About the composition of the rare sulfate - pikerengite from the Vend-Paleozoic paleoguyots from West Transbaykal region

Asavin A.M., Seduch I.A., Romashova T.V., Filimonov A.V.

Institute of Geochemistry and Analytical Chemistry, 117945, Moscow, Kosigina str.19 aalex06@inbox.ru

The existence of oceanic islands and seamounts is supposed to be the one of the main evidence of the plate tectonic theory. But there is no such acient geological object which could be so similar to rock complex of oceanic island. These structures must include the alkaline magmatism manifestation, sedimentary rocks close to recent riff limestone on the oceanic islands and sequence of  terrigenic rocks.

In the last years several possible applicants into place of paleoanalog of the oceanic hot spots were found. For example in the North West Chine Perm age oceanic island structures were found. [LAI Shaocong, Zhang Guowei, Dong Yunpeng, Pei Xianzhi & Chen Liang 2002, Wang Yongbiao 2005, Niu, ZhiJun 2008, ZhiJun Niu, Anwu Xu, JianXiong Wang, Qi Fa Duan, XiaoMing Zhao and HuaZhou Yao 2008]. On South West of  Japan Permian carbonatic-volcanogenic complex in accretic prism was found which can be interpreted as part of the paleoguyot [Tetsuji Onoue and Hiroyoshi Sano 2007]. Some Russian geologists found the interesting geological structure in Gornyi Altai and Tien Shan [Dobrecov et al 2004, I. Safonova 2006-2008, Volkova N.I., Scliarov E.V. 2007]. They believed that there is the Cambrian paleoguyots volcanic-sedimentary complex.

The one of the most interesting object on the Russian territory is the Vend-Paleozoic paleoguyots of the Djida River region. It is a part of the several close geological complex which can be combined as full oceanic island sequence. The full section includes the ultrabasic basement, pillow tholeitic flows (underwater structure of the lava flows), subalkaline and alkaline basalts (with the trace element geochemistry signs of intraplate magmatism) and sedimentary complex – limestone, dolomites, black chert.

One of very stunning features of the geological situation of area is existence of displays of huge rocks consisting from the sedimentary rocks – dolomites, cherts, limestone. The landslide interpreted as gravitas blocks from the upper part of the island structure moved to downslop of  the basement. During our field research on this rocks we have found the porosity white, white-yellower  species of the mineral which easy solutes in water. The data of the microprobe, chemical and physical analysis show that this mineral is a pickeringite  – sulfates of Fe and Mg  (Fe,Mg)Al2(SO4)4ћ22 H2O.

For diagnostics of a mineral we used  Infra-red spectrum methodic and X-Ray powder diffraction spectrum.

Geological data couldn’t give us the evidence of the paregenetic sulfate and sedimentary carbonatic rocks. The presence of manifestations of dolomites and gypsum in the carbonate top of guyot permits us to relate discovered mineralization with  the primary special features of sedimentary complex of the paleoguyot.Our researches of the trace elements composition pickeringite  showed the fitures which permitted us to make paleoreconstructions sedimentary conditions in oceanic island lagoon.

Рис. 1 а –Porous pickeringite  aggregates, б –fine-acicular pickeringite crystals forming sheaflike, or thinner frequent, radial-columnar aggregates Figure is made by Raztek MRX-10D-ST microscope in reflected light.

Table. Composition of pickeringite. 

* - H₂O calculated by the 100% norm of cations.

d.l.. – Low  the detected limit.

Conclusion.

The Paragenesis with. pickeringite (phase without natrium 0.1% wt and with noticeable potassium) proves the poor salinity of the lagoon water. The complete absence of Cl and F says the same.

At sufficiently high contents of Ba (345 ppm) the contents of Sr is extremely low (10 ppm).  Besides in all the analysis abundance Fe is observed 0.7 a.f.

The low content of Ni, Co, Cu on the level .0.0n % is uncommon for this mineral. Usually these elements are concentrated in sulfate, which are formed from mining waters in the zone of weathering. These data emphasize that the source of substance for sulfate is syngenetic with carbonate sediment.

Литература

1.                Tsutomu Nakazawa, Tsukuba Carboniferous Reef Succession of the Panthalassan Open-Ocean Setting: Example from Omi Limestone, Central Japan //FACIES 2001 I. 44 P. 25-31.

2.                H. Sano ћ M. J. Orchard Necoslie breccia: mixed conodont fauna-bearing neptunian dyke in Carboniferous-Permian seamount-capping oceanic buildup (Pope succession, Cache Creek Complex, central British Columbia) //Facies (2004) 50:133–145.

3.                Julia E. Hammer a,*, Michelle L. Coombs b, Patrick J. Shamberger a, Jun-Ichi Kimura Submarine sliver in North Kona: A window into the early magmatic and growth history of Hualalai Volcano, Hawaii //Journal of Volcanology and Geothermal Research 151 (2006) 157–188.