Flat Heavy Rare Earth Element Research Articles

Magma evolution of the South China Sea basin from continental-margin rifting to oceanic crustal spreading: Constraints from In-situ trace elements and Sr isotope of minerals

The South China Sea (SCS) has undergone a complete Wilson cycle. We studied the volcanic rocks from IODP Sites U1502, U1500, U1431, U1433, and U1434 that record the exact mantle dynamics of the SCS basin from continental rifting to oceanic crustal spreading. We present in-situ major and trace element data for minerals (olivine, clinopyroxene, and plagioclase) and melt inclusions; and in-situ Sr isotopic ratios for plagioclases from the volcanic samples. All SCS volcanic rocks belong to the tholeiite series. Most Site U1431 basalts are slightly depleted in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and similar to that of normal mid-oceanic ridge basalts (N-MORB). All clinopyroxenes and olivines, and the calculated equilibrium melts of clinopyroxene cores display depletion of LREEs and enrichment of heavy rare earth elements (HREEs) similar to N-MORB-type primary magma. Thus, these volcanic rocks are likely formed by the extension decompression partial melting of shallow depleted magma. The Sites U1502, U1500, U1433, and U1434 bulk-rocks and melt inclusions show flat LREE and HREE patterns similar to that of enriched MORB (E-MORB), likely attributed to the partial melting of a depleted mantle source with different degrees of lower continental crustal contamination. The increase in 87Sr/86Sr ratios from core to rim in Site U1500 plagioclases further prove the crustal contamination during magmatism. Site U1431 clinopyroxene and Site U1500 plagioclase show normal zoning textures and slight enrichment of incompatible elements in their rims but depletion in their cores, likely indicating significant crystallization of late-stage differentiated melts. Although some clinopyroxene and plagioclase were likely crystallized in a relatively closed environment, the existence of melt inclusions and the large Sr isotopic variations between the plagioclase rim and groundmass suggest a dynamic and open magmatic system that likely experienced melt–rock interactions and magma mixing processes. Melt supply rate changes from low to high during the SCS evolution process from initial rifting (Site U1500) to later spreading (Sites U1431 and U1433) indicating that: (1) the number of high-An plagioclase-hosted melt inclusions has gradually decreased; and (2) the zoning textures of clinopyroxene and plagioclase have gradually changed from simple to complex. The influence of mantle plumes and the upwelling of fertile subduction-related asthenospheric magma were negligible during the continental-margin breakup and initial SCS spreading, but played an important role during the late SCS spreading period, as revealed by the variation in Sr isotopic compositions of high-An plagioclase cores, the different mantle potential temperatures and source lithologies.

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.

Mineral Geochemistry of Basaltic Rocks from IODP Expeditions 334 and 344: Implications for Magmatic Processes of Cocos Ridge Segment Being Subducted Beneath the Middle America Trench

The Cocos Ridge, which is subducted beneath the Central American Volcanic Arc, has a complex tectonic evolution history due to plume-ridge interaction between the Galápagos plume and the Cocos—Nazca spreading center. This study presents major and trace element analyses of plagioclase and clinopyroxenes hosted by Cocos Ridge basaltic rocks that were drilled in three holes (U1381A, U1381C and U1414A) of Sites U1381 and U1414 on the Cocos Ridge close to the Middle America Trench during the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344. The results show that (1) plagioclases are mainly bytownite and labradorite with subordinate andesine, which are enriched in light rare earth elements (LREE) and some large-ion lithophile elements (LILE) and exhibit marked positive Eu anomalies; and (2) that clinopyroxenes are augites, which are depleted in highly incompatible elements such as LREE and LILE, have nearly flat heavy rare earth elements patterns (HREE) and lack Eu anomalies in chondrite-normalized rare earth element (REE) diagrams. During the ascent to the surface, the primary magmas experienced fractional crystallization of plagioclase, clinopyroxene, Ti-Fe oxides and possibly olivine (complete replacement of olivine by secondary minerals). The crystallization temperatures of plagioclase phenocrysts and microlites are 1050 to 1269 °C, and 866 to 1038 °C, respectively, and the pressures of plagioclase phenocrysts are 0.3–0.7 GPa. The crystallization temperatures of clinopyroxene phenocrysts/micro-phenocrysts is 1174–1268 °C, similar to those of plagioclase phenocrysts, suggesting some of clinopyroxene and plagioclase phenocrysts cotectic crystallized during early stage of magmatic evolution. In addition, the equilibrium pressures of clinopyroxene phenocrysts/micro phenocrysts are 0.02–0.97 GPa, implying that the clinopyroxene started to crystallize within the mantle, and magma evolution has undergone an early crystallization stage with clinopyroxene and no plagioclase.

