Hypabyssal Intrusions Research Articles

Quenching and hydroclastic disruption of andesitic to rhyolitic intrusions in a submarine island-arc sequence, northern Sierra Nevada, California

Devonian submarine island-arc deposits of the Sierra Buttes Formation in the northern Sierra Nevada are penetrated by numerous penecontemporaneous hypabyssal intrusions, but related extrusive lavas are absent. Significant parts of the arc succession are occupied by andesitic to rhyolitic intrusive complexes that record the quenching and disruption of large volumes of magma injected into wet sediments at shallow levels beneath the sea floor. Extensive masses of intrusive hydroclastic breccia are characterized by in situ fragmentation textures and developed by progressive brittle disintegration of quenched magma. Pillows dispersed within the breccia formed as a series of interconnected tubes that supplied magma to the growing intrusive masses. Beds of redeposited hydroclastic breccia indicate that the intrusions locally reached the sea floor to undergo slumping and debris flow. Large-scale steam explosions did not occur during magma/wet-sediment interaction, probably due at least in part to the lithostatic and hydrostatic pressure acting on the intrusions. Low-viscosity rhyolitic magma developed intrusive pillows and in situ hydroclastic breccia in the same fashion as quenched andesitic magma. This fluid behavior was favored by retention of volatiles in the rhyolitic magma under relatively deep marine conditions. Quenched and disrupted hypabyssal complexes of the type described herein may be common elements of the submarine parts of island arcs and other volcanic environments where uprise of magma through thick sequences of wet sediment is impeded due to buoyancy constraints and rapid chilling.

Литостратиграфия мезозойских пород в Восточных Родопах

Mesozoic slightly metamorphosed sedimentary and volcano- sedimentary rocks have been proven in the south-eastern part of the Eastern Rhodopes around the villages of Bregovec, Orešino, Mandrica and Dolno Lukovo. They are subdivided into two groups: Mandrica Group (new group) - with supposed Triassic or Triassic - Early Jurassic age, and Maglenica Group (new group) - with Early Jurassic and Late Cretaceous age. Mandrica Group is built up of sedimentary and sedimentary-volcanogenic rocks accompanied by basic and ultrabasic magmatic rocks, all subjected to greenschist facies to epidote-amphibolite facies regional metamorphism. Two formations are subdivided within this group: Orešinovo Formation (new formation) mainly metasedimentary, with intrusive basic igneous rocks, and Gorno-Lukovo Formation (new formation), predominantly volcanogenic. Maglenica Group is represented by two formations: Dolno-Lukovo Formation (new formation), and Meden-buk Formation (new formation). Dolno-Lukovo Formation is of sedimentaryvolcanogenic composition and Early Jurassic age. It contains olistostroms with Upper Permian and MiddleUpper Triassic silicites and organogenic limestones as well as boudinaged bodies of tholeiitic basalts - clastolavas. The degree of alteration is anchimetamorphic. Meden-buk Formation belongs to the Upper Senonian (Campanian). It is sedimenrary volcanogenic, non-metamorphic, with basaltoid andesites. It is intruded by diorite-granodioritic hypabyssal intrusions. The rocks of Mandrica Group and Dolno-Lukovo Formation from the Eastern Rhodopes on Bulgarian territory are correlated after their composition, metamorphism and age with the lithostratigraphic units Makri and Drimos-Melia, described on Greek territory in Western Thrace. The Mesozoic rocks in the Eastern Rhodopes build together with the high-grade Precambrian metamorphics an allochthonous sheet (East-Rhodope complex nappe) which has been thrust over paraautochthonous in tensely diaphthorized Precambrian ultrametamorphics. The presence of Precambrian, Paleozoic, Triassic, Jurassic and Upper Cretaceous rocks in complex allochthonous structures supports the idea about the existence of a complex collisional orogen.

