Flat Rare Earth Element Patterns Research Articles

Petrogenesis of low-pressure intra-oceanic arc granitoids: Insights from the late Neoproterozoic Avalonian–Cadomian orogen, Bohemian Massif

The processes leading to the formation of siliceous (intermediate to felsic) magmatic rocks from juvenile sources within intra-oceanic island arcs govern the formation of continental crust and tempos of crustal growth. To unravel the predominant process responsible for the formation of the intermediate to felsic intra-oceanic arc crust, we performed inverse geochemical modelling using major elements and fluid-immobile trace elements. Subvolcanic trondhjemite from a volcanic arc – Davle volcanic complex (DVC) situated in the Teplá–Barrandian unit, Bohemian Massif – that formed during the Neoproterozoic Avalonian–Cadomian orogeny was used as a proxy for the most felsic endmember of the tonalite–trondhjemite suite. The DVC trondhjemite that follows the calc-alkaline trend is enriched in several fluid-mobile elements such as Cs, Ba and U, but depleted in high field strength elements (Nb, Ti) and lithophile elements (e.g., K, Rb, Sr and Th), and exhibits flat rare earth element (REE) primitive mantle-normalized distributions with slightly negative Eu anomalies. Together with the largely radiogenic Hf-Nd isotopic signatures (ε Nd +6.0 to +8.7 and ε Hf +10.2 to +11.7), these data indicate formation in an intra-oceanic arc setting with no significant assimilation of older crustal material. In general, the composition of the studied trondhjemite markedly resembles that of other intra-oceanic arc-derived tonalites and trondhjemites worldwide, formed at low pressures below the garnet stability field. These are herein referred to as “low-pressure intra-oceanic arc granitoids” or “LP IOAG”. The performed inverse geochemical modelling revealed that dehydration partial melting or fractional crystallization of a mafic source can produce a primitive tonalitic melt. Consequently, fractional crystallization of that melt accounts for the compositional heterogeneity observed within the tonalite–trondhjemite suite. We conclude that LP IOAG, commonly forming large portions of intra-oceanic arc middle crust, can form in a nearly closed system with a negligible contribution of subduction-related melts or assimilation of older siliceous crust. Therefore, our findings suggest that the formation of LP IOAG within oceanic crust may represent the predominant process of post-Archean crustal growth. • The DVC trondhjemite has flat REE patterns resembling typical LP IOAG. • The LP IOAG play a key role in the post-Archean crustal growth. • The source composition is established by an inverse geochemical modelling. • The trondhjemite formed by fractional crystallization of a tonalitic source.

Development of Open Transport of Aqueous Fluid from Pegmatite Revealed by Trace Elements in Garnet

We investigated the fluid flow and elemental transport from a granitic body to the middle crust by determining the trace element compositions of garnet in pegmatites related to a quartz diorite intrusion and metamorphic rocks on Kinkasan Island, northeast Japan. Garnet in the pegmatites and biotite schists is characterized by spessartine– (Sps–) almandine- (Alm-) rich compositions of Sps14–69Alm22–70Prp2–14Grs1–13 and Sps16–30Alm54–66Prp9–16Grs3–6, respectively. A garnetite pod in the metamorphic unit has grossular- (Grs-) rich compositions (Sps1–4Alm8–11Prp0.1–0.4Grs80–87Adr3–4). The peak temperature ( T ) and pressure ( P ) conditions of the biotite schist during contact metamorphism were 600–650°C and 0.27–0.41 GPa, respectively. The primary fluid inclusions in quartz crystals within the pegmatites hosted by the quartz diorite and hosted by the metamorphic rocks have a wide range of homogenization temperatures (200–380°C). These correspond to the trapping temperature of 500–700°C, assuming a salinity of 4 wt.% NaClequivalent at pressure of the crystallization of the quartz diorite. Chondrite-normalized rare earth element (REE) patterns of garnets in the pegmatites in the quartz diorite and metamorphic unit are generally characterized by enrichment of heavy REEs and negative Eu anomalies with the REE contents in the schists which are systematically lower than in the pegmatites. However, garnet in the biotite schists close to the pegmatites has similar REE contents to garnet in the adjacent pegmatites. These geochemical features suggest that garnet in the biotite schists grew in response to fluid infiltration from the pegmatites. Besides, the Grs-rich garnet in the garnetite pod and its host quartz schist have flat heavy REE patterns and no Eu anomalies, which probably reflect a metasomatic process related to plagioclase replacement that produced Ca-Al-rich fluids. Our results suggest that the infiltration of pegmatitic fluids enhances elemental transport and metamorphic reactions in the middle crust.

