Dyke Swarm Research Articles

Evidence for a Single Large Igneous Province at 2.11 Ga across Supercraton Superia

Abstract The Superia supercraton palaeogeographic reconstruction (c. 2.65–2.00 Ga) is predominantly based on the shared large igneous province (LIP) record of the Superior, Hearne, Wyoming, and Karelia-Kola Archaean cratons. Palaeogeographic reconstruction relies on U–Pb geochronology (i.e. magmatic barcodes), palaeolatitudes from palaeomagnetic studies, and geometry of mafic dyke swarms as part of LIPs, as well as similarities in cover stratigraphy and/or basement geology. If contemporaneous mafic units from these cratons are indeed fragments of the same LIP, then integration of their chemistry can provide insight into the overall LIP plumbing system. A geochemical evaluation of whole-rock major and trace elements, and Nd isotopes, is presented for c. 2.11 Ga mafic units from the Marathon dykes (Superior), Griffin gabbro sills and Kazan dykes (Hearne), Bear Mountain dykes (Wyoming), and Tohmajärvi-Pirtguba dykes, Misi gabbro sills, and Oravaara and Hirsimaa volcanic rocks (Karelia-Kola). These units include the Nieminen dyke, Western Karelia, for which we present a new U–Pb CA-ID-TIMS date of 2114.4 ± 3.0 Ma. Four geochemical groups are identified: Groups 1 (mid-ocean ridge basalt-like; in Karelia) and 2 (within plate basalt; in Hearne, Wyoming, and Karelia) are distinct end-members of upper mantle and plume melts, respectively; Group 3 (in Superior, Wyoming, and Karelia) is a hybrid of Groups 1 and 2; and Group 4 (in all cratons) formed by assimilation and fractional crystallisation of Groups 1, 2, and 3. We present a model of the LIP plumbing system and define three magmatic stages by combining our geochemical interpretations with existing geochronology. The early stage (2135–2130 Ma) is limited to Group 1 dykes. The middle stage (2130–2113 Ma) includes Groups 1, 3, and 4. Onset of the late stage (2113–2101 Ma) is marked by the introduction of Group 2 but Groups 1 and 4 are also abundant. Finally, with our integrated tectonic and geochemical model, we discuss the mineralisation potential and prospectivity of the entire c. 2.11 Ga magmatic system.

A hyperextension assemblage, imbricated in Archean–Paleoproterozoic crust, at the base of the Kalak Nappe Complex in the northern Scandinavian Caledonides

The break-up of continents generates dyke swarms, basins and hyperextension assemblages. Once incorporated into new orogens, the latter can provide crucial information about origins and tectonic processes. The Nålfjell Complex in the Caledonian Skillefjord Nappe of northern Norway has many of the characteristics of a hyperextension assemblage, notably the presence of solitary serpentinite bodies exposed by the exhumation of serpentinized mantle and now embedded in schists, amphibolites, mylonites, marbles and felsic gneisses. The Skillefjord Nappe is a lithologically diverse, imbricated and discontinuous allochthon. It consists of felsic gneisses and dykes dated by zircon U–Pb to c. 3100, 2940, 2830, 2510 and 1800–1750 Ma. These rocks yield titanite ages of 2810–2700, 1750, 1660–1590 and 430–420 Ma. Metagabbro intruded at 1995 Ma. The ages and evolution of the Skillefjord Nappe are distinct from those of the structurally higher Svaertholt Terrane (>1030 Ma sediments deformed and intruded by granite at 980–950 Ma) and the Sørøy Terrane (characterized by a multi-stage 900–500 Ma evolution). The assembly of these disparate elements was completed at 420 Ma. The timing of hyperextension is uncertain. The serpentinites have a primitive Pb isotopic composition indicating old depleted mantle lithosphere. Extensional processes from the Paleoproterozoic to Ediacaran are considered. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle

Emplacement of a felsic dyke swarm during progressive heterogeneous deformation, Eastern Elba Dyke Complex (Island of Elba, Italy)

