Melting Of Lithospheric Mantle Research Articles

Mineralogical and geochemical constraints on contribution of magma mixing and fractional crystallization to high-Mg adakite-like diorites in eastern Dabie orogen, East China

The Liujiawa pluton which is located near the eastern boundary of the Dabie orogen is composed of multiple lithologic units including mainly gabbronorites, diorites, granodiorites and hornblende gabbros. Gabbronorites and hornblende gabbros occur as enclaves in dioritic hosts which show gradual contact with granodiorites. Zircon U–Pb dating indicates that gabbronorites and diorites formed coevally at ~128Ma, but they have distinct zircon Hf isotopes with εHf(t) of −26 to −23 (gabbronorite) and of −32 to −27 (diorite) respectively. Petrographic observations and rock-forming mineral compositions clearly show mixing between mafic and felsic magma end-members, which might have formed the homogeneous whole-rock Sr–Nd isotopes with εNd(t) of −17 to −25 and initial 87Sr/86Sr of 0.707 to 0.709. As revealed by zircon Hf isotopes, F concentrations in amphibole and biotite and thermodynamic modeling of crystallization, the gabbronorites represent enriched lithospheric mantle-derived magmas which evolved by fractional crystallization of orthopyroxene, clinopyroxene, magnetite and/or amphibole, whereas the granodiorites may be derived from the Dabie Archean basement. Mineralogical and geochemical data as well as major and trace element modeling show that the origin of diorites, previously interpreted as high-Mg adakites, can be explained by magma mixing between the crust-derived granodioritic magmas and the differentiation products of mantle-derived gabbronoritic magmas. As a result, the high-Mg adakite-like geochemistry of the diorites is a consequence of magma differentiation at a crustal depth, involving fractional crystallization and magma mixing, rather than an intrinsic feature of primitive melts. The mantle upwelling in the adjacent central Middle-Lower Yangtze River metallogenic (MLYR) belt during Late Jurassic–Early Cretaceous belt might have acted as a precursor and triggered the partial melting of lithospheric mantle beneath the eastern Dabie orogen and the further melting of orogenic basement, consistent with the model of Zhang et al. (2010) suggesting a magmatic link between the MLYR belt and the southeastern Dabie orogen.

Late Cretaceous crustal growth in the Gangdese area, southern Tibet: Petrological and Sr–Nd–Hf–O isotopic evidence from Zhengga diorite–gabbro

Recent studies of Gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern Tibet during the Jurassic–Early Eocene. However, the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes. Here, we report on the Zhengga diorite–gabbro suite from the Gangdese area. They consist of plagioclase (labradorite and anorthite), amphibole, clinopyroxene, biotite and minor magnetite, epidote, zircon and apatite. LA-ICP-MS zircon U–Pb dating for two samples gives a ca. 94Ma age for the Zhengga intrusive rocks, i.e., the Late Cretaceous. Apart from one high-SiO2 (52.2wt.%) diorite sample with slightly high K2O (1.72wt.%) and initial 87Sr/86Sr (0.7068) and low εNd(t) (−5.6) values, the gabbro samples are geochemically characterized by low SiO2 (39.8–50.1wt.%) and K2O (0.3–1.1wt.%), strongly negative Nb–Ta and positive Sr anomalies, and uniform initial 87Sr/86Sr (0.7043–0.7048). The gabbros can be divided into two groups: Group I gabbros with relatively low total rare earth element (REE), Rb and SiO2 contents and positive Eu anomalies, and Group II gabbros with slightly higher total REE, Rb and SiO2 contents and negative Eu anomalies. The Group I gabbros have εNd(t) (+1.7 to +4.1), and zircon εHf(t) (+6.5 to +11.1) and δ18O (5.89 to 7.24‰) values, which are slightly different to those of the Group II gabbros (−0.2 to +2.0, +2.9 to +6.5 and 6.24 to 7.05‰). Trace element compositions of amphibole and clinopyroxene grains suggest that the Zhengga mafic magmas contained a significant fluid-transported component, probably released from subducted oceanic lithosphere. We suggest that the parental magmas of the Zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts/fluids. The Group I gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas, with insignificant crustal contamination, whereas the Group II gabbros were probably produced by assimilation and fractional crystallization (AFC) processes from mafic magmas that were geochemically similar to the Group I gabbros. Pre-collisional underplating of mantle-derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late Late Cretaceous–Cenozoic granitoids of the Gangdese batholiths. This study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust.

