Continental Margin Subduction Research Articles

Devonian magmatism in the accretionary complex of southern Chile

Supposed or potential Devonian igneous rocks in the accretionary complex of southern Chile were investigated using sensitive high-resolution ion microprobe U–Pb dating of zircon, with Hf- and O-isotope analyses of selected grains. Ages of 384 ± 3 and 382 ± 2 Ma are confirmed for two igneous bodies (another having been previously dated at 397 ± 1 Ma). Detrital zircon ages in the host rocks, some associated with Devonian marine fossils, indicate maximum possible sedimentation ages of c . 330 – 385 Ma. Devonian ages of 391 ± 10 and 374 ± 3 Ma for plutonic rocks at the western edge of the North Patagonian Massif are somewhat older than those of orthogneisses in the western flank of the Andes near Chaitén (361 ± 7 and 364 ± 2 Ma). O and Hf isotopes indicate that the Devonian intrusions in the accretionary complex crystallized from mantle-derived magmas, whereas those in the North Patagonian Massif show a strong crustal influence, corresponding to oceanic and continental margin subduction environments of magma genesis, respectively. Devonian zircon provenance in the accretionary complex was from the North Patagonian Massif and not from the mantle-derived intrusions, suggesting that the accretionary complex formed an integral part of the Gondwana margin during Devonian–Carboniferous times. Supplementary material: Description of analytical methods and tables of isotope analytical data are available at http://doi.org/10.6084/m9.figshare.c.2336728 .

Formation mechanism of the global Dupal isotope anomaly

AbstractThe Dupal anomaly has been a frequently discussed feature since it was first proposed three decades ago. We here limit the distribution of the Dupal anomaly based on more than 10 000 Sr–Nd–Pb isotopic composition analyses and classify the anomaly into three types, located in three different oceans and with different mechanisms of formation. The Dupal anomaly in the East Asian Continental Margin subduction zone is related to enriched mantle II, which may originate from the recycling of the subducting plate or continental mantle. The high μ and enriched mantle I are the reason for the Dupal anomaly in the Pacific Ocean, which is related to the superplume. However, the source of the Dupal anomaly in the Indian Ocean and the Southern Atlantic Ocean is a mixture of enriched mantle I and enriched mantle II, and they come from the African superplume and the recycling of subcontinental mantle or continental crust of Gondwana. In consideration of all the above factors, we suggest that the superplume from the core–mantle boundary, the recycling subducting plate and continental mantle are the main characteristics of the global Dupal anomaly. Copyright © 2016 John Wiley & Sons, Ltd.

U–Pb Zircon Geochronology and Geochemistry of the Neoproterozoic Liujiaping Group Volcanics in the Northwest Margin of the Yangtze Block: Implications For the Breakup of the Rodinia Supercontinent

AbstractInvestigation of the petrogenesis and the origin of zircons from the volcanic rocks of the Liujiaping Group of the back‐Longmenshan tectonic belt in the northwest margin of the Yangtze Block is conducted by analysis of U–Pb geochronology and geochemistry. Results show that selected zircons are characterized by internal oscillatory zonings and high Th/U ratios (0.43–1.18), indicating an igneous origin. Geochronological results of LA–ICP–MS U–Pb dating of the Liujiaping Group zircons yield an age of 809 ± 11 Ma (MSWD = 2.2), implying that the volcanic rocks were formed in the Late Neoproterozoic. Geochemical analysis shows that the rocks are calc‐alkaline, supersaturated in Al, and metaluminous to weakly peraluminous. Rare‐earth elements are present at high concentrations (96.04–265.48 ppm) and show a rightward incline and a moderately negative Eu anomaly, similar to that of continental rift rhyolite. Trace element geochemistry is characterized by evident negative anomalies of Nb, Ta, P, Th, Ti, inter alia, and strong negative anomalies of K, Rb, Sr, et al. We conclude that the Liujiaping Group volcanic rocks resulted from typical continental crust source petrogenesis and were formed in a continental margin setting, which had no relation to subduction, and thus, were the products of partial melting of the lower crust due to crustal thickening caused by active continental margin subduction and arc‐continent collision orogeny in the northwestern Yangtze Block and were triggered by the breakup of the Rodinia supercontinent during the Neoproterozoic.

