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.