Figure 2. Location map of Himalayan Mountains and the Tibetan Plateau (from Searle et al., 1997).

Transcription

1 Nazca Plate Figure 1. Location map of Central Andes arc. This map also shows the extent of the high Altiplano-Puna plateau (from Allmendinger et al., 1997). 33

2 Figure 2. Location map of Himalayan Mountains and the Tibetan Plateau (from Searle et al., 1997). 34

3 35 Figure 3. The Palms pluton is just to the west of the Triassic and Jurassic granite found in the northwest portion of this map. The Mesozoic sheeted complex follows the southwest border (from Barth et al., 28).

4 2 7 Al2O3 (wt%) CaO (wt%) TiO2 (wt %).6.4 FeO (wt%) NA Cordilleran Central Andes SiO2 (wt %) SiO2 (wt%) Figure 4. A comparison of major oxide concentration between Cretaceous granites from the North American Cordillera and Recent rhyolite and pumice chemistry from the Central Andes. Both plots show similar trends, although the Central Andes display more scatter. NA Cordillera data from Brand, 1985 and Wooden and Barth, unpublished data. Central Andes data from Mahlburg Kay et al., 1988; Coira and Nullo, 1989, De Silva, 1989; Schreiber and Schwab, 1991; Feeley et al., 1993; Coira and Kay, 1993; Morgan et al., 1998; Haschke et al., 22; Ulrich and Heinrich, 21; Klemetti and Grunder, 28 and Vezzoli et al.,

5 MnO (wt%).1 Na2O (wt%) MgO (wt%) K2O (wt%) NA Cordilleran Central Andes SiO 2 (wt%) SiO 2 (wt%) Figure 5. A comparison of major oxide concentration between granites from the North American Cordillera and rhyolite and pumice chemistry from the Central Andes. Both plots show similar trends, although the Central Andes display more scatter. Data references are the same as figure 4. 37

6 Age = yrs Age = 38,818 yrs Age = 97,45 yrs Figure 6a. Thermal model developed using the HEAT program (Wohletz, 27) of the cooling/crystallization of the Palms pluton (first half). According to this model, it would have taken approximately 45, years for this magmatic intrusion to reach the solidus. 38

7 Age = 194,9 yrs Age = 3,839 yrs Age = 454,17 yrs Figure 6b. Thermal model of the cooling/crystallization of the Palms pluton (second half). According to this model, it would have taken approximately 45, years for this magmatic intrusion to reach the solidus. 39

8 116.3 o W o W 34.1 o N N 34. o N Sample Location Sample Location (previously collected) Quaternary Alluvium Palms Pluton (Cretaceous) Varying colors indicate zoning Mesozoic Sheeted Complex Triassic Granite Precambrian Gneiss 3 km Figure 7. Map of the Palms pluton. The warm colors are interpreted zones based on satellite imagery (Jarvis et al., 21). Additional samples were collected for whole rock geochemistry. 4

9 116.3 o W o W o N N o N Sample Location Sample Location (previously collected) Quaternary Alluvium Palms Pluton (Cretaceous) Varying colors indicate zoning Mesozoic Sheeted Complex Triassic Granite Precambrian Gneiss 3 km Figure 8. Map of the Palms pluton with location of samples where zircons were analyzed and their corresponding ages. Adapted from satellite imagery (Jarvis, et al., 21). 41

10 FeO (wt.%) Previously Collected Data New Data K 2 O (wt.%) SiO 2 (wt. %) SiO 2 (wt. %) MgO (wt.%) CaO (wt.%) SiO 2 (wt. %) SiO 2 (wt. %) TiO 2 (wt. %) Al 2 O 3 (wt.%) SiO 2 (wt. %) SiO 2 (wt. %) Figure 9. Comparison of major oxides in granite of the Palms pluton. There is systematic variation in major oxides compared to SiO 2. Previously collected data from Brand, 1985; Palmer, 25; Wooden and Barth, unpublished data. 42

11 8 7 6 Previously Collected Data New Data 5 4 Sr (ppm) CaO/SiO e e e e+5 5.8e e+5 Easting (UTM) e e e e+5 5.8e e+5 Easting (UTM) Ba (ppm) e e e e+5 5.8e e+5 Easting (UTM) Al 2 O 3 /SiO e e e e+5 5.8e e+5 Easting (UTM) Ba (ppm) MgO/SiO e e e e+5 5.8e e e e e e+5 5.8e e+5 Easting (UTM) Easting (UTM) Figure 1. Summary of spatial changes in granite chemistry (additional data from Brand, 1985; Palmer, 25; Wooden and Barth, unpublished data). Although variation is observed in major oxide chemistry (right panels), more significant variation is observed in some trace elements (left panels). This spatial variation is not in agreement with the zones observed in satellite imagery. 43

