Formation of low-temperature mylonites and phyllonites by alkali-metasomatic weakening of felsic volcanic rocks during progressive, subduction-related deformation

Dacitic to rhyolitic volcanic rocks of the Spruce Lake nappe experienced tw o phases of alkali-metasomatism as a result of fluids channelling along she ar zones. The shear zones formed during a progressive, thrust-related defor mation associated with underplating and incorporation of the volcanic rucks into the Brunswick subduction complex of northern New Brunswick. The fluid s mainly represent chemically and isotopically modified seawater released b y dewatering of the associated underthrusted shaly sedimentary rocks. Both phases of metasomatism weakened the felsic rocks, leading to strain localis ation. Albitisation of felsic volcanic rocks as a result of Na-metasomatism during underthrusting facilitated formation of mylonites near peak high-pr essure metamorphism (330-370 degreesC, 600-800 MPa). The mylonites are pref erentially preserved in the roof-thrust shear zone of the Spruce Lake nappe . Core-mantle structures. bulging and crystallographically preferred orient ations indicate that albite behaved more ductilely than K-feldspar. The duc tility of albite at these low temperatures is interpreted as a function of abundant intragranular fluids. Phengite-rich phyllonites formed after peak high-pressure metamorphism during uplift by out-of-sequence thrusting. Thes e phyllonites an generally characterised by a slight gain in K and loss of Na and are best developed in the basal sheer zones of the Spruce Lake nappe . (C) 2001 Elsevier Science Ltd. All rights reserved.