Gabbro Diorite Research Articles

Progressive left ventricular remodeling in patients with hypertrophic cardiomyopathy and severe left ventricular hypertrophy

The Central Pontides (Turkey) includes several igneous complexes, which are predominantly Permo-Carboniferous and Middle Jurassic in age. The Çangaldağ Pluton is one of the largest igneous bodies located in the northern Central Pontides, whose age and tectono-magmatic evaluation is important to constrain the geological evolution of the Northern Neotethys. The pluton tectonically overlies the Çangaldağ Metamorphic Complex, which represents an arc–back-arc complex of Middle Jurassic age. The Çangaldağ Pluton consists mainly of non-metamorphic gabbroic diorite, dacite porphyry, and lesser amount of granitic rocks. Geochemically, the various rock types are akin to volcanic arc magmas displaying Th/Nb and light rare earth element/heavy rare earth element enrichments. In-situ U–Pb age results of zircons from dacite porphyry and granite samples yield ages of 161 ± 5 Ma and 170 ± 2 Ma, respectively. Corresponding in-situ average 176Hf/177Hf initial ratios are 0.28287 ± 0.00004 and 0.28213 ± 0.00002 for the dacite porphyry and granite samples, respectively. These results are consistent with derivation from a subduction-modified mantle source. Based on Hf isotope compositions, TDM model ages vary between 624 and 1512 Ma and suggest that the arc magmatism associated with the Çangaldağ Pluton may have involved partial melting of Neoproterozoic/Mesoproterozoic crustal rocks, a common feature in Gondwana-derived terranes. The geochemical, Hf isotope, and geochronological data reported here confirm the presence of a continental arc system during the Middle Jurassic in the Central Pontides, and suggests that the Çangaldağ Pluton formed during northward subduction of the Intra-Pontide branch of the Northern Neotethys.

Late-Stage Mafic Injection and Thermal Rejuvenation of the Vinalhaven Granite, Coastal Maine

The Vinalhaven intrusive complex consists mainly of coarse-grained granite, inward-dipping gabbro–diorite sheets, and a fine-grained granite core. Small bodies of porphyry occur throughout the coarsegrained granite. The largest porphyry body (roughly 0 5 km by 2 5 km) occurs with coeval gabbro, hybrid rocks, and minor finegrained granite in the Vinal Cove complex, which formed during the waning stages of solidification of the coarse-grained Vinalhaven granite. Porphyry contacts with surrounding coarse-grained granite are irregular and gradational. Compositions of whole rocks and minerals in the porphyry and the coarse-grained granite are nearly identical. Neighboring phenocrysts in the porphyry vary greatly in degree of corrosion and reaction, indicating that the porphyry was well stirred. Thermal rejuvenation of a silicic crystal mush by a basaltic influx can explain the composition and texture of the porphyry. Comparable rejuvenation events have been recognized in recent studies of erupted rocks. Weakly corroded biotite phenocrysts in the porphyry require that hydrous interstitial melt existed in the granite during remelting. Field relations, along with thermal calculations, suggest that cooling and crystallization of coeval mafic magma could have generated the porphyry by thermal rejuvenation of granite crystal-mush containing about 20%melt. Field relations also suggest that some of the porphyrymatrix may represent new felsic magma that was emplaced during remelting.

Birimian lithological succession and structural evolution in the Goren segment of the Boromo–Goren Greenstone Belt, Burkina Faso

