Ghaleb H. Jarrar

Late- and post-orogenic Neoproterozoic intrusions of Jordan: implications for crustal growth in the northernmost segment of the East African Orogen

Approximately 90% of the upper crust exposed in southern Jordan is composed of intrusive rocks of latest Neoproterozoic age, grouped into two major subdivisions: the Aqaba (600–640 Ma) and the Araba (560–600 Ma) complexes. The Aqaba complex comprises several suites that range in composition from gabbro to high silica granite (45–80% SiO2) and follow a high-K calc-alkaline trend. This phase, which started with the emplacement of the Duheila Hornblendic Suite between 640 and 600 Ma, represents the main crust-forming stage in southwest Jordan. The Araba complex is a bimodal alkali-calcic to alkali igneous suite generated after development of a regional unconformity and deposition of the Saramuj Conglomerate. Mafic members of the Aqaba complex, the Duheila Hornblendic Suite, are enriched in LILE relative to the HFSE and are moderately enriched in REE [(La/Lu)n = 5–11], traits typical of arc basalts. Geochemical modeling suggests derivation by 10–15% melting of amphibole-bearing spinel lherzolite, possibly above a subduction zone. The granitoids of the Aqaba complex are high in Ba, Sr and LREE, have low Y and have steep REE patterns [(La/Lu)n = 20–25]. They have low initial 87 Sr/ 86 Sr (∼0.70305) and high eNd values (+2.3 to +5.0). These may have been generated by high degrees of partial melting (10–30%) of subducted oceanic crust, with or without a small proportion of ocean sediments. The mafic and intermediate end members of the Araba complex, the Araba Mafic to Intermediate Suite (48–65% SiO 2) comprises quartz-diorites, quartz-monzodiorites, monzodiorites and monzogabbros. The mafic end member of this suite is enriched in LILE compared to the Duheila Hornblendic Suite, although its primitive compositions have similar REE patterns [(La/Lu)n = 5–15]; and has been emplaced in a within-plate tectonic environment. Geochemical modeling supports formation of the Araba Mafic to Intermediate Suite by 10% partial melting of phlogopite-bearing spinel lherzolite. This hypothesis is supported by low initial 87 Sr/ 86 Sr (∼0.7035) and high eNd values (+2.4 to +4.2). The felsic end member of the Araba complex, the Humrat–Feinan Suite (68–80% SiO2), is characterized by moderate enrichments in LREE (La/Sm)n ∼ 2, flat HREE patterns (Gd/Lu)n ∼ 1, and strong negative Eu anomalies (Eu/Eu ∗ =∼ 0.07–0.53). These granites have geochemical features typical of A-type granite and can be derived by extreme fractional crystallization ( ∼80%) of the Araba Mafic to Intermediate Suite. The low initial 87 Sr/ 86 Sr (∼0.7048), high eNd values (+2.5 to +4.8), and spatial and temporal association with the Araba Mafic Suite support this model. Crustal evolution in the northernmost Arabian Nubian Shield occurred about the time of terminal collision between east and west Gondwanaland by the addition of magmas derived from mantle-derived melts. These melts changed markedly at about the time of collision, from a unimodal suite of convergent

Age determinations in the Precambrian basement of the Wadi Araba area, southwest Jordan

The Precambrian basement of Jordan belongs to the northern margin of the Arabian-Nubian Shield. Age determinations by U-Pb isotopic analyses on sized and magnetic zircon fractions, a monazite and an apatite sample and by Rb-Sr isotopic studies on whole-rocks and minerals of metasedimentary rocks, granodiorites, granites and dykes have elucidated the following events: (1)A major regional high-grade metamorphism accompanied by migmatization and synkinematic plutonism occurred at about 800 Ma according to U-Pb zircon ages of metasediments and granites. (2)During a postkinematic plutonic event between 615 and 600 Ma extensive masses of granodioritic to granitic composition and dykes were emplaced. The U-Pb data of zircons of the rocks yielded upper intercept ages with the concordia consistent with Rb-Sr biotite ages. The Rb-Sr mineral ages of the older metasedimentary rocks document the resetting of the Rb-Sr system due to the thermal pulse at this time. (3)A younger plutonic event produced diorites and dykes at about 570 Ma. The plutonic events are related to the Pan-African orogenic phase. The low initial87Sr/86Sr ratios of the plutonic rocks (0.7032–0.7046) correspond to values reported from equivalent rocks throughout the Arabian-Nubian Shield and suggest that no significant portions of ancient sialic crustal material contributed to the generation of the granitic to granodioritic magmas.

