Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms,South Africa: constraints from whole rock geochemistry

Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton, have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich (~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). ⁸⁷Sr/⁸⁶Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich (850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched in incompatible elements (La/Yb_n ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1) and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during the Proterozoic by calc-alkaline fluids/melts.     

Trace element abundances in andesites

Abundance data for Cs, Rb, Tl, Ba, Pb, Sr, the rare earths, Th, U, Zr, Hf, Sn, Nb, Mo, Mn, Cu, Co, Ni, Sc, V, Cr, Ag, Sb and the major elements are reported for two andesites and a dacite from Saipan, nine andesites and a dacite from Bougainville and two andesites from Fiji. The Saipan rocks are low-K varieties and contain notably low abundances of Rb, Ba, Th and U and have rare earth patterns subparallel to chondritic patterns. The Bougainville andesites include low-Si and high-K varieties which have higher concentrations of the large cations. The Fijian samples are close to the average circum-Pacific andesite and have rareearth patterns sub-parallel to those of sedimentary rocks.All the andesites contain characteristically low (< 20 ppm) values for Ni and have Ni/Co ratios < 1, and V/Ni ratios > 10.These data preclude derivation of calc-alkaline rocks by mixing of upper crustal material or by fractional crystallisation from basaltic parents. A two stage model is proposed involving sea-floor spreading and transportation of the oceanic crust down the dipping seismic plane into the mantle where it is remelted to form andesites.     

Do S-type granites commonly sample infracrustal sources? New results from an integrated O, U–Pb and Hf isotope study of zircon

In contrast to I-type granites, which commonly comprise infracrustal and supracrustal sources, S-type granites typically incorporate predominantly supracrustal sources. The initial aim of this study was to identify the sources of three Scottish Caledonian (~460 Ma) S-type granites (Kemnay, Cove and Nigg Bay) by conducting oxygen, U–Pb and Hf isotope analyses in zircon in order to characterise one potential end-member magma involved in the genesis of the voluminous late Caledonian (~430–400 Ma) I-type granites. Field, whole-rock geochemical and isotopic data are consistent with the generation of the S-type granites by melting their Dalradian Supergroup country rocks. While Hf isotope compositions of magmatic zircon, U–Pb data of inherited zircons, and high mean zircon δ¹⁸O values of 9.0 ± 2.7‰ (2SD) and 9.8 ± 2.0‰ for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains zircons with much lower δ¹⁸O values (6.8 ± 2.1‰), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains low-δ¹⁸O material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like δ¹⁸O. Mixing trends in plots of δ¹⁸O vs. εHf for S-type granite zircons indicate involvement of at least two sources in all three granites. This pilot study of Scottish Caledonian S-type granites demonstrates that, while field and whole-rock geochemical data are consistent with local melting of only supracrustal sources, the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic evolution involving two or more magma sources.     

新疆伊犁吐拉苏地区的线性构造及控矿特征

采用多种数值图像进行线性构造解译，其中DEM和NOAA主要用于区域尺度，TM用于勘探区尺度，地面磁法数据用于靶区范围的解译分析。研究认为该区线性构造与金矿床的空间分布关系密切，表现为：（1）区域尺度的一级控矿构造为NEE、NE和NW向构造的交汇部位；吐拉苏地区的一级控矿构造为WNW和NNW及NE方向构造的交汇部位；（2）吐拉苏地区的次级控矿构造为NNE和环形构造的交汇部位；（3）靶区的一级控矿构造为NE、SN和环形构造。线性构造异常分析结果显示，当线性构造的优益度大于40时，有较大的成矿可能性。     

New Reference Materials,Analytical Procedures and Data Reduction Strategies for Sr Isotope Measurements in Geological Materials by LA-MC-ICP-MS

Laser ablation multi-collector mass spectrometry (LA-MC-ICP-MS) has emerged as the technique of choice for in situ measurements of Sr isotopes in geological minerals. However, the method poses analytical challenges and there is no widely adopted standardised approach to collecting these data or correcting the numerous potential isobaric inferences. Here, we outline practical analytical procedures and data reduction strategies to help establish a consistent framework for collecting and correcting Sr isotope measurements in geological materials by LA-MC-ICP-MS. We characterise a new set of plagioclase reference materials, which are available for distribution to the community, and present a new data reduction scheme for the Iolite software package to correct isobaric interferences for different materials and analytical conditions. Our tests show that a combination of Kr-baseline subtraction, Rb-peak-stripping using βRb derived from a bracketing glass reference material, and a CaCa or CaAr correction for plagioclase and CaCa or CaAr + REE²⁺ correction for rock glasses, yields the most accurate and precise ⁸⁷Sr/⁸⁶Sr measurements for these materials. Using the analytical and correction procedures outlined herein, spot analyses using a beam diameter of 100 μm or rastering with a 50–65 μm diameter beam can readily achieve < 100 ppm 2SE repeatability ("internal") precision for ⁸⁷Sr/⁸⁶Sr measurements for materials with < 1000 μg g^-1 Sr.