Bimodal Volcanism Research Articles

Radiogenic isotope record of magma genesis and lithospheric geodynamics of the Rukwa Rift Basin, Tanzania, from mid Mesozoic to present

The Western Branch of the East African Rift System has experienced multiple episodes of basin development and intraplate alkaline volcanism since the Jurassic, however the geodynamic processes and lithospheric evolution involved in this protracted geological history remains poorly defined. Here, we present Sm-Nd, Lu-Hf, and Rb-Sr isotopic data of igneous minerals for three magmatic episodes that coincide with basin reactivation in the Rukwa Rift of southwestern Tanzania, respectively represented by: (1) the Jurassic to Cretaceous Panda Hill carbonatite; (2) the late Oligocene phonolitic‑carbonatitic Nsungwe Formation tuffs; and (3) the Miocene–recent bi-modal volcanism of the Rungwe Volcanic Province. Of these, the Nsungwe Formation tuffs offer a discrete record of the early phases of lithospheric disturbance associated with modern rifting of the Western Branch.Alkaline magmas erupted prior to the Miocene are isotopically distinct (ɛNd +0.5 to +1.5 and 87Sr/86Sr 0.7041) from modern Rungwe lavas and likely originate from a lithospheric mantle that experienced enrichment during the Pan-African Orogeny (~550–700 Ma). Consistent with previous studies, our results indicate that Rungwe Volcanic Province magmas were generated from a mixed, isotopically-enriched component of the lithospheric mantle that may have developed during the ca. 1.0 Ga Irumide Orogeny. The observed change in the source region of rift lavas though time is consistent with progressing melting of a compositionally layered lithosphere. Combined with existing geophysical and sedimentological data, we hypothesise that the lithosphere beneath the Rukwa-Malawi-Usangu junction of the Western Branch experienced an episode of destabilisation and foundering (lithospheric drip), during the early stages of East African Rift development.

Digital geologic map, petrography and U-Pb geochronology of the Río Santiago Shear Zone, southernmost Sierra Madre Occidental, western Mexico

The boundary between the Sierra Madre Occidental (SMO) and the Trans-Mexican Volcanic Belt (TMVB) is well exposed along the Santiago River, on the border of the states of Nayarit and Jalisco. This region includes a polydeformed belt produced by the relative motion between the Jalisco Block and the SMO, here defined as the “Rio Santiago Shear Zone” (RSSZ). In this work, we present detailed geological cartography, petrographic descriptions, and new zircon U-Pb ages that allow us to establish the stratigraphy of the region surrounding the RSSZ, providing temporal constraints to its tectonic evolution. In the mapped area, the base of the SMO stratigraphic column consists of a succession of massive andesitic lava flows intruded by felsic dikes and a felsic subvolcanic body of 27.5 and 27.2 Ma, and it is covered by a series of ignimbrites of 26.8 and 25.3 Ma. A red beds succession separates a second package of ignimbrites of ∼20-19 Ma, in turn capped by rhyolitic lava domes of 19.7 and 18.6 Ma and a rhyodacitic lava flows of 15.6 Ma, this Miocene succession is also intruded by a felsic body of 18.3 Ma west of the La Yesca dam. The SMO succession is covered by rocks belonging to the bimodal volcanism of the TMVB, emplaced between the end of the Miocene and the Pleistocene. The RSSZ developed through several deformation phases that occurred since the end of the Oligocene. The Oligocene succession of the SMO is affected by NE-SW and WNW-ESE trending lateral faults, NW-SE open folds and WNW-ESE and E-W trending reverse oblique faults. Some of the reverse oblique faults were later reactivated as normal faults. WNW-ESE trending normal faults associated with the Tepic-Zacoalco Rift affected the area south of the Santiago River during the Pliocene and Pleistocene. Normal faulting produced an accumulated vertical offset up to 700 m between the northern and southern regions of the Santiago River.

Subduction-controlled temporal and spatial variations in early Palaeozoic sedimentary and volcanic record of the Mongol-Altai Domain

The early Palaeozoic volcano–sedimentary evolution of the Khovd and Gobi-Altai zones of the Mongol–Altai Domain was investigated by quantitative petrographic analysis of sediments, whole-rock geochemistry of volcanic rocks, and U–Pb zircon dating. Three evolutionary stages were identified in the Khovd Zone: (1) Cambrian–mid Ordovician basaltic (Nb/Yb = 0.09–1.25) and rhyodacitic (Nb/Yb = 4.36) volcanism and sedimentation of immature greywackes in the fore-arc basin in front of the juvenile continental arc, (2) late Ordovician flysch-type sedimentation accompanied by a volcanic gap coincided with Barrovian metamorphism in footwall sequences reflecting a compressional stage, and (3) Silurian–Devonian within-plate bimodal volcanism (with a strong asthenospheric imprint; Nb/Yb = 4.45–20.34) followed by mature siliciclastic deposition interpreted as transformation of the former fore-arc region into a back-arc domain associated with HT metamorphism. In the Gobi-Altai Zone the three stages were: (1) Cambro–Ordovician marginal greywacke–sandstone succession adjacent to a mature continental arc (mafic rocks Nb/Yb = 0.71–4.36), (2) Silurian siliciclastic sedimentation associated with migration of the volcanic gap and Barrovian metamorphism, and (3) Lower Devonian deep-marine sedimentation followed by shallow-marine carbonate sedimentation, initially accompanied by calc-alkaline basalts (Nb/Yb = 0.74–4.57) and andesites to rhyolites (Nb/Yb = 0.69–4.87), evolving in the mid–late Devonian to within-plate alkali basalts (Nb/Yb = 11.05) thus reflecting back-arc basin development. These temporal and spatial variations in the Mongol-Altai Domain are interpreted as a diachronous sequence of retreating and advancing subduction modes instead of juxtaposition of suspect terranes as proposed in previous studies.

