INTRODUCTION

This region covers the mountain areas of the largest Denman–Scott glacier system within Queen Mary Land (QML) of East Antarctica. This region involves isolated small islands, flat plateaus, isolated cliffs, and nunataks on the west side of the Melba and Davis peninsulas and southward along the glacier head up to 140 km to the south. The region also involves Obruchev Hills and Bunger Oasis, which are significant in area and are separated by the glacier on the eastern side (Fig. 1).

Fig. 1.
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Structural–tectonic schemes of the East Antarctic and mountainous frame of the Denman Glacier. (a) General scheme of tectonic zoning of the eastern sector of the Antarctic Shield. Abbreviations: RP, Rucker Province; PCM, Prince Charles Mountains; GM, Grove Mountains; PEL, Princess Elizabeth Land; WO, Westfall Oasis; QML, Queen Mary Land; WDG, Western part of the Denman Glacier; BO, Bunger Oasis; OH, Obruchev Hills. (b) The structural–tectonic scheme of the mountainous frame of the Denman Glacier of the QML: 1, the Paleo-Mesoarchean granite–gneiss complex Davis (tonalitic ±Px-orthogneisses, Px–Amf–Bt-crystalline schists, meta-ultrabasites); 2, the Neoarchean–Paleoproterozoic Obruchev complex (the Neoarchean pyroxene-bearing, amphibole–biotite tonalitic gneisses, the Paleoproterozoic Sill–Grt–Bt-gneisses, Bt–Hb-gneisses); 3, the Wilkes Province (the Proterozoic mobile belt), Bunger Oasis; 4, the supposed area of the development of the Cape Charcot Subprovince; 5, undifferentiated areas under the Shackleton Ice Shelf; 6, Ediacaran–Cambrian subalkaline granitoids; 7, Early Cambrian (?) gabbroids; 8, the area of the development of the Denman outlet glacier; 9, supposed geological boundaries; 10, supposed boundaries of provinces and complexes (terranes); 11, supposed boundaries of the rift system of the Denman Glacier; 12, visited mountain objects, 2018 (Chugunov Island is represented by fluvioglacial, deluvial deposits); 13, mountains and islands within the region (1, Jones Rocks, 2, Cape Delay Point; 3, Cape Charcot; 4, Cape Gerlache; 5, Possession Rocks, Cape Harrison; 6, Cape Jones, 7—9, Obruchev Hills: Krainiye Hills, Cape Hoadley; 14, objects with U–Pb dating of rocks according to [3, 7, 12]).

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The western mountain framework of the glacier has remained almost unstudied due to its geographical remoteness and inaccessibility. In 2018, airborne geological work was carried out on Melba and Davis peninsulas (Hippo Island, Bezymyannoye Plateau (authors’ name), Mt. Watson, Chugunov Island).

We have identified three major structural–tectonic areas within this large region (Fig. 1):

(1) The granulite–gneiss Charcot Subprovince (1600–900 Ma?), extending westward to Mirnyi Station, acts as an independent block within the largest Proterozoic Rayner Province, which is the long-lived Circum-Antarctic Mobile Belt, the structure of which was formed from the Paleoproterozoic terranes and Archean blocks with their amalgamation into a single continental massif at the turn of 1050–1000 Ma during the formation of the supercontinent Rodinia [1, 2, 4, 9–11]. The Charcot Subprovince includes the Davis Paleo-Mesoarchaean complex (terrane) (3400–3000 Ma) on the western side of the Denman Glacier, covering the areas of the Melba and Davis peninsulas. This complex was identified by the authors on the basis of new geological and geochronological data.

(2) The Neoarchean granite–gneiss Obruchev Terrane (2690–2641 Ma [1, 4, 6]), located on Obruchev Hills, reaches the southern margin of Bunger Oasis (eastern side of Denman Glacier).

(3) The Wilkes Province is a large accretionary–collisional ensimatic mobile belt of Paleo-Mesoproterozoic age (1700–1150 Ma). It extends eastward from Bunger Oasis to the Windmill Islands and includes the Mesoproterozoic volcanogenic–sedimentary Bunger terrane [1, 11].

