The Shalap Mélange of the Salairian Alambay Ophiolite Zone (Northwestern Central Asian Orogenic Belt), Geological Structure and Compositional Features of Amphibolites and Greenstone Basalts

1. INTRODUCTION

The ophiolite zones yield clues about the oceanic basins of the past. The information about their-contained evolving oceanic basins approximately several thousand km long and geological processes in the subduction zone is constrained to a narrow band of subduction-related mélange. Geochemical features of basalts and basaltic rocks from mélange can be used to determine tectonic conditions for the formation of the oceanic crust. The Alambay ophiolite zone (AOZ) is one of the ophiolite stuctures of the Altai-Sayan Folded Area (ASFA) of the Central Asian Orogenic Belt (CAOB) to which modern research methods were least applied [Konovalova, Prusevich, 1977; Pinus et al., 1958]. It is located in an axial part of the Early Paleozoic Salairian fold-nappe structure (orogen) and includes the northern extension of the Alambay-Kaim zone, Salair, and Altai Mountains [Shokalskii et al., 2000]. The AOZ contains several en echelon lens-shaped areas. The northwestern, Tyagun-Alambay area is confined to the middle part of the Salair. The southeastern, Shalap (Martynovo-Shalap) area, forms an axial part of the southwestern flank of the Salair (Fig. 1). The Shalap area is predominantly composed of terrigenous mélange with occasional serpentinite mélange. The aim of this study is to make assumptions about formation conditions of the Shalap ophiolite mélange based on the characteristics of the geological structure and composition of metabasites therefrom.

2. GEOLOGICAL STRUCTURE OF THE SHALAP MÉLANGE

Within the ophiolite zone, there are distinguished the Alambay basalt, upper Alambay dunite-harzburgite and Shalap mélange geological complexes. Besides, the AOZ is structurally associated with a tectonic slab of amphibolites recognized as the Angurep metamorphic complex [Shokalskii et al., 2000; State Geological Map…, 2019]. The Shalap mélange complex as a mapped tectonic-stratigraphic subdivision was identified by V.N. Tokarev during cartographic mapping [State Geological Map…, 2019]. In view of the presence of the Devonian lens-shaped tectonic blocks in the mélange area field, V.N. Tokarev considered the Shalap mélange as tectonic, formed in the Permian in result of thrusting Salair slab of allochthonous material over the Kuzbass along the imbricated zone. From our point of view, the Shalap area represents the Cambrian subduction mélange, which underwent the Late Paleozoic thrust deformations as well as all the Salair orogenic formations did, so that the Devonian tectonic sediment slabs occurring in the mélange field, unlike microquartzite, limestone, serpentinite and amphibolite blocks, are not syngenetic inclusions of the ophiolite mélange. This is supported by the composition of terrigenous mélange matrix, represented by common sedimentary rocks with different degrees of tectonic reworking, the composition of blocks-inclusions in mélange, and a number of geological features which allow the Shalap mélange to be related to the category of subduction mélanges [Festa et al., 2010; Raymond, 2019]. The mélange complex has a widespread occurrence in the southern Alambay ophiolite zone, within the Shalap area (Fig. 2). It is traced as a band up to 22 km wide and about 120 km long. The band has an inhomogeneous structure and includes lens-shaped serpentinite and terrigenous mélange bodies.

According to the geophysical data, the depth distribution of mélange exceeds 5–6 km. Serpentinite mélange is studied in well core to a depth of 500 m; it is represented by serpentinite schists with several-kilometer massive serpentinized dunite and harzburgite blocks. Terrigenous mélange comprises a major part of the complex. The lumps and blocks of mélange are composed of amphibolites from the Angurep metamorphic complex, high-Ti basalts from the Alambay formation, siliceous schists and microquartzites (Fig. 3, a–c), and marbleized limestones of the Gavrilov formation, Lower Cambrian [State Geological Map…, 2019]. The terrigenous mélange matrix has non-uniform composition represented by aleurolites and volcanomictic greywacke sandstones admixed with carbonates, siliceous material, ore dust and carbonaceous matter. Mélange is intruded by the Late Permian plagiogranites of the Zhernov complex with a U-Pb zircon age of 262.2±2.3 Ma [State Geological Map…, 2019]. Most of the outcrops of the Shalap mélange represent microquartzite and clay-rich siliceous shale blocks (Fig. 3, a), containing, among other things, manganese oxide mineralization (Fig. 3, b). The microquartzite blocks vary in size from a few meters to a few hundreds of meters. The microquartzite blocks form chains elongated parallel to the structures commonly striking northeast. Microquartzites comprise a range of hills near Tselinnoye settlement. The microquartzite blocks lie in the matrix materials made up of greenish-gray aleuritic greywackes with different degrees of tectonization. Near Tselinnoye and Shalap settlements, there is a large proportion of microquartzite blocks in terrigenous mélange, with the matrix rock strongly tectonized up to the formation of folded schists. To the east, along the Yama River valley, there is a decrease in the proportion of rock inclusions in mélange and in the degree of tectonic reworking of matrix rock, so that mélange gradually re-forms into common sedimentary rock composed of fine-grained greywackes. Eastward of the Shalap area, there are some more mélange lenses and bands of smaller size. The tectonic slabs of volcanites, flysches and limestones are intercalated within the mélange zone. On the magnetotelluric (MTS) profile, intersecting the southern Salair structures, the mélange area represents the conduct anomaly with an inhomogeneous inner structure [Zhimulev et al., 2023]. The profile shows that the block with relatively high electrical resistivity, corresponding to the tectonic slab of metamorphic rock of the Angurep complex, is located within the conduction area composed of the Shalap area complexes of the AOZ. The slab of rock of the Angurep complex can be considered as a part of structural-substantial paragenesis of the paleosubduction zone.

