Migmatitic Metapelites Research Articles

An Exposed Hercynian Deep Crustal Section in the Sila Massif of Northern Calabria: Mineral Chemistry, Petrology and a P-T Path of Granulite-facies Metapelitic Migmatites and Metabasites

In the Sila massif of northern Calabria a continuous section is exposed through a segment of a Hercynian deep continental crust, which has been interpreted by previous workers as a stack of basement nappes (‘Monte Gariglione Complex’). The section consists essentially of metapelitic migmatites and subordinate metabasites and marbles, which were metamorphosed at medium-pressure–high-temperature granulite-facies conditions. A continuous metamorphic gradient through the exposed segment can be deduced from the systematic change in the compositions of ferromagnesian minerals in divariant metapelitic assemblages. This gradient is partly supported by conventional geothermobarometry and by applying the TWEEQU method. However, peak-metamorphic conditions are better defined by dehydration melting reactions, which reveal ∼4 kbar and 740°C at the top and ∼6 kbar and 770°C at the base of the section. Therefore the exposed rocks represent a former crustal depth range of ∼14–21 km at the thermal peak of metamorphism. The metamorphic evolution of the former deep crustal rocks has been reconstructed from reaction textures. The prograde P–T path took place in the stability field of sillimanite. The retrograde path is characterized by a stage of isothermal uplift after peak metamorphism to mid-crustal levels (10–15 km) followed by near-isobaric cooling to greenschist-facies conditions. The deep crustal section in northern Calabria resembles that of the Serre massif in southern Calabria, which represents the lowermost part of an exposed tilted cross-section through the Hercynian continental crust. Postkinematic mineral growth, reaction textures and isotopic age constraints indicate that the thermal conditions in the Calabrian crust during the Hercynian orogeny were mainly controlled by advective heat input through magmatic intrusions. As in southern Calabria, large granitic bodies were emplaced between the granulite-facies lower crust (‘Monte Gariglione Complex’) and the amphibolite- to greenschist-facies upper crust during regional metamorphism.

Leucosome-forming small-scale geochemical processes in the metapelitic migmatites of the Turku area, Finland

Selected outcrops of the metapelitic migmatites of the Turku area (southwestern Finland) have been investigated for their petrographic and chemical properties. Three different leucosome types could be distinguished: leucosome sills, in-situ leucosomes, and composite leucosomes. Almost all leucosomes are rich in Or-dominated alkali feldspar, only a few composite leucosomes show plagioclase-rich layers in addition. All leucosomes are of very similar composition (except the rare plagioclase-rich ones) and assumed to be generated by dehydration melting. The melt proportion formed by this reaction at peak metamorphic conditions (800°C, 5–6 kbar) is estimated from experimental data to be approximately 20 wt.%. The different leucosome types are considered to be the result of various degrees of melt segregation and transportation: the in-situ leucosomes were formed almost in place, the composite leucosomes are the outcome of segregation and transportation of partial melts over relatively short distances (<1 m), and the leucosome sills represent the most mobile melt fraction. A comparison of REE patterns of alkali feldspar–quartz-rich leucosomes and restite assemblages reveals low overall REE contents in the leucosomes with distinct positive Eu-anomalies. The Eu is inherited from the feldspars of the source rock while the light and heavy REE remained in accessory minerals and garnet in the restite. When compared to the metapelitic source rock, the restites are enriched in HREE depending on their modal garnet abundance.

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post‐metamorphic cooling in the Hercynian crustal cross‐section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma−1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.

