The palaeomagnetism and geology of the Patreksfjördur-Arnarfjördur region of Northwest Iceland

The headlands between the fjords Arnarfjördur and Patreksfjördur in northwest Iceland, consisting mostly of Upper Tertiary plateau basalt lavas, have been geologically mapped in detail. Magnetic properties of samples from some 50 lava flows and dykes in the region have been measured; the mean palaeomagnetic pole position obtained from forty normally magnetized lavas is at 88°N113°E. The application of the present work to stratigraphic correlation and magnetic anomaly interpretation in northwest Iceland is discussed.     

Stratigraphy and paleomagnetism of a 3-km-thick Miocene lava pile in the Mjoifjördur area,eastern Iceland

We have carried out stratigraphic mapping in the Upper Miocene basalt lava pile around the fjords Mjoifjördur and Seydisfjördur, eastern Iceland. The mapping is based on conventional methods including the use of interbasaltic clastic horizons and petrographically distinct lava groups. These units are also used to provide correlations with the Nordfjördur area south of Mjoifjördur. We present a 3-km composite stratigraphic column for the area between Mjoifjördur and Seydisfjördur. The geology of this area shows some differences from the classical model of Walker for the structure of eastern Iceland partly due to the fact that most of Mjoifjördur is not in the vicinity of central volcanoes. Detailed laboratory measurements of remanent magnetization were carried out on oriented core samples from 363 lavas in 10 selected profiles. The local paleomagnetic polarity stratigraphy supports correlations made on the basis of other criteria. Over 20 geomagnetic reversals are recorded in the eastern Iceland lava pile in a period approximately 13-10 Ma ago. The geomagnetic field during this period averages to a central axial dipole field, and its overall statistical properties resemble those obtained in earlier surveys in Iceland.     

Form and dimensions of dykes in eastern Iceland

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

Field characteristics of laterally emplaced dikes: Anatomy of an exhumed Miocene dike swarm in Reydarfjördur, eastern Iceland

Many dikes of the Tertiary Breiddalur dike swarm in Reydarfjördur have a great lateral extent but are vertically (upward and/or downward) discontinuous. Field relationships indicate that they are not the feeders of the lava pile they intrude. Their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading Krafla volcanic center of the neovolcanic zone. Average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other Tertiary dike swarms in eastern Iceland. It is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures.     

Longitudinal Petrochemical Variation in the Mackenzie Dyke Swarm, Northwestern Canadian Shield

Previous magnetic fabric studies of the giant, radiating, 1.27Ga Mackenzie dyke swarm concluded that flow patterns withinthe dykes support the concept of a mantle plume that is centredbeneath the swarm focus and supplies magma to overlying floodbasalts and developing radial dykes. To examine petrochemicalimplications of the model, compositional variation within thebasalt sequence is compared with that of the dykes along a ‘streamline’ of the swarm between 400 km (just beneath the lavas)and 2100 km from its focus and in a parallel segment farthereast. Evolution of tholeiitic magmas of the main sampled streamis recorded in the upward change of composition in the lavasequence from mg-numbers of 70 to 35. Underlying (feeder) dykeshave a comparable range, but outward along the swarm the rangeof compositions narrows progressively towards its more evolvedend, and at 2100 km, dyke compositions match those in upperlevels of the lava sequence. REE and other trace element abundancesshow a similar contraction in range, and a shift towards moreevolved compositions, both upward in the lava sequence and outwardalong the swarm. Apart from complications owing to crustal contamination,fundamental attributes of the magma (e.g. Zr/Y) change littlewith stratigraphic level or distance from the focus. The moreeasterly stream differs in its high proportion of alkalic compositions,suggesting the existence of distinctive subswarms. Normativemineral variation plots are consistent with fractionation inhigh- (main stream) to low-level (eastern stream) crustal magmachambers. Mackenzie magmatism is compatible with the plume model.Domal uplift related to plume activity initiated central grabencollapse and outward-extending radial fractures, thus providingaccess to plume-derived magmas and loci for magma chamber anddyke swarm development. Multiple magma chambers, forming aroundthe apex, each fed relatively independent subswarms of dykes.Uplift, accompanying fractionation, provided increasing magmatichead by which fractionating magmas could be dispatched to successivelygreater distances. This, and crystal settling in transport,accounts for the increasingly evolved nature of dykes with distancefrom the source. KEY WORDS: mantle plume; flood basalts; dyke swarm; petrochemistry; Precambrian ^*Corresponding author.     

