| 1. |
- Andersson, U.B., et al.
(författare)
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Characterization of the Paleoproterozoic Mantle beneath the Fennoscandian Shield : Geochemistry and Isotope Geology (Nd, Sr) of ~ 1.8 Ga Mafic Plutonic Rocks from the Transscandinavian Igneous Belt in Southeast Sweden
- 2007
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 49:7, s. 587-625
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Forskningsöversikt (refereegranskat)abstract
- Geochemistry and Sr-Nd isotope geochemistry of ca. 1.8 Ga, mafic intrusions of the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian shield were studied in southeastern Sweden. These rocks show LILE-LREE-enriched, HFSE-depleted, calc-alkaline, continental arc signatures in the north, grading to slightly less enriched, oceanic affinities southward.εNd(1.80) values range from +2.0 to +0.7 and 87Sr/86Sr(1.80) from 0.7022 to 0.7029 (with one outlier at 0 and 0.7033), without correlation to fractionation (e.g., Mg#) or crustal contamination, indicating sources that are mildly depleted. The most depleted ratios occur in the south, trending with the geochemistry toward more enriched compositions northward. The sources represent depleted mantle wedge material that was subjected to enrichment not long before (TDM ca. 2.0 Ga), i.e., during the preceding arc subduction (2.1-1.82 Ga), and/or during the TIB-1 magmatism itself, by hydrous fluids with a sediment and/or melt input increasing northward. The TIB-1 magmatism occurred above a south(west)ward-retreating subduction zone along the continental margin of the juvenile Svecofennian continent at 1.81-1.76 Ga.
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| 2. |
- Boskabadi, Arman, et al.
(författare)
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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
- 2017
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Ingår i: International Geology Review. - : TAYLOR & FRANCIS INC. - 0020-6814 .- 1938-2839. ; 59:4, s. 391-419
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Forskningsöversikt (refereegranskat)abstract
- 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 parts per thousand and -6.8 parts per thousand for delta C-13, +6.4 parts per thousand and +10.5 parts per thousand for delta O-18, and Sr-87/Sr-86 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 +/- N-2) and aqueous-carbonic (H2O-NaCl-CO2 +/- CH4 +/- N-2) low salinity fluid, with trapping conditions of 270-300 degrees C and 0.7-1.1kbar. 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.
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| 3. |
- Brander, Linus, 1978, et al.
(författare)
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Igneous and metamorphic geochronologic evolution of granitoids in the central Eastern Segment, southern Sweden
- 2012
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 54:5, s. 1-38
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Tidskriftsartikel (refereegranskat)abstract
- The Eastern Segment abutting the Transscandinavian Igneous Belt (TIB) mostly consists of rocks with overlapping igneous ages. In the Eastern Segment west of Lake Vättern, granitoids of clear TIB affinity exhibit strong deformational fabrics. This article presents U–Pb zircon ages from 21 samples spanning the border zone between these deformed TIB rocks in the east, and more thoroughly reworked rocks in the west. Magmatic ages fall in the range 1710–1660 million years, irrespective of the degree of deformation, confirming the overlapping crystallization ages between deformed TIB rocks and orthogneisses of the Eastern Segment. A common history is further supported by leucocratic rocks of similar ages. Prolonged orogenic (magmatic) activity is suggested by continued growth of zircon at 1.66–1.60 Ga. Six of the weakly gneissic rocks show zircons with cathodoluminescence-dark patches and embayments, possibly partly replacing metamict parts of older magmatic crystals, with 207Pb/206Pb ages dominantly between 1460 and 1400 million years, whereas three of the gneisses have zircon rims with calculated ages of 1440–1430 million years. Leucosome formation took place at 1443 ± 9 and 1437 ± 6 Ma. The minimum age of SE–NW folds was determined by an undeformed 1383 ± 4 million years crosscutting aplitic dike. Sveconorwegian zircon growth was not found in any of the samples from the studied area. To our knowledge, 1.46–1.40 Ga metamorphism affecting the U–Pb zircon system has not previously been reported this far northeast in the Eastern Segment. We suggest that the E–W- to SE–NW-trending deformation fabrics in our field area were produced during the Hallandian–Danopolonian orogeny and escaped later, penetrative Sveconorwegian reworking.