Magmatic and sub-solidus fabrics in intrusive rocks are frequently used to infer the relative timing of deformation with respect to magma emplacement and cooling. Here, we describe the relationships between strain and fabric development in leucogranite sheets (pegmatite, aplite) emplaced into shear zones that localized post-thermal peak deformation in the contact aureole of an upper crustal pluton (<0.2 GPa) on the Island of Elba, Italy. The leucogranite sheets present igneous, mylonitic, and cataclastic fabrics. Detailed meso- and microscopic structural analysis suggests that the dykes emplaced in the shear zones behaved as competent, rigid bodies during mylonitic deformation of the host rocks. Thermal modelling indicates that emplacement and cooling of the sheets occurred very rapidly (a few days to years) compared to typical tectonic strain rates and strain accumulation timescales in the host rocks. Such a fast cooling does not allow melt or magma-induced thermal softening in the host rocks during deformation. The development of mylonitic and cataclastic fabrics in the dykes was controlled by the localized activation of fluid-controlled reaction softening mechanisms (mylonitic fabric) and embrittlement during cooling in sites of high-strain (cataclastic fabric). We show that a broad spectrum of fabrics can form in igneous sheet intrusions emplaced at the same time and crustal level. The coexistence of isotropic (non-foliated igneous) versus anisotropic (mylonitic and cataclastic) fabrics in igneous sheet intrusions should therefore be evaluated in terms of tectonic strain rates, cooling rates, thermal state of the host, distribution of heterogeneous strain, and activation of strain softening mechanisms. Our observations highlight that the concepts of pre-, syn-, late- and post-tectonic fabrics in intrusive igneous rocks should be used with caution when interpreting relative timing relationships between deformation and magmatism.

El Álamo district (Baja California, México): A hint of a new Cordilleran orogenic gold belt?

The El Álamo is one of the most important historic gold mining districts in the Peninsular Ranges batholith. Gold mineralizations at El Álamo consist in fault-fill laminated quartz veins with small amounts of sulfides and oxides, and lesser amounts of gold. Veins are mainly emplaced along compressional brittle-plastic shear zones within the El Álamo quartz diorite (146.6 ± 1.6 Ma, U–Pb zircon), which is intruded by the San Marcos dike swarm (126.8 ± 1.7 Ma, U–Pb zircon). Several lines of evidence indicate that the dike swarm is older than quartz veins, and the formation of the later is related to the Peninsular Ranges orogeny (∼110−98 Ma). Gold bearing quartz veins formed during regional prograde metamorphism (greenschist facies), and the veins subsequently underwent plastic deformation, dynamic recrystallization and metamorphism. Gold remobilization and concentration seems to have been largely enhanced by metamorphism, probably accounting for very high grade, albeit sporadic, bonanza ores, typical of the district. 40Ar/39Ar dating of hydrothermal white micas from the auriferous veins yielded ages of 97.86 ± 0.17 Ma at the Ulises mine and 97.46 ± 0.17 Ma at the La Americana mine. These ages are interpreted as the minimum age of the regional metamorphism in the area. Based on their primary characteristics, the gold deposits from El Álamo district can be ascribed to the orogenic gold type. These deposits are similar to other known deposits in the same orogenic context (e.g., Julian district; southern California), and they probably conform a new Cordilleran orogenic gold belt that could extend over much of the exhumed axis of the Peninsular Ranges batholith.

1.80–1.75 Ga granite suites in the west Troms Basement Complex, northern Norway: Palaeoproterozoic magma emplacement during advancing accretionary orogeny, from field observations

The Ersfjord Granite is part of a suite of c.1.80–1.75 Ga syeno-granites in the West Troms Basement Complex, northern Norway, presumed to belong to the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian Shield. Previous data suggest the granite formed post-collisional and ascended as a batholith pluton from a source generated by delamination of mafic-intermediate lower crust. We argue that the Ersfjord Granite was emplaced initially (c. 1.80 Ga) as multiple tabular sills in an extensional setting, then as successive melt injections (c. 1.78–1.75 Ga) in an evolving Andean/Cordilleran type accretionary orogen at the waning stages of the Svecofennian orogen. Field observations indicate melt ascent initiated as successive sills (EG-I) into well-foliated Meso/Neoarchaean TTG gneisses. Some sills preserved a magmatic layering, others injected and assimilated the host rock gneisses leaving pendants of mafic gneiss/migmatite residuum in between the granite sills. The first tectonic patches of melts (EG-II) ascended into the middle/upper crust along regional shear zones and injected into ductile imbricate thrust stacks (D1 event) during NE-SW directed crustal shortening and medium grade P-T conditions, using the sills and ancestor migmatite pendants as melt pathways. Then the tabular EG-I and II granite sheets and adjacent gneisses were coaxially folded by upright macro-folds (D2 event) and steep, granitic pegmatite dyke swarms (EG-III) intruded parallel to the fold axial surface and in related D2 thrusts, at low grade metamorphic conditions. The final melt emplacement (D3 event) included granite pegmatite dykes and sills (EG-IV) along subvertical D3 fold limbs and steep strike-slip shear zones. Our provisional extension, and successive advancing accretionary orogenic emplacement model for the Ersfjord Granite may explain ascent of many other TIB-1 magmas in the Fennoscandian Shield.

