40Ar/39Ar Dating Studies of Minerals and Rocks in 

various areas in Mexico: USGS/CRM Scientific 

Collaboration (Part II) 

by Alexander Iriondo1,2,3, Michael J. Kunk1, Jeffrey A. Winick1, and CRM4 

1 U.S. Geological Survey, Denver, Colorado 

2 University of Colorado at Boulder, Boulder, Colorado 

3 Universidad Nacional Autónoma de México, Centro de Geociencias, Querétaro, México 

4 Consejo de Recursos Minerales, Pachuca, México 

Open-File Report 04-1444 

on-line edition 

2004 

This report is preliminary and has not been reviewed for comformity with U.S. Geological Survey 

editorial standards or with the North American Stratigraphic Code. 

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement 

by the U.S. Government. 

U.S. Department of the Interior

U.S. Geological Survey
INTRODUCTION

This publication contains reduced 4 0Ar/3 9Ar geochronology data of 39 rocks (47 analyses) 

collected by geologists from Consejo de Recursos Minerales (CRM) in various areas in Mexico 

(fig.1–10). Also included in this report is information on rock type and the geographical location of 

the samples (table 1). Table 1 is in alphabetical order of the different regional offices of CRM in 

Mexico. These analyses were done under Annex 2 of the Memorandum of Understanding between 

the U.S. Geological Survey (USGS) and the CRM. This is the second report produced under this 

Annex; the first report was published in 2003 (Iriondo and others, 2003). The results presented here 

are intended only to be a preliminary publication of these geochronology studies, and the data are not 

interpreted in a geological context. Therefore, users unfamiliar with argon isotopic data should use 

these results carefully. This report is primarily a detailed source document for subsequent scientific 

publications and maps that will integrate this data into a geological context. 

METHODS 

Sample Preparation 

All of the rocks provided by CRM were crushed, ground, and sized using 250, 180, and 

150μm sieves (60, 80, and 100 mesh, respectively). We used the largest size fraction possible of the 

target mineral that is free of inclusions from other mineral phases. Mineral separates of hornblende, 

muscovite, biotite, and K-feldspar (including sanidine) were produced using magnetic separation, 

heavy liquids, and hand picking to achieve a purity of >99 percent. Basalt and basaltic andesite 

samples were processed through heavy liquids and(or) magnetic separation to remove phenocrysts 

from the volcanic matrix. The resulting volcanic matrix samples were leached with 10 percent cold 

HCl to remove any traces of secondary calcite. All samples were washed in acetone, alcohol, and 

deionized water (X3) in a Branson B-220 ultrasonic cleaner to remove dust and then resieved by 

hand. 

Aliquots of sample were packaged in copper capsules (aluminum for sanidine) and sealed 

2 of 46
under vacuum in quartz tubes. The samples then were irradiated for 20 hours (package KD29) in an 

aluminum container in the central thimble facility at the TRIGA reactor (GSTR) at the USGS in 

Denver. The monitor mineral used was Fish Canyon Tuff (FCT-3) sanidine with an age of 27.79 Ma 

(Kunk and others, 1985; Cebula and others, 1986) relative to Mmhb-1 with an age of 519.4 ± 2.5 

Ma (Alexander and others, 1978; Dalrymple and others, 1981). The type of container and the 

geometry of samples and standards are the ones described by Snee and others (1988). 

Sample Analysis 

The samples were analyzed at the USGS Thermochronology laboratory in Denver, by using a 

VG Isotopes Ltd., Model 1200B or a Map 216 mass spectrometer both fitted with an electron 

multiplier using the 4 0Ar/3 9Ar step-heating method of dating or 4 0Ar/3 9Ar laser fusion dating. The 

samples were heated for 10 minutes per step and followed a schedule of three to twenty one steps per 

sample. The number and temperature of heating steps were selected in an attempt to limit the 

percentage of gas released to less than 20 percent per step for most samples. Biotite aliquots also 

were melted in a single heating to produce total fusion ages. 

Heating of samples was done in a small volume, molybdenum-lined low blank tantalum 

furnace similar to that described by Staudacher and others (1978). The temperature was monitored 

by a W5Re-W2 6Re thermocouple and controlled by a proportional programmable controller. The 

furnace and the rear manifold of the extraction system were pumped between steps with a turbo 

molecular pump. Two isolated ion pumps evacuated the front manifold and the mass spectrometer 

tube between each incremental step. The gas to be analyzed was purified in the first manifold by a 

SAES ST707 Zr-V-Fe getter (VG 1200B) or a SAES ST101 Al-Zr getter (MAP 216) operated at 

room temperature and by an incandescent tungsten filament. Gas was equilibrated with the second 

manifold with an empty cold finger (VG 1200B only) in the first manifold at LN2 temperature to trap 

water and other condensibles, then isolated and cleaned in the front manifold with a SAES ST101 AlZr getter operated at 400°C and with a Ti getter operated at 350°C. 

Prior to the admission by expansion methods of the argon dominated gas to the mass 

3 of 46
spectrometer, an activated charcoal finger (VG 1200B only) submerged in a constant boiling mixture 

of dry ice and acetone was used to remove gases with a molecular weight greater than 60 or 80 

(primarily other noble gases). A second SAES ST101 active gas getter operated at room temperature 

further purified the argon-rich gas in the mass spectrometer. Argon isotopes with masses 40 through 

36 and CO2 (mass 44) were analyzed as a function of time in five to seven analyses cycles. 4 0Ar, 3 9Ar, 

3 8Ar, 3 7Ar, and 3 6Ar peaks and baselines were measured for five to fifteen one-second integrations in 

each of the five to seven cycles. After the analysis, the mass spectrometer was evacuated. All phases 

of the sample heating, cleanup, equilibration, and sample analysis were performed under computer 

control. 

Seven sanidine samples were dated using the 40Ar/39Ar-laser fusion method with the 

MAP216 mass spectrometer. For each sample, 10 to 13 single grains of sanidine were individually 

fused using a Synrad model 48-5-28(w) 50-watt CO2 laser. The laser chamber and manifold were 

pumped between analyses with a turbomolecular pump. An isolated ion pump was used to pump 

the mass spectrometer. Prior to mass spectrometer analysis, the gas was purified in the manifold 

by two SAES ST101 Al-Zr getters—one operated at room temperature and the other at 400°C, as 

well as a hot Re filament. The argon rich gas was further purified by a third SAES ST 101 getter 

operated at room temperature in the flight tube of the mass spectrometer. 

Isotopic Data Reduction 

All the argon isotopic data from the VG 1200B were reduced using an updated version of the 

computer program ArAr* (Haugerud and Kunk, 1988). The data from the MAP 216 were reduced 

using an updated version of the computer program Mass Spec (Deino, 2001). The decay constants 

used were those recommended by Steiger and Jäger (1977). The isotopic measurements made in the 

analysis had baseline values subtracted and then were regressed, to time zero by using standard linear 

regression techniques. These regressed values and associated statistical estimates of analytical 

4 of 46
uncertainties of the time zero peak values were used in data reduction. Sample blanks measured 

before the analyses (or every fourth analysis for laser fusion data) were subtracted from the 

regressed results for 4 0Ar, 3 9Ar, 3 7Ar and 3 6Ar. Error estimates of the blanks were quadratically 

combined with the regression errors and propagated through the error equations. 

Corrections for interfering reactor-produced argon isotopes from Ca, K, and Cl in the sample 

were made by using the production ratios given in Dalrymple and others (1981) and Roddick (1983). 

Errors in calculating ages or ratios include measurement errors in the analysis, decay factor 

uncertainties, measured atmospheric or calculated initial 4 0Ar/3 6Ar ratios, the irradiation parameter J, 

the production ratios of the various reactor induced argon producing reactions, the initial 3 8Ar/3 6Ar 

ratio, and the age of the monitor (Haugerud and Kunk, 1988; Deino, 2001). 

The data tables and figures presented below include the identification of individual step ages, 

plateau ages, average ages, inverse isochron ages, total fusion ages, and total gas ages for step-heating 

results. Total gas ages represent the age calculated from the addition of all of the measured argon 

peaks for all steps in a single sample. The total gas ages are roughly equivalent to conventional K-Ar 

ages. No analytical precision is calculated for total gas ages. Plateau ages were determined using the 

definition of Fleck and others (1977) as modified by Haugerud and Kunk (1988). Average ages are 

calculated in the same manner as a plateau age but fail the definition of Fleck and others (1977). 

4 0Ar/3 9Ar laser fusion data tables include individual laser fusion ages and mean ages. Inverse isotope 

correlation analysis of the analytical data (for multistep or multigrain samples) to assess if 

nonatmospheric argon components were trapped in any samples and to calculate an inverse isochron 

age was done using the method of York (1969). For additional information on the sample data 

reduction procedure, see Haugerud and Kunk (1988), and Deino (2001). 

RESULTS 

4 0Ar/3 9Ar Geochronology Data 

The 4 0Ar/3 9Ar geochronology results in this report are presented in alphabetical order of the 

regional offices from CRM in Mexico. For each regional office the data are presented in as many as 

5 of 46
four tables (tables 3–6). Table 3 contains the results of biotite total fusion analyses that were run on 

the VG 1200B. Table 4 contains the results of step-heating runs from the VG 1200B. Table 5 

contains step-heating runs from the MAP 216, and table 6 contains results from laser total fusion 

runs from the MAP 216. In all of these tables, each sample starts with a line that gives the sample 

number, rock unit, the material analyzed, and the J-value used with its analytical uncertainty, the 

sample weight in milligrams (except for table 6 where the number of grains fused per analysis is 

listed), and the packet and package number from the irradiation. The tables include a step label for 

each analysis (except table 3), the temperature of the step, the percent of potassium derived 3 9ArK for 

each step (the total for all steps in a single sample is 100 percent, except for table 6), the radiogenic 

yield (percentage of 4 0ArR that is derived from the decay of potassium), moles of 3 9ArK, a corrected 

4 0ArR/3 9ArK ratio from which the age can be directly calculated, apparent K/Ca, and K/Cl ratios for 

each step, a calculated apparent age for the step (in millions of years), and an estimate of the precision 

of each age at the 1 sigma level. The sample precision includes estimates of the errors that are unique 

to a single sample and can be used only for comparisons with other steps of the same sample. This 

error estimate does not include the error in "J". The last line in the table represents the total gas 

results for the sample. Note that no analytical error is calculated for the age in this line. If the sample 

has a plateau age, the percentage of 3 9ArK on the plateau, the steps on the plateau, and the plateau age 

and its precision are printed. The plateau, when present, is listed in the final line of the sample. In 

table 6, a weighted mean age for those laser fusion results that agree in age within the limits of 

analytical precision is the final line of the sample entry. 

Locations of the samples presented on figures 2–10 and some of the data from tables 4, 5, 

and 6 also are presented graphically following the tabular data. The VG 1200 step-heating results are 

presented in age-spectra diagrams (fig. 11) that plot the cumulative percent 3 9ArK of the steps against 

apparent age in millions of years. The precision estimate used to construct the error boxes of each 

step is at two-sigma. The upper, smaller graph plots the apparent K/Ca ratio of each step against 

cumulative 3 9ArK released. Map 216 step-heating results are presented in much the same way on 

figure 12 but also include a plot of radiogenic yield and apparent K/Cl. The laser fusion data sets 

6 of 46
(fig. 13) replace the age spectrum diagram with an age probability distribution diagram, frequently 

referred to as an ideogram and four smaller graphs. In the ideogram, a solid line represents the 

weighted mean age of the sample, while dashed lines represent the data with outliers included. 

Immediately above the ideogram are four smaller graphs—the lowest of these plots the age of 

sample ages with error bars, the two intermediate graphs are apparent K/Ca and K/Cl, and the 

upper graph is radiogenic yield. In these graphs, filled circles represent data that are included in the 

weighted mean age, while open circles are used for those that were excluded from the weighed mean 

age. 

We have calculated isochron ages using inverse-isotope correlation diagrams that plot 

39Ar/40Ar against 36Ar/40Ar. These plots are included on figures 11, 12, and 13 adjacent to the 

respective age spectrum diagram or ideogram. These diagrams include the isochron age of the 

sample (calculated from the inverse of the x-axis intercept), the calculated initial 40Ar/36Ar ratio of 

the sample (the inverse of the y-axis intercept), the MSWD of the data (a goodness of fit 

indicator), the number of points used in the age regression, and the percentage of gas included in 

the correlation (except for fig. 13). For additional information on the sample data sets see Haugerud 

and Kunk (1988) and Deino (2001). 

ACKNOWLEDGMENTS 

We would like to thank Seth Fetters, former University of Colorado at Boulder 

undergraduate student and volunteer at the USGS, for his assistance in preparing mineral separates. 

In addition, we are thankful to both USGS internal reviewers for their helpful comments about this 

publication. 

7 of 46
REFERENCES

Alexander, E.C., Jr., Mickelson, G.M., and Lanphere, M.A., 1978, Mmhb-1: a new 4 0Ar/3 9Ar dating 

standard, in Zartman, R.E., ed., Short papers of the fourth international conference, 

geochronology, cosmochronology, and isotope geology: U.S. Geological Survey Open-File 

Report 78–701, p. 6–8. 

