11:45 AM - 12:00 PM
[SCG52-11] Geochemical and isotope constraints on post 1 Ma Yakutat slab tear magmatism in south-central Alaska (USA): the Maclaren River volcanic field and demise of the Wrangell Arc.
★Invited Papers
Keywords:subduction, volcanism, slab tear, shallow slab, Alaska, Yakutat
The relationship between arc magmatism and subducting slab features (e.g., geometry, obliquity, dip, etc.) is key to understanding melt generation in anomalous convergent margin settings such as shallow slab subduction. Shallow slab subduction is characterized by <30° subduction angles and often results in the near cessation of arc magmatism, yet significant arc magmatism can be generated along the edge and along tears of the shallow subducting slab. Also, the processes that influence magmatism and the distribution of volcanism at slab edge environments, are unclear. Therefore, studying the magmatic processes along shallow slab edges and tears can further constrain how magma is formed and erupts in generally amagmatic settings. The Maclaren River volcanic field (MRVF) of south-central Alaska consists of at least three monogenetic volcanoes that lie along the projection of the ca. ~1 Ma Yakutat slab tear. These volcanoes, which are <1 Ma, lie along a ~20 km south-north transect within the Denali Gap, a region between the Aleutian and Wrangell Arcs previously considered to be amagmatic. MRVF eruptive products (e.g., lavas and near-vent pyroclastic deposits) range from alkali basalt to subalkaline basaltic andesite and transitional basaltic trachyandesite/trachyandesite (~46-57 wt% SiO2). The studied volcanoes are named #1, #2, #3, from north to south. Volcano #1 (ca. 958 ka) lavas are porphyritic and contain olivine phenocrysts in a groundmass of clinopyroxene + plagioclase ± sparse biotite microlites. Volcano #2 lavas and scoria are porphyritic and their mineralogy suggests a more hydrated mantle source (abundant phlogopite + amphibole + olivine + clinopyroxene phenocrysts) and show evidence of minor crustal interaction (granitoid upper-crust xenoliths). Volcano #3 is located ~9 km south of volcano #2 but erupted coevally at ca. 422 ka; the rocks consist of olivine + clinopyroxene + plagioclase + opacitic amphibole and show evidence of minor crustal interaction (plagioclase + quartz xenocrysts). Sr-Nd-Pb isotopic analysis from a subset of MRVF samples, yield 87Sr/86Sri values that range from 0.70343-0.70401 and are like modern Aleutian and Wrangell Arc magmas and the Holocene, Buzzard Creek maars. This similarity is also borne out by Nd-Pb isotope data. All MRVF volcanoes display primitive mantle-like δ26Mg values (-0.20 to -0.29). Volcanoes #1 and #2 display depleted mantle-like δ11B values, while volcano #3 has a more positive δ11B, suggesting influence from altered oceanic crust. We suggest the MRVF involves magma production in a subduction-affected environment, along the geophysically imaged, nascent Yakutat slab tear (e.g., LILE enrichments, HFSE depletions, and adakite-like bulk rock chemistries). Batch melt modeling indicate three mantle sources for MRVF magmas: (1) a mantle wedge metasomatized by slab-derived sediment melt (volcano #1); (2) subcontinental lithospheric mantle also metasomatized by a slab-derived sediment melt (volcano #2); and (3) an eclogitic slab, where slab melts interacted with mantle peridotite (volcano #3). This study provides further constraints on magma generation and mantle upwelling processes along shallow slab tear/edge environments, as well as the tectonomagmatic processes that occurred at ~1 Ma in south-central Alaska, when the collision of the thickest (~30 km) segment of the Yakutat microplate occurred, jamming the trench, causing the slab tear, and initiating the demise of the adjacent Wrangell Arc.