Geología, geocronología y geoquímica de la Isla María Madre, Nayarit, México

Valerie Pompa-Mera¹*, Peter Schaaf², Teodoro Hernández-Treviño², Bodo Weber³, Gabriela Solís-Pichardo⁴, Daniel Villanueva-Lascurain¹, and Paul Layer⁵

¹ Posgrado en Ciencias de la Tierra, Instituto de Geofísica, Universidad Nacional Autónoma de México, 04510 México, D.F., México. *valerie@geofisica.unam.mx

² Laboratorio Universitario de Geoquímica Isotópica (LUGIS), Instituto de Geofísica, Universidad Nacional Autónoma de México, 04510 México, D.F., México.

³ Departamento de Geología, CICESE, Carretera Ensenada-Tijuana 3918, 22860 Ensenada, B.C., México.

⁵ Geophysical Institute and Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, AK, U.S.A.

Manuscript received: May 3, 2012
Corrected manuscript received: September 28, 2012
Manuscript accepted: October 3, 2012

ABSTRACT

Islas Marías archipelago is located 110 km NW of San Blas, Nayarit in the mouth of the Gulf of California. The archipelago is formed by San Juanito, María Madre, María Magdalena, and María Cleofas islands. The position of these islands represents a key pointfor paleogeographic and paleotectonic reconstructions of northwestern Mexico and of the tectonic evolution of Baja California Peninsula. María Madre is the largest island and covers an area of 145 km². This study presents the first detailed geological map of the island together with geochemical and geochronological data of its lithological units.

Isolated basement rocks are exposed along the western coast in the form of migmatites and orthogneisses of granodioritic to granitic compositions and middle Jurassic ages (163 -170 Ma). In the west-central part, a metasedimentary sequence, with biotite ± garnet paragneisses and folded calcsilicate rocks with or without garnet bands of unknown ages, is exposed as a roof pendant of the underlying Cretaceous intrusions (80.8 - 83.4 Ma) of tonalitic to granitic compositions. These granitoids are cut by mafic and pegmatitic dikes. We named the overall assemblage of metamorphic and plutonic rocks as "Papelillo Complex" due to the predominant outcrops in the homonymous canyon. The Papelillo Complex is overlain by ignimbrites, volcanic breccias, and lava flows of rhyolitic compositions. ⁴⁰Ar/³⁹Ar geochronology in sanidines from these rocks yielded Cretaceous (71.6 - 80.6 Ma) and Tertiary (55.4 Ma) ages, suggesting contemporaneous volcanic and plutonic activity. This inference is confirmed by similar REE patterns in both units with typical distributionsfor a subduction-related magmatic arc environment.

The lithologies of Isla María Madre are similar to those from the Los Cabos Block in Baja California Sur andfrom central Sinaloa. The overall area forms part ofa Cretaceous plutonic belt, which definitely rules out large latitudinal displacements for the southern Baja California Peninsula.

Key words: geochemistry, geochronology, paleogeography, Isla María Madre, Gulf of California, Mexico.

RESUMEN

El archipiélago de las Islas Marías se ubica a 110 km al noroeste de San Blas, Nayarit, en la desembocadura del Golfo de California. El archipiélago está formado por las islas San Juanito, María Madre, María Magdalena y María Cleofas. La ubicación de estas islas representa un punto clave para las reconstrucciones paleogeográficas y paleotectónicas del noroeste de México la evolución tectónica de la Península de Baja California. La Isla María Madre es la isla más grande y abarca una superficie de 145 km². Este estudio presenta el primer mapa geológico detallado de la isla, junto con datos geoquímicos y geocronológicos de sus unidades litológicas. Las rocas basales están expuestas a lo largo de la costa oeste y consisten en migmatitas y ortogneises de composición granodiorítica a granítica con edades del Jurásico Medio (163-170 Ma). En la parte centro-oeste, una secuencia metasedimentaria con paragneises de biotita ± granate y calco-silicatos deformados con o sin bandas de granate, de edades desconocidas, está expuesta como un colgante de los intrusivos cretácicos subyacientes (80.8-83.4 Ma) que son de composiciones tonalíticas a graníticas. Estos cuerpos están cortados por diques máficos y pegmatíticos. Al conjunto total de rocas metamórficas y plutónicas lo hemos nombrado "Complejo Papelillo" debido a sus afloramientos en el cañón homónimo. El Complejo Papelillo está sobreyacido por ignimbritas, brechas volcánicas y flujos de lava de composición riolítica. Geocronología con el método ⁴⁰Ar/³⁹Ar en sanidinos de estas rocas reveló edades cretácicas (71.6-80.6 Ma) y terciarias (55.4 Ma), sugiriendo una actividad volcánica contemporánea con el ensamble plutónico. Esta inferencia se confirmó mediante patrones similares de elementos de las Tierras Raras en ambas unidades con distribuciones típicas para un ambiente de arco magmático relacionado a subducción. La secuencia ígnea está cubierta en la parte este de la isla por depósitos marinos del Mioceno tardío - Pleistoceno? y localmente hacia el sur por una secuencia continental de areniscas del Mioceno temprano, similar a la observada en la isla vecina María Magdalena (nombrada informalmente arenisca Isla Magdalena). Los sedimentos marinos fueron nombrados informalmente "secuencia Ojo de Buey", dividida en un miembro inferior y uno superior. Edades de zircones detríticos de ambas unidades muestran picos mayores a ca. 83 Ma, que concuerdan con las edades de rocas plutónicas y volcánicas. En la formación Ojo de Buey no se encontraron edades más jóvenes. Por otra parte, zircones de la formación Isla Magdalena muestran un pico prominente a ~22 Ma, sugiriendo detritos de una fuente Ígnea distinta.

La litología de la Isla María Madre es muy similar a la del Bloque Los Cabos en Baja California Sur y a la de Sinaloa Central (Complejo Volcánico Inferior de la Sierra Madre Occidental). El área en su totalidad forma parte de un cinturón cretácico común, lo cual definitivamente excluye grandes desplazamientos latitudinales para el sur de la Península de Baja California.