Paradigm lost: Buoyancy thwarted by the strength of the Western Gneiss Region (ultra)high-pressure terrane, Norway

Subduction and exhumation of ultrahigh-pressure (UHP) metamorphic terranes are typically envisaged as short-lived processes associated with the transition from oceanic subduction to continent-continent collision. Norway’s Western Gneiss Region, by comparison, is a giant, late-orogenic UHP terrane that underwent protracted residence at UHP conditions during the Scandian phase of the Caledonian orogeny followed by relatively slow exhumation. Here, we use two-dimensional numerical thermal-mechanical models to explore the tectonics of orogens of this type and the associated controls on the size of their UHP terranes and the duration of UHP metamorphism. The models have four tectonic phases designed to capture the main stages of the Caledonian evolution: oceanic subduction and microcontinent accretion; continental margin subduction; plate quiescence; and postorogenic extension during plate divergence. Contrasting styles of exhumation are explored by varying the strength of the margin crust and investigating melt-induced weakening. The tectonic and metamorphic evolution of the Western Gneiss Region is consistent with a model in which continental margin crust was subducted beneath a thick orogenic wedge where it underwent metamorphism at (U)HP conditions for at least 15 million years (Myr) as subduction ended. The buoyant Baltican crust must have been especially strong in order to have stayed coupled to the underlying lithosphere during this phase, perhaps reflecting its refractory composition and/or a lack of fluids. Subsequent exhumation of the Western Gneiss Region can be explained by orogen-scale extension resulting from minor (∼100 km) plate divergence, with removal of the orogenic wedge by combined top-to-the-hinterland transport, normal faulting, and erosion. We conclude that large, long-duration UHP terranes are fundamentally different from transient smaller ones. The latter are often explained by the paradigm of buoyant exhumation. This paradigm is incomplete, but both types can be explained by control of the system by the exhumation number (ratio of buoyancy force to basal traction). By implication, the existence of this type of large UHP terrane is a consequence of the high strength of the subducted crust.

Influences on the positioning of mantle plumes following supercontinent formation

AbstractSeveral mantle convection studies analyzing the effects of supercontinent formation and dispersal show that the genesis of subcontinental plumes results from the formation of subduction zones at the edges of the supercontinent rather than from the effect of continental thermal insulation or thermochemical piles. However, the influence of subduction zone location on the position of subcontinental plumes has received little attention. This study analyzes 2‐D and 3‐D numerical models of supercontinent formation (in an isochemical mantle) to assess the role of subduction and mantle viscosity contrast in the generation of subcontinental mantle plumes. We find that once a critical supercontinent width is reached, plumes do not form under the center of a supercontinent. In studies featuring a low viscosity lower mantle, the surface positions of the initial plumes (arriving within 90 Myr of supercontinent assembly) become locked beneath the continent at a distance 2000–3000 km from the continental margin. However, the broad downwellings in simulations that feature a high‐viscosity lower mantle trigger plumes at a greater distance from the continental margin subduction. For all mantle viscosity profiles, subcontinental plumes show dependence on the location of supercontinent margin subduction. As theories differ on the role of core‐mantle boundary chemical piles in plume formation, it is significant that our isochemical models show that the formation of subduction zones at the margins of a supercontinent has a profound effect on subcontinental mantle dynamics. Our results may help to explain what determined the eruption sites of past (and future) large igneous provinces.

The Lamandau IOCG deposit, southwestern Kalimantan Island, Indonesia: Evidence for its formation from geochronology, mineralogy, and petrogenesis of igneous host rocks

