AMPHIBOLE-GROUP MINERALS IN THE GRENVILLE ROCKS OF ADIRONDACK MOUNTAINS AND HUDSON HIGHLANDS, NEW YORK

The main goal of EMPA systematic study of hundreds of amphiboles from the Adirondack Mountains (AM) and Hudson Highlands (HH) was to contribute to the knowledge of the naturally occurring amphibole compositions in different geological environments and to identify potential new end members of this group of minerals. An additional goal was to evaluate the fluid composition, the influence of halogens on the amphiboles stability and their geological significance in different units such as marbles, siliceous marbles, Mn-rich siliceous marbles, pegmatites and iron deposits.

Geological distribution. The amphiboles from marbles and siliceous marbles are generally F-rich/-dominant members of the Group 2 (calcic amphiboles). The Mn-rich siliceous marbles contain species of the Group 1 (Mg-Fe-Mn-Li), fluorescent and Mn-bearing amphiboles of the Group 2, and Group 5 (Na-Ca-Mg-Fe-Mn-Li) amphiboles. The amphiboles from the AM iron deposits are generally F-rich/-dominant species. In the HH iron deposits the amphiboles are K-rich/-dominant and Cl-rich or F-dominant members.

Geological significance. Substitution of F for OH in amphiboles is thought to contribute to their higher thermal stability by replacing Mg-O bonds with shorter and stronger Mg-F bonds stabilizing the amphibole to higher temperatures and pressures that characterize the upper amphibolite to granulite facies. The local bulk rock and fluid compositions are compatible with the occurrence of the Mn-bearing amphibole pods. The amphiboles from the iron deposits are the result of a regional alkali - potassium metasomatism. The newly formed products were probably generated from two compositionally distinct fluids. The formation of the K, Cl or F-rich amphiboles can be modeled assuming that initially halogen-bearing crustal fluids infiltrated the rocks and ores. These fluids subsequently replaced the Fe-rich and moderate high IVAl (hastingsite sensu lato) amphiboles, and / or by hydration reactions with pyroxenes and Fe - Ti oxides, formed K and halogen-rich amphiboles. The mineral composition and the Fe3+/Fe2+ ratio suggest a relatively low f(O2) for the fluids that generated the amphiboles in the HH iron deposits and more oxidized fluids in the AM iron deposits. The source of both halogens is not clear, but certain hyper saline brine could be considered.