**3. The eastern Australian volcanic framework**

Australia commenced separating from Antarctica some 85 million years ago, finally separating some 33 million years ago, and has been migrating northwards towards the Eurasian plate [13, 23, 33, 40, 41]. In the process, it progressively passed over a mantle hotspot on its eastern side.

Globally, volcanic activity typically is located at the edge of a tectonic plate boundary (subduction zone), or a rift, or a crustal spreading zone [35–39]. However, along the length of eastern Australia in response to the continent passing over the mantle hotspot, there is a wide 'corridor' or trackway of volcanoes and eruptive activity that is quite distant from the edge of the Indo-Australian Plate, and here volcanism appears related to a mantle plume, or a cluster of mantle plumes, or at least a mantle plume that, over time, found several proximally related weaknesses in the lithosphere through which to intrude and erupt. Sutherland [41] first suggested plate migration over magmatic upwellings with the oldest Australian volcanoes in north Queensland, and the eruptive centres have moved southwards as the Australian plate has drifted northwards over a mantle plume, forming a 'corridor' of eruptive and magmatic activity.

The volcanoes in this corridor have been active along the eastern part of Australia for at least the last 33 million years [40], showing a *series of volcanic tracks* with some starting in the north some 33 Ma ago, and others starting mid-length along the Australian eastside some 27–21 Ma ago; these various trackways have been mapped by different authors and range in age from the oldest to the north and youngest to the south [25, 40–48]. These trackways, showing a younging to the south, implicate a northward drift of the continent over a mantle hotspot as the Indo-Australian plate (a major tectonic plate that includes the continent of Australia and surrounding ocean, and extends northwest to include the Indian subcontinent and adjacent waters [43–45, 49]) migrated over a relative stable (static) mantle plume [13, 36, 42]. It has been estimated that the eastern part of the Indo-Australian Plate (Australia) is moving northward at the rate of 5.6 cm per year while the western part (India) is moving only at the rate of 3.7 cm per year due to the impediment of the Himalayas [43–46].

Within this corridor, volcanism has been expressed as eruptions determined by the thickness of the lithosphere [13]. Of interest in this Chapter, the main trackway within the corridor is what Davies *et al*. [13] termed the 'Cosgrove hotspot track' which we term the 'Cosgrove Volcano Chain' (**Figure 3**). Davies *et al*. linked the volcanoes, eruptive centres, and magmatic activity along the 'Cosgrove hotspot track' (or 'Cosgrove Volcano Chain') based on a number of criteria: *viz*., 1. standard basaltic compositions of magma in regions where lithospheric thickness is less than 110 km, 2. volcanic gaps in regions where lithospheric thickness exceeds 150 km, and 3. low volume, leucitite eruptions in regions of intermediate lithospheric thickness. Davies *et al*. found that trace-element concentrations along this track support the notion that compositional variations result from different degrees of partial melting, controlled by the thickness of overlying lithosphere, and concluded that lithospheric thickness played a dominant role in determining the volume and chemical composition of plume-derived magmas [13].

**353**

**Table 1.**

*A Globally Significant Potential Megascale Geopark: The Eastern Australian Mantle Hotspot…*

From south to north, with increasing age, the volcanoes in the Cosgrove Volcano Chain have been increasingly eroded so that with the oldest volcanoes all that often remains of the volcanic morphology is the erosion-resistant plug and the array of dykes. Clearer and more evident volcanic landscapes are present in the younger

The principles we intend to develop for the Cosgrove Volcano Chain in terms of magma mixing, nature of xenoliths and xenocrysts, the influence of lithospheric lithologies on both xenolith types and magma contamination, and the relationship of volcanic expression to crust thickness are applicable to other volcanic trackways in the eastern Australia volcanic corridor and, as will be discussed later, have

**4. The eastern Australian continent geological framework**

to syenogranite, and granite

succession of coals

and Early Devonian

succession

*Characteristics of basins/fold belts that the Cosgrove volcanic chain intersects.*

Eastern Australia contains a large number of Phanerozoic and Proterozoic sedimentary basins and fold belts that, depending on geological setting, (former) palaeoclimates, and on geologic period, have quite a variable sedimentary

**Age, thickness of the basin or fold belt and its main lithologies**

Bowen Basin [51] Permian to Triassic basin filled with 10,000 m of fluvial and lacustrine sediments

by the Middle Jurassic coal swamp environments predominated

crustal history, varied from ~20,000 m to 40,000–50,000 m Otway Basin [59] Early Cretaceous to Tertiary basin filled with thick continental, fluvio-lacustrine, and

Late Devonian to Early Carboniferous basin filled with >5000 m quartzose, felspathic and lithic sandstones, mudstone, chert, algal limestone, volcaniclastic and volcanoquartzose sandstone, and tuff overlying folded crystalline rocks of the Cambro-Ordovician Thompson Fold Belt (siltstone, fine-grained quartzose to feldspathic sandstone, phyllite, schist, cleaved mudstone, tonalite, limestone, volcaniclastic and primary volcanic deposits, felsic volcanic detritus and primary felsic volcanics, fossiliferous sedimentary rocks, rhyolitic and basaltic lavas, ignimbrite, gabbro/diorite

(quartzose sandstones, mudstone), limestones, volcanic rocks, tuffs, and thick

Early Jurassic to Early Cretaceous basin filled with 2500 m of sandstone, coal, siltstone, shale and limestone; during the Early Jurassic, deposition was mostly fluvio-lacustrine;

Late Silurian to Early Carboniferous basin, largely Devonian in age with up to 8000 m of mostly continental red-bed facies and marginal marine facies in the latest Silurian

Ordovician to Carboniferous folded/faulted and weakly metamorphosed rocks of turbidites, trench sedimentary complexes, volcanic arcs with andesites, oceanic crust and micro continents; individual rocks are mostly sandstone and shale interbedded with chert, limestones, and metavolcanics (andesites), and intrusive granites; because of folding, crustal shortening, thrusting/faulting, estimates of basin-filling thickness are difficult to determine though Collins *et al*. [58] provide estimates that, depending on

marine sediments, followed by volcaniclastic, fluvio-lacustrine deposition, and then Late Cretaceous coastal-plain, deltaic and marine deposition and, later, Cretaceous to Middle Eocene deposition of coastal plain, deltaic and shallow marine sediments, and Middle Eocene to Early Oligocene near-shore to offshore, mixed clastic and carbonate sediments, and Late Oligocene to Late Miocene, open-marine carbonate deposition, capped by Plio-Pleistocene deposition of mixed siliciclastic-carbonate

*DOI: http://dx.doi.org/10.5772/intechopen.97839*

terrain to south.

geoheritage relevance.

**Basin, or fold belts**

Drummond Basin [50]

Surat Basin [52,

Murray Darling Basin [54]

Lachlan Fold Belt [55–58]

53]

*A Globally Significant Potential Megascale Geopark: The Eastern Australian Mantle Hotspot… DOI: http://dx.doi.org/10.5772/intechopen.97839*

From south to north, with increasing age, the volcanoes in the Cosgrove Volcano Chain have been increasingly eroded so that with the oldest volcanoes all that often remains of the volcanic morphology is the erosion-resistant plug and the array of dykes. Clearer and more evident volcanic landscapes are present in the younger terrain to south.

The principles we intend to develop for the Cosgrove Volcano Chain in terms of magma mixing, nature of xenoliths and xenocrysts, the influence of lithospheric lithologies on both xenolith types and magma contamination, and the relationship of volcanic expression to crust thickness are applicable to other volcanic trackways in the eastern Australia volcanic corridor and, as will be discussed later, have geoheritage relevance.
