Zircons from the meta-gabbro are euhedral grains and show banded zones in CL images, comparable to zircons crystallized from mafic magmas (Fig. 3). Thin metamorphic rims are visible but are too narrow for U-Pb age analyses (Fig. 3). The analyzed grains display Th/U ratios of 0.5-3.0 (Table S1), which are comparable to those of magmatic zircons. Forty-two analyses from the meta-gabbro sample 17SAT13-2 yielded a concordant 206Pb/238U age range of 543–506 Ma and a weighted mean age of 518 ± 2 Ma (MSWD = 0.94, n = 40) (Fig. 3).
Zircon grains from meta-gabbro sample 17SAT13-2 display present day 176Hf/177Hf ratios of 0.282792–0.282863, equivalent to εHf(t) values of 11.1–13.6 and Hf model ages of 566–670 Ma (Fig. 4a). Zircon grains from this sample also have δ18O compositions of +2.69 ‰ – +5.7 ‰ (Fig. 4b) that are comparable to, and lower than, mantle zircons (40). Thus, zircon Hf-O isotopes argue for crystallization of zircon grains from mantle derived mafic magmas. In addition, H2O-in-zircon contents of these grains range between 109 ppmw and 1339 ppmw, with two peaks at 260 and 520 ppmw. No obvious correlation between zircon δ18O values and H2O contents has been observed (Fig. 4b).
This study focuses on the immobile elements and element ratios, considering the potential for mobilization of large ionic radius elements during metamorphism and alteration. In addition, oxides in this study were recalculated on an anhydrous (volatile-free) basis, with the oxide sum normalized to 100 %.
The meta-mafic samples display a wide compositional range with SiO2 of 45.96–52.22 wt.%, MgO of 4.75-9.91 wt.%, TiO2 of 0.1–1.08 wt.%, and Mg# of 43–75. These samples display almost flat rare earth element (REE) patterns, with no obvious Ce and Eu anomalies. They exhibit relative depletions in high field strength elements (HFSE), such as Nb and Ta (Fig. 5a, 5b). Two basaltic samples also display both LREE and HFSE depletions, whereas the gabbro sample shows minor flat light rare earth element (LREE) enrichment (Fig. 5c, 5d). The meta-intermediate samples are characterized by SiO2 of 52.98–57.32 wt.%, MgO of 3.66-5.17 wt.%, and mostly flat REE patterns and variable depletions in HFSEs such as Nb and Ta (Fig. 5a, 5b). Most of the meta-basaltic and meta-intermediate samples belong to the tholeiitic series. The ultramafic samples display a very low compositional range for TiO2 (0.1–0.09 wt.%), SiO2 (32.48–44.83 wt.%), but very high MgO (42.32-46.27 wt.%) and Mg# of 90–92. They display overall low REE contents (mostly 0.48-11.6 ppm), slight LREE enrichment and U-shaped REE patterns, with minor Nb and Ta depletions (Fig. 5c, 5d).
Less serpentinized harzburgite samples 17SAT22-1 and 18SAT41-4 and olivine pyroxenite sample 18SAT41-5 were selected for olivine and spinel compositional analyses (Fig. 2f). The olivines display a uniform composition (Fig. 6) and in the pyroxenite has Fo values of 90.9–91.7 and NiO contents of 0.256–0.524 wt.%, whereas in the harzburgite shows Fo values between 89.9 and 91.9, and NiO contents of 0.25 and 0.46 wt.% (Fig. 6b). Peridotites spinels are resistant to secondary alteration processes and preserve environmental records of peridotites formation (reference). Oxidized rims can be observed in some spinels, so analyses were only undertaken on fresh cores. The analyzed spinels display a wide compositional range. The Cr# values of spines in the olivine pyroxenite (26–77) are lower than depleted harzburgite (71-84) and a linear trend of Cr# vs Mg# values for spinels can also be observed (Fig. 6c). TiO2 contents are lower in the olivine pyroxenite (0-0.14 wt.%) than those in the harzburgites (0.11-0.20 wt.%) (Fig. 6d), whereas the NiO contents display a decrease from olivine pyroxenite to harzburgite.
Chemical duality of the Munabulake ophiolite: progressive evolution from MORB to SSZ compost ions during intra-oceanic subduction initiation
The mantle members of the Munabulake ophiolite consists of residual dunite, harzburgite, and olivine pyroxenite (Fig. 1b). The negatively correlated Cr# vs Mg# values of spinels within harzburgite and olivine pyroxenite samples suggest that the investigated rocks represent residual mantle, with the olivine pyroxenite related to low degrees of partial melting and melt extraction and the harzburgite to higher degrees (Fig. 6b). The correlation between Mg# values of olivines and the Cr# values of spinels is conformable with the olivine–spinel mantle array (Fig. 6c), also indicating the peridotite samples are residues of various degrees of melt extraction. Spinel TiO2 vs Cr# trend suggests a similar magma process (Fig. 6d). The studied residue ultramafic samples have higher Mg# (> 90) values than primitive mantle, and also display U–shaped REE patterns and that are indicative of melt introduction (Fig. 5c), consistent with high flux and volatile conditions in a supra-subduction zone setting. Melt or fluid metasomatism can also be observed from tremolite and enclosure of olivine grains in orthopyroxene in peridotite, as are their varied Zr/Hf ratios. The magma evolution thus reflects progressive source depletion coupled with increasing melt or fluid metasomatism. Therefore, compositions of ultramafic rocks and spinel-olivine minerals are consistent with progressive chemical evolution from abyssal to arc-related peridotite (Fig. 6).
