Regional-Scale Partial Melting of Granites; the Necessity of Deformation and Influx of Water
Migmatites in the centre of the Opatica Subprovince (Superior Province) developed from leucogranodiorite protoliths during late Archaean, low-temperature (680 to 740°C) medium pressure (5 to 6 kbars) anatexis. Migmatites occur in a region of back thrusts in the Opatica corresponding to a retro wedge. The morphology of the migmatites correlates with their structural position. Schollen diatexites occur in the back thrusts and metatexite migmatites between the thrusts. Water-fluxed melting of quartz, plagioclase and microcline, is inferred because these minerals, but not biotite, have corroded grain boundaries. Between 25 and 30 % partial melting occurred and terminated when K-feldspar had been consumed. Microstructures enable highly residual rocks to be distinguished from deformed plagioclase cumulates; the former contain crystal shapes that indicate the former presence of thin films of melt. Whole rock delta 18 oxygen of the protolith (average 8.2 per mil) and migmatites (average 8.3 per mil) are the same, but melt-rich rocks are 0.6 per mil higher than residual ones. The fluid that infiltrated and caused melting was probably a metamorphic one. Lithoprobe reflection profile 48 crosses the migmatites and reveals a series of south-dipping structures (the back thrusts) under the migmatites to a present depth of 20 km. These are interpreted to have been the channels along which aqueous fluids liberated by the breakdown of amphibole in the middle crust migrated to higher crustal levels where they enhanced the fertility of the leucogranodiorites there and enabled them to melt. Schollen diatexites contain flow structures and have compositions indicating that they are melt-rich; melt migrated within and possibly along, the shear zones. A new type of in situ neosome found in the metatexite migmatites is interpreted to have formed along fractures through which the aqueous fluid entered the protolith; it entered via a network of fractures rather than pervasively. The position of the metatexites relative to the conduits provided by the back thrusts indicates that the new neosome are preserved at the periphery of the zone of fluid migration. Evidence for the pathway of the fluid has not been found in the diatexites because of overprinting deformation and crystallisation of the melt.
Formation of a Granite Bodies in Depleted Granulite Terranes: the Wuluma Granite, Central Australia
The Wuluma Granite (ca.17 km2) is hosted by Palaeoproterozoic, granulite facies metasedimentary and metaigneous rocks. It is believed to have formed by in situ partial melting of quartzo-feldspathic gneisses at 1728±3 Ma due to the influx of an externally derived aqueous fluid after the granulite facies metamorphism. We have reinvestigated the Wuluma Granite and find that most contacts between the granite and the host granulites are intrusive, not gradational. Granite occurs as thin (<1m) subconcordant sheets and dykes in country rocks that contain fresh orthopyroxene and cordierite without much replacement by hydrous minerals. Screens of country rock are common within the granite, and many contain metapelitic rocks that have leucosome and melanosome structures similar those found in the country rocks. Although some of the migmatite structures in the screens still contain garnet, cordierite and orthopyroxene, in most these minerals are replaced by biotite. Biotite is the only ferromagnesian mineral in the thinnest screens of country rock. All the screens contain subconcordant sheets and dykes of granite; typically a narrow selvedge is developed between the intrusive granite and the rocks of the screen; selvedges are either rich in biotite or in quartz depending on the host rock type. Schlieren are common throughout the granite and represent the last vestiges of the country rocks in the granite; there is much morphological and mineralogical variation among the schlieren. The Wuluma granite consists of innumerable thin (less than a metre) subparallel sheets and cross-cutting dykes, that are distinguished by variations in grain size, microstructure and the proportion of minerals present. The earliest phase to be porphyritic and rich in biotite, whereas the last is leucocratic, coarse grained and locally forms dykes up to 20m wide. The centre of the granite contains large (1 cm) crystals of garnet and, more rarely, cordierite. However, in many places these have been replaced by biotite (or chlorite), although, small crystals of garnet within the matrix persist locally. Virtually all of the granite contains a magmatic foliation, and this together with the presence of small dykes of granite in shears and in fold hinges indicates that the granite body formed during regional deformation (the local D3 event). Thus, the Wuluma granite did not form by in situ partial melting. Rather, it formed at a site where small increments of anatectic melt extracted from the surrounding granulite terrane during regional deformation were able to accumulate.
Progressive melting reactions in Precambrian migmatitic complex, east-central Korea: Petrogenetic linkage between low-pressure/high-temperature metamorphism and granitic magmatism
The migmatite-leucogranite complex in the Precambrian Yeongnam Massif, Korea, shows progressive melting reactions during the anatexis and provides some clues for unraveling the genetic relationship between high- grade metamorphism and granitic magmatism. The Yeongnam Massif is characterized by the occurrence of low-pressure/high-temperature (LP/HT) schists and gneisses accompanying widespread anatexis and granitic magmatism. Metapelitic mineral assemblages define three progressive metamorphic zones pertinent to low-pressure facies series: cordierite, sillimanite, and garnet zones with increasing temperature. Migmatitic gneisses are prominent in the sillimanite and garnet zones. Metamorphic grade ranges from lower amphibolite to lower granulite facies, and peak metamorphic conditions in migmatitic gneisses reach ca. 750∼800 ° C and 4? kbar. Textural and petrogenetic relationships in leucosomes suggest that migmatitic gneisses are the product of anatexis of metasedimentary rocks. The migmatite formation during progressive metamorphism is governed initially by fluid-present melting and subsequently by biotite- dehydration melting. The large amount of leucosomes in the sillimanite and garnet zones can be accounted for by the fluid-present melting possibly triggered by an external supply of fluid. Peritectic occurrences of garnet and cordierite in leucosomes suggest that the biotite-dehydration melting is associated with the peak metamorphism. Given that leucogranites, dated at ca. 1.86 Ga using the SHRIMP analysis of zircon, occupy approximately 60∼70 % of the study area, we conclude that granitic magmatism is coeval with pervasive LP/HT metamorphism as a result of regional thermal metamorphism.
Magma Mixing and Underplating in South Gangdese, Tibet: Evidences from Zircon Hf Isotopics of Cenozoic Granitic Complex
Abstract: It is argued that magma mixing was genetically related to underplating of mantle-derived magmas beneath the Gangdese magma belt. Mo et al. (2005) and Dong et al.(2005) determined that magma mixing and underplating took place extensively in the Gangdese magma belt around 50 Ma. Underplating as one of major mechanisms for crustal growth can occur in the environments of convergent plate boundary, subduction and collision zones, and in intra-plate settings. The mantle-derived magma underplates the base of the crust, transfers mass and heat from the mantle to the crust, and promotes its partial melting. There is a giant magma belt in Gangdese southern Tibet, consisting mainly of granitoids with abundant mafic macrogranular enclaves (MME). In addition to the MME, many discrete gabbro bodies occur within the granitoid plutons, forming a mafic intrusion zone along the southern margin of the Gangdese belt. The granitoids are varied in composition, while the mafic intrusions consist of gabbro, Hb-gabbro, gabbro-pyroxinite and other mafic-ultramafic accumulates. The spatial-temporal variation and geochemical features of the igneous rocks put new constraints on the magma mixing and underplating processes. Evidences indicated that the Gangdese gabbro-pyroxenite assemblage was most likely a result of underplating of mantle-derived magma and the variable intrusive compositions were the result of magma mixing. Base on detailed field investigation, lithological and geochemical study and zircon SHRIMP II U-Pb dating of both the granitoid and mafic rocks at eleven locations, La-ICPMS Hf isotopic analysis of the zircons was systemically conducted along the Lhasa- Xigaze segment of the magma belt. In addition to the ages (47.0-52.5 Ma) and positive Nd values of the samples previously published, the 176Hf/177Hf ratio ranges from 0.282714 to 0.283222, corresponding to ¦ÅHf(t) values from 1 to 14.7, providing isotopic constraints on the Eocene magma underplating beneath the Gangdese. The Hf depleted mantle modal ages (TDM) are in the range of 138 to 632 Ma, the zircon Hf isotope crustal model ages (TDMC) range from 178 to 988 Ma. The mantle-like high and positive ¦ÅHf(t) values in these samples suggest the mass from mantle underplated blow the low Crust and input to juvenile source regions, from which the granitoid generated. There are no any significant correlations between sample lithology and their 176Hf/177Hf compositions, which implies an isotopically homogeneous processes during magma mixing. This underplating event post-dated the initiation of India-Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. It is believed that the magma mixing and underplating are of main process of mass-energy exchange between the mantle and the crust. It bursts out from magma underplating during the early stage of continental collision, resulting in partial melting in the crust and then magma mixing following the magma invading up in the crust along Gangdese belt, forming the Gangdese giant magma belt. Keywords: Granitoid, gabbro, MME, Hf isotope, magma mixing and underplating, Gangdese, Tibet