Duration Of Magmatism Research Articles

Himalayan leucogranites and migmatites: nature, timing and duration of anatexis

AbstractWidespread anatexis was a regional response to the evolution of the Himalayan‐Tibetan Orogen that occurred some 30 Ma after collision between Asia and India. This paper reviews the nature, timing, duration and conditions of anatexis and leucogranite formation in the Greater Himalayan Sequence (GHS), and compares them to contemporaneous granites in the Karakoram mountains. Himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front, such as similar timing and duration of magmatism, common source rocks and clockwise P–T paths. Despite commonalities, most papers emphasize deviations from this general pattern, indicating a fine‐tuned local response to the dominant evolution. There are significant differences in P–T– conditions during anatexis, and timing in relation to regional decompression. Further to that, some regions underwent a second event recording melting at low pressures. Zircon and monazite ages of anatectic rocks range between c. 25 and 15 Ma, suggesting prolonged crustal melting. Typically, a single sample may have ages covering most of this 10 Ma period, suggesting recycling of accessory phases from metamorphic rocks and early‐formed magmas. Recent studies linking monazite and zircon ages with their composition, have determined the timing of prograde melting and retrograde melt crystallization, thus constraining the duration of the anatectic cycle. In some areas, this cycle becomes younger down section, towards the leading front of the Himalayas, whereas the opposite is true in other areas. The relationship between granites and movement on the South Tibetan Detachment (STD) reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the Himalayas. The nature and fate of magmas in the GHS contrast with those in the Karakoram mountains. GHS leucogranites have a strong crustal isotopic signature and migration is controlled by low‐angle foliation, leading to diffuse injection complexes concentrated below the STD. In contrast, the steep attitude of the Karakoram shear zone focused magma transfer, feeding the large Karakoram‐Baltoro batholith. Anatexis in the Karakoram involved a Cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures. The impact of melting on the evolution of the Himalayas has been widely debated. Melting has been used to explain subsequent decompression, or conversely, decompression has been used to explain melting. Weakening due to melting has also been used to support channel flow models for extrusion of the GHS, or alternatively, to suggest it triggered a change in its critical taper. In view of the variable nature of anatexis and of motion on the STD, it is likely that anatexis had only a second‐order effect in modulating strain distribution, with little effect on the general history of deformation. Thus, despite all kinds of local differences, strain distribution over time was such that it maintained the well‐defined arc that characterizes this orogen. This was likely the result of a self‐organized forward motion of the arc, controlled by the imposed convergence history and energy conservation, balancing accumulation of potential energy and dissipation, independent of the presence or absence of melt.

Zircon U–Pb ages and Hf–O isotopic signatures of the Wajilitag and Puchang Fe–Ti oxide–bearing intrusive complexes: Constraints on their source characteristics and temporal–spatial evolution of the Tarim large igneous province

The Wajilitag and Puchang igneous complexes host two known economic Fe–Ti oxide deposits in the recently recognized Tarim large igneous province (TLIP). The Wajilitag complex comprises clinopyroxenite and melagabbro, whereas the Puchang complex is generally gabbroic and anorthositic in lithology apart from minor plagioclase-bearing clinopyroxenites in the marginal contact zone. The Fe–Ti oxide ores are disseminated throughout the Wajilitag complex and principally restricted to the ultramafic unit, whereas the Puchang complex contains massive to disseminated Fe–Ti oxide ores mainly hosted within the gabbroic rocks. Both secondary ion mass spectroscopy and laser ablation-inductively coupled plasma-mass spectrometry U–Pb dating of zircon grains from the Wajilitag and Puchang complexes yield U–Pb zircon ages of ca. 283Ma and ca. 275Ma, respectively, clearly indicating that there were two independent episodes of the magmatic events related to Fe–Ti oxide mineralization in the TLIP. The new zircon U–Pb ages of intrusive rocks studied here, coupled with available geochronological data from elsewhere in the TLIP, show a long duration of magmatism (up to 30Myr), although the precise age of the TLIP remains to be determined. The two complexes are late-stage events that notably postdate most, if not all, the basaltic lava flows. Furthermore, the occurrence of the earliest manifestation (e.g., ca. 300Ma kimberlitic rocks) of a proposed mantle plume in the Bachu area and the potential temporal migration of the late-stage magmatism from the Bachu and Keping areas to the edges of the Tarim Craton, indicate a possible plume centre near the Bachu–Keping district. The εHf(t) values of zircons from each complex show a range of several εHf(t) units (Wajilitag: +2.7 to +9.2, Puchang: −5.2 to +2.6), probably suggesting late-stage crustal contamination in magma chambers at the time of zircon saturation. Unlike the Lu–Hf isotopic system, the zircons may preserve the original O isotope signature of their mantle sources. The increase of O isotopic composition from the Wajilitag complex (δ18O=5.2–5.9‰) to the Puchang complex (δ18O=5.6–7.1‰), indicates a relatively high proportion of recycled subduction-related materials (e.g., eclogite and garnet pyroxenite) incorporated into the subcontinental lithospheric mantle source for the Puchang intrusive rocks. Partial melting of the refertilized subcontinental lithospheric mantle with the involvement of garnet-bearing mafic components can be of great importance for the formation of parental Fe-rich magmas and ultimately Fe–Ti oxide deposits. This observation is consistent with the occurrence of some mineralized LIPs (e.g., Emeishan) in formerly active convergent plate margins of ancient cratonic blocks, contributing to a global understanding valuable to exploration efforts.

The temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow-spreading ridge: High-precision U–Pb zircon dating of ODP Holes 735B and 1105A, Atlantis Bank, Southwest Indian Ridge

Ocean Drilling Program Hole 735B at Atlantis Bank on the Southwest Indian Ridge sampled 1508 m of plutonic oceanic crust, hosted in the footwall of an oceanic detachment fault. We present new high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U–Pb zircon dates from samples spanning the length of Hole 735B, and from the shallower adjacent Hole 1105A (158 m). The new dates provide the most complete and precise record of both the spatial and temporal distribution of magmatism during accretion of the lower oceanic crust to date. Whole rock and mineral geochemistry from Hole 735B define three main igneous series. Weighted mean 206Pb/238U dates suggest each igneous series intruded beneath the preceding series. Weighted mean 206Pb/238U dates range from 12.175 to 11.986 Ma in Series 1; 11.974 to 11.926 Ma in Series 2; and 11.936 to 11.902 Ma in Series 3 (±0.015 to 0.069 Ma). Weighted mean 206Pb/238U dates from Hole 1105A range from 11.9745 to 11.9573 Ma (±0.0082 to 0.0086 Ma). The Hole 1105A dates are coeval with Series 2 in Hole 735B, consistent with previous correlations of Fe-Ti oxide-rich layers between the two holes, suggesting individual magmatic series formed sheet-like bodies that were ≥250 m thick and extended ≥1.1 km parallel to the ridge axis (E–W) and ≥0.48 km in the spreading direction (N–S). The data suggest a total duration of magmatism in Hole 735B of ≥0.214±0.032 Ma, corresponding to accretion over a horizontal distance of ≥2.6±0.4 km. The crust at Atlantis Bank was formed during active detachment faulting, and the successive underplating of each magmatic unit may have been favored in this environment. The combined U–Pb dates, and reported Ti-in-zircon temperatures, are consistent with magmatic cooling rates of 103–104 °C/Ma over the temperature interval of 900–700 °C.

Geochronological imaging of an episodically constructed subvolcanic batholith: U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex of the Central Andes

Zircons from 15 crystal-rich monotonous intermediate ignimbrites and 1 crystal-poor rhyolite ignimbrite erupted during the 11–1 Ma Altiplano-Puna Volcanic Complex (APVC) ignimbrite flare-up record multiscale episodicity in the magmatic history of the shallowest levels (5–10 km beneath the surface) of the Altiplano-Puna Magma Body (APMB). This record reveals the construction of a subvolcanic batholith and its magmatic and eruptive tempo. More than 750 U-Pb ages of zircon rims and interiors of polished grains determined by secondary ion mass spectrometry define complex age spectra for each ignimbrite with a dominant peak of autocrysts and subsidiary antecryst peaks. Xenocrysts are rare. Weighted averages obtained by pooling the youngest analytically indistinguishable zircon ages mostly correspond to the dominant crystallization ages for zircons in the magma. These magmatic ages are consistent with eruptive stratigraphy, and fall into four groups defining distinct pulses (from older to younger, pulses 1 through 4) of magmatism that correlate with eruptive pulses, but indicate that magmatic construction in each pulse initiated at least 1 m.y. before eruptions began. Magmatism was initially distributed diffusely on the eastern and western flanks of the APVC, but spread out over much of the APVC as activity waxed before focusing in the central part during the peak of the flare-up. Each pulse consists of spatially distinct but temporally sequenced subpulses of magma that represent the construction of pre-eruptive magma reservoirs. Three nested calderas were the main eruptive loci during the peak of the flare-up from ca. 6 to 2.5 Ma. These show broadly synchronous magmatic development but some discordance in their later eruptive histories. These relations are interpreted to indicate that eruptive tempo is controlled locally from the top down, while magmatic tempo is a more systemic, deeper, bottom-up feature. Synchroneity in magmatic history at distinct upper crustal magmatic foci implicates a shared connection deeper within the APMB. Each ignimbrite records the development of a discrete magma. Zircon age distributions of individual ignimbrites become more complex with time, reflecting the carryover of antecrysts in successively younger magmas and attesting to upper crustal assimilation in the APVC. Although present, xeno­crysts are rare, suggesting that inheritance is limited. This is attributed to basement assimilation under zircon-undersaturated conditions deeper in the APMB than the pre-eruptive levels, where antecrysts were incorporated in zircon-saturated conditions. Magmatic ages for individual ignimbrites are older than the 40 Ar/ 39 Ar eruption ages. This difference is interpreted as the average minimum Zr-saturated melt-present lifetime for APVC magmas, the magmatic duration or Δ age. The average Δ age of ca. 0.4 Ma indicates that thermochemical conditions for zircon saturation were maintained for several hundreds of thousands of years prior to eruption of APVC magmas. This is consistent with a narrow range of zircon saturation temperatures of 730–815 °C that record upper crustal conditions and Zr/Hf, Th/U, Eu/Eu*, and Ti that reveal protracted magma differentiation under secular cooling rates an order of magnitude slower than typical pluton cooling rates. In concert, these data all suggest that the pre-eruptive magma reservoirs were perched in a thermally and chemically buffered state during their long pre-eruptive lifetimes. Trace element variations suggest subtle differences in crystallinity, melt fraction, and melt composition within different zones of individual magma reservoirs. Significant volumes of plutonic rocks associated with ignimbrites are supported by geophysical data, the limited compositional range over 10 m.y., the thermal inertia of the magmatic systems, and the evidence of resurgent magmatism and uplift at the calderas and eruptive centers, the distribution of which defines a composite, episodically constructed subvolcanic batholith. The multiscale episodicity revealed by the zircon U-Pb ages of the APVC flare-up can be interpreted in the context of continental arc magmatic systems in general. The APVC ignimbrite flare-up as a whole is a secondary pulse of ∼10 m.y., with magmatic pulses 1 through 4 reflecting tertiary pulses of ∼2 m.y., and the individual ignimbrite zircon spectra defining quaternary pulses of