Palabras clave: geoquímica, geocronología, paleogeografía, Isla María Madre, Golfo de California, México.

INTRODUCTION

Tectonically, the islands are placed in an intermediate position between the northwestern edge of the Middle America Trench and the southeastern limit of the Tamayo Fracture Zone of the Rivera Plate, with the Tres Marías Escarpment to the west and southwest (Figure 1a). This tectonic boundary between oceanic and continental lithosphere has a relief of more than 3000 m (Lonsdale, 1995). On the other hand, the ocean floor between the archipelago and mainland Mexico (Sinaloa and Nayarit) is a relatively shallow continental shelf with sedimentary basins (e.g., San Blas Trough, Figure 1a).

The geology of Islas Marías constitutes a key point for paleogeographic reconstructions of the Baja California Peninsula. The study and correlation between rocks of Baja California and adjacent provinces of Cordilleran Mexico also contributes to the understanding of the tectonic processes involved in the opening of the Gulf of California since Late Miocene. During the last decades, investigations on granitic rocks of Sinaloa (Henry and Fredrikson, 1987), Nayarit and Jalisco (Schaaf et al., 1995, 2000) as well as from the Los Cabos Block in Baja California Sur (Schaaf et al., 2000) pointed to a common magmatic evolution. These plutonic rocks are regarded as a southward continuation of the Cordilleran Batholiths of California (e.g., Peninsular Ranges) and Baja California (Henry et al., 2003).

In this paper, updated information on the geological units of Isla María Madre is provided, accompanied by a new geological map and a stratigraphic column of the island. Besides that, the first geochemical analyses of the most representative rock units as well as the first U-Pb and ⁴⁰Ar/³⁹Ar geochronological data are presented. The results are integrated in a preliminary model for the geological evolution of the archipelago and its paleotectonic position with respect to mainland Mexico and Baja California.

PREVIOUS WORK

Studies on the Islas Marías geology are sparse. Early works in the area are mostly paleontological studies (Grayson, 1871; Nelson, 1899). In 1925, members of the California Academy of Sciences visited the Tres Marías Islands and reported Miocene diatomites from various exposures as well as a brief description of the geology of María Madre and María Magdalena islands (Hanna and Grant, 1926; Jordan and Hertlein, 1926). Hertlein and Emerson (1959) performed a general description of macrofauna from material collected during the Puritan Expedition. Chinas (1963) published the first geological description of outcrops from all four islands, along with a geological map of Isla María Madre, and a list of fossils in Miocene sediments. The author suggested that the islands formed as the result of Mesozoic to Tertiary tectonic processes and that the granitic rocks might be contemporaneous to those from Jalisco and Baja California. Brunner (1971) identified Miocene micro-fossils in diatomitic sequences, and Ramírez-Rubio (1980) aimed at possible petroleum resources in Tertiary sediments around the Tres Marías Islands. In the last decades, several micropaleontological studies (Carreno et al., 1979; Carreno, 1985; Pérez-Guzmán, 1985; McCloy et al., 1988) focused on biostratigraphy, age, and paleoenvironments of Neogene sedimentary sequences. Repeated subsidence and uplift events that are recorded in Late Miocene to Pleistocene sediments were related to the opening of the Gulf of California (McCloy et al., 1988).

During the Deep Sea Drilling Project Leg 63 campaign, Site 473 was drilled approximately 90 km southwest of the Islas Marías Archipelago (Figure 1a). From this project resulted stratigraphical, bathymetrical, geochrono-logical, and geochemical data for the ocean floor as well as evidence for the initial opening of the Gulf of California (e.g., Bukry, 1978; Ness, 1982; Verma, 1983).

GEOLOGY OF ISLA MARÍA MADRE

A petrographic study of the different rock units was carried out in thin sections for rock classification and the results (Table 1) were also used to select samples for geo-chemical and geochronological investigations.

An outstanding morphological feature of Isla María Madre is a 600 m high, elongated mountain range in the central part, with the major axis oriented NW-SE. Steep slopes at the western part show excellent exposures of crystalline rocks. In the northern part, sedimentation and weathering processes have formed coastal deposits and terraces and, towards the east, colluvial and alluvial deposits control the morphology.

A remarkable attribute of María Madre Island is the lithological diversity found in an area of only 145 km². Metamorphic basement rocks (gneiss and metasediments), granitoids, volcanic units, and several types of sedimentary rocks are exposed (Figure 2).

Basement rocks

Outcrops of metamorphic basement rocks occur in a small segment along the western coast between Papelillo and Camaron (Figure 2) and are mainly formed by orthogneiss and paragneiss with different metamorphic textures.

Orthogneiss

Two orthogneiss types were distinguished in the field. The first one is represented by quartz-rich, folded, and banded migmatites (sample 0915, Figure 3a; sample 0939), which are coarse- to medium-grained, with 2 mm to ~5 cm thick leucocratic and mesocratic bands. The neosome is composed of quartz, potassium feldspar, plagioclase, biotite, garnet, and occasionally muscovite. The melano-some is formed by quartz, plagioclase, aligned biotite, and chlorite. Less abundant minerals are oxides, apatite, and zircon. A heterogranular mosaic with slightly strained biotite and quartz defines the texture of these rocks. Some parts of sample 0939 present millimeter- to centimeter-sized garnet cumulates.

Metasedimentary rocks

Outcrops of metasedimentary rocks, which are affected by contact metamorphism, are also restricted to the western sector of the island (Figure 2), three kilometers to the NE of Papelillo in the homonymous canyon. Four different units were identified:

1) At the slopes of the Papelillo canyon, biotite-garnet-bearing paragneisses are exposed (Figure 4a) in several locations (e.g., sample 0909). The dark grey colored rocks have a massive and homogeneous appearance in the field. In thin section they display granolepidoblastic textures with quartz, abundant biotite, potassium feldspar, garnet, plagioclase, pyrite, and zircon. This unit is in contact with a tonalitic body (Figure 4a).

