Magmatic Episodes Research Articles

Oligocene to Miocene magmatic and hydrothermal evolution in the Maricunga belt (Región de Copiapó, Chile): A new look to a long known metallogenic belt

The Maricunga metallogenic belt, herein defined to encompass the region from 25° S to 28° S along the Cordillera Frontal of northern Chile, is known for hosting a series of gold-copper and lesser copper-molybdenum porphyries and gold-silver epithermal systems that are hosted in a segment of the late Oligocene-Miocene Andean calc-alkaline magmatic arc. Based on an up-to-date geochronological database, four episodes of magmatism and related hydrothermal deposits are proposed to characterize intensity and style of volcanism and hypabyssal intrusive rocks over time. The late Oligocene episode (26-21 Ma) is characterized by stratovolcanoes, dome complexes, pyroclastic sequences, and hypabyssal intrusives of andesitic to dacitic composition that host gold-rich porphyry systems such as Caspiche, Maricunga, and La Pepa. The early Miocene episode (21–17 Ma) has similar rock types but is volumetrically reduced, hosting epithermal and porphyry deposits like La Coipa and Caserones, respectively. A reinvigoration of volcanism occurred during the third stage between 17 and 11 Ma, giving rise to large volcanic complexes and intra-to extra-caldera ignimbritic deposits that host several gold-rich porphyry deposits such as Lobo-Marte, Luciano, and Cerro Casale; as well as epithermal deposits such as Fenix. Some of the 17 to 11 Ma and younger igneous rock belong to the high-K calc-alkaline series, showing a higher alkali content compared to previous periods. The last magmatic episode, from 11 to 5 Ma, coincides with the shallowing of the subduction angle and is characterized by an eastward migration of volcanism, and more heterogeneous rocks including trachytic compositions. During this period mineralization was emplaced at the Salares Norte epithermal deposit and El Volcan porphyry Au–Cu deposit.Porphyry and epithermal systems formed throughout the evolution of the Maricunga belt. Overprint of advanced argillic alteration on porphyry style alteration is widespread albeit commonly devoid of significant addition of epithermal Au–Ag mineralization. The Au-rich nature of the majority of the porphyry systems is attributed to the shallow crustal depths of emplacement compared to Cu- and Mo-rich porphyry systems. Conversely, high-sulfidation epithermal Au–Ag deposits such as La Coipa and Nueva Esperanza are not known to be telescoped on coeval porphyry gold deposits.The location of the mineralized centers was favored by crustal weaknesses expressed by intersection of oblique- and parallel-to arc structures, the former inherited from the construction of the continent before to the formation of the Andean range. The arc oblique structures also segment the Maricunga belt into domains with unique magmatic and hydrothermal styles.

Shallow crustal imaging beneath NW Indian terrane from teleseismic P-wave coda using a Bayesian approach

Deciphering the crustal architecture of the Cretaceous Barmer-Sanchore Basin (BSB) and Neoproterozoic provinces in the NW Indian Shield presents significant challenges due to extensive sediment deposition and widespread felsic igneous rocks. To address such intricacies, we employ a transdimensional Bayesian joint inversion of teleseismic P-wave polarizations and receiver functions to resolve the detailed structure of the upper 10 km of the crust beneath 29 seismic stations deployed in the study area. The new data is analyzed separately within each of the three different backazimuthal groups to identify any strong variations in the shallow subsurface structure and derive a more robust, averaged model for every station site. The resulting 1-D models reveal heterogeneous sediment cover in BSB and Marwar Basin, with respective thicknesses ranging from ∼1.3–5.5 km and 1–1.5 km. Remarkably low shear-wave velocities (Vs < 1.0 km/s) of sediments with increased Vp/Vs (>2.3) are observed in BSB and adjacent areas, which are related to highly unconsolidated Quaternary-Tertiary deposits. The Marwar Basin is imaged to have 0.3–0.5 km of uncompacted sediments that overlie 0.5–1 km thick Marwar Supergroup sequence. The Erinpura Granites (EG) and Malani Igneous Province (MIP) are characterized by a variably thick sediment blanket, typically ranging within 1 km in thickness. The Vs of the sediments in these regions exceeds 2.4 km/s that possibly corresponds to Mesozoic and older sediments. The basement beneath EG exhibits Vs faster than 3.4 km/s, consistent with moderately metamorphosed granites. In MIP, the basement is characterized by non-uniform shear velocities, suggesting the influence of intrusions linked to magmatic episodes in NW India.

