40Ar/39Ar Geochronology Results from Volcanic … · SUSIE KYLE, Administrative Secretary I LEWIS A. LAND, Hydrogeologist ... AUSTIN, Emeritus Senior Industrial Minerals Geologist

Open File Report OF-AR-18

New Mexico Bureau of Geology and Mineral ResourcesA division of

New Mexico Institute of Mining and Technology

40Ar/39Ar Geochronology Resultsfrom Volcanic Rocks from the Alborz

Mountains, Iran

Prepared By:

Richard P. Esser and William C. McIntoshNew Mexico Bureau of Geology, Socorro, NM 87801

Prepared For:

Drs. Gary Axen and Bernard GuestDept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095

Initially prepared as:

NM Geochronology ResearchLaboratory Internal Report

NMGRL-IR 186

February 4, 2002

SOCORRO 2003

NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCESPeter A. Scholle, Director and State Geologist

a division ofNEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY

Daniel H. López, President

BOARD OF REGENTSEx Officio

Bill Richardson, Governor of New MexicoMichael J. Davis, Superintendent of Public Instruction

AppointedAnn Murphy Daily, President, 1999–2004, Santa Fe

Randall E. Horn, Secretary/Treasurer, 1997–2003, AlbuquerqueSidney M. Gutierrez, 2001–2007, AlbuquerqueAnthony L. Montoya, Jr., 2001–2003, Socorro

Robert E. Taylor, 1997–2003, Silver City

NEW MEXICO GEOCHRONOLOGY RESEARCH LABORATORY STAFF

WILLIAM MCINTOSH, GeochronologistMATT HEIZLER, Geochronologist

LISA PETERS, Argon Laboratory TechnicianRICHARD ESSER, Argon Laboratory Technician

BUREAU STAFF

BRUCE D. ALLEN, Field GeologistRUBEN ARCHULETA, Metallurgical Lab. Technician IISANDRA H. AZEVEDO, Cartographer IIALBERT BACA, Lead Maintenance CarpenterJAMES M. BARKER, Associate Director for Operations, Senior Industrial Minerals GeologistPAUL W. BAUER, Associate Director for Government Liaison, Senior Geologist, Manager of Geologic Mapping ProgramLYNN A. BRANDVOLD, Senior ChemistBRIAN S. BRISTER, Petroleum GeologistRON BROADHEAD, Associate Director for Industry Liaison, Principal Senior Petroleum GeologistRITA CASE, Administrative Secretary II (Alb. Office)STEVEN M. CATHER, Senior Field GeologistRICHARD CHAMBERLIN, Senior Field GeologistSEAN D. CONNELL, Albuquerque Office Manager, Field GeologistRUBEN A. CRESPIN, Manager, Fleet/General ServicesJEANNE DEARDORFF, Assistant EditorNELIA W. DUNBAR, Analytical GeochemistRICHARD ESSER, Senior Lab. AssociateROBERT W. EVELETH, Senior Mining EngineerKARL FRISCH, GIS TechnicianPATRICIA L. FRISCH, Assistant Curator of Mineral MuseumLEO O. GABALDON, Cartographer IINANCY S. GILSON, EditorKATHRYN E. GLESENER, Senior Cartographer/Manager, Cartography SectionDEBBIE GOERING, Business Office CoordinatorTERRY GONZALES, Information SpecialistIBRAHIM GUNDILER, Senior Extractive MetallurgistLYNN HEIZLER, Senior Lab. AssociateMATT HEIZLER, Assistant Director for Laboratories, GeochronologistLYNNE HEMENWAY, Geologic Information Center CoordinatorGRETCHEN K. HOFFMAN, Senior Coal Geologist

GLEN JONES, Assistant Director for Computer/Internet ServicesTHOMAS J. KAUS, Cartographer IPHILIP KYLE, Professor, GeochemistrySUSIE KYLE, Administrative Secretary ILEWIS A. LAND, HydrogeologistANNABELLE LOPEZ, Petroleum Information CoordinatorTHERESA LOPEZ, Administrative Secretary IDAVID W. LOVE, Principal Senior Environmental GeologistJANE A. CALVERT LOVE, Managing EditorVIRGIL W. LUETH, Assistant Director for Public Outreach, Mineralogist/Economic Geologist, Curator of Mineral MuseumMARK MANSELL, GIS SpecialistDAVID MCCRAW, Senior Geologic Lab. AssociateWILLIAM MCINTOSH, Senior VolcanologistCHRISTOPHER G. MCKEE, X-ray Facility ManagerVIRGINIA T. MCLEMORE, Minerals Outreach Liaison, Senior Economic GeologistPATRICIA JACKSON PAUL, Geologic Lab. AssociateLISA PETERS, Senior Lab. AssociateL. GREER PRICE, Senior Geologist/Chief EditorADAM S. READ, Senior Geological Lab. AssociateBEN REBACH, Cartographer IIWILLIAM D. RAATZ, Petroleum GeologistMARSHALL A. REITER, Principal Senior GeophysicistJOHN SIGDA, GeohydrologistGREGORY SANCHEZ, Mechanic-Carpenter HelperTERRY THOMAS, ICP–MS ManagerFRANK TITUS, Senior Outreach HydrologistLORETTA TOBIN, Executive SecretaryAMY TRIVITT-KRACKE, Petroleum Computer SpecialistJUDY M. VAIZA, Assistant Director for FinanceMANUEL J. VASQUEZ, Mechanic IISUSAN J. WELCH, Manager, Geologic Extension ServiceMAUREEN WILKS, Geologic Librarian, Manager of Publication Sales

EMERITUS

GEORGE S. AUSTIN, Emeritus Senior Industrial Minerals GeologistCHARLES E. CHAPIN, Emeritus Director/State GeologistJOHN W. HAWLEY, Emeritus Senior Environmental Geologist

JACQUES R. RENAULT, Emeritus Senior GeologistSAMUEL THOMPSON III, Emeritus Senior Petroleum GeologistROBERT H. WEBER, Emeritus Senior Geologist

Plus research associates, graduate students, and undergraduate assistants.

Introduction

Fifteen volcanic/hypabyssal rocks from the Alborz Mountains, Iran were submitted for4 0Ar/3 9Ar dating by Drs. Bernard Guest and Gary Axen. It was hoped that the 4 0Ar/3 9Ar ages will helpconstrain the tectonic evolution of the Alborz Mountains. The table below lists the material/mineralprepared from each sample:

Groundmassconcentrate Biotite Hornblende

19-133-1 19-141-2A 19-57-219-135-1 19-141-2B 19-141-119-137-1 20-49-3 3-87

19-141-2B BG-4 84-2A 3-8820-49-2 BG-4 84-2B BG-4 86-3

3-89

40Ar/39Ar Analytical Methods and Results

The groundmass concentrate, biotite and hornblende samples were analyzed by the furnaceincremental heating age spectrum 4 0Ar/3 9Ar method. Abbreviated analytical methods for the furnacesample is given in Table 1. Details of the overall operation of the New Mexico GeochronologyResearch Laboratory are provided in the Appendix. Figures 1-16 show the age spectrum and inverseisochron yielded by the groundmass concentrates, biotites and hornblendes. A summary of thepreferred ages yielded in this study is shown in Table 1.

Each of the six groundmass concentrate samples yielded a slightly to somewhat discordant agespectrum. For 19-133-1 (Figure 1), the first two heating steps (steps A and B) are significantly older(age ≈ 60 Ma) than the remainder of the heating steps (age ≈ 32 Ma), which exhibit a concordant agedistribution. The radiogenic yields increase gradually from a low of 2.3% for the lowest temperatureheating step to greater than 84% for the 1250°C (the 1650°C or fusion step typically has a lowerradiogenic yield resulting from a higher extraction line blank). The K/Ca values range from 0.03 (stepI) to 0.8 (step G), which are consistent with a basaltic groundmass concentrate. A weighted meanvalue for the flattest portion of the age spectrum (steps C through H; 85.7% of the cumulative 3 9ArK

released) yields an apparent age of 32.76±0.44 Ma (two sigma) with an unacceptable MSWD of 3.3.The inverse isochron for 19-133-1 does however yield an acceptable MSWD of 1.8. The age yieldedby the inverse isochron is 32.45±0.32 Ma with a 4 0Ar/3 6Ar intercept of 300±2.

