Formation of low-temperature mylonites and phyllonites by alkali-metasomatic weakening of felsic volcanic rocks during progressive, subduction-related deformation
Cr. Van Staal et al., Formation of low-temperature mylonites and phyllonites by alkali-metasomatic weakening of felsic volcanic rocks during progressive, subduction-related deformation, J STRUC GEO, 23(6-7), 2001, pp. 903-921
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.