Xigaze Ophiolite Research Articles

Evolution of the Neo-Tethys traced by Late Triassic to Early Cretaceous marine remnants in the eastern segment of the Yarlung-Tsangpo suture zone, southern Tibet

Widely distributed siliceous mélanges in the Yarlung-Tsangpo suture zone preserve significant information of the Neo-Tethys and subduction-accretion processes. Similar ophiolites and siliceous mélanges are well exposed in the Xigaze and Zedong areas, however, their relationship including age assignments, depositional environments, and accretionary process remains vague. Based on radiolarian biostratigraphy (56 species belonging to 35 genera), black cherts from the Carnian to the Norian (Late Triassic), red cherts from the Aalenian to Bathonian (Middle Jurassic), and siliceous mudstones of the Berriasian (Early Cretaceous) are distinguished from siliceous mélange in the Zedong area. Values of Al/(Al + Fe + Mn) indicate that the siliceous rocks are biogenic silica derived. The diagrams of Al2O3/(Al2O3 + Fe2O3) vs Fe2O3/TiO2 and Al2O3/(Al2O3 + Fe2O3) vs LaN/CeN indicate siliceous rocks were deposited far from the ridge. Weak positive Ce anomaly and low ratio of LaN/CeN indicate that siliceous rocks were deposited in a deep-sea basin proximal to terrigenous sources. Comprehensive summary of studies on Bainang siliceous mélange, Xigaze ophiolite, Zedong ophiolite, and Zedong siliceous mélange reconstructs the Ocean Plate Stratigraphy (OPS) and the evolution of the Neo-Tethys since the Late Triassic till the Early Cretaceous. Mélanges in Xigaze and Zedong were formed in different intra–oceanic subduction-accretion systems at different times in the Neo-Tethys Ocean during the Cretaceous.

Multistage and diverse melt-mantle interaction in dunite-harzburgite channel systems beneath oceanic slow-ultraslow spreading centers: Evidence from the Xigaze ophiolite (Tibet)

High-porosity, melt-focusing channel systems (comprising reactive dunite and its surrounding harzburgite) reflect an essential type of melt extraction within the mantle under mid-ocean ridges, proposed mainly by investigations of ophiolites produced in fast-spreading centers. However, relevant melt-migration processes from mantle wall rocks into dunitic channels under slow-ultraslow spreading centers and their effects are less well understood. We present systematic petrographic observations and whole-rock/mineral compositional analyses of a dunite-harzburgite channel system (∼250 m in width) within the Dazhuka mantle section (∼3–4 km thick) of the Xigaze ophiolite (South Tibet), produced in a Neo-Tethyan slow-ultraslow spreading center. The harzburgites, closely surrounding a dunite-rich zone, show large variations in pyroxene/olivine ratios (0.1–0.6), whole-rock MgO (41.2–45.9 wt%), Al2O3 (0.23–1.87 wt%) and CaO (0.38–2.80 wt%), as well as spinel Cr# (molar Cr3+/(Cr3++Al3+), 0.20–0.73) and pyroxene major and trace elements, almost covering the whole ranges of the Xigaze ophiolitic mantle. The harzburgites closer to the dunite-rich zone are gradually more depleted than those further ones, and finally replaced by the most depleted dunites. Modeling of decompressional melting from a depleted-MORB-mantle (DMM) source shows that the harzburgitic compositions correspond to melt depletion degrees of ∼6.9–19.2%, requiring a ∼30-km solely decompressional distance that is much greater than the sampling size of the channel system. It suggests that the wall-rock harzburgites can be produced by the reaction between high-fluxing, silica-undersaturated melts and the least-depleted harzburgites during the formation of reactive dunite-harzburgite channels. In addition, clinopyroxenes in dunites and some harzburgites show metasomatic enrichments, reflected in higher Na2O (up to 0.32 wt%), TiO2 (up to 0.17 wt%) and concentrations of Zr, Hf, light rare earth elements (LREE) and fluid-mobile elements relative to those in other harzburgites. These distinct signatures suggest the later localized metasomatism by batches of basaltic melts (some are volatile-rich) flowing through the harzburgitic wall rocks into the dunite-rich zone. We therefore propose that the dunite-harzburgite channel system archives at least the formation of reactive channels in deep upwelling asthenosphere during the high-melt-flux stage, and the subsequent metasomatic enrichments by basaltic melts reusing the channels in the shallow asthenosphere. The multistage and diverse melt-mantle interaction styles may characterize the melt extraction processes under oceanic slow-ultraslow spreading centers.

