Late Mesozoic magmatism from the Daye region,eastern China: U–Pb ages,petrogenesis, and geodynamic implications

Late Mesozoic dioritic and quartz dioritic plutons are widespread in the Daye region, eastern Yangtze craton, eastern China. Detailed geochronological, geochemical, and Sr–Nd isotopic studies have been undertaken for most of these plutons, in an attempt to provide a comprehensive understanding in the age, genesis and geodynamical control of the extensive magmatism. SHRIMP and LA-ICP-MS zircon U–Pb dating indicate that the plutons were emplaced in the range of latest Jurassic (ca. 152 Ma) to early Cretaceous (ca. 132 Ma), which was followed by dyke emplacement between 127 and 121 Ma and volcanism during the 130–113 Ma interval. Both diorites and quartz diorites are sodic, metaluminous, high-K calc-alkaline, and characterized by strongly fractionated, sub-parallel REE patterns without obvious Eu anomalies. The rocks are enriched in highly incompatible elements and large ion lithophile elements, but depleted in high field strength elements. Samples of diorite and quartz diorite have similar Sr–Nd isotopic compositions that are consistent with the early Cretaceous basalts and mafic intrusions throughout the eastern Yangtze craton. The geochemical and isotopic data, together with results of geochemical modeling, indicate an enriched mantle source for the plutonic rocks. The quartz diorites have geochemical signatures resembling adakites, such as high Al₂O₃ (15–19 wt.%), Sr (630–2,080 ppm), Na₂O (>3.5 wt.%), negative Nb–Ta anomalies, low Y (7–19 ppm), Yb (0.5–1.8 ppm), Sc (5–15 ppm), and resultant high Sr/Y (45–200) and La/Yb (31–63) ratios. Genesis of the adakitic quartz diorites is best explained in terms of low-pressure intracrustal fractional crystallization of cumulates consisting of hornblende, plagioclase, K-feldspar, magnetite, and apatite from mantle-derived dioritic magmas. Mantle-derived magmatism broadly coeval with that of the Daye region also is widespread in other regions of the eastern Yangtze craton, reflecting large-scale melting of the lithospheric mantle during the Late Mesozoic. The large-scale magmatism was most likely driven by lithospheric extension associated with thinning of lithospheric mantle beneath the eastern China continent.     

Quaternary bimodal volcanism in the Niğde Volcanic Complex (Cappadocia,central Anatolia,Turkey): age,petrogenesis and geodynamic implications

The late Neogene to Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia is one of the most impressive volcanic fields of Turkey because of its extent and spectacular erosionally sculptured landscape. The late Neogene evolution of the CVP started with the eruption of extensive andesitic-dacitic lavas and ignimbrites with minor basaltic lavas. This stage was followed by Quaternary bimodal volcanism. Here, we present geochemical, isotopic (Sr–Nd–Pb and δ¹⁸O isotopes) and geochronological (U–Pb zircon and Ar–Ar amphibole and whole-rock ages) data for bimodal volcanic rocks of the Ni?de Volcanic Complex (NVC) in the western part of the CVP to determine mantle melting dynamics and magmatic processes within the overlying continental crust during the Quaternary. Geochronological data suggest that the bimodal volcanic activity in the study area occurred between ca. 1.1 and ca. 0.2 Ma (Pleistocene) and comprises (1) mafic lavas consisting of basalts, trachybasalts, basaltic andesites and scoria lapilli fallout deposits with mainly basaltic composition, (2) felsic lavas consisting of mostly rhyolites and pumice lapilli fall-out and surge deposits with dacitic to rhyolitic composition. The most mafic sample is basalt from a monogenetic cone, which is characterized by ⁸⁷Sr/⁸⁶Sr = 0.7038, ¹⁴³Nd/¹⁴⁴Nd = 0.5128, ²⁰⁶Pb/²⁰⁴Pb = 18.80, ²⁰⁷Pb/²⁰⁴Pb = 15.60 and ²⁰⁸Pb/²⁰⁴Pb = 38.68, suggesting a moderately depleted signature of the mantle source. Felsic volcanic rocks define a narrow range of ¹⁴³Nd/¹⁴⁴Nd isotope ratios (0.5126–0.5128) and are homogeneous in Pb isotope composition (²⁰⁶Pb/²⁰⁴Pb = 18.84–18.87, ²⁰⁷Pb/²⁰⁴Pb = 15.64–15.67 and ²⁰⁸Pb/²⁰⁴Pb = 38.93–38.99). ⁸⁷Sr/⁸⁶Sr isotopic compositions of mafic (0.7038–0.7053) and felsic (0.7040–0.7052) samples are similar, reflecting a common mantle source. The felsic rocks have relatively low zircon δ¹⁸O values (5.6 ± 0.6 ‰) overlapping mantle values (5.3 ± 0.3 %), consistent with an origin by fractional crystallization from a mafic melt with very minor continental crustal contamination. The geochronological and geochemical data suggest that mafic and felsic volcanic rocks of the NVC are genetically closely related to each other. Mafic rocks show a positive trend between ⁸⁷Sr/⁸⁶Sr and Th, suggesting simultaneous assimilation and fractional crystallization, whereas the felsic rocks are characterized by a flat or slightly negative variation. High ⁸⁷Sr/⁸⁶Sr gneisses are a potential crustal contaminant of the mafic magmas, but the comparatively low and invariant ⁸⁷Sr/⁸⁶Sr in the felsic volcanics suggests that these evolved dominantly by fractional crystallization. Mantle-derived basaltic melts, which experienced low degree of crustal assimilation, are proposed to be the parent melt of the felsic volcanics. Geochronological and geochemical results combined with regional geological and geophysical data suggest that bimodal volcanism of the NVC and the CVP, in general, developed in a post-collisional extensional tectonic regime that is caused by ascending asthenosphere, which played a key role during magma genesis.     

