Composite Dykes Research Articles

Post-orogenic extension in the eastern part of the Jiangnan orogen: Evidence from ca 800–760 Ma volcanic rocks

We present a systematic geochronological and geochemical study on ca 800–760Ma volcanic rocks in the eastern part of the Jiangnan orogen. The Xucun composite dykes are dated at ca 805Ma; the mafic components have OIB-like trace-element patterns and positive anomalies in Zr and Hf. The least-contaminated sample has relatively depleted Nd isotopic features, suggesting the Xucun mafic dykes may have been generated from the partial melting of OIB-like asthenosphere with later crustal contamination. The Xucun felsic dykes have decoupled Nd–Hf isotopes, and the Hf-isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early Neoproterozoic juvenile crustal materials, with minor incorporation of Paleoproterozoic crustal components. The ca 800–790Ma Shangshu volcanics include two compositional series: calc-alkaline and tholeiitic. The Shangshu calc-alkaline volcanics in the Minjiawu area have low abundances of LILE, HFSE and high Na2O contents and Sr/Y ratios, similar to adakitic rocks. The evident arc-like geochemical features and radiogenic Nd isotopes (ɛNd(t) values of +3.7 to +4.8) suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by Na-rich melts released from the subducted slab. The tholeiitic mafic rocks from the Shangshu bimodal volcanics represent two different magma sources. The partial melting of metasomatized lithospheric mantle led to the formation of arc-like basalts with low TiO2 contents, negative anomalies in Zr and Hf, and high values of Mg# and ɛNd(t) (+6.2), whereas the partial melting of asthenospheric mantle generated volcanic rocks with high TiO2 contents and low positive ɛNd(t) (+1.4 to +2.7), without negative anomalies of Nb, Ta, Zr and Hf. The Shangshu felsic rocks were formed by the reworking of early Neoproterozoic juvenile arc crustal materials. The ca 760Ma mafic rocks from the Puling bimodal volcanics generally have low TiO2 contents (<0.9wt%), nearly flat REE distributions and arc-like trace-element patterns. They may have been generated from the high-degree partial melting of metasomatized lithospheric mantle. One sample has a high TiO2 content (2.41wt%) and high ɛNd(t) (+6.2), with overall OIB-like trace-element patterns, implying the local partial melting of asthenospheric mantle. The occurrence of significant volumes of bimodal volcanics in the eastern part of the Jiangnan orogen suggests an extensional setting in the period ca 800–760Ma. The evident partial melting of newly-metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post-orogenic extension shortly after the Neoproterozoic orogenesis may be a better explanation for the genesis of the mid-Neoproterozoic magmatic rocks in the eastern part of the Jiangnan orogen. Post-orogenic extension may be diachronous along the whole orogenic belt, and probably has no direct relationship with the Rodinia rifting event. A more detailed model is presented to illustrate the evolution of the eastern part of the Jiangnan orogen.

The Longwood Igneous Complex, Southland, New Zealand: A Permo-Jurassic, intra-oceanic, subduction-related, I-type batholithic complex

The Longwood Igneous Complex (LIC) is located in Southland, New Zealand on the eastern side of the Carboniferous to Cretaceous, I-type, Median Batholith. Intrusives of the Complex range in age from Permian to Jurassic and show trace element characteristics typical of subduction-related magmas. Gabbro, gabbroic diorite and basaltic dyke rocks show trace and minor element patterns and isotopic compositions indicating that they represent magmas generated in an intra-oceanic subduction system. Radiometric ages decrease across the LIC from 254 Ma in the east to 142 Ma in the west and mineral chemistry and mineral phase relationships indicate emplacement at depths between 15 and 25 km. Thus the petrology and geochemistry of the LIC provides the basis for evaluating the composition of lower–middle crust assembled above a long lived intra-oceanic subduction system and we estimate this to be andesitic and similar to bulk continental crust. Rocks of the LIC range in composition from troctolite and gabbro through diorite to trondhjemite and granite. All of the ultramafic rocks and most of the gabbros have petrographic and geochemical features consistent with a cumulate origin and mineral chemistry shows similarities with arc cumulate sequences from elsewhere. Few of the plutonic rocks making up the LIC have direct analogues among modern intra-oceanic volcanic rocks. The latter are the end products and the former the leftovers from magmatic processes that included fractional crystallisation, crustal assimilation and magma mixing and mingling. Longwood intrusions do not represent magma chambers. They formed as crystal cumulates and mushes left over from the processes that generated magmas erupted at the contemporary volcanic arc. A correlation between decreasing age of emplacement and Sr and Nd isotopic compositions and inheritance in zircons dated by ion probe are indications of crustal recycling. The generation of felsic rocks in the Longwood intra-oceanic arc involved crustal anatexis and, over the 100 million year history of the arc, the crust evolved towards a composition similar to bulk continental crust and average andesite. Dioritic rocks of the LIC contain abundant mafic enclaves, which are argued to represent fragments of mafic magma, derived by fractional crystallisation from basalt, which was intruded into a hot but solid or near solid diorite. Heating and remobilisation of the dioritic host disrupted and disaggregated the intruding mafic magma to form enclaves and zones of intrusion breccia that show every variation from liquid–liquid to liquid–solid mingling and mixing. They were then further modified chemically and mineralogically by diffusion of H 2O, Na, P, Ba, REE, and, to a lesser extent, Rb. Mafic dykes occur throughout the Complex and a number of these are composite with compositions ranging from dolerite through andesite to dacite. The components of composite dykes do not define unequivocal linear mixing trends and hybridisation processes that took place within them have only localised significance; mingling and hybridisation in the composite dykes do not appear to have controlled geochemical variation among the major intrusive units of the Complex.

