The Middle Fork Plutonic Complex: a plutonic association of coeval peralkaline and metaluminous magmas in the north-central Alaska Range

Abstract

The 57 m.y. Middle Fork Plutonic Complex (M FPC) intrudes Paleozoic metasedimentary rocks south of the Farewell Fault zone in the north-central Alaska Range. Though spatially related to the late Cretaceous - Early Tertiary subduction-related Alaska Range batholith, MFPC is more characteristic of an extensional or anorogenic setting. A swarm of basalt, hawaiite and rhyolite dikes east of the complex intruded, and was intruded by, the plutonic rocks. Approximately 30% of the exposed rock in the 125 km² complex is hedenbergite - fayalite syenite, ≃20% is peralkaline arfvedsonite - biotite alkali-feldspar granite (AF granite), and ≤20% is pyroxene - olivine - biotite gabbro. The rest is a mixed unit including clinopyroxene - biotite - amphibole diorite, and hornblende - biotite granite (HB granite). K-Ar and Rb-Sr radiometric dating of rock types shows that they are coeval. Their close spatial and temporal relationships led to complex magmatic interactions. Calculated initial ⁸⁷Sr/⁸⁶Sr for gabbro and diorite group around 0.705 to 0.706. HB granites are heterogeneous, but fall mostly around 0.707 to 0.708. Hypersolvus syenites and AF granites form an isochron with initial ⁸⁷Sr/⁸⁶Sr of 0.70965. These groupings suggest that at least three different magmas formed the MFPC; scatter of isotopic data reflects mu- tual contamination and assimilation. Trends in whole rock and mineral chemistry also reflect interactions between the magmas. Consanguinous hypersolvus syenite and AF granite mineralogy appears to be controlled by fluorine in the magma chamber. Magmatic mineral assemblages reflect increasingly reduced magmatic conditions; subsolidus oxidation, indicated by mafic mineralogy and cathodoluminescence, is due to magmatic process rather than introduction of external components. Eruptive stratigraphy, as predicted by intrusive history of MFPC, compares favorably with volcanic stratigraphies of peralkaline volcanic systems worldwide, and MFPC may be modelled as the root zone of a peralkaline volcanic system. The common absence of mafic volcanism during peralkaline volcanic activity may be due to entrapment of mafic magmas within the peralkaline magma chamber. The result of this entrapment is seen at the present level of erosion of MFPC, as extensive mafic enclave swarms and pull-apart dikes within syenites in the pluton's core.