Intrusive rocks of the Holden and Lucerne quadrangles, Washington— The relation of depth zones, composition, textures, and emplacement of plutons

Abstract

The core of the northern Cascade Range in Washington consists of Precambrian and upper Paleozoic metamorphic rocks cut by numerous plutons, ranging in age from early Triassic to Miocene. The older plutons have been eroded to catazonal depths, whereas subvolcanic rocks are exposed in the youngest plutons. The Holden and Lucerne quadrangles span a -sizeable and representative part of this core. The oldest of the formations mapped in these quadrangles is the Swakane Biotite Gneiss, which was shown on the quadrangle maps as Cretaceous and older in age. The Swakane has yielded a middle Paleozoic metamorphic age, and also contains evidence of zircon inherited from some parent material more than 1,650 m.y. old. In this report, the Swakane is assigned an early Paleozoic or older age. It consists mostly of biotite gneiss, but interlayered with it are scattered layers and lenses of hornblende schist and gneiss, clinozoisite-epidote gneiss, and quartzite. Thickness of the Swakane is many thousands of meters, and the base is not exposed. The biotite gneiss is probably derived from a pile of siliceous volcanic rocks containing scattered sedimentary beds and basalt flows. Overlying the Swakane is a thick sequence of eugeosynclinal upper Paleozoic rocks metamorphosed to amphibolite grade. The sequence includes quartzite and thin layers of marble, hornblende schist and gneiss, graphitic schist, and smaller amounts of schist and gneiss of widely varying compositions. The layers have been tightly and complexly folded, and, in places, probably had been thrust over the overlying Swakane prior to metamorphism. Youngest of the supracrustal rocks in the area are shale, arkosic sandstone, and conglomerate of the Paleocene Swauk Formation. These rocks are preserved in the Chiwaukum graben, a major structural element of the region. Of uncertain age, but possibly as old as any of the intrusive rocks in the area, are small masses of ultramafic rocks, now almost completely altered to serpentine. These occur either as included irregular masses in later intrusives or as tectonically emplaced lenses in metamorphic rocks. Also of uncertain age but probably much younger, perhaps as young as Eocene, are larger masses of hornblendite and hornblende periodotite that grade into hornblende gabbro. These are exposed on the surface and in the underground workings of the Holden mine. Oldest of the granitoid intrusives are the narrow, nearly concordant Dumbell Mountain plutons, having a radiometric age of about 220 m.y. They consist of gneissic hornblende-quartz diorite and quartz diorite gneiss. Most contacts consist of lit-par-lit zones, but some are gradational or more rarely sharp. The plutons are typically catazonal. Closely resembling the Dumbell Mountain plutons in outcrop appearance, but differing considerably in composition, are the Bearcat Ridge plutons. These consist of gneissic quartz diorite and granodiorite. The Bearcat Ridge plutons are not in contact with older dated plutons, but because their textural and structural characteristics so closely resemble those of the Dumbell Mountain plutons, they are considered to be the same age. Their composition, however, is suggestive of a much younger age. Cutting the Dumbell Mountain plutons is the Leroy Creek pluton, consisting of gneissic biotite-quartz diorite and trondjhemite. The gneissic foliation in the Leroy Creek is characterized by a strong and pervasive swirling. Cutting both the Dumbell Mountain and Leroy Creek plutons are the almost dikelike Seven-fingered Jack plutons. These range in composition from gabbro to quartz diorite; associated with them are contact complexes of highly varied rocks characterized by gabbro and coarse-grained hornblendite. Most of the rocks are gneissic, but some are massive and structureless. Dikes, sills, small stocks, and irregular clots of leucocratic quartz diorite and granodiorite are abundant in the Swakane Biotite Gneiss and are locally abundant in the Seven-fingered Jack and other plutons. Although the leucocratic rocks vary little in appearance or composition, some, particularly those in the Swakane, were formed by metamorphic segregation, whereas the others are probably felsic differentiates of intrusive rocks. The Tenpeak and White Mountain plutons are closely similar and are probably connected at depth. Rocks of the Tenpeak are more varied and range in composition from gabbro to granodiorite; quartz diorite is not only most common in the Tenpeak, but also constitutes, by far, the greatest bulk of the White Mountain pluton. Much of the rock in both plutons is somewhat gneissic, but some is nongneissic. The north end of the White Mountain pluton is bordered by a contact complex similar to those associated with the Seven-fingered Jack and other plutons in the area. Maximum potassium/argon ages determined on hornblende from the Tenpeak are about 90 m.y. Probably somewhat younger than the Tenpeak and White Mountain plutons is the Sulphur Mountain pluton. Only a small part of the pluton extends into the Holden quadrangle, and here the rocks are gneissic granodiorite. The High Pass and Buck Creek plutons are tabular, somewhat silllike masses of granodioritic and quartz dioritic composition. Rocks in the Buck Creek pluton are entirely gneissic, whereas rocks only near the contacts or in the thinner parts of the High Pass are gneissic. In most places, the contacts of both plutons consist of lit-par-lit zones, but numerous dikes of the High Pass are present in the host rocks, especially the Sulphur Mountain pluton. The Riddle Peaks pluton consists of layered and unlayered hornblende gabbro. Inasmuch as the pluton is cut on all sides by later intrusive rocks, its age relative to older plutons is not known, but it is almost certainly pre-Tertiary. Much of the gabbro is rhythmically layered, but some is unlayered or only vaguely layered. In places, thick sheets of unlayered gabbro alternate with equally thick sheets of rhythmically layered gabbro. The rock is mostly fresh and unaltered or recrystallized. No pseudosedimentary structures other than the gravity-stratified rhythmic layers were seen. The Cardinal Peak pluton has a core consisting largely of granodiorite and quartz diorite and a rim of contact complexes containing large amounts of gabbro and coarse-grained hornblendite. The northern part of the intrusive is highly protoclastic throughout and contains numerous spindle- and canoe-shaped masses a third of a meter to several meters across and a few meters to several tens of meters long that are either partly or completely enveloped by rinds of fine-grained protoclastic material. These masses seem to have behaved as roller bearings during intrusion of the partly crystalline magma.