The history of investigation of the Silurian rocks of the Dunquin inlier, Dingle Peninsula, County Kerry, Ireland is briefly reviewed. The Dunquin Group of upper Wenlock (Homerian) to middle Ludlow (Ludfordian) sedimentary and volcanic rocks is divided into five formations: the Ferriters Cove, Clogher Head, Mill Cove, Drom Point and Croaghmarhin Formations, the last of which reaches the Ludlow and passes transitionally into the succeeding continental Dingle Group, probably largely Přídolí in age. The Dunquin group is of varied shallow marine sediments with a continental episode probably represented in the Mill Cove Formation. The biostratigraphy of the other four formations is considered in relation to a full faunal list. Details of representative sections and exposures are provided. There is brief comment on the palaeogeographical setting of the inlier.

The North Qôroq centre consists of a number of units of syenite, all undersaturated, with later units in sharp intrusive contact with earlier ones. Chemical and mineralogical characteristics have been determined by primary crystal-silicate liquid equilibria and also later, extensive, metasomatic alteration probably involving an alkali-rich aqueous phase. Cathodoluminescence studies on feldspars, the dominant phase present, reveal the development of red-luminescing albite in all the rocks affected by a late-stage fluid phase in contrast with the blue luminescence shown by unaltered perthites. Spectral and chemical investigation of the feldspars confirms earlier views that red luminescence is caused by Fe3+, an impurity in the feldspar structure, but also indicates that, for red luminescence to be seen, the blue luminescent peak must be greatly reduced or absent. The cause of the blue luminescence is less obvious but may well be related to an oxygen defect centre. Such a centre is apparently absent in red-luminescing albites and hence not formed during growth involving late-stage fluids. The fluids evolved from highly peralkaline, undersaturated magmas and metasomatised less evolved syenites forming feldspathoids, particularly sodalite. Once equilibrium had been attained the fluid behaved principally as a passive phase on its passage through the rocks, aiding the unmixing of the alkali feldspars.

I-type granites are produced by partial melting of older igneous rocks that are metaluminous and hence have not undergone any significant amount of chemical weathering. In the Lachlan Fold Belt of southeastern Australia and the Caledonian Fold Belt of Britain and Ireland there was a major magmatic event close to 400 Ma ago involving a massive introduction of heat into the crust. In both areas, that Caledonian-age event produced large volumes of I-type granite and related volcanic rocks. Granites of these two areas are not identical in character but they do show many similarities and are markedly different from many of the granites found in Mesozoic and younger fold belts. These younger, dominantly tonalitic, granites have compositions similar to those of the more felsic volcanic rocks forming at the present time above subduction zones. The Palaeozoic granites show little evidence of such a direct relationship to subduction. Within both the Caledonian and Lachlan belts there are some granites with a composition close to the younger tonalites. A particularly interesting case is that of the Tuross Head Tonalite of the Lachlan Fold Belt, which can be shown to have formed from slightly older source rocks by a process that we refer to as remagmatisation which has caused no significant change in composition. Since remagmatisation has reproduced the former source composition in the younger rocks, the wrong inference would result from the use of that composition to deduce the tectonic conditions at the time of formation of the tonalite. Granites, particularly the more mafic ones, will generally have compositions reflecting the compositions of their source rocks, and attempts to use granite compositions to reconstruct the tectonic environment at the time of formation of the granite may be looking instead at an older event. This is probably also the case for some andesites formed at continental margins.

Several arguments can be presented in favour of a general model for the production of I-type granite sources by underplating the crust, so that the source rocks are infracrustal. Such sources may contain a component of subducted sediments with the consequence that some of the compositional characteristics of sedimentary rocks may be present in I-type source rocks and in the granites derived from them. The small bodies of mafic granite and gabbro associated with island arc volcanism have an origin that can be related to the partial melting of subducted oceanic crust or of mantle material overlying such slabs and can be referred to as M-type. These rocks have compositions indistinguishable from those of the related volcanic rocks, except for a small component of cumulative material. The tonalitic I-type granites characteristic of the Cordillera are probably derived from such M-type rocks of basaltic to andesitic composition, which had been underplated beneath the crust. Some of the more mafic tonalites of the Caledonian-age fold belts may also have had a similar origin. More commonly, however, the plutonic rocks of the older belts are granodioritic and these probably represent the products of partial melting of older tonalitic I-type source rocks in the deep crust, these having compositions and origins analogous to the tonalites of the Cordillera. In this way, multiple episodes of partial melting, accompanied by fractionation of the magmas, can produce quite felsic rocks from original source rocks in the mantle or mantle wedge. These are essential processes in the evolution of the crust, since the first stages in this process produce new crust and the later magmatic events redistribute this material vertically without the addition of significant amounts of new crust.

