motis@vms.huji.ac.il
The geochemistry and tectonic setting of three dyke suites from the very last stage of the Pan African orogeny (~ 580-540 Ma) in the Arabian Nubian Shield (ANS), and alkali basalts from the overlying Phanerozoic section are used to constrain the composition and formation of the sub-continental lithosphere in this region. The dykes were sampled in Elat and Amram massifs (southern Negev desert, Israel) and are comprised of calc-
alkaline, andesitic rhyolitic suite dissected by two sets of tholeiitic doleritic dykes. Similar suites are reported from other parts of the northern ANS. The Phanerozoic alkali basalts (PAB) erupted in three major cycles, in the late Triassic, Lower Cretaceous and Neogene-Quaternary times.
The dykes share uniform REE {(La/Yb)n = 7 to 9} and
flat HREE suggesting derivation from the spinel stability zone. The PAB show fractionated HREE patterns which indicate derivation from the garnet stability zone (Stein and Hofmann, 1992). The variations of incompatible trace element ratios (normalized to primitive mantle values) with depth of magmas sources are presented in Table. 1.
The depth - elemental ratio profile of the lithosphere shows that Nb/Th and Ce/Pb ratios increase with depth, Ba/La and Rb/Th ratios decrease, and Th/U remains constant at primitive mantle value.
The history of continental growth in the ANS during the Pan African orogeny was ascribed by Stein and Goldstein (1995) to uprising of a plume head (at ~ 900 Ma) which was transformed by subduction processes to continental lithosphere (~ 870 to 650 Ma). The proximity of many trace element ratios in the late Pan African dykes and PAB to primitive mantle values suggests that the composition of the plume head was close to that of primitive mantle. The variation we detect in some trace element ratios in the source region of the dykes and the basalts may be ascribed to redistribution of trace elements in the lithosphere during time of subduction. Such a process was recently proposed by Ionov and Hofmann (1995) based on data from lithospheric xenoliths. In the case of the ANS lithosphere we can directly relate the composition of the magma sources with the geodynamic history of the shield. We suggest that during time of subduction Ba, Rb and Pb were preferentially transported by metasomatic fluids
to shallower levels of the ANS lithosphere, where they were subsequently sampled by the calc-alkaline and dolerite dykes. Nb is retained by hydrous phases in the deeper levels of the lithosphere (in the source region of the alkali basalts), and is thus depleted in the shallower source regions of the calc-alkaline dykes. U, Th and the REE are not affected by these processes, and preserve primitive mantle values.
The transition from calc-alkaline to alkaline magmatism in the ANS was accompanied by abrupt change in the stress field from compressional to extensional, along with heating and uplifting of the lithosphere (cf. Bentor, 1985). These processes may be related to extensive delamination of the cold thick lithopshere (cf. Black and Liegeois, 1993). Delamination of lower lithospheric domains with low Ba/Th, Rb/Th, and high U/Pb, Nb/Th, and their recycling back into the asthenospheric upper mantle may be the process responsible for the formation of high U/Pb, low Rb/Sr compositions in the MORB reservoir. This model may provide an additional explanation for the Pb paradox. The preservation in the lithosphere, for long periods of time, of domains with either high or low Rb/Sr and U/Pb ratios, gives rise to development of distinct isotopic reservoirs such as EM and HIMU.
Rising plume heads are the major means of juvenile mass transfer from the lower to the upper mantle (Stein and Hofmann, 1994). The newly formed lithosphere acts as sorting and storing place for the incompatible trace elements, which eventually lead to production of geochemical heterogneities in the upper mantle.
Bentor, Y., Precam. Res. 28, 1-74 (1985).
Black, R. & Liegeois, J.P., J. Geol. Soc. Lond. 150, 89-98 (1993).
Ionov, D.A. & Hofmann, A.W., Earth Planet. Sci. Lett: 131, 341-356 (1995).
Stein, M. & Goldstein, S.L., submitted (1995).
Stein, M. & Hofmann, A.W., Earth Planet. Sci. Lett. 114, 193-209 (1992).
Stein, M. & Hofmann, A.W., Nature 372, 63-68 (1994).
Table 1.
Depth Rock type Nb/Th Th/U Ce/Pb Rb/Th Ba/La
Spinel Calc-alkaline dykes 0.1 to 0.2 1 0.3 to 0.5 0.9 to 2.5 2.5 to 3
40-60 km Dolerite dykes 1.3 1 0.1 to 0.3 1.1 to 2.6 1 to 2
Garnet > 60 km PAB 1.3 1 2.8 0.8 1 to 1.3