Cebula, G.T., Kunk, M.J., Mehnert, H.H., Naeser, C.W., Obradovich, J.D., and Sutter, J.F., 1986, The 

Fish Canyon Tuff: A potential standard for the 4 0Ar/3 9Ar and fission track dating methods: 

Terra Cognita, v. 6, no. 2, p. 140. 

Dalrymple, G.B., Alexander, E.C., Lanphere, M.A., and Kraker, G.P., 1981, Irradiation of samples for 

4 0 Ar/3 9Ar dating using the Geological Survey TRIGA reactor: U.S. Geological Survey 

Professional Paper 1176, 55 p. 

Deino, A.L., 2001, Users manual for Mass Spec v. 5.02: Berkeley Geochronology Center Special 

Publication 1a, 119 p. 

Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977, Interpretation of discordant 4 0Ar/3 9Ar age spectra of 

Mesozoic tholeiites from Antarctica: Geochimica et Cosmochimica Acta, v. 41, p. 15–32. 

Haugerud, R.A., and Kunk, M.J., 1988, ArAr*, a computer program for reduction of 4 0Ar-3 9Ar data: 

U.S. Geological Survey, Open-File Report 88–261, 68 p.

Iriondo Alexander, Kunk, M.J., Winick, J.A., and Consejo de Recursos Minerales (CRM), 2003, 

4 0Ar/3 9Ar dating studies of minerals and rocks in various areas in Mexico: USGS/CRM 

8 of 46
scientific collaboration (Part I): U.S. Geological Survey, Open-File Report 03–020, 79 p. 

Kunk, M.J., Sutter, J.F., and Naeser, C.W., 1985, High-precision 4 0Ar/3 9Ar Ages of Sanidine, Biotite, 

Hornblende, and Plagioclase from the Fish Canyon Tuff, San Juan Volcanic Field, Southcentral Colorado: Geological Society of America Abstracts with Programs, v. 17, p. 636. 

Roddick, J.C., 1983, High precision calibration of 4 0Ar-3 9Ar standards: Earth and Planetary Science 

Letters, v. 47, p. 887–898. 

Snee, L.W., Sutter, J.F., and Kelly, W.C., 1988, Thermochronology of economic mineral deposits: 

Dating the stages of mineralization at Panasqueira, Portugal, by high precision 4 0Ar/3 9Ar age 

spectrum techniques on muscovite: Economic Geology, v. 83, p. 335–354. 

Staudacher, Th., Jessberger, E.K., Dorflinger, D., and Kiko, J., 1978, A refined ultrahigh-vacuum 

furnace for rare gas analysis: J. Phys. E: Sci. Instrum. v. 11, p. 781–784. 

Steiger, R.H., and Jäeger, E., 1977, Subcommission on geochronology: Convention on the use of 

decay constants in geo- and cosmo-chronology: Earth and Planetary Science Letters, v. 36, 

p. 359–363.

York, Derek, 1969, Least squares fitting of a straight line with correlated errors: Earth and Planetary 

Science Letters, v.5, p. 320–324. 

9 of 46
Table 1. Summary of rock type and sample geographical location for dated rocks. 

Sample Type of rock Geographic Location CRM Geologic Map 

Lat. Long. 1:50,000 

Oficina Regional Chihuahua 

POD-1 rhyolite 28° 54.637' 104° 04.431' Pozos 

POD-2 andesite 28° 49.602' 104° 08.430' Pozos 

DHP-12 orthogneiss 28° 57.155' 105° 12.947' Chorreras 

DHP-14 granodiorite 28° 51.889' 105° 15.096' Chorreras 

DHP-15 granodiorite 28° 52.160' 105° 14.145' Chorreras 

FDRA-001 rhyolite tuff 28° 38.092' 105° 09.493' El Jateado 

FDRA-002 rhyolite 28° 35.538' 105° 17.712' El Jateado 

FVDA-1 dacite 28° 27.849' 104° 44.081' La Coyota 

FVDA-2 rhyolite tuff 28° 22.596' 104° 43.920' La Coyota 

AMDT-01 rhyolite 30° 14.023' 105° 59.946' San Antonio El Bravo 

Oficina Regional Chilpancingo 

VH-134 gabbro 18° 01' 54.8'' 100° 08' 57.5'' Villa Hidalgo 

Oficina Regional Culiacan 

TA-35 granodiorite 26° 47.597' 108° 12.991' Tasajeras 

TA-36 tonalite 26° 48.483' 108° 09.779' Tasajeras 

TA-37 granodiorite 26° 48.752' 108° 15.737' Tasajeras 

YE-25 granodiorite 26° 28.070' 108° 04.267' Yecorato 

YE-27 rhyolite 26° 25.304' 108° 13.916' Yecorato 

Oficina Regional Durango 

BO-1012 andesite? vitrophere 23° 45' 21.4'' 105° 44' 55.9'' Borbollones 

IR-585 andesite? vitrophere 24° 33' 46.3'' 104° 01' 57.2'' Ignacio Ramirez 

SP-405 basalt 25° 01' 55.4'' 105° 24' 17.7'' Santiago Papasquiaro 

Oficina Regional Hermosillo 

FA-4 rhyolite tuff 30° 16' 53.6'' 110° 06' 53.08'' Arizpe 

FA-5 microdiorite 30° 24' 22.1'' 110° 00' 27'' Arizpe 

2D-04 andesite porphyry 30° 25' 52'' 110° 15' 55.2'' Bacanuchi 

DATA-3 rhyolite tuff 30° 32' 58'' 110° 21' 03'' Santa Teresa 

Oficina Regional Morelia 

PL-272 gabbro 18° 27.151' 102° 20.587' Playitas 

RA-104 diorite 18° 38.440' 103° 26.096' Colola 

RA-162 granodiorite 18° 30.914' 103° 24.828' Colola 

Oficina Regional Oaxaca 

PQ-04 granodiorite 16° 55' 13.0'' 96° 01' 41.4'' San Pedro Quiatoni 

PQ-06 rhyolite tuff 16° 52' 59.0'' 96° 15' 01.2'' San Pedro Quiatoni 

MR-001 andesite porphyry 16° 44' 49.4'' 96° 01' 04.8'' Totolapan 

MR-003 andesite 16° 38' 48.3'' 96° 18' 57.2'' Totolapan 

Oficina Regional Saltillo 

LED-1 basalt 27° 36.225' 101° 27.289' Nueva Rosita 

CA-CI79 diorite 29° 03.094' 101° 39.423' CD. Acuña 

CA-CI85 monzonite? 29° 16.079' 101° 49.433' CD. Acuña 

10 of 46
Table 1. Summary of rock type and sample geographical location for dated rocks.–Continued 

Sample Type of rock Geographic Location CRM Geologic Map 

Lat. Long. 1:50,000 

Oficina Regional Saltillo (cont.) 

MB-99 gabbro-diorite 29° 11.912' 103° 47.555' Manuel Benavides 

MB-114 sienite 29° 14.088' 102° 10.256' Manuel Benavides 

SMD-01 granite porphyry 28° 16.103' 103° 23.754' San Miguel 

SMD-02 basalt 28° 43.299' 102° 32.334' San Miguel 

SMD-03 sienite 28° 39.996' 102° 48.746' San Miguel 

Oficina Regional San Luis Potosi 

PV-003 trachyte porphyry 23° 22.269' 98° 09.830' Ciudad Victoria 

11 of 46
Table 2. Summary table for dated rocks from USGS/CRM Collaboration Part II. 

Sample Rock type Laboratory Mineral Type of age Age Error Comments 

number (Ma) (Ma) 

Oficina Regional Chihuahua 

POD-1 rhyolite #139KD29 sanidine total fusion 32.82 ± 0.04 — 

POD-2 andesite #125KD29 plagioclase plateau 32.35 ± 0.05 — 

#135KD29 volcanic matrix average 33.07 ± 0.05 — 

DHP-12 orthogneiss #148KD29 hornblende average 954.70 ± 4.14 — 

DHP-14 granodiorite #127KD29 K-feldspar plateau 32.69 ± 0.10 — 

DHP-15 granodiorite #163KD29 biotite total fusion 33.13 ± 0.02 — 

FDR-001 rhyolite tuff #140KD29 sanidine total fusion 32.34 ± 0.07 — 

FDR-002 rhyolite #159KD29 biotite total fusion 41.88 ± 0.03 — 

FVDA-1 dacite #142KD29 sanidine total fusion 33.32 ± 0.05 — 

FVDA-2 rhyolite tuff #136KD29 sanidine total fusion 32.77 ± 0.05 — 

AMDT-01 rhyolite #138KD29 sanidine total fusion 37.64 ± 0.05 — 

Oficina Regional Chilpancingo 

VH-134 gabbro #181KD29 hornblende isochron 94.08 ± 4.38 — 

Oficina Regional Culiacan 

TA-35 granodiorite #147KD29 hornblende isochron 56.13 ± 0.45 — 

#175KD29 biotite total fusion 58.12 ± 0.06 — 

TA-36 tonalite #145KD29 hornblende plateau 57.85 ± 0.31 — 

#167KD29 biotite total fusion 59.38 ± 0.05 — 

TA-37 granodiorite #146KD29 hornblende average 57.24 ± 0.31 — 

#179KD29 biotite total fusion 58.50 ± 0.03 — 

YE-25 granodiorite #149KD29 hornblende average 86.07 ± 0.47 — 

#173KD29 biotite total fusion 78.79 ± 0.03 — 

YE-27 rhyolite #141KD29 sanidine total fusion 33.60 ± 0.50 — 

Oficina Regional Durango 

BO-1012 andesite? vitrophere #124KD29 plagioclase plateau 23.54 ± 0.05 — 

IR-585 andesite? vitrophere #121KD29 plagioclase average 31.06 ± 0.05 — 

SP-405 basalt #133KD29 volcanic matrix average 10.95 ± 0.02 — 

Oficina Regional Hermosillo 

FA-4 rhyolite tuff #137KD29 sanidine total fusion 24.53 ± 0.05 — 

#171KD29 biotite total fusion 24.62 ± 0.09 — 

FA-5 microdiorite #161KD29 biotite total fusion 27.47 ± 0.05 — 

Oficina Regional Hermosillo (cont.) 

2D-04 andesite porphyry #122KD29 plagioclase plateau 26.89 ± 0.06 — 

DATA-3 rhyolite tuff #150KD29 biotite total fusion 25.59 ± 0.04 — 

Oficina Regional Morelia 

PL-272 gabbro #180KD29 hornblende average 215.28 ± 1.13 — 

RA-104 diorite #153KD29 biotite total fusion 61.24 ± 0.03 — 

RA-162 granodiorite #143KD29 hornblende average age 57.03 ± 0.31 — 

Oficina Regional Oaxaca 

PQ-04 granodiorite #144KD29 hornblende total gas ~16 Bad sample, bad analysis 

PQ-06 rhyolite tuff #177KD29 biotite total fusion 15.48 ± 0.02 — 

MR-001 andesite porphyry #131KD29 volcanic matrix average 17.51 ± 0.05 — 

12 of 46
Table 2. Summary table for dated rocks from USGS/CRM Collaboration part II.–Continued 

Sample Rock type Laboratory Mineral Type of age Age Error Comments 

number (Ma) (Ma) 

Oficina Regional Oaxaca (cont.) 

MR-001 andesite porphyry #123KD29 plagioclase isochron 17.09 ± 0.06 — 

MR-003 andesite #134KD29 volcanic matrix average 22.31 ± 0.03 — 

Oficina Regional Saltillo 

LED-1 basalt #132KD29 volcanic matrix isochron 2.05 ± 0.03 — 

CA-CI79 diorite #157KD29 biotite total fusion 42.51 ± 0.02 — 

CA-CI85 monzonite? #169KD29 biotite total fusion 41.13 ± 0.05 — 

MB-99 gabbro-diorite #120KD29 plagioclase plateau 32.56 ± 0.08 — 

MB-114 sienite #165KD29 biotite total fusion 30.07 ± 0.02 — 

SMD-01 granite porphyry #187KD29 white mica plateau 32.71 ± 0.05 — 

#185KD29 K-feldspar plateau 32.70 ± 0.06 — 

SMD-02 basalt #130KD29 volcanic matrix average 46.45 ± 0.07 — 

SMD-03 sienite #155KD29 biotite total fusion 44.59 ± 0.05 — 

Oficina Regional San Luis Potosi 

PV-003 trachyte porphyry #129KD29 K-feldspar total gas ~33 Bad sample, bad analysis 

13 of 46
Part I 

Part II 

Fig. 2 

Fig. 5 

Fig. 6 

Fig. 8 

Fig. 9 

Fig. 10 

Fig. 4 

Fig. 7 

Fig. 3 

14 of 46

Figure 1. Geologic map of Mexico showing areas under study for USGS/CRM Scientific Collaboration in 40Ar/39Ar geochronology. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 
30° 

102° 

DHP-12 

DHP-14 DHP-15

FDR-002 

FDR-001 

FVDA-1 

FVDA-2 

POD-01 

POD-02 

Oficina Regional Chihuahua 

28° 

104° 

san: 37.64 ± 0.05 Ma 

san: 32.82 ± 0.04 Ma 

plag: 32.35 ± 0.05 Ma 

san: 33.32 ± 0.05 Ma 

san: 32.77 ± 0.05 Ma 

san: 32.34 ± 0.07 Ma 

bio: 41.88 ± 0.03 Ma 

horn: ~950 Ma 

bio: 33.13 ± 0.02 Ma K-spar: 32.69 ± 0.10 Ma 

AMDT-01 

Figure 2. Sample location for samples collected by geologists from the Oficina Regional Chihuahua. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviations: san (sanidine); horn (hornblende); K-spar (K-feldspar); bio (biotite); plag (plagioclase). 

