Onset Of Magmatism Research Articles

The interplay between siliciclastic and carbonate depositional systems: Maastrichtian to Danian basin‐floor sediments of the mid‐Norwegian Møre Basin

AbstractSource‐to‐sink sedimentary systems associated with volcanic rifted margins serve as important archives for basin development by recording lithospheric changes affecting the depositional systems. Distinguishing between sediment transport processes and their sediment source(s) can guide the interpretation of a basin's history, and thereby inform regional paleogeographic reconstructions. In this contribution, we integrate and utilize wireline geophysical logs, detailed petrographic observations from side‐wall cores, and seismic analysis to describe and decipher a Maastrichtian to Danian‐aged basin‐floor depositional system in the deep outer Møre Basin, mid‐Norwegian margin. Well 6302/6‐1 (Tulipan) is a spatially isolated borehole drilled in 2001 that penetrates Maastrichtian and younger strata. A succession of hitherto undescribed carbonates and sandstones in the outer Møre Basin was discovered. It is investigated for sediment transport, provenance, and depositional processes on the basin floor surrounded by structural highs and ridges. The strata from the lower parts form a basin‐floor apron consisting of redeposited carbonate sourced from a westerly sub‐aerial high. The apron transitions vertically from mixed siliciclastic and carbonate into a purely siliciclastic fan with intercalated sandstone and mudstone, providing a rare high‐resolution record of how depositional environments experience a complete shift in dominant processes. The development coincides with similar latest Cretaceous‐earliest Palaeocene sequences recorded south of this region (e.g., well 219/20‐1) and may have been influenced by regional uplift associated with the onset of magmatism in the Northeast Atlantic. This study improves our understanding of a late, pre‐breakup source‐to‐sink sedimentary system developed near the breakup axis of an infant ocean, and documents what is possibly the northernmost chalk deposit in the Chalk Group.

Characterizing the nature and importance of lava-sediment interactions with the aid of field outcrop analogues

The emplacement of lava flows into poorly consolidated sediments during volcanism can lead to a wide range of lava-sediment interaction processes. Understanding these processes and their products is critical for appraising reservoir quality and connectivity in both intra- and sub-volcanic prospectivity settings. This study investigates the nature of lava-sediment interactions during the onset of magmatism in sedimentary basins with the aid of field outcrops in the Paraná-Etendeka Igneous Province (Brazil), Mull Lava Field (UK), and Midland Valley Basin (UK). Both subaerial and invasive lava flows were identified with associated features ranging from peperites, loading structures, pillow-like peperites, rootless cones, and lavas with sharp contacts. Density contrasts between liquid magma and unconsolidated sediments make lava invasion a predictable phenomena in such environments, however, variations in sediment properties such as water saturation, porosity and cohesion may effectively restrict the process of invasion, the degree of loading and more dynamic processes leading to peperite and pillow-like peperite. Surface outcrop analogues form a key component to understand the potential impact of volcanism and magma-sediment interactions on non-volcanic reservoir rocks located proximal to the basalt-sediment transition. The intimate mixing of magma with sediment is generally restricted to a few meters from the lava-sediment contact, with an associated minor impact on reservoir quality. However, the process of lava invasion may extend this impact further and may lead to partial or complete compartmentalization of associated reservoir units and may additionally impact correlation at the detailed reservoir level.

The volcanic history of Pyrgousa—volcanism before the eruption of the Kos Plateau Tuff

The islet of Pyrgousa, 8 km west of Nisyros, has andesites overlain by proximal deposits of the Kos Plateau Tuff (KPT). These andesites record the evolution of the Kos-Nisyros volcanic centre prior to the large KPT eruption at 161 ka. This precursory activity had previously been recorded only in dacites and rhyolites in the Kefalos Peninsula, farther from the volcanic centre. This study investigated the age, geochemistry, mineralogy, and petrogenesis of the andesite domes, probable flows, and associated talus breccias on Pyrgousa. Analysed samples are basaltic andesite and andesite, with strong enrichment in Ba and Sr and low values of Ti and Zr. An 40Ar/39Ar date of 1.9 ± 0.1 Ma on biotite is similar to dates from dacite and rhyolite stocks and domes in the Kefalos Peninsula, and like those dates is the maximum age due to excess 40Ar. There is no evidence on Pyrgousa for a stratovolcano precursor of the KPT eruption. The andesite domes geochemically resemble Pliocene domes in Methana and Aegina that mark the onset of magmatism in the northwestern South Aegean Arc, with an important magma component derived from subcontinental lithospheric mantle. These early andesites heated the upper crust, thus facilitating the growth of upper crustal magma chambers, which were filled by felsic crystal mush derived by differentiation of hydrous intermediate magmas with a more asthenospheric signature.

