Prograde transformations of amphibolites into eclogites and eclogite-like rocks in the low-pressure field of the eclogite facies (by the example of the Belomorian Mobile Belt) |
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| Institution: | 1. Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017, Russia;2. Saint Petersburg State University, Universitetskaya nab. 7/9, Saint Petersburg, 119034, Russia;3. Lomonosov Moscow State University, Leninskie Gory, 119899, Moscow, Russia;1. Instituto Geológico y Minero de España, Ríos Rosas 23, 28003 Madrid, Spain;2. Departamento de Petrología y Geoquímica e Instituto de Geociencias (UCM, CSIC), Universidad Complutense de Madrid, 28040 Madrid, Spain;3. Geology and Geography Department, Tomsk State University, Lenin Street 36, Tomsk 634050, Russian Federation;1. Dipartimento di Scienze della Terra, dell''Ambiente e delle Risorse, Università di Napoli Federico II, Italy;2. School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia;3. Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche, Pavia, Italy |
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| Abstract: | Plagioclase-bearing garnet-omphacite (Grt-Omp) eclogites and garnet-augite eclogite-like (Grt-Aug) schists from the amphibolite and gneiss beds of the Belomorian Mobile Belt have been studied. They are spread over a large area. In most of the studied objects, these rocks have preserved primary concordant relations with the host amphibolite and gneiss strata; they are not disturbed by late tectonic processes and are not genetically related to tectonic-melange zones. Their protoliths were amphibolite lenses in gneisses or large mafic zones composed of amphibolites. The Grt-Omp eclogites formed in the low-pressure field of the eclogite facies (P = 12.5-13.0 kbar, T = 600-630 °C), and the eclogite-like Grt-Aug rocks, at the boundary between the amphibolite and eclogite facies (P = 9.6-11.1 kbar, T = 630-700 °C), under the intense impact of metamorphic fluid on the amphibolites. The compositional evolution of the rock-forming minerals during the formation of Grt-Omp eclogites and eclogite-like Grt-Aug rocks followed the same scheme. The petrographic diversity of apoamphibolite rocks (Grt-Omp eclogites and Grt-Aug schists) might be due to the difference both in the bulk composition of the metabasic protolith and in the ratios of CaO and Na2O activities in the metamorphic fluid. The relatively low content of CaO leads to the formation of Grt-Omp paragenesis in eclogites. Higher CaO contents give rise to eclogite-like Grt-Aug rocks containing jadeite-poor clinopyroxene. |
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