Late Mesozoic intracontinental deformation and magmatism in North and NE China in response to multi-plate convergence in NE Asia: An overview and new view

We review in this paper the major late Mesozoic contractional deformation in North and NE China and establish the tempo-spatial distribution pattern of Jurassic-Cretaceous magmatism, with the aim at clarifying its link with evolving multi-plate convergent geodynamics in NE Asia. We recognize two major changes in tectonic regime, one occurring at ~170 Ma, which marks the tectonic switch from extensional to compressional regimes, while another at ~135 Ma which manifests the change from compressional to extensional settings. Accordingly, Jurassic-Cretaceous magmatism migrated from the marginal zones in ~200 -170 Ma to the continental interior in 170-135 Ma and then was followed by Mid-Cretaceous (135-120 Ma) magmatic flare-up in the continental interior, which migrated progressively eastward since ~120 Ma. Based on these observations, we make a division of three tectonic stages and discuss their plate geodynamic setting. (1) The early stage in 200-170 Ma was dominantly under extensional setting with A-type granitoids and bi-modal volcanism occurring along the eastern and northern marginal zones. (2) The middle stage in 170-135 Ma was manifested by significant multi-directional crustal shortening and rejuvenation of the ancient orogenic belts, as distant response to multi-plate convergence along the East Asia continental margins. (3) The late stage was manifested first by a thermal surge in 135-120 Ma with the formation of extensional structures and rift basins in the continental interior, then followed by alternation of tectonic compression and extension, related to the effect of paleo-Pacific slab subduction. We propose that the thickening acquired during the middle tectonic stage and subsequent lithospheric thinning in the late stage triggered large-scale crustal melting, episodic magmatism and associated metallogenic outbreak in North and NE China.

Volcanism of the Late Silurian Eastport Formation of the Coastal Volcanic Belt, Passamaquoddy Bay, New Brunswick

This field trip is an excursion through the exquisite, nearly pristine exposures of a Silurian, felsic-dominated bimodal volcanic and sedimentary sequence exposed in the Passamaquoddy Bay area of southwestern, New Brunswick (Eastport Formation). These rocks form the northwest extension of the Coastal Volcanic Belt that extends from southwestern New Brunswick to the southern coast of Maine. The sequence is significant because it is part of a large bimodal igneous province with evidence for supervolcano-scale eruptions that began to form during the close of the Salinic Orogeny (about 424 Ma), and continued into the Acadian Orogeny (421–400 Ma). The geochemical characteristic of the rocks can be explained by extension related volcanism but the specific drivers of the extension are uncertain. The Passamaquoddy Bay sequence is 4 km thick and comprises four cycles of basaltic-rhyolitic volcanism. Basaltic volcanism typically precedes rhyolitic volcanism in Cycles 1–3. Cycle 4 represents the waning stages of volcanism and is dominated by peritidal sediments and basaltic volcanics. A spectrum of eruptive and emplacement mechanisms is represented ranging from the Hawaiian and Strombolian-type volcanism of the basaltic flows and pyroclastic scoria deposits, to highly explosive sub-Plinian to Plinian rhyolitic pyroclastic eruptions forming pyroclastic density currents (PDC) and high grade rheomorphic ignimbrites. During this field trip we will examine key exposures illustrating this spectrum of eruptive and emplacement processes, and their diagnostic characteristics, along with evidence for the interaction between mafic and felsic magmas and a variety of peperitic breccias formed as a result of emplacement of flows on wet peritidal sediments. The constraints the depositional setting and voluminous bimodal volcanism places on tectonic models will also be considered.

The Serpukhovian–Bashkirian Amalgamation of Laurussia and the Siberian Continent and Implications for Assembly of Pangea

AbstractThe final assembly of Pangea is thought to be marked by the Siberia‐Laurussia amalgamation during the Late Carboniferous–Permian, but there are large uncertainties on the timing of the amalgamation in the previous paleogeographic reconstructions because there are no high‐quality Carboniferous–Permian paleomagnetic data available from the Siberian continent. The Zharma‐Saur area in the junction between the Siberia‐Laurussia amalgamation, involving the Zharma‐Saur arc on the northern margin of the Kazakhstan microcontinent, the South Chinese Altai terrane in the southern margin of the Siberian continent and the Irtysh‐Zaisan suture in between, can provide crucial geological constraints for the assembly of Pangea. In the Zharma‐Saur arc, the Tournaisian–Visean marine sequences are interbedded with thick pillow lavas, which were formed during southward subduction of the Irtysh‐Zaisan Ocean and unconformably covered by the Moscovian terrestrial bimodal volcanic associations and sedimentary successions, in which the basalts are columnar‐jointed in structure and the sandstones also contain the detritus derived from the South Chinese Altai terrane. The Serpukhovian–Bashkirian unconformity was coeval with the transition from the Tournaisian–Visean calc‐alkaline to the Moscovian–Early Permian high alkaline and bimodal volcanism. These dramatic changes in sedimentary environments, provenances and volcanism together indicate the Siberia‐Laurussia amalgamation during the Serpukhovian–Bashkirian and this event was almost simultaneous with the Laurussia‐Gondwana collision and the welding of Laurussia with the Kazakhstan microcontinent, but slightly earlier than the Laurussia‐Tarim collision. Therefore, the final assembly of Pangea could be completed in the Late Carboniferous.

