Accretion Of Island Arcs Research Articles

Rodinia to Gondwana evolution record, South Sinai, Egypt: Geological and geochronological constraints

The geological history of Sinai is complicated and differs from the other parts of the Arabian Nubian Shield. Some rocks in Sinai have a signature from the Grenvillian crust, as evidenced in Wadi Seih area. The ultimate objective of this research is to clarify the progression of episodic island arc accretion events from the amalgamation of Rodinia to the assembly of Gondwana supercontinent.Geochemical, mineralogical, U-Pb zircon geochronological, and thermobarometric studies are considered to constrain the nature and tectonic evolution of Wadi Seih gneisses. The protoliths of the orthogneisses are tonalite, trondhjemite, granodiorite, and granite, which were derived from metaluminous-calc-alkaline magma and formed during island-arc accretion. The paragneisses originated from immature Fe-sand and arkose that were deposited in active continental margin and back-arc basin. The orthogneisses and orthopyroxene-free paragneisses were metamorphosed under amphibolite to low granulite-facies conditions with temperatures from 530 to 775 °C at pressures of 3–7 kb, while the orthopyroxene-bearing paragneisses were metamorphosed under granulite-facies at a temperature of 870 °C and pressures of 5–7 kb. Zircon U-Pb geochronological data of the orthogneisses yields three distinct ages (i.e., the oldest ages are 994 ± 6 Ma and 976.6 ± 6.2 Ma, the second age is 740 ± 6.4 Ma, and the third age is 619.4 ± 4.2). Similar ages are obtained from detrital zircons in paragneisses: 1039 ± 57 Ma, 643 ± 49 Ma, 628 ± 3.7 Ma, and 627 ± 6.6 Ma. These new U-Pb data delineate three episodic Mesoproterozoic to Neoproterozoic island arc events at 1050–900 Ma, 850–700 Ma, and 650–580 Ma. The oldest 1050–900 Ma arcs potentially formed during the Rodinia supercontinent-arc collision, while the 850–700 Ma arcs may have formed during the closure of the Mozambique Ocean. The youngest 650–580 Ma arcs could reflect the Gondwana assembly. The age of metamorphism in this study is constrained by the low Th/U ratio of a zircon grain at 633 ± 10 Ma, which is consistent with other previous studies in other areas in Sinai.

The proto-type basin and tectono-paleogeographic evolution of the Tarim basin in the Late Paleozoic

The Tarim basin is a large composite and superimposed sedimentary basin that has undergone complex multi-period and polycyclic tectonic movements. Understanding the proto-type basin and tectono-paleogeographic evolution of this complex superimposed basin is important for understanding the basin-mountain coupling and dynamical mechanisms of the Paleo-Asian and Tethys tectonic systems as well as hydrocarbon exploration and development. Based on previous works, together with the recent exploration, and geological evidences, three global plate tectonic pattern maps, four Tarim proto-type basin maps (in present-day geographic coordinates) and four regional tectono-paleogeography maps (in paleogeographic coordinates) during the Late Paleozoic are provided in this paper. Based on these maps, the proto-type basin and tectono-paleogeographic features of the Tarim basin during the Late Paleozoic are illustrated. The Devonian to Permian is an important period of terranes/island-arcs accretion and oceanic closure along the periphery of the Tarim block, and a critical period when the polarity of Tarim basin (orientation of basin long-axis) rotated at the maximum angle clockwise. During the Late Paleozoic, the periphery of the Tarim block was first collisional orogeny on its northern margin, followed by continuous collisional accretion of island arcs on its southern margin: on the Northern margin, the North and South Tianshan Oceans closed from East to West; on the South-Western margin, the Tianshuihai Island Arc gradually collided and accreted. These tectonic events reduced the extent of the seawater channel of the passive continental margin in the Western part of the basin until its complete closure at the end of the Permian. The Tarim basin was thus completely transformed into an inland basin. This is a process of regression and uplift. The Southwest of the Tarim basin changed from a passive to an active continental margin, through back-arc downwarping and eventually complete closure to foreland setting. The intra-basin lithofacies range from shelf-littoral to platform-tidal flat to alluvial plain-lacustrine facies. The tectonic-sedimentary evolution of the Tarim basin is strongly controlled by peripheral geotectonic setting.

