Late Neoproterozoic alkaline magmatism in the Arabian–Nubian Shield: the postcollisional A-type granite of Sahara–Umm Adawi pluton, Sinai, Egypt

The Sahara–Umm Adawi pluton is a Late Neoproterozoic postcollisional A-type granitoid pluton in Sinai segment of the Arabian–Nubian Shield that was emplaced within voluminous calc-alkaline I-type granite host rocks during the waning stages of the Pan-African orogeny and termination of a tectonomagmatic compressive cycle. The western part of the pluton is downthrown by clysmic faults and buried beneath the Suez rift valley sedimentary fill, while the exposed part is dissected by later Tertiary basaltic dykes and crosscut along with its host rocks by a series of NNE-trending faults. This A-type granite pluton is made up wholly of hypersolvus alkali feldspar granite and is composed of perthite, quartz, alkali amphibole, plagioclase, Fe-rich red biotite, accessory zircon, apatite, and allanite. The pluton rocks are highly evolved ferroan, alkaline, and peralkaline to mildly peraluminous A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO₂, Na₂O + K₂O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, and rare earth elements (REE) and low CaO, MgO, Ba, and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal sources that has gone through a continent–continent collision (postorogenic or postcollisional), with minor contribution from mantle source similar to ocean island basalt. The assumption of crustal source derivation and postcollisional setting is substantiated by highly evolved nature of this pluton and the absence of any syenitic or more primitive coeval mafic rocks in association with it. The slight mantle signature in the source material of these A-type granites is owed to the juvenile Pan-African Arabian–Nubian Shield (ANS) crust (I-type calc-alkaline) which was acted as a source by partial melting of its rocks and which itself of presumably large mantle source. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti, and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. Crystallization temperature values inferred average around 929°C which is in consistency with the presumably high temperatures of A-type magmas, whereas the estimated depth of emplacement ranges between 20 and 30 km (upper-middle crustal levels within the 40 km relatively thick ANS crust). The geochronologically preceding Pan-African calc-alkaline I-type continental arc granitoids (the Egyptian old and younger granites) associated with these rocks are thought to be the crustal source of f this A-type granite pluton and others in the Arabian–Nubian Shield by partial melting caused by crustal thickening due to continental collision at termination of the compressive orogeny in the Arabian–Nubian Shield.     

Tonian–Ediacaran Tectonometamorphic History of the Sa’al Complex, Sinai (Egypt): Implications for the Tectonostratigraphic Framework of the Northern Arabian–Nubian Shield

The Sa''al Metamorphic Complex (SMC; southern Sinai) encompasses the oldest arc rocks in the Arabian–Nubian Shield, comprising two non-consanguineous metavolcanic successions (the Agramiya Group and the Post-Ra''ayan Formation) separated by the metasediments of the Ra''ayan Formation. It experienced three distinct deformational events (D1–D3) and two low-medium grade regional metamorphic events (M1–M2). The Agramiya Group and the Ra''ayan Formation experienced all tectonometamorphic events (D1–D3 and M1–M2), whereas the Post-Ra''ayan volcanic rocks were only affected by the D3 and M2 events. D1 is an extensional event and is connected to the late Rodinia break-up (~Tonian; 900–870 Ma). The M1 metamorphism variably affected the older Agramiya Group, the rhyolitic tuffs experiencing lower to upper greenschist facies conditions and the basic and intermediate volcanic rocks undergoing amphibolite facies metamorphism. The Ra''ayan Formation metasediments experienced upper greenschist to amphibolite facies metamorphism. The upper greenschist facies M2 affected the youngest Post-Ra''ayan volcanic rocks and other stratigraphic successions. The compressive D2 and D3 events were coeval with the accretion of dismembered terranes in the assembly of Gondwana. D2 can be linked to the Tonian–Cryogenian arc-arc assembly (~880–760 Ma; in Elat and Sinai), whereas D3 and the accompanying M2 is constrained to 622–600 Ma (Ediacaran).     

Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO₂ or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between ?8.1‰ and ?6.8‰ for δ¹³C, +6.4‰ and +10.5‰ for δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO₂±CH₄±N₂) and aqueous-carbonic (H₂O-NaCl-CO₂±CH₄±N₂) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO₂-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition.     

Lu–Hf and O isotopic compositions on single zircons from the North Eastern Desert of Egypt,Arabian–Nubian Shield: Implications for crustal evolution

Neoproterozoic juvenile crust is exposed in the Eastern Desert of Egypt, between the Nile and the Red Sea, forming the basement to Cambrian and younger sedimentary strata in the northernmost part of the Arabian–Nubian Shield (ANS). In order to reveal how the crust of this vast region was formed, four examples of widespread Neoproterozoic (653–595 Ma) calc-alkaline and alkaline intrusive rocks in the northwestern most exposures, in the NE Desert of Egypt (NED) were studied. Single zircon Hf–O isotopic compositions of these intrusives were used to characterize the Neoproterozoic syn- and post-collisional granitoids in the NED. The ~ 653 Ma Um Taghir syn-tectonic granodiorite (I-type) displays isotopic characteristics of a depleted mantle source, such as high εHf(t) (+ 9.1 to + 11.2) and mantle δ¹⁸O (mean = + 5.12‰). In contrast, the ca. ~ 600 Ma post-collision A-type granites (Al-Missikat, Abu Harba, and Gattar) show slightly higher δ¹⁸O values (+ 5.15 to 6.70) and slightly lower εHf(t) values (+ 6.3 to + 10.6, mean = + 8.6). We interpret these isotopic data to reflect melting of a juvenile Neoproterozoic mantle source that assimilated slightly older Neoproterozoic crustal material during magma mixing. The involvement of crustal component is also supported by Hf-crustal model ages (0.67–0.96 Ga) and by the occurrence of xenocrystic zircons with U–Pb ages older than the crystallization ages, indicating melting of predominantly Late Neoproterozoic crustal protoliths.     

An example of post-collisional appinitic magmatism with an arc-like signature: the Wadi Nasb mafic intrusion,north Arabian–Nubian Shield,south Sinai,Egypt

We present new data for the Neoproterozoic mafic intrusion exposed in Wadi Nasb, south Sinai, Egypt (northernmost Arabian–Nubian Shield; ANS). The Nasb mafic intrusion (NMI) intrudes metasediments, Rutig volcanics, and diorite/granodiorite, and is intruded in turn by younger monzogranite and quartz-monzonite. Available geochronological data for the country rocks of the NMI provide a tight constraint on its age, between 619 and 610 Ma, during the hiatus between the lower and upper Rutig volcanics. The NMI is neither deformed nor metamorphosed, indicating post-collisional emplacement, and uralitization by late-magmatic and sub-solidus alteration is restricted to the margins of the intrusion. A quantitative fractionation model indicates a fractionating assemblage of 61% primary amphibole, 10% clinopyroxene, 28% plagioclase, 1% biotite, 0.4% apatite, and 0.15% Fe-Ti oxide. Contrary to the recent studies, we find that the nearby diorite of Gebel Sheikh El-Arab is not co-genetic with the appinitic gabbro of the NMI. Although there are volcanic xenoliths in the NMI, we find no chemical evidence requiring contamination by continental crust. A subduction-related signature in a post-orogenic intrusion requires the inheritance of geochemical tendencies from a previous subduction phase. Given that the fine-grained gabbro of the NMI is consistent with a near-primary mantle melt, we attribute this inheritance to persistence and later melting of the slab-modified mantle domains, as opposed to partial melting and assimilation of the juvenile continental crust. The fine-grained gabbro composition indicates derivation at temperature and pressure conditions similar to the sources of mid-ocean ridge basalts: mantle potential temperature near 1350°C and extent of melting about 7%. Such temperatures, neither so high as to require a plume nor so low as to be consistent with small degrees of melting of a volatile-rich source, are most consistent with a lithospheric delamination scenario, allowing the upwelling of fertile, subduction-modified asthenosphere to depths ≤50 km.     