2) Intensely folded calcsilicates are composed of wl-lastonite, clinopyroxene, dolomite, quartz, and plagioclase (sample 0908; Figure 4b). The calcsilicate rocks together with the paragneisses mentioned above are suggestive of contact metamorphism.

3) Close to the contact with the younger plutonic rocks, outcrops of calcsilicates with a granoblastic mosaic of fine-grained wollastonite, dolomite and quartz bands were observed (Figure 4c).

4) The immediate contact of the calcsilicates with the intrusive rocks is characterized by occurrences of exo-skarn, composed of garnet, clinopyroxene (diopside), wollastonite, and quartz (sample 0910; Figure 4d). The skarn is reddish and has a granoblastic texture with monomineralic zones of garnet and wollastonite. Clinopyroxene is observed as inclusions in garnet bands or as glomerocrystals in wol-lastonite bands.

The metasedimentary assemblage of the Papelillo canyon is interpreted as a roof pendant in the granitoid rocks described in the following section.

Intrusive rocks

Outcrops of intrusive rocks are widely distributed along the western coast and in the slopes of Isla María Madre (Figure 2), and they probably underlie large parts of the island. Undeformed granites, granodiorites, and tonalites (lithologies from petrographical observations) are the most abundant plutonic rocks, and some minor hypabyssal bodies and dikes also occur.

On the other hand, granites (sensu strictu) are abundant in the lower Papelillo canyon and along the southwestern coast (Figure 5c). As with the tonalites, the granites are affected by two normal fault systems that dip to the NW (312°/59°) and to the SW (256°/71°). The granites (samples 0932 and 1004) are massive, grey-to pink-colored, fine- to medium-grained, with granular textures, and are formed by potassium feldspar, quartz, and plagioclase. Accessory components are chlorite, biotite, opaque minerals, pyroxene, and secondary muscovite and epidote. Quartz is anhedral and potassium feldspar occasionally displays microcline twinning.

Granodiorites are found along the western coast, near the entrance to the Papelillo base camp. These rocks are exposed as a massive, light grey hypabyssal body (sample 0902, Figure 5d), with granophyric texture, formed by a microcrystalline matrix of quartz aggregates (enclosing larger crystals of anhedral quartz), potassium feldspar, twinned plagioclase, opaque minerals, and hornblende. Feldspars display sericitic alteration. In the Papelillo canyon, a NNE-SSW trending structure (Figure 2) seems to separate the granites from the other plutonic units previously presented.

Abundant outcrops of 10-250 m thick mafic dikes occur at the southern coast of Isla María Madre, close to Laguna del Toro (Figure 2). A group of these dikes was macroscopically classified as diabase (sample 1002, Figure 5e) with dark grey to greenish grey colors and aphanitic textures, affected by faults, fractures, and hydrothermal processes. Plagioclase crystals are subhedral, show albite twinning, are partly sericitized, and are set in a fine-grained matrix of hornblende and pyroxene. Chlorite and oxides partly replace hornblende.

Along the road from Laguna del Toro to Papelillo, a large hydrothermally altered mafic dike is exposed (sample 0901, Figure 5f) with grey to greenish colors and a porphyric texture. Recognized mineral phases are plagioclase and amphibole, frequently replaced by chlorite. Other secondary phases are epidote, chalcopyrite, and opaque minerals in a silicic microcrystalline groundmass. Hydrothermal processes have often destroyed the original textures of these rocks, which probably represent the youngest magmatic event in the island.

Volcanic rocks and several sedimentary sequences overlie the metamorphic and plutonic rocks.

Rhyolitic volcanic rocks

Volcanic rocks are broadly distributed in the western and central part of the island and they are mainly of rhyo-litic composition (Figure 2; Table 1). Three volcanic units are distinguished: 1) ignimbrites and breccias, 2) massive, undeformed lava flows, and 3) strongly fractured and faulted lava flows.

Ignimbrites occur in the central and northwestern parts of Isla María Madre and in the Papelillo canyon (Figure 6a). The most voluminous outcrops are exposed in the highest parts of the island between Microondas and El Mirador (Figure 2) in the form of reddish brown, banded rhyolites (sample 0905 and 0904). The texture is eutaxitic with a vitreous groundmass, quartz and potassium feldspar (sanidine) as major components, and rare plagioclase. Accessory phases are pyroxene, oxides, apatite, and zircon. Similar ignimbrites occur in the northwestern part of the island (e.g., sample 0920). In the Papelillo canyon, ignimbrites are in contact with tonalites along a normal fault plane (sample 0933B; Figure 6b) that dips to the NE (021°/68°). The exposed rocks correspond to a massive, reddish brown pyroclastic (rhyolitic) breccia, composed of a vitreous groundmass, which contains some metamorphic lithics. Major mineral components are potassium feldspar, plagioclase, and quartz. Accessory phases are hornblende, pyroxene, opaque minerals, chlorite, sericite, apatite, and zircon.

Undeformed lava flows are exposed in two locations on the island: in the south-eastern part, south of El Mirador (Figure 2), massive rhyolitic lava flows of light grey color and eutaxitic texture are characterized by a vitreous matrix that contains sanidine phenocrysts, plagioclase, quartz, and alteration products such as epidote, sericite, and chlorite. Zircon, apatite and opaque minerals are present as accessory phases (sample 1010). A reddish to purple rhyolitic lava flow (sample 0945A) is exposed along the road that connects the Rehilete with Bugambilias camps. These rocks are massive, with porphyritic texture and a microcrystalline silicic groundmass with potassium feldspar, quartz, plagioclase, pyroxene, oxides, and hornblende, along with sericitic and chloritic alterations. This lava flow is strongly affected by hydrothermal alteration.

