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| 2. |
- Keulen, Nynke, et al.
(författare)
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Meso- and Neoarchaean geological history of the Bjornesund and Ravns Storo Supracrustal Belts, southern West Greenland: Settings for gold enrichment and corundum formation
- 2014
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 254, s. 36-58
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Tidskriftsartikel (refereegranskat)abstract
- To understand the Mesa- and Neoarchaean geological history of the Bjornesund region, southern West Greenland (ca. 62-63 degrees N), we have undertaken an integrated study combining field observations with U-Pb zircon geochronology by LA-ICP-MS, metamorphic petrology, and microstructural analyses. The Bjornesund region links the southern part of the Fiskenaesset anorthosite complex to the well-preserved metavolcanic rocks of the Ravn Story Supracrustal Belt and the Bjornesund Supracrustal Belt, so the development of this region is of key interest to unravelling the Archaean geology of the entire region. In this study, we couple the observed phases of gold-enrichment and corundum formation to the geological history. The leucogabbroic rocks of the Bjornesund region, which are part of the Fiskenaesset complex, intruded at ca. 2.95 Ga into the tholeiitic and calc-alkaline metavolcanic rocks of the two studied belts. Following isoclinal folding, the rocks were intruded by diorite (2.92 Ga) and granodiorite (2.91 and 2.88 Ga), which are now texturally orthogneisses. The area was affected by three major phases of folding; F1 (undated), F2 at 2.85-2.83 Ga, and F3 at 2.71-2.70 Ga. The former deformation event is associated with the gold-enrichment in alteration zones in shear folds in metavolcanic amphibolite near the contact with the orthogneiss. Intrusion of granitic sheets during the F3 deformation triggered the formation of corundum, which is locally of gem quality, at the contact between leucogabbroic and ultramafic rocks. Peak metamorphic conditions were calculated as 580-630 degrees C and 6-4 kbar at 2.85-2.83 Ga. Later retrogression in the southern part of the area at ca. 500-450 degrees C and 4.5-3 kbar, and extensional fault reactivation overprinted some of the older structures. (C) 2014 Elsevier B.V. All rights reserved.
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| 4. |
- Lin, Li'na, et al.
(författare)
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GEOCHRONOLOGIC AND GEOCHEMICAL EVIDENCE FOR PERSISTENCE OF SOUTH-DIPPING SUBDUCTION TO LATE PERMIAN TIME, LANGSHAN AREA, INNER MONGOLIA (CHINA) : SIGNIFICANCE FOR TERMINATION OF ACCRETIONARY OROGENESIS IN THE SOUTHERN ALTAIDS
- 2014
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Ingår i: American Journal of Science. - : American Journal of Science (AJS). - 0002-9599 .- 1945-452X. ; 314:2, s. 679-703
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Tidskriftsartikel (refereegranskat)abstract
- The Langshan area in Inner Mongolia is situated in the southern Altaids between the Beishan suture to the west and the Solonker suture to the east. This paper addresses the poorly known tectonic evolution that led to formation of the terminal Solonker suture. Dating of deformed porphyries and undeformed dolerites and gabbros constrains the timing of the relevant NE-E-striking and north-vergent deformation. Deformed granitic-granodioritic porphyries in this area are characterized by high SiO2 (65.38-78.00%), low TFe2O3 (1.29-5.07%), MgO (0.13-0.63%), and variable K2O (0.534.14%) and Na2O (2.05-4.62%). All samples have enriched LREE (La/Yb 6-18) and negative Nb anomalies (Nb-N/Th(N)0.09-0.48), but different Eu anomalies (Eu* <0.7 or similar to 1); these geochemical features can be ascribed to a heterogeneous source in a subduction-related environment. Gabbros and dolerites have 42.33 to 52.03 percent SiO2. All mafic samples have similar La/Yb ratios of 4 to 6 and negative Nb anomalies (Nb-N/Th-N) ratios of 0.2 to 0.8, suggestive of a subduction-related setting. Two granitic porphyries yielded U-238/Pb-206 weighted mean ages of 284.7 +/- 2.1 Ma with MSWD of 1.6 and 291.7 +/- 2: 2.1 Ma with MSWD of 1.14; these ages are consistent with U-235/Pb-207 and U-238/Pb-206 concordia ages of 281 +/- 17 Ma with MSWD of 0.87 and 289.8 +/- 9.2 Ma with MSWD of 0.66. A dolerite yielded concordia ages of 256.2 +/- 2.6 Ma with MSWD of 0.44 and 256 +/- 2.5 Ma with MSWD 0.45. The ages and geochemistry of the deformed porphyries indicate that in the early Permian there was important deformation and recrystallization in a subduction-related setting. The isotopic and geochemical signatures of all the rocks indicate that they formed during subduction-related conditions. We propose that Langshan was a Permian active continental margin arc built on the edge of the North China Craton by southward subduction, which led to closure of the ocean, concomitant formation of the Solonker suture in the late Permian-early Triassic, and termination of the accretion-subduction orogen of the southern Altaids.
