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Journal of Petrology | Volume 44 | Number 5 | Pages 815-832 | 2003
© Oxford University Press 2003
The Petrogenesis of A-type Magmas from the Amram Massif, Southern Israel
1 INSTITUTE OF EARTH SCIENCES, THE HEBREW UNIVERSITY OF JERUSALEM, GIVAT RAM, JERUSALEM 91904, ISRAEL
2 GEOLOGICAL SURVEY OF ISRAEL, 30 MALKHE YISRAEL STREET, JERUSALEM 95501, ISRAEL
3 GEOCHEMICAL INSTITUTE, UNIVERSITY OF GÚTTINGEN, GOLDSCHMIDTSTRASSE 1, GÚTTINGEN D-37077, GERMANY
Present address: Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310, USA. Telephone: 1-206-543-6221. Fax: 1-206-685-2379. E-mail: mushkin{at}u.washington.edu
RECEIVED JUNE 8, 2002; ACCEPTED NOVEMBER 6, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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The (
550–530 Ma) alkaline magmatic suite of the Amram Massif, southern Israel, was emplaced during the transition from an orogenic to an intra-plate tectonic setting in the northeastern Arabian–Nubian Shield (ANS). The suite ranges from 45·6 to 78·8 wt % SiO2, and consists of rhyolites, alkali quartz syenites, quartz syenites, monzonites, and co-magmatic mafic to felsic alkaline dikes. These rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content. The felsic members of the suite display A-type characteristics and are genetically linked through fractionation to the more mafic ones. Moderately positive initial 
Nd values (+2 ± 0·5), low initial 87Sr/86Sr values (0·7036 ± 2), high MgO and Fe2O3 concentrations (4·10–8·95 and 10·0–12·5 wt %, respectively) and relatively 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 lithospheric mantle, above the garnet stability zone. The MELTS program was used to quantitatively model the chemical evolution of the suite. Extensive anhydrous fractionation (>90%), of plagioclase, alkali-feldspar, clinopyroxene, olivine, and minor Ti-magnetite and apatite from parental mafic magmas, represented by the Amram mafic dikes, produced the rhyolitic compositions as well as the intermediate members of the suite. This suggests the presence of a large unexposed body of cumulate rocks at depth, as well as fusion of a large source-region (equivalent to an 5 km layer) in the lithospheric mantle. Regarded as a representative example for similar A-type outcrops in this region, this petrogenetic model further suggests that Neoproterozoic–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 episode was 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
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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Anorogenic alkaline granites and related alkaline magmas (termed A-type granites and magmas) commonly occur in post-orogenic, intra-plate tectonic settings (e.g. Eby, 1990
, 1992; Turner et al., 1992; Black & Liegeois, 1993). These rocks provide significant information on post-collisional magmatic processes within the continental lithosphere and their contribution to the build-up of the upper continental crust (Turner et al., 1992). Several workers have argued for the formation of A-type granites by partial melting of pre-existing crustal rocks (e.g. Collins et al., 1982; Whalen et al., 1987; Creaser et al., 1991; Landenberger & Collins, 1996), whereas others have proposed that these granites are highly fractionated products from the differentiation of mantle-derived parental mafic magmas (e.g. Stern & Gottfried, 1986; Turner et al., 1992; Beyth et al., 1994b; Kessel et al., 1998; Volkert et al., 2000). Possibly, A-type magmas do not represent a particular geological setting, but rather, similar end-products derived through different processes (e.g. Eby, 1992; Whalen et al., 1996).
In this study we examine the petrogenesis and geological significance of post-orogenic (Pan-African), Late Proterozoic to Early Cambrian A-type magmas, from the northeastern part of the Arabian–Nubian Shield (ANS) (Fig. 1). We focus on an 550–530 Ma alkaline rock suite from the Amram Massif, southern Israel, and compare it with similar outcrops in this region. The good exposure of apparently co-magmatic A-type granitoid and alkaline mafic rocks, spanning a wide and continuous range of chemical compositions (45·6–78·8 wt % SiO2), make the Amram Massif a key site to study the formation of post-orogenic A-type magmas in this part of the ANS. We use major and trace element chemistry, as well as Sr and Nd isotope compositions to constrain the petrogenesis of the Amram magmas. We then employ the ‘MELTS’ computer program of Ghiorso & Sack (1995) to quantitatively model the differentiation process. Applying this model to contemporaneous A-type suites in this region suggests that Late Neoproterozoic to Early Cambrian A-type magmatism in the northeastern ANS contributed substantial amount of juvenile magmas to the upper crust and thus represents a significant process in the history of the shield.
