1 Introduction

The Neoproterozoic era (900–544 Ma) was a critical time in the evolution of the earth, therefore oceans and atmosphere were oxygenated during the great oxidation event like to the present day (Anbar and Knoll 2002; Hurtgen et al. 2005; Alene et al. 2006). This time was accompanied by unparalleled climatic events reflected in the Snowball Earth and orogenic collision (Stern 1994; Hoffman and Schrag 2002). The East African Orogen (EAO) was formed during the Neoproterozoic era due to the closure of the Mozambique Ocean (Stern 1994). The EAO comprises two major segments i.e., the Arabian-Nubian Shield (ANS) in the northern and Mozambique Belt (MB) in the southern of EAO (Stern 1994). The ANS stretches from southern Israel, Jordan, Yemen and to northern Ethiopia (Berhe 1990; Kroner et al. 1991; Stern 1994; Teklay et al. 2001; Kroner and Stern 2004); whereas the Mozambique Belt extends south from the Arabian-Nubian Shield into southern Ethiopia, Kenya, Somalia, Tanzania, Malawi and Mozambique (Pohl et al. 1980; Collins 2006; Stoeser and Frost 2006). In Ethiopia, the Neoproterozoic basement rocks are exposed in northern, western, southwestern, southern, and eastern parts (Asrat et al. 2001; Stern 2002; Fig. 1a, b). The Neoproterozoic basement rocks of northern Ethiopia are the southern part of the Arabian-Nubian Shield (Kazmin et al. 1978; Vail 1985). They developed by plate tectonic processes involving repeated arc accretion and terrane amalgamation during the Pan-African Orogeny (Kroner et al. 1991; Shackleton 1994; Stern 1994).

Fig. 1
figure 1

a Geological map of Mai Kenetal-Negash area in Central Tigrai modified after Tadesse (1997) with the age of Granite from Miller et al. (2003) for the Negash (606 Ma) and Hawuzien (613 Ma) and Beyth (1971) for the Mai Kenetal granites (650 Ma). b Geological map of Ethiopia modified after Stern (2002)

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The Tsaliet and Tambien groups are the major rock sequence of the Northern Ethiopian basement (Beyth 1971). The Tsaliet group is the oldest and contains metavolcanic/metavolcanoclastic rocks, agglomerates, and tuff and mafic to felsic volcanic flows, while the Tambien group comprises metasedimentary rocks of clastic and carbonate succession, mainly exposed in a series of synclinal inliers overlaying the Tsaliet group (Alene et al. 2006). The Tsaliet group rocks are considered to have evolved by orogenic magmatism associated with island arc accretion (Alemu 1998; Alene and Sacchi 2000). The study area is located along the boundary between the Tsaliet and Tambien groups. Previously, according to Garland (1980) and Tadesse (1997), different studies involving geological mapping and lithological descriptions at small-scale levels were conducted while detailed mapping, geochemical characterization, petrogenesis, and the tectonic setting remained poorly understood.

To address these research gaps, petrological and geochemical data was collected to (1) describe the systematic mineralogical and geochemical variations of the whole suite, (2) characterize the petrogenesis of the main rock types, and (3) identify the regional tectonic setting. The establishment of the petrographic, major, and trace element data and their integration with field observations are the most essential to improve our knowledge and share the findings about the geological evolution of the region.

2 Regional geology

The Arabian Nubian Shield (ANS) was formed during the amalgamation of the Gondwana supercontinent at the end of the Neoproterozoic era (Vail 1985; Stoeser and Camp 1985; Johnson et al. 2014). The geochemical and isotopic signature indicates that Arabian-Nubian Shield (ANS) is dominated by mantle-derived juvenile Neoproterozoic crust and involved in intra-oceanic arc volcanism (Stern 1994, 2002; Stein and Goldstein 1996; Stoeser and Frost 2006). The Neoproterozoic sequence of northern Ethiopian (Tigrai) is the southern end of ANS and comprises low-grade metavolcanoclastic, volcano-sedimentary, mafic, and ultramafic rocks of ophiolitic character (Kazmin et al. 1978; Alemu 1998; Tadesse et al. 2000; Asrat et al. 2001; Alene et al. 2006). Extensively folded, steeply dipping beds and low-grade metamorphic rocks, which were intruded by various granitoids and mafic bodies characterize the sequences, including the Tsaliet and Tambien groups (Tadesse 1997; Alemu 1998; Tadesse et al. 2000; Asrat et al. 2004). The Tambien group is subdivided into, from oldest to youngest, Werri slate, Assem limestone, Tsedia slate, Mai Kenetal limestone, Amota slate, Didikama dolomite, and Matheos limestone (Beyth 1971; Hailu 1975; Garland 1980; Alene et al. 2006). Recently, new units have been identified as Mariam Bohkakho slate and Negash diamictite which are the upper sequences of the Tambien group (Swanson-Hysell et al. 2015).

