COUPLING PHYSICAL AND CHEMICAL STUDIES TO ASSESS TTG PETROGENESIS

Transcription

1 COUPLING PHYSICAL AND CHEMICAL STUDIES TO ASSESS TTG PETROGENESIS A Progress Report Presented by Amanda Getsinger to The Faculty of the Geology Department of The University of Vermont Accepted by the Faculty of the Geology Department, the University of Vermont, in partial fulfillment of the requirements for the degree of Master of Science specializing in Geology The following member of the Thesis Committee has read and approved this document before it was circulated to the faculty: Dr. Tracy Rushmer (Advisor) Dr. Keith Klepeis (Committee member) Dr. Daniel Savin (Committee member) Dr. Don Baker (Ext. Committee member) Date Accepted:

2 Abstract Tonalite-trondhjemite-granodiorite (TTG) and associated magmatism comprises a significant portion of the continental crust and provides information on the origin of Earth s earliest crust. In addition, these suites provide important constrains on the role of the mantle wedge in crust development and the thermal regimes present in both early Earth and post-archean subduction zones; however, the tectonic setting in which the basaltic source material undergoes partial melting is still undetermined. Major, trace, and REE compositions of both Archean TTGs and modern adakite-like magmas have been used in conjunction with batch melting experiments and models to infer source rock compositions, depths of melting, and tectonic setting, but the physical processes by which melt segregates from, and interacts with, its partially molten host may have a profound impact on the volume and composition of the segregated melt as it leaves the source region. I am examining the processes by which high melt volumes can be generated by using rock samples collected from New Zealand. This setting is important because of provides a natural example of an island arc with exposures of mafic crust that are potential sources of TTG suite magmatism. My goal is to test this possibility through experiments designed to reproduce the local changes in bulk composition that are predicted to occur in response to TTG melt segregation, which happens along a steep geothermal gradient through melt migration along grain boundaries and contemporaneous matrix compaction. To accomplish this I am performing piston-cylinder experiments on a metabasalt with conditions and compositions based on model parameters. Preliminary results suggest that I can change the geochemistry of the model system significantly by introducing a low-degree partial melt into a metabasalt source material. The resulting melt compositions in these melt segregation experiments are lower in the An component when plotted in an Ab-An-Or ternary and have higher Mg-numbers when compared with direct partial melting results. This raises the possibility that if dynamic melt segregation and equilibrium processes are active in lower mafic crust during arc growth, they may modify melt compositions and could help explain the wide range of Mg numbers observed in TTGs. This is an area that deserves further investigation to ascertain whether or not large batholiths of TTG rocks can be developed in this way. I will contribute to this investigation by determining the differences between direct partial melting of a basalt underplate (Price, 2005) and new bulk compositions by quantifying the melt percentages generated in my experiments and determining the composition of both the matrix and melt compositions. I can compare these results with natural TTG arc samples from New Zealand. Introduction Purpose of this Study Tonalite-trondhjemite-granodiorite (TTG) and associated magmatism comprises a significant portion of the continental crust and provides information on the origin of Earth s Archean crust, much of which is TTG suites. Questions concerning the rate of melt migration, melt volume, transport mechanisms, and the impact on crustal composition remain unanswered. For this project I am studying the volume of melt produced by adding low degree partial melt in

