Wang Dezi

Crust evolution in Southeast China: evidence from Nd model ages of granitoids

Nd isotopic compositions of 58 granitoids in South China have been reported in this paper. These data together with other published data reveal that granites with Nd model ages ( t DM ) greater than 1.8 Ga are distributed mainly in three areas: southwestern Zhejiang-northwestern Fujian, two sides of the Wuyi Mountain and Wanyangshan-Zhuguangshan. These granites are believed to be derived from partial melting of old crust in these areas. The Mesozoic granites with t DM 1.6 Ga are distributed in three zones: the Gangang structural zone, Nanling latitudinal structural zone and Fujian-Zhejiang coastal zone. These zones may have been an extensional tectonic setting and mantle-derived components or magmas may have been involved to different extents in the granite formation. Based on Nd model ages of granites and published chronological data of mafic and ultramafic rocks, it is believed that the crust in South China experienced episodic accretions, among which the early-middle Proterozoic is the most important period of crustal accretion.Nd isotopic compositions of 58 granitoids in South China have been reported in this paper. These data together with other published data reveal that granites with Nd model ages(tDM) greater than 1.8 Ga are distributed mainly in three areas: southwestern Zhejiang-northwestern Fujian, two sides of the Wuyi Mountain and Wanyangshan-Zhuguangshan. These granites are believed to be derived from partial melting of old crust in these areas. The Mesozoic granites withtDM < 1.6 Ga are distributed in three zones: the Gangang structural zone, Nanling latitudinal structural zone and Fujian-Zhejiang coastal zone. These zones may have been an extensional tectonic setting and mantle-derived components or magmas may have been involved to different extents in the granite formation. Based on Nd model ages of granites and published chronological data of mafic and ultramafic rocks, it is believed that the crust in South China experienced episodic accretions, among which the early-middle Proterozoic is the most important period of crustal accretion.

A Paleoproterozoic A-type Rhyolite

The rhyolites in the upper Lüliang Group of Shanxi, China, are Paleoproterozoic weakly alkaline volcanic rocks. They are characterized by high SiO2, Na2O + K2O, Zr, Nb, Ga, Y and REE contents and large FeO* /MgO, Rb/Sr and Ga/Al ratios, and low CaO, Sr and Eu contents, and share much in common with the A-type granitic rocks. They erupted in the rift setting at the continental margin. Chemical features and isotope data, as well as high Nd and low initial Sr ratios, suggest that the original granitic magma was derived from partial melting of Late Archean metamorphic rocks in the lower crust due to the influence of basaltic magma and hot fluid in response to rifting. The A-type rhyolites were finally formed after the fractional crystallization of the dominant mineral feldspar.

Ages of Lüliang Group and its metamorphism in Mt. Lüliang region, Shanxi Province: Evidence from single grain zircon U-Pb dating

Presented in this paper are the newly obtained grain zircon U-Pb ages of volcanic rocks of the Lüliang Group and associated Kuanping granitic migmatitic gneiss in Shanxi Province. The zircon U-Pb ages of bimodal volcanic rocks (basalt and rhyolite) of the Upper Lüliang Group indicate that the rocks erupted at about 2100 Ma. So the Lüliang Group was formed during the Early Proterozoic. In the area studied the second-stage metamorphism experienced by the Lüliang Group is the dominant one which took place at about 1806 Ma, i. e., during the late Early Proterozoic.

Petrology, geochemistry and genesis of Kuiqi granite batholith

The Kuiqi granite batholith outcrops in the vicinity of Fuzhou City, Fujian Province and constitutes one of the typical alkali granitic complexes in the “Belt of Miarolitic Granites” extending along the southeast coast of China. The complex is believed to have been emplaced at higher levels of the crust in a tensional fault environment. Petrographically it is composed mainly of aegirine-arfvedsonite granites with early biotite granites scattered. Miarolitic structure and granophyric texture are commonly observed. The Rb-Sr isochron age of the complex is 107.65 m.y. Both petrological and petrochemical studies show that the Kuiqi granite is of A-type. Data on chemical composition, REE pattern and transition elements reveal that there is a close genetic connection between granites and associated volcanic rocks. Thus, syntexistype (I-type) granite, A-type granite and volcanic rocks form a cogenetic “trinity”, in which the A-type granite is usually the latest member of the volcanic-intrusive series.The Kuiqi granite batholith outcrops in the vicinity of Fuzhou City, Fujian Province and constitutes one of the typical alkali granitic complexes in the “Belt of Miarolitic Granites” extending along the southeast coast of China. The complex is believed to have been emplaced at higher levels of the crust in a tensional fault environment. Petrographically it is composed mainly of aegirine-arfvedsonite granites with early biotite granites scattered. Miarolitic structure and granophyric texture are commonly observed. The Rb-Sr isochron age of the complex is 107.65 m.y. Both petrological and petrochemical studies show that the Kuiqi granite is of A-type. Data on chemical composition, REE pattern and transition elements reveal that there is a close genetic connection between granites and associated volcanic rocks. Thus, syntexistype (I-type) granite, A-type granite and volcanic rocks form a cogenetic “trinity”, in which the A-type granite is usually the latest member of the volcanic-intrusive series.

