北秦岭晚三叠世关山岩体地球化学特征及成因机制

关山地区位于北秦岭与北祁连造山带的结合部位,目前有关关山岩体的岩石成因存在争议。锆石U-Pb定年结果表明关山岩体二长花岗岩的加权平均年龄为(236.3±4.0)Ma(样本数为6个,平均标准权重偏差为2.6)。地球化学特征显示岩石具有高Si(SiO₂ 含量(质量分数,下同)高于69%)、富碱(K₂O含量高于3.84%,Na₂O含量高于4.18%)、Mg^#值较高(50.4～51.0)的特征,轻稀土元素较富集,重稀土元素亏损且相对平坦,Eu呈微弱负异常(0.75～0.82),岩石富集Sr、Rb、Ba,亏损Y、Yb等,Sr/Y值为37.5～60.7,具有埃达克岩的性质。锆石Lu-Hf同位素(除去捕获锆石)分析结果表明,二长花岗岩5颗锆石具有负ε_<sub>Hf(t)值(-21.59～-4.49),对应的二阶段Hf模式年龄为1 538～2 622 Ma,1颗锆石为正ε_<sub>Hf(t)值(0.19),对应的二阶段Hf模式年龄为1 245 Ma,指示关山岩体来源于新元古代地壳物质。岩石具有高SiO₂、Al₂O₃含量,相比于地幔明显较低MgO以及Cr、Ni等元素含量,指示关山岩体来源于地壳物质的熔融,岩石较高Mg^#值说明其源区可能有地幔组分的加入。岩相学、地球化学、同位素特征和区域地质资料综合显示,关山岩体二长花岗岩形成于碰撞挤压环境,是下地壳增厚环境下脱水部分熔融的产物,其中伴随有地幔组分的加入。

Geochemical Characteristics and Forming Mechanism of Late Triassic Guanshan Pluton in North Qinling,China

Guanshan area is located in the junction of North Qinling and North Qilian orogenic belt, and it has a lot of controversy about the genesis of Guanshan pluton at present. The zircon U-Pb dating results show that the monzogranite of Guanshan pluton is formed in(236.3±4.0)Ma(the number of sample is 6, MSWD is 2.6). Late Triassic Guanshan monzogranite in North Qinling displays the characteristics of high Si(the content of SiO₂ is more than 69%)and alkali-rich(the content of K₂O is more than 3.84%, that of Na₂O is more than 4.18%), Mg^# is on the high side(50.4-51.0), LREE is enriched, HREE is depleted and flat, Eu has a non-obvious negative anomaly(0.75-0.82). The rock is enriched in Sr, Rb, Ba, and depleted in Y, Yb and other elements, the ratio of Sr/Y is 37.5-60.7, indicating that the rock has the propertieswith adakite. Zircon Lu-Hf isotope displays that 5 grains of the monzogranite have negative ε_Hf(t)(-21.59--4.49)with corresponding two-stage Hf pattern ages of 1 538-2 622 Ma, and 1 grain has positive ε_Hf(t)(0.19)with two-stage Hf pattern age of 1 245 Ma, indicating that Guanshan pluton is derived from the melting of Neoproterzoic crustal material. The rock has high SiO₂ and Al₂O₃ contents, low MgO, Cr and Ni contents, indicating the pluton derives from the melting of crustal material. Compared with the standard value of the crust, the higher Mg^# indicates the addition of the mantle component in the source of Guanshan pluton. The petrography, geochemistry, isotopic characteristics and regional geological data comprehensively show that the monzogranite of Guanshan pluton is formed under the tectonic setting of collisional compression, and the lower crust is thickened, dehydrated and melted with the addition of mantle components.