Neodymium–strontium–lead isotopic study of Vancouver Island igneous rocks

1991 ◽  
Vol 28 (11) ◽  
pp. 1744-1752 ◽  
Author(s):  
A. Andrew ◽  
R. L. Armstrong ◽  
D. Runkle
Keyword(s):  
Mantle Source ◽  
Large Scale ◽  
Crustal Contamination ◽  
Vancouver Island ◽  
Isotopic Signature ◽  
Late Eocene ◽  
Crustal Material ◽  
Isotopic Data ◽  
Isotopic Study ◽  
Depleted Mantle

Combined neodymium, strontium, and lead isotope measurements show that Vancouver Island is made up of Phanerozoic crustal material accreted to North America in the Mesozoic and early Cenozoic, but that there are differences in the relative proportions of depleted mantle and aged, enriched crustal components in the Phanerozoic magmatic episodes that contribute to this new crust.The Devonian Sicker Group volcanic arc has an isotopic signature that can be explained by mixing mantle material with subducted continentally derived sediments. The Early to Middle Jurassic Bonanza Volcanics and Island Intrusions magmatic arc isotopic signature indicates mixing of magma from a depleted mantle source with crustal material of Sicker arc-type, rather than of continental origin. This is consistent with large-scale assimilation of Sicker Group and Karmutsen rocks by Jurassic mantle-derived magmas, or introduction of arc-derived sediments into the Jurassic mantle by subduction. Eocene calc-alkaline Flores Volcanics – Catface Intrusions may be derived from reworked Vancouver Island crust with little addition of mantle material.Late Triassic Karmutsen Formation flood basalts are similar to the lower parts of the Columbia River Basalt in all three isotope systems and in petrochemistry. Radiogenic isotopic data are consistent with the interpretation that the Karmutsen basalts were extruded in a post-arc or back-arc setting, with mantle lithosphere and depleted mantle components, and perhaps some plume source input and crustal contamination, but the latter are not provable from the radiogenic isotopic data alone.Early Eocene Metchosin basalts show a depleted mantle source, consistent with their origin as ocean islands, before Middle to Late Eocene accretion to the rest of Vancouver Island.

Postcollisional transition from subduction- to intraplate-type magmatism in the eastern Sakarya zone, Turkey: Indicators of northern Neotethyan slab breakoff

2019 ◽  
Vol 131 (9-10) ◽  
pp. 1623-1642 ◽  
Author(s):  
Abdurrahman Dokuz ◽  
Faruk Aydin ◽  
Orhan Karslı
Keyword(s):  
Mantle Source ◽  
Middle Eocene ◽  
Tectonic Setting ◽  
Crustal Contamination ◽  
Geochemical Evolution ◽  
Crustal Material ◽  
Depleted Mantle ◽  
Slab Breakoff ◽  
Sakarya Zone ◽  
Oceanic Slab

