Protracted intra- and inter-pluton magmatism during the Acadian orogeny: evidence from new LA-ICP-MS U-Pb zircon ages from northwestern Maine, USA

Devonian granitoid plutons comprise a major part of the bedrock of northwestern Maine representing the magmatic expression of the Acadian orogeny in this part of the northern Appalachian orogen. They are petrographically diverse with minerals characteristic of both I- and S-type granites, in some cases within the same intrusion, and some are compositionally zoned. New LA-ICP-MS ages presented here elucidate the timing and duration of this magmatism. The earliest phase of granitoid magmatism began around 410–405 Ma with the emplacement of the Flagstaff Lake Igneous Complex, and the presence of contemporaneous mafic rocks suggests that mantle-derived magmas were also produced at this time. Late Devonian ages, ca. 365 Ma, for many intrusions, such as the Chain of Ponds and Songo plutons, reveal that magmatism continued for 45 million years during which compositionally diverse I- and S-type magmas were produced. In addition, there is evidence that intrusive activity was prolonged within some plutons, for example the Rome-Norridgewock pluton and the Mooselookmeguntic Igneous Complex, with 10–15 myr between intrusive units. The new ages suggest a break in magmatism between 400 Ma and 390 Ma apparently separating Acadian magmatism into early and late pulses. The production of lower crustal I-type magmas appears to have been concentrated later, ca. 380–365 Ma, although several S-type granitoids were also emplaced during this period. These Late Devonian plutons display abundant zircon inheritance with ages around 385 Ma, which suggests that the crust was experiencing enhanced thermal perturbations during this extended timeframe. The new data for granitoid plutons in northwestern Maine are consistent with tectonic models for other parts of Ganderia which propose initial flat slab subduction followed by slab breakoff and delamination.

Resolving changes in arc magma volatile budgets over Myr timescales leading up to porphyry Cu formation

<p>The crustal-scale magmatic systems of Andean-style arcs produce thick volcanic deposits and abundant plutons that are emplaced into the crust. They can also generate spatially- and temporally-restricted, economically-important porphyry Cu deposits. These deposits are formed at the magmatic-hydrothermal transition and require significant amounts of volatiles and metals to be concentrated in the sub-volcanic environment. Thus, understanding the magmatic and tectonic processes acting within an arc segment and their effect on the volatile budgets of crustal magmas could be essential for identifying the constraining factors controlling the potential of a magmatic system to produce a porphyry deposit.</p><p>In this study we examine the magmatic evolution of the Rio Blanco-Los Bronces district, ~30 km northeast of Santiago, Chile, which is host to the Earth’s largest resource of Cu. Eocene to Early Miocene volcanic rocks were intruded by the Miocene San Francisco Batholith that, in turn, partially hosts intrusions related to the Late Miocene to Early Pliocene Rio Blanco-Los Bronces porphyry deposit cluster. We apply a combination of whole-rock, apatite and zircon geochemistry and zircon geochronology to the intrusive rock suite of the district to provide temporally- constrained geochemical information over the entire duration of batholith assembly and ore formation.</p><p>U-Pb geochronology reveals incremental assembly of the San Francisco Batholith by individual magma batches over >14Myr (~18 – 4 Ma), with ore formation occurring in discrete pulses in the last 3 Myr before cessation of intrusive activity within the district. Progressive changes in the trace element chemistry indicate crustal thickening and deeper magma evolution within the arc segment as a result of the subduction of the Juan Fernandez ridge. A temporal shift to elevated SO<sub>3</sub> and Cl contents is recorded by zircon-hosted apatite inclusions from the intrusions with highest values occurring in porphyry intrusions directly associated with the ore forming events. These data suggest variable volatile budgets of magmas during zircon crystallisation and hint at crustal-scale controls on the porphyry ore-forming potential of an arc segment.</p><p> </p>

Tectono-magmatic evolution of the Karoo and Kerguelen plumes and their impact onto magmatism of the East Antarctica

