Although many recent reviews emphasize a uniformity in granulite pressure–temperature (P–T) conditions and paths, granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation. A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them record P–T conditions outside the 7.5 ± 1 kbar and 800 ± 50 °C average granulite regime preferred by many authors. In particular, an increasing number of very high temperature (900−1000 °C) terranes are being recognized, both on the basis of distinctive mineral assemblages and geothermobarometry. Petrogenetic grid and geothermobarometric approaches to the determination and interpretation of P–T histories are both evaluated within the context of reaction textures to demonstrate that the large range in P–T conditions is indeed real, and that both near-isothermal decompression (ITD) and near-isobaric cooling (IBC) P–T paths are important. Amphibolite–granulite transitions promoted by the passage of CO2-rich fluids, as observed in southern India and Sri Lanka, are exceptional and not representative of fluid-related processes in the majority of terranes. It is considered, on the contrary, that fluid-absent conditions are typical of most granulites at or near the time of their recorded thermal maxima.ITD granulites are interpreted to have formed in crust thickened by collision, with magmatic additions being an important extra heat source. Erosion alone is not, however, considered to be the dominant post-collisional thinning process. Instead, the ITD paths are generated during more rapid thinning (1−2 mm/yr exposure) related to tectonic exhumation during moderate-rate or waning extension. IBC granulites may have formed in a variety of settings. Those which show anticlockwise P–T histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion, with or without additional crustal extension. IBC granulites at shallow levels (< 5 kbar) may also be formed during extension of normal thickness crust, but deeper-level IBC requires more complex models. Many granulites exhibiting IBC at deep crustal levels may have formed in thickened crust which underwent very rapid (5 mm/yr) extensional thinning subsequent to collision. It is suggested that the preservation of IBC paths rather than ITD paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust, and that hybrid ITD to IBC paths should also be observed.Most IBC granulites, and probably many ITD granulites, have not been exposed at the Earth's surface as a result of the tectonic episodes which produced them, but have resided in the middle and lower crust for long periods of time (100−2000 Ma) following these events. The eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756800022330

The origins of granulites: a metamorphic perspective

SHRIMP U–Pb zircon geochronology of Palaeoproterozoic metasedimentary rocks in the North China Craton: Evidence for a major Late Palaeoproterozoic tectonothermal event

In general, Archean rocks exhibit rather ordinary moderate-P-high-T facies series metamorphism; neither blueschists nor any record of deep continental subduction and return are documented. However, the abundance and scale of ultrahigh-temperature (UHT) metamorphic belts from the Neoarchean to the Cambrian imply a significant change in geodynamics during the Neoarchean Era, after which transient sites of high heat flow were available at intervals throughout this period of Earth evolution. Many Neoproterozoic-Cambrian UHT metamorphic belts appear to have developed in settings analogous to modern backarcs that were closed and inverted during crustal aggregation and formation of the Gondwana supercontinent. If backarcs were the general setting for UHT metamorphism, then on a hotter Earth the cyclic formation of supercratons (in the Neoarchean Era) and supercontinents (in the Proterozoic Eon) required the destruction of oceans floored by thinner lithosphere that may have generated hotter backarcs than those as...

Metamorphic Conditions in Orogenic Belts: A Record of Secular Change

Petrographic, major, trace, and rare earth element compositions of sandstones from the upper Miocene Kudankulam Formation, Southern India, have been investigated to determine their provenance, tectonic setting, and weathering conditions. All sandstone samples are highly enriched in quartz (Q) but poor in feldspar (F) and lithic fragments (L). The major-element concentrations of these sandstones reveal the relative homogeneity of their source. Geochemically, the Kudankulam sandstones are classified as arkose, subarkose, litharenite, and sublitharenite. The CIA values (chemical index of alteration; mean value 44.5) for these sandstones and the A-CN-K diagram suggest their low-weathering nature. Similarly, their Fe2O3* + MgO (mean 2.7), Al2O3/SiO2 ( 0.09), K2O/Na2O ( 2.2) ratios and TiO2 contents ( 0.3) are consistent with a passive-margin setting. The Eu/Eu* ( 0.5), (La/Lu)cn ( 21), La/Sc ( 5.9), Th/Sc ( 1.9), La/Co ( 5.7), Th/Co ( 1.8), and Cr/Th ( 5.3) ratios support a felsic source for these sandstones. Chondrite-normalized REE patterns with LREE enrichment, flat HREE, and negative Eu anomaly also are attributed to felsic source-rock characteristics for Kudankulam sandstones. Total REE concentrations of these sandstones reflect the variations in their grain-size fractions. The source rocks are probably identified to be Proterozoic gneisses, charnockites, and granites of the Kerala Khondalite Belt, which must have been exposed at least since the late Miocene. Finally, the unusual Ni enrichment in the Kudankulam sandstones, unaccompanied by a similar enrichment in Cr, Co, and V, may be related to either the presence of pyrite in the sandstones or, more likely, the fractionation of garnet from the source rocks during transportation.

