Hugh Smithies: Sanukitoids - a key to understanding crustal evolution?

Hugh Smithies talks about sanukitoids, a rare and perhaps fundamentally important group of Archean rocks that he first encountered in the Pilbara and later discovered hiding in plain sight in the Yilgarn 

Transcript

00:01 Hugh

Perhaps the most exciting were the sanukitoids. So this was back in the late nineties that we started finding the sanukitoids. They’ve really kept me captivated ever since then. I still spend a lot of time pondering over what the hell these things mean.

00:13 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 sanukitoids, a rare and perhaps fundamentally important group of Archean rocks that he first encountered in the Pilbara and later discovered hiding in plain sight in the Yilgarn.

00:51 Hugh

It would’ve been in the mid 90s. That was when the big debate about the role that plate tectonics played in Archean geological evolution started to kick off. You know, everybody was jumping on this subduction bandwagon.  The Wandoo sequence – that represented one of the really good cases for  some form of subduction process in the Archean.

01:11 Julie

Right.

01:12 Hugh

But perhaps the most exciting were the sanukitoids. So we found an unusual suite of dioritic to granodioritic rocks that intruded into the Malina Basin. So the Malina Basin is a 2 .98 Ga [billion years] sedimentary package that overlies the west Pilbara. It’s a series of turbidites. And right along the axis of the basin you get a series of these diorites and granodiorites, very hornblende-bearing and reasonably mafic. And in that respect in the Pilbara, they were very unusual. It's unusual in all of Australian TTG terranes to find a lot of hornblende-bearing tonalities and trondhjemites. It's a bit of a misnomer that the TTG series is in fact characterized by hornblende-bearing rocks because in Australia, that's certainly not the case. They're almost invariably biotite-bearing.

02:00 Julie

Right.

02:00 Hugh

But all of a sudden we're getting these, these rocks that you know, they had magnesium concentrations up to five, six percent, magnesium numbers over 60, and very high chrome, very high nickel. But they were also unusually enriched in the large ion lithophile elements, strontium, barium, and in, in lanthanum and thorium. 

02:17 Julie

Yep.

02:18 Hugh

So groups of elements that put together, form a bit of a contradiction.

02:22 Julie

Right.

02:22 Hugh

So you know, I didn't know what the hell these things were. And I remember I was driving from Whim Creek to Port Hedland to resupply at one stage and Dave Champion was on the radio and I start describing these things Dave Champion. And it just so turned out that he’d just been reading a paper by some Canadian geologists on these things that they called sanukitoids. And so he described them and I said, “Well that's, that's actually what possibly what we're looking at.” And they've got a very distinctive appearance in the field. They're almost invariably crowded with, what in fact represent cognates xenoliths, so you find a broadly granodioritic, coarse-grained hornblende-bearing sanukitoid and it will be crowded with granodioritic to dioritic, in some cases even more mafic cognate xenoliths, so xenoliths that share textural similarities, mineralogical similarities, but just are progressively more and more primitive. And sometimes you even get xenoliths in xenoliths within these things. And so that's one of the diagnostic features of these sanukitoids.

And so Dave, I think he was even reading off from the paper that these things have these distinctive cognate xenoliths suites, I thought “Bloody hell, that's probably what we got up in the Pilbara.” Yeah, and it turns out that's exactly what we had in the Pilbara. And the unique thing about the sanukitoids, at that stage they really hit the petrological news because they were the only instance where a case could be made for a felsic magma being directly derived through melting of mantle peridotite.

03:54 Julie

Oh!

03:55 Hugh

That was the answer to the compositional paradox that these two suites of trace elements were throwing at us. They had the had the high chrome, the high nickel, magnesium numbers over 60. By definition at 60 weight percent silica, they need to have chrome and nickel both over a hundred ppm. So that's a hell of a lot for felsic rock.

04:10 Julie

Yeah.

04:11 Hugh

That was clear evidence that these things, them or their parent parental magma, started often in equilibration with a mantle peridotite. But they had these huge enrichments in strontium, barium, lanthanum, light rare earth elements. They had typically had quite significant negative niobium anomalies. So they had all the hallmarks of either a subduction component or of crustal contamination.

04:33 Julie

Right.

04:34 Hugh

And so at that stage what everybody was starting to think about these rocks was that they were direct melts of a mantle that was metasomatised through subduction. So material coming off a subducting slab metasomatising a mantle.

04:474 Julie

Yep.

04:48 Hugh

And then that mantle being remelted to directly produce a felsic melt. So if that was the case then they were absolutely unique in igneous petrogenesis. 

04:58 Julie

Yep.

04:59 Hugh

That seemed to be consistent with a lot of what we saw in the Pilbara sanukitoids. You didn't see that mafic endmember that would lead you to believe that it was part of a liquid line of descent rooted in a mafic parental magma.

05:12 Julie

Yep.

05:13 Hugh

So it all seemed to be consistent with these things being direct melts of a modified mantle.

05:18 Julie

OK

05:18 Hugh

And that really played into our sorts of discoveries within that region of boninite-like rocks and of the Wandoo subduction-like signature that we could actually then build up what we thought was a really good weight of evidence case that this region was fundamentally molded through plate tectonic processes that at least were beginning to look like modern style processes. That was quite a big thing at that stage.

05:44 Julie

There’s also a link with gold mineralization?

05:48 Hugh

The connection between gold and sanukitoids was established back in the days when Dave Champion and I were working on these things up in the Pilbara, again by the Canadians. And at that stage I think it was just an empirical observation that wherever you found sanukitoids you, or syenites, you tended to get gold camps.

