HR: 1330h
AN: V22B-0584    [PDF]
TI: Origin of Poikilitic Garnet in a Leucogranite Dike
AU: Prior, D J
EM: davep@liv.ac.uk
AF: University of Liverpool, Dept of Earth and Ocean Sciences, Liverpool, L69 3GP United Kingdom
AU: Rahimi-Chakdel, A
EM: rahimiaz@yahoo.co.uk
AF: University of Liverpool, Dept of Earth and Ocean Sciences, Liverpool, L69 3GP United Kingdom
AU: * Boyle, A P
EM: apboyle@liv.ac.uk
AF: University of Liverpool, Dept of Earth and Ocean Sciences, Liverpool, L69 3GP United Kingdom
AB: Garnet is a common accessory phase in aluminous granites. Its origin has been variously attributed to: incorporation of xenocrysts; solid-state replacement; reaction between solid or early-formed phases and granitoid melt; crystallization directly from granitoid melt. Here, we describe garnets from a leucogranite dyke from the Isle of Man. The garnets are typically 0.5 mm sized, Mn-rich, poikilitic, faceted crystals occurring as skeletal clusters up to 3 mm across. Magmatic origin has been previously suggested for these garnets, but there are problems with this interpretation. The garnets are strongly poikilitic, except at contacts with muscovite where wide (300-500 microns) inclusion free faceted rims develop. Inclusions are almost entirely quartz grains, which tend to be a little smaller than matrix quartzes (86 vs. 110 microns), but have a similar frequency per unit area (c. 550 crystals per sq mm). Both matrix and inclusion quartz crystals display a weak shape alignment and a random crystal preferred orientation, determined using electron backscatter diffraction (EBSD). It is not clear if the shape alignment is an igneous or subsolidus fabric, but either way the implication is that garnet nucleation and growth must have been either very late in the magmatic history or subsolidus (i.e. after nucleation of quartz completed). Microprobe study indicates that the garnets are strongly, concentrically zoned, typical of Rayleigh fractionation during growth from a melt. EBSD indicates that garnets in a cluster have similar crystallographic orientation indicating either multiple nucleation of similarly oriented garnets or subsequent rotation of garnets into a similar orientation. The general absence of non-quartz inclusions together with the development of inclusion free rims at muscovite contacts, suggests a coupled solution-precipitation mechanism. Formation of Mn-rich garnet from a final stage granitoid melt/fluid could involve nucleation of garnet at a favourable site coupled with diffusion through the fluid phase of garnet components to that site, dissolution (Pl+Kfs+Ms) and precipitation (Grt). The skeletal garnet geometry may reflect growth along end-stage melt/fluid pathways.
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting