Two calc-alkaline plutonic complexes. Bazman and Natanz, intruded through the south-eastern active and south-western ancient continental margins of Central Iran, Have been dated by the Rb-Sr whole-rock isochron method at 74 ± 2 and 24 ± 4.5 Ma, respectively. Detailed trace element studies together with low 87 Sr/ 86 Sr initial ratios (<0.706) indicate that these complexes represent parts of an Andean-type magmatic are formed in response to subduction of Tethyan oceanic crust beneath Central Iran. Geochemical data on the igneous rocks at Bazman suggest that subduction of the Oman oceanic crust was well established by the late Cretaceous. On the evidence of the Natanz rocks, the Arabian-Central Iranian collision did not occur during the late Cretaceous, but took place during late Paleogene or early Neogene time. The Natanz low-Rb diorites and gabbros cannot be comagmatic with the more salic rocks, for they have distinctly lower 87 Sr/ 86 Sr ratios and the gabbros, at least, were intruded some 10 Ma earlier.

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During the past twenty years considerable attention has been given to the compositions of the pyroxenes from basic igneous rocks. Hess (1941) reviewed the courses of crystallization of pyroxenes from mafic magmas and drew a trend line for the lime-rich clinopyroxenes. The present investigation was undertaken primarily to determine more closely the trend in the ferroaugite region, for here Hess's trend was based on the analyses of two pyroxenes from the Skaergaard intrusion and both of these are now known to be somewhat unusual types. The Skaergaard intrusion which is a strongly fractionated gabbroic complex is Particularly suitable for this study of the ferroaugites. Fifteen new analyses have been made for this investigation and twenty-three analyses of the pyroxenes from this intrusion are now available. Using nineteen of these, new trend lines have been drawn (fig. 1) for the Skaergaard clinopyroxenes.