Metamorphic Mineral Assemblages Research Articles

Mineralogy and metamorphism conditions of manganese ore at the South Faizulino deposit, the southern Urals, Russia

The mineralogy of slightly metamorphosed manganese ore at the South Faizulino hydrothermalsedimentary deposit in the southern Urals has been studied; 32 minerals were identified. Quartz, hausmannite, rhodochrosite, tephroite, ribbeite, pyroxmangite, and caryopilite are major minerals; calcite, kutnahorite, alleghanyite, spessartine, rhodonite, clinochlore, and parsettensite are second in abundance. This mineralic composition was formed in the process of gradual burial of ore beneath the sequence of Middle Devonian-Lower Carboniferous rocks. The highest parameters of metamorphism are T ≈ 250°C and P ≈ 2.5 kbar. The relationships between minerals and their assemblages made it possible to reconstruct the succession of ore transformation with gradually increasing temperature and pressure. Manganese accumulated in the initial sediments as oxides and a gel-like Mn-Si phase. Rhodochrosite and neotocite were formed at the diagenetic stage. In the course of a further increase in temperature and pressure, neotocite was replaced with caryopilite; ribbeite, tephroite, pyroxmangite, and other silicates crystallized afterwards. In addition to the PT parameters, the formation of various metamorphic mineral assemblages was controlled by the Mn/(Mn + Si) ratio in ore and XCO2 in pore solution. The latter parameter was determined by the occurrence of organic matter in the ore-bearing rocks. Ore veinlets as products of local hydrothermal redistribution of Mn, Si, and CO2 were formed during tectonic deformations in the Middle Carboniferous and Permian.

Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust

Creep strength of the crust depends upon the rheology of the most common mineral, usually quartz. Recrystallized quartz grains in many high-grade shear zones from the middle to lower crust are typically large (millimeter sized), implying active grain boundary migration, but equivalents from old polymetamorphic and water-deficient basement sheared at similar crustal depths can be very small. For the latter, strong crystallographic preferred orientation (CPO) of quartz, with c axes aligned close to Y, progressive misorientation of crystals, subgrain and dislocation development, and core-mantle structures with recrystallized grains of 2‐8 m size, all point to dislocation glide dominantly on the prism a system with recrystallization by subgrain rotation. Recrystallized grain size piezometry of such quartz indicates high flow stress in fine-grained shear zones, while the synkinematic metamorphic mineral assemblage and the CPO are typical of amphibolite facies conditions. This is evidence that middle to lower crust is not inevitably weak due to its high temperature: water content also has an important influence.

Metamorphic evolution of the Maud Belt: P– T– t path for high-grade gneisses in Gjelsvikfjella, Dronning Maud Land, East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P– T– t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M 1) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P– T– t path for the Pan-African orogeny. Peak metamorphic (M 2b) conditions recorded by most rocks in the area ( T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M 2a). The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M 1; 1090–1030 Ma) and again during the Pan-African orogeny (M 2a, M 2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M 2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.

Turonian (Upper Cretaceous) lithostratigraphy and biochronology, southern Gulf Islands, British Columbia, and northern San Juan Islands, Washington State

Clastic strata preserved on Sidney Island, Barnes Island, and adjacent islands of the southernmost Gulf Islands of British Columbia and the northern San Juan Islands of Washington State are assigned to new stratigraphic units: the Sidney Island Formation and the Barnes Island Formation. The Sidney Island Formation consists of basal conglomerate and sandstone that grades upward through planar-stratified sandstone into hummocky cross-stratified sandstone and siltstone, all of which are deposited in relatively shallow-marine environments. The Barnes Island Formation, in contrast, consists of deep-marine conglomerate, sandstone, and mudstone that was deposited in a submarine-fan setting. Mollusk fossils from the Sidney Island Formation are of Early to Middle Turonian age, whereas ammonites and foraminifers from the Barnes Island Formation indicate a Late Turonian age. The Sidney Island Formation thus records initial marine transgression and inundation of basement rocks, followed by basin deepening that is transitional to the deep-marine submarine-fan deposits of the Barnes Island Formation. Sidney Island Formation strata have been considered previously as derived from uplift along the nearby San Juan thrust system in mid-Cretaceous time. However, the shallow-marine strata are internally well organized, and the facies succession is persistent across the formation's outcrop area. In addition, the formation lacks the distinctive detrital metamorphic mineral assemblages that are characteristic of older rocks of the San Juan Islands. These observations suggest that strata of the Sidney Island Formation did not accumulate immediately adjacent to active thrusting but rather in a more distal setting relative to the thrust system.

