Suite Of Mafic Dykes Research Articles

Early Precambrian mafic dykes in western Shandong province, North China Craton: constraints on the chronology, genetic model and tectonic significance

AbstractIn the Late Neoarchaean, the lithosphere of the North China Craton (NCC) experienced a strong extensional event, which is of great significance for understanding the evolution of the continental crust in the Precambrian. In this study, a suite of mafic dykes from Shandong province in the northeastern NCC were investigated to determine the nature, timing and source of rift-related magma activities using zircon U–Pb data, whole-rock geochemistry and Nd–Hf isotopes. Zircon U–Pb dating of four dolerites by laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) yielded weighted mean 207Pb/206Pb ages in the range 2509 ± 6.1 to 2537 ± 6.2 Ma (2σ, 95 % confidence interval). The mafic dykes are classified as alkaline rocks based on their K2O + Na2O contents (6.78–7.21 wt %) and belong to the shoshonitic series according to their K2O contents (3.23–3.36 wt %). The dolerites show low concentrations of light rare earth elements ((La/Yb)N between 7.17 and 8.55), positive Eu anomalies (Eu/Eu* between 1.12 and 1.27), positive Ba, K, Pb, Sr, Eu, Dy and Lu anomalies, and depleted U, Nb, Pr, Ta, P, Nd and Ti anomalies. The dykes are characterized by low initial (87Sr/86Sr)i (∼0.6969), positive εNd(t) values (0.2–0.8) and εHf(t) values (0.5–8.6) and relatively old mean Nd and Hf model age (2.73 Ga). Collectively, the data suggest that the mafic dykes were derived from the partial melting (10–20 %) of an isotope-depleted garnet–lherzolite mantle source that was hybridized through interaction with subducted lower crustal material. The parental magmas of these dykes underwent a certain number of crustal contaminations during magma ascent. The mafic magmatism represented in the form of the dyke swarms is considered to be a response to widespread lithospheric extension which affected the NCC at c. 2.5 Ga during the Neoarchaean.

Crust-derived felsic magmatism in the Emeishan large igneous Province: New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block, China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The εHf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding TDM1 values are in the range of 919–523 Ma. The εHf(t) values of zircon from the DSC I-type granite are between –1 and 3, with TDM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ18O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.

Mafic dykes of the southeastern Gawler Craton: ca 1564 Ma magmatism with an enriched mantle source

This study investigates the age and composition of a suite of mafic dykes in the southeastern Gawler Craton. Mafic dykes intrude the gneissic fabric of the ca 1850 Ma Donington Suite and were previously interpreted to have been emplaced at ca 1845 Ma. However, sensitive high-resolution ion microprobe U–Pb dating of zircon within one of these mafic dykes shows it was emplaced at 1564 ± 4 Ma. The sampled dyke also contains a population of inherited zircon cores with an age of 1770 ± 6 Ma, and one grain with an age of ca 1860 Ma, likely derived from country rock. Whole-rock geochemistry from this dyke and nearby dykes with similar structural and mineralogical features show that there is a suite of dykes in the region with gabbroic, low SiO2 and TiO2 contents and tholeiitic characteristics. The dykes are enriched in Ba, K and Rb, with low to moderate MgO, Ni and Cr contents, and moderate light rare earth element enrichment. Negative Nb and Ta anomalies with subtle negative Ti anomalies suggest some interaction with continental crust. Sm–Nd isotopic data have εNd(1560 Ma) between −6.8 and −2.9, and depleted mantle model ages >2.5 Ga. Th/Nb ratios are 0.21–0.73 consistent with a metasomatised, subduction modified lithospheric mantle source. The relatively primitive nature of the tholeiites suggests the crustal-like signatures are inherited from the source region, which likely represents an enriched continental lithospheric mantle in the eastern Gawler Craton. This ca 1564 Ma mafic magmatism is newly named the Daly Head Metadolerite. In addition, narrow high-strain zones overprint the dated mafic dyke and locally the Donington Suite wall rocks. A syn-kinematic granitic dyke intrusion within one high-strain zone has been dated via laser ablation-inductively coupled plasma mass spectrometry U–Pb zircon methods and was emplaced at ca 1555 Ma. This localised ca 1555 Ma deformation is a previously unrecognised tectonic event in the southern Gawler Craton. This study adds to the growing database showing magmatism and deformation in the Gawler Craton continued after the voluminous Gawler Range Volcanics and Hiltaba Suite magmatism (ca 1596–1575 Ma). These younger tectono-magmatic events typically occur around the margins of the Gawler Craton, suggesting the internal ‘core’ of the craton may have been substantially less fertile for melting during these younger thermal/structural events, likely because of the high-temperature nature of the early Mesoproterozoic Gawler Range Volcanics and Hiltaba Suite magmatism. KEY POINTS Geochronology shows mafic dykes in southeastern Gawler Craton emplaced at 1564 ± 4 Ma. Mafic dykes are tholeiitic, derived from a metasomatised, subduction modified lithospheric mantle source. Narrow high-strain zones overprint the dated mafic dyke and contain syn-kinematic granitic dyke dated at ca 1555 Ma. Localisation of post-ca 1575 Ma tectono-magmatic events occur around margins of the craton, reflecting reduced melt fertility in the centre owing to previous high-temperature events.