High Arctic Large Igneous Province Alkaline Rocks in Canada: Evidence for Multiple Mantle Components

Abstract The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada, although dominated by tholeiites (135–90 Ma), contains two main groups of alkaline igneous rocks. The older alkaline rocks (∼96 Ma) scatter around major fault and basement structures. They are represented by the newly defined Fulmar Suite alkaline basalt dykes and sills, and include Hassel Formation volcanic rocks. The younger alkaline group is represented by the Wootton Intrusive Complex (92·2–92·7 Ma), and the Audhild Bay Suite (83–73 Ma), both emplaced near the northern coast of Ellesmere Island. Fulmar Suite rocks resemble EM-type ocean island basalts (OIB) and most show limited crustal contamination. The Fulmar Suite shows increases of P2O5 at near-constant Ba–K–Zr–Ti that are nearly orthogonal to predicted fractionation- or melting-related variations, which we interpret as the result of melting composite mantle sources containing a regionally widespread apatite-bearing enriched component (P1). Low-P2O5 Fulmar Suite variants overlap compositionally with enriched HALIP tholeiites, and fall on common garnet lherzolite trace element melting trajectories, suggesting variable degrees of melting of a geochemically similar source. High-P2O5 Hassel Formation basalts are unusual among Fulmar rocks, because they are strongly contaminated with depleted lower crust; and because they involve a high-P2O5–Ba–Eu mantle component (P2), similar to that seen in alkali basalt dykes from Greenland. The P2 component may have contained Ba–Eu-rich hawthorneite and/or carbonate minerals as well as apatite, and may typify parts of the Greenlandic sub-continental lithospheric mantle (SCLM). Mafic alkaline Audhild Bay Suite (ABS) rocks are volcanic and hypabyssal basanites, alkaline basalts and trachy-andesites, and resemble HIMU ocean island basalts in having high Nb, low Zr/Nb and low 87Sr/86Sri. These mafic alkaline rocks are associated with felsic alkaline lavas and syenitic intrusions, but crustally derived rhyodacites and rhyolites also exist. The Wootton Intrusive Complex (WIC) contains geochemically similar plutonic rocks (alkali gabbros, diorites and anatectic granites), and may represent a more deeply eroded, slightly older equivalent of the ABS. Low-P2O5 ABS and WIC alkaline mafic rocks have flat heavy rare earth element (HREE) profiles suggesting shallow mantle melting; whereas High-P2O5 variants have steep HREE profiles indicating deeper separation from garnet-bearing residues. Some High-P2O5 mafic ABS rocks seem to contain the P1 and P2 components identified in Fulmar–Hassel rocks, whereas other samples trend towards possible High-P2O5 + Zr (PZr) and High-P2O5 + K2O (PK) components. We argue that the strongly alkaline northern Ellesmere Island magmas sampled mineralogically heterogeneous veins or metasomes in Greenlandic-type SCLM, which contained trace phases such as apatite, carbonates, hawthorneite, zircon, mica or richterite. The geographically more widespread apatite-bearing component (P1) could have formed part of a heterogeneous plume or upwelling mantle current that also generated HALIP tholeiites when melted more extensively, but may also have resided in the SCLM as relics of older events. Rare HALIP alkaline rocks with high K–Rb–U–Th fall on mixing paths implying strong local contamination from either Sverdrup Basin sedimentary rocks or granitic upper crust. However, the scarcity of potassic alkaline HALIP facies, together with the other trace element and isotopic signatures, provides little support for a ubiquitous fossil sedimentary subduction-zone component in the HALIP mantle source.

Synsedimentary rifting and basaltic-komatiitic volcanism in the Pontiac subprovince, Superior craton (Canada): Implications for Neoarchean geodynamics

New field observations from the Pontiac subprovince, Superior Province, supported by whole-rock geochemistry provide strong evidence for extensional magmatism and volcanism during deposition of the ca. 2685 Ma Pontiac sediments. Contacts between basaltic and komatiitic volcanic rocks and sedimentary rocks are concordant and weakly deformed, inconsistent with an allochthonous emplacement model proposed for volcanic successions in the Pontiac subprovince. Structural observations and aeromagnetic imagery show that the volcanic rocks are present as thin (<500 m), laterally continuous (>30 km) conformable successions within the turbiditic succession and were emplaced prior to the earliest deformation of the Pontiac subprovince. Sediment-matrix breccias with fluidal and blocky igneous clasts identical to the compositions of the mafic–ultramafic rocks occur along intrusive contacts with Pontiac Group wacke. These textures are interpreted as peperite, representing the interaction of magma or lava with wet, unconsolidated sediment. Peperite was observed along the margins of tholeiitic-komatiitic rocks and calc-alkaline lamprophyre intrusions, indicating that these magmas were broadly synchronous with the deposition of the Pontiac sediments. The volcanic rocks are characterized as tholeiitic basalt and Al-depleted to Al-undepleted komatiite, have high Nb/Th, flat to positive light rare earth element (REE) slopes, and flat heavy REE slopes, consistent with partial melting of a mantle plume. The calc-alkaline lamprophyre intrusions have low Nb/Th and prominent negative light and heavy REE slopes, consistent with partial melting of a previously metasomatised mantle. The presence of autochthonous komatiitic and tholeiitic rocks is consistent with the Pontiac subprovince developing as an extensional basin within the Abitibi subprovince in response to an upwelling mantle plume ca. 2.7 Ga. This model is incompatible with uniformitarian evolutionary models for the Superior craton interpreting the sedimentary subprovinces as accretionary prisms that formed during subduction processes. Non-uniformitarian models invoking stagnant-lid tectonics predict the formation of extensional basins and ascent of depleted mantle-derived magmas associated with upwelling mantle plumes, but the ability to produce source-metasomatism magmas without subduction is highly contested. The evolution of the Pontiac basin may thus reflect a transitional period in the Neoarchean where stagnant-lid and proto-subduction styles of tectonism were operating intermittently.

Opening of the West Paleo-Tethys Ocean: New insights from earliest Devonian meta-mafic rocks in the Saualpe crystalline basement, Eastern Alps

The timing of the opening of the West Paleo-Tethys Ocean in Eastern Alps remains unclear. To constrain this event, we present new zircon U-Pb ages, Hf isotopic compositions, and whole-rock major- and trace-element data for the meta-mafic rocks (amphibolites) in the southern and western Saualpe crystalline basement, Eastern Alps. Zircon U-Pb dating of three samples yield crystallization ages of 418 ± 6 Ma, 417 ± 3 Ma and 415 ± 3 Ma, indicating that they formed during the earliest Devonian. Geochemically, these meta-mafic rocks have relatively low SiO2 and MgO contents and high TiO2 contents. They are enriched in light rare earth elements (LREE), particularly in Nb and Ta, and show relatively flat heavy rare-earth elements (HREE) patterns, suggesting that they have affinities with the alkaline oceanic island basalts (OIB). The geochemical characteristics, together with the positive εHf(t) values of 0.7–11.1, imply that the OIB-like meta-mafic rocks originated from partial melting of a lherzolite source including spinel and garnet. The primary magma shows complex sources involving asthenospheric, lithospheric mantle and subducted slab components, which was subsequently modified by crustal contamination. This reveals that the magma formed in a slab window environment associated with mid-ocean ridge subduction. The contemporaneous OIB-like alkaline amphibolites were also found in the Central Austroalpine basement and in Northwestern Turkey. We suggest that the Late Silurian–earliest Devonian OIB-like magmatism is related to a back-arc extension setting along the northern margin of Gondwana leading to the separation of the European Hunic terranes and hence placing age constraints on the opening of the West Paleo-Tethys Ocean.

Geochemical characteristics of the early Neoarchean komatiite from the North China Craton: Evidence for plume–craton interaction

Komatiite is characterized by extremely high MgO contents and H2O-poor nature and plays an important role in understanding the early mantle-derived magma processes, crust–mantle differentiation of the Archean earth, but it is sparse in the North China Craton. Although komatiitic rocks have ever been discovered in the Mengyin area, their formation ages and types are still in debate. Here, we firstly report a new komatiite outcrop near Nanbaita village (Jinan city) of the western Shandong Province and present new major and trace element contents, as well as Sr–Nd and Re–Os isotopic compositions. The rocks show spinifex texture with primary olivine and pyroxene skeletal needles have been transformed into secondary minerals (e.g., serpentine, tremolite). The komatiites display extremely high MgO, but low TiO2 and Na2O + K2O. Their chondritic Al2O3/TiO2 and GdN/YbN ratios suggest they are Aluminium-undepleted komatiites which are generated by larger degrees of melting (~50%) in relatively lower pressure (ca. 5–7 GPa) with garnet absence than Aluminium-depleted komatiites. The formation age of these komatiites was no later than 2.69 Ga constrained by the Re depletion age. Flat heavy rare earth element patterns and low 187Re/188Os and Th/Yb ratios show the primary komatiitic magma was not contaminated by substantial crustal material. The γOs value of sample 18LH51 is 0.09, which is adjacent with chondrite and other contemporaneous komatiites in the world, indicating that the Nanbaita komatiites were originated from a chondritic mantle. Furthermore, the widely distribution of ~2.7 Ga ultramafic–mafic rocks in the western Shandong Province might be the reflection of a contemporaneous worldwide superplume event.

Age, provenance and tectonic setting of metasedimentary rocks of the Simlipal Complex, Singhbhum Craton, eastern India

The Simlipal Complex is a circular/elliptical structure at the eastern margin of the Singhbhum Craton in eastern India. The complex comprises metavolcanic and metasedimentary rocks and may represent the remnant of an Archean impact structure. Major and trace element abundances and zircon U–Pb ages from the metasedimentary rocks of the complex, which possibly constituted the target rocks of the impact, are used to constrain the provenance of the sediments and the tectonic environment of their deposition. The metasedimentary rocks include sandstones/quartzites and shales/slates/phyllites. CIA index and the A-CN-K discrimination diagram suggests moderate to extensive weathering of the source areas. The shales/slates/phyllites plot along the weathering trend of granite-granodiorite-tonalite, implying an average granodioritic source. The rare earth element (REE) patterns of shales/slates/phyllites are fractionated (LaN/YbN = 2.86–13.5) with nearly flat heavy-REE (GdN/YbN = 0.92–1.84) similar to Post Archean Australian shale. The Zr/Sc and Th/Sc ratios indicate zircon addition to sandstones/quartzites and mixing of mafic and felsic igneous rocks to shales/slates/phyllites. The shales/slates/phyllites are enriched in Sc, Cr, Ni, Y, Zr, Hf, Th, and U compared to Archean Upper Crust and have positive Cr and Ni anomalies suggesting contribution from mafic/ultramafic source. The major age peaks retrieved from detrital zircons in the metasediments are at 3.63 Ga, 3.46–3.26, 3.17–2.95, 2.87–2.75 Ga, and 2.54 Ga which correlate with ages reported for granitoid suites from different lithodemic units of the Singhbhum Craton. The sediments from the central and NW regions of the complex are dominated by zircons with an age of 3028 ± 15 Ma, while those from the NE regions and areas outside of it (e.g., the Kuliana region) have a preponderance of older zircon ages (ca. 3.25–3.38 Ga). The ages of the oldest metamorphic and the youngest detrital zircon population are suggestive of deposition of the sediments between ca. 2.8 Ga and 2.5 Ga. Crystallization ages and the shape of zircon crystals along with the geochemical signature of the sediments suggest that they were derived locally from the Singhbhum Craton and its surroundings.

