A mechanical interpretation of the vein and dyke systems of the S. W. England orefield

Geological plans and sections showing the traces of veins, faults and porphyry dykes associated with the Hercynian batholith, have been used to construct diagrams representing the trajectories of the minimum and intermediate principal stress axes as they were at the time of intrusion of the dykes and as the veinlode fissures formed. The dykes and veins were emplaced during a complicated series of events and the trajectory diagrams represent stress conditions during these overlapping periods of time towards the end of intrusion of the batholith. The mineralizing fluids emanated from the cores of plutons by means of pregranite faults, primary igneous joints, and structures created by intrusion, including faults and extension fissures. The trajectory diagrams indicate that the mapped configuration of veins and porphyry dykes results from fluid pressures, exerted by the mobile cores of granitic cupolas situated within a regional stress field. The stress models also indicate that fugitive dyke magma or hydrothermal fluids, emanating from a cupola, will make the mechanically easiest exit through the flanks of the intrusion. This explains the asymmetrical disposition of belts of mineralization which border the granite cupolas of southwest England.     

A cordierite-bearing granite suite from the New England Batholith,N.S.W., Australia

A suite of cordierite-bearing biotite-muscovite intrusive granites in the New England Batholith, New South Wales, outcrops over 3400 sq km and is the largest reported occurrence of granites of this type. Compositionally the granites are close to the low temperature minimum and display only limited chemical variation. The corundum-normative nature of the granites suggests a pelitic sedimentary parentage. Cordierite with an average 100 Mg/ Mg + Fe of 55 occurs as large tabular crystals and is considered to be a refractory phase brought up from the zone of partial melting. The presence of cordierite and the absence of garnet in these granites suggests a partial melting at a pressure maximum of 6 kb, equivalent to 22 km depth.     

Vein arrays, hydraulic fractures and pressure-solution structures in a deformed flysch sequence S.W. England

Vein arrays and pressure-solution cleavages are common in the sandstone units of the deformed flysch. Both structures were established prior to the folding of the sediments, and they continued to evolve during this folding. Tensile fractures and conjugate sets of shear fractures (of wrench fault type) were formed. En-echelon arrays of veins are closely associated with the formation of these principal veins. The geometry and relations of the veins are described in detail and their relation to the principal stresses at the time of formation is discussed.

Forking at vein terminations, branch fractures and offset structures in veins are described. The analogies between these and structures produced in rock deformation experiments and found in magmatic dykes are discussed. The branch fracture provides a record of the orientation of the maximum principal stress at the time of its formation.

The veins are infilled with quartz and siderite displaying drusy growth fabrics, indicating that crystal growth occurred into a fluid-filled cavity. It is suggested that the veins originated as hydraulic fractures in a flysch sequence that had developed high pore-fluid pressures during sedimentation. The material in the veins was derived by pressure-solution activity in the sandstone units, which produced a spaced pressure-solution cleavage throughout the region. The relations between veins and pressure-solution cleavages are described. Both small- and large-scale solution transfer of material was involved.     

Sandy high-energy flood sedimentation — Some criteria for recognition,with an example from the devonian of S.W. England

Many modern sand deposits resulting from episodic floods in semi-arid zones are dominated by parallel laminations, but parallel-laminated sands are not a major feature in perennial streams. Single flood events may deposit over 1.5 m of parallel-laminated sand. Hence, ancient parallel-laminated, sand-dominated alluvial deposits may be the product of ephemeral flows.The Trentishoe Formation (Hangman Sandstone Group — Middle Devonian) of North Devon consists, in part, of laterally extensive beds of parallel-laminated, fine-grained red sandstones that build multistorey sand bodies. Individual beds are little more than 1 m thick, but may extend laterally for hundreds of metres. A vertical sequence of erosion surface, sparse silt clasts, parallel laminations, silt drape, is often found, suggesting high flow stage deposition of plane-bedded sand, followed by a very rapid waning of flow with silt fallout in the final stages of the flood. This sequence bears marked similarity to certain modern ephemeral flow sands.     

On the age of the Kiruna Greenstones,Northern Sweden

Three types of zircons from an albite diabase sill within the Kiruna Greenstone Group have been investigated to determine their age and order of crystallization. The youngest, pink and transparent zircons probably crystallized in late fissures and in association with a thermal event. The intermediate, white zircon populations could not be readily dated because of significant lead loss. The lead 207/206 ages of both the pink and white zircons agree roughly with that of the Sm-Nd isochron (about 1.93 Ga) on secondary minerals from a Kiruna greenstone (Skiöld and Cliff). The oldest, brown zircons indicate an age of about 2.2 Ga, and probably reflect the initial crystallization of the rock. The formation of the early Proterozoic greenstones in the Kiruna area thus commenced about 2.2 Ga ago or earlier. These results are consistent with the proposed chronostratigraphy of the Jatulian formations in Finland (e.g., Simonen; Meriläinen). A time interval of at least 0.3 Ga is assumed for the volcano-sedimentary period which pre-dates the Svecokarelian orogeny of the Kiruna area.     

