Magma Emplacement Research Articles

Building an inversely zoned post-orogenic intrusion in the Neoproterozoic-Cambrian Araçuaí orogen (Brazil)

Post-orogenic plutons are excellent recorders of the last stages of collapsing orogens. In the Neoproterozoic-Cambrian Araçuaí orogen (Brazilian Southeast), one of the Brasiliano orogenies (Western Gondwana), voluminous amounts of Cambrian magmatism crop out. The Venda Nova pluton (VN) is located at the southern portion of the AO, exhibiting a norite/charnockitic west-ring in contact with a syenomonzonite that envelops an alkali-gabbro core with distinct geochemical signatures. To explore the tectonic significance of this post-orogenic pluton in the AO, anisotropy of magnetic susceptibility (AMS) together with a microstructural analysis were performed. AMS data reveals, majorly, a concentric magnetic foliation/lineation pattern, implying that buoyancy forces controlled the magma emplacement of syenomonzonites and gabbros, while a previous pulse of a charnockitic intrusion was deformed by their intrusion. Restrict solid-state deformation (following the regional trend) inside the pluton could result of the reactivation of an emplacement structure in the last stages of VN crystallization. The emplacement of inversely zoned post-orogenic plutons through deep conduits in the colder forelands of the AO, such as the VN, might record different stages of a large collapsing orogen in a single plutonic body.

Zircon record of an Archaean crustal fragment and supercontinent amalgamation in quaternary back-arc volcanic rocks

Magmatism has profoundly influenced the evolution of the geosphere, hydrosphere, atmosphere, and biosphere in back-arc basins. However, the timing of the magmatism in the Okinawa Trough (OT) is not well constrained by the age spectra of zircons. Here, for the first time, we carry out an integrated study combining in situ analysis of zircon U–Th–Pb and Hf–O isotopes, and trace element compositions of zircons from the volcanic rocks from the southernmost part of the OT. We found that the young (< 100 ka) zircons in these volcanic rocks have old (108 Ma to 2.7 Ga) inherited cores, which were captured as the magma ascended through the rifting continental crust. In particular, the inherited Archean zircons strongly suggest that remnants of the old East Asian continental blocks underlie the embryonic crustal rifting zone. Moreover, the ages of most of the inherited zircons correspond to five supercontinent amalgamation events. Specifically, the Archaean inherited zircons, which have positive εHf(t) and low δ18O values, correspond to the formation of juvenile continental crust. In contrast, the negative εHf(t) and high δ18O values of the post-Archaean inherited zircons indicate that their parental magma contained recycled older crust due to the enhanced crust-mantle interactions during the evolution of the early continental crust. Therefore, the inherited zircons in the back-arc volcanic rocks not only reflect the evolution of the local magmatism, but they also contain a record of the Archaean crustal fragment and of several global continental amalgamation events.

Longevity of small-scale (‘baby’) plumes and their role in lithospheric break-up

SUMMARYControversy between advocates of ‘active’ (plume-activated) versus ‘passive’ (driven by external tectonic stresses) modes of continental rifting and break-up has persisted for decades. To a large extent, inconsistencies between observations and models are rooted in the conceptual model of plumes as voluminous upwellings of hot material sourced from the deep mantle. Such large-scale plumes are expected to induce intensive magmatism and topographic uplift, thereby triggering rifting. In this case of an ‘active’ rifting-to-break-up system, emplacement of plume-related magmatism should precede the onset of rifting that is not observed in many rifted continental margins, thus providing a primary argument in favour of an antiplume origin for continental break-up and supercontinent fragmentation. However, mantle plumes are not restricted to whole-mantle (‘primary’) plumes emanating from the mantle-core boundary but also include ‘secondary’ plumes originating from the upper mantle transition zone or shallower. Over the last decades a number of such ‘secondary’ plumes with horizontal diameters of only ∼100–200 km (therefore, sometimes also called ‘baby’ plumes) have been imaged in the upper mantle below Europe and China. The longevity of such small-scale plumes and their impact on geodynamics of continental break-up have so far not been explored. We present results of a systematic parametrical analysis of relatively small thermal anomalies seeded at the base of the lithosphere. In particular, we explore the effects of variations in initial plume temperature (T = 1500–1700 °C) and size (diameter of 80–116 km), characteristics of the overlying lithosphere (e.g. ‘Cratonic’, ‘Variscan’, ‘Mesozoic’ and oceanic) and intraplate tectonic regimes (neutral or far-field extension of 2–10 mm yr–1). In tectonically neutral regimes, the expected decay time of a seismically detectable ‘baby’-plume varies from ∼20 to &amp;gt;200 Myr and is mainly controlled by its initial size and temperature, whereas the effect of variations in the thermotectonic age of the overlying lithosphere is modest. These small but enduring plumes are able to trigger localized rifting and subsequent continental break-up occurring from ∼10 to &amp;gt;300 Myr after the onset of far-field extension. Regardless of the thermomechanical structure of the lithosphere, relatively rapid (tens of Myr) break-up (observed in models with a hot plume and fast extension) favours partial melting of plume material. In contrast, in the case of a long-lasting (a few hundreds of Myr) pre-break-up phase (relatively cold plume, low extension rate), rifting is accompanied by modest decompressional melting of only ‘normal’ sublithospheric mantle. On the basis of the models presented, we distinguish two additional modes of continental rifting and break-up: (1) ‘semi-active’ when syn-break-up magmatism is carrying geochemical signatures of the deep mantle with deformation localized above the plume head not anymore connected by its tail to the original source of hot material and (2) ‘semi-passive’ when the site of final lithospheric rupture is controlled by a thermal anomaly of plume origin but without invoking its syn-break-up melting. These intermediate mechanisms are applicable to several segments of the passive continental margins formed during Pangea fragmentation.

The multiple mineralizations and geodynamic settings of the Laozuoshan Cu–Au deposit in the Jiamusi Massif, NE China: Zircon U–Pb geochronological, elemental and Hf isotopic geochemical evidence

Situated in the centre of the Jiamusi Block, the Laozuoshan deposit is a large-sized deposit with composite of vein and lenticular Au orebodies and veinlet-disseminated Cu-Au orebodies. It is characterized by two types of mineralization, skarn Au mineralization and porphyry Cu-Au mineralization. The former occurs along the contact zone between granodiorite and Proterozoic metamorphic rocks in the eastern and central ore zones, while the latter is hosted in granodiorite porphyry and diorite in the western ore zone. In this study, we present the results of geochronological and geochemical analyses of these rocks which show the following features. (1) Zircon U–Pb dating results show that the ages of granodiorite, diorite and granodiorite porphyry are 264.6±2.6 Ma, 103.2±1 Ma and 104.6±1.8 Ma, respectively. (2) Whole-rock elemental compositions indicate that the granodiorite shows potassium-rich adakitic rock properties, and the diorite has typical island arc calc-alkaline magmatic properties. They are all characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs). (3) The in-situ Hf isotope analysis results show that the granodiorite and diorite have εHf(t) values of −3.7 to −8.7 and −0.4 to 10.7, respectively, corresponding to TDMC ages of 2040–2500 Ma and 630–1650 Ma, respectively. These results indicate two periods of mineralization in the Laozuoshan deposit. Skarn mineralization occurred in the late Permian and was a result of metasomatism between partial melting magma that originated from thickened Paleoproterozoic lower crust and Proterozoic marble. Porphyry mineralization took place in the Early Cretaceous and was a result of the shallow emplacement of mid-Neoproterozoic mantle-derived magma with the addition of crustal materials. In terms of geodynamic settings, skarn Au and porphyry Cu-Au mineralization occurred in a compressional setting formed during the gradual closure of the Paleo-Asian Ocean, and a crustal-thinning setting formed from Eurasian Plate-ward subduction, retreating, and rollback of the Paleo-Pacific Plate, respectively.

Discovery of cyanobacteria fossils in the Bayan Obo Fe-Nb REE deposit and its geological significance

<p indent=0mm>The Bayan Obo deposit in China is known for its huge Fe-Nb-REE resource. The genesis of the ore-hosted H8 dolomite is of great significance to the understanding of the Fe-Nb-REE mineralization. Generally, there are two opposite views established: (1) Intense differentiated mantle-derived carbonates as intrusions or submarine volcanics, and (2) sedimentary origin which was metasomatized by extreme REE-enriched mantle fluids sourced from carbonatitic magmas. However, neither of them could well interpret the fuzzy information given by geochemistry and Fe-Nb-REE mineralization, hence a definite sign should be found. This paper reports the newly discovered cyanobacteria fossils in the massive Fe-Nb-REE ores from the western ore deposit and provides detailed petrographic studies of the cyanobacteria and the spatial relationship with the Fe-Nb-REE mineralization and minerals, aiming to give a reasonable interpretation for the genesis of H8 dolomite. The samples are massive Fe-Nb-REE ores, composed mainly of microorganism dolomite fragments, massive Fe-Nb-REE layers, and coarse-grain dolomite veins. The cyanobacteria is distributed in the “dirty” fine-grained dolomites, consisting of a row of ellipsoidal cells, with a maximum diameter of <sc>~131 μm</sc> and a length of <sc>614 μm.</sc> The outline of the fossil is long columnar, without bifurcation, with a cylindrical front end and pointed back end. The morphology of the fossil is similar to the cyanobacterial filaments. The cell structure is clear under the BSE images, with Ca-Si mineralization for the cell wall and Fe-Mg carbonatization for the cavity. Two groups of cyanobacteria could be distinguished based on their different colors under a polarizing microscope. One group shows yellow translucent probably related with the calcite mineralization and the other group is black opacity related with the Fe-Mg carbonatization. Accessory minerals in the cyanobacteria dolomites include calcite, magnetite, hematite, pyrite, REE minerals, barite, and other carbonate and sulfate minerals. Detailed works reveal that the cyanobacteria dolomites and massive Fe-Nb-REE layers are mutually contained and then filled by coarse-grain dolomite veins along the cracks, based on that (1) their host dolomites show very clear BSE boundary with different components, Mg-Ca vs. Fe-Mn, (2) fragments of the Fe-Nb-REE layers could be found in the cyanobacteria dolomites, while cyanobacterial bioclasts are developed in the Fe-Nb-REE layers, and (3) cyanobacteria fossils are included in coarse-grained dolomites by recrystallization of fine-grained dolomites. Therefore, the cyanobacteria dolomites and massive Fe-Nb-REE layers should be syngenetic sedimented in a locally turbulent shallow sea setting. However, we would like to point out that the massive Fe-Nb-REE layers are not the original composition of sedimentary, but the products of modification by igneous carbonatites and the original composition are most likely to be Fe-Mn carbonates. In this paper, we tend to consider that the Bayan Obo H8 dolomite may be composed dominantly of iron-bearing carbonate formation, and is a classic case of microbial dolomite. Furthermore, photosynthesis is needed during the growth of cyanobacteria, therefore, a depth of < <sc>50 m</sc> is the best speculation for the sedimentary of H8 dolomite. The photosynthesis of cyanobacteria was important for the alteration of the paleoenvironment. Extra oxygen produced by the microbial activity could promote the precipitation of Fe-REE minerals within the dolomite. The magmatic emplacement of Nb-REE rich carbonatites and hydrothermal fluid may have facilitated Fe deposition and contributed some iron. Therefore, the life activities of cyanobacteria may indirectly control the Fe-REE migration and mineralization process in Bayan Obo, thus forming the protore of Fe-REE mineralization.

Using self-organizing maps in airborne geophysical data for mapping mafic dyke swarms in NE Brazil

Mapping widespread dyke swarms shed light on the tectonic processes that culminate in continental fragmentation, especially in the early phases of the crustal extension and magma emplacement. Airborne magnetic data are an effective geophysical tool to clarify the extent of dyke swarms in continental-scale areas due to the expressive magnetic contrast between magmatic bodies and host rocks. However, many geological features display similar magnetic patterns, making the qualitative interpretation of magnetic anomaly maps quite subjective and ambiguous. To improve and optimize the predictive mapping of dyke swarms, this research presents a multivariate analysis of airborne geophysical surveys, applying a Self-Organizing Maps (SOM) approach using two magnetic and three gamma-spectrometric variables. This semiautomatic technique has allowed an integrated spatial analysis and the classification of the lithological units based on their magnetic and gamma-spectrometric signatures. We applied the SOM method to investigate a set of mafic dyke swarms that intruded in the Neoproterozoic Borborema Province (BP), Parnaíba Basin (PB) and the São Francisco Craton (SFC) in NE Brazil. These dykes are part of a large magmatic event associated with the continental breakup, which formed the Equatorial Atlantic Ocean in the Early Cretaceous, denominated EQUAMP. First, the SOM hyperparameters were defined by running the algorithm in a representative area in the central part of the BP. This training test area worked as a SOM template through which all data from the area were processed. The SOM analysis identified seven different populations, according to responses found in the five geophysical input variables. Two of these populations were associated with the mafic dykes, reducing the ambiguity of the magnetic anomaly interpretation. These populations represent high SOM quantization error, which means that these groups are the most anomalous data, evidenced in airborne magnetic data. These results were checked during fieldwork, revealing that dyke swarms occur more widely than were previously known, throughout the BP intruding the SFC, and showing some occurrences embedded in the Paleozoic sedimentary infill of the eastern border of the PB.

