Gabbro Stocks Research Articles

U–Pb and 40Ar/39Ar geochronology and tectonic setting of the peralkaline magmatism in the Borborema Province (NE Brazil)

Aegirine-augite granite and syenite plutons and dikes form a distinct magmatic pulse aligned in the NE-SW direction in the central and northern Borborema Province. We use SHRIMP zircon U–Pb and laser step-heating 40Ar/39Ar determinations to constrain the crystallization and cooling ages of syenites and alkali-feldspar granites and gabbros deformed by shear zones. Concordant zircon ages of the Triunfo and Cacimbas syenites vary between 596 ± 4 and 594 ± 4 Ma while sodic amphiboles from a granitic dike of Manaíra swarm yield 40Ar/39Ar plateau ages of 606 ± 5, 605 ± 6 and 599 ± 4 Ma. Muscovite from host mylonitic gneisses, in turn, yields plateau ages of 600 ± 3 Ma and 586 ± 2 Ma. Close to the Patos shear zone which limits the Central and Northern Borborema domains, foliated felsic dike of the Serra do Algodão suite yields a concordia age of 592 ± 2 Ma whereas the Catingueira granitic pluton records numerous inherited zircons. The Taperoá gabbro stock, situated next to the connection between E− and NE-trending shear zones, yields concordant ages of 592 ± 2 Ma. These results constrain the alkaline magmatism of the Central and Northern domains between 600 and 590 Ma, i.e., shortly after the continental collisions that amalgamated the province in the Ediacaran. Cooling was faster in the Triunfo-Manaíra suite and apparently slower close to the Patos shear zone. Host rock structures indicate that the Triunfo-Manaíra suite emplaced in the brittle upper crust, whereas the alkaline rocks of the Serra do Algodão suite emplaced in HT/LP ductile transtensional setting. A possible tectonic scenario includes slab break-off in a continental collision setting, partial melting of the sub-continental mantle and lower crust and magma transfer to the middle-upper crust. The distribution of these rocks next to the margins of volcanosedimentary belts suggests that their ascent and emplacement explored deep fault segments reactivated by crustal extension.

Zircon U Pb geochronology and petrology of the tholeiitic gabbro from the Kovanlık (Giresun) area: Constraints for the Late Cretaceous bimodal arc magmatism in the Eastern Pontides Orogenic Belt, NE Turkey

Mafic intrusions of the Eastern Pontides Orogenic Belt (EPOB) in NE Turkey are found rarely in varying ages, sizes and compositions. Of these, tholeiitic gabbro stocks, namely Kovanlık gabbro, consist of pyroxene gabbro and minor hornblende gabbro with a mainly granular texture. The pyroxene gabbro contains plagioclase, clino- and ortho-pyroxene, olivine, and Fe Ti oxides, whereas the hornblende gabbro includes mainly plagioclase, hornblende, rarely biotite and Fe Ti oxides. The clinopyroxene compositions of the pyroxene gabbro exhibit arc-related tholeiitic magmas. The Kovanlık gabbroic rocks have high U/Yb zircon ratios, implying continental arc-related tectonic settings. Zircon U Pb dating on the Kovanlık gabbro yielded a concordia age of ~92 Ma (Turonian). Petrochemically, the gabbroic rocks have a tholeiitic character and show arc- and MORB-like signatures. The gabbroic rocks show positive Rb, Ba, K, Pb, Sr, Ti and negative Nb, Ce, Nd, Hf, Zr anomalies in N-MORB normalized diagrams. The chondrite-normalized REE plots of the rocks show flat to feeble right-dipping REE patterns (La N /Lu N = 0.94–2.56) with positive Eu anomalies (Eu N /Eu* = 1.15–2.14; except for two pyroxene gabbro samples). The gabbroic rocks have 87 Sr/ 86 Sr (i) ratios (0.704533 to 0.706265), εNd (i) values (4.04–4.85), 206 Pb/ 204 Pb (18.56–18.68) and 207 Pb/ 204 Pb (15.61–15.64) ratios. Low Nb/La and Nb/Y ratios, as well as high Sr/Yb and Ba/Rb ratios, indicate that source regions were modified by slab-derived fluids and/or melts. The obtained zircon dating, isotopic and geochemical data, as well as regional geology, revealed that the tholeiitic mafic magma derived from a spinel-lherzolite mantle at a relatively shallow depth, and the Kovanlık gabbro in the EPOB represents one of the first mafic intrusions of Late Cretaceous bimodal arc magmatism related to the roll-back of the northwards subducting Tethys oceanic plate. • The Kovanlık tholeiitic gabbro formed in a continental arc with a MORB-like source. • The zircon U Pb dating of the gabbro gave a concordia age of ~92 Ma (Turonian). • The parent magma originated by partial melting of a spinel lherzolite mantle source. • The gabbro evolved by fractional crystallization and lesser crustal assimilation. • Cretaceous bimodal arc magmatism is related to the rollback of the subducting slab.