Tectonic Setting of the Pebble and Other Copper-Gold-Molybdenum Porphyry Deposits within the Evolving Middle Cretaceous Continental Margin of Northwestern North America

The Pebble Cu-Au-Mo deposit in southwestern Alaska, containing the largest gold resource of any known porphyry in the world, developed in a tectonic setting significantly different from that of the present-day. It is one of a series of metalliferous middle Cretaceous porphyritic granodiorite, quartz monzonite, and diorite bodies, evolved from lower crust and metasomatized lithospheric mantle melts, which formed along much of the length of the North American craton suture with the Peninsular-Alexander-Wrangellia arc. The porphyry deposits were emplaced within the northernmost two of a series of ca. 130 to 80 Ma flysch basins that define the suture, as well as into arc rocks immediately seaward of the two basins. Deposits include the ca. 100 to 90 Ma Pebble, Neacola, and other porphyry prospects along the Kahiltna basin-Peninsula terrane boundary, and the ca. 115 to 105 Ma Baultoff, Carl Creek, Horsfeld, Orange Hill, Bond Creek, and Chisna porphyries along the Nutzotin basin-Wrangellia terrane boundary. The porphyry deposits probably formed along the craton margin more than 1,000 km to the south of their present latitude. Palinspastic reconstructions of plate kinematics from this period are particularly difficult because magmatism overlaps the 119 to 83 Ma Cretaceous Normal Superchron, a period when sea-floor magnetic data are lacking. Our favored scenario is that ore formation broadly overlaps the cessation of sedimentation and contraction and the transition to a transpressional continental margin regime, such that the remnant ocean basins were converted to strike-slip basins. The basins and outboard Peninsular-Alexander-Wrangellia composite superterrane, which are all located seaward of the deep crustal Denali-Farewell fault system, were subjected to northerly dextral transpression for as long as perhaps 50 m.y., beginning at ca. 95 ± 10 Ma. The onset of this transpression was marked by development of the mineralized bodies along fault segments on the seaward side of the basins. Geochemical and radiogenic isotopic data for igneous rocks associated with the Pebble porphyry deposit suggest continuous melt derivation from enriched lithosphere of a recently metasomatized mantle. These geochemical characteristics, coupled with the arc-continent-related collisional setting, suggest that lithospheric thickening and postcollisional lithospheric melting are the most likely cause of the ore-related magmatism. Subsequent to translation of the Alaskan margin terranes and early Tertiary oroclinal bending of Alaska, the northernmost Kahiltna basin and the Pebble deposit, as well as the other porphyry systems, reached their present-day locations along southern Alaska.

The Kultuk Volcano: spatial-temporal change of magmatic sources at the western terminus of the South Baikal basin between 18 and 12 Ma

The Kultuk volcano erupted at the axial South Baikal basin of the Baikal rift zone (BRZ). Now it exhibits facies of subvolcanic bodies, land-lava eruptions and subaqueous pillow lavas and hyaloclastites. The volcano was controlled by the Obruchev fault that is currently a border of the basin which amplitude of vertical movements is rapidly decreasing in the westward direction. It is found that the Kultuk volcano was active at the beginning and end of the volcanic activity period of the Kamar, Stanovaya, and Bystrinskaya volcanic zones, which took place 18–12 Ma ago. In previous papers, it was assumed that dominant structures in the area under study were major Cenozoic shear displacements along the Main Sayan fault and/or along the Tunka rift valley; however, at the current stage of our study, linear configurations of the volcanic zones do not reveal any of such displacements. Based on analyses of distribution of volcanic rocks in the relief at the western coast of Lake Baikal, distinct vertical crustal movements are revealed; such movements started in the Early Miocene and continue to the present time. It is concluded that volcanism was controlled by the trans-tensional system of volcanic zones. Sources are identified for the shallow lithospheric mantle melt with the substantial admixture of the low-crust component and deeper asthenospheric mantle melts in the Kamar and Stanovaya volcanic zones; for the Bystrinskaya volcanic zone, only components from the deeper source are revealed. The local shallow mantle magmatism occurred only within the lithosphere extension zone beneath the South Baikal basin. The lithosphere thinning is reflected in the change of activity from the sub-lithospheric to lithospheric sources under the Kamar zone. Rifting of the axial structure is recorded at the root of the Slyudyanka lithospheric block that was subjected to the collision-related Early Paleozoic metamorphism. Geochemical characteristics of the collision-type components were inherited by the Miocene basaltic melts.

A 100 Ma bimodal composite dyke complex in the Jiamusi Block, NE China: An indication for lithospheric extension driven by Paleo-Pacific roll-back

