Meteoric-hydrothermal Alteration Research Articles

The occurrence of Neoproterozoic low δ18O igneous rocks in the northwestern margin of the South China Block: Implications for the Rodinia configuration

Low δ18O felsic igneous rocks are reported for the first time in the northwestern margin of the South China Block. Zircon U-Pb dating indicates that intermediate to felsic plutons were emplaced at 780 Ma and 750 Ma, respectively, and rhyolite was erupted at about 750 Ma. Compared with other rock-forming minerals that have variable δ18O values (−2.3‰ to 8.7‰ for quartz; −5.4‰ to 6.3‰ for plagioclase, −7.6‰ to 2.5‰ for K-feldspar), zircon δ18O values mainly range from 1.0‰ to 4.7‰ with a few negative values of −7.7‰ to −2.9‰. A two-stage 18O-depletion scenario was proposed, 1) high-temperature meteoric water–rock reaction in a continental rift setting, generating low δ18O melt from which precipitated zircons with significant lower δ18O values than normal mantle; 2) post-crystallization sub-solidus high-T meteoric-hydrothermal alteration, resulting in varying δ18O values for rock-forming minerals that leads to δ18O-disequilibrium between zircon and other minerals. In addition, zircons from these rocks show positive εHf(t) values of 4.6 to 11.2 with Hf model ages of ca. 1.0–1.4 Ga, suggesting a magma source of juvenile crust due to mantle melting during the Grenvillian subduction of oceanic lithosphere in response to accretion of the South China Block to Rodinia in the Late Mesoproterozoic. The occurrence of low δ18O felsic rocks in the northwestern margin of the South China Block indicates the development of a low δ18O magmatic belt during the rifting of the South China Block away from Rodinia at about 750 Ma. The coeval occurrence of Neoproterozoic low δ18O magmatism in NW India, South China, Madagascar and Seychelles suggests a link between these continental blocks in the Rodinia configuration.

Oxygen isotope evolution of the Lake Owyhee volcanic field, Oregon, and implications for the low-δ18O magmatism of the Snake River Plain–Yellowstone hotspot and other low-δ18O large igneous provinces

The Snake River Plain–Yellowstone (SRP-Y) hotspot track represents the largest known low-δ18O igneous province; however, debate persists regarding the timing and distribution of meteoric hydrothermal alteration and subsequent melting/assimilation relative to hotspot magmatism. To further constrain alteration relations for SRP-Y low-δ18O magmatism, we present in situ δ18O and U–Pb analyses of zircon, and laser fluorination δ18O analyses of phenocrysts, from the Lake Owyhee volcanic field (LOVF) of east-central Oregon. U–Pb data place LOVF magmatism between 16.3 and 15.4 Ma, and contain no evidence for xenocrystic zircon. LOVF δ18O(Zrc) values demonstrate (1) both low-δ18O and high-δ18O caldera-forming and pre-/post-caldera magmas, (2) relative increases in δ18O between low-δ18O caldera-forming and post-caldera units, and (3) low-δ18O magmatism associated with extension of the Oregon-Idaho Graben. The new data, along with new compilations of (1) in situ zircon δ18O data for the SRP-Y, and (2) regional δ18O(WR) and δ18O(magma) patterns, further constrain the thermal and structural associations for hydrothermal alteration in the SRP-Y. Models for low-δ18O magmatism must be compatible with (1) δ18O(magma) trends within individual SRP-Y eruptive centers, (2) along axis trends in δ18O(magma), and (3) the high concentration of low-δ18O magmas relative to the surrounding regions. When considered with the structural and thermal evolution of the SRP-Y, these constraints support low-δ18O magma genesis originating from syn-hotspot meteoric hydrothermal alteration, driven by hotspot-derived thermal fluxes superimposed on extensional tectonics. This model is not restricted to continental hotspot settings and may apply to several other low-δ18O igneous provinces with similar thermal and structural associations.

Rhyolite petrogenesis and meteoric–hydrothermal alteration at the Maghnia volcanic massif, Northwest Algeria

Volcanic rocks from the Maghnia region of northwest Algeria consist of fresh to altered rhyolitic units that were emplaced during late Miocene time. Petrographic observations and geochemical data indicate that rhyolitic rocks were derived from a hybridized peraluminous magma. Petrogenetic modeling, REE contents and oxygen isotope data of quartz and feldspar and spessartine-rich garnet allows the reconstruction of the fractional crystallization between 680 and 780°C and pressure of less than 0.5GPa. Rhyolitic units are partially weathered and form an economic deposit of montmorillonite. Spectroscopic data and stable isotopes (O, H) of glass and montmorillonite indicate that the residual glass was partially hydrated by meteoric water at temperatures between 31 and 54°C. The chemical mass balance indicates that rhyolitic rocks were altered to montmorillonite by meteoric water enriched in Ca, Mg and Sr. The combination of the chemical mass balance and the 87Sr/86Sr initial ratios of rhyolite and montmorillonite indicate that continental water/rock interaction was related to the surrounding Miocene lacustrine limestone.

Fluid-rock interaction and geochemical transport during protolith emplacement and continental collision: A tale from Qinglongshan ultrahigh-pressure metamorphic rocks in the Sulu orogen

Ultrahigh-pressure (UHP) metamorphic rocks from the Qinglongshan region of the Sulu orogen are comprehensively studied for their whole-rock geochemistry, mineral O isotopes and zirconology. The metamorphic minerals, which experienced eclogite- to amphibolite-facies metamorphism, exhibit low to negative δ18O values, suggesting that the 18O-depletion of UHP rocks was acquired from their igneous protolith due to high-T meteoric-hydrothermal alteration during the Neoproterozoic. The O isotope heterogeneity in the protolith was not homogenized during the Triassic UHP metamorphism, indicating very limited fluid flow during orogenesis. However, the fluid flow is locally significant during exhumation of the UHP rocks, resulting in the formation of quartz veins, symplectites and coronas. Geochemical transport due to fluid action is evident in whole-rock geochemistry and mineralogical composition. The UHP rocks exhibit unreasonably low 87Sr/86Sr ratios at t1 = 750 Ma but much radiogenic Sr isotopes at t2 = 230 Ma, suggesting the mobility of water-soluble elements due to hydrothermal alteration during protolith emplacement and metamorphic dehydration during continental collision. Fluid-rock interaction during exhumation would also have mobilized Al, Si, Ca and LREE, resulting in the formation of high-pressure veins in the UHP eclogites. The protolith zircon of magmatic origin underwent different types of metamorphic recrystallization in response to fluid-mineral interaction, leading to differential redistribution of trace elements and O-Hf isotopes. Newly grown zircons of metamorphic origin exhibit negative δ18O values, indicating precipitation from negative δ18O fluids that were likely generated by metamorphic dehydration of the hydrothermally altered negative δ18O rock-forming minerals during the Triassic. The metamorphic zircons exhibit relatively homogeneous Hf isotope compositions, suggesting that fluid Hf isotopes originated from the same Hf isotope composition of the protolith. Relict zircon domains of magmatic origin exhibit both positive eHf(t) and negative eHf(t) values, indicating that the protolith of UHP rocks formed by reworking of both juvenile and ancient crustal rocks.

The origin and evolution of low-δ18O magma recorded by multi-growth zircons in granite

We for the first time reported low-δ18O granites within craton in Bengbu uplift, southeast margin of North China Craton. Integrated ion microprobe study of δ18O and U–Pb age on single zircon grains and zircon populations, gives direct evidence for the origin and later magmatic evolution of the low-δ18O granites. Three types of zircon domains are recognized: inherited magmatic core (Type I), inherited metamorphic core/mantle (Type II) and overgrown magmatic rim (Type III). Type I zircons were formed in Neoproterozoic, and have the average δ18O values of 5.3±0.6‰ around mantle value. Triassic Type II zircons have extremely low and highly heterogeneous δ18O values ranging from −9.4‰ to 8.6‰, consistent with the values and variations exhibited by metamorphic zircons from Dabie–Sulu orogen. This feature argues for a Triassic metamorphic origin for the 18O-depletion signature of zircons from ultrahigh pressure metamorphic rocks in Dabie–Sulu orogen. Jurassic Type III zircons have isotopically homogeneous δ18O values of 3.3±0.5‰. Collectively, zircon populations recorded the δ18O variations from source to low-δ18O granite. The primary protolith of Jurassic granites in Bengbu uplift is a Neoproterozoic granitic intrusion in South China Block. Later high-temperature meteoric hydrothermal alteration has lowered the whole-rock δ18O values in various degrees. The low-δ18O protolith was then buried to the middle-lower crust by Triassic continental subduction and Jurassic anatexis produced the low-δ18O granitic magma. Moreover, δ18O values of Type III zircons are isotopically lighter than previously reported values of magmatic garnets in Jurassic granites, indicating later magmatic evolution of low-δ18O magma toward high δ18O values probably as a result of infiltration of high-δ18O fluid from wall-rocks.

The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe, Dabie Orogen, China

Several metabasite lenses in Ganghe, Central Dabie, that were previously described as pillow lavas are studied by elemental, Sr–Nd–Pb isotopic, and mineral oxygen isotopic analysis as well as zircon SHRIMP U–Pb dating. Zircon U–Pb geochronology results indicate that the protolith emplacement age of these metabasites is approximately 717±38Ma, consistent with the age of the volcanoclastic rocks in the same unit, and that they experienced the Triassic HP eclogite-facies retrograde metamorphism at 221±2Ma during exhumation after subduction to mantle depth and peak ultra-high pressure metamorphism. The low δ18O values of −5.5‰ to −2.0‰ indicate that the protoliths underwent high temperature meteoric-hydrothermal alteration before subduction but had no seawater interaction. These metabasites had similar formation processes, water–rock interactions and metamorphisms as other eclogite-facies rocks cropped out in the Central Dabie terrain. They showed negative abnormalities in Nb, Sr, and Ti content and positive abnormalities in Ba, Th, and Pb content; they also showed LREE enrichment. The insusceptible Sm–Nd isotopes during metamorphism yielded εNd (t)=−12 to −10 and TDM=2.2–2.8Ga for samples from lenses #1 to #3 and −7 to −6 and 2.1–2.2Ga for lens #4; the samples also showed low radiogenic Pb isotope compositions of (206Pb/204Pb)i=15.34–16.50, (207Pb/204Pb)i=15.23–15.32, and (208Pb/204Pb)i=35.93–37.04. The data suggest that the protolith sources of the metabasites were contaminated to variable degrees by old crustal materials during formation. Unlike the Maowu layered intrusions, which were contaminated by upper crust, the magmas of the metabasites were contaminated by lower crust in the magma chamber and during eruption. It can be concluded that the protoliths of these metabasites were derived from old crustal-contaminated mantle sources and initially emplaced in the crust at the Neoproterozoic and that they were altered by meteoric water at high temperatures. In this respect, they might be similar to the Neoproterozoic mafic intrusions in the North Huaiyang terrain. However, the studied metabasites experienced the Permo-Triassic subduction and metamorphism, whereas the North Huaiyang Neoproterozoic mafic intrusions did not.

Rift melting of juvenile arc-derived crust: Geochemical evidence from Neoproterozoic volcanic and granitic rocks in the Jiangnan Orogen, South China

A combined study of zircon U–Pb and Lu–Hf isotopes, mineral O isotopes, whole-rock elements and Sr–Nd isotopes was carried out for Neoproterozoic volcanics and granites from the eastern part of the Jiangnan Orogen in South China. The results are used to test controversial models of petrogenesis (plume-rift, slab-arc and plate-rift) for similar ages of magmatic rocks in South China. Zircon U–Pb dating yields two groups of ages at ∼780 and ∼825 Ma, respectively, corresponding to syn-rift and pre-rift magmatic events in response to supercontinental rifting. Both volcanic and granitic rocks show trace element features similar to those of arc-derived igneous rocks, but with more significant enrichment in large ion lithospheric elements relative to oceanic arc basalts. They have positive ɛ Hf( t) values of 3.6–6.3 for zircons, with Hf model ages of 1.12–1.21 Ga. This indicates reworking of late Mesoproterozoic juvenile crust for the origin of the Neoproterozoic magmatic rocks. Thus, oceanic arc magmatism would occur in the late Mesoproterozoic, with remarkable production of juvenile crust at the southeastern margin of the Yangtze Block. Because of the tectonic collapse of arc–continent collision orogen in the pre-rift stage, S-type magmatic rocks were generated by burial and anatexis of juvenile arc-derived crust to form the pre-rift episode of granodiorites and volcanics. In the syn-rift stage, the volcanics formed by reworking of arc-derived sedimentary rocks whereas the granite was generated by melting of the pre-rift igneous rocks. Emplacement of evolved felsic magmas along the rift tectonic zone would cause subsolidus high- T meteoric-hydrothermal alteration, resulting in varying δ 18O values for minerals from the volcanic and granitic rocks. Therefore, the plate-rift model is advanced to account for petrogenesis of all Neoproterozoic magmatic rocks in South China, with lithospheric extsension as the driving force of supercontinental rifting. Partial melting due to arc-continent collision, orogenic collapse and supercontinental rifting is also proposed as a mechanism for the chemical differentiation of continental crust towards the felsic composition.

Dehydration and melting during continental collision: Constraints from element and isotope geochemistry of low- T/UHP granitic gneiss in the Dabie orogen