The Wadi Zaghra metasediments of Sinai,Egypt: new constraints on the late Cryogenian–Ediacaran tectonic evolution of the northernmost Arabian–Nubian Shield

ABSTRACT

The La Tinta mélange is a small but singular ultramafic mélange sheet that crops out in eastern Cuba. It is composed of dolerite-derived amphibolite blocks embedded in a serpentinite matrix. The amphibolite blocks have mid-ocean ridge basalt (MORB)-like composition showing little if any imprint of subduction zone component, similar to most forearc and MOR basalts worldwide. Relict Cr-spinel and olivine mineral chemistry of the serpentinized ultramafic matrix suggest a forearc position for these rocks. These characteristics, together with a hornblende ⁴⁰Ar/³⁹Ar age of 123.2 ± 2.2 Ma from one of the amphibolite blocks, suggest that the protoliths of the amphibolite blocks correspond to forearc basalt (FAB)-related rocks that formed during the earlier stage of subduction initiation of the Early Cretaceous Caribbean arc. We propose that the La Tinta amphibolites correspond to fragments of sills and dikes of hypoabyssal rocks formed in the earlier stages of a subduction initiation scenario in the Pacific realm (ca. 136 Ma). The forearc dolerite-derived amphibolites formed by partial melting of upwelling fertile asthenosphere at the beginning of subduction of the Proto-Caribbean (Atlantic) slab, with no interaction with slab-derived fluids/melts. This magmatic episode probably correlates with Early Cretaceous basic rocks described in Hispaniola (Gaspar Hernandez serpentinized peridotite-tectonite). The dikes and sills cooled and metamorphosed due to hydration at low pressure (ca. 3.8 kbar) and medium to high temperature (up to 720ºC) and reached ca. 500ºC at ca. 123 Ma. At this cooling stage, serpentinite formed after hydration of the ultramafic upper mantle. This process might have been favoured by faulting during extension of the forearc, indicating an early stage of dike and sill fragmentation and serpentinite mélanges formation; however, full development of the mélange likely took place during tectonic emplacement (obduction) onto the thrust belt of eastern Cuba during the latest Cretaceous.     

Neoproterozoic diamictite in the Eastern Desert of Egypt and Northern Saudi Arabia: evidence of ~750 Ma glaciation in the Arabian–Nubian Shield?

The Neoproterozoic Atud diamictite in Wadi Kareim and Wadi Mobarak in the Eastern Desert of Egypt and the Nuwaybah formation in NW Saudi Arabia consist of poorly sorted, polymictic breccia, with clasts up to 1 m of granitoid, quartz porphyry, quartzite, basalt, greywacke, marble, arkose, and microconglomerate in fine-grained matrix. Stratigraphic relations indicate that the diamictite was deposited in a marine environment. Integrated field investigation, petrographic study and U–Pb SHRIMP zircon ages demonstrate that the Atud and Nuwaybah are correlative. The distribution of zircon ages indicate that ~750 Ma ages are dominant with a significant component of older materials, characterized by minor Mesoproterozoic and more abundant Paleoproterozoic and Neoarchean ages. Some matrix and metasedimentary clast zircons yield ages that are a few 10s of Ma younger than the age of the youngest clast (754 ± 15 Ma), suggesting Atud/Nuwaybah diamictite deposition ~750 Ma or slightly later, broadly consistent with being deposited during the Sturtian glaciation (740–660 Ma). The Paleoproterozoic and Neoarchean clasts have no source within the ensimatic Arabian–Nubian Shield. The distribution of the pre-Neoproterozoic ages are similar to the distribution of the pre-Neoproterozoic ages in Yemen and Saharan Metacraton, suggesting that these clasts have been transported hundreds of kilometers, maybe by ice-rafting. The Atud diamictite may represent important evidence for Cryogenian “Snowball Earth” in the Arabian–Nubian Shield.     

Geochemistry of the sandstones of Punagarh basin: Implications for two source terranes and Arabian - Nubian connection of Aravalli craton?

