Mixing Of Crust Research Articles

Petrogenesis and Geodynamic Significance of the Early Triassic Nanpo Adakitic Pluton of the Luang Prabang-Loei Tectonic Belt (Northwestern Laos) in the East Tethys Domain: Constraints from Zircon U-Pb-Hf Isotope Analyses and Whole-Rock Geochemistry

Adakites are magmatic rocks with specific geochemical characteristics and specific dynamics that provide important clues to understanding the magmatic-tectonic evolution of orogenic belts. We studied the Early Triassic Nanpo adakitic pluton of the Luang Prabang-Loei tectonic belt in the Eastern Tethys domain (Laos Sarakan) using detailed petrological, zircon U-Pb chronological, whole-rock geochemical, and zircon Lu-Hf isotope studies to constrain their petrogenesis. The rocks are predominantly diorites and granodiorites with Early Triassic zircon U-Pb emplacement ages ranging from 247.9 ± 1.0 to 249.0 ± 2.4 Ma. Moderate SiO2 (56.26–65.95 wt%) and Na2O (3.24–5.00 wt%) contents, with Na2O/K2O values between 1.76 and 2.51 and A/CNK values between 0.81 and 0.94, indicate that the rocks belong to the metaluminous calc-alkaline rock series. The high Sr content (590–918 ppm), low Y (6.30–11.89 ppm) and Yb (1.99–3.44 ppm) contents, intermediate Mg# (42–50) values, and high Sr/Y and (La/Yb) N ratios (Sr/Y = 24–41, (La/Yb) N = 6.84–13.8) are typical for adakites. Zircon Hf isotope analysis shows a significant variation in the εHf(t) values (6.7–12.0), with a mean value of 9.4 and a TDM2 of 512–845 Ma. Geochemical evidence indicates that the Nanpo adakitic rock was formed by the partial melting of the thickened lower crust in the plate-breaking environment and has an important contribution to the underplated mantle-derived magma. We propose that the Early Triassic adakites in the Luang Prabang-Loei tectonic belt formed during the transition from subduction to a continental collision, and the mixing of crust- and mantle-derived magmas is the main mechanism for the growth of continental crust in the Paleo-Tethys orogenic belt of southeastern Asia.

Ferroan A-type metagranites from the Pernambuco–Alagoas Domain, northeastern Brazil attest Tonian crustal extension

The southern Pernambuco–Alagoas (PEAL) Domain in the Borborema Province, northeastern Brazil, is a large region characterized by voluminous Cryogenian–Ediacaran (640–580 Ma) calc-alkalic granitic to granodioritic rocks, and minor basement orthogneisses. Field, petrography, geochemistry, isotopic and geochronology studies were conducted on orthogneisses basement within Santana do Ipanema Batholith, southern PEAL Domain to determine their age, petrogenesis, and magmatic setting. The granitic to tonalitic orthogneisses contain abundant mafic microgranular enclaves and exhibit a flat-lying foliation. Biotite, amphibole, titanite, epidote and magnetite are the key mafic minerals. SHRIMP U–Pb zircon age from one sample revealed a Concordia age of 958 ± 10 Ma. Chemically, orthogneisses are metaluminous to slightly peraluminous, calc-alkaline to alkali-calcic, ferroan A-type granite to tonalite. Chondrite-normalized rare-earth and trace element patterns are fractionated, LREE and LILE-enriched, and HREE-depleted. Intensive parameter calculations indicate high-temperature crystallization (∼890 °C) with near-solidus temperature between 700 and 600 °C at a depth of ∼4 kbar. Their Sr–Nd isotopic composition (87Sr/86Sr(t) = 0.7032 to 0.7045; εNd(t) = +2.5 to +3.7 and TDM = 1.1 to 1.3 Ga) suggests derivation from a relatively young lower continental crust. Crustal extension induced sub-continental lithospheric mantle upwelling, which may have driven partial melting of newly created lower continental crust during the Tonian, and fractionation of the melt with little crustal mixing toward ferroan A-type granite geochemistry.

