Stratigraphy, geochronology, and geochemistry of the Laramide magmatic arc in north-central Sonora,...

7/30/2019 Stratigraphy, geochronology, and geochemistry of the Laramide magmatic arc in north-central Sonora, Mexico

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Geosphere

doi: 10.1130/GES00679.12011;7;1392-1418Geosphere

SantacruzBernal, Elizard Gonzlez Becuar, Floyd Gray, Margarita Lpez Martnez and Rufino LozanoCarlos M. Gonzlez-Len, Luigi Solari, Jess Sol, Mihai N. Ducea, Timothy F. Lawton, Juan Pabloin north-central Sonora, MexicoStratigraphy, geochronology, and geochemistry of the Laramide magmatic arc

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ABSTRACT

The Laramide magmatic arc in theArizpe-Mazocahui quadrangle of north-central Sonora, Mexico, is composed ofvolcanic rocks assigned to the TarahumaraFormation and several granitic plutons thatintrude it. The arc was built over juxta-posed crustal basements of the Caborca andMazatzal provinces. A basal conglomerateof the >4-km-thick Tarahumara Forma-tion overlies deformed Proterozoic igneous

rocks and Neoproterozoic to Early Creta-ceous strata, thus constraining the age of acontractional tectonic event that occurredbetween Cenomanian and early Campaniantime. The lower part of the TarahumaraFormation is composed of rhyolitic ignim-brite and ash-fall tuffs, andesite flows, andinterbedded volcaniclastic strata, and itsupper part consists of rhyolitic to daciticignimbrites, ash-fall tuffs, and volcaniclasticrocks. The Tarahumara Formation showsmarked lateral facies change within the studyarea, and further to the north it grades intothe coeval fluvial and lacustrine Cabullona

Group. The age of the Tarahumara Forma-tion is between ca. 79 and 59 Ma; the mon-zonitic to granitic plutons have ages of ca.7150 Ma. The informally named El Babizoand Hupac granites, La Aurora and LaAlamedita tonalities, and the Puerta del Solgranodiorite compose the El Jaralito batho-lith in the southern part of the area.

Major and trace element composition of theLaramide igneous rocks shows calc-alkaline

differentiation trends typical of continentalmagmatic arcs, and the isotope geochemistryindicates strong contribution from a maturecontinental crust. Initial 87Sr/86Sr values rangefrom 0.70589 to 0.71369, and Nd valuesrange from 6.2 to 13.6, except for the ElGueriguito quartz monzonite value, 0.5. TheNd, Sr, and Pb isotopic values of the studiedLaramide rocks permit comparison with thepreviously defined Laramide isotopic provincesof Sonora and Arizona. The El Gueriguitopluton and Bella Esperanza granodiorite in

the northeastern part of the study area alongwith plutons and mineralization of neighbor-ing northern Sonora have isotopic values thatcorrespond with those of the southeasternArizona province formed over the Mazat-zal basement (Lang and Titley, 1998; Bouseet al., 1999). Isotopic values of the otherLaramide rocks throughout the study areaare similar to values of provinces A and B ofSonora (Housh and McDowell, 2005) and tothose of the Laramide Pb boundary zone ofwestern Arizona, while the Rancho Vaqueraand La Cubana plutons in the northernmostpart of the area have the isotopic composi-

tion of the Proterozoic Mojave province ofthe southwestern United States. These datapermit us to infer that a covered crustalboundary, between the Caborca block witha basement of the Mojave or boundary zoneand the Mazatzal province, crosses throughthe northeastern part of the area. The bound-ary may be placed between outcrops of theEl Gueriguito and Rancho Vaquera plutons,probably following a reactivated Cretaceous

thrust fault located north of the hypothesizedMojave-Sonora megashear, proposed to crossthrough the central part of the area.

INTRODUCTION

The Late Cretaceousearly Cenozoic Laramide

magmatic arc of Sonora, Mexico (Damon et al.,

1983a, 1983b; McDowell et al., 2001), is com-

posed of a thick and geographically extensive

volcanic succession and nearly contempora-

neous, mostly granitic, plutons that are part of

the Sonoran batholith (Damon et al., 1983a)(Fig. 1). These rocks formed in a continental,

Andean-type magmatic arc related to subduc-

tion of the Farallon plate beneath North America

during Late Cretaceous and early Cenozoic time

(Coney and Reynolds, 1977; Dickinson, 1981,

1989; Damon et al., 1983a; Engebretsen et al.,

1985; Stock and Molnar, 1988).

Based mostly on their own data set of pre-

dominantly K-Ar ages of plutonic rocks,

Damon et al. (1983a) proposed that the

Laramide arc in Sonora spanned 9040 Ma, a

range mostly accepted by others and broadly

supported by subsequent geochronologic stud-

ies (Anderson and Silver, 1977; Anderson

et al., 1980; Valencia-Moreno et al., 2001,

2003, 2006; McDowell et al., 2001; Housh and

McDowell, 2005; Prez-Segura et al., 2009;

Roldn-Quintana et al., 2009). In contrast, the

study of the arcs cogenetic volcanic succession

has long been neglected despite extensive expo-

sures, although some ages were published from

northern and central Sonora (Supplemental

Table 11). The name Mesa formation (Valentine,

For permission to copy, contact [email protected]

2011 Geological Society of America

Geosphere; December 2011; v. 7; no. 6; p. 13921418; doi: 10.1130/GES00679.1; 14 figures; 2 tables; 5 supplemental tables.

Stratigraphy, geochronology, and geochemistry of the

Laramide magmatic arc in north-central Sonora, Mexico

Carlos M. Gonzlez-Len1, Luigi Solari2, Jess Sol3, Mihai N. Ducea4, Timothy F. Lawton5, Juan Pablo Bernal2,Elizard Gonzlez Becuar6, Floyd Gray7, Margarita Lpez Martnez8, and Rufino Lozano Santacruz31Instituto de Geologa, Estacin Regional del Noroeste, Universidad Nacional Autnoma de Mxico, Apartado Postal 1039,

Hermosillo, Sonora, Mxico 830002Centro de Geociencias, Universidad Nacional Autnoma de Mxico, Campus Juriquilla, Quertaro, QRO, Mxico 760013Instituto de Geologa, Ciudad Universitaria, Universidad Nacional Autnoma de Mxico, Mxico, Distrito Federal 045104Geosciences Department, University of Arizona, Tucson, Arizona 85721, USA5Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA6Departamento de Geologa, Universidad de Sonora, Hermosillo, Sonora, Mxico 830007U.S. Geological Survey, GeologyEcosystem Analysis, 520 North Park Avenue, Suite 355, Tucson, Arizona 85719, USA8Departamento de Geologa, Centro de Investigacin Cientfica y de Educacin Superior de Ensenada (CICESE),

Km 107 Carretera Ensenada-Tijuana No. 3918, 22860 Ensenada, Baja California, Mxico

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Laramide magmatic arc in northern Sonora

Geosphere, December 2011 1393

1936) was first assigned to outcrops of this

succession near the town of Cananea (Fig. 1),

where it consists of a basal conglomerate and

overlying bedded tuffs, andesite flows, agglom-

erates, and subordinate rhyolite flows >1600 m

thick. Wilson and Rocha (1949) applied the

term Tarahumara Formation to the Laramide

volcanic succession and it became a commonly

used name for extensive volcanic and volcani-

clastic outcrops in Sonora; they described it as

consisting of highly altered, aphanitic interme-

diate volcanic rocks unconformably overlying

Triassic strata in its type section near the town

of Tecoripa, in central Sonora (Fig. 1). From

that locality, McDowell et al. (2001) reported

an age range of 9070 Ma for an ~2.5-km-thick

rhyolitic, dacitic, andesitic, and volcaniclastic

succession.

Other studies of partial sections of these

rocks in northern Sonora assigned local infor-

mal names, including Alcaparros formation and

Arroyo Alcaparros andesitic rocks (Gonzlez-

Len et al., 2000) and El Tuli formation (Rodr-

guez-Castaeda, 1994). To avoid confusion and

to provide uniformity in the terminology, we

follow other authors in applying the formal,

more commonly used name Tarahumara Forma-

tion to this succession throughout its region o

exposure (McDowell et al., 2001).

In this paper we provide new constraints on

the stratigraphy and geochronology of the Tara

humara volcanic succession and the petrology

and geochronology of associated plutonic bod

ies that represent the Laramide magmatic arc

within the Arizpe-Mazocahui area in northcentral Sonora (Fig. 1). The area is located

in a position that is transitional between the

classic localities of the Tarahumara Forma

tion to the south, and the Mesa formation to

the north. It includes the 15 20, 1:50,000scale topographic quadrangles named Arizpe

Nacozari (part), Santa Ana (part), Banmichi

Agua Caliente, Aconchi, Cumpas, Bavicora

and El Rodeo (part) (Fig. 2), published by

Instituto Nacional de Estadstica, Geografa e

Informtica of the Mexican government. Ou

work is based on geologic mapping conducted

at that scale and illustrated here by a general

ized geologic map (Fig. 2). This cartographyimproves and in many instances correct

some of the previous geologic maps of the

Arizpe, Banmichi, Bavicora, and Santa

Ana quadrangles (Gonzlez Gallegos et al.

2003; Quevedo Len and Ramrez Lpez

2008; Servicios Geolgicos y Cartogrfico

del Noroeste, S.A. de C.V., 1999; Corra

Gastelum and Hernndez Morales, 2008

respectively). Six measured stratigraphic col

umns and seven accompanying structural sec

tions are included to illustrate the stratigraphic

and tectonic relationships of the Laramide

rocks with older and younger geologic unitsThe ages of the Laramide volcanic succession

and the plutonic bodies are constrained by 28

U-Pb dates, one 40Ar/39Ar date, and 9 K/A

dates, none of which have been reported pre

viously. Another new 40Ar/39Ar date reported

here helps to constrain the age of the younge

unit of the basement over which the Laramide

succession was deposited as well as the age of

the younger tectonic event that deformed tha

basement in the region. We also dated detrita

zircons from four sandstone units to constrain

their maximum deposition ages. Two of these

come from the Proterozoic basement and two

others are from sandstone beds within the

Laramide succession.