Origin and re-Os geochronology of vein bitumen in the Nanpanjiang basin, SW China: Implication for the ore-forming age of Carlin-type gold deposits

The Nanpanjiang basin has the second-largest concentration of Carlin-type gold deposits in the world. Although significant progress has been made during the past two decades, the age of gold mineralization has not been well constrained. Field observation shows that these gold deposits are spatially associated with hydrocarbon reservoirs. At the Lannigou gold deposit, solid bitumens exist as films within structural fractures and cut through the orebodies, indicating that these bitumens formed after gold mineralization. In this work, we systematically studied bitumen reflectance (Rb), trace element, carbon isotope, and Re-Os isotopic compositions. These results show that these bitumens have high Rb values ranging from 2.40 to 2.71, indicating that they are high-maturity pyrobitumen. Trace element analysis reveals that bitumens in the deposit have high V/Ni, V/(V + Ni), and Zr/Sc ratios, but low Ni/Co, La/Sc ratios, and exhibit relatively flat rare-earth element (REE) patterns with no obvious Ce or Eu anomalies, comparable to the corresponding elemental ratios and REE patterns of Permian source rocks in the region. These bitumens also have similar δ13C values (-24.4 ‰ to –23.8 ‰, mean = -24.1 ‰) to the Permian source rocks (–22.4 ‰ to 24.7 ‰, mean = –23.6 ‰), indicating that they originate from the Permian source rocks. Five bitumen samples cut through the orebody yield a well-defined isochron age of 173 ± 6 Ma, indicating that the Lannigou gold deposit formed before this time. Our new data, combined with previously published geochronological data, suggest that the majority of Carlin-type gold deposits in the Nanpanjiang basin were mineralized at late Triassic to early Jurassic. The Re-Os data of this study also imply that the bitumen Re-Os chronometer has a large potential to constrain the timing of metal mineralization in low-temperature hydrothermal deposits.

Structure, geochronology, and petrogenesis of Permian peraluminous granite dykes in the southern Chinese Altai as indicators of Altai−East Junggar convergence

How and at which thermal conditions the convergence between the Chinese Altai and East Junggar operated remain poorly understood. This issue is addressed in the current study by focusing on the timing and petrogenesis of syntectonic granite dykes from the representative areas of Fuyun (convergent front) and Kalasu-Aletai (Chinese Altai interior). It is shown that Fuyun and Kalasu-Aletai dykes are fractionated I- and S-type granites, with zircon and monazite U-Pb ages of 300−291 Ma and 281−265 Ma, respectively. Geochemically, the Fuyun dykes have lower contents of aluminous (ASI: 0.97−1.13) and light rare earth element-enriched features, while the Kalasu-Aletai dykes have ASI = 1.01−2.17 and show overall flat rare earth element patterns with tetrad effects. The Fuyun dykes exhibit less evolved Sr-Nd isotopic characteristics (87Sr/86Srinitial: 0.7039−0.7048, εNd(t): + 5.7 to + 6.1) with respect to those of the Kalasu-Aletai dykes (87Sr/86Srinitial: 0.6978−0.7183, εNd(t): −7.6 to +3.0). The Fuyun and Kalasu-Aletai dykes are geochemically compatible with isotopically less evolved East Junggar arc components and heterogeneous Ordovician wedge sediment of the Chinese Altai, respectively, implying genetic links. We propose that the late Paleozoic Chinese Altai−Junggar convergence created a local perturbation of weak mantle beneath the southern Chinese Altai, causing partial melting of the underthrusting East Junggar and the overriding Altai components successively. The resulting magmas were emplaced along northward propagating syn-tectonic tensional fractures perpendicular to the Chinese Altai−East Junggar deformation front that serves as an excellent indicator of the convergent-shortening process.

Titanite Spectroscopy and In Situ LA-ICP-MS U–Pb Geochronology of Mogok, Myanmar

With the development of mineral testing technology and ore deposit geochemistry, titanite has become a hot topic in the study of accessory minerals. Two large-grained titanite crystals from Mogok, Myanmar, were used for a detailed study. In this study, the standard gemmological properties and spectral characteristics of titanite crystals were obtained by Fourier transform in-frared, micro ultraviolet-visible-near-infrared and Raman spectroscopy, respectively, which pro-vide a full set of data. Mineral major and trace elements were analysed using Electron-Probe Mi-croAnalysis (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The purpose of this study is to report spectral characteristics and major and trace elements of Mogok, Myanmar, in order to find new potential titanite standard samples. The two titanite crystals have similar major element compositions, and both grains have relatively low Al content (0.011–0.014 apfu) and Al/Fe ratios (0.157–0.222), but high Fe content (0.063–0.079 apfu). The two titanite crystals have similar chondrite-normalised rare earth element (REE) patterns with significantly Light Rare Earth Element (LREE) (La–Gd) enrichment and deletion of Heavy Rare Earth Element (HREE) (Tb–Lu). The 238U/206Pb ages of the two titanite samples are 43.5 ± 5.8 Ma and 34.0 ± 4.2 Ma, respectively. Generally, magmatic titanite has a low Al/Fe ratio, metamorphic and hydrothermal titanite crystals have extremely low Th/U ratios close to zero, with flat chondrite-normalised REE patterns or depletions in light REEs relative to heavy REEs. Different genetic types of titanite can be distinguished by the characteristics of major and trace elements. Combined chemical features such as REE differentiation, Al/Fe and Th/U ratios with formation temperature, the analysed titanite samples are considered magmatic-hydrothermal titanites. Their 238U/206Pb ages may indicate a potential stage of magmatic hydrothermal conversion.

