Megacrystic Granite Research Articles

Early Neoproterozoic tectonics in the Marwar Crustal Block, NW India, the relevance of the Phulad Shear Zone, and implications for Rodinia reconstruction

Abstract Palaeomagnetic studies in the Malani Igneous Suite (ca. 770–750 Ma) of the Marwar Crustal Block, NW India, provide essential constraints on palaeogeographic reconstructions of the Rodinia Supercontinent. The Malani Igneous Suite is intrusive into megacrystic granite and granitegneissic enclaves of the Marwar Crustal Block. A crustalscale NE–SW ductile transpressional shear zone with a southeasterly dip known as the Phulad Shear Zone (820–810 Ma) separates this Marwar Crustal Block from the rocks farther east. The preshearing tectonic evolution of the Marwar Crustal Block is poorly understood. Three phases of ductile deformations (D1, D2, and D3, with associated fabrics S1, S2, and S3) were identified in the Marwar Crustal Block. The D1 deformation is restricted to enclave gneisses. The megacrystic granite was emplaced syntectonically during D2 deformation. The S2 magmatic foliation (NNW–SSE) in the megacrystic granite is oblique to the Phulad Shear Zone. The D3 deformation in the megacrystic granite is synchronous with the Phulad Shear Zone mylonites. Another porphyritic granite (Phulad granite, ca. 820 Ma) emplaced syntectonically during D3 deformation along and across the Phulad Shear Zone. U-Pb zircon dates in the Marwar Crustal Block document ca. 890 Ma and ca. 860 Ma magmatic dates. U-Pb-Th monazite dates in the Marwar Crustal Block show a strong peak at ca. 820 Ma. By integrating critical field relations, deformational patterns, and geochronology, we argue that the Marwar Crustal Block shows distinct geological evolution prior to its accretion with the remaining parts of India. We propose that the accretion of the Marwar Crustal Block must be younger than ca. 860 Ma and culminated during ca. 820–810 Ma to form the Greater India landmass along the Phulad Shear Zone.

Tectonics, geochronology, and petrology of the Walker Top Granite, Appalachian Inner Piedmont, North Carolina (USA): Implications for Acadian and Neoacadian orogenesis

Abstract The Walker Top Granite (here formally named) is a peraluminous megacrystic granite that occurs in the Cat Square terrane, Inner Piedmont, part of the southern Appalachian Acadian-Neoacadian deformational and metamorphic core. The granite occurs as disconnected concordant to semi-concordant plutons in migmatitic, sillimanite zone rocks of the Brindle Creek thrust sheet. Locally garnet-bearing, the Walker Top Granite contains blocky alkali feldspar megacrysts 1–10 cm long in a groundmass of muscovite-biotite-quartz-plagioclase-alkali feldspar and accessory to trace zircon, titanite, epidote, sillimanite (xenocrysts), and apatite. It varies from granite to granodiorite and contains several xenoliths of biotite gneiss, amphibolite, quartzite, and in one location encloses charnockite (here formally named Vale Charnockite). New sensitive high-resolution ion microprobe U-Pb zircon magmatic crystallization ages obtained from the plutons of the Walker Top Granite are: 407 ± 1 Ma in the Brushy Mountains; 366 ± 2 Ma in the South Mountains; and 358 ± 5 Ma in the Vale–Cat Square area. An age of 366 ± 3 Ma was obtained from the Vale Charnockite at its type locality. Major-, trace-element, and isotopic chemistry indicates that Walker Top is a high-K, peraluminous granite, plotting as volcanic arc or syn-collisional on tectonic discrimination diagrams and suggests that it represents deep-seated anatectic magma with S- to I-type affinity. The alkali calcic, ferroan Vale Charnockite likely formed by deep crustal melting, and similar geochemical and trace-element compositions suggest a similar tectonic origin as Walker Top Granite. The discontinuous nature of the Walker Top Granite plutons precludes it intruded as a volcanic arc. Instead, the peraluminous nature, common xenoliths of surrounding country rock, and geochemical and isotopic signatures suggest it formed by partial melting of Cat Square and Tugaloo terrane rocks. Following emplacement and crystallization, Walker Top plutons were deformed into elliptical to linear shapes—SW-directed sheath folds—enveloped by partially melted, pelitic and quart-zofeldspathic rocks. Collectively, Walker Top and other plutons helped weaken the crust and facilitate lateral crustal flow in a SW-directed, tectonically driven orogenic channel during the Acadian-Neoacadian event. A comparison with the northern Appalachians recognizes a similar temporal magmatic and deformational history during the Acadian and Neoacadian orogenies, although while the Walker Top Granite intruded the lower plate during eastward subduction beneath the peri-Gondwanan Carolina superterrane, the northern Appalachian plutons intruded the upper plate during subduction of the Avalon superterrane westward beneath Laurentia. We hypothesize that a transform fault, located near the southern end of the New York promontory, accommodated oppositely directed lateral plate motion and different subduction polarity between the Carolina and Avalon superterranes during the Acadian and Neoacadian orogenies.

