Mobile Belt Research Articles

High temperature fluid-rock interaction recorded in a serpentinized wehrlite from eastern Singhbhum Craton, India: Evidence from mineralogy, geochemistry and in situ trace elements of clinopyroxene

A N-S trending, lenticular, and dismembered ultramafic unit is found emplaced within the multiply deformed metapelites of the North Singhbhum Mobile Belt, belonging to the Singhbhum Craton, eastern India. Their occurrence in an orogenic belt, lack of evidences of intrusive activity and close proximity to the Singhbhum Shear Zone suggest emplacement by thrust control. These rocks have experienced extensive serpentinization which has obscured and obliterated some of the features of the protolith. Relict areas of the Bangriposi wehrlite rocks indicate that the protolith might have been a cumulate clinopyroxene-dunite/wehrlite from an oceanic magma chamber. Detailed petrography and mineralogy reveal numerous metasomatic features which are separate from the serpentinization event: (i)Very high Ni-content of the relict olivines which does not correlate with Fo contents, (ii) Presence of secondary clinopyroxene (Cpx2) rims around primary olivine and replacing primary clinopyroxene (Cpx1), which have major and trace element compositions similar to clinopyroxenes from diopsidites of crustal origin (iii) Occurrence of secondary calcic hornblendes as tiny blebs within the Cpx2 rims (iv) Occurrence of secondary Mg-chlorite as interstitial patches between olivine rimmed by Cpx2 and inter-mixed with antigorite in the matrix, (v) Modifications of original Cr-spinels to ferri-chromite cores rimmed by magnetite. These features indicate these rocks have experienced fluid-rock interaction at temperatures above the serpentinization process. Primary Cpx1 grains exhibit elevated LREE compositions and can either reflect effects of cryptic metasomatism or crystallization from an enriched melt. Both groups of clinopyroxenes exhibit LILE enrichment and HFSE depletion. P-T estimates from Cpx1 and Cpx2 yield values of ~1200 °C and 0.5–1.2 GPa. The calcic hornblendes provide more realistic estimates of ~1000 °C and ~ 1.0 GPa, which suggest that the Cpx2 + hornblende assemblage formed during high temperature fluid-rock interaction. Presence of interstitial chlorite, chlorite+antigorite in the matrix and Cr-spinel recrystallization suggests successively decreasing temperatures during the fluid-rock interaction episode. Modeled trace element characteristics of fluids which are supposed to be in equilibrium with Cpx2 show similarities with high-temperature, ore-forming basinal brines. Similar basinal brines formed Cu-U ores in the Singhbhum Shear Zone and was likely the metasomatic agent of the Bangriposi wehrlite rocks. These rocks were originally cumulate ultramafic units from an oceanic crustal magma chamber, which experienced prolonged fluid-rock interaction in a retrogressive regime, until its emplacement as a dismembered unit in an orogenic belt.

The Bastar Craton of Central India: Tectonostratigraphic evolution and implications in global correlations

The Bastar Craton occupies a key position in the Peninsular India, being located between three cratonic nuclei of India, the Dharwar Craton in the southwest, the Singhbhum Craton in the northeast, and the Bundelkhand Craton in the northwest. The southeastern and the northern margins of the craton are bordered by the Proterozoic orogenic belts constituting the Eastern Ghats Mobile Belt and the Central Indian Tectonic Zone (the Satpura Mobile Belt), respectively. Analysis of the tectonostratigraphic evolution of the craton indicates the formation of the craton during Paleoarchean Era (~3582 Ma). The Archean evolution of the craton is marked by the development of granite-greenstone belts containing chemogenic sediments (Banded Iron Formations) and gold mineralisation . High-grade gneisses and granulites were developed during three orogenic episodes in the terrane (~2700 Ma Bailadila orogeny , ~2900 Ma Bengpal orogeny and ~3400 Ma Sukma orogeny). The Paleoproterozoic evolution of the craton is gleaned from the record of siliciclastic, volcaniclastics and chemogenic precipitates (carbonates, Banded Manganese Formation, and Banded Iron Formation) on the northern margin of the craton, which were deposited during the Great Oxidation Event (GOE; 2500–2300 Ma) and Paleoproterozoic glaciation (2400–2300 Ma) as well as immediately after the GOE (2050–1850 Ma). Effects of orogenic activities in the adjacent mobile belts are marked by the metamorphic and magmatic activities affecting the rocks at the margin of the craton. The Satpura orogenic event on the northern margin of the craton led to the amalgamation of the Bastar Craton with the Bundelkhand Craton at ~2250 and ~2100 Ma, forming the major part of the Peninsular India. Orogenic collapse (extension) developed basins transverse to the orogenic trend, where clastic, volcaniclastic and chemogenic sediments (Khairagarh and Chilpi Groups) were deposited. The southeastern margin of the craton has records of migmatisation and granite emplacement during the Eastern Ghats orogeny at ~1700 Ma. A number of sedimentary basins , aligned along a line parallel to the Eastern Ghats Mobile Belt, were developed during the orogenic collapse of the Eastern Ghats. Later orogenic activities along the Eastern Ghats were responsible for the development of depositional hiatus/unconformity within these sedimentary basins of the craton. Mafic dykes of different generations (2400 Ma, 2100 Ma and 1890 Ma) cutting across the Archean basement complex provide suitable markers for paleomagnetic reconstructions of the craton. The Archean and early Paleoproterozoic rock assemblages and mineralisation patterns in the craton show a broad similarity with the Western Australian Craton, indicating near-neighbourhood positions of the two ancient cratonic blocks. Paleomagnetic studies have shown the separation of these two blocks at ~2000 Ma, with the development of a paleo-ocean which closed subsequently during the amalgamation of the Bastar Craton with the East Antarctic Shield to form the Eastern Ghats Mobile Belt in three phases at ~1700–1600 Ma, ~1050–950 Ma and ~500 Ma, with extensional phase between 1450 and 1100 Ma. The Bastar Craton preserves important clues regarding the evolution of the ocean and atmospheric conditions in the early Earth. • The Bastar and Yilgarn Cratons were juxtaposed at ~2400 Ma. • Bundelkhand-Pilbara Cratons collided with Bastar-Yilgarn Cratons at ~2000 Ma. • Satpura-Singhbhum-Capricorn Orogen developed at ~2000 Ma. • No collision/accretion took place along the CITZ after ~1800 Ma. • New tectonic model for 1950–1000 Ma is proposed for Bastar-Yilgarn Cratons.

