华北太古宙两类灰色片麻岩及其地质意义

灰色片麻岩是具太古宙特征而有构造意义的英云间长质侵入体。华北太古苗灰色片麻岩可分为两种类型:其一为遵化型。它与基性麻粒岩呈“互层”产出,形成双峰式混合岩伏建造。矿物成分复杂,稀土型式变化大,有正铕异常,属原地半原地深位混合岩化成因,熔融量大(30％±),作为早期深变质绿岩带根部的板底垫托而阻止了基性岩的沉没。其二为恒山型。有岩体形态和较大幅度的上升侵位,矿物成分简单,稀土型式变化小,常无铕异常,为深变质绿岩带俯冲下插并熔融侵位的产物,熔融量小(15%±)。本文试图通过灰色片麻岩的研究为古板块构造和早期地壳演化提供重要信息。     

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 w     

冀东迁西三屯营地区早太古宙三组片麻岩的岩石地球化学及其成因

冀东迁西三屯营地区的高级变质杂岩,主要由早太古宙的三组片麻岩所组成。岩石地球化学研究表明,较早期的三屯营片麻岩为钙碱性石英闪长质岩石,除U和Th外,它不亏损不相容元素。较晚期的小关庄片麻岩和秋花峪片麻岩,分别为钙碱性石英闪长质和钙碱性英云闪长—奥长花岗质岩石,二者均发生了不相容元素的亏损。三屯营地区的岩石亏损与未亏损是源岩浆所具有的特征,可能是初始地壳形成时上地幔不均匀造成的。     

Geochronology and geological evolution of metamorphic rocks in the Field Islands area, East Antarctica

Detailed geochronological, structural and petrological studies reveal that the geological evolution of the Field Islands area, East Antarctica, was substantially similar to that of the adjacent Archaean Napier Complex, though with notable differences in late and post Archaean times. These differences reflect the area's proximity to the Proterozoic Rayner Complex and consequent vulnerability to tectonic process involved in the formation of the latter. Distinctive structural features of the Field Islands are (1) consistent development of a discordant, pervasive S₃ axial-plane foliation; (2) re-orientation of S₃ axial planes to approximate to the subsequent E-W tectonic trend of the nearby Rayner Complex; (3) selective retrogression by a post-D₃ static thermal overprint; and (4) relatively common development of retrogressive, E-W-trending, mylonitic shear zones. Peak metamorphic conditions in excess of 800°C at 900 ± 100 M Pa (9 kbar) were attained at one locality following, but probably close to the time of D₂ folding. D₃ took place in late Archaean times when metamorphic temperatures were about 650°C and pressures were about 600 MPa (6 kbar). Later, temperatures of 600 ± 50°C and pressures of 700 MPa (7kbar) were attained in an amphibolite-facies event, presumably associated with the widespread granulite to amphibolite-facies metamorphism and intense deformation involved in the formation of the Rayner Complex at about 1100 Ma. The area was subsequently subjected to near-isothermal uplift. Rb-Sr isotopic data indicate that the pervasive D₃ fabric developed at about 2400–2500 Ma, and this age can be further refined to 2456⁺⁸_-5 Ma by concordant zircon analyses from a syn-D₃ pegmatite. All zircons were affected by only minor (<7–10%) Pb loss and/or new zircon growth during the Rayner event at about 1100Ma. Thus the 450–850 μg/gU concentrations of these zircons were too low to cause sufficient lattice damage over the 1350 Ma (from 2450 Ma) for excessive Pb to be lost during the 1100 Ma event. The emplacement of pegmatite at 522 ± 10 Ma substantially changed the Rb-Sr systematics of the only analysed rock that developed a penetrative fabric during the 1100 Ma event. Monazite in this pegmatite contains an inherited Pb component, which probably resides in small opaque inclusions. A good correlation is found between Rb-Sr total-rock ages and rock fabric. U-Pb zircon intercepts with concordia also mostly correspond to known events. However, in one example a near perfect alignment of zircon analyses, probably developed by mixing of unrelated components, produced concordia intercepts that appear to have no direct geochronological significance.     