A 125m-wide bimodal composite dyke complex, consisting of rhyolite and dolerite dykes, was emplaced into Cretaceous volcanic strata of the Songmuhe Formation in the Jiamusi Block of NE China. The dolerite dykes are sub-vertical, strike north-south, and intruded into both the country rocks and rhyolite dykes soon after the latter solidified. SHRIMP zircon U–Pb dating shows that the rhyolite dykes were emplaced at 100±2Ma and the dolerite dykes were also most likely emplaced at 100±2Ma. The rhyolite is characterized by enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and depletion in high-field strength elements (HFSE). It shows a significant negative Eu anomaly, and has εNd(t) values ranging from 0.49 to 1.66 and two groups of initial 87Sr/86Sr ratios at 0.7045 and 0.7061. The rhyolite displays the compositional signature of Peraluminous Ferroan Granitoid, indicating it was derived by either differentiation of basalt and/or low pressure partial melting of crust. The dolerite is also characterized by enrichment in LILE and LREE, and depletion in HFSE. It has a weak negative Eu anomaly and has εNd(t)=−1.22 to +3.26, and (87Sr/86Sr)i=0.7057–0.7074. The dolerite originated from partial melting of lithospheric mantle which was affected by subducted oceanic crust, and experienced different amounts of crustal contamination. Such bimodal dyke complexes are an important indicator of crustal extension under the influence of mantle processes. Thus the dyke complex in the Jiamusi Block indicates mid-Cretaceous intra-plate extension in NE China related to the subduction of the paleo-Pacific plate along the eastern Eurasian continental margin. When compared with Mesozoic bimodal magmatism further to the west, our new data support a temporal eastward migration of magmatism over a distance >1000km from ~160Ma to ~100Ma. This was most likely associated with roll-back of the paleo-Pacific Plate and consequent upwelling of asthenospheric mantle.

Mesoproterozoic differential metasomatism in subcontinental lithospheric mantle of Central Indian Tectonic zone: Evidence from major and trace element geochemistry of padhar mafic-ultramafic complex

Metasomatism above subduction zone is an important process capable of producing a heterogeneous mantle wedge with locally varying characters. This, in turn, leads to variation in character of the mafic and intermediate rocks produced through partial melting of lithospheric mantle. The Padhar mafic-ultramafic complex is situated in western part of the Betul belt, Central India, shows enrichment in Rb, Ba, Th and Pb and depletion in Nb, Hf and Zr. Major and trace element data along with REE, confirms the presence of a metasomatized mantle above a Mesoproterozoic subduction zone. Differential petrological evolution, corresponding to the different magma batches, as reflected in almost all of the binary element / oxide variation diagrams, testifies small scale metasomatic heterogeneity in the underlying suprasubduction zone mantle wedge.

Plume scar in the mantle lithosphere beneath the Seychelles revealed by seismic imaging

Continental flood basalts are commonly thought to form by direct melting of lithospheric mantle, either during rifting episodes or when a mantle plume impinges on the base of the lithosphere. If correct, then significant alteration of the chemical and physical properties of the lithospheric mantle should be observable beneath such provinces. To date, however, this hypothesis remains unproved through lack of direct evidence. Here we present the results from a seismic experiment conducted in the Seychelles, a region that separated from India at the time of the eruption of the Deccan Traps. The analysis shows a stratified upper mantle with exceptionally low S-wave velocities at depths between 50 and 190km. It is difficult to explain these anomalies without invoking the presence of melt. However, the depth extent of the low velocities rules out the presence of silicate melt. We argue that the most probable interpretation is that these layers contain sulphide melt trapped from earlier interactions of the lithosphere with episodes of continental-flood basalt generation. The presence of this melt over a depth interval of ∼130km suggests that it is stagnant, and has remained coupled with the Seychelles lithosphere.

Two types of alkaline volcanics in the southwestern Iberian margin: The causes of their diversity

The diverse geodynamic conditions of the parental magma’s melting are responsible for the compositional diversity of the alkaline volcanics near the southwestern margin of Iberia. The petrological-geochemical data show that the volcanics of the Gorringe Bank originated within the continental plate. The parental melilitite melts depleted in silica and anomalously enriched with trace elements could have been generated only in deep settings with a low degree of metasomatically enriched mantle matter melting. The volcanic melilitite-nephelinite-phonolite series is widespread in alkaline provinces of the Eurasian, African, and other continental plates. The Ampere, Josephine, and other seamounts and islands of the region are largely composed of volcanic rocks belonging to the picrobasalt-hawaiite-mugearite association. Their parental magmas were generated within the oceanic plate at shallower depths under a higher degree of moderately enriched oceanic lithospheric mantle melting. Both series of volcanics were formed under the influence of mantle plumes.

Adakitic rocks derived from the partial melting of subducted continental crust: Evidence from the Eocene volcanic rocks in the northern Qiangtang block

