Most previous geochemical studies on basalts from the Southwest Indian Ridge (SWIR) were based on the analysis of bulk rocks, and those on phenocrysts are rare. We conducted bulk rock and mineral analyses of two rock samples of plagioclase-rich basalts from Segment 27, SWIR, where the Duanqiao hydrothermal field is located. The SiO2
and MgO contents of the two samples (34IV-TVG07 and 30III-TVG14) are 49.16% and 6.76%, and 49.50 and 6.52%, respectively. Their trace elemental patterns are similar to typical N-MORB (normal mid-ocean ridge basalts). The EPMA analysis show that the An (% of anorthite) of the plagioclase phenocrysts vary in the range of 76.2 to 87.9, and most are above 80, which is significantly greater than those of plagioclase in the Mount Jordanne basalts, indicating that the An-rich plagioclase phenocrysts at 50.4°E are not derived from the lower oceanic crust of the Mount Jordanne. In addition, the Petrolog3 modeling shows that they could not crystallize directly from the mother magma. By combining the experimental constrains and previous evidence for ancient mantle wedge-like component entrained beneath this ridge, we believe that the An-rich plagioclase in Segment 27 basalts were most likely crystallized from magma due to partial melting of an ancient depleted sub-arc mantle.