内蒙集宁地区孔兹岩系中大理岩的形成时代——长英质片麻岩中LA-ICP-MS锆石U-Pb测年的证据

研究区位于华北克拉通西部孔兹岩带东段的集宁一带,区内的孔兹岩系由下部的榴云片麻岩组合和上部的大理岩组合组成。在大理岩组合的底部发现了数层与薄层大理岩互层的长英质片麻岩,岩相学、锆石特征和原岩恢复指示该长英质片麻岩原岩为长石砂岩。对长英质片麻岩进行LA-ICP-MS锆石U-Pb测年分析,在70个锆石颗粒中获取了72组锆石数据。其中:碎屑锆石数据中年龄最小的为(2 091±20)Ma,可以代表长英质片麻岩原岩沉积时代的下限;19组集中在1.8～1.9Ga的变质锆石207Pb/206Pb表面年龄的加权平均值为(1 852±15)Ma(MSWD=0.7),该年龄值可以代表长英质片麻岩原岩沉积时代的上限。结合前人研究成果可以认为:集宁地区孔兹岩系中大理岩形成于(2 091±20)～(1 852±15)Ma的古元古代,并在(1 852±15)Ma遭受了区域变质作用,同时也进一步说明大理岩为孔兹岩系的重要组成部分;横贯西部陆块的孔兹岩带中,孔兹岩系原岩的源区曾存在大规模的花岗质岩浆活动,但是带内各个部位有着不尽相同的物源区;孔兹岩系不是由附近太古宙岩石提供碎屑物质的原地沉积,而是古元古代晚期构造作用叠置与附近太古宙岩石产在一起的外来构造岩片。     

华夏古陆古元古代变质火山岩的地球化学特征及其构造意义

根据元素地球化学特征,华夏古陆闽浙地区晚古元古代斜长角闪岩可以划分成两组。第１ 组斜长角闪岩的 Ｎｂ／ Ｙｂ比值较高, Ｌ Ｒ Ｅ Ｅ富集, 具有板内玄武岩的微量元素分布特征, 类似于过渡性和碱性玄武岩, 与偏碱性的变质酸性火山岩（变粒岩） 构成板内 “双峰”式火山岩; 第２ 组斜长角闪岩的 Ｎｂ／ Ｙｂ 比值较低, Ｌ Ｒ Ｅ Ｅ平坦或略亏损, 具有洋中脊玄武岩的微量元素分布特征, 类似于洋中脊拉斑玄武岩。这些变质火山岩形成的构造环境很可能类似于现代红海型的裂谷初始洋盆环境。     

滇中易门地区约1.85Ga凝灰岩的厘定及其地质意义

在滇中易门-罗茨断裂带以西前人划分的昆阳群美党组中发现了大量的中酸性凝灰岩。采用锆石激光剥蚀法(LAICP-MS)对易门县铜厂一带的英安质凝灰岩、石英晶屑凝灰岩进行了年龄测定,获得锆石U-Pb年龄加权平均值为1842±26Ma(MSWD=7.6,n=29)和1860±25Ma(MSWD=2.5,n=11)。2件样品的锆石Th/U值为0.2～1.0,且均具清晰的振荡环带结构,为岩浆成因锆石。该套凝灰岩产于一套厚度超过1000m的火山-细碎屑岩组合中,其下发育厚约800m的黑色板岩-碳酸盐岩组合,表明滇中易门地区存在厚度超过2000m的古元古界。此年龄值与全球性的Columbia超大陆事件一致,所以,在进行Columbia超大陆重建时需要充分考虑扬子陆块的位置及意义。     

High-pressure mafic granulites in the Trans-North China Orogen: Tectonic significance and age

High-pressure mafic granulites (including retrograded eclogites) have been reported from the Trans-North China Orogen, a Paleoproterozoic orogenic belt along which two discrete continental blocks, referred to as the Eastern and Western Blocks, were amalgamated to form the North China Craton. Extensive metamorphic investigations and geochronology carried out over the last few years provide important insights into the age and significance of these high-pressure granulites, which are critical in understanding of the timing and tectonic processes involved in the assembly of the North China Craton.Most high-pressure mafic granulites in the Trans-North China Orogen preserve the high-pressure granulite facies assemblage garnet + plagioclase + clinopyroxene + quartz, the medium-pressure granulite facies assemblage garnet + plagioclase + clinopyroxene + orthopyroxene ± quartz, the low-pressure granulite facies assemblage orthopyroxene + clinopyroxene + plagioclase ± quartz, and the amphibolite facies assemblage hornblende + plagioclase. Minor high-pressure granulites preserve the early eclogite facies mineral assemblage of garnet + quartz + omphacite pseudomorph (clinopyroxene + Na-rich plagioclase), indicating that they are retrograded eclogites. These mineral assemblages and their P–T estimates define a clockwise P–T path involving near-isothermal decompression and cooling following the peak high-pressure metamorphism, which suggests that they formed during continent–continent collision. Field mapping and geochronology indicate that the precursors of these high-pressure granulites were mafic dykes which were emplaced at 1915 Ma and underwent high-pressure granulite facies metamorphism at 1.85 Ga. Taken together, the high-pressure granulites in the Trans-North China are considered to have resulted from final collision between the Eastern and Western Blocks to form the North China Craton at 1.85 Ga, not at 2.5 Ga as recently proposed by some authors.     