Paleomagnetic data from Sarawak, Malaysian Borneo, and the Late Mesozoic and Cenozoic tectonics of Sundaland

Paleomagnetic data from 231 samples from 31 sites in rocks of Upper Jurassic to Miocene age in Sarawak (Malaysian Borneo) reveal a trend of increasing counterclockwise (CCW) declination deflection with age. Six sites in Tertiary hypabyssal intrusions show 8° to 52° of CCW deflection. The intrusion deflected 52° CCW was K‐Ar dated at 26 m.y. (Upper Oligocene), while one deflected 22° CCW gives a 17 m. y. age (Lower Miocene). Three sites in the Upper Eocene to Miocene(?) Silantek Formation show an average 40° of CCW deflection. Prefolding directions, showing 90° of CCW deflection, are isolated in 4 sites (including two positive fold tests) in Upper Jurassic and Cretaceous rocks of the Bau Limestone and Pedawan Formations. A postfolding, Cenozoic remagnetization with an average of 60° of CCW deflection is found in five Bau Limestone sites. Three sites in the Upper Jurassic Kedadom Formation show an average of 50° of CCW deflection. CCW declination deflections found in Mesozoic and Cenozoic rocks as far as 400 km east and 150 km south of Sarawak, in Kalimantan (Indonesian Borneo), also fit the trend of deflection versus age. On the basis of the regional consistency of declination deflection versus age, along with geologic evidence the data are considered to be evidence of a regional (rather than a local block or distributed shear) rotation. The domain of CCW rotation extends into West Malaysia, suggesting that West Borneo and the Malay Peninsula may have been a stable block during the latest Cretaceous and Cenozoic. West Malaysia and Borneo may have had different histories in the rest of the Mesozoic. The data imply up to 108° CCW rotation of Borneo with respect to stable Eurasia, sometime during the Cretaceous and Cenozoic. Cenozoic rotation may also have occurred between Indochina and Borneo. The sense of rotation shown by the data does not support the “propagating extrusion tectonics” model for Cenozoic Southeast Asia.

Provenance of Lower Old Red Sandstone conglomerates, SE Kincardineshire: evidence for the timing of Caledonian terrane accretion in central Scotland

Northerly-derived conglomerates dominate much of the thick Lower Old Red Sandstone (ORS) succession of the NE Midland Valley of Scotland. These contain a diverse suite of granitoid boulders, the provenance of which can be used to constrain the timing of the post-Caradoc displacements which juxtaposed the Highland Border Complex against the Grampian terrane. Rb-Sr muscovite-whole rock ages for two-mica granite boulders from the Crawton Group are 473–457 Ma, confirming the persistence of an Ordovician two-mica granite source first recognized in the Dunnottar Group. The Rb-Sr muscovite ages are supported by a U-Pb age of 475 ± 5 Ma for monazite separated from one of the clasts. A Rb-Sr biotite age of 412 ± 4 Ma for a clast of porphyritic biotite granite and geochemical evidence linking this clast to many of the other biotite granite clasts, suggests that late Silurian granitoids and related hypabyssal intrusions have also made an important, hitherto unrecognized, contribution to the conglomerates. Mica-whole rock ages for accompanying psammitic clasts suggest the source included metamorphic rocks (of up to at least staurolite grade) whose uplift overlapped with the intrusion of crustally-derived melts. The chemistry, chronology and Sr and Nd isotopic compositions of the various igneous clasts are shown to resemble closely the Ordovician through to Silurian magmatic history of a small area of the Grampian terrane in NE Scotland. This is taken as strong evidence for a pre-Lower ORS juxtaposition of the Highland Border Complex and the Grampian terrane, to within several tens of kilometres of their present position. Ashgill–Silurian terrane displacements are invoked to achieve this. Discrepancies in the proportions and types of clast occurring in the conglomerates are explained by: uplift and unroofing of the Grampian terrane prior to docking; the involvement of polycyclic sediment, perhaps as a local veneer of pre-existing sediment on the Grampian terrane; and the likelihood that the source involved parts of the local Grampian terrane which are no longer exposed.

