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

Highlights

• Neoform zircon in anorthosite reveals the age of mafic dykes at intrusive contacts.

• Dyke swarms in Chiapas suggest Large Igneous Province in NW Amazonia at ca. 615 Ma.

• Oaxaquia was possibly the conjugate margin of Baltica during Rodinia breakup.

Abstract

Final fragmentation of Rodinia occurred during the Ediacaran Period as Amazonia, Baltica and Laurentia drifted apart to form the Iapetus Ocean. Accompanying rift-related mafic dyke swarms of the Central Iapetus Magmatic Province (CIMP) were emplaced between 0.62 and 0.55 Ga, which are preserved in Laurentia and Baltica, whereas no coeval mafic rocks are known from Amazonia. First evidence for the CIMP extending into Oaxaquia (Rodinia-type basement of Mexico) was reported as tholeiitic dykes that intruded the Novillo gneiss, NE Mexico, at 619 ± 9 Ma. In Chiapas, SE Mexico, amphibolite dykes that are chemically similar to the Novillo dykes intruded anorthosite and gneiss. In this paper, a new dating approach to obtain mafic dyke intrusion ages is presented by targeting contact metamorphic zircon with the UPb method, employing Secondary Ion Mass Spectrometry. Zircon that crystallized in anorthosite at intrusive contacts to mafic dykes and at temperatures exceeding 700 °C (Ti-in-zircon thermometry) yields ages between 615 ± 7 Ma and 608 ± 12 Ma, reflecting the time of dyke intrusion. Zircon chemical and isotopic (Hf, O) characteristics suggest a diachronous sequence of metamorphic processes involving Zr release from FeTi oxides, breakdown and recrystallization of other phases, and fluid-mitigated reactions during Ordovician metamorphism. Zircon δ¹⁸O values of granulites from Oaxaquia range from +6.2‰ to +9.8‰, whereas Tonian (~0.92 Ga) metamorphic zircon from SE Chiapas yielded low δ¹⁸O values from +2.0‰ to +2.8‰ that are explained by the reactivation of major tectonic boundaries during Tonian gravitational collapse. The observations increase the known extent of the CIMP in Mexico, suggesting that a Neoproterozoic superplume was still active during the Early Ediacaran producing a Large Igneous Province that extended over Amazonia, Baltica and Laurentia. The results further suggest that Oaxaquia at the northern edge of Amazonia formed the conjugate margin of Baltica during rifting.

Introduction

The Neoproterozoic Era starts with the final assemblage of the Rodinia supercontinent, followed by tectonic relaxation and the onset of extension in the eponymously named Tonian Period (Fairchild et al., 2018; Li et al., 2008; Lyons et al., 2018). Global geodynamic modeling suggests that below a supercontinent, mantle convection leads to formation of two antipodal superplumes, which are the driving force for dismembering the supercontinent (e.g. Zhong et al., 2007), as suggested for the breakup of the Rodinia during most of the Neoproterozoic (Li et al., 2008; Li and Zhong, 2009). The superplume below Rodinia started at high latitudes around 825 Ma and rotated to low latitudes between ca. 800 and 780 Ma, possibly by a true polar wander event (Li et al., 2013; Swanson-Hysell et al., 2012). The superplume extensional forces and associated magmatism led to continental rifting and Rodinia breakup, followed by the pan-Rodinian, Sturtian, “Snowball Earth” glaciation at ca. 720 Ma, which marks the beginning of the Cryogenian Period (Fairchild et al., 2018; Hoffman et al., 1998; Macdonald et al., 2010). Rodinia's breakup was nearly complete by the end of the Cryogenian when the “Marinoan” glaciation at 635 Ma, the second “Snowball Earth” event, ended (Hoffman et al., 1998; Hoffman and Schrag, 2002).

The predominant geologic evidence for plumes impacting ancient supercontinent breakup are Large Igneous Provinces (LIPs, Ernst et al., 2008) such as the Franklin LIP of northwestern Canada (~720 Ma, Macdonald et al., 2010), now preserved as dyke swarms on the eroded surface of the continental crust. There is increasing recognition of the role of LIPs, especially continental flood basalts, in major glaciation events, since extensive surface weathering of basaltic rocks under humid climate at low latitudes sequestrates atmospheric CO₂ and consequently lowers global temperature, plunging Earth's climate into widespread glaciation (Ernst and Youbi, 2017; Goddéris et al., 2003, Goddéris et al., 2007).