Volcanology, geochemistry and geodynamic setting of the Neoarchean Sunrise volcanogenic massive sulfide deposit, Beaulieu River volcanic belt, Slave craton, Northwest Territories, Canada

The Sunrise volcanogenic massive sulfide (VMS) deposit is a banded, polymetallic Zn-Pb-Cu-Ag sulfide lens hosted by felsic volcanic rocks of the Neoarchean Beaulieu River volcanic belt in the western Slave craton in the Sunset Lake area of the Northwest Territories, Canada. With historic indicated resources of 1.52 Mt at 5.99% Zn, 2.39% Pb, 0.08% Cu, 262 g/t Ag, and 0.67 g/t Au—it is the largest known VMS deposit in the Northwest Territories.The footwall stratigraphy is interpreted to be the product of subaqueous, largely effusive, bi-modal volcanism with periods of pyroclastic eruptions and concomitant subsidence. It consists of basalt to basaltic-andesite lavas with minor associated volcaniclastic rocks, rhyolite lobe-hyaloclastite flows and rhyolite domes, and felsic volcaniclastic rocks. Though hosted within felsic volcaniclastic lithofacies, the mineralization is spatially associated with the thickest portions of a quartz-feldspar porphyritic lobe-hyaloclastite flow and rhyolite dome. Felsic tuff to lapillistone units, characterized by fine ash, pumice and broken crystal clasts indicate derivation from pyroclastic volcanism and deposition by eruption-fed density currents. Felsic tuff breccia units are characterized by block-sized fragments of quartz-feldspar porphyritic rhyolite and were deposited during mass wasting events along fault scarps or by the oversteepening of domes. Massive to stringer-style sulfide mineralization is dominantly within felsic (seldom heterolithic) tuff to tuff breccia units, but laterally discontinuous argillite units occur at the top of the footwall and commonly contain stringer to semi-massive sulfide mineralization, indicating a hiatus in volcanism before the onset of basaltic andesite lavas that form the hanging wall to the deposit.Volcanic stratigraphy and architecture, and the occurrence of a volcanogenic massive sulfide deposit, combined with major and trace element geochemistry and Nd, Hf, and Pb isotopic data indicate the volcanic rocks and the Sunrise deposit formed on rifted Mesoarchean crust.

The Castle Rock and Ironside Mountain calderas, eastern Oregon, USA: Adjacent venting sites of two Dinner Creek Tuff units—the most widespread tuffs associated with Columbia River flood basalt volcanism

Abstract The Dinner Creek Tuff is an important unit of mid-Miocene rhyolite volcanism contemporaneous to flood basalts of the Columbia River magmatic province. Field mapping along with analytical data of tuff samples identify two calderas, the Castle Rock and Ironside Mountain calderas, as the venting sites of two widespread ignimbrites of the Dinner Creek Tuff. Both calderas lie within the area of the proposed general storage sites of main-phase Columbia River Basalt magmas. The Castle Rock caldera formed during the eruption of the 16.16 Ma Dinner Creek Tuff unit 1. The northwestern boundary of the caldera is roughly defined by the juxtaposition of over 300 m of densely welded rheomorphic intra-caldera tuff and tuffaceous mega-breccia deposits against Mesozoic Weathersby Formation shale and pre-Miocene Ring Butte trachybasalt lavas. Following caldera collapse, fluvial and lacustrine volcaniclastic sediments were deposited on the caldera floor, and outflow tuffs of the Dinner Creek Tuff units 2 and 4 were deposited into the caldera. Aphyric basaltic andesite and icelandite (Fe-rich andesite), which correlate stratigraphically to upper Grande Ronde Basalt lavas, intrude the caldera floor deposits, and lavas are interbedded with sediments and Dinner Creek Tuff unit 4. The Ironside Mountain caldera formed during eruption of the 15.6 Ma Dinner Creek Tuff unit 2, which lies ~15 km north of the Castle Rock caldera. The caldera is an 11 × 6 km depression wherein over 900 m of intra-caldera, rheomorphic, and partially welded tuff are bound by Weathersby Formation shale and Tureman Ranch granodiorite. Post-caldera collapse, basaltic andesite and icelandite dikes and sills that are also stratigraphically correlative to upper Grande Ronde Basalt lavas intruded into the tuff, mostly along the margins of the caldera, which altered much of the tuff. Mafic lavas within the study area that closely pre- and post-date Dinner Creek Tuff units were correlated with regional units of the Columbia River Basalt Group. Porphyritic and aphyric mafic lava flows underlying Dinner Creek Tuff unit 1 at Castle Rock are correlated with Picture Gorge Basalt and Grande Ronde Basalt. Aphyric basaltic andesite and icelandite that intrude into and overlie the Dinner Creek Tuff units 1 and 2 are westward extensions of fractionated tholeiitic magmas as seen in late-stage Grande Ronde Basalt units such as the Hunter Creek Basalt. Finally, porphyritic basalt lava flows that overlie the Hunter Creek Basalt and volcaniclastic sediments at the Castle Rock caldera are correlative with the 13.5 Ma Tim's Peak Basalt. At Castle Rock, pre-caldera Columbia River Basalt Group lavas appear to lap onto a mid-Miocene topographic high that stretches northward and westward for tens of kilometers based on stratigraphic data, and it may be related to regional uplift at initial impingement of the mantle upwelling to produce the Columbia River Basalt Group. The Castle Rock and Ironside Mountain calderas exemplify bimodal volcanism of the Columbia River magmatic province. Eruption of rhyolites is closely pre- and post-dated by the eruption of local and regional tholeiitic lavas belonging to the Columbia River Basalt Group. The local eruption of evolved tholeiitic lavas likely concealed calderas, but these lavas also illustrate the close proximity of mafic and rhyolitic magmas at depth at these rhyolite centers. Consequently, the stratigraphy of both the Castle Rock and Ironside Mountain calderas somewhat differs from that of rhyolite calderas dominated by silicic and calc-alkaline intermediate pre- and post-caldera volcanism.