Groundmass concentrate 19-135-1 (Figure 2) yields an age spectrum that is also discordantfor the first few heating steps. While steps A and B are anomalously older than the remainder of theage spectrum, steps C and D are anomalously younger than the remainder of the age spectrum. Thisundulatory behavior is confined to the heating steps below 800°C, as those steps above 800°C are

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isochronous about 32 Ma. The radiogenic yields and K/Ca values for 19-135-1 are nearly identical tothose observed for 19-133-1. The weighted mean for the flattest portion of the age spectrum (steps Ethrough I; 90.2% of the cumulative 3 9ArK released) yields an apparent age of 32.74±0.58 Ma with anunacceptable MSWD of 5.6. The inverse isochron yields an age of 32.5±2.2 Ma (MSWD=7.5) witha 4 0Ar/3 6Ar intercept of 298±11.

Groundmass concentrate 19-137-1 (Figure 3) yields the most well-behaved age spectrum of allthe groundmass concentrate samples. Only the first three steps show any anomalous behavior andthus are not included in the plateau weighted mean age (8.72±0.11 Ma; 87% of the 3 9ArK released).The radiogenic yields are slightly lower (<41%) than the previous samples but do increase from low tohigh temperature. The K/Ca ratios range from 0.06 to 2.3. The inverse isochron yields an age of8.66±0.22 Ma with a 4 0Ar/3 6Ar intercept of 296±3 (MSWD=2.5).

The age spectrum for groundmass concentrate 19-141-2B (Figure 4) is very discordant,exhibiting what is commonly called a “saddle-shaped” spectrum. Apparent ages for the lowesttemperature steps (A through C) and the highest temperature steps (G through I) are older (rangingfrom 8.5 to 27 Ma) than the steps in the middle of the age spectrum (D through F; ~7.3 Ma). Theradiogenic yields range from 6% to 52% while the K/Ca ratios range from 0.5 to 5.8. A weightedmean for the three youngest heating steps (D through F) yields an age of 7.26±0.10 Ma(MSWD=0.1). The inverse isochron is highly discordant when all of the heating steps are plotted(MSWD>1000), but drops to an acceptable value when only four of the steps are plotted (A, D, E andF; MSWD=0.2). The intercept age for the acceptable isochron is 7.04±0.12 Ma and the 4 0Ar/3 6Arintercept is 304±2.

The age spectrum for groundmass concentrate 20-49-2 (Figure 5) exhibits anomalous behaviorat the end of its heating schedule, rather than at the beginning. The first six heating steps (A throughF) are nearly isochronous at approximately 0.33 Ma. The remaining three heating steps range in agefrom 0.07 to 1.57 Ma. The radiogenic yields for this sample are low (<18%), while K/Ca ratios arehigher (~3) than those observed in previous samples. The weighted mean age for the first six steps(A-F) is 0.33±0.05 Ma (72.9% of the 3 9ArK released) with an unacceptable MSWD of 3.1. Theinverse isochron age is 0.33±0.14 Ma with a 4 0Ar/3 6Ar ratio of 295±9 (MSWD=5.5).

The age spectrum for the 3-89 groundmass concentrate sample (Figure 6) only exhibits verysmall scale discordance at the beginning and end of the heating schedule. Radiogenic yields and K/Cavalues are very similar to those observed for the previous groundmass concentrate (20-49-2). Aplateau weighted mean age for the flattest portion of the age spectrum yields an age of 0.25±0.02 Ma(79.2% of the 3 9ArK) with an acceptable MSWD of 0.6. The inverse isochron age is 0.25±0.02 with a4 0Ar/3 6Ar ratio of 296±2 and a MSWD of 1.1.

The five biotite age spectra yield more precise results than the six groundmass concentratesamples. Biotite 19-141-2A yields a very flat age spectrum (Figure 7) with an apparent plateau over

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90% of 3 9ArK released. Radiogenic yields range from about 40% to greater than 90% for the majorityof the spectrum. K/Ca values are initially high (>10), but then drop below 10 for those temperaturesteps 1180°C and higher. The weighted mean age for the flattest portion of the age spectrum (steps Cthrough K) is 6.87±0.08 Ma (95.6% of the 3 9ArK released) with a MSWD of 1.5. The inverseisochron yields a very similar age of 6.89±0.03 Ma (4 0Ar/3 6Ar intercept=294±2; MSWD=1.3).

Biotite 19-141-2B yields an age spectrum (Figure 8) very similar to that of 19-141-2A.However, while the radiogenic yields are only slightly higher for 19-141-2B, the K/Ca values areapproximately 4 times higher than those observed for 19-141-2A. The weighted mean age for steps Dthrough K is 6.89±0.05 with 90.9% of the 3 9ArK released (MSWD=1.4). The inverse isochron yieldsan age of 6.91±0.06 Ma (4 0Ar/3 6Ar intercept=294±2; MSWD=1.5).

The age spectrum for biotite 20-49-3 (Figure 9) does exhibit some discordance, but still yieldsa precise weighted mean age. The age spectrum discordance is mainly confined to the lowertemperature heating steps (<1180°C) where ages range from 0.23 to 0.92 Ma (excluding the A step).Following the anomalous behavior at low temperatures, the age spectrum becomes more isochronouswhere ages range from 0.41 to 0.59 Ma. The radiogenic yields for 20-49-3 are low and consistent fora significant quantity of the gas released, ranging from 1.3 to about 10%. Only steps J and K haveradiogenic yields greater than 20%. The K/Ca values are greater for the lower temperature steps (~12)than the high temperature steps (<6). A weighted mean for the flattest portion of the age spectrum(steps G through K) yields an age of 0.51±0.03 Ma with 86.8% of the 3 9ArK released (MSWD=2.7).The inverse isochron yields an age of 0.49±0.08 Ma (4 0Ar/3 6Ar intercept=296±6; MSWD=2.2).

The final two biotite samples from this study (BG-4 84-2A and BG-4 84-2B; Figures 10 and11, respectively) yield results very similar to one another. In both cases, initial ages are anomalouslyyounger than subsequent ages. For BG-4 84-2A, the youngest age (excluding the A step) is 4.69 Ma,while BG-4 84-2B’s youngest age is 4.82±0.86 Ma. Radiogenic yields and K/Ca values for the twosamples are also very similar, positively correlating to the shape of the age spectra. The weighted meanage for BG-4 84-2A is 7.06±0.08 Ma (steps E through K) with 89.0% of the 3 9ArK released(MSWD=2.9). The weighted mean age for BG-4 84-2B is 7.31±0.07 Ma (steps D through K) with92.0% of the 3 9ArK released (MSWD=1.5). The inverse isochron for BG-4 84-2A yields an age of7.07±0.13 Ma (4 0Ar/3 6Ar intercept=295±10; MSWD=2.6). The inverse isochron for BG-4 84-2Byields an age of 7.33±.010 Ma (4 0Ar/3 6Ar intercept=293±6; MSWD=2).

The hornblende samples from this study yield poor 4 0Ar/3 9Ar results when compared to thegroundmass concentrates and the biotites. In nearly every case, the hornblendes degassed the bulk oftheir argon in only 2 to 3 heating steps. For hornblende 19-57-2 (Figure 12), approximately 85% ofthe 3 9ArK was released in steps H and I. The initial heating steps (A through G) yielded ages rangingfrom 2.50 to 21.36 Ma. Radiogenic yields range from 9.6 to 33.5%. K/Ca ratios range from 0.006 to

4

2.4. The weighted mean age for steps H and I is 1.39±0.21 Ma with a MSWD of 9.9. The inverseisochron yields an age of 0.69±2.3 Ma with a 4 0Ar/3 6Ar intercept of 360±246 (MSWD=42).

Hornblende 19-141-1 (Figure 13) also degassed predominantly in two heating steps, but itsage spectrum does not exhibit the discordance present in 19-57-2. Steps H and I contain almost 95%of the 3 9ArK released for a weighted mean age of 6.71±0.08 Ma (MSWD=0.2). The radiogenic yieldsfor those steps containing greater than 2% of the 3 9ArK are greater than 33%. The K/Ca ratios for allof the steps is consistent at approximately 0.15. The inverse isochron yields an age of 6.74±0.08 Ma(4 0Ar/3 6Ar intercept=289±3; MSWD=1.2).

Like the 19-141-2B groundmass concentrate, the 3-87 hornblende age spectrum (Figure 14) isdistinctly saddle-shaped. Although the initial ages (steps A through C) appear to be consistent atapproximately 9.5 Ma, the following 3 steps (D through F) immediately spike to ages in excess of 11Ma. Steps G through I return to a comparatively isochronous distribution with a weighted mean ageof 2.85±0.83 (77.4% of the 3 9ArK released) and a high MSWD of 41.9. The final two heating steponce again increase in age to greater than 30 Ma. Radiogenic yields and K/Ca values are positivelycorrelated to the shape of the age spectrum. The results for the inverse isochron are very discordantwhen all of the heating steps are plotted (MSDW=940). With only the three plateau steps plotted, theisochron yields an age of 3.5±1.8 (4 0Ar/3 6Ar intercept=274±43; MSWD=9.4).

The age spectrum results for 3-88 (Figure 15) exhibit some anomalous behavior at theintermediate heating steps. Steps D through G are slightly older and less precise than the rest of theage spectrum. However, this gas only represents ~10% of the total 3 9ArK released and therefore doesnot significantly influence the results of a weighted mean from steps B through I (age=0.34±0.32;94.8% of the 3 9ArK; MSWD=4.7). Radiogenic yields for sample 3-88 are very low and in some casesare negative (over corrected for extraction line blank). K/Ca ratios range from 1.0 to less than 0.1.The inverse isochron also yields very imprecise results (age=0.32±0.35 Ma; 4 0Ar/3 6Arintercept=296±16; MSWD=3.8).

The age spectrum for the BG-4 86-3 hornblende (Figure 17) is very similar to that of the 19-141-1 hornblende. Steps G through I yield a weighted mean age of 6.67±0.08 Ma with 95.8% of the3 9ArK released and a MSWD of 0.2. Radiogenic yields for steps G through I range from 52 to 86%while the K/Ca ratios are relatively constant at approximately 0.1. The inverse isochron yields an ageof 6.67±0.08 Ma with a 4 0Ar/3 6Ar intercept of 295±4 and a MSWD of 0.7.

Discussion

For the vast majority of samples dated in this report, the weighted mean or plateau age isinterpreted to represent the age of eruption of the rock/mineral in question. The one exception to thisis groundmass concentrate sample 19-133-1, where the inverse isochron yields the preferred age

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(32.45±0.32 Ma). The first two heating steps for 19-133-1 are significantly older than the remainderof the age spectrum. These anomalously old ages are likely caused by either small amounts ofxenocrystic contamination or excess argon. Excess argon is non-atmospheric 4 0Ar within a samplethat is derived by a process other than the in situ radioactive decay of 4 0K (McDougall and Harrison,1999). Most commonly, excess argon refers to trapped 4 0Ar/3 6Ar compositions greater than 295.5 (thepresent day 4 0Ar/3 6Ar composition). In the case of the 19-133-1 groundmass concentrate, smallamounts of excess argon may have been incorporated into glass and/or mineral phases at elevatedargon partial pressures (i.e. at depth or in a magma chamber). In many cases, an inverse isochron isemployed to test for trapped 4 0Ar/3 6Ar compositions greater than 295.5. The inverse isochron for the19-133-1 groundmass concentrate yields a trapped 4 0Ar/3 6Ar composition (300±2) only slightlygreater than 295.5. The slightly elevated 4 0Ar/3 6Ar trapped component prevents an unequivocalassessment of the scale of excess argon contamination, if it is present at all. However, given that theisochron data yields in an acceptable MSWD (1.8) while the plateau does not (3.3), we conclude thatthe inverse isochron age (32.45±0.32 Ma) is the best estimate for the age of eruption for this sample.

The age spectra for groundmass concentrate sample 19-135-1 and, to a lesser extent, 19-137-1suggest minor amounts of 4 0Ar loss. The anomalously young ages for the initial heating steps,coupled with the very low radiogenic yields for those steps, indicate the presence ofalteration/hydration products. Despite the existence of this alteration, the majority of the age spectrumfor both 19-135-1 and 19-137-1 does not appear to be greatly influenced. Therefore, we interpret theweighted mean ages for 19-135-1 (32.74±0.58 Ma) and 19-137-1 (8.72±0.11 Ma) as the preferredage of these samples.

The 19-141-2B exhibits the characteristic age spectrum shape often attributed to excess argon.Assuming that the oldest ages in 19-141-2B are those most influenced by excess argon and that theyoungest ages are those least influenced by excess argon, it is desirable to assign an age to a samplebased solely on the youngest age. In the case of 19-141-2B, the youngest age is yielded by theweighted mean of steps D through F (7.26±0.10 Ma). However, it is difficult to ascertain the quantityof excess argon that may or may not be present in steps D, E and F, especially given the poor qualityof the inverse isochron results. Therefore, while we state that the weighted mean age for this sample isthe preferred age, we must also state that this age should be considered a maximum age. Thepresumed contamination by excess argon is supported by a biotite mineral separate from the 19-141-2B sample (see below). The 19-141-2B groundmass concentrate yields a plateau weighted mean ageapproximately 350,000 years older than the 19-141-2B biotite plateau weighted mean age. Given thatthere are no signs of argon loss (e.g. alteration) in the biotite age spectrum, the source of the agediscrepancy is undoubtedly the presence of excess argon in the 19-141-2B groundmass concentrate.

Excess argon also appears to be influencing the higher temperature heating steps of the 20-49-2 groundmass concentrate age spectrum. However, unlike the 19-141-2B age spectrum, the lower

6

temperature steps of the 20-49-2 age spectrum do not appear to be significantly influenced by excessargon. Therefore, the preferred age of 20-49-2 is the plateau weighted mean age of 0.33±0.05 Ma.

Groundmass concentrate sample 3-89 does not appear to be influenced by either excess argonor alteration products (as indicated by both the shape of the age spectrum and the close agreementbetween the plateau weighted mean age and the inverse isochron age). Therefore, the preferred age ofthis sample is the plateau weighted mean age of 0.25±0.02 Ma.

For each of the five biotite samples in this study, the plateau weighted mean age yields the bestestimate for the age of eruption for each respective sample.

Biotite Sample Age ± Error (2s)19-141-2A 6.87±0.08 Ma19-141-2B 6.89±0.05 Ma20-49-3 0.51±0.03 Ma

BG-4 84-2A 7.06±0.08 MaBG-4 84-2B 7.31±0.07 Ma

Only two biotite age spectra suggest any anomalous behavior: BG-4 84-2A and BG-4 84-2B. Both ofthese age spectra yield anomalously young ages in the lowest temperature heating steps. Likegroundmass concentrate samples 19-135-1 and 19-137-1, biotites BG-4 84-2A and BG-4 84-2B mayhave small amounts of alteration products (chlorite?) present. However, despite the presumed presenceof some alteration products, any 4 0Ar loss seems to be confined to the lowest temperature portions ofthe samples. Therefore, the plateau weighed mean ages are still the preferred ages for these samples.

Although the hornblende samples were degassed in only 2 or 3 heating steps (as opposed tothe 5 to 9 steps for most of the groundmass concentrate and biotite samples from this study) anddespite some minor discordance with their age spectra, we interpret the hornblende plateau weightedmean ages as being the best estimates of their age of eruption.

Hornblende Sample Age ± Error (2s)19-57-2 1.39±0.21 Ma19-141-1 6.71±0.08 Ma

3-87 2.85±0.83 Ma3-88 0.34±0.32 Ma

BG-4 86-3 6.67±0.08 Ma

Many of the disturbances observed in the hornblende age spectra can be attributed to excess argon.For example, in sample 19-57-2, the age spike at the beginning of the age spectrum is probably theresult of mineral inclusions or groundmass exposed on the surface of the hornblende separate, assuggested by the increase in K/Ca ratio for those steps. The same can also be said for the lowtemperature steps for hornblende sample 3-87. The highest temperature steps (J and K) for 3-87 mayalso be caused by mineral inclusions. In the case of hornblende 3-87, where a plateau is straddled byhigher ages (similar to groundmass 19-141-2B) it is difficult to assess how much influence the excess

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argon is imparting on the plateau age. Therefore, like 19-141-2B, we must conclude that the age of 3-87 is a maximum age.

Figure 17 shows an age summary plot for the sixteen samples dated in this study. The moststriking feature of the data is the apparent grouping of samples into three distinct pulses ofmagmatism. The earliest phase determined from this sample suite is at approximately 32.5 Ma, asrecorded by samples 19-133-1 and 19-135-1. Following the 32.5 Ma activity, about 25 million yearspasses before more magmatic activity is recorded at approximately 8.7 to 6.7 Ma, as recorded by sevensamples (eight with the 19-141-2B duplicate). At least another 3 million years of no activity passesuntil the final pulse begins at approximately 2.8 Ma, as recorded by six samples. It must be noted,however, that sampling procedures as well as incomplete exposures can potentially result in anapparent cluster of magmatic activity.

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References Cited

Cande, S.C., and Kent, D.V., 1992. A New Geomagnetic Polarity Time Scale for the Late Cretaceousand Cenozoic. Journal of Geophysical Research, 97, 13,917-13,951.

