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Carboniferous-Permian rifting and magmatism in southern Scandinavia, the North Sea and northern Germany: A review
Abstract and Figures
During the Late Carboniferous and Early Permian an extensive magmatic province developed within northern Europe, intimately associated with extensional tectonics, in an area stretching from southern Scandinavia, through the North Sea, into northern Germany. Within this area magmatism was unevenly distributed, concentrated mainly in the Oslo Graben and its offshore continuation in the Skagerrak, Scania in southern Sweden, the island of Bornholm, the North Sea and northern Germany. Available geochemical (major- and trace-element, and Sr-Nd isotope, data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. Peak magmatic activity was concentrated in a narrow time-span from c. 300 to 280 Ma. The magmatic provinces developed within a collage of basement terranes of different ages and lithospheric characteristics (including thicknesses), brought together during the preceding Variscan orogeny. This suggests that the magmatism in this area may represent the local expression of a common tectono-magmatic event with a common causal mechanism. Available geochemical (major and trace element and Sr-Nd isotope data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. The magmatism covers a wide range in rock types both on a regional and a local scale (from highly alkaline to tholeiitic basalts, to trachytes and rhyolites). The most intensive magmatism took place in the Oslo Graben (ca. 120 000 km 3 ) and in the NE German Basin (ca. 48 000 km 3). In both these areas a large proportion of the magmatic rocks are highly evolved (trachytes-rhyolites). The dominant mantle source componet for the mildly alkali basalts to subalkaline magmatism in the Oslo Graben and Scania (probably also Bornholm and the North Sea) is geochemically similar to the Prevalent Mantle (PREMA) component. Rifting and magmatism in the area is likely to be due to local decompression and thinning of highly asymmetric lithosphere in responses to regional stretching north of the Variscan Front, implying that the PREMA source is located in the lithospheric mantle. However, as PREMA sources are widely accepted to be plume-related, the possibility of a plume located beneath the area cannot be disregarded. Locally, there is also evidence of other sources. The oldest, highly alkaline basaltic lavas in the southernmost part of the Oslo Graben show HIMU trace element affinity, and initial Sr-Nd isotopic compositions different from that of the PREMA-type magmatism. These magmas are interpreted as the results of partial melting of enriched, metasomatised domains within the mantle lithosphere beneath the southern Olso Graben; this source enrichment can be linked to migration of carbonatite magmas in the earliest Paleozoic (ca. 580 Ma). Within northern Germany, mantle lithosphere modified by subduction-related fluids from Variscan subduction systems have provided an important magma source components.
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... The last stages of the Variscan orogeny were characterized by crustal extension, high heat flow, post kinematic emplacement of Late Carboniferous-Early Permian granitoids, and widespread late orogenic volcanism. The latter is mostly located in the Oslo rift and north-eastern Germany, but also recorded across much of Europe, for example, in Spain, the Italian Alps, Liguria, Sardinia, among others (McCann et al., 2006;Neumann et al., 2004;Wilson et al., 2004). By the Late Permian, the bulk of Pangea was already formed and the Neotethys Ocean (also called Mesotethys) started to open along the peri-Gondwana margin . ...
... Fertile source rocks not producing much sand can also bias a detrital zircon U-Pb record (Caracciolo, 2020) and the widespread late Carboniferous-Early Permian extensional volcanism (McCann et al., 2006;Neumann et al., 2004;Wilson et al., 2004) might be an example of that. Detrital zircons from plutonic rocks associated to this magmatism are the main component in the Glomma and Po river catchment areas (Fig. 3). ...
With the aim of testing the representativity of detrital zircon U-Pb datasets, we combine new and published data from 31 large river systems in Europe and compare this information with the well-known geology of the continent. Detrital zircon ages range from Archean to Cenozoic and age clusters can be linked to different orogenic cycles and the formation of supercontinents thus representing all relevant geological events in Europe at the continental scale. The Variscan Orogen is the largest episode of crustal reworking in Europe and consequently Variscan detrital zircons occur in all rivers. Other important age clusters are the Alpine 40–25 Ma, post-Alpine 10–0.2 Ma, the Caledonian 490–400 Ma, and the Avalonian-Cadomian 650–540 Ma. Detrital zircons of 1170–930 Ma and 1700–1400 Ma are significant in Scandinavia, as well as 2900–2500 Ma and ca. 1850 Ma in east Europe.
