Cratonic Blocks Research Articles

The birth, growth and ageing of the Kaapvaal subcratonic mantle

The Kaapvaal craton and its underlying mantle is probably one of the best studied Archean entity in the world. Despite that, discussion is still vivid on important aspects. A major debate over the last few decades is the depth of melting that generated the mantle nuclei of cratons. Our new evaluation of melting parameters in peridotite residues shows that the Cr2O3/Al2O3 ratio is the most useful pressure sensitive melting barometer. It irrevocably constrains the pressure of melting (melt separation) to less than 2 GPa with olivine (ol), orthopyroxene (opx) and spinel (sp) as residual phases. Garnet (grt) grows at increasing pressure during lithosphere thickening and subduction via the reaction opx + sp → grt + ol. The time of partial melting is constrained by Re-depletion model ages (TRD) mainly to the Archean (Pearson and Wittig 2008). However, only 3% of the ages are older than 3.1 Ga while crustal ages lie mainly between 3.1 to 2.8 Ga for the W- and 3.7 to 2.8 Ga for the E-block. Many TRD-ages are probably falsified by metasomatism and the main partial melting period was older than 3.1 Ga. Also, Nd- and Hf- model ages of peridotitic lithologies from the W-block are 3.2 to 3.6 Ga old. The corresponding very negative eNd (−40) and eHf values (−65) signal the presence of subducted crustal components in these old mantle portions. Subducted components diversify the mantle in its chemistry and thermal structure. Adjustment towards a stable configuration occurs by fluid transfer, metasomatism, partial melting and heat transfer. Ages of metasomatism from the Lu-Hf isotope system are 3.2 Ga (Lace), 2.9 Ga (Roberts Victor) and 2.62 Ga (Finsch) coinciding with the collision of cratonic blocks, the growth of diamonds, metamorphism of eclogites and of Ventersdoorp magmatism. The cratonic lithosphere was stabilized thermally by the end of the Archean and cooled since then with a rate of 0.07 °C/Ma.

Eastern Ghats Belt, Grenvillian-Age Tectonics and the Evolution of the Greater Indian Landmass: A Critical Perspective

The configuration of the Greater Indian Landmass was achieved during the late Proterozoic era (Grenvillian-age) through tectonic cycles involving cratonic blocks of India and East Antarctica in the broad framework of the assembly of the supercontinent Rodinia. Geological evidences are recorded from orogenic belts separating southern, northern and western cratonic blocks of India and its transcontinental neighbor East Antarctica. Eastern Ghats Belt of India played a pivotal role in the continental amalgamation process and it evolved in tandem with the Central Indian Tectonic Zone and the Aravalli Delhi Mobile Belt. We have collated geological and geochronological evidences from the cratonic blocks and the bounding orogenic belts to trace back the Grenvillian-age tectonics surrounding India and its eventual manifestation as the configuration of the Greater Indian Landmass. The status of the Greater Indian Landmass as a part of Rodinia is debated and unresolved issues are highlighted.

São Francisco–Congo Craton break-up delimited by U–Pb–Hf isotopes and trace-elements of zircon from metasediments of the Araçuaí Belt

Detrital zircon U–Pb geochronology combined with Hf isotopic and trace element data from metasedimentary rocks of the Araçuaí Belt in southeastern Brazil provide evidence for break-up of the Congo–São Francisco Craton. The U–Pb age spectra of detrital zircons from metasediments of the Rio Doce Group (RDG) range from 900–650 Ma and define a maximum depositional age of ca. 650 Ma. Zircon trace element and whole rock data constrain an oceanic island arc as source for the deposition setting of the protoliths to the metasediments. Zircon εHf(t) values from these rocks are positive between +1 and +15, supporting previous evidence of a Neoproterozoic extensional phase and oceanic crust formation in a precursor basin to the Araçuaí Belt. Recrystallization of detrital zircon at ca. 630 Ma is compatible with a regional metamorphic event associated with terrane accretion to the Paleoproterozoic basement after transition from an extensional to a convergent regime. The juvenile nature, age spectra and trace element composition recorded in detrital zircons of metasediments from the Araçuaí Belt correspond with zircons from metasedimentary rocks and oceanic crust remnants of other orogenic belts to its south. This suggests that rifting and oceanic crust formation of the entire orogenic system, the so-called Mantiqueira Province, was contemporaneous, most likely related to the opening of a large ocean. It further indicates that the cratonic blocks involved in the orogenic evolution of the Mantiqueira Province were spatially connected as early as 900 Ma.