Tectonics of the Greater India Proterozoic Fold Belt, with emphasis on the nature of curvature of the belt in west-central India

The Greater India Proterozoic Fold Belt (GIPFOB) is a curved highly tectonized zone dominated by Early Paleoproterozoic to Early Neoproterozoic magmatic and metamorphic rocks extending from NW India (Aravalli Delhi Fold Belt, ADFB) through central India (the Satpura Mobile Belt, SMB) to eastern India (the Chottanagpur Gneiss Complex, CGC). The continuity of the crustal domains within GIPFOB is obscured by the Gondwana Formation, the Mesozoic Deccan basalts, the intertrappean Bagh Beds and the infratrappean Lameta Formations, and younger alluvium. In supercontinent reconstructions the GIPFOB is speculated to be continuous with Proterozoic mobile belts in Western Australia and the Trans North China Orogen. The NNE-striking western arm (ADFB) of the GIPFOB is flanked by the North India Block (NIB) in the east and the Marwar Craton in the west, whereas the E-striking southern arm (CGC-SMB) is sandwiched between the South India Block (SIB) and the NIB. In the Godhra-Chhota Udepur sector (west-central India) the two arms converge. We synthesize and compare existing data on mesoscale structures, U-Pb (zircon) and monazite chemical ages, and magmatic and metamorphic histories in the Precambrian crystalline rocks in the three crustal blocks to constrain the accretion dynamics in the GIPFOB with an emphasis on the origin of the curvature in the Godhra-Chhota Udepur sector. The CGC and the central and southern domains of the SMB share considerable homogeneity in chronology and regional structures. In the CGC, the early, N-striking steep-dipping tectonic fabrics in ~1.6 Ga anatectic gneisses with ~1.45 Ga and 1.05–0.9 Ga granitoids are modified into a carapace of shallowly-dipping tectonic mélange of recumbently folded basement gneisses and granitoid mylonites thurst over by allochthonous supracrustal rocks at ~0.95 Ga. The central and southern domains within SMB also exhibit the shallowly-dipping foliations and the emplacement of 1.05–0.9 Ga granitoids. The unmodified basement and the overlying tectonic mélange in the CGC-SMB are traversed by 1.0–0.9 Ga E-striking steep-dipping sinistral (dominant) and dextral (uncommon) shear zones that accommodated the Early Neoproterozoic transpressive deformation induced by the NIB-SIB oblique collision. The mesoscale structures in the rocks of the Godhra-Chhota Udepur sector intruded by Early Neoproterozoic granitoids and followed by oblique collision are similar to CGC-SMB. In the ADFB, by contrast, the basement rocks are older (Archean to Early Paleoproterozoic), high-grade metamorphism and felsic magmatism are older (1.8–1.7 Ga), the expansive ~1.45 Ga granitoids are absent, nappe structures are locally present, and expansive domains of the shallowly-dipping foliations are lacking. Overall, the tectonic evolution of the ADFB is incoherent with those in CGC-SMB and the GC sector. We suggest that the structures in the N/NNE-striking western accretion arm terminate against the E-striking southern arm. The Early Neoproterozoic (1.0−0.9 Ga) integration of the crustal domains within the GIPFOB resulted due to broadly contemporaneous convergence of the NIB, the SIB and the Marwar Craton (?) during the Rodinia supercontinent assembly, but the accretion along the southern arm post-dated the accretion in the western arm. [482 words]

