Margin Terranes Research Articles

Upper Triassic igneous rocks of the southern Kenai Peninsula, Alaska—prelude to Early Jurassic subduction along the Western Wrangellia composite terrane margin

New U–Pb zircon geochronology identifies a latest Triassic (ca 214–201 Ma) igneous suite of tuff, hypabyssal dikes, and a pluton on the southern Kenai Peninsula, Alaska. The igneous suite was emplaced within Upper Triassic sedimentary rocks along the southern margin of Western Wrangellia, the western-most fragment of the Wrangellia composite terrane. The igneous rocks range from mafic (50.6% SiO2) to felsic (78.3% SiO2), characteristically have less than 1.55% K2O, and generally have low trace element abundances. The tonalitic and trondhjemitic magmas were largely sourced in mafic-rich lower crust and incompletely assimilated quartz and other mineral xenocrysts are common. Fractionation involving plagioclase and amphibole is indicated for some magmas and composite intrusions and igneous xenoliths indicate magma mixing was possible. Paleozoic and Precambrian inherited zircons and initial 87Sr/86Sr (0.704103–0.705609) and 143Nd/144Nd (0.512396–0.512777) ratios indicate that the Western Wrangellia crustal sources are heterogeneous and contain sialic components. The latest Triassic magmatism reflects processes that preceded Early Jurassic subduction along the Wrangellia composite terrane and Pacific Ocean plate boundary. These processes involved heating and melting of mantle lithosphere and lower crust as mantle instabilities accompanied the breaking of the plate boundary linkages. The Late Triassic transition to subduction along the Wrangellia composite terrane margin coincided with the transition to subduction cessation in the Late Triassic arcs of the western Intermontane terranes of Canada. The shift to subduction along the outboard Wrangellia composite terrane margin marks the beginning of the Pacific Ocean–Cordillera plate interactions that came to dominate the tectonic evolution of the northern Cordillera from the Early Jurassic to today.

Structural control of Cambrian paleotopography and patterns of transgression in western Laurentia

Abstract The Cambrian transgression across the Great Unconformity produced one of the largest expansions of shallow marine habitats and associated diversification of marine invertebrate faunas in Earth history. However, identification of the underlying controls on the pattern of transgression of Cambrian seas has been hampered by imprecise or inaccurate age assignments for many formations. Recovery of an Ehmaniella Zone trilobite fauna from the Lodore Formation in northwestern Colorado (United States) revises the age of this unit to be significantly older, specifically middle Miaolingian (upper Wuliuan). This expands the established distribution of thick Miaolingian deposits of the northern Rocky Mountains to within 90 km of a broad region of central Colorado where Miaolingian strata are missing and Furongian successions rest directly on basement. The boundary between these two regions marks the position of an ~200 km east-west offset within the generally north-south–trending Cambrian paleoshoreline of western North America. The offset is co-located with a Precambrian continental suture zone (Cheyenne belt) at the northern Yavapai terrane margin and is directly east of an offset of similar magnitude and latitude in the early to middle Paleozoic shelf edge in Nevada. We thus posit that Precambrian deep-seated crustal-scale features controlled the Cambrian paleotopography of western Laurentia, strongly influencing the patterns of Cambrian transgression and structure of the shallow marine ecosystem established during continental submergence. These continental-scale structural elements remained a major control on marine paleogeography for >200 m.y. into the late Paleozoic.

Cenozoic kinematic histories of the Tidding and Lohit thrusts in the northern Indo-Burma Ranges: Implications for crustal thickening and exhumation of Gangdese lower arc crust along the Indus-Yarlung suture zone

Crustal thickening has been a key process of collision-induced Cenozoic deformation along the Indus-Yarlung suture zone, yet the timing, geometric relationships, and along-strike continuities of major thrusts, such as the Great Counter thrust and Gangdese thrust, remain inadequately understood. In this study, we present findings of geologic mapping and thermo- and geochronologic, geochemical, microstructural, and geothermobarometric analyses from the easternmost Indus-Yarlung suture zone exposed in the northern Indo-Burma Ranges. Specifically, we investigate the Lohit and Tidding thrust shear zones and their respective hanging wall rocks of the Lohit Plutonic Complex and Tidding and Mayodia mélange complexes. Field observations are consistent with ductile deformation concentrated along the top-to-the-south Tidding thrust shear zone, which is in contrast to the top-to-the-north Great Counter thrust at the same structural position to the west. Upper amphibolite-facies metamorphism of mélange rocks at ∼9−10 kbar (∼34−39 km) occurred prior to ca. 36−30 Ma exhumation during slip along the Tidding thrust shear zone. To the north, the ∼5-km-wide Lohit thrust shear zone has a subvertical geometry and north-side-up kinematics. Cretaceous arc granitoids of the Lohit Plutonic Complex were emplaced at ∼32−40 km depth in crust estimated to be ∼38−52 km thick at that time. These rocks cooled from ca. 25 Ma to 10 Ma due to slip along the Lohit thrust shear zone. We demonstrate that the Lohit thrust shear zone, Gangdese thrust, and Yarlung-Tsangpo Canyon thrust have comparable hanging wall and footwall rocks, structural geometries, kinematics, and timing. Based on these similarities, we interpret that these thrusts formed segments of a laterally continuous thrust system, which served as the preeminent crustal thickening structure along the Neotethys-southern Lhasa terrane margin and exhumed Gangdese lower arc crust in Oligocene−Miocene time.

