Petrogenesis and tectonic setting of Late Devonian high-Mg andesites in the Beishan orogen, NW China

High-Mg andesites (HMAs) are crucial for the reconstruction of plate tectonics, continental margin formation and lithospheric evolution. In this study, we present new fossil age, whole-rock geochemical and Sr–Nd isotope data on the newly discovered Dundunshan Group HMAs in the Dundunshan area of the Beishan orogen (central-southern Central Asian Orogenic Belt). The Dundunshan HMA samples are characterized by high MgO (6.47–7.02 wt%) contents and high Mg# values (67.27–68.77), with SiO2 (58.57–62.13 wt%), Al2O3 (14.49–16.07 wt%) and CaO (5.05–6.24 wt%) resembling typical HMAs. The Dundunshan HMA samples are calc-alkaline and strongly enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), with slightly negative Eu anomaly and high-field-strength element (HFSE) depletions. Their (87Sr/86Sr)i ratios (0.7041–0.7057) and ϵNd(t) (3.73–5.59) indicate that the Dundunshan HMAs were mainly formed by the interactions between subducted oceanic sediment-derived melts and mantle peridotites. Fossil evidence and published radiometric age data constrain the formation of the Dundunshan HMAs to early Late Devonian time. Sedimentological features of the Middle Devonian Sangejing Formation and regional tectonic correlation suggest that the Hongliuhe–Niujuanzi–Xichangjing Ocean in the Dundunshan area was likely closed during late Middle Devonian time, and that the Dundunshan HMAs were formed in a post-collision extensional setting.

Depletion of the upper mantle by convergent tectonics in the Early Earth

Partial melting of mantle peridotites at spreading ridges is a continuous global process that forms the oceanic crust and refractory, positively buoyant residues (melt-depleted mantle peridotites). In the modern Earth, these rocks enter subduction zones as part of the oceanic lithosphere. However, in the early Earth, the melt-depleted peridotites were 2–3 times more voluminous and their role in controlling subduction regimes and the composition of the upper mantle remains poorly constrained. Here, we investigate styles of lithospheric tectonics, and related dynamics of the depleted mantle, using 2-D geodynamic models of converging oceanic plates over the range of mantle potential temperatures (Tp = 1300–1550 °C, ∆T = T − Tmodern = 0–250 °C) from the Archean to the present. Numerical modeling using prescribed plate convergence rates reveals that oceanic subduction can operate over this whole range of temperatures but changes from a two-sided regime at ∆T = 250 °C to one-sided at lower mantle temperatures. Two-sided subduction creates V-shaped accretionary terrains up to 180 km thick, composed mainly of highly hydrated metabasic rocks of the subducted oceanic crust, decoupled from the mantle. Partial melting of the metabasic rocks and related formation of sodic granitoids (Tonalite–Trondhjemite–Granodiorite suites, TTGs) does not occur until subduction ceases. In contrast, one sided-subduction leads to volcanic arcs with or without back-arc basins. Both subduction regimes produce over-thickened depleted upper mantle that cannot subduct and thus delaminates from the slab and accumulates under the oceanic lithosphere. The higher the mantle temperature, the larger the volume of depleted peridotites stored in the upper mantle. Extrapolation of the modeling results reveals that oceanic plate convergence at ∆T = 200–250 °C might create depleted peridotites (melt extraction of > 20%) constituting more than half of the upper mantle over relatively short geological times (~ 100–200 million years). This contrasts with the modeling results at modern mantle temperatures, where the amount of depleted peridotites in the upper mantle does not increase significantly with time. We therefore suggest that the bulk chemical composition of upper mantle in the Archean was much more depleted than the present mantle, which is consistent with the composition of the most ancient lithospheric mantle preserved in cratonic keels.