Lithological and structural investigations indicate that the Goren segment of the Boromo–Goren Greenstone Belt in the Sanmatenga District is dominated by conformable successions of meta-sedimentary rocks that are interbedded with volcanic deposits. These successions are intercalated by a package of mafic-intermediate volcanic rocks, volcanic breccias and tuffs. The intercalated clastic–volcanoclastic–volcanic assemblage indicates interplay of marine-deltaic sedimentary processes and volcanic activity. Deposition is interpreted to have occurred within a marginal marine setting adjacent to an emergent volcanic centre or island arc setting. An angular unconformity between a volcanic-sedimentary package (the Lower Birimian) and a volcanic–volcanoclastic–clastic package (the Upper Birimian) does not occur within the Sanmatenga District. The supracrustal rocks were subjected to several phases of deformation and are metamorphosed to greenschist facies. Gabbro–diorite–pyroxenite bodies, and several granodiorite–tonalite and granite plutons have intruded them. Structural studies have indicated that the Goren segment underwent at least four phases of deformation, D1–D4. D1 resulted in the formation of a gently north-westerly plunging macroscopic anticline (F1), which is crosscut by D2 shear zones and D3 thrusts. D2 involved a period of north–south crustal shortening that is correlated with the Eburnean Orogeny at ∼2.1 Ga. It is characterised by the development of NE-trending shear zones (sinistral reverse) and folds (F2), and NW-trending (dextral) strike-slip faults. Deformation in D2 was concomitant with emplacement of the Baskoudré Granite, and the granodiorite–tonalite plutons (Banta, Yargo, Kob, Tantogo, Tompelse, Zitenga, Goudren and Loudo granodiorites, and Koukoundi and Soubsi tonalites). It was succeeded by emplacement of the Silmidougou Megacrystic Granite and aplite dykes. D3 involved a period of E–W crustal shortening that resulted in the formation of a series of NW to NNW-trending thrusts and northerly-plunging mesoscopic folds (F3). Deformation culminated in the repetition of stratigraphy through stacking of crustal blocks. The final deformation, D4, resulted in the formation of a series of WNW-trending thrusts (dextral reverse) collectively termed the Wabi-Tampelse Shear Zone. The zone is significant in that it marks a period of north–south crustal shortening that postdates the Eburnean Orogeny.

A bimodal volcanic–plutonic system: the Zarembo Island extrusive suite and the Burnett Inlet intrusive complex

The Zarembo Island volcanic rocks and the Burnett Inlet plutonic complex in central southeastern Alaska were investigated to determine if they are genetically related. The Zarembo Island volcanic suite consists of basalt, andesite, and rhyolite lava flows, which exhibit features that suggest simultaneous eruptions of mafic and felsic lavas. Five kilometres to the southeast, the broadly layered Burnett Inlet plutonic complex consists of gabbro–diorite and granite plutons that also show characteristics of contemporaneous mafic and felsic magmatism. These bimodal volcanic and plutonic rocks are similar in age, ranging from 18.5 to 21.5 Ma. Both suites show a gap in silica concentration between 60 and 65 wt.% and have similar major, trace, and rare-earth element composition. Both suites also show igneous layering, either as interlayered basalt and rhyolite flows or as alternating gabbro and granite sheets. Additionally, both groups contain magma mingling and mixing textures, including mafic enclaves in felsic members and quartz xenocrysts rimmed by clinopyroxene in enclaves. These characteristics suggest that the Burnett Inlet intrusive complex and the Zarembo Island volcanic suite represent an eroded, shallow-level plutonic center and its eruptive cover. The style of volcanism and the bimodal nature of magmatism suggest that igneous activity occurred during crustal extension and thinning that accompanied strike-slip tectonic motion in southeastern Alaska during the Tertiary. The volcanic–plutonic rock associations now exposed at the surface indicate that at least 7° of post-20 Ma crustal tilting has affected the region and can help to explain aberrant paleomagnetic poles in mid-Cretaceous intrusions of the Cordillera Coast belt.

TEMORA 1: a new zircon standard for Phanerozoic U–Pb geochronology

The role of the standard is critical to the derivation of reliable U–Pb zircon ages by micro-beam analysis. For maximum reliability, it is critically important that the utilised standard be homogeneous at all scales of analysis. It is equally important that the standard has been precisely and accurately dated by an independent technique. This study reports the emergence of a new zircon standard that meets those criteria, as demonstrated by Sensitive High Resolution Ion MicroProbe (SHRIMP), isotope dilution thermal ionisation mass-spectrometry (IDTIMS) and excimer laser ablation–inductively coupled plasma–mass-spectrometry (ELA–ICP–MS) documentation. The TEMORA 1 zircon standard derives from the Middledale Gabbroic Diorite, a high-level mafic stock within the Palaeozoic Lachlan Orogen of eastern Australia. Its 206Pb/ 238U IDTIMS age has been determined to be 416.75±0.24 Ma (95% confidence limits), based on measurement errors alone. Spike-calibration uncertainty limits the accuracy to 416.8±1.1 Ma for U–Pb intercomparisons between different laboratories that do not use a common spike.

