Petrogeochemical characteristics of mantle sources of volcanic rocks in the southern and middle Mariana Trough

The Mariana Trough, as a typical active back-arc basin, is an ideal place to study the effects of subduction on magmatism and crust-mantle dynamics. The petrogeochemical characteristics of the samples from two areas of the trough were revealed based on the published petrogeochemical data, from which the degree of mantle enrichment/depletion, the degree of mantle melting, the depth of mantle melting, and the degree of subduction material incorporation in the southern and middle Mariana Trough were clarified. Results show that a set of medium-low potassium calc-alkaline series basalt and basaltic andesite occur in the southern and middle Mariana Trough. The volcanic rocks are rich in large ionic lithophile elements (LILE) and light rare earth elements (LREE) while deficient in high field strength elements (HFSE) and heavy rare earth elements (HREE). The Mariana Trough could be divided into three sectors along spreading center, and the mantle-melting degree and the depth of each sector were calculated and the effect of mantle heterogeneity eliminated. The correlation between mantle-melting degree and the depth in each sector was found negative near 15°N and 18°N, but positive in the other areas, which proves that there are two mantle-melting modes in the trough. Volcanic rocks in the southern and middle Mariana Trough are influenced by multiple subduction-components and there may be another water-rich melt end-member in the southern part of the trough that may be resulted from the faster spreading rate of the trough. Calculations of the extent of subduction accretion show that the influence of subduction components weakens near 15°N and 18°N. The variation of volcanic rocks in the Mariana Trough may be caused by the mixing of an N-MORB-like mantle source involved with an island arc-like mantle source. Therefore, subduction material is an important factor on mantle-melting degree, and spreading rate and mantle enrichment/depletion degree are also play an essential roles.