Sandstones of Punagarh basin of Trans Aravalli region, NW Indian shield were analyzed for their major and trace element contents in conjunction with petrographic modes. The Punagarh basin comprises four formations (Sojat, Bambolai, Khamal and Sowania), amongst which the Sojat Formation in uncoformable contact with overlying formations, is significantly older and enjoys separate status as rest of the formations constitute Punagarh Group. Petrographic attributes suggest that Sojat sandstones contain distinct modal abundances like high content of quartz and low content of feldspar, mica, matrix, rock fragments and cement. In general there is a decrease in the average modal abundance of quartz and mica with concomitant increase of feldspar, chert and rock fragments from base to top in Punagarh sandstones. Sojat sandstones are also geochemically distinct as they possess high SiO₂/Al₂O₃, Th/U and Cr/Th ratios coupled with lowest Na₂O/K₂O ratio and Zr content, least fractionated LREE, more fractionated HREE and largest Eu* anomaly compared to Punagarh sandstones. The weathering indices suggest intense chemical weathering for Sojat sandstones and low to moderate for Punagarh sandstones. Compositionally all the sandstones of the Punagarh basin come under the category of quartzarenite. Compared to PAAS and UCC, both suite of sandstones are generally depleted in REE, HFSE (with exception of Zr, Hf and Ta), and enriched in ferromagnesian trace elements particularly Cr and Co. The chemical data indicate that the sediments were derived from the source(s) of mixed felsic - mafic composition, with the former being dominant. The source rocks were granites, TTG, basalts, and rhyolites. The Sojat sandstones owe their source form Archean crust while Punagarh sandstones got detritus from Mesoproterozic crust. Immobile element ratios and REE abundances of Sojat sandstones closely match with Paleoproterozoic metagreywackes of Ghana of African craton while Punagarh sandstones show near geochemical characteristic with Gogunda, Kumbalgarh and Vindhyan quartzites of Aravalli craton. This geochemical similarity of Sojat sandstones provides credence to the hypothesis that Trans–Aravalli region of India had been an integral part of Arabian - Nubian shield. The chemical data advocate the deposition of these sandstones in a tectonic setting comparable to modern back arc setting.     

Tectonically-driven underfilled–overfilled cycles,the middle Cretaceous in the northern Cordilleran foreland basin

It is shown that the middle Cretaceous succession in the northern Cordilleran foreland basin consists of several-million-year tectonically-driven cycles comprising two components: strata deposited in an underfilled basin with a prominent forebulge zone and strata deposited in an overfilled basin lacking evidence of a forebulge. The episodic thrusting of the Cordilleran orogenic wedge and its rich sediment supply to the basin are two main controlling factors for the formation of these cycles. A qualitative model of several-million-year tectonically-driven underfilled–overfilled cycle for migration and stratigraphic fill in this basin is proposed. During the early underfilled period (orogenic loading period), due to orogenic loading of emplaced thrust sheets, flexural subsidence is created in the region proximal to the mountain belt and a prominent forebulge is developed. During the late underfilled period (early orogenic unloading period), as the cratonward migration of the subsidence center of sediment loading in the foredeep zone, forebulge zones and backbulge zones migrate cratonwards, forming a diachronic erosion surface in the central basin. During the overfilled period (late orogenic unloading period), a prominent erosion forms in the proximal basin and a peripheral sag develops above the forebulge area of the previous underfilled period. This model may provide a pattern to subdivide sedimentary successions in the Cordilleran foreland basin. Using this model, alternative interpretations are suggested for some important, but controversial stratigraphic phenomena in the Cretaceous Cordilleran foreland basin: traditionally defined eustatic highstands, wide sedimentation area of the basin, erosion surfaces and widespread subtle topographic uplifts in the central basin, high-frequency coarsening-up cycles, extensively distributed erosive-based sandstones and conglomerates enclosed in marine mudstones.     