The study also incorporates 35 whole rock

geochemical analyses and 12 isotope analyse

of Sm/Nd, Rb/Sr, and Pb/Pb of the Laramide

plutonic and volcanic rocks, and 2 from the

Proterozoic plutons. All of the analyzed sam

ples have age and stratigraphic control. The

field and analytical data help to document the

stratigraphic, tectonic, and temporal frame

work of the Laramide magmatic arc for thi

part of the Cordillera, and the new cartography

11100

3100

11400

Puerto

Penasco

Guaymas

Ures

SantaAna

Agua Prieta

Sonoita

SONORA

A RIZONA

Gulf

of

Calif

ornia

11200

+

+

Nogales

A

B C

D

Cananea

Arizpe

Nacozari

MEXICO

Laramide volcanic rocks(Tarahumara Formation)

Laramide plutonic rocks(Sonoran batholith of Damon et al., 1983a)

Cabullona Group

A ierra Los Ajos

B a Lmina

C erro Prieto

D

S

L

C

El Crestn

Trace of Mojave-Sonora megashear

10900

U S A

SONORA

2700

N

C. Obregon

Navojoa

Sahuaripa

100 km

Mazatn

MAZATZALPROVINCE

CABORCABLOCK

Caborca

HERMOSILLO

Bacoachi

Moctezuma

Studyquadrangle

Figure 2

Ycora

3100

2800

Cucurpe

Tecoripa

Mazochui

Figure 1. Map outcrop distribution of plutonic and volcanic rocks of the Laramide magmaticarc in Sonora. The distribution of outcrops of the Late Cretaceous Cabullona Group, locali-ties mentioned in the text, and inset map for location of Figure 2 are shown. The boundarybetween the Mazatzal province and Caborca blocks is indicated by the trace of the hypo-thetical Mojave-Sonora megashear as presented by Anderson and Silver (2005, their fig. 4).

1Supplemental Table 1. Bibliographic compila-tion of geochronologic ages of Laramide volcanicand plutonic rocks of Sonora between isochronesof 70 and 50 Ma, as indicated in Figure 14. Histo-gram of plutonic ages in Figure 13 was drawn withthis information. If you are viewing the PDF of thispaper or reading it offline, please visit http://dx.doi.org/10.1130/GES00679.S1 or the full-text article onwww.gsapubs.org to view Supplemental Table 1.



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and isotope data place constraints on possible

delimitations of the Caborca and Mazatzal

crustal blocks.

ANALYTICAL METHODS

U-Pb ages by laser ablationmulticollector

inductively coupled plasmamass spectrometry(LA-MC-ICP-MS) were determined at the Ari-

zona Laser ChronCenter of the Geosciences

Department of the University of Arizona (results

are reported in Supplemental Table 22) and at

Centro de Geociencias, Universidad Nacional

Autnoma de Mxico (UNAM) (Supplemental

Table 33) (procedures are described in Appen-

dices 1 and 2, respectively). K-Ar analyses on

biotites from igneous rocks were conducted at

the Instituto de Geologa, UNAM. Procedures

are described in Appendix 3 and results are in

Supplemental Table 44. The 40Ar/39Ar dating

was performed at the Geochronology Labora-

tory of the Departmento de Geologa, Centrode Investigacin Cientfica y de Educacin

Superior de Ensenada (CICESE), and proce-

dures were described in Gonzlez-Len et al.

(2010). A hornblende separate from sample

04ES-8 was analyzed by the resistance-furnace

incremental-heating age spectrum method at

the New Mexico Geochronology Research

Laboratory. (Details of the method and over-

all operation of the laboratory are provided at

http://www.ees.nmt.edu/Geol/labs/Argon_Lab

/Methods/Methods.) Geochemical analyses for

major and trace elements were done by X-ray

fluorescence and with a SIEMENS SRS 3000spectrometer in the Laboratorio Universitario

de Geoqumica Isotpica, UNAM, and by high

resolution ICP-MS at the Department of Geol-

ogy, University of WisconsinEau Claire, USA.

Radiogenic isotopic and select trace element

concentrations were determined at the Isotopic

Laboratory of the Geosciences Department of

the University of Arizona following procedures

reported in Appendix 4.

REGIONAL GEOLOGIC SETTING

The Proterozoic basement of Sonora wasrecognized as part of crustal southwestern

North America by Damon et al. (1962) on the

basis of geochronology data. Silver and Ander-

son (1974) noted that it could be divided into

a northern block with ages between 1.7 and

1.6 Ga and a southern block with ages from 1.8

to 1.7 Ga that were later assigned to the North

America (or Mazatzal) and Caborca terranes,

respectively (Campa and Coney, 1983) (Fig. 1).

However, the nature, age, and location of the

crustal boundary are debatable. On one side,

the Caborca block is interpreted as a piece of

crustal southwestern USA translated to its pres-

ent position by a major left-lateral fault assignedeither to the Jurassic Mojave-Sonora megashear

(Silver and Anderson, 1974; Anderson and Sil-

ver, 2005) (Fig. 1) or to the PermianTriassic

California-Coahuila transform fault (Dickinson

and Lawton, 2001). On the contrary, Poole et al.

(2005) argued that the Mojave basement and its

Neoproterozoic sedimentary cover wrap around

the Laurentian margin in the southwestern USA

to continue southeast into the Caborca block,

without major structural displacement. Simi-

larly, Arvizu et al. (2009) depicted the Mojave

and Mazatzal blocks in Sonora separated by a

Nd isotope Yavapai crustal province.The Pinal Schist, the basement of the Mazatzal

province, crops out in Sierra Los Ajos (Fig. 1)

and nearby areas of northern Sonora, where it

is dated as 1.69 Ga (Anderson and Silver, 2005)

and 1.64 Ga (U-Pb, zircon) (Page et al., 2010).

Proterozoic granites with ages near 1.4 Ga

intrude the Pinal Schist (Anderson and Silver,

2005), and the nearest outcrop of this granite

to the study area is in the town of Bacoachi

(Fig. 1; our own observations). Other granites

and gabbros that are assigned to the basement

of the Caborca block because of their isotopic

signatures and ages of ca. 1.7 Ga crop out in

near localities of Cerro Prieto (Anderson and

Silver, 2005), Rancho La Lmina (Amato et al.,

2009), and El Crestn (Valenzuela-Navarro

et al., 2005) (Fig. 1). Based on the occurrence

of Caborcan Proterozoic granites in Rancho

La Lmina, Amato et al. (2009) inferred that if

present, the trace of the hypothetical Mojave-

Sonora megashear might be located north of that

locality (Fig. 1).

Proterozoic, mostly clastic, strata assigned to

the Las Vboras and El Aguila Groups by Stewart

et al. (2002) crop out within a few kilometers to

the west of the study area. These units exceed a

combined thickness of 3.5 km and overlie the

igneous basement of the Caborca block. Super

jacent Paleozoic clastic and carbonate strat

(Stewart et al., 1997, 1999) are >4 km thick

This ProterozoicPaleozoic succession of the

Caborca block is lithologically different from

the CambrianPermian sedimentary successionthat unconformably overlies the basement of the

Mazatzal province and that correlates with and

resembles the Paleozoic formations of south

eastern Arizona (e.g., Hayes and Landis, 1965)

The nearby outcrops of this Paleozoic succes

sion occur in the town of Bacoachi (Stewart and

Poole, 2002) and in the vicinity of Cananea

where the successtion is 1.2 km thick (Gonzlez

Len, 1986; Page et al., 2010).

Mesozoic rocks of the neighboring region to

the north in Sonora include the Lower Jurassic

Basomar Formation (Legget et al., 2007) and

the Middle Jurassic Rancho San Martn (Mauel

2008), Elenita (Valentine, 1936; Wodzicki1994), and Lily (McAnulty, 1970, Gonzlez

Len et al., 2009) Formations, and a few dated

Middle Jurassic granites (Anderson et al.

2005). These formations make up a clastic and

volcanic succession that was deposited within

a continental magmatic arc that developed in

northern Sonora (Riggs et al., 1993, Anderson

et al., 2005). Marine strata of the Upper Juras

sic Cucurpe Formation (Villaseor et al., 2005

Mauel et al., 2011) overlie the Basomari and

Rancho San Martn successions near the town

of Cucurpe. The combined thickness of the

Jurassic formations is at least 3.5 km, and theyare unconformably overlain by strata of the Bis

bee Group, which in this region is documented

to be Early Cretaceous in age and ~3 km thick

(Peryam, 2006; Peryam et al., 2011).

A major Middle to Late Jurassic tectonic

event of extensional deformation formed a rif

basin, termed the Altar-Cucurpe Basin, where

the Cucurpe Formation was deposited (Peryam

2006; Mauel, 2008; Mauel et al., 2011). Alterna

tively, formation of this basin has been assigned

to development of the left-lateral displacemen

of the Mojave-Sonora megashear (Anderson

and Nourse, 2005; Anderson and Silver, 2005)

A younger, contractional tectonic event affected

the Bisbee Group and older strata during early

Late Cretaceous time (Rangin, 1986). The age

of the clastic continental succession of the

Cocspera Formation (Gilmont, 1978) depos

ited during this tectonic event (Gonzlez-Len

et al., 2000) is constrained by a 40Ar/39Ar age o

93.3 0.7 Ma (Fig. 3) obtained from an inter

bedded andesite in outcrops ~2 km northwest o

the study area (Lawton et al., 2009). Following

a period of uplift and erosion, deposition of the

Laramide volcanic arc succession commenced.

