Epidote-amphibolite Facies Research Articles

GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)

The latest geological research carried out during seasonal work in 2017 revealed that metamorphosed layered gabbroids, which partially outcrop along the Antarctic Peninsula Coast on Cape Tuxen and Rasmussen Island, compose sheet-like intrusive body of total dimensions more than 3 km2. Significant part of the Tuxen-Rasmussen Gabbroid Intrusion (TRGI) is submerged under Waddington Bay. Main objectives of the study were the identification of geological position of the TRGI and the clarification of its geological age. In addition to the field methods of the geological research the samples collected during this and the previous years were studied using optical and electron microscopy techniques in order to identify petrographic and mineralogical features of the gabbroids. Results of the studies confirmed previous observation that TRGI was embedded in Upper Jurassic Volcanic Group (UJVG) of Antarctic Peninsula and it was responsible for contact metamorphism of the UJVG and suffered contamination by volcanic material. Gabbroids and volcanic rocks were later intruded by granites of Late Cretaceous age, which belong to the Andean Intrusive Suite (AIS) of Graham Land. Authors define geological age of TRGI as Early Cretaceous. Previous U-Pb isotope datings of the gabbroids are believed to be “rejuvenated”. It was discovered that TRGI strikes in northeastern direction and dips steep in northwestern direction. It is assumed that intrusive body continues underwater and can outcrop at the northern shore of the Waddington Bay and at Barros Rocks direction to the southwest. Petrographical researches showed that gabbroids underwent metamorphic alteration in conditions of epidote-amphibolite facies. They bear, nevertheless, relict structural-textural features and mineral associations of mafic igneous rocks. Authors drew a conclusion that geological position and petrographical peculiarities of TRGI correspond to hypabyssal level of crystallization. Primary igneous origin is proved for the microrhythmic layering of the gabbroids. Relict mineral associations allow to identify the rocks as olivine gabbro-norites. Potential Fe-Ti-V ore specialization is emphasized according to the revealed patterns of crystal fractionation and accumulation of ilmenite and magnetite.

Progressive evolution of whole‐rock composition during metamorphism revealed by multivariate statistical analyses

AbstractThe geochemical evolution of metamorphic rocks during subduction‐related metamorphism is described on the basis of multivariate statistical analyses. The studied data set comprises a series of mapped metamorphic rocks collected from the Sanbagawa metamorphic belt in central Shikoku, Japan, where metamorphic conditions range from the pumpellyite–actinolite to epidote–amphibolite facies. Recent progress in computational and information science provides a number of algorithms capable of revealing structures in large data sets. This study applies k‐means cluster analysis (KCA) and non‐negative matrix factorization (NMF) to a series of metapelites, which is the main lithotype of the Sanbagawa metamorphic belt. KCA describes the structures of the high‐dimensional data, while NMF provides end‐member decomposition which can be useful for evaluating the spatial distribution of continuous compositional trends. The analysed data set, derived from previously published work, contains 296 samples for which 14 elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Rb, Sr, Zr and Ba) have been analysed. The KCA and NMF analyses indicate five clusters and four end‐members, respectively, successfully explaining compositional variations within the data set. KCA indicates that the chemical compositions of metapelite samples from the western (Besshi) part of the sampled area differ significantly from those in the east (Asemigawa). In the west, clusters show a good correlation with the metamorphic grade. With increasing metamorphic grade, there are decreases in SiO2 and Na2O and increases in other components. However, the compositional change with metamorphic grade is less obvious in the eastern area. End‐member decomposition using NMF revealed that the evolutional change of whole‐rock composition, as correlated with metamorphic grade, approximates a stoichiometric increase of a garnet‐like component in the whole‐rock composition, possibly due to the precipitation of garnet and effusion of other components during progressive dehydration. Thermodynamic modelling of the evolution of the whole‐rock composition yielded the following results: (1) the whole‐rock composition at lower metamorphic grade favours the preferential crystallization of garnet under the conditions of the garnet zone, with biotite becoming stable together with garnet in higher‐grade rock compositions under the same P–T conditions; (2) with higher‐grade whole‐rock compositions, more H2O is retained. These results provide insight into the mechanism suppressing dehydration under high‐P metamorphic conditions. This mechanism should be considered in forward modelling of the fluid cycle in subduction zones, although such a quantitative model has yet to be developed.