Anorthosites from an Archean continental arc in the Dharwar Craton, southern India: Implications for terrane assembly and cratonization

Anorthositic rocks have attracted attention in terms of their possible role as the primordial crust of our planet, and also as markers of Archean-Paleoproterozoic plate tectonic regimes. Here we investigate the Konkanhundi gabbro-anorthosite suite from Peninsular India, adjacent to the major collisional suture between the Western and Eastern Dharwar cratonic blocks. The anorthosites display high Al2O3 contents with negative correlation against MgO, high CaO and Sr contents, and positive Eu anomalies indicating plagioclase flotation in the magma chamber. The pyroxenite and gabbroic rocks of the suite show high Ni, Cr, Co contents and geochemical features typical of pyroxene cumulation. LA-ICPMS U-Pb analyses of magmatic zircon grains yield weighted mean 207Pb/206Pb ages of 2601 ± 12 Ma for pyroxenite, 2616 ± 12 Ma for gabbro, 2615 ± 13 Ma and 2627 ± 14 Ma for anorthositic gabbro, 2594 ± 16 Ma for anorthosite, and 2605 ± 27 Ma for microgabbro. The age data from the different rock types in the anorthosite suite are broadly consistent, marking the emplacement time as ca. 2.6 Ga, and providing insights on one of the oldest anorthosite complexes in Peninsular India. Zircon grains in the surrounding TTG gneisses into which the gabbro-anorthosite suite was emplaced show weighted mean 207Pb/206Pb age of 3321 ± 11 Ma suggesting Mesoarchean basement. The zircon grains display high Th/U values and REE patterns typical of magmatic crystallization with enriched HREE, positive Ce and Sm anomalies, and negative Pr, Nd, Eu anomalies. Zircon Lu-Hf analysis yield negative εHf(t) values of −4.9–−0.7 with TDMC (two stage model ages) of 3123–3376 Ma for pyroxenite, −4.5–−1.7 and 3192–3366 Ma for gabbro, −5.3–0.9 and 3034–3410 Ma for anorthositic gabbro, and −3.9–-2.4 and 3221–3311 Ma respectively, suggesting the incorporation of Mesoarchean components within the Neoarchean magmatic suite. However, zircon grains in the TTG gneiss possess more ‘juvenile’ εHf(t) values in the range of −0.2–1.3 with TDMC in the range of 3554–3646 Ma. The zircon Hf isotopes and trace element data, together with the whole rock geochemical features suggest that the parent magma of the Konkanhundi gabbro-anorthosite suite was derived from subduction-related depleted mantle source that also incorporated continental crustal components. The time of emplacement of the gabbro-anorthosite complex broadly correlates with widespread arc magmatism and crust production as well recycling in other domains of the Dharwar Craton. We envisage multiple convergence of microblocks during craton assembly at the end of Archean in Dharwar, with the greenstone belts marking zones of paleo-ocean closure.