Petrogenesis and tectonic setting of Early Cretaceous magmatism in the Jiwa area, central Lhasa Terrane, Tibet

ABSTRACTNew zircon LA-ICP-MS U–Pb ages, Sr-Nd isotopic data, and whole-rock major and trace element data from Early Cretaceous volcanic rocks are reported for the Jiwa area in the southern central Lhasa Terrane of Tibet. These mainly silicic volcanic rocks and subordinate intermediate-basic volcanic rocks have long been considered to be Pliocene (Wuyu Group) or Eocene (Pana Formation) in age. However, our new zircon U–Pb ages constrain the timing of eruption to the Early Cretaceous (124.6 ± 1.6–126.1 ± 1.1 Ma); thus, we have redefined these volcanic rocks as the Lower Cretaceous Zenong Group. The silicic volcanic rocks feature high-K calc-alkaline to shoshonitic compositions and are mostly strongly peraluminous, rich in Rb, Th, and light rare earth elements (LREEs), and depleted in Nb, Ta, P, and Ti. They are also characterized by negative whole-rock εNd(t) values (–9.1 to –13.1) and variable 87Sr/86Sr ratios. Thus, the geochemical and zircon U–Pb age data of the Jiwa volcanic rocks suggest that these rocks are associated with a continental arc and are mostly likely derived from anatexis of ancient continental crustal material and minor basalt-derived melts. The discovery of Early Cretaceous volcanic rocks in the southern central Lhasa Terrane extends the duration of magmatism triggered by southward subduction of the Bangong-Nujiang oceanic lithosphere from the Late Jurassic to the Early Cretaceous. The spatial distribution of magmatism is also extended 70–80 km to the south. The Lower Cretaceous volcanic rocks in the Jiwa area are proposed to be a result of bi-directional subduction, with southward subduction of the Bangong-Nujiang oceanic crust and northward subduction of the Yarlung Zangbo oceanic crust. The bi-directional subduction of the oceanic lithosphere and gravitational sinking led to slab retreat at ca. 125 Ma. The roll-back of the slab would have then led to back-arc extension and asthenospheric upwelling. The subduction-induced decompression melting of the mantle led to the generation of widespread rhyolitic volcanism with continental arc geochemical signatures.

Book review: Layered Intrusions, ed. by Bernard Charlier, Olivier Namur, Rais Latypov, and Christian Tegner

Book review: Layered Intrusions. Edited by Bernard Charlier, Olivier Namur, Rais Latypov, Christian Tegner (2015) Springer, 748 p. $179 (Hardcover), $139 (eBook), ISBN 978-94-017-9651-4. The book entitled Layered Intrusions is a well-sized collection of 15 chapters written by 36 authors—each being an expert in their prevailing sub-discipline. Hence, the book covers all aspects of the emplacement and evolution of large mafic to ultramafic magma chambers. All chapters provide a broad introduction to their topics and are richly illustrated. References appear at the end of each chapter and support the readers’ further education on particular topics. The book is well written and intends to aid in further training of graduate and undergraduate students; however, the addition of access to illustrations and figures online would increase its value as a teaching resource. All chapters are available as e-books, so with a little extra effort and expense, if you have the hard cover version, images and figures for teaching can be obtained from these. Each of the 15 chapters are available separately for $29.95. Photographs and photomicrographs are mostly color images, where grayscale images are used, these are labelled to avoid any confusion that might arise. The book is organized into two main parts that are summarized in the two paragraphs below: the first part includes reviews and new views on the formation processes of layered intrusions. The second part reviews the state of recent cutting-edge research of layered intrusions in the field. The book begins with the geochronology of layered intrusions. Chapter 1 provides information about advantages and disadvantages of well-known dating methods, e.g., U-Th-Pb, K-Ar and which of these best applied to layered intrusions from different periods in the Earth’s history. Cooling rates of the intrusions are determined by choosing various mineral species with different closing temperatures, the temperature at which radiogenic elements are retained in the crystal lattice. The case studies suggest that duration of magmatism of layered intrusions can be restricted to brief …