Formation of the Xigaze Metamorphic Sole under Tibetan continental lithosphere reveals generic characteristics of subduction initiation

Metamorphic soles found under allochthonous oceanic lithosphere, or ophiolites, are interpreted as derived from lower plate oceanic crust material accreted to upper plate mantle during intraoceanic subduction initiation. Their metamorphic evolution is inferred to reflect the thermal structure at the site of subduction nucleation, with granulite-bearing soles linked to initiation at hot spreading centers. Here we present garnet Lu-Hf geochronology for the granulite-bearing sole of the Xigaze ophiolite in South Tibet, whose oceanic crust formed ∼130 Ma through continental forearc extension. Our study shows that sole metamorphism was ongoing by 144 Ma, implying that north-directed subduction began at least 14 million years before oceanic forearc spreading. The upper plate at the time of subduction initiation was thus continental, not oceanic. Our results demonstrate that metamorphic characteristics of soles are independent of the specific tectonic setting at the subduction nucleation site and rather provide generic constraints on the subduction initiation process.

Generation of ultraslow-spreading oceanic crust traced by various mafic blocks from ophiolitic mélange in the Xigaze Ophiolites, southern Tibet

Generation of ultraslow-spreading oceanic crust traced by various mafic blocks from ophiolitic mélange in the Xigaze Ophiolites, southern Tibet

Magnesium–oxygen isotope constraints on the origin of rodingites in oceanic lithosphere

Rodingite is produced by Ca-rich fluid metasomatism of mafic igneous rocks in oceanic lithosphere. Despite its importance for the composition of subduction zone magmas, its formation mechanism still remains obscure. To address this issue, we investigated the major-trace element and Mg–O–Sr–Nd isotope compositions of rodingites in serpentinites from the Xigaze ophiolite, southern Tibet. Coexisting gabbros and rodingites show consistently depleted SrNd isotope compositions, suggesting their formation in a mid-ocean ridge setting. The rodingites exhibit variable δ26Mg values from −0.61‰ to −0.14‰, with an average of −0.37‰ ± 0.28‰ (n = 19; 2 SD), mostly lower than those of −0.26‰ to −0.13‰ for the protolith gabbros. Compared to the relatively homogeneous δ18O values of 6.1‰–6.7‰ for the gabbros, the rodingites show highly variable δ18O values from 3.8‰ to 9.4‰. The rodingites can be subdivided into two groups based on mineral assemblages and formation conditions. Group I rodingite is prehnite-dominated and hydrogarnet-free, formed at temperatures of 200–300 °C, and has higher δ18O values of 8.1‰ ± 1.4‰ (n = 15) and δ26Mg values of −0.32‰ ± 0.19‰ (n = 12). Group II rodingite consists mainly of hydrogarnet and clinopyroxene, formed at temperatures of 300–400 °C, and has lower δ18O values of 5.4‰ ± 2.5‰ (n = 7) and δ26Mg values of −0.44‰ ± 0.35‰ (n = 7). Modeling of the fluid–rock interactions between the protolith gabbro and fluid was undertaken using the MgO isotope compositions of the serpentinizing fluid inferred from the associated serpentinites. Based on the modeling results, we propose that the metasomatism of gabbros by Ca-rich, low-δ26Mg fluids produced during serpentinization would result in the MgO isotope compositions of the rodingites. Such rodingites with low δ26Mg values would generate the geochemical heterogeneity in the mantle wedge overlying the subducting oceanic lithosphere and eventually give rise to Ca-rich, low-δ26Mg basaltic magmas. The results indicate that low δ26Mg values of basaltic rocks cannot be unambiguously ascribed to carbonated mantle sources.

Shearing-enhanced deep fluid circulation induces seismic anisotropy in the lower crust at slow-spreading oceanic ridges

AbstractAlthough long-lived detachment faulting plays an important role in fluid circulation and in accommodating tectonic extension at slow-spreading oceanic ridges, it is still unclear how the fluid-enriched faults contribute to the observed seismic anisotropy in the lower crust. We investigated sheared and altered gabbros along the detachment fault zones from the Xigaze ophiolite in the southern Tibetan Plateau. Results demonstrate that the positive feedback between fluid circulation and shearing, linked by dissolution-precipitation creep of amphibole, resulted in fluid enrichment during strain localization along the fault zones. Based on this shearing-enhanced fluid circulation model, our calculations of the seismic properties show that amphiboles (de)formed by dissolution-precipitation creep along the fault zones largely contribute to the seismic anisotropy (P and S waves) and S-wave delay time in the lower crust at slow-spreading ridges, with the polarization directions of fast shear waves being subparallel to the ridges. The strength of resulting seismic anisotropy is largely a function of crustal thickness, fault zone attitude, and metasomatism intensity. This study provides a novel explanation for the origin of seismic anisotropy in the lower oceanic crust at slow-spreading ridges. The conclusion may also have implications for the origin of seismic anisotropy at fast-spreading ridges where there are high melt supplies.

