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| 2. |
- Appelquist, Karin, 1979, et al.
(author)
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The Palaeoproterozoic Malmbäck Formation in S Sweden: age, composition and tectonic setting
- 2009
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In: GFF. - 1103-5897. ; 131, s. 229-243
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Journal article (peer-reviewed)abstract
- The Paleoproterozoic Malmbäck Formation in southern Sweden comprises mafic to felsic volcanic rocks and syn-eruptive volcaniclastic and volcanogenic sedimentary deposits. Field observations indicate terrestrial to fluvial deposits – predominantly with massive to highly vesicular lavas, but also with ignimbrites, ashes, redeposited volcanic rocks and conglomerates. Three groups of volcanic rocks can be distinguished geochemically; (1) primitive basalts to basaltic andesites, (2) dacites-andesites and (3) rhyolites. The groups constitute a calc-alkaline to shoshonitic series. LILE and LREE enrichment, combined with depletion in HREE and HFSE; particulary in Nb (and Ta), is characteristic of typical arc rocks. Discriminant diagrams suggest emplacement along a one-time active continental margin, which is now marked by the Transscandinavian Igneous Belt (TIB), a more than one thousand kilometres long array of ~1.85-1.65 Ga granitic to gabbroic plutonic rocks and associated volcanic rocks. In the TIB, the Malmbäck Formation forms mega-xenoliths and other inclusions. U-Pb SIMS dating of zircon from a rhyolite of the Malmbäck Formation yielded a Concordia age of 1796±7 Ma, which is interpreted as the crystallization age of the rhyolite and the age of deposition of the Malmbäck Formation, during a relatively early part of the formation period of the TIB. Altogether, the setting, composition and age of the Malmbäck Formation and TIB confirm the existence of an Andean type active continental margin in the present southern part of the TIB at c. 1.8 Ga.
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| 3. |
- Rimsa, Andrius, et al.
(author)
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Brittle fracturing and fracture healing of zircon: An integrated cathodoluminescence, EBSD, U-Th-Pb, and REE study
- 2007
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In: American Mineralogist. - : Mineralogical Society of America. - 0003-004X. ; 92:7, s. 1213-1224
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Journal article (peer-reviewed)abstract
- The entire population of magmatic oscillatory zoned zircons in a migmatitic granite from the Tjarnesjo intrusion, southwest Sweden, reveal fine-scale brittle fracturing. The oscillatory zoned fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Hydraulic fracturing is the most probable mechanism. The internal structure of fractured zircons and the LREE-enriched, low Th character of CL-bright zircon both suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by CL-bright zircon, precipitated from hydrothermal fluids. Zircon fracturing and crack-sealing has been dated by SIMS ion-probe and U-Th-Pb isotopes to 920 +/- 51 Ma (lower intercept age, 2 sigma, MSWD = 1.09) with a limit for the Youngest possible age of 960 +/- 16 Ma (Pb-207/(206) Pb, 2 sigma, MSWD = 0.23) dated by sector-zoned rims forming overgrowths on the fractured cores.
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| 5. |
- Rimsa, Andrius, et al.
(author)
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Constraints on incipient charnockite formation from zircon geochronology and rare earth element characteristics
- 2007
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In: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 154:3, s. 357-369
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Journal article (peer-reviewed)abstract
- We present high spatial resolution ion-microprobe U-Th-Pb geochronology and rare earth element (REE) data combined with cathodoluminescence (CL) and back-scattered electron (BSE) imaging for complex zircons in incipient charnockites from Sondrum, SW Sweden. Examination of closely paired samples across the dehydration zone demonstrates that incipient charnockite formation at Sondrum is a zircon-forming process. We determined the age of the dehydration event (i.e. charnockitisation) to 1,397 +/- 4 Ma (2 sigma, MSWD = 1.7). This is the first successful attempt to date incipient charnockite formation using U-Pb systematics of zircon. Internal structure, chemical and isotopic characteristics of zircon indicate that the granitic pegmatite in the core of the incipient charnockite is a melting zone. Commonly observed bulk rock HREE depletion in incipient charnockites is not caused by zircon dissolution but by involvement of garnet as a reactant in the dehydration reactions. Moreover, REE patterns of the newly formed zircon are HREE-enriched, indicating non-concurrent growth and suggesting that the degree of charnockite depletion in HREE might be controlled by the volume of newly formed zircon.
