Basic Dyke Swarms Research Articles

Hf isotopic characteristics of the Tarim Permian large igneous province rocks of NW China: Implication for the magmatic source and evolution

The Tarim large igneous province (TLIP) in northwestern China, covering an area of ca. 250,000km2, includes large volumes of basalts, basic dyke swarms, mafic-ultramafic intrusion and minor picrite. Here we report systematic Hf isotope data from basalt, diabase, olivine pyroxenite and syenitic porphyry from the TLIP and address the source components and magma evolution. The subdivision of the Tarim basalts shows that the Group 1 and Group 2 basalts are clearly differentiated based on different Nb/Y values, with two subgroups (Group 1a, Group 1b) identified based on distinct P2O5 vs. Mg# trends. The 176Hf/177Hf isotopic composition of the basalts ranges from 0.282584 to 0.282837. The εHf(t) of the basalts belonging to the Group 1a, Group 1b and Group 2 are −0.4–4.4, 0.5–2.1, and 1.9–3.1, respectively, and those of the intrusive suite of olivine pyroxenite, diabase and syenitic porphyry show a range of 6.0–6.5, 4.5–5.7, and 6.5–8.3, respectively. The TLIP basalts generally show a good positive correlation between 176Hf/177Hf and 143Nd/144Nd, and fall in the field of the oceanic island basalts (OIBs) with low 176Hf/177Hf and 143Nd/144Nd, comparable to the basaltic lavas of the Pitcairn hotspot. These features, together with the enriched signature of the Tarim basalts might reflect the incorporation of partial melting of lithospheric mantle source in the early stages of the plume activity. The TLIP basalts show low εNd(t) and moderate εHf(t) OIB-like source. The 176Hf/177Hf-143Nd/144Nd values of the Group 1a and Group 1b basalts of the TLIP basalts are comparable to those from the Karoo high-Ti basalts, and those of the Group 2 basalts are comparable to the features of the Karoo low-Ti basalts and diabases. The Group 1a and Group 1b basalts fall in the same Hf–Nd array, whereas the Group 2 basalts fall in a different array with much higher εNd(t). The olivine pyroxenite, diabase and syenitic porphyry fall in the higher εNd(t) and εHf(t) field, with the olivine pyroxenite and diabase having features close to OIB-like source. Our new Hf isotopic results suggest distinct sources for the Tarim basalts (285–290Ma) and the intrusive rocks (274–284Ma). Furthermore, the εHf(t) vs. εNd(t) plots show that the basalts and intrusive rocks might correspond to two different periods of magmatic activity in the Tarim Basin during the Early Permian, being comparable to the temporal evolution of different rock units in the TLIP. The εHf(t) and εNd(t) combined with other evidences address that the basalts could be explained by being derived from the asthenospheric (or plume) mantle and having interaction with lithospheric mantle source by mainly lower degree of partial melting in the early stage before the eruption, and should be much less proportion of crustal contamination during the period of 285–290Ma, and the intrusive rocks might be derived from the primary magma and/or OIB-like mantle sources and underwent a magma process mainly by fractional crystallization and/or cumulation during the period of 274–284Ma.

Detrital zircon U–Pb geochronology, Hf-isotopes and geochemistry—New clues for the Precambrian crustal evolution of Cathaysia Block, South China