A combined study of petrography, whole-rock major and trace elements as well as Rb−Sr and Sm−Nd isotopes, and mineral oxygen isotopes was carried out for two groups of low- T/UHP granitic gneiss in the Dabie orogen. The results demonstrate that metamorphic dehydration and partial melting occurred during exhumation of deeply subducted continent. Zircon δ 18O values of − 2.8 to + 4.7‰ for the gneiss are all lower than normal mantle values of 5.3 ± 0.3‰, consistent with 18O depletion of protolith due to high- T meteoric-hydrothermal alteration at mid-Neoproterozoic. Most samples have extremely low 87Sr/ 86Sr ratios at t 1 = 780 Ma, but very high 87Sr/ 86Sr ratios at t 2 = 230 Ma. This suggests intensive fluid disturbance due to the hydrothermal alteration of protoliths during Neoproterozoic magma emplacement and the metamorphic dehydration during Triassic continental collision. Rb–Sr isotopes, Th/Ta vs. La/Ta and Th/Hf vs. La/Nb relationships suggest that Group I gneiss experienced lower degrees of hydrothermal alteration, but higher degrees of dehydration, than Group II gneiss. The two groups of gneiss have similar patterns of REE and trace element partition. Group I gneiss displays good correlations between Nb and LREEs but no correlations between Nb and LILEs (Rb, Ba, Pb, Th and U), indicating differential mobilities of LILEs during the dehydration. Thus the correlation between Nb and LREEs is inherited from protolith rather than caused by metamorphic modification. Relative to Group I gneiss, Group II gneiss has stronger negative Eu anomaly, lower contents of Sr and Ba but higher contents of Rb, Th and U. In particular, Nb correlates with LILEs (e.g., Rb, Sr, Ba, Th and U), but not with LREEs (La and Ce). This may indicate decoupling between the dehydration and LILEs transport during continental collision. Furthermore, dehydration melting may have occurred due to breakdown of muscovite during “hot” exhumation. Group II gneiss has extremely low contents of FeO + MgO + TiO 2 (1.04 to 2.08 wt.%), high SiO 2 contents of 75.33 to 78.23 wt%, and high total alkali (Na 2O + K 2O) contents (7.52 to 8.92 wt.%), comparable with compositions predicted from partial melting of felsic rocks by experimental studies. Almost no UHP metamorphic minerals survived; felsic veins of fine-grain minerals occurs locally between coarse-grain minerals, resulting in a kind of metatexite migmatites due to dehydration melting without considerable escape of felsic melts from the host gneiss. In contrast, Group I gneiss only shows metamorphic dehydration. Therefore, the two groups of gneiss show contrasting behaviors of fluid–rock interaction during the continental collision.

Geochronology and Stable Isotope Geochemistry of UHP Metamorphic Rocks at Taohang in the Sulu Orogen, East-Central China

Zircon U-Pb dating, mineral Sm-Nd isochron dating, and O and H isotope analyses were carried out for ultrahigh-pressure (UHP) eclogite and granitic gneiss from Taohang in the Sulu orogen. Besides heterogeneous 18O depletion on an outcrop scale, mineral-pair O isotope thermometry indicates that refractory garnet and zircon attained and preserved equilibrium fractionations at about 820 to 560°C under eclogite-facies conditions. Zircons from the UHP metamorphic rocks have low δ18O values of -1.3 to 4.2‰, variably lower than δ18O values of 5.3 ± 0.3‰ for normal mantle zircons. U-Pb discordia dating of 18O-depleted zircons yields a protolith age of 770 ± 23 Ma and a metamorphic age of 214 ± 9 Ma. Therefore, the 18O-depleted zircons crystallized from a mid-Neoproterozoic low-18O magma whose precursor experienced high-T meteoric-hydrothermal alteration prior to melting in an active rifting zone. Both H isotope composition and H2O concentration were measured by the TCEA-MS online technique. The results show δD values of -121 to -58‰ for nominally anhydrous minerals and -101 to -62‰ for hydroxyl-bearing minerals, consistent with incorporation of meteoric water into protoliths of UHP meta-igneous rocks by high-T alteration and remelting. Hundreds to thousands of ppm H2O were detected in the forms of both molecular water and structural hydroxyl to be present in the nominally anhydrous minerals, providing an important budget of water content (besides hydrous minerals) in deeply subducted continental crust. A Gt-Wr-Pl Sm-Nd isochron age of 214 ± 10 Ma was obtained, in agreement with the zircon U-Pb age and corresponding to the state of O isotope equilibrium between the isochron minerals. Thus both ages are interpreted to represent the time of high-pressure eclogite-facies recrystallization during the initial exhumation. A fluid-present process for zircon overgrowth and Nd-O isotopic reequilibration is evident for this episode of retrogression. On the other hand, a Gt-Kfs Sm-Nd isochron age of 164 ± 11 Ma was obtained, corresponding to the state of O isotope disequilibrium between garnet and K-feldspar. This age postdates the Triassic collision orogeny, and thus has no relevance to the processes of both continental subduction and exhumation, suggesting limited fluid activity in the post-collisional stage. Therefore, the state of O isotope equilibrium or disequilibrium between coexisting minerals in high-grade metamorphic rocks provides a direct test for the validity of the mineral Sm-Nd chronometer in either case.

