Accretion Of Island Arcs Research Articles

Upstream controls on incised-valley dimensions and fill: Examples from the Upper Mississippian Mauch Chunk Group, Central Appalachian Basin, USA

Subsurface mapping and facies and architectural element analysis on outcrop, combined with published detrital zircon geochronology and sandstone petrography, are used to reconstruct a source to sink system for two incised-valley fills in the Upper Mississippian Mauch Chunk Group. The lower Stony Gap incised valley is relatively broad and filled with quartzarenite fluvial facies consisting of downstream accreting elements with minimum thicknesses of 6m; cross-sets within elements average 62.5cm thick. The lower Princeton incised valley is relatively narrow and filled with sublitharenite fluvial facies consisting of channel elements that range in thickness from 0.25 to 3.6m; cross-sets within elements average 35cm thick.Quantitative estimates of drainage areas, using regional hydraulic geometry curves for comparable climatic settings, are consistent with order-of-magnitude differences for the lower Stony Gap Sandstone (>700,000km2) and the lower Princeton Formation (<100,000km2). Statistical analysis of the detrital zircon age data reveal that the Stony Gap and Princeton sample populations are not the same and were derived, at least in part, from provenances of different age. Drainage areas are consistent with detrital zircon geochronology and petrographic data and suggest that the lower Stony Gap fluvial sandstone was derived from distal, northern and northwestern cratonic sources including, uniquely, the Archean Superior Province and Proterozoic Penokean-Yavapai-Mazatal provinces, possibly through recycling from older sedimentary rocks. Sediment was dispersed into a NE-SW-oriented, axial drainage system. Inferred channel depths and drainage areas of the lower Stony Gap Sandstone are comparable to the modern Brahmaputra River. In contrast, the younger lower Princeton fluvial sandstone was derived from proximal tectonic highland sources along the eastern margin of the Appalachian Basin that included uplifted older sedimentary rocks and accreted island arc terranes. Sediment was dispersed into a transverse drainage system that was deflected by the existent foreland-basin topography into an axial drainage which is preserved in the study area. Drainage area estimates for the Lower Pennsylvanian Corbin Member in the Appalachian Basin follow the same scaling relationship as the lower Stony Gap Sandstone and lower Princeton Formation incised valleys, where dimensions of incised valleys and their internal fill elements scale to the size of their drainage areas. This study has demonstrated that valley dimensions derived from field and subsurface mapping can be used in conjunction with petrographic data and detrital zircon age spectra to constrain drainage basin area and location, at least to a first order.

Seismic evidence for secular evolution and alteration of Archaean crust in Indian shield

The mechanisms through which the Archaean continental crust evolved are debatable. The end member models advocate horizontal accretion of island arcs or vertical accretion due to differentiation of magmatic material above hotspots. Whether both the processes operated together or separately is hard to ascertain. Also, there is no consensus on the processes that govern secular change in the character of the crust in Archean, as revealed by the seismological and petrological data. In order to address these key issues, we use converted wave data to extract the bulk crustal properties of the Indian cartons. Our analysis dominated by data from the Dharwar craton, reveals that most of the crust is formed in the early Archean. Soon after its formation, it gradually altered, making it mafic-to-intermediate in bulk composition. Further, the present day heat-flow values, which are higher in late-Archaean compared to the early, correspond to regions of thinner crust, implying that the crustal formation prevails at much higher temperatures predominantly through vertical accretion initially and then by slab melting. This suggests that some form of plate tectonics was in operation even in the Archean time. However, in the later stage, horizontal accretion dominantly contributes to crustal evolution. During this stage, large part of the lower crust seems to be degenerated making it thin and intermediate in composition.

Crustal structure of the Mid Black Sea High from wide-angle seismic data

Abstract The Mid Black Sea High comprises two en echelon basement ridges, the Archangelsky and Andrusov ridges, that separate the western and eastern Black Sea basins. The sediment cover above these ridges has been characterized by extensive seismic reflection data, but the crustal structure beneath is poorly known. We present results from a densely sampled wide-angle seismic profile, coincident with a pre-existing seismic reflection profile, which elucidates the crustal structure. We show that the basement ridges are covered by approximately 1–2 km of pre-rift sedimentary rocks. The Archangelsky Ridge has higher pre-rift sedimentary velocities and higher velocities at the top of basement ( c. 6 km s −1 ). The Andrusov Ridge has lower pre-rift sedimentary velocities and velocities less than 5 km s −1 at the top of the basement. Both ridges are underlain by approximately 20-km-thick crust with velocities reaching around 7.2 km s −1 at their base, interpreted as thinned continental crust. These high velocities are consistent with the geology of the Pontides, which is formed of accreted island arcs, oceanic plateaux and accretionary complexes. The crustal thickness implies crustal thinning factors of approximately 1.5–2. The differences between the ridges reflect different sedimentary and tectonic histories.