Sedimentary rocks

The most widespread rocks in Isla María Madre are sedimentary sequences of marine and continental origin that are informally grouped into the Isla Magdalena sandstone and Ojo de Buey sequence.

Clastic sedimentary rocks (Isla Magdalena sandstone)

Exposures of sandstone strata in the southern part of Isla María Madre (Figure 7a) are correlated in this work to a sedimentary sequence observed in the neighboring María Magdalena Island, and is designated here as Isla Magdalena sandstone. In María Madre, these rocks crop out at the beach section located south of Laguna del Toro (sample 1003; Figure 2). Strata planes dip to the NE (70°/28°) and the overall sequence is affected by normal faults that dip to the SW (148780°, Figure 7b). The sandstones show terrigenous components such as quartz, potassium feldspar, plagioclase and minor calcite, biotite, and chlorite as well as some reworked foraminifera fragments in a silicic and slightly consolidated clay matrix. Hydrothermal processes (Figure 7c) affect the western segment of these sedimentary rocks.

Ojo de Buey sequence

Due to its characteristic type locality, we named a group of sedimentary sequences, studied previously by Carreno (1985) and McCloy et al. (1988), as "Ojo de Buey" sequence (Figure 8a). These authors investigated in detail three specific sections in the NE part of the island called Balleto Road, Playa Caleras, and Arroyo Hondo (Figure 2). In this work, we divided the Ojo de Buey sequence according to its deformation structures and stratigraphic position into 1) lower Ojo de Buey sequence and 2) upper Ojo de Buey sequence (Figure 8b).

Marine strata of the lower Ojo de Buey sequence crop out mostly in the northeastern part of the island. Along the northern coast (Figure 2), it is folded and affected (Figure 8b and c) by two normal major fault systems that dip 033°/42° NE and an inverse fault system that dips 029759° NE. It is composed of fine- to medium-grained, sandstone with quartz, plagioclase, potassium feldspar, minor biotite, chlorite, epidote, and scarce fragments of reworked foraminifera (sample 0917). All these components are sparite-cemented and show some intercalated carbonaceous horizons. This part of the unit can be correlated with the sequence exposed on the road from Caleras Beach to Arroyo Hondo (McCloy et al., 1988). In the Caleras Beach section (Figure 2; Carreno, 1985), strata are composed of a light brown to white, folded, micritic-cemented limestone formed by clamshells and bioclasts with fragments of foraminifera, bryozoans, mollusks, red algae, echinoderms, and minor biogenic silica (sample 0948). The contact between the lower Ojo de Buey sequence and the overlying upper Ojo de Buey sequence is exposed as an angular unconformity (Figure 8b).

Outcrops of the upper Ojo de Buey sequence are mainly located along the eastern and southern coasts of the island and consist of light brown, medium-grained sandstone (Figure 8d) that dips gently to the NW (296712°). Strata are made up by mid to lower bathyal sandstone, siltstone and foraminiferous limestone (McCloy et al. 1988). Along the unpaved road from Balleto to Aserradero, micrite-sparite cemented fine-grained sandstones (sample 0944) are gently folded with strata that dips to the SW (091°/10°). This sequence is formed by quartz, plagioclase, sanidine, biotite, altered amphibole, and oxides together with minor foraminifera fragments. Occasionally, ash layers were observed in this unit.

Further to the south, at Defines beach, near Laguna del Toro, a coarse grained, well-sorted and indurated polymictic conglomerate forms part of the Upper Ojo de Buey sequence (Figure 8e). The clasts are of volcanic rocks and subordinate plutonic and metamorphic components, supported by a micrite-cemented sandy matrix with the occurrence of reworked fragments of foraminifera (sample 0929) as well as mollusks and gasteropods. Chinas (1963) assigned a Plio-Pleistocene age to these rocks.

GEOCHEMISTRY

Geochemical, isotopic and petrological aspects of Islas Marías rocks will be discussed in detail in a forthcoming paper, but some preliminary geochemical data of igneous and metamorphic units from Isla María Madre are presented here as an overview.

Analytical procedures

Sampling locations are shown in Figure 2 and coordinates are given in supplementary Table A1. Only fresh rocks were processed by crushing in a jawbreaker, grinding in a roller mill, and pulverizing with a steel mill set. Analyses of major elements were performed for 35 samples by X-ray fluorescence spectrometry using a Siemens SRS-3000 instrument at the Laboratorio Universitario de geoquímica Isotopica (LUGIS), Instituto de Geología, Universidad Nacional Autonoma de Mexico (UNAM), according to the procedures described by Lozano-Santa Cruz and Bernal (2005). For 28 of these samples we obtained trace element data by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Table A1) using a Thermo Series XII instrument at the Centro de Geociencias (CGEO), Juriquilla, UNAM, following the sample preparation and measurement procedures described by Mori et al. (2007).

Basement and plutonic rocks

Six orthogneiss samples of granodioritic to granitic composition (SiO₂: 61.6 - 75.0 wt.%, Table A1), a biotite-garnet-paragneiss (sample 0909; SiO₂: 67.4 wt.%), and a biotite-paragneiss (sample 0912; SiO₂: 62.3 wt.%) from the metamorphic basement were analyzed. Nine plutonic rocks, including a pegmatitic dike (sample 0941), have dioritic, tonalitic, granodioritic, and granitic compositions (SiO₂ 56.2-71.7 wt.%) in the total alkali vs. silica diagram (TAS; Cox et al., 1979, Figure 9a) . Most rocks plot in the sub-alkalic field, with the exception of a leucocratic, nearly aplitic alkaline granite (sample 0932), which shows sericitic and chloritic alterations. Mafic dikes and inclusions (samples 0901, 0923, 0924A, 0952, and 1002) generally show gabbroic compositions. With respect to trace elements, 13 intrusive rocks (granite, granodiorite, tonalite, and mafic dikes) plot in the subduction-related Volcanic-Arc-Granite field in the Y+Nb vs. Rb discrimination diagram (Figure 9b; Pearce et al., 1984). Chondrite-normalized rare earth element (REE) patterns for these samples show enrichment for Light REE, frequently seawater derived negative Ce anomalies, and the presence (granite 0932 and pegmatitic dike 0913) or absence (0914 and 0902) of negative Eu anomalies (Figure 9c). The latter corresponds to tonalite and granodiorite samples with obviously less magmatic differentiation with respect to the granite and pegmatitic dike samples. The overall REE spectrum is typical for subduction related continental arc granites. The Laguna del Toro mafic dike (sample 0901; Figure 2) shows less enriched LREE concentrations and no negative Eu anomaly, common in less evolved magmas.

