Hugh Smithies: Probing the beginnings of plate tectonics on Earth

Hugh Smithies talks about key discoveries made during GSWA mapping of the Pilbara 

Transcript

00:01 Hugh

So you get a pattern of these rounded granite domes that are rising between these sinking steep synclinal keels. You can see these things from space. It’s one of the most distinctive geological patterns on Earth.

00:14 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 key discoveries made during GSWA mapping of the Pilbara.

00:46 Hugh

The Pilbara’s where I really got to apply my background in geochemistry. I started in the Pilbara, it would’ve been the mid 90s. It was the geochemistry that always really captivated me. 

1:00 Julie

What’s special about the geology of the Pilbara?

1:04 Hugh

In many respects that work was one of those sort of pivotal points in my career. Not only is the Pilbara an absolutely beautiful place to go from the scenic perspective, the exposure is absolutely exceptional. The geology is just out of this place, especially for people interested in Archean geology, there is no better place in the world. Just the opportunities, both from the mapping perspective and from the research perspective that the Pilbara geology offered was absolutely outstanding.

And then there was the team that we had gathered up there at that at the time. It was led by Arthur Hickman and Arthur has just had this absolutely incredible memory. First of all his experience during the first phase of mapping up there was immense. But I don't think he's ever forgotten anything in his life. He was just a mobile dictionary of the first phase of mapping. The dates would just roll off his off his tongue. There's no rock that he's ever forgotten, no road into really hard to get to outcrops that he's ever missed. It was just phenomenal.

Arthur gave me a lot of rope to go out and collect chemical samples and actually try and do something with them.

And then we had just an absolutely incredible team of really enthusiastic and absolutely outstanding geologists both from GSWA and from Geoscience Australia. And so that's where I really started.

I'd known Dave Champion back in my Canberra days, but quite fortuitously when I started mapping in the goldfields, he also, from Geoscience Australia, started mapping in the goldfields. And so when I moved to the Pilbara, GA started a project up in the Pilbara and he also moved to the Pilbara. So Dave and I had a very long history of collaborative work. And really I guess looking back on the sort of work that Dave started doing in the goldfields with his regional granite data sets

2:59 Julie

Yep

3:00 Hugh

and the benefit that he was getting out of that really turned me to thinking about doing the same up in the Pilbara with both the mafic and the felsic rocks. And I guess that's where I really started to appreciate what could be done with large, well located, well understood, well analyzed whole rock geochemical data sets.

And so a lot of my time, apart from the regional mapping, was spent on collecting regional geochemical data sets. And the Pilbara’s just one of these places where no one had really worked on these data sets. So whatever you collected was new.

3:35 Julie

Yep

3:35 Hugh

And in a lot of cases in a really exciting way. And you know, we were out there. We weren't just mapping the region, but we were critically trying to delve into the crustal evolution of this region. And at a time when that that plate tectonic debate, whether or not it was operating or not, was in full swing. And so that was always in the back of the our mind as we tried to unravel the geology.

You know, geology, it's an amazing science, field geology. It's like forensics. You look at the map pattern and that's the scene of a crime. And there's just clues strong strewn all over the place and it's your job to try and pick these things apart and try and reconstruct what's actually happened.

This group of guys, we just had such a good relationship, such a good time, such a good spread of expertise. You know we had Martin van Kranendonk up there and Dave Houston. Dave Houston was just, is just an absolutely fantastic economic geologist. Great guy. Just so free with his knowledge. Dave Champion, I regard as probably the best felsic geochemist in Australia over the last couple of decades. We had Richard Blewitt up there. Him and Martin were at loggerheads more often than not over their alternative structural views of the evolution of the region

4:50 Julie

Yeah.

4:50 Hugh

But I guess, you know, it was just a bit of a dream team really.

4:54 Julie

Yeah.

4:55 Hugh

And so that was when the big debate about the role that plate tectonics played in Archean geological evolution started to kick off. Everybody was jumping on this subduction bandwagon, particularly so in Australia, it seemed. It got to a ridiculous stage where almost simply the identification of a rock proved that subduction was in operation 3 billion years ago. Obviously I’m exaggerating there, but it really did get quite ridiculous at some stage. 

5:25 Julie

To clarify, can you just explain very briefly?

5:28 Hugh

It's founded on the principle of uniformitarianism that geological processes that we see today molded the geological evolution throughout Earth's history, which obviously is not the case because the Earth started off as a ball of magma. So at some stage in geological history, the principle of uniformitarianism has to break down. And so the great debate was, in the first instance, whether it operated in the Archean at all. And if it did how far back you could take it. And there was a core opinion that it went as far back as we could as the rock record could possibly be traced. Another, side of the argument that no, that even what we see in the Neoproterozoic, a much younger period, as young as a thousand to 500 million years, was significantly different to even doubt  the plate tectonic processes, so subduction, the pushing of rigid plates under each other and forming arcs where new crust is evolved, that these sorts of processes even operated then.

And geologists being what we are, we’re perhaps of the entire scientific community, we have a tendency to polarize ourselves to a particular view more than more than anybody else. So we had people running around the place trying to prove that subduction was happening as far back as 3.5 billion years ago and another group running around trying to prove that that plate tectonics had absolutely nothing to do with the evolution of the Pilbara. I was all part of that debate and on the fence at some stage. Over the 10 years that I was up there, I probably jumped off that fence to either side a few times.

6:57 Julie

Yep.

6:58 Hugh

I guess the geochemistry of the rocks up in the Pilbara really played into the debate in quite a heavy way in that some of the sequences that we uncovered in the central Pilbara, I guess proved to be, and are still recognized as perhaps the best weight of evidence argument in terms of complete stratigraphic sections comprising rocks that individually and in different ways reflect subduction processes. It's where the most complete and convincing section was found. So this is the Wandoo section, which represents volcanic rocks formed at I think it's three point one-two billion years ago. These things became regarded as probably the best evidence for some form of plate tectonics operating. And we jumped and down about that and claimed that this was evidence that plate tectonics had begun by that stage, which is possibly correct. We probably went a bit too far and to say that this was the beginning of plate tectonics on Earth because it possibly wasn't that. I guess now I would probably prefer to say that that it was evidence for some form of mantle metasomatic enrichment. Certainly not plate tectonics as we know it. There was no evidence whatsoever that stage for sort of linked global scale interconnected zones of subduction and both oceanic plate construction. No interconnected organized series of mid-ocean ridges, for example. Nothing to suspect that anything like that had developed on Earth. But I guess we preferred to think of it, after having thought about it for a while as maybe just a more local conspiracy of rigid plates.

8:36 Julie

Right.

8:37 Hugh

I guess that's where we set for quite some time on the role of plate tectonics in the Archean, specifically in the Pilbara. You know, we had the eastern Pilbara which were this typical keel and dome structure, which is it's not found in modern geological situations.

8:51 Julie

Yep.

8:52 Hugh

And I think consensus has settled on the eastern Pilbara, at least up until about up three point one [billion years] being a case of vertical tectonics non plate tectonics, a terrain derived through non plate tectonic processes.

9:03 Julie

Could you give a very general description of vertical tectonics?

9:07 Hugh

So vertical tectonics is, if you envisage a region of felsic crust under relatively high heat flow, so hot crust, if that's then blanketed by a thick sequence of basalts, a couple of things happen. The thick sequence of basalts thermally insulates the felsic crust so that the felsic crust heats up and becomes more ductile. The thick basaltic sequence cools down. And so you get an inherently gravitationally unstable situation. And so the upper dense mafic basaltic crust starts to sink.

9:45 Julie

Yep.

9:46 Hugh

The felsic crust starts to melt, becomes even more buoyant. And it rises up. And so you get a pattern that's really exemplified by the east Pilbara, of these rounded granite domes that are rising between these sinking keels, these really steep synclinal keels. You can see these things from space. It's one of the most distinctive geological patterns on Earth. That's viewed as the alternative to subduction. So if you view subduction as, horizontal tectonics, or the forces are really, in a rough sense anyway, operating in a horizontal way

10:18 Julie

Yep.

10:18 Hugh

whereas in vertical tectonics, it's up and down, its buoyancy driven.

So you have that in the east and then you have the sedimentary basins, the Malina Basin and the central Pilbara which is, I guess has these touches of more Phanerozoic type geology in them. And then you have the far western Pilbara, which again is a bit older and it's more in line with the eastern dome and keel geology.

So you've got this diversity of geology. And then it's covered by the Fortescue from 2.7 billion years younger, which is a flat lying sequence. Again, that's very unique within the Archean world. You know, we look at these highly deformed Archean and belts in Canada and the Yilgarn, extremely deformed, elongate greenstone belts, and we wonder how the hell these things formed. And we forget that just up north and in the Pilbara we have more or less the same rock types in these in these beautiful flat lying sequences. And I sometimes think that we forget that a lot of the answers to the evolution of the deformed greenstone belts really might quite simply be sitting there and these flat-lying sequences. But we've paid them very little attention really. 

