Multiple Magmatic Events Research Articles

Os–Nd–Sr isotopes in Miocene ultrapotassic rocks of southern Tibet: Partial melting of a pyroxenite-bearing lithospheric mantle?

Miocene post-collisional ultrapotassic rocks in the southern and central parts of the Lhasa Terrane of southern Tibet provide an opportunity to explore the deep processes and lithospheric evolution of the Tibetan Plateau. The magmatic source of the ultrapotassic rocks is still debated. However, the source can be identified using the Re–Os isotopic system. In this paper, we provide comprehensive data on the Re–Os isotopic compositions of ultrapotassic rocks from Mibale and Maiga areas in southern Tibet, and we refine the age of the Mibale ultrapotassic rocks to 12.5Ma. The Os isotopic data demonstrate that crustal assimilation affected the Os isotopic compositions of some ultrapotassic rocks with low Os contents, but samples with high Os contents have little or no evidence of crustal contamination. The initial 187Os/188Os ratios of the least-contaminated ultrapotassic rocks are higher than those of primitive upper mantle (PUM). The ultrapotassic rocks show a weak correlation between initial 187Os/188Os ratios and Mg# values, a negative correlation between εNd(t) and Mg# values, and high Ni contents and FeO/MnO ratios. These observations indicate that the ultrapotassic rocks were derived from a pyroxenite-bearing lithospheric mantle. Simple calculations indicate <20% pyroxenite in the lithospheric mantle, which is consistent with the pyroxenite xenoliths found in the ultrapotassic rocks of southern Tibet. The Os model ages for the ultrapotassic rocks in the south Lhasa Terrane range from 75 to 541Ma, indicating that the lithospheric mantle beneath southern Tibet underwent multiple magmatic events. We conclude, therefore, that convective removal of a pyroxenite-bearing lithospheric mantle or break-off of the Indian continental lithospheric mantle could have resulted in the generation of the ultrapotassic rocks in southern Tibet.

Petrology and geochemistry of the Tertiary Suez rift volcanism, Sinai, Egypt

The Tertiary basaltic rocks of Southwestern Sinai, situated along the Wadi Nukhul–Wadi Matullah–Wadi El Tayiba join, were selected to study the Gulf of Suez rift related-lavas and their geochemical and petrological relation with the rifting process. Whole rock samples were studied petrographically and analysed for major and trace elements. The samples from dykes, sills and flows from multiple magmatic events display a large variety in texture and in modal mineral compositions. They range from olivine dolerites and olivine-bearing basalts to vitrophyric, texturally heterogeneous basalts and crystal lithic tuffs. The transitional tholeiitic basalts display low compatible element concentrations and an enrichment of the whole spectrum of the incompatible elements. Major, trace and Rare Earth Element data suggest that the melts formed by 5% melting of mantle peridotite at the spinel–garnet transition zone (80–90km depth), in the presence of 2–4% residual garnet. During the melt ascent, the fractionating phases were olivine, clinopyroxene and, to a lesser extent, plagioclase. Thermobarometric calculations indicate the presence of two crystallization levels beneath the Gulf of Suez rift: a shallower stage at 15–20km and a deeper stage at depths of 25–30km. The mantle source consists of streaks and blobs of enriched mantle, preserved in the geochemical signatures of these rocks. The enriched mantle sources melted preferentially compared to the surrounding ambient mantle and thus led to a preferential enrichment of the sources of the Gulf of Suez rift.

Characterization and tectonic implications of Mesozoic-Cenozoic oceanic assemblages of Costa Rica and Western Panama

We present a new model to explain the origin, emplacement and stratigraphy of the Nicoya Complex in the NW part of the Nicoya Peninsula (Costa Rica) based on twenty-five years of field work, accompanied with the evolution of geochemical, vulcanological, petrological, sedimentological and paleontological paradigms. The igneous-sedimentary relation, together with radiolarian biochronology of the NW-Nicoya Peninsula is re-examined. We interpret the Nicoya Complex as a cross-section of a fragment of the Late Cretaceous Caribbean Plateau, in which the deepest levels are exposed in the NW-Nicoya Peninsula. Over 50% of the igneous rocks are intrusive (gabbros and in less proportion plagiogranites) which have a single mantle source; the remainder are basalts with a similar geochemical signature. Ar39/Ar40 radioisotopic whole rock and plagioclase ages range throughout the area from 84 to 83 Ma (Santonian) for the intrusives, and from 139 to 88 Ma (Berriasian-Turonian) for the basalts. In contrast, Mn-radiolarites that crop out in the area are older in age, Bajocian (Middle Jurassic) to Albian (middle Cretaceous). These Mn-radiolaritic blocks are set in a “matrix” of multiple gabbros and diabases intrusions. Chilled margins of magmatites, and hydrothermal baking and leaching of the radiolarites confirm the Ar39/Ar40 dating of igneous rocks being consistently younger than most of the radiolarian cherts. No Jurassic magmatic basement has been identified on the Nicoya Peninsula. We interpret the Jurassic-Cretaceous chert sediment pile to have been disrupted and detached from its original basement by multiple magmatic events that occurred during the formation of the Caribbean Plateau. Coniacian-Santonian (Late Cretaceous), Fe-rich radiolarites are largely synchronous and associated with late phases of the Plateau.

Emplacement of Jurassic-Lower Cretaceous radiolarites of the Nicoya Complex (Costa Rica)

We present a new model to explain the origin, emplacement and stratigraphy of the Nicoya Complex in the NW part of the Nicoya Peninsula (Costa Rica) based on twenty-five years of field work, accompanied with the evolution of geochemical, vulcanological, petrological, sedimentological and paleontological paradigms. The igneous-sedimentary relation, together with radiolarian biochronology of the NW-Nicoya Peninsula is re-examined. We interpret the Nicoya Complex as a cross-section of a fragment of the Late Cretaceous Caribbean Plateau, in which the deepest levels are exposed in the NW-Nicoya Peninsula. Over 50% of the igneous rocks are intrusive (gabbros and in less proportion plagiogranites) which have a single mantle source; the remainder are basalts with a similar geochemical signature. Ar 39 /Ar 40 radioisotopic whole rock and plagioclase ages range throughout the area from 84 to 83 Ma (Santonian) for the intrusives, and from 139 to 88 Ma (Berriasian-Turonian) for the basalts. In contrast, Mn-radiolarites that crop out in the area are older in age, Bajocian (Middle Jurassic) to Albian (middle Cretaceous). These Mn-radiolaritic blocks are set in a “matrix” of multiple gabbros and diabases intrusions. Chilled margins of magmatites, and hydrothermal baking and leaching of the radiolarites confirm the Ar 39 /Ar 40 dating of igneous rocks being consistently younger than most of the radiolarian cherts. No Jurassic magmatic basement has been identified on the Nicoya Peninsula. We interpret the JurassicCretaceous chert sediment pile to have been disrupted and detached from its original basement by multiple magmatic events that occurred during the formation of the Caribbean Plateau. Coniacian-Santonian (Late Cretaceous), Fe-rich radiolarites are largely synchronous and associated with late phases of the Plateau.

The recognition of multiple magmatic events and pre-existing deformation zones in metamorphic rocks as illustrated by CL signatures and numerical modelling: examples from the Ballachulish contact aureole, Scotland

The combination of cathodoluminescence (CL) analysis, temperature and temperature–time calculations, and microstructural numerical modelling offers the possibility to derive the time-resolved evolution of a metamorphic rock. This combination of techniques is applied to a natural laboratory, namely the Ballachulish contact aureole, Scotland. Analysis of the Appin Quartzite reveals that the aureole was produced by two distinct magmatic events and infiltrated by associated fluids. Developing microstructures allow us to divide the aureole into three distinct regions. Region A (0–400 m, 663°C < T max < 714°C) exhibits a three-stage grain boundary migration (GBM) evolution associated with heating, fluid I and fluid II. GBM in region B (400–700 m, 630°C < T max 700 m of contact, T max < 630°C) is characterised by healed intragranular cracks. The combination of CL signature analysis and numerical modelling enables us to recognise whether grain size increase occurred mainly by surface energy-driven grain growth (GG) or strain-induced grain boundary migration (SIGBM). GG and SIGBM result in either straight bands strongly associated with present-day boundaries or highly curved irregular bands that often fill entire grains, respectively. At a temperature of ~620°C, evidence for GBM is observed in the initially dry, largely undeformed quartzite samples. At this temperature, evidence for GG is sparse, whereas at ~663°C, CL signatures typical for GG are commonplace. The grain boundary network approached energy equilibrium in samples that were at least 5 ka above 620°C.

