Lithos 284–285 (2017) 545–559

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Neoproterozoic Rosetta Gabbro from northernmost Arabian–NubianShield, south Jordan: Geochemistry and petrogenesis

Ghaleb H. Jarrar a,⁎, Robert J. Stern b, Thomas Theye c, Najel Yaseen a, Victoria Pease e, Nathan Miller f,Khalil M. Ibrahim d, Cees W. Passchier g, Martin Whitehouse h

a Geology Department, The University of Jordan, P.O. Box 13633, 11942 Amman, Jordanb Geosciences Department, U. Texas at Dallas, ROC 2.1, 800 W. Campbell Rd, Richardson, TX 75080, USAc Institut fuer Mineralogie und Kristallchemie, Universtität Stuttgart, Azenberg 18, D-70174 Stuttgart, Germanyd Department of Earth Sciences and Environment, The Hashemite University, Zarqa, Jordan.e Department of Geological Sciences, Stockholm University, SE-10691 Stockholm, Swedenf The University of Texas at Austin, Jackson School of Geosciences, 2275 Speedway Stop C9000, Austin, TX 78712-172, USAg Institute of Geological Sciences, Johannes-Gutenberg University, Mainz, Germanyh Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden

⁎ Corresponding author.E-mail address: jarrargh@ju.edu.jo (G.H. Jarrar).

http://dx.doi.org/10.1016/j.lithos.2017.05.0080024-4937/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 January 2017Accepted 15 May 2017Available online 21 May 2017

AnEdiacaranmafic intrusion of south Jordan is a distinctive appinitic igneous rockwith a possibly unique texture,characterized by spherical clots up to 40 mm in diameter composed of amphibole cores from which plagioclaseeuhedra radiate; we call it the Rosetta Gabbro. It is exposed as a small (ca. 750m2) outcrop in the Neoproterozoicbasement of south Jordan. A second outcrop of otherwise similar gabbro is located about 400 m to the north ofthe Rosetta Gabbro, but it lacks the distinctive texture. The Rosetta Gabbro could represent a magma pipe.It intrudes the Aqaba Complex (~600 Ma) granitoids and metasediments of the Janub Metamorphic Complex(633–617 Ma). The gabbro is an Ol- to QZ tholeiite with the following chemical characteristics: SiO2 =46.2–47.8 wt.%; Al2O3 = 16.4–17.7 wt.%, TiO2 = 1.70–2.82 wt.%, Na2O = 1.27–2.83 wt.%. K2O = 0.82–1.63 wt.%; Mg# 58–63; Σ REE = 70–117 ppm; La/Yb ~6 to 8; and Eu/Eu* = 1.05–1.2. The investigatedgabbro has the geochemical features of a continental flood tholeiitic basalt emplaced in a within-plate tectonicsetting. Two varieties of amphiboles are found: 1) large, 3–5 mm, brown ferri-titanian-tschermakite(K0.09Na0.28)(Na0.20Ca1.80)(Mn0.04Fe3+1.1Mg2.34Fe2+0.90Ti0.29Al0.22)(Al1.85Si6.15)O22(OH)1.95 of the calcic amphi-bole group which is riddled with opaques; and 2) acicular yellowish-light green ferrian-magnesiohornblende(K0.04Na0.153)(Ca1.755Na0.245) (Fe

3+0.66Mn0.01Fe

2+1.01Mg3.03Ti0.06Al0.22)(Al1.03Si6.97)O22(OH)1.95. Scattered flakes

of phlogopite also occur. Tabular radiating plagioclase (An64–79) are complexly twinned, with broad lamellaethat show no zoning. Laser-ablation ICP-MS analyses of amphibole and plagioclase reveal considerable variationin trace element abundance, in spite of more subtle major element variations except for TiO2 in amphibole. TheREE in the amphibole shows anorder ofmagnitude variationwith a concave-downward pattern and a positive Euanomaly Eu/Eu* = 0.6–2, though far less pronounced compared to the Eu/Eu* = 5–45 of plagioclase. The 3Ddandelion-like texture of the rosettas is broadly similar to “Chrysanthemum Stone”, which is a diagenetic growthin sedimentary rock, but we can find no description of similar textures in igneous rocks. The formation ofthe rosettas is thought to reflect loss of magmatic water resulting in supersaturation of plagioclase, whichgrew rapidly around amphibole andmay have floated in themagma. This impliesmagmatic evolution in shallow(10 to 12 km deep) crust where temperatures were nevertheless in the range of ca. 750 to 900 °C.

© 2017 Elsevier B.V. All rights reserved.

Keywords:AppiniteRosetta GabbroWater-rich magmaAmphibole microchemistryLA–ICP-MSArabian–Nubian shield

1. Introduction

The Neoproterozoic basement complex of Jordan forms the north-ernmost extremity of the Arabian–Nubian Shield (ANS) and crops outover an area of about 1400 km2 east, northeast and north of the Gulfof Aqaba as far as the southern shore of the Dead Sea (Fig. 1A)

(Bender, 1968). The ANS is one of the largest exposures of juvenile con-tinental crust that has been formed as a consequence of plate tectonicprocesses. It evolved as the northern segment of the NeoproterozoicEast African Orogen (EAO, 900–530 Ma) that extends from south tonorth for about 6000 km (Stern, 1994). It is generally regarded as acollage of juvenile volcanic arc terrains and associated ophiolite rem-nants which were amalgamated during the assembly of Gondwana(e.g. Johnson et al., 2004; Meert, 2003; Stern, 1994). During the EastAfrican Orogeny, the ANS experienced multiphase deformation and

Fig. 1. A. The northernmost outcrops of the Arabian–Nubian Shield. B. Part of the Janub Metamorphic Complex which hosts the investigated gabbroic stocks (Ain Alhashim gabbro).C. A detailed map of the Rosetta Gabbro locality. The gabbroic bodies shown in B and C are not mappable at the scale of Ayn-Al Hashim map (map 3048I; scale 1:50,000); thus theywere not shown on this map.

546 G.H. Jarrar et al. / Lithos 284–285 (2017) 545–559

metamorphism, ranging in grade fromgreenschist to upper amphibolitefacies conditions (e.g. Abu-El-Enen and Whitehouse, 2013; Jarrar et al.,2013a). This was accompanied and followed by extensive plutonic andsubordinate volcanic activity (Jarrar et al., 2003).

We document an interesting new occurrence of appinites from theANS of southernmost Jordan. Appinite is defined as includingmelanocratic hornblende-rich syenite, monzonite, or diorite. In a recentreview, Murphy (2013) discussed the role of water in the generation,emplacement, and crystallization history of mafic to felsic magmas andnoted that a common feature of appinites is that they reflectcrystallization from unusually water-rich mafic magmas. Appinitesoccur as stocks, pipes, dikes and sills and commonly form soon afterthe cessation of subduction. These rocks are genetically related tohornblendites and spessartite lamprophyres. Murphy (op.cit) suggesteda broader definition for appinite suites as “a group of coeval plutonic andor hypabyssal rocks, ranging from ultramafic to felsic in composition, inwhich hornblende is the dominant mafic mineral and typically occurboth as large prismatic phenocrysts and in the finer grained matrix”.

