Multiple Magmatic Events Research Articles

Geochronology and petrogenesis of multiple magmatic events in the Tianbaoshan orefield, NE China: Implications for tectonic evolution

Recent studies have shown that multistage magmatic and metallogenic events in NE China were dominated by the Paleo-Asian Ocean tectonic regime and the Paleo-Pacific Ocean tectonic regime. However, outstanding questions remain on the petrogenesis and fertility of ore-causative magma in each metallogenic event. In this study, we report new geochronologic and geochemical data on ore-causative intrusions from the Tianbaoshan orefield in the east Jilin-Heilongjiang belt (EJHB), aiming to identify their petrogenesis, magma fertility, and their implications for the geodynamic evolution of the EJHB. Middle Permian Lishan ore-causative quartz monzodiorites with abundant mafic microgranular enclaves (MMEs) were emplaced at ca. 264.9 ± 2.6 Ma. Petrographic and geochemical characteristics (87Sr/86Sr(i) = 0.7049–0.7053, εNd(t) = 2.9–3.7, and εHf(t) = 4.7–11.7) indicate a juvenile crust source injected by a slab-metasomatized mantle component. The Early Jurassic Beishan monzogranites (ca. 192.5 ± 1.8 Ma) were generated by the partial melting of the juvenile underplating basaltic lower crust with subsequent fractional crystallization in response to their highly evolved geochemical features, combined with their depleted Sr-Nd-Hf isotopic signature (ISr = 0.7032–0.7037, εNd(t) = 2.8–3.3, and εHf(t) = 7.2–12.2). Based on the zircon trace element geochemistry, we infer that the Middle Permian quartz monzodiorites had a high oxygen fugacity (Ce4+/Ce3+ = 40–216), potentially generating the Pb-Zn-Cu mineralization. Early Jurassic Beishan monzogranites had a lower magmatic oxygen fugacity (Ce4+/Ce3+ = 14–106), which may account for the Mo-dominated mineralization. A combination of this study and previous results corroborates that the Tianbaoshan orefield records the superposition of different tectonic regimes during metallogenesis.

Episodic intraplate magmatism fed by a long-lived melt channel of distal plume origin.

In the past decade, marine geophysical observations have led to the discovery of thin channels at the base of oceanic plates with anomalous physical properties that indicate the presence of low-degree partial melts. However, mantle melts are buoyant and should migrate toward the surface. We show abundant observations of widespread intraplate magmatism on the Cocos Plate where a thin partial melt channel was imaged at the lithosphere-asthenosphere boundary. We combine existing geophysical, geochemical, and seafloor drilling results with seismic reflection data and radiometric dating of drill cores to constrain the origin, distribution, and timing of this magmatism. Our synthesis indicates that the sublithospheric channel is a regionally extensive (>100,000 km2) and long-lived feature that originated from the Galápagos Plume more than 20 Ma ago, supplying melt for multiple magmatic events and persisting today. Plume-fed melt channels may be widespread and long-lived sources for intraplate magmatism and mantle metasomatism.

Geology, geochemistry and genesis of the world-class Shizhushan wollastonite deposit, Mengshan area, South China

The recently discovered Shizhushan wollastonite deposit in the Mengshan district contains about 54 Mt of wollastonite, making it the world’s largest wollastonite deposit. Zircon U–Pb geochronological results indicate that the Mengshan composite pluton evolved during a prolonged magmatic history (ca. 230–218 Ma). The pluton and the surrounding carbonate units of the Maokou Formation have distinct initial 87Sr/86Sr ratios, with values of 0.7107–0.7162 and 0.7070–0.7075, respectively. Pure wollastonite crystals in the Maokou Formation are characterized by uniform 87Sr/86Sr values of 0.7070 that are indistinguishable from the carbonates, suggesting that the wollastonite was formed during contact metamorphism. Limestones of the Maokou Formation are characterized by variable δ13CV-PDB (−5.2 to 4.0‰) and δ18OSMOW (10.3 to 22.6‰) values. Carbonates with the lowest δ13CV-PDB and δ18OSMOW values (and elevated 87Sr/86Sr ratios) occur in the vicinity of a regional structure, suggesting that magmatic–hydrothermal fluids and CO2 were channeled along this fault.Based on new geological, geochronological and geochemical data, we propose that the world‐class wollastonite deposit formed as a result of several factors related to a relatively stable depositional environment during the Late Paleozoic as well as structural deformation and magmatic events during the Early Mesozoic. These features include: 1) juxtaposition of abundant sediment-hosted chert (SiO2) and limestone (CaCO3) units, 2) a prolonged heat source provided by multiple magmatic events, 3) CO2 degassing along the regional structure promoted wollastonite-forming reactions, and 4) the proximity of the Si- and Ca-rich sedimentary layers subparallel to the intrusion contact. Our study documents the important roles of Early Mesozoic deformation and magmatism in the genesis of non-metallic mineral deposits in south China.

Thermal History of the Naruo Porphyry Deposit in the Duolong Ore District, Western Tibet: Evidence from U‐Pb, 40Ar/39Ar and (U‐Th)/He Thermochronology

AbstractThe Naruo porphyry copper deposit containing more than 2 Mt of copper is located in the Duolong ore district in the west of the Bangongco–Nujiang belt in central Tibet. New zircon U‐Pb, biotite 40Ar/39Ar, zircon (U‐Th)/He ages, published age data together with thermal modeling were presented in this paper to investigate the thermal history of Naruo deposit. Thermal modeling reveals a prolonged magmatic‐hydrothermal evolution firstly cooling from ∼700°C to ∼350°C at 120 Ma, then cooling to 230°C at 106 Ma and maintaining at 200°C from 106 to 90 Ma which is attributed to multiple magmatic events and thermal effect of strike‐slip fault. Affected by thrust nappe structure, the sample was consistent with 120°C from 70 to 63 Ma. The Naruo deposit started to experience exhumation at a rate of ∼0.07 km/Myr since 60 Ma which is related to India‐Asia collision. The prolonged magmatic‐hydrothermal evolution process might have important influence on the Naruo deposit. The ore‐related intrusions preserved in the foot walls of strike‐slip fault and thrust nappe structure are the objects of future exploration in the Duolong ore district.

Evidence of secular variation in Archean crust formation in the Eastern Indian Shield

Understanding the dominant crustal accretion model in any Archean craton is the key to understanding the dominant geodynamic process responsible for early crust formation during the Hadean (> 4.0 Ga) and Archaean (4.0–2.5 Ga). The continental crust has been proposed to have formed through either horizontal/vertical accretion related to subduction or mantle plume tectonic processes. Here, the Moho depths and average crustal Vp/Vs ratios are modelled at 16 broadband stations in the Eastern Indian Shield (EIS) through HK stacking of radial P-receiver functions (PRFs). These modelled parameters are used to test both plume and subduction models, which might have played a key role in the crustal accretion of the EIS throughout the Archean. We observe a correlation between crustal age and composition within the ellipsoidal Paleoarchean cratonic domain in the Singhbhum-Odisha-Craton (SOC), which reveals an increase in age from the younger granitoid core of the SOC (with thinning of felsic crust) to the surrounding older greenstone belts (with thickening of felsic crust). A thinner mafic crust resulting from multiple magmatic events characterizes the neighbouring Meso-Proterozoic Chotanagpur Granitic Gneissic terrain (CGGT). The Common Conversion Point (CCP) image of radial PRFs reveals northward subduction of the Paleoarchean SOC below the Meso-Proterozoic CGGT.

An ancient continental crustal source for Mo mineralisation in the eastern Central Asian Orogen: A case study of the Bilugangan Mo deposit

The Bilugangan porphyry Mo deposit is located in the southern Great Xing'an Range, and the Bilugangan area near the Hegenshan-Heihe Fault in the eastern part of the Central Asian Orogen (CAO). The deposit is hosted by a NE-trending, partly concealed, composite granite consisting of highly altered porphyritic granite (G1), porphyritic monzogranite (G2), fine- to medium-grained monzogranite (G3), medium -grained monzogranite (G4), and monzogranitic porphyry dykes (G5). The Mo mineralisation forms hydrothermal stockworks or disseminated sulphides in the G2 and G3 phases and the adjoining hornfels. Magmatic zircons from the intrusions yield 206Pb/238U weighted mean ages of 295 ± 2 Ma for G1, 240 ± 1 Ma for G2, 235 ± 2 Ma for G3, 230 ± 1 Ma for G4, and 100 ± 2 Ma for G5,indicating that multiple magmatic events have been active during the Early Permian to Early Cretaceous emplacement of the Bilugangan Granite. The age of the Mo is constrained between the Middle and Late Triassic and the magma-related hydrothermal activity proceeded in at least two pulses over a period of around 5 Ma. The first pulse has a molybdenite Re-Os weighted mean age of 238 ± 2 Ma, and the second dates at 236 ± 1 Ma. These dates are coeval with those of G2 and G3, indicating a genetic association between them and the mineralisation.The Sr-Nd-Hf isotopic systematics shows that the Bilugangan Granite was probably formed by partial melting of at least two sources. The first source is a dominant Late Mesoproterozoic to Early Carboniferous juvenile lower crust indicated by the whole rock εNd(t) values of −1.9 to 5.1 and zircon εHf(t) values of 3.5 to 14.3 with two-stage depleted mantle Nd (TDM2) and Hf (tDM2) model ages of ca. 1086 to 350 Ma, which is possibly related to the evolution of the Palaeo-Asian Ocean. The second source includes Neoarchaean to Palaeoproterozoic crustal components and a subordinate Mesoproterozoic crustal component. The Neoarchaean to Palaeoproterozoic component is indicated by the presence of ca. 2700 to 2600 Ma xenocrystic zircons, negative εHf(t) values of −16.4 to −10.1 with tDM2 ages of ca. 2300 to 1900 Ma. The Mesoproterozoic component yields εHf(t) values of −4.5 to 1.1 and tDM2 ages of ca. 1600 to 1200 Ma. These data show that pre- Mesoproterozoic substrate is present beneath the southern Great Xing'an Range. Although the post-mineralisation G4 and G5 phases have comparable positive εHf(t) values with the earlier granitic phases, the G2 and G3 phases have a more complex source with distinctly negative εHf(t) values of −16.4 to –1.3, indicating that the older crustal components in the magma’s source are also probable sources for the Mo mineralisation.

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.

Mesozoic multiple magmatic events in Bangor area: Constraints on the tectonic evolution of Bangong–Nujiang Tethys Ocean

The Bangor area is located in the middle part of the Bangong–Nujiang Suture Zone (BNSZ), where a large number of Middle Jurassic‐Early Cretaceous magmatic rocks are exposed, and is a key area for studying the tectonic evolution of the Bangong–Nujiang Tethys Ocean (BNO). However, the arc magmatic system triggered by the southward subduction of the BNO has not been systematically identified in the northern margin of the Lhasa Terrane, and the timing of initial subduction and closure is still controversial. In this study, four granite, two amphibolite, and one gabbro samples from the Bangor area yield two clusters intrusive ages of 162–168 Ma and 121–133 Ma. The Stages I and II granites show whole‐rock geochemistry similar to those of continent arc magmatism, characterized by enrichment in large ionic lithophile elements (LILEs; e.g., Th, Rb, K) and depletion in high field strength elements (HFSEs; e.g., Nb, Ta, P, Ti). The Stage I magmatic rocks consist of granodiorite, amphibolite, and gabbro and have similar zircon εHf(t) values of −10.1 to −3.5, which may be formed by partial melting of enriched mantle wedge related to the southward subduction of the BNO. The Stage II highly differentiated I‐type granites show positive zircon εHf(t) values of +1.5 to +9.9, which are considered the product of highly differentiated processes via the mixed magma derived from depleted mantle and lower crustal rocks. The Stage II I‐type granites show negative zircon εHf(t) values of −9.2 to −0.7, which are derived from a mixed magma source generated by partial melting of lower crust and enriched lithospheric mantle. On the basis of data from this study and previous research studies, a four‐stage tectonic evolutionary model of the Bangong–Nujiang Tethys Ocean is constructed: Stage 1 (190–145 Ma), the northward and southward subduction of BNO formed a series of arc magmatism; Stage 2 (145–135 Ma), a magmatism lull occurred on the north side due to the subduction effect of the ocean plateau, while the southward subduction still continued; Stage 3 (135–116 Ma), the roll‐back of subducted slab enhanced the magmatism in this stage; and Stage 4 (<116 Ma), the closure of BNO lead to the formation of syn‐collisional magmatism.

Timing of multiple magma events and duration of the hydrothermal system at the Yu’erya gold deposit, eastern Hebei Province, China: Constraints from U–Pb and Ar–Ar dating

The Yu’erya deposit is a large granite–hosted gold deposit in the eastern Hebei Province classified as an intrusion–related gold system. The Yu’erya complex contains the causative intrusion which was the source of hydrothermal metal-bearing fluids. This composite granite complex contains miarolitic pegmatite(–like) cavities and pegmatite veins. A combination of detailed field observations and U–Pb dating by SHRIMP and LA–ICP–MS is used to document multiple magmatic events which occurred at 210–220, 175–173, 171–170, 167–166 and ~161 Ma formed the Yu’erya granite complex. The initiation of the hydrothermal system, as determined via Ar–Ar dating, is closely related to the exsolution of a magmatic volatile phase from the G1 granite at ca.176 Ma. The hydrothermal system operated for ~16 my (176–160 Ma) and produced multiple mineralising events (~176, 172–168 and 162–160 Ma) linked to emplacement of repeated magma pulses from 175 Ma to 161 Ma. Our results suggest that multiple magmatic–hydrothermal events over a time period of several million years may be a fundamental factor responsible for the enrichment process during formation of intrusion-related gold deposits and thus may be a requirement for forming economic orebodies.