The age of continental crust in the northern Prince Charles mountains (East Antarctica) as evidenced by zircon xenocrysts from cretaceous alkaline-ultramafic rocks

Cretaceous alkaline-ultramafic stocks in the northern Prince Charles Mountains (PCM) contain zircon xenocrysts entrained during magma ascent to the surface. These xenocrysts provide the only evidence for the age of the underlying crustal rocks in the area. The zircons display a range of ages from ca 2700 Ma to ca 300–260 Ma, with the most dominant ages of ca 900–800 Ma and ca 660–500 Ma. These data indicate that the presently exposed ca 1000–950 Ma metamorphic rocks in the northern PCM are underlain by other geological unit(s). The presence of ca 2700 Ma zircons indicates that a Neoarchaean component is present in the lower crust and that the presumed Proterozoic Rayner Province is not entirely composed of Proterozoic protoliths, but rather is a tectonic mixture of Neoarchaean and younger rocks. This finding questions a Cambrian (Kuunga-age) collisional suture running through the Ruker Province, which may thus be further south within the sub-ice terrain. Nevertheless, the presence of ca 900–800 Ma and ca 600–500 Ma zircons argue for more widespread post-1000 Ma metamorphic events than are observed at the surface. Ca 300–260 Ma zircons, which could not have been derived from any known crystalline rocks in this region, indicate within-crustal chambers parent to mafic dyke suites.

Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite facies metamorphism (M2, ~800 °C and ~9 kbar) and develop pervasive planar fabric (S2) during a continent-continent collision event at 1000–950 Ma. Mafic dykes cut the S2 fabric of its basement it intrudes. Together with the S2, the mafic dykes have been deformed by three sets of folds viz. DkF1, DkF2 and DkF3. A conspicuous NS trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF1–2. The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase rim + quartz) constrain the depth of emplacement of the mafic dykes (15–18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at ~700 °C and ~7.5 kbar (M3 metamorphism).The estimated physical conditions during the M3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M2 (ca. 964 Ma) and M3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000–950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes.

Mesoarchean exhumation of the Akia terrane and a common Neoarchean tectonothermal history for West Greenland

