Crust sequences of the Xigaze ophiolite in South Tibet: The water effect on crust accretion at a slow-spreading oceanic ridge and its implications

The tectonic setting of the Xigaze ophiolite of southern Tibet has been debated ever since it was first described 1981. It lies in the central part of the Yarlung Zangbo but differs from adjacent ophiolite by having a relatively thin crustal section, an oceanic core complex and complex geochemistry. It is overlain by radiolarian cherts suggesting and open ocean environment but also show geochemical evidence of suprasubduction zone magmatic activity. Our detailed study of this massif shows that the crustal sequence was formed by three stages of magma injection derived from a common parental source. Stage 1 is represented by heavily altered and deformed gabbros that form dikes and pods in the upper part of the peridotites, whereas stage 2 includes dolerite sills and extruded basalts, which form >80% of the crustal section. Finally, stage 3 is composed of dolerite dikes and gabbro intrusions, which crosscut rocks of stage 1 and 2 at high angles. The youngest rocks of stage 3 are plagioclase wehrlite intrusions, which are characterized by having anomalously high calcium plagioclase (An = 99–100) which was preceded by clinopyroxene. We suggest that the multiple magma injection were formed during tectonic extension and magma-fluid interaction at a slow-spreading rift in a distal fore-arc environment during the Early Cretaceous. Our study implies that caution must be exercised in applying subduction initiation models to ophiolite genesis, particularly when the geochemistry of the rocks contradicts the field observations and stratigraphic evidence.