GEOCHRONOLOGY AND SR-ND ISOTOPE GEOCHEMISTRY OF LATE PALEOZOIC COLLISIONAL GRANITOIDS OF UNDINSKY COMPLEX (EASTERN TRANSBAIKAL REGION)

There are several geodynamic models of the Central Asian Orogenic Belt (CAOB) development [Sengor et al., 1993, Zorin, 1999; Parfenov et al., 1999, 2003; Willem et al., 2012; and others]. The Mongol-Okhotsk Orogenic Belt (MOB) represents important part of CAOB. All geodymanic models of Late Riphean to Paleozoic structures of CAOB emphasize significance of subduction processes along Northern Asian craton margin at that time. Collage of CAOB terrains formed as a result of accretion of island arc, accretionary wedge, turbidite, and continental margin terrains to the Siberian paleocontinent.

Secular change in lifetime of granitic crust and the continental growth: A new view from detrital zircon ages of sandstones

U-Pb ages of detrital zircons were newly dated for 4 Archean sandstones from the Pilbara craton in Australia, Wyoming craton in North America, and Kaapvaal craton in Africa. By using the present results with previously published data, we compiled the age spectra of detrital zircons for 2.9, 2.6, 2.3, 1.0, and 0.6 Ga sandstones and modern river sands in order to document the secular change in age structure of continental crusts through time. The results demonstrated the following episodes in the history of continental crust: (1) low growth rate of the continents due to the short cycle in production/destruction of granitic crust during the Neoarchean to Paleoproterozoic (2.9–2.3 Ga), (2) net increase in volume of the continents during Paleo- to Mesoproterozoic (2.3–1.0 Ga), and (3) net decrease in volume of the continents during the Neoproterozoic and Phanerozoic (after 1.0 Ga). In the Archean and Paleoproterozoic, the embryonic continents were smaller than the modern continents, probably owing to the relatively rapid production and destruction of continental crust. This is indeed reflected in the heterogeneous crustal age structure of modern continents that usually have relatively small amount of Archean crusts with respect to the post-Archean ones. During the Mesoproterozoic, plural continents amalgamated into larger ones comparable to modern continental blocks in size. Relatively older crusts were preserved in continental interiors, whereas younger crusts were accreted along continental peripheries. In addition to continental arc magmatism, the direct accretion of intra-oceanic island arc around continental peripheries also became important for net continental growth. Since 1.0 Ga, total volume of continents has decreased, and this appears consistent with on-going phenomena along modern active arc-trench system with dominant tectonic erosion and/or arc subduction. Subduction of a huge amount of granitic crusts into the mantle through time is suggested, and this requires re-consideration of the mantle composition and heterogeneity.

Lithospheric velocity model of Texas and implications for the Ouachita orogeny and the opening of the Gulf of Mexico

AbstractA 3‐D shear wave velocity model of Texas has been developed from Rayleigh wave phase velocities by using ambient noise data recorded at the USArray stations. In the upper crust, the Ouachita front separates high velocity in the Laurentia to its west from low velocity in the east and south Texas basins. The Ouachita belt is characterized as a high‐velocity zone with local maximums coinciding with known uplifts, which we interpret as accreted island arcs during the Ouachita orogeny. Our model evidences a strong Ouachita lithosphere that helped to buffer crust thinning from the Mesozoic rifting. A significantly low‐velocity anomaly is present in southeast Texas in the lower crust and upper mantle. We associate this anomaly with a past asthenosphere upwelling that likely originated from the edge of the subducted slab during the Ouachita collision and was potentially responsible for the opening of the Gulf of Mexico.

Mesoproterozoic island arc magmatism along the south-eastern margin of the Indian Plate: Evidence from geochemistry and zircon U-Pb ages of mafic plutonic complexes

The Prakasam Igneous Province within the Nellore Schist Belt (NSB) preserves important imprints of mafic magmatism along the south-eastern margin of the Indian plate. Here we report petrology, geochemistry and zircon U-Pb age data from three gabbro plutons namely Purimetla, Kanigiri and P C Palle which intruded into the high grade rocks of the region. LA-ICP-MS U–Pb data on zircons from the three plutons reveal prominent late Mesoproterozoic ages of 1334±15Ma, 1338±27Ma and 1251.2±9.4Ma. The cumulative 207Pb/206Pb mean age of 1315±11Ma is interpreted to represent the timing of mafic magmatism in the Prakasam Igneous Province. These rocks show adcumulus to mesocumulus and poikilitic textures indicating fractional crystallization of plagioclase and clinopyroxenes in the Purimetla pluton whereas the Kanigiri and P C Palle intrusions possess hornblende and biotite suggesting the role of water during partial melting. The rocks show LREE enrichment (∑LREE/∑HREE=2.2–15.0), marked Eu-anomalies (Eu/Eu∗=0.8–2.2) and fractionated patterns (LaN/YbN=3–79). Primitive mantle normalised trace element spider diagrams indicate subduction modified arc signatures with LILE enrichment and depletion of Nb, Ti and Zr relative to Th and La. Tectonic discrimination diagrams show arc magmatic affinities for the three gabbro plutons consistent with subduction zone setting. We propose a tectonic model involving intra oceanic island arc accretion during late Mesoproterozoic along the eastern margin of the Indian continent.