Петрология и геохимия массива Ташлы-Тау, худолазовский дифференцированный комплекс, Южный Урал

Введение: Массив Ташлы-Тау представляет собой небольшой шток оливин-роговообманковых габбро, отвечая наиболее ранним по времени проявления продуктам дифференцированного магматизма худолазовского комплекса. Комплекс имеет сложное геологическое строение, поэтому детальные петролого-геохимические исследования отдельных массивов с привлечением численного моделирования параметров кристаллизации позволяют выявить особенности формирования худолазовского комплекса в целом. Цель статьи – реконструкция петрогенезиса габброидов массива Ташлы-Тау, имеющая важное значение ещё и в связи с его сульфидным Cu-Ni оруденением. Методика: Породообразующие минералы габброидов исследовались в оптическом микроскопе Axioskop 40 и на сканирующем электронном микроскопе Tescan Mira 3. Химический состав пород определялся с помощью рентгенофлуоресцентного анализа и масс-спектрометрии с индуктивно-связанной плазмой. Изотопный состав Sr и Nd изучался с помощью термоионизационной спектрометрии. Для разработки модели петрогенезиса использовалось численное COMAGMAT-моделирование и метод геохимической термометрии. Результаты и обсуждение: Выполнено петролого-геохимическое изучение пород массива Ташлы-Тау. В них отмечается широкое развитие роговой обманки – титанистого магнезиогастингсита (~50 мас. %). Габброиды характеризуются относительной обогащённостью крупноионными литофильными элементами (Cs, Rb, Sr, Ba) и обеднённостью высокозарядными элементами (Nb, Ta, Zr, Hf, REE). На основе геохимических данных сделан вывод о том, что магматический расплав являлся слабодифференцированным. Численное моделирование показало, что породы формировались из магмы, состоящей из ~20 мас. % вкрапленников оливина (Fo 76) и водонасыщенного базальтового расплава (~1.7 мас. % H2O), характеризующегося высокой глинозёмистостью, железистостью и умеренной магнезиальностью. Sr-Nd изотопный состав пород свидетельствует в пользу слабой коровой контаминации исходного расплава. Для них характерны высокие положительные значения εNd(Т) (+5.2...+10.3) и пониженное содержание радиогенного стронция. В качестве источника материнского расплава мог служить мантийный резервуар типа PREMA. Заключение: Полученные результаты важны для понимания природы формирования сложно дифференцированного худолазовского комплекса, специализированного на сульфидное оруденение. Для Урала этот комплекс является уникальным. Показано, что применение численного моделирования для водонасыщенных базальтовых систем может быть вполне эффективно до момента кристаллизации гидроксилсодержащих породообразующих минералов (роговой обманки).

