Granodiorite Dykes Research Articles

The Early Neoproterozoic Andean-Type Orogenic and Within-Plate Magmatic Events in the Northern Margin of the Yangtze Craton during the Convergence of the Rodinia Supercontinent

Although considered a crucial component of the Rodinia supercontinent, it remains uncertain how the Yangtze craton relates to the accretion and breakup of Rodinia. Here, the Huanglingmiao granitic complex (HGC), an intermediate-acid rock series that intruded on the southern Kongling terrane of the northern Yangtze craton margin, is investigated to help resolve this conundrum. Our analysis indicates that these rocks consist of tonalite, trondhjemite, granodiorite, oligoporphyritic granodiorite, porphyric biotite granodiorite, and fine- to medium-grained granodiorite dyke compositions. Collectively, this assemblage is further subdivided into two categories by their temporal, spatial, and geochemical features into early TTG-like and later granitic–dioritic units, which are composed of tonalite, trondhjemite, granodiorite, porphyritic granodiorite, and the fine- to medium-grained granodiorite dykes, respectively. Zircon U-Pb dating yields ages of 865~850 Ma for the TTG-like rocks, 844~825 Ma for the porphyritic granodiorites, and ~800 Ma for the granodiorite dykes. Combined with geochemical evidence, the data suggest that the early- and late-series rocks were formed by a partial melting of Mesoproterozoic and Paleoproterozoic crustal materials, respectively, suggesting that the vertical layering of the crust controlled the composition of the independent units. In addition, isotopic evidence points to different sources for the various rocks in the Kongling terrane and that mantle-derived materials influenced the early-series lithologies. Combined with previous studies on the northern margin of the Yangtze craton, it is inferred that the early-series rocks formed in an active continental margin environment, while the late-series rocks display within-plate boundary formation characteristics. The multiple magmatic activities revealed by this study record sequential partial melting with tectonic transition characteristics from an Andean-type to within-plate magmatism in the northern margin of the Yangtze craton. Taken together, these observations point to a strong association between these rocks, convergence, and incorporation of the northern Yangtze craton margin into the Rodinia supercontinent during the Tonian Period.

Early to Middle Jurassic San Andrés-Cedros plutonic suite, western coast of Baja California, Mexico: Geochemical and isotopic evidence for an island arc extending to the central peninsula

In the Vizcaíno peninsula and Cedros Island region, west-central part of the Baja California peninsula, México, Upper Triassic ophiolitic sequences are intruded by arc-related Mid-Jurassic plutons. We analyze the ~150 Ma old San Roque pluton (SRp) in the Vizcaíno peninsula and plutonic rocks (~165 Ma) of the Punta Norte zone (PN) in the Cedros Island. The modal composition of the almost 10 km2 extending San Roque pluton varies from biotite-hornblende quartz diorite to biotite-hornblende tonalite intruded by granodioritic dykes. Intrusive rocks are subalkaline, calcic/calc-alkaline and metaluminous, akin to island arc granite setting. The primitive initial εNd values of +8.9 and + 9 of SRp quartz diorite and tonalite are indicative of a depleted mantle source without crustal assimilation. The sequence intrudes amphibolites that are interpreted as metavolcanic remnants of the oceanic-type crust of the Vizcaíno peninsula ophiolite. One amphibolite sample is characterized as magmatically poorly evolved, with flat REE patterns, and MORB-type initial εNd values of +10.1. The PN plutonic rocks of Cedros Island crop out at its northern edge. Their modal compositions are hornblende quartz diorite, hornblende leucotonalite, and microtonalitic dykes. These rocks are classified as subalkaline, magnesian, and calcic, typical island arc granites. The REE patterns tend to be subparallel, with progressive enrichments in REE abundances, suggesting fractional crystallization resulting from the evolution of a single magmatic system. Microtonalite dykes and leucotonalite yield initial εNd values of +7.8 to +8, indicating that the PN rocks were produced from magmas associated with a depleted mantle source. These values are in agreement with the correlation of εSr(165 Ma) vs. εNd(165 Ma), where all samples plot in the depleted mantle field. The PN plutonic rocks intruded pillow basalts with basaltic dykes. Their initial εNd and εSr plot in the depleted mantle field indicative of island arc tholeiites.The SRp and PN plutonic rocks are correlated with the coeval El Arco system (~160 Ma) in the central part of the Baja California peninsula. They show the same magmatic evolution in a primitive oceanic island arc environment derived from a depleted mantle with no crustal assimilation. We suggest that all the Jurassic sequences of the southern Peninsular Ranges batholith belong to the same arc.

