Comparative analysis of basaltoids of volcanic sequences of different ages in the Yuliya Mine area (Kuznetsk Alatau)
Abstract Generated abstract
This paper compares basaltoids from three volcanogenic strata in the Yulia Mine area of the Kuznetsk Alatau, assigned to the Upper Proterozoic, Middle Cambrian, and Lower Devonian, in order to clarify whether disputed volcanic units belong to one or several stratigraphic suites. Using petrographic, mineralogical, petrochemical, and physical property data, the study contrasts phenocryst assemblages, groundmass structures, alteration minerals, chemical indices, magnetic properties, and normative compositions. The comparison shows that each stratum has a distinct set of features: Middle Cambrian basaltoids are consistently porphyritic and relatively magnesian, Lower Devonian rocks show better preservation of primary minerals and higher sodium alkalinity, and Upper Proterozoic rocks differ in titaniferous character and alteration patterns. These differences support treating the three volcanic strata as separate complexes related to different tectonic stages.
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COMPARATIVE ANALYSIS OF BASALTOIDS FROM VOLCANIC STRATA OF DIFFERENT AGES IN THE AREA OF THE YULIA MINE
(KUZNETSK ALATAU)
D. I. TSAREV
(Presented by the interdepartmental scientific seminar)
In the area of the Yulia mine (Batenev Ridge of the Kuznetsk Alatau), among the volcanogenic rocks, the author has distinguished three effusive strata of different ages, occupying a definite stratigraphic position: Upper Proterozoic, Middle Cambrian, and Lower Devonian. The age of the first is determined from its position among paleontologically characterized strata. It extends as a belt in an east-northeasterly direction from the valley of the Karysh River through Mount Romanovskaya and Podtemnyi Log. Its thickness reaches 2000 m.
The Middle Cambrian age of the Katyushka effusive stratum, about 500 m thick, was established from the trilobite fauna collected from interbeds of sedimentary rocks among the effusives by I. I. Koptev (1968).
Lower Devonian volcanogenic rocks occupy areas along the banks of the Sukhoi Erba River (Bograd Bridge) and the Bolshoi Erba River (Bol’she-Erba effusive stratum). These volcanic rocks unconformably overlap faunistically characterized sedimentary strata of the Middle Cambrian and, in material composition and degree of alteration, correlate well with the volcanites of the Bograd and Voroshilovo-Troitsk sections of the Minusinsk Depression, according to I. V. Luchitsky (1960).
Unlike the Upper Cambrian and Middle Cambrian volcanic strata, which are essentially basaltoid in composition, the Lower Devonian stratum, in addition to basaltoids, contains lava flows and beds of volcanoclastics of intermediate, acidic, and subalkaline composition. The thickness of this stratum reaches 850 m.
The inevitable subjectivism in geological mapping leads different researchers to different interpretations of the stratigraphic position and, consequently, the age of the same volcanic strata. Thus, A. A. Mossakovsky (1963), E. V. Shirokova (1968), and others combined the volcanogenic strata of Mount Romanovskaya and the Katyushka stratum into one suite. The former called it the Syiskaya suite ($Cm_{1-2}$), the latter the Berikul’skaya suite ($Cm_2$). As for the effusives of Bograd Bridge and the Bolshoi Erba River, some geologists considered them Middle Cambrian, others Lower Devonian.
In order to resolve more objectively the question of whether the various volcanogenic strata belong to one or to different suites, the author of the present work studied in considerable detail the material composition and the petrographic and physical properties of their rocks, and carried out a comparative analysis of these properties for rocks of the basalt group. The indices of the properties, in the form of arithmetic mean values, are presented in the following tables.
