THE NORTHWESTERN EDGE OF WRANGELLIA: STRATIGRAPHIC AND INTRUSIVE LINKS BETWEEN CRUSTAL BLOCKS FROM THE PENINSULAR TERRANE TO BROAD PASS

Geologic mapping of a transect in the northern Talkeetna Mtns. has redefined relationships between Wrangellia and its outboard (Peninsular) and inboard (Kahiltna, Susitna) terranes. Stratigraphic and intrusive ties between structural blocks suggest a more closely shared geologic history than previously interpreted,

Wrangellia in the Talkeetna Mtns. includes Middle-Late Triassic Nikolai basalt and co-eval dolerite sills. Nikolai dolerites intrude a km-thick continental-margin sedimentary sequence of Mississippian to Triassic siliceous siltstones with lesser fossiliferous limestone and rare quartz-pebble conglomerate. Northwest of the exposed Nikolai, Jurassic to Cretaceous flysch derived from Wrangellia sources was deposited in apparent conformity over Late Triassic to ?Early Jurassic basalt and limestone (the former "Susitna" terrane). This timing suggests that collision of Wrangellia with the continental margin was initiated shortly after deposition of the Nikolai. Pre- Late Triassic sediments below the flysch are siliceous mudstones and quartz sandstones; wall rock pendants in Paleocene plutons derived from melted flysch have quartzose protolith compositions. The abundance of silica in all Paleozoic to Triassic sedimentary rocks of the northern Talkeetna Mountains and the siliceous character of the Jurassic to Cretaceous flysch suggests that the segment of Wrangellia exposed in the Talkeetna Mountains was not far from a continental silica source before or after deposition of the Nikolai Greenstone.

No subduction-related plutonism is preserved in front of the leading edge of Wrangellia. However, most Nikolai mafic rocks have been thermally overprinted in the Middle Jurassic (reset hornblende Ar^40/39ages of 156-171Ma). The overprint is identical in age (168-150Ma Ar^40/39) to a composite diorite-granodiorite-tonalite batholith previously assigned to the Peninsular terrane. This Middle Jurassic plutonism along the outboard edge of Wrangellia indicates the development of a subduction zone below Wrangellia early in its collision history. An early (mid Jurassic) Wrangellia collision is compatible with recent work by Ridgway and others which indicates that deformation of flysch shed northwestward from Wrangellia was complete by early to mid-Cretaceous time.