April 2016 LIP of the Month

Geochemistry of the Ediacaran (c. 570 Ma) Volyn flood basalt province, south-western East-European platform

Leonid Shumlyanskyy

M.P. Semenenko Institute of geochemistry, mineralogy and ore formation, Kyiv, Ukraine

Palladina ave. 34, 03680, Kyiv, Ukraine

lshumlyanskyy@yahoo.com

Introduction

The Volyn continental flood basalt (VP) province represents a relatively well-preserved example of the Neoproterozoic continental large igneous province. It developed during the break-up of the Rodinia supercontinent and is coeval or nearly coeval with numerous other manifestations of basaltic volcanism in Baltica (Bingen et al., 1998; Andréasson, 1994, and references therein) and Laurentia (Puffer, 2002, and references therein). The importance of the VP for understanding of the processes of break-up of the Rodinia supercontinent was discussed in our previous publication (Shumlyanskyy et al., 2007). However, direct geochemical comparison of the Volyn continental flood basalts with other LIP events related to the Rodinia break-up has been hampered by the absence of systematic geochemical information regarding the VP. Results of geochemical studies of rocks of the VP were reported by many authors (Bakun-Czubarow et al., 2002; Bialowolska et al., 2002; Nosova et al., 2005; 2008). However, none of these works dealt with all varieties of rocks present in the province. So, our efforts were focused on obtaining of the geochemical data-set that will embrace the whole range of volcanic rocks present in the Volyn continental flood basalt province. The obtained results allowed us to trace the evolution of volcanism in the VP, and to reach some conclusions about origin of initial melts. These results are presented in this communication. Our further efforts will be concentrated on geochemical comparison of the VP with other manifestations of the Ediacaran mafic volcanism in Baltica and Laurentia in order to establish the general features of evolution of magmatism during Rodinia break-up.

Tectonic setting and stratigraphy

The Volyn continental flood basalt province is located in western Ukraine, eastern Poland, and south-western Belarus. The core part of this area (80 000 km²) is occupied by basaltic flows, whereas volcanoclastic sediments (tuffs) expands the area of the province to 200,000 km² (Kuzmenkova, 2009). The province can be traced over a distance of 770 km from Bialystok in Poland to Chernivtsy in Ukraine. Its width exceeds 300 km in the central part while the maximum thickness (up to 400–600 m) is confined to the axial part of the Volyn-Orsha aulacogen. The VP is located on the western margin of the East European platform, where it straddles the boundary between two main segments of Baltica, namely Sarmatia and Fennoscandia (Fig. 1).

Rocks that compose the VP rest upon the Neoproterozoic Polissya Series and Brody suite, and in places, directly overlay the crystalline basement (Figs. 1 and 2). The volcanic and volcanocalstic rocks of the Volyn continental flood basalt province constitute the Volyn Series and can be subdivided into a lowermost part (40–50 m), termed the Gorbashy suite, which is composed mainly of conglomerates and sandstones, while the main part of the Volyn Series consists of basic effusive and pyroclastic rocks (Fig. 2). Thin picrite flows present among clastic sediments of the Gorbashy suite. The volcanogenic sequence of the Volyn Series is subdivided into three parts in Ukraine (from bottom to top) called the Zabolottya, Babyne and Ratne suites (Birulyov, 1968). These rocks are poorly exposed and mainly available from numerous drill holes, while tuffs and basalts of the uppermost Ratne suite crop out in several quarries (Fig. 3) and were extensively investigated. Effusive rocks of the lowermost Zabolottya suite are less abundant and found almost entirely within the Volyn-Orsha aulacogen. They consist of one to eight basaltic flows with a subordinate (<25% of the total suite thickness) amount of pyroclastic material. The maximal thickness of the suite reaches 166 m. The Babyne suite is the most widely distributed and composed mainly of volcanoclastic sediments. The suite comprises layered, mainly fine- to medium-grained vitroclastic basaltic tuffs with abundant interlayers of fine-grained vitro-crystalloclastic and coarse lithoclastic tuffs. One to two basaltic flows occur in the central part of the Babyne suite, the thickness of which reaches 180 m. The Ratne suite is the uppermost one and is the best studied (Bialowolska et al. 2002; Nosova et al. 2008; Shumlyanskyy 2008). Tuffs are rare while basaltic flows vary in thickness from 5–10 to 50–60 m. There are up to ten basaltic flows and the total thickness of the suite can exceed 220 m. Intermediate and felsic (andesite-dacite) rocks appear locally in the north-western part of the Volyn province known as the Brest depression in Belarus (Makhnatch 1968). The maximum thickness of the felsic rocks is 112 m, but due to a small areal occurrence, they only constitute a few per cent of the total volume of the Volyn province.