The nature and origin of cratons constrained by their surface geology

AbstractCratons, defined by their resistance to deformation, are guardians of crustal and lithospheric material over billion-year time scales. Archean and Proterozoic rocks can be found in many places on earth, but not all of them represent cratonic areas. Some of these old terrains, inappropriately termed “cratons” by some, have been parts of mobile belts and have experienced widespread deformations in response to mantle-plume-generated thermal weakening, uplift and consequent extension and/or various plate boundary deformations well into the Phanerozoic.It is a common misconception that cratons consist only of metamorphosed crystalline rocks at their surface, as shown by the indiscriminate designation of them by many as “shields.” Our compilation shows that this conviction is not completely true. Some recent models argue that craton formation results from crustal thickening caused by shortening and subsequent removal of the upper crust by erosion. This process would expose a high-grade metamorphic crust at the surface, but greenschist-grade metamorphic rocks and even unmetamorphosed supracrustal sedimentary rocks are widespread on some cratonic surfaces today, showing that craton formation does not require total removal of the upper crust. Instead, the granulitization of the roots of arcs may have been responsible for weighing down the collided and thickened pieces and keeping their top surfaces usually near sea level.In this study, we review the nature and origin of cratons on four well-studied examples. The Superior Province (the Canadian Shield), the Barberton Mountain (Kaapvaal province, South Africa), and the Yilgarn province (Western Australia) show the diversity of rocks with different origin and metamorphic degree at their surface. These fairly extensive examples are chosen because they are typical. It would have been impractical to review the entire extant cratonic surfaces on earth today. We chose the inappropriately named North China “Craton” to discuss the requirements to be classified as a craton.

COMMENTS ON THE ARTICLE AUTHORED BY M.V. MINTS AND K.A. DOKUKINA – THE BELOMORIAN ECLOGITE PROVINCE (EASTERN FENNOSCANDIAN SHIELD, RUSSIA): MESO-NEOARCHEAN OR LATE PALEOPROTEROZOIC?

The comments are given on the article authored by M.V. Mints and K.A. Dokukina – The Belomorian Eclogite Province (Eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic? (Geodynamics & Tectonophysics 2020, 11 (1), 151–200). The Belomorian (White Sea) province of the Fennoscandia Shield is a key site for studying the tectonics of the early periods because numerous Precambrian eclogites have been found there. It was not anticipated, but the problem of age determinations of the eclogite metamorphism of gabbroids in the White Sea mobile belt has turned out to be extremely relevant not only for this region, but also for the Precambrian geology in general. The reason is that a number of authors determine the age of eclogites as Archean (2.7–2.8 Ga), which makes the White Sea mobile belt the only example of the Archean eclogite metamorphism in the world and, therefore, the only dated evidence in support of the plate tectonic model of the evolution of the Earth’s crust at the earliest stage of its formation. The article consistently provides a critical analysis of the arguments put forward by the supporters of the Archean age of the eclogites of the White Sea mobile belt. Special emphasis is made on the isotope geochronological and geochemical features of the composition of zircons from eclogite samples, as well as on the phase and chemical compositions and distribution patterns of mineral inclusions. Considering the age of eclogite metamorphism that led to the formation of eclogites in the White Sea mobile belt, we propose our interpretation based on a set of independent isotope geochemical dating methods, including the local U- Pb method for heterogeneous zircons with magmatic cores and eclogite rims, the Lu-Hf and Sm-Nd methods for the minerals of eclogite paragenesis (garnet and omphacite). And this age interpretation is fundamentally different from the one described in the commented article: all the three methods independently determine the eclogite metamorphism as Paleoproterozoic and yield the same age of circa 1.9 Ga. According to our data, the eclogites of the White Sea mobile belt are among the most ancient high-pressure rocks, their reliably established age of metamorphism is circa 1.9 Ga, and the age of the magmatic protolith is the range of 2.2–2.9 Ga.

New Insights into Pre-to-Post Ediacaran Zircon Fingerprinting of the Mamfe PanAfrican Basement, SW Cameroon: A Possible Link with Rocks in SE Nigeria and the Borborema Province of NE Brazil

The pre- to post-Late Neoproterozoic geological histories in the south to southwestern part of Mamfe Basin (SW Cameroon) were reported following analysis of the zircon crystals from their host rocks. A genetic model was developed for the zircon host rocks’ formation conditions, and the registered post-emplacement events were presented. The obtained ages were correlated with the data available for rocks in the Cameroon Mobile Belt, SE Nigeria, and the Borborema Province of NE Brazil. Separated zircons from Araru black to whitish gneiss, Araru whitish-grey gneiss, and Mboifong migmatite were analyzed for their morphology and texture U-Th-Pb composition, and U-Pb ages. Published U-Pb zircon ages for Otu granitic pegmatite, Babi mica schist, and Nkogho I-type anatectic granite were updated. Zircon ages in Araru black to whitish gneiss; Araru whitish-grey, Mboifong migmatite, Babi mica schist, Nkogho I-type anatectic granite, and Otu granitic pegmatite date the Eburnean tectono-magmatic/metamorphic event in Cameroon and SE Nigeria. The Late Paleoproterozoic to Early Mesoproterozoic ages record extensional (continental rift) settings and anorogenic magmatism in the Borborema Province in the NE of Brazil. These ages date collisional phases between the São Francisco–Congo and West African cratons and the Saharan metacraton with metamorphism and magmatism in Cameroon. They also date the Kibarian tectono-magmatic/metamorphism and PanAfrican tectono-magmatic/metamorphism in SE Nigeria. The Late Paleoproterozoic to Early Mesoproterozoic ages date the Cariris Velhos orogeny in the Borborema Province in NE Brazil, with Early Tonian crustal rifting, magmatism, and metamorphism and the collisional phase of the Brasiliano orogeny with syn-collisional plutons and extensive shear zoning and post-collisional granite intrusions.

