Restoring Proterozoic deformation within the Superior craton

Geometrical patterns of Paleoproterozoic dyke swarms in the Superior craton, North America, and paleomagnetic studies of those dykes, both indicate relative motion across the Kapuskasing Structural Zone (KSZ) that divides the craton into eastern and western sectors. Previous work has optimized the amount of vertical-axis rotation necessary to bring the dyke trends and paleomagnetic remanence declinations into alignment, yet such calculations are not kinematically viable in a plate-tectonic framework. Here we subdivide the Superior craton into two internally rigid subplates and calculate Euler parameters that optimally group the paleomagnetic remanence data from six dyke swarms with ages between 2470 and 2070 Ma. Our dataset includes 59 sites from the Matachewan dykes for which directional results are reported for the first time. Our preferred restoration of the eastern Superior subprovince relative to the western subprovince is around an Euler pole at 51°N, 85°W, with a rotation angle of 14° CCW. Although we do not include data from the KSZ in our rigid-subplate calculations, we can align its dyke strikes by applying a 23° CCW distributed shear that preserves line length of all dykes pinned to the western margin. Our model predicts approximately 90 km of dextral transpressional displacement at ca. 1900 Ma, about half of which is accommodated by distributed strain within the KSZ, and the other half by oblique lateral thrusting (with NE-vergence) across the Ivanhoe Lake shear zone. We produce a combined apparent polar wander path for the early Paleoproterozoic Superior craton that incorporates data from both western and eastern subplates, and that can be rotated to either of the subplates’ reference frames for the purposes of Archean-Paleoproterozoic supercraton reconstructions.     

国际新元古代年代地层学研究进展与发展趋势

国际地层委员会末元古系分会1989年正式成立以来,经过各国地层古生物学家近十五年的努力,国际末前寒武纪年代地层学研究已取得很大进展.全球末元古系的正式名称--埃迪卡拉系(Ediacaran)以及埃迪卡拉系的底界层型已经国际地科联批准,在国际地层委员会2004年版"国际地层表"中被正式应用(Gradstein et al.,2004).本文综合近年来国际地层委员会末元古系分会有关文件和部分委员的有关建议,对国际新元古代年代地层学研究的进展与未来发展动向作一概要介绍,以期引起国内晚前寒武纪地层古生物学者的广泛关注.     

The distribution of microfossil assemblages in Proterozoic rocks

A biostratigraphic model of the temporal distribution of distinctive Proterozoic microfossil assemblages is suggested, based on studies of upper Precambrian chert-embedded and compression-preserved organic-walled microfossils from the reference sections of Eurasia, North America and Australia. Microfossils from 2.0 to 0.542 Ga can be divided into seven successive informal global units which can be compared to standard units of the International and Russian time scales. Each unit is characterized by a particular association of taxa, typified by the fossil assemblage that gives it its name. These form broad biostratigraphic units comparable to assemblage zones of Phanerozoic successions; in general (but with minor differences) they correspond to chronostratigraphic units accepted by the Internal Commission on Stratigraphy. The units are: (1) Labradorian, the upper part of the Paleoproterozoic (Orosirian and Statherian), 2.0–1.65 Ga; (2) Anabarian, lower Mesoproterozoic (Calymmian–Ectasian)/Lower Riphean–lower Middle Riphean, 1.65–1.2 Ga; (3) Turukhanian, upper Mesoproterozoic (Stenian)/upper Middle Riphean, 1.2–1.03 Ga; (4) Uchuromayan, lower Neoproterozoic (late Stenian–Tonian)/lower Upper Riphean, 1.03–0.85 Ga; (5) Yuzhnouralian, upper Neoproterozoic (Cryogenian)/upper Upper Riphean, 0.85–0.63 Ga; (6) Amadeusian, lower Ediacaran/lower Vendian, 0.63–0.55 Ga; (7) Belomorian, upper Ediacaran/upper Vendian, 0.55–0.542 Ga.     