The composite North American Craton, Superior Province: Deep crustal structure and mantle-plume model of Neoarchaean evolution

This paper aims to synthesize geological and geophysical data on Superior Province and, in general, Archean crust of the composite North America Craton, relying on experience with deep geological and geophysical studies of composite East European Craton. By onset of the Neoarchean, Archean provinces of the North American Craton were parts of the single stable continent. Intensely reworked in the Neoarchean, crustal rocks were combined into a round-oval domain, wherein distribution of geophysical, structural, and metamorphic parameters displays concentric zoning. The Central zone occupies Hudson Bay basin located immediately above the lithospheric keel. The Internal zone is mainly composed of granulite facies plutonic, volcanic and sedimentary rocks. The External zone encompasses southern part of the Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of southern Superior Province obtained through synthesis of geological maps with a new geological interpretation of seismic sections along LITHOPROBE Geotransects and of the eastern Abitibi Belt based on higher resolution reflection seismic images. Neoarchean evolution of the North American craton represents plate-tectonic processes initiated by a superplume. It is only one of series of similar phenomena recorded ∼2.75Ga ago in a number of regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58Ga; (2) frequently repeated combination of high-grade (granulite and high-temperature amphibolite facies) and low- or moderate-grade (greenschist and epidote–amphibolite facies) metamorphic rocks; (3) lower crustal granulite-basaltic layer (reflectivity zone displayed in seismic images of crust) formed or transformed at the final stage of endogenic activity.

Mineralogy and metabasites formation conditions from ophiolite association at northern slope of Kuznetsk Alatau

The ophiolites of the northern slope of the Kuznetsk-Alatau Ridge are the most representative fragments of the sub-oceanic crust in the region. In this article, we present the results of a study of the metabasites from ophiolite paragenesis of the Severnaya, Zelenaya, Barkhatnaya mountains and Semenovsky massif. The selected samples from the gabbroid and subvolcanic complexes were studied in microsections, and the compositions of the main rock-forming minerals (plagioclases, amphiboles) were investigated in detail on an electron microscope equipped with micro-dispersion analyzer. It was established that feldspars are represented by a wide spectrum of composition: from albite to bitovnite; not less common amphiboles are also characterized by a wide diapason: edenite, ferro-edenit, ferropargasit, magneziogastingsit, magnesian, ferrous and actinolite hornblendes. Chlorite, which observed in all samples, represents ripidolit and piknochlorite. Accessories are apatite, ilmenite, sphene and, in rare cases, rutile. The data obtained by the phase relationship, and changing chemical composition of essential minerals (feldspars, amphiboles) allow suppose several steps of the transforming investigated metamorphic rock because signs of both progressive and retrograde metamorphism are found. The behavior of accessory minerals also expresses the variability of the thermodynamic parameters of metamorphism. This is confirmed by the traditional geothermobarometry methods (using the plagioclase-amphibole mineral pair). Observed retrograde metamorphic processes reflect the movement of ophiolite mass in the upper layers of the crust, and this makes it possible to connect metamorphism with obduction of ophiolites. For most of the investigated regions the early stage of ophiolite obduction in terms of location the “Riphean” oceanic island arc can be associated with episode of epidote-amphibolite facies of regional metamorphism at (T = 500-700°C and P = 2-6 kbar). The collision of Kuznetsk-Altai island arc to the structures of the Central Asia and Siberian Craton superterrane in Late Cambrian-Ordovician was controlled by vertical movement along submeridional tectonic discontinuities and resulted the metamorphism at low temperatures (350-510°C) and high pressure (6-9 kbar). The completion of these processes is characterized by a decrease in pressure (2-6 kbar) at the same temperatures. The result of these events is the formation of serpentinite melange on the Semenovsky massif.

Cretaceous high-pressure metamorphism and low pressure overprint in the Sistan Suture Zone, eastern Iran: Additional temperature estimates for eclogites, geological significance of U-Pb zircon ages and Rb-Sr constraints on the timing of exhumation

The Sistan Suture Zone, eastern Iran, includes blocks and lenses of eclogite-, blueschist- and/or epidote amphibolite-facies rocks that provide an excellent opportunity to examine the exhumation history of oceanic HP/LT rocks and their retrograde derivatives. Zr-in-rutile thermometry of eclogites corroborates previous interpretations suggesting metamorphic temperatures of ca. 550–600°C during the HP stage in the Sistan area. Flat HREE distribution patterns and Ti-in-zircon temperatures of ca. 500–600°C document that zircon in eclogite is of metamorphic origin. REE patterns of zircon from felsic meta-igneous rocks do not allow to distinguish between a magmatic or metamorphic origin, but relatively low temperatures indicated by Ti-in-zircon thermometry (ca. 500–600°C) and the close similarity of zircon (U–Pb) and white mica (Rb–Sr, Ar–Ar) ages favor a metamorphic zircon origin. Previously published isotopic ages of the felsic rocks cannot unambiguously be linked to the eclogite- and/or blueschist-facies P–T conditions due to the absence of unequivocal mineralogical and petrological evidence. Instead, these rocks may record contemporaneous metamorphic processes that took place at a different depth within the subduction complex, or may indicate active ridge subduction and/or melt formation in the subduction zone at relatively low pressures. Biotite-based internal Rb–Sr isochrons of newly dated epidote amphibolite and biotite-albite gneisses indicate ages of ca. 74–80Ma, either dating fluid-infiltration-induced formation of biotite during relatively fast uplift, or the time of final passage through the effective biotite closure temperature. Rb–Sr ages of phengite from both an epidote amphibolite and a biotite-albite gneiss yield ages that correspond to the HP/LT stage. This outcome, combined with textural evidence for derivation from eclogitic precursors documents that white mica ages of some strongly overprinted Sistan rocks are compromised by inheritance and do not record later exhumation.

An integrated EPMA-EBSD study of metamorphic histories recorded in garnet

Growth histories recorded in garnet grains in metasedimentary rocks from the Sanbagawa belt in Japan and the Mogok belt in Myanmar were analyzed using an effective combination of electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) data. Garnet in the Sanbagawa metapelite has inner and outer zones that formed in the eclogite and epidote-amphibolite facies stages, respectively. Based on EPMA element mapping, this garnet appears to have grown as a single crystal with a temporal break in growth between the inner and outer zones that occurred during exhumation. The EBSD data, however, document that the garnet grain is composed of four domains. The misorientation angles of crystallographic orientations between the domains are as large as 59°, and domain boundaries crosscut the growth zoning and the compositional boundary between the inner and outer zones. Sets of quartz grains included in the garnets on either side of the domain boundaries sometimes share the same crystallographic orientation with misorientation angles less than 4°. The garnet grains formed via a three-step process of prograde crystallization of polycrystalline garnet during the eclogite facies stage (inner zone) → resorption around garnet rims and along domain boundaries during exhumation → crystallization of the outer zone and in the domain boundaries during the prograde epidote-amphibolite facies stage. The garnet porphyroblasts in the Mogok pelitic gneisses, which formed during prograde metamorphism to the upper amphibolite-granulite facies (0.6–1.0 GPa/780–850 °C), are now separated into segments of various sizes by mosaic or symplectite aggregates of biotite, plagioclase, and quartz or monomineralic biotite veins. The segment texture formed at about 0.3–0.4 GPa/610–650 °C or lower-grade conditions. The EBSD analysis shows that most of the segments share the same crystallographic orientation with misorientation angles less than 4° and show no evidence of deformation and/or rotation processes after segmentation. These data suggest that the Mogok sample did not experience dynamic deformation of the garnet grains after the resorption and segmentation stage and may have been exhumed under static conditions from depths of 9–12 km.

Late Devonian to early Carboniferous arc-related magmatism in the Baolidao arc, Inner Mongolia, China: Significance for southward accretion of the eastern Central Asian orogenic belt

