Abstract
Silicocarbonate pegmatites from the southern Grenville Province have provided exceptionally large crystal specimens for more than a century. Their mineral parageneses include euhedral calc–silicate minerals such as amphibole, clinopyroxene, and scapolite within a calcite matrix. Crystals can reach a meter or more in long dimension. Minor and locally abundant phases reflect local bedrock compositions and include albite, apatite, perthitic microcline, phlogopite, zircon, tourmaline, titanite, danburite, uraninite, sulfides, and many other minerals. Across the Adirondack Region, individual exposures are of limited aerial extent (<10,000 m2), crosscut metasedimentary rocks, especially calc–silicate gneisses and marbles, are undeformed and are spatially and temporally associated with granitic pegmatites. Zircon U–Pb results include both Shawinigan (circa 1165 Ma) and Ottawan (circa 1050 Ma) intrusion ages, separated by the Carthage-Colton shear zone. Those of Shawinigan age (Lowlands) correspond with the timing of voluminous A-type granitic magmatism, whereas Ottawan ages (Highlands) are temporally related to orogenic collapse, voluminous leucogranite and granitic pegmatite intrusion, iron and garnet ore development, and pervasive localized hydrothermal alteration. Inherited zircon, where present, reflects the broad range of igneous and detrital ages of surrounding rocks. Carbon and oxygen isotopic ratios from calcite plot within a restricted field away from igneous carbonatite values to those of typical sedimentary carbonates and local marbles. Collectively, these exposures represent a continuum between vein-dyke and skarn occurrences involving the anatexis of metasedimentary country rocks. Those of Ottawan age can be tied to movement and fluid flow along structures accommodating orogenic collapse, particularly the Carthage-Colton shear zone.
Highlights
Introduction and Geological SettingThe southern Grenville Province (Figure 1) contains important mineral localities that have produced exceptional crystal specimens that grace exhibits in many natural history and mineral museums, as well as private collections worldwide
We find both Shawinigan and Ottawan crystallization ages, generally separated by the Highlands-Lowlands boundary (CCsz; Figure 2), and conclude that the occurrences represent a continuum between classic skarn mineralization and vein-dykes
Based on the data available, inherited zircon xenocrysts, we suggest that the silicocarbonate occurrences in the Adirondacks represent emplacement of melts formed by localized anatexis of calc–silicate and marble-rich lithologies of the Grenville Supergroup [7]
Summary
Introduction and Geological SettingThe southern Grenville Province (Figure 1) contains important mineral localities that have produced exceptional crystal specimens that grace exhibits in many natural history and mineral museums, as well as private collections worldwide. A unique subset of these localities, termed “vein-dykes” [1], are primarily located in the Central Metasedimentary Belt of Ontario and Quebec, with similar occurrences in the Adirondack Mountains of northern New York (Figure 1). Whereas these localities are well-known to collectors and mineral enthusiasts, they have received relatively little attention from geologists despite the potential constraints they offer on the tectonic evolution of the region [2,3]. Numerous origins have been proposed for the vein-dykes including as skarns, anatexis of marble/calc–silicate country rock, carbonatites, and silicocarbonate melts of crustal or mantle origin, and are summarized in Joyce [4]
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