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Lindline, Jennifer, 1997

Magmatic history of the southeastern Petersburg Quadrangle, Alaska

Bibliographic Reference

Lindline, Jennifer, 1997, Magmatic history of the southeastern Petersburg Quadrangle, Alaska: Bryn Mawr College, Ph.D. dissertation, 214 p.


Petrographic data, major-, trace-, and rare-earth-element analyses, and strontium isotope information were gathered for igneous rocks emplaced into Alexander terrane and Gravina belt rocks of the Petersburg Quadrangle (latitude 56 degrees 00 minutes to 56 degrees 30 minutes; longitude -132 degrees 00 minutes to -133 degrees 00 minutes) in an effort to understand the origin of compositional diversity displayed in the magmatic rocks. The study area contains three magmatic suites that record distinct post-terrane accretion magmatic episodes. (1) The 110-100 Ma Blashke Island ultramafic complex includes gabbro and ultramafic rock types. The mineralogical and chemical variation in the complex was controlled largely by olivine and clinopyroxene fractionation. Trace- and rare-earth-element characteristics suggest origin of the ultramafic complex in a island arc or back arc basin setting. (2) The 100-89 Ma epidote-bearing tonalite and granodiorite plutons are low in potassium (average 2.91 weight % K2O), high in alumina (>15 weight % Al2O3), and display a range of silica values (46.80-68.90 weight % SiO2). Major- and trace-element characteristics suggest a mixed basalt-high silica magma genesis and differentiation by hornblende and plagioclase fractionation. REE patterns suggest garnet, which is stable at pressures >22 kbar, was a residual phase in the source. A two-stage crystallization history is suggested by the presence of magmatic garnet and magmatic epidote (intermediate pressure crystallization) in the plutonic rocks and andalusite (shallow level emplacement) in the contact aureole. (3) A 20 Ma granite-gabbro complex was emplaced into the Gravina belt. Despite the common pillowing of basalt in the granite and the intrusion of composite dikes in the gabbro, the granite and gabbro rocks evolved along separate lines of differentiation with little geochemical evidence for magma mixing. Overlapping biotite and plagioclase compositions, and similar K2O and Rb content between basalt-granite couples indicates local chemical interaction between the magma types. The REE characteristics of the 20 Ma granite rocks indicate partial melting of crustal rocks at pressures in the stability field of plagioclase (<15 kbar). The change from high-pressure garnet-bearing sources for the 90 Ma tonalitic magmas to lower pressure plagioclase-dominated sources for the 20 Ma granite is tied to a change from a compressive to a transpressive tectonic regime.

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