Quartz Porphyry Dykes Research Articles

Mineralogy of the Imalia Au-Sn-bearing polymetallic sulfide deposit, Mahakoshal belt, Central India

The Au-Sn-bearing, polymetallic sulfide deposit at Imalia in the western part of Mahakoshal belt, Central India is hosted by recrystallized dolostone and subsidiary phyllitic dolostone. These host rocks are transected by shallow level intrusion of quartz porphyry dykes. There are two major NS trending ore zones (∼1.5 km cumulative length) and a subsidiary-one, confined to fractures and shears in the dolostones or along its contact with the intrusive dykes. Polyphase mineralization include: irregularly disseminated pyrite crystals of diagenetic/metamorphic origin, patchy to stringery Pb-Zn sulfide ores showing pervasive metamorphic fabric, dominant massive vein type pyrite-arsenopyrite ores with significant amounts of invisible gold and tin. Other ore minerals include cassiterite, molybdenite, wolframite and roquesite in a magnetite-rich oxidic halo, and electrum, enargite/luzonite, wittichenite, ourayite, eclarite, aikinite and idaite in the sulfidic veins. Some tentatively identified ore minerals in these veins include calaverite, proustite, famatinite, sakuraiite, and tenorite. Pyrite contains structurally located invisible gold as high as 17.6 ppm whereas arsenopyrite contains a maximum of 20–24 ppm Au. Pyrite also contains unusually high Sn (upto 2673 ppm). The mineralogical assemblage of rare phases and their paragenesis in the Imalia vein type ores reflect the high sulfur (log fS2 = −9 to −13 bar) and oxygen (logfO2 = −32 bar) fugacities at a temperature range of 350 °C to 250 °C (according to thermometric calculations) during their emplacement.

Petrogenesis of low-δ18O quartz porphyry dykes, Koegel Fontein complex, South Africa

This paper investigates the origin of low-δ18O quartz porphyry dykes associated with the 144–133 Ma Koegel Fontein Igneous Complex, which was intruded during the initial phase of breakup of Africa and South America. The 25-km diameter Rietpoort Granite is the largest and youngest phase of activity, and is roofed by a 10-km diameter pendant of gneiss. Quartz porphyry (QP) dykes, up to 15 m in width, strike NW–SE across the complex. The QP dykes that intruded outside the granite have similar quartz phenocryst δ18O values (average 8.0‰, ± 0.7, n = 33) to the granite (average 8.3 ± 1.0, n = 7). The QP dykes that intruded the roof pendant have quartz phenocrysts with more variable δ18O values (average 1.6‰, ± 2.1, n = 55). In some cases quartz phenocrysts have δ18O values as low as − 2.5‰. The variation in δ18O value within the quartz crystal population of individual dykes is small relative to the overall range, and core and rim material from individual quartz phenocrysts in three samples are identical within error. There is no evidence that quartz phenocryst δ18O values have been affected by fluid–rock interaction. Based on a ∆quartz−magma value of 0.6‰, magma δ18O values must have been as low as − 3.1‰. Samples collected along the length of the two main QP dykes that traverse the roof pendant have quartz phenocryst δ18O values that range from + 1.1 to + 4.6‰, and − 2.3 to + 5.6‰, respectively. These δ18O values correlate negatively (r = − 0.96) with initial 87Sr/86Sr, which can be explained by the event that lowered δ18O values of the source being older than the dykes. We suggest that the QP dykes were fed by magma produced by partial melting of gneiss, which had been variably altered at high temperature by 18O-depleted meteoric water during global glaciation at ~ 550 Ma. The early melts had variable δ18O value but as melt pockets interconnected during melting, the δ18O values approached that of average gneiss. Variable quartz phenocryst δ18O values in the same dyke can be explained by vertical emplacement, at variable rates of ascent along the dyke. The lateral variation in quartz, and hence magma δ18O value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke, and the ‘age’ of the particular magma batch.