Geochemical studies of hybrid granite from Madugulapalli area, Eastern Dharwar Craton, Southern India: Implications for crustal mixing

Geochemical studies of hybrid granite from Madugulapalli area, Eastern Dharwar Craton, Southern India: Implications for crustal mixing

Insight into volcanic garnet: origin and significance of garnet as exemplified by a detailed petro-mineralogical study of the Breziny andesite (Central Slovakia Volcanic Field, Western Carpathians, Central Europe)

Almandine-rich garnets from a Neogene andesite of Slovakia can be divided into two main types. Garnet megacrysts are magmatic and form a chemically homogeneous group that contains, on average, about 5 wt% CaO and 4.5 wt% MgO as petrogenetically significant components. Garnets occurring in lithic fragments and garnets aggregated in garnetite lenses are characterised by Ca-poor cores (CaO <= 2 wt%) that testify for a two-step history and correspond respectively to inherited pre-anatectic and peritectic garnets. Available experimental data show that the composition of magmatic garnet megacrysts is compatible with a peritectic origin, through the fluid-absent melting of an immature metasedimentary protolith or a tonalitic gneiss. However, thermal evolution evidenced by zircons shielded in garnet rather suggests that garnet nucleated and grew by cooling of a hybrid magma pool, resulting from the complete mixing of crust- and mantle-derived melts.

Polygenetic mélange in the retrowedge foredeep of an active arc-continent collision, Coastal Range of eastern Taiwan

The Plio-Pleistocene Lichi Mélange in the Coastal Range of eastern Taiwan offers an excellent opportunity to study processes of mélange development at the continent-ocean interface of an active arc-continent collision. This paper presents new results of detailed geologic mapping, lithofacies analysis, magneto-biostratigraphy, paleocurrent, and paleoslope analyses in the southern Coastal Range to investigate the origins and significance of this mélange. The results show that the Lichi Mélange consists of mass-transport deposits including well-stratified block-in-matrix beds (olistostromes), extra-formational blocks (olistoliths), and broken formation with abundant soft-sediment deformation features that transition laterally into distal mega-slump beds and pebbly mudstones (subaqueous debrites). Abundant observations of depositional contacts and interbedding of mélange with contemporary (ca. 4–1 Ma) flysch units of the Fanshuliao and Paliwan formations confirm their sedimentary origin. Compacted sedimentological shear fabrics in olistostromal facies are broadly parallel to internal stratification and bedding, and are readily distinguishable from cross-cutting brittle fault zones related to post ~1 Ma west-vergent thrust faults. Paleoslope and paleocurrent analyses record down-slope gravity-driven transport toward the east and southeast.The data provide evidence for a polygenetic origin of the Lichi Mélange, in which sedimentary mass-wasting deposits are overprinted by younger tectonic shear zones. Slide blocks, conglomerate clasts, and detrital sand were all derived from an eroding source in the east-vergent eastern retrowedge of the Taiwan collisional orogen. The source area included tectonically accreted fragments of the two converging plates that represent shallow-crustal equivalents of the Miocene Yuli Belt and Eastern Slates exposed in the modern Central Range. Reconstructed stratigraphic panels record eastward progradation of olistostromal facies over distal basinal flysch deposits, which we infer resulted from eastward (oceanward) migration of a steep submarine slope at the leading edge of the retrowedge orogenic front. Thus, the Coastal Range basin evolved as a migrating retro-foredeep basin that formed on top of older, pre-collisional volcanic arc and forearc crust. These results demonstrate a unique type of sedimentary basin that is formed and then rapidly inverted at a convergent continent-ocean interface during the transition from intra-oceanic subduction to arc-continent collision. This revised history of the Lichi Mélange provides a new perspective on the dynamics of rapid crustal mixing and tectonic recycling at the convergent suture of an active arc-continent collision system.

