Geological Survey Professional Paper 604 On Batholiths and Volcanoes—Intrusion and Eruption of Late Cenozoic Magmas in the Glacier Peak Area, North Cascades, Washington

INTRUSION AND DIFFERENTIATION

The main pluton probably did not emplace itself by stopping, for there is a conspicuous lack of engulfed roof rocks in it in both the Holden (Cater, 1969, p. 48) and the Glacier Peak quadrangles. The lack of marginal contamination, even where inclusions are abundant, and the presence of sharp chilled contacts as stressed by Cater (1969, p. 7-8) argue against assimilation. Furthermore, the discordant irregular shape of the batholith (fig. 2), together with the apparent lack of lateral shouldering aside of wallrock, suggests that room was made by lifting the roof, not by pushing the wall back—an appropriate hypothesis for a shallow pluton. The eastward bulge of the Cascade Crest over the batholith supports this lifting (sec p. 55). The hypothesis of the rise of the batholith relative to its host rocks on the northeast flank (fig. 26) where the dislocation is marked by a zone of chilled rocks and breccias as proposed by Cater (1969, p. 47) clearly requires that the roof has been raised.

If the batholith raised its roof, the roof rocks should be more deeply eroded near and over the pluton than they are where not uplifted. Both Crowder (1959, p. 832, 834) and Cater (1969, p. 48) noted that well-segregated and swirled gneiss and associated leucocratic dikes (alaskites) and migmatites are more abundant above and near the pluton than they are to the southeast. Crowder, assuming that the greatest uplift and deepest erosion had been on the Cascade Crest, suggested that the migmatites and associated rocks are of relatively deep-seated origin. Cater, assuming the deep-seated origin of these rocks, infered that they had been uplifted by the batholith. Aside from the obviously circular nature of these arguments, there are two processes other than uplift that can account for the concentration of leucocratic rocks and associated features: (1) In the Glacier Peak area, we have shown that some of the leucocratic dike material (alaskite) is derived from the batholith and is not of deep-seated metamorphic origin; a concentration of such material may indicate only that the pluton lies buried nearby. (2) In the Holden quadrangle, leucocratic rocks are concentrated in or near biotite gneiss and biotite tonalite-gneiss not because the rocks were formed in a deeper environment but because these lithologies are particularly susceptible to metamorphic differentiation (Crowder, 1959, p. 861). Similar biotite gneisses characterized by abundant leucocratic material occur in the biotite gneiss belt in the southwest part of the Glacier Peak quadrangle (fig. 12) and in the biotite gneiss along Lake Chelan (Cater and Wright, 1967).

Differentiation of the Cloudy Pass magma cannot be related to a conventional sequence of rock emplacement. Once the granodiorite or tonalite magma was in place at the level now exposed near Glacier Peak, an adamellite cap (the light-colored phase) formed by crystallization differentiation as early formed mafic minerals settled from the top of the chamber. The smooth curves in figure 19 do not prove crystallization differentiation occurred,¹ but they do illustrate the changing composition. Furthermore, the composition of the adamellite approaches the composition of a magma in the minimum-melting trough (fig. 20) as derived by crystallization differentiation. Some of the light-colored rocks that occur sporadically within the batholith in the Holden quadrangle (Cater and Crowder, 1967) may be the roots of this adamellite cap, although most of them are thought by Grant (1966, p. 42) and Cater (1969) to have formed where potash-rich and silica-rich vapors rising from the crystallizing core replaced the early formed overlying tonalite.

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¹Chayes (1964) showed that a Harker variation diagram itself may be of little use in proving that crystallization differentiation has produced suites of igneous rocks. In such diagrams where components are summed to 100 percent, the major constituents other than silica are bound to decrease as the silica increases. A genetic relation between samples is implied, only if points are very near the curves.

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FIGURE 20.—Ternary diagram of orthoclase (Or), quartz (G), and albite (Ab) showing normative position of alaskite and light-colored phases of the Cloudy Pass batholith in relation to minimum-melting trough of Tuttle and Bowen (1958, p. 55). The light-colored phases contain 9-10 percent anorthite as well as other components, which may account for their displacement to the left of the trough, which is that of a simple ternary system. Dashed line is 760° C isograd at pressure PH2O = 1000 kg/cm2 (kilograms per square centimeter). Numbers indicate samples listed in table 3.

The present position of the adamellite cap on the northwest side of the batholith is explained by assuming it was intruded by the core (fig. 26). This occurred when both the cap and the core were still largely molten, for their contacts are gradational. Dikes of hornblende tonalite porphyry presumably derived from the core were injected into more brittle parts of the cap, but were soon broken apart by movement of the still-plastic host. Interstitial melt in the cap (alaskite in composition and the likely residual product of crystallization differentiation) (figs. 19 and 20) may have been pressed out into the host rocks by the core intrusion. Locally, this alaskite was intruded into the more solid parts of the cap itself. With this event, the plutonic history of the Cloudy Pass batholith at the level now exposed came to an end.