Partial melting of the continental crust Migmatites-granulites, S SpainNanogranite, India Antonio Acosta-Vigil – IACT, CSIC-Univ. Granada Migmatites-granulites, S SpainNanogranite, India First talk: the continental crust and crustal anatexis Second talk: melt inclusions Partial melting of the continental crust Antonio Acosta-Vigil – IACT, CSIC-Univ. Granada Definition/nature of the continental crust ! Continental crust: defined based on seismic studies (Mohorovicic, P-waves). ! Composition: andesitic, with compositional features similar to subduction zones. ! Thickness: 20-60 km (40 km). Divided in three parts (seismics): 1.Upper continental crust: intermediate to acid (granodioritic). 2. Middle continental crust: intermediate (andesitic). 3. Lower continental crust: intermediate to basic (andesitic to basaltic). Data from Rudnick & Gao (2013) 12 km 23 km 40 km Metamorfic/ígneous (intermediate: diorites, tonalites, trondhjemites, granodiorites. Qtz, Pl, Bt, Amph, Px, Grt, Ms, Als. Metaigneous granulites, intermediate to basic. Qtz, Pl, Amph, Px, Bt. Sedimentary, metamorfic, and ígneous rocks (intermediate/acid). Qtz, Kfs, Pl, Grt, Bt, Ms, Crd, Als, Amph, Px. Composition of the continental crust Upper Middle Lower SiO2 66.6 63.5 54.3 TiO2 0.64 0.69 0.97 Al2O3 15.4 15.0 16.1 FeOt 5.04 6.02 10.6 MgO 2.48 3.59 6.28 CaO 3.59 5.25 8.48 Na2O 3.27 3.39 2.79 K2O 2.80 2.30 0.64 ASI 1.03 0.85 0.78 Data from Rudnick & Gao (2003) Generation of continental crust Hawkesworth et al. (2010) Numbers refers to Km3/yr Into the crust: 1.65 km3/yr -- Back to mantle: 1.3-1.8 km3/yr Cawood et al. (2009) Arndt & Davaille (2013) Zrn granitos orogénicos Zrn detríticos Key geodynamic settings today Back in time Early continental crust Oldest continental crust: zircons in metasedimentary sequences, Jack Hills, Australia (≈4.4-4.2 Ga), and some TTG, Acasta orthogneiss, Canada (≈4.2-4.0 Ga) Greenstone belts: supracortical sequences of komatiites-basalts and metasedimentsmetavulcanites; G-UHTM, hot weak lithosp. TTG: intermediate to acid Na gneisses, rich in LREE and Sr, poor in HREE, Y; Ep, 8-10 Kb TTG Greestone belts Arndt (2013) Cagnard (2011) A B 5 km 50 um 50 um Basin-and-dome orogens A B Early continental crust Davies (1992) Moyen and Martin (2012) STAGNANT LID REGIME-2b PLATE TECTONIC REGIME-2a Arndt (2013) Archaean hot subduction model modern subduction OC: oceanic crust CC: continental crust Red: melted crust ¿First nuclei of continental crust? Origin of early continental crust 1 2a 2b 2b Why is partial melting important? Partial melting: key process in the evolution of the Earth. core mantle EARTH CC: continental crust Inicio a los 4.4-4.2 Ga Moyen and Martin (2012) Hot subduction model Stagnant lid model Continental crust: produced by partial melting of the mantle since the Archean (4.4-4.2 Ga), and hence is key to understand the evolution of Earth.Not to scale Why is crustal melting important? CC: continental crust Inicio a los 4.4-4.2 Ga Moyen and Martin (2012) ! Importance of the study of origin of continental crust (4.4-4.2 Ga): formed progressively since the very beginning of the Earth: key to understand the history and geochemical evolution of the planet. ! The continental crust is not static, but changes/evolves/is reproccessed, after its generation. Compositional differentiation (vertical evolution in composition). Hence to know its origin it is necessary to characterize its evolution in time, and crustal melting (ANATEXIS), together with melt extraction and ascent, is thought to be one of the main processes producing compositional evolution. Brown (2004) Brown (2013) Problems genesis of continental crust andesitic basaltic rhyolite andesite basalt ultramafic (Ol, Px) modified from Stern (2002) ! Continental crust: formed by partial melting of, and melt extraction from the mantle through geologic time, mean andesitic composition (Taylor & McLennan, 1985; Rudnick & Gao, 2003). ! Genetic models: 1. “Andesite model”: addition of andesites in subduction zones (Taylor, 1967). 2. “Basalt imput model”: addition of basalts in subduction zones, plus reprocessing of the basaltic material to make it andesitic (Davidson & Arculus 2006). Davidson & Arculus (2006) andesitic basaltic rhyolite andesite basalt ultramafic (Ol, Px) modified from Stern (2002) Problems genesis of continental crust 1a 1b 2 Crustal anatexis !  