The upper Oligocene San Rafael intrusive complex (Eastern Cordillera, southeast Peru), host of the largest-known high-grade tin deposit

The upper Oligocene San Rafael intrusive complex (SRIC), located at the southwest margin of the Eastern Cordillera of southeast Peru, is an exceptional case study to investigate the petrogenesis of a large composite granitic pluton associated with the largest known high-grade Sn deposit (>1 Mt. Sn). The granitic rocks are quasi-continuously exposed over more than 1.4 km vertically, from the cupola at surface to deeper levels in the underground mine workings, thus providing access to the upper part of a large pluton. We describe new field relationships and petrographic observations of the main granite varieties of the SRIC, and we report quantitative mineral analyses, whole rock geochemistry (major and trace elements, radiogenic Sr, Nd, and Pb isotopes), and in situ UPb dating and trace element analysis of zircons. The SRIC is dominantly composed of a biotite-cordierite -bearing K-feldspar megacrystic granite, which contains comagmatic enclaves and dismembered dikes of fine-grained and medium-grained granites, and subordinate lamprophyres. While the granitic rocks are widely affected by variable degrees of pervasive hydrothermal alteration, the least altered samples are characterized by moderately fractionated peraluminous S-type compositions, with similar enrichments in incompatible elements (Li, B, Ga, Rb, Sn, Cs, Th, and U), relatively high Zr/Hf and Nb/Ta ratios, and strongly crustal Sr, Nd, and Pb isotopic signatures. In situ UPb dating and trace element analysis of zircons indicate that the granitic magmas crystallized contemporaneously within age uncertainty at 24.02 ± 0.25 Ma (megacrystic granite), 24.17 ± 0.28 Ma (fine-grained granite), and 24.26 ± 0.26 Ma (medium-grained granite) at Ti-in-zircon temperatures from 638° to 840 °C (avg = 759 ± 49 °C). The lamprophyres have high contents of lithophile elements (Li, Rb, Cs, Sn, W) while showing more primitive SrNd isotopic compositions, indicating a metasomatized subcontinental lithospheric mantle source contaminated by crustal assimilation and/or mixing with granitic magmas. Tin contents of the main San Rafael granites are moderate (3–40 ppm; avg. = 13 ppm Sn) and fall in the same range of values as reported for “tin granites” from the Central Andean tin belt. Based on our petrological and geochemical data, we conclude that the SRIC results from multiple injection and mingling/mixing of several types of coexisting felsic and mafic magmas in a shallow-crustal reservoir. The granitic magmas were largely derived from partial melting of metasedimentary protoliths and hybridization with mantle-derived mafic melts, in a regional geodynamic setting dominated by anomalous asthenospheric heat flow beneath the southern Peruvian Altiplano during the Oligocene to Miocene. Based on the moderately fractionated peraluminous character of the SRIC, we conclude that the granitic pluton was a passive host for Sn mineralization and only provided the structural focusing for metal-rich magmatic fluids derived from a deeper part of the granitic magma reservoir. We propose a model in which the injection of high-temperature mafic melts into a more reduced, fractionated granitic magma reservoir triggered late-magmatic oxidation, crystal mush reactivation, and exsolution (second boiling) of metal-rich magmatic fluids. The latter ascended toward the upper part of the crystallized granitic pluton and were focused along subvertical dilational structures, where they cooled down and mixed with downwelling meteoric waters, thus generating the world-class San Rafael Sn (Cu) deposit.

U-Pb zircon geochronology of the Dete-Kamativi Inlier, NW Zimbabwe, with implications for the western margin of the Archaean Zimbabwe Craton

We report new in-situ U-Pb zircon ages which were obtained from a variety of lithologies within the Dete-Kamativi Inlier of western Zimbabwe using both LA-ICP-MS and SIMS. These include igneous or metamorphic zircon grains from granitoids and granitic gneisses as well as detrital zircon grains from a number of paragneisses. The oldest ages are from two highly deformed migmatitic granitoid orthogneisses, which yielded Neoarchaean concordia ages of 2723 Ma and 2694 Ma, which are interpreted to be crystallisation ages. These are the first Archaean ages from the Dete-Kamativi Inlier, indicating that the Archaean Zimbabwe Craton extends much farther west than previously recognised. Our new maximum ages of deposition of 2.31, 2.0 and 2.07 Ga for the Malaputese, Inyantue and Tshontanda Formations, respectively, match closely with previously determined maximum ages of 2.29, 2.07 and 2.16 Ga for the Deweras, Lomagundi and Piriwiri Groups, thus supporting the lithostratigraphic correlation of the Dete-Kamativi Inlier with the Magondi Supergroup. Foliated and unfoliated K-feldspar megacrystic granites and granodiorites have yielded concordia ages ranging from 2.08 to 2.01 Ga. They contain abundant Archaean inherited zircon grains ranging in age from 3.34 to 2.66 Ga, implying the involvement of Archaean crust in their petrogenesis. These results suggest the formation of a calc-alkaline continental magmatic arc on the western edge of the Archaean Zimbabwe Craton, some 80 Ma prior to the onset of the Palaeoproterozoic Magondi Orogeny.

