Provenance of the Lower Mesoproterozoic Sandstones, East Anabar Shield: U-Pb Dating and Chemical Composition of Detrital Zircons and Sm-Nd Isotopic Studies
Sergey Malyshev1, Andrei Khudoley1, Anatoliy Molchanov2, and Nataliya Berezhnaya2
1St.Petersburg State University, St.Petersburg, Russian Federation.
2Russian Geological Research Institute (VSEGEI), St. Petersburg, Russian Federation.
Isotopic and geochemical studies are important in the provenance and paleogeographic restorations. Here we present results of study of sandstones from the lowermost unit of Mesoproterozoic succession of the Anabar shield. Both U-Pb dating and geochemical studies of detrital zircons were done by SHRIMP-II (VSEGEI, St. Petersburg, Russia).
Total age distributions yield three populations of detrital zircons at 2.9 - 2.4 Ga, 1.8 - 2.0 Ga and 1.8 - 1.69 Ga. The youngest grain with age 1690 Ma indicates that sandstone deposition in the Anabar shield started after this time. High abundance of 1.8 - 2.0 Ga ages shows that most of Archean rocks of the Anabar shield were metamorphosed around this age.
For most zircons distribution of REE, Th and U do not show correlation with grains age, color and morphology, and interpretation of chemical data is equivocal. However, one sample from the northern Anabar shield contains poorly rounded needle-shaped zircon grains with Th and U content typically varying from 40 to 150 ppm, and the average content of REE is around 500-1000 ppm. Th/U ratios typically range from 0,8 to 1,5. Other types of zircons typically have higher concentration of Th and U, and less Th/U ration value. Needle-shaped zircons also have significant negative Eu anomaly in comparison with other grains. Needle-shaped zircon grains likely have a sub-volcanic origin from local source. They are ca. 1700-1730 Ma in age and are the youngest among dated detrital zircons. However, latest Paleoproterozoic felsic volcanics were never reported from the northern Siberia.
Similar in age zircons have been reported from rhyolites of the Khakdon Group, Kolyma region, the Ulkan Complex from Aldan shield and, probably, from Okhotsk Massif (Khudoley et al. 2007 and references therein). Sm-Nd study of sandstones from the Anabar shield and Kolyma region show similar characteristics in terms of e(t) and model ages. Model ages vary from 2.4 to 2.65 Ga and are similar in sandstones with and without needle-shaped grains pointing to erosion of old crust. Wide distribution of ca. 1700-1730 Ma magmatic event in the Siberian craton and adjoining crystalline terranes with associated intense erosion of old crust likely reflect the beginning of a craton-scale rifting resulted in formation of Mesoproterozoic sedimentary basins.
AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.