bluuebunny/crossref_metadata_2025 huggingface dataset:
Two centuries after Hutton's ‘Theory of the Earth’: the status of granite science
Authors: Clarke D. Barrie
Published: November 2011

Granite is the final product of the high-temperature, magmatic, predominantly endogenic, chemical differentiation of the earth. Our understanding of the origin and evolution of granitoid rocks comes from a combination of direct observation, analogue experimentation and numerical modelling. A brief historical overview shows an exceptional level of such research activity over the last 50 years. The number and complexity of questions have resulted in both an absolute and a relative growth of the science since the plate tectonic revolution, largely consisting of refining the current magmatic paradigm within its overarching context. Current research activity involves large components of mineralogical–petrological–geochemical and structural–tectonic work, with much lower levels of experimental, geophysical and geochronological investigations. Many important questions concerning the thermal, physical and chemical aspects of the origin and evolution of granites remain. In keeping with the general progress of science, the complexity of the questions, the declining financial support and the revolution in information technology, directions of granite research in the foreseeable future will change from concrete and qualitative to abstract and quantitative, from expensive and active to cheap and armchair, from reductionist to holistic, and from periodic communication to continuous communication.
Articulated thelodonts (Agnatha) of Scotland
Authors: Märss T. , Ritchie A.
Published: November 2011

Articulated thelodonts,Loganellia scotica(Traquair),Shieliagen.n.taiti(Stetson),Lanarkia horridaTraquair,L. spinulosaTraquair andTurinia pagei(Powrie) from the Silurian and Devonian of Scotland are re-described. A new species,Lanarkia lanceolatasp.n. from the Wenlock, Lower Silurian, is established. For each species, diagnosis is specified, varieties of scale morphology are given, the branchial area is described and body morphology is detailed. A paired ventral fin inShieliagen.n.taitiandLanarkia lanceolatasp.n. has been discovered. A new reconstruction ofLoganelliaandShielia, and diagnosis for thelodonts from Scotland are given. The locality list of Scottish agnathans is reviewed and the biostratigraphical distribution discussed.
The braincase of a primitive shark
Authors: Coates M. I. , Sequeira S. E. K.
Published: November 2011

The Devono-Carboniferous chondrichthyanStethacanthusis best known for its striking 'spine-brush' complex. Here we provide the first detailed description of the neurocranium, using several well-preserved specimens from Bearsden, Scotland (Serpukhovian; Pendleian E1). Significant features include an otico-occipital region of about the same length as the ethmosphenoid division; short but distinct lateral otic processes; a short, broad endolymphatic fossa; a persistent otico-occipital fissure; Y-shaped basicranial canals indicating origin of the lateral aortae anterior to the occipital level; and minimal suborbital shelves limited to small, ethmoidally located outgrowths. Using placoderm, osteichthyan and acanthodian neurocrania for outgroup comparison, PAUP analysis of a matrix of 23 neurocranial characters produced the following branching sequence:(Helodus (Stethacanthus, Cobelodus (Tamiobatis (Xenacanthus (Cladodus wildungensis (Tristychius, Hybodus)))))).The location ofCladoselachein this analysis is uncertain and probably results from missing data and a morphologically restricted character set. Plesiomorphic conditions for chondrichthyan braincases are found to include the presence of a persistent otico-occipital fissure, projection of the median dorsal aorta anterior to the occipital level, and absence of suborbital shelves. These results may have important implications with regard to primitive gnathostome interrelationships, and especially the phylogenetic position of placoderms.
Alkaline basaltic volcanism in the Tertiary of central East Greenland—the Trekantnunatakker
Authors: Brown P. E. , Evans I. B. , Becker S. M.
Published: November 2011

The Trekantnunatakker rise from the inland ice NW of the head of Kangerlussuaq fjord on the Blosseville Coast of central East Greenland. In these nunataks the main sequence of Tertiary flood basalts is overlain by picritic and ankaramitic lavas. These mildly alkaline lavas have lower143Nd/144Nd and higher87Sr/86Sr than the tholeiitic flood basalts. Their isotopic and incompatible element characteristics are similar to those of moderately enriched ocean island basalts, and comparable to the alkaline lavas of the nearby Prinsen af Wales Bjerge. The Trekantnunatakker lavas were the result of small-volume deep melting and it is concluded that an enriched OIB-type source was present as a component of the East Greenland plume.
Crystallisation of fine- and coarse-grained A-type granite sheets of the Southern Oklahoma Aulacogen, U.S.A.
Authors: Hogan John P. , Gilbert M. Charles , Price Jon D.
Published: November 2011

A-type felsic magmatism associated with the Cambrian Southern Oklahoma Aulacogen began with eruption of voluminous rhyolite to form a thick volcanic carapace on top of an eroded layered mafic complex. This angular unconformity became a crustal magma trap and was the locus for emplacement of later subvolcanic plutons. Rising felsic magma batches ponding along this crustal magma trap crystallised first as fine-grained granite sheets and then subsequently as coarser-grained granite sheets. Aplite dykes, pegmatite dykes and porphyries are common within the younger coarser-grained granite sheets but rare to absent within the older fine-grained granite sheets. The older fine-grained granite sheets typically contain abundant granophyre.

