The Impact of Melt Segregation on TTG Petrogenesis
Abstract
Tonalite-trondhjemite-granodiorite (TTG) and associated magmatism comprises a significant portion of the continental crust and provides a window into the origin of Earth's earliest crust, the role of the mantle wedge, and the thermal regimes present in both early Earth and post-Archean subduction zones. However, TTG magmatism is an enigma because the tectonic setting in which the basaltic source material undergoes partial melting is still unresolved. Major, trace and REE compositions of both Archaean TTG's 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 however may have a profound impact on the volume and composition of the segregated melt as it leaves the source region. We are currently testing this process through experiments which are designed to reproduce the local changes in bulk composition that are predicted to occur in response to melt segregation, which occurs along a steep geothermal gradient through melt migration along grain boundaries and contemporaneous matrix compaction, by the numerical model of Jackson et al (2005; Lithos, 79, 43-60). Piston-cylinder experiments have been performed at 1.4 GPa and at temperatures between 900°C and 1000°C on a metabasalt with conditions and compositions based on the model parameters. We add low degree hydrous partial melt compositions (5%,10%,15%) to the starting bulk metabasalt to simulate the compositional evolution of migrating melt chemically interacting and thermodynamically equilibrating with its source material; these partial melt compositions were previously determined by direct partial melting experiments on the same metabasalt starting material (Price, MSc, 2005). The partial melts are added in varying modal amounts (10%-50%) along with 2.5 to 4.0 wt% H2O. Preliminary results suggest we have changed the geochemical system significantly by introducing a low- degree partial melt into a metabasalt source material. The resulting melt compositions in these melt segregation equilibration (MSE) 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 (DPM) results. Modally, the charges have changed: hornblende and plagioclase stability are reduced and garnet and clinopyroxene modes increase as a function of temperature. One possibility raised by this preliminary study is 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. Thus it is possible that high Mg numbers in adakites might be a segregation effect rather than a direct indication of mantle contamination (after slab melting). We reason this is an area that deserves further investigation and will ascertain whether or not large batholiths of TTG rocks can be developed in this way.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V11F..05G
- Keywords:
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- 3619 Magma genesis and partial melting (1037);
- 3630 Experimental mineralogy and petrology;
- 3640 Igneous petrology