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

Published online by Cambridge University Press:  03 November 2011

W. E. Stephens
Affiliation:
W. E. Stephens, Department of Geography & Geology, University of St Andrews, St Andrews, Fife KY169ST, 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 SiO2 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 infracrustal 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 H2O contents (ηS-typeI-type and ρS-type≤ρI-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.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 1992

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