The Central Asian orogenic belt, formed in response to consumption of the Paleo–Asian Ocean, is one of the largest and most complex accretionary collages in the world and was responsible for considerable Phanerozoic juvenile crustal growth in Central Asia. The timing of subduction-accretion processes and closure of the Paleo–Asian Ocean is controversial. The Xilingol complex, composed of deformed quartzofeldspathic rocks and lenticular or quasi-lamellar amphibolites, is located on the northern section of the eastern Central Asian orogenic belt in Inner Mongolia, China. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and geochronology of an amphibolite and an epidote amphibolite from the complex. The protolith of the amphibolite is a gabbro or gabbroic diorite with a laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb age of 382 ± 4 Ma and uniform eHf(t) values (–1.06 to +1.25). The protolith of the epidote amphibolite is a quartz diorite with a LA-ICP-MS zircon U-Pb age of 327 ± 5 Ma and uniform positive eHf(t) values (+0.78 to +4.11). The primitive magma of the Devonian gabbroic dike was generated by partial melting of a spinel lherzolite lithospheric mantle that was modified by fluids and melts from subducted slab components. A newly enriched lithospheric mantle is a possible source region for the Devonian mafic rocks. Fractionation of olivine and hornblende played a dominant role in magma differentiation with little or no crustal contamination. Amphibolite-facies metamorphism affected the Devonian gabbroic rocks at 321.6 ± 3.1 Ma, and the quartz diorite underwent epidote amphibolite-facies metamorphism at 279.4 ± 5.3 Ma, based on hornblende 40Ar/39Ar dating. The Devonian to Carboniferous intrusive rocks in the eastern Central Asian orogenic belt likely formed during the collapse of a mature arc at the southern margin of the South Mongolian microcontinent. Combining our results with previous data, we identify an initial phase of postcollisional extension (382–340 Ma) that occurred after earlier compression related to intra-oceanic subduction (484–469 Ma), ridge subduction (440–434 Ma), and arc-continent collision (427–383 Ma). We also constrain increasing extension accompanying extensive collapse of the mature arc (340–309 Ma), northward subduction of the forearc oceanic crust (322–274 Ma) coeval with development of the Hegenshan back-arc basin (354–269 Ma), and final collision (246–228 Ma). The presence of an accretionary belt along the southern margin of the South Mongolian microcontinent reflects the importance of continental growth by accretion of an arc chain during the Late Cambrian–Middle Triassic in the eastern Central Asian orogenic belt.

Eclogite varieties and petrotectonic evolution of the northern Guatemala Suture Complex

ABSTRACTField and petrologic characteristics of two new eclogite localities within the Guatemala Suture Complex (GSC) north of the Motagua Fault are presented. The Tuncaj Hill locality exposes a coherent body of retrogressed eclogite hundreds of metres long that is associated with serpentinite of the North Motagua Unit. The Tanilar River locality exposes numerous bands and lenses of eclogite hosted in sialic gneisses of the Chuacús Complex. The Tuncaj eclogite has a two-stage prograde evolution containing the peak assemblage Grt + Omp + Ttn + Czo + Zo ± Am, formed at temperatures <720°C. In contrast, eclogites of the Tanilar unit are characterized by the paragenesis Omp + Grt + Rt ± Phg ± Qtz ± Ep giving higher peak conditions of T = 720–830°C and P = 2.1–2.7 GPa, near the stability field of coesite. Previously obtained data and our thermobaric calculations suggest distinct petrotectonic evolutions for the various types of eclogites within the suture. The lawsonite eclogites south of the Motagua Fault were probably produced in a mature Farallon subduction zone during the Early Cretaceous. The northern high-pressure (HP) blocks in serpentinite mélange and coherent amphibolite bodies with eclogite relics were generated by the Early Cretaceous subduction of the proto-Caribbean lithosphere under the Great Caribbean Arc. A continental block, the North American passive margin, reached the arc’s trench in the Campanian and was subducted to ca. 80 km depth, producing the eclogites of the Chuacús Complex. As the slab was delaminated and partially exhumed, the continental Chuacús became tectonically juxtaposed with HP blocks of the proto-Caribbean that had been accreted to the Caribbean plate forming the North Motagua Unit. The juxtaposed group migrated to mid-crustal level and was contemporaneously retrogressed under epidote-amphibolite facies conditions.

Geochemical, isotopic, and geochronological evidence for subsynchronous island-arc magmatism and terrigenous sedimentation (Predivinsk terrane of the Yenisei Ridge)

In this study we present data on the geologic setting, geochemical and isotopic compositions, timing and P-T conditions of metamorphism of Neoproterozoic terrigenous metasediments, and associated island-arc metavolcanics of the Predivinsk terrane of the Yenisei Ridge. Relatively immature terrigenous rocks were eroded from a local source which is associated with island-arc magmatic complexes. The geochronological constraints indicate that the terrigenous rocks were eroded from juvenile crustal sources represented primarily by magmatic rocks, which are similar to those of the Predivinsk terrane. This is supported by a similar range of model ages, positive £Nd values of terrigenous and magmatic rocks, and correspondence between the concordant ages of detrital zircons from metasedimentary rocks (610-640 Ma) and the U-Pb ages of zircons from rhyolites (ca. 620-640 Ma) from two suites within different sequences. The P-T conditions for volcanosedimentary rocks of the Predivinsk terrane correspond to the epidote-amphibolite facies and the transition from epidote-amphibolite to amphibolite facies. The most likely age of metamorphism due to Vendian accretion/collision events is given by Ar-Ar dates of 600-610 Ma.