Evidence and implications of an extensive early Mesozoic rift basin and basalt/diabase sequence beneath the southeast Coastal Plain

Regional seismic reflection profiles across the Georgia and South Carolina Coastal Plain provide a detailed view into the internal structure and stratigraphy of the buried Triassic-Jurassic South Georgia basin. Reflection data indicate that the basin is a complex composite of smaller, individual basins which vary drastically along strike. These basins are relatively wide along north-south profile (maximum >100 km) but appear to be more restricted along strike. In cross section, the basins are asymmetric, being typically bounded on their south sides by an interpreted north-dipping master normal falult. The thickness of interpreted basin fill below the Cretaceous-Tertiary overlap also varies drastically, locally reaching 6 km. Analysis and correlation of the reflection profiles, when integrated with drill-hole and offshore reflection data, suggest the presence of an areally extensive basalt flow and/or diabase sill sequence of probable Early-Middle Jurassic age, covering a large part of the Triassic-Jurassic South Georgia basin. The sequence is traceable on seismic reflection profiles across much of the South Georgia basin and may extend as far as from western Georgia to offshore South Carolina. The implied areal extent of this sequence would be more than 100,000 km 2 , ranking it among the great basalt flows and/or sills of the world. On seismic reflection profiles over the South Georgia basin, the basalt/diabase sequence is also important in marking a post-rift unconformity which separates tilted and apparently faulted lower Mesozoic rift-related strata below from a thinner, more widespread, and generally unfaulted sequence above. This configuration is interpreted to mean that basalt extrusion and/or hypabyssal intrusion occurred after the main episode of Late Triassic-Early(?) Jurassic normal faulting and associated localized subsidence (syn-rift phase) and prior to a later, Jurassic episode of more widespread uniform subsidence (post-rift phase) accompanied by relatively minor normal faulting. This two-stage evolutionary sequence is analogous to that of many rift basins throughout the world and suggests a hypothesis that the South Georgia basin was an area of incipient crustal spreading associated with the early formation of the central Atlantic Ocean.

The upper Hadrynian Jeffers Group, Cobequid Highlands, Avalon zone of Nova Scotia: A back-arc volcanic complex

The upper Hadrynian Jeffers Group of the Cobequid Highlands (Avalon zone of northern Nova Scotia) consists of volcanic rocks of the Gilbert Hills Formation, interfingering deeper-water argillite of the Humming Brook Formation, and overlying turbidite lithic arkose and siltstone of the Cranberry Lake Formation. The Jeffers Group has a spatially inhomogeneous flat-lying cleavage, associated with thrust faults of late Hadrynian age that were in part reactivated during Carboniferous strike-slip motion on the Cobequid fault. The basal rocks of the Gilbert Hills Formation consist of felsic pyroclastics and minor mafic and felsic flows of the Lakelands facies, locally passing up into sandstone and marble, overlain by the Humming Brook Formation. Thrust over this sequence is the Harrington River facies of the Gilbert Hills Formation, comprising flows, ranging in composition from subalkalic basalt to rhyodacite, and pyroclastic rocks. Two groups of igneous rocks are distinguished geochemically. High-Ti basalts of the Lakelands facies and associated hypabyssal intrusions have tholeiitic character; enrichment in Ba, Rb, and K; flat light-REE spectra; little or no depletion in Nb and Ta; and high FeO t and Ti. They are associated with voluminous felsic rocks. The Harrington River facies has a calc-alkaline differentiation series from basalt to rhyodacite but with more enrichment in FeO t , P, Nb, and Zr than is found in typical calc-alkaline rocks. These two groups are also represented in hypabyssal rocks. The Jeffers Group rocks are compared geochemically with other predominantly continental back-arc basins in the Aegean Sea and Bransfield Strait. All three basins show the presence of a rock phase derived from partial melting of mantle only slightly modified by subduction processes (the Lakelands facies tholeiites of the Jeffers Group). They all also contain a more differentiated series showing more subduction-related chemical characteristics (the Harrington River facies of the Jeffers Group). Geochemical differences between these rocks and typical calc-alkali series are related to the lesser role of hydrous fluids in the back-arc basins. As a result, titanomagnetite and amphibole are not important fractionating phases, and Nb and Ta are not significantly depleted.

