Mid-Paleozoic age of granitoids in enclaves within Early Cretaceous granulites, Fiordland, southwest New Zealand

Orthogneisses of granite, quartz monzonite, monzonite, and tonalite, collectively termed granitoid, occur locally as isolated enclaves within the Early Cretaceous granulite terrain (Western Fiordland Orthogneiss--WFO) in Fiordland, southwest New Zealand. Discordant U-Pb zircon isotopic data (7 fractions) from four granitoid samples from enclaves at George Sound, define an upper intercept age of 341 ± 34 Ma that is interpreted as approximating the time of formation of the granitoid suite. The lower intercept age of 93 ± 37 Ma is interpreted as approximating the time of zircon isotopic disturbance by major episodic Pb loss. Isotopic disturbance probably accompanied high-temperature conditions at the time of incorporation of the granitoid rocks into the WFO protolith, and during subsequent granulite meta-morphism; both occurred in the interval c. 120-110 Ma. Five whole-rock samples from enclaves at George Sound yield an apparent Rb-Sr isochron age of 391 ± 48 Ma, in agreement with the zircon data, and a 87Sr/86Sr initial ratio of 0.7044 ± 10. The low 87Sr/86Sr initial ratio indicates that these mid-Paleozoic granitoids were derived from an isotopically primitive source, distinct from the mature source typical of mid-Paleozoic Karamea Suite granitoids abundant elsewhere in western New Zealand. The granitoid enclaves within WFO show influences of several different sources: most of the rocks have A-type compositions suggestive of high-temperature crustal anatexis; others have compositions corresponding to normal calc-alkaline and highly fractionated varieties thereof. One granitoid rock has a Sr isotopic composition and inherited zircon component compatible with melting of a Paleozoic sedimentary source; others have Sr isotopic compositions compatible with Early Cretaceous mixing of this crustal component with a mantle-derived mafic component. The granitoids provide evidence linking WFO to a mid-Paleozoic country rock similar to the central Fiordland metasediments. (author). 57 refs., 8 figs., 3 tabs