Paleocontinental Margin Research Articles

Evidence for paleo-continental margin and folding of relic fragment of oceanic lithosphere, northeast Indian subcontinent

The processes of continental breakup, reassembling of landmasses, and huge deltaic sediment depositions contribute to reconfiguring the Earth's surface. Here, we show a classic example of contiguous structures of paleo-continental margin on the eastern Indian Shield and an ancient fragment of the oceanic lithosphere beneath Bangladesh. In the present work, we collated gravity data from Bangladesh and adjoined parts of the Indian shield and the northern Bay of Bengal to decipher rocks' nature and long-term behavior. The gravity map illustrates N-S oriented alternate bands of ∼150 km wide positive and negative anomalies in the Bangladesh region, besides showing signatures of prominent geomorphic features such as the Shillong Plateau, Sylhet Trough, Burmese Arc, etc. The crustal models reveal the presence of paleo-continental margin segments on the eastern edge of the Singhbhum craton and south of the Shillong Plateau, ∼150 km wide folded oceanic crust beneath Bangladesh, and a low slab dip of the subducting Indian plate beneath the Burma platelet. The inferred continental margin evolved after the breakup occurred between the Rajmahal-Sylhet Line and continental fragments (Elan Bank and southern parts of the Kerguelen Plateau) at about 120 Ma. Subsequently, the sediments carried by two great Himalayan river systems were excessively deposited on the margin and over an older fragment of the oceanic lithosphere, resulting in the progradation of the Bengal delta towards the ocean. The oceanic crust beneath Bangladesh was folded with approximately 150 km wavelength as the compressive stress regime is locally operative due to the eastward moment of the Indian plate and westward propagation of the Indo-Burman Wedge.

Ancient roots of tungsten in western North America

Abstract The highly irregular and localized distribution of tungsten deposits worldwide constitutes a supply challenge for basic industries such as steel and carbides. Over Earth's history, tungsten has preferentially accumulated at paleocontinental margins formed during the breakup of supercontinents. Later crustal thickening of these paleogeographic regions and the magmas they produce are associated with large tungsten districts. However, all of the largest tungsten deposits in the modern North American Cordillera, which preserves over 3 b.y. of geologic record in a paleocontinental margin with abundant crustal magmatism, are limited to the narrow Canadian Tungsten Belt in northwestern Canada. We use neodymium isotopic compositions of scheelite (CaWO4) from the Canadian Tungsten Belt and the paleogeographic distribution of tungsten deposits in the North American Cordillera to constrain the factors that control tungsten distribution. We document that tungsten is specifically associated with materials that, on average, were derived from the mantle during the Mesoarchean to Paleoproterozoic. Weathering and erosion of the supercontinents Columbia and Rodinia favored pre-enrichment of tungsten in sediments. The orogenic heating of pre-enriched sediments produced reduced melts that were capable of efficiently scavenging tungsten and formed the largest deposits in North America.

Laramide Orogenesis Driven by Late Cretaceous Weakening of the North American Lithosphere

AbstractThis paper investigates the causes of the Late Cretaceous transition from “Sevier” to “Laramide” orogenesis and the spatial and temporal evolution of effective elastic thickness (EET) of the North American lithosphere. We use a Monte Carlo flexural model applied to 34 stratigraphic profiles in the Laramide province and five profiles from the Western Canadian Basin to estimate model parameters which produce flexural profiles that match observed sedimentary thicknesses. Sediment thicknesses come from basins from New Mexico to Canada of Cenomanian–Eocene age that are related to both Sevier and Laramide crustal loads. Flexural models reveal an east‐to‐west spatial decrease in EET in all time intervals analyzed. This spatial decrease in EET may have been associated with either bending stresses associated with the Sevier thrust belt, or increased proximity to attenuated continental crust at the paleocontinental margin. In the Laramide province (i.e., south of ~48°N) there was a coeval, regional decrease in EET between the Cenomanian–Santonian (~98–84 Ma) and the Campanian–Maastrichtian (~77–66 Ma), followed by a minor decrease between the Maastrichtian and Paleogene. However, there was no decrease in EET in the Western Canada Basin (north of ~48°N), which is consistent with a lack of Laramide‐style deformation or flat subduction. We conclude that the regional lithospheric weakening in the late Santonian–Campanian is best explained by hydration of the North American lithosphere thinned by bulldozing by a shallowly subducting Farallon plate. The weakening of the lithosphere facilitated Laramide contractional deformation by focusing end‐loading stresses associated with flat subduction. Laramide deformation in turn may have further reduced EET by weakening the upper crust. Finally, estimates of Campanian–Maastrichtian and Paleogene EET are comparable to current estimates indicating that the modern distribution of lithospheric strength was achieved by the Campanian in response to flat subduction.