Petrology and geochemistry of multiphase post-granitic dikes: A case study from the Gabal Serbal area, Southwestern Sinai, Egypt

Abstract Variable single and/or swarms of post-granitic dikes are widespread at Gabal Serbal, Southwestern Sinai, Egypt. The present article aims to identify and discriminate these multiphase dikes through detailed geological, petrographical, and geochemical examinations. These dikes are classified into two subphases: (1) acidic dikes (porphyritic dacite, microgranite, granophyre, and alkaline granophyre dikes); and (2) basic dikes (basalt and dolerite dikes). They range from vertical or steeply inclined bodies, 0.5–15 m wide, pink to black color, and NE–SW to N–S directions. Acidic dikes with different mineralogical constituents have medium to high k-characters, originating from calc-alkaline magma and extruded in a volcanic arc environment. In contrast, basic dikes have medium k-characters, originating from tholeiitic magma and developing within a plate environment. Basic dikes are enriched with opaque minerals, where the basaltic dike contains iron oxides (magnetite and hematite), such as apatite in addition to copper minerals. Dolerite dike comprises magnetite, titanomagnetite, and pyrite.

Skarn formation and Cu[sbnd]Ag mineralization in the McKenzie Gulch area, northern New Brunswick, Canada: Implication for the applications of mineral chemistry in exploration for porphyry copper and skarn deposits

The McKenzie Gulch (MG) copper–silver skarn occurrences are associated with the Middle Devonian intermediate to felsic dyke swarms. Mineralization occurs as veins and stockwork of veinlets, disseminations, patchy, and locally as replacement of calc-silicate skarns in argillaceous limestone. This skarn is interpreted to have developed in three stages: the earliest contact metamorphic stage resulting in hornfels developed from calcareous mudstone and very fine-grained clastic sedimentary rocks (Stage I); this was followed by metasomatic replacement resulting in prograde anhydrous skarn containing grossular-andradite (grandite), diopsidic pyroxene, and wollastonite (Stage II); increasing fluid/rock interaction results in retrograde skarn dominated by epidote, calcite, green amphibole, chlorite, sulfide minerals, titanite, andradite and hedenbergite commonly along veins and veinlets (Stage III). Subordinate sphalerite and pyrite occur in late veins cross-cutting both porphyry dykes, skarns and hornfels zone.Prograde garnets in the study area belong to the grossular-andradite solid solution that ranges from Adr16Gr82 to almost pure andradite - Adr99Gr1, whereas other garnet end-members constitute less than 5% collectively. Pyroxenes form a diopside-hedenbergite solid solution with composition between Di31Hd60 and Di93Hd7, while other pyroxenes constitute less than 10% collectively. These garnets and pyroxenes exhibit chemical zonation patterns generally characterized by a rim of Fe enrichment relative to cores of grains, which may suggest possible change in salinity (or other factor that promotes Fe transport). This means grossular and diopsidic cores are rimmed by more andradite and hedenbergitic compositions for garnet and pyroxene, respectively. In general, there is a tendency for increasing Fe component in both garnet and pyroxene from the prograde to retrograde stages.Epidote formed during the retrograde stage exhibits oscillatory zoning similar to retrograde andradite and indicates Fe enrichment increasing from core to rim. Based on the major element composition, textural and optical characteristics of garnets, we conclude that grandites (Al-rich) formed under low water/rock ratios, in equilibrium with metasomatic fluids whose composition was locally buffered by the host rocks, whereas andradite (Fe-rich) resulted from relatively high water/rock ratios that were in equilibrium with a magmatic-derived fluid. The results of this study indicate that there is a relationship between the composition of pyroxenes and garnets of the skarn alteration facies and the dominant metal of the mineralized skarns. These facies plot in the compositional field of Cu-dominated skarns. As a result, this relationship may be applied in exploration for skarn and porphyry copper mineralization.

