Petrographic Evidence for Excess Silica in Antiperthitic Plagioclase of the Montpelier Anorthosite, Virginia, and a Link with Extensive Myrmekite Development

The Montpelier anorthosite in the Piedmont Province of Virginia ranges from undeformed and massive to gneissic and completely recrystallized.  Plagioclase in undeformed rocks is highly antiperthitic (bulk compositions ~An₂₆Or₂₀).  In many cases, K-spar lamellae in antiperthite contain additional segments of quartz, providing plausible evidence for excess silica in these original high-T feldspars.  The anorthosite also locally displays a unique type of “myrmekite,” consisting of sodic plagioclase (~90 vol%), vermicular quartz (~8 vol%), and minor K-spar (≤ 2 vol%).  The scale of myrmekite development varies from extensive areas up to several cms across adjacent to antiperthite to isolated “blebs” (rods?) within antiperthite.  Many myrmekite regions show an enrichment in K-spar at the boundary with antiperthite.  Plagioclase grains in antiperthite (~An_32.6Or_1.2) and myrmekite (~An_33.5Or_1.6) have identical compositions considering the respective standard deviations.  Thus, this myrmekite type differs from the calcic variety described from other anorthosites, suggesting a different formation mechanism.  It also differs from the more typical myrmekite in granites, which develops in contact with K-spar.  At Montpelier, myrmekite appears to be derived directly (perhaps isochemically) from antiperthite based on:  (1) bulbous protrusions of myrmekite into antiperthite;  (2) similarity of plagioclase compositions in antiperthite and myrmekite;  and (3) small relic (?) blebs of K-spar within myrmekite as well the K-spar-rich margins.  Our current thinking regarding this unusual variety of myrmekite is that it represents the vermicular breakdown product of a feldspar containing excess silica, perhaps as an intermediate step on the path to complete recrystallization.  Quartz in the exsolved lamellae in antiperthite remains as the vermicular quartz in myrmekite;  some K-spar remains within myrmekite but much is concentrated at grain boundaries, either due to granular exsolution or “expulsion” from the growing myrmekite.  The apparent rarity of this myrmekite type may reflect the unique composition of Montpelier feldspars coupled with the deformational history.