Leucogranites from the Eastern Part of the South Mountain Batholith, Nova Scotia

The South Mountain Batholith is a peraluminous granitic complexranging in composition from biotite granodiorite to muscovite-topaz‘leucogranite’. Leucogranitic rocks (with generally<2% biotite) form a minor part ({small tilde}1•5%) ofthe batholith, and are of two types: (1) ‘associated leucogranites’occurring as relatively small zones in fine-grained leucomonzogranites;and (2) ‘independent leucogranites’ forming generallylarger bodies having no particular spatial association withother rock types. Mean chemical compositions of these two typesof leucogranite are as follows (associated, independent): Na₂O(3•46,3•83),K₂O(4•40,4•09),andP₂O₅ (0•26, 0•45)in wt.%;Li(149, 281), F(1199, 2712),Rb (393, 725), U (7•4, 4•4), Nb (12•8, 23•4),Ta (2•9, 7•1), and Zr (52, 31) in ppm. Rare earthelements also differ between the two types (associated, independent):REE (34•1 ppm, 19•9 ppm); and in the degree and variabilityof heavy REE fractionation (Gd_N/Yb_N=4•62•2, 2•00•7).In addition, associated leucogranite has REE compositions similarto those of its host rocks. Mean ¹⁸O values (associated +ll•21•2,independent +ll•40•5; relative to SMOW) are comparablewith the mean for the entire South Mountain Batholith (+l0•80•7).Radiometric dating (⁴⁰Ar/³⁹Ar on muscovite) shows that bothtypes of leucogranite have identical ages of 3723 Ma, equivalentto ages determined by other techniques for granodiorite andmonzogranite samples elsewhere in the batholith. Field relationsand geochemistry suggest that the associated leucogranite resultsfrom an open-system interaction between a fluid and its hostleucomonzogranite, whereas the independent leucogranite bodiesare discrete intrusions of highly fractionated melts that underwentclosed-system, late-magmatic to post-magmatic fluid alteration.Where mineralized, the associated leucogranite characteristicallyhosts greisen-type or disseminated polymetallic mineralization,whereas the independent leucogranite hosts pegmatitic or disseminatedpolymetallic mineralization.     

THE LAST ICE SHEET IN NORTH-WEST SCOTLAND: RECONSTRUCTION AND IMPLICATIONS

Recent models of the last Scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation. This proposition has been investigated on 140 mountains over an area of 10,000 km² in NW Scotland. Outside the limits of the later Loch Lomond Readvance in this area there is evidence for a single high-level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris. This limit occurs at altitudes ranging from 425 to 450 m in the Outer Hebrides to >950 m on the mainland, and is best developed on lithologies that resisted breakdown after ice-sheet downwastage. Interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by: (1) joint-depth and Schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit; (2) a much greater representation of gibbsite (a pre-Late Devensian weathering product) in the clay fraction of soils above the limit; (3) cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit; (4) the sharpness of the limit at some sites and its regular decline along former ice flowlines; and (5) shear stress calculations based on the inferred altitude and gradient of the former ice surface. Reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from >900 m to ca. 550 m at the north coast. Independent dispersion centres fed broad ice streams that occupied major troughs. On Skye an ice dome >800 m deflected the northwestwards movement of mainland ice, but the mountains of Rum were over-ridden by mainland ice up to an altitude of ca. 700 m. The Outer Hebrides supported an independent ice cap that was confluent with mainland ice in the Minches. Extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too conservative, and suggests that low-gradient ice streams extended across the Hebridean Shelf offshore. Wider implications of this research are: (1) that blockfields and other periglacial weathering covers are not all of the same age or significance, depending on the resistance of different lithologies to frost weathering; (2) that the contrasting degree of glacial modification in the Western and Eastern Highlands of Scotland may reflect a former cover of predominantly warm-based ice in the former and predominantly cold-based ice in the latter; and (3) that the approach and techniques developed in this study have potential application for constraining ice-sheet models, not only in areas peripheral to the main centres of ice accumulation in Britain and Ireland, but also in other mountain areas where nunataks protruded through warm-based Late Pleistocene ice masses.     

