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

Abstract

Knowledge of mass transfer is critical in improving our understanding of crustal evolution, however mass transfer mechanisms are debated, especially in arc environments. The Pembroke Granulite is a gabbroic gneiss, passively exhumed from depths of >45 km from the arc root of Fiordland, New Zealand. Here, enstatite and diopside grains are replaced by coronas of pargasite and quartz, which may be asymmetric, recording hydration of the gabbroic gneiss. The coronas contain microstructures indicative of the former presence of melt, supported by pseudosection modeling consistent with the reaction having occurred near the solidus of the rock (630-7108C, 8.8-12.4 kbar). Homogeneous mineral chemistry in reaction products indicates an open system, despite limited metasomatism at the hand sample scale. We propose the partial replacement microstructures are a result of a reaction involving an externally derived hydrous, silicate melt and the relatively anhydrous, high-grade assemblage. Trace element mapping reveals a correlation between reaction microstructure development and bands of high-Sr plagioclase, recording pathways of the reactant melt along grain boundaries. Replacement microstructures record pathways of diffuse porous melt flow at a kilometer scale within the lower crust, which was assisted by small proportions of incipient melt providing a permeable network. This work recognizes melt flux through the lower crust in the absence of significant metasomatism, which may be more common than is currently recognized. As similar microstructures are found elsewhere within the exposed Fiordland lower crustal arc rocks, mass transfer of melt by diffuse porous flow may have fluxed an area >10,000 km 2 .

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

“…Relict igneous pyroxene is variably replaced by a hydrous assemblage of coronitic blue–green pargasite and quartz symplectite. The symplectite minerals show no evidence of internal deformation or recrystallization, and have been interpreted to have formed under static conditions post‐D 1 due to melt–rock interaction during diffuse porous melt flow along grain boundaries (Stuart et al ., ). The GRZ form during a second melt–rock interaction event where garnet pseudomorphs coronitic pargasite.…”

The recognition of former melt flux through high‐strain zones

“…The Pembroke Granulite, Fiordland, New Zealand (Figure ), is a relatively low‐strain portion of the Median Batholith (Figure , inset), a composite regional batholith comprising Carboniferous to Early Cretaceous plutons from the lower crust of a Cordilleran magmatic arc (Blattner, ; Mortimer et al., ). The gabbroic protolith to the Pembroke Granulite was emplaced at 136–129 Ma (Hollis, Clarke, Klepeis, Daczko, & Ireland, ; Stowell, Tulloch, Zuluaga, & Koenig, ) and had an igneous assemblage of enstatite, diopside, brown‐green pargasite, plagioclase, and ilmenite (Stuart, Piazolo, & Daczko, ). Though commonly gabbroic, the whole rock composition of the protolith varies by several weight percent for all major element oxides and has some minor grain size variations (Stuart et al., ).…”

“…The gabbroic protolith to the Pembroke Granulite was emplaced at 136–129 Ma (Hollis, Clarke, Klepeis, Daczko, & Ireland, ; Stowell, Tulloch, Zuluaga, & Koenig, ) and had an igneous assemblage of enstatite, diopside, brown‐green pargasite, plagioclase, and ilmenite (Stuart, Piazolo, & Daczko, ). Though commonly gabbroic, the whole rock composition of the protolith varies by several weight percent for all major element oxides and has some minor grain size variations (Stuart et al., ). The igneous assemblage was partially recrystallized during D 1 , forming a gneissic foliation (S 1 ) striking generally NE and dipping steeply to the north and south.…”

Plutonic xenoliths from Martinique, Lesser Antilles: evidence for open system processes and reactive melt flow in island arc crust

“…Exposed roots of volcanic arcs, such as the Kohistan palaeo-island arc, Pakistan (Bouilhol et al 2015), Talkeetna, Alaska (Greene et al 2006), Fjordland, New Zealand (Stuart et al 2016) and exposed ultramafic complexes, Alaska (Murray 1972;Irvine 1974) allow for direct observations of the lower crust, and provide an analogue to Martinique plutonic xenoliths. In Kohistan, kilometre scale magmatic conduits are present in the lower crust, which melt rose through and reacted with the existing cumulate assemblages (Bouilhol et al 2015).…”

“…McBirney and Noyes 1979;Morse 1996), or in an open system involving the input of percolating reactive melts or liquids (Reiners 1998;Coogan et al 2000Coogan et al , 2001Meurer and Claeson 2002;Leuthold et al 2014;Smith 2014;Bouilhol et al 2015;Stuart et al 2016). It has been shown that the products of reactive liquid flow will differ from one produced by simple crystallisation along a liquid line of descent (Reiners 1998) and the addition of low degree melts cause large variations in incompatible trace elements.…”