Revision of F. R. C. Reeds Ordovician trilobite types from Myanmar (Burma) and western Yunnan Province, China.

The field collections made from Burma (Myanmar) by the Geological Survey of India, and described by F.R.C. Reed more than a century ago, still provide the only ground truthing for an important region of the Ordovician marginal terranes fringing Gondwana. A revision of these faunas is overdue, particularly as it is likely that further collections cannot be made in the northern Shan State in the near future. The specimens, stored in the Geological Survey of India collections in Kolkata, cannot be loaned. Sixteen species are fully revised herein; another twelve species are left under open nomenclature because of inadequacies in the material. Several of Reeds species subsequently became type species of genera that have proved to be widespread: Birmanites Sheng, 1934, Encrinurella Reed, 1915, Neseuretinus Dean, 1967, and Pliomerina Chugaeva, 1956. Reeds Ordovician trilobite collections came from two main areas: northern Shan State (Myanmar), and westernmost Yunnan (China). The Burmese (Myanmar) collections are from the Upper Ordovician (Katian) while Yunnan specimens are from the Middle Ordovician (Darriwilian), though Upper Ordovician trilobites also occur in the area. Both collections are predominantly from clastic strata. Based on a small new Katian collection from Pupiao, we report Neseuretinus birmanicus (Reed, 1906) in common between the northern part of the Shan State and western Yunnan. A few genera (Dionide Barrande, 1847, Phorocephala Lu, 1957, Lonchodomas Angelin, 1854, Nileus Dalman, 1827) are distributed worldwide, and include pelagic (Phorocephala) or deeper benthic (Dionide) taxa. The palaeogeographic comparisons offered by the other taxa are mostly peri-Gondwanan and extend from southwest China westwards (present geography) as far as the Iberian Pennsula. Birmanites is the type genus of a subfamily (Birmanitinae Kobayashi, 1960, revived herein) widely distributed over Ordovician Gondwana, and absent from Laurentia, Baltica and North China/Siberia. Mioptychopyge Zhou, Dean, Yuan Zhou, 1998, probably belongs with the same group and is otherwise known from South China. Parillaenus Jaanusson, 1954, is also peripheral Gondwanan, as is Prionocheilus Rouault, 1847. The Reedocalymeninae Kobayashi, 1951 (Neseuretinus, Reedocalymene Kobayashi, 1951) are similarly diagnostic of periGondwanan sites. However, some genera (Pliomerina, Encrinurella, Ovalocephalus Koroleva, 1959) have been associated with other oriental and Australian occurrences in particular, with outliers in certain terranes in Kazakhstan, i.e. palaeotropical Gondwana.

Variscan magmatic evolution of the Strandja Zone (Southeast Bulgaria and northwest Turkey) and its relationship to other north Gondwanan margin terranes

The Strandja Zone, straddling the border between Bulgaria and Turkey, is often assigned to either the Balkanide or the Pontide thrust belts of the Alpine orogen in the Black Sea region. Previous studies have considered this zone, which originated on the North Gondwanan margin, as part of a Late Carboniferous to Triassic magmatic arc associated with the subduction of the Paleo-Tethys Ocean beneath Eurasia. Magmatism has been regarded as representing one continuous or two separate stages occurring under different tectonic settings. We present new LA-ICP-MS U-Pb zircon ages together with field, petrographic and geochemical studies of variably deformed granites from the Sakar Batholith and Levka Pluton of the Sakar Unit in the Strandja Zone. The new U-Pb ages from Sakar Batholith (ca. 306 Ma) and Levka Pluton (ca. 319 Ma) demand a re-evaluation of previously published U-Pb crystallisation ages from these magmatic bodies. The U-Pb age reported for the Levka Pluton also provides an upper age limit for the timing of Variscan metamorphism. The Late Carboniferous to Early Permian magmatic evolution of the Strandja Zone displays a strong resemblance to that of the Sredna Gora Zone. Both units, probably together with Serbo-Macedonian Metamorphic Complex and Sakarya Zone, were part of the metamorphic core of the Variscan Orogen.

