 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; June 2001; v. 29; no. 6; p. 543–546; 3 figures; Data Repository item 2001057. 543 Geochemical evolution of arc magmatism during arc-continent collision, South Mayo, Ireland Amy E. Draut Woods Hole Oceanographic Institution–Massachusetts Institute of Technology Joint Program in Oceanography, Woods Hole, Massachusetts 02543, USA Peter D. Clift Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA ABSTRACT The Lough Nafooey arc, in the western Irish Caledonides, collided with Laurentia dur- ing the Early Ordovician. This event is recorded in the stratigraphy of the South Mayo trough, the preserved forearc basin of this system. Lavas at the base of the oldest Lough Nafooey Group show intraoceanic arc composition.  Nd(t) decreases and light rare earth elements (REEs) become more enriched upsection in the Lough Nafooey Group (ca. 495– 481 Ma), reflecting early collision with Laurentia. The subsequent Tourmakeady Group (ca. 481–470 Ma) is rhyolitic, light REE enriched, and has strongly negative  Nd(t) . These rocks were erupted during the Grampian orogeny. The Rosroe and Mweelrea Formations (470 Ma) show wide scatter of La/Sm and Nb/Zr values, suggesting mixed mantle sourc- es. This correlates with rapid exhumation of the adjacent Connemara metamorphic ter- rane. The chemical evolution of the arc supports models of collision, orogeny, and tectonic unroofing within 15 m.y., and shows that genesis of magmas more enriched than con- tinental crust can occur during arc-continent collision, clarifying the potential role of arc volcanism in continental crust formation. Keywords: Caledonides, Iapetus, Ireland, subduction, geochemistry. Figure 1. A: Outline of British Isles showing major Caledonian terrane boundaries and location of South Mayo. B: Simplified geologic map of South Mayo, showing transects along which rocks were sampled. Connemara metamorphic terrane is immediately south of South Mayo. INTRODUCTION The origin of continental crust has long been a fundamental problem for earth scien- tists, and is still strongly debated (e.g., Taylor and McLennan, 1985; Rudnick and Fountain, 1995). While some authors favor rapid crustal formation shortly after Earth’s accretion, oth- ers suggest that the crust is built up by pro- gressive accretion of volcanic arc complexes to cratonic cores (Ellam and Hawkesworth, 1988). Intraoceanic arcs are, however, typical- ly too mafic and depleted in light rare earth elements (REEs) to be simple continental building blocks (Kay, 1985). Holbrook et al. (1999) speculated that the process of arc ac- cretion may change the bulk composition of the arc crust to be more continental, possibly by delamination and loss of mafic lower crust. Such models are difficult to test because the record of arc accretion is rarely preserved. In the Caledonian suture zone of western Ireland, the South Mayo trough (Dewey and Shackle- ton, 1984) provides an exceptional record of forearc sedimentation and associated volca- nism prior to and during a collision event and subsequent subduction polarity reversal (Fig. 1), allowing analysis of arc crust evolution. The South Mayo trough is unusual in being preserved in the suture zone without having undergone strong deformation or metamor- phism. Although it has been variously inter- preted in the past, there is a consensus that the trough represents the intraoceanic Lough Na- fooey arc and its forearc (Dewey and Shack- leton, 1984). It is an along-strike equivalent of the Lushs Bight and Betts Cove ophiolites of Newfoundland (e.g., Coish et al., 1982) and probably of elements of the Shelburne Falls arc of New England (Karabinos et al., 1998). The adjacent Connemara Dalradian metamor- phic terrane is considered to be a metamor- phosed fragment of the Laurentian margin (e.g., Harris et al., 1994), and is one of the best-dated segments in the Caledonian-Appa- lachian orogen. The biostratigraphy of South