Chemical variations of chromium spinel in Medeksky, Medvezhiy Log, Malaya Shita and Tartay differentiated ultrabasic bodies, Eastern Sayan Mountains

Benedyuk Y. P.

A.P. Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia

[e-mail]

Chromium spinel is a typical mineral of basic and ultrabasic rocks. It holds information on the features of melt formation, it composition, nature and type of mineralization due to various chemical composition and early crystallization from melt.

The composition of chromium spinel was analyzed in four differentiated ultrabasic intrusions, e.g. Medeksky, Medvezhiy Log, Malaya Shita and Tartay. Cu, Ni, EPG-mineralization is associated with the massifs situated in the middle of the Eastern Sayan Mountains, between the Biryusa and Uda Rivers. The intrusions occur within the Alhadyrsky terrane which are in folded frame of the Siberian Platform foundation. Host rocks are primarily sedimentary metamorphosed and volcanogenic rocks, such as gneisses, schists, marbles, amphibolites, etc [1]. The intrusions are referred to the Barbitaysky intrusive complex of ultrabasic rocks derived by picritic magmas. The complex is dated as Neoproterozoic. Investigations of ultrabasic rocks accessory minerals composition were carried out only in south-east and middle part of The Alhadyrsky terrane [2, 3].

Compositions of chromium spinel of intrusion-forming rocks (dunite, wehrlite, plagiowehrlite and olivine gabbro) were examined by electron-probe micro analyzer Superprobe JXA-8200 (JEOL).

In the massifs studied chromium spinel occurs in all rock types in the form of rounded grains and crystals of octahedral habit. Chromium spinel makes up 5 % of rocks. There are two types of chromium spinel reflecting the order of crystallizations [2]: (I) smaller (0.05 – 0.25 mm) rounded or octahedral inclusions in olivine and (II) large (0.25 – 1 mm) irregular shape grains in rock-forming minerals interstices. Some look as homogeneous, zonal grains of chromium spinel and some others show the structures of ilmenite solid solution decay [2].

Chromian spinel in the intrusions of the Medeksky massif have highest Cr₂O₃ (23.3–57.5 mass. %, average 41.7 mass. %), relatively high MgO (2.1–14.1 mass. %, average 8.4 mass. %), Al₂O₃ (1.4–31 mass. %, average 19.4 масс. %) and FeO_tot (20.2–48.7 mass. %, average 28.2 mass %). Chromium spinel of The Medvezhiy Log massif has highest content of Al₂O₃ (5–36.3 mass. %, average 21.3 mass. %), MgO (2.5–16.5 mass. %, average 8.4 mass. %) and FeO_tot (18.3–65.3 mass. %, average 33 mass. %) and quite high content of Cr₂O₃ (17–50.2 mass. %, average 35.1 mass. %). Chromium spinel of The Malaya Shita have relatively low content of Al₂O₃ (9.2–31.2 mass. %, average 19.3 mass. %), MgO (2.2–11.4 mass. %, average 5.5 масс. %), Cr₂O₃ (18.3–41.2 mass. %, average 32 mass. %) and high FeO_tot (20.5–57 mass. %, average 38 mass. %). Chromium spinel of The Tartay massif has the following content: 5.4–27.6 mass. % Al₂O₃ (average 19.9 mass. %); 4.1–10.1 mass. % MgO (average 6.5 mass. %); 33.7–53.3 mass. % Cr₂O₃ (average 40.3 mass. %); 23.1–40.7 mass. % FeO_tot (average 30.6 mass. %).

Projection on the chromite (Cr) – picrochromite (Pc) – hercynite (Hc) – spinel (Sp) plane (Fig.c) shows that spinel widely varies in composition from chromite sensu stricto (FeCr₂O₄) to spinel sensu stricto (MgAl₂O₄).

According to data presented above chromium spinel isomorphism follows the scheme Cr⇄Al⇄Fe³⁺ and Mg⇄Fe²⁺.