11:27 Julie

And what was particularly interesting that came out of the geochemistry?

11:31 Hugh

One of the things that really caught my eye to start off with, and this again was at the time when everyone was talking about adakites. These dacitic volcanic rocks that are found in modern subduction settings. They have the chemistry that you would relate to garnet retention in a mafic source rock. So they have really strongly fractionated rare earth element patterns that reflect the fact that the melting is leaving behind garnet. And they're very sodic, which reflects the fact that these things are melting a basaltic source.

And people started at that stage looking at TTGs. So these are the tonalite-trondhjemite-granodiorite series that forms the fundamental basis of, of Archean granite domains. They started making these comparisons between adakites and TTGs. And the comparisons are indeed, they're really close. TTGS, by definition, are very sodic granitoids and they have very steep rare earth element patterns with high strontium-yttrium ratios suggesting that the source didn't contain any plagiocase, but did retain garnet. These things were formed through melting at depths of around 50 kilometres or more.

And so people started thinking, these things actually have the same composition as adakites. We strongly suspect that adakites are formed through deep melting. So it's melting of the slab as it gets subducted below a continent or below another slab as it goes down. It's hot. So it's already amenable to melting. But by the time it gets down to 50 or so kilometres, it starts to melt and it produces these felsic rocks with extremely unusual chemistries.

So, you know, being at the time when the plate tectonic debate was kicking off this was just too convenient, you know. Obviously these TTGs looked like adakites, adakites are formed at subduction. Game over. The TTG's are formed by subduction and Archean sequences are no different to modern-day plate tectonics.

So I started looking at the chemistry of the TTGs we were finding up in the Pilbara. And I compared them with the adakites in detail. And I found that whilst they were in general similar – the TTGs in fact had slightly different compositions that varied through time. So in fact one of the hallmarks of an adakite is that they do have in fact have reasonably high magnesium numbers. They attain that as these magmas traverse through the mantle wedge.

13:54 Julie

Right.

13:55 Hugh

But the earlier TTGs, so the ones in the eastern Pilbara at about 3.4 or so, they showed no similarity in that respect. They had very, very low magnesium numbers, really low chrome, really known low nickel. So say they showed no indication of any sort of interaction what might have been a mantle wedge.

14:12 Julie

Right.

14:13 Hugh

But then as you got younger in Earth history, you got into the Mesoarchean and into the Neoarchean, the TTGs seem to change in composition and they did become a bit more magnesium-rich, had higher magnesium numbers, higher chrome, higher nickel.

And so we made the suggestion that that no, you can't really relate TTGs to adakites, certainly not the Paleoarchean TTGs. They weren't similar in detail. They were different in very important ways.

14:40 Julie

Yep.

14:40 Hugh

But that there might be a case for these sorts of processes to be beginning by the Neoarchean.

14:46 Julie

Right.

14:47 Hugh

That was probably the first real scientific discovery we made. And, in fact, well the next one was I guess the identification of boninite-like rocks in the central and western Pilbara. Boninites are these high magnesium mafic rocks. But they’re second melts. So they're basically when you, have a subduction zone. Earlier on in the subduction history near subduction initiation, in the extensional part of subduction history, you introduce a fluxing component into an already depleted mantle source, and you actually flux melt at quite high temperatures in the depleted source. And you produce quite an unusual mafic rock that shows evidence of a prior depleted source with evidence of a flux enrichment. So you get these, if you look at the rare earth patterns and the incompatible trace element patterns, you get these these quite unusual u-shape patterns.

And we started  seeing similar sorts of things in the central and western Pilbara. And in fact, one of those areas that we did find these sorts of patterns turned out to be the Wandoo sequence, where we later went on to demonstrate that that represented one of the really good cases for some form of subduction process in the Archean.

15:57 Julie

Right.

15:58 Hugh

So that was another unique suite of rocks that we that we found. But perhaps the most, the most exciting were the, the sanukitoids.

16:015Julie

But more on sanukitoids in a later episode.

16:08 Hugh

I think from the scientific perspective, by the sort of mid to, early to mid-2000s, I think we’d collectively elevated the Pilbara to basically to the forefront of the debate regarding the nature and timing of horizontal versus vertical plate tectonics and the onset of what might be construed or misconstrued as modern style tectonics.

16:30 Julie

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