The Lamandau region of Kalimantan Island, Indonesia is located in Sandaland, in the southern part of the Kuching tectonic belt. A series of Cenozoic epithermal gold deposits and Fe–Cu–Au deposits are located in the Kuching belt. The Lamandau Fe–Cu–Au deposit is hosted by diorite porphyry. In-situ zircon U–Pb dating of the diorite porphyry shows that it formed between 82.1±1.7Ma and 78.7±2.3Ma. Geochemical data indicate a depletion of high field strength elements (HFSE) in the diorite porphyry and related basalt is similar to that of arc-related igneous rocks. The diorite porphyry and basalt were probably derived from typical arc magmas related to continental margin subduction and thus are characterized by light rare earth element (REE) enrichment and HFSE depletion. The sub-chondritic Nb/Ta ratios for the basalt in the Lamandau region indicate that the subducted Pacific slab began partial melting at depths where amphibole was the major residual phase, with some residual rutile. The basalt was derived from a depleted mantle source. The composition of apatite and zircon in the diorite porphyry indicates that the dioritic magma was produced from the subcontinental mantle after it was metasomatized by slab-derived fluids. The magma had a high oxygen fugacity, thus and therefore it was particularly conducive to the precipitation of Cu, Au and other ore-forming elements. The composition of magnetite indicates that it was of volcanic origin. The magnetite has a low REE content, and a high Cu–Au content. The deposit may be classified as an IOCG mineral system. In summary, the ore-related diorite porphyry in the Lamandau region might have formed in an extensional environment during rollback of the subducting western Pacific plate. The convergent velocity between the Philippine Sea and Eurasian plates was at a minimum during the rollback, so that the margin of East Asian began to undergo rifting with associated magmatism.

Petrogenesis and zircon U-Pb dating of skarnified pyroxene-bearing dioritic rocks in Bisheh area, south of Birjand, eastern Iran

در منطقه بیشه واقع در شرق بلوک لوت، هورنبلند دیوریت های پورفیری در سنگهای رسوبی و آذرین (توف‌ها و سنگهای دیوریتی پیروکسن‌دار) نفوذ کرده و به‌ترتیب تشکیل اگزواسکارن و اندواسکارن داده است. بخشهای تغییر نیافته سنگهای دیوریتی پیروکسن‌دار، دارای ماهیت کالک آلکالن پتاسیم بالا و متاآلومین (78/1 A/NK≥ و 95/0(A/CNK≤ هستند. نمودار چند عنصری به‌هنجار شده با گوشته اولیه نشان دهنده غنی شدگی از عناصر LIL مانند Rb، Ba، Cs و تهی شدگی از عناصر HFS نظیر Nb، P، Ti، Y و Yb است. نمودار عناصر نادر خاکی بیانگر مقادیر (La/Yb)N از 75/7 تا 63/8 و بی هنجاری منفی ضعیف Eu می باشد. این خصوصیات به همراه مقادیر بالای Th/Yb و Ta/Yb نشان‌دهنده ماگماتیسم وابسته به مناطق فرورانش حاشیه قاره هاست. تهی‌شدگی مشخص Nb و Ti ، میزان نسبتا بالای Mg#، نسبتهای اولیه 87Sr/86Sr ( 70606/0)، 143Nd/144Nd ( 512424/0) و Nd e (05/3-) نشان می دهد که ماگمای سازنده سنگهای مورد مطالعه از گوشته غنی شده منشأ گرفته و دچار آغشتگی پوسته ای شده است. مقادیر بالای Rb، Th، K و مقدار پایین P و Ti در سنگهای مورد بررسی مؤید آلودگی ماگما در پوسته فوقانی طی تکامل ماگما است. میزان Ba/Rb پایین (4/7-6/4) و Rb/Sr بالا (22/0-17/0)، نشاندهنده حضور فلوگوپیت در گوشته منشأ است. با توجه به میزان بالای CaO+ Al2O3 نمونه های مورد مطالعه (متوسط 3/22 درصد)، میتوان گارنت پیروکسنیت فلوگوپیتدار را به‌عنوان منشأ ماگما در نظر گرفت. سن‌سنجی به روش زیرکن-اورانیم-سرب بر روی پیروکسن دیوریت پورفیری نشان می‌دهد که این ماگماتیسم در 07/44 میلیون سال پیش یعنی در ائوسن میانی(Lutetian) رخ داده است.

