The assembly–breakup of the Columbia/Nuna supercontinent is one of the most important issues in the Precambrian geology. The reconstruction of the Indochina Block in the Columbia supercontinent is poorly constrained by far, due to the deficiency of available geochronological and geochemical data for the exposed Precambrian rocks. The Mesoproterozoic plagioclase amphibolites in the Kontum Complex have significant implication for the reconstruction of the Indochina Block within the Columbia supercontinent. This study presents detailed petrological, zircon U–Pb geochronological and Lu–Hf isotopic, and whole-rock geochemical analyses for the plagioclase amphibolites. The plagioclase amphibolite protoliths were formed at ~1432–1403 Ma and experienced metamorphism at 486–457 Ma, suggesting the preservation of a Mesoproterozoic basement in the Kontum Complex. The samples are strongly enriched in LREEs and LILEs but depleted in Nb, Ta, and Ti. They have relatively low (87Sr/86Sr)i ratios (from 0.705055 to 0.708728), positive εNd (t) values (from +2.6 to +2.8), and positive zircon εHf (t) values (from +9.9 to +17.1). Such signatures suggest that they were derived from a mantle wedge that has been metasomatized by sediment-derived melts in an arc setting and caused the breakup of the Columbia supercontinent. The Kontum Complex from the Indochina Block, along with Laurentia and East Antarctica, was distributed at the Columbian periphery.
The assembly–breakup of supercontinents during the earth’s history has been a hot topic for a long time (e.g., [1–18]). The assembly–breakup of the Columbia supercontinent is a global Precambrian issue that has significant implication for the Precambrian evolution (e.g., [1–6, 19–23]). The Columbia supercontinent was composed of many ancient Precambrian blocks that were assembled in the Paleoproterozoic (~1.9-1.7 Ga) and split in the Mesoproterozoic (~1.6-1.4 Ga) (e.g., [1–6, 19–23]). Previous studies mainly focused on the geological records of Laurentia, Australia, India, East Antarctica, North China, Baltica, and Siberia blocks, and revealed their spatial patterns within the Columbia supercontinent (e.g., [2, 3, 7–9, 24–27]). In South and East Asia, more attention has been paid to the Precambrian basement in North China, South China, and Indian cratons (e.g., [2, 3, 7–9, 19, 24, 27–29]). According to the previous studies on the main blocks in Columbia supercontinent, Laurentia, North China, Greenland, Baltica, and East Antarctica are considered in the periphery, while SW Hainan, Australia, and Cathaysia are located in the interior (e.g., [2–6, 21]). However, Precambrian basement rocks were poorly reported in the Indochina Block, which is a major block in Southeast Asia but its paleo-position in the Columbia reconstruction was poorly known (e.g., [24, 30, 31]).
The Indochina Block drifted from the Gondwana margin during the Paleozoic and finally accreted to Asia through assemblage with the South China Block during the Triassic (Figure 1(a); e.g., [8, 9, 24, 32–38]). This process resulted in the extensive development of the Phanerozoic igneous and metamorphic rocks and the Ordo–Silurian and Triassic tectonothermal events in the Indochina Block (e.g., [8, 32–37]). However, the Precambrian evolution remains poorly constrained (e.g., [30, 31, 39, 40]). Available data show that two Precambrian units of the Phu Hoat and Kontum complexes have only been identified in the Indochina Block. The Phu Hoat Complex is suggested to be of Neoproterozoic origin (e.g., [30, 31, 41–43]). The Kontum Complex in Central Vietnam is considered to be Mesoproterozoic or possibly older, but the precisely geochronological and geochemical constraints on mafic rocks are relatively poor (e.g., [24, 30, 31, 41–43]). Previous work mainly focused on the Precambrian inherited zircon grains in the Phanerozoic (meta-) igneous rocks from the Kham Duc and Kannak units of the Kontum Complex (e.g., [24, 35, 36, 41, 42]). On the contrary, little attention has been paid to the Ngoc Linh unit with the amphibolite-facies metamorphism, which is the main rock unit of the Kontum Complex in Central Vietnam (Figure 1(b); e.g., [30, 31]). To better constrain the formation age of the Kontum Precambrian rocks and reconstruct the location of the Indochina Block within the Columbia supercontinent, this paper presents detailed petrological, zircon U–Pb geochronological, and Lu–Hf isotopic results, as well as whole-rock elemental and Sr–Nd–Pb isotopic data for the plagioclase amphibolites from the Ngoc Linh unit of the Kontum Complex.
