Regional Geobarometry in the Contact Aureoles of the Anorthositic Nain Complex, Labrador

The 1.4 Gyr anorthositic Nain complex was emplaced by multipleintrusion into Archean and Aphebian rocks whose metamorphicgrades range regionally from greenschist to granulite and whosetemperature at the time of emplacement was probably no higherthan 200–300 °C. Using experimental data on Ga-Cdand Hy-Ol-Qz equilibria, compositional studies allow the determinationof contact metamorphic pressures and temperatures for samplesover an area > 10,000 km². The results indicate that forthese specimens temperatures ranged from 645° to 915 °Cand pressures ranged from about 3.7 to 6.6 kbar. Locally, originalgarnet porphyroblasts have partially reacted to produce symplectitesof cordierite+hypersthene, resulting in a corona-like texture.These textures and associated mineral zoning indicate that someof the specimens have experienced a polymetamorphic history.This poly metamorphism presumably resulted from the multipleintrusions of magmas of the Nain complex. The regional distributionof pressures appears systematic and suggests an arch in theancient pressure surfaces (paleo-isobars). This postulated arch,which trends WNW-ESE, roughly coincides with a proposed grabenstructure in the Conrad discontinuity based on gravity studies(Stephenson, 1974). The association of these two features canbe rationalized with a mechanism of incipient continental rifting.The emplacement of the unmetamorphosed northern Labrador anorthositemassifs in a non-orogenic environment of low temperature andmoderate depth suggests that the metamorphosed anorthositessouth of the Grenville Front may have been accidental participantsin a post-anorthosite Grenville orogeny and metamorphism.     

Rain-induced outflow from deep snowpacks in the central Sierra Nevada,California

Abstract

In many mountainous areas of the Pacific coast of North America, rainfall onto snowpacks causes massive floods, probably the single greatest cause of changes in channel morphology and lotie habitats. To understand and model this hydrometeorological phenomenon better, process-response hypotheses were developed for snowpack outflow amount, duration and rate and the time lags from the beginning of rainfall to initial and peak outflow. The hypotheses were evaluated by correlation and regression analyses based on measurements of 20 rainon-snow events monitored between 1984 and 1990 at forested and open plots near Lake Tahoe, California. Outflow amount correlated significantly with precipitation amount, duration and rate, snow depth and melt potential. Many of these variables also correlated significantly with outflow duration and rate and lag time to peak outlfow. Regression models expalined 80–90% of the variation in outflow amount and duration. Significant differences were not identified between the forest and open plots for any of the outflow attributes.     

On the deformation sequence in the New Harbour Group of Holy Island,Anglesey, North Wales

Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D₂ age. An earlier preferred orientation (S₁) is commonly preserved as crenulations within the Q-domain microlithons of the S₂ schistosity and is demonstrably non-parallel to bedding. F₃ folds are widely developed in S₂ and, to a lesser extent, in bedding. S₃ crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D₃ age since it clearly deforms S₂ schistosity and is consistent with the vergence of F₃ minor structures. All planar structures are deformed by folds of D₄ age. © 1997 John Wiley & Sons, Ltd.