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周庵超镁铁质岩体位于扬子板块北缘南秦岭地区,岩体主要由二辉橄榄岩组成.岩体中选出的锆石呈宽板状,具明显振荡环带和扇状环带,Th/U比值介于0.8～10.6之间,具有基性岩浆结晶锆石的特征.锆石206Pb/238U年龄平均值为637±4Ma(2,n=15),代表岩体的形成时代.锆石δ18O值介于5.2‰～7.0‰,平均值为5.8±0.4‰(1,n=33),与锆石的δ18O地幔值接近.锆石初始176Hf/177Hf(t)比值介于0.282410～0.282594之间,εHf(t)值在+1.3～+7.6之间,低于同期亏损地幔εHf(t)值(～+15),显示富集地幔特点,富集地幔源区的形成与俯冲洋壳物质交代岩石圈地幔有关.结合该地区新近发现的多个～635Ma镁铁-超镁铁质岩体以及同期大陆裂谷型耀岭河群双峰式火山岩,我们认为,周庵岩体是裂谷环境岩浆活动的产物,～635Ma的岩浆活动代表扬子板块北缘新元古代Rodinia超大陆裂解过程最晚期的产物.
The ultramafic intrusions in Zhou’an are located in the southern Qinling area on the northern margin of the Yangtze block and consist mainly of lherzolites.The zircons selected from the plutons are broadly plate-shaped with distinct oscillatory and fan-shaped annuli, Th / U ratio between 0.8 and 10.6, and has the characteristics of basic magmatic zircon: the 206Pb / 238U zircon ages are 637 ± 4Ma (2, n = 15) With a mean of 5.8 ± 0.4 ‰ (1, n = 33), which is close to δ18O mantle of zircons.The initial 176Hf / 177Hf (t) ratios of zircons range from 0.282410 to 0.282594 , the value of εHf (t) is between + 1.3 and + 7.6, which is lower than the εHf (t) value (~ + 15) of the depleted mantle over the same period, indicating the characteristics of enriched mantle, the formation of enriched mantle source area and the replacement of subducted oceanic crust materials Lithospheric mantle.Combined with the recent discovery of a number of ~ 635Ma mafic-ultramafic intrusions in the area and the bimodal volcanic rocks of the continental rift-type YaoLinghe group in the same period, we believe that the Zhou’An massif is a rift environment The magmatic activity of ~ 635 Ma, the product of magmatic activity, represents the latest product of the Neoproterozoic Rodinia supercontinental pyrolysis in the northern margin of the Yangtze block.