Channell, R., McMillan, N.J., Lawton, T.F., Heizler, M., Esser, R.P. and McLemore, V.T., 2000.Magmatic History of the Little Hatchet Mountains, Hidalgo and Grant Counties, SouthwesternNew Mexico. New Mexico Geological Society, Guidebook 51, p. 141-148..

Deino, A., and Potts, R., 1990. Single-Crystal 4 0Ar/3 9Ar dating of the Olorgesailie Formation,Southern Kenya Rift, J. Geophys. Res., 95, 8453-8470.

Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977. Interpretation of discordant 4 0Ar/3 9Ar age-spectra ofMesozoic tholeiites from Antarctica, Geochim. Cosmochim. Acta, 41, 15-32.

Giletti, B.J., 1974. Studies in Diffusion I: Argon in phlogopite mica. In Geochemical transport andkinetics (ed. A.W. Hofmann, B.J. Giletti, H.S. Yoder, Jr., and R.A. Yund), pp. 107-115.Carnegie Inst. Of Wash. Publ. 634.

Harrison, T.M., 1981, Diffusion of 4 0Ar in hornblende: Contributions to Mineralogy & Petrology, v.78, p. 324-331.

Mahon, K.I., 1996. The New “York” regression: Application of an improved statistical method togeochemistry, International Geology Review, 38, 293-303.

Samson, S.D., and, Alexander, E.C., Jr., 1987. Calibration of the interlaboratory 4 0Ar/3 9Ar datingstandard, Mmhb-1, Chem. Geol., 66, 27-34.

Steiger, R.H., and Jäger, E., 1977. Subcommission on geochronology: Convention on the use ofdecay constants in geo- and cosmochronology. Earth and Planet. Sci. Lett., 36, 359-362.

Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in PhysicalMeasurements,. Univ. Sci. Books, Mill Valley, Calif., 270 p.

McDougall, I., and T. M. Harrison, 1988, Geochronology and thermochronology by the 4 0Ar/3 9Armethod: Oxford Monographs on Geology and Geophysics, v. 9, p. 212.

York, D., 1969. Least squares fitting of a straight line with correlated errors, Earth and Planet. Sci.Lett., 5, 320-324.

Table 1. 40Ar/39Ar summary table and analytical procedures.

Sample L# Irrad Mineralage

analysis n %39Ar MSWD K/Ca Age ±2s19-133-1 52210 NM-137 48.78 mg groundmass concentrate isochron 8 n.a. 2 n.a. 32.45 0.3219-135-1 52211 NM-137 46.98 mg groundmass concentrate plateau 5 90.2 5.6** 0.4 32.74 0.5819-137-1 52208 NM-137 47.52 mg groundmass concentrate plateau 5 87.0 1.5 0.9 8.72 0.1119-141-2B 52209 NM-137 49.83 mg groundmass concentrate plateau 3 23.2 0.1 2.0 7.26 0.1020-49-2 52291 NM-137 42.43 mg groundmass concentrate plateau 5 72.9 3.1** 3.0 0.33 0.053-89 52224 NM-137 33.31 mg groundmass concentrate plateau 4 79.2 0.6 2.5 0.25 0.0219-141-2A 52192 NM-137 8.54 mg biotite plateau 9 95.6 1.5 14.4 6.87 0.0819-141-2B 52194 NM-137 10.46 mg biotite plateau 8 90.9 1.4 58.7 6.89 0.0520-49-3 52196 NM-137 8.96 mg biotite plateau 5 86.8 2.7 4.5 0.51 0.03BG-4 84-2A 52245 NM-137 9.88 mg biotite plateau 7 89.0 2.9** 9.2 7.06 0.08BG-4 84-2B 52199 NM-137 4.01 mg biotite plateau 8 92.0 1.5 5.8 7.31 0.0719-57-2 52197 NM-137 22.18 mg hornblende plateau 2 85.1 9.9** 0.1 1.39 0.2119-141-1 52289 NM-137 21.70 mg hornblende plateau 2 94.2 0.2 0.1 6.71 0.083-87 52195 NM-137 14.71 mg hornblende plateau 3 77.4 41.9** 0.1 2.85 0.833-88 52198 NM-137 12.78 mg hornblende plateau 8 94.8 4.7** 0.3 0.34 0.32BG-4 86-3 52193 NM-137 21.26 mg hornblende plateau 3 95.8 0.2 0.1 6.67 0.08** MSWD outside 95% confidence interval

Notes:Sample preparation and irradiation: Samples provided by Drs. Bernard Guest and Gary Axen of the University of Los Angeles, California.Groundmass concentrate was separated using standard techniques (crushing, sieving, magnetics and hand-picking).Biotite and hornblende were separated using standard techniques (crushing, sieving, magnetics, heavy liquids and hand-picking).Samples were packaged and irradiated in machined Al discs for 7 hours in D-3 position, Texas A&M University Research Reactor.Neutron flux monitor Fish Canyon Tuff sanidine (FC-1). Assigned age = 27.84 Ma (Deino and Potts, 1990) relative to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987).

Instrumentation: Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.All samples were step-heated in a double-vacuum resistance furnace.Reactive gases from the biotites, horblendes and groundmass concentrates were removed during 12 to15 minute reaction with 3 SAES GP-50 getters, 2 operated at ~450°C and 1 at 20°C. Gas also exposed to a W filament operated at ~2000°C and a cold finger operated at -140°C.

Analytical parameters: Electron multiplier sensitivity averaged 1.3x10-16 moles/pA for those samples analyzed by the furnace.Total system blank and background for the incrementally heated samples averaged 2440, 5.9, 7.3, 4.8, 15.6 x 10-18 molesJ-factors determined to a precision of ± 0.1% by CO2 laser-fusion of 4 single crystals from each of 4 or 6 radial positions around the irradiation tray. Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows: (40Ar/39Ar)K = 0.0002±0.0003; (36Ar/37Ar)Ca = 0.00028±0.000011; and (39Ar/37Ar)Ca = 0.00089±0.00003.

Age calculations: Total gas ages and errors calculated by weighting individual steps by the fraction of 39Ar released.MSWD values are calculated for n-1 degrees of freedom for plateau and preferred ages.Isochron ages, 40Ar/36Ari and MSWD values calculated from regression results obtained by the methods of York (1969).Decay constants and isotopic abundances following Steiger and Jäger (1977). All final errors reported at ±2s, unless otherwise noted.

Table 2. 40Ar/39Ar step-heating results for the groundmass concentrate, biotite and hornblende samples.

ID Temp40Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39ArK K/Ca

40Ar* 39Ar Age ±1s

(°C) (x 10-3) (x 10-16 mol) (%) (%) (Ma) (Ma)

19-133-1, 48.78 mg groundmass concentrate, J=0.0006998±0.10%, NM-137, Lab#=52210-01A 625 2719.1 1.902 8994.8 8.76 0.27 2.3 2.9 75.8 15.0B 700 1114.7 1.648 3604.9 21.5 0.31 4.5 10.2 61.6 5.8C 750 207.8 1.510 612.1 15.0 0.34 13.0 15.2 33.9 1.0D 800 135.7 1.412 368.6 22.0 0.36 19.8 22.6 33.67 0.78E 875 88.09 1.301 206.8 33.2 0.39 30.7 33.8 33.90 0.44F 975 54.40 0.9766 94.51 35.3 0.52 48.8 45.6 33.24 0.26G 1075 38.53 0.6375 42.83 51.3 0.80 67.3 62.9 32.45 0.19H 1250 30.77 4.375 17.44 98.1 0.12 84.4 95.9 32.60 0.15I 1650 41.32 20.80 63.20 12.2 0.025 59.0 100.0 30.95 0.32total gas age n=9 297.4 0.36 36.2 2.4*plateau MSWD=3.3** n=6 steps C-H 254.8 0.38 85.7 32.76 0.44*isochron MSWD=1.8 n=8 40Ar/36Ar=300±2* 32.45 0.32*

19-135-1, 46.98 mg groundmass concentrate, J=0.0006995±0.10%, NM-137, Lab#=52211-01A 625 16837.9 2.355 55747.2 1.29 0.22 2.2 0.4 410.6 69.4B 700 465.2 1.581 1468.6 7.88 0.32 6.7 3.0 39.1 2.3C 750 215.1 1.181 649.0 8.13 0.43 10.9 5.7 29.4 1.4D 800 163.0 1.127 468.0 12.5 0.45 15.2 9.8 31.04 0.94E 875 133.1 1.347 359.2 26.3 0.38 20.3 18.4 33.80 0.60F 975 56.40 0.9240 101.9 24.2 0.55 46.8 26.4 33.00 0.31G 1075 52.80 0.5856 89.23 44.0 0.87 50.2 40.8 33.13 0.26H 1250 33.47 1.882 25.25 161.3 0.27 78.2 93.7 32.76 0.16I 1650 49.48 20.70 89.31 19.1 0.025 50.1 100.0 31.48 0.31total gas age n=9 304.8 0.39 34.44 1.28*plateau MSWD=5.6** n=5 steps E-I 275.0 0.39 90.2 32.74 0.58*isochron MSWD=7.5** n=9 40Ar/36Ar=298±11* 32.5 2.2*