Mafic–intermediate plutonic rocks of the Salmas area,northwestern Iran: their source and petrogenesis significance

The Salmas area, in the northernmost part of the Sanandaj–Sirjan zone of Iran, contains a crystalline mafic–intermediate complex that intrudes into the Precambrian metamorphic basement complex and is composed of gabbroic and gabbrodiorite cumulates and fine-grained non-cumulate gabbronorites and diorites. These rocks have fine- to coarse-grained texture and are mainly composed of plagioclase, pyroxenes, and amphibole. Major element geochemistry indicates that the pluton has a low-K with tholeiitic affinity. Variations of major and trace elements on Harker diagrams, including negative correlations MgO, Fe₂O₃, CaO, and Co and positive correlations Na₂O, K₂O, Rb, Ba, and La, with increasing SiO₂ and chondrite-normalized REE patterns, suggest that fractional crystallization of gabbroic rocks could have played a significant role in the formation of evolved rocks. The chondrite-normalized REE patterns are not fractionated (La_N /Lu_N = 1.3–5.4) and display strong Eu anomalies (Eu/Eu* = 1.15–1.76) in cumulate rocks, which we attributed to cumulus plagioclase. Sr and Nd isotopic ratios vary from 0.704698 to 0.705866 and from 0.512548 to 0.512703, respectively. Gabbronorites with high ¹⁴³Nd/¹⁴⁴Nd ratios, low ⁸⁷Sr/⁸⁶Sr ratios, and high MgO, Ni, and Cr contents indicate that they were generated from relatively primitive magmas. We used petrogenetic modelling to constrain sources. Trace element ratio modelling indicates that the gabbroic rocks were generated from a spinel-peridotite source via 5–20% degrees of fractional melting at a depth of ～52 km. Major and REE modelling shows that the diorites are the products of fractional crystallization of gabbronorites.     

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.     

Geochronological framework of Mesozoic volcanic rocks in the Great Xing’an Range,NE China,and their geodynamic implications

Mesozoic volcanic rocks are widespread throughout the Great Xing’an Range, NE China. However, precise data constraining the exact eruption ages are limited, especially for those from the southern Great Xing’an Range, which severely hampers our understanding of the petrogenesis and geodynamics of these rocks. In this paper, we report precise in situ LA-ICPMS zircon U–Pb age measurements for these volcanic rocks. Volcanic rocks in the southern Great Xing’an Range were divided into four units from bottom to top, namely, the Manketouebo, Manitu, Baiyingaolao and Meiletu formations. The previous studies suggested that these volcanic rocks were mainly formed in the Late Jurassic. Our data demonstrate that the Manketouebo formation erupted during Late Jurassic to Early Cretaceous time, whereas the other formations are all of Cretaceous age. The southern Great Xing’an Range age dataset, along with recently obtained precise ages for volcanic rocks from the northern Great Xing’an Range indicate that Mesozoic volcanism throughout the Great Xing’an Range commenced in Late Jurassic, but peaked during the Cretaceous. They formed under an extensional tectonic setting which resulted from closure of the Mongol–Okhotsk Ocean and subsequent orogenic collapse. The globally elevated mantle temperature in Cretaceous may provided thermal contributions to the generation of the volcanisms.     

