ALBERT

Description: The potential of igneous quartz for providing a better understanding of magmatic processes is demonstrated by studying late-Hercynian rhyolites and granites from central and western Europe. Cathodoluminescence (CL) reveals growth patterns and alteration structures within igneous quartz reflecting the magma crystallisation history. The relatively stable and blue-dominant CL of zoned phenocrysts is principally related to variations in the Ti concentration, which is a function of the crystallisation temperature. The Al/Ti ratio of igneous quartz increases with progressive magma differentiation, as Ti is more compatible, compared to Al, Li, K, Ge, B, Fe, P during magma evolution. The red-dominant CL of the anhedral groundmass quartz in granite is unstable during electron bombardment and associated with OH- and H2O-bearing lattice defects. Thus, CL properties of quartz are different for rocks formed from H2O-poor and H2O-rich melts. Both groundmass and phenocrysts in granites are rich in alteration structures as a result of interaction with deuteric fluids during cooling, whereas phenocrysts in extrusive rocks do not usually contain such structures. The combined study of trace elements along with the analysis of quartz textures and melt inclusion inventories may reveal detailed PTX-paths of granite magmas. This study shows that quartz is a sensitive indicator for physico-chemical changes during the evolution of silicarich magmas. Common growth textures show a wide variety in quartz phenocrysts in rhyolites and some granites. This paper presents a classification of textures, which formed as a result of heterogeneous intra-granular lattice defects and impurities. The alternation of growth and resorption microtextures reflects stepwise adiabatic and non-adiabatic magma ascent, temporary storage of magma in reservoirs and mixing with more mafic, hotter magma. The anhedral groundmass quartz overgrowing early-magmatic phenocrysts in granites is free of growth zoning.

Description: This paper describes three mid-Tertiary intrusions from the Henry Mountains (Utah, USA) that were assembled from amalgamation of multiple horizontal sheet-like magma pulses in the absence of regional deformation. The three-dimensional intrusion geometries are exceptionally well preserved and include: (1) a highly lobate sill; (2) a laccolith; and (3) a bysmalith (a cylindrical, fault-bounded, piston-like laccolith). Individual intrusive sheets are recognised on the margins of the bodies by stacked lobate contacts, and within the intrusions by both intercalated sedimentary wallrock and formation of solid-state fabrics. Finally, conduits feeding these intrusions were mostly sub-horizontal and pipe-like, as determined by both direct observation and modelling of geophysical data.%The intrusion geometries, in aggregate, are interpreted to reflect the time evolution of an idealised upper crustal pluton. These intrusions initiate as sills, evolve into laccoliths, and eventually become piston-like bysmaliths. The emplacement of multiple magma sheets was rapid and pulsed; the largest intrusion was assembled in less than 100 years. The magmatic fabrics are interpreted as recording the internal flow of the sheets preserved by fast cooling rates in the upper crust. Because there are multiple magma sheets, fabrics may vary vertically as different sheets are traversed. These bodies provide unambiguous evidence that some intrusions are emplaced in multiple pulses, and that igneous assembly can be highly heterogeneous in both space and time. The features diagnostic of pulsed assembly observed in these small intrusions can be easily destroyed in larger plutons, particularly in tectonically active regions.

Description: The anisotropy of magnetic susceptibility (AMS) is widely and routinely used to measure the preferred orientations of Fe-rich minerals in undeformed and weakly deformed granite plutons. The interpretation of the mapped AMS fabrics depends on rock-textural observations, on the map patterns of the fabrics in plutons, and on comparisons of the pluton fabrics to tectonic structures in the country rocks. The AMS may document emplacement-flow related fabrics, but the emplacement fabrics may be reworked or completely overprinted by rather weak tectonic strains of the magma mush or the cooling pluton, especially in syntectonic intrusions. The Late Devonian Canso granite pluton is an excellent example of overprinting of emplacement fabrics by weak tectonic strains. The Canso pluton was emplaced ca. 370 Ma along the boundary between the Meguma and Avalon tectonic terranes, in the northern Appalachian orogen. The AMS was mapped along two traverses that cross the pluton and that are perpendicular to the terrane boundary. Textural evidence suggests the rocks underwent very modest post-full crystallisation strains. The AMS records the dextral transcurrent shearing that occurred on the terrane boundary during emplacement and cooling of the Canso pluton, supporting interpretations that weakly deformed syntectonic granites can be used as indicators of regional bulk kinematics. AMS fabrics in Late Devonian granites of the Meguma Terrane suggest partitioning of the non-coaxial shearing into the terrane bounding fault, with pure-shear dominated deformation further from the fault. Numerical simulations suggest that the kinematics recorded by the fabrics in the Canso pluton was simple-shear, or transpression or transpression with small components of pure shear oriented perpendicular to the bounding shear zone.

Description: A comparative study of three main igneous rock associations that plot in the K2O–SiO2 diagram shoshonite field: shoshonite series absarokites–shoshonites–banakites (henceforth referred to as shoshonites s.l.), vaugnerites, and potassic lamprophyres, reveals that similarities between the associations are superficial. Vaugnerites and lamprophyres are more magnesian, richer in large ion lithophile and high field strength elements and have higher light rare earth/heavy rare earth ratios than shoshonites. Furthermore, shoshonites have low radiogenic heat production, typical of subduction-related rocks, but most vaugnerites and some lamprophyres are highly radioactive. Relative to bulk-Earth, shoshonites have depleted, asthenospheric mantle-like Sr and Nd isotope signatures, whereas vaugnerites and potassic lamprophyres have enriched, crust or lithospheric mantle-like compositions. Though vaugnerites and some lamprophyres show evidence of crustal contamination, the contaminated magma was not originally shoshonitic. Their composition is consistent with derivation from a metasomatised upper mantle source enriched long before melting, thus precluding an active subduction setting. In conclusion, the term shoshonite, implying late-stage arc magmas, cannot be applied to a rock series simply because it plots into the K2O–SiO2 diagram shoshonite field. Shoshonites with a subduction-related source may, however, be identified by discriminant function analysis.

