Abstract:
The chemical components involved in the mantle source of the North Luzon lavas have been an issue of debate. In existing mixing models, the depleted end-member was assigned to MORB source, while EMI, subducting sediments, and metasomatic fluids and melts were considered as enriched components. These models were all developed from chemical and isotope compositions of lavas from entire North Luzon arc without a detailed investigation on chemical variations of lavas in individual islands. In this study, the chemical compositions and Sr, Nd, and Hf isotope ratios of Lutao volcanic rocks are used to address the process and source controls on lavas chemistry. Twenty-eight volcanic rock samples were collected from Lutao. Based on their SiO2 and total alkali contents, these samples were classified into basalt (B), basaltic andesite (BA), andesite (A) and dacite. Basalts and andesites were subdivided into isotopically enriched groups (BE and AE, respectively) and depleted groups (BD and AD, respectively). The andesite (group A) samples have similar Sr, Nd, and Hf isotope ratios with inverse correlations between MgO and incompatible element contents, reflecting fractional crystallization after partial melting from a common source. Based on trace element variations, the group A samples are modeled to be residual melts after intensive fractional crystallization. The two group AE samples have enriched 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf ratios, but their abundance ratios of incompatible elements resemble those of the group A samples. A plausible explanation is that the mantle sources of the groups A and AE lavas were metasomatized by same materials; however, the metasomatic process occurred earlier in the source of AE lavas, leading to enriched 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf ratios. The group AD samples plot closer to MORB in the 87Sr/86Sr-143Nd/144Nd- 176Hf/177Hf space. Their Nb/La and Ti/HREE ratios are lower than MORB values implying subjection to metasomatism, an inference consistent with their Sr, Nd, and Hf isotope data. 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf ratios of the group BE samples are similar to those of the group AE samples. However, model calculations show that their incompatible element abundances can not be explained by derivation from a common source by different degrees of partial melting. An incompatible element enriched source is required for the group AE lavas. The groups BD and AD samples have similar 143Nd/144Nd and 176Hf/177Hf ratios, but group BD has higher Sm/Nd and Lu/Hf ratios. These results imply that the source of the group BD samples was metasomatized later than that of the group AD samples. Overall, the composition and isotope data require that the mantle sources of Lutao lavas were subjected to various metasomatic agents at different time. Based on the mixing models of Kuo (2007), the 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf ratios of the Lutao samples are best explained by mixing sub-equal amounts of siliceous melts derived from subducted sediments and oceanic crust.Keywords:
Fractional crystallization (geology)
Incompatible element
Metasomatism
Dacite
Trace element
Andesites
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