NEW GEOCHEMICAL AND GEOCHRONOLOGIC CONSTRAINTS ON THE ORIGIN AND TECTONIC EVOLUTION OF THE POPPLE HILL GNEISS IN THE ADIRONDACK LOWLANDS, NEW YORK

The Popple Hill Gneiss (PHG) is the predominate NE-SW striking gneissic unit in the Adirondack Lowlands and its protolith is interpreted as having been deposited in a developing foredeep during the contractional phase of the Trans-Adirondack Basin between ca. 1250-1225 Ma. Stratigraphically, it lies between carbonate-dominated sequences, the Lower Marble and Upper Marble (UM), which were deposited in shallow, marine waters. During deposition of the UM, hypersaline conditions repeatedly occurred between episodic Pb-Zn exhalative deposition from heated brines. The PHG primarily consists of fine- to medium-grained (possibly graded in areas) biotite-bearing quartzofeldspathic gneisses with occasional garnet and sillimanite where highly deformed (mylonitic), and varies in metamorphic grade from upper amphibolite to incipient granulite facies with derived migmatites. Isoclinal folding, boudinage of quartzofeldspathic layers, and transposition of foliation are common structural elements. It is widely intruded by linear belts of granitic gneisses (fine-grained to porphyritic) of the Hermon Granite (HG), ca. 1182 Ma, and an earlier suite of thin tholeiitic to calc-alkaline sill-like amphibolitic layers. The PHG: 1) ranges from rhyolitic to andesitic; 2) has calc-alkaline and other differentiation trends consistent with arc volcanics; 3) does not contain conspicuous metapelite aluminous phases such as garnet and sillimanite; 4) is geochemically more similar to estimates of the Upper Continental Crust than well documented shales; 5) has a CIA index indicative of a partially weathered or rapidly exhumed source; 6) contains monazites of several ages - >1250 Ma (detrital) and 1190, 1154, 1112, and 1083 Ma (growth during the Shawinigan and Ottawan thermotectonic events); and 7) is the likely anatectic source of the HG, which has monazite ages of 1146, 1119, and 1065 Ma. This data is consistent with the interpretation that the PHG represents a rapidly exhumed volcaniclastic arc source into the contracting Trans-Adirondack Basin, although intercalations of volcaniclastic and terrigenous siliciclastic sediment are not precluded. The likely source is slightly older (1350-1250 Ma) arc rocks of the Southern Adirondacks. The Ordovican Taconic foredeep is a possible analog for the PHG-UM foredeep.