Rocks Of Intermediate Composition Research Articles

Lithologic mapping of the Mordor, NT, Australia ultramafic complex by using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)

Spectral measurements made in the Mordor Pound, NT, Australia study area using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), in the laboratory and in situ show dominantly Al-OH and ferric-iron VNIR–SWIR absorption features in felsic rock spectra and ferrous-iron and Fe,Mg-OH features in the mafic–ultramafic rock spectra. ASTER ratio images, matched-filter, and spectral-angle mapper processing (SAM) were evaluated for mapping the lithologies. Matched-filter processing in which VNIR + SWIR image spectra were used for reference resulted in 4 felsic classes and 4 mafic–ultramafic classes based on Al-OH or Fe,Mg-OH absorption features and, in some, subtle reflectance differences related to differential weathering and vegetation. These results were similar to those obtained by match-filter analysis of HyMap data from a previous study, but the units were more clearly demarcated in the HyMap image. ASTER TIR spectral emittance data and laboratory emissivity measurements document a wide wavelength range of Si-O spectral features, which reflect the lithological diversity of the Mordor ultramafic complex and adjacent rocks. SAM processing of the spectral emittance data distinguished 2 classes representing the mafic–ultramafic rocks and 4 classes comprising the quartzose to intermediate composition rocks. Utilization of the complementary attributes of the spectral reflectance and spectral emittance data resulted in discrimination of 4 mafic–ultramafic categories; 3 categories of alluvial–colluvial deposits; and a significantly more completely mapped quartzite unit than could be accomplished by using either data set alone.

The Early Cretaceous San Juan Plutonic Suite, Ecuador: a magma chamber in an oceanic plateau?

Sections through an oceanic plateau are preserved in tectonic slices in the Western Cordillera of Ecuador (South America). The San Juan section is a sequence of mafic–ultramafic cumulates. To establish that these plutonic rocks formed in an oceanic plateau setting, we have developed criteria that discriminate intrusions of oceanic plateaus from those of other tectonic settings. The mineralogy and crystallization sequence of the cumulates are similar to those of intra-plate magmas. Clinopyroxene predominates throughout, and orthopyroxene is only a minor component. Rocks of intermediate composition are absent, and hornblende is restricted to the uppermost massive gabbros within the sequence. The ultramafic cumulates are very depleted in light rare-earth elements (LREE), whereas the gabbros have flat or slightly enriched LREE patterns. The composition of the basaltic liquid in equilibrium with the peridotite, calculated using olivine compositions and REE contents of clinopyroxene, contains between 16% and 8% MgO and has a flat REE pattern. This melt is geochemically similar to other accreted oceanic plateau basalts, isotropic gabbros, and differentiated sills in western Ecuador. The Ecuadorian intrusive and extrusive rocks have a narrow range of εNdi (+8 to +5) and have a rather large range of Pb isotopic ratios. Pb isotope systematics of the San Juan plutonic rocks and mineral separates lie along a mixing line between the depleted mantle (DMM) and the enriched-plume end members. This suggests that the Ecuadorian plutonic rocks generated from the mixing of two mantle sources, a depleted mid-oceanic ridge basalt (MORB) source and an enriched one. The latter is characterized by high (207Pb/204Pb)i ratios and could reflect a contamination by recycled either lower continental crust or oceanic pelagic sediments and (or) altered oceanic crust (enriched mantle type I, EMI). These data suggest that the San Juan sequence represents the plutonic components of an Early Cretaceous oceanic plateau, which accreted in the Late Cretaceous to the Ecuadorian margin.

Rare Earth Elements in Coal-Bearing Deposits: Distribution Features and Geochemical Significance

The examination of Lower Cretaceous coals and clayey fractions of terrigenous host rocks of the Zyryanka Basin has revealed a generally negative type of the relative REE distribution (with a predominance of LREE over HREE) and a similar behavior of Th and U. Analogous characteristics are typical for metamorphic and igneous rocks of intermediate composition dominating in the provenance. In terms of the concentration and distribution of REE, Th, and U coal-bearing terrigenous rocks of the Zyryanka Basin are close to Phanerozoic clays and silty sandstones of the Russian Platform, while coals of the Zyryanka Basin are similar to metal-free coals of various basins (Kizel, Pennsylvanian, and others). Average REE and Th contents in the studied coals are five times lower than those in host terrigenous rocks. The coals differ from these rocks by relatively elevated HREE concentrations largely due to adsorption mechanism of the HREE accumulation. Data obtained on the REE, Th, and U distribution in coals and clayey fractions of host rocks are of fundamental significance for the comparitive analysis of the behavior of these elements in metal-bearing coals of various basins.

