Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O in the Stability of Amphibole

Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An_26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H₂O. This reflectsthe fact that conditions of low aH₂O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH₂O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt     

Magnetic mineral transport and sorting in the swash‐zone: northern Lake Erie,Canada

A combined field and laboratory study in northern Lake Erie has provided new insights into the origin and dynamics of heavy mineral placer deposits on beaches consisting primarily of non‐magnetic sediment. Work was conducted on the cross‐shore and longshore transport of heavy magnetic minerals using magnetic susceptibility and fluorescent paints to trace the movement, in the field, of samples of magnetic (magnetite) and non‐magnetic (quartz and calcite) grains, respectively. Laboratory experiments examined how the burial of small, dense magnetic minerals is affected by the grain size of the non‐magnetic host material, and how grain burial affects magnetic susceptibility measurements at the surface. The field experiments demonstrated that the magnetic mineral tracers were buried rapidly beneath coarser, non‐magnetic grains under low to moderate wave conditions, and subsequently were unable to move in the longshore or cross‐shore directions. The laboratory experiments showed that the magnetic susceptibility rapidly decreased with the rate and depth of burial of the magnetic minerals, and that magnetic grain burial was most effective beneath coarser rather than finer non‐magnetic sand and, for the latter sediments, under less rather than more energetic conditions. The results imply that magnetic mineral concentrations develop in this area through magnetic grain burial under fairly mild conditions, and subsequent settling, exposure and concentration in the upper swash zone during more energetic periods, when the non‐magnetic grains are eroded. It is probably during these erosional periods, when the magnetic minerals are exposed in fairly homogeneous deposits, that longshore and cross‐shore transport takes place.     

Polyphase deformation in Oscar II Land, central western Svalbard

Pre-Carboniferous rocks in central western Svalbard can be divided into two rock packages that have experienced three phases of deformation. The lower package includes upper Proterozoic sediments, the upper package is of upper Ordovician to lower Silurian age. Pre-upper Ordovician (Caledonian) deformation is characterized by north-south fold-axes, and is recorded in the lower package only. Following the Ordovician, east-west fold axes developed in the upper package but only locally in the lower. The third phase, probably Tertiary in age, produced easterly directed thrusting, and refolding about sub-horizontal, north-south fold axes in both packages. This complex structural history has resulted in tectonic thickening of the rock sequences by refolding and thrusting which has not been recognized by previous workers.     

The Lower Zone-Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study

The Lower Zone–Critical Zone boundary of the BushveldComplex is an intrusion-wide, major stratigraphic transitionfrom ultramafic harzburgite and pyroxenite in the Lower Zoneto increasingly plagioclase-rich pyroxenites and norites inthe Critical Zone. Quantitative textural and compositional datafor 29 samples through this transition show the following: LowerZone orthopyroxene grains are larger, have higher aspect ratios,are better foliated and have a lower trapped liquid componentthan those of the Critical Zone. The larger grain size of theLower Zone results in crystal size distribution plots that arerotated to lower slopes and intercepts relative to those inthe Critical Zone. Although all rocks show differing amountsof foliation, mineral lineations are weak to absent. These dataare consistent with significant compaction-driven recrystallizationin the study section. Numerical modeling of concurrent compactionand crystallization provides a quantitative model of how theLower Zone–Critical Zone transition may have formed: plagioclaseis rare in the Lower Zone because compaction removes interstitialliquid before it reaches plagioclase saturation. However, asthe crystal pile grows, plagioclase saturation is reached inthe interstitial liquid before compaction is complete in moreevolved pyroxenites, producing more abundant but still modestamounts of plagioclase characteristic of the Lower CriticalZone. It is concluded that both the textures and the modal mineralogyare largely controlled by compaction and compaction-driven recrystallization;primary magmatic textures are not preserved. KEY WORDS: Bushveld Complex; compaction; crystal size distributions; crystal aging; igneous textures     

SOME APPLICATIONS OF THEORY AND EXPERIMENT TO THE STUDY OF BEDDING GENESIS

The origin of many sedimentation units deposited in granular cohesionless materials can be rationally explained by utilizing the concept of the profile of equilibrium. This basic concept can be formally expressed in terms of the variables, or groups of variables, that characterize a sediment transport system. A change in one or more variables or a shift in local base level will generally cause a shift in the spatial position of the profile, resulting in either aggradation or degradation. As shown by flume experiments, the rate of shift of the profile is a critical factor in bedding genesis. A relatively large and rapid upward shift of the profile results in the deposition of a tabular or wedge-shaped unit of cross-bedding, i.e., a laboratory delta. On the other hand, a gradual upward shift of the profile results in the deposition of a sequence of horizontal bedding that is commonly associated with intercalations of ripple or dune cross-bedding. For the intermediate case of a moderately rapid shift of the profile, the depositional sequence includes trough units and poorly-defined tabular units of cross-bedding with numerous intercalations of horizontal bedding. The concept of the profile of equilibrium therefore provides a rationale for considering the depositional framework for sedimentary structures produced by current flow.     

The geology of the Morecambe gas field

The subsurface of the Irish Sea contains one of the United Kingdom's most important gas fields, the Morecambe Field, owned by British Gas. Approximately 10% of the UK's daily consumption of natural gas will be supplied by this giant accumulation trapped within fluvial deposits of the Lower Triassic Sherwood Sandstone Formation.     

Mineralogy of meteorite groups: An update

Abstract— Twenty minerals that were not included in the most recent list of meteoritic minerals have been reported as occurring in meteorites. Extraterrestrial anhydrous Ca phosphate should be called merrillite, not whitlockite.