Abstract:The ultramafic_mafic dyke swarms on the southern piedmont of the Tongbai orogenic belt in Suizhou-Zaoyang area consists of two remarkably different types: troctolitic dyke (sill) swarms and metamorphosed mafic dyke ones. The troctolitic dyke swarms did not experience any metamorphism, so its emplacement age might be not earlier than that of the HP-UHP metamorphism (220～240 Ma), belonging probably to the product of late- to post-orogenic magmatism. Nevertheless, the metamorphosed mafic dyke swarms are geochemically very similar to Late Proterozoic basalt of Huashan Group in the area and are thus likely to be acognate phase. The troctolitic dyke swarms consist chiefly of olivine, plagioclase, orthopyroxene and clinopyroxene with minor phlogopite. Olivine is chrysolite (Fo72～78). Plagioclase crystals are mainly bytownite with minor labradorite.Clinopyroxene includes augite and diopside. The TiO2 content of phlogopite (3.36%) is high and indicates a Ti-rich variety. The olivine-augite equilibrium tempera-tures mainly range from 1 120 to 1 165℃, and the crystallization temperatures of augites range from 971 to 1?079℃. The rocks are characterized by high Al203 (17.25%～20.20%), MgO (9.56%～15.30%) and MgO/FeO* (1.26～1.76), and low TiO2 (0.33%～0.58%) and alkaline (Na2O+K2O=1.64%～2.23%). The transitional element contents (Cr=70～125 μg/g, Ni=251～518 μg/g, Co=54～75 μg/g) and the Mg numbers (Mg#=54～67) are relatively high in the rocks, which may approximately represent partial melting of the parental magma in the upper mantle. The rare earth element contents of the troctolitic swarms are low (ΣREE=18.66～35.42 μg/g) and are characterized by relative strong fractionation between LREE and HREE ［(La/Yb) N=4.4～4.7］ and especially by prominent features of strong Ba, Sr and Eu positive anomalies (Eu/Eu*=1.31～1.49), thus suggesting relative enrichment of Ca-plagioclase in the swarms. There exist no HFS (high field strength) elements (Nb, Ta, U, Th, Zr, Hf) anomalies in the troctolitic dyke swarms. Such phenomena are quite different from geochemical characteristics of the enrichment-type upper mantle indicated by the early Paleozoic basalt in the area, but similar to the features of Paleogene basaltic rocks in the orogenic belt, the foreland and the backland. It is thus considered that, with the delamination of the mountain root, intense interaction occurred between the subcontinental lithospheric mantle and the lower mantle. The development of troctolitic dyke (sill) swarms means a strong extensional event in the back part of the subducted plate after the delamination of the mountain root.
