Major Magmatic Events Research Articles

The evolution of the Massif Central Rift; spatio-temporal distribution of the volcanism

Abstract The Massif Central area is the largest magmatic province of the West-European Rift system. The spatial-temporal distribution of Tertiary-Quaternary volcanism in the Massif Central, France, shows that three magmatic phases can be defined, each of them characterized by different volumes and different locations. The first event, termed the pre-rift magmatic event, is very scarce and restricted to the north of the Massif Central. It is suggested that this could result from lithospheric bending of the European lithosphere ahead of the incipient Alpine chain during the Paleocene. The second event, termed the rift-related magmatic event, is located in the north of the Massif Central only and is spatially connected with zones of high crustal thinning (i.e. the Limagne graben). It immediately follows Oligocene graben formation and associated sedimentation, and is represented by more than 200 scattered monogenic edifices. This second event can be attributed to partial melting as a consequence of lithospheric thinning that affected the north of the Massif Central during the rifting event. The lack of volcanism in the south during the same period of time is probably related to the very slight lithospheric thinning during the Oligocene. The third event, termed the major magmatic event, started first in the south in the upper Miocene at about 15 Ma, well after the end of the sedimentation. It is unrelated to any extensional event. This major magmatic event reached the north of the Massif Central at about 3.5 Ma, following a pause in volcanism of about 6 Ma after the rift-related magmatic event. These two episodes of the major magmatic event are spatially and temporally associated with the two main periods of uplift, suggesting a common origin for volcanism and uplift processes. The major magmatic event can be attributed to late thermal erosion of the base of the lithosphere above a mantle diapir, as suggested by seismic tomography data. This general magmatic evolution drawn from data at the Massif Central scale may apply to the Eger graben as well, as the three magmatic events described in this study (pre-rift magmatic event, rifting event and post-Miocene volcanic event) are also reported in the literature. This suggests that a single cause should explain the formation of the entire western European rift surrounding the Alpine mountain belt.

Age and magmatic history of the Antananarivo Block, central Madagascar, as derived from zircon geochronology and Nd isotopic systematics

We report single zircon ²⁰⁷ Pb/ ²⁰⁶ Pb evaporation and SHRIMP ages, combined with whole-rock Nd isotopic systematics for granitoid rocks from the Antananarivo Block (terrane), one of five tectono-metamorphic units making up the Precambrian basement of central and northern Madagascar. Our data reveal three distinct age groups at approximately 560 to 530, approximately 820 to 720, and 2520 to 2500 Ma respectively that reflect major magmatic events and correlate with similar events in various parts of East Africa and Sri Lanka but not in southwestern India. A widespread high-grade metamorphic event at approximately 550 Ma transformed many of the earlier granitoid gneisses into enderbite-charnockite assemblages. This granulite-facies event is common to Madagascar, East Africa, and southernmost India/Sri Lanka and reflects the final amalgamation of East and West Gondwana. Contrary to previous interpretations, there is a distinct lack of Kibaran-Grenvillian magmatism or metamorphism in Madagascar, making it unlikely that the island played a major role in the accretionary history and amalgamation of the supercontinent Rodinia. The widespread and voluminous granitoid magmatism at approximately 824 to 720 Ma remains enigmatic, and the tectonic scenario with which it is associated is difficult to reconstruct due to severe tectonic transposition of most gneisses. The Nd isotopic systematics as well as abundant zircon xenocrysts attest to extensive remelting of Archean and Paleoproterozoic crust. On presently available data the approximately 740 to 820 Ma granitoids are either related to magmatic underplating following plume generation, subcrustal mantle delamination during break-up and dispersal of Rodinia, or to continental arc magmatism related to subduction of the Mozambique ocean. They were emplaced into the ancient crust of central Madagascar as it lay either attached to East Africa or formed a microcontinent within the Mozambique ocean.

