The origin and emplacement of the Serpentine Hill Complex, Western Tasmania

Abstract

The Serpentine Hill Complex is an ophiolite dismembered by tectonic emplacement and subsequent faulting. It consists of ultramafics, gabbros, dolerites, and volcanics (mainly basaltic). The layered ultramafics (dominantly orthopyroxenite and harzburgite) and layered hypersthene gabbro are thought to be cumulates, and have been intruded by pegmatitic gabbro and microgabbros. Three types of serpentinite can be recognized in the western part of the complex, black serpentinites, green serpentinites, and sheared contact serpentinites. The black serpentinites are generally unsheared, consist essentially of lizardite, and have preferentially replaced olivine‐bearing ultramafics. The sheared contact serpentinites consist mainly of chrysotile and antigorite, and may have formed during tectonic emplacement. The formation of the green serpentinites (lizardite‐chrysotile mixtures) appears to have accompanied localized deformation which occurred after tectonic emplacement of the complex. It is postulated that the Serpentine Hill Complex was emplaced as a thrust slice before the Middle Cambrian Dundas Group, which unconformably overlies it, was deposited. Two lenses of foliated amphibolites near the basal contact in the western part of the complex may have formed by deformation of ultramafics and gabbros before, or in the early stages of, tectonic emplacement.