15 of 46
Oficina Regional Chilpancingo 

VH-134 

17° 

101° 

18° 

99° 

horn: 94.08 ± 4.38 Ma 

Figure 3. Sample location for samples collected by geologists from the Oficina Regional Chilpangingo. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviation: horn (hornblende). 

16 of 46
Oficina Regional Culiacan 

YE-27 

25° 

109° 

27° 

107° 

YE-25 

horn: 56.13 ± 0.45 Ma 

san: 33.60 ± 0.50 Ma 

horn: 57.24 ± 0.31 Ma horn: 57.85 ± 0.31 Ma

horn: 86.07 ± 0.47 Ma 

TA-36 

TA-35 

TA-37 

Figure 4. Sample location for samples collected by geologists from the Oficina Regional Culiacan. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviations: san (sanidine); horn (hornblende). 

17 of 46
Oficina Regional Durango 

25° 

106° 

BO-1012 

SP-405 

IR-585 

24° 

104° 

plag: 23.54 ± 0.05 Ma 

plag: 31.06 ± 0.05 Ma 

vm: 10.95 ± 0.02 Ma 

Figure 5. Sample location for samples collected by geologists from the Oficina Regional Durango. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviations: plag (plagioclase); vm (volcanic matrix). 

18 of 46
Oficina Regional Hermosillo 

31° 

° 

2D-04 

30° 

109° 

san: 25.59 ± 0.04 Ma 

plag: 26.89 ± 0.06 Ma 

san: 24.53 ± 0.05 Ma 

bio: 27.47 ± 0.05 Ma 

111 DATA-3 

FA-4 

FA-5 

Figure 6. Sample location for samples collected by geologists from the Oficina Regional Hermosillo. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviations: san (sanidine); bio (biotite); plag (plagioclase). 

19 of 46
Oficina Regional Morelia 

18° 

104° 

RA-104 

RA-162 

PL-272 

19° 

102° 

horn: 57.03 ± 0.31 Ma 

bio: 61.24 ± 0.03 Ma 

horn: 215.28 ± 1.13 Ma 

Figure 7. Sample location for samples collected by geologists from the Oficina Regional Morelia. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviations: horn (hornblende); bio (biotite). 

20 of 46
Oficina Regional Oaxaca 

16° 

97° 

MR-003 

17° 

95° 

MR-001 

PQ-04 

PQ-06 horn: ~16 Ma 

plag: 17.09 ± 0.06 Ma 

vm: 22.31 ± 0.03 Ma 

bio: 15.48 ± 0.02 Ma 

Figure 8. Sample location for samples collected by geologists from the Oficina Regional Oaxaca. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviations: horn (hornblende); bio (biotite); plag (plagioclase); vm (volcanic matrix). 

21 of 46
Oficina Regional Saltillo 

27° 

104° 

MB-99 

29° 

100° 

SMD-02 

SMD-01 

SMD-03 

CA-CI85 

CA-CI79 

LED-1 

plag: 32.56 ± 0.08 Ma 

bio: 30.07 ± 0.02 Ma 

bio: 41.13 ± 0.05 Ma 

bio: 42.51 ± 0.02 Ma 

vm: 46.45 ± 0.07 Ma 

wm: 32.71 ± 0.05 Ma 

bio: 44.59 ± 0.05 Ma 

vm: 2.05 ± 0.03 Ma 

MB-114 

Figure 9. Sample location for samples collected by geologists from the Oficina Regional Saltillo. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviations: bio (biotite); plag (plagioclase); vm (volcanic matrix); wm (white mica). 

22 of 46
Oficina Regional San Luis Potosi 

22° 

99° 

24° 

98° 

K-spar: ~33 Ma 

PV-003 

Figure 10. Sample location for samples collected by geologists from the Oficina Regional SL Potosi. 

Background geologic map corresponds to Carta Geologica de la Republica Mexicana (1:2,000,000). 

Abbreviation: K-spar (K-feldspar). 

23 of 46
Table 3. 40Ar/39Ar biotite total fusion data obtained using the VG 1200 spectrometer. 

Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles x 10-12) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Chihuahua 

DHP-15 granodiorite biotite TF J = 0.005197 ± 0.50% wt = 6.4 mg #163KD29 

1450 100 90.4 0.375431 3.566 32.6 25 33.13 ± 0.02 

FDR-002 rhyolite biotite TF J = 0.005203 ± 0.50% wt = 6.3 mg #159KD29 

1450 100 92.9 0.414443 4.514 344.1 73 41.88 ± 0.03 

Oficina Regional Culiacan 

TA-35 granodiorite biotite TF J = 0.005173 ± 0.50% wt = 5.7 mg #175KD29 

1450 100 96.1 0.193665 6.329 6.7 181 58.12 ± 0.06 

TA-36 tonalite biotite TF J = 0.005202 ± 0.50% wt = 4.0 mg #167KD29 

1450 100 94.2 0.224037 6.433 17.4 37 59.38 ± 0.05 

TA-37 granodiorite biotite TF J = 0.005129 ± 0.50% wt = 6.1 mg #179KD29 

1450 100 96.6 0.352050 6.426 17.7 51 58.50 ± 0.03 

YE-25 granodiorite biotite TF J = 0.005175 ± 0.50% wt = 5.4 mg #173KD29 

1450 100 99.3 0.319902 8.627 31.3 134 78.79 ± 0.03 

Oficina Regional Hermosillo 

FA-4 rhyolite tuff biotite TF J = 0.005197 ± 0.50% wt = 2.3 mg #171KD29 

1450 100 78.9 0.128676 2.644 46.3 91 24.62 ± 0.09 

FA-5 microdiorite biotite TF J = 0.005201 ± 0.50% wt = 5.6 mg #161KD29 

1450 100 92.7 0.261125 2.950 4.3 55 27.47 ± 0.05 

DATA-3 rhyolite tuff biotite TF J = 0.005203 ± 0.50% wt = 4.5 mg #150KD29 

1450 100 89.0 0.264577 2.745 50.6 144 25.59 ± 0.04 

Oficina Regional Morelia 

RA-104 diorite biotite TF J = 0.005200 ± 0.50% wt = 4.0 mg #153KD29 

1450 100 96.5 0.219440 6.640 22.0 22 61.24 ± 0.03 

Oficina Regional Oaxaca 

PQ-06 rhyolite tuff biotite TF J = 0.005130 ± 0.50% wt = 6.0 mg #177KD29 

1450 100 64.7 0.313195 1.680 99.5 76 15.48 ± 0.02 

Oficina Regional Saltillo 

CA-CI-79 diorite biotite TF J = 0.005204 ± 0.50% wt = 6.5 mg #157KD29 

1450 100 96.4 0.411054 4.582 20.4 203 42.51 ± 0.02 

CA-CI-85 monzonite? biotite TF J = 0.005200 ± 0.50% wt = 6.1 mg #169KD29 

1450 100 80.3 0.253222 4.435 91.2 138 41.13 ± 0.05 

MB-114 sienite biotite TF J = 0.005203 ± 0.50% wt = 3.1 mg #165KD29 

1450 100 79.0 0.180365 3.231 54.5 191 30.07 ± 0.02 

SMD-03 sienite biotite TF J = 0.005198 ± 0.50% wt = 5.3 mg #155KD29 

1450 100 86.5 0.266389 4.815 15.6 38 44.59 ± 0.05 

Ages calculated assuming an initial 40Ar/36Ar = 295.5± 0.

All precision estimates are at the one sigma level of precision.

24 of 46
Table 4. 40Ar/39Ar step-heating data obtained using the VG 1200 spectrometer. 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles x 10-12) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Chihuahua 

DHP-12 orthogneiss hornblende J = 0.005099 ± 0.50% wt = 248.0 mg #148KD29 

A 1100 18.2 97.6 0.095233 52.889 0.09 19 430.73 ± 0.40 

B 1125 16.2 98.6 0.084824 92.634 0.06 18 697.88 ± 0.62 

C 1150 32.7 98.9 0.170670 135.530 0.04 24 947.75 ± 1.02 

D 1250 19.0 99.2 0.099280 132.522 0.04 19 931.32 ± 1.03 

E 1450 13.8 99.1 0.072300 140.043 0.04 19 972.13 ± 0.73 

Total Gas 100.0 98.7 0.522307 113.549 0.05 21 824.03 

65.5% of gas on plateau-like in 1150 through 1450 steps Average Age = 954.70 ± 4.14 

Oficina Regional Chilpancingo 

VH-134 grabbro hornblende J = 0.005096 ± 0.50% wt = 218.0 mg #181KD29 

A 1000 4.2 52.7 0.040099 9.239 0.30 152 82.99 ± 0.34 

B 1100 10.5 80.0 0.099434 11.073 0.04 71 99.03 ± 0.22 

C 1125 4.2 77.5 0.039427 10.707 0.06 39 95.84 ± 0.55 

D 1150 6.9 89.1 0.065663 11.292 0.06 29 100.93 ± 0.36 

E 1175 12.7 90.1 0.120716 11.922 0.07 29 106.41 ± 0.17 

F 1200 11.8 93.2 0.112037 11.730 0.07 38 104.74 ± 0.18 

G 1250 18.1 92.7 0.171483 11.933 0.07 36 106.50 ± 0.16 

H 1450 31.5 89.9 0.298368 12.588 0.06 30 112.16 ± 0.16 

Total Gas 100.0 87.6 0.947227 11.814 0.07 42 105.47 

No Plateau Age 

Oficina Regional Culiacan 

TA-35 granodiorite hornblende J = 0.005100 ± 0.50% wt = 252.9 mg #147KD29 

A 1100 10.8 69.8 0.066388 5.652 0.04 16 51.27 ± 0.24 

B 1125 31.2 86.6 0.191543 6.107 0.04 11 55.33 ± 0.08 

C 1150 32.5 92.8 0.199185 6.157 0.05 10 55.77 ± 0.10 

D 1250 25.4 93.2 0.155959 6.149 0.04 11 55.71 ± 0.09 

Total Gas 100.0 88.5 0.613075 6.085 0.05 11 55.13 

No Plateau Age 

TA-36 tonalite hornblende J = 0.005155 ± 0.50% wt = 240.2 mg #145KD29 

A 1100 18.6 63.0 0.092512 6.052 0.03 3 55.42 ± 0.68 

B 1125 27.6 86.2 0.137557 6.393 0.04 4 58.49 ± 0.23 

C 1150 21.3 92.9 0.105963 6.336 0.04 3 57.98 ± 0.12 

D 1250 17.2 91.3 0.085450 6.301 0.03 3 57.67 ± 0.17 

E 1450 15.3 84.8 0.076126 6.269 0.03 3 57.38 ± 0.41 

Total Gas 100.0 84.0 0.497608 6.283 0.04 3 57.50 

53.8% of gas on plateau in 1150 through 1450 steps Plateau Age = 57.85 ± 0.31 

TA-37 granodiorite hornblende J = 0.005157 ± 0.50% wt = 248.3 mg #146KD29 

A 1100 6.6 64.6 0.062433 6.056 0.05 6 55.48 ± 0.20 

B 1125 19.1 85.2 0.179368 6.270 0.06 5 57.41 ± 0.11 

C 1150 42.3 94.3 0.397725 6.414 0.06 5 58.71 ± 0.08 

D 1175 9.3 93.5 0.086993 6.083 0.06 5 55.72 ± 0.10 

E 1250 12.4 93.4 0.116073 6.234 0.06 5 57.08 ± 0.12 

F 1450 10.3 87.0 0.097179 6.244 0.05 5 57.17 ± 0.15 

Total Gas 100.0 89.6 0.939771 6.292 0.06 5 57.61 

100% of gas on plateau-like in 1100 through 1450 steps Average Age = 57.24 ± 0.31 

25 of 46
Table 4. 40Ar/39Ar step-heating data obtained using the VG 1200 spectrometer.–Continued 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles x 10-12) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Culiacan (cont.) 

