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.     

Magma mixing model for the genesis of middle crust in the Izu–Bonin–Mariana arc: evidence from plutonic rocks in the Mineoka-Setogawa ophiolitic mélange,central Japan

ABSTRACT

A Paleogene accretionary complex, the Mineoka–Setogawa belt is distributed adjacent to the northern portion of the collision zone between Honshu and Izu–Bonin–Mariana (IBM) arcs in central Japan, comprising a mélange of ophiolitic fragments of various sizes. The Eocene-Oligocene plutonic rocks in this belt (gabbro, diorite, and tonalite) have been interpreted as fragments brought from the deep crust beneath the IBM arc through tectonic collisions. The geochemical characteristics of the gabbro and associated basaltic dike are similar to those of the Eocene IBM tholeiitic basalt; thus, the gabbro was likely formed via the crystallization of the Eocene tholeiitic basaltic magmas, which was produced by the partial meltings of a depleted mantle wedge. A comparison with experimental results and geochemical modeling indicates that the tonalite was generated by 10–30% dehydration melting of the gabbro. Actually, Eocene–Oligocene felsic veins, which are coeval with the plutonic rocks, occur in the Mineoka–Setogawa gabbro. Plagioclase crystals in the diorite comprise Ca-rich and -poor parts in a single crystal. Their compositional characteristics are consistent with those of plagioclase in the gabbro and tonalite, respectively. The textures and chemical composition of plagioclase indicate that the diorite was formed by the mixing between mafic and silicic magmas. The whole-rock composition of the diorite also indicates the evidence for the mixing between basaltic magmas which were fractionated to variable degrees and homogeneous silicic magma. The mixing model proposed from the first direct observations of the IBM middle crust exposed on the Mineoka–Setogawa belt is applied to the genesis of the Eocene to present intermediate rocks in the IBM arc. If the continental crust were created at intra-oceanic arc settings such as the IBM arc, the magma mixing model would be one of the most likely mechanisms for the genesis of the continental crust.     

中国南天山晚石炭世—早二叠世花岗质侵入岩的岩石成因与地壳增生

晚石炭世—早二叠世在中亚南天山造山带形成了大量的花岗质侵入岩.中国境内这些岩体以晚石炭世Ⅰ型花岗岩、早二叠世Ⅰ型和S型花岗岩以及最晚期的A型花岗岩为代表.不同类型的岩石在源区特征和岩浆形成的温压条件上存在一定的差异,而这些差异性反映了该时期内构造环境的演化过程.本文选取铁列克岩体、盲起苏岩体、英买来岩体、川乌鲁杂岩体、...     

Experimental and geochemical evidence for derivation of the El Capitan Granite,California, by partial melting of hydrous gabbroic lower crust

Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850°C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635–661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486–1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening.     

http://www.sciencedirect.com/science/article/pii/S1674987113000856

Sulphide inclusions, which represent melts trapped in the minerals of magmatic rocks and xenoliths, provide important clues to the behaviour of immiscible sulphide liquids during the evolution of magmas and the formation of NieCueFe deposits. We describe sulphide inclusions from unique ultramafic clots within mafic xenoliths, from the mafic xenoliths themselves, and from the three silica-rich host plutons in Tongling, China. For the first time, we are able to propose a general framework model for the evolution of sulphide melts during the evolution of mafic to felsic magmas from the upper mantle to the upper crust. The model improves our understanding of the sulphide melt evolution in upper mantle to upper crust magmas, and provides insight into the formation of stratabound skarn-type FeeCu polymetallic deposits associated with felsic magmatism, thus promising to play an important role during prospecting for such deposits.     

华北燕山带:构造、埃达克质岩浆活动与地壳演化(英文)

埃达克质火成岩在中国东部,包括燕山带是很常见的,一般认为它们是下地壳不均匀的镁铁质岩石及/或富集的上地幔岩石在高压(≥1.5 GPa)下部分熔融的结果。在燕山带内埃达克岩浆的形成有一个很长的时间(约190～80 Ma),然而岩浆活动的峰期却与约170～130 Ma间有基底岩石卷入的陆壳收缩期相一致。尽管埃达克质岩浆活动的历史很长,但那种把岩浆活动与岩石圈的拆沉效应相联系的模式似乎是不适当的。在该带内,埃达克质与非埃达克质岩浆活动有一部分是同时的,而且在地理分布上也是相间的,这说明了在下地壳和上地幔岩石的部分熔融中成分是相当不均匀的。侏罗纪及白垩纪熔融作用的热源应当是与古太平洋板块俯冲相关的中生代板底垫托的玄武岩浆。除了局部例外,在燕山带,埃达克质岩浆活动的终结和碱性岩浆活动的开始约在130～120 Ma,在此时期收缩作用使东亚大达200万km~2以上的地区发生了NW—SE向的区域性伸展作用。强烈的地壳伸展仅局限于华北克拉通北缘分布的少数几个变质核杂岩中。陆壳的伸展减薄合理地解释了130～120 Ma间发生高压埃达克质熔融条件的终结,尽管还有局部年轻的埃达克火山活动(约120～80Ma)可以在伸展规模有限而厚的地壳依然存在的地区继续出现。燕山区早白垩世的碱性侵入体中的锆石不存在前寒武纪?     

Ultramafic and Mafic Inclusions from Adak Island: Crystallization History, and Implications for the Nature of Primary Magmas and Crustal Evolution in the Aleutian Arc

Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.     

Geochronologic-petrochemical studies of the Hongshishan mafic–ultramafic intrusion,Beishan area,Xinjiang (NW China): petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting.

Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ?_Nd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.     

Geodynamic settings and history of the Paleozoic intrusive magmatism of the central and southern Urals: Results of zircon dating

The main stages of the Paleozoic intrusive magmatism in the Urals, 460–420, 415–395, 365–355, 345–330, 320–315, and 290–250 Ma, as well as two virtually amagmatic periods, 375–365 Ma (Frasnian-early Famennian) and 315–300 Ma (Late Carboniferous), are recognized. The Cambrian-Early Ordovician pause predated the onset of igneous activity in the Ural Orogen, while the Early Triassic pause followed by an outburst of trap magmatism postdated this activity. The interval from 460 to 420 Ma is characterized by mantle magma sources that produced ultramafic and mafic primary melts. The dunite-clinopyroxenite-gabbro association of the Platinum Belt and miaskite-carbonatite association are specific derivatives of these melts. The rift-related (?) Tagil Synform functioned at that time. The volcanic-plutonic magmatism in this oldest magmatic zone of the Uralides comprises gabbro, gabbro-granitoid, and gabbro-syenite series and comagmatic volcanic rocks. After a break almost 20 Ma long, this magmatism ended in the Early Devonian (405–400 Ma) with the formation of small K-Na gabbro-granitoid plutons. The magmatic intervals of 415–395, 365–355, and 320–315 Ma are characterized by the mantle-crustal nature. The first interval accompanied obduction of the oceanic lithosphere on the continental crust. The subsequent magmatic episodes presumably were related to the subduction of the island-arc (?) lithosphere beneath the continent and to the collision. The intense granitoid magmatism started 365–355 Ma ago. As in the following interval 320–315 Ma, the tonalite-granodiorite complexes, accompanied by hydrous basic magmatism, were formed. Amphibole gabbro and diorite served as a source of heat and material for the predominant tonalite and granodiorite. The Permian granitic magmatism had crustal sources. Thus, the mantle-derived Ordovician-Middle Devonian magmatism gave way to the mantle-crustal Late Devonian-Early Carboniferous plutonic complexes, while the latter were followed by the crustal Permian granites. This sequence was disturbed by rifting and formation of continental arcs accompanied by specific Early Carboniferous Magnitogorsk gabbro-granitoid series and Early Permian Stepnoe monzodiorite-granite series, which deviate from the general evolutional trend.     

