Schist Research Articles

Early neoproterozoic supracrustal successions in the central Korean Peninsula: Implications for the paleotectonic history of East Asia

The Early Neoproterozoic supracrustal metasedimentary succession from the Paju–Yangju–Dongducheon area in the central Korean Peninsula is distributed along the northwestern margin of the basement gneisses in the Middle Paleoproterozoic Gyeonggi Massif. The metasedimentary succession includes quartzite, quartz–mica schist, mica schist, calc schist, marble, and calc-silicate, showing alternating layering and large-scale isoclinal folds with Middle Paleoproterozoic gneisses. The basement gneisses yield zircon LA–MC–ICPMS U-Pb ages of 1.87–1.86 Ga. Detrital zircon U-Pb ages of eight quartzites, seven quartz–mica schists and four calc schists suggest the following features. (1) Early Neoproterozoic (ca. 0.92 Ga) sedimentation, (2) prominent age peaks in the Late Paleoproterozoic to Mesoproterozoic range, (3) sandy proximal and muddy to calcic distal provenances, and (4) age population under extensional and trailing edge settings. The detritus of the quartzites were mainly derived from the peripheral Middle Paleoproterozoic basement. In contrast, quartz–mica schists and calc schists show detritus derived from distal Late Paleoproterozoic to Mesoproterozoic clastic sources in the southeastern North China Craton and central Korean Peninsula where magmatic rocks related to the extensional tectonics associated with the disruption of the Columbia supercontinent occur. The detrital zircon records and litho-tectonic features indicate that the Early Neoproterozoic rocks in the Paju–Yangju–Dongducheon area, as well as the central Korean Peninsula, show similarities with the southeastern North China Craton. We therefore interpret that the Early Neoproterozoic supracrustal metasedimentary successions were deposited in marginal shallow marine to fluvial environments from the marginal part of the linked central Korean Peninsula and the southeastern North China Craton which were located at the southwestern margin of western part of the Columbia supercontinent during the Early Neoproterozoic (ca. 0.92 Ga). However, metamorphic overgrowths on zircons and large-scale isoclinal folding in the alternation of the Early Neoproterozoic metasedimentary and the Middle Paleoproterozoic rocks in the Paju–Yangju–Dongducheon area are related to Late Permian to Triassic compressional collision tectonics in the central Korean Peninsula after the Early Neoproterozoic sedimentation.

Tectonic Evolution of the Southern Dabie Orogenic Belt, China: Insights from Peak P–T Conditions and U–Pb Zircon Dating of the Susong Metamorphic Complex

The Susong metamorphic complex (SSC) in the southern margin of the Dabie orogenic belt (DOB) in central-eastern China is a key metamorphic unit for understanding subduction and exhumation processes in the DOB. However, the formation age and metamorphic grade of the SSC remain uncertain, hampering our understanding of the mechanism of the formation of the DOB. An integrated study of field survey, regional metamorphic petrology, geothermobarometry, and U–Pb dating of zircon was carried out in this study. Our results reveal that the SSC was metamorphosed under epidote amphibolite- to amphibolite-facies conditions with average metamorphic P–T values of 0.98 ± 0.07 GPa and 531 ± 35 °C. The smooth spatial variation in peak P–T conditions and an apparent geothermal gradient of ~17 °C/km indicate that the SSC as a whole fall into Barrovian-type metamorphic environments. Zircon U–Pb dating for garnet–mica schists of sample ZT003, ZT005 and ZT006 yield five (Groups I to V), six (Groups I to VI) and five (Groups I to V) age groups, respectively, concentrating on the Meso-Neoarchean, early-middle Paleoproterozoic, middle Mesoproterozoic, early Neoproterozoic, Palaeozoic and Triassic-lower Jurassic. Therein, a 259–190 Ma (Group V) from zircons with Th/U ratios of <0.1 in sample ZT006 record the timing of both peak and retrograde metamorphism for the SSC. All other ages are detrital zircon ages, and from age provenances in the DOB or the Yangtze Block (YZB), indicating the YZB affinity of the SSC. The two youngest age populations of 427–415 Ma (Group VI) and 475–418 Ma (Group V) from samples ZT005 and ZT006, respectively, suggest that the formation age of the SSC could be Middle Devonian. The similarity of formation age and peak P-T conditions of the SSC to Foziling Group, located in the northernmost DOB, implies that both units formed the sedimentary cover on the passive continental margin of the YZB during the late Palaeozoic, and subducted into the middle-lower crust of 20–40 km depth as a whole, corresponding to the shallow subduction. Compared to the deep subduction defined by high-pressure (HP) and ultrahigh-pressure (UHP) units, larger differences in peak P–T conditions, age and geothermal gradient between two different tectonic environments happen. Accordingly, it is speculated that a transitional subduction from shallow to deep levels occurred at Moho depths during the Early Triassic, and is due to a change in subduction dip angle.

