The Border Ranges ultramafic and mafic complex: plutonic core of an intraoceanic island arc
Stanford University, Palo Alto, CA
150 p., Illus., Maps
Rasmuson Library: ALASKA QE84 B67 B87 1983a
A discontinuous, elongate zone of mafic and ultramafic plutonic rock crops out in south-central Alaska for a distance of more than 1000 km (fig. 1.1). Intermediate and detailed-scale geologic mapping, petrographic study, and compositional data suggest that the plutonic rocks are compositionally, petrologically, and mineralogically distinct from rocks in mid-ocean ridge and back-arc basin ophiolites. The mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc of probable calc-alkaline affinity. The mafic-ultramafic zone, referred to as the Border Ranges ultramafic and mafic complex, is composed of ultramafic cumulates, gabbronorite cumulates, and massive gabbronorites. A thick sequence of andesitic volcanic rocks, the Talkeetna Formation of Early Jurassic age, lies to the north of and structurally above the mafic-ultramafic zone. Voluminous calc-alkaline plutons composed of quartz diorite, tonalite, and minor granodiorite intrude both the mafic plutonic complexes and the andesitic volcanic rocks. The composition and mineralogy of the ultramafic and mafic rocks in the Border Ranges mafic complex suggest that they represent the fractional crystalization products of the magmas that produced the island arc volcanic rocks. Field relationships indicate that intrusion of the Border Ranges mafix complex was also temporally associated with the intrusion of the quartz diorites, tonalites, and granodiorites.
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