Recent geochronological and petrographic studies in the Nuuk region, West Greenland, have established that modern-style tectonic processes were active in the Neoarchean. However, more limited work has addressed Neoarchean processes elsewhere in West Greenland. In particular, conflicting models have been proposed for the tectonothermal history of the Akia terrane, to the north of the Nuuk region. In this study, the depositional and metamorphic history of supracrustal rocks in the Akia terrane have been investigated using field relationships, petrography, phase equilibria modelling, and zircon and apatite U-Pb geochronology. The Akia terrane is a Palaeo- to Mesoarchean tonalite-trondhjemite-granodiorite (TTG) gneiss terrane with subordinate supracrustal rocks, mafic to ultramafic intrusions – all metamorphosed at amphibolite to granulite facies – Proterozoic mafic dyke suites, Neoproterozoic and Jurassic kimberlitic rocks, and a Mesoarchean and a Jurassic carbonatite. Some of the supracrustal rocks are known to be Mesoarchean, whilst others lack age constraint. New detrital and metamorphic zircon ages show that the metasedimentary rocks studied – the Kangerluarsuk Supracrustal Belt – were sourced from TTG gneisses within the host Akia terrane and deposited in the period ≤2877 Ma to ≥2857 Ma; this requires exhumation of the Akia terrane after c. 3010–2970 Ma magmatism and low-pressure granulite facies metamorphism. After exhumation, the Kangerluarsuk Supracrustal Belt was buried and underwent prolonged high-temperature metamorphism, reaching ∼820–850 °C and 8–10 kbar during regional ductile deformation in the period 2857–2700 Ma. The same rocks grew metamorphic zircon and neoblastic apatite at c. 2630 Ma and >450 to <850 °C, based on U-Pb geochronology and Pb diffusion modelling respectively. This evidence of Neoarchean metamorphism of the Akia terrane shows that a large part of West Greenland was in tectonic communication and experienced common high temperature conditions in the latest Mesoarchean to early Neoarchean and by c. 2700 Ma. The recognition of prolonged Neoarchean high-temperature metamorphism and partial melting during regional ductile deformation raises questions about the veracity of textural evidence for a giant bolide impact at ≥3000 Ma in the same region as such evidence would likely have been obliterated through such pervasive metamorphic and tectonic reworking.

In situ U-Pb geochronology and geochemistry of a 1.13 Ga mafic dyke suite at Bunger Hills, East Antarctica: The end of the Albany-Fraser Orogeny

Antarctica contains continental fragments of Australian, Indian and African affinities, and is one of the key elements in the reconstruction of Nuna, Rodinia and Gondwana. The Bunger Hills region in East Antarctica is widely interpreted as a remnant of the Mesoproterozoic Albany–Fraser Orogen, which formed during collision between the West Australian and Mawson cratons and is linked with the assembly of Rodinia. Previous studies have suggested that several generations of mafic dyke suites are present at Bunger Hills but an understanding of their origin and tectonic context is limited by the lack of precise age constraints. New in situ SHRIMP U-Pb zircon and baddeleyite dates of, respectively, 1134 ± 9 Ma and 1131 ± 16 Ma confirm an earlier Rb-Sr whole-rock age estimate of ca. 1140 Ma for emplacement of a major mafic dyke suite in the area. Existing and new geochemical data suggest that the source of the dyke involved an EMORB-like source reservoir that was contaminated by a lower crust-like component. The new age constraint indicates that the dykes post-date the last known phase of plutonism at Bunger Hills by ca. 20 million years and were emplaced at the end of Stage 2 of the Albany-Fraser Orogeny. In current models, post-orogenic uplift and progressive tectonic thinning of the lithosphere were associated with melting and reworking of lower and middle crust that produced abundant plutonic rocks at Bunger Hills. A major episode of mafic dyke emplacement following uplift, cooling, and plutonic activity with increasing mantle input, suggests that the dykes mark the end of a prolonged interval of thermal weakening of the lithosphere that may have been associated with continued mafic underplating during orogenic collapse. If the undated olivine gabbro dykes with similar trend, geochemistry and petrology at Windmill Islands are coeval with the ca. 1134 Ma dyke at Bunger Hills, this would suggest the presence of a major dyke swarm at least 400 km in extent. In such case, the dykes could have been emplaced laterally from a much more distant mantle source, possibly a plume, and interacted with the locally heterogeneous and variably metasomatised lithosphere.