Crustal anatexis (=partial melting of the continental crust): 1. Where, when, why? 2. How, i.e. mechanisms of anatexis: protoliths, melting reactions, P-T-XH2O conditions. 3. Mechanisms of segregation, extraction, ascent and emplacement. 4. Melt composition, extent of melt-solid (=residue) equilibration. 5. Connection between anatectic terranes-crustal granites: nature and intensity of crustal differentiation, role of mantle (heat-mass: crustal differentiation to crustal growth ratio). ❏  Sources of information: 1. Anatectic terranes and enclaves in volcanic rocks: field, petrology and geochemical studies. 2. Experimental petrology. 3. Thermal-thermodynamic-thermomechanic modeling. When does the continental crust melt? The geothermal gradient Melting reactions in the continental crust Clemens (2006) and references thereinAnatexis does not happen in old and stable continental crust, e.g. cratons When does the continental crust melt? East African Continental rifts Himalaya collisional belt Los Andes orogenic arc we need particular situations for anatexis Crustal anatexis !  Crustal anatexis (=partial melting of the continental crust): 1. Where, when, why? 2. How, i.e. mechanisms of anatexis: protoliths, melting reactions, P-T-XH2O conditions. 3. Mechanisms of segregation, extraction, ascent and emplacement. 4. Melt composition, extent of melt-solid (=residue) equilibration. 5. Connection between anatectic terranes-crustal granites: nature and intensity of crustal differentiation, role of mantle (heat-mass: crustal differentiation to crustal growth ratio). ❏  Sources of information: 1. Anatectic terranes and enclaves in volcanic rocks: field, petrology and geochemical studies. 2. Experimental petrology. 3. Thermal-thermodynamic-thermomechanic modeling. Field, petrology of anatectic terranesBrown (2013) Migmatite (Sawyer 2008): Rock in medium-, high-grade area, heterogeneous at micro-, macroscale, consisting of two petrographically different parts. One formed by anatexis. All parts petrogenetically related by partial melting and segregation from the solid fraction. It is a genetic-, partial meltingbased definition. The anatectic part is light-colored, larger in grain size, and rich in Qtz and feldspars; the other petrographic part is dark-colored and rich in ferromagnesian minerals. Types (morphology, funct. % melt and effect/interplay of deformation): Metatexites and diatexites. Acosta-Vigil et al. (2014) Metatexite Diatexite Field, petrology of anatectic terranes Parts of migmatite (Sawyer 2008): Neosome: part newly formed by partial melting. May or may not Have undergone segregation. Protolith: rock out of the anatectic area that will become neosome. Paleosome, resister: non-neosome part of a migmatite. Residuum: solid fraction part of the Neosome, left after melt extraction. Leucosome: part of the migmatites Derived from the segregated melt. May not have the composition of the primary melt. In situ, in source, leucocratic vein or dike. Sawyer (2008) In situ leucosome Melanosome Paleosome Paleosome Neosome Paleosome Neosome Paleosome Field, petrology of anatectic terranes 2. Mechanisms of anatexis: protoliths, melting reactions, P-T-XH2O conditions Qtz + Pl + Kfs + H2O = melt; 730 ºC, 0.5-0.6 GPa; Archean TTG, Brazil Carvalho et al. (2016) Qtz + Pl + Bt = Opx + Ilm + melt; Archean metagreywacke, Canada Sawyer (2001) Experimental petrology 4. Melting reactions, melt composition, equilibrium conditions, melt distribution P-T: 700-900 ºC at 4-15 kbar, dehydration-melting or fluid absent reactions. 2 Ms+Pl+Qtz→Melt+Als+Kfd+Bt 3 Bt+Pl+Als+Qtz→Melt+Crd/Grt±Kf 4 Bt+Pl+Qtz→Melt+Opx±Ilm 5 Hbl+Pl→Melt+Grt ±Cpx±Ilm HP-UHP: 900-1000 ºC at 15-50 kbar; UHT: 900-1100 ºC at 10 kbar Fluid present: 650 ºC 1 Qtz+Pl+Kfd+Ms+H2O→Fundido Sawyer et al. (2011) 1 2 3 4,54,5 Crustal anatexis !  Crustal anatexis (=partial melting of the continental crust): 1. Where, when, why? 2. How, i.e. mechanisms of anatexis: protoliths, melting reactions, P-T-XH2O conditions. 3. Mechanisms of segregation, extraction, ascent and emplacement. 4. Melt composition, extent of melt-solid (=residue) equilibration. 