Radiogenic isotopes document the start of subduction in the Western Pacific

Abstract

Subduction initiation is one of the least understood aspects of plate tectonics. In an effort to obtain the first in situ magmatic record of subduction initiation, the International Ocean Discovery Program Expedition 352 drilled at four sites in the inner trench wall of the Bonin Trench to recover 1.22 km of oceanic upper crust accreted within a few m.y. of subduction initiation. The two sites nearer to the trench (U1440 and U1441) yielded axial and off-axis fore-arc basalts (FAB), while those c. 15 km further from the trench (U1439 and U1442) yielded axial low-silica boninites and high-Mg andesites overlain by off-axis high-silica boninites. This study uses Hf-Nd-Sr-Pb isotope analyses from c. 50 stratigraphically representative core samples to trace the evolution of the mantle source during the brief period of FABthrough-boninite magmatism immediately following subduction initiation. Results show that: 1) the FAB have high εHf relative to εNd and were derived from variably depleted mantle of 'Indian' provenance with no detectable subduction input; 2) the axial boninites follow mixing trends between a residual FAB mantle source and a subduction component derived from shallow (amphibolite facies) melting of oceanic crust of 'Pacific' provenance; and 3) the off-axis boninites define mixing trends between a hybrid mantle wedge (residual mantle + slab melt) and an additional subduction component with lower εNd and higher 207 Pb/ 204 Pb that requires a significant contribution from pelagic sediment. This incoming of pelagic sediments may signify a change from an accretionary to non-accretionary margin as subduction evolves. The results thus indicate a rapidly evolving system in terms of geodynamics, magma genesis and crustal accretion immediately following subduction initiation.

Subduction initiation in the southeastern Palaeo‐Asian Ocean: Constraints from early Permian adakites in suprasubduction zone ophiolites, central Inner Mongolia, North China

“…By comparing the formation models of the initiation of subduction in the IBM system (Li et al, ; Reagan et al, ), a model for subduction initiation in the Meilaotewula intra‐oceanic arc during the late Carboniferous to early Permian can be deduced (Figure ). Late Carboniferous intra‐oceanic subduction initiation in the southeastern PAO began along a transform fault or fracture zone (Figure a).…”

“…Evolution of the late Carboniferous to early Permian Meilaotewula intra‐oceanic forearc system in central Inner Mongolia, based on the subduction infancy model (Li et al, ; Reagan et al, ; Whattam & Stern, ). The region may record initiation of subduction in the southeastern PAO.…”

Reply to referee Y. Kusano's comment

“…On the basis of the Suhaylah Village section, it has long been held that Geotimes locally tops the volcanic sequence, being directly overlain by Suhaylah Formation sediments, and therefore that the Alley strata are discontinuous (Fleet and Robertson, 1980;Lippard et al, 1986). However, the presence of Geotimes at the top of this section was apparently not confirmed by geochemical analyses.…”

“…The first regional-scale geological map of the Semail ophiolite with a differentiated lower and upper extrusive sequence was compiled by Lippard et al (1986) at a scale of 1 : 250 000. This work was expanded upon by the 1 : 50 000 and 1 : 100 000 maps produced by Japanese and French teams together with the Ministry of Petroleum and Minerals (BME, 1987a, b, c;BRGM, 1986aBRGM, , b, 1993a.…”

Subduction initiation and igneous petrogenesis

“…Based on isotopic compositions, Li et al (2018) found that LSB and HSB magmas are likely mixtures between a melt generated from a variably depleted Indian mantle source, and at least 2 with depleted mantle to produce LSB is likely derived from Pacific oceanic crust . In a similar manner, a mixing relationship between the residual mantle (post-LSB extraction) and a component derived from Pacific pelagic sediment was used to show that HSB likely involve melting sediment in addition to melting of basaltic crust .…”