Metamorphic Grade Research Articles

The Cerro Uyarani Metamorphic Complex on the Bolivian Altiplano: New constraints on the tectonic evolution of the Central Andean basement between ∼1.8 and 1.0 Ga

The Cerro Uyarani Metamorphic Complex (CUMC) in Bolivia provides a record of complex Paleoproterozoic to Mesoproterozoic tectono-magmatic evolution, as revealed through U–Pb isotopic analysis of zircon from three litho-domains (DI – DIII). The Paleoproterozoic crystalline basement was subject to two stages of Mesoproterozoic reworking. This detailed study of zircon textures and geochronology, including determination of first Hf-isotope data, from felsic granulites, banded granulites and amphibolites reveals a first Paleoproterozoic event at 1.81–1.74 Ga in magmatic and some metamorphic zircon cores, with zircon ϵHfT values of +6.3 to −0.2. A subsequent Mesoproterozoic event can be divided into 2 cycles: a first one at 1.19–1.17 Ga is reflected by recrystallized rims and cores of zircon with ϵHfT values of −6.8 to −3.6. This cycle is here associated with migmatization of the basement, during which the Paleoproterozoic crust was differentiated into DI-leucosome rich migmatite, DII-leucosome rich migmatite, DIII – our best proxy of Paleoproterozoic crust, melanosome, and paleosome. The second cycle at ∼1.1–1.0 Ga is recorded in metamorphic rims and by newly grown metamorphic zircon with ϵHfT values of −15.1 to −2.6. This event was responsible for the transformation of the CUMC migmatitic basement into granulite facies rocks. We interpret that the CUMC was part of the southernmost portion of the Arequipa terrane from, at least, ∼1.74 Ga and that it then became attached to the Paraguá terrane and Amazon craton at 1.19–1.17 Ga. This collision generated migmatites, and then, with the arrival of Laurentia passing through the Arequipa terrane front (with CUMC at the Arequipa terrane), the conditions of metamorphism were raised to granulite facies. A younger event of lower metamorphic grade and likely associated with fluid-driven alteration is recognized in the partial retrograde transformation of mafic granulites into amphibolites. Our new U–Pb and εHfT data indicate that the Paleo-to Mesoproterozoic history of the CUMC is, thus, similar to that of the Arequipa terrane, Rio Apa terrane, the SW Amazon craton, and other Grenville-age inliers; but it was distinct from the evolution of the Sierra de Maz and Pie de Palo of the hypothetical MARA craton.

Metamorphic evolution of the North Delhi Fold Belt, implications on Delhi orogeny and the Rodinia connection

Medium‐grade metamorphic rocks representing metapelites, calc‐silicates and metavolcanics constitute the rocks of Alwar and Khetri sub‐basins of the North Delhi Fold Belt (NDFB) in southern Haryana and adjacent western Rajasthan. Using modern petrological tools including pseudosection modelling, we describe the metamorphism of the rocks representing these two sub‐basins and show that the reported westward increasing grade of metamorphism in NDFB is non‐tenable. We show that, if anything, the grade of metamorphism decreases from the central Alwar sub‐basin to the western Khetri sub‐basin. Clockwise segments of P‐T paths are constructed using effective bulk controlled pseudosections from these two sub‐basins allowing inference of a schematic P‐T‐t path for the NDFB after integration of the reported and new age data. A subduction setting for this orogeny is indicated. Published data from the South Delhi Fold Belt (SDFB) corroborate the same‐age orogeny for both segments of the Delhi Fold Belt. The previously proposed continuation of the Delhi Fold Belt suture with the Central Indian Tectonic Zone is supported by the present work.