Tectonic evolution and copper-gold metallogenesis of the Papua New Guinea and Solomon Islands region

Papua New Guinea and the Solomon Islands are in one of the most prospective regions for intrusion-related mineral deposits. However, because of the tectonic complexity of the region and the lack of comprehensive regional geological datasets, the link between mineralization and the regional-scale geodynamic framework has not been understood. Here we present a new model for the metallogenesis of the region based on a synthesis of recent studies on the petrogenesis of magmatic arcs and the history of subduction zones throughout the region, combined with the spatio-temporal distribution of intrusion-related mineral deposits, and six new deposit ages. Convergence at the Pacific-Australia plate boundary was accommodated, from at least 45 Ma, by subduction at the Melanesian trench, with related Melanesian arc magmatism. The arrival of the Ontong Java Plateau at the trench at ca. 26 Ma resulted in cessation of subduction, immediately followed by formation of Cu-Au porphyry-epithermal deposits (at 24–20 Ma) throughout the Melanesian arc. Late Oligocene to early Miocene tectonic reorganization led to initiation of subduction at the Pocklington trough, and onset of magmatism in the Maramuni arc. The arrival of the Australian continent at the Pocklington trough by 12 Ma resulted in continental collision and ore deposit formation (from 12 to 6 Ma). This is represented by Cu-Au porphyry deposits in the New Guinea Orogen, and epithermal Au systems in the Papuan Peninsula. From 6 Ma, crustal delamination in Papua New Guinea, related to the prior Pocklington trough subduction resulted in adiabatic mantle melting with emplacement of diverse Cu and Au porphyry and epithermal deposits within the Papuan Fold and Thrust Belt and Papuan Peninsula from 6 Ma to the present day. Subduction at the New Britain and San Cristobal trenches from ca. 10 Ma resulted in an escalation in tectonic complexity and the onset of microplate tectonics in eastern Papua New Guinea and the Solomon Islands. This is reflected in the formation of diverse and discrete geodynamic settings for mineralization within the recent to modern arc setting, primarily related to upper plate shortening and extension and the spatial relationship to structures within the subducting slab.

Discovery of a meteoritic ejecta layer containing unmelted impactor fragments at the base of Paleocene lavas, Isle of Skye, Scotland

Evidence for meteorite impacts in the geological record may include the presence of shocked minerals, spherule layers, and geochemical anomalies. However, it is highly unusual to find unmelted crystals from the actual impactor within an ejecta layer. Here we detail the first recorded occurrence of vanadium-rich osbornite (TiVN) on Earth, from two sites on Skye, northwest Scotland, which are interpreted as part of a meteoritic ejecta layer. TiVN has only previously been reported as dust from comet Wild 2, but on Skye it has been identified as an unmelted phase. Both ejecta layer sites also contain niobium-rich osbornite (TiNbN), which has not previously been reported. An extraterrestrial origin for these deposits is strongly supported by the presence of reidite (a high-pressure zircon polymorph), which is only found naturally at sites of meteorite impact. Barringerite [(Fe,Ni)2P], baddeleyite (ZrO2), alabandite (MnS), and carbon-bearing native iron spherules, together with planar deformation features and diaplectic glass in quartz, further support this thesis. We demonstrate through field relationships and Ar-Ar dating that the meteorite strike occurred during the mid-Paleocene. This is the first recorded mid-Paleocene impact event in the region and is coincident with the onset of magmatism in the British Palaeogene Igneous Province (BPIP). The Skye ejecta layer deposits provoke important questions regarding their lateral extent at the base of the BPIP and the possibility of their presence elsewhere beneath the much larger North Atlantic Igneous Province.

