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A sheared and altered pendant in the Cassiar batholith, headwaters of the Stikine River, B.C. Taplin, Arthur Cyril

Abstract

This thesis represents a field and petrographic study of an elongate, tabular 'roof pendant' within the Cassiar Batholith. The general area lies between the headwaters of the Stikine and Finlay rivers in north-central British Columbia. Since no previous geological mapping had been carried out in this area, some details on the extent, lithology, and structure of the various formations are presented. These consist of the Takla Group of volcanics and marine sediments ranging from upper Triassic to upper Jurassic in age; the Cassiar Intrusions representing one general period of batholithic invasion; and the Sustut Group, composed of conspicuously bedded continental strata of upper Cretaceous and Paleocene age. Minor basic intrusives of probable Tertiary age are also present. The pendant occurs within intrusives of gabbroic composition. A wedge-shaped body along the western border consists of quartz gabbro. Bluish green hornblende, comprising some 25% of the rock, shows a lineation which is also visible in the orientation of the calcic plagioclase. Hornblende is the main mafic constituent, quartz comprises less than 10% of the total composition. The linear structures plus the presence of numerous foreign xenoliths within this more basic border phase, lead to the conclusion that the early intrusion was forceful, that emplacement was by piecemeal stoping, and that flowage of the magma occurred during crystallization. The main intrusive east of the pendant consists of massive granogabbro, composed of some 50% calcic plagioclase, 20% microcline, 20% quartz and 5% biotite. The emplacement of this intrusive was more passive. Phases intermediate to these two extremes are present, mainly along the eastern border of the pendant. These are of hornblende bearing granogabbro. The intrusives represent differentiated phases of a common magma. The pendant is composed of a conformable sequence of pyroclastics and calcareous sediments. The pyroclastics (tuffs) are predominantly of basaltic composition and form the flanks of the tabular structure. The calcareous sediments represent alternating deposition of tuffaceous and sedimentary material. The succession at three localities is presented in some detail. It is shown that thermal metamorphism of the basaltic tuffs has been negligible and that these beds have insulated the central portion of the pendant from laterally spreading thermal and metasomatic effects. The thermal metamorphism of xenoliths of basaltic tuff in the quartz gabbro is shown to be insignificant, whereas sedimentary xenoliths have been converted to an amphibole hornfels assemblage in equilibrium with the magma. Dynamic metamorphism has been active within the intrusive and older rocks, rendering those within the pendant to a schistose condition. Effects of earlier, ascending thermal metamorphism have been largely obscured by dynamic metamorphism. Hydrothermal alteration of the fault and shear zones to aggregates of quartz-ankerite and chromian muscovite, represents the final stages of the crystallization of a granitic magma. Field and petrographic criteria of ‘granitization’ are presented and critically examined. The coarse grained gabbros are shown to represent crystallization from a true magma. Maps and plates illustrate the main features. The appendix includes determinative mineralogy and petrographic descriptions of the intrusives.

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