查看更多>>摘要：A major electrically conducting crustal structure has been imaged since the 1980 s using widely-spaced long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) sites across the Archean to Paleoproterozoic Eyre Peninsula, South Australia. The structure extends from the southern continental margin, trending SW-NE for at least 300 km. To constrain depth and lateral extent of the anomaly a transect of 39 broadband MT sites were collected with ti 2 km separation across lower Eyre Peninsula. Twodimensional inversion images a low-resistivity conductor at depth 2-10 km, 20 km wide and dipping ti 10 degrees to the east, spatially correlated with a topographic high. The conductor is bound on the eastern margin by the transpressional Kalinjala Shear Zone along the Paleoproterozoic Donington Suite, and on the western margin by the Archean Sleaford Complex. The low resistivity structure, with conductance of 20,000 S occurs in Paleoproterozoic carbonaceous marine sediments of the Darke Peak and Cleve groups that were thickened to amphibolite-grade metamorphic depths of at least 15 km and 550 degrees C during the Kimban Orogeny (1730-1690 Ma), converting carbon to flake graphite in metapelites. Tight folding and exhumation of amphibolite-grade metapelites during the Kimban Orogeny, and dextral strain along the Kalinjala Shear Zone resulted in a laterally continuous electrical crustal conductor which outcrops as economic-grade graphite deposits adjacent to Kalinjala Shear Zone mylonites. In addition, a ti 40 km wide lower crustal conductor of integrated conductance 2000 S is imaged west from the Kalinjala Shear Zone. This lower crustal conductor may also be graphitic and represents deeply buried metapelites that have not been exhumed. We argue that the Paleoproterozoic Eyre Peninsula Anomaly results from graphitisation of sediments with significantly enhanced amount of organic carbon compared to any other time in Earth's history, during the ti 2 Ga Lomagundi-Jatuli Event, which is associated with the great oxidation event.(c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Gun, ErkanPysklywec, Russell N.Gogus, Oguz H.Topuz, Gultekin...
17页
查看更多>>摘要：Terranes are passengers within drifting oceanic plate, and ride with the plate to subduction plate boundaries. Although oceanic lithosphere readily subducts into the mantle at the plate boundary, terranes may resist sinking or not, depending on a variety of controlling factors that are not very well understood. Further, the tectonic development of terranes prior to their arrival at a subduction zone is not well documented. We performed numerical experiments to explore these unknowns of terrane geodynamics during the whole pre- to post-collisional period. Our analyses reveal that terranes can undergo considerable extension prior to their arrival to subduction plate boundaries owing to the pull force exerted by sinking oceanic slabs. Increasing terrane crustal thickness, decreasing terrane width or imposed convergence velocity, and an intra-oceanic subduction setting aggrandizes the pre-collisional terrane extension. The numerical models identify increasing terrane crustal thickness and width, and decreasing imposed convergence velocity as factors that promote terrane accretion. Additionally, having a continental overriding plate at the subduction boundary increases the propensity for terrane accretion. Some intra-oceanic subduction experiments demonstrate ablative subduction and subduction polarity reversal events in connection with terrane collisions/subduction. We compare our models to the evolution of a controversial Tethyan terrane, the Nilufer oceanic plateau. Our models suggest that the Nilufer terrane may have undergone pre-collisional extension owing to the pull of the sinking Tethys Ocean plate. Further, uninterrupted subduction of the Tethys Ocean, despite the accretion of the Nilufer terrane, is illustrated by our results which have implications on other regions along the Tethyan orogenic belt. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：The record of Meso-to Neoproterozoic magmatism and sedimentation from the western Xing'an-Airgin Sum Block (XAB), eastern Central Asian Orogenic Belt (CAOB), provides the first key evidence for the close affinity between XAB and southern Laurentia, as well as the linkage with the supercontinent cycles. The ti 1.4 Ga calc-alkline granite-rhyolite association within the western XAB was probably the product of extension on a long-lived retreating convergent margin during the Columbia breakup, remarkably similar to the coeval Granite-Rhyolite Province in the southern Laurentia. Siliciclastic, mixed clastic and carbonate sequences of the Airgin Sum Group accumulated on the western XAB during two stages at 1.00-0.92 and 0.87-0.75 Ga, which were comparable with the syn-to post-orogenic successions on the southern Laurentia margin representing two dominant phases of the Grenville orogenic cycle in the Rodinia assembly. The Columbia breakup to Rodinia assembly also correlated well with the shift of zircon Hf isotope from a long-term juvenile crustal growth (1.6-1.2 Ga) to the rapid crustal reworking (1.1-0.8 Ga), and with the changes of zircon trace elements over time in the western XAB. The evergrowing new discovery of predominant 1.5-1.3, 0.9-0.8 Ga detrital zircon age peaks and coeval magmatism in many blocks worldwide suggests that they recorded much more intense magmatic activity, exhumation and deposition than previous thought. Juvenile crustal growth underestimated in previous researches, occurred extensively in blocks within the CAOB and Columbia at ti 1.5-1.3 Ga, and crust reworking significantly dominated the crustal evolution during the Rodinia assembly in 1.1-0.8 Ga.(c) 2021 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.
McCoy-West, A. J.Mortimer, N.Burton, K. W.Ireland, T. R....
18页
查看更多>>摘要：The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261-243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 +/- 2.5 Ma, 256.0 +/- 1.8 Ma, 247.8 +/- 2.7 Ma and 243.2 +/- 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/YbN < 10), unradiogenic 87Sr/86Sr(t) (0.7029 to 0.7032) and radiogenic e143Nd(t) (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (d146/144Nd = 233 ppm) compared to global plutonic rocks (d146/144Nd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km2/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：Ophiolite nappes, encompassing dismembered oceanic lithosphere sections, in the Eastern Desert of Egypt are mainly confined to Tonian-Cryogenian inter-terrane sutures. A cohesive geodynamic model of these ophiolites remains elusive in light of the variably obliterated field relationships and lack of ample geochronological information. Available geochemical data of the well-studied ophiolites lead to different interpretations of mid-ocean ridge, back-arc and/or fore-arc affinities. Relevant geochronological data are likewise heterogeneous in terms of methods and the dated lithologic units. Here, we attempt to constrain the timing and genesis of some Eastern Desert ophiolites by using new SIMS zircon U-Pb and oxygen isotope data of gabbros from four different ophiolites. The new zircon U-Pb ages and oxygen isotope values integrated with available geochemical data constrain ages of 737 & PLUSMN; 6 Ma and 720 & PLUSMN; 6 Ma and fore-arc origin of the Wadi El-Sid and Gabal Abu Dahr-Abu Siayil ophiolites, respectively. The delta O-18((Zrn)) values of these fore-arc ophiolites (+2.5 to +8.5 parts per thousand) deviate significantly from primitive mantle values, suggestive of substantial degrees of subduction-induced metasomatism. The Gabal El-Rubshi and Wadi Ghadir back-arc ophiolites returned younger and mutually similar ages (702 & PLUSMN; 5 Ma and 698 & PLUSMN; 4 Ma, respectively) and less heterogeneous delta O-18((Zrn)) values (+3.1 to +6.1 parts per thousand). Excluding outliers, most measured delta O-18((Zrn)) values are indistinguishable from the unaltered MORB values and the restricted mantle-like delta O-18 range. Results of the present study suggest multiple arc-fore-arc accretion events and an extensive late Tonian back-arc basin opening event in the evolution of the Eastern Desert accretionary belt. Subduction tectonics continued to stabilize the immature arc crust ~750-720 Ma and a tapered back-arc spreading ridge engendered at ~700 Ma. The final terrane accretion and suturing could have been associated with subduction-related melts that circulated and locally metasomatized back-arc ophiolites. This model is in broad agreement with the recently advocated ~720 Ma plate reorganization in the closure of the Mozambique Ocean during the course of Gondwana assembly. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：In order to better constrain the Late Paleozoic tectonic evolution of the SW Central Asian Orogenic Belt, we conducted field structural investigations, zircon U-Pb dating, whole-rock geochemical and Sr-Nd iso-topic studies on the meta-sedimentary rocks and intruding granites and mafic dykes in the Dahalajunshan area, southern Yili Block of NW China. Our results allow us to recognize five tectonic/mag-matic events. Field-scale and microscopic structures of the low-grade meta-sedimentary rocks showed a regional top-to-the-SE thrusting (Event 1). Undeformed granites that contain dioritic enclaves (Event 2) crosscut the regional foliations and thus post-date Event 1 after ca. 379-369 Ma. Mafic dykes intruded all these rocks after-328 Ma (Event 4-1), posterior to brittle-ductile thrusting (N-S contraction) (Event 3) and prior to normal faulting (N-S extension) (Event 4-2). All the magmatic rocks formed in a subduction-related setting. The granites and dioritic enclaves derived from a mixing of crustal-and mantle-derived sources variously modified by slab-derived hydrous melts. The primary magma of the diabase dykes derived from a mantle source modified by the slab fluids with 5% crustal contamination. Variation from regional compression (ductile and brittle-ductile thrusting) to extension (normal faulting) and increasing mantle input in arc-related magmatism reveals a transition from Early Paleozoic advancing subduction (events 1 to 3) to Late Paleozoic retreating subduction (events 4-1 and 4-2) of the Junggar oceanic plate beneath the Yili Block.(c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：Diachronous opening of the Rheic Ocean and separation of Avalonian-Cadomian terranes from Gondwana began with a change from an active to passive margin in the late Ediacaran to early Cambrian. During the Cambrian, extension within these terranes was recorded by magmatism and by the development of sedimentary basins. However, the timing, style, and kinematics of this transformation still remain poorly understood and plate-scale models vary significantly. To address this issue, the Pribram-Jince basin in the Bohemian Massif was chosen as a case study since it preserves an excellent record of Cambrian rifting. Here, after Cadomian subduction ceased at similar to 527 Ma, extension initiated and a thick pile of continental siliciclastics was deposited in the basin between similar to 515 Ma and similar to 499 Ma, interrupted by marine transgression at similar to 506-503 Ma. Our field, paleocurrent, and rock-magnetic data suggest that the source areas were located to the similar to ESE and to the similar to SW of the basin during the deposition of the lower and upper formations, respectively. Sediment sources changed accordingly from distant metamorphic basement (Gondwana?) and Cadomian volcanic arcs and an accretionary wedge underlying the basin. This redirection marked a change in the tectonic evolution of the basin from orthogonal to dextral oblique extension that enlarged the basin into a pull-apart structure. Integrating this depositional and tectonic record into a large-scale picture, we suggest that strike-slip movements along the former Avalonian-Cadomian belt controlled the diachronous opening of the Rheic Ocean. An inherited suture in the Avalonian ribbon terrane facilitated complete rifting and rift-drift transition while the Cadomian terranes remained attached to Gondwana. The kinematics of this event remains controversial. Either it was opposite along the westerly (sinistral) and easterly (dextral) segments of the belt, which may be explained by interaction with an intervening spreading center, or it was the same dextral transtension. (C) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Villares, F.Blanco-Quintero, I. F.Reyes, P. S.Proenza, J. A....
