Lamprophyre or Lamproite Dyke in the SW Tarim Block? -Discussion on the Petrogenesis of These Rocks and Their Source Region

The mineralogical, petrological and geochemical studies on Keliyang () potassic dykes have been carried out to understand their rock types, the petrogenesis and the nature of their mantle sources. They are potassic lamprophyre, not lamproites as the previous researchers believed. In this study, the whole-rock major and trace element compositions of another 6 lamproite dykes recently discovered are reported. Major elements were determined by X-ray fluorescence spectrometry (XRF) techniques, while REE and trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS). They can be classified into phlogopite-diopside lamprophyre, leucite-diopside lamprophyre and granular carbonatite-bearing diopside lamprophyre on the basis of their mineral components. They are all characterized by relatively low SiO2 (41. 31%-44. 84%), TiO2 (0.75%-0.86%) and high MgO (7.30%-11.33%), K2O (4.01%-6.01%) concentrations with K2O/KNa2O ratios of 2.77-12.49. In addition, they display enrichment in large-ion lithophile elements (LILEs, e. g., Rb, Sr, Ba) and LREE, but a relative depletion in high-field-strength elements (HFSEs, e. g., Nb, Ta, Zr, Hf and Ti). They display similar chondrite-normalized REE patterns with slight negative Ea anomalies (δEu=0.64-0.82), and high initial 87Sr/86 ratios, which resemble those of high K/Ti and low-Ti potassic magmas formed in subduction-related settings. Consequently, we suggest that the parental magma was generated by partial melting of the phlogopite-amphibole-bearing garnet Iherzolite within the lithospheric mantle that might have been metasomatized by a potassium-bearing fluid released from a subduction oceanic crust.