The Petrogenesis of A-type Magmas from the Amram Massif, Southern Israel

The Petrogenesis of A-type Magmas fromthe Amram Massif, Southern Israel

AMIT MUSHKIN1,2*, ODED NAVON1, LUDWIK HALICZ2,GERALD HARTMANN3 AND MORDECHAI STEIN1,2

1INSTITUTE OF EARTH SCIENCES, THE HEBREW UNIVERSITY OF JERUSALEM, GIVAT RAM,

JERUSALEM 91904, ISRAEL

2GEOLOGICAL SURVEY OF ISRAEL, 30 MALKHE YISRAEL STREET, JERUSALEM 95501, ISRAEL

3GEOCHEMICAL INSTITUTE, UNIVERSITY OF GOÈ TTINGEN, GOLDSCHMIDTSTRASSE 1,

GOÈ TTINGEN D-37077, GERMANY

RECEIVED JUNE 8, 2002; ACCEPTED NOVEMBER 6, 2002

The (�550±530Ma) alkaline magmatic suite of the AmramMassif, southern Israel, was emplaced during the transitionfrom an orogenic to an intra-plate tectonic setting in the north-eastern Arabian±Nubian Shield (ANS). The suite rangesfrom 45�6 to 78�8 wt % SiO2, and consists of rhyolites, alkaliquartz syenites, quartz syenites, monzonites, and co-magmaticmafic to felsic alkaline dikes. These rocks define a continuouschemical evolutionary trend and reveal a correlation betweendecreasing stratigraphic age and increasing silica content. Thefelsic members of the suite display A-type characteristics and aregenetically linked through fractionation to the more mafic ones.Moderately positive initial eNd values (�2� 0�5), low initial87Sr/86Sr values (0�7036 � 2), high MgO and Fe2O3 con-centrations (4�10±8�95 and 10�0±12�5 wt %, respectively) andrelatively flat rare earth element patterns [(La/Yb)n � 6�4 �0�9] in the Amram mafic dikes (45�6±49�5 wt % SiO2),suggest their derivation from the sub-continental lithosphericmantle, above the garnet stability zone. The MELTS programwas used to quantitatively model the chemical evolution of thesuite. Extensive anhydrous fractionation (490%), of plagio-clase, alkali-feldspar, clinopyroxene, olivine, and minor Ti-magnetite and apatite from parental mafic magmas, representedby the Amram mafic dikes, produced the rhyolitic compositionsas well as the intermediate members of the suite. This suggeststhe presence of a large unexposed body of cumulate rocks atdepth, as well as fusion of a large source-region (equivalent toan �5 km layer) in the lithospheric mantle. Regarded as arepresentative example for similar A-type outcrops in thisregion, this petrogenetic model further suggests that Neoproter-ozoic±Early Cambrian A-type magmatism in the northeastern

ANS represents a significant post-orogenic addition of mantle-derived material to the juvenile crust. This magmatic episodewas of a similar magnitude to that of the Cenozoic, extension-related, alkaline volcanism of the Arabian plate.

KEY WORDS: A-type granites; fractional crystallization; MELTS;

Arabian±Nubian Shield

INTRODUCTION

Anorogenic alkaline granites and related alkaline mag-mas (termed A-type granites and magmas) commonlyoccur in post-orogenic, intra-plate tectonic settings(e.g. Eby, 1990, 1992; Turner et al., 1992; Black &Liegeois, 1993). These rocks provide significant infor-mation on post-collisional magmatic processes withinthe continental lithosphere and their contribution tothe build-up of the upper continental crust (Turneret al., 1992). Several workers have argued for theformation of A-type granites by partial melting ofpre-existing crustal rocks (e.g. Collins et al., 1982;Whalen et al., 1987; Creaser et al., 1991; Landenberger& Collins, 1996), whereas others have proposed thatthese granites are highly fractionated products fromthe differentiation of mantle-derived parental maficmagmas (e.g. Stern & Gottfried, 1986; Turner et al.,1992; Beyth et al., 1994b; Kessel et al., 1998; Volkert

JOURNAL OF PETROLOGY VOLUME 44 NUMBER 5 PAGES 815±832 2003

*Corresponding author. Present address: Department of Earth andSpace Sciences, University of Washington, Seattle, WA 98195-1310,USA. Telephone: 1-206-543-6221. Fax: 1-206-685-2379. E-mail:[email protected]

Journal of Petrology 44(5) # Oxford University Press 2003; all rightsreserved.

et al., 2000). Possibly, A-type magmas do not representa particular geological setting, but rather, similarend-products derived through different processes (e.g.Eby, 1992; Whalen et al., 1996).In this study we examine the petrogenesis and geo-

logical significance of post-orogenic (Pan-African),Late Proterozoic to Early Cambrian A-type magmas,from the northeastern part of the Arabian±NubianShield (ANS) (Fig. 1). We focus on an �550±530Maalkaline rock suite from the Amram Massif, southernIsrael, and compare it with similar outcrops in thisregion. The good exposure of apparently co-magmaticA-type granitoid and alkaline mafic rocks, spanning awide and continuous range of chemical compositions(45�6±78�8 wt % SiO2), make the AmramMassif a keysite to study the formation of post-orogenic A-typemagmas in this part of the ANS. We use major andtrace element chemistry, as well as Sr and Nd isotopecompositions to constrain the petrogenesis of theAmram magmas. We then employ the `MELTS' com-

puter program of Ghiorso & Sack (1995) to quantita-tively model the differentiation process. Applying thismodel to contemporaneous A-type suites in this regionsuggests that Late Neoproterozoic to Early CambrianA-type magmatism in the northeastern ANS contri-buted substantial amount of juvenile magmas to theupper crust and thus represents a significant process inthe history of the shield.

GEOLOGICAL BACKGROUND

The Amram Massif, located in the southern Negevdesert of Israel, is one of the northernmost outcrops ofthe Arabian±Nubian Shield (ANS) (Fig. 1). The ANSextends over large areas in NE Africa and Arabia andis considered to be the product of one of the mostintensive episodes of continental crust formation inEarth history (Reymer & Schubert, 1984; Stein &Hofmann, 1994). The geological history of the north-ern ANS has been divided into four main phases[adapted from Bentor (1985)]. Phase I (�900±870Ma) was characterized by the formation ofa several kilometer thick sequence of tholeiitic basalts,which probably erupted in an oceanic environmentand formed an oceanic plateau (Reischmann et al.,1982; Stein & Goldstein, 1996; Stein, 1999). Phase II(�870±650Ma) was characterized by mainly calc-alkaline, island arc magmatism, followed by extensivemetamorphism. Phase III (�630±600Ma) was gov-erned by intrusion of calc-alkaline granitic batholiths(mainly in the northern segments of the ANS). PhaseIV, which extended into the Cambrian (�600±530Ma), was characterized by intrusion and extrusionof alkaline magmas. Outcrops of phase IV alkalinemagmas form a small part of the ANS volume, yetthey are scattered over the entire shield (Bentor,1985; Stern et al., 1988; Black & Liegeois, 1993).In the northeastern ANS, i.e. southern Israel, south-

western Jordan and the Sinai Peninsula, phase IValkaline magmas were generated during the transitionfrom orogenic activity to intra-plate stable conditions.This period (�600±530Ma) was also characterized byextension-related tectonics, dike emplacement and sig-nificant erosion [see summary by Garfunkel (1999)].The onset of stable platform conditions in this region ismarked by the deposition of Early Cambrian sand-stones (e.g. Parnes, 1971). Intra-plate sedimentationand recurring cycles of alkaline magmatism character-ized this region through most of the Phanerozoic (e.g.Garfunkel, 1989). The present-day exhumation ofbasement rocks in this region is associated with theCenozoic tectonic activity along the Dead SeaTransform and the Red Sea spreading center (e.g.Garfunkel, 1970, 1980).

Fig. 1. (a) Location of the Arabian±Nubian Shield (ANS).(b) Neoproterozoic±Early Cambrian alkaline magmatism in south-ern Israel and the Sinai Peninsula. (1) Timna Massif, (2) NeshefMassif, (3) Iqna, (4) Katherina ring complex, (5) Wadi Kid,(6) Mandar, (7) Zahara.

