T148 Reconstructing Supercontinents and the Global Large Igneous Province (LIP) Record

Tuesday, 11 October 2011
8:00 AM - 12:00 PM
Room 102D-F

HIGH-PRECISION U-PB ZIRCON GEOCHRONOLOGY OF THE FERRAR LARGE IGNEOUS PROVINCE: CAN SILL EMPLACEMENT BE LINKED WITH THE PLIENSBACHIAN-TOARCIAN EXTINCTION EVENT?

Paper number: T148-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

Initial breakup of Gondwana is marked by the Karoo-Ferrar large igneous province, which records the eruption and emplacement of ~2.5x106 km3 of basaltic magma. This period of Earth history also saw major global changes in biodiversity and seawater chemistry (O, C, Sr, Os) associated with the Pliensbachian-Toarcian boundary. Establishing whether voluminous magmatism associated with Gondwana breakup triggered major environmental change is critically dependent on knowing precisely when each occurred. The extinction spans a maximum of 6 Ma, from the beginning of the Kunae ammonoid zone to the Crassicosta zone, an interval that includes the Pliensbachian-Toarcian boundary at 183.0 ± 1.5 Ma (Gradstein et al. 2004). Geochronology published over the past two decades for Karoo-Ferrar magmatic rocks could be interpreted to indicate a protracted eruption/emplacement history over some 12 Ma. Variable precisions and different chronometers make direct detailed comparison of datasets difficult and limit the possibility of assessing the role of magmatism as a significant trigger of environmental change. Although recent publications have improved constraints on the timing of magmatism, at present it remains largely unclear whether Karoo and Ferrar magmatism was contemporaneous, whether it was episodic or continuous, whether the sill complex pre or post-dates the extrusive component of the magmatism, and whether a significant volume of the magmatism occurred prior to, during, or after the mass extinction event.

We present a new high-precision dataset from Ferrar intrusive rocks extending ~3000 km across the pre-breakup Gondwana continent. To achieve the maximum possible precision, zircons were subjected to thermal annealing and chemical abrasion (CA-TIMS) prior to dissolution and equilibration with the 202Pb-205Pb EARTHTIME tracer solution. This approach has allowed us to achieve a precision on a weighted mean calculated date for a small population of grains on the order of < 0.03% of the age. This level of precision tightly constrains the intrusive timescale of a significant portion of Ferrar magmatism and allows for direct comparison between the emplacement of this material and the onset of chemical and biological events occurring near the Pliensbachian-Toarcian boundary.