Deep-seated pegmatites of the Emiytas mafic-ultramafic complex on Big Lyakhov Island, New Siberian Islands, and their age: 40Ar/39Ar and SHRIMP data

• Published in: Geochemistry international Vol. 47; no. 2; pp. 186 - 198
• Main Authors: Kuzmichev, A. B., Ponomarchuk, V. A., Konilov, A. N., Paderin, I. P.
• Format: Journal Article
• Language: English
• Published: Dordrecht SP MAIK Nauka/Interperiodica 01-02-2009; Springer Nature B.V
• Subjects: Crystallization; Crystals; Earth and Environmental Science; Earth Sciences; Evolution; Geochemistry; Jurassic; Mesozoic; Mineralogy; Paleozoic; Plate tectonics; Precambrian; Shellfish; Triassic; Early Jurassic; 206Pb; Geochemistry International; Zircon; 86Sr
• ISSN: 0016-7029; 1556-1968
• DOI: 10.1134/S0016702909020062

Pegmatites of the Emiytas basic-ultrabasic metamorphic complex have a granodiorite-granite composition. Their genetic relations with the host amphibolites follow from the low initial
87
Sr/
86
Sr ratios of 0.7028 and from the
P-T
conditions (650°C and 10 kbar), which are close to those of the host rocks. Amphibole, biotite, and muscovite megacrysts analyzed by the
40
Ar/
39
Ar method yielded plateau ages of 209.7 ± 0.9, 203.0 ± 0.9, and 178.5 ± 1.5 Ma, respectively. The former two minerals contain excess Ar, whereas the K-Ar system of the muscovite is undisturbed. The cooling of the complex to the closure temperature of this system was likely controlled by its exhumation to a shallower depth level. Zircons from the Emiytas pegmatites occur as polyhedral equant crystals with weakly contrasting sectorial zoning, very low concentrations of U (4–8 ppm in the enriched domains), and low Th/U ratios (0.002–0.003), which suggest that the mineral crystallized at significant depth in the presence of fluid. Tentative SHRIMP II measurements (five analyses) yielded a zircon age of 202 ± 17 Ma. The applying a specialized approach to the analysis of young low-U zircons on an ion probe is discussed. In spite of the small number of analyses, new geochronologic data leave no doubt that the crystallization age of the pegmatites is Late Triassic-Early Jurassic and invalidate earlier ideas that the Emiytas complex is Precambrian-Early Paleozoic. This conclusion makes the Emiytas amphibolites to be one of the various oceanic and suprasubduction complexes related to the Mesozoic South Anyui suture, which is important for reconstructions of the tectonic evolution of the East Siberian Arctic shelf.