Emplacement of the giant Kunene AMCG complex into a contractional ductile shear zone and implications for the Mesoproterozoic tectonic evolution of SW Angola

Massif-type anorthosites are voluminous, plagioclase-dominated batholiths that occur worldwide, but only during the Proterozoic Eon. Granitoids are closely associated with the anorthosites, and they collectively may form an AMCG suite (i.e., anorthosite, mangerite, charnockite, granite). Many fundamental questions regarding the origin of the AMCG complexes remain unresolved, including their tectonic setting and mode of emplacement. Here, we present new data from reconnaissance structural mapping, U–Pb LA-ICP-MS zircon dating, and 40Ar/39Ar amphibole and mica dating from a 500 km2 area straddling the western margin of one of the world’s largest (≥ 15,000 km2), but also least studied AMCG suites, the Kunene Complex (KC) in Angola. The oldest recognized (pre-Kunene) deformation event D1 produced a steep WNW–ESE-striking S1 fabric in supracrustal rocks, migmatite and Paleoproterozoic (1800–1780 Ma) granodiorite before ~ 1680 Ma. This was folded by the D2 event, resulting in steep, N–S-striking S2 leucosomes and gneissosity in the rocks surrounding the KC. Concordant magmatic layering and lamination, and high- to low-temperature gneissic foliation in the marginal Kunene anorthosite and coeval megacrystic granite are parallel with S2 fabrics in the Paleoproterozoic country rocks. Mineral stretching lineations L2 in all rock types across intrusive contacts are steep and associated with west-side-up (thrust) shearing. Overlapping U–Pb zircon/baddeleyite ages from the Kunene megacrystic granite and Kunene anorthosite, with 40Ar/39Ar mica ages from the mantling Paleoproterozoic country rocks, attest to progressive contraction, thrusting and exhumation of the Paleoproterozoic country rocks onto the Kunene anorthosite in the interval 1410–1380 Ma. We suggest that thermal softening of the country rocks caused by heat dissipation from the earliest phase (1500–1410 Ma) of ponding of hot anorthositic magma in the deep crust localized D2 deformation at the margin of the KC. Our data point to a tectonic model whereby anorthosite sheets were emplaced at a high angle to the shortening direction and along host-rock S2 anisotropy deep in the crust and cooled during progressive ductile thrusting and exhumation of the country rocks. These results question the current view that Proterozoic AMCG suites are emplaced in extensional environments as large diapiric bodies and favor formation in a continental arc setting.

Geology of the Acropolis prospect, South Australia, constrained by high-precision CA-TIMS ages

Acropolis is an Fe-oxide–copper–gold prospect ∼20 km from Olympic Dam, South Australia, and marked by near-coincident gravity and magnetic anomalies. Prospective Fe-oxide–apatite ± sulfide veins occur in Mesoproterozoic and Paleoproterozoic volcanic and granitoid host units beneath unmineralised sedimentary formations. We have produced a geological map and history of the prospect using data from 16 diamond drill holes, including LA-ICPMS and high-precision CA-TIMS ages. The oldest unit is megacrystic granite of the Donington Suite (ca 1850 Ma). A non-conformity spanning ca 250 My separates the Donington Suite and felsic lavas and ignimbrites of the Gawler Range Volcanics (GRV; 1594.03 ± 0.68 Ma). The GRV were intruded by granite of the Hiltaba Suite (1594.88 ± 0.50 Ma) and felsic dykes (1593.88 ± 0.56 Ma; same age as the Roxby Downs Granite at Olympic Dam). The felsic dykes are weakly altered and lack Fe-oxide–apatite–sulfide veins, suggesting that they post-date the main hydrothermal event. If correct, this relationship implies that the main hydrothermal event at Acropolis was ca 1594 Ma and pre-dated the main hydrothermal event at Olympic Dam. The GRV at Acropolis are the same age as the GRV at Olympic Dam and ca 3–7 My older than the GRV exposed in the Gawler Ranges. The gravity and magnetic anomalies coincide with sections through the GRV, Hiltaba Suite and Donington Suite that contain abundant, wide, Fe-oxide veins. The GRV, Hiltaba Suite and Donington Suite are unconformably overlain by the Mesoproterozoic Pandurra Formation or Neoproterozoic Stuart Shelf sedimentary formations. The Pandurra Formation shows marked lateral variations in thickness related to paleotopography on the underlying units and post-Pandurra Formation pre-Neoproterozoic faults. The Stuart Shelf sedimentary formations have uniform thicknesses.KEY POINTSFe-oxide–apatite ± sulfide veins are hosted by the Gawler Range Volcanics (1594.03 ± 0.68 Ma), the Hiltaba Suite granite (1594.88 ± 0.50 Ma) and Donington Suite granite (ca 1850 Ma).The age of felsic dykes (1593.88 ± 0.56 Ma) interpreted to be post-mineralisation implies that the main hydrothermal event at Acropolis was ca 1594 Ma.The Gawler Range Volcanics at Acropolis are the same age as the Gawler Range Volcanics at Olympic Dam and ca 3 to 7 My older than the Gawler Range Volcanics exposed in the Gawler Ranges.

