Shoshonitic lamprophyre dykes and their relation to mesothermal Au---Sb veins at Hillgrove, New South Wales, Australia

The Hillgrove mineral field, in the southern part of the New England Orogen of northeastern New South Wales, Australia, contains numerous mesothermal Au---Sb vein systems. Calc-alkaline (shoshonitic) lamprophyre (CAL) dykes are also associated with mineralisation with dilational lode structures acting as conduits for dyke intrusion, which has occurred before and after major quartz-stibnite veining. Dykes include minette and vogesite compositions and were emplaced in the late Permian (247–255 Ma), at the same time as regionally extensive I-type magmatism in the New England Orogen. Least-altered dykes are enriched in Mg, K, Ba, Rb, Sr, Zr, Th, Cr and Ni relative to I-type intrusives although chemical affinities are evident between lamprophyres and the more mafic members of the high-K Moonbi Plutonic Suite.

Hillgrove lamprophyres are commonly enriched in Sb, As, Hg, Au, W and Bi with respect to average CAL compositions. Evidence indicates this is most likely due to contamination of magma during intrusion through mineralised structures, rather than a primary magmatic feature. Partially resorbed xenocrystic stibnite occurs in dykes which have intruded lode structures, probably facilitated by the low melting point of stibnite (550°C) and its incorporation into the magma. Carbon and oxygen isotopic data from carbonates in least-altered, post-lode lamprophyres are indistinguishable from carbonate in altered dykes and veins, implying that hydrothermal interaction continued after dyke intrusion. Although it is unlikely that lamprophyre dykes have been a direct source for mineralisation at Hillgrove, the close temporal and spatial relation of dykes, mesothermal Au---Sb veins and I-type intrusions are interpreted to be manifestations of the post-collisional setting and influx of mantle-derived heat and partial melts into the New England Orogen during the late Permian.     

Crystallization Conditions and Mineral Chemistry in the East of Tafresh,Central Iran,with Insights into Magmatic Processes

The Tafresh granitoids are located at the central part of the Urumieh-Dokhtar Magmatic Arc(UDMA) in Iran. These rocks, mainly consisting of diorite and granodiorite, were emplaced during the Early Miocene. They are composed of varying proportions of plagioclase + K-feldspar + hornblende ± quartz ± biotite. Discrimination diagrams and chemical indices of amphibole phases reveal a calc-alkaline affinity and fall clearly in the crust-mantle mixed source field. The estimated pressure, derived from Al in amphibole barometry, is approximately 3 Kb. The granitoids are I-type, metaluminous and belong to the calc-alkaline series. They are all enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements and display geochemical features typical of subduction-related calc-alkaline arc magmas. Most crystal size distribution(CSD) line patterns from the granitoids show a non-straight trend which points to the effect of physical processes during petrogenesis.The presence of numerous mafic enclaves, sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids. Moreover, the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years, which is indicative of shallow depth magma crystallization.     

Geophysical characterisation of a blind I-type pluton emplaced within the Bundarra Suite S-type granites of the New England Batholith

Geophysical data are presented that characterise a blind pluton, the Mountain Home Pluton (MHP), which intrudes the southern portion of the Bundarra Suite (BS), 30 km northeast of Bendemeer, New South Wales. A positive magnetic anomaly within the non-magnetic granites of the BS (Banalasta and Pringles Monzogranites) was previously identified as a sub-surface intrusion. Interpretation of new gravity data and analysis of aeromagnetic data are used to infer the depth, size, density, magnetic susceptibility and likely petrology of the pluton. The best-fit model indicates that the MHP is very similar to the Looanga Monzogranite, a felsic member of the Moonbi Suite of the New England Batholith (NEB) that intrudes the BS 5–7 km southeast of the MHP. The top of the MHP is inferred to lie about 1 km beneath the surface and the pluton extends to a depth of at least 6 km. Our model furthermore suggests that the southwestern margin of the MHP is subvertical, whereas a shallower dip (<45°) towards the north is proposed for the northeastern surface of the pluton. A north-trending dyke swarm, identified on the basis of linear positive magnetic anomalies, may be related to the MHP. This swarm of more than 20 relatively magnetic dykes extends out to about 10 km north from the pluton. Magnetic modelling of the dykes indicates that susceptibility values of the dykes are probably very similar to the range of the MHP, and also suggests the width of individual dykes (also not known to be exposed at the surface) to be at most a few tens of metres. A petrographic examination of the intruded BS granites at the surface suggests that metamorphic zoning as seen in mineralogical characteristics may be related to the underlying pluton.     