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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The Amram Massif, located in the southern Negev desert of Israel, is one of the northernmost outcrops of the Arabian–Nubian Shield (ANS) (Fig. 1). The ANS extends over large areas in NE Africa and Arabia and is considered to be the product of one of the most intensive episodes of continental crust formation in Earth history (Reymer & Schubert, 1984
; Stein & Hofmann, 1994). The geological history of the northern ANS has been divided into four main phases [adapted from Bentor (1985)]. Phase I (900–870 Ma) was characterized by the formation of a several kilometer thick sequence of tholeiitic basalts, which probably erupted in an oceanic environment and formed an oceanic plateau (Reischmann et al., 1982; Stein & Goldstein, 1996; Stein, 1999). Phase II (870–650 Ma) was characterized by mainly calc-alkaline, island arc magmatism, followed by extensive metamorphism. Phase III (630–600 Ma) was governed by intrusion of calc-alkaline granitic batholiths (mainly in the northern segments of the ANS). Phase IV, which extended into the Cambrian (600–530 Ma), was characterized by intrusion and extrusion of alkaline magmas. Outcrops of phase IV alkaline magmas form a small part of the ANS volume, yet they are scattered over the entire shield (Bentor, 1985; Stern et al., 1988; Black & Liegeois, 1993).
In the northeastern ANS, i.e. southern Israel, southwestern Jordan and the Sinai Peninsula, phase IV alkaline magmas were generated during the transition from orogenic activity to intra-plate stable conditions. This period (600–530 Ma) was also characterized by extension-related tectonics, dike emplacement and significant erosion [see summary by Garfunkel (1999)]. The onset of stable platform conditions in this region is marked by the deposition of Early Cambrian sandstones (e.g. Parnes, 1971). Intra-plate sedimentation and recurring cycles of alkaline magmatism characterized this region through most of the Phanerozoic (e.g. Garfunkel, 1989). The present-day exhumation of basement rocks in this region is associated with the Cenozoic tectonic activity along the Dead Sea Transform and the Red Sea spreading center (e.g. Garfunkel, 1970, 1980).
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GEOLOGY OF THE AMRAM MASSIF |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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Bordered on all sides by Cenozoic faults, Amram Massif covers an area of over 6 km2, and comprises a late Neoproterozoic–Early Cambrian alkaline magmatic sequence, which is unconformably overlain by Early Cambrian sandstones (Wurtzburger, 1959
; Mushkin et al., 1999). The main rock units in the massif are (Fig. 2): alkali quartz syenite (holocrystalline, with 70–75% subhedral alkali feldspar, 15–20% anhedral quartz and <10% opaque minerals and chlorite in varying ratios); porphyritic rhyolite (up to 30% phenocrysts of alkali feldspar and quartz in a microcrystalline groundmass of similar mineralogy); monzonite (holocrystalline, with 40% plagioclase, 25% alkali feldspar, 10% clinopyroxene, 10% opaque minerals, 5% quartz, minor apatite and up to 10% secondary chlorite); quartz syenite (holocrystalline, with 65% alkali feldspar, 15% quartz, 20% clinopyroxene and accessory chlorite, amphibole, plagioclase and opaque minerals); and dikes of mafic to felsic compositions. Some of the dikes are composite, and consist of mafic margins with a felsic center. Kaolinitization and sericitization of feldspars and chloritization of mafic phases is common in all rocks and is extensive in the monzonite and the quartz syenite. The relatively small crystal size (up to 3 mm) of the plutonic rocks, their granophyric textures, and the perthitic nature of the alkali feldspars in some of the intrusions, all indicate relatively shallow depths of crystallization.
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Field relations, petrography, and geochronology of the Amram rocks reveal the following sequence of magmatic events, which occurred between
550 and 530 Ma [see Mushkin et al. (1999) for field mapping and detailed description of the units]: (1) deposition of volcaniclastic units and intrusion of hypabyssal alkali quartz syenite; (2) eruption of rhyolites on top of the exposed alkali quartz syenite; (3) intrusion of mafic dikes followed by the emplacement of monzonite and then quartz syenite; (4) intrusion of composite dikes (mafic margins and a rhyolitic center); (5) intrusion of dolerite dikes into the volcanoclastic unit. No field relations could be established between the doleritic dykes and the other rock units in Amram. However, in the nearby Timna Massif a similar doleritic dike cuts the mafic dikes (Baer & Beyth, 1990) and both Amram and Timna dolerite dikes are geochemically distinct from all other Amram rocks (Beyth et al., 1994a; Stein et al., 1997). This magmatic stratigraphy has been subdivided by Mushkin et al. (1999) into two cycles (I and II), whereas the doleritic dikes constitute a younger 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, discussed later in this paper, suggests that fractionation was the main factor governing this geochemical evolution, and that crustal assimilation was not significant. Normal faulting accompanied the emplacement of the Amram magmas during both magmatic cycles (Mushkin et al., 1999).