YE-25 granodiorite hornblende J = 0.005101 ± 0.50% wt = 246.9 mg #149KD29 

A 900 45.9 93.9 

B 1000 27.7 97.3 

C 1100 10.5 96.8 

D 1250 5.6 95.9 

E 1450 10.4 95.5 

Total Gas 100.0 95.5 

0.654103 9.832 0.09 9 88.28 ± 0.10 

0.394367 9.604 0.09 9 86.29 ± 0.04 

0.149163 9.429 0.09 9 84.74 ± 0.13 

0.079456 9.150 0.09 9 82.30 ± 0.14 

0.148627 9.610 0.09 9 86.33 ± 0.16 

1.425716 9.666 0.09 9 86.82 

100% of gas on plateau-like in 900 through 1450 steps Average Age = 86.07 ± 0.47 

Oficina Regional Morelia 

PL-272 grabbro hornblende J = 0.005092 ± 0.50% wt = 217.0 mg #180KD29 

A 900 45.3 69.1 

B 1250 18.6 71.4 

C 1450 36.1 76.4 

Total Gas 100.0 72.2 

0.046767 24.291 0.01 13 210.38 ± 1.92 

0.019208 22.901 0.01 16 198.98 ± 1.06 

0.037295 26.232 0.01 15 226.17 ± 0.81 

0.103270 24.733 0.01 14 213.99 

100% of gas on plateau-like in 900 through 1450 steps Average Age = 215.28 ± 1.13 

RA-162 granodiorite hornblende J = 0.005153 ± 0.50% wt = 247.1 mg #143KD29 

A 1100 2.8 83.1 

B 1150 31.8 95.5 

C 1175 36.9 98.1 

D 1250 13.9 97.3 

E 1300 14.6 95.9 

Total Gas 100.0 96.4 

0.037229 5.652 0.08 24 51.79 ± 0.24 

0.423400 6.255 0.08 18 57.23 ± 0.10 

0.491769 6.238 0.08 17 57.08 ± 0.08 

0.185859 6.140 0.08 18 56.20 ± 0.19 

0.195247 6.106 0.08 18 55.89 ± 0.54 

1.333504 6.194 0.08 17 56.68 

97.2% of gas on plateau-like in 1150 through 1300 steps Average Age = 57.03 ± 0.31 

Oficina Regional Oaxaca 

PQ-04 granodiorite hornblende J = 0.005103 ± 0.50% wt = 115.5 mg #144KD29 

A 1125 21.3 100.0

B 1150 33.5 87.2

C 1175 19.2 86.4

D 1200 6.6 73.9

E 1250 19.4 75.9

Total Gas 100.0 86.7 

0.083221 2.018 0.05 8 18.48 ± 8.32 

0.130537 1.852 0.06 2 16.97 ± 0.02 

0.074868 1.542 0.07 6 14.14 ± 0.18 

0.025845 0.998 0.07 7 9.16 ± 0.55 

0.075533 1.628 0.06 7 14.93 ± 0.34 

0.390004 1.721 0.06 5 15.77 

No Plateau Age 

Ages calculated assuming an initial 40Ar/36Ar = 295.5± 0.

All precision estimates are at the one sigma level of precision.

Ages of individual steps do not include error in the irradiation parameter J.

No error is calculated for the total gas age.

26 of 46
Oficina Regional Chihuahua 

K/Ca 

DHP-12 

hornblende 

#148KD29 

A 

B 

C D 

E 

954.70 ± 4.14 Ma 

Average Age 

0.1 

Oficina Regional Chihuahua 0.01 

0 

0 0.006 0.012 0.018 0 20 40 60 80 100


Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Chilpancingo 

A 

B 

C 

D

E 

#148KD29 

± 374.22 Ma 

[

40Ar/36Ar]i

 = 12375.53 ± 12940.18 

MSWD = 4.295 

Steps C through E with 65.5% of 39ArK 

DHP-12 

hornblende 

#181KD29 

VH-134 

hornblende 

K/Ca 

A 

B C 

D 

E F G H 

Isochron Age = 631.92 

1


1100
 0.00008

1000


900


800


700


600


500


400


300


200


Apparent Age (Ma) Apparent Age (Ma) Apparent Age (Ma)

36Ar/40Ar 36Ar/40Ar 0.00004 

0.1 

Oficina Regional Chilpancingo 0.01 

A 

B 

C 

E D

GF H 

± 4.38 Ma 

[

40Ar/36Ar]i

 = 694.1 ± 129.13 

MSWD = 2.498 

Steps E through H with 74.2% of 39ArK 

#181KD29 

VH-134 

hornblende 

Isochron Age = 94.08 

140


0.003 

120


100


80


60


36Ar/40Ar

0.002 

0.001 40


20


0 0

0 0.02 0.04 0.06 0.08 0 20 40 60 80 100


Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Culiacan 

* 

K/Ca 

hornblende 

#147KD29 

A 

B C D 

TA-35 

0.1 

0.01 

0.003 

Oficina Regional Culiacan 

A 

B 

C D 

± 0.45 Ma 

[

40Ar/36Ar]i

 = 267.39 ± 17.61 

MSWD = 0.088 

Steps B through D with 89.2% of 39ArK 

#147KD29 

hornblende 

Isochron Age = 56.13 

TA-35 

80


60
 0.002 

40


0.001 

20


0 0 

0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15


Cumulative % 39ArK Released 39Ar/40Ar 

Figure 11. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected


by CRM geologists from different areas in Mexico. These data were produced using the VG 1200


spectrometer at the USGS Thermochronology laboratory in Denver, Colorado.


27 of 46
Oficina Regional Culiacan 

K/Ca 

hornblende 

#145KD29 

A B C D E 

57.85 ± 0.31 Ma 

TA-36 

Plateau Age 

0.1 

Oficina Regional Culiacan 0.01 

0.003 

* 

A 

B 

C D 

E 

hornblende 

#145KD29 

± 0.93 Ma 

[

40Ar/36Ar]i

 = 309.12 ± 36 

MSWD = 0.936 

Steps B through E with 81.4% of 39ArK 

TA-36 

Isochron Age = 57.63 

80 

60 

Apparent Age (Ma) Apparent Age (Ma) Apparent Age (Ma) 

36Ar/40Ar 36Ar/40Ar

0.002 

40 

0.001 

20 

0 0 

0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Culiacan 

* 

A 

B 

C E D

F 

± 0.31 Ma 

[

40Ar/36Ar]i

 = 276.84 ± 2.45 

MSWD = 34.231 39ArK 

hornblende 

#146KD29 

K/Ca 

hornblende 

#146KD29 

A B C D E F 

57.24 ± 0.31 Ma 

Isochron Age = 57.62 

Steps A through F with 100% of 

TA-37 

TA-37 

Average Age 

0.1 

Oficina Regional Culiacan 0.01 

0.003 

0 0 0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Culiacan 

A 

B 

C 

D 

E 

± 0.45 Ma 

[

40Ar/36Ar]i

 = 512.94 ± 13.78 

MSWD = 48.907 39ArK 

YE-25 

hornblende 

#149KD29 

K/Ca 

YE-25 

hornblende 

#149KD29 

A B C D E 

86.07 ± 0.47 Ma 

Isochron Age = 84.23 

Steps A through C with 84% of 

Average Age 

0.1 

80 

60 

0.002 

40 

0.001 

20 

Oficina Regional Culiacan 0.01 

0 

0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.10 0 

Cumulative % 39ArK Released 39Ar/40Ar 

Figure 11. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected 

by CRM geologists from different areas in Mexico. These data were produced using the VG 1200 

spectrometer at the USGS Thermochronology laboratory in Denver, Colorado.–Continued 

140 

120 0.00018

100 

36Ar/40

80 

Ar

0.00012

60 

40 0.00006

20 

28 of 46
Oficina Regional Morelia 0.01 

Oficina Regional Morelia 0.001 

K/Ca 

#180KD29 

Average Age 

215.28 ± 1.13 Ma 

C 

A B 

PL-272 

hornblende 

#180KD29 

A B 

C 

PL-272 

hornblende 

[

Isochron Age = 91.04 ± 54.60 Ma
 40Ar/36Ar]i

 = 734.8 ± 192.49


MSWD = 49.302


Steps A through C with 100% of 39ArK


280 

0.003 

240 

200 

160 

120 

Apparent Age (Ma) Apparent Age (Ma) 

36Ar/40Ar 36Ar/40Ar 36Ar/40Ar

0.002 

0.001 80 

40 

0 0 0 20 40 60 80 100 0 0.006 0.012 0.018 0.024 0.030 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Morelia 

A 

K/Ca 

RA-162 

hornblende 

#143KD29 

A 

B C D E 

57.03 ± 0.31 Ma 

Average Age 

0.1 

E 

D

C 

Oficina Regional Morelia 0.01 #143KD29 

B 

0.00054 100 

80 

0.00036 RA-162 

hornblende 

[

Isochron Age = 56.14 ± 0.62 Ma 40Ar/36Ar]i

 = 417.01 ± 81.77 

MSWD = 3.27 

Steps B through E with 97.2% of 39ArK 

60 

40 

0.00018 

20 

0 0 

Apparent Age (Ma)

0 20 40 60 80 100 0 0.03 0.06 0.09 0.12 0.15 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Oaxaca 

K/Ca 

PQ-04 

hornblende 

#144KD29 

A 

B 

C 

D 

E 

0.1 

40 

30 

20 

10 

0 

Oficina Regional Oaxaca 0.01 

D 

C 

A 

E

B 

#144KD29 

PQ-04 

hornblende 

[

Isochron Age = 47.60 ± 488.53 Ma 40Ar/36Ar]i

 = -3620.15 ± 50124.87 

MSWD = 0.054 

Steps A through E with 100% of 39ArK 

0.0008 

0.0004 

0 

0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 

Cumulative % 39ArK Released 39Ar/40Ar 

Figure 11.

40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected 

by CRM geologists from different areas in Mexico. These data were produced using the VG 1200 

spectrometer at the USGS Thermochronology laboratory in Denver, Colorado.–Continued 

29 of 46
Table 5. 40Ar/39Ar step-heating data obtained using the MAP 216 spectrometer. 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Chihuahua 