Although some authors raised concerns on whether Baltica was connected with Laurentia during the early Neoproterozoic (Slagstad et al., 2019, and references therein), many paleogeographic models (e.g., Pisarewsky et al., 2008; Li et al., 2013; Merdith et al., 2017) identify Amazonia, Baltica, and Laurentia as the last major cratonic remnants of Rodinia at the beginning of the Ediacaran Period. Several pulses of mafic and carbonatite magmatism spanning between ca. 620 and ca. 550 Ma, also referred to as the Central Iapetus Magmatic Province (CIMP), marked the tectonic separation of Baltica and Laurentia, and the opening of the Iapetus Ocean (e.g., Ernst and Bell, 2010; Youbi et al., 2020). Whether the abovementioned superplume had waned by the end of the Cryogenian due to ongoing fragmentation of Rodinia, or if it was still active during the Ediacaran and the driving force for the opening of the Central Iapetus Ocean is still under debate (Ernst and Bell, 2010; Li et al., 2013). Furthermore, geological evidence connecting the CIMP to Amazonia or related peri-Gondwanan terranes was only discovered recently (Weber et al., 2019). The first rifting stages of the CIMP between the three cratonic masses has been mainly documented from mafic dyke swarms, namely the Egersund dykes from Norway (Baltica) yielding a UPb baddeleyite (ZrO₂) age of 616 ± 3 Ma (Bingen et al., 1998), the Long Range dykes from Labrador (Laurentia) yielding a baddeleyite age of 615 ± 2 Ma (Kamo et al., 1989; Kamo and Gower, 1994) – both dated with isotope dilution thermal ionization mass spectrometry (ID-TIMS) – and the Novillo dykes intruding Rodinia-type basement in Mexico (formerly part of Amazonia) recently dated by in-situ micro-baddeleyite secondary ion mass spectrometry (SIMS) at 619 ± 9 Ma (Weber et al., 2019). These new data suggest the existence of a basaltic LIP across the three principal landmasses of Rodinia.

In southern Mexico, the Chiapas Massif Complex of mainly Permian age (Estrada-Carmona et al., 2009; Schaaf et al., 2002; Weber et al., 2005, Weber et al., 2007) contains basement inliers of Late Mesoproterozoic to Ordovician age, also referred to as the El Triunfo Complex (Estrada-Carmona et al., 2012; Weber et al., 2018). Rocks of particular interest for reconstructing the extent of the CIMP are amphibolites that intruded the basement prior to Ordovician metamorphism (Weber et al., 2018). However, direct dating of amphibolite igneous precursors is typically very challenging and hampered by the lack of magmatic baddeleyite or zircon due to intense poly-metamorphic recrystallization along the complex Phanerozoic tectonothermal history of the CMC.

Our new dating approach is based on the observation that massif-type anorthosites intruded by mafic dykes in the El Triunfo Complex contain zircon with ages of ca. 600 Ma that grew from metamorphic reactions involving Zr exsolution processes after Ti-rich phases (e.g., Cisneros de León et al., 2017). In this contribution we report new UPb ages using Secondary Ion Mass Spectrometry (SIMS) of neoform metamorphic zircon microcrysts from anorthosite samples taken directly from anorthosite-amphibolite dyke contacts. Our novel results indicate metamorphic zircon growth as the result of contact metamorphism during mafic dyke intrusion at ca. 615 Ma. Most of this zircon, however, recrystallized during the Ordovician metamorphism. We further report chemical differences in Ediacaran and Ordovician metamorphic zircon including Ti-in-zircon thermometry as well as zircon oxygen isotopes (SIMS) and LuHf isotopes (LA-MC-ICPMS). Additionally, we present the first zircon oxygen isotope data for other, previously dated Rodinia-type basement rocks from Mexico including gneisses from the El Triunfo Complex. Finally, on the basis of previously published UPb ages coupled with Hf isotopes and the new zircon oxygen isotope data, we interpret a unique Tonian (ca. 920 Ma) metamorphic event that was reported from the El Triunfo Complex gneisses spatially associated to anorthosites (Weber et al., 2018) in terms of extensional tectonics and reactivation of a former structural boundary between two major crustal precursors.