The effect of pre‐tectonic reaction and annealing extent on behaviour during subsequent deformation: insights from paired shear zones in the lower crust of Fiordland, New Zealand

Abstract

Granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow (<20 mm) felsic dykes, in Fiordland, New Zealand, forming~10-80 mm wide garnet reaction zones. The metamorphic reaction changed the abundance of minerals, and their shape and grain size distribution. The extent of reaction and annealing (temperature-related coarsening and nucleation) is greatest close to the dykes, whereas further away the reaction is incomplete. As a consequence, grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres. The aspect ratios of clusters of S1 pyroxene and pargasite in the orthogneiss, which are variably reacted to post-S1 garnet, decrease from high in the host, to near equidimensional close to the dyke. Post-reaction deformation localized in the fine-grained partially reacted areas. This produced a pattern of 'paired' shear zones located at the outer parts of the garnet reaction zone. Our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction. The weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage.

The recognition of former melt flux through high‐strain zones

“…D 2 involved flow of melt through a series of sub‐vertical, rectilinear fractures, forming plagioclase‐, garnet‐ and quartz‐bearing dykes which cut the partially hydrated S 1 foliation. Melt‐rock interaction dehydrated the host assemblage in layers (cm‐dm) around the dykes, transforming the partially hydrated S 1 assemblage to pale pink layers comprising the D 2 assemblage garnet, clinopyroxene, plagioclase, and rutile (Blattner, , ; Clarke, Daczko, Klepeis, & Rushmer, ; Clarke, Klepeis, & Daczko, ; Daczko, Clarke, & Klepeis, ; Daczko & Halpin, ; Schröter et al., ; Smith, Piazolo, Daczko, & Evans, ). Termed garnet reaction zones (GRZ), they represent peak metamorphism in the Pembroke Granulite, forming at conditions of 680–815°C and 11–14 kbar (Daczko & Halpin, ; Stowell et al., ).…”

Mass transfer in the lower crust: Evidence for incipient melt assisted flow along grain boundaries in the deep arc granulites of Fiordland, New Zealand

“…The S 1 assemblage is defined by enstatite, diopside, pargasite, plagioclase, and ilmenite; published P‐T estimates for orthopyroxene bearing assemblages vary from < 6 to < 11 kbar and 650–850°C [ Clarke et al ., ; Daczko and Halpin , ; Stowell et al ., ]. Following the formation of the foliation, garnet reaction zones (GRZ) formed in narrow (cm – dm) zones in a mostly isochemical garnet granulite facies reequilibration of the host rock, thought to be triggered by the channelized intrusion of trondhjemitic melts [ Clarke et al ., ; Daczko et al ., ; Schröter et al ., ; Smith et al ., ] or sodic dehydration fluids [ Blattner , ]. Formation of GRZ was likely to be long lived, with dates smearing from 126 to 122 Ma [ Stowell et al ., ] and estimated P‐T conditions ranging between 11–14 kbar and 680–815°C [ Daczko et al ., ; Stowell et al ., ].…”

“…Formation of GRZ was likely to be long lived, with dates smearing from 126 to 122 Ma [Stowell et al, 2010] and estimated P-T conditions ranging between 11-14 kbar and 680-8158C [Daczko et al, 2001b;Stowell et al, 2010]. Subsequent deformation during exhumation was highly localized, preserving the lower crustal assemblage and microstructures throughout the majority of the Pembroke Granulite [Blattner, 1991;Daczko et al, 2001a;Gardner et al, 2016;Smith et al, 2015].…”

Local partial melting of the lower crust triggered by hydration through melt–rock interaction: an example from Fiordland, New Zealand

“…Outcrops of the Pembroke Granulite are dominated by two-pyroxene-pargasite (PP) gneiss that hosts narrow felsic dykes with centimetre-to decimetre-wide GRZ (for regional geology and detailed studies of the Pembroke Granulite see Blattner, 1991Blattner, , 2005Clarke et al, 2000Clarke et al, , 2005Daczko et al, 2001aDaczko et al, ,b, 2002aDaczko et al, , 2016Schr€ oter et al, 2004;Stevenson et al, 2005;Allibone et al, 2009b;Smith et al, 2015;Gardner et al, 2016;Stuart et al, 2016). The study area (044°35 0 06.30″S 167°53 0 20.94″E) represents a low D 3 strain area preserving a variably transposed and pseudomorphed S 1 foliation.…”