According to reference [5] chromium spinel of layered intrusions shows high TiO₂ and Fe₂O₃ tending to ferrichromite – chromium magnetite – magnetite trend on Cr–Al–(Fe³⁺+2Ti) plot. Such a pattern of distribution is due to a regular increase of iron index of chromium spinel during substitution by the scheme Al→Cr→Fe³⁺. However, such law is not typical for chromium spinels of researching intrusions. There are three trends of spinel on the ternary plot: (a) chromium spinel – ferrichromite – chromium magnetite, (b) less chromium spine – ferrichromite – chromium magnetite and (c) alumochromite – ferrichromite –chromium magnetite.

The large concentration of points is observed at the chromium spinel-alumochromite border. Points of the chromium spinels (II) (inclusions in olivine) also located in this fields. This fact allows to consider spinel – alumochromite border as field of early magmatic cumulates.

It is a matter of common knowledge that during basic-ultrabasic magma differentiation Cr always enriches early differentiates, concentrating in spinel group minerals, but Ti enriches the latest differentiates of basic-ultrabasic liquids during the magmatic process [6]. The fact suggests that trends in ternary plot represent distribution of different crystallization stages of chromium spinels. Thus, (a) chromium spinel – ferrichromite – chromium magnetite trend was formed by the earliest cumulates, (b) less chromium spinel – ferrichromite – chromium magnetite trend has been formed by later ones and (c) alumochromite – ferrichromite –chromium magnetite trend shows the composition of the latest species. These trends appeared to each of the intrusions differently.

Composition of chromium spinel of Medeksky massif corresponds to alumochromite and ferrichromite (Fig. a). Single grains comply with chromium spinel sensu stricto and chromium magnetite. Trend b) looks most clearly, trends (a) and (c) are less distinct.

Composition of chromium spinel of Medvezhiy Log massif varies from chromium spinel sensu stricto and alumochromite to feerichromite and chromium magnetite (Fig. b). Trends (b) and (c) look most clearly, trend (a) is less clear.

Composition of chromium spinel of Malaya Shita massif is similar to that of Medvezhiy Log, forming the range from chromium spinel sensu stricto to chromium magnetite (Fig. c). Trend (c) and part of trend (a) are belomg to this intrusion.

Fig. Ternary plot of trivalent cation content of each intrusions cr-sp. Fields of [7]: 1 – spinel, 2 – chromium spinel,

3 – alumochromite, 4 – chromite, 5 – ferrichromite, 6 – chromium magnetite, 7 – magnetite, 8 – alumomagnetite, 9 – ferrispinel

Composition of chromium spinel of Tartay corresponds to chromium spinel sensu stricto, alumochromite and ferrichromite (Fig. d), forming a clear trend (b) and illegible trends (a) and (c).

According to the data presented in Fig. it is concluded that the intrusions were derived by similar processes and under similar conditions.

As mentioned above there are zonal grains of chromium spinel along with homogeneous ones in rocks of intrusions. It is known that zoning of crystals may vary in nature. On the one hand, zoning may be due to sequential crystallization of complex oxide phases from the changing composition of crystallized melt or in consequence with the reaction between formed crystals and intercumulus liquid in a prolonged crystallization [5]. On the other hand, zoning formed due to metamorphic processes, in particular serpentinization, which is accompanied by removal of Cr₂O₃, Al₂O₃, MgO from grain margins with a simultaneous enrichment with FeO and TiO₂ [5].

Based on the zonal chromium spinel some types of geochemical zoning are identified.

(I). Increase of Cr content, decrease of Al content with a constant sum of (Fe³⁺+2Ti). Such zoning occurs in chromium spinel of the Medeksky, Medvezhiy Log and Malaya Shita intrusions. The crystals with such zoning occur in dunite, wherlite and olivine gabbro. Decrease of Al₂O₃ content from core to margin of a grain may be caused by its removal from the liquid during pyroxene and plagioclase mass crystallization [5].

(II). Decrease of Cr content, increase of Al with almost constant sum of (Fe³⁺+2Ti). Such zoning occurs in chromium spinel of the Medvezhiy Log and Tartay intrusions. The grains with such zoning occur in dunite and wherlite. Decrease of Cr₂O₃ content and increase Al₂O₃ content from core to margin of a crystal is explained by a general tendency of increasing Al₂O₃ relative to Cr₂O₃ in the residual melt [5].