SHRIMP Zircon U‐Pb Dating of Alkaline Dykes in the Pobei Area,Beishan Rift,Xinjiang Autonomous Region,China: Implications for Tectonic Setting and Mantle Plume Events

In the Beishan rift in the eastern Tianshan orogen, Xinjiang Province, a N-S-trending dyke swarm is present in the Pobei area. The swarm cuts through the 270–290 Ma mafic-ultramafic intrusions associated with Ni-Cu sulphide mineralization. These mafic-ultramafic intrusions are typically found along E-W major faults in the Tianshan orogenic belts. We report SHRIMP U-Pb dating of zircons from a dyke of alkaline composition, which yielded a mean age of 252±9 Ma. Alkaline dykes of the same age are found in the Altay region of Siberia. This age is younger than the 270–290 Ma intraplate magmatic events that produced the mafic-ultramafic intrusions in the region, but in general agreement with the 250–260 Ma Permian plume event that gave rise to the Siberian traps and the Emeishan flood basalts in SW China. We suggest that there is a link between the Emeishan event and the dyke swarm in the Beishan rift and that the intraplate magmatism at 270–290 Ma reflects an early stage of mantle plume activity. The N-S trending dyke swarm in the Beishan rift may represent a later stage in the evolution of mantle plume activity in the NW and SW of China. We also speculate that in Beishan rift and possibly elsewhere in the Tianshan region, the dykes fed basaltic volcanism, whose products have since been eroded due to the strong uplift of the Tianshan orogen as a result of the India-Eurasia collision in the Cenozoic.     

Petrogenetic Evolution of the Bayan Gol Ophiolite— Geological Record of an Early Carboniferous "Red Sea Type"Ocean Basin in the Tianshan Mountains, Northwestern China

The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous “Red Sea type” ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcanosedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous tiffing and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2)younger (late Early Carboniferous, -324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).     

Zircon U‐Pb Ages and Geochemistry of Permo‐Carboniferous Mafic Intrusions in the Xilinhot Area,Inner Mongolia: Constraints on the Northward Subduction of the Paleo‐Asian Ocean

The Central Asian Orogenic Belt(CAOB) resulted from accretion during the Paleozoic subduction of the PaleoAsian Ocean. The Xilinhot area in Inner Mongolia is located in the northern subduction zone of the central-eastern CAOB and outcropped a large number of late Paleozoic mafic intrusions. The characteristics of magma source and tectonic setting of the mafic intrusions and their response to the closure process of the Paleo-Asian Ocean are still controversial. This study presents LA-ICPMS zircon U-Pb ages and geochemical features of mafic intrusions in the Xilinhot area to constrain the northward subduction of the Paleo-Asian Ocean. The mafic intrusions consist of gabbro, hornblende gabbro, and diabase. Their intrusion times can be divided into three stages of 326–321 Ma, 276 Ma and 254 Ma by zircon U-Pb ages. The first two stages of the 326–276 Ma intrusions mostly originated from subduction-modified continental lithospheric mantle sources that underwent a variable degree partial melting(5–30%), recording the subduction of oceanic crust. The third stage of the 254 Ma mafic rocks also show arc-related features. The primary magma compositions calculated by PRIMELT2 modeling on three samples of ～326 Ma and two samples of ～254 Ma show that these mafic samples are characterized by a variable range in SiO_2(47.51–51.47 wt%), Al_2O_3(11.46–15.55 wt%), ΣFeO(8.27–9.61 wt%), MgO(13.01–15.18 wt%) and CaO(9.13–11.67 wt%), consisting with the features between enriched mantle and lower continental crust. The source mantle melting of mafic intrusions occurred under temperatures of 1302–1351°C and pressures of 0.92–1.30 GPa. The magmatic processes occurred near the crust-mantle boundary at about 33–45 km underground. Combined with previous studies, it is concluded that Carboniferous to early Permian(～326–275 Ma) northward subduction of the Paleo-Asian oceanic crust led to the formation of the mafic magmatism in the Baolidao arc zone. The whole region had entered the collision environment at ～254 Ma, but with subduction-related environments locally. The final collision between the North China craton and the South Mongolian microcontinent may have lasted until ca. 230 Ma.     