High‐pressure mineral assemblage in granitic rocks from continental units, Alpine Corsica, France

AbstractThe Popolasca–Francardo area of northern Corsica contains an assemblage of continental tectonic units affected by an Alpine deformation. In one of these units, Unit II, previously regarded as weakly metamorphosed, a metamorphic mineral assemblage characterized by sodic amphibole, phengite, quartz, albite and epidote has been found in an aplite dyke that cuts the dominant granitoids. Peak‐metamorphic temperature and pressure conditions of 300–370°C and 0.50–0.80 GPa, respectively, have been determined. This finding indicates that a continuous belt of continental slices, characterized by high‐pressure, low‐temperature metamorphism of Tertiary age, extends from the Tenda Massif in the north to the Corte area in the south, thus placing additional constraints on the tectonic evolution of Alpine Corsica. Copyright © 2005 John Wiley & Sons, Ltd.

The effect of zoned garnet on metapelite pseudosection topology and calculated metamorphic P-T paths

P-T pseudosections, constructed in MnNCKFMASH and adjusted for chemical compositional changes resulting from zoned garnet growth (chemical fractionation) in a pelitic rock, show negligible changes in the position of the peak metamorphic mineral assemblage field (garnet + biotite + plagioclase + sillimanite + quartz) compared to the position of this field calculated with the bulk-rock composition. Pelitic rock samples with less than 5% modal garnet were modeled using bulk-rock chemical compositions (unfractionated), and compositions adjusted for 1, 2, and 5% garnet growth, in order to model the effects of changes in effective composition on pseudosection topology. Differences in the location of mineral mode zero lines along the garnet growth P-T path and in the peak mineral assemblage field are generally less than 10 °C and/or less than 0.3 kbar. However, at some P-T conditions, significant changes in topology are observed. For example, at pressures above 9 kbar, large temperature shifts in the zoisite mode zero line change the pseudosection topology so that biotite+zoisite stability in the pseudosection with 5% garnet fractionation has a larger temperature range (>120 °C) than in the unfractionated pseudosection (<50 °C). The effects of porphyroblast growth-induced fractionation of bulk-rock chemistry can be determined from mineral chemistry and mineral modes; pseudosections can be constructed with the adjusted chemical compositions to resolve whether fractionation affects the pseudosection topology in the P-T range of interest. In the case of the North Cascades samples discussed here, garnet fractionation is estimated to have minimal effects on P-T paths determined from pseudosections. Therefore, pseudosection modeling based on bulk-rock chemistry can be used to estimate peak metamorphic P-T conditions and constrain parts of metamorphic P-T-t paths once the effects of fractionating minerals are understood.

Formation and chemical evolution of magnesium chloride brines by evaporite dissolution processes—Implications for evaporite geochemistry

