Search

Early Jurassic mafic–ultramafic igneous rocks are sporadically exposed in the eastern part of northeastern China. Understanding their petrogenesis and geological implications is essential for elucidating the evolution of magma in subduction zones and the evolutionary history of the Paleo-Pacific Ocean. This study presents a detailed petrological, zircon U-Pb geochronological, and whole-rock geochemical analysis of the Jurassic hornblende gabbro in the Yanbian area of northeastern China. The emplacement age of the hornblende gabbro is constrained to 175.3 ± 1.6 Ma. All samples exhibit low SiO2 content, metaluminous properties, and geochemical characteristics indicative of a subduction environment. By integrating trace elemental ratios, such as Th/Sm, Th/Ce, Ba/La, Rb/Nb, Hf/Sm, Ba/Nb, and εNd(t), our data propose that the hornblende gabbro originated from a mixture of depleted mantle and approximately 10%–20% continental sediment, and that the magmatism is generated by processes involving metasomatic addition to the source mantle wedge by 'supercritical' fluids. Contrasting with several recent hypotheses, the magma of the Early Jurassic gabbro originated from the mantle wedge, incorporating contributions from sediment melts and 'supercritical' fluids, and formed during the westward subduction of the Paleo-Pacific Ocean. The Yanbian area in Jilin Province was primarily influenced by this westward subduction during the Early Jurassic. [ABSTRACT FROM AUTHOR]

The Early Proterozoic gabbros of the Velimyaki intrusion of the Northern Ladoga region contain titanomagnetite ore, which has been mined as early as the end of the 19th century. Titanomagnetite horizons are enriched in phosphorus in form of apatite reaching 10 vol %. Isotopic Pb–Pb dating indicates that apatite was likely redeposited during superimposed metamorphism that was significantly separated in time from the magmatic stage of gabbros and clinopyroxene–titanomagnetite ores. Mineralogical, petrological, and isotope-geochemical criteria for the superimposed nature of the mineral formation with apatite recrystallization are the relationship of this mineral with the formation of other metamorphic minerals (hornblende, biotite, sodic plagioclase), the isotopic age of apatite (1790 ± 5 Ma), and the lower temperature (620–710°C) of its formation compared to the crystallization temperatures (900–1260°C) of magmatic minerals. The Pb–Pb age of apatite coincides with the age of metamorphic minerals from other rocks of Late Svecofennian region, as well as with the Rb–Sr ages of biotite and amphibole from host supracrustal rocks. Based on the data obtained, it was concluded that recrystallization of apatite and resetting of the U–Pb system occurred during the Late Svecofennian regional metamorphism. [ABSTRACT FROM AUTHOR]

Amphibole is an important mineral during the differentiation of arc magmas but rarely as a phenocryst in arc lavas or eruptive pyroclastic rocks. The Sanggeda complex, intruded into the ophiolite of the Indus–Yarlung Zangbo Suture Zone (IYZSZ), Zedong, southern Tibet, mainly consists of amphibole-rich, fine-grained, and porphyritic gabbros. The complex provides an opportunity to study the differentiation of arc magmas through amphibole crystals. Four distinct amphibole crystal populations can be recognized according to petrographic observations, EMPA, and LA–ICP–MS analysis. The first ones (Type 1) are fined-grained and euhedral, are crystallized during ascent, and are the product of the shallow emplacement of host magma. The second ones (Type 2) are euhedral, with slight negative Eu and Sr anomalies, and crystallize from an evolved magma that previously experienced plagioclase fractionation. Type 3 amphiboles have similar morphological characteristics to Type 2 but are without Eu and Sr anomalies. Type 4 crystals are shown as pseudomorphs, formed by the reaction–replacement between the clinopyroxene and melt. Type 1 crystals are autocrysts. Other amphiboles within host magma, whether presented as phenocrysts or cumulate nodules, are antecrysts. Based on the amphibole crystal population developed in the complex, in this study, a trans-crustal magma plumbing system is proposed, containing at least three magma reservoirs located at different crust depths: the shallow emplaced crust (~4.8 km), the mid-crust (~12.9 km), and the lower crust (~21.8–24.9 km). Early amphibole crystallization is an effective process to generate silicic residual melts. Gravity could help in that sense. Precursor amphibole and clinopyroxene can efficiently delaminate back into the mantle and promote the generation of silicic continental crust. [ABSTRACT FROM AUTHOR]

Amphibole is an important mineral during the differentiation of arc magmas but rarely as a phenocryst in arc lavas or eruptive pyroclastic rocks. The Sanggeda complex, intruded into the ophiolite of the Indus–Yarlung Zangbo Suture Zone (IYZSZ), Zedong, southern Tibet, mainly consists of amphibole-rich, fine-grained, and porphyritic gabbros. The complex provides an opportunity to study the differentiation of arc magmas through amphibole crystals. Four distinct amphibole crystal populations can be recognized according to petrographic observations, EMPA, and LA–ICP–MS analysis. The first ones (Type 1) are fined-grained and euhedral, are crystallized during ascent, and are the product of the shallow emplacement of host magma. The second ones (Type 2) are euhedral, with slight negative Eu and Sr anomalies, and crystallize from an evolved magma that previously experienced plagioclase fractionation. Type 3 amphiboles have similar morphological characteristics to Type 2 but are without Eu and Sr anomalies. Type 4 crystals are shown as pseudomorphs, formed by the reaction–replacement between the clinopyroxene and melt. Type 1 crystals are autocrysts. Other amphiboles within host magma, whether presented as phenocrysts or cumulate nodules, are antecrysts. Based on the amphibole crystal population developed in the complex, in this study, a trans-crustal magma plumbing system is proposed, containing at least three magma reservoirs located at different crust depths: the shallow emplaced crust (~4.8 km), the mid-crust (~12.9 km), and the lower crust (~21.8–24.9 km). Early amphibole crystallization is an effective process to generate silicic residual melts. Gravity could help in that sense. Precursor amphibole and clinopyroxene can efficiently delaminate back into the mantle and promote the generation of silicic continental crust.

Late Paleozoic–early Mesozoic intrusive rocks are distributed widely along the northern margin of the Qaidam Basin in the northern Tibetan Plateau. To constrain the tectonic evolution, we carried out petrological, chronological, and geochemical studies of the Chahannuo gabbros. LA-ICP-MS Zircon U–Pb dating yields an age of 255.0 ± 0.9 Ma for the gabbros, which confirms the existence of Indosinian tectono-magmatic activity on the northern margin of the Qaidam Basin. The Chahannuo gabbros have low whole-rock SiO2, Fe2O3 contents, and high Al2O3 contents, which suggests a calc-alkaline affinity. In addition, the gabbros have high MgO, Cr, and Ni contents and Mg#, similar to those predicted of the regional basaltic melts, and indicating that they were affected mainly by fluid from the subducted slab. The Chahannuo gabbros are characterized by arc-like trace element patterns, with enrichment in LREE and LILE, and depletion in HREE and HFSE. No obvious negative Eu anomalies also indicate that no significant magmatic differentiation has occurred. The low Nb/La ratio and Ti content in gabbros samples suggests that the Chahannuo gabbros were partially contaminated by the crust during their formation. The Chahannuo gabbros have high incompatible element ratios (Rb/Sr, Th/Nd, and Th/La), which are closer to the category of enriched mantle. Combing our data with previous data from contemporaneous magmatism in the region, we suggest that the Chahannuo gabbros formed in a continental arc environment related to the northward subduction of the Paleo-Tethyan oceanic plate. [ABSTRACT FROM AUTHOR]

Introduction: The North Qilian orogenic belt, as the Northern branch of the original Tethys tectonic domain, is important for reconstructing the tectonic evolution of the ancient Tethys. However, the tectonic history of the North Qilian orogenic belt remains controversial. This study addresses this issue from a geochemical perspective.Methods: In this study, a comprehensive analysis of the geochronology, whole-rock geochemistry, clinopyroxene mineral geochemistry, zircon Ti crystallization temperature, and gabbromagma temperature and pressure in the Yushigou ophiolite of the North Qilian orogenic belt was conducted to provide constraints on its tectonic evolution.Results and Discussion: Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating results reveal that the gabbros have ages of 519 ± 3 Ma and 495 ± 4 Ma, belonging to the Cambrian period. Most of the studied gabbros exhibited geochemical characteristics of tholeiitic basaltic rocks with normal mid-ocean ridge basalt and island arc tholeiite dual geochemical affinities. The gabbros are interpreted to have formed by a high degree of partial melting of the depleted mantle spinel lherzolite. These results suggest that the back-arc basin of the North Qilian tectonic belt may have evolved to a relatively mature stage from 519 to 495 Ma. Overall, this study contributes to our understanding of the tectonic evolution of the North Qilian orogenic belt through geochemical analyses.

The Abu Ghalaga pluton in the southern eastern desert of Egypt (SED) consists of undeformed norite and hornblende gabbro intruding island arc metavolcanics, ophiolitic rocks, and syn-tectonic older granitic rocks. The Fe-Ti ore (mainly ilmenite) occurs in the form of lenses at the base of the norite. Based on microtextures, mineralogy, and chemical composition, three types of ilmenites are recognized: massive high-Mg type, disseminated high-Mg type, and disseminated low-Mg ilmenite. The three ilmenite types are chemically distinguished by their MgO, MnO, TiO2, Fe2O3, FeO, and V2O3 contents. The variations in the concentration of these oxides are interpreted as a function of variations in T, fO2, and the composition of the parental magma. Reconstruction of the orthomagmatic conditions shows that T and fO2 of the magma from which the norite and its Fe-Ti deposit formed decrease during the course of magmatic differentiation, whereas the hornblende gabbro and its Fe-Ti mineralization have formed from a more evolved magma under relatively lower T and fO2. The REE patterns and variation diagrams support a model of crystal fractionation as the primary means of formation of the Fe-Ti deposits. Chemical features also suggest that the parent magma of both types of gabbroic rocks was generated in the mantle, but in two different pulses. Subduction zone, MORB, and WPB chemical signatures for these gabbroic rocks suggest a complex tectonic history consistent with compression followed by within plate extension possibly contributing to the development of a back-arc/intra-arc basin. [ABSTRACT FROM AUTHOR]

The annular Richat Structure, in Mauritania, is among the most striking geological features on Earth visible from space. Nicknamed "the Eye of Africa", its intriguing concentric shaping has drawn attention for several decades. Formerly hypothesized as an astrobleme, it is now consensually recognized as a complex igneous intrusion, involving two ring-like gabbroic bodies, a central breccia and carbonatite dykes. However, the coexistence of until now undated tholeiitic gabbros and 99 Ma carbonatites akin to alkaline magmatism remained enigmatic. We provide the first plagioclase 40Ar/39Ar age determination and new geochemical data on the gabbroic bodies. Although no robust age was obtained, numerical modelling of our results suggests that the gabbros were intruded between 230 and 200 Ma. This age bracket is compatible with the ∼200 Ma Central Atlantic Magmatic Province (CAMP). Moreover, the compositions of these tholeiitic gabbros match the two chemical groups of the CAMP the best represented in northwest Africa. Therefore, we argue that the Richat gabbros correspond to two CAMP sills intruded conformably into the Late Proterozoic/Lower Paleozoic sedimentary sequences of the Taoudenni basin. About 100 My later, the gabbros and their country rocks were locally uplifted by an alkaline intrusion (expressed now on surface by the carbonatites), resulting in a circular doming, 40 km in diameter. The subsequent erosion led to the actual flattened concentric structure. Therefore, the Richat Structure represents a two-stage igneous-history, separated by a time lapse of ∼100 My, resulting in two CAMP gabbroic sills mimicking ring-like bodies. • The annular Richat structure is one of the most striking geological features on Earth. • Magmatic rocks are present as carbonatites and tholeiitic gabbros. • We provide the first plagioclase 40Ar/39Ar age determination on gabbros. • Gabbros were intruded ≈ 200My ago, contemporaneously with the CAMP. • The Richat structure formed during two magmatic events separated by ≈100 My. [ABSTRACT FROM AUTHOR]

As a natural laboratory for the study of crustal growth and accretionary tectonics, the Lhasa terrane was once a microcontinent with a Precambrian basement, which was transformed into a juvenile terrane by remarkable Phanerozoic crustal reworking and growth. Here, we report the geochronological, mineralogical, and geochemical data of the Numa hornblende gabbros and Dongga cumulate hornblende gabbros in the southern Lhasa subterrane (SLS). Zircon U-Pb dating yields ages of 163.7 ± 2.2 Ma and 182.8 ± 2.2 Ma for the Numa hornblende gabbros and Dongga cumulate hornblende gabbros, respectively, which suggests generation in the Early-Middle Jurassic. Both the Numa hornblende gabbros and Dongga cumulate hornblende gabbros are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), and they have positive εHf(t) (+11.1 to +17.3) and εNd(t) (+3.6 to +6.0) values. Whole-rock geochemical and isotopic data indicate that their primary magma was derived by the partial melting of a depleted mantle wedge metasomatized by hydrous fluids. Our data, combined with previous research results, indicate that the Numa-Dongga gabbros formed in a continental margin arc setting related to the northward subduction of the Neo-Tethys oceanic slab. In addition, comprehensive studies reveal that the Early Jurassic to Late Cretaceous were the most important periods for crustal growth in the SLS and crustal thickening in the Mesozoic was concentrated mainly in the Early Jurassic (ca. 190–170 Ma) and early Late Cretaceous (ca. 100–80 Ma), which is related to the magmatic flare-up induced by slab roll-back. The recycling of subducted oceanic crust and the underplating of mantle-derived magmas play important roles in the growth and evolution of the continental crust in the SLS. [ABSTRACT FROM AUTHOR]

The Manzanillo Plutonic Complex (MPC) forms part of the Mexican Cordilleran plutonic belt at ~ 19°N. A special feature of the MPC is more than 125 km2 outcrops of gabbroic rocks, thus constituting the largest body of this lithology along the belt. The overall MPC composition is heterogeneous including gabbros and granitoids with or without hybridization structures and minor alkali-rich granites. In contrast to other Cordilleran plutons, MPC intrusives are often strongly deformed and foliated heterogeneous structures with evidence of magma mingling and mixing. U-Pb emplacement and Rb-Sr and K-Ar cooling ages are between 59 and 73 Ma. Four U-Pb gabbro ages from 63 to 65 Ma define the peak of the hybridization event. Although geochemical analyses display considerable differences within MPC members, their Sr-Nd isotopic signatures are highly homogeneous with magmatic primitive εNd parameters between +5 and +6 and low 87Sr/86Sr around 0.7035. Geothermobarometric data provide information for a hot environment for the gabbroic pulse at 3.4–4.9 Kb and >900°C, indicative for depths ~15 km. On the other hand, granitoids show typical Cordilleran characteristics with lower temperatures and shallow emplacement conditions. The MPC shows a unique petrogenetic evolution in the Cordilleran arc; there is no evidence for a relation to other events such as the Early Cretaceous Alisitos arc. [ABSTRACT FROM AUTHOR]

The Hacımahmutuşağı area (Aksaray/Turkey) is located in the western part of the Central Anatolian Crystalline Complex (CACC). Gneiss and marble compose the basement units, while intrusive rocks are gabbros and granitoids. The pegmatitic hornblende gabbros contain pegmatitic to fine-grained hornblendes, plagioclase, clinopyroxene, and accessory opaque minerals. The fine-grained gabbros, on the other hand, are composed of plagioclase, hornblende, and biotite as major components whereas the apatite and opaque minerals are present in accessory content. Granitic– granodioritic rocks are the common intrusive rock types in the area, and constitute quartz, orthoclase, plagioclase and biotite, and accessory zircon and opaque minerals. Leucogranites, comprising quartz, orthoclase, plagioclase with minor biotite, hornblende, and with accessory apatite and opaque minerals, are found as dykes intruding the marble and the granitic–granodioritic rocks. Strontium–neodymium isotope data of gabbros and granitoids have high 87Sr/86Sr(i) ratios (0.7076 to 0.7117) and low ɛNd(i) values (−5.0 to −9.8) point out enriched source and pronounced crustal contribution in their genesis. In the Hacımahmutuşağı area, it is plausible that the heat increase caused by the hot zone, which was generated by underplating mafic magma along with the hydrous mafic sills in the lower crust, might have resulted in partial melts from crystallized mafic sills and older crustal rocks. It can be suggested that these hybrid melts adiabatically rose to the shallow crust, ponded and crystallized there and formed the magma source of the intrusive rocks within the Hacımahmutuşağı area and the other hybrid granitic rocks with crustal signatures in the CACC. Geochemical data indicate that granitoids and gabbros are collision to post-collision related sub-alkaline rocks derived from an enriched source with extensive crustal inputs.

At oceanic spreading centers, the interactions between the igneous system that builds the crust, and the hydrothermal system that cools it govern the plumbing system architecture and its thermokinetic evolution. At fast‐spreading centers, most of those interactions occur around the axial magma lens (AML) that feeds the upper crust, and possibly part of the underlying mushy igneous reservoir. Heat extracted from crystallizing AML is transferred through a conductive boundary layer to the overlying hydrothermal system. Quantifying the AML physical and thermal evolutions and its interactions with hydrothermal system is therefore essential to understand oceanic accretion. Those general issues were the rationale of drilling ICDP OmanDP Hole GT3A, and we present herein the geological, structural, and petrological data that were used as a site survey to select its location. GT3 area enables observations in three dimensions of fossilized AMLs and overlying dikes. The new field data and corresponding mineral compositions are used together with thermokinetic and thermodynamic models to deliver an integrated dynamic model for the AML/hydrothermal system interactions. Results attest that the isotropic gabbro interval is composite, with gabbro bodies intruding and reheating both gabbros and dikes (up to 1,040°C). We show that AMLs should be considered as transient igneous bodies that likely crystallize from primitive MORBs in decades, releasing heat to the intruded hosts, and feeding high temperature vents on the seafloor. We show for the first time that the thermal gradient recorded in AML roof is consistent with the heat fluxes reported at active hydrothermal vents. Key Points: At fast‐spreading ridges magma‐hydrothermal interactions are extensive at roof of axial magma lenses (AML) that are fed by primitive MORBAML are transient igneous bodies that provide heat to hydrothermal vents on the seafloor through conductive boundary layersAML crystallize to form the isotropic gabbros in decades, and released heat modifies the intruded dike and gabbro hosts [ABSTRACT FROM AUTHOR]

The early Paleozoic tectonic evolution of the Xing'an–Mongolian Orogenic Belt is dominated by two oceanic basins on the northwestern and southeastern sides of the Xing'an Block, i.e., the Xinlin–Xiguitu Ocean and the Nenjiang Ocean. However, the early development of the Nenjiang Ocean remains unclear. Here, we present zircon U–Pb geochronology and whole-rock elemental and Sr–Nd isotopic data on the gabbros in the Xinglong area together with andesitic tuffs and basalts in the Duobaoshan area. LA-ICP-MS zircon U–Pb dating of gabbros and andesitic tuffs yielded crystallization ages of 443–436 ​Ma and 452–451 ​Ma, respectively. The Early Silurian Xinglong gabbros show calc-alkaline and E-MORB affinities but they are enriched in LILEs, and depleted in HFSEs, with relatively low U/Th ratios of 0.18–0.36 and ε Nd (t) values of −1.6 to +0.5. These geochemical features suggest that the gabbros might originate from a mantle wedge modified by pelagic sediment-derived melts, consistent with a back-arc basin setting. By contrast, the andesitic tuffs are characterized by high MgO (>5 ​wt.%), Cr (138–200 ​ppm), and Ni (65–110 ​ppm) contents, and can be termed as high-Mg andesites. Their low Sr/Y ratios of 15.98–17.15 and U/Th values of 0.24–0.25 and moderate (La/Sm) n values of 3.07–3.26 are similar to those from the Setouchi Volcanic Belt (SW Japan), and are thought to be derived from partial melting of subducted sediments, and subsequent melt-mantle interaction. The Duobaoshan basalts have high Nb (8.44–10.30 ​ppm) and TiO 2 contents (1.17–1.60 ​wt.%), typical of Nb-enriched basalts. They are slightly younger than regional adakitic rocks and have positive ε Nd (t) values of +5.2 to +5.7 and are interpreted to be generated by partial melting of a depleted mantle source metasomatized by earlier adakitic melts. Synthesized with coeval arc-related igneous rocks from the southeastern Xing'an Block, we propose that the Duobaoshan high-Mg andesitic tuffs and Nb-enriched basalts are parts of the Late Ordovician and Silurian Sonid Zuoqi–Duobaoshan arc belt, and they were formed by the northwestern subduction of the Nenjiang Ocean. Such a subduction beneath the integrated Xing'an–Erguna Block also gave rise to the East Ujimqin–Xinglong igneous belt in a continental back-arc basin setting. Our new data support an early Paleozoic arc–back-arc model in the northern Great Xing'an Range. Image 1 • A ~452–451 ​Ma association of high-Mg andesitic tuffs and Nb-enriched basalts were identified in the Duobaoshan. • Such an association revealed continental arc magmatism related to northwestern subduction of the Nenjiang Ocean. • The subduction gave rise to ~443–436 ​Ma Xinglong gabbros in a back-arc basin setting. • An early Paleozoic arc–back-arc system existed in the northern Great Xing'an Range. [ABSTRACT FROM AUTHOR]

Svecofennian orogen that contain ultramafic rocks. New samples were collected from the picritic and basaltic rocks as well as spatially associated gabbroic rocks, and their major and trace element compositions are presented and discussed here. Although the metavolcanic rocks have experienced primary alteration and two metamorphic events, elements that are insensitive to alteration (rare earth elements, Zr, Nb, Ni) have been used to infer their source and evolution. Based on the similar shapes of the rare earth element patterns in the metabasalt and metapicrite, basaltic melt derived from picritic one by fractional crystallization. The high Ni and Mg contents, Ni/MgO and Zr/Nb ratios, and multielement patterns make a slightly enriched garnet lherzolite a likely source for the metapicrite. With the exception of synvolcanic gabbros within the metavolcanic rocks, the gabbroic intrusions at Salittu have no genetic link to the metavolcanic rocks. Geochemical comparison with modern basalts suggests that the picritic and basaltic melts were generated in a mature arc environment during a rifting event. Picritic melt rose from convective mantle to the crust and formed a magma chamber. Basaltic melt fractionated in the chamber and extruded upon an earlier formed volcanic pile as basalt and synvolcanic gabbro. Soon after extrusion of the basalt, picritic melts, similar in composition to the earlier picrite, rose through the crust and extruded on top of the basalt. Comparison with three other metapicrite occurrences in southern Finland suggests that although the occurrences may be considered broadly comagmatic, each had their specific sources and probably also tectonic environments during emplacement.

Because of the relatively low metamorphic grade, low strain and well-preserved early structures in volcanic and volcaniclastic rocks, the Orijärvi triangle (in the center of the larger Orijärvi area) is one of the few locations in the Svecofennian orogen of southern Finland where lithostratigraphy has been determined. The geochemistry of the picritic- basaltic metavolcanic rocks of the Salittu Formation, located in the northeastern part of the Orijärvi area, has been characterized but otherwise the bedrock and structures have been barely touched. After remapping we give an interpretation of structural evolution, provide new age data to constrain duration of volcanism at Salittu, and refine the stratigraphy in the Orijärvi area. The original stratigraphy is visible at Salittu: metabasalt overlies migmatitic gneisses, and metapicrite is on top. The rocks were folded during early Svecofennian D 1 –D 2 deformations, and the large synformal structures developed as D 2 –D 5 interference structures formed during late Svecofennian D 5 deformation. The structural pattern at Salittu is much the same as in the Orijärvi triangle. The new age data, combined with earlier published data, constrains the Salittu volcanism at ca. 1875 Ma. The stratigraphy in the Orijärvi area consists of the early (1.90–1.89 Ga) volcanic Orijärvi Formation, overlain by the sedimentary Vetio Formation, the volcanic Kisko Formation, the volcanic-sedimentary Ahdisto Formation, the volcanic Toija Formation, and on top the Salittu Formation, all emplaced at 1.88–1.87 Ga. We propose a model in which the Orijärvi Formation represents magmatism at the margin of a microcontinent, and the overlying package represents sedimentation and magmatism above a subduction zone during an initial stage of microcontinental accretion. D 1 deformation occurred in an advanced stage of accretion, after emplacement of the volcanic rocks of the Salittu Formation.

The data on the distribution of rare earth elements (REE) in peridotites - abyssal (oceanic) and orogenic (continental), in mafic granulites of the base of the continent crust and gabbro of the base of the oceanic crust, in the oceans (MORB) and continental flood basalts (CFB) are compared. Two types of komatiites - Archean aluminum depleted komatiites (ADK) and Paleoproterozoic Aluminum Undepleted (ANK) have different characteristics of the content and distribution of REE: ADK have a chondritic distribution of REE; ANK-distribution with depletion of light rare earth elements (LREE). Such differences are inherited from the ANK and ADK in continental and oceanic rock groups. Given the early Precambrian formation of the ADK and the ANK, it can be assumed that the differences in the distribution of REE are inherited by the post-Cambrian igneous rocks of the continents and oceans from the early Precambrian. As a consequence, the spatial location of the continents and oceans could in that case remain stable (fixed), beginning with the early Precambrian. [ABSTRACT FROM AUTHOR]

Sr isotope analysis of plutonic rocks is an important method for deriving the provenance and age of ancient magmatic episodes, and the petrogenetic processes operating during intrusive activity. We determined the 87 Sr/ 86 Sr isotopic compositions of fresh and variably altered feldspars from the 1.14 Ga gabbroic Great Abitibi Dyke, Ontario, Canada, through the use of Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICPMS). The data show an alteration trend between fresh, low-Rb plagioclase with 87 Sr/ 86 Sr ratios of 0.7045 and high-Rb alkali feldspar with 87 Sr/ 86 Sr ratios of 0.7059, which may be used to constrain the 87 Sr/ 86 Sr ratio of the mantle source. This trend is incompatible with simple mixing and instead has age significance. As post-intrusive alteration through hydrothermal circulation starts immediately following emplacement, LA-ICPMS analysis allows rapid estimation of initial 87 Sr/ 86 Sr ratios and intrusive age, suitable for sample screening and selection for more precise dating. For the Great Abitibi Dyke, the Rb-Sr age estimate of emplacement of 1.10 ± 0.23 Ga (2σ) is concordant with the precise U-Pb emplacement age of the dyke (1140.6 ± 2.0 Ma). The most altered high-Rb alkali feldspars return younger ages, suggesting that post-intrusive alteration lasted several hundred million years. This study highlights the need for microanalysis to differentiate between alteration and magmatic processes on basis of Sr isotopic signatures, and outlines a rapid method to determine initial 87 Sr/ 86 Sr ratios and to estimate Rb-Sr ages of plutonic rocks in terranes with long histories of multiple intrusive magmatic episodes. [ABSTRACT FROM AUTHOR]

The Xiangshan mafic-ultramafic complex is one of the major Early Permian mafic-ultramafic intrusions in eastern Tianshan (Xinjiang, NW China), and consists of two major intrusive phases. The first intrusive phase is mainly gabbroic rocks hosting ilmenite mineralization, while the second intrusive phase is mainly lherzoilite associated with Ni-Cu sulfide mineralization. The Xiangshan ilmenite orebodies hosted in the Fe-Ti oxide-bearing gabbro occur along the contact between hornblende gabbros and leucogabbros. The hornblende gabbros and Fe-Ti oxide rich gabbros at Xiangshan are newly dated to be Early Permian (280.1 and 279.2 Ma, respectively). Major and trace element compositions of zircons and whole rocks from Xiangshan hornblende gabbro and Fe-Ti oxide gabbro have been measured by in situ excimer laser ablation ICP-MS. Zircon Ce4+/Ce3+ ratios based on lattice-strain model and Ti-in-zircon temperatures of hornblende gabbro and Fe-Ti oxide gabbro of the Xiangshan complex are calculated to evaluate the physicochemical variations during the ilmenite mineralization. Whole-rock geochemistry and zircon trace element geochemistry suggest that Fe-Ti oxide gabbros were formed from a basaltic parent magma which had undergone a transfromation from being H2O-rich to H2O-poor. During the magmatic evolution, primitive, H2O-poor basaltic melts may have been replenished into the system, increasing its solidus temperature and decreasing its oxygen fugacity and H2O contents. This may have supperessed the Ti-rich poikilitic hornblende fractionation and promoted the plagioclase fractionation, which consequently concentrated the ore-forming components in the residual melts and generated the ilmenite mineralization. [ABSTRACT FROM AUTHOR]

Efforts to map the vertical distribution of mafic and ultramafic igneous rocks in the Earth's crust and uppermost mantle have long been hampered by the lack of precise geobarometers for the appropriate mineral assemblages. The average P (avP) method (Powell and Holland 1994) is a multiple-reaction approach that uses a least-squares minimization to average the pressures derived from individual mineral equilibria, taking into account both their uncertainties and correlations. We applied average P to a carefully selected database of published phase-equilibrium experiments in dry to H2O-saturated, andesitic to basaltic and peridotitic systems at P = 0.6-9.3 kbar, T = 940-1240 °C, with log fO2 from NNO-2.6 to NNO+3.6 log units (where NNO is nickel-nickel oxide buffer). We made minor modifications to the thermodynamic models of clinopyroxene, spinel, and olivine to improve the accuracy and precision of the results given by the avP method. Tests on the experimental database, using the modified thermodynamic models and spinel + clinopyroxene + olivine + plagioclase equilibria, showed that average P can reproduce the experimental P, within the calculated 1s uncertainties (0.9-2.6 kbar; 1.6 kbar on average), for 67% of the database. No systematic deviations of the calculated pressure (P) with temperature (T) or mineral compositions are observed. Given the large compositional range of the experimental database, these results suggest that the method can be applied to any gabbroic, pyroxenitic, or peridotitic rocks that contain the appropriate phase assemblage clinopyroxene + olivine + plagioclase ± spinel. For assemblages equilibrated at P < 5 kbar, the calculated P shows a slight dependence on T, which therefore needs to be well constrained to keep the overall P uncertainties as low as possible. T can be estimated using either available independently calibrated geothermometers or a simple calculation routine suggested in this work. Application of average P to gabbroic xenoliths from Dominica, Lesser Antilles, and to gabbroic and peridotitic xenoliths from Wikieup, Arizona, demonstrates the ability of the method to produce precise P estimates for natural assemblages equilibrated at both mid- and lower crustal conditions, respectively. Depending on the errors on mineral composition, appropriateness of the T estimate, and attainment of equilibrium of the assemblage, P uncertainty for natural rocks is ≤1.0 kbar. Such a level of precision can help to discriminate between rival petrogenetic processes in subduction zone, intra-plate, and mid-oceanic ridge settings. [ABSTRACT FROM AUTHOR]

Centimeter-size intrusions of gabbros and plagiogranites occur in mantle peridotites of the Myitkyina ophiolite, Myanmar. The gabbros mainly consist of plagioclase and clinopyroxene, whereas orthopyroxene occasionally occurs. The plagiogranites are mainly composed of plagioclase, quartz and amphibole, with small amount of accessory minerals, such as zircon, apatite and rutile. Plagioclase in the gabbros varies from andesine to anorthite (An 37–91 ), whereas plagioclase in the plagiogranites is less calcic (An 1–40 ). Clinopyroxene in the gabbros is pervasively altered to hornblende. The gabbros contain 42.97–52.88 wt% SiO 2 , which show negative correlations with Al 2 O 3 , CaO and MgO, but positive correlations with Na 2 O, P 2 O 5 and TiO 2 . Microtextural relations reveal the crystallization of clinopyroxene prior to plagioclase in the Myitkyina gabbros. This suggests that the gabbros were crystallized from hydrous melts, which is also supported by the occurrence of orthopyroxene and anorthitic plagioclase in some gabbros. The gabbros have slightly enriched Sr-Nd isotopes, with initial 87 Sr/ 86 Sr ratios of 0.703938–0.706609 and ε Nd(t) values of + 2.4–+7.2, and relatively variable Hf isotopes, with ε Hf (t) values of + 13.4–+24.9. A subduction component is required to explain the decoupled Nd-Hf isotopes of the gabbros. Binary mixing suggests that addition of ca 2% subducted sediments to a depleted mantle can account for the Nd-Hf decoupling. Therefore, both petrological and geochemical data of the gabbros support that the Myitkyina ophiolite was originated in a supra-subduction zone setting. The plagiogranites have compositions of tonalites and trondhjemites, containing 56.93–77.93 wt% SiO 2 , 1.27–10.79 wt% Na 2 O and 0.05–0.71 wt% K 2 O. They are slightly enriched in LREE over HREE and display positive anomalies in Eu, Zr, Hf but negative Nb anomalies. Very low TiO 2 contents (0.03–0.2 wt%) of the plagiogranites suggest that they were not products of fractional crystallization of MORB but were generated by hydrous melting of gabbros. This process is also supported by the nearly constant contents of both La and Yb with increasing SiO 2 of the plagiogranites. The Sr-Nd-Hf isotopes of the plagiogranites indicate the involvement of crustal component in their genesis. We proposed that the Myitkyina plagiogranites were generated by partial melting of the mafic protoliths in the subducted slab. [ABSTRACT FROM AUTHOR]

The Salittu Formation is one of the few metavolcanic successions in the Svecofennian orogen that contain ultramafic rocks. New samples were collected from the picritic and basaltic rocks as well as spatially associated gabbroic rocks, and their major and trace element compositions are presented and discussed here. Although the metavolcanic rocks have experienced primary alteration and two metamorphic events, elements that are insensitive to alteration (rare earth elements, Zr, Nb, Ni) have been used to infer their source and evolution. Based on the similar shapes of the rare earth element patterns in the metabasalt and metapicrite, basaltic melt derived from picritic one by fractional crystallization. The high Ni and Mg contents, Ni/MgO and Zr/Nb ratios, and multielement patterns make a slightly enriched garnet lherzolite a likely source for the metapicrite. With the exception of synvolcanic gabbros within the metavolcanic rocks, the gabbroic intrusions at Salittu have no genetic link to the metavolcanic rocks. Geochemical comparison with modern basalts suggests that the picritic and basaltic melts were generated in a mature arc environment during a rifting event. Picritic melt rose from convective mantle to the crust and formed a magma chamber. Basaltic melt fractionated in the chamber and extruded upon an earlier formed volcanic pile as basalt and synvolcanic gabbro. Soon after extrusion of the basalt, picritic melts, similar in composition to the earlier picrite, rose through the crust and extruded on top of the basalt. Comparison with three other metapicrite occurrences in southern Finland suggests that although the occurrences may be considered broadly comagmatic, each had their specific sources and probably also tectonic environments during emplacement. [ABSTRACT FROM AUTHOR]

Using the SIMS, ID-TIMS and Sm-Nd isotopic methods and the electron microprobe, we have studied several differentiated mafic intrusions of the c. 2.2 Ga gabbro-wehrlite association (GWA) from four Paleoproterozoic schist belts and the Archean Kuhmo Greenstone Belt. Back-scattered electron images and electron microprobe analyses revealed that zircon crystals vary from well-preserved to turbid and highly altered with individual grains often displaying irregular, hydrated, CaO-bearing domains. In the most pristine domains, suitable for establishing the crystallization ages, SIMS 207Pb/206Pb ages fall in the range of 2210–2220 Ma, which is consistent with the most concordant ID-TIMS U-Pb ages. One of the studied intrusions that had previously yielded a conventional U-Pb date of less than 2.0 Ga, could be shown by spot analysis to belong to the 2.2 Ga family. In contrast to the well-preserved zircon domains, altered domains exhibit a variable and often strong U-Pb discordance up to 70 % and have distinctly lower 207Pb/206Pb ages. Some zircon grains record isotopic resetting at the time of the Svecofennian orogeny (ca. 1.8–1.9 Ga), while the most discordant ones project in the concordia diagram to late Paleozoic lower intercept ages indicating a relative recent Pb loss. The mineral chemistry of zircon suggests that the leakage of radiogenic Pb can be ascribed to an open-system behavior related to hydrothermal alteration via action of CaCl2-bearing fluids. Common albitization of plagioclase in the GWA intrusions has caused this mineral to behave as an open system with regard to the Sm-Nd isotopic systematics. Despite this uncertainty, our Nd isotopic data indicate that the magma that produced the GWA intrusions in various parts of northern and eastern Finland was isotopically homogeneous and had an initial εNd(2220 Ma) value of c. +0.6 precluding significant upper crustal contamination upon emplacement and subsequent fractional crystallization.

Guleman ofiyolitine ait gabro ve plajiyogranitlerin petrografik ve jeokimyasal özellikleri belirlenmiş olup, gabroların dalma-batma zonu üstündeki bazik kökenli kayaçlar olabileceği, gabroların içerisinde yer alan plajiyoklazca zengin lökokratik kayaçların (plajiyogranit) ise; magma odasındaki, bazik bir magmanın fraksiyonel kristalleşmesi ile oluşan ayrımlaşma ürünü olduğu sonucuna varılmıştır.

Zircons from a felsic volcanic rock in the Toija Formation and a synvolcanic gabbro intrusion in the Salittu Formation within the Orijärvi area were dated by U-Th-Pb SIMS in order to provide depositional constraints on these formations. Zircon crystals from the felsic rock preserve a two-stage crystallisation history with zoned core domains and homogeneous rim domains. Inner domains yield a 1878 ± 4 Ma concordia age, interpreted to determine the crystallisation of this rock. Rims yield a 1815 ± 3 Ma concordia age interpreted to determine the regional metamorphism. Small rounded zircon grains from the Salittu gabbro, located within the Jyly shear zone, yield a concordia age of 1792 ± 5 Ma. We interpret the grain textures to suggest that they recrystallised from inherited zircon seeds during the heat and fluid flow into the shear zone. Although no direct ages for the Salittu Formation have been recovered, field relationships imply that it was deposited between 1878−1875 Ma.

Sm-Nd data were determined for eight mafic-ultramafic intrusions from the Svecofennian (1.88 Ga) Kotalahti Nickel Belt, Finland. The intrusions represent both mineralized and barren types and are located at varying distances from the Archaean/Proterozoic boundary. The samples for the 23 Sm-Nd isotope analyses were taken mostly from the ultramafic differentiates. Results show a range in initial εNd values at 1880 Ma from -2.4 to +2.0. No relationship can be found between the degree of Ni mineralization and initial εNd values, while a correlation with the geological domain and country rocks is evident. The Majasaari and Törmälä intrusions, which have positive εNd values, were emplaced within the Svecofennian domain in proximity to 1.92 Ga tonalitic gneisses, which have previously yielded initial εNd values of ca. +3. In contrast, the Luusniemi intrusion, which has an εNd value of -2.4 is situated close to exposed Archaean crust. Excluding two analyses from the Rytky intrusion, all data from the Koirus N, Koirus S, Kotalahti, Rytky and Kylmälahti intrusions, within error limits, fall in the range -0.7 ± 0.3. The results support the concept of contamination by Archaean material in proximity to the currently exposed craton margin. The composition of the proposed parental magma for the intrusions is close to EMORB, with initial εNd values near +4.

Prakasam Igneous Province (PIP) is an important geological domain in the Eastern Dharwar Craton (EDC), found in the junction zone between the EDC and Eastern Ghat Mobile Belt (EGMB). The Pedda Cherlo Palle (PCP) gabbros are massive, leucocratic-mesocractic, and show cumulus textures with minerals plagioclase, cpx, and amphiboles. Compositionally, plagioclase is a labradorite-bytownite, cpx is diopside to augite, olivines are hyalosiderites and amphiboles are magnesiohornblendes. PCP gabbros have normal SiO2, high Al2O3, moderate to high TiO2, Na2O and medium Fe2O3, so, classified as subalkaline tholeiitic gabbros. Fractionated rare earth element (REE) patterns, high abundance of large ion lithofile elements (LILE) and transitional metals coupled with light REE (LREE) relative enrichment over heavy REE (HREE) and Nb are characteristics of partial melting of depleted mantle and melts that have undergone fractional crystalisation. These partial melts are enriched in LREE and LILE, due to the addition of slab derived sediment and fluids. PCP gabbros contain low abundance (5.1 to 24.6 ng/g) of platinum group elements (PGE), and show an increase in the order Ir>Os>Pt>Ru»Pd>Rh. We propose that the subduction related intraoceanic island arc might have accreted to the southeastern margin of India to the east of Cuddapah basin in a collisional regime that took place during Ur to Rodinia amalgamations.

Because of the relatively low metamorphic grade, low strain and well-preserved early structures in volcanic and volcaniclastic rocks, the Orijärvi triangle (in the center of the larger Orijärvi area) is one of the few locations in the Svecofennian orogen of southern Finland where lithostratigraphy has been determined. The geochemistry of the picriticbasaltic metavolcanic rocks of the Salittu Formation, located in the northeastern part of the Orijärvi area, has been characterized but otherwise the bedrock and structures have been barely touched. After remapping we give an interpretation of structural evolution, provide new age data to constrain duration of volcanism at Salittu, and refine the stratigraphy in the Orijärvi area. The original stratigraphy is visible at Salittu: metabasalt overlies migmatitic gneisses, and metapicrite is on top. The rocks were folded during early Svecofennian D1-D2 deformations, and the large synformal structures developed as D2-D5 interference structures formed during late Svecofennian D5 deformation. The structural pattern at Salittu is much the same as in the Orijärvi triangle. The new age data, combined with earlier published data, constrains the Salittu volcanism at ca. 1875 Ma. The stratigraphy in the Orijärvi area consists of the early (1.90-1.89 Ga) volcanic Orijärvi Formation, overlain by the sedimentary Vetio Formation, the volcanic Kisko Formation, the volcanic-sedimentary Ahdisto Formation, the volcanic Toija Formation, and on top the Salittu Formation, all emplaced at 1.88-1.87 Ga. We propose a model in which the Orijärvi Formation represents magmatism at the margin of a microcontinent, and the overlying package represents sedimentation and magmatism above a subduction zone during an initial stage of microcontinental accretion. D1 deformation occurred in an advanced stage of accretion, after emplacement of the volcanic rocks of the Salittu Formation. [ABSTRACT FROM AUTHOR]

The Cretaceous Xigaze ophiolite is best exposed at the central part of the Yarlung–Zangbo Suture Zone, Tibet Plateau. It consists of a thick section of mantle peridotites, but a relatively thin mafic sequence. This study presents geochronological and geochemical data for intrusive dykes (both mafic and felsic) and basalts to revisit the formation setting of the Xigaze ophiolite. The rodingites are characterized by high CaO and low Na 2 O contents relative to mafic dykes and show big variations in trace element compositions. Both gabbros and diabases have similar geochemical compositions, with MgO contents of 6.42–11.48 wt% and Mg# of 0.56–0.71. They display REE patterns similar to N-MORB and are variably enriched in large ion lithophile elements. Basalts have fractionated compositions and display LREE-depleted patterns very similar to N-MORB. They do not show obvious enrichment in LILE and depletion in high-field-strength elements, but a negative Nb anomaly is present. The studied plagiogranites have compositions of trondhjemite to tonalite, with high Na 2 O and low K 2 O contents. They have low TiO 2 contents less than 1 wt%, consistent with melts formed by anatexis of gabbros rather than by differentiation of basalts. Zircons from seven samples, including three rodingites, three plagiogranites, and one gabbro, have been dated and yielded U–Pb ages of 124.6 ~ 130.5 Ma, indicating the Xigaze ophiolite was formed during the Early Cretaceous. They have mantle-like δ 18 O values of + 4.92 ~ + 5.26‰ and very positive ε Hf (t) values of + 16 ~ + 13.3. Ages of the rodingites and less altered gabbros indicate that serpentinization was occurred at ~ 125 Ma. Occurrence of both gabbroic and diabase dykes within the serpentinites suggests that the mantle lithosphere of the Xigaze ophiolite was rapidly exhumed. Both mafic and felsic dykes have slightly more radiogenic 87 Sr/ 86 Sr ratios relative to MORB, but depleted Hf–Nd isotpe compositions. They have a limited range of ε Nd (t) values of + 7.9 ~ + 8.9 but variable ε Hf (t) values ranging from + 9.9 to + 16.7, which are similar to the global MORB. This indicates that the intrusive dykes within the Xigaze ophiolite were derived from a depleted mantle source, which has not been obviously affected by recycling of subducted materials. The MOR-type basalts in this study, combined with the basalts with SSZ signatures previous reported in the literature indicate the diversity of basalts in the Xigaze ophiolite. We apply the forearc hyperextension model to reconcile the occurrence of both MOR- and SSZ-type basalts in the Xigaze ophiolite. In this model, the SSZ-type basalts were produced by melting of the metasomatized mantle wedge during exhumation, whereas the MOR-type basalts were derived from the upwelling asthenosphere triggered by forearc hyperextension. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic Misi region forms the northeastern part of the Peräpohja Schist Belt in northern Finland. The area comprises mafic volcanic land sedimentary rocks, differentiated gabbros, and late-orogenic granitoids. Three geochemically different mafic volcanic units were recognised: LREE-depleted amygdaloidal lavas, slightly LREE-enriched lavas, and mafic tuffs that have a flat REE pattern. Sedimentary rocks include arkosites, mica gneisses, dolomitic marbles, quartzites, tuffites, mica schists, calc-silicate rocks and graphite-bearing schists. Two types of gabbros were identified: one with a LREE-enriched pattern and another with flat REE pattern. The age of the former is according to Perttunen and Vaasjoki (2001) 2117 ±4 Ma, whereas there is no age determination for the latter. The granitoid intrusions belong to the ca. 1800 Ma late-orogenic group of the Central Lapland Granitoid Complex. The geochemistry and the stable isotope data on mafic lavas and dolomitic marbles show similarities with the mafic volcanic rocks and marbles of the lower plart of the Kivalo group in the western part of Peräpohja Schist Belt. Peak metamorphic conditions in the region vary from upper-greenschist to upper-amphibolite facies. Three major stages of deformation were distinguished: N-S compressional D1 with ductile deformation, NE-SW compressional D2 with ductile to brittle-ductile deformation, and E-W compressional D3 with brittle deformation. Several magnetite occurrences are known in the region and four of those have been mined for iron. The ores are mainly composed of magnetite with minor haematite pyrite, chalcopyrite and bornite. Besides iron the ores contain small amounts of P, S and V as well as trace amounts of Cu, Co, Te and Au. The magnetite bodies are hosted by skarnoids within the ca. 2220-2120 Ma dolomitic marble-quartzite sequence, and highly differentiated, intensely albitised, LREE-enriched gabbro. Multistage and -type alteration is characteristic for the entire region. The styles of alteration in the region are: scapolitisation, regional and local abitisation, sericitisation and silicification associated with a major shear zone, and late carbonatisation and carbonate veining associated with brecciation of the ores and their wall rocks. Local intense albitisation and formation of skarnoids and magnetite ores took place during the pre-D1 or D1 faulting or shearing which postdate the intrusion of 2120 Ma gabbros. The iron was mobilized from the mafic to intermediate country rocks and/or marble sequence which possibly contained sedimentary iron formation. Regional alteration with the local intense albitisation and ore formation show similar features to the iron oxide-copper-gold type deposits, although the known deposits in the Misi region only contain trace amounts of gold and copper.