Petrography, geochemistry, and geochronology of the Sc-enriched Kiviniemi ferrodiorite intrusion, eastern Finland

The Kiviniemi mafic intrusion, near the eastern margin of the Paleoproterozoic Central Finland Granitoid Complex, is both spatially and temporally associated with post-kinematic Fe-Ti-P-enriched Svecofennian orogenic mafic magmatism. The main rock types in this small (~ 15 ha) intrusion are garnet-bearing fayalite ferrodiorite, leucoferrodiorite, ferromonzodiorite, and pyroxene diorite. The garnet-bearing fayalite ferrodiorite and leucoferrodiorite contain 50–281 ppm Sc, 275–5600 ppm Zr, and 58–189 ppm Y (n = 42), delineating a mineralized deposit some 2.5 ha in extent. Overall, these rocks show an evolved (iron-enriched) tholeiitic character; low values of Ni (< 20–40 ppm), Cr (< 20 ppm), and Cu (< 20–80 ppm); and high contents of Zn (213–700 ppm). The rock-forming minerals in the ferrodioritic rocks are (ferro)hedenbergite, plagioclase (~ An40), ferropargasite and ferroedenite, almandine garnet, and fayalite (Fo1–4). Accessory minerals include zircon, ilmenite, fluorapatite, biotite, pyrite, pyrrhotite, potassium feldspar, grunerite, and clinoferrosilite. Some relict cumulate textures have been preserved, but primary magmatic features have largely been overprinted by strong recrystallization and corona formation. The main carriers of Sc are amphibole, clinopyroxene, and apatite. The remarkably strong enrichment of Sc in ferromagnesian silicates and apatite, rather than in specific Sc-minerals, implies magmatic enrichment. Post-kinematic mafic intrusions in central Finland constitute a bimodal association with co-existing granitoid counterparts. The Kiviniemi mafic intrusion is associated with a coarse megacrystic granite and the two rock type display mingled contacts, indicative of contemporaneity of the two magmas. This conclusion is in accord with the coincident U-Pb zircon ages for the ferrodiorite, at 1857 ± 2 Ma (multigrain ID-TIMS) and the megacrystic granite, at 1860 ± 7 Ma (single-crystal LA-MC-ICP-MS). The initial εNd value of the ferrodiorite and the granite are + 0.1 and − 2.5, respectively. These Nd isotope compositions probably reflect a chondritic mantle source for the ferrodiorite and suggest incorporation of some Archaean crustal material into the granite in the course of magmatic evolution. The resource estimation calculated for Kiviniemi intrusion by using 40 g/t Sc cut off value is 13.4 Mt of rock with an average grade of 162.7 g/t scandium, 1726 g/t zirconium, and 81 g/t yttrium.

Emplacement of the Zindeng phonolitic lava flow (West-Cameroon) in the Cameroon volcanic line: Constraints from the anisotropy of magnetic susceptibility (AMS)

The Zindeng phonolitic flow forms a NW-SE elongated body located on the SE flank of Mount Bambouto (West-Cameroon), along the Cameroon Volcanic Line. It covers basalt emplaced on top of the Dschang biotite megacrystic granite. The magnitude of the magnetic susceptibility (Km) in the phonolitic lava flow varies between 196 μSI and 15769 μSI, indicating a dominantly ferromagnetic sensus lato behavior (Km > 1000 μSI; 89% of the stations) due to the presence of pseudo-single to multi-domain magnetite grains as deduced from hysteresis data. The low values of the degree of magnetic anisotropy, the dominantly oblate shape of the AMS ellipsoids and the concordance between lava plane and magnetic foliation point to flow-related magnetic fabrics. The pattern of the magnetic foliations and lineations indicate that the SE part of the phonolite was emplaced as a dome, whereas the NW part corresponds to a flow domain sensus stricto. It is suggested that the viscous phonolitic magma rose from a feeding conduit below the southeastern part of the lava flow and propagated laterally towards the NW.

Geochronology, Oxidization State and Source of the Daocheng Batholith, Yidun Arc: Implications for Regional Metallogenesis