5. Connection between anatectic terranes-crustal granites: nature and intensity of crustal differentiation, role of mantle (heat-mass: crustal differentiation to crustal growth ratio). ❏  Sources of information: 1. Anatectic terranes and enclaves in volcanic rocks: field, petrology and geochemical studies. 2. Experimental petrology. 3. Thermal-thermodynamic-thermomechanic modeling. Field, petrology of anatectic terranes 3. Mechanisms of segregation, extraction, ascent and emplacement In order to differentiate the continental crust during crustal anatexis, melt must separate from solid residuum and ascend to upper crustal levels. Field studies: Migmatites separate deeper levels of residual granulites with rare leucosomes, from shallower levels rich in allochthonous granites. Migmatite terranes are complex, polygenetic, in that they represent levels where melting has occurred, from which melt has been drained, in which melt has accumulated, and through which melt has transferred (Brown 2013). With field and petrology studies we may recognize the presence of melt at the microscopic and macroscopic scale, and start to understand how melt segregate and ascend from melt production sites, and emplace into upper crustal levels. Field, petrology of anatectic terranes 3. Mechanisms of segregation, extraction, ascent and emplacement Low melt fractions at the microscopic scale: pore pseudomorphs, shown by melt films, cuspate terminations. Barich et al. (2014) Sawyer (2001) Holness & Sawyer (2007) Experimental petrology Laporte y Provost (2000) Laporte et al. (1997), Holness (2011) Laporte (1994) Qtz gl Dihedral angle 3. Mechanisms of segregation, extraction, ascent and emplacement Rosemberg y Handy (2005) Permeability thershold Critical Melt Fraction Field, petrology of anatectic terranes 3. Mechanisms of segregation, extraction, ascent and emplacement Low melt fractions at the microscopic scale: pore pseudomorphs 1 mm Qtz Pl+ Qtz Migmatite Holness (2008) Field, petrology of anatectic terranes 3. Mechanisms of segregation, extraction, ascent and emplacement It is easy to recognize and map melt distribution at the macroscopic scale once melt has amalgamated into cm-, dm-, m-scale leucosomes. Sawyer (2001) Field, petrology of anatectic terranes 3. Mechanisms of segregation, extraction, ascent and emplacement Leucosomes typically form a network that changes form in the direction of melt movement. At melt production sites, small scattered paches of in situ leucosomes. Next, they link together to feed into wider, longer leucosomes, commonly along foliation planes or shear bands, to form in source leucosomes. These connect with discordant planar/circular structures that extract the melt from the anatectic terrane. Driving force for melt segregation: P gradients during differential stress on heterogenous crust (Sawyer 2014). Sawyer (2001) Field, petrology of anatectic terranes 3. Mechanisms of segregation, extraction, ascent and emplacement Brown (2004) Brown (2013) Driving force for melt ascent: Differences in density (e.g. Brown 2010). Brown (2004) Field, petrology of anatectic terranes 3. Mechanisms of segregation, extraction, ascent and emplacement Brown (2013) Brown (2007) Driving force for melt ascent: Differences in density (e.g. Brown 2010). Brown (2004) Field, petrology of anatectic terranes 3. Mechanisms of segregation, extraction, ascent and emplacement Plutons (103-104 Km3) by addition of many magma batches, emplaced while the previously intruded magma is partially crystallized (mush), hence contacts between batches are cryptic. Melt is extracted periodically. Plutons form in 106-107 yrs: end product. Processes is balanced at crustal scale. Brown (2013) Field, petrology of anatectic terranes 4. Melt composition, extent of melt-solid (=residue) equilibration Leucosomes are the part of migmatite that today approach more closely to the melt during anatexis, whereas melanosomes are those that approach more closely to the residuum. Hence, in order to obtain melt compositions, we analyze leucosomes; and to check the extent of equilibrium, we compare leucosomes with minerals in melanosomes. However, this approach has problems associated with nature of migmatites: - Leucosomes rarely represent pure melts, particularly primary melts. - Fractional crystallization, melt segregation: cumulates, evolved melts. - Upon cooling, melt-residue reaction, e.g. by