21页
查看更多>>摘要：The Tampanchi Ultramafic-Mafic Complex (TUMC), located in the central segment of the Cordillera Real (Ecuador), is an oval-shaped intrusive body of approximately 18 km2 emplaced within a Cretaceous metavolcano-sedimentary sequence. Field investigations, zircon geochronology, mineral and whole-rock elemental and isotopic compositions constrain the nature of the parental magma as well as the physical conditions of emplacement, age of crystallization and tectonic setting of formation of the TUMC. The Complex consist of wehrlite and olivine-hornblende clinopyroxenite crosscut by hornblende gabbros and minor dikes/veins of diorite and granite. Hornblendites formed mainly at the contact between the olivine-hornblende clinopyroxenite and the intrusive hornblende gabbros by reaction-replacement processes. Geochemical data for both the wehrlites and pyroxenites define a trend with dominantly olivine and clinopyroxene accumulation, whereas a second trend is formed by hornblende gabbros that differentiated to leucocratic rocks, with amphibole as the dominant fractionating mafic phase. The trace elements show enrichment in large-ion lithophile elements, depletion in high field strength elements (Nb, Zr, Ti) and P, and enrichment in Pb, Sr and Ba relative to primitive mantle, indicating subduction-related parental liquids. The Nd and Sr isotopic composition suggests a uniform mantle source metasomatized by subducted crustal components. Thermobarometric estimations constrain emplacement and crystallization at intermediate to shallow continental crust levels (~& nbsp;12.5 km depth) of hydrated basaltic melt under oxygen fugacity conditions above the nickel-nickel oxide buffer (delta NNO = 0 to 2). Zircon SHRIMP U-Th-Pb age data constrain crystallization at 75.1-76.0 Ma, with scarce inherited zircons that record limited recycling of the Cordillera Real basement. These results confirm magmatic arc activity at the continental margin of South America during the late Cretaceous as a result of an E-dipping subduction zone prior to the accretion of the Ecuadorian-Colombian-Caribbean oceanic plateau, and support simultaneous double subduction at the northwest margin of South America during the Cretaceous. (C)& nbsp;2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
查看更多>>摘要：The late Permian to Early Triassic volcano-sedimentary succession associated with the Emeishan Large Igneous Province (LIP) is well preserved in the Binchuan area, Southwest China. This paper presents coupled zircon U-Pb age, trace element, and Hf isotope analyses from the succession, together with whole-rock geochemical compositions, to reveal the origin and evolution of volcanic rocks of the LIP. Zircons from the matrix and a rhyolite clast in the Lower Triassic conglomerate bed yield identical U-Pb ages of ca. 260 Ma and have geochemical affinities to those crystallized from within-plate magmas. These features, combined with the dominance of rhyolite clasts from this horizon, imply a source related to Emeishan silicic volcanic rocks. Zircons from silicic ignimbrite and rhyolite in the upper volcanic succession of the LIP display high epsilon(Hf)(t) values (+4.2 to +12.9), low Th/Nb and U/Yb ratios, and similar epsilon(Nd)(t) values (-0.17 to +0.47) to high-Ti basalts. These chemical and isotopic characteristics are consistent with the eroded silicic volcanic rocks, indicating that the late-stage silicic volcanic rocks were generated by fractionation of high-Ti basaltic magmas without significant crustal contamination. Although the late Permian zircons from the lower and upper low-Ti basalt successions all show arc-like geochemical characteristics with high Th/Nb and U/Yb ratios, the former has much lower epsilon(Hf)(t) values (-11.7 to-3.6) than the latter (+4.4 to +11.6), and the zircons from the upper low-Ti basalt succession show positive epsilon(Hf)(t) values similar to the studied rhyolites. Samples from the lower low-Ti basalt succession have relatively higher epsilon(Nd)(t) values (-2.66 to +3.38) than those from the upper low-Ti basalt succession (-4.48 to-0.82). These geochemical features indicate the early-stage low-Ti basalts may be mainly derived from a previously enriched subcontinental mantle lithosphere, whereas the late-stage low-Ti basalts may be generated from the increasing involvement of asthenospheric mantle source with crustal contamination.& nbsp;Crown Copyright (C)& nbsp;2021 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. All rights reserved.
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