P–T path and timing of crustal thickening during amalgamation of East and West Gondwana: A case study from the Hafafit Metamorphic Complex, Eastern Desert of Egypt

The southeastern sector of the Hafafit Metamorphic Complex, southern Eastern Desert of Egypt comprises infrastructural orthogneisses of tonalite and syenogranite parentage, amphibolites, and a volcano-sedimentary association. These are overthrust by an obducted suprastructural ophiolite nappes via the Nugrus thrust. The protolith of the biotite–hornblende-gneisses was formed during island-arc accretion, while that of the garnet–biotite gneisses were formed in a within-plate regime, consistent with a transition to a post-collisional setting. The volcano-sedimentary association comprises interbedded and intercalated highly foliated metapelitic schists, metabasites, and leucocratic gneisses, deposited in a back-arc basin. The metapelites and the leucocratic gneisses originated from immature Fe-shales and arkoses derived from intermediate-mafic and acidic igneous rocks, respectively, via weak chemical weathering in a tectonically active island arc terrane. The intercalated amphibolites were derived from tholeiitic basalts generated in a back-arc setting.The volcano-sedimentary association was metamorphosed under upper-amphibolite facies conditions with pressures of 9–13kbar and temperatures of 570–675°C, as derived from conventional geothermobarometry and pseudosection calculation. A steep, tight clockwise P–T path is constrained and a geothermal gradient around 20°C/km is estimated for the peak metamorphism. We assume that deformation and metamorphism are due to crustal thickening during the collision of East and West Gondwana, where peak metamorphism took place in the middle to lower crust at 33km average crustal depth. This was followed by a subsequent quasi-isothermal decompression due to rapid exhumation during wrench tectonics. Sinistral transcurrent shearing with extensional denudation resulted in vertical ductile thinning that was accompanied by heat input from magmatism, as indicated by a higher geothermal gradient during retrograde metamorphism and exhumation of the complex.U–Pb data from magmatic zircons yields protolith ages of 731±3Ma for the biotite–hornblende gneisses and 646±12Ma for the garnet–biotite gneisses. Conforming to field evidence, our geochronology data point to a depositional age of the volcano-sedimentary cover at around 650Ma. The age of metamorphism is constrained by a low Th/U ratio of a zircon grain crystallized at an age of 597±6Ma.

THE COMPOSITION AND RECONSTRUCTION OF THE SOURCE AREA FOR THE LATE PRECAMBRIAN TERRIGENOUS ROCKS OF THE OSELKOVAYA SERIES (BIRYUSA PRISAYANIE)

We present results from petrographic and lithogeochemical studies of the Late Precambrian terrigenous rocks (sandstones, gravelites, and aleuritic sandstones) from the Oselkovaya series of Prisayanie. The studies were conducted to reconstruct the primary composition of the rocks in the source area. It has been found that the rocks in the lower part of the cross-section of this series (Marninskaya suite, and the lower part of the Udinskaya suites) are represented by more coarse-grained terrigenous rocks (gravelites, and sandstones) as compared to the upper part of the cross-section (the upper part of the Udinskaya suite, and the Aisinskaya suite) with sandstones and aleuritic sandstones. Gravelites and sandstones from the lower part of the Oselkovaya series show indicators of epigenetic changes that are less intensively expressed in the rocks from the upper part of the cross-section. The upper and lower parts of the Oselkovaya series are significantly different in terms of lithogeochemistry. The lower rocks show quite low contents of Na 2 O and ratios K 2 O/Na 2 O ranging between 10 and 75. In the terrigenous sediments of the upper part, values of K 2 O/Na 2 O do not exceed 1–2. Sandstones and gravelites in the lower part of the Oselkovaya series are characterized by reduced concentrations of radioactive, rare-earth, and highly charged elements, as well as lower concentrations of Ni and Co relative to concentrations of these elements in sandstones and aleuritic sandstones of the Oselkovaya series. The petrographic and lithogeochemical characteristics of the terrigenous sediments of the lower and upper parts of the Oselkovaya series suggest different sources of the denudation of these rocks into the sedimentation basin. It is suggested that acid rocks were the denudation source of the terrigenous rocks in the lower part of the series, and the sandstones and aleuritic sandstones in the upper part of the series were sourced from rocks of a mixed (acid–base) composition. The composition of the rocks in the source area was reconstructed, and the published ages of detrital zircons from the sandstones of the upper and lower parts of the Oselkovaya series were taken into account. The reconstruction suggests that the lower part of the Oselkovaya series resulted from the destruction of the basement rocks in the Siberian craton. The upper part of the Oselkovaya series seems to have formed in the basin, wherein the denudation took place from the orogen formed as a result of the accretion of micro-continents and island arcs of the Paleo-Asian Ocean to the south-western margin of the Siberian craton.