Geochronology of porphyries and related rocks in northern and western Dalarna, south-central Sweden

Volcanic rocks and granites belonging to the Transscandinavian Igneous Belt (TIB) form important constituents of the bedrock in the northern and western parts of the Province of Dalarna, south-central Sweden. The volcanic rocks comprise rhyolitic to quartz-trachytic porphyries as well as mainly intermediate to mafic porphyrites, and contain intercalations of sedimentary rocks. U-Pb zircon datings of 11 porphyries, 2 porphyrites, and 3 granites suggest a chronostratigraphic subdivision into two major magmatic events, at 1.8 and 1.7 Ga. We propose that the porphyries of these two events should be called Older and Younger TIB porphyries, respectively. Some of the porphyries and granites representing the younger event have a distinct rapakivi affinity, as defined by both chemical and textural criteria, whereas in others such features are lacking or only weakly developed. The rocks formed during the older event lack rapakivi characteristics. Our new chronological sub-division conforms with the age pattern in other parts of the TIB, although the youngest phase of the belt (TIB 3 at 1.65 Ga) is poorly represented in the Province of Dalarna. This subdivision should replace the previously suggested lithostratigraphy which included Gothian porphyries, a Lower Dala group, and an Upper Dala group. Nd isotopic signatures (εNd between +0.2 and +1.7) indicate that the 1.8 and 1.7 Ga igneous rocks belonging to the TIB in Dalarna and adjoining areas do not in-corporate significant amounts of Archaean crustal material.

SHRIMP U–Pb zircon dating of Grenville‐age events in the western part of the Musgrave Block, central Australia

Shrimp U–Pb zircon dating of structurally constrained felsic orthogneiss samples in the western Musgrave Block has been used to delineate discrete magmatic and metamorphic events at c.1300 and c.1200 Ma. The dating of pre‐D1 and post‐D1 felsic orthogneiss constrains D1 to have occurred at 1312±16 to 1324±4 Ma. This is the first geochronological study to identify such a metamorphic and deformation event in the Musgrave Block. D1 was accompanied by a major magmatic event involving the emplacement of voluminous felsic orthogneiss between 1296 and 1324 Ma. Zircon overgrowths on numerous igneous zircon cores give a consistent age of c.1200 Ma, reflecting zircon growth during a second high‐grade metamorphic event (D2). This c.1200 Ma metamorphic event was followed by the intrusion of a c.1190 Ma megacrystic granite. The c.1300 and c.1200 Ma events in the Musgrave Block can be tentatively correlated with metamorphic events in the Albany‐Fraser Orogen, and the Windmill Islands and Bunger Hills in east Antarctica. A major continuous Grenville‐age orogenic belt joining these areas may have represented a plate boundary between the pre‐Rodinian proto‐Australian continent and proto‐Antarctica during the formation of Rodinia in the Mesoproterozoic.

Thermal evolution of the Phlegraean magmatic system

A series of 2-D conductive/convective numerical models show a rather limited range of possible magma chamber configurations that predict the present thermal regime at Campi Flegrei. These models are calculated by HEAT, which allows continuous adjustment of heterogeneous rock properties, magma injection/replenishment, and convective regimes. The basic test of each model is how well it reproduces the measured thermal gradients in boreholes at Licola, San Vito, and Mofete reported by AGIP in 1987. The initial and boundary conditions for each model consists of a general crustal structure determined by geology and geophysics and major magmatic events: (1) the 37 ka Campanian Ignimbrite; (2) smaller volume 37–16 ka eruptions; (3) the 12 ka Neapolitan Yellow Tuff; (4) recent magmatism (e.g., Minopoli at ∼10 ka and Monte Nuovo in 1538 AD). While magma chamber depth is well constrained, magma chamber diameter, shape, volume, and peripheral convective regimes are poorly known. Magma chamber volumes between 200 and 2000 km 3 have been investigated with cylindrical, conical (funnel-shaped), and spheroidal shapes. For all reasonable models, a convective zone, developed above the magma chambers after caldera collapse, is necessary to achieve the high gradients seen today. These models should help us understand recent bradyseismic events and future unrest.

