The Kodaikanal region of the Madurai Block in southern India exposes a segment of high-grade metamorphic rocks dominated by an aluminous garnet–cordierite–spinel–sillimanite–quartz migmatite suite, designated herein as the Kodaikanal Metapelite Belt (KMB). These rocks were subjected to extreme crustal metamorphism during the Late Neoproterozoic despite the lack of diagnostic ultrahigh-temperature assemblages. The rocks preserve microstructural evidence demonstrating initial-heating, dehydration melting to generate the peak metamorphic assemblage and later retrogression of the residual assemblages with remaining melt. The peak metamorphic assemblage is interpreted to be garnet + sillimanite + K-feldspar + spinel + Fe–Ti oxide + quartz + melt, which indicates pressure–temperature (P–T) conditions around 950–1000 °C and 7–8 kbar based on calculated phase diagrams. A clockwise P–T path is proposed by integrating microstructural information with pseudosections. We show that evidence for extreme crustal metamorphism at ultrahigh-temperature conditions can be extracted even in the cases where the rocks lack diagnostic ultrahigh-temperature mineral assemblages. Our approach confirms the widespread regional occurrence of UHT metamorphism in the Madurai Block during Gondwana assembly and point out the need for similar studies on adjacent continental fragments. 相似文献
A detailed geochemical and geochronological study of anatectic migmatites from the Namche Barwa Massif (NBM), southern Tibet, has been carried out to place important constraints on the thermal and tectonic evolution of the eastern Himalayan syntaxis. SHRIMP zircon U/Pb dating indicates that the granulite-facies metapelite underwent metamorphism at 21.8 ± 0.7 Ma and 24.5 ± 0.7 Ma, respectively. The latter is similar to the timing of partial melting and the formation of Ca-rich leucosomes at ~ 24-25 Ma. These leucosomes are characterized by (1) high CaO, Na2O, and Na/K ratios; (2) radiogenic Sr (87Sr/86Sr(t) = 0.7407-0.7904) but unradiogenic Nd (εNd(t) = − 7.0 to − 21.2) isotope compositions; (3) depleted HFSE, and (3) variable but depleted HREE relative to their host pelites. Some of the leucosomes show large degrees of Nd isotopic disequilibrium, up to 10 epsilon units with respect to their hosts. These high CaO and Na2O leucosomes were derived from fluxing melting of metapelite at high pressures. A similar process could have operated during the formation of the Himalayan leucogranites and contributes to the heterogeneities in such granites. 相似文献
The metamorphic evolution of a key sector of the western Mediterranean internal Alpine orogenic belt (southern Calabrian Peloritani Orogen) is identified and described by means of P–T pseudosections calculated for selected metapelite specimens, showing evidence of multi-stage metamorphism.Attention focused on the two lowermost basement nappes of the Aspromonte Massif (southern Calabria), which were differently affected by poly-orogenic multi-stage evolution. After a complete Variscan orogenic cycle, the upper unit (Aspromonte Peloritani Unit) was involved in a late-Alpine shearing event. In contrast, the several underlying metapelite slices, here grouped together as Lower Metapelite Group, show exclusive evidence of a complete Alpine orogenic cycle.In order to obtain reliable P–T constraints, an integrated approach was employed, based on: a) garnet isopleth thermobarometry; and b) theoretical predictions of the P–T stability fields of representative equilibrium assemblages. This approach, which takes into account the role of the local equilibrium volumes in controlling textural developments, yielded reliable information about P–T conditions from early to peak metamorphic stages, as well as estimates of the retrograde trajectory in the pseudosection P–T space.According to inferred detailed P–T paths, the evolution of the Aspromonte Peloritani Unit is characterised by a multi-stage Variscan cycle, subdivided into an early crustal thickening stage with P–T conditions ranging from 0.56 ± 0.05 GPa at 570 ± 10 °C to 0.63–0.93 GPa at 650–710 °C (peak conditions) and evolving to a later crustal thinning episode in lower P–T conditions (0.25 GPa at 540 °C), as documented by the retrograde trajectory.Conversely, the prograde evolution of the rocks of the Lower Metapelite Group shows evidence of a HP-LT early Alpine multi-stage cycle, with P–T evolving from 0.75–0.90 GPa at 510–530 °C towards peak conditions, with pressure increasing northwards from 1.12 ± 0.02 GPa to 1.24 ± 0.02 GPa, and temperatures of 540–570 °C.A late-Alpine mylonitic overprint affected the rocks of both the Aspromonte Peloritani Unit and the Lower Metapelite Group. This overprint was characterised by an initial retrograde decompression trajectory (0.75 ± 0.05 GPa at 570–600 °C), followed by a joint cooling history, ranging from 0.38 ± 0.14 at temperature from 450 to 520 °C.These inferred results were then used: a) to interpret the Lower Metapelite Group as a single crystalline basement unit exclusively affected by a complete Alpine orogenic cycle, according to the very similar features of P–T paths, comparable petrography and analogous structural characteristics; b) as a tool for more reliable correlations between the Aspromonte Massif, the other Calabrian terranes and the north African Orogenic Complexes. They may therefore consider a contribution to the geodynamic modelling of the western Mediterranean. 相似文献
