Simandite the new type of the high ferrous of picrite (Africa. Guinea)

Asavin A.M.*, Senin V.G.*, Rudko D.

* Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, alex@geokhi.ru

 

We revealed the new type of porphyritic basalts similar to picrite, which contain the phenocrysts of the elongated crystals of olivine. These species were found during mapping of the iron ores deposit on the Simandou ridge (in the East part of the Guinea republic, Africa).

This very fresh of dark gray colour rock consists of bubbly glass like slag. The glassy rock consists from the long olivine phenocrysts (up to 1-2 mm) and microphenocrysts embedding in 20-40% black glass. The bodies of basalts are sills and stock up to 100 m² on the area. There are contacts the basalt with the host rocks (a series Simandou (Proterozoic) -  itaberite, the quartzites and the phyllite), with narrow zones of contact alteration.

On the geological map of Guinea these species are referred to the dolerite of Mesozoic - Karoo series (Fig.1) Typical dolerite - diabase Karoo series also are observed in the investigated region, in the form of sills and dikes (Fig.1, right part). The dolerite are characterized by more paleotypical appearance. There are not glass sections in the groundmass in the dolerite. The contact of normal dolerite with the unusual picrite is not found. Therefore the age relationships between them are not established. The age of dolerite Karro in this region is evaluated inconsistency. On the early maps they relate to the Mesozoic magmatism of approximately 200 M.a.; however, in the last publication of geological atlas Africa they are referred to the Precambrian formations. In the absence the direct isotopic estimations of the age of magmatic formations, the problem of the dating of young magmatism remains opened.

 

Fig.1. Geological map of Guinea. The light zones in the distinguished part are the PZ structures of Simandou mountain range. The star indicates the region of the find of the simandite samples.

 

A petrographic analysis of picrite revealed the presence of the quenched crystals of olivine and good fresh glass with the spinifex-textured crystals of olivine into the glass. That is the evidence of fast crystallization the ultrabasic flow with overheated of  liquidus line temperatures (Fig. 2). In spite of mono mineral olivine composition of phenocrysts the content of MgO in the species is extremely low (<1%) (Tabl. 1). It is explained by the uncommonly high content of fayalite’s end-member - Fa_98-100  in olivine phenocrysts (Tabl. 2). The sum of alkaline in the rocks is not great, potassium considerably predominating above sodium. The main characteristic the studied species are very high the content of FeO - more than 20 wt.%. By using these criterion (FeO>19%) and low content of MgO we selected similar analyses in the database of GEOROCK (http://georoc.mpch-mainz.gwdg.de/georoc/). The selected compositions are shown on the classification diagram Le Bas (Fig. 3). Mainly points of compositions enter in the regions of tholeiitic basalts and foidite. Author's the name of the selected species the most different from basalts to the dacite. Specifically, these unusual features make it possible to isolate this species as new petrographic type. We propose to give name to this rock as simandite, by the place where this type of rock was discovered - Simandou range. In composition these basalts relate to the type of highly ferrous and low magnesium picrite.

The genesis of this rocks remains for us incomprehensible. The high content of glass, the quenched structures, and the quenched nonequilibrium of phenocrysts testify about superheated and rapid of the effusion of liquid to the surface. However, in this region recently historical volcanic structures are not known. Usually high- ferrous basalts - islandite and tristanite, the formed during the processes of differentiation crystallization have the higher contents of silica and sodium and the lower content of iron. It opposites to simandite composition peculiarities.

 

Fig 2. Structure of simandite in the back-scattered electrons image. Quenching crystals of olivine (light) in the glass.	Fig.3. Classification diagram with the compositions of high-ferrous basalts: black rhombs - compositions of simandite glass, gray points - literature data.

 

Tabl.1 Glass composition wt.%

Point	SiO2	TiO2	Al2O3	FeO	MnO	MgO	CaO	Na2O	K2O	V2O3	Cr2O3
1 / 1 .	43.05	0.74	13.89	30.37	0.05	0.06	7.50	0.01	1.24	0.045	0.033
2 / 1 .	44.64	0.72	14.93	27.25	0.05	0.04	8.27	0.00	1.39	0.018	0.033
3 / 1 .	43.63	0.70	14.47	28.90	0.05	0.04	7.94	0.00	1.30	0.038	0.026
7 / 1 .	43.16	0.61	13.51	30.43	0.05	0.04	7.63	0.00	1.25	0.026	0.033
8 / 1 .	42.99	0.62	13.78	30.57	0.04	0.06	8.06	0.02	1.26	0.027	0.02
9 / 1 .	43.09	0.63	13.11	31.11	0.06	0.06	8.32	0.03	1.09	0.014	0.011
10 / 1 .	42.94	0.68	13.45	30.79	0.05	0.06	8.33	0.00	1.26	0.007	0.002
11 / 1 .	43.65	0.64	13.56	29.43	0.04	0.04	8.73	0.00	1.31	0.016	0.02
2 / 1 .	41.97	0.70	12.57	34.31	0.03	0.09	6.56	0.04	0.89		0.08
4 / 1 .	42.48	0.71	13.08	33.48	0.06	0.02	6.66	0.03	1.01		0.05
6 / 1 .	42.00	0.53	12.30	34.31	0.06	0.04	6.94	0.04	0.64		0.07

 

Tabl.2 Olivine phenocrysts composition wt.%

Point	4 / 1 .	5 / 1 .	6 / 1 .	12 / 1 .	13 / 1 .	14 / 1 .
SiO2	28.799	28.877	28.953	28.872	28.761	28.659
TiO2	0.049	0.038	0.008	0.031	0.013	0.051
Al2O3	0.082	0.097	0.066	0.069	0.053	0.069
FeO	67.301	67.567	67.253	66.987	67.469	67.23
MnO	0.104	0.113	0.104	0.108	0.096	0.097
MgO	0.644	0.547	1.092	1.115	1.107	1.03
CaO	0.455	0.411	0.346	0.353	0.368	0.351
K2O	0.008	0.006	0.007	0.001	0
V2O3		0.009	0.017		0.019	0.007
Cr2O3	0.036	0.024	0.021	0.019	0.027	0.026

 

The absence in the species of phenocrysts of titano-magnetite supports opinion about the very unusual regime of oxygen was in the magma. Obviously that magma was crystallized in the reducing atmosphere with the approximately two orders lower than the buffer QFM. Possibly, critical process which explains the appearance of these unique conditions was the process of melting of itaberite and iron ores. However, where to take powerful thermal energy source for this process is incomprehensible. Furthermore the melting of iron ores contradicts the current idea about the age of ores as to the young quaternary crust of weathering, and it makes it necessary to carry them to the ancient crusts of the weathering of K_1-2 age.