Magmatic unconformities and stratigraphic relations in the Peridotite zone, Stillwater Complex, Montana

New stratigraphic relations between cumulate layers and units are documented in the Peridotite zone of the Stillwater Complex, a late Archean mafic to ultramafic layered intrusion exposed along the northern edge of the Beartooth uplift, Montana. First, low-angle unconformities occur between all types of successive stratigraphic units and at all scales within the Peridotite zone. Second, new stratigraphic units and divisions, based on the unconformities and type of cumulate layering, within the Peridotite zone are proposed and are correlated with published stratigraphic columns. The stratigraphic units, identified on the basis of rock type(s) and contacts with adjacent units, from smallest to largest a cyclic units, multicyclic units, megacyclic groups, and subzones. The three larger units are usually bounded by low-angle magmatic unconformities. The three subzones and five of the six megacyclic groups identified in the Peridotite zone can be traced over a distance of 17.7 mi (28.3 km). The stratigraphic relations in the Peridotite zone indicate that low-angle crosscutting magmatic unconformities occur at all scales and that lateral variations within and between the identified stratigraphic units are common. The relations suggest an extremely dynamic and rapidly changing environment during formation of the Peridotite zone. Overprinted on the stratigraphic relations is a larger evolution of cumulus rock types in the Peridotite zone from more primitive stratigraphic units dominated by olivine and olivine–bronzite cumulates to more evolved stratigraphic units dominated by bronzite and bronzite–olivine cumulates. Several major magmatic, or possibly structural, events appear to have affected the Peridotite zone during its formation, as indicated by thickness-variation patterns in megacyclic groups and subzones. The thickest massive chromite layers in the Peridotite zone are associated with two of the major magmatic events.

Formula omitted] chronology of tertiary magmatic activity in Southern Yemen during the early Red Sea-Aden rifting

The early opening of the Red Sea rift was accompanied by magmatism that formed lava flows, plutons and dyke swarms. Some of these formations were sampled in Southern Yemen during a preliminary field survey and were investigated by geochemistry and 40Ar 39Ar geochronology. Ten plateau ages and more disturbed age spectra on plagioclase, alkali feldspar, biotite and whole rocks are discussed in detail, and allow us to establish the first chronological succession of magmatic events in this area immediately adjacent to the well-studied Afar region. An approximately 2000-m-thick section from the middle part of the traps gives ages ranging from 28.9 ± 0.1 Ma at the bottom to 26.5 ± 0.8 Ma at the top (lower part of rhyolitic lava flows). These ages are concordant with the oldest precisely dated alkaline lava flows from the harrats (Harrat Hadan) of Saudi Arabia. The majority of the analysed samples from two distinct dyke swarms mainly trending N120–150 °E give ages of around 25.5 Ma and 16–18.5 Ma, respectively. These dyke swarms do not reflect any drastic change in stress regime in this region during this period, but show, in the vicinity of the Gulf of Aden, the existence of volcano-tectonic trends similar to those of the Red Sea Rift in Saudi Arabia. Geochemical analysis (trace element and isotopic ratio) of these samples show a break at 21–22 Ma contemporaneous with a major magmatic event of tholeiitic affinity occurring over nearly 1700 km along the Red Sea Rift in Saudi Arabia.

Time constraints on the evolution of the Tennant Creek Block, northern Australia

Recent ion-microprobe UPb zircon age determinations on crustal components of the Palaeoproterozoic Tennant Creek Inlier have identified a relatively short (∼ 30 m.y.) history for the major magmatic events in the area, including those related to the Barramundi Orogeny. This work independently confirms the revisions of the Tennant Creek stratigraphy proposed by Donnellan et al. (1991). The oldest known rocks from the area are the volcaniclastic turbidites of the Warramunga Formation, which were deposited in a rapidly subsiding basin at 1862 ± 9 to 1859 ± 13 Ma. Subordinate zircon populations in the greywackes indicate earlier magmatic activity at 1930 ± 12 and 1908 ± 15 Ma. The regional shortening (D1) and contemporaneous intrusion of voluminous and chemically homogeneous granites (and associated felsic porphyry dykes), known across northern Australia as the Barramundi Orogeny, occurred here between 1858 ± 12 and 1845 ± 4 Ma. Following uplift, the felsic volcanics and volcaniclastics of the Flynn Subgroup were extruded between 1845 ± 4 and 1827 ± 9 Ma. During this interval, felsic porphyry dykes and minor granite bodies probably related to the Flynn Subgroup volcanism intruded the Warramunga Formation sediments. Magma source regions contain components as old as 3.0 Ga, as revealed by zircon inheritance in most of the analysed rocks. The youngest recognised igneous activity in the area are the intrusion of the “Warrego granite” at 1700-1650 Ma and the Gosse River East granite at 1712 ± 5 Ma. The chronostratigraphy presented here is remarkably similar to that of parts of the well-exposed Palaeoproterozoic Trans-Hudson and Wopmay orogens of northern America.