Geochemical Features and Tectonic Setting of Greenstones from Kunimiyama, Northern Chichibu Belt, Central Shikoku, Japan

Abstract. We report whole‐rock chemical data for the greenstones from the Kunimiyama area in the Northern Chichibu Belt and their implications on the tectonic setting of these rocks. The Kunimiyama greenstones are associated with stratiform fer‐romanganese deposits or bedded cherts in the northern part of the study area, but are closely associated with a thick limestone block or bedded cherts in the southern part. The constituent minerals of greenstones are albitized plagioclase, clinopy‐roxene, chlorite, calcite, epidote, pumpellyite, prehnite, quartz, celadonite, sericite and opaque minerals such as iron oxyhy‐droxide and hematite. These mineral assemblages, epidote + pumpellyite + chlorite and chlorite + pumpellyite + prehnite, suggest that the metamorphic grade of greenstones from the Kunimiyama area is prehnite‐pumpellyite facies. The whole‐rock chemical compositions of greenstones associated with ferromanganese deposits are generally similar to those of normal mid‐ocean ridge basalt (N‐MORB). In contrast, the chemical compositions of the greenstones associated with the limestone block are comparable to those of ocean island alkaline basalt. Greenstones associated with bedded cherts are of enriched MORB and ocean island basalt, as well as N‐MORB origins, suggesting they probably formed as a result of plume‐related MOR volcanism in the Panthalassa Ocean in Early Permian and by tectonic mixing of ocean island basalts with oceanic ridge crustal fragments during accretion/subduction processes. These geological and geochemical lines of evidence suggest that the Kunimiyama greenstones are allochthonous blocks of accreted oceanic crust and seamounts. The ferromanganese deposits are frequently accompanied by reddish greenstones. Compared to common greenish greenstones, the reddish greenstones are characterized by high MnO and rare earth element contents and distinct negative Ce anomalies, implying a slight contribution of hydro thermal component forming the ferromanganese deposits.     

IRON DEPOSITS OF S. W. HUPEH

INTRODUCTION Iron deposits are widely distributed in S. W. Hupeh, especially in the districts of I Tu (宜都) and Changyang (长陽) where native mining industry has been carried on since very ancient time. Production from such mines     

The flandrian sequence of north Wirral (N. W. England)

The correlation of borehole logs, together with evidence from temporary exposures, shows the relationship between glacial deposits and the overlying Flandrian strata. The courses of buried channels in-filled with Flandrian strata have been plotted. Radiocarbon assays and pollen and diatom analyses have mainly been confined to samples taken from near the main lithological boundaries, but some radiocarbon assays have been undertaken on in situ tree stumps and on bone. New thick buried channel sequences are described, and an outline chronology from c. 6420 years BP is proposed for the Flandrian coastal sequence of the North Wirral. The data can be applied to the interpretation of sea-level change and coastal archaeology.     

OBITUARY NOTE ON MR.S.W.WANG

The untimely death of Mr.S.W.Wang on Feb.10th 1936 iscertainly a serious loss to the Chinese geology.Although his name is notso well known as others in the geological circle,Mr.Wang has indeedmade remarkable contribution to the study of Chinese crystals and     

Petrology of the Nandewar Volcano,N.S.W., Australia

The Nandewar Mountains, N.S.W., Australia, are the remains of a Miocene continental alkaline volcano whose products range from olivine basalts to comendites and alkali rhyolites. Intermediate hawaiites, mugearites and benmoreites predominate in the shield, in which olivine basalts are rare, and the trachytic rocks form many intrusions into the shield. The Nandewar alkaline series shows extreme fractionation of a relatively differentiated alkali olivine basalt magma, saturated with silica, to yield extremely oversaturated peralkaline comendites and peraluminous alkali rhyolites. The nature of the ferromagnesian phases forming was controlled by low oxygen fugacities. Throughout the series clinopyroxenes range from diopsidic augite, through sodic ferrohedenbergites to hedenbergite-acmite solid solutions. Riebeckite-arfvedsonite solid solutions appear in the trachytes and comendites, and aenigmatite appears in some of the peralkaline rocks. The feldspars in the series fractionate from calcic labradorite through potash oligoclase and calcic anothoclase towards the minimum melting alkali feldspar composition, Ab₆₅Or₃₅. The compositions of the alkali rhyolites approach the minimum in the system SiO₂-KAlSi₃O₈-NaAlSi₃O₈. All the mineralogical and chemical evidence points to the development of the Nandewar series by the processes of extreme crystallization differentiation of an alkali olivine basalt parent magma. No significant contamination occurred, xenoliths and xenocrysts are absent, and volatile transfer and metasomatism played a minor role.     

