1. EHaz Convergent Margins Class Adakites: 10 April 2008 1 Constraints on Adakite Existence Colin Macpherson University of Durham

2. EHaz Convergent Margins Class Adakites: 10 April 2008 2 Hi! You have covered a fairly diverse range of topics so far in this class that employ a wide range of techniques to understand magmatism and volcanism at convergent plate margins. I hope that you will be up for some more geochemistry. I realise that not everyone is conversant in trace element ratios and isotopes but all arguments surrounding adakites rely on their geochemistry. I have tried to incorporate sufficient explanatory text or figures to help you understand these. The two papers you really need to read are: 1. Defant & Drummond (1990) from Nature, and 2. Macpherson et al (2006) from EPSL. Several others are mentioned in this file for the really keen!

3. EHaz Convergent Margins Class Adakites: 10 April 2008 3 “Normal” Arc Magmatism subducted lithosphere releases hydrous fluids and, possibly, silicate melts, these infiltrate the overlying mantle wedge lowering the solidus of the mantle peridotite there, partial melting of peridotite produces basaltic magma.

4. EHaz Convergent Margins Class Adakites: 10 April 2008 4 many processes operate within arc lithosphere to produce diverse compositions from the primary basaltic flux. in general, though, most arc lavas lie along the basalt-andesite-dacite-rhyolite differentiation trend as a result of differentiation at crustal pressures.

5. EHaz Convergent Margins Class Adakites: 10 April 2008 5 So, What is an Adakite? Defant and Drummond (1990) suggest that distinctive geochemical trends in some dacites and andesites cannot be produced by low pressure differentiation. This group of rocks, which they termed adakites, have: relatively high alumina content, intermediate silica content low concentrations of heavy rare earth elements, and elevated Sr/Y ratios

6. EHaz Convergent Margins Class Adakites: 10 April 2008 6 Why Does This Geochemical Signature Matter? are exactly the characteristics that would be expected of a magma extracted from hydrous basaltic crust by partial melting. relatively high alumina content, intermediate silica content low concentrations of heavy rare earth elements, and elevated Sr/Y ratios The animation over the next few pages explains why Sr and the heavy rare earth elements (to which Y is closely related) would be affected in this way.

7. EHaz Convergent Margins Class Adakites: 10 April 2008 7 Sr Y Plag Amph Gt In subducted crust (mostly greenschist facies rock) amphibole hosts Y (+ HREE) while plagioclase hosts Sr.

8. EHaz Convergent Margins Class Adakites: 10 April 2008 8 Sr Y Plag Amph Gt Phase changes occur as the slab subducts. The transformation of greenschist to blueschist has relatively little effect on Y & Sr.

9. EHaz Convergent Margins Class Adakites: 10 April 2008 9 Y Y Sr Plag Amph Gt As P increases plagioclase destabilises, so Sr is “homeless”. Garnet, in which Y is very compatible, appears.

10. EHaz Convergent Margins Class Adakites: 10 April 2008 10 Y Y Sr Plag Amph Gt If the (now eclogite facies) slab melts then Y (+HREE) will be retained in amphibole and garnet but Sr will go into melt.

11. EHaz Convergent Margins Class Adakites: 10 April 2008 11 Y Y Sr Plag Amph Gt Therefore: residue= Y-rich Sr-poor melt= Y-poor Sr-rich i.e. melt has high Sr/Y and low Y

12. EHaz Convergent Margins Class Adakites: 10 April 2008 12 What Support for Slab Melting? Defant and Drummond noted that all the rocks they called adakites were associated with subducted slab that were young. Therefore, they claimed that these slab were more likely to melt because they retained a lot of heat from their (recent) formation. This diagram shows their evidence base of ~ a dozen adakitic suites. Adakites Not Adakites Of slab

13. EHaz Convergent Margins Class Adakites: 10 April 2008 13 So What? Why all the fuss? Why does it matter if slab melts reach the surface (or arc crust)? CA Adakite Phanerozoic Archean Adakites show many similarities to tonalite, trondhjemite, and granodiorite (TTG) suites which are a defining feature of Archean terranes. The two figures A compare data for ordinary arc lavas (CA) with adakites. The figures B show TTG suites. So, adakites are a potential analogue for Archean TTG that would help understand Archean tectonics and crust generation (next slide).

14. EHaz Convergent Margins Class Adakites: 10 April 2008 14 Martin (1999) Hot Slab follows geotherm 1 Archean Cool Slab follows geotherm 3 Phanerozoic Basalt slab melts Basalt slab dehydrates Hydrated peridotite melts

15. EHaz Convergent Margins Class Adakites: 10 April 2008 15 OK – Let’s Recap 1 Adakites defined in 1990 based on their geochemical similarity to expected slab melts and association with subduction of slabs <25Ma. Adakitic rocks resemble important Archean crustal components The model is nice – it is simple, quite intuitive and makes profound predictions about how the early Earth operated.