The Rosetta Gabbro of southernmost Jordan is an appinite, butone with an unusual spheroidal texture that has not previously been

reported. Spheroidal textures, generally referred to as globules,orbs, ocelli, varioles, and rapakivi are reported from diverse igneouslithologies; nevertheless, their origins remain a matter of debate(e.g. Ballhaus et al., 2015 and references therein). Liquid immiscibilityhas been advocated as a mechanism to explain spheroidal texturesin some igneous rocks. Ballhaus et al. (2015) proposed that globularand orbicular textures my result from exsolution of a fluid phaseat elevated temperature and pressure from H2O saturated maficmagmas.

We present new field observations and U–Pb zircon geochronologywhich constrain the age of the Rosetta Gabbro. In addition, petrography,major, trace and rare element chemistry, including laser ablationinductively coupled plasma mass spectrometric (LA–ICP-MS) mineralchemistry of amphibole and plagioclase, are used to evaluate theRosetta Gabbro appinitic suite alongwith its peculiarmagmatic texture.The major and trace element compositions of the whole rocks andconstituent minerals are used to constrain how the Rosetta Gabbroformed. Our results contribute to understanding the diversity of igneousstructures and textures and how these form in water-rich maficmagmas.

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2. Geologic setting and age constraints

The Neoproterozoic basement complex of south Jordan is comprisedof igneous, sedimentary and metamorphic rocks that formed in theperiod 550–870 Ma (e.g., Ibrahim and McCourt, 1995; Jarrar, 1985;Jarrar et al., 2003, 2004, 2008, 2013a; Yaseen et al., 2013). These rocksare broadly subdivided into two lithostratigraphic and intrusivedivisions; namely, the older Aqaba and younger Araba complexes(Ibrahim andMcCourt, 1995) that are separated by the Araba Unconfor-mity at about 605 Ma (Powell et al., 2015). The Janub MetamorphicSuite (JMS) is part of the Aqaba Complex (Fig. 1B). The JMS is composedof meta-conglomerates, meta-arkoses, cataclasites, mylonites, andcordierite-biotite hornfelses (Hassuneh, 1994). Using a limited databaseof mineral chemistry, Habboush and Jarrar (2009) constrained thetemperature of the contact metamorphism to be 500–545 °C. Mineralassemblages of themetasedimentary rocks indicate low-pressuremeta-morphism b4 kbar. These P–T conditions are consistent with uppergreenschist to lower amphibolite facies.

The Rosetta Gabbro is one of three gabbroic lithologies forming theso-called Ayn-Al Hashim Gabbroic Suite (Abdelhamid et al., 1994).According to our field observations, the Rosetta Gabbro intrudes theJanub Metamorphic Complex and is intruded by Abu Jedda Granite(Fig. 2). Brook and Ibrahim (1987) dated the latter granite by Rb/Srwhole rock at 582 ± 4Ma. The Abu Jedda granite is a sub-solvus biotite

Fig. 2. A. The intrusive contact of Abu Jedda Granite (AJG) into the Janub MetamorphicComplex (H). B. The Rosetta Gabbro (RG) intruding into the Janub MetamorphicComplex and intruded by Abu Jedda Granite (AJG). C. A close up of the Rosetta Gabbroas seen in outcrop.

granite and consists of albite + microcline with rare perthite batches,quartz, biotite, and accessory minerals including opaques, zircon andapatite. The CL images of zircon show oscillatory zoning and Th/U ratios(0.48–0.72) are typical of magmatic zircon (Belousova et al., 2006). Fivedata points with low common Pb and discordance b5% yield an ageof 596 ± 5 Ma (Fig. 3). This age is almost identical to the average207Pb/206Pb age of 596 ± 8 Ma, and is interpreted to be the crystalliza-tion age of the Abu Jedda granite (Table 1). Therefore, the Ayn-AlHashim Gabbroic Suite must be younger than 596 ± 5 Ma and belongsto the Araba Complex.

The investigated Ayn Al Hashim gabbro with the amphibole–plagioclase rosettas intrudes as a small (ca. 750 m2) stock into theJanub Metamorphic Complex (Fig. 1B and C). It was first described byAbdelhamid et al. (1994) as orbicular gabbro of the Ayn Al-HashimGabbroic Suite. A similar hornblende gabbro but lacking rosettasintrudes the same complex ca. 400m to the north (Fig. 1B). The rosettasare spherical to elliptical in shape and range in size from fewmillimetersup to about 5 cm, typically consisting of tabular plagioclase grainsradiating from an amphibole core. Photographs of the gabbro in thefield (Fig. 2C) and on polished slabs show both equatorial (withamphibole nuclei) and non-equatorial (lacking amphibole nuclei)sections through the rosettas (Fig. 4). To the best of our knowledge,this interesting structure has never been described before.

3. Analytical techniques

Four whole rock samples representative of the Ayn Al-HashimGabbroic Suite were selected for detailed study. Three of these (HC-42, -43, and -44) are from the Rosetta Gabbro and an additional sample(HC-31) from the northern gabbro is without rosettas. These sampleswere studied in thin section. The software ImageJ-2 (Schneider et al.,2012) was used to calculate the ratio of plagioclase to amphibole inthe polished rock slabs. The four samples were analyzed for theirmajor, trace and rare earth elements at the Australian LaboratoryServices Arabia (ALS Arabia) in Jeddah, Saudi Arabia. Mineral chemistrywas obtained using an electron microprobe at the Institut fürKristallchemie und Mineralogie, Universität Stuttgart, Germany. Theplagioclase and the amphibole of the rosettas were also analyzed fortheir major, trace and REE using the Laser-Ablation MS facility at theUniversity of Texas at Austin, USA. More details about these methodsare provided below and raw data are presented in the supplementarymaterials (Tables S1, S2, S3, S4).

Fig. 3. U–Pb inverse (Terra and Wasserburg) Concordia plot for Abu-Jedda granite (HC-41). MSWD (c + e) = 1.5, Prob. (c + e) = 1.5 probability, c = concordance, e =equivalence).

Table 1Zircon U–Th–Pb analyses of the Abu Jedda Granite.

Analysis [U] ppm [Th]ppm

[Pb]ppm

Th/U (calc)a f206 (%)b 238U206Pb

±s% 207Pb206Pb

±s% 207-corrage (Ma)c

±s Disc. %

n3938-13 443 318 45 0.62 2.68 12.809 0.71 0.06023 2.14 482.5 3.9 -21.6n3938-09 1548 2134 180 1.13 13.39 12.357 1.14 0.05820 6.89 501.0 11.5 -6.9n3938-14 347 209 40 0.57 1.27 11.055 0.72 0.06128 1.97 556.5 4.1 -14.6n3938-10 404 271 45 0.56 3.05 11.014 0.77 0.05874 3.93 560.3 4.9 0.5n3938-03 229 127 25 0.37 1.27 10.919 0.71 0.05919 2.81 564.7 4.1 -1.7n3938-05 531 340 62 0.52 0.67 10.463 0.72 0.05932 1.52 588.6 4.2 1.7n3938-18 237 113 27 0.50 0.12 10.429 0.72 0.05939 1.03 590.4 4.1 1.6n3938-08 848 523 100 0.52 0.10 10.339 0.73 0.05945 0.86 595.4 4.3 2.1n3938-12 244 122 29 0.46 {0.02} 10.269 0.72 0.06047 0.89 598.6 4.2 -3.6n3938-02 230 94 26 0.36 {0.06} 10.170 0.72 0.06009 1.32 604.6 4.3 -0.4

a Ratios calculated from 207Pb/206Pb age and measured ratios assuming concordance.b Numbers in parentheses indicate insignificant common Pb and no correction is needed for these analyses; values b1% are excluded from the final synthesis.c Age based on projecting a line in inverse Concordia space from assumed common Pb through the total uncorrected ratios onto Concordia (Ludwig, 2009).

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3.1. Whole-rock analyses

Sample powders were prepared using an agate ball mill at theGeology Department at the University of Jordan. The major elementswere determined using Inductively Coupled Plasma-Atomic EmissionSpectroscopy (ICP-AES), while trace elements, including REE weremeasured by inductively coupled plasma mass spectrometry (ICP-MS)after lithium borate fusion. Precision and accuracy were monitoredagainst international reference standards and duplicate analyses, andkept around 1% for major elements and better than 5% for traceelements. Whole rock analytical results are reported in Table 2.

3.2. Electron microprobe analyses

Mineral compositions were determined using a CAMECA SX100electron microprobe at the Institut für Mineralogie und Kristallchemie,

Fig. 4. A. The Rosetta Gabbro as seen in the field. B. Polished slabs showing rosettas ofvarious sizes, note the equatorial and non-equatorial sections in both. C. Expandedsingle rosetta showing the tabular shape and to some extent fan-shaped growth of theplagioclase.

Universität Stuttgart, Germany. Operating conditions were 15 kV accel-erating voltage and 10 nA beam current. Naturalminerals and syntheticcompounds were used as standards. Counting time was 20 s for bothpeak and background. Calculated uncertainties range between 0.01and 0.15 wt.%. Representative mineral analyses are reported in Table 3.

3.3. LA–ICP-MS analysis

Trace element concentrations of Rosetta Gabbro amphibole, biotiteand plagioclase were measured in two analytical sessions (4/24/12,5/3/12) by LA–ICP-MS at the University of Texas at Austin, using aNew Wave Research UP 193-FX fast excimer (193 nm wavelength, 4–6 ns pulse width) laser system coupled to an Agilent 7500ce ICP-MS.The laser system is equipped with a large format, two-volume lasercell, for direct sampling of the ablation plume with fast (b1 s) washouttimes. Laser ablation parameters optimized from representative testablations were 45 s ablations using a 50 μm diameter spot at 20% laserpower (fluences of 4.06 and 3.63 J/cm2), 10 Hz repetition rate, and aHe cell flow of 300 mL/min. Pulse-to-pulse laser power variationswere b4.6% over the analysis period. All spots were pre-ablated for 2 susing a 50 μm spot, 7% laser power, and 10 Hz repetition rate to removesurface contamination. The ICP-MS operated at an RF power of 1600Wwith Ar carrier gas flows of 1.14 or 1.25 L/min. Oxide production rates,as monitored by ThO/Th for NIST 612, were ≤0.29%. The quadrupoletime-resolved method involved measurement of 33 analytes at1-point per spectral peak, using the integration times of 10 ms (23Na,24Mg, 25Mg, 27Al, 29Si, 31P, 39K, 43Ca, 44Ca, 45Sc, 47Ti, 51V, 53Cr, 55Mn,57Fe, 60Ni, 85Rb, 88Sr, 89Y, 90Zr, 93Nb, 137Ba) and 30 ms (139La, 140Ce,146Nd, 147Sm, 153Eu, 157Gd, 163Dy, 166Er, 172Yb, 175Lu, 178Hf) and 30 ms.The resulting sampling period (0.6102 s) corresponded to 90.1%detection time, enabling 73 measurements to be made within the 45 sdwell interval, which were viewed as conditions suitable for robustmeasurements (Longerich et al., 1996). A 30 s gas blank interval wasused between all laser measurements. Unknown spot analyses werebracketed hourly by triplicate analyses of laser ablation standardsNIST-612 and NIST-610 (Jochum et al., 2011). Measured analyte inten-sities were converted to elemental concentrations using Iolite software(Hellstrom et al., 2008), with NIST-612 as the primary calibrationstandard and 29Si as the internal standard reference. Rosetta Gabbro Siconcentrations for dark-colored amphibole and biotite (18.80 wt.%)and light-colored amphibole and plagioclase (23.37 wt.%) wereassigned based on stoichiometric estimates of 40 and 50 wt.% SiO2,respectively. Representative trace element analyses are given in Table 2.

3.4. Secondary ion mass spectrometric analyses

Zircon separated from the granite is translucent, yellowish andeuhedral. Ten zircon grains were mounted with the 1065 ± 0.3 MaGeostandard 91500 reference zircon (Wiedenbeck et al., 1995) in a

Table 2Whole rock chemical data of the Ayn-Al Hashim Gabbro.

Sample Hornblende Gabbro Rosetta Gabbro

HC-31 HC-42 HC-43 HC-44

Major oxidesSiO2 49.1 46.2 46.7 47.8Al2O3 16.35 17.2 17.7 16.4Fe2O3 10.4 11.4 8.89 10.95CaO 8.14 9.51 10.5 9.01MgO 6.38 9.56 7.6 7.48Na2O 2.19 1.27 1.69 2.83K2O 2.36 1.62 1.63 0.82Cr2O3 0.04 0.02 0.01TiO2 1.51 1.7 2.43 2.82MnO 0.24 0.24 0.21 0.22P2O5 0.3 0.19 0.24 0.3SrO 0.08 0.06 0.09 0.1BaO 0.04 0.03 0.03 0.02LOI 2.17 2.62 2.45 1.61Total 99.3 101.62 100.17 100.36

Trace elementsBa 370 243 271 172Cs 3.36 1.68 2.64 1.74Hf 5.3 3 4.2 5.3Nb 9.8 6.3 8.3 10.2Ta 0.7 0.5 1 0.7Pb 6 4 5 5Rb 85.6 63 64.4 30.1Sr 686 460 709 847Th 1.78 0.84 1.16 1.3U 0.65 0.33 0.44 0.52V 252 234 299 410Zn 96 159 185 154Zr 218 116 162 200Y 21.6 15.6 24.6 31Co 45.8 42.2 70.8 42.9Cr 280 180 70 20Ni 88 142 94 71Ga 20.5 17.6 17.7 19.3

Rare earth elementsLa 15.4 10 13 14.9Ce 38.9 24.4 32.8 37.7Pr 5.68 3.63 5.08 5.81Nd 24.1 15.9 23 27.1Sm 5.52 3.97 6.27 7.46Eu 1.79 1.53 2.17 2.8Gd 5.03 3.84 6.36 7.77Tb 0.76 0.57 0.94 1.15Dy 4.2 3.09 5.13 6.46Ho 0.85 0.61 1.02 1.26Er 2.2 1.59 2.59 3.21Tm 0.32 0.21 0.33 0.41Tb 0.76 0.57 0.94 1.15Yb 1.85 1.27 1.93 2.45Lu 0.31 0.2 0.31 0.39SUM 107.36 71.18 101.56 119.63