Petrogeochemical characteristics, zircon SHRIMP U—Pb ages and Lu—Hf isotopic compositions of Late Carboniferous A‐type granitoids, Yili area, Inner Mongolia (China)

A large Late Palaeozoic A‐type granite belt lies along the Hegenshan‐Nenjiang‐Heihe belt in the Central Asian Orogenic Belt (CAOB). The Kuleqi, Naimanhe, and Gejiadian plutons at Yili are developed within this A‐type granite belt. The genesis and formation environment of these A‐type granites are controversial. The results of zircon SHRIMP UPb dating analyses indicate that the A‐type granites at Yili formed during the Late Carboniferous (302–307 Ma), and their geochemical data indicate that they principally represent a weakly peraluminous (A/CNK 1.01–1.21), high‐K calc‐alkaline series. These granites are enriched in HFSEs, Th, Rb, and K and depleted in LILEs, Ba, Nb, Sr, P, and Ti, which suggests that they record signatures of evolution of continental magma arc. The Nb/Ta ratios of these granites range from 6.84 to17.51, with an average value of 11.65, indicating that they represent crustal‐sourced magma. Their Zr/Hf ratios range from 37.05 to 88.70, with an average value of 59.72 (all values >39), suggesting that these rocks are dominated by mantle‐sourced components. The zircon Hf isotopic compositions of these rocks show εHf(t) values ranging from 6.8 to 12.5 (all positive), with an average value of 9.9, and depleted mantle two‐stage model ages ranging from 518 to 887 Ma, implying that these zircons may contain Early Palaeozoic–Neoproterozoic depleted mantle material. Based on these data, combined with their trace element characteristics, we conclude that the A‐type granitoids at Yili were mainly produced by the mixing of crust and mantle materials; together with their regional tectonic characteristics, these data indicate that these granitoids are related to the collision of the Xing'an and Songliao blocks, which was triggered by the subduction of the Palaeo‐Asian Ocean.

Geochemistry and petrogenesis of Late Carboniferous igneous rocks from southern Mongolia: Implications for the post-collisional extension in the southeastern Central Asian Orogenic Belt

Late Carboniferous is a critical period in terms of the tectonic evolution of the Central Asian Orogenic Belt (CAOB). In this study, we report the petrology, geochronology and geochemistry of Late Carboniferous mafic to felsic rocks from southern Mongolia, to better characterize the Late Carboniferous magmatism in the southeastern CAOB. Three types of igneous rocks have been identified, including diabase dykes, porphyritic granites and quartz monzonite dykes. LA-ICP MS zircon U-Pb dating of these rocks shows that the diabase dykes and porphyritic granites were simultaneously emplaced at 306Ma. The quartz monzonite dykes were subsequently emplaced at 300Ma. The diabases belong to the high-K calc-alkaline series and show strong enrichment of large ion lithophile elements (LILE; e.g., Rb, Ba and Sr) and depletion in high field strength elements (HFSE; Nb, Ta and Ti), with enriched mantle isotopic signature (εNd(t)=−2.6 to +0.5). They were likely derived from the partial melting of metasomatized sub-continental lithospheric mantle (SCLM) and experienced variable degrees of crustal contamination. The porphyritic granites are characterized by high SiO2 contents, strong negative Eu anomalies with low εNd(t) values of −3.0 to −2.7, suggesting that they were derived from crustal anatexis and experienced strong fractional crystallization. Compared with the porphyritic granites, quartz monzonites display lower SiO2 contents, weaker negative Eu anomalies with higher and more variable εNd(t) values (−0.3 to +4.2). This indicates that the quartz monzonites were probably generated by the mixing of crust- and mantle-derived magmas. The petrographic and geochemical data suggest that all the mafic to felsic rocks in the southern Mongolia formed in a post-collisional extensional setting.

Geochemistry of late Palaeozoic granitoids of the Balkhash metallogenic belt, Kazakhstan: implications for crustal growth and tectonic evolution of the Central Asian Orogenic Belt