A tectonic model for the location of Palaeozoic ring complexes in Aïr (Niger, West Africa)

The Air region in Niger is one of the largest peralkaline granite provinces in the world. In addition to the granites, some plutons are characterized by an abundance of plagioclase-rich cumulates (gabbros, troctolites and anorthosites), with subordinate metaluminous granites and quartz syenites. These anorogenic ring complexes were intruded along a 400 km zone near the 9°E meridian. The province formed during a major magmatic event, newly dated at around 407 ± 8Ma, and comprises 28 plutons, which range from 0.8 to 65 km in diameter and show no correlation between distribution and age. Remote sensing over the whole Aïr massif demonstrates the existence of two main trends of faults or lineaments: N50°E–N90°E and N120°E–N150°E. Autocorrelation analysis of the ring complexes reveals three large, parallel, high-density strips orientated N20°E. Based on the structural and geological setting, coupled with map analysis, a new tectonic model for the location of the complexes is presented. We propose a dextral N5°E shear zone model, where the emplacement of the ring structures is controlled by N20°E dextral Riedel shear ( R) with a secondary N50°E tensional gash ( T). This tectonic model implies a control by lithospheric structures for the emplacement of the ring complexes, as well as a relationship between a transtensional tectonic regime and intra-plate alkaline magmatism. The new geochronological data on the Aïr massif allow us to derive a Silurian-Devonian palaeomagnetic pole (representative for Gondwana) which fits data for the other continents better than previous estimates.

KAr and RbSr geochronology og igneous rocks from the Sierra de Paiman, northwestern Argentina

Epizonal igneous and metamorphic rocks in northwestern Argentina are exposed in the Sierra de Paiman. The metamorphic rocks are quartzites, phyllites, and slates with soft-body impressions and fossil traces that suggest a late Precambrian-Cambrian age. The igneous rocks were intruded during two major magmatic events according to KAr and RbSr data. The older event is represented by different kinds of granitoids and gabbroids, intruded 437–459 Ma. These rocks were emplaced syntectonically in a shear zone that remained active after emplacement, causing extensive mylonitization on the east side of the range. The granitoids show Sr isotopic disturbances possibly related to magma mixing events. Petrologic, geochemical, and isotopic data for these rocks suggest a volcanic-arc setting, probably related to the back arc of the eastward-dipping continental arc of the Famatina Belt. During the younger event ( ca. 379 Ma) stocks and dikes of leucogranites were emplaced post-tectonically with respect to the last episode of mylonitization. The leucogranites have syn-collisional signatures and may thus represent the culmination of the volcanic arc of the Famatina Belt.

Magmatism, tin mineralization and tectonics of the Main Range, Malaysian Peninsula: Consequences for the plate tectonic model of Southeast Asia based on Rb-Sr, K-Ar and fission track data