Crustal growth during island arc accretion and transcurrent deformation, Natal Metamorphic Province, South Africa: New isotopic constraints

The Natal Metamorphic Province consists of, from north to south, the Tugela, Mzumbe, and Margate Terranes. These were accreted to the southeastern margin of the Kaapvaal Craton in the late Mesoproterozoic, and followed by intrusion of a large suite of A-type granitoid bodies. New U–Pb data from zircon, titanite, and monazite further constrains the temporal framework of these geological events.The Tugela and Mzumbe Terranes record protracted magmatism in an island arc complex from ∼1200Ma to 1160Ma, followed by the accretion of these terranes onto the southern margin of the Kaapvaal Craton at ∼1150Ma. Arc magmatism in the Margate Terrane continued until ∼1120Ma and was followed by extension and bimodal volcanism immediately prior to accretion to the Kaapvaal/Mzumbe continental margin at ∼1090Ma. This accretion was accompanied by high-pressure and high-temperature metamorphism, juxtaposition of the Mzumbe and Margate Terranes along the Melville Thrust, and the formation of a number of syntectonic intrusive units derived from melting of the pre-existing arc crust. After accretion, extensional collapse is evidenced by the intrusion of mafic/ultramafic and alkaline intermediate magmatic suites at ∼1085Ma, resulting from mafic underplating and/or lower crustal delamination. Nd and Hf isotopic data imply the magmatic rocks of the Natal Metamorphic Province were derived from relatively juvenile continental crust, initially generated by island arc magmatism and subsequently reworked during the accretion event(s). The combined Kaapvaal-NMP region (the southern margin of the enlarged Kalahari Craton) then experienced extensive sinistral transcurrent deformation centred along a series of discrete steep shear zones that are found from the Kaapvaal cratonic margin to the southernmost portion of the Natal Metamorphic Province. This deformation is accompanied by low-pressure, (ultra)high-temperature metamorphism, isobaric cooling, and intrusion of the voluminous A-type Oribi Gorge Suite porphyritic granites and charnockites throughout the Mzumbe and Margate Terranes.

Chapter 15 Evolution of c. 3.5–2.5 Ga basins of the Dharwar Craton

Abstract The geology and basin evolutionary history of the Dharwar Craton is discussed. The Dharwar Craton comprises western (WDC) and eastern (EDC) subdivisions (possibly separated by the Closepet granite), predicated on lithological contrasts and inferred metamorphic and magmatic evolution. A postulated genesis of the WDC comprises early, c. 3.5 Ga protocrust, which possibly formed as basement to the c. 3.35–3.2 Ga Sargur Group greenstone belts. The latter are thought to have formed through accretion of plume-related ocean plateaux. The approximately coeval Peninsular Gneiss Complex possibly originated from beneath plateau remnants, leading to metamorphism of Sargur Group belts at c. 3.13–2.96 Ga. At c. 2.9–2.6 Ga, the Dharwar Supergroup, comprising lower Bababudan (mainly braided fluvial, glaciomarine and subaerial volcanic strata) and upper Chitradurga (marine clastic, chemical sedimentary and subaqueous volcanic rocks) groups developed. This supergroup formed younger greenstone belts characterized by two distinct magmatic events, at 2.7–2.6 and 2.58–2.54 Ga; the latter was approximately coeval with c. 2.6–2.5 Ga granitic magmatism, which marked final cratonization of the WDC. The EDC consists of 2.7–2.55 Ga tonalite–trondhjemite–granodiorite gneisses and migmatites, essentially coeval greenstone belts (mainly volcanic lithologies), with minor inferred remnants of an older, c. 3.38–3.0 Ga crust, and voluminous 2.56–2.5 Ga granitoids (including the Closepet). An east–west accretion of EDC island arcs (or possibly of an assembled arc-granitic terrane) on to the WDC is postulated, and the Closepet granite perhaps accreted earlier on to the WDC to form a ‘central Dharwar’ terrane. A final voluminous granitic cratonization event affected the assembled Dharwar Craton at c. 2.5 Ga.

