Regional geology and sampling.
The CAOB extends from the Uralides in the West to the Pacific margin in the East and is bound by the Siberian Craton to the North and the Tarim–North China Craton to the South17,18(Fig. 1). Mongolia is located in the heart of CAOB and therefore provides an important geological record of how the CAOB developed and how the various tectonic terranes relate to one another. The Mongolia has been subdivided into two tectonic domains – the “Caledonian” domain to the North and the “Hercynian” domain to the South, separated by the Main Mongolian Lineament32. The northern domain mainly consists of Precambrian microcontinents, Neoproterozoic-Cambrian subduction-accretionary complexes, early Paleozoic metamorphic rocks, and early Paleozoic sedimentary basins. In contrast, the southern domain comprises various mid- to late Paleozoic island arcs and ophiolite fragments, and Late Carboniferous to Permian volcanic rocks17,32.
The Ulgey study area is located between the Altay and Khovd terranes, and is bounded by the Tolbo–Nuur and Khovd faults33 (Supplementary Fig. S1). These two terranes mainly consist of Cambrian–Ordovician succession of clastic, volcaniclastic rocks and intermediate-mafic volcanic rocks, which are generally interpreted as a coherent accretionary volcano-sedimentary wedge developed during the subduction of oceanic lithosphere underneath Mongolian microcontinents34,35. The Ulgey area mainly consists of Early-Middle Devonian volcanics and flysch-like sediments33. Several dismembered ophiolitic fragments occur as inliers in fault contact with the Early–Middle Devonian formations33. The ophiolite fragments mainly comprise serpentinized and carbonated harzburgite, pyroxenite, layered gabbro, isotropic gabbro, metabasalt, basaltic andesite and chert (Supplementary Fig. S2).
The Trans-Altai study area includes the Gurvan Sayhan and Zoolen ophiolitic mélanges, which occur as dismembered fragments in a matrix dominated by Ordovician-Silurian greenschist-facies sedimentary and volcaniclastic rocks28,32 (Supplementary Fig. S3). The ophiolites consist of serpentinized harzburgite and lherzolite, wehrlite, gabbro, diabase, basalt, and minor diorite–tonalite–trondhjemite28,32 (Supplementary Fig. S4). Hornblendites and diorites from the Gurvan Sayhan–Zoolen mélanges yielded zircon U–Pb ages of 520–511 Ma28. Geochemical data obtained from the ophiolite fragments so far do not provide exclusion regarding tectonic settings.
Whole-rock major and trace-element compositions (15 samples from the Ulgey ophiolite, 10 samples from the Gurvan Sayhan ophiolite and 11 samples from the Zoolen ophiolite) were determined by X-ray fluorescence (XRF) and ICP-MS, respectively. The results are presented in Supplementary Table S1–S3. The major element compositions of the Cr-spinels from the ultramafic rock of the Gurvan Sayhan ophiolite were analyzed on a JEOL JXA-8100 Electron Probe, and the results are presented in Supplementary Table S4. To obtain age estimates for the crust of the three ophiolites in West Mongolia, a gabbro from the Ulgey ophiolite, a plagiogranite from the Gurvan Sayhan ophiolite and a high-Mg diorite from the Zoolen ophiolite (detailed sample locations shown in Supplementary Fig. S1 and S3) were selected for in-situ U-Pb zircon dating using secondary ionization mass spectrometry. The zircon U-Pb age results are given in Supplementary Table S5. Zircon Hf isotope analyses were performed for the gabbro sample from the Ulgey ophiolite and were performed on the same spots as U-Pb age dating. The results are listed in Supplementary Table S6. The detailed analytical procedures can be found in the Methods Section.
Whole-rock Geochemistry.
The Ulgey ophiolite
The ultramafic rocks have high loss on ignition (LOI) values (8.68–13.56 wt. %), indicating variable degrees of serpentinization. The ultramafic rocks are characterized by low SiO2 (39.85–42.30 wt. %) and alkalinity (Na2O + K2O) concentrations (0.04–0.17 wt.%), and high MgO (33.15–38.75 wt.%) and Fe2O3T concentrations (7.57–9.24 wt.%), with Mg# [= molar Mg/(Mg + Fe2+)] ranging from 0.89 to 0.92. All samples have low total rare earth element (REE) concentrations (1.04–5.63 ppm), and the rocks are depleted in high field strength elements, e.g., exhibiting Nb concentrations less than 0.1 ppm. The samples have 2013–2455 ppm Cr and 794–2068 ppm Ni.