The Malaysian Peninsula belongs to the Southeast Asian tin belt. It is divided by a sutUre into two different magmatic provinces of s- and I-type characteristics. Three areas within the S-type granitoid province of the Main Range have been studied. New K­ Ar mica and fission track zircon and apatite ages were established, and Rb-Sr whole­ rock data from literature have been reinterpreted in the light of these new data. The results were compared with existing Rb-Sr isochrons of the I-type granitoid province and with the one reported from the northern extension of the belt stretching into Thailand. This integrated study led to the better understanding of: a) the behavior of the different isotopic systems in different geological environments, and b) the complex magmatic, tectonic and cooling history of Southeast Asian granitoids. Applying the intrusion extrapolation method, based on the evolutionary trend of decreasing Rb-Sr ages and increasing initial (87186)Sr (Kwan et al., paper submitted), periods of 300, 250, 210, and 90 Ma for the Main Range and of 260, 240, 210 and 90 Ma for the I-type granite province became evident. The best evaluated initial (87186)Sr for the source regions of the granitoids are 0.708 to 0.709 and 0.704 respectively. the major magmatic event in the Main Range dates of the late-Permian. The close time range of individual magmatic periods and their spatial distribution can best be explained in the context of plate tectonics by northward motion of compositionally different arcs; by their collision to one another and to the East Asian Continent; subsequent deformation; subduction reversal; following the model of Hamilton (1988). The most crucial event for the Main Range granitoids was the intrusion of the highly evolved, late-Triassic post-collision granites. They induced a regional hydrothermal convection system, which is considered to have lasted for a maximum of 40 Ma dependent upon the crustal level. This hydrothermal system was responsible for the major tin mineralization processes and the entire granite alteration. Biotite of the crystallized granitoids is considered to be the main Sn supplier. The K-Ar and fission track ages indicate a slow post-orogenic cooling in the range of 1- 5°ClMa. With FT zircon ages different crustal levels can be distinguished within the Main Range, and the discordant K-Ar mica and the differently reset Rb-Sr whole-rock ages can be correlated with depth. After the early-Cretaceous the Main Range has been dominated by tectonic activities, such as differential vertical uplift of the Main Rage combined with thejuxtapositioning of blocks, thrust faulting close to the Bentong-Raub suture and Tertiary left-lateral displacement, which is inferred from isotopic and petrological evidences.

The Pb-Zn-Cu-Ag deposits of the Granadena Mine, San Francisco del Oro-Santa Barbara District, Chihuahua

Mid-Tertiary polymetallic quartz sulfide vein deposits of the San Francisco del Oro-Santa Barbara district are among the largest Pb-Zn-Cu-Ag deposits in Mexico. The veins of this district are hosted by the Cretaceous Parral shale, and since 1650 have produced over 13 billion g (440 million oz) of silver. Significant production has come from vein ores and from several vein-related massive sulfide replacement bodies.Vein formation occurred in at least four distinct stages that define an unusual paragenetic sequence when compared to similar systems. Early massive sulfide veins are cut by veins rich in calc-silicates, quartz, and late sulfides. This group is cut by two stages of postore quartz-fluorite-calcite veins with minor sulfides. Early sulfides and late calc-silicates represent a paragenetic sequence which is the reverse of that normally observed in systems containing calcsilicates and sulfides.Stage 1 veins are characterized by abundant sphalerite and galena, and minor quartz, with weak alteration halos of epidote, axinite, chlorite, quartz, and minor andradite. Stage 1 vein quartz was precipitated from 250 degrees to 320 degrees C solutions with salinities ranging from 2 to 14 equiv wt percent NaCl. Vein-related replacement bodies were also formed during this stage of hydrothermal activity. Stage 2 veins are characterized by an abundance of calc-silicate minerals within the veins, sulfides (dominantly chalcopyrite-pyrite assemblages), a small amount of gold mineralization, and a pervasive, calc-silicate-rich alteration assemblage containing manganoan hedenbergite, manganoan ilvaite, quartz, minor andradite, and hematite. Homogenization temperatures of fluid inclusions in late stage 2 quartz range from 180 degrees to 300 degrees C and salinities range from 1 to 14 equiv wt percent NaCl. Stage 3 and 4 veins are characterized by quartz, calcite, and fluorite. These veins were formed by 220 degrees to 260 degrees C fluids with salinities ranging from 1 to 3 equiv wt percent NaCl.Fluid inclusion studies reveal a highly complex ore-forming history. Ore-bearing fluids varied widely in temperature (from 180 degrees -320 degrees C) and chloride concentration (from 2.0), Ag/Pb (from 50), and Ag/Zn (from 70) within stage 1 veins.Dating of igneous rocks in the district indicates a major andesitic magmatic event ca 32 m.y. ago, and a rhyolitic magmatic event ca 26 m.y. ago. Geologic relations between ore deposits and volcanic rocks indicate that ore deposition predates and postdates rhyolite emplacement.The variable temperature and salinity found in fluid inclusions, the distinct mineralogic differences observed between vein stages 1 and 2, and the variable ge relations between veining and igneous activity suggest that the deposits of the San Francisco del Oro/Santa Barbara district were probably emplaced during several episodes and that these ores may represent the superposition of at least two distinct hydrothermal systems.