This paper reports some new results from U–Pb geochronological, Hf isotopic and REE geochemical studies of detrital zircons in the Ordovician sandstones from South Jiangxi within Cathaysia. 426 groups of U–Pb age determinations define five major age populations: 2560–2380 Ma (a peak of 2460 Ma), 1930–1520 Ma (a peak of 1700 Ma), 1300–900 Ma (a major peak at 970 Ma and two subordinate peaks at 1250 Ma and 1130 Ma), 850–730 Ma (a prominent peak of 780 Ma) and 670–530 Ma (a major peak at 540 Ma and a subordinate peak at 650 Ma). We also report zircon U–Pb concordia age of 3.96 Ga, which is the oldest age so far obtained from Cathaysia. The age peak at 2460 Ma correlates with similar ages reported for Neoarchean global continental growth. The 1930–1520 Ma population broadly overlaps with the time of amalgamation and disruption of the Columbia supercontinent. The major age peak at 970 Ma and two secondary peaks at 1250 Ma and 1130 Ma reflect multiple tectonothermal events associated with the assembly of Rodinia. Similar ages are widely reported from the South China Craton (SCC). Our study reveals that the 850–730 Ma population is consistent with the breakup period of Rodinia, suggesting that the SCC within Rodinia began to break up since 850 Ma. Geologically, the evidence for this breakup event is widespread and presented by Neoproterozoic granites, bimodal igneous rocks, basic dyke swarms and formation of continental rift type basins. Our study also reveals a 670–530 Ma population that correlates well with the assembly of Gondwana during end Neoproterozoic. However, direct geological evidence for this event has not yet been found within the studied area. Furthermore, the Hf isotopic model age data suggest two major stages of crustal evolution within Cathaysia. The first is the event dated at 1.6–2.8 Ga and the second one at 3.5–3.9 Ga. The zircons show a large range of εHf(t) values from + 8.64 to − 30.54, suggesting that they have different origins with a similar age of crystallization. The fact that most detrital zircons show negative εHf(t) values suggests the ancient provenances of Cathaysia were dominated by reworked crustal materials.

Precambrian tectonic evolution of the Tarim Block, NW China: New geochronological insights from the Quruqtagh domain

The Tarim Block is an important tectonic unit to understand the Proterozoic tectonic framework of the Central Asian Orogenic Belt and the supercontinent Rodinia. The granitic, dioritic, gabbroic intrusive rocks and volcanic–volcanoclastic rocks are widely distributed in the Quruqtagh domain of NE-Tarim. The precise ages of these rocks and their tectonic implications in this part of the world are not well understood. This paper reports geochronological data of gabbro, diorite and granitic rocks from Quruqtagh. LA ICPMS U–Pb zircon ages suggest that numerous of gabbroic and granitic rocks were mainly crystallized at ca. 800 Ma. New geochronological data from the magmatic zircons of gabbro, granite and paragneiss can be preliminarily divided into four groups, which are (1) 2469 ± 12 Ma or 2470 ± 24 Ma, (2) 933 ± 11 Ma to 1048 ± 19 Ma, (3) 806 ± 8 Ma, 798 ± 7 Ma, 799 ± 24 Ma, 698 ± 51 Ma (lower intercept age of the paragneiss), and (4) 1930 Ma (upper intercept age of the paragneiss), respectively. These age data are consistent with four tectono-thermal events that took ever place in the Tarim Block. The 93 U–Pb age data (seven for average Concordia age from seven igneous plutons, 86 for xenocrystic and metamorphic ones) from eight samples can be divided into four evolutionary stages: 2360–2550 Ma (peak of 2510 Ma), 1800–2020 Ma (peak of 1870 Ma), 860–1140 Ma (peak of 920 Ma) and 680–840 Ma (peak of 800 Ma), respectively. The age peak of 2500 Ma, consistent with characteristic period of a global building-continent event, indicates that the late Neoarchean–early Paleoproterozoic magmatism had been ever taken place in Tarim. Two peaks at 1870 Ma and 920 Ma, being two assembly periods of the middle Paleoproterozoic Columbia and the Neoproterozoic Rodinia supercontinents, suggest that Tarim had connections with both Columbia and Rodinia, whereas structural evidence of these two events is absent in Tarim. Notable peak of 800 Ma is interpreted as a response to the break-up of Rodinia supercontinent. The geological indicators of break-up such as Neoproterozoic granite, bimodal igneous rocks, composite magmatic flow, basic dyke swarm and continental rift type basins are well developed, followed by a large-scale of late Neoproterozoic glacier event. Four stages of magmatism mentioned above constitute major events in the Precambrian evolution of Tarim and Central Asian. These data, combining with previous U–Pb ages from igneous rocks, provide a significant line of evidence for understanding the Rodinia evolution of Tarim and the relationships with South China, east India and east Antarctica, Lesser and Great Himalaya blocks in Proterozoic. A primary reconstruction of the Tarim Block, connecting it with South China, east India and east Antarctica, Lesser and Great Himalaya blocks, is finally proposed.

