An enormous rifting occurred causing massive floods of basaltic lavas and the release of large amounts of gas. this is a thought to have been the cause of which mass extinction

The correct answer is End-Triassic extinction.  

This is also known as Triassic-Jurassic extinction, it was a worldwide extinction incident that took place at the termination of the Triassic Period, that is, about 252 to 201 million years ago. It led to the destruction of almost 76 percent of all the terrestrial and marine species and about 20 percent of all the taxonomic families.  

It is considered that this incident took place due to climate change and elevation in sea levels leading to sudden discharge of huge concentrations of carbon dioxide. The discharge of carbon dioxide due to volcanic activity would have strengthened the worldwide greenhouse effect that elevated average air temperatures around the globe and acidified the oceans.