06:04 Julie

OK

06:04 Hugh

But certainly when that connection was pointed out to the explorers, to people studying the same sorts of systems in Australia, the connection I guess was assumed to be a fortuitous one, that the gold mineralization and the sanukitoids simply, they simply both like the same cracks. That's, that was basically it.

06:21 Julie

Yep.

06:22 Hugh

After mapping in the Pilbara and then subsequently in the Musgraves, I then moved back to the eastern goldfields.

What we're doing there is, it's a major geochemical sampling program that we that we refer to as the Archean greenstone barcoding program. And we're looking at geochemically profiling, barcoding if you like, the Archean greenstone stratigraphy at various locations, in the first instance as a way of producing a chemostratigraphy that might help the exploration industry locate where they are stratigraphically based on limited drilled chip, geochemical information for example.

06:57 Julie

Right.

06:57 Hugh

I started this program in the Kalgoorlie-Kambalda region because that's where we’re quite confident about the stratigraphy, or we think we are anyway. And one of the rocks that we started collecting a lot of samples from was the Black Flag Group, which is a, it's essentially two point six nine billion years old. It overlies the main basaltic greenstone component. It's a sequence of felsic volcanic, volcaniclastic and subvolcanic porphyry intrusions, very abundant and locally very thick. So we started collecting a whole lot of these. And once we got all the geochemical data back we started to see geochemical trends in these that very, very closely mimic the trends that we'd seen in the Pilbara rocks. And sure enough when we followed these leads up and a bit further, it turns out that the Black Flag Group, almost in its entirety, is basically a volcanic equivalent of the sanukitoid intrusions that we saw in the Pilbara.

07:55 Julie

Oh!

07:56 Hugh

And so that was huge because the Black Flag Group had been looked at before and various models had been proposed for what it represented, the main one being that it was just a volcanic example of a tonalite-trondhjemite-granodiorite series, possibly in a subduction setting again.  And so the revelation that these things were indeed sanukitoids was quite significant, particularly because you're looking at a bit of the world geologically, between Kambalda and Kalgoorlie, that's probably been more studied, more walked over, more looked at, more analyzed than any other equivalent sized bit of ground in the world. But no one really had sort of bothered to analyze enough of these rocks to recognize the sanukitoid heritage.

08:39 Julie

Right.

08:39 Hugh

I just found that quite amazing and it just makes you wonder. You know, you never give up, never give up in any area because there's always something more to learn. 

08:46 Julie

Yeah.

08:46 Hugh

You never take anything for granted.

08:48 Julie

Yeah.

08:49 Hugh

So what the Black Flag Group actually offered us was a really complete geochemical liquid line of descent. We could actually trace these rocks. We had our in our rhyolitic equivalents, we had our dacitic equivalents, we had andesitic equivalents. And we started getting basalts that had distinct sanukitoid-like compositions. Then we even started finding some rocks that had 10, 12 weight percent magnesium. And they started to merge with the lamprophyres, which are abundant in that region. So these are getting down towards the ultramafic spectrum.

09:18 Julie

Right.

09:18 Hugh

In those in those datasets we could actually prove a liquid line of descent that connected an ultramafic mantle-derived melt to the felsic derivatives. So based on that evidence, they were derived from an enriched mantle lithospheric source, that certainly didn't change. But the sanukitoids were a derivative of those. So that, at least as far as we're concerned, throws out the theory that these things were a direct felsic extract of a periodotitic mantle.

09:46 Julie

OK.

09:47 Hugh

One of the things about sanukitoids is the oldest one that's ever been found was in fact the ones we found in the Malina Basin and they were 2.95 [billion years old]. No sanukitoid older than that had been found anywhere else in any Archean terrane. Just about every Archean terrane does contain sanukitoids.

10:01 Julie

OK.

10:02 Hugh

They never amount to more than 5% of the total granitic volume.

10:05 Julie

Right.

10:06 Hugh

And so they were, for a long time, and probably still are by a lot of people considered to be a phenomena entirely restricted to the Neoarchean. We've subsequently found some in the Pilbara that are dated at around three point three [billion years old], rocks that they’re sort of maybe in the strictest of strictest of the definition, might not cut the cut the grade completely in terms of sanukitoids. But they’re certainly on their way. And so what we're suggesting at the moment is that maybe sanukitoids have always been formed through one process or another, in the Neoarchean perhaps by some form of plate tectonics that's approaching a modern style. And in the Paleoarchean by maybe some other process that's enriching the mantle lithosphere.

But I strongly suspect that they have formed one way or another throughout the entire Archean eon. But what in fact has changed has been the lower crustal rheology, that's allowed these things in the Neoarchean, through translithospheric brittle fracturing, has allowed these things to transit all the way to the near surface. But in the Paleoarchean basically meant that these things stalled in the lower crust.

And that, in fact, has some quite important consequences because these things are so hydrated, it provides a fantastic way of introducing water

11:23 Julie

Right.

11:24 Hugh

into the mafic crust at that base of Paleoarchean terranes. And that may well indeed be one way of inducing that lower crust to produce TTG's in the first place.

11:36 Julie

Oh.

11:36 Hugh

So maybe this group of unusual rocks that's being considered to be rare and restricted to the Neoarchean may end up in fact being one of the most fundamentally important groups of rocks that we have in the Archean and maybe even find them on the most fundamentally important rocks in terms of continental crustal evolution full stop.

As you can see, I love sanukitoids.

Western Australia is still full of places where you could go and, from the geological perspective, if you care to spend the time to investigate it long enough and you're interested long enough there are still these things out there to be discovered. 

12:10 Julie

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