Abstract
Strontium isotopic analyses are reported for ophiolitic and associated rocks of Mesozoic and Tertiary age from the Drocea Mountains. The samples and their average Sr87/Sr86 and Rb/Sr ratios, in order, include: (a) ultramafics (partly serpentinized or uralitized peridotite, peridotite-melagabbro). 0.7043, 0.106; (b) gabbros, dolerites and anemasites (including magnetite-bearing, quartz dolerite, hornblende and normal gabbros), 0.7032, 0.021; (c) basalts (amygdaloidal, hyalopyroclasite), 0.7030, 0.040; and (d) granophyric and albitic vein rocks, 0.7046, 0.058. Also analyzed were (e) basalt-spilites of a younger intrusive cycle 0.7042, 0.046 (f) banatites, 0.7064, 0.542. Two Quaternary volcanics were analyzed from outside the Drocea Mountains: (g) augite-hypersthene andesite from Mt. GutĂi, 0.7083, 0.247, and olivine basalt from Racoş, 0.7043, 0.056.
The data for the ophiolite suite show highest Sr87/Sr86 and Rb/Sr ratios in the ultramafics which suggests a two-stage origin with the ultramafics derived from a more primitive mantle than the later gabbros and basalts. Initial Sr87/Sr86 ratios range from 0.7021 to 0.7045 in gabbro and basalt and 0.7035 to 0.7056 in peridotite which is well within the limits found in oceanic tholeiites and suggests an origin for the complex as a spreading oceanic ridge. Cross-cutting felsic granophyric and albititic rocks as well as the late-stage basalts (a) have relatively low Sr87/Sr86 and Rb/Sr ratios, (b) represent a small volume, and (c) are intimately related to the ophiolites. They appear to have developed largely by late-stage differentiation and fractional crystallization of a tholeiitic magma. The higher ratios for the banatites and andesite from Mt. GutĂi suggest that significant amounts of sialic crustal material were involved in their formation. The basalt from Racoş is from the vicinity of a deep fracture zone; its relatively low Sr87/Sr86 ratio suggests a direct link to a mantle source with little or no crustal contamination.
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Paper presented at the IAVCEI International Symposium on Volcanism and Associated Metallogenesis, Bucharest, Rumania, September 1973.
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Herz, N., Jones, L.M., Savu, H. et al. Strontium isotope composition of ophiolitic and related rocks, Drocea Mountains, Rumania. Bull Volcanol 38, 1110–1124 (1974). https://doi.org/10.1007/BF02597109
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DOI: https://doi.org/10.1007/BF02597109