Hi Colin, maybe you can explore the next paper,

Paterson et al., 2011. Magma addition and flux calculations of incrementally constructed magma chambers in continental margin arcs: Combined field, geochronologic, and thermal modeling studies. GSA

Cheers,

Pablo

Thanks Pablo

Hi Colin,

Depending on who you beleive -- Scott Paterson and his group would probably propose that it takes only a handful of intrusions to make a pluton (such as Tuolomne); whereas other people (the Stellenbosch group) would argue that a pluton can be made of 100s or 1000s of individual batches.

Quoting from one of my email to the granite-research list, shortly after the 2011 Hutton:

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 I believe indeed that felsic eruptions are perfectly relevant to the discussion (you might want to argue that the fact that rhyolites do erupt does, precisely, demonstrate that they were different from granites that stay at depth -- but for the time being, let's assume that we are indeed looking at mobile magma batches, similar to the one forming granitic intrusions). 

Following this line, we can have a look at the size of these batches. Wikipedia, for want of a better source, gives a list of the tephra volumes of the largest eruptions (http://en.wikipedia.org/wiki/List_of_largest_volcanic_eruptions); these are tephra volumes as far as I understand the article, so the dense rock equivalent would be anywhere between 1/3 and 1/5 of that. As you can see in this table, the largest eruption apears to have a tephra volume of above 8,000 km3 (no idea how many cubic feet that may be ), which means some 1500-2500 km3 DRE. 2,000 km3 is a 5 km thick, 20 by 20 km pluton: a respectable one for sure, but by no mean extraordinary large (the world-famous TIC is, what, 1,000 sq.km2 in surface, more than double in map view). In other world, a massive eruption has a volume comparable to a medium-sized pluton.

It's hard to give average eruption size, or even typical ones; I'm willing to bet that the distribution is log-normal or power law, such that averages are meaningless. In the table I'm referring to, we have 1 eruption > 8000 km3, 3 > 7000, 5 > 6000, 7 > 5000, 8 > 4000, 15 > 3000, and so on; this does not look like a gaussian distribution to me. Sure enough, the size distribution of plutons is comparable. On this page (pardon my french !) : http://jfmoyen.free.fr/spip.php?article211, scroll down to fig. 10, direct link (http://jfmoyen.free.fr/IMG/jpg/TN_fig10-lfb_size.jpg), I plotted the map size of 150 plutons of the Lachlan Fold Belt in Australia (the other center of the granitic universe !). You can see, again, a power law distribution (this was actually pointed out to me by Paul Bons at the '07 Hutton).

Now -- it would be interesting to compare the two distributions. It requires a bit of stats (as you want to plot as fraction of the total population, etc.), which I have not done. Just looking at the top of the curve:

- For the LFB, the largest intrusion is nearly 5,000 km2 in surface, so 25,000 km3 assuming a thickness of 5 km;

- For the eruptions, the largest is < 2,500 km3 in volume.

Again, I did not check the rest of the distribution; but surely, this suggests to me that the size distribution of felsic eruptions (== the size distribution of mobile magma batches at a given time, following Scott's point) could well be an order of magnitude below the size distribution of plutons. The inescapable conclusion, then, becomes that it takes an average of 10 large batches (+ as many small one as you want, potentially !) to form a pluton...

--

A key point, I believe, is that the size distribution of magma batches is likely to be power-law or log-normal, so the volume of a pluton is probably dominnted by a very small number of large magma batches.

Hi Colin 

Have a look to:

Farina F., Dini A., Innocenti F., Rocchi S., Westerman S.D., 2010. Rapid incremental assembly of the Monte Capanne pluton (Elba Island, Tuscany) by downward stacking of magma sheets. Geological Society of America Bulletin, 122, 1463-1479.

Rocchi S., Westerman S.D., Dini A., Farina F., 2010. Intrusive sheets and sheeted intrusions at Elba Island (Italy). Geosphere, v. 6, 3, 225-236.

The ca. 110 km3 Monte Capanne pluton is made of three mappable batches. The smallest is probably 20 km3. Obviously each batch may be formed incrementally by amalgamation of more and smaller batches...who knows.

hi..have a look at these papers..

Michel et al., 2008: http://geology.gsapubs.org/content/36/6/459.short

Leuthold et al., 2012: http://www.sciencedirect.com/science/article/pii/S0012821X12000593

Hi Colin,

we tried to interpret something along the lines of what you are asking about in the Memeti et al. 2010 paper http://gsabulletin.gsapubs.org/content/122/11-12/1912.abstract

Check out also Economos et al. 2009: Causes of compositional diversity in a lobe of

the Half Dome granodiorite, Tuolumne Batholith, Central Sierra Nevada, California.

We argued in both papers that the lobes are potentially representing the maximum size of a magma batch that intruded into the Tuolumne intrusion. We suggest that since what we are observing is the final stage of the intrusive history, it is not clear if it took one large batch (3x10km) to form the Half Dome lobe or multiple smaller batches that amalgamated very quickly to produce the interconnected magma mush in the lobe. And we extrapolate this to the overall 1000km2 Tuolumne Intrusion.

Jean-Francois, please note that Scott Paterson is a co-author on both of these papers, and that he would agree that it is possible that lots of smaller batches may produce one big unit in the Tuolumne (it could be 10, 100 or 10000 pulses), however, the important point is that they will have to come in quite frequently to produce interconnected magma mush - we see lots of evidence for that at the emplacement level. Personally, I think it is just difficult to find the evidence for an individual pulse at the emplacement level since typically any magma batch appears to be interacting (above the solidus) with whatever magma is already there (mix and mingle) and continue to differentiate. I am working on going after this issue now with crystal scale geochemistry.

Cheers

Vali

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