Petrography , Geochemistry and Petrogensis of Basal Flow from Ar-Rabba Area , Central Jordan

Thirty basaltic rock samples collected from a central Jordan at Ar-Rabba area were studied. The samples cover about 20 km2 from the Al-Rabba basalt flow. The Al-Rabba Basalt (RB) introduced Miocene to Pleistocene period. Petrography, Geochemistry and Petrogenesis investigations were carried out for the RB. The petrography analyses of the RB rocks are composed of plagioclase, pyroxene, olivine, opaque minerals (magnetite), and including secondary minerals calcite, iddingsite, and clay. The RB rocks have low range of major and trace elements concentrations, and under sodic alkaline magma series. The geochemical analysis data of RB indicated that RB was derived from a slightly fractionation magma as reflected by high MgO concentration range between (5.4 to 11.7 wt%), and Mg number from 42 to 63.8, and high concentration Cr (18 385 ppm), Ni (160 364 ppm) and low silica content ( 41.79 49.87 wt%). The chemical classification of RB is divided into basaltic, calc-alkaline to alkali basalt. The tectonic setting of RB is explained by using discrimination diagrams, Ti-Zr-Y, Ti-Zr-Sr and MgO-FeO(tot)-Al2O3, the RB plotted within the plate, calc-alkali and continental basalt respectively. The Rayleigh fractionation equation modeled for Sr and Ba vector diagram indicated the RB had fractionation for clinopyroxene, orthpyro-xene, olivine and trace of plagioclase.


Introduction
The regional structures of the region as a result to the tectonic evolution of the Arabian plate which includes the African and Arabian rift system, and collision of the Arabian and Eurasian plates, produce to the Arabian dome [1]- [4].These structure evolutions led to different fracture and rifting system, i.e.East-African rift, opening of the Red Sea, and Dead Sea rift system.The tectonics evolutions were controlling to the intraplate volcanic activities in the Arabian Peninsula, which are sporadically found over an area that covers a distance of 7,000 Km from Yemen in the south through Saudi Arabia, Jordan, Syria, and up to Turkey in the north [5]- [7].This intraplate volcanism occurred after the break of Africa and Arabia, opening the Red Sea and resulting in Oligocene flood volcanism in Yemen and Ethiopia [6] [8].Furthermore, an extensive late Cenozoic intraplate volcanism covers the central Jordan, which may reflect the lithosphere extension [9].
This study focused on the intraplate volcanic field in central Jordan Ar-Rabba Basalt (RB).The volcanism in Jordan occurs as sporadic volcanic centers along the eastern side of the Jordan Rift (Figure 1).These volcanoes are clearly associated with continental rifting and inception of the Dead Sea plate boundary.The relationship between magmatism and tectonic of the intraplate volcanism has been reported by [10], indicating that alkaline volcanism in Jordan is similar to the Arabian intraplate volcanic fields, which erupt through two main fissure systems along the eastern margin of the Dead Sea rift in the east-west direction [11] [12].The volcanism might have probably commenced during the Miocene period and continued to Pleistocene [13].The K-Ar age of Jordan basaltic activity is divided into three episodes: Oligocene to early Miocene , Middle to late Miocene , and late Miocene to Pleistocene (6.95 -0.15 Ma) [14]- [16].The Jordan basalt is distributed in three main regions based on their mode of occurrences, within Jordan rift (e.g.Zara basalt), central Jordan (e.g.El-Lajjun, Ar-Rabba (Shihan) basalt and NE-Jordanian Harrat (with an area 11,400 km 2 ) which is a part of the largest Harrat Al-Shaam and NW Jordan Ash-Shuna Ash-Shamaliyya Area [17]- [19].
The basalts in central Jordan have been found to occur in seven places, namely, Tafila, Wadi Dana, Jabel Shihan (RB), El-Lajjoun, Jurf Al-Darawish, Ghor Al-Katar, and Wadi Zarqa-Ma'in, in the form of plateau basalts, local flows (wadi fills), or individual volcanic bodies (cones, plugs, and dikes) [19].Some doloritic dykes observed to occur along the major faults, such as Karak-Al-Fayha fault zone, Wadi Dana, Tafila, and Ar-Rabba plateau [13].The objective of this study is to investigate the petrography, geochemistry, and petrogenesis evolution of the intercontinental basaltic flow at Ar-Rabba area, which is in the east of Dead Sea in central Jordan; to determine the origin and type of the parental magma; and to investigate the tectonic setting evolution of the RB region.

