查看更多>>摘要：Wadi Ghadir ophiolite in the Egyptian Eastern Desert, which forms the northern part of the Nubian Shield, is considered as one of the best-preserved segments of the Neoproterozoic oceanic lithosphere on Earth. Primary melt calculations and thermodynamic modeling of new geochemical data for the Wadi Ghadir pillow lavas and dike complexes, integrated with comprehensive petrographic investigations, are here employed to unravel the geodynamic evolution of this ophiolitic section.& nbsp;Whole-rock geochemical and clinopyroxene data indicate that the pillow lavas and sheeted dike complexes preserve geochemical signatures of N-MORB, E-MORB and OIB-like basalts. Less abundant, discrete dikes have a distinctly LILE-enriched island arc tholeiite (IAT) composition. The pillow lavas and sheeted dike complexes replicate mixed liquid lines of descent, whereas the discrete dikes exhibit a calc-alkaline differentiation trend. The pillow lavas were produced by limited (5-7%) mantle partial melting as a result of isothermal decompression over a wide pressure range (5 - 19 kbar) and restricted mantle potential temperatures (Tp & AP;1260-1300 C), suggesting ponding of rising melts from depths of ~60 to 15 km in dry mantle. Primary melts of the sheeted dike complexes were produced by 9-11% partial melting at P & AP;9.7 & PLUSMN; 2.5 kbar, Tp & AP;1290 C, and P & AP;14.7 & PLUSMN; 1.4 kbar, Tp & AP;1325 C, suggesting a role of varying hydrostatic pressure. The magma source for the discrete dikes occurred at 10.7 & PLUSMN; 0.6 kbar and Tp of 1230-1300 C by 11-13% partial melting of a metasomatized mantle region.& nbsp;The calculated Tp ranges for the different melts are consistently lower than temperatures of sub-ridge ambient mantle. Together with the variable concentrations of K2O and other incompatible elements, the estimated low temperatures of melt generation emphasize mixed sources and wet peridotite melting. The modeled primary melts designate crustal growth during back-arc basin opening and closure. During the basin opening, decompression melting of mildly enriched mantle produced high-Ti tholeiitic (pillow lavas) and transitional (sheeted dike complexes) melts, whereas subduction initiation during basin closure promoted melt-peridotite interaction. Lowering of the solidus by subduction-related components triggered melting of a spinel lherzolitic mantle and produced low-Ti calc-alkaline melts that sourced the discrete dikes. The mixed MORB and SSZ geochemical characteristics of the studied volcanic rocks, coupled with the various modeled melts and lack of significant thermal anomalies in the mantle conditions, are here interpreted as manifestations of mantle source heterogeneity in a marginal oceanic basin during the accretionary stages of the Arabian-Nubian Shield.
Lei, LidanOuyang, GuangZhang, ZihuDodd, Matthew S....
13页
查看更多>>摘要：The Ediacaran Period (-635-539 Ma) witnessed the largest negative inorganic carbon isotope (delta C-13(carb)) excursion in Earth's history (i.e., the Shuram Excursion), which is characterized by decoupling from the organic carbon isotope (delta C-13(org)) record. The cause(s) of this event remains highly debated. Here, we report a major (-8-9-Myr-long) episode of strong barium (Ba) accumulation during the Shuram Excursion in the form of barite, as recorded in the Ediacaran Doushantuo Formation (-635-551 Ma) of South China. The inner-shelf Zhangcunping section exhibits minimal Ba enrichment, while the slope Siduping section shows maximal Ba enrichment with the intrashelf basinal Jiulongwan section in the middle. The Siduping section contains-5 mu m-diameter, ellipsoidal barite crystals of marine origin; and the Jiulongwan section contains large (> 50 mu m), euhedral barite crystals and cements that are partly replaced by pyrite, pointing to a diagenetic origin with barite formation within the sulfate-methane transition zone. The barite delta S-34 is-10%o higher than delta S-34 of carbonate-associated sulfate at Jiulongwan in contrast to similar values of these two components at Siduping, suggesting a limited influence of methane oxidation, if any, on the formation of Shuram Excursion at Jiulongwan. The Ce/Ce* exhibits a lateral gradient among study sections which is reverse to the Ba enrichment, supporting the hypothesis that local surface-water productivity controlled dissolved oxygen levels in the Ediacaran surface ocean. Based on these findings, we attribute the highest Ba enrichment at Siduping to oceanic upwelling which enhanced local marine productivity. Furthermore, we propose that episodic oceanic upwelling in the Ediacaran shelf regions likely transported phosphorus and dissolved organic carbon (DOC) to shallow waters, increasing their productivity and facilitating the oxidation of DOC, contributing to the largest negative carbon isotope excursion in Earth's history.
