西秦岭新生代钾霞橄黄长岩的地球化学及其岩浆源区性质

Geochemistry of the Cenozoic Kamafugites from west Qinling and its constraint for the nature of magma source region.

西秦岭礼县地区新生代钾霞橄黄长岩具有贫SiO_2和Al_2O_3,富CaO、Mgo、TiO_2及K_2O+Na_2O的特征,矿物组合中除橄榄石、辉石外,普遍含有霞石、黄长石、白榴石和磁铁矿等矿物。火山岩的∑REE为97.82×10^(-6)～639.1×10^(-6)。岩石以强烈富集大离子亲石元素(LILE,如Rb、Ba、K、Sr)和高场强元素(HFSE,如Nb、Th、Ta、P、Zr、Hf等)为特征具有Pb和Ti的负异常。火山岩具有相对高的^(143)Nd/^(144)Nd比值(0.512007～0.512130,ε^(Nd)=+3.4～+5.8)和相对低的^(87)Sr/^(86)Sr比值(0.703374～0.704726)。^(206)Pb/^(204)Pb、^(207)Pb/^(204)Pb和^(208)Ph/^(204)Ph的比值范围分别为18.088～19.112,15.476～15.626和38.071～39.680。岩石的微量元素和Sr、Nd和Pb同位素特征显示其与洋岛玄武岩(OIB)的地球化学特征相似。源区组成显示了亏损地幔端元DM、HIMU端元和富集地幔端元EMII这三种地球化学端元混合的特征。

The Cenozoic kamafugites in Lixian County of West Qinling have lower SiO2 and Al2O3 contents and high CaO, MgO, TiO2 and K2O ＋ Na2O compositions. The modal mineral assemblage composed mainly of olivine, clinopyroxene, nepheline, melilite, leucite and magnetite. The total REE composition （ ∑ REE） ranges from 97.82 × 10^-6 to 639.1 × 10^-6. The rocks are enriched in LILE （such as Rb, Ba, K, and Sr） and high field strench elements （ HFSE, such as Nb, Th, Ta, P, Zr and Hf）, with negative Pb and Ti anomalies. The rocks have high ^143Nd/^144d ratios （0. 512007 -0. 512130,εNd = ＋3.4 - ＋5. 8） and lower Sr isotopic ratios （^87Sr/^86Sr =0.703374 -0. 704726）. The ^206Pb/^204Pb, ^207Pb/^204Pb, and ^208Pb/^204pb ratios are in the ranges of 18. 088 - 19. 112, 15. 476 - 15. 626, and 38. 071 -39. 680, respectively. These trace elements, and Sr-Nd-Pb isotopic compositions of the kamafugitic rocks show distinct geochemical features similar to those of OIB and suggest an evolvement of three geochemical end-members （ DM, HIMU, and EMⅡ） into the magma source region.