Neodymium Isotope Constraints on the Origin of TTGs and High-K Granitoids in the Bundelkhand Craton, Central India: Implications for Archaean Crustal Evolution

Abstract The Bundelkhand craton in central India consists mainly of abundant high-K granitoids formed at the Archaean-Proterozoic boundary and several enclosed rafts of TTGs (tonalite-trondhjemite-granodiorites) up to 3.5 Ga. Therefore, the Bundelkhand craton is a key locality for studies on Archaean crustal growth and the emergence of multisource granitoid batholiths that stabilised a supercontinent at 2.5 Ga. Based on their geochemical characteristics, the high-K granitoids are divided into low silica–high Mg (sanukitoids and hybrids) and high silica–low Mg (anatectic) groups. We aim to provide new insights into the role of juvenile versus crustal sources in the evolution of the TTG, sanukitoid, hybrid, and anatectic granitoids of the Bundelkhand craton by comparing their key geochemical signatures with new Nd isotope evidence on crustal contributions and residence times. The ages and geochemical signatures as well as εNd(t) values and Nd model ages of TTGs point towards partial melting of a juvenile or short-lived mafic crust at different depths. Paleoarchaean TTGs show short crustal residence times and contributions from the newly formed crust, whereas Neoarchaean TTGs have long crustal residence times and contributions from the Paleoarchaean crust. This may reflect the transition from melting in a primitive oceanic plateau (3.4-3.2 Ga) in plume settings, resulting in a Paleoarchaean protocontinent, to 2.7 Ga subduction and island arc accretion along the protocontinent. The 2.5 Ga high-K granitoids formed at convergent subduction settings by partial melting of the mantle wedge and preexisting crust. Sanukitoids and hybrid granitoids originated in the mantle, the latter showing stronger crustal contributions, whereas abundant anatectic granitoids were products of pure crustal melting. Our Nd data and geochemical signatures support a change from early mafic sources to strong crust-mantle interactions towards the A-P boundary, probably reflecting the onset of supercontinent cycles.

Zircon isotope–trace element compositions track Paleozoic–Mesozoic slab dynamics and terrane accretion in Southeast Asia

Variation in zircon trace element (TE) and isotopic composition data have been used to infer changes in global-scale processes; however, few studies have explored zircon TE data as a comprehensive means to track regional shifts in tectonic regimes. In this contribution, we explore the utility of zircon TE data to record the well-documented Late Paleozoic–Mesozoic tectonic history in Southeast Asia, which includes transitions from continental magmatism to island-arc extension, subsequent compression and island-arc accretion, and finally continental collision. We present new U-Pb ages, along with Lu-Hf isotopic and TE data, from 2,478 detrital zircon grains extracted from 17 modern river sediment samples collected across two major suture zones in Thailand. Our results show that zircon Th/U and Nb/Ta mean values increase after ∼300Ma, whereas the percentage of classified ‘felsic’ zircon decline corresponding with the initiation of Nan back-arc basin extension and isolation of the Sukhothai Arc from Indochina. From ∼255 to 235 Ma, zircon εHf(t), LREE/HREE, (Eu/Eu*)N, (Ce/Ce*)N, Nb/Ta, and Th/U mean values decline, whereas Dy/Yb, U/Yb, and the proportion of classified S-type and ‘felsic’ zircon increase, showing the transition from I-type to S-type, and from more juvenile to more evolved magmatism. This transition reflects a shift from arc extension to compression via the closure of the Nan back-arc basin and the collision of the Sukhothai Arc with western Indochina, and the subsequent collision of Sibumasu with the Sukhothai Arc demarking the regional closure of the Paleo-Tethys ocean. Our data also show linear correlations between εHf(t)-DM and trace elemental data, particularly (Eu/Eu*)N, that reflect increasing sediment contamination within the melts. Lastly, our data illustrate that the Sibumasu–Sukhothai Arc collided ∼20 million years earlier in Thailand than in the Malay Peninsula, demonstrating along-strike diachroneity in Paleo-Tethys closure. Thus, time-integrated detrital zircon TE data may reliably track subduction slab dynamics and tectonic evolution beyond U-Pb and Hf isotope data alone.