The meta-basaltic and intermediate samples from the Munabulake ophiolite have high MgO contents (5.12-9.95 wt.% for mafic samples and 3.54-5.27 wt.% for intermediate samples), low TiO2 (< 1 wt.%), mostly flat REE patterns, along with obvious depletions of Nb and Ta, and very minor Ti depletions for some samples. LREE depleted patterns are also observed in two samples. In addition, the investigated mafic-intermediate samples are characterized by elevated Th in the Nb/Yb vs. Th/Yb diagram (Fig. S1a), where most of the samples lie above the mantle array, indicating Th enrichment in their mantle source, which is consistent with fluid flux melting and was likely derived from subducted slab sediment. Moreover, U/Th-Th/Nb and Ba/Th-La/Sm diagrams (not shown) also favor involvement of hydrous fluids from altered oceanic crust. This chemical evolution trend indicates increasing source metasomatism by slab-derived material (elevating Th, U, and some LREE).
The analyzed samples plot in the SSZ type ophiolite field in various tectonic discrimination diagrams, including Ta/Yb vs. Th/Yb and Ti vs. V (Fig. S1c). Their lower TiO2 (< 1.25 wt. %), Zr/Y (< 3), Nb/La (< 0.5), Hf/Th (mostly < 2), and Ta/Yb (< 0.1), along with higher Th/Nb (> 0.2) and Th/Yb (> 0.1) (Table S2), are comparable to those of oceanic arc basalts (41). Although these compositional features indicate metasomatism by a subducted slab, their flat REE patterns argue against an island arc setting (Fig. 5a), as do their mostly tholeiitic compositions. In addition, even compared to lavas of nascent oceanic island arcs such as Saipan-Rota-Guam (42), the investigated mafic-intermediate samples lack marked Eu and Ti anomalies (Fig. 5b). This indicates no plagioclase fractionation and very minor rutile fractionation. These signatures suggest that no normal arc or felsic crust had formed at the time of ophiolite formation. In addition, the higher Y/Zr values of these igneous rocks argue for derivation from depleted mantle source (11), whereas Hf/Nb versus Zr/Nb suggests magma enrichment by subducted fluids (43). The overall chemical signatures and evolution trends are comparable to crustal members of the IBM and Tethyan ophiolites (Fig. 5, S1; 13-14), consistent with the observations from Munabulake mantle end-members. More importantly, H2O contents in mantle zircons within the ophiolitic gabbro indicate similar processes, with two populations at 160-320 ppmw and 480-640 ppmw (Fig. 4b). The zircons with low H2O content are comparable to MORB zircons (44), whereas the zircons with higher water content indicate possible progressive magma hydration due to formation of a subduction zone. In addition to the mantle zircons in the ophiolite, zircons with lower δ18O isotopes (+2.69 ‰ – +5.0 ‰, peaked at 4.7‰) are also observed, which are comparable to altered oceanic crust (45). All the observations suggest linkages between progressive source depletion and metasomatism due to slab-derived fluids, which is commonly expected for a subduction initiation ophiolite and is consistent with observations from mantle end-members. However, it is noteworthy that the mafic-intermediate units occur within a ductile shearing zone and were subjected to multi-stage higher grade metamorphism and shearing. Thus, detailed reconstruction of overall chemo-stratigraphy is difficult.
Direct record of the oldest oceanic subduction initiation at ca. 518 Ma
The south Altyn margin is characterized by the following: 1) Munabulake ophiolite dated at 518 Ma; 2) the 508-475 Ma UHP-HP metamorphism (27); 3) ca. 508-505 Ma arc related magmatism at a few localities to the north of the Munabulake ophiolite (author unpublished data); 4) ca. 510-500 Ma MORB type mafic-ultramafic rocks and ca. 503 Ma adakite-diorite (Fig. 5e, 5f, 5g, 5h, S1a; 37 and reference therein); 5) ca. 517 Ma oceanic type adakite, along with calc-alkaline granitoids dated at ca. 503–497 Ma occurring in various localities (Fig. 5g, 5h, S1a; e.g. 34-38); and, 6) sinistral shearing along the fault zone at southern margin of the Munabulake ophiolite sometime after ca. 235 Ma (Fig. 1b) (26, 31).