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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After petrographically characterizing the Amram rock units, 19 representative samples were selected for chemical analysis. Major elements were analyzed by inductively coupled plasma atomic emission spectrometry [ICP-AES; Perkin–Elmer Optima-3300, at the Geological Survey of Israel (GSI)] after fusion with lithium metaborate. Loss on ignition (LOI) was determined by measuring weight loss upon heating to 1100°C. Trace element concentrations were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES; Jobin Yvon JY-48 at the GSI), and by inductively coupled plasma mass spectrometry (ICP-MS; Perkin–Elmer–SCIEX Elan 6000 at the GSI), after sintering with sodium peroxide. Standard rock reference materials (BHVO-1, BE-N, MA-N, and NIM-G) were analyzed repeatedly and compared with the data of Govindaraju (1994)
to determine accuracy and precision. Precision for major elements was found to be better than 1%, excluding P2O5, for which a 10% error is assumed, as a result of low concentration. Precision for trace element concentrations is estimated as better than 5%. Most samples were also analyzed for major and trace element concentrations by X-ray fluorescence spectrometry (XRF; Phillips PW 1480 on fused glass pellets) at the University of Goettingen. Precision for these measurements is better than 1% for major elements, and better than 5% for trace elements including Rb, Sr, Sm and Nd (Hartmann, 1994). Discrepancy between the two techniques for major 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 thermal ionization mass spectrometry (TIMS, Finnigan MAT 262 RPQ equipped with a multi-collector system operating in static mode). Rock powders (100 mg) were dissolved in HF–HNO3 in pressurized Teflon reaction vessels. Sr and Nd were chemically separated by standard procedure at the University of GÎttingen. Mass fractionation was linearly corrected using 86Sr/88Sr = 0·1194, and 146Nd/144Nd = 0·7219. Repeated measurements of the NBS-987 standard yielded 87Sr/86Sr = 0·710263 ± 20 (2
, n = 10). The measured mean 143Nd/144Nd ratio for the La Jolla standard at the time of measurements was 0·511840 ± 7 (2, n = 10). The errors on the 87Rb/86Sr and 147Sm/144Nd ratios are 7%.
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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Major elements
Major element concentrations of the Amram rocks are listed 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 trends in major element variation diagrams (Fig. 5).
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Al2O3, Fe2O3, and TiO2 concentrations decrease monotonously with increasing SiO2 concentration. MgO decreases along a hyperbolic curve, reaching near-zero concentration in the more felsic members. CaO varies widely in the mafic members (1·74–5·47 wt %), but decreases monotonously in the more felsic members. Na2O and K2O vary considerably within otherwise petrologically and chemically well-defined rock units. Nevertheless, the total alkali concentrations are uniform within each rock unit (5·10–7·64 wt % in mafic dikes, 6·80–7·64 wt % in monzonite, 8·45–11·7 wt % in alkali quartz syenite and quartz syenite, and 7·94–8·95 wt % in rhyolite and composite dikes) (Fig. 4). Agron & Bentor (1981)
suggested post-emplacement, metasomatic replacement of Na by K in similar rocks of the Neshef Massif (the westward continuation of the Amram Massif, Fig. 1). They also observed that the metasomatic process affected only some of the rocks. Where K replaced Na, SiO2 content was increased by 2–3%, whereas other element concentrations were not significantly altered. Based on this, alkali concentrations in samples displaying K2O >8 wt %, and Na2O <1 wt %, have been discarded in the subsequent discussion. The rest of the K2O data show a monotonous increase until 65–70 wt % SiO2 followed by a decrease of K2O concentrations in the higher SiO2 range (Fig. 5).
Trace elements
Trace element concentrations of the Amram rocks are listed in Table 1. Selected elements are plotted against SiO2 content in Fig. 6. Ni, V, and Sr concentrations decrease whereas Zr, Y, Nb, Rb, U, Th, and La concentrations increase with increasing SiO2 content. Ba concentrations decrease only after 67 wt % SiO2, similar to K2O. Rare earth element (REE) profiles of the Amram suite resemble each other, with moderate enrichment 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 negative Eu* anomaly (0·13–0·41) is apparent only in the silica-rich members of the suite (i.e. rhyolites and felsic centers of the composite dikes). Rocks from the two magmatic cycles of the Amram Massif (Mushkin et al., 1999) display similar major and trace element concentrations (Figs 5 and 6), as well as matching REE patterns. These geochemical similarities suggest that rocks of the two cycles evolved through similar magmatic processes (elaborated below). The dolerite dikes, however, are chemically distinct from the rocks of the two earlier cycles. In relation to the Amram mafic dikes, the dolerites have higher TiO2 (4·18–4·51 vs 1·79–2·27 wt %), CaO (7·22–8·14 vs 1·74–5·47 wt %), and Nb (35–41 vs 16–19 ppm) concentrations, and higher FeO/MgO ratios (2·9–3·1 vs 1·2–1·8). Stein et al. (1997) related these dolerites to a younger magmatic event, in which magmas were derived from a different source than the Amram sequence.
