40Ar/39Ar age studies of some glauconites from well-known stratigraphical horizons reveal unexpectedly high ages compared with the conventional K–Ar age determinations. These have been interpreted in terms of nuclear and physico-chemical reactions sustained by the glauconites in the reactor. These impose severe limitations on the use of glauconite in the 40Ar/39Ar irradiation technique.

We describe the geometry and kinematics of a Jurassic to Early Cretaceous transpressive sinistral strike-slip system within a metamorphic basement inlier of the Mesozoic magmatic arc near Bahia Agua Dulce at latitudes 31–32°S in north-central Chile and discuss possible relations with the Atacama Fault System further north. Sinistral transpression overprints structures of an accretionary system that is represented by the metamorphic basement. Sub-vertical semi-ductile NNW-striking strike-slip shear zones are the most conspicuous structures. Chlorite and sericite grew, and white mica and quartz dynamically recrystallized, suggesting low-grade metamorphic conditions during semi-ductile deformation. Folds at the 10–100 metre scale developed before and during strike-slip shearing. The folds are deforming a former sub-horizontal transposition foliation that originated during prior accretion processes. The folds have axes sub-parallel to the strike-slip shear zones and sub-vertical axial surfaces indicating a component of shortening parallel to the shear-zone boundaries, suggesting an overall transpressive deformation regime. Transpressive strike-slip deformation also affects Middle Triassic (Anisian) basal breccias of the El Quereo Formation. 40Ar–39Ar laser ablation ages of synkinematically recrystallized white mica in one of the shear zones provide an age of 174–165 Ma for the waning stages of semi-ductile strike-slip shearing. The semi-ductile shear zones are cut by mafic and rhyolite dykes. Two rhyolite dykes yield 40Ar–39Ar ages of 160.5 ± 1.7 Ma and 131.9 ± 1.7 Ma, respectively. The latter dyke has been affected by brittle faulting. Fault-slip analysis shows that the kinematics of the faulting event is similar to the one of the semi-ductile shearing event, suggesting that sinistral transpression continued after ~130 Ma. Timing, kinematics and geographic position suggest that the shear zones at Bahia Agua Dulce represent a southern continuation of the prominent Atacama Fault System that affected the Jurassic/Early Cretaceous arc over its ~1400 km length.

Folds of a 12 mm thick calcite-rich vein, embedded in slate and with a primary planar fabric of parallel, thin and plate-like calcite crystals, or laminae, have a vein thickness/arc length ratio near 1. Folds were first entirely dependent for development on intra-laminar strain and inter-laminar sliding. As limb dips increased, however, pressure solution began to thin laminae in limb regions only. As adjacent inter-laminar surfaces met at fold inflexion points, where laminae were first cut through, loss on fabric-cutting surfaces began to replace inter-laminar pressure solution. The resulting stylolites grew across the vein in fold limbs, at the limit eliminating them. In contrast, in folded equigranular calcite rocks stylolites are widely developed in both limbs and hinges, and eventual fold collapse through late stage concentration of pressure solution takes place in fold hinges and not limbs.

terrane (ter-rane’) A fault-bounded body of rock of regional extent, characterized by a geological history different from that of contiguous terranes. A terrane is generally considered to be a discrete allochthonous fragment of oceanic or continental material added to a craton at an active margin by accretion. A. G. I. Glossary of Geology. 3rd edition, 1987.

New conventional K–Ar dating of virtually unaltered pargasite phenocrysts extracted from a metabasalt of the uppermost Tremadocian Rhobell Volcanic Group, cropping out near Dolgellau, Gwynedd, north Wales, yields an age of 508 ± 11 Ma. This result provides a valuable new time-scale calibration point although incompatible with some other versions of the radiometric time-scale for the Lower Palaeozoic which have indicated ages of less than 508 ± 11 Ma for the Tremadoc/Arenig boundary.

