Retrograde Hydration Research Articles

The Fate of Pyroxenes in Mafic Xenoliths from the Kinnaur Kailash Granite, Sutlej Valley, NW Himalaya: Effect of Retrograde Hydration and Insights on the Rare Occurrence of High-Grade Metamorphic Rocks in the Himalayan Orogen

Abstract Pseudosection modelling of post-peak mineral assemblages in metamorphosed mafic xenoliths from the Kinnaur Kailash Granite, Northwest Himalaya, has provided new insights into pre-Himalayan metamorphism and constrained the P–T conditions of hydration. P–T and P–MH2O pseudosections constructed in the NCFMASH system show that retrograde hydration occurred during destabilisation of orthopyroxene and clinopyroxene in mafic granulites after decompression to 0.3 GPa pressure, and as a result, water-containing phases such as cummingtonite and hornblende formed. Results from two of the studied xenoliths indicate that the secondary mineral assemblage is controlled not only by P and T, but also by the amount of water available for hydration. In contrast, another example indicates the development of cummingtonite from hornblende via dehydration reaction after decompression to conditions appropriate for cummingtonite stability. The study shows that the Higher Himalayan Crystalline Sequence (HHCS), from which the xenoliths have been derived, must have been subjected to pre-Himalayan high-grade metamorphism, although it is not evident because of pervasive overprinting by Himalayan metamorphism. The present-day metamorphic pattern of the HHCS is expectedly the result of both pre-Himalayan and Himalayan metamorphism, and careful petrological investigation is required to distinguish their signatures so that a better understanding of the tectonothermal evolution of the Himalayas can come into picture. Isotopic dating of the xenoliths is required to establish the timings of the pre-Himalayan metamorphism and decompression.

Differential Fluid Activity in a Single Exhumed Continental Subduction Unit from Local P-T-M(H2O) Records of Zoned Amphiboles (North Muya, Eastern Siberia)

The behavior of the continental lithosphere in the Alpine-type subduction zones, which primarily depends on its thickness, thermal regime of subduction and availability of fluids/melts, remains an important issue for both metamorphic petrology and geochemistry as well as for resolving the thermomechanical properties of subduction paleo-interfaces. Rehydrated (amphibole- and zoisite-bearing) eclogites from the Neoproterozoic North Muya high-pressure complex (northern Central Asian Orogenic belt, eastern Siberia) were studied in order to assess their peak burial depths, degree of prograde dehydration, and further retrograde hydration extent within a subducted and exhumed continental unit. Three medium-grained eclogites from different localities of HP complex show similarly dry peak assemblages of pyrope-almandine-grossular garnet (XGrs up to 0.30, XPrp up to 0.25) + Na-rich omphacite (up to 44 mol % of jadeite) + rutile + quartz, which are variably replaced by secondary plagioclase + clinopyroxene ± amphibole symplectitic aggregate. The eclogites were subjected to burial at similar peak depths (up to ~17–21 kbar) but different peak temperatures (within ~600–730 °C) with or without notable heating and re-equilibration due to crustal thickening. Variable degrees of exhumation-induced pervasive rehydration led to growth of individual zoned porphyroblastic barroisite-hornblende amphibole ([B]Na = 0.03–0.45) ± zoisite over the primary eclogitic assemblage or after notable thermally-driven development of symplectitic aggregate after omphacite. Amphibole compositions together with the zoisite presence/absence in different samples reflect continuous rehydration by addition of ~0.5–1.5 wt.% at different exhumation conditions, from nearly peak eclogitic P–T (~17–21 kbar) to granulite- and amphibolite-facies depths within the plagioclase stability field (<14 kbar). This diversity most likely required irregular distribution of internally sourced, low-volume, hydrous metamorphic fluid (i.e., from host felsic rocks or metasediments) acting at different depths of the subduction interface. From the performed PTX calculations, I suggest that nearly isochemical (i.e., without any significant modification of the bulk-rock composition other than incorporation of additional H2O), retrograde hydration by only at lower- to middle-crust conditions did not significantly influence the density and the rheology of the subducted continental slices due to both (1) a limited abundance of dense metabasic rocks, which are commonly more fluid-rich (e.g., due to chlorite or amphibole alteration), and (2) the initially dry nature of mafic and felsic continental rocks. The limited dehydration and rehydration scales exemplified by the North Muya eclogites and therefore low availability of hydrous metamorphic fluids may have accounted for the high buoyancy of the eclogitic crust and explained the absence of contemporaneous suprasubduction magmatism in the regional context at ca. ~630 Ma.

