Showing papers on "Terrane published in 2003"

Gold Deposits in Metamorphic Belts: Overview of Current Understanding,Outstanding Problems, Future Research, and Exploration Significance

Abstract: Metamorphic belts are complex regions where accretion or collision has added to, or thickened, continental crust. Gold-rich deposits can be formed at all stages of orogen evolution, so that evolving metamorphic belts contain diverse gold deposit types that may be juxtaposed or overprint each other. This partly explains the high level of controversy on the origin of some deposit types, particularly those formed or overprinted/remobilized during the major compressional orogeny that shaped the final geometry of the hosting metamorphic belts. These include gold-dominated orogenic and intrusion-related deposits, but also particularly controversial gold deposits with atypical metal associations. Orogenic lode gold deposits of Middle Archean to Tertiary age are arguably the predominant gold deposit type in metamorphic belts, and include several giant (>250 t Au) and numerous world-class (>100 t Au) examples. Their defining characteristics and spatial and temporal distributions are now relatively well documented, such that other gold deposit types can be compared and contrasted against them. They form as an integral part of the evolution of subduction-related accretionary or collisional terranes in which the host-rock sequences were formed in arcs, back arcs, or accretionary prisms. Current unknowns for orogenic gold deposits include the following: (1) the precise tectonic setting and age of mineralization in many provinces, particularly in Paleozoic and older metamorphic belts; (2) the source of ore fluids and metals; (3) the precise architecture of the hydrothermal systems, particularly the relationship between first- and lower-order structures; and (4) the specific depositional mechanisms for gold, particularly for high-grade deposits. Gold-dominant intrusion-related deposits are a less coherent group of deposits, which are mainly Phanerozoic in age, and include a few world-class, but no unequivocal giant, examples. They have many similarities to orogenic deposits in terms of metal associations, wall-rock alteration assemblages, ore fluids, and, to a lesser extent, structural controls, and hence, some deposits, particularly those with close spatial relationships to granitoid intrusions, have been placed in both orogenic and intrusion-related categories by different authors. Those that are clearly intrusion-related deposits appear to be best distinguished by their near-craton setting, in locations more distal from subduction zones than most orogenic gold deposits and in provinces that also commonly contain Sn and/or W deposits; relatively low gold grades (<1–2 g/t Au); and district-scale zoning to Ag-Pb-Zn deposits in distal zones. Outstanding problems for intrusion-related deposits include the following: (1) lack of a clear definition of this apparently diverse group of deposits, (2) lack of a definitive link for ore fluids and metals between mineralization and magmatism, (3) the diverse nature of both petrogenetic association and redox state of the granitoids invoked as the source of mineralization, and (4) mechanisms for exsolution of the CO 2 -rich ore fluids from the relatively shallow level granitoids implicated as ore-fluid sources. Gold deposits with atypical metal associations are a particularly diverse and controversial group, are most abundant in Late Archean terranes, and include several world-class to giant examples. Most are probably modified Cu-Mo-Au porphyry, volcanic rock-hosted Zn-Pb-Ag-Au massive sulfide, or Zn-Pb-Ag-Au or Ba-Au-Mo-Hg submarine epithermal systems, overprinted or remobilized during the events in which orogenic gold deposits formed, but there is lack of consensus on genesis. Outstanding problems for these deposits include the following: (1) lack of a clear grouping of distinctive deposits, (2) lack of published, well integrated studies of their characteristics, (3) generally a poorly defined timing of mineralization events, and (4) lack of assessment of metal mass balances in each stage of the complex mineralization and overprinting events. Both orogenic gold deposits and gold deposits with atypical metal associations contain a few giant and numerous world-class examples, whereas the intrusion-related group contains very few world-class examples, and no giants, unless Muruntau is included in this group. Preliminary analysis suggests that the parameters of individual world-class to giant gold deposits of any type show considerable variation, and that it is impossible to define critical factors that control their size and grade at the deposit scale. However, there appears more promise at the terrane to province scale where there are greater indications of common factors such as anomalous subduction-related tectonic settings, reactivated crustal-scale deformation zones that focus porphyry-lamprophyre dike swarms in linear volcanosedimentary belts, complex regional-scale geometry of mixed lithostratigraphic packages, and evidence for multiple mineralization or remobilization events. There are a number of outstanding problems for all types of gold deposits in metamorphic belts. These include the following: (1) definitive classifications, (2) unequivocal recognition of fluid and metal sources, (3) understanding of fluid migration and focusing at all scales, (4) resolution of the precise role of granitoid magmatism, (5) precise gold-depositional mechanisms, particularly those producing high gold grades, and (6) understanding of the release of CO 2 -rich fluids from subducting slabs and subcreted oceanic crust and granitoid magmas at different crustal levels. Research needs to be better coordinated and more integrated, such that detailed fluid-inclusion, trace-element, and isotopic studies of both gold deposits and potential source rocks, using cutting-edge technology, are embedded in a firm geological framework at terrane to deposit scales. Ultimately, four-dimensional models need to be developed, involving high-quality, three-dimensional geological data combined with integrated chemical and fluid-flow modeling, to understand the total history of the hydrothermal systems involved. Such research, particularly that which can predict superior targets visible in data sets available to exploration companies before discovery, has obvious spin-offs for global- to deposit-scale targeting of deposits with superior size and grade in the covered terranes that will be the exploration focus of the twenty-first century.

