Contrasting Proterozoic basement complexes near the truncated margin of Laurentia, northwestern Sonora–Arizona international border region

Abstract

We utilize new geological mapping, conventional isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses, and whole-rock radiogenic isotope characteristics to distinguish two contrasting Proterozoic basement complexes in the international border region southeast of Yuma, Arizona. Strategically located near the truncated southwest margin of Laurentia, these Proterozoic exposures are separated by a northwest-striking Late Cretaceous batholith. Although both complexes contain strongly deformed Paleoproterozoic granitoids (augen gneisses) intruded into fi ne-grained host rocks, our work demonstrates marked differences in age, host rock composition, and structure between the two areas.The Western Complex reveals a >5-km-thick tilted section of fi nely banded felsic, intermediate, and mafi c orthogneiss interspersed with tabular intrusive bodies of medium-grained leucocratic biotite granite (1696 ± 11 Ma; deepest level), mediumgrained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphyritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and transposition of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies Nourse, J.A., Premo, W.R., Iriondo, A., and Stahl, E.R., 2005, Contrasting Proterozoic basement complexes near the truncated margin of Laurentia, northwestern Sonora-Arizona international border region, in Anderson, T.on May 31, 2015 specialpapers.gsapubs.org Downloaded from 124 J.A. Nourse et al. metamorphism, when crystalloblastic textures developed. Deformation and regional metamorphism occurred before emplacement of 1.1 Ga(?) mafi c dikes.Throughout the Eastern Complex, meta-arkose, quartzite, biotite schist, and possible felsic metavolcanic rocks comprise the country rocks of strongly foliated medium-and coarse-grained biotite granite augen gneisses that yield mean 207 Pb/ 206 Pb ages of 1646 ± 10 Ma, 1642 ± 19 Ma, and 1639 ± 15 Ma. Detrital zircons from four samples of host sandstone are isotopically disturbed; nevertheless, the data indicate a restricted provenance (ca. 1665 Ma to 1650 Ma), with two older grains (1697 and 1681 Ma). The pervasively recrystallized Paleoproterozoic map units strike parallel to foliation and are repeated in south-trending folds that are locally refolded about easterly hinges. Southeasterly lineation developed in augen gneiss and host strata becomes penetrative in local domains of L-tectonite. Regional metamorphism associated with this tectonism persisted until ca. 1590 Ma, as recorded by metamorphic growths within some zircon grains. Mesoproterozoic intrusions that crosscut the Paleoproterozoic metasediments and augen gneisses include coarsely porphyritic biotite granite (1432 ± 6 Ma) and diabase dikes (1.1 Ga?). Emplacement of the granite was accompanied by secondary hig...

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California

“…Initial Pb-Sr and Nd ratios from the Proterozoic basement west of the suture zone within the Caborca terrane (Iriondo et al 2004;Farmer et al 2005;Nourse et al 2005) are a good fit for the continental crustal component of the La Posta plutonic rocks . SHRIMP analyses documented xenocrystic cores of Proterozoic zircon within La Posta plutons that also support the probability that the Proterozoic terranes or sedimentary rocks derived from them could have been the crustal component in the plutons .…”

“…12) and the thrust beltcontains distinctive Neoproterozoic and Paleozoic strata that are a detailed match for strata found today in eastern California-western Nevada (Stewart 2005) and in the San Bernardino Mountains near Los Angeles (Cameron 1982;Stewart et al 1984). Crystalline basement is also exposed within the terrane west of the thrust belt in Sonora, where it comprises 1725-1696 Ma orthogneiss, tabular granitoid bodies (Nourse et al 2005), and anorthositic complexes dated at around 1100 Ma (Espinoza et al 2003). In the San Bernardino Mountains the basement to the metasedimentary section is 1750 ± 15 Ma (Silver 1971).…”

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

“…All errors are at one sigma level expressed in %, i.e., they are relative standard deviation expressed in % (%RSD, see Verma, 2005). For more details on SHRIMP results, see Nourse et al (2005). Table 3).…”

“…For U/Pb geochronology a sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) instrument at the Stanford University was used; dating techniques in zircons were those reported by Williams (1998) and Nourse et al (2005). 40 Ar/ 39 Ar dating was carried out at the USGS in Denver, following the methodology described by Iriondo et al (2004).…”

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.…”