Jian-Tang Peng

Samarium–neodymium isotope systematics of hydrothermal calcites from the Xikuangshan antimony deposit (Hunan, China): the potential of calcite as a geochronometer

Abstract The Xikuangshan antimony mine located at Central Hunan, China, is the largest antimony deposit in the world. Hydrothermal calcites from the Xikuangshan Mine display relatively large variations in REE abundance and Sm/Nd ratios and are characterized by MREE- and HREE-enriched and LREE-depleted patterns. Sm–Nd isotopic compositions of syn-sulfide calcites fall on well-defined lines in the isochron plot. Calcites from early- and late-stage mineralization form two distinct isochrons corresponding to 155.5±1.1 and 124.1±3.7 Ma, respectively. The isochron ages suggest that Sb mineralization took place during the Late Jurassic–Early Cretaceous Period in the Xikuangshan district, consistent with previous geological observations and with independent isotope data. Samarium–neodymium dating of calcites shows potential for determining ages of hydrothermal mineralization.

Cenozoic high Sr/Y volcanic rocks in the Qiangtang terrane, northern Tibet: geochemical and isotopic evidence for the origin of delaminated lower continental melts

Geochemical and Sr–Nd–Pb isotopic data are presented for volcanic rocks from Zougouyouchaco (30.5 Ma) and Dogai Coring (39.7 Ma) of the southern and middle Qiangtang block in northern Tibet. The volcanic rocks are high-K calc-alkaline trachyandesites and dacites, with SiO 2 contents ranging from 58.5 to 67.1 wt % The rocks are enriched in light REE (LREE) and contain high Sr (649 to 986 ppm) and relatively low Yb (0.8 to 1.2 ppm) and Y (9.5 to 16.6 ppm) contents, resulting in high La/Yb (29–58) and Sr/Y (43–92) ratios, as well as relatively high MgO contents and Mg no., similar to the compositions of adakites formed by slab melting in subduction zones. However, the adakitic rocks in the Qiangtang block are characterized by relatively low e Nd (t) values (−3.8 to −5.0) and highly radiogenic Sr (( 87 Sr/ 86 Sr) i = 0.706–0.708), which are inconsistent with an origin by slab melting. The geochemistry and tectonics indicate that the adakitic volcanic rocks were most likely derived from partial melting of delaminated lower continental crust. As the pristine adakitic melts rose, they interacted with the surrounding mantle peridotite, elevating their MgO values and Mg numbers.

He and Ar isotopic compositions and genetic implications for the giant Shizhuyuan W–Sn–Bi–Mo deposit, Hunan Province, South China

The Shizhuyuan ore deposit in southern Hunan Province is a world-class W–Sn–Bi–Mo occurrence hosted by Devonian limestone in the thermal aureole of the Qianlishan granite. Mineralization coincided with intrusion of the granite pluton during Mesozoic crustal extension in South China. We present new He and Ar isotope data for volatiles released from pyrite in the Shizhuyuan deposit. Concentrations of 40Ar range from 0.21 to 2.38 × 10−6 cm3 STP 40Ar/g, and 4He concentrations range from 0.8 to 65.1 × 10−6 cm3 STP 4He/g. 3He/4He ratios vary from 0.06 to 1.66 Ra (where Ra is the 3He/4He ratio of air = 1.39 × 10−6) and 40Ar/36Ar ratios range from 293 to 1072. The isotopic compositions of He and Ar indicate that the ore-forming fluids were mantle-derived, modified by air-saturated crustal fluids. Shallow-level boiling increased 3He concentrations, whereas crustal contamination decreased 3He/4He ratios in the magmatic fluids. The occurrence of mantle-derived components in the magmatic fluid indicates that the associated Qianlishan granite is not a typical S-type pluton that formed entirely by crustal melting. We propose that the mineralization was related to mantle upwelling and Mesozoic lithosphere extension of the South China Block.

Os isotopic compositions of a cobalt-rich ferromanganese crust profile in Central Pacific

Abstract187Os/188Os ratios in all sub-layers of the Co-rich ferromanganese crust MP5D17 from Line Seamount in Central Pacific have been systematically determined. The 187Os/188Os values in this crust are highly variable, ranging from 0.4262 to 1.0155. From the bottom to the top, the osmium isotopic ratios of the crust display a regular variation trend, which correlates well with its changes in structure and texture in the vertical profile. In comparison with the evolution curve of Os isotopic composition in seawater over the past 80 Ma, a similar evolution tendency can be found with respect to the Os isotopic composition of the crust in this study. It is preliminarily concluded that the crust MP5D17 began to grow at about 72 Ma, and underwent three growth hiatuses, corresponding to the geological ages of 64, 53 and 37 Ma, respectively.

Island-arc geochemical signatures of Cenozoic alkali-rich intrusive rocks from western Yunnan and their implication

Alkali-rich intrusive rocks in western Yunnan were derived from an enriched lithospheric mantle (EM II) source. The data available indicated they are alkali-rich (K2O + Na2O > 8wt%) and shoshonitic. Although formed in a within-plate environment they exhibit signatures of arc magmatic rocks, such as high amounts of LILE and LREE relative to the HFSE and HREE, and thus high Ba/Nb, Ba/Zr, Sr/Y, La/Yb ratios as well as mimic chondrite-normalized REE and primitive mantle-normalized trace element patterns of subducted sediments, and they fall in the collision- or arc-related tectonic setting field on all discrimination diagrams. This might suggest the enrichment be related to the substantial extent of sediment contamination by the Mesozoic Tethyan subduction processes.Alkali-rich intrusive rocks in western Yunnan were derived from an enriched lithospheric mantle (EM II) source. The data available indicated they are alkali-rich (K2O + Na2O > 8wt%) and shoshonitic. Although formed in a within-plate environment they exhibit signatures of arc magmatic rocks, such as high amounts of LILE and LREE relative to the HFSE and HREE, and thus high Ba/Nb, Ba/Zr, Sr/Y, La/Yb ratios as well as mimic chondrite-normalized REE and primitive mantle-normalized trace element patterns of subducted sediments, and they fall in the collision- or arc-related tectonic setting field on all discrimination diagrams. This might suggest the enrichment be related to the substantial extent of sediment contamination by the Mesozoic Tethyan subduction processes.