USING HIGH RESOLUTION MAPPING OF CRYSTALLIZED MELT INCLUSIONS TO UNDERSTAND THE EVOLUTION OF S AND I-TYPE VOLCANICS FROM THE LACHLAN FOLD BELT, AUSTRALIA

Volcanics from the Paleozoic Lachlan Fold Belt (LFB) have been altered and deformed through post extrusive processes, but quartz-hosted melt inclusions (MIs) may provide insights on their magmatic compositions. The LFB is the type locality for S-type and I-type granitoids (Chappell & White 1974), but only minimal attention has been paid to volcanics. Extrusive rocks warrant further study at this location, as they have the potential to yield unique insights into magmatic composition when compared to corresponding intrusives. Plutonic samples, for example, may record compositions significantly obscured by processes modifying bulk composition during crystallization or other secondary processes (diffusive resetting, fluid alteration, crystal accumulation, etc.).

Traditional studies of crystallized MIs require the experimental melting and quenching of the MIs into a glass, but this method can compromise the MI’s composition, even if they are perfectly sealed (Zajacz et al. 2009). Crystallized MIs of the Hawkins dacite (S-type) and Mountain Creek rhyolite (I-type) were studied to assess whether homogenization is always necessary for compositional studies. High resolution (1 µm²/pixel) X-ray WDS maps of MIs (5 -30 µm diameter) were created using EMPA, complimented by quantitative spot analyses of prevailing mineral phases. Semi-quantitative element maps were produced with the XMapTools code for MATLAB (Lanari et al. 2014), allowing for the extraction of phase fractions and micro scale chemical variations within each phase. The minerals within unaltered MIs are dominantly feldspathic (Hawkins dacite: An80-90, Ab85-95, Or90-99, Mt. Creek rhyolite: An75-90, Ab85-95, Or90-99), associated with quartz, biotite, and orthopyroxene, and accessories (including apatite, Fe-Ti-oxides). Average compositions show similarities to whole rock compositions.

While the potential of this method lies probably within probing the chemical variability and zonation of minerals within crystallized MIs mapping a statistically large number of MIs provide first-order insights on the chemical evolution of magmatic systems.

Chappell & White (1974) Pacific Geology 8, 173-174

Lanari et al. (2014) Computers and Geosciences 62, 227-240

Zajacz et al. (2009) Geochimica et Cosmochimica Acta 73(10), 3013-3027