Metapelitic Migmatites From Pangidi Granulite Complex, Eastern Ghats Belt, India: Petrological Constraints on Crustal Melting and Tectonic Exhumation

Metapelitic migmatites in the Pangidi Granulite Complex, Eastern Ghats belt preserve a range of high-grade mineral assemblages that vary with bulk composition. Reaction textures preserved in the migmatites indicate that the crystallized granitic melt was formed in situ by biotite dehydration-melting reactions. Decompression occurred during an episode of partial melting and melt crystallization, and was synchronous with exhumation of migmatites along shear zones. Retrograde reaction textures and the calculated positions of the mineral reactions and discrete P-T points obtained by thermobarometry reflect ∼3 kbar (11 km) of exhumation from near peak P-T conditions of ∼9 kbar and 950°C. Decompression has been traditionally ascribed to be the cause of dehydration melting in many orogens, but the bulk of melting reactions in Pangidi were ultimately driven by elevated temperatures. Simple pressure release during exhumation did not force melting reactions, instead as the present study reveals, it is more likely that the presence of melt triggered deformation in the migmatites and facilitated exhumation.

Metapelitic Migmatites from Brattstrand Bluffs, East Antarctica—Metamorphism, Melting and Exhumation of the Mid Crust

Metapelitic Migmatites from Brattstrand Bluffs, East Antarctica—Metamorphism, Melting and Exhumation of the Mid Crust

Structural and metamorphic evolution of the Nanga Parbat syntaxis, Pakistan Himalayas, on the Indus gorge transect: The importance of early events

AbstractField relationships are as important as radiometric age data for unravelling the history of polycyclic metamorphic regions. Field and petrographic data were used to investigate such a region, the Nanga Parbat syntaxis ‐ a major N‐trending transverse structure in the Pakistan Himalayas. Continental rocks of the Indian plate are exposed in the core of this structure due to uplift relative to the surrounding and structurally higher amphibolites of the overthrust Kohistan Island Arc. In the Indus gorge, the Indian plate gneisses are migmatitic, highly deformed and cut by Himalayan granite sheets. Field relationships show that the migmatization pre‐dates the emplacement of a suite of basic intrusive sheets (now amphibolites) which are themselves unlikely to be Himalayan; these amphibolite sheets are cut by the Himalayan granite sheets. There is therefore no genetic link between the granites and the migmatization: the latter might be Precambrian. During the pre‐Himalayan evolution, an eastern group of metapelitic migmatites underwent biotite dehydration melting at granulite facies conditions, leading to the development of assemblages with garnet, orthoclase and kyanite. Melt was not completely lost and back‐reacted during cooling to give biotite‐kyanite coronas around garnets. A western group of orthogneisses is characterized by foliated muscovite and biotite; orthoclase is never found with either garnet or aluminium silicate. In this second group, partial melting and small‐scale segregation gave rise to the migmatitic appearance, but melt was not lost and back‐reacted fully to regenerate the micas.The migmatites and basic sheets were strongly deformed at amphibolite facies during Himalayan collision, producing LS tectonites, mylonitic in places. This strong fabric was then tightly folded on large scale. The NNE trending fold is slightly oblique to the earlier N‐S stretching lineation; this has been reoriented on the steep limbs, but its plunge remains shallow. No new fabric is seen associated with this folding. On the western margin, steeply plunging lineations are associated with shear bands in gneisses, indicating relative uplift of the syntaxis. In contrast, on the steep eastern margin, relative uplift of the syntaxis appears to have been due to rotation without the development of new fabrics. There is no evidence for further melting during this Himalayan evolution, so the granite sheets cannot have been derived locally. This study highlights the importance of distinguishing pre‐Himalayan and Himalayan events in this polymetamorphic region, and the usefulness of field relationships in addition to geochronological constraints in such a task.