2Supplemental Table 2. U-Pb data of detrital zirconfrom sandstone and tuffaceous sandstone samples,and Laramide volcanic and plutonic rocks of thestudy area. Analyses were done at the Arizona LaserChron Center of the University of Arizona by LA-ICP-MS method. If you are viewing the PDF of thispaper or reading it offline, please visit http://dx.doi.org/10.1130/GES00679.S2 or the full-text article on

www.gsapubs.org to view Supplemental Table 2.3Supplemental Table 3. U-Pb data for volcanicand plutonic rocks of the study area done at the Cen-tro de Geociencias, Universidad Nacional Autnomade Mxico by the LA-ICP-MS method. If you areviewing the PDF of this paper or reading it offline,please visit http://dx.doi.org/10.1130/GES00679.S3or the full-text article on www.gsapubs.org to viewSupplemental Table 3.

4Supplemental Table 4. K-Ar age data of datedsamples of the study area. If you are viewing the PDFof this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00679.S4 or the full-textarticle on www.gsapubs.org to view SupplementalTable 4.



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Gonzlez-Len et al.

1396 Geosphere, December 2011

Regionally, the Laramide magmatic arc ofSonora has been documented mostly through a

large geochronology database and geochemi-

cal and isotopic studies of the plutonic rocks

(Anderson and Silver, 1977; Anderson et al.,

1980; Damon et al., 1983a, 1983b; Wodzicki,

1994; McDowell et al., 2001; Valencia-Moreno

et al., 2001, 2003, 2006; Valencia et al., 2005;

Roldn-Quintana et al., 2009; Prez-Segura

et al., 2009; Prez-Segura and Gonzlez-Partida,

2010), but far fewer data are available for the

volcanic rocks (McDowell et al., 2001). Some

ages are published for the Laramide volcanic

rocks north of the study area, in the Cananea-

Nacozari region (Wodzicki, 1994; Valencia et al.,

2005; Cox et al., 2006; Page et al., 2010), and 640Ar/39Ar ages between 73 and 66 Ma are listed

in Supplemental Table 1 (see footnote 1). Based

on Sr and Nd isotopic variations and trace ele-

ment compositions, Laramide granites of north-

ern and central Sonora with ages between 57

and 68 Ma were assigned to northern and central

granites by Valencia-Moreno et al. (2001). On

the basis of Sr, Nd, and Pb isotope geochemistry,

other granites with ages between 59 and 67 Ma

were considered as belonging to provinces A

and B by Housh and McDowell (2005), whoalso included isotopic characteristics of Oligo-

cene and Miocene volcanic rocks to define their

provinces. Geographically, the Laramide gran-

ites of provinces A and B roughly occupy the

same region as the northern and central granites

of Valencia-Moreno et al. (2001), while rocks of

the study area are within the geographic domains

of the northern granites and province A.

Younger regional events consist of Late

OligoceneMiocene magmatism, core complex

formation, and basin-fill continental sedimen-

tation of the Bucarit Formation that occurred

associated with Basin and Range extensional

deformation. Basin and Range deformation

structurally dismembered the Laramide arc and

older basement (Nourse et al., 1994; Wong

and Gans, 2008; Gonzlez-Len et al., 2010;

Wong et al., 2010).

GEOLOGY OF THE STUDY AREA ANDPREVIOUS STUDIES

Within the study area the older rocks are El

Jacaln diorite and the Santa Margarita gran-

ite (Rodrguez-Castaeda, 1994) that crop

out in the Santa Ana quadrangle (Fig. 2). The

diorite and granite have U-Pb (zircon) ages of

1702 Ma and 1104 Ma, respectively (Anderson

and Silver, 2005). A gneissic zone developed

in the El Jacaln diorite is spatially associated

with the El Jucaral normal fault of postEarly

Cretaceous age (Fig. 2) and is not a separate

Proterozoic lithostratigraphic unit, as previouslyinterpreted (the El Alamito unit of Rodrguez-

Castaeda, 1994). A thick Proterozoic succes-

sion (>1 km thick) of mostly sandstone that

locally overlies the igneous basement crops out

in the Banmichi and Santa Ana quadrangles. It

was named the Los Changos orthoquarzite, of

supposed Paleozoic age by Rodrguez-Casta-

eda (1994) and later reassigned to the Protero-

zoic Las Vboras Group by Stewart et al. (2002)

(Fig. 2). Detrital zircons from a sample of the

middle part of this succession in the Banmichi

quadrangle yielded U-Pb peak ages at 1.2, 1.47,

1.67, and 1.87 Ga (Figs. 2 and 4A) (Plascencia

Corrales, 2008), similar to a sample collectedfrom this succession that unconformably over-

lies the El Jacaln granodiorite in the Santa Ana

quadrangle (Figs. 2 and 4B).

Isolated outcrops of schist, recrystallized

limestone, and quartz-rich sandstone that occur

as roof pendants in the Laramide plutons in the

Sierra El Jaralito may be Proterozoic and/or

Paleozoic in age (Peabody, 1979, in Roldn-

Quintana, 1989; Mead et al., 1988) (Fig. 2).

Cambrian to Permian formations in the Naco-

zari and Agua Caliente quadrangles (Fig. 2)

are typical of Paleozoic strata that overlie the

Mazatzal basement in northeastern Sonora.The Mesozoic rocks in the study area include

incomplete sedimentary successions of the Lily

(Gonzlez-Len et al., 2009) and Cucurpe For-

mations of Jurassic age, the Morita, Mural and

Cintura Formations of the Early Cretaceous

Bisbee Group, and the previously undated

Cocspera Formation (Fig. 2). Assignment of

a Jurassic age by Rodrguez-Castaeda (1994)

and Corral Gastlum and Hernndez Morales

(2008) to widespread outcrops of the Bisbee

Group and the Tarahumara Formation in the

Santa Ana quadrangle are herein corrected

on the basis of our geochronology. However,

most of the area is occupied by outcrops of the

volcanic and plutonic rocks of the Laramide

magmatic arc; volcanic rocks, sedimentary

strata, and a few rhyolitic domes of Oligo-

ceneMiocene age; and by younger alluvial

deposits (Fig. 2).

Previously published geochronologic infor-

mation of the Laramide volcanic rocks in the

study area include a U-Pb (zircon) age of 76 Ma

from the Santa Ana quadrangle (McDowell

et al., 2001), a 40Ar/39Ar (biotite) age of 58.67

0.17 Ma from the Arizpe quadrangle (Gonzlez-

93.3 0.7 Ma(MSWD = 1.7)

%

Radiogenic

1

K/Ca

1

0.1

0.01

0

Cumulative % Ar released39

k

Integrated age = 90.9 0.7

10 20 30 40 50 807060 10090

50

60

70

80

90

100

110

0

80

40

Apparent

age

(Ma)

1125

F1080

D

1105

E 1155

G

1200

H

1300

I

1050

C

A

1

700

J

L# 55854: 04ES-8, Hornblende

965

B

120

Figure 3. 40Ar/39Ar age spectrum of hornblende separate of sample 04ES-8 from an andesiteinterbedded with conglomerate of the Cocspera Formation. This sample was collected nearRancho San Antonio, ~2 km northwest of the study quadrangle (Universal Transverse Mer-cator locality 12R 562410E 3382580N). MSWDmean square of weighted deviates.



7/28



Len et al., 2000), and a K-Ar (whole rock)

age of 40.6 1.1 Ma from the El Rodeo quad-

rangle (Damon et al., 1983a). Freshwater,

non-age-diagnostic fossils including plants

(Hernndez-Castillo and Cevallos-Ferriz, 1999),

algae (Beraldi-Campesi et al., 2004), diatoms

(Chacn-Baca et al., 2002), and other micro-

fossils (Beraldi-Campesi and Cevallos-Ferriz,

2005) were reported from local, interbedded

lacustrine strata of the Tarahumara Formation in

the Aconchi quadrangle.

Ages reported for the plutonic rocks include

a 40Ar/39Ar (biotite) age of 56.7 Ma for the

Rancho Vaquera pluton (pluton names derive

from our informal terminology described in the

following; Fig. 2) (Gonzlez-Len et al., 2000),

a K-Ar (biotite) age of 56.4 Ma (Mead et al.,

1988), and a 40Ar/39Ar (biotite) age of 57.3 Ma

(Zuiga Hernndez, 2010) for the Las Cabecitas

granodiorite (Fig. 2), and K-Ar ages reported by

Damon et al. (1983b) of 51 Ma (orthoclase) for

the San Felipe porphyry and 55.9 Ma (biotite)

for the Bella Esperanza granodiorite (Fig. 2).

The Bella Esperanza granodiorite was also

dated as 56.9 Ma (K-Ar, whole rock) by Housh

and McDowell (2005). Stocks of monzonite

and diorite in the Cumobabi Mine (Fig. 2

yielded K-Ar ages between 63 and 56 Ma on

biotite (Scherkenbach et al., 1985) and 51 Ma

(40Ar/39Ar; Zuiga Hernndez, 2010).