Devonian to Triassic tectonic evolution and basin transition in the East Kunlun–Qaidam area, northern Tibetan Plateau: Constraints from stratigraphy and detrital zircon U–Pb geochronology

ABSTRACT The late Paleozoic to Triassic was an important interval for the East Kunlun–Qaidam area, northern Tibet, as it witnessed prolonged subduction of the South Kunlun Ocean, a major branch of the Paleo-Tethys Ocean whose closure led to the formation of Pangea. However, the geologic history of this stage is poorly constrained due to the paucity of tectonothermal signatures preserved during a magmatic lull. This article presents a set of new provenance data incorporating stratigraphic correlation, sandstone petrology, and zircon U–Pb dating to depict changes in provenance that record multiple stages of topographic and tectonic transition in the East Kunlun–Qaidam area over time in response to the evolution of the South Kunlun Ocean. Devonian intra-arc rifting is recorded by bimodal volcanism and rapid alluvial-lacustrine sedimentation in the North Qaidam Ultra High/High Pressure Belt, whose sources include the Olongbuluke Terrane and southern North Qaidam Ultra High/High Pressure Belt. Southward transgression submerged the East Kunlun–Qaidam area during the Carboniferous prior to the rapid uplift of the Kunlun arc, which changed the provenance during the Early Permian. This shift in provenance for the western Olongbuluke Terrane and thick carbonate deposition throughout the North Qaidam Ultra High/High Pressure Belt in the late Early Carboniferous indicate that the North Qaidam Ultra High/High Pressure Belt should have been inundated, terminating an ~95 m.y. erosion history. The closure of the South Kunlun Ocean in the late Triassic generated a retroarc foreland along the Zongwulong Tectonic Belt, which is represented by the development of a deep-water, northward-tapering flysch deposystem that was supplied by the widely elevated Kunlun–Qaidam–Olongbuluke Terrane highland. This new scenario allows us to evaluate current models concerning the assembly of northern Tibet and the tectonic evolution of the Paleo-Tethys Ocean.

Ediacaran bimodal volcanism in the southernmost Dom Feliciano Belt, Uruguay: Implications for the evolution of SW Gondwana

New zircon U-Pb-Hf-O, whole rock geochemical and Sr-Nd-Pb isotope data of Ediacaran volcanic rocks from the Cuchilla Dionisio (CDT) and Nico Pérez (NPT) terranes in Uruguay are presented. High-K calc-alkaline rhyolite from the Cerros de Aguirre Formation in the southern CDT formed at an active continental margin at 569 ± 5 Ma. A metasomatized mantle source is invoked for the formation of the rhyolitic melts. Cogenetic andesite and basaltic andesite have Sr-Nd isotope compositions that are close to that of enriched mantle 1, which is consistent with melt formation in the subcontinental mantle in the presence of crustal material. Coeval rhyolite in the northern CDT is inferred to have formed distally to the active margin. Rhyolite and trachyte of the Sierra de Ánimas Formation in the NPT formed at 589 ± 3 Ma and 586 ± 3 Ma, respectively. The NPT basalts originated in the mantle and may be related to a mantle plume. During ascent they were variably contaminated by continental crust. Zircon in NPT rhyolite have very low δ18O values that are ascribed to hydrothermal alteration and melting at shallow crustal levels. We propose that the late Ediacaran igneous activity in the NPT was related to anorogenic, extensional processes in an intraplate setting but the c. 20 million years younger volcanic activity in the southern CDT occurred in a supra-subduction setting above the subducting Brazilides ocean. We ascribe the bimodal intraplate volcanism in the NPT to far-field forces caused by slab-pull in response to eastward-directed subduction of the Brazilides ocean underneath Arachania, which is a rifted-off segment of the Kalahari Craton. Thus, late Ediacaran subduction-induced lithospheric stretching and horizontal extension of the eastern margin of the Río de la Plata Craton is proposed as the main cause for bimodal volcanism and the deposition of coeval volcano-sedimentary successions at the end of the Neoproterozoic.

Petrogenesis and tectonic implications of two‐stage Cretaceous bimodal volcanism in the coast of Zhejiang Province, South‐east China

