Late Dikes Research Articles

Petrography of the Mesozoic Alkaline Rocks of the Taezhniy Massif (South Yakutia, Aldan-Stanovoy Shield, Leglier Ore Cluster)

The article is concerned with the petrographie compositions of the Mesozoic alkaline igneous rocks of the Taezhniy massif and its small bodies (dikes). The Taezhniy massif is located in the central part of the Nimnyr block (Aldan-Stanovoy shield). According to our field observations, it was first determined that the Taezhniy massif has a two-phase structure, its rocks of contain syenite-porphyry and leucocratic syenites. The rocks of the massif intrude the Archean granites, and are themselves intruded by late dikes of bostonites and vogesites. Based on petrographic studies, we also identified two phases of intrusion in the Taezhniy massif – syenite-porphyry and leucocratic syenites. The difference of them is as follows: in the syenite-porphyry rocks, plagioclase predominates over K-feldspar, and the content of dark-colored minerals reaches 10%. The rocks of the second phase are characterized by the predominance of K-feldspar over plagioclase, with the content of dark-colored minerals up to 5%. The order of intrusion of the massif formations is determined by the presence of xenoliths of syenite-porphyry rocks in leucocratic syenites. The rocks of the bostonite dike cutting the massif are characterized by the absence of amphibole and an increased content of pyroxene. This is most likely due to the fact that, the rock contains xenoliths of the host rocks of biotite-pyroxene composition. The dikes also conyain xenoliths of the second phase rocks, which indicates a later age of this dike. The bostonite dike located in the immediate vicinity of the massif is almost identical to the dike found in the massif, except an amphibole in the composition of the rocks. The only dike of vogesites that intersects the body of the massif is characterized by the presence of two varieties of amphibole: common hornblende and a sufficient amount of barkevikite, which phenocrysts stand out clearly against the background of the total microcrystalline mass. Also, in the course of crystal-optical studies, it was noted that in the structure of the massif there is an increase in the leucocratic magmatism from the early phase to the late, but with a decrease in this indicator, taking into account the introduction of vogesite dikes at the late stages of magmatism development. Based on this, the conclusion is made about the antidromic development of the rocks of the Taezhniy massif. When comparing the evolution of magmatism of the Taezhniy massif with the Ryabinoviy massif, a possible gold-ore specialization of the studied object is suggested.

The Late Jurassic Zedong ophiolite: A remnant of subduction initiation within the Yarlung Zangbo Suture Zone (southern Tibet) and its tectonic implications

The Zedong ophiolite is the largest ophiolite massif east of Dazhuqu in the Yarlung Zangbo Suture Zone in the southern Tibetan Plateau. However, its age, geodynamic setting and relationship to the Xigaze ophiolite remain controversial. New zircon U–Pb ages, whole-rock geochemical and Nd–Pb isotopic data from ophiolitic units provide constraints on the geodynamic and tectonic evolution of the Zedong ophiolite. U–Pb zircon geochronology of dolerite lavas and late gabbro–diabase dikes yield weighted mean ages of 153.9 ± 2.5 Ma and 149.2 ± 5.1 Ma, respectively. Strong positive εNd(t) and positive Δ7/4Pb and Δ8/4Pb values indicate derivation from a highly depleted mantle source with an isotopic composition similar to that of the Indian MORB-type mantle. The geochemistry of ophiolitic lavas and early dikes are analogous to typical island arc tholeiites whereas late dikes are similar to boninites. The geochemistry of these rock types suggests multi-stage partial melting of the mantle and gradually enhanced subduction influences to the mantle source through time. Combined with the MORB-like 162.9 ± 2.8 Ma Luobusha ophiolitic lavas, we suggest that the Luobusha lavas, Zedong lavas and early dikes originated in an infant proto-arc setting whereas late dikes in the Zedong ophiolite originated in a forearc setting. Together, they represent a Neo-Tethyan subduction initiation sequence. The Late Jurassic intra-oceanic proto-arc to forearc setting of the Zedong ophiolite contrasts with the continental margin forearc setting for the Xigaze ophiolite, which suggests a laterally complex geodynamic setting for ophiolites along the Yarlung Zangbo Suture Zone.