Petrogenesis of early–middle paleozoic granitoids in the qilianblock, northwest China: Insight into the transition from adakitic to non-adakitic magmatism in a post-collisional extensional setting

To better understand geodynamic implications for the transition from adakitic to non-adakitic magmatism in a continental collision setting, an integrated study including zircon U–Pb ages and whole-rock geochemistry was performed for the Gahai and Erhai granitoid intrusions within the Qilian Block. The ca. 450–428 Ma Gahai biotite granite and Erhai muscovite-bearing granite are peraluminous S-type granites with adakitic geochemical characteristics, and have low Mg# (41.2–31.7) and εNd(t) (−10.8 to −6.5) values with two–stage Nd model ages of 2.05–1.72 Ga, implying their derivation almost entirely from an ancient crustal source under a thickened lower crust condition. The geochemical characteristics of the granites indicate that the Gahai and Erhai S-type granites were derived from partial melting of meta-greywacke and meta-pelite sources, respectively. In comparison, the ca. 415 Ma Gahai granodiorite belongs to metaluminous I-type granitoid and has low Sr/Y ratios as well as high Y and heavy rare Earth element (REE) concentrations, with nearly flat heavy REE patterns, implying a shallow source region. The granodiorites were most likely generated by partial melting of a K-rich basaltic magma source, with some contributions from mantle-derived melts. Synthesized data from this and previous studies suggest that the ca. 450–415 Ma Gahai and Erhai granitoids within the Qilian Block were generated in a post-collisional extensional regime triggered by the break-off of the northward subducting South Qilian Ocean slab beneath the Qilian Block. The geochemical transition from adakitic to non-adakitic intermediate-acidic magmas indicated that the thickened continental crust of the Qilian Block had experienced significant extension and thinning after ca. 420 Ma.

Mineralogical and geochemical characteristics of near-vent metalliferous sediments: Implications for hydrothermal processes along the southern Mid-Atlantic ridge (12°S–28°S)

In this study, we present mineralogical and geochemical analyses of 50 unconsolidated submarine surface sediments collected from separate locations within six active hydrothermal fields (Zouyu, Taij, Caifan, Deyin, Xunmei, and Tongguan) along the southern Mid-Atlantic Ridge (SMAR) at 12°S–28°S. The samples consisted of 33 metalliferous sediments (MS) and 17 non-metalliferous sediments (non-MS). The study area was divided into a northern segment that contained the Zouyu, Taij, Caifan, and Deyin hydrothermal fields and a southern segment containing the Xunmei and Tongguan hydrothermal fields. The presence of chalcopyrite, octahedral pyrite, and cubic pyrite without streaks on the surface of the sediments reflected a stable high temperature mineralization environment with a small temperature gradient in the Xunmei hydrothermal field. Cubic pyrite with growth striations co-existing with pentagonal dodecahedon-form pyrite indicated an unstable ore-forming environment with large temperature gradient in the early stage and a more stable ore-forming environment in the late stage in the Tongguan hydrothermal field. Major and trace element geochemistry data indicated that the most important material source for the abiogenic component of the sediments along SMAR 12°S–28°S was hydrothermal materials, the second most important source was lithogenic components, and the third source was elements scavenged from seawater. The Xunmei and Tongguan hydrothermal fields may have recorded phase separation, a process that would have resulted in the formation of two separate hydrothermal fluids, thereby forming two types of sulfide-rich MS with rare earth element (REE) patterns with positive and absent Eu anomalies, respectively. The Si-Fe-Mn oxide-rich MS had positive Eu anomalies, which also indicated that these oxides in sediments were directly precipitated from initial hydrothermal fluids at high temperature. The chlorite-rich MS exhibited nearly flat REE patterns similar to the volcanic material-rich samples and mid-ocean ridge basalt (MORB), which reflected the origin of the hydrothermal alteration of volcanic materials. In the northern segment, most of the MS were derived from plume fall-out Fe oxide particles. In addition, the chlorite-rich MS was likely derived from hydrothermal alteration. In the southern segment, the hydrothermal materials in the MS were primarily derived from the collapse of sulfide-bearing chimneys and Si-Fe-Mn oxide debris directly precipitated from hydrothermal fluids.

Rift‐related multistage evolution of the Mangalwar Complex, Aravalli Craton (NW India): Evidence from elemental and Sr–Nd isotopic features of Proterozoic amphibolites

The Banded Gneissic Complex (BGC) of the Aravalli Craton (India) comprises Archean BGC‐I (3.3–2.5 Ga) and Proterozoic BGC‐II. The BGC‐II is a mosaic of amphibolite facies namely, (a) Mangalwar Gneissic Complex (MGC), (b) Mangalwar Metasedimentary Complex (MMC), and (c) granulite‐facies Sandmata Metamorphic Complex. Here we present field, petrography and geochemical study of the Proterozoic amphibolites from the MGC and MMC. Based on field and geochemical data, the amphibolites have been characterized into three types related to rift settings (G1, G2 and G3). The G1 type occurs as dykes in the MGC and bears ocean island basalt‐type rare earth element (REE) patterns along with negative Nb and Ti anomalies, negative to positive values of εNd(t) (−0.02 to +3.96) and slightly variable initial 87Sr/86Sr (ISr) ratios. They are derived from deep mantle sources and correspond to the pre‐rift magmatic phase. The G2 type occurs as isolated patches associated with chert and is characterized by light REE (LREE) depleted and almost flat heavy REE (HREE) patterns suggesting that they were emplaced in an oceanic setting and were derived from a shallower mantle bearing positive εNd(t) (+2.87 to +6.27) and ISr = 0.7002–0.7083. This phase corresponds to the opening of the Mangalwar sedimentary basin (MMC). The G3 type occurs intercalated with metasedimentary rocks of the MMC and marked by LREE‐enriched and HREE‐depleted to flat patterns that resemble Upper Continental Crust signature, their εNd(t) mostly negative values and variable ISr also corroborate this explanation. They are believed to be derived from heterogeneous sources and represent syn‐sedimentary volcanic phases. All these signatures indicate that the amphibolites distinctly represent three phases of magmatism that occurred during pre‐rift (1.72 Ga), opening of basin (1.62 Ga) and syn‐sedimentary volcanism (1.6–1.3 Ga) in the rift‐basin and they were formed during the Proterozoic. These rifting events might have been connected with the fragmentation of Columbia.