Variations in axial processes on the Mid-Atlantic Ridge: The median valley of the MARK area

Submersible observations and photogeology document dramatic variations in the distribution of young volcanic rocks, faulting, fissuring, and hydrothermal activity along an 80 km-long segment of the Mid-Atlantic Ridge south of the Kane Transform (MARK Area). These variations define two spreading cells separated by a cell boundary zone or a small-offset transform zone. The northern spreading cell is characterized by a median ‘neovolcanic’ ridge which runs down the axis of the median valley floor for 40 km. This edifice is as much as 4 km wide and 600 m high and is composed of very lightly sedimented basalts inferred to be < 5000 years old. It is the largest single volcanic constructional feature discovered to date on the Mid-Atlantic Ridge. The active Snake Pit hydrothermal vent field is on the crest of this ridge and implies the presence of a magma chamber in the northern spreading cell. In contrast, the southern cell is characterized by small, individual volcanos similar in size to the central volcanos in the FAMOUS area. Two of the volcanos that were sampled appear to be composed of dominantly glassy basaltic rocks with very light sediment cover; whereas, other volcanos in this region appear to be older features. The boundary zone between the two spreading cells is intensely faulted and lacks young volcanic rocks. This area may also contain a small-offset ( < 8 km) transform zone. Magmatism in the northern cell has been episodic and tens of thousands of years have lapsed since the last major magmatic event there. In the southern cell, a more continuous style of volcanic accretion appears to be operative. The style of spreading in the southern cell may be much more typical for the Mid-Atlantic Ridge than that of the northern cell because the latter is adjacent to the 150 km-offset Kane Transform that may act as a thermal sink along the MAR. Such large transforms are not common on the MAR, therefore, lithosphere produced in a spreading cell influenced by a large transform may also be somewhat atypical.

Implications of new UPb zircon chronology to early proterozoic crustal accretion in northern Sweden

UPb ages ranging from 1.89 to 1.78 Ga are reported for zircon populations formed during major magmatic events in the Proterozoic development of northern Sweden. The studied area includes parts of the continental and the marine Svecokarelian domains of the Baltic Shield. The ages record crustal accretion and reactivation off the margin of an exposed Archaean craton and show that magmatism was broadly coeval throughout the entire north-Swedish portion of the Svecokarelian province, irrespective of regions involved. A detailed chronostratigraphy is presented for the Skellefte district of northern Sweden, and comparisons are made with the evolution in other parts of the Svecokarelian province. Two main magmatic phases commencing at 1.89 and 1.79 Ga ago are recognized. The first represents large-scale volcanism and plutonism, which in the Skellefte district was characterized by the introduction of mainly mantle-derived magmas or by magmas formed from rocks with short crustal residence time. In the vicinity of the Archaean craton some distance to the north, the crust-generating processes included the mobilization of Archaean sources and appear to have continued until c. 1.85 Ga ago. A relatively long period of consolidation and denudation separates this magmatic phase from the one at c. 1.79 Ga ago. The generation of these later, migmatite-associated granites in the Svecokarelian province coincided in time, and was possibly associated with, those processes which gave rise to the initial plutonism in the neighbouring Transscandinavian Granite-Porphyry Belt.