3. ANGUREP METAMORPHIC COMPLEX

Garnet amphibolites of the Angurep metamorphic complex eastward of the Shalap mélange area comprise a 2–4 km thick tectonic slab that can be traced in rare outcrops over a distance of about 60 km in accordance with the geophysical data. The slab has a sublatitudinal strike in the northern part and a submeridional strike in the southwestern part. Metamorphic rocks in geophysical fields are accompanied by positive gravity anomalies. On the north and northwest, the tectonic slab of metamorphic rocks is superimposed with a slab of the Lower Cambrian volcanites, on the south and southeast – with a stratum comprising a wide range of materials such as volcanomictic sandstones, conglomerates, limestones and volcanites, interpreted as an olistostrome in the accretionary wedge [Dobretsov et al., 2004]. The fault zones, bordering a slab of metamorphic rocks, are traced by foliated serpentinite and graphitic milonite bodies.

Metamorphic rocks are represented by garnet-bearing and non-garnet amphibolites; the cross-section exhibits both fine- and coarse-grained varieties (Fig. 3, d–f). Plagioclase in amphibolites has partly saussuritized (Fig. 4). Amphibolites in some areas are intensely migmatized. Numerous granitoid segregations in a stratum of amphibolite are of two morphological types: 1) conformable striation directions, twisted veinlets, irregularly shaped lenses and segregations with twisted contacts (see Fig. 3, d), 2) dikes with smooth intrusive contacts intersecting banded amphibolites (see Fig. 3, e). According to S.A. Kargopolov’s data on amphibolite mineral chemistry [Vladimirov et al., 1994], three types of rocks can be distinguished based on the composition of coexisting garnets within the garnet-bearing unit: 1) with high-magnesial garnet (Рrp>20–30 %), 2) with grossular-almandine (Рrp – 10–15 %), 3) with manganous glossular-almandine (Рrp<10 %). The highest Mg garnet samples also show the highest aluminum concentrations in amphiboles (up to 17 wt. % of Al₂0₃). Such amphiboles have low alkaline and Ti contents and belong to the actinolite-chermakite group. The Mg garnet samples include rutile; the association of magnesian garnet, aluminiferous amphibole and rutile is considered as a high-pressure metamorphism product. A widespread occurrence of migmatites and granitiod veins in a stratum of amphibolites is indicative of the partial melting during metamorphism. In accordance with S.A. Kargopolov’s estimates, the thermodynamic parameters of metamorphic amphibolites both from the Angurep slab and Shalap mélange are 600–650 °С and 8–9 kbar [Vladimirov et al., 1994; Zhimulev et al., 2020]. The age of metamorphic amphiboles from the Angurep complex was determined by A.V. Travin and S.A. Kargopolov through ⁴⁰Ar/³⁹Ar isotopic dating of chermakite amphibole from the garnet-bearing amphibolite sample. High-pressure paragenesis revealed from this sample includes Qz+Pl+Hbl1+Grt+Rt. Secondary minerals are presented by Ep+Ms+Chl+Hbl2 association. Secondary amphibole (Hbl2) occurs rarely as actinolitic hornblende and actinolite. Dating was performed on a monofraction of chermakite amphibole, typical of high-pressure paragenesis. On the age spectrum diagram of the sample there is a plateau characterized by 520±2 Ma and a 60 % share of released ³⁹Ar [Vladimirov et al., 1994; Zhimulev et al., 2020]. The closure temperature of K/Ar isotopic system of amphibole (500–550 °C) is lower than the temperature of the described mineral paragenesis, so that the obtained value can be considered as the upper age limit for peak metamorphism.

