Isotope provinces in Laramide and mid-Tertiary igneous rocks of northwestern Mexico (Chihuahua and Sonora) and their relation to basement configuration

Abstract

We have identified five distinct isotope provinces in northwestern Mexico in a study of the Sr, Nd, and Pb isotope geochemistry of Laramide and mid-Tertiary igneous rocks in Sonora and Chihuahua as well as adjacent portions of southwestern United States. Province A, in northern Sonora, is characterized by very unradiogenic Nd and Pb isotopic compositions and radiogenic Sr isotopic compositions. Unradiogenic Nd and radiogenic Sr and Pb isotopic compositions characterize Province B, in central Sonora. Provinces A and B are flanked to the east and south by Provinces C, D, and E, which are characterized by more radiogenic Nd and less radiogenic Sr isotopic compositions than observed in Provinces A and B. Rocks from Provinces C, D, and E have similar Sr and Nd isotopic compositions, although the variation observed in rocks from Province E is more restricted than in Provinces C and D. These provinces are unique, however, in their Pb isotopic compositions; rocks from Province C are the least radiogenic whereas those from Province E are the most radiogenic. The identified isotope provinces correlate well with the constraints imposed by the geology of the southwestern United States and northwestern Mexico. The primary factor controlling the geographic distribution of isotope provinces in northwestern Mexico appears to be the age of the crust that the magmas passed through and with which they interacted. Provinces A and B are interpreted to reflect the area underlain by old, early Proterozoic basement in northwestern Sonora; the significance of the difference in Pb isotopic compositions between rocks in Provinces A and B is not clear. We interpret Province C, located in northern Chihuahua and adjacent regions of Texas, to reflect the distribution of mid-Proterozoic crust of the North American craton that has been overprinted by Grenvillian deformation. Province D is located in a belt running from central Chihuahua into southwestern Texas and is interpreted to reflect the distribution of exotic crust that was accreted to the southern margin of the North American craton during Ouachita convergence; this crustal block consists of a Paleozoic arc sequence developed on Proterozoic basement. Lastly, we interpret Province E, located in southern Sonora and Chihuahua, as exotic Jurassic-Cretaceous arc crust accreted during the late Mesozoic. The observed distribution of isotope provinces in northwestern Mexico is not consistent with the Mojave-Sonora megashear hypothesis. In particular, the NNE-trending eastern boundary of the two western isotope provinces (Provinces A and B) does not show the 700-800 km of left-lateral offset that would be expected if this boundary was intersected by the Mojave-Sonora megashear.

94 samples from this reference