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DISTRIBUTION OF THE PLATINUM GROUP ELEMENTS IN THE AUTONOMOUS ANORTHOSITES OF THE SOUTH-EASTERN RIM OF THE NORTH-ASIAN CRATON

Research article
Buchko I.V.
Sorokin A.A.
Palessky V.V.
Rodionov A.A.
Issue: № 12 (31), 2014
Published:
2015/01/16
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Бучко И.В.1, Сорокин А.А.2, Палесский В.В.3, Родионов А.А.4

1Доктор геолого-минералогических наук, Амурский государственный университет, Институт геологии и природопользования ДВО РАН, 2Доктор геолого-минералогических наук, Институт геологии и природопользования ДВО РАН, 3Кандидат геолого-минералогических наук, Институт Геологии и минералогии СО РАН, 4Аспирант, Институт геологии и природопользования ДВО РАН

РАСПРЕДЕЛЕНИЕ ЭЛЕМЕНТОВ ПЛАТИНОВОЙ ГРУППЫ В АВТОНОМНЫХ АНОРТОЗИТАХ ЮГО-ВОСТОЧНОГО ОБРАМЛЕНИЯ СЕВЕРО-АЗИАТСКОГО КРАТОНА

Аннотация

 Распределение ЭПГ в анортозитах архейских массивов обусловлено “законами” кристаллизации высокотемпературных mss, которые приводят к концентрации Ni, Fe, S, Ru и Rh в твердой фазе и Cu и Pd - в расплавах. Характер распределения спектров ЭПГ в палеопротерозойских анортозитах обусловлен более низкими температурами кристаллизации mss и близок к образованиям офиолитовых комплексов. В то же время, абсолютное содержание PGE типично для базальтов островных дуг.

Ключевые слова:  анортозиты, ЭПГ, спектры распределения, модель формирования, офиолиты, вулканические дуги.

Buchko I.V.1, Sorokin A.A.2, Palessky V.V.3, Rodionov A.A.4

1Doctor of geologo-mineralogical Sciences, Amur state University, Institute of Geology and Nature Management FEB RAS,2Doctor of geologo -mineralogical Sciences, Institute of Geology and Nature Management FEB RAS, 3Candidate of geologo-mineralogical Sciences Institute of Geology and Mineralogy SB RAS, 4Aspirant, 1Institute of Geology and Nature

DISTRIBUTION OF THE PLATINUM GROUP ELEMENTS IN THE AUTONOMOUS ANORTHOSITES OF THE SOUTH-EASTERN RIM OF THE NORTH-ASIAN CRATON

Abstract

PGE distribution in anorthosites of the Archean massifs is caused by the “laws” of crystallization of the high temperature mss which lead to the concentration of Ni, Fe, S, Ru and Rh in the solid phase and Cu and Pd concentration in melts. The character of the PGE distribution spectra in the Paleoproterozoic anorthosites is associated with the lower temperatures of mss crystallization and is close to the rocks from the ophiolite complexes. At the same time the absolute PGE contents are typical to those in basalts of island arcs.

Keywords: anorthosite, PGE, distribution spectra, models of the formation, ophiolite, island arcs.

Problem Formulation

Within the Dzhugdzhur-Stanovoy and Selenga-Stanovoy superterranes of the south-eastern rim of the North-Asian craton two stages of development of the autonomous anorthosites [1, 2] are established: the Neoarchean (2.63-2.62 Ga) [1, 5, 10] and Paleoproterozoic (1.86-1.74 Ga.) [2, 8]. The Kalar association and the Khorogocha massif belong to the first stage and the typical representatives of the second stage are the Dzhugdzhur and Kengurak associations and the Baladek massif. Recently there appeared a numerous publications interpreting mineralogical, petrologic-geochemical and geochronological features of the above formations [1, 2, 5, 8 etc.]. The results of those publications made it possible to reconstruct the probable geodynamic environments and to establish the models of the formation of intrusions. At the same time the peculiarities of PGE distribution in the sulphide free occurrences of the autonomous anorthosites of the south-eastern rim of the North-Asian craton are not studied.

Peculiarities of PGE distribution in anorthosites

It should be noted that a behaviour of all macro-and microcomponents in melts depends on the environments of the intrusions' formation and models of their crystallization. A crucial role in the formation of the autonomous anorthosite massifs [14-17] is played by the processes of interrelation between the mantle Fe-Ti melts of the tholeite composition generated over the plumes of the first generation [13] and the matter of the lower crust. It should be noted that in the composition of the lower crust no PGE with the exception of Pd were identified [12]. These data allow to consider that the contents and peculiarities of PGE distribution in anorthosites of the autonomous massifs are due to the composition of the initial mantle melts.

