鞍山地区营城子古太古代片麻岩杂岩的识别与锆石U-Pb年代学研究

Identification and zircon U-Pb geochronology of the Yingchengzi Paleoarchean gneiss complex, Anshan area.

鞍山地区分布有始太古代——新太古代花岗岩杂岩,是研究早期地壳物质组成及其演化的经典地区之一。通过大比例尺岩性调查工作,新近在营城子识别出一套片麻岩杂岩和奥长花岗岩组合,为本区古老岩系的对比、构造格局恢复、太古代地壳形成与演化研究提供了新的依据和线索。片麻岩杂岩主要由条带状黑云斜长片麻岩、(脉状)奥长花岗质片麻岩以及黑云母片岩组成,呈不同规模的包体产出于细粒、均匀块状的奥长花岗岩之中。片麻岩杂岩的岩石组合以及复杂多变、明显不均一的岩石组构特征表明具有深熔混合岩的成因特点。SIMS锆石U-Pb定年表明,片麻岩杂岩中条带状黑云斜长片麻岩和黑云母片岩形成年龄分别为3312±14Ma、3304±7Ma和3324±7Ma～3235±14Ma,明确反映古太古代热事件,此外个别样品中存在3.68～3.60Ga和～3.47Ga继承锆石。细粒黑云奥长花岗岩的年龄为3.14～3.13Ga,与全区中太古代岩浆热事件一致。区域对比分析表明:营城子片麻岩杂岩的岩石组合、产状关系和年代学特征与东山杂岩和深沟寺杂岩十分相似,为古太古代(3.33～3.24Ga)深熔作用的产物。营城子片麻岩杂岩是鞍山地区另一个保留有古老锆石信息和太古宙早期地壳岩石的地质体。

Eoarchean-Neoarchean granitite complexes are widely distributed in the Anshan area, which is a classical place for studying early crustal material composition and evolution. We newly identified a suit of the Yingchengzi gneiss complex and trondhjemite assemblages through large-scale field lithology survey work. This provides new evidences and clues for comparing regional ancient rock series, recovering the tectonic structures and revealing the Archean crustal formation and evolution in the Anshan area. The gneiss complex mainly consists of stromatic biotite plagioclase gneiss, vein trondhjemitic gneiss and biotite schist, which occur as xenolithes in different sizes in the trondhjemite with fine texture and homogenous massive structure. The rock assemblage, complicated and heterogeneous rock fabric of the gneiss complex show characteristics of an anatectic migmatite. SIMS zircon U-Pb dating results indicate that the stromatic biotite plagioclase gneiss and biotite schist have the ages of 3312±14Ma, 3304±7Ma and 3324±7Ma～3235±14Ma respectively, indicating an Paleoarchean thermal event. In addition, several samples contain inherited zircons with ages of 3.68～3.60Ga and～3.47Ga age. The fine grained biotite trondhjemite formed 3.14～3.13Ga, which is consistent with the regional Mesoarchean magmatic events. Regional comparative study reveals that the rock assemblage, occurrence relationship and chronological characteristics of the Yingchengzi gneiss complex are similar with the Dongshan complex and Shengouzi complex, all of which were the products of the Paleoarchean （3.33～3.24Ga） anatexis. The Yingchengzi gneiss complex in the Anshan area is another geologic body which preserved information of ancient zircons and Early Archean crustal rocks.