Late Paleozoic Granitoids Research Articles

U–Pb dating of zircons from PZ3–MZ igneous complexes of Transbaikalia by sector-field mass spectrometry with laser sampling: technique and comparison with SHRIMP

U–Pb zircon dating by laser ablation and sector-field mass spectrometry with inductively coupled plasma (LA–SF-ICP-MS) is an accessible local method with easy sample preparation. At the Geological Institute, Ulan-Ude, this method was applied using a Thermo Scientific Element XR single-collector SF ICP mass spectrometer and a UP-213 (New Wave Research) laser ablation system. Measurements for standard zircons showed the error of dating less than 2%. The results of U–Pb LA–SF-ICP-MS dating of zircon from Late Paleozoic granitoids of western Transbaikalia confirm the overlapping of the time intervals of formation of the Barguzin (330–290 Ma), Chivyrkui (305–285 Ma), and Zaza (305–285 Ma) intrusive complexes.

The formation processes and isotopic structure of continental crust of the Chingiz Range Caledonides (Eastern Kazakhstan)

According to this paper, the juvenile crust of the Chingiz Range Caledonides (Eastern Kazakhstan) was formed due to suprasubduction magmatism within the Early Paleozoic island arcs developed on the oceanic crust during the Cambrian–Early Ordovician and on the transitional crust during the Middle–Late Ordovician, as well as to the attachment to the arcs of accretionary complexes composed of various oceanic structures. Nd isotopic compositions of the rocks in all island-arc complexes are very similar and primitive (eNd(t) from +4.0 to +7.0) and point to a short crustal prehistory. Further increase in the mass and thickness of the crust of the Chingiz Range Caledonides was mainly due to reworking of island-arc complexes in the basement of the Middle and Late Paleozoic volcanoplutonic belts expressed by the emplacement of abundant granitoids. All Middle and Late Paleozoic granitoids have high positive values of eNd(t) (at least +4), which are slightly different from Nd isotopic compositions of the rocks in the Lower Paleozoic island-arc complexes. Granitoids are characterized by uniform Nd isotopic compositions (<2–3 e units for granites with a similar age), and thus we can consider the Chingiz Range as the region of the Caledonian isotope province with an isotopically uniform structure of the continental crust.

Source-inherited compositional diversity in granite batholiths: The geochemical message of Late Paleozoic intrusive magmatism in central Calabria (southern Italy)

The Serre Batholith, in central Calabria, is a Late Paleozoic granitoid complex that makes up the middle portion, ca. 13km thick, of a continuous and nearly complete section of the continental crust. The batholith displays a large compositional variety, with granitoid rocks ranging with continuity from quartz diorite to syenogranite, a distinct group of leucotonalites also occur. The granitoids are on the whole magnesian and calcic to calc-alkalic, with only some of the more evolved rocks showing a ferroan calc-alkalic to alkali-calcic composition. Quartz diorites and tonalites are metaluminous to weakly peraluminous, while granodiorites and granites are weakly to strongly peraluminous, with two-mica porphyritic types being the only population with a genuine strongly peraluminous character. Fe*-number, MALI and ASI features highlight a strong affinity of the Serre Batholith rocks with Cordilleran granitoids, inherited from the compositions of the source rocks rather than reflecting the real tectonic environment of the magmas. Major trace element and existing Sr-Nd data are consistent with an origin of the Serre Batholith from the assembling of several batches of magmas with specific original compositions derived by fluid-absent melting of different crustal sources.Quartz diorites and tonalites originated from a metabasaltic magma source, whereas metagraywackes with various mafic and pelitic contents appear the most likely sources of weakly peraluminous granodiorites and strongly peraluminous granodiorites and granites. Biotite±amphibole granodiorites could also have been derived from mafic-intermediate metaigneous sources. Two-mica porphyritic leucogranites are the only rock types representing pure crustal melts, resulting from melting of mafic pelitic sources. The other granitoid compositions are too silica-poor and MgO+FeOt rich to represent pure melts, so they need to include other components, such as solid restitic/peritectic material entrained from the magma source.Mantle-derived magmas do not appear to have played a role in the geochemical diversity of the Serre Batholith granitoids; their inherited arc signature resulted from partial melting of crustal material of magmatic arc derivation, such as magnesian igneous rocks and sediments derived from their rapid erosion. This study suggests that post-collisional granitoid magmatism is likely not to be associated with the direct generation of new continental crust; all the granitoid rock types appear to represent recycled and reworked crustal material.