Cenozoic Volcanism in Tibet: Evidence for a Transition from Oceanic to Continental Subduction

Abstract: Geochronological (K---Ar or Ar/Ar), major and trace element, Sr---Nd---Pb isotopic and mineral chemical data are presented for newly discovered Cenozoic volcanic rocks in the western Qiangtang and central Lhasa terranes of Tibet. Alkali basalts of 65---45Ma occur in the western Qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers [100 Mg/(Mg‡Fe)](54---65),Cr (204---839 ppm) and Ni (94---218 ppm) contents, and relatively low ratios of Sr/Sr (0 7046---0 7061), Pb/Pb (18 21---18 89), Pb/Pb (15 49---15 61) and Pb/Pb (38 42---38 89), and high ratios of Nd/Nd (0 5124---0 5127). In contrast, younger volcanic rocks in the western Qiangtang terrane ( 30Ma) and the central Lhasa terrane ( 23, 13 and 8Ma) are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source. They are characterized by very high contents of incompatible trace elements, negative Ta, Nb and Ti anomalies, and radiogenic Pb isotopic compositions (Pb/Pb ˆ 18 43---19 10; Pb/Pb ˆ 15 64---15 83; Pb/Pb ˆ 39 14---39 67). Sr/Sr (0 7088---0 7092) and Nd/Nd ( 0 5122) ratios of the western Qiangtang terrane potassic lavas are similar to those of 45---29Ma potassic volcanic rocks in the north---central Qiangtang terrane, whereas Sr/Sr (0 7167---0 7243) and Nd/Nd ( 0 5119) ratios of central Lhasa terrane lavas are similar to those of 25---16Ma ultrapotassic volcanic rocks in the western Lhasa terrane. The 65---45Ma alkali basalts in the western Qiangtang terrane, along with widespread calc-alkaline volcanic rocks of this age in the Lhasa terrane, may be related to roll-back of a previously shallow north-dipping slab of Tethyan oceanic lithosphere beneath Tibet. Subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in Tibet because of its occurrence in distinct, east---west-trending belts (45---29Ma in the Qiangtang terrane; 25---17Ma in the northern Lhasa terrane; 16---8Ma in the southern Lhasa terrane) and temporal and spatial relationships with major thrust systems.

