Synchronous magmatic cycles during the fragmentation of Gondwana: radiometric ages from the Levant and other provinces
Introduction
Since the Early Mesozoic, several igneous events have occurred in the Levant (Fig. 1). Igneous rocks are distributed over an area of ca 150×500 km in outcrops and the subsurface of Syria, Lebanon, Israel, Jordan and Sinai, comprising various volcanic sequences and small hypabyssalic intrusions. Contemporaneous magmatic activity is known from adjacent areas, such as the Arabian Peninsula (Baker et al., 1996, Sebai et al., 1991a, Zumbo et al., 1995a) and NE Africa (Egypt, Libya, Sudan, Ethiopia) (Cahen et al., 1984, Ebinger et al., 1993, George et al., 1998, Hofmann et al., 1997, Wilson and Guiraud, 1998, Zumbo et al., 1995b). Basalts ranging in composition from weakly sub-alkaline to alkaline are the dominant rock type, together with nephelinites, basanites, micro-gabbros, quartz-syenites and trachytes (Stein and Hofmann, 1992), all enriched in incompatible elements (e.g. LREE) and related to rifting and hot spots or mantle plumes (Garfunkel, 1989, Stein and Hofmann, 1992, Stein and Hofmann, 1994, Wilson and Guiraud, 1998). These commonly caused doming ∼1–2 km above the surrounding surface with subsequent thinning and fracturing of the lithosphere.
Widespread igneous occurrences are crustal emplacements of predominantly ferromagnesian extrusive and intrusive rocks that originated in mantle plume processes (Coffin and Eldholm, 1994). Initially, the term plume was used for hot buoyant material rising from the mantle, creating volcanoes or hotspots such as Iceland and Hawaii (e.g. Morgan, 1971, Morgan, 1981, Vogt, 1972), and was modeled as having a mushroom-shaped head about 100 km in diameter (Ribe and Christensen, 1994, Turcotte and Schubert, 1982) and a narrow cylindrical tail. The model of Griffiths and Campbell (1990), however, predicts that a plume head derived from a deep mantle source (2800 km) attains a diameter of 800–1200 km. Furthermore, plumes also refer to larger (thousands of kilometers) mantle convection cells with ascending hot material being displaced by descending cold material (Turcotte and Schubert, 1982, White and McKenzie, 1989). In the present paper, the term plume refers to the upwelling of a large-scale mantle convection cell (approximately thousands of kilometers in size), and the term igneous province is the overall area where extrusive (commonly in enormous quantities) and intrusive activity took place. The area where the igneous rocks have a plume signature is the plume province. Laboratory experiments on the lateral extension and flattening of a plume head beneath the lithosphere, after the time of maximum uplift, suggest that this extension and flattening continues for at least 20 m.y. (Griffiths et al., 1989). This time span is slightly shorter than the magmatic periods defined in the Levant.
The above-mentioned deep mantle plume theory has gained widespread acceptance in the literature (Kent, 1994), although other models, such as ‘convective partial melting’ (Mutter et al., 1988) or ‘plate tectonic reorganization’ (Anderson, 1994a, Anderson, 1994b) have been suggested for the origin of continental flood volcanics (CFVs) and other igneous provinces.
The stratigraphic evidence and isotopic age determinations (K/Ar from Lang et al., 1988, Lang and Steinitz, 1989, Recanati et al., 1989; and Rb/Sr from Lang et al., 1988, Steinitz, 1980) indicate that igneous activity began in Early Mesozoic times and continued episodically until the Senonian (Lang et al., 1988, Lang and Steinitz, 1989). Following a quiescence of several tens of million years, magmatic activity resumed during the Miocene (Steinitz et al., 1978), and continued up to subrecent times (Heimann et al., 1996, Mor, 1986). Due mainly to analytical problems, earlier interpretations of whole-rock (WR) K/Ar and Rb/Sr data revealed broad ranges of magmatic activity, and the duration of events thus remained uncertain. By applying the 40Ar/39Ar dating technique to some of the WR samples (Heimann et al., 1996, Kohn et al., 1993, Lang and Steinitz, 1994, Lang and Steinitz, 1996), the range of the K–Ar dates became significantly constrained, but still did not establish a reliable time interval for specific magmatic events.