Formation of the trachybasalt–trachyte bimodal series of the Malo-Khamardaban volcanotectonic complex, southwestern Transbaikalia: Role of fractional crystallization and magma mixing

The Malo-Khamardaban volcanogenic association is one of the largest and typomorphic structures for the Jurassic–Cretaceous rift magmatism of the Western Transbaikalia. It is mainly made up of bimodal volcanogenic trachybasalt–trachyte series with subordinate amount of trachyandesites, trachyrhyodacites, and trachyrhyolites. Geochemical modeling gives grounds to conclude that the formation of trachytic melts was controlled by crystallization fractionation of trachybasaltic magma. Mineralogical–petrographic, mineralogical–geochemical, and geochemical features of trachyandesites and some syenite porphyries (subvolcanic analogues of trachytes) indicate their hybrid nature. The trachyandesites were formed by mixing of trachybasaltic magma with trachytic magmas (endohybridisation) and trachybasaltic magma with contaminated trachytic magma. Hybrid syenite porphyries were derived by contamination of trachytic magma with host rocks (Late Paleozoic granitoid rocks of the Zaza complex). It is suggested that the trachybasaltic magma prior to generation of daughter trachytic melt and contamination of the latter by host granitoids evolved in a zoned magma chamber, where differentiated salic melt of the apical part interacted with the framework rocks. The hybrid trachyandesite magma was produced by mixing of basaltic and trachytic magmas during their simultaneous injection in one magma chamber.

Provenances of the paleozoic terrigenous sequences of the Oldoi terrane of the Central Asian orogenic belt: Sm-Nd isotope geochemistry and U-Pb geochronology (LA-ICP-MS)

The results of Sm-Nd isotope geochemical investigations of the Paleozoic terrigenous sequences of the Oldoi terrane and U-Pb dating of detrital zircons by the LA-ICP-MS technique showed that the clastic material was mainly derived from Late Precambrian granitoids and Early to Middle Paleozoic granitoids and volcanics, which were formed owing to the reworking of the Late Precambrian continental crust. During the Silurian, the main source of terrigenous material in the sedimentation basin was the erosion of Late Precambrian and Early Paleozoic granitoids. In the Devonian, provenances became more diverse, and the sedimentation basin was additionally supplied by the decomposition products of Middle Paleozoic granitoids and silicic volcanics. The age ranges obtained for detrital zircons from the sandstones of the Middle-Late Devonian Oldoi Formation and the Early Carboniferous Tipara Formation are almost identical to the stratigraphic ages of the formations, which indicates that terrigenous sedimentation in the Oldoi terrane had been accompanied, at least since the Middle Devonian, by vigorous magmatic activity and occurred in an environment of a mature island arc or an active continental margin.

New geophysical data on the depth structure of the Tien Shan intermontane depressions

New results of a detailed study of the Issyk Kul intermontane depression system by electromagnetic and geomagnetic methods are presented. A profile-based magnetometric survey across the water area of Lake Issyk Kul revealed a positive 30-km-wide magnetic anomaly at approximately +100 nT in intensity. The nature of this anomaly can be explained by the exposure of Late Paleozoic granitoid intrusions. The problems of the construction of a regional geoelectric model along the Barskaun magnetotelluric profile, which crosses the central Issyk Kul Depression in the meridional direction, are discussed. On the basis of generalization of previously obtained data from complex studies along the MANAS geophysical transect, a structural-geodynamic lithosphere section was constructed along this transect. In this section, the interrelationships between the surface and deep seismic and geoelectric structures are clearly reflected in the lithosphere of the Naryn-Atbashi intermontane depression system.

