PETROGENESIS, RARE EARTH ELEMENT CONTENTS, AND TECTONIC IMPLICATIONS OF A LATE DEVONIAN ALKALINE INTRUSION, SOUTHEAST ALASKA

The Corner Bay pluton is part of the Sitkoh alkaline complex that locally has high rare earth element (REE) contents on Chichagof Island in southeast Alaska. The alkaline complex is part of the Alexander terrane, which is composed in part of Neoproterozoic and Paleozoic metavolcanic and metasedimentary rocks that have previously documented oceanic arc geochemical and isotopic signatures. Late Silurian-Early Devonian juxtaposition of oceanic arcs resulted in an orogeny with associated metamorphism, calcalkaline plutons, pillow basalt, conglomerate, and redbeds. This orogenic belt hosts Upper Devonian alkaline basalt and rhyolite dike swarms, flows, and the Sitkoh alkaline complex.

The Corner Bay pluton is composed of amphibole-biotite metaluminous syenite, monzonite and minor gabbro, and has a ~367 Ma U-Pb zircon age. These rocks have 46 to 63 weight percent silica, and high alkali and large-ion-lithophile element (Rb, Sr, Ba, U) contents. Main accessory minerals include apatite, fluorapatite, titanite and allanite that contain elevated concentrations of REE. The rocks have relatively uniform Ɛ_Nd(t) values (+3.4 to +3.6), Neoproterozoic Nd model ages of 650-700 Ma, and initial ⁸⁷Sr/⁸⁶Sr ratios of ~0.704. We interpret generation of the intrusive rocks by fractional crystallization of an alkali basaltic parent magma formed by partial melting of an amphibole-phlogopite-bearing peridotite of the sub-arc lithospheric mantle. Geothermobarometers document emplacement of the parental magma at mid-crustal levels (8.5-10 kb/31-37 km) and polybaric crystallization from ~ 1,200°C to solidus. Crystallization of amphibole took place at ~880-980°C and 2.4-5.5 kb under hydrous conditions. Zircon, rutile, and apatite provide consistent saturation temperatures at ~800°C.

The chemical and isotopic composition of the Corner Bay pluton is consistent with emplacement in a rift. Nd isotopic data suggest the source of mafic rocks was Neoproterozoic sub-arc lithospheric mantle with no contributions from juvenile Devonian mantle, implying that rifting was of limited extent and sub-arc lithospheric mantle was unaffected by upwelling juvenile asthenosphere beneath the rift. The Late Devonian alkaline belt that contains the Corner Bay pluton is part of a metallogenic province of rare metals in the Alexander terrane.