Rare Earth Element Distributions in Continental Shelf Sediment, Northern South China Sea

A total of 388 surface sediment samples taken from the northern South China Sea (SCS) continental shelf were analyzed to characterize the signature of their rare earth elements (REEs). The average REEs concentration was 192.94 μg/g, with a maximum of 349.07 μg/g, and a minimum of 32.97 μg/g. The chondrite-normalized REEs pattern exhibits a remarkably light REEs accumulation, a relatively flat heavy REEs pattern, and a negative Eu anomaly. We subdivided the study area into three zones using the characteristics of REEs and statistical characteristics. Zone I: continental shelf off western Guangdong Province. Here, the sediment provenance is mainly river-derived from the Pearl River, Taiwanese rivers, and those in the adjacent area. Zone II: Qiongzhou Strait and Leizhou Peninsula. Here, the sediment provenance consists of the Qiongzhou Strait and the Hainan Island. Zone III: Hainan Island and SCS slope sediments are dominated. The REEs compositions are mainly controlled by source rock properties, hydrodynamic conditions, and an intensity of chemical weathering. We reconstructed the sediment dispersal and transport route using the REEs compositions, grain size, and other geochemical characteristics throughout the study area.

Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based onP-T-tpaths of the eclogite-bearing Baqing metamorphic complex

AbstractSubduction and exhumation processes, interacting with each other, play a key role in crustal recycling. Downgoing oceanic lithosphere constitutes the dominant input at subduction margins, but subduction erosion, the removal of crustal material from the overriding plate, may add additional ingredients and complexity to the subduction factory. Different exhumation models have been proposed to explain how subducted materials are exhumed and therefore contribute to crustal recycling, e.g., exhumation up the subduction channel versus diapiric rise through the mantle wedge that overlies the subducted plate. The recently discovered Baqing eclogite-bearing high-pressure metamorphic complex, central Tibet, China, provides an excellent opportunity to decode the exhumation process, the origin of subduction-related magmatism, and the crustal structure of the North Qiangtang block, in addition to elucidating processes of crustal recycling. Pressure-temperature-time (P-T-t) paths and zircon U-Pb ages and trace-element compositions for Baqing high-pressure rocks were used to evaluate exhumation processes and to determine the geochemical and tectonic affinity of the Baqing metamorphic complex.The Baqing metamorphic complex is mainly composed of eclogite, gneiss, and schist. It is located between two geologically distinct terranes—the South Qiangtang block, which has early Paleozoic basement, and the North Qiangtang block, which has Proterozoic basement. In the schist, zircon cores with steep heavy rare earth element (HREE) slopes and oscillatory zoning yielded inherited ages that are similar to detrital zircon ages for the South Qiangtang block schist; in contrast, zircon rims with flat HREE slopes yielded metamorphic ages of 224 Ma that are similar to the metamorphic ages obtained for the Baqing eclogite. In contrast, zircons from the gneiss yielded an upper-intercept age of 1033 ± 32 Ma (interpreted as the crystallization age) and a lower-intercept metamorphic age of 198 ± 4 Ma. Field relations indicate that gneiss and eclogite/amphibolite were exhumed together, so the ∼20 m.y. gap between the gneiss and the metabasite metamorphism may indicate a long exhumation duration. In the region, Proterozoic ages of ca. 1000 Ma are known only from the North Qiangtang block; we thus propose that the Baqing gneiss originated from North Qiangtang block Proterozoic basement, which, along with North Qiangtang block Triassic arc magmatic rocks and the discrepancies between ancient and current arc-trench distances, results in estimates of ∼20–170 km of Triassic subduction erosion. Results of P-T analyses show that most eclogite, amphibolite, and schist shared a similar clockwise P-T path, different from that of the gneiss, which records a higher geothermal gradient. The clockwise P-T trajectory, long exhumation duration, lack of significant heating during exhumation, and the South Qiangtang block affinity of the schist (host rock of the Baqing eclogite) are consistent with subduction-channel exhumation rather than diapiric rise through the mantle wedge. Geochemical similarities between the North Qiangtang block Triassic subduction-related rocks and the Baqing gneiss may signal the involvement of unexhumed Baqing metamorphic complex in the recycling of the Qiangtang crust.

Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China

Lamprophyre dykes that crosscut different types of ultrahigh pressure (UHP) metamorphic rocks are widely distributed in the Triassic Sulu UHP orogenic belt. Although abundant studies have been performed on these dykes, their origin and petrogenesis remain topics of controversy. This study presents the results of a detailed field-based study of petrology, whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes on lamprophyre dykes exposed in the central Sulu UHP zone, aiming at shedding lights on their petrogenesis and providing clues on the geological evolution of eastern continental China during the Cretaceous. The lamprophyres are typically porphyritic, with phenocrysts dominantly composed of amphibole and clinopyroxene set in a lamprophyric matrix. The dykes have moderate SiO2 (47.70 wt.%–60.44 wt.%), variably high MgO (2.58 wt.%–8.28 wt.%) and Fe2O3T (4.88 wt.%–9.26 wt.%) contents with high Mg# of 49–66. Geochemically, they have enriched light rare earth element (REE) and flat heavy REE patterns ((La/Gd)N=5.14–10.56; (Dy/Yb)N=1.43–1.54) with negligible Eu anomalies (Eu/Eu*=0.83–1.10), and they show enrichment in large ion lithophile elements (e.g., Ba and K) but depletion in high-field strength elements (e.g., Nb, Ti and P). In-situ zircon U-Pb geochronology reveals that the lamprophyres have concordant ages of 120–115 Ma, demonstrating that the dykes emplaced in the Early Cretaceous. These zircons have eHf(t) values ranging from −26.0 to −11.0. Inherited zircons that occur in the dykes are dated to be Neoproterozoic, in line with the protolith ages of their host (i.e., the UHP rocks). An integration of these data allows us to propose that the lamprophyres were generated during the Cretaceous, by melting of subcontinental lithospheric mantle-derived metasomatite with enriched chemical compositions underneath the North China Craton. The metasomatite was formed mainly by peridotite-fluid/melt reactions, with the fluids/melts mainly liberated from subducted Yangtze continental crust during the Triassic. Regional extension, lithospheric thinning and mantle upwelling caused by rollback of the subducted paleo-Pacific plate is considered to account for the generation of the lamprophyres as well as the extensive arc-like magmatic rocks in eastern continental China during the Early Cretaceous.

Petrogenesis of heterogeneous mantle peridotites with Ni-rich olivine from the Pujada Ophiolite, Philippines

Nickel (Ni) enrichment (>0.40 wt%) in olivine was previously recognized in mantle xenoliths and volcanic rocks. Though this enrichment is often attributed to subarc metasomatic processes, the occurrence of Ni rich olivines in mantle peridotites remains to be established. We found peridotites that randomly contain unusually high-Ni olivines (up to Ni = 0.53 wt%) from the mantle section of the Pujada Ophiolite, Philippines. The Pujada mantle peridotites can be classified into three groups based on their occurrence and spinel Cr#: Group I - massive lherzolites and harzburgites with Cr# = 0.10–0.40, Group II - lherzolite-dunite and harzburgite-dunite interlayers with Cr# = 0.10–0.40, and Group III - interlayered harzburgites and dunites with Cr# >0.40. Rare earth element composition of clinopyroxenes shows general light rare earth element (LREE) depletion and relatively flat heavy rare earth element (HREE) with minor LREE enrichment. The high-Ni olivines are associated to the occurrence of secondary orthopyroxenes, high modal abundance of orthopyroxenes (>30%) and LREE enrichment. The Ni enrichment in olivine is the result of the diffusion of Ni from a pyroxenitic melt. The crystallization of a hybridized melt formed by the combination of pyroxenite melt, primitive MORB melt and siliceous slab-derived melt, shows the same case of formation for the high Ni-olivine phenocrysts in alkali basalts from Hawaii, Kamchatka and the Mexican Volcanic Belt.

WITHDRAWN: Petrogenesis of heterogeneous mantle peridotites with Ni-rich olivine from the Pujada Ophiolite, Philippines

Abstract Nickel (Ni) enrichment (>0.40 wt%) in olivine was previously recognized in mantle xenoliths and volcanic rocks. Though this enrichment is often attributed to subarc metasomatic processes, the occurrence of Ni rich olivines in mantle peridotites remains to be established. We found peridotites that randomly contain unusually high-Ni olivines (up to Ni = 0.53 wt%) from the mantle section of the Pujada Ophiolite, Philippines. The Pujada mantle peridotites can be classified into three groups based on their occurrence and spinel Cr#: Group I - massive lherzolites and harzburgites with Cr# = 0.10–0.40, Group II - lherzolite-dunite and harzburgite-dunite interlayers with Cr# = 0.10–0.40, and Group III - interlayered harzburgites and dunites with Cr# >0.40. Rare earth element composition of clinopyroxenes shows general light rare earth element (LREE) depletion and relatively flat heavy rare earth element (HREE) with minor LREE enrichment. The high-Ni olivines are associated to the occurrence of secondary orthopyroxenes, high modal abundance of orthopyroxenes (>30%) and LREE enrichment. The Ni enrichment in olivine is the result of the diffusion of Ni from a pyroxenitic melt. The crystallization of a hybridized melt formed by the combination of pyroxenite melt, primitive MORB melt and siliceous slab-derived melt, shows the same case of formation for the high Ni-olivine phenocrysts in alkali basalts from Hawaii, Kamchatka and the Mexican Volcanic Belt.

