October 2015 LIP of the Month

Lithospheric Controls on Volcanic Outcrop and Magma Composition along Earth’s Longest Continental  Hotspot-track

D. R. Davies1, N. Rawlinson2, G. Iaffaldano1,3, and I. H. Campbell1

1Research School of Earth Sciences, The Australian National University, Canberra. Emails: rhodri.davies@anu.edu.au; Ian.Campbell@anu.edu.au

2School of Geosciences, University of Aberdeen, Scotland,  UK.

3Department of Geosciences, University of Copenhagen, Denmark.

Around 50 volcanic hotspots have been identified at Earth’s surface [1, 2, 3, 4]. Of these, only 20% occur on continents and, hence, most of our understanding of mantle plumes comes from hotspot-tracks in oceanic settings. However, oceanic lithosphere is regularly recycled into the mantle through  subduction, so if we are to understand plume-related volcanism prior to 200 Ma, we must learn: (i) how plumes interact with continental lithosphere; and (ii) how this interaction affects the chemical composition and erupted volume of lavas at the  surface.

It has long been recognised that lithospheric thickness limits the rise height of plumes [5, 6, 7] and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas [8], although unambiguous evidence of this on continents has, thus far, been lacking. In our recent study, published in Nature [9], observational constraints from surface geology, geochronology and plate-motion histories are combined to identify Earth’s longest continental hotspot-track, a 2000 km long track in eastern Australia that displays a record of volcanic activity between 33 and 9 Ma [10, 11], which we call the Cosgrove track (see Fig. 1).

Constraints from seismology and geochemistry are subsequently integrated to ascertain how regional lithospheric thickness variations influence the volume and composition of plume-derived magmas along this track. We first generate a map of lithospheric thickness, by combining constraints from the 3-D body- wave tomography results of Davies & Rawlinson (2014) [13] and Rawlinson et al. (2015) [14] with the regional AuSREM tomographic model of Kennett et al. (2013) [15] (Fig. 2). Intriguing trends are evident along the Cosgrove track: (i) volcanic gaps occur in regions where lithospheric thickness exceeds  150 km; (ii) the basaltic and felsic central-volcanos of central Queensland occur in regions where lithospheric thickness is less than 110 km; and (iii) low-volume, leucite-bearing volcanism to the south occurs, exclusively, in regions of intermediate lithospheric thickness, with volcanic gaps within the leucitite-suite also coinciding with regions of thicker lithosphere.

These unambiguous trends suggest that the thickness of overlying lithosphere is dictating the volume and composition of plume-derived magmas, by limiting the rise-height of the underlying plume and, hence, the degree of partial-melting. We infer that the underlying mantle plume: (i) cannot rise to shallow enough depths to induce decompression melting in regions where lithospheric thickness  exceeds 150 km, thus providing an explanation for the volcanic gaps along the Cosgrove track and placing the first observational constraint on the maximum melting depth of mantle plumes beneath continents (excluding ultra-volatile melts that form kimberlites and carbonatites); (ii) undergoes high-degree partial- melting beneath comparatively thin lithosphere to produce basaltic and felsic central-volcanos along the northern segment of the Cosgrove track; and (iii) undergoes very low-degree partial-melting in regions of intermediate lithospheric thickness, thus facilitating the production of low-volume leucite-bearing volcanics towards the southern end of the Cosgrove  track.

Figure 1: The distribution and classification of eastern Australian Cenozoic volcanic centres, following Wellman & Mc- Dougall (1974) [12], where black, grey and red denote central-volcanos, basaltic lava-fields and low-volume, leucite-bearing volcanics, respectively. Earth’s longest continental hotspot-track, the Cosgrove track, extends across the Australian continent from Cape Hillsborough (∼ 33 Ma) to Cosgrove (∼ 9 Ma), and incorporates both the central volcanos of central-Queensland and the leucitite-suite of New South Wales and Victoria.

To ascertain whether or not these inferences are compatible with geochemical observations, we collated previously published trace-element data from volcanic outcrops along the Cosgrove track [16, 17]. These support the notion that the compositional variations along the Cosgrove track result from different degrees of partial-melting, which is controlled by the thickness of overlying lithosphere.

Our results, therefore, provide direct observational evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas. Further details can be found here: http://www.nature.com/nature/journal/v525/n7570/full/nature14903.html

Figure 2: The volcanic centres of Fig. 1, plotted above an estimate of lithospheric thickness, highlighting a clear correlation between lithospheric thickness and volcanic outcrop, classification and composition along the Cosgrove hotspot-track.


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