Field Relationships,Geochemistry,Zircon Ages and Evolution of a Late Archaean to Palaeoproterozoic Lower Crustal Section in the Hengshan Terrain of Northern China

The Hengshan complex forms part of the central zone of the North China Craton and consists predominantly of ductilely-deformed late Archaean to Palaeoproterozoic high-grade, partly migmatitic, granitoid orthogneisses, intruded by mafic dykes of gabbroic composition. Many highly strained rocks were previously misinterpreted as supracrustal sequences and represent mylonitized granitoids and sheared dykes. Our single zircon dating documents magmatic granitoid emplacement ages between 2.52 Ga and 2.48 Ga, with rare occurrences of 2.7 Ga gneisses, possibly reflecting an older basement. A few granitic gneisses have emplacement ages between 2.35 and 2.1 Ga and show the same structural features as the older rocks, indicating that the main deformation occurred after -2.1 Ga. Intrusion of gabbroic dykes occurred at -1920 Ma, and all Hengshan rocks underwent granulite-facies metamorphism at 1.88-1.85 Ga, followed by retrogression, sheafing and uplift. We interpret the Hengshan and adjacent Fuping granitoid gneisses as the lower, plutonic, part of a late Archaean to early Palaeoproterozoic Japan-type magmatic arc, with the upper, volcanic part represented by the nearby Wutai complex. Components of this arc may have evolved at a continental margin as indicated by the 2.7 Ga zircons. Major deformation and HP metamorphism occurred in the late Palaeoproterozoic during the Luliang orogeny when the Eastern and Western blocks of the North China Craton collided to form the Trans-North China orogen. Shear zones in the Hengshan are interpreted as major lower crustal discontinuities post-dating the peak of HP metamorphism, and we suggest that they formed during orogenic collapse and uplift of the Hengshan complex in the late Palaeoproterozoic （〈1.85 Ga）.

Stagnant lids and mantle overturns:Implications for Archaean tectonics, magmagenesis,crustal growth, mantle evolution, and the start of plate tectonics

The lower plate is the dominant agent in modern convergent margins characterized by active subduction,as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight.This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle.As geological and geochemical data seem inconsistent with the existence of modernstyle ridges and arcs in the Archaean,a periodically-destabilized stagnant-lid crust system is proposed instead.Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle,perturbing Earth's heat generation/loss balance,eventually triggering mantle overturns.Archaean basalts were derived from fertile mantle in overturn upwelling zones(OUZOs),which were larger and longer-lived than post-Archaean plumes.Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods,allowing basal crustal cannibalism,garnetiferous crustal restite delamination,and coupled development of continental crust and sub-continental lithospheric mantle.Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB(mid-ocean ridge basalt)mantle.Only after the start of true subduction did sequestration of subducted slabs at the coremantle boundary lead to the development of the depleted MORB mantle source.During Archaean mantle overturns,pre-existing continents located above OUZOs would be strongly reworked;whereas OUZOdistal continents would drift in response to mantle currents.The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion,imbrication,subcretion and anatexis of unsubductable oceanic lithosphere.As Earth cooled and the background oceanic lithosphere became denser and stiffer,there would be an increasing probability that oceanic crustal segments could founder in an organized way,producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga.Plate tectonics today is constituted of:(1)a continental drift system that started in the Early Archaean,driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons;(2)a subduction-driven system that started near the end of the Archaean.

Extensive reworking of Archaean crust within the Birimian terrane in Ghana as revealed by combined zircon U-Pb and Lu-Hf isotopes

Located in the southeastern parts of the BaouléMossi domain of the West African Craton,the Birimian terrane in Ghana is known to consist of predominantly juvenile crust emplaced during the Eburnean orogeny.Here we present novel coupled zircon U-Pb and Lu-Hf isotope data from nine Birimian granitoids intruding different domains from northwestern,western and southeastern Ghana.Sub-chondriticε_(Hf)-values indicating reworking of Archaean crust are recorded in zircon from both northwestern and southeastern Ghana.As a conservative estimate 71%of all analysed zircon grains spanning in age from2220 Ma to 2130 Ma require contribution from a reworked Archaean source,contradicting the common belief that the Birimian terrane consist of predominantly juvenile crust.A minimumε_(Hf)-value of-10.5 at2139 Ma suggests a Palaeoarchaean to late Mseoarchaean component as the contributing ancient source.Combined with previously reported heavy zirconδ^(18)O signatures from Birimian river zircon in Ghana,our new data suggests reworking of Archaean aged sediments during subduction initiated crustal growth in the vicinity of one,or possibly in between two Archaean aged cratons.

Archaean Greenstone Belts in China

Chinese Achaean greenstone belts are mainly distributed along the northern and southwestern margins of the North China platform. In terms of their geological characteristics, the greenstone belts in China are comparable to those in other countries but at the same time have unique features of their own. In view of their geochemistry, the Chinese greenstone belts may be grouped into three types: the Jiapigou type, Qingyuan type and Xiaoqinling type. The greenstone belts were formed possibly in a rift-type palaeo-tectonic setting, similar to that of the modern island are-continental margin mobile belts.

Two Types of Archaean Grey Gneiss in North China and Their Geological Significance

Grey gneisses are tonalitic intrusive rocks that have Archaean characteristics and tectonic significance. Archaean grey gneisses in North China may be classified into two types: the Zunhua type and the Heagshan type. Grey gneisses of the Zunhua type alternate with basic granulites, forming a bimodal migmatitic suite. The rock is characterized by complex mineral composition, highly varied REE patterns and positive Eu anomaly and originated from autochthonous or partly autochthonous deep-level migmatization with a high degree (30%±) of fusion. As the sub-stratum in the root of the early-stage high-grade greenstone belt, they prevented basic rocks from sinking. Grey gneisses of the Hengshan type are characterized by a relatively distinct intrusion form, large-amplitude upward emplacement, rather simple mineral composition, only slight/changes of REE patterns and absence of Eu anomaly. The rock is a product of subduction and underthrusting of the high-grade greenstone belt and then fusion and emplacement with a low degree (15%±) of fusion. In this paper an attempt is made to furnish important information about palaeoplate tectonics and early crustal evolution by means of a study of grey gneisses.

The Dabieshan Coesite-bearing Eclogite Terrain-A Late Archaean Ultra - high- Pressure Metamorphic Belt

A U -Pb zircon age of 2774±24 Ma for eclogite from the Bixiling rock body of Anhui Province, central China, indicates that the Dabieshan coesite-bearing eclogite was probably formed in the Late Archaean. A phengite Ar-Ar isochron age of 662±13 Ma for the eclogite confines also an upper limit age of its subsequent retrograde metamorphism in the Precambrian. The results of isotopic dating for such type of eclogite coincide with the geological features of its restricted occurrence within the Archaean metamaorphic terrain composed of the Dabie Group. It is believed that the Dabieshan coesite-bearing eclogite terrain might be a Late Archaean ultra-high-pressure metamorphic belt. The Dabie Mountains area was the eastward extension of the southern Qinling structural belt during the Triassic. Both the Dabie Group and the coesite-bearing eclogite hosted therein underwent a late-stage dynamic metamorphic event. The present authors have obtained a muscovite Ar-Ar isochron age of 192.6±2.8 Ma from plagioclase gneiss and a hornblende Ar-Ar plateau age of 230.7±4.6 Ma for the low amphibolite in eclogite respectively, which represent the Indosinian reworking ages of the original metamorphic rocks of the Dabie Group gneiss and coesite-bearing eclogite.

Highly refractory Archaean peridotite cumulates:Petrology and geochemistry of the Seqi Ultramafic Complex,SW Greenland

This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km^2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 ± 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite(s.s.),peridotite(s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by refractory dunite, with highly forsteritic olivine with core compositions having Mg# ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth's mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust.Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian(Mg# ~ 80), near-anhydrous magma, as olivinedominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting(>40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle(SCLM) in this region,which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra-depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth's first continents and the corresponding SCLM.