Vorontsovka Carlin-style gold deposit in the North Urals: Mineralogy, fluid inclusion and isotope data for genetic model

The Vorontsovka gold deposit in the Tagil zone, North Urals, hosts 101 t Au, averaging 7 g/t Au and including 30 t Au in the regolith, and 120 t Ag, averaging 8 g/t Ag. Early skarn-magnetite mineralisation related to a dioritic intrusion of the shoshonitic series was formed in the Late Emsian. Water and volatiles (mainly CO2) released from host metapelites and limestones were mixed with the metalliferous magmatic fluid during catagenesis to contact metamorphism. The modified fluid extracted ore components from host rocks and syn-volcanic stratiform mineralisation and became enriched in Fe, Mn, Ba, Zn, Pb, Au, Ag and S to deposit these components as sulphide dissemination in the footwall of siliciclastic unit and underlying limestone both brecciated along the trust. At the final Early Eifelian stage, there was an additional inflow of deep mantle/magmatic fluid, enriched in Au, Hg, As, Sb and Tl. The mixing of this fluid with metamorphic water resulted in Carlin-style gold-realgar-stibnite (+Tl minerals) mineralisation mainly in carbonate breccia. The main gold endowment associates with gold‑arsenic ores located in brecciated silicified limestone (jasperoid) and argillised tuff-siltstone. Early ore assemblages were formed at 510–240 °C (including magnetite skarn), whereas overprinting Carlin-style gold-(Fe, As, Sb, Hg, Tl) sulphide mineralisation was deposited at decreasing temperatures mainly from 260 to 110 °C. Fluid inclusion data revealed aqueous‑carbonic, Ca-, Na-, Mg- and K-chloride compositions of ore-bearing fluid of 4.8–9.3 wt% eq. NaCl. Changing fluid pressure corresponded to near lithostatic (0.5–0.6 kb) to hydrostatic (0.15–0.2 kb) conditions. Based on the δ34S ranges of sulphides determined for siliciclastic rocks (−2.3 … +1.8‰) and skarns (−2 … +1.6‰) it is assumed that most of the sulphur derived from magmatic reservoir associated with the mantle. The lead isotopic compositions of sulphides show a relatively narrow range and continue the linear trend of the Early Silurian VMS deposits corresponding to the mixing line of mantle and crustal lead sources. Model PbPb ages for the Vorontsovka sulphides are grouped into two clusters: 425–416 and 398–388 Ma. The second period is close to the UPb isotope age of the final stage of the Auerbakh pluton as well as in a good agreement with the ArAr isotope age of 391.1 ± 4.9 Ma of hydromica from the late gold-arsenopyrite assemblage. The specific features of the Vorontsovka deposit indicative of the Carlin-style mineralisation are as follows: the paleo-continental margin and trust-related position; causing high-K/shoshonitic igneous activity; the fine-disseminated sulphide mineralisation in the highly brecciated carbonate-clastic sequence; jasperoid and argillic alteration accompanying gold orebodies with assay boundaries; multiple fluid source; Au-As-Hg-Tl geochemical association; the abundance of As- and As-Fe-sulphides and AsSb sulphosalts; the occurrence of Hg-, Tl-sulphides and sulphosalts; the association of gold with arsenian pyrite and arsenopyrite overgrowing previously forming pyrite; and the predominance of submicroscopic gold in the ores. A LA-ICP-MS analysis of pyrite confirms that elevated concentrations of Au regularly correlate with higher contents of As, Ag, Sb or Tl.