U–Pb Zircon Ages and Geochemistry of the Wuguan Complex and Liuling Group: Implications for the Late Paleozoic Tectonic Evolution of the Qinling Orogenic Belt, Central China

The tectonic evolution of the Qinling orogen is key to understanding the process of convergence between the North China Block (NCB) and the South China Block (SCB). The Wuguan Complex and Liuling Group, situated along the southern margin of the Shangnan–Danfeng suture zone (SDSZ) between the North Qinling Terrane (NQT) and the South Qinling Terrane (SQT), are important indicators of the late Paleozoic tectonic evolution of the Qinling orogen. In this paper, the detrital zircon U–Pb geochronology and geochemical analysis of the Wuguan Complex and Liuling Group are carried out. Detrital zircons from two metasedimentary rock samples of the Liuling Group yield a major age peak at 460 Ma and two subordinate peaks at 804 Ma and 920 Ma, with a few older grains having formed between 1000–2549 Ma. One metasedimentary rock sample of the Wuguan Complex has a similar age spectrum as that of the Liuling Group, which shows the main age peak at 440 Ma and two subordinate peaks at 786 and 927 Ma, indicating all detrital zircon age results have the same source area. Geochemical analyses suggest that the sedimentary rocks of the Liuling Group and part of the Wuguan Complex were deposited in the tectonic setting of the continental island arc (CIA), while the geochemical characteristics of the other group of sedimentary rocks of the Wuguan Complex indicate the mixing of basic rock sources. The protolith of garnet amphibolite and hornblende schist, which were collected from the Wuguan Complex, were classified as andesite and basalt, with the nature of arc andesite and oceanic island basalt, respectively. In combination with regional data, we suggest that the Liuling Group and the Wuguan Complex were deposited in a fore-arc basin. Additionally, the Wuguan Complex was subsequently incorporated into the tectonic mélange by the northward subduction of the Paleo-Qinling Ocean. Zircons from the subduction-related metamorphic igneous rocks in the Wuguan Complex yielded a weighted mean age of 365 ± 19 Ma, indicating that the Paleo-Qinling Ocean between the SQT and NQT was still subducted at the end of Devonian.

Metamorphism and geochronology of garnet mica schists from the Kuluketage area: Implications for reconstructions of the Tarim Craton in supercontinent Columbia

The Tarim Craton has been considered a part of the Columbia supercontinent. However, the paleogeographic location of the Tarim Craton remains ambiguous or has not been considered in the reconstruction model. This problem is partly due to the dispute over the collisional time between northern Tarim and major Columbia blocks. The Xingditage Group in the Kuluketage area records important information on the Paleoproterozoic evolution of northern Tarim during the Columbia supercontinent convergence, in which the garnet mica schist unit is key to constraining the collision time. We carried out petrographic, phase equilibrium modeling, and zircon U-Pb chronological studies for two garnet mica schist samples (LJS-1 and LJS-2) from the Lower Xingditage Group. This study attempted to constrain the collision time of the northern Tarim in the late Paleoproterozoic from the metamorphic evolution perspective. Petrography shows that both samples are rutile-kyanite-garnet micaschist. The phase equilibrium modeling reveals two clockwise P-T paths. For the sample LJS-1, the peak stage was constrained at ∼ 780 °C/12 kbar and subsequently decompressed to ∼ 760 °C/4 kbar. For sample LJS-2, the peak pressure is relatively higher, reaching ∼ 790 °C/16 kbar, followed by decompression to ∼ 796 °C/3 kbar. Both P-T paths have a near isothermal decompression path, indicating a rapid exhumation process. Zircons in this study record a metamorphic age of 1907 ± 8 Ma, which can represent the collision age of northern Tarim in the late Paleoproterozoic. We propose that the northern Tarim experienced a long-lived orogenic event in the late Paleoproterozoic, and the northern Tarim became a part of the Columbia supercontinent at ∼ 1.91 Ga. In comparison with orogenic belt evolution, an affinity with southwest Siberia was established for northern Tarim during the assembly of the Columbia supercontinent.

