Geodynamic implications of crustal lithologies from the southeast Mariana forearc

Abstract

The deep submergence research vehicle Shinkai 6500, diving on the Challenger segment of the Mariana forearc, encountered a superstructure of nascent arc crust atop a younger mantle with entrained fragments of metamorphosed crust. A plutonic block from this crust collected at 4900 m depth has a crystallization age of 46.1 Ma and mixed boninitic-arc tholeiitic geochemical signatures. A hornblende garnetite and two epidote amphibolites were retrieved from depths between 5938 m and 6277 m in an area dominated by peridotite. The garnetite appears to represent a crystal cumulate after melting of deep arc crust, whereas the amphibolites are compositionally similar to enriched mid-ocean ridge basalt (MORB). The initial isotopic compositions of these crustal fragments are akin to those of Eocene to Cretaceous terranes along the periphery of the Philippine plate. The garnetite achieved pressures of 1.2 GPa or higher and temperatures above 850 °C and thus could represent a fragment of the delaminated root of one of these terranes. This sample has coeval Sm-Nd, Lu-Hf, and 40 Ar-39 Ar ages indicating rapid ascent and cooling at 25 Ma, perhaps in association with rifting of the Kyushu-Palau arc. Peak P-T conditions were lower for the amphibolites, and their presence on the ocean floor near the garnetite might have resulted from mass wasting or normal faulting. The presence of relatively fusible crustal blocks in the circulating mantle could have contributed to the isotopic similarity of Mariana arc and backarc lavas with Indian Ocean MORB.

Glimpses of oceanic lithosphere of the Challenger Deep forearc segment in the southernmost Marianas: The 143°E transect, 5800–4200 m

“…Further sampling and study will be needed to address this point. Trace element compositions of the samples we studied along with young basaltic glasses (Stern et al, 2014) and previously reported 46.1 ±0.7 Ma diorite and tonalite (Reagan et al, 2018) are shown in Figure 7 and Figure 8; fields for SEMFR basalts to the east are also shown in Figure 8. These results are broadly consistent with inferences made from major elements above.…”

“…Comparing (a) and (e), note similar LREE depletion of 47.2 ±0.4 Ma of ol 2-pyx hornblendites and hornblende bearing basalts, suggesting these are comagmatic. Note also that both hornblendites and related basalts are more depleted than young basaltic glasses tonalite (6K-1236-R14) reported by Reagan et al (2018). Gabbros are common if not abundant components of the Mariana forearc dredged between 16 and 18 N (Bloomer, 1998).…”

“…Solid narrow lines reflect dive tracks reported in this study, dashed dive tracks are from previous studies, also 6K-1364 which recovered only peridotite. Also shown are where metamorphic garnetite and amphibolite from 6K-1232 and diorite/ tonalite from 6K-1236 were reported by Reagan et al (2018). Also shown are where three samples of volcanic breccia recovered by Shinkai 6500 dive 6K-1363 were reported by Stern et al (2014).…”

“…F I G U R E 7 Rare earth element patterns (left) and primitive mantle-normalized trace element patterns for samples studied here. Patterns for young basaltic glasses (Stern et al, 2014) and 46.1 ±0.7 Ma diorite and tonalite (Reagan et al, 2018) are also shown. In (b) note remarkable enrichment of K-708-R7 altered peridotite in Ba, U, and Sr.…”

“…Figure 8a shows that the trace element composition of igneous rocks studied here are mostly MORB-like and that 6K-1431 and 6K-1432 samples are similar in this regard to young basaltic volcaniclastic rocks (Stern et al, 2014) and SEMF basalts . Figure 8b shows that these samples show little subduction zone enrichment except for 6K-1432 andesite and diorite/tonalite studied by Reagan et al (2018).…”