Shijin Xu

Chemistry of Hf-rich zircons from the Laoshan I- and A-type granites, Eastern China

Abstract Zircon commonly occurs as one of important accessory HFSE-bearing minerals in A-type granite. A detailed electron microprobe study was carried out on zircon from the Laoshan complex, Eastern China, which is composed of I- and A-type granites. Zircon from the I-type rocks is relatively poor in trace elements (HfO2<2 wt.%, UO2, ThO2 and Y2O3<1 wt.%), but that from the A-type rocks is richer in Hf, U, Th and Y. Hafnian zircon with a HfO2 content of up to 12.37 wt.% was found in the arfvedsonite granite, which is considered the most evolved facies in the A-type suite. Enrichment in Hf is generally observed at the rims of zircon crystals relative to the cores. The Hf enrichment in zircon, and the association of exotic REE- and HFSE-bearing minerals are linked to hydrothermal activity, suggesting that during the last stage of crystallization of the A-type magma, fluids enriched in REE, HFSE, F-, CO32- and PO43- were released.

Accessory mineral record of magma-fluid interaction in the Laoshan I- and A-type granitic complex, Eastern China

Abstract The Laoshan granitic complex (LGC) in East China is composed of many intrusive units which can be divided into two distinct suites, I-type and A-type. There exist different assemblages of accessory minerals between the two units. In the I-type suite, the quartz monzonite contains the accessory mineral assemblage of titanite + magnetite + zircon, the biotite monzogranite is titanite + magnetite + ilmenite + zircon, whereas the accessory mineral assemblage in the quartz syenite is rutile + magnetite + zircon. In the alkali granite of the A-type suite, accessory mineral assemblage consists of Fe-Ti minerals including titanite, magnetite, ilmenite and rutile in addition to frequently-observed zircon and sparse pyrochlore. In the arfvedsonite granite, however, magnetite, titanite and rutile disappear, Nb-rich pyrophanite-dominant ilmenite becomes the only Ti mineral. In addition, large amounts of mineral containing REE and HFSE were crystallized. Zircon is also present in the all two facies of the A-type suite, but with different compositions. Especially, zircon in the arfvedsonite granite is enriched in Hf but poor in U and Th relative to the alkali granite. The change in accessory mineral assemblage in the different facies of both I-type and A-type suites reflects variation in temperature and/or fO2 during crystallization of every magma chamber. Particularly, presence of the large amount of HFSE- and REE-bearing minerals in the arfvedsonite granite may be attributed to the accumulation of fluids at the end of evolution of A-type magma. Geochemically, the fluid is enriched, on one hand, in REE and HFSE, and on other hand, in volatiles (F, P and CO2).

Geochemical characteristics and genesis of Neoproterozoic granitoids in the northwestern margin of the Yangtze Block

Abstract Geochemistry of nine Neoproterozoic granitoid bodies in the northwestern margin of the Yangtze Block (YB) has been studied in this paper. Their ages range from 876 Ma to 786 Ma based on U-Pb zircon dating. These granitoids can be divided into three groups in terms of major and trace elements. Rocks of Group I are alkaline series granites characterized by evident negative Eu anomaly (Eu/Eu* = 0.31 to 0.41) with total REE concentrations between 274 to 122 ppm. Group III includes migmatized granites characterized by low REE concentrations (14 to 45 ppm) and positive Eu anomaly (Eu/Eu* = 1.1 to 2.5). Group II comprises tonalite and diorite with REE between the above two groups (ΣREE = 105 to 212 ppm, Eu/Eu* = 0.73 to 0.79). Granitoids of Group II and III belong to calc-alkaline series and I-type, which were formed during the Jinning Orogeny before 820 Ma related to subduction or collision between the Yangtze Block and oceanic Qinghai-Yunnan-Tibet Plate. The Group I granites were formed after 805 Ma in the late stage of or post the Jinning Orogeny. The Neoproterozoic granitoids have e Nd (T) values ranging from −4.3 to +4.5 and initial 87 Sr/ 86 Sr ratios ⩽ 0.705, similar to those of the Neoproterozoic granitoids in the other margins of the YB, but different from those of coeval granitoids within the YB which have e Nd (T) of −8.1 to −14.2 and initial 87 Sr/ 86 Sr of 0.705 to 0.708. The difference in geochemistry of the three groups was due to difference in their sources. The Neoproterozoic granitoids of this study were formed by magmas probably derived from sources with different proportions of juvenile crust and Meso- to Paleo-Proterozoic crust. The granites of Group III were derived probably from the lower crust. The crust sources for Group I granitoids probably contain less amounts of juvenile crust component and have higher maturity when compared with those for Group II.

Yttrium zoning in garnet from the Xihuashan granitic complex and its petrological implications

The Xihuashan granitic complex is characterized by enrichment in rare-earth elements (REE). In particular, the second-stage granite (G-b) is markedly enriched in yttrium, and therefore contains complex associations of Y-bearing minerals. In this granite, garnet displays specific yttrium zoning with an Y-rich core and a “clean” rim. Besides minute inclusions of Y-bearing minerals, garnet involves a striking amount of Y and HREE in its central area. It is suggested that enrichment in Y in the garnet core accords with that in the melt as a result of REE magmatic fractionation. However, the “clean” rim may be the direct result of accumulation of fluid phases in the magma, which is virtually unfavorable for the entrance of REE in the garnet structure.

Differentiation and accumulation of fluids in A-type granites: Evidence from accessory mineral study

The Laoshan A-type granite is a highly evolved granite body. The study on accessory minerals indicates that there exist differentiation and accumulation of fluids during the late stage of evolution of the granite. The released fluids are rich in rare-earth elements (REE), high-fieldstrength elements (HFSE, such as Nb, Ta, Zr, Hf, Th, U, and Y) and volatiles (F, P, CO2, etc.). Owing to the presence of fluid, accessory mineral assemblages have changed during the evolution of A-type granite, and are especially characterized by large amounts of independent REE-Nb-Y-Th minerals present in the late facies. Late-accumulation of fluids may be of the general feature of A-type granites, at least of highly evolved ones.

First occurrence of preiswerkite in the Dabie UHP metamorphic belt

This note reports the preiswerkite that was found as the first occurrence in the UHP metamorphic belt of the Dabie Mountain. It occurs in the quartz jadeitite, and coexists with plagioclase, taramite and magnetite as retrograde symplectite. The calculated temperature of formation for preiswerkite is about 600 – 620 °C (at P=1.0 – 1.5GPa), suggesting that the preiswerkite is an indicator mineral of high-grade greenschist facies retrograded from eclogitefacies rocks.

U-Pb dating of zircon grains from the ore-bearing plagioclase amphibolite in the Dashuigou Te deposit

The Dashuigou Te deposit is a unique independent one found in the world, and hosted in the plagioclase arnphibolite. The upper intercept age of single zircon from the arnphibolite can be considered to be the crystallization age of the source-rock of the amphibolite (tholeiitic basalt), while the lower intercept is nearly consistent with the time of the tectono-thermal event.