TRACKING STORAGE, MIXING, AND DIFFERENTIATION OF A MANTLE MELT IN THE UPPER CRUST: A CASE STUDY FROM LAMPROPHYRE MEGACRYST COMPOSITIONS

Three Camptonite lamprophyre dikes of the Mesozoic Hartford Basin each contain a unique suite of mineral textures and compositions. Mineral major and trace element compositions were determined via electron microprobe analysis. A dike located in Wallingford, CT is composed of a groundmass of albite, amphibole, and minor ulvospinel and titanaugite. Phenocrysts consist of kaersutite and minor augite. A dike from Mt. Higby in Middlefield, CT is composed of a groundmass of alkali feldspar, titanaugite, and minor kaersutite. It lacks megacrysts typical of lamprophyres but contains abundant ocelli nodules composed of variable amounts of albite, k-feldspar, plagioclase (AN60), amphibole, biotite, spinel, and strontium-rich calcite. A dike in Durham, CT is composed of a groundmass of hornblende, alkaline and albitic feldspar, and ilmenite. Phenocrysts consist of Na-rich kaersutite. In all 3 dikes phenocrysts exhibit complex zoning, mineral-melt reaction rims, and decompression rims. Normally zoned (high Mg core and high Fe rims) phenocrysts can be found in the same sample containing reversely zoned (high Fe core and high Mg rims) phenocrysts. There is no correlation between K/Ti values and Mg/Fe values in the kaersutite cores and rims. This suggests the mixing of primitive melts with more evolved ones. The Wallingford dike contains kaersutite amphiboles that were partially dissolved and then recrystallized. The recrystallized hydrous-free phases are composed of omphacite, k-feldspar and spinel. This occurs when the dissolved volatiles in the melt become saturated, form their own phase, and are then released. Nearly all phenocrysts contain a thin (3-10 micron wide) low-Mg, euhedral rim. This rim is consistent with rapid, low-pressure crystallization during accent. A combination of textural observations and chemical analysis provides a means to track the processes that affect mantle melts. We suggest that even low-volume mantle melts stall, undergo differentiation, mix with more primitive melts, and are periodically stripped of their volatiles during accent. The existence of relatively small magma chambers that may be complexly spatially related and periodically recharged and tapped are required to produce the textural and chemical variability observed in the lamprophyre megacrysts.