PaleoProterozoic deformation, metamorphism, and 40Ar/39Ar thermal history of the 1.65 GA Manzanita Pluton, Manzanita Mountains, New Mexico
Cynthia L. Brown, Karl E. Karlstrom, Matthew Heizler, and Dan Unruh

Abstract:

The 1.65-Ga Manzanita pluton and its aureole record an interaction of Paleoproterozoic plutonism, deformation, and The 1.65-Ga Manzanita pluton and its aureole record an interaction of Paleoproterozoic plutonism, deformation, and metamorphism in New Mexico. Field observations and microscopic analysis indicate that the pluton was emplaced during regional NW–SE shortening. Synchronous plutonism and contact metamorphism are indicated by an increase in temperature from regional metamorphic greenschist grade rocks to contact metamorphic amphibolite grade rocks. Peak contact metamorphic temperatures and pressures of 600-620°C and 2-3 kbar are shown by assemblages with Fe-rich andalusite + K-feldspar + biotite + quartz + white mica + oxides. Synchronous deformation and metamorphism is indicated by contact metamorphic mineral-matrix relationships including the growth of garnet before, during, and after development of the regional S2 foliation, sillimanite that is both randomly oriented and aligned along S2, and hornblende that is dynamically recrystallized along S2. Synchronous plutonism and regional deformation is indicated by parallel magmatic and solid-state fabrics, dynamic recrystallization of feldspar indicating high-T solid-state deformation, variably deformed dikes and veins that cross-cut, and are also folded by, regional foliation, and dike orientations consistent with a regional kinematic framework of NW–SE shortening. New radiometric dates constrain the timing of synpluton tectonism. The Manzanita pluton has been dated by U-Pb-zircon techniques at 1645 ± 16 Ma. In addition, a post-S2 40Ar/39Ar rhyolite dike to the south of pluton cooled through 300°C at 1428 ± 2.0 Ma, determined through 40Ar/39Ar techniques. Hornblende in the contact aureole of the pluton yields variable 40Ar/39Ar ages of circa 1.4 Ga and 1.67 Ga, interpreted as hornblende growth at 1.65 Ga and reheating at 1.4 Ga to 400-500°C.metamorphism in New Mexico. Field observations and microscopic analysis indicate that the pluton was emplaced during regional NW–SE shortening. Synchronous plutonism and contact metamorphism are indicated by an increase in temperature from regional metamorphic greenschist grade rocks to contact metamorphic amphibolite grade rocks. Peak contact metamorphic temperatures and pressures of 600-620°C and 2-3 kbar are shown by assemblages with Fe-rich andalusite + K-feldspar + biotite + quartz + white mica + oxides. Synchronous deformation and metamorphism is indicated by contact metamorphic mineral-matrix relationships including the growth of garnet before, during, and after development of the regional S2 foliation, sillimanite that is both randomly oriented and aligned along S2, and hornblende that is dynamically recrystallized along S2. Synchronous plutonism and regional deformation is indicated by parallel magmatic and solid-state fabrics, dynamic recrystallization of feldspar indicating high-T solid-state deformation, variably deformed dikes and veins that cross-cut, and are also folded by, regional foliation, and dike orientations consistent with a regional kinematic framework of NW–SE shortening. New radiometric dates constrain the timing of synpluton tectonism. The Manzanita pluton has been dated by U-Pb-zircon techniques at 1645 ± 16 Ma. In addition, a post-S2 40Ar/39Ar rhyolite dike to the south of pluton cooled through 300°C at 1428 ± 2.0 Ma, determined through 40Ar/39Ar techniques. Hornblende in the contact aureole of the pluton yields variable 40Ar/39Ar ages of circa 1.4 Ga and 1.67 Ga, interpreted as hornblende growth at 1.65 Ga and reheating at 1.4 Ga to 400-500°C.


Citation:

  1. Brown, Cynthia L.; Karlstrom, Karl E.; Heizler, Matthew; Unruh, Dan, 1999, PaleoProterozoic deformation, metamorphism, and 40Ar/39Ar thermal history of the 1.65 GA Manzanita Pluton, Manzanita Mountains, New Mexico, in: Albuquerque Country, Pazzaglia, Frank J.; Lucas, S. G.; Austin, G. S., New Mexico Geological Society, Guidebook, 50th Field Conference, pp. 255-268.

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