Editing Subduction (section)

==Beginnings of subduction on Earth==
{{See also|Archean subduction}}

Modern-style subduction is characterized by low [[geothermal gradient]]s and the associated formation of high-pressure low-temperature rocks such as [[eclogite]] and [[blueschist]].<ref name=Xuetal2018/><ref name=Stern2005>{{cite journal |last1=Stern |first1=Robert J. |title=Evidence from ophiolites, blueschists, and ultrahigh-pressure metamorphic terranes that the modern episode of subduction tectonics began in Neoproterozoic time |journal=Geology |volume=33 |issue=7 |pages=557–560 |doi=10.1130/G21365.1 |bibcode=2005Geo....33..557S |year=2005 |s2cid=907243 }}</ref> Likewise, rock assemblages called [[ophiolites]], associated with modern-style subduction, also indicate such conditions.<ref name=Xuetal2018/> [[Eclogite]] [[xenolith]]s found in the [[North China craton|North China Craton]] provide evidence that modern-style subduction occurred at least as early as 1.8&nbsp;[[Gigaannum|Ga]] ago in the [[Paleoproterozoic|Paleoproterozoic Era]].<ref name=Xuetal2018/> The eclogite itself was produced by oceanic subduction during the assembly of supercontinents at about 1.9–2.0 Ga.

[[Blueschist]] is a rock typical for present-day subduction settings. The absence of blueschist older than [[Neoproterozoic]] reflects more [[mafic|magnesium-rich]] compositions of Earth's [[oceanic crust]] during that period.<ref name=PaWh2016/> These more magnesium-rich rocks metamorphose into [[greenschist]] at conditions when modern oceanic crust rocks metamorphose into blueschist.<ref name=PaWh2016/> The ancient magnesium-rich rocks mean that [[mantle (geology)|Earth's mantle]] was once hotter, but not that subduction conditions were hotter. Previously, the lack of pre-Neoproterozoic blueschist was thought to indicate a different type of subduction.<ref name=PaWh2016>{{cite journal |last1=Palin |first1=Richard M. |last2=White |first2=Richard W.|date=2016 |title=Emergence of blueschists on Earth linked to secular changes in oceanic crust composition |url= https://ora.ox.ac.uk/objects/uuid:48630722-57a2-4dd7-8101-92ea1c8df8a1|journal=Nature Geoscience |volume=9 |issue=1 |pages= 60|doi= 10.1038/ngeo2605|bibcode=2016NatGe...9...60P |s2cid=130847333 }}</ref> Both lines of evidence refute previous conceptions of modern-style subduction having been initiated in the [[Neoproterozoic|Neoproterozoic Era]] 1.0 Ga ago.<ref name=Xuetal2018>{{cite journal |last1=Xu |first1=Cheng |last2=Kynický |first2=Jindřich|last3=Song |first3=Wenlei|last4=Tao |first4=Renbiao|last5=Lü |first5=Zeng|last6=Li |first6=Yunxiu|last7=Yang |first7=Yueheng|last8=Miroslav |first8=Pohanka|last9=Galiova |first9=Michaela V.|last10=Zhang |first10=Lifei|last11=Fei |first11=Yingwei  |date=2018 |title=Cold deep subduction recorded by remnants of a Paleoproterozoic carbonated slab |journal=Nature Communications |volume= 9|issue= 1|pages= 2790|doi= 10.1038/s41467-018-05140-5|pmid=30018373 |pmc=6050299 |bibcode=2018NatCo...9.2790X }}</ref><ref name=PaWh2016/>