Reactivation and Strain Localisation in Bhavani Shear Zone, South India

Abstract: Shear markers and shear sense indicators from the Bhavani shear zone, a member of the shear system separating the northern Archaean and southern Pan African granulite terranes of the South Indian shield, with differently exhumed crustal blocks, suggest multiphase reactivation. This is revealed by textural and geochemical characteristics of mylonites which indicate an event of prograde epidote-amphibolite facies metamorphism of the previously retrograded protolith of shear zone rocks. The amphibole varies progressively from an initial magnesio-hornblende to ferrotschermakitic hornblende with increasing Al and K while the plagioclase maintains oligoclase composition (An21-25). P-T estimates suggest metamorphic conditions of 634°C - 720°C at 3.6±0.5 to 5.8±0.5 kbar. Strain partitioning during the reactivation of the shear zone prior to the Pan African event, is responsible for the gradation in the intensity of mylonitic fabric as well as some of the opposing shear sense indicators.

THE USE OF THERMAL ANALYSIS IN THE EVALUATION OF PROBABLE RESOURCES OF GOLD IN CARBONACEOUS DEPOSITS OF THE BELORETSK METAMORPHIC COMPLEX (SOUTHERN URALS)

The article briefly describes the geological structure of the zonal Beloretsk metamorphic complex; listed suites, which includes carbonaceous deposits, is given all available data sampling of carbonaceous deposits of gold. Based on the thermal method of studying the carbonaceous shales of zigazin-komarov suite established that the exothermic effect in them occurs in the temperature range 630-730 °C, which corresponds to the greenschist and epidote-amphibolite facies of metamorphism. Mapping the temperature distribution within the Beloretsk metamorphic complex, as well as the analysis of patterns of gold-ore objects within it, allowed to bind gold mineralization to a specific temperature interval. This gave the possibility with minimal cost to assess the prospects for carbonaceous deposits of the Beloretsk metamorphic complex on the gold and select a specific area on it prospecting. It is shown that the largest gold mineralizations are confined to tectonically intensively destructed, silicified and sulfidation rocks of black-shale formation, located in an external high-temperature zone of the greenschist facies of metamorphism. The established regularity is very important search value in the further prospecting work on the periphery of the Beloretsk metamorphic dome and confirms correctness of the model of metamorphic-hydrothermal gold mineralization in black-shale strata. In such geological setting was discovered Otnurok gold mineralization in which by test of carbonaceous deposits was identified a layer of ore-bearing rocks with the thickness of six meters. This gold bearing zone is traced in a north-easterly direction for several kilometres along strike in a series of high-grade metal contents, which allowed to calculate probable resources of gold on P 2 categories.

Reconstruction of the metamorphic P–T path from the garnet zoning in aluminous schists from the Tsogt Block, Mongolian Altai

The paper presents original authors’ data on aluminous schists in the Tsogt tectonic plate in the Southern Altai Metamorphic Belt. The nappe includes a medium-temperature/medium-pressure zonal metamorphic complex, whose metamorphic grade varies from the greenschist to epidote-amphibolite facies. The garnet and garnet–staurolite schists contain three garnet generations of different composition and morphology. The P–T metamorphic parameters estimated by mineralogical geothermometers and geobarometers and by numerical modeling with the PERPLEX 668 software provide evidence of two successive metamorphic episodes: high-gradient (of the andalusite–sillimanite type, geothermal gradient approximately 40–50°/km) and low-gradient (kyanite–sillimanite type, geothermal gradient approximately 27°/km). The P-T parameters of the older episode are T = 545–575°C and P = 3.1–3.7 kbar. Metamorphism during the younger episode was zonal, and its peak parameters were T = 560–565°C, P = 6.4–7.2 kbar for the garnet zone and T = 585–615°C, P = 7.1–7.8 kbar for the staurolite zone. The metamorphism evolved according to a clockwise P–T path: the pressure increased during the first episode at a practically constant temperature, and then during the second episode, the temperature increased at a nearly constant pressure. Such trends are typical of metamorphism related to collisional tectonic settings and may be explained by crustal thickening due to overthrusting. The regional crustal thickening reached at least 15–18 km.

