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Depending who you ask you will get two answers: Early Cretaceous or Permian.

The rationale is as follows: the passive margin stratigraphy of the Tibetan Himalaya shows evidence for upper Carboniferous-Permian rifting, and contains upper Permian volcanics that are interpreted as break-up volcanism, see the work of Edo Garzanti on this subject. Had there been no intra-oceanic subduction within the Neotethys, then this was the lithosphere that subducted below southern Tibet just prior to the Tibetan Himalaya-Lhasa collision.

There is, however, a band of ophiolites overlying the Tibetan Himalaya with supra-subduction zone, arc, or back-arc geochemical signatures and ~130-110 Ma ages (see review by Hebert et al, Gondwana Research 2012; I’m bluntly including the Kohistan arc in this sequence here), which demonstrate that there has been intra-oceanic subduction since that time between India and Asia. There are two schools of thought on where that subduction occurred. One suggests this occurred at the equator, one suggests that this intra-oceanic subduction occurred immediately adjacent to the Lhasa terrane, in the forearc between the Tibetan trench and the Gangdese arc.

If subduction below the ophiolite occurred at the equator, the Permian oceanic lithosphere was consumed then and there, and the age of the lithosphere that subducted just prior to Tibetan Himalaya-Lhasa collision was Early Cretaceous in age. Otherwise, you can still use Permian.

I would choose the Permian option for the following reasons. The arguments for an equatorial intra-oceanic subduction zone were based on paleomagnetism (Abrajevitch et al 2005) and seem consistent, at least in NW India, with mantle structure inferred from tomography. We have recently re-visited the pmag of the ophiolites near Xigaze and shown that the apparent low latitudes are a paleomagnetic artefact due to compaction: stratigraphic and paleomagnetic constaints strongly suggest that those ophiolites formed immediately adjacent to the Lhasa terrane (Huang et al., EPSL 2015; Maffione et al.. Geology 2015). Paleomagnetic data from the Tibetan Himalaya show that it was at the equator ~70-65 Ma (see overview in Huang et al., GRL 2015), so if ophiolites were emplaced at the equator, it must have happened around that time. This was originally interpreted to be the case based on the finding of mafic debris in Tibetan Himalaya sediments of this age (Searle 1987), but Garzanti and Hu (Gondwana Research 2015) recently showed that this mafic debris has a chemistry consistent with hotspot volcanism instead of the ophiolites. Pmag from the Kohistan arc was recently quoted to show an equatorial position (Jagoutz et al Nature Geo 2015), but the original authors of those data (e.g. Zaman and Torii GJI 1999) argue that the sediments from which these equatorial latitudes were measured were derived from north of the Shyok suture, i.e. from Tibetan terranes. Their results require 4000 km of intra-Asian convergence accommodated north of the Shyok suture in the Cenozoic, which is inconsistent with pmag from southern Tibet (Huang et al GRL 2015 and refs therein). In any case, the jury for the collision history of Kohistan and Indian in the NW is still out as far as I’m concerned; East of Kohistan, along most of the suture, a Permian age of the oceanic crust subducting just prior to the Tibetan HImalaya-Lhasa collision seems to me the best estimate.

Hope this helps!

Cheers

Douwe