Sediment melting during subduction initiation: Geochronological and geochemical evidence from the Darutso high‐Mg andesites within ophiolite melange, central Tibet

Abstract

In addition to fluids, the concept of sediment‐derived melts infiltrating the fore‐arc mantle during subduction initiation has been proposed based on studies of modern subduction zones and ophiolite mélange. However, outcrops that contain the products of such melts are rare, especially in conjunction with boninite. New U‐Pb zircon dating reveals that the Darutso volcanic rocks (DVRs) within ophiolitic mélange in the Beila area, central Tibet, crystallized at ∼164–162 Ma. This is the first recognition of Jurassic volcanic rocks in the middle section of the Bangong‐Nujiang Suture Zone. Geochemically, the DVRs are high‐Mg andesites with moderate SiO2 (59.03–63.62 wt %) and high MgO (3.74–6.53 wt %), Cr (up to 395 ppm), and Mg# (50.3–67.9). They also have high Th contents, (La/Sm)N ratios, and (87Sr/86Sr)i values (0.7085–0.7147); low Ba/Th, U/Th, and Sr/Y ratios; and negative values of εNd(t) (−8.7 to −9.8) and zircon εHf(t) (−7.4 to −9.9). The εNd(t) values of the DVRs overlap those of regional sediments. Detailed analyses of these geochemical characteristics indicate that the DVRs were derived from partial melting of subducted sediments and subsequent interaction with overlying mantle peridotite in a shallow and hot setting. In combination with the regional geology, in particular adjacent ophiolites that contain MORB‐like and boninite mafic lavas, these rocks collectively recorded the evolution of a fore‐arc setting during the initiation of the northward subduction of the south branch of the Bangong‐Nujiang Ocean. Therefore, the results provide direct evidence for sediment melting during subduction initiation and constrain the Jurassic tectonic evolution of the Lhasa terrane.

Permian lateral crustal growth and reworking along the Solonker suture zone (North China) constrained by high εHf(t) igneous rocks

“…The Elitu samples are high‐Mg andesites. For the high‐Mg andesitic magmas, three different petrogenetic origins have been proposed, involving (a) the delaminating eclogitic crust (Tatsumi, ; Yang et al, ; Zeng, Chen, Xu, Wang, & Huang, ), (b) a young and hot slab (Li et al, ; Suzuki & Tatsumi, ) or (c) slab‐derived fluid/melt metasomatised source (Ma et al, ; Suzuki & Tatsumi, ; Zeng et al, ).…”

“…Some Cenozoic high‐Mg andesites are formed by partial melting of delaminating eclogitic crust in continental rifting or extension settings (Tatsumi, 2006; Yang et al, 2015; Zeng et al, ). These rocks often show high SiO 2 contents and enriched zircon Lu‐Hf compositions.…”

“…(Li et al, 2013;Suzuki & Tatsumi, 2006) or (c) slab-derived fluid/melt metasomatised source (Ma et al, 2016;Suzuki & Tatsumi, 2006;Zeng et al, 2016). Some Cenozoic high-Mg andesites are formed by partial melting of delaminating eclogitic crust in continental rifting or extension settings (Tatsumi, 2006;Yang et al, 2015;Zeng et al, 2016). These rocks often show high SiO 2 contents and enriched zircon Lu-Hf compositions.…”

“…Red circles mark the analytical site [Colour figure can be viewed at wileyonlinelibrary.com] 5 | PETROGENESIS OF THE EARLY PERMIAN IGNEOUS ROCKS 5.1 | High-Mg andesite The Elitu samples are high-Mg andesites. For the high-Mg andesitic magmas, three different petrogenetic origins have been proposed, involving (a) the delaminating eclogitic crust(Tatsumi, 2006;Yang et al, 2015;Zeng, Chen, Xu, Wang, & Huang, 2016), (b) a young and hot slab…”

Geochemistry and Origin of the Eastern Sayan Ophiolites, Tuva-Mongolian Microcontinent (Southern Siberia)

“…Compositions of the Dunzhugur dikes reported by [Sklyarov et al, 2016] are shown. Dunzhugur dikes [Sklyarov et al, 2016] La/Yb (PM) εNd ported in Oman ophiolite [Haase et al, 2015] and some Tibetian ophiolites [Zeng et al, 2016]. The studied Eastern Sayan ophiolites might record similar case of sediment melt participation in ophiolite petrogenesis.…”