From richer to poorer: zircon inheritance in Pomona Island Granite,New Zealand |
| |
| Authors: | J M Scott J M Palin A F Cooper M W Sagar A H Allibone A J Tulloch |
| |
| Institution: | (1) Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand;(2) Rodinian Pty, Queanbeyan, Canberra, NSW, Australia;(3) GNS Science, Dunedin, New Zealand |
| |
| Abstract: | Previous U–Pb zircon dating of the Pomona Island Granite (PIG) pluton (South Island, New Zealand) yielded either Permo-Carboniferous
or Late Jurassic ages for five samples essentially indistinguishable in their field, petrographic, and geochemical characteristics.
Detailed cathodoluminescence imaging and LA-ICP-MS dating of zircon in new and previously dated samples reveal that portions
of the pluton contain either delicately oscillatory-zoned Late Jurassic zircon grains with rare Permo-Carboniferous cores,
or Permo-Carboniferous grains with ubiquitous but thin Late Jurassic rims. Based on zircon dissolution-overgrowth textures,
zircon rim and core trace element compositions, and the limited extent of sub-solidus rock recrystallisation textures, the
bipartite age distribution is unlikely to reflect variable Pb-loss or metamorphic re-equilibration. Magmatic Zr-saturation
temperatures were ≥851°C for samples dominated by Jurassic zircon and ≤809°C for samples with a predominance of Permo-Carboniferous
zircon. Together, these data are consistent with PIG magmas having been derived from partial melting of a Permo-Carboniferous
felsic igneous source at variable temperature wholly in the Late Jurassic (157 ± 3 Ma). The lowest temperature melts would
have been incapable of dissolving significant amounts of pre-existing zircon and consequently generated inheritance-rich magmas,
with the very thin rims on the pre-existing zircon grains the only evidence of the Late Jurassic magmatic age. As the partial
melting temperature increased and nearly all pre-existing zircon grains dissolved into the magma, an inheritance-poor batch
of melt was generated, which precipitated new zircon grains upon crystallisation. Concentrations of major and many trace elements
in both magma batches may have been buffered by retention of residual quartz and feldspar in the source, which would explain
the limited geochemical differences between inheritance-rich and inheritance-poor portions. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