Genesis of the Paleoproterozoic NICO iron oxide–cobalt–gold–bismuth deposit, Northwest Territories, Canada: Evidence from isotope geochemistry and fluid inclusions

The NICO deposit is located in the southern portion of the Paleoproterozoic Great Bear magmatic zone, Northwest Territories, Canada. The majority of the mineralization lies within the Bowl Zone, hosted by the Treasure Lake Group (TLG), where heavily altered precursor rocks are interpreted to be carbonate-rich wackes and siltstones. These rocks are crosscut by a set of pre-ore quartz±calcite+amphibole+K-feldspar veins (S1). The Co–Au–Bi (±W–Cu–Mo) mineralization at NICO is contained in stratabound ore lenses within the Bowl Zone, and comprises a prograde assemblage of Co-rich arsenopyrite (arsenopyrite I) and loellingite, cobaltite, pyrite, actinolite, ferrohornblende, biotite and rare scheelite (±molybdenite) along with minor magnetite and amphibole. Retrograde assemblages resulted from re-crystallization of the Co-bearing phases to form arsenopyrite II and III, along with precipitation of marcasite, pyrite, hastingsite, native Bi–Au (±Te) and minor bismuthinite and magnetite. The latest stage of retrograde mineralization comprises chalcopyrite, hastingsite, chlorite, and hematite (±emplectite). Two sets of barren quartz±dolomite+amphibole+K-feldspar+chalcopyrite veins (S2 and S3) post-date the mineralization. The Southern Breccia zone hosts minor U–Cu–Mo mineralization and is interpreted to be the deepest portion of the NICO system. Two molybdenite samples from the Bowl Zone and the Southern Breccia yielded Re–Os ages of 1865±9 and 1877±8Ma, respectively, consistent with the interpreted ca. 1870Ma age of the NICO deposit. This age is also synchronous with the onset of magmatism in the Great Bear magmatic arc (ca. 1875–1850Ma).Ore mineral δ34S values (3.3–6.7‰, sulfides) indicate that crustal sulfur was assimilated by upwelling felsic magmatic melts. The δ18O values of the fluids precipitating magnetite and Co-rich arsenopyrite (6 and 8‰), and from pre- and syn-mineralization magnetite (−0.8 to 1.5‰) support a magmatic-hydrothermal origin of the fluids. Two out of three calcite samples from pre-(S1) and post-ore (S3) veins have also δ13C values consistent with a magmatic origin (−5.5 to −3.6‰). However, one calcite sample from the S3 veins has a value that indicates a reduced sulfur source (−15.6‰). This value is similar to those of the much younger (<1843Ma) giant quartz veins cutting the GBMZ rocks. The quartz δ18O values suggest that S1 (12.7‰) formed at higher temperatures than S2 (13.2–19.14‰) and S3 (9.4–17.1‰), or that in the latter two generations of veins, 18O was enriched during fluid/rock equilibration.Secondary trails of native Bi in S1 vein quartz are associated with liquid–vapor (LV) and liquid–vapor–halite (LVS) inclusions, which indicates that Bi, and possibly Au, were transported in saline to hyper-saline brines (LV-Bi, 2–16wt.% NaCl equiv., 8–22wt.% CaCl2 equiv.; LVS-Bi, >37wt.% NaCl equiv.), with homogenization temperatures of 137–216°C and 192 to >350°C for LV-Bi (ThL+V→L) and LVS-Bi (ThL+S→L), respectively. The presence of calcium-rich fluids might indicate extensive equilibration of those solutions with the host TLG rocks. If a pressure correction is applied to the LV inclusions using a minimum entrapment temperature of 271.4°C (the Bi melting point), a minimum crystallization depth of between approximately 5 and 8km is indicated.Trace element analyses carried out in this study and compiled from Acosta-Góngora et al. (2014) show that the least altered metasedimentary TLG rocks contain up to six times more As (Carbonate unit, 30.5ppm) than the average upper continental crust. Conversely, concentrations of Au (<2ppb), Co (10ppm) and Cu (12ppm) are lower than the crustal values. As such, it is possible that the TLG was a source of As, but is a less likely source of Au, Co and Cu for the NICO deposit; this further supports a magmatic-hydrothermal origin for the metals. Nonetheless, the plurikilometer alteration halo of the NICO system indicates that large amounts of elements could have been leached from the TLG, and potentially some were incorporated to the system. However, detailed studies on structural geology, geochemical modeling, and mass balance calculations need to be carried out to consider if such a scenario is feasible.