Breaking of Henry’s law for sulfide liquid–basaltic melt partitioning of Pt and Pd

Platinum group elements are invaluable tracers for planetary accretion and differentiation and the formation of PGE sulfide deposits. Previous laboratory determinations of the sulfide liquid–basaltic melt partition coefficients of PGE ($${D}_{PGE}^{SL/SM}$$ D P G E S L / S M ) yielded values of 102–109, and values of >105 have been accepted by the geochemical and cosmochemical society. Here we perform measurements of $${D}_{Pt,\,Pd}^{SL/SM}$$ D P t , P d S L / S M at 1 GPa and 1,400 °C, and find that $${D}_{Pt,\,Pd}^{SL/SM}$$ D P t , P d S L / S M increase respectively from 3,500 to 3.5 × 105 and 1,800 to 7 × 105, as the Pt and Pd concentration in the sulfide liquid increases from 60 to 21,000 ppm and 26 to 7,000 ppm, respectively, implying non-Henrian behavior of the Pt and Pd partitioning. The use of $${D}_{Pt,\,Pd}^{SL/SM}$$ D P t , P d S L / S M values of 2,000–6,000 well explains the Pt and Pd systematics of Earth’s mantle peridotites and mid-ocean ridge basalts. Our findings suggest that the behavior of PGE needs to be reevaluated when using them to trace planetary magmatic processes.

Water contents of nominally anhydrous orthopyroxenes from oceanic peridotites determined by SIMS and FTIR

This study presents new secondary ion mass spectrometry (SIMS) reference materials (RMs) for measuring water contents in nominally anhydrous orthopyroxenes from upper mantle peridotites. The enstatitic reference orthopyroxenes from spinel peridotite xenoliths have Mg#s between 0.83 and 0.86, Al2O3 ranges between 4.02 and 5.56 wt%, and Cr2O3 ranges between 0.21 and 0.69 wt%. Based on Fourier-transform infrared spectroscopy (FTIR) characterizations, the water contents of the eleven reference orthopyroxenes vary from dry to 249 ± 6 µg/g H2O. Using these reference grains, a set of orthopyroxene samples obtained from variably altered abyssal spinel peridotites from the Atlantic and Arctic Ridges as well as from the Izu-Bonin-Mariana forearc region was analyzed by SIMS and FTIR regarding their incorporation of water. The major element composition of the sample orthopyroxenes is typical of spinel peridotites from the upper mantle, characterized by Mg#s between 0.90 and 0.92, Al2O3 between 1.66 and 5.34 wt%, and Cr2O3 between 0.62 and 0.96 wt%. Water contents as measured by SIMS range from 68 ± 7 to 261 ± 11 µg/g H2O and correlate well with Al2O3 contents (r = 0.80) and Cr#s (r. = -0.89). We also describe in detail an optimized strategy, employing both SIMS and FTIR, for quantifying structural water in highly altered samples such as abyssal peridotite. This approach first analyzes individual oriented grains by polarized FTIR, which provides an overview of alteration. Subsequently, the same grain along with others of the same sample is measured using SIMS, thereby gaining information about homogeneity at the hand sample scale, which is key for understanding the geological history of these rocks.

Arc-related peridotite blocks exhumed to the Eastern Block of the North China Craton prior to 2.47 Ga

The Songling and Majiayu peridotite blocks occur in the Eastern Block, North China Craton (NCC). Geothermobarometry data show that the Songling peridotites began exhumation from a depth of c. 70 km (c. 23 kbar). During exhumation, the Songling peridotites were intruded by an upper-crust-derived, high-δ18O (up to +7.28‰ in zircon) trondhjemitic dyke at 2.47 Ga and experienced granulite-facies metamorphism. The Songling peridotites have hybrid mantle wedge (HMW) -like high SiO2 (> 45 wt%), high FeOt (c. 10 wt%) content, high modal orthopyroxene abundance (> 35%) and high ϵNd(t) (+18.4 to +21.4), which were generated by the reaction between peridotite and eclogite-derived melts. The clinopyroxenes from the Songling peridotites were in equilibrium with a Nb-, Zr- and Ti-depleted arc-like magma. The Majiayu peridotites are characterized by depletion of Nb, Zr and Hf and are highly enriched in FeOt, Th and light rare earth elements (LREEs), which can be interpreted as an open system reaction between hydrous melts and fore-arc mantle peridotites. These two peridotite blocks are considered to be arc-related mantle peridotites that experienced melt extraction and metasomatism in different styles. They were exhumed to the north margin of the North China Craton during the c. 2.47 Ga arc–continent collision along the Zunhua structural belt.