POD-02 andesite plagioclase J = 0.005160 ± 0.50% wt = 51.0 mg #125KD29 

A 650 0.0 0.8 2.59E-16 8.096 0.12 1 73.82 ± 49.85 

B 750 0.8 25.7 5.05E-15 3.332 0.26 8 30.75 ± 1.30 

C 850 1.7 77.8 1.07E-14 3.558 0.15 95 32.82 ± 0.54 

D 950 5.7 90.9 3.65E-14 3.576 0.12 170 32.98 ± 0.17 

E 1050 11.6 95.0 7.40E-14 3.504 0.11 200 32.32 ± 0.10 

F 1150 20.6 96.2 1.31E-13 3.503 0.11 200 32.32 ± 0.07 

G 1250 23.0 97.4 1.47E-13 3.513 0.11 157 32.41 ± 0.07 

H 1350 17.7 94.9 1.13E-13 3.506 0.10 150 32.34 ± 0.08 

I 1450 9.4 91.4 5.99E-14 3.502 0.11 120 32.31 ± 0.12 

J 1550 6.9 86.9 4.40E-14 3.473 0.10 82 32.04 ± 0.15 

K 1650 2.6 68.2 1.64E-14 3.381 0.09 67 31.20 ± 0.39 

Total Gas 32.34 

82.3% of gas on plateau in 1050 through 1450 steps Plateau Age = 32.35 ± 0.05 

POD-02 andesite volcanic matrix J = 0.005153 ± 0.50% wt = 59.6 mg #135KD29 

A 550 0.3 0.9 5.76E-15 2.589 0.58 8 23.91 ± 11.65 

B 650 1.9 8.7 4.26E-14 2.042 2.14 25 18.88 ± 0.92 

C 750 2.5 38.4 5.48E-14 3.070 2.38 42 28.31 ± 0.28 

D 850 7.8 79.5 1.72E-13 3.976 1.92 79 36.58 ± 0.09 

E 950 21.7 93.7 4.77E-13 3.758 1.48 120 34.60 ± 0.05 

F 1050 25.0 97.2 5.50E-13 3.600 1.66 117 33.16 ± 0.04 

G 1150 19.9 95.8 4.38E-13 3.572 2.88 68 32.90 ± 0.05 

H 1250 14.6 88.0 3.22E-13 3.570 3.03 34 32.88 ± 0.06 

I 1350 3.8 73.0 8.39E-14 3.567 0.56 25 32.85 ± 0.13 

J 1450 1.9 53.3 4.10E-14 3.597 0.12 16 33.13 ± 0.26 

K 1550 0.5 41.5 1.09E-14 3.617 0.21 9 33.31 ± 0.74 

L 1650 0.1 31.7 2.27E-15 3.524 0.22 18 32.46 ± 3.20 

Total Gas 33.22 

65.2% of gas on plateau in 1050 through 1450 steps Average Age = 33.07 ± 0.05 

DHP-14 granodiorite K-feldspar J = 0.005200 ± 0.50% wt = 4.9 mg #127KD29 

A 550 0.7 1.1 3.34E-15 6.920 0.47 1 63.77 ± 23.89 

B 600 0.0 -0.7 1.22E-16 -0.434 1.03 3 -4.08 ± 59.86 

C 650 0.0 16.8 2.43E-16 5.094 -8.92 3 47.16 ± 29.72 

D 700 0.1 21.0 4.03E-16 4.080 1.47 3 37.88 ± 17.77 

E 750 0.2 21.7 7.63E-16 2.819 1.95 4 26.25 ± 9.48 

F 800 0.3 36.4 1.70E-15 3.094 4.50 6 28.79 ± 4.26 

G 850 0.6 46.9 3.22E-15 3.429 4.38 11 31.89 ± 2.27 

H 900 1.3 51.4 6.55E-15 3.472 16.08 23 32.28 ± 1.14 

I 950 2.2 59.4 1.12E-14 3.436 8.03 43 31.94 ± 0.67 

J 1000 3.3 56.5 1.68E-14 3.417 13.81 44 31.77 ± 0.47 

K 1025 3.0 74.8 1.51E-14 3.458 7.54 86 32.15 ± 0.49 

L 1050 2.8 75.6 1.43E-14 3.481 11.93 125 32.37 ± 0.51 

M 1075 3.8 60.0 1.94E-14 3.459 14.46 73 32.16 ± 0.41 

N 1100 2.6 87.5 1.31E-14 3.510 20.42 211 32.63 ± 0.56 

O 1150 4.7 56.4 2.37E-14 3.527 20.40 78 32.79 ± 0.35 

P 1200 6.8 58.1 3.43E-14 3.526 24.17 94 32.78 ± 0.26 

Q 1250 4.4 62.2 2.24E-14 3.555 19.76 162 33.05 ± 0.36 

R 1350 16.3 64.7 8.29E-14 3.516 23.97 54 32.68 ± 0.15 

S 1450 20.0 59.8 1.02E-13 3.522 20.04 33 32.74 ± 0.16 

T 1650 26.9 49.4 1.36E-13 3.591 17.39 32 33.38 ± 0.20 

Total Gas 32.99 

64.4% of gas on plateau in 1025 through 1450 steps Plateau Age = 32.69 ± 0.10 

30 of 46
Table 5. 40Ar/39Ar step-heating data obtained using the MAP 216 spectrometer.–Continued 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Durango 

BO-1012 andesite? vitrophere plagioclase J = 0.005164 ± 0.50% wt = 58.0 mg #124KD29 

A 650 0.3 0.9 1.55E-15 3.218 0.43 1 29.74 ± 15.23 

B 750 0.8 25.3 5.02E-15 2.347 0.64 2 21.73 ± 1.23 

C 850 1.2 55.1 7.27E-15 2.420 0.32 7 22.41 ± 0.82 

D 950 3.3 76.6 1.96E-14 2.523 0.18 44 23.35 ± 0.31 

E 1050 7.2 87.7 4.32E-14 2.546 0.15 130 23.57 ± 0.14 

F 1150 11.9 91.3 7.13E-14 2.537 0.14 212 23.48 ± 0.10 

G 1250 17.2 94.5 1.03E-13 2.545 0.14 204 23.55 ± 0.07 

H 1350 22.9 92.7 1.37E-13 2.548 0.15 193 23.58 ± 0.06 

I 1450 20.0 91.1 1.20E-13 2.581 0.16 170 23.89 ± 0.07 

J 1550 10.4 84.8 6.22E-14 2.619 0.15 135 24.23 ± 0.11 

K 1650 4.8 64.7 2.85E-14 2.637 0.13 128 24.40 ± 0.23 

Total Gas 23.71 

64.8% of gas on plateau in 650 through 1350 steps Plateau Age = 23.54 ± 0.05 

IR-585 andesite? vitrophere plagioclase J = 0.005163 ± 0.50% wt = 52.0 mg #121KD29 

A 650 1.1 11.0 7.64E-15 4.896 1.41 4 45.03 ± 1.99 

B 750 3.0 63.4 2.17E-14 4.710 1.18 6 43.35 ± 0.36 

C 850 3.3 67.0 2.37E-14 3.140 0.44 28 29.02 ± 0.32 

D 950 6.5 79.1 4.65E-14 3.212 0.22 84 29.67 ± 0.17 

E 1050 8.8 91.3 6.38E-14 3.431 0.17 161 31.68 ± 0.12 

F 1150 14.2 95.3 1.02E-13 3.443 0.16 207 31.79 ± 0.09 

G 1250 21.7 95.7 1.57E-13 3.355 0.16 202 30.98 ± 0.07 

H 1350 22.4 93.4 1.62E-13 3.320 0.16 193 30.66 ± 0.07 

I 1450 13.2 91.8 9.50E-14 3.326 0.16 196 30.72 ± 0.09 

J 1550 4.6 80.4 3.30E-14 3.339 0.16 133 30.84 ± 0.23 

K 1650 1.2 42.5 8.86E-15 3.423 0.14 96 31.61 ± 0.84 

Total Gas 31.43 

80.3% of gas on plateau-like in 1050 through 1450 steps Average Age = 31.06 ± 0.05 

SP-405 basalt volcanic matrix J = 0.005153 ± 0.50% wt = 57.6 mg #133KD29 

A 550 0.4 1.4 6.99E-15 1.572 0.52 2 14.55 ± 4.88 

B 650 2.6 20.0 4.92E-14 1.244 1.01 8 11.53 ± 0.31 

C 750 3.9 52.8 7.35E-14 1.278 1.52 10 11.84 ± 0.15 

D 850 9.7 86.1 1.85E-13 1.285 2.20 15 11.90 ± 0.06 

E 950 30.3 94.8 5.76E-13 1.211 2.91 45 11.23 ± 0.02 

F 1050 12.2 90.8 2.32E-13 1.171 1.19 52 10.86 ± 0.05 

G 1150 7.2 93.9 1.37E-13 1.158 0.82 31 10.73 ± 0.07 

H 1250 18.6 91.6 3.54E-13 1.140 0.59 23 10.57 ± 0.03 

I 1350 14.2 83.4 2.71E-13 1.130 0.13 8 10.48 ± 0.05 

J 1450 0.7 62.2 1.37E-14 1.165 0.13 12 10.80 ± 0.70 

K 1550 0.2 32.0 2.93E-15 1.156 0.11 14 10.71 ± 3.25 

L 1650 0.0 8.2 9.11E-16 1.386 0.10 19 12.84 ± 10.49 

Total Gas 11.02 

82.5% of gas on plateau-like in 950 through 1350 steps Average Age = 10.95 ± 0.02 

31 of 46
Table 5. 40Ar/39Ar step-heating data obtained using the MAP 216 spectrometer.–Continued 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Hermosillo 

2D-04 andesite porphyry plagioclase J = 0.005167 ± 0.50% wt = 58.2 mg #122KD29 

A 650 0.2 0.8 6.20E-16 4.516 0.15 1 41.61 ± 25.08 

B 750 2.1 13.4 7.79E-15 3.741 0.17 7 34.54 ± 1.24 

C 850 6.0 49.4 2.21E-14 2.952 0.14 96 27.31 ± 0.25 

D 950 13.8 79.6 5.15E-14 2.921 0.10 214 27.02 ± 0.11 

E 1050 19.2 86.9 7.17E-14 2.897 0.08 204 26.80 ± 0.08 

F 1150 14.5 86.5 5.42E-14 2.898 0.07 202 26.81 ± 0.10 

G 1250 7.3 78.6 2.71E-14 2.914 0.08 135 26.96 ± 0.16 

H 1350 7.8 45.2 2.91E-14 2.902 0.09 103 26.85 ± 0.24 

I 1450 16.9 14.2 6.30E-14 2.955 0.06 51 27.34 ± 0.77 

J 1550 9.4 17.0 3.52E-14 3.186 0.06 47 29.46 ± 0.70 

K 1650 2.8 19.5 1.05E-14 3.467 0.06 47 32.03 ± 0.78 

Total Gas 27.55 

85.5% of gas on plateau in 850 through 1450 steps Plateau Age = 26.89 ± 0.06 

Oficina Regional Oaxaca 

MR-001 andesite porphyry volcanic matrix J = 0.005185 ± 0.50% wt = 61.0 mg #131KD29 

A 550 0.3 0.5 1.08E-15 3.036 0.17 4 28.18 ± 25.82 

B 650 1.4 4.8 5.22E-15 2.551 0.23 9 23.71 ± 2.33 

C 750 3.1 26.3 1.17E-14 2.115 0.25 37 19.67 ± 0.39 

D 850 9.1 39.0 3.46E-14 1.712 0.25 44 15.95 ± 0.17 

E 950 17.1 57.0 6.47E-14 1.946 0.34 210 18.11 ± 0.10 

F 1050 13.5 72.0 5.11E-14 1.881 0.34 261 17.51 ± 0.08 

G 1150 8.5 69.9 3.23E-14 1.881 0.33 174 17.51 ± 0.09 

H 1250 17.6 62.7 6.69E-14 1.837 0.31 138 17.10 ± 0.09 

I 1350 17.5 33.7 6.64E-14 1.861 0.09 57 17.32 ± 0.19 

J 1450 9.8 19.2 3.72E-14 1.961 0.07 43 18.25 ± 0.41 

K 1550 1.5 13.6 5.56E-15 2.164 0.08 30 20.13 ± 0.79 

L 1650 0.7 13.8 2.79E-15 2.623 0.09 34 24.37 ± 1.27 

Total Gas 17.71 

74.2% of gas on plateau-like in 950 through 1350 steps Average Age = 17.51 ± 0.05 

MR-001 andesite porphyry plagioclase J = 0.005165 ± 0.50% wt = 49.0 mg #123KD29 

A 650 0.0 -0.9 8.71E-17 -6.999 0.67 3 -66.42 ± 57.92 

B 750 0.7 7.1 5.86E-15 2.471 0.30 20 22.88 ± 1.56 

C 850 2.4 31.8 2.02E-14 1.805 0.34 95 16.74 ± 0.26 

D 950 5.0 53.0 4.13E-14 1.814 0.32 180 16.82 ± 0.13 

E 1050 9.0 63.3 7.51E-14 1.848 0.24 231 17.14 ± 0.09 

F 1150 10.2 72.4 8.49E-14 1.839 0.20 228 17.05 ± 0.07 

G 1250 9.6 75.5 7.96E-14 1.846 0.29 225 17.12 ± 0.07 

H 1350 11.9 62.6 9.87E-14 1.822 0.56 164 16.90 ± 0.08 

I 1450 24.4 33.2 2.02E-13 1.842 0.29 96 17.08 ± 0.18 

J 1650 26.7 24.6 2.22E-13 2.074 0.09 55 19.22 ± 0.29 

Total Gas 17.65 

72.5% of gas on plateau-like in 850 through 1450 steps Average Age = 17.03 ± 0.04 

32 of 46
Table 5. 40Ar/39Ar step-heating data obtained using the MAP 216 spectrometer.–Continued 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Oaxaca (cont.) 

MR-003 andesite volcanic matrix J = 0.005148 ± 0.50% wt = 45.5 mg #134KD29 

A 550 0.9 3.3 1.55E-14 1.746 0.99 12 16.15 ± 2.20 

B 650 1.4 34.1 2.47E-14 1.477 1.16 33 13.66 ± 0.21 

C 750 1.4 68.9 2.48E-14 1.907 0.99 39 17.62 ± 0.12 

D 850 5.6 89.2 9.66E-14 2.455 0.80 99 22.65 ± 0.05 

E 950 23.3 96.4 3.99E-13 2.506 0.68 232 23.13 ± 0.03 

F 1050 28.2 98.1 4.84E-13 2.409 0.86 234 22.24 ± 0.03 

G 1150 21.1 96.8 3.61E-13 2.384 1.20 138 22.01 ± 0.03 

H 1250 11.6 92.7 1.99E-13 2.346 1.04 41 21.66 ± 0.04 

I 1350 4.1 82.5 7.00E-14 2.144 0.54 23 19.80 ± 0.06 

J 1450 1.5 66.8 2.57E-14 1.975 0.40 21 18.25 ± 0.12 

K 1550 0.5 59.1 8.50E-15 1.981 0.41 22 18.30 ± 0.32 

L 1650 0.3 39.5 4.42E-15 2.045 0.38 24 18.89 ± 0.60 

Total Gas 21.92 

89.9% of gas on plateau-like in 850 through 1250 steps Average Age = 22.31 ± 0.03 