The basement sequence of northern Ethiopia is also classified into six tectono-stratigraphic blocks (Fig. 1a). It includes from east to west: Mai Kenetal, Adwa, Chila, Adi-Nebrid, Adi-Hageray and Sheraro blocks (Fig. 1a; Tadesse 1997; Tadesse et al. 1999). These blocks are structurally bounded and have unique internal stratigraphy. The Mai Kenetal block is occupied by variegated slates (Bliato, Logmitti, and Segali slate) and weakly metamorphosed limestone(Tselim Imni and Filafil limestone) units whereas, the Adwa block is dominated by low-grade metavolcanic (intermediate to acidic) and metasediment units. Chila block has a thick succession (> 3000 m) of predominantly fine-grained metasedimentary rocks (phyllite, pelitic schist, and recrystallized chert) whereas, the Adi-Nebrid block comprises a thick, broadly southeast dipping but the locally upright folded sequence of basic to intermediate metavolcanic, pyroclastic rocks and associated with immature volcanoclastic metasediments. Clastic metasediments make up the Sheraro block whereas thick (> 5000 m) greenschist facies metasedimentary and metavolcanic rocks make up the Adi-Hageray blocks. This block is primarily southeast dipping and consists of heavily deformed rock. The boundaries between these blocks are defined by one or a combination of the shear zones, unconformities, and metamorphic (tectonic) discontinuities (Tadesse 1997; Fig. 1a, b).

The present study area is located at the boundary between the Mai Kenetal and Adwa blocks (Fig. 1a). The Adwa block comprises low-grade metavolcanic/clastic and metasediment units intruded by composite granitoids of batholitic size. It is dominated by a series of NE-SW synclinal basins filled by a weakly metamorphosed and deformed Tambein group of shale and limestone units (Tadesse et al. 1999). Moreover, the Mai Kenetal block is composed of Tselim Imni limestone, Bliato limestone and slate, Logomiti slate, Filafil limestone, and Segali slate units from the late to earlier stage of formations that are parts of the Tambien group (Tadesse 1997).

3 Methodology

3.1 Fieldwork and sampling

The field study was conducted for geological mapping, and collecting samples for petrographic and geochemical analysis. Chip samples were collected from each rock type considering their mineralogical, lithological, and color variations using a geological hammer. The collected samples of ~ 1 kg weight each were coded and placed into plastic bags for laboratory investigation.

3.2 Analytical methods

Sixteen thin-section (five from acidic, six from intermediate, and five from basic metavolcanic) samples were prepared in the central laboratory of the Geological Survey of Ethiopia (GSE), Addis Ababa. These samples were studied in the petrographic laboratory of School of Earth Science, Addis Ababa University using a petrological microscope. A total of fifteen (five from each acidic, intermediate, and basic metavolcanic) samples were selected considering macromineralogical and color variations for whole-rock geochemical analysis. These analytical results were used to clear the doubt about the field naming of rocks and identify mineral assemblages. They were analyzed at the Australia Laboratory Service (ALS) in Ireland, using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) for major oxides and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for trace elements. The samples were fused using lithium metaborate (LiBO2). A 0.20 gm sample was mixed with lithium metaborate flux and fused at 1000 °C. The melt was cooled and dissolved in a strong acid of 100 mL of 4% HNO3 and 2% HCl solution. The detection limits for the analytical methods range between 0.01% and 100% for major elements and 0.01–10,000 ppm for trace elements.

4 Results

4.1 Occurrences and petrography

The Neoproterozoic sequence of the Tahtai Logomiti area comprises a series of overlaying metavolcanic and meta-sedimentary units including basic metavolcanic, intermediate metavolcanic, acidic metavolcanic, meta-agglomerate, metatuff and metagraywacke, lower slate, lower metalimestone, upper slate and upper metalimestone (Fig. 2a, c).

