Chen Daogong

Preliminary study of microscale zircon oxygen isotopes for Dabie-Sulu metamorphic rocks: Ion probe in situ analyses

Abstract151 in situ analyses of oxygen isotopes were carried out by ion micro-probe for zircons from 8 localities of HP-UHP metamorphic rocks including eclogites in the Dabie-Sulu terrane. The results show significant heterogeneity in δ18O values, with variation in different rocks from −8.5‰ to +9.7‰ and within one sample from 2‰ to 12‰. No measurable difference in δ18O was observed between protolith magmatic (detrital) zircons and metamorphic recrystallized zircons within analytical uncertainties from the ion micro-probe measurements. This indicates that the metamorphic zircons have inherited the oxygen isotopic compositions of protolith zircons despite the HP to UHP metamorphism. According to their protolith ages from zircon U-Pb in situ dating by the same ion micro-probe, two groups of oxygen isotope composition are recognized, with one having δ18O values of 6‰–7‰ for old protolith of 1.9–2.5 Ga ages and the other 0‰–2‰ for young protolith of 0.7–0.8 Ga ages. The latter anomalously low δ18O values of zircons indicate that the magma has had the obvious involvement of meteoric water when forming the young protolith of high-grade metamorphic rocks. This may be correlated with the snowball Earth event occurring in South China and the world elsewhere during the Neoproterozoic.

The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region ——The Os isotope model age of mantle-derived peridotite xenoliths

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The187Os/188Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC) and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the187Os/188Os-Al2O3 alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to early Pro-terozoic. The two samples with the lowest187Os/188Os ratio (0.119 and 0.117) have the TRD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.

Chemical and isotopic characteristics of gabbroic xenoliths from Hannuoba, China

Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd, Sr and Pb isotopes. The results show that the gabbroic xenoliths are of more mafic basaltic composition. Their abundances show narrow variations in major elements. The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths. The gabbroic xenoliths are rich in Nd (0.51159–0.51249), Sr (0.70491–0.70768) and low in radiogenic Pb(16.283–17.046, 15.191–15.381 and 36.999-37.476), significantly different from basalts and lherzolites in isotopic space. The calculated Nd and Pb model ages are about 3.0–3.5 Ga. The rocks have relatively low equilibriumT(~850°C) andP (0.8–0.9 GPa). They could be interpreted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle. Their chemical and isotopic variations can be ascribed to different degrees of melting, segregation and long-term evolution.

K-Ar ages and Pb, Sr isotopic characteristics of Cenozoic volcanic rocks in Shandong Province, China

Abstract28 samples of Cenozoic volcanic rocks collected from Shandong Province have been dated by K-Ar method. They are mainly Neogene with an age range of 4–19 m. y. The basalts from Linqu and Yishui in west Shandong Province are Miocene and those from Penglai and Qixia in east Shandong Province are Miocene and Pliocene in age. The basalts from Wudi in north Shandong Province are Middle-Early Pleistocene in age. In each area the duration of volcanic eruption was estimated at about 2–3 m. y.Pb and Sr isotopic compositions and U, Th, Pb, Rb, Sr, and major elements in most of the samples were determined. The isotopic compositions are:206Pb/204Pb—16.92-18.48,207Pb/204Pb—15.30-15.59,208Pb/204Pb—37.83-38.54, and (87Sr/86Sr)i—0.70327-0.70632. There are some positive or negative linear correlations between206Pb/204Pb and207Pb/204Pb, Pb isotopes and Pb content, Pb isotopes and Sr isotopes, and Sr isotopes and other elements. The basaltic rocks from east and west Shandong Province have somewhat differences in isotopic composition and element content. The basalts probably are products of multi-stage evolution of the mantle. They have preserved the primary features of the source, although they were influenced, to some extent, by the contamination of crustal materials.