ABSTRACTThe Balkhash metallogenic belt (BMB) in Kazakhstan is a famous porphyry Cu–Mo metallogenic belt in the Central Asian Orogenic Belt (CAOB). The late Palaeozoic granitoids in the BMB are mainly high-K calc-alkaline and I-type granites, with shoshonite that formed during a late stage. Geochemical analyses and tectonic discrimination reveal a change in the tectonic environment from syn-collision and volcanic arcs during the Carboniferous to post-collision during the Permian. The late Palaeozoic granitoids from the Borly porphyry Cu deposit formed in a classical island-arc environment, and those from the Kounrad and Aktogai porphyry Cu deposits and the Sayak skarn Cu deposit are adakitic. The εNd(t) values for the late Palaeozoic granitoids are between −5.87 and +5.94, and the εSr(t) values range from −17.16 to +51.10. The continental crustal growth histories are different on either side of the Central Balkhash fault. On the eastern side, the εNd(t) values of the granitoids from the Aktogai and Sayak deposits are very high, which are characteristic of depleted mantle and suggest that crustal growth occurred during the late Palaeozoic. On the western side, the εNd(t) values of the granitoids from the Borly and Kounrad deposits are slightly low, which suggests the presence of a Neoproterozoic basement and the mixing of crust and mantle during magmatism. The granitoids have initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.335–20.993, 15.521–15.732, and 38.287–40.021, respectively, which demonstrate an affinity between the late Palaeozoic magmatism in the BMB and that in the Tianshan, Altai, and Junggar orogens.

Similar crustal evolution in the western units of the Adrar Souttouf Massif (Moroccan Sahara) and the Avalonian terranes: Insights from Hf isotope data

The Adrar Souttouf Massif is located at the western margin of the West African Craton and consists of several NNE–SSW trending units. Of them, the two westernmost have been interpreted to be linked with the Avalonian terrane assemblage and Meguma, respectively. New Hf isotopic data corroborates the Avalon correlation but has no impact one way or another on the possible Meguma connection, as there is no Hf data available from the latter. The obtained pattern of εHf(t) values vs. zircon age of the likely Avalonia related Oued Togba unit is similar to published data from Avalonia. Zircons of this unit show characteristic patterns of crustal mixing at 0.7 to 1.3Ga and 1.75 to 2.25Ga, while juvenile crust was likely formed around 0.6 to 0.75Ga, from 1.2 to 2.2Ga, and between 2.5 and 3.2Ga. The zircons of the Sebkha Gezmayet unit reveal crustal mixing for the entire Palaeozoic and Neoproterozoic, from 2.05 to 2.11Ga, and 2.8 to 2.9Ga. Juvenile crust formation is interpreted to have occurred from 0.5 to 0.7Ga, at around 2.1Ga, and at ca. 2.9Ga. As Mesoproterozoic zircons are abundant in the likely Avalonia-like Oued Togba unit, but uncommon at the West African Craton, their origin has to be found elsewhere. A comparison of available Hf data from Amazonia and Baltica, the two potential source cratons of Avalonia, shows similarities but is hampered by the lack of available data from Amazonia. Finally, a few grains from both units have Eoarchaean model ages. Among similar grains from other peri-Gondwanan terranes, they give indication of partial recycling of Eoarchaean crust in the vicinity of the northwestern West African Craton.

Mesozoic tectonic setting of rift basins in eastern North China and implications for destruction of the North China Craton

Destruction of the North China Craton (NCC) in the Mesozoic due to subduction of the Paleo-Pacific Plate has attracted a lot of recent interest, with numerous studies focusing on regional tectonics and associated magmatism. Although the peak ages for this event have been established, the timing of its initiation remains poorly understood. In this paper, two rift basins in the northern Hebei Province of China, the Diaoe Basin (DB) and Houcheng Basin (HB), are studied in order to constrain the timing of destruction of the northern margin of the NCC. Both NNE-striking basins developed on Proterozoic basement. The DB is a graben controlled by normal faults on its two margins, and the HB is a half-graben bounded by normal faults on its eastern side. Basin fills include detrital sediments and volcanic rocks, which are (from bottom to top) the Houcheng Formation (Fh), the Zhangjiakou Formation (Fzh), the Shijiayao Formation (Fs), and the Huajiying Formation (Fhj). The Fh is composed mainly of detrital sediments interlayered with andesite in its lower section, and interlayered with felsic volcanics (rhyolite) in its upper section. There exists a transitional change from the Fh to the thick felsic volcanic strata of the Fzh. Geochemically, the andesite layers in the lower Fh were most likely derived from mixing of crust and mantle melts, whereas the felsic rocks were derived from melting of the lower crust. U–Pb dating of zircons by LA-ICP-MS yielded ages for the lower Fh andesites of ca. 165.7Ma in the HB and ca. 157.4Ma in the DB. The felsic rocks at the base of the Fzh yielded ages of ca. 155.1Ma in the HB and ca. 149.2–143.4Ma in the DB. The Fs and Fhj in the DB both yielded similar ages of ca. 136Ma. The development of rift basins, together with the occurrence of massive felsic volcanic rocks, indicates a period of significant extension and thinning of the NCC. The ca. 165.7–155.1Ma age for andesites not only represents the initial timing of crustal extension, but also the timing of thermally induced melting of the lower lithosphere. We conclude, therefore, that the destruction of the northern margin of the NCC started by the Middle Jurassic or earlier.

