Mineralogy of the combustion metamorphic rocks from the Dashgil mud volcano, Azerbaijan

Kokh S.N.¹, Rashidov T.M.²

1 V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia, 2 Geology Institute of Azerbaijan National Academy of Sciences, Baku, Azerbaijan

s.n.kokh@gmail.com

The active Dashgil mud volcano is located in the Gobustan region, central Azerbaijan. This mud volcano had major eruptions at 1882, 1902, 1908, 1926, and 1958 years [1]. The eruption at June 8, 1926 was accompanied with gas ignitions and slight ejection of mud breccia on the south-east of the crater. This resulted to a 200 m row from 12 “red” mounds along a foult-line running E-W. The hills have similar size, each measuring about 4 m high, 25-20 m long, and 10-15 m wide at the base. Mounds consist of mud breccia, fragments of reddish brown clinkers and dark paralavas [5, 6]. The clinkers and paralavas are formed from the mixture of marl with high clay component and the sand. The protolith is partially or completely melted by combusting hydrocarbon gases.

The Dashgil paralavas are high-temperature products of combustion metamorphism at ambiente pressure. They occur as rock fragments (10-25 cm thick) with the ropy-mamillary surface indicating flow process. The paralavas are fresh, often glassy and variable in color from black to dark brown. Xenoliths of sandstone and marl occur in the paralavas. All paralavas contain oval vesicles; their abundance does not exceed 10 % of rock volume. They are empty and vary in size from a few microns to a few millimeters.

The paralavas consist of fine crystals (2–70 μm) in glass matrix that composes up 15–20% of the rock. Wollastonite, clinopyroxene, and plagioclase are the dominant phases; pyrrhotite, rutile and ilmenite are sporadic accessory phases. All paralavas contain grains of relic quartz and K-feldspar. A petrographic study of paralava textures suggests the following crystallization sequence: wollastonite → clinopyroxene → plagioclase → glass ± fine pyrrhotite emulsion. Mineral and glass compositions are shown in Table.

Wollastonite (1T-Ca₃Si₃O₉) forms elongate lath-like crystals up to 70 μm long. Major oxides are in the ranges of 44.25-44.67 wt% CaO, 51.44-52.41 wt% SiO₂. Electron-microprobe analyses reveal that the wollastonite contains notable amounts of iron (1.99-2.17 wt% FeO), magnesium (0.48-0.68 wt% MgO), aluminum (0.10-0.97 wt% Al₂O₃), and titanium (0.11-0.17 wt% TiO₂), which is quite characteristic for wollastonites crystallizing from rapidly quenched [7].

Clinopyroxene forms hopper-like (5 μm) or subhedral (30–70 μm) crystals. Optical and chemical zoning are typical for large crystals. In thin sections the color varies from pale green to bottle green. Typically the zoning is characterized by Mg-rich core (Mg# = 54-58), surrounded by a Fe-rich rim (Mg# = 22-30). Small hopper-like pyroxenes are Fe-rich (Mg# = 40). Impurities include (in wt %) 0.07–2.21 TiO₂, 0.20–0.78 MnO, 0.01–0.17 K₂O and up to 0.33 P₂O₅.

Plagioclase forms elongate unzoned laths (10-30 μm) or occurs as needle-like (2-5 μm) microliths in the glass. It has a composition (An_75-95Ab_4.5-23Or_0.5-2) that corresponds to bytownite-anorthite. Electron microprobe analysis show that most of the plagioclase contain around 0.85-1.37 wt% FeO. The concentrations of MnO, MgO, and P₂O₅ impurities are negligible.

Sulfides are present as fine (<2 μm) emulsion of globules in glass and represents quenched immiscible sulfide liquid. Electron microprobe data indicate compositions (Fe/S = 1, i.e., stochiometric FeS pyrrhotite) corresponding to troilite.

Rutile and ilmenite are anhedral (1-3 μm) and typically form segregations (50-20 μm) in glass.

