MANTLE SOURCES FOR CENTRAL ATLANTIC MAGMATIC PROVINCE BASALTS FROM HF ISOTOPES

The Central Atlantic Magmatic Province (CAMP) was one of the most voluminous LIP events in Earth history and likely triggered the end-Triassic mass extinction. The tectonic and mantle processes that produced such significant magmatic emplacement are thus of broad geologic importance. To further explore the origins of CAMP, we present new ¹⁷⁶Hf/¹⁷⁷Hf isotope data for a broad geographic sampling of dikes, sills, and basalt flows. We find that basaltic intrusions from the Carolinas in Eastern North America trend along a shallower slope than the terrestrial array on a diagram of ¹⁷⁶Hf/¹⁷⁷Hf vs. ¹⁴³Nd/¹⁴⁴Nd. This trend may reflect the presence of recycled upper and lower crustal materials in the melt region, consistent with previous suggestions that the CAMP melt source has been modified by the addition of crustally derived material.

Distinct from the Carolina trend, we observe that low- and high-TiO₂ basalts from Amazonia (Brazil) exhibit ¹⁷⁶Hf/¹⁷⁷Hf ratios along the terrestrial array. The high-TiO­₂ basalts are overall isotopically depleted but trend towards EM1-type compositions, and may be asthenospheric melts that have experienced the addition of melt from local subcontinental lithospheric mantle (SCLM). For comparison, high-TiO₂ CAMP basalts from Sierra Leone may likewise have incorporated melts of locally variable lithospheric mantle [Callegaro et al., in press, Jour. Petrology]. In contrast, low-TiO₂ basalts from the same region in Brazil and of similar age to the high-TiO₂ basalts have considerably lower Hf isotope ratios, indicative of a time-averaged source relatively enriched in incompatible elements. It remains unclear, however, whether the hypothesized SCLM sources represent intact lithospheric remnants, or if this material has reentered the convecting asthenosphere by delamination prior to melting. Neither ENA nor Amazonia CAMP require a mantle plume source to explain their Hf isotope systematics.