Paper No. 10
Presentation Time: 4:30 PM
LITHIUM-CESIUM-TANTALUM PEGMATITES THROUGH TIME, THEIR OROGENIC CONTEXT, AND RELATIONSHIPS TO THE SUPERCONTINENT CYCLE
BRADLEY, Dwight C., U.S. Geological Survey, 11 Cold Brook Rd, Randolph, NH 03593, MCCAULEY, A., USGS, 4210 University Drive, Anchorage, AK 99508, BUCHWALDT, R., Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Building 54-1126, Cambridge, MA 02139, SHEA, E.K., Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 and BOWRING, S., Dept. of Earth, Atmospheric & Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, dbradley@usgs.gov
Pegmatites account for ~30% of the world’s Li production, ~90% of the Ta, and virtually all of the Cs. Pegmatites enriched in these elements (LCT pegmatites) are the extreme fractionation products of S-type granitic melts. We present a refined global age distribution of LCT pegmatites that is similar to those of orogenic granites and detrital zircons. LCT pegmatite maxima at ca. 2650, 1800, 525, 350, and 100 Ma correspond to times of collisional orogeny and, except for the comparatively minor peak at 100 Ma, to times of supercontinent assembly. Between these pulses were long intervals of few or no LCT pegmatites. Global minima in LCT pegmatite abundance overlap with supercontinent tenures at ca. 2450–2225, 1625–1000, 875–725, and 250-200 Ma, as established from plate reconstructions and from passive-margin and detrital-zircon abundances.
The fundamental connection between LCT pegmatites and tectonics—that they are products of plate convergence—is well established, but ties to specific tectonic processes remain elusive. A few LCTs are known from convergent margins (e.g., Coast Ranges, CA), but examples from collisional orogens are far more numerous. New CA-TIMS U-Pb zircon dates are here reported for LCT pegmatites in the Appalachian orogen; most are characterized by complex zircon systematics and inheritance. Preliminary data suggest at least four ages or age groups of pegmatites in New England, at ca 275 Ma, 326, 341, and 371-381 Ma. An age of 275 Ma coincides with the final increment of Alleghanian convergence during assembly of Pangea. An age of 326 Ma corresponds to an otherwise quiet time; 341 Ma is at the end of the Neoacadian phase; and 371-381 Ma is at the end of the main Acadian phase. Causes of pegmatite-related magmatism could include: ordinary arc processes; overthickening during collision or subduction; slab breakoff or delamination during or after collision; and late collisional extensional collapse and decompression melting.
