Metamorphism in the Adirondacks. I. Petrology, Pressure and Temperature

Grenville Supergroup sediments and suites of pre- and syn-tectonicigneous rocks have been metamorphosed to the upper amphiboliteand granulite facies in the Adirondacks of northern New Yorkduring the Grenville orogeny about one billion years ago. Magnetite-ilmenite, alkali feldspar-plagioclase, calcite-dolomiteand garnet-clinopyroxene thermometry indicate that metamorphictemperatures (T) increase from about 650 ?C in the area westand northwest of Gouverneur to 700–750 ?C near Coltonand along the Lowlands-Highlands boundary to 750–800 ?Cin areas within and around the Marcy anorthosite massif. Thepresence of grossular-rich garnet + quartz without wollastonite+ plagioclase in calc-silicate rocks and the apparent absenceof metamorphic ferropigeonite in charnockites restrict maximummetamorphic T to less than 800–850 ?C. Metamorphic pressures (P), determined from coexisting pyrite-pyrrhotite-sphalerite,garnet-rutile-sillimanite-ilmenite-quartz, fayalite-quartz-ferrosilite,fayalite-anorthite-garnet, ferrosilite-anorthite-garnet-quartz,kyanite-sillimanite, anorthite-garnet-sillimanite-quartz andthe stability of akermanite, are 6?5–7?0 kb near Gouverneurand increase to 7?5–8?0 kb in the central Adirondack Highlands. The above P-T data deduced from diverse mineralogical/chemicalsystems are interpreted as peak or near-peak conditions forAdirondack metamorphism. The compositions of thin retrograderims on garnets indicate a post-peak-metamorphic P-T path forthe Adirondacks with appreciable cooling (200–300?) beforedecompression. Peak and retrograde P-T conditions inferred forthe Adirondacks are similar to numerous other granulite terranessuggesting that similar tectonothermal events are necessaryfor the formation of many granulite belts.     

Lunar Pyroxene-Phyric Basalts: Crystallization Under Supercooled Conditions

Pyroxene-phyric basalts constitute a distinct petrologic groupin samples from lunar maria at both Apollo 12 and Apollo 15sites. Textures of pyroxene-phyric basalts from both sites aresimilar, but bulk compositions and mineralogy are somewhat different.Pyroxene-phyric basalts are characterized by large pyroxenephenocrysts with cores of magnesian pigeonite and mantles ofaugite grading to ferroaugite, usually set in a distinctly finergroundmass of iron-rich pyroxene, plagioclase, ilmenite, andother minor minerals. Olivine is scarce or absent modally andnormatively. Controversy has arisen over whether the porphyritictexture is a result of (1) a two-stage cooling history, e.g.phenocrysts formed at depth, and groundmass formed on extrusion;or (2) single stage, rapid cooling under supercooled conditions(cooling history here refers to cooling conditions imposed byexternal factors, and is not to be equated with ‘crystallizationhistory’). A study of six rocks belonging to this groupfrom Apollo 15 rake samples is reported here. A considerablerange of textures is present in these rocks, and they may beranked in order of decreasing late-stage cooling rate (15125,15666, 15682, 15118, 15684, 15116) on the basis of groundmasscrystal size. The same ranking is obtained from delta-beta (measuredin X-ray precession photos) of pigeonite and augite exsolvedfrom once homogeneous crystals, or of epitaxially overgrownaugite and pigeonite. The size of the phenocrysts in these rocks tends to be positivelycorrelated with the coarseness of the groundmass. Furthermorethe same correlation is evidently present in almost all otherApollo 12 and Apollo 15 pyroxene-phyric rocks. This constitutesa strong argument in favor of a single-stage cooling historyfor all pyroxene-phyric rocks, because the correlation wouldbe fortuitous for a two-stage cooling history. On the basisof this and many other arguments advanced previously and inthis paper, it is concluded that all the pyroxene-phyric rocksoriginated in a single-stage cooling process. A crystallization model for pyroxene-phyric rocks accounts forthe bimodal distribution of crystal size by two episodes ofsupercooling and nucleation during the continuous cooling process,the first of pyroxene, which nucleates homogeneously, the secondof plagioclase, which nucleates heterogeneously on the pyroxenephenocrysts. The more rapidly cooled rocks attained greaterdegrees of supercooling in both stages, hence greater nucleationrates and smaller crystals.     

Past changes in the geomagnetic field caused by glaciations and deglaciations

Contributions have been made to the theory of the relation between terrestrial magnetism and glaciations. The results are applied to the ice sheets of high-latitude regions. A review of Pleistocene palcomagnetic dating is given.     

Mineralogy and Petrology of a Tactite near Helena, Montana

The aluminous pyroxene, fassaite, occurs in two small tabularbodies within mafic plutonites of the Boulder Batholith nearits north-east margin twelve miles east of Helena, Montana.First described by Knopf & Lee (1957), the bodies are contact-metasomatizedlimestone septa, now magnesian-tactites, consisting chieflyof fassaite, spinel, garnet, vesuvianite, and clintonite. Lesscommon minerals include pargasite, diopside, wollastonite, sphene,perovskite, anorthite, forsterite, calcite and chlorite. Sometwenty-five microprobe analyses of the fassaite show it is variablein composition and largely consists of the components CaMgSi₂O₆(53–83 per cent), CaAl₂SiO₆ (7–25 per cent), CaFeAlSiO₆(8–28 per cent), and CaTiAl₂O₆ (0–7 per cent). Thestoichiometry generally requires that most of the iron is ferric,consistent with Mössbauer data taken on a typical sample.If fassaite analyses from these and other contact metamorphicrocks are plotted on a triangular diagram with Ca(Mg,Fe)Si₂O₆,CaAl₂SiO₆ and CaFeAlSiO₆ as end-members, the distribution ofpoints offers no positive evidence for a solvus gap betweenfassaite and diopside as proposed by Ginzburg (1969). The mostaluminous fassaites occur with spinel-clintonite ± grossularand have 25 per cent of the Si replaced by Al, making them truepolymorphs of a garnet (i.e. Gr₄₂And₂₃Pp₃₅). No unusual cationordering is detected in these fassaites by single-crystal X-rayphotographs or Mössbauer measurements. Smede's (1966) estimate of 3–4 km of stratigraphic coverfor the Boulder Batholith indicates pressures of approximately1 kb, in agreement with the occurrence of andalusite + K-feldsparin a hornfels at the Kokaruda Ranch complex. The partial assemblagesof grossular, epidote, perovskite, anorthite-wollastonite, anorthite-calcite,and fassaite-calcite require X_CO2 = 0·12 ± 0·08and T = 570 ± 10 °C at these pressures. These pressuresand temperatures place this occurrence in the upper portionsof the hornblende-hornfels facies after Turner (1968), althoughthe low pressures and water-rich fluids permit assemblages (wollastonite,calcite-forsterite-diopside) that Turner lists as characteristicof the pyroxene-hornfels facies.     

Landscape Change and Environmental Histories

Historical geography was once a popular element of university curricula in New Zealand. It was also a conspicuous focus of research. Today however there is only one identifiable course in historical geography in New Zealand's university calendars – at Massey – and few writers have maintained an active research interest rooted in the sub‐discipline. This Comment suggests some reasons why now is a good time for New Zealand's geographers to reassess this state of affairs, and outlines five themes that might be pursued in the construction of more explicit historical geographies at the start of the third millennium.     

Uplifted marine terraces of the Akamas Peninsula (Cyprus): evidence of climatic conditions during the Late Quaternary highstands

An outcrop study of uplifted marine terraces provides information on the climate of past sea‐level maxima, supplementing existing palaeoclimatological archives. The western coast of the Akamas Peninsula shows several uplifted and intricately stacked Quaternary marine terraces. This study focuses on the sedimentology, petrography and sequence stratigraphy of the last three recent terraces and provides palaeoclimatological reconstruction and chronological framing. All three terraces display basically the same stratigraphic succession, which consists of regressive sequences ranging from bioturbated, storm‐influenced, subtidal high‐energy sands to subaerial exposure and aeolian deposition. Each sequence differs in petrography, reflecting contemporaneous climatic conditions. The first studied sequence (Marine Isotope Stage 9 or 11?) was deposited in a warm, arid climate, with oligotrophic water favouring ooid formation. The second sequence (Marine Isotope Stage 7) displays colder but humid conditions, with lower carbonate production and strong detrital input. The third sequence (Marine Isotope Stage 5e) records warm, humid conditions, with high carbonate production combined with significant detrital input. These littoral terraces offer high‐quality outcrops of glacioeustatic‐dominated, littoral sedimentology, together with evidence indicative of the regional climate during the late Pleistocene.     

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.     

Shallow-burial dolomite cement: a major component of many ancient sucrosic dolomites

Dolomite cement is a significant and widespread component of Phanerozoic sucrosic dolomites. Cements in dolomites that were never deeply buried are limpid, have planar faces (non‐saddle forms), often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores. Five samples of sucrosic dolomites, interpreted as having had mostly lime‐mudstone or wackestone precursors in four carbonate aquifers, provide insights into the abundance of planar cements in sucrosic dolomites. Such cement comprises 11% to 45% (32% mean) of peritidal to sub‐tidal dolomites on an outcrop in the Edwards aquifer (Early Cretaceous) of central Texas; 19% to 33% (25% mean) of ramp dolomites in the Hawthorn Group (Oligo‐Miocene) and 50% to 70% in shelf dolomites of the Avon Park Formation (Eocene) in the Upper Floridan aquifer of sub‐surface peninsular Florida; 18% to 45% (32+% mean) of sub‐tidal shelf dolomites in quarry sections of the Burlington‐Keokuk Formation (Early Mississippian) in south‐eastern Iowa; and 18% to 76% (50% mean) in shallow cores and outcrops of outer‐shelf dolomites from the Gambier Limestone (Oligo‐Miocene) of South Australia. Backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening, pore‐space reduction, induration and general ‘maturation’ of these dolomites. Most pre‐Holocene dolomites are multiphase crystalline rocks composed of: (i) seed crystals or ‘cores’; (ii) crystal cortices that concentrically enlarged the cores; and (iii) free‐space, syntaxial precipitates of limpid cement around the crystals. Remaining CaCO₃ grains and micrite can be replaced by dolomite, but typically they are dissolved between stages (ii) and (iii), creating systems of intercrystal and mouldic pores typical of sucrosic dolomites. Networks of cement overgrowths, aided by water‐filled pore systems under hydrostatic to lithostatic pressure, are judged to slow or prevent compaction in sucrosic dolomites. It can be argued that cortex growth involves both replacement of CaCO₃ particles and microcementation of their interparticle pores. This interpretation, and the abundance of cements in so many dolomites, would obviate the controversy over the volumetrics of ‘replacement dolomitization’. Limpid, planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters, at low temperatures (<30 to 35 °C) and shallow burial depths (<100 m), in water‐saturated networks of dolomite ‘silt’ and ‘sand’. Cements in many dolomites in island and continental–aquifer systems appear to result from event‐driven processes related to sea‐level highstands. Cementation events can follow ‘replacement dolomitization’ events by time intervals ranging from geologically ‘instantaneous’ to tens of million years.