Abstract Postcollisional magmatism in the eastern Sakarya zone was recorded by voluminous basic volcanism and repeated plutonism during the early Cenozoic. The temporal and geochemical evolution of these magmatic rocks is important for understanding the possible geodynamic history of the Sakarya zone. Here, we investigated three representative plutons lying between the towns of Çamlıhemşin (Rize) and İspir (Erzurum), Turkey. These are largely composed of medium-K gabbroic diorites (Marselavat Pluton), shoshonitic monzonites (Güllübağ Pluton), and high-K granites (Ayder Pluton). We present whole-rock geochemistry, 40Ar/39Ar geochronology, and Sr, Nd, and Pb isotope analyses from the plutons to constrain the timing of variations in magmatism and source characteristics, and we provide a new approach to the proposed geodynamic models, which are still heavily debated. The 40Ar/39Ar geochronology reveals a cooling sequence from ca. 45 Ma for the Marselavat Pluton through ca. 41 Ma for the Güllübağ Pluton to ca. 40 Ma for the Ayder Pluton. Whole-rock geochemistry and Sr, Nd, Pb isotopes suggest that crustal contamination was not an important factor affecting magma compositions. Although there was no arc-related tectonic setting in the region during the middle Eocene, the Marselavat Pluton shows some subduction affinities, such as moderately negative Nb and Ta anomalies, and slightly positive Pb anomalies. These signatures were possibly inherited from a depleted mantle source that was modified by hydrous fluids released from the oceanic slab during Late Cretaceous subduction. Geochemical traces of the earlier subduction become uncertain in the Güllübağ samples. They display ocean-island basalt–like multi-element profiles and Nb/Ta, Ce/Pb, and La/Ba ratios. All these point to a mantle source in which earlier subduction signatures were hybridized by the addition of asthenospheric melts. Melting of calc-alkaline crustal material, probably emplaced during the first phase of middle Eocene magmatism (Marselavat), led to the formation of granitic plutonism (Ayder Pluton). Our data in conjunction with early Eocene adakite-like rocks show that melt generation, as in the given sequence, was most probably triggered by breakoff of the northern Neotethyan oceanic slab, ∼13 m.y. after the early Maastrichtian collision between the Sakarya zone and Anatolide-Tauride block, and continued until the end of the middle Eocene. A shallow-marine transgression occurred contemporaneously with the middle Eocene magmatism throughout the Sakarya zone. An extension in this magnitude seems unlikely to be the result of orogenic collapse processes only. The main cause of this extension was most probably related to the northward subduction of the southern Neotethys Ocean beneath the Anatolide-Tauride block. The result is a volumetrically larger amount of middle Eocene magmatism than that expected in response to slab breakoff.

The age of the Ritscherflya Supergroup and Borgmassivet Intrusions, Dronning Maud Land, Antarctica

1995 ◽  
Vol 7 (1) ◽  
pp. 87-97 ◽  
Author(s):  
A.B. Moyes ◽  
J.R. Krynauw ◽  
J.M. Barton
Keyword(s):  
Mantle Source ◽  
Crustal Contamination ◽  
Unconsolidated Sediments ◽  
Rock Types ◽  
Geological Age ◽  
Depleted Mantle ◽  
Wide Range ◽  
Dronning Maud Land ◽  
Depositional Age ◽  
Nd Model Age

The Ahlmannryggen-Borgmassivet area of western Dronning Maud Land comprises a relatively undeformed, unmetamorphosed sequence of sedimentary-volcanogenic rocks, the Ritscherflya Supergroup, intruded by a suite of continental tholeiites, the Borgmassivet Intrusions. New Rb-Sr and Sm-Nd whole rock data from the Högfonna Formation at Grunehogna indicate a depositional age of ≈1080 Ma, the first reported direct dating of any member of the Ritscherflya Supergroup. These rocks are interpreted as a molasse-type deposit following the Kibaran orogeny at 1200–1100 Ma, and correlation is made with the Umkondo and Koras groups of southern Africa. The Ritscherflya Supergroup is intruded by the Grunehogna and Kullen sills; the ≈1000 Ma Grunehogna sill intruded unconsolidated sediments, causing partial melting of the sediments. Rb-Sr data from the Kullen sill yield an age of 1429 Ma, clearly inconsistent with these data. Combined Sr and Nd data are compatible with crustal contamination of this sill, producing a Rb-Sr pseudo-isochron with no geological age significance. By comparison with other outcrops of the Borgmassivet Intrusions at Robertskollen and Annandagstoppane, it is concluded that contamination and pseudo-isochrons may be responsible for the wide range in reported ages older than 1000 Ma. Thus the intrusive age of the Borgmassivet Intrusions is concluded to be ≈1000 Ma old. Nd model age data indicate that all rock types were ultimately derived from material separated from a depleted mantle source at ≈2200 Ma.

U–Pb and Hf isotopic study of detrital zircons from the Lüliang khondalite, North China Craton, and their tectonic implications

2009 ◽  
Vol 146 (5) ◽  
pp. 701-716 ◽  
Author(s):  
XIAOPING XIA ◽  
MIN SUN ◽  
GUOCHUN ZHAO ◽  
FUYUAN WU ◽  
LIEWEN XIE
Keyword(s):  
North China Craton ◽  
North China ◽  
Detrital Zircons ◽  
Crustal Material ◽  
Isotopic Data ◽  
Isotopic Study ◽  
Metasedimentary Rocks ◽  
The North ◽  
Wide Range ◽  
Western Block

AbstractTwo types of metasedimentary rocks occur in the Trans-North China Orogen of the North China Craton. One type consists of highly metamorphosed supracrustal rocks with protoliths of mature cratonic shale, called khondalites, as found in the Lüliang Complex; rocks of the other type are also highly metamorphosed but less mature, as represented by the Wanzi supracrustal assemblage in the Fuping Complex. U–Pb isotopic data for detrital zircons from khondalites show a provenance dominated by 1.9–2.1 Ga Palaeoproterozoic rocks. These detrital zircons display a wide range of εHfvalues from −16.0 to +9.2 and give Hf isotopic model ages mostly around 2.3 Ga. The high positive εHfvalues approach those for the depleted mantle at 2.1 Ga, highlighting a juvenile crustal growth event in Palaeoproterozoic times. Hf isotopic data also imply thatc.2.6 Ga old crustal material was involved in the Palaeoproterozoic magmatic event. These data are similar to those for the khondalitic rocks from the interior of the Western Block of the North China Craton, suggesting a common provenance. In contrast, other metasedimentary rocks in the Trans-North China Orogen, such as the Wanzi supracrustal assemblage in the Fuping Complex, have a source region with both Palaeoproterozoic and Archaean rocks. Their detrital zircon Hf isotopic data indicate reworking of old crustal material and a lack of significant juvenile Palaeoproterozoic magmatic input. These rocks are similar to the coevally deposited meta-sedimentary rocks in the interior of the Eastern Block. We propose that the Lüliang khondalites were deposited on the eastern margin of the Western Block in a passive continental margin environment and were thrust eastward later during collision with the Eastern Block. Other metasedimentary rocks in the Trans-North China Orogen were deposited on the western margin of the Eastern Block in a continental arc environment. Our data support the eastward subduction model for the Palaeoproterozoic tectonic evolution of the North China Craton.

Aphebian mafic–ultramafic magmatism in the Labrador Trough (New Quebec): its age and the nature of its mantle source

1993 ◽  
Vol 30 (8) ◽  
pp. 1582-1593 ◽  
Author(s):  
M. -L. Rohon ◽  
Y. Vialette ◽  
T. Clark ◽  
G. Roger ◽  
D. Ohnenstetter ◽  
...  
Keyword(s):  
Mantle Source ◽  
Upper Mantle ◽  
Crustal Contamination ◽  
Magma Source ◽  
Mantle Heterogeneity ◽  
The South ◽  
Zircon Age ◽  
South Central ◽  
Depleted Mantle ◽  
Intrusive Rocks

The magmatic events occurring within the two main cycles in the south-central part of the Labrador Trough (New Quebec) have been dated. In cycle 1, a granophyre dike related to the Cramolet Lake gabbro sill, which intrudes the Seward subgroup, has a U–Pb zircon age of 2169 ± 2 Ma. In cycle 2, the tholeiitic basalts of the Willbob (Hellancourt) Formation and the related mafic–ultramafic sills are dated at ca. 1900 Ma by the Pb–Pb method. These data confirm the existence of at least two main magmatic cycles separated by about 270 Ma. The magma source was depleted upper mantle, and the magma did not experience any significant crustal contamination, as indicated by the μ1 ratio (7.9) and [Formula: see text] (+4) for the tholeiitic basalts. The μ1 value for the intrusive rocks (8.03) and the average [Formula: see text] value for the gabbroic rocks of cycle 2 (+2.8) and for the granophyre of cycle 1 (+1.05) could be the result of slight crustal contamination or of mantle heterogeneity. Whatever the cause of these values, the data indicate the prolonged presence of a depleted mantle source.