Metamorphism and deformation of mafic and felsic rocks in a magma transfer zone,Stewart Island,New Zealand

In the mingled mafic/felsic Halfmoon Pluton at The Neck, Stewart Island (part of the Median Batholith of New Zealand) some hornblende gabbros and diorites retain magmatic structures, whereas others show evidence of major changes in grain and inclusion shapes, and still others are amphibolite‐facies granofelses with few or no igneous relicts. These mafic to intermediate magmas crystallized in felsic magma relatively quickly, with the result that most deformation occurred at subsolidus conditions. It is suggested that mafic‐intermediate rocks with predominantly igneous microstructures spent less time in the magmatic system. The metamorphism of the mafic rocks appears to be ‘autometamorphic’, in the sense that elevated temperatures were maintained by magmatic heat during subsolidus cooling. Elevated temperatures were maintained because of repeated sheet injection and subconcordant dyke injection of hot basaltic and composite mafic‐felsic magmas, into a dominantly transtensional, km‐scale, outboard‐migrating, magmatic shear zone that operated semi‐continuously for between c. 140 and c. 130 Ma. Complete cooling occurred only when the system evolved to transpressional and the locus of magmatism migrated inboard (southward) between c. 130 and c. 120 Ma, associated with solid‐state mylonitic deformation. Intermingled granitic rocks escaped metamorphism, because they remained magmatic to lower temperatures, and experienced shorter and lower‐temperature subsolidus cooling intervals. However, the felsic rocks underwent relatively high‐temperature solid‐state deformation, as indicated by myrmekite replacing K‐feldspar and chess‐board subgrain patterns in quartz; locally they developed felsic mylonites. The felsic rocks were deformed in the solid state because of their high proportion of relatively weak minerals (quartz and biotite), whereas the mafic rocks mostly escaped subsolidus deformation, except in local high‐strain zones of hornblende‐plagioclase schist, because of their high proportion of relatively strong minerals (hornblende and plagioclase). We suggest that such contrasting microstructural features are diagnostic of long‐lived syntectonic magma transfer zones, and contrast with the more typical complex, batholith‐scale magma chambers of magmatic arcs.     

Thermal gradient and timing of high‐T–low‐P metamorphism in the Wongwibinda Metamorphic Complex,southern New England Orogen,Australia

The Wongwibinda Metamorphic Complex (WMC) is a high‐temperature, low‐pressure (HTLP) belt in the southern New England Orogen. It is characterized by a high metamorphic field gradient exposed in variably metamorphosed siliceous turbidites. The Abroi Granodiorite and the Rockvale and Tobermory adamellites, S‐type granitoids of the Hillgrove Plutonic Suite, intrude the metaturbidites. Six samples of metaturbidite were studied from an ～3 km long field traverse. Integrated petrography, mineral chemistry, and mineral equilibria modelling indicate a peak metamorphic temperature of 350–450 °C in the lowest grade rocks and ～660 °C in the highest‐grade rocks. Maximum pressure does not exceed 3.5 kbar anywhere, implying a maximum depth of 12 km and indicating an average vertical gradient of at least 55 °C km^?1, though our calculations suggest this is not linear. Metamorphic isograds show no apparent relationship with distance to the exposed margins of the Hillgrove Suite granitoids. Electron microprobe U–Th–Pb monazite data indicate a date of 296.8 ± 1.5 Ma for the thermal peak of the HTLP metamorphism. Laser ablation inductively coupled plasma mass spectrometry indicates a zircon U–Pb crystallization age of 290.5 ± 1.6 Ma for the Abroi Granodiorite, confirming that the pluton post‐dates the peak HTLP metamorphism. Consequently, magmatic advective heat transfer from depth via emplacement of a large volume of granitoid is unlikely to be the key local driver of the high‐grade metamorphism. It is concluded that published evidence of an extensional geodynamic setting around the Carboniferous–Permian boundary supports conductive heat transfer as the key driver of HTLP metamorphism for the WMC. It is not possible to exclude magmatic advective heat transfer via emplacement of mantle derived basaltic magmas in the deeper crust.     