Chronoframe of Archaean in Anshan-Benxi Area,Liaoning Province,China

The Anshan- Benxi area is one of the important places of the Precambrian in the North China craton, in which a lot of old geological bodies were found, including the Baijiafen mylontized granitic gneiss of 3.8 Ga ,the Chentaigou gneissic granite of 33 Ga and the supracrustal rocks and associated granites of relatively younger ages .Based on the age data and other geological information obtained in recent yeais, mis paper establishes fourfold-divison scheme of the Archaean in the Anshan-Benxi area .The time boundaries are 2.5,2.8,3.2 and 3.6 Ga, corresponding to those recommended by the International Sub-commission on Precambrian Stratigraphy . This paper also discusses some questions related to the crustal evolution of the early history, and indicates that it is possible to find out more residual crustal materials of older than 2.8 Ga.

Exposed Cross-section of the Archaean Lower Crustin the Shanxi-Hebei-Inner Mongolia Border Region:Problems and Prospects

The Archaean lower crust represented by granulite facies rocks, which is rare in China, is found to be exposed in the Shanxi-Hebei-Inner Mongolia border region. Studies of the regional structure and deformation and metamorphism of the region indicate that there occurred at least two phases of deformation and metamorphism in the region. Early-phase nearly E-W-directed deformational structure is well preserved in the Zhangjiakou-Xuanhua area. Observations of the features of the geological structure from north to south (in the Hengshan metamorphic terrain) have revealed a possible exposed cross-section through the Archaean lower crust. The structure was superimposed by a NE-SW-trending high-temperature ductile shear zone in the Datong area in the late phase, thus reworking the Archaean sequence.

The Main Geological Events of the Chinese Archaean and Proterozoic Eons

A systematic review and comparison of the time, sequence, nature, and characteristics of the major geological events of worldwide Archean and Chinese Precambrian and their interrelationships reveals that these events reveals the spatial-temporal pattern of the events and their significance in geological evolution. The study also touches upon relationship between Early Precambrian Era and plate tectonics. A comparison between the Precambrian major geological events and those elsewhere on the earth proves that the crustal evolution is characterized by striking irreversibility, gradualism, and synchronism. The geological events in China have their unique features, which shows that China has a relatively complete record of formation and evolution of rocks, strata, structures, and organisms spanning 3800 Ma to 540 Ma. The Archean underwent multiple-stage complex metamorphism and deformation;metamorphism is on the higher end;komatiite is absent in the greenstone belts;cratonization occurred rather late and varied between the north and the south. The research results above provide main theoretical basis for deeply systematic researching the early dynamic evolution laws of the earth and the dynamic evolution characteristics of the tectonic plates.

DISCOVERY OF 3.3 Ga ARCHAEAN ROCKS IN NORTH TARiM BLOCK OF XINJIANG, WESTERN CHINA

Geologists have been paying much attention to the characteristics and age of the basement of the Tarim Block. Recently, some isotopic ages for the Precambrian rocks in the margin areas of the Tarim Block have been obtained. The oldest age presented by the above study is 2487±5.1 Ma determined with evaporated Pb-isotope method for zircons of the blue quartz-bearing granite from the Kuruktag area. However, the possibility of the Archaean basement of Tarim Block was only deduced. In this study the age of 3.3 Ga

Zircon SHRIMP age of Mesoarchaean meta-argilloarenaceous rock in the Anshan area and its geological significance

In order to better understand the early continental evolution of the Anshan area, one of the typical Precambrian distribution areas of the North China Craton, the geochronology and REE composition of the zircons from the meta-argillo-arenaceous rock occurred as enclave in 3.1 Ga Lishan trondhjemite are studied by using SHRIMP II ion microprobe. It is indicated that the Paleoarchaean is a very important continental formation period in the Anshan area and 3.2 Ga can be regarded as the boundary between the Paleoarchaean and Mesoarchaean.

