INVESTIGATING THE ORIGIN OF AN ORBICULAR GRANITE: KARAMEA, NEW ZEALAND

Orbicular rocks are metamorphic, migmatic and igneous rocks that have concentric shells (orbicules), defined by modally distinct layers of crystals, surrounding a central core. Though generally rare, Karamea, New Zealand is a known location of orbicular rocks. A boulder (35cm x20cm) found in an active river terrace is the focus of this study.

This research attempts to answer how orbicular rocks form as hypotheses vary with the type of rock. The methods used in this study include 3D imaging to identify whether the morphology of orbicules is due to deformation or growth, analysis of crystal orientations, and PXRF to analyze the chemical relationships between various shells and the orbicules and host rock.

Ten orbicules are identified, ranging in diameter from 32-86 mm. Orbicules are generally spherical (from serial sections) with some distortions. Analyses of mineral abundance and crystal orientations show alternating plagioclase-rich (92% plagioclase) and biotite-rich (90% biotite) shells. Other minerals identified include quartz and muscovite, also in varying abundances. Alignment of plagioclase crystals along the c-axis indicates radial growth, while micas align themselves perpendicular to plagioclase. Major element compositions indicate that the core of orbicules have the same composition as the shells and host rock. The indistinct compositional boundary between host rock and the orbicules suggests that the orbicules crystallized in situ (though deformation may have occurred later). The host rock has three varying textures from coarse- to fine-grained, with distinct boundaries between each of the textures. The fine-grained host rock has non-euhedral plagioclase and crystals that are finer than 1mm in diameter. The texture of the host rock indicates orbicule growth conditions similar to that of aplite, with parts of the fine-grained textured host rock indicating a part of the chilled margin from the aplite.

The different orientations of the plagioclase-rich and the biotite-rich shells can indicate radial growth of the plagioclase shells followed by periods of biotite sedimentation. Further work for this project includes measuring the orientation of the biotite to provide more details to their settling, and further geochemical analyses of the host rock contact.