Evidence for iron-rich sulfate melt during magnetite(-apatite) mineralization at El Laco, Chile

Abstract The origins of Kiruna-type magnetite(-apatite) [Mt(-Ap)] deposits are contentious, with existing models ranging from purely hydrothermal to orthomagmatic end members. Here, we evaluate the compositions of fluids that formed the classic yet enigmatic Mt(-Ap) deposit at El Laco, northern Chile. We report evidence that ore-stage minerals crystallized from an Fe-rich (6–17 wt% Fe) sulfate melt. We suggest that a major component of the liquid was derived from assimilation of evaporite-bearing sedimentary rocks during emplacement of andesitic magma at depth. Hence, we argue that assimilation of evaporite-bearing sedimentary strata played a key role in the formation of El Laco and likely Mt(-Ap) deposits elsewhere.

Late Devonian syenitic intrusion from southeastern Alaska: Petrogenesis, tectonic implications, and rare metal metallogeny

The Late Devonian Corner Bay pluton is a part of the Sitkoh alkaline plutonic complex on the Chichagof Island, southeastern Alaska, which is located in the Alexander terrane of the North American Cordillera. The intrusion, about 12 km long and 4 km wide, is composed predominantly of amphibole-biotite-bearing syenitic/ monzonitic rocks with minor alkaline gabbros and has a U-Pb zircon age of ~367 Ma interpreted as the crystallization age of the pluton. The rocks are metaluminous and have silica contents ranging from 47 to 63 wt% with a silica gap at 52–58 wt% and have high contents of alkalis, typical of shoshonitic rocks. They are rich in large-ion-lithophile elements (including Rb, Sr, Ba, U) and their isotopic composition is characterized by relatively uniform ƐNd(t) values (+3.4 to +3.7), Neoproterozoic Nd depleted mantle model ages (650–700 Ma) and initial 87Sr/86Sr ratios (~0.704). The rocks were generated by fractional crystallization of alkali basaltic magma formed by partial melting of an amphibole-phlogopite-bearing peridotite of the sub-arc lithospheric mantle. The mantle-source underwent a Neoproterozoic metasomatic enrichment event and does not show a contribution from juvenile Devonian mantle indicating that rifting associated with the magma emplacement was of limited extent as it did not replace the old lithospheric mantle. Several geothermobarometers document emplacement of the parental magma at mid-crustal levels (8.5–10 kb) and polybaric crystallization from ~1200 °C to solidus. Crystallization of amphibole took place at ~880-980 °C and 2.4–5.5 kb. Subsequently the rocks underwent recrystallization (feldspars: 170 Ma – 40Ar/39Ar age) and re-equilibration (biotite: 129 Ma (40Ar/39Ar) at ~600-650 °C and 2 kb). Zircon, rutile and apatite saturation temperatures provide consistent results (~800 °C). The intrusion is a part of a metallogenic province of rare metals, which stretched across the islands in the southeastern most part of Alaska.

Diagenetic Modifications and Reservoir Heterogeneity Associated with Magmatic Intrusions in the Devonian Khyber Limestone, Peshawar Basin, NW Pakistan

In the present study, an attempt has been made to establish the relationship between diagenetic alterations resulting from magmatic intrusions and their impact on the reservoir properties of the Devonian Khyber Limestone (NW Pakistan). Field observations, petrographic studies, mineralogical analyses, porosity-permeability data, and computed tomography were used to better understand the diagenetic history and petrophysical property evolution. Numerous dolerite intrusions are present in the studied carbonate successions, where the host limestone was altered to dolomite and marble, and fractures and faults developed due to the upwelling of the magmatic/hydrothermal fluids along pathways. Petrographic studies show an early phase of coarse crystalline saddle dolomite (Dol. I), which resulted from Mg-rich hydrothermal fluids originated from the dolerite dykes. Coarse crystalline marble formed due to contact metamorphism at the time of dolerite emplacement. The second phase of dolomitisation (Dol. II) postdates the igneous intrusions and was followed by dedolomitisation, dissolution, and cementation by meteoric calcite. Stable isotope studies likewise confirm two distinct dolomite phases. Dol. I exhibits more depleted δ18O (-15.8 to -9.1‰ V-PDB) and nondepleted δ13C (-2.05 to +1.85‰ V-PDB), whereas Dol. II shows a relatively narrow range of depleted δ18O (-13.9 to -13.8‰) signatures and nondepleted δ13C (+1.58 to +1.89‰ V-PDB). Dolomitic marble shows a marked depletion in δ18O and δ13C (-13.7 to -8.5‰ and -2.3 to 1.95‰, respectively). The initial phase of dolomitisation (Dol. I) did not alter porosity (5.4-6.6%) and permeability (0.0-0.1 mD) with respect to the unaltered limestone (5.6-6.9%; 0.1-0.2 mD). Contact metamorphism resulted in a decrease in porosity and permeability (3.3-4.7%; 0.1 mD). In contrast, an increase in porosity and permeability in Dol. II (7.7-10.5%; 0.8-2.5 mD) and dolomitic marble (6.6-14.7%; 8.2-13.3 mD) is linked to intercrystalline porosity and retainment of fracture porosity in dolomitic marble. Late-stage dissolution and dedolomitization also positively affected the reservoir properties of the studied successions. In conclusion, the aforementioned results reveal the impact of various diagenetic processes resulting from magmatic emplacement and their consequent reservoir heterogeneity.

Metasomatized eclogite xenoliths from the central Kaapvaal craton as probes of a seismic mid-lithospheric discontinuity

The central region of the Kaapvaal craton is relatively understudied in terms of its lithospheric mantle architecture, but is commonly believed to be significantly impacted by post-Archean magmatism such as the ca. 2056 Ma Bushveld large igneous event. We investigate a collection of 17 eclogite xenoliths from the Cretaceous Palmietfontein kimberlite at the Western Limb of the Bushveld Complex for their mineralogical compositions (major and trace elements, plus Fe3+ contents), as well as stable oxygen and radiogenic Pb isotopic compositions to gain further insights into the nature and evolution of the central Kaapvaal cratonic mantle lithosphere. New U/Pb age determinations on mantle-derived zircon yield a magma emplacement age of ca. 75 Ma for the Palmietfontein Group-1 kimberlite, which means that the entrained eclogite xenoliths may record a protracted metasomatic history from the Proterozoic through to most of the Phanerozoic eon.Garnet δ18O values of up to 6.9‰ and positive Eu anomalies for the bulk rocks suggest seawater-altered oceanic crustal protoliths for the Palmietfontein eclogite xenolith suite, which is typical for the eclogitic components of the Kaapvaal root and other cratonic mantle sections worldwide. However, several features of the Palmietfontein eclogites are commonly not observed in other mantle-derived eclogite xenolith suites. Firstly, the samples studied yield relatively low equilibration pressures and temperatures between 2.7 and 4.5 GPa and 740–1064 °C, indicating a relatively shallow residence between 90 and 150 km depths. Secondly, euhedral coarse amphibole is present in several eclogite nodules where it is in equilibrium with ‘touching’ garnet, supporting eclogite residence within the amphibole stability field at uppermost lithospheric mantle conditions. Thirdly, primary omphacitic clinopyroxene is often overgrown by diopside, and is significantly enriched in incompatible trace elements. The clinopyroxene is also characterized by elevated 206Pb/204Pb of 17.28–19.20 and 207Pb/204 Pb of 15.51–16.27, and these Pb isotopic compositions overlap with those of Mesozoic Group-2 kimberlites from the Kaapvaal craton.Our results show that eclogites reside at ~85 km depth beneath the central Kaapvaal craton as part of a layer that corresponds to an approximately 50 km thick seismically-detected mid-lithospheric discontinuity. Mid-lithospheric discontinuities have been interpreted as metasomatic fronts formed by focussed crystallization of hydrous mineral phases from enriched volatile-bearing melts, and as such the strongly overprinted amphibole-bearing eclogite xenoliths from Palmietfontein may represent a physical expression of such seismically anomalous metasomatic layer at mid-lithospheric depth. Our Pb isotope data suggest that the focussed metasomatism can be attributed to volatile-rich melts reminiscent of potassic Group-2 kimberlites, which have been invoked in MARID-style metasomatic overprinting of the lower lithospheric mantle beneath the western Kaapvaal craton. However, the relatively low fO2 recorded by the Palmietfontein eclogites (minimum FMQ-4.5) suggests that the metasomatism at mid-lithospheric depth was less protracted compared to the more intensive and oxidizing metasomatism typically observed near the base of cratonic mantle roots. While it is possible that Proterozoic magmatic events were responsible for the focussed mid-lithospheric metasomatism of the Kaapvaal mantle, on the basis of the Pb isotope constraints the Palmietfontein eclogites were most likely overprinted during ca. 120 Ma Group-2 kimberlite magmatism.

U - Pb geochronology and composition of zircon mineral from granodiorite in the G18 Gold mine, Quang Nam and its significance in regional tectonics

The Tam Ky - Phuoc Son suture zone (TPSZ) is located in central Indochina and is referred to as a amalgamation site between the Truong Son and Kon Tum terranes during the early Paleozoic. The amphibolite and ultramafic rocks within the region were considered as a part of the Tam Ky - Phuoc Son ophiolitic complex. In this study, the authors present results of the U - Pb dating and trace element composition of the zircon grains derived from a granodioritic sample collected in the G18 gold mine in Quang Nam province in order to clarify the timing of magma emplacement and tectonic setting. The U - Pb dating data indicates that the granodioritic rocks formed at 447,4±2,9 Ma while the U/Yb ratio is ̴1 (average: 1.32) and the Sc/Yb ratio is ̴ 1.04, high Hf content (Average Hf: 10937 ppm) and low Yb content (Average Yb: 308 ppm). These geochemical values are comparable with the zircon, which is formed in the continental magmatic arc. Combination with the pre - existing data allowed us to confirm the existence of two contrast magma members in the North Kontum massif: The Middle Cambrian island arc complex and the Middle Ordovician continental complex.

Tectono - Magmatic and stratigraphic evolution of final rifting and breakup: Evidence from the tip of the southwestern propagator in the south China sea

In this study, we explore the tectono-sedimentary and magma evolution of final rifting and continental breakup along the SW conjugate margins of the South China Sea (SCS). Based on new seismic observations, we define and map three rift domains, referred to as “box-shaped”, “wedging” and “proto-oceanic”. Each of these domains is characterized by: (1) the shape and thickness of the crust; (2) the specific architecture of the syn-rift mega-sequence that includes three system tracts each one including syn- and post-rift packages; and (3) different deformation styles.Results are synthetized in two Wheeler diagrams that provide a complete description of the tectono-magmatic and stratigraphic evolution of final rifting and breakup within two neighboring segments of the SW-SCS. We conclude that: 1) localization of deformation occurs simultaneous and perpendicular to the migration of magma-emplacement in front of a propagating spreading system, 2) the arrival of magma controls continental breakup, and 3) the final rifting stage and breakup are diachronous. The relationships between extension, sedimentation and magmatic processes observed in the SW-SCS differ significantly from the classical models derived from the N-Atlantic or the Alpine Tethys. Main differences include the way the crust is thinning/necking, the sharp transition to breakup and the style of magma emplacement linked to crustal breakup.

Zircon U–Pb geochronology, Hf isotope composition, and petrochemical characteristics of Paleocene granitoids in the western Gangdese Belt, Tibet

Abstract —The research team studied the petrology, whole-rock geochemistry, zircon U–Pb age, and stable isotopic characteristics of the Rongguo Longba and Garongcuo granites of the Nuocang area to understand better the impact of Neo-Tethys ocean subduction and In-dia–Eurasia continental collision on Paleocene tectonomagmatic processes along the southern margin of the Gangdese Belt. The Rongguo Longba granite and Garongcuo granite porphyry formed at 61.86 and 62.17 Ma, respectively. The Nuocang granitoids are characterized by (1) high SiO2, NaO2, and Al2O3 contents and low FeOtot, MgO, and TiO2 contents; (2) LREE and LILE enrichment and HREE and HFSE (Nb, P, and Ti) depletion; and (3) obvious negative Eu anomalies. These features indicate that the Nuocang granites are of the high-K calc-alkaline and peraluminous granite types. Furthermore, their zircon Hf isotope characteristics suggest that the magma source region has an ancient crystalline basement. The basaltic andesitic crystal tuff is the product of garnet–peridotite partial melting and crust contamination from rising magma emplacement.

Tectonic significance of the late Eocene (Bartonian) calc-alkaline granitoid body in the Marivan area, Zagros suture zone, northwest Iran

ABSTRACT The Marivan granitoid body of the northern Sanandaj–Sirjan Zone (N-SaSZ) of northwest Iran was emplaced into Cretaceous sediments, and is associated with minor amounts of iron skarn-type mineralization along the contact between the intrusion and surrounding country rocks. The granitoids within the intrusion include (hornblende-rich) diorite, granodiorite, and granite phases, and zircon U–Pb dating of four samples from this intrusion indicate they were emplaced during the late Eocene (Bartonian; 37.7 ± 1.0 Ma). These plutonic rocks can be classified into two main types, granitic and dioritic rocks, according to petrography and whole-rock major element geochemistry. Both the granitic and dioritic rocks are high-K, and some of the granitic rocks have an I-type affinity and peraluminous to metaluminous compositions. Both the granitic and dioritic rocks have εNd(t) values that cluster around zero (–3.3 to +3.0) and low 87Sr/86Sr(i) values (0.7040–0.7076), suggesting derivation from a mafic juvenile crustal or depleted mantle source, which was possibly mafic calc-alkaline rocks or amphibolites within the lower crust. The new zircon U–Pb ages and petrological and geochemical data presented in this study suggest that the Palaeocene–early Eocene collision of the Arabian and Iranian plates caused the ascent and emplacement of hot mafic magma through a slab window into the crust. This increased the geothermal gradient within the root zone of the developing magmatic system at the base of the continental crust, causing the partial melting of lower crust in this region. This generated mafic to intermediate magmas that were either directly emplaced into the upper crust or mixed with varying amounts of upper crustal material prior to emplacement, which formed the diorite to leucogranite range of compositions within the Marivan granitoid intrusion.

Copper-mineralised porphyries sample the evolution of a large-volume silicic magma reservoir from rapid assembly to solidification

Volatile-rich intermediate to silicic magmatic systems can feed devastating volcanic eruptions but also generate valuable magmatic-hydrothermal ore deposits that supply most of the world's copper. Understanding the geometry, dynamics and timescales of these magmatic systems is critical in developing models for predicting the occurrence of ore deposits and future large volcanic eruptions. Here, we use zircon petrochronology from an equigranular monzonite and successively emplaced porphyry dykes to reconstruct the time – temperature – composition evolution of the magma that sourced the giant Cu-Mo-Au deposit at Bingham Canyon (USA). Combining high-precision CA-ID-TIMS geochronology with in-situ trace element analyses by LA-ICP-MS shows the intra-grain, inter-sample and temporal geochemical changes recording the evolution of the magmatic system over 817 ± 62 kyr. Systematic variation of zircon chemistry with time indicates crystallisation from a coherent magma reservoir. After reservoir assembly its thermal and chemical state was controlled by protracted monotonous crystallisation over ∼650 kyr with rapid cooling over the first 200 kyr followed by a longer period approaching the granite solidus. Porphyry Cu-Au ore formation occurred after the early drop in magma temperature that resulted in large-scale fluid saturation and expulsion into the sub-volcanic environment but main Mo-mineralisation occurred after protracted low-temperature magma storage and the emplacement of the last porphyry. Zircons do not quantify the depth of this reservoir but integrating independent geophysical evidence with 2-D thermal modelling indicates that the time – temperature evolution recorded by the zircons is consistent with rapid incremental assembly of this large pluton (magma emplacement rate ≳ 0.0065 km3/yr) by initially zircon-undersaturated monzonitic magma into pre-heated upper crust. Our results indicate that massive fluid expulsion from rapidly-formed, large magma reservoirs containing mobile but mushy magma (>40 wt.% melt) can occur in the upper crust, favouring the formation of giant porphyry copper deposits.

Unveiling ductile deformation during fast exhumation of a granitic pluton in a transfer zone

Exhumation and cooling of upper crustal plutons is generally assumed to develop in the brittle domain, thus determining an abrupt passage from crystallization to faulting. To challenge this general statement, we applied an integrated approach involving meso- and micro-structural studies, thermochronology, geochronology and rheological modeling to the Miocene syn-tectonic Porto Azzurro pluton on Elba (Tuscan archipelago – Italy). This pluton is emplaced in an extensional setting, and we have realized that its fast exhumation is accompanied by localized ductile shear zones, developing along dykes and veins, later affected by brittle deformation. This is unequivocally highlighted by field studies and EBSD-aided microstructural analysis. To constrain the emplacement and exhumation rate of the Porto Azzurro pluton we performed U–Pb zircon dating and (U + Th)/He apatite thermochronology, which resulted in a magma emplacement age of 6.4 ± 0.4 Ma and an exhumation rate of 3.4–3.9 mm/yr. Thermo-rheological modeling established that localized ductile deformation occurred at two different time steps: within felsic dykes when the pluton first entered into the brittle field at 380 kyr, and along quartz-rich hydrothermal veins at c. 550 kyr after pluton emplacement. Hence, the major conclusion of our data is that ductile deformation can affect a granitic intrusion even when it is entered into the brittle domain in a fast exhuming extensional regime.

Genesis and age of Pb−Zn mineralization from the Ningi-Burra ring complex, North Central Nigeria: Constraints from zircon morphology, U−Pb dating and Lu−Hf isotopes

The Ningi-Burra Pb−Zn mineralization forms part of the Nigerian Sn-Nb ± (Pb-Zn-Mo)-rich anorogenic alkaline province, also known as the Nigerian Younger Granite Complexes. Six distinct E-W trending magmatic centres have been identified in the Ningi-Burra complex, but the Pb−Zn mineralization is confined to highly evolved biotite granite in Koluki (centre 6). In this paper, a combination of geochronological and geochemical data of magmatic and hydrothermally-altered zircon from the Ningi-Burra complex are used to constrain magmatic and hydrothermal processes that facilitated the formation of Pb−Zn mineralization. Zircon trace element and Hf isotopes (εHf(t) = −5.3 to −10.3) indicate that the parental magma of these ore-bearing granites were largely derived from lower crustal sources with contributions from the upper mantle. The hydrothermal alteration that accompanied the Pb−Zn mineralization include silicification, chloritization and limited sodic metasomatism. Zircon U/Pb dating reveal that magma emplacement and subsequent hydrothermal process in Ningi-Burra complex occurred at (~192 Ma) and (<191 Ma), respectively. Zircon grains from unmineralized and hydrothermally altered rock suites have similar 176Hf/177Hf but distinct 176Lu/177Hf and 176Yb/177Hf ratios which suggest a magmatic source for the hydrothermal fluids responsible for the formation of the mineralization. This study highlights the application of zircon trace element and U-Th-Hf isotopes as robust tools for constraining magmatic and mineralization process that facilitate the formation of economically significant mineralization in highly differentiated granites.

Forced Fold Amplitude and Sill Thickness Constrained by Wireline and 3-D Seismic Data Suggest an Elastic Magma-Induced Deformation in Tarim Basin, NW China

Disparities between fold amplitude (A) and intrusion thickness (Hsill) are critical in identifying elastic or inelastic deformation in a forced fold. However, accurate measurements of these two parameters are challenging because of the limit in separability and detectability of the seismic data. We combined wireline data and 3-D seismic data from the TZ-47 exploring area in the Tarim Basin, Northwest China, to accurately constrain the fold amplitude and total thickness of sills that induced roof uplift in the terrain. Results from the measurement show that the forced fold amplitude is 155.0 m. After decompaction, the original forced fold amplitude in the area penetrated by the well T47 ranged from 159.9 to 225.8 m, which overlaps the total thickness of the stack of sills recovered by seismic method (171.4 m) and well log method (181.0 m). Therefore, the fold amplitude at T47 area is likely to be elastic. In contrast, the outer area of the TZ-47 forced fold is characterized by shear-style deformation, indicating inelastic deformation at the marginal area. It is suggested that interbedded limestone layers would play an important role in strengthening the roof layers, preventing inelastic deformation during the emplacement of intrusive magma.

Fault inversion contributes to ground deformation above inflating igneous sills

Magma emplacement is commonly accommodated by uplift of the overburden and free surface. By assuming this deformation is purely elastic, we can invert the shape and kinematics of ground deformation to model the geometry and dynamics of underlying intrusions. However, magma emplacement can be accommodated by viscoelastic and/or inelastic processes. We use 3D seismic reflection data to reconstruct how elastic bending and inelastic processes accommodated emplacement of a Late Jurassic sill offshore NW Australia. We restore syn-emplacement ground deformation and compare its relief to sill thickness, showing that: (i) where they are equal, elastic bending accommodated intrusion; but (ii) where sill thickness is greater, inversion of a pre-existing fault and overburden compaction contributed to magma accommodation. Our results support work showing inelastic processes can suppress ground deformation, and demonstrate magmatism can modify fault displacements. Reflection seismology is thus powerful tool for unravelling links between magma emplacement, ground deformation, and faulting.

Episodic Crustal Extension and Contraction Characterizing the Late Mesozoic Tectonics of East China: Evidence From the Jiaodong Peninsula, East China

AbstractDuring the Late Mesozoic, East China is characterized by widespread magmatism, thrusting and folding, extensional doming, strike‐slip faulting, and block rotation. The Jiaodong Peninsula provides a key area located in East China to understand the episodic intracontinental extension and contraction and associated granitoid emplacement. Based on our structural analysis, magnetic fabric measurement, and gravity modeling, polyphase deformation and magma emplacement have been recognized within the Queshan‐Kunyushan‐Yuangezhuang‐Sanfoshan (QKYS) massif of the central Jiaodong Peninsula. A significant Late Jurassic D1 event, developed in the northern margin of the massif, was expressed by a high‐temperature, top‐to‐the‐NE shearing. Late Jurassic plutons display magnetic fabrics corresponding to the D1 structural fabrics and several NW–SE‐trending feeder zones at depth. These results link the syn‐kinematic emplacement of Late Jurassic plutons with regional NE–SW extensional tectonics. In the south of the massif, a lower‐temperature, top‐to‐the‐SW contractional deformation (D2) resulted from NE–SW contraction. The D3 shear zone with a top‐to‐the‐WNW kinematics is a rolling‐hinge type detachment fault that exhumed the massif, indicating NW–SE regional extension. Finally, Early Cretaceous plutons emplaced into the upper crust with a fast‐cooling rate and formed inverted drop shapes with concentric magnetic foliations and variably oriented magnetic lineations. In light of the previous geochronological results, the timing of these tectonic events is discussed. The tectonic evolution of the QKYS massif indicates a process from crustal thickening to lithospheric foundering in response to the Late Mesozoic plate convergences.