Fig. 2
figure 2

a Geological map of Tahtai Logomiti area, b geological cross-section of the study area representing the nature of the geology which is prepared using global mapper and arc GIS software and enhanced using CorelDRAWX4 and c vertical sectional view of the study area with their regional group name (after Alene 1998)

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4.1.1 Metavolcanic rocks

These are exposed in the northwestern part of the study area covering a large areal extent (Fig. 2a). The base of the metavolcanic rocks is not recognized because it is penetrative towards a depth (Fig. 2b) but it has a conformable lithologic contact with the overlying unit. This suite consists of variably interbedded lithologies of basic metavolcanic, basic metavolcanoclastic, intermediate metavolcanic, and acidic metavolcanic rocks.

Basic metavolcanic rock This unit forms smooth and gentle undulating ridges covered by blocks and debris of variable dimensions. It is characterized by dark green, brown, dark to black, fine to medium grained and contains visible phenocrysts of clinopyroxene (Fig. 3a). Going along the strike from NE to SW, it changes the foliation from less to moderately foliated. Randomly oriented quartz veins and exfoliation weathering are also common. The mineralogical proportion of the rock units was determined using a petrographic microscope by point counting technique. Accordingly, this rock unit consists of augite (clinopyroxene) (~ 29%), olivine (~ 26%) and plagioclase (~ 21%) relict phenocrysts set in groundmass of quartz (5%), epidote (~ 5%), serpentine (~ 4%), chlorite(~ 3%), calcite (~ 2%), sericite (~ 2%), muscovite (2%)and opaque (~ 1%) minerals which represent lower stage metamorphosed porphyritic clinopyroxene-olivine rich basalt (Fig. 3e, i). Nevertheless, the proportion of quartz and muscovite minerals is low (Fig. 3e, i). The development of secondary minerals such as epidote, sericite, serpentine, calcite, and chlorite is evidence of the stated metamorphism. Well development of foliation is defined by the alignment of secondary minerals such as chlorite, sericite, albite, and muscovite (Fig. 3i).

Fig. 3
figure 3

Field photos of the lithological units: a basic metavolcanic (TR2-19). b Basic metavolcanoclast (TR7). c Intermediate metavolcanic (TR4-27). d Acidic metavolcanic (TR5-36) concerning their microphotograph (e-l) (Chl chlorite, Ep epidote, Pl plagioclase, Px pyroxene, Ol olivine, Cal calcite, Act actinolite, Qtz quartz, Op opaque, Ser sericite, Ms muscovite)

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Basic metavolcanoclastic rock This unit forms gentle ridges covered by variable dimensions of debris. It is characterized by dark gray to green, light gray to dark, and epidotaization type of alteration. It has fine to medium grain size and massive to moderately foliated texture. It is composed of angular to sub-angular clasts of fine to medium-grained with variable composition and nearly random aligned quartz veins of different generations (Fig. 3b). Under the microscopic, plagioclase (albite) (~ 38%) and relict pyroxene (~ 33%) are the relict phenocryst minerals with accessory quartz (~ 9%) and opaque (~ 3%) minerals (Fig. 3f). While, epidote (~ 5%), chlorite (~ 4%), actinolite (~ 3%), sericite (~ 3%), and calcite (~ 2%) are the secondary minerals (Fig. 3f). In addition to that, the minerals show poikiloblastic texture due to the presences of fine-grained inclusion (quartz, sericite and epidote) engulfed on relict phenocrysts of plagioclase and pyroxene (Fig. 3f, j).

Intermediate metavolcanic rock It forms parallel ridges, has a massive appearance, porphyritic nature (relicts of pyroxenes), and containing elongated vesicles. The rocks are dark, dark green, light gray to brown, and gray red and contain angular to sub-angular variable clasts of intermediate composition (Fig. 3c). This unit is dominated by quartz veins, dike, mesoscopic scale folds, and several sets of joints. In thin-section, relict phenocrysts of plagioclase (albite) (~ 28%) and pyroxene (~ 26%) are set in a groundmass of quartz (~ 22%), epidote (~ 7%), chlorite (~ 6%), actinolite (~ 4%), muscovite (~ 3%), sericite (~ 2%) and K-feldspar (~ 2%) (Fig. 3g). It also shows well developed S1 foliation resulted due to the alignment of chlorite, sericite, actinolite, muscovite and albite; however the relict phenocrysts are less foliated (Fig. 3g, k).

