The Tasmanian ultramafic-gabbro and ophiolite complexes

A string of ophiolite and ultrnmafic gabbro complexes in western and northern Tasmania constitutes the Main Belt, while a second group in the Adamsfield area are composed entirely of ultramafics. These complexes are associated with Eocambrian and Cambrian sequences of mudstones, wackes, conglomerates and volcanic rocks lying between or flanking Proterozoic blocks. The Heazlewood River Complex, the largest and least dismembered in the Main Belt, is an ophiolite consisting of layered ultramafics intruded by gabbros and overlain by extrusive rocks. Dolerite dyke swarms intrude the ultramafics and part of the volcanic sequence, while tonalites, trondhjemites and granophyres intrude all other rock types. The rock contacts of the ultramafics with the Eocambrian Luina Beds are faulted and/ or have formed by solid flow of serpentinite. Contacts between the extrusive rocks and the Luina Beds are poorly exposed, but on the basis of established faults and melange zones it is tentatively concluded that the ophiolite is an allochthon. The Nineteen Mile Creek Dunites occurring along the NW margin of the Heazlewood River Complex are tectonites, some of which may be deformed cumulates. The rest of the ultramafics in the Hleazlewood River Complex and the other Main Belt bodies are dominantly layered orthopyroxenites, harzburgites, lherzolites and dunites, many of which contain interstitial plagioclase. Considered together, the layering styles, textures, and compositional ranges (Fo79-89, En79-89) of these rocks are consistent with a cumulative origin. It is suggested that nucleation (as opposed to mechanical sorting processes) and postcumulus diffusion are important controls on the layering. The layered ultramafics and gabbros of the Main Belt are believed to have formed by crystal accumulation from tholeiitic or high-Ilg tholeiitic magmas. Lack of olivine-plagioclase reactions suggest crystallization occurred at pressures less than 6 kb. The extrusive rocks of the Heazlewood River Complex are probably mainly basalts, but high-Ng basalts ao well as some intermediate and acid types also occur. The dolerites, which have suffered less than the volcanics from the pervasive hydrothermal or burial metamorphism, are quartz tholeiites which grade into granophyres and tonalites. Although similar to Cainozoic mid-ocean ridge basalts in Cr and Ni contents and being relatively low in the incompatible elements, the dolerites and volcanic rocks are dominantly quartz tholeiites rather than olivine tholeiites and have unusually low Ti02 contents. Nevertheless it is concluded that the Heazlewood River Complex (and perhaps the other Main Belt complexes) may have formed as Eocambrian lithosphere at a mid-ocean ridge, marginal sea, or similar spreading environment. Detritus of ophiolite rocks in Cambrian sedimentary rocks in a number of localities suggests that the Main Belt complexes were tectonically emplaced prior to the commencement of Middle Cambrian sedimentation. It is suggested that the lenses of foliated amphibolites which occuur along the contacts of several complexes, may have formed by metamorphism of ophiolite rocks prior to, or in the early stages of the initial tectonic emplacement. As a result of a series of faulting, serpentinization and tectonic re-emplacement events, several of the Tasmanian Complex now occur as serpentinite lenses strung out along major fault zones, and in contact with sedimentary rocks as young as Silurian. The first serpentinization phases affecting the complexes preferentially replaced dunites and harzburgites by massive lizardite serpentinites, and altered gabbro layers and dykes to rodingites or amphibole-prehnite rocks. Characteristic massive and sheared varieties of green waxy-lustred serpentinites formed in later phases associated with deformation. These green serpentinites are lizardite-chrysotile mixtures, and in many localities contain cross-fibre asbestos veins which typically surround residual pyroxenite kernals.