The origin of Kenya rift plateau‐type flood phonolites: Evidence from geochemical studies for fusion of lower crust modified by alkali basaltic magmatism

Abstract

Geochemical investigations support the petrogenesis of Kenya rift plateau‐type flood phonolites (14–11 Ma) by partial melting of an alkali basaltic material at lower crustal pressures. High‐pressure/high‐temperature experiments on a natural plateau phonolite (Hay and Wendlandt, this issue) document multiple saturation of augite, andesine, titanomagnetite, and phlogopite at 0.7 GPa, 1000°C, XCO2 = 0.42, with amphibole appearing at 975°C. A least squares solution to major element modeling, involving subtraction of the compositions of near‐liquidus augite, andesine, titanomagnetite, and olivine (Fo67; hypothesized product of incongruent melting of hydrous phases) from a Kenya Miocene alkali basalt composition, indicates that plateau phonolites can be derived by 15 wt % fusion of this hypothetical parental material (∑R2 = 0.07). Alkali basaltic magmas may have injected and/or underplated the lower crust in southern Kenya during prior rift‐related basaltic volcanism (23–14 Ma). Bulk Earth values of (87Sr/86Sr)i and εNd near zero for four plateau phonolite samples are consistent with a mantle‐derived parental composition. Three of these four samples reflect little, if any, postmelting modification; one sample may have evolved by fractional crystallization (high Rb/Sr, low Ba, Sr and Mg #). A fifth sample may show evidence of assimilation and fractional crystallization processes (elevated radiogenic Sr and Pb, large negative Eu anomaly, and low Ba, Sr, and Mg #). Much of the geochemical variation among plateau phonolite lavas, however, can be ascribed to melting of a predominantly alkali basaltic source with contributions from a lower crustal protolith. A mantle‐derived source is also supported by Sr‐Nd‐Pb isotope data for the phonolites, which indicate that the alkali basaltic source can be described in terms of high U/Pb (HIMU) and enriched mantle (EM1 and EM2) components.

Geochronologic and petrochemical evidence for the genetic link between the Maomaogou nepheline syenites and the Emeishan large igneous province

“…Seismic tomography reveals a high-velocity lower crust (V p = 7.1-7.8 km/s) in the Panxi area [48] , which has been attributed to the underplating of plume-derived basaltic melts in the crust-mantle boundary [9] . The pressure condition and basaltic composition of the lower crust in the Panxi area are very similar to those of partial melting model put forward by Hay et al [46,47] . On the other hand, positive Sr anomaly of the Maomaogou intrusion implies that its source may contain plagioclase-rich cumulates.…”

“…On the basis of experiments and geochemical elucidation, Hay et al [46,47] proposed that Kenya rift plateau-type flood phonolites were generated by partial melting of alkali basaltic material under lower-crustal pressures. Seismic tomography reveals a high-velocity lower crust (V p = 7.1-7.8 km/s) in the Panxi area [48] , which has been attributed to the underplating of plume-derived basaltic melts in the crust-mantle boundary [9] .…”

The East African rift system

“…There is a predominantly alkali basaltic source with contributions from a lower crustal protolith (Hay et al, 1995). According to Macdonald (1994), all major sequences show evidence of extensive polybaric fractionation within the upper mantle and lower crust.…”

Combined transient electromagnetic and magnetotelluric study of the southern Kenya rift valley

“…Some of the most recent geophysical work from the Kenya Rift can be found in Prodehl et al (1994a) and Fuchs et al (1997). Geological, geochemical and petrological perspectives are discussed by Latin et al, (1993), Macdonald et al, (1994), Hay et al, (1995a), Hay et al, (1995b), Wendlandt et al, (1995). Only some of the most important results will be briefly outlined here.…”

“…To explain the estimated basaltic magma (924,000 km3) generated beneath the rift zone in the past 30 Ma Latin et al, (1993) infer that the asthenospheric mantle must continually upwell through the melting region (extending from 70 to 150 km in depth) with a vertical velocity of between 40 and 140 mm/yr. Further geochemical studies (high-pressure / high-temperature experiments on phonolites; Hay et al, 1995a) suggested that alkali basaltic magmas may have injected and/or underplated the lower crust in central Kenya prior to the rift-related basaltic volcanism at the surface, between 23-14 Ma. Hay et al (1995b) integrated geophysical data (seismic refraction and gravity) and petrological results to model an extensive lens of anomalously high density / high velocity material in the lower crust beneath the rift axis representing material of basaltic composition; the model supports the idea that the subrift crust was extensively modified by underplating and/or basaltic intrusion and subsequent partial melting.…”