ON THE DEPTH FACIES AND THERMODYNAMIC CONDITIONS OF THE FORMATION OF THE LEBEDEV PLUTON

(Gorny Altai)

V. I. Yarygin

(Presented by the scientific seminar of the Department of Petrography)

The study of questions of facies character and of the thermodynamic conditions of formation of granitoid plutons is necessary not only from a theoretical point of view, but also in connection with assessment of the potential possibilities of such massifs with respect to mineralization. The depth of the erosional section is also of great importance, since the apical part of an intrusion usually contains significant ore bodies. Proceeding from these premises, the deciphering and tracing of those external conditions and physicochemical processes as a result of which the rocks of the Lebedev pluton—located in the basin of the Lebed River at the junction of Gornaya Shoria and Gorny Altai—were formed are of considerable interest. This article deals with questions of the depth and thermodynamic conditions of formation of the pluton. A quantitative and qualitative consideration of these characteristics will make it possible subsequently to model the principal features of its petrogenesis.

The depth of formation of massifs is of the greatest importance for the rate of cooling of magma, the behavior of volatiles, the formation of postmagmatic solutions, and so forth. The scientific basis for distinguishing depth facies was given by M. A. Usov (10), and was then continued by Yu. A. Kuznetsov (4), V. K. Monich (5), N. D. Sobolev (8), and others. According to modern classifications of the facies of magmatic rocks, the Lebedev pluton belongs to plutons of the intermediate-depth batholithic facies. The massifs of the pluton are characterized by the development of marginal hybrid zones with an aureole of pyroxene–amphibole–plagioclase hornfelses and skarns, and by series of aplite–lamprophyre veins. In the plagiogranites and trondhjemites, myrmekitic structures are observed, as well as graphic structures of silicification at the contact with the enclosing rocks. All these features of the internal processes of the magma, in combination with its interaction with the surrounding rocks, are qualitative facies indicators of intermediate depths of massif formation. The quantitative magnitude of the depth of formation of the Lebedev pluton may be judged from its relationships with the enclosing strata. It is known that, at the present erosional level, in the north the intrusion cuts the Kourin (Lower Kaim) deposits of the Middle Cambrian, while in the south it cuts the underlying Kondom (Kayanchin) suite and is transgressively overlain by deposits of the Lower Ordovician. Consequently, above the level of the present section of the intrusion there lay rocks with a thickness of about 5000 m, if the total thickness of the strata not cut by the Mundybash—

of the [[unclear: suite name]] $\approx 4450\ m$ (Upper Kaimskaya) $\mathrm{Cm}_2$ and the lower part of the Bolshekitatskaya $\approx 500\ m$ (Elandinskaya) $\mathrm{Cm}_2$ suites developed in the area. Intrusions with a thickness of about $5000\ m$ of roof rocks, according to the classification of depth, correspond to medium-depth intrusions (6). Proceeding from an estimate of the depth of formation, the pressure of the roof is approximately equal to 1250 atm. The present-day erosional section of the southern part of the pluton is probably at greater depths than the erosional section of the northern part, as is indicated by the intensive processes of contact metamorphism observed in the south (development of crystalline schists and amphibolites), micropegmatitic silicification, and myrmekitization. In addition, the ratio of magnesium to potassium in plagiogranites is greater (4–8) than in the granodiorites (1.5–2) of the northern part of the pluton, which reflects a tendency toward accumulation of potassium in the upper (apical) zones of the intrusive body, and magnesium—in the lower zones [1]. The temperature of crystallization of the magma was determined from the type of contact-metamorphic facies and by T. Barth’s feldspar method.

The hornfelses of the northern and eastern contacts of the pluton belong to the group of hornblende-hornfels facies [9]. The most representative associations are: quartz–muscovite–plagioclase and plagioclase–hornblende—diopside—tremolite. Turner and Verhoogen believe that, under the depth conditions of the Lebedev pluton, such associations should have formed at temperatures of 550°–700°C.

Determination of the temperatures of magma crystallization from the graph of the dependence of the distribution coefficient of the albite component in feldspars on temperature, proposed by T. Barth [2], may contain significant errors, since in constructing this graph the reference points were chosen very subjectively, without taking into account equilibrium associations and the composition of the feldspars. Recently experimental data have begun to appear; relying on these, I. D. Ryabchikov [7] calculated a temperature diagram of the equilibrium distribution of the albite component between coexisting feldspars. In our investigations we used Ryabchikov’s diagram.

It is evident from Table 1 that Ryabchikov’s diagram gives considerably lower temperatures and a smaller scatter of values than Barth’s graph. Since the temperatures determined from Ryabchikov’s diagram correspond best to the temperatures of formation of the hornfels facies of the Lebedev pluton, the temperature values of 650–700°C for granodiorites and quartz monzonites; 600°C for trondhjemites and plagiogranites; and 520°C for aplite-granites may be regarded as relatively reliable.

An important characteristic of the state of the system is also the amount of water dissolved in the magma. The water content in the melts of the Lebedev intrusions can be judged from the character of the partial pressure of oxygen and from data on the solubility of $\mathrm{H_2O}$ vapor in silicate melts. Osborn experimentally demonstrated, in terms of the system $\mathrm{FeO—Fe_2O_3—MgO—SiO_2}$ [13], two different trends of crystallization of basaltic magma as a function of the character of the partial pressure of oxygen, and transferred them to a simplified diagram (Fig. 1).