The Daocheng batholith consists of granite, granodiorite and K-feldspar megacrystic granite, which is located in the north Yidun Arc. It is a barren batholith in contrast to plutons of the same age that contain major copper deposits, such as Pulang to the south. In the Daocheng, abundant mafic microgranular enclaves (MMEs) mainly developed within granodiorite and K-feldspar megacrystic granite, which are characterized by quenched apatite, quartz eyes and plagioclase phenocrysts. LA-ICP-MS zircon U–Pb dating of host granodiorite yielded ages ranging from 223 Ma to 210 Ma, with a weighted mean of 215.3 ± 1.8 Ma. Zircons from MMEs yielded ages ranging from 218 Ma to 209 Ma, with a weighted mean of 214.2 ± 1.4 Ma. Geochemical analyses show that granodiorite is high-K, calc-alkaline and I-type, with SiO2 contents ranging from 67.90% to 70.54%. These rocks are metaluminous to marginally peraluminous (A/CNK = 0.98–1.00) and moderately rich in alkalis with K2O ranging from 3.28% to 4.59% and Na2O ranging from 3.18% to 3.20%, with low MgO (1.08%–1.29%), Cr (12.7 ppm–16.8 ppm), Ni (5.19 ppm–6.16 ppm) and Mg# (35–49). The MMEs have relatively low SiO2 contents (56.34%–60.91%), higher Al2O3 contents (16.06%–17.98%), higher MgO and FeO abundances and are metaluminous (A/CNK = 0.82–0.83). The MMEs and host granodiorite are enriched in light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs), with slightly negative Eu anomalies, and enriched in Th, U and large ion lithophile elements (LILEs; e.g., K, Rb and Pb), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P and Ti), showing affinities typical of arc magmas. The zircon εHf(t) values (−6.28 to −2.33) and ancient two-stage Hf model ages of 1.92 to 1.25 Ga, indicating that the magmas are generally melts that incorporated significant portions of Precambrian crust. The relatively low silica contents and high Mg# values of the MMEs, and the linear patterns of MgO, Al2O3 and Fe2O3 with SiO2 between the MMEs and host granodiorite, showing the formation of MMEs are genetically related to magma mixing. The Daocheng granodiorite is characterized by much lower zircon Ce4+/Ce3+ (average of 3.53) and low fO2 value (average of ∆FMQ = –10.84), whereas the ore-bearing quartz monzonite porphyries in the Pulang copper deposit are characterized by much higher zircon Ce4+/Ce3+ (average of 52.10) and high fO2 value (average of ∆FMQ = 2.8), indicating the ore-bearing porphyry intrusions had much higher fO2 of magma than the ore-barren intrusions considering that the high oxygen fugacity of the magma is conducive to mineralization.

Emplacement of the Dschang granitic pluton (west-Cameroon): Constraints from microstructures and magnetic fabrics

Microstructures and Anisotropy of Magnetic Susceptibility (AMS) data were investigated in the Pan-African Dschang granitic pluton and its basement rocks in order to decipher its emplacement and time relationship within the Pan-African evolution of the Central Cameroon Shear Zone (CCSZ). The Dschang granitic pluton, emplaced in biotite-hornblende gneiss country rocks, is made of biotite megacrystic granite and fine-grained biotite granite. The pluton has mostly low susceptibility magnitude (Km ≤ 500 μSI) implying that AMS susceptibility is mostly controlled by biotite. The foliation of the pluton is underlined by platy alkali-feldspar megacrystals that strike ENE-WSW to E-W or WNW-ESE with moderate to high dips, associated with biotite schlieren and spindle-shaped xenoliths. The magnetic foliations also strike dominantly NE-SW to E-W or NNW-SSE to NNE-SSW directions with moderate to steep dips. Magnetic lineations have low to moderate plunges and show radiating pattern in the Schmidt projection diagram. The microstructures within the pluton point to continuous deformation from magmatic to sub-magmatic state. The Jelinek shape parameter Tj shows abundant triaxial and oblate shape AMS ellipsoids and uncommon negative values (42% of the stations). Low to very low values of Pj parameter associated to (i) mostly triaxial and oblate shape AMS ellipsoids and (ii) dominantly magmatic to submagmatic state microstructures suggest a syn-to post-kinematic emplacement of a high viscosity granitic magma by flow mechanism. Magnetic fabrics infer that the Dschang granitic pluton was emplaced in a ENE-WSW to E-W oriented fracture initiated during the transcurrent strike-slip CCSZ.

Multiple intrusive phases in the Leinster Batholith, Ireland: geochronology, isotope geochemistry and constraints on the deformation history

The formation of granite batholiths, commonly by incremental assembly of small magma batches, and their correlation with tectonic events, on both local and regional scales, is crucial to understanding the evolution of the Earth's continental crust. However, these correlations often rely on assumptions about the detailed relationship and timing of mapped units. Here we report how an integrated geochronological, structural and isotope geochemical study in only one key locality from the late Caledonian Leinster Batholith in SE Ireland provides the potential for unravelling essential questions on batholith petrogenesis. The Northern Unit of the Leinster Batholith intruded incrementally, as demonstrated by three crosscutting granite varieties at Glenmacnass Waterfall. The oldest, foliated granite, yielded a zircon U–Pb age of 417.4 ± 1.7 Ma. Following deformation, equigranular granite intruded at 409.8 ± 1.9 Ma and was cut by megacrystic granite at 404.9 ± 2.6 Ma. Hence, batholith formation lasted up to 16.8 myr, challenging the widely accepted 405 Ma age for the entire batholith. Multi-isotope geochemical investigations permit derivation of the three granites from the same sources, including both Mid- to Upper Ordovician peri-Gondwanan arc-related magmatic and Lower to Mid-Ordovician Ribband Group metasedimentary rocks. Zircon inheritance and Lu–Hf isotope geochemistry also reveal reprocessing of older granite wall rock in the younger magmas. Supplementary Material: Data tables are available at https://doi.org/10.6084/m9.figshare.c.3898330

Geological relationships and laser ablation ICP-MS U-Pb geochronology of the Saint George Batholith, southwestern New Brunswick, Canada: implications for its tectonomagmatic evolution

