Abstract
In this paper, we provide insight into the evolution of syenite magmas based on geological data and petrographic, geochemical, and O-Nd isotope parameters of rocks of the Saibar intrusion located within the Minusinsk Trough, Altay-Sayan area. The intrusive suite includes predominant syenites, few bodies of melanocratic and leucocratic nepheline syenites (foyaites), and granites. In addition, dykes of granites and mafic rocks are present. The U-Pb zircon age from the melanocratic foyaites was determined to be 457 ± 10 Ma? Examined rocks show fractionated light rare earth element patterns, normalized to chondrite, with (La/Sm)n varying from 4 to 9, and a weakly fractionated distribution of medium and heavy rare elements, with (Dy/Yb)n from 0.35 to 1.23 and (Sm/Yb)n from 0.63 to 2.62. The spidergram normalized to the primitive mantle shows negative Ba, Sr, Nb, Ta, Ti, and Eu anomalies (Eu* = 0.48–0.60) and positive Rb, Th, and U anomalies. The δ18O values vary within 6.3 to 10.2‰, and εNd(t) from +4.1 to +5.0. We observe gradual transitions from syenites to foyaites. Assimilation by syenite magma of the host carbonate rocks was followed to transition from silica-saturated to silica-undersaturated conditions and removal of anorthite from the melt, which then led to nepheline. Granites of the main phase show depleted lithophile incompatible elements in comparison with syenites and foyaites. They originate via interaction of magmas at the marginal part (endocontact zone) of the intrusion, corresponding to north contact of the granites with the host felsic rocks. In comparison, the rock composition of granite dykes is enriched in lithophile incompatible elements, except for Zr, Hf, and Ti. These rocks are formed due to the differentiation of syenite magma without a significant effect of host rock assimilation. Mantle magmas must be used as parent magmas for syenites based on analysis of the formation model of other alkaline intrusions, which are similar in age to the Saibar intrusion. In the line of syenite intrusions of the Altai-Sayan province, the Saibar intrusion is no exception, and its origin is related to the evolution of mafic magmas that arose during the melting of the mantle under the influence of a mantle plume.
Highlights
1.1.IntroductionThemain main characteristics syenites sodium alkaline pyroxenes and amphibolesThe characteristics of of syenites areare sodium alkaline pyroxenes and amphiboles [1].[1]
These rocks are geochemically close to A-type granites [73], which are characterized by enrichment in high field strength elements (HFSEs) and heavy rare earth elements (HREEs) in combination with low Sr and Ti concentrations, which is reflected in a spidergram
The obtained geological and isotope-geochemical data for the Saibar intrusion rocks showed the following: 1. The Saibar intrusion is composed of dominant syenites, as well as few bodies of melanocratic and leucocratic nepheline syenites and endocontact granites
Summary
The characteristics of of syenites areare sodium alkaline pyroxenes and amphiboles [1]. Foyaites have melanocratic and leucocratic varieties and banded structures These are alternating bands with varying ratios of alkali feldspar, albite, aegirine, arfvedsonite, and nepheline. Quartz, and albite, dyke granites contain infrequent grains of aegirine–augite and arfvedsonite along with accumulations of magnetite, accessory apatite, and fluorite on the boundaries. Mafic rocks make up infrequent dykes intruding syenites in the central part of the Saibar intrusion. The contact of these dykes with the host rocks is sharp and clear. The rocks underwent lowtemperature hydrothermal alteration, which caused the replacement of labradorite by sericite, mafic minerals by chlorite and epidote, as well as the formation of secondary calcite
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