JOURNAL OF PETROLOGY VOLUME 44 NUMBER 5 MAY 2003

816

GEOLOGY OF THE AMRAM MASSIF

Bordered on all sides by Cenozoic faults, AmramMassifcovers an area of over 6 km2, and comprises a lateNeoproterozoic±Early Cambrian alkaline magmaticsequence, which is unconformably overlain by EarlyCambrian sandstones (Wurtzburger, 1959; Mushkinet al., 1999). The main rock units in the massif are(Fig. 2): alkali quartz syenite (holocrystalline, with70±75% subhedral alkali feldspar, 15±20% anhedralquartz and 510% opaque minerals and chloritein varying ratios); porphyritic rhyolite (up to 30%phenocrysts of alkali feldspar and quartz in amicrocrystalline groundmass of similar mineralogy);monzonite (holocrystalline, with 40% plagioclase,25% alkali feldspar, 10% clinopyroxene, 10% opaqueminerals, 5% quartz, minor apatite and up to 10%secondary chlorite); quartz syenite (holocrystalline,with 65% alkali feldspar, 15% quartz, 20% clino-pyroxene and accessory chlorite, amphibole, plagio-clase and opaque minerals); and dikes of mafic tofelsic compositions. Some of the dikes are composite,and consist of mafic margins with a felsic center.Kaolinitization and sericitization of feldspars andchloritization of mafic phases is common in all rocksand is extensive in the monzonite and the quartzsyenite. The relatively small crystal size (up to 3mm)of the plutonic rocks, their granophyric textures, andthe perthitic nature of the alkali feldspars in some of theintrusions, all indicate relatively shallow depths ofcrystallization.Field relations, petrography, and geochronology of

the Amram rocks reveal the following sequence of

magmatic events, which occurred between �550 and530Ma [see Mushkin et al. (1999) for field mappingand detailed description of the units]: (1) deposition ofvolcaniclastic units and intrusion of hypabyssal alkaliquartz syenite; (2) eruption of rhyolites on top of theexposed alkali quartz syenite; (3) intrusion of maficdikes followed by the emplacement of monzonite andthen quartz syenite; (4) intrusion of composite dikes(mafic margins and a rhyolitic center); (5) intrusion ofdolerite dikes into the volcanoclastic unit. No fieldrelations could be established between the doleriticdykes and the other rock units in Amram. However,in the nearby TimnaMassif a similar doleritic dike cutsthe mafic dikes (Baer & Beyth, 1990) and both Amramand Timna dolerite dikes are geochemically distinctfrom all other Amram rocks (Beyth et al., 1994a; Steinet al., 1997). This magmatic stratigraphy has beensubdivided by Mushkin et al. (1999) into two cycles(I and II), whereas the doleritic dikes constitute ayounger and separate phase. In both cycles the SiO2

content of the rocks increases as their stratigraphic age(determined by field relations) decreases (Fig. 3).Major and trace element geochemical variation, dis-cussed later in this paper, suggests that fractionationwas the main factor governing this geochemical evolu-tion, and that crustal assimilation was not significant.Normal faulting accompanied the emplacement ofthe Amram magmas during both magmatic cycles(Mushkin et al., 1999).

Fig. 2. Simplified geological map of the magmatic rock units in theAmram Massif [from Mushkin et al. (1999)]. Rhyolites and shallowplutonic bodies comprise the northern block; similar rhyolites andolder volcaniclastic units comprise the southern block. Alkaline dikesof various size and compositions intrude both blocks.

Fig. 3. Magmatic cycles in Amram Massif. Two magmatic cycles ofincreasing silica content with decreasing age are revealed by plottingthe average SiO2 content of the Amram rock units according to theirrelative age, as determined by field relations. Cycle 1 is representedby its felsic members: alkali quartz syenite and rhyolite. Cycle 2includes mafic dikes, monzonite, quartz syenite and compositedikes. The dolerite dikes represent a younger magmatic episode.Both magmatic cycles are constrained to �550±530Ma (Mushkinet al., 1999).

MUSHKIN et al. A-TYPE MAGMAS, AMRAM MASSIF

817

ANALYTICAL METHODS

After petrographically characterizing the Amram rockunits, 19 representative samples were selected for chem-ical analysis. Major elements were analyzed by induc-tively coupled plasma atomic emission spectrometry[ICP-AES; Perkin±ElmerOptima-3300, at theGeolog-ical Survey of Israel (GSI)] after fusion with lithiummetaborate. Loss on ignition (LOI) was determined bymeasuring weight loss upon heating to 1100�C. Traceelement concentrations were measured by inductivelycoupled plasma atomic emission spectrometry (ICP-AES; Jobin Yvon JY-48 at the GSI), and by induc-tively coupled plasma mass spectrometry (ICP-MS;Perkin±Elmer±SCIEX Elan 6000 at the GSI), aftersintering with sodium peroxide. Standard rock refer-ence materials (BHVO-1, BE-N, MA-N, and NIM-G)were analyzed repeatedly and compared with the dataof Govindaraju (1994) to determine accuracy and pre-cision. Precision for major elements was found to bebetter than 1%, excluding P2O5, for which a 10%error is assumed, as a result of low concentration. Pre-cision for trace element concentrations is estimatedas better than 5%. Most samples were also analyzedfor major and trace element concentrations by X-rayfluorescence spectrometry (XRF; Phillips PW 1480 onfused glass pellets) at the University of Goettingen.Precision for these measurements is better than 1%for major elements, and better than 5% for traceelements including Rb, Sr, Sm and Nd (Hartmann,1994). Discrepancy between the two techniques formajor elements was typically of the order of 1±2%,and never exceeded 5%.Isotope ratios of Sr and Nd in selected samples were

determined at the University of Goettingen by thermalionization mass spectrometry (TIMS, Finnigan MAT262 RPQ equipped with a multi-collector system oper-ating in static mode). Rock powders (100mg) weredissolved in HF±HNO3 in pressurized Teflon reactionvessels. Sr and Nd were chemically separated by stan-dard procedure at the University of GoÈ ttingen. Massfractionation was linearly corrected using 86Sr/88Sr �0�1194, and 146Nd/144Nd� 0�7219. Repeated measure-ments of the NBS-987 standard yielded 87Sr/86Sr �0�710263 � 20 (2s, n � 10). The measured mean143Nd/144Nd ratio for the La Jolla standard at thetime of measurements was 0�511840 � 7 (2s, n � 10).The errors on the 87Rb/86Sr and 147Sm/144Nd ratiosare �7%.

RESULTS

Major elements

Major element concentrations of the Amram rocks arelisted in Table 1. The rocks span a wide range of SiO2

content (45�6±78�8 wt %), are of alkaline affinity(Fig. 4) and generally plot along well-defined trendsin major element variation diagrams (Fig. 5).Al2O3, Fe2O3, and TiO2 concentrations decrease

monotonously with increasing SiO2 concentration.MgO decreases along a hyperbolic curve, reachingnear-zero concentration in the more felsic members.CaO varies widely in the mafic members (1�74±5�47wt %), but decreases monotonously in the more felsicmembers. Na2O and K2O vary considerably withinotherwise petrologically and chemically well-definedrock units. Nevertheless, the total alkali concentrationsare uniform within each rock unit (5�10±7�64 wt % inmafic dikes, 6�80±7�64 wt % in monzonite, 8�45±11�7wt % in alkali quartz syenite and quartz syenite, and7�94±8�95 wt % in rhyolite and composite dikes)(Fig. 4). Agron & Bentor (1981) suggested post-emplacement, metasomatic replacement of Na by Kin similar rocks of the Neshef Massif (the westwardcontinuation of the Amram Massif, Fig. 1). They alsoobserved that the metasomatic process affected onlysome of the rocks. Where K replaced Na, SiO2 contentwas increased by 2±3%, whereas other element con-centrations were not significantly altered. Based onthis, alkali concentrations in samples displaying K2O48 wt%, and Na2O51 wt%, have been discarded inthe subsequent discussion. The rest of the K2O datashow a monotonous increase until 65±70 wt % SiO2

followed by a decrease of K2O concentrations in thehigher SiO2 range (Fig. 5).