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A-type affinity of the Amram rocks
The felsic rocks of the Amram Massif (alkali quartz syenite, quartz syenite and rhyolite, SiO2 >60 wt %) display characteristics of A-type magmas as defined by Eby (1990)
. They occur in a post-orogenic setting, exhibit anhydrous, hypersolvus mineralogy, an alkaline chemical affinity (Fig. 4) and low CaO concentrations (0·18–3·16 wt %, Table 1). High concentrations of Zr, Nb, Ce and Y distinguish the Amram felsic rocks from I- and S-type granites, and classify them as A-type magmas, according to the 10 000 x Ga/Al vs Zr + Nb + Ce + Y discrimination diagram of Whalen et al. (1987).
Sr and Nd isotopic composition
Nine samples were analyzed for their Sr and Nd isotopic compositions (Table 2). The cycle II rock samples yield an Rb–Sr isochron age of 526 ± 22 Ma and an initial 87Sr/86Sr ratio of 0·7036 ± 2 (Mushkin et al., 1999). This age fits, within error, the Rb/Sr isochron age of 548 ± 5 Ma obtained by Bielski (1982) for the nearby, closely related Neshef and Elat volcanics. A combined isochron for the Amram cycle II rocks and the Neshef and Elat volcanics yields an isochron of 550 ± 7 Ma with an initial 87Sr/86Sr ratio of 0·7034 ± 2 (Mushkin et al., 2000). Although the cycle I Amram rhyolites lie slightly above these isochrons, initial Nd values for both cycle I and cycle II rocks were calculated at 550 Ma. The initial Nd values are all moderately positive and range from +1·5 to +3·0. Considering analytical uncertainties it appears that all samples can be placed at Ndi = +2·5 ± 0·5. This value is significantly lower than the Ndi = +4 to +5 values that characterize the early metavolcanics and calc-alkaline granites from the northern ANS, which are regarded as representative of the isotopic composition of the juvenile ANS crust (see Stein & Goldstein, 1996). The Amram dolerite dikes, however, yielded Nd(530 Ma) = +3·6 ± 0·5, which is similar to the values of the Phanerozoic alkali basalts from the Arabian plate, or the ‘juvenile’ calc-alkaline magmas from the ANS (Stein & Hofmann, 1992; Stein & Goldstein, 1996).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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The lithospheric-mantle origin of the Amram mafic magmas
The low silica contents (45·6–49·5 wt %), and relatively high concentrations of Fe2O3 and MgO (10·0–12·5 wt % and 4·10–8·95 wt %, respectively) in the Amram mafic dikes suggest that they were derived from an ultramafic source. Nevertheless, moderate Mg/(Mg + Fe) values (0·44–0·62) and low Ni concentrations (37–74 ppm) indicate that the Amram mafic dikes do not represent primary magmas, and that they may have experienced some fractionation, most probably of olivine before their emplacement. Crustal rocks can be ruled out as possible sources for the Amram mafic members because production of such mafic magmas by melting of any of the older, exposed crustal rocks, or estimated lower-crustal mafic lithologies in this region (e.g. Stern & Gottfried, 1986
; Stein, 1987; McGuire & Stern, 1993; Moghazi et al., 1998) requires unreasonably high degrees of partial melting. Hence, the mantle seems to be the most likely source for the Amram mafic magmas.
Based on the low and relatively uniform 87Sr/86Sr and Ndi values of magmas from the various stratigraphic stages in the evolution of the ANS, and on their distinction from the mid-ocean ridge basalt (MORB)-type Gebel Gerf ophiolite, Stein & Goldstein (1996) proposed a lithospheric mantle source for these magmas. The Amram A-type magmas, however, are characterized by somewhat lower Ndi values than the older juvenile ANS magmas (Ndi = 2·5 ± 0·5 vs 4·0 ± 1·0, respectively). We propose that this could reflect derivation of the Amram magmas from a source characterized by a Sm/Nd ratio that was lower than chondritic. Stein et al. (1997) suggested a chromatographic model for the transport of trace elements in the mantle wedge (above the ANS subduction zone). According to this model the upper zones in the ‘chromatographic column’ will be enriched in the incompatible and mobile elements such as Rb, Pb and the LREE. The slight enrichment of the Amram mafic magmas in Rb/Sr and Nd/Sm (reflected by the isotope ratios) is consistent with their derivation from such an enriched part of the lithosphere. This scenario is also supported by the flat HREE profiles of the Amram mafic rocks (Fig. 7) indicating the absence of garnet in the residual assemblage, which would imply derivation from the spinel stability zone, i.e. the upper part of the lithospheric mantle.