Infrastructure and provenance of Ganderia: Evidence from detrital zircon ages in the Brookville terrane, southern New Brunswick, Canada

The Brookville terrane in southern New Brunswick contains some of the oldest known sedimentary rocks of the Ganderian microcontinent of the northern Appalachian orogen. It includes the stromatolitic metacarbonate- and quartzite-dominated Ashburn Formation and metasiltstone-dominated Martinon Formation, in mylonitic contact with the Brookville Gneiss, an assemblage of low-pressure/high-temperature paragneiss and tonalitic orthogneiss. All of the metasedimentary units were intruded by the ca. 550–528Ma subduction-related Golden Grove Plutonic Suite. Analysis by LA-ICP-MS and TIMS of zircon grains from the Ashburn Formation quartzite yielded no <10% discordant ages younger than ca. 1200Ma, a predominance of Meso- and Paleoproterozoic ages back to ca. 2100Ma, and a few Neoarchaean ages. A sample from the metasiltstone matrix of a carbonate olistostrome in the Martinon Formation contains rounded grains with ages mainly between ca. 1000Ma and 2200Ma, indicating provenance similar to that of the Ashburn Formation. However, the sample also contains euhedral grains with ages averaging around 650Ma, indicating the maximum depositional age. A quartzite/calc-silicate sample from the Brookville paragneiss yielded detrital zircon grains with most ages between 1150Ma and 2000Ma, indicating provenance similar to that of the Ashburn and Martinon formations. An orthogneiss sample yielded euhedral (igneous) zircon grains with a concordia age of 615±4Ma, indicating an interval of continental margin subduction at least 60Ma older than that represented by the Golden Grove Plutonic Suite. The Ashburn and Martinon formations and Brookville paragneiss are evidence for development of a Late Cryogenian passive margin on the Proto-Andean-Caribbean edge of Amazonia after it had separated from Laurentia by ca. 650Ma to open the Puncoviscana Ocean. By 615Ma at least part of this margin had become a subduction zone which remained active during the Late Neoproterozoic-Early Cambrian, part of the Pampean arc system. Brookville and related terranes rifted away in the mid-Cambrian to form the foundation of Early Paleozoic Ganderia, which ultimately collided with composite Laurentia during the Salinic orogeny.

Neoarchaean crustal growth by combined arc–plume action: evidence from the Kadiri Greenstone Belt, eastern Dharwar craton, India

Abstract Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites–rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66–0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites–rhyolites are silicic rocks (SiO 2 =61–72 wt%) with low Cr and Ni, K 2 O/Na 2 O mostly 0.5–1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U–Pb age of 2353±32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers. Supplementary material: Details of whole-rock major and trace element determination, Nd isotope analysis and zircon U–Pb dating and trace element analysis, the geographical coordinates of the samples and the values of the international rock standards analysed are available at http://www.geolsoc.org.uk/SUP18660

Paradigms, new and old, for ultrahigh-pressure tectonism

Regional ultrahigh-pressure (UHP) metamorphic terranes exhibit a spectrum of lithological, structural and petrological characteristics that result from the geodynamic processes that formed and exhumed them. At least six geodynamic processes can be envisioned to have carried continental rocks to mantle depths: i) continental margin subduction, ii) microcontinent subduction, iii) sediment subduction, iv) intracontinental subduction, v) subduction erosion, and vi) foundering of a crustal root. Most of these processes have been investigated through numerical or analog models and most have been invoked for one or more specific occurrences of UHP rocks. At least six geodynamic processes can be envisioned to have exhumed UHP continental rocks: i) eduction, ii) microplate rotation, iii) crustal stacking, iv) slab rollback, v) channel flow, and vi) trans-mantle diapirs. Most of these processes have also been investigated through numerical or analog models and all have been invoked to explain the exhumation of at least one UHP terrane. More-detailed and systematic field investigations are warranted to assess the predictions of numerical models, and more-sophisticated and realistic numerical models are required to replicate and explain the petrological, structural, and chronological data obtained from UHP terranes.

Precambrian geology of China

Precambrian geology of China

Age and tectonic setting of the Bavanat Cu–Zn–Ag Besshi-type volcanogenic massive sulfide deposit, southern Iran

