科研之友
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摘要
The Yangba pluton, located in Bikou area, southern margin of Mianlue Suture,central China,were formed during middle Indosinian. The data of occurrence, petrography, geochemistry and zircon U-Pb chronology are reported, the source and geological significance of the pluton are discussed in this paper. Lithologically, this pluton consists mainly of granodiorite with mafic microgranular enclaves. Under microscope, acicular apatite in the mafic microgranular enclaves suggested quenching textures. Zircon LA-ICP-MS dating for the granodiorites yields an age of 215.4 +/- 8.3Ma, indicating they were formed during the late-orogenic or post-collisional stage( <= 242 +/- 21Ma) of south Qinling Mountain Belt. Geochemically, the granodiorites have the partial characteristics of adakites, with SiO2 >= 56%, high Al2O3 content( > 15%), Na2O > K2O, Mg-# (50.8 similar to 54.5) > 47, and enriched in LREE and LILE, Sr > 900 mu g/g, Sr/Y ratio(65 similar to 95) > 65, slightly negative Eu anomaly( delta Eu =0.84 similar to 0.89), depleted in HREE, Y and high field strength element(HFSE) (Y =9.51 similar to 14.5Wg, Yb = 0.74 similar to 1.20 mu g/g, Y/Yb = 11.12 similar to 15.10), obvious differenatiation between LREE and HREE( ( La/Yb)(N) =22.18 similar to 29.51). However, the geochemical features of high concentration of K2O(3.22% similar to 3.84%) and flatter HREE pattern are more similar to the C-type adakites from eastern China of Mesozoic, which imply they were originated from dehydration melting of thickened mafic lower continental crust( LCC), the feature of high Mg-# demonstrates that they were contaminated by mantle-sourced materials. The mafic microgranular enclaves are characterised by shoshonite with SiO2 <= 63%, sigma(4.54 similar to 6.18) > 3.3, high K2O content (4.22% similar to 6.04%), most of the samples' K2O/Na2O > 1; in the K2O - SiO2 diagram, all the samples poltted in the shoshonite area, enriched in LILE and LREE, with obvious Nb, Ta negative anomaly, these geochemical features indicated that the dioritic magmas were derived from an enriched mantle, which maybe once metasomatized by subduction fluids. The variation trends of major and trace element between enclaves and host rocks suggest a mixing and mingling process in their petrogenesis. On a whole, Integrated geological and geochemical data suggested the Yangba granodiorite and their mific microgranular enclaves were generated by mixing of enriched mantle-derived magma and thickened lower crustal felsic magma which was triggered by the former. Combined with previous studies of Qinling Mountain Belt, the Yangba granodiorites were likely to represent underplated mantle-derived magmas activities and delamination of lower crust during the late orogenic stage in Western Qinling area.
- 出版日期2005-5
- 单位西北大学