The differences between fine-grained and coarse-grained granite sheets can largely be attributed to a progressive increase in the depth of the crustal magma trap as the aulacogen evolved. At low pressures (<200MPa) a small increase in the depth of emplacement results in a dramatic increase in the solubility of H2O in felsic magmas. This is a direct consequence of the shape of the H2O-saturated granite solidus. The effect of this slight increase in total pressure on the crystallisation of felsic magmas is to delay vapour saturation, increase the H2O content of the residual melt fractions and further depress the solidus temperature. Higher melt H2O contents, and an extended temperature range over which crystallisation can proceed, both favour crystallisation of coarser-grained granites. In addition, the potential for the development of late, H2O-rich, melt fractions is significantly enhanced. Upon reaching vapour saturation, these late melt fractions are likely to form porphyries, aplite dykes and pegmatite dykes.

For the Southern Oklahoma Aulacogen, the progressive increase in the depth of the crustal magma trap at the base of the volcanic pile appears to reflect thickening of the volcanic pile during rifting, but may also reflect emplacement of earlier granite sheets. Thus, the change in textural characteristics of granite sheets of the Wichita Granite Group may hold considerable promise as an avenue for further investigation in interpreting the history of this rifting event.
A sustained felsic magmatic system: the Hercynian granitic batholith of the Spanish Central System
Authors: Villaseca Carlos , Herreros Víctor
Published: November 2011

A batholith of around 10,000 km2 was formed during the Hercynian orogeny in the Spanish Central System (SCS). Geochronological data indicate concentrated magmatic activity during the period 325-284 Ma. This late-orogenic magmatism is essentially granitic with only minor associated basic rocks (< 2% in outcrop). The SCS is a remarkably homogeneous batholith showing a restricted range of geochemical granite types without any evolutionary pattern related to time. These peraluminous granites show a limited variation in Na2O/K2O) (0·60-0·95), K/Rb (140-240), (La/Yb)n (6-13), and Eu/Eu* (0·34-0·62) ratios. This constancy in chemical characteristics is also reflected in their isotopic signatures: most monzogranites have initial 87Sr/86Sr ratios in the range of 0·7073-0·71229, initial εNd values vary between —5·4 and —6·6 and δ 18O values group in the restricted range of 8·9-9·6‰. The lack of significant differences among SCS granitoids, maintained during a long geological period, suggests constancy in the nature of their source regions and conditions of magma generation. (1) Limited range of crustal sources: an essentially magmatic recycling during Hercynian orogen is suggested. Mantle-derived components are very limited and restricted to a minor role in the origin of the batholith. Geochemical and isotopic features of SCS granitoids are compatible with felsic lower crustal sources. (2) Constraints in melt conditions: uniformity in residual mineral assemblages (feldspars and garnet are always present in the granulitic residua) combined with a lack of attainment of equilibrium conditions during accessory phase dissolution in the crustal melting process is suggested. Granitic melts never reach saturation in some trace elements (REE, Th, Y, Zr), restricting their chemical variability. (3) Homogenisation in magma chambers: long-lived magmatic systems whose successive pulses accumulate into large magma chambers have the opportunity to mingle, thus reducing source differences.
Relationships between granitoids and mineral deposits: three-dimensional modelling of the Variscan Limousin Province (NW French Massif Central)
Authors: de Veslud C. Le Carlier , Cuney M. , Royer J. J. , Floc'h J. P. , Améglio L. , Alexandrov P. , Vigneresse J. L. , Chèvremont P. , Itard Y.
Published: November 2011

Multidisciplinary three-dimensional modelling, involving geophysical, structural and geochemical data, has been used to study the relationships between magmatism, tectonics, fluid circulation and mineralisation in the northern Limousin, and to provide P-T-Z-t paths constrained by the available dating. The ore deposit occurrence displays little spatial relationship with granites emplaced in the 360-320 Ma period, probably because the low global permeability and tectonic regime did not allowed vertical fluid exchanges to be established. In contrast, the change in the tectonic regime induced by the delamination of the lower lithosphere (320-300 Ma), and characterised by the passage to general extension, has played a major metallogenic role. However, the ore deposit processes appear to be specific to each metal. Most of the W-Sn deposits appear to be synchronous with rare metal granites emplacement, atc.310 Ma, that allowed the focus of fluids of different origins towards the apex of plutons. In contrast, for Au and U, the whole mineralisation process covers several tens of-millions of years. It is controlled by the regional tectonic evolution of the Limousin area during the same period, and especially by a rapid exhumation of the ductile crust which occurred in the 310-300 Ma period.
The trilobite fauna of the Middle Ordovician Doularg Formation of the Girvan district, Scotland, and its palaeoenvironmental significance
Authors: Ingham J. K. , Tripp R. P.
Published: November 2011

