http://www.sciencedirect.com/science/article/pii/S1674987112000618

The Moyar Shear Zone(MSZ) of the South Indian granulite terrain hosts a prominent syenite pluton (～560 Ma) and associated NW-SE to NE-SW trending mafic dyke swarm(～65 Ma and 95 Ma). Preliminary magnetic fabric studies in the mafic dykes,using Anisotropy of Magnetic Susceptibly(AMS) studies at low-field,indicate successive emplacement and variable magma flow direction.Magnetic lineation and foliation in these dykes are identical to the mesoscopic fabrics in MSZ mylonites,indicating shear zone guided emplacement.Spatial distribution of magnetic lineation in the dykes suggests a common conduit from which the source magma has been migrated.The magnetic foliation trajectories have a sigmoidal shape to the north of the pluton and curve into the MSZ suggesting dextral sense of shear.Identical fabric conditions for magnetic fabrics in the syenite pluton and measured field fabrics in mylonite indicate syntectonic emplacement along the Proterozoic crustal scale dextral shear zone with repeated reactivation history.     

Geophysical constraints of shear zones and geometry of the Hiltaba Suite granites in the western Gawler Craton,Australia

The emplacement of the ca 1590–1575 Ma Hiltaba Suite granites records a large magmatic event throughout the Gawler Craton, South Australia. The Hiltaba Suite granites intrude the highly deformed Archaean‐Palaeoproterozoic rocks throughout the craton nuclei. Geophysical interpretation of the poorly exposed central western Gawler Craton suggests that the region can be divided into several distinct domains that are bounded by major shear zones, exhibiting a sequence of overprinting relationships. The north‐trending Yarlbrinda Shear Zone merges into the east‐trending Yerda Shear Zone that, in turn, merges into the northeast‐trending Coorabie Shear Zone. Several poorly exposed Hiltaba Suite granite plutons occur within a wide zone of crustal shearing that is bounded to the north by the Yerda Shear Zone and to the south by the Oolabinnia Shear Zone. This wide zone of crustal shearing is interpreted as a major zone of synmagmatic dextral strike‐slip movement that facilitated the ascent of Hiltaba Suite granite intrusions to the upper crust. The aeromagnetic and gravity data reveal that the intrusions are ～15–25 km in diameter. Forward modelling of the geophysical data shows that the intrusions have a tabular geometry and are less than 6 km deep.     

Lower Carboniferous post-orogenic granites in central-eastern Sierra de Velasco,Sierras Pampeanas,Argentina: U–Pb monazite geochronology,geochemistry and Sr–Nd isotopes

The central-eastern part of the Sierra de Velasco (Sierras Pampeanas, NW Argentina) is formed by the large Huaco (40 × 30 km) and Sanagasta (25 × 15 km) granite massifs and the small La Chinchilla stock (2 × 2 km). The larger granites intrude into Ordovician metagranitoids and crosscut Devonian (?) mylonitic shear zones, whereas the small stock sharply intrudes into the Huaco granite. The two voluminous granites are biotitic-muscovitic and biotitic porphyritic syeno- to monzogranites. They contain small and rounded tonalitic and quartz-dioritic mafic microgranular enclaves. The small stock is an equigranular, zinnwaldite- and fluorite-bearing monzogranite. The studied granites are silica-rich (SiO₂ >70%), potassium-rich (K₂O >4%), ferroan, alkali-calcic to slightly calk-alkalic, and moderately to weakly peraluminous (A/CNK: 1.06–1.18 Huaco granite, 1.01–1.09 Sanagasta granite, 1.05–1.06 La Chinchilla stock). They have moderate to strong enrichments in several LIL (Li, Rb, Cs) and HFS (Nb, Ta, Y, Th, U) elements, and low Sr, Ba and Eu contents. U–Pb monazite age determinations indicate Lower Carboniferous crystallization ages: 350–358 Ma for the Huaco granite, 352.7 ± 1.4 Ma for the Sanagasta granite and 344.5 ± 1.4 Ma for the La Chinchilla stock. The larger granites have similar ?Nd values between ?2.1 and ?4.3, whereas the younger stock has higher ?Nd of ?0.6 to ?1.4, roughly comparable to the values obtained for the Carboniferous San Blas granite (?1.4 to ?1.7), located in the north of the sierra. The Huaco and Sanagasta granites have a mainly crustal source, but with some participation of a more primitive, possibly mantle-derived, component. The main crustal component can be attributed to Ordovician peraluminous metagranitoids. The La Chinchilla stock derives from a more primitive source, suggesting an increase with time in the participation of the primitive component during magma genesis. The studied granites were generated during a post-orogenic period in a within-plate setting, possibly as a response to the collapse of the previous Famatinian orogen, extension of the crust and mantle upwelling. They are part of the group of Middle Devonian–Lower Carboniferous granites of the Sierras Pampeanas. The distribution and U–Pb ages of these granites suggests a northward arc-parallel migration of this mainly post-orogenic magmatism with time.     