Plutonic rocks of intermediate composition on Gran Canaria: the missing link of the bimodal volcanic rock suite

Analysis of abundant plutonic fragments in the voluminous Miocene rhyolitic–basaltic composite ignimbrite P1, the initial felsic cooling unit overlying the basaltic shield on Gran Canaria, helps to solve the problem of bimodal volcanism. Syenogabbros dominate the plutonic suite and provide evidence that magmas of intermediate composition formed abundantly at depth but did not erupt. The entire suite of plutonic fragments comprises (1) pyroxenitic to gabbroic cumulates that directly influenced the liquid line of descent of P1 magmas, (2) gabbro, syenogabbro, syenodiorite and quartz–syenite fragments showing moderate degrees of hydrothermal alteration (chloritization, partial melting), and (3) diverse xenoliths differing significantly in bulk-rock composition and texture from the erupted P1 magmas. The compositions of plutonic series (2) overlap with both bulk-rock and mineral compositions of the erupted magmas, whereas series (1) rocks show lower alkali and silica contents. Compositional variations among the plutonic rocks are compatible with fractional crystallization as the major petrogenetic process, locally modified by magma mixing and selective element contamination. Variations in trace element concentrations of the plutonic rocks, however, are inferred to be the result of evolved interstitial melts penetrating into, or draining out of, crystalline mushes during slow intratelluric solidification. The presence of F-rich amphibole suggests that crystalline mushes were invaded by F-bearing fluids. Overall compositional similarity, and the fact that selective contamination effects are similar, support the interpretation that the series (2) plutonic fragments represent solidified portions of the magmatic system that ultimately produced the erupted P1 magmas. This system extended from the zone of underplating at the mantle–crust boundary through the lower and into the upper crust of Gran Canaria and was roughly vertically zoned in composition as shown by geobarometric calculations.

Sphene (titanite): phase relations and role as a geochronometer

Useful U–Pb isotopic data may be obtained from sphene (or titanite, CaTiSiO 5) because: (1) it is a widespread accessory mineral, (2) it can incorporate uranium in its structure, and (3) it has a high closure temperature. In igneous rocks, sphene is most abundant in relatively oxidized rocks, such as metaluminous rocks of intermediate composition. These rocks have the high Ca/Al ratios wherein sphene is stabilized relative to ilmenite+quartz or ilmenite+anorthite. In metamorphic rocks, sphene is stable to the highest temperatures in mafic and calc-silicate rocks. It is found mostly in greenschist, blueschist, and amphibolite facies, although in calcic rocks its stability may extend into granulite facies. Recent studies show that the closure temperature for sphene lies at the upper limit of amphibolite facies. Because sphene reacts readily during metamorphism, U–Pb sphene ages are likely to yield the age of metamorphic crystallization, rather than resetting by simple diffusion. For this reason, metamorphic sphene may yield complex U–Pb systematics that contain information on the whole metamorphic history of the rock. Sphene from igneous rocks and orthogneisses has initial U contents ranging from 10 to over 100 ppm and ratios of initial U to common Pb ranging from 10 to 1000, ratios that may potentially yield high precision U–Pb ages. Sphene in marbles, calc-silicates, and metagraywackes has a similar range in composition to that from igneous rocks, but sphene from metabasites may have initial U contents of less than 1 ppm and ratios of initial U to common Pb lower than 1, making them unsuitable for geochronology. These low-U sphenes are most commonly found in weakly metamorphosed metabasites. Strategies to extract age information from sphene with moderate initial U/common Pb ratios include estimation of common Pb isotopic composition of sphene from coexisting low-U phases, use of U–Pb and Pb–Pb isochron plots, and step-wise leaching methods to improve 206Pb/ 204Pb spread. By correlating sphene compositions to metamorphic or hydrothermal reactions, age determinations on sphene can be used to directly date metamorphism, deformation, and hydrothermal alteration.

