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NARROW
GeoRef Subject
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minerals
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minerals (1)
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Primary terms
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crystallography (1)
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electron microscopy (1)
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geochemistry (1)
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minerals (1)
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Abstract An U–Pb isotopic investigation combined with rare earth element data for titanites, zircons and coexisting accessories has been undertaken to gain insight into the formation of the Archean peralkaline granites of the northeastern Fennoscandian Shield and to test the stability of titanite during metamorphic and hydrothermal processes. The obtained set of isotope data shows that whereas more stable zircon retains a memory of the major episodes of granite evolution, the coexisting titanite provides additional information on crystallization, subsequent growth, cooling and alteration of the plutonic complexes. In addition to titanites formed at the magmatic stages of c. 2710 and c. 2650 Ma, the peralkaline granites contain titanite populations which have undergone major resetting at c. 1870 Ma during the burial metamorphism related to the Svecofennian orogeny and the 1760 Ma hydrothermal alteration near contemporaneous with regional metamorphism of c. 1780 Ma. The peralkaline granites which contain c. 2710 Ma titanites also include inherited titanite grains of an age of 2795 Ma. These data support the concept that the titanite can remain a closed system to Pb diffusion at temperatures of peralkaline granite melt (<800°C) as long as the crystals escape magmatic or metamorphic recrystallization.
Abstract: Over the past decade several restorations of the hypothetical Mesoproterozoic to Neoproterozoic supercontinent have been suggested. In this paper, we summarize the most recent data on Mesoproterozoic to Neoproterozoic geology of the Siberian craton to test different restorations. U-Pb zircon ages of the Siberian craton basement are typically older than 2300 Ma or have Paleoproterozoic ages between 2050 Ma and 1700 Ma, whereas Sm-Nd model ages (T DM ) are typically older than 2100 Ma. A similar radiometric age profile has been reported from basement of crystalline massifs exposed to the east of the Siberian craton. An important feature for paleocontinental correlation is a widespread anorogenic magmatic belt (ca. 1740-1700 Ma) that links the Aldan shield, the Okhotsk massif, and the Prikolyma terrane. Mesoproterozoic magmatic events are represented by mafic dike swarms with chemistry typical of continental flood basalts. Most of the dated examples come from the Anabar shield and yield ages of ca. 1500 Ma, 1410-1380 Ma, and 1320 Ma. In contrast, two Neoproterozoic magmatic events (1000-930 Ma and 760-720 Ma) are typical of the eastern and southern margin of the craton. These areas contain significant portion of MORB-like basalts, implying extensive rift events. The most complete Mesoproterozoic to Neoproterozoic type section is located in the Sette-Daban Range of southeast Siberia and contains several hiatuses. The largest is between the Vendian Yudoma Group (younger than 620 Ma) and the Uy Group (older than ca. 930 Ma). Sedimentary successions on the western and southern margins of the craton contain rock units younger than ca. 720 Ma (Baikalian Complex) that are absent in the type section. On the margins of the Anabar shield below the Yudoma group, only rock units older than ca. 1380 Ma have been reported. The Mesoproterozoic to Neoproterozoic sedimentary cover of the Okhotsk and Omolon massifs correlates well with successions on the eastern margin of the Siberian craton. The Yudoma Group typically unconformably overlies underlying sedimentary rock units and locally rests on the crystalline basement. The data show inconsistency between Mesoproterozoic to Neoproterozoic sedimentary and magmatic evolution with paleocontinental restorations that plot Siberia as a single continent within the World Ocean without connection to other continental masses. Reconstructions that juxtapose southern Siberia to northern Laurentia (cf. Rainbird et al., 1998 ) or eastern Siberia to western Laurentia (cf. Sears and Price, 2003 ) show better fit with geological data, but more isotopic studies are necessary for a final assessment.