Shipu volcanic area is located at the east of the Jiangshan‐Shaoxing Fault zone, an important part of the Mesozoic volcanic belt in the coastal south‐east China, which provides an excellent opportunity to study the origin of rarely outcropped and coexisting bimodal volcanic rocks and bimodal dyke swarms. Two stages of bimodal volcanic rocks and dyke swarms yield zircon U–Pb ages of ca. 118–121 Ma and ca. 95–93 Ma, respectively. Both bimodal magmatic rocks show enrichment in LILEs and LREEs, but depletion in HFSEs. Basalts of the early‐stage bimodal volcanism have whole‐rock (87Sr/86Sr)i ratio of 0.71121, εNd(t) value of −8.0, and zircon εHf(t) values of −19.3 to −11.7, while the contemporaneous rhyolites show whole‐rock (87Sr/86Sr)i ratios of 0.71023 to 0.71106, εNd(t) values of −7.5 to −6.6 and zircon εHf(t) values of −15.3 to −7.6 with TDMC of 0.96–2.11 Ga. Basaltic dykes of the late‐stage bimodal volcanism have whole‐rock (87Sr/86Sr)i ratios of 0.70898 to 0.70908, εNd(t) values of −5.8 to −5.7 and zircon εHf(t) values of −10.5 to 2.9, while coexisting rhyolitic dykes display whole‐rock (87Sr/86Sr)i ratios of 0.69618 to 0.71172, εNd(t) values of −3.1 to −3.0 and zircon εHf(t) values of −9.2 to 11.9 with TDMC of 0.37–1.71 Ga. These features suggest that the early‐stage basalts could be derived from partial melting of an enriched lithospheric mantle metasomatized by slab‐derived fluids, and its corresponding felsic end‐member is the product of partial melting of Proterozoic middle‐lower crust due to underplating of basaltic magmas with minor addition of mantle materials. As for the late‐stage bimodal volcanism, the basaltic dykes are also derived from an enriched mantle similar to the early‐stage counterpart. However, the rhyolitic dykes mainly originated from partial melting of ancient lower crustal materials with abundant input from the depleted mantle. As a whole, two stages of bimodal magmatism in Shipu area provide direct constraints on regional tectonic transition from an extensional arc setting to a back‐arc setting.

Volcanic facies architecture of early bimodal volcanism of the NW Deccan Traps: Volcanic reservoirs of the Raageshwari Deep Gas Field, Barmer Basin, India

AbstractThe Deccan Traps large igneous province (LIP) comprises one of the largest continental flood basalt provinces on Earth with the main phase of volcanism spanning the Cretaceous‐Palaeogene boundary. The oldest volcanism of the province is encountered in the northwest of modern‐day India where Deccan stratigraphy is often buried beneath thick Cenozoic sedimentary sequences. The Raageshwari Deep Gas (RDG) Field, located onshore in the central Barmer Basin, NW India, produces gas from the early Deccan Raageshwari Volcanics which are subdivided into two members, the Agni Member and the overlying Prithvi Member. The RDG comprises a globally important example of a producing volcanic reservoir whilst also offering unique insights into the early volcanism of the Deccan with the aid of extensive high quality sub‐surface data. Within this study, the volcanic facies of the RDG sequences are investigated from five cored intervals (total 160 m). Core‐based facies determinations are compared with geochemical analyses, petrophysical analyses of the cores (density, porosity and permeability), and wireline data including micro‐resistivity borehole images (FMI) and Nuclear Magnetic Resonance (NMR) data. A wireline based volcanic lithofacies scheme is developed and applied to the uncored parts of the sequence which in turn is compared to 3D seismic data. Results of the study reveal the Agni Member to comprise a compositionally bimodal (basalt through to trachyte), dominantly alkaline series with mixed volcanic facies including spectacular felsic ignimbrites, basic‐intermediate simple lava flows, volcaniclastic units and newly identified shallow intrusions. The Prithvi Member in contrast is dominated by tholeiitic basalt compositions with less common basic‐intermediate alkaline compositions and comprises a sequence dominated by classic tabular lava flow facies inter‐digitated with boles, volcaniclastic units, rare compound braided lava facies and evolved tuffaceous ash layers. In one interval of the Prithvi Member, evidence for agglutinated spatter is recorded inferring potential proximity to a palaeo‐eruption site within the area. Comparison between core data and volcanic facies reveals a first order control of volcanic facies on reservoir properties highlighting the importance of volcanic facies appraisal in the development of volcanic reservoirs.

Origin, Accretion, and Reworking of Continents

AbstractThe continental crust is unique to the Earth in the solar system, and controversies remain regarding its origin, accretion and reworking of continents. The plate tectonics theory has been significantly challenged in explaining the origin of Archean (especially pre‐3.0 Ga) continents as they rarely preserve hallmarks of plate tectonics. In contrast, growing evidence emerges to support oceanic plateau models that better explain characteristics of Archean continents, including the bimodal volcanics and nearly coeval emplacement of tonalite‐trondjhemite‐granodiorite (TTG) rocks, presence of ∼1600°C komatiites and dominant dome structures, and lack of ultra‐high‐pressure rocks, paired metamorphic belts and ophiolites. On the other hand, the theory of plate tectonics has been successfully applied to interpret the accretion of continents along subduction zones since the late Archean (3.0–2.5 Ga). During subduction processes, the new mafic crust is generated at the base of continents through partial melting of mantle wedge with the addition of H2O‐dominant fluids from subducted oceanic slabs and partial melting of the juvenile mafic crust results in the generation of new felsic crusts. This eventually leads to the outgrowth of continents. Subduction processes also cause softening, thinning, and recycling of continental lithosphere due to the vigorous infiltration of volatile‐rich fluids and melts, especially along weak belts/layers, leading to widespread continental reworking and even craton destruction. Reworking of continents also occurs in continental interiors due to either plate boundary processes or plume‐lithosphere interactions. The effects of plumes have proven to be less significant and cause lower degrees of lithospheric modification than subduction‐induced craton destruction.

Neoproterozoic tectonics of the Jiangnan orogen: The magmatic record of continental growth by arc and slab-failure magmatism from 1000 to 780 Ma

The Neoproterozoic NE-striking Jiangnan orogen in the South China Block (SCB) separates the Cathaysia microcontinent on the southeast from the Yangtze craton in the northwest. The origin and evolution of the Jiangnan orogen is unresolved, with individual models explaining part of the history, such as various magmatic, deformational, metamorphic, and sedimentary events including extensional, contractional, and transpressional modes. Here, we present detailed geochronological and Nd-Hf isotopic data for Neoproterozoic mafic and felsic intrusions in the Jiangnan orogen and identify three episodes of Neoproterozoic magmatism. The first stage ranging from ~1013–942 Ma is preserved predominantly in the eastern segment of the orogen, including a series of mafic rocks with ages of ~1010–952 Ma, contrasting with weak magmatic activity in the western segment with a peak at ~997 Ma. The second stage ranges from ~906 Ma to ~820 Ma, with intensive and extensive bimodal magmatism occurring in the eastern and western parts. The last stage mainly occurred in the western Jiangnan orogen in the period from ~800 to 780 Ma, associated with drastic mafic magmatism and weakly bimodal volcanic activities. The bimodal volcanism in the eastern segment exhibits a striking weak trend after ~823 Ma, distinctly different from the western segment where the magmatism lasted until ~785 Ma, indicating its syn- to post-collisional in nature. The pre-800 Ma magmatic rocks in western Jiangnan orogen have a dominant negative Nd isotopic signature but exhibit a significant positive trend after 800 Ma, different from those in east, which have a decreasing trend. Both parts display a roughly decoupled Nd-Hf isotopic character presented by a dominant positive pattern in Hf isotopes of the Neoproterozoic intrusive rocks, indicating a significant addition of previous slab-derived components or accretionary wedges in the pre-800 Ma magma evolution, and more depleted materials involved in the source in the later stages. Oceanic subduction under the northwestern Cathaysia Block occurred around ~1.0 Ga, slightly earlier than the subduction to the southeastern Yangtze. A gradual tectonic transition from post-orogenic extension to intracontinental rifting dominated the tectonics in Jiangnan orogen in the middle-late Neoproterozoic, followed by slab failure after ~790 Ma. Arc magmatism was the dominant source of crustal growth before 820 Ma in the SCB, but was exceeded in volume by later slab failure magmatism after ~790 Ma.

Refining the Paleoproterozoic tectonothermal history of the Penokean Orogen: New U-Pb age constraints from the Pembine-Wausau terrane, Wisconsin, USA

Abstract An enduring problem in the assembly of Laurentia is uncertainty about the nature and timing of magmatism, deformation, and metamorphism in the Paleoproterozoic Wisconsin magmatic terranes, which have been variously interpreted as an intra-oceanic arc, foredeep or continental back-arc. Resolving these competing models is difficult due in part to a lack of a robust time-frame for magmatism in the terranes. The northeast part of the terranes in northern Wisconsin (USA) comprise mafic and felsic volcanic rocks and syn-volcanic granites thought to have been emplaced and metamorphosed during the 1890–1830 Ma Penokean orogeny. New in situ U-Pb geochronology of igneous zircon from the volcanic rocks (Beecher Formation), and from two tonalitic plutons (the Dunbar Gneiss and Newingham Tonalite) intruding the volcanic rocks, yielded crystallization ages ranging from 1847 ± 10 Ma to 1842 ± 7 Ma (95% confidence). Thus, these rocks record a magmatic episode that is synchronous with bimodal volcanism in the Wausau domain and Marshfield terrane farther south. Our results, integrated with published data into a time-space diagram, highlight two bimodal magmatic cycles, the first at 1890–1860 Ma and the second at 1845–1830 Ma, developed on extended crust of the Superior Craton. The magmatic episodes are broadly synchronous with volcanogenic massive sulfide mineralization and deposition of Lake Superior banded iron formations. Our data and interpretation are consistent with the Penokean orogeny marking west Pacific-style accretionary orogenesis involving lithospheric extension of the continental margin, punctuated by transient crustal shortening that was accommodated by folding and thrusting of the arc-back-arc system. The model explains the shared magmatic history of the Pembine-Wausau and Marshfield terranes. Our study also reveals an overprinting metamorphic event recorded by reset zircon and new monazite growth dated at 1775 ± 10 Ma suggesting that the main metamorphic event in the terranes is related to the Yavapai-interval accretion rather than the Penokean orogeny.

An approach to reconstruct the thermal history in active magmatic systems: Implications for the Los Humeros volcanic complex, Mexico

Reconstructing the thermal history in active volcanic complexes characterized by multiple magmatic events is challenging due to the limited knowledge of the nature and extent of the transient heat sources. Although understanding of the geometry and architecture of a magmatic system is of prime importance for accurate temperature assessments, it is still one of the most uncertain parameters in numerical models. In this work, we presented a methodology for thermal assessment in active volcanic systems, whereby field-based geological, geochemical and petrological data are integrated to define the transient heat sources of a magma plumbing system. This time-varying heat source conceptual model is applied in the Los Humeros Volcanic Complex, an active Quaternary caldera complex in the Trans Mexican Volcanic Belt, for evaluating the thermal footprint related to the major volcanic events. The site is characterized by two caldera-forming eruptions, the Los Humeros (164 000 years ago) and the Los Potreros (69 000 years ago) and numerous episodes of post-caldera bi-modal volcanism during Holocene period (8 000 – 3 000 years old). The transient nature of the heat sources is implemented as time-varying temperature boundary conditions and the complete temporal evolution for a period of 182 000 years is simulated in 13 modeling stages. The thermal impact due to the voluminous caldera-forming events and the later short-lived magma pockets of Holocene ages is simulated by emplacing heat sources in the numerical model distributed heterogeneously in space and active at different instants of time. The depth, volume and age of the magma pockets are constrained from geochemical, petrological, geochronological and thermo-barometric analysis of erupted material. The present temperature state obtained from this approach agrees well with the temperature data recorded in the geothermal wells. The thermal footprint of the individual volcanic events indicates that almost 80 % of the present-day thermal contribution results from the massive caldera-forming events. The post-caldera Holocene magma pockets had additionally increased temperatures locally by 10 % - 20 % depending on the volumes and ages of the magma pockets. The present-day thermal regime of the younger Holocene magma pockets suggests existence of super-hot resources at shallow depths in the southern part of the geothermal field, making it a potential site for future exploration activities.

Geochemical and seismic tomography constraints of two-layer magma chambers beneath the bimodal volcanism: A case study of late Cenozoic volcanic rocks from Ulleung Island and Mt. Changbai (Paektu)

We take two typical Northeast Asia bimodal volcanoes as examples to explain the general features of Cenozoic bimodal ocean island basalt (OIB)-type volcanism in Northeast Asia. We present mineralogical, petrological, geochemical, isotopic, and full-waveform seismic tomographic evidence for the existence of two-layer magma chambers of Late Cenozoic volcanic activity beneath Ulleung Island and Mt. Changbai (Paektu). Ulleung Island volcanic rocks, which are composed of alkaline basalt, phonotephrite, trachyte, and phonolite, belong to the alkaline magma series and display enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), slight depletion of heavy rare earth elements (HREEs), enriched 87Sr/86Sr (0.70475–0.70507) and 143Nd/144Nd (0.51250–0.51255) isotopic values, and enriched 207Pb/204Pb (15.544–15.626) and 208Pb/204Pb (38.750–38.954) values, similar to the geochemistry of OIB. Ulleung Island felsic volcanic rocks are characterized by significant negative Ba, Sr, P, Eu, and Ti anomalies (δEu = 0.14–0.35) and positive Pb anomalies, slightly higher 87Sr/86Sr isotopic ratios relative to those of mafic volcanic rocks, although the mafic and felsic samples have similar Sr, Nd, and Pb isotope compositions without significant differences. Ulleung Island and Mt. Changbai volcanic activities are likely related to the involvement of subduction-related compositions, but parts of the Mt. Changbai samples have been contaminated by crustal components to a certain extent. Mafic volcanic rocks of Ulleung Island are segregated from a deeper mantle source within a pressure range of 10.1–21.2 kbar compared with felsic volcanic rocks, which exhibit fractional crystallization of clinopyxene, spinel, plagioclase, and olivine. To explain this, a two-layer magma chamber beneath Ulleung Island with depths of 30–40 and 60–80 km is proposed, which is supported by mineral crystallization pressure and a three-dimensional full-waveform seismic tomography model. We also suggest that similar magma eruption processes and a two-layer magma chamber with depths of ~10 and 40–60 km also exist beneath Mt. Changbai. Taking the typical Cenozoic bimodal samples from Ulleung Island and Mt. Changbai as examples, we argue that two-layer magma chambers exist beneath Cenozoic bimodal OIB-type volcanic activities in Northeast Asia.

A unique period of bimodal volcanism at 130–110 Ma in the northern Sanandaj-Sirjan Zone: Evidence for an extensional setting

The Sanandaj-Sirjan metamorphic-magmatic Zone (SaSZ) represents the Mesozoic active continental margin of the Neotethys Ocean between the Arabian plate and the Iranian plateau. The zone evolved at the margin of ancient Ediacaran basement, as documented in this paper with an age of 557 Ma for a trondhjemite. The Mesozoic activity of the SaSZ was most intense with sedimentation and magmatism in the Jurassic between 170 and 150 Ma, but this period was then followed by a 20 million years gap with almost no magmatic activity, which has been attributed to flat slab tectonics. The present paper is concentrated on the geochemistry, and zircon CA-ID-TIMS geochronology, of a bimodal, but mainly basaltic, volcanic and subordinate plutonic succession that represents the resumption of magmatism in a deep and gradually shallowing basin in the northern SaSZ between 130 and 110 Ma. The mafic to intermediate volcanic rocks are predominantly calc-alkaline and have εNd values of +0.3 to +3.9. The felsic volcanic rocks are dominantly high-K calc-alkaline with εNd values of −0.5 to +1.9. A suite of Cretaceous trondhjemites yields εNd values of −4.6 to −4.9. Together, the geochemical, lithological and geochronological indications suggest that the thick Barremian-Aptian bimodal magmatic suite represents magmatism in a continental arc to back-arc setting, controlled by extension and probably related to foundering of the flat slab and slab roll-back. These events initiated melting of the mantle in the northern SaSZ, heralding a period of extension and the subsequent development of ophiolites all along the Neothetyan suture.

