Abstract
The Ross Orogenic Belt is in the Antarctica Transantarctic Mountains. North Victoria Land Granite Harbour Intrusive complex (GHI) records the tectonic-magmatism evolution of Ross orogeny. Extensively developed post-collisional granites around this margin of early Paleozoic magmatism can provide insights into the growth of continental crust through accretionary orogenesis. We provide geochemical and geochronological data from syenites from Terra Nova Bay, north Victoria Land in order to constrain its tectonic evolution and setting. The syenite belongs to the potassium-alkaline, calc-alkaline series and is characterized by high concentrations of rare Earth elements and large ion lithophile elements (LILE), and low content in high field strength elements (Nb, Ta, P, Ti). The petrographic and geochemical signatures show a possible island-arc granite affinity. LA-ICP-MS zircon U-Pb dating results suggest that the Inexpressible Island syenite was emplaced at ca. 471.8 ± 1.8 Ma and 477.3 ± 1.7 Ma, respectively. Zircon εHf(t) values range from −7.4 to −9.1; average −8.2 and whole-rock εNd (t) values range from −8.5 to −10.3, indicating that formed by the partial melting of the lithospheric mantle enriched with subduction slab fluids and subcontinental lithosphere. Whereas, the syenite has a strong positive Eu anomaly and a positive Sr anomaly, suggesting that plagioclase cumulate crystallization occurred in the magma source area. Furthermore, through integration with previous studies, we suggest that syenite is a result of the melting zone of an older previously subduction enriched layer of the subcontinental lithospheric mantle (SCLM). To enable syenite emplacement we suggest a tectonic-magmatic model that invokes alternating phases of extension and contraction in the overriding plate. Finally, we report the youngest age of (post-orogenic) magmatism occurred during extension in the overriding plate ca. 478–471 Ma.
Introduction
Accretionary orogenesis is characterized by repeated cycles of subduction–accretion at interoceanic and continental convergent plate margins (; ; ). The composition of accretionary orogenic belt complexes includes trench-arc-basin systems, seamounts, oceanic crust, and other relic geological records. It is also important for plate (especially micro-continental blocks) collision, assembly, and growth () Repeated subduction–accretion cycles of rock units from continental and oceanic magmatic arcs, supra-subduction zone backarcs, and forearcs loaded with continent-derived materials form magmatic arcs. These magmatic igneous rocks record key information about the evolution of the subduction regimes of orogenic belts, such as subduction-dominating slab rollback and backarc opening, with mantle modifications occurring throughout. The origin of arc magmas involves multiple stages (; Rocchi et al., 2015), components, and processes, including, 1) the nature of the mantle source, 2) the activity and type of subduction components, and 3) the degree and depth of partial melting (; Rocchi et al., 2015). This variety of materials and processes makes orogenic igneous complexes a rich source of information. The identification of magmatic arcs in ancient accretionary orogens is thus important for understanding the structure and history of orogen genesis.
The Ross Orogen exposed in Victoria Land, Antarctica, is located at the Southern Ocean termination of the Transantarctic Mountains. It represents the along-strike continuation of the southeastern Australia margin prior to the Cretaceous breakup of Gondwana (; ; Paulsen et al., 2007; ; Vaughan and Pankhurst, 2008; ). The Ross Orogen formed in the early Paleozoic within the framework of the convergence of the Paleo-Pacific oceanic plate and the Gondwana continental margin (). The Late Proterozoic to Early Paleozoic was characterized by widespread igneous activity in response to convergence along the Cambro-Ordovician margin of Gondwana (). In northern Victoria Land, Antarctica, large amounts of felsic to mafic plutonic rocks, known as the Granite Harbour Intrusive complex (GHI) (), were emplaced during the Cambro-Ordovician Ross Orogeny. In the coastal area of Terra Nova Bay, where Inexpressible Island is located, it is referred to as the Terra Nova Intrusive Complex (TeNIC) (), shows variable geochemical characteristics, suggesting the involvement of various distinct source rocks during the melting process (; Rocchi et al., 2015).