Origin and significance of the ophiolitic high-P mélanges in the northern Caribbean convergent margin: Insights from the geochemistry and large-scale structure of the Río San Juan metamorphic complex

The Río San Juan metamorphic complex exposes a segment of a high-pressure accretionary wedge, built during Late Cretaceous intra-oceanic subduction below the Caribbean island-arc. Detailed mapping, large-scale structural analysis, U–Pb/Ar–Ar geochronology, in situ trace element analyses of clinopyroxene, and bulk-rock geochemical data show that the northern half of the complex is composed of arc- and oceanic-derived units, whose large-scale internal structure consists of an imbricate stack of mainly high-P rocks. In the lower structural levels, the Gaspar Hernández serpentinized peridotite–tectonite is composed of massive, serpentinized harzburgite that grades into surrounding sheared serpentinite or gouge. Gabbroic rocks and dolerite sills with N-MORB geochemical signatures are sparse and altered to greenschists facies. U–Pb data indicate a Lower Cretaceous magmatic age (136.40±0.32Ma). These rocks are interpreted as ophiolitic fragments of the proto-Caribbean Ocean. In the intermediate Jagua Clara serpentinite-matrix mélange, the massive serpentinite with relict peridotite textures was overprinted by schistose and sheared serpentinite, and serpentinite gouge, recording various degrees and conditions of internal deformation. Serpentinite schistosity surfaces warp around mélange hard blocks (<1m to 2.5km). Blocks are massive serpentinized peridotite and exotic high-P crustal rocks. The protoliths of the mafic metaigneous rocks originated from arc-like and non-arc-like magmas. The Jagua Clara serpentinite-matrix mélange represents the deep subduction channel, formed during intra-oceanic subduction. In the upper structural levels, the Morrito unit is composed of high-P Puerca Gorda and Guineal Schists. Protoliths are basic-intermediate volcanic rocks with IAT, low-Ti IAT and boninitic geochemical signatures, similar in composition to the Lower Cretaceous Caribbean island-arc. The Morrito basal fault-zone thrusted the Puerca Gorda Schists onto the Jagua Clara mélange. The geochemical characteristics of many mafic metaigneous rocks found within the Jagua Clara mélange, and particularly in the Morrito unit, indicate that they occupied a supra-subduction zone position at some time in their history and, therefore, the mélange reflect the position of the Late Cretaceous intra-oceanic subduction zone. The presence in the mélange of exotic blocks plucked from both the upper plate (Caribbean island-arc) and the lower plate (proto-Caribbean Ocean) suggests that the arc crust was eroded at its base by tectonic processes in the Late Cretaceous. Regional structural data indicate that the Morrito basal fault-zone now forms the suture between the Caribbean island-arc terranes and the accreted units of the paleo-continental margin of North America.

An ice grounding-line wedge from the Ghaub glaciation (635 Ma) on the distal foreslope of the Otavi carbonate platform, Namibia, and its bearing on the snowball Earth hypothesis

Our detailed examination of the Ghaub Formation (possibly 635 Ma) on the distal foreslope of the Otavi carbonate platform is part of a regional study of the Congo paleocontinental margin in northwestern Namibia. Detrital carbonates of the Ghaub Formation disconformably overlie the Franni-aus Member of the Ombaatjie Formation, a coarsening-upward stack of carbonate turbidites and oolite-clast debris-flow breccias interpreted to be a glacioeustatic falling-stand wedge. Within the main Ghaub Formation, carbonate diamictites are interleaved with mesoscale, laminated to cross-laminated (climbing rippled) grainstones and mudstones, and conglomeratic carbonates. Amalgamation of diamictite units is observed where interleaved facies (grainstones/mudstones) are laterally discontinuous due to reactivation of erosion, followed by renewed deposition. The diamictite package is progradational overall and 80 m thick on average. It is overlain by the 5–15-m-thick Bethanis Member, which is unique in its lateral continuity, composite fining-upward trend, and distinctive interbedding of turbidite grainstones, argillaceous siltstones, climbing-rippled mudstones, and meter-scale stromatolite dropstones. Dropstones are ubiquitous within the finer-grained (Ghaub) lithofacies, and their presence, along with the facies context for subglacial and near grounding-line deposition, indicates a glacigenic origin for the Ghaub Formation, despite its subtropical paleolatitude and distal foreslope setting. We infer a glacial maximum represented by the sub-Ghaub disconformity, followed by the main Ghaub interval when an ice grounding line on the distal foreslope experienced abrupt step backs and readvances of limited magnitude, terminated by the Bethanis episode of unusually widespread iceberg calving and slope instability. The Bethanis Member is overlain conformably by the Keilberg Member of the Maieberg Formation. Reconstruction of the foreslope places the Ghaub grounding-line wedge >1.3 km vertically below the rim of the platform, implying an enormous base-level change upon deglaciation, when the platform was drowned below wave base for a period far exceeding the time scale for isostatic adjustment. The magnitude of base-level change supports the pan-glacial hypothesis that dynamic (thick) ice sheets existed simultaneously on virtually all continents. The snowball hypothesis that the oceans were also covered by glacial ice (sea-glacier) provides a simple explanation for the main Ghaub-to-Bethanis transition—terminal deglaciation was triggered by collapse of the sea-glacier.