Proterozoic crustal evolution of the granitoids from the northern margin of the Chotanagpur Gneissic Complex (CGC), Bihar

Since the Proterozoic, considerable expansion of the continental crust has taken place. A significant amount of granitic plutons were deposited on the earth’s crust during this time. The dynamic structure of the earth plays a close role in the genesis of granites and related rocks. Information about the formation and development of the continental crust can be understood from the study of these rocks. The emplacement of large scale anorthosite massif in shield areas was another process that contributed to the Proterozoic expansion of the continental (Mukherjee, et al.,, 2005). One of the primary sources of information regarding the formation and evolution of the continents over geological time is the genesis of granite. Enclaves of supracrustal meta-sediments intrude the Chotanagpur Gneissic Complex (CGC), which is primarily composed of granites, granitoid gneisses, and migmatites exhibiting a wide range of structure, texture, and mineralogy variations.The granitic plutons exposed in the northern fringe of the Chotanagpur Gneissic Complex, CGC, and the associated anorthosites of the Barabar Anorthosite Complex along the northen margin of Bihar holds information about the various magmatic episodes that have taken place in that area millions of years ago.

Petrology and Geochemistry of an Unusual Granulite Facies Xenolith of the Late Oligocene Post-Obduction Koum Granodiorite (New Caledonia, Southwest Pacific): Geodynamic Inferences

Pressure–temperature estimates of a xenolith found within a post-obduction granodiorite in southern New Caledonia provide evidence for subcrustal, granulite facies, peak crystallisation conditions (ca. 850 °C—8.5 ± 1.0 kbar), followed by isobaric cooling to 700 °C, and final decompression with partial rehydration at ca. 650 °C—3.5 kbar. The xenolith, dated at 24.7 Ma (U-Pb zircon), i.e., the same age as the granodiorite host rock, has low SiO2 (35.5 wt%) and high Al2O3 (33.2 wt%) contents, suggesting that it is the restite of a previous melting episode, while the elevated Ca (Ba and Sr) contents suggest mantle metasomatism. Although the concentrations of Rb, K, Ca, Ba, and Sr have been strongly modified, some geochemical (REE patterns and some “immobile” trace element ratios) and isotopic (Sr and Nd isotopic ratios, U-Pb zircon age) characteristics of the granulite facies xenolith are similar to those of the xenoliths found in other Late Oligocene intrusions in southern New Caledonia; therefore, this rock is interpreted to be related to an early magmatic episode. The rock protolith was emplaced and equilibrated at the base of the crust where it underwent ductile deformation. Younger ascending magma picked it up and they eventually crystallised together at a shallow crustal level, near the tectonic sole of the ophiolite. The recrystallisation and ductile deformation at ~8.5 kbar suggest that a rheological discontinuity existed at about 25–28 km, probably representing the Moho. It is concluded that a continental crust of normal thickness must have existed beneath New Caledonia at about 24 Ma, i.e., 10 Ma after obduction.

Evolution of Silurian to Devonian magmatism associated with the Acadian orogenic cycle in eastern and southern Newfoundland Appalachians: Evidence for a three-stage evolution characterized by episodic hinterland- and foreland-directed migration of granitoid magmatism

The migration and character of magmatism over time can provide important insights into the tectonic evolution of an orogen. We present evidence for three separate stages of compositionally distinct granitoid magmatism associated with the Acadian orogenic cycle in the eastern and southern Newfoundland Appalachians. The interpretations are based on new zircon U-Pb ages, geochemical data, and Sr-Nd-Hf-O isotopic data for 18 samples from 15 Silurian and Devonian granitoid plutons, combined with previously published data. The three stages outline hinterland- and foreland-directed migration trends and represent subduction (435−420 Ma), syncollision (415−405 Ma), and postcollision (395−370 Ma) settings in the Acadian orogenic cycle. The Silurian plutons (435−420 Ma) of the first stage consist mainly of quartz diorite, tonalite, granodiorite, monzogranite, and syenogranite, with high-K calc-alkaline and enriched Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −5 to −2; εHf[t] = −3 to −1; δ18O = +6‰ to +8‰). They are interpreted to record the subduction of oceanic lithosphere of the Acadian seaway that separated the leading edge of composite Laurentia, represented by the Gander margin, and Avalonia. Early Devonian plutons (415−405 Ma) of the second stage contain more voluminous monzogranite and syenogranite; they have calc-alkaline to high-K calc-alkaline features, adakite-like compositions, and more depleted Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −6 to 0; εHf[t] = +1 to +3; δ18O = +5‰ to +6‰). Plutons of this stage occur mostly to the northwest of the Silurian granitoids, indicating a regional-scale northwestward (hinterland-directed) migration of magmatism with a rate of &amp;gt;9 km/m.y. The migration is interpreted to have been related to the progressive shallow underthrusting of Avalonia beneath the Gander margin (composite Laurentia) at least as far as 90 km inboard. The Middle to Late Devonian plutons of the third stage (395−370 Ma) consist mainly of monzogranite, syenogranite, and alkali-feldspar granite, which are silica- and alkali-rich granites with large negative Eu anomalies. These rocks are concentrated along both sides of the Dover−Hermitage Bay fault zone, which represents the boundary between Avalonia and composite Laurentia, to the southeast of the Silurian and Early Devonian igneous rocks. This stage of magmatism represents a foreland-directed (retreating) migration. The Early Devonian and Middle to Late Devonian episodes of magmatism were separated by a gap between 405 Ma and 395 Ma and recorded an evolution from (high-K) calc-alkaline to alkaline compositions, ascribed to partial delamination of Avalonian lithospheric mantle in a postcollisional setting.

