Petrology and Structure of the Eldorado Peak Area, North Cascades, Washington

In this thesis the problem of the timing and mechanism of orogeny in the Skagit Gneiss of the North Cascades, Washington, is addressed by investigating plutonism and metamorphism in the vicinity of Eldorado Peak. The primary finding of this project is that the 88-90 Ma Eldorado pluton was intruded, not faulted, into the Cascade River Schist and Skagit Gneiss. Intrusion is indicated by cross-cutting of foliations in country rock by the pluton, apophyses of Eldorado pluton in the country rock, and xenoliths of local country rock in the pluton. Detailed mapping shows that the Eldorado pluton does not cross-cut younger internal contacts within the Skagit Gneiss as previous reconnaissance mapping had indicated. Shallow emplacement of the Eldorado pluton is indicated by several features: 1) presence of andalusite (now pseudomorphed) in a contact aureole (P \u3c 3.7 kb); 2) garnet - alumino silicate – plagioclase (GASP) barometry on garnet cores indicates low pressure (3-4 kb) in the contact aureole; 3) lack of magmatic epidote in the pluton (P \u3c 6 kb); and 4)aluminum content of magmatic hornblende in the pluton (P = 4-5 kb). Subsidence of the Eldorado Peak area and the subsequent high-pressure metamorphism of the Skagit Gneiss and Cascade River Schist occurred after intrusion of the Eldorado pluton. Evidence for this includes: 1) overprinting of andalusite by kyanite in the Eldorado pluton contact aureole; 2) GASP barometry on garnets in the contact aureole indicates that the rims grew under high pressure conditions (P =7.2 kb); and 3) intrusion at deep level of the adjacent Marble Creek pluton at 75 Ma and the Haystack pluton at 71 Ma, both of which are epidote bearing. These findings constrain the timing of subsidence in this part of the Skagit Gneiss belt to a period between 90 and 75 Ma. This event is later than subsidence of high-grade rocks south of the Entiat fault, 30-50 km to the southwest, which contain 93-96 Ma epidote-bearing plutons and was uplifted by 70-80 Ma. This study indicates that the subsidence and uplift of the North Cascades Crystalline Core are localized and diachronous and therefore cannot be explained by a single regional over-thrust event. Mapping of the Cascade River Schist shows that it is gradational with the Skagit paragneiss. This finding is in agreement with the view of Misch (1966) that the Skagit paragneiss is a higher metamorphic grade equivalent of the Cascade River Schist. Lithologic units in the Cascade River Schist are interpreted to be part of a submarine fan complex with lithologic units in the Cascade Pass area representing a proximal portion of the fan and lithologic units along the Marble Creek - Newhalem Creek divide representing a more distal portion of the fan. Foliations in the area are predominantly northwest-striking and dip steeply to the southwest. Lineations are not common, but where they are present they are subhorizontal and southeast-northwest trending. Conglomerate clasts indicate a strong flattening fabric in the high- grade schists. This flattening fabric is overprinted by discrete dextral shear zones of both high and low metamorphic grade