Late Paleoproterozoic ultrahigh-temperature metamorphism in the Korean Peninsula

The Paleoproterozoic rock record from the Sino-Korean Peninsula was fundamental to the models on evolution of the Asian continental collage and its relationship with the amalgamation and disruption of Proterozoic supercontinents such as Columbia and Rodinia. In this study, we present results from a comprehensive investigation on the khondalites in the Hongseong area, along the western part of the Gyeonggi massif within the central Korean Peninsula involving petrography, phase equilibria forward modeling, and zircon U-Pb geochronology, REE geochemistry and Lu – Hf isotopes. The P-T pseudosections show that the peak mineral assemblage of garnet + sillimanite + quartz + plagioclase + K-feldspar + ilmenite in the khondalite is stable at 7.5–9.6 kbar and 895–990 °C, corresponding to ultra-high temperature (UHT) metamorphism. We also report the first finding of corundum and quartz inclusions within sillimanite enclosed in garnet porphyroblast further confirming UHT metamorphism. The interpreted P-T path involves decompression and exhumation along a clockwise trajectory as suggested by the retrograde textures and mineral phase equilibria modeling.The detrital zircon U – Pb data from the khondalite show multiple age peaks for the magmatic population suggesting Neoarchean – Paleoproterozoic provenance. Metamorphic zircons from the khondalites yield ages of 1931 ± 33 Ma, 1871 ± 11 Ma and 1866 ± 10 Ma. The spot ages range from 1923 Ma to 1832 Ma suggesting either a long-lived metamorphic event or two distinct thermal events. The REE patterns of the zircon grains also clearly distinguish the magmatic and metamorphic populations. The garnet + sillimanite assemblage is considered to represent the peak UHT assemblage and the cordierite + biotite represents the post-peak retrograde assemblage. The ca. 1923–1832 Ma ages obtained in our study from metamorphic zircon domains correlate well with similar ages reported for metamorphism associated with the amalgamation of crustal blocks within the North China Craton. Our study suggests that the Late Paleoproterozoic UHT metamorphism in the central Korean Peninsula might be part of a common event within the Sino-Korean Craton related to the tectonics of final assembly of cratonic blocks within the Columbia supercontinent. Although a substantial volume of the Paleoproterozoic basement of the Korean Peninsula was destroyed and recycled during the Neoproterozoic and Phanerozoic tectonic events, the preserved remnants provide insights into the contiguity of the basements of the Korean Peninsula and the North China Craton.

The Paleoproterozoic Vishnu basin in southwestern Laurentia: Implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave crustal province

The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8–1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ∼1.87 and 2.4–2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ∼1.8 to 1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ∼1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791 ± 15 to 1691 ± 15 Ma. Plutons show a systematic change in Hf-isotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8–2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7–1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.

Mantle heterogeneity, plume-lithosphere interaction at rift controlled ocean-continent transition zone: Evidence from trace-PGE geochemistry of Vempalle flows, Cuddapah Basin, India

This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases. Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle (SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSE-REE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by (i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent, (ii) interaction between plume-derived melts and SCLM, (iii) their rift-controlled intrabasinal emplacement through Archean–Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone (OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.

Tectonic evolution of the Juvenile Tonian Serra da Prata magmatic arc in the Ribeira belt, SE Brazil: Implications for early west Gondwana amalgamation

The evolution of the Ribeira belt resulted from the progressive amalgamation of several terranes against the eastern margin of the São Francisco Craton between ca. 620 and 580Ma. This work brings new field, U-Pb geochronology, geochemistry and isotopic (Sm-Nd and Sr) data on the evolution primitive rocks from the Serra da Prata magmatic arc and their relationships with the previously described Rio Negro arc. The new U-Pb data allow the distinction of two episodes of arc generation: the Serra da Prata Arc (856–838Ma) and the Rio Negro Arc (790–620Ma). Rocks from the oldest stage are composed of metaluminous calc-alkaline diorites, tonalites and granodiorites, and geochemical signatures compatible with magmatic arc scenarios. Their rocks are associated to a metamorphosed volcano-sedimentary of intra or back-arc basin setting platform carbonates, amphibolites (basaltic lavas) and psammitic rocks of the Italva group. Whole-rock Nd and Sr isotope data indicate more primitive contribution than earliest stage: initial εNd=−3.7 to +5.2, TDM=1.68–0.92Ga and 87Sr/86Sr initial ratios between 0.7061 and 0.7113. The second stage – Rio Negro arc – yielded more mature arc signatures: initial εNd=−8.4 to −2.5, TDM=1.93–1.33Ga and 87Sr/86Sr initial ratios between 0.7098 and 0.7211. The new data have been interpreted as an evolution of a Tonian primitive intra-oceanic stage of the magmatic arc generation, followed by more continental or transitional arcs during the Rio Negro stage. The data from both arc stages contrast with the younger Serra da Bolívia and Rio Doce continental arcs (570–590Ma) developed in a proximal location. The data are similar to other Tonian-Ediacaran magmatic arcs: the Goiás arc in the Brasília Belt (ca. 862–630Ma) and the São Gabriel arc (ca. 840–690Ma), located respectively along the western margin of the São Francisco and Rio de La Plata cratons. In a Western Gondwana scenario, the juvenile signature indicates intra-oceanic tectonic settings. The combination of the older Tonian arcs with the more evolved Cryogenian to Ediacaran arcs within the Neoproterozoic belts, suggests more than 200m.y. of subduction around the older cratonic blocks that made up Western Gondwana.

