车排子油田南部火山岩地质时代及成因

Geological Age and Petrogenesis of Volcanic Rocks in Southern Chepaizi Oilfield

车排子油田南部石炭系主要为杏仁状玄武岩、玄武安山质集块岩、安山质角砾凝灰岩及少量玄武安山质角砾凝灰岩,为确定其地质时代,进行同位素和古生物样品分析,并与盆缘露头区石炭系对比。在C47井石炭系下部砂岩中获得丰富的孢粉化石,在C68井流纹质含角砾玻屑熔结凝灰岩中获得锆石U-Pb年龄为314.6±2.1 Ma,化石地质时代和同位素年龄均为晚石炭世,可与准噶尔盆地西北缘山前哈拉阿拉特组对比。火山岩总体为钙碱—拉斑系列,相对富Al_(2)O_(3),具有弱的Eu正异常,强烈亏损Nb、Ta、Hf等高场强元素,相对富集Ba、Rb、K等大离子亲石元素,有较明显的Nb-Ta槽,其岩浆源区可能是受流体交代的尖晶石二辉橄榄岩亏损地幔,形成于俯冲相关的岛弧构造环境。车排子油田南部井区大断裂西北盘石炭系总体与阿腊德依克赛组可对比,东南盘火山岩地层可与西北缘山前哈拉阿拉特组对比。

The Carboniferous in southern Chepaizi oilfield is mainly composed of amygdaloidal basalt,basaltic andesitic agglomerate,andesitic breccia tuff and a small amount of basaltic andesitic breccia tuff.In order to determine the geological age of these rocks,isotopic and biofossil analyses were carried out,and then the analytical results were compared with the Carboniferous rocks in the outcrop area in the basin margin.It is found that there are abundant sporopollen fossils in the bottom of the Carboniferous sandstone in Well C47,and the zircon U-Pb age of the rhyolitic breccia-bearing vitric tuff from Well C68 is 314.6±2.1 Ma,suggesting that both the geological age and isotopic age of the volcanic rock are Late Carboniferous,which are comparable to the Hala’alat formation in the piedmont at the northwestern margin of the Junggar basin.The volcanic rocks are generally calc-alkali-tholeiitic series and relatively rich in Al_(2)O_(3),with a weak positive Eu anomaly.Moreover,the volcanic rocks are strongly short of high field-strength elements such as Nb,Ta,and Hf,and relatively rich in large ion lithophile elements such as Ba,Rb,and K.There exists an obvious Nb-Ta trough,and the magma source area may be the depleted mantle of spinel peridotite because of metasomatism,and formed in the subduction-related island arc tectonic environment.For the well blocks in southern Chepaizi oilfield,the Carboniferous in the northwest wall of the large fault can be compared with the Aladeyikesai formation,and the volcanic rock formation in the southeast wall can be correlated with the Hala’alat formation in the piedmont of the northwest margin.