Accessory mineral petrochronology reveals 30 m.y. of partial melting during the separation of Zealandia from eastern Gondwana

Abstract

Granitic pegmatites emplaced within the Alpine Schist of New Zealand provide an opportunity to assess the relationship between metamorphism and melting during the separation of Zealandia from eastern Gondwana. We combined monazite, xenotime, and zircon petrochronology from 12 pegmatite samples with whole-rock geochemistry to link pegmatites to the conditions and mechanisms of partial melting of the Alpine Schist. Monazite dates define a broad range from ca. 85 to ca. 50 Ma, interpreted to represent primary crystallization, remelting of existing pegmatites, and fluid-assisted recrystallization. Xenotime and zircon dates match monazite dates from the same sample, except where recrystallization or sampling of inherited age domains has led to younger or older dates, respectively. The total age range of primary, unmodified igneous monazite is 79.5 ± 0.4–49.8 ± 0.2 Ma, extending the known lower time limit for pegmatite emplacement by 17 m.y., and indicating that the generation and crystallization of melts occurred over a period of 30 m.y. Whole-rock compositions indicate that water-fluxed melting was the dominant melting mechanism, although minor dehydration melting may have contributed to the earliest melts. Partial melting initiated immediately prior to the youngest record of garnet growth in the Mataketake Range region and persisted for 28 m.y. after garnet growth ceased in this region. The timing, duration, and mechanisms of partial melting and their relationship to the timing of metamorphism suggest that short-lived melting events were driven by episodic water fluxing from ca. 80 to 50 Ma in the Alpine Schist. Late Cretaceous to Paleogene intraplate deformation focused between the NW and SE regions of Zealandia is proposed as a potential mechanism for this prolonged period of partial melting and melt emplacement within a geographically restricted area. LITHOSPHERE; v. 11; no. 1; p. 169–189; GSA Data Repository Item 2019030 | Published online 20 December 2018 https://doi.org/10.1130/L1021.1