19-137-1, 47.52 mg groundmass concentrate, J=0.0006997±0.10%, NM-137, Lab#=52208-01A 625 736.2 0.4789 2448.5 12.6 1.1 1.7 2.1 16.0 3.5B 700 139.9 0.2822 452.1 25.8 1.8 4.5 6.4 7.98 0.66C 750 95.05 0.8515 300.5 28.5 0.60 6.7 11.2 7.99 0.48D 800 69.10 1.355 209.6 40.9 0.38 10.5 18.0 9.16 0.34E 875 42.97 0.5659 121.5 54.7 0.90 16.5 27.1 8.95 0.23F 975 26.59 0.2253 66.34 62.0 2.3 26.4 37.4 8.83 0.16G 1075 19.40 0.2377 42.73 86.4 2.1 35.0 51.9 8.556 0.098H 1250 17.05 2.768 35.01 277.6 0.18 40.7 98.2 8.744 0.077I 1650 54.36 9.202 160.8 10.8 0.055 14.0 100.0 9.63 0.39total gas age n=9 599.2 0.87 8.87 0.48*plateau MSWD=1.5 n=5 steps D-H 521.5 0.85 87.0 8.72 0.11*isochron MSWD=2.5 n=9 40Ar/36Ar=296±3* 8.66 0.22*



40Ar* 39Ar Age ±1s

(°C) (x 10-3) (x 10-16 mol) (%) (%) (Ma) (Ma)

19-141-2B, 49.83 mg groundmass concentrate, J=0.0006999±0.10%, NM-137, Lab#=52209-01A 625 182.5 0.0923 581.3 38.9 5.5 5.9 5.4 13.54 0.89B 700 22.25 0.0883 45.58 131.4 5.8 39.5 23.5 11.06 0.11C 750 14.52 0.1364 24.88 53.0 3.7 49.4 30.8 9.03 0.11D 800 12.05 0.1964 21.46 35.4 2.6 47.5 35.7 7.21 0.12E 875 11.08 0.2935 18.04 52.6 1.7 52.1 43.0 7.275 0.087F 975 11.77 0.2613 20.39 79.7 2.0 49.0 54.0 7.269 0.075G 1075 17.47 0.1656 36.42 108.1 3.1 38.5 68.9 8.470 0.085H 1250 18.14 0.2743 34.12 200.9 1.9 44.5 96.6 10.171 0.086I 1650 47.58 1.115 88.48 24.4 0.46 45.2 100.0 27.00 0.28total gas age n=9 724.4 3.1 10.07 0.28*plateau MSWD=0.10 n=3 steps D-F 167.8 2.0 23.2 7.26 0.10*isochron MSWD=0.2 n=4 40Ar/36Ar=304±2* 7.04 0.12*

20-49.2, 42.43 mg groundmass concentrate, J=0.0007659±0.09%, NM-137, Lab#=52291-01A 625 361.7 0.3606 1220.4 1.34 1.4 0.3 0.2 1.5 3.2B 700 3.040 0.2294 9.754 52.0 2.2 5.8 7.9 0.244 0.051C 750 2.232 0.2044 6.867 47.1 2.5 9.9 15.0 0.304 0.037D 800 1.637 0.1757 4.920 86.0 2.9 12.1 27.8 0.273 0.027E 875 1.486 0.1578 4.178 168.6 3.2 17.8 52.9 0.365 0.017F 975 2.791 0.1645 8.628 136.1 3.1 9.1 73.1 0.352 0.028G 1075 10.49 0.2438 35.41 73.9 2.1 0.5 84.1 0.065 0.071H 1250 15.11 1.707 47.77 67.7 0.30 7.5 94.2 1.57 0.12I 1650 20.36 2.730 67.90 38.8 0.19 2.5 100.0 0.72 0.14total gas age n=9 671.6 2.4 0.45 0.11*plateau MSWD=3.1** n=5 steps B-F 489.8 3.0 72.9 0.33 0.05*isochron MSWD=5.5** n=8 40Ar/36Ar=295±9* 0.33 0.14*

3-89, 33.31 mg groundmass concentrate, J=0.0007034±0.10%, NM-137, Lab#=52224-01A 625 3541.6 0.2500 11988.1 0.223 2.0 0.0 0.0 -1.1 26.0B 700 5.791 0.2217 18.87 18.7 2.3 4.0 3.8 0.297 0.079C 750 2.173 0.1773 6.655 19.8 2.9 10.2 7.8 0.280 0.063D 800 1.409 0.1594 4.402 44.0 3.2 8.6 16.6 0.153 0.030E 875 1.066 0.1477 3.012 90.6 3.5 17.6 34.8 0.238 0.016F 975 1.213 0.1635 3.473 149.7 3.1 16.5 64.9 0.254 0.012G 1075 5.403 0.2808 17.83 90.6 1.8 2.9 83.1 0.199 0.044H 1250 13.34 1.030 44.73 63.1 0.50 1.5 95.8 0.26 0.10I 1650 14.30 3.688 47.99 20.8 0.14 2.9 100.0 0.54 0.14total gas age n=9 497.5 2.5 0.25 0.11*plateau MSWD=0.6 n=4 steps E-H 394.0 2.5 79.2 0.25 0.02*isochron MSWD=1.1 n=8 40Ar/36Ar=296±2* 0.25 0.02*



40Ar* 39Ar Age ±1s

(°C) (x 10-3) (x 10-16 mol) (%) (%) (Ma) (Ma)

19-141-2A, 8.54 mg biotite, J=0.0006943±0.10%, NM-137, Lab#=52192-01A 650 586.2 0.1809 1969.5 2.22 2.8 0.7 0.9 5.2 3.8B 750 130.6 0.3387 424.1 5.60 1.5 4.1 3.2 6.70 0.93C 850 26.21 0.0469 70.75 14.9 10.9 20.2 9.3 6.63 0.25D 920 12.52 0.0259 24.22 19.4 19.7 42.9 17.2 6.71 0.11E 1000 12.68 0.0313 24.08 25.0 16.3 43.9 27.5 6.96 0.11F 1075 13.02 0.0463 25.86 28.5 11.0 41.3 39.1 6.73 0.10G 1110 13.54 0.0468 27.77 16.3 10.9 39.4 45.8 6.67 0.13H 1180 10.93 0.0743 18.84 22.7 6.9 49.1 55.1 6.71 0.11I 1210 7.512 0.0892 6.779 19.7 5.7 73.4 63.1 6.896 0.062J 1250 6.511 0.0796 3.332 43.3 6.4 85.0 80.8 6.917 0.047K 1300 6.023 0.0165 1.725 44.2 31.0 91.6 98.8 6.894 0.041L 1650 15.70 0.1334 33.00 2.84 3.8 38.0 100.0 7.45 0.32total gas age n=12 244.6 13.8 6.81 0.28*plateau MSWD=1.5 n=9 steps C-K 234.0 14.4 95.6 6.87 0.08*isochron MSWD=1.3 n=12 40Ar/36Ar=294±2* 6.89 0.03*

19-141-2B, 10.46 mg biotite, J=0.0006941±0.10%, NM-137, Lab#=52194-02A 650 2170.7 0.2716 7396.8 0.343 1.9 -0.7 0.1 -18.9 15.6B 750 272.5 0.0368 909.7 8.02 13.9 1.3 2.8 4.6 1.5C 850 46.53 0.0187 137.9 15.8 27.3 12.4 7.9 7.21 0.35D 920 11.42 0.0102 19.92 30.7 50.2 48.5 18.0 6.918 0.094E 1000 10.47 0.0120 16.54 30.2 42.6 53.3 28.0 6.979 0.083F 1075 10.00 0.0116 15.79 29.2 43.9 53.4 37.6 6.673 0.087G 1110 8.944 0.0280 11.45 20.3 18.2 62.2 44.2 6.953 0.079H 1180 8.019 0.0204 8.460 29.0 25.0 68.8 53.8 6.899 0.064I 1210 7.093 0.0150 5.391 21.3 33.9 77.6 60.8 6.875 0.068J 1250 6.276 0.0079 2.454 44.1 64.5 88.5 75.3 6.938 0.047K 1300 5.921 0.0049 1.446 71.7 104.0 92.8 98.9 6.866 0.040L 1650 11.21 0.1901 18.21 3.44 2.7 52.2 100.0 7.31 0.28total gas age n=12 304.0 55.2 6.82 0.28*plateau MSWD=1.4 n=8 steps D-K 276.4 58.7 90.9 6.89 0.05*isochron MSWD=1.5 n=12 40Ar/36Ar=294±2* 6.91 0.06*

