A CAUTIONARY NOTE ON THE AGE OF THE HAROHALLI DIKES PALEOMAGNETIC POLE

Radiometric dating of mafic intrusive and extrusive rocks poses a problem due to the paucity of dateable minerals. U-Pb isotopic analyses from zircons and baddeleyites yield U-Pb concordant ages with little or no ambiguity introduced by inherited components, Pb loss or common Pb. Paleomagnetic results from Precambrian dike swarms in southern India (especially the alkaline Harohalli dikes) lack robust isotopic ages to enable meaningful comparisons between the continents during the Proterozoic. Two suites of dikes are present in the Harohalli region. The first, a series of ~E-W trending doleritic dikes were dated by Rb-Sr whole rock methods at 2370 ± 230 Ma and have been more recently dated by U-Pb methods at 2365.5 ± 1.1 Ma and 2367 ± 1 Ma. Younger dikes, trending in ~N-S directions were dated to 814 ± 34 Ma (Rb-Sr whole rock), 810 ±25 Ma (K-Ar) and 823 ± 15 Ma (Rb-Sr whole rock-mineral pair).

The younger suite of dikes yields a paleomagnetic pole at 28°S, 260°E. We have attempted to date the younger alkaline suite of dikes by U-Pb methods on uranium-bearing minerals using LA-MC-ICP-MS methods. One of the dikes yielded fragmentary pieces of zircon with highly discordant ages between 2300-3200 Ma. A second N-S trending dike near Harohalli yielded several well-faceted zircons along with fragments.

Five analyses from two different zircons yielded a concordant U-Pb age of 1181 ± 14 Ma (2σ; MSWD=2.4). Eight analyses on 3 fragments yielded discordant ages ranging from from 580-1139 Ma. A discordia using these grains yielded a lower/upper intercept ages of 508 ± 17 Ma and 1224 ± 24 Ma. The lower age may represent Pb-loss event associated with the final assembly of Gondwana and the upper intercept is interpreted as being close to the crystallization age of the dikes.

While not definitive, our preliminary results suggest that paleomagnetists should use caution when interpreting the age of the alkaline Harohalli dikes as ~825 Ma. Not a single zircon (concordant or otherwise) fell in the range from 800-900 Ma. Although we also urge caution in using our preliminary ages, we do note that a number of new geochronologic results have been derived from the Indian subcontinent (either as detrital zircon or primary zircon ages) in the 1.0-1.2 Ga range. The implications of this igneous activity in relationship to Rodinia will be presented along with additional U-Pb results from the Harohalli dikes (work in progress).