Hydrogen diffusion in clinopyroxene: dehydration experiments

Hydrogen diffusivity of one clinopyroxene megacryst from Nüshan, eastern China parallel to [001]* crystallographic direction was measured by high-temperature sequential dehydration experiments at 950üC and 850üC, which was (6.5ü1.5)×10-12(m2/s) and (1.8×0.7)×10-12(m2/s) respectively. From these results and the previous published data, it seems that the correlation between hydrogen diffusivity and Fe content suggested by Dyar et al. (1996) possibly does not exist. Clinopyroxene megacrysts from alkali basalts appear to be possible reliable samples for the study of the water evolution and hydrogen isotope compositions of the upper mantle.

Zircon Hf isotope composition of metamorphic eclogite from Xindian, Dabie Terrain

Hf isotope measurement has been carried out for UHP metamorphic eclogite from Xindian by using LA-MC-ICP-MS technique. The result indicates that metamorphic growth zircon has high 176Hf/177Hf (0.282544–0.282612) and low 176Lu/177Hf (0.000004–0.000211) ratio, inherited and recrystallized protolith zircon has low 176Hf/177Hf (0.282266–0.282466) and high 176Lu/177Hf (0.000090–0.002144) compositions. The low 176Lu/177Hf of growth zircon comes from its decreasing of Lu and increasing of Hf during UHP process. The high 176Hf/177Hf deduced from high radiogenic 176Hf, which was produced from long-term evolution of high Lu/Hf ratio minerals. Partial recrystallization of protolith zircon would not cause reworking of Lu/Hf isotope in zircon. Compared to U-Pb, zircon Lu-Hf system has better stability. The initial Hf isotope composition of metamorphic growth zircon may represent the Hf isotope composition of whole rock system at the same time. The initial εHf of +3.0 for metamorphic precursor formation of Xindian eclogite indicates that the source material mainly derived from weak depleted mantle or mixing of depleted mantle with old crust.

Constraints on the early cooling rates of Mt. Dabie from diffusion modeling of chemical zoning of garnet

Diffusion modeling of zoning profiles in eclogite garnets from three different tectonic units of Mt. Dabie, UHPM unit, HPM unit and northern Dabie, was used to estimate the relative time span and cooling rates of these rocks. Modeling result for the Huangzhen eclogite garnet shows that the maximal time span for the diffusion-adjustment process is about 22 Ma since the peak-temperature metamorphism, which is the maximum time span from amphibolite facies metamorphism to greenschist facies metamorphism. The Bixiling eclogites had subjected to a cooling process at a rate of ∼10°C/Ma from 750°C to 560°C during 20 Ma. The second cooling stage of the Raobazhai eclogite following granulite-facies metamorphism is an initial fast cooling process at a rate of about 25°C/Ma and then slowed down gradually. All these belong to a coherent Dabie collision orogen with differences in subduction depth and exhumation/uplifting path.

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guangdong and its tectonic significance

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guangdong Province have been obtained by ion micro-probe. δD and H contents are constant both among different samples and within single sample, demonstrating that their formation condition is very stable. High δD values suggest the presence of a component recycled from crust which is possibly related to the subcluction of Pacific Plate beneath Eurasian Plate in Mesozoic.

Trace element geochemistry of Cenozoic volcanic rocks in Shandong Province

The Cenozoic volcanic rocks of Shandong Province are mainly alkalic and strongly alkalic basaltic rocks. The contents of major and trace elements including transitional, incompatible and rare-earth elements were determined. The chemical characteristics of major and trace elements indicate that these basaltic rocks were derived from a mantle source and probably represent a primary magma, i. e., unmodified partical melts of mantle peridotite, in terms of Mg values, correlations between P2O5 and Ce, Sr, Ni and Rb concentrations, mantle xenoliths, etc. The abundances of trace elements vary systematically from west to east. The compatible transition elements such as Co, Ni, and Cr show a remarkable depletion, whereas the incompatible and rare-earth elements are abundant as viewed from the chondrite-nor-malized patterns. The chemical composition and correlation are consistent with the tectonic setting.According to the batch and fractional partial melting theory, the trace elemant contents of Shandong volcanic rocks can be calculated from the two-component mixing model.