Isotopic composition of igneous rocks from DSDP Site 462

The tholeiitic basalts and microdolerites that comprise the Cretaceous igneous complex in the Nauru Basin in the western equatorial Pacific have moderate ranges in initial 87Sr/86Sr (0.70347 - 0.70356), initial 143Nd/144Nd (0.51278 - 0.51287), and measured 206Pb/204Pb (18.52 - 19.15), 207Pb/204Pb (15.48 - 15.66) and 208Pb/204Pb (38.28 - 38.81). These isotopic ratios overlap with those of both oceanic island basalts (OIB) and South Atlantic and Indian mid-ocean ridge basalts (MORB). However, the petrography, mineralogy, and bulk rock chemistry of the igneous complex are more similar to MORB than to OIB. Also, the rare earth element contents of Nauru Basin igneous rocks are uniformly depleted in light elements (La/Sm(ch) < 1) indicative of a mantle source compositionally similar to that of MORB. These results suggest that the igneous complex is the top of the original ocean crust in the Nauru Basin, and that the notion that the crust must be 15 to 35 m.y. older based on simple extrapolation and identification of the M-sequence magnetic lineations (Larson et al., 1981, doi:10.2973/dsdp.proc.61.1981; Moberly et al., 1985, doi:10.2973/dsdp.proc.81.1984) may be invalid because of a more complicated tectonic setting. The igneous complex most probably was extruded from an ocean ridge system located near the anomalously hot, volcanically active, and isotopically distinct region in the south central Pacific which has been in existence since c. 120 Ma.位于西赤道太平洋瑙鲁盆地的白垩纪火成杂岩体由拉斑玄武岩（tholeiitic basalts）和细粒辉绿岩（microdolerites）组成，其初始87Sr/86Sr比值（0.70347~0.70356）、初始143Nd/144Nd比值（0.51278~0.51287）以及实测206Pb/204Pb（18.52~19.15）、207Pb/204Pb（15.48~15.66）和208Pb/204Pb（38.28~38.81）均处于中等范围区间内。这些同位素比值与洋岛玄武岩（Oceanic Island Basalts, OIB）以及南大西洋和印度洋洋中脊玄武岩（mid-ocean ridge basalts, MORB）的同位素比值均存在重叠。然而，该火成杂岩体的岩石学、矿物学及全岩地球化学特征与洋中脊玄武岩（MORB）的相似性要高于洋岛玄武岩。此外，瑙鲁盆地火成岩的稀土元素（rare earth element）含量整体均表现为轻稀土元素亏损（球粒陨石标准化La/Sm比值<1），表明其地幔源区的地球化学组成与洋中脊玄武岩的源区相似。上述结果表明，该火成杂岩体即为瑙鲁盆地原始洋壳的顶部；而此前基于简单外推法及M序磁异常条带识别得出的“该洋壳年龄应老15~35百万年”的观点，由于构造背景更为复杂，可能并不成立（Larson等，1981，doi:10.2973/dsdp.proc.61.1981；Moberly等，1985，doi:10.2973/dsdp.proc.81.1984）。该火成杂岩体极有可能形成于中太平洋南部的洋脊系统，该区域自约120 Ma以来便处于异常高温、火山活动活跃且同位素组成独特的状态。