查看更多>>摘要：Morphologically diverse multicellular algae preserved with anatomical details are recovered from the chert nodules in the member II of the early Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China. They are taxonomically identified as Archaeophycus yunnanensis, Sarcinophycus papilloformis, Thallophyca ramosa, Thallophyca sp., Thallophycoides phloeatus, Wengania exquisita, W. globosa, W. minuta and two unnamed species. An individual growth of spheroidal thallus is evidenced by a series of specimens of Wengania minuta. Archae-ophycus yunnanensis and Sarcinophycus papilloformis are possibly green algae, while the others are interpreted as red algae. The radiation of multicellular algae likely enhanced the efficiency of the biological pump, influencing the ocean ecology and environment during the early Ediacaran.
查看更多>>摘要：The Precambrian basement of the Lhasa terrane provides important information for understanding the formation and evolution of the Tibetan Plateau. However, due to the paucity of exposure, the properties of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon U-Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses in the Milin area of the southeastern Lhasa terrane, southern Tibet. Inherited magmatic zircon cores of these gneisses yield protolith crystalline ages of 1556-1517 Ma, whereas the zircon rims give metamorphic ages of 1197-1192 Ma, both could be local provenance for the Mesoproterozoic detrital zircons in the Neoprotemzoic-Paleozoic strata in the Lhasa terrane. Geochemical data indicate that the protoliths of the Mesoproterozoic rocks are continental arc-like granitoids with bulk-rock epsilon(Hf)(t) values of -3.7 to +0.7 (T-DM2 = 1.99-2.34 Ga) and zircon core epsilon(Hf)(t) values of +1.8 to +8.6 (T-DM2 = 1.71-2.17 Ga), suggesting their most likely derivation from partial melting of Paleoproterozoic crustal materials. We propose that the Mesoproterozoic gneisses in the Milin area represent the Precambrian metamorphic basement of the Lhasa terrane, likely related to the growth of Columbia supercontinent. Local exposure of the Precambrian metamorphic basement may be caused by crustal rapid uplift and erosion in the late Cenozoic and their inhomogeneous distribution.
Sotiriou, PaulPolat, AliWindley, Brian F.Kusky, Timothy...
31页
查看更多>>摘要：Temporal variations in the incompatible trace element geochemistry of volcanic rocks in Archean greenstone belts have major implications for the style of tectonics that operated in the early Earth, and if and when plate tectonic processes occurred in the Archean, which are still subjects of substantial debate. Comparing the geochemistry of Archean volcanic rocks with that of Phanerozoic arc volcanic rocks has the potential to shed light on these questions. Geochemical data from 8,249 Eoarchean to Neoarchean volcanic rocks and 20,099 Phanerozoic arc volcanic rocks were compiled from the literature to address the above questions through the application of temporal incompatible trace element ratio variations, N-MORB-normalised trace element diagrams, and tectonic setting discrimination diagrams. The sampled rocks range in composition from ultramafic through basaltic and andesitic to dacitic/rhyolitic. Most of the incorporated Archean volcanic rocks were deemed to have been unaffected by significant alteration or crustal contamination in the literature and, therefore, to reflect their provenance, a feature that was corroborated by this study. Most of these Archean volcanic rocks plot in the plate margin, oceanic arc and continental arc fields in classification and tectonic setting discrimination diagrams, with the remainder plotting in the alkaline arc, mid-ocean ridge and oceanic island fields. Comparison between N-MORB-normalised trace element diagrams of volcanic rocks from well-studied Archean greenstone belts in Greenland, Canada, South Africa, China, Australia, India, Brazil and Finland and volcanic rocks from well-studied modern arcs demonstrates that their trace element patterns are remarkably similar. This indicates that the former formed in arc-related settings by modern-style plate tectonic processes that operated throughout the Archean. The Pb and Nb anomalies of most Archean volcanic rocks are fully consistent with an arc-related setting. The temporal variations in the incompatible trace element ratios of Archean volcanic rocks, coupled with their lithological associations, demonstrate that intra-oceanic arc magmatism was prominent in the Eoarchean before a shift in these ratios in the Paleoarchean signified the beginning of Andean-style continental arc magmatism between 3500 and 3200 Ma. Modern-style plate tectonic processes were a far more important contributor to Archean crustal growth and evolution than sagduction-driven vertical tectonic processes.
查看更多>>摘要：Deeply eroded collisional orogens show complex structural and inverted stratigraphic relations with juxtaposition of rock stacks from distinct crustal levels, origins and ages, hence with contrasting P-T-t paths during convergent tectonics. This paper presents petrochronological data on Paleoproterozoic (basement) and Ediacaran (cover) gneisses tectonically interleaved during the Ediacaran-Cambrian Btizios Orogeny, in southeastern Brazil. U-Pb in zircon and EPMA U-Th-Pb in monazite data, coupled with geothermobarometric data, plus Zr-in-rutile, reveal that at a first orogenic stage (ca. 530-520 Ma), these units were at distinct crustal levels. Samples within the Paleoproterozoic basement show metamorphic near-peak conditions of similar to 800 degrees C and 10 kbar, at a depth of c.37 km. Contrastingly, Ediacaran kyanite-orthoclase-garnet-biotite granulite with retrometamorphic sillimanite (cover) reached near-peak conditions of 15 kbar and 818 degrees C-785 degrees C at depths of c. 55 km, in high-pressure granulite facies. This deep burial of Ediacaran sediments in less than 20 m.y. would be compatible with a low angle subduction zone active from ca. 550 to 530 Ma. Intrusion of ca. 550 Ma tholeiitic dykes in the Paleoproterozoic gneiss indicates a high geothermal gradient for this subduction setting, which is consistent with a low subduction rate. In a second orogenic stage (ca. 520-500 Ma), these distinct stratigraphic units were placed tectonically side by side during a fast exhumation, preserving an inverted metamorphic stack. The cover underwent retrometamorphic conditions of 800 degrees C and 10 kbar on a clockwise return path due to decompression. It is proposed here that the contact between reworked units within a Paleoproterozoic continental crust and Ediacaran magmatic and sedimentary units represent the suture of an Ediacaran NW-subduction of the Angola continental paleomargin below the Oriental Terrane of the Ribeira belt. This paper reports the highest pressure recorded in Ediacaran-Cambrian metamorphic rocks from the Brasiliano belts along the actual South Atlantic continental margins. The suture we propose here is aligned along strike, with a medium to high-pressure Ediacaran metamorphic occurrence 700 km to the SW, in the Curitiba Terrane.
查看更多>>摘要：The Central Asian Orogenic Belt is sandwiched between the Siberian, East European (Baltica), Tarim, and North China cratons, which contain several Precambrian micro-continental blocks, such as the Kazakhstan-Yili-Central Tianshan blocks, the Junggar Block and the Amuria Block. The tectonic nature and positions of these micro continental blocks are unclear and disputed in many supercontinent reconstructions. In particular, the position of the Amuria Block in the eastern Central Asian Orogenic Belt, which is important for the reconstructions of the Precambrian Rodinia supercontinent, remains unknown because of the lack of late Palaeoproterozoic to Neoproterozoic geological and geochronological data. The Erguna Block, a part of the Amuria Block in NE China, has been considered as a constituent block of the Rodinia supercontinent. However, the late Palaeoproterozoic to Neoproterozoic tectonic evolution of the Erguna Block with respect to the assembly and break-up of Rodinia remains poorly understood. Here we present zircon U-Pb-Hf-O isotopic data and whole-rock elemental data of the newly discovered late Palaeoproterozoic and Neoproterozoic records in the northern Erguna Block. The gneissic granites have protolith and metamorphic ages of 1839 & PLUSMN; 24 and 497 & PLUSMN; 6 Ma, respectively, and relatively uniform two-stage Hf model ages (TDM1) of 2.67-1.26 Ga, indicating that the Erguna Block has a Palaeoproterozoic crystalline basement. The zircon U-Pb ages for the Neoproterozoic intrusive rocks define two stages of magmatism in early Neoproterozoic (Stage 1; i.e., ca. 946 Ma) and middle Neoproterozoic (Stage 2; i.e., 833-737 Ma), respectively, implying that the Erguna Block had switched to an intra-continental extensional setting (833-737 Ma) after the early Neoproterozoic (ca. 946 Ma) convergence-accretion. A comparison of data from the Erguna Block and its adjacent units reveals that the Erguna and Central Mongolia blocks were likely in a single crustal entity with the Tarim Craton associated with the Rodinia assembly at ca. 900 Ma. This composite block then detached from the Tarim Craton at ca. 800 Ma following the break-up of Rodinia, and drifted southwards to the vicinity of the Siberia Craton during the Cambrian.
查看更多>>摘要：The Trans-North China Orogen (TNCO) is suggested to form under a transition from subduction to collision from 2500 Ma to ~ 1850 Ma, and the Taihua TTG gneiss in the Xiong'ershan district is thought to have formed in an Andean-type continental arc setting or island arc setting during this geological event. However, to date, the geochemical record of the subducted oceanic slab is still lacking. This study focuses on Hf-O isotopic analyses of zircon xenocrysts from the Qi189 hornblende monzogranite, Qiyugou Au orefield, TNCO. The results reveal that zircon xenocrysts with ages from 2304 Ma to 1934 Ma generally have low-delta O-18 values from 2.66 parts per thousand & nbsp; to 3.83 parts per thousand & nbsp; and high-epsilon Hf(t) values from 2.7 to 7.8. These zircon grains were generated from remelting of high-temperature altered gabbroic rocks of the subducted oceanic slab during the transition from compression to extension. In this paper, we present new evidence for Paleoproterozoic oceanic slab subduction in the TNCO.
Ndonfack, Kevin Igor AzeudaXie, YulingZhong, RichenYomeun, Bovari Syprien...
19页
查看更多>>摘要：The Betare Oya and Woumbou-Colomine-Kette gold districts, located in the East Metallogenic Province of Cameroon, are characterized by a complex Neoproterozoic history of metamorphism, polyphase deformation, and widespread crustal anatexis and magmatism. Controversy exists on the timing of the metamorphic and magmatic events, particularly within the area of the Lom Belt, and accurate absolute ages and even geodynamic chronology pertaining to the tectono-magmatism are yet to be well constrained. New LA-ICP-MS U-Pb zircon ages, Lu-Hf isotope data, and whole-rock geochemistry of granitoids and metavolcanic-metasedimentary rocks from the Woumbou-Colomine-Kette and Betare Oya districts better constrain the tectonic evolution of eastern Cameroon. The U-Pb age data of 646.5 +/- 2.0 Ma, 643.7 +/- 1.8 Ma, 641.3 +/- 4.4 Ma, 641.4 +/- 3.5 Ma, and 620.9 +/- 2.6 Ma revealed that the granitoids from the Woumbou-Colomine-Kette district were emplaced during two main magmatic events at ca. 650-635 Ma and ca. 620 Ma. The zircon Hf(t) values between -13.80 and -6.20 with Hf-TDM model ages of 1.9-2.4 Ga, coupled with geochemical signatures show that granitoids were generated by partial melting of Paleoproterozoic supracrustal metasedimentary units and meta-igneous rocks during subduction and orogenic convergence. Metamorphic zircons from the actinolite-chlorite schist of the Lom Belt gave ages between ca. 655 and 585 Ma, implying that the metavolcanic-metasedimentary rocks of the belt were deposited prior to 655 Ma, then metamorphosed and deformed between ca. 655 and 585 Ma. Two age peaks of detrital zircons were measured at ca. 1035 Ma and ca. 784 Ma with the epsilon Hf-(t) values of -14.60 and -18.37 and +2.97 and +8.60, respectively. These data, coupled with those from previous studies, suggest that the Lom Belt may represent a suture zone between the Central and Southern Cameroon domains with an early rifting at ca. 784 Ma. In general, our U-Pb ages indicate compressional to transpressional deformational phases in the two districts at ca. 660-635 Ma (D1), 630-610 Ma (D2), and 600-580 Ma (D3). The latter D3 transpressional event was also associated with the formation of the widespread occurrences of orogenic gold, although the precise timing of gold deposition needs to be constrained.
查看更多>>摘要：Orogen-parallel extension is common in collisional settings, but its cause is debated. The Paleoproterozoic Trans -North China Orogen (TNCO) provides insights into the process of orogen-parallel extension. We present struc-tural and geochronological data from pegmatite dikes in the Xiaoqinling region of central China to constrain the time and nature of Paleoproterozoic extension in the TNCO. The pegmatite dikes strike roughly E-W, have various dip angles, and are either deformed or undeformed. A progression from ductile to brittle deformation is commonly observed in the deformed dikes, suggesting synchronous orogenic uplift probably driven by isostatic adjustments. The deformation structures demonstrate that the intrusion of the pegmatite dikes was associated with a single phase of regional extension, thus making them syn-kinematic. Both ductile fabrics and fault-slip data from the dikes indicate that N-S (orogen-parallel) extension was responsible for dike deformation. Zircon U-Pb ages for the syn-kinematic pegmatite dikes constrain the time of extension to between 1840 and 1781 Ma, suggesting a duration of ~ 59 Myr for the orogen-parallel extension. These data suggest that the extension postdated the syn-collisional shortening in the TNCO, which occurred between 1863 and 1840 Ma. The orogen-parallel extension, the synchronous isostatic uplift, and the intense magmatism in the TNCO are consistent with a gravitational collapse model in a post-collisional setting.
NSTL
Elsevier