A PALEOPROTEROZOIC RECORD OF MODERN-STYLE SUBDUCTION INITIATION? EVIDENCE FROM THE 1.9 GA PEMBINE OPHIOLITE, WISCONSIN (USA)

The theory of plate tectonics was developed based on data from active systems and the Phanerozoic rock record. Yet, significant questions remain, including: (a) how and when in Earth history plate tectonics began; and (b) how specific tectonic process have evolved since. The formation of oceanic lithosphere and its preservation as ophiolites occurs in response to seafloor spreading, subduction and continent-continent collision. Orogenic ophiolites record the magmatic products of several tectonic processes, including both the mantle rocks subject to partial melting and the volcanic products of partial melting.

Here, we focus on one of the oldest ophiolites preserved on Earth; the c. 1.9 Ga Pembine Ophiolite in Wisconsin (USA) (Holm et al. 2020). We present the results of petrography, bulk-rock geochemistry and mineral chemistry for a suite of mafic rocks. We use these data to constrain element mobility associated with metamorphism, before comparing immobile element compositions to mafic rocks from modern tectonic environments.

The mafic volcanic rocks from the Pembine Ophiolite show a geochemical progression that resembles the modern Izu-Bonin-Mariana (IBM) arc. Specifically, three chemical groups are recognized. One group of samples (n=9) — concentrated towards the stratigraphic base of the mafic unit — are HREE-enriched and light REE-depleted ([La/Lu]_ch-norm = 0.1–0.3), with these compositions resembling IBM tholeiites interpreted as associated with forearc spreading during the earliest stage of subduction initiation. A second group (n=2) are HREE-depleted, LREE-enriched ([La/Lu]_ch-norm = 0.8–1.2) and resemble IBM boninites, with these rocks spatially concentrated towards the stratigraphic top of the mafic volcanic unit. Such chemical compositions are attributed to the increasing influence of slab-derived fluids during subduction. A third group (n=6) is transitional between these end-members.

These data highlight a geochemical similarity between a c. 1.9 Ga ophiolite and the magmatic products of modern-day plate tectonics. It is therefore possible that one of the fundamental components of modern plate tectonics — subduction initiation — as well as the production of oceanic lithosphere favorable for preservation as ophiolites — was operative during the mid-Paleoproterozoic.