Oficina Regional Saltillo 

LED-1 basalt volcanic matrix J = 0.005185 ± 0.50% wt = 49.5 mg #132KD29 

A 550 0.0 0.3 1.26E-17 5.659 -0.20 1 52.17 ± 295.43 

B 650 0.5 0.6 2.64E-15 0.177 0.27 2 1.66 ± 2.09 

C 750 1.1 12.7 5.82E-15 0.378 0.38 4 3.53 ± 0.57 

D 850 2.8 24.3 1.55E-14 0.259 0.34 8 2.42 ± 0.22 

E 950 19.0 51.9 1.05E-13 0.238 0.42 22 2.22 ± 0.04 

F 1050 40.8 62.8 2.24E-13 0.226 0.45 47 2.11 ± 0.02 

G 1150 12.1 47.0 6.65E-14 0.242 0.25 27 2.26 ± 0.06 

H 1250 11.0 39.6 6.06E-14 0.231 0.18 19 2.16 ± 0.07 

I 1350 11.8 9.4 6.62E-14 0.139 0.02 16 1.30 ± 0.18 

J 1450 0.7 11.9 3.93E-15 0.360 0.03 14 3.36 ± 0.84 

K 1550 0.2 6.0 8.42E-16 0.576 0.04 15 5.38 ± 3.87 

L 1650 0.1 7.0 6.19E-16 1.793 0.07 32 16.70 ± 5.62 

Total Gas 2.11 

85.7% of gas on plateau-like in 850 through 1250 steps Average Age = 2.15 ± 0.02 

MB-99 gabbro-diorite plagioclase J = 0.005186 ± 0.50% wt = 46.2 mg #120KD29 

A 650 0.0 0.5 1.25E-16 18.129 0.05 0 162.09 ± 158.59 

B 750 0.7 6.3 2.89E-15 2.997 0.19 4 27.83 ± 3.06 

C 850 1.7 38.1 7.03E-15 3.430 0.11 17 31.80 ± 0.94 

D 950 5.2 66.0 2.15E-14 3.538 0.07 43 32.80 ± 0.32 

E 1050 10.6 86.7 4.39E-14 3.506 0.06 117 32.51 ± 0.17 

F 1150 16.7 90.6 6.88E-14 3.520 0.06 142 32.63 ± 0.12 

G 1250 15.8 88.2 6.53E-14 3.505 0.08 107 32.49 ± 0.12 

H 1350 23.7 78.0 9.73E-14 3.362 0.17 46 31.18 ± 0.10 

I 1450 17.1 72.9 7.05E-14 3.372 0.13 28 31.27 ± 0.13 

J 1550 5.9 60.5 2.43E-14 3.626 0.05 15 33.61 ± 0.31 

K 1650 2.5 28.9 1.05E-14 3.750 0.03 15 34.75 ± 0.78 

Total Gas 32.13 

50.8% of gas on plateau in 650 through 1250 steps Plateau Age = 32.56 ± 0.08 

33 of 46
Table 5. 40Ar/39Ar step-heating data obtained using the MAP 216 spectrometer.–Continued 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Saltillo (cont.) 

SMD-01 granite porphyry white mica J = 0.005130 ± 0.50% wt = 6.2 mg #187KD29 

A 550 0.1 3.1 1.25E-15 2.852 7 2 26.20 ± 7.21 

B 650 0.2 18.0 2.27E-15 3.415 16 4 31.33 ± 2.87 

C 750 0.6 50.1 5.99E-15 3.888 26 50 35.63 ± 0.69 

D 800 0.9 65.8 8.21E-15 3.797 26 95 34.80 ± 0.46 

E 850 1.7 81.2 1.62E-14 3.658 41 110 33.54 ± 0.32 

F 900 3.3 91.3 3.18E-14 3.623 72 107 33.23 ± 0.12 

G 950 5.0 94.7 4.84E-14 3.610 148 137 33.10 ± 0.12 

H 1000 4.5 91.5 4.36E-14 3.598 167 121 32.99 ± 0.11 

I 1025 2.9 87.5 2.79E-14 3.580 264 142 32.84 ± 0.14 

J 1050 3.5 82.7 3.36E-14 3.577 123 126 32.81 ± 0.15 

K 1100 40.8 91.2 3.92E-13 3.561 110 125 32.66 ± 0.06 

L 1150 28.8 95.1 2.77E-13 3.567 79 104 32.71 ± 0.05 

M 1200 3.1 93.9 2.95E-14 3.664 5 29 33.60 ± 0.15 

N 1250 4.4 91.3 4.20E-14 3.592 65 91 32.94 ± 0.12 

O 1300 0.1 39.5 4.99E-16 4.789 1 8 43.79 ± 6.86 

P 1350 0.0 29.2 3.04E-16 4.999 2 13 45.68 ± 9.09 

Q 1450 0.0 6.5 1.65E-16 1.379 2 8 12.71 ± 18.31 

R 1650 0.0 20.5 2.52E-16 3.817 1 9 34.98 ± 13.77 

Total Gas 32.83 

76.0% of gas on plateau in 1025 through 1150 steps Plateau Age = 32.71 ± 0.05 

SMD-01 granite porphyry K-feldspar J = 0.005141 ± 0.50% wt = 3.9 mg #185KD29 

A 500 0.1 3.2 2.84E-16 6.201 5 1 56.62 ± 22.97 

B 550 0.2 9.0 8.83E-16 4.408 11 2 40.43 ± 6.71 

C 600 0.4 36.7 2.00E-15 3.113 64 6 28.64 ± 2.74 

D 650 0.9 71.6 4.78E-15 3.464 90 24 31.85 ± 1.19 

E 700 2.0 89.0 1.05E-14 3.633 81 128 33.38 ± 0.48 

F 750 3.6 92.6 1.91E-14 3.562 128 168 32.74 ± 0.30 

G 800 5.7 95.5 3.00E-14 3.545 70 158 32.59 ± 0.20 

H 850 7.2 96.3 3.80E-14 3.562 68 176 32.74 ± 0.18 

I 900 7.2 96.7 3.79E-14 3.558 80 174 32.71 ± 0.15 

J 950 6.8 96.6 3.58E-14 3.551 87 186 32.64 ± 0.20 

K 1000 6.2 97.0 3.24E-14 3.557 75 177 32.69 ± 0.17 

L 1050 5.4 97.5 2.85E-14 3.553 15 79 32.66 ± 0.20 

M 1100 5.1 95.8 2.67E-14 3.549 43 160 32.62 ± 0.20 

N 1150 4.6 90.9 2.39E-14 3.560 32 130 32.72 ± 0.23 

O 1200 4.9 88.7 2.60E-14 3.597 7 49 33.06 ± 0.21 

P 1250 5.7 78.7 2.98E-14 3.575 21 105 32.86 ± 0.22 

Q 1300 11.1 81.3 5.84E-14 3.546 38 109 32.59 ± 0.13 

R 1350 15.4 78.2 8.10E-14 3.553 95 118 32.66 ± 0.13 

S 1400 5.7 69.7 2.97E-14 3.606 80 122 33.14 ± 0.22 

T 1550 1.7 47.9 9.11E-15 3.686 21 97 33.87 ± 0.89 

U 1650 0.2 35.1 9.88E-16 3.435 8 81 31.59 ± 5.78 

Total Gas 32.76 

92.4% of gas on plateau in 500 through 1350 steps Plateau Age = 32.70 ± 0.06 

34 of 46
Table 5. 40Ar/39Ar step-heating data obtained using the MAP 216 spectrometer.–Continued 

Step Temp. %39Ar Radiogenic 39Ark 

40Ar* Apparent Apparent Apparent Error 

°C of total Yield (%) (Moles) 39Ark K/Ca K/Cl Age (Ma) (Ma) 

Oficina Regional Saltillo (cont.) 

SMD-02 basalt volcanic matrix J = 0.005161 ± 0.50% wt = 52.1 mg #130KD29 

A 550 1.0 1.3 9.50E-15 1.322 0.14 6 12.27 ± 4.17 

B 650 1.5 21.1 1.47E-14 3.104 0.17 16 28.67 ± 0.64 

C 750 1.7 53.1 1.61E-14 4.995 0.34 44 45.92 ± 0.39 

D 850 4.3 82.9 4.17E-14 5.218 0.42 141 47.94 ± 0.15 

E 950 16.3 88.5 1.58E-13 5.089 0.46 208 46.77 ± 0.09 

F 1050 19.0 93.1 1.84E-13 5.060 0.65 207 46.51 ± 0.07 

G 1150 8.2 89.2 7.95E-14 5.127 0.58 153 47.12 ± 0.10 

H 1250 17.5 93.0 1.70E-13 5.054 0.58 126 46.45 ± 0.07 

I 1350 27.7 91.6 2.69E-13 4.927 0.08 66 45.30 ± 0.08 

J 1450 2.3 89.0 2.21E-14 4.947 0.11 61 45.48 ± 0.21 

K 1550 0.3 47.4 3.06E-15 4.363 0.10 78 40.18 ± 1.42 

L 1650 0.3 20.7 2.85E-15 3.481 0.10 91 32.12 ± 1.71 

Total Gas 45.62 

97.0% of gas on plateau-like in 750 through 1450 steps Average Age = 46.45 ± 0.07 

Oficina Regional San Luis Potosi 

PV-003 trachyte porphyry K-feldspar J = 0.005197 ± 0.50% wt = 5.7 mg #129KD29 

A 1050 4.6 0.3 3.71E-16 1.335 0.46 0.58 12.47 ± 23.87 

B 1075 3.9 0.6 3.12E-16 2.586 1.08 0.60 24.08 ± 23.29 

C 1100 3.6 1.9 2.89E-16 6.291 0.50 0.78 58.03 ± 22.70 

D 1150 3.7 4.8 2.98E-16 12.594 0.42 0.90 114.36 ± 18.86 

E 1200 3.6 0.2 2.89E-16 0.302 3.15 1.56 2.83 ± 17.18 

F 1250 7.2 2.2 5.77E-16 2.707 1.76 1.08 25.20 ± 9.83 

G 1350 8.3 5.0 6.65E-16 3.995 0.97 1.84 37.07 ± 7.43 

H 1450 13.5 6.5 1.08E-15 3.408 8.77 2.20 31.67 ± 4.73 

I 1650 51.5 14.1 4.13E-15 2.593 3.98 5.83 24.15 ± 1.29 

Total Gas 33.00 

84.1% of gas on plateau in 1200 through 1650 steps Plateau Age = 24.60 ± 1.20 

Ages calculated assuming an initial 40Ar/36Ar = 295.5± 0.

All precision estimates are at the one sigma level of precision.

Ages of individual steps do not include error in the irradiation parameter J.

No error is calculated for the total gas age.

35 of 46
0 

50 

50 

50 

Oficina Regional Chihuahua Oficina Regional Chihuahua 

0.3 

A 

B 

C D E F G H I J K 

#125KD29 

POD-02 

plagioclase 

32.35 ± 0.05 Ma 

Plateau Age 

0 

0.2 

0.1 

0 

80 

40 0.0032 

0.0024 

0 0 20 40 60 80 100 0 0.08 0.16 0.24 

A 

B 

C 

D EF

G H I J 

K 

± 0.10 Ma 

[

40Ar/36Ar]i

 = 321 ± 16 

MSWD = 2.8 

Steps D through I with 88% of 39ArK 

#125KD29 

POD-02 

plagioclase 

Isochron Age = 32.24 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Chihuahua Oficina Regional Chihuahua K/Cl %Rad K/Cl %Rad K/Cl %Rad 36Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

200 

100 

0 

40 

30 

20 

10 

0 

0.0016 

0.0008 

4 

A 

B 

C 

D E F G H I J-L 

POD-02 

volcanic matrix 

#135KD29 

33.07 ± 0.05 Ma 

Average Age 

0 

2 

0 150 

0 20 40 60 80 100 0 0.08 0.16 0.24 

A 

B 

C 

D 

E F

G 

H 

I 

J 

K 

L 

± 0.06 Ma 

[

40Ar/36Ar]i

 = 297 ± 2 

MSWD = 0.23 39ArK 

POD-02 

volcanic matrix 

#135KD29 

Isochron Age = 32.88 

Steps G through L with 40.8% of 

Cumulative % 39ArK Released 39Ar/40Ar 

80 

40 0.0032 

0.0024 

100 

50 

0 

0.0016 40 

30 

20 

10 

0 

0.0008 

0.0004 

0 20 40 60 80 100 0 0.08 0.16 0.24 0 

A B 

C D E 

F 

G 

H 

I 

J 

K

L 

M 

N 

O P 

Q R 

S 

T 

± 0.13 Ma 

[

40Ar/36Ar]i

 = 298.2 ± 1.2 

MSWD = 1.1 39ArK 

DHP-14 

K-feldspar 

#127KD29 

Oficina Regional Chihuahua 

Isochron Age = 32.55 

Steps A through T with 100% of 

Cumulative % 39ArK Released 39Ar/40Ar 

Figure 12. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected 

Oficina Regional Chihuahua 

A-F 

G 

H I J K L M N O P Q R S T 

#127KD29 

DHP-14 

K-feldspar 

32.69 ± 0.10 Ma 

Plateau Age 

80 

40 

30 0 

20 

0.0028 10 

0 200 

100 

0 0.0020 

40 

30 

0.0012 

20 

10 

by CRM geologists from different areas in Mexico. These data were produced using the MAP 216 

spectrometer at the USGS Thermochronology laboratory in Denver, Colorado. 

