乌拉山-集宁孔兹岩锆石激光探针等离子质谱(LA-ICP-MS)年龄——孔兹岩沉积时限的年代学研究  

LA-ICP-MS U-Pb zircon ages of the khondalites from the Wulashan and Jining high-grade terrain in northern margin of the North China Craton:constraints on sedimentary age of the khondalite

作　　者：吴昌华[1] 孙敏[2] 李惠民[1] 赵国春[2] 夏小平[2] 

机构地区：[1]天津地质矿产研究所,天津300170 [2]香港大学地球科学系

出　　处：《岩石学报》2006年第11期2639-2654,共16页Acta Petrologica Sinica

基　　金：国家自然科学基金(No.49972065);香港 RGC(HKU 7048/03P;HKU 7055/03P 等)资助项目.

摘　　要：乌拉山-集宁地区由 TTG 片麻岩、麻粒岩、斜长角闪岩、孔兹岩等组成的高级变质地体,过去一直按有序地层划分,按变质程度定时代。现在识别出深成岩后,TTG 片麻岩已广泛采用锆石 U-Pb 测年,而表壳岩测年由于小于传统预想的地层年龄,所以往往仍按变质程度定时代:如只有中-新太古代年龄的麻粒岩(兴和岩群)被定为古太古代,只有古元古代年龄的孔兹岩被定成新太古代。尤其是孔兹岩,因为常规锆石(TIMS)年龄不能分辨碎屑年龄和变质年龄,所以其沉积时代是太古还是古元古更长期争论不决。为了解决这个问题,本文依据始于上世纪90年代的国内外区分岩浆锆石、变质锆石、碎屑锆石的研究成果,以锆石阴极发光(CL)和背散射(BSE)图象,首先查明孔兹岩除了含有均匀无内部结构的变质锆石还含有具结晶韵律环带结构的锆石,因为后一种结构是岩浆成因的有力证据,它产在沉积岩中就代表碎屑锆石。碎屑锆石有老有新,只有年龄最小的碎屑锆石才能限定沉积时代,所以沉积年龄必须以大量锆石测年统计。数以百计的锆石测年以 SHRIMP 进行固然最好,然而昂贵的费用本课题无力承担,所以我们改用灵敏度也较高,而且快速简便、费用低廉的激光探针(LA-ICP-MS)进行。结果得到:乌拉山大庙片麻状花岗岩的岩浆锆石年龄为2.49Ga,显示高级变质下原岩年龄仍能保存;乌拉山-集宁的孔兹岩变质锆石一致给出≈1.8Ga 的变质年龄,与已有的 TIMS 法≈1.85Ga 的变质年龄一致;乌拉山哈德门沟石墨厂、包白铁路桃儿湾、忽鸡沟窑子湾和集宁兴和黄土窑等四处孔兹岩给出的碎屑锆石年龄有老有新,最老的≈2.3Ga,多数谐和的碎屑锆石年龄在2.2～2.0Ga 之间,因为沉积时代以最年轻的碎屑锆石年龄限定,所以孔兹岩的沉积时代不是太古而是古元古。乌拉山-集宁地区的 TTG 片麻岩年龄比孔兹岩的碎�The Wulashan and Jining Complexes in northern margin of the North China Craton are high grade terrains that are composed predominantly of TTG gneisses, granulites, amphibolites and khondalites. Traditionally, all these rocks were considered as metamorphic supracrustals and their ages were arbitrarily assigned according to their metamorphic grade, i.e. the higher grade rocks were older than those lower grade rocks. Recently, the complex has been divided into metamorphosed plutons and supracrustals, but their ages remain controversial, with most workers believing that the plutons formed in the middle Archean, whereas the khondalites formed in the late Archean. To resolve this important issue, we applied the LA-ICP-MS U-Pb zircon dating technique, combined with CL images of zircons, to determine the ages of detrital and metamorphic zircons in the khondalites from the Wulashan and Jining Complexes. The results show that the metamorphic zircons of the khondalites from the Wulashan and Jining Complexes formed at - 1.8 Ga, consistent with the metamorphic ages of - 1.85 Ga obtained using the TIMS method. Detrital zircons in the khondalites from the Hadamengou, Taoerwan and Yaoziwan areas of the Wulashan Complex yielded LA-ICP-MS U-Pb zircon ages ranging from 2.0 to 2.3 Ga, and the detrital zircons in the khondalites from the Huangtuyao area of the jining Complex yielded the same age range. These ages demonstrate that the protoliths of khondalites from the Wulashan and Jining Complexes formed in the Paleoproterozoic, not in the Archean as previously considered, in addition, a kf-granite sample collected from the Damiao area of the Wulashan Complex yielded a LA-ICP-MS U-Pb zircon age of - 2.5 Ga, similar to those ages already obtained for the granulites and TTG gneisses in the Wulashan and Jining Complexes. Although these Archean granulites, TTG gneisses and granites are spatially associated with the khondalites, the former are much older in age than the latter, which suggests that the protoliths of the khondalites were no

关 键 词：孔兹岩 LA—ICP—MS年龄 太古宙-古元古代 乌拉山-集宁高级变质地体 

分 类 号：P597.3[天文地球—地球化学]