Mead et al. (1988) first reported 40Ar/39A

ages of 46.6 Ma (hornblende) and ca. 37.1 Ma

(biotite), and a K-Ar age of ca. 39.5 Ma (bio

tite) for a granodiorite pluton in the Sierra E

Jaralito. The El Jaralito and the Aconchi batho

liths are names assigned by Roldn-Quintana

(1991) to plutonic rocks that crop out in the

Sierra El Jaralito and the Sierra de Aconchi

respectively (Fig. 2). Roldn-Quintana (1991

A 11-22-07-1

C 11-21-07-1

B 3-25-09-4

D 12-5-08-3

1000

1400

1800

2200

0.1

0.2

0.3

0.4

0 2 4 6 8

data-point error ellipses are 2

500

1500

2500

0.0

0.2

0.4

0.6

0 4 8 12 16


0

5

10

15

20

25

0 500 1000 1500 2000 2500 3000

Relativeprobability

Number

Age, Ma

~1260 Ma

~1470 Ma

~1670 Ma

~1870 Ma

0

2

4

6

8

10

12

14

0 500 1000 1500 2000 2500

Relativeprobability

Number

Age, Ma

70

74

78

82

0.0106

0.0110

0.0114

0.0118

0.0122

0.0126

0.0130

0.064 0.068 0.072 0.076

0.088

0.092

ConcordiaAge = 76.39 0.67 Ma

(2, decay-constant errors included)

MSWD(of concordance) = 2.5,

Probability(of concordance) = 0.11


~1218 Ma

~1460 Ma

~1670 Ma

~1870 Ma

0

1

2

3

4

5

6

0 500 1000 1500 2000 2500 3000

Relativeprobability

Number

Age, Ma

1000

1400

1800

2200

2600

0.05

0.15

0.25

0.35

0.45

0.55

0.65

0 4 8 12 16


1500

2500

0.0

0.2

0.4

0.6

0.8

0 10 20 30


~78 Ma

~152 Ma

~192 Ma ~1060 Ma

~1744 Ma

0

2

4

6

8

10

12

14

0 500 1000 1500 2000 2500 3000

Relativeprobability

Number

Age, Ma

20

60

100

140

180

220

260

300

0.00

0.01

0.02

0.03

0.04

0.05

0.0 0.1

0.2

0.3

207Pb/235U

206Pb/238U

207Pb/235U

206Pb/238U

207Pb/235U

206Pb/238U

207Pb/235U

206Pb/238U

Figure 4. Detrital zircon age distribution for sandstone and tuffaceous sandstone samples of the Arizpe-Mazocahui quadrangle. (A) Sample11-22-07-1 from the Proterozoic Las Vboras Group, Banmichi quadrangle. (B) Proterozoic sandstone from the Las Vboras Group in theSanta Ana quadrangle. (C) Tuffaceous sandstone from the lower part of the Tarahumara Formation in structural section DD . (D) Tuffaceous sandstone from the Tarahumara Formation in the Santa Ana quadrangle. Approximate locations of these samples are indicated inFigure 2. MSWDmean square of weighted deviates.



8/28

Gonzlez-Len et al.


noted that the El Jaralito batholith is composed

of granitic, granodioritic, quartz dioritic, and

quartz monzonite facies with ages between

ca. 69 and ca. 51 Ma, whereas he referred to

the Aconchi batholith as a two-mica, alkaline

granite with red garnet with an age of 35.96

0.7 Ma. Radelli et al. (1991) and Macias Valdz

(1992) later renamed this two-mica pluton theHupac granite. In this work we follow the sub-

sequent authors to include the Aconchi batholith

as part of the El Jaralito batholith and based on

our geochronology identify granite outcrops to

the south in the Sierra El Jaralito as the Hupac

granite. Furthermore, in this work and based

mostly on geochronology and geochemistry, we

recognize the El Jaralito batholith as a plutonic

suite that includes previously unrecognized plu-

tons (Fig. 2) (described herein).

Abundant centimeter- to meter-thick peg-

matite dikes composed of K-feldspar, quartz,

plagioclase, muscovite, biotite, garnet, and

accessory minerals that cut through the ElJaralito batholith were studied in detail by

Roldn-Quintana et al. (1989) and Macias-

Valdez (1992). Several published K-Ar and40Ar/39Ar ages obtained from K-feldspar, horn-

blende, muscovite, and biotite from the two-

mica Hupac granite, from pegmatite dikes,

and from skarn rocks range from 41.6 Ma to ca.

18 Ma. The older ages are interpreted as cooling

ages of the plutons (Mead et al., 1988), while

ages between ca. 28 to ca. 18 Ma were inter-

preted by Lugo Zazueta (2006) and Wong et al.

(2010) as cooling ages of the exhumed footwall

of the core complex that forms the Sierra deAconchi (Fig. 2).

The Laramide and older rocks of the study

area are intruded by rhyolitic and dacitic domes

with ages between 23 and 25 Ma (Fig. 2;

Gonzlez-Len et al., 2010; our data) and by

basaltic dikes with ages near 23 Ma (Wong and

Gans, 2008). These rocks are deformed by nor-

mal faults of the Basin and Range extensional

event that formed the north-south elongated

basins of the Sonora and Moctezuma Rivers,

where thick, Late Oligocene to Miocene vol-

canic and terrigenous strata of the Bucarit

Formation accumulated (Gonzlez-Len et al.,

2010) (Fig. 2).

Stratigraphy and Structural Relationshipof the Tarahumara Formation

Structural relationships of the Tarahumara

Formation with older and younger units are

illustrated along seven cross sections (Figs. 2

and 5). Tarahumara Formation stratigraphy is

described by means of six measured columns

(Fig. 6) and from its estimated thickness along

structural sections F-F and G-G (Fig. 5). Sam-

ples from different stratigraphic levels of the

Tarahumara sections were collected during field

work for petrographic, geochronologic, and

geochemical studies. A summary of the U-Pb,

K-Ar, and 40Ar/39Ar geochronology is presented

in Table 1. Geochemical and isotope analyses

were performed from samples collected at same

stratigraphic levels of the dated samples andresults are presented in Supplemental Table 55

and Table 2, respectively. The main structural

and the stratigraphy characteristics of the Tara-

humara Formation along the cross sections are

described next.

Cross-section A-A located in the Arizpe quad-

rangle crosses through the Picacho de Arizpe

peak (Figs. 2 and 5). On the eastern flank of the

Picacho, the Tarahumara Formation unconform-

ably overlies deformed strata of the tectonically

juxtaposed Mural and Cocspera Formations.

It dips homoclinally to the northeast and is

unconformably overlain by Oligocene volcanic

rocks (Gonzlez-Len et al., 2000). In the west-ern flank of the Picacho de Arizpe, the Mural

Formation is part of a block of Bisbee Group

strata that thrusts over the deformed Cocspera

Formation. The measured thickness of the Tara-

humara is 1260 m (Fig. 6A). Its lowermost part

consists of crystal-poor rhyodacitic welded

tuff and crystal-rich porphyritic dacite. Zircons

from the rhyodacite gave a U-Pb age of 75.70

+0.30/0.70 Ma (Fig. 7A; mean 206Pb/238U age,

97.3% confidence, n = 21). The remainder of

the section is composed of well-bedded rhyolite

ash-fall tuff and brown to reddish volcaniclastic

sandstone and siltstone.Along cross-section B-B, the Tarahumara

Formation unconformably overlies the deformed

Late Jurassic Cucurpe Formation, which crops

out in an erosional window of an open anticline

of the Tarahumara Formation in the El Tegua-

chi ranch area (Figs. 2 and 5). In the southern

part of this section the Cintura and Tarahumara

Formations are normally faulted against each

other across the Los Alisos fault. Between the

El Teguachi ranch and the Sierra El Juparo, the

Tarahumara Formation dips to the north and its

section is offset by the normal fault of Caada

El Potrerito. The composite stratigraphic col-

umn of the Tarahumara starts north of the El

Teguachi ranch and ends in Sierra El Juparo

(Fig. 6B). It has an incomplete thickness of

1000 m and its lower part is occupied by a

basal, 29-m-thick conglomerate that grades

upward to interbedded volcaniclastic strata,

rhyolitic to dacitic ignimbrite, ash-fall tuff and

lacustrine limestone. The overlying unit con-

sists of andesitic breccia, rhyolitic ash-fall tuff,

and ignimbrite. A normal fault at the top of this

unit omits part of the stratigraphic column, and

its upper part was measured in the southern

flank of Sierra Los Juparos. It is composed ofbedded ash-fall tuff, rhyolitic breccias, sand-

stone, and conglomerate, and its upper part is

composed of ignimbritic rhyolite and subor-

dinate interbedded rhyolitic ash-fall tuff. Zir-

cons from the upper ignimbritic rhyolite gave

a U-Pb age of 71.7 1.7 Ma (Fig. 7B; mean206Pb/238U age, n = 16).

In cross-section C-C (Fig. 5) the Tarahu-

mara Formation unconformably overlies the

El Jacaln diorite and the Morita Formation of

the Bisbee Group, both of which are juxtaposed

across the El Jucaral fault (Fig. 2). Tarahumara

rocks dip homoclinally 30NE and at Caada

La Nopalosa are unconformably overlain byCenozoic dacite, rhyolite, and conglomerate of

the Sierra Las Guijas (Fig. 5). The 880-m-thick

Caada Motepori stratigraphic column (Fig.

6C) was measured between the creek of same

name and Caada La Nopalosa. Its basal unit is

a 100-m-thick conglomerate with subordinate

coarse-grained sandstone and ash-fall tuff beds

that grade upward to volcaniclastic sandstone

and ash-fall tuffs. Its middle part is composed

of dacite to rhyolite ignimbrite and subordinate

ash-fall tuffs and its upper part consists of vol-

caniclastic sandstone, siltstone, andesite brec-

cia, fiamme-rich rhyolite ignimbrite, and daciteignimbrite. A tuff bed from the lower part of this

column did not yield zircons, but McDowell

et al. (2001) reported a U-Pb zircon age of

76 Ma for the volcanic section near El Tuli

ranch (Fig. 2) that we believe probably belongs

to our measured section.

At cross-section D-D (Figs. 2 and 5), theProterozoic Las Vboras Group thrusts over

the Early Cretaceous Mural Formation of the

Bisbee Group and the Tarahumara Formation

overlies both faulted units. Tarahumara beds

homoclinally dip to the northeast with angles

as steep as 50. The 1040-m-thick stratigraphic

column El Salmn (Fig. 6D) has in its lower

part an 80-m-thick conglomerate that is over-

lain by conglomerate, sandstone, and siltstone

beds arranged in upward-fining successions

with intercalations of rhyolitic and andesitic

ash-fall tuffs. Concordant detrital zircons sepa-

rated from a tuffaceous sandstone collected

133 m above the base of the formation yielded

a younger U-Pb peak age of 76.39 0.67 Ma,

interpreted as the maximum age of deposition

(Fig. 4C, inset; concordia age, n = 10), and

abundant Proterozoic grains.

5Supplemental Table 5. Summary of geochemi-cal analyses for the Laramide rocks and Proterozoicgranites of the study area. If you are viewing the PDFof this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00679.S5 or the full-textarticle on www.gsapubs.org to view SupplementalTable 5.



