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

Abstract

We investigate a low‐strain outcrop of the lower crust, the Pembroke Granulite, exposed in northern Fiordland, New Zealand, which exhibits localized partial melting. Migmatite and associated tschermakite–clinozoisite (TC) gneiss form irregular, elongate bodies that cut a two‐pyroxene–pargasite (PP) gneiss. Gradational boundaries between rock types, and the progressive nature of changes in mineral assemblage, microstructure and chemistry are consistent with the TC gneiss and migmatite representing modified versions of the PP gneiss. Modification is essentially isochemical, where partial modification involves hydration of the assemblage and mineral chemistry changes, and complete modification involves additional recrystallization and in situ partial melt production. Microstructures of quartz and plagioclase, including small dihedral angles, string of beads textures and films surrounding amphibole and garnet grains are consistent with the former presence of melt in modified rock types. The documented rock modification is attributed to melt–rock interaction occurring during porous melt flow of a dominantly externally derived, hydrous silicate melt. Microstructures indicate melt flow occurred along grain boundaries and field relationships show it was focused into channels tens of metres wide, with preference for following the pre‐existing foliation. Melt–rock interaction at the grain scale resulted in hydration and modification of the host PP gneiss, which resulted in localized partial melting. These relationships indicate prograde hydration during localized melt–rock interaction drove migmatization of the lower crust.

The recognition of former melt flux through high‐strain zones

“…Grain size is typically uniform within a single domain but varies widely between high‐strain zones from 100 to 1,000 μm. Low‐ and intermediate‐strain domains display coronas of magnesio‐hornblende and quartz in a matrix of plagioclase and clinozoisite (Figure a), microstructures typical of replacement of enstatite and diopside throughout the Pembroke Granulite (Stuart et al., , ). In these domains, S 3 foliation is defined by elongation and SPO of coronas.…”

“…Emplacement and D 1 were closely followed by a major magmatic flare‐up event, involving emplacement of high Sr/Y magma into the arc system (126–115 Ma; Allibone et al., ; Hollis, Clarke, Klepeis, Daczko, & Ireland, ; Hollis et al., ; Milan, Daczko, & Clarke, ; Milan, Daczko, Clarke, & Allibone, ; Tulloch & Kimbrough, ). The Pembroke Granulite was situated in the lower crust during this period, and subsequent metamorphism is interpreted to have occurred during the flare‐up, involving melt–rock interaction during the movement of these high Sr/Y magmas to higher crustal levels (Stuart, Daczko, & Piazolo, ; Stuart et al., ). Timing of individual events is not well constrained in the literature, and so relative ages as determined in previous studies are based on field relationships.…”

“…Both the partially hydrated S 1 foliation and the D 2 dykes and GRZ were hydrated by melt‐rock interaction during a second occurrence of porous flow of an externally derived melt (Stuart et al., ). This melt flow event is interpreted to be partially buoyancy‐driven (pre‐D 3 and early D 3 ) and deformation‐driven in high strain zones during D 3 .…”

“…() examined melt flux that was spatially focussed into ~10 m wide low‐strain channels, subparallel to the strike of S 1 within which the assemblage was hydrated to tschermakite, garnet, plagioclase, clinozoisite and rutile. Localised partial melting occurred in the low‐strain channels and was triggered by melt flux related hydration, where small (~5%) volumes of locally derived, in‐situ melt were produced forming migmatite (Daczko, Clarke, et al., ; Stuart et al., ). The zones of low‐strain migmatite are spatially associated with D 3 high‐strain zones suggesting that the porous flow through low‐strain channels may have been coeval with but occurred outside of the highly localised D 3 deformation.…”

“…High‐strain zones record intense contractional deformation, involving a strain magnitude ( R f ) of 4–13 (where R f is the stretch component of deformation) and up to 60% sub‐horizontal shortening (Daczko, Klepeis, et al., ). S 3 foliation (variably called S 3 and S 4 in past literature) formed at 674 ± 36°C and 14 ± 1.3 kbar and is defined by garnet, magnesio‐hornblende, clinozoisite, plagioclase, and biotite (Daczko, Klepeis, et al., ), where the garnet is interpreted as a peritectic phase indicative of minor partial melting within high‐strain zones (Figure e; Stuart et al., ).…”