Garnet Biotite Research Articles

New metamorphic constraints on the Nova-Bollinger Ni–Cu deposit, Fraser Zone, Western Australia

Metamorphic investigations were conducted using mineral chemistry, phase equilibria modelling and conventional thermobarometry on four metamorphic country rock samples from the Nova-Bollinger Ni–Cu deposit, Western Australia. New P–T constraints obtained from phase equilibria modelling of garnet-bearing granulites and cordierite–sillimanite-bearing metasedimentary rocks indicate that the Nova-Bollinger deposit formed at mid-crustal depths (0.75–0.76 GPa and 880–920 °C), along high thermal gradients around 1200 °C/GPa. Peak metamorphic conditions are consistent with those obtained from the Nova-Bollinger intrusive rocks, and with metagabbros from the southern Fraser Zone, but are slightly elevated relative to metapelites examined in the southern Fraser Zone. The peak metamorphic conditions at Nova-Bollinger reflect the combined effects of regional-scale high-T conditions during orogenesis superimposed by contact metamorphism in a thermal aureole adjacent to the Nova-Bollinger mafic–ultramafic magmas. Thermal metamorphism may have been further enhanced by local heat transfer from intruding sulfide liquid, which pervasively infiltrated country rocks on scales of tens to hundreds of metres beneath the orebodies. This additional heat source may be locally significant, despite being volumetrically limited. Retrograde P–T conditions were determined by garnet–biotite conventional thermobarometry of garnet-core–matrix biotite pairs (0.29–0.45 GPa and 550–640 °C). These P–T conditions suggest that the Nova-Bollinger country rocks cooled along a near-isobaric cooling path and reflect the effects of thermal decay following emplacement of voluminous magmas into the Fraser Zone during Stage I Albany–Fraser Orogeny.

Timing and Evolution of Gold Mineralization in the Maljavr Occurrence (NW Russia), NE Part of the Fennoscandian Shield

Gold occurrence Maljavr is the first Archean conglomerate-hosted gold mineralization found in the Fennoscandian Shield. Gold-mineralized metasomatic rocks form a set of lenses within a 10 m thick linear zone, conformable to the bedding of host conglomerates. The lenses are up to 10 m long and up to 1 m thick and they clearly exhibit three alteration envelopes: the rock in the central part consists of garnet and quartz or garnet-only; biotite, garnet, and quartz make the intermediate biotite–garnet envelope; hornblende, hedenbergite, and quartz are the principal rock-forming minerals in the outer zone of the lenses. All metasomatic rocks contain sulfide mineralization up to 15–20 vol.% and up to 0.6 g/t Au. The main ore mineral is pyrrhotite, and the minor minerals are arsenopyrite, chalcopyrite, pentlandite, löllingite, and troilite. The age of zircon from biotite gneiss in the zone of alteration is 2664 ± 18 Ma, this is considered as the time of formation of lenses of metasomatic rocks. Biotite gneiss-conglomerate and metasomatic rocks were later intruded by tourmaline granite pegmatite 2508 ± 7 Ma. The injection of pegmatite caused re-crystallization of sulfides (mainly arsenopyrite and löllingite) and redistribution of gold. Visible gold in association with Bi minerals native bismuth, ehrigite, maldonite, bismuthinite, joseite-B, and hedleyite was found in inclusions in recrystallized arsenopyrite and löllingite. Au content in the rocks with recrystallized arsenopyrite and löllingite is >1 g/t, up to 30 g/t in hand samples. The 2508 Ma pegmatite is interpreted as synchronous with formation of gold mineralization in its present form. The linkage of gold mineralization with pegmatite and geochemical association Au-As-Se-Te-Bi in the mineralized rocks agree with characteristics of intrusion-related gold deposits worldwide. Biotite gneiss–metaconglomerate, hosting the mineralized altered rocks, was the probable primary source of arsenic and gold for mineralization.

Granulites of the Larba Block of the Dzhugdzhur–Stanovoi Superterrane: Reconstruction of the Formation Conditions