The evolution of magnesium chloride brines with high bromide contents via a multistage reaction and dissolution process has been studied in brine seeps of a German potash mine. The observed chemical trends and phase equilibria can be modeled and interpreted in terms of a NaCl solution (cap rock brine) infiltrating into a potash zone characterized by the metamorphic mineral assemblage kieserite + sylvite + halite + anhydrite. Establishment of a persistent, stable equilibrium assemblage and constant fluid composition in the invariant point IP1 of the six component (Na-K-Mg-Ca-Cl-SO 4-H 2O) system of oceanic salts is prevented by the perpetually renewed input of NaCl-brine and by the intermittent exposure of incompatible kieserite. Instead, the solutions develop towards the metastable invariant point IP1(gy), with the mineral assemblage carnallite + polyhalite + sylvite + halite + gypsum, where gypsum takes the place of anhydrite (stage I). The temporary exposure of kieserite and the ensuing formation of polyhalite effectively buffer the solutions along the metastable polyhalite phase boundary during stages II and III. Eventually, in stage IV, polyhalite becomes depleted and admixture of more NaCl brine leads to low sulfate solution compositions, which are now only constrained by carnallite + sylvite + halite, and the once hexary system degenerates to a quaternary one (Na-K-Mg-Cl-H 2O) in point E. Bromide in brines shows equilibrium partitioning with respect to the wall rock minerals. The pattern of evolving brine compositions may serve as a model for similar brine occurrences, which in some cases may have been misinterpreted as remains of fossil, highly concentrated and chemically modified seawater. Similar magnesium chloride brines of salt lakes (e.g., Dead Sea, Dabusun Lake) show subtle differences and are constrained by fewer mineral equilibria (more degrees of freedom), and their low sulfate contents are due to gypsum precipitation, driven by calcium chloride input from dolomitization reactions. Finally, the observed reaction sequence is generalized, and a model for the formation of magnesium sulfate depleted, chloride-type potash salts and bischofite deposits by leaching of sulfate-type evaporites is proposed.

Neoproterozoic–Early Paleozoic gravitational tectonic collapse in the southern part of the Arabian-Nubian Shield: The Bulbul Belt of southern Ethiopia

We present new structural and metamorphic data from the Bulbul Belt in southern Ethiopia indicating that the latest phase of the Neoproterozoic evolution of the southern part of the Arabian-Nubian Shield is characterized by the development of low-angle oblique normal-slip shear zones manifesting regional gravitational tectonic collapse. The ∼100 km long, ∼1–5 km wide, N-trending Chulul Shear Zone which represents the southern part of the Bulbul Shear Zone is characterized by the presence of moderately E-dipping mylonitic fabric, SE-plunging stretching lineation and kinematic indicators showing top-to-the-southeast tectonic transport. This shear zone separates the structurally and metamorphically contrasting Melka Guba Domain (the southern extension of the Alghe terrane) in the west from the Chulul Domain (the southern extension of the Bulbul terrane) to the east. The Melka Guba Domain comprises migmatites, hornblende–biotite gneisses and quartzo-feldspathic gneisses with metamorphic assemblages that indicate upper amphibolite metamorphism ( T of ∼650 °C and P of ∼6 kbar). In addition, the Melka Guba Domain preserves E-W trending relic gneissic and migmatitic layering that is not evident elsewhere in the region. This older planar fabric is discordantly overprinted by a regional gneissic and migmatitic layering that was subsequently folded about NNE-trending upright folds. The Chulul Domain, on the other hand, is dominated by amphibolite schist, mafic and ultramafic rocks and meta-diorites interpreted as an island arc-ophiolitic-plutonic assemblage. Metamorphic mineral assemblages in the Chulul Domain suggest greenschist facies metamorphism ( T of ∼450 °C and P of ∼3 kbar). Deformation in the Chulul Domain is dominated by N-trending schistosity similar in orientation to that in the Melka Guba Domain to the west. We explain these structural and metamorphic contrasts using a model of gravitational tectonic collapse (∼550–500 Ma) involving detachment along the Chulul Shear Zone. The collapse follows the collision between the Melka Guba and the Chulul Domains at ∼820–580 Ma after the consumption of a possibly E-dipping subduction zone. Our model offers little support to the theory that the Neoproterozoic N-trending belts in southern Ethiopia were roots of northward expulsion of the Arabian-Nubian Shield from the Mozambique Belt.