The Doularg Formation is defined within the Albany Group (Tappins Complex); the formation consists of the unfossiliferous Fence Member at the base, succeeded by the Gorse Member (formerly the “Albany mudstones with nodular limestones”), the Separation Sandstone and the Jubilation Member, a bedded, calcareous, silty mudstone. The last named has yielded the rich assemblage of trilobites described, representing thirty genera and including one new genus and seven new species—Bronteopsis matutina, Bumastoides? rivulus, Agerina laurentica, Tretaspis eximia, Ampyxina medici, Stiktocybele (gen. nov.) bathytera, Platycalymene metoeca: Nileus is the most common trilobite. The trilobite fauna indicates that the Doularg Formation should be correlated with some part of the Benan Conglomerate, the uppermost formation of the Barr Group to the north of the Stinchar Valley, and is probably of Llandeilo age. The trilobite association is most closely allied to that of the slightly older basal Superstes Mudstone, reflecting the strong ecological control affecting deeper water faunas. The affinity of the Jubilation Member trilobites with those of the middle Table Head Formation (Llanvirn) of western Newfoundland, is evidenced by the presence in both of Nileus, Cybelurus, Peraspis, and Bronteopsis. These are the four most common genera in the Jubilation Member, comprising over half the total trilobite specimens, and provide striking evidence of the longevity of deeper water genera.
Reply to ‘Geochemistry and original nature of Precambrian khondalites in the Eastern Ghats, Orissa: a discussion’
Authors: Dash B. , Bowes D. R.
Published: November 2011

(1) The interpretation of Dash et al. (1987) that the quartz–sillimanite–garnet (khondalite) assemblage of Orissa represents a metamorphosed deeply weathered soil profile(s) is based on a considerable body of new geochemical data: 59 new major and trace element rock analyses and 9 new major element rock analyses(ca. 1800 determinations) compared with the previous basis of 2 rock analyses for major elements only from Orissa supported by 3 for major elements only from other parts of India and 4 from Sri Lanka (ca 90 determinations—table 4).
Dolomites, a review of origins, geometry and textures
Authors: Braithwaite C. J. R.
Published: November 2011

Dolomites form in a range of environments by processes able to drive large volumes of magnesium-rich waters through existing carbonate sediments or rocks. These fluids need not be of unusual composition but there are kinetic barriers to crystallisation which is influenced by factors such as the Mg/Ca ratio, salinity, temperature, the /Ca2+ ratio and time. Dolomite is able to form at near-surface temperatures and pressures within a few thousand years.

Textures in dolomitic rocks are controlled by their conditions of formation. A large proportion are replacive but few of these are mimetic, preserving primary structures. Crystals vary from euhedral to anhedral with boundaries ranging from planar to consertal. Solution chemistry and temperatures influence the density and distribution of nuclei together with growth rates and crystal morphology. There is still doubt whether irregular crystal faces are products of high or low temperatures or saturation. Dolomite cements are more important than has previously been realised in massively dolomitised rocks. Differential dissolution of aragonite, calcite, or evaporite minerals leaves space for these cements to occupy. Dolomitisation may also be allied to compaction, generating stylolitic rocks which are progressively enriched in dolomite. Dolomite may be replaced by calcite or it may be dissolved and the resulting pores filled with a calcite cement.

There is no general correlation between any set of petrographic features and particular geological models for dolomitisation. Similar physicochemical conditions are reproduced in a range of environments and the most effective guides to origin are in the geometry and regional petrographic variation of dolomite bodies.
Pattern and process in the evolution of the Odontopleuridae (Trilobita). The Selenopeltinae and Ceratocephalinae
Authors: Ramsköld Lars
Published: November 2011

The systematics of parts of the Odontopleuridae are revised using character analyses tracing homologous structures, and a computerised cladistic analysis. The choice of outgroup is shown to affect the result of the analysis. Several synapomorphies placeSelenopeltis, the type genus of the Selenopeltinae Hawle & Corda, 1847 as sister genus toDicranurus. The latter's close relationship toMiraspis, type genus of the Miraspidinae Richter & Richter, 1917 is confirmed. The Selenopeltinae therefore becomes a senior subjective synonym of the Miraspidinae. The Selenopeltinae includes about 90 of the over 380 named odontopleurid species known to date (excluding accepted synonyms). The Ceratocephalinae is recognised, including about 30 species. The paired, large pygidial border spines present in most odontopleurids are in some species not homologous, and a falsifiable hypothesis is proposed for the homology of the ‘true major border spines’. This structure is the posterior pleural spine of the tenth postcephalic segment in selenopeltines, ceratocephalines and odontopleurines, and in acidaspidines and apianurines it is the serially homologous spine of the eleventh postcephalic segment. The spine belongs to the first pygidial segment in all taxa except ceratocephalines, where it is on the last thoracic segment. The homology in selenopeltines of the progressive restructuring of the cheek border is reviewed, explaining the supramarginal appearance of the genal spine. The presence and homology of the sublobation of L1 in odontopleurids is discussed, and it may be homologous with the sublobation in lichids. The strongly differentiated thoracic segmental lengths (exsag.), with maximum length reached in the anterior part of thorax, and the much reduced length of the posterior segments, are aspects of a single character-complex, uniquely derived within the Selenopeltinae. The ontogenetic origin of the anterior and posterior pleural spines is reviewed. The presence inCeratocephalaof two instars in one meraspid degree is discussed. A cladistic analysis of selenopeltine and ceratocephaline genera does not entirely resolve the topology of the phylogenetic tree of these taxa, but it confirmsSelenopeltisas sister taxon toDicranurus. These two genera belong in a clade also includingMiraspis, Selenopeltoides, andCeratonurus. Two new genera are erected,CeratocaraandArchaeopleura, the latter with type speciesA. kazakhensissp. nov.
Source region of a granite batholith: evidence from lower crustal xenoliths and inherited accessory minerals
Authors: Miller Calvin F. , Hanchar John M. , Wooden Joseph L. , Bennett Victoria C. , Harrison T. Mark , Wark David A. , Foster David A.
Published: November 2011

Like many granites, the Late Cretaceous intrusives of the eastern Mojave Desert, California, have heretofore provided useful but poorly focused images of their source regions. New studies of lower crustal xenoliths and inherited accessory minerals are sharpening these images.

