## 5 Discussion
### The Provenance of the Songhua River  
The variable size, roundness, and morphology of particles (Figure 3a-c) indicate that the protoliths of the Songhua River sand belonged to multiple terranes that involved weathering and sorting. [PERSON] and [PERSON] [36] provided a convenient scheme to determine the provenance of sediments, in which concentrations of major elements were used. In the discriminant function diagram after [PERSON] and [PERSON] [36], most of the samples fall within the quartzose sedimentary field (Figure 8), and part of the samples fall within the felsic igneous provenance field. Sorting and weathering would make provenance deviate from felsic igneous provenance [37], so the provenance may contain more felsic igneous materials.  
The tetrad effect in geochemical processes has aroused the interest of many scientists, which can be attributed to some genetic models such as mineral fractionation, REE complex, fluid/rock interaction, surficial weathering, and hydrothermal alteration [38]. The computed values for each tetrad are T1 (La-Nd), T2 (Gd-Ho), and T3 (Er-Lu), which were computed using the following equation [39]:  
\[\mathrm{Ti}=\sqrt{\frac{1}{2}\times\left(\frac{X_{\mathrm{Bi}}}{X_{\mathrm{Ai} }^{2/3}X_{\mathrm{D}}^{1/3}}-1\right)^{2}+\left[\frac{X_{\mathrm{Ci}}}{X_{ \mathrm{Ai}}^{1/3}X_{\mathrm{Di}}^{2/3}}-1\right]^{2}}\]  
where \(X_{\mathrm{Ai}}\) is the chondrite-normalized concentration of the first element in tetrad i, \(X_{\mathrm{Bi}}\) and \(X_{\mathrm{Ci}}\) are the concentrations of the central elements, and \(X_{\mathrm{Di}}\) is the chondrite-normalized concentration of the last element in the tetrad. The value of Ti was calculated from all three tetrads. The T1, T2 and T3 values for samples are in the range of 0.02-0.20 (average of 0.10), 0.03-0.27 (average of 0.11), and 0.01-0.13 (average of 0.05), respectively, demonstrating a slight and inappreciable tetrad effect for the samples. Therefore, the REE could effectively be used to constrain provenance of the sediments from the Songhua River.  
Figure 7: The Th/Sc vs. Zr/Sc ratios plot for the Guxiangyun and Wenquanhe Formations.  
Low contents of V and Sc imply that the sands were sourced from the continental crust or craton [1]. Chondrite-normalized REE patterns of the sandstones (Figure 5) are similar to those for the upper continental crust [40]. An upper continental crust provenance can be recognized as the main source of the sandstones, based on trace element and REE compositions. The sediments have similar REE contents to UCC, enrichment in LREEs (LREE/HREE = 7.7), no Ce anomaly, and negative Eu anomalies (\(\delta\)Eu = 0.67). All evidences prove that the provenance of the Songhua River sediments is a suite of strongly differentiated rocks, part of which may undergo secondary transport. During the transport by the river, primary classic fragments would be mixed with newly added materials. The source for the large river, such as the Songhua River, would be variable along the river. As far as the Songhua River, the lithtic fragment of granite shown in the Figure 3d is a reliable evidence of felsic provenance. Meanwhile, the quartz and feldspar grains with moderate psephicity may be products of secondary deposition. According to the location of the research area, it was concluded that both the Lesser Khingan Mountains and Songliao Basin provide the provenance for the Songhua River.