(III). Increase of Cr content, decrease sum of (Fe³⁺+2Ti) with minor variations in Al. The zoning occurs in chromium spinel of the Medeksky, Medvezhiy Log and Malaya Shita massifs. The species with such zoning occur in wherlite, plagiowherlite and olivine gabbro. Decrease of FeO, Fe₂O₃ and TiO₂ content may be due to post-cumulus reaction between chromium spinel, intercumulus liquid and primary silicates [5].

(IV). Decrease of Cr content with variations of Al and sum of (Fe³⁺+2Ti). This type of zoning is observed in a single grain of chromium spinel from wherlite of the Tartay intrusion. It was found that FeO, Fe₂O₃ and TiO₂ content increases and Cr₂O₃ and Al₂O₃ decreases in chromium spinel with decreasiong temperature [5].

The compositions of chromium spinel of Tokty-Oi intrusion are plotted on the ternary plot for comparison. This zoning is produced by metamorphic alteration, in particular serpentinization of ultramafic rocks.

Conclusions:

1. Probably similarity of the trends indicates the similar physical and chemical conditions of the massif formation.

2. It is likely that different trends of chromium spinel reflect the change of physical and chemical parameters of the mineral forming environment during crystallization process.

3. Zoning of chromium spinel has magmatic origin.

References:

1. T. F. Galimova and L. A. Bormotkina (1983) Precambrian stratigraphy of the Biryusa Block, Precambrian stratigraphy of the Middle Siberia, Nauka, 125 – 134pp

2. Y. P. Benedyuk, T. B. Kolotilina, A. S. Mekhonoshin (2010) Accessory chrome-spinel of the Medeksky massif (the Eastern Sayan), Izvestiya, Geology, search and prospecting of ore deposits, 2 (37), ISTU, 72 – 76 pp

3. T. B. Kolotilina, A. S. Mekhonoshin, L. A. Pavlova (2002) Genetic characteristics os spinel-group minerals and ilmenite composition of ultrabasic rocks, south-east part of the Biryusa Block, Petrologu of magmatic and metamorphic complexes, 4, 68-77pp

4. Power M. R., Pirrie D., Andersen J. C., Wheeler P. D. (2000) Testing the validity of chrome spinel chemistry as a provenance and petrogenetic indicator, Geology, 28, 1027-1030pp

5. A. N. Plaksenko (1989) Accessory chromium spinel typomorfism of ultramafic-mafic magmatic formations, IVU, 222pp

6. A. S. Mekhonoshin, O. M. Glazunov, G. V. Burmakina (1986) Geochemistry and ore-bearing of metagabbro, Eastern Sayan, Nauka, 102pp

7. A. V. Okrugin (2005) Otechestvennaya geologiya, 5, 3-10pp

Geochemical heterogeneity of ophiolites from the Severnaya, Zelenaya and Barkhatnaya mountains (Kuznetsky Alatau)

Dugarova N.A.

Tomsk State University, Tomsk, Russia

[e-mail]

The Kuznetsky Alatau ridge represents a NW segment of the Altai-Sayan folded belt (ASFB). Due to its specific composition, the Kuznetsky Alatau terrain is considered as a Caledonian collisional system consists of three paleotectonic assemblages (from the top to the bottom): a) fragments of the Late Proterozoic suboceanic crust; b) Early – Middle Cambrian island arc complexes; c) Late Cambrian – Early Ordovician continental marginal and accompanying Early Paleozoic riftogenic or intraplate magmatic formations [4]. A specific signature of this area is spatially varied development of carbonate and terrigenous and volcanic rocks of Late Proterozoic and Cambrian time actively dislocated, as well as of subcontinental Middle Paleozoic volcanic sediments [1]. Tectonics of region was accompanied by dynamic magmatic activity and forming high alkali complexes. In general, “mosaic” (or block-type) structural style conditioned by developing tectonic deformations of the NW and submeridianal trend is typical for the region [2].