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| 5. |
- Rantakokko, Nina E., et al.
(författare)
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Neoproterozoic evolution of the eastern Arabian basement based on a refined geochronology of the Marbat region, Sultanate of Oman
- 2014
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Ingår i: Tectonic Evolution of the Oman Mountains. - Bath : Geological Society Publishing House. - 9781862393783 ; 392, s. 107-127
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Bokkapitel (refereegranskat)abstract
- New high spatial resolution secondary ion mass spectrometry (SIMS) U-Pb zircon data from the Sadh gneiss complex and the intruding Marbat granodiorite of the Marbat region, southern Sultanate of Oman, yield Cryogenian magmatic protolith ages for gneisses ranging from c. 850 to 830 Ma. Zircon ages record a c. 815-820 Ma period of deformation and migmatization, followed by intrusion of a hornblende gabbro/diorite and the undeformed Marbat granodiorite at c. 795 Ma. Following break-up and rifting of Rodinia at c. 870 Ma, crustal growth in the Marbat region occurred via arc accretion at c. 850-790 Ma, possibly in the easternmost part of the Mozambique Ocean based on earlier cessation of accretion here compared to the Arabian-Nubian Shield. Similarity of the new zircon geochronology to peaks of detrital zircon ages in the unconformably overlying Ediacaran Marbat sandstone suggests relatively local derivation from uplifted basement for the latter.
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| 6. |
- Szilas, Kristoffer, et al.
(författare)
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Complex calc-alkaline volcanism recorded in Mesoarchaean supracrustal belts north of Frederikshab Isblink, southern West Greenland: Implications for subduction zone processes in the early Earth
- 2012
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 208, s. 90-123
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Tidskriftsartikel (refereegranskat)abstract
- We present new geochemical data for three Mesoarchaean supracrustal belts (Ravns Storo, Bjornesund and Perserajoorsuaq) situated north of Frederikshab Isblink in southern West Greenland for which we propose the collective name 'The Ikkattup Nunaa Supracrustal Association'. They comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc-alkaline andesite composition. Both lithological units are cut by aplite sheets of tonalite-trondhjemite-granodiorite (TTG) composition with U-Pb zircon ages of c. 2900 Ma. Lu-Hf and Sm-Nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 +/- 41 Ma and 3020 +/- 78 Ma, respectively, which are within error of the age of the Fiskenaesset Complex situated 5-25 km to the north. Leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and TTG gneisses, as the end-members. By assimilation-fractional-crystallisation (AFC) modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a TTG-like end-member and another group of high P2O5, La and Nb leucoamphibolites can be explained by contamination involving a hypothetical low-silica adakite end-member. However, the leucoamphibolites are juvenile with epsilon Nd-(2970 Ma) from +2.1 to +3.5 and epsilon Hf-(2970 (Ma)) of +3.5 to +4.3. Thus, the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time (<60 Ma) as the volcanism took place. Our preferred interpretation of the geochemical and isotope data is that the protoliths of the supracrustal rocks formed in an island arc setting, where early tholeiitic volcanism gave way to calc-alkaline volcanism in a maturing arc. The apparent AFC trends are thus explained by in situ partial melting of basaltic arc crust to form juvenile TTG- and adakite-melts that mixed with mafic magmas or contaminated their mantle source to produce the calc-alkaline leucoamphibolite protolith. This model has important implications for the general interpretation of other Archaean supracrustal belts, because AFC and geochemical mixing trends towards a TTG end-member are not uniquely diagnostic of crustal contamination, but may rather reflect processes operating at lower crustal or source levels in arcs, such as melting-assimilation-storage-homogenisation (MASH) or slab-melt (low-silica adakite) metasomatism of the mantle source, respectively. (C) 2012 Elsevier B.V. All rights reserved.
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| 7. |
- Szilas, Kristoffer, et al.
