Epizonal I- and A-type granites and associated ash-flow tuffs, Fogo Island, northeast Newfoundland

Magmatic activity of Silurian-Devonian age is widespread in the Appalachian-Caledonian Orogen. A marked characteristic of this magmatism is the composite nature of the igneous suites, which range from peridotite to granodiorite in single plutonic bodies. Such a suite of intrusive rocks, ranging in composition from minor peridotite to granodiorite, intrudes an openly folded sequence of Silurian volcanogenic sandstones and ash-flow tuffs on Fogo Island, northeast Newfoundland. Two units, the Rogers Cove and Hare Bay microgranites, consist of fine-grained hastingsite granites with spherulitic and flow-banded textures. These rocks have an A-type granitoid affinity. A third and the most voluminous granitic unit, the Shoal Bay granite exhibits mineral parageneses similar to the microgranites, but chemical characteristics more typical of calc-alkaline, I-type granitoids. Volcanic-sedimentary sequences spatially associated with the granitic rocks include dense, welded, high-silica, hastingsite-bearing ash-flow tuffs with compositions that suggest they represent erupted equivalents of fractionated end members of the Shoal Bay granite. -from Authorspublished_or_final_versio

Adult Neurogenesis: Ultrastructure of a Neurogenic Niche and Neurovascular Relationships

The first-generation precursors producing adult-born neurons in the crayfish (Procambarus clarkii) brain reside in a specialized niche located on the ventral surface of the brain. In the present work, we have explored the organization and ultrastructure of this neurogenic niche, using light-level, confocal and electron microscopic approaches. Our goals were to define characteristics of the niche microenvironment, examine the morphological relationships between the niche and the vasculature and observe specializations at the boundary between the vascular cavity located centrally in the niche. Our results show that the niche is almost fully encapsulated by blood vessels, and that cells in the vasculature come into contact with the niche. This analysis also characterizes the ultrastructure of the cell types in the niche. The Type I niche cells are by far the most numerous, and are the only cell type present superficially in the most ventral cell layers of the niche. More dorsally, Type I cells are intermingled with Types II, III and IV cells, which are observed far less frequently. Type I cells have microvilli on their apical cell surfaces facing the vascular cavity, as well as junctional complexes between adjacent cells, suggesting a role in regulating transport from the blood into the niche cells. These studies demonstrate a close relationship between the neurogenic niche and vascular system in P. clarkii. Furthermore, the specializations of niche cells contacting the vascular cavity are also typical of the interface between the blood/cerebrospinal fluid (CSF)-brain barriers of vertebrates, including cells of the subventricular zone (SVZ) producing new olfactory interneurons in mammals. These data indicate that tissues involved in producing adult-born neurons in the crayfish brain use strategies that may reflect fundamental mechanisms preserved in an evolutionarily broad range of species, as proposed previously. The studies described here extend our understanding of neurovascular relationships in the brain of P. clarkii by characterizing the organization and ultrastructure of the neurogenic niche and associated vascular tissues

Geochemistry and U-Pb and 40Ar- 39Ar geochronology of The Man of War Gneiss, Lizard complex, SW England: Pre-Hercynian arc-type crust with a Sudeten-Iberian connection

The Man of War Gneiss is a variably deformed meta-igneous body that ranges in composition from gabbro to tonalite and comprises part of the basal structural slice of the Lizard ophiolite complex, SW England. U-Pb dating of zircon demonstrates that the parental magmas crystallized in the earliest Ordovician (499 +8 -3 Ma), but 40Ar- 39Ar step-heating analysis of amphiboles suggests that at least the northernmost zone of the gneiss was subjected to an amphibolite-grade metamorphic event at c. 374 Ma, probably associated with thrust emplacement of the ophiolite. The gneiss exhibits major, trace and rare earth element variations in conformity with an origin through partial melting of an amphibolitic source containing hornblende and plagioclase, probably with minor garnet and zircon. The elemental abundances and variations are characteristic of granitoids formed in supra-subduction zone environments. A granitoid sill within the adjacent Old Lizard Head Series schists, previously inferred to be related to the Man of War Gneiss, is trondhjemitic and exhibits higher SiO 2, TiO 2 and MgO, but lower Zr, V, Ba, and Sr contents, and therefore is probably not genetically related to the latter. The sill may record local melting during ophiolite emplacement. The Man of War Gneiss probably represents a structural inlier of pre-Hercynian basement incorporated in the Hercynian nappes of SW England during NNW-directed thrusting. We emphasize the geological and geochronological similarities between variably deformed granitoid units of the Lizard complex, the Sudeten Massif of southern Poland, and the allochthonous complexes of northern Portugal and NW Spain. These granitoid rocks may represent dispersed remnants of Tremadoc arcs that formed oceanward from the northern margin of Gondwana. This is further convincing evidence that at this time Eastern Avalonia was separated from northern Gondwana by an ocean basin that may have been connected with the Tornquist Sea.link_to_subscribed_fulltex