Recycling and chemical weathering in tectonically controlled Mesozoic–Cenozoic basins of New Zealand

Sedimentation in molasse basins is controlled by tectonics, however, recycling and chemical weathering play a critical role in the compositional evolution of a sedimentary succession. The Cretaceous to Pliocene molasse deposits of Central Otago, New Zealand are excellent examples of tectonically related deposits that were governed by the effects of chemical weathering and recycling. Preserved in fault-controlled basins floored by flysch deposits of the Otago Schist, the clastic successions contain ubiquitous unconformities and lithofacies consistent with alluvial, fluvial and lacustrine depositional settings. Textural analysis of Central Otago sandstones establishes a general quartz enrichment and increased mixing of angular and well-rounded quartz varieties up-section, consistent with a history of sediment recycling. Rare earth element (REE) patterns, which reflect upper crustal compositions, are similar for the flysch-type Otago Schist (Permian–Early Cretaceous), a palaeo-weathering profile, and the overlying molasse deposits. The development of quartz arenites is also consistent with high degrees of chemical weathering, and erosion of the schist basement, which contains numerous quartz veins. Although recycling has occurred, SiO₂ and TiO₂ do not consistently show a negative correlation over time. This reflects erosion of previously deposited quartz-rich sediment and the Otago weathering profile, which produced an inverse stratigraphy. CIA values range from 52 for lithic-rich, coarse-grained sandstones and polymictic conglomerate matrices, to 93 for coarse-grained to pebble-rich quartz arenites. Individual samples were split into finer- and coarser-grained pairs (<2·5φ and 2·5 to −1φ) and were analysed separately. The results show that finer-grained samples contain higher REE abundances and less SiO₂, but the coarser-grained Miocene–Pliocene samples have higher CIA values than their finer-grained counterparts. These coarse-grained deposits are quartz-rich and plot erratically on tectonic discrimination diagrams, implying that using SiO₂-poor samples is more reliable for geochemical analysis. Overall, the petrographic and geochemical results indicate that the main factors controlling the composition of the Central Otago molasse deposits were source composition, chemical weathering and recycling. Studies of this nature can be conducted in Archaean tectonically controlled molasse basins that are affected by similar allocyclic factors.     

Sr and Nd isotope systematics of Francistown plutonic rocks, Botswana: implications for Neoarchaean crustal evolution of the Zimbabwe craton

The Francistown plutonic rocks at the south-western margin of the Zimbabwe craton consist of three igneous suites: Sanukitoid, Tonalite–Trondhjemite–Granite (TTG) suites and High-K granites. The TTG suite is subdivided into High Aluminum-TTG (HA-TTG) and Low Aluminum-TTG (LA-TTG) sub-suites. Their Rb–Sr isotope systems were partially homogenized by post-crystallization thermo–tectonic events, in which hydrothermal solutions and migmatization played an important role. Therefore, the Rb–Sr isochron age of 2427±54 Ma can only be regarded as a lower limit to the Francistown plutonic rock age. The large errors in the Sm–Nd isochron dates of Francistown granitoids indicate that these dates are not really constrained. In this study we compared the rock types of Francistown and adjacent areas, adopting the precise U, Th–Pb single zircon SHRIMP ages from the Vumba area as references. For TTG and Sanukitoid suites, the age we adopted is ca. 2.7 Ga, which is close to their depleted-mantle Sm–Nd model ages (T _DM). For High-K granites, the age adopted is ca. 2.65 Ga, which is also close to their Sm–Nd isochron age. The highest ε _Nd ^t values of Sanukitoids and TTG are +2.1 and +2.3, respectively. The positive ε _Nd ^t values and trace element geochemistry support partial melting of a depleted mantle and young oceanic crust for the genesis of Sanukitoid and the TTG suites respectively. The lowest ε _Nd ^t values of Sanukitoids and TTGs are −1.0 and −1.1, respectively, indicating contamination by continental crust, up to 10 and 14%, respectively. The ε _Nd ^t values of TTG decrease with decreasing Al₂O₃ and Sr contents and increasing Eu negative anomalies (Eu*–Eu), suggesting that the TTG magmas underwent a coupled fractionation crystallization and crustal contamination, and that the LA-TTG was the product of the fractionation and contamination of the HA-TTG sub-suite. In contrast, negative ε _Nd ^t values for the High-K granites (from −0.4 to −3.5) indicate the involvement of LA-TTG and some materials from an old continental crust in their genesis. The products of partial melting of both oceanic and continental crusts at the south-western margin of the Zimbabwe craton occurred within a short time interval (from 2.7 to 2.65 Ga ago) suggesting that the Francistown plutonic rocks were formed in a active continental margin environment, where a young ocean plate (Limpopo oceanic plate) subducted underneath an old continental plate (Zimbabwe craton).     