Recording the transition from flare-up to steady-state arc magmatism at the Purico–Chascon volcanic complex, northern Chile

The long-term evolution of continental magmatic arcs is episodic, where a few transient events of high magmatic flux or flare-ups punctuate the low-flux magmatism or “steady state” that makes up most of the arc history. How this duality manifests in terms of differences in crustal architecture, magma dynamics and chemistry, and the time scale over which transitions occur is poorly known. Herein we use multiscale geochemical and isotopic characteristics coupled with geothermobarometry at the Purico–Chascon Volcanic Complex (PCVC) in the Central Andes to identify a transition from flare-up to steady state arc magmatism over ∼800 kyr during which significant changes in upper crustal magmatic dynamics are recorded.The PCVC is one of the youngest volcanic centers related to a 10–1 Ma ignimbrite flare-up in the Altiplano–Puna Volcanic Complex of the Central Andes. Activity at the PCVC initiated 0.98±0.03 Ma with the eruption of a large 80–100 km3 crystal-rich dacite ignimbrite. High, restricted 87Sr/86Sr isotope ratios between 0.7085 and 0.7090 in the bulk rock and plagioclase crystals from the Purico ignimbrite, combined with mineral chemistry and phase relationships indicate the dacite magma accumulated and evolved at relatively low temperatures around 800–850 °C in the upper crust at 4–8 km depth. Minor andesite pumice erupted late in the ignimbrite sequence records a second higher temperature (965 °C), higher pressure environment (17–20 km), but with similar restricted radiogenic bulk rock 87Sr/86Sr = 0.7089–0.7091 to the dacites. The compositional and isotopic characteristics of the Purico ignimbrite implicate an extensive zone of upper crustal mixing, assimilation, storage and homogenization (MASH) between ∼30 and 4 km beneath the PCVC ∼1 Ma.The final eruptions at the PCVC <0.18±0.02 Ma suggest a change in the magmatic architecture beneath the PCVC. These eruptions produced three small <6 km3 crystal-rich dacite lava domes with radiogenic bulk rock 87Sr/86Sr ratios ranging from 0.7075 to 0.7081, that contain abundant basaltic-andesite inclusions with relatively low bulk rock 87Sr/86Sr ratios of 0.7057–0.7061. Plagioclase and amphibole in the host lava of Cerro Chascon, the largest of the domes, record two distinct magmatic environments; an upper crustal environment identical to that recorded in the Purico ignimbrite, and a second deeper, ∼15–20 km depth, higher temperature (∼922–1001 °C) environment. This deeper environment is recorded in textures and compositions of distinct mineral phases, and in intracrystalline isotope ratios. Plagioclase cores in the host dacite lava and mafic inclusions have in situ87Sr/86Sr isotopic compositions of 0.7083 to 0.7095, broadly similar to plagioclase from the Purico ignimbrite. In contrast, plagioclase rims and microphenocrysts in the mafic inclusions are isotopically distinct with lower 87Sr/86Sr isotope ratios (0.7057 to 0.7065 and 0.7062 to 0.7064, respectively) that overlap with the regional isotopic “baseline” compositions that are parental to the modern arc lavas.The textural and compositional characteristics of the PCVC attest to two distinct stages in its history. At ∼1 Ma the system was broadly homogeneous and dominantly dacitic recording extensive upper crustal magmatism. By ∼0.2 Ma the PCVC had transitioned to a more compositionally heterogeneous, smaller volume, mixed dacite to basaltic-andesite system, coinciding with the appearance of less-enriched “baseline” compositions. The evolution of PCVC is a microcosm of the Central Andean arc in this region where, from 10 to 1 Ma, upper crustal MASH processes resulted in the production and eruption of large volumes of homogeneous crystal-rich dacite during a regional ignimbrite flare-up. Since ∼1 Ma, decreasing explosivity, smaller eruptive volumes, increasing heterogeneity, and the emergence of less isotopically enriched basaltic-andesite to dacite composite volcanoes signal a return to steady-state arc volcanism.We posit that the transition from flare-up to steady state captured at the PCVC tracks the waning of the arc scale “thermal engine”. High magmatic fluxes during the flare-up would lead to elevated geothermal gradients and efficient crustal processing leading to a dominantly “crustal” magmatism feeding the large volume Purico ignimbrite. This upper crustal MASH zone would act as an efficient filter to any parental compositions precluding them from the eruption record. As magmatic flux and thermal energy wanes, crustal isotherms would relax leading to greater thermal contrast between parental magmas, upper crust, and remnant felsic magmas stored in the upper crust. These changes are manifested in the preservation of textural and compositional heterogeneity and the survival of less isotopically enriched magmas in the upper crust. The chemical imprint of these arc-scale changes in magma dynamics is recorded at all scales from bulk rock to intra-crystalline. The distinct magma dynamics and chemical signatures of the two modes of arc magmatism detailed here should provide a model for investigations of mature continental arc evolution through time and space.