Feeder and post Deccan Trap dyke activities in the northern slope of the Satpura Mountain: Evidence from new 40Ar-39Ar ages

We present new 40Ar-39 Ar plagioclase crystallization ages from the dykes exposed at the northern slope of the Satpura Mountain range near Betul-Jabalpur-Pachmarhi area,~800 km NE of the Western Ghats escarpment.Among the two plateau ages,the first age of 66.56±0.42 Ma from a dyke near Mohpani village represents its crystallization age which is either slightly older or contemporaneous with the nearby Mandla lava flows(63-65 Ma).We suggest that the Mohpani dyke might be one of the feeders for the surrounding lava flows as these lavas are significantly younger than the majority of the main Deccan lavas of the Western Ghats(66.38-65.54 Ma).The second age of 56.95±1.08 Ma comes from a younger dyke near Olini village which cuts across the lava flows of the area.The age correlates well with the Mandla lavas which are chemically similar to the uppermost Poladpur,Ambenali and Mahabaleshwar Formation lavas of SW Deccan.Our study shows that the dyke activities occurred in two phases,with the second one representing the terminal stage.     

Anisotropy of magnetic susceptibility and rock magnetic applications in the Deccan volcanic province based on some case studies

Anisotropy of Magnetic Susceptibility (AMS) as a tool has been explored here to investigate the nature of petrofabrics in Deccan Volcanic Province (DVP) of west-central Indian region by representative sampling in typical pahoehoe and rubbly pahoehoe lava flows, dykes within flows, shear zone and the impact crater units. The rock magnetic analysis indicate varying degree of concentration of titanomagnetite compositions dominated by multi domain (MD) to pseudo single domain (PSD) grains favoring shape anisotropy of minerals that form primary fabrics. The pahoehoe type lava flows shows planar oblate fabrics without any preferred orientation of principle susceptibility axis (K₁) depicting crystal settling (of magnetic grains) as chief mechanism of fabric development. The rubbly pahoehoe type lava flow exhibit prolate fabrics with well clustered maximum susceptibility axis within horizontal to sub-horizontal planes depicting their response to viscosity shear. The dykes show well clustered K₁ parallel to it’s plane locked during rapid contractional cooling. The sampling at Lonar impact crater was unable to trace any clear fabric due to impact/shock induced deformation and rather preserve the primary fabrics. Further, the shear zone depict random fabrics demanding more detailed and systematic sampling in both the cases. The present investigation infer that the magnetic mineralogy and magnetic fabric variations in the DVP are controlled by the flow mechanism and style of cooling that is characteristic of the given flow unit or dyke and any secondary or superimposed fabric needs to be examined by critical sampling strategy. While more detailed attempts are required to establish the AMS as a tool to record various aspects including the flow dynamics and rate of effusion in the vast terrain of DVP; the present approach is useful to characterize and correlate the lava flow units and dyke occurrences.     

Multi-stage zeolite facies mineralization in the Hvalfjördur area,Iceland

The Hvalfjördur area, 30 km north of Iceland’s capital Reykjavik, belongs to the sequence of Late Tertiary to early Quaternary flood basalts with minor intercalations of hyaloclastites and rhyolites. The basalts are affected by progressive low-temperature metamorphism, caused by the burial of the lava succession and higher heat flow from nearby central volcanoes. Low-grade zeolite facies metamorphism of basaltic lavas in the Hvalfjördur area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile. Stage Ia represents syn-eruptive near-surface alteration in which celadonite and silica were precipitated along primary pores. During regional burial metamorphism (stage Ib), hydrolysis of olivine and glass led to the formation of mixed-layer chlorite/smectite clays. The chlorite content of stage Ib phyllosilicate vesicle rims increases with increasing burial depth and temperature. Stage II occurred after the burial and is marked by zeolite mineralization caused by higher heat flow from the Laxárvogur and Hvalfjördur central volcanoes. Altogether 11 different zeolites were found in the Hvalfjördur area: analcime, chabazite, epistilbite, heulandite, laumontite, levyne, mesolite, stilbite, stellerite, thomsonite and yugawaralite. In total, three separate depth and temperature-controlled “zeolite zones” occur in the Hvalfjördur area.     