Tectonic evolution and multi–episodic metallogenesis of the Sanjiang Paleo-Tethys multi-arc-basin-terrane system, SW Tibetan Plateau

The Sanjiang Tethys region in SW China lies in the tectonic junction of the Tethys domain, and represents a key precious/base metal province in China. Based on a compilation of studies from the past decades on the formation, evolution, and metallogenesis of the tectonic units in the region, we proposed a multi-arc-basin-terrane (MABT) structural framework for the Paleo-Tethys Sanjiang region. Convergence between South China and Indochina-Pu'er blocks had led to the accretion of many island arcs and micro-continental terranes, which is different from the traditionally direct convergence between two continents. Such prolonged terrane accretion process likely formed the various metallogenetic belts that contain diverse deposit types and mineral assemblages. In this contribution, we divided the Sanjiang region into ten metallogenetic belts, according to their characteristic lithologic assemblages, structures, and interpreted tectonic settings . Subsequently, we discussed the regional tectonic-metallogenic evolution of these ten ore belts and propose their metallogenetic models. This study reveals a genetic link between the MABT system and the regional metallogenesis, which is important for future mineral exploration in this region.

Two-stage crustal growth in the Arabian-Nubian shield: Initial arc accretion followed by plume-induced crustal reworking

Island-arc accretion during the assembly of Gondwana has been widely regarded as the main mechanism for Neoproterozoic crustal growth in the Arabian-Nubian Shield (ANS). However, processes involved to transform the newly accreted juvenile terranes into a typical continental crust remain unclear. Here, we present geochemical, isotopic, and U-Pb geochronological data from the El-Shadli volcanic province (80 km × 35 km and > 10 km thick) in the Eastern Desert of Egypt, which lies in the north-western part of the ANS, and overlies strongly deformed, previously accreted arc terranes. The El-Shadli volcanic province consists mainly of a mafic-felsic bimodal suite and subordinate intermediate rocks that intrude the mafic rocks of the suite. The bimodal suite rocks are tholeiitic, whereas the intermediate rocks have a calc-alkaline affinity. The bimodal suite and the intermediate rocks both yield U-Pb zircon ages of ~ 700 Ma implying they are coeval. High zircon εHf(t) values for the bimodal suite (average εHf(t) = +11.46) as well as the intermediate rocks (average εHf(t) = +9.76) indicate they were either magmatic extractions derived directly from a depleted mantle source, or the products of remelting of juvenile crust. Oxygen isotope data for zircon yield similar δ18O values for both the bimodal suite (average δ18O = 4.94‰) and intermediate rocks (average δ18O = 4.79‰). These are lower than typical mantle values indicating the parental magma in both cases interacted with hydrothermal fluids. Based on the petrological, geochemical, and isotopic data, we suggest that the El-Shadli bimodal suite and the intermediate rocks were produced by reworking of MORB-like and arc-like oceanic lithosphere, respectively, most likely driven by a mantle plume during the break-up of Rodinia. The recognition of the El-Shadli volcanic province as a likely mantle plume-induced post-kinematic magmatism provides a mechanism for the transformation of newly accreted juvenile crustal terranes into a chemically stratified normal continental crust. In addition, such plume events may result in new mantle extractions that are converted into new continental crust.

Late Cambrian calc-alkaline magmatism during transition from subduction to accretion: Insights from geochemistry of lamprophyre, dolerite and gabbro dikes in the Dzhida terrain, Central Asian orogenic belt