元古宙岩体型斜长岩的特征及研究现状

斜长岩是指斜长石含量>90%的岩浆岩,可分为6类。其中,岩体型斜长岩仅赋存于前寒武纪变质地体中,形成时代主要为元古宙（2.1~ 0.9 Ga）,代表地球演化史上很重要的构造-热事件。岩体呈穹隆状或层状产出,具典型堆晶结构,有含钾长石和斜长石出溶片晶的巨晶斜长石和富铝辉石。巨晶的出溶指示了岩浆由高压至低压的变压结晶过程,体现了斜长岩体深成、浅侵位的特点。关于斜长岩的源区,之前普遍认为源于幔源玄武质岩浆,而近10年来更趋向于源区为下地壳,母岩浆的成分为纹长苏长岩和铁闪长岩等新认识;其成因模式以底侵模式和地壳舌状物熔融模式最具代表性。岩体型斜长岩时空上常与奥长环斑花岗岩共生,构成AMCG（Anorthosite Mangerite Charnockite Granite）岩石组合,被认为属非造山岩浆作用的产物,可能代表大陆裂谷环境。然而,新近一些年龄结果显示,它们形成于造山作用的后期阶段,暗示岩体产出于碰撞后环境。斜长岩体中常赋存有Fe Ti V氧化物矿床,有的富含P及Cu,Ni硫化物等,属典型的岩浆矿床。对此,目前主要有结晶分异过程、早期堆晶过程及不混熔分离3种成因机制解释。由此对今后研究中值得关注的问题提出了一些看法。     

Reconciliation of conflicting tectonic models for Proterozoic rocks of northern New Mexico

Two major Proterozoic tectonic events are documented in the Taos Range of northern New Mexico. Regional structures involving the tectonic interleaving of c.   1.65  Ga granitoids with supracrustal rocks are interpreted to have formed before 1.42  Ga and probably during collisional assembly of island arc crust into new (1.7–1.6  Ga) continental lithosphere. Supracrustal rocks record 650–750  °C, 6–8  kbar metamorphism (M₂); these high temperatures may have been reached during sandwiching between c.   1.65  Ga granitoids. However, the early history has been obscured by renewed tectonism at c.   1.4  Ga that resulted in partial melting, fabric reactivation and new mineral growth at 4  kbar (M₃). Metamorphic temperature variations from uppermost-amphibolite to amphibolite facies rocks may be associated with c.   1.65 and/or 1.4  Ga plutonism, but not to a 1.4  Ga extensional shear zone as previously proposed. Syn- and post-1.4  Ga contraction is suggested by high- and low-temperature microstructures showing top-to-the-south-east thrusting. This work reconciles conflicting models by suggesting that the geometry of the structures was mainly established by c.   1.65  Ga, but that the present fabric also records 1.4  Ga tectonism involving high- T  metamorphism and fabric reactivation.     

第四纪沉积物测年新进展

由于沉积环境和沉积物类型与成因的差异,第四纪沉积物岩性、组成会随时间变化,并且结构和厚度在空间上也可能发生明显变化,使得长期以来难以测得其准确年代。论文对K/Ar法和⁴⁰Ar/³⁹Ar法、玻璃陨石法、铀系定年、氨基酸外消旋法、宇宙成因核素法、电子自旋共振定年等各种方法的定年范围和测定对象进行了介绍,分析了第四纪测年方法新进展存在的主要问题和改进途径。要提高所测年代结果的可靠性和准确度,不仅需要丰富的地质工作经验,还需要选择最恰当的定年方法,并且尽可能用多种定年方法进行交叉对比。随着第四纪环境演变及全球气候变化等方面的研究日益受到地质学者的关注,相信第四纪沉积物定年方法在全球气候变化、环境演化等研究领域具有更加广阔的发展前景。     

Microstructural constraints on the timing of Proterozoic deformation in Central New Mexico

The nature and extent of deformation associated with 1.4 Ga tectonism in the south-western USA are poorly understood. Two models have been proposed. Both agree that Proterozoic crustal accretion occurred at 1.65 Ga and that the rocks remained at mid-crustal conditions ( c . 12 km depth) until 1.4 Ga. However, one model suggests that 1.4 Ga deformation was regionally extensive, the other that it was localized around 1.4 Ga plutons. Following 1.4 Ga tectonism, the crust cooled below 300 °C. Detailed studies of quartz mylonite microfabrics in samples both adjacent to and removed from 1.4 Ga plutons in the Manzano Mountains, central New Mexico, are used to discriminate between these models of mid-Proterozoic thermotectonic history. In this area, as in much of northern New Mexico, the metamorphic conditions prior to emplacement of 1.4 Ga plutons were 500 °C and 4 kbar. The quartz mylonite microfabrics include ribbon grains, recrystallized grains with serrated boundaries, and strong c-axis crystallographic preferred orientations, which indicate no post-deformational modification. All of these microfabrics are consistent with deformation at upper greenschist/lower amphibolite facies conditions, and could have formed during either 1.65 or 1.4 Ga tectonism. Microfabrics formed during 1.65 Ga tectonism, however, should have been substantially modified by annealing recrystallization during residency in the middle crust and/or thermal/mechanical effects associated with 1.4 Ga tectonism. The observed microstructures are consistent with regional deformation associated with metamorphism at 1.4 Ga. The effects of deformation at 1.4 Ga in New Mexico are therefore more widespread than previously thought.     

Tectonic evolution of Proterozoic rocks in the Cimarron Mountains, northern New Mexico, USA

Abstract Portions of three Proterozoic tectonostratigraphic sequences are exposed in the Cimarron Mountains of New Mexico. The Cimarron River tectonic unit has affinities to a convergent margin plutonic/volcanic complex. Igneous hornblende from a quartz diorite stock records an emplacement pressure of 2–2.6 kbar. Rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4–5 kbar and 330 ± 50° C. The Tolby Meadow tectonic unit consists of quartzite and schist. Mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 ± 20° C. A low-angle ductile shear zone separates this succession from gneisses of the structurally underlying Eagle Nest tectonic unit. Gneissic granite yields hornblende pressures of 6–8 kbar. Pelitic gneiss records regional metamorphic conditions of 6–7 kbar and 705 ± 15° C, overprinted by retrogression at 4 kbar and 530 ± 10° C. Comparison of metamorphic and retrograde conditions indicates a P–T path dominated by decompression and cooling. The low-angle ductile shear zone represents an extensional structure which was active during metamorphism. This extension juxtaposed the Tolby Meadow and Eagle Nest units at 4 kbar and 520° C. Both units were later overprinted by folding and low-grade metamorphism, and then were emplaced against the Cimarron River tectonic unit by right-slip movement along the steeply dipping Fowler Pass shear zone. An argon isotope-correlation age obtained from igneous hornblende dates plutonism in the Cimarron River unit at 1678 Ma. Muscovite associated with the greenschist facies metamorphic overprint yields a ⁴⁰ Ar/³⁹ Ar plateau age of 1350 Ma. By contrast, rocks within the Tolby Meadow and Eagle Nest units yield significantly younger argon cooling ages. Hornblende isotope-correlation ages of 1394–1398 Ma are interpreted to date cooling during middle Proterozoic extension. Muscovite plateau ages of 1267–1257 Ma appear to date cooling from the low-grade metamorphic overprint. The latest ductile movement along the Fowler Pass shear zone post-dated these cooling ages. Argon released from muscovites of the Eagle Nest/Tolby Meadow composite unit, at low experimental temperatures, yields apparent ages of c. 1100 Ma. Similar ages are not obtained north-east of the Fowler Pass shear zone, suggesting movement more recently than 1100 Ma.     

中国南方海、陆相白垩系研究进展

自21世纪以来,中国南方海、陆相白垩系研究有显著进展。依据我国南方大地构造环境和白垩系发育特点,将其白垩系划分为5个地层区和8个地层分区;文中在分析全球年代与生物地层研究进展的基础上,对我国南方海、陆相白垩纪年代地层划分、底界界线层型及陆相白垩系上/下统的界线作了扼要介绍,并指出我国西南特提斯区海相侏罗系/白垩系界线研究已接近国际水平。     

环境演变的矿物标识研究新进展

天然矿物是自然演变的产物，在其形成和变化的整个过程中，不同时间和空间尺度上的环境变化都会在矿物中留下烙印，使得矿物成为环境演变信息的载体。本文对黄土、河流、湖泊、海洋、大气等自然环境体中的天然矿物所标识的环境演变规律进行了系统的概括，揭示了矿物的外部微形貌、内部微结构、化学组成、化学性质、物理性质、谱学特征、成因及其共生组合等方面特征与环境演变的关系，显示出矿物标识环境演变的信息载体作用。     

The Late Proterozoic Continental Accretionary History of the Southeastern Margin of Jiangnania

The newly obtained Sm-Nd isochron ages are 1034 Ma and 935 Ma for ophiolites from northeasternJiangxi and Fuchuan, southern Anhui respectively. There exist two unconformity surfaces under the initialLate Proterozoic volcanics as well as the Sinian rocks. The Xiuning intrusive body which was intruded into theShangxi Group in southern Anhui yields a whole-rock Rb-Sr isochron age of 963 Ma. There occurs a belt ofLate Proterozoic calc-alkali volcanic rocks extending from northeastern Jiangxi to northwestern Zhejiang. Inthe light of the above facts, the authors consider that the southeastern margin of Jiangnania is an ancient islandarc. At about 1000 Ma ago, the Huanan (South China) oceanic crust was subducted along the line linking Dex-ing and Hangzhou, thus starting the accretion of the island arc system to Jiangnania. At 800 Ma ago Cathaysiaand Jiangnania converged together along the Jiangshan-Shaoxing line, marking the end of the accretion.     