3 varying modal abundances to a source rock and comparing this to results from direct partial melting experiments (Price, 2005). This approach allows me to observe the rock types generated by melt refertilization processes, which is how TTGs may be created. In conjunction with the numerical model developed by M. Jackson (Imperial College) and N. Petford (Kingston University; the Jackson/Petford model), I am testing the hypothesis that the introduction of small amounts of low volume melt into the source region (which makes it more fertile for arc crust genesis) can produce large volumes of evolved arc crust (Jackson et al., 2005). The Jackson/Petford model is a theoretical and numerical approach which can predict the pressure, temperature, and composition changes in time and space conditions for a given rock type and melt/matrix compositions. The model predicts that this process can produce a suite of tonalitetrondhjemite-granitic rocks with the most evolved, and buoyant, of these (granite) nearest the Earth s surface. Geologic Setting The majority of continental crust is made in magmatic arcs and subduction zones (Rudnick, 1995). Granites and other arc rocks form commonly by partially melting the lowermost crust, and where the partial melt (a granitic magma) migrates, it is emplaced at progressively higher levels in the upper crust (Brown and Rushmer, 2006). This is the main way that the crust differentiates and evolves, creating rock suites of TTGs. TTGs comprise a large amount felsic magmatic rock (Rollinson, 2006), so in order to understand the formation of the early continental crust, we must ascertain how we produce these TTG rock suites. TTGs are characterized by high Na, Al, Sr, Sr/Y > 40, and low Y compositions (Tulloch, 2003). The restite for TTGs is comprised of garnet, clinopyroxene, and amphibole (Muir et al., 1995).

4 The SC samples selected for this study are metamorphosed calc-alkaline basalt from a mafic dike in Selwyn Creek, New Zealand, and this is part of the Darran Complex (Figure 1). These samples were collected for Robert Price s UVM Masters thesis on direct partial melting experiments (2005). These mafic dikes represent a possible source rock emplaced at a depth during early Cretaceous subduction (Hollis et al., 2003). The tectonic setting of the rock sample could include a metabasaltic underplate of amphibolite and eclogite, a dike emplaced at shallow crustal levels and subsequently buried during convergence and crustal thickening, or some other setting (Hollis et al., 2003). The Selwyn Creek field site lies on the western margin of a major batholith called the Darran Complex. The batholith is composed of mostly unmetamorphosed gabbroic, dioritic and granitic rocks emplaced into the upper crust Ma (Kimbrough et al., 1993; 1994; Muir et al., 1998; Nathan et al., 2000). Klepeis et al. (2004) suggest that a belt of high grade rocks called the Selwyn Creek Gneiss (Fig. 2) represents the deformed western margin of the Darran Complex. Marcotte et al. (2005) suggest that underthrusting of the Darran complex beneath the Gondwana margin (see also Muir et al., 1998) was accompanied by the migration of the source of magmatism continentward as the outboard terrain converged with the continental margin. We chose rock samples from New Zealand because it is thought that the Darran Complex is associated with a primitive island arc setting (Hollis et al., 2003). The Separation Point batholith, which occurs mostly in Westland region of the South Island of New Zealand, has a TTG mineralogical composition, a Sr/Y signature of > 40 and no Eu-anomaly; this is similar to Archean TTGs (Tulloch, 2003). Additionally, the isotopic compositions are very different from MORB (Muir et al. 1995). The Western Fiordland Orthogneiss (WFO) is considered to be the lower crustal equivalent of the Separation Point batholith (because they are compositionally and isotopically similar) (Muir et al. 1995). The

5 Darran Complex is older than these two rock suites and contains metabasaltic dikes compositionally similar to the mafic material found in the WFO. There are two predominant models for generating the mafic source material underplating and slab melting. For this study we will assume a mafic underplate provided both the material and the heat necessary for melting. The physical and chemical characteristics of the Separation Point batholith provide good evidence for crust underplating in this area (Muir et al., 1995). Underplating is the ponding of mafic material at the base of the continental crust. This mafic underplate could supply the material and heat source necessary for melt to migrate to progressively higher levels in the crust. The more traditional model of TTG formation involves slab melting, but rocks produced by slab melting are chemically different from TTGs; these rock suites are adakites. Because the Separation Point rocks lack the MORB signature, the protolith for this TTG suite cannot be a melted, young, subducted oceanic slab (Muir et al., 1995). Though we assume an underplate model to eliminate a variable, the results of this study will apply to both tectonic settings since both settings involve melting a mafic source rock. Experimental Methods I mix reagent grade chemicals together in the proportions that comprise the starting bulk material (SC5) and the major element melt compositions obtained from batch melting experiments (Price 2005) at different temperatures. SC5-6 is melt material produced from melting SC5 at 850 C (which produces 5% melt and 95% solid residue), SC5-7 from 925 C (10% melt, 90% solid), and SC5-8 from 975 C (15% melt, 85% solid). To create the bulk and synthetic glasses I combine the chemicals (Table 1), crush them with a mortar and pestle in alcohol for at least 30 minutes, melt them at 1550 C for 2 hours in a 1-atm furnace, and powder the glass produced from melting. I repeat this twice to achieve sample homogeneity.

6 With the starting bulk material (SC5) and glass (SC5-6, SC5-7, and SC5-8) powders I fill the sample capsules for experimental runs in the piston cylinder (PS) apparatus. I make these samples by combining different modal abundances of starting bulk material and glass. I weigh the sample amounts (making a total of 0.2g sample powder), crush them for at least 30 minutes, and load them into the AuPd capsule with de-ionized 2.5-4% H 2 O because low degree partial melts are naturally hydrous (Clemens, 2006). I then weld the capsule closed and store in a 120 C drying oven for 20 minutes before re-weighing it to determine if there is weight loss, which would indicate that there is a hole in the capsule through which water escapes (we cannot use such a sample). Prior to loading in the PS apparatus, the powdered samples are encased in a capsule, which is a 14mm cylinder of crushable aluminum (with hand drilled holes for the capsules) and a graphite furnace. All of this is encased in a small glass tube and a salt cell. We simultaneously run two samples in the same apparatus. To tell the difference between them we wrap the top of one AuPd capsule with a Pt wire and place a Cu bar near one sample before encasing them in epoxy. The runs are labeled JP1-10; they are all conducted at 1.4GPa, which represents the base of the island arc (approximately 42 km depth in the crust). The temperatures are altered to reflect the steep geothermal gradient through which the melt is assumed to ascend during melt migration along mineral grain edges. Because the samples in the capsules need enough time to attain equilibrium, the 1000 C capsules remain in the apparatus for five to seven days; the 975 C capsules for seven to ten days. The lower temperature runs require a longer time in the piston cylinder to reach equilibrium than the higher temperature runs (Price, 2005). We consider homogeneity of melt composition to be an indication of equilibrium (Price, 2005). To produce the highest volumes of melt the

7 experiments are quenched between three to seven seconds (isobaric) (Price, 2005). The temperature to which the capsules are heated in the PS apparatus represent the temperature where that melt is produced during crust refertilization (the addition of a low volume partial melt) of the lower crust at progressively higher (more felsic) levels; for example, SC5-8 (15% melt), produced at 975 C, is run at temperatures no less than 975 C (because we would not produce this amount of melt at a lower temp.). Early experiments were run under conditions that did not produce the melt at that temperature, but these experiments are interesting because we can compare more realistic data to them. Data The rock compositions are plotted in figures 2 and 3. Figure 2 shows the results of the experimental runs thus far plotted on an Ab-An-Or diagram with the direct partial melting experiments conducted by Price (2005). This diagram shows graphically what rock types we produce in terms of albite-anorthite-orthoclase content. Most of the rocks produced are granodiorites the most felsic of the TTG suite. With increasing melt percentage (15%) added to the bulk starting material, I generated granites. At lower temperatures (925 C) we make granite with both the low (SC6-5%) and high (SC8-15%) degree partial melt added to the starting bulk material. We produce more anorthitic (calcic) melt with low degrees of partial melt, regardless of temperature. The more melt added to the metabasalt, the more albitic (sodic). The data in the granodiorite region of the Ab-An-Or ternary plot in the same area as the Separation Point Batholithic Complex natural data (Tulloch and Kimbrough, 2003). On the TAS diagram (Fig. 3) the composition of the melt is also granodioritic at lower melt percentages (5%). The 15% melt compositions have less silica and more alkaline material. These results plotted with the data for the direct partial melting of the metabasalt source rock show that adding melt (in all percentages) increases the amount of silica in the new experimental

