Phase relationships in peraluminous granites : a field, experimental and theoretical study

Knowledge of phase relationships in granitic magmas is of major importance in the understanding of granite petrogenesis. It has implications for models of the nature, provenance, evolutionary processes in and metallogenetic potential of magmas. Peraluminous compositions were chosen because there is substantial information about their subsolidus equilibria on which to base a magmatic study. A review of the efficacy of five commonly proposed magmatic differentiation processes reveals that each has only limited application in the light of the physicochemical properties of granite magmas. Granitic rocks from the Furneaux Islands in the northern portion of the Blue Tier Batholith (NBTB) of eastern Tasmania were studied. Six major suites were identified, using chemical, geological and mineralogical criteria. They include hornblende-bearing I-type granodiorites (the Wybalenna Suite), cordierite- and garnet-bearing S-type adamellites and granites (the Musselroe and Boobyalla Suites) and andalusite- and cordierite-bearing A-type alkali feldspar granites (the Babel Island Suite). be high-level granites. Porphyries are common. Their textures indicate that most melts were phase multi-saturated. Parageneses constructed on the basis of petrography, indicate probable magmatic assemblages for each suite and implicate the existence of magmatic reactions consonant with the appearance and disappearance of phases. Compositional data of phases in porphyries are consistent with the operation of magmatic processes which lead to progressive changes in the compositions of the crystalline phases. However, these have been variously modified by subsolidus reactions, especially those in coarser-grained rocks. In addition to the first order whole-rock chemical characteristics of each suite, there are second-order features which correlate with sample type. Phenocryst-poor whole rocks and matrix-separates form trends which are distinguishable from those of the phenocryst-rich rocks from the same suite. This difference is attributed to differences between the compositional trend for melts and that for the bulk-phenocryst assemblages within the magmas. Estimates of intensive variables operative during the petrogenesis of NBTB granites were obtained by applying four barometers and seven thermometers to the compositions of matrix separates and minerals. The cores of phenocryst phases such as the feldspars and garnet, retain chemical features of earlier, high-PT equilibria whereas their rims and the other mafic phases record the chemical features of later, low-P subsolidus equilibria. From these, partial PT paths were constructed for five suites. Source conditions for Boobyalla Suite magmas are estimated to have been -0.6 GPa P\\(_‚Äöv†v´\\), P\\(_W\\)/P\\(_‚Äöv†v´\\) = ~0.4 and ~850¬¨‚àûC. Solidus pressures were ~0.15a¬¨¬±0.1 GPa P\\(_W\\). f0\\(_2\\)-conditions estimated using a new oxygen fugacimeter for (biotite+vapour)-bearing assemblages are close to the QFM-buffer at equilibrium biotite/garnet temperatures of ~550¬¨‚àûC. An experimental study of Fe*Mg\\(_{-1}\\) exchange equilibria amongst magmatic peraluminous phases was made in a simple-system analogue to that of the natural peraluminous granites: the KFMASHO chemical system, using a pelitic starting composition. Watersaturated experiments were co,n ducted from 800 to 1000¬¨‚àûC and at pressures from 0.5 to 1.5 GPa. Addition of Ti0\\(_2\\) to this system influences Al-Fe*-Mg-equilibria. However its major effect is to substantially increase the f0\\(_2\\) within the charges. Data of this study, together with that of Ellis (1986) for the simpler, FMASHO chemical system, were used to construct Fe*Mg\\(_{-1}\\) exchange thermometers for biotite+garnet, garnet+melt, cordierite+melt and orthopyroxene+melt assemblages. The biotite/garnet thermometers of Holdaway & Lee (1977), Thompson (1976b) and this work predict similar temperatures. Low-temperature granitic liquids are predicted to be more magnesian than equilibrium garnet, on the basis of the garnet/melt thermometer. PT and P\\(_‚Äöv†v´\\), P\\(_W\\)T grids of magmatic equilibria in the KFMASH analogue simple-system were constructed using the methods of Schreinemakers. A PLPWT loop through the 3D grid for this system, yields a paragenesis which resembles that for the Boobyalla Suite granitic rocks. Isobaric liquidus surfaces have been constructed for assemblages saturated in alkali feldspars, quartz and vapour. Major-element phase-relationship modelling of the Boobyalla Suite trend for porphyritic whole-rocks was undertaken, assuming to to have been a magma trend in which the melt compositions are given by the Babel Island whole-rock trend. The results indicate all th~ magmas to be crystal-rich and that magmatic differentiation occurred by a combination of restite unmixing and peritectoid fractionation. The model requires a side-wall accretionary mechanism. Differentiation by these means has implications for granite classification, the nature of granite magmas, magmatic processes, provenance and metallogensis.