Calc-alkaline Volcanism Research Articles

U–Pb dating on single detrital zircon grains from the Triassic Songpan–Ganze flysch (Central China): provenance and tectonic correlations

The Songpan–Ganze flysch belt (Central China) covers a huge triangular area of more than 200,000 km 2 and is bounded by the continental blocks of South China, North China and the Tibetan plateau. Detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the U–Pb method. Two samples of Middle Triassic sandstones, collected at different locations in the belt, provide identical results, which suggests similar source regions. The detrital zircons yield a wide range of ages and indicate their principal derivation from Mid-Proterozoic (1.8–2.0 Ga) source rocks with minor contribution from late Archean (ca. 2.5–2.6 Ga) material. The discordance and Pb loss patterns from low-U zircons indicate disturbances during a subsequent event which may be of Caledonian age, as suggested by concordant zircon grains at ca. 420 and 450 Ma. One sample collected within the Palang Shan Pass zone provides concordant zircon grains at around 230 Ma (231±1 Ma and 233±1 Ma). These Triassic ages are synchronous to flysch deposition and suggest intense geological activity (calc-alkaline volcanism?) at that time in the area close to the basin. The data support an origin of the clastic material mainly from a northeastern landmass, corresponding to the southern margin of the Sino–Korean craton. To a lesser degree, inputs from the Yangtze craton and possibly from the northern margin of the basin (Kunlun arc) are also detected. The age spectrum from the Upper Triassic sandstone is significantly different and shows predominance of Sinian (ca. 760 Ma) grains, probably derived from the Yangtze craton. This change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late Triassic collision between South China and North China. In the Middle Triassic, while subduction of the Songpan sea northward beneath the North China plate was still taking place, continental subduction of South China in the Dabie region was responsible for uplift of the overriding plate (i.e. the Sino–Korean craton) which supplied large volumes of sediments. During the Late Triassic, clockwise rotation of the South China block uplifted the Indo-Sinian part of the Qinling belt and closed the basin. As the accretionary wedge was thickening along the southern margin of North China, detritus derived from this source region were unable to reach the flysch basin. The age spectrum presented by detrital zircons indicates predominance of Sinian material derived from source area located on the northern margin of the Yangtze craton; a source region which was until this period swamped by Luliang material from the Sino–Korean craton.

Evolution of arc crust and relations between contrasting sources: U-Pb (age), Nd and Sr isotope systematics of the ophiolitic terrain of SW Norway

The U/Pb dating of ophiolite and arc com- plexes in the Caledonides of SW Norway has demon- strated that these spatially associated rocks are also closely related in time. A sequence of tholeiitic island arc volcanics, and an unconformably overlying sequence of calc-alkaline volcanics have been dated as 494 a 2 Ma (2r) and 473 a 2 Ma respectively. Ophiolitic crust formed both prior to, and during the first 10 Ma after the tholeiitic island arc volcanism. Boninitic and island arc tholeiitic dyke swarms intruded the ophiolites soon after they formed and represent a second phase of spreading-related magmatism in the ca 20 Ma period that separated the tholeiitic and the calc-alkaline island arc volcanism. The magmatism ended with the forma- tion of alkaline, ocean island basalt (OIB)-like magmas. Quartz dioritic and S-type granitic plutons, dated to 479 a 5 Ma and 474 +3/)2 Ma respectively, intruded into the base of the arc crust during and subsequent to the boninitic magmatism, and at the time when calc- alkaline volcanic centres developed. The quartz dioritic and the granitic rocks contain inherited zircons of Pre- cambrian age which prove the involvement of a conti- nental source. This together with the geology of the terrain and the geochemistry of these plutons suggests that the granitic magmas were partly derived from subducted clastic sediments. The Sr and Nd isotope systematics indicate that the same continental source was a component in the boninitic and the calc-alkaline magmas. While the calc-alkaline magmas may have gained this continental component at a crustal level by assimilation, both geology and isotope systematics sug- gest that the continental component in the boninitic rocks was introduced by source contamination - possi- bly by a direct interaction between the mantle source and the S-type granitic magmas. A modified mid ocean ridge basalt-like mantle source was the principal source during the earliest and the main crust forming stage. This source became replaced by an OIB-like source during the later stages in the evolution of this ancient arc.