Character and origin of lithofacies in the conduit of Unzen volcano, Japan

Unzen, western Kyushu, Japan, is an andesitic to dacitic, polygenetic volcano that reaches an elevation of 1486 m above sea level. A 1996-m-long hole has been drilled on a slanted trajectory passing beneath the volcano, penetrating the conduit zone of the volcano at 30–150 m below sea level. Spot drill cores, totalling 75 m in length, were recovered between lengths 1582 to 1996 m of the hole. The principal lithofacies of the cores are polymict volcanic breccia (74 vol.% of total drill cores), coherent dacite (13 vol.%), coherent andesite (6 vol.%), partly brecciated coherent dacite (5 vol.%), and volcaniclastic veins (2 vol.%). The polymict volcanic breccia is poorly sorted, non-graded and made up of various clasts of andesite/dacite composition, 1–120 cm across, in a matrix of andesite/dacite fragments, up to 5 mm across. The clasts are subangular to subrounded, non-vesicular to vesicular, and contain 62–66 wt.% SiO 2. This facies is interpreted as forming the subvertical body of a diatreme, and to have been produced by fragmentation of vent–conduit wall rocks by explosive eruptions and associated gravitational failure. The coherent dacite (SiO 2 = 66–67 wt.%) is uniform to flow-banded and commonly has chilled margins. The dacite is porphyritic containing phenocrysts of plagioclase, hornblende, biotite and minor quartz in a non-vesicular groundmass. This facies is interpreted as representing dykes that have intruded into the polymict volcanic breccia. The coherent andesite (SiO 2 = 59 wt.%) and partly brecciated coherent dacite (SiO 2 = 69 wt.%) are massive to fractured, vesicular and porphyritic. These facies are interpreted to be lavas extruded during the old stage (500–300 ka) of the evolution of the Unzen volcano. The volcaniclastic veins occur within all the lithofacies described and range from 0.1 to 250 mm wide. The veins consist of volcanic lithic and mineral fragments up to several millimetres across, and are inferred to have formed by injection of high-temperature fluid and entrained particles into temporarily opened fractures. Below 30–150 m sea level the conduit zone is at least 350 m wide in the north–south direction, and consists of polymict volcanic breccia and east–west-striking subvertical dykes. Both single dykes, 4–8 m wide, and composite dykes, 26 – 44 m wide, are present. Numerous volcaniclastic veins intrude both the breccia and dykes.

Mafic and Felsic Magma Interaction in Granites: the Hercynian Karkonosze Pluton (Sudetes, Bohemian Massif)