Nd–Sr–Hf–O isotope provinciality in the northernmost Arabian–Nubian Shield: implications for crustal evolution

Multi-isotope study including whole-rock Nd–Sr, single zircon Hf, and SIMS δ¹⁸O analyses of zircons sheds light on magma sources in the northernmost Arabian–Nubian Shield (ANS) during ~820–570 Ma. Reconnaissance initial Nd and Sr isotope data for the older rocks (~820–740 Ma) reaffirms previous estimates that early crustal evolution in this part of the shield involved some crustal contamination by pre-ANS material. Prominent isotope provinciality is displayed by post-collisional calc-alkaline and alkaline igneous rocks of ~635–570 Ma across a NW-SE transect across basement of the Sinai Peninsula (Egypt) and southern Israel. Silicic rocks of the NW-region are characterized by lower εNd(T)–εHf(T) and higher Sr_i and δ¹⁸O compared with rocks of the SE-region, and the transition between the regions is gradual. Within each region isotope ratios are independent of the extent of magma fractionation, and zircon cores and rims yield similar δ¹⁸O values. Comparison with southern segments of the ANS shows that the source for most ~635–570 Ma rocks can be modeled as the isotopically aged lower-intermediate crust in the ANS core (SE-region) and its northern, more contaminated ANS margins (NW-region). Nevertheless, Nd–Sr isotope enrichment of the lithospheric mantle is indicated by some basic magmas of the NW-region displaying the most enriched Nd–Sr isotope compositions. Comparison of Nd and Hf depleted mantle model ages for rocks of the SE-region may indicate that crustal formation events in the ANS geographical core took place at 1.1–1.2 Ga and were followed by crustal differentiation starting at ~0.9 Ga.     

Facies and depositional sequences of the Late Pleistocene Göçü shoreline system,Konya basin,Central Anatolia: Implications for reconstructing lake-level changes

Well-developed coarse-grained palaeo-shoreline deposits are found along the rising margins of the Konya basin, marking the former extent of a now desiccated Late Pleistocene lake. This study evaluates the depositional environments and the sequential evolution of a shoreline system that developed at the northern margin of the Konya palaeolake near Göçü. Several laterally continuous quarry sections provided an excellent opportunity for studying spatial and temporal changes of depositional environments and related lake-level fluctuations. Eight principal sedimentary facies and six major lithostratigaphic units have been identified in these deposits representing progradational and retrogradational episodes of shoreline development. The lowest sequence is an aggradational unit formed by wind-driven currents and waves in a sand-dominant lake bottom above the wave base. It is overlain by a convoluted palaeosol ¹⁴C dated to ca 28,300 bp representing a major lowering of lake levels. Following an unconformity, the next sequence is characterised by large-scale gravelly clinoforms that progressively offlap/downlap onto the underlying sequence, and correspond to progradation of a foreshore resulting from storm-originated oscillating and unidirectional currents, avalanching processes and minor subaqueous debris flows. It is overlain by an areally extensive lensoid body of structureless clays comprising a thin organic layer, abundant rootlets and freshwater mollusc shells, formed from suspension fallout in a quiet, very shallow freshwater lagoonal environment. This phase, representing a more minor lake regression, has been ¹⁴C dated to ca. 21,960–20,730 bp. The final sequences include large-scale sand waves and bars, which developed by storm-originated wave surges and strong shoreline currents, and prograding delta foresets. These sequences indicate a renewed lake transgression to higher water levels, before a final regression after 17,500 bp. Lack of tectonic deformation and the overall sedimentary characteristics of the beach system at Göçü clearly suggest that the sedimentary evolution of the system is closely related to lake-level fluctuations resulting from long- and short-term hydro-climatic changes. Successive stages of lake-level rises and large amounts of supply of coarse grained material imply a positive hydrological balance and relatively high rates of sediment discharge from the adjacent hillslopes.     