Devonian miospores have been discovered in the previously poorly dated Old Red Sandstone volcanic sequence of Papa Stour. They occur at two sites in minor sedimentary deposits between the lavas, and fossil fish remains are also present. The age range of the miospores is mid Eifelian to early Givetian, probably more specifically late Eifelian and from a position close to the Achanarras horizon. This allows a correlation of the Papa Stour volcanic sequence with that of the Upper Stromness Flags of Orkney and not the tuffaceous horizons in the Eday Sandstones. The good preservation and composition of the miospores indicate a close similarity to other Orcadian Basin sediments and support the view that the Old Red Sandstone sequences W of the Melby Fault have affinities with the Orkney and Caithness successions rather than with Shetland. The age of the volcanic sequence also provides a valuable datum point for plate tectonic models based on the geochemistry of Old Red Sandstone lavas.

The available data from Newfoundland, the British Isles and Scandinavia suggest that by late Ordovician–early Silurian times the ocean separating Laurentia from Eastern Avalonia and Baltica had partly closed with the consumption of intervening oceanic crust. Marine seaways, however, persisted until the middle or late Silurian. Phases of crustal transtension and transpression, predominantly under a major sinistral shear couple, occurred throughout the Silurian and early Devonian until the remnant Iapetus Ocean was completely destroyed. The most appropriate Recent plate tectonic models for Silurian sedimentation between Eastern Avalonia and Laurentia are probably the deep-marine foreland basins between Timor and the northern Australian margin, or between Taiwan and mainland China.

Ring complex granites of the Peruvian Batholith are tabular bodies with flat roofs emplaced by cauldron subsidence. Marginal precursory ring dykes extend upwards above roof level and a typical intrusion is “H”-shaped in cross-section. Advance of magma by repeated subsidence would give a ladder-shaped profile for such intrusions above the brittle-ductile transition. Close relationships exist between intrusion geometry, emplacement process and petrogenetic evolution. Initially a granodioritic magma chamber lay beneath the present erosion level, trapping a rising mass of dioritic magma. Expansion of granodioritic liquid resulted in the injection of microgranite and tuffisite cone sheets accompanied by roof uplift within a ring fault. Next, during subsidence within the ring fault, liquids from deeper levels in the underlying chamber rose by stoping along the outer margin of the fault to form a ring dyke. Prior to intrusion, this liquid was vertically zoned from rhyodacite downwards to diorite and these liquids became partially mixed during emplacement. Finally, granodioritic magma rose to the present level by subsidence of a roof slab bounded by the ring fault. The precursory ring structures preserve evidence of significant but transient events in the underlying chamber. Liquid differentiation may be significant in the evolution of many large plutons.

A global review of new and existing data on the distribution of uppermost Ordovician (Hirnantian) brachiopods indicates the existence of at least three biogeographically distinct faunas. The typical Hirnantia fauna characterised subtropical and temperate latitudes and comprised a variety of ecological associations; the fauna reached its acme during the bohemicus and uniformis zones. Atypical Hirnantia faunas, developed marginal to Gondwana, are of low diversity and have few species in common with the typical Hirnantia fauna; their spatial distribution probably marked the margin of the polar ice sheets. The extinction of the Hirnantia fauna occurred in response to changes in sea level. Diverse and quite different faunas, including those from the Midcontinent of North America, Kolyma, the Oslo Region and probably Anticosti Island, occupied equatorial latitudes during the latest Ordovician. The Holorhynchus fauna, on evidence to date, predates the Hirnantia fauna.

