Geochemistry and Geochronology of Early Paleozoic Intrusive Rocks in the Terra Nova Bay Area, Northern Victoria Land, Antarctica

: The Terra Nova Intrusive Complex (TNIC) in northern Victoria Land, Antarctica, results from widespread magmatism during the Early Paleozoic Ross Orogeny. According to ﬁeld relationships, geochemistry, and geochronology data, the northern part of the TNIC comprises the Browning Intrusive Unit (BIU), which is associated with an arc crustal melting including migmatization of the Wilson Metamorphic Complex, and the later Campbell Intrusive Unit (CIU), which is attributed to the mantle and crustal melting processes. Zircon U-Pb ages suggest Late Neoproterozoic to Early Cambrian protolith with Late Cambrian metamorphism (502 ± 15 Ma) in the WMC, Late Cambrian formation (~500 Ma) of the BIU, and Early Ordovician formation (~480–470 Ma) of the CIU. Sr-Nd isotopic characteristics of the BIU indicate predominant crustal component ( ε Nd (t) = − 8.7 to − 8.9), whereas those of the CIU reﬂect both mantle ( ε Nd (t) = 1.8 to 1.6) and crustal ( ε Nd (t) = − 4.0 to − 7.5) compositions. These results suggest that the northern TNIC magmatism occurring at ~500–470 Ma originated from partial melting of the mantle–maﬁc crust components and mixing with felsic crust components. By integrating the results with previous studies, the TNIC is considered to be formed by a combination of the mantle and maﬁc crust melting, crustal assimilation, felsic crust melting, and magma mixing during the Ross Orogeny.

Magma intrusion and emplacement in continental margin environments are affected by many factors, such as the source material and its interactions during a multi-stage, multicomponent, and multi-process event. The Terra Nova Intrusive Complex (TNIC: [12,13]) in NVL contains a number of magma batches emplaced in a continental arc during the early stage of the Paleozoic Ross Orogeny. Multiple emplacements of magma pulses in the TNIC are revealed by several petrology and geochemistry data from mafic to felsic intrusive rocks (e.g., [14]). However, in previous studies, the northern part of the TNIC has been neglected probably because of the limited outcrops, inaccessibility, and harsh weather conditions. Figure 1. Geologic map of the study area (modified from [15]) showing zircon U-Pb ages (in Ma) obtained in the present study and the associated sample numbers. Values in green, blue, and red denote deposition, metamorphic, and igneous ages, respectively.

Wilson Metamorphic Complex (Precambrian-Early Cambrian Protoliths)
The study area, which is in the northern part of the TNIC, comprises the WMC and the GHI (Figures 1 and 2). The WMC in the study area contains mostly migmatitic gneisses and (garnet)-biotite schists (hereafter biotite schists), with minor amphibolite and amphibole schist, as well as biotite-hornblende gneisses, which have experienced complex metamorphic events (Figure 3a,b and Figure 4a,b) [27]. Although leucogranite dikes are rare in the geologic map (Figure 1), small scales of outcrops are abundant in the whole area.  Biotite schists exposed along the western cliff of the Gerlache Inlet and to the southeast of the Browning Pass are relevant with schists in the Wilson Terrane [29] and the amphibolite facies metasediments in the WMC [13,30]. These biotite schists are characterized by extensive leucogranite intrusions parallel or at high angles to foliations (Figure 3a). Major foliation planes created by biotite and muscovite alignment display NW strikes and SW dips (Figures 1 and 4a). Principal mineral assemblages in the biotite schists are garnet, biotite, plagioclase, and quartz, with minor muscovite. The major foliation envelops garnet porphyroblasts, suggesting pre-to synkinematic formation. These biotite schists display smaller grains and contain lower metamorphic grade mineral assemblages compared to the migmatitic gneisses. Migmatitic gneisses exposed in the Mt. Browning area with the 1st Peak near the Jang Bogo and Gondwana stations were intruded by the polyphase leucogranites (Figure 3a,b,d). These leucogranites are either concordant or discordant to the major foliation, although some veinlets extend into granitic neosomes of the migmatitic gneisses. The mineral assemblage of the migmatitic gneisses mainly comprises sillimanite, cordierite, garnet, biotite, muscovite, plagioclase, K-feldspar, quartz, and rare spinel ( Figure 4b). Fibrolitic sillimanite is common and coexists with biotite, cordierite, plagioclase, and K-feldspar. Cordierite exhibits signs of replacement by muscovite and chlorite (pinitization) during retrogression, while the rare spinel occurs as an anhedral relict phase in the cordierite.

