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PEGMATITES AND PEGMATITES, JUST LIKE GRANITES AND GRANITES: APPLES AND ORANGES ARE JUST NOT THE SAME! Robert F. Martin1 Caterina de Vito2 Maria Sokolov3 1 Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada, bobm@ eps.mcgill.ca 2 Dipartimento di Scienze della Terra, Università di Roma 1 “La Sapienza”, P.le Aldo Moro 5, I-00185 Roma, Italia 3 Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada Keywords: orogenic suite, anorogenic suite, granitic pegmatite, source characteristics, contamination, oxygen isotope geochemistry, pegmatites by anatexis, pegmatites by fractional crystallization. THE BIG GRANITES PICTURE WITH There is still a strong tendency out there to lump! To many, the formation of a granitic magma is strictly synonymous with subduction of crust or overthickening of crust owing to collision of continental plates. The magmas are calc-alkaline in character, and the plutonic and volcanic rocks are metaluminous to peraluminous, depending on the proportion of mantlederived and crust-derived components in the source. At the end of an orogeny, however, there is commonly a tendency for the overthickened crust to founder, and to allow hot, asthenospheric mantle to rise relatively quickly, such that a change in tectonic setting can occur within a short time! The diapiric rise of this asthenospheric mantle is accompanied by a metasomatic forerunner, a wave of fluid affecting the uppermost mantle (which was affected earlier in many cases, one must recall, by subduction-related Estudos Geológicos v. 19 (2), 2009 phenomena) and the crust, consisting in part of young trench-fill sedimentary units that have not been through an earlier episode of prograde metamorphism. The influx of hot fluid advecting in front of the rising diapir causes anatexis in the upper mantle and in the lower to middle crust in a regime of extensional tectonics. Thus one generates by anatexis a variety of granitic and syenitic magmas of A type. At the same time, the rising hot mantle is subject to decompression melting, such that a slightly alkaline basic magma can form, and can differentiate to give a felsic magma of A type. These two subtypes of A-type granite are both distinct in many ways from granitic magmas of orogenic (calc-alkaline) associations (Martin 2006). THE BIG PICTURE WITH LCT PEGMATITES Voluminous batholiths of metaluminous to peraluminous calcalkaline granitic magma are generated at crustal margins undergoing subduction 11 PEGMATITES AND PEGMATITES, JUST LIKE GRANITES AND GRANITES: APPLES AND ORANGES ARE JUST NOT THE SAME! or collision. The ensuing pegmatites are coded LCT because they are enriched in Li, Cs and Ta, and in a host of other elements that behave incompatibly in the presence of the rock-forming minerals, and that are progressively enriched in the residual magma by crystallization of quartz and the feldspars. Whereas some pegmatite occurrences have a purely crustal signature, others reveal a mixed crust– mantle signature, just like their parental batholiths. Pegmatites of LCT type that have a strictly mantle signature are not expected, nor are pegmatites in which the a(CO2) was important. The characteristics of the source of LCT pegmatites are relatively well understood because these are not very variable, and the petrogenetic underpinnings of the suite are generally well taught in university courses. THE BIG PICTURE WITH NYF GRANITIC PEGMATITES The same cannot be said of NYF granitic pegmatites, which are enriched in Nb, Y and F, as well as in a host of other high field-strength elements, and iron. These elements were mobilized upward by the hot H2O and CO2 coming in from the mantle; the fluids fertilized the crust from below. Just as A-type granites can have arisen by fractional crystallization of basic magmas generated by decompression melting of the rising diapir OR by direct anatexis of the metasomatized sialic basement, so can the NYF pegmatites derived in this setting reveal a mantle or a crustal isotopic signature. To understand NYF pegmatites, it is thus essential to think “outside the box” in terms of open-system behavior PRIOR TO the formation of the granitic melt. One quickly comes to realize that in such a context, f(O2), a(SiO2), a(F), a(CO2) are all important variables governing the “signature” of the final product. For example, as a carbothermal fluid is unable to 12 transport SiO2, influx of a CO2-rich fluid is likely to promote the formation of nepheline syenite by anatexis, and silica-undersaturated pegmatites or even carbonate-dominant pegmatites may well be juxtaposed with coeval standard NYF pegmatites elsewhere along the same belt. NYF PEGMATITES: CASES OF CRUSTAL DERIVATION, AND CASES OF MANTLE DERIVATION Our recent studies have focused on the oxygen-isotope geochemistry of NYF pegmatites. We have acquired δ18O values on amazonitic K-feldspar and the coeval albite in the subsolvus pocket assemblage at the Anjanabonoina pegmatite, in central Madagascar, earlier investigated by De Vito et al. (2006). Values of δ18O attain +15.3‰, which is unusually high for a felsic igneous rock, and indicative of a purely crustal source (Martin et al. 2008). Similarly for the Sakavalana NYF pegmatite, located 100 km southwest of Anjanabonoina, values on feldspar and coexisting minerals are in the range 12.6 to 15.4‰. These data prove that NYF pegmatites of anatectic origin do exist, contrary to the predictions of some that all NYF pegmatites are