diffusion through melt. The way to partially solve these problems: thorough and systematic petrological-geochemical study of the anatectic terrane (Sawyer 1998, Carvalho et al. 2016). Field, petrology of anatectic terranes 4. Melt composition, extent of melt-solid (=residue) equilibration Understanding processes operating during anatexis. Anatexis of a TTG gneiss. Melt segregated before and during crystallization, producing the variety of lithologies. GD are residual. L primary and fractionated. Level of exposure: has gained melt. GD SD L HD Carvalho et al. (2016) Field, petrology of anatectic terranes 4. Melt composition, extent of melt-solid (=residue) equilibration Leucosomes in metatexites from metasediments (pelites, greywackes) range in composition from monzogranites to tonalites, though in most cases are monzogranites and granodiorites. Leucosomes in mafic metatexites range from monzodiorites to tonalites. Leucosomes from metasedimentary rocks are too leucocratic (low in Fe, Mg, Ti) to be the source of many crustal granites (mafic S-type granites). Kfs Pl Qtz Normative compositions wt % oxides Sawyer (2008) Cesare et al. (2008) Experimental petrology Kfs Pl Normative compositions Sawyer (2008) 4. Melting reactions, melt composition, equilibrium conditions, melt distribution Dehydration melting metasediments Dehydration melting-fluxed melting mafic rocks Fluxed melting metasediments Qtz Composition of the continental crust Upper Middle Lower SiO2 66.6 63.5 54.3 TiO2 0.64 0.69 0.97 Al2O3 15.4 15.0 16.1 FeOt 5.04 6.02 10.6 MgO 2.48 3.59 6.28 CaO 3.59 5.25 8.48 Na2O 3.27 3.39 2.79 K2O 2.80 2.30 0.64 ASI 1.03 0.85 0.78 Data from Rudnick & Gao (2003) Field, petrology of anatectic terranes 4. Melt composition, extent of melt-solid (=residue) equilibration Extent of equilibrium mostly (only?) calculated regarding the trace elements controlled by accessory minerals (Zrn: Zr; Mnz: LREE, Th). In most cases, disequilibrium has been reported, implying rapid segregation of melt from its residuum (102-104 yrs). Much more difficult to establish equilibrium regarding major minerals/oxides. Watt et al. (1996), Montel (1993), Bea (1996), Watson (1996) Crustal anatexis !  Crustal anatexis (=partial melting of the continental crust): 1. Where, when, why? 2. How, i.e. mechanisms of anatexis: protoliths, melting reactions, P-T-XH2O conditions. 3. Mechanisms of segregation, extraction, ascent and emplacement. 4. Melt composition, extent of melt-solid (=residue) equilibration. 5. Connection between anatectic terranes-crustal granites: nature and intensity of crustal differentiation, role of mantle (heat-mass: crustal differentiation to crustal growth ratio). ❏  Sources of information: 1. Anatectic terranes and enclaves in volcanic rocks: field, petrology and geochemical studies. 2. Experimental petrology. 3. Thermal-thermodynamic-thermomechanic modeling. Thermal modeling colisional orogens Collins (2000) 5. Connection migmatites-crustal granites - Nature/intensity of crustal differentiation, - Role of mantle, crustal differentiation/ crustal growth ratio. Not a simple answer, depends on: - Geodynamic setting: subduction, collision, extension, and - Associated granite type: S-, I-, A-type. Hawkesworth et al. (2010) Granite types, mantle role Pro-mantle role in I-type and mafic S-types: Collins (1996, 2000), Kemp et al. (2007, 2009): geochemistry, bulk rock isotopes, O-Hf isotopes of Zrn. Patiño-Douce (1999): experiments and geochemical modeling. Pro-crust role in I-type: Chappell et al. (1987, 2004), Chappell (1996): petrology, geochemistry. Clemens et al. (2011): petrology, geochemistry. S- and I-types (Chappell & White 1974): S-type: setimentary protolith, low Na, ASI>1.1, mostly felsic, irregular trends. I-type (cordilleran): igneous protolith, high Na, ASI<1.1, mafic to felsic, linear trends. Peraluminous leucogranites, two-mica granites (e.g. Barbarin 1999; PatiñoDouce 1999): S-type low in Fe, Mg and Ti, abundant primary Ms. Mafic S-types, peraluminous S-type granites (e.g. Montel & Viezeuf 1997): S-type high in Fe, Mg, Ti, high ASI, rich in Crd and/or Opx (higher T, lower P). A-type (anorogenic, anhydrous) granites (e.g. Eby 1990): High in Si, Na+K, Fe/Mg, Zr, Nb, Ga, Y, Ce, low in Ca,and Sr. Nanogranite, India Second talk: melt inclusions