Ages of Zircons from Basalt of Erdaogou Formation and Diabase Dyke Swarms in Northern Liaoning

The drainage area of Fanhe River around the Tieling City in the northern part of Liaodong Peninsula, is located in the north of the Tancheng-Lujiang fault zone, and belongs to the northeastern end of North China Block in the Sino-Korean plate. In this area, there is huge-thick basalt (Erdaogou Formation) and diabase dyke swarms, which are thought to be Mesoproterozoic basic magmatism. By means of zircon dating with sensitive high-resolution ion microprobe (SHRIMP), the eruption time of the Erdaogou Formation basalt is considered to be 36.4 ± 1.8 Ma, the Eocene Epoch. The emplacement time of the diabase swarms is considered to be 93.5 ± 1.4 Ma, the Late Cretaceous. The basic dyke swarms and basalts in the northern Liaoning were determined as Mesozoic and Cenozoic, respectively. This shows that the northern boundary that divides North China Block and Jiao-Liao-Korean Block within Sino-Korean Plate (i.e. the northern part of the Tancheng-Lujiang fault zone), was in a stretch-tensile tectonic environment in EW direction in Late Cretaceous. In addition, this tectonic environment resulted in the intrusion of the diabase swarms. In Paleogene Period, the stretching direction was changed to near SN, and the formation of the stretching tectonics led to the eruption of the huge-thick Erdaogou Formation basalt. The new SHRIMP zircon U-Pb ages in this article does not support the traditional opinion that the Erdaogou Formation basalt was formed in Mesoproterozoic.

Amphibole and lower crustal seismic properties

Measurements of seismic anisotropy frequently offer insights of the deformation kinematics in-situ within the Earth. In the continental crust, seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation (LPO) of mica crystals. However, determinations of composition suggest that the deep continental crust contains rather little mica. In this contribution, the role of amphibole, a far more plausible constituent, especially for deep continental crust of basic composition, is assessed. A field analogue approach is adopted, where the LPO are measured, correlated with the strain state of the rocks, and used to calculate the seismic properties. The seismic properties may be up-scaled through the specimen using the modal proportions of the constituent mineral phases. To examine the role of composition in controlling the intensity of seismic anisotropy, seismic properties are computed using an array of modal compositions (so-called ‘rock recipes’). The case history comes from the Central Block of the Lewisian of NW Scotland, where a regional basic dyke swarm (the Scourie dykes) is locally deformed within amphibolite facies shear zones, conditions that are representative of the deep crust. Shear strain (γ) profiles across a 100 m wide shear zone, using deflected banding in the host gneisses, yield values of 0<γ<15 or greater. A Scourie dyke deflected into the shear zone exhibits hornblende with distinct preferred dimensional orientations representative of such strains. In contrast, plagioclase (and quartz) clots are almost totally insensitive to deformation (due to grain-size sensitive creep deformation) and yield approximately constant apparent strains of γ=1–2. Hornblende–plagioclase–quartz LPOs, combined in their modal proportions and modulated by their individual single-crystal elastic properties, define the seismic properties of the shear zone. Rock recipe modelling demonstrates that it is the hornblende that dominates the seismic response and that plagioclase and quartz serve simply to dilute the intensity of anisotropy. The values calculated indicate significant (up to ~13%) seismic anisotropy for strongly sheared amphibolites. Thus amphibole, aligned through deformation, is likely be a major contributor to the seismic anisotropy of the deep continental crust.

