New data on the age of lamproite-lamprophyre magmatism in the Urals

Abstract

Several occurrences of lamproites on the eastern slopeof the Urals are objects of our investigation (Fig. 1). Thelamproite occurrence situated in the central and easternpart of the Paleozoic Magnitogorsk island-arc system ata few tens of kilometers northeast from Magnitogorskwas described in [4, 9]. These lamproites make up apetrogenetic series with lamprophyres (monchiquite,camptonite, minette, and kersantite) identified as theKalymbai Complex [9]. The second lamproite occur-rence (Pervomaiskii area) is located in the eastern partof the East Ural Uplift 40 km south of Chelyabinsk nearPervomaiskii Settlement [3, 7]. This area includes anumber of pipe-shaped magnetic anomalies, as well asdiatremes and dikes that crosscut the Devonian rocks. Inaddition, we previously studied the lamproite occurrencenear Skalistyi Settlement 25 km west of the town ofTroitsk in the Transural region [6], where lamproite dikescrosscut adamellites of the Lower Sanarka pluton.Based on Rb–Sr and K–Ar datings, the ages of thelamproites mentioned above vary from 198 to 295 Ma[1–4]. Lamproites of this age are related to the postcol-lision extension of the Earth’s crust [9], tectonomag-matic reactivation of the young platform along latitudi-nal transverse faults [4], or manifestation of an epicon-tinental hot spot [11].Lamprophyres have been investigated in the Urals toa lesser extent than lamproites, although they occur inmany places and vary in age. The best studied and mosteasily accessible lamprophyre related to the alkaline–ultrabasic potassic magmatism is represented by a dikein the Shartash granitic pluton on the outskirts of Yeka-terinburg [5]. This occurrence is situated at the conti-nental margin west of the Murzinka–Adui Block of theEast Ural Uplift. The K–Ar age of the Shartash dike,which is appreciably altered at the contact with granite,ranges from 261 Ma (reaction amphibole at the contact)to 274 Ma (phlogopite) [5].In this communication, we present the first results of