Detrital zircon U–Pb geochronology and fluvial basin evolution of the Liuqu Conglomerate within the Yarlung Zangbo Suture Zone: A critical geochronometer for the collision tectonics of the Tibetan-Himalayan Orogenic Belt

We present new U-Pb detrital zircon ages, depositional history and tectonic model for the Liuqu Conglomerate (LQC) in southern Tibet that represents a critical geochronometer for the collision history of the Tibetan-Himalayan Orogenic Belt. LQC is a ∼5 km–thick, late Mesozoic–Cenozoic molasse deposit occurring strictly within the Yarlung Zangbo Suture Zone (YZSZ) and is tectonically overlain to the north by the Cretaceous Xigaze ophiolite and to the south by the Mesozoic Tethyan Himalaya sequence. It consists of matrix- and clast-supported conglomerates with sandstone intercalations, and its matrix includes poorly to moderately sorted sandstone and mudstone. New U–Pb detrital zircon dating of LQC sandstones has revealed a youngest zircon age of 307 ± 13 Ma and an oldest zircon age of 3362 ± 51 Ma. The age spectrum of zircons displays a prominent peak of ∼935 Ma, two large peaks at ∼516 Ma and 1474 Ma, and two small clusters of ∼2429 Ma and ∼2772 Ma that point to East Gondwana as the likely provenance for the LQC depocenter. The LQC represents fluvial deposits of an axial river system, which developed in an orogen-parallel, transtensional accommodation space within the YZSZ, after the collision of the Late Jurassic-Early Cretaceous Trans–Tethyan arc–trench system with the northern edge of India in the latest Cretaceous. The Indian subcontinent with the accreted Tethyan ophiolites and the intra–suture LQC depocenter arrived at and collided with the active margin of Eurasia during the latest Oligocene (∼23 Ma). The LQC depocenter started receiving clastic material and zircons for the first time from the Gangdese Magmatic Belt and the Xigaze forearc basin to the north by ∼20 Ma. The ensuing continent–continent collision resulted in significant crustal uplift across the collision zone, and in the inversion and rapid exhumation of the LQC strata by the early–Middle Miocene. The depositional and exhumation history of the fluvial LQC formation within the YZSZ involved two discrete collision events during the evolution of the Tibetan-Himalayan Orogenic system.

Accretion Processes of Oceanic Crust in Slow‐spreading Ridges: Plagiogranite Perspective of the Xigaze Ophiolite, South Tibet

AbstractStructural and petrological data suggest that the Xigaze ophiolite from the Yarlung Zangbo Suture Zone (YZSZ) in south Tibet was a typical slow‐spreading ridge. A new field, geochemical, mineral, and U‐Pb zircon dataset of plagiogranite intrusions were used to constrain the dynamic processes of oceanic accretion in this slow‐spreading ridge. Plagiogranites mainly occur as dykes or intrusions intruded into the whole sequence of the ophiolite and have a similar orientation to the dolerite dykes developed in the late stage of detachment faulting. U‐Pb zircon ages of 122–123 Ma were obtained for two types of plagiogranites and associated dolerite dykes. Detailed geochemical and mineralogical examinations suggest that the plagiogranites are the product of low‐pressure (2–3 kbar) fractional crystallization of mid‐ocean ridge basalt‐like magma and unlikely to have been derived from the partial melting of hydrous gabbroic rocks. The complex cross‐cut relationship between the plagiogranites and ophiolite sequence reflects that they are controlled by small discontinued melt lenses rather than a big magma chamber under the ridge axis and reveals multiple injections during the oceanic crust accretion. The formation of plagiogranites possibly reflects the complex characteristic of oceanic accretion at slow‐spreading ridges, time‐dependent on structural (external) and magmatic (internal) processes.

