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Geophysical review and composition of the Coast Plutonic Complex, south of latitude 55° N

J. A. Roddick
J. A. Roddick
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January 01, 1983

The continental margin is unmistakably indicated by the steep decline in Bouguer values from 150 mgal to 30 mgal from west to east across the continental slope. A decline of almost equal magnitude marks the western side of the Coast Plutonic Complex and is possibly an expression of a former continental margin. The "seismic" crust appears to differ from the "gravity" crust in most places, but this is best accounted for by the lack of constraints on the interpretation of both types of data. The depth to the Moho discontinuity beneath the western margin of the Coast Plutonic Complex is thought to be between 23 and 27 km, and beneath the eastern margin, about 33 km. The Moho appears to deepen easterly beneath the Intermontane Belt. Heat production in the plutonic rocks is fairly constant across the Coast Plutonic Complex. A strong magnetic anomaly over the western margin of the Coast Plutonic Complex is the most striking feature revealed by geophysical data. Its source is large and deep, a mass at least 250 km long, about 50 km wide, and possibly as much as 40 km deep. It is thought to be a near-surface segment of lower crust rather than an intrusive body or a zone mineralized with magnetite.

Modal study of about 8000 specimens of plutonic rock from the southern Coast Mountains (south of latitude 52°N) confirms that the average plutonic rock of the Coast Plutonic Complex is a quartz diorite. When the specimens are divided, each according to its source in one of three belts parallel with the regional trend, a marked quartz deficiency is revealed, not in the western zone where expected, but in the axial belt; quartz diorite is seen to be relatively more abundant in the eastern, rather than in the western, belt. Also contrary to expectations, granodiorite becomes progressively less abundant from west to east. Most common plutonic rock types were found to become denser and more mafic-rich from east to west.

About 600 chemical analyses from the Coast Plutonic Complex between latitudes 51° and 54° N showed that the average composition of the plutonic rock of the Coast Plutonic Complex is nearly identical to that of typical continental crust in SiO2, FeO, TiO2, and P2O5, and similar in MgO and CaO. The Coast Plutonic Complex, however, is markedly richer in AI2O3, somewhat richer in Na2O, and distinctly poorer in K2O. Chemically, the volcanic equivalent to the average plutonic rock in the Coast Plutonic Complex is a tholeiitic andesite.

The specimens for chemical analyses were selected so as to form three widely spaced cross-sections of the Coast Plutonic Complex. The most conspicuous feature shown by the chemical cross-sections is a marked depression in SiO2 values near, but not at, the western edge of the Complex. The chemical analyses show no systematic increase in SiO2 or K2O from west to east across the Coast Plutonic Complex.

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GSA Memoirs

Circum-Pacific Plutonic Terranes

J. A. Roddick
J. A. Roddick
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Geological Society of America
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Publication date:
January 01, 1983




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