36 of 46
Figure 12. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected 

by CRM geologists from different areas in Mexico. These data were produced using the MAP 216 

spectrometer at the USGS Thermochronology laboratory in Denver, Colorado.–Continued 

37 of 46
0 

0 

50 

50 

0 

Oficina Regional Hermosillo Oficina Regional Hermosillo 

A 

B 

C D E F G H I J K 

2D-04 

plagioclase 

#122KD29 

26.89 ± 0.06 Ma 

Plateau Age 

80 

40 

0.2 0 

0.1 0.0028 

A 

B 

C 

D 

EF 

G 

H 

I J 

K 

#122KD29 

± 0.06 Ma 

[

40Ar/36Ar]i

 = 298.5 ± 0.9 

MSWD = 1.6 

Steps C through J with 94.9% of 39ArK 

2D-04 

plagioclase 

Isochron Age = 26.82 

K/Cl %Rad K/Cl %Rad K/Cl %Rad

36Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

0 200 

100 

50 0 

40 

30 

20 

10 

0.0020 

0.0012 

0.0004 

0 20 40 60 80 100 0 0.08 0.16 0.24 0.32 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Oaxaca Oficina Regional Oaxaca A 

A 

B C 

D E F G H I J K 

L 

#131KD29 

MR-001 

volcanic matrix 

17.51 ± 0.05 Ma 

Average Age 

40 

0.4 0 

0.2 

0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 0.5 

B 

C 

D 

E 

FG 

H 

I 

J 

L K

± 0.09 Ma 

[

40Ar/36Ar]i

 = 298 ± 2 

MSWD = 19 

Steps E through I with 74.2% of 39ArK 

MR-001 

volcanic matrix 

#131KD29 

Isochron Age = 17.44 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Oaxaca Oficina Regional Oaxaca 

80 0.0032 

0.0024 

300 

200 

100 

0 

0.0016 

40 

30 

0.0008 20 

10 

0 

0.8 

A 

B 

C D E F G H I J 

MR-001 

plagioclase 

#123KD29 

17.03 ± 0.04 Ma 

Average Age 

0 

0.4 

0 300 

200 

0.0008 0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 

A 

B 

C 

D 

E 

F G 

H 

I 

J MR-001 

plagioclase 

#123KD29 

± 0.06 Ma 

[

40Ar/36Ar]i

 = 293.7 ± 1.5 

MSWD = 1.8 

Steps C through I with 72.5% of 39ArK 

Isochron Age = 17.09 

Cumulative % 39ArK Released 39Ar/40Ar 

80 

40 

0.0032 

100 0.0024

0 

40 

30 0.0016 

20 

10 

0 

Figure 12. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected 

by CRM geologists from different areas in Mexico. These data were produced using the MAP 216 

spectrometer at the USGS Thermochronology laboratory in Denver, Colorado.–Continued 

38 of 46
10 

Oficina Regional Oaxaca Oficina Regional Oaxaca 

0 

0.0008 

0.0016 

0.0024 

0.0032 

36 Ar/ 40 Ar 

0 

40 

80 

0 

0.5 

1.0 

1.5 

0 

100 

200 

300 

A

B 

C 

D E F G H I 

J-L 

K/Ca 

%Rad K/Cl 

#134KD29 

MR-003 

volcanic matrix 

0 

10 

20 

30 

40 

50 

22.31 ± 0.03 Ma

Apparent Age (Ma) 

Average Age 

A 

B 

L 

Isochron Age = 22.20 ± 0.03 Ma 

[

40Ar/36Ar]i

 = 264.7 ± 1.0 

MSWD = 590 

Steps A through L with 100% of 39ArK 

C 

D 

E F

G 

H 

I 

J 

K 

#134KD29 

MR-003 

volcanic matrix 

0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Saltillo Oficina Regional Saltillo 

AB 

C 

D 

E 

F 

G 

H 

#132KD29 

LED-1 

volcanic matrix 

± 0.03 Ma 

[

40Ar/36Ar]i

 = 314 ± 5 

MSWD = 1.2 

Steps C through H with 86.8% of 39ArK 

Isochron Age = 2.05 

A-C 

D E F G H 

I 

J-L 

LED-1 

volcanic matrix 

#132KD29 

2.15 ± 0.02 Ma 

Average Age 

40 

0.6 0 

0.4 

0.2 

0 60 

80 

K/Cl %Rad

0.0032 

Apparent Age (Ma) K/Ca 

0.0024 

40 

20 

0 

K/Cl %Rad

36Ar/40Ar 

36Ar/40Ar 

8 

6 

4 

0.0016 

0.0008 

2 

0 

Apparent Age (Ma) K/Ca 

0 20 40 60 80 100 0 

Cumulative % 39ArK Released 0 1 2 3 4 

39Ar/40Ar 

Oficina Regional Saltillo Oficina Regional Saltillo 

0.2 

0.1


0


50


40


30


20


10


0


A 

B 

C D E F G H I J K 

MB-99 

plagioclase 

#120KD29 

32.56 ± 0.08 Ma 

Plateau Age 

80 

40 A 

B 

C 

J 

K 

Isochron Age = 32.56 ± 0.09 Ma 

[

40Ar/36Ar]i

 = 295.3 ± 1.2 

MSWD = 0.98 

Steps A through G with 50.7% of 39ArK 

D 

EGF

H 

I 

MB-99 

plagioclase 

#120KD29 

0.0032 

0 

150 0.0024 100 

50 

0 

0.0016 

0.0008 

20 40 60 80 100 0 

0 0.08 0.16 0.24 

Cumulative % 39ArK Released 39Ar/40Ar 

Figure 12. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected 

by CRM geologists from different areas in Mexico. These data were produced using the MAP 216 

spectrometer at the USGS Thermochronology laboratory in Denver, Colorado.–Continued 

39 of 46

0 
I 

0 

0 

20 

50 

0 

50 

0 

80 

Oficina Regional Saltillo 

A-D 

E F G H I J K L M N 

O-R 

#187KD29 

SMD-01 

white mica 

32.71 ± 0.05 Ma 

Plateau Age 

80 0.0032 

40 

300 0 

200 

100 0.0024 150 

Oficina Regional Saltillo 

0 

H 

J 

M L

N 

O 

P 

Q 

R 

± 0.09 Ma 

[

40Ar/36Ar]i

 = 301 ± 8 

MSWD = 0.69 39ArK 

SMD-01 

white mica 

#187KD29 

K 

Isochron Age = 32.66 

Steps I through L with 76.0% of 

20 40 60 80 100 0 0.08 0.16 0.24 

Cumulative % 39ArK Released 39Ar/40Ar 

Oficina Regional Saltillo Oficina Regional Saltillo K/Cl %Rad K/Cl %Rad 36Ar/40Ar 

Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca Apparent Age (Ma) K/Ca 

100 

50 

40 

30 

0 

0.0016 

0.0008 

10 

0 

A-D 

E F G H I J K L M N O P Q R S T 

U 

SMD-01 

K-feldspar 

#185KD29 

32.70 ± 0.06 Ma 

Plateau Age 

40 

150 0 

100 

50 

0 

A 

B 

C 

D 

E F GHI

J

KLM 

NO 

P Q 

R 

S 

T 

U SMD-01 

K-feldspar 

#185KD29 

± 0.07 Ma 

[

40Ar/36Ar]i

 = 300 ± 2 

MSWD = 0.71 39ArK 

Isochron Age = 32.65 

Steps A through U with 100% of 

0 20 40 60 80 100 0 0.08 0.16 0.24 

Cumulative % 39ArK Released 39Ar/40Ar 

80 

0.0032 

200 0.0024 

100 

0 

36Ar/40Ar 36Ar/40Ar 0.0016 

40 

30 

0.0008 20 

10 

0 

Oficina Regional Saltillo 

A 

B 

C D E F G H I J 

K 

L 

#130KD29 

SMD-02 

volcanic matrix 

46.45 ± 0.07 Ma 

Average Age 

Oficina Regional Saltillo 

K/Cl %Rad

A 

B 

C 

D E 

F G H

I J 

K 

L 

± 0.13 Ma 

[

40Ar/36Ar]i

 = 379 ± 7 

MSWD = 42 

Steps D through I with 93% of 39ArK 

#130KD29 

SMD-02 

volcanic matrix 

Isochron Age = 45.20 

80 

0.0034 40 

0.8 0 

0.4 

0.0026 

200 

100 

60 

40 

20 

0 

0 

0.0018 

0.001 

0.0002 20 40 60 80 100 0 0.04 0.08 0.12 0.16 

Cumulative % 39ArK Released 39Ar/40Ar 

Figure 12. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected 

by CRM geologists from different areas in Mexico. These data were produced using the MAP 216 

spectrometer at the USGS Thermochronology laboratory in Denver, Colorado.–Continued 

40 of 46

0 0.20 
100 

Oficina Regional San Luis Potosi Oficina Regional San Luis Potosi 

A 

B 

C 

I 

K-feldspar 

24.60 ± 1.20 Ma 

#129KD29 

PV-003 

Plateau Age 

K/Cl %Rad

36Ar/40Ar 

E F 

G H 

0.0028
 0 20 40 60 80 100 0 0.01 0.02 0.03 0.04 0.05 0.06


A 

B 

C 

D 

E 

F 

G 

H 

I ± 1.19 Ma 

[

40Ar/36Ar]i

 = 299 ± 2 

MSWD = 1.2 39ArK 

#129KD29 

K-feldspar 

Isochron Age = 22.70 

Steps Q through T with 80.5% of 

PV-003 

Cumulative % 39ArK Released 39Ar/40Ar 

80


40


0.0034 0 8


4


0
 6


4


2


Apparent Age (Ma) K/Ca 

0.0032 

80


60


40


20


0


0 

D 

0.0030 

Figure 12. 40Ar/39Ar age spectra and inverse-isotope correlation diagrams for samples collected


by CRM geologists from different areas in Mexico. These data were produced using the MAP 216


spectrometer at the USGS Thermochronology laboratory in Denver, Colorado.–Continued


41 of 46
Table 6. 40Ar/39Ar laser total fusion data of single-crystal sanidine obtained using the MAP 216 spectrometer. 

Hole 39Ark Radiogenic 40Ar* K/Ca K/Cl Age Error 

number (Moles) Yield (%) 39Ark (Ma) (Ma) 

Oficina Regional Chihuahua 

POD-01 rhyolite sanidine J = 0.005117 ± 0.50% #139KD29 

11 1.48E-14 94.0 3.572 18.2 69 32.68 ± 0.12 

10 3.87E-14 99.3 3.577 8.2 186 32.72 ± 0.06 

1 6.67E-14 98.7 3.582 13.4 116 32.77 ± 0.05 

2 4.82E-14 98.3 3.584 20.4 57 32.78 ± 0.06 

8 3.03E-14 96.1 3.585 7.7 40 32.80 ± 0.08 

3 3.83E-14 98.8 3.585 16.4 93 32.80 ± 0.06 

4 3.17E-14 98.8 3.586 19.5 60 32.80 ± 0.07 

7 3.32E-14 99.3 3.588 6.2 110 32.83 ± 0.07 

5 2.59E-14 98.9 3.592 6.0 91 32.85 ± 0.08 

9 2.46E-14 95.7 3.594 6.0 60 32.87 ± 0.09 

6 3.19E-14 99.0 3.620 5.6 91 33.11 ± 0.07 

Weighted Mean Age = 32.82 ± 0.04 

FDR-001 rhyolite tuff sanidine J = 0.005078 ± 0.50% #140KD29 

11 1.42E-14 49.9 3.508 5.3 170 31.85 ± 0.29 

10 8.87E-15 67.8 3.522 4.6 172 31.99 ± 0.25 

12 2.33E-14 72.3 3.547 4.5 166 32.21 ± 0.13 

1 2.25E-14 79.8 3.551 20.6 200 32.24 ± 0.12 

13 1.12E-14 75.9 3.557 3.1 152 32.30 ± 0.19 

3 1.03E-14 69.1 3.576 7.2 207 32.47 ± 0.22 

9 1.76E-14 68.5 3.581 5.8 225 32.52 ± 0.16 

7 1.78E-14 84.6 3.592 7.3 168 32.61 ± 0.13 

6 3.11E-14 77.1 3.606 6.9 197 32.74 ± 0.11 

5 1.25E-14 71.6 3.614 26.5 230 32.81 ± 0.17 

2 3.18E-14 84.1 3.616 16.1 196 32.83 ± 0.10 

8 1.48E-14 49.5 3.626 79.4 158 32.91 ± 0.27 

4 8.65E-15 80.0 3.650 3.4 200 33.13 ± 0.24 

Weighted Mean Age = 32.34 ± 0.07 

FVDA-01 dacite sanidine J = 0.005119 ± 0.50% #142KD29 

9 2.82E-14 97.4 3.636 6.7 154 33.19 ± 0.06 

4 2.70E-14 97.1 3.634 15.3 76 33.26 ± 0.07 

8 2.69E-14 96.8 3.558 3.6 155 33.28 ± 0.08 

10 4.39E-14 98.9 3.627 12.6 195 33.31 ± 0.06 

3 3.29E-14 97.9 3.641 6.9 102 33.32 ± 0.07 

2 3.73E-14 95.9 3.644 7.6 109 33.34 ± 0.07 

12 3.77E-14 98.3 3.641 8.6 126 33.35 ± 0.06 

5 3.49E-14 97.6 3.648 10.0 151 33.38 ± 0.06 

6 2.48E-14 98.7 3.650 10.9 107 33.40 ± 0.08 

1 4.57E-14 89.0 3.652 8.9 75 33.41 ± 0.08 

Weighted Mean Age = 33.32 ± 0.05 

FVDA-02 rhyolite tuff sanidine J = 0.005204 ± 0.50% #136KD29 

2 3.07E-14 81.4 3.450 8.3 28 32.11 ± 0.11 

4 2.59E-14 88.6 3.511 3.5 83 32.67 ± 0.09 

3 2.55E-14 96.0 3.513 9.7 143 32.69 ± 0.09 

7 4.20E-14 87.3 3.517 5.8 87 32.72 ± 0.08 

1 3.17E-14 90.5 3.519 7.2 78 32.74 ± 0.09 

8 3.35E-14 93.5 3.527 11.5 38 32.81 ± 0.08 

6 4.89E-14 86.6 3.533 5.5 123 32.87 ± 0.08 

9 1.72E-14 80.0 3.550 4.3 74 33.03 ± 0.15 

5 2.54E-14 78.0 3.554 3.6 96 33.06 ± 0.12 

11 3.60E-14 93.5 3.555 5.0 121 33.07 ± 0.07 

10 2.61E-14 91.8 3.630 5.0 107 33.76 ± 0.10 

42 of 46
Table 6. 40Ar/39Ar laser total fusion data of single-crystal sanidine obtained using the MAP 216 spectrometer.–Continued 