Carbonate Sedimentation in a Permian High-Latitude, Subpolar Depositional Realm: Queensland, Australia

High-latitude carbonate rocks that formed in subpolar Paleozoic oceans contain critical information about past climate, but they are neither well documented nor fully understood. Lower and middle Permian limestones that were deposited in such paleoenvironments at 45°S to 55°S along the southeastern margin of Gondwana are now exposed in eastern Australia. Such carbonates constitute a small but important and well-preserved part of the siliciclastic glacigene succession in the Bowen Basin from southern and central Queensland. Cleanest limestones accumulated on topographic highs in the eastern (seaward) parts of the basin that were isolated from siliciclastic sediment input from the western craton. Shallow-water deposits are composed of large eurydesmid bivalves, spiriferid and productid brachiopods, bryozoans, infaunal and epifaunal benthic foraminifers, calcareous sponge spicules, and abundant crinoids in the form of locally cross-bedded rudstones and grainstones. Deeper-water, muddy carbonates are dominated by bryozoans, crinoids, and productids (shells and spines). Deepest-water sediments are argillaceous floatstones or shales containing fenestrate bryozoans, thin-walled productids, and siliceous sponge spicules. When integrated with recent studies of coeval sedimentary rocks along the 2000-km-long paleocontinental margin, a clear pattern of sedimentation emerges wherein deposition took place in similar settings during three discrete periods of deglaciation: late Sakmarian, late Artinskian, and Wordian. All of the carbonates are distinguished by a low-diversity, high-abundance heterozoan biota with no phototrophs and express a recurring deepening-upward stratigraphic motif that is interpreted to reflect rising sea level accompanying glacial meltdown. They also record a strong poleward gradient of increasing ice-rafted debris, increasing skeleton size, decreasing invertebrate diversity, decreasing epifaunal calcareous benthic foraminifers, and reduction in crinoids; glendonites are confined to siliciclastic sediments deposited during glacial periods. Comparison with coeval northern hemisphere carbonates indicates a strong parallelism of facies and depositional systems. This study helps to establish these deposits as a template for a global Paleozoic high-latitude carbonate facies model.

Trachytes, comendites, and pantellerites of the Late Paleozoic bimodal rift association of the Noen and Tost ranges, southern Mongolia: Differentiation and contamination of peralkaline salic melts

The bimodal association of the Noen and Tost ranges is ascribed to the Gobi-Tien Shan rift zone and was formed 318 Ma ago at the continental margin of the North Asian paleocontinent. It is made up of volcanic series of alternating basalts and peralkaline rhyolites with subordinate trachytes, dike belts, and massifs of peralkaline granites. The association also includes a coeval massif of biotite granites. Based on Al2O3 and FeOtot contents, the peralkaline rhyolites are subdivided into comendites (FeOtot 1.5–5.7 wt %, Al2O3 10.5–15.4 wt %) and pantellerites (FeOtot 5.2–7.5 wt %, Al2O3 9.1–10.2 wt %). The peralkaline salic rocks of the bimodal association were formed by the crystallization differentiation of rift basaltic magmas combined with crustal assimilation. The comendites, pantellerites, and peralkaline granites inherited negative Nb and Ta and positive K and Pb anomalies from basalts. They are also similar to basalts in Nd isotope composition (ɛNd(T) = 5.5–7.4) and have nearly mantle oxygen isotope composition (δ18O = 5.9–7.3‰). The most differentiated and least contaminated rocks of the bimodal series of the Noen and Tost ranges are pantellerites. Calculations indicate that the fraction of the residual pantellerite melt was 8% or less of the parental basaltic magma. The comendites were derived from peralkaline salic melts by the assimilation of anatectic crustal melts compositionally similar to biotite granites. The formation of the latter within the Noen and Tost ranges is explained by the specific geodynamic position of the Gobi-Tien Shan rift zone, which was formed near a paleocontinental margin that evolved in an active margin regime shortly before the beginning of rifting.

