On the Garnet-Bearing Nepheline Syenite of the Tyrdanov Ulus Area (Kuznetsk Alatau)
Abstract Generated abstract
This paper describes garnet-bearing nepheline syenite from the Tyrdanov Ulus area in the Kuznetsk Alatau, where alkaline nepheline-bearing rocks occur in the endocontact zone of the Uybat granitoid pluton adjacent to Proterozoic carbonate rocks. Petrographic and chemical observations identify a medium-grained poikilitic rock composed mainly of microcline, nepheline, oligoclase, andradite garnet, aegirine-augite, and muscovite, with partial replacement of nepheline by cancrinite. The author argues that the nepheline-bearing and nepheline-free rocks of the endocontact zone are genetically related, based on gradual transitions, similar mineralogy, accessory-element patterns, and shared andradite and calcium-rich augite. The rocks are interpreted as products of reaction between acidic magma and carbonate country rocks, which increased alkalinity and promoted enrichment in sodium and potassium within the contact zone.
Full Text
ON THE GARNET-BEARING NEPHELINE SYENITE OF THE TYRDANOV ULUS AREA (KUZNETSK ALATAU)
A. A. MITYAKIN
(Presented at the scientific seminar of the Department of Petrography)
The area of alkaline nepheline-bearing rocks of Tyrdanov Ulus is located in the middle course of the Sukhaia Erba River, along the right side of the valley. The alkaline rocks are confined to the endocontact zone of the Uybat granitoid pluton of variegated composition, at its northern termination. The rocks of the endocontact zone are in contact with the enclosing carbonate rocks of the Proterozoic, represented by limestones and dolomites. The endocontact part of the pluton has a very varied and inhomogeneous composition, beginning with syenites and diorites and ending with hornblendites and pyroxenites, with all transitional varieties. Among these rocks there are areas of alkaline feldspathoid nepheline-bearing rocks, represented by such varieties as nepheline syenites, nepheline monzonites, nepheline diorites, and, in some places at the contact with dolomites, nepheline pyroxenites are present. Especially noteworthy are the garnet-bearing nepheline syenites, distinctive in composition.
Macroscopically this is a gray, medium-grained rock of porphyritic structure; in segregations, larger crystals of garnet of rounded form are observed, reaching 5–6 mm.
Under the microscope the rock shows a poikilitic structure with the following mineralogical composition: garnet—15%, nepheline—20%, plagioclase—20%, K-feldspar—30%, pyroxene—10%, muscovite—5%.
The brown garnet is represented by rounded crystals with corroded margins, with numerous poikilitic inclusions of nepheline, potassium feldspar, plagioclase, and also pyroxene. According to the chemical formula
\[ (\mathrm{Ca}_{2.68}\mathrm{Fe}^{\bullet}_{0.09}\mathrm{Mn}_{0.04}\mathrm{Ti}_{0.06}\mathrm{Na}_{0.04}\mathrm{K}_{0.12})_{3.03} \cdot (\mathrm{Fe}^{\prime\prime\prime}_{1.16}\mathrm{Al}_{0.82})_{1.98} \cdot [(\mathrm{Si}_{2.92}\cdot \mathrm{Ti}_{0.08})_{3}\mathrm{O}_{12}] \]
the garnet belongs to the andradite series.
Pyroxene is represented by small grains with irregular outlines, usually occurring as poikilitic inclusions in nepheline, K-feldspar, and other minerals. The mineral is green in color, with optical constants \(CNg\) from 40 to 54°, \(2V\) from \(+60\) to \(+73^\circ\). According to chemical analysis, the mineral has the formula
\[ \mathrm{Ca}_{1.05}\mathrm{Mg}_{0.82}\mathrm{Fe}^{\prime\prime\prime}_{0.36}\mathrm{Fe}^{\prime\prime}_{0.14}\mathrm{Ti}_{0.05}\mathrm{Na}_{0.07}\mathrm{K}_{0.01})_{2.10} \cdot \mathrm{Si}_{1.63}\mathrm{Al}_{0.35})_{1.98}\mathrm{O}_{6}, \]
which corresponds to aegirine-augite.
The potassium feldspar in the rock, with a \(2V\) angle from \(-76\) to \(-88^\circ\), is microcline. According to chemical-analysis data, the mineral contains about 45% of the albite component.
Plagioclase-oligoclase is represented by tabular crystals of elongate form, idiomorphic with respect to nepheline and kalinite. According to determinations on the Fedorov stage, the composition corresponds to Nos. 13–29.
Nepheline is represented by more or less isometric crystals, partly replaced by cancrinite. The mineral is often observed in intergrowths with garnet, plagioclase, and pyroxene.
Finally, muscovite is present in the rock as small flakes, isometric or slightly elongated, idiomorphic with respect to the feldspars and nepheline. With regard to the origin of this rock, as well as of other nepheline-bearing rocks of the endocontact zone of the Uibat pluton, the following may be said.
The nepheline-bearing rocks have gradual transitions to nepheline-free rocks, and outwardly they usually look the same as the nepheline-free rocks: as diorites, syenites, and other varieties. Both groups of rocks have a similar mineralogical composition and similar accessory elements. Of the latter, only three are present that are characteristic of alkaline rocks—zirconium, beryllium, and yttrium; moreover, the beryllium content is close to the Clarke value for acidic rocks. All the other accessory elements present are characteristic of acidic rocks and occur in amounts close to Clarke values for acidic rocks.
Characteristic features are the presence, in the nepheline rocks and also in the nepheline-free rocks, of garnet of the andradite series, as well as the widespread development in the rocks of augite with a high calcium content. All these features indicate that the nepheline and nepheline-free rocks of the endocontact zone of the Uibat pluton are genetically related to one another and are products of the reaction interaction of acidic magma with carbonate country rocks.
The interaction of magma with carbonate rocks led to an increase in alkalinity in the endocontact zone, which is consistent with the well-known theory of D. S. Korzhinskii, according to which, during the interaction of acidic magma with carbonate rocks, the activity of all bases increases, but especially that of strong bases such as \(\mathrm{Na_2O}\) and \(\mathrm{K_2O}\), which leads to the accumulation of these bases within the endocontact zone.