LITHOPROBE—southern Vancouver Island: Cenozoic subduction complex imaged by deep seismic reflections

1987 ◽  
Vol 24 (1) ◽  
pp. 31-51 ◽  
Author(s):  
R. M. Clowes ◽  
M. T. Brandon ◽  
A. G. Green ◽  
C. J. Yorath ◽  
A. Sutherland Brown ◽  
...  
Keyword(s):  
Large Scale ◽  
Sedimentary Rocks ◽  
Indirect Evidence ◽  
Vancouver Island ◽  
Late Eocene ◽  
San Juan ◽  
Juan De Fuca Plate ◽  
Reflective Layer ◽  
Juan De Fuca ◽  
Accreted Terranes

The LITHOPROBE seismic reflection project on Vancouver Island was designed to study the large-scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting Juan de Fuca plate. In this paper, we interpret two LITHOPROBE profiles from southernmost Vancouver Island that were shot across three important terrane-bounding faults—Leech River, San Juan, and Survey Mountain—to determine their subsurface geometry and relationship to deeper structures associated with modem subduction.The structure beneath the island can be divided into an upper crustal region, consisting of several accreted terranes, and a deeper region that represents a landward extension of the modern offshore subduction complex. In the upper region, the Survey Mountain and Leech River faults are imaged as northeast-dipping thrusts that separate Wrangellia, a large Mesozoic–Paleozoic terrane, from two smaller accreted terranes: the Leech River schist, Mesozoic rocks that were metamorphosed in the Late Eocene; and the Metchosin Formation, a Lower Eocene basalt and gabbro unit. The Leech River fault, which was clearly imaged on both profiles, dips 35–45 °northeast and extends to about 10 km depth. The Survey Mountain fault lies parallel to and above the Leech River fault and extends to similar depths. The San Juan fault, the western continuation of the Survey Mountain fault, was not imaged, although indirect evidence suggests that it also is a thrust fault. These faults accommodated the Late Eocene amalgamation of the Leech River and Metchosin terranes along the southern perimeter of Wrangellia. Thereafter, these terranes acted as a relatively coherent lid for a younger subduction complex that has formed during the modem (40 Ma to present) convergent regime.Within this subduction complex, the LITHOPROBE profiles show three prominent bands of differing reflectivity that dip gently northeast. These bands represent regionally extensive layers lying beneath the lid of older accreted terranes. We interpret them as having formed by underplating of oceanic materials beneath the leading edge of an overriding continental place. The upper reflective layer can be projected updip to the south, where it is exposed in the Olympic Mountains as the Core rocks, an uplifted Cenozoic subduction complex composed dominantly of accreted marine sedimentary rocks. A middle zone of low reflectivity is not exposed at the surface, but results from an adjacent refraction survey indicate it is probably composed of relatively high velocity materials (~ 7.7 km/s). We consider two possibilities for the origin of this zone: (1) a detached slab of oceanic lithosphere accreted during an episodic tectonic event or (2) an imbricated package of mafic rocks derived by continuous accretion from the top of the subducting oceanic crust. The lower reflective layer is similar in reflection character to the upper layer and, therefore, is also interpreted as consisting dominantly of accreted marine sedimentary rocks. It represents the active zone of decoupling between the overriding and underthrusting plates and, thus, delimits present accretionary processes occurring directly above the descending Juan de Fuca plate. These results provide the first direct evidence for the process of subduction underplating or subcretion and illustrate a process that is probably important in the evolution and growth of continents.

scholarly journals Geochemical and isotopic evidence for Carboniferous rifting: mafic dykes in the central Sanandaj-Sirjan zone (Dorud-Azna, West Iran)