Skarn-type ilmenite mineralization of the Tuzbaşi–Tunceli region, eastern Turkey

In the Tuzbaşi–Tunceli region of eastern Turkey, skarn-type ilmenite mineralizations occur within the skarn zones along the contact between marbles of the Keban Metamorphic Formation (Permo-Triassic) and gabbros and diorites of the Elazig Magmatic Complex (Coniacian–Campanian). The Elazig Magmatic Complex has characteristics typical of arc magmatism and is composed of plutonic, volcanic, subvolcanic and pyroclastic rocks. Both formations are overlain by sediments of the Alibonca Formation (Lower Miocene) and continental andesitic lava flows of the Karabakır Formation (Upper Miocene–Pliocene). Ilmenite and skarn formation developed as a consequence of plutons (granite–gabbro–diorite) of the Elazig Magmatic Complex intruding into marbles of the Keban Metamorphic Formation. This event also resulted in different types of magnetite mineralization and skarn formation, with different mineral assemblages, in neighbouring areas. Ilmenite concentrations occur as lensoid bodies in garnet–pyroxene calcic exoskarns along (gabbro–diorite)–marble contacts. Ilmenite bodies form syngenetically with skarns, and their TiO 2 contents reach up to 15.83 wt%. The predominant ore mineral is ilmenite, which is often altered to leucoxene and hematite, and occurs disseminated within skarn minerals (predominantly almandine garnet and pyroxene). The titanium of ilmenites apparently originated from the diorites and gabbros, which are otherwise Ti and Al-rich and Si-poor. The intruded marbles supplied the necessary O 2 for ilmenite mineralization.

The origin of the Cretaceous gabbros in the Fujian coastal region of SE China: implications for deformation-accompanied magmatism

The Cretaceous gabbros in the Fujian coastal region, southeastern China, experienced different magmatic and tectonic processes associated with deformation of the Changle-Nanao shear zone. Group 1 gabbros (115 Ma) show marked LILE (e.g., Sr, Ba and K) and LREE enrichment, and HFSE (e.g., Ta, Nb, Zr and Hf) depletion, strongly suggesting an island-arc affinity. The parental magma of group 1 gabbros, arguably derived from an arc-related source mantle modified by previous subduction, rose into and interacted with the coexisting syntectonic granitic magma generated during the main stage of shear zone deformation, and produced the gabbro–diorite–granite complexes. Group 2 and 3 gabbros (106–95 Ma) possibly formed by coupling of the subduction-modified source mantle with part of undepleted mantle (e.g., Nb enrichment) in an extensional environment at the end of deformation. Their parental magmas did not undergo significant magma mixing but produced a cumulate nature. All the three groups of gabbros were contaminated by crustal material during the processes of intrusion and emplacement.

Petrology of the Europe-Largest Burakovka early Paleoproterozoic layered pluton (Southern Karelia, Russia)

The Burakovka layered pluton of basic and mafic rocks is the largest intrusive massif in the Baltic Province composed of Si- and Mg-rich boninite-like rocks. The pluton consists of two individual bodies, each having its own internal structure, and contacting each other in their apical parts, known as the Aganozero and Shalozero-Burakovka bodies. Both bodies have a similar rock sequence including five dierentiated zones (upward): mafic rocks, pyroxenite, gabbro norite, pigeonite gabbro norite, and magnetite gabbro diorite (the latter found only in the Shalozero-Burakovka Body). Being generally similar to each other, these bodies dier notably in the styles of their cumulate stratigraphy and, to a lesser extent, in composition. The pluton is distinguished by the presence of markers - singular interlayers of high-temperature mafic cumulates emplaced in the sequence of lower-T formations. Their origin is believed to have been associated with the intrusion of fresh magma portions into the crystallizing magma chambers. The same mechanism is believed to have been responsible for a macrorhythmic pattern found in the southeastern portion of the Shalozero-Burakovka intrusive body. Using chemical and mineralogical data, it is shown that the bodies discussed were derived from similar high-Si and high-Mg magmas, except that the Aganozero Body was emplaced 50 million years later than the Shalozero-Burakovka intrusion: the former was dated (Sm-Nd isochron) 2372±22 Ma (Nd = 3.22 ± 0.13), and the latter, 2433±28 Ma (Nd = 3.14 ± 0.14). It is concluded that the Burakovka Pluton was a long-lived magma center which developed above a local mantle plume, the origin of which had been associated with the activity of a megaplume which had been responsible for the existence of the Baltic province throughout a period of 200 million years.

Two contrasting paleozoic magmatic belts in northern Inner Mongolia, China: petrogenesis and tectonic implications

The Solonker zone, extending from Solonker to Sonidzuoqi and Xilinhot in northern Inner Mongolia, China, has been considered as the suture between the Manchurides and Altaids. Two magmatic belts have been recognized along the suture zone: an older, variably deformed arc-related magmatic belt represented by the Baolidao magma suite, and a younger, collisional granite belt, represented by the Halatu plutons. The Baolidao suite is composed of gabbroic diorite, quartz diorite, tonalite and granodiorite. Hornblende is present in all rock types. The Halatu plutons consist mainly of two-mica adamellite and subordinate granodiorite and leucogranite. Hornblende is absent in all rock types. The bulk of Baolidao rocks were emplaced at ca. 310 Ma (zircon 206Pb/ 238U age). They are metaluminous to weakly peraluminous and possess high Na 2O/K 2O ratios from 2 to 10. They have no Eu anomalies in the REE patterns but show strong positive Sr anomalies in the primitive-mantle-normalized spidergrams. In contrast, the Halatu granites were formed at ≈230 Ma based on a whole-rock Rb–Sr isochron. They are weakly metaluminous to peraluminous and have rather uniform chemical compositions. In comparison with the Baolidao magma suite, they have much lower contents of FeO, MgO, CaO, TiO 2, Cr, V and Sr, but are richer in SiO 2, K 2O and Rb. They also exhibit distinct negative Sr, P, Ti and Nb anomalies in the spidergrams and significant negative Eu anomalies in the REE patterns, a feature typical of crustal melts. The most primitive rock type of the Baolidao suite (gabbroic diorite) has I Sr=0.7052 and ϵ Nd( T)=+2.4, whereas the other rock types show small variations in both I Sr (0.7056–0.7059) and ϵ Nd( T) (+0.7 to −0.1). Geochemical modelling suggests that the gabbroic diorite was derived from a metasomatized upper mantle and the other rock types were generated from the gabbroic diorite magma by fractional crystallization coupled with assimilation of older crustal material (AFC), and followed by hornblende-dominated fractionation in a closed system. The Halatu granites exhibit a range of I Sr (0.7048–0.7063) and ϵ Nd( T) (+1.0 to −2.2). They were derived from sources dominated by young mantle-derived rocks with variable amounts of ancient continental crust. The largely juvenile isotopic signature for both the Baolidao suite and Halatu granites indicates a significant addition of new continental crust in northern Inner Mongolia.

POST-ACCRETION MAGMATISM WITHIN THE KUIU-ETOLIN IGNEOUS BELT, SOUTHEASTERN ALASKA

Petrological and geochronological studies of Tertiary granite and gabbro plutons in southeastern Alaska provide information about the Mid-Miocene post-accretionary magmatic history of the region. The Burnett Inlet Igneous Complex is a 20 Ma bimodal granite–gabbro complex located on Etolin Island and adjacent areas in central southeastern Alaska. It consists of three main members: granite, alkali granite, and a gabbro–diorite unit consisting of intermingled mafic, hybrid and granitic rocks. Mafic magmatic enclaves are ubiquitous in the complex. They occur as isolated inclusions in the silicic plutons and as packed masses in the intermingled zones of the gabbro–diorite unit. The mafic enclaves typically display round and pillow-like shapes, igneous textures, and chilled margins, and thus denote the mingling of mafic and felsic magmas. Petrographic and geochemical features indicate that the complex developed via a combination of fractional crystallization, crystal accumulation and magma mixing. Systematic changes in mineralogy and coherent, curvilinear trends on geochemical diagrams indicate that the mafic and felsic magmas each evolved separately, mainly by crystal accumulation and fractionation. Magma-mixing textures, such as micro-inclusions and quartz xenocrysts in some mafic rocks, suggest mafic–felsic magma mixing and hybridization. However, the compositional and viscosity differences precluded bulk mixing between end-member granite and basalt magmas. Mixing probably involved intermediate magmas, whose smaller differences in composition and viscosity would have permitted hybridization. Both the mafic and felsic units have within-plate geochemical characteristics, indicating that post-accretionary magmatism occurred within an overall extensional tectonic setting. Distributions of apatite fission-track lengths and mineral-cooling curves imply that the magmatic rocks cooled to less than 100°C within 5 million years of emplacement. The complex is interpreted to have initially evolved as a shallow-level silicic magma chamber heated by underplated basaltic magma derived from partial melting of enriched upper mantle. Subsequent invasion of the silicic magma chamber by basaltic intrusions induced an explosive eruption phase, followed by rapid crystallization and cooling of the entire complex.

Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints

Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of central Cape Breton Island, Canada represent the roots of arcs active at ∼ 540–585 Ma and ∼ 440 Ma. Times of intrusion are closely dated by (1) a nearly concordant U–Pb zircon age of 553±2 Ma in diorites of the Creignish Hills pluton; (2) a lower intercept U–Pb zircon age of 540±3 Ma that is within analytical error of 40Ar/39 Ar hornblende plateau isotope-correlation ages of 545 and 550±7 Ma in the River Denys diorite; and (3) an upper intercept U–Pb zircon age of 586±2 Ma in the Melford granitic stock. On the other hand, ∼ 441–455 Ma 40Ar/39 Ar muscovite plateau ages in the host rock adjacent to the Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent with the presence of granitic dykes cutting the Skye Mountain gabbro–diorite previously dated at 438±2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace element abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The ∼ 540–585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent the youngest Neoproterozoic arc magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, which bracket the transition from convergence to extension between ∼ 540 and 505/520 Ma. This transition varies along the Avalon arc: 590 Ma in southern New England, 560–538 Ma in southern New Brunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transition is attributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and the migration of two ridge–trench–transform triple points. Following complete subduction of the ridges, remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the arc in both southern New Brunswick and central Cape Breton Island. The ∼ 440 Ma arc magmatism in the Creignish Hills extends through the Cape Breton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.

Elemental and Nd–Sr–Pb isotope geochemistry of flows and dikes from the Tapi rift, Deccan flood basalt province, India

The Deccan Traps are a large rift-associated continental flood basalt province in India, parts of which have been studied extensively in terms of geochemistry, palaeomagnetism and stratigraphy. However, the basalts of the Tapi rift in the central part of the province have been little-studied thus far. Two ENE–WSW-trending tectonic inliers of the Deccan basalts in this region, forming ridges rising from younger alluvium, are made up of basalt flows profusely intruded by basaltic dikes. Both of these ridges lie along a single lineament, although they are not physically continuous. The flows are aphyric, plagioclase-phyric and giant-plagioclase basalts, and the dikes are aphyric or plagioclase-phyric. We consider the two inliers to have been originally continuous, from the presence of bouldery remnants of a major dioritic gabbro dike along both. Samples of this dike from both ridges have previously yielded typical Deccan ages of 65.6±0.5 Ma and 65.6±0.6 Ma by the 40 Ar– 39 Ar incremental heating technique. Initial 87 Sr/ 86 Sr ratios and ε Nd( t) values, and present-day Pb isotopic ratios of most dikes indicate that they are isotopically similar to lavas of the Mahabaleshwar and Panhala Formations of the Western Ghats, about 450 km to the south. Their mantle-normalized trace element patterns have small Pb and Ba peaks. One dike has a strong Bushe Formation affinity and a Nd–Sr isotopic composition more extreme than that of any other Deccan rock yet sampled, with ε Nd( t)=−20.2 and ( 87 Sr/ 86 Sr) t=0.72315 . Its mantle-normalized element pattern shows large Pb, Th and U peaks and large Nb–Ta troughs. Its elemental and isotopic chemistry reflects substantial continental contamination. The flows cut by the Mahabaleshwar-type dikes are isotopically similar to the Poladpur Formation lavas of the Western Ghats. Their mantle-normalized element patterns show modest peaks at Rb, Ba and Pb and rather low Nb and Ta relative to La, indicating that they have been contaminated to intermediate degrees. The mantle-normalized element patterns of all the flows and dikes show enrichment in the light rare-earth elements, with small or no Eu anomalies. The entire flow-dike sequence is similar to the Wai Subgroup of the Western Ghats, in terms of its elemental and isotopic chemistry and stratigraphic relationships. Wai Subgroup-like lavas (i.e., some of the younger magma types originally identified from the southern part of the Western Ghats) are previously known from the central, northern and northeastern Deccan, and many have been thought to be far-travelled flows erupted in the southwestern Deccan. Although at least the dikes, and probably the giant plagioclase basalt flows of our study area, are locally generated and emplaced, our new data extend the known outcrop area of these widespread magma types substantially, and these magma types indeed appear to have a nearly province-wide distribution.

