Geochronological and geochemical study of mafic dykes from the northwest Chinese Altai: Implications for petrogenesis and tectonic evolution

Abstract

Fifteen zircons separated from a mafic dyke in the Chinese Altai give a concordant age population with a weighted mean 206Pb/ 238U age of 375.5 ± 4.8 Ma, suggesting a Devonian emplacement. On the basis of their mineralogical compositions and textures, the coeval dykes can be divided into gabbroic and doleritic types. They are both sub-alkaline, tholeiitic, characterized by similarly low SiO 2 contents (45.2–52.7 wt.%) and total alkaline (K 2O + Na 2O = 0.99–4.93 wt.%). Rare earth element patterns of the gabbroic dykes are similar to N-MORB (La/Yb N = 0.86–1.1), together with their high ε Nd( t) values (+ 7.6 to + 8.1), indicating that their precursor magma was mainly derived from a N-MORB-type depleted asthenospheric mantle. While the REE patterns of the doleritic dykes resemble that of E-MORB (La/Yb N = 1.12–2.28), enriched in LILEs and strongly depleted in HFSEs, with relative low ε Nd( t) values (+ 3.4 to + 5.4) and high initial 87Sr/ 86Sr ratios (0.7057–0.7060). The zircon Hf isotopic analysis of the doleritic dykes give ε Hf( t) values from + 10.7 to + 13.8. These signatures suggest that a depleted mantle wedge metasomatized by slab-derived fluids and/or melts was possibly involved in the generation of the doleritic magma. The refractory peridotite may have been melted with variable degrees caused by upwelling of the hot asthenosphere. The petrogenesis of the mafic dykes suggest a high heat flux as a result of upwelling of the hot asthenosphere and the contrast geochemical signatures can be interpreted by a ridge subduction, which could be an important tectonic control in the accretionary process of the Chinese Altai.