spe393-04 page 123 Geological Society of America Special Paper 393 2005 Contrasting Proterozoic basement complexes near the truncated margin of Laurentia, northwestern Sonora–Arizona international border region Jonathan A. Nourse* Department of Geological Sciences, California State Polytechnic University, Pomona, California 91768, USA Wayne R. Premo United States Geological Survey, Denver Federal Center, Denver, Colorado 80225, USA Alexander Iriondo Centro de Geosciencias, Universidad Nacional Autónoma de México, Campus Juriqilla, Querétaro 76230, Mexico Erin R. Stahl 164 El Camino Way, Claremont, California 91711, USA ABSTRACT We utilize new geological mapping, conventional isotope dilution–thermal ion- ization 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 north- west-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), medium- grained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphy- ritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and trans- position of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies *[email protected]. 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.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. 123–182, doi: 10.1130/2005.2393(04). For permission to copy, contact [email protected]. ©2005 Geological Society of America. 123 spe393-04 page 124 124 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 207Pb/206Pb 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 asso- ciated 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 bio- tite granite (1432 ± 6 Ma) and diabase dikes (1.1 Ga?). Emplacement of the granite was accompanied by secondary high-U overgrowths, dated at 1433 ± 8 Ma, on some of the Paleoproterozoic detrital zircons, and apparently was also responsible for resetting the whole-rock Pb isotopic systematics (1441 ± 39 Ma) within these Eastern Complex augen gneisses. Younger plutons emplaced into both Proterozoic basement complexes include medium-grained quartz diorite (73.4 ± 3.3 Ma and 72.8 ± 1.7 Ma), Late Cretaceous hornblende-biotite granodiorite, and Paleogene leucocratic biotite granite. Neogene sedimentary and volcanic strata overlie basement along unconformities that are tilted to the northeast, southeast, or southwest. A brittle normal fault, dipping gently northeast, juxtaposes Tertiary andesite with Paleoproterozoic metasandstone. These relationships suggest that the area shares a common history of mid-Tertiary extension with south- western Arizona. Later infl uence of the southern San Andreas fault system is implied by multiple dextral offsets of pre-Tertiary units across northwest-trending valleys. Our structural, geochronologic, and isotopic data provide new information to constrain pre–750 Ma Rodinia reconstructions involving southwestern Laurentia. Whole-rock U-Th-Pb and Rb-Sr isotopic systematics in both Paleoproterozoic gneiss complexes are disturbed, however, well-behaved Sm-Nd analyses preserve depleted initial εNd values (+2 to +4) that are distinct from the Mojave crustal province, but overlapping with the Yavapai and Mazatzal Provinces of Arizona. The East- ern Complex has the appropriate age and Nd isotopic signature to be part of the Mazatzal Province, but records major tectonism and metamorphism at ca. 1.6 Ga that postdates the Mazatzal orogeny. Deformed granitoids of the Western Complex have “Yavapai-type” ages and εNd but display structures discordant to the southwest- erly Yavapai trend in central Arizona. The Western Complex lies along-strike with similar-age rocks (1.77 Ga to 1.69 Ga) of the “Caborca block” that have only been studied in detail near Quitovac and south of Caborca. Collectively, these rocks form a northwest-trending strip of basement situated at the truncated edge of Laurentia. The present-day basement geography may refl ect an original oroclinal bend in the Yavapai orogenic belt. Alternatively, the western Proterozoic belt of Sonora may represent displaced fragments of basement juxtaposed against the Yavapai-Mazatzal Provinces along a younger sinistral transform fault (e.g., the Late Jurassic Mojave- Sonora megashear or the Permian Coahuila transform). Crustal blocks with these specifi c petrologic, geochronologic, and isotopic characteristics can be found in south- central and northeastern portions of the Australian Proterozoic basement, further supporting a connection between the two continents prior to breakup of the Rodinian supercontinent. Keywords: Sonora, Proterozoic, Rodinia, SHRIMP, zircon. spe393-04 page 125 Contrasting Proterozoic basement complexes 125 INTRODUCTION (presently obscured by a Cretaceous batholith) represents a Pro- terozoic suture or a younger strike-slip fault, such as the hypo- Precambrian crystalline rocks in the international border thetical Permian-Triassic “Coahuila transform” (Dickinson and region of northwestern Sonora and southwestern Arizona (Fig. 1) Lawton, 2001) or the Late Jurassic “Mojave-Sonora megashear” constitute the southwestward limit of Proterozoic basement (Silver and Anderson, 1974; Anderson and Silver, this volume). along the truncated margin of Laurentia near latitude 32°N. They To underscore the implications of various terrane juxtaposition also crop out near a poorly constrained, possibly disrupted inter- models for the confi guration of southwest Laurentia, we present section between the Mojave, Yavapai, and Mazatzal crustal prov- several alternative paleogeographic reconstructions. inces (Karlstrom and Bowring, 1988; Wooden and Miller, 1990; Wooden and DeWitt, 1991), and the Caborca block (Anderson BASEMENT GEOLOGY AND STRUCTURE and Silver, 1979, 1981; Iriondo et al., 2004; Anderson and Sil- ver, this volume). These diverse rocks and structures predate General Overview breakup of the Rodinia supercontinent at ca. 750 Ma (Stewart, 1972; Ross et al., 1989; Karlstrom et al., 2000). They occupy a Proterozoic crystalline rocks underlie rugged ranges on both strategic position with regard to paleogeographic reconstructions sides of Highway 2 in northwestern Sonora and compose several of the Rodinian and the Laurentian cratons. Integration of our small mountains or isolated hills north of the international bor- geological mapping, geochronology, and isotopic analyses with der in the Cabeza Prieta region (Fig. 2). These dark-weathering recent work on Proterozoic basement at Quitovac (Iriondo, 2001) exposures contrast markedly with light-pink Late Cretaceous– yields a new data set useful for evaluating which continent, e.g., early Tertiary biotite ± muscovite granite plutons of the Gunnery Antarctica (Moores, 1991), Australia (Karlstrom et al., 1999), Range batholith (Shafi qullah et al., 1980). Conspicuous, tilted Siberia (Sears and Price, 2000), or south China (Li et al., 1995) nonconformities separate the crystalline rocks from overlying was attached to southwestern Laurentia in the controversial Neogene sections and Quaternary basalt fl ows that become Rodinia reconstructions. The data also constrain the confi gura- increasingly abundant from west to east. tion of certain blocks of Proterozoic crust in Sonora (Fig. 1), and The Proterozoic exposures are geographically divided into offer a means to assess possible