Chemical Composition of the Primitive Mantle Source of Peridotite Inclusions at Mount Lianshan, Liuhe County,Jiangsu Province

: This paper presents a study of the major and trace element compositions of fresh mantle-derived spinel lherzolite and harzburgite inclusions from Cenozoic alkaline basalt in Mount Lianshan and Mount Panshi, Liuhe County, Jiangsu Province. An estimation is made of the contents of the major elements and some of the trace elements in the primitive mantle source region of the area, from which the authors have obtained MgO/Al2O3= 7.86. The contents of MgO and Al2O3 are also obtained as 37.58% and 4.78% respectively based on the correlation of MgO-Al2O3. Then, the contents of various elements in the primitive mantle are calculated using their regression equations with MgO, and the compositon of the primitive mantle, a basic issue in geochemistry study, is discussed on that basis.

In-situ trace element analyses of zircons from Dabieshan Huangzhen eclogite:Trace element characteristics of eclogite-facies metamorphic zircon

The internal structures of zircons in eclogite from Huangzhen have been studied by cathodoluminescence (CL) microscopy. Two growth stages were distinguished: protolith magmatic cores and metamorphic overgrowth rims. These different domains were analyzed for trace elements using LAM-ICP-MS. The protolith and the overgrowth zircons have different trace elements characteristics. The trace element contents of protolith zircons are high and very variable. The overgrowth zircons show a typical trace element feature of equilibrium with garnet, e.g. low contents of HREE (132.2-197.6μg/g) and small differential degree of HREE ((Yb/Gd)cN=8.6-11.9). The contents of Nb, Ta and the ratio of Nb/Ta are lower in the metamorphic domains (0.5-1.4μg/g, 0.7-1.5μg/g, 0.3-1.3, respectively) than in the protolith domains (3.8-19.7μg/g, 2.7-12.1μg/g, 1.0-4.6, respectively). This is the first time to give the evidence that the metamorphic zircon equilibrates with the rutile, which formed during the peak metamorphic stage.

Oxygen isotopic compositions of zircons from pyroxenite of Daoshichong, Dabieshan: Implications for crust-mantle interaction

Oxygen isotopic compositions of zircons from pyroxenite (-145 Ma) of Daoshichong, Dabieshan have been measured by an ion microprobe. Both within the single grain and among different grains, oxygen isotopic ratios are homogeneous, σ18O = (7.66‰±0.46)‰(1 SD, 1σ=0.10, n = 22). High σ18O values indicate that the mantle-derived parent magma of Daoshichong pyroxenite have undergone interaction with crustal materials. Combing with other geo-chemical constraints, the way of crust-mantle interaction is suggested to be source mixing other than crustal contamination. The time interval between crust-mantle interaction and formation of the parent magma of Daoshichong pyroxenite is less than several million years. The crustal component involving in crust-mantle interaction is mafic lower crust, and the parent magma of pyroxenite possibly contain large proportion (】37%) of such lower crust.

Zircon SIMS ages and chemical compositions from Northern Dabie Terrain:Its implication for pyroxenite genesis

We present the results of a detailed micro-scale investigation of zircons from pyroxenites, Daoshicong Northern Dabie using a combination of SIMS and ICPMS. The SIMS measurements gave ages of 134-159 Ma. Its average of (144.516.2) Ma is interpreted as the best estimate of the pyroxenite intrusion. The crystallization of zircons continued for quite a long time and underwent slow cooling. The pyroxenites are products of post-collision magmatism. The REE pattern is HREE-enriched, and its HREE concentrations fall between the magmatic and metamorphic range of gneissic zircons from the Dabie area, which indicate the involvement of crust material in its mantle source.

Oxygen isotopic heterogeneity in the mantle: Evidence from clinopyroxene megacrysts of Nüshan, eastern China

Major element, REE and oxygen isotopic com-positions of fourteen clinopyroxene megacrysts from Nushan, eastern China have been obtained. They are believed to rep-resent crystallization products of one mantle-derived alkali basaltic magma. Oxygen isotopic ratios are homogeneous within the same sample, but heterogeneous among different samples, δ18O values of these megacrysts are from 4.86‰ to 6.48‰. We suggest that the oxygen isotopic variation of cli-nopyroxene megacrysts should be inherited from their source magma, and the heterogeneity of source magma re-sults from contamination of subducted oceanic materials.

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 proto-lith zircon has low 176Hf/177Hf (0.282266―0.282466) and high 176Lu/177Hf (0.000090―0.002144) composi-tions. 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 compo-sition 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.