Abstract —We discuss the rocks of the Larba granulite block in the Ilikan zone of the Dzhugdzhur–Stanovoi superterrane. The Larba block is dominated by basic schists and garnet–biotite–orthopyroxene and garnet–biotite–cordierite–sillimanite gneisses (metabasites and metapelites). Calculation of temperatures and pressures of mineral formation was carried out by multi-equilibrium geothermobarometry, which makes it possible to evaluate the degree of equilibrium of mineral compositions along with P–T parameters. The P–T estimates have shown metamorphism of aluminous gneisses under moderate-pressure granulite facies conditions (7–8 kbar, 800–850 °C). Orthopyroxene granulites formed under granulite-amphibolite transition facies conditions. The mineral compositions and parageneses in highly ferrous metabasites permitted estimation of the conditions of metamorphism, P = 4–5 kbar and T = 630–700 °C, and show no influence of earlier granulite facies metamorphism. The bimodal P–T distribution for most samples of aluminous gneisses most probably reflects progressive and near-peak conditions of granulite metamorphism. The time of enderbite magmatism is determined from the upper intercept of discordia with concordia at 2546 ± 52 Ma and should be verified. The age of metamorphic rims over enderbite zircon is 1882 ± 11 Ma. The model Nd age of enderbites, TNd(DM) = 2.57–2.58 Ga, is close to the age of the core of enderbite zircon and differs significantly from the model Nd age of the host metamorphic rocks (2.8–3.0 Ga). The Paleoproterozoic metamorphism of the Larba block rocks regionally coincides with the third stage of collisional granitoid magmatism of the southeastern Siberian craton and records the formation of this structure.

Garnet-Staurolite-Mica Schist from Southern Sonbhadra District, U.P.: Constraints from Geothermobarometry and P-T Pseudosection Modelling

Abstract The area around Renukoot town shows the exposure of garnetstaurolite- mica schist belonging to the Chhotanagpur Granite Gneiss Complex (CGGC). The characteristic mineral assemblage present in the rock are garnet–biotite–staurolite–quartz along with minor opaque minerals like rutile, ilmenite and hematite. For the constituent minerals, the calculated XMg value decreases in the order: chlorite>biotite>staurolite>garnet. The metamorphic conditions of the garnet-staurolite-mica schist were determined using the winTWQ and Perple_X programmes, and the peak pressure-temperature conditions for the rock are estimated to be 5.3 kbar and 570°C. The metamorphic episode in the CGGC during Mesoproterozoic time may be attributed to the global level Columbian Super-continental accretionary orogeny.

P–T conditions of metamorphic and hydrothermal events at Tick Hill gold deposit, Mount Isa, Queensland, Australia

Tick Hill gold deposit is a unique gold mineralisation style in the Mary Kathleen Fold Belt in the Mount Isa Inlier. The gold at Tick Hill is generally pure without silver and was formed during two discrete metamorphic-deformation events (D1 and D3). Early gold was observed as inclusions or coarse grains hosted within D1 peak-metamorphic diopside, scapolite and hornblende from the garnet–biotite–hornblende (tschemakite)–plagioclase (andesine)–quartz assemblage. Late gold is closely associated with bismuth selenide, chlorite, albite, sericite and K-feldspar, and formed during D3. The syn-D1 garnet–plagioclase–hornblende–quartz–biotite assemblage was used to constrain the pressure and temperature (P–T) conditions of metamorphism and mineralisation using the garnet–plagioclase–hornblende–quartz barometer and the hornblende–plagioclase, garnet–biotite and garnet–hornblende thermometers. The results show that peak metamorphism at Tick Hill reached P–T conditions of 6.0–7.6 kbar and 720–760 °C. These P–T conditions together with gneissic and migmatisation textures recorded in different rock types at the Tick Hill area indicate that the peak metamorphism preserved in the area occurred at the amphibolite–granulite facies, compared with the amphibolite facies at the southern Mary Kathleen and Eastern Fold Belt. The D3 chlorite, which formed during stage 2 to stage 4 mineralisation events, displays a wide range of compositions reflecting a gradual retrograde temperature change from ∼380 °C to ∼130 °C. The pressures during D3 could not be reliably determined, but the presence of various Bi-selenides suggests that towards the waning stages of D3, the rocks may have been exhumed to a pressure less than 1 kbar.

Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif, Austria).

Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet-biotite-sillimanite-plagioclase-K-feldspar-quartz-granitic melt ±kyanite ±ilmenite ±rutile, recording peak metamorphic conditions of 0.9-1.1 GPa and 780-820°Cestimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite-spinel and cordierite-quartz reaction textures at former garnet-sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching 0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite-spinel and cordierite-quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4-0.5 GPa and 750-800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP-UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at 340Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of 335Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.

Ti‐in‐zircon and Ti‐in‐quartz thermometry using electron probe microanalysis: A promising approach in retrieving thermal conditions of granulite facies metamorphism and felsic magmatism in the Sandmata Complex, Aravalli Craton (NW India)

