Shen Weizhou

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.

Nd isotopic composition and material source of pre- and post-Sinian sedimentary rocks in Xiushui area, Jiangxi Province

Reported in this paper are the Nd isotopic compositions of the pre-Sinian and Sinian-Cambrian sedimentary rocks in the Xiushui area, Jiangxi Province. Significant differences are noticed between them in their Nd isotopic compositions. As for the pre-Sinian lightly metamorphozed sedimentary rocks,143Nd/144Nd=0.512000 ∼ 0.512214, ɛNd (T) = −2.57 ∼ 3.91, and TDM=1563 ∼ 1859 Ma;143Nd144Nd=0.511710 ∼ 0.511902, ɛNd(T)= −8.04 ∼ −9.99, and TDM= 1833 ∼ 2426Ma are suggested for the Sinian-Cambrian sedimentary rocks. These differences would reflect the diversity of material source for the sedimentary rocks deposited before and after the Sinian period. Mantle material appears to have been involved in the formation of the pre-Sinian sedimentary rocks while the post-Sinian sedimentary rocks are composed mainly of recycled detritus from the continental crust.Reported in this paper are the Nd isotopic compositions of the pre-Sinian and Sinian-Cambrian sedimentary rocks in the Xiushui area, Jiangxi Province. Significant differences are noticed between them in their Nd isotopic compositions. As for the pre-Sinian lightly metamorphozed sedimentary rocks,143Nd/144Nd=0.512000 ∼ 0.512214, ɛNd (T) = −2.57 ∼ 3.91, and TDM=1563 ∼ 1859 Ma;143Nd144Nd=0.511710 ∼ 0.511902, ɛNd(T)= −8.04 ∼ −9.99, and TDM= 1833 ∼ 2426Ma are suggested for the Sinian-Cambrian sedimentary rocks. These differences would reflect the diversity of material source for the sedimentary rocks deposited before and after the Sinian period. Mantle material appears to have been involved in the formation of the pre-Sinian sedimentary rocks while the post-Sinian sedimentary rocks are composed mainly of recycled detritus from the continental crust.

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.

Rb-Sr and U-Pb Dating of the Daguzhai and Luobuli Granitic Massifs in South China

Rb-Sr and U-Pb isotopic studies of the two contrasting granite types of the Daguzhai and Luobuli massifs in South China provide new constraints on the interpretation of isotopic age data for plutonic igneous rocks. A Rb-Sr internal isochron age of 146±7Ma for the Luobuli adamellite is interpreted to represent the age of magma crystallization, whereas the whole rock Rb- Sr isochron yields an older apparent age of 161±10Ma which is regarded as resulting from contamination processes affecting the petrogenesis of this adamellite. In the Daguzhai granite the marked scatter of whole- rock Rb-Sr data in isochron diagram is ascribed to the open system behavior of Rb during postmagmatic autometasomatism. Uniformity of initial87Sr /86Sr ratio in this granite is indicated in a plot of87Sr versus86Sr. The autometasomatism has also affected zircon U-Pb system, resulting in a spread of data along the concordia curve between 165 and 125Ma. This spread is regarded as indicating the duration of the autometasomatism.Rb-Sr and U-Pb isotopic studies of the two contrasting granite types of the Daguzhai and Luobuli massifs in South China provide new constraints on the interpretation of isotopic age data for plutonic igneous rocks. A Rb-Sr internal isochron age of 146±7Ma for the Luobuli adamellite is interpreted to represent the age of magma crystallization, whereas the whole rock Rb- Sr isochron yields an older apparent age of 161±10Ma which is regarded as resulting from contamination processes affecting the petrogenesis of this adamellite. In the Daguzhai granite the marked scatter of whole- rock Rb-Sr data in isochron diagram is ascribed to the open system behavior of Rb during postmagmatic autometasomatism. Uniformity of initial87Sr /86Sr ratio in this granite is indicated in a plot of87Sr versus86Sr. The autometasomatism has also affected zircon U-Pb system, resulting in a spread of data along the concordia curve between 165 and 125Ma. This spread is regarded as indicating the duration of the autometasomatism.