8 melt (at all three melting temperatures (850 C, 925 C, and 975 C), SiO 2 is the primary constituent of the melt (~60% to ~76%), which comes from melting of mainly quartz and plagioclase (Table 1)). Additionally, the volume of melt generated by adding these low degree partial melts is greater than the melt created solely by direct partial melting of a source rock. This is important because the model predicts that we should generate large volumes of melt in this manner. TTGs are defined as having low Mg numbers (Condie, 2005). Mg#s are calculated 100Mg/(Mg +Fe) on the molecular level (Winter, 2001). The resulting melt compositions in these experimental new glasses have higher Mg#s when compared with direct partial melting Mg#s. Future Work My future goals for this project are to further constrain the model by conducting experiments of varying modal abundances of bulk starting material and glass. Experiments thus far have a large ratio of starting material to melt (50:50). This ratio is intentionally high because it provides us with a starting point for studying melt generation processes. By lessening the modal abundance of melt, we can observe the rock range over which my synthetic samples plot and compare this to TTG suites. In addition to altering the modal abundances of bulk and glass, other experimental adjustments that could alter the results are changing the melt percentages combined with the starting bulk material (other than 5%, 10%, or 15%), varying the amount of water, and increasing or decreasing the pressure or temperature. I have begun this suite of experiments by changing the temperatures, and I will begin testing some of the other experimental parameters by changing the modes; so far we have only experimentally run samples in a modal abundance; however, I have mixed powders and prepared sample capsules with modal abundances of 60:40

9 and 75:25 of bulk SC5 and partial melt SC5-7 (10% melt). This capsule will be in the PS apparatus at 975 C for 7 days. This will be useful because we can then compare the modal amounts to those predicted by the numerical model developed by Jackson et al. (2005). Whether or not the results from these experiments match those predicted, I will further constrain the model by eliminating the processes that do not generate TTGs. Dehydration melting occurs at temperatures greater than 800 C, and there is approximately 4% water contributed from hydrous minerals (amphibole and biotite). Though I am as careful as possible with water addition, it is difficult to add precisely 4% water because we lose water when welding the capsule closed. Because water significantly changes the amount of melt produced, I shall try to quantify how much water is lost during welding to enhance precision for future experiments (though if I am unable to do this, it will not affect my results). I will examine further Mg numbers generated for each melt. Calculating Mg numbers will enable us to chemically compare our experimental rocks to those found in nature, and we may be able to further constrain the tectonic setting that produces TTGs. Adakites (rocks thought to be formed through slab melting (Condie, 2005) and MORBs have high Mg numbers (>50 and >65, respectively), and TTGs have lower Mg#s (Winter, 2001). The data generated from these piston cylinder experiments will enable us to examine the changes in whole rock (restite) and melt (glass) compositions in the context of the Jackson/Petford (2005) numerical model. This model is important because it may quantifiably assess the genesis of continental crust in an island arc setting (Davidson and Arculus, 2006). It may explain large volumes of arc crustal melts (usually produced by low degree partial melting) without leaving significant amounts of residual eclogite. If our experimental results corroborate or deviate from what we expect, we constrain the model.

10 Timeline October/November continue piston cylinder experiments every Wednesday at McGill University in Montreal, Quebec - create and load capsules for 60:40 (JP11) and 75:25 (JP12) bulk to melt samples (SC5 and SC7) - continue to examine data for previous experiments (JP1 JP10) by plotting more of each sample on Ab-An-Or and TAS diagrams - determine Mg#s November 27 progress report December - February run new samples JP11 and JP12in the piston cylinder apparatus. Every piston cylinder run can accommodate two capsules. I will run one of each (JP11 and JP12) at 975 C and 950 C. Both runs will be in the apparatus for one week. - make new synthetic samples of similar modal abundances but with SC6 and SC8 added to the bulk starting material for future runs (either for my research time permitting or for subsequent researchers) March May obtain new melt and matrix data from the microprobe at McGill for comparison with not only Price s partial melting experiments (2005) but also with my previous melt-added experiments - potentially run additional samples of varying modal abundance - 60:40 and 75:25 bulk to melt samples (SC5 and SC6; SC5 and SC8) June/early July process data by plotting them on Ab-An-Or and TAS diagrams and determining Mg#s - assess my results in the context of the Jackson/Petford model, Late July/August write thesis End of August defend thesis