Zircon SHRIMP U-Pb dating for olivine gabbro at Wangmuguan in the Beihuaiyang zone and its geological significance

Zircon SHRIMP U-Pb dating was performed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its country rock (garnet-bearing epidote-mica-quartz schist). The results show that the gabbro was crystallized at 635 ± 5 Ma, in the late Neoproterozoic rather than in the late Paleozoic as previously suggested; its country rocks formed at 464±7 Ma, younger than the enclosed gabbro. The U-Pb age for the gabbro is in good agreement with ages for tuff interbedded with sediments from the Doushantuo Formation in the South China Block and late-Neoproterozoic basic dyke swarms distributed on a large scale over areas of Suizhou to Zaoyang of Hubei Province in the northern margin of the South China Block. This suggests a large-scale magmatic activity occurred at the late Neoproterozoic in the South China Block, so that the gabbro at Wangmuguan in the western segment of the Beihuaiyang zone is geotectonic affinity to the northern margin of the South China Block. Since the olivine gabbro occurs within the schist of Ordovician protolith with tectonic contact between them but forming in different tectonic settings, it is concluded that the late-Neoproterozoic gabbro was detached from the Precambrian basement of the South China Block during the Triassic subduction of the South China Block, and tectonically thrusted over the metamorphosed rocks in the southern margin of the North China Block.

SHRIMP zircon U-Pb dating and its geological significance of Chibaisong gabbro in Tonghua area, Jilin Province, China

SHRIMP zircon U-Pb dating for Chibaisong No.1 gabbro in the Tonghua area, Jilin Province, is discussed in the paper. The cathodoluminescence (CL) images of the zircons indicate that they can be divided into two major types, i.e. euhedral-subhedral one with striped absorption and round one with obvious oscillatory zoning rims. The dating results of the zircons suggest that Chibaisong gabbro in the Tonghua area was formed at 134 ± 7Ma—the Early Cretaceous. The concordia ages of 2497 ± 13Ma, 787 ± 35Ma, 321 ± 10Ma, 217 ± 11Ma of zircons imply that the Tonghua area might geologically be located in the convergent region of the Yangtze Block (YB) and the North China Block (NCB) in the early Mesozoic and the zircons should be caught during magma intrusion. The existence of the Early Cretaceous basic dyke swarm in the Tonghua area shows that an intensive extensional environment occurred in the Early Cretaceous. It is suggested that the magmatism and lithospheric extension in the Tonghua area should be related to the subduction of the Pacific plate.

O SISTEMA TAFROGÊNICO TERCIÁRIO DO SALIENTE ORIENTE NORDESTINO NA PARAÍBA: UM LEGADO PROTEROZÓICO

This paper presents data on the E-W to ENE-WNW-trending graben and horst system in the easternmost part of Paraíba State, Brazil. The installation of this Tertiary tectonic unit was performed following previous structural trends of the Precambrian basement, comprising as a typical case of brittle-ductile structures reactivation. The graben and horst system are positioned in the easternmost part of the Borborema Province, whose main evolutionary phase occurred from the Late Neoproterozoic to the Cambrian, superposing lithostructural units of the Neoproterozoic Alto Moxotó terrane (to the south), composed of orthogneisses and peraluminous supracrustal rocks, and of the Eo-Neoproterozoic Alto Pajeú Terrane (to the north), comprising volcano-sedimentary supracrustal rocks metamorphosed under amphibolite facies. During the closing events of the Brasiliano Cycle, these terranes were penetrated by Cambrian fissural basic intrusives that crop out along the WNW-ESE direction (most of them). This basic dyke swarm is reported here for the first time. During the Tertiary, the most import period for the regional geomorphological evolution, the main structural Brasiliano trends were reactivated as extensional faults, leading to the formation of the taphrogenic system discussed here. At that time, some new faults of extensional character were also formed. As similar occurrences have been described elsewhere (e. g. Rio Grande do Norte State), these facts bring new elements for the discussion of the regional geology of Borborema Province, especially in terms of geomorphological evolution (certainly, a new aspect to he considered) and groundwater resources (heavily demanded in such semi-arid area). Additionally, the occurrence of such brittle Tertiary tectonism in the (next) interior of the continent necessarily opens again the problem of the structural behavior of the Cretaceous coastal basins, in order to investigate the formation of structural traps for hydrocarbon during this period.

