大兴安岭北段博克图地区晚石炭世花岗岩的成因:来自锆石U—Pb年龄、地球化学及Lu—Hf同位素证据

Petrogenesis of Late Carboniferous granites in Boketu region of northern Great Xing'an Range:evidence from zircon U—Pb age,geochemistry and Lu—Hf isotopes

大兴安岭北段博克图地区高吉山岩体的岩石类型为碱长花岗岩,LA—ICP—MS锆石U—Pb定年结果表明高吉山岩体形成于晚石炭世(～311 Ma)。地球化学数据显示碱长花岗岩具有高硅,富钾,贫铁、镁的特点,且轻重稀土元素分馏作用明显[(La/Yb)_N=5. 00～14. 19],相对富集大离子亲石元素(Rb、K),亏损高场强元素(Nb、Ta、Ti)及P元素等,具有负铕异常(Eu/Eu~*)=0. 47～0. 72,∑REE为49. 07×10^(-6)～139. 30×10^(-6),LREE/HREE=7. 93～13. 98。锆石Lu—Hf同位素特征显示碱长花岗岩具有正的ε_(Hf)(t)值(+8. 46～+12. 94),和较年轻的Hf二阶段模式年龄(t_(DM2)=501～783 Ma)。结合相关区域地质研究,认为大兴安岭北段晚石炭世高吉山岩体是新元古代从亏损地幔中新增生地壳物质的部分熔融的产物,可能与古亚洲洋的俯冲作用有关。

Gaojishan pluton in Boketu area of northern Great Xing’an Range is composed of alkali feldspar granites. LA-ICP-MS zircon U-Pb dating result shows that Gaojishan pluton formed in Late Carboniferous(~ 311 Ma). The geochemical data indicates that alkali feldspar granites are characterized by high SiO2 and K2O,but low total Fe2O3 and MgO,with significant fractionation of HREE relative to LREE [( La/Yb)N= 5. 00 ~ 14. 19],and they are enriched in large ion lithophile elements( e. g.,Rb,K) and depleted in high field-strength elements( e. g.,Nb,Ta,Ti) and P,with negative Eu anomalies( Eu/Eu*= 0. 47 ~ 0. 72),∑REE = 49. 07 × 10-6~139. 30 × 10-6 and LREE/HREE = 7. 93 ~ 13. 98. Zircon Lu-Hf isotope characteristics show that the alkali feldspar granites have positive εHf( t) values of + 8. 46 ^+ 12. 94,and young Hf two-stage model ages of 501 ~ 783 Ma. Combined with regional geological investigation,it is suggested that the Late Carboniferous Gaojishan pluton in northern Great Xing’an Range was product of the partial melting of newly accreted juvenile crustal material during Neoproterozoic,which possibly related to the subduction of the Paleo-Asian oceanic plate.