The titanium (Ti) concentrations in zircon and quartz and corresponding thermometers have been regarded as powerful monitors that constrain petrogenetically meaningful temperature conditions of crystallization and metamorphism. The precise measurement of trace elements by electron probe microanalyzer (EPMA) is quite challenging as their x‐ray intensities are only slightly higher than the Bremsstrahlung background, leading to significant errors while quantifying the elemental abundances (e.g., Ti‐in‐zircon). We present EPMA‐based protocols for analysing Ti concentrations in zircon and quartz by considering recent analytical and instrumental improvements. High counting times at peak and background positions, simultaneous acquisition of Ti in multiple (three) spectrometers and data processing using the non‐central chi2 test for risk determination in sub‐counting method were employed in Ti‐in‐zircon and Ti‐in‐quartz protocols. Applying these protocols to the Mongolian garnet, San Carlos olivine and NIST 610 glass standards yielded Ti concentrations of 0.60 ± 0.02 wt%, 18 ± 3 ppm and 437 ± 26 ppm (in ±2σ), respectively. The Ti concentrations obtained from these standards are consistent with those published based on high‐precision analytical methods. We use the existing Ti‐in‐zircon and Ti‐in‐quartz thermometers to understand the thermal evolution of various lithologies in the Sandmata Complex, Aravalli Craton (north‐western India). The investigated samples include garnet–biotite gneiss, migmatite gneiss, mafic granulite and Anjana granite. The Ti‐in‐zircon temperatures calculated from the patchy‐zoned zircon cores yield HT–UHT conditions (892–959°C) for garnetiferous granite gneiss, migmatite gneiss and mafic granulite, whereas the zircon overgrowths yield temperatures of 725–871°C. The homogeneous zircon cores from Anjana granite yield temperatures (914–984°C) comparable to zirconium saturation conditions (920–960°C), indicating zircon crystallization from “hot” granitic melt. The oscillatory overgrowths on the zircon core obtain variable temperature conditions for inner overgrowth (811–877°C) and outer overgrowth (714–782°C), suggesting the episodic growth of zircon grains from different magma pulses. Additionally, the application of pressure‐dependent Ti‐in‐quartz thermometry yields quartz recrystallization conditions for garnet–biotite gneiss (430–556°C), migmatite gneiss (423–593°C), mafic granulite (430–679°C) and Anjana Granite (444–533°C). The combination of Ti‐in‐zircon thermometry, Ti‐in‐quartz and conventional thermobarometry demonstrates cooling histories for various high‐grade metamorphic and magmatic rocks in the Sandmata Complex. Based on this study, we emphasize that trace element thermometers can decipher high‐temperature metamorphism, which is otherwise erased in conventional thermometers due to diffusional readjustments during long‐lasting near‐peak and post‐peak metamorphism.

P–T–t Path of Unusual Garnet–Kyanite–Staurolite– Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology

Abstract Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.

Detailed Chemical Analysis of Unusually Weathered Rocks

Objectives: This study aimed to conduct a detailed chemical analysis of the Samanalawewa dam to provide information on its weathering conditions. Method: The mineralogical composition of the dam was analyzed by powder Xray diffraction (PXRD), atomic absorption spectroscopy (AAS), gravimetric, and X-ray fluorescence (XRF) analysis in order to identify chemical changes that have taken place. Findings: Widespread discoloured (brick red) and crushable areas in the dam and the quarry site indicate weathering of the compositional rocks. Each weathered rock sample showed a similar compositional pattern to each other as well as charnockite (CHA) rather than the other structural rocks of the dam; biotite gneiss (BIO), garnet biotite sillimanite gneiss (GAR), and limestone (LIM) indicating that only CHA has been weathered rapidly. Since LIM is not affected and the contact water (pH; reservoir 6.79 - 7.12 and holes in the quarry site 6.43-6.78) is almost neutral, this rapid weathering has not been governed by general chemical weathering processes like acid rains or stormwater. The relative contents of aluminium and magnesium have decreased during the weathering process while increasing the relative content of iron as well as sodium, which is a mobile element. The potassium content also has not decreased significantly. Moreover, chemical weathering indices like chemical index of alteration (CIA) and chemical index of weathering (CIW) values for completely weathered rocks were less than 50 revealing their unsuitability to assess the degree of weathering of the Samanalawewa dam. Novelty: Charnockite in the Samanalawewa dam has been weathered rapidly and chemical weathering indices; CIA and CIW are not valid to assess its degree of weathering. Keywords: Metamorphic; Charnockite; Rock weathering; Rock composition; Chemical weathering indices

Magnetite as a provenance and exploration tool for metamorphosed base-metal sulfide deposits in the Stollberg ore field, Bergslagen, Sweden