Formation of the Amram felsic (A-type) magmas
The occurrence of co-magmatic mafic and felsic rocks of A-type affinity is not unique to the Amram Massif. Similar sequences in the ANS have been previously associated with fractionation of mafic magmas (e.g. Stern & Gottfried, 1986; Chazot & Bertrand, 1995) or with significant fusion of older crustal rocks by the mafic magmas (Jarrar et al., 1992). Here, we examine the genetic relation between the mantle-derived mafic members of the Amram suite and its co-magmatic A-type felsic members.
The evolution of magmatic systems by fractionation combined with wall-rock assimilation (AFC) was postulated as far back as Bowen (1928), and further developed in later studies (e.g. DePaolo, 1981; Devey & Cox, 1987; Marsh, 1989). As assimilation of wall rock by hot magma probably occurs in most cases, it is important to determine whether such assimilation was significant enough to affect the chemical evolution of the studied suite. In Fig. 8, the Y/Nb ratios of the Amram rocks are plotted against their SiO2 content, as well as the Y/Nb ratios for older, upper-crustal rocks in this region, which would be the potential assimilants during the evolution of the Amram suite. As the Y/Nb ratio of all the potential assimilants is distinct from that of the Amram mafics, significant incorporation of these rocks during the formation of the Amram felsic members would be manifested by modification of the Y/Nb ratio in the felsic rocks (in relation to the mafic rocks) and a deviation from the horizontal trend displayed by the Amram suite in the direction of the assimilant. The fact that no such deviation is observed for the Amram suite (Fig. 8) suggests that fusion, or assimilation of crustal rocks was not significant during the formation of the Amram felsic magmas. A fractionation-dominated genetic relation between the Amram mafic and felsic magmas is also supported by the similar initial Sr and Nd isotopic ratios in all the Amram rocks (Table 2).
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Following the fractionation-dominated model for the formation of the Amram felsic magmas, it is important to determine whether the compositionally intermediate members of the suite represent mixing products between its mafic and felsic members, or whether they represent midway products of the fractionation process itself. According to the mixing scheme, recharge of a fractionating magma chamber containing residual felsic magmas by new mafic melts derived from the same source and subsequent mixing could produce the intermediate members of the Amram suite and still satisfy the trace element and isotopic constraints described above. This possibility can be further assessed by examining the behavior of major and trace elements in the suite. In a diagram of incompatible trace element (e.g. Th) vs a compatible element (e.g. V) (Fig. 9), mixtures between two end-members should plot along the straight line connecting them. Fractional crystallization products, however, should plot along a hyperbolic curve approaching the x-axis at low compatible element concentrations. As shown in Fig. 9, the Amram rocks plot along the fractional crystallization path and thus suggest that the intermediate members of the suite represent midway products of the fractionation process. Furthermore, the initial increase of K2O and Ba concentration with increasing SiO2 content, followed by a decrease in their concentration in the high-silica members of the suite (Figs 5 and 6) cannot be produced by mixing, but rather is consistent with the fast removal of K2O and Ba upon onset of K-feldspar fractionation at the late stages of the fractionation process (see Beyth et al., 1994b
).
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To summarize, we suggest a two-stage model for the formation of the A-type magmas in the Amram Massif. During the first stage, alkaline mafic magmas derived from the upper part of the lithospheric mantle were emplaced within crustal-depth magma chambers. During their ascent, these mafic magmas experienced some fractionation, and possibly minor crustal contamination. During the second stage, the mafic magmas fractionated to produce the intermediate and more felsic members of the suite. Crustal contamination during this stage was not significant. The Amram rocks represent magma batches that were separated from the fractionating magma chamber and emplaced at shallower depths where they crystallized. This mechanism, of fractionation in a deep crustal magma chamber accompanied by injection of some magma batches into shallower levels, is similar to previously suggested models for the petrogenesis of hypersthene-normative ocean-island and continental alkalic suites (Nekvasil et al., 2000
).
A quantitative model for the evolution of the Amram magmatic suite—and the production of its A-type magmas
The MELTS program (Ghiorso & Sack, 1995) was used as the basis for a model that quantitatively describes the second stage in the petrogenesis of the Amram suite, i.e. the formation of the Amram felsic magmas by fractionation of parental mafic melts. The petrography and chemical composition of the Amram rock units were used to constrain a best-fit model.