Eocene is a critical time for the elevation of Tibetan Plateau and global climate change, and previous studies suggested that the Eocene elevation was caused by intra-continental subduction of the Songpan–Garze block beneath the Qiangtang block. This paper reports zircon U–Pb age and geochemistry of the Eocene volcanic rocks from the Zuerkenwula mountain area in the northern part of Qiangtang block, and proposes that both slab break-off of the Neo-Tethys oceanic slab along the Bangong–Nujiang suture and intra-continental subduction of the Songpan–Garze block beneath the Qiangtang block caused the extensive partial melting of lithospheric mantle and subducted Songpan–Garze continental crust, which resulted in the significant elevation of the Tibetan Plateau. The volcanic rocks have LA-ICP MS U–Pb zircon age of 40.25±0.15Ma (MSWD=2.1, 2σ), which is contemporaneous with the Eocene eclogites in the Great Himalayan and K-rich lavas in the southeastern Tibet. They display some adakitic characteristics with SiO2=57.44 to 68.72%, TiO2=0.38 to 0.81%, Na2O=2.89 to 4.35%, K2O=2.77 to 4.48%, Al2O3=13.92 to 18.22%, A/CNK=0.69 to 1.03, MgO=0.27 to 5.86% with Mg# ranging from 13.2 to 72.0, strongly depleted in heavy rare earth elements (HREEs) (Yb=0.92 to 1.51ppm and Y=10.1 to 24.1ppm), in combination with their positive Sr anomalies, high Sr/Y ratios and no significant Eu anomalies, which suggest a garnet-in and plagioclase-free source residue. These volcanic rocks can be divided into high-Mg# (>45) and low-Mg# (<45) groups. Both of the two groups share evolved Sr–Nd–Pb isotopic compositions with 87Sr/86Sr=0.707412–0.708284; εNd(t)=−4.0 to −5.7; 206Pb/204Pb=18.7499–18.8189, 207Pb/204Pb=15.7189–15.7384; 208Pb/204Pb=39.166–39.262. The geophysical data and regional geological setting suggest that the low-Mg# adakitic rocks were derived from the decompression melting of a subducted lower continental crust, when low-Mg# adakitic melts in the overlying peridotite mantle wedge captured some olivine crystals, resulting in their elevated Mg# and MgO values.

Geochronology, petrogenesis and tectonic implications of Triassic granitoids from Beishan, NW China

Five Triassic granitic plutons have been identified from the Beishan orogen by zircon U–Pb dating that defines two age groups: 240–238Ma and 225–217Ma. They belong to the high-K calc-alkaline and shoshonitic series and have alkalic and alkali-calcic characteristics, and are weakly peraluminous–metaluminous. These granitoids are mainly highly fractionated I-type granite and A-type granite. They have high Isr values (0.7049–0.7096) and negative εNd(t) values of −0.9 to −2.8, with Nd model ages of 0.89–1.13Ga, suggesting a crustal contribution. They also exhibit relatively high zircon εHf(t) values (−1.3 to +6.1) and two-stage Hf model ages of 1.34–0.87Ga, suggesting a mixture of mantle and various amounts of crustal components. The mantle component probably originated from underplated magmas produced by partial melting of lithospheric mantle metasomatized by fluids or melts, possibly derived from pre-existing subducted materials. The mid-Triassic (240–238Ma) pluton has relatively high Sr/Y (15.9–41.8) ratios and MgO (1.22–3.77%) contents, with negligibly to weakly negative Eu anomalies (δEu=0.57–0.80), indicating adakite-like features. This was likely the result of partial melting of crustal components in response to voluminous underplated mantle-derived magmas, probably linked to lithospheric delamination and asthenospheric upwelling. The Late Triassic (217–225Ma) granitic plutons show strong negative anomalies of Eu, Ba, Nb, Ta, Sr, P and Ti. These were likely the result of fractional crystallization and crustal contamination for the granitic magma emplaced in a post-orogenic setting.

A Paleogene extensional arc flare‐up in Iran

Arc volcanism across Iran is dominated by a Paleogene pulse, despite protracted and presumably continuous subduction along the northern margin of the Neotethyan ocean for most of Mesozoic and Cenozoic time. New U‐Pb and 40Ar/39Ar data from volcanic arcs in central and northern Iran constrain the duration of the pulse to ∼17 Myr, roughly 10% of the total duration of arc magmatism. Late Paleocene‐Eocene volcanic rocks erupted during this flare‐up have major and trace element characteristics that are typical of continental arc magmatism, whereas the chemical composition of limited Oligocene basalts in the Urumieh‐Dokhtar belt and the Alborz Mountains which were erupted after the flare‐up ended are more consistent with derivation from the asthenosphere. Together with the recent recognition of Eocene metamorphic core complexes in central and east central Iran, stratigraphic evidence of Eocene subsidence, and descriptions of Paleogene normal faulting, these geochemical and geochronological data suggest that the late Paleocene‐Eocene magmatic flare‐up was extension related. We propose a tectonic model that attributes the flare‐up to decompression melting of lithospheric mantle hydrated by slab‐derived fluids, followed by Oligocene upwelling and melting of enriched mantle that was less extensively modified by hydrous fluids. We suggest that Paleogene magmatism and extension was driven by an episode of slab retreat or slab rollback following a Cretaceous period of flat slab subduction, analogous to the Laramide and post‐Laramide evolution of the western United States.