Geochemical zoning, mingling, eruptive dynamics and depositional processes — the Campanian Ignimbrite, Campi Flegrei caldera, Italy

The Campanian Ignimbrite (CI) is a large-volume trachytic tuff erupted at 37 ka from the Campi Flegrei and composed of a fallout deposit overlain by ignimbrite. The ignimbrite was spread over an area of about 30,000 km 2 including the Campanian Plain and the Apennine Mountains, with ridges over 1000 m a.s.l. The pumice fragments of the CI range in composition from trachyte to phonolitic-trachyte (DI = 75–90). They do not show any systematic compositional variation with stratigraphic height, but the analyzed sections can be divided into three groups on the basis of chemical composition of pumices. Least-evolved pumices (DI = 75–83) occur in the ignimbrite in the central sector of the Campanian Plain up to 30 km from the vent, while the most-differentiated pumices (DI = 88–90) characterize the cogenetic fallout deposit and the ignimbrite in the western sector of the Campanian Plain, on the Tyrrhenian scarp of the Apennines between Caserta and Mt. Maggiore, on Roccamonfina volcano, and on the Sorrento Peninsula, up to 50 km from the source. Pumice fragments of intermediate composition (DI = 84–87) occur in the ignimbrite on the Apennine Mountains and Roccamonfina volcano, up to 65 km from the vent. In one exposure at Mondragone, at the base of a calcareous ridge, an ignimbrite with pumices of most-evolved composition is overlain by an ignimbrite with pumices of intermediate composition. The observed compositional variation between most- and least-evolved ignimbrite was generated in part by crystal-liquid fractionation, although other magmatic processes such as syn-eruptive mingling between most- and least-evolved magmas accounts for the mineralogical disequilibria and for the bimodality of the glass compositions in the intermediate-composition rocks. Pumice Sr-isotope ratios are positively correlated with degree of differentiation. Feldspar crystals separated from pumices of different compositions have a homogeneous Sr-isotope composition similar to that of the least-evolved pumices. Interaction between fluids and strongly fractionated Sr-poor less-dense magma can account for these isotopic features. Geochemical, mineralogic, stratigraphic and volcanologic data, together with the stratigraphic relations between most-, intermediate- and least-evolved ignimbrite as detected at Mondragone and from bore-hole drillings suggest that: (1) the CI magmatic system was composed of two distinct magma layers — the upper layer was more differentiated and homogeneous in composition, while the deeper was less evolved and slightly zoned; and (2) the CI was mostly emplaced in three main pulses of pyroclastic flows that tapped the chamber at variable levels and with distinct withdrawal dynamics. The eruption began with emission of the most differentiated magma, which gave rise to the fallout deposit. It continued with generation of expanded, turbulent pyroclastic flows that reached the Sorrento Peninsula in the southeast and Roccamonfma volcano in the northwest. These flows, whose thickness was greater than the overtopped relief, were able to travel over the water of the bay of Naples. Subsequently an intermediate-composition magma resulting from mingling of different portions of the magma chamber generated similar flows that spread radially and traveled not less than 65 km from the vent. During the last pulse the least-evolved magma was tapped and generated flows that spread within the Campanian Plain. Variation in eruptive dynamics and composition of magma during the course of the eruption likely reflected variations of both geometry of vent and plumbing system, and efficiency of water/magma interaction, which in turns affected the dynamics of the magma chamber and the withdrawal mechanism, and resulted from the dynamics of the caldera collapse.

The 'Daly gap' as a magmatic catastrophe

IGNEOUS rocks very commonly show a strongly bimodal distribution of compositions, one mode corresponding to basalt and the other to felsic magmas1–3. As fractional crystallization of basaltic parents produces a continuum of compositions, the paucity of rocks of intermediate composition—commonly called the Daly gap—has puzzled petrologists since the time of Daly. Gravitational or viscous trapping4,5, large crystal loads restraining convection6,7, and re-melting of deep volcanic layers2 are among the processes that have been offered as physically meaningful explanations of magmatic gaps. Here we propose an alternative interpretation, transposed from chemical reactor control theory8: at large undercooling, thermal feedback in a continuously fed and differentiating magma reservoir promotes the existence of competing thermochemical steady states. Small variations in magma residence time and cooling rate induce a large thermal and chemical swing (magmatic bifurcation or catastrophe), which interrupts the liquid line of descent, leading to bimodal erupted products.

The 40Ar/39Ar ages and tectonic setting of the middle Eocene northeast Nevada volcanic field

Widespread middle to late Eocene calc‐alkalic volcanism, which formed the Northeast Nevada Volcanic Field, marks the earliest Tertiary volcanism in the northern Basin and Range. The central part of this major field in northeast Nevada and adjacent Utah is herein defined by 23 40Ar/39Ar ages that range from 42.6 to 39.0 Ma, rock chemistry from 12 localities, stratigraphic position of the volcanic rocks above a regional middle Eocene unconformity, volcanic setting, and lithology. The type area is at Nanny Creek, in the northern Pequop Mountains, Nevada, where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs. The intermediate composition rocks are locally derived throughout the volcanic field, whereas the sources for rhyolite ash flow tuffs are unknown. The uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region. In the central part of the field the middle Eocene volcanic rocks rest with depositional angular discordance on deformed middle Paleozoic to Triassic strata; ostracode‐bearing limestone, probably of early Eocene age, is locally present below the volcanic rocks. In the western and southeastern parts of the field these middle Eocene volcanic rocks rest with depositional angular discordance on lower Eocene lacustrine strata of the Elko and White Sage Formations, respectively. This angular discordance documents a middle Eocene deformational event previously unrecognized in the region.