Oxygen isotope study of the Suzhou granite

Oxygen isotope studies of the Suzhou granite have been made based on drill-hole samples. In the inner part, the δ18O values are less variable either in the whole-rock, quartz or in feldspar. Oxygen isotopic compositions are in equilibrium between coexisting quartz and feldspar. Data points from the inner part are plotted in a small area in the δ18OQ-δ18Of diagram, indicating that this part has not been affected by meteoric hydrothermal fluids. But the whole-rock δ18O values of the marginal part vary greatly. Oxygen isotopic compositions are in extreme disequilibrium between quartz and feldspar. Data points from the marginal part are displayed with a nearly vertical slope in the δ18OQ-δ18Of diagram, implying that rocks of this part are affected by the meteoric hydrothermal fluids. Extreme water-rock interactions lead to mineralizations of rare-elements (Nb, Ta, as well as Zr, Hf, Th) in the marginal part. Source materials of the Suzhou granite are also discussed in this paper.

Geochronology and isotope geochemistry of the Xiqiu spilite- keratophyre in Zhejiang Province, China

Spilite- keratophyre is a key member of the Shuangxiwu Group volcanic series in Zhejiang Province. Presented in this paper are the reliable Sm- Nd internal isochron age (1012±28 Ma, ЄNd(T)=4.4±0.1) and whole- rock Rb- Sr isochron age (972±40 Ma,ISr=0.70327±8) obtained for the first time by the authors. From the available Nd, Sr and O isotope data in conjunction with the major and trace elements data it is suggested that the Xiqiu spilite- keratophy re is the product of island- arc volcanism during the late Middle Proterozoic.

The Tin-Bearing Topaz-Quartz Porphyry, Yangbin Area, Taishun County, Zhejiang Province A Fluid Inclusion Study

The tin-bearing topaz-quartz porphyry in the Yanbin area is subvolcanic in origin, derived from granitic residual magma through strong crystallization differentiation. The rock contains various types of inclusions (hydroxyl-melt, vapor-rich, halite-bearing multiphase, and liquid-rich), which permits us to trace the evolutionary path of the fluid. It is suggested that immiscibility took place in a pulsating manner between melts and fluids during the rock-forming process with the homogenization temperatures ranging from 580°C to 180°C and the salinities varying from low through high to low (5.1–10. 5»6. 9-21.4»30-48»16-22»2-10 in wt% NaCl). The hydroxyl-melt inclusions are considered as evidence of magma-hydrothermal transition.

The geochemical evolution of the proterozoic continental crust in northwestern Jiangxi and western Zhejiang, China

The petrochemistry and Sm-Nd isotopic compositions and- the geochemical characteristics of REE, U,Th, etc. in the Late Proterozoic and Early Palaeozoic strata in northwestern Jiangxi and western Zhejiang provinces are described in this paper. It seems to be sure that the middle Proterozoic strata of southeastern China are low in the degree of maturation. The strata contain much mantle-derived material. At the end of Late Proterozoic there was an abrupt turn with respect to the crustal geochemical evolution of the eastern part of South China. Since then, the geochemical environment has undergone a change from a simple reducing environment to a complex oxidizing-reducing environment, which would be geochemically beneficial to the formation of Late Sinian to Early Cambrian U-bearing formations.

Isotope and trace element geochemistry of Fuchuan ophiolite suite, Anhui, China

The Fuchuan ophiolite suite in Shexian County, Anhui Province, was formed in the Middle-Late Proterozoic. It is characterized by varying Nd [∈Nd (T) =0.7−3.8], Sr [∈Sr(T) = 30.7−53.9] and O(δ18O=3.2−11.0%.) and low ratios of Nd/La (<0.8), Ti/Y (<350) and Ti/V (<30). These characteristics, in combination with geological features, indicate that the ophiolite suite was formed in the axial part of the back-arc basin of the Jiangnan ancient island arc at the southeastern margin of the Yangtze Plate. The varying ∈Nd (T) was caused by the contamination of the underlying inmature sialic crust during the formation of the ophiolite and variations in ∈Sr(T) and δ18 O may have resulted from hydrothermal alteration by seawater during or shortly after its formation.