PETROGENESIS OF A COMPOSITE PLUTON IN A COLLISIONAL TERRANE SUTURE ZONE, NORTHERN TALKEETNA MOUNTAINS, ALASKA

Granitic and tonalitic rocks of the Brusk composite pluton (10’s km²) were emplaced in the suture zone between the former southern margin of Alaska and the Wrangellia composite terrane (WCT). Previously mapped as a single unit of Tertiary granitic rocks (K/Ar age of 57.2 Ma), the pluton consists of two distinct units: tonalite to quartz diorite intruded by granite to granodiorite. The average modal mineralogy of the tonalitic unit is 14% quartz, 2% orthoclase, 48% plagioclase, 8% clinopyroxene, 3% orthopyroxene, 16% biotite, 6% hornblende, and 2% chlorite and for the granitic unit is 37% quartz, 14% orthoclase, 45% plagioclase, and 4% biotite. Rocks from both units have trace percentages of zircon, apatite, and sphene. Both units also contain plagioclase with a wide range of zoning and disequilibrium textures, indicating a complex cooling history with possible magma mixing.

Granitic samples are peraluminous, enriched in some large ion lithophile elements (LILE) as well as Th and U, are depleted in Ba, Sr, P, and Ti, and show a pronounced Nb-Ta paired depletion. The granitic rocks also show a wide range in La/Yb and LILE to high field strength element ratios (e.g., Ba/Nb ~25 to 230) and range in geochemistry between volcanic arc and intraplate (syn-collisional) magmas. Tonalitic samples are metaluminous, show less LILE enrichment, no depletion in Ba, Sr, P, and Ti, and lower Ba/Nb ratios than the granitic unit and have relatively high TiO2 (>1.5%).

Other igneous rocks in the WCT suture zone have similar petrographic and geochemical trends and were formed in part by partial melting and assimilation of flysch that forms the shallow crust in the suture zone. The Brusk pluton is unique because it records multiple phases of magma evolution, first of a more primitive tonalitic unit followed by a more highly evolved granitic unit. Magmas that formed the tonalitic unit were likely derived from a combination of enriched mantle and crustal sources; the parental magmas would have been influenced by the last vestige of subduction-processes and by the early stage of a “transitional” style of magmatism related to terrane collision. The granitic magmas may have formed by fractional crystallization from the tonalitic magmas together with crustal assimilation and were then intruded into the tonalitic unit to form the composite pluton.