登封地区早前寒武纪地壳演化—地球化学和锆石SHRIMP U-Pb年代学制约

Early Precambrian Crustal Evolution in the Dengfeng Area,Henan Province(eastern China):Constraints from Geochemistry and SHRIMP U-Pb Zircon Dating

河南中部登封地区早前寒武纪基底主要由新太古代登封群、古元古代嵩山群和早前寒武纪花岗质岩石组成。登封群主要由斜长角闪岩、角闪变粒岩、黑云变粒岩、云母石英片岩及少量磁铁石英岩(BIF)组成,变质原岩主要为基性火山岩、中酸性火山岩和碎屑沉积岩。3个登封群变质酸性火山岩样品给出2.51～2.53Ga岩浆锆石年龄,存在2.61～2.69Ga残余锆石。它们高SiO2,tDM(Nd)和εNd(t)分别为2.52～2.79Ga和0.51～4.41。嵩山群主要由石英岩和片岩组成,经受绿片岩相变质。石英岩碎屑锆石年龄峰值为2.5Ga,与华北克拉通孔兹岩系变泥砂质岩石中存在大量2.1～2.3Ga碎屑锆石明显不同。石英岩中可靠的最年轻碎屑锆石年龄为2.45Ga,部分碎屑锆石年龄大于2.65Ga,最大达3.26Ga。会善寺奥长花岗岩、大塔寺英云闪长岩、路家沟钾质花岗岩和石秤二长花岗岩形成时代分别为2.55Ga、2.53Ga、2.51Ga和1.78Ga,会善寺奥长花岗岩中存在2621～2638Ma残余锆石,并有～2.51Ga变质增生边存在。花岗质岩石在元素地球化学组成上存在较大变化,但具有类似Nd同位素组成,tDM(Nd)为2.60～2.80Ga(钾质花岗岩样品除外)。根据研究,可得出如下结论和认识:①登封群形成于新太古代末期(2.51～2.53Ga),与前人认识一致。②不同类型花岗质岩石,与登封群表壳岩一道,形成于一个相对较小的时间范围(2.50～2.55Ga),结合岩石组合和地球化学组成特征,推测其形成与板底垫托作用有关。③在岩石组合和形成时代等方面,五台和箕山地区花岗绿岩带与登封地区的十分类似,可能为同一大型花岗绿岩带的不同部分。④可把嵩山群形成时代限制在2.0～2.45Ga之间,与五台地区高凡群(2.14～2.47Ga)或滹沱群(2.08～2.14Ga)对比。⑤石秤花岗岩是华北克拉通古元古代之后拉张构造体制下壳内岩浆作用的产物。

Early Precambrian basement in the Dengfeng area, central Henan Province, is composed mainly of the Neoarchaean Dengfeng Group, Paleoproterozoic Songshan Group （＂group＂ is used in the sense of regional map divisions, and does not necessarily have stratigraphic meaning） and Neoarchaean granitoids. The Dengfeng Group consists of amphibolite, fine-grained hornblende gneiss, fine-grained biotite gneiss and banded iron formation, and the protoliths are basaltic and mediate-acid volcanosedimentary rocks and sediments, forming a greenstone belt intruded by granitoidic rocks. SHRIMP U-Pb dating of zircons from 3 meta-acid volcanic rock samples yields magmatic ages of 2.53-2.51 Ga, with some inherited zircons of 2.69-2.61 Ga. They are high in SiO2, with tDM（Nd） and εNd（t） being 2.52-2.79 Ga and 0. 51-4.41, respectively. The Songshan Group mainly comprises quartzite and schist and underwent greenschist facies metamorphism. The quartzite contains detrital zircons with a distinct age peak of -2. 5 Ga, being very different from the meta-sediments of Paleoproterozoic khondalite series in the North China Craton, in which there are many detrital zircons of 2.3-2. 1 Ga. The youngest detected detrital zircon grain has a ^207Pb/^206Pb age of 2.45 Ga and some grains of 3. 26- 2. 65 Ga. The Huishanshi trondhjemite,Datashi tonlite, Lujiagou potassium granite and Shichen monzosyenite give intrusion ages of 2.55 Ga, 2.53 Ga, 2.51 Ga and 1.78 Ga, respectively. The Huishanshi trondhjemite contains some inherited zircons of 2.64 -2. 62 Ga and shows overgrowth rims of - 2. 51 Ga. All the granitoids are different in element composition to each other, but are similar in Nd isotope composition, with tDM（Nd） being 2.80-2.60 Ga （ excepting the potassium granite）. Based on the data of this study and others, following conclusions can be drawn： （1） the Dengfeng Group was formed at the end of Neoarchaean （2.53-2.51 Ga）, confirming the earlier studies） （2） different types of granitoids, together with the supracrustal rocks of the Dengfeng Group, were formed in a small time gap between 2.55 and 2.50 Ga. Combined with rock association and geochemical features, the supracrustal and graniticc rocks of the Dengfeng Group are considered to have formed in an extensional environment related to mafic underplating. （3） The Wutai and Jishan granitegreenstone belts are similar in rock association and formation age to the Dengfeng granite-greenstone belt, all probably belonging to one and the same huge belt. （4） The deposition of the Songshan Group can be limited between 2.45 and 2.0 Ga and can be compared with the Gaofan Group （2.47-2.14 Ga） or the Hutou Group （2. 14-2. 08 Ga）, both being distributed in the Wutai area, several hundreds kilometers north from Dengfeng. （5） The ca. 1.78 Ga Shichen monzosyenite is a product of intracrustal magmatism under an extensional tectonic regime following on from a compressive collisional orogeny earlier in the Paleoproterozoic in the North China Craton.