The MELTS program calculates the chemical evolution of magmatic systems on the basis of thermodynamic considerations. The program is executed in a stepwise manner, with increments of falling temperatures. At each step the Gibbs' free energy of the system is minimized, and the thermodynamic and chemical evolution of its various components (e.g. residual magma and fractionating assemblage) is calculated accordingly. The thermodynamic database for the MELTS version used in this study is incomplete for hydrous phases. Nevertheless, the low abundance of such phases in the Amram rock suite makes it possible to use the MELTS program for quantitative modeling of the evolution of this magmatic suite and the formation of its A-type magmas.
Basic assumptions
We assume that the Amram rock units represent magma batches sampled from a large fractionating magma chamber during the course of its evolution. Rocks belonging to the second magmatic cycle sampled a complete evolutionary cycle of the magma chamber, i.e. mafic magmas (mafic dikes 45·6–49·5 wt % SiO2) evolving through fractional crystallization into more felsic (A-type) magmas (i.e. monzonite 50·6–53·0 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, and rhyolite 72·9–78·8 wt % SiO2) evolve along a petrogenetic path that is similar to that of the more evolved part of the second magmatic cycle.
The primary melt in this model is an anhydrous alkaline mafic magma, which is similar in composition to the average composition of the Amram mafic dikes. 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%, CaO 5·95%, K2O 2·5%, P2O5 0·52%, and H2O 0·2%.
Model results
Calculated liquidus temperature for the primary magma was 1265°C. This magma was allowed to fractionate while cooling from 1265 to 805°C, at 3 kbar pressure (10 km), and at an oxygen fugacity that is one log unit below the quartz–fayalite–magnetite (QFM) buffer. The predicted differentiation trend for most major element concentrations in the magma chamber (i.e. residual magma in the model) is in agreement with the observed data for the Amram suite (Fig. 10). MgO concentrations fall sharply as a result of olivine fractionation (Fig. 11), whereas Al2O3, Fe2O3, CaO, and TiO2 concentrations first rise and then decrease with the appearance of clinopyroxene and titanomagnetite. K2O and Na2O concentrations increase as olivine, plagioclase, clinopyroxene, magnetite and later apatite crystallize. When the SiO2 content of the residual magma reaches 60%, the potassium content of the plagioclase feldspar quickly grows towards anorthoclase (0·38 of the sanidine end-member) and K2O and Na2O concentrations decrease. At 955°C and 69% silica, sanidine appears as an additional phase but its initial composition is close to that of the plagioclase. With subsequent cooling and fractionation, it grows richer in potassium, but the anorthoclase becomes more albitic, so that the combined feldspar fractionation remains at average sanidine end-member content of 0·38 and the moderate decrease in K2O and Na2O concentrations continues. Similar trends were described by Nekvasil et al. (2000) for feldspars growing during the fractionation of continental and oceanic alkalic magmas. The change in the chemistry of the fractionating plagioclase at 60% silica coincides with a second pulse of fractionation (Fig. 11), the first pulse being the onset of plagioclase and pyroxene fractionation from the mafic magmas. In contrast to most major element trends, the model does not reproduce the observed decrease in phosphorus concentrations. However, assuming that P2O5 concentrations are governed solely by crystallization of apatite, the behavior of the latter was adjusted manually to fit the observed P2O5 concentration. This adjustment also improved the model fit of the CaO data and is used in the trace element modeling discussed below.
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) and model differentiation trends (black line) predicted by the MELTS program for the residual magma. Refinement of apatite crystallization to fit the P2O5 data results in an improved fit for the CaO data (gray lines; see text for further discussion).
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Chemical compositions consistent with those of the Amram monzonite (
50–53% SiO2) were produced after 5–40% (by volume) of the parental magma was removed by fractionation. Accordingly, alkali quartz syenite and quartz syenite compositions (60–67% SiO2) were produced after 66–87% of the original volume was fractionated, and rhyolite compositions (72–78% SiO2) were produced after >90% fractionation of the original volume of the parental mafic magma. The calculated fractionating assemblage contains roughly 44% plagioclase, 18% alkali feldspar, 13% clinopyroxene, 12% olivine, 9% spinel, and minor apatite and Fe–Ti oxides.