Geology, geochemistry, and ore deposits of the Oligocene Hog Heaven volcanic field, northwestern Montana

The Oligocene Hog Heaven high potassium dacite-rhyolite volcanic field covers 50 km 2 of northwestern Montana. Its composite flow domes and dikes, ash-flow deposits, volcaniclastic rocks, and sparse alkali basalt dikes intrude or unconformably overlie the Late Proterozoic Belt Supergroup near the crest of the Purcell anticlinorium. North-northwest-, northeast-, and northwest-trending extensional faults and fractures controlled dome and dike emplacement and subsequent Ag-Au-Pb-Cu-Zn-As high sulfidation mineralization.The felsic volcanic rocks are similar mineralogically, have silica contents between 65.2 and 72.9 wt percent, and are meta- to peraluminous. They are anomalously high in Sr and Ba and have low initial 87 Sr/ 86 Sr ratios of approximately 0.704. Both rock and ore leads have identical and relatively unradiogenic lead isotope compositions.Petrography, whole-rock geochemistry, and Sr and Pb isotopy of the felsic volcanic rocks are most compatible with an origin by partial melting of ancient, perhaps Late Archcan, high alumina, lower crustal rocks of intermediate composition. Lead was apparently derived from the same lower crustal source; the other ore elements could have been derived from one or more sources.Known mineral deposits are restricted to the east dome complex. They are numerous, small except for the Hog Heaven mine, and occupy several stratigraphic and structural settings. Deposit types include open-space fill, replacement, vein and stockwork, and breccia hosted. Precious metal deposition accompanied silicic and subsequent advanced argillic alteration. The Ag-rich Hog Heaven mineralized system is typical of other high sulfidation deposits with respect to (1) the close temporal and spatial relationship to dome emplacement, (2) wall-rock alteration assemblages, (3) the relatively small deposit size compared with some adularia-sericite systems, and (4) high ore Pb/Cu ratios. The deposit has the highest known Ag/Au ratio of any high sulfidation system (2,233/1) and is unusual in that Pb + Zn + Cu halos surround Ag + Au-enriched zones. Deposit types include open-space fill, replacement, vein and stock-work, and diatreme hosted. Precious metal deposition accompanied silicic and subsequent advanced argillic alteration.

Magma mixing in alkaline magmas: An example from Tenerife, Canary Islands

The central region of Tenerife (Canary Islands) offers a wide variety of alkaline rocks of intermediate composition between basalts and more differentiated magmas. A recent (less than 0.1 Ma old) eruption on the southwestern slopes of Teide volcano produced separate felsic (SiO 2∼59%) and mafic (SiO 2∼47%) rocks as well as abundant hybrid inclusions (SiO 2: 50–57%) that show a wide range of geochemical and mineralogical variation between the end members. Mineralogical evidence (reverse-zoning, skeletal and patchy feldspars, incompatible assemblages, mineral disequilibrium) and geochemical data (characteristic trends of major and trace elements, REE ratios, major and trace mass-balance calculations) support the generation of a mixing process. We present a model in which basaltic magma entered a trachyte magma chamber, which resulted in a stratified chamber of trachytic above more mafic (hawaiitic) melts. The formation of hybrid magmas between these two end-members is believed to involve some chemical diffusion across the interface between the two magmas before mechanical mixing. A lowering of temperature due to the cooling effect of the trachytic magma and, consequently, a decrease in density by crystallization of plagioclase permitted the basaltic magma to penetrate through the trachyte, which caused the eruption. This natural example is in good agreement with the results obtained in experimental work on interdiffusion of silicate melts.