Facies Analysis and Sequence Stratigraphy of the Qal’eh Dokhtar Formation (Middle–Upper Jurassic) in the West of Boshrouyeh, East Central Iran

The study area is located in the east Tabas Block in Central Iran. Facies analysis of the Qal’eh Dokhtar Formation (middle Callovian to late Oxfordian) was carried out on two stratigraphic sections and applied to depositional environment and sequence stratigraphy interpretation. This formation conformably overlies and underlies the marly-silty Baghamshah and the calcareous Esfandiar formations, respectively. Lateral and vertical facies changes documents low- to high energy environments, including tidal-flat, beach to intertidal, lagoon, barrier, and open-marine. According to these facies associations and absence of resedimentation deposits a depositional model of a mixed carbonate–siliciclastic ramp was proposed for the Qal’eh Dokhtar Formation. Seven third-order depositional sequences were identified in each two measured stratigraphic sections. Transgressive systems tracts (TSTs) show deepening upward trends, i.e. shallow water beach to intertidal and lagoonal facies, while highstand systems tracts (HST) show shallowing upward trends in which deep water facies are overlain by shallow water facies. All sequence boundaries (except at the base of the stratigraphic column) are of the no erosional (SB2) types. We conclude eustatic rather than tectonic factors played a dominant role in controlling carbonate depositional environments in the study area.     

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.     

Cretaceous A-type volcanic–intrusive rocks and simultaneous mafic rocks along the Gan-Hang Tectonic Belt,Southeast China: petrogenesis and implications for the transition of crust–mantle interaction

ABSTRACT

Abundant evidence points to the Cretaceous crust–mantle interaction and plate subduction in the Gan-Hang Tectonic Belt (GHTB), southeastern China, but the evolutionary process remains poorly constrained. Here we conduct a comprehensive study on Daqiaowu granitic porphyry and diabase dikes in the eastern GHTB, in conjunction with previous studies on simultaneous felsic and mafic rocks along the GHTB, to demonstrate their petrogenesis and geodynamic evolutionary process. The Daqiaowu granitic porphyry (125 Ma), as well as the coeval granitic rocks, exhibits high zircon saturation temperatures, alkalis, 10⁴*Ga/Al ratios, and Zr + Nb + Ce + Y contents, concluding a distinctive belt of the Early Cretaceous (~137–125 Ma) A-type volcanic–intrusive rocks in the GHTB. Their ε_Nd(t) and zircon ε_Hf(t) values gradually increased through time from approximately ?9.0 to ?1.0 and ?10.0 to +4.0, respectively, implying increasing contribution of mantle-derived components to their formation, and hence progressively intensified crust–mantle interaction in an intra-arc rift environment (a geodynamic transition stage from continental arc to back-arc) during the Early Cretaceous. This plausibility is further supported by the Early Cretaceous Daqiaowu diabase dikes and coeval mafic rocks which exhibit arc-like magmatic signatures and were derived from mantle wedge. In contrast, the Late Cretaceous mafic rocks show ocean island basalt-like geochemical characteristics, reflecting a depleted asthenosphere mantle source. This discrepancy of mantle sources concludes that the geodynamic setting in the GHTB may have basically transferred to back-arc regime in the Late Cretaceous. Thus, the Cretaceous geodynamic evolutionary process in the GHTB can be defined as the Early Cretaceous gradually intensified crust–mantle interaction in a geodynamic transition stage (from continental arc to back-arc extension) and the Late Cretaceous back-arc extensional setting.     

Nd-isotope and geochemistry of an early Palaeoproterozoic high-Si high-Mg boninite–norite suite of rocks in the southern Bastar craton,central India: petrogenesis and tectonic significance

ABSTRACT

Nd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg boninite–norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4–2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger T_DM model ages than the high-Ca boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle metasomatism during ancient subduction event cannot be ignored. Trace-element modelling suggests that the high-Ca boninites may have crystallized from a magma generated by a comparatively greater percentage of melting of a lherzolite mantle source than the source for the other two varieties. Furthermore, the high-Ca boninite rocks are most likely derived from an Archaean subduction process (the Whundo-type), whereas the other two types are the products of the interaction of subduction-modified refractory mantle wedge and a plume, around the Neoarchaean–Palaeoproterozoic boundary. The emplacement of the high-Mg norites and the high-Mg diorites may be linked to crustal thickening and associated cratonization at the end of the Archaean.     

Magmatic and geodynamic evolution of Urumieh–Dokhtar basic volcanism,Central Iran: major,trace element,isotopic, and geochronologic implications

Basic volcanic rocks from the West Nain area of the Urumieh–Dokhtar Magmatic Assemblage demonstrate significant subduction-related geochemical characteristics; these along with the new age data obtained for the volcanic rocks shed new light on the geodynamic evolution of the Iranian segment of Alpine–Himalayan orogeny. The late Oligocene (26.5 Ma) high-Nb basic volcanic rocks are likely to represent a transient rather enriched asthenospheric mantle underlying the otherwise dominantly Eocene–early Oligocene West Nain island arc. Lithospheric mantle geochemical signatures of the low-Zr volcanic rocks (20.6 Ma) and high-Th volcanic rocks (19.7 Ma) imply replacement of the underlying mantle. The substitution of asthenospheric mantle by a lithospheric mantle wedge might have been associated with – or perhaps caused by – an increase in the subduction rate. Culmination of the West Nain magmatism into slab melting that produced the early Miocene (18.7 Ma) adakitic rocks is compatible with subsequent ascent that triggered slab decompression melting.     

Olivine-hosted melt inclusions in Pliocene–Quaternary lavas from the Qorveh–Bijar volcanic belt,western Iran: implications for source lithology and cooling history

ABSTRACT

Silicate melt inclusions (SMIs) are small droplets of magma that become trapped in minerals during crystal growth. SMIs in olivine crystals can provide critical information on the range of melt compositions and processes that occur during melt generation, evolution, transport, and eruption. The Pliocene–Quaternary volcanic rocks in the Qorveh–Bijar volcanic belt of western Iran show porphyritic and microlithic textures, with olivine and clinopyroxene being the dominant minerals. Magnesian olivines in these volcanic rocks contain primary SMIs. The composition and characteristic of olivine-hosted SMI of these rocks are investigated to constrain the source lithology for mafic volcanism. Bulk compositions of the SMIs overlap those of their host rocks and extend to higher CaO/Al₂O₃ values. The estimated entrapment pressures and temperatures of the studied SMIs are 9.1–10.3 kbar and 1220–1355°C. The calculated mafic parental melt contains 42.36 wt.% SiO₂, low total alkalis (3.22 wt.%), and high MgO (16.1 wt.%). Exploratory calculations using pMELTS show that this parental composition underwent variable degrees of fractional crystallization, as reflected by the variable compositions of the SMIs. Several lines of evidence including pyroxene xenocrysts and high FeO/MnO, FC3MS (FeO/CaO – 3*MgO/SiO2), and Zn/Fe ratios (14–21), suggest that a metasomatized pyroxenitic source contributed to the genesis of the parental melt. Amphibole in the SMIs indicates a high volatile content in the parental melt, which we conclude was generated from a metasomatized lithospheric mantle source. The pyroxenite source also contained garnet. Our geochemical results lead us to propose a new petrogenetic model. Specifically, we infer that a dense and unstable portion of the lithosphere underwent localized laminar detachment and downward flow, i.e. lithospheric drip. This drip underwent volatile-enhanced partial melting during descent through the underlying hot asthenosphere and generated the studied volcanic rocks.     

