Magnetic susceptibilities of Westland-Nelson plutonic rocks: Discrimination of Paleozoic and Mesozoic granitoid suites

Abstract

A portable magnetic susceptibility meter allows rapid in situ estimation of the magnetite content of granitoid rocks, and hence provides a guide to their 1-or S-type character.Except for some muscovite-bearing granites above about 71 % SiO and some orthogneisses of the Charleston Metamorpbic Group, the susceptibility measurements clearly discriminate between Westland-Nelson granitoids of known Mesozoic (magnetite-bearing, I-type) and Paleozoic (magnetite-free, S-type) age. Although some Mesozoic granites lack magnetite, no magnetite-bearing Paleozoic granites have been recognised.Precise susceptibility measurements of a suite of S-type biotite granites show that their very weak susceptibilities are proportional to modal biotite content. Ilmenite and pyrrhotite in these S-type granites have no measureable affect on susceptibility measurements. Deformation locally reduces susceptibility on an outcrop scale, whereas contact metamorphism/hydrothermal alteration may increase or decrease susceptibility.Aeromagnetic anomalies in the Westland-Nelson region of New Zealand are associated more generally with magnetitebearing I-type granitic rocks than with lamprophyre dikes (and hypothetical magnetic source bodies), and it is considered that several anomalies are due to the granitic rocks, one variety of which is apparently associated with lamprophyre swarms.

SHRIMP U‐Pb geochronology of Cretaceous magmatism in northwest Nelson‐Westland, South Island, New Zealand

“…1). The pluton also corresponds to a small north-south-trending positive aeromagnetic anomaly (Tulloch 1989). The dominant rock type is a medium to coarse grained, pale grey, equigranular, biotitehornblende granodiorite.…”

Basement geology of Taranaki and Wanganui Basins, New Zealand

“…Although we do not necessarily accept that these anomalies are all as linear as Davy claims (cf. Hunt 1978 andDavy 1992), we do regard the high contrast as typical of magnetic signatures of the Western Province, which contains a variety of highly magnetic plutons (Tulloch 1989). This variable magnetic character of Western Province and MTZ units means that magnetic anomalies are of limited use in distinguishing these units from Brook Street and Maitai Terranes.…”

Gneissic rocks of the Bonar Range, central Westland, New Zealand

“…All samples collected have low magnetic susceptibility indicating an absence of magnetite (see Tulloch 1989). Also occurring in both units are extremely flattened metasedimentary enclaves, some with layering, and consisting of fine-grained quartz + calcic oligoclase/andesine + biotite + muscovite ± garnet ± sillimanite schist (e.g., P67583).…”