Rare Element Enrichment in Lithium Pegmatite Exomorphic Halos and Implications for Exploration: Evidence from the Leinster Albite-Spodumene Pegmatite Belt, Southeast Ireland

Pegmatitic deposits of critical metals (e.g., Li, Ta, Be) are becoming increasingly significant, with growing interest in understanding metal enrichment processes and potential vectors to aid the discovery of new resources. In southeast Ireland, the Leinster pegmatite belt comprises several largely concealed Li-Cs-Ta albite-spodumene-type pegmatites. We carried out detailed mineralogical characterization and whole-rock geochemical analyses of six drill cores intersecting pegmatite bodies and their country rocks. Exomorphic halos 2–6 m thick, enriched in Li, Rb, Be, B, Cs, Sn and Ta, are identified in both mica schists and granitic rocks adjacent to spodumene pegmatites. Metasomatism in wall rocks visible to the naked eye is restricted to a few tens of centimeters, suggesting country rock permeability plays a key role in the dispersion of these fluids. We propose that halos result from the discharge of rare element-rich residual fluids exsolved near the end of pegmatite crystallization. Halo geochemistry reflects the internal evolution of the crystallizing pegmatite system, with residual fluid rich in incompatible elements accumulated by geochemical fractionation (Be, B, Cs, Sn, Ta) and by auto-metasomatic resorption of spodumene and K-feldspar (Li, Rb). The possibility of identifying rare-element enrichment trends by analysis of bedrock, stream sediments and soils brings opportunities for mineral exploration strategies in Ireland and for similar albite-spodumene pegmatites worldwide.

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.

Records of the oceanic propagator closure at the southern splay of the Palaeo‐Tethys

The Palaeo-Tethys is known to have diverged into several splays throughout East and South-east Asia. Several researchers have widely studied the importance and geometries of the Palaeo-Tethys' branches. Plate reconstruction geometries for the southern Palaeo-Tethys splay suggest that the ocean either connects to the proto-Pacific or dies out. We review and discuss the Late Palaeozoic to the Early Mesozoic lithologies attributed to the Palaeo-Tethys, from the Himalayan Syntaxis in the north to the Bangka and Belitung islands in western Indonesia to the south. Relics of oceanic crust and deep-sea sediments related to the main Palaeo-Tethys Basin can be traced from the South China (Yunnan)-Thailand border to Peninsular Malaysia. A sub-parallel back-arc suture exists in eastern Thailand in the north and vanishes towards the south at the latitude of the Gulf of Thailand. Towards the south of Peninsular Malaysia, the material that constitutes the suture zone of main Palaeo-Tethys is found offshore in the Riau Archipelago, Indonesia, and has been extensively eroded. Thus, we rely on gravity signatures to extrapolate the extension of the structures to the Bangka and Belitung islands, which show only minor serpentine and pillow basalts occurrences along a line that curves towards Borneo. The metamorphic rocks associated with the suture zones range from high grade (gneiss) in Thailand to low grade (mica schist and minor amphibolite) in Peninsular Malaysia. The syn- and post-collisional granitic bodies that form large batholiths in Thailand and Malaysia are comparatively smaller in Indonesia, particularly on Belitung Island, suggesting the lack of large underplating of the Sibumasu crust within the region. We interpret the stratigraphic and tectonic observations of the southern Palaeo-Tethys to support the theory of the southward (present orientation) opening of a propagating oceanic basin from Devonian to Carboniferous, which began to close from the Permian to Triassic period.

Detrital zircon analysis of metasedimentary rocks of the Staré Město Belt, Sudetes: implications for the provenance and evolution of the eastern margin of the Saxothuringian terrane, NE Bohemian Massif

The Staré Město Belt (SMB) in the Central Sudetes forms a Variscan tectonic boundary zone that is located between the Saxothuringian and Brunovistulian terranes of the Bohemian Massif. The three thrust-bounded upper, middle and lower lithotectonic units of the SMB are composed of metasedimentary and Late Cambrian metavolcanic rocks. A new LA-ICPMS zircon geochronology supported by zircon typology studies of the mica schists of the upper unit and the migmatitic paragneisses of the middle unit provides new insights into the provenance and evolution of the SMB. Our new data were obtained from metasedimentary rocks and compared to the previously published zircon ages of the SMB metavolcanic rocks. The results indicate that the metasedimentary and bimodal metavolcanic rocks in the separate lithotectonic units of the SMB originally formed Late Cambrian volcano-sedimentary successions. The source areas of the sedimentary basins studied were dominated by Neoproterozoic and Paleoproterozoic crystalline rocks that were presumably located near the West African Craton of Gondwana. A comparison of the detrital age spectra obtained with those previously published from the region indicates a strong association of the entire SMB with the Saxothuringian terrane of the Bohemian Massif. During partial melting of the metasedimentary rocks of the middle unit of the SMB, Cambrian and older zircon grains were affected by solid-state transformations that caused partial resetting of the U-Pb dates, changes in internal zircon textures and reductions in Th/U ratios.