Element mobility and Mg isotope fractionation during peridotite serpentinization

Serpentinite plays a crucial role in element mobility during seawater hydrothermal alteration of mid-ocean ridge peridotite on the seafloor and fluid metasomatism of peridotite in the subduction zone. However, element mobility and Mg isotope fractionation during serpentinization of peridotites are still uncertain. In order to address this issue, we present a geochemical study of serpentinites and their associated peridotites in the Xigaze ophiolite from southern Tibet. The serpentinites have lower MgO contents and Mg/Si ratios than the peridotites. The low Mg/Si ratios in serpentinites, along with the Isocon calculation results, indicate the Mg loss during serpentinization. In terms of trace elements, the serpentinites have higher contents of Ba, U, Sr and Pb than the peridotites, though both types of ultramafic rocks exhibit similar REE distribution patterns and HFSE contents. The serpentine-magnetite O isotope thermometry yields a relatively low serpentinization temperatures of 200–300 °C. The trace element composition of the serpentinites is distinct from that of forearc serpentinites but consistent with that of mid-ocean ridge ones. The peridotites show mantle-like δ26Mg values of –0.27 to –0.23‰ with an average of –0.25 ± 0.03‰ (2SD, n = 6). In contrast, the serpentinites have systematically higher δ26Mg values of –0.21 to –0.09‰ with an average of –0.16 ± 0.09‰ (2SD, n = 8). The Mg isotope composition of serpentinites shows no correlations with either the chemical weathering proxy or the talc modal content, indicating that the Mg isotope fractionation between serpentinite and peridotite is not caused by chemical weathering of the serpentinites. Instead, it probably resulted from the serpentinization of peridotite on the seafloor. Thus, there is the loss of not only element Mg but also isotopically light Mg during the serpentinization of peridotites. The isotopically heavy Mg serpentinites can be carried to oceanic subduction zones, giving rise to higher δ26Mg mantle sources for various basalts. The isotopically light Mg can be released into the seawater, regulating the Mg isotope composition of seawater in geological history. Taking into account the sources and sinks of Mg in subduction zone fluids, the formation of isotopically heavy Mg serpentinites during seafloor serpentinization can be probably an important process in changing the Mg isotope composition of mantle sources.

Crust sequences of the Xigaze ophiolite in South Tibet: The water effect on crust accretion at a slow-spreading oceanic ridge and its implications

The tectonic setting of the Xigaze ophiolite of southern Tibet has been debated ever since it was first described 1981. It lies in the central part of the Yarlung Zangbo but differs from adjacent ophiolite by having a relatively thin crustal section, an oceanic core complex and complex geochemistry. It is overlain by radiolarian cherts suggesting and open ocean environment but also show geochemical evidence of suprasubduction zone magmatic activity. Our detailed study of this massif shows that the crustal sequence was formed by three stages of magma injection derived from a common parental source. Stage 1 is represented by heavily altered and deformed gabbros that form dikes and pods in the upper part of the peridotites, whereas stage 2 includes dolerite sills and extruded basalts, which form >80% of the crustal section. Finally, stage 3 is composed of dolerite dikes and gabbro intrusions, which crosscut rocks of stage 1 and 2 at high angles. The youngest rocks of stage 3 are plagioclase wehrlite intrusions, which are characterized by having anomalously high calcium plagioclase (An = 99–100) which was preceded by clinopyroxene. We suggest that the multiple magma injection were formed during tectonic extension and magma-fluid interaction at a slow-spreading rift in a distal fore-arc environment during the Early Cretaceous. Our study implies that caution must be exercised in applying subduction initiation models to ophiolite genesis, particularly when the geochemistry of the rocks contradicts the field observations and stratigraphic evidence. • Provides new information on the crustal structure of slow-spreading ridges. • Detachment faults served as fluid conduits changing the fractional crystallization. • Geochemical changes do not necessarily reflect the change of tectonic setting. • Both ridge spreading and subduction dynamics were active in a SSZ environment.

Petrogenesis of mafic rocks from the Xigaze ophiolite, Southern Tibet: Insights into forearc extension induced by Neotethyan rollback

The Xigaze ophiolite in the central Yarlung Zangbo suture zone (southern Tibet) provides evidence of the tectonic processes that formed the Neotethyan Ocean between the Indian and Eurasian plates. In this study, we present zircon U Pb geochronological, mineralogical, and geochemical data for mafic rocks from the Xigaze ophiolite to determine their petrogenesis and tectonic environment. The crustal section of the Xigaze ophiolite includes mainly gabbro lenses, amphibolite dikes, dolerite sills or dikes, and basaltic lavas. The absence of a well-developed gabbroic crust and sheeted dike complexes in the ophiolite indicates slow to ultraslow-spreading conditions and a heterogeneous structure of the oceanic lithosphere, which is not atypical for layered ophiolites. Zircons from the amphibolite and dolerite dike reveal crystallization ages of 125.0 ± 1.8 Ma to 123.8 ± 1.3 Ma, respectively. Geochemically, all mafic rocks have mid-ocean ridge basalt (MORB)-like affinities with enrichment in large ion lithophile elements (LILEs, U) relative to high field strength elements (HFSEs, Nb, Ta and Ti) and light rare earth elements (LREEs). They also exhibit high εNd(t) values ranging from +8.4 to +8.8 at 206 Pb/ 204 Pb = 16.75–17.64. Moreover, chromite and clinopyroxene grains from the mafic rocks have a wide compositional spectrum and are mainly plotted in the supra-subduction zone (SSZ) fields. These features indicate that MORB-like magmatism was triggered by decompression melting of the upwelling asthenospheric mantle with minor mass transfer from the subducting slab. The Xigaze ophiolite lacks boninite, high‑magnesium andesites, island arc tholeiites, and calc-alkaline lavas, which indicates that it is incompatible with the complete Izu-Bonin-Mariana forearc sequence. Combined with previous studies, we speculate that the Xigaze ophiolite represents an ancient SSZ-type oceanic lithosphere formed in the Gangdese forearc during the Early Cretaceous, and the upper plate extension is linked to the southward retreat and rollback of the Neotethyan slab. Thus, the Xigaze and other Neotethyan SSZ ophiolites with no systematic change in composition with stratigraphic age provide new insights into the diversity of melt evolution in the forearc crust. • Amphibolite and dolerite dikes in the Xigaze ophiolite were formed during the Early Cretaceous. • The mafic rocks were derived from upwelling asthenosphere with minor slab input. • The Xigaze ophiolite originated from the Gangdese forearc setting. • The SIR model appears likely to be oversimplified for variable forearc ophiolites.

