Spatial, compositional and rheological constraints on the origin of zoning in the Criffell pluton, Scotland

Abstract

The Criffell pluton in southwestern Scotland (397 Ma, a Newer Granite of late Caledonian age) is concentrically zoned with outer granodiorites of typically I-type aspect passing into inner granite with more evolved characteristics. The zonation is examined in terms of the compositional surfaces of bulk parameters such as Si0 2 and Rb/Sr and compositional variation is best modelled as multi-pulse, there being greater variation in bulk composition between pulses than within pulse. Published variations in Sr, Nd and O isotopes reflect the derivation of the pulses from separate and isotopically distinct sources. Other evidence for open-system behaviour includes mingling with mafic magmas to form enclaves, whereas closed-system behaviour is indicated by restite separation in the early granodiorites, and fractional crystallisation in the late granites. A dominant infracrustai I-type magma formed the first pulse followed by magma derived from more evolved crustal rocks (mainly metasediments of varying ages and maturities). Experimental fluid-absent melting of amphibolite and metapelite at about 900°C has shown that significant quantities of melt can be generated, respectively with I-type and S-type characteristics. Despite having similar bulk compositions, these melts have very different viscosities and densities for the same H,0 contents (rj s ., ypc > rj,_ type and /O s . lype < Pi-type)-It is argued that the rheological controls on magma escape from the source region along complex and tortuous pathways favour the more fluid I-type melts over the more viscous (and only slightly less dense) S-type melts. This constraint could have the effect of reversing the expected buoyancy-driven emplacement sequence, and may represent an alternative rheological differentiation mechanism for the formation of some zoned plutons.

The Neoproterozoic to Mid-Devonian evolution of Scotland: a review and unresolved issues

“…In many cases, they are zoned with S-type interiors and I-type outer margins and contain mafic enclaves throughout (e.g. the Criffel pluton (Stephens & Halliday 1984;Stephens 1992)). The Trans-Suture Suite was emplaced on both sides of the Iapetus Suture, was coincident with Acadian orogenic deformation in Britain and Ireland, and post-dates an episode of transtension that followed the Caledonian Orogeny between 410 and 400 Ma (Dewey & Strachan 2003).…”

Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon

“…Corrioux (1987) suggested that the formation of a foliation in the 151 outer zone of the pluton resulted from the later intrusion of the inner zones, providing 152 evidence for at least two stages of emplacement. WR isotopic variations and discontinuous 153 compositional zones (Stephens et al 1985;Stephens, 1992) provide further evidence for 154 multiple sources and emplacement episodes. Mafic enclaves are a conspicuous feature of 155 zones 1, 2 and 3; they are isotopically distinct from their host rocks, indicating that they are 156 not the product of crystal settling but instead represent different magmas whose relationship 157 to the WR compositions of the granitic rocks remains unclear (Holden et al 1987;Holden, 158 1991).…”

Stepwise magma migration and accumulation processes and their effect on extracted melt chemistry

“…Each of these batches may have a different chemistry, due to local differences in source rock, differences in the melting time and regime, and accumulation history (Stephens 1992;Hobden et al 1999;Soesoo 1999;Sobolev et al 2000). The major element chemical composition of the melts generated at the different stages of partial melting can be calculated using the existing modelling packages (i.e.…”