OXYGEN ISOTOPE COMPOSITION OF GARNET IN THE PENINSULA GRANITE, CAPE GRANITE SUITE, SOUTH AFRICA: CONSTRAINTS ON MELTING AND EMPLACEMENT MECHANISMS

Garnet is an accessory mineral in the Cape Granite Suite, and garnet delta O-18 values in the Peninsula Granite range in from 10.0 to 11.4 parts per thousand (mean 10.6 +/- 0.6 parts per thousand, n = 15). These values are consistent with the garnet being produced during incongruent melting of a metapelitic source that has a similar O-isotope composition to the Malmesbury Group. Peninsula Granite quartz delta O-18 values range from 13.2 to 14.0 parts per thousand (mean 13.6 +/- 0.3 parts per thousand, n = 17), at the high end of the range previously observed for the Cape Granite Suite. These high delta O-18 values are consistent with the source of the Peninsula Granite magma having a greater component of clay minerals, which have inherently high delta O-18 values. Garnet has a high closure temperature (>800 degrees C) to oxygen diffusion and its delta O-18 value should, therefore, correlate closely with that of the source. Quartz has a significantly lower closure temperature (similar to 550 degrees C) than garnet; and sub-solidus oxygen isotope re-equilibration between quartz and feldspar during slow cooling ought to result in a greater variation in quartz delta O-18 values compared to that of garnet. That the reverse is the case suggests that granite magmas were derived from a moderately heterogeneous source, as expected for metasedimentaly rocks. This source underwent melting to produce different batches of granitic magma containing entrained garnets of slightly different delta O-18 value. Magma batches were subsequently mixed and homogenized before and/or during the emplacement process, resulting in a narrower spread of quartz delta O-18 values.