Element associations and distributions through a Lower Palaeozoic graptolitic shale sequence in the Southern Uplands of Scotland

The Lower Palaeozoic graptolitic sequence in the Southern Uplands of Scotland (Moffat Shales) is a highly condensed succession of marine shales, with much of the Ordovician and Early Silurian being represented by about 100 m of shales, mudstones and cherts. A total of 133 samples were collected from five main inliers and zoned by their graptolitic assemblages. These were analysed for the elements Si and Al (94 samples), Mg (80 samples), Ti, Fe, Mn, Ca, Na, and K (33 samples), and Cu, Pb, Zn, Rb, Sr, Y and Zr (133 samples). X-ray diffraction studies reveal the presence of quartz, illite, chlorite, carbonate and pyrite and these can be related to element associations brought out by correlation and multivariate statistical techniques. These studies reveal a systematic change up the succession based on an increasing contribution of clay to these sediments. Interpretation of the principal components indicates the main control of sedimentation to be physiographic, with physico-chemical conditions and decay of organic matter also being important.     

Palaeozoic sequences and evolution of the Calabrian-Peloritan Arc (Southern Italy)

Three Palaeozoic sequences belonging to three different basins crop out in the Calabrian–Peloritan Arc. Their age covers the time span from middle (?) Cambrian to early Carboniferous. The sequences comprise terrigenous, volcanic and carbonate rocks, and show low-grade metamorphism. The basement is of pre-middle Cambrian age, crops out in the Calabrian–Peloritan Arc, and was metamorphosed prior to the opening to the Palaeozoic basins. The Palaeozoic basins existed no later than the middle Carboniferous, with inferred maximum crustal extension in the Cambro-Ordovician. By Devonian time, the tectonic regime was compressional overall with middle Devonian island-arc type volcanic activity that continued until the closure of the basins. Approximately 330 Myr, the Palaeozoic sequences experienced low-pressure greenschist facies metamorphism and continuing subduction controlled the Variscan tectonogenesis with 280 Myr island-arc type intrusive magmas. Subduction ceased and late-stage 280–270 Myr granitic magmas were emplaced during continental collision. The crustal sector carrying the Palaeozoic basins is interpreted as comprising fragments of an active continental margin, one of the several microplates, located at the southern margin of the Euro-Asia continent which faced a large (at least 1500 km in width) ocean basin.     

川西松潘-甘孜弧前盆地的形成及演化

地处柴南缘昆中蛇绿杂岩带与羌塘地块北缘可可西里-金沙江古缝合线之间的松潘-甘孜褶皱带(包括东昆仑构造带),其主体应属古特提斯洋晚石炭世-晚三叠世时期向其北侧的柴达木古陆南缘俯冲过程中在活动陆缘弧-沟间隙之间增生形成的一个大型弧前构造带。具有由弧前盆地沉积楔和基底增生杂岩构成的双重结构特点,其形成与冈瓦纳大陆北缘若尔盖"三角"地块的楔入及俯冲带向南迁移有关。大致经历了晚石炭世-早三叠世狭窄弧前盆地和中晚三叠世宽阔弧前盆地两个主要演化阶段。     

Non-axial planar S1 cleavage in the Hawick Rocks of the Galloway area,Southern Uplands,Scotland

S₁ cleavage in the Hawick Rocks of the Galloway area is non-axial planar, cutting obliquely across the F₁ folds in a predominantly clockwise sense. Individual S₁ cleavage planes within cleavage-fans in F₁ folds strike clockwise, locally anti-clockwise, of axial surfaces, and the mean plane to the S₁ cleavage-fans dips predominantly more steeply than the axial surface. F₁ folds investigated at scattered localities in Silurian and Ordovician rocks north of the Hawick Rocks are also transected by the S₁ cleavage, indicating that non-axial planar S₁ cleavage is widespread in the Southern Uplands. The S₁ cleavage is a composite fabric. Objects deformed within sandstones and tuffs indicate oblate strain. F₁ fold plunge varies from NE to SW and fold hinges locally are markedly curvilinear. Steeply plunging and locally downward-facing F₁ folds are present along the southeast margin of the Hawick Rocks. The non-axial planar S₁ cleavage relationships persist in the steeply plunging F₁ folds. Synchronous development of the non-axial planar S₁ cleavage and the variably plunging F₁ folds is proposed.     

