Archean–Ediacaran evolution of the Campos Gerais Domain — A reworked margin of the São Francisco paleocontinent (SE Brazil): Constraints from metamafic–ultramafic rocks

The Campos Gerais Domain (CGD) in southeastern Brazil is an approximately 180 km × 35 km area of Archean–Proterozoic rocks located southwest of the São Francisco Craton (SFC). The Archean–Paleoproterozoic evolution of the CGD — alongside its potential correlation with the SFC or other cratonic blocks in the region — is currently poorly-constrained. We present the results of systematic petrography, bulk-rock geochemistry, mineral chemistry and geochronology for a suite of scarcely studied mafic–ultramafic rocks from the CGD. We also provide a compilation of previously reported bulk-rock geochemical and spinel group mineral chemical data for mafic–ultramafic rocks throughout the CGD, and geochronological information for various lithotypes in the region. The CGD records a protracted Mesoarchean to Statherian (3.1–1.7 Ga) crustal evolution, which we interpret to share a common history with the southern SFC and their related reworked segments, suggesting that it is a westward extension of this cratonic terrain. The metavolcano-sedimentary rocks of the Fortaleza de Minas and Alpinópolis segments represent a Mesoarchean greenstone belt that is stratigraphically and chemically comparable to Archean greenstone belts worldwide, and that is broadly coeval with a local suite of tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites. U-Pb SHRIMP zircon data from a subalkaline metagabbro yielded a concordia age of ca. 2.96 Ga, revealing a previously unrecognized phase of Archean magmatism in the CGD that can be chrono-correlated with metakomatiite and TTG generation elsewhere in the São Francisco paleocontinent. Our data contradict a hypothesis whereby the metavolcano-sedimentary rocks of the Jacuí-Bom Jesus da Penha and Petúnia segments represent an ophiolite, as previously suggested, instead presenting features that point to formation in association with a continental arc. Coupled with a U-Pb (SHRIMP) crystallization age of ca. 2.13 Ga recorded by zircon grains from a metaultramafic rock, these data highlight that a magmatic event was chrono-correlated with the main accretionary phase of the Minas Orogeny, and with the Pouso Alegre/Amparo and São Vicente complexes. Finally, a U-Pb (SHRIMP) concordia age of ca. 590 Ma — obtained from metamorphic-textured zircon grains from a metaultramafic rock — points to a late metamorphic overprint related to upper amphibolite conditions, brittle fault activation and the juxtaposition of crustal blocks in association with the latest stages of western Gondwana’s assembly in the southern SFC, with later retrogression to greenschist-facies.     

华北板块南缘中段中上三叠统软沉积变形构造的发现及其地质意义

豫西地区中上三叠统主要出露在济源、义马、伊川和南召一带。印支期秦岭造山运动控制和影响着邻区中上三叠统盆地属性和沉积地层展布。通过观察研究区野外露头剖面,发现中上三叠统发育一系列典型的软沉积变形构造,包括液化底辟、枕状构造、液化柱、液化卷曲变形、负载、球-枕、火焰状构造、液化角砾岩。这些软沉积变形在垂向上表现出明显的复现性,在侧向上表现为沿地层连续展布,表明该时期研究区内地层受到频繁强烈的地震影响。通过对这些软沉积现象的识别和形成机制解释,判定其触发因素为秦岭造山过程中伴生的地震。区内地层中震积岩的存在即是对秦岭造山过程中构造活动的响应。     

Taimyr Peninsula( Arctic Siberia) in Late Precambrian:tectonic thrust sheets and geodynamic evolution

Structural relationships between the Neoproterozoic rock complexes of a continental massif,island arc and back-arc basin geodynamic affinities are described and considered in this work based on field observations within the northeastern segment of the Central Taimyr tectonic zone distinguished in the late Hercynian foldthrust belt of Taimyr Peninsula. As is established for the first time,rock complexes of the continental massif with the early Late Riphean( Tonian-Cryogenian) volcanogenic-sedimentary cover occur in the study region as the allochthonous syn- and post-sedimentary thrust sheets buried in or thrust over deposits of a back-arc basin,which accumulated in the terminal Late Riphean( Cryogenian)--initial Vendian( Ediacaran). These and other results of the large-scale structural observations elucidate important details of the region tectonic development in the Late Precambrian,when two lateral ensembles of the Neoproterozoic structures originated in the region. In the first half of the Neoproterozoic,the regional tectonic ensemble included the oceanic plate abut on the continental massif with a primitive volcano-plutonic belt. The subsequent system of an island arc and marginal backarc basin originated in the second half of the Neoproterozoic and existed approximately till the mid-Vendian( Ediacaran) phase of the intense formation of thrust sheets in the region.     

秦岭环斑花岗岩的年代学研究及其意义

本文对秦岭环斑花岗岩带进行了系统的测年工作,通过对实测Ｕ－Ｐｂ,Ｒｂ－Ｓｒ,＾４０Ａｒ／＾３９Ａｒ年龄测定结果研究,认为２１０－２１７Ｍａ是秦岭造山带主造山阶段结束的时间,为秦岭主造山期结束提供了确切的语气具有重要的科学意义。     