Although the observed distribution of U-Pb detrital zircon ages is qualitatively representative of the geology of Europe at the continental scale, it is not always the case at the basin scale. Importantly, quantitative representativity for both, basin and continental scales, is not achieved. Variable zircon fertility and the sand generation potential of different lithologies highly bias the U-Pb age distributions. For instance, the magma poor Alpine orogen is relevant in minor age clusters in the Po and Danube rivers, but not in rivers draining to the west and north of the Alps, such as the Rhone and Rhine. Similarly, detrital zircons in the range of 10–0.2 Ma occur only in sediments from the Garonne and Loire rivers, although the Cenozoic igneous provinces are widespread in Europe. Detrital zircons from the post-Variscan Permian rocks were mostly found in the Po and Glomma rivers, where Permian granitoids are more abundant than volcanics of this age. Other natural causes of bias, such as transport and recycling can be identified base on the analyses of size and shape of detrital zircons. In the case of west Europe, our data shows that U-Pb detrital zircon patterns and zircon shape are mainly affected by recycling. Moreover, we tested corrections to the U-Pb age distribution for each river basin based on simple geomorphological parameters obtained from Digital Elevation Models. This attempt to overcome the overrepresentation of certain source regions resulted in a considerable change in age peak proportions towards a better quantitative representativity. However, full representativity seems unachievable, even in studies of modern sedimentary systems with known geology.
... During the Permian, the Oslo Rift Graben was formed, accompanied by widespread volcanism in several stages with the main episode between 295 and 275 Ma (Neumann et al., 2004). A prominent suture zone to the south of the Oslo Graben is the Sorgenfrei-Tornquist Zone (STZ) with its southern extension as the Teisseyre-Tornquist-Zone (TTZ). ...
... This might cause a long-lived heat contribution. Another candidate for re-heating could be the rifting and magmatic episodes during the Permian that affected the entire North Sea region south-east of southern Norway and created the Oslo Graben (Neumann et al., 2004). An additionally thermal perturbation might occur near the time of the North Atlantic breakup related to the Iceland plume (e.g., Rohrman and van der Beek, 1996). ...
We present a new 3D shear-wave velocity model and Moho map of Scandinavia, which is based on the inversion of the merged phase dispersion curves from ambient noise and earthquake-generated Rayleigh waves. A classic two step inversion scheme is used where first maps of phase velocities at different periods are derived, and then a 1D transdimensional Bayesian method is applied to determine the VSV-depth structure. We assess the question of what compensates for the unusual high Scandes mountains and aim to identify the different tectonic domains of the adjacent continental lithosphere (Baltic Shield).
While the southern Scandes lacks a pronounced crustal root, we observe a crustal root below the northern Scandes that is decreasing towards the central Scandes. A ∼10 km thick high-density lower crustal layer is present below the northern Scandes and generally thickening to the east below the Baltic Shield. The lithosphere-asthenosphere boundary (LAB) below the Scandes is deepening as well from west to east with a sharp step and a strong VSV decrease with depth of 9% in the north and of 5.5% in the south. The LAB of the thinner lithosphere is at 150 km depth in the north and varies from 90 to 120 km depth in the south. Both LAB steps coincide with the mountain front. The central area shows rather smoothly varying structures (170 km LAB depth, −4% VSV with depth) towards the east and no clear spatial match with the front. We infer therefore distinct uplift mechanisms along the Scandes. The southern Scandes might sustain their topography due to dynamic support from the mantle, while the northern Scandes experience both crustal and mantle lithosphere isostasy. In both cases, we suspect a dynamic support from small-scale edge-driven convection that developed at the sharp lithospheric steps.
Beneath the Archean Karelia craton in northern Finland, we find low-velocity areas below 150 km depth while a 250 km deep lithospheric keel is imaged below the Paleoproterozic southern Finland. The Norrbotten craton in northern Sweden can be identified at mantle depths as a unit different from the Karelia craton, Scandes and Paleoproterozic central Sweden.
... Both ranges form the rift shoulders of the Cenozoic Rhine Graben, consisting of Variscan crystalline basement and carrying the erosional remnants of late Paleozoic intramontane basins. Starting in the south, the Sankt-Peter Formation in the Ronchamp-Giromagny-Breisgau Basin (Fig. 3B) has long been known to provide hematite-encrusted, dark silicified woods from arkoses that are intercalated with red and gray pelites and volcanics (Schnarrenberger 1906;Frentzen 1931;Nitsch and Zedler 2009). The Sankt-Peter Formation remains stratigraphically roughly constrained to an interval somewhere in the Kasimovian-Gzhelian based on regional comparisons and widely scattered Rb-Sr radioisotopic data from a volcanite . ...