Advances on microLIBS and microXRF mineralogical and elemental quantitative imaging

Mineralogical and petrographic studies require analytical methods capable to underline the repartition of major to trace elements within geological samples. The EPMA (Electron Probe Micro Analyzer) conventional method used for such investigation, but on restrictive zones, is on the verge to be reached by μLIBS (micro Laser-Induced Breakdown Spectroscopy) and μXRF (micro X-Ray Fluorescence) techniques allowing the elemental imaging on thin rock sections or even larger samples in only several hours. These spectroscopic methods with extremely fast acquisition speed (~10 ms/pixel) are perfectly adapted to perform multi-elemental imaging of major to trace elements down to the ppm-level. Here, on a mica schist thin section that displays a wide paragenesis of minerals, μLIBS and μXRF quantitative elemental mapping are obtained using EPMA spot analyses as internal reference compositions. We exhibit the precision of the μLIBS and μXRF quantitative elemental imaging for major and trace elements repartition in an entire thin/thick rock section. According to these oxide weight contents, a rapid mineral classification is obtained, with a good discrimination between minerals even for those with similar compositions (alumino-silicates such as andalusite and kyanite) and within a complex matrix (Fe-oxides, quartz, micas, feldspars…).

Tourmaline as a recorder of the magmatic-hydrothermal evolution in the formation of pegmatite: In-situ elemental and boron isotopic compositions of tourmaline from the Qinghe pegmatite, Chinese Altay orogen

• Tourmalines record the magma-hydrothermal evolution of pegmatite-forming magma . • Extremely low REEs of tourmaline indicate direct melting of metapelite to form pegmatite . • Boron isotopic variation reflects the scale of fluid exsolution during pegmatitic evolution. We conducted elemental and boron isotope studies on tourmalines from a single pegmatite dyke of the Altay orogen to investigate its origin and evolution. The pegmatite dyke is composed of border, intermediate and core zones. The border and intermediate zone tourmalines mostly belong to schorl, while the core zone and magmatic hydrothermal tourmalines to dravite. Most tourmalines follow the (Na + Mg) (Al + X vac ) -1 , FeMg -1 and MnMg -1 exchange vectors. The core zone tourmalines show positively correlated FeO t and MgO, and negatively correlated FeO t and Al 2 O 3 , suggesting a rise of f O2 in this zone. Moreover, they show higher B, Mg, Sr and Eu than other tourmalines due to magmatic evolution. Tourmaline from the Altay pegmatite is characterized by extremely low REE abundances (<3 ppm) when compared with those from granites, suggesting an origin from direct melting of metapelitic rocks. The border and intermediate zone tourmalines have comparable boron isotopic compositions with δ 11 B = −14.0 to −12.2‰, which are similar to the mica schist (−13.2 ‰ to −12.2‰), but slightly heavier than the core zone and magmatic hydrothermal tourmalines (−14.4‰ to −13.3‰). This suggests derivation of the pegmatite from melting of the metapelite without causing boron isotope fractionation, whereas late-stage fluid exsolution caused somewhat boron fractionation. This may be indicative of only small amounts of fluids being released from the barren pegmatite-forming magma. Tourmalines from mineralized pegmatites basically exhibit heavier boron isotope compositions than those from barren pegmatites, due probably to more involvement of evaporate in the source of the former.

Paleo-Mesoproterozoic magmatic-sedimentary events in eastern Bainaimiao micro-block: Implications for Precambrian tectonic evolution of a fragment from Columbia supercontinent

