Late Proterozoic Rodinia breakup in Mexico - A new approach using SIMS dating of zircon coronas and baddeleyite
Final Report Abstract
Laurentia, Baltica and Amazonia formed the last fragment of the Rodinia supercontinent that broke apart during the Early Ediacaran. Essential to establish such reconstructions are coeval dyke swarms that occur across formerly adjacent cratons defining a Large Igneous Province (LIP). Whereas ca. 615 Ma plume-related dykes from Baltica (Egersund) and Laurentia (Long Range) are known for long, similar dykes have not been recognized in Amazonia or in the Oaxaquia Terrane (Mexico), a Mesoproterozoic block located in Rodinia reconstructions between Amazonia and Baltica. New Secondary Ion Mass Spectrometry (SIMS) ages presented here indicate a previously unrecognized LIP that produced plume- related magmas across all cratonic masses involved in the final Rodina breakup and the birth of Iapetus Ocean. Our evidence comes from two distinct regions: (1) Plume-related subvolcanic dykes in Oaxaquia that are known from the Novillo Gneiss of northeastern Mexico. For these dykes, in-situ U-Pb micro-baddeleyite dating yielded reliable intrusion ages of 619 ±9 Ma, coeval with Chiapas E-MORB amphibolites, Egersund and Long Range dykes; and (2) E-MORB magmas intruded ca. 1.0 Ga metamorphic basement rocks (including massif anorthosite) of the Chiapas Massif (southeastern Mexico) prior to Ordovician high-grade metamorphism. Since metamorphism precludes direct dating of these mafic dykes, we used metamorphic zircon that crystallized after thermally activated Zr exsolution from rutile and ilmenite in anorthosite at the intrusive contact with the dyke. The corresponding metamorphic zircon zones that crystallized at T>700ºC (Ti-in-zircon thermometry) yielded U-Pb ages between 615 ±7 Ma and 608 ±12 Ma. Zircon chemical and isotopic (Hf, O) characteristics suggest a diachronous sequence of metamorphic processes involving Zr release from Fe-Ti oxides, breakdown and recrystallization of other phases, and fluid-mitigated reactions during Ordovician regional metamorphism. Zircon δ18O values of previously dated granulites from Oaxaquia range from +6.2‰ to +9.8‰, whereas zircon from Stenian-Tonian gneisses from SE Chiapas tends to display lower values. One sample with Tonian (~0.92 Ga) metamorphic zircon yielded δ18O from +2.0‰ to +2.8‰. Low δ18O values are explained by reactivation of major tectonic boundaries possibly during gravitational orogenic collapse. In this scenario, Oaxaquia at the northern edge of Amazonia may have formed the conjugate margin of Baltica during rifting of both continental fragments.
Publications
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(2019). Coeval Early Ediacaran breakup of Amazonia, Baltica and Laurentia: evidence from micro-baddeleyite dating of dykes from the Novillo Canyon, Mexico. – Geophysical Research Letters 46, 2003-2011
Weber, B., Schmitt, A.K., Cisneros de León, A., González-Guzmán, R.
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(2020). Neoproterozoic extension and the Central Iapetus Magmatic Province in southern Mexico – new U-Pb ages, Hf-O isotopes and trace element data of zircon from the Chiapas Massif Complex. – Gondwana Research
Weber, B., Schmitt, A.K., Cisneros de León, A., González-Guzmán, R., Gerdes, A.
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(2020). Permo-Triassic metamorphism in the Mérida Andes, Venezuela – New insights from geochronology, O-isotopes, and geothermobarometry. – International Journal of Earth Sciences
Tazzo-Rangel, D., Weber, B., Schmitt, A.K., González-Guzmán, R., Cisneros de León, A., Hecht, L.