The behavior of trace elements during magma fractionation
The MELTS program yields the identification of the mineral phases and their place in the crystallization sequence of the initial mafic magma. These results depend only on thermodynamic considerations with no attempted adjustments to the analyzed chemical data (as done in least-square techniques). In this section we examine the results of the MELTS calculation in light of the trace element data obtained from the Amram 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 anomaly in the high-silica members; Fig. 7) and some uniform trace element concentration ratios, e.g. La/Nb and Y/Nb (e.g. Fig. 8). At first glance, this is a surprising observation, as uniform patterns and ratios are not expected for elements with different partition coefficient in magmatic processes, e.g. La and Yb, or Y and Nb. It appears that the crystallizing mineral assemblage is buffering the trace element concentration in such a way that the concentration ratios remain uniform. This pattern was reproduced in Fig. 12a for La and Yb (for which partition coefficients are available; see the Appendix). During the early stages, when olivine and pyroxene crystallize, Yb is more compatible, but with the introduction of plagioclase and titanomagnetite, the effect of pyroxene is balanced, leading to similar, moderate compatibility of both elements throughout the rest of the fractionation process.
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Europium shows no anomaly and even some small positive anomalies at low to moderate silica content (SiO2 <70%), and a prominent negative anomaly develops only in the high-silica members of the suite (Fig. 7). This feature is somewhat unusual in view of the important role of plagioclase removal throughout most of the fractionation process, and calls for further explanation. We suggest that titanomagnetite in the early stages and apatite in the intermediate stages of the fractionation process balance the effect of plagioclase. Eu is taken preferentially by the plagioclase, but is more incompatible than Sm and Gd in titanomagnetite and apatite (e.g. Jarrar, 2001
). Hence, the negative Eu anomaly develops only when K-feldspar crystallization becomes significant, at 70% silica content in the residual magma. The evolution of the Eu anomaly (Fig. 12b) is successfully simulated, along with the variation in Sm, Eu and Gd concentrations (Fig. 12c) by using the relevant partition coefficients (see Appendix) and the fractionating assemblage calculated by the MELTS fractionation model (Fig. 11).
To summarize, major and trace element evolution of the Amram magmatic sequence is simulated successfully by the fractional crystallization model presented above. This model suggests crystallization of a common water-undersaturated alkaline mafic magma, at 3 kbar (10 km), and fO2 defined by the QFM – 1 buffer. This evolution is driven by fractionation of plagioclase, pyroxene and magnetite, joined later by apatite and K-feldspar. The different lithologies of the Amram Massif represent samples separated from this deep magma chamber, whereas the residual fractionated phases accumulated at depth. The rhyolites were produced after extensive fractionation (90%) of the parental mafic magmas. Hence, the silica-rich A-type magmas in the Amram Massif would require the existence of an unexposed residual mafic body 10 times their own volume.
Post-orogenic A-type magmatism in the northeastern ANS
Orogenic-related calc-alkaline magmatic activity dominated the northern ANS until 600 Ma, while the onset of platform conditions in this region is marked by an Early Cambrian (Parnes, 1971; Beyth & Heimann, 1999) regional unconformity. This transition, from orogenic activity to stable platform conditions, was characterized by the occurrence of intra-plate A-type magmatism and an extensional tectonic regime suggested by dike swarm emplacement, uplift, erosion and graben formation (e.g. Garfunkel, 1999; Mushkin et al., 1999). In the northeastern ANS A-type magmas related to this transitional stage crop out in small, well-scattered bodies (Fig. 1), and span a period of 70 Myr from 600 to 530 Ma (Table 3). In cases where the petrography and magmatic stratigraphy of these outcrops were studied, sequences similar to that of the Amram Massif were revealed [as summarized by Mushkin et al. (1999)]. Furthermore, the chemical compositions of most of the northeastern ANS A-type outcrops plot along the same geochemical evolutionary path as that of the Amram suite, and their isotopic compositions (87Sr/86Sr = 0·7028–0·7045, = +1·5 to +5·3, Table 3), like those of the Amram rocks, suggest derivation from mantle sources with minor or no incorporation of older crustal material (e.g. Stein & Goldstein, 1996). Hence, we suggest that the petrogenetic model postulated for the Amram sequence may be applicable to the other mantle-related A-type outcrops from the northeastern part of the ANS.