Pan-African adakitic rocks of the north Arabian–Nubian Shield: petrological and geochemical constraints on the evolution of the Dokhan volcanics in the north Eastern Desert of Egypt

The Precambrian basement of Egypt is part of the Red Sea Mountains and represents the north-western part of the Arabian–Nubian Shield (ANS). Five volcanic sections are exposed in the Egyptian basement complex, namely El Kharaza, Monqul, Abu Had, Mellaha and Abu Marwa. They are located in the north Eastern Desert (ED) of Egypt and were selected for petrological and geochemical studies as they represent the Dokhan volcanics. The volcanics divide into two main pulses, and each pulse was frequently accompanied by deposition of immature molasse type sediments, which represent a thick sequence of the Hammamat group in the north ED. Compositionally, the rocks form a continuum from basaltic andesite, andesite, dacite (lower succession) to rhyodacite and rhyolite (upper succession), with no apparent compositional gaps. These high-K calc-alkaline rocks have strong affinities to subduction-related rocks with enriched LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and negative Nb anomalies relative to NMORB. The lower succession displays geochemical characteristics of adakitic rocks with SiO₂ >53 wt%, Al₂O₃ >15 wt%, MgO >2.5 wt%, Mg# >49, Sr >650 ppm, Y <17 ppm, Yb <2 ppm, Ni >25 ppm, Cr >50 ppm and Sr/Y >42.4. They also have low Nb, Rb and Zr compared to the coexisting calc-alkaline rhyodacites and rhyolites. The highly fractionated rhyolitic rocks have strong negative Eu anomalies and possess the geochemical characteristics of A-type suites. Trace element geochemical signatures indicate a magma source consistent with post-collisional suites that retain destructive plate signatures associated with subduction zones. The adakitic rocks in the northern ANS are generated through partial melting of delaminated mafic lower crust interacting with overlying mantle-derived magma. The Dokhan volcanics were likely generated by a combination of processes, including partial melting, crystal fractionation and assimilation.     

Deformation in the Neoproterozoic Smalfjord Formation,northern Norway: an indicator of glacial depositional conditions?

Recent studies on Neoproterozoic climate change have prompted renewed interest in Neoproterozoic glacial deposits and renewed debate over the criteria used to identify the nature of glacial influence on sedimentation. Analyses of soft sediment deformation structures have provided important clues to distinguish between competing palaeoenvironmental interpretations of Quaternary glacial deposits; a similar approach is presented here in the analysis of Neoproterozoic glacial deposits of the Smalfjord Formation, northern Norway. A detailed sedimentological and structural analysis at several sites in the Varangerfjorden area reveals complex soft sediment deformation at various scales in conglomerate, sandstone and diamictite. Deformation is predominantly ductile and includes anticlinal and synclinal folding, flow noses, flame structures, recumbent folding and shear structures. The deformed sediments are associated predominantly with conglomerate and sandstone, which record glaciofluvial and deltaic depositional conditions. Some deformations can be attributed to rapid deposition and slumping, whereas others appear to record shear stress associated with overriding ice. The scale, style and range of deformation, together with the coarse-grained nature of the deformed sediments and facies associations, suggest that these were unfrozen outwash sediments that were overridden by ice and resedimented in a dynamic ice-proximal setting. Whereas recent studies of diamictite-bearing strata of the Smalfjord Formation had revealed no clear evidence of glacial influence on deposition, deformation structures documented here suggest that glacial conditions prevailed on the basin margin during deposition of Smalfjord Formation sediments, with sedimentary facies and deformation structures typical of temperate ice-proximal settings.     

Characteristics and genesis of diachronous Carboniferous volcano-sedimentary sequences: insights from geochemistry,petrology and U–Pb dating in the North Junggar basin,China

