Carbonatites in collisional settings and pseudo-carbonatites of the Early Paleozoic Ol’khon collisional system

Abstract

Carbonatites associated with syenites and subalkalic mafic rocks (lamprophyres) occur in the Himalayan continental collision zone, and this suggests their possible existence in other Phanerozoic collisional settings. The Early Paleozoic Ol’khon collisional system in Western Cisbaikalia is considered one of the possible occurrences. Subalkalic gabbroids as well as peculiar carbonate (brucite marbles) and calc-silicate rocks were found here, within the Tazheran massif of alkalic and nepheline syenites. Alkalic syenites, nepheline syenites, and calciphyres were dated at 471 Ma, 451–464 Ma, and 466 Ma, respectively, and their ages correspond to the main collisional events in the system. A geochemical description of igneous and carbonate rocks in the massif is provided. Close mapping showed unusual syenite and brucite marble combinations and the frequent vein or pipe-like form of carbonates and calciphyres corresponding to their magmatic intrusion. But carbonatite nature of the marble mentioned above does not fit their typical crustal geochemical features. Not ruling out the possibility of total change of geochemical signatures of mantle carbonatite in the collisional medium, we offer two other possible explanations for these facts: (1) melting of carbonate masses by syenite and mafic magmas, followed by carbonate melt intrusion into the upper crust, and (2) protrusion of carbonates into syenites and gabbroids at the late stages of the contact action of a silicate magma. In this case the above-mentioned carbonate structural features result from late recrystallization, whose mechanism is yet to be explained.

Isotope (U–Pb, Sm–Nd, Rb–Sr) geochronology of alkaline basic plutons of the Kuznetsk Alatau

“…Note that products of Early Paleozoic alkaline magmatism are also present in other parts of the CAFB. Close ages were established for some intrusive massifs in Cisbaikalia and western Transbaikalia (~470-520 Ma), East Sayan (~490-520 Ma), Gorny Altai (~510 Ma), and Tuva (~490 Ma) (Doroshkevich et al, 2012;Kozakov et al, 2003;Nikiforov and Yarmolyuk, 2007;Sklyarov et al, 2009;Vrublevskii et al, 2012a).…”

The Overmaraat-Gol Alkaline Pluton in Northern Mongolia: U–Pb Age and Preliminary Implications for Magma Sources and Tectonic Setting

“…Feldspathoid-rich (foidolites, nepheline syenites) igneous rocks in the western Central Asian Orogenic Belt (CAOB) are mostly from the Paleozoic ages. Such magmatism culminated at 520-470 Ma, 405-385 Ma, and 310-260 Ma in the northeastern Kuznetsk Alatau, western Baikal, western Transbaikalia, southeastern Tuva, and southeastern Russian Altai regions [5,7,[12][13][14][15][16] (Figure 1a). Judging by their petrography, the rocks mostly belong to a differentiated magma series of subalkaline gabbro and theralite-foidolite-nepheline and alkaline syenite.…”

New insights into the origin of the Evate apatite-iron oxide-carbonate deposit, Northeastern Mozambique, constrained by mineralogy, textures, thermochronometry, and fluid inclusions

“…Compared to the content of critical metals in other carbonatites, the Evate deposit is basically barren and may be similar to collisional pseudo-carbonatites which originate through the remelting of carbonate crust (e.g. Sklyarov et al 2009). …”