| Summary: | Includes bibliographical references (pages [118]-125) The hybrid series of the Newark Island Layered Intrusion contains a complex association of mafic, silicic, and hybrid rocks. The whole-rock δ¹⁸O for the felsic rocks fall in the range 7.6-8.7. Based on mineral-separate data, the felsic suite has undergone subsolidus open- system exchange with a fluid of high δ¹⁸O (5.4-8.3 permil) at moderately high temperatures (≈550°C). The calculated oxygen-isotopic composition of the fluid falls within the range of tf180 for magmatic fluid. It is hypothesized that the fluid entered the chamber along with the felsic magma, reacted with the newly-crystallized granite, and was driven from the system by the large amount of heat within the intrusion. In contrast to the felsic rocks, the associated mafic suite appears to have remained closed to isotopic exchange with an external fluid since the time of its crystallization. This is the first known example of disparate subsolidus isotopic histories between closely associated mafic and felsic rocks within a single intrusion. Among the mafic rocks, the positive correlation of initial ⁸⁷Sr/⁸⁶Sr to δ¹⁸O for both the low-Ti0₂ and high-Ti0₂ suites suggests that they have undergone crustal contamination or assimilation/fractional crystallization. The overlapping ranges of δ¹⁸O, however, suggest that these two suites of rocks derive from separate sources. Some of the mafic samples from the hybrid series appear to have equilibrated oxygen isotopes without chemical mixing with a crustal component. The high δ¹⁸O of a plausible parent liquid for the layered series in conjunction with the elevated initial ⁸⁷Sr/⁸⁶Sr (0.70636-0.70810) for these rocks suggests that the parental magma for the layered series was affected by crustal contamination prior to crystallization within the NILI. M.S. (Master of Science)
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