The Hercynian, post-collisional Karkonosze pluton contains several lithologies: equigranular and porphyritic granites, hybrid quartz diorites and granodiorites, microgranular magmatic enclaves, and composite and lamprophyre dykes. Field relationships, mineralogy and majorand trace-element geochemistry show that: (1) the equigranular granite is differentiated and evolved by small degrees of fractional crystallization and that it is free of contamination by mafic magma; (2) all other components are affected by mixing.The end-members of the mixing process were a porphyritic granite and a mafic lamprophyre.The degree of mixing varied widely depending on both place and time. All of the processes involved are assessed quantitatively with the following conclusions. Most of the pluton was affected by mixing, implying that huge volumes (475 km) of mafic magma were available. This mafic magma probably supplied the additional heat necessary to initiate crustal melting; part of this heat could have also been released as latent heat of crystallization. Only a very small part of the Karkonosze granite escaped interaction with mafic magma, specifically the equigranular granite and a subordinate part of the porphyritic granite. Minerals from these facies are compositionally homogeneous and/or normally zoned, which, together with geochemical modelling, indicates that they evolved by small degrees of fractional crystallization (520%). Accessory minerals played an important role during magmatic differentiation and, thus, the fractional crystallization history is better recorded by trace rather than by major elements.The interactions between mafic and felsic magmas reflect their viscosity contrast. With increasing viscosity contrast, the magmatic relationships change from homogeneous, hybrid quartz diorites^granodiorites, to rounded magmatic enclaves, to composite dykes and finally to dykes with chilled margins.These relationships indicate that injection of mafic magma into the granite took place over the whole crystallization history. Consequently, a long-lived mafic source coexisted together with the granite magma. Mafic magmas were derived either directly from the mantle or via one or more crustal storage reservoirs. Compatible element abundances (e.g. Ni) show that the mafic magmas that interacted with the granite were progressively poorer in Ni in the order hybrid quartz dioritescgranodioritescenclavesccomposite dykes.This indicates that the felsic and mafic magmas evolved independently, which, in the case of the Karkonosze granite, favours a deep-seated magma chamber rather than a continuous flux from mantle. Two magma sources (mantle and crust) coexisted, and melted almost contemporaneously; the two reservoirs evolved independently by fractional crystallization. However, mafic magma was continuously being intruded into the crystallizing granite, with more or less complete mixing. Several lines of evidence (e.g. magmatic flux structures, incorporation of granite feldspars into mafic magma, feldspar zoning with fluctuating trace element patterns reflecting rapid changes in magma composition) indicate that, during its emplacement and crystallization, the granite body was affected by strong internal movements.These would favour more complete and efficient mixing. The systematic spatial^temporal association of lamprophyres with crustal magmas is interpreted as indicating that their mantle source is a fertile peridotite, possibly enriched (metasomatized) by earlier subduction processes.

Anisotropy of magnetic susceptibility studies in Tertiary ridge-parallel dykes (Iceland), Tertiary margin-normal Aishihik dykes (Yukon), and Proterozoic Kenora–Kabetogama composite dykes (Minnesota and Ontario)

Mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow during intrusion. In Iceland, ridge-parallel basaltic dykes were sampled on each side of the active tectonic boundary. The dykes are < 10 m wide along a 1–2 km strike, and are the result of a single intrusion from 1–2 km deep magma chambers in oceanic crust. Thirteen samples were collected (7 N. American plate; 6 European) and 153 cores were analyzed by AMS and preserve a vertical K max orientation indicating vertical emplacement. The Eocene Aishihik dyke swarm intrudes the Yukon–Tanana terrane in the Yukon province, Canada over an area ~ 200 by 60 km. These dykes were intruded normal to the accretionary margin, are porphyritic andesites, and have an intermediate geochemical signature based on major and trace element analyses. Ten dykes were sampled and 111 cores analyzed using AMS, and the dykes preserve a vertical K max orientation, indicating intrusion was vertical through ~ 30 km of continental crust. The 2.06 Ga Kenora–Kabetogama dykes in northern Minnesota and western Ontario crosscut a variety of Archean terranes (thickness ~ 50 km) in a radiating pattern. The unmetamorphosed basaltic dykes are 1–120 m wide, 10–110 km in length, are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion (composite) dykes. AMS data preserve a vertical K max orientation for the southerly locations (2 dykes, n = 53) and horizontal K max for the remainder to the northwest (15 dykes, n = 194). Maximum magnetic susceptibility axes (4 dykes, n = 92) for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin. Almost all of our results are “normal” in that, the magnetic foliation (the plane containing K max and K int, normal to K min) is parallel to the dyke planes, which gives us confidence that the magnetic lineations (i.e., K max orientations) are parallel to magmatic flow.

A 1.78 Ga large igneous province in the North China craton: The Xiong'er Volcanic Province and the North China dyke swarm

The 1.78 Ga Xiong'er Volcanic Province (XVP) and coeval North China giant mafic Dyke Swarm (NCDS) are the most important magmatic events occurring after the amalgamation of the North China craton (NCC). The XVP consists of 3–7 km of extrusive volcanics and some feeder dykes/sills located along the southern margin of the NCC and extending over an area > 0.06 M km 2. Compositions vary from basalt to rhyolite, but are predominantly intermediate in terms of silica content. There are also minor sedimentary intercalations and pyroclastic units. The sedimentary interlayers indicate an environment changing from continental-facies to oceanic-facies up-section. The XVP is characterized by fractional crystallization from an EM I type mantle source, and both continental arc (Andean-type) and rift environments have been proposed. The NCDS is widespread in the central NCC with an outcrop area > 0.1 M km 2, and are exposed at variable depths up to 20 km (deepest in the north). Dyke compositions vary from basalt to andesite and dacite, but are dominantly mafic, and comprise two series of magmatism. Previous studies revealed that the NCDS recorded assimilation and fractional crystallization of an EM I type magma source, with a minor DM contribution in the younger magmas. Both syn-collisional and intra-continental anorogenic environments have been proposed. Spatial and petrogenic correlations suggest a cogenetic relationship between the NCDS and XVP, and considered together, they define a Large Igneous Province (LIP) of > 0.1 M km 2 in area and > 0.1 M km 3 in volume, which is also notable for its continuous compositional range from mafic to felsic (with no gap at intermediate compositions). The petrology is explained by a common magma source that undergoes a silica-poor and iron-enriched fractionation trend at depth followed by a silica-rich and iron-poor fractionation trend in shallow-level magma conduits (dykes) and surface lavas. A mantle plume is favored as the cause of this ∼ 1.78 Ga North China LIP.