Description: India–Asia collision resulted in crustal thickening and shortening, metamorphism and partial melting along the 2200 km-long Himalayan range. In the core of the Greater Himalaya, widespread in situ partial melting in sillimanite+K-feldspar gneisses resulted in formation of migmatites and Ms+Bt+Grt+Tur±Crd±Sil leucogranites, mainly by muscovite dehydration melting. Melting occurred at shallow depths (4–6 kbar; 15–20 km depth) in the middle crust, but not in the lower crust. 87Sr/86Sr ratios of leucogranites are very high (0·74–0·79) and heterogeneous, indicating a 100 crustal protolith. Melts were sourced from fertile muscovite-bearing pelites and quartzo-feldspathic gneisses of the Neo-Proterozoic Haimanta–Cheka Formations. Melting was induced through a combination of thermal relaxation due to crustal thickening and from high internal heat production rates within the Proterozoic source rocks in the middle crust. Himalayan granites have highly radiogenic Pb isotopes and extremely high uranium concentrations. Little or no heat was derived either from the mantle or from shear heating along thrust faults. Mid-crustal melting triggered southward ductile extrusion (channel flow) of a mid-crustal layer bounded by a crustal-scale thrust fault and shear zone (Main Central Thrust; MCT) along the base, and a low-angle ductile shear zone and normal fault (South Tibetan Detachment; STD) along the top. Multi-system thermochronology (U–Pb, Sm–Nd, 40Ar–39Ar and fission track dating) show that partial melting spanned ̃24–15 Ma and triggered mid-crustal flow between the simultaneously active shear zones of the MCT and STD. Granite melting was restricted in both time (Early Miocene) and space (middle crust) along the entire length of the Himalaya. Melts were channelled up via hydraulic fracturing into sheeted sill complexes from the underthrust Indian plate source beneath southern Tibet, and intruded for up to 100 km parallel to the foliation in the host sillimanite gneisses. Crystallisation of the leucogranites was immediately followed by rapid exhumation, cooling and enhanced erosion during the Early–Middle Miocene.

Description: Foraminiferal, algal and problematica assemblages from the Mississippian (late Viséan and early Serpukhovian) Lower Limestone Formation have been studied in order to validate lithostratigraphical correlations of limestones within the central and western parts of the Midland Valley of Scotland. Analysis of more than 100 outcrops allows recognition of four calcareous microfossil assemblages, which span the late Brigantian and early Pendleian, and enables a detailed correlation to be made within the Central Coalfield (north Lanarkshire) and with the thinner sequences to the west (north Ayrshire), to the south (Douglas area, south Lanarkshire), and to the east (Bathgate area, West Lothian). The age of the Lower Limestone Formation is modified because the upper part of this formation is now assigned to the Pendleian (due to the first occurrences of new foraminiferans and the co-occurrence with the Namurian goniatites), and some individual limestone horizons within the formation are repositioned, or their precise correlation with other limestones is established. A refined stratigraphical framework is proposed for the above noted areas, and a correlation between them and the Pennine region in northern England is proposed, passing through the Archerbeck Borehole sequence in the Scottish Borders.

Description: The Cape Granites are a granitic suite intruded into Neoproterozoic greywackes and slates, and unconformably overlain by early Palaeozoic Table Mountain Group orthoquartzites. They were first recognised at Paarl in 1776 by Francis Masson, and by William Anderson and William Hamilton in 1778. Studies of the Cape Granites were central to some of the early debates between the Wernerian Neptunists (Robert Jameson and his former pupils) and the Huttonian Plutonists (John Playfair, Basil Hall, Charles Darwin), in the first decades of the 19th Century, since it is at the foot of Table Mountain that the first intrusive granites outside of Scotland were described by Hall in 1812. The Neptunists believed that all rocks, including granite and basalt, were precipitated from the primordial oceans, whereas the Plutonists believed in the intrusive origin of some igneous rocks, such as granite. In this paper, some of the early descriptions and debates concerning the Cape Granites are reviewed, and the history of the development of ideas on granites (as well as on contact metamorphism and sea level changes) at the Cape in the late 18th Century and early to mid 19th Century, during the emerging years of the discipline of geology, is presented for the first time.

Description: The Dalradian Supergroup contains three distinct glacigenic units, formerly termed ‘Boulder Beds’, which are correlated with widespread Neoproterozoic glaciations. The oldest and thickest unit, the Port Askaig Formation, marks the Appin–Argyll group boundary of the Dalradian Supergroup and has been correlated with the Middle Cryogenian (Sturtian) glaciation. The Auchnahyle Formation, a diamictite-bearing sequence near Tomintoul in NE Scotland, exhibits strong lithological similarities to the Port Askaig Formation. Both these glacigenic ‘Boulder Bed’ units contain abundant dolomite clasts in their lower parts and more granitic material at higher levels. Both metadiamictite units are overlain by thick shallow-marine quartzite units. C isotope data from Appin Group carbonate strata below the Auchnahyle Formation support this correlation. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) detrital zircon data from the Auchnahyle Formation metadiamictite differ slightly from the Port Askaig Formation, but are similar to detrital zircon spectra obtained from the Macduff Formation, a diamictite unit in the younger Southern Highland Group of the Dalradian Supergroup; both apparently reflect derivation from local basement rocks. No detritus younger than 0·9 Ga is observed, so the data do not constrain significantly the depositional age of the glacial strata. A thin tholeiitic pillow basalt unit in the lower part of the Auchnahyle Formation is geochemically distinct from pre-tectonic metadolerite sills and from basic metavolcanic rocks up-section. A Sturtian (c. 720–700 Ma) age for the Auchnahyle Formation metadiamictite would imply that this basaltic volcanism represents the oldest recorded volcanic activity in the Dalradian Supergroup and is inferred to represent an early, local phase of proto-Iapetan rifting within the Rodinian supercontinent.