11 Table 1: Normalized weight percent compositions of starting materials. SC5 is metamorphosed basaltic dike sample collected at Selwyn Creek, Fiordland, New Zealand. SC5-6, 7 and 8 are synthetic glasses. SC5 Bulk composition SC5-6* SC5-7** SC5-8*** SiO TiO Al2O FeO MnO MgO CaO Na2O K2O P2O SrO negligible Total *SC5-6: Partial melt at 850 o C and 1.4 GPa representing 5 vol% melt from bulk SC-5 **SC5-7: Partial melt at 925 o C and 1.4 GPa representing 10 vol% melt from bulk SC-5 ***SC5-8: Partial melt at 975 o C and 1.4 GPa representing 15 vol% melt from bulk SC-5

12 Figure 1: Map of northern Fiordland at Milford sound. (Hollis et al., 2003). The natural metabasalt came from a mafic dike in the Selwyn Gneiss area (Price, 2005).

13 Figure 2: Ab-An-Or ternary diagram of piston cylinder experiments (SC5 data from Price, 2005, Separation Point data from

14 Figure 3: TAS diagram of piston cylinder experiments (data in the shaded key from Price, 2005)

15 REFERENCES Atherton, M. P., Petford, N Generation of a sodium-rich magma from newly underplated basaltic crust. Nature, 362: Brown, M., Rushmer, T Introduction to Evolution and Differentiation of the Continental Crust. Ed. Michael Brown and Tracy Rushmer. Cambridge University Press. p Clemens, J. D., Melting of the continental crust: fluid regimes, melting reactions, and source-rock fertility. Chapter 9 in Evolution and Differentiation of the Continental Crust. Ed. Michael Brown and Tracy Rushmer. Cambridge University Press. p Clemens, J. D., The importance of residual source material (restite) in granite petrogenesis: A comment. Journal of Petrology. 30: Daczko, N.R., K.A. Klepeis, G.L. Clarke., Evidence of early Cretaceous collisional-style orogenesis in northern Fiordland, New Zealand and its effects on the evolution of the lower crust. Journal of Structural Geology, 23: Davidson, J. P., Arculus, R The significance of Phanerozoic arc magmatism in generating continental crust. Chapter 5 in Evolution and Differentiation of the Continental Crust. Ed. Michael Brown and Tracy Rushmer. Cambridge University Press. p Hollis, J. A., G. L. Clarke, K. A. Klepeis, N. R, Daczko, T. R. Ireland, Geochronology and geochemistry of high-pressure granulites of the Arthur River Complex, Fiordland, New Zealand: Cretaceous magmatism and metamorphism on the paleo-pacific Margin. Journal of Metamorphic Petrology, 21: Jackson, M. D., Cheadle, M.J., Atherton, M. P., Quantitative modeling of granitic melt generation and segregation in the continental crust. Journal of Geophysical Research. 108 (B7): Jackson, M.D, K. Gallagher, N. Petford, M.J. Cheadle, Towards a coupled physical and chemical model for tonalite-trondhjemite-granodiorite magma formation. Lithos (79): Kimbrough, D.L., Tulloch, A.J., Geary, E., Coombs, D.S., Landis, C.A. (1993) Isotope ages from the Nelson region of South Island, New Zealand: structure and definition of the Median Tectonic Zone. Tectonophysics 225, Kimbrough, D.L., Tulloch, A.J., Coombs, D.S., Landis, C.A., Johnston, M.R., Mattinson J.M. (1994) Uranium-lead zircon ages from the Median Tectonic Zone, New Zealand. New Zealand Journal of Geology and Geophysics 37,

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17 Wickham, S. M., The segregation and emplacement of granitic magmas. Journal of the Geologic Society (London). 144: Winter, John D., An Introduction to Igneous and Metamorphic Petrology. Prentice-Hall Inc., Upper Saddle River, NJ. p