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《中国科学院地质与地球物理研究所二○○三学术论文汇编·第一卷(地球动力学)》 2003年
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Geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two Variscan S-type granites in the NW Bohemian massif

W.Siebel  M.Satir  
【摘要】:正Geochemical and Nd-Sr isotopic compositions and U-Pb zircon ages of two Variscan granites(Neunburg and Oberviechtach) from southern Oberpfalz,NW Bohemian massif,have been investigated in order to place constraints on their formation and on the crustal reworking. Both granites exhibit similar mineralogical,chemical and isotopic characteristics.They have peraluminous compositions(A/CNK ratios 1.2-1.3) and display high K_2O/Na_2O ratios of 2.2-2.3,consistent with typical S-type granites.In terms of trace elements,they show an enrichment of LREE and strong fractionation between LREE and HREE(La_N/Yb_N ratios 46 to 60).Compared with the primordial mantle,distinct negative anomalies of several trace elements(Ba,Sr,Nb and Ti) are also observed in both granites.They are further characterised by low initialε_(Nd)-values of -6.2 to -8.2 and high initial ~(87)Sr/~(86)Sr ratios of 0.7114 to 0.7147.Zircon U-Pb data indicate that the intrusion of both granites shortly postdates the HT-LP metamorphism of the Moldanubian basement and crystallised at about 320 Ma.The samples studied contain zircons mostly having xenocrystic cores with diverse morphologies.These inherited zircons have Early Proterozoic to Early Palaeozoic ages.This points to melting of sources comprising substantial sedimentary rocks.The La_N/Yb_N and Tb_N/Yb_N ratios of both granites are the highest so far reported from granitoids within this region.Melting of lower crustal rocks leaving garnet as a restite phase in the source provides a viable mechanism to reproduce the REE characteristics.

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