Form and dimensions of dykes in eastern Iceland

A study was made of about 700 dykes in eastern Iceland. The majority of these belong to three swarms. About 73% dip within 10° of the vertical. Most strike between 10° and 40°NE. The strike of the dykes within the southernmost swarm (Alftafjordur) changes along its trace, from almost N at the north end to NNE-SSW southward along the swarm. The average thickness of the dykes is about 4.1 m, and the thickness does not change notably along the Alftafjordur swarm. The thinner dykes tend to have smaller grains than the thicker dykes. Of five dykes followed laterally, the longest is over 22 km. The thickness of individual dykes changes irregularly along their length, and the dyke is often offset where its thickness changes. Many dykes appear to be completely discontinuous, but some parts are connected by veins. Where the dykes end in a vertical section, most of them simply taper away. Only about 10.5% of the dykes occupy faults. The mechanical and thermal effects of the dykes on the country rock are small. Many of the dykes appear to be non-feeders, i.e. dykes that never reached the surface to feed lava flows. Using the length/width ratio, the depth of origin of three dykes has been estimated. The maximum depth of origin of these three dykes is 7.5 km, 9 km and 10 km below the original surface.     

Late Neoproterozoic alkaline magmatism in the Arabian–Nubian Shield: the postcollisional A-type granite of Sahara–Umm Adawi pluton, Sinai, Egypt

The Sahara–Umm Adawi pluton is a Late Neoproterozoic postcollisional A-type granitoid pluton in Sinai segment of the Arabian–Nubian Shield that was emplaced within voluminous calc-alkaline I-type granite host rocks during the waning stages of the Pan-African orogeny and termination of a tectonomagmatic compressive cycle. The western part of the pluton is downthrown by clysmic faults and buried beneath the Suez rift valley sedimentary fill, while the exposed part is dissected by later Tertiary basaltic dykes and crosscut along with its host rocks by a series of NNE-trending faults. This A-type granite pluton is made up wholly of hypersolvus alkali feldspar granite and is composed of perthite, quartz, alkali amphibole, plagioclase, Fe-rich red biotite, accessory zircon, apatite, and allanite. The pluton rocks are highly evolved ferroan, alkaline, and peralkaline to mildly peraluminous A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO₂, Na₂O + K₂O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce, and rare earth elements (REE) and low CaO, MgO, Ba, and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal sources that has gone through a continent–continent collision (postorogenic or postcollisional), with minor contribution from mantle source similar to ocean island basalt. The assumption of crustal source derivation and postcollisional setting is substantiated by highly evolved nature of this pluton and the absence of any syenitic or more primitive coeval mafic rocks in association with it. The slight mantle signature in the source material of these A-type granites is owed to the juvenile Pan-African Arabian–Nubian Shield (ANS) crust (I-type calc-alkaline) which was acted as a source by partial melting of its rocks and which itself of presumably large mantle source. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti, and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. Crystallization temperature values inferred average around 929°C which is in consistency with the presumably high temperatures of A-type magmas, whereas the estimated depth of emplacement ranges between 20 and 30 km (upper-middle crustal levels within the 40 km relatively thick ANS crust). The geochronologically preceding Pan-African calc-alkaline I-type continental arc granitoids (the Egyptian old and younger granites) associated with these rocks are thought to be the crustal source of f this A-type granite pluton and others in the Arabian–Nubian Shield by partial melting caused by crustal thickening due to continental collision at termination of the compressive orogeny in the Arabian–Nubian Shield.     