Trace elements

Trace element concentrations of the Amram rocks arelisted in Table 1. Selected elements are plotted againstSiO2 content in Fig. 6. Ni, V, and Sr concentrationsdecrease whereas Zr, Y, Nb, Rb, U, Th, and La con-centrations increase with increasing SiO2 content. Baconcentrations decrease only after �67 wt % SiO2,similar to K2O. Rare earth element (REE) profiles ofthe Amram suite resemble each other, with moderateenrichment of the light REE [LREE; (La/Yb)n �4�81±12�8] and a relatively flat heavy REE profile[HREE; (Tb/Yb)n � 1�31±2�52, Fig. 7]. A negativeEu* anomaly (0�13±0�41) is apparent only in the silica-rich members of the suite (i.e. rhyolites and felsiccenters of the composite dikes). Rocks from the twomagmatic cycles of the Amram Massif (Mushkin et al.,1999) display similar major and trace element con-centrations (Figs 5 and 6), as well as matching REEpatterns. These geochemical similarities suggest thatrocks of the two cycles evolved through similar mag-matic processes (elaborated below). The dolerite dikes,however, are chemically distinct from the rocks of thetwo earlier cycles. In relation to the Amram mafic


818

Table 1: Representative chemical analyses of Amram Massif rocks

Sample: MR-24 MR-32 MR-42 MR-79 MR-30 MR-6 MR-203 AM-151

Rock type: Mafic dike Mafic dike Mafic dike Mafic dike Mafic dike Dolerite Dolerite Dolerite

wt %

SiO2 47.2 47.0 47.2 48.3 45.6 47.5 47.3 46.6

TiO2 1.79 2.27 2.13 2.23 2.30 4.18 4.26 4.51

Al2O3 15.6 15.2 15.8 16.0 15.7 14.2 14.4 13.8

Fe2O3(t) 10.3 12.1 12.5 11.3 11.9 14.9 13.9 13.4

MnO 0.26 0.99 0.46 0.29 0.19 0.19 0.16 ÐÐ

MgO 5.63 8.16 6.32 8.00 8.95 4.26 4.35 5.08

Na2O 4.26 3.22 3.78 3.61 2.76 3.29 3.28 3.07

CaO 4.30 2.27 5.47 3.02 1.74 7.22 7.55 8.14

K2O 3.38 3.06 1.44 1.49 3.79 1.59 1.76 1.45

P2O5 0.40 0.39 0.49 0.46 0.99 0.94 1.03 1.00

LOI 5.20 3.99 2.87 4.17 4.58 1.33 1.91 3.26

Total 98.26 98.67 98.42 98.86 98.41 99.63 99.90 100.23

ppm

Ba 403 1974 1083 423 557 607 613 536

Cu 27 56 38 33 b.d.l. 118 123 180

Co 42 45 43 39 39 47 49 49

Ni 53 60 37 74 38 72 73 79

Zn 680 279 264 932 856 175 144 337

Rb 179 72 81 53 137 29 33 30

Sr 293 343 754 310 225 520 525 583

Y 24 26 28 28 36 44 44 44

Zr 196 199 169 197 304 321 335 297

Nb 16 17 16 17 19 39 41 35

V 203 237 247 258 170 276 289 198

Pb 61 625 19 32 55 14 7 777

Th 3.1 1.7 2.1 1.8 ÐÐ 4.3 ÐÐ 4.3

U 0.8 0.4 0.8 0.5 ÐÐ 1.3 ÐÐ 0.9

La 21 17 20 20 ÐÐ 40 ÐÐ 48

Ce 46 37 46 41 ÐÐ 91 ÐÐ 117

Pr 6.7 6.1 6.1 6.5 ÐÐ 12.0 ÐÐ 13.1

Nd 26.6 23.9 27.5 25.5 ÐÐ 52.0 ÐÐ 60.8

Sm 6.1 5.9 6.0 6.0 ÐÐ 11.0 ÐÐ 12.8

Eu 1.4 1.6 2.2 1.7 ÐÐ 3.5 ÐÐ 3.7

Gd 5.1 4.8 6.4 6.2 ÐÐ 11.0 ÐÐ 10.7

Tb 0.8 0.9 0.9 0.9 ÐÐ 1.5 ÐÐ 1.4

Dy 3.7 3.9 4.9 4.0 ÐÐ 7.5 ÐÐ 7.7

Ho 0.8 0.9 0.9 0.9 ÐÐ 1.4 ÐÐ 1.4

Er 2.3 2.3 2.6 2.5 ÐÐ 3.8 ÐÐ 3.6

Tm 0.3 0.3 0.3 0.3 ÐÐ 0.5 ÐÐ 0.5

Yb 1.9 1.9 2.2 2.0 ÐÐ 2.9 ÐÐ 3.1

Lu 0.3 0.3 0.3 0.3 ÐÐ 0.4 ÐÐ 0.5

Ta 0.9 0.5 22.0 0.7 ÐÐ 3.0 ÐÐ 3.0

Ga 25.0 26.0 23.0 40.0 24.0 26.0 30.6


819

Table 1: continued

Sample: MR-24 MR-32 MR-42 MR-79 MR-30 MR-6 MR-203 AM-151

Rock type: Mafic dike Mafic dike Mafic dike Mafic dike Mafic dike Dolerite Dolerite Dolerite

Mg-no. 0.56 0.61 0.54 0.62 0.64 0.40 0.42 0.47

Y/Nb 1.50 1.53 1.75 1.65 1.89 1.13 1.07 1.26

La/Nb 1.33 0.99 1.27 1.20 ÐÐ 1.03 ÐÐ 1.36

(La/Yb)n 7.42 5.92 6.17 6.76 ÐÐ 9.22 ÐÐ 10.12

(Gd/Yb)n 2.10 2.04 2.32 2.45 ÐÐ 3.02 ÐÐ 2.72

Eu* 0.76 0.90 1.09 0.88 ÐÐ 0.98 ÐÐ 0.98

Sample: MR-13 MR-75 MR-16 MR-26 MR-28 MR-18 MR-53 MR-54

Rock type: Monzonite Monzonite Qtz syenite Qtz syenite Qtz syenite Alkali qtz syenite Alkali qtz syenite Alkali qtz syenite

wt %

SiO2 50.6 53.0 67.5 60.1 64.9 65.6 60.0 66.4

TiO2 2.39 2.33 0.65 1.07 0.77 0.75 0.77 0.68

Al2O3 15.3 14.8 13.1 14.4 12.7 13.5 12.5 12.9

Fe2O3(t) 11.6 10.3 5.36 8.23 7.79 5.89 6.90 6.76

MnO 0.14 0.17 0.13 0.19 0.14 0.24 0.30 0.18

MgO 4.34 3.85 0.83 0.77 1.54 0.60 1.80 1.32

Na2O 4.84 5.12 3.27 4.18 3.27 4.22 2.76 3.21

CaO 4.49 3.94 1.15 3.16 1.00 2.06 1.70 1.08

K2O 1.96 2.52 5.90 4.55 5.18 4.99 8.92 5.80

P2O5 1.45 1.22 0.21 0.37 0.28 0.23 0.20 0.22

LOI 2.21 1.92 2.40 1.15 1.62 0.96 2.72 1.85

Total 99.28 99.19 100.49 98.20 99.15 99.03 98.57 100.38

ppm

Ba 1252 1185 931 1437 676 941 705 771

Cu 21 16 3 5 ÐÐ b.d.l. 2 5

Co ÐÐ ÐÐ ÐÐ 7 ÐÐ 7 ÐÐ 5

Ni 25 13 7 b.d.l. 7 b.d.l. 6 b.d.l.