Complex magmatic and impact history prior to 4.1 Ga recorded in zircon from Apollo 17 South Massif aphanitic breccia 73235

Sample 73235 is one of several aphanitic impact melt breccias collected by the Apollo 17 mission at stations 2 and 3 on the slopes of the South Massif. This study presents a detailed investigation of internal structures and U-Pb ages of large zircon grains from this breccia sample. New data combined with the results of previous studies of zircon grains from the same location indicate that most zircon clasts in breccias from stations 2 and 3 formed during multiple magmatic events between 4.37 and 4.31 Ga, although the oldest zircon crystallized at about 4.42 Ga and the youngest at 4.21 Ga. In addition, zircons from the aphanitic breccias record several impact events prior to the ∼3.9 Ga Late Heavy Bombardment. The results indicate that the zircons probably crystallized at different locations within the Procellarum KREEEP Terrane and were later excavated and modified by several impacts and delivered to the same locality within separate ejecta blankets. This locality became a source of material that formed the aphanitic impact melt breccias of the South Massif during a ∼3.9 Ga impact. However, the zircons, showing old impact features, are not modified by this ∼3.9 Ga impact event suggesting that (i) this common source area was located at the periphery of excavation cavity, and (ii) the > 3.9 Ga ages recorded by the zircon grains could date large (basin-forming) events as significant as major later (∼3.9 Ga) collisions such as Imbrium and Serenitatis.

Magmatic evolution of the ultramafic–mafic Kharaelakh intrusion (Siberian Craton, Russia): insights from trace-element, U–Pb and Hf-isotope data on zircon

The ultramafic–mafic Kharaelakh intrusion in the northwestern part of the Siberian Craton (Russia) hosts major economic platinum-group-element (PGE)–Cu–Ni sulphide deposits. In situ U–Pb, REE and Hf-isotope analyses of zircon from these rocks, combined with detailed study of crystal morphology and internal structure, identify four zircon populations. U–Pb ages of these populations cover a significant time span (from 347 ± 16 to 235.7 ± 6.1 Ma) suggesting multiple magmatic events that cluster around 350 and 250 Ma, being consistent with two recognised stages of active tectonism in the development of the Siberian Craton. The oldest zircon population, however, represents previously unknown stage of magmatic activity in the Noril’sk area. Epsilon-Hf values of +2.3 to +16.3 in the analysed zircons reflect a dominant role of mantle-derived magmas and suggest that juvenile mantle material was the main source for the ultramafic–mafic Kharaelakh intrusion. A significant range in initial 176Hf/177Hf values, found in zircons that cluster around 250 Ma, indicate mixing between mantle and crustal magma sources. Our findings imply that economic intrusions hosting PGE–Cu–Ni deposits of the Noril’sk area have a far more complex geological history than is commonly assumed.

Chemical fingerprinting of multiple large-scale magmatic events in the Mesoproterozoic Bangemall Supergroup, Western Australia

Three major igneous events, dated at ∼1465, ∼1070 and ∼500 Ma, are represented in the Proterozoic of central Western Australia, yet their extent is poorly understood. The compositions of dated mafic rocks from the western Bangemall Supergroup of Western Australia have been used to establish a chemical fingerprint for the ∼1465 Ma and ∼1070 Ma intrusive events, and to assign sills of an unknown age to one of the two events. A similar approach has been used to identify the extent of ∼500 Ma magmatism. Low-pressure fractionation or accumulation has exerted a strong influence on the chemistry of rocks from all magmatic events, but distinctive trace-element ratios (e.g. Th/Nb, Nb/Zr) and rare-earth element (REE) chemistry (e.g. Eu/Eu*, (Gd/Yb)CN) can discriminate different events. These ratios remain constant regardless of the degree of fractionation or accumulation, and reflect the chemistry of the respective mantle sources. Based on chemistry, ∼500 Ma igneous rocks are not found in the Bangemall Supergroup. Neodymium model ages for ∼1465 Ma sills overlap with crystallisation ages of subduction-related felsic intrusive rocks from the adjacent Gascoyne Complex, and this, combined with trace-element and REE chemistry, suggests that the mantle source for these sills underwent ∼5% crustal contamination approximately 450 Ma prior to melting in a subduction zone environment. Unrealistically large amounts of contaminant are required to explain the chemistry of most ∼1070 Ma sills, and their chemistry is better explained by melting of a heterogeneous mantle source, consistent with the overlap of Nd model ages and crystallisation ages. However, the relatively low ϵNd(T) and high 87Sr/86Sr(T), elevated light REE, Rb and Zr concentrations, and high Th/Nb of some ∼1070 Ma eastern Bangemall Supergroup sills are indicative of crustal contamination. These sills are relatively depleted in chalcophile elements and platinum-group elements, consistent with coeval precipitation of sulfides and crustal contamination. The overlap in the maximum depositional age of host-rocks with the crystallisation age of some sills, the confining of most ∼1465 Ma sills to older parts of the stratigraphy, and field evidence showing that some sills belonging to both the ∼1465 and ∼1070 Ma events were intruded into wet sediments, indicate a close temporal relationship between sedimentation and sill intrusion.