2017 ◽  
Vol 68 (3) ◽  
pp. 229-247 ◽  
Author(s):  
Farzaneh Shakerardakani ◽  
Franz Neubauer ◽  
Manfred Bernroider ◽  
Albrecht Von Quadt ◽  
Irena Peytcheva ◽  
...  
Keyword(s):  
Mantle Source ◽  
Crustal Contamination ◽  
Early Carboniferous ◽  
Main Trend ◽  
Geodynamic Setting ◽  
Mafic Dyke ◽  
Mafic Dykes ◽  
Enriched Mantle ◽  
Depleted Mantle ◽  
Sanandaj Sirjan Zone

Abstract In this paper, we present detailed field observations, chronological, geochemical and Sr–Nd isotopic data and discuss the petrogenetic aspects of two types of mafic dykes, of alkaline to subalkaline nature. The alkaline mafic dykes exhibit a cumulate to foliated texture and strike NW–SE, parallel to the main trend of the region. The 40Ar/39Ar amphibole age of 321.32 ± 0.55 Ma from an alkaline mafic dyke is interpreted as an indication of Carboniferous cooling through ca. 550 °C after intrusion of the dyke into the granitic Galeh-Doz orthogneiss and Amphibolite-Metagabbro units, the latter with Early Carboniferous amphibolite facies grade metamorphism and containing the Dare-Hedavand metagabbro with a similar Carboniferous age. The alkaline and subalkaline mafic dykes can be geochemically categorized into those with light REE-enriched patterns [(La/Yb)N = 8.32–9.28] and others with a rather flat REE pattern [(La/Yb)N = 1.16] and with a negative Nb anomaly. Together, the mafic dykes show oceanic island basalt to MORB geochemical signature, respectively. This is consistent, as well, with the (Tb/Yb)PM ratios. The alkaline mafic dykes were formed within an enriched mantle source at depths of ˃ 90 km, generating a suite of alkaline basalts. In comparison, the subalkaline mafic dykes were formed within more depleted mantle source at depths of ˂ 90 km. The subalkaline mafic dyke is characterized by 87Sr/86Sr ratio of 0.706 and positive ɛNd(t) value of + 0.77, whereas 87Sr/86Sr ratio of 0.708 and ɛNd(t) value of + 1.65 of the alkaline mafic dyke, consistent with the derivation from an enriched mantle source. There is no evidence that the mafic dykes were affected by significant crustal contamination during emplacement. Because of the similar age, the generation of magmas of alkaline mafic dykes and of the Dare-Hedavand metagabbro are assumed to reflect the same process of lithospheric or asthenospheric melting. Carboniferous back-arc rifting is the likely geodynamic setting of mafic dyke generation and emplacement. In contrast, the subalkaline mafic sill is likely related to the emplacement of the Jurassic Darijune gabbro.

Petrology and age determinations of the ultramafic (lamproitic) rocks from the Yakokut complex, Aldan Shield, Eastern Siberia

1995 ◽  
Vol 59 (396) ◽  
pp. 409-428 ◽  
Author(s):  
U. Mues-Schumacher ◽  
J. Keller ◽  
V. Konova ◽  
P. Suddaby
Keyword(s):  
Mantle Source ◽  
Aldan Shield ◽  
Eastern Siberia ◽  
Isotopic Data ◽  
Depleted Mantle ◽  
Heterogeneous Mantle ◽  
Crystallization Conditions ◽  
Alkaline Complexes ◽  
Subcontinental Lithosphere ◽  
Age Determinations

AbstractMineralogical, geochemical and isotopic data including K-Ar age determinations are presented for one of the largely unknown, Mesozoic ultramafic (lamproitic) occurrences of the Aldan Shield. Ultramafic, ultrapotassic rocks occur as isolated pipes or as dykes in alkaline complexes as the Yakokut complex. K-Ar dating of phlogopites from different ultramafic dyke rocks of Yakokut give cooling ages of 133.3 ± 1.3 Ma. These rocks generally contain olivine, clinopyroxene, phlogopite and spinel phenocrysts. Olivines are forsterite-rich (Fo86–94) and undeformed, suggesting that they are phenocrysts. The low Ti and Al contents of clinopyroxenes are close to those of lamproites. Phlogopite cores are rich in Al2O3 and TiO2 relative to the rims with two different zonation trends caused by distinct crystallization conditions after emplacement. Spinels are Ti-bearing magnetites and Ti-Al-magnesiochromites with high Cr/(Cr+Al) ratios (> 0.9), indicating their crystallization from a lamproitic melt.Geochemically, the rocks are ultrabasic — basic with high mg# values, low Ca, Al, Na and strong enrichments of Rb, Ba and K. Their CaO/Al2O3, Zr/Nb or Ba/Sr ratios indicate their lamproitic nature and origin in the subcontinental lithosphere of a depleted mantle source which had undergone metasomatic enrichment. As the rocks all show strong negative Nb anomalies and low Ti, Y and Yb contents, the enrichment is attributed to subduction zone fluids. The Sr-Nd-data (87Sr/86Sr0 0.70573−0.70605 ± 0.00003; ɛNd − 10.2 ± 0.7) indicate the origin by partial melting of a heterogeneous mantle source with relatively low Rb/Sr ratios and an early enrichment. The evolution model comprises a depletion of ‘basaltic elements’, leading to a harzburgitic source which was enriched by an early LREE contribution during the stabilization of the Archaean to Proterozoic Aldan Shield. It is further suggested that the Mesozoic northwesterly directed subduction of the Ochotsk-Chukotsk belt influenced the subcontinental lithosphere underneath the Aldan Shield, leading to the observed subduction-related signature of the Yakokut lamproites.

scholarly journals New zircon radiometric U-Pb ages and Lu-Hf isotopic data from the ultramafic-mafic sequences of Ranau and Telupid (Sabah, eastern Malaysia): Time to reconsider the geological evolution of Southeast Asia?

Geology ◽  
2021 ◽  
Author(s):  
Basilios Tsikouras ◽  
Chun-Kit Lai ◽  
Elena Ifandi ◽  
Nur’Aqidah Norazme ◽  
Chee-Hui Teo ◽  
...  
Keyword(s):  
Ultramafic Rocks ◽  
Crustal Material ◽  
Isotopic Data ◽  
Backarc Basin ◽  
Crustal Component ◽  
Sulu Sea ◽  
Potential Source ◽  
Depleted Mantle ◽  
Geological Evolution ◽  
Mantle Reservoir

New zircon U-Pb geochronology from a peridotite suite near Ranau and the Telupid ophiolite in Sabah, eastern Malaysia, contradict previous studies, which assumed that the Sabah mafic-ultramafic rocks are largely ophiolitic and Jurassic–Cretaceous in age. We show that these rocks formed during a magmatic episode in the Miocene (9.2–10.5 Ma), which is interpreted to reflect infiltration of melts and melt-rock reaction in the Ranau subcontinental peridotites during extension, and concurrent seafloor spreading forming the Telupid ophiolite further south. Older zircons from the Ranau peridotites have Cretaceous, Devonian, and Neoproterozoic ages. Zircon Lu-Hf isotopic data suggest their derivation from a depleted mantle. However, significant proportions of crustal components have been incorporated in their genesis, as evidenced by their less-radiogenic Hf signature compared to a pristine mantle reservoir. The involvement of a crustal component is consistent with our interpreted continental setting for the Ranau peridotite and formation in a narrow backarc basin for the Telupid ophiolite. We infer that the Sulu Sea, which was expanding throughout much of the Miocene, may have extended to the southwest into central Sabah. The Telupid oceanic strand formed during the split, collapse, and rollback of the Sulu arc due to the subduction of the Celebes Sea beneath Sabah. Incorporation of the Sulu arc in the evolving Miocene oceanic basin is a potential source to explain the involvement of crustal material in the zircon evolution of the Telupid ophiolite.

scholarly journals Origin of the High-K Tertiary magmatism in Northern Greece: Implications for mantle geochemistry and geotectonic setting.

