Plagioclase Xenocrysts Research Articles

40Ar/39Ar geochronology of the Drakensberg continental flood basalts: Understanding large argon isotopic variations in mafic groundmass and plagioclase size fractions

The extrusive component of the Karoo Large Igneous Province, the Drakensberg Continental Flood basalts of South Africa and Lesotho, is mainly composed of altered tholeiitic basalts without uraniferous mineral phases that are suitable for high-precision dating (e.g. baddeleyite or zircon), and thus dating is restricted to 40Ar/39Ar groundmass and plagioclase. Previous 40Ar/39Ar dates of plagioclase span from 178.9 ± 4.2 Ma to 189.2 ± 3.3 Ma, while most dates range between 180 and 183 Ma (Jourdan et al., 2007b; Moulin et al., 2011; Moulin et al., 2017). Dating mafic rocks can be hindered by the presence of plagioclase xenocrysts (inherited 40Ar), and the ubiquitous presence of alteration phases. Separation protocols and acid leaching are used to minimize the effect of these issues, although previous 40Ar/39Ar studies of the Drakensburg lavas did not interpret their dates within a petrochronological context, and thus they are open to inaccuracy. We have used cathodoluminescence imaging, QEMSCAN analysis and optical petrology of thin sections to show that two distinct generations of plagioclase occur within the lavas, and at least two different phases of K-rich (adularia and K-rich clays) and Ca- alteration have occurred. 40Ar/39Ar dates have been obtained from some acid-leached and unleached aliquots of groundmass and both plagioclase generations. Our results show that i) large, cluster-forming plagioclase yield older 40Ar/39Ar dates and are interpreted to be xenocrystic, ii) apparently unaltered plagioclase in thin sections hosts secondary Ar reservoirs related to alteration, and iii) alteration and leaching can enhance the effects of 37,39Ar recoil in plagioclase. We conclude that the most accurate date for the Drakensburg lavas that applies to the entire thickness of the lava stacks is 184.4 ± 1.2 Ma, which was obtained from the matrix-forming plagioclase that contains the highest proportion of non-inherited radiogenic 40Ar, is least affected by secondary alteration and did not apparently suffer the effects of Ar isotope recoil.

Unravelling the hidden evidences of magma mixing processes via combination of in situ Sr isotopes and trace elements analyses on plagioclase crystals

Granites from the ~580 Ma oxidized A-type Salto Rapakivi Granite (Itu Granitic Province, SE Brazil) bear abundant felsic microgranular enclaves (FMEs) and scarce cm-sized mafic microgranular enclaves (MMEs) with whole-rock Sr and Nd isotope signatures identical to those of their host rapakivi granites. In order to unravel the sources involved in the formation of both enclave types we conducted in situ trace elements and Sr isotopic analyses on plagioclase megacrysts from representative rocks of rapakivi granites, porphyry granites and both microgranular enclave varieties. Plagioclase from host rapakivi granites have 87Sr/86Sr ~0.7068, and are relatively more radiogenic than those from the enclaves. Plagioclase xenocrysts from the FMEs typically register a rimward decrease in 87Sr/86Sr ratios, with core 87Sr/86Sr up to 0.7069, whereas rims are usually in equilibrium with whole rock (~0.7060–0.7063). Crystals with U-shaped Sr isotopes profiles (less radiogenic cores with rimward increase in 87Sr/86Sr ratios) are rare, as are crystals with strongly radiogenic signatures observed in partially digested xenoliths found within the enclaves (87Sr/86Sr ~0.709–0.710). Plagioclase from MMEs show more primitive signatures (87Sr/86Sr 0.7055–0.7058). Crystals from the MMEs are distinguished by higher Sr (~1000 ppm) and lower Eu (< 0.5 ppm) and light rare earth elements (LREE, 6–12 ppm) compared to crystals from FMEs, porphyry and granite, which have Sr = 200–600 ppm, and LREE = 20–80 ppm. Our results allowed us to infer that FMEs were formed by mixing between a resident melt similar to that of rapakivi granites with a less radiogenic melt similar to that which originated the MMEs. Therefore the combined use of Sr isotopes and trace element contents in plagioclase crystals from A-type granites might shed light on the sources of rock varieties that have similar whole-rock isotope signatures.

History of Subduction Polarity Reversal During Arc‐Continent Collision: Constraints From the Andaman Ophiolite and its Metamorphic Sole

AbstractSubduction polarity reversal during arc‐continent collision has been proposed as a key mechanism to initiate new subduction zones. Despite often interpreted, well‐exposed geological record that document the reversal is sparse. The ophiolitic lithounits of the Andaman and Nicobar Islands have been proposed to have formed during the initiation of a new subduction zone following the collision of the Woyla Arc of Sumatra with Sundaland (Eurasia). We here present new field, petrological and geochronological data to evaluate the timing of the initiation of Andaman subduction. We targeted the previously inferred but unstudied metamorphic sole of the Andaman ophiolites that witnessed juvenile subduction. Thermodynamic modeling reveals that the exposed amphibolites of the sole formed at around 0.9 GPa and 675 °C. We dated two samples of the metamorphic sole using the Ar/Ar method on amphibole, giving cooling ages of 106.4 ± 2.1 and 105.3 ± 1.6 Ma. This is similar to published ages from plagioclase xenocrysts in recent Barren Island volcanics and in zircons from a gabbro sample from the Andaman ophiolite, which we interpret as the age of the original ophiolite formation. The Ar/Ar ages are considerably older than arc magmatic gabbros and plagiogranites of the overlying ophiolite previously dated at 99–93 Ma and thought to reflect the ophiolite age but recently reinterpreted as a volcanic arc built on the ophiolite. Combined with the ages of Woyla‐Sundaland collision, we argue that subduction polarity reversal occurred in a transient period of perhaps some 10 Myr, similar to recent settings.