Browning Intrusive Unit (Late Cambrian)
Intrusive rocks in the study area belong to the BIU-CIU (Figures 1 and 2), and these units are characterized by distinct foliations. The BIU comprises two separate plutons: foliated porphyritic granites and two-mica leucogranites, which intruded the WMC. This unit was in turn intruded by gabbros, diorites, and biotite granites of the Early Ordovician CIU (Figure 3c), which implies the BIU was formed during the Late Cambrian.
The foliated porphyritic granites, which are characterized by weak to moderate foliations and medium-to coarse-grained textures as well as feldspar phenocrysts (plagioclase and K-feldspar), locally display a heterogeneous distribution of minerals. Foliated porphyritic granites which were previously assigned to the WMC [30,31] are distinguished from the surrounding metamorphic rocks by the lack of metamorphic differentiation which produces leucocratic and melanocratic layers. Although this porphyritic granite commonly shows foliations that distinguish it from the biotite granite of the CIU (Figure 3e), it also locally exhibits massive or weakly foliated structures. Occasionally, foliated mafic xenoliths are also seen in this granite, and these are often biotite-enriched remnants of the host schists or gneisses. Major minerals in these foliated porphyritic granites are biotite, plagioclase, K-feldspar, and quartz ( Figure 4c). Plagioclase and K-feldspar phenocrysts show lengths varying between 10-30 mm, whereas biotite and other minerals are unevenly distributed at the outcrop scale. Quartz grains in the granites show undulatory extinction and sutured boundaries, with some recrystallization into subgrains along margins. In addition, fractures are common in the plagioclase and K-feldspars ( Figure 4c). Two-mica leucogranites intruded biotite schists and migmatitic gneisses of the WMC along foliations as dikes or veinlets, thereby frequently forming lens-shaped boudins ( Figure 3d). Foliations in the two-mica leucogranites depend on the alignment of biotite and are consistent with those of the host metamorphic rocks. Therefore, axial planes of the folded leucogranite and the major foliation plane of the metamorphic rocks yield similar strikes and dips. The two-mica leucogranite comprises mostly quartz and feldspars, with minor biotite and muscovite. This leucogranite also locally contains cordierite and garnet, and these are often aligned parallel to foliations. In addition, narrow veinlets of the leucogranite occasionally extend into felsic neosomes of the migmatites. Although foliations in the leucogranites are associated with magmatic emplacement, some foliations were formed after solidification. Muscovite is commonly present in the pressure shadow zones of garnets but it is also occasionally wrapped in fibrolitic sillimanite. In addition, the foliated leucogranites exhibit planar textures containing lath-like K-feldspar, plagioclase, and elongated quartz. Most of the minerals are characterized by fractures, except for micas.