Rhyolitic volcanic rocks

Nine volcanic rocks were analyzed for major elements and additional trace element data were obtained for seven of these rocks (Table A1). In general, the Isla María Madre volcanic sequence is composed of rhyolitic rocks (SiO₂ between 61.6 and 76.7 wt.%), with the exception of one trachydacite (Table A1; Figure 10a). Since rhyolite sample 0945A shows influence of hydrothermal alterations, it was not plotted in the TAS diagram of Figure 10a.

GEOCHRONOLOGY

Analytical procedures

U-Pb analyses

For age determinations with the U-Pb method, we selected zircons from a total of seven samples, including

two gneisses (0915 and 0916), two plutonic rocks (0911B and 0932), two sandstones from the Ojo de Buey sequence (0917B and 0948) and a sample from the Isla Magdalena sandstone, south of Laguna del Toro (1003). Sample locations are shown in Figure 2 and Table A1. The rocks were crushed, milled and sieved, and zircons were separated from the <80 mesh fraction by standard procedures at LUGIS-UNAM, by using a Wilfley® table or a gravitational zircon separation technique, a Frantz® isodynamic separator, heavy liquids, and handpicking techniques.

U-Pb geochronology of zircons was conducted by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the Arizona LaserChron Center (Gehrels et al., 2008). The analyses involve ablation of zircon with a Photon Machines Analyte G2 Excimer laser (samples 0911B, 0915, 0916, and 1003) or a New Wave UP193HE Excimer laser (samples 0932, 0917B, and 0948) using a spot diameter of 30 microns. The ablated material is carried in helium into the plasma source of a Nu HR ICPMS, which is equipped with a flight tube of sufficient width in order to analyze U, Th, and Pb isotopes simultaneously. All measurements are made in static mode, using Faraday detectors with 3×10¹¹ Ohm resistors for ²³⁸U, ²³²Th, ²⁰⁸Pb-²⁰⁶Pb, and discrete dynode ion counters for ²⁰⁴Pb and ²⁰²Hg. Each analysis consisted of one 15-second integration on peaks with the laser off (for backgrounds), 15 one-second integrations with the laser firing, and a 30-second delay to purge the previous sample and prepare for the next analysis.

For each analysis, the errors in determining ²⁰⁶Pb/²³⁸U and ²⁰⁶Pb/²⁰⁴Pb result in a measurement error of ~1-2% (at 2-sigma level) in the ²⁰⁶Pb/²³⁸U age. The errors for ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁶Pb/²⁰⁴Pb also result in ~1-2% (at 2-sigma level) uncertainty in age for grains that are >1.0 Ga, but are substantially larger for younger grains due to low intensity of the ²⁰⁷Pb signal.

The interference of ²⁰⁴Hg on ²⁰⁴Pb was accounted for by measuring ²⁰²Hg during laser ablation and subtraction of ²⁰⁴Hg according to the natural ²⁰²Hg/²⁰⁴Hg ratio of 4.35. This Hg correction is not significant for most analyses because the Hg backgrounds are low (generally ~150 counts per second at mass 204).

Inter-element fractionation of Pb/U is generally ~5%, whereas apparent fractionation of Pb isotopes is generally <0.2%. In-run analyses of fragments of a large zircon crystal (generally every fifth measurement) of a known age of 563.5 ± 3.2 Ma (2-sigma error) were used to correct for this fractionation. The uncertainty resulting from the calibration correction is generally 1-2% (2-sigma) for both ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁶Pb/²³⁸U ages. Concentrations of U and Th were calibrated relative to a Sri Lanka zircon standard, which contains ~518 ppm of U and 68 ppm Th.

The analytical data are reported in supplementary Table A2. Uncertainties shown in this table are at the 1-sigma level, and include only measurement errors.

The resulting interpreted ages are shown on Pb*/U concordia diagrams and weighted mean diagrams (igneous and metaigneous rocks) or relative age-probability diagrams (detrital zircons) using the routines in Isoplot (Ludwig, 2008). The weighted mean diagrams show the weighted mean (weighting according to the square of the internal uncertainties), the uncertainty of the weighted mean, the external (systematic) uncertainty that corresponds to the ages used, and the final uncertainty of the age (determined by quadratic addition of the weighted mean and external uncertainties).