NATURE AND SIGNIFICANCE OF IGNEOUS ROCKS CORED IN THE STATE 2–14 RESEARCH BOREHOLE: SALTON SEA SCIENTIFIC DRILLING PROJECT, CALIFORNIA

The State 2–14 research borehole of the Salton Sea Scientific Drilling Project penetrated 3.22 km of Pleistocene to Recent sedimentary rocks in the Salton Sea geothermal system, located in the Salton Trough of southern California and northern Baja California, Mexico. In addition, three intervals of igneous rocks were recovered; a silicic tuff and two sills of altered diabase. The chemical composition of the silicic tuff at 1704 m depth suggests that it is correlative with the Durmid Hill tuff, cropping out 25 km NW of the geothermal system. In turn, both of these tuffs may be deposits of the Bishop Tuff, erupted from the Long Valley caldera of central California at 0.7 Ma. The diabases are similar to basaltic xenoliths found in the nearby Salton Buttes rhyolite domes. These diabase are interpreted as hypabyssal intrusions resulting from magmatism due to rifting of the Salton Trough as part of the East Pacific Rise/Gulf of California transtensional system. The sills apparently intruded an already developed geo‐thermal system and were in turn altered by it.

Mineralization controls and source of metals in the Lufilian fold belt, Shaba (Zaire), Zambia, and Angola

The Pan-African Lufilian fold belt hosts extensive and rich polymetallic mineralization, dominated by Cu-Co and Zn-Pb sulfides, U oxides, and noble metals. Stratiform, vein, and skarn deposits are present in the late Proterozoic rocks of the Katangan Supergroup. Mineralization is regionally widespread.The sedimentary basin evolution included three phases. The first two, characterized by rift-fault-controlled rapid subsidence followed by slower thermal subsidence, were associated with bimodal magmatism and probably related to two thermal events. The third phase reflects the onset of tectonic deformation and uplift in the southern part of the basin.The stratiform mineralization, emplaced before the folding of the basin, is concentrated in lithologically varied permeable rocks of the Roan Group that underlie the impermeable rocks of the Lower Kundelungu Group. The stratiform mineralization was emplaced by convective circulation of basinal brines forced by a thermal gradient created by a wedge-shaped lithosome of Lower Kundelungu pelitic rocks with low thermal conductivity. Hypabyssal igneous intrusions and volcanic and volcaniclastic rocks were the source of metals. The metals were introduced to the aquifer horizons saturated with basinal brines by hydrothermal circulation associated with the hypabyssal intrusions and by compactional dewatering of a 5,000-m-thick body of pelitic rocks rich in volcaniclastic material.Vein mineralization, located on major late faults, resulted from regional metamorphism-driven dewatering of the folded basin. The skarn-metasomatic mineralization is associated with small post-Katangan acid intrusions and blocks of thermally rejuvenated basement.Details of the distribution of mineralization are stratigraphically and structurally controlled.

Petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the Solea graben, northern Troodos ophiolite, Cyprus

Hydrothermal mineral zonations and O isotope patterns of the northern Troodos complex do not parallel the ophiolite pseudostratigraphy, but reflect the convective geometry of an Upper Cretaceous seawater hydrothermal system. Large areas of the sheeted intrusive complex (SIC), including the subaxial region of the Solea graben, are composed of 18O‐rich, subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge. Other areas of the SIC are recrystallized to distinctive epidosite rocks: granular, high‐variance assemblages of epidote + quartz ± chlorite that are depleted in 18O, Al2O3, Na2O, K2O, Zr, Cu, and Zn and are enriched in CaO and Sr compared with other mafic volcanic and dike rocks of the Solea graben. Epidosite alteration occurred at temperatures of ∼310–370°C and involved fluids with δ18O values and salinities similar to those of Upper Cretaceous seawater. The epidosite zones are discordant with earlier, mineral/O isotope zonations and with the axis of spreading in the Solea graben, suggesting a postspreading, off‐axis origin. The seawater hydrothermal system responsible for Solea graben massive sulfide deposits was probably driven by hypabyssal intrusions (not exposed), emplaced in a terminal, failed spreading episode. The geometries of O isotope surfaces within the Solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits. Depletions in base metals show that epidosite alteration liberated Cu and Zn to mineralizing fluids within the fossil upflow zone.

