从变质作用观看板块构造何时在华北克拉通开始

了解板块构造在地球上何时和怎样开始的是地球科学领域还没有解决的重要问题之一。作为板块运动的最终结果,大陆碰撞造山带是识别地球历史演化中板块构造机制起主导作用的重要标志。大陆碰撞带变质作用一般以顺时针p-T轨迹演化为特征,尤其伴有峰期变质之后的等温减压过程。这样,具有峰后等温减压过程的顺时针p-T轨迹是识别地球早期的板块构造作用的重要标志之一。作为世界上最古老的克拉通陆块之一,华北克拉通基底岩石变质作用p-T演化在过去几年已得到广泛深入的研究,使得该克拉通可能成为应用大规模变质作用p-T轨迹途径来探讨构造环境和构造演化过程的最佳场所。构造上,华北克拉通可划分为三个小的陆块(东部陆块、阴山陆块和鄂尔多斯陆块)和三个古元古代活动带(华北中部碰撞带、孔兹岩带和胶—辽—吉带)。东部陆块和阴山陆块新太古代基底岩石变质作用具有等压冷却型逆时针p-T演化轨迹特征,反映变质作用热源与大量地幔岩浆底板垫托或侵位有关。尽管理论上这样大规模的地幔岩浆可形成在大陆岩浆弧、地幔柱或大陆裂谷环境,只有地幔柱模式才能合理地解释东部陆块和阴山陆块新太古代基底岩石时空分布、岩石组合和构造特征。这样,地幔柱可能是主导东部陆块和阴山陆块新太古代地壳形成和演化的主要构造机制,而板块构造在晚太古宙并不是其主要的构造机制。古元古代孔兹岩带和华北中部碰撞带基底岩石变质作用均具有等温减压型顺时针p-T演化特征,反映两造山带都经历地壳加厚和随后的隆升剥蚀构造过程。这样的构造过程是板块构造体制下的碰撞造山带的典型标志。古元古代胶—辽—吉带可进一步划分为南部带和北部带,其中南部带基底岩石具有逆时针p-T演化特征,而北部带基地岩石具有顺时针p-T演化特征,也反映板块构造机制下的产物。现代规模的板块构造在华北克拉通上的启动时间可以由三个活动带中最老的与板块俯冲有关的新生地壳形成时间来大致标定。目前,华北克拉通内部三个活动带中可识别出来的最老的与板块俯冲有关的新生地壳是华北中部碰撞带2·56Ga五台花岗岩,它们的形成可能大致标志着现代样式的板块构造在华北克拉通大规模作用的开始。     

Isotopic geochronological evidence for the Paleoproterozoic age of gold mineralization in Archean greenstone belts of Karelia, the Baltic Shield

The Rb-Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite-greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Au-bearing beresite and quartz-carbonate veins are located in the shear zone and replace Mesoarchean (～2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar-Kobozero greenstone belt. At the Taloveis ore occurrence in the Kostomuksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (～2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Aubearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Karelia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb-Sr isochrons based on whole-rock samples and minerals from ore-bearing and metasomatic wall rocks and veins yielded ～1.7 Ga for all studied objects. This age is interpreted as the time of development of ore-bearing tectonic zones and ore-forming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0–1.7 Ga) crust-forming tectonic cycle, which gave rise to the formation of the Svecofennian and Lapland-Kola foldbelts in the framework of the Karelain Craton. The oreforming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was probably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations.     

3D model of deep structure of the Early Precambrian crust in the East European Craton and paleogeodynamic implications

The integral 3D model of the deep structure of the Early Precambrian crust in the East European Craton is based on interpretation of the 1-EU, 4B, and TATSEIS seismic CDP profiles in Russia and the adjacent territory of Finland (FIRE project). The geological interpretation of seismic images of the crust is carried out in combination with consideration of geological and geophysical data on the structure of the Fennoscandian Shield and the basement of the East European platform. The model displays tectonically delaminated crust with a predominance of low-angle boundaries between the main tectonic units and the complex structure of the crust-mantle interface, allowing correlation of the deep structure of the Archean Kola, Karelian, and Kursk granite-greenstone terrane with the Volgo-Uralia granulite-gneiss terrane, as well as the Paleoproterozoic intracontinental collision orogens (the Lapland-Mid-Russia-South Baltia orogen and the East Voronezh and Ryazan-Saratov orogens) with the Svecofennian accretionary orogen. The lower crustal “layer” at the base of the Paleoproterozoic orogens and Archean cratons was formed in the Early Paleoproterozoic as a result of underplating and intraplating by mantle-plume mafic magmas and granulite-facies metamorphism. The increase in the thickness of this “layer” was related to hummocking of the lower crustal sheets along with reverse and thrust faulting in the upper crust. The middle crust was distinguished by lower rigidity and affected by ductile deformation. The crust of the Svecofennian Orogen is composed of tectonic sheets plunging to the northeast and consisting of island-arc, backarc, and other types of rocks. These sheets are traced in seismic sections to the crust-mantle interface.     