Geological, geochemical characteristics and isotope systematics of the Longqiao iron deposit in the Lu-Zong volcano-sedimentary basin, Middle-Lower Yangtze (Changjiang) River Valley, Eastern China

The Middle-Lower Yangtze (Changjiang) River Valley metallogenic belt is located on the northern margin of the Yangtze Craton of eastern China. Most polymetallic deposits in the Changjiang metallogenic belt are clustered in seven districts where magmatism of Mesozoic age (Yanshanian tectono-thermal event) is particularly extensive. From west to east these districts are: E-dong, Jiu-Rui, Anqing-Guichi, Lu-Zong, Tong-Ling, Ning-Wu and Ning-Zhen. World-class iron ore deposits occur in the Lu-Zong and Ning-Wu ore clusters, which are mainly located in continental fault-bound volcanic-sedimentary basins. One of these deposits is the Longqiao iron deposit, discovered in the northern part of the Lu-Zong Basin in 1985. This deposit consists of a single stratabound and stratiform orebody, hosted in sedimentary carbonate rocks of the Triassic Dongma'anshan Formation. A syenite pluton (Longqiao intrusion) is situated below the deposit. The iron ore is massive and disseminated and the ore minerals are mainly magnetite and minor pyrite. Wall rock alteration mostly consists of skarn minerals, such as diopside, garnet, potassic feldspar, quartz, chlorite, phlogopite and anhydrite. Thin sedimentary siderite beds of Triassic age occur as relict laminated ore at the top and the margin of the magnetite orebody. These sideritic laminae are part of Triassic evaporite-bearing carbonate deposits (Dongma'anshan Formation). Sulfur isotopic compositions show that the sulfur in the deposit was derived from a mixture of magmatic hydrothermal fluids and carbonate–evaporite host rocks. Similarly, the C and O isotopic compositions of limestones from the Dongma'anshan Formation indicate that these rocks interacted with magmatic hydrothermal fluids. The O isotopic compositions of the syenitic rocks and minerals from the deposit show that the hydrothermal magnetite and skarn minerals were formed from magmatic fluids. The Pb isotopic compositions of sulfides are similar to those of the Longqiao syenite. Phlogopite coexisting with magnetite in the magnetite ores yielded a plateau age of 130.5 ± 1.1 Ma (2 σ), whereas the LA-ICP MS age of the syenite intrusion is 131.1 ± 1.5 Ma, which is slightly older than the age of phlogopite. The Longqiao syenite intrusion may have crystallized from a parental alkaline magma, generated by partial melting of lithospheric mantle, during extensional tectonics. The ore fluids were probably first derived from magma at depth, later emplaced in the sedimentary rocks of the Dongma'anshan Formation, where it interacted with siderite and evaporite-bearing carbonate strata, resulting in the formation of magnetite and skarn minerals. The Longqiao iron deposit is a skarn-type stratabound and stratiform mineral system, genetically and temporally related to the Longqiao syenite intrusion. The Longqiao syenite is part of the widespread Mesozoic intracontinental magmatism (Yanshanian event) in eastern China, which has been linked to lithospheric delamination and asthenospheric upwelling.

Melting of enriched Archean subcontinental lithospheric mantle: Evidence from the ca. 1760 Ma volcanic rocks of the Xiong’er Group, southern margin of the North China Craton

The Xiong’er Group is an important geologic unit in the southern margin of the North China Craton. It is dominated by the volcanic rocks, dated at 1763 ± 15 Ma, that have SiO 2 contents ranging from 52.10 wt% to 73.51 wt%. These volcanic rocks are sub-alkaline and can be classified into three subgroups: basaltic andesites, andesites and rhyolites. They unexceptionally show enrichment of light rare earth elements (LREE) and share similar trace element patterns. Depletions in Nb, Ta, Sr, P and Ti relative to the adjacent elements are evident for all the samples. The volcanic rocks are evolved with low MgO contents (0.29–5.88 wt%) and accordingly low Mg # values of 11–53. The Nd isotopes are enriched and show a weak variation with ɛ Nd( t) = −7.12 to −9.63. Zircon Hf isotopes are also enriched with ɛ Hf( t) = −12.02 ± 0.45. The volcanic rocks of the Xiong’er Group are interpreted to represent fractional crystallization of a common mantle source. The volcanic rocks might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the Late Archean. This brings a correlation with the subduction-modified lithospheric mantle in an extensional setting during breakup of the Columbia supercontinent in the late Paleoproterozoic, rather than in an arc setting. The elevated SiO 2 contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar Nd isotopic characteristics to the subcontinental lithospheric mantle. The existence of extensive Xiong’er volcanic rocks (60,000 km 2) indicates an early large-scale subduction-related metasomatism in the area and probably suggest a flat subduction model for the plate-margin magmatism in the Late Archean.

