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The Xuelongbao plutonic complex in the eastern margin of the Tibetan Plateau is dated at 748 ± 7 Ma using the SHRIMP zircon U–Pb method and represents part of the Neoproterozoic igneous assemblage of South China. Rocks in the complex include tonalite and granodiorite and have SiO2 ranging from 62.0 to 74.8 wt.% and Al2O3 from 14.3 to 20.9 wt.%. Their Na2O contents range from 4.2 to 6.7 wt.% and K2O from 0.47 to 1.96 wt.%, indicating that they belong to the Na-series. These rocks show chondrite-normalized REE patterns depleted in HREE and variably enriched in LREE. They have positive Sr and negative Nb and Ti anomalies in the primitive mantle-normalized trace elemental spider diagram. Their Sr contents range from 320 to 780 ppm and Y contents are lower than 10 ppm, resulting in high Sr/Y ratios (52–320), characteristic of typical adakites. Their eNd (t) and initial Sr isotopic compositions range from + 0.36 to + 2.88 and from 0.7033 to 0.7054, respectively. The geochemical features of the Xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab. Together with arc signatures of other granites and mafic intrusions in the region, the 750 Ma Xuelongbao adakitic complex provides evidence for a major, subduction-related Neoproterozoic magmatic event of South China.

Previous studies have suggested that there were two mass extinction events in the Late Permian: one that occurred at the Permo-Triassic (P/T) boundary (251 Ma) and a second, smaller mass extinction that occurred 5–8 Myr earlier at the end of the Guadalupian. Many workers have argued that there is a causal relationship between large-scale volcanic activity and mass extinctions. The major mass extinction event at the P/T boundary coincides with the outpouring of huge quantities of lava that formed the Siberian flood basalt province in Russia. Courtillot et al. [Earth Planet. Sci. Lett. 166 (1999) 177–195] and Wignall [Earth Sci. Rev. 53 (2001) 1–33] suggested that the earlier Late Permian mass extinction coincided with the eruption of the lavas that formed the Emeishan flood basalt (EFB) province in SW China. However, the age of eruption of the EFB lavas is poorly constrained. Using the Sensitive High-Resolution Ion Microprobe to analyze zircons, we have established the age of the Xinjie intrusion, believed to be a feeder to the main phase of EFB volcanism, to be 259±3 Ma. Hence, the formation of the EFB is coincident with a proposed extinction event at 256–259 Ma. This result supports a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction.

The magmatic and tectonic history of the Yangtze Block and its possible affinity with other Neoproterozoic arc terranes are important in the reconstruction of Neoproterozoic plate tectonics. In the Panxi Belt, adjacent to the eastern margin of the Tibetan Plateau, there are many metamorphic complexes associated with Neoproterozoic granites. These are granitic gneisses of upper greenschist to amphibolite metamorphic facies, which have traditionally been considered the Archean basement of the Yangtze Block, although their origin and age of formation were poorly understood. This study provides the first reliable, SHRIMP U–Pb zircon dating results for the gneissic complexes and the Neoproterozoic granites. Three samples of the Kangding gneissic complex yielded identical ages of 797±10, 795±13 and 796±14 Ma. The Gongcai gneissic complex has zircons dated to be 824±14 Ma with metamorphic rims of 177±3 Ma, whereas the Gezong granite has an older age of 864±8 Ma. Other gneissic complexes include the Miyi complex that has a younger age of 764±9 Ma. Geochemical data show that the Kangding gneissic complex has arc signatures, representing metamorphic products of Neoproterozoic, arc-related acidic plutons. This scenario suggests subduction of oceanic lithosphere eastward (present-day orientation) underneath the Yangtze Block. There is a well-defined arc assemblage with an identical Neoproterozoic age along the eastern margin of the Yangtze Block. Thus, during Neoproterozoic time, both the western and eastern margins of the block were active arcs separated by the Trans-Yangtze basin. The Yangtze Block must, therefore, have been an isolated continent, although it was presumably located near the Rodinian supercontinent.