O SISTEMA TAFROGÊNICO TERCIÁRIO DO SALIENTE ORIENTE NORDESTINO NA PARAÍBA: UM LEGADO PROTEROZÓICO

This paper presents data on the E-W to ENE-WNW-trending graben and horst system in the easternmost part of Paraiba State, Brazil. The installation of this Tertiary tectonic unit was performed following previous structural trends of the Precambrian basement, comprising as a typical case of brittle-ductile structures reactivation. The graben and horst system are positioned in the easternmost part of the Borborema Province, whose main evolutionary phase occurred from the Late Neoproterozoic to the Cambrian, superposing lithostructural units of the Neoproterozoic Alto Moxoto terrane (to the south), composed of orthogneisses and peraluminous supracrustal rocks, and of the Eo-Neoproterozoic Alto Pajeu Terrane (to the north), comprising volcano-sedimentary supracrustal rocks metamorphosed under amphibolite facies. During the closing events of the Brasiliano Cycle, these terranes were penetrated by Cambrian fissural basic intrusives that crop out along the WNW-ESE direction (most of them). This basic dyke swarm is reported here for the first time. During the Tertiary, the most important period for the regional geomorphological evolution, the main structural Brasiliano trends were reactivated as extensional faults, leading to the formation of the taphrogenic system discussed here. At that time, some new faults of extensional character were also formed. As similar occurrences have been described elsewhere (e. g. Rio Grande do Norte State), these facts bring new elements for the discussion of the regional geology of Borborema Province, especially in terms of geomorphological evolution (certainly, a new aspect to be considered) and groundwater resources (heavily demanded in such semi-arid area). Additionally, the occurrence of such brittle Tertiary tectonism in the (next) interior of the continent necessarily opens again the problem of the structural behavior of the Cretaceous coastal basins, in order to investigate the formation of structural traps for hydrocarbon during this period.

Study on the Sr, Nd, Pb and O isotopes of basic dyke swarms in the Wudang block and basic volcanics of the Yaolinghe Group

The Sr, Nd and Pb isotopic characteristics of the Wudang basic dyke swarms and basic volcanics of the Yaolinghe Group show that they were derived from the same multi-component mixing source in the mantle. The Wudang basic dyke swarms have (87Sr/86Sr)i = 0.6905 ∼0.7061, eNd(t)=−1.9∼5.0, Δ208Pb/204Pb = 35.49∼190.26, Δ207Pb/204Pb = 4∼85, low Th/Ta and a wide range of La/Yb ratios; and the basic volcanics of the Yaolinghe Group have (87Sr/86Sr)i = 0.6487∼0.7075, eNd(t) = 0.11∼3.94, Δ208Pb/204Pb=−81.58∼219.95, Δ207Pb/204Pb = 4.44∼16.68 and higher Th/Ta and La/Yb ratios, indicating that their source is a mixture of DM and EMII, and the basic volcanics of the Yaolinghe Group were contaminated by crust materials en rout to the surface. Based on the geochemical features of continental tholeiitic basalts and being products of different facies derived from the same source, it can be concluded that an important rifting event in the South Qinling basement block occurred during Neoproterozoic, followed by a setting of oceanic basin in the Early Paleozoic.

Rifting of Wudang Block During Chenjiang Period of Neoproterozoic in South Qinling of China: Geochemical Evidence from Wudang Basic Dyke Swarm

Rifting of Wudang Block During Chenjiang Period of Neoproterozoic in South Qinling of China: Geochemical Evidence from Wudang Basic Dyke Swarm

Pre‐collision Granites and Post‐collision Intrusive Assemblage of the Kelameili‐Harlik Orogenic Belt

AbstractThe main types of intrusive rocks in the Kelameili‐Harlik Hercynian orogenic belt include calc‐alkaline granites, diabase dykes, kaligranites and alkaline granites. Investigation in field geology, petrology, mineralogy and geochemistry shows that the calc‐alkaline granites belong to the syntexis‐type (or I‐type) and were formed in a pre‐collisional magmatic arc environment. In consideration of the fact that kaligranites have many features of alkaline granites with higher consolidation temperatures than the calc‐alkaline granites and show a discontinuity of minor element and REE evolution in respect to the calc‐alkaline granites, they could not have been derived by differentiation of magmas for the calc‐alkaline granites, but are likely to have been generated in an environment analogous to that for alkaline granites. The triplet of basic dyke swarms, kaligranites and alkaline granites could be regarded as a prominent indication of the initial stage of post‐collisional delamination and extension. These rocks might have originated from underplating and intraplating of mantle‐derived magmas at varying levels with varying degrees of partial melting, mixing, and interchange of crustal and mantle materials

Organic-walled microfossils from the Proterozoic Thule Supergroup, Northwest Greenland

The Proterozoic rocks of the intracratonic Thule Basin on the northern margin of the Canadian–Greenlandic shield have recently been referred to as the Thule Supergroup. The succession comprises unmetamorphosed, unfolded sedimentary and magmatic rocks, which are, with few exceptions, not deformed. Five groups have been defined and fine-grained siliciclastic samples from the uppermost three (Baffin Bay, Dundas and Narssârssuk groups), yielded moderately well-preserved acanthomorphic and non-acanthomorphic acritarchs, prasinophytes and filamentous microfossils. The age of the supergroup is quite poorly constrained, at the base by a 1268 Ma basaltic sill, and at the top by a basic dyke swarm with a K/Ar age range between 630 and 725 Ma, that cuts the entire succession. Biostratigraphical studies further constrain the age of the Thule Supergroup and suggest that the succession is of a late Mesoproterozoic/earliest Neoproterozoic age. This age assignment is corroborated by independent stable isotope data and correlation with the Bylot Supergroup to the south. This leads us to propose that the Thule Supergroup was deposited between c. 1300 and c. 1200 Ma. Microfossil taxa previously interpreted as exclusively occurring in pre-Varangerian, Neoproterozoic rocks (i.e. Simia annulare Jankauskas, Tasmanites rifejicus Jankauskas and possibly Vandalosphaeridium varangeri Vidal) are consequently re-evaluated. Process-bearing acritarchs, herein attributed under open nomenclature to ? Comasphaeridium sp., ` Trachyhystrichosphaera truncata' and ? Vandalosphaeridium sp., are of special palaeobiological importance since acanthomorphic acritarchs have rarely been reported from pre-Neoproterozoic successions.

Thoughts on the Jiaodong Gold Province of China: Towards a Tectonic Model

Abstract The Jiaodong gold province is situated in the eastern Sino‐Korean Platform within the so‐called Jiaoliao Uplift. The basement rocks are Archaean and Proterozoic metamorphic rocks. Mesozoic sedimentary and volcanic cover occur within extensional basins. Intrusive rocks are dominated by Mesozoic granitoid, with intermediate‐acid and basic dyke swarms. The structures form an E‐W‐trending anticlinorium in the basement complex, and large‐scale NE‐SW‐ and NNE‐SSW‐trending fault zones of Mesozoic age. The gold mineralization is associated with the Mesozoic faults and related secondary fractures in the granites or granite‐basement contacts. The mineralization types are quartz‐vein type and wall‐rock alteration type. Wall‐rock alteration is very well developed around the ore zones. Alteration minerals include quartz, sericite (and fuchsite), pyrite, calcite, chlorite, hematite, rutile and graphite. The ore assemblage is uniform in all deposits, including pyrite, chalcopyrite, galena, sphalerite, arsenopyrite, pyrrhotite, gold, electrum, hessite, petzite, magnetite, molybdenite, tetrahedrite and wolframite. Mesozoic collision and subduction between the South China and North China continental blocks contributed to formation of the Mesozoic granitoid intrusions. The granitic magma is considered to be derived from partial melting of the crust through underplating processes. Gold was remobilised from basement rocks and deposited in fracture zones by the high‐temperature fluids associated with these processes.

The basic dyke swarms in the Wudang block and its geological significance

The basic dyke swarms, which have been deformed and metamorphosed, are widespread in the Wudang block. They are geochemicaiiy characterized by the continental tholeiites, and their Sm-Nd isochron age is (782 ±164) Ma. All of these indicate that there was an ancient continental block In the Proterozoic and a rifting occurring on this continental block during ±800 Ma.

Palaeomagnetism of three dyke swarms in Nansen Land, north Greenland (83° N)

Three basic dyke swarms of post-Ellesmerian (post-Early Carboniferous) age in Nansen Land (83° N, 43° W) are still not dated numerically, but cross-cutting relationships show Group 1 to be older than Group 2, while Group 3 is the freshest and likely the youngest. Group 1 (the most northerly swarm) strikes N-S; Group 2 NW-SE, and Group 3 (the most southerly swarm) E-W. From more than 200 dykes 234 specimens from 28 sites were investigated palaeomagnetically. Group 1 dykes show unexpected shallow inclinations with a cleaned mean direction of (Dm, Im) = (151°, −5.8°), N = 7, k = 18.5, α95 = 13.9°. They show hydrothermal alterations, some remagnetization by lightning, and the low inclination indicates a low palaeo latitude. The palaeopole is (Plat, Plon) = (8.9° S, 14.0° W) with (dp, dm) = (7°, 14°), and is close to the North American Early Carboniferous mean pole, suggesting a syn- or early late-tectonic dyke injection. The polarity is reverse. Groups 2 and 3 of presumed Cretaceous or Tertiary age show dominantly normal and reverse polarities, respectively. Their mean directions per polarity are well grouped, with (Dm, Im) = (−30.6°, 76.7°), n = 13, k = 191.4, α95 = 3.9°, and (Dm, Im) = (133.4°, −76.7°), n = 10, k = 87.5, α95 = 5.9°, respectively. They are antipodal within 95% significance, and combining both swarms gives (Dm, Im) = (−37.5°, 76.8°), n = 23, k = 124.3, α95 = 2.7°, corresponding to a mean pole of (Plat, Plon) = (70.0° N, 185.1° E) with (dp, dm) = (4.7°, 5.0°), for which the spline of Late Cretaceous-Tertiary poles for all Greenland indicates a palaeomagnetic age of 57 ± 10 Ma. This pole (in present-day coordinates) is very close to the Late Cretaceous North American pole, in accordance with the fact that Greenland belongs to the North American craton, and that the two younger swarms are essentially postdating the opening of Baffin Bay.

Tectono‐magmatic control on vertical dip slip basement faulting: An example from the Fennoscandian Shield

ABSTRACTVertical dip‐slip basement faults play an important role in the evolution and structuring of the Earth's crust. The Proterozoic anorogenic rapakivi‐anorthosite setting of the Fennoscandian Shield in southern Finland exhibits a widespread pattern of vertical dip‐slip basement faults that are deeply eroded. The Porkkala‐Mantsala (PM)‐fault, located c. 30 km W of Helsinki is part of a system of crustal lineaments that closely follows the outcrop pattern of Mid‐Late Proterozoic anorogenic crustal elements, such as basic dyke swarms, the outline of rapakivi granites and remnants of sediment‐filled grabens. These lineaments are formed by low‐grade dip‐slip faults that overprint Svecofennian shear zones. Structural analysis of the PM‐fault supports an interpretation in terms of reactivation of a high‐grade ductile wrench zone. Successive stages of brittle deformation visible in as well the PM‐fault and the Obbnäs granite demonstrate that brittle deformation in the PM‐fault is coeval with the intrusion of the anorogenic Obbnäs rapakivi granite. Based on the spatial and temporal relationship of anorogenic magmatism and block faulting, a genetic relationship is proposed.

