High‐pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east‐central Alaska

Abstract

Abstract Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east‐central Alaska. These terranes differ in protoliths, structural level and cooling ages. A structurally complex zone of gently north‐dipping tectonites separates the two terranes. The northern, structurally higher Taylor Mountain terrane includes garnet amphibolite, biotite ± hornblende gneiss, marble, quartzite, metachert, pelitic schist and cross‐cutting granitoids of intermediate composition (including the Late Triassic to Early Jurassic Taylor Mountain batholith). Lithological associations and isotopic data from the granitoids indicate an oceanic or marginal basin origin for the Taylor Mountain terrane. 40Ar/39Ar metamorphic cooling ages from the Taylor Mountain terrane are latest Triassic to earliest Middle Jurassic. The southern, structurally lower Lake George subterrane of the Yukon‐Tanana terrane is made up of quartz‐biotite schist and gneiss, augen gneiss, pelitic schist, garnet amphibolite and quartzite; we interpret it to comprise a continental margin and granitoid belt built on North American crust. Metamorphic cooling ages from the Lake George subterrane are almost entirely Early Cretaceous.Geothermobarometric analysis of garnet rims and adjacent phases in garnet amphibolite and pelitic schist from the Taylor Mountain terrane and Lake George subterrane indicate peak metamorphic conditions of 7.5‐12 kbar at 555‐715° C in the northern part of the Taylor Mountain terrane, in which NNE‐vergent shear fabrics are preserved; 6.5‐10.8 kbar at 520‐670° C within the contact zone between the two terranes, in which NW‐vergent shear fabrics predominate; and 6.8‐11.8 kbar at 570‐700° C in the Lake George subterrane of the Yukon‐Tanana terrane, in which NW‐vergent shear is recorded in the northern part of the study area and SE‐vergent shear in the southern part. Where the two shear‐sense directions occur together in the northern Lake George subterrane and, locally, in the contact zone, fabrics that record NW‐vergent shear are more penetrative and preceded fabrics that record SE‐vergent shear.We interpret the pressure, temperature, kinematic and age data to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile‐Slide Mountain terrane. High‐ to intermediate‐pressure metamorphism of the Taylor Mountain terrane took place within a SW‐dipping (present‐day coordinates) subduction system. High‐ to intermediate‐pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW‐directed overthrusting of the Taylor Mountain, Seventymile‐Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. The difference in metamorphic cooling ages between the Taylor Mountain terrane and adjacent parts of the Lake George subterrane is best explained by Early Cretaceous unroofing of the Lake George subterrane caused by crustal extension, recorded in its younger top‐to‐the‐SE fabric.