Lower crustal resistivity signature of an orogenic gold system

Orogenic gold deposits provide a significant source of the world’s gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade faces, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world’s foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with > 1 t production and a < 20 Ω m low-resistivity region at crustal depths > 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS2) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth, releasing HS− ligands for Au, and CO2. We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic-carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world’s richest orogenic gold provinces.

Using Silica Activity to Model Redox-dependent Fluid Compositions in Serpentinites from 100 to 700 °C and from 1 to 20 kbar

AbstractSerpentinization is a metamorphic process that can stabilize highly reduced hydrogen-rich fluids. Previous measurements of elevated CH4 and H2 concentrations in ultramafic-hosted submarine springs indicate that active serpentinization occurs along mid-ocean ridge systems at seafloor pressures (∼&amp;lt;500 bar) and temperatures (∼&amp;lt;350 °C). Serpentinites also exist at higher pressures in subduction zones; for example, during retrograde hydration of the forearc mantle wedge and during prograde deserpentinization within the subducted slab. However, many studies demonstrating the thermodynamic stability of reduced serpentinite fluids have been limited to terrestrial seafloor conditions. To investigate the redox state of serpentinite fluids at elevated pressures, phase equilibria and fluid compositions were computed for 100–700 °C and 1–20 kbar using aqueous silica activity (aSiO2(aq)) as a governing parameter. Silica-sensitive, redox-buffering assemblages were selected to be consistent with previously proposed reactions: SiO2(aq)–fayalite–magnetite (QFM), SiO2(aq)–Fe-brucite–cronstedtite, SiO2(aq)–Fe-brucite–Fe3+-serpentine, plus the silica-free buffer Fe-brucite–magnetite. Fluid species are limited to simple, zerovalent compounds. For silica-bearing redox reactions, aSiO2(aq) is buffered by coexisting ultramafic mineral assemblages in the system MgO–SiO2–H2O. Silica activity and fO2 are directly correlated, with the most reduced fluids stabilized by the least siliceous assemblages. Silica activity and fO2 increase with pressure, but are more strongly dependent on temperature, leading to greater silica enrichment and more oxidized conditions along shallow, warm subduction paths than along steeper, colder paths. Reduced fluids with mCH4/mCO2 &amp;gt; 1 and fO2 below QFM are present only when serpentine is stable, and are favored along all subduction trajectories except shallow P–T paths at eclogite-grade. Values of mH2 and mCO/mCO2 depend strongly on P and T, but also on the choice of redox buffer, especially whether the Fe-serpentine component is cronstedtite or Fe3+-serpentine. Methane and H2S production are thermodynamically favored throughout the P–T range of the serpentinized forearc mantle and in other settings with similar conditions; for example, deep planetary seafloors. The model offers a generalized technique for estimating the redox state of a fluid-saturated serpentinite at elevated P and T, and yields results consistent with previous petrographic and thermodynamic analyses. High-pressure serpentinization may be an important source of reduced species that could influence prebiotic chemistry, support microbial life in the deep biosphere or in deep planetary oceans, or promote greenhouse warming on early Earth.

Hypozonal orogenic gold mineralization in the Giyani Goldfield, Northern Kaapvaal Craton/Limpopo Complex

Abstract The paper reviews published and unpublished geological data pertaining to the structural and metamorphic controls, rock types, characteristic features, source, and timing of hypozonal orogenic gold mineralization in the Giyani Goldfield. The Giyani Goldfield includes the NW domain of the &amp;gt;3.0 Ga Giyani greenstone belt (GGB) at the northern edge of the Kaapvaal Craton and the southern retrograde hydrated domain of the juxtaposed Southern Marginal Zone (SMZ) of the ca. 2.72 Ga Limpopo Complex (LC). Mineralization at all gold mines and gold prospects of the Giyani Goldfield is structurally controlled and closely associated with the Hout River shear zone (HRSZ) and associated smaller shear zones suggesting a specific tectonic setting. This tectonic setting is the direct consequence of thrusting the SMZ of the LC against and over the adjacent GGB at the position of the steeply north-dipping (south verging) HRSZ during the exhumation stage of the ca. 2.72 to 2.69 Ga Limpopo orogeny followed by regional retrograde hydration of the southern part of the SMZ at ca. 2.68 to 2.62 Ga. This tectonic setting offers an explanation for a deep-seated crustal source for gold and for the concentration of orogenic gold mineralization within specific structural features located within the Giyani Goldfield. This tectonic setting also explains the lithological, structural and metamorphic complexity, metasomatic alteration and post-peak metamorphic timing of gold mineralization. Finally, it provides important clues with regards to a crustal source for gold mineralizing fluids and the identification of new potential targets for gold exploration in the Giyani Goldfield.