4. GEOLOGICAL SETTING OF THE SELECTED SAMPLING AREAS

In the Shalap mélange distribution field there were selected samples 19-261, 19-262, 19-263, and 20-290-1 (see Fig. 2). A more than 10-m lens of serpentinite mélange with a greenstone aphyric basalt block was penetrated in the Yama River valley slope (see Fig. 3, c; Fig. 4). Greenstone basalts are composed of plagioclase microcrystals plunged into the matrix of chloritized volcanic glass. The rock is cataclastic. A block of garnet-bearing amphibolites from the Angurep metamorphic complex in the terrigenous mélange unit was quarried in the right bank of the Shalap River. The block has a size of more than 100 m. The rocks are represented by garnet amphibolites (samples 19-261, 19-262), medium- and fine-grained, dark-green massive or indistinctly banded (Fig. 4). Garnet content varies along the cross-section, from zero or single grains to 15 %. At the eastern edge of the quarry, the amphibolites are intersected by a yellowish-gray fine-grained mica-quartz vein, presumably produced by regressive metamorphism of the granitoid vein (sample 19-263). Northwest of the Shalap mélange area, near Kytmanovo Village, the quarry penetrated the mélange area of the same name, represented by lens-shaped blocks of limestones and green-stone basalts of the Alambay formation (see Fig. 1, sample 20-331,) in the matrix of schistose greywacke and serpentine schists. The basalts contain numerous calcite veinlets and segregations. Metamorphic rocks of the Angurep complex were quarried near Popovichi Village along the Angurep and Anamas rivers (samples 20-307, 20-311, 20-313, 20-322). The AOZ gabbro and dolerite samples (19-241, 19-243, 19-243-1) were taken from the Tyagun-Alambay area of the AOZ. The data on composition of the rock samples and their sampling sites are presented in App. 1, Table 1.1 and in Fig. 1 and 2.

5. METHODS

Contents of petrogenic elements in the rocks were determined by X-ray fluorescence analysis, and those of rare and rare-earth elements – by the ICP-MS method using Shared Research Facilities of Multi-Element and Isotope Researches of SB RAS (Novosibirsk). The major task of petrochemical and geochemical analyses was to extract typical features of photoliths which yielded amphibolites. This involved the elements immobile in metamorphism (Ti, Y, Zr, Nb, REE). The sampling did not include migmatized varieties which were mismatched with the original composition of protoliths. The muscovite age was determined by the ⁴⁰Ar/³⁹Ar step-heating for mineral fractions, after [Travin, 2016].

6. GEOCHEMICAL COMPOSITION OF ROCKS

Based on the original data, consideration was given to the petrochemical and geochemical features of metabasalt and amphibolite samples from tectonic blocks in the Shalap ophiolite mélange, and a comparative compositional analysis was made on gabbro and dolerites of the Togul-Sungai ophiolite mélange area (App. 1, Table 1.2).

Some of the selected basite samples are significantly transformed by secondary processes as reflected in high loss on ignition. Their geochemical features were extracted based on the data on rare elements; since the rocks studied were involved in metasomatic and metamorphic transformations, use has been made of the diagrams with low-mobility elements (Th, Nb, Ti, V) proposed to interpret ophiolites in different geodynamic settings [Pearce, 2008, 2014].

The studied samples are partially transformed by low-temperature metamorphic processes; in particular, basalt sample 20-331 contains the calcite veins which leads to high loss on ignition (7.1 wt. %). According to total alkalinity, metabasalts belong to subalkaline series and have high titanium (more than 2 wt. %) and magnesium (9.5–11.1 wt. %) contents. Amphibolites have 43 to 56 % silica content and belong to the rocks of normal, less often moderate alkalinity; unlike the metabasalts, the amphibolites of the Angurep complex are characterized by low TiO₂ content ranging from 0.13 to 1.76 wt. %. There are distinguished two groups of amphibolites – with moderate (1.6–1.8 wt. %) and low (less than 0.8 wt. %) content of TiO₂. On Zr/Ti–Nb/Y diagram for low-mobility highly charged elements, imaging points for amphibolite composition fall within the basalt field and are comparable to those for composition of gabbro from the tectonic slab contacting with apodunite serpentinites of the Togul-Sungai massif in the west. These data allow the protoliths of amphibolites to be assigned to tholeitic basalts of normal alkalinity. Unlike those, the metabasalts have high Nb/Y contents and fall within the alkaline basalt field (Fig. 5, а).

The geodynamic setting of the formation of ophiolites can be judged from the data on Nb/Y-Zr/Y diagram [Condie, 2005]. Composition points of the amphibolites and gabbro considered lie in the area of magmatic systems unaffected by the plume source (NPS) in the mid-ocean ridge basalt field and are close to the data on the Woodlark back-arc basin. The metabasalts, on the contrary, are plume-related OIB – intraplate ocean island basalts (Fig. 5, b). On Nb/Yb-Th/Yb diagram, used to interpret ophiolite complexes [Pearce, 2008, 2014], the metabasalts have high Nb/Yb and Th/Yb ratios and lie in the composition field of basalts unrelated to subduction zones, between E-MORB – OIB points (Fig. 5, c), whereas the amphibolite points lie higher than this field of basalts whose formation was influenced by subduction processes – they lie in the field of intra-oceanic island arc basalts, being associated with the data on the back-arc basalts of the Mariana trough and the Late Mesozoic supra-subduction zone-related Troodos ophiolitic lavas [Pearce et al., 1984]. The imaging points for amphiboilite composition are widely scattered over Ti – V diagram and geochemically comparable to proximal island-arc tholeites and distal back-arc basin and N-MORB basalts (Fig. 5, d).

The distribution of rare-earth elements shows that metabasalt spectra have a negative slope (with a predominance of light lantanoids) and are most close to oceanic island basalt plots (Fig. 6, а). The distribution plots of immobile elements normalized to N-MORB also show their belonging to the rocks from the enriched mantle source similar to oceanic-island or E-MORB basalts (Fig. 6, b). The distribution plots of chondrite-normalized rare-earth elements in garnet amphibolites of the Shalap ophiolite mélange have a subhorizontal spectrum or a shallow negative slope (Fig. 6, c). This can be attributed to island-arc tholeite basalts (IATB) and back-arc basin basalts (ВАВВ). Compared to them, the rare-earth spectra in the amphibolites of the Angurep tectonic slab have lower REE contents and coincide with the BABB spectra. According to the distribution of immobile elements, the amphibolites of the Angurep complex form a compact field with a Th-, Nb-, Hf- and Ti-element content, similar to that plotted for the supra-subduction zone-related ophiolites, and indicate the amphibolite formation either near the island arc or in the back-arc basin (Fig. 6, d).

The rare-earth spectra of the gabbro and dolerite samples from the Togul-Sungai massif are identical to the spectra of the amphibolites of the Angurep complex and typical for melts from supra-subduction zones. Amphibolite sample 20-313 has the lowest rare-element content which testifies to the fact that it was derived from the source more depleted than the MORB mantle; its protolith may be a metamorphosed cumulative rock. This sample is characterized by the highest Al₂O₃ and CaO contents and a low TiO₂ content (0.13 %), not typical of mafic volcanic rocks.

According to their geochemical composition, the basalts, occurring as blocks in the Shalap mélange, were plume-derived and may be the fragments of oceanic islands (OIB). Geochemical features of gabbro of the Togul-Sungai ophiolite massif imply that it is supra-subduction zone-related. The basites that served as the protoliths for amphibolites were produced by melting out of a similar magmatic source, in the paleogeodynamic setting of oceanic island arc.

7. ARGON-ARGON DATING

The argon-argon dating was performed on muscovite from the mica-quartz apogranite vein intersecting the garnet amphibolite block in terrigenous facies of the Shalap mélange. Muscovite has differently oriented occurrence patterns, without deformational structures, and represents the latest post-tectonic mineral formed in granitoid veins at the stage of low-temperature diaphthoresis of the terrigenous mélange rocks. The argon-argon age of muscovite determined using the plateau method (App. 1, Table 1.3; Fig. 7) is 500.0±6 Ma and can be considered as the upper time limit for the completion of retrograde metamorphic evolution in the complex.