2. Geological Background and Sample Descriptions
Vietnam is located in the eastern part of the Indochina Block, facing Hainan Island to the east (Figure 1(a); e.g., [24, 31, 35, 36]). The Kontum Complex is adjacent to the Khorat Plateau in the west and the Tam Ky–Phuoc Son zone in the north (e.g., [35, 36, 49–52]). The Kontum Complex is traditionally considered an ancient continental core within the Indochina Block (e.g., [41, 42]). Available data show that the Kontum Complex is mainly characterized by the Precambrian Complex (also named the Kontum terrane) and Cambrian–Silurian and Triassic igneous rocks with strong brittle–ductile deformation (e.g., [24, 41–44, 49]).
The Kontum Complex is composed of the granulite-facies Kannak unit, amphibolite-facies Ngoc Linh unit, and greenschist-facies Kham Duc and the Hai Van migmatite units. It is dominated by greenschist, paragneiss, orthogneiss, migmatite, granulite, and amphibolites (e.g., [24, 30, 31, 35, 36, 49, 53]). Recent zircon U–Pb geochronological data have documented that the Kontum Complex might be characterized by a Mesoproterozoic basement with ages ranging from 1468 Ma to 1350 Ma (e.g., [24, 30, 31, 35, 36]). A line of evidences show that the Kontum Complex is unconformably overlain by the Cambrian–Silurian schist, siltstone, sandstone, and limestone (e.g., [24, 30, 31, 41–43, 52]). The Middle Devonian–Permian quartz sandstone, siltstone, shale, and limestone are unconformably underlain by the Precambrian and Lower Paleozoic packages . In addition, the bedded Permian–Middle Triassic volcano–sedimentary rocks are underlain by the Lower Paleozoic strata and overlain by the Upper Triassic–Jurassic conglomerate, sandstone, and siltstone across the angular unconformity. In the Kontum Complex, all the Pre-Cretaceous packages are unconformably overlain by the Cretaceous red beds . The Phanerozoic intrusive rocks are dominated by gabbro, diabase, diorite, granodiorite, and tonalite, which defined three age clusters of Ordo–Silurian, Triassic, and Jurassic–Cretaceous, respectively (e.g., [24, 32, 33, 35, 36, 38, 49, 52, 55, 56]).
The Ngoc Linh unit of the Kontum Complex is mainly composed of amphibolites, granulite, and gneiss, which have experienced metamorphism at Ordo–Silurian and Triassic with a clockwise P–T path (e.g., [24, 33, 38, 43, 49]). In this study, the representative plagioclase amphibolite samples (e.g., 20VN-38A, 20VN-38B, and 20VN-51) were collected from this unit (Figure 1(b)). They are mingled with paragneiss and schist and have the mineral assemblages of plagioclase, amphibole, and quartz (Figures 2(d)–2(f)). Sample 20VN-38A and -38B consist of ~50–60 vol. % plagioclase with polysynthetic twins and ~44–50 vol. % amphibole and minor accessory minerals (Figures 2(d)–2(f)). For sample 20VN-51, plagioclase contents are up to ~65 vol. % (Figure 2(f)).
3. Analytical Methods
3.1. Zircon U–Pb Dating, Rare Earth Elemental, and Lu–Hf Isotopic Analyses
Zircon grains were segregated using standard heavy liquid and magnetic separation techniques. They were selected and mounted in epoxy resin, polished, and finally coated with gold. The internal texture of grains was examined by cathodoluminescence (CL) images using a scanning electron microprobe at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. All in-situ zircon U–Pb geochronological, rare earth elements (REEs), and in-situ Lu–Hf isotopic analyses were carried out at the Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University.
In-situ zircon U–Pb dating and REEs analyses were undertaken using an iCAP RQ inductively coupled plasma mass spectrometer (ICP-MS) coupled with an ArF-193 nm Geolas HD laser ablation system. The spot size of 32 μm with a laser repetition rate of 5 Hz was used for zircon ablation. Helium was used as a carrier gas, to enhance the transport efficiency of the ablated materials. Standard zircons Plešovice and 91500 were used as external isotopic calibration standards and SRM 610 as an elemental external calibration standard. Detailed analytical methods were described by Wang et al. . Off-line raw data were processed by GLITTER software , and concordia diagram and weighted mean age of each sample were illustrated and calculated by the Isoplot program . 207Pb/206Pb ages were used for apparent ages >1000 Ma and 206Pb/238U ages for <1000 Ma.
In-situ zircon Lu–Hf isotopic analyses were conducted using a Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) with an ArF-193 nm Geolas HD laser ablation system. Helium and Argon gas were invoked as carriers, to transport ablated materials. The spot size of 44 μm with a laser repetition rate of 6 Hz was utilized for ablating zircons. Standard zircon 91500 was used as an external standard. Data was normalized to 176Hf/177Hf =0.7325, using exponential correction for mass bias . εHf (t) values were calculated by the chondrite ratios of 176Hf/177Hf =0.282772 and 176Lu/177Hf =0.0332 [61, 62].
3.2. Whole-Rock Elemental and Isotopic Analyses
Fresh whole-rock samples were crushed in a steel mortar and then pulverized to 200 mesh for elemental and isotopic analyses. All the whole-rock geochemical analyses were carried out at the Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University. Major oxide contents were measured by an ARL-Perform’X 4200 X-ray fluorescence spectrometer. Relative standard derivation for major oxides is lower than 5%. Trace element concentrations were analyzed by an iCAP RQ ICP-MS. Standard W-2a was used as a reference and crossed checked with BHVO-2. Detailed analytic methods were described by Wang et al. . Whole-rock Sr–Nd–Pb isotopic separation and purification were performed at Guizhou Tongwei Analytical Technology Co. Ltd. Details of analytical methods are presented in Yang et al. , and the total procedure blanks were in the range of 200–500 pg for Sr, ≤50 pg for Nd, and ≤50 pg for Pb, respectively. Whole-rock Sr–Nd–Pb isotopic analysis was analyzed by a Neptune Plus MC-ICP-MS. NIST SRM 987, ALFA-Nd, and NIST SRM 981 are the reference materials for the Sr, Nd, and Pb isotopic analyses, respectively.
4.1. Zircon U–Pb Dating, Rare Earth Elemental, and Lu–Hf Isotopic Results
Three fresh samples (20VN-38A-1, 20VN-38B-8, and 20VN-51-8) were selected for zircon U–Pb geochronological, rare earth elemental, and Lu–Hf isotopic analyses (Tables 1–3). Separated grains are translucently colorless in color and euhedral in shape.
Forty-one analytical spots (both rims and cores) from plagioclase amphibolite 20VN-38A-1 yield 206Pb/238U apparent ages of 1449–462 Ma, constituting a discordia Pb-loss line with the upper and lower intercept ages of 1429 ± 18 Ma and 457 ± 15 Ma, respectively (Figure 3(a)). Thirty analytical spots adjacent to the upper intercept age have Th/U ratios of 0.18–2.15 and yield a 207Pb/206Pb mean age of 1419 ± 19 Ma (MSWD = 0.25). Their corresponding εHf ( = 1400 Ma) values range from +9.9 to +17.1 (Figure 4). As shown in Figure 3(d), the grains with the upper intercept ages have similar REE-normalized patterns with typical igneous zircons that are enriched in Ce (Ce/Ce = 14.9–98.9) and HREEs but depleted in La and Eu (Eu/Eu = 0.03–0.68). The remaining analytical spots on the discordia line, reflective of Pb loss, have lower Ce/Ce (5.35–42.82) and Eu/Eu (0.02–0.17) ratios than the upper intercept spots (e.g., [64, 65]).
Forty-five analytical spots from 20VN-38B-8 form a discordia line with apparent 206Pb/238U ages ranging from 1438 Ma to 466 Ma. They define the upper and lower intercept ages of 1432 ± 31 Ma and 473 ± 8 Ma (MSWD = 1.1), respectively (Figure 3(b)). Four spots with the 206Pb/238U ages of 545–496 Ma are not in the calculation of the intercept ages since they are far from the discordia line. Eight analyses near to the upper intercept yield a mean age of 1418 ± 35 Ma (MSWD = 0.27) with Th/U = 0.10–1.02, whose left-sloping REE-normalized patterns are given with enriched Ce (Ce/Ce = 13.67–30.91) and depleted Eu (Eu/Eu = 0.03–0.08), suggestive of their igneous origin (Figure 3(e); e.g., [64, 65]). The remaining spots have Ce/Ce ranging from 2.50 to 51.54 and Eu/Eu from 0.01 to 0.27.
Forty-five analytical spots from plagioclase amphibolite (20VN-51-8) give the 206Pb/238U apparent ages of 1452–473 Ma, yielding the upper and lower intercept ages of 1403 ± 24 Ma and 486 ± 10 Ma, respectively (Figure 3(c)). Fifteen spots plotting at the upper intercept define a 207Pb/206Pb mean age of 1398 ± 31 Ma (MSWD = 0.23) with Th/U = 0.23–1.04. They show left-sloping REE-normalized patterns with Ce positive (Ce/Ce = 15.99–73.90) and Eu negative (Eu/Eu = 0.04–0.11) anomalies, resembling those for the igneous grains.
4.2. Whole-Rock Elemental and Isotopic Results
The samples have SiO2 contents of 51.14–58.34 wt. %, MgO of 2.52–5.45 wt. %, CaO of 3.61–9.38 wt. %, TiO2 of 0.86–1.33 wt. %, and FeOt of 5.04–8.48 wt. % (t herein is short for total). They show high Al2O3 contents of 17.71–20.77 wt. %, with Mg# (100 × Mg2+/(Mg2+ + Fe2+)) values ranging from 47 to 55. In the TAS diagram (Figure 5(a)), these samples plot in the fields of gabbro and diorite. In the MgO–Nb/La diagram (Figure 5(b)), they fall in the arc igneous field with Nb/La = 0.15–0.56.
Figures 6(a) and 6(b) show the REE-normalized and multi-elemental primitive mantle-normalized patterns, respectively. These samples show the sub-parallel right-sloping REE-normalized patterns with strong enrichment in light rare earth elements (LREEs). Their (La/Yb)N, (La/Sm)N, (Gd/Yb)N, and Eu/Eu range from 6.04 to 47.84, 2.13 to 6.19, 1.82 to 5.03, and 0.57 to 0.68, respectively. These samples have arc-like PM-normalized patterns with enrichment in LILEs and depletion in Nb–Ta and Ti (Figure 6(b)). Their (Nb/La)PM ratios range from 0.14 to 0.54, (Ti/Gd)PM ratios from 0.31 to 0.65, and (Th/Ta)PM ratios from 3.37 to 14.74. In comparison with the contemporaneous intermediate–mafic igneous rocks from Hainan, Yukon, and Yilgarn in the Columbian interior, these samples have higher LREEs and LILEs, which are yet similar to those from Laurentia and East Antarctica in the Columbian periphery [4, 30, 31, 72–74].
Sr–Nd–Pb isotopic results for representative samples are listed in Table 4. They have initial 87Sr/86Sr ratios of 0.705055–0.708728 and positive εNd (t) values ranging from +2.6 to +2.8. The Sr–Nd isotopic compositions overlapped the fields of the Precambrian arc-like igneous rocks in South China, East Antarctica, and Laurentia (Figures 7(a) and 7(b)). Their corresponding (206Pb/204Pb)i, (207Pb/204Pb)i, and (208Pb/204Pb)i ratios range from 18.49 to 21.11, 15.67 to 15.81, and 40.14 to 42.59, respectively. In the plots of (206Pb/204Pb)i and (207Pb/204Pb)i and (208Pb/204Pb)i (Figures 7(c) and 7(d)), they are close to the field of EMII source.
5.1. Constraints on the Mesoproterozoic Mafic Rocks in the Kontum Complex
Previous studies considered that the Kontum Complex contained an Archean core [41, 42]; however, the previously mapped oldest meta-igneous rocks in the Kontum Complex were recently dated at ~480–420 Ma of the Early Paleozoic period (e.g., [24, 32, 33, 35, 36, 38, 49–52, 55, 56]). Available structural, magmatic, and metamorphic data revealed the development of an Early Paleozoic tectonothermal event or the tectonic overprinting on the older rocks, thus suggesting the existence of the Precambrian basement in the Kontum Complex (e.g., [24, 41, 42]). Wang et al. and Nakano et al. reported the 1470–1350 Ma orthogneiss and granulite in the Kham Duc unit and 1424 Ma for the protolith of the granulite from the Kannak unit in the Kontum Complex [30, 31]. Wang et al. also identified the Neoproterozoic meta-sedimentary rocks by the detrital zircon U–Pb dating method . However, zircon U–Pb geochronological data have been poorly reported for the Ngoc Linh unit, the biggest unit of the Kontum Complex.
Our plagioclase amphibolites from the Ngoc Linh unit yield the upper intercept ages of 1432–1403 Ma for the zircon grains (Figure 3). The upper intercept ages might be interpreted as the crystallization ages of the protolith of plagioclase amphibolites, based on the following observations for the grains with the upper intercept ages: (a) these grains show weak zonation in CL image and have Th/U ratios of 0.10–2.15, similar to those with the magmatic origin; (b) the zircons show strongly left-sloping REE patterns with high Ce/Ce and low Eu/Eu (0.03–0.68), resembling the igneous grains [64, 65]; (c) their trace element compositions are generally similar with those from the mafic rocks (Figure 8; e.g., ). Such data suggest the perseverance of the ~1.4 Ga mafic basement in the Ngoc Linh unit. In combination with the published results, it suggests the development of the Mesoproterozoic (~1470–1350 Ma) crystallized basement in the Kontum Complex of the Indochina Block.
Zircons from the plagioclase amphibolites yield the lower intercept ages of ~486–457 Ma (Figure 3). The analyses near the lower intercept ages are justly spotted on the rims of the zircons, further suggesting that the Early Paleozoic ages are reflective of the metamorphic ages of the plagioclase amphibolites. Our results are also consistent with the reported Early Paleozoic metamorphic event for the Indochina Block (e.g., [24, 49]). In addition, a line of evidences show the development of abundant Early Paleozoic igneous rocks with the formation ages ranging from ~480 Ma to ~410 Ma, of angular unconformity of the Upper Paleozoic strata with the pre-Devonian package in the Kontum Complex (e.g., [24, 36, 38]). Such data, along with the available observations (e.g., [35, 36]), collectively suggest the Ordo–Silurian assemblage in the Indochina Block, a segment of the Prototethyan accretionary orogenesis along the East Gondwana periphery (e.g., [35, 85, 86]). The metamorphic event defined by the lower interpret ages for the plagioclase amphibolites might be the response to the Early Paleozoic accretionary orogenesis.
