A NEW LOOK AT THE DIABASE OF THE SOUTHWEST LAURENTIA LARGE IGNEOUS PROVINCE (SWLLIP): SPEEDING LAURENTIA AND STAGNANT SLABS

We propose a new model for the origin of the diabase of the ~1.1 Ga SWLLIP defined by Bright et al. (2014). They compiled and screened diabase age dates and chose five U/Pb ages as the most reliable intrusive ages, and we add a U/Pb age from the Kingston Range of 1104±7 Ma for a total of 6 ages which range from 1104 to 1080 Ma. This range overlaps the period of very rapid Laurentian plate motion from 1108 to 1096 Ma and is nearly coincident with Keweenawan LIP magmatism from 1109 to 1084 Ma (Swanson-Hysell et al. 2019, 2020). Ivanov (2015), among others, noted the association of continental flood basalt (CFB) provinces and fast subduction and proposed a deep water cycle model in which subducted slabs stagnate in the mantle transition zone (MTZ), warm and then release diapirs of hydrous fluids that rise and act as a flux in the asthenosphere, thereby producing melts that pond beneath a thick craton (such as Laurentia) at the lithosphere-asthenosphere boundary [LAB] until rapidly erupting due to stress-induced drainage events.

The SWLLIP diabase has features compatible with this model. Its water-rich nature has been previously reported. Multielement diagrams show compositions similar and generally intermediate to ocean island basalt (OIB) and continental arc basalt (CAB). All samples have negative Nb anomalies indicative of a subduction component but lack the positive Sr anomalies of CAB. Most have high TiO_2, as do OIB. The diabase is most OIB-like in the northwest and becomes more CAB-like to the southeast, presumably closer to the subduction zone. Initial isotope compositions show a comparatively wide range from radiogenic to unradiogenic ¹⁴³Nd/¹⁴⁴Nd coupled with unradiogenic to radiogenic ⁸⁷Sr/⁸⁶Sr, including values indicative of crustal input.

Rapid subduction under Laurentia’s leading southeastern (present coordinates) margin would set up conditions for stagnation of water-rich slabs and consequent ponding of magma at the LAB. At 1096 Laurentia slowed and changed direction. Whatever the cause of the slowing, convergence-caused compressional stress along the leading margin could produce northeastern extension now recorded by NW trending diabase dikes and coeval NW striking normal faults (e.g. Howard, 1991). The LAB magma pond would begin draining about 1104 Ma with the volume increasing until 1096 Ma and waning by 1080 Ma.