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    SHRIMP zircon U–Pb geochronology, elemental, and Nd isotopic geochemistry of the Neoproterozoic mafic dykes in the Yanbian area, SW China [查看] Wei-GuangZhua、HongZhong、Xian-HuaLi、Hai-LinDeng、De-FengHe、Kong-WenWu、Zhong-JieBai
    Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb zircon ages, geochemical and Nd isotopic data are reported for two groups of Neoproterozoic mafic dykes in the Yanbian area of Sichuan Province, SW China. The Group 1 and Group 2 dykes were dated at 792±13Ma and 761±14 Ma, respectively. While Group 1 and Group 2 rocks are commonly high in Fe2O3 and TiO2 contents with similar MgO contents and Mg numbers, they exhibit distinct trace element features. Group 1 rocks with high εNd(T) values (+5.8 to +7.2) and total rare earth element (REE) contents (109–171 ppm), are characterized by LREE-enriched and “humped” trace-element patterns with slightly negative Nb–Ta anomaly, similar to typical alkaline basalts associated with continental rifts.Group 2 rocks have relatively lowtotal REE contents (50.4–103 ppm) with relatively flat REE patterns and can be further divided into two sub-types. Group 2a rocks showMORB-like trace element patterns depleted in highly incompatible elements such as Th, Nb, Ta, La and Ce, whereas the Group 2b rocks exhibit flatter REE patterns than those of the Group 2a. The εNd(T) values of the Group 2a and Group 2b rocks are +5.4 to +7.2 and +4.5 to +5.3, respectively.The Group 1 and Group 2 dykes display different magma crystallization trends. Group 1 rocks were crystallized from a high-Ti alkaline basaltic magma that was generated by low degrees of partial melting of OIB-like mantle source in the garnet-bearing field. In contrast, the parental magma of Group 2 rocks is similar to those parental to low-Ti tholeiitic basaltic melt generated by relatively higher degrees of partial melting of the asthenospheric mantle source in the spinel-stable field. Group 1 and Group 2a rocks show the intraplate basalts affinity. It is therefore suggested that they originated from an intra-continental rift regime, which was likely related to a mantle plume underneath the supercontinent Rodinia.
    Shrimp U-Pb zircon geochronology, geochemistry, and Nd-Sr isotopic study of contrasting granites in the Emeishan large igneous province, SW China [查看] HongZhong、Wei-GuangZhu、Zhu-YinChu、De-FengHe、Xie-YanSong
    The Cida A-type granitic stock (∼4 km2) and Ailanghe I-type granite batholith (∼100 km2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261±4 Ma) was emplaced shortly before the Ailanghe granites (251±6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites.The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios (.εNd(.t)=−0.25 to +1.35 and (87Sr/86Sr)i=0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic.εNd(.t) ratios (−6.34 to −6.26) and high (87Sr/86Sr)i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida Atype granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe Itype granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks. In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal materialinvolved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.
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