Jun 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.

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.

In the convergence area of the southeastern margin of the Tibetan Plateau and the northwestern South China Sea (SCS), the Beibu Gulf Basin (BGB) has experienced a complex geological evolution profoundly influenced by the lateral extrusion of the India‐Asia collision and the rifting of the SCS continental margin. Nonetheless, late Cenozoic basalts are widespread in the BGB following the cessation of SCS seafloor spreading, with controversial dynamic mechanisms. To investigate the mantle dynamics beneath these late Cenozoic basalts, we aim to investigate the high‐resolution lithospheric structure of the BGB region. Our study utilizes data from 162 seismic stations, including three temporarily deployed dense arrays, to investigate the seismic structural features of the Moho and Lithosphere‐Asthenosphere Boundary (LAB). P‐to‐S receiver function analysis reveals a significantly thinned crust with high Vp/Vs ratios beneath the late Cenozoic basalt regions, coinciding with the occurrence of deep crustal earthquakes. Using S‐to‐P receiver functions, a shallow, strong, and broad LAB is observed beneath the BGB, implying the presence of ponded partial melt. These seismic structural features indicate that the mantle upwelling beneath the BGB likely interacted with the crust, leading to mafic crustal composition, deep earthquakes, and magmatic eruptions. These findings provide new evidence for mantle dynamics in the northwestern SCS, enriching our understanding of interactions between the mantle and crust in the context of intraplate magmatism and deepening seismicity.

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.

Methods, Tables S1–S14, and Figures S1–S9.

Methods, Tables S1–S14, and Figures S1–S9.

In recent studies of crustal growth using global zircon Hf–O isotopic datasets, high-δ¹⁸O zircons are typically attributed to intra-crustal reworking during which very little juvenile mantle-derived magmas were added to the crust. Although arc magmas may originate from a high-δ¹⁸O mantle wedge, it has been difficult to decipher the contribution of high-δ¹⁸O mantle to zircon-saturated felsic magma due to superimposed intra-crustal processes. We address this issue by combining the data from high-δ¹⁸O zircon-bearing ultramafic cumulates and coeval lavas from a Cretaceous magmatic arc in southern Tibet. The cumulates mainly consist of different proportions of cumulus olivine and intercumulus amphibole. Amphibole analyses show a transition from increasing to decreasing Zr with increasing SiO₂ (50–74 wt.%) contents in the intercumulus melts, indicating zircon saturation in late-stage interstitial melts. The εNd(t) values (2.4 ± 1.4) of the apatite grains crystallised before and after zircon remain almost constant. Interstitial zircons have δ¹⁸O (6.1–7.2‰) values similar to the earliest crystallised olivine (δ¹⁸O = 6.3–7.1‰) in the cumulates. The coeval lavas may represent the intercumulus melts extracted from amphibole-rich cumulates at different depths. Both the lavas and cumulates were ultimately derived from high-δ¹⁸O arc mantle modified by small amounts (18O zircons therefore are not products of crustal reworking, but record crustal growth during their crystallisation (110 ± 2 Ma). Our study shows that the combination of zircon and olivine oxygen isotopes for ultramafic to felsic rocks is more effective than zircon data alone in evaluating the role of crustal growth vs. reworking in an arc system. The implication is that global zircon-based crustal evolution models that attribute all high-δ¹⁸O zircons to crustal reworking may conceal recent crustal growth.

The drift history of the Lhasa terrane from Gondwana to Asia plays a crucial role in understanding the Tethys evolution and true polar wander (TPW). However, few reliable paleomagnetic results from Jurassic strata are currently available for reconstructing its northward journey. We performed a combined paleomagnetic and geochronological study on Bima Formation strata in the Xigaze area. Combined with previous results from the Sangri area, our results reveal a paleolatitude of 8 ± 4°S at ∼180 Ma for the reference point (29.3°N, 90.3°E). Along with other paleomagnetic results from the Triassic to Cretaceous, our new results suggest that the Lhasa terrane motion accelerated from ∼2 cm/yr during ∼220–180 Ma to ∼17 cm/yr during ∼180–170 Ma. Paleolatitude information of the North Qiangtang terrane and Tethyan Himalaya is calculated from paleopoles that meet five criteria, which include (a) structural control, (b) well‐determined rock age, (c) stepwise demagnetizations, (d) a minimum of 25 specimens or 8 sites are contained, and (e) robust field or reversal tests are provided. Both terranes also show significant acceleration during their northward motion, which may be related to oceanic slab subduction. Thus, all Gondwana‐derived microcontinents seem to share a significant acceleration during their northward motion. In addition, recent paleomagnetic results from volcanic rocks dated at ∼155 Ma subdivide the overall northward motion during ∼170–130 Ma into two stages, which include a southward drift during ∼170–155 Ma followed by northward motion during ∼155–130 Ma. These results support the fast Late Jurassic TPW during a ∼10 Myr time span.