Concordant U–Pb SHRIMP ages of U-rich zircon in granitoids from the Muruntau gold district (Uzbekistan): Timing of intrusion, alteration ages, or meaningless numbers

For decades, the U–Pb isotope system of zircon is routinely used in geosciences for the determination of intrusion ages of felsic magmatic rocks. It is, however, well-known that zircon and the related U–Pb isotope system may be influenced by secondary hydrothermal alteration and new (hydrothermal) zircon growth. As a result, age determination may yield meaningless or alteration ages. An example is given here for intensely altered late Paleozoic granitoid rocks from the Muruntau area, Central Kyzyl Kum, Uzbekistan. Pb–Pb single grain evaporation and U–Pb SHRIMP isotope systematics of zircon from six granitoid rocks yield roughly consistent results defining ages ranging between 288 and 303Ma. At first glance, these ages may be understood as intrusion ages of the magmatic rocks hosting the zircon. However, there is an apparent contradiction between the sequence of the measured U–Pb SHRIMP ages and the relative age sequences established in the field. Evaluation of data from comprehensive mineralogical and petrological studies reveals that the U–Pb system of magmatic zircon is complicated by inheritance in I-type granitoids, recrystallization, new hydrothermal zircon growth, and subsequent secondary alteration. Widespread albitization of the granitoids led to the formation of new, U-rich, hydrothermal zircon forming overgrowths on older grains or even new whole single crystals. Later alteration events caused direct and (apparent) reverse U–Pb isotope discordances. These late disturbances of the U–Pb system are mainly due to widespread sericitization. Consequently, precise concordant U–Pb zircon SHRIMP ages around 290–294Ma defined for U-rich, largely undisturbed crystal areas constrain the timing of albitization rather than that of the intrusion. The reliability of the concordant U–Pb SHRIMP ages measured on U-rich zircon is confirmed by a U–Pb SHRIMP age of hydrothermal monazite (292±8Ma). This monazite associates with extremely U-rich zircon (U–Pb SHRIMP age: 291±3Ma) in a sample of the Murun granite recovered beneath the giant Muruntau gold deposit by super-deep drilling. Obviously, detailed mineralogical studies are important for a correct interpretation of zircon ages when hydrothermal alteration and late re-mobilization of zirconium and uranium were significant as often observed in areas with intense ore mineralization. The extensive Au mineralization found at Muruntau is similar in age to albitization but apparently somewhat younger. Likewise, the intrusion age of Permian granites is at least somewhat older than the hydrothermal zircon ages confirmed by U–Pb ages of monazite. It is, however, difficult to constrain further granite intrusion ages based on presently available data.

Late Paleozoic granitoids in western Transbaikalia: sequence of formation, sources of magmas, and geodynamics

The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara–Vitim batholith) at the early stage through transitional from calc-alkaline to alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara–Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle–crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume impact, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara–Vitim batholith structure and with the post-Barguzin age of peralkaline rocks of the Vitim province.

Mafic microgranular enclaves in Late Paleozoic granitoids in the Burgasy quartz syenite massif, western Transbaikalia: Composition and petrogenesis