The giant Carlin gold province: a protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary

Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit “trends.” Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the trends. Interestingly, the large Cretaceous Alaska–Yukon intrusion-related gold deposits (e.g., Fort Knox) are associated with the northern extension of the same lithospheric margin in the Selwyn basin, which experienced an analogous series of geologic events.

Tectonic evolvement of metamorphic complexes at Jilin paleocontinental margin during the transition from late Archaean to early Proterozoic

The kinematics and dynamical process of tectonic evolvement of metamorphic complexes at the interim from late Archaean to early Proterozoic is one of the key problems in geosciences. For the disputation on the genesis of metamorphic complexes at the margin of Jilin palaeocontinent, this paper takes the example of Banshigou region, Jilin Province to discuss the dynamical evolution of palaeocontinent during the transition from late Archaean to early Proterozoic (2600-2000 Ma). On the time sequence, from center to palaeocontinental margin, it shows a series of dynamical movements including underplating, horizontal movement, subduction, intraplate extension and separation. And its corresponding sequence of kinematical modes is as follows: vertical movement, horizontal movement, extension and shearing in contact zone, uplift-sliding movement in paleocontinental margin and interformational sliding, resulting in such tectonite sequence, tectonic gneiss, gneissic complex, gneissic complex-mylonite, mylonite and fracture cleavage-mylonite, which consist of the main body of metamorphic complexes. Their palaeostresses are: < 20, 20.40, 21.72, 28.80 and 30.8-69.8 MPa respectively. The deformational metamorphic temperature is between hornblende and low-grade greenschist facies. The general deformational characters of Jilin palaeocontinent reflect a complete dynamic system of crust evolution, which indicates that the formation of the metamorphic complexes and the tectonic evolution are altered from vertical movement to compression to extension. It also indicates a continuous tectonic transformation from deep to shallow, and from ductile to brittle. The transformation between different dynamic mechanisms not only forms tectonic rocks, but also benefits the linking up, exchange and enrichment with rock-forming minerals and ore-forming elements. This research is helpful to classifying regional tectonic events and making further study on the evolution of palaeocontinental dynamics.

Significance of dextral reactivation of an E‐W transfer fault in the formation of the Pennsylvania orocline, central Appalachians

Three hypotheses for orocline development including (1) primary plate boundary shape, (2) arc rotation, and (3) arc overprinting were evaluated for the formation of the Pennsylvania orocline, Maryland, USA, using foliation intersection/inflection axes preserved within porphyroblasts (FIA). The distribution and the timing of sequences of FIA data measured from garnet porphyroblasts within the Loch Raven formation are statistically the same across the Pennsylvania orocline, suggesting that all subregions of the arc have experienced the same deformation history. This may suggest that the geometry of the paleocontinental margin controlled the basic shape of this orocline. However, if the orocline is treated as three separate regions, defined by the SW‐WE‐NE trending portions of its slightly staircase‐shaped outline, a much higher proportion of the NNE‐SSW trending FIAs (set II) are preserved in the W‐E trending region than those to either side. This geometric relationship could have resulted from WNW‐ESE directed bulk stress causing a zone of dextral shear along what is now the W‐E trending portion of this orogen. It appears that what could have been an early formed W‐E trending sinistral transform shear zone was preferentially dextrally reactivated during the Taconian orogeny.