Lithospheric thinning of the North China craton: Insights from Early Cretaceous intermediate–mafic dyke swarms in Jiaodong peninsula

The coupling/decoupling relationships between shallow crustal deformation and deep mantle tectono-magmatic activities, during lithospheric thinning and destruction of cratons, remain hotly debated in the last decades. Multidisciplinary studies were done on intermediate–mafic dyke swarms diffusively located in southern Jiaodong peninsula, southeastern North China craton (NCC) to explore the crustal and lithospheric mantle responses to regional tectonic extension. Uniform ages (122–121 Ma) of the dykes indicate intensive magma generation at the mantle depth around 122 Ma related to large-scale tectonic extension of regional lithospheric mantle. Systemic anisotropy of magnetic susceptibility (AMS), geochemical, and Sr-Nd isotope analyses suggest that the dykes were originated from enriched lithospheric mantle sources and emplaced along crustal fractures formed in a WNW-ESE oriented extensional stress field. We show that the Jiaodong lithosphere experienced two stages of Early Cretaceous tectonic evolution, i.e., an early stage (135–122 Ma) lithospheric thinning characterized by large-scale tectonic extension of the crust and limited magmatic activities of crustal sources, and a late stage (122–108 Ma) lithospheric root collapse dominated by intensive tectonic extension and magmatism in the lithospheric mantle level. The decoupling crust-mantle evolution during lithospheric thinning and destruction of the NCC is ascribed to continuous tectonic extension in WNW-ESE direction.

Geochemistry and Petrogenesis of Shoshonitic Dyke Swarm in the Northeast of Meshkinshahr, NW Iran

The study area is located in Ardabil province in the northeast of Meshkinshahr city. More than 200 small and large Eocene-age dykes form outcrops in this area. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U–Pb zircon analyses yield a consistent age of 44.3 ± 1.8 Ma for the dyke swarms. These dykes include tephritic, andesitic and basaltic compositions, and show enrichment in LREEs (relative to HREEs) and are characterized by enrichment in LILEs and depletion in HFSEs. Petrological observations, along with major, rare earth and trace elements geochemistry, suggest that the dykes have a shoshonitic signature. All the rocks are highly enriched in incompatible trace elements and have variable Sr–Nd isotopes. Enrichment in incompatible elements and other geochemical features for the dyke swarm rocks suggest that a metasomatized subcontinental lithospheric mantle is the magma source. The negative Nb–Ta–Ti anomalies in the rocks are comparable with the features of subduction-related magmatism and contamination with ancient crustal components. The radiogenic 87Sr/86Sr isotopic values of the rocks imply the involvement of slab terrigenous sediments and/or a continental lithosphere. Isotopically, the volcanic rocks exhibit a binary trend, representing 1–5% mixing between the primary mantle and sediment melts. Our melting models suggest that there are residual garnet + spinel in the source, which are incompatible with the partial melting of amphibole- and/or phlogopite-bearing lherzolites. The geochronological, geochemical and isotopic data for the northeast Meshkinshahr dyke swarms suggest that these Late Eocene magmas were derived from a small degree of partial melting of a subduction-metasomatized lithospheric mantle source in a post-collisional setting.

Multiphase Topographic and Thermal Histories of the Wallowa and Elkhorn Mountains, Blue Mountains Province, Oregon, USA

AbstractThe Mesozoic and Cenozoic history of Western North America is characterized by terrane accretion, volcanism, and orogenesis. This history complicates the interpretation of paleogeography in northeastern Oregon and adjacent Idaho, where the age of mountainous topography in the Blue Mountains Province is important for validating hypothesized Miocene to present geodynamics. In this study, we analyze the distribution of Columbia River Basalt and low‐temperature apatite and zircon (U‐Th)/He thermochronometry collected from the Wallowa and Bald Mountain Batholiths to refine regional landscape history. These batholiths underly the Wallowa and Elkhorn Mountains respectively, two of the most prominent mountain ranges within the Blue Mountains Province. We find that low‐temperature thermochronometry data from the Bald Mountain and Wallowa Batholiths record distinct thermal histories associated with unroofing and magmatism from the Cretaceous to present. We propose that reheating during intrusion of the Chief Joseph dike swarm led to partial resetting of thermochronometers but did not completely overprint recorded Mesozoic to present thermal histories in the Wallowa Batholith. Using modeled thermal histories and the present‐day distribution of Columbia River Basalt, we conclude that the Wallowa and Elkhorn Mountains have distinct topographic histories. We propose that the Elkhorn Mountains began to form in the Eocene and that the Wallowa Mountains are geologically young, forming as a result of relief generation after the Miocene eruption of Columbia River Basalt.

Tectonic timeline in the Mongol-Okhotsk oceanic regime: Insights from early Jurassic and early cretaceous mafic-intermediate dyke swarms in central Inner Mongolia, North China

Chronicling sequential mafic dyke swarms along convergent plate margins presents a key to outlining the tectonic timeline of complete Wilson orogenic cycles for any regional paleotectonic and global supercontinental reconstructions. This study documents two episodes of Mesozoic mafic dyke swarms from central Inner Mongolia in the combined northern China-southern Mongolia (NCSM) plate: The Gadabuqi (GDBQ) suite with an age of ca. 181 Ma from East Ujimqin and the Houtoumiao (HTM) suite with an age of ca. 131 Ma from West Ujimqin. The GDBQ suite consists of hornblende-rich appinitic rocks and ranges in SiO2 from 49.6 to 52.6%. They are calc-alkaline to high calc-alkaline and show enrichment in large ion lithophile element (LILE) and light rare earth element (LREE) but depletion in high field strength elements (HFSE), with whole-rock initial 87Sr/86Sr of 0.704608 to 0.706048, positive ƐNd(t) of +1.2 to +2.4 as well as zircon ƐHf(t) of +2.5 to +5.6 and δ18O of 5.4 to 6.5‰. These features attributed the GDBQ suite to a juvenile fluid-metasomatized lithospheric mantle source. By contrast, the HTM suite mainly comprises gabbronorites with SiO2 from 55.1 to 56.4% and shows a transitional character from high calc-alkaline to shoshonitic. They are variably enriched in LREE and LILE but depleted in HFSE, with whole-rock initial 87Sr/86Sr of 0.705656 to 0.706543, ƐNd(t) of −1.1 to +3.4 and zircon ƐHf(t) of +2.9 to +9.1 and δ18O of 6.1 to 7.8‰. These elemental and isotopic traits argued for a parental magma derivation from partial melting of a fertile asthenospheric mantle and syn-magmatic crustal assimilation and fractional crystallization. Besides representing the first recognition of two distinct basaltic underplating events in central Inner Mongolia, these two contrasting mafic dyke swarms are disposed to monitor a tectonic shift from Early Jurassic linear slab breakoff to Early Cretaceous pervasive lithospheric delamination in the Great Xing'an Range. In conjunction with polyphase magmatic records and multifarious manifestations of structural, metamorphic and sedimentological dimensions, the refined Mesozoic tectono-thermal sequence in the NCSM tract tends to encapsulate insightful testimonies for chronicling the tectonic timeline from pre-Jurassic ongoing subduction and slab roll back to prolonged post-collisional aftermath of the Jurassic to early Cretaceous times in the Mongolia-Okhotsk oceanic plate domain.

Casting a vote for shifting the Statherian: Petrogenesis of 1.70 and 1.62 Ga mafic dykes in the North China Craton

The Statherian Period (1.8–1.6 Ga), currently the final period of the Paleoproterozoic Era, is named “stable, firm” for the interval following orogenesis (the Orosirian Period) that culminated in the assembly of likely Earth's first supercontinent, Columbia. Thus, the Paleoproterozoic–Mesoproterozoic boundary is supposed to demarcate increased continental “stability”, defined as a transition from orogenesis and arc magmatism to unmetamorphosed shallow-water cover sequences and intraplate magmatism. It has become apparent, however, that (i) the assembly of Columbia appears to be highly diachronous and (ii) many stable cover sequences once mapped as Mesoproterozoic appear to pre-date 1.6 Ga quite significantly. These discrepancies have led to the recent suggestion by a large group of Precambrian geologists that the Statherian Period belongs more to the Mesoproterozoic Era and thus that the end of the Paleoproterozoic Era should be redefined at ca. 1.8 Ga. North China Craton is one such craton that has pre-1.6 Ga stable sedimentary covers, but dating the base of the basins have proven difficult and thus other indicators of continental stability must be sought.Here we present new geochronological and geochemical data from two dyke swarms at 1.70 and 1.62 Ga from western Shandong Province (Luxi area) of the North China Craton that can be used to constrain the tectonic setting at this critical interval in question. Precise U-Pb SIMS analysis yields baddeleyite 207Pb/206Pb ages of 1700 ± 5 Ma (n = 9, MSWD = 0.14) and 1620 ± 4 Ma (n = 14, MSWD = 0.56). The new dated ca. 1.62 Ga Xiaobeizhuang dyke (330° trending, >15 m wide) belongs to the coeval Taishan dyke swarm. The ca. 1.70 Ga swarm, newly discovered in the North China Craton, is referred to as Ximaiyao dyke swarm (340–345° trending, 0.4–13 m wide). Paleogeographic affinities of the North China Craton with the São Francisco Craton in supercontinent Columbia are considered in light of this newly identified swarm. Given that both dyke swarms pre-date 1.6 Ga and that their geochemistry implies intraplate magmatism, our new data support the suggestion that this time is more Mesoproterozoic than Paleoproterozoic in character. These pre-1.6 Ga intraplate dyke swarms, including the 1.78 Ga, the 1.73 Ga, and later 1.70 Ga and 1.68–1.62 Ga swarms, thus provide local support from North China Craton for redefining the base of the Mesoproterozoic Era as ca. 1.8 Ga and including the Statherian Period, consistent with the ages of platform cover sequences.

