Concurrent MORB-type and ultrapotassic volcanism in an extensional basin along the Laurentian Iapetus margin: Tectonomagmatic response to Ordovician arc-continent collision and subduction polarity flip

Abstract

Arc-continent collision, followed by subduction polarity flip, occurs during closure of oceanic basins and contributes to the growth of continental crust. Such a setting may lead to a highly unusual association of ultrapotassic and mid-ocean ridge basalt (MORB)-type volcanic rocks as documented here from an Ordovician succession of the Scandinavian Caledonides. Interbedded with deep-marine turbidites, pillow basalts evolve from depleted-MORB (εNdt 9.4) to enriched-MORB (εNdt 4.8) stratigraphically upward, reflecting increasingly deeper melting of asthenospheric mantle. Intercalated intermediate to felsic lava and pyroclastic units, dated at ca. 474−469 Ma, are extremely enriched in incompatible trace elements (e.g., Th) and have low εNdt (−8.0 to −6.6) and high Sri (0.7089−0.7175). These are interpreted as ultrapotassic magmas derived from lithospheric mantle domains metasomatized by late Paleoproterozoic to Neoproterozoic crust-derived material (isotopic model ages 1.7−1.3 Ga). Detrital zircon spectra reveal a composite source for the interbedded turbidites, including Archean, Paleo-, to Neoproterozoic, and Cambro-Ordovician elements; clasts of Hølonda Porphyrite provide a link to the Hølonda terrane of Laurentian affinity. The entire volcano-sedimentary succession is interpreted to have formed in a rift basin that opened along the Laurentian margin as a result of slab rollback subsequent to arc-continent collision, ophiolite obduction and subduction polarity flip. The association of MORBs and ultrapotassic rocks is apparently a unique feature along the Caledonian-Appalachian orogen. Near-analogous modern settings include northern Taiwan and the Tyrrhenian region of the Mediterranean, but other examples of strictly concurrent MORB and ultrapotassic volcanism remain to be documented.

Collision‐Induced Subduction Polarity Reversal Explains the Crustal Structure of Northern Borneo: New Results From Virtual Deep Seismic Sounding (VDSS)

“…This entire sequence of events took place over a period of <20 million years, which is comparable to the Laurentian SPR episode discussed by Gasser et al. (2022) and the 2‐D SPR numerical simulation of the Solomon back‐arc basin by Wang et al. (2022).…”

“…(2022). The continent‐continent collision‐induced SPR in northern Borneo sets it apart from observations and numerical models of arc‐continent and arc‐plateau collision (e.g., Almeida et al., 2022; Gasser et al., 2022; Hagke et al., 2016; Stern & Gerya, 2018; Wang et al., 2022) in several ways, including localized back‐arc rifting of continental crust and a more substantial mountain range.…”

“…Useful constraints can also be found in the geological record; for instance, Gasser et al. (2022) study the Laurentian Iapetus margin and find evidence for all key stages of SPR from a detailed petrological analysis of Ordovician outcrop. Although SI is largely thought to be compression‐driven, numerical modeling suggests that slab break‐off can induce mantle flow, which can cause back‐arc lithosphere to rupture and be pulled down into the mantle (Wang et al., 2022).…”

SASSIER22: Full‐Waveform Tomography of the Eastern Indonesian Region That Includes Topography, Bathymetry, and the Fluid Ocean

“…In particular, we observe a westward dipping slab beneath the southern end of the Philippine Trench and an eastward dipping slab that exhibits a reversal in subduction polarity from 100 km down to the mantle transition zone. Such a subduction polarity flip has previously been suggested for settings where two oceanic plates collide (Gasser et al., 2021; Su et al., 2019). Furthermore, P.‐F.…”

SASSIER22: Full-waveform tomography of the eastern Indonesian region that includes surface topography and the fluid ocean

“…Such a subduction polarity flip has previously been suggested for settings where two oceanic plates collide (Su et al, 2019;Gasser et al, 2021). Furthermore, Chen et al ( 2020) suggest an east-dipping slab from 80 to 300 km depth at 1 -6 • N latitude based on the ISC-EHB (Engdahl et al, 2020) earthquake distribution for this region.…”