... The embedding strata consist of prevailing gray to red arkoses and conglomerates, followed by gray mudstones, subordinate coals, pyroclastics and cherts. The rocks reach from the Barruelian to ''Saberian'' (upper Kasimovian) based on macrofloras, palynomorph assemblages and one volcanic Ar-Ar age of 300 Ma (Hess et al. 1983;Maass and Vogellehner 2005;Nitsch and Zedler 2009). ...
By colonizing drylands, plants fundamentally changed continental deposition and, thus, intensified the interaction between life and sediments. Fossil large woody debris in epiclastic strata is a key archive of this environmental turnover, although its interpretation remains challenging due to taphonomic biases. We review voluminous fluvial red-bed successions with sizeable silicified trunks that characterize Middle Pennsylvanian–lower Permian strata of east-central Europe. The stratigraphic occurrence, petrography, architecture of the deposits, and the preservation and nature of the fossil wood are discussed in the context of the tectono-climatic and vegetational evolution of the central-Pangean low latitudes. The log-bearing successions are assigned to five distinct, regionally traceable stratigraphic levels between the middle Moscovian and early Asselian. Up to 20 m long, mostly decorticated trunk fragments occur isolated in more or less feldspathic channel deposits, the architectures and dimensions of which point to large-scale river systems with highly variable discharge. Wood anatomy and floodplain adpression-fossils show that the trunks were derived from cordaitaleans, conifers, and arborescent sphenopsids in more diverse, gymnosperm-dominated dryland floras. The fossil record is biased towards successions formed in large-catchment river systems and, thus, does not accurately document the genuine nature of plant-distribution patterns. Rather, the strata show that large woody debris preservation depended on fluvial style and hydrological regime, hence turning the woody deposits into climate archives. The strata elucidate the climate development in equatorial Pangea, paralleling the acme of the Late Paleozoic Ice Age.
... (Stampfli et al., 2013 ;Franke, 2014 ;Cochelin et al., 2017). Localisation des bassins et du magmatisme permiens d'après (Lago et al., 2004b ;Neumann et al., 2004 ;Lundmark et al., 2018 ;Ducassou et al., 2019). ...
Longtemps éclipsées par l’engouement de la recherche sur les Alpes, les Pyrénées n’en demeurent pas moins un laboratoire naturel de premier ordre permettant de mieux comprendre le système Terre. Par la grande diversité et la richesse géologique que la chaîne pyrénéenne procure ainsi que par la longue et complexe histoire tectonique dont elle résulte, l’étude de cette dernière nous permet d’aborder de nombreuses questions scientifiques fondamentales qui ont jadis animé et qui animent toujours la communauté des sciences de la Terre. Déclenchée par le soulèvement tectonique alpin, l’érosion de la couverture mésozoïque à cénozoïque, au cours du Tertiaire, a permis la mise à l’affleurement d’une grande partie du socle paléozoïque de la chaîne pyrénéenne. De prime abord, l’origine et la structuration tectonique de ce socle sont héritées en grande partie d’un cycle orogénique antérieur: l’orogenèse varisque.
... A series of large-scale magmatic events roughly during 300 to 280 Ma formed a Large Igneous Province (LIP) around the Oslo Graben in Norway (Neumann et al., 2004), producing a range of tholeiitic and alkalic basalts, as well as trachytes and rhyolites during rifting. The Skagerrak-Centered Large Igneous Province (SCLIP) is thought to have been tied to a mantle plume rising from the African Large Low Shear Velocity Province (LLSVP) close to the core-mantle boundary at ~300 Ma (Torsvik et al., 2008;Fig. ...
... 273 Ma, U-Pb thermal ionization mass spectrometry zircon ages) (Corfu and Larsen, 2020) during a prolonged period of rift-related magmatic activity associated with the Oslo Rift from ca. 300-260 Ma ( Fig. 1) (Corfu and Dahlgren, 2008;Corfu and Larsen, 2020). The Oslo Rift is located in the foreland of the Variscan Orogen, formed by NW-SE lithospheric extension associated with accumulated tectonic stress during the Laurasia-Gondwana collision (Neumann et al., 2004). It has preserved a series of rift-related sedimentary and igneous rocks, among which the Øyangen Caldera formed in its late-stage development ( Fig. 1A) (Neumann et al., 1992;Larsen et al., 2008;Corfu and Larsen, 2020). ...