Mesozoic felsic dikes in the Jiaobei Terrane, southeastern North China Craton: Constraints from zircon geochronology and geochemistry, and implications for gold metallogeny

The Jiaobei Terrane in the southeastern North China Craton (NCC) witnessed intense magmatic pulses during Mesozoic in response to lithospheric thinning. These include sporadic felsic-porphyritic dikes which preserve important information on the magmatic processes and of its implications on lithospheric thinning. Here we present a detailed investigation of representative felsic-porphyritic dikes based on whole-rock geochemical analysis, zircon LA-ICPMS U-Pb geochronology, trace elements and hafnium isotopic compositions. Our results identify stages of felsic dikes in the Jiaobei Terrane with ages in the range of 131–127 Ma and 123–116 Ma. The early stage felsic dikes (ESFDs) have negative zircon ɛHf(t) (−22.4 to −11.1), high zircon EuN/EuN* (0.41–0.92) and Ce4+/Ce3+ (62–944), low Ti-in-zircon crystallization temperatures (524–738 °C), and are geochemically comparable to the coexisting Guojialing granodiorite. Abundant inherited zircons and scattered zircon Hf, Yb/Gd, Th/U, and Ce/Sm values suggest that the magmas from which the dikes formed involved crustal contamination. They are interpreted to be the hypabyssal phase of the Guojialing suite sourced from the Paleoproterozoic lower crust of the eastern NCC with input of mantle components, associated with extensive crust-mantle interaction during the peak lithospheric thinning. The late stage felsic dikes (LSFDs) are characterized by variable zircon ɛHf(t), EuN/EuN* and Ce4+/Ce3+ values (−20.1 to −12.2, 0.38–0.88 and 64–956 for the granodiorite- and quartz-porphyritic dikes; −27.6 to −16.2, 0.17–0.75 and 22–503 for the monzogranite-porphyritic dikes), low Ti-in-zircon crystallization temperatures (562 to 774 °C), and display geochemical similarity with the Aishan granitoids. Systematic variations of zircon Hf, Yb/Gd, Th/U and Ce/Sm suggest that fractional crystallization of hornblende played a significant role in the formation of the late dikes. The LSFDs originated from the Neoarchean-Paleoproterozoic lower crust of the NCC with heterogeneous involvement of lesser amounts of mantle materials than the early dikes, consistent with weak crust-mantle interaction during the waning stage of lithospheric thinning. In the LSFDs, the granodiorite and monzogranite porphyries are broadly coeval (123–118 Ma) with large scale gold mineralization in the area as also favored by the high zircon Ce4+/Ce3+ ratios. Our study also provides new insights into the formation of gold deposits.

Discovery of the Hendou-abad copper mineral district and its association to dikes: A reconstruction scenario for exploration of Cu-porphyry, northeast Isfahan, Iran

The Hendou-abad copper mineral district is located along the eastern margin of the central part of Urumieh–Dokhtar Magmatic Arc (UDMA). The mineralization is hosted in the Late Eocene basaltic andesite and trachybasalt, where it is associated with epidote, calcite, and quartz as alteration minerals, and red, white, and green breccias. Two main events of mineralization with alteration have been distinguished: hypogene and supergene. Abundant parallel dike swarms intrude Late Eocene volcanic rocks of the Henduo-abad. The Hendou-abad dikes are seen as a few cm to 4m wide and <1200m long. The purpose of this work was to survey their petrogenesis and the tectonic implication of the dikes. Further, to investigate the relationship between dikes and known copper mineralization in Hendou-abad, and then examine exploration potential in adjacent areas. The Hendou-abad dikes, based on trend, relative age, and geochemical composition, are divided into two groups: early and late dikes. The geochemical character of these dikes displays an arc setting and the late dikes show higher contents of Nb, Zr, P, K, Rb, Ba, and slightly elevated Ti. The geochemical composition of the dikes suggests two magmatic stages over time: a subduction-modified mantle source is thought of for the host basaltic andesites and the early dikes in a primitive arc stage, while an enriched asthenospheric mantle source is pictured for the late dikes in a later, more mature arc stage. Structural evidences from field-work of dikes, strike-slip and thrust faults, and microscopic studies of the associated hypogene mineralization imply that they may have formed through north-south directed extension during an uplift event. The pattern of dike orientations, their intense propylitic alteration, and the spatially association with mineralized breccias, sheeted calcite, and mineralized quartz veins, could suggest emplacement of a porphyry stock at depth. Our data show that the east-trending green dikes are associated with red and green breccias in southern Ardestan and that they could be used as an exploration target for copper mineralization.

