The formation of Earth's early felsic continental crust by water-present eclogite melting

[摘要] ENGLISH ABSTRACT: The sodic and leucocratic Tonalite, Trondhjemite and Granodiorite (TTG) granitoid series ofrocks characterise Paleo- to Meso- Archaean felsic continental crust, yet are uncommon in thepost-Archaean rock record. Consequently, petrogenetic studies on these rocks providevaluable insight into the creation and evolution of Earth's early continental crust. The highpressure(HP)-type of Archaean TTG magmas are particularly important in this regard as theirgeochemistry requires that they are formed by high-pressure melting of a garnet-rich eclogiticsource. This has been interpreted as evidence for the formation of these magmas by anatexisof the upper portions of slabs within Archaean subduction zones. In general, TTG magmashave been assumed to arise through fluid-absent partial melting of metamafic source rocks.Therefore, very little experimental data on fluid-present eclogite melting to produce ArchaeanTTG exist, despite the fact that water drives magmatism in modern arcs. Consequently, thisstudy experimentally investigates the role of fluid-present partial melting of eclogite-faciesmetabasaltic rock in the production of Paleo- to Meso-Archaean HP-type TTG melts.Experiments are conducted between 1.6 GPa and 3.0 GPa and 700 ºC and 900 ºC usingnatural and synthetic eclogite, and gel starting materials of low-K2O basaltic composition.Partial melting of the natural and synthetic eclogite occurred between 850 ºC and 870 ºC atpressures above 1.8 GPa, and the melting reaction is characterised by the breakdown of sodicclinopyroxene, quartz and water: Qtz + Cpx1 + H2O ± Grt1 = Melt + Cpx2 ± Grt2. Theexperimental melts have the compositions of sodic peraluminous trondhjemites and havecompositions that are similar to the major, trace and rare earth element composition of HPtypeArchaean TTG. This study suggests that fluid-present eclogite melting is a viable petrogenetic model for this component of Paleo- to Meso-Archaean TTG crust. The nature ofthe wet low-K2O eclogite-facies metamafic rock solidus has been experimentally defined andinflects towards higher temperatures at the position of the plagioclase-out reaction. Therefore,the results indicate that a crystalline starting material is necessary to define this solidus toavoid metastable melting beyond temperatures of the Pl + H2O + Qtz solidus at pressuresabove plagioclase stability. Furthermore, this study uses numerical and metamorphic modelsto demonstrate that for reasonable Archaean mantle wedge temperatures within a potentialArchaean subduction zone, the bulk of the water produced by metamorphic reactions withinthe slabs is captured by an anatectic zone near the slab surface. Therefore, this geodynamicmodel may account for HP-type Archaean TTG production and additionally providesconstraints for likely Archaean subduction. The shape of the relevant fluid-present solidus issimilar to the shape of the pressure-temperature paths followed by upper levels of theproposed Archaean subducting slab, which makes water-fluxed slab anatexis is verydependant on the temperature in the mantle wedge. I propose that cooling of the upper mantleby only a small amount during the late Archaean ended fluid-present melting of the slab. Thisallowed slab water to migrate into the wedge and produce intermediate compositionmagmatism which has since been associated with subduction zones.

[发布机构] Stellenbosch University