Gabbroic Cumulates Research Articles

Petrography, chemical composition, and age constraints of mafic intrusions from the Mesoproterozoic Soisson Intrusive Suite in the southeastern Churchill Province (Canada)

Several kilometre-scale gabbroic intrusions with well-preserved cumulate textures occur over a ∼180 km northwest–southeast strike length area in the Core Zone, within the southeastern Churchill Province. These intrusions (1312–1311 Ma) are devoid of structural and metamorphic overprint and are mainly composed of olivine gabbro. Their overlapping petrographic, geochemical, and mineralogical characteristics suggest that all of these gabbroic intrusions are co-genetic and form the Soisson Intrusive Suite. The Soisson intrusions formed from basaltic parental magmas with low MgO (∼6–7 wt.%) and high Fe–Ti contents. Overall, they are similarly fractionated at the regional scale and incorporated various amounts of upper crustal material, as indicated by field (e.g., xenoliths, hybrid zones) and geochemical (e.g., high HILE/MILE, depletion in Nb±Ta) evidences. The Soisson Intrusive Suite has petrological, geochemical, and mineral chemistry similarities with the troctolite and olivine gabbro cumulates of the Voisey’s Bay and Mushuau intrusions from the ca. 1363 to 1289 Ma Nain Plutonic Suite (NPS), and appears to be contemporaneous with its younger gabbroic–troctolitic intrusion phase, suggesting that the Soisson Intrusive Suite and the gabbroic–troctolitic intrusions of the NPS were formed by similar petrological processes. Furthermore, like many gabbroic–troctolitic intrusions of the NPS (e.g., Voisey’s Bay intrusion), the Soisson intrusions contain Ni–Cu–Co magmatic sulfide mineralization. Among the Soisson intrusions, the Papavoine intrusion located in the southern part of this intrusive suite may be the most prospective intrusion as a Ni, Cu, and Co exploration target. Overall, the Soisson Intrusive Suite’s similarity with the NPS raises questions about their potential genetic link and magmatic connectivity at depth.

Trace element constraints on the parental melt of gabbroic cumulates from the Naga Ophiolite Complex, North-East India

Trace element constraints on the parental melt of gabbroic cumulates from the Naga Ophiolite Complex, North-East India

Neogene Alkali Basalts from Central Slovakia (Ostrá Lúka Lava Complex); Mineralogy and Geochemistry

Ostrá Lúka basalts are a product of the final phase of Neogene volcanism in Central Slovakia. Their major and trace elements composition is alkaline, a feature confirmed by light rare earth elements (LREE) relative enrichment and some incompatible trace elements ratios. The basalts contain rare surrounded gabbro cumulates. Their peculiarity is a strong zonation of olivines and clinopyroxene. The significant zonation probably indicates a short stop of the melt at shallow depth. The Sr, Nd and Pb isotope compositions indicate an origin from a moderately depleted mantle source. The mineral and chemical composition of the basalts is similar to the Pannonian basin alkali basalts and the Western and Central Europe alkali basalts.

Petrography and Geochemistry of the Leucocratic Rocks in the Ophiolites from the Pollino Massif (Southern Italy)

In the Tethyan realm, leucocratic rocks were recognized as dikes and layers outcropping in the ophiolitic rocks of the Western Alps, in Corsica, and in the Northern Apennines. Several authors have suggested that the origin of leucocratic rocks is associated with partial melting of cumulate gabbro. Major and trace elements composition and paragenesis provided information about the leucocratic rocks genetic processes. This research aims at disclosing, for the first time, the petrographical and geochemical features of Timpa delle Murge leucocratic rocks, Pollino Massif (southern Italy), in order to discuss their origin and geodynamic significance through a comparison with other Tethyan leucocratic rocks. These rocks are characterized by high amounts of silica with moderate alumina and iron-magnesium contents showing higher potassium contents than plagiogranites, due to plagioclase alteration to sericite. Plagioclase fractionation reflects negative Eu anomalies indicating its derivation from gabbroic crystal mushes. The chondrite normalized REEs patterns suggest the participation of partial melts derived from a metasomatized mantle in a subduction environment. The results reveal some similarities in composition with other Tethyan leucocratic rocks, especially those concerning Corsica and the Northern Alps. These new data provide further clues on the origin of these leucocratic rocks and the Tethyan area geodynamic evolution.