P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

ABSTRACTThe Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite–magmatite core rimmed by a lower grade metamorphic envelope of andalusite and cordierite‐bearing schists. The oldest structure in the migmatite–magmatite core is a subhorizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn‐tectonic, with up to several tens of metres thick granitoid sills. Sillimanite–ilmenite–magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6–7 kbar at 710–780 °C. Reorientation of sillimanite into the S2 fabric indicates that the S1–S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3–4 kbar and 680–750 °C during development of the S2 steep fabric, and post‐tectonic andalusite indicates further decompression to 2–3 kbar and 600–650 °C. Widespread crystallization of post‐tectonic muscovite is explained by the release of H2O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5 to 3 kbar and 500–530 °C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5–3 kbar and 530–570 °C; replacement of cordierite by fine‐grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3–4 kbar and 540–620 °C. Monazite from a migmatite analysed by LASS yielded elevated HREE concentrations. The grain with the best‐developed oscillatory zoning is 356 ± 1.0 [±7] Ma (207Pb‐corrected 238U/206Pb), considered to date the crystallization from melt in the cordierite stability ~680 °C and 3.5 kbar, whereas the patchy BSE‐dark domains give a date of 347 ± 4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest sub‐horizontal fabric is associated with the onset of magmatism and peak of P–T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite–migmatite core with supracrustal rocks. This study provides information about tectono‐thermal history and time‐scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn‐orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems.

Two-phase subduction and subsequent collision defines the Paleotethyan tectonics of the southeastern Tibetan Plateau: Evidence from zircon U-Pb dating, geochemistry, and structural geology of the Sanjiang orogenic belt, southwest China

A curved belt consisting of terranes or blocks and associated late Paleozoic to early Mesozoic sutures are features of the southeastern part of the Tibetan Plateau. The southeastern Tibetan Plateau Paleotethyan belt is divided into a northwestern segment (central Tibetan Plateau) and a southern segment (the Sanjiang orogenic belt). The Paleotethyan tectonic history of this belt remains a subject of considerable debate. A major controversy concerns whether or not it evolved as a single genetic element. This paper provides new geochronological, geochemical, and structural data to constrain the Paleotethyan tectonics of this belt. A detailed field study across three sections of the Sanjiang orogenic belt has identified two volcaniclastic successions separated by a regional angular unconformity in the northwestern segment, and one continuous volcaniclastic succession in the southern segment. New zircon U-Pb ages reveal that: (1) the lower succession of the northwestern segment was produced by three volcanic phases at ca. 272 Ma, 253–247 Ma, and 240–235 Ma; (2) the upper succession of the northwestern segment formed during a shorter period along the Yushu–Yidun arc from ca. 227 to 211 Ma; and (3) the single volcaniclastic succession of the southern segment was produced continuously from ca. 260 to 210 Ma. Whole-rock geochemical data including trace elements show that the late Paleozoic to early Mesozoic magmatic activity has a continental arc–like affinity. The spatial distribution and field relationships of the magmatic rocks define two continuous volcanic arcs in the southeastern Tibetan Plateau, which correspond to two phases of arc-like magmatism. The Late Triassic Yushu–Yidun arc belt is exposed along the northern half of the northwestern segment to the south of the western Jinsha–Garze–Litang suture. Southward subduction along the western Jinsha–Garze–Litang suture is likely to have generated the Yushu–Yidun arc. The second arc (Jomda–Weixi–Yunxian) was diachronous and is continuous through the southeastern Tibetan Plateau. The northwestern part of the Jomda–Weixi–Yunxian arc belt is overlain by the Yushu–Yidun arc from the north. From northwest to southeast, the cessation of magmatism in the Jomda–Weixi–Yunxian arc varies from the Middle Triassic (ca. 235 Ma) to the Late Triassic (ca. 210 Ma), whereas the onset of magmatism appears to have been in the Early Permian throughout the arc. Zircon Lu-Hf isotopic data suggest that large degrees of melting of a depleted mantle source produced the earliest magmas in each segment of the Jomda–Weixi–Yunxian arc. The similarity in zircon ages of the ophiolitic units and their distribution imply that the Longmu Co–Shuanghu suture is a continuation of the Changning–Menglian suture. The spatial relationship between the Jomda–Weixi–Yunxian arc and the Longmu Co–Shuanghu–Changning–Menglian suture suggests that diachronous eastward subduction of part of the Paleotethys along the suture produced the Jomda–Weixi–Yunxian arc belt. Widespread folds of the continental arc–volcaniclastics with orogen-parallel hinges reflect compression perpendicular to the orogen, subsequent to arc magmatism. As such, the Paleotethyan tectonics of the southeastern Tibetan Plateau are characterized by two-phase subduction and subsequent two-phase continent-continent collision.