The Moffat Shale Group is a condensed, variable and partly pelagic sequence of mudrocks of Llandeilo—Llandovery age. The sequence has a five-fold lithological subdivision based mainly on the occurrence of grey mudstones within a succession otherwise dominated by fully euxinic black graptolitic mudrocks. Associated with the black mudrocks, especially in the Llandovery, are metabentonite beds which achieve a climax, both in thickness and in number, within the top quarter of the mudrock sequence. A geochemical and mineralogical study has confirmed a volcanic origin for the metabentonites. Major element data highlight a carbonate-dominated environment above the gregarius—convolutus Zones boundary. Phosphorus levels reach a peak at the same boundary, as well as at the Caradoc—Ashgill boundary where phosphorite horizons are known from Wales and Norway. Immobile trace elements have highlighted regular changes in source magma composition. Prolonged periods of crystal fractionation in magmas of intermediate composition gave rise, on eruption, to large volumes of silicic ash which had a deleterious effect on graptolite species and led to local extinctions. Regular fluctuations in ash composition from silicic to intermediate are ascribed to alternating fractionation and magma mixing cycles.

The Peninsular Ranges Batholith of southern and Baja California is the largest segment of a Cretaceous magmatic arc that was once continuous from northern California to southern Baja California. In this batholith, the emplacement of igneous rocks took place during a single sequence of magmatic activity, unlike many of the other components of the Cordilleran batholiths which formed during successive separate magmatic episodes. Detailed radiometric dating has shown that it is a composite of two batholiths. A western batholith, which was more heterogeneous in composition, formed as a static magmatic arc between 140 and 105 Ma and was intrusive in part into related volcanic rocks. The eastern batholith formed as a laterally transgressing arc which moved away from those older rocks between 105 and 80 Ma, intruding metasedimentary rocks. Rocks of the batholith range from undersaturated gabbros through to felsic granites, but tonalite is the most abundant rock throughout. Perhaps better than elsewhere in the Cordillera, the batholith shows beautifully developed asymmetries in chemical and isotopic properties. The main gradients in chemical composition from W to E are found among the trace elements, with Ba, Sr, Nb and the light rare earth elements increasing by more than a factor of two, and P, Rb, Pb, Th, Zn and Ga showing smaller increases. Mg and the transition metals decrease strongly towards the E, with Sc, V and Cu falling to less than half of their value in the most westerly rocks. Oxygen becomes very systematically more enriched in18O from W to E and the Sr, Nd and Pb isotopic systems change progressively from mantle values in the W to a more evolved character on the eastern side of the batholith. In detail the petrogenesis of the Peninsular Ranges Batholith is not completely understood, but many general aspects of the origin are clear. The exposed rocks, particularly in the western batholith, closely resemble those of present day island arcs, although the most typical and average tonalitic composition is distinctly more felsic than the mean quartz diorite or mafic andesite composition of arcs. Chemical and isotopic properties of the western part of the batholith indicate that it formed as the root of a primitive island arc on oceanic lithosphere at a convergent plate margin. Further E, the plutonic rocks appear to have been derived by partial melting from deeper sources of broadly basaltic composition at subcrustal levels. The compositional systematics of the batholith do not reflect a simple mixing of various end-members but are a reflection of the differing character of the source regions laterally and vertically away from the pre-Cretaceous continental margin.

The Watch Hill Grits, in the lower Ordovician Skiddaw Group of the Lake District, are dated as latest Tremadoc or earliest Arenig (older than the Didymograptus deflexus Biozone) by means of acritarchs and graptolites. This places them lower in the Skiddaw sequence than previous authors supposed, and they are thought to have been thrust southwards over younger Arenig rocks. The interval inferred for the Watch Hill Grits brackets the Tetragraptus approximatus Biozone and its equivalents in Australasia, S China, N America and Scandinavia. The dating of the Watch Hill Grits suggests that equivalents of the Tetragraptus approximatus Biozone are present in N England and that sedimentation continued in the Lake District during a period of global eustatic regression. Other beds in the Lake District and on the Isle of Man have yielded similar acritarch assemblages and are probably correlatives. Five new acritarch taxa are described: Caldariola gen. nov., Acanthodiacrodium? dilatum, Stellechinatum sicaforme and Striatotheca prolixa spp. nov., and Tetraniveum arenigum cumbriense subsp. nov.; one new combination, Caldariola glabra (Martin) comb, nov., is proposed.

