Mobile Belt Research Articles

Genetic constraints and possible mechanism of gold mineralization within the carbonaceous metasedimentary units of the Dalma volcano‐sedimentary belt, North Singhbhum Mobile Belt, eastern India: Implications from pyrite geochemistry and carbon and sulfur isotope studies

In the present work, gold‐bearing carbonaceous phyllites in the Dalma volcano‐sedimentary belt (DVSB), occupying the central region of the North Singhbhum Mobile Belt in eastern India were studied to understand the nature and mode of occurrence of gold mineralization. We propose that the protolith of the carbonaceous phyllites, that is, organic‐rich black shale, was the major source of Au in the phyllite‐hosted gold mineralization reported from the DVSB. LA‐ICP‐MS analysis of pyrite in this study reveals elevated Au contents in the early‐formed sedimentary pyrite (syngenetic and diagenetic pyrite). Trace element systematics confirms the presence of invisible Au with in the sedimentary pyrite as solid solution (invisible gold). Petrographical evidences suggest a close spatial relationship of Au‐enriched pyrite and carbonaceous material. Stable carbon isotope values confirm the organic nature of the carbonaceous matter. Further, the high organic carbon content of these samples falls well within the favourable range of an auriferous carbonaceous host rock. These evidences suggest that the organic carbon played a key role in fixation of gold in the carbonaceous shales. Subsequent diagenesis resulted in the release of Au from the organic matter and its incorporation onto the coeval sedimentary pyrite. Field observations and reports by previous workers indicate that these rocks were subjected to a late‐stage metamorphic/hydrothermal event. Paragenesis of pyrite coupled with its trace element systematics reveals that this late‐stage event resulted in the formation of younger euhedral pyrite and pyrrhotite and released invisible gold from the pre‐existing sedimentary pyrite followed by its precipitation in the quartz reefs. Based on these studies, we propose a two‐stage model of gold mineralization, wherein gold was initially incorporated in black shale‐hosted sedimentary pyrite and later released and precipitated in quartz veins.

Crustal architecture and rift tectonics across the Visakhapatnam Bay basin, central-east Indian margin: Insights from multichannel seismic and potential field data

The Visakhapatnam Bay (VB) basin is a passive margin rift basin located at the non-basinal segment of the Eastern Continental Margin of India (ECMI) and formed during the rift–drift events associated with the breakup of eastern Gondwanaland. In the present study, integrated analysis of potential field and multi-channel seismic reflection (MCS) data were carried out to understand the rift tectonics, crustal configuration and onshore–offshore structural continuity across this basin. The study revealed the following: (i) crustal models derived through joint gravity-magnetic modelling show limited stretching with 36–40 thick crust below the Eastern Ghat Mobile Belt (EGMB) thinning down to 16–20 km at the Ocean Continent Transition (OCT), (ii) extension of Charnockitic basement associated with the Eastern Ghat Mobile Belt (EGMB) into the offshore region. Comparison of the crustal configuration across the VB basin with that across the adjacent thick sedimentary area of the Krishna–Godavari shows that upper crustal configuration is significantly different in the VB area. The observed limited rift related structuration (horst-graben morphology), the mapped high angle break-away fault with large offset in the seismic data and narrow width (70–90 km) of extended crustal domain in VB basin suggests that this segment acts as transfer zone between Krishna–Godavari and Mahanadi rift zones. Further, long curve-linear trend of magnetic anomalies associated with Pudimadaka Lineament (PKL) demarcate the VB basin from the adjacent Krishna–Godavari rift zone.

Using the Active Faults of Eurasia Database for solving tectonic problems

The article describes principles, methods and tasks of tectonic studies using computer processing of the Active Faults of Eurasia Database. This new database contains more than 30000 objects that are geographically linked, equipped with attributes of the kinematic type, estimated movement rates and activity confidence ranks. As an exam‐ ple, we consider processing of the data on several tectonic regions of the Alpine‐Himalayan mobile belt and construction of rose‐diagrams of faults for a comparative analysis of their Late Cenozoic kinematics. The processed data set also covered the Caucasus‐Anatolian region and the entire central part of the mobile belt, and fields of shortening/lengthening and shearing were mapped to assess the patterns of these processes in different areas and to determine the characteristics of the tectonic flow of the upper crust material. Prospects are discussed for the database processing with the use of all the available attributive information for structural‐kinematic and geodynamic analysis, including processing of the database in combination with independent remote and geophysical data.

