Kaersutite megacrysts and associated crystal inclusions from the Cima volcanic field, San Bernardino County, California

Abstract

Amphibole megacrysts from alkalic basalts in the Cima volcanic field are exclusively kaersutite. The refined cell parameters for a kaersutite with 0.73 Ti atoms per formula are: a=9.810(1)Å, b=18.055(2)Å, c=5.316(1)Å and beta=105.04(2)°. For 14 kaersutite megacrysts, the range of Ti per formula unit is 0.50 to 0.74 (average 0.65) and the Fe/(Fe+Mg) ratio ranges from 0.29 to 0.40. For one sample, a colorimetric technique gives Fe 2O 3/FeO as 0.28(3), possibly lower than any reported for kaersutite from alkalic basalts. Calculated Fe 2O 3/FeO ratios for the other kaersutites average 0.24. Silicon averages 5.89 atoms/formula and Al 2.40 atoms/formula. Zoning is not apparent in these megacrysts. Plagioclases are the most common inclusions in kaersutite megacrysts with compositions from An 29 to An 52 (average An 41). One small contact plagioclase crystal is An 64. Plagioclase inclusions more calcic than An 45 are compositionally similar to those we analyzed from gabbroic xenoliths in the Cima volcanic field. Plagioclase inclusions less calcic than An 45 are similar to plagioclase megacrysts from lava and tephra from the Cima volcanic field. Pyrrhotite, with less than 2% Ni, is the only sulfide inclusion. Other inclusions are augite (average Wo 46En 41Fs 13), olivine (Fo 73 and Fo 74) and magnetite (Fe 1.80Al 0.45Ti 0.37Mg 0.30Cr 0.07O 4). Two of the augite inclusions have compositions similar to Al-augite xenoliths and one has exsolved orthopyroxene and a host composition like that of green-pyroxene xenoliths. Some inclusions represent crystals which could have been incorporated within the kaersutite from gabbroic or pyroxenitic wall rock, indicating a spatial and possibly a genetic relationship. It is possible that some of these megacrysts may have crystallized from derivative mobile fluids rich in iron, titanium and alkalis in open systems similar to those found in pegmatite dikes.