Conjugate zone between the northern termination of the Baikal-Patom Foldbelt and the Siberian Craton: Structure of the crystalline basement

New data on the deep structure of the crystalline basement in the conjugate zone between the northern termination of the Baikal-Patom Foldbelt and the Siberian Craton are discussed. Digital processing of the data of a high-precision airborne geophysical (magnetic, gravity, radiometric) surveys on a scale of 1: 50000 allows us to construct 3D models of the petrophysical properties of the rocks and to ascertain the main features of the deep structure of the pre-Riphean crystalline basement overlapped by a thick sedimentary cover. The Archean and Paleoproterozoic metamorphic and plutonic petrophysical complexes are identified, and their vertical and horizontal relationships are outlined. Horizontal tectonic movements and overthrusting for many tens of kilometers are interpreted in the internal structure of the crystalline basement. The tectonic overriding of the marginal part of the Siberian Craton by nappes of the Baikal-Patom Foldbelt is proved. In particular, the completely detached, allochthonous position of the Kotuikan Zone along which the Magan and Daldyn terranes cojugate with the Aldan Province in the south of the craton is established. This compels us to consider a re-evaluation of the currently accepted concept concerning the structure and formation history of the basement of the ancient Siberian Craton.     

胶北地块前寒武纪基底研究新进展

胶北地块位于鲁东的沂沭断裂和五莲—烟台断裂之间,以出露不同变质程度的太古宙—古元古代的结晶基底为特征,其岩石组合为TTG质(英云闪长岩-奥长花岗岩-花岗闪长岩)片麻岩和少量变质表壳岩(变质火山岩-变质沉积岩),它们构成了胶北的花岗-绿岩体。对变质(超)基性岩、TTG岩系的岩石组合、地球化学特征和同位素年代学的研究表明,胶北地块造壳的主要时期为新太古代的2.8～2.7 Ga;在经历了2.5 Ga左右构造事件后,胶北地块进入稳定陆壳沉积阶段,并接受了古元古代的孔兹岩系沉积;最后于1.8 Ga左右发生变质。与苏鲁造山带不同,胶北地块前寒武纪基底具有许多亲华北板块的特征,二者的界线可能是五莲—烟台断裂。     

南美洲圭亚那地盾北部绿岩带地质特征、典型金矿床及金成矿作用

圭亚那地盾位于南美洲亚马孙克拉通的北部,其北部广泛发育太古宙典型的花岗岩-绿岩带,主要岩石组成为花岗岩、片麻岩和英云闪长岩,古元古代变质超基性-基性火山岩-侵入岩、其他变质火山岩等。由于独特的地质构造位置和复杂的构造演化历史,形成了良好的绿岩型金成矿地质背景,产生了大量的金矿床。通过总结圭亚那地盾北部绿岩带的岩石特征、成岩时代和构造演化历史,对委内瑞拉埃尔卡劳金矿、圭亚那欧迈金矿和苏里南罗瑟贝尔金矿进行典型矿床剖析,系统研究了该区绿岩带型金矿的成矿地质背景、成矿流体及成矿年代学特征,建立了区域绿岩带型金矿成矿模型,并在分析找矿要素的基础上,提出圭亚那地盾北部的金矿找矿方向。     

The terrane concept and its manifestation by deep reflection studies in the Variscides

The terrane concept is understood as an important extension of plate tectonics and is based on the recognition of allochthonous, mobile geological units. The concept is successfully applied to the Variscides with their wide range of collisional belts. It is mainly supported by the dense deep-seismic network of DEKORP, which reveals certain reflectivity patterns and succeeds in mapping old and new deep fault zones between the terranes. Variscan terranes are rooted in the ductile lower crust and seem to consist of continental crust only, partly exclusively of rigid upper crust. Oceanic terranes, on the other hand, are always rooted in the asthenosphere. The development of continental terranes and their boundaries depends strongly on their thermal and rheological history. In the case of post-orogenic collapse with heating and extension of the lower crust, seismic lamellae develop and often truncate former thrust faults.     