On the origins of the Iapetus Ocean

The Iapetus Ocean opened during the fission of the supercontinent Rodinia, from the breakup of three of its core continental constituents: Laurentia, Baltica and Amazonia. The timing of Iapetus opening is still much debated, with estimates ranging from 700 to 550 Ma. Similarly debated is exactly how Laurentia, Baltica and Amazonia were positioned relative to each other immediately before their breakup. In this study, we reconsider the timing and framework of Iapetus opening by integrating the fragmentary mid-Neoproterozoic to early Cambrian observational records from these continents. We first demonstrate that paleomagnetic data, despite being both sparse and probably contaminated by some global-scale, non-uniformitarian process in Ediacaran time, support the existence of a wide ocean between these continents by 575 Ma. However, the paleomagnetic data alone are insufficient to allow the formulation of more specific conclusions concerning the timing and paleogeography of Iapetus opening. We therefore conduct an extensive review of the mid-Neoproterozoic to Cambrian geology of eastern Laurentia, western Baltica and western Amazonia which, jointly interpreted with the paleomagnetic constraints, allow us to construct a self-consistent and geodynamically feasible plate tectonic model. In this model, the breakup of Laurentia, Baltica and Amazonia was polyphase, involving the spalling of multiple marginal terranes from Laurentia and the successive opening of several oceans, including a composite ‘Iapetus Ocean’. The first phase of continental breakup occurred between eastern Laurentia and western Amazonia at 750–700 Ma, leading to the opening of the Puncoviscana Ocean. This was followed by the opening of the eastern branch of the Iapetus Ocean, between Laurentia and Baltica, at ~590 Ma, which may have been instigated by emplacement of the Central Iapetus Magmatic Province. The western branch of Iapetus subsequently opened at ~550 Ma by the detachment of marginal terranes from eastern Laurentia, following a protracted phase of rifting. We contend that our preferred scenario is the simplest solution given the presently available evidence but throughout this review we underline key outstanding questions and the attendant uncertainties in our preferred model.

U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic Tectonic Evolution of Southern Wrangellia

Abstract Wrangellia is a late Paleozoic arc terrane that occupies two distinct coastal regions of western Canada and Alaska. The Skolai arc of northern Wrangellia in south-central Alaska and Yukon has been linked to the older, adjacent Alexander terrane by shared Late Devonian rift-related gabbros and also by Late Pennsylvanian postcollisional plutons. Late Devonian to Early Permian Sicker arc rocks of southern Wrangellia are exposed in uplifts on Vancouver Island, southwestern British Columbia, surrounded by younger strata and lacking physical connections to other terranes. Utilizing the detrital zircon record of Paleozoic and Cretaceous sedimentary rocks, we provide insight into the magmatic and depositional evolution of southern Wrangellia and its relationships to both northern Wrangellia and the Alexander terrane. 1422 U-Pb LA-ICPMS analyses from the Fourth Lake Formation (Mississippian–Permian) reveal syndepositional Carboniferous age peaks (344, 339, 336, 331, and 317 Ma), sourced from the Sicker arc of southern Wrangellia. These populations overlap in part known ages of volcanism, but the Middle Mississippian cumulative peak (337 Ma) documents a previously unrecognized magmatic episode. Paleozoic detrital zircons exhibit intermediate to juvenile ƐHft values between +15 and +5, indicating that southern Wrangellia was not strictly built on primitive oceanic crust, but instead on transitional crust with a small evolved component. The Fourth Lake samples yielded 49 grains (3.4% of the total grains analyzed) with ages between 2802 Ma and 442 Ma, and with corresponding ƐHft values ranging from +13 to -20. In age—ƐHft space, these grains fall within the Alexander terrane array. They were probably derived from sedimentary rocks in the basement of the Sicker arc. By analogy with northern Wrangellia, this basement incorporated rifted fragments of the Alexander terrane margin as the combined Sicker-Skolai arc system advanced ocean-ward due to slab rollback in Late Devonian to Early Mississippian time. Ultimately, data from detrital zircons preserved in the Fourth Lake Formation provides significant information allowing for an updated tectonic model of Paleozoic Wrangellia.