All paralavas contain rounded grains of relic detrital quartz (10–50 μm) that are often rimmed by perlitic cracking, presumably due to the inversion from high to low quartz at 573°C (Grapes, 2006). K-feldspar is presented as partially melted relic grain (50 μm) in glass and corresponds to Na-bearing orthoclase (Or₇₈Ab₂₀An₂).

In the Dashgil paralavas, glass is either optically homogeneous or contains cryptocrystalline microlithes of plagioclase. In plane polarized light, the glass varies from a colorless transparent variety to a milky or brownish translucent one. Glass compositions within even one sample can be highly variable, typically ranging in weight percent from 67-82% SiO₂, 3-8% CaO, 8-11% Al₂O₃, 1-1.7% Na₂O, 0.2-4.3% K₂O, 1-4% FeO, ~0.5% MgO, <0.5% P₂O₅, < 1% (TiO₂ + MnO) in wt.%. In spite of the fact that total of analyses are ~ 98-100 wt%, it may be suggested that glasses contain some quantity of water.

Clinopyroxene, wollastonite, plagioclase and pyrrohotite exhibit morphological features typical for crystallization from a melt. Hopper-like crystal habits indicate very rapid crystal growth from a melt, similar to that described by Cosca et al [2] for paralavas and by Deer et al. [3] for magmatic rocks. The pyrrhotite emulsion in the siliceous glass is the result of liquation and the separation of silicate and sulfide liquids.

In Dashgil paralavas the pseudowollstonite is absent; therefore crystallization temperatures must have been less than that of the wollastonite-pseudowollastonite transition at 1125°C at the atmospheric pressure. The fragments of paralava samples were melted at air atmosphere in Al₂O₃ ceramic crucibles using a PK-01 furnace. The samples were heated at a rate of 20°C/min and treated during 7,5 hours at 950°C, 1000°C, 1050°C, 1100°C with following quenching at a rate of 10°C/min. According to our preliminary experiments, the samples start to melt at 1000°C while at 1100°C the melting was nearly completed.

So, the thermal alteration that produced the Dashgil paralavas occurred between 1000ºC and 1125ºC.

This study was financially supported by grant MK-5754.2012.5 from the President of the Russian Federation.

Phase:	wollastonite			clinopyroxene			plagioclase			glass
SiO2	51.74	51.58		45.40	43.36		45.52	44.36		82.55	72.64
TiO2	0.11	0.12		1.15	1.68		0.05	0.06		0.10	6.56
Al2O3	0.97	0.23		5.71	6.28		33.86	34.85		8.48	10.51
FeO	2.00	1.99		15.52	22.70		1.34	1.24		1.71	3.79
MnO	0.45	0.37		0.36	0.54		0.02	0.04		0.07	0.09
MgO	0.50	0.50		7.81	3.65		0.00	0.04		0.04	0.49
CaO	44.25	44.67		23.57	21.91		19.11	14.77		2.45	5.53
Na2O	0.00	0.00		0.14	0.43		0.54	0.53		1.42	1.57
K2O	0.06	0.01		0.00	0.16		0.11	0.02		3.22	3.68
P2O5	0.05	0.06		0.13	0.21		0.03	0.02		0.00	0.08
SO3	0.04	0.00		0.01	0.06		0.01	0.00		0.03	0.01
Total	100.16	99.62		99.81	100.98		100.58	99.91		100.07	98.96
O=	3	3		6	6		8	8
Si	0.996	1.002		1.778	1.738		2.103	2.061
Ti	0.002	0.002		0.034	0.051		0.002	0.002
Al	0.022	0.005		0.264	0.297		1.844	1.909
Fe	0.032	0.032		0.508	0.761		0.052	0.048
Mn	0.007	0.006		0.012	0.018		0.000	0.000
Mg	0.014	0.015		0.456	0.218		0.000	0.003
Ca	0.913	0.930		0.989	0.941		0.946	0.935
Na	0.000	0.000		0.010	0.033		0.048	0.047
K	0.001	0.000		0.000	0.008		0.007	0.001
P	0.001	0.001		0.004	0.007		0.000	0.000
Mg#				47.30	22.28

Table. Representative analyses of wollastonite, clinopyroxene, plagioclase and glass from Dashgil paralavas

References:

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