Integrated elemental and Sr-Nd-Pb-Hf isotopic studies of Mesozoic mafic dykes from the eastern North China Craton: implications for the dramatic transformation of lithospheric mantle

Evolution of the lithospheric mantle beneath the North China Craton (NCC) from its Precambrian cratonic architecture until Paleozoic, and the transformation to an oceanic realm during Mesozoic, with implications on the destruction of cratonic root have attracted global attention. Here we present geochemical and isotopic data on a suite of newly identified Mesozoic mafic dyke swarms from the Longwangmiao, Weijiazhuang, Mengjiazhuang, Jiayou, Huangmi, and Xiahonghe areas (Qianhuai Block) along the eastern NCC with an attempt to gain further insights on the lithospheric evolution of the region. The Longwangmiao dykes are alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N >4.3) and EM1-like Sr-Nd-Pb-Hf isotopic signature ((87Sr/86Sr) i >0.706; ε Nd (t) < −6.3, (206Pb/204Pb) i > 16.6, (207Pb/204Pb) i >15.4, (208Pb/204Pb) i >36.8, ε Hf (t) < −22.4). The Weijiazhuang dykes are sub-alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 3.7), and display similar EM1-like isotopic features ((87Sr/86Sr) i >0.706; ε Nd (t) < −7.0, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i >15.4, (208Pb/204Pb) i >36.9, ε Hf (t) < −23.3). The Mengjiazhuang dykes are also sub-alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 2.4) and EM1-like isotopic features((87Sr/86Sr) i >0.706; ε Nd (t) < −18.4, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i >15.4, (208Pb/204Pb) i >36.9, ε Hf (t) < −8.6). The Jiayou dykes also display sub-alkaline affinity with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 3.7) and EM1-like Sr-Nd-Pb-Hf isotopic features ((87Sr/86Sr) i >0.706; ε Nd(t) < −15.3, (206Pb/204Pb) i > 16.7, (207Pb/204Pb) i >15.4, (208Pb/204Pb) i >36.9, ε Hf (t) < −18.4). The Huangmi dykes are alkaline (with Na2O+K2O ranging to more than 5.9wt.%)) with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N > 9.3) and EM1-like isotopic composition ((87Sr/86Sr) i >0.705; ε Nd (t) < −15.1, (206Pb/204Pb) i > 16.9, (207Pb/204Pb) i >15.5, (208Pb/204Pb) i >36.9, ε Hf (t) < −12.2). The Xiahonghe dykes are alkaline with LILE (Ba and K)- and LREE-enrichment ((La/Yb) N=2.12-2.84) and similar EM1-like Sr-Nd-Pb-Hf isotopic signature ((87Sr/86Sr) i > 0.705; ε Nd (t)<-18.0, (206Pb/204Pb) i > 16.9, (207Pb/204Pb) i >15.5, (208Pb/204Pb) i >36.9, ε Hf (t)<−8.6). Our data from the various mafic dyke suites suggest that the magmas were derived from EM1-like lithospheric mantle, corresponding to lithospheric mantle modified by the previously foundered lower crust beneath the eastern NCC. Our results suggest contrasting lithospheric evolution from Triassic (212 Ma) to Cretaceous (123 Ma) beneath the NCC. These mafic dykes mark an important phase of lithospheric thinning in the eastern North China Craton.

The Paleoproterozoic magmatic–metamorphic events and cover sediments of the Tiekelik Belt and their tectonic implications for the southern margin of the Tarim Craton, northwestern China