Description: Conodont faunal dynamics and high-resolution biostratigraphy in the lithologically and faunally anomalous ‘Täljsten’ succession, which spans the DarriwilianLenodus variabilis–Yangtzeplacognathus crassusZone boundary, were investigated in a 2·5 m-thick section on Mt Kinnekulle that includes an interval yielding fossil meteorites and extraterrestrial chromite. The previous interpretation that this interval reflects a regression is consistent with the occurrence and abundance patterns of some conodont taxa. The disappearance of e.g.,Periodon, suggests that the regression began prior to the deposition of the grey ‘Täljsten’. The transition from red to grey limestone coincides with a conspicuous faunal re-arrangement. The lower half of the ‘Täljsten’ reflects a gradual shallowing favourable for some taxa, such asLenodus, and the immigration ofMicrozarkodinacf.ozarkodellaandHistiodella holodentata. In the middle of the ‘Täljsten’ interval, coinciding with the appearance of abundant cystoids, conditions became less hospitable for conodonts, resulting in a low diversity and low abundance fauna, which occurs to the top of the interval. The overlying red limestone, apparently deposited during a deepening event, marks a return to pre-‘Täljsten’ conditions with a re-organised fauna. The close correlation between the lithologic shifts and conodont faunal changes demonstrates the usefulness of conodonts as environmental indicators.

Description: Two regional-scale, recumbent folds control the structure of the Beinn Udlaidh area, Tyndrum, Perthshire. They reached their maximum development during D2, following the regional metamorphic peak, and are part of a stack of larger SE-facing recumbent folds formed during the ∼470 Ma Grampian Orogeny. The rocks belong to the Neoproterozoic–Lower Ordovician Dalradian Supergroup, and preserve a sedimentary transition between the Grampian Group and the overlying Appin Group. The latter occupies the core of the S-facing, recumbent Beinn Udlaidh Syncline (D2) which, with the underlying complementary Glen Lochy Anticline, is gently folded by a regional-scale structure, the Orchy Dome. The recumbent folds postdate an early fabric (S1), which is generally obliterated by the D2 imprint, but preserved as inclusion trails in regional metamorphic garnets, that are highly oblique to, and wrapped by, S2. It is concluded that the Dalradian rocks described here from below the Iltay Boundary Slide are in structural continuity with those of the Tay Nappe above, and that the Slide represents a structurally-modified disconformity between the Leven Schist (Appin Group) and the overlying Ben Lui Schist (Argyll Group). The Orchy Dome probably influenced the spatial distribution of minor intrusions and explosion vents of the lamprophyre suite.

Description: Dispersed spore assemblages have been recovered from the Am Binnein Sandstones from the upper part of the ‘Lower Old Red Sandstone’ sequence on the island of Arran, Scotland. The spore assemblages belong with theEmphanisporites annulatus–Camarozonotriletes sextantii(AS) Spore Assemblage Biozone (SAB), indicating an Early Devonian, Emsian (but not earliest Emsian or latest Emsian) age. This is the first reliable age constraint for the ‘Lower Old Red Sandstone’ of Arran, and enables correlation with the more extensive sequence developed on the mainland in the Midland Valley of Scotland. The Am Binnein Sandstones are confirmed as correlatives of the Strathmore Group.

Description: The authors revise the occurrences of burrow networks with striated walls having dominantly transverse to oblique striae, which have been assigned to the ichnogeneraSpongeliomorphaSaporta, 1887, andSteinichnusBromley & Asgaard, 1979. The taxonomic status of the ichnogenusSteinichnusBromley & Asgaard, 1979 is examined and it is suggested that this ichnogenus is a subjective junior synonym ofSpongeliomorphaSaporta, 1887.Spongeliomorphais best reserved for an unlined network of burrows having distinct surface ridges or grooves of different orientation and massive filling. The diagnosis ofSpongeliomorphais emended accordingly and the proposed ichnospecies revised for consistency with the diagnostic features of the ichnogenus.Spongeliomorpha milfordensisMetz, 1993a is considered a subjective junior synonym ofSpongeliomorpha carlsbergi(Bromley & Asgaard, 1979) after a visual comparison and statistical analysis of the angle of striation with respect to the burrow midline in the type material. Nevertheless, the use of statistical techniques is not advocated for distinction of ichnotaxa, but may support observations.Spongeliomorpha carlsbergiis considered as an indicator of nonmarine settings and was probably produced by burrowing insects. Proposed ichnospecies ofSpongeliomorphathat fit the emended diagnosis includeS. sudolica(Zaręczny, 1878);S. ibericaSaporta, 1887;S. siculaD’Alessandro & Bromley, 1995;S. chevronensisMuñiz & Mayoral, 2001; andSpongeliomorphaisp. nov. aff.siculaLewy & Goldring, 2006.

Description: The morphology of the amphibamid temnospondyl amphibian Platyrhinops lyelli from the Middle Pennsylvanian of Linton, Ohio is reassessed from previously described and undescribed specimens. Newly reported or newly significant features include the presence of bicuspid marginal teeth, anteriorly widened frontals, elongate choanae, a broad rhomboidal sphenethmoid and a pair of flattened blade-like ceratohyals. One specimen shows an unusual distribution of tooth sizes along the premaxillaries. No derived characters can be found to justify reference of the species lyelli to the genus Amphibamus as represented by its type species A. grandiceps, but Platyrhinops does belong to the ‘Amphibamus branch’ of the Amphibamidae.

Description: Though typically exhibiting considerable scatter, geochemical variations in granitic plutons and silicic volcanic deposits are commonly modelled as products of differentiation of originally homogeneous magmas. However, many silicic igneous bodies, particularly those classified as S-types, are internally heterogeneous in their mineralogy, geochemistry and isotope ratios, on scales from hundreds of metres down to one metre or less. The preservation of these heterogeneities supports recent models for the construction of granitic magma bodies through incremental additions of numerous batches (pulses) of magma derived from contrasting sources. Such pulses result from the sequential nature of the melting reactions and the commonly layered structure of crustal magma sources. Internal differentiation of these batches occurs, but not generally on the scales of whole magma chambers. Rather than being created through differentiation or hybridisation processes, at or near emplacement levels, much of the variation within such bodies (e.g. trace-element or Mg# variation with SiO2 or isotope ratios) is a primary or near-source feature. At emplacement levels, the relatively high magma viscosities and slow diffusion rates of many chemical components in silicic melts probably inhibit processes that would lead to homogenisation. This permits at least partial preservation of the primary heterogeneities.