Origin of infracrustal (I-type) granite magmas

ABSTRACTI-type granites are produced by partial melting of older igneous rocks that are metaluminous and hence have not undergone any significant amount of chemical weathering. In the Lachlan Fold Belt of southeastern Australia and the Caledonian Fold Belt of Britain and Ireland there was a major magmatic event close to 400 Ma ago involving a massive introduction of heat into the crust. In both areas, that Caledonian-age event produced large volumes of I-type granite and related volcanic rocks. Granites of these two areas are not identical in character but they do show many similarities and are markedly different from many of the granites found in Mesozoic and younger fold belts. These younger, dominantly tonalitic, granites have compositions similar to those of the more felsic volcanic rocks forming at the present time above subduction zones. The Palaeozoic granites show little evidence of such a direct relationship to subduction. Within both the Caledonian and Lachlan belts there are some granites with a composition close to the younger tonalites. A particularly interesting case is that of the Tuross Head Tonalite of the Lachlan Fold Belt, which can be shown to have formed from slightly older source rocks by a process that we refer to as remagmatisation which has caused no significant change in composition. Since remagmatisation has reproduced the former source composition in the younger rocks, the wrong inference would result from the use of that composition to deduce the tectonic conditions at the time of formation of the tonalite. Granites, particularly the more mafic ones, will generally have compositions reflecting the compositions of their source rocks, and attempts to use granite compositions to reconstruct the tectonic environment at the time of formation of the granite may be looking instead at an older event. This is probably also the case for some andesites formed at continental margins.Several arguments can be presented in favour of a general model for the production of I-type granite sources by underplating the crust, so that the source rocks are infracrustal. Such sources may contain a component of subducted sediments with the consequence that some of the compositional characteristics of sedimentary rocks may be present in I-type source rocks and in the granites derived from them. The small bodies of mafic granite and gabbro associated with island arc volcanism have an origin that can be related to the partial melting of subducted oceanic crust or of mantle material overlying such slabs and can be referred to as M-type. These rocks have compositions indistinguishable from those of the related volcanic rocks, except for a small component of cumulative material. The tonalitic I-type granites characteristic of the Cordillera are probably derived from such M-type rocks of basaltic to andesitic composition, which had been underplated beneath the crust. Some of the more mafic tonalites of the Caledonian-age fold belts may also have had a similar origin. More commonly, however, the plutonic rocks of the older belts are granodioritic and these probably represent the products of partial melting of older tonalitic I-type source rocks in the deep crust, these having compositions and origins analogous to the tonalites of the Cordillera. In this way, multiple episodes of partial melting, accompanied by fractionation of the magmas, can produce quite felsic rocks from original source rocks in the mantle or mantle wedge. These are essential processes in the evolution of the crust, since the first stages in this process produce new crust and the later magmatic events redistribute this material vertically without the addition of significant amounts of new crust.

Ti-rich plagioclase-phyric dykes of southern West Greenland

The investigation of Proterozoic basic dykes in southern West Greenland stemmed from the programme of systematic mapping of the Archaean craton in that region by the Geological Survey of Greenland (GGU). This work began in the southern Frederikshåb region in the early 1960s (Jensen, 1968, 1969) and progressed northwards, from bases in the Fiskenæsset (Kalsbeek &amp; Myers, 1973; GGU, 1976), Godthåb (Allaart et al., 1977) and Sukkertoppen areas (Allaart et al., 1978). The results of most of this mapping work were summarized by Bridgwater et al. (1976) and compiled onto a 1:500 000 scale geological map sheet by Allaart (1982). The distribution of the major Proterozoic dykes which cut the entire region is shown on this map. While the basic dykes are individually minor intrusions, many are up to 50 metres wide and continuous for several tens of kilometres, and collectively they represent a major magmatic event. As many of the Archaean terrains of the world possess Proterozoic basic dyke swarms, their compositions are crucial to a correlation of events from one craton to another and to an understanding of crustal and mantle evolution after the world-wide late Archaean sialic crust-forming event.

Paleozoic and Lower Mesozoic magmas from the eastern Klamath Mountains (North California) and the geodynamic evolution of northwestern America