Experimental study on soda granites of the Singhbhum Craton, eastern India, and its petrogenetic significance

The soda granite (~Paleoproterozoic) of the Singhbhum Craton (SC) of eastern India occurs along the entire stretch of the Singhbhum Shear Zone (SSZ; or Copper Belt Thrust Zone). The SSZ (marking a thrust contact between the adjacent North Singhbhum mobile belt and SC) serves as the important locale to host apatite‐magnetite, copper sulphide, and uraniferous mineralization. Typical field occurrence of soda granite is either as tongues (or patches) within a shear zone or as lit‐per‐lit style injections into surrounding country rocks which are generally highly sheared and mylonitized. Experimental studies on granite bodies can potentially throw light on their mineralogical and petrogenetic aspects; however, these studies are hitherto unreported in case of soda granites for almost the last 60 years. In this view, during the present research, experimental studies on soda granites have been carried out and these decipher the nature of the stability relationship of K‐feldspar (and transgression to more disordered anorthoclase), amphibole (both Ca‐amphibole and Fe–Mg–Mn amphibole and breaking down of Ca‐amphibole to Fe–Mg–Mn amphibole + non‐quad pyroxene + quartz at an appropriate temperature), biotite, muscovite, and albite (high albite‐ low albite displacive transformation). Whole‐rock chemistry (deduced from homogenized glass compositions) indicates its peraluminous, S‐type nature with an affinity towards Caledonian post‐collisional plutons. Characteristic geochemical plots suggest a pelitic source material for melting (and subsequent internal differentiation of the melt) and this has given rise to both migmatite as well as patchy soda granite exposures in the field. It is suggested that partial melting of mainly metapelitic (and minor mafic magmatic rocks) source (belonging to Singhbhum Series) gave rise to soda granites in this region.

Origin and characterization of clay deposits in the Dupi Tila Formation of the Bengal Basin, Bangladesh

Clay is one of the most important minerals having both geological and industrial significance. Clay lenses in the Dupi Tila Formation of the Bengal Basin have been investigated to characterize and determine their origin in terms of geological environment. Clay samples have been collected from the fresh cropped surface from the areas of Dupigaon (Jaintiapur Clay), Birisiri (Bijoypur Clay), and Bandarban (Bandarban Clay) in Bangladesh. Characterizations of the clay deposits have been carried out by field investigation, scanning electron microscope (SEM), X-ray diffraction (XRD), laser particle size analysis techniques, and X-ray fluorescence (XRF). Field investigation shows that clays are white, yellowish to variegated in color with a thickness of 15 cm to about 4 m, and moderately soft. The X-ray diffraction analysis reveals that kaolinite is the predominant clay mineral in all samples with minor amounts of quartz and feldspar. SEM analysis indicates that abundant tiny disc and irregular shape of kaolinite crystals are mixed with sparse gathers of K-feldspar and detrital quartz grains. The presence of silt in all the clay samples has been verified by the gran size ranges from laser particle size analysis. The grittiness is the result of the deposition of silt considering the highest percentage in Bandarban and Bijoypur and lowers in Jaintiapur. The XRF analysis shows that SiO2 (68–75%) is the most abundant among major oxides, Al2O3 and Fe2O3 come next, along with the lower percentage of MnO, MgO, CaO, Na2O, K2O, TiO2, P2O5, and SO3. The average concentration of different trace elements has been ranging from 3 to 430 ppm. Several geochemical indices suggest that the sediments might be derived from the Shillong plateau and the Himalayas in the north and the Naga-Lushai-China Hill-Arakan Yoma mobile belt in the northeast, which are highly weathered sedimentary terrains. Provenance studies and elemental ratio supports the evidence of the felsic origin of the studied sample. The Chemical Index of Alteration (CIA) value of 93% indicates the occurrence of significant weathering in different sediment recycling phases. From the results of the aforementioned analyses, it is clear that clay lenses in the Dupi Tila Formation are in-situ in origin and parent materials are detrital rather than authigenic nature confirmed by the presence of detrital quartz and reviewing the depositional environment, which gives us the insights of the geological condition of that time as well as the possible mode of sediment transportation.

Petrography and geochemistry of the granitoids and associated volcanic rocks of the northern part of Kushaka and Birnin Gwari schist belts, NW Nigeria: implications for provenance and geological setting