Petrogenesis of Miocene igneous rocks in the Tafresh area (central Urumieh‐Dokhtar magmatic arc, Iran): Insights into mantle sources and geodynamic processes

Cenozoic tectono‐magmatism in Iran is widely considered to be related to subduction of the Neo‐Tethys Ocean. We employed whole‐rock and mineral geochemistry and isotopic data of intrusive rocks from Tafresh, central Urumieh‐Dokhtar magmatic arc, to evaluate the role of the mantle in magmatism, to assess the timing of emplacement, and to interpret the tectonic setting. Rock compositions range from gabbro or gabbro‐diorite (plagioclase + pyroxene ± olivine), to diorite (plagioclase + amphibole ± pyroxene), to granodiorite (quartz + plagioclase + K‐feldspar + amphibole + biotite), exhibiting high‐alumina calc‐alkaline affinity. Major oxide and trace element variations vary systematically from less to more evolved rocks suggesting a major role for fractional crystallization processes. Zircon LA‐ICP‐MS U–Pb ages of major rock types are in the range of 24–19 Ma, whereas those of gabbroic dikes are ~17.5 Ma. ԐNd values range between −1.8 and 3.7, and (87Sr/86Sr)i is narrowly restricted to 0.705–0.706, suggesting a common mantle source. The enrichment in light rare earth element (REE) enrichment and flat heavy REE patterns couple depletion of Nb–Ta–Ti indicate that subducting oceanic crust had interacted with the overlying mantle wedge. High‐alumina, mid‐Mg# Tafresh plutonic rocks formed from hydrous melts from which Ca‐pyroxene and magnetite crystallized earlier than plagioclase, whereas late‐crystallizing zircon nucleated while magma traversed through lithospheric mantle and Cadomian crust. Modelling of isotope and incompatible‐element patterns suggests the contribution of no more than ~5% molten sediment or other crustal components in Tafresh magma, at a developmental stage before most plagioclase and amphibole had crystallized. The Miocene Tafresh plutons originated during the final stages of subduction, before the collision between the Arabian and Eurasian plates.

S-type like granites and felsic volcanic rocks in the Mahabad area, NW Iran: Late Neoproterozoic extensional tectonics follow collision on the northern boundary of Gondwana

The Late Neoproterozoic basement is exposed in the Mahabad area of northwest Iran, and it is classified into three main groups. The first is metamorphic rocks that include gneiss, schist, phyllite, and slate. The second is sedimentary rocks, which comprise shale, sandstone, and dolomite and the third is igneous rocks including granite, rhyolite, felsic tuff, and ignimbrite, which are the subject of this research. Zircon UPb ages for three igneous samples show that crystallization occurred at 558–552 Ma in the late Neoproterozoic (Ediacaran). The felsic magmatic rocks have SiO2 contents of 64.3 to 78.9 wt%, Al2O3 contents of 12.0 to 16.8 wt%, low TiO2 contents (0.08 to 0.69 wt%), and large variations in K2O (0.05 to 5.46 wt%) and Na2O (1.17 to 6.70 wt%). The felsic magmatic rocks are enriched in light rare earth elements (REEs) such as La and Sm, have flat heavy REE patterns, and strong negative Eu anomalies. They are mainly weakly to strongly peraluminous and calc-alkalic to alkali-calcic rocks. A wide range of initial 87Sr/86Sr(556Ma) ratios (0.7056 to 0.7203) and negative εNd(t) values (−6.0 to −2.9) do not show clear trends of mixing/contamination among mantle, slab fluids, and/or altered oceanic crust. The Mahabad magmatic rocks were produced mainly by partial melting of graywacke, psammite, and shale at high temperatures (~800 °C), based on the zircon saturation index. S-type like granites usually form in syn- to post-collisional systems, but the Mahabad felsic rocks, which have almost the same ages as the associated sedimentary rocks, were generated in an extensional regime according to geochemical discrimination diagrams. This can be explained as follows: sediments were deeply buried during the late Neoproterozoic amalgamation of microcontinents at a subduction zone along the northern boundary of Gondwana, and then erosion of the roof and/or thinning owing to extension led to upwelling of the asthenosphere and intrusion of mafic magma, which was responsible for partially melting the buried metasedimentary rocks to generate S-type like granites and felsic volcanic rocks.

