TIMING AND CONDITIONS OF LOWER CRUSTAL METAMORPHISM FIORDLAND, NEW ZEALAND

Garnet Sm-Nd ages for the Pembroke granulite and Western Fiordland Orthogneiss (WFO) constrain the timing of high-grade metamorphism and lower-crustal melting throughout western Fiordland to 126 - 109 Ma. The entire range of ages is found within mafic to intermediate granulite from Pembroke valley in northern Fiordland and requires slow cooling of these rocks over 15 - 20 m.y. The youngest Pembroke garnet age of 109 Ma is interpreted to result from diffusional re-equilibration at high temperature. Garnet from mafic granulite grew below solidus temperatures; however, garnet in intermediate granulite clearly grew at temperatures above the solidus. A Sm-Nd isochron age constructed for euhedral garnet porphyroblasts from unmelted mafic lithologies is 126.0±2.1 Ma, which indicates initiation of high-grade metamorphism. Euhedral garnet crystals from leucocratic layers in the Pembroke migmatites yield an age of 122.6±2.0 Ma for melting and peritectic garnet growth, coeval with published U-Pb zircon ages and crystallization of the northern WFO. A new Sm-Nd age of 109.7±1.9 Ma for a garnet-bearing leucosome in the WFO from central Fiordland (Resolution Island) indicates that partial melting was later in the south.

Migmatites from Pembroke valley and the southern WFO have similar mineralogy and textural characteristics. These lithologies contain the assemblage Hbl + Cpx + Grt + Qtz + Pl + Rt + Ilm ± relict OPx. Unzoned garnet (Alm₅₆Prp₂₃Sps₃Grs₁₈) is present in leucocratic veins and reaction zones between these and unmelted zones.

Unmelted mafic rocks of the Pembroke granulite contain the assemblage Hbl + Grt + Czo + Pl + Rt ± CPx ± relict OPx. Garnet is zoned from Alm₄₉Prp₂₁Sps₃Grs₂₈ in the core to Alm₄₆Prp₂₂Sps₂Grs₃₀ at the rim. Previous studies indicate that Pembroke garnet porphhyroblasts grew at > 750°C and 12 - 16 kbar. Forward modeling (TheriakDomino) of metamorphic phase equilibria and compositions in the unmelted Pembroke rocks indicate a minimum P-T of 675°C @ 10.8 kbar. Analogous modeling for unmelted WFO to the south suggests minimum P-T conditions of 690°C @ 10.4 kbar. Comparable model results for these two rocks suggest widespread high pressure metamorphism and partial melting for a large area of lower crust 126-109 Ma.