Devonian volcanics in the Voronezh Crystalline Massif, East European Platform: Evolution of the melts and characteristics of crustal contamination

• Published in: Petrology 2017-05, Vol.25 (3), p.241-271
• Main Authors: Yutkina, E. V., Nosova, A. A., Sazonova, L. V., Larionova, Yu. O., Kondrashov, I. A., Shumlyanskyy, L. V., Albekov, A. Yu, Savko, K. A.
• Format: Article
• Language: English
• Subjects: Area; Asthenosphere; Basalt; Boreholes; Cambrian; Computer simulation; Contamination; Copper; Crystal structure; Crystallinity; Crystallization; DDT; Devonian; Diapirs; Earth and Environmental Science; Earth Sciences; Fields; Fractional crystallization; Geochemistry; Kimberlites; Lava; Magma; Melts (crystal growth); Mineralogy; Niobium; Olivine; Peridotite; Petrography; Petrology; Plagioclase; Sandstone; Shoulder; Silica; Silicon dioxide; Strontium 87; Strontium isotopes; Tholeiite; Volcanic rocks; Zinc
• ISSN: 0869-5911; 1556-2085
• DOI: 10.1134/S0869591117020060

The rift system of the Dnieper–Donets trough (DDT) is the largest magmatic area in the East European Platform. Basalts of the Voronezh Crystalline Massif (VCM) are spatially constrained to the eastern shoulder of DDT and occur far away (at a distance of 150–200 km) from the rift axis. The rocks are hosted in the Paleoproterozoic Vorontsovskii terrane and are grouped in a few fields within an area of 200 × 100 km. Basalts at most of the fields were erupted at the boundary between mid- and late Frasnian time in the Late Devonian and can be studied exclusively in core material recovered by boreholes. Newly obtained mineralogical, geochemical, and isotopic-geochemical data show that the Devonian volcanic rocks in VCM are tholeiites ( Bas ) and basaltic andesites/andesites ( ABas ). The geological section was examined most exhaustively in the Novokhopersk area (Borehole 175). The bottom of the vertical section is made up of basaltic andesites and andesites ( ABas ) (thickness 34 m), which rest on an eroded surface of late Frasnian sandstones. The rocks are overlain by a thin (8 m thick) tholeiite sheet ( Bas2 ), which gives way to ABas (13 m) upsection. The top portion of the vertical section is composed of tholeiites with petrography and geochemical evidence of crustal contamination ( Bas1 ) (apparent thickness 5 m). Geochemical parameters of Bas (mg# 42–52 at SiO 2 47–51 wt %) are typical of continental tholeiites. The rocks have ( 87 Sr/ 86 Sr) 0 = 0.7043–0.7048 and ε Nd (372) = 2.1–3.5. ABas (mg# 28–31 at SiO 2 52–60 wt %) are enriched in Y (48 ppm), and possess Nb/Nb* = 0.7–0.8 and high Zn/Cu = 1.9–2.3. The rocks have ( 87 Sr/ 86 Sr) 0 = 0.7034–0.7048 and Nd–ε Nd (372) = 0.1. Some portions of Bas melts assimilated the upper crustal material, which was similar to Paleoproterozoic granites, and ABas are contaminated in the lower crust with derivatives of Early Cambrian alkaline mafic melts. Petrographic data and simulations of fractional crystallization show that olivine and high-Mg clinopyroxene were the first to crystallize from the melt. After this, clinopyroxene and plagioclase simultaneously crystallized at temperatures from 1070 to 1020°C in Bas and at 1040–900°C at f O2 below QFM + 1 in ABas . The source of ABas was likely a network of hornblendite or amphibole pyroxenite veins in peridotite in the lithospheric mantle or amphibolized peridotite cumulate in an underplating zone; and Bas were derived from spinel peridotites of an asthenospheric diapir.