AbstractMagnetite is a common mineral in the Paleoproterozoic Stollberg Zn–Pb–Ag plus magnetite ore field (~6.6 Mt of production), which occurs in 1.9 Ga metamorphosed felsic and mafic rocks. Mineralisation at Stollberg consists of magnetite bodies and massive to semi-massive sphalerite–galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite and magnetite) hosted by metavolcanic rocks and skarn. Magnetite occurs in sulfides, skarn, amphibolite and altered metamorphosed rhyolitic ash–siltstone that consists of garnet–biotite, quartz–garnet–pyroxene, gedrite–albite, and sericitic rocks. Magnetite probably formed from hydrothermal ore-bearing fluids (~250–400°C) that replaced limestone and rhyolitic ash–siltstone, and subsequently recrystallised during metamorphism. The composition of magnetite from these rock types was measured using electron microprobe analysis and LA–ICP–MS. Utilisation of discrimination plots (Ca+Al+Mn vs. Ti+V, Ni/(Cr+Mn) vs. Ti+V, and trace-element variation diagrams (median concentration of Mg, Al, Ti, V, Co, Mn, Zn and Ga) suggest that the composition of magnetite in sulfides from the Stollberg ore field more closely resembles that from skarns found elsewhere rather than previously published compositions of magnetite in metamorphosed volcanogenic massive sulfide deposits. Although the variation diagrams show that magnetite compositions from various rock types have similar patterns, principal component analyses and element–element variation diagrams indicate that its composition from the same rock type in different sulfide deposits can be distinguished. This suggests that bulk-rock composition also has a strong influence on magnetite composition. Principal component analyses also show that magnetite in sulfides has a distinctive compositional signature which allows it to be a prospective pathfinder mineral for sulfide deposits in the Stollberg ore field.

Assessment of Natural Radionuclide Distribution Pattern and Radiological Risk from Rocks in Precambrian Oban Massif, Southeastern Nigeria

A gamma spectrometric analysis of rock samples collected from the Precambrian Oban Massif, southeastern Nigeria, was performed to determine some primordial radioelements’ activity concentrations: U-238, Th-232, and K-40, and their associated radiological hazards. The mean activity of the primordial radionuclides was determined to be 1073.06 ± 0.65, 160.74 ± 1.32, and 250.76 ± 0.91 Bq·kg−1 for 40K, 238U, and 232Th, respectively, showing that they are inhomogeneously distributed, with activity changing with the rock type and location. The activity concentrations are highest in biotite garnet schist, quartz diorite, and biotite gneiss rock domains. The mean values of the radiological hazards are 0.08 Bq·kg−1 (alpha index), 2.15 Bq·kg−1(gamma index), 2.06 Bq·kg−1(internal) and 1.63 Bq·kg−1 (external4.30 Bq·kg−1(representative level index6), 602.23 Bq·kg−1, (radium equivalent), 780 nGy·h−1 (total absorbed dose rate), 270.91 nGy·h−1 (indoor), 509.78 nGy·h−1 (outdoor), 624.99, 1329.07 and 1954.06 mSv·yr−1 (outdoor, indoor and total annual effective dose equivalent, respectively), 6448.40 (cumulative excess lifetime cancer risk) and 248.94–3761.47 Bq·kg−1 (annual gonadal dose equivalent). These results are higher than their various permissible limits (except at Ako Community) and suggest that rocks in the area may be unsuitable for constructing dwelling places. It is strongly advised that basic safety standards and precautionary measures recommended by the European Commission should be strictly adhered to while dealing with these rocks.

Evidence of Eoarchean crust beneath the Yakutian kimberlite province in the Siberian craton

The Siberian craton is one of the largest Precambrian cratons in the world. However, unlike other cratons, the Siberian craton can only provide a relatively limited amount of geochronological data since a large part of it is covered by younger sedimentary deposits. Xenoliths from numerous kimberlite and lamprophyre pipes and alkali basalts allow us to investigate the composition and evolution of the Siberian Craton crust. The available isotopic and geochemical data show that the crust of the Anabar tectonic province of the Siberian craton is not age stratified, and consists of variably reworked Paleoarchean rocks and juvenile Proterozoic rocks at all crustal levels. Hence, a detailed geochronological study of crustal xenoliths (mainly zircon U-Pb age and Hf isotopic composition) in the kimberlite pipes of the Yakutian kimberlite province is required. In this study, we present U-Pb ages and Hf isotope data of zircons derived from crustal xenoliths in the Zarnitsa (Daldyn kimberlite field) and the Nyurbinskaya (Nakyn kimberlite field) kimberlite pipes. The obtained data provided grounds for identifying four main tectono-thermal events during the evolution of the Archean crust in the Anabar tectonic province of the Siberian craton. The first event at 3.2 Ga denotes the beginning of zircon crystallisation. These zircons are eventually recrystallised during the second event at 2.9 Ga, and radiogenic Hf (formed from 176Lu decay in the host rock) was introduced into the system. The third event at 2.8–2.7 Ga featured further recrystallisation of zircons and the incorporation of additional radiogenic 176Hf. The minor addition of radiogenic Hf during the final event at 1.9 Ga indicates a relatively large range for the initial 176Hf/177Hf ratio. Our data indicate that Eoarchean or even Hadean crust, which were reworked in the Paleoproterozoic, may be present beneath the Daldyn and Nakyn kimberlite fields. Zircon grains with low 176Hf/177Hf (0.28047–0.280661) were found in the xenoliths of the mafic garnet granulite (Zar-1–02) and garnet biotite gneisses (Nrb-13 and Nrb-65).

Metamorphic evolution of graphite in the Paleoproterozoic Savo Schist Belt (SSB), Central Finland: Constraints from geothermetric modeling

Flake graphite deposits of the Savo Schist Belt (SSB), Central Finland are mainly hosted by quartz-mica schist, hornblende biotite gneiss, and to a minor extent in amphibolite. These rocks represent high-grade metamorphic conditions of the SSB. Raman spectroscopy measurements on graphite flakes provide quantitative information on the degree of graphitization, which can be correlated to the grade of metamorphism. The metamorphic conditions of the graphite-bearing metapelites have been determined by P-T pseudosection modelling and three types of geothermometers: garnet-biotite (GB), Ti-in biotite and Raman spectroscopy of carbonaceous material (RSCM) geothermometry. The calculated P-T pseudosections indicate peak P-T conditions of ca. 5–6 kbar and 650 °C for two garnet-bearing hornblende biotite gneiss samples, and slightly higher conditions of ca. 5–6 kbar and 700 °C for a quartz mica schist sample. The GB thermobarometer yielded a temperature and pressure range of 600–725 °C and 5.2–8.0 kbar. The Ti-in-biotite applied to the same samples produced temperature ranging between 570 and 735 °C, and the Raman (RSCM) geothermometers indicated temperatures between 550 and 670 °C. For the garnet-bearing hornblende biotite gneiss samples there is no significant (<10–50 °C) difference between temperatures determined using the P-T pseudosections and mean temperatures calculated using RSCM, GB and Ti-in-biotite methods. Although the peak temperature in some highly crystalline graphite flakes of studied samples calculated by (RSCM) geothermometer yield temperatures of 641 °C with the Beyssac et al. 2002 method and 737 °C with the Rahl et al. (2005) method, which are the highest temperature for this calibration. Raman spectroscopy is now part of a wide selection of tools that can be used to identify the ordering of the graphite crystallinity, which is including the number of layers, quality of layers, doping level and confinement in graphene structures. The results of this work show that Raman spectroscopy is a simple and fast way to determine the peak metamorphic temperature of carbonaceous material CM in amphibolite facies metasedimentary rocks.

Zircon U–Pb Dating and Lu-Hf Isotope of the Retrograded Eclogite from Chicheng, Northern Hebei Province, China

We report zircon U–Pb ages and Lu-Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu-Hf isotopic of εHf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448 Ma), Late Paleoproterozoic (1900–1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495–234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5–325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.

Late Permian medium-pressure metamorphism in the eastern Songnen Massif, eastern Central Asian Orogenic Belt (NE China): Implications for the final closure of the Paleo-Asian Ocean

The nature of late Paleozoic metamorphism in the eastern Central Asian Orogenic Belt (CAOB) is controversial. This paper reports mineral chemistry, petrological, and zircon and titanite U–Pb age data for representative rocks in the Dongfengshan Group from the eastern margin of the Songnen Massif. We use these data to determine the rock associations, P–T conditions, and timing of metamorphism. The P–T conditions were estimated from 54 mineral pairs in representative rocks, including garnet–biotite schist, garnet–two-mica schist, garnet–biotite–plagioclase gneiss, and garnet amphibolite, and indicate the Dongfengshan Group experienced metamorphism at P = 5.5–7.1 kbar and T = 592–630 °C (i.e., a geothermal gradient ranging from 25 to 30 °C/km) under medium-pressure lower amphibolite-facies conditions. The biotite–plagioclase gneiss and garnet amphibolite yielded titanite U–Pb ages of 251 and 260 Ma, respectively, which are interpreted to represent the regional metamorphic age (260–251 Ma). The consistency of these dates and ages of metamorphic rims in zircons from a biotite plagiogneiss (252 Ma) confirm that medium-pressure metamorphism occurred in the late Permian rather than the Neoproterozoic, as assumed previously. Based on metamorphic, magmatic, stratigraphic, and paleontological constraints, as well as the regional tectonic history, we propose that late Permian medium-pressure metamorphism in the Dongfengshan Group occurred during the collision and amalgamation of the Siberian and North China cratons following the final closure of the eastern Paleo-Asian Ocean.

