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Two generations of garnet are recognized in a granite and a pegmatite from the Gangdese orogen in southeastern Tibet on the basis of a combined study of petrography, major and trace element profiles, and garnet O isotopes. Zircon U-Pb dating and Hf-O isotope compositions also help constrain the origin of both granite and pegmatite. The first generation of garnet (Grt-I) occurs as residues in the center of garnet grains, and it represents an early stage of nucleation related to magmatic-hydrothermal fluids. Grt-I is dark in backscattered electron (BSE) images, rich in spessartine, and poor in almandine and grossular. Its chondrite-normalized rare earth element (REE) patterns show obvious negative Eu anomalies and depletion in heavy REE (HREE) relative to middle REE (MREE). The second generation of pegmatite garnet (Grt-II) occurs as rims of euhedral garnets or as patches in Grt-I domains of the pegmatite, and it crystallized after dissolution of the preexisting pegmatite garnet (Grt-I domains) in the presence of the granitic magma. Compared with Grt-I, Grt-II is bright in BSE images, poor in spessartine, and rich in almandine and grossular contents. Its chondrite-normalized REE patterns exhibit obvious negative Eu anomalies but enrichment in HREE relative to MREE. The elevation of grossular and HREE contents for Grt-II relative to Grt-I domains indicate that the granitic magma had higher contents of Ca than the magmatic-hydrothermal fluids. The garnets in the granite, from core to rim, display homogenous profiles in their spessartine, almandine, and pyrope contents but increasing grossular and decreasing REE contents. They are typical of magmatic garnets that crystallized from the granitic magma. Ti-in-zircon temperatures demonstrate that the granite and pegmatite may share the similar temperatures for their crystallization. Grt-II domains in the pegmatite garnet have the same major and trace element compositions as the granite garnet, suggesting that the pegmatite Grt-II domains crystallized from the same granitic magma. Therefore, the pegmatite crystallized at first from early magmatic-hydrothermal fluids, producing small amounts of Grt-I, and the fluids then mixed with the surrounding granitic magma. The U-Pb dating and Hf-O isotope analyses of zircons from the granite and pegmatite yield almost the same U-Pb ages of 77–79 Ma, positive εHf(t) values of 5.6 to 11.9, and δ18O values of 5.2 to 7.1‰. These data indicate that the granite and pegmatite were both derived from reworking of the juvenile crust in the newly accreted continental margin prior to the continental collision in the Cenozoic. [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]

Amphibolite- and granulite-facies metamorphic rocks are common in the eastern Himalayan syntaxis of southeastern Tibet. These rocks are composed mainly of gneiss, amphibolite and schist that underwent various degrees of migmatization to produce leucogranites, pegmatites and felsic veins. Zircon U–Pb dating of biotite gneiss, leucocratic vein and vein granite from the syntaxis yields consistent ages of ∼49 Ma, indicating crustal anatexis during continental collision between India and Asia. Garnets in these rocks are categorized into peritecitc and anatectic varieties based on their mode of occurrence, mineral inclusions and major- and trace-element zoning. The peritectic garnets mainly occur in the biotite gneiss (mesosome layer) and leucocratic veins. They are anhedral and contain abundant mineral inclusions such as high-Ti biotites and quartz, and show almost homogeneous major-element compositions (except Ca) and decreasing HREE contents from core to rim, indicating growth during the P - and T -increasing anatexis. Peak anatectic conditions at 760–800°C and 9–10·5 kbar are well constrained by phase equilibrium calculations, mineral assemblages, and garnet isopleths. In contrast, anatectic garnets only occur in the vein granite. They are round or subhedral, contain quartz inclusions, and exhibit increasing spessartine and trace-element contents from core to rim. The garnet–biotite geothermometry and the garnet–biotite–plagioclase–quartz geobarometry suggest that the anatectic garnets crystallized at ∼620–650°C and 4–5 kbar. Some garnet grains show two-stage zoning in major and trace elements, with the core similar to the peritectic garnet but the rim similar to the anatectic garnet. Mineralogy, whole-rock major- and trace-element compositions and zircon O isotopes indicate that the two types of leucosomes were produced by hydration (water-present) melting and dehydration (water-absent) melting, respectively. The leucocratic veins contain peritectic garnet but no K-feldspar, have lower whole-rock K2O contents and Rb/Sr ratios, higher whole-rock CaO contents and Sr/Ba ratios, and show homogeneous δ18O values that are lower than those of relict zircons, indicating that such veins were produced by the hydration melting. In contrast, the vein granite contains peritectic garnet and K-feldspar, has higher whole-rock K2O contents and Rb/Sr ratios, lower whole-rock CaO contents and Sr/Ba ratios, and shows comparable δ18O values with those of relict zircons, suggesting that this granite were generated by the dehydration melting. Accordingly, both hydration and dehydration melting mechanisms have occurred in the eastern Himalayan syntaxis. [ABSTRACT FROM AUTHOR]

Garnet in high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions, which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction-zone conditions. It provides an important target to retrieve physicochemical changes in subduction-zone processes, including those not only in pressure and temperature but also in the durations of metamorphism and anatexis. Garnet from different compositions of HP to UHP metamorphic rocks may show different types of major and trace element zonation, as well as mineral inclusions. Discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the HP to UHP metamorphic rocks. Magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction-zone conditions, with spessartine-increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim. Metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation, with spessartine-decreasing from core to rim if the intracrystalline diffusion is not too fast. The compositional profiles of metamorphic garnet in the abundances of grossular, almandine and pyrope are variable depending on the composition of host rocks and co-existing minerals. Peritectic garnet grows through peritectic reactions during partial melting of HP to UHP rocks, with the composition of major elements to be controlled by anatectic P-T conditions and the compositions of parental rocks and anatectic melts. Trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages, the garnet-forming reactions and the property of metamorphic fluids or anatectic melts. Mineral inclusions not only present key clues to identify the different origins of garnet, but also serve as sound candidates for the temporal constraint on garnet growth. [ABSTRACT FROM AUTHOR]

A combined study of whole-rock major-trace elements and Sr–Nd isotopes as well as zircon U–Pb ages, trace elements and Lu–Hf isotopes was carried out for high-pressure (HP) and ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks in the Hong'an orogen, China. The results provide insights into the tectonic evolution from oceanic subduction to continental collision during the closure of Paleotethyan ocean between the South and North China Blocks. Based on the whole-rock geochemistry, eclogites in the Hong'an orogen are categorized into continental- and oceanic-type, respectively. The continental-type eclogites widely occur in the whole orogen, exhibiting general enrichments of LILE and LREE but depletion of HFSE and HREE. Zircon U–Pb dating yields protolith ages of about 750 to 1200 Ma, demonstrating a tectonic affinity to the South China Block. They have both positive and negative ε Nd (t) and ε Hf (t) values for whole-rock and zircons, suggesting that their protoliths were originated from both juvenile and ancient crustal rocks. The oceanic-type eclogites only occur in the northwestern edge of the orogen, exhibiting mid-ocean ridge basalt (MORB)-like flat REE patterns and arc-like trace element patterns. Zircon U–Pb dating on the relict zircon cores of magmatic origin yields protolith ages of ~ 420 Ma. They have variable ε Nd (t) and positive ε Hf (t) values and slightly high ( 87 Sr/ 86 Sr) i ratios. These geochemical features suggest that their protoliths are equivalent to backarc basin basalts (BABB) in a continental margin. The eclogite-facies metamorphic ages and grade are also different for these two types of eclogites. The oceanic-type eclogites were only metamorphosed under HP conditions in the Carboniferous whereas the continental-type eclogites were mostly metamorphosed under HP to UHP conditions in the Triassic. Thus, the two types of eclogites record the tectonic transition from oceanic subduction to continental collision during the closure of the Paleotethyan ocean between the South and North China Blocks. It is possible that the backarc basins were developed in the northern margin of the SCB during the Early Paleozoic, and then became a new subduction zone in which both backarc basin basalts and overlying terrigenous sediments were carried to a depth in the Carboniferous for the HP eclogite-facies metamorphism. Afterwards the continental crust would start to subduct northwards, eventually leading to the HP to UHP eclogite-facies metamorphism in the Triassic. Therefore, the Hong'an orogen is a composite one that is tectonically different not only from the Qinling–Tongbai orogens to the west but also from the Dabie–Sulu orogens to the east. The occurrence of both oceanic-type and continental-type eclogites in the Hong'an orogen suggests that the subduction of continental crust would be gravitationally pulled by the subduction of oceanic crust at the same subduction zone and the HP eclogite-facies metamorphic melànge would be exhumed together along the same subduction channel. [ABSTRACT FROM AUTHOR]

Abstract: Simultaneously in-situ analyses of U–Pb isotopes and trace elements were carried out for zircons, in combination with the in-situ analyses of trace elements in coexisting minerals, from low-T/UHP metagranite in the Dabie orogen. The results provide geochemical evidence for the existence of supercritical fluid during continental subduction-zone metamorphism. The zircons are categorized into three types based on their patterns of REE distribution. Type I zircons show increasing enrichment from La to Lu, with prominent positive Ce anomalies and negative Eu anomalies, which are typical of magmatic zircon. Some of them display regular or blurred oscillatory-zoned texture and apparent 206Pb/238U ages of 341 to 780Ma, suggesting metamorphic modification by solid-state recrystallization with no significant involvement of metamorphic fluid. Type II zircons share similar Th, U and HFSE contents and REE patterns to Type I zircons. However, they exhibit blurred oscillatory-zoned texture or are unzoned, have apparent 206Pb/238U ages of 348 to 709Ma, and are LREE-enriched relative to Type I zircons. This suggests that they underwent metamorphic reworking by replacement recrystallization in the presence of metamorphic fluid. The LREE enrichment is due to the presence of microscale LREE-bearing mineral inclusions (such as apatite, monazite or epidote) in the zircons. Type III zircons, representing the majority of the present analyses, are characterized by spongy texture and consistent enrichment of LREE, HREE, Th, U and HFSE relative to Type I zircons. They yield nearly concordant U–Pb ages close to the discordia lower-intercept. The consistent enrichment of trace elements relative to the magmatic zircon indicates involvement of a special UHP metamorphic fluid that has a strong capacity to extract significant amounts of LREE, HREE, Th, U and HFSE from such accessory minerals as allanite, garnet, rutile and zircon. Because these minerals are stable in the field of hydrous melt in granite–water systems, they are not able to be decomposed during the exhumation of deeply subducted continental crust. Thus, a supercritical fluid is suggested to transport the LREE, HREE, Th, U and HFSE in the accessory minerals to recrystallized zircons. The mechanism of dissolution recrystallization is responsible for the spongy texture and the very high concentration of trace elements in this type of metamorphic zircons. Therefore, the action of supercritical fluid is evident under the low-T/UHP metamorphic conditions. [Copyright &y& Elsevier]

Abstract: Different types of metamorphic zircon are recognized by a combined study of petrography, U–Th–Pb and Lu–Hf elements and isotopes in zircons from eclogite-facies metagranites in the Dabie orogen. The results provide petrological distinction between metamorphic growth and recrystallization with respect to protolith inheritance, fluid and melt effects. Zircon U–Pb dating for the metagranites yields two groups of ages at 778±13 Ma and 223±4 Ma, respectively, corresponding to protolith formation in the Neoproterozoic and metamorphic modification in the Triassic. Metamorphically grown zircons from the aqueous fluid are characterized by concordant Triassic U–Pb ages, relatively high U contents but low Th contents, low Th/U and 176Lu/177Hf ratios, and elevated Hf isotope ratios. Metamorphically grown zircons from the hydrous melt show concordant Triassic U–Pb ages, very high contents of both Th and U, elevated 176Hf/177Hf ratios, but almost unchanged 176Lu/177Hf ratios. Metamorphic recrystallization is commonly associated with discordant U–Pb ages between Neoproterozoic and Triassic, but availability of fluid/melt dictates the extent to which internal structure, morphology, U–Th–Pb and Lu–Hf element and isotope systems of protolith zircon were modified by metamorphic dehydration and partial melting. While the zircon U–Th–Pb isotope systems can be variably reset by solid-state recrystallization, its initial Hf isotope signature keeps unchanged at the same conditions. On the other hand, dissolution recrystallization causes almost complete resetting of the U–Th–Pb chronometric systems to concordant ages at the metamorphic time, but it does not significantly change the Lu–Hf isotope compositions. Replacement recrystallization resets the zircon U–Th–Pb and Lu–Hf isotope systems to variable degrees, depending on the activity of metamorphic fluid/melt. Consequently, the five types of metamorphic zircon are distinguished not only between growth from the aqueous fluid and the hydrous melt but also between the recrystallization via the solid-state, replacement and dissolution mechanisms. This provides insights into the behavior of zircon during subduction-zone metamorphism, particularly that concerning dehydration melting during exhumation. [Copyright &y& Elsevier]