Date post: | 18-Jan-2016 |
Category: |
Documents |
Upload: | randolf-day |
View: | 217 times |
Download: | 0 times |
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