How old is the Earth’s youngest exhumed pluton?
I don’t have the answer to this…I’m actually posting this to get help from my geoscience blogging colleagues. Lab Lemming and/or Thermochronic, in particular, might have some good leads for me.
This is a question that I’ve been pondering this evening as I write up this last chapter on some detrital zircon data. We are looking at sedimentary rocks that were deposited in the Late Cretaceous (spanning from ~85 to 70 million years ago). The source area for this basin was an active continental arc. There’s a boatload of volcanic and volcaniclastic grains in the sandstones and we are finding zircons that are only slightly older than the suspected depositional age. Furthermore, we are finding populations of nearly concurrent zircons as we go up through the stratigraphy. That is, the next younger formation includes a population of zircons younger than the depositional age of the formation below it. This is a pattern that is being found in other arc-sourced detrital records as well.
One issue is that the biostratigraphic control is poor and we don’t have any ashes for absolute dating. The detritals are actually helping constrain the timing of deposition.
This made me wonder … just how quickly can a zircon go from crystallization in an arc pluton to being a grain of sediment? Or, put another way, what is the age of the Earth’s youngest exhumed pluton? This seems like one of those things that other geologists probably know off the top of their head. Along those lines, how common is it for zircons that crystallize at depth to be transported to the surface via volcanism?
Any references, resources, papers, etc. appreciated.
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UPDATE: Ron found a paper discussing a ~1 million year-old exhumed pluton in Japan (see full citation in comments below). This is the winner so far.
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Clastic Detritus 
I think Sierra del Paine is 12.5ish Yellowstone and the California caldera whose name eludes me probably erupt young inherited grains- search the lit for zircon and the names of those volcanoes. I’ve heard of single digit stuff, maybe PNG or southern Andes?
For young single grains, if you are looking at arkoses (weathered plutons), you might have better control with K-Ar on detrital micas.
So stop writing and do another round of lab work.
;)
I’ll have to do some digging, but I do want to mention something related. One thing I have been amazed by recently is how fast material can be brought to the surface. See
Monteleone, B.D., Baldwin, S.L., Webb, L.E., Fitzgerald, P.G., Grove, M., and Schmitt, A., 2007, Late Miocene to Pliocene eclogite facies metamorphism, D Entrecasteaux Islands, SE Papua New Guinea, Journal of Metamorphic Geology, 25, 245-265, doi: 10.1111/j.1525-1314.2006.00685.x.
They show material that is at the surface now, that was in Ultra-high pressure conditions only a few Ma ago. Amazing I think.
I’ll have to do some looking, I’ll report back.
Thermochronic beat me to mentioning Papua New Guinea!
Another place where there’s really rapid exhumation is New Zealand, along the Alpine Fault, I believe. Maybe Chris Rowan knows those ages? I don’t know if any really young plutons are exposed… I get confused about what’s been arc and what hasn’t in a country that I haven’t visited (but really, really want to).
Meanwhile, in my back yard… hmmm. I don’t know if there are zircons erupted from the Jemez caldera. I don’t believe any of the magma chamber itself is exposed.
As far as I’m aware all of the stuff being exhumed along the Alpine Fault is quite old, dating from the Cretaceous or earlier.
If you need better age control, maybe something like strontium isotopes would do the trick? Or palaeomag (although the reversal rate is fairly slow in the Late Cretaceous). Not that you need us telling you to do more work at this stage of course ;-)
I don’t have a reference off the top of my head, but I’m pretty sure there are ~5 Ma plutons exposed in the Himalayas. I seem to recall seeing this in a Geology or GSA Bull cover photo a couple of years ago.
Given the stated amount of (presumably first generation) volcaniclastic material, I’d be surprised if you didn’t find zircons of an age approaching the sedimentary depositional age. Why wouldn’t eruptions in a volcanic arc sample zircons that are crystallizing in the magma chamber at the time of the eruption (effectively starting the clock)? Many erupted zircons would be robust enough to make it to your site of deposition. I suspect the time limiting factor is the sediment erosion-transport-deposition process rather than the exhumation process, unless you’re talking about zircons that demonstrably come from plutonic rocks.
