Timing and duration of garnet granulite metamorphism in magmatic arc crust, Fiordland, New Zealand

摘要

Pembroke Granulite from Fiordland, New Zealand provides a window into the mid- to lower crust of magmatic arcs. Garnet Sm-Nd and zircon U-Pb ages constrain the timing and duration of high-P partial melting that produced trondhjemitic high Sr/Y magma. Trace element zoning in large, euhedral garnet is compatible with little post growth modification and supports the interpretation that garnet Sm-Nd ages of 126.1 +/- 2.0 and 122.6 +/- 2.0 Ma date crystal growth. Integration of the garnet ages with U-Pb zircon ages elucidates a history of intrusion(?) and a protracted period of high-temperature metamorphism and partial melting. The oldest zircon ages of 163 to 150 Ma reflect inheritance or intrusion and a cluster of zircon ages ca. 134 Ma date orthopyroxene-bearing mineral assemblages that may be magmatic or metamorphic in origin. Zircon and garnet ages from unmelted gneiss and garnet reaction zones record garnet granulite facies metamorphism at 128 to 126 Ma. Peritectic garnet and additional zircon ages from trondhjemite veins and garnet reaction zones indicate that garnet growth and partial melting lasted until ca. 123 Ma. Two single fraction garnet ages and young zircon ages suggest continued high-temperature re-equilibration until ca. 95 Ma. Phase diagram sections constrain orthopyroxene assemblages to <0.6 GPa @ 650 degrees C, peak garnet granulite facies metamorphic conditions to 680-815 degrees C @ 1.1-1.4 GPa, and a P-T path with a P increase of >= 0.5 GPa. These sections are compatible with water contents >= 0.28 wt.%, local dehydration during garnet granulite metamorphism, and <0.3 GPa P increases during garnet growth. Results demonstrate the utility of integrated U-Pb zircon and Sm-Nd garnet ages, and phase diagram sections for understanding the nature, duration, and conditions of deep crustal metamorphism and melting. Geochronologic and thermobarometric data for garnet granulite indicate that thickening of arc crust, which caused high-pressure metamorphism in northern Fiordland, must have occurred prior to 126 Ma, that loading occurred at a rate of ca. 0.06 GPa/m.y., and that garnet granulite metamorphism lasted 3-7 my. Locally-derived partial melts formed and crystallized in considerably less than 10 and perhaps as little as 3 m.y.