Spatially controlled Fe and Si isotope variations: an alternative view on the formation of the Torres del Paine pluton

We present new Fe and Si isotope ratio data for the Torres del Paine igneous complex in southern Chile. The multi-composition pluton consists of an approximately 1 km vertical exposure of homogenous granite overlying a contemporaneous 250-m-thick mafic gabbro suite. This first-of-its-kind spatially dependent Fe and Si isotope investigation of a convergent margin-related pluton aims to understand the nature of granite and silicic igneous rock formation. Results collected by MC-ICP-MS show a trend of increasing δ⁵⁶Fe and δ³⁰Si with increasing silica content as well as a systematic increase in δ⁵⁶Fe away from the mafic base of the pluton. The marginal Torres del Paine granites have heavier Fe isotope signatures (δ⁵⁶Fe = +0.25 ± 0.02 2se) compared to granites found in the interior pluton (δ⁵⁶Fe = +0.17 ± 0.02 2se). Cerro Toro country rock values are isotopically light in both Fe and Si isotopic systems (δ⁵⁶Fe = +0.05 ± 0.02 ‰; δ³⁰Si = ?0.38 ± 0.07 ‰). The variations in the Fe and Si isotopic data cannot be accounted for by local assimilation of the wall rocks, in situ fractional crystallization, late-stage fluid exsolution or some combination of these processes. Instead, we conclude that thermal diffusion or source magma variation is the most likely process producing Fe isotope ratio variations in the Torres del Paine pluton.     

Petrology of calc-alkaline/adakitic basement hosting A-type Neoproterozoic granites of the Malani igneous suite in Nagar Parkar,SE Sindh,Pakistan

The Nagar Parkar area contains three distinct groups of rocks, from oldest to youngest, (1) basement rocks ranging in composition from mafic to (quartz)diorite, tonalite, granite, and younger granodiorite, (2) granite plutons similar in general features to those of the Malani Igneous Suite of Rajasthan, and (3) abundant mafic, felsic and rhyolitic dykes. The basement rocks show strong brittle and local plastic deformation, and epidote amphibolite/upper greenschist facies metamorphic overprint. The chemistry of the basement rocks contrasts the commonly agreed within plate A-type character of the Neoproterozoic granites (group 2) that are emplaced into them. The basement rock association is calc-alkaline; the granodiorite displays the compositional characteristics of adakites, whereas the tonalite has intermediate composition between typical adakite and classical island arc rocks. This paper presents detailed petrography of the basement rocks and compares their geochemistry with those of the group 2 granites as well as with rocks from other tectonic environments. It is proposed that the Nagar Parkar basement is part of a 900–840 Ma magmatic arc that was deformed before it was intruded 800–700 Ma ago by the A-type continental granitic rocks followed by mafic to felsic dykes.     

Felsic dykes in the Neoproterozoic Nagar Parkar Igneous Complex,SE Sindh,Pakistan: geochemistry and tectonic settings