CIPW normative mineralogyQ 0.04 0.00 0.04 0.00or 14.49 9.76 9.92 4.95ab 19.25 10.96 14.73 24.45an 28.89 37.16 36.97 30.24di 8.91 7.77 11.81 10.59hy 20.13 17.20 15.33 17.20ol 8.67 0.01mt 4.53 4.73 5.87 6.39il 2.98 3.29 4.75 5.47ap 0.72 0.45 0.57 0.71

Trace element ratiosLa/Nb 1.57 1.59 1.57 1.46Y/Nb 2.20 2.48 2.96 3.04Zr/Nb 22.24 18.41 19.52 19.61Th/Yb 0.96 0.66 0.60 0.53Ta/Yb 0.38 0.39 0.52 0.29

ADD any table notes:

549G.H. Jarrar et al. / Lithos 284–285 (2017) 545–559

resin desk, polished and gold-coated. Cathodoluminesence (CL)and secondary electron (SE) were generated using an Hitachi S4300scanning electronmicroscope (SEM) at the SwedishMuseum of NaturalHistory (SMNH). The internal structure of the polished zircon grainsrevealed by the SEM images were used to select locations for isotopicanalyses. U–Th–Pb analyses were performed using a secondary ionmass spectrometer (CAMECA IMS 1280) at SMNH. Analytical conditionsand instrumental set up followedmethods detailed byWhitehouse et al.(1999) and Whitehouse and Kamber, 2005. Recent lead contaminationduring sample preparationwas assumed for the common Pb correction.The composition of common Pb assumes the Stacey and Kramers(1975) model (for the present day = 0.83). In this study, the 207Pbcorrected (238U/206Pb) ages are used to interpret the isotopic results(calculated by projecting the uncorrected data to concordia from anassumed common 207Pb/206Pb composition). Age interpretation andconcordia diagrams were made using Isoplot/Ex software (Ludwig,2009).

4. Results

Below we summarize our results, presenting first the petrographicsimilarities and differences between the rosetta and non-rosetta typesof Ayn-Al Hashim gabbro, the results of whole-rock geochemical analy-ses, and then the results of mineral analyses.

4.1. Petrography

The Rosetta Gabbro consists primarily of brown tschermakite exten-sively replaced by pale green magnesiohornblende (Figs. 4A and 6A, B;see Section 4.2.2 for mineral chemistry). These are mantled by radiallygrown homogenous bytownite plagioclase (Figs. 5A and 6C). Thebrown tschermakite contains many opaques. Some hypersthene andorthoclase grains are present. The opaques include both magnetiteand ilmenite. Secondary chlorite and calcite are also present. Aciculargrains of apatite up to 0.5 mm in length are common. The tabularbytownite crystals clearly grew after the magnesiohornblende.

The non-rosetta hornblende gabbro also consists of browntschermakite extensively replaced by light green magnesiohornblende(Fig. 4B), andesine to labradorite plagioclase, subordinate orthoclase,and ilmenite. Secondary chlorite is common. Zoned plagioclase withlabradorite core is abundant. Ophitic and subophitic textures arecommon (Fig. 6D); therefore, we infer that plagioclase growth startedbefore amphibole and then both grew together at the eutectic, incontrast to the inferred crystallization of amphibole before plagioclasefor the Rosetta Gabbro. We calculated the amount of plagioclase asvolume percentage on polished rock slabs of Rosetta Gabbro (Fig. 7).The amount of plagioclase ranges from 32 (for large rosettas) to 36.5%(rosettas plus groundmass plagioclase with grain size ≥0.1mm). There-fore, the ratio of plagioclase:hornblende ~ 1:2.

4.2. Geochemistry

4.2.1. Bulk chemistryThe chemical data for representative samples are listed in Table 2.

This gabbro is an OL- to QZ tholeiite with SiO2 = 46.7–50.6 wt.%,Al2O3 = 16.6–18.1 wt.%, and Na2O = 1.28–2.87 wt.% on a volatile-freebasis. The gabbros are relatively unfractionated, with up to 9.56 wt.%MgO (Mg# = 55–63) and up to 142 ppm Ni and 280 ppm Cr. In spiteof their relatively unfractionated compositions, the gabbros areenriched in incompatible elements, as is typical for igneous rocksof the Araba Complex, with TiO2 = 1.51–2.82 wt.%, K2O = 0.82–2.36 wt.%, total REE = 71–120 ppm and with strong light rare earthelement enrichment [(La/Yb)N ~ 6 to 8]. Trace elements show evidenceof plagioclase accumulation in some of the samples, with high Sr con-tents (460–847 ppm), elevated Sr/Y (27–32), and positive europiumanomalies (Eu/Eu* = 1.05–1.2]. The major element data as well as

Table 3Electron microprobe mineral analyses of gabbro samples.

Sample No. Hornblende Gabbro Rosetta Gabbro Hornblende Gabbro Rosetta Gabbro Hornblende Gabbro Rosetta Gabbro

Tschermakite Magnesiohornblende Tschermakite Magnesiohornblende Feldspar Feldspar Ilmenite Magnetite Chlorite

HC31-25 HC31-57 HC31-18 HC31-24 HC43-132 HC43-102 HC43-63 HC43-92 HC31-39 HC31-76 HC31-62 HC43-114 HC43-120 HC31-1 HC43-162 HC43-75 HC43-81

SiO2 40.86 42.76 51.37 49.08 43.11 42.66 47.47 47.32 57.5 52.19 63.01 49.31 50.07 SiO2 0.044 1.03 27.98TiO2 1.36 1.5 0.33 0.64 0.97 1.43 0.47 0.46 TiO2 50.236 51.442 0.08Al2O3 11.83 10.79 3.47 5.6 11.03 12.47 8.04 7.98 26.08 30.15 17.91 31.32 31.21 Al2O3 17.07FeO 18.59 18.84 13.71 15.17 14.17 14.12 12.91 13.03 0.53 0.32 0.31 0.14 0.03 FeO 46.56 46.89 78.264 17.22MnO 0.36 0.42 0.41 0.43 0.3 0.28 0.26 0.25 MnO 2.245 1.671 0.09MgO 9.17 9.51 14.82 13.16 12.54 12.1 14.59 14.77 MgO 0.032 0.062 0.152 22.62CaO 11.12 11.33 11.95 11.6 10.92 10.85 11.02 11 8.36 13.1 0.04 16.3 15.29 CaO 0.135 0.016 0.907 0.09Na2O 1.45 1.33 0.36 0.77 1.81 2.01 1.26 1.33 5.77 3.86 0.37 2.74 3.19 Na2O 0.006K2O 0.62 0.56 0.18 0.15 0.2 0.17 0.16 0.14 1.26 0.28 17.91 0.05 0.13 NiO 0.719 0.041Total 97.332 99.125 98.654 98.639 97.071 98.216 98.262 98.352 99.5 99.9 99.55 99.86 99.92