Magmatic rifting of Pangaea linked to onset of South American plate motion

The causes of the transition from amagmatic to magmatic rifting during continental break-up are not always clear and have been often linked to the break-through of a plume. However, stress fields recorded in the crust may offer new insights into the relationship between changes in the stress field and the onset of magmatism. In this paper stress fields recorded in the crust of North America are used to test possible causes of the break-up of Pangaea and the transition from amagmatic to magmatic rifting. Finite element models reveal that the most likely scenario for the break-up involves the initial northwest motion of North America at 230 Ma, followed by a south-southeast motion of South America at 200 Ma with the initiation of magmatism, and finally a weakened area between North and South America sometime soon after 200 Ma which resulted in North America being dominated by northwest motion once again. It was also determined that plate boundary structure and orientation play a large part in the recorded stress fields and must be taken into consideration when modeling continental break-ups and rifting.

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The widespread extension occured within the Variscan orogen and its northern foreland during late Carboniferous to Early Permian times. This was associated with magnetism and with a fundamental change, at the Westphalian-Stephanian boundary, in the regional stress field, coincident with the termination of orogenic activity and onset of dextral translation between North Africa and Europe. Rifting propagated across basement terranes with different ages and thermal histories. Most of the roft basins developed on relatively thin lithosphere; however, the highly magnetic Oslo graben initiated within the edge of a craton. Early stephanian regional uplift is contemporaneous with the onset of magmatism; inviting speculation that it might have been induced by a thermal anomaly within the upper mantle. The contributions to this volume suggest that the geodynamic setting in which magmatism occurred was complex, involving wrench tectonics, slab detachment, and delimination or thermal erosion of the base of the lithosphere.

Evolution of Paleocene sediment dispersal systems in the Foinaven Sub-basin, west of Shetland

Abstract Heavy mineral assemblages in deepwater Paleocene sandstones of the Foinaven Sub-basin (west of Shetland) reflect the influence of three main provenance and dispersal systems. The Schiehallion system has relatively uniform characteristics and persisted through much of the early-mid Paleocene. Its sphere of influence was centred on the Schiehallion Field, but it progressively encroached into the Foinaven area with time. Its main source was the Triassic Foula Formation, with minor supply from Lewisian and Moine basement rocks. The Foinaven system shows marked changes in character, related to evolution of the source area. The earliest sands were derived from a heavily weathered late Cretaceous regolith, and represent the onset of uplift and exhumation of the Shetland Platform. Progressive unroofing led to the incorporation of increasing amounts of Lewisian- and Moine-sourced detritus. Following deposition of the main reservoir sandstones in Foinaven and Schiehallion, there was a progressive change in provenance, with a gradual increase in the amount of sediment shed directly from metamorphic basement. Pulsing of the proto-Icelandic plume has been proposed as the mechanism for repeated influx of sand to the basins around Scotland. However, on the basis of available geochronological data, there does not appear to be a direct link between events in the British Tertiary Igneous Province (BTIP) and changes in provenance in the Foinaven Sub-basin. The initial influx of sediment from a weathered land surface may have been coeval with the onset of magmatism in the BTIP, but the cessation of supply through the Foinaven system at c. 59 Ma does not appear to be related to magmatic events in the BTIP. The waning of magmatism in the BTIP around 58 Ma is broadly coincident with a gradual increase of first-cycle basement detritus.