Geology Setting
The RB is located about 13 km Northeast of Al-Karak city at 31˚15'3468 to 31˚15'3488N and 35˚30'752 to 35˚30'760E (Figure 1).The RB flow covers an area of about 20 km 2 with arc like strip width about 0.2 to 0.5 km with an average thickness 1 -5 m [13].The exposure strata along the sides of Wadi Abu-Jubayba, Wadi ad Dahalya, Wadi imra, Wadi ad Dafall, Wadi al Hidan and Wadi adh Dhira area.This basalt belongs to the middle Pleistocene and consistence with the second stage of the opening of the Red Sea over the last 5 Ma [17] [20].The RB seen in the field cover by Al-Hisa Phosphorite unite (Belqa Group) of Campanian age [21] [22], and coverd by fluviatile, lacustrine gravels and soil.
The study area is affected by different structures, such as Siwaqa fault with east-west direction and extends from the Dead Sea to Wadi Sirhan, and Ed dhira fault are two major trending NE-SW.The N-S trending fractures are parallel to the Dead Sea transform fault, where as the E-W fractures lie parallel to Siwaqa fault Hasa fault and Zarqa-Ma'in fault [23].

Sampling and Analytical Techniques
A total of 30 representative rock chip samples were collected from the outcropping AR-Rabba Basalt (RB).The samples crushed and powdered using a stainless steel Jaw Crusher and an Agate Ball Mill machine, to obtain grain size less than −80 μ.The samples were quartered in order to get a statistically representative (splitter) fraction and powdered using two geochemical techniques at the Natural Resources Authority (NRA) labs.
The major elements were analyzed on fused glass discs-like pellet (bead) by using a Phillips X-Ray Florescence Spectrometry (XRF) Majex PW-2424 Model at the Al al-Bayt University.A total of 2 gram of the powder samples were mixed with 8 gram of lithium tetra borate and fused in platinum crucibles over gas burners (1000˚C) for 1 h.The melts were poured into a mold to create glass disks.The Loss on Ignition (LOI) was determined by the weight lost after melting at 1000˚C.The trace elements were analyzed by decomposition using Ione Inductively Conductive Coupled Plasma Emission Spectroscopy (ICP-AES) at Natural Resources Authority Labs.A total of 1 gram of the powdered <80 μ sample mixed with 3 gram of sodium peroxide (Na 2 O 2 ) were placed in a zirconium crucible, and fused by heating it to 450˚C for 45 mints, to obtain a sinter.Subsequently, 72 ml of deionized water was added to it, and stirred for a few mints; then 28 ml of diluted HCl in a ratio 1:1 was added to obtain clear solutions that were used to determine the trace element concentrations.Thin sections were prepared at the University of Al al-Bayt and petrographically investigated via polarizing microscope type Lico proccer with different magnifications.
The geochemical data were processed and pictorially represented by using the computer program Igpet 32.CIPW-Norm calculations were carried out by using the Excel sheet [24].

Plagioclase
Plagioclase crystals are forming more than 40 vol% for modal value of the basaltic phenocrysts and miro-phenocrysts in the ground mass, and average 49.08 vol% for norm analysis (Table 1).They occur in variety of forms and size, subhedral to anhedral, and range in size from medium to fine phenocrysts to very fine acicular to needle microlites in the groundmass.The crystals are showing simple twining and normal zoning (Figure 2(a)).The extinction angles on plagioclase phenocrysts ranged between 15˚ to 22˚, indicating to oligoclase to andesine composition (An30 -An40), determined by using [26] method.Plagioclase crystals are clear in plane polarized light (PPL) and gray interference color in XPL and slightly withered to calcite.The elongated slight alignment plagioclase crystals exhibited orientation to olivine and pyroxene crystals, presenting a trachytic texture (Figure 2(b)).Some of the plagioclase crystals enclosed in pyroxene occurs ophitic to supophitic texture (Figure 2(c)).The glomerphyrritic texture occurs with associated plagioclase, pyroxne and olivine are enclosed in fin-grained ground mass in clusters of four crystals (Figure 2(d)).