Crustal Contamination and Hybridization of an Embryonic Oceanic Crust during the Red Sea Rifting (Tihama Asir Igneous Complex, Saudi Arabia)

AbstractThe Red Sea rift system represents a key case study of the transition from a continental to an oceanic rift. The Red Sea rifting initiated in Late Oligocene to Early Miocene (24–23 Ma) and was accompanied by extensive magmatism throughout the rifted basin, from Afar and Yemen to northern Egypt. Here, we present a petrological and geochemical study of two gabbro bodies and associated basalts from the Tihama Asir igneous complex, which formed at 24–20 Ma within the rifted Arabian-Nubian Shield (ANS). The Tihama Asir is therefore an ideal location to study the initial phase of syn-rift magmatism and its influence on the geodynamic evolution of the Red Sea rift system. The most primitive olivine gabbros present modal, bulk and mineral compositions consistent with formation from Mid-Ocean Ridge Basalt (MORB)-type parental melts, whereas the evolved olivine-free gabbros and oxide-bearing gabbros show saturation of phlogopite and a crystal line of descent diverging from fractional crystallization trends. In detail, whole-rock and mineral compositions in the most evolved lithologies show high Light over Middle Rare Earth Elements (LREE/MREE) ratios (LaN/SmN = 0.89–1.31) and selective enrichments in Sr, K and highly incompatible elements (Rb, Ba, U, Th). We relate these geochemical characteristics to a process of progressive assimilation of host continental crust during the emplacement of the gabbroic plutons. Interestingly, high LREE/MREE ratios (LaN/SmN = 1.45–4.58) and high Rb, Ba, Th and U contents also characterize the basaltic dike swarms associated to the gabbros. Incompatible trace element compositions of these basalts approach those of the melts that formed the most hybridized gabbros. Therefore, we propose that the dike swarms represent melts partially contaminated by assimilation of continental crust material, extracted from the underlying gabbroic crystal mush. Our results suggest that early syn-rift magmatism led to the partial replacement of the thinned continental crust by MORB-type gabbroic bodies, in turn suggesting that oceanic magmatism started prior to continental break-up. Extensive syn-rift magmatism is consistent with the interpretation of the southern Red Sea rift system as a volcanic rifted margin. One possible implication of this study is that extensive but diffuse syn-rift magmatism possibly hampered continental break-up, leading to a protracted rifting stage.

Eocene dike orientations across the Washington Cascades in response to a major strike-slip faulting episode and ridge-trench interaction

Abstract The northern Cascade Mountains in Washington (USA) preserve an exceptional shallow to mid-crustal record of Eocene transtension marked by dextral strike-slip faulting, intrusion of dike swarms and plutons, rapid non-marine sedimentation, and ductile flow and rapid cooling in parts of the North Cascades crystalline core. Transtension occurred during ridge-trench interaction with the formation of a slab window, and slab rollback and break-off occurred shortly after collision of the Siletzia oceanic plateau at ca. 50 Ma. Dike swarms intruded a ≥1250 km2 region between ca. 49.3 Ma and 44.9 Ma, and orientations of more than 1500 measured dikes coupled with geochronologic data provide important snapshots of the regional strain field. The mafic Teanaway dikes are the southernmost and most voluminous of the swarms. They strike NE (mean = 036°) and average ~15 m in thickness. To the north, rhyolitic to basaltic dikes overlap spatially with 49.3–46.5 Ma, mainly granodioritic plutons, but they typically predate the nearby plutons by ca. 500 k.y. The average orientations of five of the six dike domains range from 010° to 058°; W-NW–to NW–striking dikes characterize one domain and are found in lesser amounts in a few other domains. Overall, the mean strike for all Eocene dikes is 035°, and the average extension direction (305°–125°) is oblique to the strike (~320°) of the North Cascades orogen. Extension by diking reached ~45% in one &amp;gt;7-km-long transect through the Teanaway swarm and ranged from ~5% to locally ~79% in shorter transects across other swarms, which corresponds to a minimum of ~12 km of extension. The dominant NE-striking dikes are compatible with the dextral motion on the N- to NW-striking (~355–320°) regional strike-slip faults. Some of the W–NW- to NW-striking dikes were arguably influenced by pre-existing faults, shear fractures, and foliations, and potentially in one swarm where both NE- and lesser W-NW–striking dikes are present, by a switch in principal stress axes induced by dike emplacement. Alternatively, the W-NW–to NW-striking dikes may reflect a younger regional strain field, as ca. 49.3–47.5 Ma U-Pb zircon ages of the NE-striking dikes are older than those of the few dated W-NW–to NW-trending dikes. In one scenario, NE-striking dikes intruded during an interval when strain mainly reflected dextral strike-slip faulting, and the younger dikes record a switch to more arc-normal extension. Diking ended as magmatism migrated into a N-S–trending belt west of the North Cascades core that marks the initiation of the ancestral Cascade arc.

