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Zircon is a key accessary mineral for metamorphic geochronology and geochemical tracing, but it has been a challenge to interpret its complex chemical zoning and age record acquired during multiple episodes of anatectic metamorphism in collisional orogens. This is illustrated by a combined study of petrography, phase equilibrium modeling and metamorphic P-T-t determination for granulites from the Bohemian Massif in the Variscan Orogen. These rocks record multiple episodes of zircon growth during anatectic metamorphism. They started from the compressional heating for prograde metamorphism to high-pressure (HP) to ultrahigh-pressure (UHP) eclogite facies with low degrees of partial melting. Afterwards, they underwent a decompressional stage from UHP eclogite facies to HP granulite facies for dehydration melting. These were followed by a further decompressional stage either to kyanite granulite facies or to sillimanite granulite facies at ultrahigh-temperature (UHT) conditions. Episodes of zircon growth are linked to specific metamorphic conditions for peritectic reactions on the basis of zoning patterns, trace element signatures, index mineral inclusions in dated domains and textural relationships to coexisting minerals. The results indicate that relict zircon domains are preserved even at UHT granulite facies conditions. A few zircon domains in the kyanite granulite grew during the prograde to peak UHP metamorphism, possibly corresponding to consumption of biotite and plagioclase but growth of garnet. During the decompressional exhumation to the HP granulite-facies, relict or prograde zircon domains were mostly dissolved into anatectic melts produced by muscovite breakdown. Most zircon grains grew during this transition to the HP granulite-facies in the kyanite granulite and are chemically related to continuous growth of garnet, whereas abundant zircon grains grew subsequently at the UHT granulite facies in the sillimanite granulite and are chemically related to garnet breakdown reactions. Another peak of zircon growth occurred at the final crystallization of anatectic melts in the sillimanite granulite rather than in the kyanite granulite, and these zircon grains mostly show oscillatory zoning, low HREE + Y contents and significantly negative Eu anomalies. In terms of the inference for protolith nature, it appears that zircon in metasedimentary rocks can grow at a short timescale in different stages of anatectic metamorphism, and its dissolution and growth are mainly dictated by anatectic conditions and extent, the property of peritectic reactions, and the stability of Ti-rich minerals. [ABSTRACT FROM AUTHOR]

To understand the temporal sequence of geological processes such as magmatism, water‐rock interaction, and metamorphism in fossil continental rifts, a combined study of petrography, mineral geochemistry, in situ garnet O isotopes, in situ zircon U‐Pb ages and O isotopes, and pseudosection calculations was conducted for metagranites from a Neoproterozoic continental rift generated during the Rodinia breakup. The results provide insights into the operation of hydration and dehydration during continental rifting. In the metagranites from the northern margin of South China, three types of garnet (Garnet‐I to ‐III) are distinguished. They were sequentially produced by hydrothermal alteration, metamorphic dehydration, and fluid metasomatism. All of these garnets show negative δ18O values of −19.3‰ to −14.5‰, in contrast to mantle‐like δ18O values for magmatic zircon. The extremely negative δ18O values of hydrothermal Garnet‐I require infiltration of the continental deglacial meltwater during the continental rifting, and before that zircon crystallized from normal δ18O magmas. Once the rocks were hydrothermally altered, the extreme 18O depletion was retained in all later products such as metamorphic Garnet‐II and metasomatic Garnet‐III. Pseudosection calculations indicate that the metamorphic dehydration occurred at 1.0–3.0 kbar and 630–690 °C during a reheating stage, corresponding to high thermal gradients of >60 °C/km. The high‐temperature/low‐pressure metamorphic rocks produced by such high thermal gradients are indicative of the continental rift setting. The mineral geochemistry records not only the temporal sequence of rift magmatism, water‐rock interaction, and rift metamorphism but also the evolution of temperature and water action in the crust during the continental rifting. Key Points: Garnets record sequential operation of hydrothermal alteration, metamorphic dehydration, and albitization during the continental riftingPenetration of the surface water occurred at a time between the magmatic zircon growth and subsequent rift metamorphismMetamorphic rocks that record high thermal gradients are indicative of continental rifting [ABSTRACT FROM AUTHOR]

Two generations of garnet are recognized in ultrahigh-pressure (UHP) metagranite from the Dabie orogen by a combined study of petrography, major and trace element profiles in garnet, and phase equilibrium modeling for metagranite. The results enable distinction between metamorphic and peritectic garnet on the basis of BSE images, and major and trace element compositions. Our research provides new insights into the growth of anatectic garnet due to dehydration melting of UHP metamorphic rocks during exhumation from mantle depths. The first generation of garnet (Grt-I) occurs as a broad domain in the center, which is related to metamorphic growth during prograde subduction. This garnet is dark in BSE images, rich in grossular and poor in almandine and pyrope. The chondrite-normalized rare earth element (REE) patterns show LREE depletion and flat MREE-HREE patterns. The second generation of garnet (Grt-II) occurs as a rim of euhedral garnet, or as patches in Grt-I domains, recrystallized after dissolution of preexisting metamorphic garnet in the presence of anatectic melts during exhumation. It is bright in BSE images, poor in grossular, and rich in almandine and pyrope contents. Trace element analyses on Grt-II domains yield high contents of Sc, Cr, Y and HREE and low contents of Ti and MREE. The chondrite-normalized REE patterns exhibit LREE depletion, and steep MREE-HREE patterns. Based on REE partitioning between garnet and zircon/titanite, the last growth times for metamorphic and anatectic garnets are constrained by zircon and titanite U–Pb ages to be ~ 240 Ma and ~ 220 Ma, respectively. Based on anatectic microstructures and a modeled P-T pseudosection, it is suggested that dehydration melting occurred at 2.0–2.5 GPa during exhumation. Melting occurred through the breakdown of phengite via the peritectic reaction: garnet (I) + phengite + plagioclase + quartz → garnet (II) + biotite + K-feldspar + melt. [ABSTRACT FROM AUTHOR]

Abstract: Partial melting at continental lithosphere depths plays an important role in generating geochemical variations in igneous rocks. In particular, dehydration melting of ultrahigh-pressure (UHP) metamorphic rocks during continental collision provides a petrological link to intracrustal differentiation with respect to the compositional evolution of continental crust. While island arc magmatism represents one end-member of fluid-induced large-scale melting in the mantle wedge during subduction of the oceanic crust, the partial melting of UHP rocks can be viewed as the other end-member of fluid-induced small-scale anatexis during exhumation of the deeply subducted continental crust. This latter type of melting is also triggered by metamorphic dehydration in response to P–T changes during the continental collision. It results in local occurrences of hydrous melts (even supercritical fluids) as felsic veinlets between boundaries of and multiphase solid inclusions in UHP metamorphic minerals as well as local accumulation of veinlet-like felsic leucosomes in foliated UHP metamorphic rocks and metamorphically grown zircons in orogenic peridotites. Thus, very low-degree melts of UHP rocks provide a window into magmatic processes that operated in continental subduction zones. This article presents a review on available results from experimental petrology concerning the possibility of partial melting under conditions of continental subduction-zone metamorphism, and petrological evidence for the occurrence of dehydration-driven in-situ partial melting in natural UHP rocks during the continental collision. Although the deeply subducted continental crust is characterized by a relative lack of aqueous fluids, the partial melting in UHP rocks commonly takes place during decompression exhumation to result in local in-situ occurrences of felsic melts at small scales. This is caused by the local accumulation of aqueous fluids due to the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals in UHP rocks during the exhumation. The dehydration melting of UHP rocks would not only have bearing on the formation of supercritical fluids during subduction-zone metamorphism, but also contribute to element mobility and ultrapotassic magmatism in continental collision orogens. Therefore, the study of dehydration melting and its effects on element transport in UHP slabs, rocks and minerals is a key to chemical geodynamics of continental subduction zones. [Copyright &y& Elsevier]