The Paleozoic to Early Mesozoic geology of the eastern Klamath Mountains (N California) is characterized by three major magmatic events of Ordovician, Late Ordovician to Early Devonian, and Permo-Triassic ages. The Ordovician event is represented by a calc-alkalic island-arc sequence (Lovers Leap Butte sequence) developed in the vicinity of a continental margin. The Late Ordovician to Early Devonian event consists of the 430–480 Ma old Trinity ophiolite formed during the early development of a marginal basin, and a series of low-K tholeiitic volcanic suites (Lovers Leap Basalt—Keratophyre unit, Copley and Balaklala Formations) belonging to intraoceanic island-arcs. Finally, the Permo-Triassic event gave rise to three successives phases of volcanic activity (Nosoni, Dekkas and Bully Hill) represented by the highly differentiated basalt-to-rhyolite low-K tholeiitic series of mature island-arcs. The Permo-Triassic sediments are indicative of shallow to moderate depth in an open, warm sea. The geodynamic evolution of the eastern Klamath Mountains during Paleozoic to Early Mesozoic times is therefore constrained by the geological, petrological and geochemical features of its island-arcs and related marginal basin. A consistent plate-tectonic model is proposed for the area, consisting of six main stages: 1. (1) development during Ordovician times of a calc-alkalic island-arc in the vicinity of a continental margin; 2. (2) extrusion during Late Ordovician to Silurian times of a primitive basalt-andesite intraoceanic island-arc suite, which terminated with boninites, the latter suggest rifting in the fore-arc, followed by the breakup of the arc; 3. (3) opening and development of the Trinity back-arc basin around 430–480 Ma ago; 4. (4) eruption of the Balaklala Rhyolite either in the arc or in the fore-arc, ending in Early Devonian time with intrusion of the 400 Ma Mule Mountain stock; 5. (5) break in volcanic activity from the Early Devonian to the Early Permian; and 6. (6) development of a mature island-arc from the Early Permian to the Late Triassic. The eastern Klamath Mountains island-arc formations and ophiolitic suite are part of the “Cordilleran suspect terranes”, considered to be Gondwana margin fragments, that have undergone large northward translations before final collision with the North American craton during Late Mesozoic or Cenozoic times. These eastern Klamath Mountains island-arcs could be associated with the paleo-Pacific oceanic plate that led to accretion of these allochthonous terranes to the American margin.

Seamount magnetic polarity and Cretaceous volcanism of the Pacific Basin

In this paper we examine the magnetic polarity for Pacific Basin seamounts exclusive of the Hawaiian Ridge. On Pacific crust older than 65 Ma, seamounts of normal polarity predominate. On crust younger than 65 Ma, seamounts exhibit normal and reversed polarity as well as mixed polarity. We show that viscous components do not dominate seamount magnetization, and we suggest that a large number of normal seamounts were formed during a major magmatic event postulated for the period 110–70 Ma. This period overlaps with the long Cretaceous normal period (118–84 Ma) and corresponds to 85% normal polarity for Earth9s magnetic field. The correspondence of a period of high magmatic activity with a long period of normal polarity supports the possible correlation between those processes responsible for magma production and for magnetic field generation.

Age and origin of the major syenite centers in the Gardar province of south Greenland: Rb-Sr studies

Rb-Sr whole-rock isochron data for nine intrusive centers (including the major oversaturated and undersaturated syenite bodies) in the Precambrian Gardar alkalic igneous province of south Greenland complete a systematic isotopic study of the major complexes of the region (15 distinct igneous centers). The new data show that magmatic activity occurred throughout the region over a period in excess of 150 m.y. and that there is a clear distinction between two major magmatic events — an early Gardar (∼1,300 m.y. old) event and a more widespread late Gardar (∼1,160 m.y. old) event, which also involved extensive east-northeast-trending dike emplacement. Two relatively small intrusions in the western part of the province have middle Gardar (∼1,250 m.y. old) ages. There does not appear to be any relationship between emplacement age and geographic location, mode of emplacement, or the recognized petrologic division into saturated or undersaturated types; indeed, some adjacent complexes with strikingly similar petrologic characteristics have originated during different events, and, conversely, adjacent complexes with very different compositions are chronologically indistinguishable with present techniques. Initial 87 Sr/ 86 Sr ratios for the Gardar rocks are broadly divisible into three groups. Low (0.702 to 0.704) ratios are characteristic of most of the intrusions in the province, including most of the syenites (both undersaturated and oversaturated) and gabbros —which indicates a primitive (mantle) origin for the parental Gardar magmas. Intermediate ratios (0.704 to 0.707) are observed in fewer centers, particularly in the Nunarssuit complex (gabbro, oversaturated syenites, and granites). For these centers, the possibility of closed-system enrichment in 87 Sr (by decay of 87 Rb during a complex molten history extending throughout 150 m.y. of Gardar time) is rejected as unlikely, as is a direct origin by partial melting of older crust; field, chemical, and isotopic evidence presented here strongly support a bulk-assimilation hypothesis. Only two complexes have high (>0.707)ratios; both show evidence of introduction of radiogenic 87 Sr into their magmas at a late stage of development through selective leaching of radiogenic 87 Sr from older crust, and both of these complexes are associated with mineralization.