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Mechanisms of differentiation of mafic to ultramafic magmas in closed and open magma chambersтезисы доклада
- Авторы: Ariskin A., Koptev-Dvornikov E., Barmina G., Nikolaev G.
- Сборник: XI L.L. Perchuk International School of Earth Sciences Abstract volume
- Тезисы
- Год издания: 2017
- Место издания: Miass I.S.E.S.
- Первая страница: 8
- Последняя страница: 8
- Аннотация: Spatial structure of mafic to ultramafic intrusions and large layered complexes presents a genetic record of magma differentiation processes operating in closed and open magma chambers. We treat ‘the closed system’ as that formed by a single impulse of a heterogeneous (melt + crystals) magma (for small differentiated sills), or as a combined result of compositionally similar crystal-laden injections, which produced a larger magma body for geologically short time interval. Further solidification of the intrusive systems is governed by magma crystallization, due to heat loss through the chamber boundaries coupled with a large-scale downward transport of the crystallizing material, mostly as high-density (crystal enriched) suspension flows from the above and along inclined wall- rocks. Such combination of in-situ magma crystallization, vigorous convection (magmatic currents), and crystals accumulation is known as a ‘Convection-Accumulation Model’ (CAM) of the formation of layered intrusions (Frenkel et al., 1989). In this model, local injections of more fresh or evolved magmas into the magma chamber are assumed to be of a subordinate importance. Using the INTRUSION-routine of the COMAGMAT magma crystallization model, it is shown that the proposed CAM mechanism generates the so-called S-shaped distribution of Mg, Ni, and Cr through vertical sections of the modeled magma bodies, observed in many intrusions, as well as an upward enrichment of the modeled rocks in incompatible elements towards a boundary, where the upper crystallization front meets the uppermost part of the evolving cumulate pile (Ariskin, Yaroshevsky, 2006). The demonstration examples include a number of differentiated sills from the Siberian platform (100-150 m thick), and two large intrusions of the Olanga Group (Northern Karelia, Russia). Evolution of open magma chambers includes two major scenarios. The first one implies a magma-staging system, through which large amounts of crystal-laden magmas have passed, leaving behind a cumulate succession composed of crystallized minerals and primitive to evolved intercumulus melt. The second scenario suggests the formation of a thick olivine-rich cumulate pile is to be accompanied by its compaction, giving rise to upward infiltration and expulsion of the residual melts from the magma chamber. The Yoko-Dovyren massif (YDM) in Northern Transbaikalia (Russia) is an example whereby both types of the above processes proceeded simultaneously. The dunite- troctolite-gabbronorite intrusion (728 Ma) is composed of a diversity of olivine orthocumulates to olivine-plagioclase and gabbroic adcumulates. Comparisons of complete cross-sections of the YDM in its centre and at the NE and SW margins demonstrate differences in the cumulate succession and geochemical structure. Combining petrochemical reconstructions and calculations using the COMAGMAT, it is shown that the central and peripheral parts of the YDM formed by olivine-laden parental magmas ranged in their temperature from 1290oC (11 wt% MgO, olivine Fo88) to 1190oC (8 wt% MgO, olivine Fo86), see (Ariskin et al., 2016). Despite these differences, the geochemical structure throughout the whole intrusive body demonstrates C-shaped distribution of TiO2, K2O, P2O5, and incompatible trace elements, which are 3-5 fold depleted in the cumulate rocks from the inner horizons of the intrusion with respect to the lower and upper contact zones. In addition, a marked misbalance between estimates of the average composition of the YDM and that of the proposed olivine-laden parental magmas is established. This evidences a significant deficit of the YDM in incompatible elements, which argues that 60-70% of basaltic melts had to have been expelled from the Dovyren magma chamber during its consolidation. We acknowledge support from the Russian Science Foundation (grant No. 16-17-10129). References Ariskin A.A., Yaroshevsky A.A. (2006) Crystallization differentiation of intrusive magmatic melt: Development of a Convection-Accumulation model. Geochemistry International 44: 72-93. Ariskin et al (2016) Cu-Ni-PGE fertility of the Yoko-Dovyren layered massif (Northern Transbaikalia, Russia): thermodynamic modeling of sulfide compositions in low mineralized dunites based on quantitative sulfide mineralogy. Mineralium Deposita 51: 993-1011. Frenkel et al (1989) Convective-cumulative model simulating the formation process of stratified intrusions. In: Magma-crust interactions and evolution. Athens-Greece: Theoph. Publ. 3-88.
- Добавил в систему: Коптев-Дворников Евгений Владимирович