Age of the Volyn continental flood basalt province

The stratigraphic age of the Volyn flood basalt sequence has been defined as Early Vendian (Kruglov and Tsypko, 1988). Shumlyanskyy et al. (2016) presented the review of the radiometric ages and results of the new U-Pb zircon and baddeleyite dates of rocks of the VP. According to these results, magmatic zircons from the high-Ti basalt crystallized at 573 ± 14 Ma, whereas the rhyolitic dacite yielded an age of 571 ± 13 Ma. Baddeleyite from the olivine dolerite sample gave a ²⁰⁶Pb/²³⁸U age of 626 ± 17 Ma, whereas the ²⁰⁷Pb/²⁰⁶Pb weighted average age of 567 ± 61 Ma is close to the zircon ages. Hence, age of the Volynian continental flood basalt province can be currently defined as c. 570 Ma, that correspond to the middle to late part of the Ediacaran period.

Geochemistry of the main rock varieties

A comprehensive geochemical investigation of the all rock varieties present in the Volyn continental flood basalt province allowed recognition of the six main types of mafic rocks. These rock types, from top to bottom, are: (1) high-Ti tholeiite basalts of the upper part of the Ratne Suite; (2) low-Ti, low-Nb tholeiite basalts of the lower part of the Ratne suite; (3) low-Ti Al-rich basalts of the Babyne suite; (4) olivine basalts of the Zabolottya suite; (5) picrites that locally occur among clastic sediments of the Gorbashy Suite; (6) sheet-like intrusive bodies of dolerites and gabbroic dolerites among clastic sediments of the Polissya Series.

According to Shumlyanskyy et al. (2016), chemical compositions of minerals and rocks vary regularly in the vertical section of the Volyn Series. Calcic plagioclase (up to An₈₆), magnesian orthopyroxene (En₈₂Fs₁₃Wo₅) and clinopyroxene (En₅₉Fs₉Wo₃₂) are characteristics for picrites. These rocks contain completely altered olivine phenocrysts while the opaques are represented by chromite. Upwards in the section, plagioclase becomes more sodic and pyroxene more ferrous. Olivine is still present in the Babyne suite basalts but disappears in basalts of the Ratne suite. Plagioclase phenocrysts found in the Ratne suite basalts are very close in composition to plagioclases present in groundmass of picrites or Zabolottya suite basalts. Intrusive dolerites correspond in mineral chemistry to the Ratne suite basalts but contain olivine instead of orthopyroxene.

The whole-rock #Mg, ranging between 69 and 73 in picrite, gradually decreases upwards and reaches 35–60 in high-Ti basalts of the Ratne suite and intrusive dolerites. REE abundances and degree of their fractionation gradually increase upwards in the section (Fig. 4). A negative Eu anomaly is characteristic for picrites and for some of the Babyne suite basalts, while a weak positive Eu anomaly is found in low-Ti basalts of the Ratne suite and in dolerites.