Retrograded eclogites in the Archaean North China Craton and their geological implication

High-pressure metamorphic eclogite, as a window of study for depth level of lowermost crust and mantle, has an important indicating significance to composition, property and accretion of continental lower crust. Archaean high-pressure metamorphic rocks of eclogite facies are very rare. According to this fact, it is suggested that there is a difference in principle between the mechanisms of formation and evolution of early Precambrian and Phanerozoic continental crust. Smelov and Beryozkin first reported that some

Tectonic evolvement of metamorphic complexes at Jilin paleocontinental margin during the transition from late Archaean to early Proterozoic

The kinematics and dynamical process of tectonic evolvement of metamorphic complexes at the interim from late Archaean to early Proterozoic is one of the key problems in geosciences. For the disputation on the genesis of metamorphic complexes at the margin of Jilin palaeocontinent, this paper takes the example of Banshigou region, Jilin Province to discuss the dynamical evolution of palaeocontinent during the transition from late Archaean to early Pro- terozoic (2600—2000 Ma). On the time sequence, from center to palaeocontinental margin, it shows a series of dynamical movements including underplating, horizontal movement, subduc- tion, intraplate extension and separation. And its corresponding sequence of kinematical modes is as follows: vertical movement, horizontal movement, extension and shearing in contact zone, uplift-sliding movement in paleocontinental margin and interformational sliding, resulting in such tectonite sequence, tectonic gneiss, gneissic complex, gneissic complex-mylonite, mylonite and fracture cleavage-mylonite, which consist of the main body of metamorphic complexes. Their palaeostresses are: < 20, 20.40, 21.72, 28.80 and 30.8-69.8 MPa respectively. The deforma- tional metamorphic temperature is between hornblende and low-grade greenschist facies. The general deformational characters of Jilin palaeocontinent reflect a complete dynamic system of crust evolution, which indicates that the formation of the metamorphic complexes and the tec- tonic evolution are altered from vertical movement to compression to extension. It also indicates a continuous tectonic transformation from deep to shallow, and from ductile to brittle. The trans- formation between different dynamic mechanisms not only forms tectonic rocks, but also benefits the linking up, exchange and enrichment with rock-forming minerals and ore-forming elements. This research is helpful to classifying regional tectonic events and making further study on the evolution of palaeocontinental dynamics.

添加蔗糖对苜蓿青贮中古生菌和甲烷产生菌群的影响

采用宏基因组测序技术研究了不添加蔗糖(对照)和添加2%蔗糖对高水分紫花苜蓿青贮中古生菌和甲烷产生菌群的影响。结果表明:对照发酵品质较差,添加蔗糖显著增加了乳酸含量(P<0.05),改善青贮品质。氨氧化古菌是苜蓿原料上最丰富的古生菌种,马氏甲烷八叠球菌和氨氧化古菌分别是对照和蔗糖处理的优势种。基因phnJ是苜蓿青贮中甲烷合成的功能基因。大肠杆菌和肺炎杆菌是对照和蔗糖处理中的优势产甲烷菌,蔗糖添加显著降低肺炎杆菌的相对丰度。马氏甲烷八叠球菌相对丰度与丁酸和氨态氮的含量呈正相关。综上所述,马氏甲烷八叠球菌主导青贮中古生菌群,肠杆菌科细菌是苜蓿青贮中潜在的优势甲烷产生菌,蔗糖处理在一定程度上抑制马氏甲烷八叠球菌和肠杆菌科细菌的繁殖,显著改善高水分苜蓿青贮发酵品质。

CONFIRMATION OF THE JIAPIGOU ARCHAEAN HIGH GRADE METAMORPHIC TERRAIN AND ITS GEOLOGICAL SIGNIFICANCE

Whether a typical Archaean greenstone belt exists in China is still a question at issue. A lot of researchers firmly believe that the Jiapigou gold mine area situated on the north margin of N. China platform is a typical Archaean granite-greenstone terrain, but the detailed field and indoor work made by the authors in this area combined with the on-the-spot investigation on some typical Archaean greenstone belts in Yilgarn block, W. Australia, show that the Jiapigou area is a typical Archaean high grade metamorphic terrain but not an Archaean greenstone belt. The following is an outline of the main evidence and geological significance for the confirmation.

