HIGH-PRESSURE METAMORPHISM CAUSED BY MAGMA LOADING IN FJORDLAND, NEW-ZEALAND

Cretaceous granulite facies metamorphism in the Fiordland area of New Zealand has distinctive mineralogical, textural and structural feature s that set it apart from most other regional metamorphic belts. The me tamorphism, developed over a 30 x 150-km area and the consequence of a 20-km-thick increment to crustal thickness, is closely associated in space and time with a large plutonic complex, the Western Fiordland Or thogneiss (WFO). Although temperatures and pressures as high as 700 de grees C and 12 kbar were attained, the metamorphic overprint on earlie r low-pressure assemblages is weak and incomplete. Little strain accom panied the metamorphism. The temperature threshold at which metamorphi c recrystallization is recorded is over 500 degrees C. Zoned garnets a re preserved at unusually high temperatures, indicating duration of me tamorphism on the order of 10 times shorter than in most other regiona l terranes. This pattern of features bears close similarity to metamor phism in the Coast Plutonic Complex in North America, where a mechanis m of 'magma loading' has been invoked. In Fiordland, the high-pressure metamorphism can be explained by depression of country rock under a c rustal zone that is inflated by intrusion of the WFO. Regional structu re of the WFO as a horizontally sheeted complex suggests that the plut on was emplaced by vertical displacement of country rock, and supports the magma loading model.