The genesis of subduction-related magmas in the Andean region of South America and the Antarctic Peninsula is considered in relation to the Palaeozoic to Cenozoic granitoids belts which are thought to parallel palaeo-coastlines. Their Sr-Nd isotope systematics show a wide range of initial compositions (87Sr/86Sr0 0·7038 to >0·710; εNd, +4 to –10) requiring material input from both depleted mantle and continental crust. In local transects there are consistent trends with time of emplacement, from enriched (crustal) to depleted (mantle) sources, regardless of the sense of migration of magmatism (towards or away from the continent). These trends represent mixing between mantle-derived material and anatectic melts of the lower crust: in each case the crustal end-member reflects the age and isotopic composition of the local deep crustal basement (Precambrian in the easternmost Andes, Palaeozoic in the W and in the Antarctic Peninsula). The depleted end-member could be derived by melting within the subducted oceanic crust, the overlying mantle or previously crystallised mafic underplating. One of the most important factors controlling the mixing process is the angle of subduction, resulting in magma generation under variable tectonic conditions.

The pursuit of a comprehensive theory for the origin and evolution of granitoids is hindered by our incomplete understanding of the nature of the source and the mechanisms by which the magma is segregated and transported. This paper is a collection of three largely independent and necessarily incomplete perspectives on these outstanding issues. Lower to mid-crustal regions, which contain the principal source material for granitoid magmas, are highly heterogeneous. Consideration of available transfer mechanisms suggests that (1) this heterogeneity survives all foreseeable lower crustal processes; (2) closure is on very different scales for different chemical systems (e.g. Pb, Nd, Sr and O isotopes); in almost all cases, however, closure scale is much smaller than the scale of magma extraction zones for plutons; and (3) pluton-wide homogenisation of magmas by diffusion is precluded by low diffusivities in felsic melts. Thus, granitoid magmas begin life as aggregates of small, isolated chemical domains; homogenisation occurs only through (and on the scale of) effective stirring by convection. Because of variability in local conditions as well as in bulk composition, crustal regions undergoing anatexis must be patchworks with variable melt fractions and melt compositions. The way in which magma is extracted from and coalesces with this patchwork exerts a critical influence on the nature of granitoid magmas. Decoupling and unusual coupling of compositional parameters and isotopic heterogeneity within plutons are to be expected in crust-derived granitoids and do not require contamination. Granites image their sources, but these sources are ill-defined and do not correspond to simple, easily-recognised materials. Extent and patterns of heterogeneity remaining in crystallised plutons may be effective indicators of the ascent process.

The efforts of materials scientists in characterising the nature and evolution of solid-phase interconnectivity in partially-molten materials may offer some insights into crustal magmatic processes. In particular, the rheological properties of partially-molten crustal rocks are probably strongly affected by the contiguity of the solid grains in the system (i.e. the fraction of their surface area that is shared with other grains). Theory and experimental data for simple alloy systems reveal that contiguity depends principally upon melt fraction and upon the characteristic wetting angle (θ) of the system. Measured θ's in granitoids (∼50° on average) imply contiguities as high as ∼0·2 for melt fractions of 0·5 or greater. This value in turn suggests that, at least under static conditions, a continuous skeleton of solid grains is maintained to quite high degrees of melting in the crust. Consequently, regions consisting of 50% or more of melt can, in principle, maintain not only high yield strength, but also high viscosity (provided the strain rate is sufficiently low to avoid disrupting contiguity).

Despite the fact that on some time scale the continuous solid skeleton of a partially-molten region resists deformation, it is itself subject to textural evolution that could lead to the upward migration of melt. Occasional detachment of grains from the skeleton and subsequent “microsettling” within the partially-molten column may lead eventually to compaction of the solid (without plastic deformation) and net upward displacement of melt.

