Mafic-ultramafic subvolcanic metabasalt sills intrude Ediacaran and Cambrian strata of the La Ciénega Formation of Sonora, Mexico. Sills are typically 1–7.5 m thick, with minor associated thin (∼10 cm thick) intrusions. Sills are typically bed parallel with upper and/or lower tempered (quenched) borders and porphyritic textures characterized by abundant olivine and pyroxene pseudomorphs that have been transformed to clay and carbonate minerals. Metamorphic halos in adjacent dolostones and quartzites occur at the bottom and top contacts, and metabasalt sills contain carbonate veins typical of hydrothermal alteration and mobilization. Unlike surface flows of the overlying Cerro Rajón Formation, all sills in the La Ciénega Formation lack evidence of crystal settling, yet evidence for assimilation of country rocks is present. Assimilation is more typical of sills rather than flows and is diagnostic of sills in the El Arpa Formation, a stratigraphically lower unit in the succession. Actinolite-chlorite-albite metamorphic mineral paragenesis suggests low-grade greenschist facies metamorphism for these metabasaltic rocks. Although their primary mineralogy is obscured due to alteration, olivine and clinopyroxene pseudomorphs are present and their major and trace element geochemistry suggest they were derived from alkaline basaltic magmas with low (normalized) SiO2N (22.7–43.1 wt%), high MgON (7.25–16.6 wt%), and high TiO2N (4.04–6.27 wt%). Immobile trace elements suggest these sills were originally alkaline rift-related basalts with OIB-type signatures. Tectonic discrimination diagrams suggest continental rift magmatism with low geochemical evolution ratios and relatively low mantle melting rates when compared to contemporaneous rift related magmatism. Sr-Hf-Nd-Pb isotopes suggest the metabasalts originated from an Enriched Mantle I (EMI) reservoir and melting occurred during adiabatic decompression. These geochemical characteristics are remarkably similar to volcanic rocks of the Cerro Rajón Formation. The inferred magmatic composition, mantle melting rates, petrography and field relations of the La Ciénega Formation metabasalt are consistent with a low viscosity feeder system like that envisaged to have sourced volcanic rocks of the overlying Cambrian (Terreneuvian) Cerro Rajón Formation. Together, evidence from these two volcanic units is consistent with a Cambrian continental rift event close to the onset of the Sauk transgression.
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To determine the primary conditions of the metabasalt sills, major- and trace-element analyses were conducted on eighteen least-altered samples from Cerro Rajón, Cerro Los Apaches and Cerro Clemente. Samples are treated by locality rather than by unit because Unit 1 and Unit 3 sills exhibit similar geochemical characteristics. Details of major and trace element analyses are in Supplementary File 3. Major element geochemistry exhibits high TiO2 values (4.04–6.27 wt%.), low Na2O values (0.01–0.59 wt%) and high loss on ignition (LOI) (12.8–22.3 wt%). High LOI values suggest significant alteration of the rocks and/or the abundance of volatiles (e.g. CO2 from carbonate) impacting the accuracy of major element concentrations. However, some trace elements, especially the high field strength elements, appear to be immobile, especially at the mineral scale as is indicated by previous analyses of volcanic clasts from the overlying Cerro Rajón Formation (Barrón-Díaz et al., 2019a). La Ciénega Formation metabasalts have high Cr (<1520 ppm) and Ni (<882 ppm) values with little variation; it is a characteristic of primary magma composition (Fig. 5B). Multi-element diagrams of La Ciénega Formation samples, normalized to primitive mantle (Sun and McDonough, 1989) show a positive hump-shaped spectrum (Fig. 6A), with positive anomalies of Nb–Ta (159–180 ppm, respectively), which is a common characteristic of ocean island basalt (OIB)-type magmatism and intraplate settings. Variable amounts of Ba (36–3270 ppm.) and negative anomalies of Sr (2.7 ppm) and Pb (10.8 ppm) were observed (Fig. 6A); these values suggest a possible crustal contamination/assimilation and/or alteration. Moreover, the La Ciénega Fm REE diagram (Fig. 6B), normalized by a PRIMA (Primitive Mantle) from Sun and McDonough (1989) shows high LREE (La = 131 ppm; Ce = 113 ppm), and very low HREE (Lu- 1.6 ppm; Yb-1.7 ppm) values and absence of Eu anomalies. The latter spectra are also related to OIB type magmatism and the absence of an Eu anomaly supports the lack of plagioclase fractionation. This relationship is also observed in the Cerro Rajón Formation extrusive volcanics (Barrón-Díaz, 2019), where flows have ankaramitic textures and lack plagioclase.Tectonic discrimination diagrams based on relatively immobile trace elements demonstrate that the La Ciénega Formation metabasalts cluster together and mainly plot in the OIB fields or intracontinental settings with enriched mantle signatures (Fig. 7). Analyses of Th, Zr and Nb (Wood, 1980) place these rocks in an alkaline within-plate setting (Fig. 7A). This characteristic is further supported by employing a Y–La–Nb diagram (after Cabanis and Lecolle, 1989); La Ciénega Formation samples plot in the within-plate basalt and the alkaline-anorogenic domain, with few samples plotting in the late and post-orogenic intracontinental domains (Fig. 7B). Moreover, plotting the same immobile elements in a Pearce (2008) diagram supports an OIB affinity (Fig. 7C and D); here the primary condition of the magmatism is also observed (note the absence of deep crustal recycling in Fig. 7C). Similarly, Th/Yb vs Nb/Yb comparison of La Ciénega samples also plot inside the OIB and alkali basalt fields from Shervais (2022). La Ciénega Formation samples that fall outside of this field in the TiO2/Yb vs Nb/Yb diagram from Pearce (2008; Fig. 7D), may be explained due to an enrichment in TiO2, a primary characteristic also noted in high-TiO2 diopsides from the overlying Cerro Rajón Formation (Barrón-Díaz et al., 2019a). The Shervais (2022) Ti/V vs Th/Nb diagram supports the placement of La Ciénega Formation rocks between the OIB and alkali basalt fields (Fig. 7E). Considered together, these characteristics suggest that the La Ciénega Formation metabasalts represent a rift-related volcanic event that developed in early Cambrian time and that a genetic correlation exists between the metabasalt sills of the La Ciénega Formation and the extrusive volcanism of the Cerro Rajón Formation.
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