In the northern Victoria Land area, most of magma intrusive activity spanned ca. 545–485 Ma (; Tonarini and Rocchi, 1994; Rocchi et al., 1998; ; Tiepolo and Tribuzio, 2008; ). Similar series of intrusion rocks in Australia which have the oldest age up to 514 Ma (Weis et al., 2006). In the central Transantarctic Mountains, calc-alkaline magmatism may have initiated as early as 590 Ma, and persisted for over 100 Ma (). As most of Antarctica is covered by ice and snow, it is difficult to know the age of the oldest magmatism. The age and geochemical patterns of intrusive rocks suggest oblique convergence along a tectonically segmented margin (; Rocchi et al., 1998; ; Stump et al., 2003; ; ). The most recent magmatic processes formed the Vegetation Unit (∼475 Ma), Abbott Unit (508 Ma), Irizar granite complex (∼490 Ma) which have different source regions and intrusion depths but possess nearly identical rock compositions including granite, syenite, and lamprophyre (; ; Perugini et al., 2005; ). Research suggests that the Abbott Unit and Irizar complex was derived from partially melted mantle wedge above the subduction zone mixing with the continental lithospheric mantle, whereas the Vegetation Unit Lamprophyre was derived from partially melted ancient sub-continental lithospheric mantle mixing with crustal material under thinning lithosphere within the orogen ().
In this study, we conducted Sr-Nd isotope on two samples (N=2), major and trace element analysis of the whole rock (N=7), and zircon U-Pb dating as well as Hf isotopes ratios on two samples (N=2, n=40), from the early Ordovician syenite in the Inexpressible Island, northern Victoria Land, to further determine the nature, geochemical characteristics and petrogenesis of magmatism in the extensional environment of the late Ross orogeny.
Geological setting and sample characteristics
Convergence between the Antarctic part of Gondwana and the Paleo-Pacific oceanic lithosphere during the early Paleozoic () formed the Ross orogenic belt, which deformed and metamorphosed sedimentary rocks and granitic (sensu lato) plutons exposed along the Transantarctic Mountains (; Stump, 1995) (Figure 1). The Proterozoic history and plate tectonic reconstruction of cratonic Antarctica as part of the Rodinia supercontinent are still a matter of debate (; ; ; Pisarevsky et al., 2003). Northern Victoria Land lies at the Pacific termination of the Transantarctic Mountains. It is comprised of three different crustal blocks () (Figure 1): 1) the Robertson Bay Terrane, with a Cambrian to early Ordovician thick flysch-type sequence (; Stump, 1995; Rocchi et al., 1998; ); 2) the Bowers Terrane, a Cambrian complex of volcanic rocks and related sediments (Weaver et al., 1984; ); and 3) the Wilson Terrane (Stump et al., 1983; Rossetti et al., 2006b; ), a metasedimentary sequence including remnants of a polymetamorphic granulite complex (; Talarico and Castelli, 1995; Talarico et al., 1995) that experienced low-pressure low-to high-grade metamorphism during the Cambrian-Ordovician Ross Orogeny (; Palmeri et al., 1994; Palmeri, 1997). An extensive association of mantle-derived calc-alkaline magmas and crustal melts that intruded the Wilson Terrane during the Ross Orogeny are collectively named the GHI (; ; ; ; Vetter and Tessensohn, 1987; ; ; Stump, 1995; ; Rocchi et al., 2004). Recent work (; ; Rocchi et al., 2015, 2011) lead some models of tectonic evolution of the Ross Orogeny in the North Victoria Land (Rocchi et al., 2009; Rocchi et al., 2015). Studies show that the convergent margin of Gondwana land consists of a main continuous subduction zone coupled with local plate and transient subduction zones. These transient subduction zones are related to the continuous ribbons of outboard pieces of stretched forearc regions (Rocchi et al., 2011). The Ross Orogeny in Victoria Land was the result of many stages of advancing and retreating subduction zone(s) (Rocchi et al., 2015, 2011). Thus, as an important part of the boundary between the Wilson arc and the forearc ribbon underwent subduction accretion and detachment, there is an abundance of magmatism in the North Victoria Land.
