Postorogenic Dike Complexes and Implications for Metallogenesis
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Abstract The extent of the Reefton lamprophyre dike swarm is reviewed in the light of recent geological mapping in the areas of the Karamea Batholith east of Reefton. In 1 place, a small but distinct swarm occurs, and is informally named the Mt Albert-Mt Ross lamprophyre dike swarm. Dikes of this swarm have dominant WNW trends which are radially arranged with respect to a circular feature in the area. Petrologically the dikes are camptonite and vosgesite. Dikes that occur within the cataclastic contact zone of a granitic pluton (western margin of Karamea Batholith) show intense deformation indicating that the pluton was tectonically uplifted after the emplacement of the dikes.
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The tectono-sedimentary evolution of the Itararé Subgroup (Late Paleozoic) in the southern flank of the Ponta Grossa arch, States of Santa Catarina and Paraná, Brazil, is interpreted through stratigraphic analysis of outcropping beds.Its evolution seems to have been influenced by faulting causing rising and falling of the arch.The section analyzed runs some 50 km SE-NW, from Mafra (SC)-Rio Negro (PR) to Lapa (PR) and includes about 700 m thickness of glacio-clastic beds assigned to the Campo do Tenente and Mafra formations.Paleocurrent orientation, sense of the movement of the gravity driven Paraná glacial lobe, and stratigraphic data indicate a basin paleoslope initially dipping 7' N during deposition of the Campo do Tenente Formation.Isopach data shows that the unit fills a large trough trending NW, resting on abraded and striated rocks of the Paraná Group (Devonian).This interpretation implies a tectonically negative behavior of the Ponta Grossa arch during this time, also shown by isopach maps of palynobiostrati-graphic intervals G and H 1 (Santos et al. 1996.
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The Avila batholith of central Spain is composed of upper Carboniferous peraluminous granitoids that were preceded by volumetrically insignificant bodies of mafic‐ultramafic hybrid magmas and postdated by several dike swarms of camptonitic lamprophyres. Rb‐Sr dating indicates continuous magmatic activity from $$\sim $$ 350 Ma to $$\sim $$ 280 Ma, starting with the mafic precursors and a few midcrustal anatectic leucogranites, followed by massive autochthonous and allochthonous granodiorites and granites, and ending with the camptonitic lamprophyres. Early hybrid mafic magmas ($$\varepsilon ^{340\:\mathrm{Ma}\,}\mathrm{\,Sr}\,\sim 25$$ ; $$\varepsilon ^{340\:\mathrm{Ma}\,}\mathrm{\,Nd}\,\sim -1.5$$ ) were produced in small batches during or immediately after the main deformation phase, probably by the partial melting of a mixture of $$\sim $$ 8 : 2 mantle and biotite‐bearing crustal rocks at the crust‐mantle interface. These magmas were emplaced in the middle crust at $$\sim $$ 340 Ma, advecting a negligible amount of heat. The generation of crustal granites during the main deformation phase was very scarce and limited to highly fertile protoliths, rich in heat‐producing elements, affected by strong shear zones. The generation of crustal granitoids on a batholithic scale took place from $$\sim $$ 330 Ma to $$\sim $$ 290 Ma, during the main extensional period. Granites ($$\varepsilon ^{310\:\mathrm{Ma}\,}\mathrm{\,Sr}\,\sim 45$$ –150; $$\varepsilon ^{310\:\mathrm{Ma}\,}\,\mathrm{Nd}\,\sim -2.1\,\mathrm{to}\,\,-9$$ ) were produced by the partial melting of fertile crustal rocks ($$\varepsilon ^{310\:\mathrm{Ma}\,}\,\mathrm{Sr}\,\sim 48$$ –218; $$\varepsilon ^{310\:\mathrm{Ma}\,}\,\mathrm{Nd}\,\sim -2.2\,\mathrm{to}\,\,-9$$ ), characterized by high heat production ($$\sim $$ 2.5–3 μW m−3). The zone of partial melting, $$\sim $$ 15–22 km in depth, was heated by thermal conduction from below after crustal thinning, but the contribution of radiogenic heat and the fertility of source rocks would have been essential for anatexis. The fast thinning of the crust from $$\sim $$ 310 Ma to $$\sim $$ 285 Ma released lithostatic pressure in the upper mantle and caused decompressional melting of the metasome layer at $$\sim $$ 60–85 km in depth, producing camptonitic melts dated at $$\sim $$ 283 Ma. The existence of a fertile metasome layer implies that the lithospheric mantle beneath central Iberia was not actively involved in subduction during the Variscan orogeny.
