Yeba Formation Research Articles

New Middle Jurassic Paleomagnetic and Geochronologic Results From the Lhasa Terrane: Contributions to the Closure of the Meso‐Tethys Ocean and Jurassic True Polar Wander

AbstractThe drift history of the Lhasa terrane is crucial for understanding the tectonic evolution of Tethyan Oceans and Jurassic true polar wander. However, high‐quality Middle Jurassic paleomagnetic data from the Lhasa terrane are limited in number. Here we report a combined paleomagnetic and geochronologic study on the Yeba Formation volcanic rocks, dated at ∼170 Ma, from the Lhasa terrane. Robust field and reversal tests indicate that the characteristic remanent magnetizations are primary. Our results provide a reliable Middle Jurassic (∼170 Ma) paleopole at 29.8°N, 180.7°E with A95 = 5.7° and a paleolatitude of 14.4 ± 5.7°N for the Lhasa area. Compared with previous paleomagnetic and geologic evidence, we propose that the Meso‐Tethys Ocean probably began to close in the eastern part at ∼168 Ma and that the Lhasa terrane underwent a ∼2,900 km southward “monster shift” during the Late Jurassic.

Geochronology and petrogenesis of the Yeba formation volcanic rocks in the Epingsong area, southern Lhasa terrane, Tibet: record of the Neo-Tethys subduction

ABSTRACT The Yeba Formation volcanic rocks preserve important information of the early Neo-Tethys subduction. Here, we report the discovery of these volcanic rocks at Epingsong (southeast of the Yeba Formation), which fill the gap of the Yeba Formation at Sangri. Timing of volcanism in the eastern and western Yeba Formation is discussed. Systemic studies on petrology, whole-rock geochemistry, zircon U-Pb age, whole-rock Sr-Nd isotopes and in situ zircon Lu-Hf isotopes reveals that the rocks are calc-alkaline andesitic to felsic, and were emplaced in the Early Jurassic (ca. 191.4–174.0 Ma). The andesitic rocks have medium Mg# value (28.3–50.2, avg. 40.1), low Cr (4.68–36.01 ppm) and Ni (3.63–12.70 ppm) contents. They have slightly positive Eu/Eu* anomalies (1.0–1.13), and display continental arc affinity, as featured by enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g. Rb, K, Ba), but depletions in high-field strength elements (HFSE; e.g. Nb, Ta, Ti and Hf). The Yeba andesite has largely positive whole-rock εNd (+3.6) and zircon εHf (−1.3 to +10.9) values. The felsic rocks have low Mg# values (19.2–42.3), MREE depletion (e.g. Tb, Dy and Ho), and negative Eu/Eu* anomalies (0.76–0.96). They have varying εNd (0.9–2.9) and εHf (2.1–10.4) values, with two-stage model ages (tDM2) of 560–1098 Ma. The results indicate that the andesitic-felsic rocks were probably sourced from partial melting of the lower crust by mantle-derived magma underplating, and mixed with subducted sediment-derived melts (2.0–7.5%) and assimilated ancient crystalline basement. The Yeba volcanic rocks likely record an important tectonic transition from subduction compression to extension.

Temporal-spatial coincidence may not always discern causality: Insights from two skarn deposits in Anqing and Zhibula, China

The genetic links between skarn deposits and their causative intrusions are often difficult to establish with in situ dating techniques since ultra-high temporal resolution is required (i.e., < 0.1%). Strontium isotope can be applied to further test their causality, because of its sensitivity to water-rock interaction and country-rock contamination. Here we demonstrated the effectiveness of this approach using two skarn deposits (Zhibula and Anqing) as an example. While in situ garnet/zircon U-Pb dating indicates contemporary intrusions for both deposits, but the genetic links between mineralization and intrusions are contested owing to the limited temporal resolution. We further tested these apparent genetic links via in situ Sr isotope discrimination between mineralization-related epidote and igneous apatite from hypothesized causative intrusions. For Zhibula, the U-Pb ages of garnet (17.0 ± 0.4 Ma) and zircon (16.9 ± 0.3 Ma) support an apparent genetic link between Cu mineralization and the Zhibula granodiorite. This observation has been further supported by the 87Sr/86Sr ratio of hydrothermal epidote (0.70657 ± 0.00025), which can be interpreted by binary mixing between the granodiorite (0.71136 ± 0.00253) and the wall rock (Yeba formation; 0.70456 ± 0.00118). For Anqing, the U-Pb ages of garnet (138.1 ± 1.9 Ma) and zircon (137.1 ± 1.3 Ma) also support an apparent genetic link between Cu-(Fe-Au) mineralization and the Yueshan diorite. However, the 87Sr/86Sr ratio (0.71468 ± 0.00356) of hydrothermal epidote is too radiogenic to be explained by mixing between the Yueshan diorite (0.70654 ± 0.00012) and the Triassic carbonates (0.70810 ± 0.00015), which implies the presence of an unknown or unexposed causative intrusion. Our study highlights the risk of solely relying on in situ radiometric dating to establish genetic links between mineralization and spatial-related intrusions. Instead, integrating Sr isotopic fingerprinting with geochronology and deposit geology could be more informative.

