INSIGHTS INTO THE SUB-CONTINENTAL LITHOSPHERIC MANTLE FROM GEOCHEMICAL ANALYSIS OF LAMPROPHYRE DIKES AND ENTRAINED XENOLITHS FROM WESTERLY, RHODE ISLAND

Monchiquite variety lamprophyre dikes are found intruding the Narragansett Pier granite and the Westerly granite in Westerly, RI (Leavy and Hermes, 1979). These dikes are characterized by their distinct mineralogy, large (up to 8cm) upper-mantle derived xenoliths, and megacrysts within the dikes that are derived from those xenoliths. These dikes are part of a larger series of shallow mafic intrusions ranging from lamprophyres to dolerites following the breakup of Africa from North America; however, these are the only lamprophyres in the vicinity of Rhode Island and Connecticut and do not appear to be similar to or related to the dolerite dikes in general (McHone, 1992). K-Ar dating determined that the dikes are approximately 175 Ma (Hermes et al., 1984). These rocks thus provide us with an insight into the state of the sub-continental lithospheric mantle, specifically during late stage continental rifting. This is true for both the lamprophyre and the entrained xenoliths, which are not genetically related to their host (Leavy and Hermes, 1979). The host dike mineralogy consists of primary microphenocrysts of titaniferous augite, kaersutite, phlogopite, and apatite. The xenolith mineralogy consists of olivine, clinopyroxene, orthopyroxene, and spinel. Due to extensive alteration, bulk chemical analyses are not particularly insightful; however, investigations of the geochemistry of melt and fluid inclusions within the phenocrysts of the lamprophyre and the xenoliths will provide an unadulterated view of the mantle. Additionally, the trace element composition of the xenolith phases are also less likely to show signs of chemical alteration compared with the bulk rock major element chemistry. Electron microprobe and laser ablation-inductively coupled plasma mass spectrometry of the phenocrysts and their inclusions were conducted to provide a better view into the conditions under which these rocks formed.