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A-type rocks in the northeastern ANS cover a total area of
900 km2, which constitutes <10% of the exposed basement rocks in this region (Fig. 1). Assuming an average thickness of 0·5 km for these rocks (e.g. Eyal & Hezkiyahu, 1980), their total volume is estimated at 450 km3. Most of these rocks are felsic with 70–75 wt % SiO2. Thus, according to the Amram petrogenetic model, they would be derived by 90% fractionation from mantle-derived parental mafic magmas. Even if some crustal assimilation did occur, so that the felsic (A-type) magmas of the northeastern ANS were produced, on average, by only 80% fractionation, they would require a volume of 2300 km3 of parental mafic magma. Assuming 5% partial melting of the mantle source to produce these magmas, we infer that the A-type magmas of the northeastern ANS were derived from a voluminous mantle source of 46 000 km3. After normalizing this volume to the total area of basement outcrops in this region (i.e. 10 000 km2) we find that a sub-continental mantle column of average thickness of 5 km below the northeastern ANS was partially melted to produce the post-orogenic A-type magmas of this region. For comparison, we carried out a similar calculation for the rift-related, mantle-derived, Late Cenozoic alkali basalts of the Arabian plate (e.g. Altherr et al., 1990; Camp & Roobol, 1992). Covering 80 000 km2, with an average thickness of 0·2 km (i.e. a volume of 16 000 km3), and assuming only 3% partial melting at the source with only minor crustal contamination before emplacement, these basalts can be related to a mantle source of 500 000 km3. Normalizing this volume to an area of 160 000 km2 we infer that this magmatic episode represents partial fusion of 5 km of the sub-continental mantle beneath this part of the Arabian plate. Hence, these two magmatic episodes in the ANS, the post-orogenic, Neoproterozoic–Early Cambrian, A-type magmatism and the rift-related, Cenozoic volcanism, represent geological processes of a similar magnitude.
What caused the extensive A-type magmatism in the northern ANS? Similar suites of A-type magmas appear in the closing or post-orogenic stages of Precambrian or Phanerozoic shields in other parts of the world (e.g. the Himalayas, South Australia). Turner et al. (1992) proposed that A-type magmas from Australia were derived from the lithospheric mantle and that they represent a significant addition of juvenile material to the crust (Turner, 1996). They further argued that the magmatism is induced by delamination and thinning of the lithospheric mantle as a response to the thickening during the previous collisional–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 penetration of numerous dikes, normal faulting and graben formation. This tectonic environment as well as the chemical and isotopic evidence for the lithospheric origin of the ANS magmas concurs with the model of Turner et al. (1992). As in South Australia, the A-type magmatism in the northern ANS also represents significant addition of juvenile material from the lithospheric mantle to the crust.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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Field relations, petrography, chemistry and isotopic composition constrain a two-stage model for the formation of the Neoproterozoic–Early Cambrian Amram sequence. During the first stage, mafic magmas most probably derived from the upper part of the lithospheric mantle were emplaced in a deep crustal magma chamber. During their ascent, these magmas may have undergone limited fractionation and possibly minor contamination by crustal material. The second stage was characterized by fractionation of the mafic magmas in the magma chamber to produce the more felsic members of the suite. Crustal assimilation was not significant during this stage. The various rock units of Amram Massif represent batches of magma separated from the magma chamber and emplaced at shallower crustal levels.
A quantitative model (based on the MELTS program) relates the formation of the Amram A-type magmas to deep crustal fractionation of mafic magmas represented by the Amram mafic dikes. Removal of olivine, plagioclase, clinopyroxene, and magnetite from the parental magma produced the intermediate compositions of the suite (<60 wt % SiO2). Continued removal of clinopyroxene, plagioclase, apatite and, finally, alkali feldspar produced the felsic members of the suite (>60 wt % SiO2). High-silica A-type rocks (SiO2 >70 wt %) were produced after 90% fractionation of the parental mafic magmas, and thus imply a large unexposed volume of mafic cumulates at a 10 km depth.
Similar magmatic sequences, whole-rock chemistry and isotopic compositions relate most of the post-orogenic A-type magmatism of the northeastern ANS to a common petrogenetic process. These A-type rocks suggest the presence of much larger unexposed mafic bodies, all derived from the sub-continental mantle beneath the ANS. Despite its relatively small areal expression, post-orogenic A-type magmatism in the northeastern ANS marks a significant lithospheric melting event during the maturation stages of the newly formed continental lithosphere. As suggested by Turner et al. (1992) for post-orogenic 490 Ma A-type granites from South Australia, the A-type magmas of the northeastern ANS represent a considerable volumetric addition of mantle material to the continental crust, during the final phase of a large orogenic event.
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APPENDIX: PARTITION COEFFICIENTS USED FOR REE MODELING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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ACKNOWLEDGEMENTS |
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The manuscript was significantly improved by the critical comments of M. Beyth, S. Turner, J. B. Whalen and an anonymous reviewer. We are grateful to M. Wilson for her thorough review and to M. Eyal for discussions. The study was supported by the ISF (Grant No. 503/99 to M. Stein and Z. Garfunkel), by the Israel Ministry 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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REFERENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDGEOLOGY OF THE AMRAM...ANALYTICAL METHODSRESULTSDISCUSSIONSUMMARY AND CONCLUSIONSAPPENDIX: PARTITION COEFFICIENTS...REFERENCES |
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