Sequence stratigraphy, tectonostratigraphy and basin analysis

Abstract The Dyfed Supergroup of SW Wales ranges from Terreneuvian to Furongian in age and represents a prolonged and nearly continuous phase of siliciclastic sedimentation on the southern margin of the Cambrian Welsh Basin. The lower 564 m of the Supergroup are included in an extended Caerfai Group (six formations), embracing strata previously assigned to the Solva Group. Predominantly arenaceous, the Caerfai Group has important units of conglomerate and pebbly sandstone at its base, middle and top. The Caerfai Group is overlain by 687 m of mainly argillaceous sedimentary deposits of the Porth-y-rhaw Group (five formations), a newly erected unit that includes strata previously assigned to the ‘Menevian Group’ and ‘Lingula Flags’. This two-fold division of the Dyfed Supergroup is comparable with the distinction of the laterally equivalent Harlech Grits and Mawddach groups exposed around the Harlech Dome in North Wales. High resolution sequence-stratigraphical techniques, constrained by biostratigraphical data wherever possible, are applied to the Dyfed Supergroup across southern Britain: the revised lithostratigraphy is thereby integrated with a slightly modified version of the Avalonian chronostratigraphy initially developed for the western Avalonian successions of maritime Canada. Sequences 1 and 2 are not recognized in the Dyfed supergroup of SW Wales, reflecting the inner platform setting of the area. Those sequences are represented in North Wales however, which was situated on the outer platform. The relative ease with which the Cambrian successions of southern Britain can be incorporated into the general Avalonian framework reflects the shared epeirogenic history, sediment sources and accumulation history of a microcontinent unified by early Cambrian times. The gross lithological similarities that exist between Cambrian successions across Avalonia, and between SW Wales and North Wales in particular, are better understood when basin chronostratigraphy is placed within a sequence stratigraphic framework defined by systems tracts. This is particularly evident during Unnamed Cambrian Series 3 (C3) and Furongian times, when clear systems links are demonstrated between paralic depositional environments in SW Wales and deeper basin turbidites in North Wales. Sequences 3–7 (Terreneuvian–C3) in the Welsh Basin and its hinterland were dominated by siliciclastic deposition on an epeirogenically active platform. An extensional rift-like tectonic regime is proposed, where elongate basins dominated by mudstone deposition have time equivalents formed on areas of apparent uplift that were probably tilt-block highs. Local accumulation history, the development of regional unconformities, the extent of marine onlap, and the secular succession of lithofacies were controlled principally by phases of fault-accommodated subsidence along the NE-trending lineaments that bounded these basins and their intervening horsts or platforms. In southern Britain, movements first along the Menai Straits Fault System then the Welsh Borderlands Fault System, both of which are terrane boundaries, were responsible for the dominantly coarse-grained arenaceous sedimentation of pre-Drumian times. In the Welsh Basin, this is highlighted by a change in sediment source from the Monian Superterrane to the Wrekin Terrane. A rapid switch from marginal to inner platform source areas accompanied a major phase of tectonic reorganization of the Avalonian Superterrane during the development of the boundary between sequences 3 and 4. Although the role of secular variations in eustatic sea level as a control on sequence development and architecture has been dismissed previously, prominent lowstands such as those recorded during the basal Ptychagnostus gibbus Biozone, ‘ Solenopleura ’ brachymetopa Biozone (mid- Paradoxides forchhammeri Biosuperzone), and the upper part of the forchhammeri Biosuperzone also influenced the depositional sequence stratigraphy. The broad subdivision of Cambrian Avalonian stratigraphy into tectonically active and passive phases of sedimentation allows two megasequences to be distinguished. Megasequence 1 (sequences 1–7) represents the transform termination of Avalonian subduction following oblique convergence, and the accretion of island arcs onto the northern periphery of Gondwana. Under transpressional regimes, late Precambrian arc-related basins were inverted and recycled into pull-apart successor basins. Initially, the margins of these were dominated by alluvial fans and coarse-grained fan-deltas built by flood-generated sediment dispersal processes. In the later transform stage of Cadomian–Avalonian orogenesis, there was a switch to sediment supplied from highlands to the east. Sequence 8, represented in Pembrokeshire by the Aber Llong and Ogof Velvet formations, lies at the base of Megasequence 2 (late C3–early Ordovician). Excellent sections occur through these formations and their equivalents in North Wales, and their interpretation significantly improves understanding of younger Cambrian sedimentary basins in Avalonia. These successions reflect passive margin sedimentation and the culmination of the Avalonian–Cadomian orogenic cycle. Deposition occurred in part of a subaqueous delta platform at the mouth of a huge river system that drained a substantial part of West Gondwana, with Avalonia acting as a sink for vast quantities of fine-grained sediment. Secular variations in eustatic sea level and/or sediment supply, rather than active tectonism, were the main mechanisms controlling sequence architecture and depositional environments at this time.

Beyond Moa's Ark and Wallace's Line: extralimital distribution of new species of Austronothrus (Acari, Oribatida, Crotoniidae) and the endemicity of the New Zealand oribatid mite fauna.

The genus Austronothrus was previously known from three species recorded only from New Zealand. Austronothrus kinabalu sp. nov. is described from Sabah, Borneo and A. rostralis sp. nov. from Norfolk Island, south-west Pacific. A key to Austronothrus is included. These new species extend the distribution of Austronothrus beyond New Zealand and confirms that the subfamily Crotoniinae is not confined to former Gondwanan landmasses. The distribution pattern of Austronothrus spp., combining Oriental and Gondwanan localities, is indicative of a curved, linear track; consistent with the accretion of island arcs and volcanic terranes around the plate margins of the Pacific Ocean, with older taxa persisting on younger island though localised dispersal within island arc metapopulations. Phylogenetic analysis and an area cladogram are consistent with a broad ancestral distribution of Austronothrus in the Oriental region and on Gondwanan terranes, with subsequent divergence and distribution southward from the Sunda region to New Zealand. This pattern is more complex than might be expected if the New Zealand oribatid fauna was derived from dispersal following re-emergence of land after inundation during the Oligocene (25 mya), as well as if the fauna emanated from endemic, relictual taxa following separation of New Zealand from Gondwana during the Cretaceous (80 mya).

A review of the inferred geodynamic evolution of the Dharwar craton over the ca. 3.5–2.5 Ga period, and possible implications for global tectonics

The geological history and evolution of the Dharwar craton from ca. 3.5–2.5 Ga is reviewed and briefly compared with a second craton, Kaapvaal, to allow some speculation on the nature of global tectonic regimes in this period. The Dharwar craton is divided into western (WDC) and eastern (EDC) parts (separated possibly by the Closepet Granite Batholith), based on lithological differences and inferred metamorphic and magmatic genetic events. A tentative evolution of the WDC encompasses an early, ca. 3.5 Ga protocrust possibly forming the basement to the ca. 3.35–3.2 Ga Sargur Group greenstone belts. The latter are interpreted as having formed through accretion of plume-related ocean plateaux. The approximately coeval Peninsular Gneiss Complex (PGC) was possibly sourced from beneath plateau remnants, and resulted in high-grade metamorphism of Sargur Group belts at ca. 3.13–2.96 Ga. At about 2.9–2.6 Ga, the Dharwar Supergroup formed, comprising lower Bababudan (largely braided fluvial and subaerial volcanic deposits) and upper Chitradurga (marine mixed clastic and chemical sedimentary rocks and subaqueous volcanics) groups. This supergroup is preserved in younger greenstone belts with two distinct magmatic events, at 2.7–2.6 and 2.58–2.54 Ga, the latter approximately coincident with ca. 2.6–2.5 Ga granitic magmatism which essentially completed cratonization in the WDC. The EDC comprises 2.7–2.55 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses and migmatites, approximately coeval greenstone belts (dominated by volcanic lithologies), with minor inferred remnants of ca. 3.38–3.0 Ga crust, and voluminous 2.56–2.5 Ga granitoid intrusions (including the Closepet Batholith). An east-to-west accretion of EDC island arcs (or of an assembled arc – granitic terrane) onto the WDC is debated, with a postulate that the Closepet Granite accreted earlier onto the WDC as part of a “central Dharwar” terrane. A final voluminous granitic cratonization event is envisaged to have affected the entire, assembled Dharwar craton at ca. 2.5 Ga. When Dharwar evolution is compared with that of Kaapvaal, while possibly global magmatic events and freeboard–eustatic changes at ca. 2.7–2.5 Ga may be identified on both, the much earlier cratonization (by ca. 3.1 Ga) of Kaapvaal contrasts strongly with the ca. 2.5 Ga stabilization of Dharwar. From comparing only two cratons, it appears that genetic and chronologic relationships between mantle thermal and plate tectonic processes were complex on the Archaean Earth. The sizes of the Kaapvaal and Dharwar cratons might have been too limited yet to support effective thermal blanketing and thus accommodate Wilson Cycle onset. However, tectonically driven accretion and amalgamation appear to have predominated on both evolving cratons.

Rhyolite–granite association in the Central Taimyr zone: evidence of accretionary-collisional events in the Neoproterozoic

The Central Taimyr accretionary belt includes two granite-metamorphic terranes: Faddey and Mamont-Shrenk, which include the oldest igneous formations of the Taimyr folded area in the Arctic framing of the Siberian craton—granitoids and granite-gneisses with U–Pb zircon ages of 900–830 Ma. The [FeO*/(FeO* + MgO)]-enriched granitoids of these terranes are products of highly fractionated I-type magmas. This paper presents results of new petrographic, geochemical, geochronological, and paleomagnetic investigations of acid rocks from a volcanic-plutonic association (in the region of the Leningradskaya River) in the Faddey terrane in the northeastern Taimyr area. These rocks formed during the final stage of continent–island arc accretion and collision that occurred at approximately 870–820 Ma. We established that the studied rocks belong to a long granitoid belt extending from Mamont-Shrenk to Faddey terrane, where all the igneous bodies are deformed and oriented uniformly. The paleomagnetic pole we calculated differs significantly from the apparent polar-wander path interval of corresponding age for Siberia. The 33.8°±5.4° angular distance between the poles indicates that the formation of this volcanic-plutonic association took place at a significant distance from the Taimyr margin of the Siberian paleocontinent.

Architecture and evolution of accretionary orogens in the Altaids collage: The early Paleozoic West Junggar (NW China)

Understanding the development and evolution of accretionary orogens is crucial for characterizing continental crust growth in time and space. In the Altaids tectonic collage of Central Asia, conflicting geodynamic models have been proposed about the Paleozoic evolution, during which juvenile continental crust has been extensively formed. This study focuses on West Junggar (NW China), a key region that has not been extensively studied yet. A multidisciplinary approach, including detrital zircon provenance study, geochemistry, and field structural analysis, provides new constraints on the evolution of three Early Paleozoic units of West Junggar. Oceanic subduction predominates in Early Paleozoic time, as indicated by ophiolitic melanges, fore-arc volcaniclastic turbidites and magmatic arc suites. However, the development of an olistostrome supplied by a sub-contemporaneous carbonate platform, the occurrence of continental molasse, and the chronology and geochemistry of magmatic events reveal interruptions of subduction. Discrete collisions of volcanic arcs and micro-continent are inferred from the tectonic structure of the belt, which displays allochthonous units rooted in the suture zone. Early Paleozoic magmatic and sedimentary rocks of West Junggar bear some resemblance to contemporaneous analogues in Eastern Kazakhstan, and suggest a lateral connection. On the basis of these new results, we propose a geodynamic evolution in four steps: 1) Early to Middle Ordovician intra-oceanic subduction forming island arcs, 2) Late Ordovician tectonic accretion of island arcs against the Kazakhstan continental margin, 3) Early to Middle Silurian resumption of subduction and active continental margin magmatism, and 4) Late Silurian-Early Devonian micro-continent collision. This scenario, consistent with the formation of the Kazakhstan Block, supports a model of multiple episodic accretions for the Altaids tectonic collage rather than a continuous continental margin evolution. These results also suggest that, in Central Asia, continental crust growth results from alternating vertical and horizontal growth episodes throughout the Paleozoic.

The geodynamic evolution of Mesoarchean anorthosite complexes inferred from the Naajat Kuuat Complex, southern West Greenland

Layered anorthosite complexes are typical components of Archean crustal domains. However, the geodynamic settings in which they were emplaced are still discussed as geological relationships are often ambiguous. Here we report major, trace element and high-precision high-field-strength-element (HFSE) data for the recently discovered well preserved Naajat Kuuat Anorthosite Complex from the inner Ameralik fjord region, southern West Greenland. The dataset is complemented by the first combined Hf–Nd isotope analyses for Archean layered anorthosite complexes and U–Pb zircon geochronology. The data contribute to the small database on Archean layered anorthosite complexes and are used to unravel the origin of these complexes and the tectonic regime involved. Fractional crystallisation of olivine, pyroxene, plagioclase and possibly amphibole controls major and trace element variations in the layered intrusion. There are two groups of amphibolites: (1) a group with primitive mantle normalized trace element patterns are similar to those of MORB-like basalts and (2) typical island-arc tholeiites (IAT), apparently indicating an island-arc setting. Lu–Hf regression lines yield an age of 2985±59Ma (MSWD 4) within the error of the Sm–Nd regression age of 2929±110Ma (MSWD 17). The initial ɛHf(2985) for the Naajat Kuuat rocks range from +1.6 to +5.8 and the initial ɛNd(2985) range from +0.4 to +3.9, either indicating variably depleted mantle sources or variable degrees of crustal contamination. In contrast to most mafic assemblages, ratios of Nb/Ta are highly variable (7.85 to 18.6), reflecting fractionation and accumulation of amphibole, ilmenite and pyroxene. The MORB-like parental liquids have the highest Nb/Ta of ca. 18, consistent with a mantle source overprinted by melt-like components from subducting oceanic crust with high Nb/Ta. The accumulation of plagioclase forming the anorthosites and the primary fractionation of amphibole as well as the occurrence of high-Al basalts within the Naajat Kuuat complex argue for hydrous parental liquids in support of an island-arc related setting. Zircon U–Pb geochronology from the anorthosite and adjacent tonalites reveal major tonalite intrusion into the complex at ca. 2802Ma and a second regional event at ca. 2710Ma, in accord with crustal heating due to micro-continent amalgamation and crustal thickening. Altogether, the geochemical data can be interpreted with a geodynamic model, where anorthosite-complex associated rocks intrude into tectonically thickened island-arc crust. Crustal thickening is possibly triggered by island-arc accretion, leading to the emplacement of TTG bodies that further thickened the crustal pile. Further collision and amalgamation with other proto-crustal assemblages might have led to enhanced crustal magmatism and granulite facies metamorphism.

Middle-Late Ordovician arc-type plutonism in the NW Chinese Tianshan: Implication for the accretion of the Kazakhstan continent in Central Asia

