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The tertiary volcanic rocks of the Tamar Trough, Northern Tasmania


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Sutherland, FL 1968 , 'The tertiary volcanic rocks of the Tamar Trough, Northern Tasmania', Research Master thesis, University of Tasmania.

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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.

Item Type: Thesis - Research Master
Authors/Creators:Sutherland, FL
Keywords: Geology, Tasmania, Tamar River Valley, Igneous rocks
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Master of Science thesis

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