The present paper summarizes new mainly reliable radiometric data from T. Weissbrod, A. Segev and Y. Kapusta, Y. Kolodny and H. Ron, M. McWilliams and H. Ron, Z. Lewy, B. Lang and A. Segev (unpublished), adds reinterpreted 40Ar/39Ar WR dates from Heimann (1990), Kohn et al. (1993), Lang and Steinitz (1994), Lang and Steinitz (1996), Baer et al. (1995), Teutsch et al. (1996) and Heimann et al. (1996), together with a few valid Rb/Sr results from Steinitz (1980) and Lang et al. (1988), mainly from Israel, and reveals 13 magmatic events in the Levant area during the past 245 m.y. These magmatic events are correlated with available reliable and accepted geochronological data (mainly 40Ar/39Ar, Rb/Sr and U/Pb results) of magmatic events along the dispersed provinces of Gondwana (central and southern Atlantic Ocean, Indian Ocean, Karoo, Marie Byrd Land-Eastern Australia and Balleny).
The synthesis of the above-mentioned geochronological data enables the hypothesis of ‘global synchronous magmatic episodes’ to be tested and sheds light on the evolution of a lower mantle plume head.
Section snippets
Methods
Most of the samples for 40Ar/39Ar dating from Israel were prepared at the Geological Survey of Israel (GSI) laboratories and were irradiated in the IRR-1 reactor at the Soreq Nuclear Center, Israel (Heimann et al., 1992). Of the published results (see Table A1 in Appendix A), 22 samples remain unchanged, 16 were reinterpreted, and five samples from Weissbrod, Segev and Kapusta have not yet been published. The samples were fused by inductive furnace and analyzed by an MM-1200B mass spectrometer
New and reinterpreted 40Ar/39Ar and K/Ar ages of Mesozoic–Cenozoic igneous rocks in Israel
A list of published (part of them reinterpreted) 40Ar/39Ar WR and mineral ages (including unpublished data from Weissbrod, Segev and Kapusta, Fig. 2m) of magmatic rocks in Israel is given in Appendix A and Fig. 2. Unpublished data from Kolodny and Ron, and McWilliams and Ron are given in Appendix B, Fig. 2n–q, and from Lewy, Lang and Segev in Table C1 (Appendix C).
The refinement of the 16 disturbed WR igneous samples yields differences of up to 4% from the published 40Ar/39Ar ages, which
Ages of Jurassic–Recent magmatic events in igneous provinces inside Gondwana
The Levant area represents a relatively limited part of the northern Gondwana super-continent, from which widespread magmatic occurrences are known from its drifted fragments.
Widespread igneous occurrences or events include crustal emplacements of predominantly ferromagnesian extrusive and intrusive rocks consisting mainly of continental flood volcanics (CFV) and non-orogenic complexes, passive margin volcanics and oceanic plateaus.
A series of magmatic events throughout a period and along a
Discussion and conclusions
Large parts of the igneous provinces described, particularly those with CFVs, were suggested to have formed by deep mantle plume activity, and the location of these postulated mantle plume heads are depicted on each of the igneous provinces (Fig. 4; after Cox, 1989, Lanyon et al., 1993, Schilling et al., 1992, Weaver et al., 1994, White and McKenzie, 1989, Wilson and Guiraud, 1998). Most of the igneous rocks within the plume provinces are geochemically characterized by a strong mantle signature.
Acknowledgements
I wish to thank Y. Kolodny, M. McWilliams and H. Ron for their kind permission to publish their 40Ar/39Ar ages, and likewise, Z. Lewy and B. Lang for the K–Ar data. I also thank Y. Kapusta for his fruitful discussions, P. Kotlarsky for his assistance with the data processing and calculations, and B. Katz for editing the text. The paper benefited greatly from discussions and reviews of T. Weissbrod, M. Stein, M. Abelson and R. Weinberger. The paper was considerably improved by the comments of
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