IGNEOUS PROVINCES IN WESTERN UNITED STATES 



OOi 



xenoliths. Most are fragments of graywacke, silty argillites, greenstones, 

 and basalts. Some show little change, others have been coarsely re- 

 crystallized and complexly modified by the enclosing magma. The 

 abundance of inclusions in the andesites, and their near absence from the 

 Eocene and Miocene basalts are noteworthy (Waters, 1955). 



Much olivine basalt was erupted in the main growth of the Pliocene- 

 early Pleistocene shield volcanoes and also in the late Pleistocene and 

 Recent fissure eruptions. It is therefore evident that tholeiitic and olivine 

 basalt kindreds are in close association and that magmas resulting from 

 certain amounts of assimilation and subsequent fractional crystallization 

 also played a role. After the volcanic rocks of the adjacent Coast Ranges 

 of Oregon and Washington have been discussed, the origin of this com- 

 plex suite will be considered. 



Intrusive Rocks. Refore leaving the extrusive rocks of the Cascade 

 Range an intrusive group must be mentioned. According to Waters 

 (1955): 



Numerous stocks and small batholiths of granodiorite and diorite cut the 



volcanic rocks. The largest is the Snoqualmie batholith, a composite mass of 



pyroxene quartz diorite, hornblende granodiorite, and granophyric quartz 



j monzonite about 20 miles in diameter. The stocks occur in a linear belt along 



the core of the range [map, Fig. 36.1]. 



Most of these intrusives are rather mafic augite— hornblende granodiorites 

 and quartz diorites. Phenocrysts of plagioclase commonly show complex oscilla- 

 tory zoning similar to that in the andesites. In places the Snoqualmie granodiorite 

 lis chilled against the enclosing andesites, but elsewhere the andesite is coarsely 

 recrystallized at the contact and intimately penetrated by granodiorite. Miaro- 

 litic cavities are common. Parts of the granodiorite are altered; ferromagnesian 

 J Jminerals are decomposed to chlorite, the rock is cut by stringers of quartz and 

 epidote, plagioclase is saussuritized, and albite, quartz, and epidote form ir- 

 regular impregnations and replacements. These features suggest solidification 

 under only a thin cover. 



The plutonic activity is not closely dated. The Snoqualmie batholith invades 

 die Guye formation which contains fossil plants originally thought to be 

 Miocene (Smith and Calkins, 1906) but now regarded as Eocene. The 

 batholith had been deroofed by erosion before the building of the Mount 

 Rainier stratovolcano whose basal lavas rest on grandiorites believed to be 

 judiers of the Snoqualmie mass. The Shellrock Mountain instrusion of the 

 Columbia River gorge cuts the Columbia River basalt and is overlain uncom- 

 ormably by Quaternary andesites. 



According to the classification proposed in this chapter, the stocks 

 and batholiths of the Cascade Range are of the second cycle, whereas 

 the batholiths of the Nevadan belt are of the first cycle. 



Ratholiths of two ages have recently been noted in the Vancouver area 

 (Mount Garibaldi map area) by Mathews. The older underlies most of 

 the area and is a heterogeneous assemblage of foliated and unfoliated 

 quartz diorites and diorites. It is overlain unconformably by mid-Upper 

 Cretaceous sedimentary rocks. The younger intrusive rocks consist of 

 two plutons, one of which is a quartz diorite and trondhjemite and the 

 other a quartz-rich granodiorite and quartz monzonite. Neither of the 

 younger batholiths are in contact with the Upper Cretaceous beds, but 

 they have escaped the extensive deformation which has tilted and 

 block-faulted the stratified rocks, and are therefore considered younger 

 than mid-Late Cretaceous. The potassium-argon age determination made 

 by Follinsbee et al. ( 1957) appears to have come from the older batholith, 

 for which an age of 105 m.y. is given. This is about Mid-Cretaceous and 

 is consistent with the age indicated by the overlying mid-Upper Creta- 

 ceous beds. 



The younger batholiths may correlate with the Snoqualmie batholiths of 

 the Cascades of Washington, which according to Waters above, is post- 

 Eocene and possibly as young as Miocene. 



Coast Ranges Spilite and Keratophyre Province 



Oregon-Washington Field. According to the classification of petro- 

 graphic provinces proposed at the beginning of this chapter the Coast 

 Range spilite and keratophyre province belongs to the eugeosynclinal 

 class of "Andesite provinces." The western half of Oregon and Washing- 

 ton was a trough area of subsidence in which a great volume of volcanic 

 rocks accumulated in Eocene and early Oligocene time (see Chapter 

 29). Weaver (1945b) estimates that more lava is represented here than 

 the Columbia River basalt field, and Waters (1955) notes that more 

 than 60,000 square miles were covered by the flows, and that in the 

 northeastern Olympics the lavas are over 15,000 feet thick and in the 

 Oregon Coast Ranges in a number of sections are over 6000 feet thick. 



