Qiang Wang

The mechanisms responsible for the initial uplift of the Tibetan Plateau remain poorly constrained, given that most existing models emphasize its later episodic rise and outward expansion. At the heart of this uncertainty lies the Qiangtang block, which formed the core of the proto-plateau during the Eocene. Its crustal thickening history holds critical clues to understanding early uplift, yet the lack of direct lower-crustal material has long hindered insights into the deep processes involved. Here we report newly discovered Eocene (37.3–36.5 Ma) low-Mg adakitic rocks from the northeastern Qiangtang block that originated from partial melting of newly underplated juvenile lower crust. Their depleted isotopic signatures [(87Sr/86Sr)i = 0.7051–0.7054; εNd(t) = −0.23 to +0.28; εHf(t) = +5.3 to +9.5; δ18O = 4.8‰–6.0‰] provide geochemical evidence for a juvenile lower-crustal source. These rocks offer a rare opportunity to quantify the vertical contribution of magmatic underplating and crustal melting to surface uplift. By integrating petrology, geophysical imaging, paleoaltimetry, and density-based isostatic modeling, we estimate that magmatic underplating and crustal melting contributed up to ~32% of the early plateau uplift. These findings challenge conventional interpretations that attribute early plateau growth solely to crustal shortening and instead underscore the key role of deep crustal magmatism in shaping the proto-Tibetan landscape.

Beyond its andesite-like bulk composition, the continental crust is also markedly enriched in large-ion lithophile elements such as Ba and Sr. The relative importance of the two mechanisms proposed to explain the enrichment of these elements, mantle metasomatism versus magmatic differentiation in the crust, is debated. Newly identified Eocene high Ba−Sr volcanic rocks from the Bandaohu area of northern Qiangtang, central Tibet, provide a rare opportunity to shed light on this debate. These rocks span from primitive shoshonitic lavas to evolved trachytes, yet all display extreme Ba (2148−5998 ppm) and Sr (2182−3625 ppm) concentrations, enriched Sr−Nd isotopic signatures [(87Sr/86Sr)i = 0.70843−0.70886; εNd(t) = −8.5 to −7.3], and elevated zircon δ18O values (6.8‰−8.6‰). Trace-element ratios (high Th/Yb, low Ba/Th) and isotopic modeling indicate derivation from lithospheric mantle metasomatized by Triassic sediments of the subducted Songpan−Ganzi terrane. Fractional crystallization of ∼30%−35% clinopyroxene further magnified Ba−Sr enrichments, while the absence of plagioclase crystallization preserved them. These results establish a hierarchy of controls, with source enrichment as the primary driver and differentiation as a secondary amplifier. We propose a two-stage geodynamic model in which intracontinental subduction relaminates Songpan−Ganzi sediments at the base of the Qiangtang lithosphere and subsequent lithospheric thinning triggers partial melting of the resulting ultra-enriched mantle domains, producing high Ba−Sr magmas. This mechanism links sediment influx, mantle enrichment, and magmatic differentiation, offering new insights into crust-mantle interaction and continental growth during the India-Asia collision.

Interactions between orogenic plateau growth and environmental change, which play a key role in shaping Earth, are most pronounced in the Tibetan Plateau, affording the opportunity to explore how tectonics is coupled to surface processes during plateau uplift. The Linzhou and Wuyu depocenters of the regional intermontane basin in the Gangdese arc record a similar paleodrainage and surface evolution at ca. 60−48 Ma and 31−0 Ma, respectively, as detected by detrital zircon and apatite provenance analysis. A switch in detrital zircon and apatite populations from old and complex (polycyclic) to predominantly post–70 Ma sources in both sections highlights a decrease in sediment supply from external sources, likely caused by high-relief topography arising from surface uplift blocking distal headwaters. Abundant syndepositional zircon and apatite document rapid exhumation at ca. 54−51 Ma and 15−8 Ma, respectively, followed by a return to old (pre–70 Ma) sources with polycyclic zircon implying reactivated distal headwaters due to subsidence. The similar surface uplift and subsidence histories of the two locations in the Gangdese arc basin can be best explained by detachment of the Neo-Tethyan oceanic slab and Indian continental lithosphere, respectively, consistent with crustal thinning documented by Eu anomalies in detrital zircon. Lithospheric foundering led to asthenospheric upwelling and removed downward slab pull beneath Lhasa, triggering magmatism and uplift followed by surface subsidence linked to ongoing underthrusting. Consequently, two similar episodes of paleoelevation variations resulted from these two phases of lithospheric detachment.

The Tibetan Plateau is currently undergoing lateral extrusion and expansion, but it remains unclear whether such orogen-parallel growth occurred during the plateau’s early uplift. Additionally, it is uncertain how this process developed after the formation of the proto-Tibetan Plateau. Constraining both these processes will help improve our understanding of the plateau’s uplift history and mechanisms of formation. This study focuses on the Qiangtang terrane of western China, the core of the proto-Tibetan Plateau, and integrates new low-temperature thermochronological data from Eocene granites with published datasets to reconstruct the uplift history from the perspective of spatiotemporal exhumation patterns. Results reveal that prior to 40−35 Ma, the Qiangtang Plateau experienced not only orogen-perpendicular growth but also east−west-trending expansion. Following establishment of this proto-Tibetan Plateau, localized exhumation persisted in northern Qiangtang. Based on the correlation between apparent exhumation rates and fault activity, geomorphic indices, precipitation, and magmatism, we suggest that compressive thickening was the primary mechanism governing the initial uplift, while crustal channel flow drove the northeastward expansion of the Qiangtang Plateau. Conversely, extensional lithospheric thinning and climate-related erosion processes may have only played a role in localized regions. This study reconstructs the uplift history of the Qiangtang Plateau, from its initial formation as a mountain range to a flat plateau. The findings confirm that lateral growth was likely prevalent during the early stage of uplift and provide thermochronological evidence supporting the topographic transition driven by crustal melting.

Early fractionation of amphibole is an essential process in the evolution of hydrous sub-arc mantle-derived magmas. However, the petrogenesis of hornblende-rich cumulates in the deep crust and their significant role in arc crustal evolution remain unclear. In this contribution, we focus on Late Jurassic mafic–ultramafic intrusive rocks from the Zedong terrane (central–eastern Gangdese area, South Tibet) and use detailed petrographic, mineralogical and geochemical data to determine the origin of hornblende-rich cumulates with textural and chemical heterogeneities. The clinopyroxene (Cpx) hornblendite with adcumulate texture contains three distinct types of amphibole (cumulus, poikilitic, and minor interstitial). These amphiboles display homogeneous major and trace element compositions, consistent with crystallization from a liquid of constant composition. Rock fabric analysis further suggests that they formed by in situ nucleation and overgrowth at the interface between the mush layer and the convecting parental magma within the magma reservoir. Type I hornblende (Hb) gabbros have an orthocumulate texture with the lowest whole-rock MgO and Mg# values. Intercumulus melt evolution revealed by mineral assemblage and compositional variations indicate that Type I gabbros formed through a peritectic reaction between clinopyroxene precursors and evolved interstitial melts, followed by melt migration and fluid exsolution in a semi-open system. Type II gabbros have the highest whole-rock MgO and Mg# values and are characterized by extensive disequilibrium features. Geochemical modeling indicates that their formation entails a melt-flushing process involving the upward percolation of deep-recharge melts through the crystal mush. This study provides petrological evidence for the multiple origins of hornblende-rich cumulates within a common middle-lower crustal magma reservoir. Their textural and chemical heterogeneities may reflect spatial variations in crystallization regimes across distinct reservoir domains. The Zedong hornblende-rich cumulates have more depleted whole-rock Sr–Nd isotopic compositions and zircon εHf(t) values than those cumulates from Early Jurassic and Cretaceous Gangdese arc. This indicates that their parental magma was derived from a more juvenile mantle wedge and records significant growth in the middle-lower crust. The Cpx hornblendite has the lowest concentrations of SiO2 but the highest Fe2O3T and (Dy/Yb)N ratios. The differentiation trends between the cumulate rocks and coeval non-cumulate rocks within the Zedong terrane indicate that the evolution of the Late Jurassic arc crust was mainly controlled by amphibole fractionation. Modeling shows that melts in equilibrium with the cumulus and brown amphiboles are compositionally analogous to coeval intermediate-acid rocks, and it implies the existence of a trans-crustal magma plumbing system in this arc.

Silicon cycling between Earth’s reservoirs provides critical insights into how the Earth operates. While average crust and the bulk silicate Earth (BSE) share similar silicon isotope (δ30Si) compositions, some mantle-derived magmas exhibit lower δ30Si values than the BSE, implying the existence of an unidentified mantle reservoir with complementary higher δ30Si values. We present silicon isotope data from Cenozoic lamproites and their hosted mantle pyroxenite xenoliths from the Himalaya–Tibet orogen. These mantle-derived rocks have higher δ30Si values than the BSE, which resulted from reaction between mantle peridotite and 30Si-rich silicate melts from subducted Indian continental crust. Our results demonstrate that slab melting can produce high-δ30Si melts and complementary low-δ30Si residues. These products are unevenly distributed in the mantle, with high-δ30Si melts stored as metasomatic veins in the lithospheric mantle while low-δ30Si residues are recycled into the deep mantle. This study provides evidence that mantle metasomatism by high-δ30Si slab melts creates heavy silicon reservoirs in the lithospheric mantle above continental slabs or cool mantle wedges above oceanic slabs.

The uplift history of the Gangdese arc in Southern Tibet is fundamental to understanding when and how the plateau attained its current elevation and topographic relief. Detrital multi‐mineral provenance analysis of the Oligocene–Miocene strata in the intermontane Wuyu Basin provides new key information on the paleodrainage evolution in response to topographic changes. A Qiangtang source affinity is inferred by detrital apatite and zircon provenance, indicating a distal major eastern source at 31 Ma. The west‐flowing drainage system implies Oligocene lowlands in the Gangdese arc, probably related to regional subsidence. The shift of provenance to Linzizong volcanic rocks indicates that the Gangdese arc underwent uplift since 23 Ma, which became rapid at 15 Ma due to complete delamination of the subducted Indian continental lithosphere, as inferred by abundant syn‐sedimentary zircon and apatite. The accelerated uplift and topographic changes provide independent constraints on the Tibetan Plateau growth and associated geodynamic processes.

Establishing the northern limit of Indian lithosphere under the Tibetan Plateau is key to understanding India–Asia convergence mechanisms. Its location is disputed due to conflicting seismic interpretations and in particular the inability to distinguish between the subducted Indian and thickened Tibetan lithospheres in tomographic images. We report the results of a new approach based on the petrology and geochemistry of Miocene (ca. 11 Ma) lamproites recently discovered in northern Qiangtang of the Himalayan-Tibetan orogen. They are clearly distinguished from other orogenic lamproites by lead isotope ratios plotting to the left of the geochron, and their Pb-Nd-Os isotopes indicate an Archean (>2.7 Ga) Indian cratonic mantle source. This is compelling evidence that Indian cratonic mantle underthrust northward for ~600 km and had reached below northern Qiangtang in eastern Tibet at least 11 Ma. Such large-scale underthrusting of buoyant cratonic lithosphere resolves competing geophysical interpretations and plays an important role in shaping intraplate topography.