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We present petrographic, geochemical (major, traces and REE) and isotopic (Sm-Nd and Rb-Sr) data from ca. 1685 Ma mafic rocks of the Willyama Supergroup in the Broken Hill Inlier of western NSW, Australia. The mafic rocks occur throughout the lower Willyama Supergroup stratigraphy and are interpreted here as shallowly emplaced sills that were metamorphosed to upper amphibolite and granulite facies during the Olarian Orogeny (ca. 1600-1580 Ma). Our data indicate that the metabasites originated by variable degrees of partial melting of a depleted mantle source, only weakly more enriched in incompatible elements compared to present day N-type MORB. This was followed by simple crystal fractionation or by an AFC process involving only small degrees of crustal assimilation (r = 0.05-0.2). Crystal fractionation proceeded along a tholeiitic trend of extreme primary iron and titanium enrichment, leading to melts with up to 25 wt % of total iron as Fe2O3 and 4.2 wt % of TiO2. Some intermediate rocks were derived from this fractionation, but the bulk of the contemporary felsic magmatic rocks (Alma, Rasp Ridge and Potosi Gneisses) are not linked by fractional crystallization to the mafic melt that produced the meta-igneous amphibolites, and are products of anatexis of crustal material from the Willyama sedimentary pile. Based on the occurrence of bi-modal magmatism, a depleted mantle source, partial melting modelling and minimal crustal contamination of the mafic rocks, we infer that the Broken Hill Inlier (ca. 1685 Ma) was the extensional axis and depositional centre of an advanced stage intra-cratonic rift with relatively thin crust and lithosphere. Data from the neighbouring Olary Inlier, in contrast, that imply smaller degrees of partial melting, relatively thicker lithosphere and more crustal contamination, are consistent with placing this Inlier on a rift margin. Active faulting during the rift stage coupled with submarine sedimentation and an anomalous geothermal gradient driven by lithospheric thinning, provide an ideal theoretical environment for the formation of the Broken Hill Pb-Zn-Ag orebody.
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The Idaho batholith and spatially overlapping Challis intrusive province in the North American Cordillera have a history of magmatism spanning some 55 Myr. New isotopic data from the ∼98 Ma to 54 Ma Idaho batholith and ∼51 Ma to 43 Ma Challis intrusions, coupled with recent geochronological work, provide insights into the evolution of magmatism in the Idaho segment of the Cordillera. Nd and Hf isotopes show clear shifts towards more evolved compositions through the batholith's history and Pb isotopes define distinct fields correlative with the different age and compositionally defined suites of the batholith, whereas the Sr isotopic compositions of the various suites largely overlap. The subsequent Challis magmatism shows the full range of isotopic compositions seen in the batholith. These data suggest that the early suites of metaluminous magmatism (98–87 Ma) represent crust–mantle hybrids. Subsequent voluminous Atlanta peraluminous suite magmatism (83–67 Ma) results primarily from melting of different crustal components. This can be attributed to crustal thickening, resulting from either subduction processes or an outboard terrane collision. A later, smaller crustal melting episode, in the northern Idaho batholith, resulted in the Bitterroot peraluminous suite (66–54 Ma) and tapped different crustal sources. Subsequent Challis magmatism was derived from both crust and mantle sources and corresponds to extensional collapse of the over-thickened crust.
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Contributions of heat and/or mass from mafic magmas are commonly invoked in models of voluminous granodiorite and andesite generation in magmatic and volcanic arcs worldwide. However, mafic intrusions are a volumetrically minor component in most arc batholiths. This is the case in the Sierra Nevada batholith, California, USA, where gabbro and diorite plutons are smaller and less abundant than the granitoid suites that make up the bulk of the batholith. Here, we constrain the timing and geochemistry of mafic intrusions in the Sierra Nevada batholith to assess the role of these compositions in arc batholith construction. Previous detailed studies on a limited number of mafic intrusions demonstrate that they formed penecontemporaneously with the felsic batholith, but there is a need for a broader survey of mafic plutons using modern geochronological techniques. New U-Pb zircon ages for 13 gabbro to diorite plutons and geochemistry from 17 mafic intrusions in the eastern Sierra Nevada batholith document two main episodes of mafic magmatism in the eastern Sierra Nevada batholith, from 168 Ma to 145 Ma and from 100 Ma to 89 Ma. These episodes overlap with the ages of granitoid plutons in the eastern Sierra Nevada batholith, including the Late Jurassic Palisade Crest and Late Cretaceous John Muir intrusive suites, in addition to other felsic plutons dated in the eastern Sierra Nevada batholith. Non-primitive mineral compositions in the mafic bodies indicate that their parental magmas are the evolved products of mantle-derived basalts that first differentiated in the lower crust prior to ascent and crystallization in the upper crust. The presence of rocks with cumulate textures, as well as a wide range of bulk-rock compositions (SiO2 wt% 38−64, Mg# 39−74), show that magmatic differentiation continued within each mafic body after intrusion into the upper crust. Sr/Y ratios in melt-like (i.e., non-cumulate) mafic samples suggest that the crustal thickness of the Sierra Nevada batholith was roughly 30 km in the Early Jurassic and increased to ∼44 km by the Late Cretaceous. Concomitant intrusion of mafic melts along with voluminous granitoid plutons supports mantle melting as a major contributor of heat and magmatic volumes to the crust during construction of the eastern Sierra Nevada batholith.
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Formation of the Idaho batholith is thought to have been promoted by prolonged injection of high-temperature mantle magmas that caused partial melting of continental crustal rocks to form granitic magma. The mafic magmas are now represented at the surface by numerous synplutonic mafic dikes in the batholith and by early tonalite and quartz diorite plutons forming the western part of the batholith and scattered elsewhere around its margins. Variable degrees of mixing between the mafic magmas and the granites produced quartz diorite complexes, intermediate and composite dikes, and mafic-rich, inhomogeneous parts of the main-phase granites and granodiorites.
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