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

Abstract

We have identifi ed fi ve 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 unradiogenicNd 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 fl anked 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 identifi ed 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 refl ect the area underlain by old, early Proterozoic basement in northwestern Sonora; the signifi cance 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 refl ect 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 southwesternTexas and is interpreted to refl ect 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-trendHoush, T.B., and McDowell, F.W., 2005, Isotope provinces in Laramide and mid-Tertiary igneous rocks of northwestern Mexico (Chihuahua and Sonora) and their relation to basement confi guration, in Anderson, T.H., Nourse, J.A., McKee, J.W., and Steiner, M.B., eds., The Mojave-Sonora megashear hypothesis: Development, assessment, and alternatives: Geological Society of America Special Paper 393, p. 671-692.

Genetic implications of new Sr and Nd isotopic data of the intrusive rocks from the Laramide Arc in Northern Sonora, Mexico.

“…The sample shows very low CaO (0.11%) and high K 2 O (6.4%) with relatively high loss on ignition (LOI-2.6%). The value of initial 87 Sr/ 86 Sr in sample 7 as well as of sample 3 are somewhat higher compared to other samples reported from the area (Damon et al, 1983;Housh and McDowell, 2005; see also Table 2), differences could be caused by hydrothermal alteration as it has been observed in oceanic basalts and in the volcanic geothermal field of Los Azufres (Verma, 1992;Verma et al, 2005). The isotopic values of Sr and Nd from La Caridad and Cananea are very similar suggesting the same origin for the magmas in both districts.…”

“…To the north of the limit of the Sonora Allochthonous (Poole et al, 2005) (Mcdowell et al, 1999). This, allow us to speculate on the absence of Sonoran proterozoic basement in this zone and relate the isotopic data to a pre-Laramidic geologic history similar to that of central Chihuahua (Housh and Mcdowell, 2005).…”

“…1 and Table 1) from previous works (Damon et al, 1983, Mead et al, 1988Wodzicki, 1995;Espinosa-Perea, 1999;Schaaf et al, 1999;Valencia-Moreno et al, 2001, 2003Housh and McDowell, 2005 Nd, eSr, eNd values and the model ages for Nd in relation to depleted mantle (DM) were calculated using the equations published by DePaolo (1981) and Farmer and DePaolo (1983). For samples 1 and 2 new isotopic ages determined in this work were used; for samples 3 and 4 (Lucy and Can-12) ages were taken from the Cuitaca Granodiorite published by Anderson and Silver (1977); for samples 5 and 6 (IntrCan and IntrMaria, respectively), the ages are from the Wodzicki (1995) work; and for sample 7 (Car-5) the age used was from the granodiorite reported by Valencia et al (2005) and Barra et al, 2005).…”

Laramide to Miocene syn-extensional plutonism in the Puerta del Sol area, central Sonora, Mexico

“…Outcrops of these rocks are widespread in Sonora, and recent contributions have improved the knowledge of their regional stratigraphy, geochronology, geochemical and radiogenic isotopic composition, to better constrain the timing, nature and implication of basement variations, associated mineralization, and petrogenesis (Roldán-Quintana, 1991, McDowell et al, 2001, Valencia-Moreno et al, 2001, Valencia-Moreno et al, 2003, Iriondo et al, 2004, Housh and McDowell, 2005, Nourse et al, 2005, Noguez-Alcántara et al, 2007, Roldán-Quintana et al, 2009, Pérez-Segura et al, 2009, González-León et al, 2011, Pérez-Segura et al, 2013, Del Río Salas et al, 2013.…”

“…U-Pb zircon ages were obtained by laser ablation inductivelycoupled plasma mass spectrometry (LA-ICPMS) in the Centro de Geociencias, UNAM following procedures described in Solari et al (2010Solari et al ( , 2015 [results are reported in Supplemental Table A], and by laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) at the Arizona LaserChron Center of the Department of Geosciences, University of Arizona, using the procedures described by Gehrels et al (2006) (Housh andMcDowell, 2005: González-León et al, 2011) Nd and Sr isotope values of Laramíde rocks in the Caborca block of central Sonora (Valencia-Moreno et al, 2001, Housh andMcDowell, 1995;González-León et al, 2011) Plutons of the study quadrangle (Wooden et al, 1988;Wooden and DeWitt, 1991) showing plots of the El Palofierral orthogneiss and the Laramide and syn-extensional plutons of the study area. Black stars are Pb isotope values reported from Paleoproterozoic granites of the Caborca terrane by Farmer et al (2005) and González-León et al (2011).…”