The paper presents original data on the inner structure, mineralogy, and geochemistry of the Late Paleozoic Burgasy quartz syenite massif in western Transbaikalia and mafic microgranular enclaves (MME) in its rocks. The composition of the mafic microgranular enclaves is close to that of phase-1 monzonitoids of this pluton, but the enclaves are not xenoliths of these rocks but were produced by the crystallization of an individual portion of dispersed hybridized basalt melt. The basaltoid nature of the enclaves follows, first of all, from the relict assemblage of calcic plagioclase (An73–60) and clinopyroxene and from the magmatic dolerite and microgabbro textures of the rocks. The monzonitoid composition of the enclaves was caused by hybridism, which was responsible for the crystallization of quartz, potassic feldspar, and sodic plagioclase due to the introduction of silica, potassium, and some other components. Hybridism was restricted to a boundary crystallization layer in the deep portion of the magmatic chamber (near its bottom). The scatter of the enclaves throughout the whole volume of the pluton is explained by the density inversion of the hybrid layer and material transfer by convective flows. The mafic enclaves crystallized from basaltic melt of within-plate geochemical type. In spite of intense hybridism, the enclaves preserved typical compositional signatures of mafic magma related to the generation of granites in western Transbaikalia in the Late Paleozoic. The basaltoid nature of the mafic enclaves of the Burgasy Massif testifies that magma was simultaneously generated in the mantle and crust during the development of the Late Paleozoic province in the area.

Late paleozoic granitoids of Central Chukotka: Structural position and age constraints

Late paleozoic granitoids of Central Chukotka: Structural position and age constraints

The first geochronological evidence for late paleozoic granitoid magmatism in the Bureya terrane (East of the Central Asian Fold Belt)

The first geochronological evidence for late paleozoic granitoid magmatism in the Bureya terrane (East of the Central Asian Fold Belt)

Magnetic fabric and microstructures of Late Paleozoic granitoids from the North Patagonian Massif: Evidence of a collision between Patagonia and Gondwana?

Widespread Late Paleozoic magmatism in northern Patagonia is a target to test hypotheses on the long standing question over the origin of Patagonia. In recent years, a dispute over whether it is an accreted crustal block that collided with Gondwana in Paleozoic times or an autochthonous part of South America has taken place. As part of a multidisciplinary study, an integrated microstructural and magnetic fabric study was carried out on the Late Carboniferous Yaminué Complex and the Early Permian Navarrete Plutonic Complex, both exposed in the northeastern corner of the North Patagonian Massif (40.5°S, 67.0°W). Other investigated units are the Late Carboniferous Tardugno Granodiorite, the newly defined Cabeza de Vaca Granite and the Late Permian San Martin pluton. Over 300 oriented cores from 60 sites were collected for anisotropy of magnetic susceptibility (AMS) measurements. A systematic analysis of around 100 petrographic thin sections was performed to characterize the microstructures of the different magmatic units. Microstructures in the Yaminué Complex are indicative of a transition from magmatic to solid-state deformation. Microstructures of the orthogneiss of tonalitic composition suggest an early stage in the emplacement history of this complex. The Cabeza de Vaca Granite, intrusive in Yaminué Complex, is the most evolved unit and records less intense high-temperature solid-state deformation which suggests that the stress field that controlled the emplacement of the Yaminué Complex outlasted it. According to petrologic and structural considerations, the Navarrete Plutonic Complex has been subdivided into three facies, i.e. Robaina, Guanacos and Aranda, respectively. Microstructures of the Navarrete Plutonic Complex are mostly magmatic to submagmatic, versus the solid-state fabric that characterizes the Robaina facies at the contact with the Yaminué Complex. Combined analyses of AMS and microstructural data lead us to suggest that the Yaminué Complex, Cabeza de Vaca Granite and possibly Tardugno Granodiorite were intruded during a major compressional event associated with top-to-the-SSW thrusting. This event is most likely related to a frontal collision of the North Patagonian Massif and the southwestern Gondwana margin at around 300 Ma. The Navarrete Plutonic Complex and San Martin pluton were emplaced after that tectonic event, which must have ended by 281 Ma. Previous magmatic, geochronological and paleomagnetic data that suggest close connection of the North Patagonian Massif with the South American Gondwana blocks during the Paleozoic, can be reconciled with a Late Paleozoic collision by a model of a para-autochthonous North Patagonian Massif that rifted away from Gondwana after the Ordovician and collided again in the Late Carboniferous–Early Permian.

