Whole-rock geochemistry and Sr-Nd isotopic composition of ... - CPRM

More documents

Recommendations

Info

66 Table 4 Sm and Nd isotopic ratios of the Camamu Basin samples. Error values of the isotope ratios are expressed as absolute SD. Sample Camamu 1 147 144 Sm/ Nd Error 143 144 Nd/ Nd Error TDM (Ga) εNd(0) εNd(150 Ma) VA-03 2.7125 0.0161 0.512050 0.000011 11.48 VA-09 Camamu 2 0.8126 0.0052 0.511536 0.000009 21.50 VA-23 0.1630 0.0016 0.511323 0.000015 25.66 25 VA-29 Camamu 3 0.0900 0.0006 0.511441 0.000008 2.0 23.35 21.3 VA-33 0.0897 0.0006 0.511458 0.000013 2.0 23.03 21.0 VA-37 Camamu 4 0.1085 0.0007 0.511569 0.000008 2.1 20.84 19.2 VA-42 0.1234 0.0008 0.511570 0.000012 2.5 20.83 19.4 VA-48 Camamu 5 0.0734 0.0005 0.511547 0.000012 1.6 21.29 18.9 CA-02 0.1437 0.0010 0.511547 0.000011 3.3 21.28 20.3 CA-12 0.0822 0.0005 0.511677 0.000007 1.6 18.74 16.6 Log( Al 2O3 /K2O) 2 1 0 Fe-shales Fe-sands Shales Wackes Lithic arenites Arkose Sub-lithic arenites Sub-arkose Quartz arenites -1 0 0,5 1 1,5 2 2,5 Log(SiO2/Al2O3) The results obtained can be explained by the abundant presence of detritic muscovite in the samples, even in the fine fraction. Mica contributed with radiogenic Sr from the source rock, and, moreover, its structure does not allow Sr isotopic homogenization (Mizusaki, 1992), thus, contributing to the anomalously old results. The results might be, alternatively, attributed to a possible contribution of K-feldspar, according to results of the X-ray diffraction analyses. Table 2 shows that 87 Sr/ 86 Sr values are high, evidencing contributions from fragments of old continental rocks, whose source will be verified next. 4.3. Characteristics of the whole-rock geochemistry camamu 1 camamu 2 camamu 3 camamu 4 camamu 5 Capianga Fig. 5. Chemical classification of the analyzed rocks (modified from Herron, 1988). The geochemical composition of sedimentary rocks is a complex result of various variables such as source material, weathering, transportation, physical sorting, and diagenesis (Middleton, 1960; Piper, 1974; Bhatia, 1983; McLennan, 1989; Cox and Lowe, 1995). 87 Sr/ 86 Sr 0.820 0.800 0.780 0.760 0.740 0.720 0 5 10 15 20 87 Rb/ 86 Sr Fig. 6. 87 Rb/ 86 Sr versus. 87 Sr/ 86 Sr diagram. D.R.A. Silva et al. / Journal of South American Earth Sciences 39 (2012) 59e71 Camamu 1 Camamu 2 Camamu 3 Camamu 4 Camamu 5 r S / r S Examples of studies using geochemical data from sediments to understand sedimentary processes such as weathering, provenance, diagenesis, sorting, and recycling are increasing in the literature because of the sensitiveness of some key trace elements in identifying minor components that are not readily recognized petrographically (e.g. Hiscott, 1984; Garver et al., 1996). The chemical composition of samples represents their primary mineralogy, which frequently underwent intense pre- and post- 87 Sr/ 86 Sr 0.80 0.78 0.76 0.74 0.72 0 2 4 6 8 10 12 0.83 0.80 0.77 0.74 + Camamu 2 Camamu 3 Camamu 4 + 0.71 0 0.01 0.02 0.03 1/Sr + + Rb/ Sr Fig. 8. 87 Sr/ 86 Sr (ratio) versus 1/Sr (concentration) diagram. + Age = 470 ± 17 Ma Initial Sr/ Sr = 0.7154 ± 0.001 MSWD = 54 Fig. 7. isochron for the samples of the Camamu Basin, according to Ludwig’s model (2003). + camamu 1 camamu 2 camamu 3 camamu 4 camamu 5
depositional chemical weathering processes (Nesbitt and Young, 1982). According to Button and Tyler (1979), cations such as Al, K, Mg, and Rb are always retained in weathering profiles, while cations of smaller ionic range, such as Na, Ca, and Sr, are rapidly SiO2 Fe O 3 MgO a O N 2 2 P O5 2 D.R.A. Silva et al. / Journal of South American Earth Sciences 39 (2012) 59e71 67 O a C O K 2 TiO2 MnO Fig. 9. Variations of major elements (%) versus Al2O3 (%). leached and removed from the system. In addition, the amount of elements that is lost is proportional to the extent of the weathering processes. Table 1 shows the results of XRF analyses of samples from selected outcrops of the Camamu Basin.
Page 1 and 2: Whole-rock geochemistry and SreNd i
Page 3 and 4: Fig. 2. Stratigraphic chart of the
Page 5 and 6: Table 1 Chemical composition of the
Page 7: Table 2 (continued ) Sample 87 86 R
Page 11 and 12: The diagrams were initially drawn b
Page 13: Andersson, P.O.D., Worden, R.H., Ho

Whole-rock geochemistry and Sr-Nd isotopic composition of ... - CPRM

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?