Age and tectonic setting of Mesozoic metavolcanic and metasedimentary rocks, northern White Mountains, California

Mesozoic metavolcanic and metasedimentary rocks in the northern White Mountains, eastern California and western Nevada, are separated from lower Paleozoic and Precambrian rocks by Jurassic and Cretaceous plutons. The large stratigraphic hiatus across the plutons is called the Barcroft structural break. Recent mapping and new U/Pb zircon ages of 154 +3/−1 Ma and 137 ±1 Ma. from an ash-flow tuff and a hypabyssal intrusion, respectively, indicate that part of the Mesozoic section and the Barcroft structural break are younger than the 160–165 Ma Barcroft Granodiorite, in contrast to previous interpretations. The Barcroft Granodiorite has been thrust westward over most of the Mesozoic section. It is everywhere in fault contact with overturned metasedimentary rocks on the west side of the range, rocks which were previously thought to be upright and the oldest part of the Mesozoic section. The McAfee Creek Granite, which has a 100 ±1 Ma U/Pb zircon age, postdates thrusting; therefore, the Barcroft structural break is primarily Early Cretaceous in age.

Tectonic significance of an Early Proterozoic two-province boundary in central Arizona

A compilation of U-Pb zircon dates for lower Proterozoic rocks in central Arizona shows that, although rocks tend to be older in the northwest (1800−1696 m.y.) than the southeast (1738−1630 m.y.), there is no single boundary separating distinct geochronologic provinces in Arizona. Instead, the distribution of isotopic ages reflects the presence of two major tectonic provinces separated by a regionally subhorizontal boundary or boundaries. The northwestern part of central Arizona contains the Yavapai Series (1800−1755 m.y.) and calc-alkaline batholiths (1750−1696 m.y.), both believed to represent oceanic island-arc materials. The southeastern part of central Arizona is dominated by the Alder, Red Rock, and Mazatzal Groups and related hypabyssal intrusions (1710−1692 m.y.), with voluminous rhyolitic ash-flow tuffs and quartz arenite believed to record a relatively stable continental tectonic setting. Two working hypotheses emerge to explain the juxtaposition of representatives of these two tectonic provinces over a 100-km-wide zone in central Arizona. One interpretation (model 1) suggests that rocks of the southeast province were deposited with angular unconformity on newly accreted continental crust composed of northwest province rocks. A second interpretation (model 2) suggests that the two areas represent allochthonous terranes that evolved separately and were juxtaposed by large subhorizontal movements on thrusts and strike-slip faults. An important new constraint is that the 1699-m.y.-old strongly peraluminous Crazy Basin Quartz Monzonite was emplaced in the northwest province during ductile deformation at depths greater than 8 km at the same time that rhyolitic ash-flow tuffs and quartz arenite were being deposited in the southeast province. For model 1, this implies a rapid change of tectonic regimes about 1700 Ma, from convergence to uplift, erosion, sedimentation, and possibly extension. For model 2, the differences in crustal level, structural style, and petrologic affinity between ∼1700-m.y.-old rocks in both provinces are believed to result from juxtaposition of different crustal blocks after 1700 Ma.

Petrogenesis of a Late Precambrian (575?600 Ma) bimodal suite in Northeast Africa