Backarc spreading, rifting, and microplate rotation, between transform faults in the Manus Basin

The Manus Basin in the eastern Bismarck Sea is a fastopening backarc basin behind the New Britain arc-trench system. Within the basin, motion between the Pacific and Bismarck plates about a pole located at 11° S, 145° E, occurs along three major leftlateral transform faults and a variety of extensional segments. We interpret SeaMARC II sidescan and other geophysical data to show that a Brunhes age plate reorganization created new extensional boundaries and a microplate between the NW-trending Willaumez, Djaul, and Weitin transforms. Two linked spreading segments formed in backarc basin crust between the Willaumez and Djaul transforms: the ESE-trending extensional transform zone (ETZ) in the west and the Manus spreading center (MSC) in the east. Positively magnetized crust on the MSC forms a wedge varying in width from 72 km at its southwest end to zero at its northeast tip, with corresponding Brunhes spreading rates varying from 92 mm/yr to zero. The MSC forms the northwestern boundary of the 100 km-scale Manus microplate and opens at 51°/m.y. about a pole near its apex at 3°02′S, 150°32′E. Opposite the MSC, bordering the arc margin of New Britain, the microplate is bound by a zone of broadly distributed strike slip motion, extension, and volcanism. Within this area, the Southern Rifts contain a series of grabens partially floored by lava flows. Left-lateral motion between the Pacific and Bismarck plates appears to drive the counterclockwise pivoting motion of the Manus microplate and the complementary wedge-like opening of the MSC and the Southern Rifts. The pivoting motion of the microplate has resulted in compressional areas along its NE and SW boundaries with the Pacific and Bismarck plates respectively. East of the microplate, between the Djaul and Weitin transforms and within the arc margin of New Ireland, another zone of broad extension referred to as the Southeast Rifts takes up opening in a pull-apart basin. There, en echelon volcanic ridges may be the precursors of spreading segments, but erupted lavas include calcalkaline volcanics. Kinematic modeling and marine geophysical observations indicate that the responses to similar amounts of extension in the eastern Manus Basin have varied as a function of the different types of pre-existing crust: arc crust tectonically stretched over a broad area whereas backarc crust underwent relatively little stretching before accommodating extension by seafloor spreading.

Cyclic evolution of convergent plate margins indicated by time sequence of volcanism and subduction

The time succession and migration of calc-alkaline volcanism in the regions of convergent plate margins is explained by a model presented in this paper. This model is based on (1) the assumption that calc-alkaline volcanism is generated primarily in the intermediate-depth aseismic gap of the subducted lithosphere; and (2) on the combination of the specific distribution and time sequence of volcanism with the geometry of the Wadati-Benioff zone in the Tonga region and Andean South America. The distribution of earthquake foci in subduction zones and the migration of calc-alkaline volcanism indicate that the process of subduction runs in cycles connected with the creation of a system of downgoing slabs descending successively in or against the direction of the ocean floor spreading. The cyclic evolution of convergent plate margins implicates a new interpretation of metallogenic processes in continental wedges overlying subduction zones.

Spinel-peridotite xenoliths from Kapfenstein (Graz Basin, Eastern Austria): A geochemical and petrological study

Ultramafic xenoliths from the basaltic tuff of Kapfenstein (Graz Basin) are mainly protogranular and protogranular- to- porphyroclastic spinel lherzolites. In addition to these common rocks, a coarse-grained spinel harzburgite and a heterogeneous spinel peridotite consisting of a clinopyroxene-poor harzburgite and a clinopyroxene-rich lherzolite have been found. Interstitial amphiboles (pargasite and kaersutite) are present in most xenoliths. The magmatic activity which brought these xenoliths to the surface is part of the Plio-Pleistocene extension-related alkali basalt volcanism which took place in the Carpatho-Pannonian Region, post-dating the subduction-related calc-alkaline volcanism which formed the Inner Carpathian arc.

Sumatran segment of the Indonesian subduction zone: morphology of the Wadati-Benioff zone and seismotectonic pattern of the continental wedge

The results of a detailed study on the geometrical distribution of earthquake foci in the Sumatran region allowed the authors to distinguish the foci belonging to the recent Wadati-Benioff zone from those occurring in the continental wedge. The morphology of the Wadati-Benioff zone, its main geometrical parameters and the variable depth of its penetration into the upper mantle were established. The existence of an intermediate-depth aseismic gap in the Wadati-Benioff zone was discovered and its spatial relationship with young calc-alkaline volcanism was confirmed. The subduction process was correlated with the stratigraphy and geology of the Sumatran region. The duration of the present cycle of subduction was estimated to be 6–8 Ma. The occurrence of Oligocene volcanism and the locations of deep earthquakes point to the existence of a Tertiary subduction zone underlying the present slab. The seismotectonic pattern of the continental wedge was described by 11 seismically active fracture zones, the orientation and tectonic function of which were checked by fault plane solutions.

Recent geodynamic evolution of convergent plate margins and the reconstruction of fossil plate boundaries

The discovery of paleoplates buried in the upper mantle leads to an interpretation of the subduction as a discontinuous process running in cycles and shifting the place of its operation in or against the direction of ocean floor spreading. This mechanism explains the distribution of calc-alkaline volcanism of different age in fossil convergent plate boundaries. The establishment of regular spatial correlation of the aseismic gap in the Wadati-Benioff zones with the distribution of calc-alkaline volcanism enables to reconstruct fossil plate boundaries and to define allochtonous terranes in apparently homogeneous continental plates. The hampering effect of the ocean floor morphology and of the fragments of continental plates approaching the trench, which substantially influences the rates of subduction and the geodynamic history of active continental margins in different domains along the trench, allows us to understand the complicated geological development of continental wedges in fossil convergent plate margins. The establishment of the segmented nature of active subduction zones and the dramatic morphology of the lower limit of the active subducted slab along the trench help us to interpret extensive lateral gaps in volcanic chains overlying active as well as fossil subduction zones.

The Cerro Mencenares volcanic center, Baja California Sur: Source and tectonic control on postsubduction magmatism within the Gulf Rift

At the end of active subduction along the North American plate (12.5 Ma), in the Baja California peninsula arc magmatism was replaced mainly by alkali-rich high-Mg basaltic andesites and andesites with peculiar and unusually high K/Rb, Sr/Rb, and LREE/HREE ratios defined “bajaite” by Rogers et al. (1985) and “Baja alkalic suite” by Sawlan (1991). Two interesting features of this magmatic change are represented by (i) emplacement of transitional, high-K calc-alkaline andesites that preceded, in places, the Baja alkalic suite magmatism and (ii) subduction unrelated medium-K calc-alkaline magmatism in Baja California Sur, active in areas within the Gulf of California Rift, for example, Tres Virgenes–Reforma caldera, Isla Coronado. A previously unknown example of this calc-alkaline activity was studied in the area of Cerro Mencenares located north of Loreto, Baja California Sur. The Mencenares volcanic complex is located along the Gulf coast, east of the main Gulf escarpment. Activity of the Mencenares volcanic complex occurred in three stages: (i) an initial phase of rhyolitic dome emplacement; (ii) andesitic to dacitic volcanism that produced the Mencenares stratovolcano; (iii) late-stage emplacement of silicic domes and flows, mostly dacite and rhyolite, along north-south–trending normal faults. Tephra and lava flows interfinger with marine sediments of the Pliocene Loreto Basin and the youngest volcanic products were probably emplaced during Quaternary times. This volcanism is coeval with the Baja alkalic suite magmatism west of the Gulf escarpment. In the latter areas, Baja alkalic suite magmas ascended rapidly with only limited differentiation. In the Mencenares volcanic complex, a complex tectonic framework favored the formation of crustal magma chambers where more complex phenomena such as extensive differentiation and mixing could take place. A change from medium-K calc-alkaline toward Baja alkalic suite–like chemical characteristics is present in the younger products of the Mencenares volcanic complex and mixing possibly occurred between medium-K calc-alkaline and Baja alkalic suite magmas. Magma evolution in the Loreto area suggests that (i) Baja alkalic suite and medium-K calc-alkaline magmas can coexist in time and interact in the same volcanic field; (ii) if a relic, subduction-related component is the main cause of medium-K or high-K calc-alkaline volcanism then this component appears to decrease from east to west; and (iii) although prior calc-alkaline magmatism may not be unquestionably considered a necessary condition for the generation of Baja alkalic suite magmas, episodes of medium-K or high-K calc-alkaline melt extraction from mantle sources precede or accompany Baja alkalic suite magmatism in many instances.

