Melting Dynamics of Late Cretaceous Lamprophyres in Central Asia Suggest a Mechanism to Explain Many Continental Intraplate Basaltic Suite Magmatic Provinces

Abstract

Intraplate small‐volume mafic magmatism is spatially and temporally widespread in central and eastern Asia, but the relevant melting dynamics have remained enigmatic. Here, we report Ar‐Ar ages, mineral and whole‐rock compositions on newly found ∼81 Ma lamprophyre dykes from central Asia, aiming to constrain the source characteristics and the melting dynamics in this intraplate setting. Mineral chemistry of the lamprophyres shows that pre‐emplacement magmas equilibrated at 970–1060°C (probably at the base of the crust) and contained 1.4–2.1 wt% water. The source region of the lamprophyres is shown to have H contents equivalent to H2O = ∼150 ppm, and to be lithologically heterogeneous with silica‐deficient pyroxenite embedded in peridotite. Because of the thermodynamic complexities involved in calculating melting scenarios with major elements for such mantle domains, we have conducted a grid search with a forward‐modeling methodology that can simulate the adiabatic decompression melting of a lithologically heterogeneous mantle using incompatible trace elements. The modeling results indicate that original melting occurred at a potential temperature of ∼1400°C under a thinned lithosphere (∼70 km, corresponding to a final melting pressure of 2.3 GPa). Combining these conditions with constraints from regional geology and the tectonic history, the lamprophyres are inferred to have formed by decompression melting induced by small‐scale asthenospheric upwellings due to the edge effects under a corrugated lithospheric lower boundary. We suggest that this scenario has global relevance and represents a likely mechanism for the initiation of relatively small‐scale intraplate magmatism within continents.

Phantom recycled carbonates and mantle pyroxenites: insight from the low-δ26Mg intraplate basaltic province across central-eastern Asia

“…The results show that the Mg-isotopic characteristics of the Langshan basaltic rocks and most low-26 Mg Cenozoic basalts from eastern Asia can be well explained by the adiabatic melting of source domains containing variable fractions of isotopically light pyroxenites, where the fraction is 2-5% for the Langshan basalts and up to 50% for the lamprophyres. Notably, the high pyroxenites fractions in the source of the lamprophyres align well with and thus cross-validate by our previous estimates (55 wt.% pyroxenites in the source) using trace elements (Dai et al, 2021a).…”

“…S1). The basaltic associations were considered to originate from highly similar pyroxenite-bearing asthenospheric sources fluxed by components from the subducted Paleo-Asian oceanic igneous crust (Dai et al, 2021a). The samples previously used for whole-rock major-, trace-element and Sr-Nd-Pb isotope analyses were selected here for Mg-isotope determination.…”

“…Moreover, radiogenic Pb-isotope compositions (e.g., 206 Pb/ 204 Pb>18.5, 208 Pb/ 204 Pb>38.5), typical of ancient recycled components, have now been recognized in basaltic rocks on both sides of the gravity lineament (Fig. 1A, e.g., Langshan, Dai et al, 2021a;Niutoushan, Sun et al, 2017;Wudi, Qian et al, 2020). Accordingly, the asthenospheric sources of the vast post-110 Ma diffuse basaltic province should have been pervasively infiltrated by recycled components from older subduction episodes before the subduction and stagnation of (Paleo-) Pacific slab beneath eastern China.…”

“…S1), including low silica and high alkali-element contents, oceanic island basalt (OIB)-like trace-element patterns and moderately depleted Sr-Nd isotope compositions (Sheldrick et al, 2020). Their mantle sources are now considered to be pyroxenite-bearing asthenospheric domains polluted by recycled slab materials from multiple subduction episodes that contributed to the construction of the vast central-eastern Asian landmass (Xu et al, 2018;Dai et al, 2021a). In particular, the basaltic rocks east of the N-S gravity lineament cutting the North China Craton (directly above the stagnant (Paleo-) Pacific slab, Fig.…”

Petrogenesis of Early Cretaceous alkaline basalts in the West Qinling: Constraints from olivine chemistry

“…shows that the majority of these samples are plotted in the WPA area, by steps in lithosphere structure combined with plate movement (Dai et al, 2021;Ma et al, 2011;Missenard & Cadoux, 2012;Moufti et al, 2012). Nevertheless, most intraplate volcanic rocks in the West Qinling Orogenic Belt occur with small volumes, and the mantle potential temperatures beneath the Ganjia area vary from 1215 to 1330 C, lower than the temperature of the plume-type mantle ($1600 C; Putirka, 2016).…”