南岭早侏罗世稀有金属成矿作用研究——以闽西南大坪花岗斑岩为例

华南地区是我国重要的稀有金属矿产区,绝大部分具经济规模的稀有金属矿床均与高演化的富Li-F花岗岩有成因联系。大坪花岗斑岩位于南岭构造带最东缘福建省永定地区,与区内Nb-Ta矿床形成有关。该岩体SIMS和LA-ICP-MS锆石U-Pb定年结果分别为186.7±1.2Ma和190.7±1.1Ma,是华南少见的早侏罗世(200~180Ma)侵入的花岗岩侵入体,也是华南最早报道的早侏罗世稀有金属成矿事件。大坪花岗斑岩具高钾低镁、准铝质到弱过铝质的特征,属于高钾钙碱性花岗岩,并显示A型花岗岩的地球化学特征,如富硅(72.81%~76.44%)、高10000×Ga/Al比值、高FeO^T/MgO和高的Zr+Nb+Ce+Y含量、亏损高场强元素和Eu负异常明显等。全岩体系低的Zr/Hf、Nb/Ta比值,指示岩浆具有较高的分异演化程度,Nb 2 O 5和Ta 2 O 5均含量达到了花岗岩型稀有金属矿床的工业品位。花岗斑岩中锆石Hf-O同位素分析结果显示,其具有比较亏损的Hf同位素与比较均一的O同为素组成(εHf(t)=-2.4~3.4,δ^18 O=6.0‰~6.6‰)。结合微量元素地球化学特征,大坪花岗斑岩源区主要来源于软流圈地幔,并有约20%~30%壳源岩浆的加入,在成岩过程中发生了显著的分离结晶。晚期富氟的流体出熔并向上迁移可能对于Nb和Ta的再次富集与分异具有重要作用。大坪花岗斑岩与闽西南地区同时期的火山岩,如藩坑组双峰式火山岩,在空间上可与前人提出的“南岭山脉早侏罗世发育的东西向裂谷岩浆岩带(OIB型玄武岩、辉长岩和A型花岗岩组合)”相对应,是该裂谷带向东的延伸。

Early Jurassic rare metals mineralization in Nanling Region: A case of Daping granite porphyry in Southwest Fujian

South China is an important region for rare metals\producing in China.Most of the deposits in this region are related to the high-evolution Li-F granites.The Daping granite porphyry is situated in the east edge of Nanling Range in Fujian Province.In this study,SIMS and LA-ICP-MS zircon U-Pb dating results yield ages of 186.7±1.2Ma and 190.7±1.1Ma for the Daping granite porphyry,which is the first report of Early Jurassic(200~180Ma)magmatism and Nb-Ta mineralization in this area.Geochemically,they show metaluminous-peraluminous and display typical A-type granite characteristics,such as high SiO 2(72.8%~76.4%),K 2 O and Zr+Nb+Ce+Y contents,high 10000Ga/Al,FeO^T/MgO ratios and low Zr,Hf,Ba,Sr,Ti and Eu contents.The granite underwent high degree of crystallization differentiation,and it is enriched in Nb 2 O 5 and Ta 2 O 5,which achieved the industrial grades.In-situ zirconεHf(t)values range from-2.4 to 3.4,and their δ^18 O range from 6.0‰to 6.6‰.Based on regional geological data,it is proposed that the Daping granite porphyry is originated from the asthenosphere mantle,with 20%~30%of crustal materials.During the evolution of the granites with enriched rare metals,the contents of F,Rb and Li in the melts increased with the increasing degree of their magmatic evolution.We chose a drill hole(ZK001)as the object of study to understand the enrichment processes of the Nb and Ta in this granite.As the depth of drill hole decreases,the ratios of Nb/Ta,Zr/Hf and Th/U goes down,while the ratio of Rb/Sr goes up,and the concentrations of Nb and Ta gradually increase.The rocks with high Nb and Ta concentrations normally contain high F contents(up to 0.8%),with high ratios of Y/Ho which also indicates that strong interactions occurred between the fluid and the magma.Late fluorine-rich fluids exsolving from the magmatic melts may play an important role in the concentration and fractionation of the Nb and Ta.In combination with the E-W trending Early Jurassic rock association(OIB-type gabbro and basalt,A-type granite)in the Nanling Range,the Daping granite and Fankeng bimodal volcanics are located along the east extension of the E-W trending rift belt of whom a great dispute has been put forward about its formation.Some people believe that this belt is a product of post-orogenic continental after the Triassic collision of Indochina block and the South China block;while others argue that the subduction of the paleo-Pacific plate played an important role in the formation of this belt.This event just occurred at the time and place in the transition from the Tethys tectonic domain to the paleo-Pacific tectonic domain.Through this study,we can deepen our understanding of regional geological evolution and the formation of this rift belt and its vocalnic rock assemblage.