Mesozoic age of the Gilyui Metamorphic Complex in the junction zone of the Selenga–Stanovoi and Dzhugdzhur–Stanovoi superterranes, Central Asian fold belt

The Gilyui Complex includes sedimentary and volcanic rocks metamorphosed to amphibolite and epidote–amphibolite facies, which constitute blocks confined to the main structural sutures of the Dzhugdzhur–Stanovoi superterrane in the Central Asian fold belt. In recent stratigraphic scales, they are considered as being Neoarchean in age with Nd model age values of 1.5–3.0 Ga. The youngest detrital zircons from metamorphosed mudstone of the Gilyui Complex yield a date of 285 ± 4 Ma, which determines the lower age limit for the formation of its protolith. The age of crystallization of rhyolites from the Gilyui Complex is determined to be 231 ± 4 Ma. If the rhyolites form volcanic flow units or sills, the Gilyui Complex is approximately 230 Ma or 231 ± 4 to 285 ± 4 Ma old, respectively.

Origin of Charnockites Along Suravaram and Lingagudem Villages, Khammam District, Telangana State, India

An area of about 150 sq km has been mapped during the years 2013 and 2014 on 2 cm to 1 km scale in Khammam district of Telangana State. The region lies between Long. 80° 30’ E-80° 41’ E and Lat. 17° 15’ N-17° 21’ N of Survey of India toposheet no: 65 C/11. About 200 rock specimen were collected representing the different types and out of them 100 thin sections were prepared, they have been studied for their mineralogical and textural relationships. Lithologically, the area represents an amazing diversity of regional metamorphic rocks ranging from the epidote-amphibolite facies to the granulitic facies of metamorphism.

The structure and metamorphism of the Red Point Metamorphic Complex—A newly discovered high-pressure metamorphic complex from the south coast of Tasmania

This study presents new data on the deformational and metamorphic history of previously unstudied Cambrian high-pressure metamorphic rocks exposed on the remote south coast of Tasmania. The Red Point Metamorphic Complex consists of two blocks of high-pressure, medium-grade metamorphic rocks including pelitic schist and minor garnet-bearing amphibolite, which are faulted against a sequence of low-grade phyllite and quartzite. The Red Point Metamorphic Complex records five phases of deformation, all of which except the first are expressed at a mesoscopic scale in both the medium- and low-grade rocks. Peak metamorphic conditions in the high-pressure epidote–amphibolite facies is recorded by medium-grade schist and amphibolite and was synchronous with the second major deformation event, which produced a pervasive schistosity and mesoscale isoclinal folds. The juxtaposition of the low- and medium-grade rocks is interpreted to have first occurred prior to the development of upright, opening folding associated with the third deformation. However, the present contacts between the two contrasting metamorphic sequences formed during widespread faulting and ductile-shear zone development associated with the fourth and fifth deformation events. The new data from the Red Point Metamorphic Complex provide insights into the structural and metamorphic history experienced by the medium-grade rocks of Tasmania during the Cambrian Tyennan Orogeny. This study demonstrates that Cambrian medium-grade metamorphic rocks are more widespread throughout Tasmania than previously realised, which represents an important step towards understanding the large-scale architecture of the Tyennan Orogen.

The mechanism of infiltration of metamorphic fluids recorded by hydration and carbonation of epidote-amphibolite facies metabasaltic sills in the SW Scottish Highlands

In this study we investigate a group of metabasaltic sills from the SW Scottish Highlands metamorphosed at epidote-amphibolite facies conditions that provide useful insight into the mechanisms and characteristics of fluid infiltration during metamorphism. The sills are amphibole and garnet bearing and exhibit a strong foliation in the sill margins that developed pre- to syn-peak metamorphism. Fluid infiltration caused hydration and carbonation in the sills, expressed as (1) replacement of garnet and amphibole by chlorite and calcite and (2) replacement of amphibole and epidote to form chlorite and calcite. Using garnet-amphibole and garnet-chlorite geothermometers we show that these reactions occurred after peak metamorphism at T = 290 to 400 °C. Reaction textures show that the fluid infiltration into the sill that caused hydration and carbonation occurred in the absence of deformation. The fluid infiltration was mineralogically controlled with greater fluid access in areas of abundant fine-grained elongate minerals such as amphibole and chlorite. The replacement of garnet by chlorite most likely occurred by an interface-coupled dissolution-precipitation mechanism as evidenced by perfect pseudomorphic textures of garnet, porosity generation behind the reactive interface and fracturing ahead of this interface. Porosity generated in the product chlorite enhanced fluid access to the replacement front. The study shows that deformation was not required for extensive fluid infiltration and alteration during metamorphism. Fluid flow uses a pre-existing foliation to gain access to the rock, taking advantage of the anisotropic shape of the aligned minerals.