The effect of metamorphism on the aggregate properties of gabbroic rocks

Granitic rocks are durable materials sought after for the production of road and railroad aggregates. Granitic bedrock commonly, however, includes gabbroic components, which may enhance or decrease the aggregate performance. This study evaluates the variation in resistance to fragmentation (Los Angeles value, LA) and wear/abrasion (micro-Deval value, MDE) for the fraction 10/14 mm of gabbro in different metamorphic states. Samples were collected along a 150-km profile where metamorphic conditions grade from epidote–amphibolite to high-pressure granulite-facies, and the degree of metamorphic recrystallization varies with the amount of hydrous fluid. Rocks with no or incipient metamorphic recrystallization preserving their primary igneous fabric and interlocking texture meet the criteria for both asphalt base course and track ballast in railroad, with LA and MDE values below 25% and 14%, respectively. Mafic granulite and fine-grained amphibolite have LA values below 25% and can be used in unbound layers. Mafic granulites crystallize at high temperatures but commonly preserve a relict igneous texture due to limited hydration. Coarse-grained amphibolite and migmatitic amphibolite have the poorest performance. They recrystallized at hydrous conditions, leading to complete recrystallization, grain coarsening, and loss of interlocking igneous texture. This study shows that both temperature and infiltration of hydrous fluids significantly affect the technical properties. Even at high metamorphic temperatures, gabbroic rocks may yield aggregates of high technical performance. At hydrous conditions, however, recrystallization results in rock aggregates suitable for unbound layers only. The variation in metamorphic grade and hydration is easily assessed by the geologist in the field and by using standard petrographic microscopy.

Zircon as a Recorder of Trace Element Changes during High-Grade Metamorphism of Neoarchean Lower Crust, Shevaroy Block, Eastern Dharwar Craton, India

AbstractSystematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry are seen along a ca. 95-km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidised granulite-facies rocks near the magnetite–haematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-grade charnockite. Cathodoluminescence and back-scattered electron sub-grain imaging and sensitive high-resolution ion microprobe analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasingly depleted compositions between 2530 and 2500 Ma. With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted metamorphic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally derived, oxidising, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean.

Recognition of Detrital Carbonaceous Material in the Ryoke Metamorphic Belt by Using Raman Thermometry: Implications for Thermal Structure and Detrital Origin

Abstract Raman spectroscopy of carbonaceous material (RSCM) thermometry is an effective method to determine the thermal structure of metasedimentary domains with high spatial resolution. The mean value of numerous measurements in a single sample is used for temperature estimation. A potential complicating factor is the presence of detrital carbonaceous material (CM) that can introduce bias in the results. Detrital CM generally shows a high degree of crystallinity making it difficult to discriminate detrital and metamorphic CM in high-grade metamorphic domains. Application of the RSCM thermometry to 100 metamudstone samples from the Hongusan area of the Ryoke metamorphic belt reveals multimodal degrees of crystallinity of CM in many samples indicating the common presence of detrital CM grains. Application of a new procedure to filter out the effects of detrital CM reveals a thermal structure that can be explained as the result of the overprinting of two distinct thermal regimes related to regional and subsequent contact metamorphism. The ability to recognize the presence of detrital CM and measure its crystallinity can be used to infer the metamorphic grade present in the hinterland from which the detrital material is derived. A combination of this information on metamorphic temperature with geochemical and radiometric age data reported in the literature indicates that the detrital CM was derived from basement rocks of the Korean Peninsula.

3D structural control and spatial distribution of Zn-Pb-Cu grades in the Palmeirópolis VMS deposit, Brazil

ABSTRACT The Palmeirópolis Zn-Pb-Cu volcanogenic massive sulfide deposit consists of four areas, presenting distinct features regarding host rocks, the geometry of ore bodies, hydrothermal alteration, deformation intensity, and metal proportions. The seven modeled Zn-Pb-Cu ore bodies occur mainly along with the contact between the Mesoproterozoic acid and basic metavolcanics within amphibolite facies. Some characteristics such as the relation of higher Zn-Pb and Cu grades to a higher proportion of acid and basic metavolcanic host rocks respectively, the same metamorphic grade and the gradational contact between volcanic host rocks, and distribution of alteration halo suggest that the original stratigraphy is still preserved. A smooth folding superposing the initial tight folds that define the ore bodies indicates at least two deformation events after the Zn-Pb-Cu mineralisation in the Palmeirópolis deposit. Finally, the geometry of the 3D ore bodies in Palmeirópolis are discussed against other VMS deposits around the world with 3D data available in terms of age, metamorphic, and deformation grades.

