K–Ar and 40Ar/39Ar dates are presented for locations in the Izu–Bonin – Mariana (IBM) forearc (Ocean Drilling Program (ODP) sites 786 & 782, Chichijima, Deep Sea Drilling Program (DSDP) sites 458 & 459, Saipan), and Palau on the remnant arc of the Kyushu–Palau Ridge. For a number of these locations, the 40Ar/39Ar plateau and 36Ar/40Ar versus 39Ar/40Ar isochrons give older ages than the K–Ar results. The most important results are: (i) at site 786, initial construction of the proto‐IBM (now forearc) basement occurred at least by ca 47–45 Ma, consistent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at ca 35 Ma by K–Ar could not be confirmed by 40Ar/39Ar analysis; (ii) 40Ar/39Ar ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu–Bonin portion, for example at site 458, initial construction commenced at least by ca 49 Ma and at ca 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combination of K–Ar and 40Ar/39Ar ages indicate continued boninite magmatism in the Izu–Bonin forearc (and remnant arc at Palau) until ca 35 Ma. Subduction inception including boninite series rocks along most of the exposed length of the IBM system, clearly preceded by some 5 million years the Middle Eocene (ca 43.5 Ma) change in Pacific plate motion. Boninitic series magmatism persisted at locations now exposed in the forearc for ∼ 15 million years after arc inception concurrently with low‐K tholeiitic series eruptions from a subaerial arc system, established at ≥ 40 Ma, on the Kyushu–Palau Ridge. For the Mariana portion of the IBM system, reconstruction of the proto‐arc places this activity adjacent to the concurrent but orthogonally spreading Central Basin Ridge of the West Philippine Basin. It is possible that a combination of subduction of a young North New Guinea Plate beneath newly created back‐arc basin crust may account for some of the features of the Mariana system. It is clear, however, that the understanding of the processes of subduction initiation and early IBM arc development is incomplete.

The discovery of unexpectedly young tephra, with K‐Ar dates of 1.50 ± 0.18 and 1.38 ± 0.13 Ma BP, in the Lake Eǧirdir area sheds new light on the time‐span and magnitude of explosive volcanism in west‐central Anatolia and on the physical behaviour of pyroclastic currents. The Gölcük maar near Isparta, widely thought to have ceased its activity in Pliocene time, was apparently still erupting with strong explosions in the Early Pleistocene. The Eǧirdir tephra, 34 km NE from the volcano, indicate deposition from powerful pyroclastic surges directed by the topography. The depositing currents were fully turbulent and carried unusually large lithic clasts, up to 2–3 cm in diameter, in suspension, which is incompatible with the existing physical model for pyroclastic currents. Based on the tephra characteristics, a conceptual model of the pyroclastic current's segmentation, or lateral dynamic partition, is suggested and used further to explain the current's specific response to an encountered topographic barrier. The widely held concept of a pyroclastic current's vertical separation, or sharp rheological decoupling, is thought to describe a common secondary phenomenon.

A K-feldspar megacrystic biotite granite phase of the Burgeo intrusive suite, southern Newfoundland, has been dated by the Rb–Sr whole-rock method. The isochron age of 412 ± 5 Ma (initial 87Sr/86Sr ratio = 0.7086 ± 0.0004) is consistent with the younger of two U–Pb zircon ages obtained from other phases of the Burgeo intrusive suite. The southern margin of the granite is intensively deformed in a shear zone that also affects amphibolitic gneisses of the Grey River enclave. Data from deformed granite samples plot below the 412 Ma isochron and may have resulted from reaction and partial reequilibration of the samples with a hydrothermal fluid of low initial 87Sr/86Sr ratio during metasomatic reactions accompanying shear zone deformation. However, alternative hypotheses such as crustal contamination or derivation from two separate and isotopically distinct magmas cannot be discounted. Mica ages between 390 and 370 Ma associated with tungsten mineralization at Grey River confirm that the mineralization was unrelated to the Burgeo intrusive suite. They support previous conclusions, based on geological and geochemical data, that the mineralization was related to posttectonic leucogranite intrusions.

ABSTRACTThe K–Ar ages of 117 clay concentrates from samples associated with mineral deposits in Ireland indicate that most, if not all of the major ore deposits were formed during Carboniferous times or earlier. The Avoca orebodies are pre-400 Ma in age. Many of the vein deposits traversing Palaeozoic rocks were either formed or rejuvenated during the Hercynian orogeny c. 300 Ma ago. The SW Ireland Cu–Ba ores are 290 Ma old, and the major, frequently stratiform, base metal deposits stratabound in the Lower Carboniferous were, at least largely, formed during the Carboniferous period. In the case of the Navan orebody, an early Carboniferous age is indicated. Previous models invoking major mineralisations during Mesozoic or Tertiary times are rendered invalid. However, in some areas there appears to have been hydrothermal activity during the Triassic and possibly the Permian. The Triassic event is thought to be genetically related to coeval hydrothermal avtivity found throughout the N Atlantic regions. The data suggest that probably <10 km of cover has been removed from the majority of Ireland since Lower Carboniferous times. K–Ar dating of clays is shown to be a powerful method for constraining the ages of oredeposits.

Measurements of noble gas element abundances in igneous rock samples from the Lesser Antilles island arc (LAIA) show an enrichment of Ne, Kr and Xe relative to Ar, when compared to the atmospheric inert gas composition. The element concentrations vary as a function of time and a cyclic variation appears to occur in the pre-Pliocene stage of arc development. The pattern of elemental abundances closely resembles that of ocean floor basalts, suggesting that, (1) the origin of the inert gases in these samples is related to the subduction of oceanic lithosphere responsible for the formation of the LAIA, and (2) the subduction process does not modify the noble gas patterns of the materials being subducted.

Synopsis The Hoy Lavas are alkaline olivine basalts, occurring at the base of presumed Upper Old Red Sandstone, and resting unconformably on Middle Old Red Sandstone strata. Mean conventional K-Ar ages of the lavas are discordant and range from 300 to 333 Ma. A cross-cutting monchiquite dyke yields a conventional K-Ar age of 235 Ma. 40 Ar– 39 Ar stepwise degassing analyses of two samples indicate an age of approximately 370 Ma for the lavas. A revision of the age of the Givetian/Frasnian boundary is suggested.

Conventional K-Ar analyses of fifteen adularia feldspars from the Lizard Complex yield ages between 181 ± 2 Ma and 220 ± 3 Ma (with one exception from Holseer Cove yielding an age of 156 ± 3 Ma). The feldspars are dominantly monoclinic but most contain a minor but constant proportion of triclinic material with variable triclinicity. An inverse correlation exists between triclinicity and K-Ar age interpreted as representing argon loss related to the structural state. Extrapolation of the data to zero triclinicity indicates a true age of 210–220 Ma.40Ar-39Ar stepwise degassing analyses yield plateau ages related to the K-Ar ages and not the crystallization age. The results can be explained in terms of postcrystallization Al-Si ordering resulting in argon loss from lattice sites which under normal diffusion conditions requires a range of activation energies. The Holseer Cove sample is monoclinic and an40Ar-39Ar stepheating analysis suggests a later crystallization at 160–170 Ma. Neither primary crystallization nor spontaneous ordering through time explain the origin of the triclinic adularia. It is suggested that ordering has been induced by the passage of low-temperature alkaline solutions at a subsequent point in geological time. Argon loss by ordering could explain the low ages found using both K-Ar and40Ar-39Ar stepheating methods when applied to feldspars in general. The 210–220 Ma, and 160–170 Ma hydrothermal events affected much of southwest England and are probably related to major geotectonic movements in western Europe.

A palaeomagnetic pole at 56.9°N, 273.6°E ( dp = 13.3°, dm = 13.8°) has been determined from dykes and gneisses from Sagdlerssuaq and adjacent islets; this magnetization was probably acquired at a time given by a K-Ar age of 1620 ( ±50) Ma. Dykes at Itivdleq have given a palaeomagnetic pole at 18.0°N, 287.2°E (dp = 5.4°, dm = 7.2°); these dykes were probably magnetized at about 1750 Ma. It seems likely that the magnetizations were imposed upon cooling during uplift following the Nagssugtoqidian metamorphism.

Palaeomagnetic and isotopic results from the Kaoko lavas, Hoachanas basalts and dolerite sills of South-West Africa indicate that the Upper Triassic-Lower Jurassic Stormberg flows of South Africa may have extended into SW-Africa and that younger igneous events of Lower Cretaceous age were simultaneous with the Serra Geral volcanism in Brazil. Five analyses on three samples of the Keetmanshoop sills gave K-Ar ages between 178 ± 4 and 199 ± 4 Ma, four analyses of two samples of the Hoachanas basalts gave ages between 161 ± 3 and 173 ± 2 Ma and eight analyses of five samples of Kaoko basalt gave ages between 110±4 and 128 ± 2 Ma.

SummaryPotassium–argon and rubidium–strontium isotopic age studies of the Wolf Rock phonolite of SW England confirm its age as 131 Ma, with an initial87Sr/88Sr ratio of 0.703. The existence of a Mesozoic volcanic province of this age on the margin of the Labrador–Biscay rift is discussed with special reference to its implications for the early evolution of the N.Atlantic Ocean, and the general relationship of alkaline igneous activity to major periods of plate reorientation.

SummaryPotassium–argon age studies on the central intrusion complexes of Ardnamurchan, Western Scotland, indicate that they were emplaced at a time between 59 and 61 Ma ago. The time period associated with the cooling was probably less than of the order of a million years. The temporal setting of the Ardnamurchan complexes in the context of the other British Tertiary igneous centres is examined in the light of the new data.

Samples collected from three Jurassic igneous formations in South Morocco carry a stable component of magnetic remanence, giving a mean palaeomagnetic pole-position at 61° S, 71° E, A95 = 14°. Two of the formations were situated in the stable Saharan Shield region, and one within the Central High Atlas fold-belt. There is no systematic difference between the pole-positions derived from these two regions of contrasting tectonic history, and furthermore, no significant difference between the mean Jurassic pole-position for Morocco and that for South-east Africa. It is concluded that no significant post-Jurassic relative motion has occurred between NW and SE Africa.

Basalts, radiometrically dated at 263 ± 5 m.y., have stable remanent magnetization with normal polarity. This demonstrates the existence of a previously unrecognized event within the Late Palaeozoic Interval of reversed geomagnetic polarity.