松树沟元古宙蛇绿岩Nd、Sr、Pb同位素地球化学特征

松树沟蛇绿岩中镁铁质岩的Sm-Nd全岩等时年龄为1030±46Ma,∈Nd(t)=+5.7,模式年龄在1422-1271Ma之间,矿物的内部等时年龄为983±140Ma。全岩等时年龄为其形成上限,矿物内部等时年龄为其变质年龄,说明该蛇绿岩形成于中元古代中、晚期。镁铁质岩的∈Nd(t)在+4.2-+6.9之间,是DMM与EMI两个地幔端元的混合产物;206Pb/204Pb在18.06-18.66之间,207Pb/204Pb和208Pb/204Pb值较高,分别在15.55-15.60和37.59-38.38之间,207Pb/204Pb和208Pb/204Pb对206Pb/204Pb的关系表明,岩浆源于具DUPAL异常的源区,(207Pb/204Pb)i相对(208Pb/204Pb)i更偏离NHRL可能是变质流体作用的反映。∈Sr(t)值较高且变化大,可与蛇绿岩类比,87Sr/86Sr比值变化大,可能与海水蚀变作用有关。据此推测,松树沟蛇绿岩代表洋壳残片。

Nd, Sr AND Pb ISOTOPIC CHARACTERISTICS OF THE PROTEROZOIC OPHIOLITE FROM SONGSHUGOU AREA

The Sm-Nd whole rock isochron age of the mafic rocks in Songshugou ophiolite is 1030±46Ma,which represents the upper limit of the formational age.Their t_DM are about 1271-1422Ma.The mineral isochron age,being a metamorphic age,is 983±140Ma.All above evidences suggest that Songshugou ophiolite was formed in the mid late stage of the Mid-Proterozoic.The ∈_Nd(t) of mafic rocks are about +4.2 -+6.9 that suggests that these rocks were the mixing between DMM and EMII.Their ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and 208Pb/²⁰⁴Pb are 1806-1866,15.55-15.60 and 37.59-38.38,respectively.Pb isotopic geochemistry indicates that the magma source has DUPAL abnormal.(²⁰⁷Pb/²⁰⁴Pb)i ratios are more deviated from NHRL than (²⁰⁶Pb/²⁰⁴Pb)i ratios that may be resulted from metafluid.The higher values and variation in ∈ _Sr (t) are similar to those of ophiolite.The variation ⁸⁷Sr/⁸⁶Sr ratios may be related to the seawater alteration.Evidences above suggest that Songshugou ophiolite is a relic of an oceanic crust.