Acidic metavolcanic rock This rock has small areal coverage relative to the others and forms gentle to steep slope topographic features. It is hard, compacted, conchoidal fractured, multiple jointed sets that show the cross-cutting relationship and thinly to thickly flow banded. It is also characterized by light grey to grey, light pinkish color, and fine-grained (Fig. 3d). In thin section, the rock contains quartz (~ 61%) and plagioclase (~ 24%) relict phenocrysts with fine-grained groundmass of sericite (~ 5%), muscovite (~ 3%), K-feldspar (~ 3%), chlorite (~ 2%) and opaque (~ 2%). It is foliated due to the alignment of sericite, muscovite, and chlorite, and exhibits blastoporphyritic texture due to the relict phenocrysts of quartz and plagioclase (Fig. 3h, l). Both relict phenocrysts show the effect of deformation in the form of wavy extinction and mortaring. The area also contains small exposures of meta-agglomerate, metatuff, and meta-greywacke rocks. They are found interspersed within the predominantly basic and intermediate metavolcanic (Fig. 2a).

Meta agglomerate is mostly found within intermediate metavolcanoclast, which is characterized by fine grain size, light greenish to whitish and light grey to a greenish color, angular to subangular, and subrounded clasts (1 mm–10 cm), massive to moderately foliated unit. It has an intermediate groundmass and shows light greenish and dark to light grey color due to the alteration effect and contains primary volcanic textures (blasto porphyritic) due to the presence of relict pyroxene minerals. In the northeastern part of the study, metatuff shows a primary layering with clasts size < 2 mm and light grey to grey and light greenish color. The degree of compactness of this unit varies from welded to unwelded tuff of moderately foliated. The meta-greywacke is also composed of quartz and feldspars fragments of sub-angular to sub-rounded in shape.

4.1.2 Metasedimentary rocks

Different types of low-grade metasedimentary rocks are exposed in the southeastern part of the study area (Fig. 2). This group of rocks has lithological contact with the underlying metavolcanic rocks and comprises lower slate, lower metalimestone, upper slate, and upper metalimestone. The lower slate occurs overlying the basic metavolcanic and metatuff units. The unit forms a subdued topographic feature. They are NE- SW strike, parallel and dissected hills with undulating small ridges. Lower slate is soft, fine-grained, strongly foliated, steeply dipping, strongly deformed rocks and show variegated color(light gray to dark, light yellow, and reddish) (Fig. 4a). They contain preserved bedding, thin laminations, kink bands, minor folds, pencil structure, slaty cleavage and closely spaced joints (Fig. 4a). This rock unit consists of calcite, sericite, quartz, feldspar, chlorite and opaque minerals. The minerals are fine-grained and contain large, sub-rounded quartz and feldspar.

Fig. 4
figure 4

Outcrop photo of metasedimentary rocks representing a linear/pencil structure development within foliated and jointed lower slate unit. b Bedding, stromatolitic lamination in the lower metalimstone unit. c The upper slate shows thin veins, foliation, and pencil/linear structure. d Metalimestone unit dominated by dextral movement of en echelon calcite veins

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The lower metalimestone overlays the lower slate and underlying the upper slate. It forms rugged and ridge topography with elephant skin appearances weathered surface. It is characterized by gray to dark color, abundant karstification, stromatolitic laminations, and ribbonite facies, it also shows cross bedding and stylolitic structures as well as dominant echelon calcite veins (Fig. 4b). The rock is composed of calcite, quartz, feldspar and opaque. The upper slate is found overlying the lower metalimestone and intercalated with the upper metalimestone, forming a subdued topographic feature, exposed near the core of Mai Kenetal syncline and has well-developed pencil structures (Fig. 4c). It has a light gray, dark gray and light yellowish color and thinly bedded. It is composed of quartz, opaque, plagioclase, chlorite, and sericite minerals and shows lamination. The upper metalimestone is extensively exposed at the core of the Mai Kenetal syncline. It forms ridges and subdued topographic features. It contains molar-tooth and stylolite structures, en echelon calcite veins, and also intraclast breccia (Fig. 4d).

4.2 Whole rock geochemistry

4.2.1 Major elements characteristics

The whole rock major and trace element compositions of metavolcanic rocks help in understanding the compositional diversity, sources, and petrogenetic processes involved and can be linked to the plausible tectonic setting. The major element concentrations of metavolcanic rocks such as SiO2, Fe2O3, CaO, and MgO, show wide compositional variations (Table 1).