Campbell Intrusive Unit (Early Ordovician)
The CIU contains Early Ordovician igneous rocks that intruded both the WMC and BIU (Figure 1). This unit comprises three separate plutons: gabbroic and dioritic rocks, K-feldspar megacryst leucogranites, and biotite granites, which exhibit massive or weakly foliated structures.
The gabbroic and dioritic rocks (hereafter gabbroic diorites) are dark greenish gray, massive, and fine-to medium-grained rocks including comagmatic gabbro, monzodiorite, monzonite, and diorite, whose rock names are based on whole-rock chemical composition ( [32], see Section 5). Although the gabbroic diorite pluton is enclosed by the biotite granite pluton (Figure 3e), the contact relationship between the two plutons is unclear because of the lack of continuous outcrops. Nevertheless, rock types in the two plutons are roughly discriminated in outcrops based on color (Figure 3e). These gabbroic diorites primarily contain hornblende, biotite, plagioclase, and quartz, with minor titanite (Figure 4e).
The K-feldspar megacryst leucogranites occur as dikes and veinlets, which commonly crosscut foliations in schists and gneisses of the WMC. Although some veinlets are folded, no evidence of penetrative deformation was observed in outcrops. Biotite selvages are common between the leucogranites and migmatitic gneisses. These Campbell leucogranites contain mainly K-feldspar, plagioclase, and quartz, with minor cordierite, garnet, and biotite ( Figure 3d). Temporally, the relationship between the K-feldspar megacryst (CIU) and two-mica (BIU) leucogranites was unresolved by field observations. In some outcrops, however, K-feldspar megacryst leucogranites (CIU) intruded two-mica leucogranites (BIU) (Figure 3d).
Biotite granites enclosing the gabbroic diorite pluton are massive to weakly foliated and medium-to coarse-grained. These granites also occur as dikes that intruded foliated porphyritic granites ( Figure 3c) and biotite schists exposed along the Browning Pass ( Figure 1). The biotite granites contain abundant gabbroic diorite and migmatitic gneiss xenoliths as well as dioritic microgranular enclaves ( Figure 3f). The biotite granites consist of biotite, plagioclase, K-feldspar (microcline), and quartz as the principal minerals, and minor allanite and titanite ( Figure 4f).

Analytical Methods
Forty-five samples were analyzed for major and trace elements. Major element compositions were determined using a Shimadzu MXF-2400 X-ray fluorescence (XRF) spectrometer in the Korea Institute of Geoscience and Mineral Resources (KIGAM). Trace elements in the samples were analyzed using XRF glass bead, with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) (Nexion 350, Perkin Elmer attached with NWR 193UC, ESI) in KIGAM. Standard reference glass NIST 612 was used as an external standard, and trace element concentrations were calculated with 44 Ca as an internal standard.
Zircon grains were separated from two metamorphic and six intrusive rock samples using standard procedures, including density and magnetic separations, followed by handpicking under a binocular microscope. Internal textures of individual zircon grains were examined in the back-scattered electron and cathodoluminescence (CL) modes using a scanning electron microscope (JEOL JSM-6610LV) in the Ochang Center of the Korea Basic Science Institute (KBSI). Zircon cores were analyzed for schist and gneiss samples, while magmatic rims displaying oscillatory zoning in CL were used for granitoid samples. In situ U-Pb zircon ages were obtained using a sensitive high-resolution ion microprobe (SHRIMP) IIe housed in the KBSI (Ochang Center) and an LA-ICP-MS installed in the KIGAM. Zircon standards including SL13 (U = 238 ppm), FC1 (1099 Ma; [33]), and 91500 ( 206 Pb/ 238 U age = 1065.4 Ma; [34]) were used to calibrate the U concentration and Pb/U ratios. The SHRIMP analysis was performed according to procedures described in previous studies (e.g., [35]), and data were reduced using the Squid Exel macro [36]. For U-Pb age determination with LA-ICP-MS, the laser repetition rate, energy density, and spot diameter were set to 10 Hz, 4 J/cm, and 35 µm, respectively. Finally, both SHRIMP and LA-ICP-MS data were plotted using the Isoplot/Ex (v.3.6) [37]. We utilized reduced data that passed the statistical t-test to report at the 95% confidence level, and 204 Pb corrected isotope ratios were employed for all plots and weighted mean age calculations.
Ten rock samples were also analyzed for Rb-Sr and Sm-Nd isotopic compositions using a Thermo-Fischer Triton plus, which was operated in the static mode in KIGAM. Samples were dissolved in an acid mixture (ultra-pure HF:HNO 3 = 2:1) in Savillex Teflon vessels, which were sealed after adding 87 Rb-84 Sr and 150 Nd-149 Sm mixed spikes. Rare earth element (REE) fractions were separated using conventional cation-exchange, while Nd and Sm were separated through a second cation-exchange process involving 2-ethylhexyl phosphoric acid (HDEHP)-coated Teflon powder in HCl medium. Mass fractionations of Sr and Nd isotope ratios were normalized to 88 Sr/ 86 Sr = 8.375209 and 146 Nd/ 144 Nd = 0.7219, respectively, and these were corrected further for contributions of the spikes added. Replicate analyses of NBS987 and La Jolla standards yielded 87 Sr/ 86 Sr = 0.710265 ± 0.000004 (2σ, n = 20) and 143 Nd/ 144 Nd = 0.512099 ± 0.000003 (2σ, n = 20), respectively. Total procedural blanks showed less than 20, 100, 10, and 50 pg correspondingly for Rb, Sr, Sm, and Nd.

