Metamorphic Minerals Research Articles

Multiple mineralization events recorded in the Dahenglu Cu[sbnd]Co deposit, eastern North China Craton: Constraints from Re[sbnd]Os and U[sbnd]Pb geochronology

The formation of sediment-hosted CuCo deposits remains controversial, in part due to the lack of a precise age for CuCo mineralization. The Dahenglu deposit is located in the eastern part of the Jiao–Liao–Ji Belt (JLJB), one of the famous sedimentary–metamorphic-type CuCo deposits in China, that is mainly hosted within Palaeoproterozoic epimetamorphic rocks. The orebodies occur as stratoid, lenticular and veined forms in conformity with the carbonaceous sericite phyllite. In this study, we present new ReOs and monazite UPb geochronological data for carbonaceous sericite phyllite, and zircon UPb geochronological data for diorite porphyry associated with the deposit. The ReOs geochronology on carbonaceous rocks from the Dalizi Formation provides a direct and reliable depositional age of 1967 ± 23 Ma. The UPb analyses of metamorphic monazite intergrown with cobaltite yield an age of 1812 ± 8 Ma. Magmatic zircon UPb dating from ore-bearing diorite porphyry yields a weighted mean age of 121.8 ± 0.8 Ma. These results constrain three periods of mineralization in the Dahenglu deposit. Syngenetic sedimentary mineralization and metamorphic mineralization occurred during the Palaeoproterozoic, and magmatic–hydrothermal mineralization occurred during the Early Cretaceous. We suggest that ReOs and UPb geochronology on carbonaceous-bearing epimetamorphic rock and metamorphic monazite can serve as robust chronometers to establish the age and temporal evolution of sedimentary hosted CuCo mineralization.

The Mousty Formation (Brabant Massif, Belgium): state of the art

The Mousty Formation has unique and particularly interesting lithological, mineralogical, sedimentological, metamorphic and tectonic characteristics which have been the subject of much research in connection with the new geological map of Wallonia, but also within the framework of a large amount of academic research, sometimes unpublished. An effort to synthesise and upgrade it in an updated framework seemed necessary. After a history of its definition, mapping and outcrop areas, the following topics are successively addressed in detail and illustrated with diagrams and photos: its lithostratigraphy with the probable discovery of its base near the Court-St-Etienne anticline, its well-constrained biostratigraphy and chronostratigraphy, a new estimate of its minimum thickness, its sedimentology—a subject little addressed until now—, and the resulting depositional sedimentary environment. Its geochemistry, never addressed, is comparable to black shale international standards, except for abnormally high manganese and very low calcium contents; high radon levels in buildings are related to the black slate presence in outcrops. Its mineralogy is rich in manganese-bearing metamorphic minerals, its metamorphism in which we go beyond the strict framework of the formation to deal with the entire southern outcropping rim of the Brabant Massif. Finally, the relationship between the Mousty Formation and the tectonic is discussed, with additional field data, in the context of the innovative and unifying concept of a low-angle extensional detachment called the Asquempont Detachment System. All these observations, some old and some very recent, discussed in the updated framework of the geology of the Caledonian basement of Brabant and in the global stratigraphy and palaeogeography of the lower Palaeozoic, allow us to renew in depth the vision we had of the Mousty Formation and of its place in the Brabant Massif.

Metamorphosed Plagiogranite Veins In Salma Eclogites, Belomorian Eclogite Province

