安徽铜陵地区中生代侵入岩成因及成矿意义

安徽铜陵矿集区是我国最著名的铜、金、铁产地之一,成矿与岩浆作用关系密切.本次对铜陵地区中生代侵入岩进行了系统的矿物学、岩石学和元素地球化学研究.结果表明:①本区岩浆岩主要为辉石(二长)闪长岩( SiO2≤55％)、石英(二长)闪长岩(SiO255％～65％)和花岗闪长岩(SiO2≥65％)三种岩石组合,其矿物成分主要为...     

Mesozoic collision-related magmatism and crust–mantle interaction along the Anhui segment of the Yangtze River Valley,east-central China

The middle segment of the Yangtze River Deep Fault Belt, located in the foreland of the Dabie orogen, contains widely exposed volcanic–intrusive complexes that formed during two episodes of magmatism (post-collisional and post-orogenic), reflecting crust–mantle interactions during the Late Jurassic (J₃) to Early Cretaceous (K₁). This article summarizes research on the Mesozoic igneous suites and xenolith suites in the area along the Yangtze River. ‘Post-collisional magmatism’ occurred during lithospheric extension at ～145–130 Ma. Its beginning and end are marked by gabbroic xenoliths and pyroxene cumulates within intrusions at Tongling, and by alkali-rich magmatic rocks. The association includes peraluminous silicic rocks and metaluminous mafic–felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic compositions. Taking the Tongling region as an example, quartz monzodiorite yields a sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb age of 139.5 ± 2.9 Ma, and granodiorite yields an age of 135.5 ± 4.4 Ma. These intrusive rocks contain 52.79–66.46 wt.% SiO₂, 13.12–17.73 wt.% Al₂O₃, 1.37–4.62 wt.% MgO, 3.86–6.84 wt.% FeO^T, and 4.71–7.87 wt.% total alkalis (Na₂O?+?K₂O). ACNK values range from 0.62 to 1.20, and ANK values from 1.45 to 3.48. ‘Post-orogenic magmatism’ occurred during lithospheric delamination at ～130–120 Ma. The start of magmatism was marked by the formation of gabbro containing spinel lherzolite xenoliths in the Nanjing–Wuhu Basin (NWB), and its end was marked by the generation of feldspathoid phenocryst-bearing phonolite in the NWB and the Lujiang–Zongyang Basin (LZB), respectively. The association that formed during this episode ranges from alkaline to peralkaline. Taking the Niangniangshan Formation in the NWB as an example, the Nosite phonolite yields a whole-rock monomineral Rb–Sr isochron age of 120 ± 9 Ma, and contains 49.92–60.09 wt.% SiO₂, 17.67–20.65 wt.% Al₂O₃, 0.08–2.45 wt.% MgO, 1.32–6.62 wt.% FeO^T, and 9.24–13.92 wt.% total alkalis (Na₂O?+?K₂O). ACNK values range from 0.72 to 1.24, and ANK values from 1.03 to 1.35.

The two magmatisms correspond to two episodes of crust–mantle interaction. The first involved intensive interaction between middle–lower crust and underplated basaltic magma derived from the upper mantle lithosphere, whereas the second involved minor interaction between the middle–lower crust and basaltic magma derived from the lower lithospheric mantle.     

Elemental mobility in subduction metamorphism: insight from metamorphic rocks of the Franciscan Complex and the Feather River ultramafic belt,California