Geochronology and phase equilibria modelling of ultra‐high temperature sapphirine + quartz‐bearing granulite at Usilampatti, Madurai Block, Southern India

Sapphirine‐bearing granulite from Usilampatti in the Madurai block of southern India preserves a variety of mineral textures and reactions that help in reconstructing a three‐stage metamorphic evolution. Corroded biotite, sillimanite and quartz inclusions within garnet represent relics from the prograde history. Peak metamorphic conditions were attained with the development of sapphirine + quartz in textural equilibrium (Stage 1). This was followed by nearly isothermal decompression, leading to the formation of sapphirine + cordierite at Stage 2. Subsequent retrograde hydration (Stage 3) is only locally evident. Using the Perple_X software and the model system NCKFMASH, the peak P‐T conditions were estimated from core compositions, and the retrograde evolution was deduced from rim or symplectite compositions of different minerals as computed by isopleths of XMg garnet, XCa garnet, XMg orthopyroxene, XMg sapphirine and XMg biotite. The P‐T conditions for Stage 1 thus obtained, and supported by thermodynamic modelling using the winTWQ programme, is approximately 9 kbar and 940°C. Stage 2 conditions were constrained as 6.7 kbar and 900°C. Dating of zircon and monazite in the sapphirine‐bearing granulite and associated gneisses by the U‐Pb method using LA‐ICP‐MS indicates metamorphic overprint of zircon (lower intercept ages of discordant data arrays) at 546 ± 8 and 547 ± 11 Ma and metamorphic growth of monazite between 542 ± 3 and 551 ± 2 Ma. Upper intercept ages for zircon point to zircon growth at approximately 2514 ± 66 Ma. Although it remains unclear whether the metamorphic age data refer to Stage 1 or Stage 2 or, most likely, a continuum between both, they clearly document a late Ediacaran age for ultra‐high temperature (UHT) metamorphism in the area, which, based on the obtained P‐T path, was most likely the result of crustal thickening followed by uplift and erosion. Thus, it is concluded that the sapphirine‐bearing granulites formed in response to Pan‐African orogeny that led to the collision of the western and eastern Madurai domains, whereas initial zircon growth probably took place during late Neoarchaean arc magmatism that formed much of the western domain. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Contrasting fracture patterns induced by volume-increasing and -decreasing reactions: Implications for the progress of metamorphic reactions

Hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock, and some of these reactions proceed with fracturing. For this paper we have developed a model for coupled hydraulic–chemical–mechanical processes, using the distinct element method, in order to understand the relationships among chemical reactions, fluid flow, and fracturing during metamorphism. The model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure. With the help of the model, volume-decreasing dehydration reactions and volume-increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions, respectively. Both types of reaction proceed inwards from drained boundaries or fractures, but they show contrasting fracture patterns. A volume-decreasing dehydration reaction produces tree-type fractures, with the new fractures generated as branches of a pre-existing fracture. A volume-increasing hydration reaction produces a polygonal network of fractures, where new fractures nucleate at sites far from the pre-existing fracture, and extend to form T-junctions. These contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient. In the case of the tree-type fractures, the fractures are continuously generated at the reaction front, and the reaction proceeds smoothly by positive feedbacks between reaction, fracturing, and fluid flow. In contrast, in a volume-increasing hydration reaction, fracturing initially occurs in response to the irregular boundary shape, but as the reaction progresses, a compressive stress field is generated, which inhibits further fracture generation. The compressive stress field also prevents fluid flow by closing the pre-existing fractures, which slows down the reaction. These contrasting feedback systems between volume-decreasing dehydration and volume-increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively, and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved.