Petrogenesis of Indosinian volcanic rocks in Songpan-Garze fold belt of the northeastern Tibetan Plateau: New evidence for lithospheric delamination

In the Songpan-Garze fold belt of the northeastern Tibetan Plateau, an Indosinian lithospheric delamination model has been proposed, based on previous investigation of widespread granitoids. However, this model lacks comparable information from volcanism in the area. During the Indosinian delamination in the Songpan-Garze fold belt, whether partial melting of lithospheric mantle taken place is debated. This paper reports U-Pb zircon LA-ICP-MS ages, geochemical and Sr-Nd-Hf isotopic compositions from the Aba and Wasai calc-alkaline volcanic rocks in the central Songpan-Garze fold belt. Obtained magma crystallization ages are 210±3 Ma for the Aba andesite and 205±1 Ma for the Wasai andesite. These are consistent with magma crystallization ages of the late Indosinian granitoids in the Songpan-Garze fold belt that formed in a post-collisional tectonic setting. The Aba and Wasai andesites have distinct geochemical singnatures. The former has higher Al2O3, K2O, Rb but lower Na2O, Ba and Sr contents, suggesting differences in their magmatic evolution. The Aba andesites have ISr values of 0.7070–0.7076 and ɛNd(t) values of −3.9 to −5.3, and the Wasai andesites have ISr values of 0.7075–0.7077 and ɛNd(t) values of −3.6 to −3.9. Zircons show ɛHf(t) values of −3.7 to 0.3 for the Aba andesites and −2.7 to 5.5 for the Wasai andesites. Geochemical and Sr-Nd-Hf isotopic compositions indicate that fractional crystallization and crustal assimilation processes are not key roles for their magma evolution, implying that their chemical compositions are those of primary melts. We suggest that the magma of the Aba andesites originated predominantly from a crustal source, with a minor mantle-derived component. The source region of the magma was likely at the crust-mantle boundary. The magma of the Wasai andesites resulted from partial melting of lithospheric mantle, which was probably metasomatized by fluids so that it was amphibole bearing. The petrogenesis of the Aba and Wasai andesites provides an additional evidence for the lithospheric delamination in the the Songpan-Garze fold belt, indicating that this process invoked mantle asthenosphere upwelling and caused the partial melting of remaining lithospheric mantle.

Geochemical and scintillometric characterization and correlation of amethyst geode-bearing Paraná lavas from the Quaraí and Los Catalanes districts, Brazil and Uruguay

AbstractGeochemical studies of the six lowermost lava flows of the Cretaceous Serra Geral Formation (Paraná volcanic province) in Quaraí (Brazil) and Artigas (Uruguay) were combined with flow-by-flow field studies of structures and scintillometric profiles to establish a consistent regional stratigraphic framework over at least 100 km. This greatly improves exploration capability for amethyst and agate geodes. A basalt, colada Mata Olho (Alegrete facies, Serra Geral Formation), was the first lava to flow over the ancient Botucatu desert in the region, but an andesite, colada Catalán, overstepped this basalt in many places, perhaps palaeohighs. Four basaltic andesites complete the lava stratigraphy in this formation, adding up to 300 m of lavas. The stratigraphic sequence of contrasting lava compositions is 51.0 wt% SiO2 in the first lava, followed by 57.5, 52.5, 56.0, 53.0 and finally 54.5 wt% SiO2. Overall MgO variation is between 2 and 7 wt%. All lavas in the two districts are low-Ti (&lt;2.0 wt% TiO2) of the Gramado type. The characteristic contents of most major and trace elements (124 rock samples analysed) allow the ready identification of each lava. Contrasting rock chemistry also results in strong variation in scintillometric values (270 points measured in the field and nineteen continuous borehole profiles); from bottom to top of the stratigraphy, the cps values are 49±3.2, 123±10.3, 62±4.7, 94±4.6, ~45 and ~85. Colada Catalán has the structure of aa lava, particularly the contorted igneous banding and autobreccias in the upper and lower crusts. In some places, a 2 m thick, silicified sandstone layer lies on top of some coladas, and silicified sandstone forms breccias with volcanic rocks. Geochemistry of the six lavas indicates complex evolution, involving melting of lithospheric mantle, injection into the crust and assimilation of crust followed by fractional crystallization. This study indicates the possibility of world-class deposits of amethyst geodes on the Brazilian side of the border with Uruguay.

Origin of anorogenic ‘lamproite-like’ potassic lavas from the Denizli region in Western Anatolia Extensional Province, Turkey

