A COMPILATION OF AGE DATA FOR CONTINENTAL FLOOD BASALT PROVINCES TO EVALUATE POTENTIAL FOR DISCRETE ERUPTIVE PULSES IN THE COLUMBIA RIVER BASALT GROUP (CRBG)

Continental flood basalt (CFB) eruptions can be characterized by eruption flux and duration of volcanic activity, where the associated eruptive behavior can be identified as two endmembers: continuous or pulsed. If an eruption is pulsed through time, the associated volcanic products erupt in discrete periods which produce a bimodal or multimodal distribution of age data. These bimodal to multimodal distributions of age data has been previously proposed in different CFB’s including the Deccan Traps (largest CFB) and more recently in the Picture Gorge Basalt (PGB), a main phase unit of the Columbia River Basalt Group (CRBG, smallest CFB). A bimodal to multimodal distribution of age data is interpreted to represent discrete pulses in eruptive activity, which has implications for eruption dynamics within large igneous provinces (LIP).

In the Deccan Traps, 40Ar/39Ar data combined with recent more precise U/Pb ages indicates a potential temporal gap or ‘hiatus’ (significant decrease in eruptive rate) in the eruptive timeline with a longevity ~0.1 Ma. Within the PGB main phase unit of the CRBG, a bimodal distribution of existing age data indicates a temporal gap of ~0.4 (16.62 to 16.23 Ma) suggesting pulsed eruptive activity. Observations of all published CRBG age data (n=285, determined via 40Ar/39Ar and U/Pb methods) indicates a potential for a bi-to-multimodal distribution of ages. As a first order observation, this is unlikely an artifact of analytical precision, as age populations persist regardless of whether the errors are reported at 1-sigma (65% confidence) or 2-sigma (95% confidence), and with varying bin sizes (0.05 and 0.10 Ma). However, for ages collected using recent 40Ar/39Ar methods, the data reduction and age calculation process introduces variability in the final age (i.e., application of different decay constants and age standards). This study presents age data from different CFBs to examine how they behave overtime and contrast it to potential eruptive patterns observed in CRBG age data. To evaluate a temporal gap, we utilize volcanic stratigraphy to quantify timing and eruptive flux of a CFB progressing through the main-phase eruption. We focus on potential major (> 0.1 Ma) to minor (< 0.1 Ma) eruptive pulses, interpreted from age distribution data to tell us about eruptive behavior and dynamics over the course of CFB volcanic activity.