Martín Valencia-Moreno

A chemical and isotopic study of the Laramide granitic belt of northwestern Mexico: Identification of the southern edge of the North American Precambrian basement

Along the Laramide belt of northwestern Mexico, granitic rocks of similar bulk composition show isotopic and trace element signatures that help to delineate the position of the southern edge of the North American Precambrian basement. In the northern part, the Laramide plutons (the ‘‘northern granites’’) intruded Proterozoic crystalline rocks and a thick Late Proterozoic through Paleozoic miogeoclinal cover of North American affinity. In the central part, the granitic bodies (the ‘‘central granites’’) were emplaced into a sequence of Paleozoic eugeoclinal rocks overlain by Late Triassic clastic units. The southern part of the belt (the ‘‘southern granites’’) intruded a less-known crust characterized by middle to late Mesozoic island-arc‐related volcanic and sedimentary rocks of the Guerrero terrane. Data from a suite of metaluminous to slightly peraluminous calc-alkalic granitic rocks along the belt display north-to-south geochemical and isotopic variations, which could correlate with the type of intruded basement. The northern and central granites are characterized by strongly fractionated, light rare earth element (REE)‐enriched patterns, which display generally pronounced negative europium anomalies, whereas the southern granites have lower total REE enrichments and much flatter chondrite-normalized slopes displaying almost no europium anomalies. Isotopic results also suggest regional variations, as shown by the following initial Sr and eNd ranges: 0.7070 to 0.7089 and24.2 to25.4, respectively, for the northern granites; 0.7060 to 0.7079 and23.4 to25.1 for the central granites; and 0.7026 to 0.7062 and20.9 to 14.2 for the southern granites. On the basis of their isotopic similarities, the Proterozoic mafic to intermediate lower crust revealed by xenoliths from young volcanic flows in southern Arizona and northern Mexico is interpreted as a reasonable parental source for the northern and central granites; however, mantle-derived melts are not excluded. The more primitive southern granites are interpreted to come from a source that lacked Proterozoic basement. Instead, they were probably derived by mixing of juvenile mantle melts with partial melts of the lower parts of the Guerrero terrane. In general, the north-to-south compositional variations of the Laramide granitic rocks of northwestern Mexico reflect the crustal structure underneath the batholiths. The Sr and Nd data indicate that the edge of the North American Precambrian basement extends approximately southeastward from the coastal batholith of central Sonora; then, about 200 km south of Hermosillo in southern Sonora, the edge bends eastward and continues to the east beneath the Sierra Madre Occidental volcanic province.

Cretaceous and Tertiary sedimentary, magmatic, and tectonic evolution of north-central Sonora (Arizpe and Bacanuchi Quadrangles), northwest Mexico

The Arizpe and Bacanuchi Quadrangles provide a geologic history representative of the north-central part of Sonora, where lithologies are dominated by late Mesozoic and Cenozoic igneous rocks. In this study, new geologic mapping, 40Ar/39Ar dating, and geochemical analyses have been combined to provide a stratigraphic framework for this area. Ten lithostratigraphic units and several igneous and tectonic events can be recognized. The oldest outcropping rocks are Lower Cretaceous strata of the Bisbee Group, which along with the Picacho conglomerate record a middle Cretaceous compressive tectonic event and associated sedimentation. Laramide igneous activity is widespread and represented by (1) highly altered andesitic flows and volcaniclastic rocks (Arroyo Alcaparros andesitic rocks) of late Campanian to Maastrichtian age, (2) less altered andesitic and dacitic flows (Cerro Las Jarillas volcanic rocks) of late Paleocene age, and the intrusive bodies of (3) Sierra El Manzanal granodiorite and (4) Rancho Vaqueria quartz monzonite. The Sierra El Manzanal granodiorite was emplaced at ca. 68 Ma on the basis of a 40Ar/39Ar biotite age (67.97 ± 0.19 Ma) and cooled relatively rapidly according to less precise 40Ar/39Ar hornblende and K-feldspar ages from the same sample (64.8 ± 1.0 Ma and 62.8 ± 0.3 Ma, respectively). The Cerro Las Jarillas volcanic rocks are slightly younger (40Ar/39Ar biotite age of 58.67 ± 0.17 Ma). The Rancho Vaqueria quartz monzonite was emplaced at ca. 57 Ma (40Ar/39Ar biotite age of 56.73 ± 0.14 Ma and a less precise 40Ar/39Ar hornblende age of 55.0 ± 0.7 Ma); a protracted cooling history of this pluton is indicated by the age spectrum of K-feldspar from the same sample. A probable magmatic lull and denudation seem to have occurred between middle and late Eocene time and probably until the early Oligocene. Subsequently, rhyolitic to mafic volcanism began close to late Oligocene time and lasted until the early Miocene. Felsic volcanism is represented by the Cerro Cebadehuachi volcanic rocks, from which 40Ar/39Ar hornblende ages of 27.25 ± 0.09 and 27.32 ± 0.06 Ma and a biotite age of 26.97 ± 0.06 Ma were obtained at three different localities. The Mesa Pedregosa volcanic rocks represent the transition to younger, mafic volcanic activity that occurred during the late Oligocene, as indicated by a sanidine 40Ar/39Ar age of 25.48 ± 0.05 Ma. This late Oligocene and early Miocene magmatism was paired by two episodes of extensional deformation. The first phase is characterized by northwest-striking normal faults and folds, which expose the deepest structural levels of the area, and by the related basin fill, the Bacanuchi conglomerate. The second phase is represented by north-striking normal faults and by the syntectonic basin fill, the Arizpe conglomerate. Basaltic andesite volcanic flows at the base of the Arizpe conglomerate yielded 40Ar/39Ar (whole-rock) ages of 23.52 ± 0.17 and 21 ± 0.20 Ma. The extensional deformation (27 to 23 Ma) in the study area is coeval with the development of metamorphic core complexes in neighboring areas of Sonora and with the onset of extension in southern Sonora. The mafic volcanic rocks and clastic sedimentary units associated with this extension resemble the basin fills that in other parts of Sonora are assigned to the Baucarit Formation. Geochemical information from samples representing each of the igneous events displayed high-K calc-alkalic and mostly metaluminous compositions. The older units including the Arroyo Alcaparros andesitic rocks, the Cerro Las Jarillas volcanic rocks, the Sierra El Manzanal granodiorite, and the Rancho Vaqueria quartz monzonite are characterized by steep chondrite-normalized REE (rare earth element) slopes and generally well-developed negative Eu anomalies, suggesting garnet and plagioclase removal in the source. The younger igneous events including the Cerro Cebadehuachi and Mesa Pedregosa volcanic rocks, and the basaltic flows associated with the Arizpe conglomerate, showed basin-shaped REE slopes with no Eu anomalies, suggesting clinopyroxene or amphibole fractionation.

