MAGMATIC AND STRUCTURAL EVOLUTION OF THE MAGDALENA-MADERA METAMORPHIC CORE COMPLEX, SONORA, MEXICO

We present the original geologic map of the Magdalena-Madera core complex (MMCC; Nourse, 1989) with new ICP-MS analyses of zircon that constrain its magmatic and structural evolution. The lower plate of the MMCC occupies several ranges north of the Magdalena de Kino (Sierras Magdalena, Guacomea, Madera, and Pinito). Framework rocks exposed in central and north areas include mid Jurassic granite (171.4±1.1 Ma), rhyolite porphyry (174 Ma; Anderson and Silver, 2005), and quartz arenite, overlain by Upper Jurassic (Glance equivalent) and Late Cretaceous (<84 Ma) conglomerate, and intruded by Late Cretaceous granodiorite (78±3—Anderson et al., 1980; and 65.9±2.9 Ma). These rocks preserve a penetrative N5-25E (and locally N90E) stretching lineation related to NNE-directed Laramide compression. Farther south the framework rocks are pervaded by multiple generations of biotite granite, 2-mica granite and pegmatite that display ubiquitous gently-dipping SW-vergent mylonitic S-C fabric with unidirectional S40-50W lineation. These granites were generated by melting of Proterozoic, Jurassic, and Late Cretaceous crust, producing high-U zircon overgrowths distinguished by dark rims visible in CL images. Age distributions and overprinting textures record emplacement of anatectic granite plutons at 61.2±1.8 Ma and 59.8±2.2 Ma, followed by growth of zircon at 41.7±0.5 Ma and 32.5±1.3 Ma, interpreted to represent magmatic culminations associated with intrusion of peraluminous leucogranite and pegmatite. We have also identified an isolated pendant of coarse-grained Grenville granite (1070.3±6.8 Ma) enveloped by mylonitized Tertiary granites in the southern Sierra Magdalena. Displaced hanging wall rocks include folded Lower Cretaceous Bisbee Group strata, Late Oligocene andesite, and early Miocene basalt and rhyolite (Miranda and De Jong, 1992). The MMCC thus preserves a history typical of southern Cordilleran metamorphic core complexes. Its footwall of preserves widespread Laramide fabrics developed in Late Cretaceous, Jurassic, and Proterozoic rocks. Crustal anatexis of the southern region is recorded by intrusion of mid Paleocene and mid Eocene-early Oligocene granite that preceded mylonitization and exhumation of the footwall along the normal-slip Magdalena-Madera detachment fault at ~25-20 Ma.