Issues of Geological Structure of Synnyr Massyf

 

In the work summarizing a certain stage in the study of the Synnyr massif(V.P. Kostyuk, A.Ya Zhidkov, L.I. Panina, M.P. Orlova, T.Yu.Bazarova, 1990), the formation of an intrusive buildup was considered in two successive stages , –
intrusion of layered laccolith of feldspathoid syenites, mesocratic syenites and shonkinites, which breaks through with a later stock-like body of alkaline syenites. At the stage of autometasomatic transformation of feldspathoid syenites, large fields of light-mica microcline rocks appeared, occupying up to 25-30% of the development area of the first intrusive phase. In the section, according to the aforementioned authors, the rocks of the first phase form rather steep (on average 50-70 degrees) angles of incidence towards the conditional center of the massif.

 

The geological exploration work carried out by OOO Baikal Nedra Geo in the period from 2018 to the present time at the Kalyumnoye subsurface site and traversing outside it, together with the analysis of data from previous geological surveys, allow us to present a picture of the formation of the Synnyr massif as follows:

 

  1. Introduction of layered intrusion of ultra-, high-potassium feldspathoid syenites and alkaline gabbroids (the main body of the massif is up to 70-75% of the area). At this stage, kalsilite and nepheline-kalsilite syenites (synnyrites), several varieties of nepheline syenites, shonkinites, and melanocratic syenites associated with them arose.

  2. Occurrences of vein series of high-potassium alkaline syenites, as well as shonkinites and meso-, melanocratic syenites.

  3. Formation of sodium-calcium metasomatites (sodalite, sodalite-cancrinite, scapolite and albite) along the zones of large diagonal tectonic faults.

  4. The formation of the massif took place in a very turbulent tectonic settings, which led to the appearance of various crushing zones and linear areas of development of microcline, microcline-zeolite metasomatites, as well as biotite-pyroxene titanite formations.

  5. Development of areal light-mica-microcline metasomatites over felspathoid syenites of the main layered intrusive phase.

  6. Formation of intrusive bodies of alkaline syenites and alkaline granitoids in the central part of the massif.

  7. Injections of granite complex intrusions of high alkalinity in the southwestern and northeastern at the contact zones of the massif.

  8. Occurrence of a dike complex, including lamprophyres, alkaline gabbroids, grano-syenites and some other formations.

  9. Formation of a few highly silicate formations of quartz-feldspar and pure quartz composition. 

 

 

 

Geologically, the rocks of the first intrusive phase form a conditionally circular structure around the central ellipsoid body of syenites of the so-called second intrusive phase with “thin” western and southwestern parts and large amplification in the north, east and southeast. The northern and western outer contours of the intrusion are a very winding line. In the same areas, there are numerous xenoliths of host rocks, some of which are protrusions of underlying terrigenous-sedimentary and volcanogenic formations, and some are rock pendant. Therefore, it can be assumed that the minimum thickness of alkaline rocks is in the western, northwestern, and northern segments of this structure, and the maximum is in the east and northeast.

 

Noteworthy is the presence in feldspathoid and alkaline syenites (including synnyrites) from the western part of the massif of a number of specific minerals (garnet, diopside, high-magnesium dark micas, forming discontinuous linear zones with a thickness from a few millimeters to several centimeters). The origin of them can be explained only by the borrowing of material from the enclosing Precambrian formations, which also testifies in favor of a small supply of magmatic material in this part of Synnyr.

The processes of layering of the main intrusive phase have different degrees of occurrence in different parts of the massif. Within the inner ultrapotassic synnyr-bearing zone, the widest northeastern part is distinguished, where the maximum thickness of alkaline rocks of the Synnyr complex is noted. Here, under the conditions of a long cooling period of large volumes of intrusive melts, several successive rhythms were formed, containing ultrapotassium kalsilite and nepheline-kalsilite syenites (synnyrites) and nepheline syenites (leucocratic component), as well as alkaline syenites of different melanocraticity, shonkinites, and extremely rare biotitized pyroxenites (meso-melanocratic component). The bedding of rocks of the layered series, determined by the overall angle of incidence of contacts between the bodies of leucocratic and melanocratic rocks, can be estimated within the license area and the adjacent territory as very gently sloping with angles of 15-30 degrees northwest points, changing to subhorizontal with distance from the massif center. Leucocratic feldspathoid rocks have some petrographic-mineralogical differences depending on their position in the section of the layered series. Thus, syenites of relatively high-lying bodies are characterized by the maximum amount of pseudo-leucite symplectite structures in the ratio of potassium feldspar and feldspathoids and the minimum content of dark-colored minerals; and nepheline-kalsilite syenites. Then, with depth, rocks of a partially or completely granular structure appear, the amount of pyroxene and biotite slightly increases, and the deepest horizons are composed of granular kalsilite and nepheline-kalsilite syenites. Also with a noticeable increase in the content of dark-colored minerals, apatite becomes a secondary rock-forming with a content of up to 2-4 volume percent. Meso-melanocratic syenites and shonkinites form bodies of two morphological types. The former are represented by large (up to tens of meters in vertical section) lenticular formations with numerous swellings, narrowings, and apophyses, which can form rather extended horizons of variable thickness. The latter form linear bodies consistent in thickness (first meters), the occurrence of which also fits into the general picture of the spatial orientation of the rocks of the layered series. With a similar mineralogical composition, the linear bodies of melanocratic rocks are characterized by an increased content of apatite and sulfides. It should be noted that our surveys on the northern and eastern flanks of the licensed area and beyond show a wider distribution of synnyrites here than previously thought, which makes it possible to significantly expand the development zone of ultrapotassic rocks in the northeastern part of the Synnyr massif.