⁴⁰Ar/³⁹Ar analyses

For ⁴⁰Ar/³⁹Ar analysis, we selected four volcanic rocks (0921, 1010, 0904, and 0905) that were crushed and sieved. From these units sanidine crystals were separated using a Frantz® isodynamic separator and heavy liquid techniques at LUGIS-UNAM. Subsequently the samples were submitted to the Geochronology Laboratory at the University of Alaska in Fairbanks, where they were further purified. The monitor mineral MMhb-1 (Samson and Alexander, 1987) with an age of 513.9 Ma (Lanphere and Dalrymple, 2000) was used to monitor the neutron flux (and to calculate the irradiation parameter J). Samples and standards were wrapped in aluminum foil and loaded into aluminum cans of 2.5 cm diameter and 6 cm height. The samples were irradiated in position 5c of the uranium enriched research reactor of McMaster University in Hamilton, Ontario, Canada, for 20 Megawatt-hours. Upon their return from the reactor, the samples and monitors were loaded into 2 mm diameter holes in a copper tray that was then loaded in an ultra-high vacuum extraction line. The monitors were fused and samples heated, using a 6-Watt argon-ion laser, following the technique described in York et al. (1981), Layer et al. (1987) and Layer (2000). Argon purification was achieved using a liquid nitrogen cold trap and a SAES Zr-Al getter at 400°C. The samples were analyzed in a VG-3600 mass spectrometer at the Geophysical Institute, University of Alaska, Fairbanks. The measured argon isotope ratios were corrected for system blank and mass discrimination, as well as calcium, potassium and chlorine interference reactions following procedures outlined in McDougall and Harrison (1999). System blanks generally were 2×10^-16 mol ⁴⁰Ar and 2×10^-18 mol ³⁶Ar, which are 10 to 50 times smaller than fraction volumes. Mass discrimination was monitored by running both calibrated air shots and a zero-age glass sample. These measurements were made on a weekly to monthly basis to check for changes in mass discrimination.

A summary of the ⁴⁰Ar/³⁹Ar results is given in Table 2, with all ages quoted to the ± 1 sigma level and calculated using the constants of Steiger and Jäger (1977). The integrated age is the age given by the total gas measured and is equivalent to a potassium-argon (K-Ar) age. The spectrum provides a plateau age if three or more consecutive gas fractions represent at least 50% of the total gas release and are within two standard deviations of each other (Mean Square Weighted Deviation less than ~2.5). Each sample was run twice to both confirm age determinations and to optimize analytical precision.

Results

U-Pb single zircon ages

The migmatitic orthogneiss (sample 0915) in the western part of Isla María Madre contains zircons with heterogeneous morphologies, including prismatic and medium-elongated crystals. The separated grains range from 50 to 300 Lim in size (Figure 11a and 11b). A total of 36 zircon grains were analyzed (Table A2). Twenty concordant grains with apparent ²⁰⁶Pb/²³⁸U ages between 152 and 171 Ma yield a concordia age of 162.7 ± 2.9 Ma (95% confidence including 1.5% systematic error; Figure 11c), which is interpreted as the time of igneous protolith crystallization in the Middle Jurassic (Callovian). Concordant U-Pb data of two spots on zircon rims probably reflect the time of metamorphism between 83 and 87 Ma, whereas six spots with apparent ²⁰⁶Pb/²³⁸U ages from 116 to 145 Ma document either Pb-loss or zircon zones with mixed ages. Another eight laser spots contain varying amounts of inherited components yielding mostly discordant ages. The minimum ages of these spots, however, indicate inherited zircon components from late Mesoproterozoic to early Paleozoic sources.

The concordia diagram for a tonalite (sample 0911B), that intrudes the metasedimentary sequence exposed in the papelillo canyon, is shown in Figure 13a. The apparent ²⁰⁶Pb/²³⁸U ages of 22 analyzed spots range from 70.5 to 83.3 Ma. No older components were found. The crystallization age of this sample is interpreted from these 22 spots at 80.1^+1.4/_-1.3 Ma (Campanian, Figure 13b) by using the TuffZirc algorithm (Ludwig and Mundil, 2002) and adding a systematic error of 1.5%.

The leucocratic granite 0932 was collected from the western coast between Laguna del Toro and papelillo (Figure 2). Apparent ²⁰⁶pb/²³⁸U ages of 27 analyzed spots range from 74.4 to 90.5 Ma (Figure 13c; Table A2). Another eight analyzed spots reflect inherited components yielding apparent ²⁰⁶Pb/²³⁸U ages from about 97 to 316 Ma. The crystallization age of this granite is interpreted from 23 spots at 84.2 ^+3.1/._-2.5 Ma (calculated with TuffZirc algorithm, adding 1.9% of systematic error; Figure13d), which is within errors of the age of the tonalite 0911B.

Both granitoid rock ages are consistent with the ages obtained from metamorphic rims of the orthogneiss samples (see above).

A coarse to medium grained sandstone from the lower Ojo de Buey sequence (sample 0917B) was selected for detrital zircon provenance analyses. Sixty out of 65 analyzed zircon grains (Table A2) have Cretaceous ²⁰⁶pb/²³⁸U ages between 68 and 121 Ma (Figure 14a). The most prominent peak in the relative probability distribution is at 83.4 Ma (Figure 14b), which is similar to the age obtained for the granitoids (Figure 13c, d). Another important group of 21 detrital zircons show Aptian-Albian ages from ~100 to ~121 Ma with two peaks at ~103 and – 111 Ma. The five remaining grains have paleozoic and Mesoproterozoic ages from 249 to 451 Ma, and at 1282 and 1498 Ma (Table A2).

From the overlaying upper Ojo de Buey sequence, a fine to medium-grained sandstone (sample 0948) was collected from the Arroyo Hondo Section (Carreno, 1985). Seventy-eight detrital zircon grains were analyzed and their isotopic ratios are plotted in a Tera-Wasserburg diagram (Figure 14c). Apparent ²⁰⁶Pb/²³⁸U ages of 73 zircon grains range from 70 to 100 Ma and define a prominent peak at ca. 83 Ma (Figure 14d), suggesting that the zircons in these sediments almost entirely originated from the Upper Cretaceous igneous rocks. The remaining five analyzed zircons document Aptian-Albian and Middle Jurassic sources.

A sandstone from the Isla Magdalena sandstone (sample 1003) was collected south of Laguna del Toro. Twenty-two out of 25 grains present Tertiary ²⁰⁶Pb/²³⁸U ages between 20 and 42 Ma (Figure 14e). The relative probability plot (Figure 14f) shows a prominent peak at 22 Ma and two minor peaks at 27 Ma and 34 Ma. In contrast to the Ojo de Buey sequence, only one zircon has an apparent Upper Cretaceous ²⁰⁶Pb/²³⁸U age of ~87 Ma. Based on field observations we suggest that this clastic unit forms part of the Isla Magdalena sandstone, a dominant sedimentary sequence of the homonymous island to the southeast.

