班公湖-怒江成矿带多龙矿集区北缘早白垩世究浅西Ⅰ型花岗岩的成因

Petrogenesis of the early Cretaceous Jiuqianxi Ⅰ-Type Granitic Pluton,Bangonghu-Nujiang metallogenic belt,northern Tibet

多龙矿集区北侧的究浅西铜金矿床与早白垩世究浅西岩体有关,该岩体主要岩性为石英二长斑岩,LA-ICP-MS方法测得石英二长斑岩锆石206 Pb/238 U年龄加权平均值为120±1 Ma(MSWD=1),属于早白垩世,与扎普-多不杂岩浆弧内的普让(116±2 Ma)、青草山(122±1 Ma)、多不扎(121.6±2.0Ma)以及波龙(119~120 Ma)等岩体侵位时间基本一致。该岩体含角闪石、黑云母、磁铁矿等富铁镁质矿物,无堇青石、白云母等富铝矿物,高SiO_2(63.66%~67.12%),高K_2O(43.60%~4.65%),总碱(K_2O+Na_2O)含量高(7.94%~8.96%),低Al_2O_3(1.83%~2.33%),中等CaO含量(2.94%~4.15%)的特征。里特曼指数(σ43)为2.71~3.31,A/CNK值为:0.84~0.93。在CIPW标准矿物计算中,出现了透辉石而未出现刚玉。在ACF图解上,样品落于I型花岗岩范围内,Th、Y与Rb,Pb与SiO_2含量正相关,P_2O_5与SiO_2含量负相关,表明究浅西石英二长岩为高钾钙碱性准铝质的I型花岗岩。稀土元素和微量元素配分曲线均位于羌塘南缘主要早白垩世花岗岩的范围内。球粒陨石配分曲线具有向右倾斜的"海鸥"形特征,轻稀土曲线较陡,重稀土曲线较平缓,具明显的Eu异常,基本无Ce异常。原始地幔标准化蛛网曲呈像右倾斜的起伏曲线。相对富集Rb、Th、U、K等大离子亲石元素,相对亏损Nb、Ta、Zr、Hf、P、Ti等高场强元素。具有较典型的岛弧岩浆岩的地球化学特征。结合区域地质演化综合分析认为,究浅西花岗岩为班公湖-怒江洋壳北向俯冲的背景下,在温度较高的条件下地壳物质发生部分熔融作用的产物。

The Jiuqianxi copper-gold deposit on the northern side of the Duolong ore concentration area has relationship with the Early Cretaceous Jiuqianxi rock mass, Quartz Monzonite Zircon is measured by LA- ICP-MS method which is mainly composed of Quartz Monzonite Zircon belongs to the Early Cretaceous, and a weighted mean 206 pb/2as U age of 120± 1 Ma （MSWD = 1） for Zircon, and the age of its emplacement is basically the same as PuRang （116±2 Ma） ,Grass Mountain （122±1 Ma） ,DuoBuza （121.6±2.0 Ma） and Duolong （119±120 Ma） in the arc magmatic rocks of ZaPu-DuoBuza. This type of rock body has characteristics of Fe ＆ Mg-rieh mineral （such as amphibole, biotite and magnetite etc. ） and Al-rich mineral （such as cordierite and white mica etc. ） ,high SiO2 （63.66% -67.12%） ,high K2O （43. 60% -4. 650/00）, high alkali （K2O＋Na2O） （7. 94%-8.96%）,lower Al2O3（1.83%-2.33%）,medium CaO （2.94-4.15%）. Rittman index （σ43） is 2. 71-3. 31, and the A/CNK value is 0. 84-0. 93. In the calculation of CIPW standard mineral,diopside is appeared, but corundum not appeared, the ACF diagram of sample is in the range of I type granite, Th and Y are positively correlated with the content of Rb, Pb is positively correlated with the content of SiO2, and P2O5 is negatively correlated with the content of SiO2, which indicate that the Jiuqianxi quartz Monzonite is I type granite with highly kalium ＆ calc-alkaline metaluminous. REE （Rare Earth Elements） and trace element distribution curves are all in the range of the main early cretaceous granite in the southern margin of Qiangtang. The REE distribution curves of chondrite show the type of ＂seagull＂ with right-dipping, steep LREE distribution curve, fiat HREE distribution curve, obvious Eu abnormity,no Ce abnormity. The original mantle normalized spider web appears as a fluctuating curve of right inclining. Relative rich in large-ion lithophile elements（LIL）, such as Rb, Th, U and K etc. , relative poor in high field strength elements （HFS）, such as Nb, Ta, Zr, Hf, P and Ti etc.. And indicating the typical geochemical characteristics of arc magmatic rocks. Combined with the regional geologic evolution analysis,it is concluded that Jiuqianxi granite is the product of partial melting of crustal materials under conditions of higher temperature,and in the background of northward subduction of Bangong lake-Nujiang oceanic crust.