Petrology

New trace element abundances and isotope compositions for more than 100 mid‐ocean ridge basalts from 5.5°N to 19°N on the East Pacific Rise show step function variations in isotopic composition along the ridge axis that coincide with ridge discontinuities. Transform faults, overlapping spreading centers, and devals (deviation from axial linearity) mark the separation of individual clusters of distinct isotopic composition and trace element ratios that indicate source variations. This correlated chemical clustering and morphological segmentation indicates that source composition and segmentation can be closely related even on a fine scale. Substantial chemical variations within a segment are related to source composition. This suggests that even within segments the magma transport is mainly vertical, and there is limited along‐ridge transport, and there is little evidence for magma chambers that are well mixed along strike. Trace element concentrations show good correlations with isotopic compositions on a segment scale but less so on a regional scale. The trace element and isotopic variability along the northern East Pacific Rise can be explained by three mantle components: a depleted peridotite endmember, an enriched peridotite endmember, and a recycled gabbro‐like component. The gabbroic component has an isotopic signature indicating an ancient origin. The high‐resolution sampling indicates that within a segment the chemical variability is largely binary but that the endmembers of the binary mixing change from segment to segment. The endmembers of the binary variation within a segment are a combination of three of the endmembers.

Northwest Zhejiang Province (NWZJ) is located in the southeastern Lower Yangtze River Belt, southeastern China. Here we document the occurrence of both magnesian (149–131 Ma) and ferroan (162–121 Ma) granitoids in NWZJ. The magnesian granitoids are calc-alkalic peraluminous in composition, with a wide range of SiO₂(58–72 wt%) contents. They have high K₂O/Na₂O, Sr/Y, and (La/Yb)_N ratios, with insignificant Eu anomalies, whereas the calc-alkalic peraluminous ferroan granites have high SiO₂ (76–77 wt%) contents, Fe indices (FeO∗/(FeO∗+MgO)FeO*/(FeO*+MgO)), and Ga/Al ratios. The ferroan granites also have low Ce/Pb and Nb/U ratios, with strong Ba, Sr, and Eu negative anomalies. Most of the rocks have similar zircon Lu-Hf isotopes (εHf(t)=−6.0εHf(t)=−6.0 to −0.7). However, rocks from two ferroan granitic bodies (Huangshitan and Jiuligang) have more depleted Hf isotopes, with ε_Hf(t) ranging from −1.9 to 5.9. The whole-rock Nd isotopes of the ferroan granites (εNd(t)=−6.5εNd(t)=−6.5 to −3.2) are slightly more depleted than those of magnesian granitoids (εNd(t)=−8.8εNd(t)=−8.8 to −5.1). In addition, all ferroan granites show similar and high present-day whole-rock Pb isotopic ratios (18.3–18.8 for ²⁰⁶Pb/²⁰⁴Pb, 15.6–15.7 for ²⁰⁷Pb/²⁰⁴Pb, and 38.5–39.0 for ²⁰⁸Pb/²⁰⁴Pb). On the basis of published data and our new results, we propose that the magnesian granitoids were generated by partial melting of lower-crustal materials, whereas the ferroan granites were derived from a similar source but some more-depleted materials were added into their source after ∼135 Ma. The water contents of the magma may have played an important role in determining the different geochemical affinities of the felsic magmatism. The felsic magmatism occurred under an extensional setting during the period 162–121 Ma. The extension of the lithosphere was further enhanced and followed by upwelling of asthenospheric mantle after ∼135 Ma. This study suggests that a change in the tectonic regime occurred at ∼135 Ma in NWZJ, which may have been triggered by the rollback of the subducted Paleo-Pacific Plate.