Investigations of an intrusive contact, northwest Nelson, New Zealand--I. Thermal, chronological and isotopic constraints

Details Investigations of an intrusive contact, northwest Nelson, New Zealand--I. Thermal, chronological and isotopic constraints Investigations of an intrusive contact, northwest Nelson, New Zealand--I. Thermal, chronological and isotopic constraints

Dec 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44.1985h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 44, Issue 12, pp.1985-2003

Mathematics

Logic

38

Scientific paper

In the Nelson area of New Zealand, intrusion of the Separation Point Batholith 114 Ma ago caused thermal metamorphism in the adjacent Devonian Rameka Gabbro that allowed radiogenic 40 Ar to diffuse partially out of hornblendes in the gabbro. Comparison of mineral ages in the batholith (obtained by the K-Ar, 40 Ar / 39 Ar , Rb-Sr, U-Pb and fission track methods) with closure temperature estimates for the various isotopic systems has allowed thermal histories of several samples to be resolved. These curves indicate that, following emplacement, the batholith cooled at an exponentially decreasing rate reaching 100°C by about 75 Ma ago. Oxygen isotope measurements on minerals from the gabbro and granodiorite give calculated equilibrium water compositions of +7 and +9 of 18 O SMOW respectively. Measured porosities from samples of the gabbro yield a model permeability of 3 × 10 -17 m 12 . These results confirm that heat transfer from the batholith to the gabbro was predominantly by conduction. A numerical heat flow model has been developed for the region, and shows that samples in the gabbro near the contact reached ~600°C. whereas 5 km away from the contact, the samples were only raised ~ 50°C above the ambient temperature. Excess 40 Ar was identified as causing a significant scatter in the K-Ar ages of hornblendes from the gabbro. Though obscured by this effect, these data suggest a crystallization age of the Rameka Gabbro of 367 Ma, which is consistent with geological evidence.

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