Mesozoic and Cenozoic tectonic evolution of the Shiquanhe area of western Tibet

Abstract: [1] In the Shiquanhe area of far-western Tibet, mid-Cretaceous strata lie unconformable on ophiolitic melange and Jurassic flysch associated with the Bangong-Nujiang suture zone. On the basis of our mapping and geochronologic studies, we suggest that these Cretaceous strata were shortened by >57% over a north south distance of 50 km during Late Cretaceous-early Tertiary time. The Late Cretaceous Narangjiapo thrust placed Permian strata >20 km over ophiolitic melange and Cretaceous strata. North of the Narangjiapo thrust, >40 km of shortening was accommodated by the Late Cretaceous-early Tertiary south directed Jaggang thrust system that involves Jurassic flysch and Cretaceous strata, and roots into a decollement within ophiolitic melange. The most recent shortening was accommodated to the south of the Narangjiapo thrust, along the north dipping Shiquanhe thrust. The Shiquanhe thrust cuts flat-lying 22.6 ± 0.3 Ma volcanic rocks and underlying folded, Tertiary nonmarine strata in its footwall and was likely active during slip along the Oligocene Gangdese thrust system of southern Tibet. Ophiolitic melange and structurally overlying Jurassic flysch near Shiquanhe are interpreted to represent remnants of a subduction-accretion complex and forearc basin, respectively, that were obducted southward onto the margin of the Lhasa terrane during Late Jurassic-Early Cretaceous closure of the Bangong-Nujiang Ocean. Subsequent imbrication of the obducted sheet could have produced the two east-west trending belts of ophiolitic melanges, separated by ∼100 km, in western Tibet. Late Cretaceous-early Tertiary thin-skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of Lhasa terrane rocks beneath the obducted ophiolitic melange and the Qiangtang terrane to the north.

The Saghand Region, Central Iran: U-Pb geochronology, petrogenesis and implications for Gondwana Tectonics

Abstract: The Saghand area of East-Central Iran exposes rocks that comprise the substratum of the Central Iranian continental terrane, as part of the larger Alpine-Himalayan orogenic system. Our new U-Pb ages and geochemical data from the magmatic, metamorphic and siliciclastic rocks of the Saghand area unravel three main episodes of orogenic activity in the latest Neoproterozoic-Early Cambrian, the Late Triassic, and the Eocene. Geologic events in the oldest episode include in chronological order, low- to medium-grade metamorphism, calc-alkaline plutonism, rhyolitic to andesitic volcanism, and widespread trondhjemite emplacement, from 547 Ma to 525 Ma. The Late Triassic event (approximately 220-213 Ma) is characterized by the emplacement of granite-tonalite plutons. The extensive, high-grade metamorphic rocks, migmatites and post-kinematic intrusions of Eocene age (47-44 Ma) occur in a distinct domain, in the western part of the Saghand area. These rocks previously were thought to represent the Precambrian basement of the Central Iranian Terrane. The terminal Neoproterozoic-Early Cambrian orogeny in central Iran was related to a broad-scale magmatic arc that developed along the Proto-Tethyan margin of the Gondwanaland supercontinent. The fragmented remains of that margin occur as displaced terranes, including the Central Iranian Terrane, now embedded within the Alpine-Himalayan orogenic system. The newly recognized Late Triassic intrusions of the Saghand area are indicative of a tectonomagmatic episode of possible collisional nature, in accord with the previously identified Early Kimmerian (Cimmerian) event in the region. The extensive Eocene metamorphic and magmatic activities correspond to the early Alpine Orogeny, which resulted from the convergence between Arabian and Eurasian plates, and the Cenozoic closure of the Tethys oceanic tract(s) by subduction.

Detrital-zircon geochronology of the northeastern Tibetan plateau

Abstract: U-Pb geochronologic analyses have been conducted on 413 detrital-zircon grains collected from 16 samples in the Altun Shan, Nan Shan, and Qilian Shan. The samples come primarily from quartz arenites and metaturbidites of Middle to Late Proterozoic age and from feldspathic and volcanic clast-rich sandstones of early Paleozoic age. Zircon grains in Proterozoic strata resting on Tarim basement yielded mainly 2.0–1.9 Ga ages, whereas Proterozoic strata of the Qaidam and Qilian terranes yielded mainly ca. 930–820 Ma and ca. 1.9–1.1 Ga ages. The younger grains were apparently shed from local igneous rocks, whereas the grains older than 1.1 Ga were shed from an undetermined continental source. Grains in the lower Paleozoic strata are mainly ca. 500–430 Ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting. Our detrital-zircon ages are consistent with a model (first proposed by E.R. Sobel and N. Arnaud) in which early Paleozoic magmatism occurred within a single northeast-facing magmatic arc that was constructed across an assemblage of Middle to Late Proterozoic accretionary complexes, remnants of magmatic arcs, and shallow-marine strata. This arc system was accreted to the Tarim and Sino-Korean cratons during Silurian–Devonian time. The resulting suture has been reactivated as Tertiary thrust faults that currently define the structural and topographic margin of the Tibetan plateau. Our data also provide two new estimates for the offset along the eastern Altyn Tagh fault. A belt of Middle Proterozoic shallow-marine strata is offset by ∼400 km, whereas a belt of 490–480 Ma magmatic arc rocks is offset by ∼370 km. These values are generally similar to the 350–400 km offset reported in most previous studies.

