Contrasting plutonic rock suites of the Yukon‐Koyukuk Basin and the Ruby geanticline, Alaska
Authors: Miller Thomas P. 
Published: February 2008

The volcanogenic Yukon‐Koyukuk basin (YKB) and the Ruby geanticline (RG), a fault‐bounded miogeanticlinal assemblage of metamorphic‐plutonic rocks, are adjacent geologic provinces whose origins and mutual relationships have been enigmatic. The petrogenesis of the Cretaceous plutonic rock suites that occur in both provinces is a possible key to deciphering the complex regional geology and to placing constraints on the nature of the underlying crust and mantle. YKB plutons consist of two suites of distinctive compositions, age, and distribution forming opposite ends of a 500‐km‐long belt intruded into andesitic volcanic rocks of the Koyukuk terrane, a subduction‐related intraoceanic arc of Jurassic and Early Cretaceous age that collided with continental North America in Early Cretaceous time. Plutons in the western YKB yield mid‐Cretaceous K‐Ar ages of 99–113 Ma, have a high‐K2O character and consist of Si‐saturated to oversaturated monzonite, syenite, quartz syenite, and biotite granite with a considerable range in SiO2. Associated with the western YKB plutonic suite is an ultrapotassic rock province consisting of at least 12 intrusive complexes and dike swarms of feldspathoid‐bearing hypersolvus plutonic rocks. Their most striking chemical characteristic is consistently high K2O (to 16.6%), a K2O/Na2O ratio ≫ 1, and <60% SiO2. The western YKB plutonic suite extends into the southeastern Seward Peninsula, St. Lawrence Island, and eastern Siberia. Eastern YKB plutons in contrast form an epizonal, compositionally expanded calcalkaline suite ranging from tonalite to granite and characterized by high Na2O/K2O and Fe2O3/FeO ratios, abundant hornblende, mafic inclusions, and coeval volcanic rocks. They yield K‐Ar ages of 78–89 Ma, indicating a Late Cretaceous age of emplacement. The northeast trending Ruby geanticline contains the largest suite of plutonic rocks in west central Alaska. The plutons are composed chiefly of potassic, weakly to moderately peraluminous, hornblende‐free, ilmenite series biotite granite, two‐mica granite, and granodiorite. The plutons yield late Early Cretaceous K‐Ar and U‐Pb ages of 99–112 Ma and intrude (1) Precambrian‐Paleozoic greenschist facies metasedimentary rocks of the Ruby terrane, and (2) a thrust‐faulted sequence of mafic oceanic rocks of late Paleozoic to Jurassic age (the composite Angayucham‐Tozitna terrane). The compositional differences between the YKB plutonic suites suggests fundamental differences in source materials. The composition and geologic setting of the eastern YKB plutonic suite is typical of plutons formed by melting of mantle material or andesitic volcanic supracrustal material derived from the mantle with little involvement of continental crust or lithosphere. The western YKB plutonic suite extends into the continental rocks of the eastern Seward Peninsula, indicating similar source material beneath both provinces, possibly of a subcontinental character. This subcontinental mantle under the western YKB may wedge out to the east since K2O content of YKB plutonic rocks gradually decreases eastward across the basin. The RG plutonic suite differs from the adjacent eastern YKB plutonic suite in composition and age. The composition, geologic setting, and volume of the RG plutons indicates an origin by melting of continental crust material. The magmatic processes that caused melting may have resulted from crustal thickening, postcollisional relaxation following arc‐continent collision and suturing, or underthrusting of the Koyukuk terrane beneath the RG.