甘肃龙首山成矿带青山堡岩体地球化学特征及成因意义

Geochemistry and Petrogenesis of Qingshanbao Pluton of Longshoushan Metallogenic Belt in Gansu

青山堡岩体位于阿拉善地块西南缘,龙首山隆起带与走廊过渡带交界处,产有少量淋积型铀矿床及矿化点,为揭示其成因意义,并初步探讨其构造环境,对青山堡岩体进行地球化学研究。所采样品经野外与显微镜下鉴定分别为肉红色中粗粒钾长花岗岩、灰白色中粗粒二长花岗岩、黑云母花岗闪长岩、石英二长岩。通过对样品进行主量元素和微量元素的测试分析发现; SiO_2质量分数变化范围为62. 79%～78. 51%,全碱(K_2O+Na_2O)含量除一个二长花岗岩样品外其他均大于7. 02%,钾长、二长花岗岩和花岗岩闪长岩里特曼指数为1. 01～2. 47,为钙碱性岩列,石英二长岩里特曼指数3. 39～3. 56,为碱性岩性;在AR-SiO_2图解上,大多样品点投影到钙碱性-碱性系列过渡区域;在微量元素组成上,石英二长岩、花岗闪长岩的稀土总量较高,为204. 0×10^(-6)～289. 5×10^(-6),而钾长花岗岩、二长花岗岩的稀土总量仅为41. 8×10^(-6)～85. 5×10^(-6),稀土元素球粒陨石标准曲线均均为右倾。样品微量元素均富集Th、U、Nb、Zr、Hf等大离子亲石元素(LILE)、稀土元素La、Ce、Nd等稀土元素、亏损Ba、Ta、Nb、Ti、P、Y等高场强元素(HFSE),暗示具有岛弧火山岩的特征。应用主、微量元素构造判别图解,并结合前人对该区构造-岩浆演化的研究,青山堡岩体形成于晚奥陶世北祁连洋岛弧与阿拉善地块碰撞,和之后早志留世造山带根部的岩石发生拆沉,引起造山带上不同块体的伸展、滑塌的构造环境。

The Qingshanbao rock mass is located at the southwestern edge of the Alxa Block,at the junction of the Longshoushan uplift zone corridor transition zone,and produces a small amount of uranium deposits and mineralization sites.This is to reveal the significance of the uranium deposit and to explore its tectonic setting,to conduct geochemical studies on Qingshanbao rock mass.The samples were identified by field and microscopy as The red medium coarse-grained long-grained granite,the gray-white medium coarse-grained monzonite granite,the biotite granodiorite,and the quartz monzonite.Through the analysis of the Major elements and trace elements in the sample,it was found that the SiO 2 mass fraction ranged from 62.79%to 78.51%,and the total alkali(K 2O+Na 2O)content was more than 7.02%except for one monzogranite sample.The Rittmann Index,K-feldspar granite and monzogranite granite diorite are 1.01~2.47 for calc-alkaline rocks,quartz monzonite 3.39~3.56 for alkaline lithology;In the AR-SiO 2 diagram,most of the sample points are projected onto the calcium alkaline-alkaline series transition region;In terms of trace element composition,the total rare earth content of quartz monzonite and granodiorite is higher than that of 204.0×10^-6~289.5×10^-6,but the total amount of rare earths of K-feldspar granite and monzonite granite is only 41.8×10^-6~85.5×10^-6,the standard curve of rare earth element chondrite are all right-dip.The trace elements in the sample are enriched with Th,U,Nb,Zr,Hf and other large ion lithophile elements(LILE)and rare earth elements such as La,Ce,Nd and other rare earth elements(such as La,Ce,Nd and other rare earth elements),lean Ba,Ta,Nb,Ti,P,and Y.The field strength element(HFSE)implies the characteristics of the island arc volcanic rock.Using the discriminant plots of major and trace elements,combined with previous research on tectonic-magmatic evolution of this area,Qingshanbao rock mass in the collision of the Late Ordovician North Qilian island arc with the Alxa block,and later the Silurian orogen Rocks with roots have undergone demolition,causing structural environments in which different blocks on the orogenic belt stretch and slide.