Magmatic history of the northeastern Tibetan Plateau

Abstract: [1] The northeastern margin of the Tibetan Plateau is underlain by the Qaidam and Qilian terranes, which consist primarily of mid-Proterozoic through lower Paleozoic oceanic and arc-type assemblages that have been accreted to the southern margin of the Tarim/Sino-Korean craton. Most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the Tarim/Sino-Korean craton during early Paleozoic time. The main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins, or as a broad expanse of accretionary complexes. Our geochronologic data support a model, suggested by Sobel and Arnaud [1999], in which the Qaidam and Qilian terranes are separated from the Tarim/Sino-Korean craton by a mid-Paleozoic suture that closed along a southwest dipping subduction zone. The basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of ∼920–930 Ma granitoids. Early Paleozoic arc-type magmatism occurred between ∼480 and ∼425 Ma, apparently sweeping southwestward across much of the Qilian and Qaidam terranes. Accretion-related magmatism along the inboard margin of the Qilian terrane occurred between ∼423 Ma and ∼406 Ma. Following Silurian-Devonian accretion, the region has experienced late Paleozoic and Mesozoic uplift and erosion and has been severely overprinted by Tertiary thrusting, uplift, and strike-slip motion along the Altyn Tagh fault. Correlation of geologic features and magmatic histories between the Altun Shan and the Nan Shan suggests that the eastern Altyn Tagh fault has a total left-lateral offset of ∼375 km.

Supra-subduction zone ophiolites: The search for modern analogues

Abstract: The development of ideas on modern analogues for ophiolite complexes can be divided into three parts. In the first (1963 to 1972), ophiolites were mainly viewed as on-land analogues of the crust and upper mantle formed at mid-ocean ridges. The second (from 1972 to 1984) is a period of paradigm shift to the view that the majority of ophiolites forms above subduction zones, and can be grouped as the class known as supra-subduction zone ophiolites. The third (from 1984 onward) is a period that has led to a much better understanding of what these supra-subduction zone settings represent. The realization that many ophiolites were linked to subduction resulted from a combination of advances in geochemical discrimination methods, exploration through dredging and drilling of arc-basin systems of the Western Pacific and South Atlantic, and further field studies in ophiolite terranes. The transition in ideas from a mid-ocean ridge to a subduction origin was prolonged and much-debated, primarily because many of the best-developed ophiolites had the geochemical characteristics of subduction environments, yet none of the geological characteristics (arc volcanics and overlying volcanogenic sediments) thought to characterize those environments. This in turn led to a need for a non-generic term, supra-subduction zone, to describe this ophiolite type. Since the supra-subduction zone concept became established, precise analogues continued to be debated. Subduction initiation and ridge-trench intersections are probably the most important settings, with back-arc rifting and spreading and oblique subduction also significant.