Rates of Retrograde Metamorphism and their Implications for the Rheology of the Crust: an Experimental Study

We have carried out experimental studies of the rate at which water is consumed by hydration reactions under mid-crustal conditions. Both pelitic and mafic assemblages are susceptible to extensive hydration in the laboratory on a time scale of weeks to months. Quantitative hydration rate determinations were made using enstatite–oligoclase ± diopside powder mixtures and a natural hypersthene hornfels. Under all conditions, the main hydration product was saponite clay with variable amounts of talc according to the initial proportion of enstatite to plagioclase. The experiments yield consistent rates for water consumption of around 10−8 g H2O per m2 of mineral surface per second at 400°C and 300 MPa (3 kbar). Additional experiments were run at 300°C and 500°C and at lower pressures (40 MPa), as well as with NaCl; rates appear to be faster at higher temperatures and in the presence of salt, but slower at low pressure. Comparison of powder and core experiments on the natural hornfels indicates that it is primarily the outer surface of the rock core that is available for hydration, with only minor infiltration along grain boundaries. The hydration rates reported here appear to be typical for the types of lithology that demonstrate moderate to high degrees of retrogression along joints and deformation zones in crystalline rocks of the upper crust. Assuming that the surface roughness and damage effects in a natural fault zone are comparable with those of the materials used here in the experiments, the measured hydration rates imply that a natural fracture in crystalline rocks of the middle crust that becomes filled with a water film 0·2 mm in thickness will dry out through incorporation of the water into hydrous phases on a time scale of the order of 10–100 years. This clearly implies that free water has only a short residence time in crystalline rocks of the middle crust or deeper, provided they have cooled below their original temperature of formation and therefore have the potential to undergo retrograde hydration. We infer that the strength of retrograde shear zones in the middle to lower crust will fluctuate through time, with episodes of water infiltration resulting in short periods of water weakening before the water is fully consumed and the rocks become stronger once more.

Vein graphite deposits: geological settings, origin, and economic significance

Graphite deposits result from the metamorphism of sedimentary rocks rich in carbonaceous matter or from precipitation from carbon-bearing fluids (or melts). The latter process forms vein deposits which are structurally controlled and usually occur in granulites or igneous rocks. The origin of carbon, the mechanisms of transport, and the factors controlling graphite deposition are discussed in relation to their geological settings. Carbon in granulite-hosted graphite veins derives from sublithospheric sources or from decarbonation reactions of carbonate-bearing lithologies, and it is transported mainly in CO2-rich fluids from which it can precipitate. Graphite precipitation can occur by cooling, water removal by retrograde hydration reactions, or reduction when the CO2-rich fluid passes through relatively low-fO2 rocks. In igneous settings, carbon is derived from assimilation of crustal materials rich in organic matter, which causes immiscibility and the formation of carbon-rich fluids or melts. Carbon in these igneous-hosted deposits is transported as CO2 and/or CH4 and eventually precipitates as graphite by cooling and/or by hydration reactions affecting the host rock. Independently of the geological setting, vein graphite is characterized by its high purity and crystallinity, which are required for applications in advanced technologies. In addition, recent discovery of highly crystalline graphite precipitation from carbon-bearing fluids at moderate temperatures in vein deposits might provide an alternative method for the manufacture of synthetic graphite suitable for these new applications.

Contrasting metamorphic records and their implications for tectonic process in the central Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

Abstract Metamorphic rocks in the central part of Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, are classified into three types based on petrological characteristics. (i) The Austkampane area preserves c. 800 °C and 0.5–0.6 GPa peak metamorphic conditions followed by decompression and subsequent isobaric cooling and later hydration (A-type). (ii) The Brattnipene and eastern Menipa area preserve peak P–T conditions of c. 800 °C and 0.7–0.8 GPa with subsequent isobaric cooling and later hydration (B-type). (iii) The area including Lunckeryggen, southern Walnumfjella and western Menipa preserves an amphibolite–facies peak metamorphic condition with signatures of prograde metamorphism (L-type), which are typically unaffected by the retrograde hydration event. Peak granulite–facies metamorphism of A- and B-type rocks are contemporaneous at c. 640–600 Ma, but a difference in the P–T paths between these rocks can be explained by thrusting of the A-type rock unit onto the B-type rock unit. By contrast, the timing of the metamorphism of the L-type rocks is significantly younger at c. 550 Ma, possibly related to the intrusion of pegmatites and granitoids. These metamorphic records in the central part of the Sør Rondane Mountains can be a test ground for the regional tectonic processes proposed for the orogeny related to Gondwana formation. Supplementary material: Representative mineral compositions are listed at www.geolsoc.org.uk/SUP18623

Garnet-bearing ultramafic rocks from the Dominican Republic: Fossil mantle plume fragments in an ultra high pressure oceanic complex?

