Gore Mountain Garnet
| Gore Mountain Garnet | |
|---|---|
| Type | Geologic Formation |
| Lithology | |
| Primary | Garnet Amphibolite |
| Location | |
| Region | Adirondack Mountains |
| Country | United States |
| Extent | New York |
| Type section | |
| Named for | Gore Mountain |
 Geologic Map of Gore Mountain and the surrounding area. | |
Gore Mountain Garnet, found in the
Geography
The Gore Mountain Garnet outcrop is located in the south central Adirondack mountains in northern New York.[1][3] The mountains themselves, which are part of the Canadian Grenville Province, are composed of three main parts, the first of which is a dome of anorthosite.[2][3] This dome underlies a mantle of syenite gneiss, which in turn underlies a metasedimentary sequence of marble, quartzite, amphibolite, and other types of gneiss.[3] The rock formation that Gore Mountain is famous for, the garnet amphibolite, is located 2600 ft up its northern slope.[4] Said rock outcrop measures about 50 m by 600 m and trends east-west, grading into garnet-bearing meta-gabbro to the east.[1][5] To the south of the garnet amphibolite is a fault that runs parallel to the contact line of a meta-syenite formation.[5] The garnet amphibolite also borders the southern margin of an olivine meta-gabbro formation.[5]
Geological Overview
The garnet amphibolite in Gore Mountain formed during the Ottawan Orogeny via metamorphism.[1] Past studies suggested that the growth of garnets was aided by magma intrusion and partial melting, but that hypothesis has since been disproven by experiments.[4] The abnormal size of the garnets is attributed to high temperatures and fluid flow introduced by faults.[1][2]
Metamorphism
The protolith is a
Aside from the introduction of fluids, metamorphic transformation was isochemical as evidenced by the homogeneity of the garnets.[3] Having all chemical constituents continuously available and constant pressure and temperature conditions during formation is most likely what allowed isochemical transformation and homogeneity in the garnets.[5] The garnets underwent two growth phases and one reabsorption phase.[8] The first growth phase was homogenous, while the second phase was marked by the enrichment of yttrium and titanium in the garnets.[8] In the reabsorption phase, garnets partitioned yttrium but were depleted in titanium.[8] Two periods of cooling occurred, a fast one (91 °C/Ma) and a slow one (26 °C/Ma).[1] Advection dominated cooling happened during topography driven lower crustal flow, while conduction dominated cooling happened one flow stopped.[1]
Petrological description
The rock formation that bears the large garnets in Gore Mountain is an amphibolite that has been enriched in yttrium and lithium.[1][8][9]
Mineralogy
Minerals in the garnet amphibolite include:[5]
There are some minor cases of sulfides, and there are no olivines.[4]
The garnets in Gore Mountain are abnormally large, with the largest being 1 m in diameter.[1] They have a hardness of 8-9 on the Mohs scale and an average density of 3.95 gm/cm3.[5]
The composition of the garnets are as follows:[5]
- 43% pyrope
- 40% almandine
- 14% grossular
- 2% andradite
- 1% spessartine
Trace element geochemistry
The garnet amphibolite is strongly enriched in
Trends in the garnet amphibolite include:
- Hornblende and biotite growing bigger with the garnets[5]
- Manganese concentrations increasing towards the outer rim of garnets due to reabsorption[1]
- Enrichment of yttrium and titanium during garnet growth[8]
- Depletion of titanium and enrichment of yttrium during garnet reabsorption[8]
- Overall enrichment of lithium in the whole rock and overall depletion of copper, cesium, thorium, and uranium[9]
- Green-spinel included plagioclase becoming white plagioclase with no inclusions.[5]
Structures
The garnets bear the inclusions in this rock unit, which range from minerals such as acicular rutile, pyrite, plagioclase, pyroxene, hornblende, ilmenite, apatite, and biotite.[5] The most common of these is the acicular rutile.[5] Also shown in the garnets is well-developed tectonic parting.[5]
Viewing the garnet amphibolite as a whole, there is a fault to the south that runs parallel to the contact with a meta-syenite formation.[5] Lineation in the garnet amphibolite is formed by the parallel alignment of hornblende, elongate mafic and felsic minerals, plagioclase pressure shadows, and few instances of elongate garnet.[5]
References
- ^ .
- ^ .
- ^ ISSN 0361-0128.
- ^ a b c Axler, Jen (2011). "The Formation of Large Garnets at Gore Mountain, New York: Experimental partial melting of amphibolite and meta-gabbro" (PDF) – via Department of Geosciences of Smith College.
{{cite journal}}: Cite journal requires|journal=(help) - ^ a b c d e f g h i j k l m n Kelly, William (2002). "Geology and mining history of the Barton garnet mine, Gore Mt. and the NL ilmenite mine, Tahawus, NY with a temporal excursion to the MacIntyre iron plantation of 1857" (PDF). New England Intercollegiate Conference 94th Annual Meeting and New York State Geological Association 74th Annual Meeting Field Trip Guidebook. B: 1–B3.
- ^ S2CID 129087018.
- ^ a b Morgan, Elizabeth R., "Geochemical and Petrographic Analysis of Gore Mountain Garnets, Adirondacks NY" (2011). Honors Theses. 1036. https://digitalworks.union.edu/theses/1036
- ^ a b c d e f Denny, Alden. "Determination of the Duration of Retrograde Metamorphism at Gore Mountain and Ruby Mountain, NY."
- ^ a b c Hollocher, Kurt, et al. "Petrology of Big Garnet Amphibolites, North Creek–Warrensburg Area, Adirondacks, NY." Geological Society of America Abstracts with Programs,. Vol. 40. No. 2. 2008.