Table 1
1. Indicators of intratelluric crystallization
a. Frequency of occurrence of macrostructures
| Volcanic strata | Porphyritic (%) | Aphyric + sporadophyric (%) |
|---|---|---|
| Upper Proterozoic | 26 | 74 |
| Middle Cambrian | 100 | — |
| Lower Devonian | 43 | 57 |
b. Frequency of occurrence of pyroxene and plagioclase phenocrysts in basaltoids of porphyritic structure
| Volcanic strata | Pyroxenes (%) | Plagioclases (%) |
|---|---|---|
| $Pt_3$ | 29 | 100 |
| $Cm_2$ | 70 | 100 |
| $D_1$ | 35 | 100 |
c. Content and morphology of porphyritic phenocrysts
| Volcanic complexes | Maximum content of porphyritic segregations (%) Pl | Maximum content of porphyritic segregations (%) Py | Maximum dimensions of porphyritic segregations, mm Pl | Maximum dimensions of porphyritic segregations, mm Py | Maximum relative lengths of columnar segregations Pl | Presence of traces of zoning Pl | Presence of traces of zoning Py | Frequency of occurrence (%) melting and resorption Pl | Frequency of occurrence (%) melting and resorption Py | Frequency of occurrence (%) glomerocrysts of plagioclase |
|---|---|---|---|---|---|---|---|---|---|---|
| $Pt_3$ | 35 | 10 | 1.9 | 1.2×0.6 | 4.5 | units | — | 8 | — | 6 |
| $Cm_2$ | 21.5 | 5 | 1.54 | 2.6×1.7 | 4.0 | units | units | 25 | — | 32 |
| $D_1$ | 40 | units | 2.6 | 1.2×0.8 | 6 | units | units | 24 | — | 33 |
d. Primary composition of intratelluric phases
| Volcanic complexes, strata | Average composition of pyroxenes (%) Mg | Average composition of pyroxenes (%) Fe+Mn | Average composition of pyroxenes (%) Ca | Average indices of primary plagioclases $N^\circ$ | Average indices of primary plagioclases $2V$ |
|---|---|---|---|---|---|
| $Pt_3$ | 37 | 21 | 42 | 51 | 78 |
| $Cm_2$ | 23 | 29 | 48 | 54 | 75 |
| $D_1$ | 31 | 34 | 35 | 60 | 33 |
Table 2
2. Indicators of late crystallization of lavas
a. Structural types of groundmasses
| Volcanic strata | Felsitic | Pilotaxitic | Allotriomorphic-granular | Intersertal | Hyalopilitic | Microlitic | Tholeiitic | Microleritic |
|---|---|---|---|---|---|---|---|---|
| $Pt_3$ | 5 | 33 | 5 | 22 | 28 | 7 | — | — |
| $Cm_2$ | 12 | 6 | — | 20 | 59 | 3 | — | — |
| $D_1$ | — | — | — | 19 | 36 | 3 | 31 | 6 |
Table 3
3. Indicators of greenstone alteration
and early postvolcanic mineralization
| Volcanic strata | Preservation of the original volume of the plagioclase phase (%) — in phenocrysts | Preservation of the original volume of the plagioclase phase (%) — in the groundmass | Preservation of pyroxenes (%) |
|---|---|---|---|
| $Pt_3$ | 22 | 12 | 15 |
| $Cm_2$ | 30 | 22 | 35 |
| $D_1$ | 67 | 57 | 42 |
| Minerals | By plagioclase of phenocrysts $Pt_3$ | By plagioclase of phenocrysts $Cm_2$ | By plagioclase of phenocrysts $D_1$ | By pyroxenes $Pt_3$ | By pyroxenes $Cm_2$ | By pyroxenes $D_1$ | In the groundmass $Pt_3$ | In the groundmass $Cm_2$ | In the groundmass $D_1$ | In amygdule fillings $Pt_3$ | In amygdule fillings $Cm_2$ | In amygdule fillings $D_1$ |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sericite | 63 | 82 | 78 | — | — | — | 12 | 17 | 43 | — | — | — |
