Dr. Ian Nicholls

Supervisor/s: Ian Nicholls 
Field of Study: Field relations, volcanology of mafic-intermediate lavas and pyroclastics; petrology, geochemistry
Support Offered: Field and analytical costs
Collaborating Organisation: GeoScience Victoria
Preferred Program: Honours

In the Gisborne area 60 km northwest of Melbourne there are a number of ~3.5 million years B.P. or younger (Pliocene-Pleistocene) small lava/spatter cones of unusual hypersthene-bearing trachybasalts and trachyandesites.  Available geochemical evidence indicates that these were formed from tholeiitic basalt parent magmas by contamination with abundant xenocrystic feldspar and quartz, possibly from unexposed granitic bodies and Palaeozoic sediments.  These trachyandesites also contain small igneous enclaves indicating mingling/mixing between more and less contaminated magmas.  Some of them have common small plagioclase-spinel-garnet xenoliths of possible mid-crustal origin. Further work is needed on field relations and especially on the origin of the abundant xenocrysts and xenoliths in these rocks.  The origin of the plagioclase-spinel-garnet xenoliths is of particular interest.  They could represent fragments of felsic crustal material from beneath the thick Lachlan Fold Belt sedimentary sequences, suggesting that the LFB substrate is not exclusively oceanic, or they could have been sampled from unexposed “S-type” igneous rocks of Late Devonian age (the Mt. Macedon Igneous Complex, with thick dacitic ignimbrites and granites, lies only 10 km further north). Studies of the volcanology, petrology and geochemistry of these enigmatic rocks are needed, with emphasis on mineralogy, mineral chemistry (by electron microprobe analysis) and conditions of origin of phenocrysts, xenocrysts and xenoliths.  The depth and temperatures of origin of the apparent crustal xenoliths should be of particular interest for models of the architecture of the Lachlan Fold Belt.
 

Supervisor/s: Ian Nicholls 
Field of Study: Field relations/volcanology of mafic-intermediate lavas/pyroclastics; petrology, geochemistry

Support Offered: Field and analytical costs
Collaborating Organisation: GeoScience Victoria
Preferred Program: Honours

Other trachyandesite lavas, very close in chemical composition to those at Gisborne, are known from 40 km further N, in the area SW of Kyneton (especially the 3.25+ million year old (?) Spring Hill volcanic complex). Here they associated with a range of basaltic lavas and pyroclastics. Very little work has been done on these rocks, apart from a LaTrobe Earth Sciences Honours-level geochemical reconnaissance over 15 years ago. The trachyandesites are best exposed around the Upper Coliban reservoir, the wall of which was constructed around 1902. (A spillway bridge was designed by Sir John Monash’s engineering firm). At the time, some localities in the lavas were sampled before they were covered by the dam wall and later water. Since the reservoir is now ~20% full, some localities are probably exposed for the first time in ~110 years. Samples which ended up in the Australian Museum included apparent “xenoliths” in the lavas, some of them unusual granites which have recently been zircon dated as Late Proterozoic (~700 Ma). These xenoliths have been interpreted as mid-crustal fragments representing Proterozoic crustal substrate from beneath the Lachlan Fold Belt sediment pile, in particular the “Selwyn Block” of Western Tasmanian-type crust and lithospheric mantle proposed by GeoScience Victoria geologists as underlying the Melbourne tectonic Zone. The problem is that the trachyandesite lavas came up through locally well exposed Permian glacial deposits which contain Proterozoic “erratic” blocks, including gneisses and granites, derived from what is now Antarctica. It is quite possible that the “xenoliths” are of very shallow origin, rather than from the mid-crust! The Spring Hill and neighbouring lavas and pyroclastics need detailed studies of their distribution, petrography, geochemistry and also the occurrence, origin and significance of the supposed crustal “xenoliths” and possible glacial deposit sources.
 

Supervisor/s: Ian Nicholls & Roberto Weinberg 
Field of Study: Granite emplacement mechanisms, petrology & geochemistry
Support Offered: Field and analytical costs
Preferred Program: Honours

The Tynong Granite complex is a large area of Late Devonian granitic rocks, consisting of 5 closely spaced plutons, in the Tynong-Neerim area north of Warragul. One of the best exposures is in a large working quarry near Tynong North, just off the Princes Hwy. Here, a zone of mingling/mixing between more mafic and more felsic magma phases is spectacularly exposed, with the mafic phases forming large pillow-like enclaves. This occurrence is suitable for detailed mapping and petrological/geochemical study. This may provide evidence as to whether the occurrence formed within the lower zone or near the sides of an existing partially crystallized granitic pluton. In addition, other plutons within the Tynong complex have a range of more felsic rock types whose distribution and petrology are worth a study. This work could be carried out in conjunction with a new PhD student who will be working on the larger-scale aspects of the Tynong complex, its emplacement mechanisms and petrology/geochemistry