DETRITAL ZIRCON ANALYSIS OF BASEMENT IN THE COST NO. G-1 WELL, GEORGES BANK, OFFSHORE MASSACHUSETTS: FRAGMENTS OF WEST AFRICAN CRUST?

The COST No. G-l well is the only one of several petroleum exploration wells in the Georges Bank that penetrated basement rocks below overlying Mesozoic and younger strata. We conducted detrital zircon U-Pb laser ablation inductively coupled mass spectrometry (LA-ICPMS) on two hand specimens from quartzofeldspathic layers taken between depths of 16061–16063 ft in the generally phyllitic core in order to test whether this sequence could represent (1) sedimentary rocks of the northern Appalachian Avalonia or Meguma terrane, (2) Amazonian or West African crust forming basement to Avalonia, (3) West African crust forming basement to Meguma or (4) West African crust juxtaposed with eastern North America during formation of the supercontinent Pangea.

The detrital zircon age spectrum including results from both samples shows a dominant peak between 2.2 and 2.0 Ga encompassing 79 of 92 concordant analyses and minor peaks at ~3.1 to 2.5 Ga and at ~1.9 Ga. The chemical compositions of all these grains are typical of zircon from arc magmas. One zircon at ~1.49 Ga and one at ~70 Ma are likely contaminations introduced by drilling from higher in the well and are not interpreted further.

The age spectrum is unlike those available from sedimentary rocks in Avalonia or Meguma, which have a majority of dates <1.9 Ga. The detrital zircon signature may be consistent with Amazonian sources, although it does not show any of the <1.75 Ga zircon populations typical of Amazonia. If Amazonian, the sequence would have to pre-date the ~1.75-1.50 Ga Jurena/Rio Negro orogeny and post-date youngest zircon in the core (~1.9 Ga). The strong 2.2–2.0 Ga probability peak overlaps ages documented in Birimian/Eburnean assemblages of the West Africa Craton, but the presence of < 2.0 Ga zircon suggests that the core transects slightly younger deposits. One candidate is the earliest passive margin sequence overlying the West Africa Craton in Morocco’s Anti-Atlas region, where dates from the Taghdout Group are between ~2.94 and ~1.81 Ga with major peaks at ~2.20 and ~2.09 Ga. In any case, the core ends in units that may have become part of North America either as West African basement to Avalonia or Meguma during the Acadian or Neo-Acadian orogenies, respectively, or more likely, as West African crust during the Alleghanian orogeny.