The primordial mantle-normalized patterns of trace elements in rocks of the VP are shown in Fig. 5. There is a general tendency for the incompatible trace element abundance to increase upwards in the section of the Volyn continental flood basalt province, whereas content of compatible elements sharply decreases (Fig. 6). The gradual increase in the concentration of incompatible elements is disrupted by the Babyne Suite basalts, which have the highest content of incompatible trace elements.

The most incompatible elements (Rb, Th, U, K) demonstrate a wide range of variation that is explained by mobility of these elements in the processes of secondary alteration. In general, there is a gradual decrease of the primordial mantle-normalized concentrations from the most incompatible elements towards the moderately incompatible. All rock types demonstrate negative spikes of Sr and P of variable intensity, and positive Ba spike. A Nb-Ta anomaly is absent in the most trace-element-enriched rock types, but is very distinctive (negative anomaly) in the trace-element-poor rocks that compose lower parts of the Volyn Series.

Sr and Nd isotope compositions

All of the measured samples of rocks of the VP have negative values of εNd and positive values of εSr, calculated at 550 Ma (Fig. 7), and hence, demonstrate crustal isotope characteristics rather than mantle composition. The most prominent “crustal” composition is characteristic for picrites whereas upwards in the section the isotope composition of the rocks gradually shifts towards values more characteristic for the mantle-derived rocks. For instance, εNd in rocks gradually increases upwards from –12 in picrites to between –1 and –6 in high-Ti Ratne basalts and reaches values of –1 to –3 in dolerites. There is a pronounced link between chemical and isotope composition for the rocks in the Volyn continental flood basalt province. For instance, Nd isotope composition reveals a strong positive correlation with TiO₂ concentration (Fig. 7).

Origin of rocks of the Volyn continental flood basalt province

The regular variations of the mineral and rock chemistry in a section of the VP suggest that a simple crystal fractionation model may relate all rocks of the province to each other. As was assumed by Cox (1980), the initial melts for continental basalts must have a picritic composition in order to be in equilibrium with the mantle restite. Such rocks are present in the very bottom part of the VP, and could represent initial melt for the whole volcanic succession, whereas basaltic rocks and dolerites occur as successive differentiates of the primary picritic rocks. The composition of plagioclase, augite and Ti-magnetite phenocrysts found in basalts of the Ratne Suite supports this assumption. These phenocrysts differ sharply in composition from their counterparts present in the groundmass, indicating that phenocrysts had crystallized from a melt of different composition than the melt from which groundmass crystallized. At the same time, the composition of phenocrysts in the Ratne Suite basalts closely matches the composition of the minerals that compose groundmass of picrites and olivine basalts of the Zabolottya Suite. Apparently, phenocrysts started to crystallize from the more primitive melts of picritic composition whereas the groundmass crystallized from the evolved low-Mg# melt that had experienced fractionation.

However, geochemical and isotope data clearly indicate that rocks mentioned above having different geochemical types cannot be derived from a single melt. For instance, the most chemically primitive rocks (picrites) have the most evolved Sr and Nd isotope composition. Chemical fractionation of these mantle-derived initial melts cannot produce rocks with the more primitive isotope composition. This statement is clearly supported by the difference in isotope composition of the picrites (isotopically most evolved rocks of the VP) and olivine basalts of the Zabolottya Suite that immediately overlay the picrites and which have the most primitive Sr and Nd isotope compositions (Fig. 7). Hence, these rocks have crystallized from different batches of the mantle melts that possibly originated from different sources. The chemical composition of the Babyne Suite basalts represent additional evidence of the variability of mantle sources for Volynian continental flood basalts. The Babyne Suite basalts are rich in compatible Ni and incompatible Nb, LREE, K, Ba, and relatively poor in Cr (Fig. 4-6). These peculiarities of chemical composition make Babyne basalts different from all other rocks of the VP and cannot occur due to fractional crystallization. Apparently, the Babyne basalt originate from a specific batch of the mantle-derived melt.

