柴北缘阿木尼克山滩间山群火山岩地球化学特征及地质意义

柴达木盆地北缘阿木尼克山北坡滩间山群火山岩呈带状出露,广泛分布。本文通过对该套火山岩岩石组合、地球化学、锆石测年等研究,查明其岩性以流纹岩、英安质晶屑凝灰岩为主,间有碳酸盐岩、碎屑岩夹层。火山岩相包括溢流相、爆发沉积相。火山岩w(SiO2)=61.22%~83.88%,属中酸性岩范畴;里特曼指数(δ)均小于3.3,属于钙碱性岩,且A/CNK均小于或等于1.1。火山岩大离子亲石元素K、Rb、Ba及强不相容元素Th富集,高场强元素Ta、Nb亏损明显。中酸性火山岩稀土总量w(ΣREE)值介于75.58×10^-6~174.10×10^-6,w(LREE)/w(HREE)值介于7.09~13.31间,轻稀土明显富集,重稀土亏损,轻重稀土分馏程度中等到强烈;稀土分配曲线右倾,重稀土部分相对平坦。该区火山岩地球化学特征与赛什腾山地区吻合度很高,说明具有相似的大地构造属性,判断为岛弧环境。英安质晶屑凝灰岩和流纹岩两件样品206Pb/238U年龄分别是464.5Ma±8.5Ma、510Ma±15Ma,分别对应早-中奥陶世、早-中寒武世;通过综合研判认为柴达木盆地北缘地区早古生代秦祁昆多岛洋主域裂解离散阶段可能在早-中寒武世已经开始,该区滩间山群火山岩为洋壳俯冲的产物,发生的大规模的洋-陆俯冲发生在奥陶纪。

Geochemical characteristics and geological significance of volcanic rocks in the Tanjianshan Formation of the Amunik mountains in the northern margin of the Qaidam Basin

The volcanic rocks of the Tanjianshan group on the northern slope of the Amunik Mountain in the northern margin of the Qaidam Basin are exposed in bands and widely distributed.Study on the volcanic rock combination,geochemistry and zircon dating shows that the lithology is mainly rhyolite and dacitic crystal tuff then intercalated carbonate and clastic rocks.The volcanic facies includes the overflow phase and the explosive sedimentary facies with w(SiO2)=61.22%~83.88% and the Litman index (δ) less than 3.3 and A/CNK less than or equal to 1.1 belonging to calc-alkaline rock.The intermediate-acid volcanic rock is rich in LILE of K、Rb、Ba and the strong incompatible element of Th and the high field strength elements of Ta and Nb evidently depleted.The w(ΣREE)of the intermediate-acid volcanic rocks is between 75.58 to 174.10,w(LREE)/w(HREE)7.09 to 13.31,the light rare earth obviously enriched,the heavy rare earth depleted thus fractionation of the light and heavy rare earth moderate to strong.The rare earth distribution curve is right-dip and the heavy rare earth portion is relatively flat.The geochemical characteristics of the volcanic rocks are highly consistent with the Sashten Mountain area indicating their similar geotectonic arc environment,i.e.island arc.The 206Pb/238U ages of the two samples of dacitic crystall tuff and rhyolite are 464.5±8.5 Ma and 510±15 Ma,respectively,corresponding to the Early-Middle Ordovician and Early-Middle Cambrian Epoch.According to the mentioned above it is inferred that the splitting and dispersing stage of the main oceanic domain of Qinqikundao Islands in the northern margin of Qaidam Basin may have begun in the Early-Middle Cambrian Epoch and the volcanic rocks of the Tanjianshan group in this area are the products of subduction of oceanic crust;and that the large-scale ocean-land subduction occurred in the Ordovician Epoch.