The granitoids and the associated volcanic rocks of the northern part of Kushaka and Birnin Gwari schist belts were emplaced in the ca. 3.5 – 1. 0 Ga remobilized basement complex terrain composed of metasedimentary and metaigneous rocks that later underwent medium- to high-grade metamorphism during the Pan-African thermo-tectonic event. They comprise dominantly of diorite, granodiorite, granite, granite gneiss and basalt, and are product of metasomatism and injections. The diorite and granodiorite occur as paleosome and the granite as leucosome with the development of high temperature minerals, locally attaining granulite facie metamorphism. Plagioclase, biotite, hornblende, pyroxene and olivine fractionation played an important role during their genesis through fractional crystallization of basaltic magma and partial melting of older dioritic-granodioritic source rock in the deep crust which were themselves ultimately derived through fusion of mantle materials contaminated by continental crust and enriched by fluids derived from oceanic crust in an arc setting. Geochemical characteristics have revealed different chemical trends in granitoids and basalts. The granitoids are calc-alkaline, ferroan and magnesian, metaluminous and peraluminous in character. They also exhibit I- and S-type signatures with enrichment in LILE, radioelements (Th and U), depletion in Nb, Sr, P and Ti, high LREE fractionation factors (La/Yb) (1.05 to 77.20), and pronounced negative Eu anomalies (Eu/Eu* = 0.34 to 1.10). Similar patterns of spidergrams show that the rocks are genetically related and were emplaced in a volcanic arc and syn-collisional setting. The basalt is tholeiitic, metaluminous and high in Fe and Mg with relative enrichment in LILE, HFSE, low and near flat LREE and HREE, low fractionation [(La/Yb)N = 1.4] with Eu/Eu* value of 1.10. It is evidently a back arc cum mid-ocean ridge (MORB) basalt. The consistent decrease in the content of MgO, Fe2O3 MnO, CaO, Sc, Cr and V of the basalt, diorites, granodiorites, and granites indicates continuous igneous crystallization process. It seems that extrusion of basaltic magmas from the sub-circular Kushaka Complex derived from subduction of oceanic crust resulted in complete change in the genesis of the magmas at the time, in this region. The granitoids and the basalt may have formed behind subducted Pan-African plate due to effects of compressional and tensional forces caused by oceanic plate roll-back which resulted to a zone of extension, parallel to the island arc. The granitoids present similar chemical characteristics to those in the other areas underlain by the basement complex and schist belts in the north and eastern parts of the Pan-African mobile belt, while basalts are similar to ophiolites and amphibolites in other schist belts of Nigeria forming a lateral continuation of the same mobile belt.Â

Geodynamic processes inferred from Moho and Curie depths in Central and Southern African Archean Cratons

Central and Southern Africa is home to major Archean Cratons, flanked by Proterozoic mobile belts. The region is characterized by anomalously high topography and shallow Curie depth (Zb). Factors controlling topography and Zb anomalies in old continental crusts remain poorly understood. In this study, we present high-resolution Moho undulation (Zm) derived with gravity spectral technique. Archean Cratons and Proterozoic belts are mostly characterized by deep Moho (>36 km), whereas the Moho depths in Mozambique belts is mostly shallow (~26 to 41 km). Extremely shallow Zb (<10 km) is dominant in Archean Cratons, whereas intermediate Zb (10–25 km) is prevalent in the mobile belts. The Curie points are mostly above the Moho. Low surface heat flow (Qs) in Archean Cratons indicate that Zb is not correlated with thermal perturbation. Shallow Zb are mostly found in areas with low Vp/Vs ratio, predominantly felsic lower crust and sharp Moho. Shallow Zb also correlate with intense metasomatism and mineral enrichment, as well as depletion in basaltic components and geochemical overprinting by multiple episodes of melt and fluid infiltration. Crustal composition is the main factor that controls shallow Zb in Central and Southern Africa Archean Cratons. The regional elevated topography is not due to thermal perturbation as mostly argued by mantle plume hypothesis, but may be caused by regional thermo-chemical mantle upwelling dominated by composition.

Provenance and Depositional Setting of Black Shales from the Dhalbhum Formation, North Singhbhum Mobile Belt, Eastern India

Abstract The Paleoproterozoic black shales of the Dhalbhum Formation exposed to the north of the Dalma metavolcanic suite is comprised of siliciclastic rocks and a minor amount of carbonaceous rocks. Geochemical studies were carried out to study the provenance and depositional setting of black shales. The average values of Co, V, U, and Cr/V (1.29) and Ni/Co (2.77) ratios in the black shales of Dhalbhum Formation are higher as compared to those of the Post-Archean Australian shale (PAAS) values (Cr/V=0.73, Ni/Co= 2.39). The chondrite normalized rare earth elements (REEs) patterns, indicate the enrichment in LREE (ƩLREE/HREE =10.73) and flat HREEs. The immobile element ratios Th/Sc (1.14), Th/Co (1.120), Cr/Th (16.161), and La/Sc (2.80) of studied samples indicate that sediments were derived from felsic rocks, which is also corroborated by La-Th-Sc ternary diagram. The sediments deposited at the active continental margin generally show REE pattern intermediate between a typical andesite and PAAS. A continental margin tectonic setting is distinguished by La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10, which is further corroborated by Yb vs Th/Ta diagram where the samples plot in the field of active continental margin.

