Arabia Geology Abstracts
The Arabian continental alkali basalt province: Part III. Evolution of Harrat Kishb, Kingdom of
Saudi Arabia
Issn: 0016-7606 Journal: Geological Society of America Bulletin Volume: 104 Issue: 4 Pages: 379-396
Authors: CAMP, VICTOR E., ROOBOL, M. JOHN, HOOPER, PETER R.
Article ID: 10.1130/0016-7606(1992)104<0379:TACABP>2.3.CO;2
ABSTRACT
Harrat Kishb is a 5,892 km2 lava field in western Saudi Arabia with an overall K-Ar age range from 2
Ma to prehistoric (at least 4,500 to 2,000 yr B.P.). It contains three stratigraphic units: the Diakah, Nafrat,
and Hil basalts. Harrat Kishb differs from the coeval, mildly alkaline harrats along the 600-km-long
Makkah-Madinah-Nafud volcanic line to the west in that its lavas are nodule-bearing and considerably
more silica-undersaturated. The nodules are most abundant in basanite from the central vent zone and
include both Type I and Type II mantle xenoliths.
Harrat Kishb is fundamentally a bimodal lava field that is dominated, at one end, by alkali olivine
basalt (AOB) and basanite, with subordinate hawaiite and olivine transitional basalt (OTB), and at the
other end, by phonolite. The few intermediate phonotephrites are compositionally and texturally
heterogeneous, and regarded as hybrid lavas of basalt and phonolite.
Although none of the basaltic lavas is a primary melt, their chemistry was controlled largely by partial
melting. From bottom to top, the stratigraphic units become less voluminous and increasingly more
undersaturated, reflecting decreasing degrees of partial melting with time. Fractional crystallization of
basanites (the smallest-degree partial melts) probably occurred by the plating of pyroxene and spinet (±
olivine) along the walls of narrow conduits during their ascent through the subcontinental mantle
lithosphere (flow crystallization). Fractional crystallization of OTB and AOB (the larger-degree partial
melts) may have occurred in reservoirs at the crust-mantle boundary, a density filter for rising magmas.
The relatively higher volatile content of the basanites, a consequence of smaller degrees of partial
melting, may have allowed many of them to accelerate through the crust-mantle density barrier carrying
their load of mantle nodules rapidly to the surface.
Major-element mass-balance calculations, and trace-element enrichment factors, demonstrate that the
phonolites were probably derived from basanitic magmas via about 62% fractional crystallization of a
clinopyroxene-dominated, feldspar-bearing mineral assemblage. There is a lack of evidence for fractional
crystallization in static, high-level magma chambers, and a preferred model for the basanite-to-phonolite
link involves the variable removal of feldspar by continued "flow crystallization" during the ascent of
basanitic magmas through the crust. Such a model requires that these parental basanites had slower flow
rates than did the nodule-entrained basanites extruded at the surface.
Field evidence for the ascent of basaltic and phonoltic magmas in shared conduits is consistent with
periodic magma recharge during "flow crystallization." Although there is little evidence for the mixing of
these diverse magma types at depth, they were mixed at the vent by clastogenic processes to produce
hybrid lavas of phonotephrite composition.
The Arabian continental alkali basalt province: Part II. Evolution of Harrats Khaybar, Ithnayn, and
Kura, Kingdom of Saudi Arabia
Issn: 0016-7606 Journal: Geological Society of America Bulletin Volume: 103 Issue: 3 Pages: 363-391
Authors: CAMP, VICTOR E., ROOBOL, M. JOHN, HOOPER, PETER R.
Article ID: 10.1130/0016-7606(1991)103<0363:TACABP>2.3.CO;2
ABSTRACT
Three coalesced basaltic lava fields, Harrats Khaybar, Ithnayn, and Kura, constitute the largest
contiguous area of Cenozoic basalt in Saudi Arabia, similar in extent (20,564 km2) and volume (1,850 km3)
to Harrat Rahat, which is situated only 25 km to the south. Harrat Kura, with an age range from about 11 to
5 Ma, is the oldest of these harrats; it contains a single stratigraphic unit, the Kura basalt, primarily
composed of alkali olivine basalt (AOB) with subordinant basanite and hawaiite and a single cluster of four
phonolite domes. In contrast, Harrats Khaybar (5 Ma to Present) and Ithnayn (3 Ma to Present) are
composed of more mildly alkaline basalt types. The two oldest units on Harrat Khaybar, the Jarad and
Mukrash basalts, are dominated by dictytaxitic-to-ophitic olivine transitional basalt (OTB) flows, which
were extruded primarily from major arterial lava tubes to form unusual morphological features, here
designated "whaleback lava flows." The lava tubes emanate from a 50-km-long, north-trending fault scarp
along the western edge of the central vent zone, which marks the site of voluminous outwelling of OTB
lava. The fault scarp ceased to be a major site of extrusion at the end of Mukrash time.