Samples of Proterozoic iron-enriched mantle from the Premier kimberlite

Two populations of mantle xenoliths from the Proterozoic Premier kimberlite show an absence of potassic metasomatism common in Phanerozoic kimberlites. The Premier samples are relatively enriched in Fe and Ti, and contain Fe mica and aluminous amphibole instead of Mg-phlogopite and K-richterite. These features are consistent with a recently identified ρ wave anomaly beneath this part of the Kaapvaal craton ascribed to refertilization of the mantle. Upwelling of sublithospheric mantle to produce the Bushveld Igneous Complex is considered to be the source of silicate melt available for metasomatism. The resultant refertilized Fe-, Ti-, and Al-enriched mantle composition resembles that which is required to form Proterozoic troctolitic magmas.     

A structural synthesis of the Proterozoic Arabian-Nubian Shield in Egypt

Detailed structural geological and related studies were carried out in a number of critical areas in the Proterozoic basement of eastern Egypt to resolve the structural pattern at a regional scale and to assess the general characteristics of tectonic evolution, orogeny and terrane boundaries. Following a brief account of the tectonostratigraphy and timing of the orogenic evolution, the major structural characteristics of the critical areas are presented. Collisional deformation of the terranes ended about 615-600 Ma ago. Subsequent extensional collapse probably occurred within a relatively narrow time span of about 20 Ma (575 – 595 Ma ago) over the Eastern Desert and was followed by a further period of about 50 Ma of late to post-tectonic activity. The regional structures originated mainly during post-collisional events, starting with those related to extensional collapse (molasse basin formation, normal faulting, generation of metamorphic core complexes). Subsequent NNW-SSE shortening is documented by large-scale thrusting (towards the NNW) and folding, distributed over the Eastern Desert, although with variable intensity. Thrusts are overprinted by transpression, which was localized to particular shear zones. Early transpression produced, for example, the Allaqi shear zone and final transpression is documented in the Najd and Wadi Kharit-Wadi Hodein zones. Two terrane boundaries can be defined, the Allaqi and South Hafafit Sutures, which are apparently linked by the high angle sinistral strike-slip Wadi Kharit-Wadi Hodein shear zone with a tectonic transport of about 300 km towards the W/NW. In general, the tectonic evolution shows that extensional collapse is not necessarily the final stage of orogeny, but may be followed by further compressional and transpressional tectonism. The late Pan-African high angle faults were reactivated during Red Sea tectonics both as Riedel shears and normal faults, where they were oriented favourably with respect to the actual stress regime.     

Discovery of Early Proterozoic Carbon Isotope Shifts

The authors have detailedly and systematically studied the carbon isotopic composition of Early Proterozoic carbonate rocks. Samples which are all dolomicrite were taken from the Jianancun, Daguandong and Huaiyincun Formations of the Hutuo Group in Wutai County Shanxi Province, North China. A total of 209 samples were analysed for their carbon isotope compositions, and the mean sampling interval was 6.9 m. Carbon isotope analysis clearly shows δ13C shifts at the boundary between the Jian'ancun Formation and Daguandong Formation and near the boundary between the Daguandong Formation and Huaiyincun Formation. Like carbon isotope shifts at the Cretaceous-Tertiary, Permian-Triassic and Precambrian-Cambrian boundaries, the discovery of δ13C shifts in the Early Proterozoic has important significance in further studies on Early Proterozoic biotic evolution, regional and global stratigraphic correlation, and carbon geochemical cycles.     

祁连山东段的元古宙陇山群

陇山群是从祁连山东段原“牛头河群”哑岩系下部分解出来的变质岩系，它包括从下到上的大理岩和片麻岩两个岩组。依其片麻岩组先后同位素年代学的研究，主要最早一组年龄值为８１３－８１１Ｍａ，相当中－上元古代。作者从该岩群的出露和展布，认为在构造体系上，它属祁连山带的基底隆起部分，并讨论了赋存在片麻岩组内陈家调铁铜矿床的特征及其地质意义。     

Five hundred million years of punctuated addition of juvenile crust during extension in the Goochland Terrane,central Appalachian Piedmont Province

ABSTRACT

The Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane.     

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.     