Pyroxene
The clinopyroxene (cpx) is the second dominant mineral in RB, form more than 30 vol%.It is occurred as colorless at (PPL), and second to their order interference color (XPL) with subhedral to anhedral.The crystal had size between (2 to 6 mm), with perfect two set cleavage intersect at ~90˚ in the cross-section.The clinopyroxene had an inclined extinction, indicating the presence of clinopyroxene of diopside.The normative RB indicating to the diopside content was average 22.5 vol.% (Table 1).The clinopyroxene intersected with plagioclase crystals to form ophitic to subophtic texture (Figure 2(c)), and the crystal zoning was present in clinopyroxene (Figure 2(e)).

Olivine
The olivine phenocrsts are occurred subhedral to anhedral crystas, and up to 3 mm in diameter in the groundmass and forming morthan 20 vol% for modal and average 6.48 for norm (Table 1).The olivine crystals were colorless and displaying seriate texture with high degree of alteration to iddingsite (Figure 2(f)).The aggregate crystals exhibit glomeroporphyritic texture.Iddingtization was common particularly the edge (rim) and fracture of the crystals, and some crystals were partially to completely pseudomorphosed to brown iddingsite (Figure 2(g)).The alteration produced a result from relatively low temperature deuteric alteration process [19], and the embayment of olivine crystals as a result of interaction between melt and olivine crystals during the crystallization process (Figure 2(g)) [27].

Opaque and Secondary Minerals
Opaque minerals were commonly found in RB, forming about 10 vol% of the rocks and ranging from 0.5 to 4

Major Oxides
The results of major and trace element chemistry of RB volcanic rocks is listed in Table 1.The RB rocks exhibit a narrow range of silica (SiO 2 ) saturation between (41.79 to 54.55 wt%) with an average of (46.52 wt%), which is within the average value reported for alkali basalt and basanite by many authors [11] [19] [28]- [32], and it can be classified as basalt using the Total Alkalis-Silica classification scheme [33] (Figure 3).The Al 2 O 3 contents in the RB samples vary from 9.60 to 14.90 wt%, meanwhile CaO varies between 7.80 and 17.8 wt%.The binary plot of SiO 2 versus Al 2 O 3 and CaO exhibits the inverse relationships between both oxides and SiO 2 (Figure 4).The MgO content of the RB ranged from 5.4 wt% to 11.7 wt% with an average of 8.5 wt%.The Mg number (Mg≠), defined as the molecular proportion of (100MgO/[MgO + Total Fe]) [30].It is usually used as a petrogenetic indicator for magma fractionation and its primitive volcanic rocks [34].The RB exhibited a high Mg≠, ranging between 42 and 63.8, with an average of 55.72.The Mg# of the RB indicates evolved to moderately basalt.As shown in Figure 5, Mg# decrease with increasing SiO 2 .This general trend suggests that fractional crystallization probably plays a role in decreasing Mg-number as a function of increasing SiO 2 [35].The Mg≠ calculation considers the Fe content in the rocks.It has been reported that values of Mg≠ > 70 can be considered as a threshold that characterizes primitive magmas [36].According to [37] it has been suggested that a Mg≠ of 65 is a distinct value.Moreover, the Fe 2 O 3 content of the RB ranged between 10.4 wt% and 15.2 wt%, with an average of 13.12 wt%, indicating that the rocks were enriched in Fe. [11] has suggested that SiO 2 under saturated magma had a high FeO content of >11 wt% and high MgO content of >7.[34] and [38] reported that rocks with high Mg# (>60) exhibit lowest contents of Nb and Zr and higher contents of Cr and Ni.All the present studies for the above mentioned, were explained to indicate that the rocks undergo smaller degree of partial melting at high pressures [39].The RB has been an average Na 2 O and K 2 O content of 2.93 and 0.92 wt%, respectively.The total Na 2 O + K 2 O values were variable in all the samples, exhibiting an average value of 3.84 wt%.The average ratio of Na 2 O/K 2 O was 3.63, and Na 2 O vs. K 2 O indicating the sodic affinity of the rocks [40] (Figure 6), and that of Al 2 O 3 /TiO 2 was 16.12 which suggested the basic affinity of the rock.

