Detrital monazite geochronology, Upper Jurassic–Lower Cretaceous of the Scotian Basin: significance for tracking first-cycle sources

Abstract

AbstractMonazite geochronology was applied to an east–west transect of latest Jurassic and Lower Cretaceous deltaic sandstones of the Scotian Basin, to assess sediment sources and dispersal pathways. More than 200 detrital monazite grains yielded 694 electron microprobe age determinations with 1σ errors <±20%. Based on age, external morphology, zoning, inclusions and major element chemistry (rare earth element [REE], Th, Y), monazite grains represent more than 20 discrete sources. Similar proportions of euhedral and subhedral compared with irregular and rounded monazite grains in most age classes, together with comparison with detrital muscovite and zircon geochronology, suggest that most monazite is first cycle. Six types of REE distribution are recognized (A–F). Many igneous monazites show chemical zoning, contain sparse euhedral inclusions, and have REE distributions of types A and E. Many metamorphic monazites contain inclusions, commonly aligned, are generally rounded–subhedral to rounded, and have REE distributions of types B, C and D. Monazite geochronology shows important supply to the Scotian Basin from the Labrador rift shoulder as early as Tithonian; from Avalonian sources in the Tithonian; from Ordovician sources in northern New Brunswick, apparently via the Chaswood River; and from the inner continental shelf, particularly in the Hauterivian–Barremian.