Tectonic Assembly of the Northern Andean Block

Abstract: Based primarily on geologic field observations as recorded by numerous geoscientists over the last three decades, backed by more recent geochemical, seismic, gravity, magnetic, tomographic, and satellite-based techniques, an integrated synthesis and interpretation of the tectonic assembly of the entire Northern Andean Block (the Andes of Ecuador, Colombia, and Venezuela) is presented. Tectonic reconstruction is based on the identification and characterization of more than 30 distinct lithotectonic and morphostructural units (including terranes, terrane assemblages, physiographic domains, etc.) and their bounding suture and fault systems, which, based on geologic, geophysical, and dynamo-tectonic considerations, define four distinct tectonic realms representing the entire Northern Andean region. These include the Guiana Shield Realm (GSR), the Maracaibo subplate Realm (MSP), the Central Continental subplate Realm (CCSP), and the Western Tectonic Realm (WTR). The GSR provided the backstop for the progressive, accretionary continental growth of northwestern South America in the middle–late Proterozoic, in the middle Paleozoic, and finally during the Mesozoic-Cenozoic Northern Andean orogeny. Middle Cretaceous through Miocene time slices illustrate how, beginning in the Aptian, the sequential dextral-oblique accretion of the allochthonous oceanic WTR along the Pacific margin acted simultaneously with the northwest migration of the MSP (a detached segment of the Guiana Shield) into and over the Caribbean plate, exerting enormous transpression upon the CCSP trapped between them. Each tectonic realm contributed distinct tectonic mechanisms during Northern Andean cause and response orogenesis, and each realm records a unique internal deformational style, which in large part provides the basis for realm definition. Additionally, based on lithologic, geochemical, and paleomagnetic data and paleogeographic reconstructions, the intimate and complementary Mesozoic-Cenozoic history of the Northern Andean Block and the Caribbean plate are recognized. The migratory path of the Caribbean plate along the western and northern margin of the South American craton, as recorded by the accretionary history of the allochthonous WTR, has been instrumental in the modern-day configuration of the Northern Andean Block. Throughout this paper, the importance and contribution of underlying Proterozoic through middle Mesozoic geostructural elements in the development of Mesozoic-Cenozoic Northern Andean orogeny-phase tectonic configuration (structural style, uplift mechanisms, basin development, magmatism, etc.) are stressed. Additionally, the complex reality of Northern Andean Block assembly is contrasted with classical Central Andean Cordilleran-type orogenic models, and numerous differences are illustrated that render the application of typical Cordilleran-type models unacceptable. These differences are exemplified by the highly oblique collision/accretion/subduction tectonics of allochthonous oceanic terranes in the WTR, the detachment, migration and plis de fond–style of deformation in the MSP and the unique, transpressive pop-up of the Eastern Cordillera in the CCSP, all of which have no geologic analog in the Central Andes.

The Himalayan Yulong porphyry copper belt : Product of large-scale strike-slip faulting in eastern Tibet

Abstract: The Yulong porphyry copper belt is the most significant porphyry copper belt in Tibet and is located in the Qiangtang terrane of the Himalayan-Tibetan orogen The terrane is a collage of continental blocks joined by ophiolitic sutures and volcano-plutonic arc complexes The Yulong belt is approximately 300 km long and 15 to 30 km wide, contains one giant, two large, and two medium- to small-sized porphyry copper deposits, and more than 20 mineralized porphyry bodies The Yulong belt is located in the Changdu continental block that comprises Proterozoic to early Paleozoic crystalline folded basement and middle to late Paleozoic platform facies carbonate and clastic sedimentary rocks similar to the Yangtze continent The porphyry belt is closely associated with Tertiary potassic volcanic rocks and alkali-rich intrusions in the area and controlled by north-south-north-northwest, large-scale, strike-slip faults, which are perpendicular to the collision zone between the Indian and Asian continents Isotopic age determinations of the ore-bearing porphyries indicate that the magmatism occurred over at least three stages, peaking around 52, 41, and 33 Ma, respectively The timing of middle and late shallow-level emplacement of these magmas is consistent with the ages of associated potassic volcanism and alkali-rich magmatism in the area Although the porphyry deposits in the Yulong belt were developed in the intracontinental convergent environment, their mineralization styles and features are comparable to porphyry copper deposits in arc environments Compared to ore-bearing calc-alkaline porphyries in island arcs or continental margin arcs, the porphyritic intrusions in the Yulong belt are characterized by high K2O contents and enrichment in Rb and Ba, suggesting a shoshonitic magmatic affinity Strong negative anomalies for Nb, Ta, P, and Ti and positive anomalies for Rb, Ba, Th, and LREE, normalized by chondrite, are characteristic of arc magmas These intrusions yield a narrow 143Nd/144Nd range varying from 051243 to 051253 and 87Sr/86Sr values from 07065 to 07077, which are transitional between type II enriched mantle and mid-ocean ridge basalt (MORB) values and closer to the former in terms of epsilon Nd- epsilon Sr This suggests that the porphyritic magmas were derived either from a hydrous-enriched mantle metasomatized by components such as H2O, K, Rb, Ba, Th, and LREE or by melt derived from the subducted oceanic slab of the Paleozoic Jinshajiang oceanic plate The hypothesis is supported by Pb isotope data for the intrusions Large-scale strike-slip faults in eastern Tibet, which accommodated the compressive strains produced by the Asian-Indian continent collision, also localized the porphyry Cu mineralization North to north-northeast-directed convergence and collision produced a dextral strike-slip fault system around 60 to 70 Ma Northeast-directed wedging of the Indian continent and subsequent collision with the Yangtze continent during the Paleocene- Eocene produced conjugate strike-slip fault zones The transition from a dextral strike-slip fault system to conjugate strike-slip zones resulted in stress relaxation and formation of strike-slip pull-apart basins Crustalscale strike-slip faulting may have caused upwelling and partial melting of the hydrous-enriched mantle by decompression and facilitated the rise of a large volume of volatile-enriched porphyry magma that had ponded near the base of the lithosphere during this period