Tectonic driving of Neoproterozoic glaciations: Evidence from extreme oxygen isotope signature of meteoric water in granite

The global context of glaciation in the Neoproterozoic has been hypothesized to result from the massive reorganization of the Earth's land and ocean systems due to breakup and dispersal of the supercontinent Rodinia at about 750 Ma. This hypothesis gains support from unusually light O isotope records of hydrothermally altered rocks in a rift tectonic zone at that time, which is indicative of interaction with meteoric water of low mean annual temperature, and thus tectonic driving of cold paleoclimate at the time of hydrothermal alteration. Very negative δ 18O values of − 14.4 to − 10.0‰ are found for garnet from Neoproterozoic granite in South China, which are the lightest O isotope record so far reported for minerals from igneous rocks. Negative δD values of − 129 to − 109‰ are obtained for garnet, magnetite and zircon. Thus high-T meteoric-hydrothermal alteration occurred during magma emplacement. SHRIMP U–Pb dating for magmatic and hydrothermal zircons yields two groups of ages at 782 ± 3 Ma and 748 ± 3 Ma, respectively, responsible for granite crystallization and hydrothermal alteration. The garnet is in O isotope disequilibrium with zircon, indicating differential effects of subsolidus hydrothermal alteration on the minerals of different O diffusion rates. Occurrence of the unusually negative δ 18O granite at mid-low paleolatitudes provides a geochemical proxy for a cold paleoclimate at 748 ± 3 Ma, possibly a continental glaciation corresponding to the Kaigas iceage. It suggests a tectonic link to the climatic effect of uplifted rift flanks due to the Rodinia breakup at about 750 Ma, and thus the ice–fire interaction by syn-rift magmatism of low δ 18O imprints in association with the mantle event of asthenospheric upwelling. Hence the tectonic driving is evident for regional glaciations in supercontinental rift basins.

Melting of subducted continent: Element and isotopic evidence for a genetic relationship between Neoproterozoic and Mesozoic granitoids in the Sulu orogen

Zircon U–Pb and hornblende Ar–Ar ages, major and trace elements, and Sr, Nd and O isotope compositions of Neoproterozoic and Mesozoic granitoids from the Wulian region in the Sulu orogen of China demonstrate that post-collisional granitoids were generated by Early Cretaceous melting of subduction-thickened continental crust that has geochemical affinities to Neoproterozoic protolith of ultrahigh-pressure metaigneous rocks that were derived from the Yangtze Block. The Mesozoic granitoids share the following features with the Neoproterozoic granites: (1) occurrence of Neoproterozoic U–Pb ages in zircon; (2) strong LREE enrichment but HFSE (Nb, P and Ti) depletion; (3) variable δ 18O values for constituent minerals; (4) significantly negative ε Nd( t) values with Paleoproterozoic Nd model ages. Thus the two ages of granitoids have a genetic relationship in source nature. However, they differ significantly in both the O isotope composition of zircon and the concentration ratios of fluid-mobile to fluid-immobile elements. These differences are interpreted to reflect differences in the depth of magma sources, and in the nature of subsequent water–rock alteration. The Neoproterozoic granites were derived from hydration melting of Paleoproterozoic crust during breakup of the supercontinent Rodinia at ca. 780 to 740 Ma along the northern margin of the Yangtze Block, with subsequent overprinting of high- T meteoric-hydrothermal alteration and rifting-induced low- 18O magmatism. In contrast, the Mesozoic granitoids were derived from dehydration melting of subduction-thickened crust that was unaffected by meteoric-hydrothermal alteration. The source of the Mesozoic granitoids may be coeval middle–lower crustal counterparts of the Neoproterozoic granites.

Fluid-Rock Interaction in UHP Phengite-Kyanite-Epidote Eclogite from the Sulu Orogen, Eastern China

Combined petrographic, minerochemical, fluid inclusion, and stable isotope studies have been carried out on phengite-kyanite-epidote (Phe-Ky-Ep) eclogites from Hushan and Qinglongshan (Donghai area, UHP Sulu orogen, China) to unravel their metamorphic evolution and fluid-rock interaction. A complex metamorphic evolution, from coesite-eclogite (P = 3.5-4.0 GPa and T = 840 ± 50°C) to greenschist facies conditions (P ≈ 0.2 GPa and T ≈ 350°C), through quartz-eclogite and HT amphibolite-facies conditions, was recognized. The associated fluids are aqueous and show a progressive change in salinity and composition, reflecting different metamorphic stages. Stable isotope data confirm that, prior to subduction, the Sulu eclogite protolith experienced a meteorichydrothermal alteration, and indicate that during metamorphic evolution the rocks recrystallized without pervasive fluid infiltration.

Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U–Pb dating and oxygen isotope

There are two types of gneisses, biotite paragneiss and granitic orthogneiss, to be closely associated with UHP eclogite at Shuanghe in the Dabie terrane. Both concentration and isotope composition of bulk carbon in apatite and host gneisses were determined by the EA-MS online technique. Structural carbonate within the apatite was detected by the XRD and FTIR techniques. Significant 13C-depletion was observed in the apatite with δ 13C values of −28.6‰ to −22.3‰ and the carbon concentrations of 0.70–4.98 wt.% CO 2 despite a large variation in δ 18O from −4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ 13C and δ 18O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in 13C primarily, but subjected to overprint of 13C-rich CO 2-bearing fluid after the UHP metamorphism. The 13C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric–hydrothermal alteration before plate subduction. Both low δ 13C values and structural carbonate in the apatite suggest the presence of 13C-poor CO 2 in the UHP metamorphic fluid. The 13C-poor CO 2 is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism. Zircons from two samples of the granitic orthogneiss exhibit low δ 18O values of −4.1‰ to −1.1‰, demonstrating that its protolith was significantly depleted in 18O prior to magma crystallization. U–Pb discordia datings for the 18O-depleted zircons yield Neoproterozoic ages of 724–768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie–Sulu orogen. Therefore, the meteoric–hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated with the Rodinia supercontinental breakup and the snowball Earth event. It is thus deduced that the igneous protolith of the granitic orthogneiss and some eclogites would intrude into the older sequences composing the sedimentary protoliths of the biotite paragneiss and some eclogites along the northern margin of the Yangtze plate at mid-Neoproterozoic, and drove local meteoric–hydrothermal circulation systems in which both 13C- and 18O-depleted fluid interacted with the protoliths of these UHP rocks now exposed in the Dabie terrane.

Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime

Discovery of coesite, diamond, and extreme 18O-depletion in eclogites from the Dabie–Sulu orogen in central-east China has contributed much to our understanding of subduction of continental crust to mantle depths and its subsequent exhumation. Hydrogen, oxygen, and carbon isotope distributions were systematically investigated in the past 8 years for ultrahigh pressure (UHP) eclogites, gneisses, granulites, marbles, and peridotites from this exciting region. The available data show the following characteristic features: (1) variable δ 18O values of −11‰ to +10‰ for the eclogites and gneisses, with both equilibrium and disequilibrium fractionations of oxygen isotopes among minerals; (2) disequilibrium fractionation of hydrogen isotopes between mica and epidote from both eclogites and gneisses, with low δD values up to −127‰ to −100‰ for phengite; (3) negative δ 13C values of −28‰ to −21‰ for apatite as well as host-eclogites and gneisses; (4) positive δ 13C values of +1‰ to +6‰ for coesite-bearing marble associated with eclogites; (5) zircons from metamorphic rocks of different grades show a large variation in δ 18O from −11‰ to +9‰, with U–Pb ages of 700 to 800 Ma for the timing of low- δ 18O magma crystallization. It appears that the UHP metamorphic rocks exhibit ranges of δ 18O values that are typical of potential precursor protolith rocks. Preservation of the oxygen isotope equilibrium fractionations among the minerals of the UHP eclogites and gneisses suggests that these rocks acquired the low δ 18O values by meteoric-hydrothermal alteration before the UHP metamorphism. Thus, the UHP metamorphic rocks largely reflect the δ 18O values of their premetamorphic igneous or sedimentary precursors. The stable isotope data demonstrate that basaltic, granitic, and sedimentary protoliths of the eclogites, orthogneiss, and paragneiss in the orogen were at or near the earth's surface, and subjected to varying degrees of water–rock interaction at some time before plate subduction. The low- δ 18O rocks were isolated from water–rock interactions during their descent to and return from mantle depths. It appears that the oxygen, hydrogen, and carbon on the earth's surface were recycled into the mantle at depths of >200 km by the continental subduction. A fried ice cream model is advanced as an analogy to the rapid processes of both plate subduction and exhumation, with a short residence time of the UHP slab at mantle depths. The entire cycle of subduction, UHP metamorphism, and exhumation is estimated to take place in about 10 to 20 Ma. The 18O-depleted zircons and other minerals acquired their oxygen isotope compositions from low- δ 18O magmas that incorporated the isotopic signatures of meteoric water in rifting tectonic zones prior to solidification. The U–Pb discordia dating for the 18O-depleted zircons revealed that the meteoric water–rock interaction occurred at Neoproterozoic, a time being much earlier than the UHP metamorphism at Triassic, but correlated with the Rodinian breakup, positive carbon isotope anomaly in carbonates, and the snowball earth event. The unusually low δ 18O values can be acquired from either the meteoric water of cold paleoclimates or the melt water of glacial ice or snow. Neoproterozoic rift magmatism along the northern margin of the Yangtze craton may have provided sufficient heat source to trigger the meteoric-hydrothermal circulation. It is possible that the unusual 18O-depletion in the meta-igneous rocks occurs at some time prior to the snowball earth event, when there is a transition from a very cold earth with continental glaciers to a widely glaciated earth where bulk of the earth is covered by sea ice as defined for the snowball earth. The heterogeneity of oxygen isotope compositions at outcrop scales demonstrates the absence of pervasive fluid infiltration during prograde, peak UHP, and retrograde metamorphism; most rocks appear to have recrystallized under virtually closed system conditions characterized by widespread lack of an aqueous fluid phase. Volatiles may not escape from the rock series during the rapid subduction of the continental crust, resulting in a general lack of syn-collisional arc-magmatism in this orogen. Big differences in pressure and time from the peak UHP stage to the retrograde HP eclogite-facies stage cause significant release of aqueous fluid by dehydration from decompressing slabs during exhumation. As a result, fluid flow occurred in a channellized way on small scales subsequent to the UHP metamorphism, with very limited mobility of fluid at peak UHP conditions. The fluid for retrograde reactions was internally buffered in stable isotope compositions. While some fluids were locally derived from the surrounding gneisses, more fluid was probably derived from internal dehydration of the rock units in question. In addition to the breakdown of hydroxyl-bearing minerals, exsolution of structural hydroxyl dissolved in nominally anhydrous minerals due to abrupt decrease in pressure may have been an important source for the retrograde fluid.