∼2.1 Ga intraoceanic magmatism in the Central India Tectonic Zone: Constraints from the petrogenesis of ferropicrites in the Mahakoshal supracrustal belt

Picrites with anomalous high-Fe (ferropicrites), have erupted from the Archean to recent, however, their incidences are extremely limited. Particularly, post-Archean occurrences are scarce. Experimental studies indicate that ferropicrites are derived from partial melting of the upper mantle at moderate pressures (≤4GPa). The origin of the anomalous Fe-enrichment in their source, however, remains elusive, and in part attributed to the presence of silica-deficient pyroxenite.In this contribution we present a detailed account of ferropicrites in the Chitrangi Formation of the Paleoproterozoic Mahakoshal supracrustal belt in the Central India Tectonic Zone. The Lu-Hf isotope systematics of these rocks imply an age between ∼2.3Ga and ∼1.8Ga. Compared to the Lu-Hf isotopic system, the Sm-Nd isotopic system appears to be disturbed. Nevertheless, the combined Hf and Nd depleted mantle model ages of these lavas are consistent at ∼2.1Ga, which we propose as the maximum age.Geochemically, the rocks have moderate to high concentrations of Fe2O3(T) (13–19wt%) and MgO (6–18wt%), with moderate to low SiO2 (49–42wt%) and Al2O3 (14–9wt%) contents, and low Al2O3/TiO2 (<8) ratio. They exhibit fractionated chondrite normalized rare earth element patterns (La/Yb)N ∼10. The primitive mantle normalized positive Nb anomalies in combination with key trace element ratios, and their Hf, Nd isotope systematics preclude contamination by Archean upper continental crust. Further, their juvenile Hf, Nd isotope systematics are inconsistent with long lived recycled crustal or sedimentary components in the mantle source of the Mahakoshal ferropicrites. In contrast, the enrichments in the incompatible lithophile elements suggest that the source of the Mahakoshal ferropicrites was refertilized by melts, possibly from an upwelling mantle plume shortly prior to eruption.The preserved pillow structures imply that magmatism took place in an ocean basin. Therefore, we argue for the existence of a proto-ocean prior to the amalgamation of the northern cratonic block consisting of Aravalli and Bundelkhand proto-continents, and the southern cratonic block consisting of Singhbhum, Bastar and Dharwar proto-continents, which were still adrift at ∼2.1Ga.

Crustal structure of the Amazonian Craton and adjacent provinces in Brazil

The study of the crust using receiver functions can provide valuable geological information, such as average crustal composition, its formation dynamics and the tectonic evolution of a region, as well as serve as an initial reference for the generation of seismic wave velocity models to improve earthquake location. To fill in gaps in information on the crust of the Amazonian Craton and adjacent provinces in Brazil, we used receiver functions and H-k stacking to estimate crustal thicknesses and the VP/VS ratios. The results indicate that the crust of the study region is predominantly felsic, with an average VP/VS around 1.73 and an average thickness of 38.2 km, with a range of 27.4–48.6 km. Minimum curvature interpolation of the crustal thickness values has made it possible to delimitate of the Amazonian Craton, which corresponds to the area with an average thickness equal to or greater than 39 km. In addition, it was possible to identify its potential cratonic blocks, as well as the Paranapanema Block of Paraná Basin. The geometry of the craton, defined by its crustal thickness, is corroborated by the distribution of natural seismicity that accompanies its edges. These are related to suture zones between the Amazonian, São Francisco/Congo and Paranapanema paleocontinents. The sedimentary basins that have undergone rifting processes have a thinner crust, usually less than 37 km thick. Due to the great variability of the results, it was not possible to determine a characteristic value of crustal thickness or VP/VS ratio for each structural province located in the study region.