Hole 39Ark Radiogenic 40Ar* K/Ca K/Cl Age Error 

number (Moles) Yield (%) 39Ark (Ma) (Ma) 

FVDA-02 (cont.) rhyolite tuff sanidine J = 0.005204 ± 0.50% #136KD29 

13 3.28E-14 84.6 3.658 11.5 118 34.02 ± 0.10 

12 2.88E-14 91.6 3.663 10.5 117 34.06 ± 0.09 

Weighted Mean Age = 32.77 ± 0.05 

AMDT-01 rhyolite sanidine J = 0.005204 ± 0.50% #138KD29 

5 3.07E-14 97.1 4.039

1 2.78E-14 96.9 4.042

6 2.14E-14 95.9 4.047

13 1.22E-14 95.7 4.048

8 2.10E-14 98.5 4.049

9 1.15E-14 99.1 4.053

7 1.99E-14 96.0 4.054

3 3.88E-14 97.6 4.054

12 1.28E-14 99.1 4.054

2 2.38E-14 93.5 4.057

10 1.64E-14 98.3 4.058

4 3.36E-14 98.5 4.059

11 1.72E-14 97.5 4.061

7.7 193 37.52 ± 0.08 

5.2 174 37.56 ± 0.08 

8.4 201 37.60 ± 0.10 

4.9 185 37.61 ± 0.14 

552.5 198 37.62 ± 0.09 

37.9 177 37.65 ± 0.14 

9.2 190 37.66 ± 0.10 

6.3 170 37.67 ± 0.07 

2.7 242 37.67 ± 0.15 

15.5 181 37.69 ± 0.09 

78.7 162 37.70 ± 0.12 

4.4 163 37.71 ± 0.07 

10.5 164 37.73 ± 0.12 

Weighted Mean Age = 37.64 ± 0.05 

Oficina Regional Culiacan 

YE-27 rhyolite sanidine J = 0.005086 ± 0.50% #141KD29 

6 3.26E-16 15.7 1.768195 2.0 5.7 16.15 ± 5.03 

5 1.88E-16 36.4 3.3956789

1 1.82E-15 58.8 3.4854468

8 1.03E-15 20.5 3.5621535

3 1.65E-15 82.9 3.6625415

2 8.69E-16 33.9 3.6783932

11 1.17E-15 42.8 3.7382216

7 2.00E-15 85.0 3.8667884

10 7.57E-16 39.4 4.1508013

0.3 8.6 30.89 ± 8.62 

3.5 25.8 31.70 ± 0.93 

0.2 2.0 32.39 ± 2.10 

6.8 7.5 33.30 ± 0.96 

0.8 4.9 33.44 ± 2.04 

1.6 5.1 33.98 ± 1.69 

1.5 31.5 35.13 ± 0.86 

0.2 4.4 37.69 ± 2.53 

9 9.46E-16 32.9 4.7609672 0.6 4.5 43.16 ± 2.07 

4 8.20E-17 55.4 5.7476304 0.1 2.0 51.98 ± 19.31 

Weighted Mean Age = 33.60 ± 0.50 

Oficina Regional Hermosillo 

FA-4 rhyolite tuff sanidine J = 0.005204 ± 0.50% #137KD29 

9 4.87E-14 78.1 2.594 15.9 191 24.19 ± 0.08 

12 3.33E-14 97.2 2.622 5.9 203 24.45 ± 0.06 

11 2.71E-14 88.5 2.630 5.9 179 24.52 ± 0.09 

1 5.26E-14 91.9 2.635 3.9 182 24.57 ± 0.06 

13 1.58E-14 97.2 2.638 4.2 198 24.60 ± 0.10 

10 1.62E-14 96.5 2.679 5.5 126 24.97 ± 0.11 

7 4.39E-14 92.7 2.763 8.3 156 25.76 ± 0.06 

8 6.39E-14 96.0 2.791 10.8 172 26.01 ± 0.05 

4 2.88E-14 84.2 2.869 4.1 180 26.73 ± 0.10 

6 1.16E-14 91.8 3.192 25.1 165 29.72 ± 0.16 

5 1.24E-14 94.9 3.318 1.9 150 30.88 ± 0.15 

3 1.67E-14 91.5 3.486 2.7 174 32.43 ± 0.12 

2 1.76E-14 69.0 3.843 1.0 184 35.72 ± 0.19 

Weighted Mean Age = 24.53 ± 0.05 

Analyses in gray italics are not used to calculate the weighted mean age 

43 of 46
80


40


0


Oficina Regional Chihuahua 

#139KD29 

POD-01 

32.82 ± 0.04 Ma 

Prob. 

K/Cl 

%Rad. 

K/Ca 

Analyses 

Total fusion sanidine 

20


10


200
 0 

100


0 12


8


4


0


40


20


0 

30 31 32 33 34 35 36 37 38 39 40


Age (Ma) 

Oficina Regional Chihuahua 

80


40


0


#140KD29 

FDR-001 

32.34 ± 0.07 Ma 

Prob. 

K/Cl 

%Rad. 

K/Ca 

Analyses 

Total fusion sanidine 

80


40


300
 0


200


100


0 15


10


5


0


15


10


5


0 

20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50


Age (Ma) 

Oficina Regional Chihuahua 

80


40


0


#142KD29 

FVDA-01 

33.32 ± 0.05 Ma 

Prob. 

K/Cl 

%Rad. 

K/Ca 

Analyses 

Total fusion sanidine 

20


10


0 200


100


0 12


8


4


0


40


20


0 

20 22 24 26 28 30 32 34 36 38 40


Age (Ma) 

36Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Oficina Regional Chihuahua 0.00022 

0.00018 

0.00014 

0.00010 

0.00006 

0.00002 0.254 0.262 0.270 0.278 

1, 3, 4 & 5 

2 

6 

7 

8 

9 

10 

± 0.05 Ma 

[

40Ar/36Ar]i

 = 295 ± 14 

MSWD = 2.6 

#139KD29 

POD-01 

11 

Isochron Age = 32.82 

11 sanidine grains 

Total fusion sanidine 

39Ar/40Ar 

Oficina Regional Chihuahua 

0.0017 

0.0013 

0.0009 

0.0005 

1 

2 

3 

4 

5 

6 

7 

8 

9 10 

12 

13 

± 0.12 Ma 

[

40Ar/36Ar]i

 = 295 ± 2 

MSWD = 2.4 

9 sanidine grains 

FDR-001 

#140KD29 

11 

Isochron Age = 32.37 

Total fusion sanidine 

0.13 0.15 0.17 0.19 0.21 0.23 

39Ar/40Ar 

Oficina Regional Chihuahua 

0.00034 

0.00026 

0.00018 

0.00010 

0.00002 0.235 0.245 0.255 0.265 0.275 

1 

2 

3 

4 

5 

6 

8 

9 

10 

12 

± 0.05 Ma 

[

40Ar/36Ar]i

 = 303 ± 7 

MSWD = 1.1 

10 sanidine grains 

#142KD29 

FVDA-01 

Isochron Age = 33.29 

Total fusion sanidine 

39Ar/40Ar 

Figure 13. 40Ar/39Ar probability diagrams (ideograms) and inverse-isotope correlation diagrams for 

samples collected by CRM geologists from different areas in Mexico. These data were produced using 

the MAP 216 spectrometer (laser technique) at the USGS Thermochronology lab in Denver. 

44 of 46

0.286 
Oficina Regional Chihuahua


80


40


0 

#136KD29 

FVDA-02 

32.77 ± 0.05 Ma 

Prob. 

K/Cl 

%Rad. 

K/Ca 

Analyses 

Total fusion sanidine 

15


10


5


150
 0


100


50


0 15


10


5


0


20


10


0 

30 31 32 33 34 35 36 37 38 39 40


Age (Ma) 

Oficina Regional Chihuahua 

80


40


0


#138KD29 

37.64 ± 0.05 Ma 

Prob. 

K/Cl 

%Rad. 

K/Ca 

Analyses 

AMDT-01 

Total fusion sanidine 

40


20


300
 0


200


100


0 15


10


5


0


40


20


0 

30 32 34 36 38 40 42 44 46 48 50


Age (Ma) 

Oficina Regional Culiacan 

80


40


0 

#141KD29 

YE-27 

33.60 ± 0.50 Ma 

Prob. 

K/Cl 

%Rad. 

K/Ca 

Analyses 

Total fusion sanidine 

8


4


40
 0


20


0
 12


8


4


0 

1.0 

0.5 

0 

0 10 20 30 40 50 60 70 80 90 100


Age (Ma) 

36Ar/40Ar 36Ar/40Ar 36Ar/40Ar 

Oficina Regional Chihuahua 

0.0007 

0.0005 

0.0003 

0.0001 

1 

2 

3 

4 

5 

6 7 

8 

9 

10 12 

13 

± 0.08 Ma 

[

40Ar/36Ar]i

 = 308 ± 5 

MSWD = 0.91 

8 sanidine grains 

FVDA-02 

#136KD29 

11 

Isochron Age = 32.62 

Total fusion sanidine 

0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 

39Ar/40Ar 

Oficina Regional Chihuahua 

0.00022 

0.00018 

0.00014 

0.00010 

0.00006 

0.00002 0.222 0.226 0.230 0.234 0.238 0.242 0.246 0.250 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

12 

13 

± 0.07 Ma 

[

40Ar/36Ar]i

 = 294 ± 13 

MSWD = 0.52 

13 sanidine grains 

#138KD29 

11 

Isochron Age = 37.65 

AMDT-01 

Total fusion sanidine 

39Ar/40Ar 

Oficina Regional Culiacan 

0.0032 

0.0024 

0.0016 

0.0008 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

± 0.60 Ma 

[

40Ar/36Ar]i

 = 294 ± 5 

MSWD = 1.7 

8 sanidine grains 

#141KD29 

YE-27 

11 

Isochron Age = 33.70 

Total fusion sanidine 

0 0.04 0.08 0.12 0.16 0.20 0.24 0.28 

39Ar/40Ar 

Figure 13. 40Ar/39Ar probability diagrams (ideograms) and inverse-isotope correlation diagrams 

for samples collected by CRM geologists from different areas in Mexico. These data were produced using 

the MAP 216 spectrometer (laser technique) at the USGS Thermochronology lab in Denver.–Continued 

45 of 46
Oficina Regional Hermosillo Oficina Regional Hermosillo 

80 

40 

#137KD29 

24.53 ± 0.05 Ma 

Prob. 

K/Cl 

%Rad. 

K/Ca 

Analyses 

FA-4 

Total fusion sanidine 

0.0010


0
 30 

20 

10 0.0008 

0 200 

100 

15 0.0006 0


10


5


0 0.0004


15 

10 0.0002 

5 

0 0 20 21 22 23 24 25 26 27 28 29 30 0.16 0.20 0.24 0.28 0.32 0.36 

1 

2 

3 

4 

5 

6 7 

8 

9 

10 12 13 

#137KD29 

± 0.08 Ma 

[

40Ar/36Ar]i

 = 305 ±

MSWD = 1.1 

4 sanidine grains 

11 

FA-4 

Total fusion sanidine 

Isochron Age = 24.47 

 11 

Age (Ma) 39Ar/40Ar 36Ar/40Ar 

Figure 13. 40Ar/39Ar probability diagrams (ideograms) and inverse-isotope correlation diagrams 

for samples collected by CRM geologists from different areas in Mexico. These data were produced using 

the MAP 216 spectrometer (laser technique) at the USGS Thermochronology lab in Denver.–Continued 

46 of 46