川西长征穹窿高分异花岗岩地球化学、锆石U-Pb定年、Lu-Hf同位素特征：对区域稀有金属成矿背景的限定

Geochemistry, zircon geochronology and Lu-Hf isotopic characteristics of highly fractionated granite from Changzheng dome in western Sichuan and their constraint on mineralization setting of rare metal deposit

新近在川西松潘-甘孜造山带东南缘长征穹窿发现一处花岗伟晶岩型铷铍矿产地。为阐明与成矿花岗伟晶岩具密切成因关系的花岗岩的特征及其成矿背景,在详细野外地质调查的基础上,采集穹窿核部出露的花岗岩样品开展了镜下鉴定、元素地球化学及锆石LA-(MC)-ICP-MSU-Pb和Lu-Hf同位素测试工作。结果表明,长征穹窿核部出露的两处花岗岩岩枝分别由二长花岗岩和正长花岗岩组成,岩石高硅、富铝、高钾、富碱、低钙、低Na/K比值,为高钾钙碱性岩石系列,A/CNK值均大于1.1,为过铝质花岗岩;岩石相对富集大离子亲石元素K、Rb和放射性元素U,而亏损Ba、Sr、Ti等微量元素;岩石稀土元素总量为30.9×10^-6~69.0×10^-6,LREE/HREE为4.2~8.0,轻稀土元素富集明显,δEu为0.12~0.57,具明显的负铕异常;锆石U-Pb定年结果分别为(202.2±1.3)Ma(MSWD=0.42,n=15)和(202.2±1.5)Ma(MSWD=1.18,n=9),长征穹窿核部花岗岩岩浆结晶时代为晚三叠世末期;锆石Lu-Hf同位素εHf(t)和TDM2分别为-15.42^-4.71和2.22~1.54Ga,花岗岩可能起源于下地壳。综合分析认为,长征穹窿核部花岗岩为典型的高分异花岗岩,源于下地壳的初始岩浆在晚三叠世末期上升侵位过程中混染了较多围岩成分而形成;区域伟晶岩至少存在3期成矿作用,稀有金属矿化为多期或多次岩浆作用的产物,预测区域可能存在一期194Ma左右的伟晶岩稀有金属成矿事件;区域与稀有金属成矿作用有关的花岗岩为多期或多次岩浆作用晚期的产物,形成于同碰撞构造向碰撞后伸展构造转换的相对稳定环境。

A granite-pegmatite-type rubidium-beryllium orefield has recently been discovered in the Changzheng dome on the southeastern margin of the Songpan- garzê orogenic belt in western Sichuan. In order to clarify the metallogenic geological background, the authors collected granite samples in the dome core which is closely related to the generation of mineralized granite pegmatite for microscopic identification, element geochemistry and zircon LA-(MC)- ICP-MS U-Pb and Lu-Hf isotope testing on the basis of detailed field geological survey. The results show that the two granites in the core of the Changzheng dome are syenogranite and monzonite, characterized by high silicon, aluminium, potassium, alkali, low calcium and low Na/K ratios. They belong to high potassium calc-alkaline rock series. the A/CNK ratios are all higher than 1.1, suggesting peraluminous granites. The rocks are relatively enriched in large ion lithophile elements K, Rb and radioactive elements U, but depleted in trace elements such as Ba, Sr and Ti. The ΣREE is 30.9×10^-6~69.0 ×10^-6, and LREE/HREE is 4.2~8.0, with obvious enrichment of light REE elements. The δEu is 0.12~0.57, with obvious negative Eu anomaly. The zircon U-Pb dating results are (202.2+1.3) Ma (MSWD=0.42, n=15) and (202.2+1.5) Ma (MSWD=1.18, n=9), suggesting that the magmatic crystallization of granite in the core of the Changzheng dome took place at the end of Late Triassic, and its zircon Lu-Hf isotope epsilon εHf(t) and TDM2 are -15.42^-4.71 and 2.22~1.54 Ga, respectively. Comprehensive analysis shows that the granite in the core of the Changzheng dome is a typical highly differentiated granite. The initial magma originated from the lower crust mixed with surrounding rock components in the process of ascending emplacement at the end of the Late Triassic period. There existed at least three stages of mineralization of pegmatite in the study area. The rare metals are the products of multiphase magmatism and the authors hold that there probably existed a pegmatite rare metal metallogenic event at ±194 Ma in the study area. The regional granites related to rare metal metallogenesis were formed in late-stage of multiphase magmatism and formed in relatively stable environments such as conversion process from syn-collisional to post-collisional extensional tectonic environment.