Geology topics

Research about northern Harrat Rahat

Source-linked reports with geographic coverage including northern Harrat Rahat.

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USGS2023

The duration and characteristics of magmatic differentiation from basalt to trachyte within the Matan volcanic center, northern Harrat Rahat, Kingdom of Saudi Arabia

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, 40 Ar/ 39 Ar ages, and 36 Cl 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.

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USGS2023

Mantle origin and crustal differentiation of basalts and hawaiites of northern Harrat Rahat, Kingdom of Saudi Arabia

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.

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USGS2023

Isotopic and geochemical evidence for the source of volcanism at Harrat Rahat, Kingdom of Saudi Arabia

Pleistocene and Holocene basalts, hawaiites, mugearites, benmoreites, and trachytes from the northern part of the Harrat Rahat volcanic field, Kingdom of Saudi Arabia, were analyzed for Sr, Nd, Hf, and Pb isotopic compositions. Evolved trachytes with Mg number <0.1 (Mg# = Mg/[Mg+Fe 2+ ], molar) have relatively radiogenic Sr isotopic compositions indicating that they were influenced by contamination probably in the upper crust. Volcanic rocks with Mg# >0.1, consisting chiefly of alkali basalts but encompassing hawaiites, mugearites, and benmoreites, show a limited range in Hf, Nd, Sr, and Pb isotopic compositions. Although the total Pb isotope variation is only 1 percent, the Pb isotope values correlate with Mg#, where the least radiogenic Pb is in samples with the lowest Mg#. The trend formed in Pb isotope space points toward an unradiogenic Pb composition that is similar to the Pb isotopic composition of lower crust of the Precambrian Arabian-Nubian Shields, as well as to feldspars and galena in the upper crust of the western Arabian Shield. This trend is interpreted as progressive but overall minor (no more than 5 weight percent) assimilation of shield rocks, or their partial melts, during fractional crystallization. Isotopic compositions of the least evolved northern Harrat Rahat magmas are most similar among analyzed Arabian harrats to depleted spreading-ridge basalts of the active Red Sea rift, but isotopic values are displaced toward those of spreading-ridge basalts of the Gulf of Aden that are proximal to the site of the Afar mantle plume. The Pb isotopic compositions very near the Northern Hemisphere Reference Line indicate no discernable lithospheric contribution to yield the parental basalts of northern Harrat Rahat, and their isotopic compositions are consistent with derivation predominantly from depleted Northern Hemisphere asthenosphere with a subordinate (20–30 weight percent) component from the Afar mantle plume. Trace-element variations show that appreciable portions of melting were in the garnet stability field, confirming the sub-lithospheric origin of the magmas, and that melting extents were low, accounting for the alkalic, trace-element-enriched character of the suite. The presence of possible Afar mantle beneath the western part of the Arabian Shield and its absence beneath the Red Sea rift may result from capture and channelized flow along high-relief structures incised into the base of the sub-continental lithosphere, as revealed by geophysical images.

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USGS2023

Depth to basement and crustal structure of the northern Harrat Rahat volcanic field, Kingdom of Saudi Arabia, from gravity and aeromagnetic data

New gravity data reveal a prominent negative anomaly along the main vent axis of the northern Harrat Rahat volcanic field in the Kingdom of Saudi Arabia. The gravity low continues north of the volcanic field onto exposures of Proterozoic rocks, indicating that the low is caused not only by the volcanic field (and possibly underlying Cenozoic sediments), but also the underlying Proterozoic basement. An inversion of the gravity field guided by analysis of aeromagnetic data indicates (1) a broad depression of the basement surface that is deeper along the main vent axis in the eastern part and in the southwest part of the volcanic field and (2) less dense basement beneath the vent axis. Low densities within the basement most likely arise from lithologic variations in the basement, predating Cenozoic volcanism, although our analysis does not rule out small volumes of partial melt and higher temperatures or extensive fracturing at depth.

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USGS2023

Magnetotelluric investigation of northern Harrat Rahat, Kingdom of Saudi Arabia

Volcanism within the harrats (Arabic for “volcanic field”) of the Kingdom of Saudi Arabia includes at least one historical eruption occurring close to the holy city of Al Madīnah in 1256 C.E. As part of a volcanic- and seismic-hazard assessment of northern Harrat Rahat, magnetotelluric (MT) data were collected to investigate the structural setting of the area, the presence or absence of melt within the crust, and the mantle-derived magmatic source. Collected MT data were modeled in both two dimensions, where anisotropy can be estimated, and three dimensions. Interpretation of the preferred resistivity model includes a shallow sediment-filled graben beneath northern Harrat Rahat lavas, a melt-free upper crust, and a region of decompression melting in the asthenosphere below 60–70 kilometers depth. Models in two dimensions image the lower crust as anisotropic, demonstrating that a series of elongate conductivity anomalies with a strike of N. 10° E. within the lower crust of the three-dimensional model are artifacts of inverting anisotropic data with an isotropic modeling algorithm. Careful examination of the resistivity models, in combination with regional geological and geophysical data, suggests an anisotropic lower crust that is free of large zones of melt. Azimuthal anisotropy in the lower crust extends well beyond the limits of Harrat Rahat volcanic rocks, with a conductive direction oriented N. 10° E. and an anisotropy factor of 2–5 between the most and least conductive directions. Enhanced conductivity is likely caused by interconnected grain-boundary graphite, where the direction of anisotropy reflects either frozen-in fabric from the Neoproterozoic stabilization of the Arabian Shield or ductile deformation driven by channelized asthenospheric flow coupled with a thin rigid mantle lid. Asthenospheric melt is interpreted to transect the crust largely through diking, with limited melt storage and short residence times within the crustal column.

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