Characterising the Distinct Crustal Protoliths of Roberts Victor Type I and II Eclogites

Abstract The origin of the eclogites that reside in cratonic mantle roots has long been debated. In the classic Roberts Victor kimberlite locality in South Africa, the strongly contrasting textural and geochemical features of two types of eclogites have led to different genetic models. We studied a new suite of 63 eclogite xenoliths from the former Roberts Victor Mine. In addition to major- and trace-element compositions for all new samples, we determined 18O/16O for garnet from 34eclogites. Based on geochemical and textural characteristics we identify a large suite of Type I eclogites (n = 53) consistent with previous interpretations that these rocks originate from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted mid-ocean ridge basalt (MORB) mantle. We identify a smaller set of Type II eclogites (n = 10) based on geochemical and textural similarity to eclogites in published literature. We infer their range to very low δ18O values combined with their varied, often very low zirconium-hafnium (Zr-Hf) ratios and light rare earth element-depleted nature to indicate a protolith origin via low-pressure clinopyroxene-bearing oceanic cumulates formed from melts that were more depleted in incompatible elements than N-MORB. These compositions are indicative of derivation from a residual mantle source that experienced preferential extraction of incompatible elements and fractionation of Zr/Hf during previous melting.

The Fuchuan Ophiolite in South China: Evidence for Modern‐Style Plate Tectonics During Rodinia Breakup

AbstractSubduction initiation processes, such as those evidenced in the Izu‐Bonin‐Mariana (IBM) forearc crust are a key feature of modern‐style plate tectonics. However, the evidence for this process in the Precambrian is limited, due to the scarcity of ophiolite suites and in particular a lack of typical complete ophiolitic sequences similar to those observed in Phanerozoic ophiolites. Mineral analyses of olivine, orthopyroxene, and chrome spinel from the harzburgite and two types of clinopyroxene from the cumulate gabbro of the Fuchuan ophiolite, South China suggest formation in a Neoproterozoic subduction system similar to the IBM. High temperature (1,000–1,200°C), low pressure along with a high degree of melt extraction (>18%) have been recorded in the harzburgite. This evidence, in conjunction with magma evolution from forearc basalt to boninite to hydrous calc‐alkaline magmatism, recorded in the crustal section of the Fuchuan ophiolite indicates a transition from initial to mature subduction in a forearc setting. Statistical analysis of global ophiolites through time suggests that IBM‐like subduction systems may have successfully operated since at least the latest Neoproterozoic (ca. 1,000 Ma) and be pervasive on Earth from ca. 850 Ma. Additional evidence for Neoproterozoic ophiolites from India, Siberia, and Arabian‐Nubian Shield, which are also similar to the IBM forearc ophiolite, suggests that cold and deep subduction became much more common in the Neoproterozoic (ca. 850 Ma) around the periphery of the Rodinia supercontinent. This IBM‐like subduction system may have acted as the geodynamic trigger for the break‐up of Rodinia.

Eucrite‐type achondrites: Petrology and oxygen isotope compositions†

AbstractWe report petrologic studies and oxygen isotope analyses of normal and anomalous eucrites, termed eucrite‐type achondrites. Petrologically anomalous eucrite‐type achondrites can have normal oxygen isotope compositions, and vice versa. Two basaltic eucrites with normal oxygen isotope compositions contain pyroxenes with anomalous Fe/Mn engendered by parent body processes acting on normal eucrites: solid‐state reduction by S gas in EET 87542, and reduction during crystallization by magmatic S in QUE 94484. Cataclastic basaltic breccias PCA 82502 and PCA 91007 are paired (petrology, anomalous oxygen). Although isotopically like Pasamonte, they are petrologically distinct. We confirm the petrological and isotopic anomalies of cumulate gabbro EET 92023; likely formed by impact melting of mixed cumulate and basaltic materials. Many main group eucrites include plagioclases that retain near‐liquidus compositions despite metamorphic overprinting. Stannern group eucrites contain more sodic plagioclase, which is consistent with the melt hybridization hypothesis for Stannern group magma formation. The lack of more calcic plagioclase suggests reactive exchange of the anorthite component of the primary melt with the albitic component of the crust. Asteroids that are modestly different in composition can produce virtually indistinguishable basalts, providing a ready explanation for the eucrite‐type achondrite suite. Small stochastic variations in petrologic evolution can cause substantial differences in rocks produced on an asteroid.