Zn 145 122 125 102 126 101 216 168

Rb 47 54 100 66 93 79 96 124

Sr 751 426 94 188 76 147 114 99

Y 34 36 36 34 39 43 37 48

Zr ÐÐ ÐÐ ÐÐ 268 ÐÐ 304 ÐÐ 885

Nb 22 26 27 20 28 26 24 26

V 145 160 14 34 ÐÐ 23 13 17

Pb 5 12 24 5 12 9 62 18

Th 3.2 3.7 7.4 4.3 7.6 9.2 5.4 6.8

U 1.2 1.2 2.0 1.0 2.2 2.7 3.2 12.2

La 51 51 38 25 43 52 35 38

Ce 116 116 86 57 95 108 76 85

Pr 15.3 15.5 11.0 7.5 12.4 14.0 10.0 11.2

Nd 66.2 67.4 45.8 32.6 52.4 59.5 42.8 47.8

Sm 12.7 13.0 9.2 7.0 10.6 12.5 8.9 10.2

Eu 3.6 3.7 2.5 3.8 2.9 4.4 2.8 3.1

Gd 11.5 11.9 9.0 7.1 10.1 12.5 8.9 10.1

Tb 1.5 1.5 1.3 1.0 1.5 1.9 2.3 1.5

Dy 7.0 7.7 7.2 5.7 8.0 10.0 7.2 8.3

Ho 1.3 1.4 1.4 1.1 1.6 2.0 1.4 1.6

Er 3.5 3.9 4.0 3.2 4.6 5.7 4.1 4.6

Tm 0.5 0.5 0.6 0.4 0.7 0.8 0.6 0.6


820

Sample: MR-13 MR-75 MR-16 MR-26 MR-28 MR-18 MR-53 MR-54

Rock type: Monzonite Monzonite Qtz syenite Qtz syenite Qtz syenite Alkali qtz syenite Alkali qtz syenite Alkali qtz syenite

Yb 2.8 3.1 3.9 2.9 4.4 5.1 4.0 4.2

Lu 0.4 0.5 0.6 0.5 0.7 0.8 0.6 0.7

Ta 1.4 1.5 2.0 1.2 2.0 2.6 1.6 1.9

Ga ÐÐ ÐÐ ÐÐ 26.0 ÐÐ 22.0 ÐÐ 26.0

Mg-no. 0.47 0.47 0.29 0.18 0.34 0.19 0.38 0.31

Y/Nb 1.50 1.40 1.32 1.70 1.38 1.65 1.50 1.85

La/Nb 2.30 1.95 1.41 1.23 1.51 1.99 1.43 1.47

(La/Yb)n 12.23 10.89 6.55 5.67 6.46 6.79 5.82 6.07

(Gd/Yb)n 3.27 3.06 1.83 1.95 1.83 1.95 1.77 1.92

Eu* 0.92 0.91 0.85 1.66 0.86 1.08 0.97 0.94

Sample: AM-271 AM-281 MR-39 MR-45 MR-58 AM-121 AM-211 MR-48

Rock type: Alkali qtz syenite Alkali qtz syenite Rhyolite Rhyolite Rhyolite Rhyolite Rhyolite Comp. dike

wt %

SiO2 64.7 67.2 76.0 74.7 72.9 78.8 73.8 75.2

TiO2 1.01 0.76 0.19 0.24 0.26 0.19 0.23 0.20

Al2O3 13.4 12.4 10.5 11.0 10.9 10.6 10.9 12.3

Fe2O3(t) 6.58 7.39 3.14 2.90 4.49 3.14 3.06 3.26

MnO ÐÐ ÐÐ 0.03 0.15 0.03 ÐÐ ÐÐ 0.07

MgO 1.36 0.96 0.18 0.22 0.36 0.26 0.27 0.57

Na2O 3.45 ÐÐ 3.34 0.29 0.73 3.73 1.33 3.67

CaO 1.40 1.37 0.83 0.78 0.18 0.37 0.45 0.57

K2O 5.45 5.17 4.50 8.66 8.17 4.41 7.12 4.91

P2O5 0.36 0.26 0.02 0.01 b.d.l. 0.01 0.01 0.02

LOI 2.15 2.31 0.67 1.21 1.12 0.35 1.13 1.14

Total 99.86 97.82 99.30 100.18 99.14 101.86 98.30 101.85

ppm

Ba 918 850 82 306 325 29 119 132

Cu 3 33 b.d.l. 225 ÐÐ 31 2 13

Co 4 4 b.d.l. b.d.l. ÐÐ 3 1 ÐÐ

Ni 1 3 b.d.l. 6 6 2 3 8

Zn 132 81 106 27 58 122 30 118

Rb 96 118 177 209 169 176 161 102

Sr 111 98 18 17 ÐÐ 14 17 28

Y 35 36 102 58 67 104 62 45

Zr 294 383 824 446 ÐÐ 952 547 ÐÐ

Nb 23 26 54 37 41 60 40 35

V 17 14 8 6 ÐÐ 7 5 5

Pb 12 31 23 14 46 33 19 7

Th 7.5 9.6 18.8 9.4 10.9 23.4 12.3 10.5

U 2.6 3.2 5.9 2.9 1.6 5.5 2.2 3.2

La 38 39 87 46 48 91 63 48

Ce 95 94 172 104 94 223 157 108

Pr 11.0 10.7 20.9 12.9 12.6 25.7 ÐÐ 12.5

Nd 47.1 47.9 83.4 53.8 51.1 109.0 72.8 50.0

Sm 9.7 9.9 18.0 11.7 10.7 22.1 ÐÐ 10.5


821

dikes, the dolerites have higher TiO2 (4�18±4�51 vs1�79±2�27 wt %), CaO (7�22±8�14 vs 1�74±5�47 wt %),and Nb (35±41 vs 16±19 ppm) concentrations, andhigher FeO/MgO ratios (2�9±3�1 vs 1�2±1�8). Steinet al. (1997) related these dolerites to a younger mag-

matic event, in which magmas were derived from adifferent source than the Amram sequence.

A-type affinity of the Amram rocks

The felsic rocks of the Amram Massif (alkali quartzsyenite, quartz syenite and rhyolite, SiO2 460 wt %)display characteristics of A-type magmas as definedby Eby (1990). They occur in a post-orogenic setting,exhibit anhydrous, hypersolvus mineralogy, an alka-line chemical affinity (Fig. 4) and low CaO concentra-tions (0�18±3�16 wt %, Table 1). High concentrationsof Zr, Nb, Ce and Y distinguish the Amram felsic rocksfrom I- and S-type granites, and classify them asA-type magmas, according to the 10 000 � Ga/Al vsZr � Nb � Ce � Y discrimination diagram of Whalenet al. (1987).

Sr and Nd isotopic composition

Nine samples were analyzed for their Sr and Nd iso-topic compositions (Table 2). The cycle II rock sam-ples yield an Rb±Sr isochron age of 526 � 22Ma andan initial 87Sr/86Sr ratio of 0�7036 � 2 (Mushkin et al.,

Table 1: continued

Sample: AM-271 AM-281 MR-39 MR-45 MR-58 AM-121 AM-211 MR-48

Rock type: Alkali qtz syenite Alkali qtz syenite Rhyolite Rhyolite Rhyolite Rhyolite Rhyolite Comp. dike

