The first U-Pb age data of zircons from relict spreading zones in the Middle Urals

Abstract

Zones of paleospreading and crustal extension inthe Urals, as well as around the world, are clearlymarked sheeted dyke complexes. In the Middle Uralssuch a complex was first described by S.N. Ivanov andhis colleagues in 1973 [1] as a relic fragment of theoceanic spreading crust. Later, complexes of paralleldikes were distinguished and described in detail alongthe Urals fold belt, from th e Polar Urals in the north toWestern Mugodzhary Mountains in the south [2–8,etc.]. However, there are still no reliable isotope agedata for these sheeted dyke complexes. As a rule, theage of formation of dikes was determined on the basisof age data of the surrounding rocks [2, 4, etc.] or wereconsidered to be the same as the age of gabbro andbasalts from the ophiolite sections studied.We have made an attempt to obtain isotope age datafor accessory zircons from dolerite dykes of the representative ophiolite complex of Mt. Azov (MiddleUrals) (Fig. 1). Within this complex, fragments of thesheeted dyke complex, extending with breaks over60 km in the eastern framework of the Revda massif,are part of the Uralian Platinum Belt [6, etc.]. Gabbros and pyroxenites of the Revda massif and surrounding basalts were intruded by dolerite dykes. Contacts between dykes and surrounding rocks are oftencomplicated by numerous tectonic faults. The sheeteddyke complex is well exposed among basaltic andandesitebasaltic pillow lavas in the upper part of Mt.Azov (Fig. 1), located 5 km west of the town ofPolevskoi. Parallel dolerite dykes of northeasternstrike and steep southeastern or northwestern dippingare mainly 0.5–2 m thick. Dykes form swarms anddikeindike structures. In total, the volume of dykesexceeds the volume of pillow lavas approximately twofold. As noted in [1, 6, 7], in the contact zone betweendolerites and pillow lavas, hardening zones with athickness of a few centimeters and nonhardened contacts with rugged edges occur. It follows that somedykes first intruded into the hot pillow lava strata; subsequent portions of the magma intruded into thealready cooled strata. The dykes are made of finegrained gabbrodolerites and porphyritic doleriteswith plagioclase and hornblende phenocrysts, as wellas less common aphyric and fine porphyritic varieties.Dolerites were metamorphosed under the conditionsof lower greenschist facies. Microscopically, they usually consist of a amphibole–sossurite aggregate withchlorite, quartz, pumpellyite, and an ore mineral.The dyke complex and pillow lavas are presented bylowK basalts and andesite–basalts of normal alkalinity [7, etc.]. Dolerites are characterized by a lowerTi content (TiO