摘要
采用激光阶段加热^40Ar/^39Ar技术,对柴达木盆地北缘锡铁山榴辉岩退变质作用形成的榴闪岩和斜长角闪岩之角闪石进行了定年分析。09NQ44Amp来自榴闪岩,各阶段表观年龄（以现代空气氩^40Ar/^36Ar比值295.5扣除非放射性成因^40Ar）构成了单调下降的阶梯状年龄谱。在反等时线图解上,2~4阶段数据点和5~18阶段数据点分别构成了两条等时线,等时年龄分别为427.6±10Ma和425.1±2.6Ma,对应的初始40Ar/36Ar比值则分别为435.2±6.1和705.3±13。角闪石09NQ43Amp来自榴辉岩强烈退变质作用形成的斜长角闪岩,40Ar/39Ar阶段加热分析也获得单调下降的年龄谱,在反等时线图解上其数据点3~6阶段和7~16阶段分别构成了两条等时线,等时年龄分别为418.9±2.9Ma和418.1±2.1Ma,对应的初始40Ar/36Ar比值则分别为493.7±2.8和685.8±34.3。等时线截距值高于现代大气40Ar/36Ar比值,表明角闪石中含过剩40Ar。同时,由低温和中-高温阶段加热数据点分别构成两条等时年龄基本一致,截距值却明显不同的等时线,表明在角闪石热力学性质不同的源区,存在两期明显不同且未混合的初始捕获Ar组分。等时年龄425~418Ma代表的是锡铁山榴辉岩角闪岩相退变质作用发生的时间。等时线图解法虽然有效的校正了角闪石中的过剩40Ar,但仅根据表观年龄图谱和等时线图谱还无法清晰判断过剩40Ar在角闪石中的赋存状态,有待进一步探讨。
Two amphibole samples from the garnet-amphibolite and amphibolite were dated by the 4oAr/39Ar laser stepwise heating. The amphibole from the gamet-amphiholite 09NQ44Amp yielded a monotonic declining staircase-shaped age spectrum （by excluding non-radiogenic 40Ar with 4oAr/36Ar ratio of 295.5 of the modern atmosphere） , marked by anomalously old apparent ages（ to 6. 0Ga）at low temperature steps. The data points shows two linear arrays on the inverse isoehron diagram. The data of steps 2 -4 define an isochron with an age of 427.6 -＋ 10Ma and initial 4~Ar/36Ar ratio of 435.2 -＋6. 1. Those of steps 5 ~ 18 construct another isochron with an age of 425.1 -＋ 2. 6Ma and initial 4oArZ36Ar ratio of 705.3 -＋ 13. The amphibole from amphibolite （09NQ43Amp）yielded similar declining staircase-shaped age spectrum and isochron plot. Data points of steps 3 ~ 6 and 7 - 16 define two isochrons. The isochron of steps 3 - 6 corresponds to an age of 418. 9 -＋ 2. 9Ma and initial 4oAr/36 Ar ratio of 493.7 ＋ 2. 8, while the other one of steps 7 - 16 yields concordant isochron age of 418.1 -＋ 2. 1 Ma but with higher initial 4oAr/36 Ar ratio of 685.8 -＋ 34. 3. These initial 4o Ar/36 Ar ratios are much higher than that of the modem atmosphere, indicating the existence of excess 4~At in the amphibole samples. The obviously different initial ratios of the two isochrons of each amphibole sample reveal two argon reservoirs trapped inside, and concordant isochron ages probably indicate rapid exhumation happened to the Xitieshan eelogites at 425Ma and 418Ma.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2013年第9期3031-3038,共8页
Acta Petrologica Sinica
基金
荷兰皇家文理科学院基金(06CDP002)
中-荷联合培养博士生计划(ISK/3523/PhD)联合资助