Potassium distribution and isotope composition in the lithospheric mantle in relation to global Earth’s reservoirs

Recent analytical advances have provided means to measure potassium (K) isotopes in various terrestrial materials, but little is known about K distribution and stable isotope composition in the lithospheric mantle because of: (a) common low K abundances, (b) potential contamination and alteration, (c) diversity of mantle rocks and minerals hosting K in different tectonic settings. We report K abundances and δ41K values for well-studied whole-rock (WR) mantle xenoliths (spinel and garnet peridotites and pyroxenites) from mobile belts, a craton, a subduction zone, as well as for K-rich phases (mica, amphibole, silicate glass) and xenolith-bearing volcanic materials (67 samples). The xenolith materials show extremely broad ranges of K content (7 μg/g to 6.6 wt.%) and δ41K (−2.77‰ to 0.62‰). They contrast with the narrow δ41K range for host volcanic materials (−0.53‰ to −0.27‰) and literature data on oceanic basalts (melting products of upwelling asthenosphere: −0.43 ± 0.17‰, 2sd). Amphibole-bearing subduction zone peridotites show the highest WR δ41K values (0.40–0.62‰) likely inherited from fluids released into the mantle wedge from subducted oceanic crust. All other WR samples yield negative δ41K: −0.06‰ to −2.77‰ for peridotites and −0.17‰ to −0.52‰ for pyroxenites. The δ41K in K-rich mantle phases range from positive values (0.16–0.57‰) for phlogopite in strongly metasomatized peridotites to negative values (−0.27 to −0.94‰) for phlogopite, amphibole and glass pockets from other samples, which cannot be explained by equilibrium inter-mineral fractionation inferred from ab initio calculations. We attribute the broad δ41K variations to (a) isotope fractionation during fluid-rock interaction in the mantle, and (b) distinct sources of K-bearing fluids. Kinetic isotope fractionation during fluid percolation and diffusion is inferred for composite xenoliths (phlogopite and pyroxenite veins in peridotites) that have lower δ41K in the hosts than in the veins due to slower migration of 41K than 39K from the veins (former fluid channels) to host mantle. Overall, the K isotope fractionation in the lithospheric mantle appears to be greater than for magmatic fractionation in the crust. The average δ41K of normal off-craton continental lithospheric mantle calculated from the least altered fertile and lightly metasomatized lherzolites is −0.57 ± 0.28‰ (2sd). This value is within error (though a little lower) of estimates for both continental crust and MORB and OIB mantle sources indicating that these major silicate Earth reservoirs have similar bulk δ41K values, apparently due to low or negligible K isotopic fractionation during their formation by magmatic differentiation and melting. By contrast, K isotopes in modern and fossil subduction zones are fractionated via fluid-related equilibrium and kinetic processes.

PALEOZOIC GRANITOID MAGMATISM OF THE URALS: THE REFLECTION OF THE STAGES OF THE GEODYNAMIC AND GEOCHEMICAL EVOLUTION OF A COLLISIONAL OROGEN

The Ural mobile belt is an intracontinental epioceanic orogen that has already gone through all stages of the geodynamic development. Igneous rocks formed during each stage are important indicators for understanding the evolution of this belt and determining potential ore contents of its segments. We consolidated large datasets on petrogeochemistry and isotope geochronology of the Paleozoic (490–250 Ma) granitoids associated with the opening and evolution of the Ural paleoocean and the subsequent formation of the collisional orogen. Using these data, we have revised the ages of several tectono-magmatic events, clarified the paleogeodynamic settings for the generation of granitoids of different compositions, and described the roles of mantle-crust interactions and the plume factor in the formation of the mature continental crust in the study area. The results can be useful for geological mapping and improving the assessment of the potential ore contents in granitoid complexes that differ in origin and composition.

Zircon and monazite petrochronology studies reveal Mesozoic amphibolite-facies metamorphism overprinting Paleoproterozoic granulite-facies metamorphism: A case study from Rangnim Massif, North Korea

Recent studies have shown similar geological evolution processes between the Precambrian basements in North Korea and North China Craton (collectively called the Sino-Korea Craton). However, the detailed metamorphic evolution of supracrustal rocks in North Korea is still poorly understood. In this study, we investigated the petrology, mineralogy, zircon, and monazite UPb geochronology of five metapelite samples from the Jungsan Group in the Rangnim massif, North Korea, with the aim of better documenting the geodynamic evolution. The mineral assemblages and microtextures recorded two distinct metamorphic events, 1 and 2. Event 1 is characterized by a low-Ca garnet main domain, their abundant sillimanite inclusions, and equilibrium assemblages of sillimanite and biotite in the matrix, with minimum peak P-T conditions estimated at ~ 820 °C and ~ 8 kbar (with a geothermal gradient of ~ 100 °C/kbar). The timing of Event 1 was constrained to ca. 1855 Ma by zircon UPbdating. Event 2 is characterized by a high-Ca garnet narrow chemical band, staurolite, kyanite, biotite, and muscovite in the matrix, with near peak P-T conditions estimated at ~ 610 °C and ~ 7.9 kbar (with a geothermal gradient of ~ 75 °C/kbar). The timing of Event 2 was constrained to ca. 155–110 Ma based on a previous study of titanite and rutile UPb ages, but a subsequent fluid infiltration event at ca. 106 Ma by monazite UPb dating in this study. Our study indicates that supracrustal rocks in North Korea have experienced the same Paleoproterozoic high-grade metamorphism as that in the Jiao-Liao-Ji mobile belt in the North China Craton related to the Columbia Supercontinent; however, there has also been overprinting of a thermal event with mineral assemblages of amphibolite-facies related to extensive Late Jurassic–Cretaceous granitoids intrusion in the region.