The Salma subduction eclogites of the Meso-Neoarchean Belomorian Eclogite Province have compositions of N-MORB oceanic crust protoliths and contain felsic veins of plagiogranite compositions. Felsic veins consist of metamorphic minerals sush as garnet, quartz and kyanite, the earliest high-pressure inclusion of phengite and zoisite in garnet. Veins have geochemistry characteristics that are typical for island arc granites or adakites. They are compositionally comparable with eclogite-bearing tonalite-trondhjemite-granodiorite (TTG) gneisses, modern adakite, and Archaean andesite association of the Vedlozero–Segozero greenstone belt. The origin of these felsic veins can be represented as a product of melting of fluid-modified oceanic mafic slab at PT-conditions of eclogite metamorphism.Geothermometry and pseudosection calculations of metamorphic conditions in felsic veins, for eclogitic mineral assemblage of pyropic garnet, kyanite, high-silica phengite and quartz gave high pressures conditions (ca. 16–22 kbar) at relatively low temperatures (ca. 600–650 °C); for secondary decompression plagioclase ± biotite coronas between quartz, kyanite and garnet gave 14–11 kbar at 650–700 °C. This estimate corresponds “warm” subduction, which is most consistent with the parameters of the modern subduction of the base of the Cascadia plate . Mineral assemblages in eclogites, that included the felsic veins, give 15–11 kbar at 750–700 °C. Thermodynamic modelling indicates that the eclogites hosting the felsic veins did not reach partial melting conditions under the estimated PT conditions: exposure temperatures to the mafic rocks must have been significantly higher. A probable explanation for high-pressure parageneses in felsic veins may be the formation of plagiogranites as a result of the melting of the mafic slab under higher temperature and pressure conditions than were measured in this work. Thus, the high-pressure paragenesis of garnet, phengite, zoisite, and kyanite reflects the path of rock exhumation through eclogitic conditions.Zircon from the felsic veins gave several Archean and Paleoproterozoic ages ∼ 2.87, 2.78, ∼2.70 and, ∼1.9 Ga. The oldest age gave the fluid-modified oceanic zircon (∼2.9 Ga) from Fe-Ti eclogites directly near the veins. The oldest igneous zircons in the veins are represented by a population with the age of ∼ 2.87 Ga and correspond to the igneous age of some TTG gneisses, which do not contradict a hypothesis of the plagiogranite formation during the melting of the mafic slab in the subduction zone. A Neoarchean zircon population of ∼ 2.78 Ga is often present in eclogitized mafic rocks and surrounding TTG gneisses which can be associated both with the formation of TTG gneiss protoliths in the subduction zone (?) and with the recrystallization of older zircon as a result of fluid or thermal impact. The ɛHf of the older population (∼2.87 Ga) is + 3 to + 4, indicating a juvenile origin; the ∼ 2.78 Ga population has similar Hf-isotope compositions, suggesting that their ages reflect resetting (non-zero Pb loss) of the older zircons. The Sm-Nd model TDM ages of Grt-Ky veins have Meso-Neoarchean values (3.0–2.75 Ga) with positive εNd = 1.8–3.8 that may indicate a juvenile nature of sources of the plagiogranite melt. The eclogitized rocks of the Belomorian province generally underwent the granulite facies overprint at ∼ 2.74–2.64 Ga ago that is coincides with the age of granulite facies metamorphism of regional and global rank. The Paleoproterozoic evolution of the Salma eclogites and the East European Craton as a whole: at 2.5–2.4 Ga and 2.0–1.9 Ga with formation of the Lapland–Mid-Russia–South Baltia intracontinental and Svecofennian accretionary orogens was terminated in Paleoproterozoic time at ∼ 1.87–1.7 Ga. These age values correspond to closure of Pb-Pb and Sm-Nd isotopic systems of rock-forming metamorphic minerals of the felsic veins.

Multistage evolution of the Angara orogenic belt (SW Siberian craton) from granulite to ultrahigh-temperature metamorphism

An exotic block of andalusite-bearing garnet-cordierite-sillimanite gneisses was identified in the area of ultrahigh-temperature metamorphic complex of Angara-Kan block (southwestern Siberian Craton). Study of PT-conditions of these rocks indicate that peak PT-parameters correspond to granulite facies at moderate pressure (T = 820 °C, P = 6 kbar) and was followed by clockwise retrograde PT-path with changing direction from near isothermal decompression to near isobaric cooling at pressure about 3 kbar. The new U-Pb SHRIMP and LA-ICP-MS data on zircons and monazite indicate the time of granulite metamorphism at 1865 ± 5 Ma. The age data, metamorphic mineral assemblages and micro-textures indicate that garnet-cordierite-sillimanite gneisses were not affected by later ultrahigh-temperature metamorphism (1.78–1.76 Ga) widespread in the Angara-Kan block. Thus, the block of garnet-cordierite-sillimanite gneisses can be considered as a fragment of the early granulite metamorphic complex, which was tectonically combined with UHT metamorphic block of the Angara-Kan terrane. The observations prove multistage metamorphic history of the Angara-Kan block and indicate collisional event at ca. 1.86 Ga typical for Siberian craton amalgamation. This metamorphic episode caused melting out of a large volume of post-collisional granitoids and probably played a key role in pre-conditioning of the crust for achieving of UHT conditions during the following metamorphism.

Understanding Chemical and Mineralogical Composition of Nahartangi Nephrite from Goshta