Multi‐Stage Hydrothermal Veins in Layered Gabbro of the Oman Ophiolite: Implications for Focused Fluid Circulation in the Lower Oceanic Crust

AbstractAn outcrop in the layered gabbro section from the Wadi Wariyah of the Oman ophiolite has been investigated to provide new insights into the hydrothermal activities that cover a large temperature range in the lower crust at fast‐spreading mid‐ocean ridges. Our observations reveal a complex veining system composed of gabbroic dikes, hydrous mineral (amphibole, epidote, and prehnite) veins and associated alteration, which strongly overprinted the layered gabbros shortly after the solidification of the gabbroic cumulates. The general strike of the veins is roughly perpendicular to the magmatic foliation of the layered gabbro. Mineral‐based geothermometry indicates a wide temperature range (ca. 1,000−250°C) of hydrothermal activities, spanning from hydrous suprasolidus stage (VHT) through amphibolite facies (HT) and epidote‐amphibolite facies (MT) down to the prehnite‐pumpellyite facies (LT), which induced significant differences in selective element depletion and enrichment as well as in exchange of Sr and O isotopes. A key observation is the omnipresence of relics from a previous temperature stage in a given hydrothermal paragenesis formed at lower temperatures, implying that the same pathways were used for hydrothermal veining in a broad temperature interval spanning about 750°C. Our observations support a model that focused fluid circulations, in the form of combined microcracking (grain boundary spacing) and macrocracking (veining), developed shortly subsequent to the complete solidification of the layered gabbros, which may have facilitated effective cooling for magma crystallization in the lower crust.

Platinum group elements in lamprophyre, picrobasalt, gabbro and basalts of the Phenai Mata and nearby areas: implications for Fe–Ni–Cu–PGE mineralization in the Deccan Large Igneous Province

Abstract Among the basaltic large igneous provinces (LIPs) world over, the Deccan Large Igneous Province (DLIP) is considered barren of metalliferous sulfide deposits, and especially platinum group minerals (PGMs). The tholeiitic basaltic magmas of the DLIP were found to lack sulfur saturation at all stages of its evolution. However, we found some incidences of Fe–Ni–platinum group element (PGE) mineralization in the cumulate gabbros of the Phenai Mata Igneous Complex (PMIC), as well as intrusive dykes of lamprophyre, picrobasalt and basalt in adjoining areas. To examine the metal potential of the gabbros, we adopted the following approach: (a) whole-rock PGE analysis of selected rocks; (b) preconcentration of the samples by communition, sieving and gravity-magnetic separation, followed by froth floatation, Ni-sulfide fire assay and geochemical analysis of the concentrates; and (c) direct scanning of samples using an electron probe microanalyser (EPMA). The geochemical proxies – namely, Pd/Ir, Cu/Ir, Ni/Pd and Ni/Cu – indicate that PMIC has distinct geochemical signatures compared to other parts of the DLIP. The results have further indicated an interesting suite of minerals comprising metal sulfides of Fe–Cu–Zn–Pb–Co–Ni. This metal-rich suite in the otherwise ‘barren’ Deccan Trap tholeiites is a result of accidental fertilization of metals into their sulfides due to the mixing of crustal sulfur.

A non-arc tectonic setting for the evolution of Archean gabbro anorthosite Complexes: Evidence from the Singhbhum Craton, eastern India

Understanding the petrogenetic evolution of Archean gabbroic cumulates provides a window to the mantle geodynamics operated in the early Earth. Gabbroic cumulates are associated with various tectonic settings including convergent plate boundaries, mantle plumes, mid-ocean ridges, anorogenic, post-orogenic, continental rift settings and oceanic plateaus. The exposures of preserved gabbroic suites in the Archean are limited. The Mesoarchean Mayurbhanj Gabbro Anorthosite Complex (GAC) is a well-preserved mafic body situated along the eastern margin of the craton along with concomitant anorogenic Mayurbhanj granite. These rocks are devoid of deformational structures and/or mineral recrystallization. Based on their mineralogical characteristics, GAC rocks are mainly grouped into gabbronorite, leucogabbronorite, leucogabbro and gabbro. The minimal abundance of magmatic amphibole and the variation in the anorthite content (An36-An84) rules out the role of a hydrous mantle source in the evolution of these rocks. The studied samples display tholeiitic trend transitioning to calc-alkaline field, major and trace element data indicates the fractional crystallization of olivine and pyroxene. The trace element systematics of the Mayurbhanj GAC show LILE enrichment over HFSE depletion, with negative Nb-Ta and mixed Zr-Hf and Ti anomalies. The geochemical signatures such as elevated Th/Yb, Nb/Yb, variable Dy/Yb and Dy/Dy* and varying Gd/YbCN ratios suggest that parental magmas of the Mayurbhanj GAC originated from a heterogeneous depleted mantle source. The studied rocks' Sr-Nd isotopic systematics reveal that their parental magma has undergone assimilation by a Paleoarchean felsic crust. We argue that the ‘arc-like’ geochemical signatures displayed by these rocks resulted from crustal contamination and may not be due to subduction origin. Consolidating the field, geochemical and isotopic evidence suggests that the Mesoarchean Mayurbhanj GAC possibly evolved from an anhydrous heterogeneous mantle source in a non-compressive tectonic regime.

