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

W. E. Stephens*

*Corresponding author for this work

Research output: Chapter in Book/Report/Conference proceedingChapter

1 Citation (Scopus)

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 Si02 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 (n S-type > nI-type and/PS-type ≤ P I-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.

Original languageEnglish
Title of host publicationThe Second Hutton Symposium on the Origin of Granites and Related Rocks
EditorsP.E. Brown, B.W. Chappell
PublisherGeological Society of America
Pages191-199
Number of pages9
ISBN (Print)9780813722726
DOIs
Publication statusPublished - 1992

Publication series

NameGSA Special Papers
PublisherGeological Society of America
Volume272

Keywords

  • Caledonian
  • Compositional zoning
  • Granite

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