O base 6 6 32 36T sites Ti 1.992 1.965 0.012[Si 6.213 6.394 7.498 7.218 6.399 6.205 6.907 6.874 Si 2.607 2.377 2.991 2.264 2.291 FeII 1.98 1.992 7.94 3.04Al 1.787 1.606 0.502 0.782 1.601 1.795 1.093 1.126 Al 1.393 1.618 1.002 1.695 1.683 FeIII 15.269Total T 8 8 8 8 8 8 8 8 Mn 0.097 0.072 0.016M1–M3 (C) Sites Ca 0.406 0.639 0.002 0.806 0.749 Mg 0.007 0.005 0.08 6.98[Al 0.333 0.296 0.095 0.188 0.328 0.342 0.285 0.24 Na 0.507 0.341 0.035 0.244 0.283 Ca 0.074 0.001 0.345Ti 0.156 0.169 0.036 0.071 0.108 0.156 0.051 0.05 K 0.073 0.016 0.992 0.003 0.008 Si 0.002 0.365 5.79Fe3+ 1.016 0.851 0.3 0.423 1.01 1.397 0.671 0.755 An 41.18 64.16 0.2 76.55 72.06 4.078 4.035 24 15.84Mg 2.079 2.12 3.225 2.885 2.775 2.624 3.165 3.198 Ab 51.43 34.2 3.4 23.17 27.21Ni 0.008 0.002 0.001 Or 7.39 1.63 96.4 0.28 0.73Fe2+ 1.348 1.505 1.433 0.749 0.32 0.825 0.755Mn2+ 0.046 0.053 0.03 0.034Total C 4.978 4.994 5 5 5 4.882 5 5M 4 (B) SitesMn2+ 0.051 0.054 0.008 0.032 0.031Fe2+ 0.03 0.01 0.074 0.072Ca 1.812 1.815 1.869 1.828 1.737 1.691 1.718 1.712Ba 0.005Na 0.188 0.18 0.051 0.109 0.256 0.309 0.176 0.185Total B 2 2 2 2 2 2 2 2A Sites[Na 0.239 0.206 0.051 0.111 0.265 0.258 0.18 0.19K 0.12 0.107 0.034 0.028 0.038 0.032 0.03 0.026Total A 0.359 0.312 0.085 0.139 0.303 0.289 0.21 0.216Tot. Cat. 15.337 15.307 15.085 15.139 15.303 15.171 15.21 15.216Mg/(Mg + Fe2) 0.607 0.585 0.701 0.667 0.787 0.891 0.779 0.794

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Fig. 5. Textures of Rosetta Gabbro (A) and non-rosetta gabbro (B). A. Scanned thin sectionof Rosetta Gabbro (HC-43) showing a small (left center) and a large rosetta (right). Centralmagnesiohornblende (Mhb) cores, tschermakite (Ts) is dark greenish brown in color. B. Ascanned thin section of the non-rosetta hornblende gabbro (HC-31) showing both thetschermakite (brown) surrounded and replaced by magnesiohornblende withplagioclase (Pl) that is often zoned. The zoning is obvious from the weathered core witha cloudy appearance and is confirmed by electron microprobe analysis.

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HFSE classification and tectonic discrimination plots suggest that thesuite is a within-plate tholeiite (Fig. 8).

The chondrite-normalized patterns (Fig. 9A) are moderatelyenriched in LREE and show a weak positive Eu anomaly indicative ofcumulate plagioclase. The MORB-normalized multielement diagram(Fig. 9B) of these rocks is also typical of continental flood basaltictholeiites (Pearce, 1983;Wilson, 1989). The primitive mantle extendedincompatibility plot (Fig. 9C) shows a positive spike in LILE at Cs, Rb,Ba, Pb and Sr and a slight depletion at Th, Nb, Ta and P.

4.2.2. Mineral chemistryA total of 112 analyses were obtained on amphiboles from the two

gabbroic varieties (Supplementary Tables 1 and 2). Representativeanalyses of the various minerals are listed in Table 3. A wide range inmajor element composition is obvious: Compared to the magnesio-hornblende, the tschermakite is lower in silica (mean = 41.28 ± 1.04vs. 47.56 ± 2.06 wt.% SiO2), richer in alumina (mean = 11.74 ± 0.65vs. 7.25 ± 1.81 wt.% Al2O3), titanium (mean = 2.62 ± 0.89 vs. 0.58 ±0.36 wt.% TiO2), sodium (mean = 1.65 ± 0.17 vs. 1.07 ± 0.37 wt.%Na2O), and potassium (mean = 0.49 ± 0.12 vs. 0.18 ± 0.1 wt.% K2O),contains indistinguishable calcium (mean = 11.25 ± 0.24 vs. 11.20 ±0.62 wt.% CaO) and has a slightly lower Mg# (72.4 ± 0.1 vs. 74.15 ±0.07). The two varieties (Rosetta Gabbro and hornblende non-RosettaGabbro) contain tschermakite that differs in SiO2, Al2O3, TiO2, andalkalis.

The amphibole classification following recommendations by Leakeet al. (2004) is based on the general chemical formula:

A0−1B2C5VIT8

IVO22 OH; F;Clð Þ2

Since water and halogen contents are not determined by EMP, theamphibole formula is calculated on the basis of 23 oxygens and theFe3+/Fe2+ following the procedure proposed by Schumacher (1997)and using the software WinAmphcal by Yavus (2007). For amphibolesanalyzed from the Ayn-Al Hashim Gabbroic Suite, (Ca + Na)B N 1.60and NaB b 0.5, so all amphiboles are calcic with SiIV between 5.890 and7.489 apfu. Furthermore, on theMg/(Mg+ Fe2+) vs Si classification di-agram, the amphiboles from rosetta bearing and hornblende gabbrosrange from ferri-titanian-tschermakite (Fig. 10),

(K0.09Na0.28)(Na0.20Ca1.80)(Mn0.04Fe3+1.1Mg2.34Fe2+0.90Ti0.29Al0.22)(Al1.85Si6.15)O22(OH)1.95 to ferrian-magnesiohornblende

(K0.04Na0.153)(Ca1.755Na0.245)(Fe3+0.66Mn0.01Fe2+1.01Mg3.03Ti0.06Al0.22)(Al1.03Si6.97)O22(OH)1.95.

in composition, except for some analyses that fall in the actinolitefield. The Rosetta Gabbro has amphiboles with generally higher Mg/(Mg + Fe2+) (Fig. 10).

The plot of AlVI (C site) vsAlIV (T site) inAyn-AlHashimGabbroic Suite(Fig. 11) shows that Al is preferred in the tetrahedral site. Furthermore,the Ti and (Na + K)A correlate positively with tetrahedral Al (Fig. 11).These features and the negative correlation between (Na + K)A vs. Si(Fig. 11) implies the dominance of the edenite type substitution [(Na+ K)A + AlIV = Si IV + (□)A], Ti-tschermak substitution [AlIV + TiIV

= SiIV + MgVI] and Al-tschermak substitution [AlIV + AlVI = SiIV

+ MgVI] during the replacement of the tschermakite bymagnesiohornblende. Similar substitutions have been documented foramphiboles near the roof of the mafic magma chamber (Murphy et al.,2012).

