COEVAL ULTRAMAFIC, CARBONATITIC, AND FELSIC ROCKS ARE ASSOCIATED WITH MAGNETITE DEPOSITS IN THE HUDSON HIGHLANDS OF NEW YORK: A CASE OF MULTIPLE ANATECTIC MELTS

The Hudson Highlands, in southeastern New York, are the site of a lineament-controlled swarm of unusual alkali-bearing peridotites of Late Proterozoic [1009 ± 1.9 to 1001 ± 2 Ma] age, emplaced in upper-amphibolite- to granulite-facies gneissic rocks. The intrusive rocks are coarse-grained, undeformed, and cut their host rocks with a sharp contact. They are very fresh. Clinopyroxene is generally dominant, olivine is a minor phase, and amphibole is primary and interstitial. However, the peridotite is locally amphibole-dominant and pegmatitic. It contains ferroan phlogopite, magnetite and almandine. In some localities, a chlorine-dominant K-rich calcic amphibole is found, whereas elsewhere in the vicinity, fluorine is the dominant halogen. The Greenwood iron mine, in Orange County, is the type locality of fluoro-potassichastingsite, ideally KCa₂(Fe²⁺₄Fe³⁺)Si₆Al₂O₂₂F₂ (Mg# 0.33, 2.23% F, 0.61% Cl). The amphibole is LREE-enriched and isotopically of crustal derivation. Saturation in a monosulfide solid-solution is locally developed. It is a property of such amphiboles to approach the bulk composition of the residual magma after the massive extraction of clinopyroxene. Such an "amphibolic" magma also shows evidence of a buildup in calcite, forming "clots" in which crystals of the amphibole are strikingly euhedral. The relative insolubility of carbonate in a silicate magma suggests the development of globules of carbonate liquid in the ultrabasic magma. Also present are accumulations of magnetite, now largely mined out. The magnetite is poor in Ti and in Cr, and largely unaltered. There is no evidence of concentration of magnetite by gravitational settling. It seems more likely that the silicate magma reached saturation in an iron oxide melt that collected to form a discrete mappable unit. These occurrences are devoid of an obvious hydrothermal overprint that would be expected if mineralization were hydrothermal. Finally, we see evidence of a syenitic magma, possibly formed by anatexis of the gneissic precursor. We contend that conditions of relatively high P and high T promoted complex anatectic reactions in a carbonated fertile lower crust located above a rising asthenospheric mantle during the waning stages of the Grenville event.