Structural history of the Greenvale Province,north Queensland: Early Palaeozoic extension and convergence on the Pacific margin of Gondwana*

The southeastern Georgetown Inlier (Greenvale Province) consists of Early Palaeozoic metamorphic rocks in fault contact along the Lynd Mylonite Zone with the Palaeoproterozoic to Mesoproterozoic craton of northeastern Australia. It has a central assemblage of metamorphosed silicic volcanic and sedimentary rocks considered equivalent to the Late Cambrian to Early Ordovician Seventy Mile Range Group that developed in an extensional backarc in the Charters Towers Province to the southeast. In the western part of the Greenvale Province, the Oasis Metamorphics have a U – Pb zircon SHRIMP metamorphic age of 476 ± 5 Ma and are intruded by the granodioritic Lynwater Complex with U – Pb zircon ages of 486 ± 5 Ma and 477 ± 6 Ma. These ages are consistent with these rocks forming basement and intrusive equivalents to the extensional volcanic basin. Existing geochronological constraints on the Halls Reward domain, located at the eastern margin of the province, are consistent with it being basement to the extensional basin. Several domains are recognised in the Greenvale Province with either dominantly steep or low to moderate dips of the main foliation, and each experienced multiple deformation with locally up to four overprinting structural phases. Steepening of foliation in several of the domains is attributed to contractional deformation in the Early Silurian that is inferred to have overprinted low-angle foliation developed during extensional tectonics in the backarc setting. Contractional deformation related to the Early Silurian Benambran Orogeny is considered responsible for multiple deformation in the Greenvale Province and reactivation of domain-bounding faults.     

A plate tectonic scenario for the Iapetus and Rheic oceans

The tectonics, dynamics, and biogeographic landscape of the early Paleozoic were dominated by the opening and expansion of one large ocean—the Rheic—and the diminution to terminal closure of another—Iapetus. An understanding of the evolution of these oceans is thus central to an understanding of the early Paleozoic, but their chronicle also presents a rich temporal profile of the Wilson cycle, illustrating continental-scale rifting, microcontinent formation, ocean basin development, arc accretion, and continent–continent collision. Nevertheless, contemporary paleogeographic models of the Iapetus and Rheic oceans remain mostly schematic or spatiotemporally disjointed, which limits their utility and hinders their testing. Moreover, many of the important kinematic and dynamic aspects of the evolution of these oceans are impossible to unambiguously resolve from a conceptual perspective and the existing models unsurprisingly present a host of contradictory scenarios. With the specific aim to resolve some of the uncertainties in the evolution of this early Paleozoic domain, and a broader aim to instigate the application of quantitative kinematic models to the early Paleozoic, I present a new plate tectonic model for the Iapetus and Rheic oceans. The model has realistic tectonic plates, which include oceanic lithosphere, that are defined by explicit and rigorously managed plate boundaries, the nature and kinematics of which are derived from geological evidence and plate tectonic principles. Accompanying the presentation and discussion of the plate model, an extensive review of the underlying geological and paleogeographic data is also presented.     

Multi-proxy isotopic tracing of magmatic sources and crustal recycling in the Palaeozoic to Early Jurassic active margin of North-Western Gondwana

We trace source variations of active margin granitoids which crystallised intermittently over ~300 Ma in varying kinematic regimes, by combining zircon Lu-Hf isotopic data from Early Palaeozoic to Early Jurassic igneous and metaigneous rocks in the Mérida Andes, Venezuela and the Santander Massif, Colombia, with new whole rock Rb/Sr and Sm-Nd isotopic data, and quartz O isotopic data. These new data are unique in South America because they were obtained from discrete magmatic and metamorphic zircon populations, providing a high temporal resolution dataset, and compare several isotopic systems on the same samples. Collectively, these data provide valuable insight into the evolution of the isotopic structure of the continental crust in long-lived active margins.Phanerozoic active margin-related granitoids in the Mérida Andes and the Santander Massif yield zircon Lu-Hf model ages ranging between 0.77 Ga and 1.57 Ga which clearly define temporal trends that can be correlated with changes in tectonic regimes. The oldest Lu-Hf model ages of >1.3 Ga are restricted to granitoids which formed during Barrovian metamorphism and crustal thickening between ~499 Ma and ~473 Ma. These granitoids yield high initial ⁸⁷Sr/⁸⁶Sr ratios, suggesting that evolved, Rb-rich middle to upper crust was the major source of melt. Granitoids and rhyolites which crystallised during subsequent extension between ~472 Ma and ~452 Ma yield younger Lu-Hf model ages of 0.80 Ga–1.3 Ga and low initial ⁸⁷Sr/⁸⁶Sr ratios, suggesting that they were derived from much more juvenile, Rb-poor sources such as mafic lower crust and mantle-derived melts. The rapid change in magmatic sources at ~472 Ma can be attributed either to reduced crustal assimilation during extension, or a short pulse of crustal growth by addition of juvenile material to the continental crust. Between ~472 Ma and ~196 Ma Lu-Hf model ages remain mostly constant between ~1.0 and ~1.2 Ga. The large scatter and the absence of definite trends in initial ⁸⁷Sr/⁸⁶Sr ratios suggest that both mafic, Rb-poor, and evolved Rb-rich sources were important precursors of active margin magmas in Colombia and Venezuela throughout the Palaeozoic to the Early Jurassic.Previous studies have shown that the genesis of arc magmas may be stimulated by heat advection to the crust during the underplating of mantle derived melt, but the absence of permanent younging trends in Lu-Hf model ages from ~472 Ma to ~196 Ma suggests that very little new crust was generated during this period in the studied region. An overwhelming majority of the analysed igneous rocks yield zircon Lu-Hf model ages of >1 Ga which may be accounted for by documented local crustal end members of 1 Ga–1.6 Ga, and do not require contributions from the depleted mantle. Therefore, recycling of ~1 Ga and older crust was a dominant process in the north-western corner of Gondwana between ~472 Ma and ~196 Ma.This study shows that whole rock Sm-Nd and zircon Lu-Hf data can be interpreted similarly regarding the age of the source regions, whereas Rb-Sr and O isotope data from the same rocks yield valuable information regarding the geochemical nature of the source.     

The Palaeozoic evolution in the Alps: from Gondwana to Pangea

More than 50% of the Alps expose fragments of Palaeozoic basement which were assembled during the Alpine orogeny. Although the tectonic and metamorphic history of the basement units can be compared to that of the Variscan crust in the Alpine foreland, most of the basement pieces of the Alps do not represent the direct southern continuation of Variscan structural elements evident in the Massif Central, the Vosges–Black Forest or the Bohemian massif. The basement units of the Alps all originated at the Gondwana margin. They were derived from a Precambrian volcanic arc suture fringing the northern margin of Gondwana, from which they were rifted during the Cambrian–Ordovician and Silurian. A short-lived Ordovician orogenic event interrupted the general rifting tendency at the Gondwana active margin. After the Ordovician, the different blocks drifted from the Gondwana margin to their Pangea position, colliding either parallel to Armorica with Laurussia or with originally peri-Gondwanan blocks assembled presently in Armorica. From the Devonian onwards, many basement subunits underwent the complex evolution of apparently oblique collision and nappe stacking. Docking started in the External massifs, the Penninic and Lower and middle Austroalpine units in approximately Devonian/early Carboniferous times, followed by the Upper Austroalpine and the South Alpine domains, in the Visean and the Namurian times, respectively. Wrenching is probably the best mechanism to explain all syn and post-collisional phenomena since the Visean followed by post-orogenic collapse and extension. It explains the occurrence of strike-slip faults at different crustal levels, the formation of sedimentary troughs as well as the extrusion and intrusion of crustal and mantle-derived magmas, and allows for contemporaneous rapid uplift of lower crustal levels and their erosion. From the Stephanian onwards, all regions were deeply eroded by large river systems.     

Early Palaeozoic orogenic events north of the Rheic suture (Brabant,Ardenne): A review

The Lower Palaeozoic rocks exposed in the Brabant-Ardenne region (Belgium, France) recorded the Early Palaeozoic history on the southern margin of the perigondwanan microcontinent of Avalonia, north of the Rheic suture. These rocks crop out in the Brabant basement and in the Ardenne basement inliers within the Variscan Ardenne allochthon. The two main unconformities are classically associated with distinct orogenic episodes, the Late Ordovician “Ardennian” event and the Early Devonian “Brabantian” event. A review of the current state-of-knowledge with respect to the reconstruction of Early Palaeozoic geodynamics in the Brabant-Ardenne region is presented. It is demonstrated that an unconformity does not necessarily represent an orogenic event, and that the hiatus related to an unconformity does not necessarily coincide with tectonic activity, especially when tectonism is diachronous in nature. The former applies to the Ardennian unconformity, while the latter applies to the Brabantian unconformity. Finally, the well-constrained Brabantian orogeny, as well as the Ardenne-Eifel basin development, is tentatively framed within the Early Palaeozoic geodynamic context of the northern margin of the Rheic realm. By doing so, it is shown that the Brabant-Ardenne region links, both in space and time, the Rheic and Rhenohercynian ocean.     

Highly depleted isotopic compositions evident in Iapetus and Rheic Ocean basalts: implications for crustal generation and preservation

Subduction of both the Iapetus and Rheic oceans began relatively soon after their opening. Vestiges of both the Iapetan and Rheic oceanic lithospheres are preserved as supra-subduction ophiolites and related mafic complexes in the Appalachian–Caledonian and Variscan orogens. However, available Sm–Nd isotopic data indicate that the mantle source of these complexes was highly depleted as a result of an earlier history of magmatism that occurred prior to initiation of the Iapetus and Rheic oceans. We propose two alternative models for this feature: either the highly depleted mantle was preserved in a long-lived oceanic plateau within the Paleopacific realm or the source for the basalt crust was been recycled from a previously depleted mantle and was brought to an ocean spreading centre during return flow, without significant re-enrichment en-route. Data from present-day oceans suggest that such return flow was more likely to have occurred in the Paleopacific than in new mid-ocean ridges produced in the opening of the Iapetus and Rheic oceans. Variation in crustal density produced by Fe partitioning rendered the lithosphere derived from previously depleted mantle more buoyant than the surrounding asthenosphere, facilitating its preservation. The buoyant oceanic lithosphere was captured from the adjacent Paleopacific, in a manner analogous to the Mesozoic–Cenozoic “capture” in the Atlantic realm of the Caribbean plate. This mechanism of “plate capture” may explain the premature closing of the oceans, and the distribution of collisional events and peri-Gondwanan terranes in the Appalachian–Caledonian and Variscan orogens.     

The Early Palaeozoic break-up of northern Gondwana,new palaeomagnetic and geochronological data from the Saxothuringian Basin,Germany

Early Palaeozoic volcanic and sedimentary rocks from the Saxothuringian Basin (Franconian Forest, northern Bavaria) have been subjected to detailed radiometric and palaeomagnetic studies in order to determine the tectonic environment and geographic setting in which they were deposited. Two hand samples were collected from the as yet undated pyroclastic flow deposits for ²⁰⁷Pb/²⁰⁶Pb age dating. Radiometric results for these samples, obtained by the single-zircon evaporation technique, are identical within error, and the mean age of all measured grains is 478.2ǃ.8 Ma (n=11). This age is considered to be primary and firmly constrains the eruption of the ignimbrites and formation of the subaqueous pyroclastic flows as having occurred in Early Ordovician (Arenig) times. Palaeomagnetic studies were carried out on these Early Ordovician volcanic rocks, and also on the biostratigraphically dated, Late Ordovician (Ashgillian) Döbra sandstones. The volcanic rocks carry up to three directions of magnetisation. The poorly defined, low and intermediate unblocking temperature directions are thought to represent secondary overprint directions of post-Ordovician age. The high temperature component, however, identified at temperatures of up to 580 °C, is of mixed polarity and passes the fold test with 99% confidence. The overall mean direction after bedding correction is 189°/76°, !₉₅=11.6°, k=44.7 (25 samples, five sites), and is considered to be primary and Early Ordovician in origin. It yields a palaeo-south pole at 24°N and 007°E, which translates into palaeolatitudes of 63°+21.7°/-17.3° S for the Saxothuringian Terrane. Samples from the Late Ordovician Döbra sandstone are generally very weakly magnetised. A high temperature D component of magnetisation can be identified in some samples and yields a mean direction of 030°/-58°, !₉₅=18.5°, k=25.7 (15 samples, four sites) after bedding correction. The Arenig palaeomagnetic results indicate high palaeolatitudes, but separation from northern Gondwana. This is in basic agreement with data from elsewhere in the Armorican Terrane Assemblage, all of which suggest high southerly palaeolatitudes in the Early Ordovician. The geochemical signatures of these rocks indicate emplacement in an extensional environment. These new data, therefore, are interpreted as marking the onset of rifting of Saxothuringia from the north African margin of Gondwana, and the start of the relative northward migration of the Saxothuringian Terrane. Although the Late Ordovician palaeomagnetic results presented here are only poorly constrained, they suggest an intermediate palaeolatitude for Saxothuringia in Ashgillian times, in good agreement with Late Ordovician palaeomagnetic data from the Barrandian.     

Comparison of the Jurassic and Cretaceous sedimentary cycles of Somalia and Madagascar: implications for the Gondwana breakup

The Jurassic and Cretaceous sedimentary history of northern Somalia and the Morondava Basin of south-western Madagascar have been studied. Both regions display an independent facial development; however, a comparison of the sequential evolution of the Mesozoic sedimentary successions in these two presently widely separated areas reveals a surprisingly high level of similarity, which probably reflects major events during the disintegration of Eastern Gondwana during the Jurassic and Cretaceous. Although in Jurassic times the onset of transgressions and regressions in both areas compares well with eustatic development, major deviations in combination with the tectonic activities of different degrees are observed in the Early and Late Cretaceous synchronously in both regions. Transgressions are observed in Toarcian, Bajocian (not dated in northern Somalia), Callovian, Valanginian (Madagascar only), Aptian and Campanian times. Tectonism is noted before the Aptian and Campanian transgressions in northern Somalia and the Morondava Basin of south-western Madagascar.     

Phanerozoic plate history and structural evolution of the Tarim Basin,northwestern China

ABSTRACT

As the largest inland oil-bearing basin in China, the Tarim Basin is a large-scale composite basin that has experienced a complex tectonic evolutionary history from the Ediacaran to the Cenozoic. From the Ediacaran to the Ordovician, the Tarim Basin was in an extensional tectonic environment. From the Silurian to the Devonian, the Tarim Block switched from the presence of passive margins to active margins along its northern and southern edges, eventually colliding with the North Kunlun Terrane in the Silurian. From the Carboniferous to the Triassic, the transition of the Tarim Block from an independent landmass to an internal component of the Eurasian Plate resulted from collisions with the Yili-Central Tianshan Terrane to the north during the Late Carboniferous and the Qiangtang Terrane to the south during the Triassic. From the Jurassic to the Paleogene, several unconformities developed because of the subduction of the Meso-Tethys oceanic plate during the Late Jurassic and the Neo-Tethys oceanic plate during the Paleogene. After the Neogene, as a rejuvenated foreland basin, the Tarim Basin was activated along its margins and became an intermountain basin due to the intense regional compression induced by the Indian Plate. Based on a seismic profile cross-section of the basin, we conclude that the extension and shortening in the profile reflects the block amalgamation history and the structural evolution of the Tarim Basin. The structural-sedimentary evolution of this basin is closely related to the movement of the peripheral plates.     

Two-stage evolution of mantle peridotites from the Stalemate Fracture Zone, northwestern Pacific

This paper reports the results of a mineralogical study of 14 mantle peridotite samples dredged in 2009 from the eastern slope of the northwestern segment of the Stalemate Ridge in the northwestern Pacific during cruise SO201-KALMAR Leg 1b of the R/V Sonne. The sample collection included four serpentinized and silicified dunites and ten variably serpentinized lherzolites. The compositions of primary minerals (clinopyroxene, orthopyroxene, and spinel) change systematically from the lherzolites to dunites. Spinel from the lherzolites shows higher Mg# and lower Cr# values (0.65–0.68 and 0.26–0.33, respectively) compared with spinel from the dunites (Mg# = 0.56–0.64 and Cr# = 0.38–0.43). Clinopyroxene from the lherzolites is less magnesian (Mg# = 91.7–92.4) than clinopyroxene from dunite sample DR37-3 (Mg# = 93.7). Based on the obtained data, it was concluded that the lherzolites of the Stalemate Fracture Zone were derived by 10–12% near-fractional melting of a DMM-type depleted mantle reservoir beneath the Kula-Pacific spreading center. The dunites were produced by interaction of residual lherzolites with sodium- and titaniumrich melt and are probably fragments of a network of dunite channels in the shallow mantle. The moderately depleted composition of minerals clearly distinguishes the lherzolites from the strongly depleted peridotites of the East Pacific Rise and indicates the existence of slow-spreading mid-ocean ridges in the Pacific Ocean during the Cretaceous-Paleogene.     

Basement lithostratigraphy of the Adula nappe: implications for Palaeozoic evolution and Alpine kinematics

The Adula nappe belongs to the Lower Penninic domain of the Central Swiss Alps. It consists mostly of pre-Triassic basement lithologies occurring as strongly folded and sheared gneisses of various types with mafic boudins. We propose a new lithostratigraphy for the northern Adula nappe basement that is supported by detailed field investigations, U–Pb zircon geochronology, and whole-rock geochemistry. The following units have been identified: Cambrian clastic metasediments with abundant carbonate lenses and minor bimodal magmatism (Salahorn Formation); Ordovician metapelites associated with amphibolite boudins with abundant eclogite relicts representing oceanic metabasalts (Trescolmen Formation); Ordovician peraluminous metagranites of calc-alkaline affinity ascribed to subduction-related magmatism (Garenstock Augengneiss); Ordovician metamorphic volcanic–sedimentary deposits (Heinisch Stafel Formation); Early Permian post-collisional granites recording only Alpine orogenic events (Zervreila orthogneiss). All basement lithologies except the Permian granites record a Variscan + Alpine polyorogenic metamorphic history. They document a complex Paleozoic geotectonic evolution consistent with the broader picture given by the pre-Mesozoic basement framework in the Alps. The internal consistency of the Adula basement lithologies and the stratigraphic coherence of the overlying Triassic sediments suggest that most tectonic contacts within the Adula nappe are pre-Alpine in age. Consequently, mélange models for the Tertiary emplacement of the Adula nappe are not consistent and must be rejected. The present-day structural complexity of the Adula nappe is the result of the intense Alpine ductile deformation of a pre-structured entity.     

西藏南部江孜盆地上侏罗统至古近系沉积岩石学特征与盆地演化

江孜盆地紧邻雅鲁藏布江缝合带,随着特提斯洋的演化及最终消亡,盆地必然随之经历一系列演化,分析其沉积响应是了解盆地演化的一个重要手段.因而,本文以分布于江孜地区的晚侏罗世至古近纪海相沉积地层为研究对象,在前人建立的地层格架基础上,基于野外露头和镜下观察,仔细分析了其沉积岩石学特征.江孜地区晚侏罗世至古近纪沉积序列为:石英砂岩(上侏罗统维美组)-页岩夹火山岩屑砂岩(下白垩统日朗组)-黑色硅质/钙质页岩(中白垩统加不拉组黑层段、白层段)-黑色硅质岩(上白垩统加不拉组硅质岩段)-红色硅质页岩、泥灰岩夹滑塌灰岩(上白垩统床得组)-灰绿色页岩夹外来岩块(上白垩统宗卓组)-砂/页岩互层(古近系甲查拉组).从中可以看出海水从维美组至加不拉组硅质岩段为逐渐加深的过程,而至甲查拉组海水再次浅于硅质岩段沉积水深.生物沉积、悬浮沉积及块体搬运作用沉积常见,其中块体搬运作用包括砂质碎屑流、浊流、滑塌和岩崩等,沉积环境为大陆斜坡至深水洋盆.通过沉积学与沉积地球化学分析可知,底层水经历了缺氧、氧化再到缺氧的过程.根据沉积岩石学特征结合沉积环境分析,从较长的时间尺度上将盆地的演化历史划分为四个阶段:晚侏罗世至早白垩世被动大陆边缘盆地稳定发育阶段,白垩纪中期被动大陆边缘盆地持续沉降阶段,晚白垩世残留被动大陆边缘盆地阶段,白垩世末期至始新世初期为残留洋盆地阶段.     

Graptolite shales in the continental margins of the Paleozoic Yapetus and Paleothetys (Rheic) Oceans

Graptolite shales are a type of fossil shales that contain a large number of graptolite imprints and remains. These deposits are characterized by high TOC contents (C_org = 2–18%). Based on the data of many studies, graptolite shales are one of the main hydrocarbon sources that formed oil and gas fields in Paleozoic deposits around the world, e.g., the Silurian graptolite shales make up to 9–15% of all hydrocarbons that form the oil and gas fields in the largest petroleum basins.     

Timing of the progress granite,Larsemann hills: Additional evidence for early Palaeozoic orogenesis within the east Antarctic Shield and implications for Gondwana assembly

The Progress Granite is one of numerous S‐type granitoid plutons exposed in the Larsemann Hills region, southwest Prydz Bay, east Antarctica. The granite was emplaced into a migmatitised pelitic to felsic paragneiss sequence during a regional high‐grade transpressional event (D₂) that pre‐dates high‐grade extension (D₃). SHRIMP (II) U‐Pb dating for two occurrences of the Progress Granite from D₂ and D₃ structural domains gives ²⁰⁶Pb/²³⁸U ages of 516.2 ± 6.8 Ma and 514.3 ± 6.7 Ma, respectively. These ages are interpreted as crystallisation ages for the Progress Granite and confirm Early Palaeozoic orogenesis in the Larsemann Hills region. This orogen appears to have evolved during continental convergence and is probably responsible for widespread radiogenic isotopic resetting and the near‐complete exhumation of the adjacent northern Prince Charles Mountains which evolved during a ca 1000 Ma event. The identification of a major Early Palaeozoic orogen in Prydz Bay allows tentative correlation of other domains of Early Palaeozoic tectonism both within the east Antarctic Shield and other, once contiguous, Gondwana fragments and illustrates the potential complexity inherent within intercratonic mobile belts. One such possibility, tentatively offered here, suggests a continuous belt of Early Palaeozoic tectonism from Prydz Bay eastward to the West Denman Glacier region and into the Leeuwin complex of Western Australia.     

Late Jurassic terrane collision in the northwestern margin of Gondwana (Cajamarca Complex,eastern flank of the Central Cordillera,Colombia)

Medium-grade metabasites and metapelites from the Cajamarca Complex (Central Cordillera of Colombia) are in fault contact with the Jurassic Ibague batholith and show a penetrative foliation, locally mylonitic, suggesting intense dynamic–thermal metamorphism. The amphibolites are composed of calcic amphibole + epidote + plagioclase + quartz plus rutile + titanite + apatite + carbonate as accessory phases. Chlorite and albite appear as retrograde replacements. The metapelites are mainly composed of phengite + quartz + garnet + chlorite, plus epidote + albite + apatite + titanite + haematite as accessory phases. Bulk geochemistry of the amphibolites indicates basaltic protoliths with a mid-ocean ridge basalt (MORB) signature, although enrichment in the mobile large-ion lithophile elements compared to MORB suggests pre- and/or syn-metamorphic alteration by fluids. Peak pressure–temperature determinations for both types of rocks are similar, ranging 550–580°C and 8 kbar (approximately 26 km depth and an apparent geothermal gradient of 22°C/km). ⁴⁰Ar-³⁹Ar dating of amphibole from two amphibolite samples and one phengitic mica from a pelitic schist yielded plateau ages of 146.5 ± 1.1 Ma and 157.8 ± 0.6 Ma, and 157.5 ± 0.4 Ma, respectively. These Late Jurassic ages contrast with previously published (Permian)Triassic ages of metamorphism in the Cajamarca Complex. Taken together, our data indicate tectonic-driven burial of oceanic supracrustal sequences down to mid-crustal depths during Late Jurassic times and are best explained as the result of terrane collision-related metamorphism and deformation in a fore-arc/volcanic-arc environment of the active western margin of Gondwana rather than as a result of Jurassic thermal–metamorphic resetting of a (Permian)Triassic metamorphic sequence during intrusion of the Jurassic Ibague batholith. Our results represent the first report of Jurassic terrane collision tectonics involving supracrustal oceanic rocks in the northwestern margin of Gondwana in Colombia.     

Palaeozoic history of the Armorican Massif: Models for the tectonic evolution of the suture zones

The Armorican Massif (western France) provides an excellent record of the Palaeozoic history of the Variscan belt. Following the Late Neoproterozoic Cadomian orogeny, the Cambro-Ordovician rifting was associated with oceanic spreading. The Central- and North-Amorican domains (which together constitute the core of the Armorica microplate) are bounded by two composite suture zones. To the north, the Léon domain (correlated with the “Normannian High” and the “Mid-German Crystalline Rise” in the Saxo-Thuringian Zone) records the development of a nappe stack along the northern suture zone, and was backthrusted over the central-Armorican domain during the Carboniferous. To the south, an intermediate block (“Upper Allochthon”) records a complex, polyorogenic history, with an early high-temperature event followed by the first generation of eclogites (Essarts). This intermediate block overthrusts to the north the Armorica microplate (Saint-Georges-sur-Loire), to the south: (i) relics of an oceanic domain; and (ii) the Gondwana palaeomargin. The collision occurred during a Late Devonian event, associated with a second generation of eclogites (Cellier).     

华南地区早古生代沉积演化与油气地质条件

从构造角度上,现今的华南地区可以分为扬子地块和华夏地块,从成冰纪(南华纪)开始,由于受基底性质和构造活动等因素的影响,华南地区的沉积演化出现了分异,在扬子地块和华夏地块具有不同的沉积环境和沉积充填序列。总体上讲,扬子地块主体属于克拉通盆地,其多数时期为陆表海和局限浅海环境,构建了稳定的碳酸盐台地沉积和广泛分布的黑页岩沉积;而华夏地块处于构造活动环境,早期火山活动强烈,属于裂陷盆地,未形成统一的碳酸盐台地沉积,以陆源碎屑沉积充填为主体。两者不同的沉积环境和沉积序列决定了两个地区油气基本地质条件的差异:扬子地块具有丰富的烃源岩、良好的储集层以及封盖层的先天条件,而华夏地块没有较好的生油层和储集层。因此,在针对以早古生代地层为目的层的油气勘探工作部署时,应优先集中在扬子地块区。     

Petrogenesis of Ordovician magmatic rocks in the southern Chiapas Massif Complex: relations with the early Palaeozoic magmatic belts of northwestern Gondwana

The recent discovery of Early Ordovician S-type granites in the southwest of the Chiapas Massif Complex adds a new perspective to the Palaeozoic history of the Maya block, inasmuch as no rocks of such age had previously been reported in this region. New geologic mapping west of Motozintla, Chiapas, revealed pelitic to psammitic metasedimentary successions (Jocote Unit) intruded by granitoids and metabasites. The Jocote Unit is unconformably underlain by the newly defined Candelaria Unit, which comprises deformed calc-silicate rocks and interlayered folded amphibolites. The Candelaria Unit is the oldest rock succession so far recognized in the southern Maya block. We used laser-ablation multicollector inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating to determine the ages of the rock, yielding Early Ordovician (ca. 470 Ma) and Late Ordovician (ca. 450 Ma) ages.

Major and trace element geochemistry, as well as Nd and Sr isotope data, suggest that folded amphibolites of the Candelaria Unit are mantle-derived and are probably related to rifting. The Early Ordovician bimodal magmatism of the Jocote Unit is more strongly differentiated; it reflects crustal contamination and volcanic-arc chemical signatures. A granitic stock (Motozintla pluton) intruded the area in the Late Ordovician. Its geochemical composition indicates less crustal contamination and a mixed signature between volcanic-arc and within-plate settings. Magmatic rocks analogous in age and chemical character crop out in the Rabinal and the Altos Cuchumatanes areas of Guatemala, suggesting the existence of a semi-continuous Ordovician magmatic belt from Chiapas to central Guatemala. Similar but somewhat younger granites also occur in the Maya Mountains of Belize, suggesting that magmatism migrated in the Silurian from the Chiapas–Guatemala belt towards the Maya Mountains.     

Early Carboniferous anorogenic magmatism in the Levant: implications for rifting in northern Gondwana

The Variscan orogenesis in Europe peaked during the Late Devonian–Early Carboniferous times when Gondwanan terranes collided with Laurasia. Hitherto it has been thought that Carboniferous tectonics in northern Arabia and the adjacent parts of NE Africa were broad swells (‘arches’) and depressions (‘basins’) that formed as a far-field contractional effect of the Variscan compression. The discovery of a 351 ± 3 Ma (U–Pb in zircon) within-plate felsic volcanism in the Helez borehole, southern coastal Israel, suggests that the Levant Arch is, instead, extensional in origin. Felsic volcanism was associated with gabbro underplating of the crust, an extreme (~50°C/km) crustal thermal gradient, major uplift, and truncation of the ≥2.5 km section. Taken together with the recent discovery of the ~340 Ma oceanic crust in the Eastern Mediterranean, the Levant Arch is interpreted as an uplifted shoulder of a rift, preceding ocean spreading.     

The Dorn gold deposit in northern Victoria Land,Antarctica: Structure,hydrothermal alteration,and implications for the Gondwana Pacific margin

The Dorn gold deposit in northern Victoria Land, Antarctica is a fault related gold-only deposit and it represents the first described occurrence of gold mineralization in Paleozoic terranes of the Antarctic continent. The deposit is hosted in lower greenschist facies Middle Cambrian metavolcanic and metasedimentary rocks of the Bowers terrane. Gold-bearing veins are located in a brittle–ductile reverse high-strain zone, which was produced by transpressional deformation that overprints the structures of the Cambrian–Ordovician Ross Orogeny. The vein system is surrounded by a hydrothermal alteration zone that is as much as 300-m-wide, where the host rocks are partially to completely transformed into Fe–Mg carbonate-rich rocks with different degrees of replacement of the original mineralogy and texture.The type of host rock, the temperature estimates for mineralization from 290–320 °C, the quartz dominant vein system with sulfides and Fe-rich carbonates, and the controlling structures linked to a convergent margin tectonic setting together suggest that this mineralized vein system can be classified as an orogenic gold deposit. Close analogies are found with deposits of the Stawell zone in western Victoria, which is consistent with the correlation between units and hydrothermal events in northern Victoria Land and southeastern Australia.