A clearer view of crustal evolution: U-Pb, Sm-Nd, and Lu-Hf isotope systematics in five detrital minerals unravel the tectonothermal history of northern China

AbstractMuch of the global picture of crustal evolution has been constructed using zircon. While this has revealed a rich and complex history, this view is necessarily incomplete because of the lithology-specific affinity of zircon and the high temperatures needed to reset the U-Pb and Lu-Hf systems inherent within it. Here we use a five mineral, multi-isotope system approach to compare the record of crustal evolution recorded by zircon versus the picture provided by monazite, titanite, apatite, and rutile from the Yong-Ding and Luan rivers, northern China. These other minerals sample more diverse lithologies and temperature-pressure conditions that reflect additional tectonothermal events to those recorded solely by zircon. Zircon from both studied rivers predominantly reflects magmatic features, yielding age peaks at 2.6–2.3, 2.0–1.8, and 0.38–0.13 Ga, corresponding to the major magmatic events in their catchments. However, the detrital zircon record from both catchments fails to record and detail several important tectonothermal events. Specifically, the detrital monazite U-Pb ages cluster into two Paleoproterozoic peaks of ca. 1.95 and 1.85 Ga, while detrital apatite and rutile ages document unimodal and protracted U-Pb age peaks at 1.9–1.6 Ga. The different U-Pb closure temperatures of monazite, apatite, and rutile likely record two metamorphic events and the subsequent cooling history—key details that are absent from or obscured in the zircon record. The Phanerozoic mineral U-Th-Pb ages correspond to multiple magmatic events between 0.40 and 0.24 Ga and subsequent 0.24–0.20 Ga metamorphism. The 0.60–0.25 Ga rutile U-Pb ages along with 0.33–0.24 Ga U-Pb ages in some zircon grains with radiogenic Hf isotope compositions from the Luan River do not match the geological records in the North China Craton, but instead reflect the protracted subduction-accretionary history of the Central Asian Orogenic Belt. In addition to their U-Th-Pb ages, Nd model ages of monazite, titanite, and apatite, plus zircon Hf model ages provide additional constraints on regional crustal evolution. The Nd model age information is blurred by the fact that the relationship between the Sm/Nd of these minerals and their former host rocks is not precisely known. Taken at face value, the monazite Nd model ages have two Neoarchean peaks at 2.9–2.7 and ca. 2.5 Ga, that may correspond to two crustal growth episodes, while the titanite Nd model ages with predominant peaks at 2.2–1.8 and 1.5–1.3 Ga broadly correspond with those derived from the whole-rock analyses of the wide spread Phanerozoic granitoids, and hence record extensive crustal reworking. In contrast, the zircon Hf model ages are strongly skewed to a 2.9–2.7 Ga period and fail to record the post-Archean evolution of this region. These data highlight the power of integrating the U-Th-Pb age and Lu-Hf/Sm-Nd isotope compositions of multiple detrital minerals, with a broad range in geochemical behavior and closure temperatures, to gain a more complete understanding of tectonothermal history and crustal evolution than zircon alone.

40Ar/39Ar geochronological evidence for multiple magmatic events during the emplacement of Tapira alkaline-carbonatite complex, Minas Gerais, Brazil

The Alto Parnaíba Igneous Province (APIP) is a voluminous magmatic province composed of various alkaline-carbonatite complexes emplaced in the Brasilia Mobile Belt during the Cretaceous. Relative timing of emplacement of silicate and carbonate magmas in most of these complexes remains mostly unresolved due to conflicting geochronological results. To determine the duration of magmatism and to test the possible existence of multiple magmatic events, we employ 40Ar/39Ar phlogopite single crystal dating to determine the history of magma emplacement at the Tapira alkaline-carbonatite complex, Minas Gerais, Brazil. The new single crystal data indicate at least two magmatic events during the emplacement of this complex, the first at > 96.2 ± 0.8 Ma and the second at 79.15 ± 0.6 Ma. The first igneous event was responsible for emplacement of the silicate plutonic series, while the second event corresponds to the emplacement of primarily carbonatitic magmas, generating metasomatic phlogopite alteration in bebedourites. The ages of intrusion and cooling of the alkaline-carbonatite complexes in the APIP must be investigated in other complexes to determine if intrusion intervals of ~17 Ma or more are common regionally. Protracted intrusive events, if related to magma generation by passage of South America over a stationary Trindade plume, requires complex ponding and lateral magma flow below a slow-moving continent.