Geochemical constraints on the genesis of the Sarıcakaya intrusive rocks, Turkey: Late Paleozoic crustal melting in the central Sakarya Zone

The Sakarya Zone is intruded by several Late Paleozoic granitoids, of which the Sarıcakaya intrusive rocks in the central Sakarya (Eskişehir) region, is the least-studied. The Sarıcakaya intrusive rocks consist mainly of quartz diorite-granodiorite, granodiorite and granite. They are, geochemically, divided into two groups: diorites and granites. The former is medium-K and calc-alkaline (mainly I-type), whereas the latter is high-K to shoshonite and calcic (I-type). Typical minerals for both rock types are plagioclase, K-feldspar, quartz, biotite, hornblende and Fe–Ti oxides. Chondrite-normalized REE patterns for the Sarıcakaya intrusive rocks are moderately fractionated and have small negative Eu anomalies. They are enriched in LILE and LREE relative to HFSE showing characteristics of arc-related granitoids. Geochemical characteristics of the Sarıcakaya intrusive rocks indicate a hybrid origin through partial melting of lower crustal source rocks.

Age and sources of granitoids in the junction zone of the Caledonides and Hercynides in southwestern Mongolia: Geodynamic implications

The paper reports newly obtained geological, geochronological (U-Pb zircon method), Nd isotopic, and geochemical data on Middle and Late Paleozoic granitoids and metamorphic rocks from the southern slope of the Mongolian Altai and Gobi Altai and on granitoids from the Trans-Altai Gobi. Tectonically, the former rocks are hosted in the margin of a Caledonian paleocontinent, and the latter are localized among island-arc and oceanic complexes related to the development of the Hercynian Southern Mongolian Ocean. According to their geological setting, the intrusive complexes are subdivided into two major groups: (i) related to processes of regional metamorphism and (ii) separated from these processes. Geochemical data suggest that the source of most of the granitoids and metamorphic rocks contained island-arc rocks and their erosion products. Nd isotopic evidence indicates that practically all of the allochthonous granitoids, regardless of their composition, age, and structural setting, have positive ɛNd(T) values [i.e., belong to the ɛ(+) type] and could not be formed by the melting of metaterrigenous rocks widespread at the modern erosion level. These granitoids in both the Caledonian and the Hercynian structures have practically identical Late Riphean Nd model ages [TNd(DM) = 0.97–0.60 Ga], which become slightly younger in the granitoids of the Hercynides. The exception are ultrametamorphic subautochthonous ɛ(−) granites of the first group localized in the peripheral part of migmatite fields. The sources of these granitoids could be the host metaterrigenous rocks. The results obtained in the course of this research suggest, with regard for preexisting data on granitoids in the isotopic provinces in Central Asia, that the sources of the Paleozoic granitoids were the rocks of the “juvenile” Caledonian and Hercynian island-arc crust and of the older crust of cratonic blocks with a Early Precambrian and Late Riphean basement, respectively. The Late Riphean crustal material in Caledonian and Hercynian structures related to the development of the corresponding oceanic basins most probably consisted of clastic sediments or relatively small fragments of the Late Riphean crust. The occurrence of this crustal material in the sources of the granitoids can be explained by the involvement of sediments in subduction zones and the participation of these sediments and fragments of Late Riphean complexes in the accretionary-collision processes during the closure of the paleoceanic basins. Simultaneously, the subduction zones received juvenile material that could be later involved in the melting processes together with older rocks.

Late Paleozoic granitoids of western Transbaikalia: magma sources and stages of formation