Mid-Cretaceous granitic magmatism during the transition from subduction to extension in southern New Zealand: a chemical and tectonic synthesis

Regional geochronological studies indicate that mid-Cretaceous plutonism (the Hohonu Suite at 110 Ma) in the Hohonu Batholith, Western Province of New Zealand, occurred during a period of rapid tectonic change in the SW Pacific portion of Gondwana. The 30–40 m.y. preceding Hohonu Suite magmatism were dominated by the subduction-related plutonism of the Median Tectonic Zone volcanic arc. Between 125–118 Ma there was a major collisional event, inferred to be the result of collision between the Median Tectonic Zone and the Western Province. This collision resulted in melting of the Median Tectonic Zone arc underplate and generation of a distinctive suite of alkali-calcic granitoids, termed the Separation Point Suite. At 110 Ma there was another pulse of magmatism, restricted to the Buller terrane of the Western Province, and including the Hohonu Suite granitoids. This was followed almost immediately by extension, culminating in the opening of the Tasman Sea some 30 m.y. later. The Hohonu Suite granitoids overlap temporally with the last vestiges of collisional Separation Point magmas and the onset of crustal extension in the Western Province, and thus represent magmatism in a post-collisional setting. Hohonu Suite magmas are typically calc-alkaline, but retain a chemical signature which suggests that the earlier Separation Point Suite magmas and/or sources were involved in Hohonu Suite petrogenesis. A model is proposed in which rapid isothermal uplift, resulting from the post-collisional collapse of continental crust previously thickened during the Median Tectonic Zone collision, caused melting of lower continental crust to generate the Hohonu Suite granitoids. In this example, granitoid composition is a consequence of the composition of the source rocks and the conditions present during melting, and no geochemical signature indicative of the tectonic setting during magmatism is present.     

The Hohonu Batholith of North Westland, New Zealand: granitoid compositions controlled by source H2O contents and generated during tectonic transition

Geochemical studies on the Hohonu Batholith, of the West Coast, South Island, New Zealand, have recognised two distinct but chemically related suites of mid-Cretaceous granitoids. The suites are characterised by restricted radiogenic isotopic compositions (Sr_(i) = 0.7062 to 0.7085; ɛNd_(i) = −4.4 to −6.1), and represent melting of a mafic lithosphere source followed by interaction with Ordovician metasediments. The two suites (Te Kinga Suite and Deutgam Suite) are distinguished by contrasting contents of Al₂O₃, Na₂O, Sr, Ba, Eu and HREE, attributable to different residual asssemblages controlled by differing H₂O contents during melting of a metabasaltic source. The relatively mafic, metaluminous, I-type Deutgam Suite represents magmas derived by dehydration melting in equilibrium with an amphibolitic (plagioclase + amphibole) residue. In contrast, the peraluminous, high silica compositions of the Te Kinga Suite were produced by melting at higher H₂O contents, reducing the stability of plagioclase and resulting in a melt in equilibrium with a plagioclase-free eclogitic (garnet + amphibole) residue. Residual plagioclase during generation of the Deutgam Suite resulted in lower Al₂O₃, Na₂O, Sr, Ba and Eu contents, whereas residual garnet during generation of the Te Kinga suite resulted in depleted HREE contents. The mid-Cretaceous granitoids of the Hohonu Batholith were generated during a period of rapid tectonic transition from crustal thickening during collision to crustal thinning and core complex formation during extension. Received: 23 July 1996 / Accepted: 21 August 1997     

Petrological,geochemical and geochronological evidence for a Neoproterozoic ocean basin recorded in the Marlborough terrane of the northern New England Fold Belt

Petrological, geochemical and radiogenic isotopic data on ophiolitic‐type rocks from the Marlborough terrane, the largest (～700 km²) ultramafic‐mafic rock association in eastern Australia, argue strongly for a sea‐floor spreading centre origin. Chromium spinel from partially serpentinised mantle harzburgite record average Cr/(Cr + Al) = 0.4 with associated mafic rocks displaying depleted MORB‐like trace‐element characteristics. A Sm/Nd isochron defined by whole‐rock mafic samples yields a crystallisation age of 562 ± 22 Ma (2σ). These rocks are thus amongst the oldest rocks so far identified in the New England Fold Belt and suggest the presence of a late Neoproterozoic ocean basin to the east of the Tasman Line. The next oldest ultramafic rock association dated from the New England Fold Belt is ca530 Ma and is interpreted as backarc in origin. These data suggest that the New England Fold Belt may have developed on oceanic crust, following an oceanward migration of the subduction zone at ca540 Ma as recorded by deformation and metamorphism in the Anakie Inlier. Fragments of late Neoproterozoic oceanic lithosphere were accreted during progressive cratonisation of the east Australian margin.     

Petrochemistry of Granitoids Along the Loei Fold Belt,Northeastern Thailand

Petrochemical characteristics of Permo‐Triassic granitoids from five regions (i) Mung Loei, (ii) Phu Thap Fah – Phu Thep, (iii) Phetchabun, (iv) Nakon Sawan – Lobburi, and (v) Rayong – Chantaburi along the Loei Fold Belt (LFB), northeastern Thailand were studied. The LFB is a north–south trending 800 km fold belt that hosts several gold and base‐metal deposits. The granitoids consist of monzogranite, granodiorite, monzodiorite, tonalite, quartz‐syenite, and quartz‐rich granitoids. These are composed of quartz, plagioclase, and K‐feldspar with mafic minerals such as hornblende and biotite. Accessory minerals, such as titanite, zircon, magnetite, ilmenite, apatite, garnet, rutile, and allanite are also present. Magnetic susceptibilities in the SI unit of granitoids vary from 6.5 × 10⁻³ to 15.2 × 10⁻³ in Muang Loei, from 0.1 × 10⁻³ to 29.4 × 10⁻³ in Phu Thap Fah – Phu Thep, from 2.7 × 10⁻³ to 34.6 × 10⁻³ in Petchabun, from 2.4 × 10⁻³ to 14.1 × 10⁻³ in Nakon Sawan – Lobburi, and from 0.03 × 10⁻³ to 2.8 × 10⁻³ in Rayong – Chantaburi. Concentration of major elements suggests that these intermediate to felsic plutonic rocks have calc‐alkaline affinities. Concentration of REE of the granitoids normalized to chondrite displays moderately elevated light REE (LREE) and relatively flat heavy (HREE) patterns, with distinct depletion of Eu. Rb versus Y/Nb and Nb/Y tectonic discrimination diagrams illustrate that the granitoids from Muang Loei, Phu Thap Fah – Phu Thep, Phetchabun, Nakon Sawan – Lobburi, and Rayong – Chantaburi formed in continental volcanic‐arc setting. New age data from radiometric K‐Ar dating on K‐feldspar from granodiorite in Loei and Nakhon Sawan areas yielded 171 ± 3 and 221 ± 5 Ma, respectively. K‐Ar dating on hornblende separated from diorite in Lobburi yielded 219 ± 8 Ma. These ages suggest that magmatism of Muang Loei occurred in the Middle Jurassic, and Nakon Sawan – Lobburi occurred in Late Triassic. Both Nb versus Y and Rb versus (Y + Nb) diagrams and age data indicate that Nakon Sawan – Lobburi granitoids intruded in Late Triassic at Nong Bua, Nakon Sawan province and Khao Wong Phra Jun, Lobburi province in volcanic arc setting. Muang Loei granitoids at the Loei province formed later in Middle Jurassic also in volcanic arc setting. The negative δ³⁴S_CDT values of ore minerals from the skarn deposit suggest that the I‐type magma has been influenced by light biogenic sulfur from local country rocks. The Au‐Cu‐Fe‐Sb deposits correlate with the magnetite‐series granitoids in Phetchabun, Nakon Sawan – Lobburi and Rayong – Chantaburi areas. Metallogeny of the Au and Cu‐Au skarn deposits and the epithermal Au deposit is related to adakitic rocks of magnetite‐series granitoids from Phetchabun and Nakon Sawan areas. All mineralizations along the LFB are generated in the volcanic arc related to the subduction of Paleo‐Tethys. The total Al (^TAl) content of biotite of granitoids increases in the following order: granitoids associated with Fe and Au deposit < with Cu deposit < barren granitoids. X_Mg of biotite in granitoids in Muang Loei indicates the crystallization of biotite in magnetite‐series granitoids under high oxygen fugacity conditions. On the other hand, low X_Mg (<0.4) of biotite in magnetite‐series granitoids in Phu Thap Fah – Phu Thep and Rayong – Chantaburi indicates a reduced environment and low oxygen fugacity, associated with Au skarn deposit (Phu Thap Fah) and Sb‐Au deposit (Bo Thong), respectively. The magnetite‐series granitoids at Phu Thap Fah having low magnetic susceptibilities and low X_Mg of biotite were formed by reduction of initially oxidizing magnetite‐series granitic magma by interaction with reducing sedimentary country rocks as suggested by negative δ³⁴S_CDT values.     

Geochemistry of Ore-bearing Lamprophyre from the Cu-Ni Deposit in Dhi Samir, Yemen

Magmatic Cu-Ni sulfide deposits are generally associated with mafic-ultramafic rocks and it has not been reported that lamprophyre is one of the surrounding rocks of Cu-Ni sulfide deposits. The Dhi Samir deposit in Yemen, however, is a rare example of Cu-Ni deposits which are hosted in lamprophyre dikes. In this paper, comprehensive research is made on petrology, petrochemistry and isotope geochemistry for Cu-Ni-bearing rocks in the Dhi Samir area and the results show that dark rocks related to Cu-Ni orebodies are sodium-weak potassium and belong to calc-alkaline series lamprophyre, especially camptonite, characterized by enriched alkali, iron and titanium. In these rocks large-ion-lithophile elements are obviously concentrated, while high field strength elements slightly depleted, showing clear negative anomalies of Ta and Nb, and weak deficiency of Ti. The SREE is very high (225.67-290.05 ppm) and the REE partition curves are flat and right-inclined, featuring a LREE-enriched pattern with low negative Eu anomalies. Study of magmatic source areas indicates that the rocks have low (87Sr/86Sr) and high εNd(t), and the magmas were probably derived from the enriched mantle I (EM-I) end-member. Based on the LA-ICPMS on zircon U-Pb isotope dating, the lamprophyre in the Dhi Samir mining area has an age of 602±2.6 Ma, indicating that the rock was formed in the late Proterozoic and in an intraplate setting due to magmatism of an extensional environment in the post-Pan-Africa orogeny.     

云南姚安Au-Pb-Ag矿床含矿富碱岩浆岩地球化学特征及岩石成因

通过锆石U-Pb定年、全岩主微量元素、Sr-Nd-Pb和锆石Hf同位素测试, 对滇西姚安Au-Pb-Ag矿床含矿正长斑岩和粗面岩的地球化学特征进行了分析, 系统探讨了其岩浆起源和演化过程.正长斑岩和粗面岩的锆石U-Pb年龄分别为33.8±0.42 Ma和33.9±0.60 Ma, 它们与同时代滇西镁铁质火山岩和煌斑岩具有相似的稀土和微量元素配分模式和Sr-Nd-Pb同位素组成, 而与区内同时代加厚地壳来源的富碱埃达克质岩石存在Sr-Nd-Pb-Hf同位素组成的明显差异.全岩SiO₂与主微量元素关系指示正长斑岩和粗面岩总体上可由矿区内同时代的基性岩浆岩分异演化而来, 表明它们与这些基性岩浆岩起源相似, 较高的Rb/Sr(≥ 0.1)和较低的Ba/Rb(< 20)比值, 指示其源区为富金云母富集地幔, 较低的εHf和古老的模式年龄暗示源区的交代富集发生在中元古代.姚安富碱岩浆活动与矿化关系密切.正长斑岩和粗面岩较滇西镁铁质火山岩和煌斑岩具有稍高的初始Pb同位素组成, 暗示岩浆可能遭受了地壳混染, 从而提高了母岩浆中的金属含量, 增强了岩浆成矿潜力; 适中的氧逸度利于Au富集; 角闪石分离结晶和较多黑云母发育指示母岩浆含水量较高, 利于成矿流体的形成.这些特征综合起来为矿化发育提供了有利条件.      

Density,Viscosity and Velocity (Ascent Rate) of Alkaline Magmas

Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54–2.71 g/cc for shonkinite; 2.61–2.78 g/cc for lamprophyre; 2.66–2.74 g/cc for alkali basalt] and viscosities [3.11–3.39 Pa s for shonkinite; 3.01–3.28 Pa s for lamprophyre; 2.72–3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11–2.13 m/sec, 0.23–2.77 m/sec and 1.16–2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68–15.31 m/sec) > ultramafic-mafic magmas (3.81–4.30 m/sec) > intermediate-felsic magmas (1.76–2.56 m/sec). We propose that SiO₂ content in the terrestrial magmas can be modeled as a semi-quantitative “geospeedometer” of the magma ascent rates.     

First Evidence of Lamprophyric Magmatism from the Konya Region,Turkey: a Genetic Link to High-K Volcanism

In the vicinity of Konya （Turkey）,mafic,micro-porphyritic sub-volcanic rocks intrude into the Mesozoic units,which represents the only example of such a rock type in the region.40Ar/39Ar dating of two whole rock samples from the sub-volcanics gave ages of 13.72±0.13 and 12.40±0.11 Ma,suggesting temporal association to the Late Miocene-Pliocene high-K calc-alkaline volcanism in the region.The mineral chemistry and geochemical data permit us to classify the rocks as ＂minette＂ lamprophyres.They include diopside and phlogopite phenocrysts in a microcrystalline groundmass composed of sanidine,phlogopite,diopside and titano-magnetite.Segregation and ocelli-like globular structures occur commonly in the samples.In terms of major elements,the lamprophyres are calcalkaline,and potassic to ultrapotassic rocks.All the lamprophyres display strong enrichments in LILE （Rb,Ba,K,Sr）,radiogenic elements （Th,U） and LREE （La,Ce） and prominent negative Nb,Ta,and Ti anomalies on primordial mantle-normalized trace element diagrams.Geochemical data suggest that the lamprophyres and high-K calc-alkaline rocks in the region derived from a subduction-modified lithospheric mantle source affected by different metasomatic events.Lamprophyric magmatism sourced phlogopite-bearing veins generated by sediment-related metasomatism via subduction,but high-K calc-alkaline magmas are possibly derived from a mantle source affected by fluid-rich metasomatism.     

Two “S-type” granite suites with low initial 87Sr/86Sr ratios from the New England Batholith,Australia

Two “S-type” (pelitic) granite suites from the New England Batholith, N.S.W., have Upper Carboniferous ages, indicating that they predate by 40 m.y. the intrusion of hornblende biotite granites, and are the oldest plutons of the batholith. Mineralogically and geochemically both suites have “pelitic” characteristics, one suite containing an Al-rich biotite, muscovite and cordierite, the other an Al-rich biotite and rare pyrope-almandine garnet. Low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.706 for both suites probably reflect the volcanoclastic nature and young age of the sedimentary source of these granites at the time of melting. The age of the suites coincides with the last stages of (Andean type?) volcanism along an andesite/dacite volcanic chain to the west, suggesting an origin for the “S-type” granitic magma by partial melting of deformed sediments marginal to a continental region.     

The Moruya Batholith and geochemical contrasts between the Moruya and Jindabyne suites

Rocks of the Moruya Batholith range from gabbros and gabbroic diorites through quartz diorites and tonalites to granodiorites and rare adamellites. The gabbros and gabbroic diorites appear as small, early bodies intruded and enclosed by quartz diorites and tonalites. These early gabbroids are petrographically and chemically distinct from the granitoids. The latter occur as a meridionally‐oriented sequence of nine separate plutons. Mafic xenoliths are most abundant in the quartz diorites and tonalites; they are petrographically similar to their host granitoids and are chemically a more mafic extension of the variation in granitoid compositions. The various granitoid bodies are considered to be derived from similar source rocks, with the xenoliths representing modified material relict from partial melting of that source.

Comparison of chemical data from the Moruya granitoids with those of the I‐types of the Jindabyne Suite in the Kosciusko Batholith, shows that the potassium content is indistinguishable in the two suites from each side of the Moruya‐Kosciusko Province, although elsewhere it has been shown to vary systematically across an orogenic belt. The most outstanding difference is the higher Na and Ti and lower Ca in the Moruya Batholith compared with those in Kosciusko Batholith I‐type granitoids.     

Granitoids from the Moonbi district,New England Batholith,Eastern Australia

The late Palaeozoic granitoids of the Moonbi district are derived both from igneous (I‐type) and sedimentary (S‐type) sources. Field and petrographic observations and chemical data on the I‐type granitoids show that they are derived from four separate and distinct source‐rock compositions and that, consequently, these granitoids may be grouped into four suites. Mafic xenoliths and microxenoliths are relatively more abundant in more mafic I‐type granitoids. Such xenoliths are interpreted as restite, or material residual from partial melting of the source rocks. Variation within the granitoids is ascribed to varying degrees of separation of restite from the melt produced during each fusion event. The source material of the I‐type granitoids is considered to have been material underplated beneath the crust during an earlier subduction event. Two suites of S‐type granitoids can be recognized. These are derivatives of pelitic materials that have undergone only a small amount of chemical weathering.     

Magmatic changes during the stabilisation of a cordilleran fold belt: the Late Carboniferous–Triassic igneous history of eastern New South Wales, Australia

In a 60 Ma interval between the Late Carboniferous and the Late Permian, the magmatic arc associated with the cordilleran-type New England Fold Belt in northeast New South Wales shifted eastward and changed in trend from north–northwest to north. The eastern margin of the earlier (Devonian–Late Carboniferous) arc is marked by a sequence of calcalkaline lava flows, tuffs and coarse volcaniclastic sedimentary rocks preserved in the west of the Fold Belt. The younger arc (Late Permian–Triassic) is marked by I-type calcalkaline granitoids and comagmatic volcanic rocks emplaced mostly in the earlier forearc, but extending into the southern Sydney Basin, in the former backarc region. The growth of the younger arc was accompanied by widespread compressional deformation that stabilised the New England Fold Belt. During the transitional interval, two suites of S-type granitoids were emplaced, the Hillgrove Suite at about 305 Ma during an episode of compressive deformation and regional metamorphism, and the Bundarra Suite at about 280 Ma, during the later stages of an extensional episode. Isotopic and REE data indicate that both suites resulted from the partial melting of young silicic sedimentary rocks, probably part of the Carboniferous accretionary subduction complex, with heat supplied by the rise of asthenospheric material. Both mafic and silicic volcanic activity were widespread within and behind the Fold Belt from the onset of rifting (ca. 295 Ma) until the reestablishment of the arc. These volcanic rocks range in composition from MORB-like to calcalkaline and alkaline. The termination of the earlier arc, and the subsequent widespread and diverse igneous activity are considered to have resulted from the shallow breakoff of the downgoing plate, which allowed the rise of asthenosphere through a widening lithospheric gap. In this setting, division of the igneous rocks into pre-, syn-, and post-collisional groups is of limited value.     

Early Carboniferous (Viséan) emplacement of the collisional Kłodzko–Złoty Stok granitoids (Sudetes, SW Poland): constraints from geochemical data and zircon U–Pb ages

Sensitive high-resolution ion microprobe zircon U–Pb dating and geochemical data of igneous rocks from the composite K?odzko–Z?oty Stok (KZS) Granite Pluton, Sudetic Block, indicate that the granitoids represent an Early Carboniferous Viséan phase of Variscan metaluminous, high-K, I-type, syn-collisional granite magmatism within the Saxothuringian Zone of the Central European Variscides. Igneous zircon records hypabyssal magmatism that produced various granitoids and lamprophyre (spessartite) emplaced from ca. 340 to 331 Ma. The KZS granitoids have compositions ranging from granodiorite to monzonite, low A/CNK ratios (<1), and are associated with abundant mafic members. Most of them are alkaline, highly potassic, and moderately evolved. The major and trace element contents of the KZS granitoids suggest geochemical heterogeneity, and the hybrid nature of magmas derived from a range of sources in the middle crust, with a strong input of material from the upper mantle. Mixing of magmas of mantle origin with high-K material from partly melted continental crust was probably a more important factor than fractional crystallization, in controlling the evolution of the magmas. The mean Pb–U ages of the main population of igneous zircon from a quartz monzodiorite (?elazno) and hornblende monzonite (Droszków) are 340.2 ± 2.5 Ma and 339.5 ± 3.1 Ma, respectively. A slightly younger biotite-hornblende granodiorite from Chwalis?aw, 336.7 ± 2.5 Ma, was cut by a spessartite dyke at 333.1 ± 3.1 Ma. This indicates that mafic magmas were immediately intruded into fractured, probably incompletely solidified, granodiorites. The lamprophyric dyke also contains igneous zircon of Neoproterozoic age, 566.3 ± 6.4 Ma, typical of the crust in the Saxothuringian Zone. Tonalite from Ptasznik Hill near Droszków is of similar age to the spessartite, 331.5 ± 2.6 Ma. High REE contents in the tonalite and its igneous zircon indicate advanced differentiation of granitic magma, producing a more leucocratic melt associated with post-magmatic activity including abundant late crosscutting pegmatites and quartz veins, and contact metasomatic mineralization. The KZS granitoids have rather similar petrographic and geochemical characteristics to granitoids from other parts of the Central European Variscides, where a thickened orogenic root caused a substantial rise in crustal temperatures, producing granitoid magmas closely correlated with regional tectonic activity between the Saxothuringian and Brunovistulia Terranes at the NE margin of the Bohemian Massif.     