Detrital Opaque Oxides as Provenance Indicators in River Nile Sediments

ABSTRACT Heavy-mineral assemblages from the rivers and wadis in the River Nile drainage basin reflect their origins in three petrologically and tectonically distinct source terranes that supply sediment to the River Nile and delta. The White Nile carries heavy-mineral assemblages dominated by calcic amphibole, epidote, garnet, staurolite, aluminosilicates, and opaques (ilmenite and titanomagnetite) derived from amphibolite- and granulite-grade metamorphic rocks of the Central African Province. The Blue Nile and Atbara Rivers carry assemblages dominated by calcic amphibole, clinopyroxene, and opaques derived from alkali and tholeiitic basalts of the Ethiopian Highlands. Wadis (dry river beds) draining the Red Sea Hills contribute predominantly clinopyroxene, epidote, calcic amphibole, and opaqu s eroded from ophiolite complexes, calc-alkaline volcanics, and granitoids as well as minor metamorphic minerals from amphibolite-grade metapelites. Detrital ilmenite grains from the three sources are distinguished on the basis of TiO2, Al2O3, Cr2O3, MgO, ZnO, and V2O3 but not MnO contents determined by electron microprobe analysis. These data corroborate findings of earlier workers with respect to Al2O3, MgO, V2O3 and MnO in detrital ilmenite. Submicroscopic inclusions of quartz and sphene recognized on the basis of elevated SiO2 and CaO contents in ilmenite also distinguish different sources. Statistical and graphical analyses of the compositional data lea to the same conclusions about ilmenite chemistry and source areas. Opaque oxide (ilmenite and titanomagnetite) grains also have textural features unique to their sources. Though most grains are texturally homogeneous, ilmenite from the Central African Province contains exsolution lamellae in one crystallographic direction (0001) and titanomagnetite from the Red Sea Hills showing exsolution textures has octahedral {111} inter-growths. Taken together, compositions and textures of detrital opaque oxide grains from Nile River sediment sources can be used as provenance indicators of Nile delta sediments. This study also provides ties between igneous, metamorphic, and sedimentary petrology that are relevant for provenance studies of ancient river systems.

Tectonics and deep structure of the north-western Mediterranean Basin

Crustal data, derived from geophysical experiments in the Valencia Trough and the Provençal Basin, are compared and integrated in the framework of the north-western Mediterranean Basin. This basin is a Neogene extensional rift, located within a region of compression between the European and African plates. The northern margin of the Valencia Trough and the Provençal Basin can be compared with other Atlantic-type margins, whereas the southern margin is affected by calc-alkaline volcanism related to subduction. Thinned continental crust, covered by a thick volcanic layer in the deep Catalan margin, is widespread in the Valencia Trough and on both sides of the Provençal Basin, whereas oceanic-type crust is restricted to a narrow domain. The Provençal Basin may have been formed by a simple shear process behind a subduction zone. A geodynamic consequence of this simple shear model is overlap of the thin crust of the Provençal Basin by the thick crust of the Sardinia margin.

Petrogenetic significance of peralkaline rocks from Cenozoic calc-alkaline volcanism from SW Sardinia, Italy