Intracontinental reworking in the Capricorn Orogen, Western Australia: the 1680 – 1620 Ma Mangaroon Orogeny*

Structures and metamorphic mineral assemblages throughout much of the Proterozoic Capricorn Orogen have been attributed to the 1830 – 1780 Ma Capricorn Orogeny. However, in the northern part of the Gascoyne Complex at the exposed western end of the orogen, precursor sediments to pelitic gneiss and metamorphosed feldspathic sandstone were deposited after 1680 ± 13 Ma. The sediments were first deformed and intruded by voluminous granites during intracontinental reworking (the Mangaroon Orogeny) between 1680 Ma and 1620 Ma. The Mangaroon Orogeny comprises a subhorizontal gneissic layering and peak metamorphic conditions of upper amphibolite facies (D1m/M1m) followed by upright, macroscopic southeast-trending folds and an associated subvertical foliation formed at greenschist facies (D2m/M2m). Schlieric biotite – muscovite granodiorite dated at 1677 ± 5 Ma represents an anatectic melt that intruded during D2m. Cross-cutting biotite and biotite – muscovite monzogranites mainly crystallised at ca 1680 – 1660 Ma, but granite plutons were emplaced across the Gascoyne Complex until 1620 Ma. Our findings invalidate correlation of these metasedimentary rocks with lower grade rocks of the upper Wyloo Group along the northern margin of the orogen, and necessitate a reassessment of tectonic activity in the orogen attributed to the Capricorn Orogeny.

THE RIDGEWAY GOLD-COPPER DEPOSIT: A HIGH-GRADE ALKALIC PORPHYRY DEPOSIT IN THE LACHLAN FOLD BELT, NEW SOUTH WALES, AUSTRALIA—A DISCUSSION

Sir: Wilson et al. (2003), in their important paper on the Ridgeway Au-Cu deposit, describe hydrothermal alteration about the Ridgeway porphyry ore system. However, their findings are substantially more valuable if taken into account with the published works on regional burial metamorphism of the district (Smith, 1968, 1969). These works described burial metamorphism of the Ordovician volcanic sequences, including the hosts to the Ridgeway deposit, based on district and regional zonation of secondary mineral assemblages. This discussion briefly presents this context and focuses on the key issue of discriminating the porphyry-related hydrothermal alteration from the effects of burial metamorphism. Because porphyry-related hydrothermal minerals and later metamorphic mineral assemblages can be similar, their discrimination is crucial in mapping alteration styles as vectors in exploration for concealed ore …

05/00534 Preparation of solar grade silicon from optical fibers wastes with thermal plasmas: Ma, W. et al. Solar Energy Materials and Solar Cells, 2004, 81, (4), 477–483

The evolution of magnesium chloride brines with high bromide contents via a multistage reaction and dissolution process has been studied in brine seeps of a German potash mine. The observed chemical trends and phase equilibria can be modeled and interpreted in terms of a NaCl solution (cap rock brine) infiltrating into a potash zone characterized by the metamorphic mineral assemblage kieserite + sylvite + halite + anhydrite. Establishment of a persistent, stable equilibrium assemblage and constant fluid composition in the invariant point IP1 of the six component (Na-K-Mg-Ca-Cl-SO4-H2O) system of oceanic salts is prevented by the perpetually renewed input of NaCl-brine and by the intermittent exposure of incompatible kieserite. Instead, the solutions develop towards the metastable invariant point IP1(gy), with the mineral assemblage carnallite + polyhalite + sylvite + halite + gypsum, where gypsum takes the place of anhydrite (stage I). The temporary exposure of kieserite and the ensuing formation of polyhalite effectively buffer the solutions along the metastable polyhalite phase boundary during stages II and III. Eventually, in stage IV, polyhalite becomes depleted and admixture of more NaCl brine leads to low sulfate solution compositions, which are now only constrained by carnallite + sylvite + halite, and the once hexary system degenerates to a quaternary one (Na-K-Mg-Cl-H2O) in point E. Bromide in brines shows equilibrium partitioning with respect to the wall rock minerals. The pattern of evolving brine compositions may serve as a model for similar brine occurrences, which in some cases may have been misinterpreted as remains of fossil, highly concentrated and chemically modified seawater. Similar magnesium chloride brines of salt lakes (e.g., Dead Sea, Dabusun Lake) show subtle differences and are constrained by fewer mineral equilibria (more degrees of freedom), and their low sulfate contents are due to gypsum precipitation, driven by calcium chloride input from dolomitization reactions. Finally, the observed reaction sequence is generalized, and a model for the formation of magnesium sulfate depleted, chloride-type potash salts and bischofite deposits by leaching of sulfate-type evaporites is proposed.

