REE contents in whole-rock samples and constituent minerals of the Qianlishan and Baerzhe granites from China were determined using three analytical methods (ICP-AES, ICP-MS, and ID-TIMS). The REE abundance patterns of the granites show the M-type tetrad effect and strong Eu depletion in both whole-rock samples and in individual minerals samples. Fluid-melt interaction in the late stage of fractional crystallization is suggested to be the most important factor controlling the formation of REE tetrad effects in the granites.
The Xingzi Group, exposed in Lushan, southeast China, is the oldest basement in the Late Proterozoic Jiangnan orogen and composed mainly of metamorphosed psammites and pelitic schists with the grade of amphibolite facies. Eight quartz and felsic veins from two locations in this group have been chosen to investigate trace element behaviour during metamorphic dehydration. One location is at Xixiansi, where the wallrocks are completely metamorphosed psammites, and the other at Huangyansi, where the wallrocks are mainly pelitic schists. According to their field and petrologic characteristics, all the veins studied from the two locations are synmetamorphic in origin and were deposited by metamorphic fluids derived from their wallrocks. This is further indicated by the similar oxygen isotopic compositions of mineral separates between the veins and their immediate wallrocks. The estimated mean δ18O values of the fluids equilibrated with minerals are 11.2‰ at Xixiansi and 10.2‰ at Huangyansi, supporting no pervasive fluid infiltration on a relatively large scale across whole the Xingzi Group during metamorphism. The much different Nb/Ta, Zr/Hf and Pb/Nd ratios in the veins from those in their wallrocks as well as the REE tetrad effects in the veins at Huangyansi indicate fractionation of the REEs and several geochemical pairs (Nb-Ta, Zr-Hf, and Pb-Nd) during metamorphic dehydration. In addition, the large ion lithophile elements (LILEs) show a different behaviour during this process. The ratios of La/Ba, Sr/Nd and Th/Hf are similar in all veins and wallrocks. Rb/Ba, Ba/Nb, La/Nb, and La/Yb ratios are also similar in the quartz veins and wallrocks, but vary significantly in the felsic veins, suggesting their fractionation in the felsic veins. The various REE abundances in the veins confirm that the REEs in sedimentary rocks can obviously be mobilized during regional metamorphism of amphibolite facies. Based on differences in trace element characteristics of the veins between Xixiansi and Huangyansi, we conclude that the mineralogical compositions of the protoliths most likely play an important role in controlling the chemical compositions of the fluid, and in turn, controlling REE and other trace element behaviour during metamorphic dehydration.
Barium- and titanium-bearing biotites from Miocene volcanic rocks of Mezitler area, eastern Balikesir, western Turkey are studied. The chemical composition of volcanic rocks range from andesite to rhyodacite. The iron-enrichment index of micas (average I.E. = 0.40) is intermediate between annite and phlogopite. The biotite phenocrysts contain up to 1.72 wt.% BaO and 5.90 wt.% TiO2, with the average formulae (K0.807Na0.131Ca0.036Ba0.027) (Mg1.404Fe2+0.800 Fe3+0.131 Ti0.303Al0.056Mn0.023) (Si2.832Al1.167)O10[(OH)1.976Cl0.024]. The BaO content of electron-microprobe micas is positively correlated with the Al2O3, TiO2, and FeO contents, and with the I.E., and is negatively correlated with the SiO2, K2O, and MgO contents. Ba- and Ti-rich micas are generally found in potassic igneous rocks, in subalkaline and alkaline gabbroic rocks and in contact metamorphic rocks, whereas Ba- and Ti-bearing micas in this study occur in calc-alkaline volcanic rocks that hosted manganese-oxide and barite deposits. Most of the phenocrysts analyzed have deficiencies in their octahedral and partly interlayer sites. Deficiencies in the octahedral sites may arise from the Ti-vacancy and partly the Ti-tschermakite substitution. On the other hand, deficiencies in the interlayer-site are due to the replacement of K by Ba. The substitution mechanism in the Mezitler micas is characterized by Ba + 2Ti + 3Al = (K + Na + Ca) + 3(Mg + Fe + Mn) + 3Si, with an excellent correlation coefficient. In terms of aluminum and titanium contents, micas from the Mezitler area lie on a similar trend parallel to that for metasomatic phlogopites from Canary Island xenoliths, which overlap the field for micas from the Ilha da Trindade xenolith, South Atlantic. Biotite compositions from the Mezitler area fall between the quartz-fayalite-magnetite (QFM) and nickel-nickel oxide (NNO) oxygen fugacity buffers. All these show that Mezitler micas with low to moderate Ba- and Ti-contents may be formed from magmas in a subduction-enriched sub-continental lithospheric mantle environment.