The overall ages and field relations indicate that the 518 Ma Munabulake ophiolite is the oldest oceanic succession in the Southern Altyn, followed by establishment of an intra-oceanic arc system lasting some 20 Ma at least until ca. 500 Ma, as is inferred from the 510-500 Ma MORB type mafic-ultramafic suites and ca. 503–497 Ma arc type granitoids and adakite-diorite. Thus, the crustal and mantle members of the Munabulake ophiolite, along with available data across the south Altyn, agree with the subduction initiation signature of the 518 Ma ophiolite, which we conclude to be have been formed during subduction initiation of the Proto-Tethys Ocean, in a scenario similar to the IBM ophiolites (Fig. S1). The reported ca. 517 Ma oceanic type adakite was generated by partial melting of oceanic crust in a newly formed subduction zone. Moreover, given the large-scale sinistral shearing of the south Altyn fault zone, we infer that the Munabulake ophiolite should be a member of the south Altyn ophiolite belt and was offset to current location by strike-slip motion of the fault system.
Global plate re-organization at ca. 530-520 Ma during Gondwana assembly
The Munabulake ophiolite dates the subduction initiation of the Proto-Tethys Ocean in the Altyn segment, but the overall subduction initiation of this ocean is not well constrained and earlier inferred individual subduction zones have largely been treated in isolation (e.g., 19, 21, 25, 28, 46-47). In addition, the ocean has been given a number of localized names adjacent to the variety of continental and arc related blocks in East Asia that are inferred to lie within the ocean. The early Paleozoic oceanic successions in these blocks are related to the evolution of the ocean and accretion of these blocks to the northern Gondwana margin (e.g., 19-21, 47). Nevertheless, here in this study, a time-space plot of early Paleozoic ophiolites and trench-arc assemblages across East Asia blocks, along with related magmatic and metamorphic events, enables determination of overall timing of initial oceanic subduction of the Proto-Tethys Ocean (Fig. 7). In particular, elsewhere across East Asia, ages of initiation of subduction are inferred from the oldest arc magmatism, which are in accordance with stratigraphy and ages of ophiolite and metamorphic event (Fig. 7). We conclude that the main branch of the Proto-Tethys Ocean principally subducted northward and commenced at ca. 533 Ma in the West Kunlun segment (25-26), at ca. 525-520 Ma in the Altyn-Qaidam-Qilian segments (28, 46; this study), and at ca. 515 Ma in the North Qinling segment (Fig. 7, 8; 47). Locally, possible isolated subduction zones in the Qiangtang and Indochina segments commenced at sometime around 535 Ma and 490 Ma (19, 21, 49), but these dates are not well constrained. Therefore, overall timing of oceanic subduction initiation of the Proto-Tethys Ocean displays an eastward younging direction from ca. 533 Ma in the west segment to ca. 515 Ma in the east most segment.
It is noteworthy that the timing of initial subduction of the Proto-Tethys Ocean coincides well with the timing of slab roll-back of the Pacific Ocean in the southern Gondwana margin (50) (Fig. 8). The extension regime in the southern Gondwana and expression regime in the northern Gondwana was thus strictly correlated, indicative of global plate re-organization at this time, which is linked to final collisional assembly of Gondwana.
Subduction initiation ophiolites in Earth history: initiation of modern plate tectonic regime
Ophiolites formed during simultaneous subduction initiation of a modern Mariana type oceanic subduction zone include the 52 Ma Izu-Bonin-Mariana ophiolites and Tonga ophiolite in west Pacific (9, 11-12, 14), 100-90 Ma Tethyan ophiolites (13, 15), the ca. 335 Ma Paleo-Asian Ocean ophiolite (16), 490-485 Ma Appalachian-Caledonian ophiolites (17-18), and the 518 Ma Munabulake ophiolite (Proto-Tethys ophiolite) in northern Gondwana margin in this study (Fig. 8). Given the conditions required for simultaneous initiation of stable one-sided modern Mariana type oceanic subduction (2), the key factors of oceanic plate tectonic regime, slab strength in particular, must have been comparable at least since the early Cambrian time. This was controlled by progressive cooling of mantle which increased slab strength (3, 51). High slab strength, along with comparable ophiolite characteristics and scenarios of simultaneous oceanic subduction initiation since the early Cambrian, coincides well with the inferred transition from early plate tectonic regime to modern plate tectonic regime in the Neoproterozoic- Cambrian (7, 52). Earlier conclusions on the transition of tectonic regime in the Neoproterozoic- Cambrian are mostly based on records of the Phanerozoic low T/P UHP metamorphic rocks and geodynamic modelling results that indicate drops of mantle temperatures below ΔT= 80-100°C at this time (Fig. 9), and thus plate strength was strong enough for deep plate subduction (1, 6). On the other hand, the subduction initiation ophiolites discussed in this study also argue for modern plate tectonics at least since the earliest Phanerozoic, consistent with significant drops of mean thermobaric ratios at 525 Ma (Fig. 9). In addition, recent works also favor formation of Earth inner core in the Ediacaran (53), which might have feedback effects on plate tectonic regime. We suggest that following the global plate re-organization at ca. 530-520 Ma during Gondwana assembly, a plate tectonic regime with products and slab strength similar to modern day Earth was achieved.