东昆仑大水沟东地区稀土矿化石英正长岩地球化学、年代学及Hf同位素特征

Geochemistry,geochronology and Hf isotopic characteristics of rare earth-bearing quartz syenite in eastern Dashuigou,East Kunlun

东昆仑中段大格勒地区稀有和稀土矿化碱性杂岩体的发现,打开了东昆仑稀有、稀土矿找矿突破口。本次含稀土矿化石英正长岩的发现,进一步拓展了该区稀土矿找矿空间,对该区战略性矿产找矿工作和科学研究具有重要意义。本文对大水沟东地区石英正长岩的年代学、地球化学和Hf同位素研究,揭示了稀土矿化石英正长岩岩石成因、岩浆源区和构造背景。获得岩石锆石U-Pb年龄为(400.2±3.7) Ma,属早泥盆世;岩石属于准铝质碱性系列,具有高SiO_(2)(64.11%~66.20%)、富Al_(2)O_(3)(15.98%~16.19%)、富碱(Na_(2)O+K_(2)O)含量(11.31%~11.85%),尤其富钾K_(2)O(7.24%~8.18%),低Ti_(2)O(0.38%~0.44%)、CaO(1.13%~1.86%)、MgO(0.13%~0.22%)特征。A/CNK值为0.9~0.98,富集Rb、K、Ba、Th等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素,10000Ga/Al值为2.84~3.4,Zr含量为(751.30~1 034.85)×10^(-6),(Zr+Nb+Ce+Y)含量为(881.49~1 210.16)×10^(-6),锆石饱和温度为905.76~931.41℃,高的Y/Nb值(1.87~2.45)和构造图解指示,石英正长岩为A_2型花岗岩,结合区域构造岩浆演化资料表明其形成于碰撞后伸展环境。锆石ε_(Hf)(t)值为-0.76~1.41,二阶段(t_(DM2))Hf模式年龄为1 435~1 303 Ma,表明稀土矿化石英正长岩来源于幔源高温玄武质岩浆底侵中元古代长英质地壳部分熔融的产物。

The discovery of the rare metal/rare earth-bearing alkaline complex in Dagele area,middle East Kunlun was a major breakthrough in the exploration of rare metal and rare earth deposits in East Kunlun.The subsequent discovery of rare earth-bearing quartz syenite in eastern Dashuigou further expanded the scope of exploration,which is of great significance to the strategic mineral exploration and scientific research in East Kunlun.This paper reports the geochronological,geochemical,and Hf isotope studies of the quartz syenite to reveal its petrogenesis,magma source area and tectonic setting.The quartz syenite has a mean zircon(Zr)U-Pb age of(400.2±3.7)Ma corresponding to the Early Devonian.It belongs to the quasi-aluminous alkaline series,characterized by high SiO_(2) content(64.11%66.20%),enriched Al_(2)O_(3)(15.98%16.19%)and alkali(Na_(2)O+K_(2)O,11.31%11.85%),especially K_(2)O(7.24%8.18%),and low Ti_(2)O(0.38%0.44%),CaO(1.13%1.86%),and MgO(0.13%0.22%)contents,with a A/CNK value of 0.90.98.It is enriched in large ion lithophile elements(Rb,K,Ba,Th),and depleted in high field strength elements(Nb,Ta,Ti,P).Results of 10000Ga/Al value(2.83.4),Zr content((751.301034.85)×10^(-6)),(Zr+Nb+Ce+Y)content((881.491210.16)×10^(-6)),Zr saturation temperature(905.76931.41℃),and Y/Nb ratio(1.872.45),along with tectonic discrimination diagrams,indicate the quartz syenite is A 2-type granitoid.Combined with regional tectonic and magmatic evolution data,it is believed the quartz syenite was formed in post-collision extension environment;while the Zr_(εHf)(t)value(-0.76-+1.41),and the second stage(t_(DM2))Hf model age(13031435 Ma)indicate the quartz syenite is derived from partial melting of Middle-Proterozoic felsic crust during mantle-derived,high-temperature basaltic magmatic underplating.