Tracking the transition from subduction to continental collision using Ce and Eu anomaly in detrital zircons

Zircon geochemistry provides critical information on the melt from which they form. Specifically, Eu and Ce anomalies in zircons can be used to infer the evolution of average crustal thickness over time. However, they are typically influenced by multiple factors, such as the depth of magmagenetic processes, the nature of the parental magma, magma hydration, oxidation state, and the crystallization of minerals like plagioclase, apatite, and garnet. As a result, translating these data into paleo-depth is challenging and can introduce significant biases into interpretations of crustal evolution.

We tested these proxies on detrital zircons from the well-known Pan-African Anti-Atlas orogen. Current geodynamic models suggest an initial igneous phase (>800 Ma) dominated by rifting mafic magmatism, and the formation of oceanic basins and passive margins along the northern boundary of the West African Craton. Between 760 and 650 Ma, magmatic arcs developed, characterized by juvenile mantle-derived magmas. This period is followed by the closure of oceanic domains around 630 Ma and the subsequent development of syn-orogenic flysch basins. Abundant post-collisional to Cadomian felsic magmatism ignited around 610 Ma and lasted until 550 Ma.

A dataset of 827 Neoproterozoic zircons was statistically analyzed using bootstrap approach to produce chemical timeseries for both Eu and Ce anomalies. The results are the following: (i) pre-760 Ma (12% of zircons data): shows a slightly increasing trend in Eu (negative) and Ce (positive) anomalies. (ii) 760 - 710 Ma: zircon's age-frequency diagram suggests a first magmatic inflation around 750 Ma, Eu anomaly trends decrease while Ce anomaly remains constant. (iii) 710 - 630 Ma: Ce anomaly is still constant, but Eu anomaly shows a gradual decrease. (iv) 630 - 600 Ma: both proxies drop sharply and synchronously, coinciding with a negative shift from a compilation of whole-rock Nd signatures. This marks the implication of the West African Craton crust in the source of post-collision magmas. (v) 600 - 550 Ma: both proxies rise significantly and remain closely correlated.

Our analysis reveals that the paroxysm of the magmatic flare-up occurs at the transition from oceanic subduction to continental collision (at ~630 Ma) in the Anti-Atlas orogenic belt. If used as a proxy for crustal thickness, the Eu/Eu* ratio in zircons would be expected to increase around 630 Ma, as most geological markers indicate crustal thickening related to continental collision. However, it instead shows a sharp decline strongly correlated with Ce anomaly and coinciding with a major shift in magma sources—from mantle-dominated to crust-dominated. Conversely, intervals associated with variations in crustal thickness (from 760 to 700 Ma for example) exhibit a clear decorrelation between the Eu and Ce anomalies time series. This shows that magmatic changes associated with geodynamic transitions (e.g., from rifting to subduction to collision) have a significant impact on zircon trace element composition which inhibits other variations related to petrogenetic processes or crustal architecture.