Xenoliths in Tertiary dykes in this region are the residues of an extensive partial melting event. Great diversity in their composition reflects initial heterogeneity (both igneous and sedimentary protoliths) and varying amounts of melt extraction (from <10% to >70%). Mineral assemblages and thermobarometry suggest that the melting event occurred at T ≥ 750°C at a depth of about 40 km. Present-day Sr, Nd, and Pb isotopic ratios indicate a Mojave Proterozoic heritage, but unrealistic model ages demonstrate the late Phanerozoic adjustment of parent/daughter ratios. A link between these xenoliths and the Late Cretaceous granites, though not fully documented, is probable; in any case, they provide invaluable clues concerning a crustal melting event, recording information about nature of source material (heterogeneous, supracrustal-rich), conditions of melting (moderately deep, moderately high T, accompanied by partial dehydration), and melt extraction (highly variable, locally extensive).

The Old Woman-Piute granites contain a large fraction of inherited zircon and monazite. A SHRIMP ion probe investigation shows that these zircons record a Proterozoic history similar to that which affected the Mojave region. Zonation patterns in zircons, and to a lesser extent monazites and xenotimes, document multiple phases of igneous, metamorphic, and sedimentary growth and degradation, commonly several in a single grain. Low Y in portions of the cores of inherited zircons and monazites and in monazites and outer portions of zircons from the xenoliths appear to indicate growth in equilibrium with abundant garnet.
Evidence for ascent of differentiated liquids in a silicic magma chamber found in a granitic pluton
Authors: Mahood Gail A. , Cornejo Paula C.
Published: November 2011

Fluid dynamic modelling of crystallising calc-alkalic magma bodies has predicted that differentiated liquids will ascend as boundary layers and that accumulation of these buoyant liquids near chamber roofs will result in compositionally stratified magma chambers. This paper reports physical features in La Gloria Pluton that can be interpreted as trapped ascending differentiated liquids. Leucogranitic layers decimetres thick, which are locally stratified, are trapped beneath overhanging wall contacts. The same felsic magmas were also preserved where they were injected into the wall rocks as dykes and as large sill complexes. These rocks do not represent differentiated magmas produced by crystallisation along the exposed walls because the felsic layers occur at the wall rock contact, not inboard of it. Rather, we speculate that evolved felsic liquids are generated by crystallisation all across the deep levels of chambers and that initial melt segregation occurs by flowage of melt into tension fractures. Melt bodies so formed may be large enough to have significant ascent velocities as diapirs and/or dykes. The other way in which the leucogranite occurrence is at variance with the convective fractionation model is that the ascending liquids did not feed a highly differentiated cap to the chamber, as the composition at the roof, although the most felsic in this vertically and concentrically zoned pluton, is considerably more mafic than the trapped leucogranitic liquids. This suggests that these evolved liquids were usually mixed back into the main body of the chamber. Backmixing may be general in continental-margin calc-alkalic magmatic systems, which, in contrast to those in intracontinental settings, rarely produce volcanic rocks more silicic than rhyodacite. That the highly differentiated liquids are preserved at all at La Gloria is a result of the unusual stepped nature of the contact and the entirely passive mode of emplacement of the pluton, which, in contrast to ballooning in place, does not result in wall zones being “scoured”.
Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia
Authors: Haapala Ilmari , Rämö O. Tapani
Published: November 2011

The 1·65–1·54 Ga rapakivi granites of southeastern Fennoscandia represent the silicic members of a bimodal magmatic association in which the mafic members are tholeiitic diabase dykes and minor gabbroic-anorthositic bodies. They are metaluminous to slightly peraluminous A-type granites and occur as high-level batholiths and stocks in an E-W-trending belt extending from Soviet Karelia to southwestern Finland. The Soviet Karelian granites were emplaced into the contact zone between Archaean craton and Svecofennian juvenile 1·9Ga-old crust, while the Finnish granites were intruded into the Svecofennian crust. Deep seismic soundings show that the rapakivi granites and the contemporaneous, mainly WNW or NW-trending diabase dyke swarms are situated in a zone of relatively thin crust. Below the Wiborg Batholith there exists a domal structure in the lithosphere in which a transitional zone (mafic underplate) occurs between the crust and the mantle.

The Nd isotopic evolution of the rapakivi granites (εNd(T) −3·1—−0·2) corresponds to the evolution of the 1·9Ga-old Svecofennian crust, as do their Pb isotopic compositions. This implies that the Finnish granites represent anatectic melts of the Svecofennian crust. In contrast, the Soviet Karelian granites show isotopic composition indicative of substantial incorporation of Archaean lower crust material. Petrochemical modelling of one of the Finnish batholiths shows that its parental magma could have been generated byc.20% melting of a granodioritic source and that fractional crystallisation was important during the subsequent evolution of this magma.