The large mass difference (∼10%) between the two most abundant isotopes of calcium, 40Ca and 44Ca, gives Ca great potential in tracking mass-dependent fractionation during magmatic processes. Resolvable Ca-isotope fractionation during fractional crystallization of magma, particularly by feldspar in evolved melts, has been theoretically inferred but not robustly tested in nature. To further explore the effects of magmatic differentiation on Ca-isotope systematics, we studied the late-Permian alkaline igneous suite of the Øyangen Caldera, Oslo Rift, Norway, consisting of volcanic and intrusive units ranging from basanitic to rhyolitic compositions. Major and trace element variations and modeling demonstrate that the main series of samples (N = 21), including basanites, ring-dyke syenites, and central-dome syenites, likely documents a co-genetic and closed-system fractional crystallization sequence. Our data show minimal δ44/40Ca variation (< 0.05‰) in the intermediate magma and a marked increase in δ44/40Ca in the felsic magma of the Øyangen Caldera (from 0.62 ± 0.02‰ to 1.15 ± 0.03‰ relative to Ca standard, SRM915a). The systematic increase is best explained by equilibrium isotopic fractionation dominated by alkali feldspar in the fractionating mineral assemblage. This is further supported by strong correlations between δ44/40Ca, CaO, and Eu/Eu* in the main-series samples. Implementing a Monte Carlo approach, isotopic modeling of the liquid line of descent using Rayleigh fractionation is highly consistent with the observed Ca-isotope evolution. For the first time, we confirm prominent Ca stable isotope fractionation in felsic-stage differentiation of alkaline magma and constrain the isotope fractionation factors of plagioclase and K-feldspar. Integrated with extant estimations on mineral fractionation factors from the literature, our results suggest increasing fractionation effects of rock-forming minerals with decreasing Ca content. The affirmation of significant Ca-isotope fractionation in alkaline magma by feldspar empowers the application of Ca as a versatile tracer of crustal evolution, allowing further tests in other magmatic conditions across various planetary objects.
... The wedge-shaped feature in the Smeaheia Fault Block, also noted by Christiansson et al. (2000), however, is striking. The thickness corresponds to Devonian basins in the Moray Firth and the Shetland Platform (Marshall & Hewett, 2003), and could be indicative of Devonian postorogenic extension (Faerseth, 1996;Fossen et al., 2017), which has been documented in surrounding regions (Neumann et al., 2004;Scisciani et al., 2021). ...
The Permian-Triassic alluvial rift succession in the Horda Platform area is analysed to construct a refined depositional model. The study demonstrates how subsurface continental rift successions may be stratigraphically subdivided and correlated by integrating seismic and well-log data in concert with conceptual models. Regional unconformities mark the top and base of the Permian-Triassic succession, which is sub-divided into six seismic stratigraphic sequences, delineated by erosional- and non-depositional surfaces. The definition of seismic stratigraphic sequences is based on seismic facies trends and Gamma Ray log signatures. Time-thickness maps combined with geometries in cross-section display the depocentre development. During the Permian-Triassic, the Horda Platform experienced faulting, shaping the Caledonian pre-rift landscape into a series of N-S trending half-graben basins, contemporaneous to a gradual climate change from arid in the Permian to humid in the latest Triassic. The basin underwent three phases of rifting during the Late Permian-Middle Triassic; (1) disconnected heterogeneous depocentres with strain concentrated in the west; (2) depocentres expanded northward; and (3) mature half graben development with widely distributed strain. Vertical lithological changes in mudstone/sandstone dominance reflect varying rates of accommodation and sediment supply (A/S). Systematic A/S variation reflects strong climatic fluctuations, which controlled facies patterns during both tectonic quiescence in the Middle-Late Triassic, and during syn-rift sedimentation. The Permian-Triassic tectono-sedimentary development in the Horda Platform area provides valuable lessons on the influence of faulting on depocentre development, how the interplay between tectonic and climatic forcing is expressed in subsurface continental deposits, and aid the characterization of reservoirs.