Formation stages of the Early Precambrian crust in the Sharyzhalgai basement uplift,southwestern Siberian craton: Synthesis of Sm-Nd and U-Pb data

Newly obtained U-Pb and Sm-Nd isotopic data on Early Precambrian metamorphic and granitoid complexes in the southwestern margin of the Siberian craton (Sharyzhalgai basement uplift) are synthe-sized in order to elucidate the crustal evolution starting at the Paleoarchean (∼3.6 Ga) to Late Paleoproterozoic (∼1.85 Ga), evaluate the lateral extent of the Paleo-Mesoarchean crust, and identify major stages in its growth and recycling. Two crustal growth stages were determined in the Onot and Bulun granite-greenstone terranes: at 3.6–3.3 and 2.8–2.9 Ga. The earliest recycling processes (at ∼3.4 and 3.2 Ga) involved partial melting, metamorphism, and migmatization and produced a stable continental crust. Crustal growth in the Mesoarchean (∼2.8–2.9 Ga) due to basaltoid magmatism was associated with the recycling of the Paleoarchean crust, which served as a source of felsic melts and of detrital material for terrigenous sediments. The Archean crust of the Irkut granulite-gneiss terrane was formed by two pulses of intermediate-felsic and basic volcanism at ∼3.6-3.4 and ∼2.7 Ga. In the terminal Archean (at ∼2.55 Ga), the preexisting crust was involved in metamorphic and magmatic processes. Traces of recycling of the Paleoproterozoic crust are identified in the isotopic parameters of the intermediate-felsic granulites. Two discrete stages in the influx of juvenile material are identified in the Paleoarchean: at ∼2.0 and 1.88–1.85 Ga, with the latter stage associated with the large-scale recycling of the Archean crust during the origin of granitoids.     

华北中部构造带不同前寒武纪地块主要特征对比研究

古元古代是华北克拉通构造演化的关键时期,主要体现在由新太古代大陆地壳垂向增生到古元古代侧向增生的转换。华北中部构造带的构造性质是近年来华北克拉通构造演化研究争论的焦点之一,构造带中段出露恒山、五台和阜平等多个前寒武纪变质杂岩,其中发育多条大型韧性剪切带,这些剪切带的主期活动时代均为1. 85～1. 80Ga。本文以这些剪切带为构造边界将变质杂岩划分为恒山地块、繁峙地块、五台地块和阜平地块,并在早期研究中解析了繁峙地块构造挤入事件和阜平地块西部的伸展构造事件,表明这些前寒武纪地块的构造位置在古元古代末前后可能有所不同。结合前人资料,本文对这些地块的岩石组成、形成时代、变质作用以及典型构造样式等特征进行详细对比研究。结果表明,恒山地块、阜平地块和五台地块共同组成华北克拉通早前寒武纪"高级区-绿岩带"的典型壳层结构,而繁峙地块与其他地块具有明显差异,具有外来地块特征。     

华北克拉通古老大陆地壳组成及演化

对鞍本、冀东、鲁西、阴山等早前寒武纪典型地区和深部物质进行了深入研究,总结了华北克拉通早期地壳形成演化历史。揭示出鄂尔多斯地块本身强烈卷入了古元古代晚期构造热事件。首次在华北克拉通划分出3个>2.6 Ga古陆块。     

Paleoproterozoic supercontinent: Origin and evolution of accretionary and collisional orogens exemplified in Northern cratons

