Results of New Field and Geochemical Studies of the Volcanic and Intrusive Rocks of the Maymecha-Kotuy Area, Siberian Flood-Basalt Province, Russia

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Abstract
The Maymecha-Kotuy area, comprising ∼70,000 km2 of the northern part of the Siberian flood-basalt province, is of unusual interest because it appears to be the only such province in the world where high-Ti, alkaline-ultramafic rocks with associated carbonatites predominate over basaltic extrusive and intrusive rocks. New field and geochemical studies of the igneous rocks of this area were initiated with the goals of (1) correlating them with the magmatic formations of the well-studied Noril'sk area and (2) reconstructing the entire magmatic evolution of Siberian flood-basalt volcanism. This report presents the first complete stratigraphic section for the volcanic sequence of the Maymecha River Basin (the most extensive in the Maymecha-Kotuy area), based on flow-by-flow mapping and sample collection. The geochemical and lithologic characteristics of the volcanic and intrusive rocks are thoroughly documented and show an unusually broad range in composition–e.g., SiO2 and MgO contents range from 40 to 70 and from 2, in which a large, laccolithic intrusive mass of dunitic to peridotitic rocks was breached by a central volcanic edifice from which flowed ankaratrite and picritic ankaratrite lavas. Cutting the ankaratrites and picritic ankaratrites of the complex, but intimately related to it, is a variety of intrusive, alkaline rock types and two carbonatite bodies that may be related to a central feeder zone. We recognize four magma types as products of Siberian flood-basalt volcanism and refer to them herein as low-Ti-I, low-Ti-2, moderate-Ti, and high-Ti. In contrast to the Noril'sk area, where high-Ti magmas constitute <1 vol% of the igneous rocks, their proportion is ∼50 vol% in the Maymecha River Basin. These high-Ti rocks can be subdivided into trachybasaltic, melanephelinitic, and meymechitic rock series; clearly related to these series, judging from trace-element characteristics, are associated trachyandesitic, trachytic, and felsic lavas. In general, the trachyandesitic, trachytic, and felsic lavas appear to be related to the trachybasaltic series through fractionation, but some of the felsic tuffs display characteristics that suggest their relation to the melanephelinitie series. Previous investigators have concluded that meymechitic magma formed by low degrees of mantle-peridotite melting (<7%) at pressures corresponding to depths of 250 to 300 km. The distinctions between the trachybasaltic, melanephelinitic, and meymechitic rock series must relate to different sources and conditions of melting. It is remarkable that no geochemical signature of crustal contamination can be recognized for any of the high-Ti rocks, including the felsic lavas and tuffs. We conclude that the alternation of low-Ti and high-Ti magma types observed in the lower half of the volcanic sequence in the Maymecha River Basin resulted from repetitive expulsion from discrete mantle sources through independent plumbing systems.