Petrology and Geochemistry of the Grubergebirge Anorthosite and Marginal Rocks, Central Dronning Maud Land: Further Characterization of the Late Neoproterozoic Magmatic Event in East Antarctica

Abstract The Grubergebirge anorthosite, a Late Neoproterozoic massif-type anorthosite, was emplaced into Late Mesoproterozoic-aged metamorphosed orthogneisses and supracrustal rocks in the Wohlthat Mountains. Mineralogically and chemically, the marginal rocks to the anorthosite massif classify as ferromonzodiorite and ferromonzonite. Variations in trace and rare earth element abundances and normalized patterns between the anorthosite and associated marginal ferromonzodiorite (and minor ferromonzonite) and distinct differences between the ferromonzodiorite and ferromonzonite have been observed. Whereas the magmas from which the anorthosite crystallized have been slightly contaminated by incorporating crustal material (now occurring as enclaves), the marginal ferromonzodiorite represent rocks that originated due to mixing (hybridization) of injected primitive ferrodiorite magma(s) with preexisting crustal material, resulting in the hybrid ferromonzodiorite. These events at the margin of the massif anorthosite represent a widespread Late Neoproterozoic magmatic event with accompanying crustal mixing; all these rocks were thereafter metamorphosed under amphibolite- to granulite-facies conditions during the Early Cambrian Period. In contrast, unmetamorphosed dykes with a ferrodiorite mineralogy, of Late Cambro-Ordovician age, have been found that probably represent a part of the late magmatic suites of the south Petermannketten (Zwiesel).

Recent volcanism in the Siqueiros transform fault: picritic basalts and implications for MORB magma genesis

Small constructional volcanic landforms and very fresh-looking lava flows are present along one of the inferred active strike-slip faults that connect two small spreading centers (A and B) in the western portion of the Siqueiros transform domain. The most primitive lavas (picritic and olivine-phyric basalts), exclusively recovered from the young-looking flows within the A-B strike-slip fault, contain millimeter-sized olivine phenocrysts (up to 20 modal%) that have a limited compositional range (Fo 91.5-Fo 89.5) and complexly zoned CrAl spinels. High-MgO (9.5–10.6 wt%) glasses sampled from the young lava flows contain 1–7% olivine phenocrysts (Fo 90.5-Fo 89) that could have formed by equilibrium crystallization from basaltic melts with Mg# values between 71 and 74. These high MgO (and high Al 2O 3) glasses may be near-primary melts from incompatible-element depleted oceanic mantle and little modified by crustal mixing and/or fractionation processes. Phase chemistry and major element systematics indicate that the picritic basalts are not primary liquids and formed by the accumulation of olivine and minor spinel from high-MgO melts (10% < MgO < 14%). Compared to typical N-MORB from the East Pacific Rise, the Siqueiros lavas are more primitive and depleted in incompatible elements. Phase equilibria calculations and comparisons with experimental data and trace element modeling support this hypothesis. They indicate such primary mid-ocean ridge basalt magmas formed by 10–18% accumulative decompression melting in the spinel peridotite field (but small amounts of melting in the garnet peridotite field are not precluded). The compositional variations of the primitive magmas may result from the accumulation of different small batch melt fractions from a polybaric melting column.