12 Magmatic 25 2 Number Premagmatic Premagmatic Age (Ma) Figure 11. Histogram and probability plot of ages of zircons from Palms pluton. This is a plot that combines a histogram with the cumulative probability distribution obtained by summing the probability distributions of a suite of data with normally-distributed errors. The number of peaks represents the number of statistically distinct ages that exist in the zircon grains. The height of the peak represents the likelihood that additional analysis will fall in that range. This figure displays at least 3 unique populations. One population is of Proterozoic premagmatic zircons. The second premagmatic population is from Mesozoic time. The magmatic zircons are the most prevalent and are from Cretaceous time. 44

13 I 1 8 Number Age (Ma) II Num ber Age (Ma) Figure 12. Histogram and probability plot of ages of premagmatic (I) and magmatic (II) zircons from Palms pluton. This figure displays at least 3 distinct populations for premagmatic zircons and two magmatic populations at approximately 77 and 81 Ma. This observation supports the hypothesis that this pluton was formed from multiple intrusions. 45

14 data-point error symbols are 1σ 82 Mean = 79. ± 1.1 [1.3%] 95% conf. Wtd by data-pt errs only, of 1 rej. MSWD = 5.1, probability = (error bars are 2σ) JW JW221-5 Ma 78 JW221-3 JW221-9 JW221-7 JW JW JW JW JW data-point error symbols are 1σ Mean = ±.65 [.83%] 95% conf. Wtd by data-pt errs only, of 8 rej. MSWD = 1.2, probability =.28 (error bars are 2σ) 79 Ma Figure 13. Calculated magmatic age of JW221. The lower graph has excluded the 81 Ma grains. The red bars represent error bars for each spot analysis. The plots include the mean age for each of the rock samples, mean square weighted deviation (MSWD), and the probability that these spot analyses are from one population. 46

15 data-point error symbols are 1σ 83 Mean = 78.4 ± 1.5 [1.9%] 95% conf. Wtd by data-pt errs only, of 6 rej. MSWD = 1.4, probability =.24 (error bars are 2σ) 81 Ma data-point error symbols are 1σ 81 8 Mean = 78 ± 1 [1.3%] 95% conf. Wtd by data-pt errs only, of 5 rej. MSWD =.65, probability =.63 (error bars are 2σ) 79 Ma Figure 14. Calculated magmatic age of JW222. The lower graph has excluded the 81 Ma grains. The red bars represent error bars for each spot analysis. The plots include the mean age for each of the rock samples, mean square weighted deviation (MSWD), and the probability that these spot analyses are from one population. 47

16 Figure 15. Europium anomaly versus age. The samples presented here contain both precise age and geochemical data. This graph displays that the older zircon grains found in JW221 have a similar geochemistry to zircon grains found in JW224, the oldest sample. 48

17 data-point error symbols are 1σ Ma Mean = 8.61 ±.48 [.59%] 95% conf. Wtd by data-pt errs only, of 7 rej. MSWD =.73, probability =.63 (error bars are 2σ) 77 Figure 16. Calculated magmatic age of JW224. The plots include the mean age for each of the rock samples, mean square weighted deviation (MSWD), and the probability that these spot analyses are from one population. 49

18 data-point error symbols are 1σ 79 Mean = 75.5 ± 1.3 [1.7%] 95% conf. Wtd by data-pt errs only, of 8 rej. MSWD =.36, probability =.92 (error bars are 2σ) 77 Ma Figure 17. Calculated magmatic age of JW341. The plots include the mean age for each of the rock samples, mean square weighted deviation (MSWD), and the probability that these spot analyses are from one population. 5

19 data-point error symbols are 1σ Ma Mean = ±.56 [.72%] 95% conf. Wtd by data-pt errs only, of 1 rej. MSWD = 1.8, probability = 55 (error bars are 2σ) 73 Figure 18. Calculated magmatic age of 561. The plots include the mean age for each of the rock samples, mean square weighted deviation (MSWD), and the probability that these spot analyses are from one population. 51

20 Ma Figure 19. Summary of ages. There is a difference of roughly 3 my between the oldest and youngest sample, if uncertainties are included. 52

21 JW221 JW341 JW224 Fe (ppm) Ca (ppm) Ti (ppm) Al (ppm) Figure 2. Analysis of major elements in potential silicate and oxide mineral inclusions in zircon. The dashed lines represent the maximum accepted values; analyses with higher values of these elements were not included in data analysis. 53