A new activity model for Fe\u2013Mg\u2013Al biotites: II\u2014Applications in the K2O\u2013FeO\u2013MgO\u2013Al2O3\u2013SiO2\u2013H2O (KFMASH) system

The new biotite activity model and standard-state thermodynamic properties of Ann, Phl, and Eas presented in part-I were used to make pseudosections of bulk compositions representing experimental Fe–Mg exchange equilibria and (model) pelitic bulk rock compositions in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH), using mainly the software Perple_X. These pseudosection calculations (termed ‘our calculation(s)’ in the following) were compared to analogous ones performed with the solution model of biotite and thermodynamic data cited in White et al. (J Metamorph Geol 32:261–286, 2014, 10.1111/jmg.12071), termed ‘W14 calculation’. Our calculations with the experimental bulk composition used by Zhou (Ti–Mg–Fe biotites: formation, substitution, and thermodynamic properties at 650 to 900 °C and 1.1 Kb with fO2 defined by the CH4–graphite buffer. PhD thesis, State University of New York, 1994) in his experimental study of the Fe–Mg exchange between biotite (Bt) and olivine (Ol) confirm that biotite had no or only minimal octahedral Al (AlVI) in these experiments. The experimental data of Ferry and Spear (—FS78, Contrib Mineral Petrol 66:113–117, 1978, 10.1007/BF00372150) on the Fe–Mg distribution between biotite and garnet (Grt) are well reproduced by our calculations. The computed composition of biotite (XFe) in equilibrium with garnet of Alm90Py10 composition and the resulting lnKD values as a function of temperature are in good agreement with the experimental brackets. An analogous W14 calculation on the same Fe-rich bulk composition predicts too high XFeBt in order of 0.1 mol fraction. The AlVI contents of biotite of about 0.3–0.45 apfu, as measured by Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218) in similar biotite–garnet exchange experiments performed with Alm80Py20 and Alm70Py30 garnets, are well reproduced by our, as well as by W14 calculations. The extent of Tschermak substitution in biotite in the FS78 experiments, which had Fe-richer bulk compositions, has not been measured. Comparing the FS78 biotites with the ones from Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218), it is very likely that the biotites reported in FS78 contained AlVI in the same order of ca. 0.3–0.4 apfu. A T–XFe (= molar FeO/(FeO + MgO) pseudosection demonstrates the bulk composition dependence of lnKD of the Mg/FeGrt/Bt exchange reaction in high-variance fields. Further comparisons, demonstrating the application of the new biotite solution model in the KFMASH system, are presented in pseudosections constructed for an average model pelite, as well as for a natural high-T/low-P and a natural high-P metapelite. The pseudosections show that biotite according to our biotite model breaks down at lower temperatures and pressures than predicted from the W14 biotite model in the KFMASH system. This means that KFMASH biotite can break down before the wet solidus is reached, which can explain the existence of dry high-T/low-P metapelites. At higher pressures, biotite according to our calculations breaks down at lower pressures than computed with the W14 biotite model. Before biotite breaks down, however, its AlVI content based on our calculations could potentially be used for pseudosection barometry, similarly as the Si-in-phengite barometer. These trends need to be confirmed by a future extension of our model which incorporates Ti, Fe3+ and a di–tri-octahedral substitution.

Zircon and monazite dating of pelitic high-pressure granulite in the Eastern Himalayan Syntaxis and geological significance

东喜马拉雅构造结的南迦巴瓦杂岩含有广泛分布的高压麻粒岩，但由于以前获得了许多不同的年龄，对这些麻粒岩的变质与深熔时代、持续时间和成因存在不同认识。本文对泥质高压麻粒岩（蓝晶石榴黑云片岩）中的锆石和独居石进行了系统的内部结构、U-（Th）-Pb定年和微量元素分析，以求揭示这些岩石是否具有相同的演化过程。所研究的6个蓝晶石榴黑云片岩由石榴石、蓝晶石、黑云母、石英、钾长石、斜长石、夕线石、白云母、石墨和副矿物金红石、钛铁矿、锆石和独居石组成，峰期矿物组合是石榴石+蓝晶石+斜长石+钾长石+黑云母+石英+金红石。6个样品中的锆石均由继承碎屑核+变质（深熔）幔+变质（深熔）边组成。其中3个样品中的锆石幔和边较宽，均可进行原位定年，幔部给出了类似的较老年龄范围（39.6~31.6Ma、40.8~32.0Ma和38.1~31.3Ma），而边部给出了类似的较年轻年龄范围（26.8~17.3Ma、28.3~18.6Ma和28.4~18.8Ma）。另外3个样品的锆石幔部较窄，不能进行分析，其边部给出了与前3个样品锆石边部类似的年轻年龄范围（22.0~17.0Ma、20.9~16.9Ma和22.2~16.6Ma）。一个片岩样品中的独居石给出了与其锆石幔部+边部年龄类似的较宽年龄范围（38.1~17.5Ma），而另外3个样品中的独居石获得了与其锆石边部年龄相似的年轻年龄范围（26.0~18.8Ma、22.3~16.9Ma和26.4~19.4Ma）。随着年龄的减小，锆石和独居石的Th/U比值增大，Eu/Eu^*减小，独居石的HREE和Y含量减小。基于这些分析结果，笔者认为所研究的6个片岩记录了相同的、从~41Ma持续到~17Ma的进变质与深熔过程。但是，由于某些样品中的锆石和独居石在早期变质和深熔过程中形成的结晶域（锆石幔部）很窄，无法定年，导致不同的样品获得了不同的年龄范围。结合现有研究成果，笔者推测南迦巴瓦杂岩中的高压麻粒岩经历了相似的长期进变质与深熔过程。;The Namche Barwa complex in the Eastern Himalayan Syntaxis contains widely distributed high-pressure granulites. However, due to many different ages previously obtained, there are different understandings on the metamorphic and anatectic age, duration and genesis of these granulites. In this paper, zircon and monazite in high-pressure pelitic granulite (kyanite garnet biotite schist) samples are systematically analyzed by internal structure, U-(Th)-Pb isotope and trace element, in order to reveal whether these rocks have the same evolution process. The six schists studied are composed of garnet, kyanite, biotite, K-feldspar, plagioclase, quartz, sillimanite, muscovite, graphite and accessory minerals rutile, ilmenite, zircon and monazite. The peak mineral assemblages are garnet+kyanite+plagioclase+K-feldspar+biotite+quartz+rutile. The zircons in the six samples are composed of inherited detrital core+metamorphic (anatectic) mantle+metamorphic (anatectic) rim. The zircon mantle and rim in three samples are sufficiently wide and can be dated in situ. The zircon mantles give a similar older age range of 39.6~31.6Ma, 40.8~32.0Ma and 38.1~31.3Ma, respectively, while the rims give similar younger age ranges of 26.8~17.3Ma, 28.3~18.6Ma and 28.4~18.8Ma, respectively. The zircon mantles of the other three samples are narrow and cannot be analyzed, while the zircon rims give young age ranges of 22.0~17.0Ma, 20.9~16.9Ma and 22.2~16.6Ma, respectively. Monazite in one schist sample gives a wide age range of 38.1~17.5Ma, which is similar to that obtained by zircon mantle+rim from the same sample. Monazite in the other three samples obtains young age ranges of 26.0~18.8Ma, 22.3~16.9Ma and 26.4~19.4Ma, respectively, which are similar to those given by the zircon rims from the same samples. With the decrease of age, Th/U ratio of zircon and monazite increases, Eu/Eu^* of zircon and monazite decreases, HREE and Y of monazite decrease. Based on these analytical results, we suggest that the six schists recorded the same and long-lasting prograde metamorphic and anatectic process from ca.41Ma to ca.17Ma. However, the domains of zircon and monazite in some samples formed during early metamorphism and anatexis are too narrow to date via the in-situ analysis method adopted by this study. Combined with the existing research results, we speculate that the high-pressure granulites in the Namche Barwa complex have experienced similar prolonged prograde metamorphism and anatexis.

Geochemical characteristics of Precambrian Basement rocks from southeastern Nigeria: an approach to their petrogenesis and tectonic setting.

Trace and Rare-Earth element geochemical study of twenty samples of migmatitic banded gneisses, garnet biotite schists, dolerites, granites and rhyolites was carried out in a bid to determine their petrogenetic and tectonic significance in the evolution of the southeastern Basement complex of Nigeria. The data shows that partial melting (crustal anatexis) of migmatitic gneisses and schists played a significant role in the evolution of the granitic intrusions. This is supported by the high incompatible (Rb/Sr = 0.16 to 1.31 and Ba/Sr = 0.75 to 6.21) elements ratio in the granitic intrusions than that of the migmatitic gneisses and schists (Rb/Sr, 0.051 to 0.824; Ba/Sr, 0.7 to 5). High ratios of Ba/Sr and Rb/Sr and lesser values of Ba/Rb ratios in some granitic intrusions than in others suggests increasing fractionation during the anatexis. The role of partial melting is also evident in the smooth REE patterns shown by most of these rocks and the negative Eu anomaly as indicated by the values of Eu/Eu* (0.097 to 0.7). LREE enrichment is evident in the high values of Ce/YbN (12.08-174.5), La/YbN (15.2-228.4) and La/SmN (2.6-7.2) in the granitic intrusions. Tectonic discrimination diagrams of the rocks indicate that the basement rocks were most probably formed in a post-collision orogenic setting while the dolerite and the rhyolite were formed in within-plate anorogenic setting.