The Tiekelik Belt in the southwestern part of the Tarim Craton, NW China, consists of Archean–Paleoproterozoic orthogneisses, and granitoids, and younger sedimentary successions. U–Pb dating of Paleoproterozoic rocks in the Tiekelik Belt has provided new information on the southwestern margin of the Tarim Craton. Two orthogneisses from the Heluositan Complex yielded crystallization ages in the interval 2310–2260Ma and metamorphic ages of 2040–2007Ma and 1830Ma, respectively. Two syenogranites from the Buweituwei pluton yielded crystallization ages of 1900 and 1795Ma. Detrital zircon analyses from sedimentary rocks nonconformably overlying the Buweituwei and Akazi plutons indicate maximum depositional ages between 1950Ma and 1780Ma. Major detrital inputs are from the basement in the region and form prominent peaks at 1900–1800Ma and 2300–2200Ma. Our new data, combined with previous results, allow us to summarize the Archean–Paleoproterozoic tectonic evolution of the Tiekelik Belt as follows: (1) Emplacement of Archean-aged TTG magmas during the 3.14–2.76Ga interval; (2) A series of 2.41–2.26Ga magmatic events that represent an important period of crustal reworking within the Tarim craton; (3) 2.03–1.80Ga magmatic and metamorphic events that include the emplacement of a 1.9Ga mafic dyke suite into the Heluositan Complex along with several episodes of intraplate magmatism. The late magmatic events are coeval with metamorphism and may support an orogenic extensional model for the Tiekelik Belt during the 1.9–1.8Ga interval; (4) post-1.8Ga subsidence and formation of a Changcheng sequence in the Tiekelik Belt. The Paleoproterozoic ages of major tectonothermal events within the Tiekelik, North Altyn–Dunhuang block and Alashan block are almost identical and it is argued that they were connected at that time as North China and Tarim were part of the larger Columbia supercontinent.

U–Pb zircon age, geochemical and Sr–Nd isotopic data as constraints on the petrogenesis and emplacement time of the Precambrian mafic dyke swarms in the North China Craton (NCC)

Precambrian mafic dykes in the North China Craton (NCC) consist of dolerite. Geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses were performed on this suite of mafic dykes to characterise their ages and petrogenesis. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) U–Pb zircon analyses yield consistent ages ranging from 837.9±4.8Ma to 2510±18Ma for seven of the mafic dykes (SHS01, TDG01, LJZ01, LQS02, HB02, WJ01, and DST01) from NCC. Based on these, the mafic dykes can be divided into three groupings: ~2.4–2.5Ga, ~1.8–1.9Ga and 0.8–1.0Ga. The studied dykes belong to the alkaline and sub-alkaline magma series in terms of K2O+Na2O contents (2.6–6.2wt.%), and to the Tholeiitic, calc-alkaline and high-K calc-alkaline series based on their K2O contents (0.1–2.1wt.%). The mafic dykes are further characterised by low and variable light rare earth elements (~2.4–2.5Ga mafic dykes, (La/Yb)N=3.83–10.4; ~1.8–1.9Ga mafic dykes, (La/Yb)N=1.45–2.35); 0.8–1.0Ga mafic dykes, (La/Yb) N=0.66–4.27, show variable Eu anomalies (δEu=0.94–1.35, 0.77–1.21; 0.92–1.28, respectively), positive anomalies in Ba, U, La and Pb, and are depleted in Rb and high field strength elements (Nb, Ta, Ti, and minor Zr and Hf). In addition, the studied mafic dykes all display relatively low radiogenic Sr [(87Sr/86Sr)i=0.6747–0.6921; 0.7017–0.7025; 0.7005–0.7049, respectively for the ~2.4–2.5Ga, ~1.8–1.9Ga and 0.8–1.0Ga mafic dyke groups] and large εNd (t) (13.2–13.5; 10.4–11.8; 6.5–8.5, respectively). These results suggest that all the mafic rocks were derived from a depleted mantle source hybridised by foundered lower crust. The parent mafic magmas likely experienced fractional crystallisation of olivine, pyroxene, hornblende, plagioclase and Fe–Ti oxides (e.g., rutile, ilmenite, titanite, etc.) during dyke ascent, with negligible evidence for crustal contamination prior to emplacement at a high crustal level.

