7th International Symposium on Granitic Pegmatites, PEG 2015 Książ, Poland
A GEOCHEMICAL EVALUATION OF PEGMATITE PARENT GRANITE 1
Sarah L. Hanson#
1
Adrian College, Geology Department, 110 S. Madison St., Adrian, MI 49221, USA #
[email protected]
Key words: pegmatite, NYF, LCT, Pearce discrimination diagram, parent granite, tectonic affiliation The most widely cited classification system for pegmatites is strongly based on the depth of crystallization and classifies pegmatites, from deepest to shallowest, as abyssal, muscovite, muscovite – rare-element, rareelement, and miarolitic (Černý and Ercit 2005). These authors also suggest a classification system based on petrogenetic parameters for pegmatites that are derived by fractionation of a granitic melt. For the rare-element and miarolitic classes, these pegmatite families include the lithium, cesium, tantalum (LCT), niobium, yttrium and REE, fluorine (NYF) and Mixed LCT + NYF pegmatites. These authors show no strong correlation between parent granite chemistry and tectonic setting versus pegmatite family. In contrast, Martin and DeVito (2005) propose that the tectonic regime determines the nature of the parental melt, thus the geochemical characteristics of pegmatites derived from these melts. They suggest that LCT pegmatites form in compressional (orogenic) settings whereas NYF pegmatites develop in extensional (anorogenic) settings. Mixed LCT and NYF pegmatites are attributed to contamination, at either the magmatic or post-magmatic stage. Additionally, these authors propose that some pegmatites may be anatectic, resulting from partial melting of either crustal or mantle rocks. Creating a definitive chemical classification system for pegmatites is hindered by the large grain size of pegmatite minerals, as well as the internal zonation of the pegmatite that may result in partial exposures where not all minerals present are exposed. These factors preclude a direct measurement of the whole rock geochemical composition for pegmatites. For these reasons, a geochemical evaluation of well-constrained granite-pegmatite systems was evaluated using tectonic discrimination diagrams based on parent granite chemistry (Pearce et al. 1984). Data was mined via an extensive literature search that provided geochemical data for the parent granites of 16 LCT pegmatites, 24 NYF pegmatites and 6 Mixed LCT-NYF pegmatites. Only pegmatites that were convincingly attributed to a parental granite were included in this study. Potential sources of error include incorrectly assigned parental granite and poor resolution of older geochemical data. Locations for this study are as follows. LCT pegmatites: Sebago, ME, Black Hills, SD, McAllister, AL, Kings-X, WI, San Diego, CA, Brown Derby, CO (all USA locations), Lucerne, QC, Dryden Pegmatites, ON, Red Cross Lake, MB (all in Canada), Elba, Italy, Nanga Parbat, Pakistan, Sierra Pampenas, Argentina, Härnö Granite, Sweden, Manaslu, Nepal, Keketuobai, China, Pinilla de Fermoselle, Spain, and Stak Nala, Pakistan. NYF pegmatites: Bokan Mountain, AK, Lake George, CO, Mount Rosa, CO, Pikes Peak Batholith, CO, Mount Antero, CO, Searchlight, NV, Llano, TX, Mojave County, AZ, Conway granite, NH, Trout Creek Pass, CO, Wausau, WI (all USA locations) Porterillos and El Portezuelo, both in Argentina, Fellingsbro, Sweden, Naegi, Japan, Baveno, Italy, Jhargsuda, India, Georgeville Pluton, Nova Scotia, Klein Spitzkoppe and Erongo, Namibia, Königshain, Germany, Tysfjord and Evje-Iveland, Norway, and the Westford Intrusion, New Brunswick. Mixed pegmatites: O’Grady Batholith, NT in Canada, McHone Pegmatite, NC, USA, Moldanubian Zone (Třebíč pluton), Czech Republic, and Telemark (Tørdal Granite), Norway. In general, the parental granites for LCT pegmatites are peraluminous whereas granites that are genetically related to NYF pegmatites range from peraluminous to metaluminous and exhibit some overlap with the LCTpegmatite bearing granites. However, these granites can be geochemically distinguished using tectonic discrimination diagrams from Pearce et al. (1984). Orogenic granites, which can be subdivided into subduction related volcanic arc (VAG) and continental collision related (syn-COLG) granites, are generally depleted in Nb and Y relative to anorogenic within plate granites (WPG) (Figures 1 and 2). It is important note that postorogenic (post-COLG) granites, which lie within the oval in Figure 1, are compositionally identical to some synCOLG, VAG and WPG granites. This overlap arises from the origin of these granites, which are associated with post-closure uplift subsequent to collision and are generated from mantle derived magmas that have undergone significant crustal assimilation (Pearce et al. 1984). Thus, they contain components of both orogenic and anorogenic granites. For this reason, other techniques, such as age dating may be required to accurately assign a granite to a post-orogenic origin.
