A LA-ICP-MS SEARCH FOR THE PRIMARY MAGMA COMPOSITION OF THE HIGGANUM DIKE SYSTEM, CENTRAL ATLANTIC MAGMATIC PROVINCE

The compositions of the primary magmas and the origin of the crustal component in the Central Atlantic Magmatic Province has long been problematic. Two endmember models have been proposed; the first is that the crustal component is the result of subduction zone enrichment of the mantle, the second is that it is the result of AFC processes as the magmas resided at the base of, and traversed through, continental crust. The Higganum-Holden-Onway-Christmas Cove dike system that fed the Talcott basalt of the Hartford basin hosts strongly zoned clinopyroxenes. The cores are Mg- and Cr-rich, containing up to 1 wt % Cr2O3. LA-ICP-MS analyses of the cores were used to calculate liquid compositions of the most primitive component in the Higganum system. Calculated liquid compositions have flat chondrite-normalized REE patterns with (La/Yb)N values of ~ 2 and LREE abundances of ~ 20 x chondrites compared to 40-50 times chondritic values of the bulk-rocks. Multi-element diagrams show low abundances of highly incompatible elements (1-5 times primitive mantle) indicative of a relatively depleted mantle source. In contrast, these same elements are enriched up to 30-40 times primitive mantle in the bulk-rocks. Nb and Ta anomalies are absent in the calculated liquids whereas the bulk-rock analyses show prominent negative anomalies. Calculated liquid trace element ratios (i.e., Ba/Nb and Sr/Nd verus Zr/Nb) also lack an arc signature. These data suggest that the crustal signature of Higganum dike resulted from assimilation after crystallization of the clinopyroxene cores rather than being a primary signature from a mantle enriched by subduction zone processes.