Isotopic study of the origin of sulfur and carbon in Solfatara fumaroles, Campi Flegrei caldera

Isotopic study of the origin of sulfur and carbon in the hottest (Solfatara) fumaroles of Campi Flegrei caldera, Southern Italy, was carried out on gas samples collected between 1983 and 1988, i.e. during and after the 1982–1984 seismo-volcanic crisis. The results for sulfur (H 2S), the first ever reported on these gases, indicate a mean ∂ 34 S of −0.3±0.3‰ (range: −0.7 to +0.1‰ ) versus Canyon Diablo Troilite standard, consistent with an igneous derivation of this element, from either active magma degassing or/and leaching of reduced sulfur-bearing minerals in the volcanic layers. The lack of peculiar ∂ 34S variation during and after the crisis suggests that the chemical variation of H 2S and S/C ratio in the fumaroles (increase and then decrease by a factor 3) were not due to a changing origin of sulfur. The mean ∂ 13C of carbon (CO 2) over the period of survey, −1.6±0.2‰ (range: −1.9 to −1.3‰ ) versus PDB standard, is similar to the values obtained before the crisis (since 1970). Such an isotopic constancy requires a large and stable source of carbon feeding the fumaroles. The measured ∂ 13C values are much higher than those typical of primary mantle-magmatic carbon ( −6±2‰) and plot within the ∂ 13C range for marine carbonates ( 0±2‰). Such high values may reflect either (a) 13C-fractionation during degassing of CO 2 from the underlying (⩽5 km depth) magma chamber or (b) the contribution of heavy CO 2 of sedimentary origin, derived from either thermometamorphism of Mesozoic limestone series embedding the magma chamber or, possibly, past contamination of the local mantle by subducted sediments. Various arguments, among which volcanological evidence of an isolated and cooling magma reservoir (which would have been extensively degassed and, so, depleted in 13C along with time), the low 3He/ 4He ratios and the broad 13C-enrichment of volcanic fluids in the region, and geochemical evidence of crust-magma fluid interactions, suggest that a considerable fraction (⩾60%) of CO 2 in Solfatara fumaroles derives from carbonate sediments in the basement. The contribution of magma-derived CO 2 may be higher within the central part of the caldera (including Solfatara crater) than toward its western margin, where fumarolic and geothermal well gases exhibit lower 3He/ 4He ratios and still higher ∂ 13C values. Such a geochemical pattern is consistent with the central distribution of ground deformation and seismicity during the 1982–1984 crisis and with the idea of a residual magma body, confined beneath the central part of the structure. Alternatively, higher ∂ 13C and lower 3He/ 4He ratios toward the western margin may result from dilution of Solfatara-type gas during progressively deeper water boiling. Finally, accepting that Solfatara CO 2 derives from simple crustal mixing between magmatic and sedimentary carbon, its constant isotopic composition (together with the constant He isotope ratio) would restrict the possibility of magma intrusion and/or higher magmatic gas input as mechanisms responsible for the 1982–1984 events. However, this conclusion would no more hold true if the magma itself, or even its mantle source, were previously contaminated by crustal carbon.