The Tertiary volcanism of SW Sardinia is characterized by close field association of lower sequences characterized by calc-alkaline basaltic andesites, andesites and minor basalts (28.5 ± 1 to 17 ± 0.7 Ma) and by an upper sequence (up to 500 m thick) (mainly composed of ignimbrites) ranging from dacites to calc-alkaline rhyolites and comendites in the upper part (17.6 ± 0.8 to 13.8 ± 0.7 Ma).In this paper we focused on petrological aspects of the upper sequence in an attempt to understand the origin of peralkaline rocks generally characterizing intraplate volcanic settings (e.g., main Ethiopian rift) associated with rock series of transitional character. In Sardinia, comendites are associated with calc-alkaline rocks related with subduction of oceanic lithosphere. In this context, ignimbrites from the Northern sector have been interpreted as products of partial melting of continental crust.Five major groups were distinguished on the basis of stratigraphy, mineral assemblages, whole-rock and glass chemistry: 1.(a) G1 dacites (PlAn68–42 + Opx + Cpx ± altered Ol and locally PlAn58–43 + Bt + Amph ± Opx ± Cpx);2.(b) G2 rhyodacites (PlAn46–29 + SanOr66 + Opx + Cpx ± Ol ± Bt);3.(c) G3 rhyolites “transitional” to peralkaline rhyolites (PlAn40–26 + SanOr55–42 ± Anor ± Opx ± Cpx);4.(d) G4 comendites (Na-SanOr46–39 ± Anor + Qz + Na-Px + Na-Amph);5.(e) G5 post-comenditic rhyolites “transitional” to peralkaline rhyolites (PlAn26–24 + Na-SanOr42–40 ± Anor).Ti-Mt + Ilm ± Ap appear to be the common accessory phases in all groups; however, Zir + Mt were also found in the G3 and G4 groups.Petrographic data and variation of chemical compositions of the main mineral phases (i.e. % An in plagioclases and Mg# in mafic minerals) suggest that the whole sequence is genetically related. Geothermometric and geobarometric calculations gave liquidus temperatures decreasing from G1 to G4 from 1100° to ∼950°C and pressures of ∼5–7 kbar. The latter should correspond to the depth of magma evolution for the investigated rocks.The agpaitic index increases with SiO2 and shifts from subalkaline to peralkaline values at the transition from G3 to G4.The chemical evolution observed in the upper sequence could be governed by dominant crystal/liquid fractionation processes of the observed mineral assemblage, leading to residual peralkaline liquids. Particularly, trace elements show a general enrichment of incompatible elements and a strong enrichment for Zr and Nb, accompanied by depletion of compatible elements (Sr and Ba). Furthermore, the chondrite-normalized rare-earth element (REE) diagram shows a comparable pattern characterized by a progressive REE enrichment from G1 to G4 with an increasing negative Eu anomaly, which confirms that feldspar is the dominant fractionating phase.Mass-balance calculations are consistent with a magmatic system dominated by crystal/liquid fractionation processes. This model confirms that the comendite rocks can be related with the subalkaline upper sequence from SW Sardinia.(87Sr86Sr)o at 16.5 and 15 Ma for subalkaline and peralkaline units in the upper sequence are 0.70750 and 0.70640, respectively, and are well comparable with the values for the andesites. This confirms that the whole sequence from the Sulcis area is genetically related and crystal/liquid fractionation was the dominant process in the evolution of the upper sequence.

Neogene ignimbrites of the Nevsehir plateau (Central Turkey): stratigraphy, distribution and source constraints

In Anatolia (Turkey), extensive calc-alkaline volcanism has developed along discontinuous provinces from Neogene to Quaternary times as a consequence of plate convergence and continental collision. In the Nevsehir plateau, which is located in the Central Anatolian Volcanic Province, volcanism consists of numerous monogenetic centres, several large stratovolcanoes and an extensive, mainly Neogene, rhyolitic ignimbrite field. Vent and caldera locations for the Neogene ignimbrites were not well known based on previous studies. In the Neogene ignimbrite sequence of the Nevsehir plateau, we have identified an old group of ignimbrites (Kavak ignimbrites) followed by five major ignimbrite units (Zelve, Sarimaden Tepe, Cemilköy, Gördeles, Kizilkaya) and two smaller, less extensive ones (Tahar, Sofular). Other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau. Excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as, from bottom to top: Kavak, Zelve, Sarimaden Tepe, Cemilköy, Tahar, Gördeles, Sofular, Kizilkaya. Our stratigraphic scheme refines previous ones by the identification of the Zelve ignimbrite and the correlation of the previously defined ‘Akköy’ ignimbrite with the Sarimaden Tepe ignimbrite. Correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria: (1) sedimentological characterisitics; (2) phenocryst assemblage; (3) pumice vesiculation texture; (4) presence and characteristics of associated plinian fallout deposits; and (5) lithic types. The correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites: volumes range between 80 km 3 and 300 km 3 for the major ignimbrites, corresponding to 2500–10,000 km 3 in areal extent. The major ignimbrites of the Nevsehir plateau have an inferred source area in the Derinkuyu tectonic basin which extends mainly between Nevsehir and the Melendiz Dag volcanic complex. The Kavak ignimbrites and the Zelve ignimbrite have inferred sources located between Nevsehir and Derinkuyu, coincident with a negative gravity anomaly. The younger ignimbrites (Sarimaden Tepe, Cemilköy, Gördeles, Kizilkaya) have inferred sources clustered to the south between the Erdas Dag and the Melendiz Dag volcanic complex. We found evidence of collapse structures on the northern and southern flanks of the Erdas Dag volcanic massif, and of a large updoming structure in the Sahinkalesi Tepe massif. The present-day Derinkuyu tectonic basin is mostly covered with Quaternary sediments and volcanics. The fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding. Drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred, probably coalesced or nested, calderas within it.