Table 1 The major elements (wt%), trace and rare earth elements (ppm) of the metavolcanic rock samples of Tahtai Logomiti area (nd = not detected)
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Furthermore, the Mg# ranges from 54.08 to 64.90 wt% with an average of 60.50 wt%, 34.54 wt%–50.36 wt% with an average of 40.44 wt% and 27.32–38.81 wt% with an average of 32.82 wt% for basic, intermediate and acidic metavolcanic rocks respectively (Table 1). Based on the Harker variation diagram, MgO, Fe2O3 and TiO2 show a characteristic decreasing trend. While CaO and K2O show little scattered patterns that are probably related to alteration during metamorphism (Fig. 5). According to Nb/Y vs. Zr/Ti diagram for volcanic rocks (Pearce 1996), the sampled rocks from the study area can be classified as basaltic, basaltic andesite, andesite and dacite (Fig. 6a). Furthermore the ternary plot of (Y + Zr)-TiO2×100-Cr indicates that all the samples have a calc-alkaline affinity (Fig. 6b).

Fig. 5
figure 5

Harker variation diagrams showing the variety of major elements (wt%) concerning SiO2 for the metavolcanic rocks of the Tahtai Logomiti area

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Fig. 6
figure 6

Chemical classification and nomenclature of the Tahtai Logomiti metavolcanic rocks using a Nb/Y vs. Zr/Ti diagram (Pearce 1996) and b [(Y + Zr)-TiO2 × 100-Cr] ternary diagram (after Davies et al. 1979)

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5 Trace elements characteristics

In basic to acidic metavolcanic rocks, the average concentrations of Sr, Th, and Zr increased while the average concentrations of V, Ba, and Ni decreased (Table 1). Based on the Harker variation diagram Zr, Nb and Sr represent an increasing trend whereas Ni, Sc, and Rb represent a decreasing trend (Fig. 7). From the Harker variation diagram of total REE and Mg#, the samples also show a moderately dispersed pattern (Fig. 8). The (La/Yb)N ratio values of metavolcanic rocks is relatively high which ranges from 1.93 to 5.94 with an average 3.44. According to Alene and Sacchi 2000, this value indicates moderate LREE enrichment. Furthermore, the test result shows that the Eu/Eu* value ranges from 0.86 to 1.34 with an average of 0.99 Th/Ta > 3.8, La/Ta > 38, and low ratios of Th/La < 1, Nb/La < 1, and high Pb contents. In the chondrite-normalized REE patterns (Fig. 9a), all the rock samples display slight enrichment in light rare earth elements (LREE) relative to the heavy rare earth elements (HREE) with slight positive and negative Europium anomalies (Eu/Eu*). Except for one sample of intermediate metavolcanic (TR-27) showing some crosscutting relation with the other, all the samples display parallel REE pattern (Fig. 9a). In the multi-element plot (Fig. 9b), the trace elements data show enrichment in Ba, U, Pb, K, and Sr, and depletion in Rb, Th, Ta, Nb, Pr, and Ti.

Fig. 7
figure 7

Harker variation diagrams of the selected trace elements (ppm) against SiO2 (wt%) for the metavolcanic rocks of the Tahtai Logomiti area

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Fig. 8
figure 8

Harker variation diagram of Mg# and TotalREE that represent the degree of magma fractionation

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Fig. 9
figure 9

a Chondrite normalized REE patterns (Chondrite value from Sun and McDonough 1989), b multi elemental variation diagram (Normalized value from McDonough and Sun 1995) of metavolcanic rocks

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6 Discussion

All the metavolcanic rocks of the study area show variable degrees of metamorphism and deformation with relict and recrystallization of minerals. The recrystallized quartz shows undulose extinction and grain boundary migration under the microscopic observation of the metavolcanic and metavolcanoclast rocks. The relict minerals are pyroxene (augite), olivine, plagioclase, and quartz which have blastoporphyritic texture (Fig. 3e–g). Pyroxene and olivine minerals are altered into chlorite, actinolite, epidote, and serpentine. While, calcite and sericite minerals are the alteration product of Ca-rich pyroxene and plagioclase respectively (Fig. 3e–g). Recrystallization of quartz and the formation of secondary minerals cause the mobilization of major and trace elements in the rocks (Fig. 3e–g). As a result, the rocks have been subjected to low-grade metamorphism which is parts of greenschist facies and shows prograde metamorphism (Fig. 3e–g; Winkler 1979; Asrat et al. 2004; Alene et al. 2006).In addition to that, the mineral assemblages of chlorite-calcite-muscovite-sericite-quartz-feldspar and preserved primary structures [e.g., bedding (S0), cross bedding, slump structure, stromatolitic lamination, stylolitic structure] of the metasedimentary rocks are also the diagnostic features of the low-grade greenschist facies metamorphism which is in line with Tadesse (1997) and Tadesse et al. (1999) works of the region.

