Vol.:(0123456789) 1 3 Contrib Mineral Petrol (2017) 172:19 DOI 10.1007/s00410-017-1332-6 DISCUSSION Discussion: “Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust” by Wang et al. 2016 (Contributions to Mineralogy and Petrology) 171:62 Aleksandr S. Stepanov 1  · Ian Campbell 2  · Robert P. Rapp 2  · Jessica Lowczak 2  · Andrey V. Korsakov 3   Received: 28 October 2016 / Accepted: 21 January 2017 © Springer-Verlag Berlin Heidelberg 2017 Introduction The study by Wang et al. (2016a) documents an interest- ing suite of xenoliths from ultrapotassic dykes in southern Tibet. We compliment the authors on their careful study and agree that the rocks represent “an excellent example of in situ crust–mantle hybridization in the deep Tibetan crust”. However, we question their principal conclusion that the rocks are derived by partial melting of “…an old, metasomatized subcontinental lithospheric mantle”. On the contrary, the data presented by Wang et al. (2016a) show clearly that melts of shoshonitic afnity can be generated by the direct melting of crustal rocks, followed by variable interaction with mantle peridotite, as suggested by Camp- bell et al. (2014), Stepanov et al. (2014) and Stepanov et al. (2016). Major and trace elements The major and trace element compositions of ultrapotassic rocks are the key to understanding their origin. Although Wang et al. (2016a) do not present whole rock composi- tions for the volcanic rocks that host the xenoliths they stud- ied, relevant analyses are available in the literature (Turner et al. 1996; Miller et al. 1999; Ding et al. 2003; Wang et al. 2015). The ultrapotassic rocks from South Tibet can be called a shoshonite suite, as defned by Morison (1980), and are similar to those from eastern Tibet: both suites show a wide range of SiO 2 (44–78 wt.%), which correlates negatively with MgO (0.16–10 wt.%), and have a high con- centration of incompatible elements. The arguments against formation of such an association by fractional crystalliza- tion were outlined by Campbell et al. (2014). The rocks described by Wang et al. (2016a) contain abundant crustal Abstract Wang et al. (Contrib Mineral Petrol 171:62, 2016a) present data on composition of xenolith from Southern Tibet and conclude that ulrapotassic melts from the region formed by melting mantle, and complex interac- tion with a crustal component. In this discussion we dem- onstrate that numerous observations presented by Wang et al. (2016a) can be explained by partial melting of crust followed by interaction between that melt and the man- tle. We show that this model can explain the variability of magmas in such suits without evoking occurrence of coin- cidental, unrelated events. Moreover we demonstrate that our model of a crustal origin of the proto-shoshonite melts is now supported by independent lines of evidence such as geochemistry of restites after high- and ultrahigh- pressure melting and melt inclusion studies. Keywords Ultrapotassic magmas · Shoshonite · Xenolith · Crust–mantle interaction Communicated by Hans Keppler. * Aleksandr S. Stepanov sashas@utas.edu.au 1 ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Private Bag 79, Tas 7001, Australia 2 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia 3 V.S. Sobolev Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences, Koptyug Pr. 3, Novosibirsk 630090, Russia