坦桑尼亚太古界卡维隆多群杂砾岩型金矿化特征及其发现意义

坦桑尼亚太古界卡维隆多群杂砾岩主要分布于中部的恩泽加地区,少量分布于其西南部的卢帕地区,该岩系在恩泽加地区呈近东西向和北西向展布,长约50 km,宽0.5~3 km,并普遍发育金矿化,其矿化特征主要为硅化和黄铁矿化,其次为毒砂化和磁黄铁矿化。研究成果表明:①激电异常显示出明显相对低阻高极化特征;②土壤地球化学测量结果显示,Au异常规模较大,浓集中心及分带较为明显;③金矿化严格受岩层控制,局限于杂砾岩带内;④金矿化规模大,矿化体宽5~20 m;⑤矿化体金品位较低,为0.2~1.0 g/t,局部富集地段可达到2~3 g/t。

渤中26-6油田发现井钻井设计与施工

渤海油田渤中26-6构造太古界油气运聚条件优越,且潜山长期遭受剥蚀,具备良好的储层条件,非常有利于成藏。为明确该构造东高点太古界潜山储量,探索潜山成藏模式,在该构造部署了探井26-6-2井,主要目的层为太古界潜山,但该井地质条件复杂,钻井难度大。分析了该井主要钻遇岩性及钻井技术难点,认为该井存在漏失风险高、井壁易失稳、易发生井斜和井控风险高等施工难题。为此,从井身结构、钻头及提速工具、钻具组合、钻井液等方面对渤中26-6-2井进行了一系列优化设计。现场施工中,为了达到风化壳与内幕分隔测试的目的,又创新设计,增加了一开次,顺利在潜山内幕下入尾管并成功封固管鞋。渤中26-6-2井测试获得工业油气流,发现了渤中26-6亿吨级油田。相关钻井技术对渤海油田后续钻井设计及现场施工具有指导意义。

变质岩潜山储层分带特征及在地质作业中的应用——以渤中凹陷及周缘潜山为例

随钻过程中对变质岩潜山结构识别不准确制约了太古界变质岩潜山储集层随钻过程中的划分,进而影响对潜山储集层空间分布规律的认识。以渤海油田渤中凹陷太古界潜山花岗片麻岩岩屑、薄片等录井资料为基础,结合测井电阻率和成像密度等数据,将太古界变质岩潜山纵向上分为5个带,即黏土带、风化砂砾岩带、风化淋滤带、致密带和内幕裂缝带,并建立了不同分带录测井识别特征,为随钻过程中优质储层划分和油气层解释提供依据。通过在渤海油田太古界潜山勘探中应用,为卡潜山作业、优质储层识别和油气层解释等提供了重要依据,为后续类似潜山勘探提供了技术借鉴。

山西五台山—恒山太古代绿岩带金矿及其典型矿床地质特征

山西五台山—恒山太古代绿岩带金矿为华北板块区绿岩带中规模最大又十分典型的金矿床之一。矿体主要赋存于五台群柏枝岩组、金岗库组硅铁建造与鸿门岩组云母石英片岩中,典型金矿床有山西五台山东腰庄和代县小板峪金矿床等;金矿体呈似层状、透镜体状产出,矿床与太古代绿岩带硅铁建造和脆-韧性剪切带密切相关;矿石类型以含黄铁矿化绢云钠长片岩为特征,金主要赋存于黄铁矿中;硅化、黄铁矿化等围岩蚀变与金矿化关系密切;金矿床成因为与太古代绿岩带硅铁建造和脆-韧性剪切带密切相关的变质热液型。