22 1..8 a. Premagmatic 561 JW221 JW224 JW c. Magmatic 561 JW221 JW224 JW341 Eu/Eu* III IV II I Eu/Eu* b. Premagmatic Yb/Gd 561 JW221 JW224 JW d. Magmatic Yb/Gd 561 JW221 JW224 JW341 Th/U 1.5 Th/U Yb/Gd Yb/Gd Figure 21. Summary of REE patterns. The europium anomaly is quantified by dividing the Eu concentration by Eu*. Eu* is the midpoint of a calculated line from Sm to Gd; it therefore represents the value of Eu if there were no anomaly. The smaller this value, the deeper the negative europium anomaly is. Greater Yb/Gd values represent steeper HREE trends. The closed symbols on these figures represent spots analyzed from what was interpreted to be premagmatic grains and the open symbols represent analyses from magmatic zircon grains. Top graphs compare the Eu anomaly to Yb/Gd ratio. For premagmatic grains, there are four distinct populations and two (populations I and II) are consistently found in all analyzed samples. One yields high Yb/Gd values and low Eu/Eu* values. The second contains both low Yb/Gd and Eu/Eu* values. The bottom graphs compare Th/U values to Yb/Gd values. Population I from the above graph shows similar Th/U values to zircons from Proterozoic metamorphic rocks in southern California (Barth et al., 29); JW224 is an exception to this general observation because it has high Th/U values. 54

23 561 JW Magmatic Premagmatic 1..8 Magmatic Premagmatic Eu/Eu*.6.4 Eu/Eu* Yb/Gd JW Yb/Gd JW Eu/Eu*.6.4 Eu/Eu* Yb/Gd Yb/Gd Figure 22. Comparison of REE values for premagmatic and magmatic grains in each sample. Each sample contains premagmatic populations I and II. JW221 contains population III and JW341 contains population IV. 55

24 Premagmatic Magmatic Temperature ( o C) Temperature ( o C) Hf (ppm) Hf (ppm) JW224 JW Temperature ( o C) Temperature ( o C) Mz Temperature ( o C) Hf (ppm) JW Hf (ppm) Hf (ppm) Figure 23. Summary of zircon crystallization temperatures calculated from Ferry and Watson (27) plotted against Hf content (ppm). The activity coefficients used for calculations are 1 for Si and.7 for Ti. The solid line represents the solidus of granite from Piwinskii (1968) at 4 kb and 15% water content. The dashed line represents the zircon saturation temperature calculated from Watson and Harrison (1983). Generally, the premagmatic trend is shifted to the right of the magmatic trend. 56

25 km 1 Squaw Tank Smoke Tree 2 Palms Sheeted Complex Josephine Mt. 3 Waterman 4 Figure 24. Reconstructed crustal section of the North American Cordilleran arc (adapted from Palmer and others, 26). 57

26 Figure 25a. Working model of the formation of the Palms pluton and the Mesozoic sheeted complex. Green and red represent mafic and felsic igneous material, respectively. Pink represents heating and partial melting of the country rock. 58

27 Figure 25b. Working model of the formation of the Palms pluton and the Mesozoic sheeted complex, continued. The Palms pluton is located above the sheeted complex and formed from the same processes as the sheeted complex. 59

28 .5.4 Palms Squaw Tank Smoke Tree TiO 2 (wt%).3.2 CaO (wt%) K 2 O (wt%) MgO (wt%) Na 2 O (wt%) MnO (wt%) Al 2 O 3 (wt%) FeO (wt%) SiO 2 (wt%) SiO 2 (wt%) Figure 26. A comparison of major oxide concentration between Palms pluton and other nearby Cretaceous granites. Overall, the granites follow the same geochemical trends. Squaw Tank pluton has less felsic samples and trends are unclear between SiO 2 and MnO, Na 2 O, and K 2 O (additional data from Palmer, 25 ). 6

29 FeO (wt%) FeO (wt%) CaO (wt%) TiO 2 (wt%) Upper Crust Lower Crust K 2 O (wt%) SiO 2 (wt%) SiO 2 (wt%) Figure 27. A comparison of major oxide concentrations between granitic rocks above and below the sheeted complex. Compositions appear to be similar although the lower crust granitic rocks may have higher concentrations of TiO 2 and FeO (data from Brand,1985; Barth and others, 1995; Buehrer and others, 2; Palmer, 25; Wooden and Barth, unpublished data). 61

30 .5.4 Al/Zr Rb/Sr Upper Crust Lower Crust Ba/Zr Al/Zr Figure 28. A comparison of trace element concentrations between granitic rocks above and below the sheeted complex. Compositions appear similar but deeper granites are enriched in Ba. Data references are the same as figure