Evolution of the Coast batholith along the Skagway Traverse, Alaska and British Columbia.

In an investigation termed the Skagway Traverse, a geologic map was made of an area about 70 km long and l0 to 15 km wide transecting the Coast batholith (Coast Plutonic Complex of most Canadian workers) from Haines to Skagway, Alaska, to Log Cabin and Tutshi Lake, British Columbia. The batholith here consists of (l) Cretaceous or older migmatitic orthogneiss of quartz dioritic and tonalitic compositions and minor metasedimentary rocks; (2) minor, Late Cretaceous or older nonmigmatitic orthogneiss of tonalitic composition; (3) Late Cretaceous (zircon U-Pb age : 72 m.y.) feldspar-porphyritic biotitehornblende granite-the granite of Log Cabin; (4) Eocene (zircon U-Pb age : 54 m.y.) massive plutons of hornblende-biotite tonalite and granodiorite-the tonalite of Skagway; and (5) Eocene (zircon U-Pb ages are 53 and 48 m.y.) massive plutons of near-minimummelt biotite leucogranite-the granites of Clifton and Summit Lake. The orthogneisses and tonalite of Skagway show typical calc-alkaline, Pacific-margin compositions:59-7lo/o SiOr, 18.6-15.4o/o Al2Or,6-30/oCaO,l.5-3.10/o IlO, 830-480 ppm Sr, 65-45 ppm Rb, and chondrite-normalized rare-earth-element (REE) patterns of moderate slope and small negative or no Eu anomaly. The granites of Clifton and Summit Lake have higher SiOr(73.9-77.2o/o),KrO (4.74.2o/o), and Rb (85-219 ppm), less AlrO3 (14.012.3o/o), CaO (1.6-1.10/o) and Sr (210-85 ppm), and large negative Eu anomalies and flat slopes for heavy REE. The granite of Log Cabin is transitional between these two types. Initial 87Sr/t6Sr ratios for most plutonic rock units are 0.7052 to 0.7058; Summit Lake Granite is 0.7049 and 0.7060. They show no systematic variation related to rock type or location. The orthogneisses and tonalite ofSkagway show a rough trend on the normative quartzplagioclase-orthoclase diagram from near dioritic composition toward granodiorite and granite. These rocks probably originated largely by fractionation ofplagioclase, hornblende, and other phases. However, oxygen-isotope ratios indicate either exchange of the magmas with a high 'tO/I6O fluid or minor assimilation of high-6'tO crustal rocks. The granites of Log Cabin, Clifton, and Summit Lake, however, probably were derived largely by crustal melting of graywacke (flysch) or siliceous to intermediate igneous rocks.