华北克拉通中部古元古代高Mg低Ti-P镁铁质岩墙的富集地幔源区——锆石Hf同位素的制约

华北克拉通中部是古元古代岩墙群最为发育的地区。以镁铁质成分为主的岩墙群可以分为变质的和不变质的两类。不变质岩墙群是在1780～1760Ma之间大约20Ma时间内形成的,是华北克拉通古元古代最晚期铗铁质岩浆活动的产物。不变质岩墙又可以细分成高Mg-Ti低P、低Mg高Ti-P和高Mg低Ti-P等3组。本文报道了采自丰镇附近高Mg低Ti-P岩墙样品的锆石U-Pb年龄和Hf同位素组成分析结果。新获得的SHRIMP锆石U-Pb年龄为1769±4Ma,与其它不变质岩墙的锆石、斜锆石U-Pb年龄和~(40)At/~(39)Ar年龄的范围相同。高Mg低Ti-P岩墙的锆石ε_(Hf)(t)值的变化范围在-6.4～ 0.4之间,平均值为-2.2,略高于时代相近的花岗质岩石。由于镁铁质岩浆在上升、侵位和结晶过程中几乎没有受到地壳物质混染,因而锆石ε_(Hf)(t)值可以代表镁铁质岩浆的富集的岩石圈地幔源区的特征。文献资料显示,高Mg低Ti-P岩墙的岩石圈地幔源区的~(87)Sr/~(86)Sr初始比值为0.7040～0.7050,平均值为0.7046,ε_(Nd)(t)值为-5.6～-2.8,平均值为-4.4。     

湖南早元古代“仓溪岩群”及其划分

出露在湖南的早元古代变质岩,以前曾被误划入中元古代冷家溪群。究其岩性特征,确系一套“绿岩”组合。测得该“绿岩”类角闪片岩、斜长角闪片岩的Ｓｍ － Ｎｄ 等时线年龄为１ ８６０±１７ Ｍａ,因而创建“仓溪岩群”,辖“枫梓冲、斫木冲、水梁亭三个岩组”。“仓溪岩群”及其时代揭示,１ ８００ Ｍａ 前江南古岛弧业已存在。     

内蒙大青山地区黑云母花岗岩SHRIMP U-Pb定年及其构造意义

大青山黑云母花岗岩主要由钾长石（45%）、斜长石（15%~25%）、石英（25%）、黑云母（5%~10%）组成。主元素SiO₂含量69.48%~72.40%,K₂O+ Na₂O含量7.74%~9.58%,K₂O＞Na₂O,A/CNK为1.0~1.17,具过铝质-强过铝质花岗岩特征;稀土元素分馏较强,REE配分型式具中等负Eu异常;微量元素在原始地幔（PM）标准化蛛网图上显示Rb、Th、K、Nd相对富集,Nb、Sr、Ti相对亏损,岩石地球化学特征与碰撞花岗岩类似。本文对大青山地区料木山黑云母花岗岩进行高精度锆石SHRIMP U-Pb定年研究,其形成时间为2430±19Ma,可能代表鄂尔多斯陆块与阴山微陆块碰撞造山岩浆热事件年龄。区域上埃达克质岩、赞歧岩、Colcepet花岗岩同位素年龄均为2470~2330Ma, 预示着这一地区在古元古代早期可能已经出现与板块消减相关的陆-陆碰撞造山带。古元古代花岗岩的岩石地球化学和同位素年代学特征表明乌拉山-大青山麻粒岩相带是一条古元古代造山带。     

Hercynite–quartz-bearing granulites from Brejões Dome area, Jequié Block, Bahia, Brazil: Influence of charnockite intrusion on granulite facies metamorphism

In the present study, we describe and discuss the geology of aluminous–magnesian granulites and associated garnet-bearing charnockitic granulite from the Brejões Dome area, located in the Jequié Block, part of the São Francisco Craton in Bahia, Brazil. Investigation of metamorphic mineral assemblages allows the evaluation of P–T conditions for the formation of these rocks, and therefore to obtain constraints for the better understanding of the geological evolution of the area. We conclude that the rocks from the Brejões Dome area were formed under granulite facies conditions of low to intermediate pressure (5–8 kbar). Temperatures determined in samples of aluminous–magnesian granulites collected away from the dome are in the order of 850 °C, similar to those determined elsewhere in the southern part of the Itabuna–Salvador–Curaçá Orogen. However, samples of the same rock type collected close to the Brejões Dome are hercynite + quartz-bearing and record higher temperatures of about 900–1000 °C. It is suggested that the intrusion of the Brejões charnockite diapir was responsible for a local increase in temperature above the peak temperature of regional granulite metamorphism.     

Dimensional parameters of columnar stromatolites as a result of stromatolite ecosystem evolution

Columnar stromatolites representing more than a half of species described in Precambrian stromatolite assemblages reveal a regular trend of size variations during the Proterozoic and Early Paleozoic. Their dimensional parameters grew gradually during the Paleoproterozoic to attain peak values in the Early Riphean and to decline steadily afterward during the Middle-Late Riphean, Vendian, and Cambrian. Size variations are established based on statistically averaged maximum diameters of columns calculated for 230 taxa and on percentages of large, medium and small species occurring in successive units of stratigraphic scale. The units correspond to three Paleoproterozoic subdivisions (time span from 2.3 to 1.65 Ga) and to five subdivisions of the Riphean, Vendian and Early Paleozoic jointly spanning a comparable period of geologic time. The results of calculation depict a unimodal variation curve with one infliction point designating inversion of ascending and descending trends in the Early Riphean time. The inversion and cardinal changes in taxonomic composition of the entire stromatolite community across the Riphean lower boundary appear to be interrelated. Abiotic events, which certainly influenced diversity of all, especially columnar stromatolites, have no manifestation however in the size-variation curve lacking perceptible oscillations in both the ascending and descending branches. Consequently, dimension parameters of columnar stromatolites appear to be independent of direct influence of abiotic events.     

