metacarbonate rocks of the paleoproterozoic khapchan series (southeastern part of the anabar shield): mineral and chemical composition, metamorphic conditions

The mineral composition of metacarbonate rocks (silicate marbles and carbonate-silicate rocks) of the khapchan series (southeastern part of the anabar shield) was studied, and the pt (pressure and temperature)-parameters of their formation were established. silicate marbles contain calcite, dolomite, forsterite, clinohumite, spinel, enstatite, diopside, pargasite, meionite, phlogopite, and feldspars. carbonate-silicate rocks are composed of calcite, quartz, feldspars, diopside, grossular, marialite, and vesuvianite. carbonate-silicate rocks are significantly enriched in sio2, al2o3, feo, na2o, k2o, tio2 and contain less mgo, cao than silicate marbles. a difference was revealed in pt-parameters determined for silicate marbles (temperatures 700-900 °c and pressure no more than 8 kbar) and for carbonate-silicate rocks (temperatures 680-820 °c, pressures 8-15 kbar). silicate marbles have a primary sedimentary nature, as evidenced by their rare-element composition and the presence of fragments of host terrigenous rocks. there is no doubt about the primary sedimentary nature of carbonate-silicate rocks, which are very similar in ree distribution spectra and in rare-element composition to silicate marbles. a number of features indicate that metacarbonate rocks have undergone metasomatic alteration. thus, in silicate marbles, reaction rims are observed around orthopyroxene, forsterite, potassium feldspar, as well as quartz veins bordered by accumulations of phlogopite, feldspars, and diopside. in carbonate-silicate rocks, the development of secondary marialite on potassium feldspar has been established; the rare-element composition of garnet may indicate its metasomatic origin.