French Creek Granite and Hohonu Dyke Swarm,South Island,New Zealand: Late Cretaceous alkaline magmatism and the opening of the Tasman Sea*

The Hohonu Dyke Swarm and French Creek Granite represent contemporaneous and cogenetic alkaline magmatism generated during crustal extension in the Western Province of New Zealand. The age of 82 Ma for French Creek Granite coincides with the oldest oceanic crust in the Tasman Sea and suggests emplacement during the separation of New Zealand and Australia. The French Creek Granite is a composite A‐type granitoid, dominated by a subsolvus biotite syenogranite with high silica, low CaO, MgO, Cr, Ni, V and Sr and elevated high‐field‐strength elements (Zr, Nb, Ga, Y). Subordinate varieties of French Creek Granite include a hypersolvus alkali amphibole monzogranite and a quartz‐alkali feldspar syenite. Spatially associated rhyolitic dykes are considered to represent hypabyssal equivalents of French Creek Granite. The Hohonu Dyke Swarm represents mafic magmatism which preceded, overlapped with, and followed emplacement of French Creek Granite. Lamprophyric and doleritic varieties dominate the swarm, with rare phonolite dykes also present. Geochemical compositions of French Creek Granite indicate it is an A₁‐subtype granitoid and suggest derivation by fractionation of a mantle‐derived melt with oceanic island basalt ‐ like characteristics. The hypothesis that the French Creek Granite represents fractionation of a Hohonu Dyke Swarm composition, or a mantle melt derived from the same source, is tested. Major‐ and trace‐element data are compatible with derivation of the French Creek Granite by fractionation of amphibole, clinopyroxene and plagioclase from mafic magmas, followed by fractionation of alkali and plagioclase feldspar at more felsic compositions. Although some variants of the French Creek Granite have Sr and Nd isotopic compositions overlapping those of the Hohonu Dyke Swarm, most of the French Creek Granite is more radiogenic than the Hohonu Dyke Swarm, indicating the involvement of a radiogenic crustal component. Assimilation‐fractional crystallisation modelling suggests isotopic compositions of French Creek Granite are consistent with extreme fractionation of Hohonu Dyke Swarm magmas with minor assimilation of the Greenland Group metasediments.     

Petrogenesis of Lingshan highly fractionated granites in the Southeast China: Implication for Nb-Ta mineralization

Whole rock major and trace element and Sr-, Nd- and Hf-isotope data, together with zircon U-Pb, Hf- and O-isotope data, are reported for the Nb-Ta ore bearing granites from the Lingshan pluton in the Southeastern China, in order to trace their petrogenesis and related Nb-Ta mineralization. The Lingshan pluton contains hornblende-bearing biotite granite in the core and biotite granite, albite granite and pegmatite at the rim. In addition, numerous mafic microgranular enclaves occur in the Lingshan granites. Zircon SIMS U-Pb dating gives consistent crystallization ages of ca. 132 Ma for the Lingshan granitoids and enclaves, consistent with the Nb-Ta mineralization age of ∼132 Ma, indicating that mafic and felsic magmatism and Nb-Ta mineralization are coeval. The biotite granites contain hornblende, and are metaluminous to weakly peraluminous, with high initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7071–0.7219, negative ε_Nd(t) value of −5.9 to −0.3, ε_Hf(t) values of −3.63 to −0.32 for whole rocks, high δ¹⁸O values and negative ε_Hf(t) values for zircons, and ancient Hf and Nd model ages of 1.41–0.95 Ga and 1.23–1.04 Ga, indicating that they are I-type granites and were derived from partial melting of ancient lower crustal materials. They have variable mineral components and geochemical features, corresponding extensive fractionation of hornblende, biotite and feldspar, with minor fractionation of apatite. Existence of mafic microgranular enclaves in the biotite granites suggests a magma mixing/mingling process for the origin of the Lingshan granitoids, and mantle-derived mafic magmas provided the heat for felsic magma generation. In contrast, the Nb-Ta mineralized albite granites and pegmatites have distinct mineral components and geochemical features, which show that they are highly-fractionated granites with extensive melt and F-rich fluid interaction in the generation of these rocks. The fluoride-rich fluids induce the enrichment in Nb and Ta in the highly evolved melts. Therefore, we conclude that the Nb-Ta mineralization is the result of hydrothermal process rather than crystal fractionation in the Lingshan pluton, which provides a case to identify magmatic and hydrothermal processes and evaluate their relative importance as ore-forming processes.