| Saussurite | 22 | 14 | 29 | — | — | — | 5 | — | 7 | — | — | — |
| Chlorite | 50 | 36 | 9 | 83 | 69 | 60 | 88 | 79 | 60 | 75 | 95 | 65 |
| Carbonate | 12 | 50 | 4 | 68 | 72 | 36 | 43 | 61 | 9 | 50 | 52 | 62 |
| Hematite | — | — | 1 | — | — | 11 | — | — | — | — | — | — |
| Leucoxene | — | — | — | — | — | — | 12 | — | 17 | — | — | — |
| Pelite | — | 3 | 8 | — | — | — | 7 | — | 4 | — | — | — |
| Epidote, zoisite | 9 | 25 | 4 | 8 | 12 | — | 24 | 25 | 5 | 25 | 17 | — |
| Albite | 60 | 60 | 4 | — | — | — | ? | ? | — | — | — | — |
| Quartz | 6 | 18 | 1 | 42 | 14 | — | — | 18 | 3 | 34 | 47 | 8 |
| Magnetite | — | — | — | 17 | 43 | — | ? | — | ? | — | 34 | — |
| Actinolite | — | — | — | — | 3 | — | 7 | 4 | — | 9 | — | — |
Table 4
4. Average petrochemical indices of rocks of the basalt–andesite–basalt group
| Strata | $\alpha$ | $\dfrac{Na_2O}{K_2O}$ | $TiO_2$ | $FeO''$ | $Fe'$ | $\dfrac{MgO}{FeO}$ | $\dfrac{MgO}{CaO}$ |
|---|---|---|---|---|---|---|---|
| $Pt_3$ | 36.6% | 2.42 | 1.04—2.45% | 7.62—13.9% | 0.72 | 0.54 | 0.52 |
| $Cm_3$ | 34.0% | 1.34 | 1.10—1.74% | 7.37—10.22% | 0.65 | 1.04 | 0.69 |
| $D_1$ | 26.7% | 3.8 | 0.88—2.36% | 7.5—11.0% | 0.75 | 0.91 | 0.54 |
Note: $\alpha$ is the anorthite number according to D. S. Shteinberg; $FeO''$ is the total conventional ferrous oxide, according to D. S. Shteinberg $(FeO'' = Fe_2O_3 \cdot 0.9 + FeO)$; $Fe'$ is the ferruginosity
\[ Fe'=\frac{FeO+Fe_2O_3}{FeO+Fe_2O_3+MgO}. \]
The dependence of titanium dioxide on the total conventional ferrous oxide is roughly expressed by equations of a straight line with a slope coefficient, the slope coefficient decreasing from the Proterozoic to the Cambrian and to the Devonian. Thus, for the rocks of the Upper Proterozoic volcanic complex this dependence is expressed by the equation $TiO_2 = 0.345\,FeO''$, for the rocks of the Middle Cambrian complex by the equation $TiO_2 = 0.3\,FeO''$, and for the rocks of the Lower Devonian complex by the equation $TiO_2 = 0.264\,FeO''$.
Table 5
5. Indices of relationship of volcanites according to A. Rittmann (1964)
| Volcanic complexes | $\sigma$ | Ratio of weight % of alkalis | Character of the related group |
|---|---|---|---|
| $Pt_3$ | 4.5 | $\mathrm{Na_2O} > \mathrm{K_2O}$ | Atlantic transitional—to Pacific |
| $Cm_2$ | 5.85 | $\mathrm{Na_2O} > \mathrm{K_2O}$ | Atlantic weak |
| $D_1$ | 2.78 | $\mathrm{Na_2O} > \mathrm{K_2O}$ | Pacific medium |
\[ \sigma=\frac{(\mathrm{Na_2O}+\mathrm{K_2O})^2}{\mathrm{SiO_2}-43} \]
(weight % of the analysis) is the relationship index. The table includes the arithmetic mean data for each complex.
On the basis of the data presented above, the following conclusions may be drawn regarding the characteristic distinctive features of the volcanic strata studied.