Eu 3.7 3.1 0.8 0.7 1.5 0.8 1.1 0.6

Gd 7.9 7.0 18.5 11.8 11.6 17.6 12.0 9.9

Tb 1.1 1.1 2.9 1.8 2.0 2.8 ÐÐ 1.5

Dy 6.1 6.2 16.4 9.7 12.2 17.9 14.4 8.4

Ho 1.4 1.3 3.3 1.9 2.5 3.4 ÐÐ 1.7

Er 3.7 3.8 9.8 5.7 7.5 10.0 ÐÐ 5.0

Tm 0.6 0.5 1.4 0.8 1.1 1.4 ÐÐ 0.7

Yb 4.0 3.8 8.9 5.3 6.7 90.8 4.9 4.8

Lu 0.6 0.6 1.3 0.8 1.0 1.4 ÐÐ 0.7

Ta ÐÐ ÐÐ 4.6 2.5 3.0 ÐÐ ÐÐ 2.6

Ga 26.8 25.6 29.0 26.0 ÐÐ 29.9 25.7 ÐÐ

Mg-no. 0.33 0.23 0.13 0.16 0.17 0.18 0.19 0.29

Y/Nb 1.52 1.38 1.89 1.57 1.64 1.73 1.55 1.32

La/Nb 1.67 1.48 1.61 1.25 1.17 1.52 1.56 1.40

(La/Yb)n 6.40 6.72 6.53 5.83 4.81 6.20 8.58 6.73

(Gd/Yb)n 1.58 1.45 1.66 1.77 1.38 1.43 1.96 1.64

Eu* 1.30 1.14 0.13 0.18 0.41 0.13 ÐÐ 0.18

1Kessel (1995).Fe2O3(t), total Fe as Fe2O3. Subscript `n' indicates normalization to chondritic values (Nakamura, 1974). Fe3�/FeT � 0.15(Sack et al., 1980). Mg-no. �Mg/(Mg � Fe2�) molar ratio. Eu* � fEun/[(Smn � Gdn)

1=2]g. Values in italics were measuredby XRF; otherwise, major elements by ICP-AES and trace elements by ICP-MS. b.d.l., below detection limits. LOI includesgain on ignition as a result of oxidation of Fe2�.

Fig. 4. Total alkali vs SiO2 [fields drawn according to Le Bas et al.(1986); alkaline and sub-alkaline fields from Irvine & Baragar(1971)]. Tb, trachybasalt; Bta, basaltic trachyandesite; Ta,trachyandesite; Td, trachydacite.


822

1999). This age fits, within error, the Rb/Sr isochronage of 548 � 5Ma obtained by Bielski (1982) for thenearby, closely related Neshef and Elat volcanics. Acombined isochron for the Amram cycle II rocks andthe Neshef and Elat volcanics yields an isochron of550 � 7Ma with an initial 87Sr/86Sr ratio of 0�7034 � 2(Mushkin et al., 2000). Although the cycle I Amramrhyolites lie slightly above these isochrons, initialeNd values for both cycle I and cycle II rocks werecalculated at 550Ma. The initial eNd values are allmoderately positive and range from �1�5 to �3�0.Considering analytical uncertainties it appears thatall samples can be placed at eNdi � �2�5 � 0�5. Thisvalue is significantly lower than the eNdi � �4 to �5values that characterize the early metavolcanics andcalc-alkaline granites from the northern ANS, whichare regarded as representative of the isotopic composi-

tion of the juvenile ANS crust (see Stein & Goldstein,1996). The Amram dolerite dikes, however, yieldedeNd(530Ma)��3�6� 0�5, which is similar to the valuesof the Phanerozoic alkali basalts from the Arabianplate, or the `juvenile' calc-alkaline magmas from theANS (Stein & Hofmann, 1992; Stein & Goldstein,1996).

DISCUSSION

The lithospheric-mantle origin of theAmram mafic magmas

The low silica contents (45�6±49�5 wt %), andrelatively high concentrations of Fe2O3 and MgO(10�0±12�5 wt % and 4�10±8�95 wt %, respectively) inthe Amram mafic dikes suggest that they were derivedfrom an ultramafic source. Nevertheless, moderateMg/(Mg � Fe) values (0�44±0�62) and low Ni concen-trations (37±74 ppm) indicate that the Amram maficdikes do not represent primary magmas, and that theymay have experienced some fractionation, most prob-ably of olivine before their emplacement. Crustal rockscan be ruled out as possible sources for the Amrammafic members because production of such mafic mag-mas by melting of any of the older, exposed crustalrocks, or estimated lower-crustal mafic lithologies inthis region (e.g. Stern & Gottfried, 1986; Stein, 1987;McGuire & Stern, 1993; Moghazi et al., 1998) requiresunreasonably high degrees of partial melting. Hence,the mantle seems to be the most likely source for theAmram mafic magmas.Based on the low and relatively uniform 87Sr/86Sr

and eNdi values of magmas from the various strati-graphic stages in the evolution of the ANS, and ontheir distinction from the mid-ocean ridge basalt(MORB)-type Gebel Gerf ophiolite, Stein & Goldstein(1996) proposed a lithospheric mantle source for thesemagmas. The Amram A-type magmas, however, arecharacterized by somewhat lower eNdi values than theolder juvenile ANS magmas (eNdi� 2�5� 0�5 vs 4�0�1�0, respectively). We propose that this could reflectderivation of the Amram magmas from a source char-acterized by a Sm/Nd ratio that was lower than chon-dritic. Stein et al. (1997) suggested a chromatographicmodel for the transport of trace elements in the mantlewedge (above the ANS subduction zone). According tothis model the upper zones in the `chromatographiccolumn' will be enriched in the incompatible andmobile elements such as Rb, Pb and the LREE. Theslight enrichment of the Amram mafic magmas in Rb/Sr and Nd/Sm (reflected by the isotope ratios) is con-sistent with their derivation from such an enriched partof the lithosphere. This scenario is also supported by theflat HREE profiles of the Amram mafic rocks (Fig. 7)

Fig. 5. Major element oxides (wt %) vs silica content inthe Amram rocks. Circled samples may have experienced post-emplacement metasomatic replacement of Na by K (see textfor details).


823

indicating the absence of garnet in the residual assem-blage, which would imply derivation from the spinelstability zone, i.e. the upper part of the lithosphericmantle.

Formation of the Amram felsic (A-type)magmas

The occurrence of co-magmatic mafic and felsic rocksof A-type affinity is not unique to the Amram Massif.Similar sequences in the ANS have been previouslyassociated with fractionation of mafic magmas (e.g.Stern & Gottfried, 1986; Chazot & Bertrand, 1995)or with significant fusion of older crustal rocks by themafic magmas (Jarrar et al., 1992). Here, we examinethe genetic relation between the mantle-derived maficmembers of the Amram suite and its co-magmaticA-type felsic members.The evolution of magmatic systems by fractionation

combined with wall-rock assimilation (AFC) waspostulated as far back as Bowen (1928), and furtherdeveloped in later studies (e.g. DePaolo, 1981; Devey& Cox, 1987; Marsh, 1989). As assimilation of wallrock by hot magma probably occurs in most cases, itis important to determine whether such assimilation

Fig. 6. Selected trace element concentrations (ppm) plotted against silica content in the Amram rocks.

Fig. 7. Chondrite-normalized, rare earth element (REE) patternsfor the Amram rocks. Normalized to chondritic values of Nakamura(1974).


824

was significant enough to affect the chemical evolutionof the studied suite. In Fig. 8, the Y/Nb ratios of theAmram rocks are plotted against their SiO2 content, aswell as the Y/Nb ratios for older, upper-crustal rocks inthis region, which would be the potential assimilantsduring the evolution of the Amram suite. As the Y/Nbratio of all the potential assimilants is distinct from thatof the Amram mafics, significant incorporation of theserocks during the formation of the Amram felsic mem-bers would be manifested by modification of the Y/Nbratio in the felsic rocks (in relation to the mafic rocks)and a deviation from the horizontal trend displayed bythe Amram suite in the direction of the assimilant. Thefact that no such deviation is observed for the Amramsuite (Fig. 8) suggests that fusion, or assimilation ofcrustal rocks was not significant during the formationof the Amram felsic magmas. A fractionation-dominated genetic relation between the Amrammafic and felsic magmas is also supported by thesimilar initial Sr and Nd isotopic ratios in all theAmram rocks (Table 2).Following the fractionation-dominated model

for the formation of the Amram felsic magmas, it isimportant to determine whether the compositionallyintermediate members of the suite represent mixingproducts between its mafic and felsic members, orwhether they represent midway products of the frac-tionation process itself. According to the mixingscheme, recharge of a fractionating magma chambercontaining residual felsic magmas by new mafic meltsderived from the same source and subsequent mixingcould produce the intermediate members of the

Table2:

Isotopiccomposition

ofSr

andNdinselected

samplesfrom

AmramMassif

Sam

ple

no.