Late Precambrian crustal evolution in the North Eastern Desert of Egypt occurred in a strongly extensional tectonic environment and was accompanied by abundant bimodal igneous activity. The extrusive and intrusive expressions of this magmatism, known as the Dokhan Volcanics and Pink Granites, respectively, were studied in detail from two areas. The Dokhan Volcanics and associated feeder dikes consist of a “mafic” suite dominated by andesites (∼60% SiO2) and smaller volumes of basalt and a “felsic” suite composed of rhyolite tuffs, ignimbrites and hypabyssal intrusions (∼72–78% SiO2). The rocks of the mafic suite display calc-alkaline trends on an AFM diagram but are enriched in incompatibles such as TiO2, P2O5, K2O, Rb, Sr, Ba, Zr, Y, Nb, and LREE. Rare earth element patterns are steep, with (Ce/Yb)n = 7.7 to 16.8. They contain moderate Ni (60 ppm) and Cr (95 ppm), indicating limited low-P fractionation. The melts of the mafic suite are interpreted to have formed either by ≤25% batch melting of eclogite or by ∼10% batch melting of LREE-enriched garnet lherzolite. The rocks of the felsic suite include Dokhan rhyolites and the epizonal Pink Granites. These contain 72–78% SiO2, are metaluminous and peraluminous, and have the high K2O/Na2O and FeO*/(FeO*+MgO) characteristic of post-tectonic, “A-type” granites. They are moderately enriched in incompatible elements, but their REE patterns overlap with those of the mafic suite, from which they can be distinguished by deep europium anomalies (Eu/Eu*=0.08–0.64) and flat HREE patterns=((Yb/Er)n=0.90–1.16). They share with the rocks of the mafic suite isotopic characteristics of depleted mantle, precluding anatexis of much older continental crust. The europium anomalies covary with Sr contents and indicate that plagioclase control was important, while the flat HREE patterns preclude residual garnet in the source. Hence the felsic melts could not have formed by anatexis of garnet-bearing mafic lower crust. Such melts could have formed by anatexis of amphibolite-facies crust, an interpretation which is not favored because the melts are not saturated with P2O5. Alternatively, the felsic melts may have formed via low-P fractional crystallization of the mafic melts, with about 2/3 removal of mostly plagioclase and amphibole along with minor apatite and zircon. This may have been accompanied in the latest stages of magmatic evolution by liquid-state fractionation such as thermo-gravitational diffusion or halide complexing.

Pre-Acadian magmatic suites of the southeastern Gaspé Peninsula

There are two phases of post-Taconic, pre-Acadian magmatism in the eastern Gaspe Peninsula of Quebec, (A) the Middle to Upper Ordovician Pabos Suite and (B) widespread Silurian-Devonian volcanics and hypabyssal intrusions. The Pabos Suite forms two large intrusions composed of LREE-depleted, N-MORB-like gabbros and ferrogabbros. Evolution within this suite was controlled by fractional crystallization, and the presence of the intrusions implies a tensional event between the Taconic and Acadian orogenies. The Silurian-Devonian volcanic and intrusive rocks are chemically variable (tholeiitic to alkaline, basaltic to rhyolitic) and do not belong to a single fractionation series. Quartz xeno-cryst-bearing rocks are enriched in Si, K, Zr, and LREE9s, providing evidence of disequilibrium contamination by a felsic melt similar to locally occurring rhyolites. The lavas and dikes that do not carry felsic xenocrysts also show patterns of enrichment/depletion of trace elements, which suggest that felsic contamination/hybridization was an important factor in their development, but show little Si enrichment. This is interpreted to be due to maintenance of chemical equilibrium during contamination. This would have forced the composition of the magma to remain on the plagioclase + clinopyroxene + olivine cotectic and would thus have inhibited Si enrichment. The xenocryst-bearing rocks superficially resemble a calc-alkaline series, but most of the pre-Acadian Silurian-Devonian volcanics of the Gaspe Peninsula do not appear to have been directly associated with subduction. The preferred model is that of local tensional environments in an orogenic foreland.

Stratigraphy of mid-Paleozoic island-arc rocks in part of the northern Sierra Nevada, Sierra and Nevada Counties, California