HYC (McArthur River) SEDEX deposit, Australia: First paleomagnetic results

Analysis of pyrrhotite-borne paleomagnetic data from four sites on each limb of a syncline through the supergiant HYC (McArthur River) SEDEX Zn–Pb deposit in Australia indicates that the mineralization is entirely postfolding, formed within 15 Myr after deposition at 1640 Ma of the host Barney Creek Formation, and thus is either late syndiagenetic or epigenetic in origin.     

Paleoproterozoic,High-Metamorphic,Metasedimentary Units of Siberian Craton

Abstract: Sensitive, high-resolution ion microprobe zircon U–Pb ages of Paleoproterozoic, high-grade, metasedimentary rocks from the south-western part of the Siberian Craton are reported. Early Precambrian, high-grade complexes, including garnet–biotite, hypersthene–biotite, and cordierite-bearing gneisses compose the Irkut terrane of the Sharyzhalgay Uplift. Protoliths of studied gneisses correspond to terrigenous sediments, ranging from greywacke to shale. The paragneiss model Nd ages of 2.4–3.1 Ga indicate Archean-to-Paleoproterozoic source provinces. Zircons from gneisses show core-rim textures in cathodoluminescence (CL) image. Round or irregular shaped cores indicate detrital origin. Structureless rims with low Th/U are metamorphic in origin. The three age groups of detrital cores are: ≥2.7, ~2.3, and 1.95–2 Ga. The ages of metamorphic rims range from 1.86 to 1.85 Ga; therefore, the sediments were deposited between 1.95 and 1.86 Ga and derived from Archean and Paleoproterozoic source rocks. It should be noted that Paleoproterozoic metasedimentary rocks of the Irkut Block are not unique. High-grade metaterrigenous sediments, with model Nd ages ranging from 2.3 to 2.5 Ga, are widely distributed within the Aldan and Anabar Shields of the Siberian Craton. The same situation is observed in the North China Craton, where metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 to 2.1 Ga (Wan et al., 2006). All of these sedimentary units were subjected to Late Paleoproterozoic metamorphism. In the Siberian Craton, the Paleoproterozoic sedimentary deposits are possibly marked passive margins of the Early Precambrian crustal blocks, and their high-grade metamorphism was related to the consolidation of the Siberian Craton.     

中条山地区古元古代变基性岩墙的地球化学特征、 LA-ICP-MS U-Pb年龄及其地质意义

华北克拉通南部中条山地区发育有大量古元古代基性岩墙,它们对于认识华北克拉通古元古代构造演化具有重要意义。本文通过对中条山北段绛县地区变质基性岩墙进行锆石LA-ICP-MS U-Pb测年分析,结果表明该基性岩墙的变质年龄为1847±14 Ma,代表本区一期重要的变质事件,其形成时代大致限定为1.85～2.14 Ga。地球化学数据显示,岩石的SiO_2含量为45.94%～51.69%,TiO_2(0.93%～1.09%)和K_2O (0.15%～0.46%)含量低, Fe_2O_3~T(11.18%～13.43%)含量高,属于拉斑玄武岩系列;REE总量变化于60.56×10~(-6)～77.26×10~(-6),LREE、HREE分异明显,大部分样品显示Eu正异常(δEu=0.99～1.29);微量元素总体富集Rb、Ba、La等大离子亲石元素,相对亏损Nb、Ta、Zr、Hf等高场强元素,指示其属于大陆拉斑玄武岩系列,源区具有富集地幔的特征,推测其可能是在拉张环境下被先前俯冲带流体交代的地幔物质部分熔融的结果。综上,中条山地区古元古代基性岩墙可能代表着华北板块在新太古代晚期克拉通化后的一次强烈的伸展事件,随后其经历了18.5 Ga左右强烈的变质作用。     

丹东地区古元古代晚期花岗岩对胶-辽-吉带造山作用的制约

华北克拉通古元古代胶-辽-吉带造山过程保存了丰富的变质-变形、岩浆-构造热事件与成矿作用记录,带内广泛存在的高压麻粒岩变质作用演化p-T-t轨迹呈顺时针,指示胶-辽-吉带经历了俯冲-碰撞-后碰撞折返伸展造山过程,但这时期的俯冲碰撞或陆-陆碰撞与之后伸展造山作用的时代目前仍有争议.本文对丹东地区大楼房古元古代花岗岩进行了...     

五台地区雁门关早元古代镁铁-超镁铁岩体的岩石地球化学特征、成因及其大地构造意义

本文对五台地区侵入于原北金岗库组的雁门关镁铁-超镁铁岩体进行了详细的岩石学、矿物学和岩石地球化学分析研究.雁门关岩体在矿物成份上,特别是单斜辉石、尖晶石和角闪石,具有阿拉斯加型侵入岩体和俯冲带或弧相关的岩浆特征,其元素地球化学特征也可与阿拉斯加型侵入杂岩体类比,具有明显的Nd (Ta),Zr (Hf),Y和Ti负异常.从构造环境判别图来看,雁门关岩体主要为与俯冲相关的成因,具大陆边缘弧的性质.因此,雁门关岩体是侵入在大陆边缘弧环境的岩浆经历了橄榄石和单斜辉石的分离结晶作用的产物.它与恒山-五台-阜平地区广泛分布的同时代花岗岩和火山岩一起,构成了华北克拉通中部造山带内一期重要的古元古代大陆边缘岛弧岩浆活动.