Implications of the dominant LP–HT deformation in the Guanhães Block for the Araçuaí West-Congo Orogen evolution

The Araçuaí-West Congo Orogen is the result of convergence between the São Francisco and Congo cratons during the assembly of West Gondwana. Within this orogen, the Araçuaí Belt in SE Brazil reflects the western external domain. The Araçuaí Belt is characterized by major east-dipping structures and high- to medium pressure metamorphic rocks developed during crustal thickening at ca. 585–560 Ma and are juxtaposed along crustal-scale structures to the crystalline core of the orogen.The Guanhães Block in the southern Araçuaí Belt is characterized by higher metamorphic grade and dominantly shallowly west-dipping structures compared to the neighboring tectonic domains. While several studies have focused on the evolution of the axial crystalline core of the Araçuaí orogen and on the early collisional stages in the Araçuaí Belt, little attention has been given to the structural and metamorphic history of the poorly studied Guanhães Block.The supracrustal rocks in the central Guanhães Block record two tectono-metamorphic stages: D2/M2 and D3 /M3, with potential D1/M1 relicts rarely preserved within D2/M2 low-strain domains. The D2/M2 overprinting stage is associated with the development of a dominant, shallow axial planar foliation (S2) of isoclinal F2 folds under low-pressure and high-temperature (LP–HT) conditions (ca. 6 kbar and 750 °C). During this D2/M2 stage continuous segregation of syn-kinematic partial melting and generation of granitoid rocks occurred. The 530 Ma, syn- D2/M2 granitoid has negative εHf(i) at ca. −32 which indicates the source of magma from melting of older, isotopically evolved, Archaean or Paleoproterozoic crust.U–Pb geochronology on monazite, titanite and hydrothermal zircon constrain the retrograde D3/M3 event at ca. 510 Ma. The Guanhães Block uniquely records dominant deformation under LP–HT metamorphic conditions during the late-orogenic stages and its evolution could have important tectonic implications for the concurrent models suggested for the late Neoproterozoic Araçuaí-West Congo Orogen.

Mercury isotopic compositions of the Precambrian rocks and implications for tracing mercury cycling in Earth's interior

Mercury isotopes undergo unique mass-independent fractionation (MIF) during photochemical processes on Earth's surface. Studies have observed pronounced Hg-MIF signals in sedimentary and magmatic rocks, suggesting recycling of Hg from Earth's surface systems into the lithosphere via sedimentation and magmatism. However, the isotopic signature of Hg in metamorphic rocks and the geochemical fate of Hg during metamorphism remain unclear. Precambrian basements are important components of cratons or orogenic belts on Earth. Here, we study the Hg concentration and isotopic composition of Precambrian metamorphic and sedimentary rocks from the eastern Central Asian Orogenic Belt, and North and South China cratons. Metamorphic rocks show much lower Hg contents (0.21–7.8 ppb) than sedimentary rocks (2.6–694 ppb), indicating a substantial loss of Hg during metamorphism. The lack of correlation between δ202Hg values (–2.41 to 0.18‰) and metamorphic grades indicates no systematic mass-dependent fractionation (MDF) of Hg isotopes during metamorphism. The Δ199Hg/Δ201Hg ratios of ∼ 1.0 for both metamorphic and sedimentary rocks indicate Hg was sourced from Earth's surface systems. The coupling of Hg-MIF signals between the metasedimentary rocks and the sedimentary settings of their protolith suggests no Hg-MIF during metamorphism. The negative Δ199Hg values (–0.30 to –0.02‰) in the Precambrian coastal sedimentary rocks imply the input of Hg into coastal regions via soil erosion. The positive Δ199Hg values (0.06 to 0.31‰) in the Precambrian marine sedimentary rocks suggest deposition of atmospheric Hg(II) to open oceans via wet deposition. The lack of significant Hg-MIF during metamorphism and other underground geological processes shows that Hg-MIF signals can work as a reliable tracer for indicating material cycling in Earth's interior.

Electron Backscatter Diffraction Study of Ultrahigh-Pressure Tso Morari Eclogites (Trans-Himalayan Collisional Zone): Implications for Strain Regime Transition from Constrictional to Plane Strain during Exhumation

Abstract The Tso Morari Crystalline Complex (TMCC) of trans-Himalaya (eastern Ladakh, India) contains enclaves of ultrahigh-pressure eclogites that underwent deep burial (≥80 km) and subsequent rapid exhumation during continental subduction, collision, and final accretion of the Indian plate with the Eurasian plate. We present an electron backscatter diffraction (EBSD) study of eight eclogite samples to investigate the deformation mechanism and strain regimes active during peak (HP) metamorphism and subsequent postpeak rapid exhumation of the TMCC. Our study shows that the least retrogressed eclogite exhibits strong linear fabric (L tectonite) characterized by omphacite, having [001] axes parallel to and (110) poles perpendicular to lineation. These features concur with constrictional strain during peak (HP) metamorphism. A transitional planolinear fabric (LS tectonite) is shown by other eclogites that show petrographic evidence of omphacite alteration to amphibole and the presence of lower metamorphic grade minerals like actinolite and chlorite. Characteristics of lattice preferred orientation (LPO) of omphacite and quartz, indicated, respectively, by the LS and B indices, also suggest variation in strain regime from pristine eclogites to their altered counterparts. Based on these results, it is suggested that a constrictional strain regime prevailed during peak (HP) metamorphism in the TMCC due to the buoyant rise of TMCC in response to slab break-off and reverse slab pull during and after the deepest continental subduction. This buoyant rise was also facilitated by compression related to the ongoing India-Eurasia collision. This regime evolved later to plane strain that was superimposed on the UHP rocks at a shallower depth. It is plausibly associated with foliation-parallel extension during exhumation at midcrustal depths. A high-temperature prism c-slip in quartz shown by few samples is interpreted to have formed due to a subsequent granulite facies metamorphic overprint on the eclogite during collisional thickening.

Deviation between quartz‐in‐garnet elastic geobarometry and equilibrium‐based pressure–temperature modelling in Barrovian metamorphic rocks

AbstractQuartz‐in‐garnet elastic geobarometry (QuiG) pressures in rocks from two Barrovian metamorphic terranes in the western US Cordilleran hinterland exceed pressures determined using chemical thermodynamics by 3–4 kbar. For this study, 135 quartz inclusions from the Funeral Mountains, California, were analysed using QuiG in five garnets from three locations representing metamorphic grades of upper greenschist, lower amphibolite, and middle amphibolite facies. From a second Barrovian terrane, the Wood Hills in northeastern Nevada, 125 quartz inclusions were analysed using QuiG in 14 garnets from a single rock sample metamorphosed to middle amphibolite facies. Pressures determined for rocks in the Funeral Mountains using QuiG and methods rooted in equilibrium thermodynamics yielded consistent pressure differences between locations, but QuiG pressures are higher. Similarly, QuiG pressures determined for rocks in the Wood Hills are higher than pressures determined by equilibrium thermodynamic approaches. Possible explanations for the pressure differences include garnet compositions not reflecting equilibrium, sources of error in thermodynamic calculations such as thermodynamic data or a‐X models, or an unknown source of systematic error that causes QuiG to overestimate pressures of entrapment. To test Raman spectroscopy's ability to reproduce inclusion pressures, pressures were calculated using Raman spectroscopy and synchrotron X‐ray diffraction, which yielded consistent pressures and support the use of the single mode‐shift of the 464 cm−1 band of quartz for geobarometry, which simplifies the method by assuming hydrostatic compression of quartz. These results are compared with pressures obtained using Grüneisen tensors and show consistency between these different approaches.

Thorium zoning in monazite: A case study from the Ivrea–Verbano zone, NW Italy

AbstractMetamorphism and partial melting of the lower crust is commonly assumed to cause depletion in heat producing elements (HPEs; K, U, Th). In the deep crust, volumetrically subordinate metasedimentary layers, which are source to crustal granites (sensu lato), host the majority of Th ± U, primarily within the REE + Th + U + Y phosphate mineral monazite. We examine the spatial and temporal distribution of Th within monazite grains in metasedimentary rocks from the lower crustal section of the Ivrea–Verbano Zone (Italy), using textural, compositional and geochronological data. We link this to outcrop and regional scale trends described by in‐field gamma‐ray spectrometry data (in‐field GRS) for the purpose of understanding how Th distribution is controlled by progressive metamorphism and partial melting. In‐field GRS data shows that the whole rock budget of Th does not change between granulite facies rocks and their unmelted equivalents but is significantly lower in rocks that have undergone more significant melt loss at ultra‐high temperature (UHT) conditions. Concurrently, the bulk Th budget of monazite increases with metamorphic grade to granulite facies conditions and is greatly reduced in UHT samples. Monazite geochronology returns dates mostly in the range 240–320 Ma with two main peaks at circa 290 and 270 Ma. Textural and chemical constraints indicate that these dates record the timing of pre‐peak to peak metamorphic conditions. Amphibolite facies monazite compositional zones are absent from granulite facies monazite, in contrast to examples from lower‐pressure terranes. This is consistent with the expanded stability of allanite relative to monazite with increasing pressure having an important role in determining the internal structure, composition and extent of inheritance of monazite in going from amphibolite facies to granulite facies rocks. We propose high‐pressure granulites should preserve less monazite inherited from amphibolite facies conditions than low‐pressure granulites. Monazite is preserved at all metamorphic grades and presents a mineralogical mechanism for retaining Th in residual deep crust during partial melting and after melt loss.

