The origin of arfvedsonite in metabasites from the contact aureole of the Kharlovo gabbro intrusion (Russia)

In the contact aureole of the Kharlovo gabbro (NW foot of the Altay Mts, S Siberia) against rare amygdaloidal diabases, hornblende rimmed by arfvedsonite is reported; estimated P-T conditions were 1-1.5 kbar and 525-550 degrees C. EPMA results are given for minerals in these metabasic rocks, together with a 12-point traverse across a hornblende crystal rimmed (with a sharp boundary) by arfvedsonite. The arfvedsonite has alpha (brown) 1.697, beta (blue) 1.701, Gamma (dark blue) 1.703, and is considered to have formed under local saturation of pore solution by Si, Na and Fe in a very small volume as a result of a local metasomatic reaction, controlled mainly by Na and Si leaching during the incongruent dissolution of acid plagioclase. Mass balance calculations and the variation in composition and amount of coexisting minerals indicate that the transfer of the main petrogenetic components was limited to volumes as small as 0.06-0.1 mm (super 3) .