The evolution of the North American, East European, and Siberian cratons is considered. The Paleoproterozoic juvenile associations concentrate largely within mobile belts of two types: (1) volcanic-sedimentary and volcanic-plutonic belts composed of low-grade metamorphic rocks of greenschist to low-temperature amphibolite facies and (2) granulite-gneiss belts with a predominance of high-grade metamorphic rocks of high-temperature amphibolite to ultrahigh-temperature granulite facies. The first kind of mobile belt includes paleosutures made up of not only oceanic and island-arc rock associations formed in the process of evolution of relatively short-lived oceans of the Red Sea type but also peripheral accretionary orogens consisting of oceanic, island-arc, and backarc terranes accreted to continental margins. The formation of the second kind of mobile belt was related to the activity of plumes expressed in vigorous heating of the continental crust; intraplate magmatism; formation of rift depressions filled with sediments, juvenile lavas, and deposits of pyroclastic flows; and metamorphism of lower and middle crustal complexes under conditions of granulite and high-temperature amphibolite facies that, in addition, spreads over the fill of rift depressions. The evolution of mobile belts pertaining to both types ended with thrusting in a collisional setting. Five periods are recognized in Paleoproterozoic history: (1) origin and development of a superplume in the mantle that underlay the Neoarchean supercontinent; this process resulted in separation and displacement of the Fennoscandian fragment of the supercontinent (2.51–2.44 Ga); (2) a period of relatively quiet intraplate evolution complicated by locally developed plume-and plate-tectonic processes (2.44–2.0 (2.11) Ga); (3) the origin of a new superplume in the subcontinental mantle (2.0–1.95 Ga); (4) the complex combination of intense global plume-and plate-tectonic processes that led to the partial breakup of the supercontinent, its subsequent renascence and the accompanying formation of collisional orogens in the inner domains of the renewed Paleoproterozoic supercontinent, and the emergence of accretionary orogens along some of its margins (1.95–1.75 (1.71) Ga); and (5) postorogenic and anorogenic magmatism and metamorphism (<1.75 Ga).     

Metamorphic rocks of the Murmansk domain (Kola Peninsula) as compared with the oldest rock associations of the northeastern Baltic shield,Canada, and Greenland

Rock associations characterized by heterogeneous sets of petrogeochemical parameters were compared by quantifying the degree of their similarity-dissimilarity and searching for discrimination trends between them. Using procedures specially developed for this purpose, it was demonstrated for the first time that the lithotectonic complexes of the Murmansk domain are fundamentally different from those of typical granulite-gneiss terrains and resemble the granite-greenstone terrains of the Baltic shield, Greenland, and Canada. Based on the whole data set, the Murmansk domain can be considered as a deeply eroded Archean granite-greenstone terrain retaining only the tonalite-trondhjemite-gneiss basement with abundant supracrustal enclaves. A trend of the compositional difference between the older and younger rock associations is similar to that between the tholeiitic and boninitic volcanic series. It was suggested that the petrogeochemical “age” trend reflects the initial stage of the compositional evolution of the metamagmatic rocks of the region from metamorphic rocks similar to tholeiites at the early stages to the Paleoproterozoic boninite-like rocks, which are believed to be linked to the unique PGE-bearing province of the northeastern Baltic shield. This implies that the specific metallogenic features of the region emerged already in the Archean, which supported the suggestion on the long duration of geological processes in the Early Precambrian.     

The Proterozoic evolution of northern Siberian Craton margin: a comparison of U–Pb–Hf signatures from sedimentary units of the Taimyr orogenic belt and the Siberian platform

Identifying the cratonic affinity of Neoproterozoic crust that surrounds the northern margin of the Siberian Craton (SC) is critical for determining its tectonic evolution and placing the Craton in Neoproterozoic supercontinental reconstructions. Integration of new U–Pb–Hf detrital zircon data with regional geological constraints indicates that distinct Neoproterozoic arc-related magmatic belts can be identified within the Taimyr orogen. Sedimentary rocks derived from 970 to 800 Ma arc-related suites reveal abundant Archean and Paleoproterozoic detritus, characteristic of the SC. The 720–600 Ma arc-related zircon population from the younger Cambrian sedimentary rocks is also complemented by an exotic juvenile Mesoproterozoic zircon population and erosional products of older arc-related suites. Nonetheless, numerous evidences imply that both arcs broadly reworked Siberian basement components. We suggest that the early Neoproterozoic (ca. 970–800 Ma) arc system of the Taimyr orogen evolved on the active margin of the SC and probably extended along the periphery of Rodinia into Valhalla orogen of NE Laurentia. We also suggest the late Neoproterozoic (750–550 Ma) arc system could have been part of the Timanian orogen, which linked Siberia and Baltica at the Precambrian/Phanerozoic transition.     

