Chapter 4. TTG & Genesis of the Early Continental Crust Introduction In Archean terrains, 3 types of granitoids are found in association with greenstone belts: (1) gneiss complex: composition TTG (2) diapiric plutons : composition TTG (3) discordant plutons (late granite intrusions): composition granite. Question: What are TTG’s? and the relative ages between the greenstone belts and the surrounding granitic gneisses? Ref. - Martin et al., 2005. Lithos 79, 1-24 (and refs therein). (This is an excellent review paper) Highly deformed greenstone - granite terrain Amphibolites and TTG gneiss (“grey gneiss”) TTG’s - how do they look like? 灰色片麻岩 TTG gneiss intruded by late granite (highly deformed) A piece of grey gneiss from the Narryer Complex, Yilgarn Craton TTG is an acronym for “tonalite-trondhjemite-granodiorite”. These granitoids, together with greenstone belts, are a typical feature of all Archean terrains. In addition, a rarer feature of Archean terrains is the occurrence of high-Mg diorite-tonalite-granodiorite rocks, collectively called “sanukitoids”. TTG’s, sanukitoids and modern adakites have something in common. They are all interpreted to represent to have a direct or indirect link to partial melting of basaltic protolith, and implicitly related to some form of plate subduction. TTG classification O’Connor’s classification - based on normative feldspar composition (valid for rocks with normative Q ≥ 10%) Chemical definition TTG = siliceous rocks, with SiO2 = 65 - 75% Al2O3 >15% (SiO2 ≤ 70%), <14% (SiO2 ≥ 70%) (FeOT + MgO) < 3.4% FeOT/MgO = 2 - 3 CaO = 1.5 - 3% Na2O = 4 - 5.5% K2O ≤ 2%; Na2O/K2O >1 at SiO2 = 70%, if Al2O3 >15%, then called high-Al TTG if Al2O3 <15%, then called low-Al TTG REE patterns highly fractionated with HREE depletion; commonly no Eu anomaly; negative Nb-P-Ti anomaly in spidergrams (La/Yb)N vs (Yb)N plot Q - why this kind of plot? its physical meaning? Model calculations (Yb)N Melting experiments at low pressures Melting experiments at medium pressures Melting experiments at high pressures Model calculations (Yb)N Melting in subduction zones cool mantlewarm crust cool mantlecool crust warm mantle cool crust warm mantlewarm crust Melting curves and geothermal gradients What is the geotectonic significance? Why TTG’s of HREE depletion typically occur in the Archean? Comparison with sanukitoids & adakitic rocks Adakite (Defant & Drummond, 1990) - adakites form suites of intermediate to felsic rocks whose compositions range from hb-andesite to dacite and rhyolite; basaltic members are lacking. The rocks have: SiO2 >56%, Na2O = 3.5-7.5%, Na2O/K2O ≈ 2. REE highly fractionated with high (La/Yb)N >10. [Sr] content high (>400 ppm), high Sr/Y ratios. 2 groups of adakites (Martin & Moyen, 2003): HSA (high silica), SiO2 >60%, LSA (low silica), SiO2 <60%. K-Na-Ca plots Martin et al., 2005 TTGADK element plots MgO- SiO2 & MgO-K2O plots Archean TTG’s and modern HSA (heavy dotted lined field) and LSA (light dash lined field) black diamond - TTG <3.0 Ga grey diamond - TTG 3.0-3.5 Ga white diamond - TTG >3.5 Ga open circle - Archean sanukitoids soild circle - Closepet-type granites (MgO & K2O > TTG) TTG & ADK spidergrams Sanukitoids & adakites (LSA) Where do they occur, the adakitic rocks? 1. in subduction zones with young and warm subducting oceanic lithosphere (e.g., Adak Island, Aleutian Islands; Kay, 1978). 2. in thickened continental crust in which the lower part of the crust was basaltic in composition, probably formed by underplated magma (e.g., South Tibet; Chung et al., 2003, Geology) South Tibet Tibetan adakites Q - why do we use Sr/Y vs Y plot, instead of (La/Yb)N vs YbN plot? Conclusions Important questions: 1. TTG and basaltic rocks (greenstones) - which came first? (perpetual chicken and egg problem?) 2. No or little Eu anomalies in TTG REE patterns, why? 3. Plate tectonics, did it work in the Archean? (have you seen the subduction geochemical signatures?) 4. Heat flow in the continental crust - higher in the Archean than in the later times? (no clear evidence from granulite facies terranes) 5. Nature of the earliest CC? anorthositic crust?