Pluton differentiation and batholith evolution : Separation Point batholith, NW Nelson
Year:
Project type:
Subject:
Advisers:
Abstract:
A subdivision of the Separation Point Batholith, NW Nelson (part of the Separation Point Suite) into discrete plutonic units is proposed based on a combination of field and petrographic evidence in conjunction with geochemical data. Four plutons are separated from samples collected along the Abel Tasman coastline. Biotite fluorine compositions have been analysed in order to assess the degree to which the Separation Point Batholith has been contaminated with crusta! material. This reveals a moderate fluorine concentration in granite samples, which may be compariable with contamination from metasediments (potentially of the Greenland Group). At least two plutonic units can be separated from each other on the basis of differing XF/X0H ratios calculated from biotites. An estimate of granite emplacement depth has been carried out usmg the Al-inhornblende geobarometer from a sample on Sandy Bay Road, near Riwaka. Hornblende compositions produce a crystallisation pressure estimate of 3.2±1 kbar (""' 12 km). When combined with several Al-in-hornblende depth estimates from elsewhere in the batholith, this provides further evidence suggesting a progressive emplacement depth increase towards the south of the Separation Point Batholith. Epidote, considered to be of magmatic origin, is observed predominantly in the northern region of the batholith. Primary epidote grains indicate that the Separation Point Batholith magmas were emplaced from depths of 22 km or more. Based on dissolution rates of epidote within granitic melts, an estimate of the minimum magma ascent rate has been calculated at 860 m/yr. This is consistent with an emplacement process involving fracture propagation and dyke networks, but rules out emplacement by diapirism as this is several orders of magnitude slower.
Thesis description:
iv, 99 leaves : col. ill., col. maps (1 folded) ; 30 cm.
Department:
OU geology Identifier:
2004Eggers
OURArchive handle:
OURArchive access level:
Collection
Citation
Eggers, Benjamin K., “Pluton differentiation and batholith evolution : Separation Point batholith, NW Nelson ,” Otago Geology Theses, accessed February 7, 2025, https://theses.otagogeology.org.nz/items/show/427.