Figure 7: The tectonic setting of the Alxa HP Grt-websterite.

The cartoon illustrates the situation when the northern margin of the NCC collided with the southern Siberian craton at∼1.82 Ga. Websterite in oceanic crust (ophiolite) with sedimentary and other crustal rocks (marked by 1) is subducted to depth in the mantle giving rise to HP metamorphism of the Grt-websterite at a pressure of ∼2.4 GPa (marked by 2). The HP rocks were exhumed to the lower crust where granulite facies conditions record ∼0.9 GPa pressure at 1.82 Ga (marked by 3). The possible positions of the HP Alxa and Sharyzhalgai complexes are suggested. The subduction–exhumation paths from 1 to 2 to 3, correlate with those in the PT diagram in Fig. 5. The reconstructed pseudostratigraphy of the ultramafic rocks, gabbro, basalt, mudstone and limestone is comparable to that of an ophiolite. The harzburgite and lherzolite are from location C in Supplementary Fig. 1.