ЗЕМНАЯ КОРА И ВЕРХНЯЯ МАНТИЯ В ОБЛАСТИ СОЧЛЕНЕНИЯ ЦЕНТРАЛЬНО-АЗИАТСКОГО И ТИХООКЕАНСКОГО СКЛАДЧАТЫХ ПОЯСОВ

Spatial distributions of gravity sources and density contrast of geological media, which is reflected by the values of parameter μz , into the crust and upper mantle of Northeast China are analyzed. Features of rheological layering of the tectonosphere and deep spatial relationships of tectonic structures (cratonic blocks, marginal terranes, and sedimentary basins) are defined. In the density contrast distributions the formal signs of Paleozoic subduction of the North-China Craton and Mesozoic subduction of the Pacific Plate under the Amurian Plate were revealed. Crustal deformations are in sharp contrast with upper mantle deformations in structural planes resulting from different directions of tectonic stresses in the Paleozoic and Mesozoic. Thrusting of marginal terranes (Jamusi, Khanka) over the Amurian Plate lithosphere is revealed. Rheology and deep structure of North East China bear many similarities to other regions of the Pacific western margin in Asia and Australia.

Copper-Gold Fertility of Arc Volcanic Rocks: A Case Study from the Early Permian Lizzie Creek Volcanic Group, NE Queensland, Australia

Abstract The Early Permian Lizzie Creek Volcanic Group of the northern Bowen Basin, NE Queensland, Australia, has compositions that range from basalt through andesite to rhyolite with geochemical signatures (e.g., enrichment in Cs, Rb, Ba, U, Th, and Pb, depletion in Nb and Ta) that are typical of arc lavas. In the Mount Carlton district the Lizzie Creek Volcanic Group is host to high-sulfidation epithermal Cu-Au-Ag mineralization, whereas farther to the south near Collinsville (~50 km from Mount Carlton) these volcanic sequences are barren of magmatic-related mineralization. Here, we assess whether geochemical indicators of magma fertility (e.g., Sr/Y, La/Yb, V/Sc) can be applied to volcanic rocks through study of coeval volcanic sequences from these two locations. The two volcanic suites share similar petrographic and major element geochemical characteristics, and both have undergone appreciable hydrothermal alteration during, or after, emplacement. Nevertheless, the two suites have distinct differences in alteration-immobile trace element (V, Sc, Zr, Ti, REE, Y) concentrations. The unmineralized suite has relatively low V/Sc and La/Yb, particularly in the high SiO2 rocks, which is related to magma evolution dominated by fractionation of clinopyroxene, plagioclase, and magnetite. By contrast, the mineralized suite has relatively high V/Sc but includes high SiO2 rocks with depleted HREE and Y contents, and hence high La/Yb. These trends are interpreted to reflect magma evolution under high magmatic H2O conditions leading to enhanced amphibole crystallization and suppressed plagioclase and magnetite crystallization. These rocks have somewhat elevated Sr/Y compared to the unmineralized suite, but as Sr is likely affected by hydrothermal mobility, Sr/Y is not considered to be a reliable indicator of magmatic conditions. Our data show that geochemical proxies such as V/Sc and La/Yb that are used to assess Cu-Au fertility of porphyry intrusions can also be applied to cogenetic volcanic sequences, provided elemental trends with fractionation can be assessed for a volcanic suite. These geochemical tools may aid regional-scale exploration for Cu-Au mineralization in convergent margin terranes, especially in areas that have undergone limited exhumation or where epithermal and porphyry mineralization may be buried beneath cogenetic volcanic successions.

Evolving Marginal Terranes During Neoproterozoic Supercontinent Reorganization: Constraints From the Bemarivo Domain in Northern Madagascar

AbstractMadagascar is a key area for unraveling the geodynamic evolution of the transition between the Rodinia and Gondwana supercontinents as it contains several suites of c. 850–700 Ma magmatic rocks that have been postulated to correlate with other Rodinian terranes. The Bemarivo Domain of northern Madagascar contains the youngest of these units that date to c. 750–700 Ma. We present zircon Hf and O isotope data to understand northern Madagascar's place in the Neoproterozoic plate tectonic reconfiguration. We demonstrate that the northern component of the Bemarivo Domain is distinct from the southern part of the Bemarivo Domain and have therefore assigned new names—the Bobakindro Terrane and Marojejy Terrane, respectively. Magmatic rocks of the Marojejy Terrane and Anaboriana Belt are characterized by evolved εHf(t) signatures and a range of δ18O values, similar to the Imorona‐Itsindro Suite of central Madagascar. These magmatic suites likely formed together in the same long‐lived volcanic arc. In contrast, the Bobakindro Terrane contains juvenile εHf(t) and mantle‐like δ18O values, with no probable link to the rest of Madagascar. We propose that the Bobakindro Terrane formed in a juvenile arc system that included the Seychelles, the Malani Igneous Suite of northwest India, Oman, and the Yangtze Belt of south China, which at the time were all outboard from continental India and south China. The final assembly of northern Madagascar and amalgamation of the Bobakindro Terrane and Marojejy Terrane occurred along the Antsaba subduction zone, with collision occurring at c. 540 Ma.

