大兴安岭北段牙克石地区玄武质火山岩的元素、同位素地球化学特征及其地质意义
Element and isotope geochemistry of basaltic rocks in the Yakeshi area in the northern segment of the Da Hinggan Mountains, China and geological significance
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摘要: 牙克石地区出露一套早白垩世玄武质火山岩，其SiO2含量为52.81%~53.39%，K2O含量为1.86%~2.87%，岩性为玄武质粗面安山岩。富集大离子亲石元素Rb和Ba，高场强元素Nb和Ta亏损明显，Zr和Hf无明显异常，δEu为0.77~0.82。从同位素的特点看，(87Sr/86Sr)i变化于0.704762～0.704941之间，εNd(t)为2.00~2.54；在εNd(t)- (87Sr/86Sr)i图解上，样品投影点落入洋岛玄武岩(OIB)和美国盆岭省范围内。206Pb/204Pb为18.3288~18.4225，207Pb/204Pb为15.4566~15.4893，208Pb/204Pb为37.9401~38.0523；在207Pb/204Pb-206Pb/204Pb和208Pb/204Pb-206Pb/204Pb图解上，样品投影点都落在亏损洋中脊玄武岩地幔附近。综合考虑本区火山岩的地质、地球化学特点，认为其来源于被俯冲洋壳交代的岩石圈地幔。Abstract: A set of Early Cretaceous basaltic rocks is exposed in the Yakeshi area. They are basaltic trachyandesite in lithology, with the SiO2 content ranging between 52.81 and 53.39% and K2O ranging between 1.86 and 2.87%. They are enriched in the large ion lithophile elements Rb and Ba and strongly depleted in the high field strength elements Nb and Ta, with no pronounced Zr and Hf anomalies and δEu=0.77 to 0.82. In terms of the isotopic characteristics, the (87Sr/86Sr)i values have a variation of 0.704762-0.704941 and εNd(t) is 2.00-2.54. In the εNd(t)-(87Sr/86Sr)i diagram, all the samples fall in the field of oceanic island basalts (OIB) and the basin-range province of the United States. 206Pb/204Pb is 18.3288-18.4225, 207Pb/204Pb is 15.4566-15.4893, and 208Pb/204Pb is 37.9401-38.0523; in the diagrams of 207Pb/204Pb-206Pb/204Pb and 208Pb/204Pb-206Pb/204Pb, all samples plot near the depleted mid-ocean ridge basalt mantle (DMM). Judging from the geological and geochemical features of the volcanic rocks in the area, the authors conclude that they were derived from the lithosphere mantle metasomatized by subducted oceanic crust.
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