DEBRIS SOURCE AREA EVOLUTION DURING FORMATION OF THE URALIAN FOREDEEP AT THE SAKMARIAN STAGE OF THE EARLY PERMIAN ACCORDING TO U-Pb LA-ICP-MS ISOTOPE DATING OF DETRITAL ZIRCONS

Abstract

The article presents the results of studying the sources of terrigenous material deposition in the Uralian foredeep according to U-Pb dating of detrital zircons from terrigenous rocks of four stratigraphic levels of the Kapysov formation (Sakmarian stage). The Kapysov formation is one of the formations of the Lower Permian section of the Yuryuzano-Sylvenskaya depression in the Uralian foredeep. It is composed of conglomerates and sandstones and unconformably overlies the underlying formations. Samples 14110 and 14110-1 were taken from the same section exposed in the quarry near the village of Tatarsky Maloyaz, from coarse-grained sandstones of the conglomerate matrix in the lower parts of the formation; sample 15004 was taken in the quarry near the town of Mikhailovsk, from gravel sandstones in the middle part of the formation; sample 18228 was taken in the quarry at the southern end of the village of Elgildino, from sandstones of the upper part of the formation. In each sample. there were determined two zircon populations: Precambrian (mostly rounded grains) and Paleozoic (mostly unrounded grains). The relative amount of grains of the Precambrian population decreases gradually up the section from 90.9 to 31.8 %, with a corresponding increase in the amount of Paleozoic zircons. The Precambrian zircon age spectra of all samples are approximately the same, which suggests that they were derived from the same source. This assumption does not contradict the results of the Kolmogorov-Smirnov test (the coefficient varies from 0.316 to 0.999). These spectra are almost identical with the zircon age spectra in the Vendian Asha group of the Bashkir megazone from which the detrital zircon grains could have been derived. This situation suggests a very likely possibility of short-distance transportation since the complexes of the Yuryuzano-Sylvenskaya depression are overlapping directrly with the Bashkir megazone. The age spectra of the Paleozoic deterital zircon grains differ significantly for different levels. The Paleozoic zircon population gradually attains the younger age up the section of the the Kapysov formation: from the Ems to the Middle Carboniferous. Zircon grains with an age of about 400 million years could have been derived by erosion of the ophiolite and island-arc rocks of the Urals whose age is similar to that of the Baymak-Buribai complex. The Carboniferous zircon crystals are the products of the destruction of magmatic complexes in the Magnitogorsk and East Ural megazones.

Keywords

• uralian foredeep
• lower permian
• sandstones
• kapysov formation
• detrital zircon