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Study of tectonic processes and certain geochemical abnormalities in the Coast Mountains of British Columbia Culbert, Richard Revis

Abstract

An examination was made of the distribution of potassium, rubidium and strontium in rocks of the Coast Mountains batholithic complex south of Nass River, and of the form of tectonism affecting this regime as revealed by morphological analysis and geophysical evidence. The ratio K/Rb in most plutonic suites of the Coast Mountains is distinctly higher than values generally associated with rocks of the Continental Crust. This might be explained either by assuming that these alkalis have been derived in part from destruction of an oceanic crust (of typically higher K/Rb values) or that the batholith is a regime from which a volatile alkaline phase has been removed. Igneous rocks with K/Rb values greater than 400 are widespread in the central and southern portions of the project area, but in the northern region they are less abundant and appear to correlate with zones of thermal activity. In southern British Columbia, the unusual K/Rb values terminate abruptly on the eastern margin of the Coast Mountains. Average strontium concentration in the Coast Mountains plutonic complex is approximately 720 ppm, which is unusually high. Strontium content appears to be related to plagioclase content and likely controlled by magmatic fractionation. High strontium concentration proved to be typical of plutonic rocks across the Cordillera of southern British Columbia, rather than being confined to the Coast Mountains. The frequency distribution for strontium concentration is bimodal for both the plutonic rocks of the southern Coast Mountains and those of the adjacent interior of the province; furthermore, the modes match. This suggests a similar early* history for plutonic magmas in these two regions, while the history of their more mobile alkalis obviously has been different. The overall ratio of rubidium to strontium in the Coast Mountains batholithic complex is only 0.047. The Sr⁸⁷ /Sr⁸⁶ ratio in this environment is consequently changing by approximately 0.001 every 550 m.y. A few measurements were made of this ratio for plutonic rocks and gneisses in the Coast Mountains. These ranged from 0.7031 to 0.7068, the variation being expected in view of metamorphic rocks having been digested during formation of some of the plutonic suites. In the measurement of rubidium and strontium concentrations for whole rock powders by X-ray fluorescence, an extension of the Compton scattering technique was developed which allowed correction not only for matrix absorption but also for background. This system employs linear regression analysis to obtain the parameters of a largely empirical equation for the correction of readings. An apparent erosion surface of raid-Tertiary age is still evident in the similarity of summit heights over wide areas of the Coast Mountains, especially in the western portion of the range. On the theory that any lines of vertical movement active since development of the surface should have caused discontinuities in this summit envelope, a computer analysis was made of summit elevations to locate dislocations. The results showed that much of the Coast Mountains and Vancouver Island appears to be broken into blocks by such lines. Some of the major discontinuities also mark the location of other signs of Tectonic activity such as thermal centers (hotsprings and Quaternary volcanoes), strong lineaments, metamorphic screens, and abrupt changes in fjord depths. One zone running up the Coast Mountain chain for almost the entire 500 mile length of the project area shows the above features as well as seismic activity in its southern part. In the north it separates regions of different potassium-argon ages and forms one of the belts of anomalous K/Rb ratios in plutonic rocks. It is suggested that the deepest Coast Mountains fjords are drowned features, this outlook being supported by apparent tectonic control of the terminations of deep fjord segments and in a bimodal frequency distribution of depths for fjords transverse to the tectonic trend. An analysis of secondary erosion surfaces along two major summit envelope discontinuities was undertaken to find if movement postdated uplift. The best correlation available was for all movement following uplift, but the results were not entirely satisfactory. An attempt to make a quantitative interpretation of lineament patterns by laser analysis of contour maps did not prove fruitful. The general tectonic history suggested for the Coast Mountains is for early Tertiary uplift of at least part of the belt, likely as the result of active subduction. This was followed by the development of an erosion surface and then resumed uplift in the Pliocene or late Miocene. Both tectonic and igneous style of the late Tertiary however, suggest relaxation and quite likely block subsidence of the fjord zone.

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