Plume-lithosphere interaction in the generation of the Tarim large igneous province, NW China: Geochronological and geochemical constraints

Abstract

The magmatism in the early Permian Tarim large igneous province (TLIP) in NW China is represented by basaltic lava flows in Keping and ultramafic-mafic-felsic intrusions and mafic dikes in Bachu, northwestern Tarim Craton. This paper reports new Ar-Ar dating results and chemical compositions of Keping basalts and Bachu dikes, with aims of better characterizing the timing of and mantle/crustal contribution to the TLIP. The Keping basalts yield two well-defined ⁴⁰Ar/³⁹Ar plateau ages of 287.3 ± 4.0 Ma and 287.9 ± 4.1 Ma, which, together with age data from the literature, define a magmatic event at ∼289 Ma. The intrusions and dikes in Bachu are believed to have formed at ∼279 Ma based on screened literature data. Thus, they together define two magmatic episodes. The Keping basalts, representing the earlier episode, have alkaline affinity (SiO₂ = 44.0-47.9 wt.%, Na₂O + K₂O = 3.7-4.9 wt.%), low MgO (4.3-5.9 wt.%) and high TiO₂ (3.8-5.1 wt.%) contents, showing fractionated chondrite-normalized LREE and nearly flat HREE patterns [(La/Yb)_N = 6.27-7.71; (Dy/Yb)_N = 1.36-1.48] with noticeable negative Nb and Ta anomalies in the primitive mantle-normalized trace element diagram. They have negative and relatively uniform ε_Nd(t) (−2.3 to −3.8) and low (²⁰⁶Pb/²⁰⁴Pb)_i (17.43-17.57). We argue that these “crustal signatures” cannot be attributed to crustal assimilation because neither ε_Nd(t) nor (²⁰⁶Pb/²⁰⁴Pb)_i correlates with SiO₂; rather they are more likely derived from a sub-continental lithospheric mantle (SCLM) source metasomatized by subduction-related processes. The Bachu dikes, representing the later episode and confined to the margins of the Tarim Craton, have similar MgO (3.6-5.4 wt.%) and TiO₂ (3.1-4.7 wt.%) contents to the Keping basalts, and display more fractionated REE patterns [(La/Yb)_N = 10.1-14.0; (Dy/Yb)_N = 1.79-1.99]. They have variable isotope compositions [ε_Nd(t) = −0.3-4.8, (²⁰⁶Pb/²⁰⁴Pb)_i = 17.50-18.11] and display OIB-like trace element signatures. Correlations between isotopic and trace element ratios indicate that some dikes with low ε_Nd(t) and low initial Pb isotope ratios could have been subjected to crustal assimilation. We propose a model involving plume-lithosphere interaction to account for the two discrete magmatic episodes with distinct mantle sources in the TLIP. The earlier episode was formed in response to the impact of a sub-lithospheric mantle plume at the base of the SCLM. Partial melting of the metasomatized lithospheric mantle was triggered by temperature increase due to conductive heating of the impregnating mantle plume. The later episode was generated by decompression melting of the mantle plume, as a result of deflection of the plume towards the margins of the Tarim Craton with thinner lithosphere.