6.1 Petrogenesis

The (Y + Zr)-TiO2×100-Cr ternary plot, represents the tholeiitic and calc-alkaline boundaries in which the composition of metavolcanic rocks fall within the calc-alkaline field (Davies et al. 1979; Fig. 6b). The high concentration of Al2O3 in basic, intermediate and acidic metavolcanic rocks (Table 1) is similar to those of calc-alkaline series magma (Irvine and Baragar 1971). The type of magma can also evidenced by the trace element ratio: an average ratio of Nb/Y, La/Sc, and La/Y is < 1.0 in all the rock units, while La/Th is 8.79, 8.38, and 5.61 in basic, intermediate and acidic metavolcanic rocks respectively. These ratios imply that the metavolcanic rocks have calc-alkaline affinities (Garcia 1978; Lissan and Bakheit 2010).

The selected major and trace elements are plotted against SiO2 using Harker variation diagrams (Figs. 5, 7). The CaO and K2O show a nearly scattered pattern with silica because of their mobile behavior during hydrothermal activity and metamorphism (Fig. 5; Alene and Sacchi 2000). However, MgO, Fe2O3, and TiO2 are negatively correlated with SiO2 reflecting high differentiation trends of magma (Fig. 5). This correlation suggests that olivine, pyroxene, and Fe-Ti oxides were fractionating phases during the evolution of magma (Alene and Sacchi 2000; Sifeta et al. 2005). In addition to that, the harker variation diagram of total REE and Mg#, shows a moderately dispersed patternwhich confirms, the relatively high magma fractionation (Fig. 8; Sifeta et al. 2005). Furthermore, the positive trend of the incompatible elements (Y, Ce, and La vs. Zr), indicates the magma differentiation from the primitive towards the evolved ones (Okay Cemal and Kaan 2016). The effect of fractional crystallization is best observed in the REE pattern and multi elemental variation diagram that displays significant depletions in Ti, which suggest fractionation of Fe-Ti-oxides (Fig. 9a, b). Therefore, it must be noted that the rocks have evolved due to the element enrichment (Fig. 9a, b).

The enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to heavy rare earth elements (HREE), strongly differentiated pattern (moderately steep), strong LREE to HREE enrichment in fractionated ratio and negative Nb-Ti anomalies are also confirming evidence of magma differentiation (Fig. 9a, b); Rollinson 1993; Lissan and Bakheit 2010). The parallel REE patterns of the samples suggest that they have a common source (Fig. 9a; Lissan and Bakheit 2010). The average total REE is 66.34 ppm and a rather high (La/Yb)N ratio (an average of 3.44 ppm) indicates depleted magma sources (Alene and Sachi 2000). The slight positive and negative anomalies in Eu in the samples also suggest an accumulation of plagioclase in the magma during fractionation and the presence of garnet in the residue (Fig. 9a, b; Yibas and Anhaeusser 2003; Matiaba-Bazika et al. 2024). In addition to that, calc-alkaline rocks typically show moderate degrees of light rare earth element(LREE) enrichment flat heavy rare earth element (HREE) pattern and low Nb/Y ratio, indicating shallow depth mantle sources (Murphy 2007).

The average concentration of Fe2O3 (9.29 wt%, 8.4 wt%, and 3.87 wt%) is relatively higher in all the samples indicating the abundance of iron-bearing minerals in the magma: pyroxene, olivine, and iron oxides (Deer et al. 1996; Table 1). There is also evidence in the petrographic data by the occurrences of pyroxene and olivine minerals that are dominated in the basic and intermediate metavolcanic rocks (Fig. 3e–g). Furthermore, Ca and Na bearing secondary minerals (albite, actinolite, sericite, and epidote) are indicated in the mineralogical data(Fig. 3e, f). From the geochemical data, the average concentrations of CaO (8.62 wt%, 6.54 wt%, 5.63 wt%) and Na2O (2.63 wt%, 4.34 wt%, 2.65 wt%) is also relatively high which, is related to the presence of epidote, albitized plagioclase and low degree of metamorphism (greenschist facies) (Jakes and White 1972; Alene and Sacchi 2000; Lissan and Bakheit 2010). The concentration range of SiO2(41.2–50.5 wt%, 48.8–61.2 wt%, 65.9–71.7 wt%) and Al2O3(13.05–17.75 wt%, 14.35–16.85 wt% and 12.2–13.25 wt%) in basic, intermediate and acidic metavolcanic (Table 1) respectively exhibit much less variations and appear to have been much less affected by the metamorphic event. According to Abdel-Rahman and Kumarapeli (1999), the concentration of silica is still representative of the protolith and reflects the nature of basic, intermediate, and acidic rocks.