Geochemical and Sr–Nd–Pb isotopic data bearing on the origin of Pan-Africangranitoids in the Kid area, southeast Sinai, Egypt

The Kid plutonic suite in southeast Sinai, Egypt is composed of: (1) a gabbro–diorite complex; (2) quartz monzodiorite; (3) granodiorite–monzogranite association; (4) syenogranite. Major- and traceelement analyses and radiogenic isotope data (Rb/Sr, Sm/Nd and Pb/Pb) are presented for these rocks. Rb–Sr isochrons indicate ages of 581±11 Ma, 576±11 Ma and 570±4 Ma for the quartz monzodiorite–granodiorite–monzogranite association and syenogranite, respectively. The gabbro–diorite complex is a layered intrusion varying in SiO2 from 42 to 59 wt%. It shows a calc-alkaline trend, and the trace element characteristics of arc lavas. Subsequently, this magma fractionated to give quartz monzodiorite andsyenogranite magmas. The quartz monzodiorite has low initial 87 Sr/ 86 Sr ratios (0.703) and high ɛ Nd values (+4.4±0.3), with trace element characteristics of volcanic-arc granites. The granodiorite–monzograniteassociation has higher initial Sr ratio (0.7044), lower ɛ Nd (+1.5–+2.7) and relatively high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb, significantly different from those of the older rocks. Isotopic data and REE modelling suggestthat it formed by partial melting of an early Pan-African amphibolitic lower crust. The syenogranite isa typical highly fractionated post-orogenic granite, with a deep negative Eu anomaly, HFSE and REEenrichment, a 87 Sr/ 86 Sr initial ratio of 0.7033, ɛ Nd of 4.5 & 0.5 and distinct variations in the 206 Pb/ 204 Pbratio. The isotopic compositions of Sr, Nd and Pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic. Increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite. This process led to the enrichment of HFSE and HREE, to extensivefractional crystallization, to changes in the U/Pb ratios and a distinct variation in 206 Pb/ 204 Pb.

The plutonic igneous complex at Sorel Point, Jersey, Channel Islands: a high level multi-magma assemblage

Sorel Point lies on the north coast of Jersey, Channel Islands, and comprises a variety of lithologies within the compositional spectrum gabbro–diorite–granodiorite–granite. The rocks of the complex can be separated into four groups, each representing a separate intrusive episode. Each group consists of at least two lithologies which were present as coexisting magmas. The groups are separated by the time required for the preceding intrusive episode to crystallize completely. The rocks thus display a variety of contact relationships. Certain of the rocks present were produced by physical and chemical interactions between coexisting magmas. The rocks have calc-alkaline chemistry and were formed at high crustal levels during the latter stages of the Cadomian orogeny. The Sorel Point complex demonstrates the availability of a wide variety of magmas in a relatively condensed space and time and allows an insight into the formation of a constantly evolving subvolcanic magma chamber. © 1998 John Wiley & Sons, Ltd.

Naming materials in the magma/igneous rock system

The main aim of igneous petrology is to develop a complete specification of the magma/igneous rock system. This paper is a sequel to an earlier essay ( Middlemost, 1991) on the classification of igneous rocks and magmas. It explores the ways and means of developing a single, consistent method of naming all igneous materials. All modal classifications are fettered by problems arising from heteromorphism, extremes in grain size, and the presence of glass. Only chemical parameters can provide a reliable and straightforward method of classifying all the common igneous rocks and their magmas. This is undoubtedly true of glassy rocks and magmas. The potential of the TAS diagram in this new and enlarged role is evaluated, resulting in a modification of some boundaries recommended for the volcanic rocks. A new comprehensive chemical classification of the plutonic rocks is introduced. To keep it and the volcanic classification in tandem, several new terms are proposed. They include gabbroic diorite as a plutonic equivalent of basaltic andesite and peridotgabbro as a plutonic equivalent of picrobasalt. Peridotite is defined as the plutonic equivalent of picrite and by taking the idea of equivalence a step further, it is defined as a peridotgabbroic or gabbroic rock that contains more than 18% MgO and less than 2% total alkalis. The picrobasaltic and basaltic rocks that contained more than 18% MgO and more than 2% (Na 2O + K 2O) are called alkalic picrites. Their plutonic equivalents are named alkalic peridotites. A benefit of this new chemical classification of plutonic rocks is that it enables one to avoid the awkward term ultramafic. A single classification that links magmas, plutonic and volcanic rocks should be appreciated by all geochemists and petrologists who amass, and manipulate, large geochemical databases but are unwilling, or unable, to carry out quantitative modal analyses. This classification also enables geoscientists to focus on magma — the most important concept in igneous petrology.

