GEOCHEMICAL ANALYSES OF AND TECTONIC SETTING FOR TWO MAFIC INTRUSIVE BODIES IN THE NORTHEASTERN ADIRONDACK MOUNTAINS

Mafic through felsic intrusive rocks are common throughout eastern North America that have been associated with one of several periods superplume and/or rifting events along the eastern margin of Laurentia including Mid-Vendian(615-564 Ma), Late Vendian (554-550 Ma), early Paleozoic and Mesozoic ENA suites. The mid-Vendian, late-Vendian and early Paleozoic occurrences are classified as continental flood basalts (CFB), Laurentian ocean island basalts (LOIB) and normal mid-oceanic ridge basalts (MORB), respectively (Puffer 2002). This research is focused on two such mafic intrusive bodies located near Whitehall, New York with the goal of using geochemistry to classify and assign these rocks to one or more of the known periods of igneous activity. In the field, these mafic bodies intrude into Grenville-aged granulite facies metamorphic units and cannot be traced into the younger (Cambro-Ordovician /560-445 Ma) sedimentary cover. The mafic bodies consist of a narrow (3m wide) dike and a small (50m? thick) sill. The two differ both in texture (sill is gabbroic/ dike is diabasic) and in metamorphic/deformational overprint. The diabase shows no metamorphic or deformational overprint whereas the gabbro records weak deformational and mineralogic (epidote-amphibolite facies) overprints. Overall, the gabbro preserves igneous textures and plagioclase laths show igneous chemical zoning. Deformation of the gabbro consists of kinked plagioclase laths and grain boundary mortoring of plagioclase and pyroxene. The metamorphic overprint is limited to amphibole+biotite rims on pyroxene (cpx +opx) and opaque oxides, and epidote +white mica after plagioclase. The diabase records only low temperature alteration. Bulk, trace element (including REE) analyses of these two mafic units clearly show the diabase and the gabbro originated from different source magmas yet both are classified as EMORB CFB. REE and SPIDER analyses show that the gabbro is most similar to published analyses for the Mid-Vendian mafic suite. The low-grade metamorphic and deformational overprints observed are most likely Taconic in origin as this region is located along the Taconic Front. REE and SPIDER analyses of the diabase correlate well with published analyses for the ENA Mesozoic suite.