NEAR SYNCHRONOUS PLUTON EMPLACEMENT, PARTIAL MELTING, AND DUCTILE EXTENSION IN MAGMATIC ARC CRUST, FIORDLAND NZ

Mafic to intermediate plutons of the Western Fiordland Orthogneiss (WFO) intruded over <12 m.y. to form a significant volume of the lower crust in the Cretaceous arc formed along the Zealandia-Australia-Antarctica flank of Gondwana. At current exposure levels the WFO crops out over >1,500 km² of South Island, NZ. These mid to lower crustal plutonic rocks are intimately associated with ductile shear zones that accommodated extension of the lower crust during the mid to late Cretaceous and prior to opening of the Tasman Sea. Three large plutons: the ca. 126 Ma Worsley, 122-116 Ma Misty, and 116-114 Ma Malaspina intruded sequentially from north to south, respectively. The two northern plutons intruded at 6-8 kbar and were subsequently loaded to 12-14 kbar prior to or during metamorphism; however, the Malaspina Pluton intruded at 12-14 kbar and remained at this pressure during initial metamorphism. The Malaspina Pluton is flanked and capped by metasedimentary rocks which are separated from the pluton by ductile shear zones forming a gneiss dome previously interpreted as a metamorphic core complex.

Overprinting mineral textures indicate that early two pyroxene igneous assemblages were pervasively metamorphosed to hornblende granulite. Subsequently these minerals were locally replaced by garnet + clinopyroxene +/- clinozoisite during ductile strain and partial melting that produced high Sr/Y trondhjemite partial melts. Along the eastern flank of the Malaspina Pluton, highly-strained orthogneiss provides a a record of intrusion, metamorphism, and strain. U-Pb zircon ages indicate intrusion at 116-114 Ma. Euhedral garnet porphyroblasts in highly-strained gneiss and vein selvages are free of oriented inclusions and show no evidence of rotation - compatible with syn- to post-deformation growth. Sm-Nd ages for these garnet are 113.3+/-2.7, 111.9+/-3.0, and 110.6+/-1.9 Ma indicating that most or all ductile strain had ceased ca. 111 Ma. The near synchronous intrusion, garnet granulite metamorphism, subsequent partial melting, and ductile extension of the lower crust were intimately linked during orogenic collapse.