-
Among the effusive and pyroclastic rocks of the basalt group of the Lower Devonian and especially of the Upper Proterozoic complex, aphyric and sporadically phyric varieties are predominant. The basaltoids of the Middle Cambrian complex, however, have a porphyritic structure in 100% of cases.
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In porphyritic varieties of basaltoids, pyroxene phenocrysts occur most often in the Middle Cambrian complex and least often in the Upper Proterozoic. The maximum content of pyroxene phenocrysts, by contrast, is observed in Upper Proterozoic volcanic rocks, and the minimum in Lower Devonian rocks.
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Plagioclase phenocrysts are present in all basaltoids of porphyritic structure. Their maximum content and maximum sizes are observed in Lower Devonian porphyrites, and the minimum in Middle Cambrian ones. Melted and resorbed plagioclase phenocrysts in Cambrian and Lower Devonian porphyrites occur three times more often, and glomerocrysts five times more often, than in Proterozoic ones.
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The pyroxenes in Proterozoic volcanic rocks correspond in composition to augite; in Cambrian rocks, to ferrosalite; and in Devonian rocks, to pigeonite-augite.
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The anorthite content in the primary plagioclases of the phenocrysts increases from Proterozoic to Devonian porphyrites.
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The most widespread groundmass structures of the basaltoids are: in the Upper Proterozoic complex, pilotaxitic, intersertal, and hyalopilitic; in the Middle Cambrian, hyalopilitic and intersertal; in the Lower Devonian, hyalopilitic, tholeiitic, and intersertal.
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The preservation of primary minerals (plagioclases and pyroxenes) increases from Proterozoic rocks to Devonian ones. Moreover, the percentage of preservation of phenocryst minerals in Devonian basaltoids is three times higher than in Proterozoic ones, and the preservation of the groundmass is five times higher.
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Secondary mineral formations after plagioclases of phenocrysts are most developed in rocks of the basalt group: in the Proterozoic complex—sericite, chlorite, albite; in the Cambrian—sericite, chlorite, carbonate, epidote, zoisite, albite; in the Devonian—sericite, saussurite. After pyroxenes: in Proterozoic rocks—chlorite, carbonate,
quartz; in the Cambrian—chlorite, carbonate, magnetite; in the Devonian—chlorite, carbonate, hematite.
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The calcareousness of the rocks decreases from the Proterozoic to the Devonian.
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The highest proportion of sodium oxide in the sum of alkalis belongs to the Lower Devonian basaltoids, and the lowest to the Cambrian ones.
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In terms of titanium dioxide content, the Upper Proterozoic rocks of the basalt–andesite-basalt group are normally titaniferous (D. S. Shteinberg, 1964), whereas the Cambrian and Lower Devonian rocks are low-titanium.
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The ferruginosity of the rocks is highest in the Lower Devonian and Upper Proterozoic strata.
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Magnesiality is highest in the Cambrian rocks and lowest in the Upper Proterozoic rocks.
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The dependence of titanium content on iron is different for the various volcanic complexes.
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The density of rocks of the basalt group increases from the Upper Proterozoic to the Lower Devonian.
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Magnetic susceptibility and residual magnetization are highest in the Cambrian rocks and lowest in the Proterozoic rocks. The Lower Devonian rocks proved to be the most magnetically stable. The lowest magnetic stability in the Cambrian basaltoids is, in all probability, caused by significant contact metamorphism of the volcanogenic stratum.
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The basaltoids of the Upper Proterozoic and Lower Devonian strata contain normative quartz and lack normative olivine, whereas those of the Middle Cambrian stratum contain normative olivine and do not contain normative quartz.
From the comparative characterization presented, it is evident that the three volcanic strata studied have their own individual properties, distinct from one another, characteristic of particular tectonic stages in the development of the Earth’s crust: a certain degree of preservation of primary minerals, their own chemistry, and their own related groups among the products of certain types of modern volcanism.