Spot Name U-Pb ages (Ma) Plagioclase ages (Ma) Biotite age (Ma) Whole rock age (Ma)

Transcription

1 Table 1. Average U-Pb ages from this study in comparison with previous ages from Sherrod and Tosdal (1991, and references therein). Previous study ages are reported as ranges including uncertainty (i.e. given 20±2 Ma, range reported would be 18-22Ma) Ages from Sherrod and Tosdal s study not marked as spurious are in better agreement with new U-Pb age. Ages of basalts have not been updated and the age of andesitic basalt of Flat Tops remains uncertain due to lack of agreement with stratigraphic relationships. Spot Name U-Pb ages (Ma) Plagioclase ages (Ma) Biotite age (Ma) Whole rock age (Ma) Basalt of Black Mountain Flat Tops Andesitic Basalt ~33-25 ~ tuff of Black Hills (tbh) 23.3± (28.6±9.2) 628 tuff of Felipe Pass 23.8± (?) ignimbrite of Ferguson Wash 23.2± (sanidine) Quechan volcanic rocks 23.4± ±0.6 QP136 granite of Peter Kane Mountain 24.4± granite of Mount Barrow (gmb) 24.5± ±0.2 Extension begins (muscovite and biotite)* (Sherrod and Tosdal, 1991, and references therein) *(Jacobson et al., 2007)

2 Table 2. Whole rock oxide data for samples collected from tuff of Black Hills, granite of Mount Barrow, basalt of American Girl (American Girl in table), andesitic basalt of Flat Tops, and basalt at Black Mountain. 25 Sample SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 FeO* MnO MgO CaO Na 2 O K 2 O P 2 O 5 Totals tbh tbh tbh tbh tbh gmb B gmb gmb gmb American Girl Flat Tops Black Mountain * multiplied by and not included in totals

3 Table 3. Trace element data for whole rock samples. 26 Sample Rb (ppm) Sr (ppm) Y (ppm) Zr (ppm) Ba (ppm) tbh tbh tbh tbh tbh gmb B gmb gmb gmb American Girl Flat Tops Black Mountain

4 Figure 1. Generalized geologic map of the area modified from Sherrod and Tosdal (1991). Thick, black outlined box on the left of the area is the study area. 27

5 Figure 2. Arc map of sample locations. Small blue circles are samples from 08 field work including basalt, granite, rhyolite, and some andesitic samples, other symbols are keyed on map and graphs and were collected by Dr. Andrew Barth and Dr. Carl Jacobson on separate field trips 28

6 Timeline of important events Extension Basalt of Black Mountain Tuff of Black Hills Tuff of Felipe Pass Ignimbrite of Ferguson Wash Quechan volcanic rocks Granite of Peter Kane Mountain Granite of Mount Barrow Andesitic basalt of Flat Tops Time (Ma) Figure 3. Timeline of important mid-late Cenozoic events in the Chocolate Mountains region. References for extension and the basalt ages can be found in Table 1. Ages on volcanic and plutonic units are average weighted means of U-Pb zircon ages from this study. 29

7 1100 Tuff of Black Hills mineral temperatures zircon rims zircon interiors zircon cores 1000 Temperature ( C) tuff of Black Hills zircon temperatures tuff of Black Hills quartz temperatures Bishop tuff quartz temperatures 1r 11.2r 2r 4r 5.1r 6.1r 9.1r Spot 11.3i 6.2i 7.1incl 9.2c 11.1c 3c 5.2c 1100 Ignimbrite of Ferguson Wash mineral temperatures zircon rims zircon interiors zircon cores 1000 Temperature ( C) ignimbrite of Ferguson Wash zircon temperatures ignimbrite of Ferguson Wash quartz temperatures Bishop tuff quartz temperatures 1.1r 10.1r 10.3r 11.2r 12.2r 2.1r 3.2r 4.1r 13.2r 7.1r 15.1r 5.2r 6.1r 9.2r 10.4i 10.5i 12.3i 4.2incl 10.2c 11.1c 12.1c 13.1c 3.1c 14.1c 15.2c 6.2c 7.2c 9.1c Figure 4. Quartz and zircon temperature estimates for tbh and the ignimbrite of Ferguson Wash in comparison to Bishop tuff quartz (Wark et al., 2007). Errors on zircon temperatures are about symbol size, and cross-hatched areas represent overlap in quartz temperatures. Bishop tuff quartz and volcanic zircon temperatures are split based on spot location on the grain (e.g. rim), but quartz measurements from tbh and the ignimbrite of Ferguson Wash showed no appreciable rim-core zonation and are not separated. Spot 30

