MARE-STYLE IMPACT-ASSISTED VOLCANISM AT THE TRIASSIC-JUURASSIC BOUNDARY?

As some authors search for dubious links between large hypervelocity impacts and voluminous volcanic activity near extinction boundaries, we are investigating the possibility that a widely accepted process in the evolution of many planetary bodies might have played an important role in developing the Triassic-Jurassic boundary on Earth. Dietz (Meteoritics, 1986) proposed that an impact centered near the onset of Atlantic rifting could have triggered both the Tr-J extinction and CAMP magmatism. Although we disagree with that assertion, we have found evidence suggesting the plausibility that a Late Triassic impact in that region could have sufficiently fractured the crust to provide a determinative pathway for CAMP eruptions. This mechanism would be similar to the presumed emplacement of lunar maria, a process underrepresented in the terrestrial cratering record.

A suite of lamprophyric dikes that occur in SE North America, NW Africa, and SW Europe appear to slightly predate CAMP eruptions. In southeastern North America, some of the conduits containing lamprophyre were subsequently occupied by CAMP basalt. The kersantite member of the lamprophyre suite contains abundant quartz, microcline, and andesine/labradorite xenocrysts. The feldspar xenocrysts exhibit crystallographically controlled melting in a checkerboard pattern often attributed to shock at known impact structures. Quartz grains exhibit planar fracturing, planar fluid inclusion trails at acute angles to PFs, possible PDFs, and ballen. A rectilinear network of skeletal titanomagnetite pervades the mesostasis, occasionally extending into and across ghost-like plagioclase grains suggesting the hosts were completely molten just before quench. Taken together these observations seem inconsistent with even extremely rapid magma ascent and suggest that these rocks might have been emplaced from above by injection of impact melt.

If these rocks are not associated with an impact, the similarity of petrographic features to recognized impact signatures should be documented, as the dikes likely would have introduced material to Tr-J strata through both eruption and erosion. But if they were produced by an impact, they may represent the remains of a significant structure, dissected by rifting, that might have predestined the ascent of CAMP magmas.