Geochemistry, detrital zircon geochronology and Hf isotope of the clastic rocks in southern Tibet: Implications for the Jurassic-Cretaceous tectonic evolution of the Lhasa terrane

In order to reconstruct tectonic evolution history of the southern margin of Asia (i.e., Lhasa terrane) before the India-Asia collision, here we present a comprehensive study on the clastic rocks in the southern Lhasa terrane with new perspectives from sedimentary geochemistry, detrital zircon geochronology and Hf isotope. Clasts from the Jurassic-Early Cretaceous sedimentary sequences (i.e., Yeba and Chumulong Formations) display high compositional maturity and experienced moderate to high degree of chemical weathering, whereas those from the late Early-Late Cretaceous sequences (Ngamring and Shexing Formations) are characterized by low compositional maturity with insignificant chemical weathering. Our results lead to a coherent scenario for the evolution history of the Lhasa terrane. During the Early-Middle Jurassic (∼192-168Ma), the Lhasa terrane was speculated to be an isolated geological block. The Yeba Formation is best understood as being deposited in a back-arc basin induced by northward subduction of the Neo-Tethys ocean with sediments coming from the interiors of the Lhasa terrane. The Middle Jurassic-Early Cretaceous Lhasa-Qiangtang collision resulted in the formation of a composite foreland basin with southward-flowing rivers carrying clastic materials from the uplifted northern Lhasa and/or Qiangtang terranes. During the late Early-Late Cretaceous (∼104-72Ma), the Gangdese magmatic arc was uplifted rapidly above the sea level, forming turbidites (Ngamring Formation) in the Xigaze forearc basin and fluvial red beds (Shexing Formation) on the retro-arc side. At the end of Late Cretaceous, the Lhasa terrane was likely to have been uplifted to high elevation forming an Andean-type margin resembling the modern South America before the India-Asia collision.

Contrasting arc magma fertilities in the Gangdese belt, Southern Tibet: Evidence from geochemical variations of Jurassic volcanic rocks

Jurassic volcanic rocks are widely distributed along the Gangdese belt of southern Tibet, forming a westerly-extending southern belt (Bima Formation) with one large arc-related porphyry CuAu deposit (Xietongmen), and an easterly-extending northern belt (Yeba Formation) without any contemporary mineralization. A compilation of new and published data on the Bima Formation is compared to published data on the Yeba Formation in order to understand their different relationships with mineralization. The Bima Formation volcanic rocks comprise basalt, andesite, and dacite with zircon UPb ages of 195.0 Ma–165.1 Ma (including new ages of 180.2 ± 0.8 Ma to 166.7 ± 1.1 Ma). They are calc-alkaline series lava flows and show enrichments in LILE and LREE, depletions in HFSE and HREE, which are features typical of arc rocks. Coherent geochemical variations and similar depleted Sr-Nd-Hf isotopic characteristics between basaltic, intermediate, and felsic rocks (new and published data: (87Sr/86Sr)i = 0.703182 to 0.705489); εNd(t) = 3.2 to 7.1; εHf(t) = 9.3 to 16.0) suggest that they were formed from fractional crystallization of the basaltic magma. In contrast, the published Yeba Formation volcanic rocks, although with a similar age range (192.7 Ma to 168.0 Ma), have a wider compositional range (from basalt to rhyolite), and more evolved Sr-Nd-Hf isotopic compositions ((87Sr/86Sr)i = 0.700182 to 0.707723; εNd(t) = −5.1 to 4.5; εHf(t) = −18.5 to 17.6).New data on the Bima Formation andesite and dacite samples have higher zircon Eu/Eu*, Ce4+/Ce3+ and (Ce/Nd)/Y ratios, and lower titanium-in-zircon temperatures than those from the Yeba Formation, suggesting the Bima Formation magmas were more oxidized and hydrous, and therefore more fertile for porphyry copper deposit formation. The relatively reduced Yeba Formation magmas formed in a distal arc setting near the Central Lhasa old basement. Their evolved isotopic characteristics and reduced nature may have resulted from more extensive contamination by reduced crustal components, rendering them infertile for porphyry copper mineralization. Exploration for arc-related porphyry copper deposits in the Gangdese belt should therefore focus on the fertile Bima Formation.