New stratigraphic data for the mid-Paleozoic island-arc succession in the northern Sierra Nevada are presented from a well-exposed area in Sierra and Nevada Counties. The Grizzly Formation of Diller is shown to be a laterally discontinuous but regionally extensive epiclastic unit at the base of the arc succession. Newly discovered macrofossils and conodonts date the formation as Late Devonian (Frasnian–mid-Famennian), and it records a widespread erosional episode that affected the underlying Shoo Fly Complex prior to the onset of arc volcanism. The Grizzly Formation is conformably overlain by the Sierra Buttes Formation, a sequence of silicic to andesitic submarine ash-fall tuffs; tuffaceous turbidites; and thick, nonwelded, subaqueous volcaniclastic mass-flow deposits intercalated with radiolarian cherts. Coeval hypabyssal intrusions show evidence of rapid chilling against wet, unlithified sediment, and some consist of complex masses of intrusive hyaloelastite breccia and pillows. Newly discovered conodonts, together with previously described fossils, show that the Sierra Buttes Formation was deposited largely or entirely in the mid-Famennian. Andesitic tuffs and tuff-breccias of the Upper Devonian-Lower Mississippian(?) Taylor Formation abruptly overlie the Sierra Buttes Formation, with no evidence for an intervening Elwell Formation. Like the Sierra Buttes Formation, the Taylor Formation contains abundant coarse-grained subaqueous volcaniclastic mass-flow deposits interbedded with finer-grained rocks. The two formations together constitute a thick sequence of volcaniclastic debris that accumulated rapidly below storm-wave base in an active island-arc setting.

The role of pressure in control of potassium, sodium, and copper concentration in hypabyssal intrusives as demonstrated in late Precambrian dikes in southwest Jordan

The formation of dikes in southwest Jordan took place during the late Proterozoic and represents the subsequent magmatic activity of the Pan-African orogeny. Dikes studied in detail include a zoned, multiple, composite dike of rhyolitic—trachytic and latitic composition, an andesitic dike and two devitrified rhyolitic dikes. High-level (near-surface) dikes are characterized by an extreme potassium metasomatism, which is caused by pressure effects. The shallower the intrusion the more intensive will be the potassium metasomatism. Some dikes display anomalously high copper concentrations. The copper is originally incorporated in silicates and has been enriched along flow lines and vesicules by autometasomatism. The formation of secondary copper minerals (e.g., cuprite, malachite, chrysocolla) can be considered as contemporaneous with the solidification of the congealing rhyolitic—trachytic melt.

Isotopic and chemical constraints on the building of the deep Scottish lithosphere

Synopsis The Scottish late Caledonian granites define a chemical province characterised by high Na, Ba and Sr abundances, particularly towards the north and west where they also have higher Zn, REEs and La/Y and lower Rb and Th. A comparison with data for inclusions found in younger volcanics and hypabyssal intrusions and thought to have been excavated from the mantle and lower crust indicates that the source of at least some of the chemical variation in the granitic magmas lies in the lithospheric mantle. The asthenosphere or postulated subducted slab material from the Iapetus ocean floor cannot have directly contributed significant amounts of these elements to most of the granitoids. Further, isotopic data clearly indicate that lower crust-derived materials are essential components of the more evolved granites, implying that considerable melting took place near the Moho. The age of the lower crust parallels the changes in chemistry in the lithospheric mantle which suggests that the Moho is not a major tectonic discontinuity. The Mid-Grampian line probably represents the rifted continental edge during Dalradian deposition. There is no isotopic evidence for primary crust of Grenville age under Scotland; rather the lower crust appears to be a complicated mixture including some components which are very young (probably Lower Palaeozoic) under the Midland Valley. The only location where clearly enriched mantle has been identified is under the Lewisian of the Outer Hebrides. The lower crust in this region is also Lewisian, apparently metasomatised. We tentatively suggest that the chemical enrichments and depletions in the lithosphere north of the Mid Grampian line are most simply explained by a volatile flushing mechanism.