Rock

type

Mag

matic

cycle

Age(M

a)Rb(ppm)

Sr(ppm)

87Sr/

86Sr

87Rb/8

6Sr

Sm

(ppm)

Nd(ppm)

147Sm/1

44Nd

143Nd/1

44Nd

eNd

eNdinitial

MR-26

Qzsyen

ite

II526�

22�

66188

0.71169

1.04

7.0

32. 6

0.1299

0.512536

ÿ2. 0

�2. 5

MR-30

Mafic

dike

II526�

22�

137

225

0.71630

1.91

ÐÐÐÐ

ÐÐÐÐ

ÐÐÐÐ

MR-79

Mafic

dike

II526�

22�

53310

0.70734

0.51

6.0

25. 5

0.1423

0.512526

ÿ2. 2

�1. 5

MR-42

Mafic

dike

II526�

22�

81754

0.70598

0.32

6.0

27. 5

0.1320

0.512568

ÿ1. 4

�3. 0

MR-211

Comp.dike

II526�

22�

322

510.84919

19. 0

ÐÐÐÐ

ÐÐÐÐ

ÐÐÐÐ

MR-39

Rhyo

lite

I�5

50�

177

180.98303

29. 0

18. 0

83. 4

0.1306

0.512512

ÿ2. 5

�2. 0

MR-45

Rhyo

lite

I�5

50�

ÐÐÐÐ

ÐÐÐÐ

11. 7

53. 8

0.1315

0.512524

ÿ2. 2

�2. 2

MR-6

Dolerite

ÐÐ532�

8y29

520

0.70438

0.17

11. 0

52. 0

0.1280

0.512585

ÿ1. 0

�3. 6

MR-203

Dolerite

ÐÐ532�

8y33

525

0.70481

0.19

ÐÐÐÐ

ÐÐÐÐ

ÐÐÐÐ

� Mushkinet

al.(1999).

yBeyth

&Heiman

n(1999).

Fig. 8. Y/Nb vs SiO2 content for the Amram rocks and older Neo-proterozoic basement rocks in this region. These upper-crustal rocksare typical calc-alkaline magmas, which may be the potential assi-milants during the fractionation of the Amram mafic magmas. Thehorizontal trend displayed by the Amram suite suggests that crustalassimilation was not significant during its evolution. EA, EilatAmphibolite. EGG, Eilat Granitic Gneiss; EG, Eilat Granite; ES,Eilat Schist; RQD, Roded Quartz Diorite. Symbols as in Fig. 4.


825

Amram suite and still satisfy the trace element andisotopic constraints described above. This possibilitycan be further assessed by examining the behavior ofmajor and trace elements in the suite. In a diagram ofincompatible trace element (e.g. Th) vs a compatibleelement (e.g. V) (Fig. 9), mixtures between twoend-members should plot along the straight line con-necting them. Fractional crystallization products,however, should plot along a hyperbolic curveapproaching the x-axis at low compatible element con-centrations. As shown in Fig. 9, the Amram rocks plotalong the fractional crystallization path and thus sug-gest that the intermediate members of the suite repre-sent midway products of the fractionation process.Furthermore, the initial increase of K2O and Ba con-centration with increasing SiO2 content, followed by adecrease in their concentration in the high-silica mem-bers of the suite (Figs 5 and 6) cannot be produced bymixing, but rather is consistent with the fast removal ofK2O and Ba upon onset of K-feldspar fractionation atthe late stages of the fractionation process (see Beythet al., 1994b).To summarize, we suggest a two-stage model for the

formation of the A-type magmas in the AmramMassif.During the first stage, alkaline mafic magmas derivedfrom the upper part of the lithospheric mantle wereemplaced within crustal-depth magma chambers.During their ascent, these mafic magmas experiencedsome fractionation, and possibly minor crustal contam-ination. During the second stage, the mafic magmasfractionated to produce the intermediate and morefelsic members of the suite. Crustal contamination dur-ing this stage was not significant. The Amram rocksrepresent magma batches that were separated from thefractionating magma chamber and emplaced at shal-

lower depths where they crystallized. This mechanism,of fractionation in a deep crustal magma chamberaccompanied by injection of some magma batchesinto shallower levels, is similar to previously suggestedmodels for the petrogenesis of hypersthene-normativeocean-island and continental alkalic suites (Nekvasilet al., 2000).

A quantitative model for the evolution ofthe Amram magmatic suiteÐand theproduction of its A-type magmas

The MELTS program (Ghiorso & Sack, 1995) wasused as the basis for a model that quantitativelydescribes the second stage in the petrogenesis of theAmram suite, i.e. the formation of the Amram felsicmagmas by fractionation of parental mafic melts. Thepetrography and chemical composition of the Amramrock units were used to constrain a best-fit model.The MELTS program calculates the chemical evo-

lution of magmatic systems on the basis of thermo-dynamic considerations. The program is executed ina stepwise manner, with increments of falling tempera-tures. At each step the Gibbs' free energy of the systemis minimized, and the thermodynamic and chemicalevolution of its various components (e.g. residualmagma and fractionating assemblage) is calculatedaccordingly. The thermodynamic database for theMELTS version used in this study is incomplete forhydrous phases. Nevertheless, the low abundance ofsuch phases in the Amram rock suite makes it possibleto use the MELTS program for quantitative modelingof the evolution of this magmatic suite and the forma-tion of its A-type magmas.

Basic assumptionsWe assume that the Amram rock units representmagma batches sampled from a large fractionatingmagma chamber during the course of its evolution.Rocks belonging to the second magmatic cyclesampled a complete evolutionary cycle of the magmachamber, i.e. mafic magmas (mafic dikes 45�6±49�5wt % SiO2) evolving through fractional crystallizationinto more felsic (A-type) magmas (i.e. monzonite50�6±53�0 wt % SiO2, quartz syenite 60�1±67�5 wt %SiO2, and the central part of composite dikes 75�2 wt% SiO2). Rocks belonging to the first magmatic cycle(i.e. alkali quartz syenite 60�0±67�2 wt % SiO2, andrhyolite 72�9±78�8 wt % SiO2) evolve along a petro-genetic path that is similar to that of the more evolvedpart of the second magmatic cycle.The primary melt in this model is an anhydrous

alkaline mafic magma, which is similar in compositionto the average composition of the Amram mafic dikes.

Fig. 9. Th (ppm) vs V (ppm) for the Amram suite. Mixing betweentwo magmatic end-members should plot along a straight line.Fractional crystallization (FC) should plot along a hyperboliccurve, and asymptotically approach the Th-axis at advanced stagesof the fractionation. Symbols as in Fig. 4.


826

After normalization to 100%, the estimated initial com-position we use is: SiO2 49�8%, TiO2 2�25%, Al2O3

16�6%, Fe2O3 11�8%, MgO 7�35%, Na2O 3�0%, CaO5�95%, K2O 2�5%, P2O5 0�52%, and H2O 0�2%.

