Testing petrogenetic models for contemporaneous mafic and felsic-intermediate magmatism within the ‘Newer Granite’ suite of the Scottish and Irish Caledonides

Donnelly B. Archibald, Brendan Murphy, Mike Fowler, Rob Strachan, Robert S. Hildebrand

Research output: Contribution to journalArticlepeer-review

Abstract

Granitoid batholiths dominated by felsic-intermediate compositions are commonly associated with mafic plutons and enclaves; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is unclear. Here we review the age, lithogeochemical and Nd-Sr isotopic compositions of some classic plutonic rocks emplaced in the Northern Highlands, Grampian and Connemara terranes of the Caledonide orogen of Scotland and Ireland. The Northern Highlands terrane consists mostly of Neoproterozoic metasedimentary rocks of the Moine Supergroup and is located north of the Great Glen Fault. The Grampian terrane also consists of Neoproterozoic metasedimentary rocks (Dalradian Supergroup) and is located south of the Great Glen Fault in both Scotland and Ireland. Amphibolite-facies metasedimentary rocks in the Connemara terrane are correlated with the Dalradian Supergroup and the terrane is bounded by splays of the Highland Boundary and Southern Uplands faults. These three terranes were intruded by Silurian-Devonian mafic and felsic-intermediate plutonic rocks that display field evidence for mingling and mixing and have a similar range (between ~ 437 and 360 Ma) in emplacement ages. This range implies they were intruded during and after the late Caledonian Scandian orogenic event that resulted from the midto late Silurian collision of amalgamated Avalonia and Baltica with Laurentia and the final
closure of the Iapetus Ocean. Our review supports the contention that the Great Glen Fault represents a major compositional boundary in the Silurian lithosphere. Felsic-intermediate plutons that occur north of the Great Glen Fault are more enriched in light rare earth elements and Ba-Sr-K compared to those to the south. Isotopic compositions of these late Caledonian plutonic rocks on both sides of the Great Glen Fault indicate that metasomatism and enrichment of the subcratonic lithospheric mantle (SCLM) beneath the Northern Highlands terrane occurred just prior to emplacement of late Caledonian plutons. Within the same terrane, mafic and felsicintermediate rocks display similar trace and rare-earth element concentrations compatible with models implying fractionation of a mafic magma played an important role in generating the felsic-intermediate magmas. The onset of slab failure magmatism may have been diachronous
along the length of the collision zone. If so, slab failure may have propagated laterally, possibly initiating where promontories collided.
Original languageEnglish
JournalSpecial Papers of the Geological Society of America
Volume554
Publication statusAccepted for publication - 16 Jun 2021

Keywords

  • slab failure magmatism
  • granite
  • appinite
  • lamprophyre
  • Caledonian orogeny

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