The geochemical mineralogical characteristics of the Nahartangi nephrite deposit in the Goshta district of Nangarhar province are investigated in this research. Nephrite is a Cyclosilicate mineral with a high silicate content. Ca2(Mg, Fe)5Si8O22(OH)2 is its chemical formula, and it is monoclinic. It's been discovered in metamorphic rocks across the arena. The nephrite zone of Nahartangi is linked to the Jalalabad structural band and the ophiolite complex. Jalalabad is a region in eastern Afghanistan, southeast of Nuristan. As a result, this belt is associated with the upper or late Cambrian. The age of the rocks in this quarter is related to the Archean-Mesoproterozoic AR-PR2. This manuscript offers the first systematic mineralogical and geochemical observation of the Nahartangi nephrite deposit electronic probe Microanalysis, X-ray fluorescence spectroscopy (XRF) and mineralogical analysis. The Nahartangi nephrite was investigated using an XRF and petrographic microscope. According to field research, dolomitic marble is involved in Nahartangi nephrite deposits close to the intrusive granitoids. According to petrographic investigations, nephrite is mostly made up of fine-grained actinolite, schist, and dolomite, all of which are metamorphic minerals. Through every phase, nephrite samples had low Fe/(FeO+MgO) values (8.2524.17%) according to geochemical analysis, and SiO2 (81.30%), Al2O3 (3.22), and CaO (17.41%) were also low and 1.21% NiO content, as well. Chemical records analysis and interpretation of Mg+Fe percentages indicate that actinolite is the type of nephrite. The formation of veins and hydrothermal lenses in contact with serpentinite and carbonates has produced nephrite deposits. Dolomitic marble rocks are the specific rocks found near the study area. The economy and value are immediately impacted by color and its transparency. The presence of Mg elements causes the nephrite from the Goshta district to appear green.

Trace element changes in rutile from quartzite through increasing P–T from lower amphibolite to eclogite facies conditions

Abstract Low concentrations of Na, Ca, K, Fe, Mg and Al in quartzite commonly prevent the crystallization of index metamorphic minerals, inhibiting the obtainability of thermobarometric calculations. Quartzite typically contains quartz, zircon and rutile; therefore, single-element thermometers, such as Zr-in-rutile, may be applied. We investigate changes in trace-element composition of rutile from quartzite through increasing metamorphic conditions. Studied samples derive from a quartzite package (Luminárias Nappe, Minas Gerais, Brazil) where previous thermobarometric constraints on metapelites showed an increasing metamorphic grade southwards, from high-pressure lower amphibolite facies (580°C; 0.9 GPa) to eclogite facies (630°C; 1.4 GPa). Rutile from the lower-grade facies samples show a large spread in Zr concentrations, with the highest values corresponding to temperature estimates higher than metamorphic conditions affecting those units, and thus interpreted as inherited detrital signatures. A narrower spread in Zr concentration is observed in rutile grains from the higher-grade facies, and estimated Zr-in-rutile temperatures agree with previous thermobarometric constraints. Therefore, we show that at 630°C, Zr contents in detrital rutile from quartzites re-equilibrate. The comparison between the quartzite- and metapelite-hosting rutile grains from the same area shows that the resetting of the geothermometer in the latter seems to occur at slightly lower temperatures (∼50°C lower). Supplementary material: A full dataset of analized trace elements in rutile is available at https://doi.org/10.6084/m9.figshare.c.6793887

Prograde metamorphism provides gold and base metals to orogenic gold deposits in southern Tibet: Insights from thermodynamic modeling

The metamorphic devolatilization model is presently the most widely accepted model for the formation of orogenic gold deposits. According to this model, Au is likely derived from metamorphic rocks that experienced metamorphism from greenschist to amphibolite facies. Nevertheless, in recent years, some researchers have suggested that Au may have originated from different source regions, such as the mantle. As one of China's most significant orogenic gold provinces, the Indus-Yarlung Zangbo suture zone (IYZSZ) in southern Tibet plays a critical role in this discussion. Orogenic gold deposits within the IYZSZ are typically hosted in Triassic metasedimentary rocks called the Langjiexue group. This paper aims to investigate the suitability of metasedimentary rocks of the Langjiexue group as a source region of Au and base metals. Furthermore, we also examine whether the metamorphic devolatilization model is applicable to the IYZSZ. Our study involves reporting the mineral compositions and evolution of metamorphic zones (chlorite to sillimanite zones) in metasedimentary rocks obtained from the IYZSZ and providing detailed petrographic descriptions of these rocks. Thermodynamic modeling based on metamorphic mineral compositions was used to reveal the behaviors of H2O, Au, S, Cu, Pb, and Zn within prograde metamorphism. Our modeling results proved that the breakdown of chlorite and muscovite, both of which are the main mechanisms of Au and base metals release in metasedimentary rocks, occurs under different geothermal gradients. Given the observed metamorphic mineral compositions, it is the breakdown of chlorite that is the predominant reaction responsible for releasing Au and base metals under the real geothermal gradient of lower crustal metasedimentary rocks (∼25°C/km). This gradient is capable of releasing at least 1.5 ppb Au, 5.2 ppm Cu, 4.3 ppm Pb, 5.0 ppm Zn, and 20 ppm S (as part of the bulk rock) into the ore fluids. Through the mass balance calculations, we found that over 4.05 t of Au, 14040 t of Cu, 11600 t of Pb, and 13500 t of Zn can be mobilized from 1km3 of metasedimentary rocks. Therefore, lower crustal metasedimentary rocks can play a significant role in contributing to Au and base metals in the IYZSZ and other metasediment-hosted orogenic gold deposits worldwide.

