Igneous Carbonates in Dolerites of Franz Joseph Land

Carbonates showing the signs of crystallization from a melt were found in dolerites and basalts of lava covers and dikes of Alexandra Land, Heiss, and Newcomb islands. These carbonates may fill interstitials between silicate minerals, as well as constitute wormlike or amebiform separations in dolerites. In other cases, the “bulbs” are formed within acidic glass in the centers of globules consisting of pyroxene and rimmed with ilmenite crystals in cryptocrystalline basalts. Most of the carbonate separations are constituted of siderite (80–95% of siderite minal); calcites (up to 20% of siderite minal) and carbonates of calcite–dolomite isomorphic series are found less frequently. In view of the plume nature of volcanic rocks at the archipelago, the capture of carbonatite melt by silicate magma seems to be the most likely. However, the possibility of the magma capture and melting of residual siderite fragments from the underlying terrigenous formation of Mesozoic age cannot be excluded.