Initiation of ultrahigh‐pressure metamorphism and its significance on the Proterozoic–Phanerozoic boundary

Ultrahigh‐pressure (UHP) metamorphism began at 540–530 Ma near the Precambrian–Cambrian boundary, and repeated more than 10 times through the Phanerozoic, indicating one of the most prominent tectonic aspects of the Phanerozoic Earth. The preceding subduction zone metamorphism back to the Archean had been of the intermediate‐ to low‐P type. An apparent change in the P/T conditions of subduction zone metamorphism occurred at the Precambrian–Cambrian boundary at ca 544 Ma. At ∼ 200 million years earlier than the initiation of UHP metamorphism, blueschist facies metamorphism occurred along the northern margin of the Tarim craton. A transitional period between the Proterozoic and Phanerozoic at 750–540 Ma corresponds to the initiation of blueschist facies metamorphism at the subduction zone; the cooling Earth dropped its geothermal gradient which intersected the blueschist facies P–T field. An abrupt change in the geothermal gradient from 22–15 °C/km (Archean–Proterozoic) to 6–7 °C/km (Phanerozoic) to initiate UHP metamorphism may correspond to a rather abrupt increase of slab‐pull driving force at the beginning of the Phanerozoic. Large amounts of heat loss from the Earth's interior may have occurred just before 540 Ma; presumably it may correspond to the duration from the breakup of supercontinent Rodinia at 750 Ma, to the formation of the next supercontinent Gondwana at 540 Ma. Through the transient period 750–540 Ma the most extensive glaciation in Earth's history ended at ca 600 Ma, when the world's largest‐scale unconformity covered nearly all the continents on the Earth. Huge landmasses from ∼ 5 to 30% of the Earth's surface first raised above sea‐level during 600–540 Ma; such change may have initiated since ca  750 Ma, and repeated four times until 540 Ma. Emergence of large continental masses above sea‐level resulted in extensive erosion and diversity of surficial environment, and significant modification in the composition of seawater and world climates. Coincidentally or accidentally with the breakup of the latest Proterozoic supercontinent, the Cambrian life explosion began after the transient period in which the Ediacara fauna evolved along rifted seaways with hot springs in a partly rifted supercontinent. The complex scenario of these phenomena may be triggered and linked by a sudden increase in the plate‐driving force of slab‐pull during 750–540 Ma, which allowed buoyant continental crust to be subducted to a 150 km depth and subjected to UHP metamorphism. The increased slab‐pull force reflects an increase in plate thickness, and is the surface manifestation of the loss of a large amount of internal heat from the Earth's interior before the initiation of UHP metamorphism. It occurred in a short time span (750–540 Ma) from rifting of Rodinia to amalgamation of the next supercontinent Gondwana. Since then, old slabs with deep oceans survived, large landmasses raised above sea‐level, the surface environment diversified, and the explosion of life accelerated before and after the mass extinction that occurred at 550–540 Ma.