Agpaitic nepheline syenites have complex, Na-Ca-Zr-Ti minerals as the main hosts for zirconium and titanium, rather than zircon and titanite, which are characteristic for miaskitic rocks. The transition from a miaskitic to an agpaitic crystallization regime in silica-undersaturated magma has traditionally been related to increasing peralkalinity of the magma, but halogen and water contents are also important parameters. The Larvik Plutonic Complex (LPC) in the Permian Oslo Rift, Norway consists of intrusions of hypersolvus monzonite (larvikite), nepheline monzonite (lardalite) and nepheline syenite. Pegmatites ranging in composition from miaskitic syenite with or without nepheline to mildly agpaitic nepheline syenite are the latest products of magmatic differentiation in the complex. The pegmatites can be grouped in (at least) four distinct suites from their magmatic Ti and Zr silicate mineral assemblages. Semiquantitative petrogenetic grids for pegmatites in log aNa2SiO5- log aH2O - log aHF space can be constructed using information on the composition and distribution of minerals in the pegmatites, including the Zr-rich minerals zircon, parakeldyshite, eudialyte, låvenite, wöhlerite, rosenbuschite, hiortdahlite and catapleiite, and the Ti-dominated minerals aenigmatite, zirconolite (polymignite), astrophyllite, lorenzenite, titanite, mosandrite and rinkite. The chemographic analysis indicates that although increasing peralkalinity of the residual magma (given by the activity of the Na2Si2O5 or Nds component) is an important driving force for the miaskitic to agpaitic transition, water, fluoride (HF) and chloride (HCl) activity controls the actual mineral assemblages forming during crystallization of the residual magmas. The most distinctive mineral in the miaskitic pegmatites is zirconolite. At low fluoride activity, parakeldyshite, lorenzenite and wöhlerite are stable in mildly agpaitic systems. High fluorine (or HF) activity favours minerals such as låvenite, hiortdahlite, rosenbuschite and rinkite, and elevated water activity mosandrite and catapleiite. Astrophyllite and aenigmatite are stable over large ranges of Nds activity, at intermediate and low water activities, respectively.
The late Paleozoic ice age (LPIA) was the longest-lived glaciation of the Phanerozoic, and the demise of LPIA is the Earth's only recorded transition from an icehouse to a greenhouse state. In order to explore the records of chemical weathering and volcanism linked to paleoclimate, an integrated multi-proxy study from the Pennsylvanian to the earliest Cisuralian in the Western Shandong from the North China Craton (NCC) was conducted, including results of chemical weathering induces (CIA, CIW, PIA, WIP, CIX, and τNa), element indicators (Sr/Ba, Sr/Cu, and Rb/Sr), Hg concentration, total organic carbon (TOC), and organic carbon isotope (δ¹³Corg). The chemical weathering indies show a peak of chemical weathering intensity during the early Asselian (~298 Ma), indicating a warming event occurred at that time. Hg/TOC shows one obvious peak of volcanic intensity, corresponding with the rapid increase of chemical weathering. Moreover, the peak in volcanic intensity coincides with negative excursions of δ¹³Corg, hinting at volcanic drivers for the perturbations of carbon isotope and the change of paleoclimate. This phenomenon coincides with the change of conodont oxygen isotope (δ¹⁸O), a shift in atmospheric partial pressure of CO2 (pCO2), sea-level variation, and late Paleozoic deglaciation records that together document the earliest Permian climate warming-cooling perturbation with a temperature maximum. We suggest that the climate warming in the early Asselian may be driven by volcanism through released greenhouse gas. Our result provides an important contribution to enabling the correlation of volcanic and climatic events during glacial and interglacial cycles.
Late Cretaceous granitic rocks occur in the Gejiu ore district to the east and west of the N-S striking Gejiu Fault, whereas major Sn deposits are only known to occur to the east of the Gejiu Fault. Comparison of the whole-rock chemistry, the apatite trace-element chemistry, and zircon Hf and O isotope data of the various granites demonstrates that fertile granites occur to both sides of the Gejiu fault. The results demonstrate that the c. 83 Ma old granitic intrusions (i) have similar magma sources, which are dominated by metasedimentary rocks that had experienced intense chemical weathering, resulting in reduced melts, (ii) had similar melting conditions, i.e., high temperature biotite dehydration melting, and (iii) in part had experienced large extents of fractional crystallization. The most evolved granites to both sides of the Gejiu Fault have the characteristics typical of tin granites. Therefore, the absence of major deposits to the west of the Gejiu Fault is not due to the absence of fertile granites. The areas to the east and the west of the Gejiu Fault, however, have fundamentally different fault pattern, which indicates different orientation of the stress field to both sides of the Gejiu Fault at the time of the emplacement of the Cretaceous granites. Late Cretaceous dextral movement along the Ailaoshan Fault Zone resulted in a (trans)tensional setting in areas to the east of the Gejiu Fault and in a (trans)pressional setting to the west of that fault. We speculate that the tectonic setting influences the potential for mineralization because the Sn bearing fluids need efficient pathways to transporting metals from the roof zone of the batholith into the wall rocks. In contrast to regional compression, hydraulic fracturing in an overall extensional setting has the potential to develop efficient fluid pathways and, thus, may lead to major ore deposits. To the west of the Gejiu Fault, however, granite intrusions in an overall compressional setting are likely to develop no or only small mineralization.
