Margin Of Pluton Research Articles

AMS fabrics and emplacement model of Butiá Granite, an Ediacaran syntectonic peraluminous granite from southernmost Brazil

The post-collisional stage of the Brasiliano/Pan-African Cycle in Southern Brazil is marked by metaluminous and peraluminous granites controlled by a transcurrent shear zone system. In southernmost Brazil, the sinistral, NE-trending Dorsal de Canguçu Transcurrent Shear Zone (DCTSZ) is the best known structure that conditioned these peraluminous granites. Despite its poorly-developed linear fabric, the emplacement of the NNW-elongate Butiá Granite (BG) northwest of the DCTSZ is interpreted to have been controlled by a dextral transcurrent shear zone. Thus, an anisotropy of magnetic susceptibility (AMS) study was performed in the BG aiming to constrain its emplacement mechanism and the relation of the granite with the regional shear zone system. Magnetic mineralogy was investigated through hysteresis loops, thermomagnetic and IRM acquisition curves and SEM analysis. These experiments show a dominant contribution of paramagnetic phases and a small content of low-coercivity (e.g., magnetite and titanomagnetite) and high-coercivity (e.g., hematite) remanence-carrying minerals. In spite of the presence of minor ferromagnetic grains, the BG magnetic anisotropy fabric is interpreted as dominantly controlled by paramagnetic biotite crystals. Magnetic susceptibility ranges between 0.1 and 8.0 × 10−5 SI. Shape parameter (T) ranges from 0.272 to 0.908, and anisotropy degree (P) ranges from 1.073 to 1.266, increasing from the inner portion of the pluton to its margins. Magnetic fabrics, microstructures and field relations suggest that magma ascent and emplacement were controlled by a NNW-trending dextral transcurrent shear zone. The presence of S-C magmatic fabric and high temperature (ca. 650 °C), solid-state deformation at the margins confirm that the pluton was deformed during its cooling process. Close to the host-rocks, magnetic foliation dips steeply towards W or E, and magnetic lineation plunges steep to moderately, indicating dominant flattening there. Shallow-plunging lineation parallel to the NW- to NNW-striking foliation is found away from the pluton margins. Foliation becomes less steep towards the BG northeastern portion and the presence of roof pendants in this area suggests the proximity to the roof zone. The combination of buoyancy forces and the partitioning of regional strain into simple and pure shear is in accordance with a transpressive regime. These results also suggest a time-space relationship between the NNW-dextral shear zone that controlled the emplacement of the Butiá Granite (ca. 629 Ma) and the sinistral, NE-trending Dorsal de Canguçu Transcurrent Shear Zone, responsible for the emplacement of peraluminous granites during the early post-collisional stage (ca. 634 - 610 Ma) of the Brasiliano/Pan-African Cycle in southernmost Brazil.

In situ U–Th–Pbtotal dating of polychronous monazite in the Koraput anorthosite pluton, Eastern Ghats Granulite Belt (India), and implications

AbstractU–Th–Pbtotal age determinations in monazite in a noritic anorthosite at the margin of the Koraput anorthosite pluton constrain the time of emplacement and sub-solidus chemical modifications in the Grenvillian-age anorthosite pluton in the Eastern Ghats Province (EGP), India. The monazites hosted within dynamically recrystallized orthopyroxene and plagioclase grains are large (50–500 μm in diameter) and complexly zoned. Based on the textural–chemical heterogeneities, these monazites are classified into three groups. Group-I monazites exhibit a low-ThO2 core mantled by a high-ThO2 rim. By contrast, the group-II monazites exhibit high-ThO2 cores laced by ThO2-poor mantles, and outermost rims with still lower ThO2 contents. Skeletal group-III monazites at polygonized grain/phase boundaries exhibit patchy and concentric zones with decreasing ThO2 towards the margin. The U–Th–Pbtotal chemical ages obtained using electron probe microanalyses exhibit four age clusters. The oldest age population (mean 939 ± 4.5 Ma) obtained in cores in group-I, II and III monazites with patchy zones corresponds with the emplacement of the Koraput anorthosite, and this age population is consistent with 980–930 Ma emplacement ages reported in other EGP anorthosite massifs. Younger monazites mantling the cores in group-II monazites and in group-III monazites with mean ages of 877 ± 5 Ma and 749 ± 18 Ma possibly reflect episodic monazite growth by fluid-aided dissolution–precipitation culminating with the disintegration of Rodinia at ~750 Ma. The youngest age population (mean 574 ± 19 Ma) in the outermost monazite rims and monazite veins represents renewed monazite growth during the Pan African assembly of the Grenvillian-age EGP domain with the proto-Indian cratons.

Rapid exhumation of the northern Jiaobei Terrane, North China Craton in the Early Cretaceous: Insights from Al-in-hornblende barometry and U-Pb geochronology

The Guojialing and Aishan plutons in the northern Jiaobei Terrane represent the magmatic products of the lithospheric thinning of the North China Craton in the Early Cretaceous. The exhumation history of the terrane in response to the thinning process and its effect on the formation and preservation of coeval gold deposits remain poorly constrained. Granodiorites from the Guojialing pluton and monzogranites from the Aishan pluton contain zircon grains with U-Pb ages of ca. 130 Ma and ca. 116 Ma, respectively. Using the Al-in-hornblende barometry, the Guojialing and Aishan plutons had crystallization pressures of 3.2–5.4 kbar (∼12 km) and 1.5–2.6 kbar (∼5 km), respectively. Using the hornblende-plagioclase thermometry, the Guojialing and Aishan plutons had crystallization temperatures of 628–697 °C and 702–745 °C, respectively. Fluid inclusion microthermometric data show that auriferous quartz veins in the Daliuhang deposit formed at temperatures between 243 and 322 °C at a depth of 7–11 km which was similar to the emplacement depth of the Aishan pluton. These thermobarometric and geochronological data reveal ∼7 km uplift between 130 Ma and 116 Ma, but negligible exhumation for the gold deposits after its formation until 116 Ma. Combining previous results, we envisage that the exhumation magnitude of the Guojialing intrusions were nearly the same in the wide area with slight differences between the core and margin of a single pluton. Numerous gold deposits in the Jiaobei Terrane formed during or after the late stage of rapid exhumation of granitoids, and were uplifted with similar depths to the Aishan pluton since ca. 116 Ma. The rapid exhumation of the Guojialing pluton was with the regional extension, and might be genetically associated with the intensive delamination of destabilized lithospheric root and dense residues.

Petrogenesis and W-Mo fertility indicators of the Gaojiabang “satellite” granodiorite porphyry in southern Anhui Province, South China

The Gaojiabang W-Mo mineralized granodiorite porphyry occurs as a “satellite” intrusion on the northern margin of the Qingyang-Jiuhua pluton in southern Anhui Province, South China. The geology, mineral and whole-rock geochemistry of the granodiorite porphyry is I-type and formed from a reduced melt. The Gaojiabang mineralized granodiorite porphyry was derived from the transition zone between the lower crust and upper mantle, and underwent high-degree fractional crystallization during emplacement. There is no obvious genetic relationship between the granodiorite porphyry and the granodiorite pluton of the Qingyang-Jiuhua plutonic complex. The key characteristics of the W-Mo mineralized Gaojiabang granodiorite porphyry include: 1) small volume (<0.5km3), 2) an abundance of hornblende (>5vol%), 3) low ∑REE concentrations (145.50–159.46ppm) accompanying weak negative Eu anomalies (0.85–0.94), 4) high differentiation-indices degree and water contents, and 5) low oxygen fugacity. These criteria can be used to identify potential W-Mo mineralized intrusions in southern Anhui Province, South China.

Cogenetic origin of mafic microgranular enclaves in calc-alkaline granitoids: The Permian plutons in the northern North China Block

Mafic microgranular (microgranitoid) enclaves (MMEs) with fine-to medium-grain–size igneous microstructures are abundant in Permian granitoid plutons in the northern North China Block (NCB). They are mainly dioritic in composition with SiO 2 contents from 51.0 wt% to 62.7 wt% and are contiguous with their host granitoids (SiO 2 = 60.4–68.4 wt%). Their main mineral assemblage of plagioclase (oligoclase and andesine), hornblende, biotite, K-feldspar, and quartz is similar to that of their host granitoids but with different mineral proportions. Zircon laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) U-Pb dating, hornblende-plagioclase thermobarometry, and Ti-in-zircon thermometry results show that the crystallization ages and pressure-temperature ( P-T ) conditions of the MMEs in each Permian granitoid pluton are very similar to those of their host granitoids, indicating simultaneous crystallization at the middle to upper crustal levels. Petrographical, geochemical, and Sr-Nd-Hf isotopic data show that the source areas of the Permian granitoid plutons in the northern NCB are temporally and spatially different and magma mixing was likely involved during formation of some granitoid plutons. However, the whole-rock Sr-Nd and zircon Hf isotopic compositions of the MMEs in each pluton are very similar to those of their host granitoids, indicating complete isotopic equilibration between the host granitoids and their enclaves. Combined with similar mineral assemblage and mineral chemistry between the MMEs and their host granitoids and absence of coeval mantle-derived rocks in or nearby the plutons, we confirm that the MMEs were crystallized from a coeval magma that gave rise to the host granitoids, and not by mixing and/or mingling between mantle-derived mafic and crustal-derived felsic magmas or by synplutonic injections of mafic magma into the host granitoid magma. We conclude that magma isotopic equilibration between host granitoids and their enclaves was achieved prior to emplacement and the magma mixing process during formation of some Permian granitoid plutons occurred mainly in the melting, assimilation, storage, and homogenization (MASH) zone in the lowermost crust or mantle-crust transition, not in the magma conduits or chambers at middle to upper crustal levels. A two-stage model that includes rapid cooling within the cogenetic host granitoid magma operating at pluton margins to form solid to sub-solid dioritic rocks and crystal accumulations of the host granitoid magma at the bottom of the pluton to form cumulates, and dioritic rocks and cumulates then fragmentated and interacted with progressive evolved host granitoid magma to change their shapes and orientations is proposed for origin of the MMEs in the Permian granitoid plutons in the northern NCB. This model may be more widely applicable to the generation of many MMEs in granitic rocks, especially where no direct evidence exists for the presence of mafic magmas coeval with granitoids and where there is a lack of isotopic contrast between hosts and enclaves. Our new results on origin of the enclaves in the granitoid plutons in the northern NCB show that some MMEs previously used as evidence for magma mixing and/or mingling are “autoliths (cognate xenoliths)” crystallized from the same magma as their host granitoids. There is an essential need for caution in using the MMEs as evidence for crustal-mantle interaction or mixing and/or mingling between crustal-derived felsic and mantle-derived mafic magmas during formation of granitic rocks, especially where no independent evidence is found for such a process. The role of MMEs in evaluating the mixing and/or mingling processes between crustal-derived felsic and mantle-derived mafic magmas has been much more likely overestimated by some previous studies and should be reconsidered. Instead, the MMEs in this origin can provide valuable information on emplacement mechanism and incremental growth of granitoid plutons in continental crust.

The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement

Contact aureoles rimming plutonic rocks are the locus of metamorphism and deformations witnessing magma emplacement mechanisms in the crust. In this study, structural and petrological observations are combined to mineral equilibria modelling to unravel the polyphase tectono-metamorphic history of the Permian Sondalo gabbro and its host rock, the Campo unit (Eastern Alps). The Campo unit consists of Grt–St–Ms–Bt–Pl–Qtz ± Sil ± And ± Crd mica schists attesting of a Carboniferous prograde P–T path, reaching 6 kbar/600 °C and subsequently 5.6 kbar/650 °C. This metamorphism is coeval with the formation of a sub-vertical NE–SW trending foliation (S1) and its overprint by a sub-vertical NW–SE trending foliation (S2). The heat brought by the Permian intrusives subsequently caused heating of the Campo unit at around 3–4 kbar/540 °C reflected by regional static crystallization of cordierite and andalusite porphyroblasts. During the intrusion of the Sondalo gabbro, thermal peak conditions are recorded by Grt–Sil–Spl–Crd–Ilm granulitic xenoliths at ~5.5 kbar/930 °C, subsequently exhumed at ~4 kbar during the development of a new foliation (S3). This foliation is localized around the pluton and moderately dips away from the centre of the pluton. In the migmatitic contact aureole, Grt–Sil–Bt–Pl–Qtz–Ilm and Grt–Sil–Crd–Spl–Bt–Kfs–Ilm residual rocks bear the new foliation (S3) and document a decompression from 6 kbar/750 °C to 5 kbar/725 °C and from 5.2 kbar/800 °C to reach 4.8 kbar/770 °C, respectively. The P–T–d paths recorded by the host rock and the xenoliths point to a two-step magma emplacement. First the Sondalo gabbro intruded the Campo unit causing heating of the host rock without deformation at 3–4 kbar. Second, the ductile flow along the pluton margins developed a new foliation (S3) during exhumation of the pluton and its immediate contact aureole from 6 to 4 kbar. Altogether, it indicates a progressive increase in mechanical coupling between the pluton and the host rock during magma emplacement.

Early Paleozoic and Early Mesozoic intraplate tectonic and magmatic events in the Cathaysia Block, South China

The geodynamic framework of the South China Craton in the Early Paleozoic and Early Mesozoic has been modeled as developing through either oceanic convergence or intracontinental settings. On the basis of an integrated structural, geochemical, zircon U-Pb and Hf isotopic, and mica 40Ar/39Ar geochronologic study we establish that an intracontinental setting is currently the best fit for the available data. Our results suggest that widespread tectonomagmatic activity involving granite emplacement and mylonitic deformation occurred during two distinct stages: ~435–415 Ma and ~230–210 Ma. The coeval nature of emplacement of the plutons and their ductile deformation is corroborated by the subparallel orientation of the mylonitic foliation along the pluton margins, gneissose foliation in the middle part of pluton, the magmatic foliation within the plutons, and the schistosity in the surrounding metamorphosed country rocks. The 435–415 Ma granitoids exhibit peraluminous, high-K characteristics, and zircons show negative eHf(t) values (average −6.2, n = 66), and Paleoproterozoic two-stage model ages of circa 2.21–1.64 Ga (average 1.84 Ga). The data suggest that the Early Paleozoic plutons were derived from the partial melting of the Paleoproterozoic basement of the Cathaysia Block. The 230–210 Ma granites are potassic and have zircons with eHf(t) values of −2.8–−8.7 (average −5.4, n = 62), corresponding to TDM2 ages ranging from 2.0 to 1.44 Ga (average 1.64 Ga), suggesting that the Early Mesozoic partial melts in Cathaysia were also derived from basement. The geochemical distinction between the two phases of granites traces continental crustal evolution with time, with the Early Mesozoic crust enriched in potassium, silicon, and aluminum, but deficient in calcium, relative to the Paleozoic crust. Kinematical investigations provide evidence for an early-stage ductile deformation with a doubly vergent thrusting pattern dated at 433 ± 1 to 428 ± 1 Ma (40Ar/39Ar furnace step-heating pseudoplateau ages obtained on muscovite and biotite from mylonite and deformed granite) and a late-stage strike-slip movement with sinistral sense of ductile shearing at 232 ± 1 to 234 ± 1 Ma (40Ar/39Ar furnace step-heating pseudoplateau ages) along an E-W direction. The geological, geochemical, and isotopic signatures likely reflect far-field effects in response to continental assembly events at these times.

Structure, stability, and tsunami hazard associated with a rock slope in Knight Inlet, British Columbia

Abstract. Rockfalls and rockslides during the past 12 000 years have deposited bouldery debris cones on the seafloor beneath massive rock slopes throughout the inner part of Knight Inlet. The 885 m high rock slope, located across from a former First Nations village destroyed in the late 1500s by a slide-induced wave, exposes the contact between a Late Cretaceous dioritic pluton and metamorphic rocks of the Upper Triassic Karmutsen Formation. The pluton margin is strongly foliated parallel to primary and secondary fabrics in the metamorphic rocks, resulting in highly persistent brittle structures. Other important structures include a set of sheeting joints and highly persistent mafic dykes and faults. Stability analysis indicates that planar and wedge rock slope failures up to about 500 000 m3 in volume could occur. We suspect that failures of this size in this setting would have the potential to generate locally hazardous waves. As several similar rock slopes fronted by large submarine debris cones exist in the inner part of Knight Inlet, it is clear that tsunami hazards should be considered in coastal infrastructure development and land-use planning in this area.

Structure and petrography of the southwestern margin of the Biella pluton, Western Alps

This work presents a new form surface map of the southwestern margin of the Biella pluton at the scale 1:10,000. The Biella pluton is part of the Periadriatic intrusives of the Alps and is emplaced in the continental Sesia-Lanzo Zone of the western Austroalpine domain. The country rocks consist of metapelites and different metagranitoids. Pre-intrusive HP (high pressure) mineral assemblages are dominant in country rocks with the exception of metagranitoids dominated by HT (high temperature) assemblages. The plutonic rocks consist of monzonite with minor syenite. The ductile polyphasic deformation of the country rocks predates the pluton emplacement, with the exception of syn-intrusive folding and shearing, which were recorded in the country rocks of the deeper part of the pluton. Syn-intrusive deformation may be represented by brittle structures that bear mineralisation; however, the majority of the brittle deformation postdates the final emplacement of the pluton. The orientation of the pervasive foliation in the country rocks controls the space available for magma intrusion and possible magmatic flow during emplacement. The inferred diffusion of the thermal aureole in the country rocks is based on the variation in contact metamorphic minerals, which is described by microscopic analysis. The extent of the aureole appears to be controlled by the type of dominant mineral assemblages, rock permeability, and the orientation of the regional foliation in country rocks with respect to the pluton margin. The multiscale structural analysis reveals that the Biella pluton emplaced at a depth as shallow as the greenschist facies conditions or shallower.

Syn-tectonic emplacement of the Late Mesozoic Laojunshan granite pluton in the eastern Qinling, central China: An integrated fabric and geochronologic study

The porphyritic monzogranite of Laojunshan (eastern Qinling in central China) forms a W–E elongated pluton characterized by a varying abundance of cm-scale potassium-feldspar (Kfs) megacrysts (with or without plagioclase mantles), mafic microgranular enclaves (MMEs) and associated mafic dykes. The measurements of the shape preferred orientation of Kfs megacrysts and the tiling of megacrystic pairs throughout the pluton suggest that these megacrysts constitute magmatic foliations subparallel to the pluton's margin, possibly resulting from eastward non-coaxial magmatic flow. Felsic-mafic magma mixing and mingling were important during magma emplacement, as indicated by abundant MMEs, coeval mafic dykes and disequilibrium textures within Kfs megacrysts. Quantitative analyses of the crystal size distribution and the in-situ chemistry of Kfs megacrysts revealed that the temperature fluctuation in the deep magma chamber and the resultant textural coarsening are the most likely important mechanisms for the growth of Kfs megacrysts. The syn-tectonic and two-main-stage emplacement of the pluton in a sinistral transtensional regime is inferred by the mechanical coupling between magmatic–submagmatic–solid-state fabrics in the pluton and wall-rock deformations, as well as zircon U–Pb dating results, which produce eastward younging emplacement ages spanning from ∼122 Ma to ∼119 Ma. Combined with other studies, our work supports the idea that the Qinling-Dabie orogen was in a sinistral transtensional regime during the Early Cretaceous, which may correspond to the eastward extrusion tectonics in eastern China.

Placer Tin Ores from Mt. Cer, West Serbia, and Their Potential Exploitation during the Bronze Age

Tin is a rare metal that is essential for making bronze, the defining technology of the Bronze Age. The source(s) of tin for Aegean bronze is undetermined but several small Bronze Age tin mines have been documented in the circum‐Aegean region. The discovery of Bronze Age archaeological sites in West Serbia near a tin placer deposit on the flanks of Mt. Cer led to an investigation of this site as a potential additional Bronze Age tin mine in the region. Geochemical prospecting of stream sediments flowing from Mt. Cer allowed for categorization of streams based on relative tin grade. Tin grade is highest in the Milinska River, a likely combination of a broad catchment area with multiple ore‐bearing tributaries, and a topographic profile that favors the accumulation of placer deposits. A survey of cornfields along the southern pluton margin discovered archaeological sites spanning the Neolithic to the Iron Age. Unlike older and younger sites, those of the Bronze Age were found only along the Milinska and Cernica Rivers where placer tin grades are highest, but appear to be absent where tin is scarce or absent. This suggests that these sites were associated with the exploitation of the tin ore.

Tourmaline occurrences within the Penamacor-Monsanto granitic pluton and host-rocks (Central Portugal): genetic implications of crystal-chemical and isotopic features

Tourmalinization associated with peraluminous granitic intrusions in metapelitic host-rocks has been widely recorded in the Iberian Peninsula, given the importance of tourmaline as a tracer of granite magma evolution and potential indicator of Sn-W mineralizations. In the Penamacor-Monsanto granite pluton (Central Eastern Portugal, Central Iberian Zone), tourmaline occurs: (1) as accessory phase in two-mica granitic rocks, muscovite-granites and aplites, (2) in quartz (±mica)-tourmaline rocks (tourmalinites) in several exocontact locations, and (3) as a rare detrital phase in contact zone hornfels and metapelitic host-rocks. Electron microprobe and stable isotope (δ18O, δD, δ11B) data provide clear distinctions between tourmaline populations from these different settings: (a) schorl–oxyschorl tourmalines from granitic rocks have variable foititic component (X□ = 17–57 %) and Mg/(Mg + Fe) ratios (0.19–0.50 in two-mica granitic rocks, and 0.05–0.19 in the more differentiated muscovite-granite and aplites); granitic tourmalines have constant δ18O values (12.1 ± 0.1 ‰), with wider-ranging δD (−78.2 ± 4.7 ‰) and δ11B (−10.7 to −9.0 ‰) values; (b) vein/breccia oxyschorl [Mg/(Mg + Fe) = 0.31–0.44] results from late, B- and Fe-enriched magma-derived fluids and is characterized by δ18O = 12.4 ‰, δD = −29.5 ‰, and δ11B = −9.3 ‰, while replacement tourmalines have more dravitic compositions [Mg/(Mg + Fe) = 0.26–0.64], close to that of detrital tourmaline in the surrounding metapelitic rocks, and yield relatively constant δ18O values (13.1–13.3 ‰), though wider-ranging δD (−58.5 to −36.5 ‰) and δ11B (−10.2 to −8.8 ‰) values; and (c) detrital tourmaline in contact rocks and regional host metasediments is mainly dravite [Mg/(Mg + Fe) = 0.35–0.78] and oxydravite [Mg/(Mg + Fe) = 0.51–0.58], respectively. Boron contents of the granitic rocks are low (<650 ppm) compared to the minimum B contents normally required for tourmaline saturation in granitic melts, implying loss of B and other volatiles to the surrounding host-rocks during the late-magmatic stages. This process was responsible for tourmalinization at the exocontact of the Penamacor-Monsanto pluton, either as direct tourmaline precipitation in cavities and fractures crossing the pluton margin (vein/breccia tourmalinites), or as replacement of mafic minerals (chlorite or biotite) in the host-rocks (replacement tourmalinites) along the exocontact of the granite. Thermometry based on 18O equilibrium fractionation between tourmaline and fluid indicates that a late, B-enriched magmatic aqueous fluid (av. δ18O ~12.1 ‰, at ~600 °C) precipitated the vein/breccia tourmaline (δ18O ~12.4 ‰) at ~500–550 °C, and later interacted with the cooler surrounding host-rocks to produce tourmaline at lower temperatures (400–450 °C), and an average δ18O ~13.2 ‰, closer to the values for the host-rock. Although B-metasomatism associated with some granitic plutons in the Iberian Peninsula seems to be relatively confined in space, extending integrated studies such as this to a larger number of granitic plutons may afford us a better understanding of Variscan magmatism and related mineralizations.

Field and petrographical insight into the formation of orbicular granitoids from the Bonney Pluton, southern Victoria Land, Antarctica

AbstractThe diversity of orbicule types exposed within granitoids of the Bonney Pluton in southern Victoria Land attests to the dynamic and complex interplay of magmatic processes that were responsible for their formation. Orbicules formed in small pockets of H2O-rich silicate melt that was extracted from the crystallizing and fractionating Bonney Pluton magma and concentrated along the pluton margins. These pockets of melt experienced a superheating event that destroyed almost all pre-existing nuclei and a subsequent delay in crystallization, which led to undercooling conditions that promoted rapid dendritic crystal growth. Superheating was induced by the injection of hot mafic magmas, evidenced by elevated plagioclase XAn, and Mg-Al-Ti-contents in hornblende that point to a higher temperature and a more mafic composition in the melt that the orbicule shells crystallized from. Variation in the type and structure of orbicules (hornblende-rich versus plagioclase-rich shells) were likely due to repeated changes in the composition, H2O-content, temperature and PH2O at the crystallizing orbicule boundary layer in response to pulses in the movement of magma, competition between crystallizing phases, episodic vesiculation, degassing and/or second boiling which dictated the composition, texture and size of individual orbicule shells. Brittle fragmentation of orbicules occurred in response to vesiculation and fragmented orbicules are often found as cores within intact orbicules, indicating that multiple phases of orbicule formation were common. The alignment and compaction of orbicules in ‘pods’ indicates movement of orbicules within the melt-rich pockets occurred, prior to resumption of near-equilibrium crystallization in the host granite.

Paleoarchean partial convective overturn in the Singhbhum Craton, Eastern India

Analyses of fold superposition structures and kinematics of shear zones along a 120km long E–W corridor across the Singhbhum Craton (Eastern India) are combined with U–Pb–Th chemical ages of texturally constrained monazites in supracrustals, gneisses and granitoids to reconstruct the Paleoarchean tectonic history of the western part of the Craton. Metamorphic sub-greenschist facies shallow marine to platformal sediments of the Iron Ore Group (IOG) and the accordantly deformed Jagnnathpur meta-lavas were accreted onto a basement composed of >3.4Ga amphibolite facies para-amphibolites/quartzites/muscovite schists (Older Metamorphic Group, OMG) and anatectic Older Metamorphic Tonalite Gneisses (OMTG). Crustal shortening was followed by emplacement-ascent of the Singhbhum granitoid pluton, SGP (3.2–3.3Ga) synchronous with sinking of supracrustal screens along N/NNE-trending steep-dipping sinistral–normal ductile shear zones. Deformation microstructures indicate the bulk of the pluton responded to exhumation-strain by low-T (≤500°C) ductile flow of quartz and K-feldspar, with plagioclase being largely unaffected. But granitoids along the western margin of pluton emplaced concurrent with westward-retreating exhumation of the thickened crust was deformed at T≥650°C. Since all granitoids in the pluton share the same N/NNE-trending tectonic fabric, microstructural features imply phased emplacement of the pluton during exhumation strain. The transition from crustal shortening to exhumation interrupted by phased granitoid emplacement between 3.2 and 3.4Ga is explained by a two-stage partial convective overturn model involving structures typical of supracrustal-down, granitoid-up sense of movement. The tectonic restoration in the western part of the Singhbhum Craton is new and complements growing evidence that accretion and convective overturn may have operated during the Paleoarchean.

Multi-stage granitic magmatism during exhumation of subducted continental lithosphere: Evidence from the Wulong pluton, South Qinling

Detailed petrology and zircon U–Pb dating data indicate that the Wulong pluton is a zoned granitic intrusive, formed from successive increments of magmas. An age range of at least 30Ma is recorded from the 225–235Ma quartz diorite on the pluton margin, the ca. 218Ma granodiorite in the intermediate zone, and the ca. 207Ma monzogranite at the pluton center. All the granitoids display evolved Sr–Nd–Pb isotopic compositions, with 87Sr/86Sr(i) of 0.7044–0.7062, unradiogenic Nd (εNd(t) values of −6.1 to −3.0, Nd model ages of 1.1–1.3Ga, and moderately radiogenic Pb compositions (206Pb/204Pb(i)=17.500–17.872, 207Pb/204Pb(i)=15.513–15.549, 208Pb/204Pb(i)=37.743–38.001), in combination with variations in zircon Hf isotopic compositions (with εHf(t) values in each stage span 12 units) and the Hf isotopic model ages of 800–1600Ma. These features suggest that the granitoids might have been derived from the reworking of an old lower crust, mixed with Paleozoic and Proterozoic materials. The rocks also display an adakitic affinity with Sr (479–973ppm), high Sr/Y ratios (mostly >60) and negligible Eu anomalies (Eu/Eu*=0.78–0.97) but low Rb/Sr ratios, low Y (4.6–17ppm), HREE (Yb=0.95–1.7ppm), Yb/Lu (6–7) and Dy/Yb (1.9–2.4) ratios, suggesting the absence of plagioclase and presence of garnet+amphibole in their residue. Considering a large gap among their crystallization ages, we propose that the geochemical evolution from pluton margin to center was controlled mainly by melting conditions and source compositions rather than fractional crystallization. Mafic enclaves that were hosted in the quartz diorite and granodiorite are mainly syenogabbroic to syenodioritic in composition, and are metaluminous and enriched in LREE and LILEs, but are depleted in HFSE, and display an evolved Sr–Nd–Pb isotopic composition, suggesting that they may have been derived from the partial melting of an enriched mantle lithosphere, which was metasomatized by adakitic melts and fluids from a subducted continental crust.In combination with the results of the Triassic ultra-high pressure metamorphic rocks in the Dabie orogenic belt, we apply a model involving the exhumation of subducted continental crust to explain the formation of the Wulong pluton. At the first stage, a dense and refractory mafic lower crust that was trapped at mantle depth by continental subduction witnessed melting under high temperature conditions to produce the quartz diorite magma, characterized by low SiO2 (60.65–63.98wt.%) and high TiO2 (0.39–0.86wt.%). The magma subsequently interacted with mantle peridotite, leading to high Mg# (57–67) and the metasomatism of the overriding mantle wedge. At the second stage, an asthenosphere upwelling that was probably caused by slab break-off at ca. 220Ma melted the enriched sub-continental lithospheric mantle (SCLM) to produce mafic magmas, represented by the mafic enclaves that are hosted in the quartz and granodiorite, resulting in the partial melting of the shallower subducted crust, and generating the granodiorite that is distinguished by high SiO2 (69.16–70.82wt.%), high Al2O3 (15.33–16.22wt.%) and A/CNK values (mostly >1.05). At the third stage, the final collapse of the Triassic Qinling–Dabie Orogenic Belt at ca. 215–205Ma caused extensive partial melting of the thickened orogenic lower crust to produce the monzogranite, which is characterized by high SiO2 (67.68–70.29wt.%), low TiO2 (mostly <0.35wt.%) and high Sr/Y ratios of 86–151.

Petrogenesis of the Barcroft pluton, northern White-Inyo Mountains, east-central California

The White-Inyo Range lies within the regional transition from Paleozoic-Precambrian North American continental basement to outboard Mesozoic and younger accreted terranes and a superimposed Andean-type arc. In the central White Mountains, the metaluminous Barcroft granodiorite invaded a major NE-striking, SE-dipping high-angle reverse fault—the Barcroft break. Because it is a relatively isolated igneous body and is well exposed over an elevation range of 1,500–4,000 m, its thermal history and that of the surrounding superjacent section are clearer than those of nearly coeval, crowded plutons emplaced in the hotter Sierra Nevada belt. The Barcroft pluton was emplaced as a compositionally heterogeneous series of areally scattered melt pulses episodically injected over the approximate interval 167–161 Ma. The oldest dated rocks are relatively quartzofeldspathic, whereas the youngest is more ferromagnesian, suggesting progressive partial fusion of a relatively mafic protolith. Heavy rare earth-enriched zircons indicate that Barcroft melts were derived at mid-crustal depths from a previously emplaced metabasaltic protolith containing plagioclase but lacking garnet. Granodioritic magma genesis involved the possible mixing of mafic and felsic melts, as well as very minor assimilation of country rocks, but mainly by fractional fusion and crystallization. Bulk chemical, rare earth, and isotopic data suggest that analyzed Barcroft rocks are members of a single suite. Granodioritic rocks are slightly more magnetite-rich at higher elevations on the NE, nearer the roof of the pluton. Earlier thermobarometry chronicled cooling and re-equilibration of the Barcroft pluton from its margins inward, as well as from mid-crustal generation depths of ~25 km through ascent and stalling at ~10–12 km. Refractory phase assemblages crystallized along the pluton margins, whereas subsolidus minerals in the interior of the of body continued to exchange with upper crustal deuteric and surficial aqueous fluids during exhumation and cooling.

Magnetic and structural study of the Eaux-Chaudes intrusion: understanding the Variscan deformation in the Western Axial Zone (Pyrenees)

The present study provides new magnetic and microstructural data for the Eaux-Chaudes granodioritic massif (Western Axial Zone, Pyrenees) and contributes to the understanding of its geometry, internal structure and emplacement mechanism. Moreover, the geological cross-sections and field data allow to reconstruct the evolution of the whole area from Variscan to Alpine times and to integrate the emplacement of the igneous body in the context of the Variscan orogeny. The Eaux-Chaudes pluton (301 ± 9 Ma) is mainly composed by granodiorite, describing a normal compositional zoning and an approximately concentric arrangement that is consistent with the zonation of the low-field magnetic susceptibility. Magnetic foliation is subhorizontal in the inner part of the intrusion and becomes parallel to the petrographical contacts along pluton margins, roughly describing the geometry of the intrusion. Magnetic lineations are dominantly subhorizontal, with E–W to ENE–WSW directional maximum. The general parallelism between Variscan structures of the host rock and the geometry and magmatic fabric of the intrusion reveals a late syn-Variscan emplacement. The tectonic regime registered during magma emplacement is in agreement with an N–S shortening and an E–W stretching direction, consistent with the transpressive regime deduced for other Pyrenean intrusions. Alpine overprint produced a slight tilting in the southern part of the intrusion, but it can be considered that the original Variscan structure is basically unchanged.

Geochemistry, geochronology, and petro-genesis of the early Paleozoic granitic plutons in the central-southern Jiangxi Province, China

This paper reports the systematic study on petrology, geochemistry, LA ICPMS zircons U-Pb dating, and in situ Hf isotope geology of the four plutons in the central-southern Jiangxi Province, an important part of the South China Block. In the outcrops, rocks are gradually changed from wall rock (slate or schist) to pluton (gneissic granite); some residual blocks of sandy rock occur in the margin of pluton, and the foliations of residual blocks are parallel to those of both wall rock and gneissic granite. The thin-section observations show that the four plutons contain peraluminous minerals such as muscovite and sillimanite. The flattened and elongated feldspar and quartz grains are often visible in the gneissic granite, parallel to direction of lineation, suggesting that the granitic rock were subjected to a strong ductile shearing. Geochemically, the A/CNK values from 13 granitic samples are between 1.03 and 1.37 with an average of 1.16, indicating that the granites are of strongly peraluminous plutons. The REE compositions of the 13 samples are similar, showing higher ΣREE contents, with enrichment in LREEs, depletion in Eu and REE patterns with relative LREE-enrichment and negligible Eu anomalies. They show enrichment in Rb, Th, U and depletion in Ba, Sr, Nb, Ti, belonging to a low Ba-Sr type of granite. Thus, the four bodies should be derived from the same magmatic source. Zircons used as U-Pb dating mostly exhibit euhedral shape and high Th/U values from 0.52 to 1.54 with an average of 1.08, suggesting that most zircons are of magmatic genesis. The zircons from four plutons yielded rather similar 206Pb/238U vs. 207Pb/235U concordia ages: 436.1±5.7 Ma for the Tangwan granite, 440.6±4 Ma for the Jiekou gneissic granite, 435.9±6.2 Ma for the Dongbao gneissic granite, and 441.9±3.1 Ma for the Jinxi K-granite, respectively, corresponding to Silurian Llandovery. Several xenocrysts yielded U-Pb ages around 700 Ma, implying that a breakup event took place during Neoproterozoic in the South China Block. In situ Lu-Hf isotopic analysis shows that all ɛHf(t) values of zircons are negative and have two-stage Hf model ages (TDM2) from 1.4 to 3.6 Ga, indicating that the Silurian granitic magma came from the recycle of Meso-Paleoproterozoic basement and even partly Archean rocks, and had not been effected by mantle magma. Researches on regional geology suggest that an intracontinental tectono-magmatic event took place during the early Paleozoic in the study areas, which is characterized by folding and thrusting, leading to crustal shortening and thickening, up to 20 km thickness. The high geothermal temperature from thickening crust and accumulation of producing high-heat radioactive elements gradually softened crustal rocks and caused a partial melting, forming peraluminous granitic magma. Under the post-orogenic extensional and de-pressure condition, these granitic magma rose and was emplaced in the upper crust, leading to development of S-type plutons

Lifetime of an ocean island volcano feeder zone: constraints from U–Pb dating on coexisting zircon and baddeleyite, and 40Ar/39Ar age determinations, Fuerteventura, Canary IslandsThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

High-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U–Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U–Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699–988 °C) to amphibole (500–600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

Localized pluton deformation and linked focused flow of low-volume fraction residual melt in deforming plagioclase cumulates

Shear-hosted, sub-cm to 50-m-wide, Fe-Ti-Zr-REE (FTZR)–rich melt bands occur exclusively at folded margins of the Bolangir anorthosite pluton (eastern India) transverse to margin-parallel foliation in anorthosite. The N- to NNW-trending FTZR melt bands are oriented at a low angle ( 90% fractionation of anorthosites from crustally contaminated, high-Al gabbro (HAG) parent melt. At the high degree of crystal fractionation (crystal content higher than the particle-locking threshold), the FTZR melts, during last stages of crystallization, were locked within the plagioclase-dominated cumulus pile. The occurrence of FTZR melt bands exclusively at folded—as opposed to straight segments of—pluton margin is attributed to melt-enhanced strain localization in the pluton induced by the FTZR melts locked within anorthosite. Further melt-induced straining led to focusing of the FTZR melts along dilatant shear zones formed synchronously with the east-west shortening deformation; the variable widths of the melt bands correspond to the varying sizes of actively deforming magmatic plagioclase in anorthosite. The extraction and transport of locked-in, low-volume fraction melt documented here suggest long-duration, far-field stresses may segregate and transport large volumes of low-volume fraction melts in planetary interior over considerable distance.