9/28



The middle part of the column is composed

of andesite flows and breccia, while the upper

part that crops out between Cerros El Cuervo

and Caada El Cuervo (Fig. 5) is composed of

rhyolite ignimbrite, ash-fall tuff, and andesitic to

dacitic breccia and agglomerate. A rhyolite from

this section yielded a 70.02 1.5 Ma age (Fig.

7C; mean 206Pb/238U age, n = 24). The upper-

most part of this section between Caada El

Cuervo and Arroyo El Aliso (Fig. 5) has an esti-

mated thickness of 700 m and is composed in its

lower part of bedded dacitic ignimbrite, dacitic

agglomerate and subordinate volcaniclastic

sandstone with an uppermost quartz porphyritic

rhyolite ignimbrite.

Cross-section E-E preserves the most com

plete stratigraphic succession of the Tarahumara

Formation (Figs. 2 and 5), which unconformably

Picachode Arizpe

1000

1300

Arroyo ElCumarito

ArroyoLa Galera

A 75.7 MaTahuichopafault

750

1250

Arroyo LasHigueras

CerrosEl Cuervo

CerroEl Vigia

ArroyoEl Aliso

D 76 Ma 70 Ma

2 km

CaadaEl Salmn

CaadaEl Cuervo

Rhyolite,brecciaand agglomerate

Conglomerateand sandstone

TIRACUAB

Fm

)enecoi

M/enecogil

O(

Dacite flow

1000

1500 Sierra Las GuijasC

76 Ma

CaadaMotepori

CaadaEl Picacho

CaadaLa Nopalosa

1 km

1000

CerroEl Volantin

PuertoLos Mojones

LasCabecitas

150068.6 Ma

63 MaF

Sierra LaCieneguita

1500

750

CerroColorado

ArroyoMalpaso

ArroyoLos Nogales

Sierra El OsoEl Saucitofault

E74 Ma 74.8 Ma 75 Ma

900

1500

Arroyo Los

Alisos

Cerro Cajon

de Enmedio

Puerto El

Teguachi

El Teguachi

ranch

Sierra El

Juparo

B

73 MaLos Alisos

fault62 Ma

Caada

El Potrerito

Cocospera formation

Cintura Formation

Mural Formation

Morita Formation

Cucurpe Formation

Proterozoic sedimentaryrocksEl Jacalon diorite (1.7 Ga

ee

bsi

B G r o u p

aramu

hara

T-er

P

basemen

t

Angular unconformity

Granitic to dioriticplutons

Rhyolitic ignimbriteand ash-fall tuff

Dacite

Andesite

Agglomerateand breccia

Limestone

Conglomerate

Volcanosedimentaryrocks

AMGAMEDIMARAL

CRACIT

noitamro

Faramu

ha

raT

Downthrown blockin normal faultThrust fault

Mine

Unconformity

FrontRanges

fault

500

1000

MoctezumaRanchoAgua ZarcaEl Rodeo

fault

70 Ma 5 km78.7 Ma 76 Ma

72 MaG

59 Ma

A

B

C

D

E

F

G

Figure 5. Structural cross sections from the study quadrangle showing relationships of the Tarahumara Formation with other older andyounger lithologic units. Rock ages shown as reported in this contribution, except for cross-section CC , taken from McDowell et al. (2001)Locations of cross sections indicated in Figure 2. Vertical scale is in meters. Horizontal scale of 2 km is for cross-sections AA to EE .



10/28

Gonzlez-Len et al.


overlies the Proterozoic Las Vboras Group and

dips homoclinally to the east. In the western

foothills of the Sierra El Oso, the Tarahumara

is offset by the northwest-southeast El Saucito

normal fault that dips steeply to the southwest.

In the eastern part of the section the Tarahumara

is unconformably overlain by conglomerate and

rhyolite of the Bucarit Formation. Because

of the El Saucito fault offset, we measured the

stratigraphy of the Tarahumara in two columns,

named Cerro Colorado and Sierra El Oso,

located west and east of the El Saucito fault,

respectively.

The Cerro Colorado stratigraphic column

(Fig. 6E) is 1865 m thick. Its basal unit is a

50-m-thick conglomerate composed of quartz-

rich sandstone clasts that grades upward to

a volcaniclastic succession with interbedded

rhyolite and dacite ignimbrite. Zircons from a

20-m-thick, fiamme-rich rhyolite tuff located

120 m above the base of the formation yielded

an age of 74.30 1.3 Ma (Fig. 7D; mean206Pb/238 age, n = 39). Upward, the succession

is composed of rhyolite ignimbrite and ash and

lapilli tuff, whereas its middle part consists of

andesite flows and breccia, rhyolitic and dacitic

ignimbrite, volcaniclastic sandstone, and well-

bedded ash-fall tuff. Its upper part is composed

of rhyolitic and dacitic ignimbrite, interbedded

volcaniclastic sandstone, conglomerate, and

andesite ash-fall tuff. The lower part of this col-

umn located between the measured section and

the Santa Elena Mine (Fig. 2) is intruded by a

quartz phenocrystbearing rhyolite dome that

we dated as 73.56 1.3 Ma (Figs. 6E and 7E;

mean 206Pb/238U age, n = 59).

The exposed lower part of the 930-m-thick

Sierra El Oso stratigraphic column (Fig. 6F)

is a rhyolitic ignimbrite flow that was dated as

74.64 1.5 Ma (Fig. 7F; mean 206Pb/238U age,

n = 43). Most of the lower part of this succes-

sion is composed of rhyolitic to dacitic ignim-

brite and interbedded ash-fall tuffs, whereas its

middle part consists of volcaniclastic strata,

minor ash-fall tuffs, dacite, and andesitic brec-

cia. Its upper part is andesitic and rhyolitic

ash-fall tuff, trachyandesite flows, and rhyolite.

Zircons from a rhyolitic tuff of this upper part

yielded a U-Pb age of 75.10 1.2 Ma (Fig. 7G;

mean 206Pb/238U age, n = 41).

Structural cross-sections F-F and G-G in thesouthern part of the area traverse thick succes-

sions of the upper Tarahumara Formation as its

lower part is offset by normal faults and cut by

plutonic intrusions (Figs. 2 and 5). Its estimated

thickness along section F-F is more than 2 km

dipping homoclinally to the northeast, except in

its upper part where it forms an open syncline,

the northwest-southeast axis of which follows

the upper part of the Sierra Las Palomas (Fig. 2).

Outcrops of its exposed lower part are inter-

bedded andesitic volcaniclastic sandstone, con-

glomerate, and andesite flows that are overlain

by crystal- and lithic-rhyolite ignimbrite and an

interbedded porphyritic dacite that was dated as

68.50 2.0 Ma (Fig. 7H; mean 206Pb/238U age,

n = 14). The middle part of the Tarahumara For-

mation is occupied by andesitic flows and its

upper part is composed of stratified, crystal- to

Proterozoic El Jacaln diorite

500

0

880

Basal, massive to poorly bedded, clast-supported, cobble conglomerate withclasts of subrounded sandstone,quartzarenite, andesite and gneiss.Upper part composed of coarse- tom e d iu m -g r a i ne d v o l ca n i cl a s ti csandstone and bedded ash-fall tuff.

Dacite to rhyolite ignimbrite in beds up to12 m thick, and subordinate, interbedded

ash-fall tuffs. Petrographic compositionof a dacitic ignimbrite is crystal-rich withaltered plagioclase and hornblende ing lo me ro po rp hyr it ic t ex tu re , i n ahyalopilitic groundmass.

Bedded volcaniclast ic sandstone,conglomerate, siltstone and andesitebreccia in the lower part. A 40-m-thick,f iamme-r ich rhyoli te ignimbrite in themiddle part is overlain by reddishsandstone. The upper part is composedof reddish rhyoli tic ignimbrite,subordinate ash-fall tuffs and a top 50-m-thick welded dacite tuff composed offeldspar, b io ti te and a ltered mafi cminerals.

C. Caada Motepori

500

0

1000

Massive, clast-supported, angularto sub-rounded, poorly sor tedcobble- to pebble-conglomeratewi th c la st s o f q ua rt za re ni te ,andesite, sandstone and siltstone.In terbedded, coarse-gra inedvolcaniclas tic sands tone andrhyolite to dacite ignimbrite flows,andesitic ash-fall tuff and mediumbedded, dark gray, stromatolitic,oolitic and oncolitic, dolomitizedlacustrine limestone.

Andesitic breccia and ash-fall tuff,rh yo li te i gn imbr it e fl ows a ndrhyoliticash-falltuffs.

Lithic, bedded ash-fall rhyolitic tuff,r hy ol it ic b recc ia , s ands tone ,siltstone and subordinate pebbleconglomerate lenses.

Ignimbritic rhyolite flows composedof quartzphenocrysts, feldspar andfiamme, with interbedded rhyoliticash-fall tuff.

Crystal-rich ignimbritic rhyolite

flows composed of fragmented andp ar ti al ly r es or be d q ua rt zphenocrystals, feldspar and alteredbiot i te in a vitr ic eutaxit icgroundmass. Interbedded rhyoliticash-falltuff.

LateJurassicCucurpe Formation

B. Rancho Teguachi

500

0

1260

1000

Crystal-poor rhyodacitewith crystalsof quartz, feldspar, biotite andvolcanic rock fragments in a vitricgroundmass.Crystal-richporphyriticdacite with feldspar and volcanicrock fragments in a fluidal vitricmatrix.

C o ng l om e ra t e, s a nd s to n e,

stromatolitic limestone, dacitic torhyoliticbrecciaand ash-fall tuff.Thetuff contains plagioclase,biotiteandpumice fragments altered tochlorite and clay minerals, and thegroundmass consists of devitrifiedmicrocrystal line quartz and claymaterial.

Middle and upper parts of the

column are composed of rhyolitica sh - fa l l t uf f, v ol ca ni c la st i csandstone and siltstone. The tuffoccursas bedded intervalsup to 58m thick and typically has quartz,altered feldspar and biotite crystalsin a groundmass of devitrified glassshards. The volcaniclastic strata arebrown,reddish andgreen and occurasmassivebeds upto 5 m thick thatbecomemoreabundant upwards inthesection.