To determine the geological stages of formation of the continental crust of the Davis Terrane, as well as to compare the evolution of geodynamic processes with the known Paleo-Mesoarchaean craton blocks of East Antarctica, India, and Australia, the ages of crystallization and tectono-thermal reworking of the previously undated protoliths of metamorphic rocks and intrusive granitoids were estimated.

The granite–gneiss Davis Terrane, identified by the authors, is an Archean protocraton similar to other granite–granulite–gneiss cores of the early consolidation of East Antarctica. This terrane and the areas under the thick ice cover to the southwest and west were previously defined as the “Cape Charcot orthogneisses” or “Charcot craton” owing to extremely limited study based on dating only the tonalitic orthogneiss from the cape of the same name and on the basis of the age of protolith crystallization at 3003 ± 8 Ma with subsequent metamorphism at 2889 ± 9 Ma (U–Pb, SHRIMP, [3]).

Blocks of the Archean Davis protocraton are represented in isolated bedrock outcrops of the Melba and Davis peninsulas. The blocks are composed of banded, migmatite (±Px)–Bt–Amf-gneisses and plagiogneisses to layer-by-layer migmatites with subconformable interlayers and bands of apopyroxenite amphibolites, crystalline schists, and metaultrabasites. The vein material of migmatites usually corresponds to the composition of leucocratic plagiogranites, and the substrate is represented by amphibolites or pyroxene-bearing gneisses. Ultrabasite and basite sills, often boundinaged, lenses (up to a few dozen meters in length) are represented by (±Ol)–Opx-metahornblendites and olivine-bearing metawebsterites and are also established as part of the metamorphic formations on Jones Rocks, Cape Charcot [4, 6, 7]. All rocks are cut through by veins and dikes of Early Cambrian granitoids varying in composition from normal leucogranites–granites to subalkaline granosyenites.

Zircon crystals have been studied in five rock samples of the metamorphic complex and the igneous intrusions that break through it: leucocratic muscovite-bearing biotite orthogneiss (plagiogneiss), olivine-bearing metapyroxenite, and three granitoids sampled from a gneiss-crossing vein and from two large plutons, that is, biotite leucogranite, biotite–amphibole granodiorite, and pyroxene-bearing amphibole granosyenite.

Studies of zircon grains were carried out on a CamScan electron scanning microscope and by U–Pb SIMS (secondary ion mass spectrometry, SIMS SHRIMP-II) at the Karpinskii Russian Geological Research Institute, St. Petersburg. U–Th–Pb–isotope measurements were taken on a high-resolution microprobe SHRIMP-II in the single-collector mode using a secondary electron multiplier [5]. The obtained concordant ages and discordia intersections in the text are given in the two-sigma confidence interval. The results of measurements are summarized in the tables (Table 1).

Table 1. Results of U‒Th‒Pb (SHRIMP-II) geochronological studies of zircon from orthogneisses and granitoids of the Davis Terrane on the western side of the Denman Glacier
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THE ARCHEAN ORTHOGNEISESS OF THE DAVIS TERRANE

Bezymyannoye plateau, Davis Peninsula. The first plateau studied (coordinates 66°31.67′ S, 98°50.65′ E) was named by the authors as Bezymyannoe (“Unnamed”) Plateau. The rocks are represented by migmatized, heterogeneous, banded, leucocratic biotite granitogneisses and plagiogneisses (sample 63868-1a), with subconformable strata- and band-shaped bodies of amphibole–biotite crystalline schists and bimetapyroxene metapyroxenites (sample 63868-2), and by veins of the Paleozoic granitoids (leucogranites, aplites, 63868-3), which intersect metamorphites.

The chemical composition of the studied plagiogneiss corresponds to plutonic plagiogranite (Table 2). Mineral composition is represented by 40–50% oligoclase, 25–30% quartz, 10–15% orthoclase, 2–3% biotite, euhedral grains of muscovite, accessory apatite, and zircon.