Ultra high pressure (UHP) garnet-bearing ultramafic rocks from the Dominican Republic may represent the only known example where such rocks were exhumed at an ocean–ocean convergent plate boundary, and where the protolith crystallized from a UHP magma (>3.2GPa, >1500°C). This study focuses on the petrology and geochemistry of one of the ultramafic lithologies, the pegmatitic garnet-clinopyroxenite (garnet+clinopyroxene+spinel+corundum+hornblende). Three distinct types of garnet were recognized: Type-1 garnet (low Ca, high Mg) is interpreted as near magmatic (P>3.2GPa, >1500°C). Type-1′ garnet (high Ca, low Mg) is interpreted as having formed approximately isochemically from magmatic high-Al clinopyroxene. Type-2 garnet (intermediate Ca, high Mg, and low Fe+Mn) formed together with hornblende as a result of late, low-pressure retrograde hydration. Clinopyroxene is close to diopside–hedenbergite (Mg# ~88) and metasomatized by arc-related fluids. Spinel and corundum occur as microinclusions in type-1 and type-1′ garnets in the only reported natural occurrence of coexisting garnet+spinel+corundum, indicative of very high pressure. Chondrite-normalized REEs (rare earth elements) of the garnets show humped or weakly sinusoidal patterns, typically associated with garnet inclusions in diamond and garnet in kimberlite that crystallized at UHP conditions. These humped to weakly sinusoidal REE patterns developed as the result of interaction with a light REE-enriched metasomatic fluid. Partitioning of REEs between type-1′ and type-1 garnets is consistent with the former having inherited its REEs from a high-Al clinopyroxene predecessor. The partitioning preserves a record of near-solidus temperatures (~1475°C). Petrology and phase relationships independently suggest near-solidus conditions >1500°C (the highest temperature conditions reported in a UHP orogenic setting), providing evidence for an origin in a mantle plume. Therefore, the Dominican ultramafic rocks may represent the only example of exhumed “fossil fragments” of mantle plume in an orogenic setting (oceanic or continental).

Modeling metamorphism in collisional orogens intruded by magmas: II. Fluid flow and implications for Barrovian and Buchan metamorphism, Scotland

This paper presents the results of two-dimensional numerical modeling of crustal fluid flow in a collisional overthrust setting during emplacement of syn-metamorphic mafic sills. An important feature of the fluid flow in the presence of magmatic intrusions is the long-lived prograde up-temperature (up-T) flow of fluid. This upward flow is driven by dehydration reactions in the rocks underlying the intrusions and upward is the consequence of the inverted thermal profile that appears within the depth range of magmatism. If magma is intruded at mid-crustal depths following crustal collision, the up-T flow regime may persist for as much as 5 Myr. Such long-lived up-T fluid flow is not observed in models without magmatism. Strong retrograde reactions in rocks surrounding intrusions may give rise to downward up-temperature flow into the heated but rapidly cooling areas of the crust during exhumation, but the fluid fluxes in these areas are typically small. The total time-integrated fluid fluxes produced by magmatism and crustal overthickening in the middle and deep crust are substantial, in some cases reaching 3,000 m3 m−2 over 35 Myr. Early magmatic events may give rise to fluid fluxes exceeding 1,000 m3 m−2 in only 1 Myr. The strong dehydration that accompanies the emplacement of hot magma into the relatively low-temperature wall rock leads to hydrofracturing of large volumes of crustal rocks surrounding intrusions. The fractured rock region may extend over tens of kilometers from the zone of magma emplacement. More hydrofracturing is predicted for the middle and deep crust than for the shallow crust because of the exponential decrease of crustal permeability with depth that is assumed in our models. Endothermic dehydration, exothermic retrograde hydration, and the latent heat of wall rock fusion can all influence thermal budgets. In the final part of the paper we integrate the results of the fluid flow modeling presented here, the reconstructions of thermal evolution presented in Part I, and field-based data for the type locality of Barrovian and Buchan metamorphism in Scotland. Many key phenomena predicted by our numerical models are observed in the Scottish Dalradian, including the very short thermal peaks attained over a relatively narrow time span across a broad range of metamorphic zones; penecontemporaneous Barrovian, Buchan, and granulite facies metamorphism; steep Metamorphic Field Temperature Gradients (MFTGs) in the vicinity of intrusions; and petrological evidence consistent with up-temperature fluid flow. Our results suggest that some controversial aspects of regional metamorphism in the Scottish Dalradian, including heat transfer processes and the steep MFTGs, may be plausibly explained by the effects of plutonism.

