Geochemistry and petrology of komatiites of the Pioneer Ultramafic Complex of the 3.3Ga Weltevreden Formation, Barberton greenstone belt, South Africa

摘要

The 3.3Ga Weltevreden Formation, located in the northern part of the Barberton greenstone belt, contains multiple complexes that are composed largely of well-preserved, partially serpentinized komatiitic rock. The Pioneer Ultramafic Complex is a 1-km-thick section of massive and differentiated komatiites and spinifex-textured komatiitic basalts that are interbedded with current-structured komatiitic tuffs. Most komatiites were emplaced subaqueously as large sheet flows that would on occasion pond, cool slowly and internally differentiate. Between effusive eruptions, explosive komatiitic volcanism resulted in the deposition of up to 60-m-thick sections of fine-grained ash. Massive komatiites preserve primitive olivines with a maximum Mg# of 92.9, supporting the eruption of a high temperature, high magnesian liquid. Al _2O _3/TiO _2 (19-34) and Gd/Yb _N (0.84-1.15) ratios indicate that the komatiites and komatiitic basalts originated from Al-undepleted to slightly Al-enriched magma. Fresh pigeonite found in the cores of zoned pyroxenes suggests that crystallization occurred under anhydrous conditions. Similar to komatiitic augites analyzed in other areas of the Barberton greenstone belt, those in the Pioneer Complex possess both high Wo contents (0.31-0.46) and high Mg#s (75-90.2) but these features are not attributed to high dissolved water contents in the melt.