AMALGAMATION AND BREAKUP OF EASTERN RODINIA, PART II: EVIDENCE FROM U-PB AGES AND HF-ISOTOPIC RATIOS FROM DETRITAL ZIRCONS IN IAPETAN SYNRIFT STRATA, WESTERN NEWFOUNDLAND

Breakup of the supercontinent Rodinia at the end of the Neoproterozoic framed the eastern Laurentian continental margin and the departing conjugate cratons around the opening Iapetus Ocean. While this scenario is generally well established, the identity of cratons in the Rodinian assembly conjugate to specific segments of the eastern Laurentian margin remains unresolved. Several models have narrowed potential conjugates of the eastern Laurentian margin to either Baltica or Amazonia; the most prevalent places Amazonia conjugate to the entire eastern Laurentian margin. During breakup, these departing cratons left a U-Pb and Hf-isotopic fingerprint on the eastern Laurentian margin in the form of detrital zircons in synrift deposits through which the positions of Baltica and Amazonia can be tested relative to Laurentia in the Rodinian assembly.

Approximately 510 zircons from 9 samples of Iapetan rift-aged strata and underlying crystalline basement from western Newfoundland were analyzed by LA-ICP-MS at the University of Florida for U-Pb ages and Hf isotopic ratios. Five samples collected from synrift strata yielded U-Pb zircon ages from 544 to 3605 Ma with maximum frequencies typical of a Laurentian provenance (i.e., 1.0-1.2 Ga, 1.3-1.4 Ga, 2.6-2.8 Ga). A minor population of zircons with ages corresponding to the North American magmatic gap (1500-1650 Ma) was recovered from the synrift samples; only a single zircon with a ca. 2100 Ma age was discovered. To distinguish Laurentian zircons from zircons of potential exotic provenance, synrift zircons were analyzed for Hf isotopic ratios and compared against U-Pb ages and Hf ratios from zircons in two basement-cover samples of unequivocal Laurentian origin. Hf isotopic ratios of ca.1.0 Ga, 1.2 Ga, and 1.4-1.6 Ga detrital zircons from Newfoundland synrift rocks are a close match to reported Hf ratios for Baltican zircons of the same vintage. Furthermore, whole-rock isotopic analysis from basement complexes in the Blue Ridge massif of the southern Appalachians indicates that Amazonia was conjugate to southeastern Laurentia at the time of Rodinia breakup. Thus, the preponderance of data indicates that Baltica, and not Amazonia, was conjugate to northeastern Laurentia.