Fragments of suboceanic crust formed by the set of petrographic varieties, which are typical for the ophiolitic section, are mostly localized in the axial part of the Kuznetskiy Alatau Ridge. These fragments form an apparent belt. According to today’s conceptions, there are two age levels of ophiolitic genesis in the region: Late Riphean – Early Vendian; and Late Vendian – Early Cambrian. At the second level, forming the biggest ultramafic-mafic paragenesises with the most full petrographic compositions took place. These are mountain peaks: Stanovoi Ridge, Chemodan, Barkhatnaya, Zayachia, Severnaya and Zelenaya, which are traditionally considered as an ophiolitic association of the Kuznetsky Alatau. They are the most possible primary source for platinum group elements (PGE) from the placer of the Kia river basin [4]. However, the recent isotopic geochemical data indicate more ancient and probably simultaneous forming the oceanic crust products in this region.

The signature of gabbroids from the ophiolitic association is the presence of regional metamorphism signs at the level of amphibolite facies. A basite component of ophiolites from the Severnaya and Zelenaya mountain peaks is mostly represented by rocks of sodium tholeitic series. At the same time, the presence of subalkaline rocks is typical. Their origin is probably connected with developing later dikes from local zones of postorogenic activation, or with developing albitization processes under conditions of regional metamorphism. A criterion for discriminating different natures of high alkali rocks is their detailed petrographic diagnosis. Particularly, the presence of mineral paragenesis with signs of metamorphic transformations at the level of amphibolite facies completely rejects a postorogenic nature of basite veins.

Petrochemical parameters of the studied rocks vary within wide limits (10-20% Al₂O₃; 0,5-2,5% TiO₂; 3-17% MgO), that indicates their possible belonging to the “cumulative” or “layered” complex. According to the K₂O/Na₂O ratio, these rocks refer to formations of sodium and subsodium series. At the same time, majority of normal alkaline basites corresponds to the field of the MOR-type ultrasodium basalts (K₂O/Na₂O<0.1). Nevertheless, some of these basites correspond to the products of calc-alkaline series by FeO/MgO ratio. In general, according to petrochemical characteristics, rocks from the Barkhatnaya mountain peak correspond to middle- and high-titanium low-magnesium basites widely spread in the upper parts of gabbroitic section and in the parallel dike complex. The specialization of differentiated suites from cumulative and layered complexes of ophiolitic section is more typical for rocks from the Zelenaya and Severnaya Mountains. Observed heterogeneity of basites shows a possible vertical sequence of magmatic complexes in the ophiolitic section [3, 5].

Varied differentiation degree of the matter composition of ultramafic-mafic suites from ophiolitic paragenesises is also planned in the rare earth elements (REE) patterns. Particularly, a significant REE differentiation from plutonic to hypabyssal formations is not found in some mafic varieties developed on the Barkhatnaya, the Severnaya, and the Zelenaya mountains [4]. We should note that the europium maximum occurs when plagioclase appears in these rocks. In general, REE distribution of the most rocks corresponds to the MORB-type. In the other varieties, we should take into consideration the prevalence of light REE over heavy REE at the insignificant europium minimum. The shape of REE patterns for these rocks is close to patterns for island arc systems (Fig.).

Based on the whole complex petrochemical parameters, observed rocks of ophiolitic suite from the Severnaya, Zelenaya, and Barkhatnaya mountain peaks can be referred to the products of sodium tholeitic petrochemical series, which are typical for the MORB magmatism. However, complication of the apparent tholeitic specialization of basites is always observed with signs of calc-alkali derivatives. These derivatives are similar to the formations of island arc systems, back-arc spreading areas, or SSZ-type ophiolites. That assumes tectonic association of different type fragments of oceanic crust from ancient sea basins in the folded belt structures.

References:

1. Alabin L.V. Structural-formational and metallogenic zonality of the Kuznetsky Alatau / Novosibirsk: Nauka, 1983. 102p.

2. Krasnova T.S. Petrology of ultramafic massifs of the Severnaya-Zelenaya and Barkhatnaya mountain peaks (Kuznetsky Alatau) / Ph. Doctoral thesis. Tomsk: TSU, 2005. 20p.

3. Krasnova T.S., Gertner I.F. Olivine fabrics in chromite-bearing ultramafites of the Barkhatnaya mountain peak (Kuznetsky Alatau) / Dynamic metamorphism and the fabric evolution in associations of mafic-ultramafic rocks: Proceedings of scientific seminar. Tomsk: TSU, 1996. P. 68-71.