Lateglacial oceanographic conditions off Southwest Iceland inferred from shallow-marine deposits in Reykjavík and Seltjarnarnes Peninsula

New palaeoenvironmental data from the Lateglacial in Southwest Iceland add to the record of climatic events during deglaciation of the region. Recently exposed sediments on the north coast of Seltjarnarnes Peninsula in the Reykjavik area, Southwest Iceland, contain evidence of marine deposition during the Bølling Interstadial. The glaciomarine sediments contain both slightly reworked marine macrofossils and microfossils indicating normal marine salinity and subarctic climate conditions. Previous sedimentological studies and radiocarbon dating of the sporadic sediments covering the lava bedrock in Reykjavik have revealed lateglacial marine units from the Allerød, the Allerød—Younger Dryas transition, the Younger Dryas and from the Preboreal. Until now, the only Bølling evidence has consisted of scattered radiocarbon-dated redeposited shell fragments. From the Bollagardar deposits we report the first faunas dated to the Bølling chronozone preserved in marine sediments in the Reykjavik area. Recently published work in Hvalfjördur and Borgarfjördur, West Iceland, has shown that sea level was relatively high during the Bølling and that deglaciation was rapid. Bølling, Allerød and Younger Dryas deposits in the coastal areas of the Reykjavík region accumulated in a relatively open marine environment in oceanographic conditions similar to the present ones. Combined previous and present results indicate that several episodes of glaciomarine deposition occurred.     

Rift-zone development around a plume-related magma centre on the Isle of Skye (Scotland): a model for stress inversions

ABSTRACT Using the example of the Isle of Skye in Scotland, we investigate the influence of pressure variations in upper-crustal magma reservoirs on the development of rift-type normal faulting around central volcanoes. The regional synmagmatic stress regime is of strike-slip type in Scotland during the Lower Tertiary. During a prolonged period of overall high pressure in the Skye magma reservoir (gabbro intrusion stage), crustal extension results from the injection of basaltic dykes parallel to the trend of the far-field maximum stress. During a subsequent period of pressure decrease in the reservoir (granites intrusion stage) normal faults trending parallel to the dykes are initiated. These faults tilt the upper-crustal blocks along with the former dyke swarm and associated lava pile. Finite-element modelling shows that a decrease of magma pressure in a circular cavity may lead, as in Skye, to a change from a regional strike-slip to a local rift-type normal stress regime.     

An Isotope and Trace Element Study of the East Greenland Tertiary Dyke Swarm: Constraints on Temporal and Spatial Evolution during Continental Rifting

Dykes of the East Greenland Tertiary dyke swarm can be dividedinto pre- and syn-break-up tholeiitic dykes, and post-break-uptransitional dykes. Of the pre- and syn-break-up dykes, themost abundant group (Tholeiitic Series; TS) has major elementcompositions similar to the main part of the East Greenlandflood basalts. A group of high-MgO tholeiitic dykes (Picrite–AnkaramiteSeries; PAS) are much less common, and are equivalent to someof the oldest lavas of the East Greenland flood basalts. Isotopiccompositions of the TS and PAS dykes partly overlap with thosefor Iceland, but Pb isotopic compositions extend to less radiogenicvalues than those seen in either Iceland or North Atlantic mid-oceanridge basalt (MORB). The isotopically depleted source requiredto account for this isotopic variation is interpreted as subcontinentallithospheric mantle with low ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb and high     

Precambrian mafic magmatism in the Bastar craton,central India

The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i) three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature, (iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite, metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB). Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper. Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline) dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide.     

Stratigraphy and paleomagnetism of a 2.9-km composite lava section in Eyjafjördur,Northern Iceland: a reconnaissance study

Eight mountainside profiles in lava flows south of the fjord Eyjafjördur, Northern Iceland, were sampled for paleomagnetic studies. The sampling was concurrent with pilot stratigraphic mapping of the lava sequences in these and several supplementary profiles. The eight profiles are correlated with minor overlaps so that they form a composite section of 2.9 km thickness, estimated to cover the age range between about 9 and 5 million years ago (Hardarson et al. 1999). Paleomagnetic measurements made on 319 lavas generally yield primary remanence directions of high stability and within-unit consistency. Evidence for at least 17 reversals of the geomagnetic field is seen, as well as numerous field excursions. Frequent clustering of directions in successive lavas indicates that the volcanism was episodic.     

The Okavango giant mafic dyke swarm (NE Botswana): its structural significance within the Karoo Large Igneous Province

