Mantle sources and magma–crust interactions in volcanic rocks from the northern Kenya rift: geochemical evidence

Abstract

Miocene to Quaternary mafic (SiO 2<54 wt.%) and evolved (SiO 2>54 wt.%) lavas from Samburu Hills, northern Kenya rift, exhibit significant variations in major and trace element compositions, and Sr–Nd isotopic ratios. Bimodal lavas were erupted in the early and middle Miocene (20–15 Ma and 15–10 Ma) and Pliocene (4.1–3.6 Ma), whereas only mafic lavas were produced in the upper Miocene (7.5–5.3 Ma) and Quaternary (2.0–0.1 Ma). Incompatible element concentrations (Zr, Nb, Y, Rb, and K) vary widely in the Miocene mafic lavas, but are depleted in Pliocene and Quaternary equivalents. In the Miocene and Pliocene mafic lavas, initial 87Sr/ 86Sr ratios vary from 0.70315 to 0.703808, and initial 143Nd/ 144Nd ratios vary from 0.51254 to 0.51288. 143Nd/ 144Nd ratios of the Quaternary lavas are similar (0.512724–0.512873), but 87Sr/ 86Sr ratios are higher (0.70432–0.70468). On a Nd- vs. Sr-isotopic diagram, the Quaternary lavas plot near bulk earth, whereas all other mafic lavas lie within the depleted mantle quadrant and at the left end of the East African Carbonatite Line (EACL). The geochemical variations of the mafic lavas originated from heterogeneous mantle sources produced by interaction of plume and lithospheric mantle, and subsequently modified by variable degrees of partial melting. The evolved lavas are divisible into evolved high-alkali (Na 2O+K 2O>9 wt.%) early Miocene latites, trachyphonolites, and middle Miocene and Pliocene trachytes; less evolved low-alkali (Na 2O+K 2O<9 wt.%) middle Miocene mugearite, and Pliocene benmoreite and trachyte suites. The high-alkali trachyphonolites and trachytes are enriched in Nb, Zr, Rb, and K, and depleted in Ba, Sr, P, and Ti relative to latites and evolved low-alkali lavas. The evolved high-alkali lavas have higher initial Sr-isotopic (0.70372–0.705939) ratios than their mafic equivalents, but their initial Nd-isotopic ratios (0.512656–0.512777) lie within the same range. The low-alkali suites, however, have lower initial Nd-isotopic ratios (0.51235–0.51250), and slightly higher Sr-isotopic ratios (0.70347–0.70407) than their associated mafic lavas. Assimilation and fractional crystallization (AFC) of mafic magma is considered to be the dominant process for the generation of evolved lavas. The high-alkali and low-alkali evolved lavas require low and high degrees of crustal assimilation, respectively.