Table 2. Silicate composition of the studied orthogneisses and granitoids of the Davis Terrane of the western side of the Denman Glacier (element content per abs.-dry matter in %%)
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Metaultrabasite is represented by olivine–plagioclase–bearing Bt-Hbl-metapyroxenite with a combination of the heterogranoblast structure superimposed on the magmatic allotriomorphic-grained structure. The mineral composition is represented by 50–55% orthopyroxene, 35–40% clinopyroxene, 6–8% hornblende, 3–4% biotite, ~1% plagioclase, olivine grains, accessory zircon, and ore minerals. Fig. 2

Fig. 2.
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Geochronological characteristics of the studied zircons from metamorphic and intrusive rocks of the western framework of the Denman Glacier. Isotope U–Pb diagrams with concordia (a–d).

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Fig. 2.
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(Contd.)

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Sample 63868-1a is represented by leucocratic muscovite-bearing Bt-plagiogneiss. Transparent, weakly colored grains, generally prismatic with significantly rounded pyramidal faces or with an isometric habitus (L > 2 (2.5–5)) predominate among zircons. Cathodoluminescence (CL) revealed a complex structure of grains with the presence of cores and rims of different morphology (Fig. 3a). The cores mostly have a prismatic habitus, with faceting from combinations of rounded dipyramids and that, sometimes, are partially rounded or split. The cores are characterized by distinct oscillatory concentric zoning, indicating the magmatic origin of zircons. There are two types of metamorphic overgrowth (internal dark gray rims, sometimes with a concentric structure, and external light gray rims) resulting from recrystallization of magmatic zircon. In the case of zonal cores, the Th/U ratio is very typical for zircon of magmatic origin and has an average value of ~0.6. For homogeneous, dark cores, as well as for most of the rims studied (excluding the second type) of the represented zircons, the Th/U-ratio does not generally exceed 0.2–0.3, which is typical for metamorphic zircons.

Fig. 3.
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(a–b) Photo of zircon crystals in the cathodoluminescence mode. Views of the characteristic morphological types of zircons of (a) plagiogneiss and (b) metapyroxenite, Bezymyannoye Plateau. Circles with numbers are measurement points (first stage of measurements is shown by the thin line, smaller diameter; the second stage is shown by a bold or yellow line, larger diameter). Fig. 3. (c–d) Photo of zircon crystals in the cathodoluminescence mode. Views of characteristic morphological types of zircons of (c) vein leucogranite, Bezymyannoye Plateau, and (d) granodiorite, Hippo Island. Circles with numbers are measurement points (first stage of measurements is shown by a thin line, smaller diameter; the second stage is shown by a bold or yellow line, larger diameter).

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A total of 42 analyses were performed on different parts of 30 grains, of which 13 values were discordant. The age of plagiogneiss was estimated according to the results of the remaining 14 analyses of zonal cores and 15 analyses from newly formed zircons and rims. Regression lines were plotted on the basis of the 14 and five measurements of the zonal cores. In the first case, the upper intersection corresponds to an age of 3317 ± 21 Ma, when the five values in the upper intersection form a subconcordant cluster with an age of 3347 ± 11 Ma, and is interpreted as the age of protolith crystallization, while the lower intersection, corresponding to an age of 2461 ± 410 Ma, may indicate a loss of radiogenic Pb by zircon grains in the Paleoproterozoic and may be associated with metamorphism known in regions west of the study area [2, 10].

A concordant age of 3355 ± 5.4 Ma was obtained on two cores of grains, similar to the upper intersection, which is interpreted as the time of the maximum crystallization of the magmatic protolith of orthogneisses. Measurements on 15 rims and metamorphic grains gave 207Pb/206Pb ages ranging from 2886 to 3131 Ma. A regression line with the upper intersection at 3094 ± 40 Ma was plotted on the basis of 14 analyses. Similar concordant ages of 3082 ± 24 Ma and 3084.6 ± 5 Ma were obtained for two values and were interpreted as the time of the Mesoarchean tectono-thermal event.

Transparent poorly colored isometric grains or grains with poorly manifested faceting of rounded dipyramids predominate in sample 63868-2 (Bt–Hbl-metapyroxenite), in a selection of 46 zircons; their surface is smooth, without traces of mechanical abrasion, an elongation ratio (L) = 1–4. The CL images reveal several types of growth zoning, that is, broadband concentric, block (mosaic), and sectorial (Fig. 3b). The external CL rims are bright, oscillatory, and of variable thickness; they have been revealed only on some grains. The morphology and the internal structure of most of zircons (excluding a few grains) suggests its metamorphogenic origin.