Late Quaternary glacial dynamics and sedimentation variability in the Bering Trough, Gulf of Alaska

Abstract Ice dynamics, tectonic setting, and sediment supply are the key parameters controlling the architecture of high-latitude margins and the formation of trough mouth fans (TMFs). Current understanding of these archives of paleo–ice streams is based on studies of ice sheets adjacent to stable, passive margins, while the behavior of active, convergent glacier-influenced margins remains relatively unconstrained. We integrate high-resolution seismic data and chronology from Integrated Ocean Drilling Program Expedition 341 cores in southeast Alaska across the actively converging Yakutat terrane margin to examine the late Quaternary evolution of the Bering Glacier, the largest outlet glacier of the poorly understood Cordilleran Ice Sheet (CIS). We interpret at least eight glacial advances to the shelf break since the end of the mid-Pleistocene transition, showing a more dynamic CIS than hitherto realized. During the past ∼130 k.y., the temperate, meltwater-charged Bering Glacier delivered ∼925 km3 of sediment to the shelf and slope, providing one of the highest rates of sustained sediment accumulation (5–10 m/k.y.) ever reported globally. Rapid formation of a TMF, reaching ∼600 m thick in ∼130 k.y., emphasizes the extreme sediment flux that can be produced by wet-based glacial systems, and its critical role in the development of high-latitude margin stratigraphy. TMF formation despite initially steep, tectonically controlled slopes in this active setting reflects an autogenic shift in the evolution of the Bering Trough, suggesting that major transitions between sedimentary regimes need not reflect some externally driven change in climate variability.

Basin Analysis of the Albian–Santonian Xigaze Forearc, Lazi Region, South-Central Tibet

Abstract The Xigaze forearc basin records the evolution of the southern Lhasa terrane convergent margin, largely affected by Neo-Tethyan subduction processes, prior to the Paleocene Tethyan Himalaya–Eurasia collision. New geologic mapping and U-Pb detrital-zircon geochronologic data from the Lazi region, 340 km southeast of Lhasa, show that forearc basin sedimentation began ca. 110 Ma conformably atop the Yarlung–Tsangpo ophiolitic melange. By this time, the arc–trench system along the southern margin of the Lhasa terrane (Eurasia) consisted of an accretionary complex, overlying ophiolitic melange, the Xigaze forearc basin, and the Gangdese magmatic arc. There is no geological evidence in the Lazi region for more than one subduction zone between the southern Lhasa terrane margin and India. Sedimentological facies analysis from Albian to Santonian clastic and carbonate sedimentary rocks preserved in the Xigaze forearc basin indicate deep-marine sedimentation characterized by hemipelagic carbonate and volcanogenic sediment-gravity-flow deposits. Sandstone modal petrographic and U-Pb detrital-zircon geochronologic data reveal Asian continental margin, Gangdese magmatic arc, and central to northern Lhasa terrane provenance. During this time, basin fill was deposited in cycles of high and low sediment flux characterized by alternating successions of clastic turbidite inner- to outer-fan deposits and hemipelagic limestone and marlstone sequences. Along-strike differences in the timing of initial forearc basin sedimentation are likely the result of intra-basin topography and/or diachronous development of ophiolitic forearc basement.

Boron-polymetallic metallogeny of the north and northeast of the Sino-Korean Craton