Description: Trace element abundances in garnet from a polyphase migmatite were measured by secondary ion mass spectrometry (SIMS) in order to identify some of the effective variables on the trace element distribution between garnet and melanosome or leucosome. In general, garnet is zoned with respect to REE, in which garnet cores are enriched by a factor of 2–3 relative to the rims. For an inclusion-rich garnet from the melanosome, equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim. Inclusion-poor garnet from the same melanosome located in the vicinity of the leucosomes shows distinct enrichment and depletion patterns for REE from core to rim. These features suggest disequilibrium between garnet and the host rock which, in this case, could have been an in-situ derived melt. This would probably indicate a period of open-system behaviour at a time when the garnet, originally nucleated in the metamorphic environment reacted with the melt. In addition, non-gradual variation in trace element abundances between core and rim may suggest variable garnet growth rates. Inclusion-free garnet from the leucosome, interpreted to have crystallised in the presence of a melt, has a small core with high REE abundances and a broad rim with lower REE abundances. Here, crystal-liquid diffusion-controlled partitioning is a likely process to explain the trace element variation.

Description: The causes of compositional diversity in the Tuolumne Batholith, whether source heterogeneity, magma mixing, or fractional crystallisation, is a matter of longstanding debate. This paper presents data from detailed mapping and a microstructural and major element, trace element and isotopic study of an elongate lobe of the Half Dome granodiorite that protrudes from the southern end of the batholith. The lobe is normally zoned from quartz diorite along the outer margin to high-silica leucogranite in the core. Contacts are steep and gradational, except for the central leucogranite contact, which is locally sharp: magmatic fabrics overprint contacts. A striking feature of the lobe is the 18 wt SiO2 range comparable to that observed for the entire Tuolumne Batholith. Feldspar-compatible elements (Sr and Ba) decrease towards the centre, while Rb increases. Light and middle REEs show a smooth decrease towards the centre of the lobe. Calculated initial isotopic ratios of 87Sr/86Sr(i) and εNd(t) have identical values within error across the lobe, except in the central leucogranite, the most silica rich phase, which shows a slightly more crustal signature. Field, structural, geochemical and isotopic data suggest that fractionation was the dominant process causing compositional variation in this lobe. It is envisioned that this fractionation/crystal sorting occurred in a vertically flowing and evolving magma column with the present map pattern representing a cross-section of this column. Thus the areal extent of the lobe represents a minimum size of interconnected melt at the emplacement level of the Tuolumne Batholith and, given its marginal position, limited width and proximity to colder host rocks, implies that fractionation in larger chambers likely occurred in the main Tuolumne Batholith magma chamber(s).

Description: In the recently completed and formally ratified new series and stage classification of the Ordovician System, the base of the Middle Ordovician Series coincides with the base of the global Dapingian Stage. In the Global Stratotype Section and Point (GSSP) of this stage, which is located at Huanghuachang in southern China, the base of the Dapingian Stage is defined as the level of first appearance of the conodont Baltoniodus triangularis. The fact that this species, along with some other taxa present at the Dapingian GSSP, occurs in many sections in Baltoscandia makes it possible to recognise with considerable precision the level of this global stage boundary in Sweden, Estonia, northwestern Russia, and Denmark. In several, but not all, regions, especially in the East Baltic, the global stage boundary coincides with the base of the regional Volkhov Stage and can be tied to the base of the Megistaspis polyphemus Trilobite Zone. The regionally somewhat different relationships between the position of the global stage boundary and a very widespread hardground complex are probably due to the occurrence of local and/or regional unconformities in the upper Floian–lower Dapingian interval. Although biostratigraphically important graptolites are present in the study interval in some Baltoscandic sections, the precise graptolite correlation of the base of the Dapingian Stage remains somewhat unclear, although it appears to be near, or at, the base of the Isograptus victoriae victoriae Zone (Ca 2).

Description: New three-dimensionally preserved specimens of two euthycarcinoid arthropods, namely Schramixerxes gerem and Sottyxerxes multiplex, allow complete description of both the dorsal and ventral sides of the exoskeleton. The functional morphology is tentatively interpreted for the first time. In S. gerem, the ‘thirteenth somite’, or ‘monosomite’ is fully described and re-interpreted as the main articulatory process of the body, between the cephalic region and the preabdomen. The morphology and arrangement of the two parts of the process clearly indicate that the anterior cephalic region of the body could move laterally and bend ventrally, while posterior somites could only move ventrally. Unlike several other euthycarcinoid species, the ventral side of the head area exhibits one or two plates instead of mandibles; such distinct morphologies are indicative of different feeding mechanisms and behaviours among euthycarcinoid arthropods. Possible homologies with the labrum of Hexapoda support the hypothesis that euthycarcinoids have hexapod affinities.

Description: A diverse, previously undescribed trilobite fauna from northwest Iran sheds new light on the Early Palaeozoic geography of the Alborz terrane. The structural history of the Alborz region is well known, though faunal data for the area are sparse, especially for the Ordovician. Trilobites, brachiopods, conodonts, bryozoans and echinoderms occur in a red grainstone/packstone near the village of Tatavrud, 35 km southwest of Bandar-e-Anzali. The fossils indicate a Late Ordovician age, although lithologically similar outcrops in northern Iran have previously been considered Silurian. Trilobite genera occurring include Trinodus, Geragnostus, Illaenus, Panderia, Phorocephala, Ovalocephalus, Dicranopeltis, Symphysops, Cyclopyge, and Sphaerexochus. Similar assemblages have been reported from the Late Ordovician of Ireland, Spain, Poland, Norway, northwest China, Kazakhstan and the Turkistan–Alai Ridge (Uzbekistan–Kyrgyzstan border). Reconstructions of Late Ordovician geography place the Alborz terrane near the eastern margin of Gondwana, but there is doubt as to its precise location. Some workers have considered it part of the margin while others have considered it a separate terrane. Several species present, including Mezzaluna tatavrudensis n. sp. and Phorocephala cf. ulugtana (Petrunina, 1975) are very similar to taxa described from the Turkistan–Alai Ridge. In addition, the isocolid genus Paratiresias has only been reported from Iran, the Turkistan–Alai Ridge and northwest China. The presence of these taxa indicates that the Alborz terrane was in close proximity to the eastern margin of Gondwana at this time.