The ore‐forming fluids characteristics of quartz‐vein type scheelite deposits in eastern Yanbian, NE China: Evidence from in situ LA‐ICP‐MS rare earth elements of Yangjingou and Sidaogou deposits

AbstractThe quartz‐vein type scheelite deposits distributed in the Hunchun SN‐trending gold‐copper‐tungsten belt in eastern Yanbian, Jilin Province, are a group of recent discovery for the past decade. To determine properties of the ore‐forming fluids and the mineralization mechanism, in situ laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) rare earth elements (REE) analysis of the ore mineral scheelite was carried out for the selected Yangjingou and Sidaogou scheelite deposits in this metallogenic belt. The results showed that the substitution of REE3+ for Ca2+ in scheelite from the Yangjingou scheelite deposit can be accounted for the substitution mechanism: 3Ca2+ = 2REE3+ + □Ca (where □ is a site vacancy). It shows a slightly right‐dipped flat REE pattern with distinct positive Eu anomalies. The Sidaogou scheelite deposit, completes the substitution via the 2Ca2+ = REE3+ + Na+ and Ca2+ + W6+ = REE3+ + Nb5+ mechanisms. It shows left‐dipped REE pattern with relative enrichment of MREEs (mainly Dy) with negative Eu anomalies in the scheelite core and no or insignificant positive Eu anomalies in the scheelite rim. By comparing with the REE of granites which are close related to mineralization in the area, the ore‐forming fluids of these two scheelite deposits are determined to be mainly derived from magmatism. The inapparent correlation between EuN and Eu*N of the Yangjingou scheelite deposit and the high EuN/Eu*N values indicate that its ore‐forming fluids are reducing fluids. Its strong positive Eu anomalies are not entirely inherited from the mineralized tonalite, but is due to the release of Eu from the water‐rock reaction. The good correlation between EuN and Eu*N and the low EuN/Eu*N values in the Sidaogou scheelite deposit indicate that the ore‐forming fluids are oxidizing fluids. This may be caused by the mixing of the original magmatic fluid with a large amount of meteoric water. This study suggests that the water‐rock reaction is an important mineralization mechanism for the quartz vein‐type scheelite deposits in eastern Yanbian. In addition, fluid mixing is also important for the Sidaogou scheelite deposit. All these mechanisms influence the REE compositional characteristics of scheelite. The initial ore‐bearing fluids metasomatized the metamorphic rocks of Wudaogou Group, resulting in water‐rock reaction, and enriched ore‐forming materials such as Ca and W. As the ore‐forming fluids migrated upward along the NW‐trending structures, the escape of CO2 and CH4 caused by the tectonic decompression disrupted the physiochemical balance of the ore‐forming fluid system, catalyzed the combination of Ca2+ and WO42−, resulting in the precipitation and enrichment of the scheelite.

Petrogenesis of two types of gabbro from Neoproterozoic Fuchuan ophiolite complex, Jiangnan Orogen: Implication for a Japan Sea‐like back‐arc basin

AbstractThe Neoproterozoic Fuchuan ophiolite complex (FCO) represents the lithospheric remnant of the back‐arc basin in the southeastern margin of the Yangtze Craton. However, the nature and development of this back‐arc system are still confusing. This study focuses on the tholeiitic gabbros in the FCO, which can be divided into enriched and depleted types according to their geochemical characteristics. Enriched gabbros (843 ± 5 Ma) in the FCO are characterized by left‐sloping rare earth element (REE) patterns and relatively low εHf(t) (mostly in the range of 4.6–7.8). In contrast, depleted gabbros are slightly younger (838 ± 5 Ma) and isotopically more depleted (εHf(t) mostly in 7.7–11.8) than enriched gabbros, exhibiting flat REE patterns. Based on geochemical variations and numerical modeling, the primary magmas of enriched gabbros were generated by a low degree (~14 %) partial melting of the melt‐modified depleted mantle. In contrast, the more depleted isotopic composition and variable Ba/Th and V/Yb ratios of depleted gabbros suggest that their magmas have originated from a higher degree (~14–26 %) partial melting of the fluid‐modified residual depleted mantle. Considering that the most ~860–830 Ma magmatism in the eastern Jiangnan Orogen is calc‐alkaline, the Neoproterozoic back‐arc basin where the FCO formed could be sialic and built on the thinning continental crust, resembling the Japan Sea. The occurrences of tholeiitic gabbros in the FCO thus provide crucial insights into the opening of the back‐arc basin in the southeastern margin of the Yangtze Craton. The changes of mantle source nature and metasomatic agent reflect the upwelling of the depleted asthenosphere and oceanward retreading of the continental arc during the back‐arc opening. Consequently, the early continental arc in the southeastern Yangtze Craton may have been transformed into a new island arc, resembling the formation of the northeast Honshu arc during the Miocene.

Juvenile continental crust evolution in a modern oceanic arc setting: Petrogenesis of Cenozoic felsic plutons in Fiji, SW Pacific

Viti Levu, Fiji, provides one of the best exposed Phanerozoic analogues for the formation of juvenile continental crust in an intra-oceanic setting. Tonalites and trondhjemites are present in several large (75–150 km2) adjacent, mid-Cenozoic plutons. We report major and trace element data including rare earth element (REE) and high-precision high field strength element (HFSE) compositions, new Hf-Nd-Sr-Pb isotope data, and zircon U/Pb-ages, O-Hf isotopes, and trace elements, from five different plutons. The Eocene Yavuna pluton and the Miocene Colo plutons are mainly composed of tonalites and trondhjemites and represent the exposed middle crust of the former Vitiaz island arc. The plutons can be divided into three suites. One suite is light REE (LREE) depleted with some trace element ratios lower than average normal mid-ocean ridge basalts (N-MORB). A second suite has flat REE patterns similar to local island arc basalts. Both suites occur near the coast of Viti Levu, include a wide compositional spectrum from gabbro to tonalite, and can be produced mostly by fractional crystallization of mafic precursor melts. The third suite is characterized by LREE enrichments with higher LaN/YbN (2.3–4.9), higher Zr/Y (4.3–7.1), and lower Nb/Ta (9.6–12.4). They occur closer to the center of the island and are bimodal trondhjemite-gabbro intrusions. These characteristics are consistent with formation mostly by partial melting of mafic crust. Trace element modeling shows that the trace element ratios of the third suite can be produced by 10–20 % melting of the mafic crust in the presence of residual amphibole, resulting in the retention of the medium REE (MREE) and diagnostic trace element ratios including low Nb/Ta and high Zr/Y. Geochemical similarities of the LREE enriched suite to typical “low”-pressure Archean tonalites-trondhjemites-granodiorites (TTGs) imply a common petrogenetic origin and similar mechanisms for the generation of juvenile Archean and modern differentiated crust by partial melting of mafic crust with residual amphibole. In modern oceanic arcs, genetically unrelated felsic plutonic as well as volcanic rocks co-exist, and in this regard, the Fijian plutons accompany major tectonic disruptions to arc processes.

Petrology and geochemistry of ultramafic and mafic rocks in the late Silurian-early Devonian Darbut ophiolitic mélange of west Junggar (NORTHWESTERN CHINA): implications for petrogenesis and tectonic evolution

ABSTRACT New petrological and geochemical data for lherzolite, harzburgite, and gabbros in the Darbut ophiolitic mélange of west Junggar are combined to constrain the geological evolution of the Darbut ophiolite. Lherzolite, consisting of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with low Cr# values (34–39), is analogous to fertile abyssal peridotite. Harzburgite, composed of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with relatively high Cr# values (48–55), is similar to the supra–subduction zone (SSZ) peridotite. Isotropic gabbro, characterized by a flat rare earth element (REE) pattern as well as low Nb/Yb and high Ti/V ratios, is comparable to mid–ocean ridge basalt (MORB). Hornblende gabbro, displaying relative enrichments of fluid-soluble elements and elevated Th/Yb ratios, is similar to that of fore–arc basalt. Geochemical modelling of partial melting suggests that lherzolite samples are compatible with their formation after relatively low-degree (11–16%), anhydrous dynamic melting of the primitive mantle, while harzburgite samples have undergone 5–10% secondary-stage partial melting based on the already 16% depleted primitive mantle. These data suggest that the Darbut ophiolite was generated in a forearc setting. The upwelling asthenosphere triggered by the subduction initiation of the Junggar oceanic lithosphere led to low-degree, anhydrous decompression melting, producing lherzolite as well as the MORB–like melts at Late Silurian period. Increasing slab–derived fluids influx, accompanied by the progressively sinking slab, largely enhanced the partial melting degrees of the depleted mantle, and formed refractory harzburgite.

Forearc lava stratigraphy of the Beila Ophiolite, north-central Tibetan Plateau: Magmatic response to initiation of subduction of the Bangong-Nujiang Meso-Tethys Ocean

It has been suggested that boninites and associated volcanic rocks found in supra-subduction zone (SSZ) ophiolites form in a forearc setting, meaning that these rocks could provide crucial constraints on understanding the magmatic response to subduction initiation. Here, we report newly documented forearc basalts (FAB) and boninites from the Beila Ophiolite of the Bangong–Nujiang Suture Zone, north-central Tibetan Plateau. The lava stratigraphy, from bottom to up, includes FAB, basaltic boninites, and boninites, similar to lavas from the Izu–Bonin–Mariana (IBM) forearc. The FAB has geochemical characteristics similar to those of IBM forearc basalts, with nearly flat REE patterns, and shows enrichment in Rb, Ba, Pb, and Sr and depletion in Nb, Ta, and Zr, suggesting an origin by decompression melting of a depleted mantle source with minor input of subducting plate materials. The basaltic boninites and boninites show similar petrological features, as well as porphyritic textures with phenocrysts of olivine, clinopyroxene, and minor orthopyroxene. The basaltic boninites and boninites display uniform major- and trace-element characteristics that are similar to those of typical boninites, with high MgO (>10.78 wt%), Cr, and Ni contents, extremely low TiO2 (<0.21 wt%), rare earth element (REE), and high-field-strength element (HFSE) contents, and U-shaped REE patterns. In addition, the basaltic boninites and boninites have lower ɛNd(t) values than those of the FAB and display variable degrees of influence by slab melts and fluids. Geochemical modeling further suggests that the boninites were generated later during residual highly depleted mantle fluxing by the subducting plate. Together with contemporary boninitic and arc-related magmas along the Bangong-Nujiang Suture Zone and adjacent terranes (the North Lhasa and South Qiangtang), it suggests that the whole Meso-Tethys Ocean was subducted since the Middle Jurassic. Moreover, the Beila lava stratigraphy with FAB overlain by boninite is similar to that found in the IBM forearc and in many ophiolites (e.g., Troodos, Oman, and Xigaze), and represents a continuous geochemical transition from MORB-like to boninitic affinities. This similarity suggests that initiation of subduction might be commonly involved in the formation of ophiolites.