Based on published data and original investigations, it has been shown that the combination of widely known Ag, Fe, and Fe-Mn ore deposits, as well as boron and Pb-Zn world-class deposits, is typical for metallogenic zones in the north and northeast of the Sino-Korean Craton. The ore genesis was spatially inherited and lasted from the Archean to Mesozoic. The Archean metallogenic zones are related to the protocontinental margin terranes of the craton basement and they comprise banded iron ore and Cu-Zn sulfide deposits. The proterozoic-Early Paleozoic metallogenic zones are related to rift basins, where the ore-bearing Archean folded basement is overlain by volcanic and sedimentary complexes. The Proterozoic metallogenic zones host quartz veins and schistosity zone-related Au deposits, banded iron and Cu-Zn ore deposits, large sedimentary-metamorphogenic borate and magnesite deposits, Cu-W deposits in tourmalinites, exhalation-sedimentary Pb-Zn ore deposits, and large polygenic REE-Fe-Nb ore deposits. The Riphean-Cambrian terrigenous-carbonate strata are represented by stratiform Pb-Zn and fluorite deposits. Mesozoic metallogenic zones related to volcano-plutonic complexes of intraplate series coincide with zones where the folded basement is made of Precambrian ore-bearing series. Gold deposits are typical of all the metallogenic zones, but most of them are related to Mesozoic volcano-plutonic complexes.

The Russian-Kazakh Altai orogen: An overview and main debatable issues

The paper reviews previous and recently obtained geological, stratigraphic and geochronological data on the Russian-Kazakh Altai orogen, which is located in the western Central Asian Orogenic Belt (CAOB), between the Kazakhstan and Siberian continental blocks. The Russian-Kazakh Altai is a typical Pacific-type orogen, which represents a collage of oceanic, accretionary, fore-arc, island-arc and continental margin terranes of different ages separated by strike-slip faults and thrusts. Evidence for this comes from key indicative rock associations, such as boninite- and turbidite (graywacke)-bearing volcanogenic-sedimentary units, accreted pelagic chert, oceanic islands and plateaus, MORB-OIB-protolith blueschists. The three major tectonic domains of the Russian-Kazakh Altai are: (1) Altai-Mongolian terrane (AMT); (2) subduction-accretionary (Rudny Altai, Gorny Altai) and collisional (Kalba-Narym) terranes; (3) Kurai, Charysh-Terekta, North-East, Irtysh and Char suture-shear zones (SSZ). The evolution of this orogen proceeded in five major stages: (i) late Neoproterozoic–early Paleozoic subduction-accretion in the Paleo-Asian Ocean; (ii) Ordovician–Silurian passive margin; (iii) Devonian–Carboniferous active margin and collision of AMT with the Siberian continent; (iv) late Paleozoic closure of the PAO and coeval collisional magmatism; (v) Mesozoic post-collisional deformation and anarogenic magmatism, which created the modern structural collage of the Russian-Kazakh Altai orogen. The major still unsolved problem of Altai geology is origin of the Altai-Mongolian terrane (continental versus active margin), age of Altai basement, proportion of juvenile and recycled crust and origin of the middle Paleozoic units of the Gorny Altai and Rudny Altai terranes.

Tectonic Setting of the Pebble and Other Copper-Gold-Molybdenum Porphyry Deposits within the Evolving Middle Cretaceous Continental Margin of Northwestern North America

The Pebble Cu-Au-Mo deposit in southwestern Alaska, containing the largest gold resource of any known porphyry in the world, developed in a tectonic setting significantly different from that of the present-day. It is one of a series of metalliferous middle Cretaceous porphyritic granodiorite, quartz monzonite, and diorite bodies, evolved from lower crust and metasomatized lithospheric mantle melts, which formed along much of the length of the North American craton suture with the Peninsular-Alexander-Wrangellia arc. The porphyry deposits were emplaced within the northernmost two of a series of ca. 130 to 80 Ma flysch basins that define the suture, as well as into arc rocks immediately seaward of the two basins. Deposits include the ca. 100 to 90 Ma Pebble, Neacola, and other porphyry prospects along the Kahiltna basin-Peninsula terrane boundary, and the ca. 115 to 105 Ma Baultoff, Carl Creek, Horsfeld, Orange Hill, Bond Creek, and Chisna porphyries along the Nutzotin basin-Wrangellia terrane boundary. The porphyry deposits probably formed along the craton margin more than 1,000 km to the south of their present latitude. Palinspastic reconstructions of plate kinematics from this period are particularly difficult because magmatism overlaps the 119 to 83 Ma Cretaceous Normal Superchron, a period when sea-floor magnetic data are lacking. Our favored scenario is that ore formation broadly overlaps the cessation of sedimentation and contraction and the transition to a transpressional continental margin regime, such that the remnant ocean basins were converted to strike-slip basins. The basins and outboard Peninsular-Alexander-Wrangellia composite superterrane, which are all located seaward of the deep crustal Denali-Farewell fault system, were subjected to northerly dextral transpression for as long as perhaps 50 m.y., beginning at ca. 95 ± 10 Ma. The onset of this transpression was marked by development of the mineralized bodies along fault segments on the seaward side of the basins. Geochemical and radiogenic isotopic data for igneous rocks associated with the Pebble porphyry deposit suggest continuous melt derivation from enriched lithosphere of a recently metasomatized mantle. These geochemical characteristics, coupled with the arc-continent-related collisional setting, suggest that lithospheric thickening and postcollisional lithospheric melting are the most likely cause of the ore-related magmatism. Subsequent to translation of the Alaskan margin terranes and early Tertiary oroclinal bending of Alaska, the northernmost Kahiltna basin and the Pebble deposit, as well as the other porphyry systems, reached their present-day locations along southern Alaska.

Development of the Khao Khwang Fold and Thrust Belt: Implications for the geodynamic setting of Thailand and Cambodia during the Indosinian Orogeny

The Indosinian Orogeny in Thailand is often viewed as having developed between strongly linear terranes, which today trend approximately N–S. The terranes were subsequently disrupted by later tectonics, particularly NW–SE trending Cenozoic strike-slip faults. The ENE–WSW to NE–SW striking thrusts and folds in the Khao Khwang Platform area of the Saraburi Group on the SW margin of the Indochina Terrane are not easily explained in the context of this traditional view. Reversal of the clockwise rotation shown to have affected the block north of the Mae Ping Fault zone only enhances the E–W orientation of structures in the fold and thrust belt, and moves the belt further east towards Cambodia. One solution for the trend that fits better with regional understanding from hydrocarbon exploration of the Khorat Plateau is that the Indochina Terrane was actually a series of continental blocks, separated by Permian rifting. During the Early Triassic the early stages of collision (South China-Cathaysian Terrane collision with Vietnam Indochina) resulted in the amalgamation of disparate blocks that now form the Indochina Terrane by closure along the rifts. At the same time or following on from the collision there was closure of the back-arc area between Indochina and the Sukhothai zone. The rift basins, were thrusted and inverted during the early stages of the Indosinian orogeny, and only underwent minor reactivated when later Sibumasu collided with Sukhothai Zone-Indochina Terrane margin during the Late Triassic. The scenario described above requires the presence of a (minor) E–W trending suture in NW Cambodia. Evidence for this suture is suggested by the presence of Permo-Triassic calc-alkaline volcanism.

Eastern Dharwar Craton, India: Continental lithosphere growth by accretion of diverse plume and arc terranes

Greenstone belts of the eastern Dharwar Craton, India are reinterpreted as composite tectonostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new high-precision elemental data. The former are dominated by a komatiite plus Mg-tholeiitic basalt volcanic association, with deep water siliciclastic and banded iron formation (BIF) sedimentary rocks. Plumes melted at <90 km under thin rifted continental lithosphere to preserve intraoceanic and continental margin aspects. Associated alkaline basalts record subduction-recycling of Mesoarchean oceanic crust, incubated in the asthenosphere, and erupted coevally with Mg basalts from a heterogeneous mantle plume. Together, komatiites-Mg basalts-alkaline basalts plot along the Phanerozoic mantle array in Th/Yb versus Nb/Yb coordinate space, representing zoned plumes, establishing that these reservoirs were present in the Neoarchean mantle.Convergent margin magmatic associations are dominated by tholeiitic to calc-alkaline basalts compositionally similar to recent intraoceanic arcs. As well, boninitic flows sourced in extremely depleted mantle are present, and the association of arc basalts with Mg-andesites-Nb enriched basalts-adakites documented from Cenozoic arcs characterized by subduction of young (<20 Ma), hot, oceanic lithosphere. Consequently, Cenozoic style “hot” subduction was operating in the Neoarchean. These diverse volcanic associations were assembled to give composite terranes in a subduction-accretion orogen at ∼2.7 Ga, coevally with a global accretionary orogen at ∼2.7 Ga, and associated orogenic gold mineralization.Archean lithospheric mantle, distinctive in being thick, refractory, and buoyant, formed complementary to the accreted plume and convergent margin terranes, as migrating arcs captured thick plume-plateaus, and the refractory, low density, residue of plume melting coupled with accreted imbricated plume-arc crust.