Recent experimental work has shown that the homogenization of elemental concentrations can be much slower than that of isotopic ratios when there are strong concentration gradients in SiO_2 and Al_2O_3. The ramifications of this result for magma homogenization and other petrological problems related to diffusion are significant. We report here a comparison of experimental profiles of elemental concentrations and isotopic fractions of K and Ca in rhyolite-andesite (large concentration gradients) and rhyolite-rhyolite (small concentration gradients) melt couples. When the concentration profile and the isotopic profile of the same element in a single couple are compared, the former is much shorter than the latter in the rhyolite-andesite couple, consistent with other recent studies. However, the lengths of both concentration and isotopic profiles are similar in the rhyolite-rhyolite couple. Therefore, diffusion of an element or oxide may be decoupled from or coupled with isotopic ‘diffusion’, depending on whether large concentration gradients of major components are present. When the two couples are compared, the intrinsic effective binary diffusivities obtained from isotopic profiles are similar for each element in the two couples, whereas the effective binary diffusivity of K obtained from the concentration profile in the rhyolite-rhyolite couple is 37 times that in the rhyolite-andesite couple. Therefore, isotopic homogenization is roughly independent of elemental homogenization and the presence of SiO_2, Al_2O_3, and other concentration gradients, whereas elemental homogenization is strongly affected by concentration gradients. Our experimental data (isotopic and concentration profiles including uphill diffusion profiles) can be modeled quantitatively to a good approximation using a modified effective binary diffusion model in which the flux of a component is assumed to be proportional to its activity gradient instead of its concentration gradient. Therefore, the multicomponent diffusion effect in the silicate systems of our experiments seems to be largely due to contributions of non-ideal mixing to the cross-terms of the diffusivity matrix.

The concentrations of 32 elements (major, minor, REE, Ba, Hf, Sc, V, Co, Ni, Sr, Y, Zr, Nb, Ge, Th and U) were measured using an ion microprobe in olivine and orthopyroxene, and in coexisting glass in the run products produced in isothermal and dynamic crystallization experiments on chemical compositions that produce porphyritic olivine, radial pyroxene and barred olivine textures. Cooling rates in the dynamic crystallization experiments ranged from 1°C/h to 2191°C/h. The mineral/melt partition coefficients (D) calculated from the measured concentrations for both olivine and orthopyroxene showed very little change between equilibrium experiments and dynamic experiments with cooling rates of up to 100°C/h. The data appear to define reliable equilibrium D values. Olivine D's range from 2 × 10^(−5) for U to ∼ 10 for Ni, and orthopyroxene D's range from 2 × 10^(−5) for U to ∼ 8 for Mg. There are regular relationships between the ionic radius, the valence of the trace element and the partition coefficients in olivine and orthopyroxene in all experiments. While there are some differences in the D values between olivine and orthopyroxene, they follow very similar trends. For experiments at high cooling rates there is an increase in the measured concentrations for incompatible trace elements in the crystals which yields apparent partition coefficients greater by up to 3 orders of magnitude from the equilibrium values obtained for isothermal equilibrium and slow cooling experiments. In contrast, compatible trace element D values are only slightly sensitive to cooling rate, typically varying by less than a factor of 2. SEM studies showed that melt inclusions were common in orthopyroxenes. It is shown that the increase in apparent D values for pyroxene observed with high cooling rates can be quantitatively explained by trapping of melt inclusions in crystals with equilibrium D values. Incorporation of melt inclusions may also quantitatively explain the increases in apparent D values in olivine at high cooling rates. However, no inclusions of melt were identified in olivine, even at the fastest cooling rate. If the high apparent D values obtained for olivine at high cooling rates were due to incorporation of trace elements in the lattice and not due to entrapment of melt, it would require incorporation of elements independent of valence and ionic radius.

The study of planetary materials in chondritic meteorites constrains the compositional diversity of materials in different nebular environments and provides information on the degree of differentiation of early planetary bodies. We studied a unique microgabbro fragment from the Parnallee (LL3) unequilibrated ordinary chondrite. The fragment, which was originally identified by its ophitic to sub-ophitic texture, exhibits features characteristic of lunar and terrestrial tholeiitic basalts—extreme compositional zoning in clinopyroxene (Wo_(10)En_(65)Fs_(25) to Wo_(15)En_2Fs_(83)), a multiply saturated major element composition similar to mid-ocean ridge basalt with 3.1 wt% Na_2O and 0.15 wt% K_2O, and uniformly enriched rare earth elements (c.10 × C1). A high bulkMnO/FeO ratio (0.064) distinguishes the microgabbro from other basaltic rocks and suggests the precursor material formed or reached equilibrium in a reducing environment. However, the absence of Fe metal and the extreme enrichment of FeO (up to 40 wt%), in late crystallizing pyroxferroite, requires the last crystallization event to have occurred in a relatively oxidizing environment. We suggest the microgabbro formed by partial melting in a planetary body after removal of metallic Fe. Examination of possible planetary source materials, such as alkali-rich eucritic material, a volatile-depleted C1 carbonaceous chondrite or H-group chondrite, shows that multiple-stage fractionation is required to produce a melt with the FeO/MgO ratio of the microgabbro from these materials. The increasing number of planetary igneous fragments observed in unequilibrated ordinary chondrites (UOC) suggests the picture of UOC as primitive assemblages of unprocessed material is overly simplistic.