The Nagar Parkar Igneous Complex consists of Neoproterozoic igneous and metamorphic rocks dissected by mafic, felsic, and rhyolitic dykes. The latter can be classified broadly into porphyritic felsic dykes intruding gray and pink granites at Nagar Parkar and the surrounding areas, and the orthophyric felsic dykes intruding amphibolites, deformed pink granites, and the alkaline mafic dykes in the Dhedvero area, north of Nagar Parkar. The porphyritic felsic dykes are composed of perthites, quartz, and albitic plagioclase whereas the orthopheric felsic dykes contain K-feldspar (dominant), plagioclase, and minor quartz. Geochemically, the porphyritic and orthophyric felsic dykes are subalkaline and alkaline demonstrating post-orogenic A2- and OIB-A1-type characteristic on Nb–Y–Ce and Nb–Y–3Ga ternary plots, respectively. One orthophyric felsic dyke contains normative acmite and sodium metasilicate. This study suggests two distinct tectonic regimes for the origin of the felsic dykes of the area. The porphyritic felsic dykes show similarities with the ~800–700 Ma granites of the area, the rhyolite dykes of the Mount Abu, western Rajasthan in India, and the granites of the Seychelles microcontinent. The orthophyric felsic dykes show chemical resemblance with the Tavidar volcanic suite of western Rajasthan and the Silhouette and North islands of the Seychelles microcontinent. This study confirms spatial and temporal links among the Rodinian fragments exposed in the Nagar Parkar area of Pakistan, western Rajasthan of India, and the Seychelles microcontinent.     

A record of ductile syn-intrusional fabrics to post solidification cataclasis: Magnetic fabric analysis of neoproterozoic Mirpur and Mt. Abu Granitoids,NW India

The Mirpur granite body represents a relatively small (10 km²) pluton intruded along the northern margin of the adjacent Mt. Abu batholith (∼125 km²) in NW India. It is a visibly undeformed alkali feldspar rich pink granite; in contrast, the Mt. Abu is a composite granitoid body and variably deformed. Both are intruded by rhyolitic dykes and the terminal magmatic events in both the cases are mafic dykes. The AMS (Anisotropy of Magnetic Susceptibility) data identify the Mt. Abu with SE-dipping foliations and subvertical lineations as a single structural domain while the Mirpur granite body shows two domains characterized by predominantly E — W trend of magnetic foliation in the eastern part (domain I) and N — S orientations in the western part (domain II). The domain I shows magmatic fabrics, typical for the peraluminous granites of Malani Igneous Suite (MIS). Change in fabric orientation in the domain II has resulted from cataclasis wherein the samples show destruction of the original E — W fabric and complete transposition by N — S trends. The foliations in the Mt. Abu granites have been related to SE orientation of maximum horizontal stress. The same maximum stress direction can be inferred from dyke orientation in the Mirpur granite, which is interpreted as continuation of the tectonic imprint in this region during emplacement of both the granites. Age of the cataclastic overprint with a predominant N — S orientation is not yet constrained but corresponds with the trend of the nearby Sindreth basin within the Malani Igneous Suite. The Neoproterozoic tectonic scenario for the region has been interpreted in terms of an ongoing crustal convergence and granitic magma emplacement against the back stop offered by the rigid Delhi Fold Belt.     

Diorites and associated rocks in the Anglem Complex at The Neck, northeastern Stewart Island, New Zealand: an example of magma mingling

Exposures of the Anglem Complex (Cretaceous) at The Neck, northeastern Stewart Island consist dominantly of diorite, microdiorite, tonalite and granite. Dyke-rocks include microgranite, microdiorite, andesite and microgranodiorite.

A Diorite Suite composed of hornblende gabbro, diorite and quartz-bearing diorite forms a complex association of basic to intermediate lithologies with highly variable grainsize. The Heterogeneous Suite formed in part by mingling of diorite and tonalite intrudes the Diorite Suite. Enclaves within this suite show highly variable margins, from sharp sometimes crenulated and apparently chilled contacts, through to diffuse contacts. Prior to, and during intrusion of this heterogeneous magma, significant textural modifications of dioritic enclaves occurred, and some important controls on this modification process are discussed. Aspects considered include contrasts between the host melt and enclaves in temperature, viscosity, chemistry, volatile content, deformation and recrystallisation. A liquid immiscibility model is discounted in favour of the magma mingling model. Subsequent intrusion of granite was followed by a porphyritic microgranodiorite. Microdiorite and microgranite dykes intrude all of the above rocks with the exception of the microgranodiorite. Coexisting microdiorite, microgranite, and composite microdiorite-microgranite dykes in the one dyke swarm also provide compelling evidence for magma mingling.     

