Protolith Relations of the Gravina Belt and Yukon-Tanana Terrane in Central Southeastern Alaska

• Published in: The Journal of geology Vol. 100; no. 1; pp. 107 - 123
• Main Authors: McClelland, William C., Gehrels, George E., Samson, Scott D., Patchett, P. Jonathan
• Format: Journal Article
• Language: English
• Published: Chicago University of Chicago Press 01-01-1992; University of Chicago, acting through its Press
• Subjects: 580000 - Geosciences; ACTINIDE ISOTOPES; AGE ESTIMATION; ALASKA; Batholiths; CARBONIFEROUS PERIOD; CHEMISTRY; CORRELATIONS; CRETACEOUS PERIOD; DEVELOPED COUNTRIES; DEVONIAN PERIOD; GEOCHEMISTRY; GEOLOGIC AGES; GEOLOGIC FORMATIONS; Geology; GEOSCIENCES; IGNEOUS ROCKS; ISOTOPE DATING; ISOTOPES; JURASSIC PERIOD; LEAD ISOTOPES; MESOZOIC ERA; METAMORPHIC ROCKS; MINERALS; NORTH AMERICA; PALEOZOIC ERA; PETROGENESIS; Protoliths; Quartz; Quartzite; ROCKS; SILICATE MINERALS; Stone; Tectonics; URANIUM ISOTOPES; USA; VOLCANIC REGIONS; VOLCANIC ROCKS; ZIRCON; Alaska
• DOI: 10.1086/629574
• EISSN: 1537-5269
• ISSN: 0022-1376

Metamorphic rocks west of the Coast Mountains batholith in central southeastern Alaska are divided into the Gravina belt, Taku terrane, and newly defined Ruth assemblage. The Ruth assemblage comprises metapelite, quartzose metaclastic strata, quartzite, marble, felsic metatuff, mafic metavolcanic rocks, and orthogneiss. Depositional and emplacement ages of$367 \pm 10 Ma$and$345 \pm 13 Ma$inferred from discordant U/Pb zircon analyses on felsic metatuff and granodioritic orthogneiss, respectively, require that at least portions of the Ruth assemblage be Late Devonian and early Mississippian in age. The assemblage is similar in age and protolith to, and thus correlated with, the Yukon-Tanana terrane. The Gravina belt is characterized by upper Jurassic and lower Cretaceous mafic volcanic rocks and tuffaceous turbiditic clastic strata that unconformably overlie the Alexander terrane. Metamorphic rocks that structurally underlie the Taku terrane and Ruth assemblage are included in this assemblage. Trace element geochemistry and the abundance of pyroclastic flows associated with tuffaceous turbidites suggest that the Gravina belt evolved in an intra-arc basinal setting. In central southeastern Alaska, the mid-Cretaceous structure that currently separates the Ruth assemblage (Yukon-Tanana correlative) from the Gravina belt marks the fundamental boundary between the Alexander-Wrangellia terrane and inboard Yukon-Tanana and Stikine terranes.