A late Proterozoic bimodal volcanic/subvolcanic suite from Wadi Araba, southwest Jordan

Abstract Geotectonic setting, age relationships, petrography and geochemistry of a Pan-African bimodal volcanic/subvolcanic suite have been examined. The volcanic phase terminated the Pan-African molasse evolution and spans the time interval 550-540 Ma. The volcanics, forming a 70 km NNE-SSW trending belt, are distributed predominantly as rhyolitic lava flows and subordinately as trachybasalts and trachyandesites. The composition of the numerous dikes varies from basalt to trachyandesite and rhyolite. The rhyolites are typified as alkali-rhyolites and comendites. Geochemical criteria characterize a bimodal suite, which is not of cogenetic origin. The basalts are mantle-derived, fractionation -controlled products, whereas the alkali-rhyolites and comendites were generated by partial melting of continental crustal rocks. The investigated magmatic suite has been compared with equivalent suites from the Arabian-Nubian Shield.

A Pan-African alkaline pluton intruding the Saramuj Conglomerate, south-west Jordan

The geological setting, petrography and bulk mineral chemistry of a monzodiorite and a presumably consanguineous megaporphyry with large (up to 25 cm) labradorite megacrysts, both intruding the upper Proterozoic Saramuj Conglomerate in south-west Jordan (south eastern shore of the Dead Sea), were examined. The crystallization temperatures of the monzodiorite and the megaporphyry as determined from pyroxene thermometry and supported by contact metamorphic mineralogy are about 700 and 900°C, respectively. The intrusion depth of the monzodiorite is about 3–4 km. The monzodiorite was emplaced in the Saramuj Conglomerate at about 595 + 2 Ma ago according to Rb/Sr and U/Pb age determinations.The stratigraphic positions of the monzodiorite, megaporphyry and their host rock (the Saramuj Conglomerate) were compared with time-equivalent lithologies in the Arabian-Nubian Shield.

K-Ar dating, X-Ray diffractometry, optical and scanning electron microscopy of glauconies from the early Cretaceous Kurnub Group of Jordan

The glaucony of the early Cretaceous Kurnub Group in Jordan has been isotopically dated using the K‐Ar method. The glaucony occurs in an arenaceous dolomite unit, referred to here to as glaucony marker unit (GMU), located in the upper part of the Kurnub Group, that persists throughout Jordan. The glaucoliths of the heavy fraction are dark green in colour, ovoidal or mammilated, with a mainly cracked smooth surface, whereas the light fraction glaucoliths are light green in colour, irregular in shape and have a rough or porous surface. Both fractions exhibit boxwork and rosette microstructure, whereas lamellar microstructure is restricted to the heavy glaucoliths. X-ray diAraction and chemical analysis placed the glauconies of the GMU of the Kurnub Group in Odin and Matter’s evolved to highly evolved class corresponding to glauconitic mica and suggested that they should be wellclosed chronometers. On the other hand, petrographic investigation proved these glauconies to be unaltered, whereas the other altered ones are discarded from the age determination. The unweathered, highly evolved, heavy glauconies that are neither tectonized nor deeply buried best fulfil recommendations regarding appropriate samples for K‐Ar dating. The apparent age constrained, within the analytical uncertainty limits, from the most evolved glaucony is 96.1+1.1 Ma and suggests that the GMU is of Albian age. The other less evolved glauconies, which are still within the evolved to highly evolved class of Odin and Matter, yield a mean apparent age of 93.6+1.0 Ma, which is probably slightly younger than the true depositional age by 2‐3 Ma due to genetic and historical uncertainties, as indicated by the petrographic and sedimentological data. Thus the upper part of the Kurnub Group, where the GMU is located, is of Albian age. #1998 John Wiley & Sons, Ltd.

The role of pressure in control of potassium, sodium, and copper concentration in hypabyssal intrusives as demonstrated in late Precambrian dikes in southwest Jordan

Abstract The formation of dikes in southwest Jordan took place during the late Proterozoic and represents the subsequent magmatic activity of the Pan-African orogeny. Dikes studied in detail include a zoned, multiple, composite dike of rhyolitic—trachytic and latitic composition, an andesitic dike and two devitrified rhyolitic dikes. High-level (near-surface) dikes are characterized by an extreme potassium metasomatism, which is caused by pressure effects. The shallower the intrusion the more intensive will be the potassium metasomatism. Some dikes display anomalously high copper concentrations. The copper is originally incorporated in silicates and has been enriched along flow lines and vesicules by autometasomatism. The formation of secondary copper minerals (e.g., cuprite, malachite, chrysocolla) can be considered as contemporaneous with the solidification of the congealing rhyolitic—trachytic melt.

Mineral chemistry in dioritic hornblendites from Wadi Araba, southwest Jordan

The mineral chemistry of dioritic rocks from Wadi Araba, southwest Jordan, has been determined by electron microprobe, with special emphasis on the amphiboles. These are calcic amphiboles, according to Leake (1978), since all analyses have (Ca + Na)>1.34 and NaB<0.67. The observed chemical variations in the amphiboles are attributed to a combination of edenite, pargasite, hornblende and hastingsite end-member substitutions. Calculated average pressures of emplacement using four Al-in-hornblende barometers are 2.41 ± 1.33, 3.47 ± 1.60 and 4.52 ± 1.42 kbar for the pegmatitic diorite (PD), biotite-hornblende diorite (BHD) and diopside-hornblende diorite (DHD), respectively. The corresponding temperatures of crystallisation according to the Holland and Blundy (1994) thermometer are 730 ±40, 763 ± 37 and 783 ± 103°C. The variation of temperature of equilibration is almost within the uncertainty range of the applied thermometer. However, if this variation is real, the studied hornblende shows both temperature and pressure dependent substitutions. The calculated pressures fall within the estimated range (2.4 to 5.5 kbar), as deduced from the metamorphic grade of the metasedimentary rocks exposed at the same crustal level.

Quantifying the chemical variability of a Precambrian diabase from south Jordan using stochastic techniques: a proposal

Abstract A late Precambrian diabase from south Jordan is used as an introductory case study to examine the possibility of stochastically quantifying the chemical variability of given volcanic rock constituents based on their total alkali (TA) and silica (S) contents. The primary aim has been to quantify the chemical behavior of the rock constituents studied in order to evaluate whether their sample constituents are chemically interdependent, i.e., if knowing one tells us anything about the other, and if so over what range this occurred. The application of the autocorrelogram analysis revealed that most of the rock constituents exhibit specific chemical dependence between 0.40 and 0.80 wt.% along the silica direction and between 0.10 and 0.30 wt.% along the total alkali direction. The deduced autocorrelation functions were found to be correlated over certain chemical ranges with patterns of three basic types: typical; having a large zone of influence; and cyclic with nested structures. The application of semivariogram analyses, on the other hand, indicates that the rock constituents are chemically interdependent over a larger scale, that their interdependence is greater than that encountered when applying autocorrelation techniques (2–5% for major oxides and from 0.87 to 5.30% for Sr and Ni), and that many variables exhibit inherited random variability. The determined ranges of chemical dependence could be used to characterize differentiation trends which prevailed during rock formation, and to develop more precise predictive models regarding petrogenesis.