Metamorphic Evolution of Garnet-bearing Epidote-Barroisite Schist from the Meratus Complex in South Kalimantan, Indonesia

DOI:10.17014/ijog.2.3.139-156This paper presents metamorphic evolution of metamorphic rocks from the Meratus Complex in South Kalimantan, Indonesia. Eight varieties of metamorphic rocks samples from this location, which are garnet-bearing epidote-barroisite schist, epidote-barroisite schist, glaucophane-quartz schist, garnet-muscovite schist, actinolite-talc schist, epidote schist, muscovite schist, and serpentinite, were investigated in detail its petrological and mineralogical characteristics by using polarization microscope and electron probe micro analyzer (EPMA). Furthermore, the pressure-temperature path of garnet-bearing epidote-barroisite schist was estimated by using mineral parageneses, reaction textures, and mineral chemistries to assess the metamorphic history. The primary stage of this rock might be represented by the assemblage of glaucophane + epidote + titanite ± paragonite. The assemblage yields 1.7 - 1.0 GPa in assumed temperature of 300 - 550 °C, which is interpreted as maximum pressure limit of prograde stage. The peak P-T condition estimated on the basis of the equilibrium of garnet rim, barroisite, phengite, epidote, and quartz, yields 547 - 690 °C and 1.1 - 1.5 GPa on the albite epidote amphibolite-facies that correspond to the depth of 38 - 50 km. The retrograde stage was presented by changing mineral compositions of amphiboles from the Si-rich barroisite to the actinolite, which lies near 0.5 GPa at 350 °C. It could be concluded that metamorphic rocks from the Meratus Complex experienced low-temperature and high-pressure conditions (blueschist-facies) prior to the peak metamorphism of the epidote amphibolite-facies. The subduction environments in Meratus Complex during Cretaceous should be responsible for this metamorphic condition.

The Whole‐Rock Geochemical Composition of the Wudaogou Group in Eastern Yanbian, NE China–New Clues to Its Relationship with the Gold and Tungsten Mineralization and the Evolution of the Paleo‐Asian Ocean

AbstractThe late Paleozoic Wudaogou Group, one of the oldest metamorphic units in the eastern Yanbian area, has important tectonic and metallogenic significance. Here, we provide new insights into their protoliths, tectonic setting of the metamorphic rocks and their relationships with the gold and tungsten mineralization, using new petrographic and whole‐rock geochemical data for various lithologies within the Wudaogou Group. The protolith of the metamorphic rocks of the Wudaogou Group was intermediate–basic volcanic rocks (e.g. basaltic andesite, trachyandesite, and basalt) and sedimentary rocks including argillaceous rocks, quartz sandstone, arkose and clayish greywacke, as well as pyroclastic sedimentary rock, covering tuffaceous sandstone. Before undergoing late Paleozoic epidote–amphibolite facies regional metamorphism, these protoliths were formed during the middle–late Permian in an island arc setting within a continental margin collage zone. Combined with the regional tectonic evolution, it can be speculated that the formation and the subsequent metamorphism of the protoliths of the metamorphic rocks from the Wudaogou Group were influenced by the change from subduction to collision of the Paleo‐Asian Ocean. Similarities of the rare earth element (REE) patterns and parameters among the metamorphic rocks within the Wudaogou Group, auriferous ores from the Xiaoxi'nancha gold (copper) deposit, and scheelites from the Yangjingou tungsten deposit, together with the favorable metallogenic element contents within the metamorphic rock series, imply that the Wudaogou Group could provide parts of metallic material for the gold and tungsten mineralization in the eastern Yanbian area, as exemplified by the Yangjingou deposit and Xiaoxi'nancha deposit, respectively. Further, the metamorphic sedimentary rocks, especially the metamorphic sandstones, quartz schists and quartz mica schists within the Wudaogou Group, have closer genetic relationships with the Yangjingou tungsten mineralization. However, the specific lithologies within this group which control the gold mineralization are still uncertain, and need further research.