Supercontinent evolution and the Proterozoic metallogeny of South America

The cratonic blocks of South America have been accreted from 2.2 to 1.9 Ga, and all of these blocks have been previously involved in the assembly and breakup of the Paleoproterozoic Atlantica, the Mesoproterozoic to Neoproterozoic Rodinia, and the Neoproterozoic to Phanerozoic West Gondwana continents. Several mineralization phases have sequentially taken place during Atlantica evolution, involving Au, U, Cr, W, and Sn. During Rodinia assembly and breakup and Gondwana formation, the crust-dominated metallogenic processes have been overriding, responsible for several mineral deposits, including Au, Pd, Sn, Ni, Cu, Zn, Mn, Fe, Pb, U, P₂O₅, Ta, W, Li, Be and precious stones. During Rodinia breakup, epicontinental carbonate-siliciclastic basins were deposited, which host important non-ferrous base metal deposits of Cu–Co and Pb–Zn–Ag in Africa and South America. Isotope Pb–Pb analyses of sulfides from the non-ferrous deposits unambiguously indicate an upper crustal source for the metals. A genetic model for these deposits involves extensional faults driving the circulation of hydrothermal mineralizing fluids from the Archean/Paleoproterozoic basement to the Neoproterozoic sedimentary cover. These relations demonstrate the individuality of metal associations of every sediment-hosted Neoproterozoic base-metal deposit of West Gondwana has been highly influenced by the mineralogical and chemical composition of the underlying igneous and metaigneous rocks.     

A mantle or mafic crustal source for Proterozoic anorthosites?

Calculations of fractional crystallization (FC) and assimilation fractional crystallization (AFC) at 11 kb for a variety of primitive magmatic compositions and a mafic assimilant demonstrate that none of them has a bulk composition suitable to be parental to massif anorthosites. Mafic compositions thought to be parental to massif anorthosites have Mg′ values of 0.6 to 0.4 and form coherent arrays with moderately steep slopes on plots of TiO₂, K₂O, and P₂O₅ versus Mg′. The calculated liquid lines of descent (LLD) of basaltic magmas undergoing FC or AFC processes pass through the arrays of anorthosite parent magma compositions with much shallower slopes than the natural arrays, which indicates that the arrays of natural parental magmas were produced by a process other than FC/AFC. Also, by the time most crystallizing basaltic magmas with or without assimilation reach plagioclase saturation, their residual liquids have Mg′ values that are too low to be parental to anorthosites. MORB-like olivine tholeiites and high-aluminum olivine tholeiites (HAOT) from convergent plate margins do reach plagioclase saturation while sufficiently magnesian, but their Wo (Wollastonite) contents are too high such that they reach plagioclase saturation coexisting only with augite and do not reach orthopyroxene saturation (if at all) until Mg′ is too low. Calculations show it is not possible to produce a high-Al melt from typical mantle peridotites that has sufficient TiO₂ to make andesine-type anorthosite.

Calculation of partial melting for an average mafic crustal composition at 11 kbar provides a much closer match to the array of natural parental compositions in terms of minor element concentrations and proportions of mineral components. However, accounting for the entire array requires a more magnesian source composition. Such compositions exist in several crustal xenolith localities. Similar results were obtained using the bulk composition of the Stillwater Complex, which is used as a model mafic source (here the premise is that overdense crustal intrusions might sink back into the mantle). As with the terrain composition, this particular layered intrusion composition is not sufficiently magnesian, however, the fit improves when mixtures of early and late stage portions of the complex (i.e., the denser portions) were run as potential source regions.     

北秦岭构造属性与元古代构造演化

秦岭前寒武纪地质、地球化学和年代学证据表明,北秦岭具有明显高的初始εNd(t)值和Pb同位素比值特征,区别于明显低初始εNd(t)值和Pb同位素比值特征的华北陆块和扬子块体群.北秦岭与南秦岭具有明显不同的前寒武纪构造与演化特征.北秦岭是2 000 Ma左右形成于华北陆块南侧洋岛基础上的独立陆块,经历2 200~1 800 Ma和1 400~900 Ma分别以垂向加积增生和侧向加积增生为主要机制的地壳生长.北秦岭在早元古代洋盆演化基础上,首次于1 600 Ma左右拼接于华北陆块南缘,1 300~1 000 Ma发生扩张裂解,出现宽坪裂谷-洋盆构造环境,100 Ma左右再次拼贴于华北陆块南缘.同时,北秦岭南侧可能发生与南秦岭陆块群中的陡岭微陆块的拼接.