New rock magnetic and paleomagnetic results for the 1.64 Ga Suomenniemi dyke swarm, SE Finland

This study represents new petrophysical and paleomagnetic data obtained for the 1.64 Ga Suomenniemi diabase and quartz porphyry dyke swarm associated with Suomenniemi rapakivi granite. We also present petrophysical data for the coeval dykes of the Sipoo (1.63 Ga) and Häme (1.642 ± 0.002 Ga; 1.647 ± 0.014 Ga) swarms in order to address: (1) the discrepancy between the coeval Sipoo and Häme poles and (2) the proposed severe overprint in paleomagnetic data for rapakivi related dyke swarms in Fennoscandia.Demagnetization of Suomenniemi dyke samples revealed dominantly normal polarity magnetization carried by magnetite. For the first time, a few samples from the few sites have shown a trace of a reversed magnetization. The combined normal (N) polarity direction from nine dykes (=sites) is at D = 031.2°, I = 07.2°, (with α95 = 10.6°, k = 21.8) yielding a paleomagnetic pole at Plat = 27.8°N; Plong = 171.7°E (with A95 = 7.9°, K = 38.4). The Suomenniemi pole fulfils four of the seven quality criteria for paleomagnetic poles (Van der Voo, 1990). The new pole for the Suomenniemi swarm is similar to the coeval Sipoo swarm pole, but distinctly different from the nearly coeval Häme swarm pole. The relative position of the poles compared to other well-defined paleomagnetic poles for Baltica indicates that the Suomenniemi and the Sipoo poles are slightly younger than the Häme pole.We show here that the original magnetization in some of the 1.64 Ga dykes in Suomenniemi have been overprinted with a secondary magnetization component, but several dykes of the swarm lack this overprint. Moreover, the presumably primary Subjotnian remanence component for the Suomenniemi dykes can be clearly isolated due to its different coercivity and/or unblocking temperature from the overprinted component. The same applies to the Sipoo and Häme dyke swarms.

Geochemical characteristics of biotite from felsic intrusive rocks around the Sisson Brook W–Mo–Cu deposit, west-central New Brunswick: An indicator of halogen and oxygen fugacity of magmatic systems

The Sisson Brook W–Mo–Cu deposit was formed by hydrothermal fluids likely related to the Nashwaak Granites (muscovite–biotite granite, Group I; and biotite granite, Group II) and related dykes (biotite granitic dykes, Group III; and a feldspar–biotite–quartz porphyry dyke, Group IV). Chemical data obtained using EPMA and LA-ICP-MS data of primary magmatic biotites were used to investigate magmatic processes and associated hydrothermal fluids.Trace element features of biotite in the Group I two-mica granite suggest other magmatic processes along with a simple fractional crystallization. The K/Rb ratios and compatible elements (Cr, Ti, Co, V, and Ba) in biotite from Groups II, III, and IV decrease, whereas incompatible elements including Ta, Tl, Ga, Cs, Li, and Sn increase with magma fractionation. No correlation of Cu, W and Mo with K/Rb ratios is evident, suggesting that partitioning of Cu, W, and Mo into biotite may not be entirely controlled by magma fractionation.Halogen fugacity of the parental magma of the Nashwaak Granites and related dykes, calculated from zircon saturation temperature shows that Group I has high fHF/fCl ratios (broadly higher than 0), similar to the plutons at the Henderson porphyry Mo deposit. The fHF/fCl ratios of the other groups are generally lower than 0, comparable to the Santa Rita porphyry Cu deposit. The fH2O/fHCl and fH2O/fHF ratios inferred from biotite in the Nashwaak Granites and related dykes range from 3 to 5 and from 4 to 5, respectively. The inferred oxygen fugacity shows that the dyke phases (Groups III and IV) have the oxygen fugacity around the nickel–nickel oxide buffer. The plutonic phases (Groups I and II) have the oxygen fugacity around the quartz–fayalite–magnetite (QFM) buffer at high temperatures and oxidized to nickel–nickel oxide buffer at lower temperatures. This oxidation process in the plutonic phases (Groups I and II) could be caused by H2 release at or near H2O vapor saturation at high H2O/Fe2+. The magma associated with the biotite dykes (Group III) is more likely the source of the hydrothermal fluids at the Sisson Brook deposit since it has the highest differentiation degree and seems to have formed in an oxidized setting, necessary for Mo to concentrate in the late stage fluids.

Oxygen Isotope Diversity in the Anorogenic Koegel Fontein Complex of South Africa: a Case for Basement Control and Selective Melting for the Production of Low- 18O Magmas

Koegel Fontein, about 350 km north of Cape Town, is the only known early Cretaceous anorogenic igneous complex along the volcanic rifted margin of South Africa. The oldest rocks of the complex are minor granite and syenite intrusions at 144 Ma, which were followed by tholeiitic and alkaline basalt dykes, then by microsyenite and quartz porphyry dykes. The youngest and largest igneous unit is the 135 Ma Rietpoort Granite, with an exposed diameter of about 20 km. The country rocks are Mesoproterozoic gneisses of the Namaqua–Natal Province, which in many places were deformed and retrogressed by Pan-African tectonism. Whole-rock δ18O values from the Rietpoort Granite and smaller plutonic units (syenite, granite) are in the range 6–9‰ (outliers 4‰ and 17‰). Quartz δ18O values from all units are in a narrow range and indicate magma δ18O values between 6 and 8‰. In contrast to the syenites and granites, most mafic and silicic dyke units have δ18O

GEOCHEMISTRY OF THE EARLY CRETACEOUS KOEGEL FONTEIN ANOROGENIC IGNEOUS COMPLEX, SOUTH AFRICA

The Koegel Fontein Igneous Complex in the Western Cape Province of South Africa is a high-level granite-syenite intrusive suite with minor plugs and dykes of felsic and mafic composition. Age dating and field relations show that the felsic rocks intruded as two sequential series, beginning with syenites and the Kerskloof suite of felsic dykes (andesite-trachyandesite, trachyte-rhyolite) at about 144 Ma, followed by the Rietpoort granite pluton and quartz-porphyry dykes at about 134 Ma. The intrusion of the mafic dykes and plugs took place between the two felsic series. The early Cretaceous age of the Koegel Fontein associates its formation with Gondwana breakup and establishment of the volcanic rifted margin in southwest Africa. This paper reports results from the first geochemical study of the Koegel Fontein Complex. Two groups of mafic units are distinguished based on mineralogy, field characteristics and chemical composition. The tholeiitic group comprises basalt and basaltic andesites, the alkaline group ranges from basanite-tephrite to phono-tephrite and basaltic trachy-andesite. Distinctions of incompatible element concentrations (K, P, Ti) and ratios (Y/Nb, Zr/Nb) indicate the presence of two separate magma series. Regional dolerite dykes (Gariep dykes and others) associated with contemporary magmatism along the continental margin occur in and near the complex. These regional dolerites are much more homogeneous compositionally than the mafic dykes of Koegel Fontein. Compositionally, they closely resemble the less-differentiated tholeiite dykes. The felsic units at Koegel Fontein show geochemical and isotopic evidence for two magma series, which correspond to the two intrusive episodes suggested by geochronology and field relations. The first series comprises the volumetrically minor intrusions of syenites, the Kerskloof dykes and the Rooivleitjie granite, with high values (ɛNd(t) from −0.4 to −1.8) indicating a strong mantle input. Similarities of trace element signatures suggest an affinity with the alkaline mafic units. The second series of felsic magmas makes up the bulk of the Koegel Fontein complex, with the large Rietpoort granite pluton and quartz-porphyry dykes (ɛNd(t) = −4.8 to −6.9). These rocks may be related to the tholeiitic mafic series but their isotopic composition indicates a large crustal component. The granitic rocks at Koegel Fontein have geochemical characteristics regarded as typical for anorogenic (A-type) granites, including high contents of Y, Nb, U and Th. In this and in many other features, the Koegel Fontein complex resembles the contemporary intrusive complexes in the Damaraland of northwest Namibia. Like the Damaraland granites, the Koegel Fontein magmas are of mixed origin, and their isotopic composition reflect the proportion of mantle vs. crustal source contributions.

Geochemical investigation on a quartz porphyry dyke of the Kamioka Mine, central Japan.

Geochemical investigation on a quartz porphyry dyke of the Kamioka Mine, central Japan.

Interrelations between coeval mafic and A-type silicic magmas from composite dykes in a bimodal suite of southern Israel, northernmost Arabian–Nubian Shield: Geochemical and isotope constraints

Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ 18O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648. The initial 143Nd/ 144Nd ratios, expressed by ε Nd( T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [ δ 18O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of 18O/ 16O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ 18O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ 18O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally.