⁴⁰Ar/³⁹Ar sanidine ages

⁴⁰Ar/³⁹Ar geochronology was performed on sanidines from three rhyolites and a trachydacite of the Isla María Madre volcanic sequence. Results are summarized in Table 2 and the obtained age spectra are shown in Figure 15. Detailed analytical data is given in supplementary Table A3. Trachydacite (0905) and rhyolite (0904) samples correspond to the massive ignimbrites from the central and highest part of the island (Figure 2). The obtained plateau ages are 80.6 ± 1.6 Ma and 76.6 ± 1.4 Ma (1rj), respectively (Figura15a and b). Integrated ages (80.6 ± 2.0 Ma and 75.9 ± 1.5 Ma) and isochron ages (81.2 ± 1.7 Ma and 77.6 ± 3.0 Ma) are all within errors. From a rhyolitic lava flow sampled in the east-central part (sample 1010), a sanidine plateau age of 71.6 ±1.9 Ma was calculated (Figure 15c; intergrated age 82.6 ± 3.3 Ma; isochron age 63.0 ± 5.1 Ma). It is noteworthy, that these sanidine ages are comparable to the U-Pb zircon ages of the plutonic rocks, suggesting a contemporaneous or slightly younger magmatic event for the volcanic rocks, which is supported by field observations (Figure 6d).

From the fractured and deformed rhyolithic lava flow in the northwestern part of the island (sample 0921), a Lower Tertiary plateau age of 55.4 ± 2.4 Ma was obtained (Figure 15d; integrated age 55.5 ± 2.5 Ma; isochron age 57.7 ± 3.7 Ma).

DISCUSSION

The large lithological variety of Isla María María Madre rocks provides an excellent local stratigraphic record. This includes regional and contact metamorphism, a Jurassic-Cretaceous-Tertiary magmatic history and sedimentation processes with different detrital sources, witnessing multiple subsidence and uplift events related to the opening of the Gulf of California. Based on field observations and the available geochemical and geochronological data, we developed stratigraphic columns for the southern and central part of the island (Figure 16), summarized as follows:

Folded, banded, and locally migmatitic orthogneisses form the oldest exposed basement rocks in the island. These rocks were affected by regional medium- to high-grade metamorphism, locally with anatexis.

Metasedimentary rocks such as calcsilicates and paragneisses are considered as roof pendants of the underlying plutonic rocks. They are affected by contact metamor-phism evidenced by abundant skarn in calcsilicates. The age of the sedimentary precursor is unknown, but field observations suggest that it is younger than the gneissic basement.

Plutonic rocks were emplaced during the Late Cretaceous. Pegmatitic dikes represent the ultimate magmatic phase. Mafic dikes with dioritic compositions commonly fill fracture zones within the granitoids, and hence, they are younger than the granitoids.

These three lithological units constitute the "Papelillo Complex", named by its type locality in the Papelillo canyon (Figure 2).

The Papelillo Complex is unconformably overlain by silicic volcanic rocks in the shape of effusive and explosive ignimbrites and lava flows of rhyolithic composition. Effusive rocks can be observed in the highest elevations of the island and are highly tectonized in the southern part. In many locations, these rocks are affected by hydrothermal and deformation processes. According to the obtained ⁴⁰Ar/³⁹Ar ages, these rocks are contemporaneous or younger than the plutonic suite.

The Isla Magdalena sandstone constitutes the oldest sedimentary sequence of Isla María Madre. Although its detrital zircons are considerably younger than those from the overlying Ojo de Buey sequence, its lower stratigraphic position is indicated by field observations (Figure 8c).

Miocene through Pleistocene sedimentary sequences (lower and upper members of the Ojo de Buey sequence) form the most abundant rocks exposed in Isla María Madre. Their foraminiferal and diatomic fossil content characterizes these units as marine to shallow marine sequences, intercalated with some non-marine sandstones (McCloy et al., 1988).

Both sedimentary sequences (Ojo de Buey sequence and Isla Magdalena sandstone) contain different detrital material. U-Pb zircon ages from the stratigraphically older Isla Magdalena sandstones (Figure 7c) show a dominant peak at 22 Ma (Figure 14f) but no contemporaneous igneous rocks have been identified so far on María Madre and María Magdalena islands. On the other hand, Duque et al. (2012) have recently reported similar U-Pb zircon and ⁴⁰Ar/³⁹Ar biotite ages between 18.1 and 25.6 Ma for granitic rocks from Santa Catalina Island, Baja California Sur, from the Pescadero basin of the Gulf of California ocean floor, and from southern Sinaloa and Nayarit. Such rocks are potential sources for the detritus in the Isla Magdalena sandstone. It is obvious from the zircon data presented here, that these sources did not participate in the sedimentation of the Ojo de Buey sequence. In this Upper Miocene sequence (McCloy et al., 1988), most detrital zircons are of Campanian age (Figuras 14b and d) and probably shed from eroding plutonic and volcanic series of Isla María Madre.

The relative ages among basement rocks are not completely well established, but similar banded, folded, and migmatitic orthogneisses and calcsilicate metasediments were reported from southern Sinaloa (Henry and Fredrikson 1987) with Jurassic and Jurassic-Lower Cretaceous ages, respectively. Recently Cuellar-Cardenas et al. (2012) reported a 157 Ma zircon age (U-Pb) from an orthogneiss NE of Mazatlan, Sinaloa. Like those from Isla María Madre, these units were also affected by contact and possibly regional metamorphism and are associated with Cretaceous and Tertiary plutonic rocks. In the Los Cabos Block, Baja California Sur, migmatitic gneisses, deformed granitoids, and calcsilicates have been recognized (Schaaf et al., 2000) and are described in detail by Pérez-Venzor (2012).

