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Journal of the Geological Society

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The role of hybridization and crystal fractionation in the evolution of the Cairnsmore of Carsphairn Intrusion, Southern Uplands of Scotland

Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

The petrology of the Cairnsmore of Carsphairn granitic intrusion in the Southern Uplands of Scotland was the subject of several classical studies by Deer about 50 years ago. The presence of concentric zonation from dioritic margins to granitic core with petrologically transitional contacts was explained in terms of successive episodes of intrusion and hybridization. Magmas of gabbroic, granodioritic and finally granitic composition were involved, producing extensive intermediate zones of hybrid diorite and hybrid granite.

These proposals are examined in the light of recent field observations and new major and trace element data. Using suitable end-member compositions, it is shown that hybridization (or mixing) alone could not have produced the intermediate rock types, although a form of hybridization may locally account for transitional boundaries between the main rock types. It is shown that geochemical variations through all units are consistent with crystal fractionation as the dominant mechanism of magma evolution, but field observations of locally sharp contacts between different rock types and varying compositional gradients preclude a simple model of in situ fractional crystallization.

The preferred model for the pluton is that a large volume of dioritic magma was emplaced, assimilated country rocks near the margins and crystallized inwards to form the diorite/quartz diorite unit. The enclosed melt fractionated towards a granodioritic composition and broke through the partially crystallized carapace to form the zone of granodiorite. The melt continued to fractionate at depth and, after a further interval, a granitic melt was intruded through the largely crystallized granodiorite to form the inner zone of granite. The first intrusion of granite formed the finer-grained microgranite (where more rapid cooling and/or volatile loss occurred). This was intruded by a second pulse of compositionally similar melt to form the coarse-grained granite.

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