FIGURE 1
Syenite in the Inexpressible Island, Northern Victoria Land, is a confused and different part of the TeNIC unit, which is important to determining the nature, geochemical characteristics, and petrogenesis of magmatism in the extensional environment of the late Ross orogeny. We analyzed seven syenite samples (DJS-1, DJS-5a, DJS-6, DJS-7, DJS-13, DJS-14, and DJS-15) (see summary in Table 1) from the Inexpressible Island Dingjunshan area (Figure 2). A∼ 5 m wide mafic dyke occurs with an approximately N-S trend and extends over 300 m across the syenite bedrock outcrop area (Figure 3A) (
TABLE 1
| Sample name | Rock description | Location | Sr and Nd isotope ratio analysis | Photomicrographs | Bulk major and trace element analysis | Zircon cathodoluminescence (CL) images | U-Pb dating of zircon | In suit Hf isotope analysis | Mineral assemblage | ||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Lat | Long | ||||||||||
| (oS) | (oE) | ||||||||||
| DJS-1 | medium-to-coarse-grained with a granitic texture | Gray Bt syenite | −74.9194 | 163.6975 | √ | √ | — | — | — | — | Q (25%) + Pl (25%) + Mc (20%) + Pth (10%) + Bt (15%) |
| DJS-5a | Gray Bt syenite | −74.9194 | 163.6975 | √ | √ | — | — | — | — | Q (35%) + Pl (15%) + Mc (25%) + Pth (10%) + Bt (12%) | |
| DJS-6 | Gray syenite | −74.9194 | 163.6975 | √ | √ | √ | √ | √ | √ | Q (35%) + Pl (12%) + Mc (25%) + Pth (15%) + Bt (8%) | |
| DJS-7 | Gray Bt syenite | −74.9194 | 163.6975 | √ | √ | — | — | — | — | Q (35%) + Pl (25%) + Mc (10%) + Pth (10%) + Bt (15%) | |
| DJS-13 | Gray Hbl syenite | −74.9201 | 163.6943 | √ | √ | √ | √ | √ | √ | Q (30%) + Pl (13%) + Mc (13%) + Pth (25%) + Hbl (6%) + Bt (7%) | |
| DJS-14 | Gray Hbl syenite | −74.9201 | 163.6943 | √ | √ | — | — | — | — | Q (30%) + Pl (17%) + Mc (13%) + Pth (25%) + Hbl (6%) + Bt (7%) | |
| DJS-15 | Gray Hbl syenite | −74.9201 | 163.6943 | √ | √ | — | — | — | — | Q (30%) + Pl (32%) + Mc (13%) + Pth (12%) + Hbl (5%) + Bt (3%) | |
Summary of syenites analyses project and mineral assemblage.
Note: Bt: biotite, Kfs: K-feldspar, Pl: plagioclase, Q: quartz; Mc: Microcline, Pth: Perthite; Abbreviations of minerals based on
FIGURE 2

Geological and tectonic setting of the Inexpressible Island area showing the sample sites (
FIGURE 3

Representative outcrops and photomicrographs of granites from Inexpressible Island, NVL. (A) The field contact relationship between diorite and syenite is shown in the figure, which shows diorite dyke. (B) Massive syenite occurs in the field. (C) Syenite bedrock in the eastern bay region.
FIGURE 4

Representative photomicrographs of Early Ordovician syenite from Inexpressible Island. (A), (B) and (C) Photographs of the syenite. (D)–(I) polarizing microscope and Cross polarized light photomicrograph. Bt: biotite, Kfs: K-feldspar, Pl: plagioclase, Q: quartz; Mc: Microcline, Pth: Perthite.
