Hugh Smithies: A supervolcano right in the centre of Australia

Hugh Smithies talks about the Musgraves mapping project, which led to discoveries that would lay the basis for understanding the geology of a vast area under cover in the Western and South Australian border region.

Transcript

00:01 Hugh

The whole sequence formed what we began to realize was one huge caldera structure. And when we added up the volumes of felsic magma produced, this represented a fossil supervolcano right in the centre of Australia.

00:15 Julie

Welcome to The Rocks Beneath Our Feet. In this series, five geologists talk about their years devoted to working for the Geological Survey of Western Australia. From understanding early life, to the tectonic processes that shaped our planet, and making the maps that unearth our understanding of Western Australia’s geology, they reveal their shared passion for discovering the stories in the rocks beneath our feet.

I’m Julie Hollis.

In this episode, Hugh Smithies talks about the Musgraves mapping project, which led to discoveries that would lay the basis for understanding the geology of a vast area under cover in the Western and South Australian border region.

00:54 Hugh

That was the next step in my journey in the Geological Survey. I moved over I think it was in 2005. By that stage Nebo Babel had been discovered. So that's a big gabbro-hosted nickel copper sulphide deposit. And all of a sudden, a piece of ground that was, up to that stage, not perceived to be particularly perspective was hot property. The only sort of problem in that, it represents probably one of the absolute heartlands of Aboriginal Central Australia. And so the opportunities for companies to get in were very limited. But nevertheless from a purely geological perspective, it was clearly important that we got in there and we tried to understand what the region represented, for us not particularly from the economic perspective, but from the pure scientific perspective. The Musgraves, we always suspected it was a pivotal part of the Proterozoic evolution of Australia. 

We started scoping out the possibility of setting up a program in 2005. The then assistant director of the survey was Mike Donaldson. I remember Mike, when we were first setting the project up, looked at the map and said, “Oh yeah, it's about five years work, so we should be done in five years.” I think Mike didn't really come from a very strong background in regional mapping and I just smiled and nodded at him and thought, “Yeah mate, we’ll still, we’ll still be here in ten. Don't you worry about that.”

02:21 Julie

What’s the drive like?

02:23 Hugh

So we’d drive to Kalgoorlie. That's a day. And the next day we’d drive to Laverton, usually referred to by the locals as LA, a thriving metropolis, both houses. That's where the bitumen stops. So by that stage, we'd traveled well over a thousand kilometres. And the whole of the next day was spent driving to Took Ella Roadhouse, just a simply a roadhouse in the middle of nowhere. And the day after that, late that day we were on outcrop. So what's that? That's four days of continuous driving of which at least half of it was totally on gravel roads.

03:02 Julie

Yeah.

03:03 Hugh

So it was it was a long trip. Our trips were typically only three weeks long, not including the drives.

03:08 Julie

Right.

03:09 Hugh

In some cases a bit longer than that. But, we used to have a mid-trip rest stop. We'd actually drive from the Musgraves into the Northern Territory to Curtin Springs. And have a wash and a shower. Do our clothes washing, have a quiet drink. And then next day 600 kilometers all the way back to get back into the mapping.

03:28 Julie

Wow.

03:29 Hugh

Yeah, the distances were quite extraordinary.

03:31 Julie

Yeah. And what’s the country like out there where you were doing the mapping?

03:35 Hugh

Beautiful. Beautiful desert. Absolutely pristine area. Then occasionally you’d come across these forests of desert oak

03:42 Julie

Oh wow.

03:43 Hugh

which is a Casuarina species. Yeah and these things were quite extensive, quite heavily wooded, very difficult to drive through if you had to. They attracted the Major Mitchell cockatoos as well, which I'd never seen a Major Mitchell cockatoo in the bush until I got out to the Musgraves. 

So, it was heavily duned, but the strike ridges cut through the dunes. And you know, the topography, on the strike ridges, it was very rugged but not particularly high. You know when you get those granite copy outcrops, beautiful rounded granite torrs forming copies, with fig trees in the midst of a, sort of hidden, hidden in amongst sand dunes.