Description: The Palaeogene volcanic succession of the Faroe Islands in the NE Atlantic Ocean is formalised using a purely lithostratigraphic approach and following international guidelines. The Faroe Islands Basalt Group (FIBG) has a gross stratigraphic thickness of ∼6·6 km, dominated by subaerial basalt lava flows, and is subdivided into seven formations. The Lopra Formation forms the basal ∼1·1 km of the Lopra-1/1A borehole, dominated by hyaloclastites, volcaniclastic sandstones and invasive basaltic lavas/sills. It is overlain by the ∼3·25 km-thick Beinisvørð Formation, dominated by laterally extensive basalt sheet lobes separated by minor volcaniclastic lithologies. The Beinisvørð Formation is overlain by the

Description: Zavitokichnus fusiformisn. igen. et n. isp. occurs in Lower Cretaceous (Valanginian to Hauterivian) limestones of the Fatric Superunit in the Western Carpathians. Typical cross sections of this more or less spiral trace fossil are sometimes U–O–C–S-shaped. In cross-section the trace fossil passes from a simple linear form, and spreads to a wider rolled-up or rolled-out form and then it returns to a linear trace. Spreite-like lamellae are distinguishable on several cross-section examples. The trace fossil was produced by a deposit feeder and it might be classified as a fodinichnion.Z. fusiformisco-occurs with trace fossil associations ofZoophycos,Chondrites,Planolites,HormosiroideaandPalaeophycusin carbonate sediments of a deep-seated ramp along the margin of the Fatric intrashelf basin.

Description: A specific type of granitoid, referred to as sanukitoid (Shirey & Hanson 1984), was emplaced mainly across the Archaean–Proterozoic transition. The major and trace element composition of sanukitoids is intermediate between typical Archaean TTG and modern arc granitoids. However, among sanukitoids, two groups can be distinguished on the basis of the Ti content of the less differentiated rocks of the suite: high- and low-Ti sanukitoids. Melting experiments and petrogenetic modelling show that they may have formed by either (1) melting of mantle peridotite previously metasomatised by felsic melts of TTG composition, or (2) by reaction between TTG melts and mantle peridotite (assimilation). Rocks of the sanukitoid suite were emplaced at the Archaean–Proterozoic boundary, possibly marking the time when TTG-dominated granitoid magmatism changed to a more modern-style, arc-dominated magmatism. Consequently, the intermediate character of sanukitoids is not only compositional but chronological. The succession of granitoid magmatism with time is integrated in a plate tectonic model where it is linked to the thermal evolution of subduction zones, reflecting the progressive cooling of Earth: (1) the Archaean Earth’s heat production was high enough to allow the production of large amounts of TTG granitoids formed by partial melting of recycled basaltic crust (‘slab melting’); (2) at the end of the Archaean, due to the progressive cooling of the Earth, the extent of slab melting was reduced, resulting in lower melt:rock ratios. In such conditions the slab melts can be strongly contaminated by assimilation of mantle peridotite, thus giving rise to low-Ti sanukitoids. It is also possible that the slab melts were totally consumed in reactions with mantle peridotite, subsequent melting of this ‘melt-metasomatised mantle’ producing the high-Ti sanukitoid magmas; (3) after 2·5 Ga, Earth heat production was too low to allow slab melting, except in relatively rare geodynamic circumstances, and most modern arc magmas are produced by melting of the mantle wedge peridotite metasomatised by fluids from dehydration of the subducted slab. Of course, such changes did not take place exactly at the same time all over the world. The Archaean mechanisms coexisted with new processes over a relatively long time period, even if they were subordinate to the more modern processes.

Description: Though little is known of the graptoloid reproductive mechanism, graptolites with putatively sac-like appendages, supposedly ovarian vesicles, have been known from the Moffat Shales Group, Southern Uplands, Scotland, for over 150 years. Locally, these co-occur with isolated, two-dimensional, discoidal or ovato-triangular fossils. In the 1870s, Nicholson interpreted these isolated fossils as being graptoloid ‘egg-sacs’ detached from their parent and existing as free-swimming bodies. He assigned them to his genus Dawsonia, though the name was pre-occupied by a trilobite, and named four species: D. campanulata, D. acuminata, D. rotunda and D. tenuistriata. A reassessment of Nicholson’s type material from the Silurian of Moffatdale, Scotland, and from the Ordovician Lévis Formation of Quebec, Canada, shows that Dawsonia Nicholson comprises the inarticulate brachiopods Acrosaccus? rotundus, Paterula? tenuistriata and Discotreta cf. levisensis, the tail-piece of the crustacean Caryocaris acuminata and the problematic fossil D. campanulata. Though D. campanulata resembles sac-like graptolite appendages, morphometric analysis reveals the similarity to be superficial and the systematic position of this taxon remains uncertain. There is no definite evidence of either D. campanulata or sac-like graptoloid appendages having had a reproductive function.

Description: A number of approaches to interpreting geochemical data for categorising and correlating metabentonites are presented. Bulk rock geochemistry, combined with data from cognate minerals, provides the most robust means of characterising an altered ash bed. While it is not possible at present to identify one single parameter which can uniquely fingerprint an altered ash bed, it is possible to come close to an unique fingerprint, or ‘Golden Spike’, by utilising an array of geochemical parameters comprising bulk rock element ratios, Rare Earth Elements, Sr and Nd isotopic ratios in apatite, composition of sandine phenocrysts, and the chemical composition of melt inclusions.

Description: An alteration zone at the base of a Carboniferous lava flow on the island of Bute resembles modern and ancient weathering profiles, but it is ‘upside down’ in the sense that alteration is most intense at the base and decreases upwards. Values for a Chemical Index of Alteration (CIA) increase from fresh lava and corestones downwards to fine-grained structureless material. The altered material is Mg- and Cu-rich, possibly as a result of migration of these elements from underlying sediments. Rare earth elements (REE) display considerable and systematic mobility and fractionation. In general heavy rare earth elements (HREE) are concentrated during alteration, whereas the light rare earth elements (LREE) are lost. Mobility of the REE appears to be related to atomic weight, with La (the lightest REE) being the most depleted through to Lu, which is the most concentrated REE in the highly altered material. Similar systematic fractionation is shown by some weathering profiles developed on mafic igneous rocks. Movement of water into the volcanic rocks was probably driven by a steep thermal gradient between the hot lava and its sedimentary substrate.

Description: A revision of the Lower Devonian dalmanitid trilobites of the Prague Basin (Czech Republic) is presented. The subfamily Odontochilinae Šnajdr is considered a synonym of Dalmanitidae Vogdes. Twenty-one previously and five newly described (three in open nomenclature) species and subspecies occur in the Prague Basin from the lowermost Pragian (one problematic specimen possibly comes from the uppermost Lochkovian) to the Lower Emsian; the last questionable record is from the Upper Emsian. The species have been assigned to four genera and subgenera: Odontochile Hawle & Corda, Reussiana Šnajdr, Zlichovaspis (Zlichovaspis) Přibyl & Vaněk and Zlichovaspis (Devonodontochile) Šnajdr. These trilobites are considered as scavengers or opportunistic predators, living most of their lives as vagrant benthos burying in the muddy substrate to find organic remains. The first undoubted adult-like meraspid specimens are described.

Description: In geochemical diagrams, granitoids define ‘trends’ that reflect increasing differentiation or melting degree. The position of an individual sample in such a trend, whilst linked to the temperature of equilibration, is difficult to interpret. On the other hand, the positions of the trends within the geochemical space (and not the position of a sample within a trend) carry important genetic information, as they reflect the nature of the source (degree of enrichment) and the depth of melting. This paper discusses the interpretation of geochemical trends, to extract information relating to the sources of granitoid magmas and the depth of melting.%Applying this approach to mid-Archaean granitoids from both the Barberton granite–greenstone terrane (South Africa) and the Pilbara Craton (Australia) reveals two features. The first is the diversity of the group generally referred to as ‘TTGs’ (tonalites, trondhjemites and granodiorites). These appear to be composed of at least three distinct sub-series, one resulting from deep melting of relatively depleted sources, the second from shallower melting of depleted sources, and the third from shallow melting of enriched sources. The second feature is the contrast between the (spatial as well as temporal) distributions and associations of the granites in both cratons.

Description: A detailed account of the morphology and ontogeny of the late Middle Cambrian crustacean †Henningsmoenicaris scutula is presented. Ten successive ontogenetic stages could be recognised in the material collected from various localities in Sweden. Morphogenetic changes include the development of a pair of stalked lateral eyes and the increase in the number and size of appendages and their setal armature. Notably, early stages lack ‘proximal endites’ on all post-antennular appendages; such a spine-bearing endite has previously been thought to appear simultaneously on these limbs. In †H. scutula a single functional endite appears on the third limb in an advanced stage; an additional endite appears on the second limb and, subsequently, further endites appear on more posterior limbs. Furthermore, a single specimen of †Sandtorpia vestrogothiensis gen. et sp. nov. is described. Based on this new information and data of other ‘Orsten’ taxa, particularly those assigned already to the early evolutionary lineage of Crustacea, a small-scale computer-based phylogenetic analysis was performed. This resolved the basal branchings of Crustacea s. l. as follows: †Oelandocarididae (=†Oelandocaris oelandica+†H. scutula+†S. vestrogothiensis)+(†Cambropachycopidae (=†Goticaris longispinosa+†Cambropachycope clarksoni)+ (†Martinssonia elongata+Labrophora (=†Phosphatocopina+Eucrustacea))). Plotting ontogenetic data on the phylogram and comparing the ground pattern at every node led to the detection of three peramorphic heterochronic events in the evolutionary lineage towards Eucrustacea.

Description: The Sixth Hutton Symposium on the Origin of Granites and Related Rocks was held on July 2–6, 2007 at the University of Stellenbosch, South Africa, founded on granite, nestled at the feet of towering mountains and fringed by the rolling winelands of the Western Cape. This Special Issue opens with Master’s historical account of how the Cape granites influenced 18th and early 19th century thinking on the origins of these rocks. The fascinating fact is that the granites of the Western Cape were apparently the first intrusive granites recognised outside Britain. The balance of the volume contains a collection of research papers derived from the meeting and illustrates some of the important directions in which granite research may be evolving. One of the characteristics of the papers and talks presented at the meeting was that there seemed to be some shift in interest, away from the crust as a source of granitic magmas and towards mantle rocks that have been metasomatised by subduction-zone fluids or melts. Nevertheless, the crust still holds pride of place as the cradle of granite genesis.

Description: Several authors have proposed that granitic melt accumulation and transport from the source region occurs in networks of connected melt-filled veins and dykes. These models envisage the smallest leucosomes as ‘rivulets’ that connect to feed larger dykes that form the ‘rivers’ through which magma ascends through the sub-solidus crust. This paper critically reviews this ‘rivulets-feeding-rivers’ model. It is argued that such melt-filled networks are unlikely to develop in nature, because melt flows and accumulates well before a fully connected network can be established. In the alternative stepwise accumulation model, flow and accumulation is transient in both space and time. Observations on migmatites at Port Navalo, France, that were used to support the existence of melt-filled networks are discussed and reinterpreted. In this interpretation, the structures in these migmatites are consistent with the collapse and draining of individual melt batches, supporting the stepwise accumulation model.

Description: The Murotomisaki Gabbroic Intrusion is a sill-like layered gabbro emplaced in sedimentary strata of Tertiary age in southwest Japan. The zoning (including resorption structures) and the compositional variations of plagioclase from throughout the intrusion were studied, and it was found that the zoning pattern may be classified into four types, which may well correlated with the hosting rock types, the mode of occurrences and their stratigraphic positions in the intrusion. The plagioclase zoning was successfully decoded, and the sequence of events that took place during the magmatic differentiation was deduced and further interpreted in the context of a stratified basal boundary layer slowly ascending in a solidifying magma body. It was shown that various layered structures – modal layering, podiform gabbroic pegmatites and anorthositic layers – observed in the Murotomisaki Gabbro were formed within the moving basal boundary layer by flushing of H2O-rich fluid and fractionated silicate melts from below. By the fluxing of hydrous fluids, plagioclase crystals preferentially dissolved and then melt fraction increased in the basal boundary layer. Under these circumstances, plagioclase-rich fractionated melts diapirically segregated from the crystal pile. Calcic plagioclases, which are out of equilibrium in the central part of the intrusion, may have originated from the basal boundary layer in this manner.

Description: Regional scale biotite and cordierite-bearing granites (s.l.) in the Variscan of the Central Iberian Zone (CIZ) are spatially closely associated with cordierite-rich nebulites and cordierite-bearing two-mica granites, and with cordierite-rich high grade hornfelses and cordieritites (〉60% cordierite) that are relatively common in the aureoles of these granites. Building on published field evidence, petrological data are presented which, combined with new chemical and isotopic (Sr–Nd) modelling, indicate that the cordierite-bearing granites cannot be derived by simple anatexis of regional sedimentary protoliths; but the data are consistent with a process of reactive assimilation that involves the interaction of biotite granite magma with high-grade host rocks ranging from cordierite nebulites to andalusite-bearing cordieritites. The contribution of the postulated cordierite-rich contaminants to the diversity of cordierite granite compositions is modelled using the compositions of regional Lower Cambrian–Upper Neoproterozoic metasedimentary rocks that are generally chemically mature (CaO very rarely exceeds 1·4%). These rocks include specific horizons in which extreme chemical alteration is attributable to sediment reworking during eustatic falls in sea level. Such compositions may account for the presence of the high concentrations in Al that later produced cordieritites. Fractional crystallisation is also important, particularly in generating the more evolved cordierite granite and cordierite biotite muscovite granite compositions. Although assimilation in situ is normally regarded as a minor contributor volumetrically to evolving plutons, in this instance the emplacement of large volumes of granite magma into a high-T–low-P environment significantly increased the potential for reactive assimilation.

Description: Microzarkodina is a genus of mainly Middle Ordovician conodonts that has its centre of distribution in Baltoscandia, and much less commonly occurs in southern China, Australia, Argentina and Laurentia. In Baltica a series of species, Microzarkodina russica n. sp., M. flabellum, M. parva, M. bella, M. hagetiana and M. ozarkodella, established themselves successfully. The succession of species ranges from just below the base of the Middle Ordovician (M. russica) to the upper part of the Middle Ordovician (M. ozarkodella). The species are frequently used for biostratigraphical purposes. The largely contemporaneous species Microzarkodina bella and M. hagetiana probably both evolved from M. parva and mostly occurred in separate areas. Microzarkodina ozarkodella probably evolved from M. hagetiana. This present investigation is based on a total of 94,208 elements, collected from 20 sections and one drill-core site in Sweden, one drill-core site and one outcrop in Estonia and two sections in the St Petersburg area in Russia. The Microzarkodina apparatus probably consisted of 15 or 17 elements: four P, two or four M and nine S elements. The S elements include different Sa, Sb1, Sb2, and Sc element types.

Description: The regionally metamorphosed, Riphean–Cambrian Argyll Group Dalradian rocks of NW Achill Island, western Ireland are disposed in a large-scale, regionally west-facing, tight, recumbent F2 curvilinear fold, with which two ductile shear zones are associated. Clasts in conglomerates within the Dalradian sequence that are deformed by the shear zones preserve evidence for a constrictional overprint of earlier plane strain as the fold became curvilinear, while stretched clasts maintained a constant orientation as the hinge curvilinearity developed. During the constrictional overprint a crenulation fabric, S2b, overprinted a penetrative foliation, S2a, in the shear zones. The S2b has an orientation that varies systematically with that of the fold hinge. It is inferred that, although the S2b surfaces initiated as a dip-slip fabric, there was an increasing degree of strike slip on these surfaces as the fold hinge approached parallelism with the direction of tectonic transport. It is possible that many curvilinear folds have an early history involving plane strain, but that increasing constrictional strain is intrinsic to the later stages of their development.

Description: The early Cambrian Chengjiang biota in Yunnan, southern China has yielded many articulated exoskeletons of the spiny redlichiid trilobite Eoredlichia intermedia, of which some have their appendages exceptionally well preserved. Both of the paired uniramous antennae of a medium-sized holaspis consist of 46–50 short segments (articles), each of which bears a fine spine near its inner edge. Behind the antennae there are twenty-one pairs of biramous limbs: three pairs are situated underneath the cephalon, one pair underneath each of the fifteen thoracic segments, and probably three pairs underneath the small pygidium. The endopod consists of a broad basis and seven podomeres, of which the last is divided into three terminal spines. The exopod is blade-like, and according to one interpretation, is dorsally hinged to the basis of the endopod; an alternative suggestion being that both the endopod and exopod are split from the basis, the latter being independent and not forming part of the endopod. The exopod has a prominent anterior rim, and possesses about forty long filaments along the posterior margin, and short setae along the rounded distal lobe. The basic appendage features of the redlichiid trilobites, and likewise the gut, are comparable to those of other known Cambrian polymerid trilobites that belong to more distantly related clades.

Description: Partial melting of metapelitic rocks beneath the mafic–ultramafic Rustenburg Layered Suite of the Bushveld Complex in the vicinity of the periclinal Schwerin Fold resulted in a structurally controlled distribution of granitic leucosomes in the upper metamorphic aureole. In the core of the pericline, subvertical structures facilitated the rise of buoyant leucosome through the aureole towards the contact with the Bushveld Complex, with leucosomes accumulating in en-echelon tension gashes. In a subhorizontal syn-metamorphic shear zone to the southeast of the pericline, leucosomes accumulated in subhorizontal dilational structural sites. The kinematics of this shear zone are consistent with slumping of material off the southeastern limb of the rising Schwerin pericline. The syndeformational timing of leucosome emplacement supports a syn-intrusive, density-driven origin for the Schwerin Fold. Modelling of the cooling of the Rustenburg Layered Suite and heating of the floor rocks using a multiple intrusion model indicates that temperatures above the solidus were maintained for 〉600,000 years up to 300 m from the contact, in agreement with rheological modelling of floor-rock diapirs that indicate growth rates on the order of 8 mm/year for the Schwerin Fold.

Description: Trace element inversion modelling of Grenvillean anorthosite massifs and associated rocks yield NMORB-normalised trace element profiles enriched in highly incompatible elements; commonly with negative Nb and Th anomalies. Model melts can be divided into subtypes that cannot be linked through fractional crystallisation processes. Most model melts are depleted in the heavy rare-earth elements and can be explained by partial melting of arc basaltic sources (5–60 melting ) with garnet-bearing residues. Some of the model melts have flat NMORB-normalised profiles (for rare-earth elements), have high compatible element contents, and might have been derived from mantle fertilised by arc magmatism, followed by low-pressure fractional crystallisation. Intermediate Ce/Yb types may represent mixtures of these end-members, or less probably, variations in the crustal source composition and residual assemblage. The active tectonic context now favoured for the Grenville Province appears to be inconsistent with plume or thermal insulation models. The heat source for crustal and mantle melting could record either post-orogenic thermal relaxation of a tectonically-thickened arc crust, or basaltic underplating caused by delamination of a mantle root or subduction slab beneath this arc crust. In this context, pre-Proterozoic anorthosites may be lacking, because prior to ca. 2·5 Ga, the crust may have been too weak to be thickened tectonically. The absence of post-Proterozoic anorthosites may be due to the secular decrease in radiogenic heating and cooling of the mantle and crust.

Description: Many continental flood basalt provinces contain rhyolites with ‘A-type’ compositions and many studies have concluded that these higher silica rocks are crustal melts from metapelitic or tonalitic country rock. However, although many of the low-Ti continental flood basalt sequences exhibit a marked a silica gap from ∼55–65 wt. SiO2, many incompatible element ratios, and the calculated eruption temperatures (950–1100°C) are strikingly similar between the rhyolites and associated basalts. Using experimental evidence, derivation of the low-Ti rhyolites from a basaltic parent is shown to be a viable alternative to local crustal melting. Comparison of liquid compositions from experimental melting of both crustal and mantle-derived (basaltic) source materials allows the two to be distinguished on the basis of Al2O3 and FeO content. The basalt experiments are reversible, such that the same melts can be produced by melting or crystallisation. The effect of increased water content in the source is also detectable in the liquid composition. The majority of rhyolites from continental flood basalt provinces fall along the experimental trend for basalt melting/ crystallisation at relatively low water content. The onset of the silica gap in the rhyolites is accompanied by an abrupt decrease in TiO2 and FeO*, marking the start of Fe–Ti oxide crystallisation. Differentiation from 55–65 wt. SiO2 requires ∼30 fractional crystallisation in which magnetite is an important phase, sometimes accompanied by limited crustal contamination. The rapid increase in silica occurs over a small temperature interval and for relatively small changes in the amount of fractional crystallisation, thus intermediate compositions are less likely to be sampled. It is argued that the presence of a silica gap is not diagnostic of a crustal melting origin for either A-type granites or rhyolites in continental flood basalt provinces. The volume of these rhyolites erupted over the Phanerozoic is significant and models for crustal growth should take this substantial contribution from the mantle into account.

Description: The 532±5 Ma old Carion pluton is a dark, porphyritic ferro-potassic granitoid emplaced near the late Pan-African Angavo mega-shear zone. A rough normal zoning from tonalitic to granitic compositions can be recognised in the field. Steep magmatic foliations are evidenced by K-feldspar megacryst preferred orientations. Microstructures are either magmatic or typical of incipient solid-state deformation in near solidus conditions. Magnetic susceptibility magnitudes (K) range from 11 to 111×10−3 SI in the pluton and can be correlated to the petrography (highest K values in the tonalites; lowest K in the granites; granodiorites in between). The susceptibility magnitudes display a complex zoning pattern. Combined with the arrangement of magnetic foliation trajectories, it is possible to delineate four nested sub-units, regarded as four magmatic pulses successively emplaced from the west to the east of the pluton. The four pulses are characterised by very similar magma geochemistry, but variable magmatic differentiation. The highest degrees of magnetic susceptibility anisotropies (up to 1·6) are observed along internal contacts between sub-units and along the borders of the pluton. The magnetic lineations are also steeply plunging in some places in each sub-unit, possibly imaging the different feeder zones. Magma emplacement occurred at the end of the activity of the Angavo shear zone, hence avoiding re-orientation of the magmatic structures by the late Pan-African transcurrent tectonics. The diachronicity of the four magmatic pulses is consistent with previously determined palaeomagnetic data, because only the two older sub-units display a magnetic reversal sequence, whereas the two youngest sub-units lack any reversion. Emplacement of these four magmatic batches was responsible for a strain aureole and suggests a diapiric mode of ascent.

Description: Some 40 brachiopod species are known from the localities of Kilbucho and Wallace’s Cast in the Kirkcolm Formation in the Northern Belt of the Southern Uplands of Scotland. The fauna is diverse despite the relatively small numbers of brachiopod specimens (c. 180) available for study. Much of the fauna was transported downslope and is locally preserved in obtrution deposits. It represents a broad census of outer shelf and upper slope palaeocommunities around this part of the Laurentian margin during the early Katian, and is dominated by relatively small plectambonitoid brachiopods. When compared with other circum-Iapetus assemblages, the brachiopods from the Southern Uplands compare most closely with those from the Bardahessiagh Formation, Pomeroy, Northern Ireland, rather than with adjacent, well-known faunas from the Girvan district, SW Scotland. These new data suggest that this part of the Southern Uplands was located in closer proximity to Pomeroy than Girvan, and located in deep-water environments similar to those in the upper parts of the Bardahessiagh Formation.