Schlieren Structures Research Articles

Local processes involved in the generation of migmatites within mafic granulites

ABSTRACTProcesses involved in the formation and evolution of melts within the lower crustal mafic granulites are considered with reference to mafic migmatites from late Proterozoic (1200-1000 Ma) granulites of the Rauer Group, East Antarctica. Metaluminous dioritic and noritic leucocratic veins on scales of 1 cm to 1 m show agmatitic, stromatic and schlieren structures. These possible melts are compositionally distinct from charnockitic and enderbitic orthogneisses, which show intrusive contacts with the migmatites in areas of low strain.Important field relationships include the following:(a) Leucocratic veins contain plagioclase and rare quartz, coarse subhedral to euhedral orthopyroxene, ilmenite and apatite. Finer (2 cm) veins and layers are richer in mafic phases than larger (2-10 cm) veins.(b) Selvedges or melanosomes are developed between the larger melt areas and enclosing mafic gneisses. These melanosomes consist of garnet, orthopyroxene, plagioclase and biotite and are apatite-rich.(c) Pyroxene granulite palaeosomes typically display bleached zones (1-2 cm) adjacent to selvedges and veins, in which the modal proportion of clinopyroxene diminishes in favour of orthopyroxene.Geochemical and petrological studies demonstrate that localised or near-localised partial melting of the mafic granulites occurred during decompression from 8-9 kb to 7 kbar at a minimum temperature of 800-850°C. Geochemical mass balance calculations using measured vein, selvedge and palaeosome compositions indicate that near-closed system melting behaviour is likely for a large number of major and trace elements, but LILE behaviour is affected by the introduction of biotite probably associated with late stage fluids. Minor- and rare-earth element modelling predicts similar percentages of melting to those observed in the field, but yields reasonable results only when garnet is included as a minor residual phase. HREE concentrations in melanosomes do not show expected enrichments, probably as a result of later subsolidus changes including the breakdown of garnet during decompression.This study demonstrates that migmatites may form through the near-localised partial melting of basic lithologies within the granulite facies. The exact role of fluids in this case cannot be determined but melting is interpreted to be vapour-undersaturated. This process may be important in the production of volumetrically significant amounts of dioritic to tonalitic calc-alkaline magmas.

The chromitiferous rocks of the Troodos ophiolite complex, Cyprus

The Troodos massif is a well-developed ophiolite complex comprising a sequence of basaltic volcanic rocks overlying a plutonic complex of upper gabbros and lower peridotites containing podiform chromite deposits. The chromitiferous rocks range from disseminations and minor segregations of chromite in dunite to orebodies of economic size and grade composed of nearly pure chromitite. They display both primary textures, such as chromite net and occluded silicate texture of cumulate magmatic origin, and schlieren structures produced by deformation. Dendritic texture in nodular chromite is interpreted as a product of crystallization of a magma that was supersaturated and/or supercooled with respect to chromium. Unusual orbicular structures are thought to have formed by mechanical accretion of previously settled chromite grains around a nucleus of dunite.Chromite deposits occur mainly as isolated pods and layers either in the dunite near the harzburgite contact or in the harzburgite within dunite lenses or enclosed in a narrow dunitic envelope. Deposits show a wide range of reciprocal Cr-Al variation and have low iron oxidation values typical of alpine chromitites. The areal distribution of chemically and texturally different deposits of variable size and grade conforms to no recognizable pattern. Accessory chromite exhibits a trend of decreasing Cr and increasing Fe in the passage from dunite, through wehrlite, to picrite.It is concluded that the plutonic complex evolved as a differentiated body of magmatic cumulates laid down upon a basement of depleted mantle harzburgite. The chromitites originated as isolated magmatic segregation ore deposits near the base of the cumulate dunite during episodic crystallization at localized centers within the magma. Postcumulus mobilization and accompanying penetrative deformation of the lower ultramafic rocks caused the tectonic overprint of schlieren structures on earlier magmatic textures, and associated large-scale rock flowage resulted in deep infolding between lithologic units; outliers of chromite-bearing dunite were formed in this way.