Collision-related fore-arc and back-arc evolution of the Northern Sunda ARC

Comparison with other arc systems suggests that in the fore-arc area of the northern Sunda Arc emplacement of chromite and jadeite-bearing serpentinite melange diapirs and deposition of olistostromes were caused by Campanian collision with a continental fragment since underthrust eastwards beneath the arc. The age and position of east-directed thrusts and associated tin granites in the continental back-arc area implies that thrusting and generation of granites were genetically related to the collision, and that the back-thrusts were coupled to the fore-arc thrusts by a decollement beneath the fore-arc basin and magmatic arc. Subsidence and sedimentation in the fore-arc basin succeeded collision, and rise of the back-arc Shan Plateau possibly resulted from late Cenozoic relaxation of collision-related compressive stress.     

A geodynamic model of the Southern Apennines accretionary prism

The Apennines comprise a Neogen—Quaternary accretionary prism that shows several anomalies with respect to classic alpine-type mountain belts, namely (i) low elevation, (ii) a shallow new Moho below the core of the belt, (iii) high heat flow in the internal parts, (iv) mainly sedimentary cover involved in the prism, (v) a deep foredeep and (vi) a fully developed back-arc basin. The suction exerted by a relatively eastward migrating mantle can determine the eastward retreat of the subduction zone and an asthenospheric wedging at the retreating subduction hinge. Heat flow, geochemical and seismological data support the presence of a hot mantle wedge underlying the western side of the Apenninic accretionary prism. A thermal model of the belt with foreland dipping isotherms fits with deepening of the seismicity toward the east. Mantle volatiles signatures are also widespread in springs along the Apennines.     

Chitinozoan reflectance in the Lower Palaeozoic of the Welsh Basin

Vitrinite (woody matter) and spores are generally uncommon or absent in pre-Devonian sediments. To determine organic maturity in the Lower Palaeozoic alternative methods, such as chitinozoan reflectance, must be employed. Chitinozoa are marine, vase-shaped organic-walled microfossils of unknown affinity. The reflectance of incident light from polished chitinozoa walls can provide reliable and accurate maturity data and, due to their relative facies-independency, a high degree of resolution to regional maturity studies. Calibration of the chitinozoan reflectance scale with vitrinite reflectance enables palaeotemperature estimates to be made from pre-Devonian sediments.
The range of chitinozoan reflectance for the Welsh Basin is from under 1% to 8%. This corresponds to estimated palaeotemperatures of under 100°C to over 300°C. The regional spatial distribution of organic maturation recorded by chitinozoan reflectance in the Welsh Basin is attributed to palaeobasin form. The increase in reflectance with age in the rocks of the Myddfai Steep Belt and associated areas allows an estimation of a palaeogeothermal gradient. In the Silurian sequences around Llandovery the gradient is calculated to be in the region of 50°C km⁻¹. Similar gradients are derived from other studies (clay minerals, metabasite mineral assemblages, conodont colours). The results from Myddfai Steep Belt suggest the thermal peak occurred during the Acadian event.
Chitinozoan reflectivity demonstrates that, with respect to hydrocarbon generation, mature conditions occur on the palaeoplatform, where the sedimentary sequences are relatively thin. Overmature conditions are prevalent in the palaeobasin where there is a thick sedimentary pile which has been subsequently deformed.     

Geochemistry of mafic and ultramafic xenoliths from Fidra (Southern Uplands, Scotland): implications for lithospheric processes in Permo-Carboniferous times

Several types of xenoliths occur in a Permian basanite sill in Fidra, eastern central Scotland. One group consists of spinel lherzolites, which have geochemical and isotopic characteristics similar to those of lithospheric upper mantle from elsewhere in western Europe, with both LREE-depleted and LREE-enriched compositions. A separate group comprises pyroxenites and wehrlites, some of which contain plagioclase; these have compositions and textures that indicate that they are cumulates from mafic magmas. In terms of Sr and Nd isotope compositions, the pyroxenites closely resemble the host basanite and most likely formed by high-pressure fractionation of Permo-Carboniferous alkaline magmas at lower crustal depths. They also have mantle-like δ¹⁸O values. A third group is composed of granulite xenoliths that vary between plagioclase-rich and clinopyroxene-rich compositions, some of which probably form a continuum with the pyroxenites and wehrlites. They are all LREE-enriched and most have positive Eu anomalies; thus, they are also mostly cumulates from mafic magmas. Many of the granulites also have Sr and Nd radiogenic isotope ratios similar to those of the host basanite, indicating that they have formed from a similar magma. However, several of the granulites show more enriched isotopic compositions, including higher δ¹⁸O values, trending towards an older crustal component. Thus, the pyroxenites and granulites are largely cogenetic and are mainly the product of a mafic underplating event that occurred during the widespread magmatism in central Scotland during Permo-Carboniferous times.     