The Canary Islands origin: a unifying model

A new model, partially based on the three most widely cited previous hypotheses, is proposed to explain the genesis of the Canary Islands. From the hotspot hypothesis it retains the notion that the islands originated from a thermal anomaly in the mantle. From the propagating fracture hypothesis it takes the critical role of regional fractures in the onset of magmatism. The uplifted block hypothesis contributes with the notion that the islands are in their present freeboard attitude due to the action of tectonic forces.The main drawbacks of the three preceding hypotheses are solved within this unifying approach: the thermal anomaly is an upper mantle residue from an old plume, and therefore it does not carry (or does it in a highly diluted form) the typical geophysical and geochemical plume signatures; the fractures are well developed on the continental and oceanic crust, but not in the extremely thick sedimentary pile between the Canary Islands and Africa; and the Canary Islands uplift took place through transpressive shears, and not by means of purely reverse faults. This unifying model, which integrates the thermal and tectonic histories of the lithosphere and the sublithospheric mantle, is considered to be a valid approach to a number of volcanic areas where, as has been highlighted in recent years, pure hotspot or pure fracture models are found wanting to explain oceanic or (less frequently) continental volcanic lines.

Helium isotope composition of the early Iceland mantle plume inferred from the Tertiary picrites of West Greenland

Picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have 3He/ 4He ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio. These high values, measured during gas extraction by crushing in vacuum, are similar to the highest magmatic 3He/ 4He ratios found in young terrestrial volcanic rocks. By analogy with young basalts, in which crushing selectively extracts magmatic helium, any significant cosmogenic 3He appears to be absent in these picrites. Additional evidence for the absence of cosmogenic helium is provided by fusion results on the crushed olivine powders and by a single stepwise crushing experiment, in which only magmatic and radiogenic helium components are resolvable. The West Greenland picrites have Pb, Nd and Sr isotope compositions which overlap those found in picrites from Iceland and in basalts from Loihi Seamount, localities which today also have high 3He/ 4He ratios. Isotopic variations in He, Pb, Nd and Sr for the West Greenland picrites are interpreted to largely result from interaction of the early Iceland mantle plume with the upper mantle during plume ascent and dispersion beneath the continental lithosphere. The presence of high 3He/ 4He ratios in West Greenland, and the onset of magmatism across the North Atlantic Volcanic Province near 62 Ma, supports the hypothesis for very rapid dispersion (>1 m/year) of mantle plume head material during the earliest stages of plume impact, as predicted in recent numerical simulations of plume behavior during thermal mantle convection with non-Newtonian rheology.

U–Pb and Rb–Sr geochronology of magmatism and metamorphism in the Dalradian of Connemara, western Ireland

New geochronological data which clarify the timing of syn-orogenic magmatism and regional metamorphism in the Connemara Dalradian are presented. U–Pb zircon data on four intermediate to acid foliated magmatic rocks show important inherited components but the most concordant fractions demonstrate that major magmatism continued until 465 Ma whereas the earliest, basic magmatism has been dated previously at 490 Ma; a fine-grained, fabric-cutting granite contains discordant zircons which also appear to be 465 Ma old. Arc magmatism in Connemara therefore spanned a period of at least 25 Ma. Recent U–Pb data on titanite from central Connemara which gave a peak metamorphic age of 478 Ma are supplemented by U–Pb data on titanite and monazite from metamorphic veins in the east of Connemara which indicate that low- P, high- T regional metamorphism continued there to 465 Ma, i.e. at least 10 Ma later than in the central region dated previously. New Rb–Sr data on muscovites from coarse-grained segregations in different structural settings range from 475 to 435 Ma: in part this range probably also reflects differences in age from west to east, with three ages close to 455 Ma from the eastern area, which is also the site of the lowest pressure metamorphism. Thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1–2 Ma. The new dates emphasize the complexity in the spatial and temporal distribution of high-level regional metamorphism caused by magmatic activity. The relatively simple overall distribution of mineral-appearance isograds revealed by regional mapping masks the complexity of a prolonged but punctuated metamorphic history related to multiple intrusions, primarily in the southern part of Connemara. The later stages of magmatic activity followed progressive uplift and erosion after the onset of magmatism, and were localized in the eastern part of the region.

