Migration of Radiogenic Strontium during Metamorphism

Study of the concentration and isotopic composition of strontium and rubidium in hornblende diorite dikes and sills of Precambrian age from the Panamint Mountains of California showed that these rock systems were enriched in radiogenic Sr during a late Mesozoic metamorphism. Enrichments in radiogenic Sr were observed for total-rock samples which yielded apparent ages of up to 34,000 m.y., although there is no obvious petrographic evidence for such metasomatic changes. These results indicate that some caution is necessary in interpreting the Sr isotopic composition of rocks in terms of their original source or in assuming that 'total rocks' form closed systems. In general, the initial isotopic Sr composition should be determined, not assumed. Introduction. This study is an investigation of the effects of relatively recent metamorphism on the isotopic composition of Sr and the concentration of Sr and Rb in some marie igneous rocks of the Precambrian. The samples were chosen from an area in which Lanphere et al. [1963, 1964] had demonstrated that certain older Precambrian rocks had lost appreciable amounts of radiogenic Sr during a Mesozoic metamorphism. The samples studied are from the Stone Corral area in Pleasant Canyon in the southern part of the Panamint Mountains of California. A sequence of later Precambrian rocks (the Pahrump series) unconformably overlies augen gneiss and crosscutting granite of the World Beater complex. The crystalline rocks occupy the core of an anticlinal dome and are cut by dioritic dikes, some of which can be traced into sills and dikes in the lower part of the overlying Pahrump sedimentary section. Middle-grade metamorphic mineral assemblages, characterized by the widespread occurrence of biotite and the local occurrence of andalusite, cord•,er ire, garnet, and chloritoid, are developed in the manfling sedimentary rocks. However, the preservation of fine deposittonal features and the absence of metamorphic differentiation features Contribution 1256, Division of Geological Sciences, California Institute of Technology, Pasadena. are suggestive of a lack of gross recrystallization or of significant mobilization and metasomatism. The mineral assemblages and textures in the augen gneiss, the granite, and the dioritic dikes and sills are not significantly affected by the metamorphism and might be interpreted reasonably as the primary mineralogy and texture. Details of the geology and petrography are given in the report by Lanphere et al. [1964]. The results from samples of the augen gneiss and granite from this area are that the Sr•/SF • and Rb87/Sr•ø ratios for the various component minerals of a particular sample lie on a straight line on the Sr evolution diagram, with a slope corresponding to an age of 100 to 150 m.y. This indicates that essentially complete Sr isotopic equilibration took place during Mesozoic regional metamorphism. While such mineral isochrons were found for several different rocks, it was found that the total-rock samples scattered rather widely and did not lie along an isochron. Pb-U ages on zircon indicated primary ages of 1800 m.y. for the augen gneiss and 1350 m.y. for the granite, but nearly all the total-rock samples lay below a 1600-m.y. isochron, indicating younger apparent ages. These results showed that even total-rock samples as large as 85 kg were open to Sr or Rb transport during the period of isotopic redistribution. Apatite and plagioclase, which contain very little Rb, showed large Sr • enrichments; thus the observed effects must be attributed to Sr redistribution.