Specifics of geological composition, geochemistry and geochronology of rocks from the Kresty alkaline-ultrabasic massif (Maimecha-Kotui province, Polar Siberia)

In this work, we demonstrate new data that allows us to accurate geochronological ranges of formation of the Kresty alkaline-ultrabasic massif, which is considered to be a satellite of the Gulinian giant pluton. We also interpreted geological, geochemical and isotope-geochemical data obtained earlier for major varieties of this volcanic-plutonic association taking into account new geochronological results, as well as considered new aspects/information on matter source of alkaline-ultrabasic massifs from this province. One of the main aspects is interaction of Siberian super plume matter with hosting substrate of Siberian craton continental crust.

By their petrochemical parameters, the KVPS rocks conform to those of tholeiite (dunites, wehrlite, and clinopyroxenites), subalkaline (trachydolerites, trachyandesites, and syenitoids), and alkaline series (melilitolites, monticellitolites, nephelinites, and alkaline picrites), mainly of potassium-sodium specialization. These rocks, as all MKP magmatites, are characterized by high concentrations of most of trace elements comparable to numbers in oceanic island basalts (OIB), which confirms plume nature of magmatism in the province. Heterogeneity of the products of tholeiite and alkaline series is well pronounced in the behavior of the most magmatophilic elements (Nb, Ta, Th, Sr, LREE) concentrated mainly in alkaline melts. The content of REE and La/Yb ratio increase subsequently in series from olivinites and wehrlites to pyroxenites, nephelinites, and melilitolites, reaching maximum in monticellitolites. The dykes of syenites and trachydolerites show moderate concentrations of lanthanides comparable to ultrabasic rocks of normal alkalinity, which negates thereby the model of differentiation from a single parent melt. The fractionation degree of rare elements increases in following series: "volcanites→ dyke formation → main intrusive phases", at the same time obvious negative anomalies of Rb, Ba, Sr, Nb, Zr, Hf, and Ti appear in olivine-rich variations. Geochemical heterogeneity of the KVPS rocks is also traced in the parameters of the neodymium and strontium isotope composition, whose variations indicate autonomous matter sources for associations of normal and high alkalinity [3]. However, a valid Sm-Nd isochrone line was obtained only for the whole rock compositions of olivine, wherlites, and two pyroxenites, yielding the age of 251 ± 20 Ma (Ɛ Nd (T) = + 2,0). Alkaline ultramafic rocks show higher primary 143 Nd/ 144 Nd ratios (Ɛ Nd (T) = + 2,4 ÷ 3,1), whereas decreased values of this parameter (from -0.6 to -15.9) are typical for trachyandesites and syenitoids. The probable heterogeneity of the sources of rock associations is also shown in the values of the T DM model age. Compared to the small intrusions of alkaline syenites (~ 2.36 Ga), younger dates were found for melanephelenites and monticellitolites (~ 590-620 Ma). The isotope composition of strontium shows a wider variability with I 0 values ranging from 0.7035-0.7037 (for nephelinites, melilitolites, and monticellitolites) and 0.7038-0.7041 (for olivinites, wehrlites, and pyroxeniteto 0.705-0.708 (for trachydolerites and alkaline syenites). The ratios of oxygen isotopes in whole-rock compositions and in rock-forming pyroxene (δ 18 O = + 1.1 ÷ 4.9 %) suggest considerable interaction of the rocks with meteor waters permeating probably as deep as 500 m.

Analytical techniques
New U-Pb isotopic data on zircon grains was studied. It represents two intrusive phases of the Kresty massif, including one of the main rock types like kugdites and vein types of rocks like alkaline syenites. Zircons from this object were analyzed with SHRIMP-II microprobe at the Russian Geological Research Institute named after A.P. Karpinsky (St. Peterburg) using a standard technique with a spot diameter of 18 µm. The internal structure of mineral grains was determined with cathodoluminescent images taken on ABT55 scanning electron microscope in a usual operating mode. Data was processed with SQUID software. The U/Pb ratios were normalized to the value of 0.0668 of the TEMORA standard zircon. The errors of isotope ratio and age measurements were within ± 1ϭ, and the errors in calculation of concordant ages and intersections with concordia were within ± 2 ϭ. The plots with concordia were built using the ISOPLOT/Ex software.

Results and Discussion
Our study has given new proof of these very short-term intrusive events being connected to Siberian plume. Our new geochronological data is demonstrated in Table 1  Kugdite ages were determined on perovskite grains, which is one of the main accessory minerals of this rock. Despite small error, the age (249±3.4 Ma) corresponds fully to the Paleozoic-Mesozoic boundary. More accurate dating was obtained from zircons for another rock variety (alkaline syenite type), which represents one of the latest phases of dyke complex formation in this intrusive massif (Table 2, Figure. 3).     Errors are 1-sigma; Pbc and Pb * indicate the common and radiogenic portions, respectively.
Error in Standard calibration was 0.29%( not included in above errors but required when comparing data from different mounts).
(1) Common Pb corrected using measured 204 Pb. Ages of alkaline syenites determined on accessory zircon grains turned out to be more accurate, considering range of possible errors. Real age values correspond to the same values of 252 ± 1.0 Ma (Fig. 3). Patterns in isotopic evolution of KVPS rocks are obviously shown on ε Nd (T)-ε Sr (T) diagram, where composition points of petrographic varieties of Maimecha-Kotui province form composition field expanding in a fan-like pattern from moderately depleted mantle of HIMU and PREMA type into the area of enriched sources EMI and EMII. Boundaries between them coincide with lines of mixing of lower crust (LC) and upper crust (UC) matters (mixing in ratios from LC:UC = 10:1 to LC:UC = 1:10) [3]. At the same time, isotope compositions of tholeiitic and alkaline magma products show noticeable discreteness only at the base of the Mesozoic volcanogenic mass/thickness (Pravoboyarsk, Onkuchansk and Tyvankitsk series) or in separate intrusive bodies (Kugda massif) distant from assumed "finger plume" center. As they get closer to the "finger plume" center, composition fields of magmatic rocks move closer to each other and subsequently move towards area of depleted mantle substrates. Similar variations of isotope composition with certain level of Sr enrichment are also seen in flood basalts from Norilsk region [9]. We can assume that it might indicate lower permeability of mantle matter and its interaction with brines of sedimentary cover of the platform.
By their isotope composition, volcanic and plutonic KVPS varieties take intermediate position between their analogues in the Gulinian and Kugda massifs in a relatively narrow contrast range between the alkaline and tholeiite derivatives. This is quite consistent with the scale of their spatial distance from the probable center of "finger plume". The most complete coincidence of isotope