The Bavanat Cu–Zn–Ag Besshi-type volcanogenic massive sulfide (VMS) deposit occurs within the Surian volcano-sedimentary complex in the Sanandaj–Sirjan zone (SSZ) of southern Iran. The Surian complex is comprised of pelite, sandstone, calcareous shale, basalt, gabbro sills, and thin-bedded limestone. Mineralization occurs as stratiform sheet-like and tabular orebodies hosted mainly by greenschist metamorphosed feldspathic and quartz feldspathic sandstone, basalt, and pelites. The basalts of the Surian complex show predominantly tholeiitic to transitional affinities, with a few samples that are alkalic in composition. Primitive mantle-normalized trace and rare earth element (REE) patterns of the Surian basalts display depletions in light REE, negative anomalies of Nb, Ta, and Ti, and positive anomalies of P. Positive P anomalies are indicative of minor crustal contamination. Furthermore, Th enrichments in the mid-ocean ridge basalt-normalized patterns of the Surian basalts are characteristic of rifted arc basalts emplaced in continental margin subduction zones. The high MgO content (>6 wt.%) of most Surian basalts and low TiO2 content of two samples (0.53 and 0.62 wt.%) are characteristic of boninites. The aforementioned features of the basalts indicate arc tholeiites emplaced in intra-arc rift environments and continental margin subduction zones. U–Pb dating by laser ablation- inductively coupled plasma mass spectrometry of detrital zircons extracted from the host feldspathic and quartz feldspathic sandstone yields various ages that are predominantly Permian and Triassic; however, the youngest zircons give a mean Early Jurassic concordant U–Pb age of 191 ± 12 Ma. This age, together with geological and petrochemical data, indicate that VMS mineralization formed in the Early Jurassic in pull-apart basins within the SSZ. These basins and the VMS mineralization may be temporally related to an intra-arc volcano–plutonic event associated with Neo-Tethyan oblique subduction.

High-temperature deformation during continental-margin subduction & exhumation: The ultrahigh-pressure Western Gneiss Region of Norway

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction.

Neoproterozoic Jiaoziding Peraluminous Granite in the Northwestern Margin of Yangtze Block: Zircon SHRIMP U-Pb Age and Geochemistry and Their Tectonic Significance

The granite of Jiaoziding dome core is located in back-Longmenshan area of the northwestern margin of Yangtze Block. The objectives of the study carried out by applying zircon SHRIMP U-Pb geochronology and geochemistry are to constrain its formation age and petrogenesis. The results show that the zircons of the samples in deformative granite and massive granite of the Jiaoziding intrusions have internal oscillatory zoning, zircons of the Jiaoziding intrusions have higher Th/U ratios (0.52–1.23, 0.32–1.16) and are igneous in origin. The results of zircon SHRIMP U-Pb dating for deformative granite and massive granite are 793 ± 11 Ma and 792 ± 11 Ma, which indicates that the granite intrusions formed in the Nanhuan of late Neoproterozoic, and formation of their time is fully consistent. The Jiaoziding intrusions are high in SiO 2 (72.01%–77.65%) and Al 2O 3 (12.12%–14.18%) composition, and A/CNK = 0.95–1.13 (1.04 on average). It is a typical peraluminous granite, with supersaturation in Al and Si, and identified to be S-type granite. The abundance of ∑REE varies in the range of (75.81–79.97) × 10 −6 (77.89 × 10 −6 on average). The rocks show a LREE enrichment pattern and obvious weak to intermediate negative Eu anomalies. The trace element geochemistry is characterized evidently by negative anomaly of Ta, Nb, Ti, etc. and positive anomaly of Rb, Ba, Sr, etc. The granite emplaced by underplating of granitic magma, which formed through partial melting of sedimentary mainly of arenite, and is typical crust source petrogenesis. The Jiaoziding intrusions show postcollisional granite characteristics and regarded as postorogenic granites. Thus, the granite intrusions are interpreted as syncollisional granites resulted from the crustal thickening caused by active continental margin subduction and arc-continent collision orogeny during the Neoproterozoic in the northwestern margin of Yangtze Block. The granitic intrusion formed in a transitional environment from syncollision (compressional environment) to postcollision (extensional environment) and is the products of Rodinia supercontinent initial break-up.

Melange genesis and ophiolite emplacement related to subduction of the northern margin of the Tauride–Anatolide continent, central and western Turkey