<p>The Mesozoic Karoo-Maud and Kerguelen plumes had a significant influence on Gondwana and the oceanic lithosphere. Jurassic magmatism, formed under the influence of a huge Karoo plume at 184–178 Ma ago, covered large areas of the Dronning Maud Land in East Antarctica. Later, 130 – 0 m.y. ago, under the influence of the Kerguelen plume, magmatism formed in the area of the Lambert glacier, and the Gaussberg volcano (Quaternary time) appeared, located on the coast opposite the Kerguelen archipelago. We assume that the Karoo mantle plume initiated the formation of a “mega-apophyses” from the main plume manifestation area within the Karoo province in the southeastern African continet (ca. 2000 km in diameter). These mega-apophyses are represented by the Ferrar Igneous Province (ca. 3000 km long area of intrusive activity along the Transantarctic Mountains) and a supposed igneous province (ca. 1500 km long) covering the East Antarctic coast between the Lazarev and Cosmonauts Seas. Based on petrological and geochemical studies, the characteristic features of magmas of the Karoo, Dronning Maud Land, and Ferrar igneous provinces have been determined, which indicate that for all magmas associated with Karoo and Kerguelen plumes, the main source of melt enrichment is a mantle source with characteristics of the EM-II component (most typically for magmas of the Ferrar Province). It reflects the properties of an enriched, fluid-rich, ancient continental mantle, metasomatized at the early stages of the tectonic development of the region and involved in the melting process. A rarer admixture of the ancient lithospheric component (EM-I, with <sup>206</sup>Pb/<sup>204</sup>Pb = 16.5 and <sup>143</sup>Nd/<sup>144</sup>Nd = 0.5122) was revealed in both plumes. The existence of mantle plumes in the Southern Hemisphere and their long-term development had a significant impact on the structure and evolution of the East Antarctica.</p>

Chronostratigraphic Analysis of the Evolution of the Christiana-Santorini-Kolumbo Volcanic Field

<p>The Christiana-Santorini-Kolumbo (CSK) volcanic field in the South Aegean Sea is one of the most active volcanic-tectonic lineaments in Europe, having produced numerous explosive eruptions, catastrophic earthquakes, and disastrous tsunamis in the past 350,000 years. The present-day Santorini caldera is located in the centre of the NE-SW trending CSK field, which extends for more than 60 km from the extinct Christiana volcano in the southwest to the active submarine Kolumbo volcano northeast of Santorini. While the onshore architecture of Santorini has been well studied, little is known about its offshore architecture. Further, the past volcanism of Kolumbo is only known for its last eruption in 1650 AD and that of Christiana is completely unknown. Based on the available onshore datings of the volcanic formations, it has been proposed that volcanism in the CSK field initiated at Christiana, then migrated northeast towards Santorini and later to Kolumbo. This, however, has yet to be confirmed by offshore investigations. To fully constrain and understand the initiation and evolution of volcanism in the CSK field, we combine an extensive collection of high-resolution multichannel and vintage seismic data covering the entire zone.</p><p>With these seismic profiles, we are able to (1) correlate the seismo-stratigraphy of the Christiana basin west of Santorini with that of the Anhydros Basin east of Santorini, (2) identify major phases of extrusive and intrusive activity of individual volcanic vents, and (3) establish a regional chrono-stratigraphic framework in which we chronologically integrate these phases. We conclude that volcanism occurred repetitively during distinct phases of activity, which are separated from each other by periods with little or no volcanic activity. The onset of volcanism occurred without the generation of significant pyroclastic flows and was mainly characterized by shallow intrusions, clearly visible at Christiana, at its neighbouring volcanic cones, and at the southwestern flank of Akrotiri. The next phase saw the formation of the Kolumbo Volcanic Chain in the Anhydros Basin and the formation of sill intrusions in the Christiana Basin, which we find below a chaotic, presumably pyroclastic unit. This was followed by a major regional event on Santorini, during which a thick transparent subunit was deposited in all surrounding basins. The most recent phase was dominated by the volcanoclastic deposits from Santorini’s eruptive cycles and the recent eruption of Kolumbo. Using estimates of sedimentation rates, we convert this chrono-stratigraphic scheme into an approximate timeline, which implies that the initiation of volcanism occurred during Late Pliocene to Early Quaternary - much earlier than revealed by onshore dating.</p>

Periodic dike intrusions at Kīlauea volcano, Hawaiʻi

Forecasting heightened magmatic activity is key to assessing and mitigating global volcanic hazards, including eruptions from lateral rift zones at basaltic volcanoes. At Kīlauea volcano, Hawaiʻi (United States), planar dikes intrude its east rift zone (ERZ) and repeatedly affect the same segments. Here we show that Kīlauea’s upper and middle ERZ dikes in the last four decades intruded at regular intervals of ~8 or ~14 yr. Segments with shorter recurrence intervals are adjacent to faster-moving parts of the flank, and ~1–5 MPa of tension accumulates from flank spreading in the time between dike events. Intrusion frequency was neither advanced nor delayed during magma supply variations, supporting the role of long-term flank motion on the timing of dike intrusions. Although fewer historical dikes have occurred near the 2018 CE eruption site in the lower ERZ and the adjacent slowly sliding lower eastern flank, similar tension accumulated between the 1955 and 2018 eruptions. Regular dike intrusion recurrence intervals indicate the importance of including both extrusive and (commonly neglected) intrusive activity in eruption hazard analyses.

Sills Sautéing Shales: Did Karoo Intrusions into the Ecca Formation cause the Toarcian Ocean Anoxic Event?

<p>Eruptions of Large Igneous Provinces (LIP) are commonly correlated with global climate change, and environmental, as well as biological, crises. However, establishing a causative link via chemical and physical proxies for global change is more complicated and often ambiguous. As technical improvements have allowed for increasingly higher precision dates especially in U/Pb dating, it is possible to better assess hypotheses connecting LIP’s and environmental impact via their contemporaneity. Here, we focus on the early Jurassic period, which includes a period of global change known as the Toarcian oceanic anoxic event (TOAE), as well as emplacement of the Karoo Large Igneous Province (K-LIP). Previous work has tied these two events together due to overlapping chronology and observed metamorphism and degassing (e.g., Svensen et al., 2012; Sell et al., 2014), and excellent exposure allows for extensive sampling of both the intrusive and extrusive components of the K-LIP. Therefore it is possible to directly study the influence of intrusive LIP magmatism on potential climate forcing.</p><p>The K-LIP is comprised of a suite of basaltic lava flows, sills, dike swarms, centered in southern Africa. Approximately 340,000 km<sup>3</sup> of sills are interlaid within the Karoo Basin, and therefore served as significant heat source to the basin upon emplacement. While much of the sedimentary rocks of the basin are siliciclastic, the Ecca Group contains organic-rich facies and hosts 160,000 km<sup>3</sup> of basaltic sills (Svensen et al., 2012). This unit is therefore uniquely capable of generating large volumes of thermogenic gas through thermal metamorphism of the organic matter of the shale. Previous mass balance calculations indicate that between 7,000 and 27,000 Gt of CO<sub>2</sub> equivalents was released through metamorphic reactions in contact aureoles within the Ecca Group (Svensen et al. 2007). If intrusive magmatism was short lived within this formation, causing rapid volatilization and degassing from the shales, than this event could represent a mechanism to drive a short pulse of global climate change. Previous studies have shown that intrusions are coeval with the TOAE (Svensen et al., 2012; Corfu et al. 2016), however higher-precision geochronology data from the sills is necessary to determine if the flux and timing of thermogenic gases from the basin was sufficiently high to destabilize Earth’s climate. In order to test the hypothesis, we present single crystal U-Pb zircon dates from sills across the Ecca Group. These data will be used (i) to quantify the duration and flux rate of carbon gas during the intrusive event, and (ii) to better understand how and to what extent K-LIP intrusive activity and associated thermogenic gas release of Ecca wall rocks were able to drive global climate change.</p><p> </p><p>Corfu, F., et al., (2016) EPSL, 434, 349-352.</p><p>Sell, B., et al., (2014) EPSL, 408, 48-56.</p><p>Svensen, H., et al., (2007) EPSL, 3-4, 554-566.</p><p>Svensen, H., et al., (2012) EPSL, 325-326, 1–9.</p>