Interrelations between coeval mafic and A-type silicic magmas from composite dykes in a bimodal suite of southern Israel, northernmost Arabian–Nubian Shield: Geochemical and isotope constraints

Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ 18O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648. The initial 143Nd/ 144Nd ratios, expressed by ε Nd( T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [ δ 18O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of 18O/ 16O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ 18O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ 18O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally.

Replenishment, Crystal Accumulation and Floor Aggradation in the Megacrystic Kameruka Suite, Australia

Detailed field evidence indicates that the Kameruka Suite plutons of the Bega Batholith, eastern Australia, grew by crystal accumulation on the floor of a magma chamber. Depositional features in the plutons, including mafic enclave channels, asymmetric enclave pillows and exotic rafts, load casts and flame structures, and graded and trough cross-beds, indicate that the pluton built progressively upward. The general eastward dip of depositional features in the main pluton implies a lower western and upper eastern contact, consistent with a basal granite–migmatite contact in the west and a sharp hornfelsic sidewall contact in the east. Mafic, felsic and composite dykes, most common near and below the basal western contact, are interpreted as conduits for magma chamber replenishment and imply open-system behaviour during pluton construction. Textural relations are also consistent with an open-system, cumulate origin. Typically, centimetre-scale grains of quartz, plagioclase and megacrystic alkali feldspar form a touching framework with interstices filled with smaller biotite flakes and smaller intercumulus quartz and feldspar crystals. Alkali feldspar megacrysts vary from euhedral and unzoned, to mantled and partially replaced by plagioclase, to ovoid and completely pseudomorphed by quartz–albite aggregates. The common occurrence of mantled and pseudomorphed alkali feldspar in mafic enclaves, and in hybrid tonalitic rocks forming the matrix to enclave swarms, suggests that replacement or resorption of granitic primocrysts was associated with mafic replenishments. The occurrence of all megacryst types at outcrop scale implies extended alkali feldspar crystallization in different parts of the chamber, thorough stirring during, or after, periodic replenishment, and final settling in a cumulate mush. The bulk composition of the cumulate mush, represented by granodiorite, cannot represent the emplaced magma. Compositional variation can be modelled by variable degrees of crystal accumulation from a parental, silica-rich melt represented by the silicic dykes. As dykes periodically fed the magma chamber, crystals accumulated on the floor, and more evolved melts probably erupted from its roof. Thus, the average composition of the magma, and the cumulus minerals, may have remained relatively constant, and the sublinear chemical trends that typify the Kameruka Suite simply reflect differing proportions of melt and cumulate material. Sublinear chemical trends can also be explained by a restite model; however, the distinctive Ba, light rare earth element and Zr spikes at high silica can be explained only by a cumulate model, which also explains why the low-silica granites of the suite share the same chemical characteristics as the high-silica granites.

MAGMATISMO ÁCIDO-BÁSICO NA ILHA DO ARVOREDO - SC

The rocks that constitute the Arvoredo Island are alkaline granites (Arvoredo Granite) that are cut by basaltic andesite dykes with variable thickness, striking N10°-20°E. This dykes belongs to Florianopolis Dyke Swarm (Cretaceous age, gotten by Ar 40 /Ar 39 data in previous paper - Tomazzoli et al., 2005) and are associated to rhyodacites and dacites that can occur as simple dykes, composite dykes or as hypabissal bodies of irregular geometry. The composite dykes are constituted by rhyodacite in the center with borders of basaltic andesite. Composite dykes and rounded enclaves of basaltic andesite in rhyodacite are features that indicate magma mingling process. The basaltic andesites and rhyodacite are high-K tholeiitic rocks with high Ti, P and uncompatible trace element contents. Geochemical modeling shows that rhyodacites can be derived from basaltic andesites by fractional crystalization of 61% of solids, represented by andesine (45.1%), edenite (16.5%), augite (18.7%) magnetite (9.8%) and ilmenite (6.8%). The magma mixing/mingling process could have occurred after the fractional crystallization. The geochemistry of rhyolite dykes and aplite veins differ from that of the tholeiitic rhyodacites/dacites and is similar to Arvoredo Granite.

