Sapphirine/kornenipine-bearing rocks and crustal uplift history of the Limpopo belt,Southern Africa |
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| Authors: | B F Windley D Ackermand R K Herd |
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| Institution: | (1) Department of Geology, The University, LE1 7RH Leicester, UK;(2) Mineralogisches Institut der Universität, D-2300 Kiel, Germany;(3) Geological Survey of Canada, 601 Booth Street, K1A OE8 Ottawa, Ontario, Canada |
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| Abstract: | Sapphirine/kornerupine-bearing rocks occur within the anorthosites of the Messina layered intrusion in the Limpopo mobile belt of Zimbabwe. The XMg range of the major minerals is as follows: cordierite (0.98-0.93); enstatite (0.97-0.86); chlorite (0.98-0.92); phlogopite (0.98-0.90); sapphirine (0.98-0.86); kornerupine (0.94-0.88); gedrite (0.96-0.85); spinel (0.92-0.78). There are four rock types, the constituent minerals of which have different
values, which decrease in the above mineral order; other minerals are corundum, sillimanite and relict kyanite. We recognise twenty reactions without phlogopite and nine reactions involving phlogopite. The textural relations and the plots of the microprobe data of coexisting minerals in the MgO-Al2O3-SiO2-(H2O) system are consistent with the following sequence of main reactions: (1) enstatite+corundum  cordierite+sapphirine; (4) sapphirine+sillimanite cordierite+corundum; (8) kornerupine+corundum cordierite+sapphirine; (13) kornerupine cordierite+sapphirine+enstatite; (15) enstatite+spinel chlorite+sapphirine; (18) cordierite+sapphirine chlorite+corundum; (20) sapphirine chlorite+corundum+spinel. The early reactions are shown by coarse-grained reaction intergrowths, kornerupine and gedrite breakdown is shown by finer-grained symplectites, and the latest reactions by very fine-grained products in micro-fractures. These selected reactions illustrate a remarkably steep trajectory from thePT peak close to 10 kbar and 800° C to the minimum observable at 3.5–4.5 kbar and 700° C as indicated by the pure MASH system. Very rapid uplift took place under nearly isothermal conditions. The protolith of this material was possibly sedimentary, derived from altered volcanic rocks. The bulk composition is close to the composition of kornerupine or to a mixture of alunite, chlorite and pyrophyllite. These texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history. |
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