Subvolcanic gabbro–porphyrite and intrusive diorite, and sulfide mineralization of the Dzhusa volcanogenic massive sulfide deposit (Southern Urals)

Two sequentially formed groups of dikes in the gabbro–porphyrite complex have been distinguished, the ages of which are early Eifelian (early dikes) and early Givetian (late dikes). We have estimated the temperature impact of ore contact metamorphism, which is related to dikes of the Lower Carboniferous Magnitogorsk intrusive complex. A hidden zonality of microimpurities in the ore-forming minerals has been established for the first time by the LA-ICP-MS method. The ore formation age has been determined as early Eifelian–early Givetian.

Sulfide Immiscibility Induced by Wall-Rock Assimilation in a Fault-Guided Basaltic Feeder System, Franklin Large Igneous Province, Victoria Island (Arctic Canada)

The Southern Feeder Dike Complex is part of the Franklin Large Igneous Province (LIP), exposed in the Minto Inlier of Victoria Island in the Canadian Arctic. Previous field and geochemical studies on the Franklin LIP considered its igneous rocks to be prospective for Fe-Ni-Cu mineralization. The Southern Feeder Dike Complex comprises a series of NW-SE-trending gabbroic intrusions and sedimentary hosts. Field and textural relationships show that the Complex intrusions were emplaced contemporaneously with Neoproterozoic normal faulting. Faulted contact zones correspond to prominent first derivative magnetic lineaments. Gabbroic dikes have intrusive contacts against brecciated country rock, and diabasic microxenoliths in basaltic matrices indicate multiple intrusive/brecciation events. Intrusive breccias are commonly overprinted by hydrothermal greenschist facies assemblages, with calcite + pyrite veins filling open spaces between breccia fragments. Late dikes emplaced into these heterogeneous breccias contain disseminated globular and net-textured sulfides suggesting that sulfide immiscibility was triggered on a local scale by assimilation of local wall rock. This inference is supported by elevated δ34S values of sulfides in these dikes, consistent with assimilation of country rocks. Wall-rock assimilation would have been facilitated by fault-related brecciation and cataclasis, which would expose extensive xenolith surface areas to fresh magma. Gossanous and meter-scale semimassive sulfide showings associated with dikes and sills located upsection from the Southern Feeder Dike Complex suggest that immiscible sulfide liquids may have been flushed downstream (or upsection) during replenishment of composite dike systems. Fault-mediated melt ascent along northwest-southeast faults has been documented elsewhere in the Minto Inlier, providing equivalent opportunities for wall-rock assimilation and consequent triggering of sulfide immiscibility and sulfide melt redistribution. The evidence preserved in the Complex confirms the Fe-Ni-Cu potential of the Franklin LIP and informs current models of ore deposit formation in conduit-type magmatic plumbing systems.

Geochemical constraints on the tectonic setting of basaltic host rocks to the Windy Craggy Cu-Co-Au massive sulphide deposit, northwestern British Columbia

Windy Craggy is an approximately 300 Mt Cu-Co-Au volcanogenic massive sulphide (VMS) deposit in northwestern British Columbia, Canada. The Windy Craggy deposit is hosted by the Middle Tats Volcanics (MTV), a Late Triassic volcano-sedimentary sequence of intercalated mafic pillowed to massive volcanic flows and sills and calcareous argillite that are part of the Alexander terrane. The host footwall and hangingwall flows and sills are predominantly alkalic basalts (Nb/Y > 0.70). MTV alkali basalts at Windy Craggy are enriched in light rare earth elements (LREEs) >100X chondrite compared to chondrite, have steep REE patterns [(La/Yb)cn = 7.1–25.4], and generally lack the Ta and Nb depletions relative to primitive mantle (e.g. [Nb/Th]pm = 0.68–1.94) characteristic of arc environments, although most have [Nb/La]pm < 1. By contrast, volcanic rocks away from the deposit (and regionally; Lower Tats Volcanics, LTV) as well as late dikes that cross-cut all lithologies including metamorphic and deformational fabrics are sub-alkalic tholeiitic to calc-alkaline basalts and basaltic andesites that are less enriched in the LREEs (10–100X chondrite), have less steep REE patterns [(La/Yb)cn = 0.41–10.6], and show well-developed Ta and Nb depletions (arc signatures; [Nb/Th]pm = 0.20–0.79), consistent with formation in an oceanic arc environment. The co-occurrence of tholeiitic/calc-alkaline arc rocks with alkalic rocks indicates that the LTV (former) and MTV (latter) formed from melts that were influenced to varying degrees by subducted oceanic crust, and likely formed within a back-arc basin setting formed on a rifted oceanic arc. There is no geochemical or isotopic evidence for major involvement of continental crust. The LTV basalts likely were produced by progressive depletion in the source by partial melting of mantle overlying the subducting oceanic crust. The presence of the MTV alkalic Windy Craggy rocks overlying the LTV is consistent with the presence of a slab-window, perhaps related to subduction of a spreading centre, which allowed more enriched magmas to reach the surface with only minimal interaction with subduction-modified mantle. The presence of this slab-window might have provided the mechanism for the generation of anomalously high heat flow close to the seafloor, which initiated and sustained vigorous, long-lived hydrothermal activity necessary for the precipitation of large accumulations of massive sulphide. To our knowledge, this is the first example of a large VMS deposit associated with a slab-window.

Geodynamic formation conditions and magmatic sources of late cambrian sills and dikes in the daribi range (Western Mongolia)

Geodynamic formation conditions and magmatic sources of late cambrian sills and dikes in the daribi range (Western Mongolia)

Occurrence and significance of a quartz–amphibole schist xenolith within a mafic microgranular enclave in the Xiangshan volcanic-intrusive complex, SE China

The Xiangshan volcanic-intrusive complex is composed of rhyolitic crystal tuffs, welded tuffs, rhyodacite, porphyroclastic rhyolitic lava, subvolcanic rocks such as granite porphyry, and late quartz monzonitic porphyry and lamprophyre dikes. We report the first occurrence of a quartz–amphibole schist (QAS) xenolith enclosed within a mafic microgranular enclave (MME) in the Xiangshan volcanic-intrusive complex. The mineralogy of this xenolith consists of amphibole, biotite, quartz, and minor plagioclase. Petrographic and mineral composition studies indicate that the protolith of this xenolith likely originated from the metamorphic basement beneath Xiangshan. The amphibole (actinolite and magnesiohorblende) has been partially replaced by orthopyroxene at 800–1000°C and by diopside at <700°C, according to mineral thermometers; this replacement process may have taken place after the xenolith was trapped by the mafic magma host (now an MME). Studies of the QAS xenolith provide new information on the emplacement history of the mafic magma. The peak metamorphic temperature for amphibole replaced by pyroxene is higher than the crystallization temperature of the subvolcanic magma, which indicates that the heat of pyroxene formation must have been provided by the engulfing mafic melt. This magma must have emplaced to crustal level and trapped the QAS as a xenolith and then injected into the felsic magma. We suggested that the hybridization processes for the major elements of the pristine mafic magma may have been contaminated by crustal rocks to form its present composition of MME before mafic magma injection. However, the hybridization process appears not to have been formed via a single-stage process because various types of MMEs are presented in the Mesozoic magmatic rocks of SE China.

Intrusion and contamination of high-temperature dunitic magma: the Nordre Bumandsfjord pluton, Seiland, Arctic Norway

The Caledonian Nappe Complex of Arctic Norway provides rare insights into the interaction between mafic–ultramafic magmas and the deep continental crust. The Kalak Nappe Complex contains >25,000 km3 of mafic igneous rocks, mostly layered gabbros, making up the 570–560 Ma Seiland Igneous Complex. The complex has been intruded by a series of ultramafic magmatic rocks, including the Nordre Bumandsfjord pluton. Field relationships in this pluton show that extremely fluid, dry, relatively Fe-rich (Fo81) dunite magmas intruded a pile of cumulate gabbros, with block stopping and intrusive brecciation. Diking on scales from mm to metres and extensive melting and assimilation of the gabbros attest to high temperatures, consistent with a 2-km-wide granulite-facies contact aureole. Major- and trace-element trends show that the dunites were progressively contaminated by a cpx-rich partial melt of the gabbros, producing a range of lithologies from dunite through lherzolites to wehrlite. Experimental studies of natural samples at 0.8–1 GPa define the dunite solidus at 1,650–1,700 °C. In the average peridotite, contamination has produced a crystallisation interval of ca 400 °C (1,600–1,200 °C); this would provide large amounts of heat for melting and metamorphism and would maintain the fluidity of the magmas to relatively low T, consistent with field relationships. Thermochemical and dynamic modelling demonstrates that the dunitic primary magmas may represent the last melting of a rapidly ascending diapir of previously depleted subducted oceanic lithosphere. The mafic rocks of the Seiland Complex may already have been extracted from this diapir, and the late dikes of the province may reflect melting of the asthenosphere as the diapir spread out beneath the lithosphere. Ultramafic magmas, abundant in the Archean, may still be more common than usually assumed. However, they would only penetrate to the shallow crust under unusually extensional conditions, where ascent could outpace assimilation.

Geochemical and isotopic constraints on the petrogenesis of the Puesto La Peña undersaturated potassic complex, Mendoza province, Argentina: Geodynamic implications

Major and trace element and radiogenic and stable isotope data are reported for the Miocene Puesto La Peña undersaturated potassic complex, located in the Argentine Precordillera. The complex is composed of a pyroxenite core surrounded by a malignite–borolanite association, followed by radial and ring dikes composed of alkaline lamprophyres, tephrite to phonolite and intermediate varieties, porphyritic microledmorite, benmoreite, alkaline trachyte and a swarm of ultrabasic ouachitite dikes. Late trachytic to phonolitic volcanic necks and associated dikes crop out surrounding the malignitic–borolanitic massif.Major and trace element data are consistent with their derivation by partial melting from an enriched, probably lithospheric, metasomatized heterogeneous mantle involving spinel- and garnet-facies mantle sources. The trace element and isotope data indicate that all rock types are derived from a common parental magma and are thus cogenetic.Sr, Nd and Pb variations in the different studied lithologies are restricted, except for the evidence of Sr decoupling in the ouachitite samples. Their Sr–Nd isotope compositions follow the “mantle array” defined by oceanic basalts, within the OIB field, consistent with an intraplate depleted source. The genesis of the ouachitite is interpreted to be related to mixing between asthenospheric magma and melts from the lower lithosphere consisting of K-rich metasomatic layers. The Dupal like Pb isotopes signature suggests a mantle modification by introduction of continental crust material in the upper mantle; this is consistent with the Sr–Nd isotope data from the late silica-undersaturated felsic dikes and volcanic necks.Multistage mantle extraction would have occurred by the end of Neoproterozoic to lower Paleozoic times as indicated by the isotopic data. Partial melting was initiated by mantle upwelling decompression during lithosphere extension. Back-arc extensional conditions during the latest Early Miocene (19Ma), related to the geometry of the Pacific subducted plate beneath the South American plate, favored the tectonic conditions for magma ascent from a magmatic reservoir located at around 30km depth in the crust. A fractional crystallization process, primarily of clinopynoxene, plagioclase, alkali feldspar, nepheline and lesser biotite, magnetite, and apatite, first yielded the cumulate clinopyroxenite, followed by the malignite–borolanite association and the late dikes and volcanic necks. The morphology of the complex, as well as the relationship between their different facies, points to a cauldron subsidence model of emplacement.

Os diques latíticos portadores de ouro e sulfetos da Associação Shoshonítica de Lavras do Sul – RS: petrogênese e geoquímica

A Associação Shoshonítica de Lavras do Sul (ASLS), situada no extremo sul do Brasil, tem idade neoproterozóica e hospeda ocorrências importantes de Au-Cu-sulfetos, relacionadas por alguns autores a depósitos do tipo Cu-Au pórfiro. Dados de campo, petrográficos, mineralógicos e geoquímicos permitiram identificar e classificar os diques que ocorrem na ASLS cortando os monzonitos hipabissais, andesitos e lamprófiros, em três tipos principais: diques compostos autoclásticos, diques latíticos e diques latíticos com anfibólio. Sua grande variedade litológica e textural é resultado da diferenciação magmática, segregação mineral e processos de mistura de magmas. Os diques mostram afinidade shoshonítica, com teores de K2O &gt; (Na2O –2), acompanhados de conteúdos elevados de Rb, Ba e Sr e baixos a moderados de Nb, Zr e ETRP, assinatura típica da Associação Shoshonítica de Lavras do Sul. Os dados obtidos permitiram identificar dois trends composicionais: um alto-Ti, enriquecido em Fe2O3t, P2O5, HFS, LILE e ETR, com razões K2O/Na2O &lt; 0,8; outro baixo-Ti, que predomina na ASLS, com menores teores dos elementos anteriormente citados e razão K2O/Na2O &gt; 1,4. Os diques alto-Ti, com menores razões K2O/Na2O indicam uma evolução das fontes e processos no sentido da produção de magmas alcalinos saturados em sílica mais sódicos, como é comum na evolução do magmatismo pós-colisional. A cristalização dos magmas e o progressivo enriquecimento dos voláteis e de complexos de Au-S-Cl geraram uma supersaturação do sistema, causando o segundo ponto de ebulição e a vesiculação de porções dos diques. Admite-se que o magma lamprofírico pode ser a fonte principal do ouro e enxofre do sistema. A mistura dos magmas latíticos ou traquíticos com os lamprofíricos, pode ser responsável por um aumento do potencial de oxidação e dos voláteis no sistema, desencadeando assim a separação da fase volátil, a desestabilização dos complexos e a precipitação dos sulfetos magmáticos portadores de Au, além da própria formação das soluções hidrotermais.

The Koshrabad granite massif in Uzbekistan: petrogenesis, metallogeny, and geodynamic setting

The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) mafic and quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, often rimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the still unconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized by high f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product of fractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. In high f values and HFSE contents the massif rocks are similar to A-type granites. Data on the geochemical evolution of the massif rocks confirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acid melts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusion of compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronous melting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter in the dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulation in commercial concentrations.

Comparative characteristics of Middle Paleozoic and Late Riphean plume magmatism of the Siberian platform

Based on isotopic and geochemical data for Late Riphean dikes and sills and for Devonian dolerite dikes and basalt covers within the Sette-Daban rift in the western part of the Siberian platform, we proved conceptions about the participation of various deep sources in their formation. The inverse correlation in Devonian basites between concentrations of Nb, light rare earth elements, and a number of other highly incompatible elements on the one hand and Zr, Y, and other moderate incompatible elements, including heavy rare earth elements on the other hand allows us to assume that two sources participated in the formation of melts. The source of dolerites is close to the EMORB type, and the source of basalts is close to the OIB type. The compositions of Riphean rocks correspond to a trend for which magma formation occurred with the participation of a source with characteristics between NMORB and EMORB and also a component typical of subduction zones. The data obtained imply associate formation of basites with the influence of mantle plumes on the lithosphere of the southeastern part of the Siberian craton in the Late Riphean and Middle Paleozoic. In the Riphean the plume mantle was composed of a moderately depleted mantle of the EMODB type and a mantle with the composition close to the above-subduction mantle, which was metosomatically changed under the influence of fluid water, which caused the appearance of a Nb and Ta deficit in melting products. The isotopic characteristics Nd(147Sm/144Nd = 0.165 and ɛNd(T) ∼ 2.3–4.7) of rocks show the moderate depleted nature of these sources.

Geochemical Characteristics of Late Mesozoic Dikes, Jiaodong Peninsula, North China Craton: Petrogenesis and Geodynamic Setting

Widespread Cretaceous dikes in the Jiaodong Peninsula of eastern China are genetically related to gold deposits. Bulk-rock major element analyses show that they are composed of medium-K, subalkaline to shoshonitic rocks characterized by wide ranges of SiO2 (48.98-71.38%) and MgO (0.63-10.02%), and extremely high contents of Al2O3 (14.32-16.06%). They are enriched in large-ion lithophile elements (LILE, e.g., K, Rb, Sr, Ba) and light rare-earth elements (LREE), but depleted in high-field-strength elements (HFSE, e.g., Ta, Nb, Zr, Hf), and display consistent primitive mantle- and chondrite-normalized multi-element patterns on spidergrams, different from Cenozoic basalts in the Peninsula. These data suggest a magma origin by partial melting of an ancient enriched lithospheric mantle. High Th/Yb and Pb/Ce ratios and weak negative Ce anomalies reflect involvement of recycled sediments in the mantle reservoir. Strong fractionation in REE, with LREE >100 times chondrite and HREE <10 times chondrite, indicates the presence of garnet in the mantle source. Nb* values of 1.49-2.38 (>1), K* values of 0.08-0.14 (<1), and similar Zr/Hf (32.15-40.65), U/Th (0.10-0.33), and Hf/Sm (0.45-0.67) ratios in all the dikes, along with published chemical and Sr-Nd isotopic compositions of lamprophyres (Cheng et al., 1998; Sun et al., 2000; Yang et al., 2004) preclude crustal contamination during magma ascent and binary mixing between mantle and crustal magmas. The geochemical features of these dikes can be interpreted as the result of interaction of old lithospheric mantle with melt and/or fluid generated from subducted lower/middle crust of the Yangtze craton and/or Izanagi oceanic crust. The source for the Late Mesozoic dikes was completely reconstructed, enriched lithospheric mantle containing disseminated amphibole-and phlogopite-bearing pyroxenite veins surrounded by refractory peridotite wall rock. The enriched source was modified by subsequent partial melting due to sinistral transtensional strike-slip movement of the Tan-Lu fault that led to widespread dike intrusion. Trace element geochemistry and regional geology indicate that these dikes formed in a back-arc extensional environment.