U-Pb Geochronology
U-Pb isotope data for zircons obtained from the WMC (J-13-1 and J-114-1), BIU (J-17), and CIU (J-45-1, J-114-9, J-76-2, J-43-1, and J-68-1) samples were generated using the SHRIMP and LA-ICP-MS. Zircon grains from the schists and gneiss are predominantly rounded to subrounded, whereas those from the intrusive rocks are generally euhedral and prismatic. Grain sizes range between 30-300 µm for both groups of samples, while aspect ratios vary between 1:1-3:1 for the schist and gneiss samples and 1:1-4:1 for the intrusive rock samples. Spot analyses with >~1% common 206 Pb or >2500 ppm U were omitted from the age calculations. Representative CL images of the zircon grains are shown in Figure 5, while the U-Th-Pb isotopic data are presented in Tables S1 and S2.
Regarding the Browning leucogranite sample (J-17), which is related to migmatization of the WMC, the zircon grains are characterized by dark CL oscillatory-zoned domains with and without bright CL cores. Six spots in 40 grains (Th/U = 0.22-0.73) associated with inherited cores exhibit ages varying from~1120 to~950 Ma. After rejecting six high-U spots, oscillatory-zoned rims yield a weighted mean 206 Pb/ 238 U age of 499.8 ± 7.9 Ma (n = 10, MSWD = 5.2) (Figure 6c).
The U-Pb geochronology data can be summarized as follows: (1) Schist and gneiss in the WMC exhibit Late Neoproterozoic to Early Paleozoic (Early Cambrian) protolith ages and a metamorphic age of 502 ± 15 Ma. (2) Leucogranite in the BIU reveals a formation (igneous) age of 500 ± 8 Ma. (3) Gabbroic diorite (monzodiorite) and leucogranites in the CIU exhibit a formation age of~480 ± 5 Ma, while biotite granites in the CIU yield igneous ages between~475-470 Ma.
At least two types of leucogranites are identified in the study area and these include: the Browning leucogranites associated with migmatization of the WMC and/or intrusion of the complex and a later leucogranite assigned to the CIU. Based on compositional data, these leucogranites mostly fall within the segment for granite in the TAS classification scheme, with some samples exhibiting granodiorite and diorite compositions (Figure 7a). Chondrite-normalized REE plots for leucogranites display various patterns, with wideranging La/Yb N ratios (2.02-59.21) and both negative and positive Eu anomalies (Eu/Eu* = 0.09-2.15), which indicate that these samples originate from diverse plutons. A sample (J-17, BIU) age-confirmed shows positive Eu anomaly and HREE depletion, while another age-confirmed sample (J-76-2, CIU) exhibits negative Eu anomaly and higher REE contents (Figure 8c).
Biotite granites in the CIU exhibit granodiorite-granite compositions in the TAS classification scheme (Figure 7a). Chondrite-normalized REE plots reveal high LREE/HREE ratios (ave. La/Yb N = 13.23) and distinct negative Eu anomalies (ave. Eu/Eu* = 0.56) for these granites. The plots show flat HREE variation or slight inclination from the middle REE (MREE) to HREEs (Figure 8e).