Metamorphic and Thermal History of a Fore-Arc Basin: the Fossil Bluff Group, Alexander Island, Antarctica

The Himalia Ridge Formation (Fossil Bluff Group), Alexander Island is a 2.2-km-thick sequence of Upper Jurassic-Lower Cretaceous conglomerates, sandstones and mudstones, derived from an andesitic volcanic arc and deposited in a fore-arc basin. The metamorphic and thermal history of the formation has been determined using authigenic mineral assemblages and vitrinite reflectance measurements. Metamorphic effects include compaction, pore-space reduction, cementation and dissolution and replacement of detrital grains by clay minerals (smectite, illite/smectite, corrensite and kaolinite), calcite, chlorite, laumontite, prehnite, pumpellyite, albite and mica, with less common quartz, haematite, pyrite and epidote. The authigenic mineral assemblages exhibit a depth-dependence, and laumontite and calcite exhibit a strong antipathetic relationship. Detrital organic matter in the argillaceous layers has vitrinite reflectance values (R-o) ranging from 2.3 to 3.7%. This indicates considerable thermal maturation, with a systematic increase in reflectivity with increasing depth. There is good correlation of metamorphic mineral assemblages with chlorite crystallinity and vitrinite reflectance values-all indicating temperatures in the range of 140 +/- 20degreesC at the top of the sequence to 250 +/- 10degreesC at the base of the sequence. The temperatures suggest a geothermal gradient of 36-64degreesC/km and a most likely gradient of 50degreesC/km. It is suggested that this higher-than-average gradient for a fore-arc basin resulted either from rifting during basin formation or from a late-stage arc migration event.

Alleghanian orogenic-float on the Martic thrust during dextal transpression, central Appalachian Piedmont

Abstract During the Late Paleozoic Alleghanian orogeny, the mid-Atlantic Piedmont experienced transpressional deformation dominated by dextral strke-slip shear zones. The dextral displacement on these shear zones greatly influenced the geographic distribution of lithotectonic units. Transpressional deformation is evident in the Piedmont with the cogenetic development of domes and en-echelon antiforms between many of the shear zones. In the core of the Pennsylvania reentrant, major Alleghanian structures include the dextral Pleasant Grove shear zone and Tucquan-Mine Ridge antiform. Recent field mapping coupled with detailed metamorphic and deformation fabric studies have revealed that a major thrust, the Martic thrust, was also active during this time. Shear bands were identified during petrofabric analysis of the hanging wall rocks to the Martic thrust. The direction of displacement on these shear bands was parallel to the orogen, a direction contrary to earlier studies. Metamorphic mineral assemblages and ceased reaction textures, associated with ductile shear fabrics in the hangingwall rocks, are consistent with lower greenshist facies deformation. This low grade metamorphism, which is generally confined to sheared rocks, overprints the regional upper greenshist- to lower amphibolite-facies assemblages. Structural and magnetic modeling of the hangingwall block has revealed a complex geometry. A model of orogen parallel structural escape, or orogenic float, related to late Paleozoic dextral transpression is employed to explain the late reactivation on this important central Appalachian structure.