Early Cretaceous adakitic granites in the Northern Dabie Complex, central China: Implications for partial melting and delamination of thickened lower crust

To date, few adakitic rocks have been reported in direct association with contemporary intra-continental extensional structures, which has cast doubt on genetic models involving partial melting of the lower crust. This study presents Early Cretaceous (143–129 Ma, new Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages) adakitic granites, which are directly associated with a contemporary metamorphic core complex (i.e., the Northern Dabie Complex in the Dabie area). These granites exhibit relatively high Sr contents, negligible to positive Eu and Sr anomalies, high La/Yb and Sr/Y ratios, but very low Yb and Y contents, similar to subducted oceanic crust-derived adakites. They are also characterized, however, by very low MgO or Mg # and Ni values, and Nd–Sr isotope compositions ( ε Nd( t) = −14.6 to −19.4 and ( 87Sr/ 86Sr) i = 0.7067–0.7087) similar to Triassic continent-derived eclogites subducted in the Dabie–Sulu Orogen. Additionally, late granitic dikes in the adakitic intrusions exhibit low Sr contents, clearly negative Eu and Sr anomalies, low La/Yb and Sr/Y ratios, but relatively high Yb and Y contents, similar to 118–105 Ma granites in the Northern Dabie Complex. Based on composition and geochronology data of Neoproterozoic amphibolites and orthogneisses, Triassic high- to ultra-high pressure metamorphic rocks, and Early Cretaceous mafic–ultramafic intrusive rocks, and the constraints provided by experimental melt data for tonalites, metabasaltic rocks and eclogites, we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile-bearing eclogitic lower crust as a consequence of Triassic–Middle Jurassic subduction and thrusting. The late dikes probably originated from plagioclase-bearing intermediate granulites. Moreover, we suggest that late Mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of Early Cretaceous mafic–ultramafic intrusive rocks in the Dabie area, and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite. Asthenospheric upwelling in response to post-collisional delamination of lithospheric mantle was likely to have provided the heat source for the Cretaceous magmatism.

POST-VARISCAN MAFIC DIKES FROM THE LATE OROGENIC COLLAPSE TO THE TETHYAN RIFT: EVIDENCE FROM SARDINIA

Dolerite dikes cut the Lower Paleozoic medium- to high-grade metamorphic basement, the Sardinia-Corsica batholith and the Stephanian - Autunian calcalkaline effusives (1st volcanic episode, according e.g. to Bonin, 1989; Cabanis et al., 1990) with a N-S trend and subvertical dip. Their occurrence is reported from several areas of Sardinia, from the N to the SE. The basic dikes are associated with diorite to rhyolite dikes having trends ranging from E-W to N-S. A late Triassic lamprophyric dike intruded the garnet-staurolite-kyanite micaschists near Mt. Nieddu and has a 40°N direction. Two 40Ar-39Ar age determinations for the dolerite dikes were carried out on amphibole. Ages between 253.8±4.9 and 248±8 Ma (Permian - Early Triassic) probably correspond to the emplacement interval. A continental within-plate geochemical signature, with prevalent tholeiitic to transitional (dolerites) and minor alkalic (lamprophyre) terms, has been determined. The relative LILE and REE enrichment compared with the MORBs and Nd isotopic ratios (4 bulk samples; initial 143Nd/144Nd ratios between 0.512529±0.000025 and 0.513143±0.000023) is consistent with a source in a sub-continental lithospheric mantle. The remarkable compositional and isotopic variability can be explained by the partial melting of a heterogeneous source. Crustal contamination variably affects some transitional dikes; the host Arzachena monzogranite probably represents the major contaminant source. On the whole, the geochemical data support an anorogenic geochemical affinity, in accordance with the extensional regional tectonics, for the Permo-Triassic dikes in Sardinia. The E-W emplacement patterns correspond to the major fractures and primarily R and RI structures associated with the late Variscan transcurrent-extensional E-W faults. The N-S emplacement trend is consistently associated with a generalized E-W extension of the Sardinian crust: in a predrift restoration it coincides with the patterns of extension between the European and Insubric crust at the beginning of the Tethyan rift. In Sardinia, the lamprophyre dike is significantly younger than the late Variscan alkaline volcanites and dikes of the rest of the Mediterranean domain (e.g. Eastern Provence: 278-264 Ma, 40Ar/39Ar isotope dating on feldspar from rhyolites and on plagioclase from mafic flow, Zheng et al., 1992; Western Pyrenean Axial Zone: 271-266 Ma, K-Ar isotope dating on kaersutite phenocrysts from mafic dikes, Debon and Zimmermann, 1993), in accordance with the diachronism of both the first and second volcanic episodes in different areas of the Mediterranean domain. The overlap between the calc-alkaline events and the volcanic and sub-volcanic alkalic event, is not exclusive to Sardinia and Corsica (e.g. the Pyrenees and Pan-African orogen).