LA–ICP–MS U–Pb DATES FOR DETRITAL ZIRCON FROM QUARTZITES OF THE TEMRYUK FORMATION (NOVOUKRAINKA IRON DEPOSIT OF THE KORSAK BLOCK, WEST AZOV AREA)

The West Azov iron district corresponds to the Korsak synclinorium or block in which iron deposits of the Kamyana Mohyla belt (Pavlivka, Novoukrainka, and Serhiivka areas), Kuksungur, and Korsak Mohyla are located. The iron– bearing units are confined to the metamorphosed volcanogenic–sedimentary rocks of the Satchky Formation of the Central Azov Group, while rocks of the Temryuk Formation form the base of the section. The group is intruded by numerous dykes of pink microcline granites and pegmatites up to 15 to 20 m in thickness. The basement for the group consists of Archean plagioclase granitoids hosting xenoliths of gneisses, crystalline schists, and amphibolites. The iron ores of the Korsak block are characterized by the same rock association and mineral assemblage as those in the Mangush and Orikhovo–Pavlohrad structures. We have conducted geochronological study of metaterrigenous rocks of the Temryuk Formation of the Novoukrainka (Kamyana Mohyla) iron deposit. The iron deposit extends along the western limb of the Korsak synclinorium for ca. 20 km. The prevailing strike of the iron deposit is NW 350°. Metamorphic rocks form separate bodies (lenses) that are continuous along the strike for 1200–1500 m with a maximum thickness of 120– 140 m. Ferruginous quartzites are represented by banded iron formation with interlayers of biotite and biotite–amphibole gneisses (Sachky Formation). Micaceous (biotite, garnet–biotite) quartzites occur in the lower part of the section, where they are interbedded with biotite, muscovite–biotite–sillimanite, biotite–amphibole, and, rarely, pyroxene–garnet–biotite gneisses and schists (Temryuk Formation). Fifty–eight grains of detrital zircon from micaceous quartzite of the Temryuk Formation were dated by LA–ICP–MS. Four crystals were dated at 3.2–3.23 Ga, three more grains have an age of 3.08– 3.11 Ga, thirty–two crystals yielded an age of 2.9–2.97 Ga, fifteen crystals had an age of 2.82–2.87 Ga, and, finally, four grains were dated at 2.76–2.78 Ga. According to the obtained data, metasedimentary rocks of the Central Azov Group, bearing iron–rich units in the Korsak synclinorium of the West Azov area were deposited on the basement with the Mesoarchean (2.9–3.1 Ga) Tonalites–Trondjemites–Granites (TTG). Archean granites with ages of 2.82–2.87 and 2.76– 2.78 Ga as well as the remnants of older TTGs provided a minor contribution from the provenance. The Central Azov Group was intruded by the 2.2 Ga Na–K–rich granites. On the Mesoarchean Azov craton, volcanogenic–sedimentary rocks of the Huliaipole Formation in the Huliaipole brachysyncline and the Vovchansk unit in the Orekhiv–Pavlohrad structure were formed during the same time interval. Deposition of this volcanogenic–sedimentary succession with thick Banded Iron Formations (BIFs) is possibly related to submarine volcanism and marine transgression.

The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 1. Constraints on theP–Tevolution from igneous, metasomatic and metamorphic amphiboles

AbstractThe metamorphic history of the Mesoarchean Amikoq Layered Complex within the Akia terrane of SW Greenland was characterised by electron microprobe mineral data and detailed petrography on 12 representative samples, integrated with zircon U–Pb geochronology and petrology. The complex intruded into a &gt;3004 Ma supracrustal association now consisting of granoblastic metabasites with subordinate quartz-rich gneiss. Supracrustal host rocks contain a relict high-temperature assemblage of orthopyroxene–clinopyroxene (± pigeonite exsolution lamellae, exsolved at ~975–1010°C), which is interpreted to pre-date the Amikoq intrusion. Cumulate to granoblastic-textured rocks of the main Amikoq Layered Complex range modally from leuconorite to melanorite, orthopyroxenite to harzburgite/dunite and rare hornblende melagabbro. Observed mineralogy of main complex noritic lithologies is essentially relict igneous with orthopyroxene–biotite and hornblende–plagioclase thermometers yielding temperatures of ~800–1070°C. An anatectic zircon megacryst from a patchy quartzo–feldspathic leucosome hosted in an orthopyroxene-dominated Amikoq rock reflects local anatexis at peak metamorphicP–Tconditions and yields an intrusion minimum age of 3004 ± 9 Ma. Field observations indicate local anatexis of orthopyroxene-dominated lithologies, possibly indicating a post-intrusion peak temperature of &gt;900°C. The last preserved stages of retrogression are recorded in paragneiss plagioclase–garnet, biotite–garnet and host rock ilmenite–magnetite pairs (≤3 kbar and ~380–560°C).The Amikoq Complex intruded a MORB-like crustal section and the former remained relatively undisturbed in terms of modal mineralogy. Preservation of igneous textures and mineralogy are related to an anhydrous, high-grade metamorphic history that essentially mimics igneous crystallisation conditions, whereas local high-strain zones acted as fluid pathways resulting in hydrous breakdown of igneous minerals. There is no evidence of equilibration of the intrusion at sub-amphibolite-facies conditions.