invariably related to gabbro–granite sequences and fractional crystallization imposed on a basic magma. We have sought to test these surprising results by acquiring such data on other NYF pegmatites. We can report that the amazonite and the pocket tourmaline in the contaminated Minh Tien granitic pegmatite (Sokolov & Martin 2009) register above 15.8‰ and attain 16.4‰, which is the world champion for a felsic rock at present. Samples of amazonitic microcline from the Paleozoic Morefield pegmatite, in Virginia, studied by Sokolov (2007), lie in the range +7.8–8.6‰, which Estudos Geológicos v. 19 (2), 2009 Robert F. Martin et al. may indicate a purely crustal source, but more likely reflects a mixed source, i.e., possibly involving both crust and mantle components. In contrast, there is the late Grenvillian St-Ludger-de-Milot NYF granitic pegmatite, near Roberval, Quebec, in which δ18O is 3.2‰, quite consistent with a modified mantle signature. Considered in the same category are results for the Proterozoic Keivy pegmatites, in the Kola Peninsula, Russia, where the results are systematically in the region 0 to –2‰! These unexpected results may indicate two- or three-stage anatexis of a mantlederived source rock. We realize that there are amazing amazonite-bearing granitic pegmatites in Minas Gerais, for example at San Sebastião de Pouso Alegre, investigated by Coutinho (1947). A sample simply labeled Minas Gerais, purchased from a dealer in North America and characterized as “clean, green amazonite” gave a value of 3.8‰, also indicative of a modified mantle-derived source. Whether or not it is typical of all NYF pegmatites in Minas Gerais, and how these values might differ from the LCT pegmatites for which Minas Gerais is famous, are interesting avenues for future research. COMPLICATIONS As Martin & De Vito (2005) have shown, real life may be somewhat more complicated than the above two-way classification would suggest. There are instances in the world where contamination complicates immeasurably the classification of a body of granitic pegmatite. For example, a granitic pegmatite having an NYF signature defined by its primary minerals may undergo significant contamination at the final stages of consolidation, when thermal contraction or physical implosions have caused a shattering of the wall zone. Leakage of the caustic orthomagmatic fluid Estudos Geológicos v. 19 (2), 2009 may well destabilize the rock-forming minerals (plagioclase in particular) in the wallrocks. Incursions of a fluid medium from the wallrock may well lead to the juxtaposition of elemental enrichments that are rather unconventional. For example, where the wallrock contains marble, as in Anjanabonoina, Madagascar (De Vito et al. 2006) and Luc Yen, northern Vietnam (Sokolov & Martin 2009), Ca becomes an important additive at the pocket stage. The Anjanabonoina pegmatite is the type locality of liddicoatite, a calcic tourmaline, but Ca was a mere trace element in the evolved NYF granitic magma. Our results so far on the Ca-, Li- and B-bearing minerals that postdate the NYF stage of crystallization do not show δ18O results very different from the early primary minerals. TAKE-HOME MESSAGE Mineralogical and geochemical attributes of granitic pegmatites are the result of an incredibly wide array of physical and chemical variables. In this review, an effort has been made to show that all granitic pegmatites are not created equal. The challenge is to address the question of source characteristics of the felsic magma, because these characteristics are progressively amplified as the system evolves, to be fully expressed at the end stage of crystallization of the magma. It is possible to make sense out of simple LCT and simple NYF systems from the point of view of source characteristics. The likelihood that the systems become open to country-rock contaminants late in their history, however, because of physical phenomena like implosions, OR the aggressive chemical makeup of the final batches of the orthomagmatic fluid, OR both factors working in concert, make contaminated granitic pegmatites unusually challenging to understand fully. 13 PEGMATITES AND PEGMATITES, JUST LIKE GRANITES AND GRANITES: APPLES AND ORANGES ARE JUST NOT THE SAME! REFERENCES Coutinho, J.M.V. 1947. Amazonita em Minas Gerais. Mineração e Metalurgia XII, 68-70. De Vito, C., Pezzotta, F., Ferrini,V. & Aurisicchio, C. 2006. Nb–Ta–Ti oxides in the gem-mineralized and “hybrid” Anjanabonoina granitic pegmatite, central Madagascar: a record of magmatic and postmagmatic events. Canadian Mineralogist 44, 87-103. Martin, R.F. 2006. A-type granites of crustal origin ultimately result from open-system fenitizationtype reactions in an extensional environment. Lithos 91, 125-136. Martin, R.F. & De Vito, C. 2005. The patterns of enrichment in felsic 14 pegmatites ultimately depend on tectonic setting. Canadian Mineralogist 43, 2027-2048. Martin, R.F., De Vito, C. & Pezzotta, F. 2008. Why is amazonitic K-feldspar an earmark of NYF-type granitic pegmatites? Clues from hybrid pegmatites in Madagascar. American Mineralogist 93, 263-269. Sokolov, M. 2007. Characterization of Pb and selected trace elements in amazonitic K-feldspar. M.Sc. thesis, McGill University, Montreal, Canada. Sokolov, M. & Martin, R.F. 2009. A Pb-dominant member of the tourmaline group, Minh Tien granitic pegmatite, Luc Yen district, Vietnam. Fourth International Conference on Pegmatite, Recife, Brazil. Estudos Geológicos v. 19 (2), 2009