PR1- Montana Rox
Matt Crandell
Rock sample number TR07DH6 was collected during summer field camp in 2007
in the Tobacco Root Mountains in Montana. Upon a cursory glance in hand sample, it
appears to be an amphibolite. In hand sample, biotite and hornblende are the primary
minerals present (figs 1, 2).
Upon inspection of the thin section, additional minerals were discovered. The
biotite seen in hand sample was discovered not to be in the center of the rock. Biotite
appears only on the weathered surfaces of the rock. The complete mineral assemblage is
hornblende (60%), an opaque mineral likely to be magnetite (15%), augite (10%), epidote
(5%), plagioclase (5%), and siderite (5%). The hornblende is very prevalent throughout
the thin section, it is subidioblastic and medium to coarse grained (fig 3). The magnetite
is idioblastic and mostly fine grained (fig 4). The augite was subidioblastic and fine
grained (fig 5). Epidote and plagioclase in this sample were sparse and fine grained. A
low first-order interference uniaxial negative mineral was present as well, hesitantly
classified here as siderite (fig 6). These grains were subidioblastic and medium grained.
A more specific name for this rock is epidote-bearing iron-rich amphibolite. The rock
displays a slightly schistose texture defined by marginally aligned amphibole grains. This
is not prevalent throughout the sample, however. Granofels is a more probable
classification.
From these observations, the bulk composition of the rock is amphibolite. Due to
the low levels of plagioclase, it appears to be an epidote amphibolite. This would account
for the low levels of plagioclase. The protolith of this rock was a mafic igneous rock,
most likely basalt or gabbro. Likely sources of this rock are gabbro dikes or basalt
columns of an ophiolite sequence, or possibly a terrigenous flood basalt. Due to the
presence of epidote and the low levels of plagioclase, this rock metamorphosed in the
epidote amphibolite facies. More specifically, about .8 GPa, and ~ 550 oC.
Rock sample number TR07BB07 was collected during summer field camp in
2007 in the Tobacco Root Mountains in Montana. In hand sample, it resembles vein
quartz with a slightly vitreous mass of green crystal (gibbsite?) positioned on the
boundaries of the sample (fig 7).
In thin section, accompanying minerals to quartz and gibbsite were discovered.
The mineral assemblage is 80% quartz, 10% plagioclase, 5% gibbsite, 3% epidote, 2%
titanite. The author currently suffers from less-than-expert thin section production
techniques. Consequently, few images of thin sections of sample TR07BB07 are worth
presenting here. The quartz was very coarse grained and xenoblastic. Plagioclase in the
sample is medium grained and subidioblastic. In thin section, gibbsite’s interference color
is masked or extremely low; additional confusion ensues (fig 8). Epidote was sparse, fine
grained, and subidioblastic. Titanite was fine grained and approaching idioblastic. The
rock has a primarily hornfelsic texture. A more specific name for this sample is gibbsitebearing meta-vein quartz.
From these observations, the bulk composition of this rock is silicic. The most
likely protolith of this rock is a vein of quartz; the accessory minerals forming from
impurities in quartz and possibly fluid movement to incorporate additional minerals.
Ascertaining the P/T conditions of this rock’s metamorphosis is somewhat difficult,
specific ternary diagrams or P/T reaction curves for this simple a rock do not exist.
Judging by the presence of epidote, I can guess that it reached ~.7 Gpa and ~500 °C.
Figure One- TR07DH6 in hand sample.
Figure Two- TR07DH6 close up. Note biotite in lower right.
Figure Three- TR07DH6 in thin section, hornblende in plane light.
Figure Four- TR07DH6 in thin section, magnetite and hornblende in plane light.
Figure Five- TR07DH6 in thin section, augite in plane light.
Figure Six- TR07DH6 in thin section, siderite in plane light.
Figure Seven- TR07BB07 in hand sample, quartz and gibbsite (?).
Figure Eight- TR07BB07 in thin section, gibbsite in plane and crossed polar light.