Late Jurassic to Early Cretaceous age of the Daqiao gold deposit, West Qinling Orogen, China: implications for regional metallogeny

The West Qinling Orogen is endowed with more than 100 sediment-hosted gold deposits with an estimated resource of > 2000 t Au. Previous radiometric dating results have shown that most deposits formed during a Late Triassic to Early Jurassic period of contractional deformation over the orogen. However, here we show that the newly discovered Daqiao gold deposit (> 105 t at 3–4 g/t) in the southern belt of the West Qinling Orogen formed in latest Jurassic to Early Cretaceous under a different tectonic regime. The Daqiao gold deposit is hosted in weakly metamorphosed Triassic turbidites and is spatially associated with hydrothermally altered granodiorite and diorite porphyry dykes. Six granodiorite dykes have similar zircon U–Pb ages ranging from 215.0 ± 1.1 to 211.5 ± 1.5 Ma (1σ), whereas one diorite porphyry dyke has a zircon U–Pb age of 187.5 ± 2.1 Ma (1σ). The age of gold mineralization is constrained by two types of sericite: sericite aggregates coexisting with disseminated auriferous pyrite in relatively high-grade breccia ores and sericite coexisting with auriferous pyrite in weakly mineralized granodiorite dykes. Sericite aggregates from the breccia ores have 40Ar/39Ar plateau ages ranging from 150.7 ± 3.1 to 142.3 ± 2.5 Ma (2σ), whereas grains from the altered granodiorite dykes and low-grade breccia ores have 40Ar/39Ar plateau ages of 130.8 ± 3.1 to 127.2 ± 0.6 Ma (2σ). The 40Ar/39Ar ages thus suggest two periods of gold mineralization in the latest Jurassic and Early Jurassic that are likely related to repeated brecciation at Daqiao. These Jurassic-Cretaceous mineralization ages coincide with discounted ages from several other gold deposits in the region and suggest that there is an underappreciated gold event in the West Qinling Orogen that may not have been associated with the orogenic deformation but is genetically related to the far-field effects of plate reorganization during Paleo-Pacific subduction beneath the eastern Eurasian continent.

Zircon U-Pb ages and Hf isotope data from the Kukuluma Terrain of the Geita Greenstone Belt, Tanzania Craton: Implications for stratigraphy, crustal growth and timing of gold mineralization

The Geita Greenstone Belt is a late Archean greenstone belt located in the Tanzania Craton, trending approximately E-W and can be subdivided into three NW-SE trending terrains: the Kukuluma Terrain to the east, the Central Terrain in the middle and the Nyamullilima Terrain in the west. The Kukuluma Terrain, forms a NW-SE trending zone of complexly deformed sediments, intruded by the Kukuluma Intrusive Complex which, contains an early-syntectonic diorite-monzonite suite and a late-syntectonic granodiorite suite. Three gold deposits (Matandani, Kukuluma and Area 3W) are found along the contact between the Kukuluma Intrusive Complex and the sediments. A crystal tuff layer from the Kukuluma deposits returned an age of 2717 ± 12 Ma which can be used to constrain maximum sedimentation age in the area. Two granodiorite dykes from the same deposit and a small granodiorite intrusion found along a road cut yielded zircon ages of 2667 ± 17 Ma, 2661 ± 16 Ma and 2663 ± 11 Ma respectively. One mineralized granodiorite dyke from the Matandani deposit has an age of 2651 ± 14 Ma which can be used to constrain the maximum age of the gold mineralization in the area.The 2717 Ma crystal tuff has zircon grains with suprachondritic 176Hf/177Hf ratios (0.28108–0.28111 at 2717 Ma) and positive (+1.6 to +2.6) εHf values indicating derivation from juvenile mafic crust. Two of the granodiorite samples have suprachondritic 176Hf/177Hf ratios (avg. 0.28106 and 0.28107 at 2663 and 2651 Ma respectively) and nearly chondritic εHf values (avg. −0.5 and −0.3 respectively). The other two granodiorite samples have chondritic 176Hf/177Hf ratios (avg. 0.28104 and 0.28103 at 2667 and 2661 Ma respectively) and slightly negative εHf values (avg. −1.1 and −1.5 respectively). The new zircon age and isotope data suggest that the igneous activity in the Kukuluma Terrain involves a significant juvenile component and occurred within the 2720 to 2620 Ma period which, is the main period of crustal growth in the northern half of the Tanzania Craton.

Structural setting of Fimiston- and Oroya-style pyrite-telluride-gold lodes, Paringa South mine, Golden Mile, Kalgoorlie: 1. Shear zone systems, porphyry dykes and deposit-scale alteration zones

