A NEW LOOK AT THE DIABASE OF THE SOUTHWEST LAURENTIA LARGE IGNEOUS PROVINCE (SWLLIP): SPEEDING LAURENTIA AND STAGNANT SLABS
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 TiO2, 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.