Geochemistry of the mafic dykes in the Prakasam Alkaline Province of Eastern Ghats Belt, India: Implications for the genesis of continental rift-zone magmatism

Mesoproterozoic rift-zone magmatism in the Prakasam Alkaline Province of Eastern Ghats Belt, India is represented by three geochemically distinct primary mafic magmas and their plutonic differentiates. The three mafic magmas correspond to the alkali basaltic dykes, gabbroic dykes and lamprophyric dykes. The dyke activity is synchronous with the host plutons and belongs to the 1350–1250 Ma period Mesoproterozoic magmatism. Geochemical signatures suggest that the alkali basaltic dykes have a source in the thermal boundary layer, which has a history of prior melt extraction followed by enrichment. Both the gabbroic and lamprophyric dykes are derived from lithospheric sources and their geochemical variation can be explained by “vein-plus-wall-rock melting model”. Vein/wall-rock ratio is low for the sources of gabbroic dykes, whereas it is high for the lamprophyric dykes. Geochemistry of the gabbro dykes further indicates preservation of previous arc-signals by the lithosphere beneath the Prakasam Alkaline Province during the Mesoproterozoic. Geochemical signatures of lamproite, which could be a cratonic expression of the rift-triggered magmatism in the Prakasam Province, suggest a general increase in the metasomatic imprint with increasing lithosphere thickness from cratonic margin towards interior. It is found that geochemistry of continental rift-zone magmatism of the Prakasam rift is remarkably similar to that of the Gardar rift of South Greenland. It appears that the geodynamic conditions under which melting occurred in the Prakasam Alkaline Province are similar to that of a propagating rift with variable contributions from the convective mantle and subcontinental lithosphere mantle to the rift-zone magmas. The present study illustrates how fertility and chemical heterogeneity of the lithosphere play significant roles in the creation of enormous geochemical diversity characteristic of continental rift-zone magmatism.     

Contrasting stratified plutons exposed in tilt blocks, Eldorado Mountains, Colorado River Rift, NV, USA

Roof-to-floor exposures of mid-Miocene plutons in tilt blocks south of Las Vegas, NV, reveal distinct but strongly contrasting magma chamber statigraphy. The Searchlight and Aztec Wash plutons are well-exposed, stratified intrusions that show a similar broad range in composition from 45–75 wt.% SiO₂. Homogeneous granites that comprise about one-third of each intrusion are virtually identical in texture and elemental and isotopic chemistry. Mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination, whereas more felsic rocks are hybrids that are either juvenile basaltic magma+crustal melt mixtures or products of anatexis of ancient crust+young (Mesozoic or Miocene?) mafic intraplate.

Despite general similarities, the two plutons differ markedly in dimensions and lithologic stratigraphy. The Searchlight pluton is much thicker (10 vs. 3 km) and has thick quartz monzonite zones at its roof and floor that are absent in the Aztec Wash pluton. Isotopic and elemental data from Searchlight pluton suggest that the upper and lower zones are cogenetic with the granite; we interpret the finer grained, slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate. In contrast, the upper part of the Aztec Wash pluton is granite, and a heterogeneous, mafic-rich injection zone with distinct isotopic chemistry forms the lower two-thirds of the intrusion. Similar mafic rocks are relatively sparse in Searchlight pluton and do not appear to have played a central role in construction of the pluton. Large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the Aztec Wash pluton but absent in the Searchlight pluton. Thus, although both intrusions were filled by similar magmas and both developed internal stratification, the two intrusions evolved very differently. The distinctions may be attributable to scale and resulting longevity and/or to subtle differences in tectonic setting.     

Geochronology and geochemistry of early Paleozoic granitic and coeval mafic rocks from the Tannuola terrane (Tuva,Russia): Implications for transition from a subduction to post-collisional setting in the northern part of the Central Asian Orogenic Belt

Early Paleozoic magmatism of the Tannuola terrane located in the northern Central Asian Orogenic Belt is important to understanding the transition from subduction to post-collision settings. In this study, we report in situ zircon U-Pb ages, whole rock geochemistry, and Sr-Nd isotopic data from the mafic and granitic rocks of the eastern Tannuola terrane to better characterize their petrogenesis and to investigate changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal three magmatic episodes for about 60 Ma from ∼510 to ∼450 Ma, that can be divided into the late Cambrian (∼510–490 Ma), the Early Ordovician (∼480–470 Ma) and the Middle-Late Ordovician (∼460–450 Ma) stages. The late Cambrian episode emplaced the mafic, intermediate and granitic rocks with volcanic arc affinity. The late Cambrian mafic rocks of the Tannuola terrane may originate from melting of mantle source that contain asthenosphere and subarc enriched mantle metasomatized by melts derived from sinking oceanic slab. Geochemical and isotopic compositions indicate the late Cambrian intermediate-granitic rocks are most consistent with an origin from a mixed source including fractionation of mantle-derived magmas and crustal-derived components. The Early Ordovician episode reveal bimodal intrusions containing mafic rocks and adakite-like granitic rocks implying the transition from a thinner to a thicker lower crust. The Early Ordovician mafic rocks are formed as a result of high degree melting of mantle source including dominantly depleted mantle and subordinate mantle metasomatized by fluid components while coeval granitic rocks were derived from partial melting of the high Sr/Y mafic rocks. The latest Middle-Late Ordovician magmatic episode emplaced high-K calc-alkaline ferroan granitic rocks that were formed through the partial melting the juvenile Neoproterozoic sources.These three episodes of magmatism identified in the eastern Tannuola terrane are interpreted as reflecting the transition from subduction to post-collision settings during the early Paleozoic. The emplacement of voluminous magmatic rocks was induced by several stages of asthenospheric upwelling in various geodynamic settings. The late Cambrian episode of magmatism was triggered by the slab break-off while subsequent Early Ordovician episode followed the switch to a collisional setting with thickening of the lower crust and the intrusion of mantle-induced bimodal magmatism. During the post-collisional stage, the large-scale lithospheric delamination provides the magma generation for the Middle-Late Ordovician granitic rocks.     

Geochronology, petrology and geochemistry of the Nanlinshan and Banpo mafic–ultramafic intrusions: implications for subduction initiation in the eastern Paleo-Tethys

The Nanlinshan and Banpo mafic–ultramafic intrusions belong to the prominent Yunxian-Jinghong magmatic belt in the western part of the Simao Block, one of several Gondwana-derived continental fragments assembled by the closure of multiple Tethyan oceans. Different petrogenic models including ophiolites, MORB-type cumulates and Alaskan-type complexes have been proposed for these intrusions. In order to better constrain possible origins, we have undertaken an integrated geochronological, petrological and geochemical study of both intrusions. Precise CA-ID-TIMS zircon U–Pb dating indicates that the Nanlinshan and Banpo intrusions have similar ages of ~298 and 295?Ma, respectively, confirming that they are the oldest intrusive rocks in the Yunxian-Jinghong magmatic belt. A comparison between whole rock compositions and the compositions of major silicate minerals, including trace elements in clinopyroxene, reveals that (1) the ultramafic rocks of these intrusions are crystal cumulates of a relatively primitive magma, (2) the associated gabbroic and dioritic rocks are the products of more fractionated liquids, and (3) the parental magmas of these rocks were all depleted in some high field strength trace elements including Nb, Zr and Hf. Both intrusions are also characterized by elevated ε_Nd values between +3.4 and +6.6. The positive ε_Nd values coupled with negative Nb and Zr–Hf anomalies are consistent with the interpretation that these two intrusions are the products of subduction-related basaltic magmatism. The results from this study suggest that subduction of the Paleo-Tethys Ocean along the western margin of the Simao Block initiated as early as ~298?Ma and that the Simao Block and the Northern Qiangtang Block of the Tibet Plateau are separate Gondwana-derived continental fragments instead of a single fragment as previously reported.     

Cenozoic forearc gabbros from the northern zone of the Eastern Pontides Orogenic Belt,NE Turkey: Implications for slab window magmatism and convergent margin tectonics

The Eastern Pontides Orogenic Belt represents one of the best examples of fossil convergent margins in the eastern Mediterranean region. However, the origin and geodynamic setting of the late Mesozoic–Cenozoic magmatism in this belt remain controversial due to lack of systematic geological, geochemical and chronological data. The general consensus is that the late Mesozoic–Cenozoic igneous activity is related to northward subduction of oceanic lithosphere in the late Mesozoic and following collision between Tauride and Pontide blocks in the early Cenozoic. Here we present a comprehensive study focusing on the origin and geodynamic setting of gabbro bodies exposed along a narrow zone, parallel to the southeastern coast of the eastern Black Sea basin, in the Northern Zone of the Eastern Pontides Orogenic Belt.The studied gabbro bodies are hosted within late Cretaceous basaltic, andesitic, and dacitic volcanics including pyroclastic rocks and interbedded sedimentary rocks. The gabbro bodies range in size from 0.1 km² to 1.5 km², and outcrop patterns vary from round or elliptical to markedly elongate with sharp and discordant contact with the host rocks. Their mineral assemblage includes mainly clinopyroxene, plagioclase, minor olivine, amphibole, magnetite and rarely orthopyroxene, biotite, zircon and titanite. The occurrence of sutured grain boundaries on clinopyroxene and plagioclase, and the presence of reverse compositional zoning in clinopyroxene and olivine suggest mixing between magmas of contrasting compositions during mineral growth. Thermobarometric computations indicate that the temperature at the beginning of crystallization was ~ 1250 °C and crystallization was polybaric. Zircon and titanite U–Pb ages indicate that these small intrusions were emplaced into crustal rocks of the Eastern Pontides Orogenic Belt during Lutetian (45 ± 2 Ma). The depletion of HFSE is consistent with the involvement of an arc-related source in the petrogenesis of these rocks, and low to moderate enrichment Ce, Rb, Ba, K, Pb, Sr and Th suggests that involvement of subducted oceanic sediment was modest. The low Th content and low Th/Yb indicate that the role of sediment addition was nevertheless limited. The Nd, Sr and Pb isotopic data are consistent with the interpretation that the dominant source component in these gabbros is a depleted, peridotitic mantle, and that crustal contamination is relatively unimportant. We suggest that mafic magmas that produced the gabbroic intrusions were derived from melting of a depleted mantle source under the forearc region of the Eastern Pontides Orogenic Belt during southward subduction of two oceanic plates separated by a mid-ocean ridge, leading to the formation of a slab window. We also infer fractional crystallization and assimilation during both magma storage in the crust–mantle transition zone and transfer into the overlying arc crust.     

Crust-derived felsic magmatism in the Emeishan large igneous Province: New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block,China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The ε_Hf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding T_DM1 values are in the range of 919–523 Ma. The ε_Hf(t) values of zircon from the DSC I-type granite are between –1 and 3, with T_DM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ¹⁸O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.     

Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Querigut Complex, French Pyrenees

The Quérigut mafic–felsic rock association comprisestwo main magma series. The first is felsic comprising a granodiorite–tonalite,a monzogranite and a biotite granite. The second is intermediateto ultramafic, forming small diorite and gabbro intrusions associatedwith hornblendites and olivine hornblendites. A U–Pb zirconage of 307 ± 2 Ma was obtained from the granodiorite–tonalites.Contact metamorphic minerals in the thermal aureole providea maximum emplacement pressure of between 260 and 270 MPa. Petrographiccharacteristics of the mafic and ultramafic rocks suggest crystallizationat <300 MPa, demonstrating that mantle-derived magmas ascendedto shallow levels in the Pyrenean crust during Variscan times.The ultramafic rocks are the most isotopically primitive components,with textural and geochemical features of cumulates from hydrousbasaltic magmas. None of the mafic to ultramafic rocks havedepleted mantle isotope signatures, indicating crustal contaminationor derivation from enriched mantle. Origins for the dioritesinclude accumulation from granodiorite–tonalite magma,derivatives from mafic magmas, or hybrids. The granitic rockswere formed from broadly Proterozoic meta-igneous crustal protoliths.The isotopic signatures, mineralogy and geochemistry of thegranodiorite–tonalites and monzogranites suggest crystallizationfrom different magmas with similar time-integrated Rb/Sr andSm/Nd isotope ratios, or that the granodiorite–tonalitesare cumulates from a granodioritic to monzogranitic parent.The biotite granite differs from the other felsic rocks, representinga separate magma batch. Ages for Quérigut and other Pyreneangranitoids show that post-collisional wrenching in this partof the Variscides was under way by 310 Ma. KEY WORDS: Variscan orogeny; Pyrenees; Quérigut complex; epizonal magmatism; post-thickening; mafic–felsic association     

An overview of anorthosite-bearing layered intrusions in the Archaean craton of southern West Greenland and the Superior Province of Canada: implications for Archaean tectonics and the origin of megacrystic plagioclase

Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (>An₇₀) megacrystic (2–30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25–35%) in the hotter (1500–1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth.