Hornblende, the continent maker—Evolution of H2O during circum-Pacific subduction versus continental collision

Abstract

Research Article| August 01, 1999 Hornblende, the continent maker—Evolution of H2O during circum-Pacific subduction versus continental collision W. G. Ernst W. G. Ernst 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (8): 675–678. https://doi.org/10.1130/0091-7613(1999)027<0675:HTCMEO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W. G. Ernst; Hornblende, the continent maker—Evolution of H2O during circum-Pacific subduction versus continental collision. Geology 1999;; 27 (8): 675–678. doi: https://doi.org/10.1130/0091-7613(1999)027<0675:HTCMEO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Andesitic arcs are sited over convergent lithospheric plate junctions. Aqueous fluids driven off the downgoing slab are responsible for partial fusion of the warming subducted plate and/or the overlying mantle wedge, and the ascent of calc-alkaline melts. Various hydrous minerals have been proposed to be the source of this H2O. Experimental equilibrium studies show that, under subduction-zone geothermal gradients of 5–7 °C/km, clinoamphibole constitutes a major phase in deep-seated (>75 km) metabasalts; other hydrous minerals are absent or are of very minor abundance. Clinoamphiboles dehydrate at pressures of ∼2.2–2.4 GPa under equilibrium conditions (pressure overstepping is probable), so mafic blueschists and amphibolites expel H2O at magmagenic depths. Partly serpentinized mantle beneath the oceanic crust devolatilizes at comparable to slightly higher pressures. However, micas remain stable in pelitic and granitic gneisses to pressures far exceeding 4.0 GPa, so at subduction depths >100 km, micaceous lithologies characterizing the upper and middle sialic crust fail to evolve significant H2O. Deep underflow of hydrated oceanic lithosphere thus generates most of the volatile flux along and/or above a subduction zone prior to continental collision. As large masses of quartzofeldspathic material enter a suture zone, volatile evolution at deep levels nearly ceases. While small amounts of peraluminous, S-type anatectic melts may be produced, I-type calc-alkaline arcs—and the continents—owe their formation over geologic time to the sustained underflow of oceanic lithosphere. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.