2013 ◽  
Vol 47 (1) ◽  
pp. 416
Author(s):  
K. Pipera ◽  
A. Koroneos ◽  
T. Soldatos ◽  
G. Poli ◽  
G. Christofides
Keyword(s):  
Mantle Source ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Calc Alkaline ◽  
Northern Greece ◽  
Mafic Rocks ◽  
High K ◽  
Depleted Mantle ◽  
Subducted Oceanic Crust ◽  
Compositional Variations

Tertiary plutonic and volcanic rocks cropping out in the Rhodope Massif (N. Greece) are studied using existing and new geochemical and isotopic data. Most of these rocks belong to the post-collisional magmatism formed as part of the prolonged extensional tectonics of the Rhodope region in Late Cretaceous– Paleogene time. This magmatism is considered to be of mantle origin; however, the character of the mantle source is controversial. Rock bulk chemistry and compositional variations show magmas with calc-alkaline to high-K calc-alkaline and shoshonitic features associated with magmatism at convergent margins. Initial 87Sr/86Sr, 143Nd/144Nd ratios, Pb isotopes and REE composition of the mafic rocks indicate mainly an enriched mantle source, even if some rocks indicate a depleted mantle source. Low- and High-K mafic members of these rocks coexist indicating a strongly heterogeneous mantle source. The High-K character of some of the mafic rocks is primarily strongly related to mantle enrichment by subduction-related components, rather than crustal contamination. The geochemical characteristics of the studied rocks (e.g Ba/Th,Th/Yb,Ba/La, U/Th, Ce/Pb) indicate that primarily sediments and/or sediment melts, rather than fluid released by the subducted oceanic crust controlled the source enrichment under the Rhodope Massif.

Triassic magmatism in the European Southern Alps as an early phase of Pangea break-up

2020 ◽  
Vol 157 (11) ◽  
pp. 1800-1822 ◽  
Author(s):  
Angelo De Min ◽  
Matteo Velicogna ◽  
Luca Ziberna ◽  
Massimo Chiaradia ◽  
Antonio Alberti ◽  
...  
Keyword(s):  
Mantle Source ◽  
Crustal Contamination ◽  
Mineral Chemistry ◽  
Western Mediterranean ◽  
Southern Alps ◽  
Late Triassic ◽  
Geodynamic Setting ◽  
Isotopic Data ◽  
Geological Evidence ◽  
Break Up

AbstractMagmatic rocks from the Dolomites, Carnic and Julian Alps, Italy, have been sampled to investigate the origin and geodynamic setting of Triassic magmatism in the Southern Alps. Basaltic, gabbroic and lamprophyric samples have been characterized for their petrography, mineral chemistry, whole-rock major and trace elements, and Sr, Nd and Pb isotopic compositions. Geothermobarometric estimates suggest that the basaltic magmas crystallized mostly at depths of 14–20 km. Isotopic data show variable degrees of crustal contamination decreasing westwards, probably reflecting a progressively more restitic nature of the crust, which has been variably affected by melting during the Permian period. Geochemical and isotopic data suggest that the mantle source was metasomatized by slab-derived fluids. In agreement with previous studies and based on geological evidence, we argue that this metasomatism was not contemporaneous with the Ladinian–Carnian magmatism but was related to previous subduction episodes. The lamprophyres, which likely originated some 20 Ma later by lower degrees of melting and at higher pressures with respect to the basaltic suite, suggest that the mantle source regions of Triassic magmatism in the Dolomites was both laterally and vertically heterogeneous. We conclude that the orogenic signatures of the magmas do not imply any coeval subduction in the surrounding of Adria. We rather suggest that this magmatism is related to the Triassic rifting episodes that affected the western Mediterranean region and that were ultimately connected to the rifting events that caused the break-up of Pangea during the Late Triassic – Early Jurassic period.

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