Identifying tectonic positions and origins of Precambrian micro-blocks within the Central Asian Orogenic Belt (CAOB) is essential for revealing global paleogeography and evolution of the early Earth. The Bainaimiao micro-block in the southeastern CAOB preserves multistage geologic records which can better reveal Precambrian tectonic history of micro-blocks within the Phanerozoic accretionary orogenic belt. This study presents integrated research of geochronology, geochemistry, and zircon Hf isotopes on the Chenjiatun metamorphic complex in the eastern Bainaimiao micro-block, which consists of meta-sedimentary rocks (plagioclase gneisses and mica schists), amphibolites and two-mica granites. The protoliths of amphibolites, two-mica granites and meta-sedimentary rocks were formed at 1.75 Ga, 1.38 Ga and 1.39–1.34 Ga, respectively, suggesting that they represent Paleo-Mesoproterozoic basement of the Bainaimiao micro-block. The detrital zircon age spectra of meta-sedimentary rocks show two representative age peaks at 1.80 Ga and 1.40 Ga, with zircon Hf isotopes varying from −11.11 to +10.67. Characteristic mostly positive zircon Hf isotopes and geochemical features of the Chenjiatun complex indicates that the Bainaimiao micro-block probably has tectonic affinity with the Laurentia-Baltica continent. Paleoproterozoic amphibolites with positive εHf(t) (+5.67 to +15.42) were derived from metasomatized mantle wedge mixed with sediment melts. They are characterized by arc-like geochemistry such as high Th/Yb (0.73–2.77), enrichment in LILEs and depletion in HFSEs (eg. Nb and Ta), similar to coeval arc magmatism in the Laurentia-Baltica continent, which indicate the Bainaimiao micro-block was possibly in a subduction convergent setting in the late Paleoproterozoic. By contrast, Mesoproterozoic two-mica granites, with mostly positive εHf(t) (−0.29 to +11.53), belong to typical S-type granites and could be generated from partial melting of meta-pelites with participation peritectic garnets. These two-mica granites, together with ∼1.40 Ga juvenile A-type grantioids from nearby and other Precambrian micro-blocks in the southern CAOB, exhibit relatively high alkalis and HFSEs contents, young Hf model ages (1.50–1.40 Ga) and were generated at high-temperature environment. These findings reveals that the Bainaimiao micro-block was in an extensional setting at 1.40 Ga coeval with the breakup evolution of the Columbia supercontinent.

Exhumation of the high‐pressure Richarddalen Complex in NW Svalbard: Insights from 40Ar/39Ar geochronology

Abstract40Ar/39Ar dating of white mica from foliated metamorphic rocks of the Caledonian orogen of NW Svalbard was performed using the single‐grain fusion technique. Two samples, an augen orthogneiss and a mica schist, were collected from the high‐pressure Richarddalen Complex. Three additional mica schist samples were collected from the tectonically underlying Montblanc Unit. 40Ar/39Ar analysis of white mica separates yielded weighted average 40Ar/39Ar dates of 446.1 ± 1.1, 438.4 ± 1.1, 439.4 ± 1.2, 439.8 ± 1.5 and 439.3 ± 1.0 Ma, with the oldest date provided by the Richarddalen mica schist. The single‐grain fusion 40Ar/39Ar dating was accompanied by geochemical mapping of white mica using a microprobe. We interpret the oldest date as the age of cooling after a Late Ordovician high‐pressure event and the four younger dates to represent the timing of subsequent deformation associated with assembly of a Caledonian nappe stack.

Ore Genesis of the Baishitouwa Quartz–Wolframite Vein-Type Deposit in the Southern Great Xing’an Range W Belt, NE China: Constraints from Wolframite In-Situ Geochronology and Geochemistry Analyses

The Baishitouwa deposit is a medium-scale quartz–wolframite vein-type deposit in the southern Great Xing’an Range tungsten (W) belt. The W mineralization occurs mainly as veins and dissemination within the mica schist of the Mesoproterozoic Baiyunebo Group. The formation of the deposit can be divided into four stages. The wolframite yielded a lower intercept 206Pb/238U age of 221.0 ± 3.4 Ma (1σ, MSWD = 2.0), which records a late Triassic W mineralization event in the Baishitouwa deposit. In combination with previous geochronological data, we suggest that NE China may have an enormous potential for Triassic W mineralization and more attention should be given to the Triassic ore prospecting in the region. This work highlights that the chemical composition of wolframite is controlled by both the crystallochemical parameters and the composition of the primary ore-forming fluid. Trace-element compositions suggest that wolframite (I) was controlled by the substitution mechanism of 4A(Fe, Mn)2+ + 8BW6+ + B□ ↔ 3AM3+ + AN4+ + 7B(Nb, Ta)5+ + 2BN4+, whereas wolframite (II) was controlled by the substitution mechanism of A(Fe, Mn)2+ + A□ + 2BW6+ ↔ 2AM3+ + 2BN4+. Wolframite (I) contains higher concentrations of Nb, Ta, Sc, and heavy rare earth elements (HREEs), and lower Mn/(Mn + Fe) ratios than wolframite (II). Both wolframite (I) and (II) have similar trace elements and left-dipped REEN patterns, and analogical Nb/Ta ratios. They have similar Y/Ho ratios to Mesozoic highly fractionated W-mineralized granitoids in NE China. These data indicate that the W mineralization at Baishitouwa is genetically related to an underlying highly fractionated granite, and the compositional variation of fluids is likely driven by crystallization of wolframite during the processes of fluid evolution. A change of the ore-forming fluids from an oxidized to a relatively reduced state during the evolution occurred from stage 1 to 2.