The gabbro samples exhibit variable SiO2 (44.69–49.47 wt. %), MgO (4.56–5.30 wt. %), CaO (8.31–11.79 wt.%), and Al2O3 (14.25–17.31 wt.%) concentrations, but low Na2O (1.89–3.71 wt.%), TiO2 (1.41–2.44 wt.%), and P2O5 (0.31–0.63 wt.%) concentrations. Mg# values range from 0.43 to 0.51. The samples have REE concentrations between 53 and 126 ppm with no or positive Eu anomalies (Eu/Eu* =0.91–1.68). They exhibit chondrite-normalized light REE (LREE) enrichment (Fig. 2a). The gabbro samples are enriched in large-ion lithophile elements (LILEs; e.g., Sr, Ba, and U) and lack HFSE anomalies (Fig. 2b).
The two basalt samples from the Ulgey ophiolite have SiO2 concentrations of 47.51 and 51.79 wt.%, Al2O3 concentrations of 14.18 and 16.04 wt.%, MgO concentrations of 4.55 and 5.74 wt.%, Na2O concentrations of 3.93 and 4.17 wt.%, K2O concentrations of 0.03 and 0.20 wt.%, and TiO2 concentrations of 0.78 and 0.99 wt.%. The Mg# values are 0.55 and 0.56. In the Zr/TiO2 vs. Nb/Y diagram, the basalt samples plot in the basalt field, close to the forearc basalt (FAB) samples of Izu-Bonin-Mariana (IBM) forearc (Supplementary Fig. S5a). They have REE concentrations of 29 ppm, with distinct LREE depletion [(La/Yb)N of 0.44–0.60] and negligible Eu anomalies (Eu/Eu* =0.92 and 1.0) (Fig. 2a). In a primitive mantle normalized trace-element variation diagram, the samples are slightly enriched in LILEs (e.g., Sr and Ba) and show minor negative anomalies in Nb and Ta, overlapping with the IBM FAB samples (Fig. 2b).
The andesite samples have high MgO (4.04–6.64 wt. %), Mg# (0.42–0.57), Fe2O3T (10.08–13.12 wt. %), moderate SiO2 (54.63–60.46 wt.%), and low TiO2 (0.38–0.79 wt.%) concentrations. In the SiO2-MgO diagrams (Supplementary Fig. S5b), all the samples plot in the high-Mg andesite (HMA) area, close to the HMA samples of the IBM arc. The samples show REE concentrations of 13–22 ppm, and the chondrite-normalized REE patterns of the samples are slightly fractionated and display a positive slope with depletion of LREE [(La/Yb)N = 0.29–0.75)] and slightly negative Eu anomalies (Eu/Eu* = 0.79–0.96) (Fig. 2a). They exhibit negative Nb, Ta and Ti anomalies in primitive - mantle - normalized trace element diagrams (Fig. 2b).
The Gurvan Sayhan ophiolite
The two ultramafic rocks with extensive serpentinization and alteration have high LOI values (13.4 and 14.7 wt. %). They are characterized by low SiO2 (38.73 and 37.88 wt.%), alkalinity concentrations, but high MgO (38.10 and 38.20 wt.%), Mg# of 0.92. The concentrations of mantle compatible elements such as Cr and Ni are high in the ultramafic samples (2826 and 2251 ppm for Cr, and 2026 and 2098 ppm for Ni). The Cr-spinels from the ultramafic rock have relatively high Cr# (Cr# = Cr/(Cr + Al)) values (50–55) but low Mg# (Mg# = Mg/(Mg + Fe2+)) values (39–43) (Supplementary Table S4), and TiO2 concentrations of 0.23–0.35 wt.%, MnO concentrations of 0.31–0.45 wt.% and NiO of 0.04–0.15 wt.%. Gabbro samples have similar SiO2 (49.04 and 49.50 wt.%), MgO (6.67 and 6.22 wt.%), Al2O3 (14.21 and 14.80 wt. %), Na2O (3.45 and 3.69 wt.%) and TiO2 (1.11 and 1.16 wt.%) concentrations. They show flat REE patterns [(La/Yb)N = 1.21 and 1.14)] and negligible Eu anomalies (Eu/Eu* = 0.98 and 0.88) (Fig. 2c). The samples show enrichment in LILEs (e.g., Sr and Ba) and depleted in Nb, Ta and Ti (Fig. 2d). The basalt samples have SiO2 concentrations of 48.37–53.27 wt.%, and Al2O3, MgO, Na2O, K2O, and TiO2 concentrations of 14.96–19.25 wt.%, 2.96–4.66 wt.%, 2.60–4.86 wt.%, 0.14–2.78 wt.%, and 0.89–1.41 wt.%, respectively, and their Mg# range from 0.39 to 0.51. All the samples show distinct enrichment of LREE [(La/Yb)N = 4.3–7.9] relative to HREE with slightly negative Eu anomalies (Eu/Eu* =0.82–0.96) (Fig. 2c). In the primitive – mantle – normalized trace element variation diagram, these samples show enrichment in Ba, Th and Sr, but depleted in Nb, Ta and Ti (Fig. 2d).