The structural organization of a giant mafic dyke swarm, the Okavango complex, in the northern Karoo Large Igneous Province (LIP) of NE Botswana is detailed. This N110°E-oriented dyke swarm extends for 1500 km with a maximum width of 100 km through Archaean basement terranes and Permo-Jurassic sedimentary sequences. The cornerstone of the study is the quantitative analysis of N>170 (exposed) and N>420 (detected by ground magnetics) dykes evidenced on a ca. 80-km-long section lying in crystalline host-rocks, at high-angle to the densest zone of the swarm (Shashe area). Individual dykes are generally sub-vertical and parallel to the entire swarm. Statistical analysis of width data indicates anomalous dyke frequency (few data <5.0 m) and mean dyke thickness (high value of 17 m) with respect to values classically obtained from other giant swarms. Variations of mean dyke thicknesses from 17 (N110°E swarm) to 27 m (adjoining and coeval N70°E giant swarm) are assigned to the conditions hosting fracture networks dilated as either shear or pure extensional structures, respectively, in response to an inferred NNW–SSE extension. Both fracture patterns are regarded as inherited brittle basement fabrics associated with a previous (Proterozoic) dyking event. The Okavango N110°E dyke swarm is thus a polyphase intrusive system in which total dilation caused by Karoo dykes (estimated frequency of 87%) is 12.2% (6315 m of cumulative dyke width) throughout the 52-km-long projected Shashe section. Assuming that Karoo mafic dyke swarms in NE Botswana follow inherited Proterozoic fractures, as similarly applied for most of the nearly synchronous giant dyke complexes converging towards the Nuanetsi area, leads us to consider that the resulting triple junction-like dyke/fracture pattern is not a definitive proof for a deep mantle plume in the Karoo LIP.     

A geological excursion in the Wadi Fatima area, near Jiddah, Saudi Arabia

Wadi Fatima, east of jiddah on the Red Sea coast of Saudi Arabia, is a microcosm of the geology of the jiddah area. Rocks ranging in age from 800-million-year-old metamorphic rocks to Tertiary lava flows are exposed, and illustrate the geological richness of western Saudi Arabia. The valley is a major south-west-trending fault zone that has been active since Precambrian times. A major Tertiary dyke swarm in the vicinity of Wadi Fatima is related to the opening of the Red Sea as the African-Arabian landmass split apart.     

Palaeomagnetism of Pliocene-Pleistocene sediments and lava flows on Tjörnes and Flatey, North Iceland

Palaeomagnetic measurements have been carried out on lava flows and volcaniclastic sedimentary rocks of Upper Pliocene and Pleistocene age from sections on the Tjömes Peninsula and a core section from the island of Flatey off North Iceland. Earlier palaeomagnetic work in the area is reviewed, and new data on the stratigraphy of the Hoskuldsvik Group on Tjörnes are presented. A revised polarity column based on our resultsisset upfortheoutcropson Tjörnes, including twoadditionalnormalsubchronswithin the Matuyama. Our palaeomagnetic results indicate that the rocks penetrated by the Flatey drillhole accumulated during the Matuyama. Age estimates for the Upper Tjömes sequence are reviewed, and correlation alternatives indicate that the earliest glaciation of Tjörnes took place either at c.2 Ma or near the Gauss/Matuyama boundary at c. 2.5 Ma. Correlation of individual Matuyama subchrons remains uncertain. A comparison of the climatic evidence from the Tjörnes area with data on oxygen isotope and carbonate variations at DSDP site 552A is used to evaluate correlation alternatives.     

The Vetreny Poyas (Windy Belt) subprovince of southeastern Fennoscandia: An essential component of the ca. 2.5–2.4 Ga Sumian large igneous provinces

Ca. 2.5–2.4 Ga Sumian magmatism is widespread in the Karelia and Kola cratons of Fennoscandia and probably represents at least two intermixed large igneous provinces (LIPs). It is distinct from other Paleoproterozoic LIPs (Jatulian 2.22–2.1 Ga and Ludicovian 2.06–1.96 Ga) elsewhere in the Fennoscandian Shield. A poorly understood portion of Sumian magmatism is the Vetreny Poyas (Windy Belt) subprovince, which covers ∼75,000 km² in southeastern Fennoscandia. This subprovince consists of four genetically related complexes which developed at different levels in the crust: a volcanic complex (komatiitic basaltic lava flows on Golets, Levgora and Myandukha hills, and Victoria lava lake on Levgora hill), a subvolcanic complex (mafic–ultramafic sills and lopoliths including Ruiga, Kirichgora, Kozhozero and Undozero), plutonic complexes (Burakovsky and Vyzhiga) and a dyke complex (gabbronoritic Avdeyevo and Shala dykes and peridotitic Vinela and Koppalozero dykes). Similar patterns are present in other Sumian belts elsewhere in Karelia, for instance in southern Lapland and the Kola Peninsula.     