The presence of a stable “Megablock” in the southwestern North American Proterozoic craton in northern Mexico

The present study, based on combined geophysical, geochronological and isotopic data identifies a cratonic block which we call the Western Chihuahua-Mesa Central megablock (WCMB) in northern Mexico. New Bouguer and isostatic gravity anomaly maps revealed a conspicuously distinct, spatially-extensive gravity minima over a region extending from north western Chihuahua to as far south as the Mesa Central (MC). This region can be differentiated from the surrounding mobile belts with significantly higher gravity anomaly values. Gravity modeling combined with sparse seismically determined crustal thickness estimates indicate that the proposed WCMB region has crustal thicknesses ranging from 38 to 42km. The regions surrounding WCMB are the spatially constrained mobile belts that are characterized by basinal and trough-like structures and are associated with A-type granites and bimodal volcanism. Spatial distribution of enriched initial Sr isotopic data (>0.706) confirms the sialic character of the crust associated with the gravity minimum at least in western Chihuahua. The mobile belts surrounding the WCMB contain geologic records of the events since 1.8–0.9Ga. These ages from within and just adjacent to the block correlate well with the Rodinia Neoproterozoic supercontinent that was assembled 1.3–0.9Ga. We propose that the northwest-southeast directed WCMB is a cratonic region underlain by Proterozoic crust. The WCMB is most likely separated from the deformed and active southwestern part of the North American Proterozoic craton by repeated thermotectonic activities within the surrounding mobile belts. These events have helped to dislocate and transport the block to its present location through processes of repeated compression, extension, and transform faulting.

Seismic tomography of the Arctic region: inferences for the thermal structure and evolution of the lithosphere

Abstract Waveform tomography with very large datasets reveals the upper-mantle structure of the Arctic in unprecedented detail. Using tomography jointly with computational petrology, we estimate temperature in the lithosphere–asthenosphere depth range and infer lithospheric structure and evolution. Most of the boundaries of the mantle roots of cratons in the Arctic are coincident with their geological boundaries at the surface. The thick lithospheres of the Greenland and North American cratons are separated by a corridor of thin lithosphere beneath Baffin Bay and through the middle of the Canadian Arctic Archipelago; the southern archipelago is part of the North American Craton. The mantle root of the cratonic block beneath northern Greenland may extend westwards as far as central Ellesmere Island. The Barents and Kara seas show high velocities indicative of thick lithosphere, similar to cratons. The locations of intraplate basaltic volcanism attributed to the High Arctic Large Igneous Province are all on thin, non-cratonic lithosphere. The lithosphere beneath the central part of the Siberian Traps is warmer than elsewhere beneath the Siberian Craton. This observation is consistent with lithospheric erosion associated with the large igneous province volcanism. A corridor of relatively low seismic velocities cuts east–west across central Greenland. This indicates lithospheric thinning, which appears to delineate the track of the Iceland hotspot.

The Neoarchean-Paleoproterozoic basin development and growth of the Singhbhum Craton, eastern India and its global implications: Insights from detrital zircon U-Pb data

Singhbhum Craton (>3500Ma), the Archean continental nuclei of eastern India, preserves on its southern margin coexisting deep crustal granulites and shallow crustal metasedimentary lithologic units that were juxtaposed through complex transpressional tectonics. U-Pb detrital zircon ages were obtained from five samples of widely separated locations of two parallel belts, which are known as the Northern and the Southern Supracrustal Belts. In addition, zircon grains from a pegmatite from the central gneissic complex were also dated. The LA-ICP-MS derived U-Pb age data from these samples lead to the estimation of age of sedimentation and the thermal events in the provenance. The U-Pb age from these samples yielded younger than 2235±28Ma for Northern Supracrustal Belt, while that of the Southern Supracrustal Belt is consistently younger (<1835Ma). Development of these basins at the southern margin of the Singhbhum Craton was associated with four cycles of sedimentation at ca. 3100–2800Ma, ca. 2800–2450Ma, ca. 2450–2235Ma and ca. 2235–1835Ma. These cycles resulted in the southward growth of the Singhbhum Craton through successive opening and closing of basins. The ca. 2540–2480Ma zircon population of the Northern Supracrustal Belt has no proven source from the Singhbhum Craton, but distal cratonic blocks might have provided the detritus. The provenance of the sediments could also be traced in the transcontinental pre-existing cratonic block, e.g. East Antarctica.

