Drs V. E. Camp, N. J. Jackson, C. R. Ramsay, M. J. Roobol, D. B. Stoeser and D. L. White write: The recent paper by Al-Shanti and Gass (1983) describing ophiolite mélange zones in the eastern Arabian Shield reaches a number of conclusions which we consider do not accord with the evidence.

The oceanic island arc accretion model

The recognition that the Arabian Shield contains island arcs (Jackaman 1972; Greenwood et al. 1976) which have been tectonically juxtaposed (Bakor et al. 1976; Frisch & Al-Shanti 1977; Gass 1977), was a major conceptual breakthrough. However, evidence now indicates that the basic model reiterated by Al-Shanti and Gass (p. 867, ‘accretion of intra-oceanic island arc systems over the period 1100-SO0 Ma’) is an oversimplification. First, part of the eastern Arabian Shield may be underlain by continental sialic crust (e.g. Ramsay et al. 1979; Stacey et al. 1980; Delfour 1980; Stacey & Stoeser 1983). Second, the best estimate for the start of oceanic island arc develop- ment was shortly before about 900 Ma (Marzouki et al. 1982). Third, although the development of oceanic island arc systems was the dominant crustal process from about 900 to about 700 Ma (e.g. Fleck et al. 1980; Jackson & Ramsay 1980; Roobol et al. 1983), most of these arc systems were assembled into a single plate by 670 f 10 Ma ago. Following this suturing, the shield- forming magmatic processes changed from one of lithospheric magma systems dominated by subduction- related procsses, to magma systems dominated by massive

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