Geophysical Investigations of Volcanic Terrains: A Case History from the Gawler Range Volcanic Province, South Australia

Flat-lying magnetic cover rocks, especially when they comprise thick volcanic sequences, present a difficult interpretation problem. Magnetic maps of such regions essentially reflect the exposed volcanic rocks and sources within and beneath the volcanic rocks cannot be easily resolved. Yet such areas are often of economic interest. The Gawler Range Volcanic Province in South Australia represents an example of one such volcanic terrain. The Volcanics are distinctly bimodal in composition: the 'older' units are mafic and the 'younger' units dominantly felsic, with few rocks of intermediate composition. The exposed Volcanics cover approximately 25 000 km2 and drill-hole intersections confirm their existence well beyond the present limits. Magnetite is present in both mafic and felsic volcanic rocks. The province has been identified as a target area for possible epithermal gold mineralisation and volcanogenic-hosted base metal mineralisation. Regional gravity and aeromagnetic data are available over the whole province, while part is covered by a recent high-resolution aeromagnetic and aeroradiometric survey. The Gawler Range Volcanic Province is associated with a large gravity high caused probably by mantle underplating or the emplacement of mantle derivatives prior to the extrusion of the volcanics. This is consistent with isotope data which were previously interpreted to indicate a mantle component for the Volcanics. Two major gravity trends (NE and ESE) intersect within the study area. The ESE-striking trend has been modelled as a deep crustal fault with the southern block down-faulted. These two trends are also reflected in the high-resolution magnetic maps once the high frequencies have been removed from the data. Initial images of the high-resolution aeroradiometric and aeromagnetic data correlated very closely with the outcrop and joint patterns. Areas of radiating fracture patterns may represent volcanic centres. The image of the K:U Ratio reflected minor variations in the otherwise homogeneous geochemistry of the Volcanics. Through image processing and filtering, and combining the interpretation of available geophysical data, an inferred geological map for the area has been suggested. The igneous province is interpreted as extending further eastwards beneath part of the Stuart Shelfing consisting of three well-defined units. The first corresponds to the main Gawler Range Volcanic Province, the second to the south-west gabbroic extension and the third to an eastward extension consisting of a felsic core circled by a mafic ring.

Ordovician bimodal magmatism in the Ogcheon belt (South Korea): intracontinental rift-related volcanic activity

The Ogcheon belt of South Korea has been interpreted to be an oceanic suture zone; nevertheless, all lines of evidence are consistent with an intraplate evolution since early Paleozoic times. The early Paleozoic Ogcheon Supergroup is composed of a Cambrian to early Ordovician quartzite and carbonate platform sequence overlain by a thick volcano-sedimentary sequence; the volcanic rocks have been emplaced in close association with thick debris flow or flysch-like deposits. The metavolcanic complex displays a bimodal association of metabasalts or metadolerites associated with metatrachytes and rare metarhyolites; most of the basic volcanic rocks were emplaced as hypabyssal complexes (sills, dykes and laccoliths) while the acidic volcanic sequences are mainly pyroclastic flow or fall deposits and volcano-derived sedimentary rocks. The scarcity of rocks of intermediate composition, the occurrence of Fe- and Ti-rich basic facies and the high level of incompatible elements are all features of tholeiitic transitional suites. ϵNd values close to zero for basic rocks and trachyte are diagnostic of intracontinental lavas. The negative ϵNd values of the rhyolite suggest that crustal contamination occurred during magma genesis or ascent. Taking into account both the overall characteristics of the sedimentary environment and the petrological and geochemical features of the volcanic component, the sequences appear to have evolved in an intracontinental rift and do not represent oceanic remnants as has been previously interpreted. Thus, the interpretation of the Ogcheon belt as a suture zone is definitely ruled out. Consequently, the limit between early Paleozoic North and South China continents might be located elsewhere, perhaps farther north in the Imjingang belt of central Korea.

Mildly peraluminous high-silica granites in a continental rift: the Drammen and Finnemarka batholiths, Oslo Rift, Norway