The incompatible and immobile trace element ratios (e.g., Ti/Yb and Nb/Yb) are important to inspect source heterogeneity or depth of melting of the magma (Pearce 2008). The TiO2/Yb vs. Nb/Yb diagram shows differentiation between shallow and deep melting sources (Fig. 10; Pearce 2008). In this diagram, the studied samples show affinity with a shallow melting array characterized by a low Ti/Yb ratio and plot in between depleted to enriched MORB compositions (Fig. 10). The concentrations of Sr, V, and Ba are higher than the concentration of Nb, Th, Ni, and Zr in all the rock samples (Table 1). This feature suggests that the metavolcanic rocks are formed within active plate margins (Pearce and Gale 1977). The bivariant plots of Zr vs. the incompatible elements Nb, Y, Th, Ce, and La show a positive scatter pattern while Sc shows scattered negative correlations probably suggesting that the rocks have a common magmatic source (Fig. 11; Lissan and Bakheit 2010). The high ratios of (Th/Ta) > 3.8, (La/Ta) > 38, and low ratios of (Th/La) < 1, (Nb/La) < 1, and high Pb content would indicate crustal contamination of the magma (Frey et al. 2002; Wooden et al. 1993; Xia and Li 2019). The Tahtai logomti were mainly acquired through interaction with crustal rocks during the differentiation of the magma.

Fig. 10
figure 10

The TiO2/Yb vs. Nb/Yb plot diagram for the metavolcanic rocks of Tahtai Logomiti area highlighting the shallow depth magma source (after Pearce 2008)

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Fig. 11
figure 11

Zr variation diagram for various trace elements (ppm) of the Tahtai Logomiti metavolcanic rocks

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6.2 Paleotectonic setting

The Tahtai Logomiti metavolcanic rocks represent the southern continuation of the Adwa block rocks of northern Ethiopia (Fig. 1a, b) and the Nakfa volcano-sedimentary terrain in Eritrea (Tadesse et al. 1999). The Nakfa metavolcanic terrain in Eritrea and the southern continuation (Adwa block) metavolcanic rocks of northern Ethiopia have similar characteristics. The chemistry of these metavolcanic rocks is characteristically calc-alkaline and most yields arc-related geochemical signatures (Tadesse 1997; Tadesse et al. 1999) in which the character of Tahtai Logomiti metavolcanic is in line with this regional information. The presence of linear belts of mafic and ultramafic ophiolitic sequences in the Nakfa Terrane including in the Axum area of northern Ethiopia suggested the possibility of a subduction zone (Filho and Drury 1998; Tadesse et al. 1999). In northern Ethiopian, Zager and Daro Tekli mafic–ultramafic rocks are the dominate belts (Tadesse 1997; Tadesse et al. 1999). Zager mafic–ultramafic belt comprises many components of dismembered ophiolitic succession and some arc-related components (Tadesse et al. 1999). The DaroTekli mafic–ultramafic belt is predominantly greenschist facies of metamorphism that comprises some components of oceanic crust fragments such as ophiolitic assemblage supported with geochemical features that strongly suggest a subduction environment.

The trace element abundances of metavolcanic rock samples from the study area form a pattern on a primitive mantle normalized diagram, with large ion lithophile element (LILE) enrichment, depletion in HFS elements, pronounced negative Nb and Ta anomalies (Fig. 9b). This pattern (Fig. 9b) represents slight depletion in Nb relative to adjacent LIL elements (such as K, Ba, Rb, and Sr) is characteristic of subduction component in the mantle source with composition like volcanic rocks of island arcs (Alene and Sacchi 2000; Sifeta et al. 2005; Kremer and Tishin 2017). The slight LILE enrichment can be attributed either to a subduction component to a slight crustal contamination or both (Fig. 9a; Jake and White 1971; Rollinson 1993).

Apart from the matavolcanic rocks, as the field observation indicates, the study area is characterized by foliated and deformed with the variegated color of slate-phyllite and metalimestone overlining the metavolcanices. The trace element compositions of metavolcanic rocks integrated with field and petrographic observations can provide significant insights into the petrogenetic process as well as the tectonic setting (Sifeta et al. 2005). The major element concentrations of SiO2, Fe2O3, CaO, and MgO show wide compositional variations and increasing value from basic, intermediate to acidic metavolcanic rocks (Table 1). The tectonic setting of Tahtai Logomiti metavolcanic rocks was determined using alteration-resistant elements including Nb, Zr, Y, La, Hf, and Ta discrimination diagrams (Winchester and Floyd 1975; Wood 1980). Based on Zr-Nb-Y ternary discrimination diagrams (Fig. 12a) of Meschede (1986), the metavolcanic rocks are plotted in the volcanic arc basalt fields. A clear tectonic setting indication of the study of metavolcanic rocks is given by the Th-Hf-Ta triangular discrimination diagram of Wood (1980), on which different types of MORB, within plate and volcanic arc basalts can be separated (Fig. 12b). This diagram discriminates not only identifies the volcanic arc basalts, but also separates tholeiitic and calc-alkaline magma types. Almost all the metavolcanic rock samples fall inside the volcanic arc basalt field (Fig. 12b). Moreover, all those samples are distributed within the VAB field, below the line defined by Hf/Th = 3, indicating calc-alkaline magma sources (Fig. 12b). The high concentration of Al2O3, an average ratio of Nb/Y, La/Sc and La/Y is < 1.0 in all the rock units, while La/Th is 8.79, 8.38, and 5.61 in basic, intermediate and acidic metavolcanic rocks (Table 1) is similar to those of calc-alkaline series magma (Irvine and Baragar 1971; Garcia 1978; Lissan and Bakheit 2010).

Fig. 12
figure 12

Tectonic discrimination diagrams of the Tahtai Logomiti metavolcanic rocks. a Zr–Nb-Y ternary diagram after Meschede (1986) with the fields (AI: within-plate alkali basalt; AII: within plate tholeiite; B: E-MORB; C and D: volcanic-arc basalt and D: N-MORB), and b Th-Hf-Ta ternary diagram after Wood (1980). Abbreviation for all plots: N-MORB normal mid-ocean ridge basalt, E-MORB enriched mid-ocean ridge basalt, VAB volcanic arcbasalt, WPA within plate alkali basalt, WPT within plate tholeiite

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7 Conclusion

Through the integration of field observation, petrographic interpretation, and geochemical data, the following conclusions are drawn. The study area is characterized by basic to acidic metavolcanic and metasedimentary rocks. Based on the mineral assemblages, the basic metavolcanics contain primary minerals of plagioclase, olivine, and pyroxene relict phenocrysts associated with secondary minerals of epidote, sericite, actinolite, serpentine, calcite, and chlorite. The intermediate metavolcanic rock also has relict phenocrysts of plagioclase and pyroxene minerals set in a groundmass of chlorite, quartz, albite, actinolite, muscovite, sericite, calcite, and epidote. Whereas acidic metavolcanics contain quartz and plagioclase relict phenocrysts with fine-grained muscovite, sericite, K-feldspar, and chlorite. The presence of preserved relict phenocrysts (plagioclase, olivine, and pyroxene) and secondary minerals such as chlorite, actinolite, epidote, serpentine, sericite, and albite in the metavolcanic rocks represent that the area is affected by low-grade greenschist facies metamorphism. It is also evidenced by the preserved primary structures (e.g., cross bedding, bedding, stylolitic structure, stromatolitic lamination, and slump structures) in the metasedimentary rocks. Based on the major and trace elements data of the TiO2/Yb versus Nb/Yb diagram, the protolith of the metavolcanic rocks have shallow-depth melting sources. Furthermore, in the Nb/Y versus Zr/Ti bivariate and(Y + Zr)-TiO2×100-Cr ternary diagrams, the rocks of the study area are classified as basaltic, basaltic andesite to andesite and dacite units of calc-alkaline affinity (Fig. 6a, b). The high-alumina content (Al2O3), and low trace element ratios (Nb/Y, La/Y, and La/Sc) indicate calc-alkaline magma series rocks. Furthermore, the major and trace elements (Harker and bivariant of Zr), and REE patterns reflect differentiation and common magmatic source. Based on the Zr-Nb-Y and Th-Hf-Ta ternary diagram, the Tahtai Logomiti metavolcanic rocks are volcanic arc-related calc-alkaline magma type and also the negative Nb and Ti relative to LILE elements represent subduction-related magmatic source.