Early–Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province

A suite of dolerite dykes from the Ahlmannryggen region of western Dronning Maud Land (Antarctica) forms part of the much more extensive Karoo igneous province of southern Africa. The dyke compositions include both low- and high-Ti magma types, including picrites and ferropicrites. New Ar-40/Ar-39 age determinations for the Ahlmannryggen intrusions indicate two ages of emplacement at similar to 178 and similar to 190 Ma. Four geochemical groups of dykes have been identified in the Ahlmannrygggen region based on analyses of similar to 60 dykes. ne groups are defined on the basis of whole-rock TiO2 and Zr contents, and reinforced by rare earth element (REE), Sr-87/Sr-86 and Nd-143/Nd-144 isotope data. Group I were intruded at similar to 190Ma and have low TiO2 and Zr contents and a significant Archaean crustal component, but also evidence of hydrothermal alteration. Group 2 dykes were intruded at similar to 178Ma; they have low to moderate TiO2 and Zr contents and are interpreted to be the result of mixing of melts derived from an isotopically depleted source with small melt fractions of an enriched lithospheric mantle source. Group 3 dyke were intruded at similar to 190Ma and form the most distinct magma group; these are largely picritic with superficially mid-ocean ridge basalt (MORB)-like chemistry (flat REE patterns, Sr-87/Sr-86, similar to 0.7035, is an element of Nd-i similar to 9). However, they have very high TiO2 (similar to 4 wt %) and Zr (similar to 500ppm) contents, which is not consistent with melting of MORB-source mantle. The Group 3 magmas are inferred to be derived by partial melting of a strongly depleted mantle source in the garnet stability field. This group includes several high Mg Fe dykes (ferropicrites), which are interpreted as high- temperature melts. Some Group 3 dykes also show evidence of contamination by continental crust. Group 4 dykes are low-K picrites intruded at similar to 178Ma; they have very high TiO2-Zr contents and are the most enriched magma group of the Karoo-Antarctic province, with ocean-island basalt (OIB)-like chemistry. Dykes of Group I and Group 3 are sub-parallel (ENE-WSW) and both groups were emplaced at similar to 190Ma in response to the same regional stress field, which had changed by similar to 178Ma, when Group 2 and Group 4 dykes were intruded along a dominantly NNE-SSW strike.

Origin of Minette by Mixing of Lamproite and Dacite Magmas in Veliki Majdan, Serbia

Composite dykes consisting of leucominette and dacite as well as discrete dykes and flows of minette and lamproite composition, occur in the Veliki Majdan area, western Serbia. This area is part of the Serbian Tertiary magmatic province, which consists of numerous small occurrences of ultrapotassic igneous rocks. The composite dykes have leucominette margins (up to 150cm thick) enclosing a central part of dacite up to 100m in width. Between these two lithologies, a decimetre-sized transition zone may occur. Petrography, mineral chemistry and bulk-rock geochemistry, including Sr, Nd and Pb isotopes, provide evidence that the minettes and leucominettes formed by hybridization between a felsic magma similar in composition to dacite and a mantle-derived lamproitic magma. The leucominettes and minettes contain all phenocryst types (biotite, plagioclase, quartz) present in the dacites, but in partly resorbed and reacted form. The mica displays a great diversity of resorption textures as a result of partial dissolution, incipient melting and phlogopitization, suggesting superheating of the felsic melt during hybridization; the mineral modes and mineral compositions of the leucominettes and minettes resemble those in the lamproites. A model for the modification of lamproite melt towards minette is presented in which minette is formed by mixing of lamproite and530% felsic magma. The lack of any significant correlation between Pb isotopic ratios and some of the ‘mixing-indices’ (SiO2, Zr, Zr/Nb, 143 Nd/ 144 Ndi) recognized in the hybridization model for the Veliki Majdan dykes may be a result of similarity of the Pb-isotopic signature in the two end-members. Highly phlogopitized biotite xenocrysts in the minettes are ascribed to the retention of volatile components after magma mixing and crystallization of a new generation of phlogopite from the hybridized magma. The magma-mixing model explains the reverse zoning and resorption features of phlogopite macrocrysts commonly recognized in calcalkaline lamprophyres elsewhere. Therefore, this mixing mechanism may be globally applicable for the origin of minettes associated with calcalkaline granitic plutonism in postorogenic settings.