Geochronology,geochemistry, and petrogenesis of the Maçka subvolcanic intrusions: implications for the Late Cretaceous magmatic and geodynamic evolution of the eastern part of the Sakarya Zone,northeastern Turkey

The Maçka subvolcanic intrusions (MSIs) in the eastern part of the Sakarya zone, northeastern Turkey, play a critical role in understanding the petrogenetic and geodynamic processes that took place during the growth of Late Cretaceous arc crust of this region. U–Pb zircon (79.97 ± 0.97 Ma) and two ⁴⁰Ar–³⁹Ar amphibole ages (average 81.37 ± 0.5 Ma) indicate that the MSIs were emplaced in Late Cretaceous (Campanian) time into the coeval volcanic rocks. A slightly younger zircon fission track (FT) age (73 ± 9 Ma) points to a rapid exhumation and cooling after crystallization. The intrusions are observed in areas less than 1 km² in the field and contain abundant mafic microgranular enclaves (MMEs). The host rocks (HRs) are entirely composed of tonalite (SiO₂ = 63–65 wt.%, Mg# = 43–52), and the MMEs are gabbro-diorite in composition (SiO₂ = 53–57 wt.%, Mg# = 45–48). Both the HRs and the MMEs are I-type, high-K calc-alkaline in composition and display a metaluminous character. They are characterized by geochemical features typical for magmas of subduction-related environments. Chondrite-normalized REE patterns are moderately fractionated [(La/Yb)_N = 6–11] and display slightly negative Eu anomalies (Eu/Eu* = 0.7–0.9), with weak concave-upward REE patterns, suggesting that amphibole fractionation played a role during their evolution. The MMEs have slightly different I_Sr (0.7081–0.7085) and ε_Nd (?5.0 to ?5.4) values compared with those of their HRs (I_Sr = 0.7084–0.7087 and ε_Nd = ?5.7 to ?6.9), indicating that variable amounts of crustal and mantle components were involved in the generation of parental magma to these rocks. All of these data, combined with those of previous regional studies, suggest that the MSIs are hybrid in origin, produced by the mixing of enriched lithospheric mantle- and lower crust-derived melts in an extensional arc setting that was caused by slab rollback.     

Mineral chemical constraints on the petrogenesis of mafic and intermediate volcanic rocks from the Erciyes and Hasandağ volcanoes,Central Turkey

Hasandağ and Erciyes stratovolcanoes, which produced both calc-alkaline and alkaline eruptive products, are the two important volcanic complexes in Central Anatolia. There are three geochemical evolution stages in the history of the Hasandağ strato volcanic complex: (1) Keçikalesi tholeiitic, (2) Hasandağ calc-alkaline and (3) Hasandağ alkaline. Volcanologic and petrologic characteristics of the Hasandağ and Erciyes calc-alkaline series show that water played an important role on the genesis of these rocks. These rocks are phenocryst-rich with vesicular texture, and contain hydrous mineral phases. The approximate pressure and temperature estimates obtained from the mineral chemistry studies of the Hasandağ strato volcanic complex indicate crystallization temperature of 1100 °C with 2.5–3.4 kbar pressure interval for the first stage of Keçikalesi tholeiitic volcanism, and about 850 °C temperatures with 4.3–9.6 kbar pressure intervals for the second stage of Hasandağ calc-alkaline volcanism.The geochemical evolution of Erciyes volcanic complex also exhibits three distinct evolutionary stages: (1) Koçdağ alkaline, (2) Koçdağ calc-alkaline and (3) Erciyes calc-alkaline. The temperature of Koçdağ alkaline volcanism is 1097–1181 °C and in a range of 5.1–6.7 kbar pressure, for Koçdağ calc-alkaline volcanism 850–1050 °C temperature to 2.0–6.6 kbar pressure interval, and for Erciyes calc-alkaline volcanism about 950 °C temperature, to 3.2–7.9 kbar pressure intervals were calculated. Polybaric origin of magma chambers for calc-alkaline and alkaline rocks and disequilibrium parameters observed in phenocrysts indicate that the rocks were affected by magma mixing processes in crustal magma chambers. The disequilibrium features of amphibole and plagioclase phenocrysts in these rocks point the latent heat in magma chambers and periodic recharging with mafic magma chambers and also show that magmas reequilibrate before the eruption.     

Origin,petrogenesis and geodynamic implications of the early Eocene Altınpınar adakitic andesites in the eastern Sakarya Zone,northeastern Turkey

Early Cenozoic magmatism in the eastern Sakarya Zone (NE Turkey) provides an important constraint on the regional tectono-magmatic evolution of the region. Early Eocene syn-collisional adakitic rocks are observed as small stocks with outcropping areas commonly less than 10 km². This study presents petrography, whole-rock geochemistry and Sr-Nd-Pb isotope data, as well as in-situ ⁴⁰Ar/³⁹Ar age constraints on one of these adakitic andesites in the Altınpınar area of Gümüşhane, and discusses source region, petrological processes and geodynamic setting prevailed during their genesis. Andesites commonly show microlitic porphyric and vitrophyric porphyric textures, and include significant amounts of mafic microgranular enclaves (MMEs). Plagioclase, hornblende, Fe-Ti oxides and minor pyroxene are the main mineral phases. In-situ ⁴⁰Ar-³⁹Ar amphibole dating constrains the cooling age of andesites into a time span from 52.8 ± 1.3–48.8 ± 1.9 Ma. Andesites are medium to high-K calc-alkaline and display most of the signatures typical of those of the adakites. They are characterized by moderate MgO (1.7–4.1 wt%), low Y (9−14 ppm), Yb (0.9–1.5 ppm), and HREE and high Sr (325−964 ppm) contents, and high Sr/Y (36–76) ratios. ⁸⁷Sr/⁸⁶Sr_(t) (0.704948−0.705100) and ¹⁴³Nd/¹⁴⁴Nd_(t) (0.512588−0.512628) ratios are in the isotopic range of the adakites. All these geochemical and isotopic data suggest that the parental magma of adakitic andesites has been produced by partial melting of oceanic basalts under amphibole-eclogite facies conditions during the breakoff of the northern Neotethyan oceanic slab.     

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.     

Geochronology and geochemistry of Late Triassic bimodal igneous rocks at the eastern margin of the Songnen–Zhangguangcai Range Massif,Northeast China: petrogenesis and tectonic implications

Abstract

New zircon laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectroscopy U–Pb ages, and Hf isotope and whole-rock geochemical data are reported for Mesozoic igneous rocks from the eastern margin of the Songnen–Zhangguangcai Range Massif, Northeast China, in order to document the petrogenesis of the igneous rocks and reconstruct the early Mesozoic tectonic setting of the region. Zircons from five representative igneous rocks are euhedral–subhedral and display oscillatory growth zoning or striped absorption in cathodoluminescence images, suggesting a magmatic origin. The dating results indicate that granite, gabbro, and rhyolite from the eastern Songnen–Zhangguangcai Range Massif formed during Late Triassic (204–211 Ma). The Late Triassic granitoids and rhyolites have an affinity to A-type granites or rhyolites. Their zircon ε_Hf(t) values and Hf two-stage model ages range from –3.8 to +3.8 and from 999 to 1485 Ma, respectively, indicating that their primary melts were derived from the partial melting of the Meso-Proterozoic crust. The geochemistry of coeval gabbros, which reflects primary magma composition, shows a significant large ion lithophile element (e.g. Ba and Sr) enrichment and high field strength element (i.e. Zr, Hf, Nb, Ta, and Ti) depletion. Based on zircon ε_Hf(t) values (–4.2 to +2.8) and Hf single-stage model ages (746–1031 Ma), we conclude that the mafic magma is the product of partial melting of lithospheric mantle that was metasomatically enriched by fluids derived from the subducted oceanic crust. The Late Triassic magmatism along the eastern margin of the Eurasian continent has bimodal magma compositions, indicating an extensional setting after the final closure of the Palaeo-Asian Ocean rather than being related to subduction of the Palaeo-Pacific Plate beneath the Eurasian continent. The occurrence of Late Triassic igneous rocks on the eastern side of the Mudanjiang Fault suggests that this fault does not represent the suture zone between the Songnen–Zhangguangcai Range and Jiamusi massifs.     

Early Permian East-Ujimqin mafic–ultramafic and granitic rocks from the Xing’an–Mongolian Orogenic Belt,North China: Origin,chronology, and tectonic implications