The Denizli region of the Western Anatolia Extensional Province (WAEP) includes a typical example of intra-plate potassic magmatism. Lamproite-like K-rich to shoshonitic alkaline rocks erupted in the Upper Miocene-Pliocene in a tensional tectonic setting. The absence of Nb and Ta depletion, low Th/Zr and high Nb/Zr ratios and distinct isotopic values (i.e. low 87Sr/86Sr, 0.703523–0.703757; high 143Nd/144Nd, 0.512708–0.512784; high 206Pb/204Pb, 19.079–19.227, 207Pb/204Pb, 15.635–15.682, 208Pb/204Pb, 39.144–39.302) mark an anorogenic geochemical signature of the Denizli volcanics. All of the lavas are strongly enriched in large-ion-lithophile elements (e.g. Ba 1,100–2,200 ppm; Sr 1,900–3,100 ppm; Rb 91–295 ppm) and light rare-earth elements (e.g. LaN = 319–464), with a geochemical affinity to ocean-island basalts and lack of a recognizable subduction signature or any evidence for crustal contamination. The restricted range of isotopic (Sr, Nd, Pb) ratios in both near-primitive (Mg# 66.7–77.2) and more evolved (Mg# 64.6–68.7) members of the Denizli volcanics signify their evolution from an isotopically equilibrated parental mantle source. Their high Dy/Yb and Rb/Sr values also suggest that garnet and phlogopite were present in the mantle source. Their strong EM-II signature, very low Nd model ages (0.44–049 Ga) and isotopic (Sr-Nd-Pb) values analogous to those of the Nyiragongo potassic basanites and kimberlites from the African stable continental settings, suggest that the parental melts that produced the Denizli volcanics are associated with very young and enriched mantle sources, which include both sublithospheric and enriched subcontinental lithospheric mantle melts. Mantle-lithosphere delamination probably played a significant role in the generation of these melts, and could be related to roll-back of the Aegean arc, lithospheric extension and asthenospheric mantle upwelling.

Petrogenesis of Plio-Quaternary post-collisional ultrapotassic volcanism in NW of Marand, NW Iran

Post-collisional potassic and ultrapotassic (UP) alkaline magmas were erupted northwest of Marand in northern part of Urumieh-Dokhtar magmatic arc (UDMA) during Plio-Quaternary. The studied rocks display microlithic porphyritic texture with phenocrysts of clinopyroxene, leucite, and plagioclase ± biotite ± olivine. The UP volcanic rocks are mostly silica undersaturated with normative nepheline, high Mg#, and high K 2O/Na 2O ratios. They are characterized by significant enrichment in LILE and LREE and depletion in selected high-field-strength elements, such as Nb, Ta and Ti. All of the studied samples display fractionated REE pattern (Ce/Yb) N = (15.39–18.08) and negative Eu anomalies (Eu/Eu∗ = 0.71–0.78). Furthermore, they exhibit high Ba/Nb (41–60) and Ba/Ta (682–1139) ratios, which are a typical feature of subduction related-magmas. Rare-earth element modeling, Rb/Sr and Ba/Rb ratios indicate that they can be generated from low degree partial melting of lithospheric mantle with phlogopite-garnet peridotite source. We suggest that post-collisional volcanic activity during late Miocene to Quaternary occurred by rollback and slab breakoff processes after Neo-Tethys subduction. The slab breakoff mechanism would have caused the direct contact of hot asthenospheric mantle with metasomatised sub-continental lithospheric mantle and also has increased the thermal gradient, which weakened the lithosphere and assisted to lithospheric extension and magma ascent.

Petrogenesis of post-collisional A-type granitoids from the Urumieh–Dokhtar magmatic assemblage, Southwestern Kerman, Iran: Constraints on the Arabian–Eurasian continental collision

Three plutons (Deh-Siahan, Bande-Bagh and Baghe-Khoshk Sharghi, collectively referred to as the DBB hereafter) in southwestern Kerman, in the southeastern part of the Urumieh–Dokhtar magmatic assemblage (UDMA) of the Zagros orogenic belt differ from the typical calc-alkaline metaluminous, I-type intrusions of the region. The DBB intrusions have a distinct lithological assemblage varying from diorite through monzogranite and monzonite to alkali feldspar syenite and alkali granite. The DBB granitoids are metaluminous to slightly peraluminous, alkaline to shoshonitic in composition and have high total alkali contents with K 2O > Na 2O, high FeO T /MgO values, and low CaO and MgO contents. They are enriched in some LILEs (such as Rb and Th) and HFSEs (such as Zr, Y and REEs except Eu) and depleted in Sr and Ba relative to primordial mantle, and have low concentrations of transitional metals. These features along with various geochemical discriminant diagrams suggest that the DBB granitoids are post-collisional A-type granitoids, which had not been recognized previously in the UDMA. The chondrite-normalized REE patterns of the DBB granitoids show slightly enriched light REEs [(La/Sm) N = 2.26–4.13], negative Eu anomalies [(Eu/Eu*) N = 0.19–0.74] and flat heavy REE patterns [(Gd/Yb) N = 0.80–1.87]. The negative Eu anomaly indicates an important role for plagioclase and/or K-feldspar during fractional crystallization. Whole-rock Rb–Sr isotope analysis yields an isochron age of 33 ± 1 Ma with an initial 87Sr/ 86Sr value of 0.7049 ± 0.0001. Whole-rock Sm–Nd isotope analysis gives ε Nd t values from + 2.56 to + 3.62 at 33 Ma. The positive ε Nd t and low I Sr values of the DBB granitoids together with their T DM of 0.6–0.7 Ga suggest their formation from partial melting of a lithospheric mantle source, modified by fluids or melts from earlier subduction processes. Melting of lithospheric mantle occurred via a dehydration melting process at pressures below the garnet stability field, as a consequence of lithospheric mantle delamination or break-off of a subducted slab and melting of the lithospheric mantle by upwelling of hot asthenosphere. On the basis of Rb/Sr age dating and the post-collisional geochemical signatures of the DBB granitoids, along with extensive pre-collisional volcanic eruptions in Middle Eocene, we suggest Late Eocene for the time of collision between the Arabian and Central Iranian plates. This also implies that the calc-alkaline I-type intrusions in the southwestern Kerman and in other parts of the UDMA may have formed in a post-collisional context.