Re-initiation of plutonism at the Gondwana margin after a magmatic hiatus: The bimodal Permian-Triassic Longwood Suite, New Zealand

The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261–243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 ± 2.5 Ma, 256.0 ± 1.8 Ma, 247.8 ± 2.7 Ma and 243.2 ± 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/YbN < 10), unradiogenic 87Sr/86Sr(t) (0.7029 to 0.7032) and radiogenic ε143Nd(t) (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (δ146/144Nd = 233 ppm) compared to global plutonic rocks (δ146/144Nd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km2/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite.

Phase Equilibrium and Trace-Element Modeling of the Partial Melting of Basaltic Rocks under Low Pressure: Implications for Plagiogranite Generation

AbstractPhase equilibria and trace-element modeling using two previously reported basaltic bulk-rock compositions (samples D11 and 104-16), were carried out in this study, in order to better understand mechanism of low-pressure (LP) partial melting of mafic rocks and associated melt compositions. The T–MH2O pseudosections for both samples at three pressures (i.e. 0.5, 1.0 and 2.0 kbar) display that the H2O-stability field gradually increased with decreasing pressure within the T–MH2O range of 600–1100°C and 0–12 mol.%. The H2O contents of 10, 5.0, and 0.5 mol.% were selected on the basis of the T–MH2O pseudosections to calculate Pressure–Temperature (P–T) pseudosections over a P–T window of 0.1–3 kbar and 600–1100°C, so that the reactions of both the H2O-fluxed and -absent meltings at LP conditions can be investigated. The solidus displays a negative or near-vertical P–T slope, and occurs between 710 and 900°C at pressure between 0.1 and 3.0 kbar. LP melting of metabasites is attributed to the reactions of the hydrous mineral (hornblende and/or biotite) melting and anhydrous mineral (plagioclase, orthopyroxene, and augite) melting. The hydrous mineral melting is gradually replaced by anhydrous mineral melting as pressure decreasing, as the stability of hornblende decreases with falling pressure. With increasing temperature at a given pressure, the modeled melt compositions are expressed as progressions of the granite-granodiorite-gabbroic diorite fields for sample D11and granite-quartz monzonite-monzonite-gabbroic diorite fields for sample 104-16 on the total alkali–silica diagram. The modeled melts produced through the H2O-fluxed melting display higher Al2O3, CaO, MgO, and lower SiO2 and K2O than those formed by H2O-absent melting at the same P–T conditions. Furthermore, the modeled melts formed by H2O-absent melting, become richer in Al2O3, CaO, MgO, FeO, Na2O, but poorer in SiO2 and K2O as increasing water content. The results of trace-element modeling suggest that the nearly flat rare earth elements (REE) patterns of modeled bulk-rock composition are inherited by all the modeled melts, and the negative Eu anomalies and Sr depletion of the modeled melts gradually decrease as melting degree increases. Combined with the geochemical characteristics of natural oceanic plagiogranites, which have low K2O contents and flat or slightly light rare earth (LREE)-depleted REE patterns, our results imply that a bulk-rock composition with low K2O (&amp;lt;0.17 wt.%) and slightly LREEs depletion is the most likely protolith composition (e.g. basalt D11) for plagiogranites, and the compositions of modeled melts formed by LP H2O-absent partial melting of the basalt D11 at relatively high temperatures (1000–1025°C) are coincident with those of 1256D tonalites.

Isotopic and geochemical constraints on the protolith, redox state and paleo-tectonic setting of the Malagarasi Supergroup of north-western Tanzania

We report detrital zircon U–Pb ages, radiogenic Sm–Nd isotopic compositions, major and trace element compositions of the intra-plate extensional rhomb-shaped Neoproterozoic Malagarasi Supergroup of north-western Tanzania in order to elucidate the paleo-tectonic setting, source rocks chemistry and paleo-oxygenation in the basin. Assuming that provenance trace element proxy ratios, including the rare earth elements (REE) contents, behave as closed systems during sedimentation and diagenesis, we suggest that the detritus for the Malagarasi Supergroup originated from more felsic rocks than those for upper crustal composites (i.e. Post-Archean Australian Shale; PAAS, Proterozoic Shale; PS). This observation is also supported by enriched chondrite-normalized light REE (La/SmCN = 2.36–7.84; mean = 3.85), overall negative Eu anomalies (Eu/Eu* = 0.56–0.93; mean = 0.69) and flat heavy REE patterns (Gd/YbCN = 1.01–2.04; mean = 1.47). Paleo-weathering proxies using the Chemical Index of Alteration (CIA ~59–78%) protocol indicate an overall moderate weathering intensity in the basin. V/Cr ratios (mean = 1.18; range = 0.48–1.69) coupled with intra-sample Ce anomalies reveal an overall oxidizing state during deposition. Mantle extraction ages (TDM = 1594–2394 Ma) suggest a diverse mixing of protolith terranes including Archean (Tanzania craton), Eburnian (Ubendian Belt) and possibly the nearby Mesoproterozoic Kibaran Belt. Radiogenic laser ablation detrital zircon U–Pb ages of between 1826 and 2656 Ma may preclude contribution from the Kibaran. Given that Nd systematics in sedimentary rocks provide mixing ages of different hinterland source rocks (1594–2394 Ma; in our study), we suggest relatively juvenile sources such as the local post-orogenic effusive bodies farther south of the basin in the realm of Rodinia Supercontinent. Thus, a northerly flow direction of the of the proto/paleo river system in the region is inferred.

