Sm Nd isotope age and its geological significance of epimetamorphic rock series in the Qinglong area, eastern Hebei province
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摘要: 冀东青龙地区出露的浅变质岩系主要由角闪黑云变粒岩、变质基性—酸性火山岩、浅粒岩、二云石英片岩等组成，夹变质砾岩和铁英岩。姚家沟附近变质砾岩上下的斜长角闪岩的Ｓｍ Ｎｄ等时年龄为（２６９６±１８５）Ｍａ，７个样品的ｔＤＭ为２８９０～２９５５Ｍａ。西汉沟具变余枕状构造的斜长角闪岩的Ｓｍ Ｎｄ等时线年龄为（２７９３±６９）Ｍａ，其平均模式年龄为２８８４Ｍａ。两套样品相对集中的模式年龄（２．９Ｇａ）代表这套火山岩从地幔分异出的时间，等时线年龄则代表火山岩系形成的时间。Abstract: The epimetamorphic rock series exposed in the Qinglong area, eastern Hebei province, consists mainly of hornblende biotite leptynite （granulitite）, metamorphosed basic acid volcanic rocks, leucoleptynite and quartz biotite muscovite schist. Seven samples of amphibolite collected from the upper and lower layers of the conglomerate in Yaojiagou village have been determined for the Sm Nd isotopic compositions, and give an isochron age of （2 696±185） Ma （2σ）, with I Nd = 0.909 23±16 （2σ） and ε Nd （t） = +1.8±1.5, and their model ages t DM range from 2 890 to 2 955 Ma. Ten samples of amphibolite collected from Xihangou village yield an isochron age of （ 2 793 ±69） Ma （2σ）, with I Nd = 0.509 194±67 （2σ） and ε Nd （t） = +2.0. An average model age of 2 844 Ma has been calculated from seven samples with similar model age in Xihangou village.The similar Nd isotopic composition and identical Nd model ages of the upper and lower amphibolites of the conglomerate in Yaojiagou village show that the Yaojiagou conglomerate may be a volcanic interlayer conglomerate or a structure conglomerate. So the upper and lower amphibolites in the conglomerate formed at the same time. The isochron ages of 2 696 Ma and （2 793±69） Ma of the amphibolite samples from Yaojiagou and Xihangou villages indicate that the epimetamorphic rock series in the Qinglong area belongs to the Archean.
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Keywords:
- Sm Nd isotopic age /
- Archean /
- metamorphic conglomerate /
- Qinglong area
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