The tertiary volcanic rocks of the Tamar Trough, Northern Tasmania

Tertiary volcanic rocks are widespread in the Tamar Trough, a fault structure formed in late Mesozoic or early Tertiary time. The volcanic rocks are predom- inantly confined lavas, erupted from a number of centres, and fill old channels of the Tamar River System. Eruptions commenced in the Lower Tertiary, following dissection of Palaeocene-Middle Eocene sediments filling the Tamar Trough, became maximal in the Middle Tertiary, and may have continued into the Upper Tertiary. The volcanic suite ranges from strongly undersaturated lavas of olivine- nephelinite, limburgite and nepheline- basanite, through under-saturated to nearsaturated alkali olivine- basalts and tholeiitic olivine basalt. The field stratigraphy tentatively suggests the following eruptive sequence: olivine- basalts in the lower, middle and south Tamar areas in Upper Eocene-Oligocane (?) time, followed by olivine- nephelinite and nepheline- basanite in the middle Tamar area in Oligocene (?) time, and then widespread effusions of undersaturated olivine- basalts in the middle, upper and south Tamar areas in Middle-Upper (?) Tertiary time. These last include thick lavas of coarse olivine- basalt in which some di1fferentiation took place and in which pegmatitic phases developed. The relationships of olivine- nephelinite, limburgite, and tholeiitic olivine-basalt in the south Tamar area to the eruptive sequence are unknown. The Tamar lavas are similar to those in adjacent areas to the north- west, south- west and north- east, but differ somewhat from those in adjacent areas to the south and east, where tholeitic olivine- basalts predoinate. The Tamar lavas resemble the Older Volcanic Series of Victoria and the Auckland basalts in New Zealand, in their petrochemistry. The eruptive history of the Tamar laves appears to differ in some respects from that of the Older Volcanics of Victoria. Chemical variation diagrams show that the Tamar lavas essentially form an alkaline association falling generally along a similar trend to the Hawaiian alkali basalts and to some extent the Black Jack teschenite trend. The alkaline association passes into an olivine- tholeiitic association to the south- east, and the parent magmas of the two associations may have derived from differing degrees of partial melting of mantle pyrolite