20-49-3, 8.96 mg biotite, J=0.0006929±0.10%, NM-137, Lab#=52196-01A 650 64.24 0.1274 213.4 2.13 4.0 1.9 0.9 1.49 0.88B 750 19.79 0.1602 64.89 3.02 3.2 3.2 2.1 0.78 0.36C 850 13.56 0.1178 45.32 3.55 4.3 1.3 3.5 0.23 0.33D 920 9.726 0.0697 30.45 3.74 7.3 7.5 5.0 0.92 0.27E 1000 5.584 0.0412 17.84 7.14 12.4 5.7 7.9 0.40 0.17F 1075 4.948 0.0424 15.95 11.5 12.0 4.8 12.6 0.297 0.087G 1110 4.634 0.0416 14.49 10.7 12.3 7.7 16.9 0.443 0.099H 1180 5.717 0.0826 17.78 44.7 6.2 8.2 35.0 0.587 0.062I 1210 3.152 0.0984 9.590 54.0 5.2 10.4 56.9 0.408 0.038J 1250 1.922 0.1904 5.103 80.3 2.7 22.4 89.4 0.537 0.022K 1300 1.601 0.2212 4.168 24.8 2.3 24.2 99.4 0.484 0.040L 1650 14.31 0.1934 46.25 1.50 2.6 4.6 100.0 0.82 0.58total gas age n=12 246.9 5.1 0.51 0.14*plateau MSWD=2.7 n=5 steps G-K 214.3 4.5 86.8 0.51 0.03*isochron MSWD=2.2 n=12 40Ar/36Ar=296±6* 0.49 0.08*



40Ar* 39Ar Age ±1s

(°C) (x 10-3) (x 10-16 mol) (%) (%) (Ma) (Ma)

BG-4 84-2A, 9.88 mg biotite, J=0.0007058±0.10%, NM-137, Lab#=52245-01A 650 885.4 0.9152 2994.7 1.49 0.56 0.1 0.5 0.6 6.3B 750 92.21 1.790 300.1 3.13 0.29 4.0 1.6 4.69 0.96C 850 33.67 0.3421 101.5 5.75 1.5 11.0 3.6 4.73 0.33D 920 12.16 0.0586 23.50 14.9 8.7 42.9 8.8 6.63 0.14E 1000 8.146 0.0284 8.492 41.7 17.9 69.2 23.4 7.166 0.059F 1075 7.631 0.0452 6.648 61.3 11.3 74.3 44.7 7.205 0.054G 1110 8.773 0.0668 10.73 25.0 7.6 63.9 53.5 7.127 0.077H 1180 8.499 0.1256 10.06 25.9 4.1 65.1 62.5 7.034 0.072I 1210 7.412 0.1180 6.493 21.6 4.3 74.2 70.0 6.994 0.056J 1250 6.878 0.2486 4.645 32.1 2.1 80.3 81.2 7.024 0.055K 1300 6.349 0.0530 2.936 47.5 9.6 86.4 97.8 6.972 0.044L 1650 10.23 0.1822 16.06 6.34 2.8 53.7 100.0 6.98 0.14total gas age n=12 286.6 8.8 6.96 0.22*plateau MSWD=2.9** n=7 steps E-K 255.0 9.2 89.0 7.06 0.08*isochron MSWD=2.6** n=9 40Ar/36Ar=295±10* 7.07 0.13*

BG-4 84-2B, 4.01 mg biotite, J=0.0006913±0.10%, NM-137, Lab#=52199-01A 650 1447.0 0.7216 4880.0 0.317 0.71 0.3 0.3 6.3 13.1B 750 40.33 0.7916 123.6 2.09 0.64 9.6 2.4 4.82 0.86C 850 41.32 0.4477 123.5 2.00 1.1 11.8 4.4 6.07 0.67D 920 14.18 0.0987 28.66 4.64 5.2 40.3 9.1 7.12 0.28E 1000 8.052 0.0441 6.979 13.2 11.6 74.4 22.4 7.461 0.085F 1075 7.765 0.0616 6.496 20.5 8.3 75.3 43.0 7.283 0.073G 1110 7.889 0.0995 7.160 9.64 5.1 73.3 52.7 7.197 0.095H 1180 8.871 0.1471 9.931 11.0 3.5 67.1 63.8 7.41 0.11I 1210 7.763 0.1794 6.037 9.30 2.8 77.2 73.1 7.46 0.10J 1250 7.433 0.3566 5.490 12.5 1.4 78.6 85.7 7.271 0.079K 1300 6.500 0.1060 2.502 10.7 4.8 88.8 96.4 7.182 0.091L 1650 10.91 0.1413 17.74 3.55 3.6 52.1 100.0 7.08 0.27total gas age n=12 99.4 5.5 7.22 0.34*plateau MSWD=1.5 n=8 steps D-K 91.4 5.8 92.0 7.31 0.07*isochron MSWD=2 n=12 40Ar/36Ar=293±6* 7.33 0.10*

19-57-2, 22.18 mg hornblende, J=0.0006923±0.10%, NM-137, Lab#=52197-01A 800 54.10 0.3507 125.0 2.76 1.5 31.8 2.9 21.36 0.72B 850 4.704 0.2168 9.212 1.97 2.4 42.5 4.9 2.50 0.33C 950 13.65 0.4419 33.48 1.71 1.2 27.8 6.7 4.73 0.51D 1020 53.29 1.206 159.9 0.727 0.42 11.5 7.4 7.7 1.2E 1080 85.94 2.072 258.7 0.485 0.25 11.2 7.9 12.0 1.8F 1120 40.02 2.977 95.96 0.587 0.17 29.8 8.5 14.8 1.4G 1160 21.00 4.125 62.77 5.21 0.12 13.3 13.9 3.49 0.35H 1200 7.172 4.350 22.58 21.7 0.12 12.0 36.2 1.08 0.10I 1300 3.394 5.357 9.140 60.8 0.095 33.5 99.0 1.424 0.035J 1400 27.40 87.34 99.01 0.640 0.006 19.6 99.6 7.1 1.3K 1650 47.82 10.82 149.3 0.344 0.047 9.6 100.0 5.8 2.2total gas age n=11 97.0 0.21 2.34 0.28*plateau MSWD=9.9** n=2 steps H-I 82.5 0.10 85.1 1.39 0.21*isochron MSWD=42** n=11 40Ar/36Ar=360±246* 0.69 2.3*



40Ar* 39Ar Age ±1s

(°C) (x 10-3) (x 10-16 mol) (%) (%) (Ma) (Ma)

19-141-1, 21.70 mg hornblende, J=0.0007079±0.10%, NM-137, Lab#=52289-01A 800 2353.0 5.998 8086.5 0.207 0.085 -1.5 0.2 -46.9 22.1B 850 987.5 26.06 3431.9 0.169 0.020 -2.5 0.3 -32.1 10.3C 950 104.1 5.111 326.4 0.207 0.100 7.8 0.5 10.4 4.1D 1020 36.46 3.169 123.4 0.262 0.16 0.7 0.7 0.3 2.9E 1080 40.30 2.441 125.3 0.341 0.21 8.6 1.0 4.5 2.4F 1120 33.30 2.708 96.76 0.402 0.19 14.8 1.3 6.3 1.9G 1160 13.82 3.308 31.97 2.79 0.15 33.6 3.5 5.93 0.36H 1200 8.884 3.488 13.13 9.29 0.15 59.6 11.1 6.76 0.13I 1300 6.270 3.704 4.490 107.2 0.14 83.7 97.7 6.709 0.041J 1400 15.08 4.232 34.78 1.99 0.12 34.2 99.3 6.58 0.44K 1650 25.92 7.183 73.95 0.863 0.071 18.0 100.0 6.0 1.0total gas age n=11 123.7 0.14 6.53 0.28*plateau MSWD=0.2 n=2 steps H-I 116.5 0.14 94.2 6.71 0.08*isochron MSWD=1.2 n=11 40Ar/36Ar=289±3* 6.74 0.08*