The Gunung Pongkor gold-silver deposit, West Java, Indonesia

The Gunung Pongkor gold-silver deposit in West Java is one of the most recent and largest gold discoveries in Indonesia, containing more than 100 tonnes of gold and 1000 tonnes of silver. It consists of several steeply dipping quartz-veins that formed within the epithermal environment, associated with Neogene calc-alkaline volcanism. The discovery and definition of the orebody resulted from a three year long exploration programme using a variety of geological, geophysical and geochemical techniques. Production is expected to commence in 1994 at an annual rate of 2 tonnes of gold. Vein textures (colloform banding and cockade textures) and mineralogy (quartz, carbonate, adularia) are typical of those encountered in low-sulphidation vein deposits. Mineralogical and fluid inclusion evidence suggests that the mineralizing fluids were dilute (around 1 eq.wt.% NaCl), near-neutral pH brines with a moderate gas content and temperatures of around 230°C, and that they were boiling under conditions which favoured gold deposition. There is little variation in mineralogy, fluid inclusion data and gold grade throughout the drill-tested portion of the vein systems.

Geology, geochemistry, and origin of high sulfidation Cu-Au mineralization in the Nansatsu District, Japan

The Nansatsu district of southern Kyushu has been the site of calc-alkaline volcanism for the last 10 m.y., shifting eastward with time. Associated hydrothermal activity followed deposition of the volcanic host rocks by about 0.5 m.y. and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions, resulting in the formation of high sulfidation Cu-Au deposits at Kasuga, Iwato, and Akeshi. The orebodies consist of 95 wt percent SiO 2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride-sulfate waters. These are inferred to have formed when magmatic vapors containing HCl and SO 2 condensed into meteoric water. The residual silica (now quartz) orebodies are best developed where the host was initially permeable. The margins of the quartz bodies are abrupt, with narrow (1-2 m) halos representing the reaction front of acid waters isochemically dissolving the host rock. The halo comprises alunite (strongly zoned in Na and K, with P-rich cores), dickite, and/or kaolinite+ or -pyrophyllite, grading out into illite and interlayered illite-smectite clays, and finally, propylitic alteration. This pattern is characteristic of deposits of this type throughout the world, for example, at Summitville, Colorado, and Lepanto, Philippines. Mineralization occurred after initial leaching by the vapor condensates, with metals transported by a dense magmatic fluid. Mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies; ore grades are restricted to the quartz bodies. Gold is most closely associated with enargite and pyrite; later minerals include covellite and then sulfur. The last stage of activity was steam-heated, with descending waters oxidizing sulfides to goethite and locally remobilizing Au into fractures (this varies in degree between deposits). Erosion exposed the orebodies to supergene weathering, continuing the sulfide oxidation and Au remobilization. Stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid (delta 18 O = 7+ or -2 per thousand, delta D = -25+ or -5 per thousand, similar to nearby active volcanic discharges) with local meteoric water. In contrast, the clays in the marginal halo have isotopic compositions indicating a delta 18 O shift of 6 to 8 per mil from local meteoric water values, probably due to water-rock interaction, and the delta 18 O values of residual silica quartz may also be due to meteoric water domination. Fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent NaCl equiv during late quartz growth, though there is evidence in one sample for higher salinity fluid having been present, up to 30 wt percent NaCl equiv (some inclusions contain daughter minerals of halite and sulfur). The T h values of over 1,000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from 200 degrees C at Akeshi to about 200 degrees C at Iwato and 230 degrees C at Kasuga. The presence of vapor-rich inclusions, some with T hv similar to T hl , indicate the presence at times of a two-phase fluid in the center of the ore zones, with depths of about 150 to 300 m below the paleowater table. The mineralizing fluid was relatively oxidized (sulfide/sulfate ratio about 3:1), close to pyrite-alunite coexistence. Under these redox conditions, a pH of 3 and over a temperature range of 200 degrees to 300 degrees C, AuCl 2 (super -) complexing may dominate over HAu(SH) 2 at salinities above about 2 wt percent NaCl. Several conditions are conducive for high sulfidation mineralization to occur: (1) a crystallizing magma exsolves a fluid, with lower pressure conditions favoring metal fractionation from melt to fluid, (2) the exsolved fluid separates into vapor and saline liquid phases due to immiscibility, with the latter being metal rich, (3) the gas-rich (HCl and SO 2 +H 2 S) vapor ascends to the surface with at least a portion condensed into meteoric water, forming an acid fluid which leaches the host rock to create permeable zones for later mixing, and (4) the dense, metal-bearing fluid also ascends into this leached zoned and precipitates Cu sulfosalts, sulfides, and Au upon mixing with meteoric water. If the saline liquid is not released from its source adjacent to the magma, due to lack of fracturing, or if there is a strong hydraulic gradient caused by high relief, only the vapor-related stage may occur. This will leave leached barren rock which is characteristic of many eroded volcanic terranes.

Geochemical zonation of volcanic rocks and deep structure of Ecuador and southern Colombia

The chemical composition of 104 samples of young volcanic rocks from the Western and Eastern Cordilleras of Ecuador and southern Colombia was correlated with the deep structure of the Andean region bounded by latitudes 1.5° N and 2° S and longitudes 77° W and 79° W. The geochemical zonation of the volcanic products apparently depends on the depth of the primary source of calc-alkaline magma in the partially melted part of the subduction zone. It was found that the contents of K 2O, U, Th, Rb, Sr, Ba, Zr, V, TiO 2 and P 2O 5 in andesites significantly increase with the depth of the upper boundary of the intermediate depth aseismic gap in the Andean Wadati-Benioff zone. The differences in average elemental abundances for volcanic centers in southern Colombia suggest that recent calc-alkaline volcanism in Ecuador and southern Colombia are related to different subducted lithospheric plates. Analysis of the geographical distribution of andesite and dacite samples with apparent anomalous enrichment of both the compatible and incompatible elements in relation to the subduction-dependent pattern of deep-seated fracture zones showed that the geochemistry of the volcanic products in convergent plate margins may be considerably influenced by the deep fault structure of the continental wedge and need not be identical with the geochemistry of the inferred primary magma.

Geochemical, Sr-Nd Isotopic, and P-T Data on a Central Mexican Xenolith Suite: Evidence for Upper Mantle and Lower Crust Compositions

Northeast of San Luis Potosi (Central Mexico, 22~ 101~ Quaternary basanites and tuff breccias have sampled a suite of ultramafic upper mantle to lower crustal xenoliths, associated with seven phreatomagmatic maars. Xenolith distribution range from spinel lherzolites (55%), spinel pyroxenites (10%), hornblende pyroxenites (18%) to metaigneous and metapelitic granulites (17%). The SLP volcanic field is located in the back-arc region of two major volcanic belts along the southwestern border of the North American craton (Fig.l): The Oligocene calc-alkaline volcanics from the Sierra Madre Occidental in the east and the late Miocene to recent basaltic to rhyolitic volcanics f rom the Transmexican Volcanic Belt. Analysis of the geochemical and Sr-Nd isotopic data together with p-T-conditions of the xenolith profile gives hints for the thermal evolution and composition of the upper mantle segment and lower crust in Central Mexico. The latter is of

Mineral/matrix partition coefficients for orthopyroxene, plagioclase, and olivine in basaltic to andesitic systems: A combined analytical and experimental study

Mineral/matrix partition coefficients have been determined for a large suite of elements including Li, Be, Ti, V, Cr, Mo, Rb, Sr, Y, Zr, Nb, Cs, Ba, REEs, Ta, Pb, Th, and U for olivine, orthopyroxene, and plagioclase from a suite of calc-alkaline volcanics from Mt. Adams, Washington. The partition coefficients were determined via a combination of ICP-MS bulk analyses and IPMA microanalyses. The IPMA data were used to correct the larger ICP-MS data set for matrix contamination effects. Crystallization temperatures and the extent of equilibration between crystalline phases and host matrices were evaluated via 0.1 MPa phase equilibrium experiments. The result is a comprehensive set of partition coefficients, determined simultaneously on the same samples, which may be applied to studies of the petrogenesis of anhydrous calc-alkaline magmas.