The Himalayan orogen is an archetype of collisional orogens built by the continental collision between India and Asia in the Cenozoic, and it has been a natural laboratory for studying the tectonic evolution of continental collision zones. Various tectonic models have been proposed to interpret the formation and evolution of the Himalayan orogen. However, these models were primarily based on local observations of structural geology and petrology, and some of these models even contradict to each other. The widespread exposure of metamorphic rocks and their metamorphic features are crucial aspects for reconstructing the tectonic evolution of the collisional orogen, as it provides robust records of metamorphic pressure (P) and temperature (T) at different ages. In this study, we present a systematic review on the metamorphic features of crustal rocks from the Western through the Central to the Eastern Himalayas. In doing so, we outline the spatial and temporal changes of metamorphic features (including P – T paths and T/P ratios) at first and then discuss their tectonic implications. In general, the metamorphic evolution of the Himalayan orogen can be categorized into three periods: (1) pre-collisional period from the Early Jurassic to Paleocene, which is represented by low T/P Alpine type metamorphism forming high-pressure (HP) blueschists in the Western to Central Himalayas; (2) syn -collisional period from the Paleocene to Eocene, which is divided into two stages. The early stage from the Paleocene to early Eocene is characterized by continental deep subduction at low T/P ratios to form Alpine type HP to ultrahigh-pressure (UHP) eclogites in the Western Himalaya, whereas the late stage from the early to late Eocene is characterized by continental hard collision at medium T/P ratios to form Barrovian type medium-pressure (MP) amphibolite and HP granulite in the Central to Eastern Himalayas; (3) post-collisional period from the Oligocene through Miocene to Pliocene, which is also divided into two stages. The early stage during the Oligocene is represented by continuous metamorphism at medium T/P ratios. Tectonic reactivation for the transition from the moderate to high T/P metamorphism would occur locally in the early stage, but became extensive in the late stage when metamorphic T/P ratios were high enough to cause intensive anatexis in the Miocene, giving rise to Buchan type HT to UHT metamorphic rocks in the Central to Eastern Himalayas. The inspection of different metamorphic facies series throughout the entire Himalayan orogen indicates that the Indian continental crust was collisionally thickened at first and then underwent large-scale reworking through intracontinental extension in the post-collisional period. The deep subduction of oceanic and continental crust is characterized by Alpine type metamorphism at low T/P ratios, the continental hard collision is represented by Barrovian type metamorphism at medium T/P ratios, and the post-collisional reworking is indicated by Buchan type metamorphism at high T/P ratios. The difference in metamorphic T/P ratios is spatiotemporally distinguishable in the Himalayan orogen, documenting the tectonic transition from oceanic subduction through continental subduction and collision to post-collisional rifting along the convergent plate margin. Therefore, the secular change of metamorphic T/P ratios can be used to retrieve the change of geothermal gradients and dynamic regimes from syn -collisional compression to post-collisional extension in the lifetime of the convergent continental margin. As such, the orogenic wedge is a holistic model for the spatiotemporal occurrence of Himalayan sequences cross the collisional orogen, and the failed continental rifting is a viable mechanism for high T/P metamorphism and granitic magmatism in the intracontinental orogen. • Secular change of metamorphic thermobaric ratios during the Cenozoic is evident in the Himalayan orogen. • The Indian continental crust was deeply subducted to mantle depths in the Western Himalaya. • The Indian continental crust was significantly thickened during collisional compression in the Central and Eastern Himalayas. • The collisional orogen became an intracontinental orogen through active rifting in the post-collisional period. • The orogenic wedge is a holistic model for the formation and evolution of the Himalayan orogen. [ABSTRACT FROM AUTHOR]