THE TRANS-ADIRONDACK BASIN - PRECURSOR TO THE SHAWINIGAN OROGENY

The Trans-Adirondack Basin, best preserved in the Adirondack Lowlands (AL), extends from the Frontenac terrane (FT) to the southern Adirondack Highlands (AH) and records the sedimentary response from back arc rift/drift to foredeep development prior to Shawinigan orogenesis. The basin initially opened when the Elzivirian arc on the margin of Laurentia split at ~1.30-1.35 Ga. Neodymium and oxygen isotopes and the distribution of pre- (Antwerp-Rossie and Hermon; ca. 1.18-1.20 Ga) and post-assembly (AMCG; ca. 1.15-1.17 Ga) stitching plutonic suites places the margin of Laurentia along the Black Lake shear zone in the AL prior to 1.17 Ga. This boundary marks a change in sedimentary facies with predominantly shelf quartzites and marbles to the west in the FT and the occurrence of abundant pelitic gneisses to the south and east in the AL and AH. In the AL, where a structural stratigraphy has been previously developed, carbonate rocks of the Lower Marble document deposition of an extensive carbonate platform on the rifted margin of Laurentia. Development of a foredeep basin followed and accommodated the deposition of a thick sequence of fine-grained clastics (Popple Hill gneiss) that were extensively intruded by sills of MORB chemistry. Similar intrusions are known from the central AH but are hosted mainly in marbles. Filling of the basin and isolation from the ocean is recorded in the Upper Marble where shallow water siliceous carbonates and thick sequences of evaporates were deposited. Evaporitic sequences record compression and uplift and were followed by Zn-Pb sedimentary exhalative deposits as hydrothermal fluids vented within the basin. Supracrustal rocks are allochthonous but were likely deposited on oceanic crust, now recognized as highly dismembered and intruded amphibolitic bodies; in some cases overlain by sulfide-rich graphitic shales deposited in linear fault bounded troughs. The southern margin of the basin was located along Piseco Lake shear zone in the AH where plutonic rocks of arc-like character occur inboard of remnants of the original Elzivirian arc (1.30-1.35 Ga tonalitic gneisses). To the northwest a failed rift back-arc basin, defined by others using Nd model ages <1.35 Ga, developed synchronously documenting pre-Shawinigan extension across the Central Metasedimentary Belt and eastward into the AH.