 According to the presently existing concepts [7] the magmatic melt consists of two immiscible “liquids”-silicate and sulphide and crystallization of each liquid proceeded according to their proper “laws”. The earliest product of the sulphide melt crystallization is the mono-sulphide solid solution [(Fe,Ni)1+xS]mss). Thus, high temperatures at its crystallization promote the concentration of Ni, Fe, S, Ru and Rh in a solid phase and Cu and Pd are driven into a melt [11]. The presence of troilite (mss) in the composition of the ancient mantle is due to the high reducing conditions of its existence corresponding to Fo2   values being close to the equilibrium conditions with a contribution of the metallic Fe-Ni phase [3, 4].

According to experimental data [6] the high temperature series of platinoids are established which are listed in the order of increase of their ability to enter the composition of pyrrhotite (troilite) - Pt-Os-Ru-Pd-Ir-Rh. It should be noted that PGE form a stoichimetric compound with sulphur and, therefore, they easily substitute iron in the composition of troilite and pyrrhotites. With a strengthening of the oxidation environments a potentiality of PGE dispersion in pyrrhotites is reduced and this favoures their separation in the native form [7].

For anorthosites from the autonomous massifs of the south-eastern rim of the North-Asian craton the following PGE seiries were established in the order of a decrease of their normalized concentrations: the Khorogocha -Pt-Os-Pd-Ru-Ir-Rh, the Kalar -Pt-Os-Pd-Rh-Ir-Ru, the Kengurak -Pt-Os-Pd-Ir-Ru (Fig.1). Listed series of the elements are almost consistent with the experimental data [6] with the exception of Pd which is due to the concentration of the latter in residual melt [11]. It should be noted that the most ancient Archean “coarse-grained anorthosites” are characterized by higher Pt, Os contents and by lesser Rh,Ir contents which is related with their entering in the composition of troilite (pyrrhotite).

In contrast to the Archean anorthosites the spectra of PGE distribution in their Paleoproterozoic “massif type” analogues are different. (Fig.1) First of all it is associated with the abrupt uneven alteration of the composition of the mantle melts and the character of geodynamic prosseses (2.3-2.0 Ga) on the scale of the whole Earth [13] Besides, the formation of the initial melts of this type autonomous anorthosites is related to a considerable contamination of them with the matter of the lower crust and this must result in the decrease of temperatures in the magmatic chamber. It should be noted that in the course of time the mantle became more and more oxidized [3, 4].

The PGE contents in anorthosites of the Kengurak massif are at most close to those in basalts of the island arcs for which the enrichment in Pt, Pd and Re [9] was established. At the same time the character of their distribution spectra mostly corresponds to the ophiolite formations (Fig.1) and this confirms the accepted models of the formation for the “massif type” anorthosites [14-17].

m_merged23

Fig.1 - Distribution of platinum group elements normalized on chondrite C1 [18] in the autonomous anorthosites of the south-eastern rim of the North-Asian craton. Conventional signs: 1-2- the rocks of the Khorogocha massif of the “coarse-grained anorthosites”: 1-ultrabasites, 2- anorthosites; 3-4- the rocks of the Kalar massif of the “Archean coarse-grained anorthosites”: 3- ultrabasites, 4- anorthosites; 5-6- the rocks of the Kengurak massif”massif type anorthosites”; 5-gabbroids, 6-anorthosites.

Conclusion

Within the Dzhugdzhur-Stanovoy and Selenga-Stanovoy superterranes of the south-eastern rim of the North-Asian craton two stages of development of the autonomous anorthosites are established: the Neoarchean (2.63-2.62 Ga) and Paleoproterozoic (1.86-1.74 Ga.).

The contents and peculiarities of PGE distribution in anorthosites of the autonomous massifs are due to the composition of the initial mantle melts.

For anorthosites from the autonomous massifs of the south-eastern rim of the North-Asian craton the following PGE seiries were established in the order of a decrease of their normalized concentrations: the Khorogocha -Pt-Os-Pd-Ru-Ir-Rh, the Kalar -Pt-Os-Pd-Rh-Ir-Ru, the Kengurak -Pt-Os-Pd-Ir-Ru.

PGE distribution in anorthosites of the Archean Khorogocha and Kalar massifs is caused by the “laws” of crystallization of the high temperature mss which lead to the concentration of Ni, Fe,S,Ru and Rh in the solid phase and Cu and Pd concentration in melts. The character of the PGE distribution spectra in the Paleoproterozoic anorthosites of the Kengurak massif is associated with the lower temperatures of mss crystallization and is close to the rocks from the ophiolite complexes.

References

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