The Golden Mile in the 2.7 Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, has produced 385 million tonnes of ore at a head grade of 5.23 g/t gold (1893–2016). Gold-pyrite ore bodies (Fimiston Lodes) trace kilometre-scale shear zone systems centred on the D2 Golden Mile Fault, one of three northwest striking sinistral strike-slip faults segmenting upright D1 folds. The Fimiston shear zones formed as D2a Riedel systems in greenschist-facies (actinolite-albite) tholeiitic rocks, the 700-m-thick Golden Mile Dolerite (GMD) sill and the Paringa Basalt (PB), during left-lateral displacement of up to 12 km on the D2 master faults. Pre-mineralisation granodiorite dykes were emplaced into the D2 shear zones at 2674 ± 6 Ma, and syn-mineralisation diorite porphyries at 2663 ± 11 Ma. The widespread infiltration of hydrothermal fluid generated chlorite-calcite and muscovite-ankerite alteration in the Golden Mile, and paragonite-ankerite-chloritoid alteration southeast of the deposit. Fluid infiltration reactivated the D2 shear zones causing post-porphyry displacement of up to 30 m at principal Fimiston Lodes moving the southwest block down and southeast along lines pitching 20°SE. D3 reverse faulting at the southwest dipping GMD-PB contact of the D1 Kalgoorlie Anticline formed the 1.3-km-long Oroya Shoot during late gold-telluride mineralisation. Syn-mineralisation D3a reverse faulting alternated with periods of sinistral strike-slip (D2c) until ENE-WSW shortening prevailed and was accommodated by barren D3b thrusts. North-striking D4 strike-slip faults of up to 2 km dextral displacement crosscut the Fimiston Lodes and the barren thrusts, and control gold-pyrite quartz vein ore at Mt. Charlotte (2651 ± 9 Ma).

Geology, geochemistry, and genesis of the Dashui Carlin‐type gold deposit in the West Qinling orogenic belt, Gansu Province, China

The Dashui gold deposit occurs in the south of West Qinling orogenic belt where is belonged to a famous gold triangle area named “Shan‐Gan‐Chuan” in Northwest China. The presence of native Au associated with hematite alteration is unique in China. The ore bodies at Dashui mainly occur in the limestone and dolomitic limestone units within the Lower Triassic Maresongduo Formation. The granodioritic dykes that intrude the Triassic sedimentary rocks are locally mineralized because of later carbonate alteration. The ore bodies are vein, lenticular, cystic, and dendritic in shape and are controlled by NW‐, NNE‐ (near N‐S‐), and E‐W‐trending faults. Gold mineralization occurs commonly at fault intersections and contact zones between the carbonate and magmatic rocks. The NNE‐ (near N‐S‐) trending faults and associated pinnate fractures are most favorable loci for gold deposition. The metallic minerals include native Au, pyrite, cinnabar, and marcasite, which are mostly overprinted by an oxidized mineral assemblage of hematite and limonite. Gold mineralization is closely associated with decalcification, silicification, and carbonate alterations. Gold content in altered samples correlates with Si enrichment and Ca and Mg depletions. Rare earth elements (REE) increase with Al2O3 and Fe2O3 in altered carbonate rocks, indicating that an enrichment of REE may be closely related to argillic alteration and supergene weathering. The REE patterns of altered carbonate rocks are more likely consistent with granodiorite instead of the ones of fresh carbonate rocks. Moreover, δ34S values of Dashui deposit ranging from −1.8‰ to 4.1‰ indicate that the deep‐sourced magmatic hydrothermal fluids may have worked during the early stage mineralization. Altered carbonate rocks show higher δ13C and lower δ18O values than those of the original host rock. Therefore, the REE, S, C, and O data suggest that the ore‐forming fluids may be magmatic derived and later interact and exchange with host rock and meteoric water. The Dashui gold deposit is characterized by structural and stratigraphic controls; Au–As–Sb–Hg geochemical association; alteration of decalcification, silicification, and argillization; low base‐metals; high Au/Ag ratio; and low δ18O values. These features are consistent with Carlin‐type Au deposits in Western USA, suggesting that the Dashui deposit is a Carlin‐type deposit that formed in the Middle Jurassic to Early Cretaceous (170 to 135 Ma).

Mineral equilibria and zircon, garnet and titanite U-Pb ages constraining the PTt path of granite-related hydrothermal systems at the Big Bell gold deposit, Western Australia

The Big Bell deposit (75 t gold) is located in a narrow spur of the Meekatharra greenstone belt, Yilgarn Craton, Western Australia. Two ore bodies are located in a calcic-potassic contact alteration zone overprinting lineated granodiorite dykes and amphibolite: almandine-cummingtonite-hornblende skarn (1–3 g/t Au, 1700 g/t As, 330 g/t W) and the muscovite-microcline gneiss (3–5 g/t Au, 580 g/t Sb, 620 g/t W) of the Main Lode. Genetic models vary from pre- to post-metamorphic replacement. Hornblende-plagioclase pairs in amphibolite constrain peak metamorphic temperature to 670 ± 50 °C. In contrast, garnet-biotite thermometry provides estimates of 578 ± 50 and 608 ± 50 °C for garnet-cordierite-biotite schist bordering the skarn and enveloping the Main Lode. Garnet-cordierite and garnet-hornblende pairs extend the range of fluid temperature to 540 ± 65 °C, well below peak metamorphic temperature. At 540–600 °C, the alteration assemblage andalusite + sillimanite constrains pressure to 300–400 MPa corresponding to 11–14 km crustal depth. Published U-Pb ages indicate that metamorphism took place in the aureole of the southeast granodiorite-tonalite batholith (2740–2700 Ma), followed by gold mineralization at 2662 ± 5 Ma and by the emplacement of biotite granite and Sn–Ta–Nb granite-pegmatite dykes at 2625–2610 Ma. Amphibolite xenoliths in granite northwest of the deposit record the lowest temperature (628 ± 50 °C), suggesting it lacks a metamorphic aureole. The rare metal dykes are spatially associated with epidote-albite and andradite-diopside skarns (≤1.5 g/t Au), mined where enriched in the weathered zone. We analysed hydrothermal zircon intergrown with andradite. Concordant U-Pb ages of 2612 ± 7 and 2609 ± 10 Ma confirm the presence of a second granite-related system. The zircons display oscillatory zoning and have low Th/U ratios (0.05–0.08). Low-Th titanite from an albite granite dyke has a concordant but reset U-Pb age of 2577 ± 7 Ma.