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.     

Palaeomagnetism of the Loch Doon Granite Complex, Southern Uplands of Scotland: The Late Caledonian palaeomagnetic record and an Early Devonian episode of True Polar Wander

The Southern Uplands terrane is an Ordovician–Silurian back-arc/foreland basin emplaced at the northern margin of the Iapetus Ocean and intruded by granite complexes including Loch Doon (408.3 ± 1.5 Ma) during Early Devonian times. Protracted cooling of this 130 km³ intrusion recorded magnetic remanence comprising a predominant (‘A’) magnetisation linked to initial cooling with dual polarity and mean direction D / I = 237 / 64° (α₉₅ = 4°, palaeopole at 316°E, 21°N). Subsidiary magnetisations include Mesozoic remanence correlating with extensional tectonism in the adjoining Irish Sea Basin (‘B’, D / I = 234/− 59°) and minority populations (‘C’, D / I = 106/− 2° and ‘D’, D / I = 199/1°) recording emplacement of younger ( 395 Ma) granites in adjoining terranes and the Variscan orogenic event. The ‘A’ directions have an arcuate distribution identifying anticlockwise rotation during cooling. A comparable rotation is identified in the Orthotectonic Caledonides to the north and the Paratectonic Caledonides to the south following closure of Iapetus. Continental motion from midsoutherly latitudes ( 40°S) at 408 Ma to equatorial palaeolatitudes by  395 Ma is identified and implies minimum rates of continental movement between 430 and 390 Ma of 30–70 cm/year, more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander. Silurian–Devonian palaeomagnetic data from the British–Scandinavian Caledonides define a 430–385 Ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from Gondwana. They reconcile pre-430 Ma and post-380 Ma APW from this supercontinent and show that Laurentia–Baltica–Avalonia lay to the west of South America with a relict Rheic Ocean opening to the north which closed to produce Variscan orogeny by a combination of pivotal closure and right lateral transpression.     

Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales

Abstract A diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo-silicate mineralogy, illite crystallinity and b_o parameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low-pressure facies series type, with a geothermal gradient of ∼ 40°Ckm^-1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth-related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn-tectonic metamorphic event is apparent.
Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote-actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite-pumpellyite assemblages in metabasites and it is suggested that the latter represent non-buffered, and therefore non-diagnostic, assemblages.     

Strain-related differences in the crystal growth of white mica and chlorite: a TEM and XRD study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland

Abstract TEM and XRD techniques were used to study crystal growth characteristics of the fabric-forming phyllosilicates which developed in response to low-grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding-parallel. TEM-measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD-measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric-forming phyllosilicates. White micas are paragonite-poor phengites with a mean b lattice parameter of 9.037 Å, and indicate an intermediate pressure series of metamorphism with a field gradient of <25° C km^-1. Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150–320° C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain-induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low-grade metamorphism is closely associated with tectonic fabric development.     

Development of New Zealand according to the fore-arc model of crustal evolution

Analysis of New Zealand geology using a fore-arc model (Crook, 1980a) leads to the recognition of four arc terrains. The west facing Tuhua volcanic arc was active from the Late Proterozoic until the Middle or Late Cambrian. Post-subduction sediments, neritic in the east and flysch in the west, accumulated on the Tuhua accretionary prism from the Late Cambrian until the Early Devonian. Thermal equilibration, metamorphism, granitoid plutonism and penetrative deformation occurred in the Middle to Late Devonian. A small area of Permian platform cover has escaped later erosion. The east-facing Rangitata Terrain records subduction from Early Permian to late Early Cretaceous. Much of its accretionary prism consists of a submarine fan complex derived from Western Antarctica and carried sideways into the trench. The accretionary prism is thick and completely kratonized in southern New Zealand, but the thickness is more variable northwards. There the overlying Upper Cretaceous to Upper Oligocene post-subduction sequence comprises shelf sediments (implying an intermediate-thickness prism) or flysch followed by shelf sediments (implying a thin prism). During the accumulation of this sequence the Rangitata Terrain was a passive continental margin. The south-facing Jurassic-late Oligocene Northland Terrain collided with this passive margin in northern New Zealand at the end of the Oligocene, forming the Northland Allochthon. Subduction then flipped and the oldest part of the Kaikoura Terrain volcanic arc formed on the outer part of the Northland Terrain. Originally this terrain faced northeast and consumed the southwestern part of the South Fiji Basin crust, but during the Miocene the arc migrated clockwise to assume its present northeastern orientation. The fore-arc model employed here satisfactorily explains most first-order and many second-order features of New Zealand geology without requiring modification, thus attesting to the model's versatility and robustness. New Zealand provides a basis for elaborating some aspects of the model, particularly the transition from the syn- to post-subduction phases of fore-arc evolution. Combination of this study with a similar study of the southeastern Australian Paleozoic yields insights into the Phanerozoic evolution of the Australian: Pacific Plates' active margin.     