The 207Pb/206Pb ages were obtained on cores of grains over a broad time interval of 1296 ± 6.6—2984 ± 16 Ma. All values were discordant, and regression lines with upper intersections at 2874 ± 24 and 2827±6 Ma were plotted for 16 and seven analyses, respectively, which are interpreted as the time of syntectonic intrusion of metapyroxenite protoliths (recrystallization of inherited zircon grains, crystallization of the magmatic protolith). Fourteen measurements, made on the external rims and metamorphic grains, formed a concordant group with an age of 552 ± 2.3 Ma correlated with the time of the Pan-African tectonic-thermal event.

THE LATE EDIACARAN–EARLY CAMBRIAN COMPLEX OF GRANITOIDS

Bezymyannoye plateau, Davis Peninsula. We conducted 32 analyses of 21 zircons on sample 63868-3, which is a vein of biotite-bearing leucogranite intersecting metamorphic rocks. Taking into consideration the morphology of crystals, zircons of magmatic genesis predominate in the rock. These are transparent, pale pink, short–medium prismatic zircon grains, (sub-)idiomorphic, without fractures, mostly with sectorial zoning. Ten inherited cores enclosed in broad zonal rims are noted too. The cores are nonzonal and dark in CL or with mosaic zoning (Fig. 3c).

A regression line with the upper intersection at 2978 ± 14 Ma and the lower boundary at 541.1 ± 4 Ma was plotted on the basis of 18 analyses (including inherited cores) (Fig. 1d).

For a group of nine values from the inherited cores, a concordant U–Pb age of 2978.8 ± 7.2 Ma was obtained, similar to the upper intersection value; it is considered as the recrystallization age of protolith zircons. This dating can be compared with the concordant metamorphism ages for orthogneiss (3082–3084 Ma, sample 63868-1a) and possibly corresponds to the final impulse of this tectonic–thermal stage. Of the 23 values of magmatic zircons and rims, 18 were concordant and formed a compact cluster with a leucogranite crystallization age of 548 ± 2.5 Ma, which is similar to the value of a group of analyses of metamorphic zircons from metapyroxenite.

Hippo Island, Mt. Watson. Within the northern end of Melba Peninsula, granodiorite–granosyenite intrusions of Hippo and David islands, together with previously identified outcrops of gabbro–diorites and leuconorites (to the north, Delay Point and Cape Kennedy, together with previously established outcrops of gabbro–diorites and leuconorites [8]), represent exposed blocks of a large complex multiphase intrusive massif of the gabbro–diorite–granodiorite magmatic formation (Fig. 1).

Hippo Island is an extended ridge (~1 km) with a height of 200 m. It is composed of the Amf-Bt-monzodiorite–granosyenite association with narrow (up to 0.5 m) intersecting veins of light pink leucoplagiogranites (the Mesoarchaean gneisses were supposed to have been developed earlier on the island [8]). Mt. Watson is composed of a homogeneous granosyenite–syenite pluton with an area of ~0.7 km2.

Studied zircons of Hippo Island and Mt. Watson are generally represented by (sub-)idiomorphic long-prismatic (L = 2–4.5) crystals with oscillatory, sometimes, broadband zoning. Grains are transparent, poorly colored, often with distinct dipyramidal faces. The morphological and structural characteristics of zircons are typical for grains of magmatic origin (Figs. 2d–2f, 3).

In amphibole–biotite monzodiorite 63867-1 (Hippo Island), all 25 analyses of zircon grains were concordant and formed a compact group with an age of 512.6 ± 1.4 Ma, which is interpreted as the time of magma crystallization. In the sample of biotite granosyenite (Watson, Borovkova, N.V., VNIIOkeangeologiya), 17 analyses were performed. According to 12 concordant values of idiomorphic grains, the age of granitoid magma crystallization (518.8 ± 5.4 Ma) was obtained.

DISCUSSION

The established time of crystallization of the Davis orthogneiss protoliths (~3355 Ma) on the Bezymyannyi Plateau demonstrates a significant increase in the age of formation of the protocraton and also increases the area of the development of the Paleo-Mesoarchean tonalitic gneisses of the Davis Terrane to the east of the Charcot Formation by at least 27 km.