8 Plutonic and volcanic temperatures tuff of Felipe Pass tuff of Black Hills Quechan volcanic rocks ignimbrite of Ferguson Wash plutonic rocks Temperature ( C) cores rims Log Eu/Eu* Figure 5. Temperature ranges for plutonic and volcanic samples plotted against Eu anomaly. Tuff of Black Hills cores plot within the field of Quechan volcanic rocks and are represented by the open ellipse labeled cores. Plutonic rocks are represented by the oddly-shaped field encompassing tuff of Felipe Pass, cores from tbh, and the lower parts of Quechan. Tuff of Black Hills rims plot in the open ellipse marked rims. 31

9 10000 HREE similarity between tbh and gmb Log Concentration/Chondrite tuff of Black Hills granite of Mount Barrow 10 GD TB DY HO ER TM YB LU Heavy Rare Earth Elements (HREE) Figure 6. Chondrite-normalized HREE concentrations for tbh and gmb. The overlap of units combined with similar ages provoked further attention. 32

10 1000 Whole rock REE chemistry Log Concentration/Condrite tuff of Black Hills granite of Mount Barrow andesitic basalt of Flat Tops basalt of Black Mountain La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Rare Earth Elements (REE) Figure 7. Chondrite-normalized volatile-free whole rock REE chemistry comparing tbh, gmb, and basalt samples. 33

11 Figure 8. Quartz from tbh in the process of dissolution. 34

12 Quartz temperatures of Bishop Tuff 1100 Wark et al. (2007) estimates Our estimates 1000 Temperature ( C) BT-1.1 BT-1.2 BT-1.3 BT-1.4 BT-1.5E BT-1.6I BT-1.7E Spot Figure 9. Comparison of our estimate of temperature in the Bishop Tuff with Wark et al. (2007). Wark et al. found temperatures on average about 20 C higher than our study. This could mean our quartz temperature estimates are off by 20 C or that we have a sample from a cooler part of the tuff. 35

13 28 Very little HREE change with time 26 Age (Ma) tuff of Black Hills granite of Mount Barrow tuff of Felipe Pass Quechan volcanic rocks ignimbrite of Ferguson Wash granite of Peter Kane Mountain Yb/Gd Figure 10. Little correlation between chondrite-normalized calculated melt HREE and time is seen between units. Shaded areas demonstrate spread of data for individual units, with gmb and tbh symbols remaining for comparison. 36

14 28 Plagioclase fractionation over time 26 Age (Ma) tuff of Black Hills granite of Mount Barrow tuff of Felipe Pass Quechan volcanic rocks ignimbrite of Ferguson Wash granite of Peter Kane Mountain Log Eu/Eu* Figure 11. Plagioclase fractionation between units over time. Zircons from slightly older units have shallower negative Eu anomalies, which get deeper in zircons from slightly younger units. 37

15 28 No overall temperature change with time 26 Age (Ma) tuff of Felipe Pass tuff of Black Hills Quechan volcanic rocks ignimbrite of Ferguson Wash granite of Mount Barrow granite of Peter Kane Mountain Temperature ( C) Figure 12. There is not an age correlation with temperature between units. Error on temperature is about symbol size. Within tbh, temperature increases ~50 C during roughly 2.5 m.y., indicating possible either steady heating or reheating during this time frame. Quechan volcanic rocks show a general cooling, although one of the younger zircons does yield a higher temperature, generally younger grains yield cooler temperatures and older grains yield higher temperatures. 38

16 1100 Tuff of Black Hills zircon temperature paths 1000 Temperature ( C) c 11.2r 5.2c 5.1r 6.2i 6.1r 9.2c 9.1r 3c 11.3i 1r 2r 4r 7.1incl 600 Spot Figure 13. Temperature paths from core to rim of each grain with multiple measurements from tbh, as indicated by arrows. Grains with only one measurement are on the right side of the graph without arrows and are ordered from core to rim for separate grains. Grains that have two or more measurements ubiquitously show cooling from core to rim. All grains are from the same age population listed in Table 1 or have no age associated with them. 39