Slab breakoff: A model for syncollisional magmatism and tectonics in the Alps

Slab breakoff is the buoyancy‐driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision. In a recent paper Davies and von Blanckenburg [1994] have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts. Breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere, melting of its enriched layers, and thus to bimodal magmatism. Breakoff will also lead to thermal weakening of the subducted crustal lithosphere, thereby allowing buoyant rise of released crustal slices from mantle depths. In this paper we present a test of this model in the Tertiary evolution of the European Alps. In the Alps, both basaltic and granitoid magmatism occur between 42 and 25 Ma, following the closure of oceanic basins by subduction and continental collision. The granitoids are now well established to result from mixing of basalt with assimilated continental crust. To identify the tectonically crucial origin of the partial mantle melts, we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole Alpine arc. Their trace element and isotopic composition suggests that they have been formed by low‐degree melting of the mechanically stable lithospheric mantle. We see no evidence for melting of asthenospheric mantle. It was thus not decompressed to depths shallower than 50 km. Once initiated, rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust. This explains why all Alpine magmatic rocks intruded almost synchronously along a strike‐slip fault, the Periadriatic Lineament. A compilation of ages from Penninic high‐pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55–40 Ma, followed by uplift at 40–35 Ma. From the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff. The slab breakoff model fulfills its predictions in the case of the Alps and therefore supports the assumptions made in the theoretical model on a geological basis. We believe that the characteristic association of magmatic activity with the return of high‐pressure rocks to the surface allows the identification of this process in the Earth's mountain belts.

A U–Pb dating of the Askersund granite and its marginal augen gneiss

Abstract U–Pb zircon ages of 1848±15 and 1842+23 -13 Ma have been obtained for the coarse porphyritic Askersund granite (‘Filipstad type’) and for its marginal augen gneiss, respectively. The results indicate that the onset of magmatism in the Transscandinavian Igneous Belt (TIB) took place in connection with the late Svecofennian orogenic development. They also support a structural model implying that regionally deformed and folded marginal parts of older granitoid intrusions in the TIB have similar ages as the isotropic and discordant central parts. The deformation is reflected in the morphology of the zircons but does not affect the isotopic ages.

Nature and timing of Franklin igneous events, Canada: Implications for a Late Proterozoic mantle plume and the break-up of Laurentia

U Pb baddeleyite/zircon ages have been determined for gabbro sills and diabase dykes emplaced during Franklin igneous events in northern Canada. Results for seven widely separated sills from Victoria Island, Coronation Gulf and Bathurst Inlet, and for a single diabase dyke from southeastern Baffin Island, indicate that most of the Franklin igneous activity was synchronous and occurred at 723 + 4/−2 Ma. A U Pb baddeleyite/ zircon age of 718 ± 2 Ma for one of the Victoria Island sills suggests that the magmatism continued for at least 5 m.y. after the initial outburst. The short-duration and large volume of the intrusive tholeiitic magmatism and its association with the Natkusiak flood basalts suggest a link with a mantle plume-generated hotspot. The onset of magmatism may have coincided with the arrival of the plume at the base of the lithosphere. Stratigraphic relationships on Victoria Island indicate that differential uplift occurred just prior to the initial outbreak of igneous activity, which is interpreted as a response to doming above the upwelling plume. The exposed record provides no indication of significant pre-volcanic extension and rifting, although some NE-SW extension occurred after the initial eruptive phase. The 779 Ma mafic magmatism in the Mackenzie Mountains, the 723 Ma Franklin events and the 615 Ma Long Range dykes in Labrador are three precisely dated mafic events in North America that may be contemporaneous with various phases of Late Proterozoic break-up of Laurentia. These ages establish key chronological constraints for global correlation of Late Proterozoic epicontinental basins and for Precambrian plate reconstructions.