摘要
采用激光剥蚀-等离子质谱(LA-ICP-MS)分析技术,测定崆岭杂岩中斜长角闪岩包体的锆石U-Pb年龄和Hf同位素组成,以探讨黄陵结晶基底的形成及演化。崆岭杂岩主要由太古代TTG片麻岩和早元古代孔兹岩系组成,TTG片麻岩中存在少量斜长角闪岩包体。该包体中的锆石可分为岩浆结晶锆石、变质改造锆石和变质新生锆石三类。(1)第一类原生岩浆结晶锆石的U-Pb年龄为(3000±24)Ma,MSWD=2.4,代表斜长角闪岩的原岩-拉斑玄武岩的成岩时间,指示崆岭杂岩中以包体形式存在的斜长角闪岩(3.0Ga),是黄陵结晶基底和扬子克拉通中出露的最古老岩石。(2)第二类变质改造锆石的U-Pb年龄为(2715±9)Ma,MSWD=1.4,代表黄陵地区第Ⅰ期角闪岩相变质事件的时间。第Ⅰ期(2.75~2.7Ga)角闪岩相变质作用,使TTG花岗岩及其拉斑玄武质岩石包体,变质为TTG片麻岩及其斜长角闪岩包体。(3)第三类变质新生锆石的U-Pb年龄为(2558±40)Ma,MSWD=0.93,代表黄陵地区第Ⅱ期角闪岩相变质事件的时间。第Ⅱ期构造热事件(2.6~2.5Ga)与"水月寺运动"相关,造成黄陵地区太古代与元古代之间的不整合面。总之,黄陵地区第Ⅰ期和第Ⅱ期变质事件的共同作用,将黄陵花岗岩-绿岩型地体转变为晚太古代稳定陆块,并从此开始了长达500Ma的克拉通化。(4)斜长角闪岩包体中锆石的平均εHf(t)为-11.59^-3.98、平均亏损地幔模式年龄tDM2为3.4Ga,表明黄陵地区存在比崆岭群更古老(>3.2Ga)的地壳。
The Kongling complex, an important part of the basememt block for the Yangtze craton, is exposed in the northern part of the Huangling anticline in western Hubei province. It mainly consists of TTG gneisses, amphibolites, leptynites and khondalite series. So far, the oldest basement rocks recognized are the TTG gneisses with ages around 2.9 Ga in the Yangtze craton, but the amphibolites occur as enclaves in the TTG gneisses, implying the amphibolites are older than TI＇G gneisses. Zircon U-Pb ages and Lu-Hf isotopic compositions of amphibolites from the Kongling complex were analyzed by LA-ICP-MS. Accorinding to CL images and Th/U ratios, all zircons from amphibolite enclaves can be divided into three types： igneous, metamorphic alteration and metamorphic growth zircon. （ 1 ） The protolith ignous zircons show a core-mantle-rim structure. The cores have well-developed magmatic oscillatory zoning, and the mantles are thin and bright. The U-Pb spot analyses were done on cores, and the resultsform a discordia with the upper intercept at （ 3000 ± 24 ） Ma, MSWD=2.4. This age is interpreted as the eruption time of tholeiiticbasalt, which was the protolith of amphibolite enclaves. Therefore, the amphibolite enclaves are the oldest basement rocks in the Yangtze craton found to date. （ 2 ） The metamorphic alteration zircon grains still keep the same shape as their protolith igneous zircons, and show weak oscillatory zoning, sector zoning and patched zoning, which reveals that the metamorphic zircons were transformed from the igneous zircons. These zircon U-Pb analysis results form a discordia with the upper intercept at （ 2715 ＋ 9 ） Ma, MSWD--1.4, indicating the ignous zircons were transformed into metamorphic zircons at this time. Therefore, the first metamorphic event occm＇red at （ 2.75~2.7 ）Ga, and the TFG granites and tholeiitic basaltic rock enclaves were transformed into TTG gneisses and amphibolite enclaves during the first metamorphic event. （ 3 ） The metamorphic growth zircons have such internal structure as cloudy zoning, sector zoning, fire-tree zoning, planar zoning, indicating they were formed in amphibolite- facies. These zircon U-Pb analysis results form a discordia with the upper intercept at （ 2558 ± 40 ）Ma, MSWD=0.93. This age is interpreted as the timing of tectono-thermal event, related to the Shuiyuesi movement, and caused unconformable contact between Archean and Proterozoic. In conclusion, the granite-greenstone terrain was transformed to Neoarchean stable landmass by the first and second metamorphic events, and then the stage of cratonization began to last for 500 Ma. （ 4 ） The zircons have the negative εHf（t） average values of -11.59^-3.89, and the Hf model age of 3.4 Ga, iudicateing a new demonstration for the existence of 3.4 Ga crustal relicts, which is older than the Kongling Group （ 3.2 Ga ）.
出处
《高校地质学报》
CAS
CSCD
北大核心
2012年第4期589-600,共12页
Geological Journal of China Universities
基金
国家自然科学基金项目(40972050)资助
