Preliminary results, Central Gneiss Complex of the Coast Range batholith, southeastern Alaska: the roots of a high-K, calc-alkaline arc?

Abstract

The Central Gneiss Complex (CGC) of the Coast Range batholith is the oldest unit of the batholith east of Ketchikan, Alaska, being dated by the zircon UPb method (by T.W. Stern) at 128–140 Ma. Heterogeneous, layered, commonly migmatitic, orthogneiss of hornblende-biotite quartz diorite, tonalite, quartz monzodiorite and granodiorite compositions (IUGS terminology) form the major part of the CGC. These gneisses show a range of 50–65% SiO 2 and are high in Al 2O 3 (c. 15–19%), K 2O (1.5–4%) and Sr (800–900 ppm). Most major elements show coherent, typically magmatic trends with SiO 2. La and Rb show maxima at ∼ 58% SiO 2. Initial 87Sr/ 86Sr ratios are relatively high and range from 0.7052 to 0.7066. Wallrocks of the CGC are mostly metagraywacke, pelite and metavolcanic rocks at amphibolite facies; they are geochemically dissimilar to the CGC. Major and minor elements of the CGC are very similar to those of high-K orogenic, calc-alkaline andesitic suites. The CGC may have formed largely by fractionation of mantle-derived, high AlKSr basaltic liquid in an ascending diapir, having hornblende, plagioclase, and biotite as major precipitating phases. The CGC probably represents the plutonic equivalent of a continental-margin or Andean arc that formed when the Taku terrane of the Insular belt on the west collided with the previously emplaced (but also allochthonous) Stikine terrane on the east in Late Jurassic or Early Cretaceous time.