The rapakivi granites are redefined as A-type granites that show the rapakivi texture at least in larger batholiths. The field, geochemical, and seismic data indicate that the classical Finnish rapakivi granites were generated in an anorogenic extensional regime by partial melting of the lower/middle crust. The melting, and possibly also the extensional tectonics, were related to upwellings of hot mantle material which led to intrusion of mafic magmas at the base and into the crust.
The rapakivi granites of S Greenland—crustal melting in response to extensional tectonics and magmatic underplating
Authors: Brown P. E. , Dempster T. J. , Harrison T. N. , Hutton D. H. W.
Published: November 2011

Early Proterozoic rapakivi intrusions in S Greenland occur as thick sheets which have ramp–flat geometry and were intruded along the median planes of active ductile extensional shear zones. These shear zones and their intrusions were linked via transfer zones in a major three-dimensional framework. At high structural levels (c.6 km) the rapakivi intrusions developed thermal aureoles which overprint the regional assemblages, whereas at deeper levels in the regional structure they are contemporaneous with regional metamorphism. Thermobarometry on the regional and contact assemblages indicates low pressure granulite facies conditions (200–400 MPa, 650°-800°C) suggesting very high thermal gradients. The rapakivi suite and associated norites have low initial87Sr/86Sr together with positive εNdvalues, indicating the involvement of predominantly young crust and/or mantle component in the generation of the igneous suite. It is considered that the voluminous norites are closely related to the mafic melts which underplated the juvenile crust to trigger the generation of the monzonitic rapakivi suite. Taken together, the data are consistent with a model of Proterozoic lithospheric extension, thinning of relatively juvenile continental crust and compression of mantle isotherms, resulting in high crustal heat flow, mafic underplating, and crustal melting with emplacement of magmas along a linked network of extensional shear zones.
Genesis of the lower crustal garnet-orthopyroxene tonalites (S-type) of the Hidaka Metamorphic Belt, northern Japan
Authors: Shimura T. , Komatsu M. , Iiyama J. T.
Published: November 2011

The Hidaka Metamorphic Belt (HMB) in Hokkaido, northern Japan, consists of tilted metamorphic layers of an island-arc type crust from lower (granulite facies) to upper (very low-grade metasedimentary) horizons. Abundant granitic rocks, mainly S-type tonalites of crustal origin, intrude various metamorphic layers and are classified into four depth types, namely upper, middle, lower and basal. The basal orthopyroxene-garnet (S-type) tonalities were intruded into granulite facies country rocks. Textural and compositional evidence from minerals in the basal tonalite indicates that the crystallisation sequence is Grt-Pl-Opx-Bt-Qtz-Crd-Kfs, and that crystallisation took place at about 600 MPa and 900°C-700°C.

Some crystallisation experiments were carried out in an internally heated pressure vessel, using the basal tonalite, under the conditions of 300 and 600 MPa, 700-900°C, and with 0-20 wt% H2O, respectively. The results show that the primary S-type tonalite magma was at a temperature above 900°C and contained 3-4 wt% H2O at the beginning of crystallisation. In order to study the influence of normative orthoclase content on orthopyroxene crystallisation, some starting materials also included 15, 20 and 25% normative orthoclase, by adding KAlSi3O8gel to the rock powder. Normative orthoclase content has an influence on the subliquidus crystallisation limit of orthopyroxene.

The changes inP-Tconditions and chemical composition of the magma during ascent would generate the sequence from the basal to upper S-type granite. Opx-free S-type granitic magma can be generated from lower crustal Grt-Opx S-type granitic magma, by differentiation with falling magmatic temperature.
Geochemistry and Rb-Sr geochronology of Mesozoic granites from Hong Kong
Authors: Sewell R. J. , Darbyshire D. P. F. , Langford R. L. , Strange P. J.
Published: November 2011

The granites of Hong Kong comprise a variety of assemblages dominated by chemically evolved compositions. They are divided into two suites based on petrographic, geochemical, and age criteria. The oldest and most primitive intrusive units are deformed biotite-hornblende granodiorites and monzogranites of the Lamma Suite. These rocks are characterised by high CaO (1·4-2·7%), and low Nb and Y contents. The Lion Rock Suite (LRS) is dominated by relatively undeformed monzogranite with subordinate quartz syenite and comprises three subgroups. Granites of subgroup I are separated into coarse- and fine- to medium-grained lithologies. The fine- to medium-grained granites are predominantly fluorite-bearing with silica contents ranging from 75·5-78%. They are characterised by high total REE, Ga, F, Rb, Nb, and Y contents and yield a Rb-Sr whole-rock isochron age of 155 ± 6 Ma with an initial 87Sr/86Sr ratio of 0·7101 ± 0·0060 (MSWD = 4·6). Granites of subgroup II comprise a diverse range of compositions (SiO2 = 63–77%) and are characterised by highly variable trace element abundances. Coarse-grained granites yield an age of 148 ± 9 Ma with an initial 87Sr/86Sr ratio of 0·7060 ± 0·0006 (MSWD = 0·1). Granites of subgroup III are moderately to highly evolved (SiO2 = 72·5-77·9%) and the silica-rich compositions are marked by enrichment in Y, Nb, Rb and depletion in Ba and Sr. Rb-Sr whole-rock isochron ages for individual plutons vary from 138 ± 1 to 136 ± 1 and corresponding initial 87Sr/86Sr ratios are 0·7080 ± 0·0002 (MSWD = 1·2) and 0·7092 ± 0·0006 (MSWD = 0·4). Granites of the Lamma Suite and coarse-grained granites of LRS subgroup I are interpreted as synorogenic I-types, whereas those of LRS subgroups II and III are interpreted as late-orogenic to postorogenic, fractionated I-types. Fineto medium-grained granites of LRS subgroup I have distinctive A-type affinities and together with their association with quartz syenite indicate a transition from compressional to tensional tectonics.
The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia
Authors: Blevin Phillip L. , Chappell Bruce W.
Published: November 2011