Geochemical and geochronological studies of the main types of granitoids of the Angara-Vitim batholith (AVB) and granites of the Zaza complex in western Transbaikalia were carried out. U-Pb (SHRIMP-II) and Rb-Sr dating yielded the age of autochthonous gneiss-granites of the Zelenaya Griva massif (325.3±2.8 Ma), quartz syenites of the Khangintui pluton (302.3±3.7 Ma) and intruding leucogranites of the Zaza complex (294.4±1 Ma), monzonites of the Khasurta massif (283.7±5.3 Ma), and quartz monzonites of the Romanovka massif (278.5±2.4 Ma). The U-Pb and Rb-Sr dates show that the Late Paleozoic magmatism in western Transbaikalia proceeded in two stages: (1) 340–320 Ma, when predominantly mesocratic granites of the Barguzin complex, including autochthonous ones, formed, and (2) 310–270 Ma, when most AVB granitoids formed. We suggest that at the early stage, crustal peraluminous granites formed in collision geodynamic setting. At the late (main) stage, magmatism occurred in postorogenic-extension setting and was accompanied by the formation of several geochemical types of granitoids: (1) typical intrusive mesocratic granites of the Barguzin complex, similar to those produced at the first stage; (2) melanocratic granitoids (monzonitoids, quartz syenites), which were earlier dated to the early stage of the AVB evolution; (3) leucocratic medium-alkali (peraluminous) granites of the Zaza intrusive complex; and (4) some alkali-granite and syenite intrusions accompanied by alkaline mafic rocks. The diversity of granitoids that formed at the late stage of magmatism was due to the heterogeneous composition of crust protoliths and different degrees of mantle-magma participation in their formation.

Syntectonic emplacement of the Middle Jurassic Concón Mafic Dike Swarm, Coastal Range, central Chile (33° S)

The Concón Mafic Dike Swarm (CMDS) consists of basaltic to andesitic dikes emplaced into deformed Late Paleozoic granitoids during the development of the Jurassic arc of central Chile. The dikes are divided into an early group of thick dikes (5–12 m) and a late group of thin dikes (0.5–3 m). Two new amphibole 40Ar/ 39Ar dates obtained from undeformed and deformed dikes, constrain the age of emplacement and deformation of the CMDS between 163 and 157 Ma. Based on radiometric ages, field observations, AMS studies and petrographic data, we conclude that the emplacement of the CMDS was syntectonic with the Jurassic arc extension and associated with sinistral displacements along the NW-trending structures that host the CMDS. The common occurrence of already deformed and rotated xenoliths in the dikes indicates that deformation in the granitoids started previously. The early thick dikes and country rocks appear to have been remagnetized during the exhumation of deep-seated coastal rocks in the Early Cretaceous (around 100 Ma). The remanent magnetization in late thin dikes is mainly retained by small amounts of low-Ti magnetite at high temperature and pyrrhotite at low temperature. The magnetization in these dikes appears to be primary in origin. Paleomagnetic results from the thin dikes also indicate that the whole area was tilted ∼ 23° to the NNW during cooling of the CMDS. The NNW–SSE extension vectors deduced from the paleomagnetic data and internal fabric of dikes are different with respect to extension direction deduced for the Middle–Late Jurassic of northern Chile, pointing to major heterogeneities along the margin of the overriding plate during the Mesozoic or differences in the mechanisms driving extension during such period.

Mid- to late Paleozoic K-feldspar augen granitoids of the Yukon-Tanana terrane, Yukon, Canada: Implications for crustal growth and tectonic evolution of the northern Cordillera

Mid- to late Paleozoic K-feldspar augen granitoids of the Yukon-Tanana terrane, Yukon, Canada: Implications for crustal growth and tectonic evolution of the northern Cordillera

Paleozoic and Mesozoic granitic rocks in the Hotont area, central Mongolia.

The granitic rocks of the Hotont area were classified into the Late Paleozoic Delgerhaan batholithic complex and Mesozoic Egiindavaa stocks, the latter of which is further divided into normal granitoids and Sn-rich granitoids. The Delgerhaan granitoids are weakly magnetic, their oxygen fugacity being at QMF buffer, while the Egiindavaa granitoids are more reduced belonging generally to ilmenite series. The Sn-rich granites are completely magnetite free and most reduced. The Delgerhaan granitoids are shoshonitic, whereas the Egiindavaa granitoids are shoshonite to high-K series. Both the granitoids are similar in most of chemical components in the Harker’s diagrams, but the Delgerhaan granitoids are richer in P2O5 and poorer in Fe2O3, TiO2 and MgO than the Egiindavaa granitoids. Both the granitoids seem to be generated within continental crust in islandarc setting and not fractionated. The Sn-rich granite may be a later intrusion and generated in anorogenic tectonic setting, and well fractionated. Weak Au-mineralizations are related to the Late Paleozoic Delgerhaan granitic complex. The Late Paleozoic granitoids are too well exposed to have primary base-metal deposits, and the Mesozoic granitoids are not oxidized enough to have base metal mineralizations. Sn and W mineralizations and some Ti-Ta-Nb occurrences are possibly associated with reduced granitoids of the Mesozoic age, particularly with the Sn-rich granites, which may have formed in anorogenic setting.