In the normal mid-ocean ridge basalt (N-MORB)-normalized multi-element diagram, granitoids of the BIU-CIU exhibit variations highlighting high large ion lithophile element (LILE)/high field strength element (HFSE) ratios. These samples are also characterized by negative Ta-Nb, P, and Ti values and elevated K and Pb (Figure 8b,d,f). These patterns, which are similar to that of the average continental crust [46], suggest that the source materials of granitoids in the BIU-CIU originated predominantly from the continental crust.
Gabbroic to dioritic samples in the CIU show compositions for gabbro, monzodiorite, monzonite, and diorite ( Figure 7a) within a limited outcrop, and thus, these are considered comagmatic. These gabbroic diorites display distinct slopes from LREEs to HREEs (ave. La/Yb N = 12.7) in chondrite-normalized plots and weak negative or no Eu anomaly (ave. Eu/Eu* = 0.89) (Figure 8g). In N-MORB-normalized multi-element diagrams, these gabbroic diorites also display high LILE/HFSE characteristics, negative Ta-Nb, Pb, Zr, and Ti values, and positive K and Pb values (weak Pb spike). This pattern is similar to that of the average subduction zone seafloor sediment (global subducting sediment composite, GLOSS-II: [45]) or the upper continental crust (UCC: [46]) (Figure 8h). This suggests that terrigenous components were involved in producing primitive basaltic magmas from which the gabbroic diorites were derived (see Section 6.1).
In a tectonic discrimination diagram, gabbroic diorites of the CIU mostly show similarities to volcanic arc calc-alkaline basalts. Granitoids in the BIU-CIU predominantly display resemblance to volcanic arc granites, yet granites of the CIU especially show characteristics akin to post-collisional tectonic environments (Figure 7c,d).
In summary, the Campbell gabbroic diorites show initial Sr-Nd ratio characteristics of mantle components, and a Campbell biotite granite sample (J-44-1) displayed similar values. Diorite (J-46-1), biotite granite (J-43-5), foliated porphyritic granite (J-58), and leucogranite samples of the BIU-CIU show enriched initial Sr-Nd ratios, with the leucogranite sample (J-12-2) linked to migmatization of the WMC exhibiting the most enrichment in crustal components (Figure 9). Incompatible trace element contents of the gabbro-granites of the BIU-CIU produce similar patterns in an N-MORB-normalized multi-element diagram, and these patterns resemble those of samples with continental crust components (Figure 8). This suggests that the continental crust component was significantly involved during partial melting of the mantle and later acted as the main source of magma for the granitoids.  [47,48]. λ 87Rb = 1.42 × 10 −11 /yr [50]; λ 147Sm = 6.54 × 10 −12 /yr [51]. Initial isotopic ratios of the migmatitic gneiss (WMC) were calculated to be an age of 500 Ma.