Origin of dyke swarms in Wadi El Redi-Wadi Lahami area,southern Eastern Desert of Egypt

Dykes predominate within the Neoproterozoic rocks, especially granites, of Wadi El Redi-Wadi Lahami area in the southern Eastern Desert of Egypt. The dyke swarms form three major suites: from the oldest to the youngest, they are basaltic andesite—Suite 1 (E-W and ENE-WSW), rhyolite—Suite 2 (NE-SW), and andesite—Suite 3 (NNE-SSW, NNW-SSE, and NW-SE). Despite the wide ranges of the dyke compositions, the feldspar and amphibole are usually the essential forming minerals. The plagioclase arrays between Ab_0.9An_99.10 in the basaltic andesite and Ab_98.80An_0.70 in the rhyolite, while sanidine ranges from Or_44.60Ab_49.70 to Or_98.40Ab_1.60. Amphibole in Suite 1 and 3 (Al₂O₃, TiO₂, Na₂O, and K₂O are the lowest and those of SiO₂ and CaO are the highest) samples are usually magnesio-hornblende, whereas it is edenite and tschermakite in Suite 2 dykes. Despite all parent magmas have calc-alkaline affinity, some elements such as Ni show an erratic behavior against the progressing differentiation from one magma chamber and implying for an assimilation of the country rocks. The high contents of amphibole, the depletion in Ti, and the enrichment in large-ion lithophile elements (such as K, Rb, Ba, Sr, and Ba) compared to the primitive mantle composition are consistent with parent hydrous melts generated due to extension above the subduction zone. The estimated compositions of liquids in equilibrium with amphiboles and the pressures at which they crystallized (4.61–7.8 kbar for the Suite 2 and 1.5–2 kbar for the Suites 1 and 3) are greatly varied. These are indications for a difference in the source regions of the parent magmas of the studied dykes. It is supposed that the Suite 1 and 2 dykes are a conjugate set emplaced due to the NW-SE crustal extension in the Arabian-Nubian shield, whereas the Suite 3 dykes generated due to the rifting along the Red Sea.     

Magnetic fabric of a basaltic dyke swarm associated with Mesozoic rifting in northeastern Brazil

The anisotropy of magnetic susceptibility (AMS) has been studied in a 120 km long, Early Cretaceous tholeiitic dyke swarm emplaced during the early stages of rifting and opening of the equatorial Atlantic Ocean. The vertical dykes filled a set of E-trending fractures that cut the structural grain of the Precambrian basement of northeastern Brazil at a high angle. These strongly magnetic rocks contain pseudo-single domain, Ti-poor magnetite and secondary maghemite as revealed by thermomagnetic and hysteresis data. The contribution of the paramagnetic and the high coercivity antiferromagnetic fractions to the bulk susceptibility is less than 1.2%. The dykes generally show well-clustered AMS principal directions. The plunge of the magnetic lineation varies from nearly subvertical in the center of the swarm to horizontal in the west. The strike of the magnetic foliation is generally oblique to the dyke wall and exhibits a curved trend at the regional scale. This fabric pattern suggests that the magma source that fed the dykes was situated in the center of the swarm, which is presently below Tertiary sandstones.     

Magnetic fabrics and their relationship with the emplacement of the Piracaia pluton,SE Brazil

Magnetic fabric and rock-magnetism studies were performed on the four units of the 578 ± 3-Ma-old Piracaia pluton (NW of São Paulo State, southern Brazil). This intrusion is roughly elliptical (~32 km²), composed of (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), which is predominant in the pluton, (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the units. Several rock-magnetism experiments performed in one specimen from each sampled units show that for all of them, the magnetic susceptibility and magnetic fabrics are carried by magnetite grains, which was also observed in the thin sections. Foliations and lineations in the units were successfully determined by applying magnetic methods. Most of the magnetic foliations are steeply dipping or vertical in all units and are roughly parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for the whole pluton. However, for eight sites, they are steep up to vertical. Thin-section analyses show that rocks from the Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state fabric in the north of the pluton, indicating that magnetic fabrics in this area of the pluton are related to this strain. Otherwise, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the SW of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlie this area.     