Halogen and trace-element chemistry in the Gardar Province, South Greenland: Subduction-related mantle metasomatism and fluid exsolution from alkalic melts

XRF analyses of 152 magmatic dyke samples from a broad area (150×60km) of the Gardar Province in Southern Greenland span the time 1280 to 1163Ma and represent a wide compositional range (transitional olivine basalts to trachytes, alkalinity index of 0.3 to 1.5). Among those, 16 dyke samples were additionally analysed for Cl and Br.Generally, the dykes represent a continuous fractionation trend from relatively unfractionated basalts to more highly fractionated trachytes. Dykes from different areas exhibit a diverse geochemistry suggesting a heterogeneous and metasomatised mantle source. Enrichment in LILE, LREE and Sr and depletion in HFSE, Nb and Ti suggest that some of the metasomatism may have been associated with subduction processes pre-dating Gardar activity by some 600Ma (Ketilidian orogeny).The dykes are characterised by high F contents up to 1.2wt.%, particularly in the vicinity of the Ivigtut fluoride deposit. F was probably derived from partial melting of lithospheric mantle enriched in F-apatite and F-phlogopite. High Cl/Br (>500) and low Cl/F (<1) ratios of the dykes point to a fluid degassing/separation process which is supported by mineral/rock and fluid inclusion data from the Ilímaussaq and Ivigtut intrusions. There, the analysed rocks and minerals generally show high Cl/Br (>300) and low Cl/F (<1) weight ratios whereas the fluid inclusions have complementary low ratios (Cl/Br ±100; Cl/F >10). Our investigations are in accordance with experimental data which show that F is preferentially enriched in the melt whereas Cl and especially Br are lost with the fluid phase. Accordingly, the halogens show an increase in incompatible behaviour in the magma in the order of F<Cl<Br.

The influence of plume head–lithosphere interaction on magmatism associated with the Yellowstone hotspot track

Although commonly attributed to a mantle plume, time-transgressive magmatism of the Snake River Plain–Yellowstone (SRPY) province differs in important ways from that associated with typical oceanic hotspots. A fundamental question concerns the relative contributions of lithosphere vs. upwelling sub-lithospheric mantle to formation of SRPY basaltic magmas. Specifically, association of this province with initially thick and cold Archean lithosphere (Wyoming craton) poses a problem in that this lid will hinder and possibly prevent melting of rising plume material. Assuming an anhydrous peridotite mantle, melting can only occur if (1) the lid can be substantially thinned over geologically reasonable time and/or (2) the upwelling material is exceptionally warm, or (3) the lid was suitably thin to begin with. Petrologic modeling indicates that SRPY primitive basalts last segregated from mantle at conditions (< 1500 °C and ∼ 100 km depth; Leeman, W.P., Schutt, D.L., Hughes, S.S., 2009. Thermal structure beneath the Snake River Plain: implications for the Yellowstone hotspot. J. Volcanol. Geotherm. Res.) only slightly warmer than MORB-source mantle and significantly cooler than sources of many oceanic hotspot magmas. In this study, geodynamic models were developed to evaluate lithospheric thinning processes. The motivation for the modeling is the observation that if the lithosphere is initially more than 200 km thick – typical of many cratons – then thinning by at least a factor of two is required to allow decompression melting of an ascending plume, assuming low volatile content and high excess temperature (potential temperature > 1500 °C). Fully dynamic models were applied to investigate the extent and rate of lithosphere thinning assuming an initial structure representative of the Wyoming craton. We find that thermal erosion by plume impingement alone appears incapable of providing the required lithospheric thinning. Alternative models (e.g., low-angle Laramide subduction, lithospheric delamination) also conflict with geochemical evidence that SRPY basalts contain a dominant contribution of old, isotopically evolved mantle material — presumably derived from subcontinental lithospheric mantle (SCLM). We conclude that SCLM is likely to be preserved, that the thick SCLM lid prevents substantial melting of rising plume material (tomographically imaged), and that SRPY basalts are predominantly derived by melting of lithospheric mantle.