PETROLOGY, GEOCHEMISTRY AND MONAZITE U-Th-Pb AGES OF WEST ÁGUA RASA PALEOPROTEROZOIC POST-COLLISIONAL METAGRANITES FROM SOUTHERN SÃO FRANCISCO CRATON – BRAZIL

During the transition of Rhyacian to Orosirian periods a collision between the Archean Divinópolis and Campo Belo metamorphic complexes occurred in southern São Francisco Craton (SFC) followed by the collapse of the orogen. In the same region, the Statherian period is marked by a huge intraplate magmatic event that is represented in the area by the Para de Minas dyke swarm. Peraluminous granitoid occurrences are reported in the region of Formiga and Itapecerica (Minas Gerais – Brazil), surrounding high metamorphic grade khondalitic paragneisses and banded iron formations. To contribute to the understanding of the tectonic context of the southern SFC during the Paleoproterozoic era, these granitic rocks are studied based on petrographic, geochemical, and monazite U-Th-Pb geochronological analyzes. They were characterized as S-type (metasedimentary origin) peraluminous metamonzogranites, with crustal geochemical signature, and genesis related to anatexis in syn- to post-collisional environment. The geochronological results yielded two groups of Orosirian (~1,90 Ga) and Statherian (~1,78 Ga) ages. These results are related, respectively, to the collapse of the Rhyacian-Orosirian orogen and to the regional warming, associated with the Avanavero-Xiong’er LIP that is represented in the area by the Pará de Minas dyke swarm.

Petrogenesis of Mesozoic giant dike swarms and geodynamical insights about EMI-Gough flavors in the Equatorial Atlantic Magmatic Province

The Rio Ceará-Mirim magmatism produced Mesozoic giant arcuate dike swarms during the initial stage of the West Gondwana breakup near the Equatorial Atlantic Ocean (NE Brazil), where they assemble tholeiites and subordinate intermediate rocks (SiO2 > 57 wt%). A multidata approach based on petrography, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb isotopes enables the discrimination of four geochemical groups (two high-Ti and two low-Ti). High-Ti groups are composed of (1) basalts and basaltic andesites with TiO2 generally higher than 2.0 wt%, MgO > ~3 wt% (named the HT group) and rich in incompatible elements and (2) (trachy)andesites and trachytes with lower TiO2 (<2.5 wt%; Mg < 3 wt%) interpreted as fractionated magmas from the HT group, therefore named evolved HT. Both these groups have similar 87Sr/86Sr(127) (~0.706) and εNd(127) (−2.90 to −2.56) and moderately radiogenic 206Pb/204Pb(m) ratios between 18.66 and 18.21. Conversely, low-Ti tholeiites (TiO2 < 2 wt%) are (3) LT1 composed of evolved basaltic andesites, with ~Ti/Zr < 70, MgO <6 wt%, and Al2O3 > 15 wt%; and (4) LT2 composed of subalkaline (to transitional) basalts and basaltic andesites but ~Ti/Zr >70, and MgO > 6 wt%. These two low-Ti groups are, respectively, less radiogenic (LT1 εNd(127) -4.65 to −4.40) and slightly more radiogenic (LT2 εNd(127) -1.44 to −1.08) in Nd relative to the broad HT group. The geochemical groups present different degrees of enrichment in large ion lithophile and light rare-earth elements coupled with depletion in high-field-strength elements (NbTa) combined with isotopic (Sr-Nd-Pb) signatures compatible with enriched mantle sources. We showed that the EMI (Gough-like) isotope compositions of Rio Ceará-Mirim magmatism may be explained by mixing DMM with enriched SCLM-derived melts plus minor crustal assimilation or by the involvement of OIBs derived from mantle plumes of the South Atlantic area mixed with lithospheric sources. In a geodynamical perspective, given the narrow similarity of HT Rio Ceará-Mirim isotope compositions with EMI-Gough type and high-Ti magmas of Transminas swarm, a genetic connection between Equatorial Atlantic and Paraná-Etendeka provinces must be considered.