Tectonic evolution of the southern Gawler Craton,South Australia,from electromagnetic sounding

Long-period natural-source electromagnetic data have been recorded using portable three-component magnetometers at 39 sites in 1998 and 2002 across the southern Eyre Peninsula, South Australia that forms part of the Gawler Craton. Site spacing was of order 5 km, but reduced to 1 km or less near known geological boundaries, with a total survey length of approximately 50 km. A profile trending east – west was inverted for a 2D electrical resistivity model to a depth of 20 km across the southern Eyre Peninsula. The main features from the models are: (i) on the eastern side of the Gawler Craton, the Donington Suite granitoids to the east of the Kalinjala Shear Zone are resistive (>1000 Ωm); (ii) the boundary between the Donington Suite granitoids and the Archaean Sleaford Complex, which has much lower resistivity of 10 – 100 Ωm, is almost vertical in the top 10 km and dips slightly westwards; and (iii) two very low resistivity (<1 Ωm) arcuate zones in the top 3 km of Hutchison Group sediments correlate with banded iron-formations, and are probably related to biogenic-origin graphite deposits concentrated in fold hinges. Such features suggest an extensional regime during the time period 2.00 – 1.85 Ga. We suggest that the resistivity boundary between the Donington Suite and the Archaean Sleaford Complex represents a growth fault, typical for rift systems that evolve into a half-graben structure. In the graben basin, low-resistivity shallow-marine Hutchison Group sediments were deposited. Folding of the sediments during the Kimban Orogeny between 1.74 and 1.70 Ga has led to migration of graphite to the fold hinges resulting in linear zones of very low resistivity that correlate with banded iron-formation magnetic anomalies.     

Palaeomagnetism and aeromagnetic modelling of the Mesoproterozoic Ntimbankulu pluton, Kwazulu-Natal, South Africa: mushroom-shaped diapir?

The aeromagnetic signature of the Mesoproterozoic Ntimbankulu porphyritic granite-charnockite pluton, southern KwaZulu-Natal, is conspicuously circular. Surface geological mapping of the area broadly confirms such a geometry: the pluton is expressed topographically as an annulur ring of marginal hills, composed of porphyritic granite, surrounding a low-lying central core underlain by charnockite. The pluton forms one of the least deformed of ten major granite-charnockite intrusions known collectively as the Oribi Gorge Suite. The suite, which has been dated at 1050 Ma, intrudes the Mzumbe and Margate Terranes of the 1.1 Ga Natal Metamorphic Province. A palaeomagnetic study was performed on samples from the pluton and the surrounding country rock gneisses. The measured directions of magnetisation of the different sites were combined to calculate the virtual palaeomagnetic pole position of the pluton, which was determined at 27°N and 327°E. This is consistent with previously published poles from the Namaqua-Natal Metamorphic Belt. The magnetic parameters derived during the palaeomagnetic study were integrated into the aeromagnetic data to model three selected magnetic profiles across the pluton. The aim was to test the hypothesis that the pluton has a mushroom-shaped 3D form with remnants of Mapumulo Group gneiss situated underneath, and dipping towards, the pluton. It is concluded that these remnants of gneiss with their negative inclination are responsible for the ring dyke like signature of the aeromagnetic data.     

Mesoproterozoic (1.47–1.44 Ga) orogenic magmatism in Fennoscandia; Baddeleyite U–Pb dating of a suite of massif-type anorthosite in S. Sweden

The Jönköping Anorthositic Suite (JAS) in S. Sweden has characteristics typical for (Proterozoic) massif-type anorthosites. The interstitial liquid of these plagioclase-porphyritic rocks solidified at 1,455 ± 6 Ma, as determined by U–Pb isotope analysis of baddeleyite. The JAS developed during a regional 1.47–1.44 event in Fennoscandia that generated widespread mafic magmatism (basalts, and diabase dykes and sills) in the north and emplacement of felsic plutons in the south. The event of 1.47–1.44 Ga magmatism in Fennoscandia largely coincides in age with dynamic high-grade metamorphism in SW Sweden and was probably related to convergent active-margin processes during the Danopolonian orogeny.     

Age and magnetic fabric of the Três Córregos granite batholith: evidence for Ediacaran transtension in the Ribeira Belt (SE Brazil)

Anisotropy of magnetic susceptibility (AMS) and U–Pb (SHRIMP) zircon ages in the eastern part of the Três Córregos batholith (Ribeira belt, SE Brazil) indicate a well-defined fabric pattern acquired between 600 and 595 Ma. The batholith consists mostly of porphyritic granites distributed in the Ribeirão Branco, Barra do Chapéu and Itaóca plutons. Late fluorite-bearing alkaline granites, some containing Sn-polymetallic greisen-type deposits, intruded the Ribeirão Branco pluton and the low-grade metasedimentary host rocks. The magnetic fabric of the Ribeirão Branco granite is dominantly oblate and oblique to the pluton elongation while that in Barra do Chapéu is mostly concentric. On both plutons, AMS records the preferred orientation of coarse, homogeneous Ti-poor magnetite grains. Such fabric patterns indicate a partitioned strain field dominated by strike-slip left-lateral shear deformation in the Ribeirão Branco and extension in the Barra do Chapéu pluton. The zircon ages of these plutons are 600 ± 6 Ma and 595 ± 4 Ma. Likewise, the ages of alkaline plutons were in the range of 597 and 595 Ma, registering a fast transition between the typical syntectonic batholithic magmatism to the late, highly evolved and specialized magmas. These results indicate that the Ribeira belt was deformed by transtension in the Middle Ediacaran. The geological setting is consistent with a continental arc with the Três Córregos batholith emplacing at the middle-upper crust.     

Fractional crystallization and magma mixing: evidence from porphyritic diorite-granodiorite dykes and mafic microgranular enclaves within the Zhoukoudian pluton,Beijing

The Zhoukoudian pluton in the North China craton is a circular granodiorite intrusion containing porphyritic diorite dykes (PDDs), porphyritic granodiorite dykes (PGDs) and abundant mafic microgranular enclaves (MMEs), which provide an excellent opportunity to study fractional crystallization and magma mixing. The PDDs and PGDs are located in the western part of the pluton with the PDDs intruded by the PGDs. The dykes have similar mineral assemblages although plagioclase in the PDDs has higher anorthite content than the PGDs. Linear relationships between the SiO₂ and most major and trace element contents, as well as a positive trend of initial ⁸⁷Sr/⁸⁶Sr ratios and a negative trend of epsilon Nd values with increasing SiO₂ contents for the dykes suggest that both types were formed by assimilation and fractional crystallization of a common parental magma. Major oxide mass balance and trace element Rayleigh fractionation modeling points to early separation of garnet (11 %), clinopyroxene (27 %), orthopyroxene (16 %), plagioclase (25 %), biotite (19 %), and apatite (2 %) and late fractionation of hornblende (25 %), plagioclase (46 %), biotite (25 %), apatite (1 %), and magnetite (3 %). Most MMEs occur within the transitional granodiorite of the Zhoukoudian pluton. Zoned MMEs, dyke-like MME swarms, local presence of concave margins, veins and enclaves of host granodiorite within some MMEs, and several MMEs surrounded by the biotite-rich granodiorite support their formation by multiple magma mixing events, which finally resulted in different whole-rock major oxides and compatible elements, but homogeneous mineral major oxides (except zoned plagioclase), whole-rock incompatible elements and Sr-Nd isotopes between the MMEs and their host granodiorite. We suggest that multiple magma mixing events might also cause complexly zoned plagioclase in the Zhoukoudian pluton. Relative calcic, irregular or patchy cores and dusty zoned mantles from the zoned plagioclase crystals and their relatively low anorthite content indicate multiple mixing events between mafic/intermediate and felsic magmas. The mafic/intermediate end-members could be represented by the diabase dykes and the PDDs. Therefore, the dykes and MMEs in the Zhoukoudian pluton are genetically linked.     