Geochemical Features and Geological Processes Timescale of the Achaean TTG Complexes of the Ingozero Massif and the Pechenga Frame (NE Baltic Shield)

This article provides a geological review and results of the structural, metamorphic, and geochronological studies of the Pechenga frame outcrops located in the NW part of the Central-Kola terrain and the Ingozero massif outcrops situated in the northeastern part of the Belomorian mobile belt of the Kola Region (NW Baltic Shield). As a result of the work, the deformation scales and ages of the geological processes at the Neo-Archaean–Paleoproterozoic stage of the area’s development were compiled, and the reference rocks were dated. The petrochemical and geochemical characteristics of the Ingozero rocks are similar to those of tonalite–trondhjemite–granodiorite (TTG) complexes established on other Archaean shields. The isotope U–Pb dating of individual zircon grains from the biotite gneisses provided the oldest age for magmatic protolith of the Ingozero gneisses, which is 3149 ± 46 Ma. Sm–Nd model ages showed that the gneisses protolite initial melt formed at 3.1–2.8 Ga. Ages of metamorphic processes were determined by using isotope U–Pb dating ID TIMS (isotope dilution thermal ionization mass spectrometry): Biotite gneisses—2697 ± 9 Ma; amphibole–biotite gneisses—2725 ± 2 Ma and 2667 ± 7 Ma; and biotite–amphibole gneisses 2727 ± 5 Ma. Ages of granitoids, which cut the deformed gneisses, are 2615 ± 8 Ma and 2549 ± 31 Ma for plagiogranites and pegmatoid veins in gneisses, respectively. The following age sequence of geological processes was established by using U–Pb zircon dating: 2.8 Ga—The time of the garnet–biotite gneiss metamorphism; 2722 ± 9 Ma—The granodiorite crystallization time; 2636 ± 41 Ma—The aplite emplacement age and 2620 ± 16 Ma—The age of pegmatites origin, which marked final stages of the Archaean evolution; 2587 ± 5 Ma—The age of gabbros emplacement and 2507 ± 7 Ma—The age of gabbros metamorphism; 2522–2503 Ma—The origin time of the iron quartzite interpreted as the age of gabbros and biotite gneiss metamorphism.

Primary gold mineralization in the Lisungwe Valley area, Kirk Range, southern Malawi

Abstract A conspicuous paucity in gold resources in Malawi’s mineral inventory may be partially attributed to a general low resolution in the knowledge of Malawi’s geology, coupled with historical under-exploration for this important economic commodity. To address this knowledge gap, the present study focusses on developing a regional, local and microscopic understanding of the characteristics of gold occurrences in the mineralized Little Chisumbwiti river valley, located in the Kirk Range region which forms part of Southern Irumide mobile belt in southern Malawi. Regional scale interpretations are afforded by in-depth investigation of airborne geophysical data, which are then supported by ground geological mapping and by microscopic observations using optical and electron microscopy, and X-ray computed tomography (XCT). Structural mapping and regional geophysics indicate that the area has been affected by at least two periods of deformation (D1 to D2). The D1 deformation event is characterized by northwest – southeast directed crustal shortening (tentatively associated with the Southern Irumide orogeny) which produced northeast – southwest structures. Gold mineralization in the Little Chisumbwiti river valley is hosted in northeast – southwest trending quartz—sulphide vein sets, where it occurs as flakes ranging in size between 0.24 and 4 mm. The gold is associated with a paragenetically-late pyrite-chalcopyrite-sphalerite generation of sulphide precipitation, and to a lesser extent, with sericitised biotite schist wall rock. Exploration in the Kirk Range region should focus on the northeast-southwest structures, which represent potential conduits for fluid flow during the D1-related hypogene Au mineralization event, however the impacts of subsequent D2 deformation must also be considered. Locally, wall rock lithology (e.g., Fe (II)-rich biotite schists) may have an additional second order control on the siting of gold mineralization. The new insights reported in this study contribute meaningfully towards advancing the status of Malawi’s regional geology and should serve as a useful resource for future exploration efforts undertaken in the Kirk Range, and in other parts of south/central Malawi occurring within the Southern Irumide tectonic block.

Continental crustal evolution and synchronous metallogeny through time in the North China Craton

The generation and evolution of continent crust have long been matters of debates. There is evidence to support the view that the first formation of TTGs of continent was possibly before 4.4 Ga, followed by the sizable continent crust associated with BIF-bearing supra-crustal rocks at ca. 3.8 Ga. Multiple stages of continental growth include continental nuclei at ~3.1 Ga, enormous crustal growth at 2.9–2.7 Ga and stabilization and cratonization at ~2.5 Ga. Subsequently a stable super-continent was formed and the Earth was evolved into the Proterozoic eon. In the Proterozoic eon, major geological events include the great oxygen event (GOE), global rift and mobile belt event, and the Earth’s middle age event with a 1.0 billion years long period of Earth’s adjustment or silence. After that, the Rodinian super-continental break up event (the Nanhua rift) and subsequent Gondwana amalgamation have been suggested to be involved with mantle-plume and the modern style-like plate tectonics, both of which are still under debates. Recent studies suggest that the continental metallogeny has secular changes with the evolving Earth, showing synchronous evolution with the continental crust through time. The North China Craton (NCC) is one of the oldest cratons in the world, and has undergone six major tectono-metallogenic cycles: (1) the Neoarchean crustal growth and Algoman-type BIFs-greenstone belt and gold metallogenic system, (2) Paleoproterozoic GOE associated with global (Huronian) rifting- Glaciation and Superior-type BIFs-Mg-B metallogenic system, (3) Paleoproterozoic rifting-subduction-accretion-collision event (mobile belt) and Cu-Pb-Zn metallogenic system; (4) Late Paleoproterozoic – Neoproterozoic multi-stage rifting and REE-Fe-Pb-Zn metallogenic system; (5) Paleozoic orogenesis at the margins of the craton and porphyry Cu-Mo metallogenic system; and (6) Mesozoic extensional tectonics associated with lithosphere thinning and intracontinental Au metallogenic systems. Ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities. Changes of metallogenic styles with time, from the dominant BIFs in Archean to polymetallic deposits in Proterozoic and Phanerozoic, demonstrate that the nature and style of metallogeny through time change with different stages of the Earth’s evolution and are the expression of variable materials at the different stages. The changes of metallogeny through time are controlled by the irreversible continental tectonic evolution. The connotation and signification of synchronous evolution of continental crust and mineral deposit systems through time are beyond the study of mineral resources but would provide new insights into the geodynamics of the generation and evolution of the continent. The NCC provides an excellent venue to address the linkage between metallogenic processes and secular changes in the Earth System, including mantle dynamics and crust-mantle interaction on a global perspective.

Paleoproterozoic melt-depleted lithospheric mantle in the Khanka block, far eastern Russia: Inferences for mobile belts bordering the North China and Siberian cratons

The eastern part of Asia between the North China and Siberian cratons contains orogenic belts formed by the Paleo-Asian and Pacific subduction and older continental blocks. A fundamental question regarding these and all mobile belts is the fate of the continental lithospheric mantle (CLM) during their formation, i.e. whether, or to what extent the CLM may be formed, replaced or affected during orogeny. Insights into these processes can be obtained from mantle xenoliths hosted by Cenozoic basalts in the Proterozoic Khanka block in the far eastern Russia between NE China and the Pacific coast of Asia. We report petrographic, chemical, and Os-Sr-Nd isotope data for spinel peridotite xenoliths at two Khanka sites: Sviyagin and Podgelban. The modal abundances and chemical compositions suggest that the peridotites are residues of low to moderate degrees of melt extraction from fertile mantle. They show an 187Os/188Os vs. 187Re/188Os correlation with an apparent 1.9 Ga age; the 187Os/188Os ratios are positively correlated with Al2O3 and other melt extraction indices. These results provide the first robust CLM age constraints for the eastern Central Asian Orogenic Belt (CAOB). The ages suggest that the ancient CLM of the Khanka block may be roughly coeval with reworked CLM at Hannuoba (North China craton), and that it persisted through the Phanerozoic orogenies. Moreover, despite the proximity to Phanerozoic subduction zones, the Khanka CLM shows little post-melting enrichment, e.g. the clinopyroxenes are typically LREE-depleted and have Sr-Nd isotope ratios typical of the MORB mantle. We posit that the metasomatism of the CLM, earlier proposed for North China xenolith suites and ascribed to the effects of Pacific or older subduction and related mantle upwelling, may not be widespread in the CAOB. In general, Proterozoic blocks composed of residual peridotites may be more common in the CLM of the SE Siberia and northern China, and possibly other orogenic belts, than previously thought.

