Ages of Detrital Zircon from Siliciclastic Successions South of the São Francisco Craton, Brazil: Implications for the Evolution of Proterozoic Basins

In this work we report ²⁰⁷Pb/²⁰⁶Pb LA-ICPMS ages of 152 detrital zircons from lower greenschist facies quartzites from Proterozoic basin successions of the southern border of the São Francisco Craton, southern Minas Gerais State, Brazil. These are the intracratonic São João del Rei basin, the intraplate continental margin Andrelândia basin, and the Serra do Ouro Grosso sequence, developed on a crystalline basement older than 1.8 Ga, and deformed and metamorphosed during the Brasiliano Orogeny, ca. 0.59–0.50 Ga. The data constrain both the ages of the sources and the interval of sedimentation. The detrital zircons of the Serra do Ouro Grosso sequence were derived predominantly from the erosion of a Neoarchean crust, 2.5–2.8 Ga old, with only one grain showing a Paleoproterozoic age (2, 245±83 Ma) older than the Transamazonian event. Zircons extracted from a shelf quartzite of the lowermost sequence of the São João del Rei basin indicate derivation from the 1.8–2.2 Ga Transamazonian crust, with subordinate contribution from the 2.5–2.9 Ga Archean crust. The 1, 809±41 Ma age is interpreted as the maximum limit for sedimentation in this basin. The results confirm the regional correlation with the Espinhaço Rift successions. The zircons extracted from an autochthonous quartzite of the Andrelândia sequence yielded ages in the 1.0–2.2 Ga range, with a modal class at 1.2–1.3 Ga. Only two of the forty analyzed zircons yield Archean ages. The youngest zircon yields 1, 086±85 Ma. The zircons from the allochthonous quartzite yield ages between 1.0–2.7 Ga, with a modal class at 2.1–2.2 Ga. Only five of 45 analyzed grains yield Archean ages. The youngest zircon has an age of 1, 047±77 Ma. The results indicate that the detrital sediments deposited during the second marine flooding event of the Andrelândia sedimentation were mainly derived from the erosion of Mesoproterozoic and Paleoproterozic rocks. The 1, 047±77 Ma age is interpreted as the maximum depositional age for the described association.     

Systematic variations in xenocryst mineral composition at the province scale, Buffalo Hills kimberlites, Alberta, Canada

The Buffalo Hills kimberlites define a province of kimberlite magmatism occurring within and adjacent to Proterozoic crystalline basement termed the Buffalo Head Terrane in north-central Alberta, Canada. The kimberlites are distinguished by a diverse xenocryst suite and most contain some quantity of diamond. The xenocryst assemblage in the province is atypical for diamondiferous kimberlite, including an overall paucity of mantle indicator minerals and the near-absence of compositionally subcalcic peridotitic garnet (G10). The most diamond-rich bodies are distinguished by the presence of slightly subcalcic, chromium-rich garnet and the general absence of picroilmenite, with the majority forming a small cluster in the northwestern part of the province. Barren and near-barren pipes tend to occur to the south, with increasing proximity to the basement structure known as the Peace River Arch. Niobian picroilmenite, compositionally restricted low-to moderate-Cr peridotitic garnet, and megacrystal titanian pyrope occur in kimberlites closest to the arch. Major element data for clinopyroxene and trace element data for garnet from diamond-rich and diamond-poor kimberlites suggests that metasomatism of lithospheric peridotite within the diamond stability field may have caused destruction of diamond, and diamond source rocks proximal to the arch were the most affected.     

玲珑花岗岩斜长石的费氏台研究及岩体形成时代问题的初步探讨

作者在地质调查基础上,对取自玲珑花岗岩、包体和胶东群变质岩的25个有代表性的样品,用费氏台测定了113个斜长石颗粒。本文根据测定资料的统计分析,探讨了玲珑花岗岩斜长石成分、有序度、双晶类型、演化规律及其成因意义,并将玲珑花岗岩与金喆佑、舟桥三男（1972）建立的不同时代花岗岩模式进行了对比。上述研究的初步结果认为玲珑花岗岩属于元古代交代-深熔成因。     

早前寒武纪洋壳的地质记录及其板块构造意义

元古宙蛇绿岩的不断发现表明威尔逊旋回至少在早元古代已经明显起作用，部分太古宙绿岩带由不同的构造单元拼合而成，并发育不同于显生宙蛇绿岩的大洋壳岩石组合，地幔柱在早太古代构造演化过程中起重要作用，是板块构造的重要补充机制，地球早期热产量较高，可能是造成板块规模较小，洋壳较厚，板块运动速度较快的原因，并以缓倾角俯冲为特征。     