Abstract The Tauride–Anatolide continent, stretching for c . 900 km across western and central Turkey, is one of the world's best example of a subducted, exhumed passive margin within a collisional orogen. Twelve widely separated areas were studied and correlated to develop a new plate-tectonic model. A metamorphosed, rifted continental margin of Triassic–Lower Cretaceous age (Tauride–Anatolide platform) is overlain by Upper Cretaceous (Cenomanian-Lower Maastrichtian) pelagic sediments and then by both tectonic melange (subduction complexes) and sedimentary melange (foredeep gravity complexes). The melanges are overthrust by unmetamorphosed ophiolitic rocks, commonly peridotites with swarms of diabase/gabbro dykes, and are underlain by metamorphic soles. New geochemical evidence from basaltic blocks in the melange indicates predominantly subduction influenced, within-plate and mid-ocean ridge-type settings. The dykes cutting the ophiolites were probably intruded during early-stage intra-oceanic arc genesis. The metamorphosed continental margin, melanges and ophiolites in the north (Anatolides) are correlated with unmetamorphosed equivalents in the Taurides further south (e.g. Beyşehir and Lycian nappes). Oceanic crust of Triassic–Late Cretaceous age formed between the Gondwana-related Tauride–Anatolide continent in the south and the Eurasia-related Sakarya microcontinent in the north. Following Late Triassic–Early Cretaceous passive margin subsidence, the continental margin was covered by Cenomanian-Turonian pelagic carbonates ( c . 98–90 Ma). Ophiolites formed in an intra-oceanic subduction zone setting in response to northward subduction, probably within a two-stranded ocean, with the Inner Tauride ocean in the SE and the İzmir–Ankara–Erzincan ocean in the north/NW. Metamorphic soles relate to intra-oceanic subduction ( c . 95–90 Ma). Oceanic sedimentary/igneous rocks accreted to the advancing supra-subduction oceanic slab. The Tauride–Anatolide continental margin then underwent diachronous collision with the trench ( c . 85 Ma), deeply subducted and metamorphosed at HP/LT ( c . 80 Ma). Accretionary, ophiolitic and exhumed HP/LT rocks were gravity reworked into a southward-migrating flexural foredeep and progressively overridden ( c . 70–63 Ma). Slices of the upper part of the platform and its margin detached and were thrust southwards as the (Tauride) Lycian and Beyşehir nappes, together with regional-scale ophiolites. The continental margin and melange were simultaneously exhumed during Maastrichtian–Early Paleocene (70–63 Ma) and transgressed by shallow-water sediments, beginning in the Late Maastrichtian in the east ( c . 64 Ma) and the Mid?-Late Paleocene ( c . 60 Ma) further west. Remnant oceanic crust was consumed during Early Cenozoic time, followed by Mid Eocene (45–40 Ma) diachronous continental collision and a second phase of regional deformation. Rather than being progressive there were two stages of collision: first, Upper Cretaceous ophiolite emplacement driven by continental margin-subduction trench collision, and secondly Eocene collision of the Tauride and Sakarya/Eurasian continents.

Age and petrogenesis of volcanic and intrusive rocks in the Sulphur Spring Range, central Nevada: Comparisons with ore-associated Eocene magma systems in the Great Basin

Widespread base- and precious-metal anomalies, oxidized sulfi de veins, silicifi ed calcareous shales and carbonates, and altered porphyry intrusions occur in the northeastern Sulphur Spring Range, Nevada, 80 km south of important gold deposits in the Carlin trend. The small historic mines and prospects in the area are spatially and perhaps genetically related to a suite of variably altered dikes, small lava fl ows, silicic domes, and related pyroclastic rocks. New major- and trace-element data and U-Pb zircon ages show that the East Sulphur Spring volcanic suite is Eo-Oligocene in age (36‐31 Ma) and ranges in composition from high MgO- basaltic andesite to peraluminous rhyolite. The major- and trace-element compositions of the volcanic rocks are characteristic of continental margin subduction zone magmas and form a high-K, calc-alkaline suite with low Fe/Mg ratios. In addition, the rocks have negative Nb and Ti anomalies and elevated Ba, K, and Pb on normalized trace-element diagrams. Crustal melting is indicated by the eruption of a peraluminous garnet-bearing ignimbrite and as a component in hybridized andesite. The nature of this suite and its potential for mineralization is elucidated via comparisons to other Eocene age volcanic rocks associated with much larger gold and copper deposits in the Great Basin. The East Sulphur Spring suite is more similar to Eocene igneous rocks found along and near the Carlin trend than it is to those erupted while the Bingham porphyry copper deposit developed 300 km farther to the east. For example, the East Sulphur Spring suite and the Eocene magmatic rocks along the Carlin trend are less alkaline than the Bingham suite and lack its unusual enrichment of Cr, Ni, and Ba in intermediate composition rocks (58‐68 wt% silica). Nonetheless, the Bingham and East Sulphur Spring volcanic suites both preserve evidence of mixing that created intermediate compositions. For example, an andesite has obvious mineral disequilibrium with plagioclase, biotite, clinopyroxene, orthopyroxene, olivine, and amphibole coexisting with extensively resorbed megacrysts of quartz, K-feldspar, and garnet—indicative of mixing basaltic andesite or andesite and largely crystallized garnet-bearing rhyolite. On the other hand, we found no evidence for mixing with a mafi c alkaline magma like that in the Bingham Canyon magma-ore system. We conclude that: (1) an unusual tectonic setting prevailed during the Eocene and Oligocene of the western United States that promoted the production of oxidized mafi c magma in an arclike setting, but far inland as a result of the rollback of the Farallon slab; (2) the mafi c magmas intruded or erupted separately, or mixed with more silicic magma generated by fractional crystallization and assimilation of crustal materials; and (3) these mafi c magmas may have delivered signifi cant amounts of sulfur and chalcophile metals to upper crustal magma chambers and eventually to Paleogene ore deposits in the eastern Great Basin.