The degree of element mobility in subduction metamorphism has generated much debate; some workers advocate considerable mobility during metamorphism, whereas others postulate minimal mobility. We assess this issue by examination of major and trace element concentrations and Pb-, Nd-isotopic data for 39 mafic metavolcanic rocks from the Franciscan subduction complex, related units of coastal California, and the Feather River ultramafic belt of the northern Sierra Nevada, California; these samples span a wide range of metamorphic grade. We conclude that these rocks, despite their metamorphism up to eclogite facies, preserve protolith major and trace elemental compositions and isotopic ratios, with the exception of some mobile large ion lithophile elements such as Ba, Pb, and to a smaller extent La, U, and Sr. Thus subduction metamorphism of these metabasalts occurred in a largely closed system. Lack of light rare earth element enrichment in the rocks demonstrates lack of chemical exchange with subducted metasediments. Relatively low SiO₂ content (<48 wt.%) of many of the metamorphic rocks and the lack of correspondence between silica depletion and metamorphic grade suggests that the silica depletion resulted from seafloor hydrothermal alteration before subduction. In spite of demonstrated mobility of Pb, and possible mobility of Nd, isotopic ratios of Pb and Nd were not modified during subduction metamorphism. In contrast to our results from metabasaltic rocks, our analysis of actinolite-rich rinds from high-grade Franciscan mélange blocks suggests some chemical exchange between metachert and the overlying mantle. The increasing enrichment in Ba and Pb with increasing metamorphic grade suggests that Ba- and Pb-rich fluids interacted more intensely with metabasalt at the higher grades of metamorphism. Comparison of these results with studies of the active Mariana forearc suggests that fluids interacting with the mantle wedge up-dip of the region of magma genesis are derived from subducting sediments overlying the down-going plate.     

Chemical geodynamics of Western Anatolia

We report major and trace element concentrations and Nd–Sr–Pb isotopic data of 10 post-collisional volcanic domains in Western Anatolia, a seismically active part of the Alpine–Himalayan belt in the Aegean extensional province. Our objective is to provide geochemical constraints for tectono-magmatic processes shaping the late Cenozoic geodynamic evolution of Western Anatolia.

Calc-alkaline volcanic rocks occurring to the north of the Izmir–Ankara–Erzincan suture zone show arc-like trace elements and isotopes and were formed by the melting of the metasomatized Neotethyan mantle-wedge; this process was facilitated by asthenospheric upwelling resulting from slab delamination. Calc-alkaline and alkaline volcanic rocks from within the Izmir–Ankara–Erzincan suture zone also show the imprint of subduction fluids in their major and trace elements, but their isotopic compositions indicate derivation from a metasomatized lithospheric mantle followed by assimilation of ancient crust. Volcanics along the N–S-oriented Kirka–Afyon–Isparta trend were derived from the lithospheric mantle that was metasomatized by fluids from the older subduction of the African plate. Golcuk–Isparta volcanic rocks show an asthenospheric imprint; the latter was a consequence of upwelling following a tear in the subducting African lithosphere. Shoshonitic Kula volcanic rocks show very high trace element concentrations, OIB mantle-like trace elements, and Nd–Sr–Pb isotopic signatures, and were formed by partial melting of the upwelling asthenospheric mantle; this event was synchronous with the Aegean extension and possibly also with slab window formation due to ruptures in the African plate.

Inherent in the above chemical geodynamic models are the high ?_Nd(0) values (+6.4) of the end-member volcanic rocks, implying the presence of an asthenospheric source beneath Western Anatolia that is responsible for the currently observed high heat flow, low Pn wave velocities, high seismicity, and tectonic activity.     

Timing of granitic magma mixing in the southeastern Gyeongsang Basin,Korea: SHRIMP-RG zircon data

Late Cretaceous calc-alkaline granites in the Gyeongsang Basin evolved through the mixing of mafic and felsic magmas. The host granites contain numerous mafic magmatic/microgranular enclaves of various shapes and sizes. New SHRIMP-RG zircon U–Pb ages of both granite and mafic magmatic/microgranular enclaves are 75.0?±?0.5 Ma and 74.9?±?0.6 Ma, respectively, suggesting that they crystallized contemporaneously after magma mixing. The time of injection of mafic melt into the felsic magma chamber can be recognized as approximately 75 Ma by field relations, petrographic features, geochemical evolution, and SHRIMP-RG zircon dating. This Late Cretaceous magma mixing event in the Korean Peninsula was probably related to the onset of subduction of the Izanagi (Kula)–Pacific ridge.     

Thermomechanical maturation of the continental crust and its effects on the late Eocene–early Oligocene volcanic record of the Sierra Madre del Sur Province,southern Mexico

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant ¹⁴³Nd/¹⁴⁴Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO₂ at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.     