Re-evaluating the geochronology of the Permian Tarim magmatic province: implications for temporal evolution of magmatism

The Permian Tarim magmatic province has 118 published ages, ranging from 358 to 205 Ma, but the timing of mafic magmatism is not well constrained. We report two new secondary ion mass spectrometry U–Pb zircon dates on the Halahatang trachydacite and Wajilitag olivine clinopyroxenite, which are 287.2 ± 2.0 Ma and 283.2 ± 2.0 Ma, respectively. The trachydacite overlies the uppermost basalt and constrains the latest eruption age of basalt in northern Tarim. The latter is the first high-resolution date for the Wajilitag mafic layered intrusion. By screening all published ages, we identified 22 robust ages, ranging from 290.9 ± 4.1 to 261.7 ± 1.8 Ma. The robust ages together with our new data reveal a protracted period of mafic magmatism at c . 283 and c . 267 Ma. Silicic magmatism occurred from 291 to 272 Ma. Although the current known volume of Tarim basalt is too small to qualify as a large igneous province, the eroded and intrusive components, as well as pyroclastic deposits and silicic lavas, may increase the estimated volume. Further work is required to refine the duration of magmatism and the volume estimate of the province. Supplementary material: A list of all the published dates, raw data of published 40 Ar– 39 Ar dates, raw data of published U–Pb dates, published geochemistry data used for calculation and complete details of the data evaluation are available at http://www.geolsoc.org.uk/SUP18862 .

Tectonic reconstruction of batholith formation based on the spatiotemporal distribution of Cretaceous–Paleogene granitic rocks in southwestern Japan

AbstractThe spatiotemporal distribution of Cretaceous–Paleogene granitic rocks in southwestern Japan is investigated to understand the origin of the granitic batholith belt and to reconstruct the tectonic setting of emplacement. New U–Pb zircon ages for 92 samples collected from a region measuring 50 km (E–W) by 200 km (N–S) reveals a stepwise northward younging of granitic rocks aged between 95 and 30 Ma with an age‐data gap between 60 and 48 Ma. Based on the spatiotemporal distribution of granite ages, we examine two plausible models to explain the pattern of magmatic activity: (i) subduction of a segmented spreading ridge and subsequent slab melting (ridge‐subduction model), and (ii) subduction with a temporally variable subduction angle and corresponding spatial distribution of normal arc magmatism (subduction angle model). We optimize the model parameters to fit the observed magmatism in time and space, and compare the best‐fit models. As to ridge subduction model, the best‐fit solution indicates that the spreading ridge started to subduct at approximately 100 Ma, and involved a 45‐km‐wide section of the ridge segment, a subduction obliquity of 30°, and a slow migration velocity (∼1.6 cm/y) of the ridge. These values are within the ranges of velocities observed for present‐day ridge subduction at the Chile trench. On the other hand, the best‐fit solution of subduction angle model indicates that the subduction angle decreases stepwise from 37° at 95 Ma, 32° at 87 Ma, 22° at 72 Ma, to 20° at 65 Ma, shifting magmatic region towards the continental side. These results and comparison, together with constraints on the geometry of the tectonic setting provided by previous studies, suggest that the ridge subduction model better explains the limited duration of magmatism, although both models broadly fit the data and cannot be ruled out.

Piggy-back Supervolcanoes—Long-Lived, Voluminous, Juvenile Rhyolite Volcanism in Mesoproterozoic Central Australia