Model resultsCalculated liquidus temperature for the primarymagma was 1265�C. This magma was allowed to frac-tionate while cooling from 1265 to 805�C, at 3 kbarpressure (�10 km), and at an oxygen fugacity that isone log unit below the quartz±fayalite±magnetite(QFM) buffer. The predicted differentiation trend formost major element concentrations in the magmachamber (i.e. residual magma in the model) is in agree-ment with the observed data for the Amram suite(Fig. 10). MgO concentrations fall sharply as a resultof olivine fractionation (Fig. 11), whereas Al2O3,Fe2O3, CaO, and TiO2 concentrations first rise andthen decrease with the appearance of clinopyroxeneand titanomagnetite. K2O and Na2O concentrations

increase as olivine, plagioclase, clinopyroxene, mag-netite and later apatite crystallize. When the SiO2

content of the residual magma reaches �60%, thepotassium content of the plagioclase feldspar quicklygrows towards anorthoclase (0�38 of the sanidineend-member) and K2O and Na2O concentrationsdecrease. At 955�C and 69% silica, sanidine appearsas an additional phase but its initial composition isclose to that of the plagioclase. With subsequent cool-ing and fractionation, it grows richer in potassium, butthe anorthoclase becomes more albitic, so that thecombined feldspar fractionation remains at averagesanidine end-member content of �0�38 and the mod-erate decrease in K2O and Na2O concentrations con-tinues. Similar trends were described by Nekvasil et al.(2000) for feldspars growing during the fractionation ofcontinental and oceanic alkalic magmas. The changein the chemistry of the fractionating plagioclase at�60% silica coincides with a second pulse of fractiona-tion (Fig. 11), the first pulse being the onset of plagio-clase and pyroxene fractionation from the maficmagmas. In contrast to most major element trends,the model does not reproduce the observed decreasein phosphorus concentrations. However, assumingthat P2O5 concentrations are governed solely bycrystallization of apatite, the behavior of the latterwas adjusted manually to fit the observed P2O5 con-centration. This adjustment also improved the modelfit of the CaO data and is used in the trace elementmodeling discussed below.Chemical compositions consistent with those of the

Amram monzonite (�50±53% SiO2) were producedafter 5±40% (by volume) of the parental magma was

Fig. 10. Comparison between patterns of major element concentra-tion (wt %) in the Amram rocks (°) and model differentiationtrends (black line) predicted by theMELTS program for the residualmagma. Refinement of apatite crystallization to fit the P2O5 dataresults in an improved fit for the CaO data (gray lines; see text forfurther discussion).

Fig. 11. The proportion of crystallizing phases (wt%) relative to themass of magma per 1 wt% change in the silica content of the residualmagma as calculated by the MELTS program. A bimodal fractio-nating assemblage is apparent. Olivine (ol), clinopyroxene (cpx) andplagioclase (pl) dominate the crystallizing assemblage at �50 wt %silica. As spinel (sp) joins and later apatite (ap), less fractionation isrequired to increase the silica content of the melt. The second peak offractionation is related to the growing abundance of albite (ab) andsanidine (san) in the crystallizing plagioclase. K-feldspar (Kspar)joins the fractionating assemblage at 69 wt % silica (shaded area).Dashed lines represent the combined proportion of the three end-members (albite, anorthite (an) and sanidine) in both feldspars.


827

removed by fractionation. Accordingly, alkali quartzsyenite and quartz syenite compositions (�60±67%SiO2) were produced after �66±87% of the originalvolume was fractionated, and rhyolite compositions(�72±78% SiO2) were produced after 490%fractionation of the original volume of the parentalmafic magma. The calculated fractionating assem-blage contains roughly 44% plagioclase, 18% alkalifeldspar, 13% clinopyroxene, 12% olivine, 9% spinel,and minor apatite and Fe±Ti oxides.

The behavior of trace elements duringmagma fractionation

The MELTS program yields the identification of themineral phases and their place in the crystallizationsequence of the initial mafic magma. These resultsdepend only on thermodynamic considerations withno attempted adjustments to the analyzed chemicaldata (as done in least-square techniques). In this sec-tion we examine the results of the MELTS calculationin light of the trace element data obtained from theAmram rocks (Table 1).A striking feature of the Amram suite is that both

mafic and felsic rocks display similar REE patterns([La/Yb]n � 8±10, excluding the negative Eu anomalyin the high-silica members; Fig. 7) and some uniformtrace element concentration ratios, e.g. La/Nb and Y/Nb (e.g. Fig. 8). At first glance, this is a surprisingobservation, as uniform patterns and ratios are notexpected for elements with different partition coeffi-cient in magmatic processes, e.g. La and Yb, or Yand Nb. It appears that the crystallizing mineralassemblage is buffering the trace element concentra-tion in such a way that the concentration ratios remainuniform. This pattern was reproduced in Fig. 12a forLa and Yb (for which partition coefficients are avail-able; see the Appendix). During the early stages, whenolivine and pyroxene crystallize, Yb is more compati-ble, but with the introduction of plagioclase and tita-nomagnetite, the effect of pyroxene is balanced,leading to similar, moderate compatibility of both ele-ments throughout the rest of the fractionation process.Europium shows no anomaly and even some small

positive anomalies at low to moderate silica content(SiO2 570%), and a prominent negative anomalydevelops only in the high-silica members of the suite(Fig. 7). This feature is somewhat unusual in view ofthe important role of plagioclase removal throughoutmost of the fractionation process, and calls for furtherexplanation. We suggest that titanomagnetite in theearly stages and apatite in the intermediate stages ofthe fractionation process balance the effect of plagioclase.Eu is taken preferentially by the plagioclase, but is moreincompatible than Sm and Gd in titanomagnetite and

apatite (e.g. Jarrar, 2001). Hence, the negative Euanomaly develops only when K-feldspar crystallizationbecomes significant, at �70% silica content in the resi-dual magma. The evolution of the Eu anomaly (Fig.12b) is successfully simulated, along with the variationin Sm, Eu and Gd concentrations (Fig. 12c) by using therelevant partition coefficients (see Appendix) and thefractionating assemblage calculated by theMELTS frac-tionation model (Fig. 11).To summarize, major and trace element evolution

of the Amram magmatic sequence is simulated

Fig. 12. (a) Comparison between predicted variation of the La/Ybratio during fractionation based on the MELTS results (continuouslines), and the measured ratio in the Amram rocks (°). (b) The sameas (a) for Eu anomaly, Eu/(Sm�Gd)1=2. (c) Measured concentra-tions (symbols) and model results for Sm (fine black line), Eu (boldblack line) and Gd (gray line). The same simulation reproduces boththe element concentrations and the Eu anomaly plotted in (b).Partition coefficients used are listed in the Appendix. Solid diamonds,measured Eu concentrations; open squares, measured Sm concentra-tions; gray triangles, measured Gd concentrations.


828

successfully by the fractional crystallization model pre-sented above. This model suggests crystallization of acommon water-undersaturated alkaline mafic magma,at�3 kbar (�10 km), and fO2 defined by the QFM ± 1buffer. This evolution is driven by fractionation ofplagioclase, pyroxene and magnetite, joined later byapatite and K-feldspar. The different lithologies of theAmram Massif represent samples separated from thisdeep magma chamber, whereas the residual fractio-nated phases accumulated at depth. The rhyoliteswere produced after extensive fractionation (�90%)of the parental mafic magmas. Hence, the silica-richA-type magmas in the Amram Massif would requirethe existence of an unexposed residual mafic body�10 times their own volume.

Post-orogenic A-type magmatism inthe northeastern ANS

Orogenic-related calc-alkaline magmatic activitydominated the northern ANS until�600Ma, while theonset of platform conditions in this region is marked byan Early Cambrian (Parnes, 1971; Beyth & Heimann,1999) regional unconformity. This transition, fromorogenic activity to stable platform conditions, wascharacterized by the occurrence of intra-plate A-typemagmatism and an extensional tectonic regime sug-gested by dike swarm emplacement, uplift, erosionand graben formation (e.g. Garfunkel, 1999; Mushkinet al., 1999). In the northeastern ANS A-type magmasrelated to this transitional stage crop out in small, well-scattered bodies (Fig. 1), and span a period of�70Myrfrom �600 to 530Ma (Table 3). In cases where thepetrography and magmatic stratigraphy of these out-crops were studied, sequences similar to that of theAmram Massif were revealed [as summarized byMushkin et al. (1999)]. Furthermore, the chemicalcompositions of most of the northeastern ANS A-typeoutcrops plot along the same geochemical evolutionarypath as that of the Amram suite, and their isotopiccompositions (87Sr/86Sr � 0�7028±0�7045, e � �1�5 to�5�3, Table 3), like those of the Amram rocks, suggestderivation from mantle sources with minor or noincorporation of older crustal material (e.g. Stein &Goldstein, 1996). Hence, we suggest that the petroge-netic model postulated for the Amram sequence maybe applicable to the other mantle-related A-type out-crops from the northeastern part of the ANS.A-type rocks in the northeastern ANS cover a total

area of �900 km2, which constitutes 510% of theexposed basement rocks in this region (Fig. 1). Assum-ing an average thickness of 0�5 km for these rocks (e.g.Eyal & Hezkiyahu, 1980), their total volume is esti-mated at �450 km3. Most of these rocks are felsic with70±75 wt % SiO2. Thus, according to the Amram