Metapelite-derived migmatites (“bedded migmatites”) formed in the low-pressure/high-temperature (LPHT) Cooma Complex, southeastern Australia, contain magma (neosome and leucosome) confined to the metapelitic beds in which they were generated. The metapsammitic beds were more ductile than the metapelitic beds (and the metapelitic parts of graded beds), which underwent fracture and boudinage, thereby providing space for the magma, though some also occurs in axial surface folia. Transitions from bedded to stromatic migmatites can be seen, but the magma mainly remained in the metapelites, even in the most strongly deformed stromatic migmatites. This, together with boudinage and transposition of the leucosome, as well as microstructural evidence of quartz recrystallization, suggest that much or most of the stromatic layering was formed by solid-state deformation. In contrast, magmas (neosomes) formed by partial melting of feldspathic metapsammites at Cooma moved out of their parent rocks, and coalesced into veins and small intrusions of diatexite, because (1) the host rocks deformed more homogeneously, and no interboudin space was made for the melts, and (2) the melt escape threshold was exceeded, probably with the assistance of deformation. Metapsammite melting occurred after solidification of the metapelite-derived magma, and the mobile metapsammite-derived magma (diatexite) disrupted and incorporated fragments of the metapelitic migmatites. The metapsammite-derived magma, together with this solid metapelitic material, locally coalesced into bodies closely resembling the Cooma Granodiorite. 相似文献
Garnet-sillimanite gneisses, locally known as khondalites, occur abundantly in the Chilka Lake granulite terrane belonging
to the Eastern Ghats Proterozoic belt of India. Though their chemistry has been modified by partial melting, it is evident
that the majority of these rocks are metapelitic, with some tending to be metapsammitic. Five petrographically distinct groups
are present within the khondalites of which the most abundant group is characteristically low in Mg:Fe ratios — the main chemical
discriminant separating the five groups. The variations in Mg:Fe ratios of the garnets, biotites, cordierites, orthopyroxenes
and spinels from the metapelites are compatible with those in the bulk rocks.
A suite of granitoids containing garnet, K-feldspar, plagioclase and quartz, commonly referred to as leptynites in Indian
granulite terranes, are interlayered with khondalites on the scale of exposures; in a few spots, the intercalated layers are
thin. The peraluminous character of the leptynites and presence of sillimanite trails within garnets in some of them suggest
derivation of leptynites by partial melting of khondalites. Here we examine this connection in the light of results derived
from dehydration melting experiments of micas in pelitic and psammitic rocks.
The plots of leptynites of different chemical compositions in a (MgO + FeO)-Na2O-K2O projection match the composition of liquids derived by biotite and muscovite dehydration melting, when corrected for co-products
of melting reactions constrained by mass balance and modal considerations. The melt components of the leptynites describe
four clusters in the M-N-K diagram. One of them matches melts produced dominantly by muscovite dehydration melting, while
three clusters correspond to melting of biotite. The relative disposition of the clusters suggests two trends, which can be
correlated with different paths that pelitic and psammitic protoliths are expected to generate during dehydration melting.
Thus the leptynites evidently represent granitoids which were produced by dehydration melting in metapelites of different
compositions.
The contents of Ti, Y, Nb, Zr and Th in several leptynites indicate departures from equilibrium melt compositions, and entrainment
of restites is considered to be the main causative factor. Disequilibrium in terms of major elements is illustrated by leucosomes
within migmatites developed in a group of metapelites. But the discrete leptynites that have been compared with experimental
melts approach equilibrium melt compositions closely. 相似文献
The Proterozoic terrane of the Black Hills, South Dakota, includes the composite Harney Peak leucogranite and associated pegmatites that were emplaced into metamorphosed pelites and graywackes. Available dates indicate that granite generation post-dated regional metamorphism and deformation that have been attributed to collision of the Wyoming and Superior cratons at 1760 Ma. Previous radiogenic and stable isotope work indicates that the exposed metasedimentary rocks are equivalent to sources of the leucogranites. In this study, whole rock and mineral compositions of the metasedimentary rocks were used to calculate the likely average residue mineralogies and melt fractions that would be generated by muscovite dehydration melting of the rocks. These were then used to model observed trace element compositions of the granites using published mineral/melt distribution coefficients. Model trace element melt compositions using pelitic and graywacke protoliths yield similar results.
The models reproduce well the observed depletion of transition metals and Ba in the granites relative to metasedimentary protoliths. The depletion is due mainly to high proportion of biotite with variable amounts of K-feldspar in the model residue. Sr is also depleted in the granites compared to source rocks, but to a lesser relative extent than Ba. This is because of the low biotite/melt distribution coefficient for Sr and because high proportion of plagioclase in the residue is compensated by high Sr concentrations in protoliths. Rubidium, Cs and Ta behaved as slightly compatible to incompatible elements, and therefore, were not strongly fractionated during melting. Of the considered elements, only B appears to have been highly incompatible relative to residue during melting. The protoliths had sufficient B to allow tourmaline crystallization in those parts of the Harney Peak Granite in which Ti concentration was sufficiently low not to enhance crystallization of biotite.