Characteristics of the Composite Metallogenic System along the Jilin Paleocontinental Margin and the Evolution of Continental Dynamic Regimes

Abstract In view of the existing problems about the metallogenic epoch and origin of the complex along the Jilin paleocontinental margin, we propose the composite metallogenic system of the Jilin old land, based on the theory of metallogeny and from the continental dynamics and comprehensive analysis on the metallogenic characteristics. Its scientific theory is demonstrated by the following aspects: (1) Screening and analysis of the multi‐stage superimposition and sequences. The composite ore‐forming system consists mainly of three types: the divergent‐type epicontinental metallogenic system on the southern margin, the convergent‐type epicontinental metallogenic system on the northern margin and the strike‐slip‐type epicontinental metallogenic system on the northeastern margin. Their major metallogenic epochs are Archean‐Proterozoic, Variscan and Yanshanian respectively. (2) Comprehensive analysis of the genetic relations of the complicated deposits. The above‐mentioned three types all indicate a consanguinity among granites, vein rocks and gold deposits, correspondence with geochemical elements, and unicity showing the same activities of mantlederived fluids and multi‐source origin of ore‐forming substances. (3) As to the mechanisms for mineral concentration, in addition to the sources, we also focus on the co‐actions of permeability, vibration, gas‐liquid state alternation and abrupt changes of flow velocity during the movement of ore‐forming liquids, as well as the extraction and communication of multi‐sequence circulating liquids. (4) It is helpful for establishing the relationship between metallogeny and the evolution of continental dynamic regimes, i.e., the ore‐controlling correlations between the ore‐forming elements‐metallogenic series‐metallogenic systems‐composite metallogenic system and ore‐controlling structural features (stratabound, primary and branching types)‐regional faulting or secondary continental extension and collision‐transformation of different continental dynamic types (divergent, convergent and strike‐slip)‐the evolution process of the whole old land for the Jilin old land margin. All these are helpful for further researches on the continental metallogenic dynamics.

U–Pb geochronology of the southern Brası́lia belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

Abstract The Brasilia belt borders the western margin of the Sao Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U–Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the Sao Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana. Ages of detrital zircons (by ID–TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the Sao Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxa Group in the Araxa and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperanca Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean–Paleoproterozoic ages (3.4–3.3, 3.1–2.7, and 2.5–2.4 Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3–1.9 Ga, with a peak at ca. 2.15 Ga) and to the ca. 1.75 Ga Espinhaco rifting of the Sao Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the Sao Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to the individualisation of the Sao Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxa Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin. Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the Sao Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6–1.2 Ga source rocks must be present in the Sao Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambui Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasilia belt. Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event that started with westward subduction of the Sao Francisco plate and ended with continental collision against the Parana block (and Goias terrane). Continuing collision led to the exhumation and cooling of the Araxa and Passos metamorphic nappes, as indicated by monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasilia belt. Whilst continent–continent collision was proceeding on the western margin of the Sao Francisco Craton along the southern Brasilia belt, eastward subduction in the East was generating the 634–599 Ma Rio Negro magmatic arc which collided with the eastern Sao Francisco margin at 595–560 Ma, much later than in the Brasilia belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasilia belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588 Ma, as indicated by monazite age. The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545–500 Ma in the Paraguay belt and ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Parana block and Goias terrane with the Sao Francisco Craton along the Brasilia belt preceded the accretion of the Amazonian craton by 50–100 million years.

Geotectonic Settings of Large and Superlarge Mineral Deposits on the Southwestern Margin of the North China Plate

AbstractThe geotectonic setting refers to the three‐dimensional space and related events based on which a metallogenic system is formed and an ore‐forming process takes place. This paper discusses the tectonic evolution of the southwestern margin of the North China paleocontinent and related geotectonic settings in which large or superlarge deposits are formed. Emphasis is put on the geodynamic conditions of the Jinchuan nickel‐copper deposit, the Baiyin copper‐polymetallic deposit and the Hanshan gold deposit. It is significant that the three deposits occur together as a “trinity” on the same paleocontinental margin. The Jinchuan nickel‐copper deposit was formed during the early stage of rifting of the paleocontinental margin; the Baiyin copper‐polymetallic deposit was formed during the splitting stage of a continental‐margin arc. The continental‐margin arc spitting resulted in an “island arc rift” in the early stage of evolution. The Hanshan gold deposit was formed within the Altun sinistral strike‐slip fault system and its provenance is the “intraoceanic arc” volcanic rocks.