Four successive episodes of Late Pan-African dikes in the central Elat area, southern Israel

Four successive episodes of Late Pan-African dikes in the central Elat area, southern Israel

Fe-Ti basalts and propagating-rift tectonics in the Josephine Ophiolite

The Jurassic Josephine Ophiolite of northwest California and southwest Oregon is one of the largest and most complete ophiolites in North America and clearly formed in a suprasubduction-zone setting. The geochemistry of lavas and dikes is highly variable because of a range in magma types and different degrees of fractionation. Basalts rich in Fe and Ti occur in the uppermost part of the extrusive sequence and as late dikes cutting oceanic serpentinite. They have MORB (mid-oceanic-ridge basalt) affinities and appear to be unrelated to the rest of the ophiolite that has affinities dominantly transitional between IAT and MORB (IAT = island-arc-tholeiite). This late Fe-Ti-MORB suite was erupted while the ophiolite was undergoing the last half of ∼50° of tilting of the entire crustal sequence and after widespread serpentinization of ultramafic cumulates and uppermantle peridotite related to freezing of the axial magma chamber. Fe-Ti basalts are characteristic of propagating spreading centers on mid-ocean ridges and in at least one backarc basin. Geochemical, structural, stratigraphic, and regional geologic constraints suggest formation of the Josephine Ophiolite by propagation of a spreading center into rifted island-arc lithosphere that is preserved along the margin of the ophiolite. Another possible indication of propagating-rift tectonics is the presence of off-axis metalliferous sedimentary rocks that may have been deposited as hydrothermal plume fall-out from a second propagating spreading center. The Lau Basin provides modern analogues, including spreading centers propagating into rifted arc lithosphere, into older backarc ocean crust, and across the arc into the forearc.

Petrogenesis of the Red Mountain pluton, Laramie anorthosite complex, Wyoming: implications for the origin of A-type granite

Abstract The Red Mountain pluton, the youngest intrusion in the Laramie anorthosite complex, is one of five monzonitic to granitic A-type plutons intruded at 1.43 Ga in the Laramie Mountains of southeastern Wyoming. The Red Mountain pluton ranges in composition from fayalite monzonite, clinopyroxene quartz monzonite, and biotite–hornblende quartz syenite to granite. The Red Mountain pluton is petrologically and geochemically distinct from the northern and southern Sherman batholith, the Sybille intrusion, and the Maloin Ranch pluton. It has higher FeO t /(FeO t +MgO) and higher K 2 O, is more strongly enriched in REEs, and at any given silica content it has lower abundances of TiO 2 , FeO, MgO, CaO, and P 2 O 5 . The fayalite monzonite in the Red Mountain pluton crystallized at temperatures around 1000 °C and oxygen fugacities 1–2 log units below that of the FMQ buffer, similar to the crystallization conditions of the Sybille and Maloin Ranch plutons. The biotite–hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of FMQ. Most of the units in the pluton carry mantle-like Nd, Sr, and Pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the LAC, indicating that the pluton evolved mainly via differentiation. Substantial crustal assimilation occurred only in the late dikes and the granite. We suggest that all of the 1.43 Ga A-type intrusions in southeastern Wyoming are ultimately derived from tholeiites. Extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust. The differences between the Red Mountain pluton and other A-type granitoids in southeastern Wyoming are related to the extent of differentiation prior to crustal assimilation. The compositional ranges of A-type granitoids in southeastern Wyoming suggest that the geochemical spectrum of A-type granites worldwide similarly are produced from evolved mantle-derived melt that has assimilated varying proportions of crustal material.