Geochemical and petrological characteristics of volcanic rocks of Bayangol accretionary prism, Northern Mongolia

Karimov A.A.

A.P. Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia

mozies@yandex.ru

The Bayangol accretionary prism is the Paleozoic structure lying within the Dzhida terrain of the Central Asian foldbelt, located between the Tuva-Mongolian microcontinent and Khamar-Daban terrain. This terrain is referred to the accretion-collisional orogen derived by closing of the Paleoasian ocean, which proceeded in the Late Carboniferous – Permian time. Its main structural units are: (1) island arc, (2) accretionary complex, (3) passive margin complex and (4) seamount remnants.

The Bayangol accretionary prism consists of steeply dipping tectonic slices, separated by tectonic sutures. Tectonic slices consist of the following rock associations: 1- serpentinites bearing lenses of metagabbroes, metabasalts, boninites and pyroxenites; 2- tectonic mélange composed of basic tectonites (blastomylonites), limestones and dolomites; 3 - sequence of altered basalts; 4- differentiated metavolcanics made up of shale, blastomylonites with limestone lenses; 5- terrigenous sequence of siltstones, sandstones and pelite.

Volcanics were derived from serpentinite mélange. The differences were subdivided into massive lava and clastic lavas, fragmentary lava rocks. Its fragments are brought by the lower and marginal parts of streams while lava is moving capturing material from the walls of the channel, cone and apical parts of the dome.

Clastic lava commonly holds fragments of aphyric texture and porphyry ones. Phenocryst fragments of the latter are present as tabular elongated grains of plagioclase (0.5-1.5 mm) and small-sized clinopyroxene (0.3-1 mm). However, the bulk mass includes plagioclase and clinopyroxene grains both in the fragments and cementing lava. The clasts of the first type are "surrounded" by the groundmass of the second type. So the extract, consisting of the second type minerals, captured and cemented frozen clasts of the first type.

Massive lavas are divided into 3 groups with specific structural and textural features and a certain set of phenocrysts.

The first group of volcanics show porphyritic texture with minor phenocrysts (15-25%) being clinopyroxene and minority of plagioclase. The grain size of clinopyroxene varies widely from 0.5 to 5 mm, their shape is polygonal with smooth edges. Grains of plagioclase are smaller (0.5-1 mm) and occur as elongated tables. Groundmass is composed of fine crystals of plagioclase.

The second group also displays porphyritic texture. However, plagioclase crystals predominate among phenocrysts. Plagioclase grains range in size from 0.5 to 2.5 mm. Clinopyroxene grains are smaller (from 0.2 to 1 mm) compared to plagioclase ones. Both plagioclase and clinopyroxene crystals have tabular shape.

The third group of rocks is characterized by essential secondary alteration. Amphibole replaces clinopyroxene, whereas zoisite replaces plagioclase. The texture of these rocks is usually aphyric. The groundmass is constituted by fine plagioclase crystals or fine grains of amphibolized clinopyroxene.

The geochemical characteristics of massive lava, massive lavas of the first group have low contents of TiO₂ (<0.5 wt %), a wide range of concentrations of SiO₂ and CaO, and quite high concentration of MgO (5-15 wt %). Such concentrations of major oxides are common for boninitic series. In the rocks, when SiO₂ concentrations increase the MgO and CaO abundances decrease, whereas the content of TiO₂ increases. Such patterns are characteristic of mostly differentiated rocks called boninites.

Petrographic studies revealed that some of the volcanic rocks have clinopyroxene phenocrysts only, while some other volcanics contain both clinopyroxene and plagioclase. This fact suggests that clinopyroxene was the first phase to crystallize, followed by plagioclase.

Usually, boninitic series crystallize clinopyroxene first. This leads to the removal of MgO and CaO from the melt, and increase in Al₂O₃. The crystallization of plagioclase after clinopyroxene should cause the change of positive correlation of Al₂O₃ with SiO₂ to negative in the melt.

Some volcanic rocks represent clastic lavas with fragments of previously crystallized rocks, and they can not meet the chemistry of boninites.

The second-group volcanics contain about 1.2 wt. % TiO₂, 17-19 wt % Al₂O₃, and low abundances of MgO (~ 5 wt %). The concentrations of every major oxide are sparcely scattered, which might suggest the homogeneity of lavas and similar conditions for their formation.

Volcanics of the third group differ from the second one in lower concentrations of SiO₂ (<46 wt %), Na₂O (<2 wt %), extremely high CaO (15-20 wt %) and MgO (5-12 wt %), and elevated TiO₂ (up to 2 wt %).

Thus, the first group of the Bayangol volcanics is similar to boninitic series, that satisfies the following criteria: SiO₂> 52 wt %; MgO> 8 wt %; TiO₂ <0.5 wt %, Cr> 800 ppm.

The second and third groups of volcanics are basalts. The third group basalts were subjected to a significant alteration compared to the basalts of the second group. The former have extremely low concentrations of K₂O and Na₂O (<1 wt %), and high CaO (15-23%).

The Bayangol boninites are strongly depleted in trace elements in comparison with the N-MORB. The range of distribution of trace elements has a smooth appearance to the individual peaks of Cs, Ba, U, K, Pb, Sr. Increase in Tb to Lu concentrations is common for HREE .

The studied species show growing concentrations of trace elements with decreasing MgO content in the rock; but the spectrum curve does not change. Thus, it is concluded that the formation of the Bayangol boninites proceeded through fractional crystallization, i.e. the melt was enriched in incompatible elements during crystallization of clinopyroxene.

Bayangol basalts are characterized by enrichment of all elements compared to N-MORB except for the HREE; HFSE (Nb, Zr, Hf, Ti) abundances are minimum in island arc lavas.

Basalts of the third group have the same level of concentration and pattern of distribution of rare elements, such as basalts, the second group except for the absence of peaks for Rb, Ba and K. As noted above, they differ from the basalts of the second group by major element contents; they have high CaO, TiO₂, MgO, and low SiO₂, Na₂O, K₂O concentrations. However, the same distribution of trace elements (except for Rb, Ba, K) suggests a close composition of the rocks of the second and third groups of basalts and hydrothermal conversion of the third group. The interaction with hydrothermal solutions caused addition of Ca, Mg and removal of Na, K, Rb, Ba elements being mostly mobile in the aquatic environment.

Petrological boninite uniqueness lies in the fact that they are generated only in subduction geodynamic settings. This statement is confirmed by some factors.

(a) The enrichment in LILE (K, Cs, Ba, Sr, U), as compared to N-MORB, reflects the critical contribution of aqueous fluid extracted from the submerging slab.

(b) High MgO and SiO₂ contents with very low concentrations of highly incompatible elements (Nb, Ta and Ti) with respect to the concentrations of LILE (K, Cs, Ba, Sr, Th, U) require shallow melting of peridotite at depth 30 km.

(c) The crystallization of MORB-type melts, which do not contain water, occurs in the following sequence: olivine → plagioclase → clinopyroxene. The melts generated over submerging slabs, have a different order of crystallization: olivine → clinopyroxene → plagioclase. The differences in the sequence of crystallization reflect the fundamental differences in water saturation of mantle sources.

As follows from this statement, boninitic magmatism takes place only in subduction zones. Boninites are the only type of rock that might indicate clearly the origin of associated volcanic rocks in supra-subduction ensimatic island arc environmen.

Partial melting of depleted oceanic lithosphere and generation of boninites occur at depths 30 to 40 km. The basalts formed later on while the slab was submerging. Basaltic melt was derived during partial melting of the mantle, and dehydration of melts produced at depth about 100 km.

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