An Isotope and Trace Element Study of the East Greenland Tertiary Dyke Swarm: Constraints on Temporal and Spatial Evolution during Continental Rifting

Dykes of the East Greenland Tertiary dyke swarm can be divided into pre- and syn-break-up tholeiitic dykes, and postbreak-up transitional dykes. Of the pre- and syn-break-up dykes, the most abundant group (Tholeiitic Series; TS) has major element compositions similar to the main part of the East Greenland flood basalts. A group of high-MgO tholeiitic dykes (Picrite---Ankaramite Series; PAS) are much less common, and are equivalent to some of the oldest lavas of the East Greenland flood basalts. Isotopic compositions of the TS and PAS dykes partly overlap with those for Iceland, but Pb isotopic compositions extend to less radiogenic values than those seen in either Iceland or North Atlantic mid-ocean ridge basalt (MORB). The isotopically depleted source required to account for this isotopic variation is interpreted as subcontinental lithospheric mantle with low 87 Sr/ 86 Sr and 206 Pb/ 204 Pb and high eNd. The post-break-up Transitional Series (TRANS) dykes are isotopically distinct from Iceland and MORB, and are interpreted as the products of contamination of Iceland plume melts with continental crust. Comparison of the Nd---Sr---Pb isotopic and trace element compositions of dykes from different segments of the East Greenland margin indicates that there is no systematic compositional change with distance from the presumed protoIcelandic plume centre. This suggests that a northwardincreasing crustal thickness observed offshore may be attributed to active upwelling rather than a systematic rise in temperature towards the plume centre.

Origin of heterogeneous mafic enclaves by two-stage hybridisation in magma conduits (dykes) below and in granitic magma chambers

Field, petrographic and geochemical evidence from the K-feldspar megacrystic Kameruka pluton, Lachlan Fold Belt, southeastern Australia, suggests that complex, multicomponent, mafic microgranular enclaves (MME) are produced by two-stage hybridisation processes. Stage 1 mixing occurs in composite dykes below the pluton, as mafic and silicic melts ascend through shared conduits. Pillows formed in these conduits are homogeneous, fine-to medium-grained stage 1 MME, which typically range from basaltic to granitic compositions that plot as a sublinear array on Harker diagrams. Stage 2 hybridisation occurs in the magma chamber when the composite dykes mix with the resident magma as synplutonic dykes. The stage 2 hybrids also form linear chemical arrays and range from basaltic to granodioritic compositions, the latter resembling the more mafic phases of the pluton. Stage 2 MME are distinguished from stage 1 types by the presence of K-feldspar xenocrysts and a more heterogeneous nature: they commonly contain stage 1 enclaves. Subsequent disaggregation and dispersal of stage 2 hybrid synplutonic dykes within the magma chamber produces a diverse array of multi-component MME.Field evidence for conduit mixing is consistent with published analogue experimental studies, which show that hybrid thermo-mechanical boundary layers (TMBL) develop between mafic and silicic liquids in conduits. A mechanical mixing model is developed, suggesting that the TMBL expands and interacts with the adjacent contrasting melts during flow, producing an increasing compositional range of hybrids with time that are mafic in the axial zone, grading to felsic in the peripheral zones in the conduit. Declining flow rates in the dyke and cooling of the TMBL zones produce a pillowing sequence progressing from mafic to felsic, which explains the general observation of more MME in more silicic hosts.The property of granitic magmas to undergo transient brittle failure in seismic regimes allows analogies with fractured solids to be drawn. The fracture network in granitic magmas consists of through-going ‘backbone’ mafic and silicic ± composite dykes, and smaller ‘dangling’ granitic dykes locally generated in the magma chamber. Stage 1 hybrids form in composite backbone dykes and stage 2 hybrids form where they intersect dangling dykes in the magma chamber. With subsequent shear stress recovery, the host magma chamber reverts to a visco-plastic material capable of flow, resulting in disaggregation and dispersal of these complex, hybrid synplutonic dykes, and a vast array of double and multicomponent enclaves potentially develop in the pluton.

Sources of Proterozoic mafic dyke swarms: constraints from [formula omitted] and [formula omitted] ratios

Th Ta and La Yb ratios in mafic dykes provide an important constraint on their magma sources and magma evolution. These element ratios clearly indicate at least three sources must be available for some swarms and that few swarms can be accounted for by only one source. Some dyke magmas appear to reflect mixing of mantle plume sources with Ta-depleted Archean subcontinental lithosphere. Systematic secular compositional changes in Th Ta or La Yb ratios are not observed in dyke swarms ranging in age from 2.45 to 1.14 Ga from the southern Superior Province, and in the giant Mackenzie swarm (1.27 Ga) there is no change in either element ratio as a function of distance from an inferred plume source. Paleoproterozoic dyke swarms range from those dykes with low Th Ta and La Yb ratios, indicating a strong contribution of depleted or primitive mantle, probably in a plume source, to picrite/norite dykes with very high ratios, perhaps indicative of a source in the Archean subcontinental lithosphere. There is an overall shift in composition of dykes from high Th Ta ratios in the Paleoproterozoic to low ratios in the Neoproterozoic, reflecting a decrease in importance of Archean subcontinental lithospheric sources and an increase in importance of plumes containing enriched mantle components (recycled sediments and oceanic lithosphere). The delayed appearance of enriched mantle components in dyke sources until the Neoproterozoic may reflect the time it takes to recycle oceanic lithosphere through the lower mantle, beginning in the Late Archean.