The Central Asian orogenic belt (CAOB) is one of the largest orogens on Earth characterized by complex and long-term history resulted from the development of the Paleo-Asian Ocean. In Late Cambrian–Ordovician, closure of Paleo-Asian basins and accretion of microcontinents, island arcs and back-arc basins to the southern margin of the Siberian continent resulted in large-scale magmatism recorded in Southern Siberia and Mongolia. The timing of terminal stage of subduction and start of accretion-collision stage is a matter of discussion for different parts of this extensive collage of terrains. In this paper, we report new data on whole-rock and mineral composition and 40Ar/39Ar age of calc-alkaline dolerites, lamprophyres and gabbros of Bayangol river area (Northern Mongolia) from the Dzhida terrain of CAOB. The Bayangol calc-alkaline mafic rocks were previously considered as a member of boninite-basaltic sequence originated in a primitive island arc. We found that the Bayangol calc-alkaline mafic rocks compose late mafic dikes, cutting terrigenous sediments and serpentinite mélange with boninite blocks, and, therefore, calc-alkaline rocks are not related to boninites. We obtained 40Ar/39Ar age of 498.9 ± 7.7 Ma for amphibole from Bayangol lamprophyre. The calc-alkaline rocks preserve clinopyroxene (diopside, Mg# of 76–90) and amphibole (mainly pargasite and magnesiohastingsite, Mg# of 38–92), which crystallized at 1170–1210°С, 2.2–9.5 kbar and 760–1030°С, 2.0–7 kbar, respectively. Dolerites, lamprophyres and gabbros show variations in major oxides (3.3–14.9% MgO, 10.1–19.4% Al2O3, 0.09–0.92% P2O5) and immobile trace element systematics (e.g., Th and LREE enrichment, HFSE depletion) corresponding to island arc calc-alkaline series. Isotope compositions of Nd (εNd(T) = −0.7 to +2.5) and Sr (87Sr/86Sr(T) = 0.7049–0.7078) suggest 1–5% addition of recycled terrigenous sediment melts to mantle wedge source of the Bayangol mafic rocks. Relations of trace element and SiO2 abundances and NdSr isotope ratios are resulted from fractional crystallization and do not support in situ crustal contamination of Bayangol magmas. Within the Dzhida terrain, the Bayangol mafic rocks have Nd isotope compositions transitional between those of Late Cambrian island arc intrusions with juvenile signatures and Early Ordovician collisional granitoids with crustal characteristics. Therefore, the Bayangol calc-alkaline mafic rocks mark transition from subduction to the accretion-collision regime during accretion of Dzhida island arc to the Siberian continent as a result of Paleo-Asian Ocean closure in SW Transbaikalia and Northern Mongolia.

The accretion history of the South China Block at its northwest margin in the Neoproterozoic: Records from the Changba complex in the Mianlue zone

To constrain the accretion history of the South China Block (SCB), a geochronological and geochemical study was carried out for the Changba Complex within the Mianlue zone in the northwestern margin of the SCB. Zircon U-Pb dating reveals that the metagabbros and metatonalites were formed at ca. 985–950 Ma, while the bt-amp gneisses have a younger protolith age of ca. 830 Ma. The metagabbro and metatonalite are low- to medium-K tholeiitic series and are significantly enriched in LILEs (Rb, Ba, Th, U, K) and slightly enriched in LREEs, but depleted in HFSEs such as Nb, Ta and Ti. In addition, they have highly positive zircon εHf(t) values (+11.3~ +16.9) and whole-rock εNd(t) values (+3.9~ +5.5) and εHf(t) values (+11.0 ~ +11.8). These geochemical features resemble to those of the magmatic rocks in juvenile oceanic arcs. Zircon δ18O value ranges from 3.0 to 6.2‰ for metagabbro and 4.6 to 5.34‰ for metatonalite, which are slightly lower than the mantle values. Hydrothermally altered gabbros in the lower oceanic crust often have lower δ18O values than mantle and can be primary components of arc lower crust. Therefore the Changba tonalite is interpreted to have been formed by partial melting of the hydrothermally altered gabbroic lower crust. The ca. 830 Ma gneisses are enriched in LREEs and incompatible elements and depleted in HFSEs, similar to the trace element pattern of the average bulk continental crust. They have enriched Sr-Nd-Hf isotopic compositions (0.7055 ~ 0.7060, −5.1 ~ −6.3, and −8.5 ~ −8.8, respectively), which is distinct from the juvenile arc crust. Besides, variable zircon εHf(t) values of −40.6 to 0 and slightly low to elevated zircon δ18O values of 4.2 to 6.6‰, suggesting derivation from reworking of a heterogeneous crustal source. In this regard, the Changba complex records the accretion of juvenile island arc and subsequent reworking of both juvenile and ancient crustal materials, which highlights the crustal growth and geodynamical evolution of the northern margin of the South China Block during the Neoproterozoic in response to the assembly of Rodinia.

