The classic high-T-low-P metamorphism of west-central Maine: is it post-tectonic or syntectonic? Evidence from porphyroblast-matrix relations

Devonian polymetamorphism of the Central Maine belt of sedimentary rocks ha s been regarded as "static", the result of advective heat due to sequential emplacement of "post-tectonic" granite plutons. The following evidence is cited in support of pluton-driven static metamorphism: (1) a spatial relati onship between higher-grade metamorphic zones and pluton margins, and (2) a reported random orientation of euhedral porphyroblasts within matrix fabri cs. In contrast, our observations of porphyroblast-matrix relations show th at regional metamorphism was synchronous with progressive accumulation of p lastic strain. Metasedimentary rocks have penetrative grain-shape fabrics d efined by mica and quartz. Spatial variations in the style of matrix fabric s reflect contrasts in rheology as a function of lithology, stratigraphy an d metamorphic grade. Strain partitioned into zones characterized by a high degree of parallelism between steeply oriented compositional layering and f oliation (higher-strain zones) that surround zones in which variably orient ed, moderately dipping foliation is not as strongly developed (lower-strain zones). A well-developed moderately to steeply northeast-plunging mineral- elongation lineation is pervasive in both types of structure, and is define d by the same mineral assemblage at the same grade of metamorphism. Thus, w e interpret mineral growth to record the accumulation of plastic strain. Bi otite "fish" and quartz-dominated polycrystalline aggregates in asymmetrica l pressure-shadows around porphyroblasts, which are elongate in the directi on of mineral elongation, show consistent dextral-reverse displacement alon g the mineral-elongation lineation in the plane of the foliation. Andalusit e and staurolite porphyroblasts have preferred orientations, statistically parallel to matrix fabrics, and garnet and staurolite porphyroblasts includ e a foliation (S-i) that is discontinuous with matrix foliation (S-e). In s taurolite, the obliquity of S-i with respect to S-e decreases through up to three textural zones from core to rim, to record episodic interkinematic g rowth of porphyroblasts and progressive modification of the matrix. In lowe r-strain zones, granite plutons are associated with retrogressive replaceme nt of andalusite and staurolite, which reflects preferential contact metamo rphism in these structures.