An unusual Tasmanian Tertiary basalt sequence, near Boat Harbour, northwest Tasmania

Abstract

ABSTRACT. The mineralogy and petrology of basalts near Boat Harbour, NW Tasmania, are described as this sequence is unusual for Tasmanian Tertiary basalts. The rocks are more sodic and evolved basalts carry more prolific anorthoclase and zircon megacrysts than is normal in Tasmania. Older nephelinites and melilite-nephelinites (26-27 Ma) and younger nepheline hawaiites and mugearites (14-15 Ma) are present and fission track zircon ages (l3-14 Ma and 8-9 Ma) demonstrate that zircon was erupted during and after the evolved basalts. The nephelinites and some evolved basalts carry common spinellherzolite xenoliths, some with rare alkaline reaction veins. Crustal xenoliths are ubiquitous as Precambrian country rocks but include rare 2-pyroxene granulite and anorthosite. Some nephelinites show mariolitic assemblages of olivine, diopsideaugite-aegirine clinopyroxenes, nepheline, sodalite, barian sanidine, ulvospinel, titanian richterite, titanian phlogopite and carbonate.The nephelinites and melilite nephelinite represent primary melts, while the evolved basalts can produce primary basanites by addition of a cumulate wehrlite mineralogy. However, crystal fractionation models for evolving these melts only give inadequate solutions. The low Zr contents, and presence of zircon and anorthoclase megacrysts do not fit into fractionation processes.N ephelinitic geochemistry indicates 4-7% degrees of partial melting of an amphibole-dominated metasomatised garnet peridotite. Low H 2 0+C0 2 contents favour melilite nephelinite melt derived from deeper levels than for the nephelinite melts. Evolved basalts from such metasomatised sources may mark limited fractional crystallisation, while minor early initial melting may produce sodic Zr-rich felsic melts that crystallise anorthoclase and zircon without significant fractionation.The rocks show an isotopic HIMU imprint, but unlike older east Tasmanian basalts lack trace element signatures related to the Tasman Balleny plume system.

The Tamar Trough revisited: correlations berween sedimentary beds, basalts, their ages and valley evolution, North Tasmania

“…2005. This is compatible with a proposed model for quasi-continuous, but episodic Tasmania volcanism that resulted from lithospheric drift of eastern Australia over an extended Tasman Sea plume zone (Sutherland et al 1989). Furthermore, precise Tasmanian basalt ages (Sutherland et al 2004, Everard et al 2004b) and better-controlled Australian plate motion paths back to 80 Ma (Sutherland 2003) now allow more detailed testing of the Tasman margin magma source model for Tasmanian volcanism.…”

“…Tholeiitic basalts are prominent and the Tamar Trough appears no exception, particularly if the olivine tholeiite in the South Tamar also fits here. So far, no later volcanism is known in the Tamar Trough to match the last phase of alkaline activity in northern Tasmania (9-20 Ma;McClenaghan et al 1982, Baillie 1986, Sutherland et al 1996,2002, Everard et al 2004b). …”

Table Cape vent xenolith suite, northwest Tasmania: Mineralogy and implications for crust-mantle lithology and Miocene geotherms in Tasmania

“…The samples for this mineralogical study were collected by breaking up the dense glassy basalt, which provided more protected, fresher samples than those found in weathered Gee (1971), Sutherland et al (1996 and Everard (2003).…”

“…The 01 (Fo S9 ) and opx (Enss) are Mg-rich, the cpx is AI-Na-rich diopside (W0 48 En 4G PSG) and the spl is Cr-bearing pleonaste (e.g., SP78 HC l2 Cms Mtl' Usp) (table 3). This type dominates mantle xenoliths in the Table Cape region and elsewhere in Tasmania (Piestrzeniewicz 1972, Varne 1977, Sutherland et at. 1996.…”

“…The paper integrates the Table Cape vent xenolith data with Boat Harbour (Sutherland et al 1996) and provides a revised lithospheric framework for the Wynyard-Boat Harbour region. The proposed lithology is then correlated with new seismic data on crust-mantle structures under northern Tasmania (Rawlinson et al 2001(Rawlinson et al , 2004.…”