The efficiency of the LA–ICP-MS technique for mapping igneousminerals for their trace and REE element content has been recentlydocumented (e.g. Ubide et al., 2015). The early crystallizing amphibole(tschermakite) has trace element compositions that are distinct fromlate-forming magnesiohornblende (Supplementary Table 3). Relative tomagnesiohornblende, tschermakite contains more Sc (mean = 49.9 ±28.9 vs. 17.9 ± 17.8 ppm), V (mean = 529.4 ± 116.8 vs. 221.1 ±96.0 ppm), Rb (mean = 6.6 ± 12.7 vs. 0.44 ± 0.90 ppm), Sr (mean =170.3 ± 94.2 ppm vs. 38.4 ± 45.6 ppm), Y (30.5 ppm vs. 10.9 ±5.6 ppm), Zr (mean = 89.5 vs. 20.2 ± 20.3 ppm), Nb (mean = 12.2 ±3.6 vs. 2.34 ± 1.75 ppm), Ba (mean = 86.9 ± 61.0 vs. 15.3 ±45.3 ppm), Hf (mean = 3.18 ± 1.49 ppm vs. 0.70 ± 0.73 ppm, andREE (mean 94.3 ± 34.4 vs. 39.0 ± 14.8 ppm). Tschermakite containsless Cr (mean = 84.3 ± 111.5 vs. 143.8 ± 73.0 ppm) and Ni (mean =169.3 ± 102.5 vs. 285.4 ± 73.0 ppm).

A distinctive positive relationship exists between V concentrationandTiO2, Zr, and total REE (Fig. 12). Furthermore, Zr correlates negativelywith Eu anomaly, Cr and Ni. The latter elements correlate positively witheach other (Fig. 12) andwithMgO (not shown). Similar behavior of traceelements in crystallizing amphiboles has been observed by Meurer andClaeson (2002). The highest concentrations of REE are encountered inamphiboleswith the highest V and TiO2 contents, i.e.— in tschermakites.

The tschermakites are enriched in total REE relative to themagnesiohornblendes. However, the chondrite-normalized plots ofamphiboles (Fig. 13A and B) from both varieties are bowed up in themiddle with negative and positive Eu anomalies calculated using thegeometric method (Eu/Eu* ~ 0.55 to 1.25 for tschermakite and up to2.0 for magnesiohornblende). The primitive mantle normalized incom-patibility plots for amphiboles are also concaved downwards with

Fig. 6. Photomicrographs of Rosetta Gabbro (A–C) and normal hornblende gabbro (D). A. Plane-polarized light image of magnesiohornblende (Mhb) replacing tschermakite (Ts) (HC-43),the gray circle in themiddle is a 40 μmlaser ablation analysis spot. B. Cross-polarized image showing anhedral aggregate ofmagnesiohornblende forming the cores of the rosettas. C. Cross-polarized image showing the perpendicular growth of tabular plagioclase crystals radiating from the amphibole core (HC-43). D. Cross-polarized image of ophitic and subophitic texture innon-rosetta gabbro (HC-31) where plagioclase laths are partially or completely enclosed in the tschermakite and/or magnesiohornblende.

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negative anomalies at Rb, Ba, K, Sr, and Zr, thoughmore pronounced formagnesiohornblende (Fig. 13D & E).

The plagioclase of the Rosetta Gabbro is dominantly bytownite(An72–77) with some labradorite (An62–64). In contrast, the plagioclase

Fig. 7. Textural analysis for the purpose of estimating the plagioclase to amphibole ratio(Fig. 4B) prepared using the ImageJ software.

in the non-Rosetta Gabbro ranges from andesine to labradorite(An39–58). Its plagioclase has much lower total REEs (3.57–19.23;average 7.47 ± 4.15) and a pronounced positive Eu anomaly (Eu/Eu*up to 48) (Fig. 13C).

5. Discussion

The rosetta texture we describe here is unusual and testifies to adifferent style of crystallization of amphibole and plagioclase growingin mafic magma. Clearly, amphibole crystallized first, then plagioclasenucleated on its surface and grew outward. This must have happenedwhile amphiboles were suspended in magma. Outward growth of

Fig. 8. Total alkali-silica classification and AFM diagrams (LeMaitre et al., 1989; Irvine andBaragar, 1971); P2O5 vs. Zr and Zr/Y vs. Zr diagrams (Pearce and Norry, 1979). Opentriangles = Rosetta Gabbro, open circles = non-rosetta gabbro.

Fig. 9. A. Chondrite-normalized pattern; B. MORB-normalized plot; and C. multi-elementspider diagram for the investigated gabbros. Open triangles = Rosetta Gabbro, opencircles = non-rosetta gabbro.

Fig. 10. Amphibole compositions. Open triangles = Rosetta Gabbro, open circles = non-rosetta gabbro.(After Leake et al., 2004.)

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plagioclase must also have occurred while suspended inmagma. Some-what similar textures were identified by Yazgan and Mason (1988),who described layers of troctolite in orbicular gabbro from near Baskil,southeastern Turkey. They report millimeter thick layers consisting ofradiating prismatic olivine (Fo81) and plagioclase (An91).

One might be tempted to call the rosettas “spherulites”, because thelatter are also radiating arrays of acicular crystals which are common inglassy felsic volcanic rocks (Vernon, 2008), however this is inappropri-ate because spherulitic growth reflects devitrification of glassy felsicrocks such as obsidian (Ewart, 1971), which is fundamentally differentthan the magmatic growth that formed the rosettas.

Spheroidal textures in igneous rocks are called by various names:globules, orbs, orbicules, ocelli, and varioles, and rapakivi are reportedfrom diverse igneous lithologies. The range of names is appropriatebecause awide range of origins have been advanced:melt immiscibility,cavity filling by late-stage liquids, magma mingling or incompletemixing, partial assimilation, exsolution of a fluid phase from hydrousmagma, and rapid undercooling by volatile loss has been advocated as amechanism to explain spheroidal textures in some igneous rocks(Ballhaus et al., 2015 and references therein). As we show in the follow-ing, none of these terms or explanations is suitable for the Jordanianappinite, justifying introductionof a new term “rosettas” for these igneousspheroids. Below we use the information that we have collected toexplain how the rosetta texture formed.

5.1. Pressure and temperature

The temperatures obtained from plagioclase-tschermakite pairsare: 886 ± 21.62 and 905.36 ± 41 °C (Supplementary Tables 1 &2) for hornblende gabbro and Rosetta Gabbro, respectively. In contrast,plagioclase-magnesiohornblende pairs yield lower temperatures ofequilibration 746 ± 62 and 829 ± 25 °C for hornblende gabbro andRosetta Gabbro, respectively.

The pressure of crystallization as deduced from the Al content in thetschermakite amphiboles is 6.40 ± 0.76 and 6.28 ± 0.99 kbar for horn-blende gabbro and Rosetta Gabbro, respectively. On the other hand,magnesiohornblende documents lower pressures of 2.49 ± 0.89 and3.29 ± 0.62 kbar for both varieties.

Cationic substitutions in amphiboles are temperature-sensitive asdeduced from natural and experimental assemblages (e.g. Blundy andHolland, 1990; Holland and Blundy, 1994; Shane and Smith, 2013;Kiss et al., 2014). Shane and Smith (2013) concluded that nearly all

Fig. 11. AlVI, Ti and A-site vs. tetrahedrally coordinated Al. Open triangles = RosettaGabbro, open circles = non-rosetta gabbro.

