The Miyun area is located in the northwestern border of the Eastern Block of the North China Craton, where the metaultramafic rocks occur as intrusive bodies, some of which display cumulate features. The metaultramafic rock is (olivine-bearing) amphibole websterite and two generations of mineral assemblage have been identified: the magmatic mineral assemblage -consists- of clinopyroxene + orthopyroxene ± olivine ± spinel ± pyrite, whereas the subsequent metamorphic mineral assemblage comprises of hornblende + serpentine + chlorite + magnetite + zircon ± biotite ± calcite ± chromite ± calcite ± garnet. The metamorphic P-T conditions are estimated to be ~ 600–630 °C/4–6 kbar, which belongs to amphibolite facies and medium-P/T facies series. U-Pb dating of metamorphic zircon and ⁴⁰Ar/³⁹Ar dating of metamorphic hornblende indicate that the metamorphic event occurred at ~ 1.85–1.82 Ga. These data suggest that the Miyun area involved in the late Paleoproterozoic subduction-collision between the Eastern and Western Blocks, and the metamorphism possibly occurred at upper-middle crustal level with fluids.

密云地区位于华北克拉通东部地块的西北边界，那里的米托拉菲克岩以侵入体的形式出现，其中一些具有累积特征。陨石铁屑岩是（含橄榄石的）闪石韦伯石，已鉴定出两代矿物组合：岩浆矿物组合由斜辉石+正邻苯二酚±橄榄石±尖晶石±黄铁矿组成，而随后的变质矿物组合包括角闪石+蛇纹石+亚氯酸盐+磁铁矿+锆石±黑云母±方解石±亚铬酸盐±方解石±石榴石 变质PT条件估计为〜600–630°C / 4–6 kbar，属于闪长岩相和中等P / T相系列。变质锆石和变质角闪石的⁴⁰ Ar / ³⁹ Ar测年表明该变质事件发生在〜1.85-1.82 Ga。这些数据表明密云地区参与了东部和西部地块之间的晚古元古代俯冲碰撞，并且可能在地壳中上层有流体。