Seismic expressions of the Yongle Atoll and the Sansha Yongle Blue Hole in the Xisha Islands, Northwestern South China Sea

<p>Sansha Yongle Blue Hole is an oceanic blue hole and is located at the northeastern edge of the Yongle Atoll, in the Xisha Islands of the northwestern South China Sea. The 301.19 m deep makes it to be the deepest known blue hole in the world. Despite the 3-D morphology, hydrochemical properties and chemocline of the blue hole have been comprehensive investigated, its karst formation process is still enigmatic. This study presented new acquired multi-channel seismic data across the Yongle Atoll and seismic data across the Sansha Yongle Blue Hole to describe the seismic reflection characteristics of the carbonate platform and the blue hole in extensive detail. Combined with the scientific wells drilled on the atoll, our results show that carbonate sequences including Lower Miocene, Middle Miocene, Upper Miocene, Pliocene and Quaternary developed on the platform. The magmatically intrusive activity and related magmatic hydrothermal fluid flows have been very active since 5.5 Ma around/on the Yongle Atoll, and may remain active on both slopes and the carbonate platform of the Yongle Atoll at present. Seismic profiles also show that the blue hole is characterized by chaotic seismic reflections which are easily distinguished from surrounding carbonate rocks with sub-parallel, continuous, low to medium amplitude, and low to medium seismic reflections. It seems that the depth of the blue hole is deeper than that measured according to the seismic images. The results of δ<sup>18</sup>O from scientific wells show that the phreatic extent in the Xisha Islands is from 14.75 – 38.89 m to 152.06 – 183.29 m. Therefore, different from other classic karstological blue holes formed by the phreatic dissolution processes, a hydrothermal – phreatic model with magmatic hydrothermal pipes and collapse of deep seated phreatic dissolution voids was proposed to describe the formation of the Sansha Yongle Blue Hole.</p>

Mesozoic Orogenic Gold Mineralization in the Jiaodong Peninsula, China: A Focused Event at 120 ± 2 Ma During Cooling of Pregold Granite Intrusions

Jiaodong gold deposits are mainly sited along faulted contacts between Upper Jurassic Linglong granite and Precambrian basement metamorphic rocks or Lower Cretaceous Guojialing granite. Long-standing controversies relate to timing of gold mineralization and granite-gold relationships. In this study, gold-related muscovite consistently provides concordant 40Ar/39Ar plateau ages of 120 ± 2 Ma (2σ) for the Jiaojia, Sizhuang, and Luoshan deposits. Analogous 40Ar/39Ar timing constraints from gold-related muscovite are provided by total gas and high-temperature ages from Fushan, concordant high-temperature ages from Rushan, and fusion-step ages from Xiadian deposits. These new 40Ar/39Ar ages, when combined with previous reliable 40Ar/39Ar and U-Pb age constraints for mineralization, including ages of pre- and postgold dikes, define a widespread gold mineralization event at 120 ± 2 Ma (2σ). Published zircon U-Pb ages for Guojialing and Aishan granite magmatism suggest an ~8-m.y. lag between peak intrusive activity and gold mineralization. This, together with lack of both high-temperature alteration assemblages and alteration and/or metal zonation, indicates that the structurally controlled Jiaodong deposits are orogenic rather than intrusion-related deposits. Despite this, granite intrusions are considered to have provided suitable fluid trap sites. New 40Ar/39Ar analyses of biotite from the Linglong and Guojialing granites show they had cooled to about ~300° ± 50°C by ca. 123 to 124 Ma, providing pressure-temperature conditions similar to those under which most orogenic gold deposits formed close to the ductile-brittle transition. This enabled the effective ingress of fluids at supralithostatic pressures at 120 ± 2 Ma, leading to intensive brecciation, alteration, and deposition of both vein-type and disseminated gold ores. New zircon (U-Th)/He dates together with apatite fission-track data indicate that preservation of the gold province is due to slow postmineralization uplift and exhumation.