16. EHaz Convergent Margins Class Adakites: 10 April 2008 16 Some things to discuss - 1 Are you happy that Defant & Drummond (1990) exhausted all alternative explanations for the geochemical signature of adakites? Are Defant & Drummond’s adakites open or closed systems (and does this matter)? How would you recognise an adakite in the field?

17. EHaz Convergent Margins Class Adakites: 10 April 2008 17 Post-1990 Through the 1990s more rock suites with adakitic geochemistry were found in modern and ancient convergent margins. Not all of these suites were associated with young slabs. Therefore, two possible alternatives were recognised: 1. The slab melting model is wrong. 2. Slabs can reach fusion point through other ways than just being young. Many of the workers studying the rocks chose to follow option 2 leading to many slab-melting mechanisms being inferred. Some of these are illustrated on the next few slides.

18. EHaz Convergent Margins Class Adakites: 10 April 2008 18 Flat Slab Adakitic rocks have been found above some areas where the slab has a shallow dip. e.g. Gutscher et al (2000, Geology 28, 535-538) Long time at low P therefore different PTt path (~ path 1 in slide 14) and more chance to heat slab above solidus Normal PTt path (~ path 3 in slide 13) for slab

19. EHaz Convergent Margins Class Adakites: 10 April 2008 19 Subduction Initiation Adakitic rocks were found in the Philippines where the slab was old but had not been subducting long. Numerical modelling (Peacock et al., 1994, EPSL 121, 227-244) suggests that under these conditions the mantle will be hotter than mantle adjacent to more mature slabs so may provide sufficient heat to make the slab melt. e.g. Sajona et al (1993, Geology 21, 1007-1010) Mantle wedge not yet cooled by ongoing subduction High temperature adjacent to slab, therefore slab melting

20. EHaz Convergent Margins Class Adakites: 10 April 2008 20 Slab Tears Some adakitic rocks occur close to “tears” or gaps in the slab. This is assumed to expose the slab interior to relatively high temperatures so that it can melt. e.g. Yogodzinski et al (2001, Nature 409, 500-504). Mantle wedge cooled by ongoing subduction producing “normal” arc lavas High temperature adjacent to slab, therefore slab melting

21. EHaz Convergent Margins Class Adakites: 10 April 2008 21 OK – Let’s Recap 2 Adakites defined in 1990 based on their geochemical similarity to expected slab melts and association with subduction of slabs <25Ma. Several occurrences defined from 1993 to 2001 use only geochemistry to define slab melting. By early 2000s three classes of exception to the rule (that I have listed and others that I haven’t) have been introduced. The model is not so nice now – it has lost its simplicity and can no longer make specific predictions about how the early Earth operated.

22. EHaz Convergent Margins Class Adakites: 10 April 2008 22 A Case Study Many adakite “locations” based on relatively few rocks. Subduction initiation study in Surigao, Philippines (slide 19) based on three rocks. Examine Surigao example in more detail with comprehensive dataset collected in 1999 (Macpherson et al., 2006, EPSL 243, 581-593) Thorough sampling of Surigao peninsula and wide array of geochemical and petrological techniques used.

23. EHaz Convergent Margins Class Adakites: 10 April 2008 23 Philippine Sea Plate 55Ma 40Ma Mindanao Westwards subduction of Philippine Sea Plate began in north about 10Ma and has propagated south The Philippine Tectonic Context This trench propagating south

24. EHaz Convergent Margins Class Adakites: 10 April 2008 24 Philippine Sea Plate 55Ma 40Ma Mindanao Philippine Sea Plate is ~55Ma where it is subducting so is too old to melt under normal geotherm The Philippine Tectonic Context

25. EHaz Convergent Margins Class Adakites: 10 April 2008 25 Philippine Sea Plate 55Ma 40Ma Mindanao Surigao Peninsula Adakitic rocks collected from Surigao peninsula on Mindanao island. The Philippine Tectonic Context

26. EHaz Convergent Margins Class Adakites: 10 April 2008 26 Trench is over there Large strike-slip fault acts as western graben-bounding fault Reactivated back- thrusts act as eastern graben-bounding fault Down-thrown area hosting volcanic peak (star M) and shallow lake (surrounded by very recent flat-bedded sediments). Suggests crustal thinning. Surigao Peninsula Geology M

27. EHaz Convergent Margins Class Adakites: 10 April 2008 27 Northeasterly view from top of volcano M (Maniayao) with eastern graben bounding fault indicated

28. EHaz Convergent Margins Class Adakites: 10 April 2008 28 Surigao is nice, friendly place … for the most part!

29. EHaz Convergent Margins Class Adakites: 10 April 2008 29 In east, normal arc andesites, dacites and rhyolites (ADRs). In west, lots of adakites. Perfect opportunity to test: 1 Relationship between adakites and ADRs 2 Relationship within the suite of adakites 3 Origin of an adakite suite

30. EHaz Convergent Margins Class Adakites: 10 April 2008 30 1 Relationship between adakites and ADRs All trace element ratios very similar except that Y is depleted in adakites. Also resemble typical arc magmas. Suggests similar sources and processes of formation in source for adakites and ADRs. ADRs formed by normal arc processes (slides 3 and 14). Y depletion of adakites is main difference to ADRs.