Geochemical and microstructural evidence for in situ formation of pseudotachylitic Sudbury breccia by shock-induced compression and cataclasis

Pseudotachylitic breccias occur in the crater floor of large impact structures. Their origin is widely attributed either to shock melting and cataclasis during propagation of an impact shock wave, or to frictional melting and cataclasis during large slip displacements along crater collapse superfaults. By contrast, some models propose that the breccia matrices are allochthonous superheated melts from overlying impact melt sheets or from sites within the crater floor, which leaked into dilational fractures during collapse of the crater. In an effort to test these models, the compositions of pseudotachylite Sudbury breccias in the South Range of the Sudbury impact structure, Canada, were here compared to those of its associated impact melt sheet, the Sudbury Igneous Complex (SIC). The studied breccias occur along the contact between sandstone and a gabbro stock, and additional breccia bodies are located solely within these two lithologies. Major, trace element and lead isotope systematics suggest that the contact breccias are derived by mixing of comminuted gabbro and sandstone and that breccias hosted exclusively by sandstone or gabbro have compositions similar to their host rocks. The SIC has distinct chemical and isotopic compositions which argue strongly against the involvement of the impact melt sheet in the makeup of the breccias. Quartz and plagioclase in clasts and matrix of the breccias are transected by multiple microfractures and they have pervasive mosaic textures characterized by rolling and mottled undulatory extinction of micron-sized subgrain-like domains and new grains. These textures and strong local host control on chemical and clast composition of the breccias suggest that the breccias formed in situ by shock-induced compression and cataclasis of their host rocks during propagation of the shock wave.

Permian 40Ar/39Ar ages for post-Variscan minor intrusions in the Iberian Range and Spanish Central System

40Ar/39Ar dating of biotite from a calc-alkaline gabbro stock NW of Loscos in the Iberian Range yielded a slightly disturbed gas release spectrum. It has an early Permian, 288.5 ± 1.4 Ma total gas age, interpreted as the minimum emplacement age. The 40Ar/39Ar apparent age of the gabbro agrees with the Autunian stratigraphic age of related volcanic rocks. 40Ar/39Ar step-heating dating of amphibole phenocrysts from an alkaline camptonite dyke ca. 4 km SE of El Hoyo de Pinares in the Sierra Guadarrama sector of the Spanish Central System yielded a mid-Permian 264 ± 3 Ma plateau age, interpreted as the emplacement age. The age is indistinguishable from published 40Ar/39Ar amphibole ages for N-S trending camptonite dykes from the western, Sierra de Gredos segment of the Central System. This suggests that the camptonite dykes from both sectors belong to the same generation.

Geologic Evolution of the Escondida Area, Northern Chile: A Model for Spatial and Temporal Localization of Porphyry Cu Mineralization

