GEOCHEMICAL COMPARISON OF METAIGNEOUS ROCKS IN THE ADIRONDACK HIGHLANDS AND LOWLANDS: DIFFERENT ROCKS, DIFFERENT TERRANES

Statistical analysis of over 1850 chemical analyses of Adirondack metaigneous rocks confirms the existence of numerous distinct groups in both the Highlands and the Northwest Lowlands. Lowlands groups in general are less fractionated than those of the Highlands; none are clearly identifiable with any of the Highlands groups. The metaigneous suites provide no evidence for a common history for the two terranes prior to juxtaposition following the Ottawan orogeny.

Most Highlands granitoids are chemically and lithologically similar to rapakivi suites from other Proterozoic terranes. Despite a nearly 100 Ma range of U-Pb zircon ages, they appear to form a single group with an A-type, within-plate geochemical signature. Highlands mafic rocks comprise anorthosite (ANT) and three other distinct groups: strongly fractionated tholeiitic olivine metagabbros (OMG), jotunites (JOT), and ferrogabbros (FEG). JOT and FEG, both closely associated with ANT, were probably derived by fractionation of ANT parent magma(s). Most amphibolite layers in granitic and metasedimentary units appear to be related to OMG. The bimodal Highlands suite is consistent with episodic underplating of tholeiitic magmas and partial melting of underplated and lower crustal rocks during the 100 Ma prior to the Ottawan.

The Northwest Lowlands, with a higher proportion of metasedimentary rocks suggestive of affinity with the Frontenac terrane, contain at least six distinct groups of granitoids (Antwerp, Rossie, Hucleberry Mtn., Hermon, Rockport, and Hyde School). Except for the alaskitic facies of the Hyde School Gneiss, these are transitional between A-type and arc-related I-type granitoids. Lowlands mafics include amphibolites in the Popple Hill Gneiss (PHG), amphibolites in HSG, and mafic dikes in HSG. Each of these predominantly tholeiitic groups is geochemically distinct from the others, from small gabbro and diorite intrusives, and from a second, calc-alkaline set of PHG amphibolites. No clear genetic relationships are evident from the data.