DIKTYTAXITIC TEXTURE IN BASALTS: A CLUE TO "FILTER-PRESSING" IN ANORTHOSITES?

Because massif anorthosites are clearly igneous, yet cannot have formed from magma of their own composition, many workers appeal to "filter-pressing" as a means for removing residual liquid from a plagioclase-rich crystal mush. Yet rarely is the actual mechanism defined. It is difficult to envision a set of external forces that could effectively "squeeze" the liquid from a mush. I suggest that diktytaxitic texture in basalt may provide a clue. The texture refers to a network of crystals (mainly plagioclase and Fe-Mg silicates) enclosing angular cavities. Fuller (AJS, 1938) suggested that expanding volatiles expelled residual liquid, leaving an open network of crystals; vapor-driven filter-pressing has also been proposed for silicic magmas (Sisson and Bacon, 1999; Geology). A key requirement is that a semi-rigid framework of crystals form prior to vapor saturation. Exsolution and subsequent expansion of vapor can thus provide an internal force for expulsion of residual melt.

Many features of Laramie anorthosites (LA) can be explained if a similar mechanism had operated there - with a C-O rich fluid being the driving force to expel residual liquid as dikes and pods (some pegmatitic) of ferrodiorite, leaving behind a semi-rigid network of plagioclase laths. Eventually the interstitial fluid would escape, and lithostatic pressure would then close the cavities - consistent with the facts that deformed LA plagioclase has been crushed rather than sheared (LaFrance et al. CMP, 1996) and that typically anorthosite layers are much more deformed than intervening leucogabbroic layers. Based on emplacement P, T, and fO₂ of ~ 3 kbar, ~1200^oC, and ~1 log unit below FMQ, XCO in the fluid would be 0.10-0.15. Upon cooling and decompression, much of the CO would decompose to graphite + CO₂, consistent with the presence of minor graphite within the LA and major "plumbago" deposits in the surrounding host rock. Finally, it is likely that a high activity of some C-O species is essential for the Fe-enrichment and Si-depletion characteristic of the ferrodiorites and jotunites so typical of massif anorthosites - and believed by many to represent the residual melts. Thus vapor-driven "filter-pressing" is consistent with many aspects of the LA.