A program of geologic mapping and lithogeochemical and geochronological sampling has been carried out over a 745-km2 area of the Atacama Desert surrounding the porphyry Cu deposits at Escondida, Zaldivar, and Chimborazo (Cordillera de Domeyko, northern Chile). The purpose of this study was to examine the regional tectonic and magmatic setting of this preeminent porphyry Cu district for evidence of features or processes that might explain the giant scale of mineralization at Escondida and provide predictive tools for exploration in other areas. The geologic history of this area as recorded by exposed rocks begins with voluminous, intermediate to felsic Permo-Carboniferous volcanism (La Tabla Formation), and these rocks appear to constitute the crystalline basement throughout much of the porphyry belt of northern Chile. Geochemically, they are I-type in character, but the parental magmas were relatively dry, and thus did not generate effective magmatic-hydrothermal systems (few significant ore deposits are known to be associated with them). Andean cycle arc magmatism began in the Triassic, centered on the La Negra magmatic arc (now located near the Chilean coast). Farther inland, near Escondida, back-arc processes led to the eruption of intermediate to felsic lavas and tuffs and the deposition of marine sediments in rift basins. Closure of these basins in the Late Cretaceous resulted in deformation of the volcano-sedimentary sequences and was followed by emplacement of small alkali gabbro stocks and dikes. The axis of arc magmatism moved eastward in the Paleocene (Central Valley arc) and produced widespread calc-alkaline intermediate to felsic volcanism through to the Eocene. East- to northeast-directed convergence maintained a dextral transpressive regime during this period, and early movements in the West Fissure zone, a corridor of orogen-parallel faults that runs the length of the Cordillera de Domeyko (over 1,000 km), reflect this couple. At the end of the Eocene, however, stresses in the arc appear to have relaxed, and by the late Oligocene, strike-slip movement along the West Fissure zone had reversed to sinistral. This period of stress relaxation at the end of the Eocene period coincided with the voluminous emplacement of dioritic magmas at shallow crustal levels and also with porphyry development. Six samples of hornblende from these diorites yield 40Ar/39Ar dates between 38.28 ± 0.32 and 36.94 ± 0.46 Ma (2 σ ). Porphyry emplacement at Escondida, Zaldivar, and Chimborazo was coeval with this dioritic magmatism at ~38 Ma. Where plutonism was intense, the dioritic magma is interpreted to have evolved by processes of assimilation and fractional crystallization to more felsic compositions characteristic of the ore-forming porphyry intrusions. Whole-rock trace element data indicate that hornblende fractionation was an important control on chemical evolution of the diorites and attests to high-magmatic water contents (>4 wt % H2O). Volatile saturation would have occurred during further differentiation of these magmas, evidence for which is provided by the porphyry ore deposits. Porphyry emplacement was localized within a broad zone of intersection between the West Fissure zone and a regionally extensive northwest-trending structural corridor (the Archibarca lineament). It is proposed that the geometry of this junction was conducive to the formation of transtensional pull-apart structures during relaxation or reversal of dextral shear on the West Fissure zone. Such dilational structures would have focused the ascent and pooling of magma in the upper crust and maximized the potential for formation of magmatic-hydrothermal ore deposits. The formation of giant porphyry systems such as Escondida is, therefore, considered to be the result of a fortuitous coincidence of processes, including generation of suitable volumes and compositions of magma, appropriate lithospheric stress conditions, and structural focusing of emplacement; in addition, the development of thick supergene enrichment blankets has been critical to the economic value of these deposits. None of these contributory processes are in themselves unusual or rare, but because they are largely independent of one another, their constructive cooperation in ore formation is not necessarily repeatable at different places and at different times, thus explaining the relative rarity of giant porphyry deposits.

Docking of the Central Metasedimentary Belt to Laurentia in geon 12: evidence from the 1.17-1.16 Ga Chevreuil intrusive suite and host gneisses, Quebec

The Chevreuil intrusive suite (1.17-1.16 Ga) represents a chronological field marker of regional extent that intruded the Central Metasedimentary Belt in the western Grenville Province of Quebec after peak metamorphism. Style and site of magma emplacement, and extent of deformation of Chevreuil plutons and dykes permit unravelling of the early Grenvillian evolution of the belt with respect to cratonal North America. The suite comprises a series of vertically layered gabbro stocks and monzonite-diorite-gabbro sheet intrusions, and a swarm of microdiorite dykes that cut across gneisses. The dykes display systematic variations in extent of deformation across the belt. We targeted U-Pb geochronology on gneisses within the identified strain windows; they preserve the record of a ca. 1.20 Ga high pressure-temperature (P-T) metamorphic event. The sheet intrusions define magmatic corridors all along, and concordant with, the western, northern, and eastern tectonic boundaries of the belt. The concordant and elongate shape of these bodies results from emplacement, not deformation. Chevreuil magmas thus sealed the belt boundaries largely in their current positions, with the implication that docking of Elzevirian and pre-Elzevirian terranes with cratonal North America predates 1.17 Ga. We interpret the 1.20 Ga metamorphism as evidence for the initiation of Grenvillian continent-continent collision during the culmination of the Elzevirian orogeny at ca. 1.22 Ga. Emplacement-related fabrics indicate that the Chevreuil suite and the coeval Morin anorthosite suite intruded during renewed orogenesis. This orogenic pulse (Shawinigan) is not accretionary, but represents a strongly partitioned, compressive, intraplate reactivation event.

