Gore Mountain Garnet

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]

Outcrop of garnet amphibolite from Gore Mountain with scale.

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]

The protolith is a spinel-clouded plagioclase metamorphosed gabbro (metagabbro).[1] Metamorphism took place during the Ottawan phase of the Grenville orogeny, when crustal thickening was occurring.[1] The peak temperature of metamorphism was notably high, being over 900 °C.[1] Peak pressure was also quite high, ranging from 9-10 kbar.[1] The metamorphosis of the protolith into garnet amphibolite is marked by orogenic folding and plastic flow, as well as a hydration phase.[3] Garnet growth, introduction of fluid, retrograde metamorphism, and ductile deformation all occurred simultaneously.[6] The fluid flow was likely channelized, as evidence from the large variability in trace elements.[7] Fluid flow induced ion mobility and plasticity, as well as enriching the whole rock chemistry with lithium and thorium and depleting cesium.[6][7]

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]

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]

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]