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

ON BATHOLITHS AND VOLCANOES—INTRUSION AND ERUPTION OF LATE CENOZOIC MAGMAS IN THE GLACIER PEAK AREA, NORTH CASCADES, WASHINGTON

By R. W. TABOR and D. F. CROWDER

In the Glacier Peak area, the three principal episodes of Cenozoic igneous activity have been the intrusion of the tonalite-granodiorite Cloudy Pass batholith in early Miocene time, as shown by potassium-argon radiometric dates of 22 m.y. (million years); the extrusion on and near the exposed batholith of a thick pile of predominantly andesitic to dacitic breccia, tuff, and lava of Gamma Ridge between early Miocene and Pleistocene time; and, in late Pleistocene and Recent time, the growth of the dacitic Glacier Peak volcano, which was accompanied by eruption of small amounts of basalt from separate vents near Glacier Peak.

Much of the Cloudy Pass batholith in the Glacier Peak area lies under a relatively thin roof of regionally metamorphosed rocks, as shown by a retinue of stocks in a zone trending southwest away from the main pluton and by thermal metamorphism and concentrations of alaskite dikes in the roof rocks over this zone. Some of the alaskite dikes are related to earlier regional metamorphism, but many appear to be differentiates of the batholith squeezed out of an adamellite cap—exposed along the northwest margin of the pluton and in small patches farther east—by renewed intrusion of the tonalite-granodiorite core. Hornblende tonalite porphyry dikes and inclusions confined to the adamellite border are probably injections from the core which were partly disrupted by renewed intrusion. The textures of the adamellite and its chemical similarity to the experimentally determined minimum-melting composition of granite indicate that the cap is a normal crystallization differentiate enriched in felsic constituents by settling out of early formed mafic crystals.

Intrusive breccias with aphanitic and protoclastic matrices occur along steep contacts of the main pluton and of a satellitic stock and in small bodies nearby. These breccias resemble many described by F. W. Cater along Phelps Ridge in the Holden quadrangle. Clearly intruded under low lithostatic pressure, the breccias and aphanites may have been vents for erupting gas-charged magma from the core of the batholith.

By the time the coarse volcanic breccias tuffs, lithic wacke, volcanic wackes and minor lavas of Gamma Ridge erupted, the Cloudy Pass batholith had cooled at the level now exposed and had been partly deroofed by erosion. Welded tuff in the Gamma Ridge rocks and the considerable local relief under them indicate subaerial eruption in mountains. Basal inter-bedded volcanic and monolithologic breccias are similar to some of the nearby intrusive breccias that cluster around the Cloudy Pass batholith, and this similarity suggests that the Gamma Ridge rocks erupted from the batholith's still-molten core. However, present drainage patterns appear to have been set by diversion around the Gamma Ridge volcanic center, which suggests the rocks may be latest Pliocene or even earliest Pleistocene in age. If the magma of the batholith's core were the source, it would have had to remain molten an inordinantly long time.

The oldest Glacier Peak lavas pooled in valleys on the east side of Lime Ridge, a northwest-trending spur of the Cascade Crest, and now crop out as elongate, locally very thick, ridge caps, owing to inversion of topography by the subsequent erosion. Later flows cling to the sides of present valleys and are moderately dissected; the youngest flows bottom the present valleys and are little dissected. This spectrum of greatly dissected older flows to little dissected younger flows indicates continual uplift and erosion during growth of the volcano. The ridge-capping flows radiate from the present summit area of the volcano and extend nearly to the Suiattle River, which shows that the river was forced into its wide northeast loop around the Gamma Ridge eruptive rocks prior to earliest Glacier Peak time. As Glacier Peak grew by continued eruption of clinopyroxene-hypersthene dacite, the lavas eventually spilled over Lime Ridge into the White Chuck valley.

Late in the life of the volcano, an oxyhornblende—hypersthene dacite dome was extruded near the summit at Disappointment Peak. A second hornblende dacite dome is presumed to have grown and collapsed on the east side of the peak to furnish debris for a giant fan and valley fill in the just deglaciated Suiattle valley. Chocolate Creek was diverted from ancestral Dusty Creek and forced to spill over a confining lava ridge by the growth of the fill. The Suiattle River was dammed and forced farther east.

All the flows of Glacier Peak were erupted within the past 700,000 years, as shown by their normal magnetic polarity, and the latest flows, within the past 17,000 years, after alpine glaciers had retreated. About 12,000 years ago (as shown by a carbon-14 date and correlations of R. Fryxell and other workers for areas far from the Glacier Peak area) pumice was expelled and drifted far, especially to the east. During or soon after this eruption, mudflows and streams on the west side of the peak partly filled the recently deglaciated White Chuck valley with pumice lapilli and other volcanic detritus from the volcano, which formed the White Chuck fill. A nuée ardente rushed valleyward to form a thin cap of vitric tuff on this fill and was later covered by pumice that continued to wash off the mountain. Three hot springs—Kennedy, Sulphur, and Gamma—are the only evidence that hot rocks, perhaps even magma, still exist at depth.

There is some indication that the differentiation index increases in the younger eruptive rocks of Glacier Peak, but the dacite lavas making up the bulk of the volcano are remarkably uniform in composition, varying a maximum of only 8.0 percent in silica (as shown by refractive index measurements on fused glass beads) over a period of at least 10,000 years and probably more on the order of 500,000 years.

Basalt is not interlayered with the dacite flows, but late in Glacier Peak time basaltic andesite was extruded near Lightning Creek and basalt was erupted from cinder cones at Indian Pass and the upper White Chuck River. The absence of basalts or intermediate rocks in the dacite cone suggests sources for the dacites and basalts.

The intrusion of the batholith appears to have been guided by: (1) Northwest-trending regional foliation, compositional layering in the schist and gneiss host rocks, and (2) northeast-trending joints perpendicular to the regional trends and fold axes (ac joints). All Cenozoic magmas in the Glacier Park area have risen along the intersection of several regional structures: (1) The northwest-trending normal faults of the Chiwaukum graben, (2) a related (?) northwest-trending belt of small pods of sepentinized ultramafic rocks, and (3) a deep fracture (?), that trends north-northeast and, underlies a zone of dikes and eruptive rocks. This deep fracture (?) passes under the volcano and, on projection, intersects other Tertiary batholiths and Quaternay eruptive centers (for example, the Snoqualmie batholith and Mount Rainier volcano). It may have led basaltic magma to the surface from the mantle and may have helped localize, the emplacement of the more silicic magmas of Cloudy Pass, Gamma Ridge, and Glacier Peak.