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Indicator Minerals

The forward approach of melting possible source rocks under known conditions is complemented by the inverse approach of crystallizing the magmas to determine the liquidus mineralogy as a function of pressure, temperature, H2O content, and other variables. If the tonalites and trondhjemites are primary magmas from amphibolite, then for the pressure and temperature of their generation, the near-liquidus minerals should match the residual minerals in partially melted rocks. Experiments on the Nuk trondhjemite from Greenland by Drs. A. D. Johnston and S. R. van der Laan illustrate the approach. Figure 2 shows the liquidus surface in terms of pressure and temperature, with (1) a third dimension shown by the contours for H2O content (figure 2A), and (2) the distribution of liquidus mineral fields and field boundaries (figure 2B). According to these results, the trondhjemite is near multiple-saturation with the assemblage [Cpx + Pl + Hb + Ga] at 13.5 kb, 900 C, with about 7.5% dissolved H2O. The liquidus minerals match the residual minerals in area (3) in figure 1B.

The Origin of Archean Tonalites and Trondhjemites

The tonalites, trondhjemites and granites (TTGs) of the Archean grey gneisses are characterized by highly fractionated rare earth elements, and this appears to require that amphibole, or garnet or both were residual minerals during generation of the magmas. Recently, attention has been focussed on vapor-absent dehydration-melting, during which H2O released by the breakdown of amphibolite dissolves directly into a H2O-undersaturated silicate liquid, and it appears that this process can yield liquids corresponding to the Archean TTG rocks through a range of pressures and temperatures. Dr. M. B. Wolf has completed a detailed study at 10 kbar (40 km depth) of the effects of temperature, time and texture on the progressive melting of vapor-absent amphibolite. His results are consistent with the phase diagram for vapor-absent amphibolite shown in figure 1A. The new feature is the sharp backbend of the solidus near 9 kbar and 900 C, associated with the formation of garnet. This expands the field for liquid generation with garnet-amphibole residues to much lower temperatures and pressures than in the other recent experimental results. Figure 1B shows a generalized map of the residual mineral assemblages to be expected after a small fraction of melt has been removed during dehydration-melting of amphibolite.

Figure 1. (A). Selected phase boundaries for fully- hydrated, simple amphibolite. Vapor-absent melting occurs where hornblende begins to break down forming augite in the reaction interval [1], and forming garnet (and other minerals) in the reaction interval [3-5] (Wyllie and Wolf, 1993). (B) Residual minerals left behind after dehydration-melting of amphibolite and magma segregation. Abbreviations: Hb = hornblende, Pl = plagioclase, Cpx = clinopyroxene, Ga = garnet.

Experimental data are also available for the liquidus minerals on the H2O-undersaturated liquidus surfaces of two tonalites with different SiO2 contents. The liquidus field boundaries for amphibole and garnet occur in similar positions for all three magmas. One can read the pressures, temperatures and H2O contents of magmas that would leave residual garnet, amphibole or both in the source rocks. Residual amphibole requires moderate temperatures but high H2O; residual garnet requires depths greater than about 50 km, lower H2O, and higher temperatures than for residual amphibole. There is a only a limited area where garnet and amphibole coexist on the liquidi. Further refinement of these phase boundaries will place tighter constraints on processes in terms of depth, temperature and H2O contents, and help to define the tectonic environment in which the magmas were formed and emplaced.

Figure 2. The H20-undersaturated liquidus surface for the Nuk gneiss (trondhjemite). Abbreviations: see fig. 1, and Ep = epidote. (A) The liquidus surface mapped with approximate contours for H20 content. (B) Primary minerals on the liquidus surface. Compare the boundaries for hornblende, and for garnet, with the residual minerals in fig. 1 (van der Laan and Wyllie, 1992).