The Late Silurian to Late Devonian Saint George Batholith in southwestern New Brunswick is a large composite intrusion (2000 km2) emplaced into the continental margin of the peri-Gondwanan microcontinent of Ganderia. The batholith includes: (1) Bocabec Gabbro; (2) equigranular Utopia and Wellington Lake biotite granites; (3) Welsford, Jake Lee Mountain, and Parks Brook peralkaline granites; (4) two-mica John Lee Brook Granite; (6) Jimmy Hill and Magaguadavic megacrystic granites; and (6) rapakivi Mount Douglas Granite. New LA ICP-MS in situ analyses of six samples from the Saint George Batholith are as follows: (1) U-Pb monazite crystallization age of 425.5 ± 2.1 Ma for the Utopia Granite in the western part of the batholith (2) U-Pb zircon crystallization ages of 420.4 ± 2.4 Ma and 420.0 ± 3.5 Ma for two samples of the Utopia Granite from the central part of the batholith; (3) U-Pb zircon crystallization age of 418.0 ± 2.3 Ma for the Jake Lee Mountain Granite; (4) U-Pb zircon crystallization age of 415.5 ± 2.1 Ma for the Wellington Lake Granite; and (5) U-Pb monazite crystallization age of 413.3 ± 2.1 Ma for the John Lee Brook Granite. The new geochronological together with new and existing geochemical data suggest that the protracted magmatic evolution of the Late Silurian to Early Devonian plutonic rocks is related to the transition of the Silurian Kingston arc-Mascarene backarc system from an extensional to compressional tectonic environment during collision of the Avalonian microcontinent with Laurentia followed by slab break-off.

Timescales and mechanisms of batholith construction: Constraints from zircon oxygen isotopes and geochronology of the late Variscan Serre Batholith (Calabria, southern Italy)

The late Variscan Serre Batholith in central Calabria represents the middle portion, c. 13-km-thick, of a tilted crustal section continuously exposed from lower-crustal granulites to upper-crustal phyllites. The batholith is zoned, consisting of several granitoid types that were emplaced at depths ranging from c. 23 to c. 6km. Deep, strongly foliated quartz diorites and tonalites were emplaced into migmatitic metapelites, the intermediate level granitoids are weakly foliated to unfoliated porphyritic granodiorites and monzogranites, and the shallowest bodies are two-mica granodiorites and granites, grading upward to biotite ± amphibole granodiorites, emplaced into paragneisses and phyllites. Five samples, representative of the main granitoid types in terms of both composition and emplacement depth, have been dated by SHRIMP. Zircon from a lower-crustal quartz diorite gave an emplacement age of 297.3±3.1Ma; two middle-crustal strongly peraluminous K-feldspar megacrystic granites were emplaced at 296.1±1.9Ma and 294.9±2.7Ma; a middle- to upper-crustal two-mica monzogranite was emplaced at 294.2±2.6Ma and finally, an upper-crustal weakly peraluminous granodiorite from the batholith roof was emplaced at 292.2±2.6Ma. These results are consistent with the development of the batholith by incremental multipulse overaccretion, each granitoid body being in a dominantly rigid state before intrusion of the next, with little or no possibility of magma mixing. The difference in age between the oldest and youngest granitoids, from the batholith floor and roof, respectively, is 5.1±4.0Ma, providing an upper limit of about 9Ma on the time taken for batholith construction. The presence in the c. 296–294Ma granitoids of c. 305–302Ma anatectic zircon with varied Th/U and oxygen isotope compositions indicates a time interval of 8–9Ma between incipient partial melting in the lower crust and magma emplacement in the middle crust. The emplacement of the first granitoid bodies into the top of the lower crust was controlled tectonically by the activation of a deep-seated shear zone. The shallowest granodiorites were emplaced during the waning stages of the shear zone activity, producing late- to post-tectonic contact metamorphism in the upper-crustal phyllites and mylonitic paragneisses.

Age and anatomy of the Gongga Shan batholith, eastern Tibetan Plateau, and its relationship to the active Xianshui-he fault

The Gongga Shan batholith of eastern Tibet, previously documented as a ca. 32–12.8 Ma granite pluton, shows some of the youngest U-Pb granite crystallization ages recorded from the Tibetan Plateau, with major implications for the tectonothermal history of the region. Field observations indicate that the batholith is composite; some localities show at least seven crosscutting phases of granitoids that range in composition from diorite to leucocratic monzogranite. In this study we present U-Pb ages of zircon and allanite dated by laser ablation–inductively coupled plasma–mass spectrometry on seven samples, to further investigate the chronology of the batholith. The age data constrain two striking tectonic-plutonic events: a complex Triassic–Jurassic (ca. 215–159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14–5 Ma) record of monzonite-leucogranite emplacement. The former age range is attributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduction zone magmatism along the western Yangtze block of south China. The younger component may be related to localized partial melting (muscovite dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, and are among the youngest crustal melt granites exposed on the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of zircons and growth and/or resetting of allanites. The young garnet, muscovite, and biotite leucogranites occur mainly in the central part of the batholith and adjacent to the eastern margin of the batholith at Kangding, where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seismically active strike-slip fault in Tibet and is thought to record the eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites of the Gongga Shan massif and has a major transpressional component in the Kangding-Moxi region. The course of the Xianshui Jiang river is offset by ∼62 km along the Xianshui-he fault and in the Kangding area granites as young as ca. 5 Ma are cut by the fault. Our new geochronological data show that only a part of the Gongga Shan granite batholith is composed of young (Miocene) melt, and we surmise that as most of eastern Tibet is composed of Precambrian–Triassic Indosinian rocks, there is no geological evidence to support regional Cenozoic internal thickening or metamorphism and no evidence for eastward-directed lower crustal flow away from Tibet. We suggest that underthrusting of Indian lower crust north as far as the Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau.