Geochronology and Geochemistry of Gabbros from Moa‐Baracoa Ophiolitic Massif, Eastern Cuba: Implication for Early Cretaceous SSZ Magmatism

AbstractA series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto‐Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North American plates (Iturralde‐Vinent et al., 2016). However, neither their age nor their geodynamic settings are well constrained. They are generally considered to be Creataceous in age but they have been interpreted as having formed in a back‐arc basin, forearc basin or plume setting (e.g. Giunta et al., 2009; Marchesi et al., 2016; Kerr et al., 1999). The Moa‐Baracoa ophiolitic massif consists of mantle harzburgite, a Moho transition zone, layered and isotropic gabbro, and pillow basalts. The isotropic gabbros, collected west of Moa (20°35′ 17.99″N, 75°02′56.58″W), are composed mainly of subhedral plagioclase and subhedral to euhedral clinopyroxene with minor primary brown amphibole. The plagioclase grains are fresh, whereas the clinopyroxene very weakly altered and partly replaced by secondary amphibole. Cathodoluminescence (CL) images of zircon grains separated from isotropic gabbro display oscillatory zoning and broad zoning, analogous to typical igneous zircon in gabbro. Ten zircon grains with oscillatory zoning in CL images yielded a mean U‐Pb SHRIMP age of 136.7±1.8 Ma. Isotropic gabbro samples have low SiO2 (47.22 wt%–49.25 wt%), TiO2 (0.13wt%–0.26 wt%) and K2O (0.04 wt%–0.08 wt%) and high MgO (8.22 wt%–16.72 wt%), Cr (1337–2065 μg/g) and Ni (192–507 μg/g) contents. Their CaO, Al2O3 and Na2O contents vary from 13.56 wt% to 15.78 wt%, 14.98 wt% to 21.98 wt%, and 0.71 wt% to 1.87 wt%, respectively. In the SiO2 versus Zr/TiO2∗0.0001 diagram, the samples plot in the sub‐alkaline basalt field but show calc‐alkaline affinities in the FeOT/MgO vs. SiO2 diagram. The gabbros contain low REE contents (ΣREE = 4.71–7.71), and their chondrite normalized REE patterns show very weak depletion in light REE (LREE, (La/Yb)N = 0.34–0.50) and flat heavy REE (HREE, (Gd/Yb)N = 1.02–1.15) profiles, with slightly positive Eu anomalies (Eu/Eu∗ = 1.55–1.76) suggesting magmatic accumulation of plagioclase. Furthermore, all samples are depleted in Nb and Th, and enriched in Ba, Pb and Sr in the primitive mantle‐normalized trace element diagram. Geochemical features, such as high values of Ba/La (30–38) and low Th/Yb (0.02–0.03), medium values of (Ta/La)PM (0.30–0.35) and (Hf/Sm)PM (0.58–0.80), indicate enrichment by slab‐derived fluids and subduction‐related metasomatism. These isotropic gabbros are geochemically comparable with those of the Kermanshah and Neyriz ophiolites of Iran (Monsef et al., 2018) that are thought to have originated in a SSZ environment. Therefore, we suggest that the isotropic gabbros in the Moa‐Baracoa massif crystallized from early Cretaceous SSZ magmas that were most likely derived from a mantle source metasomatized by slab fluids in a suprasubduction zone environment.

Possible imprints of late Paleoproterozoic orogeny in the Dunhuang terrane, NW China: Constraints from igneous and metapelitic rocks

The Dunhuang terrane is a NEE trending tectonic belt confined by the Tarim Craton, Beishan Orogen, Alxa Block and Altyn Tagh Fault. To constrain the late Paleoproterozoic geotectonic background of this terrane, a synthetic research on petrology, geochemistry, zircon and monazite geochronology, and zircon Lu-Hf and whole-rock Nd isotopes of late Paleoproterozoic (ca. 1.9–1.8 Ga) igneous rocks and metapelites was conducted. The Sanweishan-Dongshuigou Na-rich rhyolite and granite formed at ca. 1.84–1.82 Ga exhibit enriched light rare earth elements (LREEs), depleted heavy rare earth elements (HREEs) and insignificant Eu anomalies, with low Y (3.44–6.39 ppm) and Yb (0.29–0.53 ppm) but relatively high Mg# (54.5–58.5) and Sr (368–643 ppm), and variable Cr (5.58–29.7 ppm) and Ni (2.82–15.6 ppm) contents, as well as negative to positive εHf(t) (−6.1 to +6.4) values, indicating subduction-related arc magmatism accompanied by crustal-mantle interaction. The Shuwozi granitic gneiss crystallized at ca. 1.81 Ga displays S-type granite affinity (A/CNK > 1.1) and has low Sr/Y (1.0–1.3) and (Gd/Yb)N (1.50–1.98) ratios, and negative εHf(t) (−20.3 to −5.1) and εNd(t) (−4.3) values, suggesting partial melting of crustal rocks at shallow crustal depth. The Hanxia amphibolite with protolith age of ca. 1.86–1.84 Ga is featured by depleted LREE and flat HREE patterns, insignificant Eu anomalies, low Nb/Ta (11.0–12.0), Ta/Yb (0.02–0.04), Th/Yb (0.01–0.04) and Th/Ta (0.7–1.3) but high Ti/V (22.9–26.7) ratios, and positive εNd(t) (2.7–4.0) values, resembling depleted mantle-derived N-MORB- or BABB (back-arc-basin basalt) geochemical affinities. The metapelite, characterized by mineral assemblage of garnet, biotite, sillimanite, plagioclase, K-feldspar, quartz and rutile, documented ca. 1.81 Ga upper amphibolite-facies metamorphism with peak metamorphic P-T conditions being restricted to 690–810 °C and 5.3–6.4 kbar based on conventional geothermobarometry. These magmatic and metamorphic rocks were variously altered and deformed during Paleozoic (ca. 430–360 Ma) tectonothermal event. Integrated with published data, it can be inferred that the Dunhuang terrane underwent ca. 1.89–1.81 Ga subduction-collisional orogeny related to late Paleoproterozoic (ca. 2.0–1.8 Ga) global assembly of the Columbia supercontinent, similar as those recorded in the northern Tarim Craton, Alxa Block and Quanji massif, and then overprinted Paleozoic (ca. 440–360 Ma) orogenic process, corresponding to the previously proposed composite orogeny in the Dunhuang terrane.