An enhanced chemical maturation of juvenile arc crust recorded by the Urad Houqi intrusive suite in Langshan arc, Central Asia

Continental arc magmatism is crucial in producing juvenile continental crust of andesitic to dacitic composition, yet its impact on the formation of the modern mature continental crust, which is more enriched in potassic and granitic components, remains poorly understood. We examined a suite comprised of norite gabbro, gabbroic diorite, high-Mg diorite, dioritic enclave, and medium-K to high-K granites (the Urad Houqi suite) from the Langshan arc in the southern Central Asian Orogenic Belt (CAOB). Zircon U-Pb geochronology indicates that the Urad Houqi suite was formed ca. 278−268 Ma by a continuous magmatic event, marking the final episode of the late Paleozoic continental arc magmatism in the southern CAOB induced by the subduction of the Paleo-Asian Ocean. The suite exhibits a more potassic and fertile composition compared to typical continental arc magmas, and shows a continuous increasing K2O/Na2O ratio and incompatible element concentrations, including K, Rb, Th, and U, with increasing SiO2 content. The geochemical evolution of the suite offers valuable insights into the maturation process of the juvenile arc crust. By combining zircon Hf and bulk-rock Sr-Nd-Pb isotope studies, we demonstrated that the various lithologies within the Urad Houqi suite recorded a three-stage enhancement of the geochemical fertility of the juvenile Langshan arc crust: (1) generation of the initially fertile primary melt from the mantle wedge metasomatized by the recycled subducted sediments; (2) trans-crustal open-system hybridization processes, including peritectic reaction and magma mixing, that further fertilized the derived melts in addition to fractional crystallization; and (3) diverse petrogenetic processes that contributed to the fertility of the granitic magmas, including polybaric fractionation of the primary magma, anatexis of ancient crust, and extraction of high-silica melts from the shallow mush system. The three-stage maturation of the juvenile Langshan arc crust was typically coupled with the accelerated subduction of the Paleo-Asian Ocean since the earliest Permian. Our study finds that the Urad Houqi suite can serve as an example of modern continental crust maturation at continental arc settings.

Episodic magmatism of the Gongga batholith (eastern Tibet) revealed by detrital zircon U-Pb geochronology: Insights into phased Xianshuihe fault activity and plateau growth

Understanding the onset and episodes of magmatism is essential for comprehending tectonic history, crustal extension, and geodynamic processes. However, due to physical constraints, many places have remained unexplored, which makes it difficult to understand their geological evolution. Following thorough sedimentary provenance analysis, the chronology and periods of magmatism within a drainage area can be revealed through the detrital zircon U-Pb dating method. Here, we present detrital zircon U-Pb ages (n = 1429) obtained from sediments in modern rivers of the Gongga batholith in the eastern Tibetan Plateau. Our results reveal five major magmatic episodes since the early Mesozoic. Three episodes of magmatism occurred in the early to middle Mesozoic (ca. 230–200 Ma, ca. 200–180 Ma, and ca. 180–160 Ma), followed by a protracted period of magmatic quiescence. During the Cenozoic, there were two main periods of magmatism at ca. 50–25 Ma and ca. 25–5 Ma. This is consistent with bedrock geochronological data acquired previously. We propose that the Mesozoic magmatism was most likely caused by post-collisional extension after the closure of the Paleo-Tethys Ocean. The two Cenozoic magmatic episodes are coeval with the progressive intensification of Xianshuihe fault activity. Consequently, these episodes highlight two significant phases of plateau growth in the eastern Tibetan Plateau: the northward push of the Indian plate and “lateral extrusion,” which is consistent with the ongoing subduction of the Indian plate beneath the Eurasian plate.

Timing of Hydrothermal Alteration and Au-Sb-W Mineralization, Stibnite-Yellow Pine District, Idaho