04:20 Julie

Wow.

04:21 Hugh

It's that sort of absolutely spectacular desert. Just a very special place.

04:26 Julie

Yeah. In terms of the geology, what were the big advances.

04:31 Hugh

What we had was the 1300, 1330 [million year old] calc-alkaline Wankanki Supersuite. This was a 1300 [million year old] arc sequence. It had a very, very classic Phanerozoic calc-alkaline arc, continental arc, in fact, signature to them. That was a relatively new discovery. But I think more than that was how unusual that the much more value voluminous sort of 1180 to 1220 million year old Pitjinjarra Supersuite group of rocks were. You know it was always recognized that there was a suite of charnockitic rocks out in the Musgraves. But the true relevance of those, the implications of those being there was really brought out by our work out there.

These things are essentially a group of a type A type granitic magma's particularly enriched in phosphorus and and titanium. But they included a group of orthopyroxene bearing, monzogranitic to syenogranitic rocks. It was primary orthopyroxene. So these represented true charnockitic intrusions. And they were compositionally directly related to the greater number of rocks that didn't have orthopyroxene. These are biotite, in some cases hornblende-biotite bearing granodioritic to more commonly monzogranitic rocks. These things were highly voluminous, on average very anhydrous, extremely high temperature rocks. And they were isotopically very, very primitive, very radiogenic neodymium-hafnium isotopic signatures. In fact, I think they probably still represent one of the most juvenile components of sort of mid-Proterozoic central Australia. These things related basically back to a very, very primitive source at about 2 billion years.

Chris Kirkland had joined the geological survey by that stage. He spent a lot of time out there. He was dealing with the geochronological and isotopic side of the project. And so this is where Chris Kirkland came up with the concept of this two billion year old Merling Ocean that we now believe probably separated the Western Australia, North Australian, and South Australian cratons beginning at around two billion years.

06:31 Julie

Right.

06:32 Hugh

We were working at that stage alongside the South Australian Geological Survey, looking at their part of the world and with Adelaide University, in particular Dave Kelsey. And again, we had one of these collections of really interested in really exciting geoscientists. And it basically turned out what the Pitjinjarra Supersuite represented ended up being probably the world's largest ultra-high temperature metamorphic terranes. We're looking at pressures of around 5, 6 kilobars, maybe even lower, and at temperatures in excess of a thousand degrees.

This event persisted for upwards of 40 million years and the only way we could reconcile that high heat flow over such a wide area for such a long time at that stage as, we sort of thought of it as if the rigid Archean, deep Archean keels that underlay the Western Australian craton and the North Australian craton and the South Australian craton, when these things converged, they couldn't completely converge. And so they always left this window of thin two billion-year-old dominantly mafic crust that just sat there and stewed

07:40 Julie

Right.

07:41 Hugh

And all of the mantle upwelling was channeled by these keels into this collective hotspot that just underwent hell for 40 million years.

So that was quite an extraordinary geological slant to the Musgraves. Of course on top of that we had the Warakurna Event at 1070 [million years]. The more famous manifestation of that is the Warakurna large igneous province. You get the dolerite dikes that cover a vast area of central Australia. But it also produced quite a thick series of basalts and intercalated dacitic to rhyolitic volcanics. And these things were in most cases beautifully preserved, flat lying volcanic and volcaniclastic sequences.

A lot of these things were ignimbrites, rheomorphic ignimbrites. They showed an absolute wealth of beautiful volcanic textures, just a volcanologist’s absolute dream. And also a geochemist’s dream as well, it turns out. Again, really high temperature A-type felsic volcanics. The mafic sequence, they were asthenospheric, broadly tholeiitic compositions.

The whole sequence formed what we, after a while we began to realize was one huge caldera structure.

08:53 Julie

Right.

08:54 Hugh

And when we added up the volumes of felsic magma produced, some of the specific layers, exceeded what was required for them to be interpreted as the result of supervolcano eruptions.

09:06 Julie

Yeah.

09:06 Hugh

This represented a fossil supervolcano. So a 1040 to 1070 million-year-old fossil supervolcano right in the centre of Australia.

When Chris did all of his magic with the zircons, it turned out that some of these felsic volcanics were absolutely crowded with anticrysts. So zircon crystals that were more or less genetically related to the same magmatic system. But if you follow that logic, it meant that not only was this a super volcano, but it represented one of the longest-lived super volcanic systems that the world's ever known.

And on top of that, when Chris did the oxygen isotopic work on the zircons, they were characterized by really light oxygen, which is typical of caldera systems. So it was one of the largest light oxygen systems known of. We're talking about Yellowstone sized things.

09:56 Julie

Yeah.

09:56 Hugh

So yeah, just amazing.

The 1180 to 1220 million year old Pitjinjarra Supersuite represented quite an extraordinary metamorphic environment. At that stage, you know, it was restricted to the Musgraves, but with subsequent studies have shown that it extended all the way under the, under the Eucla basement down into the into the Albany-Fraser, representing a dramatic high temperature event that Cath Spaggiari started referring to as the Maralinga Event, quite appropriately in a sense.

10:26 Julie

Yeah.

10:27 Hugh

That was really neat and at that stage Cath was endeavouring to sort of produce our first remotely-sensed, geophysical regional map. This is our first real foray into the world of mapping undercover using the available geology and as much remotely-sensed data as we can get. The real gem of that project was to put down a series of stratigraphic holes. That just emerged to be an amazingly successful project that just added so much information in terms of what underlies that huge area of southeastern Western Australia and southwestern South Australia that’s just covered by the Eucla basement. We basically established it shared a history very much like the South Australian part of the Musgraves and the Western Australian part of the Musgraves. A lot of the rocks there were again isotopically rooted in Chris Kirkland's 2 billion year old Merling Ocean

11:24 Julie

Right.

11:25 Hugh

that we could relate, based on the ages, the isotopes and the geochemistry, what we could relate to a series of closing oceanic arcs. We had everything there from real classic Phanerozoic calc-alkaline basaltic units to a series of beautiful shoshonitic intrusions. On top of that, we had the Pitjinjarra Supersuite again, this 1180 to 1200 million year old ultra-high temperature metamorphic terrane. Again, which led us to suggest that that part of the connection between Western Australia and South Australia had never been allowed to go to completion.

11:59 Julie

Right.

12:00 Hugh

Against the Archean buttress of the Western Australian craton we had preserved what, correctly I think, interpreted it as an ophiolite complex. Another one of these areas where, so much to offer in terms of geological discoveries.

12:17 Julie

It seems impressive on the basis of almost no outcrop.

12:21 Hugh

Absolutely. And what that project showed us was how with a really good geophysical interpretation with limited and in some cases virtually no outcrop input but with just a bit of diamond drill hole control, how much information you can ultimately get in these regions of Western Australia, South Australia, Northern Territory and Queensland, these vast regions that are almost entirely covered by younger sedimentary basins, if your intent is to is to investigate the basement rock and the basement rock prospectivity. A lot of our geological enterprises now are set up along these lines. In a sense, it probably formed the basis upon which the whole National Drilling initiative that’s now operating. 

13:09 Julie

And returning to the Musgraves ultra-high temperature metamorphic zone that extends south toward the Albany Fraser Orogen.

13:15 Hugh

It was quite interesting. We explained this to the traditional owners and we showed them the aeromagnetic signatures where these, what we referred to as hot rocks, disappeared under the regolith. And they were absolutely fascinated because it just so turned out, you know, I'd say coincidence, they’d say it was always meant to be, it just so turned out that some of their dreaming trails followed the buried trails of these really hot rocks. Their dreaming explained these things in terms of the passage of hot things.

13:43 Julie

Oh wow.

13:44 Hugh

Yeah, it was spookingly coincidental.

13:47 Julie

You’ve been listening to The Rocks Beneath Our Feet. You can discover more about GSWA by visiting dmp.wa.gov.au/gswa or find GSWA on LinkedIn and Facebook. If you like what you’ve heard, give them a follow.