Clastic wedge provenance in the Zemplinicum Carboniferous–Permian rocks using the U–Pb zircon age dating (Western Carpathians, Slovakia)

U–Pb (SHRIMP) detrital zircon ages from the Pennsylvanian–Permian meta-sedimentary rocks of the Zemplinicum Unit were used to characterise the provenance and the tectono-thermal evolution of the basement. The magmatic zircon ages from the contemporaneous rhyolite pyroclastics, ranging from 308 to 305 Ma, dated the Pennsylvanian sedimentary formations to the Moscovian and Kasimovian Ages. Two brakes in sedimentation within the Pennsylvanian–Permian sequence are presumed, first, flanked by Gzhelian–Asselian and second, intra-Permian. The detrital zircon age spectrum demonstrates two prominent populations: (i) Middle/Late Ordovician (age peak 459 Ma), (ii) Ediacaran–Cryogenian (age peaks 592 and 641 Ma). These, together with minor clusters from ~ 773 to 950 Ma, evidently document the Pan-African multiple magmatic events. The 1.1–1.8 Ga age gap and isolated zircons of Mesoproterozoic ages (1036–1361 Ma) are characteristic. Two populations, 1.8–2.2 Ga and 2.5–2.8 Ga, are presented within the Paleoproterozoic–Neoarchean zircons. The Zemplinicum Neoproterozoic arc crust had been affected by the extensional thermal relaxation and melting during Middle/Late Ordovician. The subsequent reworking had been connected with the Mississippian collision, followed by the Pennsylvanian/Permian extension. The presence of the Neoproterozoic detrital zircon ages including the Tonian ones permit to compare the Zemplinicum basement with the eastern peri-Gondwanan domain, which was situated at the northern margin of the Saharan Metacraton or the Arabian Nubian Shield during Neoproterozoic time.

Neoproterozoic and Early Paleozoic magmatic records from the Chalukou ore district, northern Great Xing’an Range, NE China: Implications for tectonic evolution and Mesozoic Mo mineralization

To constrain the pre-mineralization tectonic evolutionary history and the nature of the magma source of the Chalukou Mo deposit, which is located on the western margin of the Xing’an block, we recognized and studied a period of Neoproterozoic magmatism and a suite of Early Paleozoic volcanic rocks in this ore district. The magmatic zircons of quartz chlorite schist collected from the Wolegen Group yielded a concordant 206Pb/238U age of 685.1 ± 4.5 Ma (MSWD = 0.21), and they show very high εHf(t) values (+15.5 to +17.3) and young Hf modal ages (573–664 Ma) close to the formation age, indicating that the protolith was generated during the Neoproterozoic, and was most likely derived from a depleted mantle region. This Neoproterozoic magmatism might represent an important crustal growth event within the Xing’an Block. The host wallrocks in the Chalukou deposit mainly consist of rhyolite, dacite, andesite and volcanic tuff which have undergone slight deformation and metamorphism. The results of zircon LA-ICP-MS U-Pb dating indicated that they were generated during 472–475 Ma. The results of whole-rock geochemical analysis indicate that they have arc-affinity features characterized by LREE enrichment with moderate to strong fractionation between LREE and HREE (LaN/YbN = 7.7–13.6), intense negative Eu anomalies (δEu = 0.36–0.71), pronounced positive Ba, Th, U, Pb and La anomalies and negative Nb-Ta, P and Ti anomalies. Hf isotopes show various εHf (t) values ranging from −2.0 to +5.9 and slightly old two-stage model ages ranging from 1070 Ma to 1573 Ma. Combined with discrimination diagrams, these data indicate that they were derived from the partial melting of a slightly older lower crust with the addition of a juvenile crust component in an active continental margin setting related to the subduction of the Paleo-Asian Ocean between the Xing’an and Songliao blocks. The existence of Neoproterozoic magmatic activity and Mesoproterozoic lower crustal materials obtained from the Hf model ages of the Paleozoic volcanic rocks in the Chalukou ore district demonstrated that Precambrian basement exists in the Xing’an Block, but its distribution area may be limited. As the largest porphyry Mo deposit in China, the ore-forming materials might have multiple sources, considering the long-term and multiple magmatic events that occurred under different tectonic settings in the ore district. The previous modification and refertilization of the lower crust and/or lithospheric mantle by the subduction of the Paleo-Asian Ocean might be important in the generation of the fertile lower crust, which is the direct source of the ore-related porphyries.

