Ultra Potassium Trachytes from Middle Timan

Udoratina O.V.*, Burtsev I.N.*, Kulikova K.V.*, Varlamov D.A**.

*Institute of Geology Komi SC UB RAS, Syktyvkar, Russia

**Institute of Experimental Mineralogy RAS, Chernogolovka, Russia

udoratina@geo.komisc.ru

 

Ultrapotassium high-titanium rocks are known at the Middle Timan – feldspatolites, microclinites, bostonites, timanites, trachytes (Makeev et al., 2008; Malkov, 1999; Malkov et al., 2006).

There are two viewpoints on their genesis; it is considered as magmatic or metasomatic. The published data favors to both versions of genesis.

Magmatic genesis (Malkov, 1999; Malkov et al., 2006): ultrapotassium trachytes, alkaline syenite aplites-bostonites, are observed as dikes 9 to 50 m thick in rocks with active intrusive contact. The age of rocks – 270-288 Ma. The rocks are full-crystalline, with miarolitic texture, sanidine. They are characterized by high content of potassium to 14.7-15.5 wt%. The formation temperature is 900-1000°С. The genesis is magmatic, crystallization of trachyte magma, high content of potassium – supply of potassium at autometasomatosis, high content of titanium is considered as basalt contamination.

Metasomatic genesis – hydrothermal-metasomatic alumina-silica-potassium metasomatosis (Makeev et al., 2008 and others). Feldspatolites, microclinites are observed as lens-like-dike bodies in supply channels, or as numerous microinjections of feldspatolites accompanied by distinct reaction-metasomatic zones or as impregnation zones. The thickness is to 150 m. The relict ophite microstructures are observed. The relicts of diopside-hedenbergite (substitute of plagioclase and pyroxene – pseudomorphs on them), neogenic orthoclase are observed. We find complete absence of sanidine and presence of adular  due to warming and activation of pore solutions of enclosing strata. The source of titanium – basalts. Formation temperature is from 400-550° to 100-150°С. The age is Hercynian.

The rocks were studied by X-ray, petrographic methods. The data on mineralogical, chemical composition and contents of rare and rare earth elements were received.

Petrographic characteristics: the rocks are light brown, beige with porous (cavernous(?)) texture and small-grain structure. The color is conditioned by practically 90% content of potassium feldspar. The cavities (pores) seem to be leached, though observed throughout the whole depth of rocks. Under microscope the rocks has a porous (~ 10%) texture, bostonite structure, composed predominantly by potassium feldspar with elongated tabular form with irregular wavy edges. The elongated tabular crystals with size 0.2x1 mm are dominating, though smaller varieties are found. Feldspar forms simple polysynthetic twins. Thin substitution pertites are characteristic as well. Feldspar is partly pelitized and substituted by small grain aggregate, supposedly, of zeolites (by X-ray data – mordenite). Brown argillaceous material is observed in interstitions between small tablets of potassium feldspar, which, supposedly substituted glass. Inside the brown argillaceous mass, and partly inside feldspar as well, apatites needles are observed. Ore mineral (titanium magnetite) is evenly scattered in feldspar, the content does not exceed 3 vol% and forms either subhedral, or anhedral grains.

By its mineral composition and structure – we consider the rocks as trachyte (alkaline trachyte) (Petrographic Code…, 2008).

Petrochemical characteristics: The rocks are related to medium rocks 59≤SiO₂≤66 wt%, alkaline subclass, 9≤Na₂O+K₂O≤21 wt% and characterized (wt %) by utmost potassium content (K₂O–14.46), at practically complete absence of natrium (Na₂O–0.40, K₂O/Na₂O=36). The content (wt %) of silica is 59.31, and alumina - 18.57, ferrous oxide dominates over ferric oxide, high titanium content is characteristic ­– 2.30 (Table 1 (15)).

Table 1. Chemical composition (wt %) of rocks.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
SiO2	60.6	59.7	59.32	61.08	60.24	60.6	58.43	65.32	61.32	62.00	58.36	56.22	60.73	58.36	59.31
TiO2	2.1	2.1	2.65	2	2.13	1.8	0.34	0.08	0.89	0.57	2.25	2.1	2.3	2.25	2.3
Al2O3	18.23	17.4	18.39	17.62	19.07	18.46	17.84	16.31	18.43	17.44	17.44	16.12	18.71	17.44	18.57
Fe2O3	1.24	2.57	1.19	1.19	1	1.67	5.09	1.52	3.84	2.06	2.52	1.75	2.88	2.52	1.255
FeO	0.2	0.19	0.16	0.16	0.16	0.17	–	0.54	1.6	2.68	0.28	0.14	0.57	0.28	0.22
MnO	­–	0.03	–	–	–	–	0.42	–	0.01	0.12	.0.02	0.01	0.05	0.02	0.009
MgO	0.1	0.31	0.41	0.23	0.28	0.33	0.43	0.26	0.46	0.91	0.58	0.12	0.88	0.52	0.3
CaO	0.36	0.32	0.5	0.39	0.39	0.36	0.8	–	1.45	2.26	0.73	0.48	1.09	0.73	0.57
Na2O	0.07	0.12	0.05	0.12	0.14	0.16	0.38	0.43	5.75	5.86	0.19	0.1	0.23	0.19	0.4
K2O	15.52	15	14.72	15.28	15.52	15	13.9	13.94	4.94	5.17	14.52	11.42	15.24	14.62	14.46
H2O	0.08	0.66	0.86	0.18	–	0.32	0.11	–	–	–	–	–	–	–	0.38
H2O	0.84	0.96	1.64	0.82	0.72	0.39	1.05	1.56	–	–	–	–	–	–	–
CO2	–	–	–	–	–	–	–	–	–	–	–	–	–	–	0.05
P2O5	0.07	0.2	0.25	0.24	0.23	0.17	0.35	0.03	–	0.17	0.14	0.09	0.17	0.14	0.22
ППП	0.89	1.2	2.18	0.72	0.8	0.84	–	–	1.31	0.76	2.67	0.98	4.25	2.67	1.42
Total	99.41	99.69	100.3	99.51	99.95	99.5	99.2	99.98	100	100	99.72	min	max	av	99.04

Notes. 1–6 – data by B. А. Malkov (Malkov et al., 2006); 7 – trachytes from  Upper Uganda (Malkov et al., 2006); 9 – bostonite by Deli; 10 – average alkaline syenite by Deli; 11– average data from 12 analyses, data by (Makeev et al, 2008); 12–14 – data from report (Lebedev, 1998); 15 – our data. – no data, min, max, av – contents: minimal (min), maximal (max), average (av).

 

Thus, chemical composition of the rocks supports its petrographical data and allows determining it as trachyte. Though at the diagram (Na₂O+K₂O– SiO₂) the composition point is in the phonolite field, and chemically it is closer to leucitic phonolite with high contents of TiO₂ and K₂O, which are also related to the rocks with potassium alkalinity, but petrographically it lacks typical minerals. At the diagram of K₂O–SiO₂ composition points are also in the field of alkaline rocks.

   The content of rare earth elements in the rocks is not high. The diagrams of REE distribution (normalized on chondrite C1) are characterized by a steep slope and show predominance of light REE over heavy ones ((La/Yb)_N – 7) and by a clearly expressed europium minimum.

Microprobe studies of thin section on epoxy revealed that the mineral composition of the rocks was represented by potassium feldspar composing basic matrix; the chemical compositions do not have any specific character. The matrix of potassium feldspar shows also rare earth (light) phases with zirconium. Ore minerals, predominantly forming idiomorphic crystals with skeleton crystal faces. Are represented by aggregates of oxide Fe-Ti phases, which are intergrowths of the following minerals – titanium magnetite, ilmenite, rutile. The intergrowths are resulted from oxidizing decay of homogeneous phases of primary titanium-ferrous oxide solid solution (Mg,Fe²⁺)(Al,Ti,Fe³⁺)₂O₄). The further oxidizing of the decay products also forms own phases. The process of decay and oxidizing forms a number of structural features of the oxide aggregates, which we have determined in the studied samples. The titanium magnetite shows ilmenite growths forming parallel and intercrossing thin lamellas along planes (111) of primary titanium magnetite (trellis type of oxidizing decay according to Haggerty, 1991). The width of ilmenite structures varies from 2 to 20 mcm. At the subsequent oxidizing rutile is formed. The chemical composition of titanium magnetite and ilmenite is consistent, there are admixtures of vanadium and zinc. Accessory minerals are practically exclusively represented by apatite (rounded in section and acicular in shape). Apatite contains fluorine admixture (to 4.2 wt%), which relates it to F-apatite.

Thus, the obtained data allow concluding that the studied rocks are magmatic, composed of potassium feldspar with accessory apatite, ore titanium magnetite, ilmenite and rutile, secondary zeolites. Chemically the rocks correspond to medium, alkaline, and we refer it to as trachyte (trachyte-porphyry – to term dike rocks particularly). We consider their genesis as magmatic.

 

References:

Makeev А. B., Lebedev V.А., Bryanchaninova N.I. Magmatites from Middle Timan. Ekaterinburg, Ural branch of RAS, 2008. 348 p. (in Russian).

Malkov B. А. Hercynian bostonite complex from Middle Timan // Geology of European North of Russia. Coll 4. - Syktyvkar, 1999. P. 43–47. (Works of Institute of Geology Komi SC RAS, Issue 103). (in Russian).

 Malkov B. А., Philippov B.N., Shvetsova I.V. Timanite–unique high-titanium ultrapotassium variety of trachyte: Middle Timan, Late Paleozoic // Vestnik of Institute of Geology Komi SC UB RAS, 2006. №2. P. 13–21. (in Russian).

Petrographic Code of Russia. Magmatic, metamorphic, metasomatic, impact formations. Second edition, revised. S-Pb.: VSEGEI, 2008. 200 pp. (in Russian).

Haggerty S. E. Oxide textures – a mini-atlas / Oxide minerals: petrologic and magnetic significance // Reviews in mineralogy, 1991. Vol. 25. P. 129-220.