Geology and petrology of the plutonic complexes in the Wadi Fizh area: Multiple magmatic events and segment structure in the northern Oman ophiolite

Multiple magmatic events are recorded in the gabbroic unit in the Fizh area of the northern Oman ophiolite. Gabbroic blocks intruded by sheeted dike complex and upper gabbros of the main crustal sequence show the oldest event. Gabbronorite sills in the gabbroic blocks are nearly coeval with the host gabbro. Wehrlitic intrusions (wehrlite I) mark the third event of magmatism. These three magmatic events occurred at the retreating (dying) ridge axis because all these rocks are intruded by dolerite dike swarm, which is generally regarded as a precursor of advancing ridge axis. The next stage of magmatism is a main phase of oceanic crust generation in this area. Wehrlite II and then gabbronorite dikes intrude the still hot main gabbro unit. All of these above rocks have similar signatures with respect to clinopyroxene compositions and covariations between plagioclase and mafic minerals, though slight differences are present in the compositional ranges and clinopyroxene compositions of each unit. After considerable cooling of the main gabbro unit, primitive basalt dikes intrude the main gabbro unit, which may correspond to the Lasail unit. Finally, the Fizh‐South complex intrudes into considerably cooled crustal sequence, being below the brittle‐plastic transition temperatures. The Fizh‐South complex, which was regarded as a common wehrlitic intrusion, is significantly different from all of the above mentioned rocks, with respect to the covariation between plagioclase and associating mafic minerals, crystallization order, and clinopyroxene compositions. The clinopyroxenes are characterized by extremely low Ti and Na contents, comparable with those of the V2 unit (Alley volcanics), suggesting that the Fizh‐South complex correlates with the plutonic facies of the V2 unit during arc stage. Layered gabbros in the Wadi Zabin area, about 10 km north of the Fizh area, may be a northern extension of the gabbro blocks of the Fizh area, because they are intruded by numerous dolerite dikes. On the other hand, basin‐like structure of the main gabbroic unit in the northern end of the Fizh area may demonstrate a fossilized magma chamber beneath the advancing ridge axis due to the ceasing of magmatic crustal accretion. On the basis of these lines of evidence, we propose that the Fizh area indicates the northward propagation tip of the ridge axis.

The Bholghati howardite: Chemical study

Bholghati is homogeneous on a cm scale in major element composition, but heterogeneous in trace elements such as the rare earth elements (REEs). The REEs are concentrated in accessory phosphates, and their sampling can influence the REE patterns of eucritic clasts which, in turn, can govern the bulk REE patterns. Bholghati can be reasonably modeled with 56% eucritic, 45% diogenitic, and 3% dark clast components; however, an additional component (feldspathic) is required to match the bulk composition. Dark clasts represent carbonaceous CM type material that was admixed with other components during regolith formation on the eucrite parent body (EPB). One eucritic clast (fine-grained) is light REE (LREE) depleted relative to heavy REE (HREE), which is unusual in normal eucrites. The LREE depletion pattern cannot be modeled from one-stage partial melting of a chondritic source; however, it can be modeled from a non-chondritic source depleted in LREEs or multi-stage melting of a chondritic source, provided that the LREE depletion is not from undersampling of phosphates. Bholghati has a complex history that involved early multiple magmatic events, later annealing, and fragmentation and low-temperature mixing of eucritic, diogenitic, and carbonaceous components on the howardite parent body.

The Bholghati (howardite) consortium: An overview

The Bholghati (howardite) has had a complex history. The consortium studies indicate that eucrite clasts show evidence of rapid crystallization followed by prolonged subsolidus annealing. Dark clasts are carbonaceous CM2 type. Bholghati bulk composition can be modeled by 55% eucritic, 45% diogenitic, and 3% dark clast components. The eucritic clasts show a LREE depleted pattern relative to HREEs, which is not typical of a normal eucrite. The LREE depletion requires two-stage melting from a chondritic source. The volatile/mobile trace element patterns in dark and eucritic clasts are highly variable, probably due to volatile redistribution. The eucritic clast shows excess fission Xe, which is attributed to in situ decay of Pu-244. The cosmic-ray exposure age is 10–17 Ma. The Rb-Sr, Sm-Nd, and K-Ar ages of Bholghati bulk and eucritic clasts indicate 4.53 Ga as crystallization age, 2–3 Ga as metamorphic event, and probably a minor thermal event < 1 Ga. The Bholghati evolution scenario is 1) early multiple magmatic events (4.53 Ga ago), producing eucrites and diogenites; 2) a metamorphic event (2–3 Ga ago) and prolonged subsolidus annealing; 3) fragmentation and low-temperature mixing of eucrites and diogenites; 4) low-velocity impact (< 1 Ga. ago) and admixing of carbonaceous material; 5) disruption of regolith and ejection of Bholghati 10–17 Ma ago; and 6) Bholghati fell on the Earth in 1905.