Xigaze ophiolite (South Tibet) records complex melt-fluid-peridotite interaction in the crust-mantle transition zone beneath oceanic slow-ultraslow spreading centers

The formation and evolution of the crust-mantle transition zone (CMTZ) under oceanic slow-ultraslow spreading centers, compared with the well-documented examples that developed under fast spreading centers, remain largely unknown due to the lack of suitable targets. In this study, we systematically examine the CMTZ of the Xigaze ophiolite in the Yarlung Zangbo suture zone (south Tibet). This ophiolite represents a rare lithospheric fragment produced in oceanic slow-ultraslow spreading settings. We conduct detailed field mapping and petrological as well as geochemical studies on the CMTZ in the Dazhuka massif, the easternmost segment of the Xigaze ophiolite. Our aim is to characterize the lithological architecture and associated melt-fluid-peridotite interaction history of the CMTZ. The CMTZ (~1800 m thick) in the Dazhuka ophiolite consists of 4 subzones with complex lithological associations. They range successively from clinopyroxene-rich harzburgite with weak metasomatism and without amphibole in Zone 1 (the bottom) to clinopyroxene-poor harzburgite with strong melt impregnation and more amphibole in Zone 3 (the upper part, with Zone 2 transitional harzburgite in between), all cut by a wealth of dyke rocks (gabbro, dolerite and dunite). The Al2O3 contents decrease consistently in whole-rock (2.97–0.57 wt%), orthopyroxene (4.99–0.85 wt%) and clinopyroxene (5.25–1.22 wt%) for the harzburgites from Zones 1–3, contrary to the trends for the spinel Cr# (0.17–0.65) and clinopyroxene Li/Y, and bulk rare earth element (REE), Pb and Sr contents. The plagioclase-bearing peridotite (commonly enclosed in layered gabbro) in Zone 4 (the top) has remarkably elevated TiO2 contents (0.12–0.34 wt%) and Cr# values (0.45–0.53) in spinel, suggesting equilibration and strongest interaction with MORB-like melts among the CMTZ. The troctolite in Zone 4 has olivine Mg# (80.6–84.2) and NiO (0.37–0.45 wt%) and spinel Cr# (0.68–0.73). The fluid-mobile elements (U, Pb, Sr and Li) in both whole rock and clinopyroxene as well as amphibole abundances increase consistently from Zones 1–4, implying that the interaction of the mantle rocks with hydrothermal fluids became increasingly intensive from bottom to top of the CMTZ.These vertical variations collectively suggest the complex melt-fluid-peridotite interaction during the upward movement of deeply sourced melts and the downward penetration of seawater. We propose that such interactions are ultimately controlled by the relative slow spreading rates, where the less melt supply and very thin or even missing oceanic crust facilitate the downward seawater injection and enhance pervasive fluid metasomatism. This may represent a suitable explanation for the CMTZ with intriguing lithological and chemical heterogeneity under many other active or fossil oceanic slow-ultraslow spreading environments.

Provenance of Late Cretaceous accretionary complex within the Yarlung–Zangpo suture zone, Bainang, southern Tibet: Implications for the subduction–accretion of the Neo-Tethyan ocean