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Al atomic substitutions in amphiboles can be accounted for bytemperature-sensitive edenite and Ti tschermak exchange mechanismswith a minor role played by the pressure sensitive Al-tschermaksubstitution; consequently Al-amphibole barometers can produce erro-neous results. Our calculated pressures are based on the empirical andexperimental Al-hornblende barometers suggested by Hammarstromand Zen (1986) and Anderson and Smith (1995)which require a criticalmineral assemblage of plagioclase + hornblende + biotite + K-feld-spar + quartz + titanite, but the studied gabbro lacks titanite andquartz, and phlogopitic mica and K-feldspar are very rare. Larqocqueand Canil (2010) proposed an empirical barometer based on octahedralaluminum in the amphibole which has the following form:

AlVI ¼ 0:056 � P−0:008 pressure in kbarð Þ:

Krawczynski et al. (2012) updated this equation using additionalexperimental data and suggested the following barometer that is

independent of mineral assemblage and not constrained by tempera-ture:

P MPað Þ ¼ 1:675 � AlVI–48

We use octahedral Al from tschermakite and magnesiohornblendewhich for both varieties average 0.22 (apfu) to calculate the pressureof equilibration. Applying the two calibrations, we obtained 3.75 and3.20 kbar, respectively. These pressure values correspond to 12.48 to10.66 km, respectively. This depth estimate is reasonable bearingin mind the gabbro intrudes greenschist facies rocks of the JanubMetamorphic Complex.

We conclude that the amphibole-cored rosettas formed whilesuspended in a mafic magma at ~600 Ma. It is clear that tschermakiticamphibole crystallized first. As these tschermakites grew, they slowlysank in the mafic magma body. Tschermakites were overgrown bymagnesiohornblende, indicating a change in magma composition.From the trace element concentrations in the two amphiboles, itappears that early-formed tschermakite was in equilibrium with asignificantly more fractionated magma than was late-formedmagnesiohornblende. This strongly suggests that tschermakite formedin equilibrium with fractionated mafic magma and that the growth ofmagnesiohornblende resulted from recharge of the magma body withprimitive, mantle-derived melt.

5.2. Magmatic water and plagioclase crystallization

Experimental data indicates that increasedwater pressure inmagmaexpands hornblende stability relative to olivine, pyroxene and plagio-clase (e.g. Krawczynski et al., 2012; Moore and Carmichael, 1998).Greater magmatic water content also favors depolymerization of themelt and consequently reduces magma viscosity, further facilitatingrapid growth of hornblende (e.g., Meysen, 1988). The petrologicalsignificance of H2O in these magmas is clear; the source of this wateris still a matter of debate (Murphy, 2013 and references therein).Nevertheless, many investigations indicate that the mantle-derivedmafic magma inherits its water from metasomatized mantle above asubduction zone (Fowler et al., 2001).

Experimental data on amphibole stability in mafic magmas demon-strate that the first amphibole appearing at 5 kbar has an Al2O3 contentof 12.5 wt.% with lower contents at pressures of 2 kbar since itco-precipitates with plagioclase. Plagioclase crystallization depends onmagma H2O content (e.g. Sisson and Grove, 1993). There is a switch inthe order of crystallization between plagioclase and amphibole atabout 3 kbar (Krawczynski et al., 2012) and at higher pressures, plagio-clase crystallizes after hornblende. The growth of plagioclase crystalsthat nucleated on magnesiohornblende cores may have been drivenby volatile loss from a hydrous magma in a shallow magma conduit.Nucleation and growth of crystals in magmatic systems depend on thedegree of undercooling or supersaturation of the magma (e.g. Higgins,2006). The growth rate of plagioclase in basaltic systems has beenconstrained between 10−9 and 10−11 mm/s for lower and higherdegrees of undercooling, respectively (e.g. Chashman, 1993; Higginsand Chandrasekharam, 2007). The largest plagioclase in the rosettas hasa length of ca. 15 mm. Accordingly, the maximum crystallization timeof these rosettas may have varied between ~500 and ~50,000 years.Taking an average growth rate of 10−10 mm/s suggests a crystallizationtime of ~5000 years. Radial growth of plagioclase around a magnesio-hornblende nucleus implies oversaturation, perhaps caused by loss ofthe water-rich fluid phase as a consequence of hornblende crystalliza-tion and magmatic degassing.

5.3. Suspension in magma

The nearly spherical shape of the rosettas and the lack of evidencefor close packing (Fig. 2) indicate that they grew while suspended in

Fig. 12. Variation diagrams of amphibole trace element concentrations. Mangesiohornblende = open triangles; tschermiakite = solid triangles.

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magma. This is very likely because the density of plagioclase is veryclose to that of basaltic magma, especially iron-rich magmas (Scoates,1999). The rosettas may have been slightly more dense than the sur-rounding mafic magma and sunk slowly in it, or they may have beenslightly less dense than the magma and risen slowly through it. We donot know the composition of the magma and we infer that the wholerock compositions listed in Table 2 are mixtures of magma and rosettas.Ifwe take the average Fe2O3 of the 3 RosettaGabbro samples (10.4wt.%)

and assume that 30% of the rock is rosettas with 0 wt.% Fe2O3, then themagma contained ~15 wt.% Fe2O3; this would have been significantlymore dense than most plagioclase (Sparks and Huppert, 1984). Horn-blende has a density of ~3.0–3.4 g/cm3 and these would have sunk inthe magma (density ~ 2.7–2.75 g/cm3), but the density of the rosettaswould have decreased as plagioclase (An64–79) with a density of~2.6 g/cm3 grew. It is possible that the rosettas sank at first but asplagioclase continued to grow, they reached neutral buoyancy and

Fig. 13. REE in amphiboles and plagioclase (A, B, &C) and multi-element plots (D & E) of the rosetta gabbro. Magnesiohornblende = open triangles; tschermakite = solid triangles.

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then became buoyant. One can imagine a scenario whereby sinkinghornblendes slowed their descent as they became coated with plagio-clase, stopping their descent after they reach a critical size, then beginto rise in the magma. Whether or not this scenario is correct, it seemsclear that the rosettas attained their remarkable form while growingin the melt.

5.4. Tectonic setting and regional comparisons

Appinitic complexes occur throughout geologic time, e.g. — the LateArchean in the Superior Province, Canada (Stern et al., 1989); Silurian,Carboniferous, and Permian complexes of China (Ye et al., 2008).Hornblende is typically the dominant mafic mineral phase, indicatingthat these are water-rich mafic magma. Appinites usually have a LILE-enriched and HFS-depleted geochemical signature reflecting an arcsetting where sub-continental lithospheric mantle was metasomatizedby water-rich fluids derived from a subducted slab (Murphy, 2013 andreferences therein).

Classic occurrences of appinites such as in the type locality nearAppin (Scotland) and in Donegal (northwestern Ireland) form stocks,pipes, sheets, dikes and sills and are generally considered to be shallowcrustal subvolcanic complexes (Murphy, 2013 and references therein).These appinite complexes are thought to be the result of slab break-offfollowing cessation of subduction associated with the closure of theIapetus ocean, which then triggered post-collision mantle upwellingand the generation of hydrous mafic magmas (Neilson et al., 2008).Other appinitic complexes correlate with the late stages of arc-relatedmagmatism, e.g. — the Greendale of Nova Scotia intruded into thelower greenschist facies arc-related volcano-sedimentary sequence(Georgeville Group) at ca. 607 Ma when Avalonian island arcmagmatism ceased (Murphy, 2013).

In the northernmost extremity of the Arabian–Nubian Shield, theEast African Orogen (EAO) gave rise to several metamorphic complexesas exemplified by the Abu Barqa and Janubmetamorphic suites in southJordan (Jarrar et al., 2013a) and the Elat schist of southern Israel (Elishaet al., 2017). Arc volcanism to formEAO crust ended by ca. 620Mawhenterminal collision began and was followed by voluminous calc-alkalineand alkaline plutonism. These metamorphic complexes and latergranitoids were exhumed during Late Ediacaran time, accompanied byerosion of about 10 km of upper crust. During this period of uplift,erosion, and extension,widespread bimodalmagmatic activity occurredthroughout the shield and included mafic and felsic intrusions, theirvolcanic and subvolcanic equivalents of simple, composite and hybriddikes, as well as spessartite lamprophyric dikes (Ghanem and Jarrar,2013; Jarrar et al., 1992, 2003, 2008, 2013b; Wachendorf et al., 1985).This stage of intraplate igneous activity generated the so-called ArabaComplex of Jordan (ca. 605–541 Ma, this study; Jarrar et al., 2013a,2013b; Yaseen et al., 2013), including the Rosetta Gabbro.

Other appinitic rocks in the northern ANS include the Huheilahornblende-bearing suite (Ibrahim and McCourt, 1995; Jarrar, 1998)from central Wadi Araba, Jordan (611 ± 9 Ma, K-Ar whole rock age;Lenz et al., 1972) and the hornblende gabbroic intrusion at WadiNesryin in southwestern Sinai, Egypt (617 ± 19 Ma, Rb-Sr whole rockisochron age; Abu Anbar, 2009). These rocks, the products of mantle-derived magmatism intruded into different structural levels, are alsoassociated with collision-related extension and erosion of the ANSjuvenile crust in the latest Neoproterozoic (ca. 605–542 Ma; Ghanemand Jarrar, 2013; Jarrar et al., 2013a;Yaseen et al., 2013).

Although appinite complexes occur in a wide range of settings(Murphy, 2013), the great majority are associated with the final stagesof convergence, as are those from the northern ANS such as the RosettaGabbro. This is consistent with the inferred genesis of late- to post-orogenic plutonism in the northeastern Arabian Shield, where one or

Fig. 14.Model for formation of Rosetta Gabbro and non-rosetta hornblende gabbro of the Ayn-Al HashimGabbroic Suite. A: Twomafic intrusions are emplaced ~600Ma, one iswater-richbut can easily degas via volcanic conduit (left) and one is drier and/or cannot easily degas (right). Both bodies crystallize tschermakite early but plagioclase crystallization is suppressed inone body because of highmagmatic water pressures (left). The other magmatic body crystallizes plagioclase along with tschermakite (right). B: Magnesiohornblende crystallizes aroundtschermakite cores. In the case of one intrusion (left), this reflects influxofmore primitivemaficmagma from themantle,which heats fractionatedmagma and causes it to degas, leading toa decrease inmagmaticwater pressure. C: Degassedmagma becomes supersaturated in plagioclase, which nucleates on tschermakite-magnesiohornblende crystals suspended inmagma.Radial growth of plagioclase is rapid, lowering density of plagioclase–amphibole rosettas and allowing them to remain suspended in the melt. Prolonged suspension of rosettas insupersaturatedmagma lead to rapid growth of plagioclase. D: Abu Jedda granite intrudes gabbroic bodies ~596Ma. E: ~10 kmof erosion in Late Ediacaran time results in present exposure.

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more episodes of late-orogenic slab tear, break-off, or rollback has beeninvoked to induce asthenospheric mantle flow to produce A-type gra-nitic magmas (Robinson et al., 2017). Consequently, some mechanismof late-stage slab break-off associated with the cessation of subductionexplains emplacement of late-stage melts (appinites and A-type) inthe northern ANS. This genetic association is not restricted to the ANSbut is relevant to other orogens across the globe and throughout Earth.

6. Conclusions

The ~600MaRosetta Gabbro of southern Jordan is characterized by aunique igneous texture consisting of tschermakite cores surrounded byrims of magnesiohornblende, from which euhedral plagioclase lathsextend radially, creating nearly spherical bodies up to a few centimetersin diameter. We conclude that these rosettas formed while suspendedin mafic magma and may have floated in this magma.

Our understanding of the formation of the Rosetta Gabbro and non-rosetta hornblende gabbro is summarized in Fig. 14. Panel A shows twomafic intrusions that were emplaced ~600 Ma. The magma body thatwent on to become the Rosetta Gabbro was water-rich and able tolose magmatic volatiles (especially water) by degassing, perhaps via avolcanic conduit. The magma body that did not develop rosettas mayhave been drier and/or one that could not easily degas. Both magmabodies crystallized tschermakite early but plagioclase crystallizationwas suppressed in the magma body that became the Rosetta Gabbrobecause of high magmatic water pressures. The other (non-rosetta)magmatic body crystallized plagioclase along with tschermakite. Inboth magma bodies, magnesiohornblende crystallized aroundtschermakite cores. In the case of the Rosetta Gabbro intrusion, thegrowth of magnesiohornblende likely reflected influx ofmore primitivemaficmagma,which heated fractionatedmagma and caused it to degas,leading to a decrease inmagmatic water pressure. The degassedmagmabecame supersaturated in plagioclase, which nucleated on hornblendecrystals suspended in themelt. Once begun, radial growth of plagioclasewas rapid, lowering the density of hornblende-plagioclase rosettas andallowing them to remain suspended in the magma. At some point, themagma body cooled and crystallized, freezing the rosettas in place.

Shortly afterwards ~596Ma, the Abu Jedda granite intruded the gabbroicbodies. Subsequently ~10 km of erosion in Late Ediacaran time broughtthe magma chamber to the surface.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2017.05.008.

Acknowledgements

Ghaleb H. Jarrar and Najel Yaseen are grateful to the University ofJordan for logistic and financial support. This studywas partially carriedout while the senior author was on a sabbatical leave as a Fulbrightscholar at the Jackson School of Geosciences, UTA, Austin, USA. Thesupport offered to him by Elizabeth Catlos andWilliamCarlson is highlyappreciated. The stay of the senior author at NORDSIM facility wassupported by the JEBEL project and Swedish Research Council grantsto V. Pease and M.J. Whitehouse. Constructive comments and sugges-tions by reviewers, J Brendan Murphy, Lawford Anderson, and theJournal Editor Nelson Eby which helped to improve the quality of themanuscript are highly appreciated. This is NORDSIM contribution#510. This is UTD Geosciences contribution # 1298.

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