Experimental research on different metamorphic grades of coal bodies with macro–mesoscopic structure fractal characteristics

This paper studies the Daliuta mine, Xinglongzhuang mine, Qingdong mine and Qincheng mine. Here, nuclear magnetic resonance (NMR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were performed on different metamorphic grades of coal samples from 4 main mining areas in China. The coal body fissure–pore structure parameters were measured in the range of 100 nm∼5000 nm, and the fractal characteristics were obtained for the different metamorphic grades of coal bodies at macro–meso-structure scales, revealing the influence of reservoir formation on the complexity of the coal body structure. The results show that with the increase in metamorphic degree, the ratio of total pore volume occupied by pores and macropores in coal samples gradually decreases, while the volume ratio of transition pores and micropores increases; in addition, the influence of coal reservoir formation processes on fissure–pore structure parameters of coal is obtained. Based on computer graphics theory and fractal geometry theory, the relationship between the 3D volume fractal dimension of the coal bodies, as measured by NMR, MIP and other fluid intrusion methods, and the box-dimension statistical method of SEM to measure the surface fractal dimension is analyzed. The 3D volume fractal dimension calculated by NMR and MIP is consistent with the change in metamorphism degree; that is, with the increase in metamorphism degree of the coal sample, the 3D volume fractal dimension increases. In addition, the 3D volume fractal dimension measured by NMR is larger than that measured by the MIP; the difference between the 3D volume fractal dimension measured by NMR and 2D surface fractal dimension measured by SEM is approximately 1. Here, the influence of reservoir formation on the characteristics of coal structure fractal is obtained. The research results provide important scientific guidance for enhancing coalbed methane recovery and mine disaster prevention.

Distinct scheelite REE geochemistry and 87Sr/86Sr isotopes in proximally- and distally-sourced metamorphogenic hydrothermal systems, Otago Schist, New Zealand

The Otago Schist in New Zealand hosts orogenic tungsten (W) mineralization in two types. Proximally-sourced (“proximal”) mineralization in sub- to lower-greenschist facies Mesozoic metasedimentary rocks within the northern flank of the Otago Schist Belt comprise quartz-calcite veins with minor scheelite and few sulfides. Distally-sourced (“distal”) deposits are hosted in major crustal discontinuities within Mesozoic sub-greenschist to upper-greenschist facies metasedimentary rocks and are associated with abundant sulfides ± Au. In-situ trace element (particularly REE, Sr, Mo, Na, As) and in-situ 87Sr/86Sr compositions of scheelite show the scheelite occurring in these different mineralization types to be geochemically distinct. Proximal occurrences are characterised by scheelite with both heterogenous trace element and Sr isotope compositions displaying variations that can be linked to different veins, host rock types and scheelite generations. In contrast, scheelite within distal scheelite deposits tends to have fairly homogenous trace element and Sr isotopic ratios at the deposit to grain scale. The heterogenous compositions of proximal occurrences represent local derivation of diverse components from the sub-millimetre to meter scales and small extents of fluid flow, which resulted in a high sensitivity to sources and local rock compositions and thus limited large-scale equilibration of elemental and isotopic systematics within the scheelite they contain. The larger distal deposits formed by regional leaching by metamorphic fluids, probably at the greenschist-amphibolite facies boundary, followed by homogenisation as they ascended through the crust and leading to deposition of scheelite with rather uniform chemical characteristics. Our data re-enforce the model that tungsten mobility in the Otago Schist involved local-scale mobilisation of W in the shallow crust by breakdown of detrital rutile and scheelite at low metamorphic grade. Burial of these veined scheelite-bearing rocks to temperatures of ∼500 °C enabled remobilisation of W by metamorphic fluids, concurrent with scavenging of Au, As and S, followed by return of the metal-bearing fluids towards mid to upper crustal levels, where they precipitated scheelite.