40Ar/39Ar geochronological evidence for multiple magmatic events during the emplacement of Tapira alkaline-carbonatite complex, Minas Gerais, Brazil

The Alto Parnaíba Igneous Province (APIP) is a voluminous magmatic province composed of various alkaline-carbonatite complexes emplaced in the Brasilia Mobile Belt during the Cretaceous. Relative timing of emplacement of silicate and carbonate magmas in most of these complexes remains mostly unresolved due to conflicting geochronological results. To determine the duration of magmatism and to test the possible existence of multiple magmatic events, we employ 40Ar/39Ar phlogopite single crystal dating to determine the history of magma emplacement at the Tapira alkaline-carbonatite complex, Minas Gerais, Brazil. The new single crystal data indicate at least two magmatic events during the emplacement of this complex, the first at > 96.2 ± 0.8 Ma and the second at 79.15 ± 0.6 Ma. The first igneous event was responsible for emplacement of the silicate plutonic series, while the second event corresponds to the emplacement of primarily carbonatitic magmas, generating metasomatic phlogopite alteration in bebedourites. The ages of intrusion and cooling of the alkaline-carbonatite complexes in the APIP must be investigated in other complexes to determine if intrusion intervals of ~17 Ma or more are common regionally. Protracted intrusive events, if related to magma generation by passage of South America over a stationary Trindade plume, requires complex ponding and lateral magma flow below a slow-moving continent.

The Cenozoic magmatism of East Africa: Part II – Rifting of the mobile belt

Despite the foundational role East Africa has played in the advancement of our understanding of continental rifting, there remains substantial ambiguity as to how magmatism has evolved during progressive rift development. This contribution is a comparative study that explores the temporal development of magmatism in both rifts, and within the interposing Turkana Depression. Notwithstanding the independent evolution of the Main Ethiopian Rift and Kenya Rift prior to their Quaternary linkage, magmatic events within them show remarkable parallelism. Following an initial pulse of basaltic magmatism ca. 20 Ma, more evolved compositions (flood phonolites in Kenya, rhyolites in Ethiopia) dominated the landscape until ca. 12 Ma. From ca. 12 Ma to 9 Ma, a renewed phase of widespread basaltic volcanism (Mid-Miocene Resurgence Phase) impacted the entire region from the Afar margin to Kenya, though activity in the south slightly predates equivalent basaltic events to the north. Following this widespread basaltic event, silicic magmatism again dominated the now nascent rifts until a renewed phase of basaltic activity commenced ca. 4 to 1.6 Ma termed the ‘Stratoid Phase’. Following the termination of the Stratoid Phase, the modern expression of volcanism in both rifts has been dominated by central silicic volcanoes. Magmatic activity within the Turkana Depression broadly parallels that in both rifts – basaltic events are evident at ca. 20 Ma, 12 Ma, and 4 Ma, however the silicic interludes that are evident within the rifts, are generally absent. Moreover, from 4 Ma to present, there is a progressive decrease in magmatic activity from an initial fissural basalt, to a series of large shield volcanoes, to modern belts of cinder cones. The results of this synthesis suggest widespread pulsed basaltic events precede silicic flare-ups throughout the East African Rift System, and the lack of such flare-ups in Turkana may relate to the Mesozoic rifting that modified the continental lithosphere in that region.