Current state of knowledge of the gold mineralization at Imonga-Saramabila, Maniema (DR Congo): a petrographic and mineralogical study of the mineralized vein system

The Great Lakes area in Central Africa forms a large metallogenic province that hosts important deposits of gold mineralization. We present a petrographic, mineralogical and geochemical study of unique borehole samples from the Imonga-Saramabila gold deposit, a historical mine site located in the Maniema Province (DR Congo) in the Karagwe-Ankole belt (KAB) in the Great Lakes area and one of the only places in the Maniema province with accessible boreholes allowing to study the mineralization. The samples are metasedimentary rocks belonging to the Mesoproterozoic Kivu Supergroup, with bedding-parallel meta-igneous rocks. These rocks have undergone upper greenschist to lower amphibolite facies metamorphism, based on the presence of andalusite and chiastolite porphyroblasts, and are affected by hydrothermal alteration. The porphyroblasts formed during peak metamorphism and posterior to a first vein generation. Three additional vein generations were identified at Imonga based on crosscutting relationships, with the second and third events overprinting the porphyroblasts by intense chloritization, and associated with sulfide mineralization. The fourth vein generation is again barren. The first veining event formed pre-folding and the three subsequent generations postdate folding, as concluded based on the relationship of the veins with the cleavage. Only one important folding event is proposed based on the development of only one cleavage. Gold occurs as free gold or is included in pyrite in the second (and maybe third) vein generation. Based on the paragenesis, structural characteristics, and the link between veining and metamorphic minerals, the gold mineralization at Imonga is interpreted to be linked to the early Neoproterozoic (~980 Ma) compressional deformation event, associated with the amalgamation of the Rodinia supercontinent.

Metamorphic titanite–zircon pseudomorphs after igneous zirconolite

Abstract. The formation of metamorphic zircon after baddeleyite is a well-known reaction that can be used to date the metamorphism of igneous silica-undersaturated rocks. By contrast, metamorphic minerals formed after igneous zirconolite have rarely been reported. In this paper, we document metamorphic titanite + zircon pseudomorphs formed from the metamorphic breakdown of igneous zirconolite in syenodiorite and syenite, in the southeastern Sveconorwegian Province, Sweden. Water-rich fluid influx during tectonometamorphism in epidote–amphibolite-facies metamorphic conditions caused the release of silica during a metamorphic reaction involving igneous feldspar and pyroxene and the simultaneous breakdown of igneous Zr-bearing phases. Typical titanite + zircon intergrowths are elongated or platy titanite crystals speckled with tiny inclusions of zircon. Most intergrowths are smaller than 15 µm; some are subrounded in shape. Locally, bead-like grains of titanite and zircon are intergrown with silicate minerals. The precursor igneous zirconolite was found preserved only in a sample of near-pristine igneous syenodiorite, as remnant grains of mainly < 2 µm in size. Two somewhat larger crystals, 8 and 12 µm, allowed semiquantitative confirmation using microprobe analysis. Analogous with zircon pseudomorphs after baddeleyite, titanite + zircon pseudomorphs after zirconolite potentially offer dating of the metamorphic reaction, although the small size of the crystals makes dating with today's techniques challenging. The scarcity of reports of zirconolite and pseudomorphs reflects that they are either rare or possibly overlooked.

Metamorphic evolution of garnet amphibolites from the Fuping Complex: Implications for the Paleoproterozoic subduction–collision history of the Trans‐North China Orogen

The Trans‐North China Orogen (TNCO) is a major Palaeoproterozoic collisional orogen in the North China Craton and has experienced complex tectonic evolution during the assembly of the Eastern and Western Blocks. The vestiges of subducted oceanic crust preserved in this orogen can be used to gain insights into the subduction–collision process associated with the Palaeoproterozoic orogeny. In this study, we investigate garnet amphibolites from the Wanzi Group in the Fuping Complex along the TNCO through integrated petrography, mineral chemistry, phase equilibrium modelling and zircon U–Pb geochronology. Three stages of metamorphic mineral assemblages are identified as follows: (i) the prograde assemblage is preserved as plagioclase, amphibole, biotite and quartz inclusions in garnet porphyroblasts; (ii) the peak mineral assemblage includes coarse‐grained amphibole, garnet, plagioclase, ilmenite, biotite and quartz; and (iii) the retrograde assemblage is mainly represented by the corona of amphibole, plagioclase and biotite in the matrix. Petrological features and phase equilibrium modelling define a clockwise pressure–temperature path involving a prograde stage of 7.2–9.6 kbar and <670°C, peak conditions of 10.0–10.2 kbar and 690–810°C, and a retrograde stage with near‐isothermal decompression to ~7.4 kbar and cooling. Zircon U–Pb dating shows weighted mean ages of 1848 ± 22 Ma and 1836 ± 31 Ma, representing the growth of zircon in the retrograde stage during the uplift and cooling process. In combination with available data from the TNCO, the post‐peak decompression and cooling process is inferred to be long‐lived during 1.90–1.76 Ga, following collision along the TNCO that occurred during 1.97–1.90 Ga, ultimately leading to the final assembly of the North China Craton.

