The Monaro Volcanic Province (MVP) is an Oligocene-Eocene intraplate basaltic lava
field situated in the Southern Highlands of New South Wales between the towns of
Cooma and Bombala. The lava pile of the MVP consists of basal sub-alkali rocks
(olivine tholeiite, transitional basalt) capped by a number of thick ankaramite lavas,
above which lie less numerous alkali rocks including alkali olivine basalt, nepheline
basanite and olivine nephelinite. Intercalated with the lava flows are massive and
matrix-supported alkali and ankaramitic hyaloclastites, alkali pillow basalts, rare tuffs,
bauxitic weathering profiles, lacustrine sediments and reworked late Cretaceous to early
Tertiary river gravels. The lava pile is intruded through by numerous volcanic plugs
and dykes and rare maars. Volcanic centres are principally concentrated in two NW-SE
trending zones parallel to major crustal-scale fractures in the Palaeozoic basement.
Centres almost always lie over the intersections of two or more conjugate strike-slip or
transverse fractures. The stratigraphy, whole-rock geochemistry and Sr and Nd isotopic
signatures of rocks from the MVP indicate magma-genesis initially from an
asthenospheric source with EM1 characteristics, gradually becoming more lithospheric
with DM source characteristics. The long-lived nature of the MVP rules out a mantle
plume-type source for magmas. Instead, a diapiric source is envisaged. The MVP
mantle xenolith suite appears to have equilibrated at slightly higher temperatures for
given pressures than the Newer Volcanics Province suite suggesting the palaeogeotherm
for the MVP was slightly hotter than the "South East Australian" geotherm. Large
amounts of amphibole (pargasitic hornblende, pargasite, ferroan pargasite and
kaersutite) occuring within the more silica-undersaturated rocks of the MVP, and rarely
within Iherzolitic xenoliths, are interpreted to have formed as selvages on mantle veins
in contact with peridotite beneath the MVP. Amphiboles were later sampled by
magmas rising through the same conduits and were brought to the surface. MVP
ankaramite lavas feature < 2cm clinopyroxene porphyrocrysts, the cores of which are
shown to have crystallised at ca. 18 kb pressure or ca. 54 km depth. This defines the
base of the local crust within the MVP region. Data from the MVP support a landscape
evolution model based on the isostatic rise of the Southern Highlands due to
voluminous magmatic underplating since the Cretaceous. Data further support limited
denudation since the Early Tertiary based on a pulsatory but high palaeogeotherm.
| Identifer | oai:union.ndltd.org:ADTP/219294 |
| Date | January 1999 |
| Creators | Roach, Ian C., n/a |
| Publisher | University of Canberra. Applied Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Ian C. Roach |
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