An origin of ultraslow spreading ridges for the Yarlung-Tsangpo ophiolites

Abstract

As relics of ancient ocean lithosphere, ophiolites are the most important petrological evidence for marking the sutures and also play a key role in reconstructing plate configuration. They also provide valuable windows for studying crustal accretion and mantle processes occurring at modern ocean ridges. Abundant ophiolites are distributed along the Yarlung-Tsangpo suture and represent the relics of ocean lithosphere of the Neo-Tethys. They are characterized by an incomplete litho-stratigraphy, of which the mantle section is much thicker than the crustal section. Ocean crustal rocks outcropped in the Yarlung-Tsangpo ophiolites are much thinner than normal ocean crusts (~ 7 km) or even absent. Tectonic settings from which the Yarlung-Tsangpo ophiolites originated remain highly controversial, although an origin of the supra-subduction zone is prevailing. Moreover, their incomplete litho-stratigraphy has been commonly attributed to tectonic dismemberment during the late-stage emplacement after their formation. Nevertheless, such an incompleteness resembles the ocean lithosphere generated at modern ultraslow spreading ridges, such as the Southwest Indian Ridge (SWIR). In this paper, we present several lines of evidence that support the formation of the Yarlung-Tsangpo ophiolites at ultraslow spreading ridges, during which detachment faults were developed. This suggests that the Yarlung-Tsangpo ophiolites might represent the ocean core complexes (OCC) in the Neo-Tethys Ocean. The OCC with high topography in the seafloor were clogged in the trench and preserved as ophiolites through Indo-Eurasia collision. The clogging resulted in the demise of an old subduction and a new subduction was re-initiated beneath the clogged OCC.

Keywords: Subduction re-initiation; Tibetan plateau; Ultraslow spreading ridges; Yarlung-Tsangpo ophiolites.

Fig. 1

Sketch map of the Yarlung-Tsangpo ophiolites, modified after Dai et al.. Zircon U-Pb ages of representative ophiolites are compiled in , and hornblende Ar-Ar ages of metamorphic soles are from [23,25].

Fig. 2

Field photos of mafic dykes intruding mantle peridotites of the Yarlung-Tsangpo ophiolites. (a-b) Sheeted diabase sills intruding mantle peridotites, with development of chilled margins (c). (d-e) Mafic dykes have been transformed to rodingites.

Fig. 3

Field photos of gabbronorites in the Yarlung-Tsangpo ophiolites. (a-b) gabbronorite patches and (c-d) gabbronorite veins in the peridotites.

Fig. 4

Major elements of mantle peridotites from the Yarlung-Tsangpo ophiolites. (a) Whole rock MgO vs Al₂O₃ contents; (b) Spinel Mg# vs Cr# values. Data of the Yarlung-Tsangpo mantle peridotites are from [20,27,28,31,33,[35], [36], [37], [38], [39],42,58,95], abyssal peridotites are from , and forearc peridotites are from .

Fig. 5

Whole rock Os isotopes of mantle peridotites from the Yarlung-Tsangpo ophiolites. Data of the Yarlung-Tsangpo mantle peridotites are from [28,31,37,38,61] and abyssal peridotites are from , , , , .

Fig. 6

Chemical variations of gabbros from the Jiding ophiolite(a) and the Atlantis Bank, Southwest Indian Ridge(b).

Fig. 7

Clinopyroxene Mg# vs plagioclase An of gabbronorites and gabbros in the Yarlung-Tsangpo ophiolites. The fractionation trajectories are from . Data of the Purang gabbronorite dykes are from . Data of gabbronorite patches and gabbro dykes are unpublished.

Fig. 8

Zircon U-Pb ages of ocean crustal rocks of the Yarlung-Tsangpo ophiolites. Data are compiled in .

Fig. 9

Cartoon for the subduction re-initiation model. (a) At ~ 130 Ma, ocean ridges in the Neo-Tethys Ocean were proximal to the Eurasian continental margins and have been slowed down to an ultraslow spreading rate. Detachment faults were developed at the ocean ridges, which resulted in the exhumation of mantle peridotites and occasionally the gabbros at the seafloor as ocean core complexes (OCC). (b) The ocean core complexes, with incomplete ocean lithospheres, were transported into the trench due to the northward subduction. They cannot be subducted due to their higher topography relative to the ambient ocean crust, and thus clogged in the trench. The clogging led to the demise of the old subduction to the north and a new subduction was re-initiated in the south of the ocean core complexes. A metamorphic sole with high temperature metamorphism was produced beneath at the bottom of the mantle section. Meanwhile, mantle peridotites of the clogged ocean core complexes might have been modified by fluids or melts released from the slabs, giving rise to the SSZ-like geochemical features.