Silurian‐Devonian Lithospheric Thinning and Thermally Softening Along the Northern Margin of the Tarim Craton: Geological Mapping, Petro‐Structural Analysis and Geochronological Constraints

AbstractWhile the western part of northern Tarim Craton has long been considered as a Paleozoic passive margin, a pronounced Silurian‐Devonian magmatism developed on eastern part of this margin may indicate different but active margin setting. In this contribution, detailed structural mapping, petro‐structural analysis, and geochronological investigations were conducted in the Korla area, eastern part of northern Tarim Craton. Three main generations of fabrics were recognized. The earliest pervasive fabric is an originally sub‐horizontal metamorphic S1 foliation that is in part associated with migmatization characterized by high temperature/low pressure metamorphic mineral assemblages, interpreted as reflecting crustal extension. S1 foliation was affected by D2 contraction forming regional‐scale F2 upright folds associated with sub‐vertical axial planar foliation S2. D3 is marked by development of NW‐SE oriented dextral fault, asymmetric mega‐folding of S2 and spaced NW‐SE‐striking S3 foliation, likely in response to dextral transpression. Geochronological data indicate that D1 extension occurred from ca. 420 to 410 Ma, D2 contraction started around 410 Ma and lasted till 400 Ma or later, and D3 transpression was ongoing around ∼370 Ma. Integrated with regional data, an updated geodynamic model is proposed by interpreting the Central Tianshan, South Tianshan and NE Tarim Craton as an early Paleozoic supra‐subduction system. We suggest that the Silurian‐Devonian event reflects thermal softening and horizontal stretching of the supra‐subduction crust, resulting in drifting of the Central Tianshan continental arc from the proto Tarim Craton in association with opening of the South Tianshan back‐arc basin in‐between.

METAMORPHIC TRANSFORMATIONS OF ROCKS IN THE CENTRAL PART OF THE PRIMORSKY FAULT. WESTERN BAIKALIA

In this paper we compare metamorphic transformations in acid and mafic rocks in a zone of intense tectonic movements. The object of study is the exhumed part of damage zone of Primorsky fault – main collision suture, which connects the Siberian Craton and the Olkhon Terrane. The studied area is located near the Khorga River. Collected metamorphic rock samples are represented by both acid (plagiogranitegneiss) and mafic (amphibole-biotite plagiogneisses) rocks. The samples present two different parent rocks contacting each other but differ in chemical composition. The formation of identical metamorphic minerals belonging to the groups of garnet and epidote is noted in both types of rocks. Mineral formation was associated with high-pressure regional metamorphism and possibly with later collision. Studying chemical composition of rock-forming minerals has allowed to determine the P-T condition of metamorphism in these rocks via the method of multimineral thermobarometry. It was established that conditions of early metamorphism retain in amphibole-biotite plagiogneiss. Early metamorphism is characterized by high-pressure amphibolite facies of medium temperature next to eclogite facies. In plagiogranitegneisses were found conditions of later metamorphism that formed rocks at the same pressure but at a lower temperature in the paragonite-kyanite-zoisite subfacies of the eclogite facies next to the greenschist facies. Minerals formed in the early metamorphism are poorly preserved.

Electrical Resistance Evolution of Graphite and Talc Geological Heterostructures under Progressive Metamorphism

The electrical properties of isolated graphene established precedents for studies of electrical superconducting materials at room temperature. After the discovery of stabilized graphene and graphite nanoplatelets in a geological context, the interest in characterizing the properties of these minerals arose. This work evaluates the electrical resistance evolution of mineral graphite and talc heterostructures under progressive metamorphism simulated in the laboratory. The experiments were conducted on an end-loaded piston-cylinder apparatus. This equipment allows for the application of equal pressure in all sample directions (lithostatic pressure) and heating, simulating geological phenomena. The behavior of two sets of mineral samples were compared: graphite and talc in billets and powder. Samples in billets were submitted to treatments at 400 °C and 4 kbar; 400 °C and 6 kbar; and 700 °C and 9 kbar. The powder samples were subjected to 700 °C and 9 kbar, with two ways of disposing the mineral powders (mixed and in adjacent contact) beyond 900 °C and 9 kbar (in adjacent contact). The results show that the samples in billets had lower electrical resistance when compared to the powder samples. The lowest electrical resistance was observed in the sample treated at 400 °C and 6 kbar, conditions that are consistent with metamorphic mineral assemblage observed in the field. Powdered samples showed better cleavage efficiency during the experiment, resulting in thinner flakes and even graphene, as pointed out by Raman spectroscopy. However, these flakes were not communicating, which resulted in high electrical resistance, due to the need for an electrical current to pass through the talc, resulting in a Joule effect. The maximum electrical resistance obtained in the experiment was obtained in the sample submitted to 900 °C, in which talc decomposed into other mineral phases that were even more electrically insulating. This work demonstrates that electrical resistance prospecting can be an efficient tool to identify potential target rocks with preserved mineral nanometric heterostructures that can form an important raw material for the nanotechnology industry.

