Abstract
Harrat Hutaymah is a Cenozoic small-volume volcanic field in the northeastern margin of the Arabian Shield, Saudi Arabia. Mantle and lower-crustal xenoliths have been collected from the various tuff ring and maar deposits. Based on whole-rock and mineral chemistry, there are two categories of xenoliths: (1) the fertile lherzolite and moderately depleted harzburgite, (2) spinel-rich clinopyroxenite cumulates, lower crustal hornblendite, and mid-crustal gabbro-diorite. The mantle peridotite have high whole-rock Mg# values (> or =0.90), and high Fo and NiO contents for olivines (91 and 0.37 wt% on average), high average Mg# values for clinopyroxene (0.93) and orthopyroxene (0.92), and low to intermediate Cr# values for spinels (0.1-0.4). The mantle peridotite xenoliths show homogeneous unfractionated chondrite-normalized REE patterns, with slight enrichment in LREE. The compositional characteristics of the peridotite indicate that they came from a residual fertile mantle which experienced low to moderate degrees of partial melting. The estimated average equilibration temperature of the peridotites is 897 degrees C, corresponding to an average pressure of 11 kbar, which is equivalent to an average depth of 37 km beneath the relatively thin continental crust ( approximately 30 km) of the Arabian Shield. Clinopyroxenite xenoliths have low average whole-rock Mg# values (0.81), low average Fo for olivine (81), low average Mg# values for clinopyroxene (0.85) and orthopyroxene (0.84), and very low average Cr# values for spinels (0.001). The whole-rock trace element characteristics and mineral compositions of the clinopyroxenite xenoliths indicate that they were cumulates and formed from the early stage of picritic melt within an arc setting. The estimated P-T conditions of the clinopyroxenite cumulate gave an average depth of 48 km, which is relatively deeper below the mantle-crust boundary. The successive decrease of average bulk-rock Mg# values for the xenoliths from spinel-rich clinopyroxenite (0.81) to hornblendite (0.73) and gabbro-diorite (0.44), the enrichment of LILE the depletion of HFSE with characteristic negative Nb anomalies, the high La/Nb ratios, and the enrichment of LREE in the hornblendite and gabbro-diorite xenoliths suggests that these rocks formed from a highly fractionated hydrous calc-alkaline parental melt in a subduction setting. The estimated P-T conditions gave an average pressure of 6 kbar and depth of 20 km for the hornblendite xenoliths and an average pressure of 4 kbar and depth of 14 km for the gabbro-diorite suite. Thus, hornblendite xenoliths could be formed earlier and deeper than the gabbro-diorite xenoliths. The proposed crystallization process could produce the layered cumulates with decreasing amphibole and increasing plagioclase upward to the shallower depth. Geochemical and mineralogical characteristics of Harrat Hutaymah lithospheric xenoliths provide a profile to interpret the lithosphere structure of the eastern margin of the Arabian Shield.