U–Pb zircon geochronology of Silurian–Devonian granites in southeastern Australia: implications for the timing of the Benambran Orogeny and the I–S dichotomy

Despite extensive geochemical study and their importance to granite studies, the geochronology of Silurian to early-Devonian granitic rocks of southeastern Australia is poorly understood. In order to provide an improved temporal framework, new ion microprobe U–Pb zircon ages are presented from these rocks, and previous work is critically reviewed. Geochronological control is best in the Berridale Batholith, where S- and I-type granites have a close spatial relationship. In this region, there is a small volume of I-type granite that crystallised at 436 Ma, followed closely by a large volume of S-type granite at 432 Ma. I-type granite is abundant in a second peak at ca 417 Ma, although the Jindabyne pluton from the Kosciuszko Batholith is slightly older, at 424 Ma. A broader survey of S-type granite throughout the eastern Lachlan Orogen shows that the 432 Ma event is ubiquitous. There is no temporal overlap between S- and I-type granites in the Kosciuszko and Berridale Batholiths, which suggests that factors other than variations in degree of crustal contamination (which may include variation in tectonic setting, heat-flow, mass transfer across the crust–mantle boundary and/or availability in source materials) contribute to the diversity in granite types. The S-type granitic rocks occupy an aerial extent of greater than 28 000 km², and geochronological constraints suggest that the crystallisation of these granites took place over a relatively small interval, probably less than 10 m.y. This implies a magmatic flux of over 64 km³/Ma per km strike length, comparable to other high-flux granitic belts. Previous work has linked the Benambran Orogeny to the generation of the S-type granites, and so the age of these granites constrains the age of Benambran Orogenesis     

A Nd- and O-isotope study of the REE-rich peralkaline Strange Lake granite: implications for Mesoproterozoic A-type magmatism in the Core Zone (NE-Canada)

It is well established that A-type granites enriched in high field strength elements, such as Zr, Nb and the REE, form in anorogenic tectonic settings. The sources of these elements and the processes controlling their unusual enrichment, however, are still debated. They are addressed here using neodymium and oxygen isotope analyses of samples from the 1.24 Ga Strange Lake pluton in the Paleoproterozoic Core Zone of Québec-Labrador, an A-type granitic body characterized by hyper-enrichment in the REE, Zr, and Nb. Age-corrected εNd values for bulk rock samples and sodic amphiboles (mainly arfvedsonite) from the pluton range from ?0.6 to ?5.7, and ?0.3 to ?5.3, respectively. The εNd values for the Napeu Kainiut quartz monzonite, which hosts the pluton, range from ?4.8 to ?8.1. The ¹⁴⁷Sm/¹⁴⁴Nd ratios of the suite and the host quartz monzonite range from 0.0967 to 0.1659, large variations that can be explained by in situ fractionation of early LREE-minerals (Strange Lake), and late hydrothermal HREE remobilization. Oxygen isotope analyses of quartz of both Strange Lake and the host yielded δ¹⁸O values between +8.2 and +9.1, which are considerably higher than the mantle value of 5.7 ± 0.2‰. Bulk rock oxygen isotope analyses of biotite-gneisses in the vicinity of the Strange Lake pluton yielded δ¹⁸O values of 6.3, 8.6 and 9.6‰. The negative εNd values and positive δ¹⁸O values of the Strange Lake and Napeu Kainiut samples indicate that both magmas experienced considerable crustal contamination. The extent of this contamination was estimated, assuming that the contaminants were sedimentary-derived rocks from the underlying Archean Mistinibi (para-) gneiss complex, which is characterized by low εNd and high δ¹⁸O values. Mixing of 5–15% of a gneiss, having an εNd value of ?15 and a δ¹⁸O value of +11, with a moderately enriched mantle source (εNd = +0.9, δ¹⁸O = +6.3) would produce values similar to those obtained for the Strange Lake granites. Based on analogies between the Nain Plutonic Suite and the Gardar alkaline igneous province (SW-Greenland), we conclude that the Strange Lake pluton and associated REE-mineralized anorogenic bodies formed from a combination of subduction-induced fertilization of the sublithospheric mantle, crustal extension and in situ magma evolution.