Petrology of ophiolites of Memel, Nsimè-Kellé and Mapan (Yaoundé group): Evidence of the geodynamic evolution of the Pan-African orogeny in South Cameroon

Ophiolites are the remnants of ancient oceanic crust that can serve as tools to reconstruct the evolutionary history of fossil oceans. At Boumnyebel, the western part of the Yaoundé group, the sole of the lower unit (magnesian mica-schists and gneiss) of the Pan-African nappes is made up of ophiolitic rocks (talc schists, chlorite schists, serpentinites, amphibolites, pyroxenites, hornblendites). Structurally, the lower unit has a flat-lying schistosity with some recumbent folds while the underlying TTG gneisses are marked by a high-dipping foliation at Mapan. The contact between the Pan-African mica schists and the basement formations (Congo craton) is therefore discordant. In ophiolitic rocks, amphibole is actinolite, tremolite or magnesio-riebeckite. Chlorite compositions correspond to clinochlore. Talc shows notable substitution of Mg by Fe whereas substitution of Mg by Fe and Al prevails in serpentine. Spinel compositions are either Cr-rich with high FeO/Fe2O3 values (peridotitic spinels) or Cr-poor with low FeO/Fe2O3 values (melt-derived or magmatic spinels). Main mineral features indicate a metamorphic retromorphosis of the original lherzolites, harzburgites, dunites and pyroxenites to hornblendites and serpentinites. This is followed by hydrothermal alteration (at various temperatures: 250–570 °C and low pressure: < 2 kbar) that yielded chlorite schists and talc schists. Chemically, talc schists are made up of SiO2, MgO and minor FeO and Al2O3, while chlorite schists and hornblendites contain notable amounts of Al2O3 and CaO, respectively. All the ultramafic rocks display the same range of high Ni, Cr, Co contents with various LREE enrichments but flat HREE patterns. Multi-element patterns show that most talc schist are depleted in almost all elements. All the ultramafic rocks are marked by deep Nb, Ta, K, Pb, Sr, Ti, Zr and Hf troughs suggesting that the ultramafic rocks possibly derived from heterogeneous mantle sources varying from fertile to refractory peridotites. Amphibolites are generated from N-MORB, IAT or CA geotectonic settings. The compositions of spinel and ultramafic whole rocks reveal an arc setting that has evolved from divergent plate setting (oceanisation, N-MORB) to convergent plate (IAT, CA, SSZ D-MORB) settings. In summary, ophiolites emplaced in various geotectonic settings and were amalgamated during the Pan-African collision.

Paleozoic sedimentation and Caledonian terrane architecture in NW Svalbard: indications from U–Pb geochronology and structural analysis

Svalbard's Northwestern Basement Province is traditionally divided into the Albert I Land and the Biscayarhalvøya terranes. New U–Pb age data on zircon and monazite and structural and geochemical data provide first evidence of early Paleozoic deposits south of the Biscayarhalvøya Terrane indicating the possible existence of a third terrane: the Germaniahalvøya Terrane. This area is represented by a Cambro-Ordovician succession of mica schist and marble (Lernerøyane Group) and its higher-grade metamorphic equivalent (Liefdefjorden Migmatite Complex), which were affected by the Taconian phase (migmatization at c. 469 Ma) and the Scandian phase ( c. 422–415 Ma) of the Caledonian Orogeny. During the Scandian phase, the ductile Lerner Deformation Zone was formed. New isotopic data from the eclogite-bearing Richarddalen Complex of the Biscayarhalvøya Terrane imply the formation as an Ordovician–Silurian collision-related mélange dominantly composed of c. 730 to 600 Ma Timanian island-arc-derived detritus and igneous rocks, partly eclogite-facies metamorphosed at c. 656 Ma, and Tonian meta-igneous rocks. After amphibolite-facies metamorphism of the mélange matrix at c. 423 Ma, the Richarddalen Complex and the Stenian–Tonian Biscayarfonna Group were juxtaposed and mylonitized by the dextral Biscayarhalvøya Deformation Zone. Supplementary material: The complete geochemical and U-Pb isotope geochemical dataset as well as additional figures are available at https://doi.org/10.6084/m9.figshare.c.5778735

Ambient seismic noise analysis of LARGE-N data for mineral exploration in the Central Erzgebirge, Germany

