Abstract
In the Alborz Mountains of the eastern Roodbar (north of Iran), Tertiary volcanic rocks have a variety of composition between olivine basalt, basaltic andesite, pyroxene andesite and andesite. The presence of different xenoliths and xenocrysts is among the evidence of crustal contamination of these rocks. The positive correlations of CaO/Al2O3 vs to MgO and Al2O3 vs. SiO2 are of signs of the olivine and clinopyroxene fractionation in the variation trend of the area rocks. Positive correlations K2O/P2O5 vs with SiO2 and La/Sm vs. K2O/P2O5 demonstrate contamination of magma with the continental crust. The incompatible trace element patterns and their comparison with crustal contents indicate contamination of the rocks of the area with the lower and upper continental crust. Linear trends in the variation diagram of Nb/Y vs. Zr/Y, introduce two different source regions: a MORB source and the other continental crust for the rocks which are the genesis. The variations of Y/Nb vs. Zr/Nb and Rb/Y vs. Nb/Y reveal a crustal contamination of the magma originated from the MORB source. Geochemical studies represent that the area’s rocks were derived from the 15% melting of a mantle source of MORB type with spinel facies within a continental environment, which was contaminated by varying degrees of continental crustal rocks.
Highlights
Investigations of the ratios of incompatible trace elements suggest that the basic samples of the region are close to MORB asthenospheric mantle source and the trends between primary and evolved samples indicate a linear arrangement between the MORB source mantle and the continental crust, representing an interaction of the MORB mantle-derived magmas with continental crust
The most basic sample (N8) is close to the MORB source and as shown in Figure 13, the ratio of La/Sm is strongly influenced by crustal contamination and the observed linear trend resulted from crustal contamination
In the La/Sm vs. Sm/Yb diagram (Figure 18), the contents of partial melting of a MORB source are shown [21]. As it would be considered, samples of the area indicate constant ratios of Sm/Yb so that the most basic sample has the least amount of La/Sm and is closer to the MORB source
Summary
It is tried to investigate the characteristics of mantle source and the role of continental crust in the magmatic evolutions of Tertiary volcanic rocks through geochemical evidence and determine its relationship with the governing geodynamic setting at the time of the formation of these rocks. This could help us in better understanding of the tectonomagmatic setting of Tertiary rocks in Alborz and generally, Iran.
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