Immediately to the east of the fault scarp, a thin (0.7 to 1.0 km), uptilted slab of crust formed the roof of
a high-level magma chamber ( 60 x 25 km) that dominated the eruptive history of the Abyad basalt, the
youngest stratigraphic unit on Harrat Khaybar. The Abyad basalt occurs as seven stratigraphic subunits, the
two youngest of which (Qb6 and Qb7) are "post-Neolithic" and historical in age. In contrast to the Jarad
and Mukrash basalts, the Abyad basalt is dominated by AOB and hawaiite, with only minor OTB. No
Abyad differentiated rocks occur on Harrat Ithnayn; however, fractional crystallization in the high-level
magma chamber beneath Harrat Khaybar has resulted in an extreme compositional range of rock types,
from mugearite to benmoreite, trachyte, and comendite. The vent distribution at Harrat Khaybar shows a
distinct zonation indicative of a high-level, compositionally stratified magma chamber.
Whole-rock chemistry suggests that even the most primitive basalts are not primary magmas, but rather
fractionated melts derived from primary magmas of variable melt fractions generated at differing mantle
depths. The more undersaturated Harrat Kura basalts were derived from primary magmas representing the
smallest degrees of partial melting, at the greatest depth. Crystal fractionation of most primary magmas
appears to have taken place in reservoirs at the crust-mantle boundary. Chemical and petrographic data
suggest that open-system magma recharge was an important process in the high-level magma chamber
beneath Harrat Khaybar. The chemical and mineralogical heterogeneity of the Harrat Khaybar comendites,
their Sr-isotope values, and their Zr/Nb ratios all indicate that they were derived by varying degrees of
crystal fractionation (>95%), crustal melting, and halogen complexing.
The north-trending volcanic axes of Harrats Rahat, Khaybar, and Ithnayn form a single, 600-km-long
linear vent system, designated the Makkah-Madinah-Nafud (MMN)volcanic line. The three transitional-tomildly alkaline harrats extruded from this volcanic line differ from most Arabian harrats, which like Harrat
Kura to the west, are significantly more undersaturated. The relatively large degrees of partial melting
evident in Harrats Rahat, Khaybar, and Ithnayn suggest that the MMN volcanic line may overlie a northsouth thermal lineation in the mantle. The petrochemical stratigraphic sequence in these three harrats is
nearly identical; early voluminous extrusions of OTB were followed by later, less extensive extrusions of
minor OTB and abundant AOB, hawaiite, and differentiated lavas. As this petrochemical stratigraphy
evolved, it progressed northward along the MMN trend with time, beginning on Harrat Rahat about 10 Ma,
on Harrat Khaybar about 5 Ma, and on Harrat Ithnayn about 3 Ma. The interplay of open-system magmatic
processes beneath this migrating volcanic line has resulted in a classic example of a strongly fractionated,
mildly alkaline basalt series.
Calcareous Duricrust in Al-Hasa, Saudi Arabia
Issn: 0016-7606 Journal: Geological Society of America Bulletin Volume: 85 Issue: 1 Pages: 119-130
Authors: CHAPMAN, RANDOLPH W.
Article ID: 10.1130/0016-7606(1974)85<119:CDIASA>2.0.CO;2
ABSTRACT
The calcareous duricrust in al-Hasa, Saudi Arabia, covers a rugged topography developed on sandy
limestones of Miocens and Pliocene age. It is typically gray or reddish brown and concretionary, and it has
been divided into three types: very immature, immature, and mature. The very immature crust rests on the
youngest topographic surfaces and is the youngest, thinnest, and least thoroughly developed. The immature
crust rests on somewhat older surfaces, and is older, thicker, and more thoroughly developed. The mature
crust rests on the oldest topographic surface and is the oldest, thickest, and most thoroughly developed.
The crusting process involved (1) the recrystallization of original calcite in the parent sandy limestone,
(2) the introduction of additional calcite either from above or below, and (3) the replacement of quartz and
feldspar by calcite. The CaCO3 content of the three crust types varies directly with their age.
The duricrust appears to have evolved progressively, though intermittently, during the very late
Pliocene, Pleistocene, and early Holocene. Differences in the thickness of the three crust types are
attributed to differences in the climate under which they formed, and differences in their degree of
development are attributed to differences in their age.
Calcareous duricrust is recognized widely as an indicator of semiarid climate, and under certain
circumstances, it provides evidence of climatic changes. The relations in al-Hasa suggest the possibility of
dating and correlating crusted surfaces by the character of their crust in regions where surfaces and crust
have developed alternately over the same period of time.