Ancient roots of tungsten in western North America
V. Elongo; H. Falck; K.L. Rasmussen; L.J. Robbins; R.A. Creaser ...Abstract: The highly irregular and localized distribution of tungsten deposits worldwide constitutes a supply challenge for basic industries such as steel and carbides. Over Earth’s history, tungsten has preferentially accumulated at paleocontinental margins formed during the breakup of supercontinents. Later crustal thickening of these paleogeographic regions and the magmas they produce are associated with large tungsten districts. However, all of the largest tungsten deposits in the modern North American Cordillera, which preserves over 3 b.y. of geologic record in a paleocontinental margin with abundant crustal magmatism, are limited to the narrow Canadian Tungsten Belt in northwestern Canada. We use neodymium isotopic compositions of scheelite (CaWO4) from the Canadian Tungsten Belt and the paleogeographic distribution of tungsten deposits in the North American Cordillera to constrain the factors that control tungsten distribution. We document that tungsten is specifically associated with materials that, on average, were derived from the mantle during the Mesoarchean to Paleoproterozoic. Weathering and erosion of the supercontinents Columbia and Rodinia favored pre-enrichment of tungsten in sediments. The orogenic heating of pre-enriched sediments produced reduced melts that were capable of efficiently scavenging tungsten and formed the largest deposits in North America.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49801.1/612990/Ancient-roots-of-tungsten-in-western-North-America
Pothole-like depressions in the chamber floor of the Sudbury Igneous Complex, Canada
S.Yu. Chistyakova; R.M. Latypov
Abstract: The magmatic stratigraphy of the Sudbury Igneous Complex (Canada) is thought to have resulted from closed-system differentiation of an initially homogeneous impact melt sheet. The topography of its upward-growing chamber floor is therefore thought to have been planar and subhorizontal. However, we report on the discovery of a large pothole-like depression (~300 m in depth and ~550 m in width) in the chamber floor of this complex. The depression has been revealed through two-dimensional mapping of igneous layering that is defined by systematic vertical changes in cumulus assemblages and bulk rock chemistry. Although the formation of the depression by syn- to post-magmatic folding and/or slumping of chamber floor cumulates cannot be completely excluded, we favor an alternative explanation that follows from the recent recognition that the Sudbury Igneous Complex melt sheet crystallized concurrently from the floor and roof inward. The roof sequence was subsequently disrupted and collapsed as large discrete blocks onto the floor sequence. This may have resulted in local irregularities in topography of the upward-growing chamber floor so that crystal deposition onto and between the neighboring blocks produced pothole-like depressions. The phenomenon of physical disruption of roof sequences appears to provide a reasonable explanation for the common lack of the rocks that grew from the roof downward in layered intrusions.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49928.1/613388/Pothole-like-depressions-in-the-chamber-floor-of
S.Yu. Chistyakova; R.M. Latypov
Abstract: The magmatic stratigraphy of the Sudbury Igneous Complex (Canada) is thought to have resulted from closed-system differentiation of an initially homogeneous impact melt sheet. The topography of its upward-growing chamber floor is therefore thought to have been planar and subhorizontal. However, we report on the discovery of a large pothole-like depression (~300 m in depth and ~550 m in width) in the chamber floor of this complex. The depression has been revealed through two-dimensional mapping of igneous layering that is defined by systematic vertical changes in cumulus assemblages and bulk rock chemistry. Although the formation of the depression by syn- to post-magmatic folding and/or slumping of chamber floor cumulates cannot be completely excluded, we favor an alternative explanation that follows from the recent recognition that the Sudbury Igneous Complex melt sheet crystallized concurrently from the floor and roof inward. The roof sequence was subsequently disrupted and collapsed as large discrete blocks onto the floor sequence. This may have resulted in local irregularities in topography of the upward-growing chamber floor so that crystal deposition onto and between the neighboring blocks produced pothole-like depressions. The phenomenon of physical disruption of roof sequences appears to provide a reasonable explanation for the common lack of the rocks that grew from the roof downward in layered intrusions.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49928.1/613388/Pothole-like-depressions-in-the-chamber-floor-of
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