Trace element geochemistry during metamorphic dehydration: A case study from the Xingzi Group of Lushan, southeast China.

Abstract

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 δ18 O 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. Smith and Yardley, 1999). Such veins are important manifestations of fluids related to metamorphism and enable the study of the origins of metamorphic fluids and the behaviour of element during metamorphism. Previous studies on metamorphic veins mainly focused on their mineral, major element and oxygen isotopic compositions

Geochronology and geochemistry of volcanic rocks from the Shaojiwa Formation and Xingzi Group, Lushan area, SE China: Implications for Neoproterozoic back-arc basin in the Yangtze Block

“…Xingzi Group was previously regarded as the only high-grade metamorphic Precambrian crystalline basement in the southeastern Yangtze Block (BGMRJX, 1984;Zhang et al, 1999) besides the Archean to Paleoproterozoic high-grade metamorphic Kongling complex located near the Three Gorges (Gao et al, 1999;Qiu and Gao, 2000;Zhang et al, 2006). It is underlain by both the Shuangqiaoshan Group and the Shaojiwa Formation (Xie et al, 1997;Liu and Tang, 1999), although the contact is not preserved due to later shearing (Tang and Liu, 2002).…”

“…The main rock types include garnet two-mica schists, staurolite-garnet schists, quartz schists and leptites, locally intercalated with amphibolites. All these rocks have experienced amphibolite facies metamorphism and P-T calculations indicate that peak metamorphism took place at temperatures of 550-680 • C and pressures of 5-7 kbar (Tang and Liu, 2002). The Xingzi Group was intruded by Late Proterozoic gneissic granites (Li et al, 1998) and Mesozoic granites (Lin et al, 2000).…”

“…The 1500 km long Li et al, 2007;Zhao and Cawood, 2012). (c) Geological map for Lushan area showing the Shuangqiaoshan Group, Shaojiwa Formation and Xingzi Group (modified after Tang and Liu, 2002). collisional orogen, specifically in the igneous rocks.…”

Oxygen isotope and REE geochemistry of metamorphic veins within the Zhoutan Group, central Jiangxi Province

“…Due to more clay minerals in the protolith at immediate wall rocks of felsic veins, large amounts of fluids with higher CO 2 contents were produced during metamorphic dehydration. The HREEs in the protolith at immediate wall rocks of felsic veins are more easily mobilized into and transported by the metamorphic fluids, as suggested by Stable (1987), Bau (1991) and Hong-Feng Tang (2002, 2007. Therefore, REE behavior (including their mobility) during metamorphism is mainly controlled by the chemical composition of the metamorphic fluids, which, in turn, depends on the mineralogical composition of the protolith.…”

Trace and rare-earth element behaviors during alteration and mineralization in the Attepe iron deposits (Feke-Adana, southern Turkey)

“…The mobility of REE is not well understood (Smith et al, 2000;Tang and Liu, 2002;Bao, et al, 2004;Chen et al, 2006, Frei et al, 2008 and references therein). There are two different arguments on the mobility of REE during magmatic, metamorphic, and hydrothermal processes.…”