4. Krasnova T.S., Gertner I.F., Utkin Yu.V. Perspectives of platinum mineralization of the Kuznetsky Alatau ophiolites / Petrology of magmatic and metamorphic complexes. V.2: Proceedings of the 2d scientific meeting. Tomsk: CSTI, 2001. P. 229-235.

5. Sun S.S., McDonough W.F. Cmecical and isotopic systematics of oceanic basalts: implications for mantle composition and processes //Magmatism in oceanic basins. Eds. Saunders A.D. & Norry M.J.. 1989. Geol. Soc. Spec. Publ. No 42. P. 313-345.

Amphibole chemistry of quartzdiorites from Boroujerd granitoid complex (Western Iran)

Esmaeily D., Maghdour-Mashhour R.

Faculty of Geology, College of Science, University of Tehran, Iran

[e-mail]

Boroujerd Granitoid Complex of the Sanandaj-Sirjan Zone in western Iran mainly consists of three units: granodiorite, quartzdiorite and monzogranite. Amphibole, the dark-colored rock-forming inosilicate mineral, from quartzdiorites has been analyzed with electron microprobe. Composition of Amphiboles used to describe the nature of the granitic magma and estimate the pressure at which quartzdiorite unit of Boroujerd granitoid complex is emplaced. Structural formulae for hornblende were calculated on the basis of 13 oxygens. Based on the nomenclature of Leake et al. [1], they are classified as magnesiohornblende to actinolitic hornblende. Good correlation between Al(IV) and total Al in amphiboles from metaluminous I-type quartzdioritic unit of Boroujerd complex demonstrates that Al content is controlled by crystallization pressure. Among several calibrations for aluminum-in-hornblende barometry, Anderson & Smith [2], a temperature-corrected Al-in hornblende barometer, are used. This geobarometry indicate that the quartzdiorite of Boroujerd granitoid complex was emplaced at pressure of 1.04±0.6 to 1.48±0.6 Kbar. Temperature was calculated based on coexisting hornblende and plagioclase using the thermometry of Blundy and Holland [3]. Calculated Tempratures of emplacement are in the range of 750 °C to 790 °C. log(XF/XOH) versus log(XMg/XFe2+) plot of Ague and Brimhall [4] is also used in order to shed light on processes contributed in genesis of the host rock e.g. degree of contamination. log(XMg/ XFe2+) and log(XF/XOH) ranges from 0.1 to 0.3 and -2 to -0.5 respectively for the analyzed amphiboles and most of samples from qaurtzdioritic unit of Boroujerd complex fall within I-MC (Moderately contaminated I-type) field. Noteworthy that, quiet a few samples classified as I-SCR and I-WC types. These observations are consistent with previous studies on biotites and whole rock geochemistry of Boroujerd complex. In general calc-alkaline quartzdiorite magma of Boroujerd complex is emplaced at depth of of 3.64±1.8 to 5.18±1.8 Km and interpreted as having formed with moderate contributions from aluminous metasedimentary material or magmas from aluminous supracrustal material, either by assimilation or cotamination, during its petrogenesis.

References:

1. Leake, B.E., Woolley, A.R., Arps, C.E.S., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne, F.C., Kato, A., Kisch, H.J., Krivovichev, V.G., Linthout, K., Laird, J., Mandarino, J.A, Maresch, W.V, Nickel, E.H., Rock, N.M.S., Schumacher, J.C., Smith, D.C., Stephenson, N.C.N., Ungaretti, L., Whittaker, E.J.W. and Guo, Y. (1997) Nomenclature of amphiboles: Report of the subcommittee on amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names. American Mineralogist, 82, 1019.1037.

2. Anderson, J.L. and Smith, D.R. (1995) The effects of temperature and.O2 on the Al-in-hornblende barometer. American Mineralogist, 80, 549.559.

3. Blundy, J.D. and Holland, T.J.B. (1990) Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer. Contribution to Mineralogy and Petrology, 104, 208.224.

4. Ague, J., and Brimhall, G. H, (1988), Regional variations in bulk chemistry, mineralogy, and the compositions of mafic and accessory minerals in the batholiths of California, Geol. Soc. Amer. Bull., 100, 891-911.

Дата добавления: 2015-10-29