Abstract. Ambient seismic noise tomography is a novel, low-impact method for investigating the earth's structure. Although most passive seismic studies focus on structures on crustal scales, there are only a few examples of this technique being applied in a mineral exploration context. In this study, we performed an ambient seismic experiment to ascertain the relationship between the shallow shear wave velocity and mineralized zones in the Erzgebirge in Germany, one of the most important metal provinces in Europe. Late Variscan mineralized greisen and veins occurring in the Geyer-Ehrenfriedersdorf mining district of the Central Erzgebirge were mined from medieval times until the end of the 19th century. These occurrences represent a significant resource for commodities of high economic importance, such as tin, tungsten, zinc, indium, bismuth and lithium. Based on ambient noise data from a dense “LARGE-N” network comprising 400 low-power, short-period seismic stations, we applied an innovative tomographic inversion technique based on Bayesian statistics (transdimensional, hierarchical Monte Carlo search with Markov Chains using a Metropolis/Hastings sampler) to derive a three-dimensional shear wave velocity model. An auxiliary 3D airborne time-domain electromagnetic dataset is used to provide additional insight into the subsurface architecture of the area. The velocity model shows distinct anomalies down to approximately 500 m depth that correspond to known geological features of the study area, such as (a) gneiss intercalations in the mica schist-dominated host rock, imaged by a SW–NE striking low-velocity zone with a moderately steep northerly dip, and (b) a NW-trending strike-slip fault, imaged as a subvertical linear zone cross-cutting and offsetting this low-velocity domain. Similar to the velocity data, the electromagnetic data exhibit north-dipping (high-conductivity) structures in the mica schists, corresponding to the strike and dip of the predominant metamorphic fabric. An unsupervised classification performed on the bivariate 3D dataset yielded nine spatially coherent classes, one of which shows a high correspondence with drilled greisen occurrences in the roof zone of a granite pluton. The relatively high mean shear velocity and resistivity values of this class could be explained by changes in density and composition during greisen formation, as observed in other areas of the Erzgebirge. Our study demonstrates the great potential of the cost-efficient and low-impact ambient noise technology for mineral exploration, especially when combined with other independent geophysical datasets.

Boron isotopes of tourmalines from the central Himalaya: Implications for fluid activity and anatexis in the Himalayan orogen

The relationship between fluid activity and anatexis in the Himalaya is ambiguous in regard to whether it is dominated by fluid-saturated melting triggered by Lesser Himalayan Sequence (LHS) fluids or dehydration melting of Greater Himalayan Sequence (GHS) metasedimentary rocks. Boron isotopes are a sensitive probe for tracing the source of fluids and their role during partial melting. However, a lack of constraints on LHS fluid activity has limited our understanding of anatectic processes in the Himalayan orogen. Here we report the results of in situ tourmaline and muscovite B isotope analyses of a suite of samples from the LHS and GHS, including chlorite schist from the LHS, quartz–tourmaline rock from the Main Central Thrust Zone, and leucogranites, leucosomes, mica schist, and garnet amphibolite from the GHS. Tourmaline B isotopic compositions fall in the range of δ11B = −18.3‰ to −8.2‰, with tourmalines from leucogranites showing δ11B values of −17.2‰ to −8.2‰, from leucosomes of −11.8‰ to −10.7‰, from schists of −17.8‰ to −13.9‰, from quartz–tourmaline rock of −18.3‰ to −12.8‰, and from garnet amphibolite of −16.9‰ to −13.6‰. Muscovites from leucogranite and mica schist yield relatively low B isotopes of −23.3‰ to −17.2‰ and −22.4‰ to −15.4‰, respectively. Different source compositions, prograde devolatilization, tourmaline fractionation, fluid exsolution, and external fluid metasomatism are inferred to be the main mechanisms responsible for the varied B isotopes in this study. Our data suggest that the LHS is characterized by depleted 11B, with LHS-derived dehydration fluids calculated to have δ11B values of −15.2‰ to −12.0‰ at 600 °C, whereas metamorphic fluids in the GHS have δ11B values of −11.8‰ to −4.8‰. Thus, we propose that dehydration of LHS hydrous minerals produced relatively light-B fluids. Consequently, leucogranites derived from hydration melting triggered by LHS fluids exhibit light B isotopic compositions compared with those derived from dehydration melting in the GHS. To investigate the source of heavy B in GHS leucogranites, we modeled B isotopic fractionation during muscovite prograde devolatilization, with results showing that substantial fractionation (∆11B > 7‰) would occur during muscovite dehydration. Hence, B lost from micas could be a source of heavy B in the leucogranites. On the basis of these results, we propose a model whereby LHS hydrous metasediments underwent intense dehydration, with the resultant fluids migrating upward and triggering hydration melting on the edge of melt channel in the GHS. Hence, tourmalines that crystallized from the hydrated magmas inherited the low δ11B characteristic of the LHS fluids, whereas those derived from dehydration melting of micas in the interior of melt channel acquired higher δ11B values.