青藏高原碰撞造山带:Ⅰ.主碰撞造山成矿作用

大陆碰撞与成矿作用是当代成矿学研究的重要前沿。与板块构造成矿作用研究相比,大陆碰撞造山带的成矿作用研究则明显薄弱。文章以青藏高原主碰撞带为对象,研究了印度-亚洲大陆主碰撞过程与区域成矿作用的耦合关系,并初步建立了主碰撞造山成矿模型。研究表明,印度-亚洲大陆主碰撞始于65Ma,延续至41Ma,形成了以藏南前陆冲断带、冈底斯主碰撞构造-岩浆带和藏北陆内褶皱-逆冲带为特征的青藏高原碰撞造山带主体。伴随陆-陆碰撞,在冈底斯带相继发育①壳源白云母花岗岩-钾质钙碱性花岗岩组合（66-50Ma）、②＋εNd花岗岩-辉长岩组合（52-47Ma）和③幔源玄武质次火山岩-辉绿岩脉组合（42Ma）,以及大面积分布的巨厚（5000m）的林子宗火山岩系（65-43Ma）,反映深部相继发生大陆碰撞和板片陡深俯冲（65-52Ma）→板片断离（52-42Ma）→板片低角度俯冲（〈40Ma）等重要过程。在主碰撞期,初步识别出4个重要的成矿事件：①与壳源花岗岩有关的Sn、稀有金属成矿事件,在藏东滇西形成腾冲Sn、稀有金属矿集区;②与壳／幔花岗岩有关的Cu-AuMo成矿事件,在冈底斯南缘形成长达百余公里的Cu-Au矿化带;③与碰撞造山有关的剪切带型Au成矿事件,沿雅鲁藏布江缝合带分布,形成具有较大成矿潜力的A-u矿化带;④与挤压抬升有关的Cu-Au成矿事件,形成以雄村大型铜金矿为代表的斑岩型／浅成低温复合型Cu-Au矿床。在综合研究基础上,初步建立了大陆主碰撞造山区域成矿模型。     

华南早古生代陆内造山演化

华南早古生代造山带的构造属性长期缺乏统一的定论,且此次构造事件的形成机制与动力学背景研究较少。本文在前人研究成果的基础上,综合沉积环境、岩浆作用与岩浆来源、构造变形以及变质作用等地质证据,再次审视华南早古生代的构造演化过程。研究表明：1）华南广泛发育泥盆系与下伏前泥盆纪地层之间的角度不整合面,早古生代沉积相自江南区到华夏区呈过渡渐变,无同期火山碎屑岩、含鲍马序列的浊积岩等洋盆相关的记录;2）华南出露大规模弥散型面状分布的S型花岗岩,结晶年龄集中于443～423 Ma,在任何方向均无新老演化趋势,且岩浆主要来自壳源物质的熔融再循环,几乎无幔源物质加入;3）华南早古生代以低绿片岩相变质作用为主,仅陈蔡、龙游等地区零星出露绿片岩相—麻粒岩相变质岩;变质时间与岩浆结晶年龄相近,变质P-T-t轨迹均为近等温降压的顺时针轨迹;4）华南早古生代绝大部分构造线为近S-N与NE方向,变形作用以韧性流变为主,以武夷山瑞金—遂昌为轴呈双向逆冲推覆构造模式。根据以上地质事实,本文推断华南早古生代构造事件属性为陆内造山,即华南板块边缘汇聚碰撞产生的应力,经岩石圈远距离传播至大陆内部的远程响应。     

Controls on P–T–t deformation path from amphibole zonation during progressive metamorphism of basic rocks (estuary of the River Vilaine, South Brittany, France)

Abstract Edenite/tremolite and edenite/magnesio-hornblende in equilibrium with plagioclase, chlorite, epidote, quartz and vapour involve several types of reactions for which K _D can be related to T and P. Thermodynamic calculation of these equilibria leads to isopleth systems. Given knowledge of the progressive changes of end-member activities in zoned Ca–Mg amphiboles (based on microprobe analyses), it is possible to construct precise pressure–temperature–time paths ( P–T–t paths) which have been followed by metabasites during polyphase metamorphism. When applied to basic rocks from the River Vilaine area, this method allows us to construct a P–T–t path that can be compared directly to the P–T–t path constructed from interbedded acid rocks (aluminous micaschists) in the same structural unit. Through time, both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the L–S fabric system of Flinn. This allows us to construct a P–T–t deformation path for this structural unit.
These paths are interpreted in terms of an under/overthrusting continental collision belt (the Hercynian belt), and represent an illustration of the time delay caused by stacking of more than two crustal units.     

Fluid inclusion evidence for basement decompression during Permo-Triassic extension, SE New England, USA

Abstract CO₂-bearing fluid inclusions in strongly lineated but weakly foliated late Precambrian gneisses within the Hope Valley Shear zone of Connecticut and Rhode Island are of mixed composition ( X _co2± 0.1; 7 wt% NaCl equivalent) and variable density (0.59–0.86 g/ml) and occur mainly as isolated inclusions. Also present are dilute (3 wt% NaCl equivalent) aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism. Isochores for dense isolated CO₂-bearing inclusions indicate pressures of 7.5–9 kbar at 500–600° C, the estimated temperature conditions of peak metamorphism. Published ⁴⁰Ar/³⁹Ar hornblende plateau age spectra indicate cooling through about 500° C at 265 ± 5 Ma. Isochores for low-density CO₂-bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism (325–400° C) and indicate pressures of 3–4 kbar. Published ⁴⁰Ar/³⁹Ar biotite plateau ages indicate cooling through about 300° C at 250 ± 5 Ma. These data define a P–T uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0.4 and 3.3 mm/year in Permian time. Exhumation of basement gneisses was coeval with normal (west-down) motion along the regional basement–cover contact (Honey Hill–Lake Char–Willimantic fault system), and is interpreted as due to post-orogenic extensional collapse of the Alleghanian orogeny.