By the way, I interpreted similarly rapidly deposited zircons (in conglomerate clasts) in the Gualala basin. Let me know if you need the references.
thanks all…
Yeah, in my limited search to answer these questions last night, I uncovered that eclogite paper Thermochronic cited. If I remember, Taiwan is another place where things are happening fast…I think there’s blueschist facies rocks from only a few million years ago being exhumed.
The Paine Massif laccolith (actually in the same region I’m talking about here) is ~12 Ma.
Ron…5 Ma plutons in the Himalayas?…maybe I’ll focus my search a little bit to that region, thanks.
I’ll have to look at the images of our detrital zircons again, but I don’t recall even the youngest ones resembling volcanic zircons (i.e., thin, elongate little guys).
thanks all! when the defense and everything is over, i’ll post some more about this project
Okay, a quick Google search finds this:
Harayama, Satoru, 1992, Youngest exposed granitoid pluton on Earth: Cooling and rapid uplift of the Pliocene-Quaternary Takidani Granodiorite in the Japan Alps, central Japan, Geology, vol. 20, Issue 7, p.657-660.
Abstract: The Takidani Granodiorite is a pluton (13 x 4 km) situated along a major axis of the Japan Alps that was intruded into late Pliocene age (2.4 Ma) volcanic rocks. Rb-Sr, K-Ar, and fission-track dates on coexisting minerals from the pluton are 1.9-0.8 Ma. Geology and mineral dates for the Takidani Granodiorite indicate that emplacement and cooling of the pluton occurred from late Pliocene to Quaternary time, making the Takidani Granodiorite the youngest granitoid pluton exposed on Earth’s surface. Mineral ages give a nearly straight path on a temperature-time (T–t) diagram according to the closure temperature of the minerals. The linearity of the T–t path indicates high thermal input from deeper levels in the early stage and cooling through rapid uplift in the late stage. The rapid exposure of the Takidani Granodiorite resulted from more than 2000 m of Quaternary uplift along the major axis of the Japan Alps.
Still, I don’t see any reason why your zircons need to be plutonic rather than volcanic in origin.
Ron…awesome! Cool, a 1 Ma exhumed pluton…and in an arc setting.
Yeah, I will look at the images of those young zircons, but this info gives me a good base for what is a possible maximum exhumation rate.
The other question embedded in the original post was can zircons that crystallized at depth be brought up to surface in some way during volcanism?
There are Quaternary granites in the Himalayan syntaxes. GSA Today January 2001 had an article on exhumation of the Himalaya that refers to some very young plutonic rocks.
I’m looking at the 1992 Geology article now. He says that 650-ky conglomerates from the Japan Alps have none of the granodiorite, suggesting “that the pluton was first exposed at the surface during middle Pleistocene time.” Ages show a clear progression of cooling with the mineral being dated, from hornblende starting 1.9 Ma, to biotite at 1.2-1.1 Ma, and zircon fission tracks at 0.8 Ma.
Other question:
Of course they can- via crustal xenoliths, or previous magma injection inheritance, and is one of the big problems for getting super high prescision on zircons (There are zirc U disequilibria studies done at stanford on this- who was that short, aggresive woman with long curly blonde hair?)
Or was that UCLA? All the same from over here…
Also, anything these guys have published might help:
http://www.uvm.edu/~kklepeis/Andes/Research%20in%20the%20Southernmost%20Andes.html
Any ideas on the youngest detrital zircons ever found?
Andrew…great, thanks for that reference. So another one in the ballpark of a million years.
Lab Lemming…thanks for that link, I’ll check it out.
Thermochronic…good question. I guess we’d have to go to a recent volcanic deposit and try and find a volcanic zircon.
This is an abstract from this past GSA meeting discussing Mt. St. Helens zircons being a few hundred thousand years old. I guess the question is, are they considered “detrital” once they are free of their subsurface prison? Or do they have to be reworked into a sediment dispersal system first?
Thermochronic: I only worked with TIMS, but it’s gotta get tough to date very young zircons (detrital or otherwise) with the U/Pb method because they simply haven’t had enough time for a measurable quantity of radiogenic lead to accumulate – especially in single zircon studies. Maybe it’s different if you’re working with a SHRIMP or some other source type of mass spectrometer, but I have to wonder what sort of errors really young zircons would yield.