Genesis and age of zircon from alkali and mafic rocks of the Elet’ozero Complex, North Karelia

The paper presents a study of zircon from the Middle Paleoproterozoic Elet’ozero high-Ti layered ultramafic-mafic-alkaline intrusive complex with carbonatites and Sm-Nd isotope system of rock-forming minerals in ferrogabbro sample from this complex. Zircons extracted from syenite and ferrogabbro samples have similar pattern and experienced two main independent episodes of secondary transformation. The first episode was accompanied by the partial or complete replacement of the magmatic zircon with oscillatory zoning by porous zircon with a complex irregular inner structure, while a rim of CL-homogenous zircon around primary and porous zircons was produced during the second episode. The primary magmatic zircon is characterized by the elevated HREE, lowered LREE contents and positive Ce anomaly. The porous zircon is enriched in LREE, Ca, Ti, and frequently, in Th. While both varieties are mainly depleted in U. The REE distribution pattern in a homogenous zircon shell is similar to that of the magmatic zircon, but differs in much lower REE, especially HREE and MREE contents. The porous zircon contains microinclusions of thorite, barite, yttrialite, as well as other rare-metal and rare-earth minerals (calciosamarskite, pyrochlore, and hatchettolite), which indicates it was reworked by fluids containing not only F and Th, but also Y, U, Nb, Ta, Ba, Fe, Ti, and Ca. These components were presumably released during metamorphism-related replacement of magmatic minerals by metamorphic assemblages with fairly limited isomorphic capacity. U-Pb SHRIMP-II geochronological study of zircon showed its age varies within a wide range and correlates with peculiarities of its inner structure, which is explained by a style of superimposed processes. However, age estimations, especially for the domains dispersed alongside with porous zircon or “penetration” of shell material, often have younger values. The oldest ages (2086 ± 53 and 2086 ± 30 Ma for syenites and 2070 ± 24 Ma for ferrogabbroids) are presumably closest to the crystallization age of the intrusion. According to geological data, syenites intrude gabbroids. Therefore, the age obtained for syenites (2086 ± 30 Ma) seems to be the more preferable for the real formation age of the intrusion. Age values of porous zircon also widely vary, which is explained by the presence of relics (domains) of primary zircon in a porous matrix. We suggest that the minimum obtained dates (1.8–1.9 Ga) are the closest to the crystallization age of porous zircon. This age coincides with the Svecofennian tectonometamorphic processes, which presumably spanned the rocks of the Elet’ozero Complex, caused the metamorphism of all kind of rocks under the epidote-amphibolite facies conditions and formation of porous zircon. CL-light zircon shells were presumably formed later by interaction of zircon with a crustal fluid during the Meso- and Neoproterozoic, and even Caledonian events. The age of ferrogabbroids determined by Sm-Nd isochron method (1988 ± 63 Ma) is comparable within error with results of U-Pb dating of porous zircon, thus confirming that magmatic rocks of the complex underwent intense secondary transformations during the Svecofennian orogeny. The Middle Paleoproterozoic Elet’ozero Complex is the oldest manifestation of the high-Ti moderately alkaline magmatism at the Karelian Craton.

Metamorphic history of riebeckite- and aegirine-augite-bearing high-pressure–low-temperature blocks within the Siuna Serpentinite Mélange, northeastern Nicaragua

The Siuna Serpentinite Mélange (SSM) is a subduction-zone-related complex that contains diverse blocks of igneous and sedimentary origin, overprinted by various metamorphic conditions. The SSM is located at the southern border of the Chortís block and marks the boundary between continental and oceanic crusts in the western margin of the Caribbean Plate. The serpentinite matrix mainly consists of lizardite/chrysotile, Cr-rich spinel, and relict orthopyroxene that suggest a harzburgitic protolith and an upper mantle supra-subduction zone origin. Blocks within the southern and central regions range from Jurassic pelagic sediments to mafic/intermediate igneous rocks that are metamorphosed to various degrees, ranging from prehnite-pumpellyite/greenschist to likely blueschist facies (e.g. riebeckite-bearing metashale) conditions. In contrast, the northern section encloses almost exclusively epidote-amphibolite facies metabasite blocks, and minor mica- and chlorite-rich rocks of metasomatic origin, respectively. Some of the epidote-amphibolite blocks contain relic garnet-rich zones embedded in an amphibole-rich matrix. The garnets appear to record two generations of growth and contain mineral inclusions such as amphibole, apatite, titanite, aegirine-augite, and quartz. Thermobarometric estimates for the garnet-rich zones and epidote-amphibolite-rich matrix suggest a prograde blueschist facies at ~1.2 GPa and 400–450°C, an eclogite facies metamorphic peak at 1.5–1.7 GPa and 565–614°C, and a post-peak epidote-amphibolite facies metamorphism. These pressure and temperature estimates indicate a classical clockwise PT path that has been observed in many palaeo-subduction zone environments worldwide. Phengite Ar–Ar dating of mica-rich rock yields 140 Ma and suggests an Early Cretaceous exhumation along the southern edge of the continental Chortís block.