Abstract The Stibnite-Yellow Pine district of central Idaho was mined from the early 1900s until the 1990s, extracting gold, antimony, tungsten, and mercury from veins and disseminated and replacement ores in mountainous terrain along the headwaters of the Salmon River. Mining during the two World Wars supplied critical antimony and tungsten to the war efforts. Recent exploration has delineated mineral resources of over 187 metric tons Au, 274 metric tons Ag, and 93,000 metric tons Sb. Mineralization is hosted in Cretaceous Idaho batholith granitic rocks and a sequence of Neoproterozoic to Paleozoic metasedimentary strata of carbonate and siliciclastic compositions. Historic studies outlined some of the complex paragenesis but debated the absolute age of mineralization. New petrographic and geochronologic work documents a sequence of five hydrothermal events in the Stibnite-Yellow Pine district. Event 1 is related to Cretaceous magmatic and hydrothermal activity and includes events ranging in age from 86 to 75 Ma, including sparse quartz-molybdenite veins dated at 86 Ma. Disseminated gold mineralization of Event 2 is associated with sericitic alteration and sulfidation of igneous biotite and replacement of plagioclase by potassium feldspar, largely in granodiorite. Gold is present in zoned arsenian pyrite in both disseminated ores and in crosscutting carbonate-quartz veins containing pyrite and arsenopyrite. The large Yellow Pine deposit, localized at a dilatant bend in the Meadow Creek fault, hosts such disseminated and vein gold. Event 2 is interpreted as the major gold-forming event; 40Ar/39Ar ages of sericite and potassium feldspar alteration range, respectively, from 70 to 59 and 66 to 56 Ma. The long span is interpreted to reflect the age of gold mineralization and local overprinting by Event 3. A narrower range from 66 to 61 Ma is interpreted to date the peak of gold mineralization and alteration. Event 3, tungsten mineralization with scheelite, is texturally later than Event 2 gold and localized along the Meadow Creek structure. Event 3 scheelite has been dated by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb methods at 57 Ma. Event 4, best developed in the West End area, includes gold-silver–bearing quartz-carbonate-pyrite veins and breccias with epithermal textures and potassium feldspar alteration envelopes. Adularia from Event 4 yields 40Ar/39Ar plateau ages of 52 to 51 Ma. Event 5 antimony and mercury mineralization consists of stibnite veins and breccia cements at the Yellow Pine and Hangar Flat deposits as well as cinnabar veins and replacements at the peripheral Fern and Hermes deposits; it is constrained by an LA-ICP-MS U-Pb date on scheelite (ca. 47 Ma) intergrown with stibnite. Minor propylitic and argillic alteration is evident in 47 Ma igneous dikes, which do not contain economic mineralization. The Au-Sb-W ores in the Stibnite-Yellow Pine mining district formed over an extended time period from about 70 to 45 Ma in multiple pulses that were localized along the Meadow Creek fault zone. Each event corresponds to episodes of magmatism and/or hydrothermal activity in the region. Insignificant Event 1 skarn and molybdenum mineralization is similar in age to the Thompson Creek porphyry molybdenum deposit in central Idaho. Event 2 gold mineralization occurred during a magmatic gap in central Idaho but was synchronous with magmatism in the Bitterroot lobe further north; Event 2 is similar in age to orogenic gold-arsenic mineralization at the Beartrack mine in eastern Idaho. Event 3 scheelite mineralization coincides with tungsten mineralization at the Quartz Creek deposit, late magmatism in the Bitterroot lobe, and rapid exhumation of the Atlanta lobe of the Idaho batholith. Event 4 gold mineralization is coincident with the onset of regional Challis magmatism and extension. Event 5 antimony and mercury mineralization is time-equivalent to epithermal gold mineralization in the nearby Thunder Mountain volcanic field and the peak of Challis magmatism.

Constraints on the Timing and Lower Crustal Accretion at the Schulz Massif, Mohns Ridge, Arctic Mid Ocean Ridges

AbstractThe ultraslow‐spreading Mohns Ridge is a key supersegment of the Arctic Mid‐Ocean system, where it represents a boundary between the Jan Mayen hotspot in the south and the highly anomalous Knipovich Ridge to the north. Understanding the timing and mode of Plio‐Pleistocene seafloor spreading along this ridge segment is critical for establishing the recent geodynamic evolution of the Norwegian‐Greenland Sea. To investigate magmatic accretion at this ultraslow spreading ridge, we collected samples from the Schulz Massif, which is located off‐axis at 73.4°N and exposes gabbroic intrusives and mantle peridotite. The petrology and petrography of these samples indicate that the exposed crustal section underwent multiple episodes of magmatism, which are characterized by distinct crystal sizes and geochemistry. To calibrate the age of the seafloor, we combined high‐resolution and high‐precision single zircon U‐Pb geochronology. Our data suggest that seafloor spreading has been nearly symmetrical for the last ∼4.6 Myr with a time‐averaged half‐spreading rate of ∼7.4 mm yr−1. Crystal size analysis of olivine in porphyric intrusions suggests that the crustal section was fed crystal‐laden melts with recurrence rates predicted to stabilize fault‐dominated seafloor spreading. Our combined geochronological and crystal size approach gives a critical perspective on the mode of seafloor spreading in the Mohns Ridge and allows insights into accretionary mechanisms and crustal structures during symmetric seafloor spreading.

Stratigraphy, geochronology, geochemistry and Nd isotopes of the Ouarzazate Group, Anti-Atlas, Morocco: Evidence of a Late Neoproterozoic LIP in the northwestern part of the West African Craton

In the eastern and central Anti-Atlas of Morocco, the Late Neoproterozoic Ouarzazate Group (OG) forms a ∼ 2 km-thick section of volcano-sedimentary rocks deposited during intermittent magmatic activity, spanning a time period between 590 and 540 Ma. In the eastern Anti-Atlas, the stratigraphy of the OG, termed IMS (Imiter Mine succession), includes four units that reflect a gradual change from fluvial to lacustrine depositional sedimentary environments. In the central Anti-Atlas, the deposition of the OG occurs within a caldera environment, consisting of five units referred to as BMS (Bou Azzer Mine succession). UPb dating on zircon from the rhyolite lava flow at the top of the IMS constrains the latest lava flow of the IMS to 570.7 ± 2.1 Ma. In the Bou Azzer inlier, the stratigraphically correlated basal andesite and top dacite lava flows were emplaced at 590.6 ± 4.2 Ma and 555.9 ± 20.4 Ma, respectively, suggesting that the OG was constructed through successive pulses within a long-lived magmatic episode. The magmatic rocks display geochemical signatures typical of continental arc magmas, including high-K calc-alkaline to shoshonite composition, enrichment in LILE, and negative Nb, Sr, Ti and P anomalies. Nd isotopes indicate that magmas supplying the plumbing systems of the IMS and BMS constitute transitional products resulting from a combination of orogenic and intraplate volcanism, involving deep crustal recycling. The compositions of these magmas evolved through processes of crustal contamination, bulk assimilation, and minor fractional crystallization of the parent melt. High εNd values of the BMS rocks argue for a juvenile origin, whereas lower εNdt recorded in the IMS likely reflect old crustal protoliths and an overall less contribution of juvenile material in their source. The magmatic rocks of the OG are interpreted as products of a large igneous province (LIP) developed by a post-collisional delamination model that initiated tens of millions of years after the subduction-collision process stage of the Pan-African orogeny in the northwestern part of the West African Craton (WAC).

Seismic evidence of a plume conduit in a metasomatized lithosphere beneath the Barmer rift in northwestern India

Teleseismic tomography was done to investigate the lithosphere beneath northwestern India which was affected by several magmatic episodes since Neoproterozoic times. Broadband stations were deployed in phases at 29 locations in an area of 300 km × 300 km in northwestern India. About 8000 high-quality P phases corresponding to 934 events were used. Relative residuals estimated through adaptive stacking were inverted using the Fast Marching teleseismic tomography to obtain the 3D velocity perturbations. The tomograms revealed an anomalous ≈ 100 km wide, cylindrical zone of low velocities (−2% to −2.3%) extending from a depth of ≈ 60 km to ≈140 km beneath the Barmer rift. A broader zone of low velocities (−1%) encompasses the upper mantle down to a depth of at least 200 km. The anomaly lies beneath the localized, small-volume, eruptive centers of the earliest phase of Deccan volcanism within and on the flanks of the rift which falls on the projected extension of the Reunion hotspot track along the rift system of northwestern India. Explaining such large anomalies requires a significant increase in temperature, presence of melt and/or compositional changes. Geochemical studies indicate that the polychronous (early Cretaceous to Paleogene) alkaline complexes above the anomalous zone are derived from hydrated mantle peridotite and contain an abundance of phlogopites, suggesting a hydrated and metasomatized lithosphere beneath northwestern India. A thermal source impinging from below upon a metasomatized, weak, and rifted lithosphere may result in the observed signature akin to that of a plume conduit that overprints the older signatures of the Neoproterozoic Malani magmatic event.

Stratigraphy and depositional history of the Aguja Formation (Upper Cretaceous, Campanian) of West Texas, southwestern USA

Abstract Although the Aguja Formation (West Texas, southwestern USA) and its fossil vertebrate fauna have been known for over a century, its basic stratigraphic requisites (type area and type section) have not been formally documented. The formation is herein subdivided into a series of formal members, and a lectostratotype section is proposed. Lithostratigraphic and biostratigraphic subdivisions are documented and integrated with geochronologic data to provide an age model for the formation. Four terrestrial vertebrate biozones are proposed. There are at least four major depositional intervals represented in the Aguja and intertonguing Pen Formations. An initial progradational deltaic succession is recorded by the La Basa Sandstone and lower part of the Abajo Shale Members of the Aguja Formation. A second phase of deposition resulted in a retrogradational shoreface succession that includes the upper part of the Abajo Shale, overlying Rattlesnake Mountain Sandstone Member, and lower part of the McKinney Springs Tongue of the Pen Formation, up to a skeletal phosphate bed interpreted to represent the maximum flooding surface. The third phase of deposition comprises a progradational deltaic succession that includes the upper part of the McKinney Springs Tongue, Terlingua Creek Sandstone Member of the Aguja Formation, and lower part of the Alto Shale Member of the Aguja Formation. This third succession records eastward migration of the strandline and withdrawal of the Western Interior Seaway from the Big Bend region. The fourth phase of deposition comprises a series of aggradational fluvial channel and floodplain successions that form the upper part of the Alto Shale Member and is coincident with redirection of stream flow to the southeast. This interval is much thicker in the central part of the Big Bend region, thins to the southwest and northeast, and likely records initial subsidence in the Laramide Tornillo Basin. The upper part of this succession was also contemporaneous with a series of basaltic pyroclastic eruptions, the westernmost expression of the Balcones igneous province. A dramatic constriction in the southern entrance to the Western Interior Seaway through the Gulf of Mexico occurred during this final phase in deposition of the Aguja Formation and corresponds to a shift of stream flow southeastward and to an outbreak of local pyroclastic eruptions. Regional uplift associated with this episode of magmatism is likely responsible for closing the southern aperture of the Western Interior Seaway.

Paleogene mid-crustal intrusions in the Ruby Mountains–East Humboldt Range metamorphic core complex, northeastern Nevada, USA

Abstract Middle Eocene to early Oligocene intrusions, widespread in the Ruby Mountains–East Humboldt Range metamorphic core complex, Nevada, USA, provide insights into a major Paleogene magmatic episode and its relation to tectonism in the northeastern Great Basin. These intrusions, well-exposed in upper Lamoille Canyon, range in composition from gabbro to leucomonzogranite. They form small plutons, sheets, and dikes that intrude the metamorphic and granitic infrastructure of the core complex. Two types of Paleogene monzogranite were recognized. The first is exemplified by two of the larger intrusive bodies, the Snow Lake Peak and Castle Lake intrusions, which occur as sheet-like bodies near and at the structural base of metamorphosed Neoproterozoic and Cambrian Prospect Mountain Quartzite where it is inverted above Cambrian and Ordovician marble of Verdi Peak in the Lamoille Canyon nappe. Swarms of dikes are associated with these intrusions. U-Pb (zircon) ages range ca. 40–33 Ma and typically display relatively simple and minor inheritance. The rocks have the lowest εHf (zircon) and εNd (whole rock) of any of the middle Cenozoic granites. The second type of monzogranite, Overlook type, typically occurs as thin, isolated dikes and leucosome-like bodies in Late Cretaceous granites of the infrastructure, with no obvious relationship to the large monzogranite bodies. Overlook-type monzogranite displays complex zircon inheritance, yields igneous ages ca. 37–32 Ma, and has εHf (zircon) and εNd (whole rock) identical to those of Late Cretaceous granites in the core complex. These isotopic and field data indicate that Overlook-type monzogranite formed in situ through anatexis of host Cretaceous granites. A pervasive thermal event was required to stimulate this crustal melting. Gabbros from Lamoille Canyon and quartz diorite dated from 32 km away signal mantle-derived magmatism ca. 39–37 Ma (U-Pb, zircon) was a driver of crustal melting and hybridization. Eocene 40Ar/39Ar apparent ages on hornblende and biotite are consistent with syn- to post-magmatic extensional exhumation and decompression. Thus, the core complex provides a window into trans-crustal magmatism and insight into how such magmatism affected the Nevadaplano orogenic plateau. This Paleogene thermal pulse, which may relate to removal of the Farallon slab by delamination of mantle lithosphere, involved partial melting of the upper mantle and transfer of magma and heat to the Nevadaplano crust. Lower-crustal melting of Archean(?) to Paleoproterozoic rocks resulted in Snow Lake Peak–type magmas, and middle-crustal melting of granite in the infrastructure yielded Overlook-type magmas. This crustal magmatism, as exemplified by the intrusions in the core complex, likely played a role in destabilizing the Nevadaplano and its later collapse during middle Miocene extension. The Paleogene intrusions and associated structural features also provide insight into the evolution of the core complex through either the buoyant upwelling of a melt-rich diapir (gneiss-dome model) or buoyant upwelling of the melt-rich middle crust synchronous with a west-rooted mylonitic shear zone (extensional shear-zone model). We favor a hybrid that incorporates both models.

The passive margin of the southern São Francisco paleocontinent, metamorphic record and implications for the assembly of West Gondwana: Evidence from the Lima Duarte Nappe, Ribeira Orogen (SE Brazil)

In this contribution, we report new P-T (pressure–temperature) and U-Pb zircon geochronological data from the Lima Duarte Nappe and basement rocks from the Mantiqueira Complex, Central Ribeira Orogen, southeastern Brazil, highlighting their relevance to the understanding of the West Gondwana assembly. The lower metasedimentary sequence of the Lima Duarte Nappe records a passive margin environment, with a majority of Paleoproterozoic detrital zircon grains, with probable main source rocks located in the São Francisco paleocontinent. The upper foreland sequence of the Lima Duarte Nappe has a prevalence of Tonian-Cryogenian detrital zircon grains, absent in the lower passive margin sequence and source rocks were most likely in a magmatic arc developed during the convergence between the São Francisco and the Paranapanema paleoplates. Zircon rims from the Lima Duarte passive margin and foreland sequences, and zircon grains from an amphibolite of the Lima Duarte Nappe record granulite facies metamorphism at ca. 560–590 Ma (Th/U ratio < 0.1). Similar ages are recorded by CL-dark zircon rims and overgrowths in orthogneisses of the Mantiqueira Complex and are interpreted to reflect basement reworking. Metapelitic metatexites from the foreland sequence record granulite facies peak metamorphism at ∼ 780–800 °C and 8–9.5 kbar in the sillimanite-rutile stability field, constrained by phase equilibrium modeling. Rutile inclusions in garnet record minimum peak temperatures of ∼ 750 °C. The retrograde mineral assemblage in the matrix is marked by the disappearance of rutile and the presence of ilmenite and mineral compositions re-equilibrated at upper amphibolite facies. Late Tonian (ca. 850–750 Ma) metamafic rocks within the Mantiqueira and Juiz de Fora complexes might record an extensional intraplate mafic magmatism episode that culminated with the establishment of the passive margin at the south of the São Francisco paleocontinent. The Lima Duarte metasedimentary sequence along with the Canastra and Carrancas groups (western and southern portions of the Brasília Orogen, respectively) represent a Tonian proximal passive margin environment, developed at the margin of the São Francisco paleocontinent. The Lima Duarte Nappe metasedimentary units and the orthogneisses of the Mantiqueira Complex record Ediacaran syn-collisional metamorphism related to the Araçuaí-Ribeira Orogenic System.

Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology

AbstractThe oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north‐western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet‐bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (&lt;7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono‐metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean‐Mesoarchean crustal rocks in recording high T/P ratios (&gt;77.5°C/kbar).

Geochronological and Geochemical Constraints on the Magmatic Evolution of the Dun Mountain Ophiolite Belt, New Zealand

Abstract New whole-rock major and trace element geochemical, zircon U-Pb geochronological, and Hf isotopic data from gabbroic rocks in New Zealand’s mid-Permian Dun Mountain ophiolite belt (DMO) provide insight into the evolution of subduction systems and early stages of intraoceanic arc development. Fe-oxide-bearing gabbros yielded high εHf(t) values (+10.3 to +13) and zircon U-Pb ages of 271.6 ± 0.6 Ma. In contrast, Fe-Ti-oxide-bearing gabbros of 268.1 ± 0.6 Ma show more enriched geochemical characteristics, including a wide range of εHf(t) values (+15.5 to +6.8). New findings strengthen the evolutionary model for the DMO and place constraints on its youngest known magmatic episode. We infer that late magmatism fingerprinted by these gabbros, including consistent negative Nb-Ta anomalies, reflects early stages of arc development and formation of island arc tholeiites on the DMO. Our model is consistent with other existing regional geochronological and geochemical data, implying that the DMO had an early stage of normal-mid-ocean ridge basalt crustal accretion followed by an influx of slab-derived components and maturity of the subducting system between ca. 271.6 and 268 Ma. These results extend our understanding of the evolution of distinct intraoceanic systems.

Two episodes of mid-Neoproterozoic mafic magmatism in the Hong'an Terrane, Central China: Records of continental rifting during the breakup of the Rodinia supercontinent

The mid-Neoproterozoic (800–635 Ma) tectonic regime along the northern margin of the Yangtze Block is still debated, particularly the timing and mode of transition from a subduction-related setting to a rifting-related setting. The new identification of two episodes of mafic magmatism, represented by (I) 736–710 Ma Daleishan and (II) 635 Ma Hong'an intrusions in the Hong'an Terrane, central China, provides new constraints on the nature of their mantle source and tectonic evolution of the northern Yangtze Block. (I) Daleishan mafic rocks are tholeiitic in composition and exhibit N-MORB- to arc-like trace element patterns. They have variable whole-rock initial 87Sr/86Sr (0.7041 to 0.7092), εNd(t) (−2.98 to +2.17), and εHf(t) (+3.37 to +10.25) values. Their magmatic zircons preserve relatively homogeneous δ18O (+4.50 to +6.01 ‰) and highly variable εHf(t) (+1.3 to +13.1) values. Chemical compositions and modeling results suggest that Daleishan rocks were derived from a depleted asthenospheric mantle source with involvement of recycled sediment component, and the magma subsequently underwent 5–15% contamination by the wall-rocks during its emplacement. (II) Hong'an mafic rocks mainly show E-MORB- to arc-like geochemical features with more enriched HfNd isotopes [zircon εHf(t) = −3.0 to +3.6; whole-rock εNd(t) = −6.87 to −0.75, and εHf(t) = −0.50 to +2.00], suggesting they were derived from the interactions between the asthenosphere and the previously-metasomatized lithospheric mantle. Integrating the new data with previous results for the Neoproterozoic rocks in the northern margin of the Yangtze Block (including the Qinling-Dabie orogen), we emphasize that episodic extension-related asthenosphere upwelling in a continental rifting setting, coupled with interactions between the asthenosphere and the lithosphere, played a prominent role in the formation of the 780–635 Ma bimodal-type magmatic rocks and diverse geochemical features in the mafic counterparts.

Arc-arc amalgamation during accretionary orogenesis: Insights from mid-Paleozoic tectono-magmatic records in eastern Junggar, NW China

Abstract Arc-arc amalgamation occurs during the evolution of composite orogens at convergent plate margins and plays a critical role in controlling accretionary patterns and processes. The eastern Junggar terrane in the southern Central Asian Orogenic Belt underwent a long-lived subduction-accretion process in the Paleozoic, but whether and how the Yemaquan and Dananhu-Harlik arcs were amalgamated remain debatable. A systematic U-Pb–Hf-O isotopic study was conducted on zircons from Silurian granitic rocks in the Yemaquan arc. The U-Pb dating results suggest that these rocks were emplaced at 433–422 Ma and inherited abundant 536–435 Ma zircons representing a predominant magmatic episode in the Yemaquan arc. Their positive εHf(t) values and young Hf model ages indicate that the Yemaquan arc is dominated by juvenile basement with significant crustal growth during the Neoproterozoic to early Paleozoic. The variations in zircon Eu/Eu*, εHf(t), and δ18O values reveal that the Yemaquan arc experienced remarkable crustal thickening and remobilization at ca. 450 Ma, similar to the northern Dananhu-Harlik arc, and this was followed by extension that initiated at ca. 420 Ma. These features support the amalgamation of these two arcs occurring ca. 450–420 Ma. Integrated with regional data, we correlated this amalgamation event in the eastern Junggar terrane with the orogenic event in the Chinese Altai terrane, and we propose a middle Paleozoic tectonic evolution model in the eastern Junggar–Altai area from arc assembly to dispersal in association with a transition in accretionary mode. This scenario probably took place as a response to plate reorganization during the breakup of the northern margin of Gondwana.

High-precision U-Pb geochronology links magmatism in the Southwestern Laurentia large igneous province and Midcontinent Rift

Abstract The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25−0.24+0.67 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension. The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ~2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ~2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift.

Main minerals of the Araxá alkali-carbonatite complex, Minas Gerais State, Brazil

The Araxá alkali-carbonatitic complex is a significant source of niobium and phosphate and accounts for approximately 75% of global niobium production. The complex contains magmatic niobium ore minerals, such as natrium-pyrochlore and calcium-pyrochlore, and hydrothermal niobium ore minerals in the form of kenopyrochlore and natrium-pyrochlore. The magmatic micas in the complex are tetraferriphlogopites that exhibit Mg-rich cores surrounded by one or two rims of tetraferriphlogopites formed by hydrothermal alteration. These rims have lower IVAl and Mg apfu (atom per formula unit) values but higher IVFe3+ and Fe2+ contents than the original phlogopite. Microphlogopitite dikes that formed during several episodes of magmatism between 83.48 and 77.4 Ma contain tetraferriphlogopites, phlogopite, eastonite, biotite, and annite-siderophyllite. These micas exhibit variations in Mg2+ and Fe2+ concentrations owing to the magmatic differentiation and subsequent hydrothermal alteration, resulting in rims with higher Mg and lower Fe2+ concentrations than the mica core. The Araxá complex also contains magmatic magnetite that has been further altered by hydrothermal processes, resulting in the formation of ilmenite and the oxidation and crystallization of titaniferous hematite. Magnetite within the microphlogopitites displays variations in the concentrations of Fe3+ and Ti, likely owing to the timing of the intrusion of the microphlogopitites into the Araxá complex. The degree of oxidation of the oxidized ilmenite of the Araxá complex varies based on the intrusions, which occurred before or during hydrothermal activity. Apatite is concentrated in carbonatites, which form a ring around the core of the complex, and disseminated in phlogopite-rich rocks hosting norsethite carbonatite veins in the center of the complex. The apatite in the core rocks exhibits complex zonation with unexpected changes in Ca, P, and Sr concentrations. Apatite in the carbonatite ring was generated during the differentiation of the carbonatite magma, whereas that in the core rocks exsolved from the phosphorus-saturated carbonatite magma (in press). The carbonate minerals dolomite, norsethite, and calcite exhibit compositions grouped around their end-members, suggesting that they formed during different stages of magmatic source differentiation. Strontium differentiation occurs in the magmas corresponding to each endmember, and carbocernaite is exsolved from the Sr-saturated crystals, thus forming micro-inclusions in some carbonate crystals that have been displaced by late-magmatic hydrothermal fluids.