ABSTRACT

The subduction of oceanic lithosphere during the Carboniferous Period contributed to the formation of widely distributed subduction-related volcanic rocks within the Junggar basin. These volcanic rock associations contain significant clues for understanding the subduction of the Keramaili oceanic lithosphere and the filling of the remnant oceanic basin. Here, we report regional gravity and magnetic data, petrology, geochemistry, and U–Pb dating for Carboniferous volcanic rocks from the North Junggar basin (NJB). Using samples from well Y-1, we distinguish upper and lower volcanic sequences on the basis of selected geochemical data. An isochronous stratigraphic framework of Carboniferous volcano-sedimentary sequences is then constructed and the petrogenesis of these volcanic rocks is discussed. Finally, we propose an explanation for the genesis of these diachronous Carboniferous volcano-sedimentary sequences. The results show that various volcanic rocks are distributed in different areas of the NJB, and mainly consist of calc-alkaline basalt–andesite–dacite assemblages and alkaline basalt–basaltic andesite–andesite assemblages. The geochemical data also demonstrate a binary nature of the Carboniferous volcanic rocks. In the eastern NJB, the lower and upper volcanic sequences are formed during the early and late Carboniferous, respectively. However, all of these volcano-related sequences in the western of the NJB are formed during the late Carboniferous. These volcano-sedimentary sequences exhibit a ‘ladder-style’ of temporospatial evolution from east to west. The geochemical results also indicate that the upper volcanic rocks include island arc components formed in an extensional setting, whereas the lower volcanic rocks were derived from deep crustal cycling metasomatism by various mantle components in a continental arc environment. Earlier closure of the Keramaili oceanic basin and slab roll-back of the Junggar oceanic lithosphere in eastern versus western Junggar basin led to the formation of these diachronous volcano-sedimentary sequences.     

U–Pb and Lu–Hf isotopes in detrital zircon from Neoproterozoic sedimentary rocks in the northern Yangtze Block: Implications for Precambrian crustal evolution

A combined study of Lu–Hf isotopes and U–Pb ages for detrital zircons from sedimentary rocks can provide information on the crustal evolution of sedimentary provenances, and comparisons with potential source regions can constrain interpretations of paleogeographic settings. Detailed isotopic data on detrital zircons from Neoproterozoic sedimentary rocks in the northern part of the Yangtze Block suggest that these rocks have the maximum depositional ages of ~ 750 Ma, and share a similar provenance. In their source area, units of late Archean (2.45 to 2.55 Ga) to Paleoproterozoic (1.9 to 2.0 Ga) U–Pb ages made up the basement, and were overlain or intruded by magmatic rocks of Neoproterozoic U–Pb ages (740 to 900 Ma). Hf isotopic signatures of the detrital zircons indicate that a little juvenile crust formed in the Neoarchean; reworking of old crust dominates the magmatic activity during the Archean to Paleoproterozoic, while the most significant juvenile addition to the crust occurred in the Neoproterozoic. Only the Neoproterozoic zircon U–Pb ages can be matched with known magmatism in the northern Yangtze Block, while other age peaks cannot be correlated with known provenance areas. Similar zircon U–Pb ages have been obtained previously from sediments along the southeastern and western margins of the Yangtze Block. Thus, it is suggested that an unexposed old basement is widespread beneath the Yangtze Block and was the major contributor to the Neoproterozoic sediments. This basement had a magmatic activity at ~ 2.5 Ga, similar to that in North China; but zircon Hf isotopes suggest significant differences in the overall evolutionary histories between the Yangtze and North China.     

The Taraklı Flysch in the Boyali area (Sakarya Terrane,northern Turkey): Implications for the tectonic history of the IntraPontide suture zone

In the Boyal? area, northern Turkey, the tectonic units of the ?stanbul–Zonguldak Terrane and the IntraPontide suture zone are thrust over the deposits at the top of the Sakarya Terrane, known as Tarakl? Flysch. It consists of Early Maastrichtian–Middle Paleocene turbidite and mass-gravity deposits, whose source mainly corresponds to the ?stanbul–Zonguldak Terrane, and, with a lesser extent, to the IntraPontide suture zone. These deposits were sedimented in a foredeep basin developed during the convergence between Sakarya and Eurasian continental microplates. In the Late Paleocene–Early Eocene time span, the Tarakl? Flysch was deformed (D1 phase) during the closure of the foredeep basin. In the Miocene time, the strike-slip tectonics (D2 phase) related to the North-Anatolian fault produced further deformations of the Tarakl? Flysch.     