Petrochemical and petrological characteristics of 1.9 Ga old volcanics in northern Sweden

Abstract Palaeoproterozoic, ∼ 1.9 Ga old, subaerial felsic to mafic volcanics of the Kiruna-Arvidsjaur Porphyry Group (KAPG) are abundant in northernmost Sweden. The felsic-dominated KAPG is situated north of a proposed north-dipping subduction zone along the boundary between the Archaean craton in the northeastern part of the Baltic Shield and juvenile Proterozoic crust in the south. The geochemistry of the volcanics indicates that plate-tectonic and volcanic-arc processes similar to recent ones occurred in northern Sweden ∼ 1.9 Ga ago. The volcanics were formed in extensional as well compressional environments. Calc-alkaline volcanics mark compression in the south and in the east. Mildly alkaline volcanics occurring in the west were formed in an extensional setting. The same rift zone gave rise to a basin filled with a thick sequence of younger sediments. On the basis of regional differences in geochemistry and petrology, the KAPG has been divided into the Kiruna, Arjeplog. Arvidsjaur and Lulea subprovinces. The volcanics in the Skellefte ore district south of the KAPG occurrence area have a volcanic-arc character and can be related to the volcanics of the Arvidsjaur subprovince immediately to the north; however, the latter are subaerial whereas the Skellefte volcanics represent a marine facies. The northerly Kiruna subprovince represents a different tectonic environment with a mafic to intermediate, trachytic volcanism that indicates extensional environments. The pattern of the different geotectonic settings indicates that the development of the ∼ 1.9 Ga old volcanics cannot be assigned solely to the action of a subduction zone dipping northwards beneath the Skellefte district.

Initiation and evolution of the Oligo-Miocene rift basins of southwestern Europe: Contribution of deep seismic reflection profiling

Southwestern European Oligo-Miocene rift basins have recently been investigated by deep seismic reflection profiling. The study of these data, together with other geophysical and geological data, shows that the rifts, which run from the Rhinegraben to the western Mediterranean, do not form a single clearcut system. The N-trending rifts (Rhinegraben, Bresse and Limagne) were developed on a cold and rigid lithosphere affected by the Alpine collision. The NE-trending rifts (southeastern France, Gulf of Lions and Valencia Trough) were formed slightly later in a backarc basin associated with an active segment of the European-Iberian plate that was heated, affected by widespread calcalkaline volcanism and probably weakened. All the southwestern European rifts and basins together may, however, be related to a common heritage represented by the boundary between the European-Iberian and African-Apulian plates that was created in the Jurassic with the initiation of the Tethys Ocean. The present features of the southwestern European Oligo-Miocène rift basins may be explained by a combination of three geodynamic mechanisms: mechanical stretching of the lithosphere, active mantle uplifting, and subordinate lithospheric flexuring. All the rifts were probably initiated by passive stretching. A systematic discrepancy between stretching derived from fault analysis and attenuation of the crust has been observed in all the rifts. This suggests that these rifts were subsequently reworked by one or several active mantle upwelling events associated with late shoulder uplift, asthenosphere upwelling and anomalous P-wave velocities in the lowermost crust and the uppermost mantle. Crustal attenuation may have been achieved by mantle intrusion, metamorphism of the deep crust and/or its delamination. Some of the rifts were affected by lithospheric flexuring. Combinations, in various proportions, of a small number of geodynamic mechanisms probably controlled many basins in the world. This explains the unique characteristics of each basin, difficulties in basin classification and the frequent failure of single-mechanism models to explain the geological observations.

A model of ocean-crust accretion for the Superior province, Canada

One of the keys to understanding the origin of Archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones, felsic to intermediate volcanic rocks and terrigenous sediments. Traditional models for greenstone belt evolution have been based on in-situ stratigraphic relationships. Most of these models, for example an oceanic island-arc developed on oceanic basement, back-arc basins, and the recently popular plume model, predict concordant stratigraphic relationships among the various greenstone belt lithologies. However, rather than being depositional in nature, several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an allochthonous origin for most greenstone sequences. All of these latter models make analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting. In this paper, we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the Superior province, where the accretion of oceanic material can be documented, and direct comparisons are made to geological processes in Phanerozoic accretionary complexes. In the Malartic area in the southeastern Abitibi Subprovince, 3 to 4 km thick slices of komatiite and tholeiite, with intercalated terrigenous sediment, are tectonically imbricated and are overlain by calc-alkaline volcanics which postdate tectonic stacking. In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the south-eastern limit of the Wabigoon belt, slices of iron-rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment. The Malartic-Val d'Or area is considered to be an example of accretion of an Archaean oceanic plateau, while the Larder Lake and the Beardmore-Geraldton regions are potentially typical of accretion of normal oceanic crust in an arc-environment. Phanerozoic accretion of oceanic crust is accompanied by a step-back in subduction, and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late-magmatism. This scenario requires that magmas be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the plate tectonic scenario envisaged would result in the well-described “cyclic stratigraphy” of many granite greenstone sequences.