Andrew Craig Kerr

Mantle heterogeneity in lithology and geochemistry is often attributed to recycled subducted materials. While distinct mantle end‐members are identified by radiogenic isotopes, the specific recycled materials contributing to this heterogeneity remain debated. This study presents Mo‐Sr‐Nd‐Pb isotopic data for OIB‐like alkali basalts from the Maguan area in the southeastern Tibetan Plateau, focusing on slab inputs' role in mantle heterogeneity. The Miocene (ca. 13 Ma) Maguan alkali basalts are divided into two types based on petrographic and geochemical characteristics, showing similar Sr‐Nd‐Pb isotopic signatures but different Mo isotopic compositions. Type I basalts exhibit a wide δ^98/95Mo range (−0.31‰ to −1.03‰, average −0.47‰ ± 0.06‰, 2SD = 0.40‰, n = 13), while type II basalts have heavy and constant δ^98/95Mo values (−0.11‰ to −0.17‰, average −0.14‰ ± 0.01‰, 2SD = 0.05‰, n = 6). The unique low δ^98/95Mo value (−1.03‰) in type I basalts is among the lowest reported in OIB‐like continental basalts. Type I basalts likely originate from an enriched asthenospheric mantle metasomatized by melts from recycled dehydrated oceanic crust and sediments, whereas type II basalts are derived from partial melting of an enriched asthenospheric mantle metasomatized by melts from recycled serpentinized peridotites. The residual Tethys oceanic slabs in the deep mantle significantly contribute to the mantle source of the Maguan basalts. The formation of Maguan Miocene magmas may be linked to mantle upwelling induced by the subduction of the West Burma plate. This study highlights the Mo isotopic system's utility in tracing complex slab fluxes generating mantle geochemical heterogeneity.

Recycling of molybdenum isotopes in continental subduction zones remains debated. In this contribution, we re‐visit the Mo isotope compositions of the Sailipu post‐collisional ultrapotassic rocks in the Himalaya‐southern Tibet orogen. These ultrapotassic rocks have very varying δ^98/95Mo values of −0.66 to −0.07‰ and Mo/Ce ratios of 0.0008–0.005, which are lower than those of mid‐ocean ridge basalts (MORB; δ^98/95Mo = −0.20 ± 0.06‰, and Mo/Ce = 0.03) and oceanic subduction‐related (i.e., mantle source involving fluids, residual slab, or oceanic sediments) magmatic rocks (e.g., modern arc lavas, Cenozoic OIB‐type basalts in eastern China and the central Mariana Trough basalts in the back‐arc basin, syn‐collisional andesitic rocks in southern Tibet). Combined with the light Mo isotopes of the Himalayan schists and gneisses, we suggest that the light Mo isotopic signature of the Sailipu ultrapotassic rocks is derived from subducted Indian continental crust. This is consistent with the extremely low δ¹¹B (−17.4 to −9.7‰) and B/Nb (0.16–1) values and enriched Sr‐Nd‐Pb isotopes of the Sailipu ultrapotassic rocks. Thus, this study reveals the recycling of light Mo‐B isotopes during continental subduction and demonstrates that Mo‐B isotopes can effectively distinguish between continental and oceanic subduction.

Subduction initiation processes, such as those evidenced in the Izu‐Bonin‐Mariana (IBM) forearc crust are a key feature of modern‐style plate tectonics. However, the evidence for this process in the Precambrian is limited, due to the scarcity of ophiolite suites and in particular a lack of typical complete ophiolitic sequences similar to those observed in Phanerozoic ophiolites. Mineral analyses of olivine, orthopyroxene, and chrome spinel from the harzburgite and two types of clinopyroxene from the cumulate gabbro of the Fuchuan ophiolite, South China suggest formation in a Neoproterozoic subduction system similar to the IBM. High temperature (1,000–1,200°C), low pressure along with a high degree of melt extraction (>18%) have been recorded in the harzburgite. This evidence, in conjunction with magma evolution from forearc basalt to boninite to hydrous calc‐alkaline magmatism, recorded in the crustal section of the Fuchuan ophiolite indicates a transition from initial to mature subduction in a forearc setting. Statistical analysis of global ophiolites through time suggests that IBM‐like subduction systems may have successfully operated since at least the latest Neoproterozoic (ca. 1,000 Ma) and be pervasive on Earth from ca. 850 Ma. Additional evidence for Neoproterozoic ophiolites from India, Siberia, and Arabian‐Nubian Shield, which are also similar to the IBM forearc ophiolite, suggests that cold and deep subduction became much more common in the Neoproterozoic (ca. 850 Ma) around the periphery of the Rodinia supercontinent. This IBM‐like subduction system may have acted as the geodynamic trigger for the break‐up of Rodinia.

The nature and pre‐Cenozoic evolution history of crust in southern Lhasa, which is crucial for our understanding of Indo‐Asian continental collision and Tibetan uplift during the Cenozoic, remains controversial due to a “missing” pre‐Mesozoic magmatic record. In this contribution, we report petrological and geochemical data for newly identified Paleozoic bimodal magmatism in the Zhengga area of southern Tibet. The magmatism comprises Late Devonian‐Early Carboniferous (366–353 Ma) amphibolite and two‐mica gneissic granite. The protoliths of the Zhengga amphibolite were gabbro and diorite with low SiO₂ and high MgO, Cr, and Ni contents with high ε_Nd(t) values of +3.3 to +8.0, variable and positive zircon ε_Hf(t) of +0.9 to +11.2, and low zircon δ¹⁸O of 5.7 ± 0.2‰. These protoliths are proposed to have formed by decompression melting of asthenosphere during intracontinental back‐arc extension. In contrast, the granite has relatively high SiO₂ and low MgO contents with much lower ε_Nd(t) of −8.6 to −7.3, variable and negative zircon ε_Hf(t) of −10.4 to −1.3, and high zircon δ¹⁸O of 9.4 ± 0.2‰ values and was most likely derived from an ancient metasedimentary source. This magma subsequently underwent recharge with minor amounts mafic magma followed by fractional crystallization of K‐feldspar in middle‐upper crust (~10–20 km) magma chambers. Using our new data, in combination with Nd‐Hf isotopes, we present the first comprehensive picture of crustal evolution in southern Lhasa. The southern Lhasa sub‐block is likely to have been a microcontinent that underwent extensive Phanerozoic crustal reworking and growth, rather than a Mesozoic‐Early Tertiary juvenile accretionary arc terrane.