ANORTHOSITES AND RELATED MEGACRYSTIC UNITS IN THE EVOLUTION OF ARCHEAN CRUST

Abstract

Archean anorthositic complexes occur in essentially all Archean cratons and contain large equidimensional plagioclase crystals (up to 30 cm. diam.) with highly calcic compositions (An 80 to An 90 ) but are not readily amenable to determination of their parent melt compositions. However, insight into petrogenesis of the complexes is provided by megacrysts of plagioclase that are identical to those in the complexes and occur in many Archean flows, sills, and dikes whose matrices display REE and fractionation patterns that indicate tholeiitic trends and are compatible with prior subtraction of plagioclase during earlier evolution of the melts. Included blocks of anorthosite and megacrysts with very thin rims that approach the more sodic composition of lathy plagioclase in the matrices indicate an earlier stage of cryst formation under different conditions of crystallization than the matrices. The megacrystic units occur both in greenstone belts that have oceanic affinities and stable cratonic dike swarms that have continental affinities. Both major and trace element contents of the matrices of the megacrystic units differ between greenstone and cratonic dike environments; the dikes being higher in Si0 2 , TiO 2 FeO, Na 2 , K 2 O, and light REEs but lower in Al 2 O 3 and CaO. The matrices of both environments follow separate but parallel tholeiitic fractionation with high Fe-enrichment trends similar to Skaergaard liquids suggesting relatively low volatiles and f o 2 . Experimental data and projections in CMAF space suggest a multistage petrogenesis involving a relatively high-pressure fractionation of olivine and/or orthopyroxene from a primitive mafic melt followed by ascension of the fractionated, less-dense melt, probably in several pulses, to a low-pressure chamber, probably at 1 to 2 kb. The depressurization accompanied by cooling could easily place the melt composition in the plagioclase field and significantly below the liquidus resulting in several crystallization cycles of plagioclase in the low pressure chamber. The melts would crystallize as anorthositic complexes and periodically expel pulses that would form the observed megacrystic flows, sills, and dikes.