Metastable prograde mineral reactions in contact aureoles

Extrapolation of reaction paths and rates of metamorphic mineral growth from experimental to natural systems is complicated by a number of factors. Many of these factors are difficult to evaluate for natural systems. A combination of textural modeling and stable isotope analysis allows for a distinction between several possible reaction paths for olivine growth in a siliceous dolomite contact aureole. It is suggested that olivine forms directly from dolomite and quartz. The formation of olivine from this metastable reaction implies metamorphic crystallization far from equilibrium. Stable and metastable reaction paths predict the textures observed (calcite haloes around bladed olivine crystals) well. It is possible to discriminate between individual reaction paths only on the basis of the oxygen isotope compositions of the minerals involved. Products were found to be in stable isotope equilibrium, but in disequilibrium with the reactants. Only the metastable overall reaction dolomite + quartz → olivine + calcite + CO 2 produces no dolomite by local reactions, and hence agrees with the oxygen isotope data. Thus, significant mineral growth occurred far from equilibrium with respect to the thermodynamically stable reactions of the system. This amazing finding implies that metamorphism of contact aureoles has to be reinterpreted in a more complex, dynamic fashion, involving metastable reactions and metastable equilibria as well. The spatial distribution of metamorphic mineral assemblages in a contact aureole cannot be interpreted as a proxy for the temporal evolution of a single rock specimen, because each rock undergoes a different reaction path, depending on temperature, heating rate, and fluid-infiltration rate.

Chemical composition of the actinolite-hornblende assemblage as indicator for the P-T metamorphic conditions in chlorite-amphibole schists and metadiabases, Vrteska, Plackovica Mt., Eastern Macedonia

Microscopic and electronic microprobe studies carried out on the chlorite-amphibole schists and the metadiabases from the vicinity of Vrteska determined an actinolite, Mg-hornblende, biotite, epidote, chorite, albite, calcite, quartz, and ilmenite assemblage. The metamorphic mineral assemblage and structural-textural characteristics indicate that these rocks metamorphosed in conditions of regional dynamothermal metamorphism in a Barovian type series of facies which, according to the degree of metamorphism, corresponds to the transition from quartz-albite-epidote-biotite to quartz-albite-epidote-almnadine subfacies. The chemical composition of actinolite and hornblende indicate that the rocks under investigation metamorphosed in conditions of prograde metamorphism at T = 400-500°C and P = 2-5 kbar.

P– T evolution of the Pan-African Taba metamorphic belt, Sinai, Egypt: Constraints from metapelitic mineral assemblages

The Taba metamorphic belt (TMB) is one of four Pan-African metamorphic belts of the Sinai Peninsula forming the northernmost extension of the East African Orogen. The TMB underwent two main phases of ductile deformation accompanied by a single metamorphic event. Critical metamorphic mineral assemblages in the metapelitic rocks testify to regional metamorphism under amphibolite-facies conditions and form a zonal pattern (all assemblages + quartz + plagioclase + ilmenite): (i) garnet–biotite ± muscovite, (ii) staurolite–garnet–biotite ± muscovite, (iii) sillimanite/andalusite–staurolite–garnet–biotite, (iv) cordierite (pinitized)–staurolite–garnet–biotite ± muscovite ± andalusite. A P– T pseudosection calculated for a garnet–staurolite–sillimanite schist constrains the peak metamorphic assemblage garnet–staurolite–biotite–sillimanite–quartz to a narrow field extending from ≈590 to 640 °C and 5 to 6 kbar. The observed retrograde assemblage garnet–staurolite–sillimanite–chlorite–muscovite–quartz is stable at 560–590 °C and 4.5–5 kbar. This indicates a near isobaric cooling path segment during retrogression. Temperatures and pressures of 550–590 °C and 4 ± 1 kbar were also estimated with conventional geothermobarometry. The data suggest that the Taba metamorphic rocks underwent a high- T/low- P metamorphic evolution, with a peak metamorphic gradient of ≈50–60 °C/km. The results support the interpretation that the Sinai basement formed at a magmatic arc during the collision between East and West Gondwana.