The Talbot Sub-basin is one of several bimodal volcanic depositional centres of the Mesoproterozoic Bentley Basin in central Australia. It is dominated by rocks of rhyolitic composition and includes ignimbrites, some forming large to super-eruption size deposits. Ferroan, incompatible trace element enriched, A-type compositions, anhydrous mineralogy and clear evidence for local rheomorphism indicate high eruption temperatures, with apparent zircon-saturation temperatures suggesting crystallization at >900 degrees C. Comagmatic basalt is of mantle origin with minor Proterozoic basement contamination. The rhyolites cover the same range of Nd isotope compositions (epsilon(Nd(1070)) +1.24 to -0.96) and La/Nb ratios (1.2-2.1) as the basalts (epsilon(Nd(1070)) +2.1 to -1.1: La/Nb 1.2-2.3) and are compositionally far removed from all older basement and country-rock components (average epsilon(Nd(1070)) = -4, La/Nb = 10). The rhyolites and basalts are cogenetic through a process probably involving both fractional crystallization of mafic magmas and partial melting of recently crystallized mafic rock in a lower crustal intraplate, extraction of dacitic magmas to a voluminous upper crustal chamber system, and separation of rhyolite by processes involving rejuvenation and cannibalization of earlier chamber material. More than 230 000 km(3) of parental basalt is required to form the >22 000 km(3) of preserved juvenile rhyolite in the Talbot Sub-basin alone, which represents one of the most voluminous known felsic juvenile additions to intracontinental crust. Zircon U-Pb age components are complex and distinct from those of basement and country rock and contain antecrystic components reflecting dissolution-regrowth processes during periodic rejuvenation of earlier-emplaced chamber material without any significant interaction with country rock. The overall duration of magmatism was >30 Myr but can be divided into between two and four separate intervals, each probably of a few hundred thousand years' duration and each probably reflecting one of the distinct lithostratigraphic groups defined in the sub-basin. Neither the composition nor style of felsic and mafic volcanism changes in any significant way from one volcanic event to the next and the range of zircon U-Pb ages indicates that each period utilized and cannibalized the same magma chamber. This volcanism forms a component of the 1090-1040 Ma Giles Event in central Australia, associated with magma-dominated extension at the nexus of the cratonic elements of Proterozoic Australia. This event cannot be reasonably reconciled with any putative plume activity but rather reflects the >200 Myr legacy of enhanced crustal geotherms that followed the final cratonic amalgamation of central Australia.

New U-Pb Zircon and 40AR/39AR Age Constraints on the Late Mesozoic to Cenozoic Plutonic Record in the Western San Juan Mountains

New U-Pb zircon and 40Ar/39Ar crystallization ages for latest Mesozoic to Cenozoic plutonic rocks in the western San Juan Mountains provide insight into the timing and interplay of mantle and crustal magmatism over the past 80 Ma. Subduction-driven magmatism in the Laramide (75-60 Ma) generated alkalic intermediate to mafic magmas. These were emplaced as laccoliths and stocks along northeast trends that were likely controlled by crustal-scale zones of weakness with Proterozoic ancestry. The transition from Laramide subduction to slab rollback and delamination was marked by incipient regional extension and widespread emplacement of plutons. Oligocene plutons of gabbro to granite were mostly peripheral to 29-27 Ma caldera complexes that formed over a regional subvolcanic batholith. Alkaline mantle magmas that accompanied the semi-continuous intrusion of shallow felsic to intermediate plutons from 25 to 4 Ma produced diatreme-dike complexes and dike swarms across the northern San Juan Basin. Emplacement of plutons from 75-4 Ma caused localized uplift that had a major influence on landscape evolution, and often were linked to zones of mineralization. Late Mesozoic to Cenozoic plutonic events in the western San Juan Mountains record a long-term shift to more bimodal magmatism. Injection of mantle melts from 25 to 4 Ma during incipient crustal extension accompanied elevated thermal gradients over the region, as evidenced by resetting of cooling ages in some Laramide intrusive rocks. The preservation of xenocrystic zircons in 75-4 Ma plutonic rocks provides evidence for variable magmatic conditions with an overall increase in the thermal state of Oligocene melts proximal to caldera systems and all plutons emplaced after 18 Ma. High geothermal gradients caused by the long duration of magmatism and increased mantle contributions may have permitted more prolonged assembly and slower cooling in some plutons.

Mesozoic magmatism and metallogenesis associated with the destruction of the North China Craton: Evidence from U–Pb geochronology and stable isotope geochemistry of the Mujicun porphyry Cu–Mo deposit

The North China Craton (NCC) provides a classic example of lithospheric destruction and refertilization. The timing and duration of magmatism and related metallogenesis associated with the destruction process are pivotal to understanding the geodynamic controls. In this study, we present zircon U–Pb and Hf data, Re–Os ages, and He, Ar, Pb and S isotope data from the Mujicun porphyry Cu–Mo deposit in the northern Taihang Mountains within the Central Orogenic Belt of the NCC. We constrain the timing of magmatism as 144.1±1.2Ma from zircon U–Pb data on the diorite porphyry that hosts Cu–Mo mineralization. Another U–Pb age of 139.7±1.4Ma was obtained from an epidote skarn that is located in the contact zone between the porphyry and its wall rocks. These data and five Re–Os molybdenite ages that range from 142.7±2.0Ma to 138.5±1.9Ma suggest that magmatism and mineralization occurred in about five million year duration from ~143Ma to ~138Ma. The He, Ar, Pb and, Hf data suggest that magmatism involved recycled Neoarchean lower crustal components, with input of heat and volatiles from an upwelling mantle. The Mujicun porphyry and associated mineralization provide a typical example for magmatism and metallogeny associated with lithospheric thinning in the NCC.

Sources of mesozoic polyformation magmatism in the Ketkap-Yuna Igneous Province, Aldan Upland: Isotopic data

The KetkapYuna Igneous Province (KKYuIP) can be considered as a tectonotype of tectonomag� matic activation (TMA) zones within the Aldan Shield. The KKYuIP is represented by a series of mul� tiphase volcanicplutonic edifices of the central type stretching more than 300 km, from Kondyor Ridge (AyanMaya district of Khabarovsk Krai) to South Yakutia, as an arcshaped structure of igneous origin (Fig. 1). Rocks that are coeval and cogenetic to the KKYuIP rocks have been found in 12 areas within the Aldan Shield (2). Their origin corresponds in time to the intensive intraplate magmatism episode in the Late Mesozoic. The foundation age, degree and pattern of deformations, petrogenesis of igneous products, and duration of magmatism in the KKYuIP are the key points required for assessment of the validity of the existing geodynamical models describing evolution of the Siberian continent (5, 14, 15, etc.) and for reveal� ing the fundamental causes of origination and fading of magmatism in TMA zones.

Neoproterozoic island arcs in East Sayan: duration of magmatism (from U–Pb zircon dating of volcanic clastics)

Two island arcs of different ages have been reconstructed in the Neoproterozoic history of southeastern East Sayan: Dunzhugur and Shishkhid. According to earlier concepts, the Dunzhugur arc formed at ∼1020 Ma and underwent collision with the Siberian(?) continent at ∼810 Ma. The Shishkhid arc formed somewhat earlier than 800 Ma and existed till the end of the Late Baikalian (∼600 Ma, from indirect data). This primitive geologic history, when each arc existed for 200 Myr, was suggested because of the deficit of direct data, and its reconstruction cast doubt. In this work we present results of preliminary dating of detrital zircons separated from the volcaniclastic rocks composing the above arcs. We analyzed 12 zircon crystals from the Dunzhugur volcanic clastics, whose 206Pb/238U age varies from 844 ± 8 to 1048 ± 12 Ma (1σ). Five most ancient zircons form a concordant cluster with an age of 1034 ± 9 Ma (2σ). Hence, the arc formed earlier than it was assumed and existed for a long time, most likely, till its collision with the continent. We also studied two zircon samples from the volcaniclastic rocks of the Oka accretionary prism, which probably formed in the Shishkhid arc. All ten crystals of the first sample form a concordant cluster with an age of 813 ± 7 Ma (2σ). The analyzed zircons of the second sample arrange in two clusters, with an age of 775 ± 8 Ma (2σ, nine crystals) and 819 ± 17 Ma (three crystals). Thus, the Shishkhid arc formed earlier than it was assumed, at the end of the Early Baikalian, and underwent active volcanism at least till 775 Ma. Dating of detrital zircons from the volcaniclastics generated at the mature stage of the Shishkhid-arc evolution will help to reconstruct partly or completely its history in the period 775–600 Ma.

Rapid crustal accretion and magma assimilation in the Oman‐U.A.E. ophiolite: High precision U‐Pb zircon geochronology of the gabbroic crust

New high‐precision U/Pb zircon geochronology from the Oman‐United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust. The new data come from a well‐preserved and exposed crustal section in the center of the Wadi Tayin massif. Single grain and grain fragment 206Pb/238U dates from upper‐level gabbros, tonalites/trondhjemites and gabbroic pegmatites, corrected for initial Th exclusion, range from 112.55 ± 0.21 to 95.50 ± 0.17 Ma, with most data clustered between 96.40 ± 0.17 to 95.50 ± 0.17 Ma. Zircon dates from upper‐level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half‐rates of 50–100 km/Ma. Dates from tonalites/trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper‐level gabbros, suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates. Three of the dated upper‐level gabbros and a single gabbroic pegmatite from the base of the crust have >1 Ma intrasample variability in single grain dates, suggesting assimilation of older crust during the formation or crystallization of the magmas. Whole rock εNd(t) of seven samples, including the upper‐level gabbros with variable zircon dates, have tightly clustered initial values ranging from εNd(96 Ma) = 7.59 ± 0.23 to 8.28 ± 0.31. The εNd values are similar to those from other gabbros within the ophiolite, suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas. The new dates suggest that the studied section formed at a fast spreading mid‐ocean ridge between ∼96.4–95.5 Ma. The large intrasample variability in zircon dates in some samples is unexpected in this setting, and may be related to propagation of a younger ridge into older oceanic lithosphere.

