ALBERT

Description: 〈span〉Mantle-derived alkaline magmatism along major strike-slip faults provides unique insights into the lateral growth of the Tibetan Plateau. Here we use the geochemistry of perovskites from the West Qinling melilitite to probe into the nature and dynamics of sub-lithospheric mantle beneath the northeastern Tibetan Plateau. The texture and chemical composition of perovskites indicate their early crystallization from a CO〈sub〉2〈/sub〉-rich melilitite magma. Most perovskite crystals have moderately depleted Sr-Nd isotopic compositions, whereas a few grains exhibit high 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr〈sub〉i〈/sub〉 and low ε〈sub〉Nd〈/sub〉(t). Together with the bulk-rock geochemistry of the melilitite, the perovskite Sr-Nd isotope data imply that the primary magma parental to the melilitite was most likely derived from seafloor subduction–modified asthenosphere and underwent interaction with lithospheric mantle during ascent. 〈span〉In situ〈/span〉 U-Pb dating of the perovskites demonstrates the temporal correlation between the melilitite magmatism and Kunlun strike-slip faulting in the early Miocene. These findings indicate the fundamental role of India-Asia convergence in driving outward plateau growth through strike-slip extrusion and in reactivating long-lived lithospheric zones of weakness for evacuating low-volume asthenospheric melts.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Mantle-derived alkaline magmatism along major strike-slip faults provides unique insights into the lateral growth of the Tibetan Plateau. Here we use the geochemistry of perovskites from the West Qinling melilitite to probe into the nature and dynamics of sub-lithospheric mantle beneath the northeastern Tibetan Plateau. The texture and chemical composition of perovskites indicate their early crystallization from a CO〈sub〉2〈/sub〉-rich melilitite magma. Most perovskite crystals have moderately depleted Sr-Nd isotopic compositions, whereas a few grains exhibit high 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr〈sub〉i〈/sub〉 and low ε〈sub〉Nd〈/sub〉(t). Together with the bulk-rock geochemistry of the melilitite, the perovskite Sr-Nd isotope data imply that the primary magma parental to the melilitite was most likely derived from seafloor subduction–modified asthenosphere and underwent interaction with lithospheric mantle during ascent. 〈span〉In situ〈/span〉 U-Pb dating of the perovskites demonstrates the temporal correlation between the melilitite magmatism and Kunlun strike-slip faulting in the early Miocene. These findings indicate the fundamental role of India-Asia convergence in driving outward plateau growth through strike-slip extrusion and in reactivating long-lived lithospheric zones of weakness for evacuating low-volume asthenospheric melts.〈/span〉

Description: Zircons entrained in mantle-derived magmas offer a unique opportunity to identify cryptic magmatic episodes in the deep crust and thus to image lithospheric thickening and crustal evolution. We investigated zircon xenocrysts from mantle-derived ultrapotassic rocks in southern Tibet to evaluate their potential as a probe of crustal evolution. Similar age (Proterozoic–Paleozoic) distributions of these zircons and those in the Lhasa terrane detrital spectra demonstrate the continental origin of xenocrysts with high U/Yb. Time-progressive variations in zircon Hf ( t ) reveal three major magmatic pulses ca. 90, 50, and 20 Ma, suggesting significant crustal growth in the Lhasa terrane at those times. This is consistent with major mantle inputs previously documented from surface rocks in the Lhasa terrane. Increasing Dy N /Yb N and U/Yb since ca. 55 Ma are interpreted to reflect progressive crustal thickening in response to the India-Asia convergence. Zircon xenocrysts with varying U-Pb ages and heterogeneous Hf isotopes indicate assimilation of Lhasa terrane crust in the genesis of ultrapotassic magmas.

Description: 〈span〉Recent studies demonstrate that lithosphere thickness variation exerts the primary control on global seafloor basalt compositions. If the mechanism of such control, i.e., the lid effect, is indeed at work, lithosphere thickness variation must also influence basaltic compositions in continental settings. To test this hypothesis, we chose to study Cenozoic basalts in eastern continental China over a distance of ~260 km along a southeast-to-northwest traverse with a steep topographic gradient (~500 to ~1500 m above sea level) mirrored with a steep lithospheric thickness gradient (~80 to ~120 km). The basalts erupted on the thinned lithosphere to the east are characterized by lower pressure (e.g., higher Si〈sub〉72〈/sub〉, lower Mg〈sub〉72〈/sub〉, Fe〈sub〉72〈/sub〉, and [Sm/Yb]〈sub〉N〈/sub〉; subscript “72” refers to corresponding oxides corrected for fractionation effect to Mg# = 72; N—primitive mantle normalized) and higher extent (e.g., low Ti〈sub〉72〈/sub〉, P〈sub〉72〈/sub〉, K〈sub〉72〈/sub〉, Rb, Ba, Th, and ratios of more- to less-incompatible elements such as [La/Sm]〈sub〉N〈/sub〉, Ba/Zr, and Zr/Yb) of melting than basalts erupted on the thickened lithosphere to the west. Importantly, these geochemical parameters all show significant correlations with both lithosphere thickness and topographic elevation. These first-order observations are a straightforward manifestation of the lid effect. Lithospheric contamination and mantle-source compositional variation can indeed contribute to the compositional variability of these continental basalts, but these latter effects are averaged out and are overshadowed by the lid effect. This finding emphasizes the importance of evaluating the lid effect before interpreting the petrogenesis of continental basalts and mantle dynamics. Our results also indicate that the continental surface elevation is isostatically balanced above a mantle depth that is deeper than the lithosphere-asthenosphere boundary.〈/span〉