Counter-clockwise prograde P-T path in collisional orogeny and water subduction at the Precambrian-Cambrian boundary: The ultrahigh-pressure pelitic schist in the Kokchetav massif, northern Kazakhstan

A prograde pressure-temperature (P-T) path is estimated for pelitic schists from the latest Precambrian Kokchetav ultrahigh-pressure massif, Kazakhstan, using compositional zoning and mineral inclusions in coarse-grained and inclusion-rich garnets. Ti-bearing inclusions are abundant in garnet and display a zonal distribution. Ilmenite occurs in the inner-core, where most of it makes a composite inclusion with rutile, whereas monomineralic rutile occurs in the outer-core to mantle domains. In the rim region both ilmenite and rutile are present, although in small amounts. Application of the ilmenite-garnet thermometer yields a systematic temperature increase towards rim from 500 to 750 °C. The pressure-sensitive reaction: 3 Fe-Ilm (in Ilm) + Ky + 2 Qtz = 3 Rt + Alm (in Grt) yielded pressures of 1.2-1.3 GPa for the outer-core inclusions. A petrogenetic grid in the K₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O model system was used to estimate the equilibrium compositions of the garnet. The change of the grossular component along the model P-T path expected from the forward modelling is close to the observed compositional profile of the outer-core to rim domains. No constraint is available from thermobarometry in the inner-core; however, the forward modelling of garnet zoning provides information on the early stage of the P-T path during the garnet growth. The estimated P-T path is counter-clockwise in the prograde stage with a steep bend at around 700 °C and 1.2-1.5 GPa. This is similar to the metamorphic P-T gradient of the Kokchetav massif. This result contrasts markedly with the traditional clockwise P-T path in many collisional metamorphic terranes, and is regarded to represent a subduction geotherm at the Precambrian-Cambrian boundary. The P-T path proposed in this study also supports the models for the recovery of the "snowball Earth" from late-Proterozoic glaciation through effect of water in the solid Earth mantle.