Petrochemical evolution of the White Mfolozi Granite pluton: Evidence for a late Palaeoarchaean A-type granite from the SE Kaapvaal Craton, South Africa

One of the major limitations in understanding the geochemical evolution of the Kaapvaal Craton, South Africa, is the scarcity of whole rock trace element data of the granitoid and other rocks compared to the vastness of this cratonic block. Here we present new XRF major oxide and ICP-MS trace element analyses of the White Mfolozi Granitoid (WMG) pluton, SE Kaapvaal Craton, which suggest that the ~3.25Ga (U–Pb zircon age) old WMG pluton is a peraluminous A-type granite and could be equivalent to the intrusive potassic granite phase of the Anhalt Granitoid suite, occurring to the North of the WMG pluton. The pluton was generated by batch partial melting of a pre-existing TTG source in two major phases under relatively anhydrous conditions, and the heat of partial melting could have been provided by a voluminous mantle-derived mafic magma, which intruded into mid-crustal levels (c. 17km), perhaps during a period of crustal extension. The estimated pressure and temperature of generation of the WMG parent magma with average molar [or/(or+ab)] ~0.48 could be ~500MPa and close to 1000°C, respectively, when compared with the results of experimental petrology. Interstitial occurrence of relatively iron-rich biotite [Mg/(Mg+Fe) ~0.41–0.45] suggests that the final temperature of crystallization of the pluton was close to 800°C. An important magmatic event following the main phase of partial melting was limited mixing between the intrusive mafic magma and co-existing newly generated granitic melt. This magma mixing resulted in distinct variations in SiO2 and a low initial Sr isotopic ratio (0.7013) of the WMG pluton. Although both the models of partial melting of quartzo-feldspathic sources and fractional crystallization of basaltic magmas with or without crustal assimilation have been proposed for the origin of A-type granites, the model of magmatic evolution of the WMG pluton presented here can also be an alternative model for the generation of A-type granites. In this model, post-partial melting magma mixing is perhaps critical in explaining the Daly gap in composition and extreme variations in chemical (e.g., SiO2) and isotopic compositions observed in many bimodal A-type granite suites. The emplacement of the oldest known A-type granitoid suite in the Kaapvaal Craton, the WMG pluton, marks a period of stabilization of the craton before erosion and deposition of the overlying volcano-sedimentary succession of the Pongola Supergroup.

Seismic anisotropy of Precambrian lithosphere: Insights from Rayleigh wave tomography of the eastern Superior Craton

AbstractThe thick, seismically fast lithospheric keels underlying continental cores (cratons) are thought to have formed in the Precambrian and resisted subsequent tectonic destruction. A consensus is emerging from a variety of disciplines that keels are vertically stratified, but the processes that led to their development remain uncertain. Eastern Canada is a natural laboratory to study Precambrian lithospheric formation and evolution. It comprises the largest Archean craton in the world, the Superior Craton, surrounded by multiple Proterozoic orogenic belts. To investigate its lithospheric structure, we construct a frequency‐dependent anisotropic seismic model of the region using Rayleigh waves from teleseismic earthquakes recorded at broadband seismic stations across eastern Canada. The joint interpretation of phase velocity heterogeneity and azimuthal anisotropy patterns reveals a seismically fast and anisotropically complex Superior Craton. The upper lithosphere records fossilized Archean tectonic deformation: anisotropic patterns align with the orientation of the main tectonic boundaries at periods ≤110 s. This implies that cratonic blocks were strong enough to sustain plate‐scale deformation during collision at 2.5 Ga. Cratonic lithosphere with fossil anisotropy partially extends beneath adjacent Proterozoic belts. At periods sensitive to the lower lithosphere, we detect fast, more homogenous, and weakly anisotropic material, documenting postassembly lithospheric growth, possibly in a slow or stagnant convection regime. A heterogeneous, anisotropic transitional zone may also be present at the base of the keel. The detection of multiple lithospheric fabrics at different periods with distinct tectonic origins supports growing evidence that cratonization processes may be episodic and are not exclusively an Archean phenomenon.

Gold-bearing volcanogenic massive sulfides and orogenic-gold deposits in the Nubian Shield

The Nubian Shield is a large region of juvenile Neoproterozoic rocks that, together with its counterpart in the Arabian Shield, is part of a major accretionary orogen. It originated as late Tonian-Cryogenian island arcs on the site of the Mozambique Ocean formed by breakup of Rodinia. Arc collisions, subsequent magmatism, volcanism, sedimentation, and orogeny, associated with Cryogenian-Ediacaran convergence of cratonic blocks during the assembly of Gondwana, converted the region into the East African Orogen. The Nubian Shield region also has important Paleozoic-Neogene strata, including significant flood basalt, that conceal large areas of the basement. The Shield contains hundreds of gold occurrences and evidence of a 5,500-year history of gold mining. The main deposit types are orogenic gold and gold associated with volcanogenic massive sulfides (VMS). The juvenile subduction-related origin of the Shield rocks and the pervasiveness of shearing and greenschist-to-amphibolite facies metamorphism associated with late Neoproterozoic orogeny are geologic features highly favorable for the development of these types of deposits, and make the combined Arabian-Nubian Shield the Earth’s largest Neoproterozoic gold resource. Gold-bearing VMS deposits are currently mined at Bisha (Eritrea) and are soon to be mined at Hassai (Sudan). Orogenic gold is mined at Sukari and Hamash (Egypt), Qbgbih and Kamoeb (Sudan), Koka (Eritrea), and Lega Dembi and Sakaro (Ethiopia), and mining is due to start at Gupo (Eritrea) and Tulu Kapi (Ethiopia) in the near future. More than twenty companies are actively exploring for gold in the region and potentially important deposits are known in Egypt (Hamama-VMS), in northern Sudan (orogenic gold and VMS), along strike from Bisha and in the Asmara area, Eritrea (VMS and orogenic gold), in northern Ethiopia (VMS), and in western and southern Ethiopia (orogenic gold and sparse VMS).

A strong contrast in crustal architecture from accreted terranes to craton, constrained by controlled-source seismic data in Idaho and eastern Oregon

Crustal structure was derived from EarthScope Idaho-Oregon (IDOR) controlled-source seismic data across the Precambrian continental margin in the Idaho and Oregon region of the U.S. Cordillera. Refraction and wide-angle reflection traveltimes were inverted to derive a seismic velocity model that constrains the contact between oceanic accreted terranes and craton. The seismic data reveal that the boundary is a near-vertical, through-going feature of the crust, represented by the transpressional western Idaho shear zone (WISZ). The WISZ separates crust with different seismic velocities at all depths, implying a contrast in lithology, and extends to an ∼7 km offset of the Moho. The thinner, ∼32-km-thick accreted terrane crust to the west is characterized by faster seismic velocities that correspond to an intermediate composition. We interpret a high-velocity layer below a high-amplitude seismic reflection as mafic magmatic underplating associated with the feeder system of the Columbia River Basalts. The cratonic crust east of the WISZ is 37–40 km thick, with a felsic composition to ∼29 km subsurface depth, underlain by an intermediate-composition layer above the Moho. The strong contrasts in lithology and crustal thickness across the WISZ have influenced subsequent magmatism and extension in the region. The northwestern extent of the Archean Grouse Creek cratonic block beneath the Atlanta lobe of the Idaho batholith is interpreted based on continuity of crustal architecture in the seismic model. The velocity structure and crustal thickness east of the WISZ are consistent with the Atlanta lobe melting within a thickened crust.