Late Paleozoic–Mesozoic subduction and accretion of the Paleo-Pacific Plate: Insights from ophiolitic rocks in the Wandashan accretionary complex, NE China

The Wandashan accretionary complex (AC), consisting of the Raohe and Yuejinshan complexes, is located on the continental margin of Northeast Asia and represents an excellent source of information about Paleo-Pacific subduction and accretion. However, the protolith nature and tectonic evolution of the Wandashan AC are under debate. This contribution reports new geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data for ophiolitic rocks from the Wandashan AC. The 169–166 Ma plagioclasites and homogeneous gabbros from the Raohe complex are OIBs while 228–214 Ma homogeneous gabbros are continental VABs. Cumulate gabbros from the Yuejinshan complex formed at 280–278 Ma and ~220 Ma and have similar characteristics with E-MORB and N-MORB, respectively. They are BABBs and their primary magma was derived from a source region between EMI and EMII that was affected by continental crustal contamination as well as subduction-zone metasomatism. Combined with previous studies, we suggest that the onset of subduction of the Paleo-Pacific Plate was in the Early Permian. Subsequently, a back-arc basin, whose present suture is on the eastern margin of the Jiamusi Massif, formed and widened during 280–232 Ma, after which the basin closed and BABBs were emplaced to form the Yuejinshan complex during 210–180 Ma. The formation of VABs of the Raohe complex is coincident with the closure of the back-arc basin, and together with the 169–166 Ma OIBs, they constitute a major part of the Raohe complex. The accretionary process was completed during 133–131 Ma. Taken together, the ophiolitic rocks indicating multistage magmatism in the Paleo-Wandashan region recorded the formation-closure process of back-arc basin and the accretionary process of the Wandashan AC, during the westward subduction of the Paleo-Pacific plate. The back-arc basin identified in our study sheds new lights on geodynamic evolution model of subduction and accretion of the Paleo-Pacific Plate on the continental margin of NE Asia.

Cumulate gabbros in the South Andaman Island ophiolite suite (India): their bearing on the tectonic setting

The Andaman ophiolite of southeastern India is located on the outer arc of the Andaman–Java subduction zone. It is represented by thrust slices formed in the Mesozoic Neo-Tethys Ocean. Lithologically, it consists of dismembered mafic and ultramafic rocks and associated oceanic pelagic sediments. The present study focuses on the mafic cumulate rocks of the Andaman ophiolite preserved in the Kodiaghat and Munda Pahar area of South Andaman Island. The mafic cumulates are represented by olivine-bearing and olivine-free gabbros. The sequence of crystallization in the gabbros is olivines (Fo∼80) ± chromian spinels (Cr# = 57–59), plagioclases (An61–95), clinopyroxenes (Mg# = 82–89), and amphiboles (Mg–hornblende, edenite, and pargasite). Major oxide and trace element whole-rock geochemistry and mineral compositions are consistent with a hybrid signature of island arc tholeiite (IAT) – mid-oceanic ridge basalt (MORB). Geochemical modelling shows that trapped melt fractions of 0%–20% can produce the observed trace element signatures of these gabbros. Our findings suggest that the gabbroic cumulates of the Andaman ophiolite were formed in an oceanic back-arc and oceanic arc setting developed in the Neo-Tethys oceanic domain between the Indian and Burmese plates during the Late Cretaceous.

