Hi all,

I guess I should share the Bourne granite case as it is relevant to many of the posts on this topic and demonstrates how complex granite can be. This rock forms a distinctive suite of grey granitic dykes crosscutting the gneissossity of basement gneiss in the SE Canadian Cordillera (Monashee Complex). Some dykes contain magmatic monazite, other magmatic allanite, other magmatic titanite, yet they are all comagmatic in this 1.8 Ga suite. However, we could not immediately rule out the hypothesis that monazite and zircon were inherited because: 1) all the rocks structurally above were strongly deformed and partially melted in the Paleogene; 2)most dykes contain euhedral allanite rather than monazite; 3) monazite are surrounded by a coronae of apatite+thorite+allanite, which could suggest disequilibrium of inherited 1.8 Ga monazite in a Paleogene melt; 4) one dyke (more pegmatitic) contains only Paleogene monazite. The inheritance hypothesis could only be ruled out by combining field relationships, geochronological characteristics and by compairing the chemical composition of monazite and zircon in the same dyke and between dykes. Now, we know its age (1.8 Ga) but whether the Bourne granite suite is a S- or I-type granite is still up for grab!!! 

You can find that story in: Crowley et al., 2008. Assessing inheritance of zircon and monazite in granitic rocks: an example from the Monashee complex, Canadian Cordillera, Journal of Petrology, 49, 1915-29.
And the tectonic implication in : Gervais et al., 2010. Tectonic implications for a Cordilleran orogenic
base in the Frenchman Cap dome, southeastern Canadian Cordillera. Journal of Structural Geology 32, 941-959.

Félix  

____________________________________________
Félix Gervais, M.Sc., Ph.D.
Professeur adjoint
Département des génies civil, géologique et des mines
École Polytechnique de Montréal
Montréal, Québec, Canada
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(514)340-4711 ext. 4739

----- Mail d'origine -----
De: Robert Tracy <[log in to unmask]>
À: [log in to unmask]
Envoyé: Tue, 20 Sep 2011 17:11:20 -0400 (EDT)
Objet: Re: distinguishing I- and S-type granites

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
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