EarlyCretaceousBisbeeGroup

A. Picacho de Arizpe

Figure 6 (on this and following page). Measured stratigraphic columns of the Tarahumara Formation in the study area with lithologicdescription. Thickness is in meters.



11/28



lithic-rich rhyolite ignimbrite with subordinate

rhyolitic and andesitic ash-fall tuff and breccias.

A rhyolite from the uppermost part of this suc-

cession was dated as 63.5 1.4 (Fig. 7I; mean206Pb/238U age, n = 19).

Along section G-G the Tarahumara Forma-tion dips homoclinally to the east and is offset

by normal faults that bound north-trending

Cenozoic basins (Fig. 5). At the southwestern

end of this section, the Tarahumara Formation is

intruded by the La Aurora tonalite (discussed in

the following). The faulted section of the Tara-

humara Formation between the El Rodeo ranch

and the Range Front fault (Deen and Atkinson,

1988) (Figs. 2 and 5) is ~1 km thick and com-

posed of interbedded rhyolitic ignimbrite and

pebble-cobble conglomerate, and its upper part

is composed of stratified ash-fall tuff, rhyolitic

ignimbrite, conglomerate, and sandstone. A

rhyolite from the lower part of this succession

was dated as 78.7 1.3 (K-Ar, biotite, sample

1126094; Table 1). The normal-faulted Tara-

humara succession that crops out from Sierra La

Cieneguita to the town of Moctezuma (Figs.

2 and 5) consists in its lower part of interbed-

ded rhyolitic and dacitic flows and subordinate

ash-fall tuff, volcaniclastic sandstone, and con-

glomerate; the upper part, near Moctezuma, it is

composed of stratified red ignimbrite rhyolite.

A dacitic ignimbrite from the lower part of this

section yielded an age of 75.75 0.55 Ma (Fig.

8A; mean 206Pb/238U age, 94.3% confidence, n =

14) and a rhyolite from its upper part yielded an

age of 72.20 1.6 Ma (Fig. 8B; mean 206Pb/238U

age, 96.1% confidence, n = 12). The thickness

of the Tarahumara Formation in this section

may be >1.5 km.

Other U-Pb ages of the Tarahumara succes-

sion in the study area come from samples that

are not located on a measured section. From the

Santa Ana quadrangle (Fig. 2) we dated detri-

tal zircons from a volcaniclastic sandstone (Fig.

4D, n = 85) that yielded a dominant age peak

of 152 Ma and subordinate peaks of 78, 192,

1060, and 1700 Ma, and two other rhyolites that

yielded ages of 76.0 2.7 Ma (Fig. 8C; mean206Pb/238U age, n = 13) and 73.8 1.6 Ma (Fig.

8D; mean 206Pb/238U age, n = 23). A reworked,

tuffaceous rhyolite from the Aconchi quadran

gle (Fig. 2) yielded an age of 69.1 2.4 Ma (Fig

8E; mean 206Pb/238U age, n = 17).

Laramide Plutonic Rocks

Laramide plutons in the northern part of the

study area crop out as small exposures of a few

square kilometers, but they form larger outcrop

in Sierra El Jaralito batholith in the southern par

(Roldn-Quintana, 1989). Based on field obser

vation of well-exposed outcrops, most of the

plutons are apparently fresh and homogeneou

in texture and mineralogy, although detailed

examination could reveal subtle variations in

these characteristics. As also observed by othe

authors from the Laramide plutons of Sonora

(Richard et al., 1989; Roldn-Quintana, 1991)

mineralogy of the study plutons is simple with

varying proportions of plagioclase, alkali feld

spar, quartz, biotite, scarce amphibole and pyrox

ene, and accessory minerals including titanite

opaque minerals, and zircon. Feldspar, biotite

and hornblende may present slight to moderate

Basefaulted

Rhyol i te ign imbr i te w i thplagioclase, sanidine, biotiteand rock fragments in a vitricand eutaxitic groundmass withfiammefragments.

930

Crystal-rich rhyolitic, daciticand rhyodacitic ignimbrite andinterbedded dacitic to rhyoliticash-fall tuff.

Stratified, fine- to coarse-grained, thin- to medium-b e d d e d v o l c a n i c l a s t i clitharenite, ash-fall tuffs, lensesof pebble conglomerate withclasts of volcanic rocks, daciteandandesiticbreccia.

Andesitic and rhyolitic ash-falltuffand trachyandesiteflows.

500

0

F. Sierra El Oso

0

1000

1800

200

Massive, poorly sorted, clast-supported,pebble- to cobble-conglomerate.

Bedded, arkosic to lithic sandstone,siltstone and interbedded ash-fall tuff beds,pebble-conglomerate and rhyolitic to daciticash-flow tuffs. Intruded by a quartzphenocryst-bearing rhyolite dome.

Proterozoic Las Vboras Group.

Crystal- and lithic-rich rhyolite ignimbriteand interbedded ash- and lapilli-tuff beds.Crystals in rhyolite are feldspar, quartz andbiotite; lithic grains are altered volcanicrocks. Mesostasis is vitric and eutaxitic.Flatfiamme fragmentsare common.

Aphanitic andesite flows, well-bedded ash-fall tuff and minor volcaniclastic sandstone.The upper part of this interval is massive,aphaniticandesiteflowsand beds up to 5 m

thickof andesiteandrhyolitebreccias.

Fiamme-rich rhyolite ignimbrite. Crystalsare fragmented and resorbed quartzcrystals, feldspar and biotite set in aeutaxitic, vitric mesostasis.

Porphyritic dacite ignimbrite, hornblendeandesite, breccia, and volcaniclasticsandstone. Dacite in cooling units up to 40m thick is crystal-rich with plagioclase,hornblende, andesite lithics and fiammefragments in a hyalopilitic mesotasis.An de si te has hy al op il it ic te xt ur e,phenocrystsof plagioclaseand hornblende.Volcanicbreccia in massive unitsup to 50 mthickwithblocks of andesiteandrhyolite.

Rhyolite and crystal-rich dacite ignimbriteinunits up to 50 m thick, sandstone, pebbleconglomerate and andesitic tuff. Typicallyrhyolite has abundant fiamme, up to 25%volume of feldspar and biotite crystals in avitric mesostasis. Dacite consists of up to35% crystals of feldspar, andesite lithicfragments, and biotite in a eutaxitic vitricmatrix.

E. Cerro Colorado

1040

Massive, clast-supported, poorlysorted cobble conglomerate withclasts of quartzarenite. It finesupwards topebble-conglomerate.

R e d d is h b r o w n t o p u r p l evolcaniclastic, lenticular, pebbleconglomerate, sandstone andsiltstone forming upward-finingcycles up to 45 m thick. Interbeddedyellow rhyolitic ash-fall tuff beds andsubordinateandesitetuffs.

Massive, light gray porphyritichornblende andesite and andesiticbreccia.

Lithic-, and crystal-rhyolite ignimbritein beds up to 50 m thick, subordinateinterbeds of ash-fall tuff, daciteignimbrite and andesitic to daciticbreccia and agglomerate. A typical,crystal-rich rhyolite tuff from thisinterval is composed of fragmentedand resorbed phenocrysts of quartz,fe ldspar, b io ti te and f iammefragmentsin a fluidalvitricmatrix.

500

0

ProterozoicLas Vboras Group.

D. El Salmon

Volcaniclasticrocks

Conglomerate

Andesitic brecciaand agglomerate

Rhyoliteignimbrite

AndesitictuffAndesiteflows

Dacite

Ash-falltuff

Rhyolitic brecciaand agglomerate

Limestone

Unconformity

Rhyolitic dome

Pre-Tarahumarabasement rocks

Covered

Figure 6 (continued).



12/28

Gonzlez-Len et al.


TABLE 1. SUMMARY OF U-Pb, K-Ar AND 39Ar/40Ar GEOCHRONOLOGY FOR THE LARAMIDE ROCKS IN THE STUDY AREA

Sample

Age(Ma)

U-Pb (zr)

Age(Ma)

K-Ar (bi)

Age(Ma)

Ar/Ar (bi) snoitavresbOedutitla,noitacolMTU

dna,esalcoigalpcidos,ztrauqhtiwetiloyhrcitiryhproPm7741,N9605033E002795R214.15.365-90-32-4chloritized grains of biotite in a eutaxitic vitric mesostasismoderately altered to sericite.

,etitoib,esalcoigalphtiweticadcitiryhprop,hcir-latsyrCm8121,N0098923E228395R2125.8621-90-32-4and hornblende strongly altered to chlorite. Hyalopiliticmesostasis recrystallized to microcrystalline quartz,chlorite, and iron oxides.

ztrauqdetnemgarfdnadebroserhtiwetiloyhrcitiryhproPm968,N0183333E830775R215.120.072-80-5-3crystals to 3 mm long, broken plagioclase crystals,biotite altered to chlorite and scarce volcanic rockfragments. Hyalopilitic mesostasis and glass shards.

dnadedorrochtiwetirbmingicitiloyhrhcir-latsyrCm4031,N5545433E435165R217.17.172-90-82-1fragmented quartz crystals to 3 mm long, K-feldspar andsodic plagioclase and subordinate biotite in a hyalopiliticmesostasis with glass shards.

mm2otpurapsdleffoslatsyrcderetlahtiweticadoyhRm486,N0705923E347526R2102.1/06.1+02.276-90-52-11long, quartz and subordinate laths of biotite altered tochlorite. Groundmass is vitric and fluidal with fragmentsof fiamme recrystallized to quartz and calcite.

mm2otsniargztrauqdebroserhtiwetiloyhrcitiryhproPm709,N6562233E642185R213.165.374-70-61-11long, sodic plagioclase and orthoclase and subordinatechloritized biotite. Vitric mesostasis with incipientrecrystallization.