早期碰撞造山过程与板块构造：前寒武纪地质研究的机遇和挑战

普遍认为修正后的板块构造模式仍适用于新太古代地质研究，但是早期板块构造过程与后期有明显差异，包括陆块规模、造山带类型、碰撞造山过程等。典型碰撞造山带在地史上的初次形成具有划时代的构造演化意义，涉及典型板块构造初始发生过程、最早超级大陆拼合、威尔逊旋回及板块碰撞造山过程等方面。华北中部保留一条近南北向的碰撞造山带（2 600~2 500 Ma BP），保留特征的巨型复式褶皱、不同层次推覆构造、蛇绿岩混杂带等。围绕华北中部造山带及其25亿年重大构造热事件的研究，对认识华北早期构造演化及其超大陆再造具有重要意义，也是早期板块构造研究的关键突破口之一，开展其不同地壳层次构造变形及其前陆盆地的研究，将深化早期板块边界及其造山过程的科学认识。     

Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits

With very few exceptions, orogenic gold deposits formed in subduction-related tectonic settings in accretionary to collisional orogenic belts from Archean to Tertiary times. Their genesis, including metal and fluid source, fluid pathways, depositional mechanisms, and timing relative to regional structural and metamorphic events, continues to be controversial. However, there is now general agreement that these deposits formed from metamorphic fluids, either from metamorphism of intra-basinal rock sequences or de-volatilization of a subducted sediment wedge, during a change from a compressional to transpressional, less commonly transtensional, stress regime, prior to orogenic collapse. In the case of Archean and Paleoproterozoic deposits, the formation of orogenic gold deposits was one of the last events prior to cratonization. The late timing of orogenic gold deposits within the structural evolution of the host orogen implies that any earlier structures may be mineralized and that the current structural geometry of the gold deposits is equivalent to that at the time of their formation provided that there has been no significant post-gold orogenic overprint. Within the host volcano-sedimentary sequences at the province scale, world-class orogenic gold deposits are most commonly located in second-order structures adjacent to crustal scale faults and shear zones, representing the first-order ore-forming fluid pathways, and whose deep lithospheric connection is marked by lamprophyre intrusions which, however, have no direct genetic association with gold deposition. More specifically, the gold deposits are located adjacent to ～10°-25° district-scale jogs in these crustal-scale faults. These jogs are commonly the site of arrays of ～70° cross faults that accommodate the bending of the more rigid components, for example volcanic rocks and intrusive sills, of the host belts. Rotation of blocks between these accommodation faults causes failure of more competent units and/or reactivation and dilation of pre-existing structures, leading to deposit-scale focussing of ore-fluid and gold deposition.Anticlinal or antiformal fold hinges, particularly those of 'locked-up' folds with ～30° apical angles and overturned back limbs, represent sites of brittle-ductile rock failure and provide one of the more robust parameters for location of orogenic gold deposits.In orogenic belts with abundant pre-gold granitic intrusions, particularly Precambrian granitegreenstone terranes, the boundaries between the rigid granitic bodies and more ductile greenstone sequences are commonly sites of heterogeneous stress and inhomogeneous strain. Thus, contacts between granitic intrusions and volcano-sedimentary sequences are common sites of ore-fluid infiltration and gold deposition. For orogenic gold deposits at deeper crustal levels, ore-forming fluids are commonly focused along strain gradients between more compressional zones where volcano-sedimentary sequences are thinned and relatively more extensional zones where they are thickened. World-class orogenic gold deposits are commonly located in the deformed volcano-sedimentary sequences in such strain gradients adjacent to triple-point junctions defined by the granitic intrusions, or along the zones of assembly of micro-blocks on a regional scale. These repetitive province to district-scale geometrical patterns of structures within the orogenic belts are clearly critical parameters in geology-based exploration targeting for orogenic gold deposits.     

Lower precambrian of the Keivy Terrane,Northeastern Baltic Shield: A stratigraphic succession or a collage of tectonic sheets?

