Dezi Wang

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).

Th-rich zircon from peralka line A-type granite: Mineralogical features and petrological implications

The Taohuadao, Qingtian and Laoshan granites are three typical late Yanshannian peralkaline granitic plutons in the coastal area, eastern China. In this paper, internal structures and chemical compositions of zircon from these A-type granites were investigated by electron microprobe. It is shown that zircon grains are mainly composed of two distinctly separated parts. One is rich in Th (ThO2 >1 wt%, and ThO2/UO2>2), and attains ThO2 up to 10.1 wt%; such value exceeds the dissolution limit of Th in the zircon structure (ThO2=5.5±.5 wt%) determined in previous experiment. On the other hand, the other part is poor in Th (ThO2<l wt%), but contains many thorite micro-inclusions with sieved texture. By comparison, it is also implied that zircon in aluminous A-type granites is characterized by low content of ThO2 (<1 wt%), ThO2/UO2 <2 and absence of thorite inclusion. Based on mineralogical features, one is tempted to assume that the Th-rich zircon is formed during the early crystallization of deep-sourced, high-temperature and Th-enriched A-type granitic magma. Such zircon is then subjected to late dissolution owing to accumulation of fluids at the end of magmatic evolution of A-type granite. Recrystallization finally leads to formation of sieved low-Th zircon with thorite micro-inclusions, which may coexist with remnants of Th-rich zircons. The Th-rich zircon may be considered to be one of characteristic accessory minerals of peralkaline A-type granites.

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.

A survey of accessory mineral assemblages in peralkaline and more aluminous A-type granites of the southeast coastal area of China

Abstract An extensive belt of A-type granite exists along the southeast coast of China. The granites are divided into peralkaline and more aluminous subgroups which differ in mineral assemblages, mineral compositions and textures. In the peralkaline subgroup, primary magmatic Th-rich zircon is typically overgrown by Th-poor zircon containing thorite micro-inclusions. REE minerals in this subgroup are dominated by allanite-(Ce), chevkinite-(Ce), titanite and pyrochlore. Fe-Ti oxides are titanian magnetite and Mn-rich ilmenite. In contrast, in the more aluminous subgroup rocks, zircon is weakly zoned and exhibits very low Th but relatively high U contents. The REE minerals are dominated by Th-rich monazite-(Ce). Titanium-poor magnetite, pyrophanite and rutile are the major Fe-Ti oxides. These occurrences indicate that peralkaline magmas favour the formation of REE silicates, whereas magmas with higher alumina saturation stabilize REE phosphates. Peralkaline granites crystallized at temperatures 50−100°C greater than the more aluminous granites, but under lower oxidation conditions. These differences in formation conditions of the two A-type granite subgroups, deduced by accessory mineral characteristics, are inferred to be related to magma derivation at different crustal levels, with peralkaline magma deriving from a deeper crustal level with more mantle input.

The thermal history of the miarolitic granite at Xincun, Fujian Province, China

The thermal history of the late Mesozoic miarolitic granite has been studied based on zircon U-Pb dating, whole rock Rb-Sr dating and K-Ar dating of muscovite, biotite and K-feldspar from the same rock sample. From the beginning of zircon crystallization to the closure of K-Ar system of biotite, the granite body had a slow cooling rate (11.0°C/Ma) and an ascending rate (0.07 mm/a). From the end of this stage to the closure of K-Ar system of K-feldspar, the granite body increased its cooling rate (45°C/Ma) and ascending rate (0.36 mm/a). The thermal history of the Xincun granite with a slow cooling rate at the early stage and a fast cooling rate at the late stage may have been related to the fact that the Fujian coastal area had very high geothermal gradient in the late Mesozoic and evident decrease in geothermal gradient in the early Cenozoic.

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.

The Nd-Sr isotope study of Mesozoic granitoids in Jiangxi Province

Nd-Sr isotopic compositions of 12 Mesozoic granitoids from Jiangxi Province have been reported. They show low-∈ND(−13.8–−8.3), high-/Sr(0.71069–0.73981) and oldtDM (2087–1635 Ma). Comparison between the Sm-Nd isotopic compositions of the granitoids and those of the basement metamorphic racks suggests that these granitoids were mainly derived from the metasedimentary rocks. There is an east-west trend Mesozoic granitoid zone with low-tDM from S. Jiangxi to S. Fujian. The origin of these granites was preliminarily discussed.

SR-ND-PB ISOTOPES OF TERTIARY VOLCANICS OF KING GEORGE ISLAND, ANTARCTICA

1 Geological settingThe King George Island is the largest one in South Shetland Islands, West Antarctica,with an area of 1200 km 2 . Most of it is covered by ice and snow all the year round. It consistsmainly of Tertiary volcanics, associated with plutonic rocks and some late Mesozoic and Qua-...