Protolith and tectonic implications of the No. Ⅳ pluton in Xiarihamu area, East Kunlun, Qinghai Province

东昆仑造山带西段夏日哈木地区IV号岩体是镁铁-超镁铁质侵入体还是蛇绿岩单元仍存争议。本文对IV号岩体开展了野外地质特征、岩相学、地球化学及锆石U-Pb年代学的研究。IV号岩体主体为斜长角闪岩，呈透镜状产在片麻岩中。斜长角闪岩主要由角闪石、斜长石组成，含少量石英、黑云母、榍石和铁钛氧化物。具有基性岩的化学成分，属于拉斑玄武岩系列岩石，稀土配分模式中Eu显示正异常（δEu=1.3~1.6），ε_Nd（t）值为+3.5~+5.3，推测其原岩可能为堆晶辉长岩，明显不同于含铜镍矿的Ⅰ号镁铁-超镁铁岩体，与该地区出露的榴辉岩（退变榴辉岩）可以对比。斜长角闪岩中的锆石具有弱环带，采用LA-MC-ICP-MS方法得出的²⁰⁶Pb/²³⁸U加权平均年龄为421±4Ma （MSWD=2.8），Th/U比值为0.01~3.26，指示锆石为变质-深熔成因，代表了斜长角闪岩的形成时代。夏日哈木地区的斜长角闪岩和该地区出露的榴辉岩及榴闪岩的原岩可能都是早期形成于陆壳环境的基性岩（大陆边缘的基性侵入岩或喷出岩），在430Ma左右发生陆壳深俯冲作用时形成榴辉岩，420~410Ma榴辉岩发生不同程度退变形成榴闪岩和斜长角闪岩。由此推测，IV号岩体的构造属性与该地区的榴辉岩属性相同，形成于陆壳环境，其原岩不属于蛇绿岩单元。斜长角闪岩的出现指示了该地区原特提斯洋壳已俯冲消亡，深俯冲的陆壳开始折返，这对认识东昆仑早古生代的构造演化和铜镍矿形成的地质背景具有重要意义。;Whether the protolith of the No. IV pluton in Xiarihamu area in the western part of the East Kunlun Orogenic Belt belongs to a mafic-ultramafic intrusive body or a component (gabbro) of the ophiolitic unit remains debated. Here, we reported the occurrence and the petrographic, geochemical and zircon U-Pb geochronological analysis results of this pluton to clarify the above disagreement. Petrographically, this pluton is an amphibolite block occurring as lens within a set of gneisses. The amphibolite rocks mainly consist of hornblende and plagioclase, with minor quartz, biotite, titanite and Fe-Ti oxide. The samples collected from amphibolite pluton have SiO₂ contents varying from 43% to 50% with Mg^# values from 42 to 73, and chondrite-normalized REE patterns show they have a positive Eu anomaly (δEu=1.3~1.6), implying their protoliths were cumulative mafic rocks and belong to tholeiitic series. Furthermore, the amphibolite rocks have positive whole-rock ε_Nd(t) values of +3.5~+5.3, similar to those of the eclogite or retrograded-eclogite and different from those of the No. I Ni- and-Cu-bearing mafic-ultramafic rocks in Xiarihamu area. Zircon grains from the amphibolite pluton show weakly zonation on their CL images with Th/U ratios varying from 0.01 to 3.26, suggesting they have a metamorphic-anatexis origin rather than a magmatic one. The zircon grains yield a ²⁰⁶Pb/²³⁸U weight mean age of 421±4Ma (MSWD=2.8), which represent the formation time of the pluton. In other words, the formation of this pluton can be divided into two stages: the previous cumulate gabbro underwent a high-pressure metamorphism and formed eclogite in the subduction stage, then followed by a retrograde stage which led to the transformation from eclogite to amphibolite. To be more clearly, the protoliths of the amphibolite, eclogite and retrograded-eclogite in the Xiarihamu area are mafic rocks (mafic pluton or eruption) formed in a continental margin, they turned into eclogites during a deep continental subduction approximately at 430Ma, and finally became retro-eclogite and amphibolite with various retrograde metamorphism at 420~410Ma. Therefore, the protolith of the No. IV pluton is not a gabbro component of the ophiolitic unit. Our studying results indicate that the evolution of this region experienced a closing of Paleozoic Proto-Tethys oceanic basin and then an exhumation of subducted continental crust, which is important for understanding the Paleozoic tectonic evolution of East Kunlun Orogen and the geological setting of the Cu-Ni mineralization.