An overview of the geology of the Transvaal Supergroup dolomites (South Africa)

 In the Neoarchaean intracratonic basin of the Kaapvaal craton, between approximately 2640 Ma and 2516 Ma, two successive stromatolitic carbonate platforms developed. Deposition started with the Schmidtsdrif Subgroup, which is probably oldest in the southwestern part of the basin, and which contains stromatolitic carbonates, siliciclastic sediments and minor lava flows. Subsequently, the Nauga formation carbonates were deposited on peritidal flats located to the southwest and were drowned during a transgression of the Transvaal Supergroup epeiric sea, around 2550 Ma ago. This transgression led to the development of a carbonate platform in the areas of the preserved Transvaal and Griqualand West basins, which persisted for 30–50 Ma. During this time, shales were deposited over the Nauga Formation carbonates in the southwestern portion of the epeiric sea. A subsequent period of basin subsidence led to drowning of the stromatolitic platform and to sedimentation of chemical, iron-rich silica precipitates of the banded iron formations (BIF) over the entire basin. Carbonate precipitation in the Archaean was largely due to chemical and lesser biogenic processes, with stromatolites and ocean water composition playing an important role. The stromatolitic carbonates in the preserved Griqualand West and Transvaal basins are subdivided into several formations, based on the depositional facies, reflected by stromatolite morphology, and on intraformational unconformities; interbedded tuffs and available radiometric age data do not yet permit detailed correlation of units from the two basins. Thorough dolomitisation of most formations took place at different post-depositional stages, but mainly during early diagenesis. Partial silicification was the result of diagenetic and weathering processes. Karstification of the carbonate rocks was related to periods of exposure to subaerial conditions and to percolation of groundwater. Such periods occurred locally at the time of carbonate and BIF deposition. Main karstification, however, probably took place during an erosional period between approximately 2430 Ma and 2320 Ma. Received: 15 September 1996 · Accepted: 12 May 1998     

内蒙古大青山太古宙麻粒岩带的板块构造演化模式

太古宙末期在全球范围内出现的麻粒岩带是地壳早期地质演化历史中的一个重大事件，可能意味着地壳构造演化机制的重大转变。越来越多的地质事实表明，太古宙晚期大陆地壳已具有足够的刚性和稳定性。太古宙时期地幔存在强烈的热对流，具备发生板块运动的条件。太古宙麻粒岩带就是太古宙地壳发生刚性板块运动的直接产物。内蒙古大青山太古宙麻粒岩带呈东西向连续出露于大青山地区达２３０余公里，由中晚太古代乌拉山群麻粒岩相组成，其     

A review of the In Ouzzal granulitic terrane (Tuareg shield, Algeria): its significance within the Pan-African Trans-Saharan belt

The In Ouzzal terrane (IOT) or In Ouzzal granulite unit (IOGU) is an elongated Palaeoproterozoic block within the Neoproterozoic Pan-African belt of north-west Africa. The granulites derive from Archaean protoliths that include a large volume of metasediments which were deposited on a 3.2-Ga gneissic basement. Near-peak granulite facies conditions between 2.17 and 2 Ga were estimated at P=10 kbar and T rising from 800 to 1000°C. Premetamorphic orthogneisses were intruded at 2.5 Ga, and followed by the emplacement of syn- to late-kinematic charnockites, syenites and carbonatites at around 2 Ga. Cooling of the granulites occurred till 1800 Ma. Shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas, the IOT was buried beneath late Palaeoproterozoic and Neoproterozoic cover sequences, and then behaved as a rigid block. Both margins are lithospheric faults, as evidenced by the occurrence of shear-zone-related mafic and felsic plutons. Pan-African tectonothermal events were negligible in the north, but granulites in the south were significantly reworked under lower greenschist facies conditions during the northern motion of the block with respect to both the western and the eastern Pan-African terranes. The Cambrian molasse, associated with widespread alkaline volcanism and subvolcanic granites, is horizontal in the north. The IOT, which was part of a larger continental mass including its counterpart in northern Mali, is interpreted as an exotic terrane which may have docked during Pan-African plate convergence and lateral collision. The unchanged pediplain since c. 1.7 Ga in the north suggests that the IOT is underlain by thick Palaeoproterozoic lithospheric mantle, whereas its southern part is probably allochthonous and overlies Pan-African structural units.