Origin of the early Sinian Minle manganese deposit, Hunan Province, China

The Minle Mn ores occur at the base of the Early Sinian Minle Formation black shales, on the southeast limb of the Motianling anticline. These rhodochrosite ores are rich in organic carbon and fossil algae and formed in a bay or lagoon on a paleo-continental margin with adjacent island barriers. The salinity of water in the depositional basin was probably brackish. The partly restricted ocean basin was characterized by low-oxygen bottom waters and pore-water pH values ranging from 9 to 11 and neutral to negative Eh values, which produced alkaline and reducing conditions suitable for precipitation of MnCO 3. Growth of blue algae played a role in concentrating metals, especially Mn, changing the sedimentary environment (photosynthesis prior to deposition and degradation of organic matter during diagenesis), and enhancing the deposition of MnCO 3 during diagenesis. The Mn was mainly derived from the continent, with some portion supplied from submarine volcanic activity and supracrustal deep-seated hydrothermal fluids.

Cenozoic history of the Bering Sea and its northwestern margin

Tectonic reconstructions based on the geodynamic analysis of geologic, paleomagnetic, structural and kinematic data of Cenozoic age from the western Bering Sea region are proposed in the present paper. The most active tectonic and magmatic processes took place in the Komandorsky segment of the Bering Sea, exemplified by the Late Cretaceous–Early Eocene Olutorsky Arc and Eocene–Oligocene Govena–Karaginsky Arc, which was built on the structures of the Olutorsky Arc. A model of the complex collision of these two arcs with the paleocontinental margin, which considers rotations of the geological blocks from the various structural zones of the western margin of the Bering Sea in the horizontal plane (paleomagnetic data), was proposed by the authors. According to this model the collision of the flanks of the Olutorsky and Govena–Karaginsky arcs took place in the Eocene, before the collision of the central parts in the Miocene.

The Ashele Deposit: A Recently Discovered Volcanogenic Massive Sulfide Cu‐Zn Deposit in Xinjiang, China

Abstract: The Ashele Cu‐Zn deposit is a recently discovered volcanogenic massive sulfide deposit in Xinjiang, Northwestern China. It is the largest Cu‐Zn deposit in this type of deposits in China, which were formed in the early period of later Palaeozoic Era. This deposit is hosted within a suit of bimodal submarine volcanic rocks of the Ashele Formation of Lower‐Middle Devonian System formed in an environment of paleocontinental margin rift setting. Lensoid orebodies occur between spilitic rocks developed at footwall and quartz‐keratophyric tuff at hanging wall. Zonation of metal elements in the Ashele mine is one of typical volcanic‐related exhalative Cu‐Zn sulfide deposits in the world. Black ores enriched in Pb, Zn and Ag occurs on the top of the No. 1 orebody in the Ashele deposit, yellow ores enriched in Cu in the middle part, and the chalcopyritization stringer below the massive sulfide ores. Zonation of ore‐structure in the No. 1 orebody is also apparent and corresponds to the zoning of elements, i. e. lamellar and/or banded sulfide‐sulfate ores on the top, massive sulfide ores in the middle, and stockwork veinlets associated with altered breccia pipe on the bottom. Four epochs of mineralization in the Ashele deposit has been recognized. The first period of syngenetic‐exhalative deposition of sulfides is the main epoch of mineralization, and the ores deposited subsequently subjected to thermo‐metamorphism at the second epoch, superimposed by hydrothermal mineralization at the third epoch, and weathered or oxidized at the fourth epoch.More than 100 categories of minerals have been recognized in the Ashele mine, but only pyrite, chalcopyrite, sphalerite, tetrahedrite, galena, barite, quartz, chlorite, sericite, and calcite are dominant, making up various types of ores, and alteration pipes or horizons. Studies of ore petrology suggest that the massive ores were volcanogenic and deposited by exhalative process.

Nina Creek Group and Lay Range Assemblage, north-central British Columbia: remnants of late Paleozoic oceanic and arc terranes

