THE IMPORTANCE OF HP/LT AND UHP/MT ROCKS AND MINERALS FOR UNDERSTANDING WHEN PLATE TECTONICS BEGAN

Plate Tectonics is a planetary cooling style characterized by independently moving lithospheric fragments, set in motion by sinking of dense lithosphere in subduction zones. From a kinematic perspective “Plate Tectonics" (pTec) is an appropriate description, but from a dynamic perspective it is best called “Subduction Tectonics" (sTec). There is little agreement about when pTec/sTec began on Earth; estimates range from >4.2 Ga to ~0.85 Ga. The temporal distribution of HP/LT and UHP/MT assemblages provides a key constraint on when subduction-like geothermal gradients existed, or at least when rocks cooked in these environments were returned to the surface. HP/LT represents jadeite-glaucophane facies series; glaucophane (Gln) and lawsonite (Lws) are critical phases. UHP/MT is metamorphism at P to form coesite (Coe) and/or diamond (Dia) but below T for phengite melting. The presence of HP/LT rocks such as blueschist (BS), Lws-eclogite (EC), and Gln-EC and related precious stones like jadeitite, along with UHP/MT EC are key petrotectonic indicators for pTec; we exclude Dia-bearing granulite, garnet peridotite and ophiolitic chromitite from “UHP/MT”. HP/LT indicates that oceanic crust was subducted to depths of 30-80 km whereas UHP/MT indicates that continental crust was subducted to ~120-160 km depth. Here is a list of seven petrotectonic indicators and their oldest ages [Ma]: 1) Coe-EC [640]; 2) Dia-EC [530]; 3) Jadeitite [520]; 4) BS [750]; 5) Gln-EC [620]; 6) Lws-BS [560] and 7) Lws-EC [470]. All 7 indicators have maximum ages in the range 750-470 Ma. Each indicator provides an important constraint on the history of subduction, but interpreting each individually is complicated because the existence of each record requires that subducted materials were subsequently returned to the surface. Such a record is likely to be incomplete because the vast majority of subducted materials are not returned to the surface. Considering the 7 suites together provides a more compelling constraint on when evidence for subduction metamorphism first was preserved in the geologic record, sometime between 750 and 450 Ma. Experts on these and similar rocks are best-suited to evaluate these powerful petrotectonic indicators, and this community is invited to contribute their unique perspectives on when pTec/sTec began.