Single mineral Rb-Sr isochron dating applied to the Nohi Rhyolite and a quartz porphyry dyke, central Japan

Conventional (multi-)mineral Rb-Sr isochron method requires several mineral species. Not all rock-forming minerals, however, provide the same chronological information. The closure temperature and the extent of weathering differ from mineral to mineral. On the other hand, the whole-rock isochron method has ambiguities on the initial Sr isotope equilibrium. In order to eliminate these complications and to date individual events related to each minerals in a given rock sample accurately, single mineral Rb-Sr isochron method was developed and applied to two different types of Cretaceous volcanic rocks in central Japan, Agigawa welded tuff sheet (Nohi Rhyolite Complex) and quartz porphyry (a dyke intruded into the Toki granite). Precise drilling was carried out on single phenocrystic grains of K-feldspar and chlorite (latter from quartz porphyry only) in order to obtain several aliquots from a single mineral grain. The internal difference of parent/daughter isotope ratios is expected fortuitously. Rb-Sr multi-mineral isochron method failed to yield a precise age from the Agigawa welded tuff sheet, probably because of partial alteration. A single grain K-feldspar age of 72.0 ± 4.1 Ma obtained from the same sample is likely to date annealing of the K-feldspar grain by the intrusion of the Toki granite. In the case of the quartz porphyry dyke, three K-feldspar grains gave the ages of 79.9 ± 6.8 Ma, 71 ± 14 Ma and 82 ± 18 Ma. Combined K-feldspar age for all separates from the three grains is 77.0 ± 4.6 Ma, probably corresponding to the intrusion of the dyke. On the other hand, a combined single mineral isochron age of chlorites (five grains) in the same quartz porphyry dyke is 54.5 ± 4.6 Ma. It seems to date the chlorite formation, which corresponds to the subsequent hydrothermal alteration. Single mineral isochron method proved its effectiveness in the case when Rb-Sr isotope system was partially disturbed between different mineral phases.

Geology and Geochemistry of the Pre-early Cambrian Rocks in the Sandikli Area: Implications for the Pan-African Evolution of NW Gondwanaland

The pre-Early Cambrian Sandikli Basement Complex in western Central Anatolia comprises a low-grade meta-sedimentary succession (Güvercinoluk Formation) and meta-rhyolites intruded by meta-quartz porphyry rocks (Kestel Cayi Porphyroid Suite). The Güvercinoluk Formation consists of alternation of meta-siltstones and meta-sandstones with olistostromal conglomerates, rare black chert and cherty meta-dolomite lenses. The Kestel Cayi Porphyroid Suite is a deformed, highly sheared dome-shaped rhyolitic body with quartz porphyry rocks. Quartz porphyry dykes intrude both the volcanic carapace and the meta-sedimentary rocks of the Güvercinoluk Formation. Both the meta-quartz porphyry rocks and meta-rhyolites are typically mylonitic with relict igneous textures. Geochemical data indicate that the felsic rocks of the Kestel Cayi Porphyroid Suite are subalkaline and display characteristic features of post-collisional, I-type granitoids. The basement complex is unconformably overlain by variegated conglomerates, mudstones and arkosic sandstones with andesitic lavas, followed by siliciclastic rocks and carbonates that yielded Early Middle Cambrian fossils. Based on the geochemical characteristics of the felsic rocks of Kestel Cayi Porphyroid Suite and the depositional features of the associated sediments it is suggested that the Sandikli Basement Complex is related to a post-collisional extension event in NW Gondwanaland. Similar occurrences elsewhere have been related to a transition from continental plate convergence to continental plate divergence along the Pan-African Belt.