Geochemistry of Dalma metavolcanic Suite from Proterozoic Singhbhum Mobile Belt, Eastern India: Implications for Petrogenesis and Tectonic Setting

ABSTRACT The Dalma metavolcanic belt is composed of ultramafics at the base, tuffs on top and basalts in-between. It extends for about 200kms east-west in the middle of the Proterozoic Singhbhum Mobile Belt. Ultramafics are chiefly composed of actinolite, hornblende, relic pyroxene and olivine. The main mineral constituents of basalts are actinolite, hornblende, chlorite, epidote, clinozoisite and plagioclase whereas; tuffs are composed of xenocrysts of biotite, quartz, k-feldspar, plagioclase and iron oxides. Ultramafic samples have high MgO (&amp;gt; 21 wt.%) and low SiO2 (&amp;lt; 42 wt.%), Al2O3 (&amp;lt; 10 wt.%) and Na2O+K2O (&amp;lt; 1 wt.%). CaO shows large variation in these samples from 3.91 to 7.77 wt. %. In studied basalt samples SiO2, TiO2, Al2O3, MgO and alkalis (Na2O+K2O) show variation like (45.7-48.3 wt.%), (0.64-0.97 wt.%), (9.3-13.9 wt.%), (8.5-13.6 wt.%) and (0.95-4.64 wt.%) respectively. In studied tuffs the SiO2, TiO2, Al2O3, MgO and alkalis show variation from (48.9-49.8 wt.%), (0.7-0.8 wt.%), (13.6-13.9 wt.%), (8.4-9.1 wt.%) and (2.39-2.85 wt.%) respectively. In general, studied samples show light rare earth element depleted patterns and flat patterns for heavy rare earth elements similar to MORBs on chondrite normalized REE diagrams. The overall enrichment of LILEs (e.g., Rb, K and Th) and depletion of HFSEs (e.g., Nb-Ta trough) on primitive mantle normalized multi-element diagrams are similar to subduction zone or islamd arc basalts. Ratios like Ti/Y, Ti/Zr, Zr/Nb and Ti/V are also similar to those found in arc related volcanic suites. Sm/Yb vs La/Sm diagram indicates generation of melts from the spinel lherzolite mantle source. On tectonic setting discrimination diagrams like Zr vs Zr/Y, Zr vs Ti and Ti vs V majority of studied samples clearly show island arc tectonic affinity.

Magma mingling textures in granitic rocks of the eastern part of the Strzegom-Sobótka Massif (Polish Sudetes)

Many granitic intrusions display evidence of magma mixing processes. The interaction of melts of contrasting composition may play a significant role during their generation and evolution. The Strzegom-Sobótka massif (SSM), located in the Sudetes (SW Poland) in the north-eastern part of the Bohemian Massif of the Central European Variscides, exhibits significant evidence of magma mingling on the macro- and micro-scales. The massif is a composite intrusion, with four main varieties: hornblende-biotite granite (with negligible amount of hornblende) and biotite granite in the western part, and two-mica granite and biotite granodiorite in the eastern part. Field evidence for magma mingling is easily found in the biotite granodiorite, where dark enclaves with tonalitic composition occur. Enclaves range from a few centimeters to half a meter in size, and from ellipsoidal to rounded in shape. They occur individually and in homogeneous swarms. The mixing textures in the enclaves include fine-grained texture, acicular apatite, rounded plagioclase xenocrysts, ocellar quartz and blade-shaped biotite. The most interesting feature of the enclaves is the presence of numerous monazite-(Ce) crystals, including unusually large crystals (up to 500 μm) which have grown close to the boundaries between granodiorite and enclaves. The crystallization of numerous monazite grains may therefore be another, previously undescribed, form of textural evidence for interaction between two contrasting magmas. The textures and microtextures may indicate that the enclaves represent globules of hybrid magma formed by mingling with a more felsic host melt. Chemical dating of the monazite yielded an age of 297±11 Ma.

Geochemistry and chronology of the granites in Alasituo, west Tianshan Orogen: Implications for a magma mixing origin

There are intense magmatic activities in the Alasituo integrated gold exploration area, west Tianshan Orogen. On the basis of field characteristics, geochemistry features, and chronology in Alasituo, the formation mechanism of complex granite rocks was explored. The intrusive rocks are divided into 3 parts: a silicic endmember, a mafic endmember, and intermediate rocks. Mafic microgranular enclaves, as well as spotted structure, silicic veins intruding mafic rocks and plagioclase xenocrysts, developed in rocks. Under the microscope, there are many unbalanced mineral combinations, needle‐like apatites, and poikilitic structures. Futhermore, there is an obvious linear trend between the mafic endmember samples, intermediate samples, and the silicic endmember samples in Harker diagrams. All samples were evolved from magma mixing in TFe2O3‐MgO diagram. The trace element concentrations of intermediate rocks have the transitional features of both silicic and mafic endmember rocks. The above characteristics show that rocks have experienced low degree and uneven physical and chemical magma mixing. The silicic endmember rocks have a high content of K2O + Na2O, Zr, Nb, Y, and REE; high Ga/Al ratios, and; low content of Sr, Eu, Ti, and P, which have the characteristics of A‐type granites with a crustal source. The high content of MgO, TFe2O3, and low K2O/Na2O in mafic endmember rocks indicate a mantle source. The LA‐ICP‐MS U–Pb zircon age of the granodiorites is 350.3 ± 3.0 Ma. Combined with regional geological background and geochemical characteristics, the intrusive rocks were formed when the thinning lithosphere triggered mafic magma underplating, heating upper crust granites, and causing them partial melt.

Processes of the formation of mugearitic and benmoreitic magmas on Nemrut Volcano (East Turkey)

The samples of trachybasalt, mugearite, benmoreite, trachydacite, and comendite from Nemrut Volcano (East Turkey) were studied. Except for trachybasalt, each sample contains partly dissolved mineral xenocrysts from magmas with different compositions. At the precaldera stage of volcanic evolution fractional crystallization of the alkali basaltic melt contaminated with a small amount of material of the upper crust, as well as mixing of trachybasaltic and trachydacitic magmas, could result in the formation of mugearitic magma. At the precaldera stage benmoreitic magma formed as a result of the evolution of trachybasaltic and mugearitic magmas, while at the postcaldera stage—after the mixing of trachybasaltic magma and low-Fe comenditic melt. The presence of relics of postcaldera benmoreitic magma in comendite (glass of trachyrhyodacite–trachyrhyolite composition, xenocrysts of Mg-rich olivine, plagioclase, and augite) allows us to assume that repeated eruptions at the postcaldera stage of volcanic evolution were caused by its eruptions into the low-Fe comenditic magma.

The petrologic history of the Sanganguey volcanic field, Nayarit, Mexico: Comparisons in a suite of crystal-rich and crystal-poor lavas

To evaluate if intermediate magmas erupting from Volcán Sanganguey (Mexico) and the surrounding volcanic field are formed by mixing of basalts and rhyolites or if they initially exist as intermediate liquids, a detailed petrological study is presented for eight andesite and dacite magmas. Six of the samples erupted from the central edifice (four andesites and two dacites) are crystal-rich (≤50vol%), whereas the remaining two samples (one andesite and one dacite) erupted from monogenetic vents in the peripheral volcanic field and are crystal poor (≤5vol%). Despite the variation in crystallinity, all samples are multiply saturated in five to seven mineral phases (plagioclase+orthopyroxene+titanomagnetite+ilmenite+apatite±clinopyroxene±hornblende). In all samples, plagioclase spans a 30–40mol% An range in composition and orthopyroxene spans a range in Mg# of 5–10. Pre-eruptive temperatures and oxygen fugacites (relative to the NNO buffer) range from 853 (±24) to 1085 (±16) °C and −0.1 (±0.1) to 0.9 (±0.1) ∆NNO, on the basis of Fe-Ti two oxide thermometry. Application of the plagioclase-liquid hygrometer to the samples reveals maximum H2O contents that range from 1.7–6.2wt%. Comparison with phase equilibrium experiments demonstrates that all plagioclase and orthopyroxene compositions in the crystal-poor samples could have grown from their respective whole rock compositions. Comparison of crystal rich samples with phase equilibrium experiments reveals the presence of sodic xenocrysts which reflect resorption textures and an estimated excess plagioclase crystal cargo of >6vol%. The excess plagioclase crystal cargo is not distinguishable from phenocrystic plagioclase based on composition or texture, suggesting that they were also grown in intermediate melts, and are therefore described as antecrystic. No calcic plagioclase xenocrysts (>An79) typical of hydrous arc basalts are observed, thus it is likely that the excess plagioclase in the crystal-rich samples were originally formed in intermediate magmas.For the crystal-poor samples, we propose that the mechanism producing the complex phenocryst assemblages is degassing (±cooling), as it may shift equilibrium plagioclase compositions, kinetically inhibit crystal-growth, and increase melt viscosity, leading to complex textures. Notably, the hypothesis of degassing (±cooling) induced crystallization requires that the intermediate melts initially exist as liquids, prior to crystallization, supporting the hypothesis that intermediate melts are generated in the deep crust and arrive in the upper crust as liquids. For the crystal-rich samples, degassing (±cooling) may also be the mechanism generating a portion of the compositional and textural variation in the mineral assemblages and some incorporation of antecrysts or xenocrysts must occur as evidenced by an excess plagioclase crystal cargo; however, we find no definitive evidence supporting the incorporation of crystals initially grown in basalts or rhyolites. Given the similarities in phase assemblage, mineral compositions, mineral textures, and intensive variables between the crystal-poor and –rich samples, we conclude that the melts arriving into the upper crust beneath Volcán Sanganguey and the surrounding peripheral volcanic field are intermediate in composition and are initially formed (as liquids) in the deep crust.

Interaction between two contrasting magmas in the Albtal pluton (Schwarzwald, SW Germany): textural and mineral-chemical evidence

The magmatic evolution of the Variscan Albtal pluton, Schwarzwald, SW Germany, is explored using detailed textural observations and the chemical composition of plagioclase and biotite in both granite and its mafic magmatic enclaves (MMEs). MMEs probably formed in a two-step process. First, mafic magma intruded a granitic magma chamber and created a boundary layer, which received thermal and compositional input from the mafic magma. This is indicated by corroded “granitic” quartz crystals and by large “granitic” plagioclase xenocrysts, which contain zones of higher anorthite and partly crystallized from a melt of higher Sr content. Texturally, different plagioclase types (e.g. zoned and inclusion-rich types) correspond to different degrees of overprint most likely caused by a thermal and compositional gradient in the boundary layer. The intrusion of a second mafic magma batch into the boundary layer is recorded by a thin An50 zone along plagioclase rims that crystallized from a melt enriched in Sr. Most probably, the second mafic intrusion caused disruption of the boundary layer, dispersal of the hybrid magma in the granite magma and formation of the enclaves. Rapid thermal quenching of the MMEs in the granite magma is manifested by An30 overgrowths on large plagioclase grains that contain needle apatites. Our results demonstrate the importance of microtextural investigations for the reconstruction of possible mixing end members in the formation of granites.

Magma mixing in the Kalaqin core complex, northern North China Craton: Linking deep lithospheric destruction and shallow extension

The widespread Mesozoic magmatism in the North China Craton (NCC) has received considerable attention as a trigger for large scale lithospheric destruction. Here we investigate the Early Cretaceous Jiguanzi adamellite from the northern part of the NCC which is contemporaneous with shallow extensional deformation and deep lithospheric destruction. This intrusion emplaced at ca. 133Ma is located in the foot wall of the Kalaqin metamorphic core complex (MCC), and occurs as a synextensional ring complex with numerous magmatic equigranular (Group 1) and porphyritic (Group 2) enclaves. Hornblende and plagioclase from the host adamellite and xenocrysts of Group 2 enclaves show distinct inverse zoning with Mg- and Ca-rich mantle. The Group 2 enclaves are characterized by plagioclase xenocrysts hosting hornblende, biotite and apatite inclusions, quartz ocelli with fine-grained rim enriched in biotite and hornblende, and poikilitic biotite surrounded by hornblende. Geochemically, the host intrusion is calc-alkaline to alkaline and metaluminous with variable contents of SiO2 (60.70–72.20wt.%), Al2O3 (14.19–17.22wt.%), Na2O+K2O (6.16–9.42wt.%), and Mg# values (28.0–47.7), whereas the Group 2 enclaves exhibit low SiO2 (54.05–55.55wt.%), high Fe2O3 (8.18–8.64wt.%) and TiO2 (2.08–2.28wt.%), and moderate Mg# (44.0–44.1). Both the host intrusion and Group 2 enclaves are enriched in large-ion lithophile and light rare earth elements, and depleted in high field strength elements and heavy rare earth elements except that the latter has lower Ba and high Nb, Ta and Ti contents. The major and trace element contents of the Group 1 enclaves are broadly similar to those of the host intrusion. Analyses of Sr–Nd–Hf isotopes in the host intrusion, and in Group 1 and Group 2 enclaves show (87Sr/86Sr)ihost=0.70600–0.70618, εNd(t)host=−8.2 to −9.6, T2DM(Nd)host=1592–1706Ma, εHf(t)host=−9.2 to −12.0, (87Sr/86Sr)iGroup 1=0.70590–0.70635, εNd(t)Group 1=−9.6 to −10.4, T2DM(Nd)Group 1=1705–1775Ma, (87Sr/86Sr)iGroup 2=0.70586–0.70587, εNd(t)Group 2=−0.6 to −0.7, T2DM(Nd)Group 2=971–978Ma, and εHf(t)Group 2=−4.5 to −10.0. The mineralogical and geochemical features suggest that the host Jiguanzi adamellite and Group 2 enclaves were generated through synchronous mixing and fractional crystallization accompanied by mechanical and chemical exchanges. The Group 1 enclaves showing cogenetic affinity with their felsic host represent the mechanical concentrations of mafic minerals and accessory phases from evolved hybrid host magma. Magma sources of the host Jiguanzi adamellite and its Group 1 enclaves are dominated by lower crust (LCC) components, whereas those for the Group 2 enclaves are dominated by metasomatized lithospheric mantle. The magma genesis involved complex multi-stage crust–mantle interaction processes including: (1) lithospheric mantle modification induced by Triassic subduction of the Paleo-Asian Ocean slab and/or Triassic–Cretaceous asthenospheric melt underplating; and (2) melting of the ancient LCC and lithospheric mantle, and hybridization of mantle- and crust-derived melts. In conjunction with regional geological and geochemical data, we argue that the Late Jurassic–Early Cretaceous shallow extension events represented by the Kalaqin MCC triggered deep-seated multi-stage magmatism and lithospheric destruction, and the continuous generation of magma further strengthened the extension and result in more extensive lithospheric thinning.