Arc magmatism associated with steep subduction: Insights from trace element and Sr–Nd–Hf–B isotope systematics

AbstractSubduction zones are the major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting. However, the nature of slab fluids associated with steep subduction remains largely unknown. To clarify this issue, we present an integrated study for Late Paleozoic (318–312 Ma) intermediate dykes from the Beishan orogenic collage, NW China. The dykes consist mainly of dioritic and granodioritic rocks. The dioritic dykes exhibit typical subduction‐like geochemical signatures, together with relatively high Mg#, high εNd(t) and εHf(t), and low initial Sr isotopes, suggesting that they originated probably from a subduction‐modified mantle. The granodioritic dykes exhibit high Mg#, high Sr/Y, La/Yb, and Na2O/K2O ratios, low Y and Yb contents, and mid‐ocean ridge basalt‐like Sr–Nd isotopes and high zircon εHf(t), similar to slab‐derived adakite, indicating that they were likely formed by partial melting of subducted oceanic crust. The coeval adakitic and normal dioritic dykes reflect a thermal anomaly that was probably caused by rollback of subducted oceanic slab. The dioritic dykes have δ11B values from −7.7 to −6.4‰, whereas the adakitic dykes have relatively high δ11B values from −6.9 to −4.4‰. The δ11B values of adakitic dykes are lower than those of typical altered oceanic crust, in agreement with the expected loss of 11B from subducted oceanic slab during early subduction. Results of a mixing model suggest that the mantle source of the dioritic dykes has been hybridized by 11B‐depleted fluids expelled from a highly dehydrated slab at deep depth, owing to the high‐angle dip of the subducting oceanic slab.

550–490 Ma pre-to post-collisional shoshonitic rocks in the Ribeira Belt (SE Brazil) and their tectonic significance

This work presents a detailed study in a new occurrence of shoshonite rocks in the central Ribeira Belt, SE Brazil. It is the Tinguí Unit, a (hornblende) biotite orthogneiss with shoshonitic affinity, and undeformed dykes of similar composition. They crop out along the contact between Costeiro (part of Oriental Terrane, Ribeira Belt) and Cabo Frio Tectonic Domains. The Tinguí Unit is represented by mesocratic orthogneiss, ranging from quartz-diorite to granodiorite in composition. Plagioclase, biotite, quartz, hornblende, microcline, titanite, apatite, allanite, zircon, and opaque minerals are the main minerals. Tonalitic to granodioritic dykes, with similar composition, cross cut the Tinguí orthogneiss. Besides mineralogical similarities, analyzed samples from both the orthogneiss and the dykes have shoshonitic signature with crustal and mantle Nd-Sr isotope nature. They are basic to intermediate rocks with low Fe contents and high alkalis, Ba, Rb, Sr, Zr, and LREE values. High K2O/Na2O (up to 2.06) and LREE/HREE ratios along with others geochemical parameters reinforce the shoshonitic signature. Sm-Nd data point to εNd values of −13.2 to −4.38, and 1.7 to 1.2Ga TDM Nd model ages. Although the 143Nd/144Nd ratios indicate a predominance of lower continental crust heritage, the 87Sr/86Sr ratios ranging from ca. 0.705 to 0.707 suggest an enriched mantle contribution. Nevertheless, geochronological data from Tingui orthogneiss and tonalitic dykes reveal a difference of ca. 50m.y. in their crystallization ages: concordia ages of 551±3Ma, for the orthogneiss; 495±4Ma and 488±4Ma for the mesocratic and leucocratic tonalitic dykes, respectively. Comparing our new data and the tectonomagmatic events recorded in the Oriental Terrane, we propose a tectonic model emphasizing the role of this shoshonitic magma, interpreted as pre- and post-collisional intrusions regarding to the Buzios Orogeny, and possibly derived from the similar source. This magmatic activity is attributed to a long-term maintenance of high temperatures and low cooling rates in the Ribeira Belt, considered as a hot orogen. The 50m.y. crystallization age gap between the studied rocks could be assigned to two periods of melting of a similar or even same source at different tectonic stages of the orogen, pre- and post-collisional.

