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

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