The tholeiitic crystallization trend is characterized by a decrease in $P_{\mathrm{O}_2}$ and by reducing conditions of the medium. The decrease in $P_{\mathrm{O}_2}$ is caused by absorption of oxygen by the system by crystallizing silicates under conditions of rapid cooling of the magmatic chamber. The reality of such a process at the level of our knowledge is obvious, judging from natural associations of magnesian-iron minerals and calculated values of the partial pressure of oxygen on the basis of the ratio,

of ferrous and ferric iron [12]. Later, however, Kennedy [3] abandoned quantitative estimates because of the influence of carbon and hydrogen on the degree of oxidation of iron, but he noted the FeO : Fe₂O₃ ratio as a qualitative criterion. Judging from the position on

Table 1

Silicate analyses of rocks of the Lebedev pluton

Osborn’s diagram (Fig. 1), the figurative points of pyroxenite, gabbro, and ferrogabbro from the Yaman-Sadry body show that crystallization of the rocks of this body took place at negligible \(P_{\mathrm{O}_2}\), and consequently at a low water content. Under these conditions, pyroxene and olivine formed at different stages of crystallization (sample 1612б), while iron accumulated in the residual liquid (sample 1080). However, the crystallization of anhydrous silicates alone led, in the final stage, to an accumulation of water, which caused the formation of pegmatitic gabbro with amphibole. Such a crystallization process in geosynclinal regions is possible, in Bickford’s opinion [11], in the case of a magmatic chamber open to volatiles, which could have been a fracture of deep origin. Probably the gabbroid bodies enclosed within the Lebedev pluton have a similar mechanism of formation. As for the granitoids of the pluton, their figurative points on Osborn’s diagram are localized parallel to curves 2—4—5—6—7 and IV—V—VI, implying a constant or increasing \(P_{\mathrm{O}_2}\), i.e., a consistently high water content, which is ensured by its diffusion into the magma from the enclosing rocks [3, 13]. The influence of increasing \(P_{\mathrm{O}_2}\) is clearly demonstrated by the mineral composition. In the initial period of crystallization, ferric-iron minerals (spinel, magnetite) form; these are often replaced by secondary minerals. In the late stages the melt becomes enriched in silica. The dark-colored minerals contain

Fig. 1. Curves for magmatic series (after Osborn): IV—VI—andesite–dacites crystallizing with increasing \(P_{O_2}\); 2—4—5—6—7—basalt–andesites of the Cascades—increasing \(P_{O_2}\); \(Sk_1\), \(Sk_2\), \(Sk_3\)—rocks of the Skaergaard intrusion, crystallizing with decreasing \(P_{O_2}\); 1612б—777—1080—gabbroids of the Lebedev pluton; 1526а—1630—1487—end members of the granitoids of the Lebedev pluton.

hydroxyl group (amphibole, biotite). The upper limit of the water content in the granitoid melt can be estimated from the calculation of the depth of emplacement of the pluton. From Goranson’s diagram [3] it is evident that, at a roof pressure of the Lebedev pluton of 1250 atm, the content of dissolved water in the melt should not exceed 6 wt. %. From the index (the value of the ratio of the acid rim to the core in zoned plagioclases) one may judge the water content in the magma at the onset of crystallization [14]. Consequently, the melt from which the quartz monzonites, granodiorites, quartz diorites, and diorites of the Lebedev pluton formed, with an index of 1 : 10, could not have contained more than 0.6% water at the beginning of crystallization; and the melt of the plagiogranite rocks, with an index of 1 : 5, probably contained 1.2% water.

In conclusion, it may be noted that, by all thermodynamic criteria and qualitative facies features, the Lebedev pluton belongs to the batholiths of the medium-depth facies, according to Yu. A. Kuznetsov (4).

REFERENCES

1. N. N. Amshinsky. Certain regularities in the distribution of accessory elements in granitoids. Izv. AN SSSR, ser. geolog., No. 2, 1964.

2. T. F. Barth. Measurement of paleotemperatures of granitic rocks. Readings in Memory of V. I. Vernadsky, IV, Acad. Sci. USSR Press, 1962.

3. J. Kennedy. On the role of water in magma. In: The Earth’s Crust, 1957.

4. Yu. A. Kuznetsov. On the principles of distinguishing and classifying facies of magmatic rocks. In: Basic Ideas of M. A. Usov in Geology. Acad. Sci. Kazakh SSR Press, 1960.

5. V. K. Monich. On the question of facies of magmatic rocks of Kazakhstan. Izv. AN SSSR, ser. geolog., issue 5, 1952.

6. V. A. Nikolaev. On mapping and the geological study of intrusions in complexes of lower–middle stages of regional metamorphism. In: Methods of Geological Mapping of Metamorphic Complexes. Gosgeoltekhizdat, 1957.

7. I. D. Ryabchikov. Improvement of Barth’s feldspar geological thermometer. In: Mineralogical Thermometry and Barometry. Nauka Press, 1965.

8. I. D. Sobolev. Facies of granitoids and mineralization. In: Basic Ideas of M. A. Usov in Geology. Acad. Sci. Kazakh SSR Press, 1960.

9. F. Turner and J. Verhoogen. Petrology of Igneous and Metamorphic Rocks. IL Press, 1961.

10. M. A. Usov. Facies and phases of intrusives. In: Basic Ideas of M. A. Usov in Geology. Acad. Sci. Kazakh SSR Press, 1960.

11. M. E. Bickford. Petrology and structure of layered gabbro, Pleasant Bay, Maine. Jour. Geol. VII, No. 2, 1963.

12. G. C. Kennedy. Equilibrium between volatiles and iron oxides in igneous rocks. Am. Jour. Sc. V. 246, 1948.

13. E. F. Osborn. Role of oxygen pressure in the crystallization and differentiation of basaltic magma. Am. Jour. Sc. V. 257, 1959.

14. I. A. Vance. Zoning in igneous plagioclase: normal and oscillatory zoning. Am. Jour. Sc. V. 260, No. 10, 1962.