The tlikakila complex in southern alaska: A suprasubduction-zone ophiolite between the wrangellia composite terrane and north america

The Tlikakila complex is a northeast-striking ∼5-km-wide and ∼75-km-long belt of lower greenschist-facies sedimentary and igneous rocks in the Lake Clark region of south-central Alaska. It forms the only exposures of pre-Cretaceous rocks between the Peninsular terrane and the Farewell terrane. Protoliths include basalt, gabbro, ultramafic rocks, limestone, chert, mudstone, chert-pebble conglomerate, and minor quartz sandstone. Geochemical analyses of igneous rocks indicate primitive island arc compositions. Rare earth element (REE) patterns of the volcanic rocks and gabbro are flat with most elements between 5 and 15 times chondrite values. Initial ⁸⁷ Sr/ ⁸⁶ Sr isotope ratios range from 0.7042 to 0.7065. ε _Nd ranges from +9.3 to +2.6. The complex is older than 192 Ma based on a ⁴⁰ Ar/ ³⁹ Ar date of white mica from a pegmatite vein in a metasedimentary rock and younger than ca. 293 Ma based on the youngest zircon in a chert-pebble conglomerate. Detrital zircon ages suggest source rocks included the Yukon-Tanana and Wrangellia-Alexander terranes. Previously reported Norian conodonts in nearby correlative limestone indicate at least some of the complex is Late Triassic. Metamorphism reached peak temperatures of 350-450 °C based on mineral assemblages in metapelite and metabasite. Three ⁴⁰ Ar/ ³⁹ Ar dates of biotite from metapelite indicate metamorphism at 177 ± 1 Ma. We interpret the Tlikakila complex as a dismembered suprasubductionzone ophiolite that originated near a trench above a north-dipping subduction zone in Late Triassic time. This subduction eventually created the Talkeetna arc. Asecond northdipping subduction zone closed the intervening basin between the Talkeetna arc and the southern Alaska continental margin. Metamorphism and deformation of the Tlikakila complex was coeval with either collision of the arc or shallowing of this slab at ca. 177 Ma.