Garnet within Qbt

Rick Warren and Fraser Goff

Retired, Los Alamos National Laboratory

Thin sections for SWNVF

Garnet habits in volcanics

Grain shown is largest found, .091 mm (91,000 square microns)

SWNVF: GT in 60 of 313 thin sectionsJVF: GT in 10 of 76 thin sections

Garnet areas in thin section

SWNVF only131 grains

Stratigraphic units and lithologies

• All stratigraphic assemblages for SWNVF

• From base to top of Qbt for JVF

• Fractions of each lithology for SWNVF:– 62% are nonwelded tuff– 15% are welded tuff– 12% are alluvia– 7% are vitrophyric tuff– 3% are rhyolite lava

Garnet in Qbt(10 of 67 thin sections)

Subunit (location)

Thin sections

TS with GT

ppmV GT-bearing Lithics

ppmV GT in MX

5 (Plateau) 12 2 61 1.3

4u (Vent) 2 2 21300 235

4u (Other Valles)

10 4 3120 0

l (Valles) 4 2 0 1.3

Garnet in sandstone lithics

Garnet necklaces in argillite lithics

BSE image of argillite lithics

Garnet compositions

Origin of garnet in volcanics

• Necklaces require pneumatolytic origin

• Carbonates completely absent

• Argillites can provide Ca, Fe for andradite

• Andradite also forms in other host lithics

• Almandines likely detrital

• Prevalence in SWNVF suggests that pneumatolytic andradite/grossular may commonly form in rhyolite vents