Crustal structure of the transpressional Variscan orogen of SW Iberia: SW Iberia deep seismic reflection profile (IBERSEIS)

Abstract: [1] IBERSEIS, a 303 km long (20 s) deep seismic reflection profile, was acquired across the Variscan belt in SW Iberian Peninsula. The acquisition parameters were designed to obtain a high-resolution crustal-scale image of this orogen. The seismic profile samples three major tectonic terranes: the South Portuguese Zone, the Ossa-Morena Zone, and the Central Iberian Zone, which were accreted in Late Paleozoic times. These terranes show a distinctive seismic signature, as do the sutures separating them. Late strike-slip movements through crustal wedges are apparent in the seismic image and have strongly modified the geometry of sutures. The upper crust appears to be decoupled from the lower crust all along the seismic line, but some deformation has been accommodated at deeper levels. A sill-like structure is imaged in the middle crust as a 1–2 s thick and 175 km long high-amplitude conspicuous reflective band. It is interpreted as a great intrusion of mafic magma in a midcrustal decollement. Taking into account surface geological data and the revealed crustal architecture, a tectonic evolution is proposed for SW Iberia which includes transpressional collision interacting during Early Carboniferous with a mantle plume. The Moho can be identified along the entire transect as subhorizontal and located at 10 to 11 s, indicating a 30–35 km average crustal thickness. Its seismic signature changes laterally, being very reflective beneath the South Portuguese Zone and the Central Iberian Zone, but discontinuous and diffuse below the Ossa Morena Zone.

Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the India‐Eurasia collision in Myanmar

Abstract: Situated south of the eastern Himalayan syntaxis at the western margin of the Shan-Thai terrane the highgrade Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia The first sensitive high-resolution ion microprobe U-Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (~170 Ma) zircons that have partly recrystallized during ~43 Ma high-grade metamorphism A hornblende syenite from Mandalay Hill also contains Jurassic zircons with evidence of Eocene metamorphic recrystallization rimmed by thin zones of 309 plus or minus 07 Ma magmatic zircon The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time Mid- Cretaceous to earliest Eocene (120 to 50 Ma) I-type granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra Metamorphic overgrowths to zircons in the orthogneiss near Mandalay date a period of Eocene (~43 Ma) high-grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (at 65 to 55 Ma) This was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 Ma Similar syntectonic syenites and leucogranites intruded the Ailao Shan-Red River shear belt in southern China and Vietnam during the Eocene-Oligocene to Miocene, and the Wang Chao and Three Pagodas faults in northern Thailand (that most likely link with the MMB) were also active at this time The complex history of Eocene to early Miocene metamorphism, deformation, and magmatism in the MMB provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of India and resulting extrusion or rotation of Indochina

Metamorphic evolution of the coesite-bearing ultrahigh-pressure terrane in the North Qaidam, Northern Tibet, NW China

Abstract: Widespread evidence for ultrahigh-pressure (UHP) metamorphism is reported in the Dulan eclogite-bearing terrane, the North Qaidam–Altun HP–UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K-feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO2 lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites are P = 2.9–3.2 GPa, and T = 631–687 °C; the eclogite shows a near-isothermal decompression P–T path suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites: (1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn at P = 2.9–3.3 GPa and T = 729–746 °C; (2) a high-pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24–29) + Scp at P = 1.9–2.0 GPa, T = 873–948 °C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep/Czo at P = 0.7–0.9 GPa and T = 660–695 °C. The clockwise P–T path of the SDB eclogites is different from the near-isothermal decompression P–T path from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub-belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north-western China.

Life and death of the Resurrection plate: Evidence for its existence and subduction in the northeastern Pacific in Paleocene–Eocene time

Abstract: Onshore evidence suggests that a plate is missing from published reconstructions of the northeastern Pacific Ocean in Paleocene– Eocene time. The Resurrection plate, named for the Resurrection Peninsula ophiolite near Seward, Alaska, was located east of the Kula plate and north of the Farallon plate. We interpret coeval near-trench magmatism in southern Alaska and the Cascadia margin as evidence for two slab windows associated with trench-ridge-trench (TRT) triple junctions, which formed the western and southern boundaries of the Resurrection plate. In Alaska, the Sanak-Baranof belt of near-trench intrusions records a west-to-east migration, from 61 to 50 Ma, of the northern TRT triple junction along a 2100-km-long section of coastline. In Oregon, Washington, and southern Vancouver Island, voluminous basaltic volcanism of the Siletz River Volcanics, Crescent Formation, and Metchosin Volcanics occurred between ca. 66 and 48 Ma. Lack of a clear age progression of magmatism along the Cascadia margin suggests that this southern triple junction did not migrate significantly. Synchronous near-trench magmatism from southeastern Alaska to Puget Sound at ca. 50 Ma documents the middle Eocene subduction of a spreading center, the crest of which was subparallel to the margin. We interpret this ca. 50 Ma event as recording the subduction-zone consumption of the last of the Resurrection plate. The existence and subsequent subduction of the Resurrection plate explains (1) northward terrane transport along the southeastern Alaska–British Columbia margin between 70 and 50 Ma, synchronous with an eastward-migrating triple junction in southern Alaska; (2) rapid uplift and voluminous magmatism in the Coast Mountains of British Columbia prior to 50 Ma related to subduction of buoyant, young oceanic crust of the Resurrection plate; (3) cessation of Coast Mountains magmatism at ca. 50 Ma due to cessation of subduction, (4) primitive mafic magmatism in the Coast Mountains and Cascade Range just after 50 Ma, related to slab-window magmatism, (5) birth of the Queen Charlotte transform margin at ca. 50 Ma, (6) extensional exhumation of high-grade metamorphic terranes and development of core complexes in British Columbia, Idaho, and Washington, and extensional collapse of the Cordilleran foreland fold-and-thrust belt in Alberta, Montana, and Idaho after 50 Ma related to initiation of the transform margin, (7) enigmatic 53–45 Ma magmatism associated with extension from Montana to the Yukon Territory as related to slab breakup and the formation of a slab window, (8) right-lateral margin-parallel strike-slip faulting in southern and western Alaska during Late Cretaceous and Paleocene time, which cannot be explained by Farallon convergence vectors, and (9) simultaneous changes in Pacific-Farallon and Pacific-Kula plate motions concurrent with demise of the Kula-Resurrection Ridge.

Discovery of metamorphic diamonds in central China: an indication of a > 4000‐km‐long zone of deep subduction resulting from multiple continental collisions

Abstract: The Central Orogenic Belt (COB) of China is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt. Here we report discovery of microdiamonds from both eclogites and felsic gneisses in the North Qinling zone in the central portion of the COB. This discovery demonstrates that the country rocks of continental affinity shared in the ultra-high-pressure metamorphic (UHPM) event and provides a bridge connecting the two previously recognized UHPM terranes, thereby establishing the existence of a UHPM belt extending more than 4000 km. Geochronological dating yields Early Palaeozoic ages in the west and Early Mesozoic ages in the east, recording two separate continental collisions overprinted within the COB. Occurrence of UHP metamorphism during recurrent continental collision here and in the Alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional, with significant implications for processes of plate tectonic reorganization and mantle mixing over time.