5.2. Petrogenesis of the Mesoproterozoic Plagioclase Amphibolites in the Kontum Complex
As mentioned above, the plagioclase amphibolites in the Kontum Complex have experienced high-graded metamorphism and potential low-temperature alteration. Our samples have low LOI (loss on ignition) contents (1.08–1.70 wt. %) and show poor linear correlations of LOI with major oxides (e.g., Na2O, K2O, and MgO), elemental ratios (e.g., Nb/La, Th/La, and Zr/Nb), and Sr–Nd–Pb isotopic compositions (not shown). These samples additionally show the sub-parallel REE-chondrite- and multi-elemental PM-normalized patterns (Figure 6). Such signatures rule out the significant effects of metamorphism and alteration for the samples.
The samples have relatively consistent Nb/La (0.15–0.56) and low Nb/U ratios (3.40–16.31) irrespective of SiO2 contents (Figure 9(a)). They have relatively high εNd (t) values (+2.6 to +2.8), distinct from the crustal contamination trend (Figure 9(b)). Such signatures, along with poor correlation between Sr/Nd ratios and SiO2 or MgO contents (not shown), suggest that the effects of crustal contamination on the samples are negligible. Our samples have variable Mg# values ranging from 47 to 55, suggesting fractional crystallization to some degree. They have decreasing CaO contents, Al2O3/CaO ratios, and Cr and Ni contents with decreasing MgO contents (not shown), reflective of the fractional crystallization of olivine and clinopyroxene. The plagioclase fractionation is evidenced by significant Eu and Sr negative anomalies (Figure 6). However, the trend of increasing La/Sm (3.29–9.59) with increasing La (26.6–92.1 ppm) indicates that the plagioclase amphibolites are mainly in petrogenesis related to source heterogeneity or partial melting rather than fractional crystallization (Figure 9(c)).
The plagioclase amphibolite samples are strongly enriched in LILEs and depleted in HFSEs with significant Nb–Ta and Ti negative anomalies (Figure 6(b)). Such arc-like signatures potentially indicate that the mantle source has been metasomatized by subduction-related components. Questions remain as to what kind of subducted components (slab or sediment-derived fluids or melt) have been added into the mantle source. Slab-derived melts/fluids have generally high Eu, Sr, Cr, and Ni contents, high Mg#, low initial (87Sr/86Sr)i, and high εNd (t) (e.g., [34, 35, 52, 62, 73, 81]). Our samples have (87Sr/86Sr)i = 0.705055–0.708728 and εNd (t) = +2.6 to +2.8, similar to or slightly lower than slab-derived components (e.g., [35, 37, 57, 88]). However, their low Pb/Zr (0.03–0.09) irrespective of Nb/Zr (0.04–0.15), and low Ba/La (8.33–26.06) with variable Th/Yb (1.97–28.33), argues for the addition of the recycled sediment-derived melts (Figures 10(a) and 10(b)). In Figure 10(c), they have low (Ta/La)PM and (Hf/Sm)PM, falling into the field of the melt-related subduction metasomatism. Such signatures tend to suggest that the Mesoproterozoic plagioclase amphibolites in the Ngoc Linh unit originated from the mantle wedge modified by the recycled sediment-derived components (e.g., [35, 37, 57, 89, 90]).
5.3. Tectonic Implications for the Columbia Breakup
Taking into account the positions of main blocks and fragments in the Columbia supercontinent, they can be classified as two groups of Laurentian margins, with North China, Greenland, Baltica, and East Antarctica in the periphery, and SW Hainan, Australian Yilgarn, Yangtze, and Yukon in the interior (e.g., [2–6, 21, 91–94]). In the Columbian interior, the Mesoproterozoic igneous rocks usually show higher Nb/La and Ta/La ratios than those in the periphery (Figure 5(b)). They are characterized by flat REE-normalized patterns, low LREEs/HREEs, and insignificant Nb–Ta and Ti anomalies (Figure 6), and plotted in the N-MORB or OIB ranges in Figures 10(c) and 11 [4, 77, 89, 95–98]. In contrary, the Mesoproterozoic igneous rocks along the periphery are signed by the enrichment of LREEs and LILEs, depletion of Nb, Ta, and Ti, and arc-like multi-elemental PM-normalized patterns. They are usually plotted into the field of arc volcanics (Figures 6 and 11; e.g., [77, 89, 99]).