Trace Elements
The RB was found to have a high content of Cr and Ni (Table 1).The Cr content varied between 18 to 385 ppm, with an average value 246.87 ppm, and Cr/Ni ratio of 0.10 to 1.39.The Ni content ranges between 160 and 364 ppm with average 285.80 ppm, suggesting that RB fractionation by presence of olivine and clinopyroxene [41].The high content of Cr and Ni indicated that the parental magma had been derived through partial melting of peridotite mantle source [11] [36].The binary diagram (Figure 7) shows the Mg# versus Cr and Ni.The general trend is decrease of Cr and Ni with increasing Mg#, this result documented with Mudawwara-Quwayra Basaltic Dike [42].This is most probably resulted from crustal mixing and assimilation of the magma with the country rock which resulted to dilution of Cr and Ni, where Cr is related to clinopyroxene [36].
The Sr and Zr in the RB were relatively high contents, Sr ranging between 389 to 1351 ppm with average 706.4 ppm, and Zr content (80 to 334 ppm) with average 188 ppm (Table 1).The Reaer Earth Elements (REE) includes Nb, Y, La and Ce content range between (8 to 63 ppm, 13 to 60 ppm, 13 to 49 ppm), and (11 to 110 ppm) respectively (Table 1).The average Zr/Nb ratio 7.64, Y/Nb ratio 0.84 and Zr/Y ration 10.29.These ratio decomented by [43] reported for the intercontinental alkali basalt.

Discussion
The major and trace elements of the RB were used to construct discriminatory diagrams, to help for the classification, nomenclature, and interpretation of the tectonic setting of the RB.Classification for [33], the RB samples plotted in the alkaline to subalkaline rock field (Figure 8(a)).The AFM diagram shows the RB samples plotted in the calcalkaline series (Figure 8(b)).The ternary diagram for Ti-Zr-Y based on [44] was distinguished among island-arc tholeiitas, MORB, calc-alkaline, and intraplate basalt, all the RB samples were plotted within the plate basalt field (Figure 9(a)).On the other hand, Ti-Zr-Sr digram shows all the RB samples plotted in the calcalkaline basalt field (Figure 9(b)) [44].The MgO-FeO (tot) -Al 2 O 3 diagram for [45], shows the RB samples ploted within continental basalt field (Figure 10).
The low content of SiO 2 (41.79 to 54.55 wt%) and high content of MgO (5.4 to 11.7 wt%) and total FeO (10.40 -15.20 wt%) indicated the natural fractionation of the RB.The high concentration of Cr (18 to 385 ppm) with average 246.87 ppm is consistent with findings reported for primary magma |(e.g., 241 ppm [30], 257.6 ppm [11], 313.6 ppm [19], 125.3 ppm [46], 95 ppm [42].The high Mg# (average 55.66) for RB is similar to that reported for rock affected by fractionation or accumulation of clinopyroxene, orthopyroxene, and olivine and     plagioclase [34].The Rayleigh fractionation equation for [47], are used to modeled the mineral fractionation ( ) , where C l is the concentration of a trace element in the residual melt; C o is the concentration of a trace element in the original melt; F is the fraction of melt that remains, and D is the bulk partition coefficient.The partition coefficient of Sr and Ba modeled of the mineral fractionation vector diagram in Figure 11, which indicated that the RB rock samples had fractions of clinopyroxene, orthopyroxene, olivine, and traces of plagioclase.
The batch melting equation for [47] are used to study the degree of partial melting of the RB.The equation ( ) , where F is the melting degree; C o and C l are the concentrations of the elements in the source (original) and liquid, respectively, and D o is the bulk distribution coefficient of the elements in the initial assemblage and minerals entering into the liquid.The distribution coefficient model for Cr and Co, and the concentration of the large ion lithophile elements (LILE) was found to exhibit primitive composition [27].The degree of partial melting (F) calculated by using the concentrations of the oceanic crust sources.The studied RB samples shows an average partial melting degree of around 10%, which documented with the previously published results for Jordanian and Arabian interpolate basalt [11] [19] [31] [48].The geochemical information for major and trace elements are used to explain the natural source of RB, and it is used to explain the mantle sources of partial melting.This indicates by using trace element ratios, such as Zr/Nb and K/Ba ratios [49].The high content of Zr/Y and TiO 2 /Y ratios and low content of Y indicate that to the garnet -bearing source rocks [50] [51].The spider diagram for normal mid-ocean ridge basalt (NMORB) for the studied volcanic rocks (Figure 12(a)) presented enrichment of the strongly incompatible LILE such as Ba and K, depletion of Nb relatively to K, enrichment of Pb over Ce, and light rare earth elements (LREE) enrichment La over heavy rare earth elements (HREE) and Y, which showed high similarity to MORB.The mafic volcanic RB exhibited negative Nb anomalies and positive Pb anomalies, but presented higher LILE enrichments.The negative anomalies of Ba, Sr, P, and Ti may be attributed to the fractionation of feldspar for Ba and Sr depletion, apatite for P depletion, and (Fe-Ti) oxides for Ti depletion [38].The primitive mantle value of the rock (Figure 12(b)) showed a positive Nb peak, which conforms to the tertiary to recent continental alkali basalt provinces [52]- [54] and indicates that the RB is the product of lithosphere from upwelling asthenospheric mantle [19] [34] [36] [55].

