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 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-type>ηI-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.

Pluton emplacement in arcs: a crustal-scale exchange process

“…4b) and elsewhere we and others have argued that this zoning usually reflects the nesting of magma pulses rather than in situ fractional crystallisation (e.g. Bouchez & Diot 1990;Stephens 1992;Paterson & Vernon 1995). In other chambers (e.g.…”

Stepwise accumulation and ascent of magmas

“…Once initiated, a pluton will grow by the addition of subsequently arriving magma batches, which would all come through a few feeder zones, as described by, e.g., Ame Âglio et al (1997). If homogenisation is limited, the resulting pluton will display chemical, mineralogical and/or isotopic inhomogeneity, as predicted by Clemens & Mawer (1992) and observed by France-Lanord & Le Fort (1988), Stephens (1992), and many others.…”

Textural, mineralogical and geochemical variation in the zoned Timbarra Tablelands pluton, New South Wales*

“…1990); (iii) sidewall crystallisation and convective flow (Sawka et al . 1990; Pitcher 1993); (iv) thermo‐gravitational separation (Ayuso 1984); (v) vapour‐phase transport or volatile mass transfer (Hildreth 1981); (vi) multiple injection of more felsic magma pulses due to a rheological differentiation mechanism, and several stages of crystallisation (Stephens 1992); and (vii) upwelling of more fractionated magmas (Patterson & Vernon 1995). The models variously evoke in situ processes associated with a single magma pulse (points i–v) to multiple injections from a compositionally layered magma chamber at depth (points vi, vii).…”