Mafic dikes of the Mariinsky Taiga Alkaline Province, Kuznetsk Alatau terrane, southwestern Siberia: Intraplate alkaline magmatism in the Central Asian Orogenic Belt

Abstract

The Mariinsky Taiga Alkaline Province (MTAP) is located in the Kuznetsk Alatau terrane of southwestern Siberia, which is part of Altai-Sayan orogenic system, of the Central Asian Orogenic Belt. The MTAP consists of a wide distribution of Paleozoic alkaline-basite magmatism (volcanic, subvolcanic and intrusive), which has been recently characterized with the recognition of at least two main timings, ca. 500 and ca. 400 Ma (herein labeled MTAP-1 and MTAP-2, respectively). We newly characterize the petrology, geochemistry, and age constraints of NNW and NNE trending alkaline dikes which cross-cut the alkaline intrusions and host volcanogenic and carbonate units and which are inferred to belong to MTAP-2. The dikes consist of ultrabasic foidolites (urtites, urtite-porphyry, ijolite-porphyry, microijolites), basic foidolites (plagioclase ijolites), alkaline basites (gabbro-diabase, labradorite porphyrites, camptonites and tamaraites), and nepheline, analcime and alkaline syenites. Their chemical composition is characterized by a wide range in silica (SiO 2 = 38.7–60.7 wt%), and alkalinity (Na 2 O + K 2 O = 4–18.7 wt%; Na 2 O/K 2 O = 0.5–8.3 wt%), low titanium (TiO 2 = 0.07–2.15 wt%) and high alumina content (Al 2 O 3 = 11.1–28.5 wt%), which corresponds to the K-Na differentiates of the basic alkaline magmatic series. The sum of rare-earth elements (ΣREE) increases from alkaline basite (ΣREE = 103.58–218.94 ppm), to the main foidolites (ΣREE = 137.74–217.61 ppm), and then to the ultrabasic foidolites (ΣREE = 55.56–268.88 ppm). Maximum REE content occurs in nepheline syenites and microsyenites (ΣREE = 73.24–1566.96 ppm) and they are the final product of the differentiation of foidite and basite melts. All dike varieties are characterized by low concentrations of the most highly charged elements (Th, Nb, Ta, Hf, and Y), which, according to their concentrations, plot between oceanic island basalt (OIB) and island arc basalt (IAB). The presence of a negative Nb-Ta anomaly and the relative enrichment of Rb, Ba, Sr and U indicate probable interaction of a mantle plume melts with lithospheric mantle metasomatized in an earlier subduction event. The primary isotope ratio 87 Sr/ 86 Sr(t) = 0.705388–0.706025 and the range of ɛNd(t) from +3.5 to +6.53 in the rocks indicate interaction of mantle derived melts with crustal components. Sm-Nd and U-Pb dating of the alkaline-basite dikes matches the boundary of the Early-Middle Devonian (406–389 Ma) and indicates membership in MTAP-2, which can be linked to the regional ca. 400 Ma Altai-Sayan Large Igneous Province/Rift System. Both the NNW and NNE trending dikes are associated with, and probably fed from, nearby alkaline intrusions such as i.e. Kiya-Shaltyr pluton which are also ca. 400 Ma (part of MTAP-2), but are independent of older ca. 500 Ma intrusions (which belong to MTAP-1) such as the University pluton and which are suggested to belong to a older plume-generated regional event. • Characterization of NNW and NNE trending dike belts in the MTAP, western Siberia. • They formed in the Middle Paleozoic (406–389 Ma) in a plume-rifting event. • Isotopic parameters ɛSr(t) indicate a strong contamination by crustal components. • The dike represent intraplate magmas emplaced in a shear setting.