Analytical methods
Zircon cathodoluminescence images
The internal structure of zircon can be revealed by the cathodoluminescence (CL) imaging technique. CL image can reflect the difference in the abundances of some trace elements (such as U, Y, Dy, and Tb) (Wu and Zheng, 2004), which can be influenced by temperatures at which the melts crystallized (Rubatto and Gebauer, 2000; Wu and Zheng, 2004; Rubatto and Gubauer, 2007), also the change of structural parameters such as crystallinity or the presence of defect centres can be reflected in CL image (
U-Pb dating of zircon by LA-ICP-MS
40 zircon grains were collected using conventional density and magnetic separation techniques and picked out under a binocular microscope from two samples of DJS-6 and DJS-13, which finally have been used to do U-Pb dating. The grains were subsequently mounted in epoxy resin, polished to half their thickness and they were later photographed in transmitted and reflected light. We selected the location of the oscillatory-zoned rim or the location of the uniform color of cathode luminescence. And we avoided choosing cracks or inclusions area in the transmission-reflected image of zircon grains, the specified analysis spots are shown (Figure 5). U–Pb dating of zircon grains by LA-ICP-MS at the Wuhan SampleSolution Analytical Technology Co., Ltd., Wuhan, China. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as described by Zong et al. (2017). Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7900 ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. A “wire” signal smoothing device is included in this laser ablation system (Hu et al., 2015). The laser beam spot and frequency of the GeolasPro for this analysis were 32 µm and 5 Hz, respectively. Zircon isotope ratios were calibrated by standard sample Plešovice 338.15 ± 1.7 Ma (Sláma et al., 2008), and isotope ratio monitoring standard sample GJ-1,599.0 ± 1.7 Ma (Jackson et al., 2004). All time-resolved analysis data consisted of approximately 20–30 s of blank signal and 50 s of sample signal. Offline processing of the analytical data (including the selection of sample and blank signals, instrument sensitivity drift correction, and U-Pb isotope ratio and age calculation) was undertaken using the software ICPMSDataCal (
FIGURE 5

Representative cathodoluminescence images of zircons from the dated samples from Inexpressible Island showing U-Pb ages and the location of the analyzed Hf.
Whole rock major element analysis
We have analyzed the content of major elements in seven syenite samples (DJS-1, DJS-5a, DJS-6, DJS-7, DJS-13, DJS-14, DJS-15) from the Inexpressible Island. Major element analyses of whole rock were conducted on XRF (Primus Ⅱ, Rigaku, Japan) at the Wuhan Sample solution Analytical Technology Co., Ltd., Wuhan, China. The detailed sample-digesting procedure was as follows: 1) Sample powder (200 mesh) was placed in an oven at 105°C for drying of 12 h; 2) ∼1.0gdried sample was accurately weighted and placed in the ceramic crucible and then heated in a muffle furnace at 1000°C for 2 h. After cooling to 400 °C, this sample was placed in the drying vessel and weighed again in order to calculate the loss on ignition (LOI). 3) 0.6 g sample powder was mixed with 6.0 g cosolvent (Li2B4O7: LiBO2: LiF = 9:2:1) and 0.3 g oxidant (NH4NO3) in a Pt crucible, which was placed in the furnace at 1,150°C for 14 min. Then, this melting sample was quenched with air for 1 min to produce flat discs on the fire brick for the XRF analyses.
Whole rock trace element analysis
We have analyzed the content of trace and rare Earth elements in seven syenite samples (DJS-1, DJS-5a, DJS-6, DJS-7, DJS-13, DJS-14, DJS-15) from the Inexpressible Island. Whole-rock trace and rare Earth elements were analyzed by ICP-MS (Agilent 7700e). The detailed sample-digesting procedure was as follows: 1) Sample powder (200 mesh) was placed in an oven at 105°C for drying of 12 h; 2) 50 mg sample powder was accurately weighed and placed in a Teflon bomb; 3) 1 ml HNO3 and 1 ml HF were slowly added into the Teflon bomb; 4) Teflon bomb was put in a stainless-steel pressure jacket and heated to 190°C in an oven for >24 h; 5) After cooling, the Teflon bomb was opened and placed on a hotplate at 140°C and evaporated to incipient dryness, and then 1 ml HNO3 was added and evaporated to dryness again; 6) 1 ml of HNO3, 1 ml of MQ water and 1 ml internal standard solution of 1ppm In were added, and the Teflon bomb was resealed and placed in the oven at 190°C for >12 h; 7) The final solution was transferred to a polyethylene bottle and diluted to 100 g by the addition of 2% HNO3.