,esalcoigalp,ztrauqfoslatsyrchtiwffutcitiloyhRm0221,N2778333E244355R216.18.372-80-5-21subordinate lithic fragments, and biotite. Crystals are

broken and angular. Quartz is locally resorbed andplagioclase is altered to calcite. Matrix recrystallized tomicrocrystalline quartz.

htiwetiloyhretirbmingi,)slatsyrc%lov02


13/28
http://geosphere.gsapubs.org/


14/28

Gonzlez-Len et al.


Mean206Pb/238Uat

63.51.4

Ma

MSW

D(concordance)=0.2

1

4-23-09-5

200

0.04

0.06

0.08

0.10

0.12

0.14

0

20

40

6

0

80

100

120

data-pointerrorellipsesare2

64

68

72

76

80

84

Mean=74.3

01.3

Ma

MSWD=2.0

(95%confidence)

(errorbarsare2)

207Pb/

206Pb

Interceptsat

70.62.3

&1692140Ma

MSWD=5.4

9-18-07-2

238U/206Pb

D

70

80

0.04

0.06

0.08

0.10

0.12 7

2

76

80

84

88

92

96

100


64

68

72

76

80

84

Mean=73.5

61.3

Ma

MSWD=6.3

(95%confidence)

(errorbarsare2)

207Pb/

206Pb

238U/206Pb

Mean206Pb/238Uat

73.51.3

Ma

MSWD(concordance)=0.88

11-16-07-4

E

60

80

100

0.0

4

0.0

6

0.0

8

0.1

0

0.1

2

0.1

4

0.1

6

0.1

8 60

70

80

90

100

110


58

62

66

70

74

78

82

86

90

94

207Pb/

206Pb

238U/206Pb

Mean206Pb/238Uat

74.61.3

Ma


8

11-19-07-1

F

Mean=74.6

41.5

Ma

MSWD=6.9

(95%confidence)

(errorbarsare2)

MSWD

=0.3

2

4-23-09-12

Interceptsat

7228&25355Ma

400

0.0

4

0.0

6

0.0

8

0.1

0

0.1

2

0.1

4

0.1

6

0.1

80

20

40

60

80

100


207Pb/

206Pb

238U/206Pb

G70

72

74

76

78

80

Mean=75.1

01.2

Ma

MSWD=2.0

(95%confidence)

(errorbarsare2)


6

2-27-08-4

Interceptsat

74.9

60.6

7&170542Ma

207Pb/

206Pb

238U/206Pb

H

I

207Pb/

206Pb

238U/206Pb

B

Mean206Pb/238Uat

71.71.7

Ma


1-28-09-2

207Pb/

206Pb

238U/206Pb

110

90

70

0.0

4

0.0

5

0.0

6

0.0

7

0.0

8

0.0

9

0.1

0 55

65

75

85

95

105

115

data-pointerrorellipsesare

2

55

65

75

85

95

Mean=71.71.7

Ma

MSWD=0.3

69(95%confidence)

(errorbarsare2)

160

0.0

4

0.0

5

0.0

6

0.0

70

20

40

60

80

100

120

data-pointerrorellipsesare

2

80

70

60

50

0.0

4

0.0

5

0.0

6

0.0

7

0.0

8

0.0

975

85

95

105

115

125


70

78

86

0.0

7

0.0

6

0.0

5

0.0

4

0.0

8

84

88

80

76

72

238U

/206Pb

207Pb/

206Pb

4-7-08-1

Mean206Pb/238Uage

75.7

00.3

0/-0.7

0Ma

MSWD=2.0


Age

boxheights

are2

TuffZirc

Age=75.7

0

+0.3

0

-0.70

Ma

(97.3

%

confidence,

fromcoherentgroup

of21)

A

207Pb/

206Pb

238U/206Pb

M

ean206Pb/238Uat

70.01.5

Ma

MSW

D(concordance)=1.5

3-5-08-2

C280

200

120

0.0

4

0.0

6

0.0

8

0.1

0

0.1

2

0.1

4 20

40

60

80

100

120


56

60

64

68

72

76

80

84

Mean=70.0

21.5

Ma

MSWD=0.4

4(95%confidence)

(errorbarsare2)

50

60

70

80

90

Mean=68.52Ma

MSWD=0.9

6(95%confidence)

(errorbarsare2)

48

52

56

60

64

68

72

76

Mean=63.51.4

Ma

MSWD=0.7

2(95%confidence)

(errorbarsare2)

Figure7.



15/28



medium grained with plagioclase, quartz, bio-

tite, K-feldspar, hornblende, titanite, apatite,

zircon, and iron oxides. Plagioclase is euhe-

dral oligoclase in crystals as large as 7 mm and

quartz is anhedral. Biotite occurs as euhedral

phenocrysts as large as 8 mm, and K-feldspar

is euhedral orthoclase and microcline as large as

3 mm. Zircons from this pluton yielded a U-Pbage of 61.10 +0.90/0.50 Ma (Fig. 8H; mean206Pb/238U age, 95.1% confidence, n = 17). La

Aurora is the largest pluton of the study area

and varies in composition from diorite in its

western outcrops to tonalite in its eastern out-

crops. It is medium grained, holocrystalline, and

hypidiomorphic with anhedral quartz, euhedral

plagioclase (oligoclase), biotite, orthoclase, and

microcline. Accessory minerals are titanite, apa-

tite, zircon, and iron oxides. La Aurora yielded

a zircon U-Pb age of 69.65 +1.05/0.45 Ma

(Fig. 8I; mean 206Pb/238U age, 95% confidence,

n = 26), and three other samples of different

localities of the pluton gave K-Ar ages (biotite)of 60.4 1.2, 57.2 1.4, and 49.5 1.1 Ma

(Table 1).

Granodioritic plutons include the Rancho

Vaquera, Los Alisos, San Antonio, Las Cabe-

citas, Puerta del Sol, and Bella Esperanza (Figs.

2 and 9A). The Rancho Vaquera is a medium-

grained, phaneritic, hypidiomorphic rock with

plagioclase, quartz, K-feldspar, and biotite and

equal or lesser amounts of hornblende, titanite,

augite, and iron oxide. This pluton gave a U-Pb

age of 55.8 0.9 Ma (Fig. 10A; mean 206Pb/238U

age, n = 27). The Los Alisos granodiorite

(Fig. 2) is medium grained, holocrystalline, andporphyritic with euhedral plagioclase (albite-

oligoclase), subhedral quartz, and euhedral

orthoclase. Secondary minerals are biotite and

magnetite in a glomeroporphyritic texture, and

accessory minerals are titanite, apatite, and zir-

con. This pluton yielded a 40Ar/39Ar plateau age

of 61.76 0.81 Ma in biotite (Fig. 11; Table 1).

The San Antonio granodiorite is medium

grained, holocrystalline, and hypidiomorphic

with euhedral plagioclase (oligoclase-ande-

sine), anhedral quartz, K-feldspar, biotite and

lesser amounts of hornblende, titanite, magne-

tite, apatite and zircon. It yielded a U-Pb age of

67.7 1.6 Ma (Fig. 10B, mean 206Pb/238U age,

n = 22). Las Cabecitas granodiorite is medium

grained, hypidiomorphic, and granular with

commonly zoned plagioclase, quartz, K-feld-

spar, biotite, and lesser amounts of hornblende,

titanite, and magnetite. Two samples collected

at different localities in this pluton yielded ages

of 59.1 1.6 (Fig. 10C; mean 206Pb/238U age,

n = 12) and 56.3 1.2 Ma (Fig. 10D; mean206Pb/238U age, n = 18).

The Puerta del Sol pluton intrudes La Alame-

dita tonalite and the Hupac granite in its outcrop

in the eastern part of the Sierra de Aconchi, but

its more extensive outcrop is in the southwest-

ern part of the study area (Fig. 2). We assume

that these outcrops belong to the same pluton

based on their similar composition and age. The

Puerta del Sol was named by Anderson et al.

(1980) for its outcrops that extend south of the

study area, where U-Pb zircon ages of 57 3 Maand 49.1 Ma were reported by Anderson et al.

(1980) and Gonzlez Becuar (2011), respec-

tively. It is medium to coarse grained and holo-

crystalline, with quartz, plagioclase, K-feldspar,

biotite, titanite, muscovite, zircon, and iron

oxide. Quartz is anhedral, as long as 1.3 cm, and

K-feldspar is euhedral orthoclase and microcline

as long as 1.2 cm; plagioclase is albite-oligo-

clase. Biotite crystals are euhedral and musco-

vite is secondary. Samples from the two different

outcrops of the Puerta del Sol within the study

area yielded U-Pb ages of 51.26 1.0 (Fig.

10E; mean 206Pb/238U age, n = 17) and 49.95

+1.05/0.45 Ma (Fig. 10F; mean 206Pb/238U age,95% confidence, n = 26), but a biotite K-Ar age

was 23.6 1.1 Ma (Table 1). We did not study

the Bella Esperanza granodiorite that was dated

by Housh and McDowell (2005), but their iso-

topic data are referred in the Discussion.

Granites are part of the El Jaralito batho-

lith and include the leucocratic El Babizo and

Hupac granites (Fig. 2). These plutons have

meter-sized enclaves of metasedimentary rocks,

and larger blocks of these rocks are interpreted as

roof pendants. The El Babizo granite is intruded

by the La Alamedita, La Aurora, Hupac, and

Puerta del Sol plutons. It is coarse grained topegmatitic and composed of subhedral quartz,

euhedral orthoclase, microcline and oligo-

clase, biotite, muscovite, hornblende, titanite,

zircon, and iron oxide. Biotite is euhedral and

hornblende is anhedral; anhedral to subhedral

muscovite occurs as late-stage filling of micro-

fractures and along twin planes of plagioclase.

Perthite is common and some plagioclase has

myrmekitic rims. The El Babizo ranges in com-

position from monzogranite to syenogranite

(Fig. 9A). Zircons from two samples collected

at different localities of this granite (Fig. 2;

Table 1) were dated as 71.50 +0.20/0.70 Ma

(Fig. 10G; mean 206Pb/238U age, 94.3% confi

dence, n = 14) and 70.50 + 0.30/0.60 Ma (Fig

10H; sample 102-098, mean 206Pb/238U age

97.7% confidence, n = 24). A biotite separate

from sample 102-098 was also dated by K-A

as 48.7 1.0 Ma (Fig. 2; Table 1).