Accretionary complexes offer important source of information for studying the subduction history and source-to-sink system and neighboring terranes. In this study, we conducted field mapping, detrital zircon U–Pb age and trace element analyses, and electronic microprobe composition analysis of detrital Cr-spinel in sandstones from the Cretaceous Bainang accretionary complex in the central part of the Yarlung–Zangpo suture zone (YZSZ). The Bainang accretionary complex is located in a structural window that is mainly composed of Xigaze ophiolite, Late Triassic–Early Cretaceous radiolarite chert with a Late Cretaceous hemipelagic siliceous shale matrix, and Late Cretaceous sandstone. The obtained detrital zircon geochronological dataset confirms two spectra. Detrital zircons from the Canggasaba and Lianxiang sections are dominated by Precambrian grains with subordinate numbers of Mesozoic grains clustering at 243–178, 164–117, and 109–91 Ma, consistent with the mélange and trench-fill Rongmawa Formation, and all the trace elements of the Mesozoic grains match continental arc affinity. Provenance analysis suggests that detritus in these sections was mainly derived from the eastern part of the Gangdese arc and the central–northern Lhasa terrane. These results confirm the existence of the ancestral Lhasa River that transports sediments from the eastern Lhasa terrane to the subduction zone and further westward along the east–west axial system parallel to the trench. Conversely, the Qiajiu section is mainly dominated by Mesozoic detrital zircon grains, representing submarine fan deposits that were sourced from the adjacent Gangdese arc and transported through a local north–south-flowing river system. Detrital Cr-spinel compositions and detrital zircon trace elements from the Qiajiu section also revealed the contribution of ophiolites. Considering the abundance of Eurasia-affinity detrital zircons, this study supports the single subduction model that was developed along the southern margin of the Lhasa terrane.

Rodingitization records from ocean-floor to high pressure metamorphism in the Xigaze ophiolite, southern Tibet

Abundant rodingite was found within serpentinized peridotite and spatially close to gabbro and diabase dykes in the Xigaze ophiolite of the Yarlung Zangbo suture zone. Two types of rodingite are identified based on field- and textural relationships as well as on mineral- and bulk-rock compositions. Type I rodingite with porphyroblastic texture occurs along extensional fractures within massive serpentinized peridotite and contains two subtypes representing the early stage of rodingitization. Type II rodingite is associated with a foliated serpentinized host rock and contains three subtypes, characterized by prehnite-absent mineral assemblages with fine-grained porphyritic to aphanitic texture. All rodingite II subtypes experienced a higher degree of rodingitization than Type I. Type I rodingite was formed by metasomatism of CaO-rich fluids from serpentinized peridotite during ocean floor metamorphism at ~120–130 Ma. Thermodynamic modeling shows that Type I rodingite originated at low P-T conditions of <~4 kbar and ~200–300 °C during limited desilication and calcium enrichment. within a highly reducing environment (logƒO2 < −35.3). Type II rodingite originated during subduction starting at ~120 Ma with most intense desilication and calcium increase under peak P-T conditions up to ~13.5–21.5 kbar and ~450–550 °C. Modeled values of logƒO2 for the formation of rodingite II subtypes vary from highly oxidized conditions of logƒO2 = −19.7 to −18.6 to relatively reducing conditions of logƒO2 < ~−26. P-XCO2 pseudosection modelling predicted Type II rodingite to have formed under CO2-poor conditions at XCO2 < ~1 × 10-4 during subduction to the maximum depth. Hence the rodingite of the Xigaze ophiolite provides important fingerprints recording events during ocean floor spreading and different stages of subduction as well as late exhumation.

Tectonic Controls on Block Rotation and Sheeted Sill Emplacement in the Xigaze Ophiolite (Tibet): The Construction Mode of Slow‐Spreading and Ultraslow‐Spreading Oceanic Crusts

AbstractThe internal structure of oceanic crusts is not well understood due to the limitation of deep drilling. However, that of ophiolites, i.e., on‐land ancient analogs of oceanic lithosphere, could be precisely mapped and measured. The Xigaze ophiolite in Tibet has been regarded as “peculiar”, due to the sheeted sill complex in its upper crust, and non‐sheeted diabase sills/dikes crosscutting its mantle and lower crust, which are geometrically different from the primarily vertical sheeted dike complex. Based on extensive field observations, here we present petrological and geochemical data for the Xigaze ophiolite to decipher the origin of sheeted sill complex and its implications for the construction of oceanic crusts. Diabases in the Xigaze ophiolite could be subdivided into sheeted sills, Group 1 non‐sheeted dikes, and Group 2 non‐sheeted sills, based on their orientations. These diabases cut other lithologies, and hence belong to the latest‐stage products. Based on petrological, geochemical, and structural data, we highlight the important role of detachment fault in the generation of sheeted and non‐sheeted sills. During the formation of oceanic crust, large block exhumation, multi‐stage rotations, and foundering are argued here as key mechanisms for the generation of Xigaze sheeted and non‐sheeted dikes/sills, all of which are in the evolution of detachment fault systems. These processes are also not uncommon for asymmetrical segments at modern slow‐spreading and ultraslow‐spreading ridges, but are rare at symmetrical segments. Due to the evolution of detachment fault, the internal structures of (ultra)slow‐spreading ridges are more complex than those at fast‐spreading ridges.

