PALEOGEOGRAPHIC PROVENANCE OF IAPETAN SYNRIFT DETRITAL ZIRCONS FROM THE ST. LAWRENCE PROMONTORY, WESTERN NEWFOUNDLAND: A U-PB AND HF ISOTOPIC STUDY

Breakup of Rodinia at the end of the Neoproterozoic framed the eastern Laurentian margin and the departing conjugate cratons around the opening Iapetus Ocean. Along the eastern Laurentian margin, continental extension by a low-angle detachment rift system produced a zig-zag set of promontories and embayments that accumulated variable amounts of sedimentary detritus, presumably from Laurentia and the neighboring cratons. While much is known of the Iapetan rift on the eastern Laurentian margin, the identity of conjugate cratons to specific segments of the margin remains largely unresolved. Several studies have narrowed potential conjugates of the St. Lawrence promontory in western Newfoundland to either Baltica or Amazonia. Either of these cratons may have left a U-Pb and Hf-isotopic fingerprint on the promontory preserved in detrital zircons deposited as part of the synrift sedimentary record.

Approximately 510 zircons from 9 samples were analyzed by LA-ICP-MS at the University of Florida for U-Pb ages and Hf isotopic ratios. Five samples collected from the Iapetan synrift succession yielded ages that range from 544 Ma to 3605 Ma with maximum age frequencies of 1000-1200 Ma (Grenville), 1350-1450 Ma (Pinware), and 2650-2800 Ma (Superior). Although 2000-2200 Ma zircons are notably absent, a minor population of zircons ranging in age from 1500 to 1650 Ma was recovered from the synrift samples, suggesting a potential Baltican provenance for part of the synrift detritus on the St. Lawrence promontory. To isotopically distinguish Laurentian detrital zircons from zircons of an exotic provenance, two samples of Laurentian basement consisting of a granite and granitic gneiss were collected along with two samples of Early Cambrian sandstone sampled 1 meter above the two basement samples. The basement samples yielded U-Pb ages of 1044 Ma (granite) and 1495 Ma (gneiss), and the two sandstones yielded detrital zircons with age frequency peaks between 1000 and 1200 Ma. Interestingly, preliminary analysis indicates that the Hf isotopic ratios of ca.1000 Ma, 1200 Ma, and 1400-1600 Ma detrital zircons from the Laurentian synrift succession are a close match to reported Hf ratios from Baltican zircons of the same vintage, suggesting that Baltica rather than Amazonia was the conjugate to the St. Lawrence promontory.