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A Geochemical Evaluation of Pegmatite Parent Granite It is apparent from these diagrams that granites that are parental to LCT-type pegmatites exhibit orogenic signatures, and lie in either the VAG or syn-COLG granite field with only a few within the overlapping postCOLG field. In contrast, granite that is parental to NYF pegmatites generally plots either as post-collisional or anorogenic (WPG). Although there are only a few locations where mixed LCT and NYF pegmatites can be correlated to a parent granite, these granites plot on the orogenic side of the diagram, but within the post collisional (post-COLG) field of Pearce (1996). These correlations are consistent with the earlier work in classification schemes that suggest a tectonic origin for the differences in pegmatite compositions (Martin and De Vito 2005 and references therein).
Fig. 1. Rb vs Nb+Y tectonic discrimination diagram for granites parental to LCT, NYF and Mixed LCT+NYF pegmatites (after Pearce et al. 1984, Pearce, 1996). Abbreviations: Volcanic Arc Granites (VAG), syn-collisional (syn-COLG), within plate granites (WPG), post-collisional granites (postCOLG) and Ocean Ridge Granites (ORG).
Fig. 2. Nb vs. Y tectonic discrimination diagram for granites parental to LCT, NYF and Mixed LCT+NYF pegmatites (after Pearce et al. 1984).
Pegmatites that are produced from granites in subduction zones and collisional tectonic settings typically contain Li- and Cs-bearing minerals that are characteristically associated with LCT pegmatites. In contrast, granites formed in anorogenic tectonic settings are typically more enriched in HFSE and the REE, thus form pegmatites that commonly contain Nb-Ta-Ti oxide minerals, REE-bearing minerals, and, in some cases fluorite; minerals that are typically associated with NYF pegmatites. It should be noted that it is not essential that pegmatites in the NYF family be enriched in all three elements, Nb, Y and F. Thus, although LCT and NYF parental granites, thus by inference the pegmatites, can clearly be differentiated petrogenetically, classifying pegmatites by using acronyms for the chemical constituents can be misleading if all of the components are not present. Mixed LCT and NYF pegmatites plot in the collisional or subduction related field, although they compositionally lie within the post-COLG field. Martin and DeVito (2005) suggest that mixed pegmatites are NYF pegmatites that have been overprinted by an LCT assemblage at the hydrothermal stage. These authors further suggest that, due to solubility constraints, LCT pegmatites would not be expected to be over-printed by NYF mineral assemblages. If this is the case, mixed pegmatites would be post-orogenic NYF pegmatites that were either contaminated by undepleted crustal material (as suggested by Černý and Ercit 2005) or overprinted by an LCT assemblage at the hydrothermal stage (proposed by Martin and De Vito 2005). REFERENCES ČERNÝ, P. & Ercit (2005): The Classification of Granite Pegmatites Revisited. Can. Mineral. 43, 2005–2026. MARTIN, R.F. & DE VITO, C. (2005): The Patterns of Enrichment in Felsic Pegmatites Ultimately Depend on Tectonic Setting. Can. Mineral. 43, 2027–2048. PEARCE, J.A., HARRIS, N.B.W. & TINDLE, A.G. (1984): Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrology 25, 956–983. PEARCE, J.A. (1996): Sources and settings of granitic rocks. Episodes 19 (4), 120–125.
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