Feasibility of AFC models for the petrogenesis of calc-alkaline magma series |
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Authors: | Malcolm P Roberts John D Clemens |
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Institution: | (1) Department of Geology, University of Manchester, Oxford Rd, Manchester M13 9PL, UK, GB;(2) School of Geological Sciences, Kingston University, Penrhyn Rd Kingston-upon-Thames, KT1 2EE, UK, GB |
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Abstract: | One of the most popular petrogenetic concepts is that of simultaneous assimilation and fractional crystallisation (AFC).
The equations governing these processes are well known and widely applied. The power and attraction of the model are that,
with the minimum of constraints, petrogenetic schemes can be erected for essentially any group of rocks. These models are
usually based on the observed variations in a small number of trace elements and/or isotopic systems. In this paper we have
generated well-constrained AFC models for the Variscan, calc-alkaline Quérigut complex in the French Pyrénées. Our models
are based on geological constraints on the identities of the parent magma and candidate assimilants, and the Sm-Nd and Rb-Sr
isotope systematics of the rocks. The modelling is successful in mimicking the isotope array for the actual rocks. However,
the results are massively inconsistent with those produced using the major- and trace-element concentrations. Our major conclusion
is that AFC modelling should be undertaken only with robust initial constraints on the compositions of the parent magma and
the proposed assimilant, with due respect for crystal-liquid partition coefficients, in so far as they are known. The results
should then be accepted only if the isotope-based model parameters are consistent with those derived from major- and trace-element
variations.
Received: 19 August 1994/Accepted: 23 March 1995 |
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