Multiple Mixing and Hybridization from Magma Source to Final Emplacement in the Permian Yamatu Pluton, the Northern Alxa Block, China

A combined study of zircon U–Pb ages, whole-rock chemistry, Sr–Nd isotopes and in situ Lu–Hf isotopic ratios in zircons was carried out on Permian monzogranites and mafic microgranular enclaves (MME) and coeval massive gabbros in the northern Alxa block, North China. The data obtained were used to constrain magma sources and petrogenetic processes involved in the generation of this igneous suite. Zircon U–Pb dating yields ages of 271 ± 1 Ma, 270 ± 1 Ma and 276–270 Ma for the monzogranites, MME and massive gabbros, respectively. Two populations of MME, gabbroic (SiO 2 <49 wt %) and dioritic (SiO 2 >53 wt %) enclaves, are identified. They represent mafic to intermediate magmas quenched in a partially crystallized granitic (sensu lato) host; evidence to support this conclusion includes their fine-grained textures, sinuous margins and diffuse contacts with the host monzogranites. Back-veins and xenocrysts of quartz and plagioclase, as well as various disequilibrium textures and mineral assemblages, indicate mingling or mixing processes. The two magma systems, mafic and felsic, have broadly similar isotopic characteristics with whole-rock initial 87 Sr/ 86 Sr(i) ratios ranging from 0·7075 to 0·7077 in the monzogranite host and from 0·7067 to 0·7069 in the MME, with eN d (t) values ranging from –10·2 to –12·4 in monzogranites and from –8·2 to –9·9 in the MME. Zircon eHf(t) values of the monzogranites, gabbroic enclaves and massive gabbros show a wide range and significant overlap from –1·2 to –15·2, –4 to –13·3 and –5·4 to –19·5, respectively; the maximum frequency value of the Hf model age is almost coincident with the whole-rock Nd model age. Mafic, gabbroic rocks similar in composition to some enclaves form layered synplutonic intrusions several metres in thickness and more than 100 m in lateral extent. A mixing test based on mass balance for whole-rock major element compositions reveals that mixing was an efficient process between two coeval magmas. The fraction of felsic magma involved in the hybrid rocks ranged from 0·19–0·29 in the dioritic enclaves to 0·84 in granodiorite and good linear fits (r 2 > 0·9) are obtained using the average composition of gabbroic enclaves and the most felsic monzogranite as end-members. Although the monzogranites and enclaves may be derived from distinct magma sources, they share similar isotopic signatures, pointing to interaction processes in the source region. The combination of information from geochronology, petrology, whole-rock geochemistry and isotopic compositions leads us to conclude that the processes of hybridization and magma mixing were effective at both the level of emplacement in the shallow crust and at depth in the magma source region. Hybridization in a source region within the lithospheric mantle, involving mantle and crustal source rocks, produced magma bimodality with strong geochemical affinities between end-members and clear calc-alkaline arc signatures, compatible with a subduction setting. A plausible subduction erosion plus relamination model, which differs from classical models based on mafic magma underplating, is proposed. Accordingly, the Yamatu monzogranites are argued to have been generated from granitic melt segregated and ponded in buoyant silicic diapirs, which formed by melting of subducted melanges in the lithospheric mantle and eventually relaminated to the lower crust. They represent the melts that metasomatized the mantle region during a pre-Permian subduction event. The massive gabbros were formed by decompression melting of the previously metasomatized mantle. The gabbroic enclaves and gabbro layers of the Yamatu pluton are interpreted as magmas formed by decompression melting of this modified hydrated mantle. The dioritic enclaves represent hybrid liquids generated from reaction between granitic melts, which were derived from subducted melanges, and the hydrated mantle or melts derived from it.

40Ar/39Ar geochronology of subaerial lava flows of Barren Island volcano and the deep crust beneath the Andaman Island Arc, Burma Microplate

Little was known about the nature and origin of the deep crust beneath the Andaman Island Arc in spite of the fact that it formed part of the highly active Indonesian volcanic arc system, one of the important continental crust forming regions in Southeast Asia. This arc, formed as a result of subduction of the Indian Plate beneath the Burma Microplate (a sliver of the Eurasian Plate), contains only one active subaerial magmatic center, Barren Island volcano, whose evolutional timeline had remained uncertain. In this work, we present results of the first successful attempt to date crustal xenoliths and their host lava flows from the island, by incremental heating 40Ar/39Ar method, in an attempt to understand the evolutionary histories of the volcano and its basement. Based on concordant plateau and isochron ages, we establish that the oldest subaerial lava flows of the volcano are 1.58 ± 0.04 (2σ) Ma, and some of the plagioclase xenocrysts have been derived from crustal rocks of 106 ± 3 (2σ) Ma. Mineralogy (anorthite + Cr-rich diopside + minor olivine) and isotopic compositions (87Sr/86Sr 7.0) of xenoliths not only indicate their derivation from a lower (oceanic) crustal olivine gabbro but also suggest a genetic relationship between the arc crust and the ophiolitic basement of the Andaman accretionary prism. We speculate that the basements of the forearc and volcanic arc of the Andaman subduction zone belong to a single continuous unit that was once attached to the western margin of the Eurasian Plate.

Primary melt from Sannome-gata volcano, NE Japan arc: constraints on generation conditions of rear-arc magmas

The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9–11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous 87Sr/86Sr ratios (0.70318–0.70320), whereas low-MgO scoriae (MgO 0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe2+)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160–1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5–0.6 wt% H2O at 1,220–1,230 °C and at ~1.8 GPa, and that the H2O content of the primary magma was 6–7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6–0.7 wt% H2O at ~1,250 °C and at ~1.3 GPa.

강원도 고성 뒤배재 화산암의 암석화학적 특성

Duibaejae basalts from Goseong, Gangwon-do, are divided into the lower basalt and the upper basalt depending on the properties, such as occurrence, mineral compositions, and major and trace compositions of the basalts. The lower basalts have characteristics of agglomerate rocks as well as contain, crustal and mantle xenoliths, and olivine, pyroxene, and plagioclase xenocrysts. The upper basalts with columnar joints contain relatively more mantle xenolith and olivine xenocryst than the lower basalts. The major and trace element compositions suggest that the composition of the upper basalts is close to primary magma composition. Enrichment and depletion patterns of the trace and the rare-earth elements of the lower basalts are similar to those of the upper basalts, whereas the lower basalts are more LREE enriched than the upper basalts. The source magmas of the lower and upper basalts from Duibaejae volcanic edifice were generated from about 0.8-1.2% and 3.7-4.0% batch melting of garnet peridotite, respectively. The abundance of granite xenolith, and plagioclase and quartz xenocrysts with reaction rim indicates that the lower basalts, compared with upper basalts, might have been assimilated with the crustal materials during ascending to surface.

Petrology of Stewart Mountain basalt field in central Arizona, U.S.A.: A lithospheric source with small-scale trace element and isotopic heterogeneities

The Stewart Mountain basalt field in central Arizona is composed of three horizons of Miocene lavas over ~4 km 2 . The youngest lava is ~15.5 Ma. The field is in the southern Basin and Range at its transition to the Colorado Plateau. It is also at the northwestern margin of the ~8000 km 2 Goldfield-Superstition volcanic province (G-SVP), where basaltic lavas are ~20–19 Ma. Stewart Mountain basalts are alkalic, and most have from 6–8 weight percent (wt%) MgO, but more primitive and evolved lavas (10.7 and 4 wt% MgO, respectively) are also present. Most incompatible element abundances differ widely for basalts within the 6–8 wt% MgO range, and they distinguish the three horizons (e.g., ranges for P 2 O 5 are 0.5–1.4 wt%; Zr 125–250 ppm; La 40–80 ppm). One lava has quartz and plagioclase xenocrysts and even lower incompatible element abundances (e.g., P 2 O 5 0.25 wt%; La 25 ppm). All Stewart Mountain basalts, however, have Nb-Ta negative anomalies, consistent with a lithospheric mantle source that had subduction characteristics. Isotopic compositions differ across the three basalt horizons (e.g., ranges for 87 Sr/ 86 Sr are 0.7049–0.7061; 206 Pb/ 204 Pb 17.7–19.2; e Nd -3.5 to -6.2), where the xenocrystic lava has the lowest Sr and Pb isotopic ratios. Over its life, the Stewart Mountain field radiogenic isotope ratios decreased to reflect source heterogeneities, and its 206 Pb/ 204 Pb range is as wide as that formed by Oligocene–Miocene basalts collectively across the southern Basin and Range and transition zone. Incompatible-element abundances and ratios also reflect source heterogeneities, whereby the greatest differences are observed as abundances decreasing from middle to upper horizon basalts. Several abundance ratios, such as Zr/Nb, Th/Ta, Th/Nb, and Zr/Hf, record some of the source heterogeneities that are manifested over the short geologic time represented by the successive lava horizons. These temporal compositional changes likely reflect partial melts from a variably metasomatized lithospheric mantle. Compared to the compositions of the older, neighboring G-SVP basalts, Stewart Mountain lavas are generally evolved (MgO <8 wt%). The absence of mantle xenoliths in any Stewart Mountain lava and the xenocrystic lava both point to the compositional evolution having occurred in crustal reservoirs; however, based on the lowest isotopic ratios present in the xenocrystic lava, the upper crust was not a reservoir. Comparing Stewart Mountain basalt incompatible-element abundance ratios to those in the neighboring G-SVP shows enough difference to conclude that these two Miocene basalt localities had lithospheric sources with distinct trace element characteristics. The G-SVP source also had higher, distinguishing e Nd (-1 to -2). All characteristics combined, the Stewart Mountain field shows that lithospheric source heterogeneities can be manifested both temporally and spatially over only a small surface area. Stewart Mountain lithospheric source indicates that magmatism in central Arizona did not have asthenospheric sources by 15 Ma.

Magma mingling and chemical diffusion in the Taojiang granitoids in the Hunan Province, China: evidences from petrography, geochronology and geochemistry

Petrographic study and zircon LA-ICP-MS U-Pb dating reveal that the Taojiang pluton is mainly composed of Late Indosinian biotite granodiorites (216 ± 2 Ma, 217 ± 1 Ma and 217 ± 1 Ma) with contemporaneous microgranular enclaves (219 ± 3 Ma). The host rocks belong to metaluminous to peraluminous and high-K calc-alkaline granodiorite with mean K2O/Na2O ratio less than 1, while the microgranular enclaves belong to metaluminous and shoshonitic monzodiorite and quartz monzonite, with average K2O/Na2O greater than 1. The enclaves contain back-veins and xenocrysts of quartz, biotite and plagioclase, and have contents of K, Rb and total REE higher than their host rocks, indicating mingling of two different magmas and elemental diffusion from the felsic magma to the mafic magma due to temperature gradient between them. The host granodiorites have initial 87Sr/86Sr of 0.71411 ~ 0.71508, e Nd(t) values of −6.05 ~ −7.39 and Nd isotope two stage model ages $$ \left( {\mathrm{T}_{\mathrm{NdDM}}^2} \right) $$ of 1.49 Ga to 1.60 Ga, while the enclaves have initial 87Sr/86Sr of 0.71438, e Nd(t) values of −6.92 and $$ \mathrm{T}_{\mathrm{NdDM}}^2 $$ of 1.56 Ga, showing similar features. The zircon e Hf(t) values of the enclaves (−4.21 ~ 0.54) are slightly higher than those of the host rocks (−6.77 ~ −2.18), and the zircon Hf isotopic two stage model ages $$ \left( {\mathrm{T}_{\mathrm{NfDM}}^2} \right) $$ of the enclaves (mainly 1.21 Ga ~ 1.45 Ga) are accordingly slightly younger compared with those of the host rocks (1.39 Ga to 1.75 Ga). These data suggest that the host rock magma was derived mainly from partial melting of Mesoproterozoic - Paleoproterozoic crustal rocks, while the enclave magma was originated from partial melting of basic/ultrabasic rocks intruding in the crust during Mesoproterozoic period. Three inherited zircon cores from the granodiorites are dated 1512 Ma, 2325 Ma and 2458 Ma, also giving evidence for involvement of Mesoproterozoic- Paleoproterozoic crust rocks in the magma formation. The more evident negative Eu anomaly of the enclaves than their host granodiorites may have resulted from more distinct fractional crystallization of plagioclase in the enclave magma before mingling with the felsic magma.

Textural control on the weathering of basic igneous rocks: A micromorphological approach

The Puno minettes (Eastern Andean Cordillera, southern Peru) are phlogopite–phyric rocks with a groundmass consisting mainly of phlogopite, alkali feldspar and apatite microphenocrysts and/or microlites. They display the strong large-ion-lithophile-element enrichments that characterize potassic to ultrapotassic rocks of the lamprophyre clan (Rb, Ba, Sr up to 103× Primordial Mantle, REE up to 6.102× CI-chondrites). In addition to their close association with granitoids, the rocks of this series show many features of minettes occurring in arc-related collisional zones, i.e., absence of mantle xenoliths, up to 30 vol.% felsic xenocrysts and strong negative Nb and Ti anomalies. However, they are distinguished by unusual phenocryst phases, whole-rock compositions and groundmass mineralogy. Some minettes contain only a few per cent of biotite±quartz±plagioclase xenocrysts immersed in a typical minette groundmass whereas those richer in xenocrysts display the complete liquidus paragenesis of the coexisting peraluminous granite (biotite+quartz+plagioclase+alkali feldspar+cordierite+zircon+pyrite) in a high-K latitic to trachydacitic groundmass. Field relations, radiometric ages, whole-rock data and felsic xenocrysts all indicate that the Puno minettes result from a mixing process involving Al-rich lamproites and peraluminous monzogranites. The most common phenocryst assemblage (highly magnesian phlogopite+olivine±titanian Cr-spinel±Ca-rich enstatite±magnesian ilmenite±Al- and Ti-poor diopside) is similar to that reported in Al-rich phlogopite lamproites from southeastern Spain. Likewise, the host rocks have diagnostic features of Spanish phlogopite lamproites (e.g., K2O/Al2O3 ratios >9, CaO contents <2 wt.%, negative Ba and Sr anomalies in primitive mantle-normalized multi-element plots, LaN/NdN<1.5). Graphite in the groundmass and Cr-spinel and ilmenite compositions devoid of Fe3+ suggest strongly reduced conditions of formation. Another phenocryst assemblage (Ca–Al-rich enstatite+Ca-deficient highly magnesian augite+magnesian phlogopite±ilmenite±discrete olivine) is observed in one sample which derives from an Al-rich leucite lamproite.

The origin of mafic microgranular enclaves and their host granodiorites from East Kunlun, Northern Qinghai-Tibet Plateau: implications for magma mixing during subduction of Paleo-Tethyan lithosphere

Voluminous granodioritic magmatism is recorded in the East Kunlun, Northern Qinghai-Tibet Plateau. Here we present mineralogical, petrological, geochemical and Sr-Nd-Hf isotopic data and zircon U-Pb ages for the Naomuhun pluton and its numerous mafic microgranular enclaves (MMEs). Whole-rock geochemical data and regional geological studies indicate that this pluton consists of subduction-related high-K calc-alkaline metaluminous, I-type granodiorite. The MMEs have plagioclase xenocrysts and disequilibrium textures, such as oscillatory zoning and resorbed rims, indicating magma mixing. Compositions of plagioclase (An30-An49), amphibole (Mg# = 0.62 ~ 0.68), and biotite (Mg# = 0.52–0.56) of MMEs are similar to or very slightly different from equivalent minerals in the host granodiorites, suggesting nearly complete equilibration between the mafic-and felsic magmas. The zircon U-Pb age of the MMEs (263 ± 2 Ma) is identical, within analytical error, to that of the host granodiorites (261 ± 2 Ma). The MMEs have eHf(t) values of −6.83 to −3.15 (average = −4.68), whereas those of the granodiorites range from −9.00 to −3.20 (average −5.63), which is identical within analytical uncertainty. Combined with relatively homogeneous Sr-Nd isotopic compositions, we suggest the MMEs were derived from magma mixing, and their source is similar to an enriched mantle composition. The granodiorites have TDM2(Hf) model ages ranging from 1.49 to 1.86 Ga, consistent with the Nd model ages (TDM2), implying that the host magma was derived from Paleo- or Meso-proterozoic rocks, probably the Xiaomiao Group, which forms the basement of East Kunlun. We propose a model for magma formation and magma mixing in a subduction zone environment, in which subduction of an oceanic slab at ca. 260 Ma led to fluid metasomatism, inducing partial melting of an enriched lithospheric mantle to form the voluminous mafic magma. The mafic magma underplated the overlying lower crust, resulting in its partial melting to form felsic magma. The mafic magma then mixed with the felsic magma at lower crustal levels to form the MMEs by convective motion, or forceful injection into the host felsic magma. The MMEs and their host magma were then emplaced at a depth of ca. 12 km, where they crystallized at a temperature of ca. 700–770°C.

Petrological relationships among lavas, dikes, and gabbros from Integrated Ocean Drilling Program Hole 1256D: Insight into the magma plumbing system beneath the East Pacific Rise

A continuous section from extrusive lavas, through sheeted dikes, and uppermost gabbros recovered from Integrated Ocean Drilling Program Hole 1256D provides important information regarding magma plumbing systems beneath superfast spreading ridges. Petrological examination demonstrates that a model of fractional crystallization from a magma of composition similar to one of the more primitive gabbros in a shallow (?50–100 MPa) melt lens reasonably explains mineral and whole rock compositions of many lavas and dikes. Elevated concentrations of trace elements in some rocks appear to have resulted from mixing between primitive magma and highly evolved magma. About half of the dike samples have more evolved Fe-rich compositions than the extrusive lava samples. Magma densities of the Fe-rich dikes are a little higher (?30 kg/m3) than those of lavas, suggesting that these dike magmas would not reach the surface. Mineralogical investigations reveal that both lavas and dikes contain oscillatory zoned plagioclase xenocrysts, implying magma mixing caused by successive episodes of fractionation and magma replenishment in the melt lens. The plagioclase xenocrysts contain high-Anorthite sections [An: 100 × Ca/(Ca+Na) in mole percent] whose compositions are not in equilibrium with host liquids. The high-An sections were likely crystallized when primitive magmas with high CaO/Na2O were injected in the melt lens. Since the oscillatory zoned plagioclase generally forms crystal clots, they were probably accumulated in a mush zone. The petrographical examination favors a model suggesting that injection of primitive magma into the melt lens broke the mush zone and pushed out the oscillatory zoned plagioclase.

Mafic to hybrid microgranular enclaves in the Ladakh batholith, northwest Himalaya: Implications on calc-alkaline magma chamber processes

Felsic magmatisms in the north of Indus-Tsangpo Suture Zone (ITSZ) in Ladakh range of northwest Indian Himalaya, referred herein Ladakh granitoids (LG), and associated magmatic rocks constitute the bulk of the Ladakh batholith. They have been characterized as Andean-type, calc-alkaline, largely metaluminous (I-type) to a few peraluminous (S-type) granitoids derived from partial melting of subducting materials. The LG can be broadly classified into coarsegrained facies with abundant mafics (hbl-bt), medium-grained facies with low content of mafics, and fine-grained leucocratic facies with very low amount of mafics. Mesocratic to melanocratic, rounded to elliptical, fine to medium grained, mafic to hybrid microgranular enclaves (ME) are ubiquitous in medium to coarse-grained LG. ME are absent or rare in the leucocratic variety of LG. In this paper different types of ME, and their field relation and microstructures with respect to felsic host LG are documented from northwestern, central, southeastern parts of the Ladakh batholith. Rounded to elongate ME of variable sizes (a few cm to metres across, mostly d<30 cm) commonly having sharp, crenulate, and occasionally diffuse contacts of ME with felsic host LG suggest that several pulses of crystal-charged mafic and felsic magmas coexisted, hybridized, and co-mingled into subvolcanic settings. Occurrence of composite ME (several small mafic ME enclosed into large porphyritic ME) strongly point to multiple mafic to hybrid magma intrusions into partly crystalline LG magma chambers. Synplutonic mafic dykes disrupted to form subrounded to angular (brecciated) mafic ME swarms commonly disposed in strike-length suggest mafic magma injections at waning stage of felsic magma evolution with large rheological contrasts. Pillowing of mafic melt against leucocratic (aplitic) residual melt strongly suggests mafic magma intrusion in nearly-crystallized condition of pluton. Although common mineral asemblages (hblbt-pl-kfs-qtz-ap-zrn-mt±ilm) of ME (diorite, quartzdiorite) and host LG (granodiorite, monzogranite) may relate to their cogenetic relation, fine to medium grained porphyritic (hybrid) nature and lack of cumulate texture of ME strongly oppose cognate origin for ME. Presence of plagioclase xenocrysts, quartz ocelli and accicular apatite in porphyritic ME strongly indicate mingling and undercooling of hybridized ME globules into relatively crystal-charged cooler host LG magma. Grain size differences of some ME, except to those of porphyritic ones, appear related to varying degrees of undercooling of ME most likely controlled by their variable sizes. Several smaller ME, however, lack fine-grained chilled margin probably because of their likely disaggregation from a large size ME during the course of progressive hybridization (mingling to mixing) leaving behind trails of mafic schlieren. Field and microstructural evidences at least suggest that Ladakh granitoids and their microgranular enclaves are products of multistage magma mingling and mixing processes concomitant fractional differentiation of several batches of mafic and felsic magmas formed in open magma chamber(s) of subduction setting.