Conclusions
The RB introduced within Miocene to Pleistocene volcanism at central Jordan.It is produced within intraplate to continental calc-alkaline to alkali basalt.The samples study covered at Al-Rabaa basalt flow about 20 km 2 .The following is the conclusion of the present study: 1) The mineral composition of RB is as follows: plagioclase, pyroxene, olivine, and opaque minerals (magnetite), with secondary minerals such as iddingzite, calcite, and clay.The main textures observed trachytic, glomeroporphyritic, vesicular, amygdaloidal, ophtic and supohtic.
2) The chemical classification of RB uses major elements into basaltic-calc-alkaline to alkali basalt and includes into sodic series.3) The tectonic setting for the discrimination diagram showed that the RB samples were ploted within interplate basalt, calc-alkaline basaltic and continental basaltic field.
4) The vector diagram for fractional crystallization model for RB showed the presence of clinopyroxene, orthoproxene, olivine, and traces of plagioclase.Documented with the published results for Jordanian and Arabian interpolate basalt, the distribution coefficient of Cr and Co indicated a partial batch melting of RB around to 10%.
5) The spider diagram for NMORB and MORB showed enrichment of the incompatible LILE such as Ba and K, and depletion of Nb relatively to K, enrichment of Pb over Ce, and Light Rare Earth Elements (LREE) enrichment La over heavy rare earth elements (HREE) and Y. Furthermore, RB exhibited negative Nb and positive Pb anomalies, which indicated that RB was a product of lithosphere from upwelling asthenospheric mantle.

Figure 1 .
Figure 1.(a) Simplified Geological map of Jordan show the area study; (b) Geological and sample location map of the area under study after [25].

Figure 6 .
Figure 6.K 2 O vs Na 2 O showing the sodic affinity of the RB samples [44].

Figure 8 .
Figure 8.(a) Total alkali vs SiO 2 in RB; (b) AFM diagram showing the boundary between the calc-alkaline field and tholeiitic field for RB [33].

Table 1 .
Chemical composition of the samples from RB.The major oxides are given in wt%, trace elements in ppm and CIPW-wt% norm.The normative RB indicating to th Hematite content was average 13.40 vol%.Magnetite occurs a phenocrysts scattered through the rock and inclusion within plagioclase, pyroxene and olivine crystals (Figures2(a)-2(d)).The optical properties of magnetite were black color within ppl and xpl optics.The main secondary minerals occurred as a result of ultration process such as hydrothermal solution, these mineral are iddingsite, calcite and clay.