中条山上玉坡地区黑云母片岩成岩时代及成因

Geochemistry and Geochronology of Shangyupo Biotite Schist in the Zhongtiaoshan Mountains:Implications for Its Petrogenesis

中条山上玉坡背斜核部主要有北峪花岗岩、变富钾流纹岩及(角闪石/方柱石)黑云母片岩为主的变镁铁质杂岩出露.上玉坡的黑云母片岩类的归属及成因争论已久.空间上,上玉坡黑云母片岩类岩石及地球化学特征形成了一个以背斜核部为中心的环形的递变带,岩石的主要造岩矿物特征及K、Ca、Na、Rr、Sr、Ba受退变质韧性剪切带高盐度流体改造均发生系统变化,Ta及LREE也显示有明显变化.上玉坡变富钾流纹岩LA-MC-ICPMS测年获得207Pb/206Pb加权平均年龄为2164±8.9Ma,与绛县群竖井沟组富钾流纹岩(207Pb/206Pb加权平均年龄2161.3±1.5Ma)同时代产出.上玉坡产出的变富钾流纹岩-黑云母片岩与铜矿峪分布的绛县群双峰式火山建造岩性组合类似,推断上玉坡背斜核部产出的黑云母片岩产出于2.20～2.15Ga.使用在流体中保持相对稳定的高场强元素对图解,判断中条山黑云母片岩的原岩更可能为亚碱性镁铁质火成岩,并非前人认识的"超钾基性火山岩",综合中条山古元古代地质特征,该地区古元古代可能处于与俯冲活动有关的张性构造环境.

The primary compositions of the anticline core at Shan~,o3,upo are Beiyu meta-granitoid, meta-rhyolite and meta-mafic rocks. The age and petrogenesis of the biotite schist have been under debate for a very long time. The petrology features and geochemistry of Shangyupo amphibole/scapolite biotite schist are cricoid gradually distributed from the anticlinal core to its edge. The K, Ca, Na, Rr, Sr and Ba have been changed systematically by the high salinity fluid from the ductile shear belts, and Ta and LREE are also shown to have been dramatically changed in this event. LA-MC-ICPMS zircon U-Pb dating for the Shangyupo meta-rhyolite and the meta-ryholite of Jiangxian group yield the weighted mean 207 pb/206 pb age of 2 160. 5 ± 7. 8 Ma and 2 161.3 ±1. 5 Ma respectively. The Shangyupo meta-ryhorite and biotite schist may have been formed during the Jiangxian volcano event between 2. 20～2. 15 Ca, based on the chro- nology and comparison to related rocks. The HFSE pairs show that the biotite schist is a subalkaline mafic rocks instead of the alkaline mafic rocks as previously suggested. The Jiangxian event may represent a tensional event related to the subduction, based on the com- prehensive geological irdorrnation of Zhongtiaoshan area.