In north-central British Columbia, a belt of upper Paleozoic volcanic and sedimentary rocks lies between Mesozoic arc rocks of Quesnellia and Ancestral North America. These rocks belong to two distinct terranes: the Nina Creek Group of the Slide Mountain terrane and the Lay Range Assemblage of the Quesnel terrane. The Nina Creek Group is composed of Mississippian to Late Permian argillite, chert, and mid-ocean-ridge tholeiitic basalt, formed in an ocean-floor setting. The sedimentary and volcanic rocks, the Mount Howell and Pillow Ridge successions, respectively, form discrete, generally coeval sequences interpreted as facies equivalents that have been interleaved by thrusting. The entire assemblage has been faulted against the Cassiar terrane of the North American miogeocline. West of the Nina Creek Group is the Lay Range Assemblage, correlated with the Harper Ranch subterrane of Quesnellia. It includes a lower division of Mississippian to Early Pennsylvanian sedimentary and volcanic rocks, some with continental affinity, and an upper division of Permian island-arc, basaltic tuffs and lavas containing detrital quartz and zircons of Proterozoic age. Tuffaceous horizons in the Nina Creek Group imply stratigraphic links to a volcanic-arc terrane, which is inferred to be the Lay Range Assemblage. Similarly, gritty horizons in the lower part of the Nina Creek Group suggest links to the paleocontinental margin to the east. It is assumed that the Lay Range Assemblage accumulated on a piece of continental crust that rifted away from ancestral North America in the Late Devonian to Early Mississippian by the westward migration of a west-facing arc. The back-arc extension produced the Slide Mountain marginal basin in which the Nina Creek Group was deposited. Arc volcanism in the Lay Range Assemblage and other members of the Harper Ranch subterrane was episodic rather than continuous, as was ocean-floor volcanism in the marginal basin. The basin probably grew to a width of hundreds rather than thousands of kilometres.

Late Cenozoic tectonics and paleomagnetism of North Cauca Basin intrusions, Colombian Andes: Dual rotation modes

Further paleomagnetic results are reported from 18 sites in Late Tertiary hypabyssal igneous rocks adjacent to the Romeral fault system which forms the eastern boundary of the North Cauca Basin. The paleomagnetic directions in these massive igneous intrusions are distributed in vertical planes parallel to the adjacent Romeral suture zone. Two modes of rotation, involving non-coherent rotations about horizontal rotation axes and coherent rotations about vertical rotation axes, can explain the observed remanence patterns. The paleovectors have been rotated variably (non-coherently), both upwards and downwards, through up to nearly 90°, about horizontal axes which are perpendicular to the prevailing structural trend. These horizontal axis rotations are believed to be associated with shear gradients operating on rough projections or “asperities” on otherwise approximately lenticular bodies. These give rise to torques which cause variable horizontal axis rotation along splay faults of the Romeral and adjacent systems. In addition, the north segment of the zone of intrusions has been rotated coherently, i.e. through uniform angles, of about 30° counterclockwise, about vertical axes relative to the southern zone. The change in declination trend corresponds to a major change in trend of the Romeral suture zone. The vertical-axis rotations are believed to represent regional-scale kinking of vertical lenti-laminar fault panels of crust accreted to the paleocontinental margin along the Romeral suture. It is hypothesized that oblique transpression has pressed the zone of intrusions into the Caldas Re-entrant in the paleocontinental margin.

Crustal structure of the northern Yukon and Mackenzie Delta, northwestern Canada

Travel time inversion and amplitude forward modeling have been applied to two seismic refraction profiles from the northern Yukon‐Mackenzie Delta region of northwestern Canada. The two‐dimensional crustal P wave velocity models feature a near‐surface layer which is 1–7 km thick and has an average velocity of 4 km/s; this overlies three crustal units, each having an average thickness of 11–15 km and with average velocities of 5.9, 6.1, and 7.1 km/s. The Moho is at ∼37 km with little relief and overlies an upper mantle with a poorly constrained velocity. Tectonically, the study area lies between cratonic and Cordilleran North America and adjacent Mesozoic polar continental margin. The velocity models clearly illustrate a domainal crustal structure in the study area. A cratonic domain is characterized by a middle and lower crust with homogeneous velocities of 6.6–6.8 km/s. The other domain (“Yukon domain”) is characterized by midcrustal velocities near 6 km/s and a lower crustal layer with velocities near 7.1 km/s. The transition zone between these domains is well‐defined and is interpreted as a Proterozoic paleocontinental margin, supporting previous interpretations based on geological trends and potential field data. Lateral homogeneity of the crustal velocity structure within Yukon domain supports interpretations that Arctic Alaska was not emplaced into its present position on strike‐slip faults. Local variations in lower crustal thickness, together with clear wide‐angle Moho reflections, suggest a lower crustal and Moho signature possibly related to rifting, crustal extension, and magmatic intrusion and underplating during the Jura‐Cretaceous development of the Arctic Ocean and polar continental margin.