Continental outbuilding along the margin of an Archean cratonic nucleus in the North China Craton

The history of crustal growth in the Early Earth, formation of cratonic nuclei and the building of large continents have important implications in understanding the secular evolution of our planet. The North China Craton (NCC) is envisaged in recent models as a collage of several Archean microblocks which were amalgamated along multiple zones of ocean closure during the late Neoarchean, and offers window to gain insights on the origin of cratonic nuclei and their subsequent outbuilding. Here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks, the Qianhuai Block. We present petrological, geochemical and zircon U–Pb geochronological and Lu-Hf isotopic data on a suite of rocks ranging from hornblendite through gabbro and dioritic gabbro to TTG (tonalite-trondhjemite-granodiorite) and granitoids exposed in the Miyun area, and representing the cross section of the lower to mid segments of a continental magmatic arc. Geochemically, the felsic units of the suite straddle from monzonite through granodiorite to granite with dominantly metaluminous affinity, magnesian composition and arc-related features. The metagranites and TTG gneisses are characterized by negative Nb-Ta anomalies and positive K and Pb anomalies. The diorite, gabbros and hornblendite display negative anomalies of Th-U, Nb-Ta and Zr-Hf and positive anomalies at Ba, Pb and Sm with negative Eu anomalies and minor positive Ce anomalies, attesting to arc-related origin. In the tectonic discrimination diagrams, the rocks plot in the volcanic arc field, indicating formation within a subduction-related setting.The oldest magmatic population with ages of ca. 2560 and 2529 Ma from the hornblendite and the TTG gneiss represent the magma emplacement ages with fractional crystallization of underplated basalts derived from slab melts, leaving hornblendite as the cumulate. Magmatic zircon grains from the metagranite and granodiorite show upper intercept ages of 2457 ± 21 Ma and 2475 ± 12 Ma whereas those from the TTG gneisses yield ages of 2529 ± 19 Ma and 2522 ± 38 Ma. Younger age groups of 2444.4 ± 8.3 Ma, 2418 ± 16 Ma and 2161 ± 31 Ma also occur in these rocks indicating multiple thermal events. The zircon grains from diorite and gabbros show 207Pb/206Pb weighted mean ages of 2411 ± 11 Ma–2486 ± 13 Ma with younger ages of 2.0–2.37 Ga. These rocks also carry older xenocrystic grains of ca. 2.51–2.65 Ga. The zircons in hornblendite show an upper intercept age of 2560 ± 33 Ma and a lower intercept age of 1947 ± 54 Ma. Young groups of ca. 2.2 Ga and 1.8 Ga are also found in this rock. The ca. 1.9 to 1.8 Ga ages correspond to the timing of metamorphism. The Hf isotopic data on zircons from the various rock types of the Miyun magmatic suite show both negative and positive εHf(t) values ranging from −9.64 to +7.04 with Hf model ages in the range of 2606–2844 Ma (TDM1) and 2635–3147 Ma (TDM2), suggesting crustal growth through both juvenile magmas from depleted mantle sources and reworked Meso- to Neoarchean crustal components. Our study reveals that the ca. 2.7–3.0 Ga core of the Qianhuai microcontinent represents one of the ancient cratonic nuclei in the NCC that underwent extensive continental outbuilding during late Neoarchean – early Paleoproterozoic through multiple magmatic events mostly related to convergent margin arc-type processes. It is possible that similar processes operated around the other Archean microcontinents in the NCC in the process of building the major crustal blocks which were finally amalgamated at the end of the Paleoproterozoic.