The Northern Charlotte belt North Carolina and South Carolina: A deformed composite batholith

AbstractThe Charlotte belt in North Carolina and South Carolina, U.S.A. is characterized by numerous plutons with granitic to gabbroic composition that have intruded gneiss, schist, metavolcanic rocks, and metamorphosed mafic complexes (basalt, gabbro, and ultramafic rocks). The exposed rock constitutes a composite batholith which contains three superunits: (1) the Old Plutonic (metaquartz diorite, metagranodiorite, and metagabbro) approximately 550 my old; (2) the Concord‐Salisbury (gabbro and granodiorite) approximately 400 my old; (3) the Landis ('big feldspar') granite approximately 300 my old. Geophysical and geologic studies suggest small bodies of granodiorite and mafic complexes intruding metavolcanic rocks, schist, and gneiss form the upper level of the batholith; a thick metagabbro sill penetrated by gabbro stocks and underlain by poikilitic gabbro and small bodies of syenite form the middle level; and a complex of metaquartz diorite and metagranodiorite intruded by Landis granites form the lowest level. Outcrop patterns and structural profiles based on gravity suggest the batholith forms an east‐dipping monocline. The bimodal suite of gabbro and granodiorite plutons suggest a tensional tectonic environment ∼ 400 my ago, and negates subduction as the only mode of formation for the batholith.

Metchosin Igneous Complex, southern Vancouver Island: Ophiolite stratigraphy developed in an emergent island setting

The Metchosin Igneous Complex of southern Vancouver Island has three components—gabbro stocks (formerly Sooke Gabbro), a newly recognized sheeted-dike complex, and a basaltic volcanic sequence (formerly the Metchosin Volcanics). The sheeted dikes occupy a position transitional between the gabbro stocks and submarine volcanics, which they tie together in an ophiolitic pseudostratigraphy. The submarine volcanics shoal upward and pass into subaerial sheet flows. The complex formed in an area of regional extension, as an emergent island.

Paleomagnetic zones in the Oligocene East Sooke Gabbro, Vancouver Island, British Columbia

The 39-m.y.-old East Sooke polyphase gabbro stock intrudes Late Eocene Metchosin volcanics, shows no evidence of metamorphism, and is unconformably overlain by undeformed Late Oligocene sediments. A grid of samples (36 sites, 180 cores, 360 specimens) was collected in and around its 5- × 2½-mile (8 × 4 km) outcrop area at the southernmost end of Vancouver Island. Antiparallel directions, a positive contact test, thermomagnetic curves, and so forth indicate that all phases after af demagnetization retain a characteristic magnetization residing in magnetite. The stock contains a central zone of normal polarity bounded by two reverse zones. The zones are independent of petrologic boundaries. Thus all phases were solidified above the determined Curie temperatures (505° ± 10°C). Gradational direction trends in the zones show the irregular migration of isotherms in the stock while the earth's magnetic field switched polarity at least once and probably twice. The East Sooke pole position (150.9°E, 69.5°N, δm = 6.7°, δp = 5.3°) reflects a 20° ± 7° counterclockwise twist rotation of the southern Tertiary belt relative to the rest of Vancouver Island.

