Contact interaction of agpaitic magmas with basement gneisses: An example of the Khibina and Lovozero Massifs |
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| Authors: | A A Arzamastsev L V Arzamastseva G P Zaraiskii |
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| Institution: | 1.Geological Institute, Kola Research Center,Russian Academy of Sciences,Apatity, Murmansk oblast,Russia;2.Institute of Experimental Mineralogy,Russian Academy of Sciences,Chernogolovka, Moscow oblast,Russia |
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| Abstract: | Data on processes that occurred at contacts of large agpaitic syenite intrusions and basement gneisses obtained by the authors
by studying and sampling profiles across the contacts and involve the composition of minerals, analysis of mineral assemblages,
isotopic dating of the processes, and analysis of the behavior of major, volatile, and trace elements in rocks near the contact.
The contact zones of the massifs were determined to consist of products of contact interaction during the early and late magmatic
stages and provide a record of successive stages of a continuous process of gneiss transformations, starting with the filling
of the magmatic reservoir with melt and ending with late- and postmagmatic processes related to the development of a system
of alkaline veins and pegmatite bodies in the gneisses. Early alkaline metasomatic processes in the Khibina Massif were local,
controlled by diffusion, and were induced by the immediate thermal and chemical effect of alkaline melts on the gneisses.
In the Lovozero Massif, metasomatism was related predominantly to the development of postmagmatic veins at 359 ± 5 Ma, was
controlled by infiltration, and proceeded immediately after the consolidation of the main intrusive series. The metasomatic
transformations during the early and late magmatic stages under the effect of agpaitic melts on gneisses predetermined different
closure conditions and, correspondingly, different behaviors of the Rb-Sr and Sm-Nd isotopic systems during the contact processes:
while the interaction of agpaitic melts with gneisses has modified the (87Sr/86Sr)(T = 370 Ma) ratio via the enrichment of radiogenic Sr in the host Archean rocks, the Sm-Nd isotopic characteristics of the syenites in
the inner contact zone and veins preserved their mantle values, which corresponded to the average ones for rocks in the central
parts of the intrusion. Experimental data, model simulations, and natural observations testify that Nb, Ta, Zr, Hf, and REE
were mobile in the contact interaction zone with agpaitic melts. With regard for data on the fluid regime of the agpaitic
melts and the concentrations of volatile components in the contact zones, we believe that the main role in the transfer of
REE and HFSE during contact metasomatism could be played by their ligands with F−, Cl−, and SO42−. |
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