Folded Basinal Compartments of the Southern Mongolian Borderland: A Structural Archive of the Final Consolidation of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) records multiple Phanerozoic tectonic events involving consolidation of disparate terranes and cratonic blocks and subsequent reactivation of Eurasia’s continental interior. The final amalgamation of the CAOB terrane collage involved diachronous closure of the Permian-Triassic Solonker suture in northernmost China and the Jurassic Mongol-Okhotsk suture in northeast Mongolia and eastern Siberia. The distribution, style, and kinematics of deformation associated with these two terminal collision events is poorly documented in southern Mongolia and northernmost China because these regions were later tectonically overprinted by widespread Cretaceous basin and range-style crustal extension and Miocene-recent sinistral transpressional mountain building. These younger events structurally compartmentalized the crust into uplifted crystalline basement blocks and intermontane basins. Consequently, widespread Cretaceous and Late Cenozoic clastic sedimentary deposits overlie older Permian-Jurassic sedimentary rocks in most basinal areas and obscure the deformation record associated with Permian-Triassic Solonker and Jurassic Mongol-Okhotsk collisional suturing. In this report, satellite image mapping of basinal compartments that expose folded Permian-Jurassic sedimentary successions that are unconformably overlapped by Cretaceous-Quaternary clastic sediments is presented for remote and poorly studied regions of southern Mongolia and two areas of the Beishan. The largest folds are tens of kilometers in strike length, east-west trending, and reveal north-south Late Jurassic shortening (present coordinates). Late Jurassic fold vergence is dominantly northerly in the southern Gobi Altai within a regional-scale fold-and-thrust belt. Local refolding of older Permian north-south trending folds is also evident in some areas. The folds identified and mapped in this study provide new evidence for the regional distribution and kinematics of Jurassic and Permian-Triassic contractional tectonism in the southern Mongolia-northern China borderland region. The newly mapped folds are also important potential targets for hydrocarbon exploration and vertebrate paleontological discoveries.

West Africa: The World’s Premier Paleoproterozoic Gold Province

West Africa, with presently an approximate 10,000-metric ton (t) gold endowment, is one of the world's great gold provinces and the largest Paleoproterozoic gold-producing region. The gold resources are concentrated within the 2250 to 2000 Ma greenstone belts of the Man-Leo shield, forming the southern part of the West Africa craton. Most of the major orebodies are best classified as orogenic gold deposit types, although there are paleoplacer and porphyry-skarn deposits within some of the greenstone belts, and perhaps local intrusionrelated gold systems. The gold-hosting, mainly greenschist metamorphic facies greenstone belts are dominated by tholeiitic volcanic rocks, with clastic and chemical sediments filling adjacent subbasins. The Paleoproterozoic sequences formed in what was likely a rift or series of rifts in a Precambrian cratonic block; it is not clear whether significant Late Archean lithospheric roots occur below these Paleoproterozoic arcs that formed in the resulting ocean subbasins. Although diachronous across West Africa, the Eburnean orogeny is typically indicated to have been initiated at ca. 2130 Ma, with closure of the subbasins, amalgamation of the Paleoproterozoic arcs, and their accretion back to the continental margin of Archean rocks. Compressional tectonics took place for about 25 to 30 m.y., with widespread crustal thickening along orogen-parallel, commonly NE-trending, firstorder thrust fault systems. This was followed by more than 100 m.y. of transcurrent tectonism and associated exhumation; gold ores mainly formed late during the Eburnean deformation.

Imaging the lithospheric structure beneath the Indian continent

AbstractWe present a high‐resolution 3‐D lithospheric model of the Indian plate region down to 300 km depth, obtained by inverting a new massive database of surface wave observations, using classical tomographic methods. Data are collected from more than 550 seismic broadband stations spanning the Indian subcontinent and surrounding regions. The Rayleigh wave dispersion measurements along ~14,000 paths are made in a broad frequency range (16–250 s). Our regionalized surface wave (group and phase) dispersion data are inverted at depth in two steps: first an isotropic inversion and next an anisotropic inversion of the phase velocity including the SV wave velocity and azimuthal anisotropy, based on the perturbation theory. We are able to recover most of the known geological structures in the region, such as the slow velocities associated with the thick crust in the Himalaya and Tibetan plateau and the fast velocities associated with the Indian Precambrian shield. Our estimates of the depth to the Lithosphere‐Asthenosphere boundary (LAB) derived from seismic velocity Vsv reductions at depth reveal large variations (120–250 km) beneath the different cratonic blocks. The lithospheric thickness is ~120 km in the eastern Dharwar, ~160 km in the western Dharwar, ~140–200 km in Bastar, and ~160–200 km in the Singhbhum Craton. The thickest (200–250 km) cratonic roots are present beneath central India. A low velocity layer associated with the midlithospheric discontinuity is present when the root of the lithosphere is deep.