Early Paleozoic Ocean in the North Qaidam: Constraints from Kaipinggou Ophiolite

AbstractThe North Qaidam orogenic belt (NQOB) lies at the northeast margin of Tibet Plateau, between the Qilian massif to the north and Qaidam massif to the south. It consists of predominantly gneisses and marbles with minor eclogite and garnet peridotite, and is known as an Early Paleozoic continental deep subduction ultrahigh pressure (UHP) metamorphic belt in northwest China. Recently some studies suggest that the NQOB experienced oceanic deep subduction prior to the continental deep subduction. However, the records of ocean events are rarely preserved because of the complex multi‐stage transformation. Therefore, whether the Early Paleozoic ocean exists and the formation, nature and evolution of the Early Paleozoic ocean in North Qaidam are still in controversies.In this contribution, we report a new discovered SSZ‐type ophiolite suite in Kaipinggou area, the western segment of the NQOB. The ophiolite is mainly composed of peridotite, gabbro, basalt, boninite, andesitic porphyrite and plagiogranite. In which, the basalt and homogeneous gabbro are both enriched in LILE and depleted in HFSE, and display flat or LREE‐depleted REE patterns, similar to the MORB‐like geochemical characteristics of fore‐arc basalts; the boninite exhibits characteristics of fore‐arc boninites with high Si and Mg, but low Ti and Ca contents; the andesitic porphyrite is enriched in LILE but depleted in HFSE, similar to island arc magmatic rocks. Rock assemblage and their geochemical characteristics suggest that this SSZ‐type ophiolite was formed in fore‐arc setting of intra‐oceanic arc. The plagiogranite is a low‐potassium tholeiite metaluminous granite with low trace elements contents, slight enrichment of LREE, depletion of HREE and positive Eu anomaly; combining with ∊Hf (t) values (13.8–17.1) of zircons, the plagiogranite could be originated from partial melting of gabbros. LA–ICP–MS zircon U–Pb dating yielded the crystallization ages of 535±2 Ma, 513±3 Ma and 510±3 Ma for two cumulate gabbros and one homogeneous gabbro, respectively, and a formation age of 492±8 Ma for the plagiogranite.All these data suggest the existence of an Early Paleozoic ocean in the North Qaidam, and the initial subduction of the oceanic slab occurred earlier than 535Ma.

Petrogenesis of the Neoproterozoic Xinlin ophiolite, northern Great Xing’an Range, northeastern China: Implications for the evolution of the northeastern branch of the Paleo-Asian Ocean

The Xinlin ophiolite in northeastern China marks the Xinlin–Xiguitu branch of the Paleo-Asian Ocean (PAO), and provides an opportunity to reconstruct the evolution of the Xing’an–Mongolian orogenic belt (XMOB). The main ophiolitic body is composed of mantle peridotites, mafic and ultramafic cumulates, isotropic gabbros, diabase dykes, and massive basaltic lava flows, from base to top. The uppermost pelagic sediment cover is missing and was presumably eroded. The cumulate gabbro yields a concordant zircon U–Pb age of 669 ± 8 Ma and has positive zircon εHf(t) values (+10.7 to +15.6). The mafic rocks of the ophiolite have rare earth element (REE) patterns that plot between the compositions of normal (N-) and enriched (E-) mid-ocean ridge basalt (MORB). The monzogranite that intruded into the Xinlin ophiolite yields a narrow age range (574–571 Ma), and is enriched in light REE and depleted in high field strength elements (HFSE). Zircon grains from the monzogranite have negative εHf(t) values (−5.6 to −10.0), suggesting the monzogranite was formed by partial melting of the Erguna continental crust, rather than oceanic crust. The geochemical characteristics of the ophiolite and spatially associated younger arc granites show that the Xinlin ophiolite developed at a mid-ocean ridge setting, and was subsequently accreted onto the Erguna continental margin before amalgamation with the Xing’an Block.

Mineral Carbonation of CO2 in Mafic Plutonic Rocks, II—Laboratory Experiments on Early-Phase Supercritical CO2‒Brine‒Rock Interactions

The potential for mineral carbonation of CO2 in plutonic mafic rocks is addressed through a set of laboratory experiments on cumulate gabbro and gabbro-diorite specimens from the Sines Massif (Portugal). The experiments were conducted in an autoclave, for a maximum of 64 days, using a CO2 supersaturated brine under pressure and temperature conditions similar to those expected around an injection well during early-phase CO2 injection. Multiple techniques for mineralogical and geochemical characterization were applied ante- and post-carbonation experiments. New mineralogical phases (smectite, halite and gypsum), roughness increase and material loss were observed after exposure to the CO2 supersaturated brine. The chemical analysis shows consistent changes in the brine and rock specimens: (i) increases in iron (Fe) and magnesium (Mg) in the aqueous phase and decreases in Fe2O3 and MgO in the specimens; (ii) a decrease in aqueous calcium (Ca) and an increase in CaO in the cumulate gabbro, whereas in the gabbro-diorite aqueous Ca increased and afterwards remained constant, whereas CaO decreased. The geochemical model using the CrunchFlow code was able to reproduce the experimental observations and simulate the chemical behavior for longer times. Overall, the study indicates that the early-stage CO2 injection conditions adopted induce mainly a dissolution phase with mineralogical/textural readjustments on the external area of the samples studied.