Oxygen isotope geochemistry of the Granite Harbour Intrusives, Wilson Terrane, Northern Victoria Land, Antarctica

The oxygen and strontium isotope compositions of the Cambro-Ordovician granitoids cropping out in the Wilson Terrane (Granite Harbour Intrusives–GHI) constrain the petrological evolution of the magmatism in Antarctica, related to the Ross Orogeny. The measured δ18OWR values of these intrusives define three different compositional groups: the metaluminous rocks (MAG), with δ18OWR ranging from 6.9 (olivine gabbro) to 11.4‰ (monzogranite); the unaltered peraluminous granites (PAG), having δ18OWR values ranging from 10.6 to 13.2‰, and the foliated peraluminous leucogranites (SKG), characterised by δ18OWR values above 14‰. The analysis of equilibrium mineral assemblages indicates that the high δ18OWR values are magmatic and unaffected by low-temperature processes. A few peraluminous granites sampled in the vicinity of Cenozoic intrusions show anomalously low δ18OWR, due to meteoric-hydrothermal alteration. The isotopic data indicate that the coeval and spatially related metaluminous mafic and felsic intrusives forming the GHI were not comagmatic: the mafic and intermediate rocks were likely derived from lower crustal contamination of a pristine basaltic magma; their δ18OWR values were also increased during emplacement, due to the interaction with the adjacent 18O-rich hydrous felsic magmas (mixing). Oxygen isotope data indicate that the crustal sources producing the Granite Harbour intrusives were not homogeneous: the felsic metaluminous intrusives were produced by partial melting of fertile rock with possible igneous origin, whereas partial melting of a metapelitic source rock is claimed for the genesis of the peraluminous granites.

Sm-Nd dating and Nd-Sr isotopic characteristics of the Shimian ophiolite suite, Sichuan Province

By measuring the Sm-Nd and Rb-Sr isotopic compositions of harzburgite and gabbro from Shimian ophiolite suite, we got the whole rock Sm-Nd isochron age of (938±30) Ma (2σ), and the eNd of 7.6±0.8 (2σ), which shows that the ophiolite was formed at the Early Neoproterozoic. The obvious change (0.70209–0.70708) of ISr values of the ophiolite is caused by the meteoric hydrothermal alteration. The high eNd values indicate that the primitive magma was derived from the intense depleted mantle reservoir. It is suggested that this area was in a back-arc basin environment during the Early Neoproterozic.

Oxygen and neodymium isotope evidence for recycling of juvenile crust in northeast China

It has been long recognized from Nd and Sr isotopes that depleted mantle sources consist of recycled oceanic materials, but difficulty was encountered in identifying this signature by means of oxygen isotopes because of significant postemplacement hydrother- mal alteration. Zircon is expected to preserve this signature because it is resistant to high- temperature hydrothermal alteration. This effect is illustrated by a combined Sm-Nd and oxygen isotope study of whole-rock and mineral samples from a Mesozoic A-type granite at Nianzishan in northeastern China. The Sm-Nd isotope results show positive «Nd(t) val- ues of 10.86 to 14.27 with young Nd model ages of 569-846 Ma, manifesting a significant input of newly mantle derived material. The zircon d 18 O values of 3.12‰-4.19‰ are significantly lower than the d 18 O value of 5.3‰ 6 0.3‰ for the normal mantle zircon and thus appear to require remelting of hydrothermally altered oceanic crust. The combined Nd-O isotope studies not only provide compelling evidence for geochemical recycling of young juvenile crust by plate subduction, but also demonstrate that the granitic magmas can result from partial melting of mantle-derived rocks that were subjected to seawater- hydrothermal alteration before magma generation. Disequilibrium oxygen isotope frac- tionations are observed between common rock-forming minerals with significantly lower d 18 O values for alkali feldspar than seawater, corresponding to meteoric-hydrothermal alteration after magma crystallization.

