered sample (13.0 m in the profile) which may indicate the position of the fissures in which the circulation of the post-metamorphic fluids was the most intense. Muscovite is the only mica from the lower part of the profile and chlorite becomes a significant constituent, whereas paragonite, halloysite, and kaolinite are missing. Discrete smectite is present in all the samples in spite of being incompatible with the prograde evolution of the sequence. The interleaved smectite layers in chlorite and muscovite/chlorite mixed-layers show at the lattice level textural characteristics indicative of a later alteration process. After the metamorphic peak at epizonal conditions, the introduction of hot fluids through the fractures gave rise to the crystallization of Na–K white micas and muscovite/chlorite under anchizonal conditions. In a final stage of the hydrothermal process, the cooling of the circulating fluids favored the formation of halloysite and kaolinite in the areas near to the fissures, smectites in the samples far away from the fractures, and locally, goethite. The Fe2O3 content increasing upwards indicates oxidizing conditions in the late stage of hydrothermal activity and/or eventually, an influence of a younger near-surface weathering.

The juxtaposition of eclogite and mid‐crustal rocks in the Orlica–Śnieżnik Dome, Bohemian Massif

AbstractEclogite, felsic orthogneiss and garnet–staurolite metapelite occur in a 5 km long profile in the area of Międzygórze in the Orlica–Śnieżnik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high‐pressure and medium‐pressure rocks. The structural succession in all lithologies shows an early shallow‐dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallow‐dipping axial‐plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet‐rich (grs = 22–29 mol.%) and omphacite‐rich (jd = 33–36 mol.%) layers with oriented muscovite (Si = 3.26–3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of ∼19–22 kbar and ∼700–750 °C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from ∼9 kbar and ∼730 °C to 5–6 kbar and 600–650 °C. The S3 fabric contains in addition domains with albite, chlorite, K‐feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from ∼5 kbar and ∼520 °C to ∼7 kbar and ∼610 °C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to ∼7.5 and ∼630 °C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550 °C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P–T paths during the development of the subvertical fabric reflects their exhumation together.

Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone, Guatemala: insights from phase equilibria and Raman spectroscopy

AbstractLawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a progradeP–Tpath that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐Tof ∼520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ∼10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived progradeP–Tpath associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐Tsubduction zone.

In situ monazite (U–Th)–Pb ages from the Southern Brasília Belt, Brazil: constraints on the high‐temperature retrograde evolution of HP granulites

AbstractIn the southern sector of the Southern Brasília Belt, late Neoproterozoic arc–passive margin collision resulted in juxtaposition of an arc‐derived nappe (the Socorro–Guaxupé Nappe) over a stack of passive margin‐derived nappes (the Andrelândia Nappe Complex) that lies on top of autochthonous basement of the São Francisco Craton. (U–Th)–Pb monazite ages are reported from the high‐grade nappes of the Andrelândia Nappe Complex to better constrain the high‐temperature retrograde evolution. For residual HP granulites from the uppermost Três Pontas–Varginha Nappe, (U–Th)–Pb ages of c. 662 and 655 Ma from low yttrium monazite inclusions in the rims of, or associated with garnet are interpreted to date the late‐stage close‐to‐peak prograde evolution, whereas an age of c. 648 Ma from a similar low yttrium monazite inclusion is interpreted to record post‐peak recrystallization with melt via factures in garnet. For the same nappe, ages of 640–631 Ma retrieved from higher yttrium areas or cores in monazite grains that occur both as inclusions in garnet and in the matrix are interpreted to record growth of monazite either by local breakdown of garnet (±older monazite) and mass exchange with a matrix melt reservoir along cracks or growth from residual melt in the matrix as it crystallized during high‐pressure, close‐to‐isobaric cooling close to the solidus, the temperature of which, at a given pressure, varies with bulk composition of the residual granulites. (U–Th)–Pb ages in the range 620–588 Ma from lower yttrium areas in these monazite grains and from matrix‐hosted patchy monazite are interpreted to date exhumation, as recorded by close‐to‐isothermal decompression and subsequent close‐to‐isobaric cooling. Older monazite ages in this group are interpreted to record late‐stage interaction with melt close to the solidus whereas younger monazite ages are interpreted to record recrystallization of monazite by dissolution–reprecipitation owing to ingress of alkali fluid from the Carmo da Cachoeira Nappe beneath as fluid was released by crystallization of in‐source melt at the solidus. In the underlying Carmo da Cachoeira Nappe, higher yttrium areas in monazite and one single domain monazite yield chemical ages of 619–616 Ma, which are interpreted to date growth as in‐source melt crystallized close to the solidus along the high‐pressure, close‐to‐isobaric segment of the retrograde P–T evolution. Younger (U–Th)–Pb ages of 600–595 Ma retrieved from lower yttrium areas and one single domain monazite are interpreted to record recrystallization of monazite by dissolution–reprecipitation owing to release of fluid at the solidus during exhumation of this nappe. Monazite from the Carvalhos Klippe, interpreted to be correlative with the uppermost nappe, yields a wide range of (U–Th)–Pb ages: for two zoned grains, c. 619 and c. 614 Ma from higher yttrium cores, and c. 583 and c. 595 Ma from lower yttrium rims; and, 592–580 Ma from single domain grains in one sample, and ages of c. 593 and c. 563 Ma from monazite in a second sample. Ages younger than 605 Ma are interpreted to date a fluid‐induced response to the early stages of orogenic loading associated with terrane accretion in the Ribeira Belt to the southeast. The results reported here demonstrate that ages retrieved from monazite that grew close to the solidus in residual granulites from a single tectonic unit will vary from sample to sample according to differences in the solidus temperatures. Further, we show that monazite inclusions may yield ages that are younger than the host mineral and confirm the propensity of monazite to record evidence of tectonic events that are not always registered by other high‐temperature mineral chronometers.

Tectono-metamorphic history recorded in garnet porphyroblasts: insights from thermodynamic modelling and electron backscatter diffraction analysis of inclusion trails

In a Barrovian metamorphic sequence, garnetiferous mica schists document a heterogeneously developed superposition of sub-orthogonal fabrics and multiple garnet growth episodes. In the variably deformed domains, four types of garnet porphyroblasts have been defined based on inclusion trail patterns. Modelled garnet zoning in the MnNCKFMASHTO system indicates a prograde evolution from 4-4.5 kbar and 490-510 � C to 5-6 kbar and 520-550 � C in the earliest subhorizontal fabric progressing towards 6.5-7.5 kbar and 560-590 � C in the subsequent subvertical foliation. This fabric is heterogeneously deformed into a shallow-dipping retrograde foliation associated with garnet resorption. In situ electron backscatter diffraction measurements of ilmenite inclusions in individual garnet grains yield precise data on included planar and linear elements. Consistent orientations of internal foliations, lineations and foliation intersection axis sets indicate a superposition of three sub-orthogonal foliation systems. Weak variations of internal records with increasing intensity of deformation suggest that a moderate buckling stage occurred, but apparent lack of porphyroblast rotation is interpreted as a result of dominant passive flow. Coupling the orientation of internal fabric sets with P-T estimates is used to complement the tectono-metamorphic evolution of the thickened crust. We demonstrate that garnet porphyroblasts preserve features which reflect large-scale tectonic processes in orogens.