The ore-element associations of granite-related ore deposits in the eastern Australian Palaeozoic fold belts can be related to the inferred relative oxidation state, halogen content and degree of fractional crystallisation within the associated granite suites. Sn mineralisation is associated with both S- and I-type granites that are reduced and have undergone fractional crystallisation. Cu and Au are associated with magnetite- and/or sphene-bearing, oxidised, intermediate I-type suites. Mo is associated with similar granites that are more fractionated and oxidised. W is associated with a variety of granite types and shows little dependence on inferred magma redox state. The observed ore deposit-granite type distribution in eastern Australia, and the behaviour of ore elements during fractionation, is consistent with models of ore element sequestering by sulphides and Fe-Ti phases (e.g. pyrrhotite, ilmenite, sphene, magnetite) whose stability is nominally fO2-dependent. Fractional crystallisation acts to amplify this process through the progressive removal of compatible elements and the concentration of incompatible elements into decreasing melt volumes. The halogen content is also important. S-type granites are poorer in Cl than I-types. Cl decreases and F increases in both S- and I-type granites with fractional crystallisation. Low Cl contents combined with low magma fO2 in themselves seem to provide an adequate explanation for the rarity of Mo, Cu, Pb and Zn type mineralisation with S-type granites. Although such properties of granite suites seem adequately to predict the associated ore-element assemblage to be expected in associated mineral deposits, additional factors determine whether or not there is associated economic mineralisation.
Controls on ore metal ratios in granite-related ore systems: an experimental and computational approach
Authors: Candela Philip A.
Published: November 2011

Size and composition (bulk metal ratios) of magmatic hydrothermal mineral deposits are affected by a number of chemical and physical processes including the nature of the source region and mode of emplacement. At shallow levels, rising plumes of vapour bubbles + melt, and the advection of water through interconnected vapour bubbles, allows access of the magmatic aqueous phase to the upper reaches of a magma chamber. These processes are operative at shallow levels where low water solubility and high molar volume for water make these processes more efficient.

Partitioning experiments suggest that oxygen fugacity-dependent crystal/melt partitioning of ore metals leads to different efficiencies of removal of Cu, W, and Mo from silicate melts into ore-forming aqueous fluids. For example, the Mo/W ratio in magmatic hydrothermal deposits should increase as the oxygen fugacity of the magma increases. Further, Cu should behave as a crystal-compatible element in H2O-undersaturated, sulfide-saturated felsic magmas with fO2 NNO + 1 due to the strong partitioning of Cu from the melt into pyrrhotite.

Cycling of oxidised, hydrated, sulfidised and Cl-enriched oceanic crust into mantle can give rise to magmas that contain S but are oxidised (≥NNO). The combination of high oxidation state, relatively hydrous but shallow conditions and a high Cl/H2O ratio leads to saturation with respect to H2O early during crystallisation, and loss of a large proportion of magmatic Cu to the aqueous phase. Ores formed from these oxidised magmas also possess high Mo/W ratios due to the effect of oxygen fugacity on the sequestering of Mo vs W.

In less oxidised magmas, Cu and Mo are partitioned into sulfides and Ti-bearing phases, respectively, resulting in lower efficiencies of removal of Cu and Mo from melts into aqueous fluids. Further, the partitioning of W into crystallising phases is reduced, producing a more efficient removal of W into ore-forming fluids. This ultimately leads to mineral deposits with higher W/(Mo + Cu) ratios relative to deposits associated with more oxidised systems. Silicic, high-F magmas with fO2 = NNO can be found in tensional environments (e.g. rocks associated with the Climax-type deposits of the Colorado Mineral Belt). High HF/H2O activity ratios in the source regions yield melts that evolve an aqueous phase late during crystallisation, leading to relatively low ratios of compatible/incompatible elements in the melt at H2O saturation.
Feldspar crystallisation in felsic magmas: a review
Authors: Nekvasil Hanna
Published: November 2011

