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Chapter 4. TTG & Genesis of the Early Continental Crust

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

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Chapter 4. TTG & Genesis of the Early Continental Crust

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  1. Chapter 4. TTG & Genesis of the Early Continental Crust

  2. 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)

  3. Highly deformed greenstone- granite terrain Amphibolites and TTG gneiss (“grey gneiss”)

  4. TTG’s - how do they look like? 灰色片麻岩 TTG gneiss intruded by late granite (highly deformed)

  5. A piece of grey gneiss from the Narryer Complex, Yilgarn Craton

  6. 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.

  7. TTG classification O’Connor’s classification - based on normative feldspar composition (valid for rocks with normative Q ≥ 10%)

  8. 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

  9. REE patterns highly fractionated with HREE depletion; commonly no Eu anomaly; negative Nb-P-Ti anomaly in spidergrams

  10. (La/Yb)N vs (Yb)N plot Q - why this kind of plot? its physical meaning?

  11. Model calculations (Yb)N

  12. Melting experiments at low pressures

  13. Melting experiments at medium pressures

  14. Melting experiments at high pressures

  15. Model calculations (Yb)N

  16. Melting in subduction zones cool mantle- warm crust warm mantle -cool crust cool mantle-cool crust warm mantle-warm crust

  17. Melting curves and geothermal gradients What is the geotectonic significance? Why TTG’s of HREE depletion typically occur in the Archean?

  18. 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%.

  19. K-Na-Ca plots Martin et al., 2005

  20. TTG-ADK element plots

  21. 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)

  22. TTG & ADK spidergrams

  23. Sanukitoids & adakites (LSA)

  24. Where do they occur, the adakitic rocks? in subduction zones with young and warm subducting oceanic lithosphere (e.g., Adak Island, Aleutian Islands; Kay, 1978). 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)

  25. South Tibet

  26. Tibetan adakites Q - why do we use Sr/Y vs Y plot, instead of (La/Yb)N vs YbN plot?

  27. Conclusions Important questions: TTG and basaltic rocks (greenstones) - which came first? (perpetual chicken and egg problem?) No or little Eu anomalies in TTG REE patterns, why? Plate tectonics, did it work in the Archean? (have you seen the subduction geochemical signatures?) Heat flow in the continental crust - higher in the Archean than in the later times? (no clear evidence from granulite facies terranes) Nature of the earliest CC? anorthositic crust?

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