Mesoproterozoic-trans-Laurentian magmatism: A synthesis of continent-wide age distributions, new SIMS U–Pb ages, zircon saturation temperatures, and Hf and Nd isotopic compositions

• A-type magmatism across southern Laurentia between 1480 and 1340 Ma.
• Change from crustal to more juvenile source across “Nd line”.
• Magma temperatures mostly greater than 850 °C.
• Granite–rhyolite assemblages formed in broad intracratonic extensional setting inboard of convergent or transform margin.

The origin and tectonic setting of the Mesoproterozoic (1.50–1.34 Ga) granite–rhyolite Provinces of the midcontinent region of the United States, and the coeval A-type granite plutons of the southwestern United States, have been an enigma for decades. In this paper we review the history of this problem, examine the significance of a wealth of previously published U–Pb zircon and whole-rock Sm–Nd data, and present new U–Pb zircon age and Lu–Hf isotopic data from selected zircon samples. Our data show that the initiation of this magmatism spans the interval 1.50–1.40 Ga across the continent, but there is a general younging westward. Younger, 1.39–1.34 Ga magmatism also occurred across the continent, but forms a major magmatic event only in the south-central mid-continent.There is generally a good correlation between Sm–Nd and Lu–Hf isotopic compositions and inferred mantle derivation model ages, except possibly in southeastern Oklahoma. The absence of older cores in zircons is compatible with calculated zircon saturation temperatures of ca. 850 °C, and supports mantle-driven basaltic underplating and resulting crustal partial melting as a petrogenetic model for the A-type magmatism. Our new data compilation refines the existence of the “Nd-line” that extends from southwestern Texas into southeastern Michigan and the western Grenville Province in Canada; it marks an isotopic boundary between granites and rhyolites of cratonal origin to the northwest (1.8–1.6 Ga model ages) and those of more juvenile origin to the southeast (1.5–1.4 Ga model ages). Our suggested tectonic model for the Mesoproterozoic A-type magmatism across eastern and southern Laurentia involves the development of an active Laurentian margin extending from eastern Canada, across what is now the midcontinent region of the United States, and at least into the current southwestern United States. Magmatism along this margin was likely related to asthenosphere–lithosphere interactions associated with the accretion of outboard arcs and possible exotic terranes that caused complex deformational and magmatic responses in the continental interior. This activity evidently spread from northeast to southwest over a period of ca. 70 million years. The occurrence of the younger, ca. 1.39–1.34 Ga event indicates that similar processes occurred some 130 million years later, suggesting a long-lived convergent plate margin and recurring basaltic underplating in some locations, perhaps due to inboard lithospheric delamination and/or mantle upwelling.