甘肃北山大山头南基性-超基性杂岩体SHRIMP锆石U-Pb定年及其地质意义

Zircon SHRIMP U-Pb age of Dashantounan basic-ultrabasic intrusion complex in the Beishan Mountain,Gansu Province

大山头南铜镍矿化基性-超基性杂岩体位于北山褶皱带东段,侵位于长城系古硐井群变质岩中。岩相分带明显,主要包括花岗闪长岩-闪长岩相、辉长岩-橄榄苏长辉长岩相和橄榄二辉岩-纯橄岩相。铜镍矿化主要赋存于橄榄二辉岩-纯橄岩相中。岩石化学特征显示K2O、Na2O、TiO2含量较低,Mg#值介于0.74~0.85之间,MgO/FeOT的值介于1.6~3.1之间,属于铁质镁铁-超镁铁岩;AFM图解显示该岩体具拉斑玄武岩的演化趋势;稀土元素总量较低,介于6.37×10-6~37.51×10-6之间;稀土元素标准化配分曲线表现为轻稀土元素稍富集的右倾型,LREE/HREE比值介于3.03~4.11之间,轻重稀土发生了明显的分馏;采用SHRIMP锆石U-Pb方法,获得杂岩体辉长岩中锆石U-Pb年龄为374.3±3 Ma（MSWD=0.92）,表明该岩体侵位于中晚泥盆世,与形成大型镍矿床的黑山岩体年龄一致,形成于洋壳俯冲阶段。该年龄的获得不仅对北山乃至中亚造山带地质构造演化具有重要意义,而且也为北山褶皱带东段铜镍找矿提供了新的方向。

The Dashantounan basic-ultrabasic intrusion hosting Ni-Cu sulfide deposit is located in the eastern part of the Beishan fold belt on the southern margin of the Central Asian Orogenic Belt （CAOB）. Detailed field geo- logical survey indicates that only Gudongjing Group of Changcheng System occurs in the south of the intrusion. The complex is composed mainly of granodiorite, diorite, gabbro, norite bronzitite, olivine norite-gabbro, olivine websterite, lherzolite, othopyroxenite peridotite, and dunite. Among them, lherzolite, othopyroxenite peridotite and dunite are closely related to the Cu-Ni mineralization. The major element, trace element and RF.E geochemistry indicates that the values of K20, Na20 and TiO2 are low; the values of Mg# are between 0.74 and 0.85, and MgO/FeOT ratios are between 1.6 and 3.1, suggesting mafic and ultramafic rocks. The values of REE are low, varying between 6.37×10^-6 and 37.51×10^-6. The chondrite normalized REE partition curve exhibits right-inclined REE patterns, implying that the intrusion is enriched in LREE. The ratios of LREE/HREE vary between 3.03 and 4.11, and the AFM diagram shows a trend of tholeiitic magmafic differ- entiation. Through the zircon SHRIMP U-Pb dating of gabbro, the authors obtained the intrusion age of 374.3 -＋ 3 Ma, similar to ages of Dashantou and Heishan intrusion, which suggests the emplacement took place in middle-late Devonian. The age has important practical significance for Cu-Ni mineral exploration in this area.