Petrology, age, and tectonic setting of the rapakivi-bearing Margaree pluton, Cape Breton Island, Canada: evidence for a Late Devonian posttectonic cryptic silicic-mafic magma chamber

The Margaree pluton extends for &gt;40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The three rock types have similar U–Pb (zircon) ages of 363 ± 1.6, 364.8 ± 1.6, and 365.5 ± 3.3 Ma, respectively, consistent with field and petrological evidence that they are coeval and comagmatic. The rare earth elements display parallel trends characterized by enrichment in the light rare earth elements, flat heavy rare earth elements, moderate negative Eu anomalies, and, in some cases, positive Ce anomalies. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by the mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The chemical and isotopic (Sm–Nd) signature of the Margaree pluton is consistent with the melting of preexisting continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other, better exposed occurrences of rapakivi granite in the northern Appalachian orogen.

Provenance and paleo-weathering of Paleoproterozoic siliciclastic sedimentary rocks of Bijawar Group, Sonrai Basin, Uttar Pradesh, India: using a geochemical approach

Shales and sandstones of the Paleoproterozoic Bijawar Group (India) have been studied to decipher their paleo-weathering conditions and provenance based on geochemistry. Shale samples are composed of k-feldspars, mica, quartz, and fine iron-oxides, whereas sandstones are composed of quartz and k-feldspars. The predominant positive correlation of K2O vs TiO2, Na2O vs Al2O3, and K2O vs Al2O3 indicates that the elements are associated with detrital phases. Trace elements Sc, Rb, Ba,Th, Y, and Zr show a strong positive correlation with Al2O3 and K2O which indicates that the absolute abundances of these elements are controlled mainly by illite. A negative correlation of trace elements Ni, Cu, Co with Al2O3, and K2O represents their presence in iron oxides like magnetite, ilmenite, and hematite. Multi-element Post-Archaean Australian Average Shale (PAAS) normalized spider diagrams show enrichment of V, Pb, Zr, Y, Hf, Th, Rb, and depletion of Ni, Sr, Ba, Nb, U. The depletion of Sr indicates the least enrichment of plagioclase and the same is also supported by negative Eu-anomaly of the samples, though weak. The chondrite normalized rare earth element (REE) plots show a slightly light-REE (LREE) enriched and flat heavy-REE (HREE) pattern with weak negative Eu anomaly. The overall, high values of LaN/SmN (avg. 10.49), CeN/YbN (avg.0.37), LaN/YbN (avg. 34.28) and low values of GdN/YbN (avg. 1.82) and TiO2 (0.27–1.82, average 0.92 < PAAS) in most of the samples, show a relationship with the felsic source rocks. The index of compositional variability (ICV) of the samples is < 0.84 which indicates no enrichment of primary minerals. The chemical index of alteration (CIA) of present samples varies from 48.60 to 81.69, indicating low to moderate intensity of chemical weathering of the source. The plot between SiO2 and Al2O3 + K2O + Na2O depicts that majority of the samples belong to the environment from humid to semi-humid and have attained a high chemical maturity. The high SiO2/Al2O3 values (1.36–20.79) in studied samples indicate relatively high sediment maturity due to the extreme sorting of sediments in a stable cratonic regime. A stable cratonic regime, with a low upliftment rate that has allowed strong chemical weathering of protoliths, is further indicated by low CaO, Na2O, and Sr contents of the studied samples. The overall geochemical characteristics of the present samples—high SiO2, Al2O3 and K2O and low TiO2, MgO, Na2O, and CaO contents—suggest a felsic provenance. The felsic nature of the source rocks is also supported by the trace element ratios like (La/Lu)N (7.72–76.89), La/Sc (1.50–7.40), La/Co (0.39–11.48), Th/Co (0.80–15.90), and Cr/Th (0.08–5.10). Since the Bijawar Group is sited over the Bundelkhand Granite Gneiss Complex (BGGC), it is, therefore, plausible that the main source components of Bijawar Group siliciclastic rocks are the BGGC felsic rocks.

