African lithospheric structure, volcanism, and topography
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Ashwal, Lewis D.↗
Engineering topics
Ashwal, Lewis D.
Publications and source records attributed to Ashwal, Lewis D..
River Valley pluton, Ontario - A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province
This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.
Ashwal, Lewis D.↗
Sm-Nd age of the Fiskenaesset Anorthosite Complex, West Greenland
A Sm-Nd isotopic study on samples from the Fiskenaesset Anorthosite Complex in West Greenland was conducted to estimate the age of crystallization of the complex. A five-point isochron, including data for whole-rock samples of anorthosite, metagabbro, metaperidotite, and separates of calcic plagioclase and mafic matrix from a coarse megacrystic leucogabbro, corresponds to an age of 2.86 + or - 0.05 Ga, with initial sigma(Nd) of +2.9 + or - 0.4. This implies a relatively short time interval, on the order of 70 Ma, during which anorthosite formation, tonalite emplacement, and high-grade metamorphism took place.
Ashwal, Lewis D.↗
Workshop on the Growth of Continental Crust
Constraints and observations were discussed on a fundamental unsolved problem of global scale relating to the growth of planetary crusts. All of the terrestrial planets were considered, but emphasis was placed on the Earth's continental crust. The title of each session is: (1) Extraterrestrial crustal growth and destruction; (2) Constraints for observations and measurements of terrestrial rocks; (3) Models of crustal growth and destruction; and (4) Process of crustal growth and destruction.
Ashwal, Lewis D.↗
Workshop on the Deep Continental Crust of South India
The various theories in the field on the Southern Indian granulite formation are discussed in this workshop proceeding. That some widely accepted ideas about the origin of granulites, particularly relating to the role of metamorphic fluids, will have to be modified, is one outcome of the workshop. Extended abstracts of the papers presented, summaries of the attendant discussions, up-to-date accounts of the geology of the South Indian Precambrian Shield, and detailed field trip guides to all areas visited are presented. It should serve as a convenient source of information and reference to those interested in the classic Precambrian high grade terrains on Earth.
Ashwal, Lewis D.↗
Anorthosites: Classification, mythology, trivia, and a simple unified theory
An overview was presented of anorthosites. They were classified into six types: (1) Archean megacrystic, (2) Proterozoic massif-type, (3) stratiform, (4) oceanic, (5) inclusions, and (6) extraterrestrial. Some of the anorthosite mythology was discussed, such as the existence of a distinct, catastrophic anorthosite event in the late Proterozoic, the misconception that anorthosite is a major constituent of the lower continental crust, and the misconception that Archean anorthosites represent metamorphosed equivalents of mafic layered intrusions such as Bushveld or Stillwater. A general statement was offered about the origin of all anorthosites: They are cumulates of plagioclase from mantle-derived basaltic magmas.
Ashwal, Lewis D.↗
Heat transfer by fluids in granulite metamorphism
The thermal role of fluids in granulite metamorphism was presented. It was shown that for granulites to be formed in the middle crust, heat must be advected by either magma or by volatile fluids, such as water or CO2. Models of channelized fluid flow indicate that there is little thermal difference between channelized and pervasive fluid flow, for the same total fluid flux, unless the channel spacing is of the same order or greater than the thickness of the layer through which the fluids flow. The volumes of volatile fluids required are very large and are only likely to be found associated with dehydration of a subducting slab, if volatile fluids are the sole heat source for granulite metamorphism.
Morgan, Paul↗
Workshop on Techtonic Evolution of Greenstone Belts
Topics addressed include: greenstone belt externalities; boundaries; rock terranes; synthesis and destiny; tectonic evolution; rock components and structure; sedimentology; stratigraphy; volcanism; metamorphism; and geophysics.
Dewit, M. J.↗