Geochronology, petrography and Sr-Nd-Hf isotopes of Mbalizi carbonatite, southwestern Tanzania

The Mbalizi carbonatite is located in the middle of the Paleoproterozoic Ubendian Mobile Belt and the western branch of East Africa Rift, southwestern Tanzania. Calcite, dolomite, phlogopite, pyrochlore and apatite are found in the sample. Mineral chemistry studies have shown that the carbonatite phlogopite is linked to mantle-derived magmatism. The apatite is fluorapatite, means they are of magmatic origin. The analyses on two crystals of pyrochlore show high concentrations of Nb2O5, and therefore the Nb-oxide is classified as pyrochlore subspecies. Three types of zircon have been obtained from the Mbalizi carbonatite, including xenocrysts zircon, igneous zircon and metamorphic zircon. Zircon in-situ LA-ICP-MS U–Pb dating in this contribution indicates that the Mbalizi carbonatite was crystallized at ca. 116.0 ± 1.8 Ma. The εHf(t) values of igneous zircon ranging from −13.9 to +5.7, indicates that the carbonatite parental magma was originated from the sub-continental lithospheric mantle, and evolves toward HIMU and EM. The whole-rock Sr–Nd isotopic data suggest more contribution of the HIMU and EMⅠ material. We propose that the complex evolutionary history of the Ubendian Mobile Belt has stored the subduction oceanic crust which has the EMⅠ and HIMU components, forming the compositional heterogeneity mantle beneath the Ubendian Mobile Belt. At 116.0 ± 1.8 Ma, with the extension stress field, deep faults cause the pressure reduction, resulting in reactive of the upwelling of the HIMU and EMⅠ components. This provides the metamorphic conditions to induce the isotopic resetting and may result in large scatter of initial 176Hf/177Hf ratios of carbonatite melts.

Geochemical, Sr-Nd isotopic and U-Pb zircon study of 1.88 Ga gabbro-wehrlite from north-eastern Singhbhum Craton, India: Vestiges of Precambrian oceanic crust?

Dismembered, kilometer-scale slices of gabbroid and serpentinized wehrlite rocks are found emplaced within metapelites of Chaibasa Formation, belonging to the North Singhbhum Mobile Belt, Singhbhum Craton, India. Zircon grains from the gabbroid body yield an average U-Pb crystallization age of 1881.1 ± 8.4 Ma. The undeformed gabbroid body exhibits modal and cryptic layering, and both units exhibit cumulate texture. The presence of dismembered cumulate and layered mafic–ultramafic rocks within a mobile belt assemblage is enigmatic and has potentially far-reaching implications. The gabbroid rocks are moderately evolved with Mg# (100 Mg/Mg + Fe) ranging from 48 to 63, low alkali, TiO2, U, Th, Zr, and high Al2O3, Pb, Ba, Rb, with unfractionated rare earth elements patterns. The serpentinized wehrlite (Mg# 65–71) hosts secondary minerals (clinopyroxene rims, amphibole blebs and interstitial Mg-chlorite) and has LREE enriched rare earth element patterns, which suggest complex fluid-rock interaction history. The gabbroid rocks have 87Sr/86Sr(t) 0.70600–0.71099, 143Nd/144Nd(t) 0.510130–0.510294, and ƐNd(t) 1.8 to −1.4. The serpentinized wehrlite displays extreme enrichment with 87Sr/86Sr(t) 0.71045–0.72640, 143Nd/144Nd(t) 0.509120–0.509706, and almost upper crust-like ƐNd(t) of −9.7 to −21.2. The cumulate nature of the mafic–ultramafic rocks with other petrographical and geochemical evidences suggest formation in a magma chamber. Additionally, their field characteristics, stratigraphic constraints and E-MORB like character of calculated equilibrium melts of the gabbroid rocks, indicates affinity towards formation in an oceanic magma chamber and possible emplacement by obduction. Mineral chemistry, whole-rock geochemistry and clinopyroxene trace elements of the serpentinized wehrlite reveal progressive fluid-rock interaction from very high-temperature to low-temperature regimes, and deep crustal affinity of the modeled metasomatic fluid. Isotopic character of the gabbroid rocks can be explained by 30%-50% mixing between a depleted and an enriched domain in the lithospheric mantle, whereas isotopic nature of the wehrlite has been perturbed by the prolonged metasomatic event. Their occurrence within the Chaibasa intra-continental rift basin indicates rift-drift transition and formation of incipient oceanic crust at 1.88 Ga. A 1.86 Ga orogenic event caused closure of the basin and obduction of these remnant oceanic fragments onto the mobile belt assemblage. This is the first discovery of oceanic crustal fragments in Singhbhum Craton, which fits into the model of rift-dominated tectonics, widespread continental amalgamation, and ophiolite emplacement during the Columbia supercontinent cycle in Paleoproterozoic.

Methodological aspects of regional metallogenic analysis

Relevance and purpose of the work are due to the need to formulate the conceptual base and essence that determine the methodology of regional metallogenic analysis. Results. The fundamental provisions that make up the methodological and conceptual-semantic basis of the regional metallogenic analysis have been determined and concretized. The conceptual basis of regional metallogenic analysis is the provision that the ore (deposit) is included as a natural component in certain structural-material associations (or complexes), i.e., in certain formations of igneous, sedimentary and metamorphic rocks. They are objectively existing and actually mapped geological bodies, which are expressed both in the form of stratified (series, formations, strata, packs, horizons) and intrusive (massifs) formations. On their basis, structural-formation (structuralmetallogenic) zoning is carried out. An important feature of magmatic, sedimentary and ore formations is their recurrence in the geological history of mobile belts, which confirms the concept of the conservatism of metallogenic processes in the geological history of the Earth. At the same time, along with repetitive ones, there are magmatic, sedimentary and ore formations formed in the geological history of the Earth only once. Lateral and vertical rows of rock and ore paragenetic associations are characterized. Examples of Caledonian and Hercynian lateral ore-formation series of the Urals are given. The main provisions of the quantitative assessment of the predicted resources of the predicted types of minerals are formulated. Conclusions. The regional metallogenic analysis consists of the following components: petro-lithoformational analysis, structural-formational zoning which is adequate to structural-metallogenic zoning, paragenetic analysis of mineral associations and geological-industrial typification of various-scale occurrences of a mineral, and quantitative assessment of expected mineralization predicted resources.

Seismotectonic analysis of the 2017 moiyabana earthquake (MW 6.5; Botswana), insights from field investigations, aftershock and InSAR studies

The April 3, 2017 MW 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the Nubian plate and in a seismogenic region previously considered as stable. Our objective is to combine several approaches (field and remote sensing investigations) in order to adopt a multidisciplinary strategy so as to enhance our understanding of earthquake occurrence in intraplate southern Africa. We analyse the mainshock and aftershocks sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 900 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (MW 4.6 on the April 5, 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of the focal mechanisms produced results that are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes (a pair of ascending images 2017-03-30 and 2017-04-11) with two lobes with 3.86 cm–5.15 cm coseismic slip on a NW-SE elongated and 40-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~50 cm maximum slip on a fault plane striking 315°, dipping 45°, −80° rake and with Mo 3.68 × 1018 Nm. Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo Mobile Belt classifies the intraplate region as an active plate interior.

Structure and geometry of the Neoproterozoic Dom Feliciano Belt and Phanerozoic basins, southern Brazil, from magnetotelluric imaging

The investigation of the deep structures of Precambrian mobile belts is crucial for defining the tectonic evolution of Phanerozoic intraplate basins. The magnetotelluric (MT) method provides high-resolution two-dimensional (2-D) electrical resistivity models for the major deep-basement structures of a passive margin in southern Brazil. We obtained the resistivity models by applying mathematical inversion of 106 MT stations distributed along four profiles. The resistivity contrast between basement and basin and lateral discontinuity between basement terranes outline the geometry of the basins and the variation of the block physical properties and terrane of the Sul-Rio-Grandense shield. This information allowed to characterize and delimit the main structures, such as the Dorsal do Canguçu Shear Zone (DCSZ), the Passo do Marinheiro Shear Zone (PMSZ), and the depth of sedimentary cover of the onshore portion of the Pelotas Basin along the coast of the state of Rio Grande do Sul (RS), Brazil. The resistivity models extend 15 km deep, several kilometers below the top of the Precambrian basement. The top of the Precambrian basement varies from one profile to another from meters to kilometers and is covered in some places by packages of sedimentary rocks of different ages. The passive basin sediments of the Pelotas Basin can reach up to 4 km in thickness with structural control due to the inherited basement lineaments. Our MT data combined with data from wells and gravimetry data support a tectonic model for main structures of Sul-Rio-Grandense Shield in Neoproterozoic and for Phanerozoic sedimentary basin thicknesses.

Rock Magnetic Signatures of the Dalma Formation in the Singhbhum Mobile Belt, Eastern India

Abstract Petrography and rock magnetic properties of the Dalma volcanics (DV) and associated meta-sediments of Singhbhum mobile belt (SMB) are presented to describe magnetic grain size and Fe-Ti oxide mineralogy of the volcano-sedimentary sequence. Petrography revealed the presence of Ti-magnetite, hematite, ilmenite and pyrite in quartz-biotite-sericite-muscovite schists, quartzites, and volcanic rocks. The average value of magnetic susceptibility (χ) is 5.6 × 10−7 m3/kg, anhysteretic remanent magnetization (ARM) is 2.25 × 10−5 Am2/kg, saturating isothermal remanent magnetization (SIRM) is 403.33 × 10−5 Am2/kg, soft IRM is 240.2 × 10−5 Am2/kg, hard IRM is 77.56 × 10−5 Am2/kg, and natural remanent magnetization (NRM) is 54.1 mA/m. The central tendency of the inter-parametric ratio ARM/χ is 0.06 kAm−1, SIRM/χ is 111.04 kAm−1 and S-ratio is 0.8. Based on results, magnetic mineralogy indicated that the bulk magnetic carrier is stable single domain (SSD) or pseudo single domain (PSD) grains of Fe-Ti oxides with minor iron sulphides such as pyrite, greigite and pyrrhotite. Petrography and rock magnetic studies suggest that the magnetic carriers are chiefly secondary in origin and the NRM is modified over time.