17 1100 Ignimbrite of Ferguson Wash zircon temperature paths 1000 Temperature ( C) c 10.4i 10.5i 10.1r 10.3r 11.1c 11.2r 12.1c 12.3i 12.2r 13.1c 13.2r 15.2c 15.1r 6.2c 6.1r 7.2c 7.1r 9.1c 9.2r 3.1c 3.2r 14.1c 2.1r 4.1r 4.2r(incl?) 5.2r 1.1r Spot Figure 14. Temperature paths from core to rim of each grain with multiple measurements from the ignimbrite of Ferguson Wash, as indicated by arrows. Grains with only one measurement are on the right side of the graph without arrows and are ordered from core to rim for separate grains. Most grains that have two or more measurements show reheating, with two notable exceptions. All grains are from the same age population listed in Table 1 or have no age associated with them. 40

18 1100 Plutonic temperature paths 1000 Temperature ( C) c r c r r r r r r r r r r r QP c QP r QP c QP r QP c QP r QP c QP r QP c QP r QP c QP I QP r QP c QP r QP c QP r QP c QP r QP c QP r QP c QP c QP c QP r QP r Spot Figure 15. Temperature paths from core to rim of each grain with multiple measurements from the plutonic rocks, as indicated by arrows. Units are split by solid bold line, and grains with only one measurement are to the right of grains with multiple measurements for each unit. The granite of Mount Barrow only had one grain with core and rim measurements, and it records reheating. Two grains from granite of Peter Kane also record higher rim temperatures, but the temperature change is nearly within error. All other grains with multiple measurements record cooling. 41

19 Figure 16. Whole rock data from tbh, gmb, mafic lavas, and the Basin and Range (Navdat; 42

20 10 1 Zircon crystallization Chemical fractionation trends 10 1 Zircon and biotite crystallization Eu/Eu* 0.1 Eu/Eu* A 0.01 B Plagioclase crystallization Yb/Gd 10 Yb/Gd 43 Eu/Eu* 1 Zircon and biotite crystallization 0.1 Apatite and amphibole crystallization 0.01 C Plagioclase crystallization Figure17. Expected change in melt chemistry indicated by arrow, with crystallization of mineral. A. Crystallization of zircon will cause a decrease in Yb/Gd in the melt and very slight increase in Eu/Eu*. Biotite is not shown, but behaves in a similar manner. B Zircon and biotite decrease Yb/Gd of melt, but plagioclase decreases Eu/Eu* of the melt while it crystallizes. Crystallizing zircon (and/or biotite) and plagioclase would result in a vector (dotted arrow) representing a decrease in Yb/Gd and Eu/Eu*. C. Amphibole and apatite significantly increase Yb/Gd and Eu/Eu* very slightly. Example; crystallizing both apatite and zircon would have roughly no net affect on Yb/Gd, but might slightly increase Eu/Eu* if no plagioclase is growing. Yb/Gd

21 10 Chemical fractionation trends 1 cores Log Eu/Eu* 0.1 plutonic cores 0.01 cores tuff of Black Hills ignimbrite of Ferguson Wash plutonic rocks Yb/Gd Figure 18. HREE in relation to Eu anomaly. The field for tbh has been filled in with a pattern to make it more distinguishable from the plutonic samples and will be represented with the same field throughout. Core locations are labeled for the ignimbrite of Ferguson Wash, tbh and plutons. Arrows show chemical change as grains in these units were growing. 44

22 Temperature effect on plagioclase behavior tuff of Black Hills ignimbrite of Ferguson Wash plutonic rocks 1000 Temperature ( C) cores rims cores cores rims rims Log Eu/Eu* Figure 19. Eu anomaly relationship to temperature in units that show core-rim fractionation trends. Arrows show chemical and temperature changes as grains were crystallizing. 45

23 HREE enrichment with crystallization tuff of Black Hills ignimbrite of Ferguson Wash plutonic rocks Temperature ( C) rims cores cores cores rims rims Yb/Gd Figure 20. HREE enrichment with crystallization. tbh cores are in the middle of plutonic cores, which have a separate label outside the tbh ellipse, but within the irregularlyshaped plutonic field. Rims for all units have higher Yb/Gd than their cores, indicating slight HREE enrichment as zircon grows. 46

24 10 Zircon and whole rock HREE trend 1 Log Eu/Eu* 0.1 tuff of Black Hills rim tuff of Black Hills core 0.01 tuff of Black Hills whole rock granite of Mount Barrow rim granite of Mount Barrow core granite of Mount Barrow whole rock andesitic basalt of Flat Tops basalt of Black Mountain Yb/Gd Figure 21. HREE pattern changes with Eu anomaly. The basalt lavas have positive anomalies, while gmb whole rock and the core analyses of tbh have the shallowest negative anomalies. Whole rock tbh chemistry is HREE enriched relative to most zircon analyses from both units, and tbh zircon rims show HREE depletion with an increase in europium anomaly. 47

25 Figure 22. CL images of a zircon grain from VP-10. Image goes from left to right and sequential numbers represent number of polishing rounds on grain. Image 0 is an unpolished grain, image 1 shows a grain with about 3-5 microns polished off, another 10 microns are polished off for image 2, and so on. The bright rim in image 1 was a common occurrence on many zircon grains from VP-10, but not all grains displayed this. Notice the thin bright edge is visible throughout the sequence. The various scales for grains are also given; the first set of images; 0-4 were magnified 130X, but images 2-4 were altered in size along with their bar scale, and images 5-6 were taken at 120X. The apparent change in size of grains is a result of the epoxy; grains are mounted in epoxy and as they are polished down, more of the grain is exposed. 48

26 Figure 23. Zircons from tbh which display small bright areas on their rims. These spots would be good locations to begin CL-depth imaging and analyses of melt conditions during the very final stage of growth. 49

27 Appendix Table 1. Geochronologic data for all grains from each sample. 50 Spot Name U (ppm) Th (ppm) % Comm 206 Pb 238 U/ 206 PB % err 207 Pb/ 206 Pb % err 206 Pb / 238 U Age granite of Peter Kane Mountain QP QP QP QP QP QP QP QP QP QP QP QP QP QP granite of Mount Barrow (MBg) s err

28 Appendix Table 1 continued. Spot Name U (ppm) Th (ppm) % Comm 206 Pb 238 U/ 206 PB % err 207 Pb/ 206 Pb % err 206 Pb / 238 U Age 1s err ignimbrite of Ferguson Wash Quechan volcanic rocks

29 Appendix Table 1 continued. 52 Spot Name U (ppm) Th (ppm) % Comm 206 Pb 238 U/ 206 PB % err 207 Pb/ 206 Pb % err 206 Pb / 238 U Age tuff of Black Hills (BHt) s err

30 Appendix Table 1 continued. 53 Spot Name U (ppm) Th (ppm) % Comm 206 Pb 238 U/ 206 PB % err 207 Pb/ 206 Pb % err 206 Pb / 238 U Age tuff of Felipe Pass s err

31 Appendix Table 2. REE concentrations of fragments of Ceylon (CZ3) zircon. 54 Spot Name La Ce Pr Nd Sm Eu Ho Gd Tb Dy Er Tm Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ CZ

32 Appendix Table 2 continued. Spot Name La Ce Pr Nd Sm Eu Ho Gd Tb Dy Er Tm Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm CZ CZ CZ CZ CZ CZ CZ3-5.1A

33 Appendix Table 3. Raw REE concentrations for all grains from each sample. 56 Spot name La Ce Pr Nd Sm Eu Ho Gd Tb Dy Er Tm Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm granite of Mount Barrow (MBg) ignimbrite of Ferguson Wash

34 Appendix Table 3 continued. 57 Spot name La Ce Pr Nd Sm Eu Ho Gd Tb Dy Er Tm Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm

35 Appendix Table 3 continued. 58 Spot name La Ce Pr Nd Sm Eu Ho Gd Tb Dy Er Tm Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm granite of Peter Kane Mt. QP QP QP QP QP QP QP QP QP QP QP QP QP QP QP QP QP QP QP QP QP

36 Appendix Table 3 continued. 59 Spot name La Ce Pr Nd Sm Eu Ho Gd Tb Dy Er Tm Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm QP QP QP QP QP tuff of Felipe Pass tuff of Black Hills

37 Appendix Table 3 continued. 60 Spot name La Ce Pr Nd Sm Eu Ho Gd Tb Dy Er Tm Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm Quechan volcanic rocks

38 Appendix Table 3 continued. Spot name La Ce Pr Nd Sm Eu Ho Gd Tb Dy Er Tm Yb Lu ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm