High-pressure ultramafics in the lower crustal rocks of the Pekul’ney complex, central Chukchi Peninsula. 1. Petrography and mineralogy

Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul’ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610–680°C and 9–14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul’ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al₂O₃. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them.     

Composition and geodynamic setting of the volcanic rocks from ophiolites of the Ust’-Belaya Mountains,Chukchi Peninsula

In addition to ophiolites in the structure of the Otrozhnaya sheet, the igneous rocks were established within the Middle Devonian-Lower Carboniferous tuffaceous-terrigenous complex earlier considered to be the cover of the ophiolite association. In order to establish their geodynamic formation setting, the geochemical study of igneous rocks was conducted. The volcanic rocks from the ophiolite complex are similar to MORB; subvolcanic rocks of tuffaceous-terrigenous complex have a suprasubduction origin. An abundance of pyroclastic rocks and the type of sediments allow us to conclude about their formation in an island arc setting. The existence of the Middle Devonian-Lower Carboniferous island arc complex within the Ust’-Belaya Mountains gives rise to continue the Koni-Taigonos arc inside the region and testifies to its subsistence in the Devonian.     

Textural evolution of perovskite in the Afrikanda alkaline–ultramafic complex,Kola Peninsula,Russia

Perovskite is a common accessory mineral in a variety of mafic and ultramafic rocks, but perovskite deposits are rare and studies of perovskite ore deposits are correspondingly scarce. Perovskite is a key rock-forming mineral and reaches exceptionally high concentrations in olivinites, diverse clinopyroxenites and silicocarbonatites in the Afrikanda alkaline–ultramafic complex (Kola Peninsula, NW Russia). Across these lithologies, we classify perovskite into three types (T1–T3) based on crystal morphology, inclusion abundance, composition, and zonation. Perovskite in olivinites and some clinopyroxenites is represented by fine-grained, equigranular, monomineralic clusters and networks (T1). In contrast, perovskite in other clinopyroxenites and some silicocarbonatites has fine- to coarse-grained interlocked (T2) and massive (T3) textures. Electron backscatter diffraction reveals that some T1 and T2 perovskite grains in the olivinites and clinopyroxenites are composed of multiple subgrains and may represent stages of crystal rotation, coalescence and amalgamation. We propose that in the olivinites and clinopyroxenites, these processes result in the transformation of clusters and networks of fine-grained perovskite crystals (T1) to mosaics of more coarse-grained (T2) and massive perovskite (T3). This interpretation suggests that sub-solidus processes can lead to the development of coarse-grained and massive perovskite. A combination of characteristic features identified in the Afrikanda perovskite (equigranular crystal mosaics, interlocked irregular-shaped grains, and massive zones) is observed in other oxide ore deposits, particularly in layered intrusions of chromitites and intrusion-hosted magnetite deposits and suggests that the same amalgamation processes may be responsible for some of the coarse-grained and massive textures observed in oxide deposits worldwide.     

Keketuohai mafic–ultramafic complex in the Chinese Altai,NW China: Petrogenesis and geodynamic significance

Devonian magmatism was very intensive in the tectonic evolutionary history of the Chinese Altai, a key part of the Central Asian Orogenic Belt (CAOB). The Devonian Keketuohai mafic–ultramafic complex in the Chinese Altai is a zoned intrusion consisting of dunite, olivine gabbro, hornblende gabbro and pyroxene diorite. The pyroxene diorite gives a zircon U–Pb age of 409 ± 5 Ma. Variations in mineral assemblage and chemical composition suggest that the petrogenesis of the Keketuohai Complex was chiefly governed by fractional crystallization from a common magma chamber. Low SiO₂, K₂O and Na₂O contents, negative covariations between P₂O₅, TiO₂ and Mg# value suggest insignificant crustal assimilation/contamination. Thus the positive ε_Nd(t) values (0 to + 2.7) and slight enrichments in light rare earth elements (e.g., La/Yb_N = 0.98–3.64) suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle. Model calculation suggests that their parental magma was high-Mg (Mg# = 66) tholeiitic basaltic melt. The Keketuohai intrusion was coeval with diverse magmatism, high temperature metamorphism and hydrothermal mineralization, which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the Chinese Altai.     

Geochemistry and lithology of accretionary wedge in the Pekul’nei range, central Chukotka

Based on the study of various facies of ancient and modern oceanic sediments, the Ce/La versus Zr/Y criterion is proposed to discriminate the principal genetic groups of sedimentary rocks in foldbelts and to determine their geodynamic formation conditions. The structure and composition of siliceous, clayey and fineclastic tuffaceous rocks from various complexes of paleoaccretionary wedge in the Pekul’nei Range (Chukotka) are discussed. Lithological and geochemical studies of sedimentary rocks in the Pekul’nei Range allowed us to trace the regular variations in rock sections depending on (1) propagation of oceanic plate from the spreading center toward the convergent boundary for paleooceanic rock complexes and (2) sedimentation regime and provenances for rocks deposited above the convergent plate boundary.     

Geochronology and geochemistry of the Nantianwan mafic–ultramafic complex,Emeishan large igneous province: metallogenesis of magmatic Ni–Cu sulphide deposits and geodynamic setting

The Nantianwan mafic–ultramafic complex is situated in the northwest part of the Panxi district, southwest China. It consists predominantly of gabbros, gabbronorites, and lherzolites. LA–ICP–MS U–Pb zircon dating of the gabbronorites yields an age of 259.7 ± 0.6 million years, consistent with the ages of other mafic–ultramafic intrusions in the Emeishan large igneous province (ELIP). Gabbronorites and lherzolites host Cu–Ni sulphide ores. Cumulus texture is pronounced in these rocks, containing magnesium-rich olivine (up to 81.4% forsterite). SiO₂ contents of the lherzolites range from 42.93 to 44.18 wt.%, whereas those of the gabbronorites vary between 44.89 and 52.76 wt.%. Analysed samples have low rare earth element (REE) contents (23.22–30.16 ppm for lherzolites and 25.21–61.05 ppm for gabbronorites). Both lherzolites and gabbronorites have similar chondrite-normalized REE patterns, suggesting that they are comagmatic. All samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs, e.g. Nb, Ta, and Ti), very similar to those of ocean island basalts (OIBs). The presence of cumulus textures and geochemical signatures indicates that fractional crystallization played an important role in the petrogenesis of these rocks. Initial (⁸⁷Sr/⁸⁶Sr) _t (t?=?260 Ma) ratios and ?_Nd(t) values of the mafic–ultramafic suite vary from 0.70542 to 0.70763, and??0.4 to 1.7, respectively. Compared to the Cu–Ni-bearing Baimazhai and Limahe intrusions in the ELIP, which were considerably contaminated by variable crustal materials, the Nantianwan complex exhibits much lower (⁸⁷Sr/⁸⁶Sr) _t . Their ?_Nd(t) versus (Th/Nb)_PM ratios also indicate that the ore-bearing magmas did not undergo significant crustal contamination. In combination with (Tb/Yb)_PM versus (Yb/Sm)_PM modelling, we infer that the magmas originated from an incompatible elements-enriched spinel-facies lherzolite that itself formed by interaction between the Emeishan plume and the lithospheric mantle. Most plots of NiO versus Fo contents of olivine suggest that sulphides are separated from the parental magma by liquid immiscibility, which is also supported by bulk-rock Cu/Zr ratios of the lherzolites (7.04–102.67) and gabbronorites (0.88–5.56). We suggest that the gabbronorites and lherzolites experienced undersaturation to oversaturation of sulphur; the latter may be due to fractional crystallization in a high-level magma chamber, accounting for the sulphide segregation.     

Geochemistry of Early Cretaceous volcanic rocks in the Northeastern Great Xing’an Range,northeast China and implication for geodynamic setting

ABSTRACT

The mechanism that triggered large-scale Late Mesozoic magmatism in the northeastern Great Xing’an Range (NE GXAR) is strongly controversial. In this paper, we present whole rock geochemistry and zircon trace element, U-Pb and Hf isotopic data on the volcanic rocks in the Longjiang and Guanghua formations in the northeastern Xing’an Block. Zircons with ages of 120–119 Ma indicate that these volcanic rocks were formed in the Early Cretaceous. Combined with previous data, it is clear that volcanic rocks in the NE GXAR erupted between 128 and 108 Ma. The andesite samples of the Longjiang Formation show high contents of Al₂O₃, CaO, and MgO, significant negative Nb, Ta, and Ti anomalies; ε_Hf (t) values of zircons from the andesite sample vary from +4.13 to +7.67, indicating an enriched mantle source. The rhyolites of the Guanghua Formation show high SiO₂ and K₂O concentrations, low P₂O₅, MgO, Cr, and Ni contents and Mg# values. The positive ε_Hf (t) values (+5.72 to +10.58) with two-stage Hf model ages ranging from 939 to 701 Ma indicate that the rhyolites are derived from the partial melting of basaltic lower crust. Combined with the regional geological evolution, we conclude that the generation of the Early Cretaceous volcanic rocks in the NE GXAR might be triggered by the dehydration, disintegration, and foundering of the Mongol-Okhotsk Oceanic flat-slab and the subsequent upwelling of the asthenosphere.     

Geochemistry,petrogenesis and tectonic setting of Neoproterozoic mafic–ultramafic rocks from the western Jiangnan orogen,South China

The Jiangnan orogenic belt (JOB) has been interpreted as a suture zone between the Yangtze craton and Cathaysian terranes in South China. The Neoproterozoic mafic–ultramafic rocks are extensively exposed in the western JOB, providing an ideal opportunity to study the Neoproterozoic assembly and tectonic evolution of South China. We present integrated field and geochemical studies including LA-ICP-MS zircon U–Pb dating, and whole-rock major and trace element and Sm–Nd isotope analyses of the Neoproterozoic mafic–ultramafic rocks exposed in the northern Guangxi Province, South China. Geochronological results show that the magmatic events took place in two distinct periods: the early Neoproterozoic (861–834 Ma) and the late Neoproterozoic (770–750 Ma). Early Neoproterozoic ultramafic rocks of the Sibao Group have positive ε_Nd(t) values (+ 2.7 to + 6.6) whereas mafic rocks exhibit negative ε_Nd(t) values (− 5.8 to − 0.9). The basaltic rocks show TiO₂ contents of 0.62–0.69 wt.% and Mg-number of 59–65, and also display an enrichment of light rare earth elements (LREEs) and pronounced negative Nb, Ta and Ti anomalies on chondrite- and primitive mantle-normalized diagrams, consistent with subduction-related geochemical signatures. Late Neoproterozoic rocks of the Danzhou Group show ε_Nd(t) values (− 1.23 to + 3.19) for both ultramafic and mafic rocks. The basaltic rocks have TiO₂ contents of 1.01–1.33 wt.% and Mg-number of 57–60, and have a mixture of MORB- and arc-like geochemical affinities, inferred to have formed in an extensional arc environment. Geochemical signatures suggest that all rock types in this study were derived from subarc mantle wedge sources and underwent various degrees of crustal contamination. Thus, we suggest that subduction may have continued to ca. 750 Ma in the western JOB, implying that the amalgamation event between the Yangtze craton and Cathaysian terranes was later than 750 Ma.     

Petrogeochemical conditions and geodynamic settings of volcanic rocks in the Kiselyovka–Manoma accretionary complex (Russian Far East)

The Kiselyovka–Manoma accretionary complex formed at the end of the Early Cretaceous during subduction of the Pacific oceanic plate underneath the Khingan–Okhotsk active continental margin along the east of Eurasia. It is composed of Jurassic–Early Cretaceous oceanic chert, siliceous mudstone, and limestone that include a significant amount of basic volcanic rocks. The known and newly obtained data on the petrogeochemistry of the Jurassic and Early Cretaceous basalt from various parts of the accretionary complex are systemized in the paper. Based on the comprehensive analysis of these data, the possible geodynamic settings of the basalt are considered. The petrogeochemical characteristics provide evidence for the formation of basalt in different parts of the oceanic floor within the spreading ridge, as well as on oceanic islands far from the ridge. The basalts of oceanic islands are mostly preserved in the accretionary complex. The compositional variations of the basalts may be controlled by the different thickness of the oceanic lithosphere on which they formed. This is explained by the varying distances of the lithosphere from the spreading zone.     

U–Pb SHRIMP zircon geochronology,petrogenesis, and tectonic setting of the Neoproterozoic Baekdong ultramafic rocks in the Hongseong Collision Belt,South Korea

The Hongseong area, located in the western Gyeonggi Massif, South Korea, can be correlated with the northern margin of the South China block (Yangtze Craton). This area experienced Neoproterozoic igneous activity related to subduction before the amalgamation of Rodinia. Several isolated, lenticular, and serpentinized ultramafic–mafic bodies occur in the Hongseong area. The Baekdong body, one of the largest ultramafic bodies, has been highly deformed and metamorphosed to eclogite- and granulite-facies. The petrogenesis and tectonic environment of the Baekdong rocks are assessed using the composition of unaltered cores of spinel and olivine grains, and show that these rocks represent the mantle section of a suprasubduction ophiolite. The rocks originated from oceanic lithosphere that formed during the transition from nascent back-arc to mature island arc, related to subduction roll-back. During the back-arc stage, Al-rich spinel harzburgite formed through melt–rock interaction caused by the intrusion of magma. This magma was produced in small amounts, by less than 10% of partial melting of the wedge mantle. Subsequently, during the mature island arc stage, Cr-rich spinel dunite formed through melt–rock interaction caused by the intrusion of relatively evolved magma that formed by 30–35% partial melting due to a high input of volatiles from the subducted slab and sediments. The Baekdong ultramafic rocks, together with the Bibong ultramafic rocks, indicate that a suprasubduction tectonic setting prevailed before the amalgamation of Rodinia (at 860–890 Ma) in the Hongseong area, which may be an extension of the northern margin of the Yangtze Craton.     

The Volch’etundrovsky Massif of the autonomous anorthosite complex of the Main Range, the Kola Peninsula: Geological, petrogeochemical, and isotope-geochronological studies

The Volch??etundrovsky Massif occupies the middle part of the autonomous anorthosite complex of the Main Range, has a sheet morphology and marks the tectonic suture between the Kola block and the Belomorian mobile belt. The massif is characterized by homogenous structure and consists of the volumetrically dominant Main Zone including leucogabbro, leucogabbronorites, and anorthosites, and Marginal Zone made up of leuconorites and gabbronorites with subordinate plagioclasites and orthopyroxenites. Chemically, the rocks of the Volch??etundrovsky Massif are ascribed to the normal (tholeiitic and calc-alkaline) petrochemical series with typomorphic high Al₂O₃ contents (11.71?C29.32 wt %). With Al₂O₃ increase in the leuconorite-anorthosite series, the SiO₂ and TiO₂ contents show weak variations, CaO and alkalis insignificantly increase, whereas the MgO and FeO contents sharply decrease. The rocks of the Volch??etundrovsky Massif reveal significant REE fractionation and increase in total REE content in the leuconorite-anorthosite series, most approximating the Paleoproterozoic (Sumian) anorthosites of the Kola region. The anorthosites and leucogabbro are characterized by flat HREE, while the leuconorites is strongly depleted in HREE due to garnet fractionation. All rocks of the massif have significant positive Eu anomalies caused by the plagioclase accumulation. Zircons are characterized by LREE depletion and enrichment in HREE. This defines the steep positive slope of the plots complicated by the negative Eu and positive Ce (in zircons from leucogabbro) anomalies, which is typical of the REE distribution patterns in the unaltered zircons from igneous rocks. In zircons from anorthosites, the Ce anomaly is weak to absent. The trace-element distribution in the rocks of the Volch??etundrovsky Massif show positive Ba, Ta, Pb, Sr, Sc, and V anomalies, being controlled by the mineral specifics of the massif and the presence of definite accessory minerals. New U-Pb zircon data on the rocks of the Volch??etundrovsky Massif indicate that the leuconorites from the Marginal Zone were formed 2473 ± 7 Ma and 2463 ± 2.4 Ma ago, and the leucogabbro from the Main Zone, 2467 ± 8 Ma. These rocks have negative ?_Nd(T) from -1.54 up to -3.10, which indicates their derivation from enriched mantle reservoir variably contaminated by crustal material. The anorthosites of the Main Zone define an U-Pb age of 2407 ± 3 Ma and ?_Nd(T) = ?3.78, which presumably reflect the timing of hydrothermal-metasomatic alterations in the upper part of the magmatic chamber accompanied by significant crustal contamination.     

Seismic interpretation of crustal structure in the flinders‐mt lofty ranges and gulf regions,South Australia

There appears to be little correlation of earthquake epicentres with known surface geological features in South Australia. Seismic wave travel‐time residuals are used to derive corrections for the velocity and depth parameters for the simple uniform crustal model which approximates to that in South Australia. Local studies of Moho depth in the seismic zone and analysis of travel‐time station corrections from both local earthquake and teleseismic data suggest that lateral and vertical variations in the South Australian crust are small. Data presented in this paper appear to be consistent with a plate tectonic model derived from focal mechanism studies (Stewart & Mount, 1972) for the active South Australian seismic zones.     

Age and nature of the Jurassic–Early Cretaceous mafic and ultramafic rocks from the Yilashan area,Bangong–Nujiang suture zone,central Tibet: implications for petrogenesis and tectonic Evolution

The Jurassic–Early Cretaceous Yilashan mafic–ultramafic complex is located in the middle part of the Bangong–Nujiang suture zone, central Tibet. It features a mantle sequence composed of peridotites and a crustal sequence composed of cumulate peridotites and gabbros that are intruded by diabases with some basalts. This article presents new whole-rock geochemical and geochronological data for peridotites, gabbros, diabases and basalts to revisit the petrogenesis and tectonic setting of the Yilashan mafic–ultramafic complex. Zircon laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) U–Pb ages of three diabase samples are 169.6 ± 3.3 Ma, 132.5 ± 2.5 Ma, and 133.6 ± 4.9 Ma, respectively. These ages together with previous studies indicate that the Yilashan mafic–ultramafic complex probably formed during the Jurassic–Early Cretaceous. The peridotites exhibit nearly U-shaped REE patterns and are distinct from abyssal peridotites. The diabase and basalt samples show arc features with selective enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILEs; e.g. Rb, U, and Sr) and depletion in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti). The gabbro samples display cumulate features with selective enrichment in LILEs (e.g. Rb, Ba, and Sr) but depletion in LREEs and HFSEs (e.g. Nb, Zr, and Ti). Combing the positive ε_Nd(t) values (+6.1 to +10.0) and negative zircon ε_Hf(t) values (–16.5 to –11.7 and –13.6 to –0.4) with older Hf model ages for the mafic rocks, these signatures suggest that the Yilashan mafic and ultramafic rocks likely originated from an ancient lithospheric mantle source with the addition of asthenospheric mantle materials and subducted fluids coupled with limited crustal contamination in a continental arc setting as a result of the southward subduction of the Bangong–Nujiang Tethys Ocean beneath the Lhasa terrane during the Jurassic–Early Cretaceous.     

Geochemistry of Silurian–Carboniferous sedimentary rocks of the Ulaanbaatar terrane,Hangay–Hentey belt,central Mongolia: Provenance,paleoweathering, tectonic setting,and relationship with the neighbouring Tsetserleg terrane

The Hangay–Hentey belt is situated in the central Northern Mongolia, and forms part of the Central Asian Orogenic Belt (CAOB). It is internally subdivided into seven terranes, the largest of which are the neighbouring Ulaanbaatar and Tsetserleg terranes. These coeval terranes are mainly composed of Silurian–Devonian accretionary complexes and Carboniferous turbidites. Proposals for their depositional setting range from passive margin through to island arc. A suite of 19 Ulaanbaatar terrane sandstones and mudrocks (Gorkhi and Altanovoo Formations) were collected with the aim of constraining their provenance, source weathering, and depositional setting based on whole-rock major and trace element data, and for comparison with the neighbouring Tsetserleg terrane. New REE analyses were also made of 35 samples from the Ulaanbaatar and Tsetserleg terranes. Geochemically the Ulaanbaatar sandstones are classed as wackes, and most of the mudstones as shales. Geochemical parameters suggest an immature source, similar to that of the Tsetserleg terrane. Geochemical contrasts between sandstones and mudrocks in the Ulaanbaatar sediments are small, and trends on element – Al₂O₃ variation diagrams are weak. Comparison with average upper continental crust (UCC), major element discriminant scores, and immobile element ratios (Th/Sc, Zr/Sc, Ce/Sc, Ti/Zr) indicate a uniform average source composition between dacite and rhyolite. Maximum Chemical Index of Alteration value in the Ulaanbaatar terrane is ∼65 after correction for K-metasomatism, indicating minimal weathering in a tectonically active source, similar to that of the Tsetserleg terrane. REE data in both terranes show moderate LREE enrichment and flat HREE segments, with negative Eu anomalies somewhat less than those in UCC and PAAS. Chondrite-normalized patterns are very similar to that for average Paleozoic felsic volcanic rock, supporting the relatively felsic source indicated by immobile trace element ratios. Tectonic setting discriminants (K₂O/Na₂O–SiO₂/Al₂O₃, La–Th–Sc, Th–Sc–Zr) indicate an evolved continental island arc (CIA; A2) environment for both terranes, similar to several other CAOB suites of similar age. This common arc source was situated within the Mongol-Okhotsk Ocean during Silurian–Lower Carboniferous time. The present-day Aleutian arc is a possible modern analogue of the depositional setting.     

Fluid regime of platinum group elements (PGE) and gold-bearing reef formation in the Dovyren mafic–ultramafic layered complex, eastern Siberia, Russia

The Dovyren layered dunite–troctolite–gabbro massif (northern Transbaikalia region, Russia) contains precious metal mineralization related to sparsely disseminated sulfides (Stillwater type). The distribution of gases trapped in micro-inclusions and intergranular pores of the Dovyren massif has been investigated. This type of study had previously only been undertaken on the traps or peridotite–pyroxenite–norite intrusions hosting copper–nickel sulfide deposits. A novel method of analyzing trapped gases, involving the grinding of samples under high vacuum at room temperature, was employed. A modified gas-chromatography and mass-spectrometry approach was used to analyze the composition of the extracted gases. The concentrations of reduced gases (CH₄ and H₂) are higher in inclusions trapped by silicate minerals, whereas oxidized gases (H₂O, CO₂) are less common. The content of reduced gases (H₂, CH₄, CO), N₂, He, radiogenic Ar, and C₂H₆ increases upward through the layered series of the massif. The distribution of all gases, especially methane and hydrogen, show peak concentrations coincident with the PGE and gold reef type horizons. A correlation of the gas peaks and noble metal contents appears to be related to their geochemical affinities. This conclusion is supported by the experimental modeling. Received: 4 August 1999 / Accepted: 13 January 2000     

Petrogenesis of Early Cretaceous volcanic rocks of the Manketouebo Formation in the Wuchagou region,central Great Xing’an Range,NE China,and tectonic implications: geochronological,geochemical, and Hf isotopic evidence

ABSTRACT

This study presents new whole-rock major and trace element geochemistry, zircon U–Pb ages, and Hf-isotope compositions for volcanic rocks from the Manketouebo Formation of the central Great Xing’an Range, NE China. These data provide precise ages and information on the petrogenesis and source of the magmas that formed this formation, furthering our understanding of the geodynamic setting of the large-scale late Mesozoic magmatism in the Great Xing’an Range and other areas in NE China. The Manketouebo Formation in the study area is dominated by rhyolites and rhyolitic tuffs with minor trachydacites. The LA-ICP-MS zircon U–Pb dating indicates that these volcanic rocks formed between 143 and 139 Ma. The volcanic rocks contain high silica (66.70–79.91 wt.%) and total alkali (5.93–9.72 wt.%) concentrations, and low concentrations of MgO (0.08–1.15 wt.%), total FeO (0.68–4.50 wt.%), and CaO (0.10–2.56 wt.%). They are enriched in large-ion lithophile elements (LILEs; e.g. Rb, Th, and U) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs; e.g. Nb, Ta, Ti, and P) and heavy rare earth elements (HREEs), indicating that they are similar to highly fractionated I-type igneous rocks. All of the magmatic zircons from the analysed samples have high initial ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282900–0.283093), positive ε_Hf(t) values (7.48–14.19), and young Hf two-stage model ages (954–344 Ma) that suggest the primary magma that formed the volcanic rocks of the Manketouebo Formation was derived from the partial melting of Neoproterozoic to Phanerozoic juvenile crustal material, indicating in turn that significant crustal growth occurred at this time within the Xing’an Terrane. These data, combined with previous research into the spatial–temporal distribution of Mesozoic volcanic rocks in NE China, suggest that the Early Cretaceous magmatism in the Great Xing’an Range was influenced by both the subduction of the Palaeo-Pacific Plate and the closure of the Mongol–Okhotsk Ocean. This was a crucial period in the transformation from the Mongol–Okhotsk Ocean to the Palaeo-Pacific tectonic regimes. In summary, the early stages of Early Cretaceous magmatism in this area were related to the closure of the Mongol–Okhotsk Ocean, whereas the later stages of magmatism in this area and elsewhere in NE China were related to the subduction of the Palaeo-Pacific Plate.     

Genesis of the titanian andradite in alkaline rocks and their petrological and mineralization significance:A study example of the garnet from the Fanshan ultramafic. syenitic complex in North China CratonSCIEI北大核心CSCD

富钛钙铁榴石是硅不饱和碱性火成岩中的特征矿物,其Ti含量和Ti进入石榴石的方式可直接反映岩浆体系的硅饱和度和氧逸度。河北矾山超镁铁岩-正长岩杂岩体是华北克拉通北缘东西向展布三叠纪碱性岩带的重要岩体之一,其岩石中发育三种类型的富钛钙铁榴石:类型Ⅰ发育于岩体外带的石榴石辉石正长岩中,与辉石、黑云母呈岩浆共生关系,端元组成为Adr_(25-65)Mmt_(15-37)Slo_(8-28)Grs_(10-14),以高TiO_(2)含量(6.08%~18.61%)、较低的SiO_(2)含量(25.46%~33.26%)为特征,为原生岩浆成因;类型Ⅱ见于各类岩石,呈细粒他形充填于其他矿物颗粒之间,端元组成为Adr_(57-69)Mmt_(5-19)Slo_(0-6)Grs_(19-27),以高Al_(2)O_(3)含量(3.95%~5.56%)为特征,为岩浆演化后期熔体富Al_(2)O_(3)时结晶而成;类型Ⅲ亦见于各类岩石,主要呈细粒他形或细碎状发育于其他矿物内,端元组成为Adr_(68-79)Mmt_(8-14)Slo_(0-5)Grs_(6-14),以低的Al_(2)O_(3)含量(1.18%~2.89%)和较高的FeO含量(21.65%~24.62%)为特征,为岩浆期后热液成因。在矾山杂岩体的富钛钙铁榴石中,Ti主要以Si→Ti^(4+)和2Fe^(3+)→Ti^(4+)+Fe^(2+)/Mg替代方式进入到晶体结构中,反映了矾山杂岩体的母岩浆体系为二氧化硅不饱和的碱性岩浆,并具有较高的氧逸度。受石榴石的主量元素组成和相应的晶体结构控制,在微量元素组成上,矾山杂岩体中的富钛钙铁榴石富集轻稀土或中稀土、亏损重稀土,大离子亲石元素(Rb、Ba、Pb和Sr)含量较低或极低,而高场强元素(如Th、U、Nb、Ta、Zr和Hf等)含量普遍较高。本文对石榴石的成因研究结果支持矾山杂岩体不同类型的岩石形成于封闭体系下同一母岩浆系统充分的结晶-分异和堆晶作用;此演化模式可以较好地解释矾山杂岩体的同心环状特征和韵律层状结构,以及磁铁矿和磷矿的成因。