Genesis of calcite vein in basalt and its effect on reservoir quality: A case study of the Carboniferous in the east slope of Mahu sag, Junggar Basin, NW China

The characteristics and genesis of the calcite veins in Carboniferous basalt in the east slope of Mahu Sag, Junggar Basin are investigated based on observation of cores and thin sections; analyses of X-ray fluorescence, in situ major, trace and rare earth elements (REE), carbon, oxygen and strontium isotopes, fluid inclusions, as well as basin modeling. There are three periods of calcite fillings. The Period I calcite is characterized by low Mn content, flat REE pattern, strong negative cerium (Ce) anomaly, weak to moderate negative Eu anomaly, and light carbon isotopic composition, indicating the formation of the calcite was affected by meteoric water. The Period II calcite shows higher Mn and light REE contents, weak positive Ce anomaly and slight positive europium (Eu) anomaly, and a little heavier carbon isotopic composition and slightly lower strontium isotope ratio than the Period I calcite, suggesting that deep diagenetic fluids affected the formation of the Period II calcite to some extent. The Period III calcite is rich in iron and manganese and has REE pattern similar to that of the Period II calcite, but the cerium and europium enomalies vary significantly. The Period I and II calcites were formed in shallow diagenetic environment at approximately 250–260 Ma, corresponding to Late Hercynian orogeny at Late Permian. The Period III calcite was probably formed in the Indo-China movement during Late Triassic. It is believed that the precipitation of calcite in basalt fractures near unconformity was related to leaching and dissolution of carbonates in the overlying Lower Permian Fengcheng Formation by meteoric water, which destructed the Carboniferous weathering crust reservoirs in early stage. Relatively high quality reservoirs could be developed in positions with weak filling and strong late dissolution, such as structural high parts with Fengcheng Formation missing, distant strata from Fengcheng Formation vertically, buried hills inside lake basin, etc.

Geochemistry, U[sbnd]Pb geochronology, and Sr-Nd-Hf isotope systematics of a SW-NE transect in the southern Peninsular Ranges batholith, Mexico: Cretaceous magmatism developed on a juvenile island-arc crust

The Peninsular Ranges batholith (PRB) is a North American Cordillera segment with a protracted record of subduction-related magmatism (177–92 Ma). However, such a record is regionally fragmented, showing broad petrologic variations challenging to integrate into a coherent geologic evolution history. Here, we present a petrogenetic study of Cretaceous plutons in the southern PRB along the Sierra El Arco-San Francisquito transect (SW-NE) perpendicular to the paleotrench. The Middle Jurassic-Early Cretaceous (166–140 Ma) magmatism developed as an island arc with sedimentary rocks of intra-arc and oceanic marginal basins. At the Sierra El Arco, the Calmallí pluton intruded the Jurassic arc sequence. The basic to intermediate rocks of the Calmallí pluton show εSrt (average –3.6), εNdt (average +3.3), and εHft values (average +6.0) that suggest an origin from mantle-derived melts with minor contamination. These rocks display almost flat rare earth elements (REE) patterns and Eu anomalies suggesting plagioclase fractionation. East of the Calmallí pluton from the Piedra Blanca to the San Francisquito region, granodiorite and granite (100–92 Ma) are continuously exposed. The εSrt (average +3.1), εNdt (average +1.1), and εHft (average +2.6) isotopic signatures in the acid rocks suggest a mantle-derived melt genesis with increased contamination. Most granodioritic and granitic rocks display a characteristic depleted-HREE pattern without Eu anomalies, suggesting differentiation under the influence of garnet. The Jurassic-Cretaceous (166–92 Ma) magmatism displays an εSrt vs. εNdt correlation trend, indicating progressive contamination of mantle-derived magmas with a recycled crust. SrNd and NdHf isotopic mixing curves suggest the assimilation of the metasedimentary Triassic basement of NW Mexico instead of the considerably older and isotopically evolved Precambrian basement. The integration of this work with data from NW Mexico suggests a continuous magmatic activity from the Middle Jurassic to the Late Cretaceous. We consider the geochemical, isotopic, and temporal variation of the PRB indicative of natural heterogeneities along the trench axis of a long-lived arc system (crustal thickness and paleogeography) due to continuous subduction of the Farallon plate below the North American plate.