Protracted timescales of lower crustal growth at the fast-spreading East Pacific Rise

The formation of oceanic crust at mid-ocean ridges is a fundamental component of plate tectonics. A large fraction of the new crust is created when magmas in the lower crust cool to form gabbroic rocks. The duration of magmatism during formation of the new gabbroic crust is expected to vary with plate-spreading rate and has been constrained by dating gabbroic rocks at the slow-spreading Southwest Indian Ridge and Mid-Atlantic Ridge1, 2, 3, 4. Here we present high-precision U–Pb dating of zircon minerals from gabbroic rocks formed at the fast-spreading East Pacific Rise and exposed at Hess Deep. We find that the zircons formed between 1.420 and 1.271 (±0.006–0.081) million years ago. Within individual samples, the zircon minerals exhibit a range in formation dates of up to 0.124 Myr, consistent with either protracted crystallization from a magma or assimilation into the magma of older zircons from adjacent rocks. The variability of zircon dates is comparable to that measured at the slow-spreading Mid-Atlantic Ridge3. We conclude that the timescales of magmatic processes in the lower crust may be similar at slow- and fast-spreading ridges, implying that the duration of crust formation at mid-ocean ridges is not only controlled by spreading rate.

Precise age determination of mafic and felsic intrusive rocks from the Permian Emeishan large igneous province (SW China)

Radiogenic isotopic dating using different (mainly micro-beam analyses) techniques of rocks from the Emeishan large igneous province (ELIP) has yielded a range of ages from the Capitanian to the Early Triassic. In contrast, estimates based on paleontological and paleomagnetic data suggest the ELIP was emplaced rapidly (≤2Ma) and that volcanism ended by ~257Ma. New zircon CA–TIMS U–Pb ages, that have superior precision compared to ages from micro-beam analyses, were obtained from strategic intrusive rocks of the Panxi region (Inner Zone) of the ELIP. The Huangcao (258.9±0.7Ma), Daheishan (259.1±0.5Ma), Woshui (259.6±0.5Ma) and Cida (258.4±0.6Ma) syenitic and granitic plutons and three mafic dykes (259.2±0.4Ma; 259.5±0.8Ma, 257.6±0.5Ma) yielded a narrow range of ages between >257Ma and ~260Ma. The new results are consistent with the estimates from magneto-biostratigraphic data and a rapid emplacement model. The new Wuchiapinqian ages of the Huangcao and Daheishan plutons suggests that magmatism may have lasted significantly less than 10Ma as previously suggested, and the precise ages of the mafic dykes refute the hypothesis that Emeishan ‘high-Ti’ basalts represent the waning stages of the ELIP magmatism. The short duration of magmatism also implies that degassing of the host sediments (mostly carbonates), and thus releasing of greenhouse gasses occurred within a substantially shorter interval than previously thought, which significantly changes the loading of atmosphere per time unit. The latter corollary potentially has severe environmental effects that may have contributed to causing the end-Guadalupian mass extinction.

In-situ zircon U–Pb, oxygen and hafnium isotopic evidence for magma mixing and mantle metasomatism in the Tuscan Magmatic Province, Italy

In this study, we have used in-situ U–Pb, Hf and O isotopic analyses of zircon grains to gain insights into both magmatic processes and duration of magmatism in igneous rocks from the Tuscan Magmatic Province (0.1–9 Ma), Italy. Three plutonic centres have been investigated (Monte Capanne and Porto-Azzuro monzogranites in Elba and the Giglio monzogranite) as well as Capraia, the only volcanic centre in the Tuscan Archipelago. New ion microprobe zircon U–Pb data reveal a continuum of plutonic activity in Elba over 2 Ma (8.3–6.3 Ma), with coeval volcanic activity in Capraia (7.1–7.6 Ma), and plutonic activity resuming in Giglio (5.5 Ma) after a gap of 1 Ma. From these zircon data we also show that construction of the Monte Capanne pluton (Elba) may have occurred over a period of c. 0.5 Ma. A significant range of both 176Hf/ 177Hf (determined by LA–MC-ICPMS) and δ 18O (determined by ion microprobe) in zircon (~ 7 epsilon Hf units and ~ 5‰, respectively) is present, which, together with zircon morphology and trace element data (Gagnevin et al., 2010), emphasises the importance of mixing and replenishment involving magma batches with both metaluminous and peraluminous affinities. Inherited and xenocrystic zircons also occur, but are scarce. These have a wide range of 176Hf/ 177Hf and δ 18O values, further emphasising that a variety of crustal components has contributed to the genesis of the Tuscan magmas, either as contaminants or magma sources. While mixing undoubtedly occurred between mafic (metaluminous) and felsic (peraluminous) magmas, the range of Hf and O isotopic data suggests a diversity within the peraluminous component. The unradiogenic Hf composition (εHf( t) <− 4) and relatively heavy δ 18O signature (> 6‰) of the inferred mantle-derived component (represented by Capraia volcanism, and at least in part, lamproitic in composition) strongly supports the idea that the mantle source involved in Tuscan magmatism was severely modified by subduction-related, crustal-derived metasomatic fluids.

Geochronology and Geochemistry of Mafic Dikes from Hainan Island and Tectonic Implications

Abstract:In the present study, the major and trace element compositions, as well as Sr, Nd isotopic compositions and K‐Ar age data in mafic dikes from Hainan Island, China, have been analyzed. Whole‐rock K‐Ar dating yielded a magmatic duration of 61–98 Ma for mafic dikes. Mafic dikes have a very high concentration of incompatible elements, for example, Ba, Rb, Sr, K, rare earth elements, and especially light rare earth elements (LREE), and negative anomalies of Nb, Ta, and Ti in the normalized trace element patterns. The initial 87Sr/86Sr ratios and εSr(t) of the mafic dikes are 0.70634–0.71193 and +27.7 to +112.2, respectively. In the 87Sr/86Sr versus εNd(t) diagram, the Hainan Island mafic dikes plot between fields for depleted mantle and enriched mantle type 2. All these characteristics show that the mantle (source region) of mafic dikes in this area experienced metasomatism by fluids relatively enriched in LREE and large ion lithophile elements. The genesis of Hainan Island mafic dikes is explained as a result of the mixing of asthenospheric mantle with lithospheric mantle that experienced metasomatism by the subduction of the Pacific Plate. This is different from the Hainan Island Cenozoic basalts mainly derived from depleted asthenospheric mantle, and possibly, minor metasomatised lithospheric mantle. This study suggests that the Mesozoic and Cenozoic lithospheric revolutions in Hainan Island can be divided into three stages: (1) the compression orogenesis stage before 98 Ma. The dominant factor during this stage is the subduction of the ancient Pacific Plate beneath this area. The lithospheric mantle changed into enriched mantle type 2 by metasomatism; (2) the thinning and extension stage during 61–98 Ma. The dominant factor during this stage is that the asthenospheric mantle invaded and corroded the lithospheric mantle; and (3) the large‐scale thinning and extension stage after 61 Ma. The large‐scale asthenospheric upwelling results in the strong erupting of Cenozoic basalts, large‐scale thinning of the lithosphere, the southward translating and counterclockwise rotating of Hainan Island, and the opening of the South China Sea.

New SHRIMP age constraints on the timing and duration of magmatism and sedimentation in the Mary Kathleen Fold Belt, Mt Isa Inlier, Australia

Paleoproterozoic magmatic rocks from the Mary Kathleen Fold Belt of the Mt Isa Inlier record different magmatic textures and variations in tectonic strain associated with extension and the development of crustal-scale detachment zones. New SHRIMP U–Pb zircon geochronology for magmatic rocks, combined with field relationships, refine the duration of this extension to between 1780 and 1740 Ma. The initial stages of this tectonomagmatic event are coincident with mafic magmatism, basin formation and rapid sedimentation of the ∼1780–1765 Ma Myally Supersequence of the Leichhardt Superbasin in the adjacent Leichhardt River Fault Trough. The Ballara Quartzite and Corella Formation represent a period of sag-phase sedimentation during the later part this event, and facies models, sequence-stratigraphic interpretations and detrital-zircon geochronology data confirm the time equivalence of these units to the Quilalar Supersequence of the Leichhardt River Fault Trough. These correlations permit the Eastern and Western Successions of the Mt Isa Inlier to be correlated from 1780 Ma. Locally, the Corella Formation is intruded by 1740 Ma granites, suggesting that at least the lower parts of this package were deposited during the 1780–1740 Ma extensional event. By linking deep-crustal extension processes in the Mary Kathleen area with near-surface basin formation in the adjacent Leichhardt River Fault Trough, it is possible to develop crustal-scale architecture models that provide insights into the development and migration of ore-bearing fluids.

From Devonian extensional collapse to early Eocene continental break-up: an extended transect of the Kejser Franz Joseph Fjord of the East Greenland margin

SUMMARY Seismic investigations along East Greenland’s Fjord Region completed during the last decade provide fundamental insights into the region’s crustal structure and tectonic history. A summary of models along a transect through the Kejser Franz Joseph Fjord provides a view from the Precambrian Shield to the Eocene oceanic crust. We conclude that a change of rifting geometry from an upper- to a lower-plate-style margin occurred in early Mesozoic times and formed the >350-km-wide rift zone. Despite the demonstrated asymmetry of the northeast Greenland and conjugate Voring margins, the change of rift geometries and the direction of rift jumps remain debatable. A combined model for productivity and duration of magmatism is proposed for the northeast Greenland fjord region. We suggest that magmatism started slowly at 58.8 ± 3.6 Ma with a production rate of 1.5 × 10 −4 km 3 km −1 a −1 , which is similar to the productivity of onshore upper and lower lava sequences on the Geikie Plateau. A peak of 9.4 × 10 −4 km 3 km −1 a −1 for 0.5 Myr, and a subsequent productivity of 4.4 ± 0.3 × 10 −4 km 3 km −1 a −1 for 2.5 Myr between 53.3 and 50.8 Ma, produced the majority of melt, but break-up did not occur immediately afterwards. Continuous production of melt, similar to the rate of ocean spreading until C22 (∼50 Ma), contributed to massive magmatic underplating until eventual break-up at 50 Ma. The volumes and production rates show similarities to those obtained from a profile off the southeast Greenland margin but with a major difference in a smaller regional spatial extent.