Major and trace element geochemistry and 40Ar/39Ar geochronology of Laramide plutonic rocks associated with gold-bearing Fe skarn deposits in Guerrero state, southern Mexico

Abstract Fe–Au skarn deposits related to intrusive centers, mostly of granodioritic composition, are widespread in southern Mexicos Guerrero state. These intrusive rocks are largely associated with the NW–SE-oriented Laramide magmatic belt that extends across most of western Mexico. The geochemical composition and ages of representative rocks from the Mezcala mining district in central Guerrero are studied to evaluate the petrogenetic aspects of the ore-related magmas. Some major and trace elements display nearly linear silica variation trends, which suggest a possible comagmatic origin. However, other elements have scattered distributions, possibly due to irregular mantle-to-crust magma mixing ratios, heterogeneities in the composition of the assimilated crustal material, or modifications during the emplacement or postemplacement processes. Major element chemistry indicates calc-alkalic metaluminous compositions, whereas trace element data suggest a volcanic arc tectonic setting, confirming that these rocks evolved from magmas generated above a subduction zone. Compared with the Laramide granites from the northern part of the belt in northwestern Mexico, which intruded a crust underlain by Proterozoic North American rocks, the studied samples are similar but relatively low in Nb and high in Sr, the middle rare earth elements (REE), P, and Zr. They also display minor Ti enrichments and a moderate depletion in the heavy REE. These characteristics may indicate a source of basaltic composition. New 40 Ar/ 39 Ar dating of granodiorites and dacite porphyries shows a north-to-south age progression from 66.2±0.8 Ma in the northern part of the belt to 62.2±0.7 Ma in the south. Moreover, the argon dates identify a younger postorogenic igneous event 35–30 Ma ago. This event is poorly documented and may have occurred after the extinction of the Laramide arc and prior to the mid-Tertiary Sierra Madre Occidental ignimbrite flare-up. On the basis of limited geochemical data, these rocks appear to be depleted in P 2 O 5 and Sr and enriched in U relative to the studied Laramide granites. A Fe skarn deposit located in Buena Vista de Cuellar, in the north central part of Guerrero, suggests that this magmatic pulse took place after the ore development of the Mezcala district.

Geochemistry of Laramide Granitic Rocks across the Southern Margin of the Paleozoic North American Continent, Central Sonora, Mexico

The geochemistry of Laramide granitic rocks from central Sonora was studied to relate possible compositional variations to the assimilation of different crustal sources. Most of the studied rocks are granodiorites collected near the southern boundary of the Paleozoic North American continent. North of this boundary, the Laramide plutons intruded a thick section of Upper Proterozoic and Paleozoic miogeoclinal strata, whereas south of it, the intrusives were emplaced in Lower Ordovician to Permian eugeoclinal rocks accreted to the continental margin between Late Permian and Middle Triassic times. Whole-rock Na2O, TiO2, and P2O5 contents are slightly higher in plutons intruded in cratonic and miogeoclinal rocks, whereas MgO and CaO seem to be higher in plutons emplaced in eugeoclinal rocks. The samples located in the north are characterized by steeper chondrite-normal-ized REE slopes and generally well developed negative europium anomalies, whereas the group of granitoids in the south have flatter REE slopes...