Types and genetic model of Precambrian granitoids of South China

Precambrian granitoids in South China, including more than 20 bodies, are distributed on the Jiangnan ancient continent covering Guangxi, Jiangxi, Anhui and Zhejiang Provinces. The total area of granitoids is about 4300 km3, less than 2.5% of whole area of the Jiangnan ancient continent. According to the features of the representative bodies the Precambrian granitoids in South China may be divided into four genetic types: (1) mantle-derived granitoids crystallized from differentiated basic magma; (2) cordierite-bearing magmatic granitoids whose source rocks are mainly oceanic igneous rocks; (3) cordierite-bearing magmatic granitoids originated mainly from pelitic metamorphosed basement; (4) metasomatic granitoids caused by metasomatism of metamorphosed pelitic rocks. Except for the first type, the other three types essentially belong to the S-type granite in the current classification of granites. Tectonically, they are of collision granitoids irrelevant to subduction.

Geochemical characteristics and genesis of topaz-bearing porphyries in Yangbin area of Taishun County, Zhejiang Province

The host rocks of the porphyry tin deposits in the Yangbin area are principally topaz-bearing porphyry dikes about 2 km long and 2–20m wide. Three lithologie types are identified for the dikes: topaz-bearing potassium feldspar granitic porphyry, topaz-bearing monzonitic granitic porphyry and topaz-bearing quartz porphyry. The content of topaz in the rocks ranges from 10 to 20 vol.%. Porphyritic texture is characteristic, with quartz, potassium feldspar and albite as main phenocryst minerals. The phenocryst occupies 10–20 vol% of the rocks. The rock groundmass consists of subhedral topaz, quartz and protolithionite. Topaz has a unit-cell parameter b=8.797 (Å), and F:OH=1.92:0.18, indicating a F-rich variety formed at high temperature. The topaz-bearing porphyries occurring in this area are strongly peraluminous (A/NKC=1.574–12.94), with high ratios of F/Cl (146–303) and Rb/Sr (5–122). They are rich in incompatible elements (Sn, 313 × 10−6–1042 × 10−6; W, 6 × 10−6–218 × 10−6; Nb, 27 × 10−6–54 × 10−6), but poor in compatible elements (Sr, 10 × 10−6–28 × 10−6; Ba, 58 × 10−6–73 × 10−6; V, 3 × 10−6–12 × 10−6, Cl, 150 × 10−6–226 × 10−6). The rocks are also characterized by high total REE amount (281.69 × 10−6–319.76 × 10−6), with strong Eu depletion (δEu=0.01–0.03) and low ratio of LREE/HREE (0.78–0.84). In summary, the authors propose an idea of S-type genesis for the topaz-bearing porphyries with tin mineralization, instead of I-type.

Distribution and geochemical features of Hercynian-Indosinian granitoids in Southeastern China

The tectonic patterns of the Hercynian-Indosinian cycle in southeastern China can be divided into two types. The first type is the Hercynian-Indosinian eugeosynclinal system, which is situated on the outer side of the Caledonian uplift belts. The second type is the Hercynian-Indosinian fault depression zone or superimposed basin system, which is developed on the basement formed by the Caledonian orogenic movement. There are two genetically different series of granitoids corresponding to two tectonic settings in the Hercynian-Indosinian tectonic regime of southeastern China.Among the transformation-series batholiths one of the largest in the Qinzhou Gulf geosyncline is known as the Darongshan granite (NE-extending) with an outcrop area of about 7000 km2. The age of emplacement determined by whole-rock Rb-Sr isochron method ranges from 223 to 263 m.y., corresponding to late Permian, with higher Sr inital ratios (0.7179–0.7274). The granite is chemically rich in K2O but poor in Na2O with the atomic Na/K ratio<1.0 and the mole ratio Al2O3/(Na2O + K2O + CaO) > 1.0. K-feldsparg are usually predominated by medium to maximum microclines with a higher degree of Al-Si ordering characterized by the concentration of Al in T1 sites of the feldspars (t1 (o) = 0.57–0.99). Biotites are often rich in Fe but poor in Mg, belonging to Fe-biotite or siderophyllite, with a lower MF value (Mg/(Mg + Fe+3 +Fe2+ +Mn)), normally <0.4, and a lower Fe3+/Fe2+ ratio of 0.03-0.02 in their octahedron layers. This fact indicates that the oxidation degree of iron is relatively low in the transformation-series granites as compared with the syntexis-series granitoids.Granitic batholiths of the syntexis series distributed along the fault belts in the Hercynian depressions are usually more mafic, meta-aluminous granodiorites and smaller in size than those of the transformation series, with an outcrop area of 1–200 km2. The Taiping granodiorite, for instance, is situated on the southern side of the Lower Yangtze River depression. It was emplaced 223 m.y. ago as dated by whole-rock Rb-Sr isochron method, with an initial Sr ratio of 0.706. Petrochemically, it is characterized by Na/K > 1.0 (1.31), mole Al2O3/(Na2O + K2O + CaO) < 1.0(0.95) and high Fe2O3/(Fe2O3 + FeO)(0.47), evidently differing from the transformation-series granites. K-feldspars are predominated by orthoclase with a lower degree of Si-Al ordering (t1 (o) = t1(m) = 0.30). Biotites are commonly rich in Mg in their octahedron layers (MF = 0.42–0.50) with higher Fe3+/Fe2+ ratios (0.36-0.51), indicating their formation under higher oxygen fugacity conditions. All this goes to prove that the syntexis-series granitoids were probably derived from mafic igneous or metamorphic source rocks in the lower crust.