Quartz chemistry fingerprints melt evolution and metamorphic modifications in high-purity quartz deposits

In this paper, statistical quartz chemistry is combined with petrographic observations to improve our understanding of how metamorphism reworks trace elements of igneous quartz. We elucidate the key processes involved in the formation of the pegmatite-type high-purity quartz deposits in Tysfjord, Northern Norway, where produced quartz has the second-best chemical purity in the world. The Tysfjord pegmatites are the largest known intra-plutonic Nb-Y-F type, comprising Paleoproterozoic and Devonian generations. The Paleoproterozoic pegmatites show characteristics of Caledonian deformation and can be divided into a strongly-deformed type and a less-deformed type: primary internal zonation is replaced by quartz core and K-feldspar-rich intermediate zones surrounded by sheared granitic wall zones; quartz has undergone dynamic recrystallization and contains very few fluid inclusions. The Devonian pegmatites are undeformed and the quartz is rich in healed, fluid-inclusion-bearing microcracks.Trace elements in the pegmatite quartz are dominated by Al, Ti, Li, Ge and B, and total concentrations of the five elements are 29 ± 9 μg/g, 35 ± 10 μg/g and 38 ± 12 μg/g for the strongly-deformed, less-deformed and undeformed types, respectively. The quartz chemistry of the Devonian pegmatites is largely heterogeneous and shows significantly increasing Ge and B contents from early- to late-stage quartz, reflecting the fractionation enrichment of Ge and B in the evolved melts. The quartz chemistry of the Paleoproterozoic pegmatites has become homogeneous and is dominated by either Ti or Li and Al, resulting from the metamorphic modifications during the Caledonian overprint. The modifications on quartz took place via volume diffusion or dynamic recrystallization equilibrium, resulting in a resetting of Ti (at low TiO2 activity) and losses of Li and Al. Therefore, the less-evolved pegmatite-forming melts derived from the Paleoproterozoic magmatism and the moderate metamorphic modifications during the Caledonian orogeny are important for the formation of the high-purity quartz deposits. This study manifests that quartz chemistry has the potential to fingerprint igneous and metamorphic mineralization processes in tectonomagmatic terranes.

Metamorphic origin of stratiform cassiterite mineralization in the Schwarzenberg -Aue district – Clues to the metamorphic history and pre-orogenic Sn enrichment of the Erzgebirge (Germany)

Here we present petrographic and field observations, mineral and whole-rock compositions as well as conventional thermobaromerty and thermodynamic modelling of the Sn-mineralized schists near Bockau, Erzgebirge (Germany). The metamorphic mineral assemblage comprises locally abundant cassiterite (up to 2 wt%), which exhibits metamorphic features indicating pre-orogenic Sn enrichment of the sedimentary protoliths. The results imply that the composite schist sequence of Bockau underwent regional metamorphism at 0.8–0.9 GPa and 550 ± 50 °C and a subsequent relatively hot-exhumation path between 600 and 700 °C. Flat compositional zoning profiles of garnet suggest that these elevated temperature conditions were not short-lived but likely lasted for a time period of 20 to 40 Ma leading to the development of retrograde mineral assemblages randomly overgrowing cassiterite. The textural position of cassiterite indicates that its formation is predominantly related to the prograde/collisional stage of the Variscan orogeny (∼340 Ma) and therefore is linked to metamorphic reactions and hence pre-dates widespread LiSn granites of the area (325–310 Ma). The presented observations of the Bockau schists provide the first direct evidence for pre-orogenic Sn enrichment of their sedimentary protoliths. This has two important implications: first, the meta-sedimentary units of the Bockau series as a part of the Frauenbach super unit may host extensive, yet underexplored Sn resources along its entire strike length (> 40 km). Second, melting and assimilation of cassiterite-bearing Bockau schists may have significantly contributed to the formation of Sn-enriched granites within the Saxothuringian Zone of the European Variscides (and elsewhere). Hence, the presence of Sn-enriched (meta) sedimentary units could be an important prerequisite for Sn fertility of a metallogenic region. The ultimate pre-metamorphic source and the cause of sedimentary Sn-enrichment remain poorly constrained.