Understanding the controls on the evolution of natural feldspars is greatly assisted by coupling experimental determinations of feldspar/melt equilibria with thermodynamic modelling and the calculation of crystallisation paths. Such a combined approach permits the evaluation of the influence of intensive and extensive variables on feldspar compositions. Feldspar compositional paths are influenced only to a minor degree by pressure. The presence of H2O or other melt component incompatible in feldspar has a more major effect, not only in increasing the temperature interval over which feldspar crystallises, but also in decreasing the amount of Ab enrichment of the feldspar which occurs during crystallisation. The amount and behaviour of H2O in the magma has a pronounced influence on feldspar compositions when two feldspars are stable. Under conditions of high bulk H2O content or constant activity of H2O, plagioclase and alkali feldspar compositions evolve by increasing Ab content similar to the behaviour manifested in the simple binary systems. If the bulk H2O content is low, however, and the H2O content increases during crystallisation, plagioclase evolves by increasing Ab content until alkali feldspar is stabilised. From that point on, plagioclase compositions change mainly by decreasing Or content, while alkali feldspar evolves mainly by increasing Or content.
A two-parameter Margules method for modelling the thermodynamic mixing properties of albite-water melts
Authors: Blencoe James G.
Published: November 2011

A two-parameter Margules method has been developed for modelling the thermodynamic mixing properties and hypersolidus phase relations of albite-water (ab-w) melts. The method is based largely on phase-equilibrium and calorimetric data that are either currently available or readily obtainable; P-V-T data are not required.

The new modelling method has been applied to calculate: (1) thermodynamic mixing properties for ab-w melts at 2-5 kbar, 815°C; and (2) hypersolidus phase relations for the ab-w system at 2·5 kbar. Results are uniformly positive—activity-composition relations are similar to those predicted by equations based on P-V-T data, excess enthalpies are in good agreement with data obtained from calorimetry, and calculated phase relations are fully compatible with phase-equilibrium data for ab-w melts acquired at 2·5 kbar.

It is concluded that the Margules method for thermodynamic modelling of ab-w melts is a practical alternative to the P-V-T modelling method.
Crustal versus mantle sources of granitic magmas: a two-parameter model based on Nd isotopic studies
Authors: DePaolo Donald J. , Perry Frank V. , Baldridge W. Scott
Published: November 2011

Temporal and spatial variations in the Nd isotopic compositions of Tertiary caldera-forming rhyolite tuffs, and Cretaceous and Tertiary granites of the western U.S.A. are used as a basis for a model that accounts for the observed proportions of crustal versus mantle contributions to silicic magmas in terms of two parameters: the ambient crustal temperature and the rate of supply of basaltic magma from the mantle. The crustal contribution to silicic igneous rocks is measured in terms of the Neodymium Crustal Index (NCI). The relationships between crustal temperature, basalt supply and NCI are quantified using a model of a magma chamber undergoing continuous recharge, wall-rock assimilation and fractional crystallisation. From the model, a critical value of the ratio of basalt recharge-to-assimilation, (r/a)c, is deduced, which increases with decreasing crustal temperature. The r/a value must exceed (r/a)cto allow the volume of differentiated magma to increase, a prerequisite for developing large volumes of silicic magma. Strongly peraluminous (or S-type) magmas (NCI = 0·8–1), form under conditions of high crustal temperature and low basalt supply. Metaluminous or I-type granites form over a wide range of conditions (NCI = 0·1–1), generally where basalt supply is substantial. In individual long-lived volcanic centres, the large-volume high-silica ignimbrites are associated with the highest r/a and lowest NCI values, indicating that these magmas are typically differentiates of mantle-derived basaltic parents.
Some observations on the use of zircon U-Pb geochronology in the study of granitic rocks
Authors: Williams Ian S.
Published: November 2011

In situ, microscale, U-Pb isotopic analyses of zircon using the SHRIMP ion microprobe demonstrate both the potential and the limitations of zircon U-Pb geochronology. Most zircons, whether from igneous or metamorphic rocks, need to be considered as mixed isotopic systems. In simple, young igneous rocks the mixing is principally between isotopically disturbed and undisturbed zircon. In polymetamorphic rocks, several generations of zircon growth can coexist, each with a different pattern of discordance. A similar situation exists for igneous rocks rich in inherited zircon, as these contain both melt-precipitated zircon and inherited components of several different ages. Microscale analysis by ion probe makes it possible to sample the record of provenance, age and metamorphic history commonly preserved within a single zircon population. It also indicates how the interpretation of conventionallymeasured bulk zircon isotopic compositions might be improved.
Structural interpretations of the Southern Uplands Terrane
Authors: Anderson T. Bernard
Published: November 2011

Bounded by sutures and demonstrating a unique geological history and structure, the Lower Palaeozoic rocks of the Southern Uplands–Down–Longford form a definitive Caledonian suspect terrane. The geological history of the final closure of Iapetus is encrypted in their structural fabric.

Across the terrane, NW-younging turbidites predominate but graptolites invariably indicate the presence of younger sediments to the SSE. This fundamental Southern Upland paradox is soluble only by recognizing many strike-parallel faults, dividing the terrane into more than thirty tracts, each with its own variant of the stratigraphy and structure, and each having a lateral extent far in excess of what might be expected from the probable mechanical strength of the composing sediments. Structural interpretations of the terrane's unique tectonostratigraphic pattern are critically reviewed and the accretionary prism model, modified by strong sinistral transpression from the late Llandovery onward, is preferred. Transpression was apparently triggered when the converging continents of Laurentia and Avalonia made solid contact, so establishing a mechanically effective coupling of sialic crustal elements beneath and across the closing Iapetus ocean basin.