Zircon in situ Hf isotope analysis of zircon by LA-MC-ICP-MS
We choose to conduct Hf analysis in a place where the surface is relatively uniform or close to the U-Pb analysis spots, in order to obtain fairly accurate Hf isotope ratios, these analytical spots are as shown (Figure 5). 40 zircon grains collected from two samples of DJS-6 and DJS-13 have been used to do in situ Hf dating. Experiments using in situ Hf isotope ratio analysis were conducted using MC-ICP-MS in combination with an excimer ArF laser ablation system hosted at Wuhan Sample Solution Analytical Technology Co., Ltd. All data were acquired on zircon in single spot ablation mode at a spot size of 44 μm. The energy density of the laser ablation used in this study was ∼8.0 J cm−2. Each measurement consisted of 20 s of acquisition of the background signal followed by 50 s of ablation signal acquisition. The operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method were the same as described (
Sr and Nd isotope ratio analysis by LA-ICP-MS
We analyzed the content of Sr and Nd isotope ratios in two syenite samples (DJS-6, DJS-13) from the Inexpressible Island. Sr and Nd isotope analyses were performed on an MC-ICP-MS at the Wuhan Sample Solution Analytical Technology Co. Analyses of the NBS 987standard solution yielded 87Sr/86Srratio of 0.710244 ± 22 (2SD, n=32, Thirlwall M. F, 1991), which is identical within error to their published values (0.710241 ± 12, Thirlwall, 1991). All data reduction for the MC-ICP-MS analysis of the Sr isotope ratios was conducted using Iso-Compass software (Zhang et al., 2020). The USGS reference materials BCR-2 (basalt) and RGM-2 (rhyolite)yielded results of 0.705034 ± 14 (2SD, n=4) and 0.704192 ± 10 (2SD, n=4) for 87Sr/86Sr, respectively, which is identical within error to their published values (
Results
Zircon U-Pb geochronology
In this study, LA-ICP-MS zircon U-Pb dating analysis was performed on syenite samples DJS-6 and DJS-13 from the TeNIC. The results are shown in (Supplementary Table SA1) and Figure 6. All spots on the oscillatory-zoned rim domains of zircon grains from the samples tightly cluster or are close to the Concordia line, all data Concordia filter better than 95% in error ellipses. The integration time is set to 30–35s according to the recommended value of the monitoring standard sample. We allow for an age error of better than 1% (See Supplementary Table SA1). Most zircons from the syenite samples of DJS-6 and DJS-13 were dominantly euhedral to subhedral. The CL images display reduced CL contrast between bands which are sometimes broad, also sometimes appear as sector zoning, were generally 100–180 μm and 80–100 μm in size, and had length/width ratios of 1:1–3:1 and 1:1–2:1, respectively (Figure 5). In total, 40 grains were analyzed from sample DJS-6 and DJS-13, yielded Th/U ratios of 0.58–0.78 and 0.46–0.83, respectively (Supplementary Table SA1). The concordia ages of DJS-6 is 471.8 ± 1.8 Ma (2σ, MSWD = 0.87, Probability = 0.35, n=20) (Figure 6A), giving a weighted mean 206Pb/238U age of 471.8 ± 3.5 Ma (2σ, MSWD = 0.87; n=20) (Figure 6B). The concordia ages of DJS-13 is 477.3 ±1.7 Ma (2σ, MSWD = 0.59, Probability = 0.44, n=20) (Figure 6C). 206Pb/238U age of 477.3 ± 3.4 Ma (2σ, MSWD = 0.59; n=20) (Figure 6D).
FIGURE 6

Zircon U–Pb concordia diagrams (A,C), weighted mean 206Pb/238U ages (B,D) of the Inexpressible Island syenites, northern Victoria Land.
