The first finding of explosive alkaline picrites at Nadan’hada-Alin’ (China)

Abstract

During joint Russian-Chinese investigations of contiguous territories in the Nadan’khada mountain range (China), explosive micaceous picrites earlier unknown here but widespread in adjacent regions of Primor’e, where small diamonds were found in them, were discovered. The described picrites are attributed to the Yurassic alkaline-ultrabasic volcano-plutonic formation consisting of alkaline basalts, meimechites, and concentric zoned sienite-peridotite intrusions. The discovered body (m-6m) is a diatreme, which was proved by ultrabasic inclusions and original (symplektitic) forms of chrome spinel disaggregations in them. As well as in Primor’e, the body is localized in Jurassic picrite-basalts in framing of large gabbro-peridotite in Zhaokheiskii massif. The rock is composed of large (up to 2 cm) megacrystals of kaersutite and titanphlogopite, sunk into a fine mass of microlites of the same minerals and ilmenite cemented by calcite. Grains of kaersutite have zoning with enrichment of the rims with Mg and Ti and depletion with K. Calcite is of two generations: (a) the initial magmatic in the main mass, enriched with Sr, and (b) the secondary, intersecting all the above-mentioned minerals and containing only insignificant additions of Mn. Phlogopite loses Ti (from 6 to 2% TiO2) at the contact with this calcite. However, K-kaersutite transfers into glaucophane (up to 7% Na2O) at the contact with magmatic calcite. Spinel can be found only in ultrabasic inclusions replaced by talc. In the core of the grains it is moderately chromous (39.45% Cr2O3), on the rims it is replaced by titanous-manganous-flinty (up to 2% SiO2) ferrochromite. Ilmenite is highly manganous and does not contain Mg and Cr. Enrichment by both siderophile and lithophile microelements is characteristic of the entire rock. The examined picrites are fluidized explosive apophysis of large alkaline-ultrabasic intrusions in fact and deserve more detailed investigation because of small diamond findings in them.