The peraluminous Drammen batholith (650 km2) is the largest granite complex within the mainly alkaline province of the Permo-Carboniferous Oslo Rift, and peraluminous to metaluminous granites are also present in the southern part of the otherwise alkaline Finnemarka complex (125 km2). The emplacement of the Drammen granite, and probably most of the other biotite granite complexes, predate the alkaline syenites and granites. The eight separate petrographic types of the Drammen batholith range in SiO2 from 70 to 79 wt.% and have experienced variable amounts of fractionation of feldspars, biotite, zircon, apatite, titanite and Fe−Ti-oxides. The initial Sr, Nd and Pb isotopic ratios and a decoupling between the variations in the SiO2 content and the aluminum saturation index [ASI=Al2O3/(CaO+Na2O +K2O)] show that the various intrusive phases are not strictly comagmatic. The eNd values of the southern part of Finnemarka (+3.5 to +4) and the northern part of the Drammen granite (+1 to +1.5) are high and indicate insignificant (for Finnemarka) to minor Precambrian crustal or enriched mantle contributions. The very low eSr values of all of these samples (−1 to −12, outside the main Oslo Rift magmatic array), point to a time integrated Rb-depleted crustal contaminant or an EM1 mantle component. The earliest extruded alkali basalts along the southwestern margin of the Oslo Rift are the only other samples within this low eSr area, but their isotopic signature may also be linked to a mantle enrichment event (involving an EM1 component), e.g. associated with the Fen carbonatite magmatism 540 Ma ago. For a given 206Pb/204Pb, the 208Pb/204Pb ratios of the Drammen and Finnemarka batholiths are distinctly lower than those of the Skien alkaline volcanics and all other magmatic Oslo Rift rocks. This may indicate that the lithosphere of the central part of the rift had a time integrated Th-depletion. The samples from the southern part of the Drammen batholith, characterized by the presence of abundant miarolitic cavities, have eNd near 0 (−0.7 to +0.4) but strongly elevated eSr of +35 to +67. The combined Pb isotopic ratios of all the samples analyzed indicate that the Precambrian crustal anatectic contribution is in the form of time integrated Th-and U-depleted lower crust, and the high +Sr of the sourthern part of the Drammen granite results from shallow level wallrock assimilation or magma-fluid interactions. The remarkably low contribution of old crustal components to the Finnemarka and the northernmost Drammen batholiths may result from extensive late Precambrian intracustal differentiation in southwestern Scandinavia, leading to widespread upper crustal granites (≈ 900 Ma) and a correspondingly dense and refractory lower crust, in particular in a zone intersecting the central part of the rift. Liquidus phase relations and mass-balance constrainst permit derivation of the granites from mildly alkaline to tholeiitic melts by extensive crystal fractionation of clinopyroxene-and amphibole-rich assemblages. It is equally possible to form the granitic magmas by partial melting of Permian gabbros of similar composition. Either scenario is consistent with the isotopic constrainst and with the presence of dense cumulates and/or residues in the lower crust. The lack of igneous rocks of intermediate composition associated with the Drammen and Finnemarka batholiths point to an efficient upper crustal density filtering. Considerable amounts of heat would be accumulated in this region if differentiated, intermediate melts could not escape to shallower levels. Successive magma injections would therefore easily result in partial melting of already solidified mafic to intermediate melts and cumulates, and it is suggested that the peraluminous granites formed mainly by water-undersaturated anatexis of mafic material.

Geology and tectonic evolution of the Bear Mountains Fault Zone, Foothills Terrane, central Sierra Nevada, California

The Late Jurassic‐Early Cretaceous Bear Mountains fault zone (BMFZ) is the westernmost strand of the Foothills fault system in the Western Metamorphic belt of the Sierra Nevada. The tectonic significance of this 300‐km‐long fault zone has been downplayed in the past, but we contend that it is a major discontinuity within the Foothills terrane. The BMFZ is an ˜ 5‐km‐wide shear zone consisting of slate‐metagraywacke‐matrix melange that experienced intense polyphase deformation and metamorphism. Small‐scale structures indicate reverse slip along most of the extent of this steeply east dipping shear zone. A host of lithologically diverse tectonic blocks are enclosed in the BMFZ. Blocks of sandstone, metavolcanic rocks, and plutonic rocks of intermediate composition have counterparts in the adjacent eastern and western zones of the Foothills terrane, whereas other block types are exotic. Exotic blocks include ultramafites that locally contain pods of gabbro, garnet amphibolite, greenschist, sedimentary breccia, and volcaniclastic rocks. The breccia is composed primarily of clasts of amphibolite and minor garnet‐bearing impure quartzite (metachert?); it probably accumulated at the base of fault scarps. The exotic blocks comprise a broadly ophiolitic assemblage similar to that inferred to form basement to the Foothills terrane, and they record a deformation prior to incorporation in the slate‐metagraywacke matrix. Amphibolite blocks and quartzite preserved as clasts within the breccia probably were deformed during overthrusting of oceanic lithosphere in a setting similar to that beneath the Tuolumne ophiolite east of the BMFZ. The presence of exotic blocks implies large‐scale mixing and large displacements in the BMFZ, as do differences in stratigraphy across the fault zone. Metamorphic grade does not vary significantly across the BMFZ, however, suggesting that dip slip did not exceed several tens of kilometers if the fault zone originally dipped gently eastward as we propose. There is no direct evidence for major strike slip along the BMFZ as has been proposed by others; any such slip must have occurred before the reverse‐slip related structures of the zone. We thus interpret the BMFZ as an intra‐arc reverse fault of moderate to large displacement, with possible earlier strike slip, that incorporates ophiolitic basement showing an older, complicated history.