Forearc magmatic evolution during subduction initiation: Insights from an Early Cretaceous Tibetan ophiolite and comparison with the Izu-Bonin-Mariana forearc

Abstract Subduction initiation is a key process in the operation of plate tectonics. Our understanding of melting processes and magmatic evolution during subduction initiation has largely been developed from studies of the Izu-Bonin-Mariana forearc. Many suprasubduction zone ophiolites are analogous to the Izu-Bonin-Mariana forearc sequence. However, whether there are differences between Izu-Bonin-Mariana subduction initiation sequences and suprasubduction zone ophiolites remains unclear. Here, we report field geological, geochemical, and geochronological data from mafic and felsic rocks in the Xigaze ophiolite (southern Tibet) mantle and crustal section; the same types of published data from both this ophiolite and the Izu-Bonin-Mariana forearc are compiled for comparison. The ophiolite section is intruded by various late-stage dikes, including gabbroic pegmatite, diabase, basalt, and plagiogranite. The compositions of clinopyroxene and amphibole suggest that gabbroic pegmatite formed from hydrous high-SiO2 depleted melts, while whole-rock compositions of basaltic and diabase dikes show negative Nb and Ta anomalies, suggesting flux melting of depleted mantle. Along with the mafic rocks, plagiogranite has a roughly constant content of La and Yb with increasing SiO2 contents, implying hydrous melting of mafic amphibolite. Early-stage pillow basalts exhibit geochemical affinities with Izu-Bonin-Mariana forearc basalts, but they are slightly enriched. Synthesized with the regional geological setting and compared with Izu-Bonin-Mariana forearc magmatism, we propose that the transition from mid-ocean ridge basalt–like lavas to subduction-related mafic and felsic dikes records an Early Cretaceous subduction initiation event on the southern flank of the Lhasa terrane. However, the mantle sources and the magmatic evolution in the Xigaze ophiolite are more variable than those for the Izu-Bonin-Mariana forearc.

Detachment faulting in the Xigaze ophiolite southern Tibet: New constraints on its origin and implications

The structure and lithological sequences of ophiolites provide critical information for reconstructing their tectonic setting and thus, the history of the paleo-oceanic basins in which they formed. Our systematic mapping of three of the best-preserved massifs in the Xigaze ophiolite from the middle segment of the Yarlung Zangbo Suture Zone (YZSZ) in South Tibet, revealed a nearly intact structure of the ophiolitic sequence that has largely escaped emplacement and post-emplacement dismemberment. One of the most important results is the identification of a nearly E-W striking, serpentinized shear zone (SPSZ) preserved within the sequence. The SPSZ is rooted in the serpentinized front of the upper mantle and is mainly composed of meta-gabbroic and rodingite lenses floating in a serpentinite matrix. Extensive talc-serpentinite metasomatism, shearing and cataclastic deformation in the SPSZ reveal a complex interplay of fluid metasomatism and progressive ductile-brittle deformation in hundreds of meters of shear zone between the mantle and the crust. The structural and petrological features of the SPSZ are very similar to those of detachment faults within modern in situ oceanic core complexes (OCCs) in the Mid-Atlantic Ridge and the Southwest Indian Ridge. Zircon UPb ages from the footwall of the fault, and the age of radiolarian chert overlying it suggest that the detachment faulting took place between ca. 125 to 123 Ma. Combined with the stratigraphic and geochronological relationships between the ophiolite and the overlying sediments, we suggest that the Xigaze ophiolite developed in the supra-subduction zone on the southern margin of the Lhasa terrane during rollback of the Neo-Tethyan oceanic slab. Recognition of detachment faulting in the Xigaze ophiolites, along with other features, suggest that the Xigaze ophiolite formed at a spreading ridge, whereas the other large ultramafic massifs in YZSZ, may have formed in an off-axis location on the slow-spreading ridge.

The Xigaze ophiolite: fossil ultraslow-spreading ocean lithosphere in the Tibetan Plateau

The crust and mantle both in ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are critical to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogues for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it with the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dykes and sills cutting both mantle and lower crust. The gabbro units are thin ( c . 350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable with the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analogue of ultraslow-spreading ocean lithosphere. Supplementary material: Single-spot analysis and average compositions for mineral major element compositions (wt.%) of the Jiding gabbro section, Xigaze ophiolite are available at https://doi.org/10.17632/dtzpmwkscp.1

Thermodynamic modeling and elemental migration for the early stage of rodingitization: An example from the Xialu massif of the Xigaze ophiolite, southern Tibet