1.83—1.82 Ga formation of a juvenile volcanic arc—implications from U—Pb and Sm—Nd analyses of the Oskarshamn-Jönköping Belt, southeastern Sweden

The Oskarshamn-Jönköping Belt in southeastern Sweden is a geographically well-defined area comprising calc-alkaline intrusions and volcanic rocks together with units of coarse-grained clastic metasedimentary rocks. Deviating from the general composition is the Fröderyd Group with basalts of MORB character. The belt is surrounded by the 1.81-1.77 Ga Transscandinavian Igneous Belt. A conglomerate clast from the central part of the Oskarshamn-Jönköping Belt yielded a U—Pb zircon age of 1829±8. This age confirms the c. 1.83-1.82 Ga formation age of the belt. Sm—Nd whole rock analyses of various rock types throughout the Oskarshamn-Jönköping Belt show that no substantially older (>100 m.y.) continental material contributed to these rocks. Furthermore, the high positive εNd values of many of the analysed rocks points to the depleted mantle as the main component of their source. The Sm—Nd analyses also suggest that while some of the felsic units were formed by a high degree of magmatic fractionation from a mantle derived melt others were formed as the result of remobilisation of older Svecofennian crust. The new U—Pb data together with previously published ages imply that the formation of the Oskarshamn-Jönköping Belt was a rather quick process, possibly completed in c. 10 m.y. The most likely model of genesis for the area, based on the new U—Pb and Sm—Nd data, is formation at a continental margin subduction zone to the present south-west of a slightly older Svecofennian continent. In such a model the Fröderyd Group represents either a fore-arc setting or a back-arc rift.

Arc–continent collision and subduction reversal in the Cenozoic evolution of the Northwest Pacific: an example from Kamchatka (NE Russia)

The principal new feature in the Cenozoic evolution of the Northwest Pacific is based on the fact that two arc terranes of Upper Cretaceous–Paleogene age were incorporated in the Kamchatka orogen during the early Tertiary and at the end of the Miocene, respectively. These two arc terranes, so-called Achaivayam–Valaginskaya (Campanian–Lower Paleocene) and Kronotskaya (Coniacian–Eocene) arcs, are separated by a suture zone that contains deformed slices of young (Paleocene–Lower Eocene) oceanic crust. Paleomagnetic data on the original paleolatitudes of the arc positions as well as geochemical data characterized the arcs as intraoceanic, originated in the Northwest Pacific. The arcs were separated by the Vetlovka oceanic plate, suggested to be a trapped fragment of the Kula plate, in which a new spreading zone developed in the early Tertiary. The principal event in the reconstructed evolution is the Eocene collision of the extended Achaivayam–Valaginskaya arc terrane with the continental margin of Asia. The margin is interpreted as the southeastern (facing the Pacific) Cretaceous accretionary boundary of the Okhotsk microplate docked against the Asian continent at ∼55–65Ma. The arc–continent collision developed progressively northward from the end of the Paleocene–Early Eocene in Southern Kamchatka to the Middle Eocene in the Olutorka region. The different stages of the collision occurred simultaneously along strike of the Achaivayam–Valaginskaya arc. According to the geodynamic model based on geological data for Southern Kamchatka, collision included: (1) the entrance of the continental margin in the subduction zone, deformation of the overriding plate and subduction of fore-arc block; (2) arc obduction on the continental margin, blocking of continental margin subduction; and (3) subduction reversal with onset of oceanic subduction at the eastern side of the deformed arc. This subduction reversal produced deformation and reverse faulting along the eastern side of the arc, thus allowing accretion of tectonic slices of young oceanic crust of the Vetlovka plate, which began to subduct. During the subduction reversal, the subducted plate dipping to the southeast was broken off and detached at depth under the arc area. The subduction polarity reversal, the most characteristic feature of the collision model, is strongly supported by structural and geologic data of Southern Kamchatka and explains the main tectonic, metamorphic and magmatic events related to and postdating the collision.

Numerical modelling of arc–continent collision: application to Taiwan

Two-dimensional finite element numerical modelling is applied to study the deformation and failure of the overriding plate during arc–continent collision (continental margin subduction). This plate has elasto-plastic rheology with strain weakening and contains a ‘volcanic arc’ with thinned and weakened lithosphere. The overriding plate deforms due to the normal and tangential stresses applied along the interplate surface. These boundary conditions represent the friction and pressure between the plates. The latter is due to the flexural rigidity of the subducting lithosphere and the buoyancy force generated by the subducting continental crust. The modelling shows that continental margin subduction results in increasing compression and failure of the overriding plate, which occurs along the surface dipping under the arc in either of two possible directions. The failure mode is largely controlled by the two competitive factors: the rigidity of the subducting plate and thickness gradient of the subducted continental crust. A high rigidity favors failure along an ocean-vergent fault, which is followed by a subduction reversal, while a high thickness gradient favors failure in the opposite direction, which is followed by a fore-arc block underthrusting beneath the arc. Both scenarios seem to have natural analogs. We consider one of them, the ongoing arc–continent collision in Taiwan, and argue that this process occurs according to the second scenario corresponding to the fore arc underthrusting. Wavelet statistical analysis of the seismicity distribution to the south of Taiwan has clearly displayed a shallow (0–40 km) zone of high density, coherently distributed seismicity beneath the Luzon Arc. This zone, interpreted as a lithospheric-scale fault, dips from the forearc basin to the east and corresponds to the initiation of the forearc block subduction. A self-consistent, combined mechanical–gravity–topography model is used to see whether failure of the overriding plate to the south of Taiwan can be ‘captured’ by this model. By ‘tuning’ different controlling parameters, we did not succeed in obtaining realistic topography and gravity field in a model where failure of the overriding plate was not allowed. Introduction of this failure and underthrusting of the forearc block under the Luzon Arc allowed us to fit both topography and gravity data.

Plumes, orogenesis, and supercontinental fragmentation

A time–space relationship between large igneous provinces (LIPS), present day hot spots, and the fragmentation of Pangea has been documented over several decades, but the cause of fragmentation has remained elusive. LIPS are regarded either as the result of impingement of a mantle plume on the base of the lithosphere, or as the initial products of adiabatic decompression melting of anomalously hot mantle. Do LIPS therefore constitute evidence of an active role for plumes from the deep mantle in supercontinental fragmentation, or are they merely the first indications of a large-scale but near-surface tectonic process? Two long recognized and enigmatic orogenic events may offer a solution to this geologically important ‘chicken or egg’ conundrum. The reconstructed early Mesozoic Gondwanide fold belt of South America, southern Africa, and Antarctica, could have resulted from ‘plume-modified orogeny’, flattening of a downgoing lithospheric slab due to the buoyancy of a plume rising beneath a continental margin subduction zone. If so, the ∼180 Ma Karroo and Ferrar LIPS associated with the opening of the ocean basin between East and West Gondwanaland at ∼165 Ma resulted from impingement of this plume and are unrelated to the thermal insulation of the shallow mantle beneath Gondwanaland. It would then follow that the plume itself played an active, possibly critical, role in the initial breakup of the supercontinent. The Late Paleozoic ‘Ancestral Rockies’ deformation in the southwestern United States could be yet another example of orogeny driven by a plume that initiated the break-up of Pangea approximately 15 Myr earlier in the Central Atlantic region.