An oxygen and carbon isotopic study of multiple episodes of fluid flow in the Dalradian and Highland Border Complex, Stonehaven, Scotland

The carbon and oxygen systematics of rocks north of Stonehaven, Scotland were studied to provide new constraints on the nature and timing of metamorphic and post‐metamorphic fluid infiltration. Carbon and oxygen isotopic data were collected from carbonated spilites and carbonate‐bearing veins in the Highland Boundary Fault, from Dalradian metacarbonate layers and pervasively carbonated schists, and from three generations of carbonate‐bearing veins, and a quartz porphyry dyke within the Dalradian. Isotopic evidence for syn‐metamorphic fluid infiltration in the Dalradian is preserved in quartz–calcite veins in the upper chlorite zone and in staurolite zone metacarbonates. δ 18 O values for coexisting quartz (δ 18 O quartz = 13.6‰) and calcite (δ 18 O calcite = 11.6‰) in a vein from the upper chlorite zone are consistent with precipitation of both minerals from a single fluid at c. 375°C. Oxygen isotopic data from staurolite zone metacarbonate layers (δ 18 O calcite = 10.4 to 12.9‰) suggest major isotopic exchange with an external fluid equilibrated with a quartzo‐feldspathic reservoir, such as the Dalradian metasediments, at metamorphic temperatures. Low, negative carbon isotope data from these metacarbonate layers (δ 13 C calcite = −12.4 to −17.7‰) may indicate a component of oxidized organic material in the infiltrating fluid. Post‐metamorphic fluids derived from or equilibrated with a high‐temperature silicate reservoir such as felsic igneous rocks were responsible for precipitation of post‐metamorphic vein carbonate, carbonation of pelitic schists, and alteration of metamorphic feldspar porphyroblasts from Barrow’s famous spotted chloritoid schists in an area geographically associated with several carbonated quartz porphyry dykes. Some veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow. Quartz from this generation of veins preserves a metamorphic isotopic signature (δ 18 O quartz = 11.2 to 13.2‰) whereas dolomite and calcite are considered to be post‐metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic disequilibrium between vein quartz and carbonate minerals (δ 18 O dolomite = 23.7 to 24.2‰, δ 18 O calcite = 25.3‰, δ 18 O quartz = 11.2 to 13.2‰). Variably depleted carbon isotopes from carbonate minerals in these veins (δ 13 C dolomite = −8.1 to −13.4‰, δ 13 C calcite = −12.2‰) may reflect input of oxidized organic material in the mineralizing fluid. Isotopic data from carbonate veins and variably carbonated spilites from the Highland Boundary Fault are consistent with hydrothermal alteration of basalts by seawater, which had its carbon isotope systematics modified by addition of magmatic CO 2 and/or oxidation of organic carbon. Such alteration may have occurred on the seafloor or during emplacement in the Highland Boundary Fault.

Palaeomagnetic and rock magnetic investigations of the Sipoo Subjotnian quartz porphyry and diabase dykes, southern Fennoscandia

The Sipoo quartz porphyry dyke swarm (UPb zircon age 1.63 Ga) in southern Finland carries an intermediate to high coercivity remanent magnetization A N ( D = 22.2°, I = −1.8°, k = 34.2, α95 = 13.3°, n = 5 dykes) which is regarded as primary. The Sipoo diabase dykes, emplaced slightly before the granite porphyries, yield a highly scattered antiparallel direction A R ( D = 198.0°, I = −6.5°, k = 6.2, α95 = 40.1°, n = 4 dykes) isolated in intermediate to high coercivities. Based on positive contact test results, remanence A R is primary in the diabases. The best preserved diabase dyke, as determined by petrographic and bulk magnetization properties, carries both the A R magnetization and the A N magnetization as a secondary component in some samples. The most altered diabase dyke is almost completely overprinted by A N . The overprinting has taken place as a result of hydrothermal alteration and formation of new magnetite during the emplacement of quartz porphyry dykes and rapakivi granite. The quartz porphyries and diabases carry mixed SD and MD magnetic grain sizes, the large MD grains being responsible for a relatively large Present Earth's Field (PEF) magnetization. In addition, a high coercivity magnetization B ( D = 23.9°, I = 52.2°, k = 215.8, α95 = 6.3°, n = 4 dykes), presumably a chemical overprint, has been isolated in the quartz porphyries. The B magnetization is interpreted to be either an intermediate direction between A N and PEF with no geological significance, or to represent a separate reactivation of the crust at about 1.32 Ga ago.