LA‐ICP‐MS U–Pb geochronology of zircons from granitoids and modern river sands in Guyang, North China Craton and their tectonic significance

The Palaeo‐Asian Ocean (PAO) was located between Siberia and the North China Craton (NCC). The time of its closure, as well as the crustal architecture of the surrounding region, are still uncertain. In this paper, we address these problems using LA‐ICP‐MS U‐Pb ages of zircon grains in three magmatic rocks and three river sand samples of the Guyang area at the northern margin of the NCC. The magmatic rocks yield early Carboniferous crystallization ages of 340.3 ± 7.5 Ma, 331.7 ± 2.3 Ma and 341.1 ± 1.9 Ma for the main granitic pluton, a granodiorite dyke and a diorite enclave, respectively. They were the product of subduction of the PAO underneath the NCC. The detrital zircon U‐Pb ages in the river sand samples are dominated by two groups at 2550–2400 and 290–260 Ma, with subordinate groups at 2700–2600, 2000–1800 and 320–300 Ma, dispersed Early Palaeoproterozoic grains and a minor but important group of Cretaceous grains in sands of the Aibugai River. These results, combined with previous geochronological data, permit a re‐assessment of the tectonic evolution of the northern margin of the NCC. The events at ~2.5 Ga reflect major crustal reworking and cratonization of the northern margin of the NCC, which then became a stable platform lasting until the Carboniferous. The period between 2.0 and 1.8 Ga was characterized by major collisions related to formation of the Khondalite Belt and the Trans‐North China Orogen. In the Carboniferous, subduction of the PAO underneath the northern margin of the NCC developed a zoned distribution of magmatic rocks. The PAO is now constrained to have closed in the Early Permian by the collision of the NCC and Siberia. The youngest group of Cretaceous zircons reflects magmatic events related to lithosphere thinning beneath the Eastern Block of the NCC. Copyright © 2016 John Wiley & Sons, Ltd.

Paragenesis and fluid evolution of the Halasu III porphyry Cu deposit, East Junggar (NW China): Implications for the Paleozoic multiphase superimposing mineralization in the Central Asian Orogenic Belt

The Halasu III deposit, an important Cu deposit in the southernmost part of the Halasu porphyry Cu belt, is situated in the East Junggar block in NW China. Copper and Mo sulfide mineralization commonly occurs as disseminations or veinlets, and is mainly associated with Devonian granodiorite porphyry. Our detailed field geological investigation identified late-stage high grade Cu sulfide-bearing veining superimposed on the earlier porphyry Cu–Mo mineralization.Four hydrothermal alteration and mineralization stages were identified, namely the early epidote alteration (Stage I), porphyry-style alterations (Stage II comprising II-A potassic, II-B propylitic and II-C phyllic alteration sub-stages), late veining (Stage III) and supergene processes (Stage IV) in the northern- and southern mineralization zones at Halasu III. Multiphase granodiorite- and alkali granite dykes intruded the Beitashan Formation volcanic rocks, generating the early epidote alteration. Potassic alteration (Stage II-A) occurred in and around the NNW-treading granodiorite dykes, while the propylitic alteration (Stage II-B) occurred in the intrusions or volcanic wall rocks around the potassic zone. These early alteration assemblages were overprinted by the subsequent, structurally-controlled phyllic alteration (Stage II-C) confined within the granodiorite porphyry. Late veins (Stage III) include sulfide-bearing (Stage III-A) and sulfide-barren (Stage III-B) ones. Supergene process (Stage IV), represented by hematite and jarosite replacing primary sulfides, extends tens of meters down depth and is commonly fracture-controlled.Three fluid inclusion (FI) types were recognized, namely Type A (single-phase), B (liquid–vapor two-phase) and C (liquid–vapor–solid three-phase). For Type B FIs, those of potassic alteration homogenized at 183–347°C, with corresponding salinities of 6.2–17.8wt.% NaCl equiv. The propylitic and phyllic FIs homogenized at 172–425°C and 231–352°C, respectively, with the corresponding salinities of 0.9–14.0wt.% NaCl equiv. and 0.4–10.5wt.% NaCl equiv. The sulfide-bearing vein FIs homogenized at 243–410°C (Stage III-A-1), 260–362°C (Stage III-A-2) and 117–235°C (Stage III-A-3), along with the corresponding salinities of 1.9–9.2wt.% NaCl equiv., 1.1–6.7wt.% NaCl equiv. and 6.5–17.1wt.% NaCl equiv., respectively. The sulfide-barren vein FIs yielded homogenization temperatures of 260–380°C (Stage III-B-1) and 114–297°C (Stage III-B-2), with corresponding salinities of 1.7–9.8wt.% NaCl equiv. and 0.2–13.6wt.% NaCl equiv., respectively. We suggest that hydrothermal veins at Halasu III may have undergone complex and superimposing alteration due to the episodic reopening of the preexisting veins, with Stage III-A-3 veining overprinting the phyllic alteration and Stage III-A-2 veining. Stage III-B-2 veining superimposed on the potassic alteration, as well as on Stages III-A-1 and III-B-1 veining.Paragenetic sequence and fluid evolution at Halasu III indicate that these complex hydrothermal alteration stages were formed in accordance with the regional tectonic evolution. These various alteration and mineralization stages at Halasu III deposit reveal a prolonged history of magmatic and hydrothermal processes extending episodically from the Middle Devonian to Middle Permian, wherein early mineralization was overprinted by late hydrothermal events. Such superimposed mineralization may occur commonly in the Paleozoic Central Asia Orogenic Belt.