关于开展元古宙重大生命演化事件研究的建议

九十年代以来,随着生物地层学研究的深入发展,我国在元古宙生物演化的研究中,取得了一个接一个的重大进展。其中除了在南方扬子地区约600Ma晚震旦世地层中的许多发现（如庙河生物群、瓮安生物群和高家山生物群等）外,还在北方元古宙的地层中,特别是在约18亿年前的常州沟组中,不仅早已发现了迄今最古老的单细胞浮游真核生物,而且新发现了具细胞组织的宏观多细胞生物及有性生殖结构。所有这些不仅为进一步深入研究元古宙生命演化规律奠定了全新的基础,而且还表明对这一领域的研究我国具有十分优越的地质条件,因此,如果能进一步加大支持力度的话,在我国关于元古宙生物演化的研究和理论完全有可能在短期内取得全面突破,我国的科学家将可能在该研究领域取得更加重大的成果,为人类的科学事业做出更加重要的贡献。     

U–Pb zircon (TIMS and SIMS) and Sm–Nd whole-rock geochronology of the Gour Oumelalen granulitic basement, Hoggar massif, Tuareg shield, Algeria

Two major granulitic units are recognized in the Gour Oumelalen area. One of the units is composed partially of Archean gneisses (Red Gneiss complex) with U–Pb zircon SIMS and TIMS ages of approximately 2.7 Ga. Although they were formed from 3.0- to 3.2-Ga-old precursors, as indicated by Nd model ages, we find no evidence of any older history (≈3.5 Ga) as suggested by previous Pb–Pb ages. The other formation (Gour Oumelalen supergroup) is a metasedimentary sequence at least partly of Paleoproterozoic age, as indicated by zircon dates of a metavolcanic rock at approximately 2.2 Ga. A later magmatic event is recorded at approximately 1.9 Ga in both units and related to coeval granulite-facies metamorphism that affected both units. Nd model ages at approximately 2.0 Ga suggest an accretion of juvenile crust formation at that time. The existence of T_DM Nd model ages intermediate between 2.5 and 2.9 Ga could result from the mixing of 3.2 and 2.0-Ga-old material or may reflect separate events.     

福建永定地区下、中元古界变质岩基本特征及地层时代依据

闽西永定地区的中深变质岩系经调查主要岩石组合为变粒岩、片岩、变质火山岩及少量石英岩,变质程度达低角闪岩相,构造形变特征明显有别于区域内的震旦纪楼子坝群,获得同位素年龄1825—1628 Ma。据此认为,这套变质岩不属于原划定的楼子坝群,应分属于下、中元古界。对该地层所夹变质火山岩的形成环境,本文亦进行了讨论。     

扬子板块西缘早元古代俯冲体系的地球化学证据—有关变基性岩的微量元素地球化学研究

本文通过对云南早元古代哀牢山群、底巴都组与大红山群的地质地球化学研究，从地球化学角度证明早元古代扬子板块西缘存在板块运动，前特提斯域的元古大洋板块在其东北方向的俯冲作用中，与古扬子板块发生碰撞而在俯冲带大陆边缘弧与弧后盆地分别形成哀牢山群、底巴都组与大红山群的基性火山岩。     

秦-祁-昆造山带元古宙副变质岩层碎屑锆石年龄谱研究

根据秦-祁-昆造山带中北大河岩群、湟源岩群、化隆岩群、金水口岩群、陡岭岩群和秦岭岩群等副变质岩层碎屑锆石年龄谱资料,这些原定为古元古代的地层,至少包括测年样品在内的部分地层的时代应属中元古代或新元古代初期.研究资料还显示上述碎屑岩层具有大量中元古代早期的物源,而古元古代蚀源物质较匮乏.因此,上述副变质岩层显示出相近的碎屑锆石年龄谱特征.     

Discussion on Relationship Between Western Margin of Yangtze Block and Southwestern Sanjiang Region in Proterozoic

DISCUSSION ON RELATIONSHIP BETWEEN WESTERN MARGIN OF YANGTZE BLOCK AND SOUTHWESTERN SANJIANG REGION IN PROTEROZOIC