The East-Ujimqin complex, located north of the Erenhot–Hegenshan fault, North China, is composed of mafic–ultramafic and granitic rocks including peridotite, gabbro, alkali granite, and syenite. We investigated the tectonic setting, age, and anorogenic characteristics of the Xing’an–Mongolian Orogenic Belt (XMOB) through field investigation and microscopic and geochemical analyses of samples from the East-Ujimqin complex and LA-MC-ICP-MS zircon U–Pb dating of gabbro and alkali granite. Petrographic and geochemical studies of the complex indicate that this multiphase plutonic suite developed through a combination of fractional crystallization, assimilation processes, and magma mixing. The mafic–ultramafic rocks are alkaline and have within-plate geochemical characteristics, indicating anorogenic magmatism in an extensional setting and derivation from a mantle source. The mafic–ultramafic magmas triggered partial melting of the crust and generated the granitic rocks. The granitic rocks are alkali and metaluminous and have high Fe/(Fe + Mg) characteristics, all of which are common features of within-plate plutons. Zircon U–Pb geochronological dating of two samples of gabbro and alkali granite yielded ages of 280.8 ± 1.5 and 276.4 ± 0.7 Ma, placing them within the Early Permian. The zircon Hf isotopic data give inhomogeneous ε_Hf(t) values of 8.2–14.7 for gabbroic zircons and extraordinary high ε_Hf(t) values (8.9–12.5) for the alkali granite in magmatic zircons. Thus, we consider the East-Ujimqin mafic–ultramafic and granitic rocks to have been formed in an extensional tectonic setting caused by asthenospheric upwelling and lithospheric thinning. The sources of mafic–ultramafic and granitic rocks could be depleted garnet lherzolite mantle and juvenile continental lower crust, respectively. All the above indicate that an anorogenic magma event may have occurred in part of the XMOB during 280–276 Ma.     

Geochronological and geochemical investigation of the late Mesozoic volcanic rocks from the Northern Great Xing’an Range and their tectonic implications

Precise age dating and systematic geochemical investigation were performed on the widely distributed late Mesozoic volcanic rocks in the North Great Xing’an Range (NGXR). In situ zircon U–Pb age measurements indicate that the volcanic eruption commenced from 163 Ma ago and lasted to 113 Ma ago. These volcanic rocks show a wide range in compositions from basaltic andesite, trachyandesite and trachydacite to rhyolite. The majority of volcanic rocks exhibit high-K calc-alkaline affinity with the subordinate showing shoshonitic features. The volcanic rocks are characterized with low MgO contents, LILE, LREE enrichment and HFSE depletion. Elemental and isotopic variations suggest that fractional crystallization with the predominant removal of olivine and orthopyroxene play an important role in the evolution of magma. Most of the basic and intermediate volcanic rocks are generated from an enriched lithospheric mantle which was metasomatised by fluids released from subducted slabs during the closure of the Paleo-Asian and Mongol-Okhotsk oceans. The generation of such widely distributed volcanic rocks was caused by the decompressional partial melting of enriched lithospheric mantle in an extensional regime, which resulted from the gravitational collapse and upwelling of asthenosphere after the final closure of the Mongol-Okhotsk oceans in late Jurassic and from then the Mongol-Okhotsk orogen turned into the post-orogenic stage.     

U–Pb geochronology,Sr–Nd isotopic compositions,geochemistry and petrogenesis of Shah Soltan Ali granitoids,Birjand, Eastern Iran

The Shah Soltan Ali area (SSA) is located in the eastern part of the Lut Block metallogenic province. In this area different types of sub-volcanic intrusions including diorite porphyry, monzonite porphyry and monzodiorite porphyry have intruded into basaltic and andesitic rocks. Zircon U–Pb dating and field observations indicate that intermediate to mafic volcanic rocks (38.9 Ma) are older than subvolcanic units (38.3 Ma). The subvolcanic intrusions show high-K calc-alkaline to shoshonitic affinity and are metaluminous. Based on mineralogy, high values of magnetic susceptibility [(634 to 3208) × 10^?5 SI], and low initial ⁸⁷Sr/⁸⁶Sr ratios, they are classified as belonging to the magnetite-series of oxidant I-type granitoids and are characterized by an enrichment in LREEs relative to HREEs, with negative Nb, Ti, Zr and Eu anomalies. These granitoids are related to volcanic arc (VAG) and were generated in an active continental margin. Low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7043 to 0.7052) and positive εNd values (+1.48 to +3.82) indicate that the parental magma was derived from mantle wedge. Parental magma was probably formed by low degree of partial melting and metasomatized by slab derived fluids. Then assimilation and fractional crystallization processes (AFC) produced the SSA rocks. This magma during the ascent was contaminated with the crustal material.All data suggest that Middle-Late Eocene epoch magmatism in the SSA area, occurred during subduction of Neo-Tethys Ocean in east of Iran (between Afghan and Lut Blocks).