The Zoolen ophiolite
The diorite samples have SiO2 concentrations of 54.35–57.51 wt.%, Al2O3 of 10.24–17.86 wt.%, MgO of 4.37–9.54 wt.% (Mg# = 0.55–0.74), Na2O of 3.43–4.79 wt.%, K2O of 0.41–1.37 wt.%, and TiO2 of 0.52–0.61 wt.%. In the SiO2-MgO diagrams, these samples plot in the HMA area (Supplementary Fig. S5b). They exhibit highly fractionated chondrite - normalized REE patterns with high LREE/HREE ratios [(La/Yb)N of 7.40–12.83] and slight Eu negative anomalies (Eu/Eu*= 0.83–0.95) (Fig. 2e). In the primitive mantle normalized spider diagram, they are show negative Nb, Ta, Zr, Hf and Ti anomalies (Fig. 2f). The basalt samples show different concentrations of SiO2 (48.08–53.37 wt.%), Al2O3 (14.14–18.75 wt.%), MgO (3.314–6.48 wt.% (Mg# = 0.44–0.57), Na2O (3.87–7.18 wt.%), K2O (0.22–1.97 wt.%), and TiO2 (0.80–1.16 wt.%). They show moderate LREE enrichment [(La/Yb)N = 3.89–4.39] and nearly no Eu anomalies (Eu/Eu* = 0.83–0.94) (Fig. 2e). In the primitive mantle normalized spider diagram, these samples display enrichment in Ba, Th and Sr, but depleted in Nb Ta, and Ti (Fig. 2f).
Zircon U-Pb Geochronology
Zircons from the gabbro sample MAT-73 collected from the Ulgey ophiolite are euhedral to subhedral prisms that are 50–300 µm long. Most zircons show oscillatory zoning (Fig. 3a), which is characteristic of igneous zircon38. Zircon U and Th concentrations are between 236 to 1913 ppm and 126 to 3177 ppm, respectively. The Th/U values are between 0.53 and 1.98, also indicating an igneous origin38. The analyses yielded a weighted mean 206Pb/238U age of 529 ± 2 Ma (MSWD = 1.5; Fig. 3b), which is interpreted as the crystallization time of zircons. Six zircon grains were selected for Hf isotope analysis. The analysis yielded positive εHf(t) values (+ 12.5 – +15.0), which would yield young model ages TDM (537–700 Ma).
Zircon grains from the plagiogranite sample MS-45 collected from the Gurvan Sayhan ophiolite are euhedral prisms that are 100–200 µm long (Fig. 3a). Most zircons show well-developed oscillatory zoning. Twelve zircon grains were chosen for analysis. The measured U and Th concentrations vary from 104 to 661 ppm and from 14 to 190 ppm, respectively, with Th/U ratios between 0.14 and 0.39. All the analyses yielded a weighted mean 206Pb/238U age of 508 ± 8 Ma (MSWD = 1.1; Fig. 3c), which is interpreted as the zircon crystallization age.
Zircon grains from the high-Mg diorite sample SM-394 collected from the Zoolen ophiolite are relatively euhedral, and their CL images show well-developed oscillatory zoning (Fig. 3a). Fourteen zircon grains were analyzed. The U and Th concentrations are 66 to 228 ppm and 109 to 114 ppm, respectively. The Th/U values are between 0.50 and 1.72. All the analyses yielded a weighted mean 206Pb/238U age of 496 ± 4 Ma (MSWD = 1.7; Fig. 3d), which can be interpreted as the time of zircon crystallization.