Conglomerate beds occur in an Upper Ordovician—Lower Silurian clastic sequence on the northeastern side of the Newfoundland Central Paleozoic Mobile Belt. They contain scattered pebbles and cobbles dispersed in a finely laminated sandstone and siltstone matrix. Laminations are generally 5–20 mm thick and the clasts 10–300 mm across. The laminations are locally disrupted by the clasts. These laminations have subsequently been tectonically flattened into augen around the clasts and locally disrupted by boudinage. It is concluded that these conglomerate beds represent icerafted glacio-marine deposits interbedded with turbidites which were probably deposited in a locally restricted marine environment. The deposits are interpreted on the basis of scattered fossil localities as being of Late Ordovician–Early Silurian age. They are compared with deposits of similar age elsewhere in the North Atlantic region.

A water saturated melt was developed in the semipelitic Leven Schist in contact with the kentallenitic basic rocks of the Barnamuc intrusion. The partly melted rock consisted of a small volume of granitic melt with relict refractory quartz grains and layers of aluminosilicates. This material could be mobilized and mixed with phenocryst-bearing magma. Sheets of country rock breccia are associated with the mobilization and veins of breccia and mobilized hornfels invade the intrusion. Sheets of igneous rock occur along the contact zone and their emplacement is related to the mobilization of the hornfels. Loss of cohesion between solid country rock and crystallized outer parts of the plug leads to collapse and removal of early formed igneous rocks, accounting for the lack of chilled margins in some intrusions.

In a study of the rocks of the eastern end of the Newry Complex, the writer distinguished two contrasted groups of hybrid rocks— (i) the Slievegarron type, and (ii) the Seeconnell type—both of which are intimately associated with biotite-pyroxenite and biotite-peridotite. The Slievegarron hybrids comprise an augite-biotite-diorite series. The typical augite-biotite-diorite (1934, p. 611) is a highly undersaturated rock heteromorphous with certain varieties of orthoclase-basalt and leucite-basanite; it might equally well be described as biotite-essexite-gabbro. No further discussion of these rocks will be undertaken in this paper, but it seems desirable to point out that they should not be confused—as unfortunately they have been—with gabbro-diorite.

Numerous silicified and calcareous sclerites of various sizes, recovered from the latest Famennian of Thuringia (Germany), allow the description of the first complete growth series of a blind proetoid trilobite: Drevermannia richteri. In addition, the partial ontogenetic development of Drevermannia antecurvata sp. nov. and undetermined species, Drevermannia sp. 1, are described. The proetoid anaprotaspides, associated with D. richteri, illustrate that a marked increase in larval size occurred prior to the terminal Devonian extinction event. Considering the homogeneity of larval size in older Devonian proetoids, it is interpreted as evidence that the developmental strategy of these trilobites was significantly modified. Though largely speculative, two alternative hypotheses are proposed to explain this modification. Finally, all three ontogenetic sequences show that ocular structures never develop externally in Drevermannia, but also illustrate that the development of optical nerves is not completely lost in this group. This suggests that blindness in the Drevermannia lineage followed a centripetal mode of eye reduction.

Archaean komatiite volcanics at Kambalda, Western Australia have been metamorphosed to upper greenschist–lower amphibolite grade and have experienced intense though heterogeneously developed polyphase deformation. Despite this, preservation of igneous textural features is often good, particularly in areas which underwent only ‘static style’ metamorphism. Thin lavas from the Tripod Hill Member of the Kambalda Komatiite Formation over the western margin of the Hunt nickel shoot display textural elements and facies variations which are virtually identical to those found in fresher thin komatiite sequences in other Archaean greenstone belts. Four principal flow profile (facies) types are defined, comprising nine subtypes. These represent stages in a facies continuum, ranging from ‘mature’ profiles which comprise thick spinifex textured tops and close packed cumulate bases through to massive, jointed ‘immature’ profiles devoid of mesoscopic spinifex texture. The causes of textural diversity within and between profiles are many and complex. However, facies variations can be attributed mainly to the effects of lava velocity at the time of major heat loss, combined with relative lateral position within any flow. The most mature textural (and geochemical) profiles developed in parts of lavas which had become ponded prior to major heat loss, whereas the least evolved profiles developed along the lateral margins (levees) of moving lavas. The study area komatiites occur as alternating stacks of flows of similar type. This stratigraphy records temporal and spatial shifts in the locus of lava ponding over the western margin of the Hunt nickel shoot. Such shifts may have been caused by irregularities in the underlying volcanic topography and/or by synvolcanic faulting and subsidence.

The Montréal area belongs to the St Lawrence Lowlands, a Cambrian Early Ordovician passive margin of the Iapetus Ocean, later covered by Appalachian Middle to Upper Ordovician foreland deposits. A structural and palaeostress analysis has been carried out in order to reconstruct its tectonic evolution. The structural map has been revised with new data. Palaeostresses are reconstructed based on inversion of fault slip data, and these results are independently corroborated by the microstructural study of calcite mechanical twinning. Field relationships are used to establish the relative chronology of fractures and to deduce the motion on regional faults. The reconstructed structural and tectonic evolution brings to light some relationships between structural inheritance and tectonic events that have affected the area since Early Palaeozoic times. An early NW–SE extension is responsible for N040-trending faults along the northern border of the St Lawrence Lowlands, and for N090- and N120-trending faults cross-cutting the Montréal area. This extension is followed by WNW–ESE and NNW compressions, which have induced reverse motion on pre-existing faults and generated strike-slip conjugate faults. Subsequent NE–SW and NNW–SSE-directed extensions have reactivated previous faults with normal to strike-slip motions. A late NE–SW compression is recorded in the Monteregian plutons. Compressions in WNW–ESE and NNW directions are consistent with Appalachian collisional tectonism, but N040- and N090-trending faults cross-cut Appalachian folds and foreland deposits. Although the early NW–SE extension is consistent with the collapse of the Iapetan margin in Early Palaeozoic times, most of the present geometry of the St Lawrence Lowlands could be attributed to Mesozoic tectonism, recorded as nearly N–S-directed extensional events.

Cretaceous-Palaeogene sediments of the Kangerdlugssuaq area on the continental margin of Central East Greenland were deposited in an embayment of an extensive pre-NE Atlantic shelf sea. Pre-Sparnacian sediments are thin (150 m), incomplete and of siliciclastic type, formed in shallow marine waters. Sparnacian times saw the onset of vigorous basaltic vulcanicity, marking the initial rifting episode between Greenland and Eurasia. Uplift immediately prior to the vulcanicity is evidenced by an unconformity at the base of the Sparnacian, above which basement-derived arkosic sandstones and conglomerates are followed by about 1.5 km of coarse volcaniclastics, basaltic flows of dominantly picritic composition, pro-grading hyaloclastite wedges and thin siltstones with abundant organic detritus. Very rapid subsidence accompanied this early phase of vulcanicity, maintaining the top of the pile close to sea level and allowing the deposition of a further kilometre of waterlain tuffs in the embayment. Sedimentation extended northwards and eastwards on to basement rocks at this period, with the formation of a non-marine sequence which includes coals.

The overlying plateau tholeiites overlap the earlier volcanics; their depositional area was extremely extensive along the East Greenland margin and bears no relationship to the Kangerdlugssuaq sedimentary embayment, although their thickest development, 4 km or more, was attained in that region. Eruption rate of the pile exceeded subsidence for a period and it is dominantly subaerial.

This sequence of events is compared with the similar history of sedimentation and basaltic vulcanicity on the west coast of Greenland, and it is inferred that just as the East Greenland sequence records the initiation of spreading between Greenland and Rockall-Faeroe at anomaly 24−25 time, so that of West Greenland marks the propagation of the Labrador Sea spreading axis through the Davis Strait into Baffin Bay at anomaly 26−27 time.

Upper Devonian strata are distributed widely throughout Iran. A review of all available literature on the Devonian, together with information from recent studies in the Kerman region, provides a basis for correlation and description of facies successions for the whole country. In most sections the local base of the Upper Devonian sits disconformably on older rocks, whilst the upper contact with the Carboniferous is almost everywhere transitional. There is everywhere an apparent hiatus spanning a probable early and mid Devonian interval. Lower and Middle Devonian fossils have not yet been discovered, but for the Upper Devonian there are abundant fossil data indicating the widespread occurrence of Frasnian and Famennian shallow marine deposits. A palaeogeographical map for the Upper Devonian of Iran is constructed.

Owing to incomplete palæontological knowledge, the true age and correlation of the various divisions of the great Mesozoic series of New Zealand, which, together with the Maitai Series, forms such an important element in the structure of the country, has long remained a matter of uncertainty. In consequence the idea has to some extent taken hold among New Zealand geologists that the Mesozoic faunas, owing to supposed conditions of isolation, show archaic features. It was explained that certain Permian forms occurred in the Trias and that Trias forms may have persisted into Jurassic times, and to a still greater extent that a Cretaceous fauna lived on in this portion of the earth into the Tertiary period.

The general geology of the Mt. Barney Central Complex is described. This intrusive complex shows close similarity in type to the classic Tertiary ring complexes of Western Scotland and Northern Ireland. In this discussion of the complex, the similarities shown by ring dykes, cone sheets, a forcefully intruded granophyre boss, and a closed ring fault are indicated.

High-SiO2 rocks referred to as oceanic plagiogranites are common within the crustal sequences of ophiolites; however, their mode of petrogenesis is controversial with both late-stage fractional crystallization and partial melting models being proposed. Here, we present new whole-rock data from plagiogranitic dyke-like bodies and lenses from the lower and middle sections of the sheeted dyke complex of the Cretaceous Muslim Bagh Ophiolite, northwestern Pakistan. The plagiogranites have similar geochemical signatures that are inconsistent with them being the fractionation products of the mafic units of the Muslim Bagh Ophiolite. However, the plagiogranites all display very low TiO2 contents (<0.4 wt%), implying that they formed by partial melting of mafic rocks. Melt modelling of a crustal gabbro from the Muslim Bagh Ophiolite shows that the trace-element signature of the plagiogranites can be replicated by 5–10% melting of a crustal hornblende gabbro with amphibole as a residual phase, resulting in a concave-up middle rare Earth element pattern. Compositional similarities between the Muslim Bagh Ophiolite plagiogranites and Archaean TTG (trondhjemite–tonalite–granodiorite) has implications for the generation of juvenile Archaean continental crust. As the Muslim Bagh Ophiolite was derived in a supra-subduction zone, it is suggested that some Archaean TTG may have been derived from melting of mafic upper crust in early subduction-like settings. However, due to the small volume of Muslim Bagh Ophiolite plagiogranites, it is inferred that they can be instructive on the petrogenesis of some, but not all, Archaean TTG.

Feldspathic peridotites on Rhum contain teschenite segregations and veins. Bulk rock and mineral compositional trends define a path of alkali and iron enrichment related to discontinuous isolation of evolving intercumulus melts. Compositionally zoned and skeletal crystals of actinolite, zircon, apatite, sphene, epidote and chlorite are enclosed by clear analcime, and appear to have grown directly from a water-rich fluid at some undercooling, and below dry equilibrium solidus temperatures. The occurrence of late stage residua with undersaturated alkalirich characteristics in the Rhum ultrabasic complex suggests that the patental magma was alkaline rather than tholeiitic, and had similarities to the Plateau Magmas on Skye.

I Have not hitherto seen any specimen so Satisfactorily and finely illustrative of the affinity of Strophodus to Cestracion as that figured in Plate VII. and which is now in the British Museum. It consists of the major part of the dental covering of a jaw, including the posterior part of the symphysis, and shows that the principal or largest, as in Cestracion. These are followed by two rows (at least) of smaller crushing teeth, and are preceded by rows of teeth both smaller and more produced at the middle of their working surface, and in the same degree changing from the crushing molar to the conical prehensile type. This dental coating or armature is imbedded in a block of the fine Oolitic building-stone from Caen, which has taken the place of the dissolved cartilaginous support of the teeth, so as to maintain and exhibit the curve of the arch (fig.1a) by which the teeth obliquely overspanned the jaw to which they were originaly attached.