Oxygen isotope evidence for two-stage water-rock interactions of the Nianzishan A-type granite in NE China

The oxygen isotope ratios of whole-rock, common rock-forming minerals and zircon from Mesozoic A-type granitic pluton at Nianzishan in northeastern China were analyzed by the conventional BrF5 method and the laser-probe technique, respectively. Both whole-rock and rock-forming minerals show large δ18O variations up to 5.5‰ with significant oxygen isotope disequilibrium between zircon and the other minerals, whereas the δ18O values of zircon are tightly clustered between 3.12‰ and 4.19‰ and thus lower than the normal-mantle δ18O values. These results indicate that the Nianzishan A-type granite experienced two-stage water-rock interactions subsequentially. The remarkably low zircon δ18O values are genetically due to seawater exchange with granite protolith in the first stage, and the oxygen isotope disequilibrium fractionations between zircon and rock-forming minerals are caused by meteoric-hydrothermal alteration in the second stage. It is inferred that the18O-depleted A-type granitic magma was derived from partial melting of subducted lower oceanic crust which was isotopically exchanged with seawater at high temperatures. In the process of granite emplacement into the upper crust, meteoric-hydrothermal circulation was triggered to overprint crystallizing granite under subsolidus conditions.

Low- 18O silicic magmas: why are they so rare?

Low- 18O silicic magmas are reported from only a small number of localities (e.g., Yellowstone and Iceland), yet petrologic evidence points to upper crustal assimilation coupled with fractional crystallization (AFC) during magma genesis for nearly all silicic magmas. The rarity of low- 18O magmas in intracontinental caldera settings is remarkable given the evidence of intense low- 18O meteoric hydrothermal alteration in the subvolcanic remnants of larger caldera systems. In the Platoro caldera complex, regional ignimbrites (150–1000 km 3) have plagioclase δ 18O values of 6.8±0.1‰, whereas the Middle Tuff, a small-volume (est. 50–100 km 3) post-caldera collapse pyroclastic sequence, has plagioclase δ 18O values between 5.5 and 6.8‰. On average, the plagioclase phenocrysts from the Middle Tuff are depleted by only 0.3‰ relative to those in the regional tuffs. At Yellowstone, small-volume post-caldera collapse intracaldera rhyolites are up to 5.5‰ depleted relative to the regional ignimbrites. Two important differences between the Middle Tuff and the Yellowstone low- 18O rhyolites elucidate the problem. Middle Tuff magmas reached water saturation and erupted explosively, whereas most of the low- 18O Yellowstone rhyolites erupted effusively as domes or flows, and are nearly devoid of hydrous phenocrysts. Comparing the two eruptive types indicates that assimilation of low- 18O material, combined with fractional crystallization, drives silicic melts to water oversaturation. Water saturated magmas either erupt explosively or quench as subsurface porphyries before the magmatic 18O can be dramatically lowered. Partial melting of low- 18O subvolcanic rocks by near-anhydrous magmas at Yellowstone produced small-volume, low- 18O magmas directly, thereby circumventing the water saturation barrier encountered through normal AFC processes.

Fluid-rock interactions during ultra-high pressure metamorphism, Dabie Shan, China

Oxygen isotope analyses of garnet separates and whole rock samples of gneiss- and marble-hosted eclogites from the “ultra-high” pressure (UHP) Dabie Shan terrain fall in the range −6.76‰ to 10.76‰. Comparison of the isotopic compositions of the eclogites with those of their host rocks leads to the following conclusions: 1) The basaltic protolith of the eclogites had isotopic compositions in the range 2.15 to 4.99‰. It is proposed that this is the result of contamination of the parent basaltic magma with hydrothermally-altered 18O-depleted source materials. 2) 18O-enriched eclogite compositions resulted from local-scale (<3 m) interaction between this basaltic eclogite protolith and fluids produced by decarbonation of impure marble lithologies during UHP metamorphism. 3) 18O-depleted eclogite (and gneiss) compositions resulted from regional-scale meteoric-hydrothermal alteration of the basalts and their host rocks prior to UHP metamorphism. Epidote separates and whole-rocks from the cores and rims of mineralogically-zoned eclogite boudins within amphibolite-grade biotite gneisses show no distinct isotopic differences. This suggests that the preferential recrystallisation of the gneisses and the associated amphibolite-grade overprint of the rims of eclogite bodies could not have resulted from regional-scale infiltration of significant volumes of fluid during the exhumation of the Dabie Shan terrain.