On the other hand, the undeformed Isla María Madre plutonic rocks share geochemical and geochronological similarities with their counterparts in Sinaloa (post-tectonic intrusions; Henry and Fredrikson, 1987, Henry et al., 2003), the Los Cabos Block (LCB; Schaaf et al., 2000), and the Puerto Vallarta Batholith (Schaaf et al., 1995). The composition of these granitoids range also from tonalite to granite and their ages are consistent with those from the María Madre granites. In Sinaloa, Henry et al. (2003) reported continuous granite emplacement ages between 90 and 45 Ma (U-Pb and K-Ar ages), in the southeastern LCB, Fletcher et al. (2007) published U-Pb zircon ages from 75 to 90 Ma, and Pérez-Venzor (2012) obtained intrusion ages of ca. 80 Ma (Rb-Sr WR isochrons) for central LCB plutonic rocks. For the Puerto Vallarta Batholith, U-Pb, Rb-Sr, and K-Ar mineral and whole rock ages show a range between 101 and 75 Ma (Schaaf et al., 1995; Fletcher et al., 2007). All intrusions, including those from Isla María Madre, display volcanic-arc environments and are considered as a part of a continuous Cretaceous plutonic belt, extending from northern Baja California (Peninsular Ranges Batholith) to western Mexico, at least to the latitudes of the Jalisco Block. In the light of the new data on Islas Marías igneous rocks, large latitudinal displacements of Baja California Sur as proposed by Beck (1980) or Sedlock et al. (1993) can be ruled out.

The Cretaceous and Tertiary ages from María Madre volcanic rocks can be correlated with rhyolitic flows and quartz-latites from the Lower Volcanic Complex (McDowell and Keizer, 1977) of southern Sinaloa, which are described in several publications (Drumble, 1900; Weed, 1902; Hisazumi, 1929; De Cserna and Kent, 1961; Roldán-Quintana and Clark, 1974; Clark, 1976). Henry and Fredrikson (1987) considered them as volcanic equivalents of plutonic rocks. The Lower Volcanic Complex is widely distributed in northwestern Mexico (McDowell and Keizer, 1977) including rocks of the Peninsular Ranges batholiths of Baja California and its extension into Sinaloa (McDowell and Clabaugh, 1979) with ages from 90 to 40 Ma (e.g.,Damon et al., 1983).

The isolated ash layers in the Ojo de Buey sequence and mafic dikes (samples 0901 and 1002) from the southern María Madre coast are still undated, but are considered to represent the youngest volcanic event on the island.

CONCLUSIONS

This work presents for the first time a detailed geological map of María Madre Island, accompanied by an explicit description of the observed geological units and by geo-chemical and geochronological investigations. Despite its small area of only 145 km², a large variety of metamorphic basement rocks, metasediments, granitoids with pegmatitic, aplitic, and mafic dikes, volcanic rocks, and sedimentary covers is exposed.

The metamorphic basement consists of Middle Jurassic orthogneisses and migmatites. Sedimentary sequences were deposited on igneous protoliths and subsequently the entire package was deformed, metamorphosed and affected by an Upper Cretaceous (Campanian) intrusive event. The metasediments are considered as roof pendant of the plutonic suite.

The foraminifera and diatom record of Upper Miocene to Pleistocene sediments of the shallow marine Ojo de Buey sequence witness repeated subsidence and uplift events related to the opening of the Gulf of California (McCloy et al., 1988). The detrital zircons in this sequence originate almost exclusively from the Upper Cretaceous igneous María Madre units. The early Miocene? clastic sedimentary rocks exposed along the southern coast of Isla María Madre are correlated according to our field observations to the Isla Magdalena sandstone from the homonymous neighboring island. This sequence is stratigraphically older than the Ojo de Buey sequence but its detrital zircons are considerably younger (around 20 Ma), suggesting different depositional conditions for both units.

The metamorphic and igneous record of Isla María Madre strongly resembles similar Jurassic-Cretaceous plutonic and metamorphic rocks found in the Los Cabos Block of Baja California Sur and volcanic and plutonic rocks from the Mexican continental margin between Sinaloa and Jalisco. Most of these rocks are calc-alkaline and formed in a subduction-related environment.

Taking the above remarks into account, our new data are an important contribution for understanding the evolution of magmatic systems during Cretaceous and Tertiary times in this part of the North American margin. The results obtained so far characterize the María Madre igneous rocks as a part of the Cordilleran batholithic belt that extends from North America to the Isthmus of Tehuantepec and almost certainly exclude large paleotectonic displacements for the Baja California Peninsula.

Further investigations on the other islands of Islas Marías Archipelago are in progress as well as detailed geochemical and isotopic analysis of Isla María Madre rocks to constrain effectively our interpretations.

ACKNOWLEDGEMENTS

This work was supported by DGAPA-PAPIIT grants IN122509 and IN 113012 to P.S. We want to thank Secretaria de Marina, Armada de Mexico, Mazatlan; Secretaria de Seguridad Publica, Mexico (SSP), and Secretaria del Medio Ambiente y Recursos Naturales (SEMARNAT) for the facilities provided to access the study area. Thanks to Rufino Lozano Santa Cruz and Patricia Giron (LUGIS) for XRF analyses, and to Ofelia Pérez (CGEO) for ICP-MS data. Thanks to Jeff Benowitz from the Geochronology Laboratory, University of Alaska at Fairbanks, for ⁴⁰Ar/³⁹Ar analyses; to George Gehrels and Joaquin Ruiz for providing access to the Arizona LaserChron Center that is partially supported by a grant from the Instrumentation and Facilities Program, Division of Earth Sciences, NSF-EAR 0732436, and to Mark Pecha, who was in charge of the LA-MC-ICPMS instrument at Arizona LaserChron Center during our measurements. Reviews by Christopher Henry and Carlos Gonzalez-Leon helped to improve the manuscript and are greatly appreciated.

APPENDIX A. SUPPLEMENTARY DATA

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