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| 4. |
- Busby, CJ, et al.
(författare)
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The missing half of the subduction factory : shipboard results from the Izu rear arc, IODP expedition 350
- 2017
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 59:13, s. 1677-1708
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although severalsites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand theevolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporalrecord (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chainsup to ~100-km long roughly perpendicular to the arc front. Dredge samples from these aregeochemically distinct from arc front rocks, and drilling was undertaken to understand this arcasymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-kmwest of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery.We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected(~60%), and the remaining fraction of the section is much finer-grained than predicted from itsposition within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size(clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% ofthe section, and only one igneous unit was encountered, a rhyolite peperite intrusion at~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagneticdatum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zirconconcordia intercept age of (13.6 + 1.6/−1.7) Ma. Both arc front and rear arc sources contributed tothe fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcani-clastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arcasymmetry is not recorded in rocks older than ~13 Ma.
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| 5. |
- Cornell, David H., 1948, et al.
(författare)
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The Plat Sjambok Anorthosite and its tonalitic country rocks : mesoproterozoic pre-tectonic intrusions in the Kaaien Terrane, Namaqua-Natal Province, southern Africa
- 2013
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 55:12, s. 1471-1489
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Tidskriftsartikel (refereegranskat)abstract
- The Plat Sjambok Anorthosite crops out near Prieska Copper Mines in the Namaqua-Natal Province of southern Africa. It is a massif-type anorthosite, previously regarded as a late-tectonic intrusion and part of the ca. 1100 Ma bimodal Keimoes Suite. Our new ion probe U-Pb zircon data show that the Plat Sjambok massif intruded at 1259 ± 5 Ma, before the 1220 Ma Namaqua collision events and is thus approximately 150 million years older than the Keimoes Suite. Despite the proximity to Prieska Mines, the anorthosite is located in the Kaaien Terrane close to the Brakbos Fault, which is the boundary with the Areachap Terrane in which Prieska Mines is situated. We dated the Nelspoortjie Tonalite, the main country rock of the Plat Sjambok Anorthosite, by laser ablation ICPMS at 1273 ± 13 Ma. Both intrusions thus originated concurrently with the 1286-1241 Ma volcanic rocks of the Areachap Group, which developed in a subduction-related arc setting, prior to its collision with the Kaaien Terrane and Kaapvaal Craton. Metamorphic zircon rims in the Plat Sjambok Anorthosite give an age of 1122 ± 7 Ma, a time that corresponds to a quiet period in the Areachap Terrane. We propose a tectonic model in which formation of the Nelspoortjie Tonalite and Plat Sjambok Anorthosite was driven by intrusions from the mantle into a back-arc related tensional environment within the Kaaien Terrane, possibly situated above an Archaean crustal tongue. This led to heating in a thickened crustal setting in which the tonalite originated as a partial melt of amphibolite. The anorthosite then formed as a mixture of mantle-derived gabbro and Archaean crustal rocks, which explains the 2100-2600 Ma zircon-Hf crustal residence ages and the Sm-Nd trend towards an old crustal source. The anorthosite and its country rocks were only juxtaposed with the Prieska Copper Mining District by late-tectonic uplift and transpressional movements on the Brakbos Fault towards the end of the Namaqua tectogenesis.
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| 6. |
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| 7. |
- Pease, Victoria, 1959-, et al.
(författare)
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Deciphering crustal growth in the southernmost Arabian Shield through zircon U-Pb geochronology, whole rock chemistry and Nd isotopes
- 2022
-
Ingår i: International Geology Review. - : Taylor and Francis Ltd.. - 0020-6814 .- 1938-2839. ; 64:16, s. 2359-2377
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Tidskriftsartikel (refereegranskat)abstract
- New U-Pb zircon geochronology using high-spatial resolution secondary ion mass spectrometry fills a data gap and provides crystallization ages for granitoids from the Asir composite terrane in the southernmost Arabian Shield of Saudi Arabia. Ages of c. 810–685 Ma, c. 663–636 Ma, and 625–610 Ma reflect oceanic island arc genesis, subduction-related arc accretion (syn-collisional), and post-collisional stabilization, respectively. All samples have juvenile εNd(t) compositions with no evidence of older material being involved in their genesis, indicating that this part of the Arabian Shield grew through juvenile magmatic addition and that assimilation by syn- and post-tectonic magmatism involved an isotopically juvenile component(s). The crustal thickness derived from the (La/Yb)N proxy indicates significant thickening from 10–20 km to c. 70 km at c. 650 Ma, consistent with timing of orogenic uplift and increasing crustal thickness post-dating peak Nabitah orogeny. The age of an intrusion cross-cutting the Atura formation, when combined with other data, provides a well-constrained depositional age of c. 646–625 Ma for the Atura formation and indicates that erosion of the orogenic edifice in this part of the Arabian Shield began at latest by 625 Ma. Our new data indicate that denudation occurred 80–100 m.y. before the development of the prominent sub-Cambrian peneplain, consistent with previous assertions that major pulses of denudation occurred prior to the waning stages of Nabitah orogenesis.