Geomorphology and tectonics of the Dorrigo Plateau,N.S.W.

The Dorrigo Plateau is covered by basalt, which is a remnant of the 18 Ma old Ebor Volcano. The centre of this volcano is an intrusion in the Bellinger Valley. The volcano was erupted on a palaeoplain of moderate relief. Subsequent uplift and tilting led to erosion of the Nambucca Beds, together with much of the volcano, and creation of a major escarpment, part of the Great Escarpment of eastern Australia. In this area the Great Escarpment is younger than 18 Ma.     

Geochemical evolution of lithospheric mantle beneath S.E. South Australia

The record of mafic magmatism from the Proterozoic to the Holocene in southern Australia reflects episodic incompatible element enrichment of the sub-continental lithospheric mantle (SCLM) recording periodic interaction of asthenosphere and lithosphere. The composition of Jurassic and Cainozoic mantle derived magmas is strongly influenced by the geochemical impact on the SCLM of events which took place during the Neoproterozoic and Cambrian. These events include rifting, passive margin development and orogenesis.Neoproterozoic to Cambrian basalts are widespread in western New South Wales, South Australia and Tasmania and reflect mantle decompression during extension and rifting of the Australian–East Antarctic Craton during the development of the proto-Pacific passive margin. These basalts fall into two regionally extensive and very different suites: (i) a voluminous suite of tholeiites and (ii) a highly undersaturated alkaline (nephelinite–basanite) series.Both Jurassic kimberlite magmas from the Adelaide Fold Belt and highly undersaturated Quaternary analcimites and basanites from the Mt. Gambier district of S.E. South Australia, have geochemical characteristics like those of the Precambrian–Cambrian alkaline suites. They have high concentrations of large ion lithophile (LIL), rare earth (RE) and high field strength (HFS) elements, and high HFSE/LILE and LREE/HREE ratios with T_DM^Nd values of 0.5–0.8 Ga. The Jurassic kimberlites appear to sample lithospheric mantle enrichment zones of Late Neoproterozoic to Early Cambrian age. The Quaternary suites result from mixing of contemporary mantle plume components with this old lithospheric enrichment, which is also identified with the occurrence of metasomatic phlogopite, amphibole and apatite in lherzolite mantle xenoliths from a number of Cainozoic volcanoes in Western Victoria.A very different type of lithospheric mantle enrichment took place during the late stages of the Ross–Delamerian Orogeny. This yielded a crustally contaminated mantle zone that mirrors the Cambro-Ordovician position of that orogen. This zone of contaminated lithospheric mantle interacted with a large plume in the Jurassic to yield the highly anomalous Ferrar–Tasmanian–Kangaroo Island basalts and dolerites.     

Geochemical exploration methods for gold in areas with mountain glaciation in Siberia,U.S.S.R.

In the mountainous areas of Siberia intensive destruction of ore deposits occurs as a result of Pleistocene mountain glaciation and, as a consequence, element dispersion trains form in the eroded material and in the stream sediments. In these areas, cirques and glaciated valleys are incised as much as 250–600 m into ore deposits. Geochemical anomalies in the stream sediments in the glaciated valleys form as water washes through the till and lacustrine sediments. The surface erosion processes result in the removal of fine gold from talus-covered cirque walls from where it is deposited in channels and lakes. Secondary dispersion halos may either be localized entirely within cirques, or they may be found in the drift within the outwash or glaciolacustrine environments downstream of the cirques. Intermittent accumulations of gold are specifically concentrated in glacially-fed stream sediments in those glaciated valleys characterized by smooth, flat bottoms and abundant lakes: gold in such cases tends to be concentrated in the lake muds. After the lakes have silted up, gold will be found in the fluvial deposits. In general, the complete elemental composition of the ore under discussion (Au, Ag, Pb and Zn) is reflected in only short dispersion trains within the fluvial and lacustrine deposits. Gold anomalies, on the other hand, are extensive (up to 6 km) in the stream sediments within the glaciated valleys and will occur sorbed onto hydrous iron oxides and clays, as well as in the form of flattened (thin) gold particles. In the glaciated valleys, the gold geochemical anomalies in moraine, lacustrine and stream sediments properly reflect the mineralization in the drainage basin.     

Resolution of foraminiferal biotopes in Broken Bay,N.S.W.

A recent benthonic foraminiferal population consisting of 144 species from an interacting marine‐estuarine environment has been analysed by Q‐mode cluster analysis to reveal biotopes. Simplification of the total populations, both in terms of abundance and occurrence, discloses that the more significant subpopulation is composed of the widely‐occurring species. Subpopulations based on rarely occurring but locally abundant species do not define meaningful biotopes.