WIDESPREAD GARNET GRANULITE METAMORPHISM IN THE LOWER CRUST OF A CONTINENTAL MAGMATIC ARC, FIORDLAND NZ

Lower crustal metamorphism provides direct evidence for externally derived heating and possible delamination in the Cretaceous Fiordland arc. Large-scale heating of these rocks reached T >850°C at P =11 to 18 kbar. Metamorphism effected >2000 km² of metaplutonic and metasedimentary rocks. Garnet Sm-Nd and zircon ages for the Misty and Malaspina Plutons, and Breaksea Orthogneiss of the Western Fiordland Orthogneiss (WFO) indicate that granulite metamorphism was synchronous across >1300 km² at 111.7±1.0 Ma (N-rocks=18, 2 reject). Metamorphism occurred ca. 4 Ma after a high magmatic flux event (HFE) at 118 to 116 Ma, represented by emplacement of the Misty and Malaspina Plutons, and began during initial extension.

Metamorphism of the northern WFO, Worsley Pluton, dated by U-Pb on metamorphic zircon rims range from 121-116 Ma (N=6), indicating that garnet granulite metamorphism there is similar, but ranges to slightly older ages (Gebauer, 2016). Metamorphism of the southernmost WFO, Breaksea Orthogneiss, is considerably more complex than in the central WFO with old zircon U-Pb and garnet Sm-Nd ages indicating early intrusion and possible igneous garnet 124-121 Ma. Younger garnet Sm-Nd ages indicate that the 112 Ma granulite event also affected Breaksea. Central Fiordland lower crust remained hot, ca. 725°C, until 108 Ma as recorded by titanite growth during extensional collapse. This part of the crust continued cooling to Ar closure in hornblende ca. 500°C at 95 Ma (Gibson et al., 1988). However, titanite and zircon indicate that the Breaksea area to the south remained at higher T at least until 92-90 Ma.

Widespread heating of lower crustal rocks requires a significant heat source likely related to mantle derived magma intruding beneath the current exposure, or increased heat flow from the mantle. Although cryptic intrusions below the WFO cannot be ruled out, synchronous heating and subsequent cooling beginning shortly after the HFE is compatible with slab breakoff/rollback or delamination of the lowermost part of thickened arc crust, mantle flow, and increased heat flow from this upwelling mantle. We speculate that age differences between WFO intrusion, granulite metamorphism, and cooling are of appropriate duration for delamination caused HFE followed by continued high heat flow and conduction upward to the WFO.