Epilithic Microbial Community Functionality in Deep Oligotrophic Continental Bedrock.

The deep terrestrial biosphere hosts vast sessile rock surface communities and biofilms, but thus far, mostly planktic communities have been studied. We enriched deep subsurface microbial communities on mica schist in microcosms containing bedrock groundwater from the depth of 500 m from Outokumpu, Finland. The biofilms were visualized using scanning electron microscopy, revealing numerous different microbial cell morphologies and attachment strategies on the mica schist surface, e.g., bacteria with outer membrane vesicle-like structures, hair-like extracellular extensions, and long tubular cell structures expanding over hundreds of micrometers over mica schist surfaces. Bacterial communities were analyzed with amplicon sequencing showing that Pseudomonas, Desulfosporosinus, Hydrogenophaga, and Brevundimonas genera dominated communities after 8–40 months of incubation. A total of 21 metagenome assembled genomes from sessile rock surface metagenomes identified genes involved in biofilm formation, as well as a wide variety of metabolic traits indicating a high degree of environmental adaptivity to oligotrophic environment and potential for shifting between multiple energy or carbon sources. In addition, we detected ubiquitous organic carbon oxidation and capacity for arsenate and selenate reduction within our rocky MAGs. Our results agree with the previously suggested interaction between the deep subsurface microbial communities and the rock surfaces, and that this interaction could be crucial for sustaining life in the harsh anoxic and oligotrophic deep subsurface of crystalline bedrock environment.

Semiquantitative microscopic pore characterizations of the metamorphic rock reservoir in the central paleo-uplift belt, Songliao Basin

Currently, metamorphic rock is a common target for natural gas exploration, and reservoirs are the key factors restricting natural gas exploration and development in metamorphic rocks. The deep metamorphic rock gas reservoir in the central paleo-uplift of the northern Songliao Basin has good exploration and development potential. In this study, we use a combination of qualitative descriptions and quantitative analysis to comprehensively analyze the pore characteristics of the reservoir and explore the factors controlling the pore characteristics of the metamorphic rock reservoir in the central paleo-uplift belt of the Songliao Basin. The metamorphic rock reservoir in the central paleo-uplift belt contains three types of lithologies: chlorite schist, mica schist and mylonite, each with different protoliths and metamorphic histories. The results of high-pressure mercury intrusion and nitrogen adsorption indicate that the pore size distributions of the schist and mylonite differ. Compared with the mylonite, the schist has larger reservoir space, more heterogeneity, smaller pore size, larger specific surface area and larger adsorbed gas storage capacity. This paper also studies the formation process of the reservoir and divides it into four stages. Finally, this article discusses in detail the factors controlling the microscopic pore characteristics of metamorphic rock reservoirs in the central paleo-uplift belt; the metamorphic rock protolith is the most important controlling factor.

Geology and Geoelectrical Appraisal of Panda and its Environs parts of Gitata Sheet 187NE North-Central Nigeria

Geological mapping and Geophysical investigations were carried out in Panda and its environs, with the aim of studying the geology (rock types and their structural features; joints &amp; fractures) and its groundwater potentials. The area is underlain by the Basement Complex rocks of the North Central Nigeria consisting of porphyritic granodiorite, migmatized-gneiss and mica schist. The structural features associated with rocks in the study area include; exfoliation, joints, fault, pegmatitic and quartz veins. Twenty eight (28) Vertical Electrical Sounding (VES) were carried out away from cultural interferences. Interpreted data shows a dominance of KH and H type curve. The quantitative and qualitative interpretations revealed weathered and fractured basement. Iso-resistivity of the true aquifer, depth to basement, basement resistivity and, piezometric maps were delineated. Out of the twenty eight VES,only areas within the center of the study area (Kuban SE, Tanga North, Panda market, Jababe, Idur West, Idur and MusheJaba) have been delineated as the most viable locations for good water potentials due to the resistivity value of the rock materials (fractured and weathered basement) around those areas (50Ωm - 200Ωm) and some of the structural features (fractures, joints and faults) which serves as secondary reservoir for groundwater on account of their low resistivity values due to the high conductivity of groundwater that occupies the voids and fissures of the rock. The potential for groundwater in the study area are moderate at AngwanKwaro, AngwanMaje, and Chandap North, Kukuri area where depth to basement is shallow and occupied by fresh basement rocks (porphyritic granodiorite). While the least expected yield lies in AngwanPaa North and GidanSarki North. The two types of aquifer in the study area are weathered basement and fractured basement. This study will serve as guide for groundwater exploration and development in the study area.