(författare)
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Remnants of arc-related Mesoarchaean oceanic crust in the Tartoq Group of SW Greenland
- 2013
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Ingår i: Gondwana Research. - : Elsevier BV. - 1342-937X. ; 23:2, s. 436-451
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Tidskriftsartikel (refereegranskat)abstract
- The Tartoq Group, located in SW Greenland, consists of supracrustal rocks of mainly tholeiitic basaltic composition, including pillow lavas, sills/dykes and gabbros, as well as ultramafic rocks. Metamorphic grade ranges from greenschist fades to granulite facies. The Tartoq Group crops out as a series of blocks and slivers that are imbricated with originally intrusive Mesoarchaean TTG orthogneisses. The supracrustal rocks form part of a SE vergent fold and thrust belt consistent with the imbrication of TTG gneisses and supracrustal rocks along a convergent margin. LA-ICP-MS U-Pb zircon dating of an intrusive TTG sheet yields a minimum age of 2986 +/- 4 Ma for the Tartoq Group. This age is consistent with MC-ICP-MS Lu-Hf and Sm-Nd isotopic whole-rock data for mafic samples from different blocks of the Tartoq Group, which yield errorchron ages of 3189 +/- 65 Ma and 3068 +/- 220 Ma, respectively. The mafic supracrustal rocks of the Tartoq Group have chondrite-normalized REE patterns with La-CN/Sm-CN of 0.67-1.96 and rather flat primitive mantle-normalized multi-element patterns, except for scattered LILE contents, and generally negative Nb-anomalies with Nb/Nb* of 0.26-1.31. Th/Yb varies between 0.06 and 0.47 and Nb/Yb between 0.45 and 4.4 indicative of an arc affinity when compared to rocks from modern settings. The similar geochemistry of the different lithological units, together with their coeval formation, as evident from trace element geochemical trends, supports a co-magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust Accordingly, we propose that the Tartoq Group represents remnants of Mesoarchaean oceanic crust, which formed in a suprasubduction zone geodynamic environment (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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| 8. |
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| 9. |
- Wan, Bo, et al.
(författare)
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Permian hornblende gabbros in the Chinese Altai from a subduction-related hydrous parent magma, not from the Tarim mantle plume
- 2013
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Ingår i: Lithosphere. - 1941-8264 .- 1947-4253. ; 5:3, s. 290-299
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Tidskriftsartikel (refereegranskat)abstract
- In the Chinese Altai, on the northern side of the Erqis fault, the similar to 10-m-wide Qiemuerqieke gabbro is composed almost entirely of hornblende and plagioclase. Its relative crystallization sequence is olivine, hornblende, plagioclase, and it shows a narrow compositional range in SiO2 (48.7-50.2 wt%), MgO (6.33-8.54 wt%), FeO (5.27-6.46 wt%), Na2O (3.06-3.71 wt%), and K2O (0.26-0.37 wt%). These contents result in a high Mg# value (68-72) and a low K2O/Na2O ratio of similar to 0.1. It has (Sr-87/Sr-86)(i) ratios of 0.70339-0.70350, epsilon(Nd(t)) values of 4.8-6.0, and zircon epsilon(Hf(t)) of 11.4-15.8; these values demonstrate a mantle-derived source, and a whole-rock delta O-18 of similar to 6.7 parts per thousand suggests a mantle wedge origin. The presence of magmatic hornblende suggests a relatively high water fugacity, and the crystallization temperature (715-826 degrees C) calculated using Ti-in-zircon thermometry is considerably lower than that of a normal mafic melt but consistent with an origin from a water-bearing magma. The gabbro has a secondary ion mass spectrometry zircon U-Pb age of 276.0 +/- 2.1 Ma, which is coeval with the 275 Ma mantle plume in the northern Tarim craton, but the Qiemuerqieke petrological and geochemical data do not indicate an abnormally high mantle temperature or a deep mantle signature, which would commonly characterize a mantle plume source. Our results integrated with published data support a model of juvenile crustal growth by a subduction-related process.
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| 10. |
- Wan, Bo, et al.
(författare)
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Re-Os molybdenite age of the Cu-Mo skarn ore deposit at Suoerkuduke in East Junggar, NW China and its geological significance
- 2014
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 56, s. 541-548
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Forskningsöversikt (refereegranskat)abstract
- The Suoerkuduke Cu-Mo ore is a medium-sized skarn deposit in the East Junggar orogen of the southern Altaids, Xinjiang, Northwest China. We measured Re-Os isotopes of a molybdenite veinlet from the main ore body, of which eight yielded model ages ranging from 303 +/- 5 to 317 +/- 5 Ma with a well-constrained Re-187-Os-187 isochron age of 305 +/- 7 Ma, a MSWD of 4.9 and a weighted average model age of 310 +/- 4 Ma with a MSWD of 4.2. The Re-187-Os-187 isochron age is consistent with the weighted mean model age, within error, and indicates that the time of Cu-Mo mineralization was at c. 310 Ma in the late Carboniferous. This new age is close to the youngest porphyry-related Cu-Mo metallogenic epoch (c. 311 Ma) in the East Junggar region, and is not, as previously suggested, in the Permian. Considering the implications of this new age, we propose that the Suoerkuduke deposit was part of a large-scale porphyry-skarn system, which formed by the interaction between far-traveled hydrothermal fluids and the calcium-rich volcanic wall rocks.
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