Abstract This study re-examines a reported mylonitic ‘metabasic granulite’ block in a megabreccia that is the most outboard igneous rock outcrop in the Avalon terrane, near the Meguma–Avalon terrane boundary in the northern Appalachians. The block of foliated gabbro is one of several igneous rock blocks in a largely dissolved salt wall and in style of deformation, mineralogy, lithogeochemistry and Sm/Nd isotopes resembles foliated and locally mylonitized late Devonian–early Carboniferous gabbro plutons along the Cobequid Shear Zone to the north. Garnet porphyroclasts in the foliated gabbro are exceptional, with a distinctive composition of Alm 55 Pyr 25 Grs 13 And 4 Sps 3 . Inclusions of pyroxene, andesine and ilmenite, lack of zoning and corroded rims suggest the garnets are antecrysts. Elsewhere in the world, garnets of similar composition in arc-related andesites are interpreted to be from disintegration of comagmatic cumulate material at 0.8–1.0 GPa under hydrous conditions. The Clarke Head foliated gabbro has two mafic components, one resembling arc-related hydrous magma and the other with tholeiitic back-arc character, similar to coeval rocks in the Cobequid Highlands. The gabbro is a product of complex mixing in crustal magma chambers and rapid rise of magma containing lower crustal antecrysts along strike-slip faults.

Abstract The Cobequid Highlands of northern Nova Scotia lie at the intersection of two major dextral intra-continental shear zones which developed during closure of the Rheic Ocean and formation of Pangea. The Cobequid Shear Zone was an ENE–WSW transfer zone in a NE–SW-trending, orogen-parallel, shear system in the late Devonian–early Carboniferous (Neo-Acadian phase), in which syntectonic granite–gabbro plutons and volcanic strata <4 km thick were progressively deformed along multiple faults. In the Late Carboniferous–Permian, the Alleghanian collision of Africa with Laurentia formed the east–west-trending Minas Fault Zone, reactivating parts of the Cobequid Shear Zone. This deformation was mostly taken up on the master Cobequid–Chedabucto Fault. Deformation chronology is well constrained by the biostratigraphy of syntectonic sedimentary rocks, and by radiometrically dated igneous rocks and minerals in faults and veins. Early strike-slip faults were lubricated by magmatic heating, leading to dyke-to-pluton construction along multiple faults. During cooling, rooted gabbro was more resistant than ductile granite, thereby deflecting solid-state deformation. Neo-Acadian strike-slip displacement across the shear zone is estimated as >50 km. The location of Alleghanian deformation was influenced by the paucity of magmatism and the resistance offered by older stitching plutons.

Abstract Monazite 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.