The nature and origin of Western Australian tourmaline nodules ; a petrologic, geochemical and isotopic study
Shewfelt, Debbie Amy
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The origin of tourmaline nodules, bizarre spherical to irregular textures documented worldwide, remains a geologic mystery. Although previously described by numerous researchers, the physical and chemical parameters that govern their formation have yet to be resolved. Commonly containing tourmaline, quartz, and occasionally feldspar, nodules are surrounded by a halo of leucocratic host rock, and are typically eight to ten centimeters in diameter. Tourmaline nodules of the present study are contained within the Paleoproterozoic Scrubber Granite of the southern Gascoyne Complex in Western Australia. This study integrated field observations, nodule petrography, tourmaline crystal chemistry, tourmaline fluid inclusion analyses, whole rock chemistry of nodule cores, leucocratic halo zones and host granite zones, stable and radiogenic isotope signatures of tourmaline separates as well as comparisons with other tourmaline nodule studies to propose the most scientifically sound theory for the formation of tourmaline nodules in the Scrubber Granite. Numerous nodule morphologies, including spherical and C-shaped nodules, along with other features such as tube-like nodules and tourmaline veins occur in massive, porphyritic, foliated and sheared phases of the Scrubber Granite. Microscopically, tourmaline displays prismatic, sub-rounded and massive textures. Microthermometric studies completed on tourmaline fluid inclusions revealed that the nodule-forming fluid contained 14 to 15 weight percent NaCl + CaCl2. Based on stable isotope studies and homogenization temperatures, fluid temperatures were constrained between 450 and 700¢ªC. The ¥ä18O and ¥äD concentrations of the nodule-forming fluid at this temperature range plot above the typical magmatic water field. Epsilon Nd values indicate that the tourmaline nodules of the Scrubber Granite may have been disturbed by a later metamorphic event.Tourmaline nodules of the Scrubber Granite are herein proposed to have formed from the exsolution and rise of buoyant pockets or bubbles of volatile fluid derived from the crystallizing Scrubber Granite magma.
DegreeMaster of Science (M.Sc.)
SupervisorAnsdell, Kevin M.
Copyright DateJanuary 2005