Late Cretaceous evolution of the northern Sistan suture zone, eastern Iran: Implications of magnetic fabrics and microstructures in the Bibi Maryam granitoid

Anisotropy of magnetic susceptibility (AMS) survey supported by field and microstructural studies have been applied on the Late Cretaceous Bibi Maryam granitoid (BMG) in the northern Sistan suture zone (SSZ), east of Iran. The BMG is composed of quartzdiorite-tonalite with late granodiorite dykes and stocks that are surrounded by steeply SW-dipping Neh shear zone (NSZ). The magmatic fabrics are characterized by transpressional environment dominated by steep dipping foliations (mean strike: N13°W) and sub-horizontal stretching lineations (mean trend: 167°). Based on microstructural studies, it is inferred that these fabrics are related to emplacement and cooling of the pluton and the internal fabrics revealed are evidence of a deformation continuum in the granitoid from magmatic to solid state. Deformation in the region continued even after the BMG had fully crystallized, which led to development of the NW–SE foliations and lineation trend that these fabrics are subparallel with the NSZ. The BMG has emplaced in a transpressional setting that was controlled by a NW–SE stretching direction and supported the model that has proposed the relationship between granitoid emplacement and oblique intra-oceanic subduction of the Neotethys during the Late Cretaceous already recognized in this part of the SSZ.

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.

Bandas de cataclastitas en el Plutón Tambillo: Manifestaciones de la orogénesis oclóyica en el cordón de Lila, norte de Chile

Cataclastic bands from the eastern border of Tambillo Pluton are studied. Two band systems are observed: one northeast and the other northwest in strike. The northeast system is the most conspicuous and its kinematic analysis shows dextral displacement. The two directions of bands form right angle and this suggest that they formed from joints in the Tambillo Puton. A number of granodioritic dykes intruded along the bands and thus they constitute weak zones for dyke emplacement. The age of the cataclasis is undoubtely Ordovician and occurred during the Arenigian in a maximum span of time of only 11 m.y. It is assigned to the Ocloyic orogenesis.

U–Pb dating, Hf-isotope characteristics and trace-REE-patterns of zircons from Medet porphyry copper deposit, Bulgaria: implications for timing, duration and sources of ore-bearing magmatism

Precise U–Pb geochronology, Hf isotope compositions and trace element distributions in zircons are combined in the present study to define the timing and sources of the magmatism forming the Medet porphyry copper deposit, Bulgaria. ID-TIMS U–Pb-zircon dating demonstrates that ore-bearing magmatism extended for less than 1.12 Ma. As inferred from the field relationships, it started with the intrusion of a quartz-monzodiorite at 90.59 ± 0.29 Ma followed by granodiorite porphyries at 90.47 ± 0.30 and 90.27 ± 0.60 Ma and by crosscutting aplite dykes at 90.12 ± 0.36 Ma. These units were overprinted by potassic alteration and host economic copper-(Mo–Au) mineralization. The main magmatic–hydrothermal activity ceased after that, and a later quartz-granodiorite porphyry dyke, dated at 89.26 ± 0.32 Ma, only contains an uneconomic quartz–pyrite mineralization. Assimilation of Lower Paleozoic rocks with a mantle to mantle–crust signature is characteristic of the fertile magma in the Medet deposit, as defined by positive ɛ-Hf values of the inherited zircons. The positive Ce-anomalies and the higher Eu/Eu* ratios of the zircons in the mineralized Cretaceous rocks of Medet deposit argue for crystallization from a generally more oxidized magma compared to the later quartz-granodiorite porphyry dyke. A change in paleostress conditions occurred during the intrusion of the Medet pluton and its dykes. The initial stage reveals E–W extension associated with N–S compression, whereas the younger granodiorite dyke was emplaced during subsequent N–S extension. The large-scale switch of the extensional stress regime during the mineralization was favourable for ore deposition by channelling the fluids and increasing the effective permeability.

Palaeomagnetic study of the Cairnsmoor of Fleet Granite and Criffel-Dalbeattie granodiorite contact aureoles: Caledonian tectonics of the Southern Uplands of Scotland and Devonian palaeogeography

The plutons of Cairnsmoor of Fleet (392±2 Ma) and Criffel-Dalbeattie (397±2 Ma, both mineral isochron ages) comprise two of four major post-tectonic granitic complexes emplaced into the Southern Uplands, an Ordovician–Silurian back-arc and foreland basin complex formed at the northern margin of the Iapetus Suture. To expand the palaeomagnetic record of the Southern Uplands we have studied palaeomagnetism and magnetic fabrics in traverses spanning contacts of these intrusions with host mudrocks. A uniform anisotropy of magnetic susceptibility (AMS) fabric across the Cairnsmoor of Fleet contact has been enhanced by recrystallization into hornfels near the contact and records a late Acadian regional stress operative during, or soon after, emplacement of the pluton in Middle Devonian times. Magnetization during slow cooling recorded a dual polarity (‘A’) remanence in granite and hornfels with mean direction D/I = 92/−2° (α95 = 6.5°) yielding a palaeopole (Q = 6) at 2°N, 265°E linked to cooling at c. 392 Ma. Subsidiary magnetizations are overprints imparted during Variscan tectonism (‘B’, D/I = 194/6°) and Jurassic rifting within the adjoining Irish Sea Basin (‘C’, c. 160–140 Ma, D/I = 172/−52°). The Criffel-Dalbeattie pluton has more complex AMS fabrics recording both deformation and emplacement effects. Hematite of secondary hydrothermal origin is a significant feature of the rock magnetic record in the aureole, which is otherwise dominated by paramagnetism. The granodiorite is more strongly magnetized than the country rocks, accounting for a positive aeromagnetic anomaly. A fairly dispersed dual polarity remanence (mean D/I = 115/55°, α95 = 18°) in granodiorite and late tectonic porphyrite dykes is probably the oldest magnetization preserved in this pluton because it correlates with an excursion of Britain into southerly palaeolatitudes at c. 410 Ma and indicates an Early Devonian emplacement age. The palaeofield at c. 397 Ma, the currently accepted isotopic age, is recorded by a minority overprinted remanence (mean D/I = 272/2°, α95 = 12°) similar to the record in the Cairnsmoor of Fleet pluton and granites from the adjoining Lake District terrane. Granite complexes of the Southern Uplands Block collectively record regional rotation and excursion of Britain into southerly latitudes between c. 410 and 390 Ma. Comparable Silurian–Devonian palaeomagnetic poles identify common apparent polar wander (APW) in paratectonic and orthotectonic terranes from the Variscan Front in the south to the Laurentian foreland in the north following climactic Acadian deformation. APW between 430 and 390 Ma embracing the (post-closure) history of the Caledonian orogen is a loop executed at rates much higher than typical rates of plate motion and appears to record a component of true polar wander. The ∼110° arc length is identical to polar shift identified between mid-Silurian and Lower–Middle Devonian poles from Gondwana. The two paths superimpose to show that the western margin of Gondwana was in proximity to the SE margin of Laurentia during Acadian deformation in Early–Middle Devonian times and remote from the Caledonides; the residual Rheic Ocean subsequently closed by a combination of pivotal and left lateral strike-slip motions.

Convergent margin tectonics, central Wabigoon subprovince, Superior Province, Canada

Best known for its Neoarchean greenstone-granite terranes in the east and west, the Wabigoon subprovince of the Superior Province features a central transverse corridor of Meso- and Neoarchean gneissic, granitoid, and minor supracrustal rocks resembling an eroded intermontane plateau. Structural analysis and U–Pb geochronology in the north-central Wabigoon subprovince reveal a complex history of plutonism, supracrustal rock deposition, polyphase deformation and amphibolite-facies metamorphism between 3075 and 2670 Ma. The geometric and kinematic character of D 1 and D 2 deformational events recorded in 2890–2718 Ma tonalitic gneisses remain cryptic as a result of magmatic and structural (D 3–5) overprinting, but the early structures are bracketed in age between 2725 and 2713 Ma by a polydeformed gneiss and tonalite dyke. These events occurred deep within an active continental magmatic arc and correspond in age to a collision between the juvenile Wabigoon and continental Winnipeg River subprovinces recognized to the west. Clastic sediments deposited after 2701 Ma are the youngest rocks involved in subsequent regional deformation. A D 3 event is responsible for rapid burial of supracrustal units to 15 km within warm arc crust, and imposition of a regional, penetrative, S 3 foliation that accommodated some relative uplift prior to 2697 Ma, the age of a discordant granodiorite dyke. This event corresponds to variably preserved, steep, north-trending D 1 structures recognized in adjacent greenstone belts. Subsequent (D 4) ductile deformation defines the map pattern in the deep crustal rocks of the central Wabigoon subprovince. Upright, east-trending F 4 folds developed synchronously with D 2 dextral transpressive structures that formed throughout the Wabigoon subprovince prior to 2698 Ma. The final (D 5) sinistral shearing event occurred during emplacement of mantle-derived granodiorites of sanukitoid affinity (2688 Ma) and crust-derived granites (2685 Ma). This event appears to have triggered rapid cooling, reflected in 2680–2670 Ma titanite dates and late brittle increments of displacement.

Imprint of different fluid generations on granulitic gneisses from central Dronning Maud Land, Antarctica

Gneisses of Grenvillian age from Storsata and Svarthornkammen, Petermann Range, central Dronning Maud Land, East Antarctica, show a fluid imprint, characteristic of arrested charnockitization. Charnockite formation took place at Svarthornkammen, where fluid composition is dominantly pure CO2. At Storsata, P–T conditions of the amphibolite–granulite transition facies during Pan-African metamorphism were insufficient to produce orthopyroxene-bearing charnockite s. str. Typical high-density CO2–N2 fluid inclusions (0.62–0.86g/cm3) were found in garnet and quartz. At retrograde upper greenschist facies conditions a later generation of aqueous, low-salinity fluids (3.0±1.9mol% NaCleq) were entrapped. Finally, high-salinity fluids originated in connection with the late-tectonic emplacement of granitic to granodioritic dykes. These fluids leached the formerly charnockitizized gneisses which resulted in irregularly bleached rock areas.

Geology and U-Pb geochronology of rocks of the Eokuk Uplift: a pre-2.8 Ga basement inlier in the northwestern Slave Province, Nunavut, Canada

Rocks of the Eokuk Uplift have been mapped in detail along the coast of Coronation Gulf and 10 key units have been dated by U-Pb analysis of zircon, monazite, and titanite. The combined data indicate that this inlier of the Slave Province has a >3.2 Ga crustal component, evidence of a granulite-grade orogenic event predating 2.8 Ga and a lack of evidence for any significant orogenic activity corresponding to the 2.7-2.6 Ga events common in the rest of the Slave Province. The oldest rocks in the study area are a succession of granitoid and supracrustal gneisses that have been metamorphosed to amphibolite to granulite facies. From field relationships, the oldest rock is a granodiorite to tonalite orthogneiss, with a zircon crystallization age of 3254+13-6 Ma. A granite gneiss, which may be a small felsic intrusion or an anatectic melt of the tonalite gneiss, yields a zircon age of 3216+14-13 Ma. A K-feldspar megacrystic monzogranite gneiss contains old, discordant, possibly inherited zircons with 207Pb/206Pb ages ranging from 3103 to 3039 Ma, together with coexisting 2879 ± 3 Ma zircon and monazite. These high-grade gneisses are intruded by two megacrystic granite plutons, dated at 2887 ± 2 and 2881+4-3 Ma. The absence of extensive recrystallization and complex structures in these plutons indicates that this igneous event postdated the high-grade metamorphism. An amphibolite-grade synplutonic metamorphic event is dated at ~2880 Ma by new monazite in the older gneiss units. A series of variably deformed mafic to felsic dykes and pegmatites intrude both the granites and gneisses and constrain the end of penetrative deformation in the area. Of these, a boudinaged diorite dyke, with a strong internal foliation parallel to the regional fabric, is dated at 2877 ± 3 Ma. A younger granodiorite dyke that crosscuts the regional fabric at a high angle and has only a weak internal foliation yields an age of 2864+3-9 Ma. An undeformed syenogranite pegmatite, which represents a suite that intrudes all other units in the study area, has a combined zircon-monazite age of 2852 ± 3 Ma. The varying degrees of deformation in these minor intrusive rocks constrains the end of deformation in the study area to ca. 2850 Ma. This contrasts with data from the rest of the Slave Province, where the main phases of deformation, metamorphism, and synmetamorphic plutonism have been dated at ca. 2.62-2.59 Ga. Metamorphic titanite ages from the diorite and granodiorite dykes indicate two lower amphibolite to greenschist facies metamorphic events: one at ca. 2705 Ma and one at ca. 2646 Ma. The youngest Archean magmatic event in the area is represented by granite intrusions at 2594+3-2 Ma, coeval with crystallization of titanite at greenschist-grade conditions in some of the older gneissic and intrusive rocks.

Litho-structural layering within the Archean Lac du Bonnet Batholith, at AECL’s Underground Research Laboratory, Southeastern Manitoba

The Lac du Bonnet Batholith (LDBB) is one of many late to post-tectonic granites in the western Superior Province which appear to be compositionally homogeneous and structurally simple on the surface. However, mapping in the 443 m deep access shaft at the Underground Research Laboratory (URL), borehole logs, and detailed surface mapping clearly demonstrate that decametre-scale layering is widespread in the LDBB. Individual layers or litho-structural domains are distinguished by the type and abundance of xenoliths and late residual or metasomatic segregations, by auto-intrusive contacts, and by variations in style and orientation of mesoscopic fabric. The litho-structural domains are inconspicuous at surface due to their low dip and the flat topography; however, detailed mapping in the central part of the URL has identified their surface expression as a series of broad and open domes and intervening saddles. The litho-structural domains host three systems of auto-intrusive dykes, sills, and recrystallized zones whose abundance and mode of occurrence change with depth. Late magmatic granodioritic dykes are the predominant rock type below 300 m depth at the URL, but the swarm narrows rapidly up-dip, and at surface is represented only by narrow zones of alteration and ductile deformation. Late pegmatite–aplite dykes are pervasive across all rock types at surface, but are limited to the larger granodiorite dykes at 420 m depth at the URL. The distribution of inclusions, alteration and fractures in the LDBB suggest the present topographic surface is close to the original roof zone of the batholith.