petrogenetic model, they would be derived by �90%fractionation from mantle-derived parental maficmagmas. Even if some crustal assimilation did occur,so that the felsic (A-type) magmas of the northeasternANS were produced, on average, by only �80% frac-tionation, they would require a volume of �2300 km3

of parental mafic magma. Assuming 5% partial melt-ing of the mantle source to produce these magmas, weinfer that the A-type magmas of the northeastern ANSwere derived from a voluminous mantle source of�46 000 km3. After normalizing this volume to thetotal area of basement outcrops in this region (i.e.�10 000 km2) we find that a sub-continental mantlecolumn of average thickness of �5 km below thenortheastern ANS was partially melted to producethe post-orogenic A-type magmas of this region. Forcomparison, we carried out a similar calculation for therift-related, mantle-derived, Late Cenozoic alkalibasalts of the Arabian plate (e.g. Altherr et al., 1990;Camp & Roobol, 1992). Covering �80 000 km2, withan average thickness of 0�2 km (i.e. a volume of16 000 km3), and assuming only 3% partial meltingat the source with only minor crustal contaminationbefore emplacement, these basalts can be related to amantle source of 500 000 km3. Normalizing this volumeto an area of 160 000 km2 we infer that this magmaticepisode represents partial fusion of �5 km of the sub-continental mantle beneath this part of the Arabianplate. Hence, these two magmatic episodes in the ANS,the post-orogenic, Neoproterozoic±Early Cambrian,A-type magmatism and the rift-related, Cenozoicvolcanism, represent geological processes of a similarmagnitude.What caused the extensive A-type magmatism in the

northern ANS? Similar suites of A-type magmasappear in the closing or post-orogenic stages ofPrecambrian or Phanerozoic shields in other parts ofthe world (e.g. the Himalayas, South Australia).Turner et al. (1992) proposed that A-type magmasfrom Australia were derived from the lithosphericmantle and that they represent a significant additionof juvenile material to the crust (Turner, 1996). Theyfurther argued that the magmatism is induced bydelamination and thinning of the lithospheric mantleas a response to the thickening during the previouscollisional±orogenic stage.In the ANS, the A-type magmatism commenced

after a long period of crust formation through calc-alkaline magmatism. The production of the alkaline(A-type) magmas was associated with penetrationof numerous dikes, normal faulting and grabenformation. This tectonic environment as well as thechemical and isotopic evidence for the lithosphericorigin of the ANS magmas concurs with the model ofTurner et al. (1992). As in South Australia, the A-type


829

magmatism in the northern ANS also representssignificant addition of juvenile material from thelithospheric mantle to the crust.

SUMMARY AND CONCLUSIONS

Field relations, petrography, chemistry and isotopiccomposition constrain a two-stage model for the for-mation of the Neoproterozoic±Early CambrianAmram sequence. During the first stage, mafic mag-mas most probably derived from the upper part of thelithospheric mantle were emplaced in a deep crustalmagma chamber. During their ascent, these magmasmay have undergone limited fractionation andpossibly minor contamination by crustal material.The second stage was characterized by fractionationof the mafic magmas in the magma chamber to pro-duce the more felsic members of the suite. Crustalassimilation was not significant during this stage. Thevarious rock units of Amram Massif represent batchesof magma separated from the magma chamber andemplaced at shallower crustal levels.A quantitative model (based on the MELTS pro-

gram) relates the formation of the Amram A-typemagmas to deep crustal fractionation of mafic magmasrepresented by the Amram mafic dikes. Removal ofolivine, plagioclase, clinopyroxene, and magnetitefrom the parental magma produced the intermediatecompositions of the suite (560 wt % SiO2). Continuedremoval of clinopyroxene, plagioclase, apatite and,

finally, alkali feldspar produced the felsic members ofthe suite (460 wt % SiO2). High-silica A-typerocks (SiO2 470 wt %) were produced after �90%fractionation of the parental mafic magmas, and thusimply a large unexposed volume of mafic cumulates ata �10 km depth.Similar magmatic sequences, whole-rock chemistry

and isotopic compositions relate most of the post-orogenic A-type magmatism of the northeastern ANSto a common petrogenetic process. These A-type rockssuggest the presence of much larger unexposed maficbodies, all derived from the sub-continental mantlebeneath the ANS. Despite its relatively small arealexpression, post-orogenic A-type magmatism in thenortheastern ANS marks a significant lithosphericmelting event during the maturation stages of thenewly formed continental lithosphere. As suggestedby Turner et al. (1992) for post-orogenic 490MaA-type granites from South Australia, the A-type mag-mas of the northeastern ANS represent a considerablevolumetric addition of mantle material to thecontinental crust, during the final phase of a largeorogenic event.

ACKNOWLEDGEMENTS

The manuscript was significantly improved by thecritical comments of M. Beyth, S. Turner, J. B. Whalenand an anonymous reviewer. We are grateful toM. Wilson for her thorough review and to M. Eyal for

Table 3: Available Sr and Nd isotopic data for Neoproterozoic alkaline rocks in

southern Israel, southwestern Jordan, Sinai Peninsula, and the Gabal Gerf ophiolite in

southeastern Egypt

Rock type Location* Age (Ma) eNdi 87Sr/86Sri Reference

Dolerite dike Timna 5321 �5.1 0.7028 a

Cycle II rocks Amram �530 �1.5 to �3.0 0.70358 this study

Granite Mandar 5302 4.4 0.7045 b, d

Rhyolite Neshef 5482 n.a. 0.7043 b

Granite Wadi Araba 45503 �2.0 to �4.0 0.7035±0.7045 g

Syenogranite Wadi Kid 5704 �4.0 to �5.3 0.7033 e

Qtz monzodiorite Timna 5995 4.6 0.7033 f

Alkali rhyolite Wadi Araba 5536 n.a. 0.7123 c

Granite Katherina 5602 4.1 0.7062 b, d

Granite Wadi Kid 5807 n.a. 0.7028 h

N-MORB Gabal Gerf 7508 �6.5 to �8.8 0.7024±0.7029 i

�See Fig. 1 for location in the ANS.1Beyth & Heimann (1999).2Bielski (1982). 3Jarrar (2000). 4Moghazi et al. (1998). 5Beyth & Reischmann (1996).6Kroener et al. (1990). 7Bielski et al. (1979). 8Zimmer et al. (1995).(a) Beyth et al. (1994a); (b) Bielski (1982); (c) Jarrar et al. (1992); (d) Stein & Goldstein (1996); (e) Moghazi et al. (1998);(f) Beyth et al. (1994b);(g) Jarrar (2000); (h) Bielski et al. (1979); (i) Zimmer et al. (1995).


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discussions. The studywas supported by the ISF (GrantNo. 503/99 toM. Stein and Z. Garfunkel), by the IsraelMinistry of Energy and Infrastructure (Grant No. 97-17-034 to M. Stein and O. Navon), and by Nieders.Vorab der Volkswagen-Stiftung (Grant No. 25D-3-76251-99-26/95 to G. Woerner).

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APPENDIX: PARTITION

COEFFICIENTS USED FOR

REE MODELING

La Yb Sm Eu Gd

Olivine* 0.007 0.014 0.007 0.007 0.008

Clinopyroxene* 0.056 0.620 0.500 0.510 0.610

Orthopyroxene* 0.02 0.34 0.05 0.05 0.09

Spinel* 0.01 0.01 0.01 0.01 0.01

Ti-magnetite* 1.5 0.9 1.1 0.6 1.0

Plagioclasey 0.1214 0.0098 0.0478 0.6340 0.0410

K-feldsparz 0.08 0.03 0.02 3.40 0.03

Apatitex 10.15 10.78 32.20 17.85 30.73

*Jarrar (2001).yDunn & Sen (1994).zNash & Crecraft (1985).xFujimaki (1986).


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Documents

The Petrogenesis of A-type Magmas from the Amram Massif, Southern Israel