The Hupac granite is fine to mediumgrained, allotriomorphic-granular to hypidio

morphic-granular, and composed of quartz

K-feldspar, plagioclase, muscovite, biotite, gar

net, zircon, and magnetite. Quartz is subhedra

to euhedral with undulose extinction. K-feldspa

is euhedral to subhedral orthoclase and micro

cline with late-stage inclusions of quartz, plagio

clase, and muscovite. Plagioclase is mostly

euhedral oligoclase and minor albite. Muscovite

is euhedral and biotite is anhedral. Myrmekite

and perthitic intergrowths are present. Two sam

ples from different localities yielded ages of 58

+0.60/0.90 Ma (Fig. 10I; mean 206Pb/238U age

93.5% confidence, n = 11) and 54.95 1.6 Ma(mean 206Pb/238U age, n = 36); two other sam

ples yielded K-Ar biotite ages of 29.5 0.9 and

28.7 1.0 Ma (Table 1).

Other intrusive rocks that crop out in the area

and were not studied are the Rancho Viejo dio

rite, the San Felipe porphyry, local stocks o

diorite and granodiorite in the Cumobabi Mine

(Scherkenbach et al., 1985), a rhyolitic dome

near Rancho Agua Caliente (Fig. 2), and dikes

of pegmatite, microgranite, diorite, and basal

that occur throughout the area.

GEOCHEMISTRY

Elemental Geochemistry

The analyzed volcanic rocks range in composi

tion between 63 and 78 wt% SiO2. They are mostly

high-K calc-alkaline andesites to rhyolites, and

half of the samples are high-silica rhyolites in the

(K2O/SiO

2)

Ndiagram with the fields of Peccerillo

and Taylor (1976) (Fig. 9B). Major elements such

as Al2O

3, Fe

2O

3, and CaO show negative cor

relation trends with SiO2, except for K

2O (Fig

9B), typical calc-alkaline differentiation trends

Figure 8 (on following page). Histogram and concordia diagrams of U-Pb ages of the Tarahumara Formation and plutonic rocks of the Arizpe-Mazocahui area. MSWDmeansquare of weighted deviates. (A) Sample 11-26-09-3 from a dacite tuff in the lower part ofthe Tarahumara section of Sierra La Huerta, cross-section GG . (B) Sample 11-25-09-6from the upper part of the Tarahumara section of Sierra La Huerta. (C) Sample 3-3-09-7from a rhyolite tuff of the Santa Ana quadrangle. (D) Sample 12-5-08-2 from a rhyolite inthe Santa Ana quadrangle. (E) Sample 3-30-09-13 from a rhyolitic tuff from the Aconchiquadrangle. (F) Sample 3-3-08-1 from La Cubana monzonite. (G) Sample 2-27-09-8 fromthe El Gueriguito quartz monzonite. (H) Sample 10-1-09-1 from La Alamedita tonalite(I) Sample 9-30-09-4 from the La Aurora tonalite.



16/28

Gonzlez-Len et al.


Mean206Pb/238Uat

58.11.7

Ma


6

2-27-09-8

ElGueriguito

Mean206Pb/238Uat

52.7

60.9

Ma

M

SWD(concordance)=0.7

3

3-3-08-1

LaCubana

Mean206Pb/238Uat

69.12.4

Ma


9

3-30-09-13

MSWD=0.2

3

12-5-08-2

Interceptsat

73.04.1

&144333Ma

200

0.0

4

0.0

6

0.0

8

0.1

00

20

40

60

80

100

120


100

9

0

80

70

60

0.04

0.06

0.08

0.10 6

0

70

80

90

100

110

120


D G

E

C

56

60

64

68

72

76

80

84

88

92

Mean=73.81.6Ma

MSWD=0.45(95%confidence)

(errorbarsare2)

50

60

70

80

90

Mean=69.12.4Ma


(errorbarsare2)

200

0.04

0.06

0.08

0.10

0.12 0

40

80

120

160


40

50

60

70

80

90

Mean=52.760.9Ma


(errorbarsare2)

80

60

0.04

0.08

0.12

0.16

0.20

0.24

0.28

70

90

110

130

150


42

46

50

54

58

62

66

70

74

78

Mean=58.11.7Ma


(errorbarsare2)

238U/206Pb

238U/206Pb

207Pb/

206Pb 207

Pb/206

Pb

207Pb/

206Pb

207Pb/

206Pb

207Pb/

206Pb

238U/206Pb

207Pb/

206Pb


86

78

70

0.0

40

0.0

44

0.0

48

0.0

52

0.0

56

0.0

60

0.0

64

0.0

68 7

0

74

78

82

86

90

94

MSWD

=4.3

70

72

74

76

78

80

82

84

86

Age

TuffZirc

Age=75.7

5

+0.5

5

-0.8

5Ma

(94.3

%confidence,

fromcoherentgroupof14)

boxheightsare2

11-26-09-3

Mean206Pb/238Uage

75.7

5+0.5

5/-0.8

5Ma

A

238U/206Pb

238U/206Pb

238U/206P

b

238U/206

Pb

207Pb/

206Pb


200

0.0

4

0.0

5

0.0

6

0.0

7

0.0

8

0.0

9

0.1

00

20

40

60

80

100

120

Interceptsat

72.21.6

&1089190Ma

MSWD=19

74

64

68

72

76

80

84

88

Age

boxheightsare2

TuffZirc

Age=72.2

0+1.6

0/-1.2

0Ma

(96.1

%

confidence,coherentgroupof12)

11-25-09-6

B


8

3-3-09-7

Interceptsat

7

7.34.1

&140235Ma

200

0.04

0.06

0.08

0.10

0

20

40

60

8

0

100

120


55

65

75

85

95

Mean=76.02.7

Ma

MSWD=0.6

0(95%confidence)

(errorbarsare2)

238U/206Pb

207Pb/

206Pb


78

74

70

66

0.0

40

0.0

44

0.0

48

0.0

52

0.0

56

0.0

60

0.0

64

0.0

68 8

2

86

90

94

98

102

MSWD=2.0

69.6

5+1.0

5/-0.4

5Ma

64

66

68

70

72

74

76

Age

TuffZirc

Age=69.6

5

+1.0

5-0.4

5Ma

(95%

conf,fromcoherentgroupof26)

boxheightsare2

9-30-09-4

LaAurora

238U/206Pb

207Pb/

206Pb


80

70

50

0.040

0.044

0.048

0.052

0.056

0.060

0.064

0.068 8

0

90

100

110

120

130

MSWD=6.4

61.1

0+0.9

0/-0.5

0Ma

10-1-09-1

LaAlamedita

52

56

60

64

68

72

76

Age

TuffZir

c

Age=61.10+0.90-0.50Ma

(95.1%

confidence,coherentgroupof17)

boxheightsare2

60

H

IF

Mean206Pb/238Uat

Mean206Pb/238Uat

Figure8.



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6

4

2

0

CaO

Fe

O2

3

8

6

4

2

0

PeraluminousMetaluminous

Al O /(CaO + Na O + K O) (molar)2 3 2 2

Al

O

/(Na

O

+K

O)(mo

lar)

2

3

2

2

0.5 0.7 0.9 1.1 1.3 1.5 1.7

0.6

1.0

1.4

1.8

2.2

2.6

Monzonite, diorite and granodiorites

Granites

El Jacaln dioriteSanta Margarita granite

12

14

16

18

Al

O2

3

B

D

Volcanic arc

Y + N b

Rb

1 10 1000

10

100

100

1000

Syn-collisionalWithin plate

Ocean ridge

E

Diorite - tonalite

Granite

500

250

750

1000

1500 2000 2500 3000

Alkali granite

Syenogranite

Monzogranite

Granodiorite

Tonalite

DioriteMonzodiorite

Monzonite

Quartzmonzonite

1250

R1

R2

Granodiorite

Monzonite

Gabbro-Diorite

1500

Santa Margaritagranite

El Jacaln diorite

A

1000

2

4

6

0SiO2

K

O2

Medium-

K

High-K

, calc-a

lkaline

60 70 8050

Nd Dy Ho Er Tm Yb LuSm Eu Gd TbPrCeLa

Volcanic rocks

Monzonite, diorite

and granodiorites

Huepac granite

El Babizo granite C1000

100

10

1.0

0.1

Northern and central granites ofSonora (from Valencia-Morenoet al., 2001)

IAGCAG

CCG

Volcanic rocks

Plutonic rocks

Figure 9. Geochemical discrimination plots for the geochronologically dated Laramide volcanic and plutonic rocks of the studyarea. (A) Chemical classification of the dated Laramide plutons according to the R1-R2 diagram of De la Roche et al. (1980). Rockgrouping used in this paper is derived from this plot. (B) Al2O3 versus SiO2, Fe2O3 versus SiO2, and CaO versus SiO2 Harker dia-grams, as well as (K2O vs. SiO2)N diagram with fields of Peccerillo and Taylor (1976). (C) Rare earth element diagram normalizedto chondrite (McDonough and Sun, 1995) for the volcanic and plutonic rocks of the study area compared with the northern andcentral granites of Sonora reported by Valencia-Moreno et al. (2001). (D) Shands index diagram A/(NK) versus A/(CNK) fromManiar and Piccoli (1989) to characterize geochemical and tectonic environments of the studied plutons, including the ProterozoicEl Jacaln diorite and the Santa Margarita granite. Fields for island arc (IAG), continental collision (CCG), and continental arc(CAG) granites are indicated. (E) Rb versus (Y+Nb) diagram (Pearce et al., 1984) characterizing tectonic environment.



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Gonzlez-Len et al.


Chondrite-normalized rare earth element (REE)

patterns are primarily enriched in the light (L)

REEs and have unfractionated heavy (H) REE

patterns. LaN/Lu

Nratios are moderately fraction-

ated,

Stratigraphy, geochronology, and geochemistry of the Laramide magmatic arc in north-central Sonora,...

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