OCCURRENCE AND SIGNIFICANCE OF HIGH GRADE METAMORPHIC MINERAL ASSEMBLAGES (INCLUDING KYANITE) IN EASTERN MAINE

The Devonian (384 Ma) Deblois granitic pluton complex (DPC) intrudes metasediments of the Fredericton Trough (Merrimack-Harpswell terrane) and St. Croix terrane. These terranes were amalgamated in the late Ordovician-early Silurian along the Old Stream fault, assumed to be the eastern extension of the Sennebec Pond thrust fault. Rocks of the Fredericton Trough and St. Croix terrane show the widespread effects of low grade (greenschist facies, chlorite zone) metamorphism. Typical mineral assemblages include musc-qtz-plag (arenites), musc-chlor-qtz-plag (pelites), and musc-chlor-qtz-plag-calcite±ankerite (calcareous flysch). This metamorphism accompanied deformation associated with terrane collision and is presumably late Ordovician to early Silurian in age.

Higher grade minerals are developed in some gneisses and migmatites close to the DPC. These minerals include chltd, staur, gt, cd, plag, K-spar, andalusite, sill (prismatic and fibrolite) and ky in pelites, plag, amph and sphene in metabasalts, and plag, amph and cpx in calcareous lithologies. Accessory tourmaline also occurs in metapelites. This is the first report of ky in rocks from eastern Maine.

Textures in thin section are complex. Evidence of both prograde (ky®sill, musc®K-spar) and retrograde (amph®bi, bi®chlor) reactions are preserved. The foliation wraps around garnet and staurolite porphyoblasts, whereas kyanite contains kinked twin lamellae. These observations suggest that high grade metamorphism is not soley the result of thermal effects associated with intrusion of the DPC. Rather it appears that the effects of high temperature contact metamorphism may be superimposed on mineral assemblages developed during high-grade regional metamorphism. In particular, it is unlikely that ky developed in response to intrusion of the DPC at relatively low pressures (0.1-0.2 Gpa). We suggest that these high-grade rocks were uplifted as blocks along splays of the Old Stream fault during terrane collision. If so, they provide a window into conditions the deeper parts of the crust in eastern Maine in the Salinic orogeny.