PROGRADE PSEUDOMORPHS AS INDICATORS OF METAMORPHIC PROCESSES

During metamorphism, evidence of the prograde path is commonly obliterated by continued recrystallization as temperatures increase. However, pseudomorphs formed during prograde metamorphism may provide insight into this portion of the metamorphic path. As first described by Guidotti (1968, Am.Min.), prograde staurolite (st) pseudomorphs record previously encountered thermal conditions.

Seventeen sillimanite zone metapelitic samples, each containing 2-5 muscovite-rich pseudomorphs, from the Farmington Quadrangle, Maine were investigated to determine their use as indicators of prograde metamorphic conditions. SEM-CL images, X-ray maps, and electron microprobe analyses characterize the mineral distribution, modes, and compositions within the pseudomorph and the surrounding matrix. These data were used in textural modeling studies to determine the mass transport required to replicate the observed mineralogy.

Based on modal mineralogy, pseudomorphs are divided into four major types: muscovite (mus)-rich (>70%), plagioclase (pl)-mus (10-20% pl + <10% qtz,sil), sillimanite (sil)-pl-mus (10-20% pl, sil + <8% qtz), and qtz-mus (10-25% qtz + <8% pl,sil). A biotite-rich mus-poor mantle surrounds most pseudomorphs. All pseudomorphs are prograde after staurolite as demonstrated by the coarse grain size of the randomly oriented mus, their morphology, and their partial replacement at lower grades. Textural modeling indicates that st reacts to form a pseudomorph of ~75% mus, ~10% bio, ~10% pl, and ~5% qtz in a system closed to infiltration at the handspecimen scale. Mus-rich pseudomorphs are suggested to have formed in such a system. Pl-mus pseudomorphs appear to have formed due to infiltration of Na and Al. The sil-pl-mus pseudomorphs require multiple infiltration events, whereas the qtz-mus pseudomorphs likely reflect a qtz-rich initial rock composition. This study suggests that each major pseudomorph type represents a different mechanism of formation. Subtle mineralogical differences recorded in the pseudomorphs provide evidence of conditions along the prograde path.