Petrogenesis and tectonic significance of the early Paleozoic Delenuoer ophiolite in the Central Qilian Shan, northeastern Tibetan Plateau

The newly discovered early Paleozoic Delenuoer ophiolite, in the western margin of the Central Qilian Shan, is composed of serpentinized peridotite, cumulate gabbro, diabase, massive basalt, and pillow basalt. This study presents geochronological and geochemical data for the cumulate gabbro and basalt. LA–ICP–MS U–Pb dating of zircons from the cumulate gabbro yielded a magmatic crystallization age of 472 ​± ​4 ​Ma. The basalts have normal mid-ocean ridge basalt (N-MORB) compositions and a narrow range of εNd(t) values (+4.5 to ​+5.3), which indicates they were derived from a depleted mantle source. On the basis of regional geological constraints, it is proposed that the Delenuoer ophiolite is a westward extension of the South Ophiolite Belt (Yushigou–Youhulugou–Donggou–Dongcaohe Ophiolite Belt) in the North Qilian Shan. The Delenuoer ophiolite, along with the Gulangxia–Delenuoer fault, defines the westernmost part of the tectonic boundary between the North and Central Qilian Shan. This ophiolite may have formed during southward subduction of the Qilian Ocean slab during the early Paleozoic.

Petrogenesis and Lu–Hf Dating of (Ultra)Mafic Rocks from the Kutná Hora Crystalline Complex: Implications for the Devonian Evolution of the Bohemian Massif

Abstract The Lu–Hf isotope system and Sr–Nd–Hf–Os isotope systematics of mantle rocks are capable of unravelling the early processes in collision belts, especially in a hot subduction context where the Sm–Nd and U–Pb systems in crustal rocks are prone to resetting owing to high temperatures and interaction with melts during exhumation. To improve models of the Devonian–Carboniferous evolution of the Bohemian Massif, we investigated in detail mafic and ultramafic rocks (eclogite, pyroxenite, and peridotite) from the ultrahigh-pressure and ultrahigh-temperature Kutná Hora Crystalline Complex (KHCC: Úhrov, Bečváry, Doubrava, and Spačice localities). Petrography, multiphase solid inclusions, major and trace element compositions of rocks and minerals, and radiogenic isotopic data document contrasting sources and protoliths as well as effects of subduction-related processes for these rocks. The Úhrov peridotite has a depleted composition corresponding to the suboceanic asthenospheric mantle, whereas Bečváry and Doubrava peridotites represent lithospheric mantle that underwent melt refertilization by basaltic and SiO2-undersaturated melts, respectively. Multiphase solid inclusions enclosed in garnet from Úhrov and Bečváry peridotites represent trapped H2O ± CO2-bearing metasomatizing agents and Fe–Ti-rich melts. The KHCC eclogites either formed by high-pressure crystal accumulation from mantle-derived basaltic melts (Úhrov) or represent a fragment of mid-ocean ridge basalt-like gabbroic cumulate (Spačice) and crustal-derived material (Doubrava) both metamorphosed at high P–T conditions. The Lu–Hf age of 395 ± 23 Ma obtained for the Úhrov peridotite reflects garnet growth related to burial of the asthenospheric mantle during subduction of the oceanic slab. By contrast, Spačice and Doubrava eclogites yield younger Lu–Hf ages of ∼350 and 330 Ma, respectively, representing mixed ages as demonstrated by the strong granulite-facies overprint and trace element zoning in garnet grains. We propose a refined model for the Early Variscan evolution of the Bohemian Massif starting with the subduction of the oceanic crust (Saxothuringian ocean) and associated oceanic asthenospheric mantle (Úhrov) beneath the Teplá–Barrandian at ≥380 Ma, which was responsible for melt refertilization of the associated mantle wedge (Bečváry, Doubrava). This was followed by continental subduction (∼370–360 Ma?) accompanied by the oceanic slab break-off and incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle and subsequent coeval exhumation of mantle and crustal rocks at ∼350–330 Ma.

Petrographic and mineral-glass chemical dataset of igneous rock clasts from Early Oligocene Aveto-Petrignacola Formation (Northern Italy).

This dataset article contains petrographic and mineral-glass chemical data of igneous rock clasts from Early Oligocene Aveto-Petrignacola Formation (APF; Northern Italy). Methods for obtaining the dataset include optical microscopy, scanning electron microscopy and electron probe microanalysis. The APF volcanic rocks are basalts, basaltic andesites, andesites, dacites and rhyolites. Rare gabbroic cumulate nodules complete the dataset. Basalts are porphyritic, with calcic plagioclase (An72–92Ab7–27Or0–1), ferroan enstatite (En59–68Fs29–37Wo3–4) and augite (En38–39Fs18–20Wo41–44) phenocrysts, in a hypocrystalline groundmass made up of bytownite (An71–85Ab14–28Or1), augite (En37–38Fs19Wo43–44), ferroan enstatite (En62–68Fs30–35Wo1–4) and rare pigeonite (En46–50Fs37–42Wo7–17). The basaltic andesites are porphyritic to glomeroporphyritic with phenocrysts of zoned plagioclase (An44–67Ab32–55Or1), orthopyroxene, Mg-rich augite (En38–42Fs15–17Wo43–45), rare pargasite to edenite amphibole (Mg# 69–59) and very rare biotite in a hypocrystalline to holohyaline groundmass. Andesites are highly porphyritic with phenocrysts of plagioclase (An47–79Ab20–52Or0–1), pargasite to magnesio-hornblende (Mg# 72–67), Mg-rich augite (En43–46Fs12–17Wo41–43), subordinate ferroan enstatite (En68–74Fs23–29Wo3–4), biotite (Mg# 53) and Ti-magnetite (Usp29–41). Dacites (massive lavas and ignimbrites) are porphyritic, with phenocrysts and phenoclasts of plagioclase (An33–79Ab20–62Or0–4), calcic amphibole (Ti-pargasite, Mg-hornblende and edenite; Mg# 81–46), biotite (Mg# 67–56), very rare Mg-rich augite (En41–42Fs16–18Wo40–43) and resorbed quartz in hypohyaline to holohyaline groundmass with a dense mat of anhedral quartz, labradorite-andesine (An36–66Ab33–61Or1–4) and rare anorthoclase (An22Ab66Or12). Rhyolitic compositions have been found both as volcanic clasts (massive lava and ignimbrites) with andesine to oligoclase phenoclasts (An25–38Ab61–71Or1–4), quartz, biotite (Mg# 55–53) and Ti-magnetite (Usp18–77), and as interstitial glasses (residual melt drops) in other APF volcanic rocks. The cumulate nodules are olivine-gabbro and amphibole-gabbro/gabbronorite with a mineral paragenesis dominated by plagioclase (An41–73Ab26–57Or1–3), olivine (Fo68–72), Mg-rich augite to ferroan diopside (En41–45Fs12–15Wo42–45; Mg# 79–74), ferroan enstatite (En65–74Fs24–33Wo2–3; Mg# 76–68), magnetite (Usp15–28) and titanian pargasite (Mg# 67–65). The main cumulus phases are plagioclase, olivine and pyroxene, while intercumulus/postcumulus phases are titanian pargasite and magnetite. The dataset can be used to compare petrographic features and chemical compositions of calc-alkaline rocks emplaced in other subduction-related settings. Above all, it can represent a useful contribution in solving the problem linked to the identification of a hidden Early-Oligocene source of the thick volcaniclastic APF succession in the Alpine-Apennine belt geodynamic evolution.

The origin of low-MgO eclogite xenoliths from Obnazhennaya kimberlite, Siberian craton

The petrology, mineral major and trace-element concentrations, and garnet oxygen isotopic composition of low-MgO (11–16 wt%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites contain two types of compositionally distinct garnet: granular coarse garnet, and garnet exsolution (lamellae and fine-grained garnet) in clinopyroxene. The former record higher temperatures at lower pressures than the latter, which record the last stage of equilibrium at moderate pressure–temperature conditions 2.3–3.7 GPa and 855–1095 °C in the upper mantle at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like δ18O of the garnets (5.07–5.62‰) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO2 contents and depleted light REE ((Nd/Yb)N < 1) compared to modern oceanic gabbros, suggesting that they experienced partial melting. Positively inclined middle to heavy REE patterns ((Dy/Yb)N < 1) of the reconstructed bulk rocks mostly result from repeated partial melting in the eclogite stability field, based on melting model calculations. We, therefore, suggest that the Obnazhennaya low-MgO eclogites may represent the gabbroic section of subducted or foundered basaltic crust that underwent continued partial melting processes at high pressures, where garnet was the main residual phase.