Whole rock geochemistry
The whole-rock geochemical compositions of the syenite unit are listed in (Supplementary Table SA2). The rocks yielded concentrations of 60.88–62.81 wt% SiO2 with 10.02–11.72 wt% total alkalis. The K2O/Na2O ratios varied from 0.34 to 0.39 (Supplementary Table SA2). The Inexpressible Island high-potassium syenite shows different evolutionary paths (Figures 7A,B). All the samples are plotted in the shoshonite series field on the K2O vs. SiO2 diagram (Figure 7C). Our samples contain low MgO (0.36–0.53 wt%), TiO2 (0.43–0.66 wt%), and P2O5 (0.11–0.18 wt%) contents with a relatively high Al2O3 (17.46–18.46 wt%), the Al2O3/(CaO + Na2O+ K2O) (A/CNK) in molar proportion values ranging from 0.94 to 1.08, indicating that metaluminous to peraluminous (Figure 7B). Chondrite-normalized rare Earth elements (REE) concentrations of the syenite are shown in (Figure 8A) (Sun and McDonough, 1989). The ΣREE concentrations displayed negative trend than diorites, and strongly enriched Eu anomalies (0.89–5.58) (Supplementary Table SA3). On the primitive mantle-normalized multi element diagram, the syenite showed negative Th (1.60–5.78 ppm), P (4.82–8.31 ppm), and Ti (1.98–3.02 ppm) anomalies and positive Rb (134.8–269.4 ppm), Ba (3,368–5,003 ppm), Pb (14.31–34.89 ppm), K (241.0–289.2 ppm), and Zr (92.6–465.9 ppm) anomalies than the Inexpressible Island diorites (Figure 8B).
FIGURE 7

(A) Total alkali-silica diagram for the Inexpressible Island syenite (
FIGURE 8

Primitive mantle-normalized plots of incompatible elements and chondrite-normalization plots (Sun and McDonough, 1989) of trace elements, trace element data from the diorite and granite are from
Whole-rock Sr-Nd isotopes
Whole-rock Sr-Nd isotope data for the Inexpressible medium- and coarse-grained syenites are listed in Supplementary Table SA4. Whole-rock (87Sr/86Sr) i and εNd(t) values were calculated as 471.8 Ma for DJS-6 and 477.3 Ma for DJS-13. The Inexpressible syenites had similar εNd(t) values, ranging from −8.5 to −10.3, and a high and narrow range of initial 87Sr/86Sr values, ranging from 0.7104 to 0.7128.
Zircon Lu-Hf isotopes
The zircons used for the U-Pb dating were used for the in situ or counterpoint zircon Hf isotope analyses (Supplementary Table SA5). The initial 176Hf/177Hf ratios for the zircons from the DJS-13 syenite with Early Ordovician ages ranged from 0.282228 to 0.282276. Their εHf(t) ranged from −7.4 to −9.1 (Figure 9). (Supplementary Table SA5). The εHf(t) values for the zircons from the DJS-6 syenite ranged from −7.4 to −9.0.
FIGURE 9

Zircon εHf(t) vs. T (Ma) plot of the Inexpressible Island syenite and diorite (
Discussion
Timing of the magmatism
Previous published chronological studies of the Terra Nova igneous body suggest the extensive Rose Orogeny magmatism, such as, calc-alkaline granites in the eastern Mountaineers Range by Rb-Sr pseudo-isochron dating are 610 Ma ages (
FIGURE 10

Structural environment diagram of the rocks in the Inexpressible Island intrusive. (A) 104*Ga/Al vs. Zr diagram (Whalen et al., 1987); (B) K2O vs. Na2O; (C) and (D) Nb vs. Y and Rb vs. Nb+Y diagrams (Pearce et al., 1984).