Barrovian metamorphism in nominally lowermost Alpujarride Complex units: tectonic implications for the development of the orogenic wedge in the Western Mediterranean

ABSTRACT The sedimentary successions of the Permian-Triassic rifting related to the Alpine Tethys in the easternmost Lower Alpujarride units (Internal Betic Zone, south Spain) host shallow-intrusive basic bodies of Mesozoic age. These gabbroic bodies experienced low-grade Alpine metamorphic overprint, singularly recorded in three outcrops nearby Alicante city in southeast Spain: Callosa and Orihuela Ranges and Nueva Tabarca Island. Overall, the metabasic bodies still preserve magmatic ophitic/subophitic texture and relicts of magmatic augite and ferropargasite-ferroedenite-edenite and kaersutite. In contrast, metamorphic minerals include magnesioriebeckite, actinolite, albite, stilpnomelane, phengite, and chlorite. Major and trace element geochemical data suggest crystallization of the igneous protoliths from subalkaline (tholeiitic) to alkaline within-plate (continental) basaltic magmas slightly enriched in incompatible elements. These magmas with E-MORB geochemical affinity very likely intruded at shallow crustal levels in an extensional-synrift-geodynamic setting related to the breakoff of Pangea in the region. Pressure-Temperature (P-T) conditions estimated by average P-T method and pseudosection calculations suggest metamorphic peak at ca. 6 kbar at 340°C consistent with burial metamorphism down to ca. 18 km depth with intermediate thermal gradients of ca. 18°C/km. These conditions are different to the typical subduction-related blueschist facies conditions of the Lower Alpujarride units and suggest, instead, a collisional (Barrovian) event. This can be explained by a late-stage accretion of the studied units after the main subduction-related event, pointing to the dispersion of the corresponding Lower Alpujarride sections along the margin in different palaeogeographic positions before deformation and movement to their actual structural position. The metamorphic correlation of the studied units with the accreted passive margin of South Iberia (Subbetic zone) is also discussed.

Post-orogenic exhumation triggers gold mineralization in the Trans-Hudson orogen: New geochronology results from the Lynn Lake greenstone belt, Manitoba, Canada

Metamorphic devolatilization is thought to be the critical process that can liberate gold from amphibolite facies source rocks to form orogenic gold deposits. However, a growing number of studies demonstrate that orogenic gold deposits are the product of multiple hydrothermal stages over 10 s to 100 s of Myr rather than one metallogenic event. Herein we report new geochronology results for orogenic gold deposits hosted within the Paleoproterozoic Lynn Lake greenstone belt (Manitoba, Canada) to address the knowledge gap. Garnet-hosted xenotime crystals, which occur in a folded pyrite veinlet with gold, yield a weighted average 207Pb/206Pb date at 1827 ± 8 Ma. This new and more precise age demonstrates that the earliest auriferous fluids at the MacLellan gold deposit coincided with the rapid burial of ca. 1836 Ma sedimentary rocks before regional metamorphism. New garnet-biotite geothermometry results further demonstrate that the earliest auriferous veins were preserved despite being metamorphosed to amphibolite facies conditions (505–689 °C). The peak metamorphic mineral assemblage is overprinted by: (1) the main xenotime- and gold-bearing deformation fabric (S2; 1790–1827 Ma); (2) an overprinting xenotime- and monazite-bearing S3 fabric that is also associated with gold-rich arsenopyrite (1769–1790 Ma); and (3) chlorite-hosted xenotime and gold (1747 ± 12 Ma) that are coeval with the oldest U-Pb apatite dates (1691–1740 Ma) and a prominent mode of 40Ar/39Ar biotite dates at ca. 1746 Ma. We suggest that the post-orogenic gold events are incompatible with the standard devolatilization model because they post-date subduction and peak-metamorphism by up to 80 Myr. Instead, the new geochronological results suggest that gold deposition at the MacLellan deposit was linked to burial and the subsequent exhumation of chemically reactive trap rocks through favourable temperature windows that allowed gold transport as sulphide complexes. Exhumation and far-field tectonics are interpreted as the most likely drivers of post-orogenic auriferous fluids rather than regional metamorphism and active subduction. This revised mineral systems model provides an explanation for the high prospectivity of high- and low-temperature mineral assemblages and suggests that mineral exploration should focus on the earliest ore-forming structures that are reactivated during multiple stages of orogenesis.

A multistage model of preservation in fossil plants from the Llewellyn Formation (Pennsylvanian), St. Clair, Pennsylvania, U.S.A

