Origin of Mafic Pegmatoids in the Dais Intrusion, Wright Valley, Antarctica

The Dais Intrusion is a differentiated layered mafic intrusion that is part of the Ferrar dolerites in the McMurdo Dry Valleys. Mafic pegmatoids occur as irregularly-shaped bodies in the upper part of the intrusion, above its modally layered zone. These pegmatoids are dominantly plagioclase and pyroxene with minor quartz, alkali feldspar, amphibole, biotite, apatite, and oxides. We have undertaken detailed petrographic and geochemical investigations of representative samples of pegmatoid, gabbronorite taken 2.4 m from the nearest pegmatoid, and the contact between the two rocks. The surrounding gabbronorite is interpreted to be a cumulate, because it does not have the composition of a liquid saturated with plagioclase + augite + low-Ca pyroxene at low pressure, and the rock is poor in P and Ti. Relative to the surrounding gabbronorite, the pegmatoid is richer in SiO2 and FeO and enriched by a factor of 2.4 to 2.7 in incompatible elements (e.g. Ti, K, Ba, Rb, Nb). Although plagioclase compositions overlap in all three rocks, those in the pegmatoid average about 20% lower An than the surrounding rock. Plagioclase in the surrounding gabbronorite and contact rock exhibits irregular zoning characterized by wide normal zones punctuated by sudden jumps to higher An. Plagioclase in the pegmatoid is either unzoned or normally zoned, with cores similar in composition to that of the rims of plagioclase in the surrounding gabbronorite. Chlorine in apatite from the pegmatoid is lower than that in the host rocks. Despite the incompatible element enrichment of the bulk rocks, ion probe analyses reveal that plagioclase and pyroxene of the pegmatoid are not systematically enriched in incompatible elements relative to the surrounding gabbronorite. The subequal incompatible element ratios (e.g. Ba/Nb; K/Ti) in all three rock types and the apatite compositions suggest that the pegmatoid formed from the intrusion of an evolved silicate liquid extracted from the gabbronorite cumulate, and the pegmatoid also acted as a pathway for a volatile phase that originated from the underlying and surrounding cumulates. The mineral zoning profiles and trace element contents suggest that both the gabbronorite and pegmatoid are a complex mixture of preexisting crystals and evolved, hydrous silicate liquid, with the latter being dominant in the pegmatoid.