ABSTRACT Volcanic arc basalts are all characterized by a selective enrichment in
incompatible elements of low ionic potential, a feature thought to be due to the input of
aqueous fluids from subducted oceanic crust into their mantle source regions. Island are
basalts are additionally characterized by low abundances [for a given degree of fractional
crystallization) of incompatible elements of high ionic potential, as feature for which high
degrees ot'melting, stability of rninor residual oxide phases, and remelting of depleted mantle are all possible explanations. Calc-alkaline basalts and shoshonites are additionally characterised by enrichment of Th, P and the light REE in addition to elements of low ionic potential, a feature for which one popular explanation is th contamination of their mantle source regions by a melt derived from subducted sediments.
By careful selection of variables, discrimination diagrams can be drawn which highlight these various characteristics and therefore enable volcanic arc basalts to he recognized in cases where geological evidence is ambiguous. Plots of Y against Cr, K[Yb, Ce/Yb, or Th/Yb against Ta/Yb, and Ce/Sr against Cr are all particularly successful and can be modelled in terms of vectors representing different petrogenctic processes. An additional plot of Ti/Y against Nb/Y is useful for identifying 'anomalous' volcanic arc settings such as Grenada and parts of the Aleutian arc. Intermediate and acid rocks from volcanic are settings can also be recognized using a simple plot of Ti against Zr.
The lavas from the Oman ophiolite complex provide a good test of the application of these techniques. The results indicate that the complex was made up of back-arc oceanic crust intruded by the products of volcanic arc magmatism.
Numerous magmatic dykes occur at the western and north-eastern coast of Bornholm. They intruded exclusively gneisses and granites of the Precambrian basement. The majority of the dykes is approximately N-S oriented. Thus, their formation was generally coupled with W-E directed extension processes. Based on palaeomagnetic studies, they represent several magmatic periods during Proterozoic times. Only a few WNW to NW striking dykes in the north-western part of the island indicate a different extensional regime. Apart from their orientation, geochemical features suggest that these intrusions are the south-eastern continuation of a large, WNW to NW trending dyke swarm crossing the neighbouring Swedish province of Scania. The mainly tholeiitic dykes are genetically related to the Sorgenfrei-Tornquist fault zone, and point to an intra-continental rifting during Permo-Carboniferous times. This assumption is also supported by palaeomagnetic studies of ABRAHAMSEN & LEWANDOWSKI (1992), which yielded a Permian age for the WNW-ESE oriented Lindesdal dyke exposed at the western coast of Bornholm.
New chemical analyses for major elements in 76 Hawaiian rocks are presented and bring the total number of such modern analyses to about 470. Many determinations of minor elements also are becoming available. Hawaiian petrology is discussed against this total background. The three major rock suites, tholeiitic, alkalic, and nephelinic, are chemically intergradational. The main mass of the volcanoes is tholeiitic, followed by a relatively small volume (generally less than 1 percent) of alkalic lavas; the two types of lavas are interbedded in a thin transitional zone. The nephelinic lavas are separated from the others by a long time interval that is marked by a profound erosional unconformity. Variations within the rock suites are largely the result of crystal differentiation. All three rock suites probably are derived from a single type of parent magma, which varies slightly from one volcanic center to another, of olivine tholeiite composition. Crystallization of this magma in shallow magma chambers leads to eruptible magmas of tholeiitic composition. In the last stages of volcanism, consolidation of the upper part of the magma body leads to crystallization at deeper levels under higher pressure and to production of alkalic magmas. Finally, crystallization at depths of several tens of kilometers produces nephelinic magmas that are erupted after a long period of volcanic quiescence.