Kiglapait Geochemistry IV: The Major Elements

Abstract

The summed bulk composition of the Kiglapait intrusion is SiO 2 47.46 (31). TiO 2 0.81 (20), Al 2O 3 19.46 (48), Fe 2O 3 1.91 (46), FeO 9.36 (36), MnO 0.15 (01). MgO 8.04 (21), CaO 9.27 (46). Na 2O 3.13 (05), K 2O 0.22 (01), P 2O 5, 0.09 (01), and BaO 0.02 (01). total 99.98 wt% (estimated standard deviations of the last two decimals in parentheses). The CIPW norm is ap 0.34, mt 2.78, il 1.52, or 1.11, ab 26.27, an 38.64, ( fsp 66.47), di 5.38, hy 2.45, ol 21.58. Molar X An is 0.58; molar x fo is 0.65. The variation curves for oxides in calculated liquids are explained in detail by the petrographic variations shown in earlier papers of this series. The Kiglapait FeO + MnO:MgO:Alkalies (FMA) trend reaches higher iron concentrations than the Skaergaard liquid and constitutes a new limiting case of the Fenner trend of fractional crystallization, implying low oxygen fugacity and silica activity. The end-stage Kiglapait ferrosyenite lies on the FeO + MnO-Alkalies sideline, being essentially devoid of MgO. Except in K and K Rb. the Kiglapait composition is the same within experimental error as the chilled margin of the nearby Hettasch intrusion (BERG, 1980.), but slight systematic differences show that the Hettasch composition is slightly more fractionated or represents a smaller fraction of melting at the source. Both are low-augite troctolite compositions, and they require troctolitic parent liquids. Such liquids are related to common augite-rich basalts by addition of spinel, and they imply exhaustion of clinopyroxene in the source. Except for their low augite and high Al 2O 3, the Kiglapait and Hettasch compositions resemble modern MORB and suggest a depleted source.