Further evidence for 1.85 Ga metamorphism in the Central Zone of the North China Craton: SHRIMP U–Pb dating of zircon from metamorphic rocks in the Lushan area, Henan Province

This paper reports SHRIMP zircon U–Pb dating of Precambrian supracrustal and granitic rocks from the Lushan area, Henan Province, in the southern portion of the Central Zone (also referred to as the Trans-North China Orogen) of the North China Craton. A graphite–garnet–sillimanite gneiss (Sample TW0006/1) of the Shangtaihua ‘Group’ gives a range of inherited zircon ages from 2.73 to 2.26 Ga and a metamorphic zircon age of 1.84 ± 0.07 Ga. A garnet-bearing gneissic granitoid (Sample TWJ358/1), which is considered to intrude the Shangtaihua ‘Group’, gives a magmatic zircon age of 2.14 ± 0.02 Ga and a metamorphic zircon age of 1.87 Ga. The metamorphic zircon ages of 1.87–1.84 Ga obtained in this study indicate that an important tectonothermal event occurred at the end of the Paleoproterozoic in the Lushan area. This supports the southern continuation of a Central Zone in the North China Craton that workers have recently considered to result from continent–continent collision. It is also evident that the Shangtaihua ‘Group’ was formed during the Paleoproterozoic (between 2.26 and 2.14 Ga), and not during the Archean, as previously considered.     

辽吉古元古宙褶皱带构造分区与构造演化

辽吉古元古宙褶皱带是早前寒武纪时期的一条重要的褶皱造山带与成矿带。依据褶皱带在组成、结构、构造特点、成矿作用与变质作用和岩浆作用演化上的变化 ,识别出褶皱带的内部分带性。褶皱带由北部线性褶皱带和南部隆褶带构成 ,二者虽然具有诸多方面的差异 ,但却具有统一的区域构造演化并产生了统一的构造系统。文中系统讨论了褶皱带的结构、组成、构造特点与构造演化。重点讨论了褶皱带早期演化中形成的隆滑构造与岩浆核杂岩的特点 ,并对褶皱带研究与找矿工作中涉及到的一些关键问题进行了讨论。     

云南易门狮子山落雪组凝灰质白云岩锆石U-Pb年代学研究及其地质意义

落雪组作为云南易门狮子山铜矿区最主要的赋矿层位，前人对该地层沉积时代研究程度较低，缺乏确切的地层形成时代资料。本文对狮子山铜矿区落雪组一段的凝灰质白云岩进行锆石U-Pb定年，获得3组有效²⁰⁷Pb/²⁰⁶Pb年龄值，分别为：①1.7 Ga—2.0 Ga；②2.11 Ga—2.43 Ga；③2.5 Ga—3.02 Ga。其中，第一组锆石年龄代表成岩期后构造热液事件；第二组锆石自形程度较好，棱角分明，发育韵律环带，为岩浆结晶锆石，其加权平均年龄为2299 Ma±29 Ma （n=27，MSWD=8.1），指示落雪组一段凝灰质白云岩的沉积时代为古元古代；第三组锆石年龄为捕获的古老继承锆石年龄，锆石的微量元素w（Y）—w（U）、w（Y）—w（Ce）/w（Ce）^*、w（Y）—w（Yb）/w（Sm）岩性判别图解显示，该组锆石主要来自陆壳花岗质岩石中的S型花岗岩，暗示扬子板块西南缘古元古代—新太古代存在近源出露的花岗岩类岩石，存在较早的酸性成分的大陆地壳。通过区域地层岩性特征和地层沉积时代对比分析，研究认为狮子山铜矿区出露的落雪组应为滇中易门群罗洼垤组的延伸，同属古元古代的沉积建造。