Palaeomagnetism of metadolerite dykes and sills from Proterozoic Shillong basin,NE India: Implications related to the age and magmatism

The basement gneiss of the Shillong plateau and the overlying rocks of the Shillong basin have been dissected by a number of mafic dyke swarms represented by older Proterozoic dolerite dykes and sills named as Khasi greenstone and the younger set of Cretaceous dykes. The older dolerites dykes of Shillong basin are partly metamorphosed and have undergone low-grade metamorphism compared to the fresh unmetamorphosed Cretaceous dykes dominantly exposed in the BGC of West Garo hills region. The Khasi greenstones are tholeiitic in composition and range from basalt to basaltic andesite. Most of the metamorphosed mafic dykes indicate continental nature with some amount of overlapping oceanic tectonic setting. Palaeomagnetic study of the metadolerite dykes show a direction of magnetization of Dm=29, Im=38 (α₉₅ = 28.84; k = 48.33; N = 2) with a palaeolatitude of 21.3° N to the Indian sub-continent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from IRM studies is magnetite in the range of psuedosingle to multi domain (MD) states with minor contribution from hematite.     

The petrogenesis of the Kangâmiut dyke swarm,W. Greenland

Previous studies have shown that the 2.04 Ga Kangâmiut dyke swarm of SW Greenland was injected into an active tectonic environment associated with the formation of the Nagssugtoqidian orogenic belt. Major and trace element modelling of the swarm shows that its chemical evolution was controlled by simple clinopyroxene–plagioclase fractionation. However, such trends — although typical of continental flood basalts and mafic dyke swarms — are at variance with their mineralogy and petrography, which show that locally hornblende is the dominant primary ferromagnesian mineral. Modelling of intradyke fractionation alone shows that hornblende could locally have been an important crystallising phase within several dykes. Normal basaltic fractionation must have occurred before dyke injection at the exposed crustal levels, where the influx of water into the dykes is believed to be responsible for the transition from clinopyroxene–plagioclase (tholeiitic) to hornblende–plagioclase±oxides (calc–alkaline) crystallisation. Overall geochemical trends are dominated by tholeiitic fractionation because (1) hornblende fractionation tended to buffer chemical composition; (2) the presence of water in the surrounding country rocks may have resulted in the advection of heat away from the dyke and consequently resulted in rapid crystallisation, particularly in thin dykes. There is no evidence from trace element data, and particularly Pb isotopic ratios, of any significant assimilation of country rocks occurring during clinopyroxene–plagioclase fractionation, although this does not preclude contamination of the mantle source prior to magma generation. It is likely that the incompatible element enrichment within the dykes resulted from subduction-related mantle metasomatism. The Kangâmiut dyke swarm was both a syn-tectonic and thermal event, which triggered it may be linked to passage of a slab window underneath the metasomatised region, or a mantle plume ascending under a subduction zone.     

Mineral chemistry,crystallization conditions and magma mixing‐mingling at Orduzu volcano (Malatya), Eastern Anatolia,Turkey

The Middle Miocene Orduzu volcanic suite, which is a part of the widespread Neogene Yamadağ volcanism of Eastern Anatolia, consists of a rhyolitic lava flow, rhyolitic dykes, a trachyandesitic lava flow and basaltic trachyandesitic dykes. Existence of mafic enclaves and globules in some of the volcanic rocks, and microtextures in phenocrysts indicate that magma mingling and mixing between andesitic and basaltic melts played an important role in the evolution of the volcanic suite. Major and trace element characteristics of the volcanic rocks are similar to those formed in convergent margin settings. In particular, incompatible trace element patterns exhibit large depletions in high field strength elements (Nb and Ta) and strong enrichments in both large ion lithofile elements (Ba, Th and U) and light rare earth elements, indicating a strong subduction signature in the source of the volcanic rocks. Furthermore, petrochemical data obtained suggest that parental magmas of rhyolite lava and dykes, and trachyandesite lava and basaltic trachyandesite dykes were derived from subduction‐related enriched lithospheric mantle and metasomatized mantle (± asthenosphere), respectively. A detailed mineralogical study of the volcanic suite shows that plagioclase is the principal phenocryst phase in all of the rock units from the Orduzu volcano. The plagioclase phenocrysts are accompanied by quartz in the rhyolitic lava flows and by two pyroxenes in the trachyandesitic lava flows and basaltic trachyandesitic dykes. Oxide phases in all rocks are magnetite and ilmenite. Calculated crystallization temperatures range from 650°C to 800°C for plagioclase, 745°C–1054°C for biotite, 888°C–915°C for pyroxene and 736°C–841°C for magnetite–ilmenite pairs. Calculated crystallization pressures of pyroxenes vary between 1.24–5.81 kb, and oxygen fugacity range from −14.47 to −12.39. The estimates of magmatic intensive parameters indicate that the initial magma forming the Orduzu volcanic unit began to crystallize in a high‐level magma chamber and then was stored in a shallow reservoir where it underwent intermediate‐mafic mixing. The rhyolitic lava flow and dykes evolved in relatively shallower crustal magma chambers. Copyright © 2007 John Wiley & Sons, Ltd.