The origin of carbonatites and carbonatite-type rocks remains a debatable petrologic problem. Therefore, any information on findings of these rocks is valuable for the better understanding of key mechanisms of their formation. We found carbonatite tuffs among eruption products of the early Neogene Amba Volcano in southwestern Primorye (Fig. 1). The volcanic edifice occupies an area of ~20 km 2 in the middle reaches of the Amba River at a distance of 28 km from Amur Bay. The basalts of the stratovolcano base overlie the Permian carbonate and volcanosedimentary rocks and the Triassic arkose sandstones. The basalts are, in turn, overlapped by the upper Miocene volcanosedimentary rocks of the Ust-Suifun Formation and basalts of the Shufan Plateau. The central vent of the Amba Volcano is filled with explosion breccia and basaltic lavas. Pyroclastic rocks, including layers of carbonatite tuffs, compose the peripheral facies of the volcano. The eruption is characterized by a sequential transition from olivine‐clinopyroxene to spinel‐Ti-augite‐ plagioclase varieties. Spinel and clinopyroxene megacrysts (3‐8 mm across) in basalts contain rounded carbonate inclusions. In addition, the basalt groundmass contains numerous late carbonate segregations in gas cavities. The segregations are especially abundant in the vent facies, indicating predominance of carbon dioxide among volcanic gases. The basalts are characterized by subalkali, high-Ti, high-Mg, and high-Ca composition with low content of silica, high contents of Sr, Ba, Nb, and Zr, and less common Ce, Nd, Sm, and Th (Table 1). According to their HFSE and REE contents, they correspond to withinplate alkali basalts with negative slope of PM-normalized REE pattern. The carbonatite tuff layers were found in the southwestern part of the volcanic edifice on the right wall of the Vtoroi Creek, the right tributary of the Amba River. The welded carbonatite tuffs occur as two layers (2.0‐ 2.5 m thick) at the base and top of the visible section. They are separated by interlayers of tuffaceous basaltic sandstones and gravelstones with a variable carbonate admixture. The tuffs are dark gray massive, severely welded rocks with psammitic and psephitic dimensions of crystal fragments and lithic clasts. The high degree of tuff welding is related to enrichment of the natural carbonate‐silicate cement in calcareous component. Tuffs contain fragments of tree trunks and xenoliths of intrusive and sedimentary rocks from the basement. The geological observations indicate that the bedded volcaniclastic member of carbonatite tuffs was formed as a result of multiple directed volcanic explosions with formation of gas-saturated pyroclastic flows deposited in depressions (often waterlogged) around the volcano. The welded tuffs in the section are supplemented with tufflavas represented by more compact carbonate‐ silicate aggregates (20‐25 cm in size) with large spinel and clinopyroxene phenocrysts. Walls of numerous gas cavities and pores in this rock are coated with pale crystalline carbonate. The carbonatite tuffs are composed of crystal and lithic clasts. The white or light gray lithic clasts (0.2‐ 0.5 cm in diameter) have irregular or rounded (ocellar) shape. They are largely composed of carbonate material with spinel and pyroxene phenocrysts and plagioclase laths (Figs. 2, 3). The lithic clasts have an unusually high content (up to 3‐5 vol %) of rather large (up to 5‐ 12 mm) crystals of aluminous spinel and their fragments. The fassaite clinopyroxene is less abundant (Table 2). They also contain relatively large (up to 0.5 cm) orange (less frequently, transparent white or cherrycolored) euhedral and rhombohedral Fe‐Mg-calcite crystals incorporated into the cement (Fig. 3, Table 2). Crystals of spinel (with faces of dissolution), clinopyroxene, and calcite occur in both the carbonatite clasts
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Jan 1, 2009
D M Polyakov 