Longitudinal Petrochemical Variation in the Mackenzie Dyke Swarm, Northwestern Canadian Shield

Previous magnetic fabric studies of the giant, radiating, 1.27 Ga Mackenzie dyke swarm concluded that flow patterns within the dykes support the concept of a mantle plume that is centred beneath the swarm focus and supplies magma to overlying flood basalts and developing radial dykes. To examine petrochemical implications of the model, compositional variation within the basalt sequence is compared with that of the dykes along a ‘stream line’ of the swarm between 400 km (just beneath the lavas) and 2100 km from its focus and in a parallel segment farther east. Evolution of tholeiitic magmas of the main sampled stream is recorded in the upward change of composition in the lava sequence from mg-numbers of 70 to 35. Underlying (feeder) dykes have a comparable range, but outward along the swarm the range of compositions narrows progressively towards its more evolved end, and at 2100 km, dyke compositions match those in upper levels of the lava sequence. REE and other trace element abundances show a similar contraction in range, and a shift towards more evolved compositions, both upward in the lava sequence and outward along the swarm. Apart from complications owing to crustal contamination, fundamental attributes of the magma (e.g. Zr/Y) change little with stratigraphic level or distance from the focus. The more easterly stream differs in its high proportion of alkalic compositions, suggesting the existence of distinctive subswarms. Normative mineral variation plots are consistent with fractionation in high- (main stream) to low-level (eastern stream) crustal magma chambers. Mackenzie magmatism is compatible with the plume model. Domal uplift related to plume activity initiated central graben collapse and outward-extending radial fractures, thus providing access to plume-derived magmas and loci for magma chamber and dyke swarm development. Multiple magma chambers, forming around the apex, each fed relatively independent subswarms of dykes. Uplift, accompanying fractionation, provided increasing magmatic head by which fractionating magmas could be dispatched to successively greater distances. This, and crystal settling in transport, accounts for the increasingly evolved nature of dykes with distance from the source.

The role of hybridization in the genesis of Hercynian granitodis in the gredos massif, Spain: inferences from Sr-Nd isotopes

The Gredos massif is one the better exposed granitoid complexes of the Iberian massif. It is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes. The massif also contains mega-enclaves of migmatites with which the granitoids show transitional contacts. Two major magmatic associations have been distinguished in this study: (1) One comprises the granitoids with microgranular enclaves, the enclaves, and basic rocks; (2) the other is formed by leucogranites, intrusive into the former series and free of microgranular enclaves. Field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process. The Sr-Nd isotopic study reveals that this process is complex, relating different end-members of mantle and crustal affinities, and occurred around 295 Ma ago, late with respect to the main deformation phases of the Hercynian orogeny. The granitoids with microgranular enclaves (GME) are part of an overall mixing trend involving Palaeozoic mantle-derived magma and melts of older crustal material. Amphibole-bearing GME, in general, contain greater proportions of the mantle-derived component than the cordierite-bearing GME. The actual mixing processes took place on a variety of scales, sometimes between melts which were themselves hybrids. On a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke. The isotopic signatures of the basic rocks are probably, to a large degree, the result of interaction with crustal melts, though additionally the presence of an enriched mantle source cannot be elmininated. Microgranular enclaves and their immediate hosts have differing initial Sr and Nd isotopic signatures, indicating that isotopic equilibrium was not attained. This suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system. The enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees. Late-stage peraluminous leucogranites have similar initial Nd isotopic compositions to the evolved GME; a crustal source with a radically different Nd isotopic composition or age does not need to be invoked in their petrogenesis.

Geochemistry of the Grenville Dyke Swarm: role of plume-source mantle in magma genesis

Major and trace element data of samples from the approximately 590 Ma Grenville dyke swarm, in the southeastern parts of the Canadian Shield, were used to evaluate the extent of crustal contamination of the dyke magma and to model its probable mantle source. The dyke rocks are mostly saturated, quartz tholeiites. A few samples represent transitional basalts. Their overall chemistries are similar to those of continental flood basalts. Their compositions resemble those of liquids initially produced in the garnet peridotite mantle zone and then subjected to polybaric fractionation. However, fractionation models fail to mass-balance Ti, Fe, K, and P. Geochemical and isotopic criteria indicate that crustal contamination was minimal. The incompatible elements Zr, Y, K, Rb, Nb, V, Ta, Hf, and Th show hyperbolic elemental ratio-ratio distributions indicative of mixing between two end-members. One end-member has elemental ratios similar to those of a “depleted” N-MORB source and the second of a P-MORB i.e. ◂ertile” or “plume” MORB source. The more enriched dyke compositions can be successfully reproduced by mixing models in the ratio of 6:1 between the most depleted “MORB-like” dyke magma and a liquid composition similar to metasomatic glass veinlets coexisting with mica peridotite mantle xenoliths, and subsequent crystal fractionation. The Grenville dykes are probably a synrift swarm that was emplaced along a rift arm related to an Iapetan rrr triple junction that is thought to have formed over a mantle plume. The chemistry of the dyke swarm is consistent with its derivation from a plume head, in that the minor enriched source can be identified with plume-source mantle and the major depleted source with entrained mantle. The magma probably originated in the upper cooler parts of the plume head.