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| 6. |
- Rimsa, Andrius
(author)
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Understanding Zircon Geochronology - Constraints from Imaging and Trace Elements
- 2007
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Doctoral thesis (other academic/artistic)abstract
- Formation of incipient charnockite in Söndrum, SW Sweden and the Kerala Khondalite Belt (KKB), S. India is accompanied by zircon growth. The age of the dehydration event is constrained by dating newly formed zircon to 1397±4 Ma (2?, MSWD=1.7) in Söndrum and ca. 520-510 Ma in S. India by secondary ionization mass spectrometry (SIMS). The formation of incipient charnockites correlates with regional magmatic or metamorphic events. In Söndrum, SW Sweden, charnockitisation is simultaneous with the formation of the Varberg Charnockite- Granite Association (CGA). Identical ages for incipient charnockite formation in studied localities in the KKB, S. India imply that charnockitisation is a major regional event related to the latest stage of Pan-African orogeny. Commonly observed bulk rock HREE depletion in incipient charnockites is not caused by zircon dissolution but by involvement of garnet as a reactant in the dehydration reactions. The Kerala Khondalite Belt, S. India, previously believed to be an entirely supracrustal belt, in fact contains magmatic rocks as a major lithological component. Ages (SIMS U-Pb on zircon) of magmatic garnet-biotite gneiss (1877±24; 2?, MSWD=0.6), augen gneiss (1891±36; 2?, MSWD=1.04) and massive charnockite (1865±16; 2?, MSWD=0.41) identify a previously unknown, Paleoproterozoic magmatic major crust forming event in the Kerala Khondalite Belt, S.India. Two distinct metamorphic events at 580-550 Ma and 520-510 Ma, identical in age in three studied localities of garnet biotite ortho- and paragneiss and augen gneiss, have been identified. The pervasive 580-550 Ma regional metamorphic event dates deformation and migmatisation of the magmatic and sedimentary rocks in the Kerala Khondalite Belt during Pan-African orogeny. The second metamorphic event is related to formation of the incipient charnockites. The entire population of magmatic oscillatory zoned zircons in a migmatised granitic sample from the Tjärnesjö intrusion, SW Sweden, experienced brittle hydraulic fracturing and rapid healing. The oscillatory zoned zircon fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Both the internal structure of fractured zircons and the LREE-enriched, low-Th character of CL-bright zircon suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by hydrothermal CL-bright zircon. Zircon fracturing and crack-sealing has been dated by SIMS U-Pb on zircon to 920 ± 51 Ma with a limit for the youngest possible age of 960 ± 16 Ma provided by sector zoned rims that form overgrowths on the fractured cores.
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| 7. |
- Skridlaite, Grazina, et al.
(author)
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On the origin and evolution of the 1.86–1.76 Ga Mid-Baltic Belt in the western East European Craton
- 2021
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In: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 367, s. 1-21
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Journal article (peer-reviewed)abstract
- A Mid-Lithuanian Domain (MLD) was distinguished by Bogdanova et al. (2015) as part of the large Mid-BalticBelt (MBB) in the western East European Craton. Zircon U-Pb dating by SIMS, Sr- and Nd-isotope systematics and a detailed geochemical study have allowed to subdivide the MLD into two parts: NW and SE. The NW magmatic rocks have been emplaced at 1.86 to 1.83 Ga from either an enriched mantle source or from a mantle magma with presence of older crustal material. The SE MLD magmatic rocks originated from a slightly depleted mantle source at 1.87 to 1.82 Ga. At 1.79 Ga, they were intruded by gabbronorites which at 1.76 Ga were crosscut by thin veinlets of microgabbronorite. The SE MLD magmatic series with their oceanic island arc affinity correlate well with the contemporaneous Fröderyd metavolcanics of the Vetlanda-Oskarshamn belt in Sweden, while the NW MLD rocks are similar to the TIB-0 Askersund granitoids in the southern Bergslagen area. The younger (1.81–1.79 Ga) intrusives in both areas are time-equivalents of the TIB-1 magmatism on the Swedish side. Thus, the MLD as well as its counterparts on the Swedish side of the Baltic Sea may belong to the same Mid-Baltic Belt, representing an active, south-facing continental margin established at ca. 1.86 Ga. The shape and outline of the belt was affected by the Fennoscandia-Sarmatia collision at ca. 1.82–1.80 Ga, as well as by later magmatism.
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| 8. |
- Whitehouse, Martin, 1962-, et al.
(author)
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Behaviour of radiogenic Pb in zircon during ultrahigh-temperature metamorphism: an ion imaging and ion tomography case study from the Kerala Khondalite Belt, southern India
- 2014
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In: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 168:2, s. 1042-1042
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Journal article (peer-reviewed)abstract
- Zircon crystals from a locally charnockitized Paleoproterozoic high-K metagranite from the Kerala Khondalite Belt (KKB) of southern India have been investigated by high-spatial resolution secondary ion mass spectrometry analysis of U-Th-Pb and rare earth elements (RE), together with scanning ion imaging and scanning ion tomography (depth-profiled ion imaging). The spot analyses constrain the magmatic crystallization age of the metagranite to ca. 1,850 Ma, with ultrahigh-temperature (UHT) metamorphism occurring at ca. 570 Ma and superimposed charnockite formation at ca. 520-510 Ma, while the ion imaging reveals a patchy distribution of radiogenic Pb throughout the zircon cores. Middle-to heavy-RE depletion in ca. 570 Ma zircon rims suggests that these grew in equilibrium with garnet and therefore date the UHT metamorphism in the KKB. The maximum apparent Pb-207/Pb-206 age obtained from the unsupported radiogenic Pb concentrations is also consistent with formation of the Pb patches during this event. The superimposed charnockitization event appears to have caused additional Pb-loss in the cores and recrystallization of the rims. The results of depth-profiling of the scanning ion tomography image stack show that the Pb-rich domains range in size from <5 nm to several 10 nm (diameter if assumed to be spherical). The occurrence of such patchy Pb has previously been documented only from UHT metamorphic zircon, where it likely results from annealing of radiation-damaged zircon. The formation of a discrete, heterogeneously distributed and subsequently immobile Pb phase effectively arrests the normal Pb-loss process seen at lower grades of metamorphism.
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