Geochronological and geochemical constraints on Mesozoic suturing in east central Tibet

Abstract: [1] This paper reports isotopic, major and minor element geochemistry of igneous and metamorphic rocks from the Kokoxili and Yushu regions of central and eastern Tibet. The first region lies along the Kunlun suture, which separates the Bayan Har-Songpan Ganze (Songpan) terrane from the Tarim and Qaidam blocks. Two Kokoxili granitoids yield U-Pb zircon dates of 217 ± 10 and 207 ± 3 Ma (Late Triassic), which represent the time of emplacement, and Rb-Sr isochron dates of 195 ± 3 and 190 ± 3 Ma (Early Jurassic), which are interpreted as cooling ages. The geochemical signatures of these granitoids suggest that they are related to subduction continuing into the Late Triassic. In the Yushu area, three samples help constrain the age of the Jinsha suture, which separates the Songpan terranes from the Qiangtang blocks. A leucocratic granite and an orthogneiss in the suture zone yield U-Pb zircon dates of 206 ± 7 and 204 ± 1 Ma, respectively, and a paragneiss south of it, a U-Pb monazite date of 244 ± 4 Ma. The existence of coeval magmatism in both the Jinsha and Kunlun sutures suggests that the two subduction zones were simultaneously active. Combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions, we propose that the Kunlun and Qinling block boundaries, which were distinct in the Permian, subsequently formed a continuous, Late Triassic, northward subducting plate margin. Our data suggest that the Jinsha suture correlates with the Benzilan and Nan-Uttaradit sutures, which together belong to a major Late Triassic subduction zone.

Combined single-grain (U-Th)/He and U/Pb dating of detrital zircons from the Navajo Sandstone, Utah

Abstract: Radioisotopic dating of detrital minerals in sedimentary rocks can constrain sediment sources (provenance), elucidate episodes and rates of ancient orogenesis, and give information on paleogeography and sediment-dispersal patterns. Previous approaches have been restricted to the application of a single technique, such as U/Pb or fission-track dating, to detrital grains. These methods provide crystallization and cooling ages, respectively, of sediment sources (terranes). However, evidence for source regions from a single technique can be ambiguous because candidate source terranes often have similar ages for a given radioisotopic system. This ambiguity can be avoided by applying multiple radioisotopic systems to individual detrital grains. Here we present a method for measuring both (U-Th)/He and U/Pb ages of single crystals of detrital zircon, providing both formation and cooling ages (through ∼180 °C). We applied this technique to zircons from the Lower Jurassic Navajo Sandstone, which represents one of the largest erg deposits in the geologic record. A large fraction of these zircons was derived from crust that formed between 1200 and 950 Ma, but cooled below ∼180 °C ca. 500–250 Ma. This history is characteristic of Grenvillian-age crust involved in Appalachian orogenesis (and subsequent rifting) in eastern North America. Our finding requires the existence of a transcontinental sediment-dispersal system capable of moving a large volume of detritus westward (modern coordinates) throughout the late Paleozoic and early Mesozoic.

U-Pb zircon ages for a collision-related K-rich complex at Shidao in the Sulu ultrahigh pressure terrane, China.

Abstract: The Shidao complex at the eastern extremity of the Sulu ultrahigh pressure (UHP) terrane is composed of oldest pyroxene syenite, quartz syenite and youngest granite intrusives of the K-rich shoshonitic series. U-Pb zircon dating yields nearly concordant ages of 225 ± 2, 211 ± 3 and 205 ± 5 Ma for pyroxene syenite, quartz syenite and granite, respectively. The ages closely postdate the 240 to 220 Ma UHP metamorphism and correspond to the rapid cooling and exhumation of UHP rocks. A close genetic relation may exist between the formation of the Shidao intrusives and the continental collision and UHP metamorphism. However, the K-rich Shidao intrusive rocks are different from common syn-collisional granites in association of K-rich and granitic magmatism. A breakoff model is postulated to explain the formation of the complex. The breakoff of the subducting slab caused the rapid exhumation of the UHP terrane. Mantle upwelling resulted in basaltic magmatism and formation of the K-rich complex. Formation of the Shidao complex marked the cessation of the UHP metamorphism and the oldest ages of 225 ± 2 Ma of the complex is the minimum timing for the UHP metamorphism.