The Keivy Terrane in the northeastern Baltic Shield appreciably differs from the adjacent tectonic blocks. In the northwestern part of this terrane (the Serpovidny Range), an outlier of Paleoproterozoic supracrustal rocks called the Serpovidny structure is surrounded by Archean (?) Keivy high-alumina paraschists. As follows from structural and magnetic data, the Paleoproterozoic rocks are deformed into a tight sheath fold 8 × 2 km in size at the surface and 5 km in length along the sheath axis. Faults parallel to the boundaries of the layers and locally cutting them off at an acute angle are involved in folding as well. The outer boundaries of the Serpovidny structure are tectonic. This structure is complementary to a larger tectonic lens composed of the Keivy mica schists. It is concluded that all of the supracrustal rocks of the Serpovidny Range are in fact tectonic sheets and lenses deformed into sheath folds. The literature data show that kilometer-scale sheath folds occur throughout the Keivy paraschist belt and most likely were formed owing to thrusting of the Murmansk Craton onto the Keivy Terrane in the south-southwestern direction. Foliation and lineation related to thrusting have been established in the Archean silicic metavolcanics and peralkaline granites occupying the most part of the terrane. In contrast, the granitoids and gabbroanorthosites of the Archean basement, which form a block 90 × 20 km in the southwestern Keivy Terrane, were not affected by Paleoproterozoic deformation. In other words, a detached assembly of tectonic sheets composed of the upper and middle crustal rocks that underwent deformation at the initial stage of the Paleoproterozoic Lapland-Kola Orogeny and the Archean basement, which is free of this deformation, are distinguished. The depth of detachment is estimated at 20–25 km. The detachment of the upper and middle crust in the Keivy Terrane and its position in the structure of the Baltic Shield are consistent with a spatiotemporal succession that resulted in the formation of a Paleoproterozoic supercontinent and the Baltic Shield as its fragment. This succession began with the amalgamation and deformation of the Archean terranes in the northeast of the Baltic Shield during the Lapland-Kola Orogeny, the Keivy Terrane showing a record of the earliest reworking (1.97–1.93 Ga). The succession completed in the southern and southwestern parts of the shield (1.80 Ga) after the Svecofennian Orogeny, expressed in the accretion of island-arc terranes composed of Paleoproterozoic juvenile crust to the continent.     

从全球对比探讨华北克拉通早期地质演化与板块构造过程

板块构造理论为 2 0世纪取得的重大科学成就。如何在新世纪发展板块构造、推动大陆动力学研究成为新的挑战。世界古大陆前寒武纪地质研究在花岗岩绿岩带、高级变质区 ,新太古代造山作用 ,早期大洋地质记录与古板块构造 ,以及超级大陆等方面取得新进展。在此全球构造背景下 ,华北早期地质演化的相关重大问题包括 :新太古代典型造山带地质演化、碰撞过程及其盆山耦合作用。围绕新太古代蛇绿岩的研究 ,特别是豆荚状铬铁矿及地幔岩 ,将提供早期大洋岩石圈性质、扩张运移过程的重要线索 ,并提出早期板块边界划分标志及其洋陆作用过程。华北中部造山带及蛇绿岩混杂带的洲际对比 ,对认识华北与最古老超级大陆聚合过程具有重要意义     

U/Pb and Sm/Nd geochronologic studies of the eastern Borborema Province, Northeastern Brazil: initial conclusions

The Borborema Province of NE Brasil comprises the central part of a wide Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the São Luis-West Africa craton and the São Francisco-Congo-Kasai cratons. New Sm/Nd and U/Pb results from the eastern part of this province help to define the basic internal architecture and pre-collisional history of this province, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly into West Gondwana during the Pan African-Brasiliano orogeny at ca. 600 Ma.The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandão massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) the Archean to Paleoproterozoic São Francisco craton (SFC) to the south; and c) a complex domain of Paleoproterozoic to Archean basement blocks with several intervening Mesoproterozoic to Neoproterozoic fold belts in the center (south of Patos Lineament and north of SFC). The northern and central domains comprise the Borborema Province.Archean basement gneiss and Transamazonian granulite of northern SFC are exposed in the southern part of the central domain, underlying southern parts of the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; Nd model ages (T_DM) of ca. 2.6 Ga for 2.15 Ga gneisses indicate a substantial Archean component in the protoliths to these gneisses. The Caldas Brandão massif to the east yields both Transamazonian and Archean U/Pb zircon and Nd (T_DM) ages, indicating a complex architecture. Metasedimentary rocks of the Jucurutu Formation yield detrital zircons with original crystallization ages as young as 1.8 Ga, indicating that these rocks may be late Paleoproterozoic and correlate with other ca. 1.8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaço Group.Most metavolcanic and pre-Brasiliano granitic units of the Sergipano (SDS), Pajeú-Paraíba (SPP), Riacho Pontal (SRP), and Piancó-Alto Brígida (SPAB) fold belts in the central domain formed ˜ 1.0 ± 0.1 Ga, based on U/Pb ages of zircons. Nd model ages (T_DM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are commonly 1.3–1.7 Ga, indicating that rocks of the fold belts were not wholly derived from either older (> 2.1 Ga) or juvenile (ca. 1.0 Ga) crust, but include mixtures of both components. A simple interpretation of Brasiliano granite genesis and the Nd data implies that there is no Transamazonian or Archean basement underlying large parts of these fold belts or of the Pernambuco-Alagoas massif. An exception is a belt of syenitic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAB and SPP that clearly has a Transamazonian (or older) source. In addition, there are several smaller blocks of Archean to Transamazonian gneiss that can be defined within and among these fold belts. These blocks do not appear to constitute a continuous basement complex, but appear to be isolated older crustal fragments.Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming event along the northern edge of the São Francisco craton. Our data also show that significant parts of the Borborema Province are not remobilized Transamazonian to Archean crust, but that Mesoproterozoic crust is a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, suggesting that the rifting event created several small continental fragments that were later incorporated into the Brasiliano collisional orogen. We cannot at present determine if the Rio Piranhas-Caldas Brandão massifs and the older crustal blocks of the central domain were originally part of the São Francisco craton or whether some (or all) of them came from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age.     