The postshield eruptive stage of Mauna Kea volcano, Hawaii, can be divided into an early basaltic substage, the Hamakua Volcanics, containing picrites, ankaramites, alkalic and tholeiitic basalt, and a hawaiite substage, the Laupahoehoe Volcanics, containing only hawaiites and rare mugearites. Cumulate gabbroic xenoliths in Laupahoehoe Volcanics have isotopic ratios similar to the Hamakua Volcanics, and these gabbros provide constraints on the crustal evolution of Mauna Kea lavas. Because of the small variation in87Sr/86Sr (0.70335–0.70362),143Nd/144Nd (0.51297–0.51308) and206Pb/204Pb (18.306–18.440), lavas from both substages must contain relatively fixed proportions of depleted, enriched and primitive mantle components. In addition, there is Sr, Nd and Pb isotopic overlap between tholeiitic and alkalic Hamakua basalts. However, the steep207Pb/204Pb vs.206Pb/204Pb arrays of postshield lavas from Mauna Kea, West Maui and Haleakala volcanoes and the existence of rare samples with high207Pb/204Pb up to 15.548, requires an unusual component in some Hawaiian lavas. This component is unlikely to be derived from sediments or MORB lithosphere, and it may be a minor plume component. Lavas erupted during the postshield stage of Mauna Kea volcano do not define a systematic temporal trend of varying87Sr/86Sr and143Nd/144Nd. This result contrasts with the temporal trend defined by lavas from Haleakala Volcano and provides evidence for important differences between the origin and evolution of different Hawaiian volcanoes. However, the Laupahoehoe Volcanics trend to lower206Pb/204Pb ratios than the Hamakua Volcanics. As inferred for other Hawaiian volcanoes, this trend reflects a larger proportion of a depleted component in the youngest lavas. Finally, postshield lavas from Mauna Kea have relatively highCe/Pb (40 ± 4 in 28 Mauna Kea lavas) when compared with most oceanic basalts, and this ratio may be more variable in oceanic island lavas than previously recognized.

Basalts dredged from ridge axes within 70 km of the Indian Ocean triple junction in the western Indian Ocean have many geochemical and petrologic characteristics in common with depleted mid-ocean ridge basalts (MORBs) from the Atlantic and Pacific. For example there is overlap in major and trace element abundances, and in diagnostic ratios such as K/Rb (700–925) and La/Sm (less than chondritic). Also, glass inclusions in calcic plagioclase (An89–90) provide evidence for a primitive high Mg/Fe, low TiO2 melt. In contrast, basalts dredged from 250 to 400 km southwest of the triple junction on the Southwest Indian Ridge are compositionally distinct from depleted MORB. They are nepheline-normative or slightly hypersthene normative and have higher alkali metal and incompatible element abundances than depleted MORBs with similar MgO contents. All of these Indian Ocean basalts have Sr, Nd and Pb isotope ratios which corroborate previous studies showing that relative to depleted Atlantic and Pacific MORB, many Indian Ocean MORBs have low206Pb/204Pb and high87Sr/86Sr. However, individual Indian Ocean ridges have different radiogenic isotope characteristics, and basalts from the vicinity of the triple junction have unusually high87Sr/86Sr (∼ 0.7032) at low206Pb/204Pb ratios (17.3–18.2). Moreover, the shallow axial region of the Central Indian Ridge from ∼ 12°S to the triple junction (26°S) has high87Sr/86Sr (> 0.7030). Apparently, the depleted component of Indian Ocean MORBs has been contaminated by an isotopically unusual component which does not occur in Pacific and Atlantic MORBs, and is not dominant in basalts from many Indian Ocean islands. The degree of this contamination is not uniform in western Indian Ocean MORB; the most contaminated basalts occur from 12°S on the Central Indian Ridge to the triple junction (∼ 26°S) and easterly along the Southeast Indian Ridge to ∼ 72°E.