The Svecofennian layered Soukkio Complex (1.87 Ga) in Mäntsälä, southern Finland, consists of layered tholeiitic gabbro and porphyritic calc-alkaline monzonite, quartz monzonite and granite, mingled together. The gabbro belongs to a group of ten mafic-ultramafic intrusions of Mäntsälä, part of the 150 km long and 20 km wide, linear, E-W trending Hyvinkää–Mäntsälä Gabbroic Belt (HMGB), representing syn-collisional magmatism. Structures and textures related to magma mingling and mixing occur in a 1– 2 km wide zone around Lake Kilpijärvi, located at the center of the Soukkio Complex. The complex is compositionally stratified and consists of four zones: its base, found at the Western Zone, is a dynamically layered gabbro. The following tonalite is probably a result of magma mixing. Felsic amoeboid layers and pipes, alternating with or cutting the fine-grained gabbro in the Central- Western Zone, resemble those of mafic-silicic layered intrusions in general. Mafic magmatic enclaves (MMEs) and pillows form the South-Central Zone and disrupted synplutonic mafic dykes or sheets intruded the granite in the Eastern Zone. The MMEs and disrupted synplutonic mafic dykes or sheets show cuspate and chilled margins against the felsic host, quartz ocelli, corroded K-feldspar xenocrysts with or without plagioclase mantles, and acicular apatite, all typical features of magma mingling and mixing. Mixing is suggested by intermediate composition of MMEs between granitoid and gabbro, as well as by their partly linear trends in some Harker diagrams. REE composition of the MMEs is similar to that of the Soukkio Gabbro, as expected for granite hosted MMEs. The model proposed for evolution of the Soukkio Complex involves intrusion of mafic magma into the crust, causing its partial melting. This generated granitic magma above the mafic chamber. Injections of mafic magma invaded the felsic chamber and those magmas interacted mainly by intermingling. Mingling and mixing of contrasting magmas is also found elsewhere in the Mäntsälä region and HMGB, suggesting a widespread synorogenic zone of coeval mafic-silicic plutonism and crust-mantle interaction during the Svecofennian Orogeny in Finland.

Plagiogranites (albitite and albite granite dikes/lenses) occur in the western section of the Zhaheba ophiolite, middle part of the Eastern Junggar orogen. The Zhaheba albitites (498.0 ± 5.8 Ma) and albite granites (494.6 ± 6.9 Ma) were roughly coeval and with distinct petrographic textures, i.e., cumulus texture for the albitites and granitic texture for albite granites. The Zhaheba plagiogranites are Na-enriched and LILE-depleted, resembling typical plagiogranites. Geochemical characteristics and cumulus texture of the albitites indicate that they may have been formed by early stage accumulation of albite, whereas the albite granites may have been the products of the residual magma consolidation. The ε Nd ( t ) values of the albitites and albite granites vary in the ranges of 7.2–7.7 and 6.4–7.3, respectively, whereas the ε Hf ( t ) values vary in the ranges of 11.5–17.9 and 9.8–13.9, respectively. Isotopic compositions and Zr/Hf ratios of the Zhaheba plagiogranites are similar to those of typical MORB, implying a genetic relationship with the oceanic crust. The low TiO 2 , Nb, Ta content, LREE enrichment and elevated ( 87 Sr/ 86 Sr) i values of the Zhaheba plagiogranites indicate that the rocks were likely derived from the anatexis of amphibolites, which were related to hydrothermal alteration of gabbros in intra-oceanic backarc basin. U–Pb geochronology of the Zhaheba plagiogranites indicates that it is more reasonable to connect the Zhaheba–Armantai and Karamay ophiolites to the Zaysan collision zone. [ABSTRACT FROM AUTHOR]

Heterogeneous zircon populations from four different localities within the Svecofennian area in Finland demonstrate that the ca. 1890 Ma old syntectonic granitoids contain older material, probably belonging to the ca. 1950 Ma Svecofennian protocrust, the influence of which is also evident in metaturbidites deformed by the Svecokarelian orogeny.