Olivine and pyroxene composition from the porphyritic peridotite from the Guli massif as an evidence of the genetic relationship between ultrabasic intrusive phases

Nikolay A. Nekrilov*,  Victor A. Zaitsev**, Pavel Yu. Plechov*.

* Faculty of Geology, Moscow State University, Moscow, Russia; ** Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia

nekrilov.n@gmail.ru

 

The Guli massif includes a complex of intrusive ultrabasic phases; one of that contain the porphyritic peridotite subvolcanic rocks (PPR). There is the ultrabasic porphyritic rocks with olivine phenocrysts crystals and groundmass which consist from aggregate of Cpx, Phl and ore minerals.

 

Fig. 1. Simplified geological map of Guli massive [Egorov, 1991]. 1 – Mesozoic deposits, 2 – Carbonatite complex, 3 – Melelite bearing rocks, 4 – Meimechite, 5 – Syenite, quartz syenite, nepheline syenite, shonkinite, granite dykes, 6 – Ijolite, melteigite, jacupirangite, 7 – Melanephelinite, nepheline picrite, biotite-pyroxene picrite, 8 – Kos’vite, 9 – Dunite (a), Altered dunite (b).

 

Vasil’ev & Zolotuhin consider PPR as source of the dunites; in this case PPR is frozen magma on the contact with dunites. Phenocrysts olivine crystals from these rocks are the source of the cumulate olivine from dunite [Vasil’ev, Zolotuhin, 1975].

There is essential difference between chemical composition of olivine phenocrysts from the PPR rocks from western and central parts of the massive. Chemical composition of olivine from the western samples varies in the interval of 82-83% Fo, whereas olivine from the central part is represented by 93-94% Fo. CaO contents in central part olivines varies in the interval of 0,17-0,32 wt. % while in less magnesium olivine from the western samples this interval are 0,24-0,45 wt. %.

 

Fig. 2. Olivines composition from PPR (our data) and from dunites (rhombus) and meimechites (squere) [Sobolev, Sobolev, Kuzmin, Malich, Petrunin, 2009].

 

   Clinopyroxene is a basic mineral of the PPR groundmass. The common chemical composition of internal parts of clinopyroxene crystals from western samples is (Na_0,02-0,03Mg_0,4-0,44Ca_0,45-0,46Fe²⁺_0,02-0,05Fe³⁺_0,03-0,05Al^VI_0-0,03Cr_0-0,01)₂[(Si_0,9-0,97Al^IV_0,01-0,05Ti_0,02-0,05)₂O₆]; and from the central part - (Na_0,02-0,03Mg_0,46-0,47Ca_0,45-0,47Fe²⁺_0,01-0,02Fe³⁺_0,02-0,04Al^VI_0-0,01)₂[(Si_0,97-0,98Al^IV_0,01-0,02Ti_0,01)₂O₆].

   Cpx chemical composition for PPR from western part of Guli massive differ from Cpx chemical composition for PPR from central part of this massive. This difference is shown in table 1. The trend of Aeg increasing is noted in Cpx crystals from rocks from western part of Guli massive since of inclusions and finishing to external parts of Cpx crystals. This trend is not observed in Cpx from rocks from central part of massive.

 

Table 1. Cpx chemical composition:

№ An.	SiO2	TiO2	Al2O3	Cr2O3	FeO	MgO	CaO	Na2O	Total
1985-60-7	49.53	2.78	2.79	0.44	4.68	15.05	23.22	0.75	99.25	internal part
1985-60-8	53.76	0.56	0.50	0.35	4.05	16.03	22.86	1.30	99.41	external part
1985-60-9	55.39	0.00	0.00	0.00	2.23	17.69	25.69	0.20	101.19	Inclusion in Cpx
1985-60-10	55.63	0.00	0.00	0.00	1.27	18.02	25.46	0.38	100.76	Inclusion in Cpx
1985-60-11	48.91	3.33	3.54	0.00	4.91	14.54	23.26	0.79	99.28	Internal part
1985-60-12	54.30	1.17	0.54	0.84	5.21	15.18	21.58	2.07	100.88	external part
1985-60-13	52.91	1.16	0.50	0.00	4.43	16.06	22.53	1.10	98.69	inclusion in Mt
1985-60-14	50.16	2.21	2.27	0.27	4.90	15.11	23.22	0.47	98.61	internal part
1985-60-15	51.88	0.83	0.61	0.30	4.16	15.53	22.14	1.25	96.70	external part
Г-09-12-6	53.96	0.64	0.53	0.00	2.53	17.02	23.81	0.72	99.21
Г-09-12-7	53.79	0.73	0.64	0.24	2.99	17.01	23.26	0.88	99.53
Г-09-12-10	53.80	0.70	0.89	0.00	2.49	17.39	24.51	0.48	100.26
Г-09-12-11	54.44	0.44	0.93	0.28	2.50	17.12	24.13	0.62	100.46
Г-09-12-14	53.98	0.71	0.87	0.00	2.76	17.64	24.41	0.50	100.85
Г-09-12-38	54.82	0.57	0.55	0.00	3.05	17.26	23.81	0.79	100.84
Г-09-12-39	53.91	0.61	0.63	0.00	2.64	17.29	23.99	0.48	99.55
Г-09-12-40	53.67	0.54	0.28	0.00	3.10	16.95	23.47	0.74	98.75
Г-09-12-41	55.04	0.33	0.60	0.00	3.18	17.55	24.10	0.72	101.50
Г-09-12-42	54.24	0.52	0.41	0.00	2.81	17.55	23.69	0.61	99.83
Г-09-12-43	54.06	0.58	1.24	0.00	3.69	17.11	23.92	0.57	101.17
Г-09-12-44	53.78	0.49	0.76	0.00	2.33	17.26	23.71	0.60	98.92
Г-09-12-45	54.05	0.70	0.66	0.00	3.18	17.67	24.23	0.56	101.05

 

 Comparison of olivine chemical composition from PPR with the published data from meimechites and dunites (fig. 1) and comparison pyroxene chemical composition from PPR from western and central part of Guli massive (table 1) show the following conclusions:

1) PPR is a special type of intrusive phase of Guli.

2) These intrusive phases have a different condition of crystallization in central and western part of Guli massif.

3) Olivine phenocrysts in PPR from the central part of the Guli massif lie on the one trend of olivine composition from meimechites and dunites.

 

       Literature:

1. Egorov L.S. Ijolite-Carbonatite Plutonism Ð A case history of the Maimecha-Kotui complexes, Northern Siberia. Nedra, 1991. Leningrad, p. 260.

2. Sobolev A.V., Sobolev S.V., Kuzmin D.V., Malich K.N., Petrunin A.G. Siberian meimechites: origin and relations to flood basalts and kimberlites. Geology and Geophysics, 2009, Vol. 50, № 12, pp. 1293–1334.

3.  Vasil'yev Yu.R., Zolotukhin V.V. Petrology of Ultrabasics in the North Siberian Platform and some problems of their origin. Nauka, 1975, Novosibirsk, pp. 78-95.