Geochemistry of Middle Proterozoic mafic and composite mafic-felsic dykes in southeastern Sweden

Abstract Mafic and composite mafic-felsic dykes of Middle Proterozoic age occur within the general area of the Transscandinavian Igneous Belt in southeastern Sweden. The composite dykes consist of felsic, quartz-feldspar porphyritic cores and mafic margins. The mafic rocks are altered with pyroxene typically replaced by amphibole and plagioclase by sericite or epidote. Two subalkaline groups can be distinguished among the mafic dykes. A tholeiitic group comprises NE-trending mafic dykes in the southern part of the area, some NW-trending dykes further north, and all the mafic margins of the composite dykes irrespective of dyke trend. The calc-alkaline dyke geochemistry is constrained mainly to NE-trending dykes in the northern part of the studied area. Within both groups there is enrichment of LIL-elements to contents above those of normal within-plate tholeiites. This was caused by metasomatism across the dyke contacts during greenschist-facies metamorphism and to some extent probably also by crustal contamination. The REE contents are those of normal continental tholeiites. Geotectonic discrimination diagrams indicate oceanic or arc affinity which is in conflict with the ensialic setting of the dykes. This is in accordance with studies of continental tholeiites elsewhere. Field relationships suggest that the composite dykes were formed by semi-synchronous intrusion of mafic and felsic magma. However, trace elements and the REE distribution indicate that the two magmas were not derived from a single parental melt. A possible explanation is that the mafic magmatism caused anatectic melting in the lower crust. The two magmas then intruded more or less simultaneously.

The Neogene ash-flow caldera of A'ithayn: an eruptive centre upon the trap series in central North Yemen

Abstract The main type of eruptive vents on the Yemen Plateau are dykes; monogenic and composite dykes are present. In addition, a morphologically well preserved ash-flow caldera with a diameter of 10 km is identified as the eruptive centre of extensive surrounding volcanics. It is situated in the centre of the Yemen Arab Republic, 40 km south of Sana'a and NW of Ma'bar. The caldera and its products are younger than the underlying Trap Series and perhaps of Pliocene age. In comparison to the 5–6.5 Ma old calderas in South Yemen the structure shows a much better state of preservation. Striking differences exist between the silicic rocks erupted from the caldera and those in the underlying Trap Series. We emphasise that a high proportion of silicic rocks is a feature of the central Yemen Trap Series. In contrast to the Ethiopian plateaus, a dominant role for basalts is not evident at the structural levels examined.

Relationships between calc-alkaline acidic and basic (mantle-derived) magmas in Late Archaean composite dykes, Kambalda Goldfield, Western Australia

Composite dykes from the North Dome area, Kambalda, comprise basic spessartite (hornblende-plagioclase lamprophyre, ≈56% SiO 2 ), with mg numbers, Cr and Ni contents typical of primitive mantle-derived rocks, and more acidic plagioclase-phyric quartz-andesite (≈ 66% SiO 2 ), together with felsic net-veins and hornblendic patches suggesting local differentiation. The spessartites can be regarded chemically as volatile-, LILE-and incompatible element-enriched versions of siliceous high-magnesian basalts from the local greenstone sequence. A Pb-Pb isochron on one dyke yields an age of 2636±27 Ma, making this the oldest composite dyke so far dated. Field and geochemical evidence, including identical Pb initial-ratios for the constituent rock-types, virtually indistinguishable REE spectra, and tight linear trends on plots such as Y versus Ti and Ce/Cr versus Ti/Cr, suggests that the two magmas (lamprophyric and acidic) were genetically related and were emplaced contemporaneously into the same fracture. Quantitative least-squares modelling of whole-rock and mineral compositions suggests that the acidic components resulted mainly from fractionation of hornblende from parental spessartite magmas, together with a possible contribution from mixing of the same spessartite magma with crustally-derived acidic (e.g., trondjhemitic) magma; the latter is represented by separate dykes and stocks in the Kambalda area. Close similarities between the Kambalda composite dykes and Archaean-Tertiary composite dykes in other parts of the world, imply a persistent role for mantle-derived magmas such as lamprophyres in the genesis of the dominant intermediate-acidic (e.g., granitoid) magmas in many calc-alkaline provinces.