Geochronology and tectonic evolution of the Hohewarte Complex, central Namibia: New insights in Paleoproterozoic to Early Neoproterozoic crustal accretion processes

The Hohewarte Metamorphic Complex has been variously correlated with the Abbabis Complex, Kamanjab Inlier and the Rehoboth inlier, which is better studied. The Hohewarte Metamorphic Complex is wholly surrounded by the Damara Sequence that experienced peak orogenesis at circa 550Ma. The Damaran cover has been thrusted over the Hohewarte gneisses, producing subhorizontal fabrics. The Hohewarte Metamorphic Complex was already accreted onto the Kalahari craton during Damaran peak orogenesis. The Hohewarte Metamorphic Complex is composed of a variety of gneisses, ranging from amphibolites, quartz-feldspathic gneiss, plagioclase-rich striped gneisses that enclose two varieties of augen gneiss; a felsic K-feldspar-augen gneiss and an augen gneiss with plagioclase and K-feldspar augens in a matrix of biotite and hornblende rich matrix, occurring mainly to the east of the complex. To the northeast in the Seeis inlier, occurs a medium-grained granitic gneiss with a weak foliation. To the west occurs the ubiquitous banded quartz-feldspar-biotite gneisses, that were intruded by megacrystic granites that are now gneisses. The whole complex has been deformed with two deformations prior to the Damaran orogeny. The fabrics in the gneisses are vertical to subhorizontal, whereas the Damaran cover rocks show a distinct ENE trending fabrics with a consistent dip of 30–45° to the NW. Four samples were selected for single zircon U–Pb dating to establish the age relations; the amphibolite unit within the quartzo-feldspathic gneiss records an age of 1758±10Ma; the felsic augen gneiss yielded an age of 1290±10Ma. The megacrystic K-feldspar gneiss yielded an age of 1168±38Ma, whereas the granitic gneiss gave the youngest age of 1061±10Ma. εHf model ages show that all the Hohewarte Metamorphic Complex gneisses plot close to the CHUR and posses very primitive crustal signatures and have not been recycled much in the crust. A depleted mantle source is suggested for their origin.

The Bratten–Landegode gneiss complex: a fragment of Laurentian continental crust in the Uppermost Allochthon of the Scandinavian Caledonides

Abstract This report presents new field observations and geochronology (isotope-dilution thermal ionization mass spectrometry/ID-TIMS and secondary-ion mass spectrometry/SIMS) on an igneous complex and its country rocks in the Bodø area, northern Norway, traditionally interpreted to represent the (par)autochthonous crust of Baltica ( c. 1.8 Ga). Field observations however indicate that the rocks are allochthonous and comprise the uppermost tectonostratigraphic level in the area. The presence of a migmatitic megacrystic granite with an emplacement age of 946 Ma strongly supports such an interpretation and indicates that the Bratten–Landegode gneiss complex is exotic with respect to Baltica. The 946 Ma granite intrudes metasedimentary rocks. The rocks were metamorphosed in the Late Ordovician and intruded by granitic pegmatites and diorites at 430 and 427 Ma, respectively. The Bratten–Landegode gneiss complex shows a close correlation with Mesoproterozoic–Neoproterozoic rock complexes in the East Greenland Caledonides and we interpret it to be a Laurentian pre-Caledonian continental fragment. The discovery of Laurentian pre-Caledonian continental crust in the Uppermost Allochthon calls for a revision of the tectonostratigraphy of this part of the Caledonides and provides important constraints on the sequence of events on the Laurentian margin prior to continent–continent collision between Baltica and Laurentia as well as on intercontinental interactions during the Caledonian orogeny.

Late crystallization of K-feldspar and the paradox of megacrystic granites

K-feldspar crystals >5 cm in greatest dimension are common in calc-alkaline granites and granodiorites worldwide. Such megacrysts are generally interpreted as having grown to large sizes early in a magma’s crystallization history while they were largely molten, owing to field relations such as megacryst alignment and megacryst-rich clusters and to crystallographic features such as zonally arranged inclusions and sawtooth Ba zoning. These features are consistent with early growth but do not require it. In contrast, experimental petrology, mineral compositions, and natural examples of partial melting of granite demonstrate that K-feldspar is typically the last major phase to crystallize and that most K-feldspar growth occurs after the magma crosses the rheologic lock-up threshold of ~50 % crystals. The near-absence of K-feldspar phenocrysts in dacite lavas and tuffs, even in highly crystalline ones, demonstrates that natural magmas do not precipitate significant K-feldspar while they are mobile. The highly potassic compositions of megacrysts (and indeed, of K-feldspar in non-megacrystic granites as well) require exsolution of albite component down to temperatures of ~400 °C. The low Ca contents of megacrysts cannot result from exsolution of anorthite and must represent recrystallization of the crystals at low temperature. These mineralogical and experimental constraints require that K-feldspar megacrysts indicate widespread and thorough recrystallization of the host granites and granodiorites.

