The lead and strontium isotope geochemistry of metalliferous sediments associated with Upper Cretaceous ophiolitic rocks in Cyprus, Syria, and the Sultanate of Oman: Discussion

Abstract

Introduction The recent paper by Gale et al. concerning the origin of lead in some ophiolite suites accepts as proven the hypothesis that an oceanic spreading center is the tectonic environment for ophiolites and compares this environment with the East Pacific Rise. The authors are therefore forced to conclude that about half the lead in the umbers of the ores of the Troodos massif on Cyprus was derived from sea water because the lead isotopes are not characteristic of an oceanic mantle. The isotopic nature is intermediate between that of oceanic mantle and that of pelagic sediments or manganese nodules, and to visualize incorporation of lead from sediments into an ore at a spreading center is difficult as sediments are generally sparse and should not get into the zone of leaching. That the Troodos massif is a mature oceanic spreading center like the East Pacific Rise is by no means universally accepted, and material balance considerations indicate that a sea-water origin is highly problematical. The lead isotope data are very likely telling us that the Troodos massif probably did not originate at a rift in a mature oceanic environment such as an East Pacific Rise. Several other environments fit the lead isotope data better. Agreement is by no means unanimous that the Troodos massif of Cyprus is an oceanic spreading center (cf. Coleman 1977 and papers cited therein). Indeed, Miyashiro (1973) proposed that "the Troodos ophiolitic complex was probably formed in an island arc." Although the proposal has met with considerable resistance, Coleman (1977) does point out that an oceanic spreading center and island-arc volcanism can be very difficult to distinguish. Pearce and Gale (1977) classify the Troodos massif as forming in an inter-arc or marginal-basin environment and Gass (1977) concludes "Most ophiolites so far studied in detail seem to represent allochthonous fragments of the crust of small oceans or marginal seas. No convincing representative from a major ocean has been identified." Consideration of an environment other than one similar to an East Pacific Rise therefore appears legitimate. In addition, three modem metalliferous deposits in the Pacific Ocean were noted by Gale et al. These are Deep Sea Drilling Project (DSDP) site 66 in the central Pacific at 2'24'N, the Baer Deep at 9"S, and the East Pacific Rise at 17"s (see Gale et al. for references). Subsequent to the publication of the paper by Gale et al., data on a new locality-21°N on the East Pacific Rise-have been published (Brevart et al. 198 1; Vidal and Clauer 1981). All sulfides analyzed have lead isotopic compositions characteristic of the oceanic mantle although one manganese layer in basalt has isotopic ratios similar to those of sea water, as do manganese nodules. Sediments at the site also have isotopic ratios characteristic of normal pelagic sediments. We wish to add data on the sulfides from the locality at 21°N here: