Determination And Discussion On Origin Of The Shoshonite Rocks In Guangfeng And Yushan Basins

Guangfeng and Yushan basins are in the juncture between the Yangtze paleoplate andthe Cathaysian paleoplate，which are characterized by the red elastic rocks and maficvolcanic rocks.In Guangfeng basin，the volcanic-sedimentary rocks can divided into3lithostratigraphic units: Luotang group，Ganzhou group and Guifeng group，the volcanicrocks exist in the upper of Ganzhou group and Guifeng group in Yushan basin，thevolcanic-sedimentary rocks can be counted into Guifeng group，and the volcanic rocksexist in the upper group of Guifeng group.According to the SHRIMP U-Pb datingchronology，the zircon U-Pb dating is99±0.7Ma of the mafic volcanic rock of Ganzhougroup in Guangfeng basin，it belongs to the earliest Late Cretaceous. The zircon U-Pbdating is93±1Ma of the mafic volcanic rocks of Guifeng group in Yushan basin， itbelongs to the early Late Cretaceous.In Guangfeng and Yushan basins，the mafic volcanic rocks of Guifeng groupsubordinate to the shoshonite rocks by their the mineralogy and the major elementcharacteristics: they have porphyritic structure，the phenocrysts mainly are plagioclase，pyroxene，and also has olivine；the pyroxene mainly is clinopyroxene，but still hasorthopyroxene，the plagioclase has the K-feldspar shell.The hole rocks span a SiO₂rangeof49.8%⁵3.6%（the average is51.7%）；（Na₂O+K₂O）（=5.00⁶.94%，the average is6.35%）has a high content，and plotted within the field of alkaline series in the TASdiagram；（Fe₂O₃+FeO）（=7.10⁹.99%，the average is8.26%）；low MgO content（=2.30⁶.14%，the average is4.20%）；Fe₂O₃/FeO （=0.39¹2.87，the average is2.84） has a highratio，and belong to calc-alkaline series in the FAM diagram；high K₂O content（=2.40³.33%，the average is2.79%），K₂O/Na₂O （=0.68¹.08，the average is0.79） has a highratio，belonging to the shoshonite series field in SiO₂-K₂O diagram，and within the high-Kseries field in K₂O-Na₂O diagram；Al2O3（=15.8¹8.4%） has a high content and a widerange，TiO2（=1.41².07%，the average is1.69%） is relatively lower. The total amount of rare earth element （ΣREE=239³93×10-6）is relatively higher ofthe shoshonite rocks in Guangfeng and Yushan basins， which show rich LREE（（La/Sm）N=3.44⁴.55） obviously，and with no negative value of Eu （δEu=0.79⁰.97） andCe （δCe=0.89¹.06）.The lithophile element （Rb、Sr、Ba、Th） has enrichment among traceelement， but the Nd、 Ti element loss significantly，Nd/Th （=4.3⁸.2） and Ti/Yb（3088⁵444） has low ratios，which show the earth’s crust were added.There are no evidentcorrelated relationships in87Sr/86Sr（t）-SiO₂，εNd（t）-SiO₂，87Sr/86Sr（t）-1/Sr and εNd（t）-1/Nddiagrams，and all these appearances suggest that the magma has less contamination addedby the crust during rising process，but has the crustal contamination in the mantle sourcearea.The （87Sr/86Sr）ivalue of shoshonite rocks in Guangfeng and north Yushan basins is0.705856⁰.706352，εNd is from-0.07to1.9，n（206Pb）/n（204Pb） is from17.95to18.08，n（207Pb）/n（204Pb） is from15.46to15.54，n（208Pb）/n（204Pb） is from38.20to38.38；But the（87Sr/86Sr）ivalue of shoshonite rocks in east Yushan basin is0.703626⁰.703711，εNd isfrom3.40to3.86，n（206Pb）/n（204Pb） is from17.80to17.86，n（207Pb）/n（204Pb） is from15.20to15.24， n（208Pb）/n（204Pb） is from37.82to37.93.Among n（87Sr）/n（86Sr）-n（206Pb）/n（204Pb）、 n（143Nd）/n（144Nd）-n（206Pb）/n（204Pb） and n（143Nd）/n（144Nd）-n（87Sr）/n（86Sr） diagrams， the points are all between DMM and EM Ⅱ positions， becausen（87Sr）/n（86Sr） and n（206Pb）/n（204Pb） has positive correlation， n（143Nd）/n（144Nd） andn（206Pb）/n（204Pb） has negative correlation，the source mantle of the shoshonite rocksprobably has both DMM and EMⅡ magma，and higher proportion of DMM in the east ofYushan basin.Trace element and tectonic environment diagrams（Ce-Y、Zr/Al₂O₃-TiO₂/Al₂O₃、Ce/P₂O₅-Zr/TiO₂and Zr×3-Nd×50-Ce/P₂O₅）show that the shoshonite rocks generated inrear collision tectonic environment in Guangfeng and Yushan basins. During the EarlyJurassic，because of subduction of the ancient Pacific plate and southeast mainland，themantle enriched by fluid replacement in southern part of China.In Late Cretaceous，thesubduction angle of Pacific plate increased from a very low angle to a median angle，whichcaused delamination of thickened lower crust and asthenosphere upwelling under theextensional background.The shoshonite magama may be produced by the mixing ofmagma from asthenospheric-mantle and enriched lithospheric mantle. The differences ofmixing proportion and area （Guangfeng basin is suited in Guangfeng microplate andYushan basin is located in Huaiyu depression） caused the differences of the shoshoniterocks in Guangfeng and Yushan basin.