Petrology of Ultramafic and Related Rocks from Mt
PETROLOGY OF ULTRAMAFIC AND RELATED ROCKS FROM MT. LIGHTNING, NEAR GUNDAGAI, NEW SOUTH WALES, AUSTRALIA. By A. S. Ray, M.Sc. A THESIS SUBMITTED TO THE UNIVERSITY OF NEW SOUTH WALES FOR THE DEGREE OF DOCTOR OF PHILOSOPHY, 1977. ABSTRACT A rock assemblage in the Mt. Lightning area of the Coolac Serpentine Belt in south-eastern New South Wales has been mapped and studied by means of petrographic and chemical techniques. The principal lithologies include massive to locally foliated, partially serpentinized, olivine-rich harzburgites, with deformed and recrystallized textures, containing consistently Mg-rich olivine {Fo89 _92} and enstatite {En87_90 } with subordinate diopside and chrome-spine!. These rocks are associated with blocky and sheared lizardite- chrysotile serpentinites. Massive and variolitic varieties of low-potash spilites containing tholeiitic pyroxenes as inferred from chemical compositions, occur within and flanking the serpentinites. Smaller rock masses enclosed by, or flanking the ultramafic rocks, include low-iron rodingites of two genetic groups, a series of low-potash feldspathic rocks ranging from quartz-rich trondhjemites to quartz-poor albitites, and both er-rich and Al-rich chromitites. Group 1 rodingites containing grossularite, vesuvia nite, chlorite and relict diopside, and characterized by high Cr {> 200 ppm} and Ni {> 150 ppm} contents, and high Cr/V ( > 2) and Mg/Fe (> 2) ratios, are considered to be metasomatized mafic rocks. Group 2 rodingites are reaction zone products that formed at contacts of spilite and ultra- mafic rocks. They contain zoisite, prehnite, chlorite and grossularite but lack vesuvianite and relict pyroxene, have low er ( <. 40 ppm) and Ni ( <. 50 ppm) contents, and low Cr/V ( < 1) and Mg/Fe (<1.5') ratios.
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