Mantle Origin and Crustal Differentiation of Basalts and Hawaiites of Northern Harrat Rahat, Kingdom of Saudi Arabia

Professional Paper 1862-I
Jointly published with the Saudi Geological Survey [as Saudi Geological Survey Special Report SGS–SP–2021–1]
By: , and 



Quaternary volcanic rocks of northern Harrat Rahat, Kingdom of Saudi Arabia, are chiefly alkali basalts with subordinate transitional basalts, hawaiites, mugearites, benmoreites, and trachytes. Geochemical and isotopic results indicate that crystallization-differentiation, mixing, and cumulate reassimilation within the magmatic system produced most of its compositional diversity, with only minor involvement of Neoproterozoic crust. With increasing evolution, crystal assemblages that separated from and drove basalt-to-hawaiite differentiation passed from (1) dunitic or troctolitic to (2) olivine gabbroic to (3) titanomagnetite-bearing olivine gabbroic, with typical hawaiites representing about 20 weight percent residual liquids from an estimated primary alkali basaltic parent. Crystallization-differentiation simulations for midcrustal pressures yield the closest compositional match to the basalt-hawaiite suite, and stagnation in the midcrustal area (near 20 kilometers [km] depth) may result from density trapping beneath the lower density upper continental crust. Least differentiated alkali basalts have fractionation-adjusted major-oxide compositions that are consistent with separation from the deeper parts (60–70 km) of the spinel-lherzolite stability field at pressures that are close to the local lithosphere-asthenosphere boundary (60–80 km). Mantle potential temperature estimates are strongly sensitive to modeling approach, but potential temperatures (1,345–1,390 degrees Celsius [°C]) are not discernably greater than for midocean ridge basalts (MORB; 1,350–1,410 °C) if adherence to spinel-lherzolite melting relations is required. Inversion of the trace-element concentrations of the lesser fractionated basalts indicates a depleted mantle source, similar to MORB-source estimates, but one that is enriched in Sr and includes a greater relative proportion of melting in the garnet-peridotite stability field. These geochemical and thermal relations, as well as radiogenic isotopes, point to a dominantly depleted mantle asthenospheric source for Harrat Rahat basalts, admixed with subordinate materials either from the Afar mantle plume or an enriched MORB component in the ambient asthenosphere. The lithosphere-asthenosphere boundary is shallower beneath the belt of major volcanic fields on the Arabia Plate, and restoration of rifting across the Red Sea and Gulf of Aden places the south end of this belt adjacent to the northern part of the Afar region, suggesting a once-continuous structure, possibly an arch, a weakness, or a discontinuity along the base of the lithosphere. Magma generation can be ascribed to focused upwelling and decompression melting, perhaps driven by a magmatic-feedback mechanism whereby basaltic intrusions into the deep lithosphere solidify as eclogites, causing lithospheric foundering and further asthenospheric upwelling and decompression melting in a restricted region.

Suggested Citation

Sisson, T.W., Downs, D.T., Calvert, A.T., Dietterich, H.R., Mahood, G.A., Salters, V.J.M., Stelten, M.E., and Shawali, J., 2023, Mantle origin and crustal differentiation of basalts and hawaiites of northern Harrat Rahat, Kingdom of Saudi Arabia, chap. I of Sisson, T.W., Calvert, A.T., and Mooney, W.D., eds., Active volcanism on the Arabian Shield—Geology, volcanology, and geophysics of northern Harrat Rahat and vicinity, Kingdom of Saudi Arabia: U.S. Geological Survey Professional Paper 1862 [also released as Saudi Geological Survey Special Report SGS–SP–2021–1], 42 p.,

ISSN: 2330-7102 (online)

ISSN: 1044-9612 (print)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Volcanic Suite of Northern Harrat Rahat
  • Petrogenetic Interpretations
  • Tectono-Magmatic Synthesis
  • Concluding Summary
  • Acknowledgments
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Mantle origin and crustal differentiation of basalts and hawaiites of northern Harrat Rahat, Kingdom of Saudi Arabia
Series title Professional Paper
Series number 1862
Chapter I
DOI 10.3133/pp1862I
Year Published 2023
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Volcano Science Center
Description vii, 42 p.
Country Kingdom of Saudi Arabia
Other Geospatial northern Harrat Rahat
Online Only (Y/N) N
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details