Devonian volcanism in the Minusa basin in the Altai–Sayan area: geological,geochemical, and Sr–Nd isotopic characteristics of rocks

Based on geological data and the geochemical and isotopic (Sr, Nd) parameters of the Devonian volcanic associations of the Minusa basin, the main regularities of volcanism development are considered, the composition of magmatic sources is studied, and the geodynamic mechanisms of their involvement in rifting are reconstructed. The early stage of formation of the Minusa basin was characterized by intense volcanism, which resulted in differentiated and, more seldom, bimodal volcanic complexes composed of pyroclastic rocks and dolerite sills. At the late stage, only terrigenous deposits accumulated in the basin. It has been established that the basites are similar in composition and are intermediate in geochemical characteristics between intraplate rocks (OIB) and continent-marginal ones (IAB). The basites, like OIB, have high contents of all lithophile elements, which is typical of enriched mantle sources, and, like IAB, show negative anomalies of Nb, Ta, Ti, and, to a smaller extent, Rb, Th, Zr, and Hf, selective enrichment in Pb and Ba (and, sometimes, Sr), and a weak REE differentiation (7 < (La/Yb)N < 17). In contrast to the basins in other segments of the Devonian Altai–Sayan rift area, the igneous associations in the Minusa basin are characterized by a worse expressed geochemical inhomogeneity of rocks and lack of high-Ti (> 2 wt.% TiO2) basites. The Sr and Nd isotope compositions of the Minusa basites deviate from the mantle rock series toward the compositions with high radiogenic-strontium and low REE contents.This points to the melting of a mantle substratum (PREMA-type) and carbonate-rich sedimentary rocks, which were probably assimilated by basaltic magma. The correlations between the contents of trace incompatible elements in rocks with SiO2 = 53–77 wt.% testify to the assimilation of crustal substrata by parental basaltic melts and the subsequent differentiation of contaminated magmas (AFC model). We propose a model for the formation of primary melts with the simultaneous participation of magmatic sources of two types: plume and fluid-saturated suprasubductional, localized beneath the active continental margin.     

Quantifying 3D post-accretionary tectonic strain in the Arabian–Nubian Shield: Superimposition of the Oko Shear Zone on the Nakasib Suture,Red Sea Hills,Sudan

Deformed conglomeratic clasts exposed along the Neoproterozoic Nakasib Suture and the Oko Shear Zone are used to calculate three-dimensional (3D) tectonic strain associated with the latter to quantify strain associated with post-accretionary deformational belts in the Arabian–Nubian Shield. The Nakasib Suture is a NE-trending fold and thrust belt that is sinistrally offset (∼10 km) by the cross-cutting NNW- to NW-trending strike-slip faults of the Oko Shear Zone. The Nakasib Suture was formed as a result of collision between the Haya terrane and the Gebeit terrane at ∼750 Ma ago. The Oko Shear Zone was subsequently formed as a result of an E–W directed shortening of the Arabian–Nubian Shield due to collision between East and West Gondwana at ∼670–610 Ma ago. This analysis indicates the following: (1) The Nakasib Suture is dominated by flattening strain with the flattening plane of the associated strain ellipsoid oriented at 21°/77°SE. This flattening deformation is interpreted to be associated with nappe emplacement from north to south. (2) Some regions along the Nakasib Suture are characterized by constriction strain that might be due to refolding of the early nappes about NE-trending axes. (3) The Oko Shear Zone is characterized by constriction strain, with the XY plane of the strain ellipsoid oriented at 171°/68°E. The strain ellipsoid associated with the Oko Shear Zone manifests superimposition of E–W shortening on the NE-trending fold and thrust belt associated with the Nakasib Suture. (4) The tectonic strain of the Oko Shear Zone, superimposed over the structures of the Nakasib Suture, is characterized by a strain ellipsoid whose flattening plane is oriented at 21°/49°W. The strain ellipsoid of the tectonic strain has a major axis with a quadratic elongation of 3.6 and an orientation of 357°/25°, an intermediate axis with a quadratic elongation of 1.2 and an orientation of 231°/30°, and a minor axis with a quadratic elongation of 0.25 and an orientation of 115°/18°. This suggests that the post-accretionary deformation of the Arabian–Nubian Shield was superimposed as a NW–SE directed shortening that created early N–S shortening zones and late NW-trending sinistral strike-slip faults.