Discussion
As mentioned in Sections 3 and 5, the igneous activity in the study area is characterized by continuous and various compositional (mantle and crustal source components) magma intrusions, which have an age range between 500 and 470 Ma. This result may reflect the continental arc orogeny process of the northern TNIC. Thus, we intend to discuss the Paleozoic crustal evolution of the entire TNIC based on our isotopic and age data combined with literature data. First, based on the Sr-Nd isotope components of the TNIC, we consider the crustal formation of the TNIC, and then, we discuss the formation of the TNIC from the temporal and tectonic perspectives.

Magma Sources of the BIU-CIU in the TNIC
In Section 5, the trace element contents and Sr-Nd isotope characteristics of gabbroic diorites and granitoids were presented. In the N-MORB-normalized multi-element diagram, gabbroic diorites show negative troughs for Ta-Nb, P, and Ti and positive spikes for K and P, which are characteristics similar to those obtained for the average trench sediment (GLOSS-II: [45]) and the UCC [46] (Figure 8). These characteristics, which are typically displayed by arc basalts or arc basaltic andesites, are explained by the following processes: (1) a mantle wedge acquired a terrestrial signature through metasomatism involving partial melting of subducting terrigenous sediments or fluid dehydration of the subducting plate and (2) arc igneous rocks inherited crust-like geochemical characteristics from the metasomatized mantle wedge (e.g., [52][53][54][55][56][57]). Regarding the Campbell gabbroic diorite samples (J-45-1 and J-57), their initial Sr-Nd data ( 87 Sr/ 86 Sr (i) = 0.7047-0.7049 and εNd (t) = 1.6 to 1.8) possibly reflect characteristics of the mantle source. However, a Campbell diorite sample (J-46-1) exhibits an evolved crustal signature ( 87 Sr/ 86 Sr (i) = 0.7095 and εNd (t) = −7.5) (Figure 9). This crustal signature can be attributed to the following: (1) a highly enriched mantle signature, (2) crustal assimilation linked to emplacement of the primitive basaltic magma, or (3) mixing of basaltic and granitic melt components. Scenarios presented in 2 and 3 were advanced previously for the TNIC [12,13].
A two-stage magma evolution model (scenario 2-3) for the central-south TNIC involving crustal assimilation and magma mixing has been proposed [12,13]. According to this model, mafic-intermediate-felsic intrusives in the TNIC originate from the interaction of mantle melts, crustal melts, and the crust. In other words, the primitive mantle melt evolved through crustal assimilation (stage 1) and mixed with a crustal felsic melt (stage 2) to produce an intrusive complex of varying composition in the Terra Nova Bay area. Isotopic characteristics of mafic-intermediate-felsic igneous rocks in the BIU-CIU, which represent the northern TNIC appear to also follow the evolutionary stages of the two-stage model (Figure 9). However, because mafic-intermediate magmas characterized by isotopic enrichment without distinct crustal assimilation have been reported in many continental arcs, the possibility of a highly enriched mantle source (e.g., enriched subcontinental lithospheric mantle) for the TNIC intrusives also deserves attention. This requires isotope studies of the TNIC (monzo)gabbro.
Granitoids of the BIU-CIU exhibit wide-ranging 87 Sr/ 86 Sr (i) (0.7043-0.7232) and εNd (t) (1.4 to −8.9) values. The positive εNd (t) value (1.4) for the Campbell biotite granite sample (J-44-1) suggests an association with mantle components. However, characteristics of incompatible trace elements for the Campbell biotite granites indicate a dominance of crustal components (Figure 8d,f). Therefore, a young (e.g., Cambrian) mafic intrusive rock is a potential dominant source material for the Campbell biotite granites because a young mafic crust retains its source mantle isotopic characteristics. In addition, if the source material is attributed to a rock of Cambrian age, this will most likely be a mafic intrusive associated with the early phase of the Ross Orogeny. The highly enriched crustal values ( 87 Sr/ 86 Sr (i) = 0.7189-0.7232; εNd (t) = −8.7 to −10.8) for the leucogranite sample (J-12-2) and the surrounding migmatitic gneisses probably reflect the felsic crust (or upper crust) composition of the TNIC in Early Paleozoic. Based on these relationships, samples J-43-5 and J-7-3 probably formed from partial melting of the dominantly mafic crust source. Whereas, the enriched Sr-Nd isotope characteristics of sample J-58 suggest a mafic crustderived melt that assimilated felsic crust or was mixed with felsic crust-derived melt and/or a felsic crust-derived melt itself.