Two types of granitic intrusions in the Hida belt, Japan: Sr isotopic and chemical characteristics of the Mesozoic Funatsu granitic rocks

Initial 87 Sr 86 Sr ratios, major- and trace-element compositions have been determined for the Late Triassic to Early Jurassic granitic rocks (Funatsu granitic rocks) in the Hida belt, Japan. Six intrusions, which are exposed in a transverse section of the belt, were selected for analyses. Based on the characteristics of initial 87 Sr 86 Sr ratios (Sr I-values) and element concentrations, the above intrusions are classified into two types. One is the intrusion with nearly constant Sr I-values over the variation of rock compositions. The Utsubo (Sr I=0.7053) and Hirose (0.7044) intrusions in this study, and the already reported Funatsu and Shimonomoto intrusions (both 0.7048) belong to this group, and rocks in this group are interpreted to have been formed mainly by fractional crystallization from mafic magma that originated in the lower crust or upper mantle. In contrast, the rocks in the other four intrusions (Hodatsusan, Shokawa, Yatsuo and Nagarehayama) show relatively higher and variable Sr I-values (0.7047–0.7105). Two-stage models, crustal mixing by the parental mafic magma at mid-crustal levels and subsequent differentiation by fractional crystallization, are interpreted to have been the main mechanisms for producing granitic rocks in these intrusions. Two contrasting types of Sr isotopic and chemical compositions recognized in the intrusions of the Funatsu granitic rocks, which are attributed to a difference of the predominant mechanism (crustal mixing or differentiation), correlate well with two geological features observed in the Hida belt. One is the existence or absence of shear zones which contemporaneously formed with granitic emplacement, and the other is the pattern of chemical zonation of the intrusion. Intrusions with a wide range of high Sr I-values are accompanied by a dextral shear zone in one margin and show a reverse- (less normal-) type chemical zonation. On the other hand, intrusions with constant Sr I-values do not relate with shear deformation and have normal chemical zonation. Considering these correlations, it is concluded that the stress state in middle-to upper-crustal levels is one of the important factors controlling the mechanisms of magma formation and way of emplacement.

Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts

Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40–50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.

Radiogenic isotopes: the case for crustal recycling on a near-steady-state no-continental-growth Earth

Pb, Sr and Nd isotopes define rock ages and residence times in mantle and continental crust, but are not diagnostic of either crustal growth or recycling in a near-steadystate Earth. Constancy of continental freeboard and uniformity of thickness of stable continental crust with age are the only two quantitative measures of crustal volume through time and these imply negligible crustal growth since 2.9 Ga B.P. Planetary analogies, Pb isotopes, atmospheric evolution, and palaeomagnetism also argue for early terrestrial differentiation. Rates of crustal growth and recycling are sufficient to reach a near-steady state over the first 1 Ga of Earth history, before widespread cratonization. Isotope compositions and ages of rocks are quantitatively compatible with near-steady-state recycling involving multiple complete reworkings of the crust by injection into the mantle along subduction zones. The recycling process can be observed to occur on the Earth today. Continental crust is uplifted, reduced in area, and thickened in orogenic belts. It is then subject to erosion, an effective isotopic homogenization process. A fraction of the sediment flux reaches the ocean floor or accumulates along continental margins, while soluble elements and water become fixed in ocean crust and trapped in sediments. Subduction of altered ocean crust, sediment, microcontinents and fragments of basement ripped off the edges of continents completes the process whereby continental crust, enriched in radiogenic Pb and Sr and with relatively unradiogenic Nd, is returned to the mantle. Pb, Sr and Nd isotopic compositions of igneous rocks from the mantle are explainable in terms of a near-steady-state model. The mantle buffer dominates observed isotope Sr and Nd evolutions. Mixing of crust and mantle causes all the isotope evolutions to approximate single-stage growth curves. Isotopic heterogeneity of the Earth increases as the rate of mixing declines. The observed escape of primordial 3 He from the mantle is not evidence for continuing continental differentiation or against early differentiation of the Earth. Even if nearly complete equilibrium chemical differentiation occurred at 4.6 Ga B.P., some 3 He would remain dissolved in the interior and would escape as recycling continued. A true steady state cannot be achieved because the driving energy declines with time and Earth surface processes have evolved with development of life and an oxygenated atmosphere. These unidirectional changes, plus variation in subduction rates and setting and style, and variation in rates of cratonization, provide a complex overprint on the near-steady-state background of continental evolution.