Abstract

During late Precambrian time three sets of mafic dikes were emplaced in southwestern Montana south of the east–west Helena embayment of the Belt Basin. The oldest dikes, intruded approximately 1455 Ma ago into both the southern Tobacco Root Mountains and the adjoining Ruby Range, are low K tholeiite in composition. The two other sets of dikes were intruded at approximately the same time, about 1120–1130 Ma ago. Both are high K quartz normative types: one is strongly enriched in Fe and is most similar to ferrobasalt or ferrogabbro in composition, the other is low in iron and differentiated along strong alkali and silica enrichment trends. The 1455 Ma old dikes and the iron-enriched 1120 Ma dikes have initial Sr ratios in the range 0.7020–0.7030 that indicate probable derivation from mantle material that has maintained a low Rb–Sr ratio (0.024) for much of the Earth's history. This mantle source is much lower in Rb–Sr ratio than that proposed for the source of dikes in the Beartooth–Bighorn Mountain area to the southeast. The iron-poor 1130 Ma old magma has an initial ratio of 0.709, which suggests contamination by crustal Sr.A strong correlation appears to exist between the timing of mafic intrusive events in the older Precambrian rocks to the south of the Belt Basin and tectonic-intrusive events within the basin. Intrusive events are recorded at 1455–1430 Ma ago both inside and outside the basin. A 1330 Ma old mafic intrusive event in the Beartooth Mountains is associated with a period of metamorphism and (or) a period of deposition in the basin. The 1120–1130 Ma old dikes are correlated with mafic flows and sills and another major period of deposition within the Belt Basin.

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