Geochemistry of Sandstones from the Upper Miocene Kudankulam Formation, Southern India: Implications for Provenance, Weathering, and Tectonic Setting

A mineral that forms under conditions as variable as diagenesis to deep subduction, melt crystallization to low temperature alteration, and that retains information on time, temperature, trace element and isotopic signatures is bound to be a useful petrogenetic tool. The variety of conditions under which zircon forms and reacts during metamorphism is a great asset, but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure–temperature–deformation–fluid context. Under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information.
In the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology. With the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high-grade metamorphic rocks. Since then, there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures, its capacity to protect mineral inclusions, zircon thermometry, trace element patterns and their relation to main mineral assemblages, solubility of zircon in melt and fluids, and isotopic systematics in single domains that go beyond U–Pb age determinations.
Metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks, but has become a truly indispensable mineral in reconstructing pressure–temperature–time–fluid-paths over a wide range of settings. An obvious consequence of its wide use, is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial: in this chapter, reference to published works are intended as examples and not as a compilation.
This chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture, deformation structure and mineral chemistry, including trace element and isotopic systematics. Linking this information together highlights the potential of zircon as a key mineral in petrochronology.

Zircon: The Metamorphic Mineral

Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka Two major kinds of orthopyroxene-producing reactions are recognized Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic “gray gneiss” produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of “mixed-facies” exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO2 and Na2O Rb and Y are depleted in the charnockite Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz±garnet to produce orthopyroxene and K-feldspar Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Kerala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd Metamorphic conditions of both Kabbaltype and Ponmudi-type localities were 700°–800° C and 5–6 kbar Lower P(H2O) in the Ponmudi-type metamorphism was probably the definitive factor CO2-rich fluid inclusions in quartz from the Kabbaltype localities support the concept that this type of charnockite formation was driven by influx of CO2 from some deep-seated source The open-system behavior and high oxidation states of the metamorphism are in accord with the CO2-streaming hypothesis CO2-rich inclusions in graphitebearing charnockites of the Ponmudi type, however, commonly have low densities and compositions not predictable by vapor-mineral equilibrium calculations These inclusions may have suffered post-metamorphic H2 leakage or some systematic contamination Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO2 drove charnockite formation of the Ponmudi type The possibility remains that orthopyroxene and CO2-rich fluids were produced by reaction of biotite with graphite without intervention of fluids of external origin Further evidence, such as oxygen isotopes, is necessary to test the CO2-streaming hypothesis for the Ponmudi-type localities

Arrested charnockite formation in southern India and Sri Lanka

The Khondalite Belt of southernmost India is an immense section of granulite facies metasediments of probable early Proterozoic age. Dominant rock types are migmatized khondalites (garnet-sillimanite $$\pm$$ cordierite metapelites), acid biotite-garnet graphitic gneisses and charnockites (quartzofeldspathic gneisses with orthopyroxene). Throughout much of southern Kerala, the biotite gneisses are in a state of arrested conversion to charnockite. Partially decrepitated $$CO_{2}$$ inclusions in quartz at the Ponmudi mixed biotite gneiss-charnockite quarry have significant $$CH_{4}$$ and/or $$N_{2}$$ and are texturally late. The probable sequence of events in metasediments was (1) migmatization, (2) charnockite formation, (3) symplectitic cordierite formation, and (4) late $$CO_{2}$$ entrapment. Phase equilibrium diagrams of anatexis in aluminous systems and Fe-Mg exchange between garnet and biotite indicate metamorphic temperatures of 650-850°C for the khondalites. Temperatures based on garnet-orthopyroxene...

Metamorphic P-T Conditions of the Kerala (South India) Khondalite Belt, a Granulite Facies Supracrustal Terrain

Geochemistry of high-grade supracrustal rocks from the Kerala Khondalite Belt and adjacent massif charnockites, South India

Dehydration—Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India

Zircon crystals from a locally charnockitizedPaleoproterozoic high-K metagranite from the KeralaKhondalite Belt (KKB) of southern India have been investigatedby high-spatial resolution secondary io ...

Behaviour of radiogenic Pb in zircon during ultrahigh-temperature metamorphism: an ion imaging and ion tomography case study from the Kerala Khondalite Belt, southern India

G. R. Ravindra Kumar

Papers

Arrested charnockite formation in southern India and Sri Lanka

Metamorphic P-T Conditions of the Kerala (South India) Khondalite Belt, a Granulite Facies Supracrustal Terrain

Geochemistry of high-grade supracrustal rocks from the Kerala Khondalite Belt and adjacent massif charnockites, South India

Dehydration—Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India

Behaviour of radiogenic Pb in zircon during ultrahigh-temperature metamorphism: an ion imaging and ion tomography case study from the Kerala Khondalite Belt, southern India