The geometry of the terrane's internal structural fabric is analysed. Tentative area-balancing calculations indicate a crustal shortening from a basin width of at least 1,000 km to the current terrane width of 75 km. Continuing sinistral transpression was expressed in fault reactivation and the development of a major shear zone. Late Palaeozoic strike-parallel extension produced W-facing half-grabens and the associated rotation may account for the easterly plunge of most fold axes.
Silurian subduction-related assembly of fault-defined tracts at the Laurieston Fault, Southern Uplands accretionary terrane, Scotland, U.K.
Authors: Akhurst M. C. , McMillan A. A. , Kimbell G. S. , Stone P. , Merriman R. J.
Published: November 2011

Subduction-related accretion of fault-defined tracts built up the Southern Uplands terrane during the final stages of closure of the Iapetus Ocean (Llandovery to Wenlock). Contrasts in depositional environment and pronounced differences in geochemical composition, provenance studies and metamorphic grade across the Laurieston Fault between the Gala and Hawick groups, suggests that it has a greater regional significance than most other tract-bounding structures. Initiated by underthrusting, and acting as a locus for subsequent sinistral strike-slip, the fault overlies a regional gravity anomaly gradient that is interpreted to be due, in part, to a concealed NW-ward dipping shallow basement surface. This is modelled as an open ramp in the NE that steepens to a near-vertical step along-strike to the SW. A change in structural geometry noted at the Laurieston Fault, with excision of accretionary tracts, is related to a period of oblique closure of the Iapetus Ocean. The youngest Gala Group tracts were accreted during a period of intense transpression to form a regional strike-slip duplex over the shallow basement ramp with termination of the tracts at the Laurieston Fault, its surface expression. The ramp acted as an obstacle to forward-breaking thrust progress, forcing the out-of-sequence thrusting and repetitive thrust imbrication noted in the eastern Southern Uplands. Upper Palaeozoic reactivation of this basement structure may have transferred strain between extensional Permian basins.
Volcanically mediated plankton blooms in the Central Belt of the Southern Uplands, Scotland, during the Llandovery
Authors: Rigby S. , Davies S. J.
Published: November 2011

At Thirlestane Score and at four other localities in the Southern Uplands, graptolites of the LlandoverygemmatusZone occur in couplets of lithologies immediately above thin ‘high-U’ bentonites. Above the bentonites, abundant graptolites, especially siculae, and a straight-line survivorship trend implies high productivity coupled with environmentally mediated mortality: the population structure expected in the early part of a plankton bloom. In the overlying facies, fewer, larger individuals and a convex survivorship curve suggest reduced productivity and internally mediated mortality. This is consistent with the later stages of a bloom where resources were waning but the ecological structure of the system was better developed. It is likely that the introduction of trace-metals, Fe or Al, to the water column via volcanic ash increased primary productivity, suggesting that macronutrients were available in the Southern Uplands system, allowing a bloom to be stimulated by the addition of volcanic products. This process is observed in modern open oceanic systems and implies a temporal continuity of control on the plankton despite complete faunal turn-over since the Silurian. These interpretations are most consistent with an open ocean geotectonic setting for the region.
Late Ordovician brachiopod biofacies of the Girvan district, SW Scotland
Authors: Harper David A. T.
Published: November 2011

Diverse and abundant brachiopod faunas, associated with unstable outer shelf and slope environments, occur through the Upper Ardmillan Group (upper Caradoc–upper Ashgill) in the Girvan district of SW Scotland. Representatives of the deep-waterFoliomenafauna occur intermittently throughout the group, appearing in both the Whitehouse and Drummuck subgroups. This distinctive assemblage of small, thin-shelled brachiopods, includingDedzetina, Christiania, CyclospiraandFoliomenaitself, first appeared in South China during the early Caradoc but had colonised the Laurentian margins by the late Caradoc. Within the upper Caradoc–lower Ashgill Whitehouse Subgroup, theFoliomenafauna is interbedded with a variety of other less cosmopolitan deep-water assemblages including theOnniella–SkenidioidesandLingulella–Trimurellinaassociations. Shallower-water environments in the middle Ashgill Lower Drummuck Subgroup hosted theFardenia–Eopholidostrophiaassociation in sands, and theChristiania-Leptaenaassociation in muds and silts. The remarkable Lady Burn Starfish Beds in the upper part of the group contain a variety of brachiopod-dominated assemblages including theEochonetesandPlaesiomys-Schizophorellaassociations, transported from various shelf locations, within a very diverse mid-Ashgill biota. Nevertheless, elements of theFoliomenafauna persisted to near the top of the Drummuck Subgroup, occurring as rare assemblages in more muddy and silty facies. The upper Ashgill High Mains Formation contains abundant elements of the terminal OrdovicianHirnantiafauna includingEostropheodonta, HindellaandHirnantiaitself, but also some taxa more typical of the Laurentian Edgewood Province. As a whole, the changing brachiopod biofacies monitor environmental fluctuations, on part of the Laurentian margin, driven by mainly eustatic and tectonic events.