Geochronology, stratigraphy and structural evolution of Silali volcano, Gregory Rift, Kenya

Silali, 1°10'N, 36°12°E, is the largest Quaternary caldera volcano in the northern Gregory Rift and is composed predominantly of peralkaline trachytic lavas and pyroclastic deposits, and mildly alkaline to transitional basalts. Detailed mapping combined with radiometric 40 Ar/ 39 Ar age determinations is used to constrain the evolutionary development of Silali. Activity commenced at c. 400–220 ka with the construction of a low relief lava shield whose summit area was subsequently modified by alternating periods of faulting, subsidence and infilling associated with two major periods of explosive activity. This activity ceased around 133–131 ka and was probably the result of fracturing and decompression of a high level magma chamber by regional extension and the injection of basaltic dykes below the volcano. Later eruptions (c. 120 ka) along the western flanks migrated eastward with time and culminated in the eruption of viscous trachyte lavas from a circumferential fissure zone. The emplacement of a basic dyke swarm to shallow crustal levels beneath Silali resulted in the formation a broad volcanic rift zone within which large volumes of fluid basalts were erupted to mantle the flanks of the volcano. This activity mainly pre-dated, but probably also overlapped with, incremental subsidence and asymmetric downsagging of the summit area and the propagation of a circumferential fissure zone. Basalt and trachyte lavas, erupted along the circumferential fissure zone and from a major meridional summit fissure, moved outward to form a series of flat summit benches, and ponded in summit depressions before overflowing. The continuing inward collapse of the summit area and the lateral drainage of magma from a high-level reservoir finally culminated in the formation of a large caldera at c. 63 ka. Post-caldera activity utilized pre-existing weaknesses within the caldera, erupting basalt and trachyte lavas until around 7 ka, and was contemporaneous with the eruption of trachyte lava domes (9–7 ka) on the eastern flanks.

Geology and tectonics of the Betic Zone, SE Spain

The southern margin of the Iberian continent comprises a complex geological history, which can be traced back into the Proterozoic when a gneissic basement was formed. A cover of Nevado-Filabride rocks was deposited onto this Proterozoic basement during the Palaeozoic. The Permian intrusion of the Bédar granite suggests that the Palaeozoic sedimentary rocks now exposed in the Betic Cordilleras have been affected by the Hercynian orogeny, similar to that of Palaeozoic rocks in the adjoining Ossa Morena zone. The Hercynian basement was probably covered by Alpujarride carbonates in the Mesozoic, when rifting opened Tethys and intruded 140 Ma old basic dyke swarms. The Palaeozoic basement of the Betic Cordilleras comprises a suture of Late Cretaceous subduction (80–85 Ma B.P.). This can be inferred from (HP,LT)-metamorphic facies obducted in coeval thrust slices of Nevado-Filabride rocks. The Nevado-Filabride nappes include Mesozoic carbonates transformed into marbles (Macael). The suture was probably annnealed after the closure of Tethys. The Mesozoic boundary between Africa and Europe and the rifting in Tethys ceased during the Palaeogene when the Atlas Mountains were formed (36 Ma B.P.). Subsequently, relative movements between Africa and Europe in the Neogene were accommodated by strike-slip faults. These define the Iberian microplate moving eastward by Atlantic spreading and possibly subducting under Italy. Neogene deformation of Iberia is most intensive in southeastern Spain due to emplacement of a mantle diapir under the Alboran area. This occurred early in the Neogene (Aquitanian) and caused nappe spreading involving Alpujarride and Malaguide rocks. The subsequent Neogene history has now been studied in great detail and is reviewed here. It involves clear examples of calc-alkaline volcanism — most probably not related to subduction, seismic motion along strike-slip faults within the Iberian microplate, and syntectonic deposition in intermontane Neogene basins.

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.