Migration of Na, K, Ba and Sr in a quartz porphyry dyke of the Kamioka Mine, Japan, due to liquid percolation through rock.

A quartz porphyry dyke in the Kamioka Pb-Zn-Mo mining district shows large variations of Na, K, Ba and Sr concentrations with sodium negatively correlating to other three elements. These compositional changes are well explained by a model of liquid percolation through the rock, for which a mathematical formula was described by Haskin (1984). The liquid percolation brought about addition of K, Ba and Sr to the rock by compensating Na. Estimated composition for the liquid has K2O/Ba = 47 and Sr/Ba = 0.14. It is also indicated that DNa is in inverse proportion to the concentrations of K, Ba or Sr in the liquid.

Early Proterozoic Svecofennian volcanism and associated plutonism in Enklinge, SW Finland

Abstract The c. 1.9 Ga old Enklinge volcano-plutonic centre is part of the Svecofennian province of SW Finland. The rocks of the area consist of an early subaquatic extrusion of mafic/intermediate to rhyolitic volcanics underlain by marbles, graywackes and quartz-feldspar-rich volcanoclastic sediments. The volcanic sequence is deformed and syntectonically intruded by a biotite tonalite accompanied and mixed with coeval basaltic dykes. Dacitic quartz porphyry dykes and small breccia pipes occur along the margins of the tonalite intrusion. Contrary to the Bergslagen area in Sweden no large co-magmatic granites can be found in the volcano-plutonic complex and the rhyolites are much less voluminous and only slightly mineralized. The rocks of the Enklinge area are strongly deformed and the tectonic environment differs strongly from that of the almost nonorogenic and more terrestrial Bergslagen area W. of Enklinge.

Palaeomagnetism and age of the quartz-porphyry intrusions, Isle of Arran

Synopsis Most of the quartz-porphyry intrusions of Arran, many of which form the acid member of composite intrusions, have been sampled palaeomagnetically. Though both normal and reversed polarity may be found in a single intrusion it is concluded that the reversed polarities are due to overprinting by later reversely magnetised intrusions and that all the quartz-porphyries were intruded while the magnetic field had normal polarity. The age of the Drumadoon quartz-porphyry dyke and, by inference, that of all the other quartz porphyry intrusions, is 58.5 ± 0.8 (lσ) Ma. It is suggested that normal polarity may provide a stratigraphic ‘horizon’ useful for elucidating the poorly-known succession of Arran Tertiary igenous rocks, most of which are reversely magnetised. This work forms part of a comprehensive study, using palaeomagnetic polarity and radiometric dating, of the timing and duration of activity in the British Tertiary Igneous Province, and the age quoted above lies in the period of peak activity. This is the first time that a correlation of polarity with a distinctive petrographic rock type has been found.

Chronology of granite magmatism and associated mineralization, SW England

An Rb-Sr study of Hercynian magmatism in SW England has established that the major granites were emplaced c. 290–280 Ma and that the inferred sequence of intrusion shows no geographic pattern. High initial 87 Sr/ 86 Sr ratios (0.710–0.716) are compatible with the ‘S–type’ and peraluminous mineralogical characteristics of the granites. REE distributions are consistent with those for normal leucogranites, in that they have a high LREE/HREE ratio and negative europium anomaly. However, the REE profiles for Bodmin and Carnmenellis contrast markedly with those of Land’s End and Dartmoor, implying differences both in the chemistry of the source region and in conditions of crystallization. Ages obtained for the Brannel and Wherry elvans (quartz porphyry dykes) suggest more than one episode of injection during the interval c. 280–270 Ma. An independent fluid inclusion Rb-Sr age for the South Crofty Sn–W deposit (269 ± 4 Ma) has shown that in the Camborne-Redruth area, the main stage Polymetallic mineralization is significantly younger than the Carnmenellis host granite. There is little evidence to support the idea of an older Westphalian event, and thus in the broader European Hercynian context the granites show greatest affinity with the younger Sn–W granites of Portugal.

Petrogenesis of Quartz Porphyry Dykes in South-West England

It has long been recognized that elvan (quartz porphyry) dykes in Cornwall and Devon are late-stage differentiates of the Permo–Carboniferous granite batholith of that region, but until recently little attention has been paid to their mode of formation.