Abstract:
Large Igneous Provinces (LIPs) comprise enormous edifices of rapidly emplaced basaltic lava and associated intrusive rocks. Two of the largest LIPs, the Siberian Traps ( approximately 2.5X10 (super 6) km (super 3) of basalt erupted at 250 Ma) and Deccan Traps ( approximately 2.6X10 (super 6) km (super 3) of basalt erupted at 65 Ma), are temporally associated with mass extinctions. A third LIP, the Central Atlantic Magmatic Province (CAMP), has been identified in the lava and igneous intrusions of the Newark Supergroup and is also temporally associated with a mass-extinction event at the Triassic-Jurassic ( approximately 201 Ma) boundary. The CAMP may be the largest LIP, extending over 7X10 (super 6) km (super 2) , covering significant portions of four continental plates and heralds the rifting of the supercontinent Pangea and formation of the Atlantic Ocean. (super 40) Ar/ (super 39) Ar ages from the Palisade sill (PS) Newark Basin, New Jersey, and from the Iberia continental margin provide thermochronologic constraints on the kinematics of lithospheric extension and continental Breakup. All plagioclase separates but two from the PS yielded discordant saddle shaped step-heating spectra. The two remaining plagioclase samples have disturbed spectra, but yield plateau ages of approximately 193 Ma, with concordant isochron and total-fusion ages. (super 40) Ar/ (super 39) Ar step-heating measurements on biotite from the PS, however, indicate an older plateau age of 200.8 + or - 0.6 Ma, also with concordant total fusion and isochron ages. This biotite age, concordant with previously reported U-Pb age (201 Ma) and (super 40) Ar/ (super 39) Ar results (202 Ma) from a biotite bearing xenolith from within the PS, dates the igneous activity of the CAMP. The combined (super 40) Ar/ (super 39) Ar results from the PS indicate a later metasomatic event that altered the plagioclase, and accounts for the variation in previously published K-Ar and (super 40) Ar/ (super 39) Ar ages. (super 40) Ar/ (super 39) Ar results on amphibole and plagioclase from amphibolite recovered from the Iberia continental margin, ODP Leg-173 also show evidence for metasomatism and indicate uplift ages 160 to 137 Ma. Together the results from these two studies may indicate metasomatic events associated with crustal thinning and continental breakup are a general phenomenon.