BARIAN TITANIAN PHLOGOPITE FROM POTASSIC LAVAS IN NORTHEAST CHINA - CHEMISTRY, SUBSTITUTIONS, AND PARAGENESIS

Magmatic barian titanian phlogopite (BaO = 4.2-11.2 wt% and TiO2 = 10. 1-13.1 wt%) occurs in the groundmass and in a magmatic inclusion of ol ivine leucitites from northeast China. In addition, titanian phlogopit e (BaO < 0.3 wt% and TiO2 less-than-or-equal-to 6.4 wt%) formed by rea ction between olivine phenocrysts and late fluids also occurs as coron as rimming olivine phenocrysts in associated leucite basanites. Compos itions of the barian titanian phlogopite vary systematically; Al2O3 an d TiO2 increase and SiO2, MgO, FeO, and K2O decrease with increasing B aO. Substitutions deduced from such variations include Ba + (Al,Fe3+) = K + Si and 2(Mg,Fe2+) = Ti + square. Ti-O and Ti-Tschermak's substit utions may also be present in subordinate amounts. The Ba + Al = K + S i substitution is common to most magmatic barian micas, except for som e lamproitic micas. In contrast, no general Ti substitution scheme is applicable, as Ti substitutions vary with melt compositions and intens ive variables. These phlogopite samples crystallized as a late phase a t low pressures and temperatures. Differences in mineral assemblages b etween olivine leucitites and leucite basanites contribute significant ly to the observed disparities in chemical composition between the bar ian titanian phlogopite and the corona phlogopite and, in general, amo ng phlogopite from other alkalic magmatic rocks.