The reproducibility of observed trace element concentrations in the Harney Peak Granite by the models supports the often made proposition that metapelites and metagraywackes are common sources for leucogranites. This argues against mass input from the mantle into metagraywacke and metapelitic crustal sources or melting of amphibolites to generate the post-collisional Harney Peak and other similar peraluminous granite suites. 相似文献
In this contribution we have empirically calibrated the garnet–biotite–muscovite–aluminosilicate–quartz (GBMAQ) barometer using low- to medium–high-pressure, mid-grade metapelites. Application of the barometer suggests that the GBMAQ and GASP barometers show quite similar pressure estimates. Furthermore, metapelites within thermal contact aureole or very limited geographic area show no meaningful pressure diversity determined by the GBMAQ and GASP barometers which is the geological reality. The random error of the GBMAQ barometer is expected to be around ± 0.8 kbar, and this barometer shows no systematic bias with respect to either pressure, or temperature, or AlVI in muscovite, or Fe in biotite, or Fe in garnet. The GBMAQ barometer is thermodynamically consistent with the garnet–biotite geothermometer because they share the same activity models of both garnet and biotite. This barometer is especially useful for assemblages with Ca-poor garnet or Ca-poor plagioclase or plagioclase-absent metapelites. Application of this barometer beyond the calibration ranges, i.e., P–T range and chemical ranges of the minerals, is not encouraged. 相似文献
Three xenoliths erupted as ejecta during recent violent explosion of Stromboli volcano (Aeolian Islands) were investigated in this paper. They consist of high-temperature mineral association (cordierite, hercynite spinels, sillimanite, ±plagioclase, ±mullite, ±corundum) and abundant glass (10–70 vol.%), and may be classified as buchites. The peraluminous composition of the xenoliths, their trace element distribution and REE patterns support their origin from granulite- and amphibolite-facies metapelites of the Calabrian continental crust, which is considered the crystalline basement beneath Stromboli. Buchites have an isotopic composition comparable to that of Stromboli extrusives and significantly different from that of the Calabrian basement.
The glass is generally colourless and has a Si–Al–alkali-rich composition, except for one sample where a Fe–Mg–Ca-rich reddish-brown glass also occurs. These two kinds of glass show complicated textures where patches of reddish-brown glass are often surrounded by plagioclase and/or cordierite or forms streaks and swirls with the colourless glass. Cordierite, plagioclase and oxides have different compositions according to their position in the xenoliths. Ca-rich plagioclase (An72–95), Mg-poor cordierite (Mg-values 47–66) and Al-rich spinels are in the inner portions of the xenoliths and associated with colourless glass; on the contrary, close to the contact with the host lava or associated with coloured glass, cordierite shows higher Mg-values, Ti–Fe-bearing oxides occur and plagioclase is chemically similar to the basalt phenocrysts (An66–71).
The abundant and fresh glass and the idiomorphic shape of the high-temperature minerals suggest that the xenoliths were hold in the basaltic magma, before its extrusion, for a significant time to allow their partial or nearly complete melting and subsequent nucleation and growth of new phases. During this stage, the interaction between the anatectic liquid and the basaltic magma affected the original isotopic composition of the xenoliths and, in some cases, produced glass and mineral phases (cordierite, plagioclase and oxides) with different composition. 相似文献
Metagranites in the NKFMASH system require external hydration during prograde high-pressure metamorphism in order to equilibrate to ambient HP conditions by producing more siliceous muscovite. Any lack of external fluid or the disappearance of biotite stops re-equilibration and thus prevents recording of high-pressure conditions. The same hydration reactions cause dehydration during exhumation. Orthogneiss from shear zones or adjacent to metapelites and metabasites will take up external fluid during subduction and record the highest P–T conditions, but will also be the first to dehydrate upon exhumation, now hydrating other lithologies and probably refuelling shearzones.
The (de)hydration behavior of Ca-bearing metagranitoids is similar to that in the Ca-free system. However, the anorthite component of plagioclase decomposes with increasing pressure to form either (clino)zoisite or a grossular-rich garnet. Both reactions are fluid-consuming. If H2O is supplied from an external source, the garnet-bearing assemblage can record P–T conditions up to very high pressures, but dehydrates again during heating and/or decompression to form a more Fe-rich garnet and Al-rich mica(s). The garnet compositions observed in natural HP-metagranites are mostly too Fe-rich to be formed in the presence of an H2O-rich fluid.
N(C)KFMASH metapelites generally have a more complex mineralogy and succession of mineral assemblages along a P–T path. The H2O contained in hydrous silicates like chlorite and chloritoid is only partly released, but partly transferred to other minerals like paragonite, glaucophane or phengite during subduction and further dehydration during exhumation is common. The mineral assemblage preserved by the rock may then record P–T conditions way below those of the actual pressure and temperature peak of the path. Contouring of the P–T pseudosection of a specific metapelite composition with mode isopleths for H2O shows which P–T conditions along a given path are the ones most likely recorded by the rock. 相似文献
