Zirconology of nepheline syenite of the Berdyaush Massif (Southern Urals)

Abstract

Problems of the origin of rocks of the BerdyaushMassif including nepheline syenite (NS) have beenconsidered many times [1–5], which allows us torestrict ourselves to a short description of only thosedata directly related to the mentioned problem. A.N.Zavaritskii [1] distinguished two stages in the formation of the massif: the earlier stage including intrusionsof basic and granite magmas, the formation of hybridrocks and rapakivi, and the later stage comprisingintrusions of syenite and related NS. In [2] thissequence is complemented by the appearance ofalkalirich basalt, as well as the formation of dykes ofdiabasic porphyrite, orthophyre, bostonite, and NS inthe Late Riphean–Early Vendian. In essence, theseslightly different models illustrate the controversialnature of the problem of NS origin and their relationswith other rocks of the massif.Geochronological information about NS does notgive a direct answer to these questions. The K–Ar ageof amphibole from NS (1370 Ma) is considered asconfirmation of their relationships with rapakivi, anddating of 670–716 Ma is related to the processes ofalbitization and recrystallization of NS [3]. Later [4]not only new K–Ar data (1245–1347 Ma), but Rb–Srdata of 1350 ± 17 Ma (formation) and 750 Ma (metasomatic transformations) as well were obtained withNS. The first data on zircons from NS (Sample K633,Golaya Mt.) appeared in 1986 [5]. Their mineralogicalproperties were described; crushing, replacement ofearly generations by late ones with synchronousdecrease of U concentrations from 2890 to 970 ppm inthem, and geochemical heterogeneity in the distribution of Th, Y, and Fe were registered. Attention waspaid to the violation of isotopic age relationships inzircons and clearly reflected differences of theirappearance, composition, and sizes from such zirconsof other rocks of the massif significantly depleted in U(96–207 ppm). The classic U–Pb method by zirconsprovided the age parameters of the NS formation (1354 ±20 Ma) and their metamorphism (750–800 Ma).The development of new analytical methods madeit possible to date not only single crystals, but theirindividual parts as well, which significantly widenedthe possibilities to obtain age information on heterogeneous crystals. Using the possibilities of a SHRIMPmicroprobe, the age of zircons from rocks related tonepheline syenite from the Berdyaush Massif wasdetermined as 1373 ± 21 Ma [6]. Although it was closeto the previously obtained ones, the age problem wasnot solved. Besides that, the absence of the information about the composition and location of the studiedrocks in [6] did not allow us to carry out the comparative analysis of the studied rocks and estimate theirconsistence to each other. Clearly it was necessary toconfirm the age information for zircons (Sample K633) previously obtained by the classic U–Pb methodby datings of individual crystals. Using the knownSHRIMP methodology [7, 8], the results obtained forthis sample are given in table and in Figs. 1–3. It is evident that they are consistent with the previouslyobtained results and in addition more clearly reflectthe development of the secondary porous substance atthe expense of the primary homogeneous one. Isotopic analyses of both phases were performed for Crystal 5;only U and Th were measured for crystals 9 and 10. Thetotal crystal community was divided into three groups(Fig. 2): (I) nebular, porous (3, 5.1, 6, 8, and 9);(II) intermediate (1 and 5.2); (III) homogeneous,transparent (2, 4, 7, and 10). Zircons I were significantly depleted in U and Th (U