Sequence and petrogenesis of the Jurassic volcanic rocks (Yeba Formation) in the Gangdese arc, southern Tibet: Implications for the Neo-Tethyan subduction

The Yeba Formation volcanic rocks in the Gangdese arc recorded important information regarding the early history of the Neo-Tethyan subduction. To explore their magmatic evolution and tectonic significance, we performed a systematic petrological, geochronological and geochemical study on these volcanic rocks. Our data indicated that the Yeba Formation documents a transition from andesite-dominated volcanism (which started before 182 Ma and continued until ~176 Ma) to bimodal volcanism (~174–168 Ma) in the earliest Middle Jurassic. The early-stage andesite-dominated volcanics are characterized by various features of major and trace elements and are interpreted as the products of interactions between mantle-derived arc magmas and lower crustal melts. Their positive εNd(t) and εHf(t) values suggest a significant contribution of asthenosphere-like mantle. The late-stage bimodal volcanism is dominated by felsic rocks with subordinate basalts. Geochemical signatures of the basalts indicate a composite magma source that included a “subduction component”, an asthenosphere-like upper mantle domain and an ancient subcontinental lithospheric mantle component. The felsic rocks of the late stage were produced mainly by the melting of juvenile crust, with some ancient crustal materials also involved. We suggest that the occurrence and preservation of the Yeba Formation volcanic rocks were tied to a tectonic switch from contraction to extension in the Gangdese arc, which probably resulted from slab rollback of the subducting Neo-Tethyan oceanic slab during the Jurassic.

The Jurassic Yeba Formation in the Gangdese arc of S. Tibet: implications for upper plate extension in the Lhasa terrane

ABSTRACTThe formation of the Jurassic Xiongcun porphyry copper deposit, within the Gangdese magmatic belt of southern Tibet, took place coincident with subduction of the Neotethyan oceanic lithosphere. Although the Yeba Formation (coeval and co-magmatic intrusions and volcanic rocks) in the eastern Gangdese magmatic belt formed at the same time as the Xiongcun deposit, the Yeba Formation is devoid of subduction-related porphyry Cu deposits. Here, we report detrital zircon U–Pb ages for sedimentary rocks of the Yeba and Bima Formations along with a summary of all published whole-rock geochemical data and zircon epsilon Hf isotopes for the volcanic rocks in the Yeba Formation. We use these data to re-evaluate the petrogenesis, magmatic source, and tectonic setting of the volcanic rocks. Detrital zircon ages from the sedimentary rocks reveal that the volcanic rocks of the Yeba and Bima Formations are the main source for their sedimentary interlayers. In addition, the Yeba sedimentary layers contain inherited zircons sourced from older country rocks whereas the Bima Formation is devoid of ancient zircons. Geochemically, the Yeba volcanic rocks resemble a bimodal volcanic suite. Geochemical data reveal that the Yeba volcanic rocks formed in a continental arc setting, while the mafic ones were derived from the partial melting of the mantle wedge above the subducting slab rather than from the melting of the descending oceanic slab. In addition, the mafic ensemble formed in a back-arc setting, indicative of subduction-related extension in the Lhasa terrane. In contrast, the felsic ensemble may originate from the melting of basaltic lower crust. Thus, the magma source and tectonic setting for the Yeba Formation are distinct from typical subduction-related Cu-hosted porphyry and volcanic rocks in a compressional setting.

The origin and tectonic significance of the volcanic rocks of the Yeba Formation in the Gangdese magmatic belt, South Tibet

Zircon U-Pb geochronology, Hf isotope and whole-rock geochemistry were performed on the tuffs of the Yeba Formation in the Gangdese magmatic belt, South Tibet. The results are used to detail the age, source nature and tectonic processes that led to the formation of the Gangdese belt. Dating results indicate that the rhyolitic-andesitic tuffs were formed at 174-170 Ma. Positive and variable zircon e Hf(t) values of the rhyolitic tuffs reveal that the source was dominated by juvenile material, however, experienced crustal contamination. The basaltic tuffs have low HREEs, high contents of compatible elements (V and Cr) and no Eu anomaly. In contrast, the rhyolitic-andesitic tuffs show low compatible trace elements, depletion in Eu but enrichment in incompatible elements (Rb, Zr and Hf). According to the discrimination diagrams of P2O5-SiO2 and Th-Rb, the rhyolitic-andesitic tuffs show a close affinity to I-type granitoids. Moreover, these tuffs are marked by significant depletion in Nb, Ta and Ti, plotted in calc-alkaline field, and with the andesitic-rhyolitic tuffs falling into an active continental margin setting. We suggest that these tuffs of the Yeba Formation were probably generated in an active continental margin above the northward subduction of the Neo-Tethyan oceanic lithosphere.

Geochronology and geochemistry of the Early Jurassic Yeba Formation volcanic rocks in southern Tibet: Initiation of back-arc rifting and crustal accretion in the southern Lhasa Terrane

Understanding the geological history of the Lhasa Terrane prior to the India–Asia collision (~55±10Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene (~18–10Ma) adakitic magmatism with economically significant porphyry-type mineralization has been interpreted as resulting from partial melting of the Jurassic juvenile crust, but how this juvenile crust was accreted remains poorly known. For this reason, we carried out a detailed study on the volcanic rocks of the Yeba Formation (YF) with the results offering insights into the ways in which the juvenile crust may be accreted in the southern Lhasa Terrane in the Jurassic. The YF volcanic rocks are compositionally bimodal, comprising basalt/basaltic andesite and dacite/rhyolite dated at 183–174Ma. All these rocks have an arc-like signature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba and U) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr–Nd and zircon Hf isotopic compositions, pointing to significant mantle isotopic contributions. Modeling results of trace elements and isotopes are most consistent with the basalts being derived from a mantle source metasomatized by varying enrichment of subduction components. The silicic volcanic rocks show the characteristics of transitional I–S type granites, and are best interpreted as resulting from re-melting of a mixed source of juvenile amphibole-rich lower crust with reworked crustal materials resembling metagraywackes. Importantly, our results indicate northward Neo-Tethyan seafloor subduction beneath the Lhasa Terrane with the YF volcanism being caused by the initiation of back-arc rifting. The back-arc setting is a likely site for juvenile crustal accretion in the southern Lhasa Terrane.

Geochronology and petrogenesis of Miocene granitic intrusions related to the Zhibula Cu skarn deposit in the Gangdese belt, southern Tibet

The Zhibula Cu skarn deposit (19.5Mt ore @ 1.64%), near the Qulong porphyry Cu–Mo deposit, is located in the Gangdese porphyry copper belt in southern Tibet. The deposit is a typical metasomatic skarn that is related to the interaction of magmatic-hydrothermal fluids and calcareous host rocks. Stratiform skarn orebodies are mainly distributed in the contact between tuff and marble in the lower part of the Jurassic Yeba Formation. Endoskarn zonations for an outward trend are observed in the granodiorite, which grade from a fresh granodiorite to a weakly chlorite-altered granodiorite, a green diopside-bearing granodiorite, and a dark red-brown garnet-bearing granodiorite. The Zhibula granodiorite and monzogranite have similar secondary ion mass spectrometry (SIMS) zircon U–Pb ages of 16.9±0.3Ma and 17.0±0.2Ma, respectively. They exhibit different fractional crystallization from granodiorite (SiO2=64.8–69.3wt.%) to monzogranite (SiO2=72.3–76.8wt.%). Both the granodiorite and monzogranite are characterized by high Al2O3 (12.6–16.7wt.%) and K2O (1.5–5.5wt.%) contents, high Sr/Y (35–151) and La/Yb (19–48) ratios, and variable MgO (0.16–3.91) and Mg# (31–61) values. They display features of enrichment in large ion lithophile elements (LILEs, e.g., Rb, Ba, Sr, and K), depletion in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and P), and moderate negative Eu anomalies (δEu=0.58–0.98). They show restricted in situ zircon Hf isotopic compositions (+6.7 to +8.8; only one sample is +4.5) and consistent δ18O values (+6.0‰ to +6.6‰). The geochemical data indicated that the Miocene Zhibula granitic intrusions formed by the magma that were characterized by high Sr/Y ratios and were derived from the partial melting of the thickened juvenile lower crust, which may have been metasomatized by the slab melts during subduction of the Neo-Tethyan oceanic crust and were induced by the convective removal of the thickened lithosphere. In addition, the Zhibula Cu skarn deposit and the Qulong Cu–Mo deposit form a significant porphyry–skarn ore system.

Alteration and mineralization at the Zhibula Cu skarn deposit, Gangdese belt, Tibet

The Zhibula Cu skarn deposit contains 0.32Mt. Cu metal with an average grade of 1.64% and is located in the Gangdese porphyry copper belt in southern Tibet. The deposit is a typical metasomatic skarn that is related to the interaction of magmatic–hydrothermal fluids and calcareous host rock. Stratiform skarn orebodies occur at the contact between tuff and marble in the Lower Jurassic Yeba Formation. Alteration zones generally grade from a fresh tuff to a garnet-bearing tuff, a garnet pyroxene skarn, and finally to a wollastonite marble. Minor endoskarn alteration zonations are also observed in the causative intrusion, which grade from a fresh granodiorite to a weakly chlorite-altered granodiorite, a green diopside-bearing granodiorite, and to a dark red-brown garnet-bearing granodiorite. Prograde minerals, which were identified by electron probe microanalysis include andradite–grossularite of various colors (e.g., red, green, and yellow) and green diopside. Retrograde metamorphic minerals overprint the prograde skarn, and are mainly composed of epidote, quartz, and chlorite. The ore minerals consist of chalcopyrite and bornite, followed by magnetite, molybdenite, pyrite, pyrrhotite, galena, and sphalerite. Three types of fluid inclusions are recognized in the Zhibula deposit, including liquid-rich two-phase inclusions (type L), vapor-rich two-phase inclusions (type V), and daughter mineral-bearing three-phase inclusions (type S). As the skarn formation evolved from prograde (stage I) to early retrograde (stage II) and later retrograde (stage III), the ore-forming fluids correspondingly evolved from high temperature (405–667°C), high salinity (up to 44.0wt.% NaCl equiv.), and high pressure (500–600bar) to low-moderate temperature (194–420°C), moderate-high salinity (10.1–18.3 and 30.0–44.2wt.% NaCl equiv.), and low-moderate pressure (250–350bar). Isotopic data of δ34S (−0.1‰ to −6.8‰, estimated δ34Sfluids=−0.7‰), δDH2O (−91‰ to −159‰), and δ18OH2O (1.5‰ to 9.2‰) suggest that the ore-forming fluid and material came from magmatic–hydrothermal fluids that were associated with Miocene Zhibula intrusions. Fluid immiscibility likely occurred at the stage I and stage II during the formation of the skarn and mineralization. Fluid boiling occurred during the stage III, which is the most important Cu deposition mechanism for the Zhibula deposit.

Revision of the Cretaceous–Paleogene stratigraphic framework, facies architecture and provenance of the Xigaze forearc basin along the Yarlung Zangbo suture zone

New field investigation, structural analysis, review of biostratigraphy, stratigraphic framework, facies architecture, provenance, and flow direction of the Cretaceous–Paleogene sedimentary strata in southern Tibet resulted in reinterpretation of the evolution of the Xigaze forearc basin (XFB). The XFB is comprised by flysch-dominant Xigaze Group (consisting of Ngamring, Chongdoi, and Sangzugang Formations) and of the shallow marine Cuojiangding Group (Padana, Qubeiya, Quxia, and Gyalaze Formations), with their ages herein revised as the Albian–Coniacian and the Santonian–Ypresian respectively. The deep-sea flysch represented by the Ngamring Formation constitutes the early sedimentary fill of the XFB. It was subdivided into five megasequences MS1–MS5 in order: the early submarine fan facies stage, channel submarine fan stage, the mature submarine fan stage, the highstand submarine fan stage, and outer fan-pelagic stage. The paleocurrent flows were eastwards and westwards during the early and late stages of basin development and northwards and southwards during its middle stage, indicating typical forearc basin sedimentation with longitudinal supply and lateral filling. Provenance studies point to multiple sources, with andesitic debris sourced from the Jurassic–Lower Cretaceous Yeba Formation and Sangri Group formed in the volcanic island arc chains in the southern Gangdese. The four periods of the XFB development are: 1) the formation of the XFB basement by the Xigaze ophiolite, and completed before the Aptian. The coeval deposits on the shelf are the Sangzugang Formation carbonate; 2) the earliest XFB deposits of the earliest Aptian age (ca. 125/120Ma) represented by the Chongdoi Formation, related to the residual forearc; 3) composite forearc developed, represented by the late Albian–late Cenomanian flysch megasequences (MS1–MS2) and the Turonian–Coniacian flysch megasequences (MS3–MS5), respectively; and 4) the final stage of the XFB development started the terminal forearc stage represented by the late Late Cretaceous–Early Paleogene shallow marine Cuojiangding Group and terminated during middle Ypresian, when the Gyalaze Formation was deposited.

SHRIMP Zircon Age and Geochemical Constraints on the Origin of Lower Jurassic Volcanic Rocks from the Yeba Formation, Southern Gangdese, South Tibet

We present SHRIMP zircon dating, bulk-rock geochemical, and Sr-Nd-Pb isotopic results for Yeba volcanic rocks and a mafic dike from Southern Gangdese (SG), southern Tibet, in order to constrain their tectonic setting and origin. Yeba volcanic rocks span a continuous compositional range from basalt to dacite, although andesites are minor, and mafic and felsic rocks are volumetrically predominant. New SHRIMP zircon dating for a dacite coupled with previous SHRIMP zircon dating for a mafic dike and fossil constraints for the sedimentary sequence indicate that Yeba volcanic rocks were emplaced in the Early Jurassic (174-190 Ma). Yeba tholeiitic mafic rocks possess compositional diversity and are divided into three groups based on concentrations of MgO, Al2O3, and La. Mafic samples are all characterized by marked negative Nb, Ta, and Ti anomalies and positive εNd(T) values (+ 2.4 to + 4.5). Yeba calc-alkaline felsic rocks are characterized by coherent, concave-upward MREE patterns and negative anomalies in Nb, Ta, P, and Ti, with positive εNd(T) values (+ 0.3 to + 2.6). Sr-Nd-Pb isotopes overlap among the different groups of Yeba mafic rocks; Pb isotopic compositions in both mafic and felsic rocks are nearly identical. These features are consistent with a subduction-related origin, most likely in an arc built on thin, immature continental crust. Yeba volcanic rocks are interpreted as having been created by northward subduction of Neo-Tethyan oceanic crust in Early Jurassic time. Geochemical signatures and quantitative modeling indicate that fractional crystallization and crustal assimilation played insignificant roles in the generation of Yeba mafic magmas, and that their geochemical diversity was probably produced by variable degrees of partial melting from a common but heterogeneous mantle source, which had been metasomatized by variable contributions of sediments/fluids released from the subducted Neo-Tethyan oceanic crust. Yeba felsic rocks were probably generated by moderate degrees of partial melting of juvenile basaltic lower crust, which consists of dominant underplated magmas (similar to Yeba mafic rocks in composition) and variable contributions from ancient lower crust beneath the Gangdese Back-Arc fault uplift belt (GBAFUB).

Late Triassic‐Jurassic bivalves from volcanic sediments of the Lhasa block, Tibet

Mesozoic volcanic sediments known as the Yeba Formation are widespread in the northeastern region of Lhasa. Recent fossil collections in the formation allow the establishment of a biostratigraphic framework for the volcanic sediments; bivalves are assigned to 21 genera and 22 species including a new genus and 5 new species: Lhasanella lhasaensis (gen. et sp. nov.), Trigonodus xiabolangensis (sp. nov.), Isocyprina lhasaensis (sp. nov.), Gervillaria tibetica (sp. nov.), and Jurassicorbula fuersichi (sp. nov.). Three benthic assemblages have been recognised: the Trigonodus‐Isocyprina assemblage (Rhaetian‐Hettangian), the Lhasanella‐Propeamussium assemblage (Toarcian‐Bajocian), and the Jurassicorbula‐Neomiodon assemblage (Middle Jurassic). The Yeba Formation ranges in age from latest Triassic to Middle Jurassic. Volcanism in the Lhasa block occurred frequently during this period, corresponding to the fragmentation episode in eastern Gondwanaland and to the northwards drift of the Lhasa block. A strong faunal affinity between the Lhasa block and southern Qinghai, Japan, and Vietnam suggests that the Lhasa block had been located within the northern marginal region of the East Tethys since Toarcian times. The existence of a deep ocean which might serve as a barrier to fauna dispersal between the Lhasa and Qingtang blocks remains questionable.