西秦岭夏河—合作地区德乌鲁杂岩体的侵位时代、岩石成因及构造意义

Geochronological and Geochemical Constraints on the Genesis and Tectonic Setting of Dewulu Intrusive Complex in Xiahe-Hezuo District of Western Qinling

德乌鲁杂岩体位于西秦岭造山带西段、夏河—合作断裂带北侧。该岩体由德乌鲁石英闪长岩和老豆石英闪长斑岩2个侵入单元组成,其中石英闪长岩含有大量暗色微粒包体。报道了石英闪长岩、石英闪长斑岩及石英闪长岩中暗色微粒包体的锆石U-Pb年龄及全岩元素地球化学和Sr-Nd同位素组成特征,以综合限定德乌鲁杂岩体的侵位时代、岩石成因,并讨论其构造意义。LA-ICP-MS锆石U-Pb定年结果表明,德乌鲁石英闪长岩和老豆石英闪长斑岩的侵位时代分别为(238.6±1.5)、(247.6±1.3)Ma,属于早—中三叠世岩浆活动。石英闪长岩、暗色微粒包体及石英闪长斑岩的SiO2质量分数分别为64.21%～66.22%、56.28%～58.18%和62.33%～64.58%,全碱含量较高,相对富钾,Na2O与K2O质量分数比值接近或大于1。石英闪长岩及其中暗色微粒包体为准铝质,石英闪长斑岩为准铝质—弱过铝质,但它们均属于中—高钾钙碱性系列,具有较高的Mg#值(61～70)。德乌鲁杂岩体的各单元岩石具有相似的微量元素及稀土元素组成,La与Yb质量分数球粒陨石标准化后的比值w(La)N/w(Yb)N为6.88～16.55,具有中等到弱的负Eu异常(0.43～0.79)。石英闪长岩、石英闪长斑岩及暗色微粒包体的Sr-Nd同位素组成非常相近,初始Sr同位素比值为0.707 3～0.707 8,εNd(t)为-6.7～-7.6。以上元素地球化学及Sr-Nd同位素组成表明:石英闪长岩和石英闪长斑岩岩浆均可能起源于富钾的基性下地壳的部分熔融,并有幔源岩浆加入;暗色微粒包体的地球化学特征与高镁安山岩相似,其源区可能为受板片流体/熔体交代的富集岩石圈地幔。结合区域地质背景,认为德乌鲁杂岩体属于印支早期活动大陆边缘古特提斯洋向北俯冲过程中形成的弧岩浆活动产物。

The Dewulu intrusive complex, which is located in the west part of western Qinling orogenic belt and north of Xiahe-Hezuo fault zone, is composed of two intrusive units includingDewulu quartz diorite and Laodou quartz diorite porphyry, and Dewulu quartz diorite contains a lot of mafic microgranular enclaves. Zircon U-Pb ages, whole-rock element geochemical and Sr- Nd isotopic composition of quartz diorite, quartz diorite porphyry and mafic microgranular enclave in quartz diorite were presented in order to constrain the emplacement age and petrogenesis of Dewulu intrusive complex, and the tectonic significance was discussed. The results from LA-ICP- MS Zircon U-Pb dating show that emplacement ages of Dewulu quartz diorite and Laodou quartz diorite porphyry are （238. 6 ±1.5） Ma and （247.6 ± 1.3） Ma respectively, constraining their formation in Early-Middle Triassic magmatism. Mass fractions of SiO2 are 64.21±66.22% for quartz diorite, 56.28%-58.18% for mafic microgranular enclave and 62.33%-64.58%0 for quartz diorite porphyry, total alkali content is high, K is relatively rich and the ratio of mass fractions of Na20 and K2O is close to or more than 1. Quartz diorite and mafic microgranular enclave are metaluminous, quartz diorite porphyry is metaluminous-weak peraluminous, but they are of medium-high K calc-alkaline series and have high Mg＂ values （61-70）. Compositions of trace element and rare earth element are similar for each unit of Dewulu intrusive complex, ratio of mass fractions of La and Yb with chondrite normalized （ω（La）N/ω（Yb）N） is 6. 88-16. 55, and Eu negative anomaly is 0. 43 0. 79. Sr-Nd isotopic compositions of quartz diorite, quartz diorite porphyry and mafic microgranular enclave are similar, initial Sr isotopic ratios are 0. 707 3-0. 707 8, values of eNd（t） are --6. 7---7. 6. The geochemical and Sr-Nd isotopic compositions indicate that quartz diorite and quartz diorite porphyry may be generated by partial melting of K-rich meta- basaltic rocks in the lower crust, with the addition of mantle-derived magmas; geochemical features of mafic microgranular enclave is similar with high-Mg andesite, and the source may be an enriched lithospheric mantle derived by the metasomatism of plate fluid or melting. Combined with regional geological setting, Dewulu intrusive complex is a product of arc magmatism formed in an active continental margin setting, which is related to the northward subduction of Paleo- Tethys ocean during Early Indosinian.