Reconnaissance Feasibility Study of Heavy-Mineral Suites in the Fine-Grained Matrix of Several Lithostratigraphic Terranes, Southern New Zealand

Abstract

Nineteen samples of mélange matrix and volcanogenic sandstone matrix were collected from the Maitai (including the Hawtel), Caples, and Torlesse terranes, South Island, and southernmost North Island, New Zealand, in an attempt to identify contrasting provenances for these lithostratigraphic units. Heavy minerals were concentrated employing water settling columns followed by high-speed centrifugation utilizing bromoform and methyl iodide. Semiquantitative scanning-electron-microscope and quantitative-microprobe investigations of heavy-mineral concentrates support conclusions of previous workers. (1) The Torlesse Complex is a richly quartzofeldspathic, at least partially multicycle, unit derived from an evolved, well-dissected continental margin or mature island arc. It contains a diverse suite of igneous, metamorphic, and sedimentary detritus including widespread traces of clastic zircon, rutile, biotite, barite, almandine, clinopyroxene, Ca-amphibole, staurolite, chromite, and epidote. (2) Mélanges belonging to the Maitai and Caples terranes possess heavy-mineral suites suggestive of somewhat more restricted, first-cycle volcanogenic arc-like provenances, being richer in unstable rock fragments, aluminosilicates, grossular-andradite garnets, and chlorites as well as amphiboles and pyroxenes. Neoblastic hydrous, calcic aluminosilicates and layer silicates suggest that the Hawtel mélange recrystallized at lithostatic pressures of less than 3 kbar, whereas the other investigated terranes were metamorphosed at slightly higher pressures.