The geological record of base metal sulfides in the cratonic mantle: A microscale 187Os/188Os study of peridotite xenoliths from Somerset Island, Rae Craton (Canada)

We report detailed petrographic investigations along with 187Os/188Os data in Base Metal Sulfide (BMS) on four cratonic mantle xenoliths from Somerset Island (Rae Craton, Canada). The results shed light on the processes affecting the Re-Os systematics and provide time constraints on the formation and evolution of the cratonic lithospheric mantle beneath the Rae craton.When devoid of alteration, BMS grains mainly consist of pentlandite+pyrrhotite±chalcopyrite. The relatively high BMS modal abundance of the four investigated xenoliths cannot be reconciled with the residual nature of these peridotites, but requires addition of metasomatic BMS. This is especially evident in the two peridotites with the highest bulk Pd/Ir and Pd/Pt. Metasomatic BMS likely formed during melt/fluid percolation in the Sub Continental Lithospheric Mantle (SCLM) as well as during infiltration of the host kimberlite magma, when djerfisherite crystallized around older Fe-Ni-sulfides. On the whole-rock scale, kimberlite metasomatism is visible in a subset of bulk xenoliths, which defines a Re-Os errorchron that dates the host magma emplacement.The 187Os/188Os measured in the twenty analysed BMS grains vary from 0.1084 to >0.17 and it shows no systematic variation depending on the sulfide mineralogical assemblage. The largest range in 187Os/188Os is observed in BMS grains from the two xenoliths with the highest Pd/Ir, Pd/Pt, and sulfide modal abundance. The whole-rock TRD ages of these two samples underestimate the melting age obtained from BMS, demonstrating that bulk Re-Os model ages from peridotites with clear evidence of metasomatism should be treated with caution.The TRD ages determined in BMS grains are clustered around 2.8–2.7, ∼2.2 and ∼1.9Ga. The 2.8–2.7Ga TRD ages document the main SCLM building event in the Rae craton, which is likely related to the formation of the local greenstone belts in a continental rift setting. The Paleoproterozoic TRD ages can be explained by addition of metasomatic BMS during (i) major lithospheric rifting at ∼2.2Ga and (ii) the Taltson-Thelon orogeny at ∼1.9Ga. The data suggest that even metasomatic BMS can inherit 187Os/188Os from their original mantle source. The lack of isotopic equilibration, even at the micro-scale, allowed the preservation of different populations of BMS grains with distinct 187Os/188Os, providing age information on multiple magmatic events that affected the SCLM.

Post‐rift magmatic evolution of the eastern North American “passive‐aggressive” margin

AbstractUnderstanding the evolution of passive margins requires knowledge of temporal and chemical constraints on magmatism following the transition from supercontinent to rifting, to post‐rifting evolution. The Eastern North American Margin (ENAM) is an ideal study location as several magmatic pulses occurred in the 200 My following rifting. In particular, the Virginia‐West Virginia region of the ENAM has experienced two postrift magmatic pulses at ∼152 Ma and 47 Ma, and thus provides a unique opportunity to study the long‐term magmatic evolution of passive margins. Here we present a comprehensive set of geochemical data that includes new 40Ar/39Ar ages, major and trace‐element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. The Late Jurassic volcanics are bimodal, from basanites to phonolites, while the Eocene volcanics range from picrobasalt to rhyolite. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Sr‐Nd‐Pb systematics for the Late Jurassic event suggests HIMU and EMII components in the magma source that we interpret as upper mantle components rather than crustal interaction. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long‐term evolution of this margin, resulting in a “passive‐aggressive” margin that records multiple magmatic events long after rifting ended.

Nature and origin of the J‐magnetic anomaly offshore Iberia–Newfoundland: implications for plate reconstructions

AbstractThe nature and origin of the J‐magnetic anomaly along the Iberia–Newfoundland margins are controversial and its validity for plate kinematic reconstructions questioned. At present, it is interpreted as either an oceanic isochron or an edge effect of oceanic crust corresponding to lithosphere breakup. Both interpretations result in restorations that are in conflict with the current knowledge from Pyrenean and North Atlantic geology. We combine seismic interpretations and dating of magmatic additions with magnetic data to examine the nature and formation process of this anomaly and discuss its value for plate restorations. We show that the J‐anomaly is the result of polygenic and multiple magmatic events occurring during and after the formation of the first oceanic crust. Therefore, we conclude that the J‐anomaly cannot be used for plate kinematic studies and, more generally, we question the validity of using ill‐defined magnetic anomalies outside unequivocal oceanic domains for plate reconstructions.

Seismostratigraphy of the Ceará Plateau: Clues to Decipher the Cenozoic Evolution of Brazilian Equatorial Margin

The Ceara Plateau offshore Fortaleza holds some particular characteristics when compared to the other seamounts of the Brazilian Equatorial Margin (BEM). Not only it is the largest and the closest to the continent, it is also located at the boundary between the continental and the oceanic crusts, while all the others seamounts along the BEM are located on oceanic crust. Seismic imaging of the Ceara Plateau shows a “disorganized” interior, probably of volcanic origin, overlain by a series of horizontal seismic reflectors that can be interpreted as pelagic/hemipelagic sediments. As large uncertainties exist about the age of the initial formation of this seamount, three scenarios must be considered. If the age of the volcanic edifice is Coniacian (1), then the overlying pelagic/hemipelagic sedimentary succession can include an almost continuous record of the last ~90 Ma at the Equatorial Atlantic Ocean. In the case that the volcanic edifice is Eocene in age (2), the sedimentary sequence would still encompass the upper Paleogene and all the Neogene. There is also the possibility that the volcanic edifice was built during multiple magmatic events (3). In this case, it is likely that the sediments are interfingered with volcanic rocks at the edge of the structure. Although the age estimation (between Coniacian and Eocene) has an uncertainty of more than 40 Myr, the current interpretation is that it developed initially as a volcanic edifice, formed by a series of magmatic events that occurred between the Santonian and the Eocene. Since then, the topography has been leveled by pelagic/hemipelagic sedimentation. Whichever was the initial age, a continuous and constant sequence of sediments deposited onto the Ceara Plateau, at the same latitude, and thus under the same oceanographic conditions, for the last several tens of million years. This represents a unique opportunity to record a long-term history of the Atlantic Equatorial Margin.

Seismic interpretation, attribute analysis, and illumination study for targets below a volcanic-sedimentary succession, Santos Basin, offshore Brazil

During the past decade, a significant exploration effort into deeper water and deeper targets in offshore areas brought more knowledge about the distribution of igneous rocks in the sedimentary basins along many continental margins. Nonhomogeneous illumination effects may occur below shallow, high-impedance igneous rock bodies. The seismic processing, depth imaging, interpretation, and attribute analysis require a special attention when these magmatic bodies affect the illumination of deeper targets. Usually, those structures are not considered in illumination studies, and the salt diapirs govern the analysis. In this work, seismic attributes are quite relevant to constrain the geologic model used to simulate the amplitude maps of the deeper target and to quantify the shadow effects observed on it. These amplitude shadows may either create (false) or hide (true) attribute anomalies. We have modeled a Tertiary volcano-sedimentary succession (VSS) mapped in the northeastern Santos Basin, offshore Brazil, to simulate the effect in the amplitude response of deeper targets. There were multiple magmatic events through the Santos Basin Cretaceous-Tertiary sedimentary succession, intrusive and extrusive. The igneous extrusive rocks intercalated with sediments forming VSS marked by low- and high-amplitude responses with tough lateral discontinuity. The amplitude found well-preserved architectural elements interpreted as lava flows and volcanoes, contrasting with layered sediments. We have defined geobodies constrained by relative impedance and 3D edge detection to build the 3D geologic model of the igneous successions used for seismic simulation. From the geobodies, we modeled two VSS to run the P-waves ray-tracing propagation to simulate the migrated amplitude map of the deeper Lower Albian sequence top. We computed the P-velocity and the density from well logs, and the seismic acquisition geometry was similar to the original. Comparison between real and simulated amplitudes showed the impact of shadow zones caused by shallower igneous bodies.

Crustal velocity structure of the Shaowu-Nanping-Pingtan transect through Fujian from deep seismic sounding-tectonic implications

The Shaowu-Nanping-Pingtan deep seismic sounding profile is located in northern Fujian Province. High-quality seismic sounding data were acquired by five large explosive blasts received by 133 digital seismic instruments along the profile. Based on seismic facies analysis and travel-time picking on shot record sections, a model of the velocity structure of upper crust was developed by finite-difference tomography of the first breaks; the 2-D P-wave velocity structure and tectonic characteristics of the crust were interpreted further by fitting of waveforms and seismic travel times. The results show that the top of the crystalline basement is buried at depths of 2.0–4.0 km, with the deepest buried up to 4.0 km within the Fuzhou Basin. The Moho interface was found to be deeper in the west and shallower in the east (i.e., 30.0 km near the coast, increasing to 33.0 km northwestward). The lower crust on the east side of the Zhenghe-Haifeng Fault Zone has a smoothly varying gradient structure, whereas on the west side it has two distinct layers with a boundary between those layers at a depth of 23 km. Seismic velocities on the west side are generally lower than on the east side; a low velocity layer is observed with a lowest speed of 6.25 km/s at a depth of 22 km on the west side, which may consist of partially molten material. The Zhenghe-Haifeng Fault is a deep crustal fault, and should be a channel for deep material upwelling; it has a direct relationship with multiple stages of continental tectonic movements in Southern China and with multiple magmatic events that started in the Proterozoic and ended in the of late Tertiary in Fujian.