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| 8. |
- Robinson, Frank. A.
(författare)
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Variation in rhyolitic magma composition linked with fractionation from a common source : insights from the Rotoiti eruption, Taupo Volcanic Zone, New Zealand
- 2016
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 58:16, s. 1967-1982
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Tidskriftsartikel (refereegranskat)abstract
- The Taupo Volcanic Zone (TVZ), New Zealand, is a well-documented volcanic arc characterized by explosive rhyolitic magmas within a series of caldera complexes that include the Okataina Volcanic Centre (OVC). New quartz melt inclusion and volcanic glass data from the 45ka caldera-forming Rotoiti eruption within the OVC are compared to published studies. The new data are characterized by low K2O (similar to 1.5-3.5wt.%), Rb (similar to 30-70 ppm), Sr (similar to 40-90 ppm), U (similar to 0.5-2.5 ppm), and Ba (similar to 300-1000 ppm) ranges that differ significantly from other OVC systems (similar to 3.0-4.5wt.% K2O, similar to 80-150 ppm Rb, and similar to 2.5-5.0 ppm U). Most interestingly, the Rotoiti melt inclusion data measured in this study show a decrease in Rb, Sr, and U, although the fractionation trends originate from the same source point as published OVC data. This progressive decreasing trend is interpreted as an interaction with a less enriched rhyolitic melt (represented by the low Rb, Sr, and U of glasses) during fractionation processes from a common TVZ source. The established model for TVZ rhyolites is that they are extracted from a middle or upper crustal source (mush' zone) prior to eruption. Adding to this model, new melt inclusion data suggest that all TVZ rhyolites are fractionated from this common TVZ source and, prior to eruption, the Rotoiti system was rejuvenated by this source (evidenced by the low REE glasses). Exactly what triggers the common TVZ source to fractionate remains unclear, but a proposed mechanism to account for this involves the successive melting of the upper crust by upwelling mantle induced by incremental subduction.
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| 9. |
- Rutanen, H, et al.
(författare)
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1.8 Ga magmatism in southern Finland : strongly enriched mantle and juvenile crustal sources in a post-collisional setting
- 2011
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 53:14, s. 1622-1683
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Tidskriftsartikel (refereegranskat)abstract
- Whole-rock and isotope geochemistry of six similar to 1.8 Ga post-kinematic intrusions, emplaced along the similar to 1.9 Ga Southern Svecofennian Arc Complex (SSAC) and in the SW part of the Karelian Domain in Finland, was studied. The intrusive age [U-Pb secondary ion mass spectrometer (SIMS)] of one of these, the Petravaara Pluton, was determined as 1811 +/- 6 Ma. Basic-intermediate rocks are alkali-rich (K(2)O + Na(2)O > 4 wt.%) and typically shoshonitic, strongly enriched in large ion lithophile elements and light rare earth elements, but relatively depleted in high field strength elements and heavy rare earth elements. The enrichment is much higher than can be accounted for by crustal contamination and requires previously melt-depleted mantle sources, subjected to variable metasomatism by carbonate-rich fluids and sediment-derived melts. These sources are inferred to consist of phlogopite +/- amphibole-bearing peridotites from depths below the spinel-garnet transition, as shown by the high Ce/Yb ratios. (87)Sr/(86)Sr(1.8 Ga) ratios in the range 0.7027-0.7031 and 'mildly depleted' epsilon(Nd)(1.8 Ga) values (+0.1 to +1.4), with T(DM) values <2.1 Ga, suggest that mantle enrichment was associated with the previous Svecofennian subduction-accretion process, when enriched sub-Svecofennian mantle sections developed, dominantly characterized by (147)Sm/(144)Nd ratios of 0.14-0.17. The associated granitoids are diversified. One group is marginally peraluminous, transitional between I (volcanic-arc) and S (syn-collisional) types, and was derived from mixed igneous and sedimentary, but juvenile Svecofennian source rocks, as supported by near-chondritic epsilon(Nd)(1.8 Ga) and somewhat elevated (87)Sr/(86)Sr(1.8 Ga). The other group is transitional between I and A (within-plate) types in character and had dominantly igneous protoliths. The whole-rock geochemistry and isotopes suggest that the compositional variation between similar to 50 and 70 wt.