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GEO-METAMORPHISM  September 2011

GEO-METAMORPHISM September 2011

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Subject:

Re: distinguishing I- and S-type granites

From:

Robert Tracy <[log in to unmask]>

Reply-To:

Metamorphic Studies Group <[log in to unmask]>

Date:

Tue, 20 Sep 2011 17:11:20 -0400

Content-Type:

multipart/mixed

Parts/Attachments:

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text/plain (17 lines) , VTBR1-89-12BSE-2b.TIF (17 lines) , text/plain (24 lines)

Ryan et al.,

One good example I can cite is work we have done (Clayton Loehn and myself) on monazites from the Black Mountain Granite in Vermont (also Devonian). It is a pretty classic S-type and is well dated at about 360 Ma (in Appalachian tectonic terms, Late Acadian). The monazites were dated by John Aleinikoff with the SHRIMP at Palo Alto, and gave roughly 360 Ma, but there were some hints in the isotopic data of an older age lurking in some places in the grains in a few of his dated spots. Using EMP we dated the same grains that John had done, and got a 358 age for the euhedrally zoned outer parts but found some odd-looking patches in the maps that looked xenocrystic and which we dated with the microprobe (I've pasted in a BSE image of one grain below - the suspected xenocrystic stuff is near where the two EMP traverse lines indicated by spots cross in the NW part of the grain). Just to the NW of the crossing point of the two EMP traverses you can see a hole that represents the SHRIMP pit. This one gave 386 Ma, whereas his spots in the outer zones of the grains gave more like 360.

With its spatial resolution, we consistently got 385-390 Ma for those small domains using the EMP. This is the same age as the peak amphibolite-facies metamorphism in the country rocks surrounding the granite and presumably also the source rocks for the small pluton, so the inference is reasonable that these bits of inherited xenocrystic monazite from the granite protolith served as nucleation sites for well-formed magmatic monazite grains in the grainite. These older bits of monazite are volumetrically minor, but definitely there, and their ages form a consistent picture. The age relationships may be coincidental, but probably not.

Bob

Dr. Robert Tracy
Professor of Geosciences
Virginia Tech
Blacksburg VA 24061-0420
540-231-5980
540-231=3386 (F)




On Sep 20, 2011, at 3:51 PM, Ryan Ickert wrote: > That Bob Tracy suggests that inheritance might be common in monazite from S-type granite is interesting. This is not the case for the classic Siluro-Devonian locality in southeastern Australia. The bulk TIMS Pb/U isotopic work that Ian Williams did on rocks from the Berridale Batholith in the 1970's showed that while inheritance was pervasive in zircon, it was not present in monazite fractions (on a rather large scale, mind you). His 2001 paper on the Cooma Metamorphic complex (of the same age as the granites, also in southeastern Australia; Williams IS 2001; AJES v48(4)) yields a similar story, but based on ion microprobe isotopic analyses. > > In terms of zircon, though, inheritance in those late-Silurian S-type granites in SE Australia is clearly very abundant (this should not be news to anyone) and does not depend on bulk-composition. Spatially associated I-type granite lacks a significant inherited component; older zircon is present, but often you have to look very hard for it. > > As a side note, the Th-U chemical systematics of zircon from S-type granite really show the effect of monazite co-crystallisation! There is a strong negative correlation between Th/U and U abundance in single-spot analyses of melt-precipitated zircon that is not seen in, say, spatially associated I-type granite or gabbro. > > Anyways, my experience (primarily based on ion probe Pb/U analyses of zircon from a large number of Siluro-Devonian S- and I-type granites from SE Australia) is that zircon from those particular S- and I-type granites can be easily distinguished based on abundance of inheritance and Th-U systematics. Whether that observation can be generalized to other localities (or whether it is useful to anyone) is not known to me. > > -Ryan > > -- > Ryan Ickert > Post-Doctoral Fellow > Canadian Centre for Isotopic Microanalysis > Department of Earth & Atmospheric Sciences > University of Alberta

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