Proposed granite transport mechanisms are discussed, although several are viewed as having historical interest only. In the absence of tectonic transport, diapirism appears to be the most compelling of these processes. However, considerable diversity exists in the literature regarding a pivotal requirement for this mechanism. Structural studies have tended to conclude that the granite diapir must be highly crystallised in order to ascend, whereas results of physical modelling yield contradictory results. For ascent to occur in these models, the magmas must be sufficiently fluid to allow convective circulation. Indeed, heat loss associated with diapirism is so efficient as to be a significant restriction on overall ascent. The resolution of these contrasting views appears to be that they reflect different phases of the ascent/emplacement continuum. Understanding the emplacement history of a southeastern Australian pluton allows assessment, via the diapir model, of the flow properties of the rock within the deformation aureole. Results suggest rock viscosities about an order of magnitude lower than those predicted by laboratory experiments, perhaps reflecting difficulties in reproducing natural conditions in the laboratory.

The zoned McMurry Meadows Pluton has been examined for REE and trace element variations in hornblende, sphene, apatite, allanite and zircon. Mineral separates (17), were analysed by INAA from four granitoids spanning the compositional range of the pluton (60%–75% SiO2). All of the phases examined exhibit significant compositional variations, with sphene having the largest changes in chondrite normalised REE patterns. Compositional variations in these minerals are related to paragenetic sequence and, as the whole rocks become more evolved, increases in partition coefficients; not subsolidus re-equilibration. Hornblende is only a dominant site for REE in granites where sphene is a later crystallising phase, otherwise allanite (LREE only) and sphene are the dominant whole rock sites for REE. Zircon and apatite normally contribute less than 10% each to the whole rock abundance of REE. Three zircon samples contain only small compositional differences and are interpreted as having crystallised from the bulk magma prior to differentiation. Zr variation in the pluton is nonlinear, first increasing and then decreasing with whole rock fractionation. A simple process, analogous to “restite unmixing” applied to the Zr variation, defines a bulk magma composition of about 63% SiO2, before differentiation of the zoned pluton. The modelled bulk magma need only have evolved by about 2·5% silica and still have produced the entire compositional range (60–75% SiO2) observed in the pluton.

S-type granites have properties that are a result of their derivation from sedimentary source rocks. Slightly more than half of the granites exposed in the Lachlan Fold Belt of southeastern Australia are of this type. These S-type rocks occur in all environments ranging from an association with migmatites and high grade regional metamorphic rocks, through an occurrence as large batholiths, to those occurring as related volcanic rocks. The association with high grade metamorphic rocks is uncommon. Most of the S-type granites were derived from deeper parts of the crust and emplaced at higher levels; hence their study provides insights into the nature of that deeper crust. Only source rocks that contain enough of the granite-forming elements (Si, Al, Na and K) to provide substantial quantities of melt can produce magmas and there is therefore a fertile window in the composition of these sedimentary rocks corresponding to feldspathic greywacke, from which granite magmas may be formed.

In this paper, three contrasting S-type granite suites of the Lachlan Fold Belt are discussed. Firstly, the Cooma Granodiorite occurs within a regional metamorphic complex and is associated with migmatites. It has isotopic and chemical features matching those of the widespread Ordovician sediments that occur in the fold belt. Secondly, the S-type granites of the Bullenbalong Suite are found as voluminous contact-aureole and subvolcanic granites, with volcanic equivalents. These granites are all cordierite-bearing and have low Na2O, CaO and Sr, high Ni, strongly negative εNd and high 87Sr/86Sr, all indicative of S-type character. However, the values of these parameters are not as extreme as for the Cooma Granodiorite. Evidence is discussed to show that these granites were derived from a less mature, unexposed, deeper and older sedimentary source. Other hypotheses such as basalt mixing are discussed and can be ruled out. The Strathbogie Suite granites are more felsic but all are cordierite-bearing and have chemical and other features indicative of an immature sedimentary source. They are closely associated with cordierite-bearing volcanic rocks. The more felsic nature of the suite results in part from crystal fractionation. It is suggested that the magma may have entered this “crystal fractionation” stage of evolution because it was a slightly higher temperature magma produced from an even less mature sediment than the Bullenbalong Suite. The production of these S-type magmas is discussed in terms of vapour-absent melting of metagreywackes involving both muscovite and biotite. The production of a magma in this way is consistent with the low H2O contents and geological setting of S-type granites and volcanic rocks in the Lachlan Fold Belt.