Geochemical constraints on crustal anatexis: a case study from the Pan-African Damara granitoids of Namibia |
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Authors: | F McDermott N B W Harris C J Hawkesworth |
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Institution: | (1) Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK, GB |
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Abstract: | Major and trace element models of recently published vapour-absent mica dehydration melting experiments are used to identify
granitoids generated by muscovite and biotite dehydration melting, and to distinguish between plagioclase-limited and biotite-limited,
biotite dehydration melting. In the case of granitoids from the Pan-African Damara mobile belt (Namibia), many of the leucogranites
and Salem-type granitoids may be modelled by biotite dehydration melting. The low Rb/Sr granitoids (e.g. Donkerhuk Onanis,
Salem Onanis, Donkerhuk Nomatsaus, Salem Goas) probably reflect feldspar-limited, biotite dehydration melting (a pelitic source)
whereas the high Rb/Sr suites (e.g. Bloedkoppie leucogranite, Stinkbank leucogranite, Salem Swakopmund, Leucocratic Stink
bank granite) reflect biotite-limited, biotite dehydration melting (a greywacke source). Alaskites from the Damara belt have
major element compositions which are consistent with muscovite dehydration melting, and their positive Eu anomalies are linked
to high K2O reflecting K-feldspar entrainment. Combined Zr and LREE (light rare earth element) solubility models indicate that insufficient
time (probably less than 104 years) had elapsed between melt generation and melt extraction to ensure that the alaskite melts attained their equilibrium
concentrations of Zr and the LREEs. In contrast, the leucogranites and Salem-type granites have attained their equilibrium
inventories of these trace elements. Combined Fe2O3 and MgO contents in some samples from two granitoids (the Salem Goas and Donkerhuk Onanis intrusions) are higher than those
readily attainable by biotite dehydration melting indicating either: (1) that they contain a contribution from melts generated
by incipient garnet breakdown or; (2) that they contain small amounts of an entrained ferromagnesian phase.
Received: 24 April 1995/Accepted: 11 December 1995 |
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