An accretionary tectonic model for the Mesoproterozoic ca. 1500–1340 Ma tectonic evolution of the southern Laurentian margin is presented. The tectonic model incorporates key observations about the nature and timing of Mesoproterozoic deposition, magmatism, regional metamorphism, and deformation across the 5000-km-long southern Laurentian margin. This time period was one of transition in the supercontinent cycle and occurred between the breakup of Columbia and the formation of Rodinia, and the southern Laurentian margin was a significant component of a much greater accretionary margin extending into Baltica and Amazonia and possibly parts of Antarctica and Australia. However, fundamental questions and contradictions remain in our understanding of the tectonic evolution of Laurentia and paleogeography during this time interval.
It is proposed that the Pinware orogen of eastern Canada, the Baraboo orogen of the midcontinent, and the Picuris orogen of the southwestern United States delineate a previously unrecognized, ~5000-km-long, ca. 1520–1340 Ma trans-Laurentian orogenic belt. All three orogenic provinces are characterized by Mesoproterozoic sedimentation, magmatism, metamorphism, and deformation—the hallmarks of a tectonically active plate margin. Tectonism was diachronous, with the earliest stages beginning ca. 1520 Ma in eastern Canada and ca. 1500 Ma in the southwest United States. Magmatic zircon age distributions are dominated by Mesoproterozoic, unimodal to multimodal age peaks between ca. 1500 and 1340 Ma. The onset of magmatism in the Pinware and Baraboo orogens was ca. 1520 Ma, and onset for the Picuris orogen was ca. 1485 Ma. Detrital zircon age distributions within each orogenic province yield maximum depositional ages between ca. 1570 and 1450 Ma. Minimum depositional ages generally fall between ca. 1500 and 1435 Ma, as constrained by crosscutting intrusions, metatuff layers, or the age of subsequent metamorphism. Metamorphic mineral growth ages from zircon, garnet, and monazite yield peak ages between ca. 1500 and 1350 Ma and tend to be older in the Pinware and Baraboo orogens than in the Picuris orogen. The 40Ar/39Ar cooling ages for hornblende, muscovite, and biotite yield significant peak ages between ca. 1500 and 1350 Ma in the Baraboo and Picuris orogens. We propose that the Pinware-Baraboo-Picuris orogen formed in a complex, diachronous, convergent margin setting along the southern edge of Laurentia from ca. 1520 to 1340 Ma.
Garnet Sm-Nd and Lu-Hf isotopic data are increasingly used in tandem to characterize tectono-metamorphic events and their duration because the two chronometers may yield a progression from older Lu-Hf to younger Sm-Nd ages. In this contribution, we utilize garnet compositional zoning and monazite U-Pb ages to evaluate explanations for differences in garnet Sm-Nd and Lu-Hf ages for low- to mid-amphibolite facies rocks from the Picuris and Tusas Mountains, New Mexico. Ten Sm-Nd ages, eight Lu-Hf garnet ages (6 previously published), and four matrix monazite U-Pb dates span 1422–1370 Ma, 1456–1399 Ma, and 1377–1356 Ma, respectively. The weighted means of garnet ages are 1398±11 Ma for Sm-Nd (N=10/10) and 1402±4 Ma for Lu-Hf (N=6/8); however, Sm-Nd and Lu-Hf ages from three samples differ by 29, 34, and 55 m.y. with differences between 2σ age envelopes of 12, 9, and 38 m.y., respectively. In these samples, primary Lu and Sm zoning in garnet, diffusion modification of zoning, or disequilibrium is sufficient to cause older Lu-Hf ages and younger Sm-Nd ages. Alternatively, two of the three Lu-Hf ages, c. 1450 Ma, may reflect the onset of a prolonged regional event or an earlier thermal event and the three c. 1370 Ma garnet Sm-Nd ages may reflect diffusional re-equilibration. These three c. 1370 Ma ages are indistinguishable from the monazite ages and garnet have trace element zoning consistent with diffusional alteration of growth zoning in Sm and Nd but not Lu and Hf. Garnet from the other seven samples have prograde zoning with little to no evidence for diffusional relaxation of zoning in these elements. We conclude that the majority of garnet preserve primary compositions and indistinguishable garnet ages of c. 1400 Ma indicating a c. 22 m.y. garnet growth history based on weighted mean. The two oldest c. 1450 Lu-Hf and three youngest c. 1370 Sm-Nd garnet ages require an earlier thermal history and modification of garnet composition, respectively. Results of this study re-define the youngest extent of amphibolite-facies metamorphism during the Picuris Orogeny to at least c. 1370 Ma.
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