Significance of retrograde hydration in collisional metamorphism: A case study of water infiltration in the Kokchetav ultrahigh-pressure metamorphic rocks, northern Kazakhstan

Petrological analysis reveals the volume of aqueous fluids infiltrated into high-pressure–ultrahigh-pressure metamorphic (HP–UHPM) rocks during exhumation of the Kokchetav massif, Kazakhstan. The mode and extent of retrograde hydration in the massif are highly variable depending on lithology. The volume of aqueous fluids infiltrated during retrograde hydration was calculated based on a comparison of the peak and present mineral assemblages obtained by observations and thermodynamic calculations. Eclogite and metapelite absorbed about 1.0 wt.% and 0.2 wt.% of water during retrograde metamorphism, respectively, whereas orthogneiss and leptite showed no change in water content despite nearly complete retrograde recrystallisation. Based on the change in water content and the spatial coverage of each lithology, the volume of aqueous fluids infiltrated during retrograde metamorphism of the HP–UHPM unit was calculated as 0.1% of the total mass of the massif. This amount of fluid is fully covered by the dehydration of the underlying Daulet Suite during contact metamorphism by the emplacement of the HP–UHPM unit at the mid-crustal level. Our results suggest that the fluid infiltration at the exhumation stage is tectonically inevitable, and even a small amount of water infiltration less than 1 wt.% effectively enhances retrograde recrystallisation in permeable lithologies such as pelitic and felsic schists/gneisses.

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromso eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.

Retrograde hydration sequence in disordered Mg amphiboles: A TEM investigation

Acicular Mg amphibole in thermal-metamorphic serpentinites from Elba Island, Italy, is affected by polytypic disorder (with the possible coexistence of Pnmn, Pnma, and C2/m structures), chain-width defects, and retrograde hydration reactions. Based on HR-TEM nanostructures, we documented three main hydration stages: (1) formation of chain-multiplicity faults (CMFs); (2) formation of fibrous topotactic talc, elongated parallel to [001]Tlc and resulting from progressive polymerization of CMFs; and (3) pervasive serpentinization.

Titanium behavior in quartz during retrograde hydration: Occurrence of rutile exsolution and implications for metamorphic processes in the Sør Rondane Mountains, East Antarctica

In the central Sør Rondane Mountains, East Antarctica, orthopyroxene felsic gneiss (OPG) was converted to hornblende-biotite felsic gneiss (HBG) by hydration that accompanied the intrusion of pegmatite. The retrograde HBG contains exsolved rutile in quartz. The composition of orthopyroxene and clinopyroxene in OPG suggests a temperature of 840 °C (interpreted as the near-peak temperature), and the composition of hornblende and plagioclase in HBG suggests a temperature of 670–700 °C (interpreted as the temperature during hydration). Ti concentrations in quartz were measured using an electron probe micro-analyzer, and Ti-in-quartz thermometers were applied. Measured Ti concentrations were 110 ppm (equivalent to 760–820 °C) for homogeneous quartz from OPG and 35 ppm (650–700 °C) for an exsolution-free area of a quartz grain from HBG. The pre-exsolution Ti concentration in quartz from HBG was reconstructed with 100 μm beam diameter and 25 kV of accelerating voltage, giving 103 ppm, similar to the value obtained for homogeneous quartz in OPG. The temperatures obtained using a Ti-in-quartz thermometer are consistent with those estimated using other thermometers. Although analysis of the main constitute minerals in HBG yields the conditions of hydration, the reconstructed pre-exsolution Ti content in quartz within HBG yields the pre-hydration conditions. Thus, the Ti-in-quartz thermometer is a potentially powerful tool with which to identify the peak or near-peak temperature conditions, even for retrogressed metamorphic rocks.

Modeling the Magnitudes and Directions of Regional Metamorphic Fluid Flow in Collisional Orogens

We present two-dimensional numerical models that simulate fluid flow in an orogenic overthrust setting in the absence of magmatism. The modeling is intended to test hypotheses regarding the direction of regional fluid flow in the middle and deep crust, including upward, convective, sub-horizontal, up-temperature, and downtemperature flow regimes. Several geological factors that have received relatively little attention in previous numerical models of regional metamorphic fluid flow are considered: (1) retrograde hydration metamorphic reactions; (2) anisotropy and spatial heterogeneity in crustal permeability structure; (3) temporal evolution of permeability as a result of porosity production and consumption during metamorphic reactions; (4) heat of metamorphic reactions. Our results suggest that the typical fluid flow pattern in overthrust settings is towards the surface, in the direction of decreasing temperature. The flow is mainly driven by metamorphic dehydration reactions; the integrated fluxes in the deep crust below 20 km are of the order of 10 3 m 3 /m 2 over 30 Myr for a background permeability of 10 ^19 m 2 . At higher crustal permeability, the buoyancy flux generally exceeds the metamorphic flux and large-scale fluid circulation becomes possible.Transient upward flow in the up-temperature direction is observed during the first 1^3 Myr of the model evolution within the thrust core that undergoes retrograde hydration; this flow is the result of the initially inverted geotherm in the thickened crustal section. Downward fluid infiltration caused by metamorphic hydration may occur in the flanks of the model orogen, but the integrated fluxes in these areas are only � 10 m 3 /m 2 . Hydration-driven downward flow may also develop in the core of the orogen at late stages of exhumation. Low-permeability layers lead to sub-horizontal (but down-temperature) flow over distances corresponding to the lateral dimensions of such layers. Heterogeneities in permeability structure, as well as porosity^permeability variations produced by metamorphic reactions, result in focusing of fluid flow and locally elevated fluid fluxes (up to 10 4 m 3 /m 2 ). These fluxes are large enough to produce significant chemical and isotopic metasomatism, but are not sufficient to significantly affect the temperature distribution in the model orogen. However, metamorphic hydration and dehydration reactions are found to exert considerable control on the regional thermal structure. We conclude that convective, downward, or uptemperature fluid flow is probably very limited during amagmatic prograde metamorphism of the middle and lower crust, even in the presence of permeability anisotropy, high-permeability channels, or low-permeability barriers.

Eocene Gold Ore Formation at Muteh, Sanandaj-Sirjan Tectonic Zone, Western Iran: A Result of Late-Stage Extension and Exhumation of Metamorphic Basement Rocks within the Zagros Orogen

A multidisciplinary study including field geology, microstructure analysis, 40Ar/39Ar dating, and fluid inclusion microthermometry and Raman spectrometry has been carried out on the Muteh gold deposit located in a greenschist to amphibolite facies metamorphic rock complex of the Sanandaj-Sirjan tectonic zone, Zagros orogen, Iran. The Muteh gold deposit has been previously interpreted as genetically related to Precambrian granitic intrusions, as an exhalative hot-spring deposit related to Paleozoic rhyolitic-acidic tuffs, and to local metamorphic processes. Host rocks of the gold deposit are predominantly schist and gneiss, and subsidiary amphibolite and quartzite, and are intruded by leucogranites. In the vicinity of the gold deposit, the metamorphic and granitic rocks display a subhorizontal mylonitic foliation, containing a northeast-oriented stretching lineation. Field investigations in one of the producing open pits show that the gold orebodies are controlled by northwest-oriented normal faults and joints, dipping to the northeast and the southwest, and crosscutting the ductile fabric of the host rocks. Both the ductile fabric of the host rocks and the gold ore-controlling brittle structures are interpreted to have formed within a single, continuous extensional event, which started with ductile deformation and gradually changed into brittle deformation. Hydrothermal alteration associated with ore formation consists of quartz, muscovite, pyrite, dolomite-ankerite, and albite, which crosscuts the ductile fabric and overprints the metamorphic minerals of the host rocks. Pyrite is the dominant opaque mineral and is the major phase associated with gold. Chalcopyrite, marcasite, bismuth, galena, sphalerite, and pyrrhotite are subsidiary to rare phases. 40Ar/39Ar incremental-heating and in situ laser-ablation age data reveal a coherent sequence of cooling and hydrothermal events in the metamorphic complex hosting the Muteh gold deposit. Muscovite samples from the alteration zone and from one quartz vein from the orebodies yield 40Ar/39Ar ages between 55.7 and 38.5 Ma and show that gold mineralization is the youngest among the different dated events. This Eocene age is consistent with the young structural setting of the gold orebodies revealing an emplacement along northwestoriented normal faults, which can be correlated with Tertiary extensional tectonic events reported by previous field investigations. The data also indicate that gold ore formation is coeval with magmatism in the adjacent Tertiary Urumieh-Dokhtar magmatic belt and with intrusive activity within the Sanandaj-Sirjan zone, with a 40Ar/39Ar age of 54 Ma near Muteh according to our study. The remaining 40Ar/39Ar data reveal a Cretaceous to early Tertiary metamorphic, magmatic, and deformation history of the complex hosting the Muteh gold deposit. The fluid inclusion study showed that distinct fluids were present during the regional geologic evolution predating ore formation and later Eocene gold ore formation. Early regional fluids consist of an H2O-CO2-NaCl fluid with subsidiary N2 and CH4 that underwent local unmixing. The salinities of the liquid-rich inclusions resulting from unmixing range between 2.2 and 17.5 wt percent NaCl equiv, and those of the CO2-rich, vaporrich inclusions range between 2.8 and 7.2 wt percent NaCl equiv. The high salinity of the regional liquid-rich fluids is attributed to metamorphism of evaporite-bearing sedimentary rocks, magmatic intrusions, retrograde hydration reactions, metamorphism of impure marbles, and migmatization. The fluid inclusions spatially associated with the gold veins postdating regional metamorphism and ductile deformation of the host rocks contain a low-salinity aqueous fluid with a dilute, low-density CO2 component, and maximum salinities between 2.6 and 5.8 wt percent NaCl equiv. Gold deposition at Muteh occurred during or after late stages of Eocene brittle extension as a result of exhumation of the host metamorphic complex, possibly during magmatic activity in the Sanandaj-Sirjam zone. It is most likely linked to the introduction of a distinct low-salinity, low CO2 content aqueous fluid during crustal extension. These fluids are younger than, and therefore unrelated to, the highly saline and CO2-rich, regional metamorphic-magmatic fluids. Our results refute previous models advocating Precambrian to Paleozoic ages of the Muteh gold deposit.

Fluid compositions and local fluid buffering during the retrograde metamorphism of Al-bearing dolomitic calc-silicate granulites in the Limpopo Central Zone, southern Africa

Calc-silicate granulites were examined to evaluate the fluid composition and retrograde metamorphic conditions in the Central Zone of the Limpopo Belt, southern Africa. Quartz deficient assemblages are characterized by minerals such as diopside, forsterite, spinel and/or magnesiohornblende and tremolite in the presence of calcite and dolomite. Although the granulites are Al-poor (Al 2O 3 is less than or equal to 1.0 wt.%) and dolomitic in composition, they include Al-bearing phases. Phase analyses for the assemblages in the two model systems CaO–MgO–SiO 2–H 2O–CO 2 and CaO–MgO–SiO 2–Al 2O 3–H 2O–CO 2 provide constraints on fluid compositions in the granulite facies and retrograde metamorphisms in the Limpopo Central Zone. In the presence of amphiboles, isobaric T–X(CO 2) phase relations suggest that high X(CO 2) conditions were established in the calc-silicate rocks of present study. The phase relations with tschermakitic amphiboles at 0.35 GPa restrict diopside-spinel occurrences in the presence of calcite, dolomite and forsterite within very-high X(CO 2) with low a(H 2O). The fluid compositions, X(CO 2), were effectively buffered by the mineral assemblages during granulite facies metamorphism to subsequent decompression and cooling stages. The presence or absence of retrograde magnesiohornblende and tremolite appeared to be controlled not only by infiltration of H 2O-rich fluid during retrograde metamorphism but also Al content in the local bulk rock compositions. The presence of the two-amphibole phases shows that the fluid compositions were locally buffered in the Al-bearing dolomitic granulites. Comparing the calculated X(CO 2) values in the present study area and in the Alldays area, a difference of retrograde hydration effects is observed.

High-temperature crustal scale shear zone at the western margin of the Eastern Ghats granulite belt, India: implications for rapid exhumation

A mylonitic shear zone occurs along the western margin of the Eastern Ghats Granulite belt, India. In the northern sector, it separates metapelitic and mafic granulites from the granite gneisses of the Bastar craton, while in the southern sector, it separates charnockitic gneisses and mafic granulites from the granite gneisses of the Bastar craton. This boundary shear zone is characterized by a mylonitic foliation and a stretching lineation. Microstructures characterize high-grade conditions and quartz c-axis patterns show strong point maxima close to the long axes of the strain ellipsoids, indicating dominant prism [c] slip in quartz, commonly interpreted in terms of ductile deformation at high-temperatures. Quartz c-axis patterns in the recrystallized and unrecrystallized domains also imply grain boundary migration as the principal mechanism of dynamic recrystallization at high temperature conditions. A rapid exhumation during shearing in this marginal part of the convergent Eastern Ghats orogen is indicated by decompression reaction textures and retrograde hydration reactions, presumably due to hydrous fluid ingress, in the shear zone rocks.