Old Rb-Sr whole-rock isochron apparent ages from Lower Cambrian psammites and metapsammites, southeastern New York
Samples from two outcrops of Poughquag Quartzite and an outcrop of Lowerre Quartzite yield nearly linear arrays of points on Rb-Sr whole-rock isochron diagrams, but the apparent ages range between 600 and 1,000 m.y. If the Lowerre Quartzite and the Lower Cambrian Poughquag Quartzite are correlative, these ages may be interpreted as maximum deposition ages that are no younger than the time of deposition and no older than the age of the source terrane (provenance age). This behavior of metamorphosed psammitic rocks contrasts with the behavior of metamorphosed pelitic rocks in southeastern New York which tend to yield minimum depositional ages bounded by the time of deposition and the time of metamorphic closure. The difference in age is believed to be a function of (1) the abundance, in the initial sediment, of unaltered detritus versus altered and diagenetic materials, and (2) the hydrous phase present during metamorphism. Unaltered detrital grains will tend to retain provenance Rb-Sr ages, whereas diagenetic minerals and weathered grains will tend to re-equilibrate during sedimentary processes and subsequent metamorphism. During metamorphism, water produced by dehydration reactions will promote the open-system behavior of rubidium, strontium, and strontium isotopes. Psammitic rocks that lack hydrous phases are more likely to remain closed systems on the scale of a hand sample during metamorphism than pelitic rocks, allowing them to retain their premetamorphic Rb-Sr age pattern. Thus the apparent age of metasedimentary rocks can have a variety of geological interpretations. The Rb-Sr whole-rock isochron method must be applied cautiously to metamorphic rocks of uncertain origin, even where the array of analyzed points on an isochron diagram is linear.