Metamorphic P-T Path of the Southern Jilin Complex: Implications for Tectonic Evolution of the Eastern Block of the North China Craton

The Southern Jilin complex represents the northeasternmost exposure of the basement rocks in the Eastern block of the North China craton and consists of intensely foliated tonalitictrondhjemitic-granodioritic (TTG) gneisses, weakly foliated syn-tectonic potassium granites, posttectonic hypersthene-quartz diorite and clinopyroxene granodiorite and a minor amount of supracrustal rocks. The supracrustal rocks include ultramafic to felsic volcanic rocks, BIF, felsic paragneisses, pelitic gneisses, calc-silicates, and marbles metamorphosed from amphibolite to granulite facies. The petrological evidence from the mafic granulites and pelitic gneisses indicates three metamorphic mineral assemblages (M1 to M3). The early prograde assemblage (M1) is preserved as mineral inclusions within minerals of the peak assemblages, represented by hornblende + plagioclase + quartz ± biotite in the mafic granulites and biotite + plagioclase + quartz in the pelitic gneisses, with P-T conditions estimated at ~0.6 GPa and ~750°C. The peak assemblage is represented by orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the mafic granulites and garnet + sillimanite + plagioclase + quartz + biotite in pelitic gneisses, with P-T conditions estimated at 0.84-0.87 GPa and 800-850°C. The post-peak assemblage is characterized by garnet + quartz symplectic coronas in mafic granulites and kyanite replacing sillimanite in the pelitic gneisses, which probably occurred at ~0.85 GPa and 760°C. These mineral assemblages and their P-T estimates define an anticlockwise P-T path involving nearly isobaric cooling, reflecting an origin related to the intrusion and underplating of large amounts of mantle-derived magmas, which are considered to have been related to the interaction of upwelling mantle plumes with the lithosphere in the Eastern block of the North China craton in the Late Archean.

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P– T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites. Carbonic fluid (CO 2-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH 4 and/or N 2 as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N 2, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO 2 and mixed H 2O–CO 2(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO 2±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO 2, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.

Geological setting of the Rapu Rapu gold-rich volcanogenic massive sulfide deposits, Albay Province, Philippines

Gold-rich Fe–Cu–Zn volcanogenic massive sulfide deposits occur within strata of probable Jurassic age on Rapu Rapu Island in Albay Province, Philippines. Massive sulfides at the Ungay Malobago and Hixbar deposits are spatially associated with dacitic volcanic rocks within a highly-deformed sequence of mafic volcanic and quartzofeldspathic sedimentary rocks. The massive sulfide deposits formed at the stratigraphic contact between footwall dacites and hangingwall mafic volcanic and quartzofeldspathic rocks. The deposits and their host strata have undergone regional metamorphism with strong penetrative deformation. Metamorphic mineral assemblages and textural evidence suggest that peak metamorphism was upper-greenschist to lower-amphibolite grade and syn-D1 deformation. Based on the age of regional metamorphism, deformation is inferred to be mid-Tertiary in age. Deformation at Rapu Rapu resulted in reorientation of the strata into a broad antiform with strong shallow-plunging elongation fabrics, overturning of the volcanic sequence that hosts the Ungay Malobago deposit, and complex folding of the mineralized zones. The present highly linear form of the Ungay Malobago deposit is mainly a product of this ductile strain.

Rare earth and trace element mobility in mid-crustal shear zones: insights from the Mont Blanc Massif (Western Alps)

The behaviour of rare earth elements (REE) during fluid–rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO 2 (70.5–75.3 wt%) and magmatic chemical variations are of the order of 10–20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at ∼0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zone’s major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid–rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnäsite, aeschynite and tombarthite). Minor proportions (<2%) of these accessory phases, with grain sizes mostly <20 μm, account for enrichments of up to 5:1 compared to the initial granite whole-rock REE budget. The stability of the REE phases appears to be largely dependent on the altering fluid composition. REE mobility is ascribed to changes in pH and to the availability of CO 3 2−, PO 4 2−, and SO 4 2−ligands in the fluid. Such processes are likely to influence the mobility of REE, Y, Hf and Ta in shear zones.