Petrogenesis
The mineralogical and chemical composition of the syenites associated with the Inexpressible Island area, comprising of alkaline-feldspar and perthite, coupled with high content of high field strength elements such as Zr, Hf, slightly decrease in their ΣREE content (expect for Eu) (Figures 8A,B), when compared to S-type and I-type granitoid suggest that the granitoid are typically syenite and A-type (e.g.,
FIGURE 11

Harker diagrams for the medium- and coarse-grained syenites. Diorite data are derived from
FIGURE 12

εNd(t) vs (87Sr/86Sr) i plot of the Inexpressible Island intrusive rocks. Also reported for comparison are 1) intrusive complexes emplaced around 490–500 Ma in the Wilson arc; 2) Abbott gabbro and its hybridization from deep-crust melts, Vegetation Island leucogranites and their hybridization from metasedimentary upper-crust melts (
Rocchi et al. (2015) established a genetic link between mafic, diorite, and granite of GHI in the northern Victoria Land via a liquid line of decent involving fractional crystallization and/or assimilation fractional crystallization (AFC) of enrich mantle derived magmas. Parabolic trends of the Granite Harbour intrusive samples suggest mixing between old crust with low εNd(t) and high (87Sr/86Sr) i and a juvenile component with higher εNd(t) and low (87Sr/86Sr) i (Figure 12). These results are consistent with the interpretation of Rocchi et al. (2009), who suggested that the late-stage diorite with low εNd(t) in northern Victoria Land was sourced from enriched subcontinental lithospheric mantle. In addition,
The definition of “ultrapotassic rocks” introduced by (
Tectonic implications
Early Paleozoic convergence between the East Antarctic Craton and the Paleo-Pacific oceanic plate gave rise to southwest-directed subduction (present-day coordinates) and related continental and oceanic arc magmatism (Weaver et al., 1984;
FIGURE 13

Regional tectonic schematic cartoon after (A) Late Ross orogenic stage post-collisional potassic magmatism. (B) Schematic diagram of subduction zone structure. Modifications after reference (
In summary, we suggest that based on the tectonic and geochemical similarities of the Vegetation Island mafic rocks, Irizar granite dyke, Abbott gabbro, and granite with both late to post-orogenic shoshonites and lamprophyres (Turner, 1996; Turner and Foden, 1996;
Conclusion
1. The Inexpressible Island syenite samples (DJS-6 and DJS-13) zircon U-Pb ages are 471.8 ± 1.8 Ma (MSWD=0.87, n=20) and 477.3 ± 1.7 Ma (MSWD=0.59, n=20), respectively, indicating early Ordovician magmatic intrusive activity.
2. Whole-rock geochemistry shows that the Inexpressible Island syenite is weakly peraluminous metaluminous and ultra-potassium, with typical island-arc magmatic affinity. We interpret the syenite as an Ordovician intrusion associated with regional extensional in Wilson continental margin. The magma source is linked to local involvement in the melting zone of an older previously enriched layer of the subcontinental lithospheric mantle, which was further metasomatized by a more recent subduction component. Otherwise, plagioclase cumulate crystallization reflects the complex genesis of the magma source controlled by phlogopite and hornblende.
3. We support an active continental margin accretion model, which existed on the Antarctic margin of east Gondwana during the Early Paleozoic, in which multiple plates and ocean arcs were active. Plate tectonic deformation led to slab roll-back or tear of subducted plates, and upwelling asthenosphere mantle activate the subcontinental lithospheric mantle beneath the continental arc, which components be modified by subduction component before, and the derived mantle magma is significantly more evolved, ultimately intrude in the current stratum, formed the Inexpressible Island syenite.
Statements
Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.
Author contributions
LT contributed to conception and design of the study. PG organized the database. LC performed the statistical analysis. PG wrote the first draft of the manuscript. LT and LC wrote sections of the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.
Funding
This work was supported by the National Natural Science Foundation of China (No. 41976072), the Fundamental Research Funds for Second Institute of Oceanography, Ministry of Natural Resources (No. QNYC1901) and (No. JG2002).
Acknowledgments
We would like to acknowledge China’s 34th Antarctic Scientific Expedition for help with the samples and investigation. Thanks are due to LT and LC for their guidance in the study and writing process. We are grateful to Dr. Eirini Poulaki and Dr. Giovanna Rizzo, and editor Gilby Jepson whose reviews have significantly improved this work.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feart.2022.966085/full#supplementary-material
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Summary
Keywords
U-Pb geochronology, geochemistry, Rose orogeny, syenite, Inexpressible island
Citation
Gao P, Tang L and Chen L (2023) Geochemistry and zircon U–Pb ages of early Ordovician syenites from the Inexpressible Island, Antarctica and tectonic implications. Front. Earth Sci. 10:966085. doi: 10.3389/feart.2022.966085
Received
10 June 2022
Accepted
31 October 2022
Published
12 January 2023
Volume
10 - 2022
Edited by
Gilby Jepson, University of Arizona, United States
Reviewed by
Giovanna Rizzo, Università degli Studi della Basilicata, Italy
Eirini Poulaki, University of Texas at Austin, United States
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© 2023 Gao, Tang and Chen.
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*Correspondence: Limei Tang, tanglm@sio.org.cn
This article was submitted to Geochemistry, a section of the journal Frontiers in Earth Science
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