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
Graphical abstract
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.
Section snippets
Peri-Gondwanan Terranes and Rodinia-type basement of Mexico
Several decades ago, the pre-Mesozoic lithosphere of Mexico was subdivided into several tectonstratigraphic terranes of uncertain origin (Campa and Coney, 1983; Howell et al., 1985; Keppie, 2004). Since then, dozens of papers have elucidated structural, petrogenetical, and geochronogical aspects of many of those “suspect terranes” leading to a more tangible framework for the crustal evolution of Mexico (Martini and Ortega-Gutiérrez, 2018; Ortega-Gutiérrez et al., 2018) that integrates most of
Sample sites and dating approach
In silica-undersaturated rocks like MORBs or E-MORBs, magmatic zircon is absent or rare. In the El Triunfo Complex, due to subsequent medium- to high-grade metamorphism, all zircon crystals separated from such amphibolites yielded either metamorphic UPb ages or were inherited from assimilated crust (Weber et al., 2018). Similarly, baddeleyite (ZrO2), which is the most stable magmatic Zr phase in silica-undersaturated rocks, would recrystallize to metamorphic zircon (Heaman and LeChiminant, 1993
Zircon separation and documentation
Zircon crystals from samples CH16-1, CH16-3a and CH16-5a were separated at Departamento de Geología, Centro de Investigacón Científica y de Educación Superior de Ensenada (CICESE), México, using standard techniques including Wilfley table, Frantz magnetic separator, and heavy liquids prior to hand-picking under a binocular microscope. Then, zircons crystals of similar size were mounted in epoxy discs together with the AS3 and 91500 zircon standards. Zircon from previously dated samples was
UPb zircon geochronology
Cathodoluminescence (CL) images of selected zircon grains separated from anorthosite samples are shown in Fig. 4, Fig. 5. The results from UPb dating by SIMS are listed in Supplementary Table 1 and displayed in Fig. 6.
Zircon from the Mariscal anorthosite (CH16-1) rarely contains xenocrystic cores (Spot #1, 44, 62, 63, 75 in Fig. 4) that might be relics from originally magmatic or metamorphic Rodinia-type events. However, either due to extremely low U-contents (<1 ppm) or metamorphic overprint,
Timing of mafic magmatism
The UPb ages of corresponding zones from zircon separates of two samples, one from the Mariscal anorthosite (CH16-1) and another from the Soconusco anorthosite (CH16-5b) yielded identical Ediacaran ages at 615.3 ± 6.6 Ma and 614.7 ± 8.8 Ma (Fig. 6). In addition, in-situ analyses of zircon grains in a polished thin section of sample ENFORT-01 (Soconusco anorthosite), where the textural and genetical relationships of zircon to rutile, ilmenite and silicate phases are preserved (Fig. 7), yielded
Concluding remarks
- 1.
The intrusion of mafic dykes and sills without magmatic zircon could be dated with SIMS on neoform zircon that crystallized in host anorthosite during contact metamorphism.
- 2.
Subsequent amphibolite-facies regional metamorphism leading to recrystallization of much of the metamorphic zircon formed during dyke intrusion.
- 3.
Chemical and isotopic constraints suggest that neoform zircon related to dyke intrusion contact metamorphism crystallized at temperatures around 700 °C and at the expense of TiFe
CRediT authorship contribution statement
Bodo Weber: Conceptualization, Investigation, Visualization, Writing original draft. Axel K. Schmitt: Methodology, Conceptualization, Funding Acquisition, Validation, Writing review & editing. Alejandro Cisneros de León: Analyses, visualization, Writing review & editing. Reneé González-Guzmán: Resources, Data curation, Writing review & editing. Axel Gerdes: Methodology, Analyses.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This paper was supported by DFG (German Science Foundation) grant SCHM 2521/4-1 and CONACyT (Consejo Nacional de Ciencia y Tecnología) grant CB-2016-01-285638. We would like to thank Ilona Fin and Oliver Wienand (thin sections), Sonja Storm (sample preparation), and Alexander Varychev (Scanning Electron Microscope), all University of Heidelberg. We are grateful to reviewers David Chew (Trinity College, Dublin), Roelant van der Lelij (Geological Survey of Norway) and an anonymous reviewer for
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