Silicic Magma Chambers as Traps for Basaltic Magmas: The Cadillac Mountain Intrusive Complex, Mount Desert Island, Maine

The Cadillac Mountain intrusive complex, located on Mount Desert Island, Maine, provides a superb record of episodic invasion of a floored silicic magma chamber by many pulses of basaltic magma. The complex consists of three units: the Cadillac Mountain granite (CMG), the Somesville granite (SG), and gabbro-diorite (G-D). Basaltic magmas were emplaced before, during, and after the emplacement and crystallization of the granitic plutons, and the chambers of silicic magma acted as traps for basaltic magma. Basaltic magma emplaced into the surrounding country rocks crystallized as homogeneous diabase and gabbro, while basaltic magma emplaced within the perimeter of the granite displays abundant evidence for commingling with silicic magma. The G-D unit (about 1.5 km thick) consists of interlayered gabbroic, dioritic, and granitic rocks. Although thin (<1 m thick) gabbroic layers in the G-D unit are typically chilled on all margins, thicker layers of gabbro commonly form the lower parts of macrorhythmic units that may grade upward from basally chilled gabbro through hybrid dioritic rocks to granite. These macrorhythmic units provide a cumulate record of the temporary stratification of mafic and silicic magmas in the CMG chamber. Widespread, small mafic enclaves occur at all levels of the CMG and were probably generated along turbulently stirred double-diffusive boundaries between mafic and silicic magma. Heat from basaltic infusions promoted convection in the overlying silicic magma, and this convection distributed enclaves thoughout the CMG chamber. Scarce, finegrained enclaves of hybrid, intermediate rock in the CMG may represent disrupted portions of a hybrid magma that evolved near the base of the chamber due to repeated infusions of basaltic magma. Some zones of transgressive granophyre in the CMG probably represent highly silicic liquids that evolved within the CMG magma chamber. The Somesville granite may represent a mixture of magma that evolved within the CMG chamber and injections of new silicic magma from below.

Episodic Early Proterozoic granitoid plutonism in the Makkovik Province, Labrador: U–Pb geochronological data and geological implications

Episodic granitoid plutonism in the Early Proterozoic Makkovik Province of the easternmost Canadian Shield is revealed by high-precision U–Pb zircon studies of a range of intrusive rock types. The oldest granites yield ages of 1893 ± 2 and 1891 ± 5 Ma and document a previously unrecognized event that may correlate with an early migmatization of reworked Archean basement. These dates also constrain early structural reworking of the basement, the earliest deformational event grouped as part of the Makkovikian orogeny. Four samples have essentially identical zircon ages of 1801 ± 2, 1802 ± 2, [Formula: see text], and 1825–1799 Ma, and a fifth is slightly older, at [Formula: see text]. These ages suggest correlation with local volcanic sequences, dated in part at 1807 ± 4 Ma. The plutonic suites include both syn- and posttectonic granitoid assemblages and define the main magmatic pulse associated with the Makkovikian orogeny and constrain its final deformational episode. Distinctive, fluorine-enriched "A-type" granites yield an age of 1719 ± 3 Ma and represent a previously unrecognized late postorogenic to anorogenic magmatism of regional significance. Two layered, gabbro–diorite–monzonite–syenite suites yield identical zircon ages of 1649 ± 1 and 1649 ± 3 Ma. A regionally extensive granodioritic unit gives an age of 1647 ± 2 Ma, and a high-level alaskitic granite is dated imprecisely at 1640–1650 Ma. These plutonic suites correlate with volcanic rocks previously dated at 1649 ± 1 Ma.These data show that the plutonic evolution of the Makkovik Province is significantly more complex than previously supposed. The clustering of ages suggests episodic, rather than continuous, magmatism. The different age groupings can, to some extent, be correlated with compositional associations defined by major- and trace-element geochemistry. The new data also raise questions about the regional configuration of Early and Middle Proterozoic orogenic belts in Labrador. Previous correlations between the Makkovik Province, the Ketilidian Mobile Belt of Greenland, and the Svecofennian and Trans-Scandinavian belts of Sweden are supported and expanded by these new results.

Age and tectonic setting of the Duke Island ultramafic intrusion, southeast Alaska

Concordant U–Pb zircon ages on hornblende–plagioclase pegmatites and a related granophyre from marginal and roof zones of the Duke Island ultramafic intrusion, and from leucoquartz diorite internal to hornblende–clinopyroxenite of nearby Mary Island cluster between 108 and 111 Ma. Sm–Nd mineral and bulk-rock data from pegmatite and granophyre zircon samples and from wehrlites and clinopyroxenites of the main Duke Island cumulate sequence are consistent with the consanguinity of the ultramafic cumulates and the zircon-bearing feldspathic rocks, and with an Early Cretaceous igneous age for the intrusion. Wall rocks for the intrusion consist of Ordovician–Silurian amphibolite- and greenschist-grade metamorphosed plutonic, volcanic, and minor sedimentary rocks and crosscutting Late Triassic gabbro–diorite of the Alexander terrane.The Duke Island ultramafic intrusion formed as a northwest-trending elongate funnel-shaped stratiform body commensurate with extensional brittle–ductile faulting in its roof and adjacent wall rocks. Intrusion occurred in a basinal setting within the Gravina volcanic arc along the Cordilleran continental margin. During the latter phases of intrusive activity the Gravina basin began to close by regional thrust faulting, which in the Duke Island region was west-northwest directed. The intrusion may have still had local intercumulate liquid upon initiation of thrust faulting. It responded first by open folding with incipient crystal plasticity. Once completely solidified, the intrusion behaved like a large "augen" with ductile deformation concentrated along its margins and within quartzo-feldspathic and micaceous members of its wall-rock complex. Much of the intrusion's northern margin behaved as a ductile shear zone serving as a tear fault between thrust plates to the northeast and an oblique thrust complex that roots beneath the southwest margin of the intrusion. The intrusion may thus be tectonically transported relative to its original underpinnings. North-trending high-angle faults and parallel fracture cleavage also cut the intrusion in response to axial loading as a result of its initial elongate shape and orientation relative to the thrust kinematics.

Persistent mafic igneous activity in an A-type granite pluton, Cobequid Highlands, Nova Scotia

The West Moose River pluton is one of a series of latest Devonian – Early Carboniferous plutons of granite and gabbro–diorite in the Cobequid Highlands (southern edge of Avalon zone). Intrusion of minor gabbro preceded emplacement of the granite, which is fractured and cut by later mafic dykes, of which some predate and others postdate a Namurian compressive deformation. Minor hybrid phases reflect mixing of mafic and felsic magmas, which are also represented by basalt and rhyolite in the synchronous Fountain Lake Group. The mafic rocks are continental tholeiites reflecting regional Early Carboniferous extension. Later dykes are more fractionated, with high P2O5, TiO2, and other high-field-strength elements. The dykes are pervasively altered, probably by residual fluids from the granite hydrothermally circulated by the hot mafic rocks. Higher Ga/Al, Hf, and Zn content in basalts than dykes suggests upward increase in halogen complexing in the mafic plumbing system. Late-stage fluids concentrated K, Rb, and Cs in the dykes but not the lavas. Alteration in granite includes mafic partings (biotite and Fe–Ti oxides) along fractures, particularly near dykes, and local albitization. Such alteration hinders determination of granite protolith from geochemistry, but mineral composition and the regional enrichment of the granite in Nb, Y, and Ga/Al is similar to that in A-type granites. Alteration associated with the mafic dykes continued for tens of millions of years after pluton emplacement.

The age and significance of Carboniferous plutons of the western Cobequid Highlands, Nova Scotia

A series of granite plutons with minor gabbro–diorite phases outcrops immediately north of the Cobequid Fault in the western Cobequid Highlands, Nova Scotia. The plutons were cut by numerous dykes, and parts of some plutons and some dykes subsequently underwent ductile deformation. Four Rb–Sr whole-rock–K-feldspar isochron dates ranging from 338 to 356 Ma have been obtained from these plutons and the dykes that cut them. The ages of these plutons are indistinguishable from an age of 341 Ma obtained for the nearby Fountain Lake Group volcanic rocks, within estimated uncertainties. Measured Carboniferous granite ages are uncommon in the Appalachian Orogen of eastern Canada. The geochemical character of these plutons indicates that they result from crustal extension. This extension and the compression observed in the granites with ductile deformation are related to transcurrent motion between the Avalon and Meguma terranes along the Minas fault zone.Preliminary Rb–Sr whole-rock data from poorly exposed plutons in the northern part of the Cobequid Highlands suggest a Middle to Late Devonian or older age. These plutons may represent an earlier phase of crustal extension.