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

Abstract

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 SiC>2, FeO, Ti02, and P2O5, and similar in MgO and CaO. The Coast Plutonic Complex, however, is markedly richer in AI2O3, somewhat richer in Na20, 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 SiC>2 values near, but not at, the western edge of the Complex. The chemical analyses show no systematic increase in Si0 2 or K 2 0 from west to east across the Coast Plutonic Complex. 195 on July 25, 2015 memoirs.gsapubs.org Downloaded from

Chapter 23: Two traverses across the Coast batholith, southeastern Alaska

“…Crawford and others (1987) also proposed that all these tonalitic and dioritic plutons rose through the crust along steep ductile shear zones. Magnetic results (Griscom, 1977;Roddick, 1983b) indicate the existence of a large crustal mass of mafic rock under the western margin of the Coast batholith, which may be related to the western tonalites and which also may have caused the thermal pulse. The foliation of these western tonalites may have been induced by regional stress, rather than simply by flow of partly crystalline magma; relative movement between the Taku terrane and the central gneisses also may have occurred at this time.…”

“…127 Ma) either in Stikine terrane or in an oceanward terrane that is poorly preserved in the graywacke and pelites found near the central gneisses east of Ketchikan (Fig. 2), and extending south into British Columbia (Hutchison, 1982;Roddick, 1983b).…”

U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

“…There have also been a number of syntheses outlining igneous, metamorphic, structural, and tectonic aspects of the Coast Mountains batholith in coastal British Columbia (e.g., Armstrong, 1988;Arth et al, 1988;Barker and Arth, 1990;Butler et al, 2001b;Crawford et al, 2005Crawford et al, , 2009Hollister andAndronicos, 1997, 2006;Monger et al, 1982Monger et al, , 1994Roddick and Hutchison, 1974;Roddick, 1983;Stowell and Crawford, 2000;Woodsworth et al, 1992).…”

Caractérisation et transfert des magmas felsiques dans la croûte moyenne à profonde :

“…Weinberg ( Figure 7.7: Contours de la taille critique d'un dyke we, nécessaire pour que le magma migre sans se solidifier, calculés à partir de l'équation 3 de l'article de Petford et al (1993), en fonction de la viscosité du magma et du contraste de densité entre le magma et la roche encaissante (tiré de Petford et al 1993) CBB: Cordillera Blanca Batholith (Petord, 1990); PRD1, PRD2-Puscao Ring Dikes, Peru (Bussel 1988); CTG-central Chilean Tertiary Granitoids (Lopez-Escobar et al 1979); WPRB-moyenne des Batholites Peninsular Ranges (Silver et Chappell, 1988;Cough et Riddihough, 1989); GST-Gréa, Tonalité Sill, Alaska (Roddick, 1983); MGG-Mount Givens Granodiorite, Sierra Nevada (Bateman ei Noklebuig, 1978;Dodge et Bateman, 1988); BB-Baltoro Batholith (Searle et al 1992). Enfin, les calculs de la taille de la source W s amènent à se questionner sur la distance entre les fortes densités de veines et si cette distance est relative à la source (Sawyer, sous presse).…”

“…1993) CBB: cordillera Blanca batholith (Petord, 1990); PRD1, PRD2-Puscao ring dikes, Peru (Bussel, 1988); CTG-central Chilean Tertiary granitoids (Lopez-Escobar et al 1979); WPRB-moyenne des batholites Peninsular Ranges (Silver et Chappell, 1988;Cough et Riddihough, 1989); GST-Great Tonalité Sill, Alaska (Roddick, 1983); MGG-Mount Givens Granodiorite, Sierra Nevada (Bateman etNokleburg, 1978;Dodge et Bateman, 1988); BB-Baltoro batholith (Searle et al 1992) 332 Tableau 5.11: Calcul des températures selon la méthode de Perchuk et Lavrent'eva(1983).200…”