Constraints on the Timing of Co-Cu Au Mineralization in the Blackbird District, Idaho, Using SHRIMP U-Pb Ages of Monazite and Xenotime Plus Zircon Ages of Related Mesoproterozoic Orthogneisses and Metasedimentary Rocks

The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xeno-time and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 ± 4 Ma), determined on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1315 to 1270 Ma and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on 206 Pb/ 238 U data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 ± 3 and 92 ± 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events.

MAJOR THRUSTING IN THE GRANITE AND THE ROLE OF LATE INTRUSIONS IN EXPOSING THE DEEPER PARTS OF THE CENTRAL BLOCK OF THE GALWAY GRANITE BATHOLITH

New geological mapping in the centre of the Galway Granite at Camus, Co. Galway, reveals a major north—south thrust zone, the Furnace Thrust, typically dipping east at 35°, which thrust up westward deeper-crystallised granite, including two minor late intrusions, on top of a higher-level granite footwall. Al-in-hornblende geobarometry of the footwall and the hanging wall confirms the thrusting. At 2km east of the thrust, both the footwall and the hanging wall of the thrust sheet were later substantially further uplifted by the steeper Shannawona Fault. At 6km east of this fault, the granite contains post-400—395Ma, pre-370—378Ma thrusts that moved south-eastward, so a major block of the deep ∼400—395Ma Megacrystic Granite has been squeezed upward. This granite still farther east was later uplifted more by the steep Shannapheasteen Fault, which is connected with the late (?380Ma) central intrusion of the Shannapheasteen Granite, which pushed its roof upward. The uplift of the Central Block with its deeply crystallised Megacrystic Granite was therefore the result of thrusts and faults connected in a complex way with the coeval intrusive pressures of the late emplacement of the Shannapheasteen Granite and six other late granites. All seven late granites are confined to the Central Block and, having low densities, exerted protracted buoyancy uplift forces. This modifies the previous partly correct, but mechanically difficult, explanation for the origin of the Central Block of the Galway Batholith.

Late Paleozoic subduction and exhumation of Cambro-Ordovician passive margin and arc rocks in the northern Acatlán Complex, southern Mexico: Geochronological constraints

The origin and age of high pressure (HP) rocks are crucial for paleogeographic reconstructions because they either define an oceanic suture or an extrusion zone within the upper plate. HP rocks in the San Miguel Las Minas area in the northern part of the Acatlán Complex, southern Mexico have been inferred to be of early Paleozoic age and to mark oceanic sutures. However, blueschists in the northern part of the complex have yielded Mississippian 40Ar/ 39Ar plateau ages of 344 ± 5 Ma for glaucophane and 338 ± 3 Ma and 337 ± 2 Ma for muscovite. These ages are slightly younger than recently published ages: a U–Pb zircon age of 353 ± 1 Ma from associated eclogite, and a 347 ± 3 Ma muscovite age from the tectonically overlying, greenschist facies Las Minas Unit. Taken together, these data indicate rapid cooling between 700° and 340 °C in ca. 17 Myr. On the other hand, associated Ordovician Anacahuite Amphibolite cooled through ca. 500 °C at 299 ± 6 Ma ( 40Ar/ 39Ar on hornblende) suggesting a second, Permian phase of exhumation. Protoliths of the high grade rocks include Cambrian–Ordovician, rift-passive margin, psammites, pelites, and tholeiitic dykes, an Ordovician mafic intrusion (Anacahuite Amphibolite dated at 470 ± 10 Ma: U–Pb zircon) and megacrystic granite (dated at 492 ± 12 Ma: U–Pb zircon), and arc-related mafic rocks of unknown age. These rocks are interpreted to have been part of the upper plate rocks that was removed by subduction erosion and taken to depths between 35 and 55 km where they underwent blueschist–eclogite facies metamorphism. This was followed by rapid extrusion along a channel bounded by an easterly dipping, Mississippian, listric normal shear zone, and a thrust modified by a Permian dextral fault. Rocks above and below the extrusion zone are mainly Cambro-Ordovician rift-passive margin units, but a small vestige of the arc is preserved as dikes cutting rocks lying unconformably beneath the fossiliferous latest Devonian–Lower Permian Patlanoaya Group. Since faunal data indicate that Pangea had begun amalgamation by the Mississippian, at which time the Acatlán Complex lay 1500–2000 km south of the Ouachita collisional orogen between Gondwana and Laurentia, it is inferred that subduction and extrusion of the high pressure rocks occurred on the active western margin of Pangea.