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534844 
Journal Article 
Spatial, temporal and geochemical characteristics of Silurian collision-zone magmatism, Newfoundland Appalachians: An example of a rapidly evolving magmatic system related to slab break-off 
Whalen, JB; McNicoll, VJ; van Staal, CR; Lissenberg, CJ; Longstaffe, FJ; Jenner, GA; van Breeman, O 
2006 
1 
Lithos
ISSN: 0024-4937 
89 
3-4 
377-404 
English 
Silurian plutonic suites in the Newfoundland Appalachians include abundant gabbro, monzogabbro and granite to granodiorite and lesser quartz diorite and tonalite. Most are medium- to high-K, but included are some low-K and shoshonitic mafic compositions. Felsic rocks are of both alkaline (A-type or within-plate granite (WPG)) and calc-alkaline volcanic arc granite (VAG) affinity. Mafic rocks include both arc-like (Nb/Th<3) calc-alkaline and non-arc-like (Nb/Th>3) transitional calc-alkaline basalt to continental tholeiitic affinity compositions. epsilon(Nd)(T) values range from -9.6 to +5.4 and delta O-18 (VSMOW) values range from +3.1 to +13.2 parts per thousand. A rapid progression from exclusively arc-type to non-arc-like mafic and then contemporaneous WPG plus VAG magmatism has been documented using precise U-Pb zircon dating. Earlier arc-like plutonism indicates subduction, while asthenosphere-derived mafic magmas support slab break-off, due to subduction of a young, warm back-arc basin. Contemporaneous mafic magmas with arc and non-arc geochemical signatures may reflect tapping of asthenospheric and subcontinental lithospheric mantle (SCLM) sources and/or contamination of asthenosphere-derived magmas by SCLM or crust. The brevity (<5 Ma) of the mafic magmatic pulse agrees with the transient nature of magmatism associated with slab break-off. The subsequent ca. 1 to 2 m.y. period of voluminous WPG and VAG plutonism likely reflects mafic magma-driven partial melting of both SCLM and crustal sources, respectively. Continuation of VAG-like magmatism for an additional 2 to 5 m.y. may reflect lower solidus temperatures of crustal materials, enabling anatexis to continue after mantle melting ceased. East to west spatial variation of epsilon(Nd) and (La/Yb)(CN) in Silurian plutons suggests a transition from shallow melting of juvenile sources proximal to the collision zone to deeper melting of old source materials in the garnet-stability field further inboard. Previous work has demonstrated that geochemical discriminaton of post-collisional granitoid magmatism (PCGM) is difficult in the absence of other constraints. Our example should contribute to the understanding and identification of PCGM if it can be employed as a 'fingerprint' for slab break-off'-related PCGM within the Paleozoic geological record. (C) 2006 Elsevier B.V. All rights reserved. 
post-collisional; slab break-off; granite; alkaline; Newfoundland; s-type granites; canadian appalachians; continental-crust; thermal; evolution; hydrogen isotope; volcanic-rocks; trace-element; calc-alkaline; pb; constraints