Kazakhstan is one of the largest continental assemblages within the Central Asian Orogenic Belt. It was formed by accretion and amalgamation of several continental fragments and island arcs during early Paleozoic. The Aktau – Wenquan domain is one important continental constituent within the Kazakhstan and is located in the northeast of the Kazakhstan. Late Ordovician arc-type magmatic rocks were recently recognized from the Wenquan Group, which is long known to be Paleoproterozoic. Our investigation in the southern Wenquan area (NW China) suggests that within the Wenquan Group, foliated magmatic rocks are largely different from the amphibolite in terms of petrographic texture, protolith age, geochemical composition and Sm-Nd model age. New SHRIMP zircon U–Pb ages were determined for two deformed diorites, one gabbro enclave and a monzodiorite dyke at 466±4–447±6Ma, 462±5Ma and 450±9Ma, respectively. Zircons of the monzodiorite dyke contain inherited cores of ∼1000 to ∼1400Ma. The new geochemical analyses and literature data show that the magmatic rocks are of subduction-related calc-alkaline affinity. The εNd(t) values range from −2.0 to 0.8 and Nd model ages from 1.0 to 1.2Ga, indicating variable involvement of Proterozoic crust in the generation of the Early Paleozoic magmatic rocks. Consequently, these rocks were mostly likely emplaced in a continental arc environment, probably in the northern margin of the Aktau – Wenquan continental domain. Regional comparison suggests that this arc magmatism could probably be linked with the Karamai and/or North Balkhash accretionary wedges, and resulted from subduction of the Junggar–Balkhash oceanic lithosphere during Middle to Late Ordovician. Subsequent continental – arc collision and continuing accretion during Middle – Late Paleozoic in north of the Kazakhstan overprinted the structure of Early Paleozoic active continental margin in the Aktau – Wenquan domain.

The tectonomagmatic evolution of structure-lithologic complexes in the Tannu-Ola zone, Tuva, in the Late Vendian–Early Cambrian ( from geochemical, Nd isotope, and geochronological data)

In the Tannu-Ola zone, there are island-arc (Late Vendian) and subduction-accretionary (first half of the Early Cambrian) complexes whose mantle sources were generated from melts produced in suprasubduction conditions as a result of the melting of mantle wedge peridotites. The island arc, whose fragment is present in the Tannu-Ola zone, formed tentatively at 570 Ma and ceased as an individual structure at the Vendian–Cambrian boundary, though the subduction continued till the middle Early Cambrian. From the Vendian–Cambrian boundary to middle Early Cambrian, the accretion of island-arc, back-arc, oceanic, and metamorphic structure-lithologic complexes took place at this segment of the Central Asian Fold Belt. From middle to late Early Cambrian, the Tannu-Ola zone was an active continental margin, where basalts with island-arc characteristics, quartz microdiorites of adakite composition, basalts, dolerites, and quartz microdiorites of intraplate type, and diorite-tonalite-plagiogranite association of island-arc type formed. To explain the spatial and temporal conjugation of different magmatism manifestations on the active margin, we applied the slab–window model.

Evolution of the Guerrero composite terrane along the Mexican margin, from extensional fringing arc to contractional continental arc

The western margin of Mexico is ideally suited for testing two opposing models for the growth of continents along convergent margins: accretion of exotic island arcs by the consumption of entire ocean basins versus accretion of fringing terranes produced by protracted extensional processes in the upper plate of a single subduction zone. We present geologic and detrital zircon evidence that the Zihuatanejo terrane of the Guerrero composite terrane originated from the latter mechanism. The evolution of the Zihuatanejo terrane can be explained by extensional and compressional processes operating entirely within the upper plate of a long-lived subduction zone that dipped east under the Mexican margin. This process controlled crustal growth by continental margin rifting and addition of new igneous and volcaniclastic material during extension, followed by accretion and thickening of the crust during contraction. Prior to this study, all Mesozoic rocks in the western part of the Guerrero composite terrane were considered to be part of a single arc. However, we divide it into four distinctive tectonostratigraphic assemblages: (1) a Triassic–Early Jurassic accretionary complex (Arteaga complex); (2) a Jurassic to earliest Cretaceous extensional volcanic arc assemblage; (3) an Early Cretaceous extensional arc assemblage; and (4) a Santonian–Maastrichtian compressional arc assemblage. (1) The Arteaga subduction complex forms the basement to the Zihuatanejo terrane and includes Grenville, Pan-African, and Permian detrital zircon suites that match the Potosi fan of the Mexican mainland. (2) The Jurassic to earliest Cretaceous extensional volcanic arc assemblage shows a Callovian–Tithonian (ca. 163–145 Ma) peak in magmatism; extensional unroofing began in this time frame and continued into through the next. (3) The Early Cretaceous extensional arc assemblage has two magmatic peaks: one in the Barremian-Aptian (ca. 129–123 Ma), and the other in the Albian (ca. 109 Ma). In some localities, rapid subsidence produced thick, mainly shallow-marine volcano-sedimentary sections, while at other localities, extensional unroofing of all older assemblages resulted in recycling of zircon from all older units (1, 2, 3). (4) For the Santonian–Maastrichtian compressional arc assemblage, our new detrital zircon dates show for the first time that arc volcanic rocks of this age are present in the coastal Zihuatanejo terrane. The contractional arc developed atop assemblages 1–3, which were shortened between Turonian and Santonian time (ca. 93 and 84 Ma). Taken together, the western Zihuatanejo terrane records a more protracted history of arc magmatism than has yet been dated in other terranes of western Mexico, but it closely matches the history of Baja California to the northwest