Rucheng Wang

Formation history and protolith characteristics of granulite facies metamorphic rock in Central Cathaysia deduced from U-Pb and Lu-Hf isotopic studies of single zircon grains

The petrochemical as well as zircon U-Pb and Lu-Hf isotopic studies of granulite facies metamorphic rock from the Taoxi Group in eastern Nanling Range, Central Cathaysia indicate that its protolith is the sedimentary rock with low maturation index. The clastic materials are mostly from middle Neoproterozoic (~736 Ma) granitoid rocks with minor Neoarchaean and Paleoproterozoic rocks. The timing of this Neoproterozoic magmatism is in agreement with the second period of magmatism widespread surrounding the Yangtze Block. Hf isotopic data indicate that the Neoproterozoic granitoids resulted from the recycled Paleoproterozoic mantle-derived crustal materials. The sedimentary rock was deposited in Late Neoproterozoic Era, and carried into low crust in Early Paleozoic. The partial melting of the meta-sedimentary rock took place at about 480 Ma and subsequently granulite facies metamorphism occurred at ca. 443 Ma. The zircons forming during this time interval (Early Paleozoic) show large Hf isotope variations, and their eHf(t) values increase from -13.2 to +2.36 with decreasing age, suggesting the injection of mantle-derived materials during partial melting and metamorphism processes in the Early Paleozoic. Calculation results show that this metamorphic rock, if evolved to Mesozoic, has similar isotopic composition to the nearby Mesozoic high Si peraluminous granites, implying that this kind of granulite facies metamorphic rock is probably the source material of some Mesozoic peraluminous granitoids in eastern Nanling Range.

Two subgroups of A-type granites in the coastal area of Zhejiang and Fujian Provinces, SE China: age and geochemical constraints on their petrogenesis

Late Cretaceous (90–100 Ma) A-type granites are widespread in the coastal area of the Zhejiang and Fujian Provinces, SE China. According to mineralogical and geochemical characteristics, the A-type granites in this belt can be further divided into aluminous and peralkaline subgroups. The aluminous subgroup often contains aluminous-rich minerals (e.g. spessartine and Mn-rich muscovite), while the peralkaline subgroup usually contains riebeckite, arfvedsonite and aegirine. Geochemically, the aluminous A-type granites show lower Nb, Zr, Ga, Y and REE abundances, and lower FeO*/MgO and Ga/Al than the peralkaline subgroup. When they occur in the same area, the two subgroups of A-type granites display quite similar initial Nd isotopic compositions, which are indicative of mixing of ancient basement crustal rocks with variable amounts of mantle materials. Integrated geological and geochemical investigations indicate that both the aluminous and the peralkaline magmas are highly evolved and reflect the residual liquids left after high degrees of fractional crystallisation in a deep magma chamber. The present authors suggest that the mineralogical and geochemical differences between the aluminous and peralkaline subgroups are likely to have been generated via different differentiation paths controlled by varying fluorine contents of the parent magmas.

Discovery of the eclogite and its petrography in the Northern Dabie Mountain

Eclogite from the Northern Dabie Mountain is a new finding by the authors. These eclogites in foliated perdotite are enveloped by banded gneiss and occur in the mafic-ultramafic rock belt. They are mainly composed of omphacite, garnet, diopside, orthopyroxene, amphibole, plagioclase and magnetite, and a small amount of rutile, spinel, olivin and corundum. The mineral association of peak metamorphism of the eclogite is omphacite+garnet+rutile. The existence of eclogite in the Northern Dabie Mountain implies that there was an eclogitic metamorphism prior to the granulitic facies one in the mafic-ultramafic rock belt.

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.

Chemical evolution of Nb-Ta oxides and zircon from the Koktokay No. 3 granitic pegmatite, Altai, northwestern China

Abstract The Koktokay No. 3 granitic pegmatite, Altai, northwestern China, is a strongly zoned rare-element granitic pegmatite, where the petrographic zones were distinguished into two groups: outer zones (I to IV) and inner zones (V to IX). Nb-Ta oxides and zircon are investigated in this paper by using quantitative electron-microprobe analyses (EMPA) and backscattered-electron (BSE) imaging. Columbite-tantalite and zircon occur in most textural zones, whereas tapiolite and uranmicrolite are mainly restricted to zone VII. Manganocolumbite and zircon from the outer zones (zones II and IV) are homogeneous except for a few exceptions, whereas manganotantalite and hafnian zircon from the inner zones (V-VII) are obviously heterogeneous and strongly zoned. Chemically, Ta/(Nb+Ta) in columbitetantalite and Hf/(Zr+Hf) in zircon increase from the outer to the inner zones on one hand, and from core to rim in single zoned crystals on the other hand. Observations of intra-zonal variations of the chemical composition of Nb-Ta oxides and zircon in the Koktokay No. 3 granitic pegmatite may suggest that the outer zones crystallize under magmatic conditions, whereas the inner zones crystallize under fluid-rich magmatic conditions, and locally under hydrothermal conditions. The extreme enrichments of columbite-tantalite in Ta, and of zircon in Hf, as well as the occurrence of uranmicrolite and tapiolite, indicate an elevated evolution of the Koktokay No. 3 granitic pegmatite.

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.

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.