As discussed above, our data indicate that the plagioclase amphibolites were derived from the metasomatized wedge, generally occurring in a compressive-related setting (e.g., [35–37, 57, 88, 89, 101–103]). They have low TiO2 contents (0.86–1.33 wt.%) and high Ba/Nb (35.27–59.08) and La/Nb (1.80–6.87) ratios, similar to those in arc volcanics (Figure 11), further suggesting a Mesoproterozoic active continental margin setting for the Kontum Complex. Available data show that abundant Mesoproterozoic igneous rocks have been identified in Laurentia, East Antarctica, Australia, North China, Greenland, Baltica, and Hainan Island (Table 5). However, the Mesoproterozoic (∼1430 Ma) metamorphic mafic rocks in SW Hainan have been interpreted as products in an extensional setting [4–6]. Li et al.  reported the synchronous Baoban orthogneiss and metarhyolites with A-type geochemical affinity, and believed that they were formed in a rifting setting (e.g., [15, 104, 105]). In addition, detrital zircon U–Pb age-spectra of the Kontum Precambrian sedimentary rocks defined the age-peaks of ~1853–1745 Ma and~ 1434–1464 Ma, similar to those of Laurentia and East Antarctica, and those of the syn-rift sedimentary rocks from the Lower Belt–Purcell Supergroup in west Laurentia and the Lower–Middle Rocky Cape Group in Tasmania (e.g., [5, 15, 17, 30, 45, 67, 104]). A line of evidences show that the ~1.43 Ga magmatism is characterized by banded iron formation and nonorogenic igneous rocks in the Columbian interior (e.g., Hainan, Yilgarn, and Yukon), but hardly in the Columbian periphery (e.g., [2, 4–6, 17, 21, 97, 98]). The Mesoproterozoic rifting-related extensional system is created by the breakup of the Columbian interior prior to ∼1.60 Ga in spite of the continuous assemblage along the Columbian periphery (e.g., [4, 5, 45, 46, 67, 75]). Thus, these data consistently suggest that the Indochina Block was near to the margin of the East Antarctica and Laurentia at the Mesoproterozoic period, and support the Mesoproterozoic connection of SW Hainan with SW Laurentia, southwest Baltica, and southeast India (Figure 12). The Mesoproterozoic plagioclase amphibolites in the Kontum Complex have been formed in the Columbia periphery in response to the Columbian breakup.
This paper presents detailed petrological, zircon U–Pb geochronological, and Lu–Hf isotopic data, as well as whole-rock elemental and Sr–Nd–Pb isotopic results for the plagioclase amphibolites from the Ngoc Linh unit in the Kontum Complex. These data allow us to draw the following conclusions:
The plagioclase amphibolite protoliths were formed at ~1432–1403 Ma, which experienced the Ordo–Silurian metamorphism
The plagioclase amphibolites have arc-like geochemical signatures with positive εNd (t) and εHf (t) values (from +2.6 to +2.8 and+ 9.9 to +17.1, respectively), originating from a mantle wedge modified by minor subduction-related components
The plagioclase amphibolites were formed in an active continental margin setting of the Columbian periphery at the Mesoproterozoic period
Data used to support the study can be found in this manuscript.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
We would like to thank Drs. Y. Z. Zhang, Y. K. Wang, X. H. Lu, H. L. Li, Z. T. Lai, X. Q. Yu, and C. B. Luo for their help in the field survey and formal analyses. Three anonymous reviewers and chief-in-editor are thanked for their critical and constructive comments. Financial support are from the National Natural Science Foundation of China (41830211) and the Guangdong Basic and Applied Basic Research Foundation (2018B030312007 and 2019B1515120019).