RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 17, ES1001, doi:10.2205/2017ES000578, 2017
A. F. Grachev
Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
Newly obtained major-component, trace-element, and rare-earth element (REE) analyses are used to gain insight into the nature of Late Cenozoic volcanism in the Udokan Range. The composition of the lavas is analyzed in each lava flow and it is demonstrated that the rocks define a bimodal differentiated series with a Daly gap. The absence of andesite is seen not only in the summarizing diagrams but also in individual vertical sections of the volcanic rocks. The petrochemical specifics of volcanics in the Udokan Range involves broad variations in the TiO$_2$ concentrations: from 0.22 to 4.41 wt %. The REE patterns of the volcanics show a trend analogous to that of oceanic islands basalts (OIB). The compositional specifics of the rocks suggest that the basalt-trachyte series was derived from a single mantle source related to a mantle plume.
The Udokan Range in the northeastern segment of the Baikal Rift Zone affiliates with the Chara Rift and is noted for volcanic activity that gave rise to both plateau basalts and central-type volcanoes.
The very first data on the age of volcanism in the Udokan Range were obtained in the 1960s: tuff-breccia found in the bottom portion of a basalt pile contained a pollen spectrum dated at the Pliocene-Quaternary [Muzis, 1970]. Later a young (Quaternary) age of the basalt was confirmed in [Solonenko, 1966], although no new evidence had then been obtained.
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| Figure 1 |
Our studies of five vertical sections in the lowermost portion of the basalt-underlying unit resulted in finds of volcanic-sedimentary or sedimentary rocks that rest on either older basalt or granite (Figure 1). Pollen data (obtained by A. N. Smirnova, V. A. Belova, M. P. Grichuk, and V. M. Klimanova) have shown that the rocks underlying the basalt are of Quaternary age, no older than the older epoch (at most). The age of the underlying basalt was uncertain.
The very first K-Ar dates of the basalts were obtained in 1970 at a laboratory of the Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, using basalt samples from the areas of Lake Burichi and Lake Kuas: $2.5 \pm 0.5$ and $5 \pm 1.0$ Ma, respectively.
Later the values of 9.5 and 16 Ma were published in [Kiselev et al., 1979]. The 35 dates published in [Bagdasaryan et al., 1981] average at $5.55 \pm 3.35$ Ma (with two deviating values of 11.75 and 14.0 Ma). If these two dates are ignored, the average age of the Udokan basalt is $4.27 \pm 1.67$ Ma. The dates (13 measurements) obtained thereafter in [Amirkhanov et al., 1985] yield an amazing average of $24 \pm 4$ Ma, but the authors do not report their sampling sites, the composition of the rocks, or detail of the dating technique. Similarly presented data can be found in [Stupak, 1987]. The absence of information on the Ar concentrations, blanks, and details of the analytical techniques make it impossible to utilize these data.
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| Table 1 |
Later the samples dated by M.M. Arakelyants at IGEM were dated again in [Drubetskoi and Grachev, 1987] (Table 1), with all the dates consistent within the analytical errors. The average age values of the Udokan basalt is $4.40 \pm 1.55$ Ma (Table 1) and practically exactly coincide with the value of $4.27 \pm 1.67$ Ma in Bagdasaryan et al., 1981].
Several K-Ar and $^{40}$Ar/$^{39}$Ar dates presented in [Rasskazov et al., 2000] indicate that the volcanics were erupted at 14, 8.7–7.4, 4–2.4, and $<1.8$ Ma.
Below we discuss the specifics of volcanism in the Udokan Range over the past 2.5–3.0 Ma, with the total volume of the volcanic products amounting to no less than 500 km$^3$. Recently made chemical analyses of the volcanics and their concentrations of trace elements are then utilized to discuss the nature of volcanism in the Udokan Range.
The deeply eroded river valleys expose the significantly dissected pre-basalt topography of the Udokan Range, with elevations varying for as much as 400–600 m. Products of the volcanic activity have smoothed the topography by filling all lows with lava flows, so that the modern surface topography is amazingly flat and is thus contrastingly different from that before the basalt was erupted.
The topography of the lava fields in the Udokan Range is distinguished for a stepwise morphology, with three major levels at elevations of 1500, 1800, and 2100 m. These levels are eroded in the primary surface of the plateau basalts, whose relics are found in the crest portion of the Udokan Range at elevations of approximately 2400 m. Along with these major cycle surfaces (which are pediments), local steps ranging from 5–10 to 100–150 m in width are widespread, and their number depends on the relative height of the slope. A feature usually shared by these surfaces is that they taper off down the strikes of the lava flows.
Facies of the lava flows. All of the examined lava sheets were erupted in subaerial environments. Horizontal flows are most widely spread in, and are typical of, the middle and upper portions of the plateau-basalt piles in the Udokan Range. Deviations from a horizontal position in the bottom portions are explained by the pre-basalt topography and are controlled by the older primary strikes and dips of the rocks. The overlying basalt flows gradually become horizontal.
If the dips of some of the basalt flows were controlled by tectonics, the whole lava pile (from it bottom to top) shows the same strikes and dips, as for example, the 190-m-thick lava pile in the area of Lake Burichi, with all 13 flows dipping $230\mbox{°}-240\mbox{°}$ SW$<15\mbox{°}-20\mbox{°}$.
The thicknesses of the basalt flows vary from 1.5–2 to 30–40 m, with older flows being thicker than the younger ones. It is hard to estimate the true length of the flows. Comparison of selected nearby vertical sections in the crest part of the Udokan Range does not allow their correlation even if the distance between the flows is as small as 500–600 m. Away from the crest, individual flows are sometimes traced for a few kilometers, as for example, in the area of the left-hand tributaries of the Eimnakh River.
The basalt flows are clearly zoned, which is discernible, thanks to well developed and thick cinder zones of red brown color, even from great distances from the outcrops and enables one to easily count the number of individual flows in a given exposed vertical section.
Most of the flows can be subdivided into clearly discernible lower, transitional, and upper (cinder) zones, which differ from one another in the fabric and color of the lava. The lower zone usually consists of massive basalt of black or dark gray color. Depending on its thickness, the porosity of the rocks can be different. If a zone is 1–3 m thick, pores are constrained to its top portion, whereas thicker zones can be subdivided into lower and upper subzones with a unit of alternating porous and massive basalt in between.
The transitional zones show gradual color variations (lilac shades appear) and an increase in the content (up to 50%) and size of the pores. The pores are usually open, ellipsoidal, and elongate along the flow direction. The transition to the cinder zone is sharp, and the boundary is uneven.
The cinder zone is typically of red-brown color and consists of cinder, cinder-lava, and lava-breccia, whose thicknesses vary from 1–2 to 5–7 m depending on their distance from the eruption center. This zone quite often hosts volcanic bombs and granite xenoliths. The rock composing the zone sometimes shows spherical parting, with the cores of the spheres consisting of lava-breccia and with porosity increasing toward the peripheries of the spheres and with the basaltic lava-breccia grading into cinder.
The lava flows are extensively fractured, and the fractures are classified into two types: vertical, in the form of columnar and block parting, and horizontal, as platy parting. The columnar parting is different in the flows of different thickness. In thin (no thicker than 5–6 m) flows, the columns extend all the way from the bottom to top of the flows, whereas the thick flows consist of lower (columns) and upper (entablature) parts. Arrays of horizontal fractures define systems of platelets whose thicknesses do not exceed 10 cm (5–6 cm on average). The boundaries between the platelets are poorly pronounced and cannot be traced for a long distance.
The explosion facies is relatively rare and comprises lavas, agglutinates, tuff, tuff-breccia, tuff-sand, and tuff-conglomerate, which are often found in the bottom portions of plateau basalt piles, and analogous rocks produced by the Holocene volcanoes (Aku, Syni, and Chepe) [Solonenko, 1966]. The rocks were also found on the surface of the plateau basalt and are concentrated near volcanic apparatuses. The clastic material of the tuff-breccia sometimes hosts volcanic bombs and lapilli up to 15–20 cm across.
The vent facies is widespread and is found mostly as dikes and much more rare necks. The dikes occur mostly in the crest part of the Udokan Range, where they are grouped in arrays of parallel dikes trending NE 40–60°. The average thickness of the dikes is 2–3 m. Locally (in the upper reaches of the Verkhnyaya Chukchudu River) dike-in-dike systems are found. The dikes very often contain xenoliths of various rocks, up to ultramafic nodules [Grachev et al., 1973].
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| Table 2 |
Estimations of the mineral proportions of the volcanic rocks (carried out using a KONTRAST equipment) demonstrate that the rocks of different facies (Table 2) have much in common, and hence, their systematics should be underlain by the chemical composition of the rocks, and their geochemical specifics calls for studying trace elements and REE in these rocks.
Although several aspects of the chemical composition of the rocks composing the lava plateau of the Udokan Range have been described elsewhere [Grachev, 1977; Kiselev et al., 1979; Polyakov and Bagdasaryan, 1986; Rasskazov et al., 1985; Solonenko, 1966; Stupak, 1987], the nature of volcanism in the Udokan Range is so far poorly understood, considering the contrasting differences between volcanism in the southwestern flank of the Baikal Rift and in the Vitim Plateau [Grachev, 1977; Kiselev et al., 1979; Solonenko, 1966].
In attempt to solve this problem, we sampled the lavas of each flow in two reference vertical sections in the area of Lake Kuas, from which lava samples had been previously analyzed (sampled along the Verkhnyaya Ingamakit, Chukchudu, and Kanksa rivers).
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| Table 3 |
The major-component and normative compositions of the rocks are reported in Table 3. The volcanic rocks from the Udokan Range contain 52.05–58.93% SiO$_2$, their Na$_2$O concentrations are usually higher than the K$_2$O concentrations, and Mg7.47–78.58 and characterizes an obvious differentiation trend.
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| Figure 2 |
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| Figure 3 |
Factor analysis (major-component method) reveals a complete and clearly pronounced Bowen differentiation trend (factor I corresponds to a load of 54.9% and involves SiO$_2$ K$_2$O Al$_2$O$_3$ Na$_2$O/MgO, CaO, and TiO$_2$), and factor II (P$_2$O$_5$, 15.2% load) reflects the role of phosphorus, whose concentration systematically varies from 0.07 to 0.73%. The enrichment of the latter component in the melt during its late differentiation can result in the onset of immiscibility between acid and mafic derivatives [Mysen et al., 1981; Ryerson and Hess, 1980; Solonenko, 1966].
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| Figure 4 |
The petrochemical specifics of volcanics in the Udokan Range is broad variations in their TiO$_2$ content, which varies from 0.22 to 4.41% (Table 2). Sampling the rocks flow by flow in a 100-m vertical section in the area of Lake Kuas (no hiatuses were detected) led us to discover (Figure 4) that high-Ti basalt $(>4.0$% TiO$_2$) composes the bottom portion of the section and gives way upward to progressively Ti poorer rocks (with TiO$_2$ concentration decreasing to 2.5%). The decrease in the TiO$_2$ concentrations is negatively correlated with an increase in P$_2$O$_5$: the Ti-poor basalt contains no more than 0.1% P$_2$O$_2$ while Ti poorer varieties contain up to $>0.6$% P$_2$O$_5$ (Table 2).
Another important trait of the rocks detected when they were studied flow by flow in the Late Pliocene–Quaternary sequence of volcanic rocks is that alkaline basalt alternates with trachyte, as was first noted in [Kiselev et al., 1979; Stupak, 1987]. The bottom of the vertical section in the vicinity of Lake Kuas is made up of trachyte with clearly seen plate parting. These rocks are overlain by a number of olivine basalt flows (total thickness 100 m), which are overlain, again, by a 16-m trachyte unit. It is important to mention that the absent of andesite is seen not only in summarizing diagrams, as on oceanic islands [Baker, 1968; Baker et al., 1964], but also in individual vertical sections of volcanic rocks in the Udokan Range.
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| Table 4 |
Table 4 reports the concentrations of trace elements and REE. According to their Cr, Ni, and Co concentrations, the Udokan volcanics are classified into two groups: one of them comprises rocks rich in Cr ($>100$ ppm), Ni ($>80$ ppm), and Co ($>50$ ppm), and the other is defined by rocks poorer in these elements. Note that these groups also differ in TiO$_2$ concentration: the high-Cr volcanics contain up to 4.4% TiO$_2$ while the low-Cr ones bear no more than 2.8%. Factor analysis of major and trace elements shows that Cr, Ni, Co, and P$_2$O$_5$ are negatively correlated with TiO$_2$, i.e., the composition of melt in the sequence of volcanic rocks in the Lake Kuas area evolved with enrichment in P$_2$O$_5$ and depletion in Cr, Ni, Co, and TiO$_2$.
These data led us to suggest that the volcanic activity evolved cyclically and resulted in alternating volcanic rocks of different major-component and trace-element composition. The high TiO$_2$ concentration in the basalt likely reflects the content of Ti-bearing minerals in the mantle source [Xirouchakis et al., 2001]. The high Cr concentration may be explained analogously.
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| Figure 5 |
The REE patterns shows that the Udokan volcanics evolved along a trend analogous to that of OIB (Figure 5), with enrichment in LREE and depletion in HREE. In contrast to OIB, both basalts from the Udokan Range and mantle nodules in these rocks typically posses very low $^3$He/$^4$He ratios Drubetskoi and Grachev, 1987].
Volcanic rocks in the Udokan Range differ from products of Cenozoic volcanic activity elsewhere within the Baikal Rift Zone in possessing unusual facies and chemical compositions.
The volcanic rocks display the following distinguishing compositional features: (1) the lavas show a bimodal distribution of their chemical compositions, with a Bowen differentiation trend and a Daly gap; this is typical of both the plateau basalts and the rocks of the central volcanoes; (2) the rocks are rich in TiO$_2$ ($>4$%); and (3) the compositional evolution of the volcanics involves enrichment in P$_2$O$_5$ and depletion in Cr, Co, and Ni. These features of volcanism in the Udokan Range are closely similar to those of volcanic processes at such oceanic islands as, for example, the Tristan da Cunha Islands [Baker et al., 1964].
Data of chemical geodynamics show that volcanism in the southwestern part of the Baikal Rift (in the Khamar Daban Range and Tunka depression) and in the Vitim Plateau developed under the effect of mantle plumes [Grachev, 1998; Johnson et al., 2005], whose presence was inferred from geophysical data [Zorin et al., 2003]. It was also hypothesized [Zorin et al., 2003] that a mantle plume occurs in the area of the Udokan Range. This puts forth the question as to which geochemical features (if any) of the Udokan lavas may reflect the influence of this plume.
In this context, it is worth mentioning first of all the high TiO$_2$ concentrations of the lavas, which reach $>4$ wt % in the basanite and basalt (Table 2). No such high TiO$_2$ concentrations are found in either any other volcanic rocks in the southwestern flank of the Baikal Rift [Grachev, 1998] or volcanics in the Vitim Plateau [Johnson et al., 2005]. It is also important that the high-Ti basalts, containing $>4.0$ wt % TiO$_2$, compose the bottom parts of the volcanic piles and give way to Ti poorer lavas (containing no more than 2.5 wt % TiO$_2$) upsection. The systematic variations in the TiO$_2$ concentrations are correlated with those of P$_2$O$_5$: the high-Ti basalt contains no more than 0.1% P$_2$O$_5$ whereas basalts poorer in Ti are richer in P$_2$O$_5$ (contain as much as 0.6%) (Table 2). It is also worth mentioning the high MgO concentrations (up to 11%) of basanite and basalt from the Udokan Range.
Associations of lavas rich and poor in TiO$_2$ are typical of areas with flood basalts worldwide [Gibson et al., 1995; Mahoney and Coffin, 1997; and several others]. However, unlike flood basalts, whose high- and low-Ti basalts differ in Sm and Nd isotopic ratios [Gibson et al., 1995; Xiao et al., 2004], these ratios in the lavas in the Udokan Range are similar, regardless of the TiO$_2$ concentrations of these rocks [Harris, 1998].
With regard for the fact that the lavas with high and low Ti concentrations occur together and are coeval, that their geochemistries are similar, and all of the lavas contain spinel lherzolite nodules, it is reasonable to suggest that the bimodal basalt-trachyte series was derived from a single mantle source related to a mantle plume. The high TiO$_2$ concentration of the Udokan basalts may reflect the content of Ti-bearing minerals in the mantle source [Xirouchakis et al., 2001].
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Received 7 October 2016; accepted 10 November 2016; published 7 January 2017.
Citation: Grachev A. F. (2017), The nature of Late Cenozoic volcanism in the Udokan Range, northwestern segment of the Baikal Rift Zone, Russ. J. Earth Sci., 17, ES1001, doi:10.2205/2017ES000578.
Copyright 2017 by the Geophysical Center RAS.