北阿尔金早古生代同碰撞花岗质岩浆记录及其对增生造山过程的启示

本文对出露在北阿尔金的喀腊大湾似斑状花岗岩、沟口泉似斑状二长花岗岩、卓尔布拉克花岗岩和木孜萨依白云母花岗岩进行了详细的岩相学、岩石地球化学、锆石U-Pb年代学和Hf同位素研究。地球化学特征显示喀腊大湾似斑状花岗岩具有高硅和钠,低铁、镁和钙的特征,铝饱和指数（A/CNK）为1.03~1.04,属弱过铝质I-S过渡型花岗岩;沟口泉似斑状二长花岗岩和卓尔布拉克花岗岩具有相对高的钠含量和准铝质-弱过铝质特征,表现为Ⅰ型花岗岩;木孜萨依白云母花岗岩具有高硅、富碱、富集Rb、Th和LREE,亏损Ba、Sr、Ti和Eu特征,铝饱和指数大于1.1,属过铝质S型花岗岩。锆石U-Pb定年结果显示4个岩体的年龄分别为432.4±4.9Ma、432.8±4.1Ma、439.6±3.5Ma和437.3±2.4Ma。结合区域地质资料和构造判别图解,揭示这4个岩体均形成于同碰撞构造背景下,表明北阿尔金洋最终关闭以及洋陆转换的时间节点应为445~440Ma。锆石Hf同位素结果显示沟口泉似斑状二长花岗岩和卓尔布拉克花岗岩岩浆起源于新生地壳物质的部分熔融;喀腊大湾似斑状花岗岩起源于变杂砂岩部分熔融,木孜萨依白云母花岗岩起源于变泥质岩石的低程度部分熔融;但喀腊大湾似斑状花岗岩和木孜萨依白云母花岗岩ε_Hf（t）值分别为+0.18~+5.88和-6.47~+4.52,反映二者的岩浆源区也均有新生地壳物质的加入。上述这些特征与北祁连早古生代同碰撞花岗岩体具有良好的可对比性,进一步支持二者曾作为统一的整体经历了洋盆裂解-扩张-俯冲-闭合造山等构造演化过程。

     

The establishment of recumbent folds in the Lower Palaeozoic near Queanbeyan,New South Wales

Lower Palaeozoic sedimentary and volcanic rocks east of Queanbeyan, N.S.W., have undergone multiple deformation resulting in four systems of folds. The first of these consists of large isoclinal, recumbent folds (F1). The second generation folds (F2) are the most pronounced; they consist of flattened flexural‐slip folds with well developed axial‐plane slaty cleavage. Minor variants of this system are associated with meridionally‐trending faults. Third and fourth generation folds are minor kink systems.

The existence of first generation folds was established on the basis of F2 fold‐facing determinations, and their likely form was deduced from the geometrical variations of F2 folds. It is thought that all fold phases developed during the Late Silurian Bowning Orogeny.     

A sedimentological study of diamondiferous Quaternary sediments in Southern Shandong,China

A sedimentological study of the lower Quaternary sediments in southern Shandong Province suggests that they were derived from a northern highland area of Archaean gneiss and Palaeozoic sediments. Diamond-bearing kimberlites, intruded during the Ordovician, occur in the highland area and are a possible source of diamonds found in the Quaternary sediments. A warm and humid climate prevailed at the beginning of the Quaternary and braided rivers carried gravel and sand southeast towards the Yellow Sea. Cooler and drier conditions in Q2 times reconcentrated diamonds from the underlying Q1 gravels. Post-Q2 faulting deflected the drainage to the south and subsequent erosion left only small remnants of diamondiferous Q2 gravel. A study of diamond shape characteristics suggests that the diamonds in the Quaternary sediments were derived from the known kimberlites to the north, although the possibility of undiscovered kimberlites occurring beneath the Quaternary sediments cannot be ruled out.