Delineation of gold mineralization near Lawa village, North Singhbhum Mobile Belt, India, using electrical resistivity imaging, self-potential and very low frequency methods

North Singhbhum Mobile Belt (NSMB), India, is almost 200 km long elongated zone. The region has stretched out shear zones that are in E-W direction with a complex history of lithology, structure, metamorphism and tectonic settings, which closely mirrors other gold producing orogenic metamorphic mobile belts. The study area, Lawa village, is located in Nimdih Block of Seraikella-Kharsawan district, Jharkhand, India. Detailed Electrical resistivity tomography (ERT), Self-Potential (SP) and Very Low Frequency Electromagnetic (VLF-EM) surveys were carried out to investigate appropriate conducting zones associated with CuU mineralization for gold exploration. Apparent resistivity pseudo sections obtained and compared with the current density pseudo sections of VLF data achieved from KH filtering technique and SP anomaly curves to mark out considerable lateral extension of prospective source. Conventional filtering methods cannot help in eliminating non-linear and non-stationary noises that contaminated the VLF-EM data, obtained from the field. To regain signals with significant geologic information, we have used empirical mode decomposition (EMD) technique. Results obtained show an excellent correlation between ERT and VLF profiles marked by low resistivity, high current density zones. This correlation indicates that the conducting features have shallow depth and are steeply dipping with a strike along E–W direction. However, SP anomaly curves were not clear enough to harmonize with the above result. This study boosts the scope of further geo-electrical and EM investigations in this region to explore appropriate target locations for gold exploration, which may be later, utilized on a significant scale for commercial purposes.

Modeling the Inception of Supercontinent Breakup: Stress State and the Importance of Orogens

AbstractThe relative significance of various geodynamic mechanisms that drive supercontinent breakup is unclear. A previous analysis of extensional stress during supercontinent breakup demonstrated the importance of the plume‐push force relative to the dragging force of subduction retreat. Here, we extend the analysis to basal traction (shear stress) and cross‐lithosphere integrations of both extensional and shear stresses, aiming to understand more clearly the relevant importance of these mechanisms in supercontinent breakup. More importantly, we evaluate the effect of preexisting orogens (mobile belts) in the lithosphere on supercontinent breakup process. Our analysis suggests that a homogeneous supercontinent has extensional stress of 20–50 MPa in its interior (<40° from the central point). When orogens are introduced, the extensional stress in the continents focuses on the top 80‐km of the lithosphere with an average magnitude of ~160 MPa, whereas at the margin of the supercontinent the extensional stress is 5–50 MPa. In both homogeneous and orogeny‐embedded cases, the subsupercontinent mantle upwellings act as the controlling factor on the normal stress field in the supercontinent interior. Compared with the extensional stress, shear stress at the bottom of the supercontinent is 1–2 order of magnitude smaller (0–5 MPa). In our two end‐member models, the breakup of a supercontinent with orogens can be achieved after the first extensional stress surge, whereas for a hypothetical supercontinent without orogens it starts with more diffused local thinning of the continental lithospheric before the breakup, suggesting that weak orogens play a critical role in the dispersal of supercontinents.

Rayleigh wave group velocity dispersion tomography of West Africa using regional earthquakes and ambient seismic noise

West Africa could teach us much about the early tectonic history of Earth, but current seismic models of the regional crustal and lithospheric structure lack the resolution required to answer all but the most basic research questions. We have improved the resolution of group velocity maps of the West African Craton by complementing the uneven path distribution of earthquake-generated surface waves with surface waves reconstructed from ambient noise cross-correlations. Our joint dataset provides good spatial coverage of group velocity measurements from 20- to 100-s period, enabling us to reduce artifacts in our group velocity maps and improve their resolution. Our maps correlate well with regional geological features. At short periods, they highlight differences in crustal thickness, recent tectonic activity, and thick sediments. At long periods, we found lower velocities due to hot, thin lithosphere under the Pan-African mobile belt and faster velocities due to cold, thick lithosphere under the Man-Leo and Reguibat shields. Our higher resolution maps advance us a step towards revealing the detailed lithospheric structure and tectonic processes of West Africa.

The use of reflectance spectroscopy and the prehnite spectral index to target gold mineralization at the Bonfim polymetallic skarn deposit, Seridó Mobile Belt, Borborema Province, Brazil

The Seridó Mobile Belt hosts numerous mineralized skarns in the Borborema Province, northeastern Brazil. The Bonfim W-Mo-Au-Bi-Te skarn occurs within a Neoproterozoic metasedimentary sequence (Seridó Group), within marble and schist lenses, or at the contacts between both. An ASD-FieldSpec™ 3 High-Resolution spectrometer was used to characterize the mineralogy of a representative drill core interval (181 m) from the Bonfim deposit, in order to generate the spectral characterization to define the gold mineralization. Skarns are characterized by the presence of diopside, actinolite-tremolite, clinozoisite-zoisite, muscovite and prehnite. Prehnite and epidote constitute the main components of the calc-silicate assemblage associated with late-stage gold mineralization. The diagnostic absorption feature of prehnite at 1478 nm (OH) can be used to identify its presence, and the prehnite index proposed in this case study (a reflectance ratio of 1465 nm/1478 nm) allows highlighting the prehnite enriched sections along the core. Geochemical data show a positive correlation between gold and the presence of a prehnite absorption at 1478 nm. The results suggest that the prehnite spectral signature can be used as a reliable and efficient (rapid) indicator for gold skarns, perhaps under geologically similar circumstances to those at Bonfim.

Variation in vorticity of flow during exhumation of lower crustal rocks (Neoproterozoic Ambaji granulite, NW India)

The exhumation of the Neoproterozoic Ambaji granulite in the Aravalli-Delhi mobile belt, NW India, took place along NNW-SSE trending D2-shear zones. The shear zones evolved from a high temperature (>700 °C) thrust-slip shearing event in the lower-middle crust to a low temperature (450 °C) retrograde sinistral top-to-NW shearing event at the brittle-ductile-transition (BDT). The vorticity of flow (Wm) along the shear zones is estimated with the Rigid Grain Net and strain ratio/orientation techniques. The Wm estimates of 0.32–0.40 and 0.60 coincide with the high temperature event and suggests pure shear dominated deformation. The low temperature phase coincides with Wm estimates of 0.64–0.87 and ~1.0 implying two flow regimes. The shear zone was first affected by general non-coaxial deformation and gradually became dominated by simple shearing. We interpreted that the high temperature event happened in a compressive tectonic regime which led to horizontal shortening and vertical displacement of the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate a non-steady strain during exhumation of granulite. This tectonic evolution is comparable with that of the Himalayas.

Geochemistry of Dalma metavolcanic Suite from Proterozoic Singhbhum Mobile Belt, Eastern India: Implications for Petrogenesis and Tectonic Setting

ABSTRACT The Dalma metavolcanic belt is composed of ultramafics at the base, tuffs on top and basalts in-between. It extends for about 200kms east-west in the middle of the Proterozoic Singhbhum Mobile Belt. Ultramafics are chiefly composed of actinolite, hornblende, relic pyroxene and olivine. The main mineral constituents of basalts are actinolite, hornblende, chlorite, epidote, clinozoisite and plagioclase whereas; tuffs are composed of xenocrysts of biotite, quartz, k-feldspar, plagioclase and iron oxides. Ultramafic samples have high MgO (> 21 wt.%) and low SiO2 (< 42 wt.%), Al2O3 (< 10 wt.%) and Na2O+K2O (< 1 wt.%). CaO shows large variation in these samples from 3.91 to 7.77 wt. %. In studied basalt samples SiO2, TiO2, Al2O3, MgO and alkalis (Na2O+K2O) show variation like (45.7-48.3 wt.%), (0.64-0.97 wt.%), (9.3-13.9 wt.%), (8.5-13.6 wt.%) and (0.95-4.64 wt.%) respectively. In studied tuffs the SiO2, TiO2, Al2O3, MgO and alkalis show variation from (48.9-49.8 wt.%), (0.7-0.8 wt.%), (13.6-13.9 wt.%), (8.4-9.1 wt.%) and (2.39-2.85 wt.%) respectively. In general, studied samples show light rare earth element depleted patterns and flat patterns for heavy rare earth elements similar to MORBs on chondrite normalized REE diagrams. The overall enrichment of LILEs (e.g., Rb, K and Th) and depletion of HFSEs (e.g., Nb-Ta trough) on primitive mantle normalized multi-element diagrams are similar to subduction zone or islamd arc basalts. Ratios like Ti/Y, Ti/Zr, Zr/Nb and Ti/V are also similar to those found in arc related volcanic suites. Sm/Yb vs La/Sm diagram indicates generation of melts from the spinel lherzolite mantle source. On tectonic setting discrimination diagrams like Zr vs Zr/Y, Zr vs Ti and Ti vs V majority of studied samples clearly show island arc tectonic affinity.

Petrogenesis of ultramafic-mafic clasts in the Dos Hermanos Mélange, Ilocos Norte: Insights to the evolution of western Luzon, Philippines

The clasts of ophiolitic mélanges formed in orogenic margins reflect the tectonomagmatic history of a region and record the petrological signatures of oceanic lithospheres that interacted in the past. Exposed at the northwestern edge of Luzon, Philippines, the highly deformed Dos Hermanos Mélange (DHM) provides new insights on the complex history of western Luzon island. The DHM is a tectonic mélange composed predominantly of ultramafic-mafic clasts set in a sheared serpentinite matrix. The ultramafic clasts are mostly harzburgites with rare occurrences of lherzolite, dunite and chromitite. Petrographic (e.g. protogranular to equigranular texture) and geochemical characteristics (e.g. spinel Cr# = 0.17–0.60, olivine Fo content = 87–91) of the peridotites typify residual mantle peridotites which underwent low to moderately high degrees of partial melting. Mineral chemistry of some dunite and harzburgite samples (e.g. high spinel TiO2 = 0.01–0.64 wt%) further record subsequent modification of the depleted mantle material by arc-related processes (e.g. metasomatism). Most of the mafic clasts classify as gabbros and are composed of highly anorthitic plagioclase (An88–99) and Ti-poor pyroxenes which suggest derivation from arc-related melts. One troctolite clast, however, records the distinct petrographic (e.g. ophitic texture) and geochemical (e.g. low An content of plagioclase = 73–80) signatures of primitive MOR-related magma. These contrasting petrologic signatures in the ultramafic-mafic clasts of the DHM are similar to those observed in the crustal and mantle sections of the Eocene Zambales Ophiolite Complex (ZOC). This suggests that the DHM, like the ZOC, records the complex history of the convergence and emplacement of an ancient oceanic crust onto the Philippine Mobile Belt. Later tectonic processes in the region, which occurred after the emplacement of the ZOC, resulted to the extensive dissection and translation of ophiolitic blocks northwards transforming them into the DHM.

Interactions between propagating rifts and linear weaknesses in the lower crust

AbstractPre-existing structures in the crust such as shear zones, faults, and mobile belts are known to exert a significant control on the structural evolution of continental rifts. However, the influence of such features when the extension direction progressively changes over time remains uncertain. Here we present new results from three-dimensional lithospheric-scale laboratory experiments of rotational extension that provide key insights into the temporal evolution of propagating rifts. We specifically test and characterize how rifts propagate and interact with linear crustal rheological heterogeneities oriented at variable angles with respect to the extension direction. Results show that approximately rift-parallel pre-existing heterogeneities favor the formation of long, linear faults that reach near-final lengths at early stages. Low angles between the heterogeneities and the propagating rift axis may result in strong strike-slip reactivation of the pre-existing structures if they are suitably oriented with respect to the stretching direction. When the linear heterogeneities are oriented at intermediate to high angles rift branches become laterally offset as they propagate, resulting in complex rhombic fault patterns. Rift-perpendicular crustal heterogeneities do not affect fault trends during rift propagation, but cause stalling and deepening of laterally growing rift basins. Similarities between the analogue experimental results and selected natural examples provide insights on how nature finds the preferential pathway to breakup in heterogeneous continental lithosphere.

Edge-driven convection and thinning of craton lithosphere: Two-dimensional thermal-mechanical modeling

There are usually abrupt changes in lithospheric thickness at the boundaries between ancient cratons and adjacent young mobile belts. Lateral variations in temperature and density between the two can trigger small-scale mantle convection (edge-driven convection, EDC). Here, we use two-dimensional thermal-mechanical simulations to explore the EDC caused by the lithospheric step between a craton and a mobile belt, and its role in the thinning of the craton lithosphere. The results show that the impact of EDC on a craton depends on the properties of the craton lithosphere and on their contrasts with the adjacent mobile belt, given the same initial condition. When the craton lithospheric density is relatively large, a high-strength craton has strong ability to resist EDC, and craton lithospheric thinning is limited to the edge. In contrast, the ability of a low-strength craton to resist EDC is weak, and the craton lithosphere is gradually eroded by the downward flow, eventually leading to large-scale thinning of the cratonic lithosphere. When the craton lithospheric density is relatively small, regardless of the strength of the cratonic lithosphere, the craton can well withstand the impacts of EDC. In this case, upwelling flow drives cratonic lithosphere materials to the base of the mobile belt, and lithospheric thinning only occurs at the edge of the craton lithosphere. The Archean North China Craton (NCC) was surrounded by Phanerozoic mobile belts, and its lithospheric thinning first occurred on the northern and eastern margins, a process that lasted for a long time. We suggest that EDC has played an important role in lithospheric thinning of the NCC, in particular the initiation of lithospheric thinning, but we cannot rule out the additional contributions from other mechanisms.

Reappraisal of the ‘early proterozoic gabbro-anorthosite suite’ rocks from the eastern Singhbhum craton, India: Insights from field features, petrography–mineralogy and geochemistry

The present study focuses on six mafic–ultramafic units from eastern Indian Singhbhum craton: (i) gabbroic rocks of Galudih, (ii) dolerite from North of Bisoi, (iii) Bangriposi wehrlite (BW), (iv) dolerite from North of Kuliana, (v) Kuliana-layered gabbro (KLG) and (vi) dolerite of Jashipur. These rocks had been grouped earlier as a co-genetic unit titled ‘early proterozoic gabbro-anorthosite suite’. Dolerites of Jashipur, North of Bisoi and North of Kuliana are found as undeformed intrusives into the Mesoarchaean Mayurbhanj granite (MBG) unit and show hydrothermal alteration features and variable LREE enrichment. Multiply-deformed gabbroic rocks of Galudih also intrude the MBGs with similar hydrothermal alteration features and comparatively lower REE abundances. KLG and BW are found as dismembered rocks, emplaced within a multiply-deformed metasedimentary assemblage of Mid-Proterozoic North Singhbhum mobile belt. Through a critical assessment of their field features, petrography, mineralogy and geochemistry, the earlier grouping was found to be invalid. Instead, these rock units should be categorised into three groups: Group I (Galudih), Group II (Jashipur, North of Bisoi, North of Kuliana) and Group III (KLG and BW). Group I rocks are the oldest among them, but their stratigraphic affinity remains unclear. Group II mafic rocks possibly belong to the Proterozoic newer dolerite dykes, whereas Group III represents fragments of oceanic crust and mantle.

Delimiting the Neoproterozoic São Francisco Paleocontinental Block with P-wave traveltime tomography

SummaryThe São Francisco Paleocontinental Block (SFPB) represents part of the Congo-São Francisco Paleocontinent (CSFP), amalgamated around 2 Ga. In the Neoproterozoic, a branched continental rift system evolved to ocean basins around most edges of the SFPB that remained only partially linked to the Congo Paleocontinent by means of the Bahia-Gabon Continental Bridge. After the Brasiliano—Pan-African orogeny, two relatively preserved CSFP sectors formed the São Francisco and Congo cratons, surrounded by Neoproterozoic orogenic belts. Recent results of upper mantle P-wave seismic tomography allowed us to suggest a delimitation in lithospheric depths of the Neoproterozoic SFPB, which comprise the São Francisco Craton, and that this would have been connected with the Congo Paleocontinent along the Araçuaí Belt. It is characterized by high-velocity anomalies and its boundaries with other blocks are marked by low-velocity anomalies at lithospheric depths. We tested the resolution of the tomographic results through synthetic models obtained by a ray tracing scheme using the observed ray configuration. We observe that the lateral resolution is adequate, but the method used was not able to set the depth reached by the SFPB. Our results indicate that the SFPB area in lithospheric depths is larger than the surface area ascribed to the São Francisco craton, and thus, the SFPB basement deeply extends beneath neighboring orogenic regions, suggesting that these Neoproterozoic mobile belts, such as Araçuaí Orogen and the Brasilia Fold Belt, reworked the continental crust. We observe a low-velocity anomaly in the SFPB central region, corresponding to the Pirapora aulacogen. Our results have a good spatial correspondence with the low Bouguer anomalies used to define the SFPB in previous studies. The limits of the SFPB are consistent with deviation of the mantle flow, as suggested by SKS fast polarization.