Volcanological development of the Nakuru area of the Kenya rift valley

The Nakuru area lies within the inner graben of the central Kenya rift valley and has experienced continual outpourings of trachyte magma during the last 6.2 Ma. Basalts are rare, and volcanic rocks of intermediate composition are not exposed. Sedimentary rocks are intercalated with the volcanic rocks. Volcanic rocks older than 6.2 Ma must underlie the area, contributing to the total volcanic and sedimentary rift fill which is constrained by seismic data to be 4 km thick. Five major stratigraphic groups of trachytic volcanic rocks are identified. They consist of voluminous ignimbrites (at least seventeen) with individual volumes probably 20–30 km 3, lava flows, with volumes of 0.01–2.0 km 3, strombolian deposits, widespread (probably mostly sub-plinian) air-fall deposits and hydromagmatic tuff cone deposits. Because the lithology of these volcanic rocks, and apparently their typical volumes, have remained unchanged for 6 Ma, it is considered that the sub-volcanic plumbing systems in this part of the rift also remained similar. The various eruptive styles of trachyte can be explained by eruption from relatively large magma chambers, capped by trachytic magmas and emplaced in the upper crust, like that thought to exist below the Recent volcano Menengai. It is argued that no large volume trachyte lava flows that might have erupted directly maom lower crustal magma chambers exist in the Kenya rift. A caldera complex that may have overlain an earlier magma chamber emplaced in the upper crust was centered on Olongai, NW of Menengai. Menengai, the Olongai complex, and all the Quaternary volcanoes to the S of the Nukura area, lie above a ridge of high gravity that extends along the axis of the inner trough, and which represents a ca 15 km wide linear zone of Late Miocene to Recent mafic dykes and cumulate rocks that formed the roots of trachytic magma chambers.

Deep-water basins within the Mesozoic carbonate platform of Argolis, Greece

In the past the classic Argolis area of the central southern Hellenides was interpreted as comprised of essentially in situ carbonate platforms and basins, or, alternatively of far-travelled thrust sheets derived from a Mesozoic ocean basin sited well to the east (Vardar ocean). Our fieldwork, however, demonstrates that at two key localities an important unit of Middle Triassic–Early Jurassic deep-water redeposited carbonates (Asklipion Unit) passes laterally into shallow-water platform carbonates (Pantakrator Unit). The deep-water limestones represent basins within the platform, rather than far-travelled thrust sheets, while other slices of deep-water limestone record a platform margin, further east. The basins originated during Middle Triassic (Ladinian) rifting of the Neotethys and are floored by extrusive rocks of intermediate composition. Highly condensed Ammonitico Rosso pelagic limestones accumulated on an inferred volcanic seamount during the Mid–Late Triassic (Anisian–Carnian). The platforms slowly subsided in Early Jurassic time, while the basins partly filled with redeposited carbonates (Adhami Limestone). Shallow-water carbonates locally transgressed the margins of the deeper-water basins. The platforms were submerged in the Mid-Jurassic (Toarcian–Bathonian), with the deposition of pink pelagic Ammonitico Rosso, followed by marked subsidence and ribbon radiolarite deposition below the carbonate compensation depth (e.g. Koliaki Chert). In early Late Jurassic time (Kimmeridgian), the area was regionally compressed (‘Eohellenic’ phase), and the intraplatform basins, founded on thinned continental crust were inverted, and thrust up to several kilometres over the adjacent platform units. The deformed continental margin then flexurally subsided, ahead of emplacing Jurassic ophiolites, largely olistostromes (Migdhalitsa Unit). After taking account of up to 90 degrees clockwise palaeorotation of assumed Neotectonic age, kinematic indicators and facies trends suggest the ophiolite was derived from a Pindos ocean basin originally sited to the west. Early Cretaceous transgression resulted in the deposition of shallow-water limestone and was followed by Late Cretaceous deep-water limestone, then terrigenous flysch accumulation in a foreland basin related to final suturing of the Neotethyan ocean. Later, the area was dissected by a series of mainly E–W trending, high-angle Neotectonic normal faults, related to the Aegean back-arc extensional system.

