John,
I have looked at and for monazites in many, many rocks, and they are by far more common in what I would call S-type granitoids, based on mineralogy, than in other types. However, on rare occasion they can also occur in plutonic rocks that are less obviously S-type, i.e, are somewhat ambiguous as to genesis and may be such highly fractionated I-types that Ian refers to, or something transitional or mixed. And I don't think I've ever seen a monazite in peralkaline A-type granites, which are typically allanite- or titanite-bearing (see below).
I'm sure that many monazites in S-type granites are at least partly inherited, and in some case mostly inherited, from the protolith that melted. Both chemical and age zoning suggest this, but such relict metamorphic cores are commonly overgrown by magmatic monazite with euhedral zoning patterns (and magmatic ages). And I certainly agree with John and others that a healthy dose of skepticism is useful when trying to infer granite "type" on an isotopic basis or from elemental discrimination diagrams alone.
One thing I can say for sure - I have only seen one case of monazite in a rock that contained either allanite or titanite. That one case was in a granulite gneiss from western North Carolina in which large (300-600 micron) 1100 Ma Grenvillian monazites were spectacularly sheared and recrystallized in a 280 Ma Alleghanian mylonite-forming event. The Th-rich monazite reacted to form allanite + apatite + thorite/huttonite during the shearing and hydration. Otherwise, in 100% of my observations you get either monazite or allanite/titanite, but not both. This North Carolina example was also interesting in that monazite is in general a rare thing in Grenville-age rocks of eastern North America, from the Green Mountains south to the Blue Ridge of the southern Appalachians, either metamorphic gneisses or granitoids. This whole terrane appears to be singularly depleted in Th, U and LREEs, and in many cases the rare monazites one finds are so low in Th and U that they have almost strikingly low Pb, given their age.
Bob T.
Dr. Robert Tracy
Professor of Geosciences
Virginia Tech
Blacksburg VA 24061-0420
540-231-5980
540-231=3386 (F)
On Sep 20, 2011, at 11:44 AM, Clemens, JD, Prof <[log in to unmask]> wrote:
> Ian Buick pointed out to me that highly fractionated I-types might have monazite in them, if all the LREE aren't ripped out by fractionation of alanite and titanite. I'm not sure, but it is always best to steer clear of very leucocratic members of an association and make judgements based on the mineralogy of ordinary granites, monzogranites and granodiorites. The associated highly felsic rocks can then be placed in context. Yes, give me two chunks of garnet-muscovite leucogranite, one S- and one I-type, out of any geological context, and I might have some problem discriminating between them. S-types commonly have monazite included in the biotite (which crystallises early in S-types). Biotite is commonly a late mineral in I-types.
>
> John D. Clemens
> Professor in Geology & Exec. Head
> Dept of Earth Sciences, University of Stellenbosch,
> Private Bag X1, 7602 Matieland, South Africa
> tel: +27 (0)21 808 3159 fax: +27 (0)21 808 3129
> e-mail: [log in to unmask]
> web page: http://www.sun.ac.za/geo/people/clemens_e.htm
> ________________________________________
> From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of Jorge Restrepo [[log in to unmask]]
> Sent: 20 September 2011 17:31
> To: [log in to unmask]
> Subject: Re: distinguishing I- and S-type granites
>
> Good morning,
> Thank you for such precise guidelines. They are very helpful.
> Best regards,
> Jorge J. Restrepo
>
>
> ----- Original message -----
> From: "Clemens, JD, Prof <[log in to unmask]>" <[log in to unmask]>
> To: [log in to unmask]
> Date: Tue, 20 Sep 2011 17:08:21 +0200
> Subject: distinguishing I- and S-type granites
>
> Hi,
>
> Mike Brown suggested that what people need are some guidelines for
> recognising S- and I-types. Here are some simple quite reliable
> mineralogical criteria.
>
> If your rock has red-brown biotite with high Al and Ti contents or
> cordierite or monazite or pyrrhotite as the Fe sulphide then it's almost
> certainly an S-type.
>
> If your rock has hornblende or clinopyroxene, or titanite or magmatic
> epidote or the biotite is dull brown to greenish brown or the rock
> contains magnetite or alanite, you've got an I-type.
>
> Muscovite, andalusite and garnet can occur in both sorts, especially is
> you have fractionated highly felsic magmas involved. My impression is
> that muscovite is more common in fractionated I-types than it is in
> S-types.
>
> Isotope characteristics should be used with circumspection. There are
> high delta 18O I-types and I-types with very ancient sources can aslo
> have high initial Sr isotope ratios and low epsilon Nd.
>
> Likewise there are chemical indicators – Na2O/K2O is generally higher in
> I-types, as is CaO at a given SiO2 content. ASI (= A/CNK) is a tricky
> one. A lot of granites are peraluminous, but very few I-types have ASI >
> 1.1. There is commonly a rather telling variation in ASI with maficity
> (Fe + Mg). In S-types these are commonly positively correlated, due to
> the fact that mafic minerals responsible for the variation in maficity
> are peraluminous. In I-types, the trend is opposite, with the more mafic
> rocks having lower ASI. Accumulation of Hbl or Cpx does this.
>
> So, as I wrote earlier, it is the totality of evidence that points to
> the character of the source rock. Arriving at the point of knowing
> something pretty specific about the source is rather superior to knowing
> that you have an alkali-calcic granite (which could be either S- or
> I-type, actually). In short, beware of magic discriminant diagrams. They
> can work rather poorly and totally mislead you. Part of the reason for
> this, especially with tectonic discriminant diagrams, is incorrect
> assignment of the character of the rocks that were used to construct the
> field boundaries in the first place and incorrect assumptions regarding
> the sources of felsic magmas.
>
> I have spoken.
>
> JC
>
>
> John D. Clemens
> Professor in Geology & Exec. Head
> Dept of Earth Sciences, University of Stellenbosch,
> Private Bag X1, 7602 Matieland, South Africa
> tel: +27 (0)21 808 3159 fax: +27 (0)21 808 3129
> e-mail: [log in to unmask]
> web page: http://www.sun.ac.za/geo/people/clemens_e.htm
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