Petrology, Geochronology and Geochemistry of the Xar Moron River Ophiolite: Implications for the Tectonic Evolution of the Paleo‐Asian Ocean

AbstractAs the largest accretionary orogen, the crustal tectonic framework and evolution of the Central Asian Orogenic Belt (CAOB) have always been one of the hot topics among geologists (Sengör et al., 1993, 1996; Jahn et al., 2000a; Badarch et al., 2002; Windley et al., 2007; Li et al., 2009). The formation of the main part of the crust in the CAOB involved continuous lateral accretion of island arcs and accretionary complexes along the margins of the Siberian, Sino‐Korean and Tarim paleo‐continents and the final collision between these continental margins because of the subduction of the Paleo‐Asian Ocean plate since Mesoproterozoic. The ophiolites, which represent the fragments of ancient oceanic lithosphere, are the direct evidence for the study of the evolution of orogenic belts. Based on field geological survey, the mantle peridotite (serpentinite), gabbro, basalt and radiolarian bedded chert, which were deemed as the “ophiolite trinity”, were identified as isolated blocks in the matrix of pelitic siltstone and silty mudstone in the Kedanshan, Xingshuwa and Jiujingzi areas along the Xar Monron River in southeast Inner Mongolia of China. Besides, there were plenty of other exotic blocks, such as limestone and sandstone, in the matrix. Both of the matrix and blocks underwent strong foliated deformation. All of these rocks above constitute a tectonic mélange. Zircon U‐Pb dating for the gabbro blocks in the Xingshuwa and Jiujingzi ophiolites reveals that they were formed in early Permian (275–280 Ma). The ages of the gabbros, together with the middle Permian radiolaria fossils in the chert reported by Wang and Fan (1997), indicate that the oceanic basin was not closed in early‐middle Permian. The geochemical compositions of the basaltic blocks distributed in different locations in the Xingshuwa tectonic mélange display different genetic types of normal mid‐ocean ridge basalt (N‐MORB), enriched mid‐ocean ridge basalts (E‐MORB), oceanic island basalt (OIB), island arc basalt and continental marginal arc basalt, which indicates what they represented is a complex oceanic basin. Combining with the studies on regional magmatism, strata and structure data, it is suggested that the Xar Moron River Ophiolite belt represented the final suture zone of the Paleo‐Asian Ocean in the southeast Inner Mongolia, and the ocean did not close before late Permian.

Emplacement of Young Island Arc Crust Over Older Mantle Along a Cratonic Foreland: Constraints From Multiple Isotopes and Elemental Geochemistry

AbstractMany studies have revealed that mantle‐derived mafic rocks can be more isotopically enriched than coexisting crustal felsic rocks. However, its origin remains to be elucidated. Here, we report on some Early Cretaceous (ca. 130 Ma) mafic rocks in Gan‐Hang Belt (South China) that are more isotopically enriched than the temporally and spatially associated granitoids. Relatively depleted Nd‐Hf isotopic signatures [εNd(t) = −0.4 to +0.6, εHf(t) = 3.4 to 4.8, εHf(t)zircon = −2.3 to +9.3], coupled with low zircon δ18OSMOW values (5.33–6.84 ‰) and geographical proximity to the exposed island arc rocks, indicate that the isotopically depleted Daixi alkali feldspar granites were dominantly derived from the relatively young Neoproterozoic accreted Shuangxiwu island arc sequences. Yet new and published whole‐rock Sr‐Nd‐Hf‐Pb‐O isotopic data [87Sr/86Sr)i = 0.70788–0.70833; εNd(t) = −7.6 to −3.5; εHf(t) = −6.8 to −1.9; (206Pb/204Pb)i = 18.028–18.108, (207Pb/204Pb)i = 15.565–15.577, (208Pb/204Pb)i = 38.198–38.341; δ18OSMOW = 4.5–5.5 ‰] highlight that the isotopically enriched Siling hornblende gabbros were predominantly sourced from an older, metasomatically enriched SCLM of adjacent Yangtze cratonic affinity. The arc crustal allochthon is assumed to have been tectonically emplaced onto the southeast Yangtze cratonic foreland, resulting in isotope inversion with younger crust on top of the older lithospheric mantle. Our results reveal a juxtaposition geometry of continental material after collision and accretion of island arcs by using isotope methods, which is independently supported by tectonic geological and geophysical studies. It suggests that ancient lithospheres should exist at depth within certain orogens associated with island arc accretion.