The role of the crustal contamination can be estimated from the plot Th/Ta vs. εSr (Fig. 8). The upper-crustal rocks have high Th/Ta and high εSr values, in contrast to the mantle-derived rocks that have low Th/Ta and εSr. As can be seen from Fig. 8, olivine basalts of the Zabolottya Suite are the least contaminated rocks of the Volyn continental flood basalt province. High-Ti basalts of the Ratne Suite and intrusive dolerites are also moderately contaminated rocks, while picrites can be regarded as heavily contaminated rocks. The same conclusion can be reached from examination of the La/Nd vs. εNd plot (Fig. 8). In general, it may be concluded that the degree of crustal contamination gradually decreases upwards in the vertical section of the VP, with the exception of olivine basalts of the Zabolottya Suite which occur in the lower part of the volcanic succession and appear to be the least contaminated rock type.

A similar situation has been described in the Emeishan continental flood province, SW China, where the high-Ti basalts (enriched in incompatible trace elements and less contaminated) overlay low-Ti basalts (Xu et al., 2001). A gradual decrease in the degree of contamination is explained by the depletion of the walls of the feeding channels with respect to the fusible incompatible trace elements as these channels are been washed out by subsequent portions of the mantle melts. The same model may be applied to the VP, as there is a tendency for the later melts to be less contaminated. However, the case of the least contaminated Zabolottya olivine basalts indicates, that this model is oversimplified, and different portions of the melt used different feeding systems and were generated from different sites in the mantle.

A possible source of the initial melts may be assumed from the Zr/Nb vs. Ba/Nb and Dy/Yb vs. La/Yb plots (Fig. 9). The rational behind these plots is discussed by Riley et al., 2005 and Shumlyanskyy et al. (2016). As can be seen from these plots, initial melts for the earliest rocks of the VP were generated at relatively shallow depth and high degree of melting. With time, the melting site migrated to greater depth, whereas the degree of melting gradually decreased. Furthermore, rocks of the VP plot between fields of enriched mantle and lower continental crust, indicating the mixed source for the initial melts.

As noted above, most of the rocks of the Volyn continental flood basalt province have negative anomalies of Sr, P, and Ti, and some of them – Nb negative anomaly. Similar anomalies were found in Mesozoic Gondwana low-Ti flood basalts (Hergt et al., 1991). According to these authors, such anomalies are characteristic for rocks that originated from the subcontinental lithospheric mantle. This conclusion is in line with other evidences that include high degree of contamination, “evolved” Sr and Nd isotope compositions etc.

Crustal contamination may have the same effect on the melt’s chemistry as their origin from metasomatized lithospheric mantle (e.g. Pearce, 2008; Hollings et al., 2012; Ernst, 2014 and references therein). These two processes can be distinguished taking into account the intermediate bulk composition of the continental crust. Hence, crustal contamination must result in increase of SiO2 abundance and decrease of MgO, whereas melting of the metasomatized lithospheric mantle will have only minor effect on major elements composition. Indeed, there is no apparent correlation between SiO₂ (and MgO) concentrations and Nd isotope composition, which is an argument in favour of interaction with mantle rather than of crustal contamination. However, there are some facts that support crustal contamination: (1) picrites of the VP are unusually rich in SiO2 (45.2 – 50.2 %) that may be explained by contamination by the siliceous crustal material; (2) basalts of the Ratne Suite contain numerous zircon xenocrysts that vary in age from c. 1220 Ma to Archaean (Shumlyanskyy et al., 2016), that also indicate extensive crustal contamination. In general, there is a reverse correlation between the abundance of incompatible trace elements and the degree of crustal contamination: rocks with low concentrations of incompatible trace elements have lower εNd, higher εSr, and deeper negative Nb-Ta, Sr, P, and Ti anomalies. These peculiarities of their composition can be explained by crustal contamination, as the smaller amount of the incompatible trace elements is present in the melt, the easier it will be modified by the crustal contaminants.