Geodynamic setting of Neoproterozoic nickel sulphide deposits in eastern Africa

Four recently explored nickel sulphide deposits in East Africa are briefly described, and their geodynamic and palaeoenvironmental settings in the Neoproterozoic era are proposed. The area of East Africa considered lies at the southern margin of the Congo craton. This craton was assembled in the Palaeoproterozoic and microcontinental fragments were added at its southern margin in the Mesoproterozoic Irumi orogeny. In the early Neoproterozoic, the southern margin of the Congo craton was subject to rifting and widespread mafic magmatism, and island arcs may have developed off its eastern margin during this time. Final assembly of Gondwanaland in the late Neoproterozoic and Cambrian led to closure of the transient Mozambique and Zambezi oceans and widespread thermal and tectonic reactivation within the Congo craton and its adjacent marginal terranes. The Munali nickel deposit represents an early phase of basic magmatism in the early Neoproterozoic Katanga rift, hosted by a composite gabbro-ultramafic body emplaced at a high level in rift to passive margin sediments. The Mpemba, Rovuma and Nachingwea groups of deposits may also represent extensional basic magmatism at the southern margin of the greater Congo craton. However, an alternative setting for the Rovuma and Nachingwea deposits may be as small, subvolcanic intrusions in a more oceanic setting (island arc or back-arc basin) within the former Mozambique ocean.

Deep shelf trilobite biofacies from the upper Katian (Upper Ordovician) of the Grangegeeth Terrane, eastern Ireland

AbstractThe trilobite fauna of the upper Katian (mid‐Ashgill in terms of Anglo‐Welsh chronostratigraphy) Oriel Brook Formation in the Grangegeeth Terrane, eastern Ireland comprises 16 species. It is dominated numerically by mesopelagic cyclopygids (Cyclopyge cf. marginata Hawle and Corda and much rarer Symphysops sp.), which comprise 63% of the 156 identifiable sclerites known unequivocally from the formation. Other elements of the trilobite fauna include species of Staurocephalus, Phillipsinella, Nankinolithus and the agnostids Trinodus and Sphaeragnostus. The abundance of cyclopygids in a terrane interpreted as lying within the Iapetus Ocean closer to Laurentia than the Anglo‐Welsh sector of Avalonia is consistent with the Late Katian spread of cyclopygids onto low latitude shelves seen in Quebec and Maine in eastern North America, Girvan in SE Scotland and Co. Clare in the Irish Central Terrane. With the exception of the agnostid trilobites, whose mode of life is strongly debated, associated benthic trilobites lacking eyes are fairly rare and the bottom‐dwelling fauna does not conform to the atheloptic assemblages that typified many mid‐ to high latitude Ordovician outer shelf and upper slope settings. The benthic assemblage can be considered in terms of a bathymetrically arranged spectrum of faunas and is closest to that of the deep shelf Phillipsinella parabola—Staurocephalus clavifrons fauna, known from the Upper Ordovician of Avalonia and Baltica and to some extent marginal Gondwana and South China. This is the first record of such a fauna from Laurentia or its marginal terranes and reflects the Late Ordovician breakdown of provincialism in the Iapetus Ocean. The deep‐water setting is compatible with that of the associated Foliomena brachiopod fauna. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Tin deposits of the Sikhote–Alin and adjacent areas (Russian Far East) and their magmatic association

The Sikhote–Alin accretionary belt along the northwestern Pacific Plate hosts the most important tin province of Russia. Here, more than 500 ore deposits were formed between 105 and 55 Ma at transform and active subduction margins. Petrological models suggest an active role of the mantle in the mineralisation processes. The deposits can be divided into three groups according to their mineral content and associated magmatism. The first group, a cassiterite–quartz group is defined by tin-bearing greisens as well as quartz–cassiterite and quartz–cassiterite–feldspar veins and stockworks. The mineralisation shows distinct genetic relationships with S- and A-type granites. The deposits are located mainly in Jurassic accretionary prisms adjacent to the Bureya–Khanka Paleozoic continental terrane margin. The second group is represented by the economically important cassiterite–silicate–sulfide deposits, which produce about 80% of Russian tin. Mineralisation in this group is represented by metasomatic zones or veins related to I-type granitoids. The orebodies consist of cassiterite–tourmaline–quartz or cassiterite–chlorite–quartz associations and contain variable amounts of sulfides. The third group comprises tin deposits containing cassiterite and sulfides with the most complicated ore composition with abundant sulfides and sulfostannates accounting for 60–80% of the total ore mass. In some deposits, zinc, lead and silver dominate, whereas tin is sub-economic. The deposits of this group are generally associated with magmatic rocks of the Sikhote–Alin volcano-plutonic belt. The different associations are found together in the same districts and, locally, also in individual deposits. These are characterised by polychronous and polygenetic mineral systems, formed during long periods of time and in different tectonic settings. This testifies to changes in the many physico-chemical parameters of ore formation and, probably, of ore sources. We suggest that the complex mineral and element compositions of some of the ores were caused by the long-lasting composite tectono-magmatic processes.