The Bearing of Lead Isotopes on the Source of Granitic Magma

Abstract

The isotopic composition of lead was measured in Mesozoic and Cenozoic igneous rocks (including volcanic glasses, olivine basalts, and potassium feldspars from granitic rocks) of the coastal region (California, Oregon, and Washington)-where melting is possible at the bottom of a Phanerozoic sedimentary pilel—land in similar rocks of the continental interior region (Rocky Mountain provinces, southwestern New Mexico, South Dakota, and Texas) —where Precambrian rocks are shallow or surround the Mesozoic and Cenozoic igneous rocks at the surface. The coastal region is typified by a narrow range in lead isotopic composition and by isotope ratios more radiogenic than expected from closed system models (J type). The continental interior region is typified by a broad range in lead isotopic composition, even within an individual magmatic complex such as the Boulder batholith, and by at least one isotope ratio that is less radiogenic than expected from closed system models (B type) and than found in the coastal region. If, as suggested by available data in the literature, most Precambrian and Paleozoic granitic rocks have leads more radiogenic than closed system models predict by the time of the Cenozoic Era, granitic rocks, in the continental interior region at least, cannot form by complete melting of older granitic rocks. The rocks may have been formed by one of the following mechanisms (in the order of the author‘s preference): (1) Differentiation from or partial melting of a basaltic-gabbroic parent coupled with crustal assimilation. (2) Differentiation or partial melting from a basaltic-gabbroic parent having U/Pb and Th/Pb characteristics in the continental interior different from those of the coastal regions and ocean basins. (3) Partial melting of upper crustal rocks. It is difficult to choose between mechanisms 1 and 2; mechanism 3 appears to be in disagreement with results of work on Rb-Sr and therefore appears to be less likely than the other two. The differences in isotopic characteristics of the igneous rocks in the coastal and continental interior regions can be explained (1) if the predominant crustal assimilation takes place in the upper crust through partial assimilation of Phanerozoic or Preqambrian rocks, whichever happen to be present near the surface in the area or, (2) if some other mechanism occurred, such as partial melting of the mantle in coastal regions and partial melting of the lower crust in the interior regi

Keywords

Affiliated Institutions

• United States Geological Survey US

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