Structural controls of lode-gold mineralization by mafic dykes in late-Paleozoic granitoids of the Kochkar district, southern Urals, Russia

The Kochkar gold district in the East Uralian Zone of the southern Urals is located in late-Paleozoic granite gneisses of the Plast massif. Gold mineralization is associated with tabular quartz lodes that are preferentially developed along the margins of easterly trending mafic dykes. Fabric development indicates that dykes had a profound influence on the development of shear zones in granitoids. ENE- and SE-trending dykes have been reactivated as dextral and sinistral oblique strike-slip shear zones, respectively, forming a set of approximately conjugate shear zones related to the Permian, regional-scale E-W directed shortening. Dyke-shear zone relationships in the Plast massif are the result of strain refraction due to the presence of biotite-rich, incompetent dykes in more competent granite-gneisses. Deformation and the formation of associated gold-quartz lodes occurred close to peak-metamorphic, upper-greenschist to lower-amphibolite facies conditions. Strain refraction has resulted in partitioning of the bulk strain into a component of non-coaxial mainly ductile shear in mafic dykes, and a component of layer-normal pure shear in surrounding granitoids where deformation was brittle-ductile. Brittle fracturing in granitoids has resulted in the formation of fracture permeabilities adjacent to sheared dykes, that together with the layer-normal dilational component, promoted the access of mineralizing fluids. Both ore-controlling dykes and gold-quartz lodes were subsequently overprinted by lower greenschist-facies, mainly brittle fault zones and associated hydrothermal alteration that post-date gold mineralization.

Pre-Mesozoic basement rocks offshore Nova Scotia, Canada: New constraints on the accretion history of the Meguma terrane

Basement samples from offshore wells and geophysical data show that the Meguma terrane forms the basement of Mesozoic–Cenozoic sedimentary basins as far as 200 km offshore Nova Scotia. The data provide constraints on Avalon-Meguma terrane interaction during middle to late Paleozoic events in the Appalachian orogen. The basement consists of metasedimentary rocks intruded by late Paleozoic granitoid plutons, principally magnetite-bearing granodiorites, which contrast with more peraluminous ilmenite-bearing Devonian granodiorite and granite onshore. Offshore tonalites have a subduction-related trace element signature that is similar to the signature in minor Devonian gabbro and tonalite intrusions on land. Nd, Sr, and Pb isotopes suggest three sources for Meguma terrane plutons: (1) 2 Ga basement (possibly Saharan Shield) and sediments derived therefrom—this basement may be present beneath the southwestern Scotian Shelf; (2) Late Proterozoic underthrust Avalonian basement and basement autochthonous to the Meguma terrane; and (3) melts from mantle last remobilized in the Late Proterozoic Pan-African orogeny. Seismic reflection profiles show Meguma terrane thrust northward over a wedge of Avalon basement extending 30 km south of the surface suture. There is no evidence that the Devonian tonalites are directly related to subduction. The lack of either ophiolites or Paleozoic pelagic sediment suggests that convergence of the Avalon and Meguma terranes was achieved principally by strike-slip motion resulting from low-angle continent-continent convergence, with only a minor thrust component. The widespread Devonian-Carboniferous igneous activity of the amalgamated Meguma and Avalon terranes was a consequence of regional extension, which resulted in decompression melting of upper mantle and production of mafic magmas variably mixed with crustal material.