Isla San Pedro Nolasco as a Late Miocene intrusive record at the eastern margin of the Gulf of California: Insights from geological, geochemical and geochronological studies

Isla San Pedro Nolasco (ISPN) is a structural high bounded by inactive dextral oblique-slip faults in the east-central part of the Gulf of California rift zone and is composed of intrusive rocks not exposed on other Gulf of California islands. Here we present the reconnaissance results from geological mapping, as well as first geochemical and geochronological data for the ISPN intrusive complex. The intrusive rocks compose a sheet-like body of intermediate and felsic composition intruded by an intermediate and acidic dike swarm. All intrusive rocks (host and dikes) range in age from ca. 9 Ma to 10 Ma (40Ar/39Ar) and show a hydrous ferromagnesian mineral association (amphibole and biotite) with a calc-alkalic and transitional affinity. This hydrated mineralogical association has not been recognized in the coeval rocks along the onshore western margin of the North American plate (coastal Sonora). However, such hydrous mineralogical association is found in the coeval rift transitional volcanic rocks from the Baja California Microplate at Santa Rosalía and Bahía de Los Ángeles – Bahía de Las Ánimas. The ISPN continental block, at least 40 km long, has been pulled apart by transtensional faulting of the late Miocene Gulf of California shear zone before the westward migration of the North America-Pacific plate boundary at ca. 3–2 Ma. Eventually, ISPN became isolated as an island during the late Miocene flooding of the Gulf of California seaway.

Age, geochemistry, and origin of the mid-Proterozoic Häme mafic dyke swarm, southern Finland

We have reappraised the age and composition of the mid-Proterozoic Häme dyke swarm in southern Finland. The dominant trend of the dykes of this swarm is NW to WNW. Petrographic observations and geochemical data indicate uniform, tholeiitic low- Mg parental magmas for all of the dykes. Nevertheless, the variability in incompatible trace element ratios, such as Zr/Y and La/Nb, provides evidence of changing mantle melting conditions and variable crustal contamination. Our ID-TIMS 207Pb/206Pb ages for four low-Zr/Y-type dykes indicate emplacement at 1639 ± 3 Ma, whereas the most reliable previously published ages suggest emplacement of the high-Zr/Y-type dykes at 1642 ± 2 Ma. We propose that the Häme dyke swarm, and possibly also the other mid- Proterozoic mafic dyke swarms in southern Finland, records a progressive decrease in Zr/Y values due to magma generation under developing areas of thinned lithosphere. We consider that the formation of mafic magmas was most probably associated with the upwelling of hot convective mantle in an extensional setting possibly related to the nearby Gothian orogeny. The generation of tholeiitic magmas below continental lithosphere was probably promoted by the elevated mantle temperature underneath the Nuna supercontinent. We speculate that the origin of most of the relatively small mid-Proterozoic mafic dyke swarms, anorthosites, rapakivi granites, and associated rocks found across Nuna was similarly triggered by extensional plate tectonics and the convection of anomalous hot upper mantle below the supercontinent.

The statherian taphrogenesis of the South American Platform

Records of Statherian intracontinental ruptural tectonics, magmatism and sedimentation events are recognized all over the South American Platform as an immediate tectonic response to the formation of a large continental domain during Rhyacian-Orosirian orogenic collage. In this paper, we present a preliminary synthesis of the variety of tectonic-sedimentary and magmatic occurrences of these taphrogenic processes along the Statherian Period, which are recorded in all cratons, as well as in the basement of most Brasiliano orogenic belts. The Statherian rock units are better preserved over the cratons, and within the Brasiliano structural provinces they are reduced by erosional processes and discretely overprinted by Tonian and Cryogenian structures, and strongly by the Ediacaran-Brasiliano Orogeny. Clastic sediments are dominantly mature, primary structures clearly pointing to the environmental setting. Anorogenic bimodal magmatism, represented by volcanic and plutonic rocks is common, where felsic rocks are largely dominant. Their geochemical signature indicates within-plate settings and dike swarms are common within the cratons. The magmatism was very intense locally, suggesting LIP occurrences, although in need of additional geophysical and petrological support. The taphrogenic and magmatic events took place within Orosirian and Rhyacian basement, in which main tectonic-magmatic events ceased just before the Late Statherian, occasionally extending into the Calymmian.

Resende lamprophyres: new petrological and structural interpretations for a regional Upper Cretaceous alkaline mafic dyke swarm

Resende lamprophyres: new petrological and structural interpretations for a regional Upper Cretaceous alkaline mafic dyke swarm