Isotopic mapping reveals the location of crustal fragments along a long-lived convergent plate boundary

New Guinea has acted as the boundary between the Australian and Pacific plates for hundreds of millions of years. Strike-slip movement and arc–continent collisions along this boundary during the Cenozoic have shuffled rocks of different age and composition in a series of terranes along the plate boundary making mapping them a considerable challenge. Here we report results of SrNd isotopic data obtained from rock samples from western New Guinea that are representative of the different terranes. These isotopic data reveal the crustal affinity of the terranes and we have used these data to map their spatial distribution. The isotopic data show three distinct crustal domains underlying western New Guinea; Palaeozic–Mesozoic Australian continental crust (87Sr/86Sr = 0.719594 to 0.710921; εNd = −13.85 to 1.373); thinned transitional crust intruded by Miocene–Pleistocene magmatic rocks (87Sr/86Sr = 0.706524 to 0.704019; εNd = 6.67 to 2.13); and accreted island arc crust (87Sr/86Sr = 0.704053 to 0.703759; εNd = 6.63 to 4.97). These data, together with crustal contamination models, indicate that the northern-most extent of Australian continental crust exists beneath the northern-most section of western New Guinea. We also combined our isotopic data with existing data across New Guinea and used these to develop an isotopic map that shows the position of the ancient Australian–Pacific Plate boundary, producing results that are also consistent with broad-scale seismic tomography imagery. Our findings provide a framework for mapping other plate boundaries, particularly ancient systems where only fragmentary data exist.

Tectonic and sedimentary history of the neoproterozoic metavolcanic–volcaniclastic rocks of the El-Dabbah Group, Central Eastern Desert, Egypt

The El-Dabbah Group is a >7500 m-thick sequence of metavolcanic rocks, volcaniclastics, and banded iron-formation located within the Nubian Shield, Central Eastern Desert, Egypt. The sequence preserves lower greenschist-facies island arc rocks. We used detailed mapping to reconstruct the stratigraphic and tectonic history of the El-Dabbah Group and the surrounding sedimentary basins. The El-Dabbah Group was identified as deposition at central rift zone of an oceanic island arc and was overlies by the subaerial sedimentary rocks (the Atshan Formation and the Hammamat Group). Four faulting were distinguished within the El-Dabbah area: (1) top-to-the-S low-angle thrusting; (2) NNE side down normal faulting; (3) sinistral displacement on north–south faults; and (4) WNE trend N-side-down normal faulting with the Hammamat Group. We obtained the crystallization ages of a granite (638.1 ± 2.9 Ma) and a quartz porphyry (659.6 ± 3.0 Ma). The detrital zircon age of the Hammamat Group show peaks at 650, 680, and 790 Ma. These evidences suggest, three sedimentation-deformation stages were identified in this area. Stage 1 involved sedimentation of island-arc-derived volcaniclastic rocks at 770-700 Ma and to thrust related accretion at 700–680 Ma. Stage 2 involved collisional and transtensional deformation related to formation of the subaerial basin that contains the Atshan Formation at 670-640 Ma, the normal fault, the strike-slip fault and granite intrusion during 660–620 Ma. Stage 3 involved orogenic collapse, regional strike-slip-related sedimentation and deformation of the Hammamat Group at 610–570 Ma. We show that the El-Dabbah Group records a history of oceanic island arc accretion, collision, and collapse within the northern East African Orogeny during 770–570 Ma in Neoproterozoic.

The Global Factors of Lead–Zinc Ore Formation

The tectonic evolution of the Earth is the leading factor that accounts for the uneven distribution of lead and zinc reserves through geologic time. The cyclic changes in lead–zinc ore formation productivity reflected the periodic amalgamation of most continental crustal blocks and supercontinent formation, stabilization, and eventual breakup. Many features of the age spectra of lead and zinc reserves are caused by the gradual growth of the continental crust as a result of the accretion of island arcs to the cratonic nuclei, the widening extent of the ensialic environments of ore formation, and the increasing role of the continental crust in magma generation processes.

Global factors of lead-zinc ore formation

Tectonic evolution of the Earth is a principle global factor responsible for uneven distribution of lead and zinc reserves in geological time. Cyclic changes in productivity of lead-zinc ore-formation processes resulted from periodical amalgamation of most blocks of continental crust, formation, stabilization and final break-up of supercontinents. Many features of age spectrums of lead and zinc reserves are caused by gradual increase of volume of continental crust resulting from accretion of island arcs to ancient cratons, widening of distribution of ensialic environments of ore-formation and increasing role of continental crust in magmatic processes.