Geology of a Portion of the Contwoyto Lake Area, Northwest Territories: ABSTRACT

Approximately 24 square miles of Archean (?) igneous and metamorphic rocks have been mapped at a scale of 1 inch equals 1,000 feet in an area centered 5 miles south of the central part of Contwoyto Lake, N.W.T. The oldest rock unit is apparently a quartz-plagioclase-biotite-K-feldspar paragneiss containing numerous pegmatitic segregations of quartz, plagioclase, K-feldspar, and minor amounts of muscovite, tourmaline, and apatite. A sequence of low- to medium-grade meta-sediments overlies the paragneiss. In the northern part of the area, these rocks are largely quartz-chlorite-muscovite phyllites. As the grade of metamorphism increases southward, biotite, almandite, andalusite, cordierite, and staurolite become common. Approximately 20 beds of quartz-hornblende-almandite amphibolite up to 10 feet in width have been mapped in the schist sequence. The phyllites, schists, and amphibolites are believed to be correlative with the Yellowknife group. Two biotite granite bodies approximately one mile in diameter intrude the Yellowknife group metasediments. The sediments are also cut by diabase dikes 20-100 feet in width, and a gabbro stock 3/8 mile in diameter. Pyrrhotite, pyrite, and arsenopyrite are found locally in a few of the amphibolite beds. Gold mineralization is locally present in some of the tightly folded pyrrhotite and arsenopyrite-bearing amphibolites. End_of_Article - Last_Page 520------------

PETROLOGY AND METAMORPHISM OF THE WILMINGTON COMPLEX, DELAWARE, PENNSYLVANIA, AND MARYLAND

The Wilmington complex occupies about 100 square miles in Delaware and adjacent Pennsylvania and Maryland and is bordered by the Wissahickon formation (lower Paleozoic?) and the Port Deposit granodiorite. Its southern edge is covered by Coastal Plain sediments. It consists of a banded gneiss comprising mafic bands of calcic plagioclase, hypersthene, augite, and commonly hornblende and thick felsic bands of quartz and andesine with minor pyroxene. In the central portion the gneiss has been invaded by a granite, probably metasomatic, consisting of orthoclase, oligoclase-andesine, biotite, and pyroxene. To the southwest the banded gneiss grades into amphibolites. Amphibolites near the gneiss contain calcic plagioclase, hornblende, and traces of hypersthene. Those farther away are hypersthene-free, and the plagioclase is replaced in part by epidote. Small bodies of amphibolite within the adjacent schist show similar characteristics. Veins of granite, apparently related to the Port Deposit granodiorite, intrude the southwestern part of the complex. A coarse-grained undeformed gabbro stock has intruded the gneiss north of Wilmington; a similar body of gabbro occurs as a group of low hills surrounded by Coastal Plain sediments. The relationship between the chemical composition of the rocks and the mineral phases present indicates that the central granite body and the banded gneiss crystallized or re-crystallized under granulite-facies conditions. The amphibolite near the gneiss represents upper almandine amphibolite facies (probably almandine-sillimanite subfacies), and the more distant amphibolites, having undergone greater stress, probably represent the almandine-diopside-hornblende subfacies. The complex is probably along the axis of most intense metamorphism of the Appalachian(?) Revolution. Structurally, the complex is characterized by the banding ( S 1 ) and foliation ( S 2 ) which are commonly, but not always, parallel. Structures in the complex generally parallel structures in the Wissahickon formation, indicating simultaneous deformation. Recrystallization of the mafic portion of the banded gneiss was accompanied by the introduction, probably metasomatic, of the felsic material. Mafic bands may be transformed into felsic bands by the introduction of Si and Na and the removal of Ca, Fe, Mg, and Al representing a transfer of approximately 7 per cent material by volume. This “potassium-free granitization” may have a definite spatial relationship to the true granitization that produced the central granite. The origin of the mafic materials which comprise the gneiss and the amphibolites may have been (1) sedimentary (impure limestone); (2) volcanic (basaltic tuffs or flows); (3) hypabyssal (a lopolith); or (4) plutonic (a gabbro). Volcanic origin is tentatively suggested. If this is the case the rocks may be correlative with metabasalts of nearby Cecil County, Maryland, and with the Catoctin-South Mountain metabasalts in western Maryland, both of which underlie the Glenarm series.