Reconstruction of the Talkeetna intraoceanic arc of Alaska through thermobarometry

[1] The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper mantle through the volcanic carapace is well exposed. We reconstruct the vertical section of the Talkeetna arc by determining the (re)crystallization pressures at various structural levels. The thermobarometry shows that the tonalites and quartz diorites intruded at 5–9 km into a volcanic section estimated from stratigraphy to be 7 km thick. The shallowest, Tazlina and Barnette, gabbros crystallized at 17–24 km; the Klanelneechena Klippe crystallized at 24–26 km; and the base of the arc crystallized at 35 km depth. The arc had a volcanic:plutonic ratio of 1:3–1:4. However, many or most of the felsic plutonic rocks may represent crystallized liquids rather than cumulates so that the liquid:cumulate ratio might be 1:2 or larger. The current 5to 7-km structural thickness of the plutonic section of the arc is 15–30% of the original 23to 28-km thickness. The bulk composition of the original Talkeetna arc section was 51–58 wt % SiO2.