Abstract
The Nizhne-Derbinsk mafic-ultramafic complex is located between the Central Asian Orogenic Belt and the Siberian Craton and, is associated with the Ballyk fault. The largest, spatially related to each other, plutons in the central part of the complex are the Burlakski and Nizhne-Derbinsk. Rocks in the main units of these plutons are divided into three groups: peridotites (ultramafic), pyroxenites (sub-ultramafic), and gabbroic rocks (mafic). The ultramafic and sub-ultramafic cumulate series are devoid of plagioclase and contain <3 vol. % chromian spinel. The Fo content of olivine in the sub-ultramafic cumulates from both plutons ranges from Fo79 to Fo86. The En content [= Mg/(Mg + Fe + Ca) × 100 atomic ratio] of clinopyroxenes and orthopyroxenes varies from 46–56, and 63–80, respectively. Plagioclase corresponds to labradorite with An contents between 55 and 57. Hornblende is compositionally similar to pargasite. The sequence of change of rock units corresponds to the paragenesis: olivine − olivine + clinopyroxene (orthopyroxene) − clinopyroxene + orthopyroxene – clinopyroxene + orthopyroxene + plagioclase – orthopyroxene. Petrographic, mineralogical, and mineral chemical features of the Burlakski and Nizhne-Derbinsk plutons suggest that the diversity of the material composition of these plutons is due to the processes of magmatic differentiation in deep-seated conditions. Estimates of crystallization pressures and temperatures of the Burlakski and Nizhne-Derbinsk plutons suggest that they crystallized at high pressures ≥ 10kb and temperatures ranging from 1000–1400 °C. Mineralogical and petrological features suggest that the mafic-ultramafic cumulates were derived from a high-Mg basaltic magma. The presence of magmatic hornblende and hydrous mineral assemblages within the ultramafic cumulates indicates that the parental melts had been enriched in dissolved volatile constituents. Taking into account the age of the gabbronorites of the Burlakski pluton (~490 ± 11.8 Ma), the magmatism likely occurred during the Ordovician collision stage of the evolution of the Central Asian Fold Belt.
Highlights
Numerous researchers [1,2,3,4,5,6,7] have focused their efforts on characterizing the petrogenesis of mafic-ultramafic intrusions, as they are ideal environments for understanding magmatic differentiation.By characterizing the primitive magma composition of a magma series, it is possible to model the composition of the rock from which a melt was formed
Plutonic rocks of the dunite-clinopyroxenite series are especially important as they are the main constituents of the Moho transition zone (MTZ) and their petrogenesis still remains unclear [12]
This study provides new petrological data about two poorly studied and poorly exposed mafic-ultramafic plutons that contain dunite-clinopyroxenite series rocks
Summary
Numerous researchers [1,2,3,4,5,6,7] have focused their efforts on characterizing the petrogenesis of mafic-ultramafic intrusions, as they are ideal environments for understanding magmatic differentiation.By characterizing the primitive magma composition of a magma series, it is possible to model the composition of the rock from which a melt was formed. Numerous researchers [1,2,3,4,5,6,7] have focused their efforts on characterizing the petrogenesis of mafic-ultramafic intrusions, as they are ideal environments for understanding magmatic differentiation. The virtual absence of plagioclase in these cumulates containing both orthopyroxene and clinopyroxene, as early fractionating phases, is indicative of medium to high-pressure crystal fractionation of primary basaltic melts [17]. Our understanding of these plutons formation mechanisms, plays a critical role in understanding several petrogenetic issues such as parental and evolution of magma composition, metal and sulfur enrichment
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