Phase equilibria and oxygen isotopes in the evolution of metapelitic migmatites: a case study from the Pre-Alpine basement of Northern Switzerland

Metapelites that have undergone variable degrees of migmatization were encountered in two deep boreholes in Northern Switzerland. Microtextures and calculated phase diagrams yield three successive stages of metamorphism, consistent with one P-T loop: (1) Amphibolite-grade metamorphism (biotite-sillimanite-garnetoligoclase-quartz-ilmenite), P-T conditions are estimated at 4.5–6.5 kbars/550–650 °C. (2) Anatectic migmatization melting reactions. Leucosomes do not represent pure crystallized melt but rather a mixture of melt, cordierite and K-feldspar grown at the expense of biotite and sillimanite. (3) Retrograde hydration, recorded by the growth of large amounts of muscovite at the expense of cordierite and K-feldspar, P-T conditions are estimated at <2.5 kbars/ <600 °C. Oxygen isotope measurements were obtained for whole-rocks and eight mineral species. Isotopic equilibrium among three or even four minerals grown during stage 1 can be demonstrated. Calculated isotopic temperatures are consistent with phase petrology. Migmatization did not re-equilibrate the isotopic composition of the pre-existing quartz grown in stage 1 even over distances as little as 10 cm. High δ18O in migmatitic quartz is best explained by disequilibrium melting in coexistence with an infiltrating, isotopically heavy fluid. Lack of equilibration of oxygen isotopes between different quartz generations suggests that mineralogical and geochemical changes were rapid relative to diffusion rates. A meteoric-hydrothermal alteration at 300–400 °C, probably genetically linked to the intrusion of Variscan granites, strongly affected the rocks. Quartz did not exchange isotopes with hydrothermal fluids except in segregations where it is recrystallized. The δ18O values of micas, feldspars and cordierite are often very low and fractionations with quartz very large, which reflects significant hydrothermal effects that were previously detected in the Black Forest by other workers.

Geological relationships in high‐grade gneiss of the Brattstrand Bluffs coastline, Prydz Bay, East Antarctica

The Brattstrand Bluffs coastline of Prydz Bay, east Antarctica consists of Proterozoic granulite and subordinate late intrusions. Two gneissic associations have been identified, one comprising interlayered felsic and mafic orthogneiss, and the other comprising metapelite‐dominated paragneiss. The orthogneiss exhibits fabrics (S1) and migmatitic textures developed in metamorphic events not experienced by the paragneiss, which is interpreted as a cover sequence deposited onto the orthogneiss after these early events. Cover and basement were tectonically interleaved (D2), and then metamorphosed during a Proterozoic granulite event (∼ 1000 Ma). Geothermobarometry of garnet‐orthopyroxene‐plagioclase‐quartz semi‐pelite yields peak pressure‐temperature estimates of 0.6 GPa and 850°C for this event. Peak metamorphism was associated with development of a penetrative foliation (S3), and generation and extraction of leucocratic melts from metapelite. Textural studies of metapelitic migmatite indicate that partial melting proceeded through incongruent biotite melting reactions such as: biotite + sillimanite + quartz = garnet + cordierite + K‐feldspar + melt, which proceed in the absence of a volatile fluid phase. Retrograde mineral textures, such as spinel‐cordierite symplectites after garnet and sillimanite, indicate decompression from peak pressures to 0.3–0.4 GPa, which was partly synchronous with localized development of flat‐lying folds and shear fabrics (D4). This combination of decompression and subhorizontal shear is attributed to extensional collapse of overthickened crust. Further retrograde evolution was characterized by cooling, emplacement of a granite pluton and dykes, and D5 shortening to produce a regional antiform under amphibolite to granulite conditions. A pegmatite suite was intruded at ∼ 770 Ma, and mylonite development, further granite intrusion and a greenschist overprint occurred at 500 Ma.

Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa

Abstract Dehydration‐melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high‐grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of aH2,o. The element‐partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aH2o is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T± 850°C is relatively small (10–20%), and probably insufficient to mobilize a partially melted rock body.Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse‐grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration‐melting reactions: Bt + Sil + Qtz + PI = Grt ° Crd + Kfs + L (metapelite), Bt + Qtz + Pl = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + Pl + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that H2O in the melt was released as vapour on crystallization.This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.

Compositional and modal relationships and melting reactions in some migmatitic metapelites from New Hampshire and Maine

Compositional and modal relationships and melting reactions in some migmatitic metapelites from New Hampshire and Maine