## 4 Results
### Geochemistry of the Ildeus Intrusion  
The major and trace geochemical characteristics of ultramafic rocks (dunites, plagioclase-bearing dunites, wehrlites, websterites and pyroxenites) from the Ildeus intrusion are summarized in Table 1. Intrusive rocks display significant variations in silica contents accompanied by a decrease in MgO, Cr, Ni and Co contents from dunites and wehrlites to websterites and pyroxenites (Table 1). Ildeus ultramafic rocks are characterized by a wide range of Na\({}_{2}\)O contents (0.06-2.62 wt.%; Table 1) coupled with relatively low K\({}_{2}\)O concentrations of 0.02-0.15 wt.% (with the exception of a single pyroxenite sample with 2.51 wt.% K\({}_{2}\)O; Table 1). The TiO\({}_{2}\) contents are variable, but are generally low to moderate (0.11-1.07 wt.%), which is consistent with the overall low contents of high-field strength elements (HFSE) and variable concentrations of light rare earth (LREE) and large-ion lithophile (LILE) incompatible elements in the Ildeus intrusion (Table 1). Major element variations suggest that Ildeus rocks represent igneous cumulates formed via fractional crystallization of primitive, island arc-type magma (Figure 3a). All ultramafic plutonic rocks from the Ildeus intrusion exhibit clear HFSE depletions (Figure 3b) coupled with high LILE/LREE, LILE/HFSE ratios typical of convergent zone magmas [51; 52], suggestive of their derivation from subduction-modified mantle sources [36]. Hornblende-biotite dacite and HFSE-rich lamprophyre disks within the Ildeus intrusion compositionally resemble typical adakites [52; 53; 54] and high-Nb basalts [52; 54; 55; 56]. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Sample** & **001H** & **007H** & **008H** & **036H** & **069F** & **070F** & **074F** & **037F** & **040F** \\ \hline
**Rock Type** & **Pl-D** & **Pl-D** & **D** & **D** & **Pl-D** & **Pl-D** & **Pl-D** & **Whr** & **Whr** \\ \hline SiO\({}_{2}\) (wt.\%) & 40.15 & 39.36 & 38.49 & 48.55 & 41.39 & 42.18 & 44.70 & 45.47 & 45.82 \\ TiO\({}_{2}\) & 0.11 & 0.15 & 0.16 & 0.23 & 0.16 & 0.20 & 0.27 & 0.59 & 0.57 \\ Al\({}_{2}\)O\({}_{3}\) & 5.07 & 4.75 & 1.32 & 2.83 & 4.74 & 4.50 & 5.08 & 5.52 & 6.17 \\ Fe\({}_{2}\)O\({}_{3}\) & 12.12 & 11.38 & 12.40 & 12.84 & 12.55 & 12.93 & 11.80 & 10.59 & 11.83 \\ MnO & 0.16 & 0.17 & 0.18 & 0.18 & 0.18 & 0.23 & 0.23 & 0.18 & 0.19 \\ MgO & 32.29 & 34.38 & 35.17 & 32.84 & 31.72 & 31.16 & 31.52 & 24.25 & 23.61 \\ CaO & 2.85 & 3.19 & 0.52 & 1.94 & 3.54 & 3.14 & 2.34 & 11.20 & 8.91 \\ Na\({}_{2}\)O & 0.40 & 1.00 & 0.06 & 0.25 & 0.60 & 0.42 & 0.25 & 0.89 & 0.66 \\ K\({}_{2}\)O & 0.05 & 0.05 & 0.02 & 0.02 & 0.05 & 0.04 & 0.03 & 0.06 & 0.05 \\ P\({}_{2}\)O\({}_{5}\) & 0.03 & 0.03 & 0.03 & 0.02 & 0.03 & 0.03 & 0.02 & 0.00 & 0.00 \\ LOI & 6.76 & 5.54 & 11.70 & 0.24 & 5.02 & 5.14 & 3.78 & 1.24 & 2.18 \\ Total & 99.99 & 99.99 & 100.05 & 100.00 & 99.98 & 99.99 & 100.00 & 100.00 & 100.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Major (wt.%) and trace (ppm) element geochemistry of ultramafic rocks from the Ildeus intrusion (Stanovoy suture zone, Far East Russia). Figure 2: Petrographic features of olivine websterite IL-5. (**a,b**) clinopyroxene-dominated mesocumulate with amphibole as the main intercumulus phase: (**a**)—parallel nicols, (**b**)—crossed nicols. (**c**) olivine-orthopyroxene-clinopyroxene accumulate “cemented” by amphibole intercumulus (parallel nicols). (**d**) olivine-clinopyroxene-orthopyroxene mesocumulate texture with amphibole intercumulus (parallel nicols). (**e**) detail of orthopyroxene-dominated mesocumulate texture with abundant intercumulate amphibole (parallel nicols). (**f,g**) orthopyroxene-dominated orthocumulate texture with plagioclase intercumulus: g—parallel nicols, (**b**)—crossed nicols. (**h,i**) olivine-orthopyroxene-dominated (“harzburgitic”) orthocumulate texture with plagioclase intercumulus: (**h**)—parallel nicols, (**i**)—crossed nicols.