Geochemical evidence for Proterozoic continental arc and continental-margin rift magmatism along the northern margin of the Yangtze Craton, South China

Middle to Late Proterozoic volcanic-intrusive magmatism of continental island-arc and continental-margin rift nature is well developed in the Xixiang-Beiba area, Shaanxi Province, along the northern margin of the Yangtze Craton. In the Xixiang area, volcanism evolved temporally from submarine low-K arc tholeiite through calc-alkaline to subaerial alkali or shoshonitic. This was accompanied by gradational enrichment of LREE and Zr, an increasing negative Eu anomaly, as well as strong depletion of compatible elements Cr, Co, V and Sc. The associated Hannan Intrusive Complex evolved from gabbro through tonalite-trondhjemite-granodiorite to granite with the distribution of basic to felsic plutons suggesting arc polarity. There is also a systematic lateral increase in K 2 O content, K 2 O/Na 2 O ratio, and La N /Yb N ratio towards the interior of the Yangtze Craton in these rocks. This spatial framework and compositional variation resembles that found in Circum-Pacific island arcs or active continental margins. In contrast, both volcanic and intrusive rocks in the Beiba area are characterized by alkaline compositions and a bimodal habit; they are composed of tholeiite (gabbro) and alkali rhyodacite-rhyolite (alkali granite), and lack rocks of intermediate composition. They may have formed in a continental-margin rift setting. Geological and geochemical evidence further suggests that the Xixiang paleo-arc was a mature continental arc built on older sialic basement. The emplacement of voluminous tonalites and trondhjemites thickened the crust and may have marked an important continent-building event at the northern margin of the Yangtze Craton in the Middle to Late Proterozoic.

Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field, northern Rio Grande rift, New Mexico

Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO2 rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO2) to quartz latite (67% SiO2). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO2 peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km3 high-SiO2 peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.

Constraints on melting and magma production in the crust

Major intrusions of granitic rocks are found in several tectonic settings and, in all cases, crustal melts may contribute to the volumes of granitic magma. High-grade metamorphism and partial melting of the crust take place predominantly under fluid-absent conditions. We present a model for calculating the amounts of melt that may be formed by fluid-absent breakdown of micas and amphiboles in common crustal rock types (pelitic, quartzofeldspathic, intermediate and mafic). Melt proportions depend mainly on the kind of source rock, the pressure at which melting takes place, the temperature and the hydrous mineral (H 2O) content of the source. As a consequence of the pressure dependence of water solubility in silicate melts, any given source rock will produce more melt, by a given fluid-absent reaction, at lower pressure. At a given pressure, higher-temperature reactions can produce more melt from a given source rock. Based on a survey of the compositions of common rock types, we show that the amounts of melt can vary from < 10to> 50vol.%. Thus, crustal rocks vary widely in their “fertility” as magma sources, depending on the types and amounts of hydrous phases they contain. In general, muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation. Biotite breakdown in pelites occurs at higher temperature and, because most high-grade pelites (below granulite grade) are biotite-rich, can yield up to about 50 vol.% of granitoid melt. Rocks of intermediate composition and hornblende-rich mafic rocks are potentially highly fertile magma sources also, provided that the high temperatures necessary for biotite and hornblende breakdown are realized. Pyroxene-rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile. Data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with < 30and probably< 20vol.% melt. The pressure-temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria.

Late svecofennian magmatism and tectonism, Åland, Southwest Finland

The Proterozoic, late-Svecofennian, Group III Granites of Sederholm (Ava, Seglinge, Mosshaga and Lemland) form discrete complexes distributed along a NESW line across the Åland Archipelago in southwestern Finland. They were emplaced (1840-1800 Ma) after cessation of the main Svecofennian orogeny, during the subsequent long period of continuing mantle—crust interaction and repeated crustal adjustment by shearing which culminated in the major Rapakivi Granite event. Current investigations on the island of Seglinge suggest that these complexes were rapidly emplaced at high crustal levels. Over a short time interval, a composite pipe of hybrid rocks of intermediate composition was emplaced and, with the surrounding country rocks, was subsequently disrupted and invaded by porphyritic-porphyroclastic granite to form breccia screens and ring dykes. Field relationships and textures suggest that the granite was emplaced largely as gas-borne solids. The petrography and mineralogy of the hybrid rock suite is such as could have been produced by contact anatexis—assimilation where hot, basic melts were impounded beneath older, deeper porphyritic granite sheets. The granite phase at Seglinge may be a sample of the roof of such a source complex, mobilised by volatile-discharge when the surface was breached. Compositional change in transit would be minimal. The compositional similarity of the Seglinge granite and the rapakavi granites is so close as to suggest some genetic kinship. The Group III complexes may represent local, premature, tappings from the developing rapakivi source region. The composition indicated for the granites of the source region of the hybrid suites and granites of the Group III complexes differs markedly from that of the early Svecofennian granites of the region. It is, however, closely similar to granite sheets described from SW Sweden and Greenland which have been attributed to crustal depletion associated with basic intrusion. Similar lower crust—mantle processes may have operated during the long, spasmodically eventful, period which followed the major Svecofennian orogenic activity in SW Finland.