Neoarchean seafloor hydrothermal metamorphism of basalts in the Zanhuang ophiolitic mélange, North China Craton

Epidosites are suggested to form in up-flow zones located at the base of ore-forming oceanic hydrothermal systems, and are commonly preserved in ophiolitic sequences. However, occurrences of epidosites in Archean tectonic mélanges are rarely documented. In this study, we report the presence of late Neoarchean epidosites in the Zanhuang ophiolitic mélange in the North China Craton using meter-scale litho-structural and petrologic analysis, zircon U-Pb dating, major and trace element, and oxygen and strontium isotopic geochemistry. Field studies show that the epidosites are mainly preserved as the cores of meta-pillow lavas, and are distributed as blocks in different matrices within the tectonic mélange. The pillow rims are used as protolith to estimate the element mobility during the alteration process. The epidosites display strong depletion in Na2O (0.01–0.27 wt%), MgO (0.43–3.01 wt%), depletion in large ion lithophile elements (LILE) (e.g. Rb, Ba, Cs, U) and transition metals (e.g., Co, Sc, Cu, Ni, Zn), and show enrichment of CaO (13.35–21.0 wt%), Sr and Pb. The high field strength elements (HFSE) (e.g. Zr, Hf, Ta, Th) of epidosites are varied but rare earth elements (REE) are unchanged. The amphibolitic pillow rims are characterized by enrichment of LILE, flat heavy rare earth elements (HREE) patterns, and depletion in HFSE with negative Ti anomalies and positive Pb and Ba anomalies. Oxygen and strontium isotopic ratios of epidosites are 5.0–8.6‰ and 0.70284–0.70551, respectively, both of which are greater than those of fresh basalts. The major and trace elements, oxygen and strontium isotopic values and extensive element migration indicate that the protolith was altered by a submarine hydrothermal system in the absence of continental influence. Magmatic zircons from two samples of epidosites yield 207Pb/206Pb ages of 2512 ± 13 Ma (MSWD = 0.51) and 2542 ± 13 Ma (MSWD = 1.4), confirming that the epidosites were generated at ca. 2.51–2.54 Ga. Based on geochemical data and field observations, we suggest that the protolith of the epidosites formed in an arc-related tectonic setting during the late Neoarchean. This study shows that hydrothermal activity operating in the late Neoarchean was similar to that of the modern ocean floor.

Petrogenesis and tectonic implications of the Late Triassic Nangou granodiorite porphyry in the Eastern Kunlun Orogenic Belt, northern Tibetan Plateau

Triassic granitic magmatism is widespread in the Eastern Kunlun Orogenic Belt (EKOB), northern Tibetan Plateau. Some of the granitoids are characterized by high Sr and low Y contents, and consequently high Sr/Y ratios. These high Sr/Y-ratio granitoids are often interpreted as adakitic rocks, originating from the thickened continental lower crust. However, studies have shown that granitoids with high Sr/Y ratios may have formed via other geological processes. This paper reports U–Pb ages, geochemical and Sr–Nd–Hf isotopic data for newly discovered granodiorite porphyries in the Kunlun River area of the Eastern Kunlun Orogenic Belt, and discusses whether the EKOB experienced crustal thickening during the Triassic. The granodiorite porphyries crystallized at 205 Ma. They have some adakitic characteristics with SiO2 = 66.96–69.68 wt.%, Sr/Y ratios = 31–43, La/Yb = 26.9–57.9, Y = 8.47–11.3, Yb = 0.75–1.30, and MgO = 0.44–0.99 wt.%. However, the relatively flat heavy rare earth element patterns indicate that garnet was not the main residue in the magma source. In addition, combined with Nd–Hf isotopic data, these results indicate that the timing of the original generation of the crustal sources of the granodiorites should be Mesoproterozoic, with the involvement of older (Paleoproterozoic) components. The granodiorite porphyries were emplaced in a post-collisional environment after the northward subduction of Paleo-Tethyan oceanic lithosphere, and without thickening of the continental crust.

Geochemistry and petrogenesis of oligocene felsic volcanic rocks from the Pinos Volcanic Complex, Mesa Central, Mexico

The Pinos Volcanic Complex (PVC) is located in the San Luis Potosi Volcanic Field (SLPV) and southern region of the Mesa Central that includes Oligocene rhyolites and trachydacites. These felsic volcanic rocks are characterized by porphyritic textures with a main mineralogical arrangement of phenocrystals of quartz, potassium feldspar (sanidine), plagioclase and biotite embedded in a vitreous matrix with microcrystals of quartz. They display peraluminous, high-silica, alkaline (not peralkaline) composition, with flat heavy rare earth element concentrations and negative Eu anomalies values. They are enriched in large-ion lithophile elements (LILEs; Rb, Ba, K, Th, Ce) relative to highfield-strength elements (HFSE; Nb, Zr, P, Ti). Trace element ratios trend highlights variable degree of partial melting rather than the role of fractional crystallization. The petrogenetic modelling of trace elements reveal that the felsic volcanic rocks are consistent with their formation via partial melting of the middle/upper continental crust. Multi-dimensional tectonic discriminant diagrams and magmatic model indicate an extensional tectonic environment for the genesis of the felsic volcanic rocks.