The lawsonite paradox: a comparison of field evidence and mineral equilibria modelling

Lawsonite equilibria are predicted to occur over a broad P–T spectrum developed during subduction, yet lawsonite‐bearing assemblages are rare. In the context of mafic mineral equilibria modelled for the range of common crustal metamorphism (4–23 kbar, 400–750 °C) using the system Na₂O‐CaO‐K₂O‐FeO‐MgO‐Al₂O₃‐SiO₂‐H₂O and the software thermocalc , unusually high water contents are demanded by lawsonite assemblages. As a consequence, lawsonite assemblages are predicted to have difficulty forming and lawsonite equilibria to be uncommon. Metabasalt undergoing cooler subduction may experience substantial periods involving the metastable persistence of mineral assemblages because of water under‐saturation with non‐occurrence of recrystallization. If formed, lawsonite‐bearing assemblages are observed to be highly unstable; their preservation requires that exhumation be accompanied by substantial cooling. The amount of structurally bound H₂O in minerals plays a critical role in the formation and preservation of mineral assemblages, controlling key changes in rocks undergoing subduction.     

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K_D=Δ (X_Ca,g) where K_D is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low a_O2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.     

Infiltration of Aqueous Fluid and High Fluid: Rock Ratios During Greenschist Facies Metamorphism: A Discussion

Ferry's (1984) analysis of the biotite isograd in pelitic schistsof the Waterville Formation (south-central Maine, U.S.A.) ledto the conclusion that metamorphism had been caused by infiltrationof 1–2 rock volumes of aqueous fluid. We find that thisimportant observation is extremely sensitive to the assumedtemperature of metamorphism and to corresponding uncertaintiesin the thermodynamic data and solution assumptions. Uncertaintiesin the experimental calibrations of the geothermometers areon the order of ? 30?C. In addition, an analysis of the effectsof minor substituents on calcite-dolomite and garnet-biotitegeothermometers indicates that Ferry's (1980) temperatures areabout 40?C too low in the garnet zone. Correcting the phaseequilibria to higher temperatures by this latter amount lowersfluid:rock ratios by an order of magnitude to about 0?1:1. Sincesmall uncertainties have such great effects it is clearly importantto perform careful uncertainty analyses before using Ferry'sapproach to draw wide-ranging conclusions about fluid-rock interactions.Given a 40?C upward revision of temperature it is not necessaryto invoke pervasive infiltration of aqueous fluid as the majoragent of metamorphism. With these low fluid:rock ratios thewater source could reasonably be a thin sequence of underlyingmetapelites and/or the granitic intrusives which accompaniedthe Acadian orogeny.     

Potassium and Associated Elements in Tholeiitic Basalts

Selected average analyses of tholeiitic basalts, which havemarkedly differing contents of K₂O, show a broad coherence betweenK, P, Ti, Ba, Rb, Sr, and Zr. The content of K₂O in the averageanalyses of 43 tholeiitic provinces shows a wide range (0.07–1.61per cent) and has a positively skewed frequency distribution.K₂O in individual tholeiitic basalts and dolerites has a similardistribution pattern. The relationship between K₂O and MgO in the average analysesindicates that the range of K₂O content cannot be solely theresult of near-surface differentiation, nor, indeed, any differentiationprocess which is indexed by the MgO content of the differentiates. Processes which could effect an enrichment of K and associatedelements in tholeiites are assessed and it is concluded thatdegree of melting, eclogite fractionation at high pressure,and mantle wall-rock reaction are likely to be significant factors.Crustal contamination is considered not to be a generally significantfactor in the development of continental tholeiites. The concept that all other tholeiites are derived from a parentalmagma with the composition of low-K deep oceanic tholeiitesis rejected.     

REMOTE-REFERENCE MAGNETOTELLURICS: EQUIPMENT AND PROCEDURES*

During the past 3 years, major advances in the magnetotelluric technique have improved the quality of magnetotelluric data to the point where random errors in the impedance tensor and tipper are generally smaller than the uncertainty in their interpretation. The major factor in this improvement has been the introduction of the remote-reference technique, although the use of ultrasensitive magnetometers and minicomputers for in-field data processing has also been important. After a review of the remote-reference technique, this paper describes the equipment and procedures used for remote-reference magnetotellurics by the authors. Magnetometers using d.c. Superconducting Quantum Interference Devices typically have a sensitivity of 10^?14 T Hz^?1/2, a dynamic range of 10⁷ in a 1 Hz bandwidth, and a slewing rate of 3 × 10^?5.T s^?1 at 10 kHz. The electric field measurements use conventional Cu-CuSO⁴ electrodes. The remote magnetic reference signals are transmitted to the base station using FM analog telemetry. The data are collected and processed by a minicomputer based on an LSI-11 microprocessor; the essential results—for example, the apparent resistivities and the tipper components, with their probable errors—are available in the field. Practical details are given of the handling of superconducting devices, low temperature cryostats and liquid helium in the field. Various spurious noise sources are mentioned, and techniques for minimizing their effects are described.