Rare elements in granitoids from hypabyssal intrusions in Transbaykalia

Rare elements in granitoids from hypabyssal intrusions in Transbaykalia

Metamorphism and plutonism in the central part of the Okanogan Range, Washington

This report is concerned with the bedrock geology of a 1,000-km 2 area in the central part of the Okanogan Range in north-central Washington. Most of the rocks underlying this area were formed under conditions of high-grade amphibolite-facies metamorphism and/or plutonism. The metamorphism, accompanied by metasomatic granitization and anatexis, occurred between Late Triassic and late Early Cretaceous time. Plutonic rocks in the area are grouped into three temporal suites: (1) an early premetamorphic suite represented by inclusions of mafic to granitic basement rock in gneisses of a trondhjemitic batholith which also is believed to have served as basement to supracrustal rocks deposited in a late Paleozoic-early Mesozoic geosyncline; (2) a synmetamorphic suite of gneisses, including both concordant and discordant intrusions; and (3) a post-metamorphic suite of massive rocks which generally are discordant. The igneous rocks range in composition from peridotitic to granitic; trondhjemites and granodiorites are most abundant. Many of these units are somewhat leucocratic for their respective compositions. Younger intrusive rocks are more felsic than older ones. After the plutonic rocks had been deroofed, probably in early Tertiary time, the eastern part of the area was covered by quartz-keratophyre flows. These flows, widely scattered diatremes, and other hypabyssal intrusives are the youngest bedrock elements. Structures generated during metamorphism and plutonism generally trend northwest and indicate lateral compression. Some younger (middle and late Tertiary?) structures followed the established northwest grain, but most formed athwart it. These structures represented a basin-and-range type block-faulting episode in the region—a period of presumed lateral extension. The strike of these fault-block structures seldom varies more than a few degrees from north.

The (?) mid Triassic volcanic rocks of Lakonia, Greece

SummaryThick subaerial volcanic sequences of probable mid Triassic age rest unconformably on Permo-Carboniferous limestones in the ‘Phyllite Series’ within the Gavrovo-Tripolitsa zone of the external Hellenide nappes. The volcanic rocks are varied in character. Pyroclastic rocks (often reddened) predominate, but minor basalt or andesite and rhyolitic hypabyssal intrusions and flows are also found. The rocks have experienced low-grade metamorphism and the only relict primary minerals are pyroxene and rare plagioclase in basic rocks and potash feldspar in acid rocks. The most important metamorphic phases are albite, chlorite, potassium mica, epidote, hematite, quartz and pumpellyite. Twenty-seven whole-rock major-element analyses suggest that there has been some exchange of Na2O and K2O for CaO during metamorphism, but that other elements have been relatively stable. A genetic interpretation of the rocks is attempted using published geochemical discriminator diagrams. The basic rocks are tholeiitic in major element chemistry and pyroxene composition. Trace element (Hf, Ta, Th, Ce, Yb) distribution suggests magma generation at a destructive plate margin.

Very low grade metamorphism of Triassic volcanics, West Hellenic nappes, southern Peloponnese, Greece: Summary

The Stefania Volcanics are a series of (?)mid-Triassic basalt, andesite, rare dacite, and rhyolite hypabyssal intrusions and flows, together with voluminous pryoclastic and volcaniclastic deposits (Pe-Piper and others, 1981). They crop out in central Lakonia in the southern Peloponnese (Fig. 1), forming part of the Tyros Beds, which, together with more highly metamorphosed rocks (including the Krokee Metamorphic Complex) make up the “Semi-metamorphosed” or Phyllite Series (Panagos and others, 1979). The type locality of the Tyros Beds lies 50 km to the northeast at Tyros, and includes volcanic rocks very similar to the Stefania Volcanics. The Phyllite Series forms part of the West Hellenic nappe sequence (Jacobshagen and others, 1978). It may have originally formed the basement to the Mesozoic limestones and Cenozoic flysch of the Gavrovo-Tripolis isopic zone (Richter, 1974) or may have accumulated in a more westerly isopic zone (Jacobshagen and others, 1978). The Stefania Volcanics have suffered very low grade metamorphism. The Stefania Volcanics unconformably overlie limestones with Permo-Carboniferous fossils of the Tyros Beds near Faros (Panagos and others, 1979) and appear to conformably overlie Carnian sediments near Molaoi (Brauer and others, 1980). They are overlain by the Norian and younger Tripolitsa Limestone (Thiebault and Kozur, 1979). The metamorphism of the Tyros Beds, including the Stefania Volcanics, is clearly Alpine (Paraskevopoulos, 1964).