Exceptionally preserved plant fossils from St. Clair, Pennsylvania, have long been part of museum and private collections. Fossil leaf fragments and stems from the underclay of the Buck Mountain Coal (Reading Anthracite Company quarry), lowest Llewellyn Formation (late Pennsylvanian, Virgilian), combine outer layers of coalified organic material (phytoleim) with strikingly white kaolinite, quartz, and the low-grade metamorphic mineral pyrophyllite in the interior. The surficial dark, coalified material appears to preserve the dense outer layers (cuticle, epidermis, and palisade layer) and some cells of the spongy mesophyll in the fossilized leaves. XRD spectra of separated phytoleim contain graphite peaks. Raman spectra with peaks near 1600 and 1320 cm−1 indicate that the phytoleim is composed of disordered graphitic material. ID/IG ratios from ca. 0.3 to 0.45 are consistent with greenschist facies metamorphic conditions forming the anthracite coal. The interior pyrophyllite is highly crystalline, with large crystals often oriented perpendicular to leaf surfaces. Hydrothermal formation of pyrophyllite requires relatively limited groundwater flow conditions and temperatures between ca. 275 and 350 °C. Although many fossil leaves are compressed, some are near their original thickness. We infer that the remains were replaced prior to significant burial and compression of the sediment by an early-forming mineral phase, possibly pyrite, which was in turn replaced by kaolinite and then pyrophyllite at higher temperature and pressure. Woody structures such as cuticle and veins, which had resisted replacement by the initial low temperature phase, were coalified at higher temperatures.

Oceanic subduction to continental collision in the NE Proto-Tethys revealed by early Paleozoic eclogites with high-temperature granulite-facies overprinting in the East Kunlun orogenic belt, northern Tibet

The East Kunlun orogenic belt in the northern Tibetan Plateau records a long-term accretionary and collisional history in the northeastern Proto-Tethys Ocean, which is important for reconstructing the paleogeography of early Paleozoic East Asia. Here, we present an integrated study combining petrology, geochemistry, geochronology, and metamorphic pressure−temperature (P−T) data of newly found eclogites in the middle Nuomuhong segment of the East Kunlun orogenic belt. The eclogites are composed mainly of garnet, omphacite and low-sodium clinopyroxene, amphibole and plagioclase with minor orthopyroxene, biotite, quartz, accessory rutile, ilmenite, titanite, and zircon. Detailed petrographic observations, conventional geothermobarometry, and phase equilibrium modeling point to the presence of five metamorphic mineral assemblages with corresponding P−T conditions related to: (1) prograde M1 stage P−T estimates at >14.0 kbar/∼470−506 °C; (2) Pmax M2 eclogite-facies stage P−T conditions of ∼26 kbar/∼570 °C; (3) early retrograde M3 high-pressure granulite-facies stage; (4) subsequent M4 retrograde medium-pressure granulite facies at Tmax of ∼860−900 °C; and (5) later M5 retrograde amphibolite-facies stage P−T conditions of <6.2 kbar/∼710−730 °C. These P−T estimates define a clockwise P−T path characterized by heating during the Pmax formation of the eclogite facies, to the Tmax exhumation stage of granulite-facies lithologies, the latter of which is identified for the first time in retrograde eclogites from the East Kunlun orogenic belt. Whole-rock geochemical compositions indicate a mid-oceanic-ridge basalt (MORB) affinity for the eclogite protoliths and a fragmented oceanic crust origin. Sensitive high-resolution ion microprobe (SHRIMP) zircon U−Pb isotopic analyses of the eclogite yielded two groups of weighted mean 206U/238Pb ages of 464 ± 8 Ma and 419 ± 4 Ma, which are interpreted as the age of the eclogite protoliths and the lower threshold for peak eclogite-facies metamorphism, respectively. Our new data, together with regional eclogite-facies metamorphism, suggest a ca. 520−460 Ma age for the subduction of the eastern Kunlun oceanic crust, within the northern Proto-Tethys Ocean, to a depth of ∼83 km, with early subduction−accretionary orogenesis at ca. 419 Ma. Overprinting by high-temperature granulite-facies points to exhumation of oceanic crust to the middle to shallow crustal level at this time. Collectively, the preserved eclogite and high-temperature granulite mineral assemblage provide new constraints on the tectonic evolution and the detailed accretionary-to-collisional orogenesis of the Proto-Tethys Ocean. They suggest that the ca. 428−411 Ma subduction-collisional event marked the termination of the Proto-Tethys Ocean and the eventual formation of an ∼500-km-long, high- to ultra-high−pressure metamorphic belt in the East Kunlun orogenic belt.

Mineralogical and spectral characteristics of Changbai jade, Jilin province, Northeast China

Changbai jade is a type of jade ore with good economic value that is found in the tuff of the Upper Triassic Changbai Formation of the Mesozoic period in Jilin Province, China. However, the mineral composition of the kaolinite group in Changbai Jade has not been definitively identified, and there has been a lack of systematic mineralogical and spectral analysis. To analyze the mineralogical and spectroscopic characteristics of Changbai jade, this study utilized several modern testing methods, including X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Fourier transform infrared absorption spectroscopy (FTIR), Raman spectrum test (Raman), and Ultraviolet visible (UV-vis). Mineralogical and spectroscopic analyses were conducted on Changbai jade ore samples CB21 and CB22. The results indicated that the main metamorphic minerals of the two samples are dickite and the color-causing factor of the yellow part is pyrite. This research provides basic theoretical research data for jade processing technology and the geological origin of Changbai jade.