The sub ject of the study are the re la tion ships be tween the Late Tri as sic A-type gran ites of the Baga-Gazriin Chuluu pluton in Cen tral Mon go lia and the mag matic bod ies lo cated ei ther ad ja cent to or a short dis tance from this in tru sion. We com pare new geo chem i cal data from the most rep re sen ta tive sam ples of the Baga-Gazriin Chuluu pluton with pub lished data from the Me so zoic granitoids of the Egiindavaa com plex and Tsagduul mas sif in the Hotont area. The geo chem i cal anal y ses of all these granitoids are also com pared with the geo chem i cal char ac ter is tics of trachyandesites of un known age which neigh bour the Baga-Gazriin pluton. The Baga-Gazriin gran ites, most of the Tsagduul gran ites and a few sam ples from Egiindavaa are A-gran ites with strong crustal af fin ity. By con trast, most of the Egiindavaa granitoids in clude con sid er able man tle-de rived ma te rial and thus seem ingly formed in dif fer ent con di tions, with a higher water con tent in the magma. The trachyandesites ad ja cent to the Baga-Gazriin Chuluu pluton have been re ferred to a Perm ian vol cano-sed imen tary com plex, mak ing them dis tinctly older than the granitoids. All the A-type gran ites dis cussed were emplaced in extensional con di tions, which de vel oped af ter ces sa tion of the compressional re gime re sult ing from the clo sure of the Mon gol-Okhotsk Ocean. The shal low-ly ing source for their gen er a tion was con nected with slab break-off or the pres ence of a hot spot. The granitoids, in the late stage of crystallisation, were en riched in com plex ions that were REE car ri ers, as shown by a dis tinct "tetrad ef fect".
Katarzyna Machowiak, In sti tute of Civil En gi neer ing, Poznañ Uni ver sity of Tech nol ogy,
In the Variscan Bohemian Massif, orthogneiss complexes of different sizes, derived from early Palaeozoic granitoids, are accompanied by metavolcano-sedimentary successions. They are common in the Sudetes, SW Poland, and belong mainly to the Saxothuringian Terrane. In order to better characterize the evolution of the latter, new U-Pb and O isotopic zircon analyses, combined with zircon typology and new U-Th-Pb isotopic monazite analyses, were performed. The present data show that the S-type granitic precursors of the Doboszowice orthogneisses (Fore-Sudetic Block) and the Śnieżnik gneisses (Sudetes) were formed at ca. 495 Ma from differently evolved magmas. Protoliths of the Doboszowice orthogneisses developed entirely in the crust, whereas the precursors to the Śnieżnik gneisses received some mantle input. Metasediments that accompany the Doboszowice orthogneisses reveal zircon spectra, which point to a late Cambrian-Early Ordovician sedimentary age. These spectra, in addition to predominant grains with Ediacaran ages, contain up to 10% of zircons dated at ca. 1.0 Ga. Therefore, it is suggested that the parent basin was supplied with detritus coming from areas, located in the Libyan-Nubian part of North Africa. Using also earlier published data, such a provenance is assigned to the units that now occur in the eastern part of the Fore-Sudetic Block. Late Cambrian-Early Ordovician granite intrusions and the concurrent accumulation of sediments originated in an extensional setting of the peri-Gondwana rifted continental margin or a back-arc setting. The Pb/U and Pb/Th monazite data constrain the ca. 346-341-Ma peak of the Variscan regional metamorphism in the eastern Saxothuringian Terrane and the prolonged juxtaposition of now adjacent tectonic units at least until 330 Ma.
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