8. RESULTS AND DISCUSSION

As a result of research in serpentinite mélange of the Shalap area of the AOZ, there were found basalt blocks with petrogeochemical characteristics of ocean island basalts (TiO₂=2.0 %, P₂O₅=0.22 %, Nb=16 ppm, Zr/Nb=7.6, La/Yb_n=5.54). The metamorphic rocks, represented by garnet-bearing and non-garnet amphibolites of the Angurep complex, compose a narrow tectonic slab, lying between the island-arc acid effusive rock slab and the olistostrome unit, and the blocks in the terrigenous facies of the Shalap mélange. In the mélange, there is a superimposition of rock blocks comprising large tectonic slabs which lie on both sides of the AOZ; these are metabasalts of the Alambay formation, microquartzites, including manganiferous, comprising the Kivdinskaya tectonic slab, amphibolites of the Angurep complex, and limestones of the Gavrilov formation. The tectonic slab package as a whole can be considered as an accretionary complex, and the Shalap mélange area – as an axial part of the structure corresponding to the paleosubduction zone. Amphibolites of the AOZ correspond in their petrogeochemical features to the island-arc tholeitic basalts and contrast sharply with intraplate basalts including the basalts of the Alambay formation. This is thought to be a feature by which the rocks of the Angurep complex differ from most of the subductional metamorphites of the ASFA, characterized by the presence of metabasites with a chemical composition corresponding to intraplate basalts [Volkova, Sklyarov, 2007]. If further studies confirm the island-arc nature of the protolith of amphibolites of the Angurep complex, it will then be possible to talk about tectonic erosion of basalt basement of the Early Cambrian island arc [Safonova, Perfilova, 2022], changed to the episode of exhumation of the island-arc metabasite slab. The ideas of the existence of tectonic erosion in the Early Cambrian Salair subduction zone allow resolving one of the geological contradictions in the region: most of the Lower Cambrian Salair volcanic sections are composed of acid and intermediate rocks (Pecherkino formation and its analogs), not typical of the oceanic island arcs. During tectonic erosion, the basalt basement of the oceanic island arc could be largely subducted. The fact that the AOZ is the northern continuation of the Katun accretionary complex of the Altai Mountains [Dobretsov et al., 2004; Shokalskii et al., 2000] suggests the subducted terrigenous and serpentinite mélanges were both formed and exhumed during the transformation of the subduction zone as a result of collision between the oceanic island and island arc and the subsequent reformation of the island-arc system. The Lower Cambrian marmorized limestone blocks in mélange can be the fragments of carbonate sedimentary cover of the oceanic island. The Vendian–Early Cambrian and Late Cambrian–Ordovician Salair island-arc volcanic stages, separated by the late Lower and Middle Cambrian amagmatic periods of carbonate and terrigenous deposition (Suenga and Kinterep formations) and a large structural unconformity [State Geological Map…, 2015; Shokalskii et al., 2000; Vetrova et al., 2022], are in good agreement with this model. The Vendian–Lower Cambrian Salair island-arc volcanites could be the protolith of garnet amphibolites of the Angurep complex.

The detailed geochronological studies of subduction metamoirphites of the Angurep complex are the challenges of the future; the present-day peak metamorphism can be estimated approximately at ~520 Ma, or at somewhat earlier date, considering that the closure of amphibole isotopic system occurred after some rock cooling from metamorphic peak conditions. By 500 Ma, the rocks were brought to the upper crustal depths. The 520–500 Ma interval corresponds to the time of collision of the island-arc system with oceanic islands and to the Salair Pacific-type orogeny.

9. CONCLUSIONS

As a result of the research, the authors made the following conclusions.

1. In the Shalap mélange, there are basalt blocks of the Alambay formation with petrogeochemical features corresponding to oceanic island basalts (OIB).
2. Metamorphic rocks of the Angurep complex, represented by garnet-bearing and non-garnet amphibolites, compose a tectonic slab incorporated into the imbricated accretionary complex. Metamorphic rocks also comprise blocks in polymictic facies of the Shalap mélange. Amphibolites of the Angurep complex are similar in their petrogeochemical features to intracontinental island-arc tholeitic basalts and identical to gabbroids from basite-hyperbasite complexes of the Alambay ophiolite zone.
3. The Shalap mélange is a fragment of the Cambrian Salair paleosubduction zone. The mélange complex formation is related to the Early Cambrian island-arc collision with oceanic islands. The processes of subduction and exhumation in this paleosubduction zone stopped around 500 Ma.

10. ACKNOWLEDGEMENTS

The authors express their gratitude to I.S. Novikov, Dr. Sc. (Geol.-Min.), and N.S. Tychkov, Cand. Sc. (Geol.-Min.), for assistance in conducting fieldwork, to S.A. Kargapolov, Cand. Sc. (Geol.-Min.), and V.N. Tokarev, Cand. Sc. (Geol.-Min.), for the consult on Salair geology, to E.S. Zhimuleva, Cand. Sc. (Tech.), for help in preparing manuscript, and to V.A. Belyaev, Cand. Sc. (Geol.-Min.), and the anonymous reviewer for the revision notes.

11. CONTRIBUTION OF THE AUTHORS

All authors made an equivalent contribution to this article, read and approved the final manuscript.

12. DISCLOSURE

The authors declare that they have no conflicts of interest relevant to this manuscript.

APPENDIX 1