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Geochemical parameters and isotopic indices of Precambrian silicic igneous rocks record largely ensialic crustal environments during 2.5 to 1.0 b.y. ago. Simatic crustal regimes have evidently coexisted (i.e., southwest and central United States, Fennoscandia, Venezuela) though their extent remains unclear. The significance of initial Sr87/86 ratios (Ri) and large-ion-lithophile (LIL) element data is considered: both tend to be higher in Proterozoic than in Archean silicic igneous suites, suggesting, respectively, predominance of ensialic and ensimatic anatectic processes. The ensialic nature of middle Proterozoic crustal reworking is supported by Sm/Nd study of composite North American samples yielding Archean model...

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