Neoproterozoic Magmatism in the Southern Arabian-Nubian Shield: Implications for Petrogenesis and Tectonic Setting

The metavolcano-sedimentary assemblages and the associated granitoid intrusions in Central Eritrea, part of the southern Arabian-Nubian Shield, were primarily formed by accretion of juvenile island arcs during the Neoproterozoic. The metavolcanic rocks comprise mafic to felsic rocks, which show tholeiitic affinities. They exhibit variable SiO2 (41.63–78.4 wt%), Al2O3 (8.51–23.41 wt%), MgO (0.05–13.98 wt%), Fe2O3 (3.11–15 wt%), Cr (2.83–834 ppm), Ni (1.63–244 ppm), and Zr (16.4–128 ppm) that form linear correlations. They have low ΣREE (20–144 ppm) and display flat chondrite-normalized REE patterns with negative and positive Eu anomalies (Eu/Eu* = 0.67–1.36). They are generally rich in LILE (Ba, Pb, and U) and depleted in HFSE (Ta, Nb, and Ti), and have low initial 87Sr/86Sr (0.70079–0.70525), positive eNd values (+ 4.0 to + 6.4) and moderate Pb isotope compositions (206Pb/204Pb = 17.598–19.446, 207Pb/204Pb = 15.518–15.639, and 208Pb/204Pb = 37.175–38.811). These lines of chemical evidence indicate that slab-derived fluids and melts significantly modified the mantle source. In contrast, the ca. 850 Ma Emba-Derho granites display restricted compositions which show high SiO2 (71.28–73.64 wt%), Al2O3 (14.93–15.89 wt%), Na2O (3.16–4.15 wt%), and low K2O (1.25–2.04 wt%). These samples exhibit highly fractionated REE patterns (e.g., La/Yb = 14–30), high Sr/Y (42–86), and low Y (3.75–6.03 ppm), as well as high eNd (+ 4.7 to + 7.8), suggesting that they were partial melts of the subducting oceanic crust. Both the metavolcanic rocks and the granitic intrusions are suggested to have been formed in a forearc setting.

Geodynamic setting and models of the emplacement of ophiolitic allochthons in the structure of accreted island arcs of the pacific north-western framing

Geodynamic setting and models of the emplacement of ophiolitic allochthons in the structure of accreted island arcs of the pacific north-western framing

New Crustal Framework in the Amazon Craton Based on Geophysical Data: Evidences of Deep East-West Trending Suture Zones

The Tapajos mineral province (TMP), in the Brazilian Amazon Craton, comprises NW-SE Paleoproterozoic insular magmatic arcs accreted to the Carajas Archean Province (CAP). We present new geological and geophysical data pointing toward a different evolutionary model for the TMP. Results obtained from magnetic data indicate that NNW-SSE trending structures occur at shallow crustal levels. Furthermore, an E-W structural framework shows up at 15.4 km depth, in disagreement with the accreted island arc orientation. These E-W structures are associated with north-dipping blocks, reflecting ductile compressive tectonics, similar to the tectonic setting found in the CAP. We interpret these E-W structures of the TMP as the continuity westwards of similar structures from the CAP, under the Paleoproterozoic volcanic rocks of the Uatuma Supergroup. Based on this evidence, we propose that Paleoproterozoic arcs have been formed in an Archean active continental margin, instead of in island arcs. This novel tectonic setting for the TMP has significant implications for the tectonic evolution and the metallogenic potential of the southern portion of the Amazon craton, particularly for Paleoproterozoic magmatic-hydrothermal (epithermal and porphyry) precious and base metal systems.

Composite basement along the southern margin of the North Australian Craton: Evidence from in-situ zircon U-Pb-O-Hf and whole-rock Nd isotopic compositions

Hafnium-isotope compositions of emplacement-age zircon grains are integrated with zircon δ18O and whole-rock Nd-isotope data in order to characterise the age and composition of the sources of Paleoproterozoic granites along the southern margin of the North Australian Craton. The petrogenesis of the granitic rocks is interpreted in terms of partial melting of distinct crustal components with variable crustal residence ages, at different crustal levels, reflecting the local Paleoproterozoic crustal architecture. This finding implies a composite nature of the unexposed deep crust, which comprises Paleoproterozoic supracrustals and subordinate, discontinuous and local Neoarchean meta-igneous components. The limited lateral extension of Neoarchean deep crust does not support tectonic models for the Paleoproterozoic amalgamation of the craton that consider intracratonic and/or continent-continent collision settings. The composite nature of the basement and the presence of cryptic boundaries are consistent with the accretion of Paleoproterozoic supracrustal elements, as back-arc and island-arc, to a Neoarchean proto-craton. The dominant Paleoproterozoic reworking of Archean-derived supracrustal materials indicates that accretionary orogenesis was an important setting for crustal reworking and craton stabilisation in the Paleoproterozoic.

EARTH SYSTEM PROCESSES OF THE WALLACEA AND THEIR IMPLICATIONS TO THE INDONESIAN THROUGHFLOW AND BIOGEOGRAPHY

The Wallacea with the total land area of 347,000 km designates a biogeographical group of Eastern Indonesian islands separated by deep water straits from the Asian and Australian continental shelves, and occupies a region with the world's most complex geological evolution and the accompanying extreme endemism. It also encloses remnants of emerged and submerged lands as well as dispersed and accreted island arcs and fragments of the continents resulting from a relatively continuous Cenozoic convergence of the Indo-Australian, Eurasia and Pacific-Philippine Sea plates. This paper discusses the Cenozoic spatial and temporal evolution of terranes in the region and explores how the Earth System Processes have eventually shaped up the Indonesian Throughflow and highlights the biogeographically unique and rich Wallacea. It will be demonstrated that the Wallace's line may have inherited an evolved multi-origin Early Cenozoic deep water barrier and the present Wallacea has been the resultant of significant Neogene convergent tectonics and significant modification by glacially-related sea level and climatic changes during the Quaternary. The gradual and systematic changes of the respective ocean regimes, the corresponding oceanometeorologic evolution, the emergence and disappearance of islands and land bridges, the dispersal and docking of some terranes have all resulted in the present configuration of the areas of endemism within the Wallacea. These changes correspond with the Neogene closure of the Indonesian Seaways, a narrow passage formed by the collision of the three major plates and the vanished pre-Cenozoic Tethyan ocean formed by the otherwise connected Indian and Pacific Oceans.

Early–Middle Ordovician volcanism along the eastern margin of the Xing’an Massif, Northeast China: constraints on the suture location between the Xing’an and Songnen–Zhangguangcai Range massifs

ABSTRACTWe present new zircon U–Pb and Hf isotopic as well as whole-rock geochemical data for volcanic rocks from the eastern margin of the Xing’an Massif, Northeast China, in order to further our understanding of the suture location between the Xing’an and Songnen–Zhangguangcai Range massifs. Zircon secondary ion mass spectrometry U–Pb dating indicates that the volcanic rocks formed during the Early–Middle Ordovician (473–463 Ma). Compared with the coeval Moguqi basalts (rare earth element [REE] = 171–183 ppm; εHf(t) = +0.3 to +2.7; TDM1 = 1074–977 Ma), the Duobaoshan andesites exhibit lower overall REE abundances (109–131 ppm) with relatively high heavy REE contents, stronger high-field-strength element depletion, higher εHf(t) values (+13.0 to +14.8), and much younger TDM1 ages (559–484 Ma). This suggests that the primary magma for the andesites was generated by the partial melting of a relatively depleted mantle wedge that was metasomatized by subduction-related fluids. The primary magma for the basalts in the Moguqi area was probably derived from the partial melting of a relatively enriched lithospheric mantle that was also modified by fluids sourced from a subducted slab. These interpretations suggest that the andesites in Duobaoshan formed in a newly accreted island arc setting, whereas the coeval basalts in Moguqi formed along an active continental margin. We therefore attribute the Early–Middle Ordovician volcanism along the eastern margin of the Xing’an Massif to the northwestward subduction of the Nenjiang–Heihe oceanic plate beneath the Xing’an Massif. Furthermore, considering coeval igneous activity in the southern parts of the Xing’an Massif, we suggest that a magmatic arc existed along the margin of the Xing’an Massif in the early Palaeozoic (490–420 Ma). We conclude that the location of the suture between the Xing’an and Songnen–Zhangguangcai Range massifs runs from Airgin Sum, via south of Xilinhot, to Ulanhot, Moguqi, Nenjiang, and finally Heihe.