中国北部古元古代地壳尺度的伸展拆离和硅铝壳内活动带:北东向线性航磁异常的地质构造解释

早前寒武纪地质构造和分辨率大于120km的长波航磁异常特征表明:塔南—华北—朝鲜北部(狼林地块)具有类似的太古宙结晶基底,它们可能曾经是太古宙统一克拉通的组成部分。20~120km分辨率的中波航磁异常凸现一组北东东—北北东走向、改造太古宙高级变质结晶基底的线性航磁异常,它们分别对应于地壳尺度的长寿断裂。古元古代阶段沿着它们发生了大规模的左行韧性伸展剪切运动,在使塔南、朝鲜北部陆块分别向南西和北东伸展拆离的同时,华北克拉通内部也出现了有限的裂解,并伴随着硅铝壳内活动带的发育。本文将这些古元古代阶段韧性伸展剪切带划分为塔南与华北陆块之间的西部剪切带系统、华北与狼林陆块之间的东部剪切带系统和华北陆块内部的青龙—太行山—中条山等3个独立的剪切带系统。西部韧性剪切系统由阿尔金、大同—环县、以及它们之间的狼山—吉兰泰、雅布赖山等次级剪切带组成,控制古元古代阿拉善群和上集宁群的孔兹岩系为代表的硅铝壳内活动带的沉积—构造演化。东部韧性剪切系统包括沂水和新宾—桦甸等韧性剪切带,相关的古元古代活动带以胶东地区的粉子山群、荆山群和辽吉地区的辽河群为代表。中部韧性剪切带系统以太行山地区阜平群、赞皇群中出现的角闪岩相糜棱片麻岩、眼球状糜棱岩带为代表,发育b-型线理,指示近水平或低角度斜滑性质的左行韧性剪切,相关的古元古代活动带包括双山子—青龙河、五台—滹沱、甘陶河—东焦、吕梁、中条以及济宁等。不同规模的古元古代活动带彼此并不相连,以发育变质沉积—火山岩建造为主体,岩石磁化率明显低于太古宙高级变质结晶基底,与之相关的韧性剪切带则以其透入性矿物线理产生了增强的磁化率各向异性,两者共同构建了地壳尺度北东东—北北东向线性航磁异常带。古元古代末的中条运动使上述硅铝壳内活动带褶皱回返,塔南—华北—朝鲜北部陆块重新焊接,形成近纬向展布逐渐稳定的中轴大陆克拉通,上述地壳尺度的韧性伸展剪切带在中元古代以后的地质历史中,大多被后期不同性质的脆性断裂所追踪。     

Introduction to tectonics of China

The continental crust of China is a mosaic of cratonic blocks and orogenic belts, containing small cratons and terranes with various tectonic settings. They have diverse origins and complex histories of amalgamation, and often suffered repeated reworking after multiple episodes of amalgamation. In the last three decades, extensive geological, geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts, producing an abundant amount of new data and competing interpretations. This provides important insights into understanding the formation and evolution of the Chinese continents. The papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in China. Complex tectonic histories were experienced not only by the large-scale cratonic blocks and orogenic belts, but also by small-scale terranes and orogens between and inside these blocks. Nevertheless, our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in China. It has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks. An overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones.