Pyroxenite Xenoliths Record Complex Melt Impregnation in the Deep Lithosphere of the Northwestern North China Craton

Abstract

Transformation of refractory cratonic mantle into more fertile lithologies is the key to the fate of cratonic lithosphere. This process has been extensively studied in the eastern North China Craton (NCC) while that of its western part is still poorly constrained. A comprehensive study of newly-found pyroxenite xenoliths from the Langshan area, in the northwestern part of this craton is integrated with a regional synthesis of pyroxenite and peridotite xenoliths to constrain the petrogenesis of the pyroxenites and provide an overview of the processes involved in the modification of the deep lithosphere. The Langshan pyroxenites are of two types, high-Mg# [Mg2+/(Mg2++Fe2+)*100 = ∼ 90, atomic ratios] olivine-bearing websterites with high equilibration temperatures (880 ∼ 970 oC), and low-Mg# (70 ∼ 80) plagioclase-bearing websterites with low equilibration temperatures (550 ∼ 835 oC). The high-Mg# pyroxenites show trade-off abundances of olivine and orthopyroxene, highly depleted bulk Sr-Nd (ƐNd = +11.41, 87Sr/86Sr = ∼0.7034) and low clinopyroxene Sr isotopic ratios (mean 87Sr/86Sr = ∼0.703). They are considered to reflect the reaction of mantle peridotites with silica-rich silicate melts derived from the convective mantle. Their depletion in fusible components (e.g., FeO, TiO2 and Na2O) and progressive exhaustion of incompatible elements suggest melt extraction after their formation. The low-Mg# pyroxenites display layered structures, convex-upward rare earth element patterns, moderately enriched bulk Sr-Nd isotopic ratios (ƐNd = -14.20 ∼ -16.74, 87Sr/86Sr = 0.7070 ∼ 0.7078) and variable clinopyroxene Sr-isotope ratios (87Sr/86Sr = 0.706-0.711). They are interpreted to be crustal cumulates from hypersthene-normative melts generated by interaction between the asthenosphere and heterogeneous lithospheric mantle. Combined with studies on regional peridotite xenoliths, it is shown that the thinning and refertilization of the lithospheric mantle was accompanied by crustal rejuvenation and that such processes occurred ubiquitously in the northwestern part of the NCC. A geodynamic model is proposed for the evolution of the deep lithosphere, which includes long-term mass transfer through a mantle wedge into the deep crust from the Paleozoic to the Cenozoic, triggered by subduction of the Paleo-Asian ocean and the Late Mesozoic lithospheric extension of eastern Asia.

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

“…In this context, marked lithospheric thinning and rifting are unlikely in the Late Cretaceous, and thus could not account for the coeval magmatism in that region. Edge‐driven convection. The lithosphere in the northwestern NCC probably had been thinned to <80 km before the eruption of the ∼89 Ma Langshan basalts, based on the absence of garnet‐bearing xenoliths (Dai et al., 2018, 2020; Wu, Liu, et al., 2017; Zhang et al., 2020) and the calculated low melting pressures of the regional xenolith‐bearing basalts (Dai et al., 2019; Xu et al., 2017). This is comparable to the present lithospheric thickness revealed by surface‐wave tomographic imagery (Huang et al., 2009).…”

“…Third, the pre‐emplacement temperatures of the magma system (see Section 5.2) overlap with the solidus (∼1010°C) of the lower continental crust (e.g., Rudnick & Gao, 2014) as constrained by Rhyolite‐MELTS modeling (Gualda et al., 2012), suggesting limited crustal assimilation. Fourth, energy‐constrained simultaneous assimilation and crystallization (EC‐AFC) modeling (Bohrson et al., 2014), using the averaged Langshan basalts (Dai et al., 2019) as the starting material, has shown that crustal assimilation has been very limited (<7.3%, Dai et al., 2020). Crustal assimilation of this extent, if present, will have little effect on the trace‐element concentration of the lamprophyres because the lamprophyres are much more enriched in incompatible elements than lower continental crust (Figure 8).…”

“…The absence of garnet‐bearing xenoliths indicates a thinned lithospheric mantle compared to the adjacent thick blocks, for example, the Ordos block in the south (Chen et al., 2014) and the Gobi block in the north (Zhang et al., 2017). The lithospheric thinning in this region is inferred to have occurred in the Early Cretaceous or before (Dai et al., 2020); this thin‐lithosphere corridor remained 70–80 km thick from the Late Cretaceous to the present, as indicated by the evolution of xenolith‐bearing basalts in this period (Dai et al., 2019; Xu et al., 2017) and geophysical observations (Huang et al., 2009; He, 2015).…”

“…The basalt-lamprophyre pair investigated here is located in the northwestern NCC (Figure 1b). The lithospheric mantle in this region, represented by spinel lherzolite and pyroxenite xenoliths from the ∼89 Ma basalts (Dai et al, 2018(Dai et al, , 2020 and regional Cenozoic basalts (Wu, Ling, et al, 2017;Zhang et al, 2020), is fertile and has depleted trace-element and isotopic compositions. The absence of garnet-bearing xenoliths indicates a thinned lithospheric mantle compared to the adjacent thick blocks, for example, the Ordos block in the south (Chen et al, 2014) and the Gobi block in the north (Zhang et al, 2017).…”

Thermodynamic modeling of melt addition to peridotite: Implications for the refertilization of the non-cratonic continental mantle lithosphere

“…Given the Os isotope evidence that large-scale refertilization can be temporally associated with the melting event that led to harzburgite depletion, refertilization is sometimes viewed as an integral part of the melting process, and has been described as "autometasomatism" (Rudnick and Walker, 2008). When melt circulation in the mantle is more channelized, cumulative pyroxenites may form (Bodinier et al, 2008;France et al, 2015;Tilhac et al, 2016;Borghini et al, 2020;Dai et al, 2021); such pyroxenites are among the most fertile mantle lithologies, and could thus be important components in any potential subsequent partial melting episode (e.g., Lambart et al, 2013;France et al, 2015).…”