Publication Type:Journal Article
Source:LithosLithos, Volume 236-237, p.173-189 (2015)
Keywords:Albite, Fluids, Leucogranite, Mongolia, Ongonite, Topaz
Ongonites were defined at their type locality at Ongon Khairkhan, central Mongolia, as pristine magmatic topaz-bearing albite-quartz-keratophyres with up to 4wt. % F and containing phenocrysts of albite, K-feldspar, quartz and rare mica and topaz hosted in a groundmass composed of the same minerals. However, detailed petrographic and SEM-EDS studies indicate that these rocks underwent considerable subsolidus exchange with deuteric fluids, as evidenced by the presence of albitic plagioclase (Ab~100) and end-member orthoclase (Or~100), secondary Li-Fe-rich mica (zinnwaldite) enriched in rare metals (Sn, W, Ta, Nb), pitted feldspars containing fluid inclusions, and disseminated fluorite. The ~120Ma old dyke rocks, emplaced at a high structural level in the crust, are strongly peraluminous leucogranites characterized by high Al and alkalis that are also enriched in Rb, Cs, Ga and Ta, depleted in Mg, Ca, Zr, Ba, Sr and Eu, and have anomalous K/Rb, Rb/Sr, Zr/Hf and Nb/Ta ratios compared to the average continental crust. However, the suite has Nd isotopic ratios (εNd(120) ~-1) similar to those of contemporaneous A-type granites of the Mongolian-Transbaikalian igneous province of the Central Asian Orogenic Belt. The inferred primary δ18O (~+6 to +7‰) and Pb isotopic values are consistent with a granitic parent magma and interaction with orthomagmatic fluids. The ongonites and constituent minerals record (1) an extensive and protracted crystal fractionation history, in part due to the presence of volatiles (particularly F) which depressed the solidus temperature of the felsic rocks and extended its duration of crystallization and (2) subsolidus exchange with fluids which includes late flux of heated meteoric water as indicated by modified whole rock δ18O values (+0.5 to +2.7‰). The interaction of the ongonites with internally derived orthomagmatic fluids is considered to result in enrichment and/or redistribution of several incompatible elements, but not to have greatly modified their original major element chemistry which indicates that this suite represents the last stages of fractionation of a highly differentiated, F-rich granitic magma. The escape of these evolved melts from the apical part of the underlying pluton is now represented by the ongonite dykes. Late-stage magmatic, water-rich fluids enriched in incompatible elements including Nb, Ta, Sn and W were responsible for the late- to post-magmatic alteration and associated W mineralization.