TIMING OF HIGH PRESSURE METAMORPHISM AND PARTIAL MELTING FIORDLAND, NEW ZEALAND

Euhedral garnet porphyroblasts, garnet reaction zones, and leucocratic melt veins are common throughout much of western Fiordland. These textures have been interpreted to indicate partial melting at lower crustal conditions. In northern Fiordland (Pembroke valley), large garnet porphyroblasts grew in the Arthur River Complex during emplacement of high Sr/Y [HiSY] Western Fiordland Orthogneiss [WFO] magmas and likely records crustal heating by these intrusions at high pressure. Early garnet growth at 126.0±2.1 Ma [6 points] is recorded in mafic amphibolite that shows no evidence of melting. These garnet are dominantly unzoned with Alm=49, Prp=21, Grs=27, & Sps=3; however, narrow rims are Alm=45, Prp=24, Grs=30, & Sps=1. These late-stage rims are inferred to have grown synchronous to later peritectic garnet at 122.6±2.0 Ma [5 points] that occurs in intermediate gneiss with abundant evidence for melting. Peritectic garnet exhibits little or major element zoning with Alm=55, Prp=22, Grs=18, & Sps=5. LA-ICPMS maps of peritectic garnet indicate little to no zoning in Sm & Nd and concentrations that match those from mass spectrometry [1.5 to 4 ppm]. However, there is significant zoning in other trace elements. For example, Lu & Yb form broad annuli with concentrations of ca. 5 and 30 ppm, respectively. These annuli have concentrations less half that of adjacent core and rim. In southern Fiordland, peritectic garnet and melt occur within WFO plutons. On Resolution Island, garnet grew at 109.7±1.8 Ma directly after emplacement of the plutons. Garnet ages indicate that 1) peak metamorphism and partial melting, which produced distinctive HiSY melts, varied in age across Western Fiordland; 2) garnet grew during emplacement of HiSY WFO magmas; and 3) partial melting locally records crustal heating by WFO intrusions at high pressure. The timing of WFO emplacement and partial melting in granulite suggests multiple pulses of HiSY magma production during construction of the Median batholith along the margin of Gondwanaland.