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Geochemical Indicators of Plate Tectonic Processes in Old Rocks Julian Pearce (Cardiff University)

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Presentation on theme: "Geochemical Indicators of Plate Tectonic Processes in Old Rocks Julian Pearce (Cardiff University)"— Presentation transcript:

1 Geochemical Indicators of Plate Tectonic Processes in Old Rocks Julian Pearce (Cardiff University)

2 Bob’s Smoking Guns for Archean Subduction (Blueschists, UHP rocks, Ophiolites)

3 However, Subduction Fluxes are Forever Even if the arc is overwritten by collision or eroded away, inherited subduction signals remain in the mantle lithosphere and can be reactivated later

4 Finding Evidence of Subduction is the Key to Knowing when Plate Tectonics started But present-day subduction is a complicated processes; essentially mantle flow and subduction input are the geochemical indicators of plate tectonics, while crustal interactions tend to mask these indicators. The key to identifying arc lavas in the Archaean is separating subduction signals from crustal signals. Pearce & Peate (1995)

5 Geochemical Indicators of Plate Tectonic Processes Indicators of Plate-driven Mantle Flow Indicators of Subduction At subduction zones, these two processes act together. LT HT UHT

6 Geochemical Tracing of Subduction Input Progressive subduction leads to sequential release of: LT elements (Rb, Ba etc) HT elements (LT elements plus L- MREE, Th, P) UHT elements ( HT elements plus Nb, Ta, Zr, Hf). In the Archean, we would expect this sequence to take place at shallower depths than at present LT HT UHT

7 Geochemical Tracers for Subduction Input Shallow subduction components cannot be investigated in Archaean rocks because of alteration- sensitivity Deep subduction components are more robust. Th/Nb (an indicator of negative Nb anomalies also an effective tracer of deep subduction input that is robust to upper amphibolite facies

8 Geochemical Tracing of Mantle Flow Flowing mantle undergoing decompression can drastically change its chemical composition Pearce (2005)

9 Geochemical Tracing of Mantle Flow Tonga-Lau System: collabration w. Pam Kempton, Jim Gill Results indicate that mantle entering subduction systems progressively loses incompatible elements by melt extraction while flowing to the sub-arc region

10 Geochemical Tracers for Mantle Flow Thus Nb/Yb acts as a Good proxy for mantle fertility Nb/Yb gradients provide a means of tracing mantle flow Isotope ratios reach plateaus, so trace element ratios are more effective for mapping. Most effective for subduction systems are VICE/MICE ratios based on immobile elements

11 Th/Yb-Nb/Yb Fingerprinting Pearce and Peate, 1995 At the present day, MORB and IOB plot in a well defined array, along the mantle flow axis (Nb/Yb); arc lavas are displaced to higher Th/Nb ratios. The overall dispersion of arc lavas is parallel to the MORB array indicating the importance of melt extraction during mantle flow in magma genesis.

12 Th/Yb-Nb/Yb Fingerprinting: Role of Mantle Flow HT subduction melt extraction two components to a first approximation

13 Th/Yb-Nb/Yb Fingerprinting: Interaction of Mantle Flow and SZ input Location of data and shapes of trends indicate process 1= Add SZ component before melt extraction 2= Add SZ component during melt extraction 3. Add SZ component without or after melt extraction 4. UHT SZ component adds Nb as well as Th

14 Types of Subduction Zone North Tonga: Oceanic plume- subduction interaction Cascades: Continental plume- subduction interaction IBM Eocene: Intra-oceanic subduction initiation Japan Miocene: Intra-continental subduction initiation Various Localities: Ridge subduction Taiwan, SE Indonesia: Syn- collision Mariana: Arc Rifting Anatolia: Subduction component reativation plus steady state subduction Each type of Subduction Zone shows a different topology on the Th/Yb-Nb/Yb diagram

15 Example: W. Pacific Eocene

16 Example: North Tonga

17 Example: Kamchatka

18 Misinterpretation of Subduction Zones: Amphibolite-facies metamorphism HT fluids/melts from sediments impregnated and metamorphosed the lavas. The result is that some normally-immobile LIL elements (e.g. Th) are enriched Broken Hill cores

19 Misinterpretation of Subduction Zones: granulite facies metamorphism The lower crust loses a melt fraction (which can be seen in places ‘escaping’ from the rock) leaving a granulite residue This depletes the residue in LIL elements. David Waters increasing melt depletion

20 Misinterpretation of Subduction Zones: Crustal Contamination The crustal contamination vector is typically parallel to the UHP subduction vector.

21 Crustal Contamination is also part of continental arc dispersion

22 Proposed Archean Subduction-Related Rocks Komatiites Boninites BADR volcanic series Adakite lavas and TTGs Aim is to assess these subduction signals

23 Testing Archean Komatiite Subduction Models Parman et al. (1997, 2001), following the work of Grove, controversially argue that Barberton lavas were wet rather than hot – i.e. subduction related. However, crustal contamination is more consistent with the data

24 Testing Archean Boninite Subduction Models Dispersion is along a crustal contamination vector, not mantle flow vector; however the Isua boninites do require source depletion.

25 Alternative Model for Archean Boninites and Related Rocks Arculus et al. (1992) from IODP Leg 125 Phanerozoic boninites result from shallow, wet melting (opx = ol + siliceous melt)

26 Alternative Model for Archean Boninites and Related Rocks Arculus et al. (1992) from IODP Leg 125 But Archean boninites could be explained by komatiite-crust interaction Komatiite crust

27 Testing Archean BADR Subduction Models The proposed BADR series shows increasing Th/Nb with increasing silica content and trend parallel to crustalcontamination trends. Wawa lavas do not however have an end member in the mantle array: a difficult call.

28 Turkish Analogue Post-collision magmas erupted at the site of the ‘dead’ arc have subduction signatures Post-collision magmas erupted where there was no arc have intraplate signatures ( with crustal assimilation) Reactivation of sub-arc lithosphere following collision can be evaluated using Nb anomalies

29 Testing Archean Adakite/TTG Subduction Models Maybe all four! But subduction not essential. All agree on melting of mafic material but how? Flat subduction (lower crust melting) Hot subduction (slab melting) Delamination (lower crust melting) Magma chambers (mid-crust melting)

30 Crustal Processing With no plate tectonics, could Archean volcanic terranes have undergone intracrustal reprocessing like present-day Collision Zones, so explaining subduction-like chemical signatures?

31 Conclusions Many Archean boninites and other evolved high-Mg magmas could be explained by interactions between komatiite (and related) magmas and crust rather than by subduction. Archean basalt-andesite-dacite-rhyolite (BADR) series require substantial magma- crust interaction and may not all additionally have subduction components. Adakites could (as is well known) involve melting of mafic rocks in the crust as well as in subduction zones. Unlike modern arc lavas, Archean ‘arc’ lavas do not exhibit a geochemical indication of plate-driven flow into and within a mantle wedge. If there was subduction in the Archaean, it must have involved variable ‘adakitic’ addition to a homogneous mantle Personally, I would start subduction around 2.7 Ga but work still needs to be done separating crustal and subduction signals.

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