 

 

 

 

 

Further, the ultrapotassium main synnyritic-bearing part of the massif is followed by an intermediate zone with very vague contours, composed mainly of apofellespathic light-mica-potassium feldspar metasomatites, nepheline syenites, synnyrites and an insignificant amount of melanocratic rocks; in the marginal parts of the massif there are only large fields of nepheline and nepheline-bearing syenites with relatively small areas of secondary microcline-muscovite rocks and rare thin bodies of synnyrites, mesocratic syenites and shonkinites.

 

The vein-dike rocks associated with the layered series of the first intrusive phase in more than 90% of cases are represented by leucocratic microcline syenites, which are most often occurred in the inner parts of the intrusion. Here, there are also thin crosscutting bodies of meso-melanocratic syenites and shonkinites, alkaline pegmatoid rocks are extremely rare. Spatially, most of the bodies of leucocratic microcline syenites gravitate towards the areas of junction of felspathoid syenites and mesocratic syenites.

 

Sodalite and sodalite-cancrinite syenites form a large, diffusely contoured field of distribution south of the main zone of development of kalsilite and nepheline-kalsilite synnyrites at the intersection of large fault structures of northeastern and southwestern strikes. Small local occurrences of these rocks are also noted in other parts of the massif, however all of them are confined to areas of diagonal tectonic faults. At the moment, some questions of the origin of these formations remain not fully clarified, although most of the data support their metasomatic genesis – in the bulk of the studied samples, the replacement of primary nepheline by sodalite and cancrinite is established. Outcrops of poorly studied albitites and scapolitites are also confined to the tectonic diagonal zones of northeastern and northwestern striking, which indicates the occurrence of sodium-calcium metasomatism along it, the place of which in the sequence of formation of the rocks of the Synnyr massif remains open. It should be noted that areas of enrichment with zircon, monocyte, and thorite are associated with albitized rocks.

 

The main part of faults within the ultrapotassium complex are accompanied by crushing zones of different thickness, along which halos of metasomatically altered rocks are observed. Very often, the solid substitution of feldspathoid syenites by microcline-zeolite with kaolin and light hydromica rocks gradually gives way to focal microclinization with distance from the axes of tectonic faults. In shonkinites and associated melanocratic syenites, there is an intensive development of iron hydroxides, the appearance of secondary biotite, very rarely amphibole. In addition, there are crushing zones (from the first centimeters to the first meters of thickness), in which the fragmental synnyrite substrate is cemented by melanocratic biotite-pyroxene-titanite material. It should be noted that the Synnyr massif is located in the junction zone of two large tectonic structures (northeast and northwest striking), the formation of which occurred in the Early Proterozoic, while individual occurrences of activity along them are traced to the present day. Therefore, disintegration processes within the rocks of the ultrapotassium complex began immediately from the moment of its formation, constantly renewing itself as the intrusion developed.

 

At the stage of areal metasomatic alteration of the rocks of the previous intrusive stage of the formation of the Synnyr magmatic buildup, light-mica-potassium feldspar formations appeared, in which the preservation of “shadow” textural and structural features of the primary feldspathoid-bearing rocks can be observed. Despite significant differences in the compositions of various nepheline and calsilite syenites, the metasomatites arising from them are characterized by stable chemical and mineralogical parameters – the rocks contain about 70% microcline and up to 30% light mica, with almost complete absence of dark-colored minerals.

 

The next major stage in the formation of the Synnyr massif was considered to be the introduction in the central part of the intrusive buildup of an ellipse-like stock-like body in a horizontal section of alkaline syenites, often trachytoid potassium feldspar-pyroxene-amphibole rocks with biotite and an insignificant admixture of nepheline rocks. Previous authors have consistently indicated that the trachytoidity of the central stock always agrees or subconforms to the contour line of this magmatic body. However, more than 80% of the area here does not have a bedrock exposure, and where it is found, there is often a sharp change in the textural and structural characteristics (including the orientation of trachytoidity) even within small outcrops. In addition, massive varieties are relatively widespread. A number of works in the past and recent studies show that the formation of rocks of the so-called second magmatic phase occurred through the introduction of at least two large intrusive bodies, while alkaline trachytoid syenites occupy up to 80% of the central magmatic buildup in the central, southern and eastern parts, and the western and the northwestern segments are filled with quartz-bearing syenite and alkaline granite.

 

In the zones of the southwestern and northern contacts of the Synnyr massif, rather large bodies of alkaline granites are noted, which have crosscutting relationships both with the host Precambrian strata and with feldspathoid syenites. The question of whether the granitoid bodies belong to an alkaline high-potassium complex remains open, but we can confidently speak of a single geological-tectonic position of these intrusive formations and similar age parameters.

 

 
 
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