% SiO(2) may be explained by hybridization between strongly enriched mantle-derived magmas and anatectic granitic magmas from the juvenile Svecofennian crust. One intrusion in the east contains a significant portion of Archaean, mostly igneous protolithic material (epsilon(Nd)(1.8 Ga) = -2.8 and epsilon(Hf)(t) for zircons between +2.8 and -11.9, with an average of -4.9). The similar to 1.8 Ga post-kinematic intrusions were emplaced within the SSAC subsequent to the continental collision with the Volgo-Sarmatia craton from the SE, during a shift from contraction to extension, that is, in a post-collisional setting.
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| 10. |
- Topuz, Gültekin, et al.
(författare)
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Origin of late Carboniferous highly fractionated high-K calc-alkaline I-type granites from the eastern Pontides (Şiran, northern Turkey)
- 2023
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Ingår i: International Geology Review. - 0020-6814 .- 1938-2839.
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Tidskriftsartikel (refereegranskat)abstract
- The Sakarya Zone (northern Turkey) is characterized by the emplacement of voluminous granitoids during the late Carboniferous. In this study, we present geological, U-Pb zircon age, elemental abundance and Sr-Nd-Pb isotopic data on two highly evolved Carboniferous granite bodies within the Early to Late Jurassic volcaniclastic rocks in the Şiran region. The largest body forms a roughly E-W trending ~30 km long and 0.5–2.5 km wide stripe the southern boundary of which is a southvergent thrust, and the other one a subcircular outcrop, ~5 by ~3 km, unconformably overlain by Lower Jurassic volcaniclastic rocks. Their emplacement ages are constrained by LA-ICP-MS U-Pb zircon dating to 310–313 ± 7 Ma (2σ, late Carboniferous). The granitic rocks consist of quartz, microperthitic K-feldspar, plagioclase, and minor biotite, and are characterized by highly evolved compositions with high concentrations of SiO2, Na2O, K2O and Ba, and low concentrations of TiO2, Fe2O3*, MgO, CaO, P2O5 and Sr. Geochemical characteristics indicate a highly fractionated high-K calc-alkaline peraluminous I-type affinity. Chondrite-normalized rare-earth element (REE) patterns are slightly fractionated, with light REEs enriched with respect to middle and heavy ones, and showing variable negative Eu anomalies. On the N-MORB-normalized multi-element variation diagrams, samples display negative anomalies of Ba, Nb, Sr, P, Zr, Eu, and Ti, and positive anomalies of K, and Pb. Initial 87Sr/86Sr and εNd values are 0.70432–0.70610 and −6 to −11, respectively, similar to other Carboniferous high-K calc-alkaline granites and mafic-ultramafic intrusions in the region. The compositional characteristics of the Şiran granites can be explained mainly by fractional crystallization involving hornblende, plagioclase, and biotite and without the involvement of significant amounts of garnet. There is no need to invoke magma mixing and/or melting of a heterogeneous source to account for the geochemical variation. We propose that the late Carboniferous granites formed by remelting of middle- to high-K calc-alkaline mafic rocks at lower crustal depths, followed by extensive fractionation at the upper crustal depths. A thorough review of data from the literature reveals that highly fractionated granites constitute a significant component of the late Carboniferous granites in the Eastern Pontides.
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| 11. |
- Whitehouse, Martin J., 1962-, et al.
(författare)
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Neoproterozoic crustal growth at the margin of the East Gondwana continent - age and isotopic constraints from the easternmost inliers of Oman
- 2016
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 58:16, s. 2046-2064
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Tidskriftsartikel (refereegranskat)abstract
- The Jebel Ja'alan and Qalhat inliers of Oman represent the easternmost exposures in the Arabian peninsula of the Neoproterozoic basement associated with the East African Orogen (EAO) and the assembly of East and West Gondwana. These inliers expose tonalitic gneisses and metasediments intruded by granodiorites and granites of the Ja'alan batholith. Zircons from the gneisses yield U-Pb SIMS ages of ca. 900-880Ma, which are interpreted as crystallization ages. These represent the oldest magmatic events associated with the closure of the Mozambique Ocean reported to date. Zircon of this age is also the dominant component in the metasediments. The Ja'alan batholith yields ages of ca. 840-825Ma. Nd isotopes indicate that both the gneisses and the batholith range from juvenile to slightly more evolved, with epsilon(Nd)(t) of +6 to +1.5 interpreted to reflect variable contamination by older, evolved continental material; this is also indicated by >900Ma detrital zircon from the metasediments. The Nd data also contrast with the uniformly juvenile signature of younger, ca. 840Ma, rocks of the Marbat region of southern Oman that lie structurally to the west. The Ja'alan and Qalhat inliers thus document eastward increasing age and continental influence, consistent with the progressive development of arc rocks onto the western margin of East Gondwana, although the location and nature of the eastern continental block remain elusive.
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| 12. |
- Zhang, Wen, et al.
(författare)
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From the Libyan border to the Nile - Neoproterozoic magmatism and basement evolution of southern Egypt
- 2019
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 61:16, s. 2057-2079
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Tidskriftsartikel (refereegranskat)abstract
- In southern Egypt, Neoproterozoic granitoids intrude the crust from Libya in the west to the Nile River in the east. Best exposed at Gebel Uweinat, Gebel Kamil, Bir Safsaf, Gebel El Asr and Gebel Umm Shaghir, these rocks represent the transition between the rejuvenated Archean to Paleoproterozoic crust of the east Saharan craton and the juvenile Neoproterozoic crust of the Arabian-Nubian Shield to the east. We present U-Pb zircon geochronology and whole rock geochemistry from 16 Neoproterozoic (c. 628-580 Ma) granitoids across southern Egypt which document the synchronous production of diverse compositions indicative of an intraplate or intracontinental tectonic setting. Our data better constrain the age relationships, petrogenesis, and Pan-African crustal growth associated with the amalgamation of Gondwana and contribute to understanding the crust-modifying processes that led to the development of the Saharan metacraton in the Ediacaran.
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| 13. |
- Zhang, Wen, et al.
(författare)
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Pre-Neoproterozoic basement evolution of southwestern Egypt
- 2019
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 61:15, s. 1909-1926
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Tidskriftsartikel (refereegranskat)abstract
- The only surviving Archaean fragment of the Saharan Metacraton is exposed in the Uweinat-Kamil Mountains of Egypt, Libya, and Sudan. To expand our knowledge of this rare Archaean crustal fragment, we investigate protolith ages and the timing of metamorphic events associated with it via U-Pb zircon geochronology and geochemistry of 10 samples from the Egyptian part of the Uweinat-Kamil region. Our work confirms that the orthogneisses represent a tonalite, trondhjemite, granodiorite (TTG) and granite suite of Archaean-early Paleoproterozoic age (3.25-2.49 Ga). Late Paleoproterozoic rocks include 2.15-2.00 Ga paragneiss, 2.00 Ga metagranite and 1.99 Ga trondhjemitic leucosome. Neodymium isotopes from the 3.25 Ga orthogneiss record an entirely juvenile mantle signature, whereas the 2.91 and 2.49 Ga orthogneisses record significant juvenile mantle input, and 2.86-2.68 Ga orthogneisses reflects recycling of older crustal rocks. At c. 2.0 Ga, a magmatic and metamorphic event associated with anatectic partial melting occurred in northern Africa, assimilating both granitic and leucocratic gneiss of the Archaean TTG crust. Detrital input contributing to the paragneiss was mainly from Uweinat-Kamil TTG rocks and sources to the east-southeast. The oldest 3.42 Ga detrital zircon from the paragneiss probably reflects the conjoined Saharan Metacraton-Congo craton.
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| 14. |
- Zhang, Wen, et al.
(författare)
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Timing, petrogenesis, and setting of granites from the southern Beishan late Palaeozoic granitic belt, Northwest China and implications for their tectonic evolution
- 2015
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Ingår i: International Geology Review. - : Informa UK Limited. - 0020-6814 .- 1938-2839. ; 57:16, s. 1975-1991
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Tidskriftsartikel (refereegranskat)abstract
- Late Palaeozoic granites are widely distributed in the southeastern Beishan area, which is located in the central part of the southern Central Asian Orogenic Belt (CAOB). U–Pb zircon dates of five late Palaeozoic granitic plutons from the southeastern Beishan area yield Permian ages: 285 ± 2 Ma (Shuwojing and Western Shuwojing plutons), 269 ± 3 Ma (Jianquanzi and Jiuquandihongshan plutons), and 260 ± 1 Ma (Jiujing pluton). The early Permian Shuwojing pluton, an unfractionated calc-alkaline biotite monzogranite with slightly positive εNd(t) (+0.7 and +0.6) and low initial 87Sr/86Sr (0.70722 and 0.70712), and the early Permian Western Shuwojing pluton, a high-K calc-alkaline biotite granite with slightly negative εNd(t) (−0.2 and −0.1) and low initial 87Sr/86Sr (0.70390 and 0.70919), are likely derived from a mixture of depleted (juvenile) mantle and upper continental crustal (or sedimentary) material. The mid-Permian Jianquanzi and Jiuquandihongshan monzogranites have highly fractionated potassium-rich calc-alkaline characteristics with negative εNd(t) (−4.3) and very high initial 87Sr/86Sr (0.71949), reflecting a continental crustal component. The compositionally diverse Jiujing pluton and the granodiorite and high-Sr monzogranite phases display adakite-like compositions with relatively low εNd(t) (−0.1 and −2.2) and high initial 87Sr/86Sr (0.70822 and 0.70913). The Jiujing low-Sr monzogranite has higher initial 87Sr/86Sr (0.73464) and lower εNd(t) (−2.8), indicating a significant continental crustal component in its genesis. This work, combined with the regional geology and previous studies, suggest that the early to middle Permian southern Beishan plutons formed in a post-collisional environment, probably an intracontinental rift environment linked to asthenospheric upwelling in response to the break-off of a subducted slab. In the late Permian, the southern Beishan area was in a compressive tectonic regime and thickening of the continental crust resulted in the formation of the Jiujing adakite-like granite.
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| 15. |
- Savelev, Aleksandr D., et al.
(författare)
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980-970 Ma Sette-Daban event of the Siberian craton : new geochronological and geochemical data, relationship to LIP and potential connection with other LIPs
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Ingår i: International Geology Review. - 0020-6814.
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Tidskriftsartikel (refereegranskat)abstract
- The paper presents new geological, geochronological, geochemical and Nd-Sr isotopic data on the Meso- Neoproterozoic dolerites of the Siberian Craton. New U-Pb baddeleyite and apatite ages of a E-W-trending dyke and two sills from southeastern Siberia are 982 ± 11, 977 ± 7, 970 ± 31 and 972 ± 60 Ma, respectively, extending the area of distribution of the event by more than 100 km to the north. The Sette-Daban intrusions are subalkalic mostly low-Ti dolerites, although high-Ti dolerites have been locally documented as well. Trace element abundances in dolerites vary from typical to E-MORB to OIB with arc-like signatures represented by high Th/Yb and low TiO2/Yb ratios. All dolerite samples display moderately positive εNd(t) values varying from +3.3 to +7.7 and indicating the magmas were derived from a depleted mantle source. E-MORB and OIB intrusions are attributed to the different degree of interaction of magma between the depleted asthenospheric mantle and regions within the sub-continental lithospheric mantle (SCLM) that were metasomatically enriched during earlier subduction events. Available data on Sette-Daban event distribution, composition and duration satisfy the characteristics of a typical Large Igneous Province (LIP). Similar age 1000–950 Ma mafic magmatism is also recognized in the Baltic and Amazonian cratons and can be potentially correlated with the Sette-Daban event.
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