Tectonic Environment for the TNIC Magmatism
Tectonic interpretations for the formation of NVL from the Ross Orogeny have been advanced based on strike-slip movements, geochemical characteristics of the volcanics and granitoids, and sedimentary events [22,[66][67][68][69]. Kleinschmidt and Tessensohn [70] proposed a tectonic model involving two parallel westward-dipping subduction zones. This model can be summarized in the following steps: (1) An island arc (the Glasgow volcanics) formed earlier in an intraoceanic region was transported toward the continental Wilson Terrane between 530 and 500 Ma. (2) During the Ross Orogeny, widespread granitization mainly occurred at~500 Ma, and this was accompanied by deformation events until~480 Ma.
(3) Devonian granitoids and related volcanics were then formed after a long tectonic lull. In subsequent models, the possible formation of a continental arc at~530-520 Ma and a continent-continent collision at~500-490 Ma [71][72][73] were proposed.
Our geochronological and geochemical data reveal two distinct magmatic ages of 500 Ma for the BIU and~480-470 Ma for the CIU, which reflect the complete magma evolu-tion especially for mantle components (Figure 9) [12,13]. Based on these data, magmatism in the Wilson Terrane margin in NVL can be reconstructed. During the Early Cambrian, calc-alkaline granitoids associated with eclogite-facies metamorphism were emplaced at 530 Ma, and this indicates an active continental margin existed at~530-520 Ma [2,74,75]. The wide-ranging magmatic/metamorphic zircon ages from the studied schists imply a continuous supply of detrital materials in the Early Cambrian [76]. During the Late Cambrian, widespread magmatism and migmatization occurred due to a subduction of the Paleo-Pacific Plate or a continent-arc (i.e., Wilson Terrane-Bowers Terrane) collision (e.g., [77]). The compressional regime of the Ross Orogeny produced numerous intrusive bodies, such as the Confusion (~526-513 Ma), Browning (~500 Ma), and Abbott (~495 Ma) units, which are possibly of intermediate to extensive depths, in the TNIC. During Early Ordovician, the Ross Orogeny experienced a transitional regime toward post-collisional (or post-orogenic) extension, and the Inexpressible Syenite (~485 Ma) and Campbell (~480-470 Ma) and Vegetation (~475 Ma) units, likely associated with shallow depths, were formed in the TNIC (Figure 2) (e.g., [23,71,78,79]). Therefore, the TNIC magmatism at~510-470 Ma characterized by distinctively wide-ranging 87 Sr/ 86 Sr (i) and εNd (t) values could reflect its various magmatic sources and a changing tectonic environment in NVL.

Conclusions
In the present study, we conducted geochronological and geochemical investigations of igneous rocks from the northern TNIC in NVL, Antarctica, and the main findings are as follows: 1.
Two distinct intrusive units are identified in the TNIC. The BIU (Late Cambrian) originates from arc crustal melting, and also is contributed to the migmatization of the WMC. The CIU (Early Ordovician), which comprises later igneous bodies, is attributed to mantle-derived melts intrusions and associated crustal anatexis.

2.
Gneiss and schist in the WMC exhibit Late Neoproterozoic to Early Paleozoic (Early Cambrian) protolith ages and a metamorphic age of 502 ± 15 Ma. A leucogranite from the BIU reveals the emplacement age of 500 ± 5 Ma which is synchronous with the metamorphism of the WMC. Relatedly, gabbroic diorites and leucogranites from the CIU produce ages of~480 ± 5 Ma, while biotite granite (later intrusive in the CIU) samples yield igneous ages between~475-470 Ma. The distinct wide range of 87 Sr/ 86 Sr (i) and εNd (t) associated with the BIU-CIU are attributed to mantle and mafic crust melting, crustal assimilation, subsequent felsic crust melting, and magma mixing, which are processes typical in continental arcs. These processes well explain the Ross orogenic arc crustal building of the TNIC in NVL.

Supplementary Materials:
The following are available online at https://www.mdpi.com/article/10 .3390/min11070787/s1: Table S1. SHRIMP zircon U-Pb isotopic data for samples from the northern TNIC, NVL, Antarctica; Table S2. LA-ICP MS zircon U-Pb isotopic data for samples from the northern TNIC, NVL, Antarctica; Table S3. Representative whole-rock major and trace element data for BIU-CIU rocks in the northern TNIC, NVL, Antarctica.

Data Availability Statement:
The data presented in this study are available in the Supplementary Materials.