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

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


### **Table 1.** *Characteristics of basins/fold belts that the Cosgrove volcanic chain intersects.*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**3. The eastern Australian volcanic framework**

chemical composition of plume-derived magmas [13].

over a mantle hotspot on its eastern side.

eruptive and magmatic activity.

complex sedimentary basin.

To account for geochemical heterogeneity in hotspot and flood basalt lavas, Farnetani & Richards [37] suggest either inherent plume-source heterogeneity or contamination from the lithosphere through which the primary magma ascended. We accord with the latter, *i.e.*, contamination from a heterogeneous lithosphere, particularly when the magma rises through a thick lithosphere [13] or through a

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

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

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

### **Figure 4.**

*Map of eastern Australia showing occurrence of the sedimentary basins and fold belts (modified from geoscience Australia https://www.ga.gov.au/\_\_data/assets/image/0020/13943/GA14654.gif) that the trackway of the Cosgrove volcanic chain will have intersected. Dominant lithologies that will yield xenoliths/xenocrysts are listed in Table 1.*

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

history, metamorphic history, and structural history, *viz*., 1. mainly siliciclastic, 2. carbonate-dominated, 3. changing mega-stratigraphically between siliciclastic and carbonate-dominated, 4. basins with sedimentary sequences as 1–3 above but with insertion of coal measures and/or volcanic events (**Table 1**). This is the geological framework that mantle plumes, in ascending towards the surface, have intersected. Thus, along eastern Australia, the range of mantle plumes (or mantle hotspots) from northernmost Australia to Tasmania (the Cosgrove Volcanic Chain trackway is shown in **Figure 3**) needed to transgress these various heterogeneous lithological

As mentioned earlier, we have opted to focus only on the Cosgrove Volcanic Chain (trackway) to illustrate what this volcanic plume needed to transgress lithologically, what it needed to cross in basin thickness, and what it encountered in stratigraphic/lithologic heterogeneity. The Cosgrove Volcano Chain crosses a number of sedimentary basins and a fold belt (**Figure 4**) that are of various ages, thicknesses, and lithologies (**Table 1**) and, as such, the trackway had the potential to interface with a variety of rocks that could be involved in melting/mixing and melt contamination, and yielding of discrete xenoliths and xenocrysts. Given that the volcanic trackway may not have been strictly linear, and may have deviated from the trackway that is shown in **Figures 3** and **4**, the most probable xenolith and xenocryst contributions to the evolution of the Cosgrove mantle plume are from lithologies from the following basins and tectonic zones (from north to south): Drummond Basin, Bowen Basin, Surat Basin, Murray Darling Basin, the Lachlan Fold Belt, and the Otway Basin (**Table 1**). **Table 1** list the, thickness of the basin or fold belt, its main lithologies, and its age. The deepest part of each basin often rests on metasediments or on crystalline rocks though the bulk of the sedimentary fill of

**5. Geoheritage significance of the eastern Australian Cosgrove volcanic** 

The Cosgrove Volcano Chain provides a globally-unique system to explore an intra-plate migratory volcanic system and, additionally, in this context, with its globally-distinct suite of sedimentary basins that it has over time intruded through, it has global geoheritage significance. From north to south, in its younging direction, the Cosgrove Volcano Chain has a rich history and expression of volcanic activity with a range of lava types and eruptive volcanic rocks (dominantly basaltic lava [alkali olivine basalts, hawaiites, mugearites], but with occurrences of leucitite, trachyte, rhyolite, andesite, andesitic basalt, trachyandesite, and, where explosive and where expressed pyroclastically, tuff, breccia and agglomerate). Geomorphically, these volcanic eruptions are expressed as a range of primary volcanic landforms as well as eroded-residual geometry types (shield volcanoes, stratovolcanoes, domes, plugs, spires, *etc*.), and stratiform ash deposits, dykes, plugs, and sills. There are also valley fills, conic accumulations of tephra (*e.g.*, accretionary

In a sub-global context, the Cosgrove Volcano Chain presents a range of magma types, volcanic expressions, a history of interactions with the variable but regionally diagnostic lithosphere (*i.e.*, regionally-specific and lithologically-diagnostic sedimentary basins). It also presents a landscape-*cum*-climate response as the various volcanic and eruptive centres passed progressively through a climate gradient as Australia migrated from artic/boreal climates through to subtropical/tropical

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

the basins tend not to be metamorphosed.

pyroclastic cones), ash sheets, and lava tubes.

systems.

**chain**

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

history, metamorphic history, and structural history, *viz*., 1. mainly siliciclastic, 2. carbonate-dominated, 3. changing mega-stratigraphically between siliciclastic and carbonate-dominated, 4. basins with sedimentary sequences as 1–3 above but with insertion of coal measures and/or volcanic events (**Table 1**). This is the geological framework that mantle plumes, in ascending towards the surface, have intersected. Thus, along eastern Australia, the range of mantle plumes (or mantle hotspots) from northernmost Australia to Tasmania (the Cosgrove Volcanic Chain trackway is shown in **Figure 3**) needed to transgress these various heterogeneous lithological systems.

As mentioned earlier, we have opted to focus only on the Cosgrove Volcanic Chain (trackway) to illustrate what this volcanic plume needed to transgress lithologically, what it needed to cross in basin thickness, and what it encountered in stratigraphic/lithologic heterogeneity. The Cosgrove Volcano Chain crosses a number of sedimentary basins and a fold belt (**Figure 4**) that are of various ages, thicknesses, and lithologies (**Table 1**) and, as such, the trackway had the potential to interface with a variety of rocks that could be involved in melting/mixing and melt contamination, and yielding of discrete xenoliths and xenocrysts. Given that the volcanic trackway may not have been strictly linear, and may have deviated from the trackway that is shown in **Figures 3** and **4**, the most probable xenolith and xenocryst contributions to the evolution of the Cosgrove mantle plume are from lithologies from the following basins and tectonic zones (from north to south): Drummond Basin, Bowen Basin, Surat Basin, Murray Darling Basin, the Lachlan Fold Belt, and the Otway Basin (**Table 1**). **Table 1** list the, thickness of the basin or fold belt, its main lithologies, and its age. The deepest part of each basin often rests on metasediments or on crystalline rocks though the bulk of the sedimentary fill of the basins tend not to be metamorphosed.
