Minerals of the rh?nite-kuratite series in paralavas from a new combustion metamorphic complex in the Choir–Nyalga basin (Central Mongolia): composition, mineral assemblages and formation conditions

摘要

This is the first description of rare minerals found in paralavas from a recently discovered combustion metamorphic complex in the Choir-Nyalga basin, Central Mongolia. The identified minerals contain strongly variable concentrations of Si,Ti, Mg, Fe2+ and Fe3+ and most commonly have compositions intermediate in a series from kuratite Ca4Fe102+Ti2O4[Si8Al4O36] and rh?nite Ca4(Mg,Fe2+)8Fe23+Ti2O4[Si6Al6O36] to low-Ti kuratite and unnamed Ti-free Fe2+-analogue of rh?nite Ca4Fe82+Fe43+O4[Si8Al4O36]. The minerals crystallized in residual Si-Al-K and Si-Al-Ca-Fe immiscible melts after spinel, anorthite-bytownite, melilite, Al-clinopyroxene ± Mg-Fe olivine, together with Fe3+-bearing hercynite, Ca-rich fayalite, kirschsteinite, pyrrhotite ± native iron, wüstite, magnetite, celsian, hyalophane, Ba-orthoclase and fresnoite, but before nepheline ± kalsilite, and later sulfates, carbonates, an unidentified ‘X-mineral’ close to Al- and Fe-rich tobermorite and goethite. Micro-Raman spectroscopy of kuratite shows five bands near 133-155 (strong), 399-401, 545-566, 684-693 (strongest) and 828-839 cm?1. The kuratite-bearing Nyalga paralavas have bulk compositions with MgO/(MgO+FeO+Fe2O3), mol.% ～0.5 and a CIPW normative ratio of Ne/(Ne+Lc) = 0.23-0.76. Minerals of the rh?nite-kuratite series formed during paralava crystallization at ～1100°C. The diversity of the paralava mineral assemblages might result from local composition variations of Ca-rich silica-undersaturated melts derived from Fe-bearing carbonate-silicate sediments which were affected by nearby coal combustion sources at reducing conditions (IW-WM-QFM buffers) and at a nearly atmospheric pressure.