Constraints on the age and geodynamic setting of the iron formations and anhydrite Fe-(Ba) deposits in the Bulunkuole Group of the Taxkorgan area, NW China

• Bulunkuole Group (BG) and associated mineral deposits is both early Cambrian in age. • Volcanic-sedimentary rocks in the BG reveal a back-arc basin depositional setting. • Iron formations deposited in newborn back-arc basin from redox-stratified water column. • Anhydrite Fe-(Ba) deposits formed in restricted sulfate-rich water during back-arc basin collapse. Here we present detailed field observations, zircon U-Pb-Hf isotopes, whole-rock geochemical, and Sm-Nd isotopic data for the volcanic-sedimentary rocks within the Bulunkuole Group (BG) in the Taxkorgan area, northwestern China in order to reveal significant information on the formation age and tectonic setting of the iron formations (IFs) and anhydrite Fe-(Ba) deposits hosted by this group. Exposed rocks in the BG are broadly grouped into three subgroups: the lowermost Subgroup 1 is a bimodal volcanic suit (basalts and dacites) interbedded with IF layers; the overlying Subgroup 2 is a clastic sedimentary sequence hosting abundant anhydrite Fe-(Ba) mineralization; and the uppermost Subgroup 3 is dominated by carbonate rocks. Zircon U-Pb ages provide constraints on the formation age of the BG. Specifically, the bimodal volcanism occurred at ~ 540–521 Ma and the sedimentary rocks deposited after 540 Ma. Basalts from the BG bimodal suit are characterized by slightly enriched LREE patterns and pronounced to insignificant negative Nb anomalies when normalized to primitive mantle compositions. These features are comparable with those of modern N-MORB and back-arc basin basalts. Combined with highly heterogeneous Nd isotopes ( ε Nd (t) = 0.2–4.4), these suggest that their magmatic precursors were derived from partial melting of a depleted mantle modified by slab-derived fluids, and interacted with minor crustal components during ascending. By comparison, the BG dacites display gently sloped to flat LREE patterns with a FII-type felsic affinity and a range of negative to positive ε Nd (t) values (-5.8~+0.7). They are inferred to be generated by partial melting of the old continental crust at relatively shallow levels with variable addition of juvenile material. The BG clastic sediments were dominated by greywacke and arkose. Their diagnostic geochemical features, detrital zircon age spectra, and in situ zircon Hf isotopic data indicate that they were mainly derived from weathering of the early Cambrian felsic rocks. Together, these characteristics above, integrated with previous studies, suggest that the BG and related IFs and anhydrite Fe-(Ba) deposits were formed on an active continental margin (possibly back-arc basin) during the southward subduction of the Proto-Tethys Ocean beneath the Taxkorgan-Tianshuihai Terrane.

Geochemical evolution of the Lebowa Granite Pluton in western Bushveld Igneous Complex, South Africa: More insight into the evolution of bimodal A‐type granitoid

The ~2.05 Ga Bushveld Igneous Complex (BIC) in the Kaapvaal Craton, South Africa, with an areal extent of ~65,000 km2, represents one of the largest bimodal A‐type granitoid suites in the world. The igneous activity of this complex begun with the Rooiberg Group of basalt–rhyolite bimodal volcanism, followed by the emplacement of the ultramafic–mafic Rustenburg Layered Suite (RLS), and finally the granitoid rocks of the Rashoop Granophyre (RG) and Lebowa Granite Suite (LGS). In this study, we present our new borehole description and whole‐rock chemical data on the LGS from the relatively unknown western BIC to constrain the origin of the LGS and try to evaluate their geochemical relationship to the Rooiberg Group felsic volcanics. In the western BIC, the LGS is locally represented by the hydrothermally altered Paalkraal Granite and the Kenkelbos Granite that together form a central dome‐like feature. The Veekraal Granite forms the peripheral part of the dome. These granitoids are, in general, coarse‐grained, porphyritic, or equigranular, showing granophyric intergrowth, and consist of quartz, perthitic microcline, and albitic plagioclase with minor opaque and interstitial hornblende/biotite. Our whole‐rock chemical data on western LGS, along with those from the Groblersdal area, eastern BIC (taken as reference), form a cluster in the ‘granite’ field in the normative Ab–An–Or diagram with mean Or/(Ab+Or) ~0.5 (0.11), and are peralkaline to weakly peraluminous in composition. These A‐type granitoids are strictly ‘ferroan’ and calc‐alkalic to alkali‐calcic in nature, and evolved through partial melting as evident mainly in process identification plots. In bulk chemical composition, the LGS and high‐SiO2 Rooiberg Group rhyolites overlap with the experimental melts generated during dehydration melting of tonalite–granodiorite, indicating the Archaean Kaapvaal TTG as their possible source. Constraints from experimental petrology further suggest the pressure and temperature of this relatively anhydrous (≤1 wt% H2O) melting were 2–1 kbar (i.e., ≤7.5 km crustal depth) and ~920°C, respectively. As suggested before, the parent magma of the voluminous mafic–ultramafic body (RLS) of the BIC, forming a voluminous magma chamber in shallow crustal depth, provided the heat for this partial melting event. The following igneous event as evident from field description in the literature, our new borehole description and whole‐rock chemical data, were a limited chemical mixing between the parent basaltic magma of the Rooiberg Group and the parent LGS magma under anorogenic condition to generate the metaluminous felsic magma represented by the Rooiberg Group dacite at present. This anorogenic mixing, perhaps guided by chemical diffusion and lasted for ≤950 ka, modified the major oxide compositions (except Na), and/or transitional elements (Sc, V, Co, and Ni), LILEs (Rb, Sr, and Ba), REEs, HFSE (Y, Zr, Nb, and Ta), actinide (Th and U), and possibly Sr‐isotopic compositions of both the parent LGS and Rooiberg Group basaltic magmas. The petrogenetic model observed for the BIC is perhaps unique in explaining geochemical and isotopic variations observed in bimodal A‐type granitoid/volcanic suites like the Karoo Igneous Province.