The presented Davis complex is a Paleo-Mesoarchaean protocraton that is close in its formation to other granite–gneiss cores of the early consolidation of East Antarctica. In terms of the geological development, the protocraton in many respects is synchronous and syngenetic with the Archean granite–greenstone Rucker area in the southern part of the Lambert Glacier, with granulite–charnockite–enderbite Napier Block of Enderby Land or with the Vestfold Hills [2, 10].

Interpretation of the obtained data makes it possible to conduct a temporal, structural–formational correlation with the above-mentioned terranes of East Antarctica, as well as to correlate the Archean domains of India and Australia, which are compared in tectonic reconstructions within the framework of the problem of the formation and disintegration of ancient supercontinents. The issue of geodynamic models of the formation of the Proterozoic–Early Cambrian terranes within the paleocontinents Rodinia and Gondwana, considered as blocks of juxtaposition of the India–Antarctica or Australia–Antarctica crust, remains controversial.

It is known that the Rayner orogeny in East Antarctica is divided into two close stages of tectono-thermal activity in the intervals 1300–1150 and 1200–900 Ma [1–3, 11]. This asynchrony of tectono-magmatic processes, revealed by many studies, made it possible later to identify independent provinces to the east and west of the Denman–Scott glacier system within the Circum-Antarctic Mobile Belt, that is, the Wilkes and Rayner provinces, corresponding to the Australia–Antarctica and India–Antarctica blocks, respectively. These provinces differ in the history of the geological development at the Proterozoic–Early Cambrian stage [1, 4, 9, 11].

The presence of Early Cambrian granitoids also differs significantly from the Davis Complex from the terranes on the eastern side of the Denman Glacier (Obruchev Hills, Bunger Oasis), where no evidence of late Pan-African tectono-thermal events has been established in metamorphic and intrusive formations, except for a few dates from dolerite dikes [1, 8]. It can be assumed that the Charcot and Wilkes domains occupied different levels of the Earth’s crust and/or were affected by opposite stress regimes in independent geodynamic conditions in isolation from each other at the Late Proterozoic–Early Cambrian stage of geological development.

As a result of this research, the following was established:

(1) The most ancient rocks of the Paleo-Mesoarchaean Davis Terrane are represented by Paleoarchean plagiogneisses; the age of crystallization of the magmatic protolith of this complex is 3355 ± 5.4 Ma. Metamorphic zircon grains and zircon rims of tonalitic orthogneisses of the Davis Complex, as well as inherited zircon cores from the Early Paleozoic granitic vein (2978.8 ± 7.2 Ma) demonstrate the age interval of an ancient Mesoarchean metamorphic event of 3100–3000 Ma.

(2) The minimum time of magmatic protolith crystallization during syntectonic intrusion of olivine-bearing metapyroxenites is estimated at 2827 ± 6 Ma, which can be compared with the age of granulite metamorphism and deformation of the Cape Charcot orthogneisses (2889 ± 9 Ma, [3]).

(3) The presence of metamorphosed ultramafic–mafic rocks in the composition of the Davis Terrane is typical of areas with the development of divergent processes and suggests possible rifting of the Paleoarchean crust at the Meso–Neoarchean boundary (2900–2800 Ma), as has been suggested for some Archean blocks of East Antarctica [2, 10].

(4) The formation of vein granitoids and granosyenite–granodiorite plutons with an age of zircon crystallization in the range of 550–510 Ma is compared with the time of the manifestation of the Pan-African (Late Ediacaran–Cambrian) tectono-thermal domain.

(5) The Davis Complex is a granite–gneiss area of the Paleoarchean formation (Paleoarchean protocraton) with age markers indicating at least three stages of granulite and amphibolite metamorphism (3100–3000, 2900–2800, and 550–510 Ma).

(6) The Paleoarchean age of the Davis protocraton is very close to the time of formation of the ancient cores of the protocontinental crust of India and Australia. The data obtained allow us to compare the stages of the geological development of the region with the evolution of the Early Archean craton blocks Singhbhoom, Bastar and Pilbara, Yilgarn, including the stages of the formation of Rodinia in the Neoproterozoic (~1 Ga) and Gondwana in the Cambrian (~500 Ma).