Early subduction–exhumation and late channel flow of the Greater Himalayan Sequence: implications from the Yadong section in the eastern Himalaya

Based on metamorphic studies of the Yadong high-pressure (HP) granulite and multiple thermochronological investigations of granitoids from both upper and lower parts, the Yadong section in the eastern Himalaya constrains the Cenozoic tectonic evolution of the Greater Himalayan Sequence (GHS). The Yadong HP granulite, located at the top of the GHS, underwent a peak-stage HP granulite facies metamorphism and two stages of retrograde metamorphism. Granulite and hornblende facies retrograde metamorphism took place at 48.5 and 31.8 Ma, respectively, marking the time of exhumation of the subducted Indian slab to lower and middle crustal levels. Subsequently, an average young zircon U–Pb age obtained from the Yadong HP granulite indicated that this unit was captured by its surroundings in a partially molten condition at 16.9 Ma. In addition, three granitoids from both the lower and the upper parts of the GHS yielded biotite ⁴⁰Ar/³⁹Ar ages of 11.0, 11.3, and 11.5 million years. These consistent ages suggest that the GHS along the Yadong section was laterally extruded and synchronously cooled to ～300°C at ～11.3 Ma. Furthermore, the granitic gneisses yield apatite fission track ages of ～7 million years, documenting the cooling of the GHS to ～110°C. A two-stage model describes the Cenozoic tectonic evolution of the GHS: (1) the Indian slab had subducted under Tibet before ～55 Ma, and was exhumed to the lower crust (50-40 km) at 48.5 Ma, and to the middle crust (22-15 km) at 31.8 Ma; and (2) the partial melting occurred at middle crustal levels during the period 31.8 to 16.9 Ma, causing channel flow. In the late stage, the GHS was laterally extruded by ductile mid-crustal flow during the period 16.9 to ～7 Ma, characterized by a fast cooling rate of ～2 mm per year.     

Subduction of a fossil slow–ultraslow spreading ocean: a petrology-constrained geodynamic model based on the Voltri Massif,Ligurian Alps,Northwest Italy

Slow–ultraslow spreading oceans are mostly floored by mantle peridotites and are typified by rifted continental margins, where subcontinental lithospheric mantle is preserved. Structural and petrologic investigations of the high-pressure (HP) Alpine Voltri Massif ophiolites, which were derived from the Late Jurassic Ligurian Tethys fossil slow–ultraslow spreading ocean, reveal the fate of the oceanic peridotites/serpentinites during subduction to depths involving eclogite-facies conditions, followed by exhumation.

The Ligurian Tethys was formed by continental extension within the Europe–Adria lithosphere and consisted of sea-floor exposed mantle peridotites with an uppermost layer of oceanic serpentinites and of subcontinental lithospheric mantle at the rifted continental margins. Plate convergence caused eastward subduction of the oceanic lithosphere of the Europe plate and the uppermost serpentinite layer of the subducting slab formed an antigorite serpentinite-subduction channel. Sectors of the rather unaltered mantle lithosphere of the Adria extended margin underwent ablative subduction and were detached, embedded, and buried to eclogite-facies conditions within the serpentinite-subduction channel. At such P–T conditions, antigorite serpentinites from the oceanic slab underwent partial HP dehydration (antigorite dewatering and growth of new olivine). Water fluxing from partial dehydration of host serpentinites caused partial HP hydration (growth of Ti-clinohumite and antigorite) of the subducted Adria margin peridotites. The serpentinite-subduction channel (future Beigua serpentinites), acting as a low-viscosity carrier for high-density subducted rocks, allowed rapid exhumation of the almost unaltered Adria peridotites (future Erro–Tobbio peridotites) and their emplacement into the Voltri Massif orogenic edifice. Over in the past 35 years, this unique geologic architecture has allowed us to investigate the pristine structural and compositional mantle features of the subcontinental Erro–Tobbio peridotites and to clarify the main steps of the pre-oceanic extensional, tectonic–magmatic history of the Europe–Adria asthenosphere–lithosphere system, which led to the formation of the Ligurian Tethys.

Our present knowledge of the Voltri Massif provides fundamental information for enhanced understanding, from a mantle perspective, of formation, subduction, and exhumation of oceanic and marginal lithosphere of slow–ultraslow spreading oceans.     