The analysis of early stage rodingite from the ultramafic rocks of the Xialu Massif in the Xigaze Ophiolite, Tibet, in China shows that the rodingitization involved continuous changes in fluid composition during different stages of subduction. The early stage prehnite-bearing rodingite was produced at low pressures and temperatures along extensional fractures. Samples of rodingite were collected along a profile from the center to the margin of a rodingitized intrusive igneous rock (~10 m × 30 m), and they record wide variations in bulk composition, mineralogy, and texture. The mineral assemblages, from center to margin, vary from (1) relics of primary clinopyroxene (Cpxr) and primary amphibole (Ampr) + newly formed late amphibole (Act) + primary plagioclase (Plr) + clinozoisite + prehnite + albite + chlorite + titanite + ilmenite (R1 rodingite), through (2) relics of primary clinopyroxene (Cpxr) + newly formed late clinopyroxene (Cpxn) + primary and late amphiboles (Ampr + Act) + clinozoisite + prehnite + albite + chlorite + titanite (R2 rodingite), to (3) newly formed late clinopyroxene (Cpxn) and amphibole (Act) + clinozoisite + prehnite + albite + chlorite + titanite (R3 rodingite). As a result of the metasomatic process of rodingitization, the content of CaO in the whole rock chemical composition from R1 to R3 increases, SiO2 decreases, and Na2O + K2O is almost completely removed. Mass-balance diagrams show enrichments in large ion lithophile elements such as Rb, Cs, Ba, and Pb as well as Ni during rodingitization. The central part of the rodingitized intrusion (R1 rodingite) was only slightly affected by metasomatism. On the other hand, the contents of the rare earth elements (REEs), high field strength elements (HFSEs; e.g. Zr, Nb, Ta, Hf, and Y), and some highly compatible elements such as Cr and Sc decreased slightly during rodingitization. Thermodynamic modeling based on equilibrium mineral assemblages indicates that the rodingite of the Xialu Massif formed in an H2O-saturated, CO2-rich environment. The estimated conditions of metamorphism were ~281–323 °C and 0.4–3.9 kbar, representing the subgreenschist facies. In this environment, REEs and HFSEs were soluble in the fluids and partly removed. Moreover, these prehnite rodingites formed in a progressively reducing and less alkaline environment, as indicated by decreases in f(O2) and bulk-rock Fe3+/Fe2+ ratios, and the records of fluid ΔpH from the center to the margin of the studied rodingitized intrusion.

Geochemical evidence for forearc metasomatism of peridotite in the Xigaze ophiolite during subduction initiation in Neo-Tethyan Ocean, south to Tibet

Subduction initiation is one of the key processes in the operation of plate tectonics. Our understanding of this process is largely derived from geochemical studies of basaltic rocks in ophiolites at continental margins. Because of the petrological inheritance in forearc basalts, it is important to acquire geochemical signatures for fluid metasomatism of the nascent mantle wedge at forearc depths. This may be provided by the study of peridotite in ophiolites, which is least affected by tectonic emplacement of the ophiolites onto continental margins. For this purpose, we have carried out a comprehensive study of whole-rock and mineral geochemical compositions for harzburgites from the Xigaze ophiolites in the Yarlung Zangbo suture zone, southern Tibet. The results provide insights into the forearc metasomatism during subduction initiation before the closure of Neo-Tethyan Ocean for the India-Asia continental collision in the Cenozoic. The harzburgites show high spinel Cr# values and strong whole-rock depletion in CaO, Al2O3 and REE, suggesting high degrees of melt extraction (~10–20%) during seafloor spreading to form the Neo-Tethyan mid-ocean ridge basalts. They exhibit positive LILE anomalies in trace element distribution patterns, indicating metasomatism by subduction zone fluids. This is supported by the occurrence of tremolite and dolomite in some of the harzburgites. Based on the presence or absence of carbonate minerals, two types of metasomatic fluids are distinguished. One is the aqueous solutions, leading to the formation of tremolite and the enrichment of LILE in clinopyroxene. The other is the carbonate-bearing fluids, resulting in the formation of dolomites, tremolite and low Mg# (79–82) olivine. The metasomatism also results in the high contents of structural hydroxyl in pyroxenes and elevated δ18O values for the harzburgites. It is inferred that the fluid metasomatism was induced by incipient subduction of the juvenile Neo-Tethyan oceanic slab to forearc depths shortly after conversion of the Neo-Tethyan mid-ocean ridge into the trench. During this period of subduction initiation, the fluids were produced by heating dehydration of the sinking ridge crust to the forearc depths. It is these fluids that would metasomatize the overlying nascent mantle wedge, leading to superimposition of the geochemically enriched signature on the lithochemically depleted harzburgites in the ophiolites. Therefore, the forearc metasomatism of peridotite in ophiolite can be used as a geochemical proxy for subduction initiation. Furthermore, the Xigaze ophiolites were produced by the ridge-trench conversion for subduction initiation in the Mesozoic and they were emplaced onto the southern margin of the Asian continent in the Cenozoic.