3-87, 14.71 mg hornblende, J=0.0006937±0.10%, NM-137, Lab#=52195-01A 800 65.19 0.7779 194.9 2.69 0.66 11.8 3.7 9.58 0.83B 850 24.27 0.4178 55.66 1.94 1.2 32.4 6.4 9.81 0.60C 950 16.92 0.5865 31.62 2.47 0.87 45.0 9.8 9.51 0.39D 1020 29.85 0.7778 47.13 1.39 0.66 53.6 11.8 19.91 0.67E 1080 32.35 0.9718 46.71 1.22 0.53 57.6 13.5 23.18 0.74F 1120 37.78 2.372 96.62 1.79 0.22 25.0 16.0 11.78 0.61G 1160 16.71 3.790 50.52 9.90 0.13 12.5 29.7 2.62 0.19H 1200 21.69 4.200 70.99 18.9 0.12 4.9 55.9 1.33 0.18I 1300 6.408 4.819 14.60 27.0 0.11 38.9 93.3 3.128 0.074J 1400 31.90 4.490 26.68 2.55 0.11 76.5 96.8 30.36 0.40K 1650 31.08 1.046 20.88 2.28 0.49 80.4 100.0 31.04 0.41total gas age n=11 72.1 0.22 5.95 0.46*plateau MSWD=41.9** n=3 steps G-I 55.8 0.12 77.4 2.85 0.83*isochron MSWD=9.4** n=3 40Ar/36Ar=274±43* 3.5 1.8*

3-88, 12.78 mg hornblende, J=0.0006918±0.10%, NM-137, Lab#=52198-01A 800 65.54 1.188 220.9 2.63 0.43 0.6 4.7 0.46 0.78B 850 7.641 0.4953 26.77 3.30 1.0 -3.0 10.5 -0.29 0.26C 950 4.825 0.5846 17.08 4.08 0.87 -3.6 17.7 -0.22 0.20D 1020 8.042 0.8558 24.45 2.49 0.60 11.0 22.1 1.11 0.31E 1080 17.09 1.537 57.21 0.861 0.33 1.8 23.6 0.39 0.86F 1120 24.57 2.282 87.57 0.534 0.22 -4.5 24.6 -1.4 1.2G 1160 32.50 3.476 105.9 2.32 0.15 4.6 28.7 1.86 0.58H 1200 25.11 4.062 86.27 8.50 0.13 -0.2 43.7 -0.06 0.28I 1300 7.756 5.218 26.51 31.5 0.098 4.6 99.4 0.444 0.077J 1400 82.65 24.99 284.8 0.226 0.020 0.7 99.8 0.7 3.2K 1650 138.1 14.43 493.3 0.108 0.035 -4.7 100.0 -8.2 7.3total gas age n=11 56.5 0.26 0.33 0.48*plateau MSWD=4.7** n=8 steps B-I 53.6 0.25 94.8 0.34 0.32*isochron MSWD=3.8** n=11 40Ar/36Ar=296±16* 0.32 0.35*



40Ar* 39Ar Age ±1s

(°C) (x 10-3) (x 10-16 mol) (%) (%) (Ma) (Ma)

BG-4 86-3, 21.26 mg hornblende, J=0.0006943±0.10%, NM-137, Lab#=52193-01A 800 428.6 1.672 1434.1 0.478 0.31 1.2 0.6 6.3 4.2B 850 533.0 3.086 1838.6 0.051 0.17 -1.9 0.6 -12.7 14.8C 950 241.2 4.167 762.2 0.072 0.12 6.8 0.7 20.4 10.3D 1020 104.7 4.429 343.8 0.080 0.12 3.3 0.8 4.4 8.2E 1080 51.85 4.582 151.3 0.233 0.11 14.5 1.1 9.4 3.0F 1120 24.32 4.755 64.40 0.559 0.11 23.4 1.7 7.1 1.2G 1160 10.05 4.773 17.54 3.14 0.11 52.3 5.4 6.60 0.21H 1200 9.175 4.683 14.46 15.5 0.11 57.7 23.4 6.636 0.099I 1300 6.208 4.736 4.306 63.8 0.11 85.8 97.5 6.682 0.043J 1400 14.45 5.067 34.07 1.63 0.10 33.2 99.4 6.02 0.52K 1650 36.58 4.337 107.2 0.549 0.12 14.4 100.0 6.6 1.3total gas age n=11 86.1 0.11 6.66 0.28*plateau MSWD=0.2 n=3 steps G-I 82.4 0.11 95.8 6.67 0.08*isochron MSWD=0.7 n=11 40Ar/36Ar=295±4* 6.67 0.08*

Notes:Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interferring reactions.Individual analyses show analytical error only; plateau and total gas age errors include error in J and irradiation parameters.Analyses in italics are excluded from final age calculations.Discrimination = 1.00684±0.00098 a.m.u.†=analyses excluded from plateau weighted mean age.ø=analyses excluded from inverse isochron age.K/Ca = molar ratio calculated from reactor produced 39ArK and 37ArCa.* 2s error** MSWD outside of 95% confidence interval

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.040

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034

Isochron age = 32.45 ± 0.32 Ma40Ar/36Ar Intercept = 300 ± 2MSWD = 1.8, n = 8

AB

C

D

E

F

G

H

I

39Ar/40Ar

36A

r/40

Ar

0

40

80

L# 52210: 19-133-1, 48.78 mg groundmass concentrate

0.01

0.1

1

0 10 20 30 40 50 60 70 80 90 10025

30

35

40

45

50

55

60

A

B700

C750

D800

E875

F975 G

1075H

1250 I

32.76 ± 0.44 Ma

Integrated Age = 36.2 ± 2.4 Ma

Cumulative 39ArK Released

App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

Figure 1. 40Ar/39Ar age spectrum and inverse isochron for the 19-133-1 groundmass concentrate.The preferred age of this sample is the inverse isochron age (32.45±0.32 Ma). All errors are 2σ.

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.040

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


AB

CD

E

F

G

H

I

39Ar/40Ar

36A

r/40

Ar

0

40

80

L# 52211: 19-135-1, 46.98 mg groundmass concentrate

0.01

0.1

1

0 10 20 30 40 50 60 70 80 90 10020222426283032343638404244

B

C750

D800

E875

F975

G1075

H1250 I

32.74 ± 0.58 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

Figure 2. 40Ar/39Ar age spectrum and inverse isochron for the 19-135-1 groundmass concentrate.The preferred age of this sample is the plateau weighted mean age (32.74±0.58 Ma). All errors are 2σ.

0

40

80L# 52208: 19-137-1, 47.52 mg groundmass concentrate

0.010.1110

0 10 20 30 40 50 60 70 80 90 10002468

1012141618202224

B C750

D800

E875

F975

G1075

H1250

I

8.72 ± 0.11 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.02 0.04 0.06 0.08 0.1 0.12 0.140

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


AB

C D

E

F

GH

I

39Ar/40Ar

36A

r/40

Ar


0

40

80L# 52209: 19-141-2B, 49.83 mg groundmass concentrate

0.1

1

10

0 10 20 30 40 50 60 70 80 90 1000

5

10

15

20

25

30

A625 B

700 C750 D

800E

875F

975

G1075

H1250

I

7.26 ± 0.10 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


A

B

C

DEF

G

HI

39Ar/40Ar

36A

r/40

Ar

Figure 4. 40Ar/39Ar age spectrum and inverse isochron for the 19-141-2B groundmass concentrate.The preferred age of this sample is the plateau weighted mean age (7.26±0.10 Ma). All errors are 2σ.

0

40

80

L# 52291: 20-49.2, 42.43 mg groundmass concentrate

0.1

1

10

0 10 20 30 40 50 60 70 80 90 100

-0.4-0.2

00.20.40.60.8

11.21.41.61.8

2

B700

C750

D800

E875

F975

G

H1250

I

0.33 ± 0.05 Ma*



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.5 1 1.5 2 2.5 3 3.5 40

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


AB

C D

E

F

G

HI

39Ar/40Ar

36A

r/40

Ar


0

40

80

L# 52224: 3-89, 33.31 mg groundmass concentrate

0.1

1

10

0 10 20 30 40 50 60 70 80 90 100

-0.4

-0.2

0.2

0.4

0.6

0.8

1

1.2

1.4

B700

C D800

E875

F975 G

1075 H1250

I

0.25 ± 0.02 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0

Figure 6. 40Ar/39Ar age spectrum and inverse isochron for the 3-89 groundmass concentrate.The preferred age of this sample is the plateau weighted mean age (0.25±0.02 Ma). All errors are 2σ.

H

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


A

BC D

EF

GI

39Ar/40Ar

36A

r/40

Ar

6.87 ± 0.08 Ma

0

40

80

L# 52192: 19-141-2A, 8.54 mg biotite

1

10

100

0 10 20 30 40 50 60 70 80 90 1003

4

5

6

7

8

9

10

11

B

C850

D920

E1000 F

1075G

1110H

1180I

1210J

1250K

1300L



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


AB

C

DE

FG

H

I

J

K

L

39Ar/40Ar

36A

r/40

Ar

Figure 7. 40Ar/39Ar age spectrum and inverse isochron for the 19-141-2A biotite. The preferred age ofthis sample is the plateau weighted mean age (6.87±0.08 Ma). All errors are 2σ.

F

0

40

80

L# 52194: 19-141-2B, 10.46 mg biotite

1101001000

0 10 20 30 40 50 60 70 80 90 1004

5

6

7

8

9

10

C850

D920

E1000 F

1075

G1110

H1180

I1210

J1250

K1300

6.89 ± 0.05 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


A

B

C

D

EG

H

I

J K

L

39Ar/40Ar

36A

r/40

Ar

Figure 8. 40Ar/39Ar age spectrum and inverse isochron for the 19-141-2B biotite. The preferred age ofthis sample is the plateau weighted mean age (6.89±0.05 Ma). All errors are 2σ.

0

40

80

L# 52196: 20-49-3, 8.96 mg biotite

1

10

100

0 10 20 30 40 50 60 70 80 90 100-1

-0.5

0

0.5

1

1.5

2

E1000

FG

1110

H1180 I

1210

J1250 K

1300

0.51 ± 0.03 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2. 2.2 2.4 2.60

0.00020.0004

0.00060.0008

0.001

0.0012

0.00140.0016

0.0018

0.002

0.0022

0.00240.0026

0.0028

0.003

0.0032

0.0034


ABC

D EFGH I

J K

L

39Ar/40Ar

36A

r/40

Ar

Figure 9. 40Ar/39Ar age spectrum and inverse isochron for the 20-49-3 biotite. The preferred age of thissample is the plateau weighted mean age (0.51±0.03 Ma). All errors are 2σ.

0

40

80L# 52245: BG-4-84-2A, 9.88 mg biotite

0.1110100

0 10 20 30 40 50 60 70 80 90 1003

4

5

6

7

8

9

10

C

D920

E1000

F1075 G

1110H

1180I

1210J

1250K

1300 L

7.06 ± 0.08 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


AB

C

D

EF

G H

I

J

K

L

39Ar/40Ar

36A

r/40

Ar

Figure 10. 40Ar/39Ar age spectrum and inverse isochron for the BG-4 84-2A biotite. The preferredage of this sample is the plateau weighted mean age (7.06±0.08 Ma). All errors are 2σ.

0

40

80

L# 52199: BG-4 -84-2B, 4.01 mg biotite

0.1110100

0 10 20 30 40 50 60 70 80 90 1003

4

5

6

7

8

9

10

C

D920

E1000 F

1075 G1110

H1180

I1210 J

1250 K1300 L

7.31 ± 0.07 Ma*



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.160

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034

Isochron age = 7.33 ± 0.10 Ma40Ar/36Ar Intercept = 293 ± 6MSWD = 2, n = 12

A

B

C

D

EF

H

I J

K

L

G

39Ar/40Ar

36A

r/40

Ar

Figure 11. 40Ar/39Ar age spectrum and inverse isochron for the BG-4 84-2B biotite. The preferred age ofthis sample is the plateau weighted mean age (7.31±0.07 Ma). All errors are 2σ.

0

40

80

L# 52197: 19-57-2, 22.18 mg hornblende

0.001

0.1

10

0 10 20 30 40 50 60 70 80 90 100-5

0

5

10

15

20

25

A

B850

C G1160 H

1200I

1300

1.39 ± 0.21 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

Isochron age = 0.69 ± 2.3 Ma40Ar/36Ar Intercept = 360 ± 246MSWD = 42, n = 11

A

B

CF

I

J

39Ar/40Ar

36A

r/40

Ar

Figure 12. 40Ar/39Ar age spectrum and inverse isochron for the 19-57-2 hornblende.The preferred age of this sample is the plateau weighted mean age (1.39±0.21 Ma). All errors are 2σ.

0

40

80

L# 52289: 19-141-1, 21.70 mg hornblende

0.01

0.1

1

0 10 20 30 40 50 60 70 80 90 1002

3

4

5

6

7

8

9

10

11

12

G

H1200

I1300

J

6.71 ± 0.08 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

0.0002

0.0004

0.00060.0008

0.001

0.00120.0014

0.0016

0.00180.002

0.0022

0.0024

0.00260.0028

0.003

0.00320.0034

0.0036


A

B

C

D

E

F

G

H

I

J

K

39Ar/40Ar

36A

r/40

Ar

Figure 13. 40Ar/39Ar age spectrum and inverse isochron for the 19-141-1 hornblende.The preferred age of this sample is the plateau weighted mean age (6.71±0.08 Ma). All errors are 2σ.

0

40

80

L# 52195: 3-87, 14.71 mg hornblende

0.1

1

10

0 10 20 30 40 50 60 70 80 90 100Cumulative 39ArK Released

App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

-10

-5

0

5

10

15

20

25

30

35

A B850

C

D

F

G1160 H

1200

I1300

J K

2.85 ± 0.83 Ma


0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

39Ar/40Ar

36A

r/40

Ar

00.00020.00040.00060.0008

0.0010.00120.00140.00160.0018

0.0020.00220.00240.00260.0028

0.0030.00320.00340.00360.0038


A

B

C

D

E

F

G

H

I

J

K

Figure 14. 40Ar/39Ar age spectrum and inverse isochron for the 3-87 hornblende.The preferred age of this sample is the plateau weighted mean age (2.85±0.83 Ma). All errors are 2σ.

0

40

80L# 52198: 3-88, 12.78 mg hornblende

0.010.1110

0 10 20 30 40 50 60 70 80 90 100

-6

-4

-2

0

2

4

6

8

A800

B C950

D

E

F1120

G

H1200

I1300

0.34 ± 0.32 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

39Ar/40Ar

36A

r/40

Ar

0 0.5 1 1.5 2 2.5 3 3.5 40

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


Figure 15. 40Ar/39Ar age spectrum and inverse isochron for the 3-88 hornblende.The preferred age of this sample is the plateau weighted mean age (0.34±0.32 Ma). All errors are 2σ.

0

40

80L# 52193: BG-4-86-3, 21.26 mg hornblende

0.01

0.1

1

0 10 20 30 40 50 60 70 80 90 1003

4

5

6

7

8

9

10

G H1200

I1300

6.67 ± 0.08 Ma



App

aren

tAge

(Ma)

%R

adio

geni

c

K/C

a

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0.0022

0.0024

0.0026

0.0028

0.003

0.0032

0.0034


AB

C

D

E F

GH

I

J

K

39Ar/40Ar

36A

r/40

Ar

Figure 16. 40Ar/39Ar age spectrum and inverse isochron for the BG-4 86-3 hornblende.The preferred age of this sample is the plateau weighted mean age (6.67±0.08 Ma). All errors are 2σ.

0 10 20 30 40

Apparent Age (Ma)

19-133-1

19-135-1

19-137-1

19-141-2B

20-49-2

3-89

19-141-2A

19-141-2B

20-49-3

BG-4 84-2A

BG-4 84-2B

19-57-2

19-141-1

3-87

3-88

BG-4 86-3

32.45±0.32 Ma

8.72±0.11 Ma

7.26±0.10 Ma

6.87±0.08 Ma

0.33±0.05 Ma

32.74±0.58 Ma

0.25±0.02 Ma

6.89±0.05 Ma

0.51±0.03 Ma

7.06±0.08 Ma

7.31±0.07 Ma

1.39±0.21 Ma

6.71±0.08 Ma

2.85±0.83 Ma

0.34±0.32 Ma

6.67±0.08 Ma

Figure 17. Summary diagram of the 40Ar/39Ar ages yielded by the samplesfrom Alborz Mountains, Iran. Note the apparent clustering of ages at~32.5, ~7 and again at ~1 Ma. All errors are two-sigma.

Gro

undm

ass

Con

cent

rate

sB

iotit

esH

ornb

lend

es

Documents

40Ar/39Ar Geochronology Results from Volcanic … · SUSIE KYLE, Administrative Secretary I LEWIS A. LAND, Hydrogeologist ... AUSTIN, Emeritus Senior Industrial Minerals Geologist