CHEMICAL CHARACTERIZATION OF ZIRCON AND APATITE AS A STRATIGRAPHIC CORRELATION TOOL: EXAMPLE FROM THE ORDOVICIAN BENTONITES OF THE ST LAWRENCE PLATFORM IN THE MONTREAL AREA, QUEBEC

Ordovician successions of the St. Lawrence Platform of southern Quebec (Canada) contain altered volcanic ash beds, known as K-bentonites, which are found throughout eastern North America. U-Pb ages, combined with chemical and isotopic characteristics of the zircon and apatite contained in these bentonites can provide a fingerprint for individual ash layers, which can be used to correlate the bentonites across and between sedimentary basins. In the Montreal area, the stratigraphy of the St. Lawrence Platform contains Cambrian sandstones of the Potsdam Group at the base, overlain by the Ordovician Beekmantown, Chazy, Black River, Trenton, Utica and Lorraine Groups, which are a series of alternating limestones, shales and dolomites that grade into rhythmites and turbidites in the Utica-Lorraine. Volcanic ash beds are hosted by the Chazy and Trenton groups. Our work to date has identified 8 cm-scale K-bentonite horizons interbedded in the Trenton Group. Preliminary geochemical analyses of zircon and apatite of these horizons, as well as of bentonite samples from the Deicke and Millbrig horizons (Kentucky and Tennessee), have been carried out. The minerals have been imaged using BSE and CL detectors on an SEM, the composition of the apatite crystals has been measured using an electron microprobe to constrain their volatile element concentrations (Cl, F, OH). Trace element concentrations of zircon and apatite have been determined by LA-ICP-MS. These data allow us to chemically fingerprint these bentonites. Zircons from Trenton Group and those from Deicke and Millbrig horizons show only slight geochemical differences. Apatite from each individual bentonite have distinct compositions except one of the Trenton Group, which appears to have a similar composition to the Millbrig horizon. The use of zircon and apatite analyses from the bentonites of the St. Lawrence platform presents an interesting potential for the characterization of volcanic sources and precise stratigraphic correlation in Montreal area, and possibly over greater distances. Additional high-precision dating will provide temporal constraints on deposition of the different bentonites.