31. EHaz Convergent Margins Class Adakites: 10 April 2008 31 2 Relationship within the suite of adakites Significant contrast in behavior of Y between adakites and ADRs. In adakites Y shows strong negative correlation with SiO 2. Consistent with Y depletion during differentiation, possibly by fractional crystallisation of amphibole or garnet. How can we determine which is involved? Adakites (west): open circles ADRs (east): black circles

32. EHaz Convergent Margins Class Adakites: 10 April 2008 32 Fractionation of middle (e.g. Dy) from heavy (e.g. Yb) REE is different for removal of amphibole as opposed to garnet from a melt. Davidson et al. (2007)

33. EHaz Convergent Margins Class Adakites: 10 April 2008 33 Fractionation of middle (e.g. Dy) from heavy (e.g. Yb) REE is different for removal of amphibole as opposed to garnet from a melt. Davidson et al. (2007)

34. EHaz Convergent Margins Class Adakites: 10 April 2008 34 Fractionation of middle (e.g. Dy) from heavy (e.g. Yb) REE is different for removal of amphibole as opposed to garnet from a melt. Davidson et al. (2007)

35. EHaz Convergent Margins Class Adakites: 10 April 2008 35 2 Relationship within the suite of adakites Adakite suite shows positive correlation of Dy/Yb with SiO 2. This is consistent with fractional crystallisation of garnet from mafic melt. ADRs are more consistent with fractional crystallisation of low- pressure crystal assemblage (amphibole + plagioclase).

36. EHaz Convergent Margins Class Adakites: 10 April 2008 36 3 Origin of an adakite suite If Surigao adakites are slab melts then should have isotope composition of Philippine Sea Plate ADRs and adakites indistinguishable in isotope ratios. Both different to PSP. Suggests similar sources for adakites and ADRs in metasomatised mantle wedge peridotite.

37. EHaz Convergent Margins Class Adakites: 10 April 2008 37 OK – Let’s Recap 3 The geochemistry of Surigao adakites is very similar to many other adakitic suites. Adakitic rocks are largely indistinguishable from near-contemporaneous ADRs. Trace element ratios and isotope ratios point to similar sources in mantle wedge metasomatised by the same type of slab fluids found in most subduction zones. The simplest explanation for Surigao adakites is that they are the produced by differentiation of hydrous basaltic magma at sufficient pressure to include garnet in the fractionating assemblage. In Surigao the crust is estimated to be ~ 25km thick while garnet require pressures equivalent to ~33km or more. Differentiation must be sub- Moho.

38. EHaz Convergent Margins Class Adakites: 10 April 2008 38 Implications On slide 17, I said the presence of adakites in subduction zones with old slabs could be interpreted in one of two ways: 1. The slab melting model is wrong. 2. Slabs can reach fusion point through other ways than just being young. In 1993 the Surigao was interpreted as an “expectional” case of slab melting. The more thorough study outlined in slides 22 to 37 suggests that Surigao adakites can be produced without slab melting. Is Surigao an oddity, or are there other examples where high pressure differentiation of basaltic arc magma can be invoked?

39. EHaz Convergent Margins Class Adakites: 10 April 2008 39 Ecuador Suites of different ages that appear to be derived from similar parents but differentiate at different depth. Chiarardia et al (2004, Mineralium Deposita 39, 204-222)

40. EHaz Convergent Margins Class Adakites: 10 April 2008 40 Chile Longaivi volcano. Early granet- dominated phase (mafic enclaves) followed by lower pressure differentiation. Rodriguiz et al (2007, Journal of Petrology 48, 2033-2061)

41. EHaz Convergent Margins Class Adakites: 10 April 2008 41 Other non-Slab-Melt Models Differentiation: Castillo et al. (1999, CMP) Camiguin Island, Philippines. Garrison & Davidson (2003, Geology) Nothern Volcanic Zone, Andes Prouteau & Scaillet (2003, J. Pet) Pinatubo 1991 dacite Very low-degree partial melting of hydrous peridotite Eiler et al. (2007, G-cubed)

42. EHaz Convergent Margins Class Adakites: 10 April 2008 42 Some things to discuss - 2 Does the slab melt? What conditions would be required for those melts to reach the surface? How can the deep differentiation model be reconciled with other “classic” adakite occurrences? What are the implications of the deep differentiation model for the adakite - TTG analogue?