Tracing wedge-internal deformation by means of strontium isotope systematics of vein carbonates

AbstractRadiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a central role in the tectonometamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. In addition, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units of the central European Alps (Glarus Alps, Switzerland), which were accreted to the North Alpine fold-and-thrust belt during the early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade. This relationship reflects the progressive equilibration of the pore fluid with the host rock and allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including bedding-parallel shearing, imbricate thrusting, folding, cleavage development, tectonic mélange formation and extension. The strontium isotope systematics of vein carbonates provides new insights into the prograde to early retrograde tectonic evolution of the Alpine fold-and-thrust belt and helps to understand the relative timing of deformation events.

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

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

Distribution and characterization of heavy minerals in Meghna River sand deposits, Bangladesh

Heavy mineral (HM) concentrations in sand bar deposits of the Meghna River were evaluated using optical microscopy, X-Ray Diffraction (XRD), X-Ray Fluorescence spectrometry (XRF), Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA) to determine the mineralogy, geochemistry and likely source rocks of the sands, and to examine the distribution and HM potential of the valuable heavy minerals, ilmenite and garnet. The Meghna sands were dominated by the light minerals quartz and feldspar and grainsize analysis results showed that>78 wt.% of the sand population fell within the size range of fine to very fine sand (−250 + 63 μm). The HM component made up around 9 wt.% and consisted mainly of amphibole (∼17–46 wt.%), epidote (∼7–30 wt.%) and garnet (∼3–21 wt.%). Fe-Ti oxide minerals including ilmenite, rutile, and titanomagnetite were present at levels of 0.7–7 wt.% with the magnetite content at 0.9–7 wt.%. The textures (moderate sorting, sub angular) and significant feldspar (6–15 wt.%) content in the sands indicated low degrees of hydraulic sorting and low amounts of alteration/weathering. The HM assemblage indicated a likely mixed source area of varying metamorphic grade (low to high) facies and igneous rocks. EPMA results from the detrital ilmenites and garnets showed that for ilmenite, TiO2 contents were between 42.9 and 54.5 wt.% (ave. 51 wt.%), with overall low impurities, while the garnet compositions were dominated by Fe-rich, almandine types. From the chemical analysis of individual garnet and ilmenite grains, it is likely the Meghna River sands have good potential for commercial extraction of garnet and ilmenite.

PETROLOGY, GEOCHEMISTRY AND MONAZITE U-Th-Pb AGES OF WEST ÁGUA RASA PALEOPROTEROZOIC POST-COLLISIONAL METAGRANITES FROM SOUTHERN SÃO FRANCISCO CRATON – BRAZIL

During the transition of Rhyacian to Orosirian periods a collision between the Archean Divinópolis and Campo Belo metamorphic complexes occurred in southern São Francisco Craton (SFC) followed by the collapse of the orogen. In the same region, the Statherian period is marked by a huge intraplate magmatic event that is represented in the area by the Para de Minas dyke swarm. Peraluminous granitoid occurrences are reported in the region of Formiga and Itapecerica (Minas Gerais – Brazil), surrounding high metamorphic grade khondalitic paragneisses and banded iron formations. To contribute to the understanding of the tectonic context of the southern SFC during the Paleoproterozoic era, these granitic rocks are studied based on petrographic, geochemical, and monazite U-Th-Pb geochronological analyzes. They were characterized as S-type (metasedimentary origin) peraluminous metamonzogranites, with crustal geochemical signature, and genesis related to anatexis in syn- to post-collisional environment. The geochronological results yielded two groups of Orosirian (~1,90 Ga) and Statherian (~1,78 Ga) ages. These results are related, respectively, to the collapse of the Rhyacian-Orosirian orogen and to the regional warming, associated with the Avanavero-Xiong’er LIP that is represented in the area by the Pará de Minas dyke swarm.

Geochemistry of Brines and Oil Occurrences in the Udachnaya Kimberlite Pipe (Siberian Platform)

Abstract —Results of a geochemical study of brines and oil occurrences in the Udachnaya kimberlite pipe are presented. Like other intrusions in the Daldyn–Alakit diamondiferous region, this diamond deposit is a unique cryohydrogeologic microstructure differing from the host sedimentary rocks and other diamond pipes of the Yakutian diamond-bearing province. Two waterlogged zones distinguished in the section of orebodies at the explored depths of the deposit correspond to the upper and middle Cambrian aquifers. Predominantly acidic (average pH = 5.5) Cl–Ca and Cl–Ca–Na brines with TDS from 94.3 to 391.3 g/dm3 are widespread within the orebodies and host rocks. The brine mineralization and contents of major salt-forming components increase with depth, to the horizon at the -365 m elevation, where TDS reaches 391 g/dm3, while below, at the –650 m level with noted hydrogeochemical-field inversion, TDS is 253 g/dm3. The mineralization of Cl–Ca, Cl–Ca–Na, Cl–Ca–Mg, and Cl–Ca–Mg–Na brines in the upper Cambrian rocks varies from 102.9 to 192.9 g/dm3, and the pH values, from 4.9 to 6.2, averaging 5.6. Among the microcomponents, the highest average concentrations (mg/dm3) are found for Br1292.8 > S875.7 > Sr453.7 > Fe79.7 > Li53.4 > B32.7 > I13.3 > Si10.8 > Mn6.4 > Se3.6 > Rb2.3. The values of genetic coefficients vary widely: The rNa/rCl coefficient ranges from 0.18 to 0.31; rCa/rMg, from 1.03 to 3.60; Ca/Cl, from 0.2 to 0.3; and the integrated metamorphism index S (according to S.L. Shvartsev) varies from 193 to 277. The middle Cambrian rock complex, containing more saline brines, has been examined in much more detail. It hosts Cl–Ca, Cl–Ca–Na, Cl–Ca–Mg, and Cl–Na–Mg brines with TDS from 94.3 to 391.3 g/dm3 and high average concentrations (mg/dm3) of microcomponents: Br2224.9 > Sr1024.9 >S500.1 > B202.9 > Li147.1 > Fe97.0 > I33.2 > Rb11.4 > Si9.6 > Se9.5 > Mn3.6 > Ni1.7. As compared with brines in the overlying rocks, the middle Cambrian brines show a wider variation in element ratios: rNa/rCl from 0.14 to 0.34, rCa/rMg from 0.66 to 9.71, and Ca/Cl from 0.03 to 0.45. These brines are also characterized by a significantly higher metamorphism grade, which is indicated not only by the rNa/rCl and rCa/rMg ratios but also by the S index varying from 278 to 316. The composition of stable isotopes δD and δ18O) and dissolved inorganic carbon δ13C) of the brines was investigated. The studied waters are assumed to be of sedimentary–metamorphic origin. Their isotopic composition reflects the climatic conditions existing at the time of their burial, which were probably aggravated by the contribution of the oxygen isotope exchange with water-bearing rocks. The δ13C values of carbon dioxide dissolved in water allow an inference about its biogenic origin. The biogenic carbon isotope exchange is governed by the relationship between methanogenic and SMT processes. Analysis of the 87Rb/86Sr and 87Sr/86Sr isotope ratios of the studied brines has revealed affinity between the isotopic compositions of waters in the Cambrian deposits and in ancient seawaters. The mass chromatograms of saturated-hydrocarbon (HC) fractions show at least two individual types of oils and malthas (naphthides). The third variety resulted from their mixing at different stages of migration. The fourth is from the contact zone; it changed during the explosion of kimberlites. The first, most common, type of naphthides (“postexplosive”) is similar in all geochemical parameters to oils from the Nepa–Botuobiya anteclise, in particular, to those from the Mirnyi arch. Oils of the second (pre-explosive) type are found only in the Udachnaya Formation, within the depth range 1130–1430 m.

