The Duration and Characteristics of Magmatic Differentiation from Basalt to Trachyte within the Matan Volcanic Center, Northern Harrat Rahat, Kingdom of Saudi Arabia

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



A fundamental goal of igneous petrology is to quantify the durations required to produce evolved magmas following influx of basalt into the crust. However, in many cases, complex field relations and (or) the presence of a long-lived magmatic system make it difficult to assess how basaltic inputs relate to more evolved magmas, therefore precluding calculation of meaningful timescales. Here we present field relations, geochemistry, 40Ar/39Ar ages, and 36Cl surface-exposure ages for volcanic rocks from the Matan volcanic center, located in the northern part of the Harrat Rahat volcanic field, in the Kingdom of Saudi Arabia. These data document a systematic and repeated temporal progression from alkali basalt to trachyte for the youngest eruptive products. From ~155–17 thousand years ago, the following eruptive sequence occurred four times: (1) alkali basalt, (2) hawaiite, mugearite, or benmoreite, and (3) trachyte. We interpret each eruptive sequence to result from injection of basalt into the crust, and its subsequent differentiation and eruption of progressively evolved magmas. We use the interval time between successive eruptions within a given sequence to calculate the timespans required to produce trachyte from alkali basalt. Differentiation from alkali basalt to intermediate compositions (hawaiite, mugearite, and benmoreite) took ≤3 thousand years (k.y.) on average. Differentiation from intermediate compositions to trachyte took a maximum of 6.7±3.6 to 22.9±1.7 k.y. Thus, the total duration of differentiation was ~10–25 k.y. Timescales presented here are independent of the processes evoked to drive differentiation because they are based solely on the ages and compositions of eruptive products from a system characterized by a simple, repeated differentiation sequence.

Suggested Citation

Stelten, M.E., Downs, D.T., Dietterich, H.R., Mahood, G.A., Calvert, A.T., Sisson, T.W., Witter, M.R., Zahran, H.M., and Shawali, J., 2023, The duration and characteristics of magmatic differentiation from basalt to trachyte within the Matan volcanic center, northern Harrat Rahat, Kingdom of Saudi Arabia, chap. F 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], 56 p.,

ISSN: 2330-7102 (online)

ISSN: 1044-9612 (print)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Geologic Background
  • Analytical Methods
  • Results
  • Discussion
  • Conclusions
  • Acknowledgments
  • References Cited
  • Appendix 1. Detailed Description of Analytical Methods
  • Appendix 2. Location of Samples from the Matan Volcanic Center
  • Appendix 3. 36Cl Results
  • Appendix 4. Argon Age Spectra and Isochron Diagrams
Publication type Report
Publication Subtype USGS Numbered Series
Title The duration and characteristics of magmatic differentiation from basalt to trachyte within the Matan volcanic center, northern Harrat Rahat, Kingdom of Saudi Arabia
Series title Professional Paper
Series number 1862
Chapter F
DOI 10.3133/pp1862F
Year Published 2023
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Volcano Science Center
Description Report: vii, 56 p.; 2 Data Releases
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