Lawsonite- and glaucophane-bearing blueschists from NW Qiangtang,northern Tibet,China: mineralogy,geochemistry, geochronology,and tectonic implications

The occurrence of high-pressure (HP) blueschists within the central Qiangtang terrane of northern Tibet has a significant bearing on plate-suturing processes. In order to contribute to the ongoing debate on whether the central Qiangtang metamorphic belt represents an in situ suture within the Qiangtang terrane, we examined lawsonite- and glaucophane-bearing blueschists from the northwest Qiangtang area (84° 10′–85° 30′ E, 34°10′–34° 45′ N). All studied rocks are metapelites, metasandstones, or metabasalts, characterized by lawsonite + glaucophane + phengite, lawsonite + glaucophane + epidote + albite + quartz, or glaucophane + phengite + quartz assemblages. The meta-mafic rocks contain very high TiO₂ and low Al₂O₃ contents. They are typified by abundant ferromagnesian trace elements, and an absence of Eu anomalies and Nb–Ta deletions; all the above features indicate that these mafic rocks represent oceanic island basalt (OIB) protoliths. Most of the metasediments contain high SiO₂, moderate Al₂O₃ + K₂O, and low TiO₂ + Na₂O. They display high CIA (chemical index of alteration) values (74% ± 5%) and distinctly negative Eu anomalies (Eu/Eu* = 0.64 ± 0.05). This, along with their high field strength elemental characteristics, indicates that they were deposited in a passive continental margin environment, intercalated with OIB-type basalts. We estimate the peak metamorphic conditions for these blueschists as T = 330–415°C and P = 9–11.5 kbar. This HP event occurred at ca. 242 Ma, indicated by a well-defined ⁴⁰Ar/³⁹Ar plateau age for glaucophane. Retrograde metamorphism occurred at T = 280–370°C, P = 6.5–9.5 kbar, t = ca. 207 Ma (⁴⁰Ar/³⁹Ar dating of phengite). Therefore, a cold subduction (geotherm ～8°C/km) attended the passive continental margin during the Triassic when the eastern Qiangtang collided with the western Qiangtang. The northwest Qiangtang HP metamorphic belt is an extension of the central Qiangtang metamorphic belt that defines the suture between eastern and western Qiangtang, and indicates an anticlockwise, diachronous closure of the Shuanghu Palaeo-Tethys.     

Palaeozoic multiphase magmatism at Barleik Mountain,southern West Junggar,Northwest China: implications for tectonic evolution of the West Junggar

The West Junggar lies in the southwest part of the Central Asian Orogenic Belt (CAOB) and consists of Palaeozoic ophiolitic mélanges, island arcs, and accretionary complexes. The Barleik ophiolitic mélange comprises several serpentinite-matrix strips along a NE-striking fault at Barleik Mountain in the southern West Junggar. Several small late Cambrian (509–503 Ma) diorite-trondhjemite plutons cross-cut the ophiolitic mélange. These igneous bodies are deformed and display island arc calc-alkaline affinities. Both the mélange and island arc plutons are uncomfortably covered by Devonian shallow-marine and terrestrial volcano-sedimentary rocks and Carboniferous volcano-sedimentary rocks. Detrital zircons (n = 104) from the Devonian sandstone yield a single age population of 452–517 million years, with a peak age of 474 million years. The Devonian–Carboniferous strata are invaded by an early Carboniferous (327 Ma) granodiorite, late Carboniferous (315–311 Ma) granodiorites, and an early Permian (277 Ma) K-feldspar granite. The early Carboniferous pluton is coeval with subduction-related volcano-sedimentary strata in the central West Junggar, whereas the late Carboniferous–early Permian intrusives are contemporary with widespread post-collisional magmatism in the West Junggar and adjacent regions. They are typically undeformed or only slightly deformed.

Our data reveal that island arc calc-alkaline magmatism occurred at least from middle Cambrian to Late Ordovician time as constrained by igneous and detrital zircon ages. After accretion to another tectonic unit to the south, the ophiolitic mélange and island arc were exposed, eroded, and uncomfortably overlain by the Devonian shallow-marine and terrestrial volcano-sedimentary strata. The early Carboniferous arc-related magmatism might reflect subduction of the Junggar Ocean in the central Junggar. Before the late Carboniferous, the oceanic basins apparently closed in this area. These different tectonic units were stitched together by widespread post-collisional plutons in the West Junggar during the late Carboniferous–Permian. Our data from the southern West Junggar and those from the central and northern West Junggar and surroundings consistently indicate that the southwest part of the CAOB was finally amalgamated before the Permian.