Effects of ultrathin igneous sill intrusion on the petrology, pore structure and ad/desorption properties of high volatile bituminous coal: Implications for the coal and gas outburst prevention

As the one of the most catastrophic mining hazards, coal and gas outburst seriously threatens the mining safety of collieries. Previous studies indicate that lots of outbursts are closely related to igneous intrusion. Thus, to further understand the influence of igneous intrusion on coal’s physical properties and outburst risk (especially the impacts from ultrathin igneous sill intrusion, which is hard to be detected underground), the combined analyses of petrographic, proximate, pore structure and methane ad/desorption are applied. Results indicate that with the approaching to the ultrathin igneous sill, physical properties of coal change significantly. Due to thermal evolution effect from intrusion, metamorphic grade of coal increases from the high volatile bituminous coal (Ro = 0.6949) of unaffected normal region to the low volatile bituminous coal (Ro = 1.6684), and the altered range of thermal evolution zone is larger than 3 times of the sill thickness but of which the range of strong thermal evolution zone only reaches 0.8 times of the sill thickness. Within the thermal evolution zone: moisture content of coal decrease while the ash content increase when approaching to the sill; micropore volume and specific surface area of the coal are nearly unchanged in the weak thermal evolution zone but reduced in the strong thermal evolution zone, however, methane adsorption capacities of the coal are generally enhanced when comparing to the normal region sample; the transition pores and mesopores pore of the sample close to distal boundary of thermal evolution zone is seriously suppressed (which are 49.21–68.32 % and 20.14–63.31 % of the normal sample, respectively). By comprehensively considering the impact of igneous intrusion on coal, coal locates near the distal boundary of strong thermal evolution zone (about 0.5 times of sill thickness away from the intrusion boundary) seems to have the most serious threat of outburst risk. Additionally, the moisture, ash contents and the gas desorption property of coal has the feasibility to be used as the signs for the hard-detectable small-scale igneous intrusion ahead during roadway tunneling process. Meanwhile, due to the correlation between igneous intrusion and outburst, which can also be regarded as the early warning indicator for outburst prevention.

Fracture Orientation Significance in the Study of Pb-Zn Mineralization of Lower Benue Trough, Abakaliki Se Nigeria

The Lower Benue Trough, in South-eastern Nigeria, hosts several lead-zinc deposits. The mineralization is frequently hosted by Cretaceous black shales that appear to have undergone low grade metamorphism, and subordinate sandstones. Most parts of the deposits have been epigenetically formed, and occur as blocky shaped, discordant veins, fracture, and possibly open-space fillings, of hydrothermal origin. For this reason, structural evaluation constitutes an important criterion for the investigation of the mineralization. In particular, the orientation of fractures occurring at the Pb-Zn deposits can be useful in the differentiation of the relative ages of the mineral vein deposits. A total of one thousand, one hundred and forty-seven fractures around mineralized zones, at the study localities, were measured and analysed. Results indicate that the vein deposits are structurally controlled, have four main orientations, and most likely formed by crack-seal process. The most prominent fracture sets show NW-SE, NNE-SSW to N-S orientations, while the less prominent ones are displayed in the NE-SW and ESE-WNW directions. The NW-SE, NNE-SSW to N-S fracture sets is interpreted as ‘ac’ extension fractures from two different deformation episodes that affected the trough, while the NE-SW, ESE-WNW to E-W sets is the two ‘bc’ tensile fractures, respectively parallel to the axes of F1 and F2 folds which occur in the trough. Small variations of the NNE-SSW and ESE-WNW brought about the occurrences of N-S and E-W sets respectively. Indication are that the two deformation episodes affected the trough: the first episode produce the NE-SW (F1) fold axes, while the second less intensity episode produce the ESE-WNW (F2) folds. The most dominant mineral vein trends in the study area are the NW-SE and NNE-SSW to N-S orientations, in which mineral veins are loaded in the ‘ac’ extension fractures. The less dominant mineral vein trends are the NE-SW and ESE-WNW orientations, which are in the ‘bc’ tensile fractures. At the NW-SE trend, vein minerals mainly occur along 150-160o from N, although it ranges from 135-175o from N. Vein width up to 7.0m can be obtained from both the NW-SE and NNE-SSW directions. The Lower Cretaceous sediments appear to be characterized by the NW-SE (major) and NE-SW (minor) sets of veins, while the upper Cretaceous sequence is most likely characterized by the NNE-SSW (major) and ESE-WNW (minor) vein sets. The widest Pb-Zn vein in the study area is 11.5m wide, located in the Enyigba axes sub-area and trends in the NE-SW direction with evidence of slickensides which indicates faulting.