Metamorphic evolution of the Chotanagpur Granite Gneiss Complex of the East Indian Shield: current status

Abstract Geological information on the Chotanagpur Granite Gneiss Complex (CGGC) has been reviewed with a view to: (a) identifying the different metamorphic episodes; (b) developing an event stratigraphy in the high-grade blocks; and (c) correlating the different metamorphic episodes with the globally extensive orogenic processes. Integrating the existing geological information, the geological evolution of the high-grade block of the CGGC has been divided into four stages associated with four distinct metamorphic events ( M I − M IV ). The earliest metamorphic event ( M I ) that is recorded in granulite enclaves in the regionally extensive felsic gneisses culminated in ultrahigh temperatures (&gt;900 °C at c . 5–8 kbar) at around 1.87 Ga. In the second stage, voluminous felsic magmas were intruded – the M I granulites – and were metamorphosed to form migmatitic felsic gneisses ( M II ) within about 1.66–1.55 Ga. The third stage witnessed intrusions of a suite of anorthosite and porphyritic granitoids ( c . 1.55–1.51 Ga), followed by high-grade metamorphism (700±50 °C, 6.5±1 kbar, M III ) during approximately 1.2–0.93 Ga. The fourth stage ( M IV ) is marked by the intrusion of a suite of mafic dykes, followed by infiltration-driven metamorphism (600–750 °C at 7±1) during 0.87–0.78 Ga. The proposed metamorphic events have been correlated with the supercontinental cycles in the Proterozoic time.

Resetting of the U–Pb Zircon System in Cambro-Ordovician Intrusives of the Deep Freeze Range, Northern Victoria Land, Antarctica

A detailed in situ isotopic (U–Pb, Lu–Hf) and geochemical study of zircon populations in a composite sequence of foliated to massive Cambro-Ordovician intrusions in the Deep Freeze Range (North Victoria Land, Antarctica), has highlighted great complexity in zircon systematics. Zircons in deformed granitoids and tonalites display complex internal textures, a wide spread of concordant U–Pb ages (between 522 and 435 Ma) and unusual trace-element compositions (anomalous enrichment of light rare earth elements, U, Th and Y) within single zircon grains. In contrast, zircons from undeformed samples display a limited range of U–Pb ages and trace-element compositions. Zircons from all age and textural populations in most of the deformed and undeformed samples show a relatively narrow range of εHf values, suggesting that the Lu–Hf system remained undisturbed. Inferred emplacement ages cover a time interval of about 30 Myr: from 508 to 493 Ma for the oldest strongly foliated synkinematic Howard Peaks megacrystic monzogranites and high-K calc-alkaline mafic to intermediate rocks of the ‘Corner Tonalite’ unit; from about 489 to 481 Ma for the younger massive shoshonitic mafic dyke suite and the high-K calc-alkaline Keinath granite. The observed isotopic and chemical variations in zircon are attributed to a sub-solidus recrystallization under hydrous conditions and varying temperature, in a setting characterized by a transpressional to extensional stress regime.

Geochemistry, mineral chemistry and petrogenesis of a Neoproterozoic dyke swarm in the north Eastern Desert, Egypt

Dyke swarms traverse Neoproterozoic rocks in the Hawashiya region in the extreme northern part of the Eastern Desert of Egypt. They are a suite of basaltic andesite and andesite mafic dykes, and dacitic and rhyolitic felsic dykes. The mafic dyke suite is more abundant in the younger granites (577 ± 6 Ma) than in the older granitoids (614 Ma), in which the felsic dykes are the most common. The dyke swarms trend predominantly NE–SW, and the felsic dyke suite is older than the mafic dyke suite. Both dyke suites are calc-alkaline (alkaline dykes are rare) and are relatively poor in TiO2 and Nb but enriched in the incompatible elements and HFSE. The felsic dyke suite is enriched in REE and is strongly LREE fractionated relative to the mafic dyke suite. Although the Hawashiya dykes were emplaced at the end of the Neoproterozoic era in an extensional tectonic setting, they have geochemical characteristics that are consistent with a subduction-related regime. These chemical signatures were inherited from the lithospheric rocks that produced their host Hawashiya granitoids. The felsic dyke suite magma may be derived from crustal rocks (essential source component) by partial melting. The mafic dyke suite magma was generated from a lithospheric mantle and has undergone fractional crystallization of plagioclase, amphibole, clinopyroxene and magnetite, as documented by major and trace elements fractionation modelling.

Overlap of Karoo and Ferrar Magma Types in KwaZulu-Natal, South Africa

A suite of mafic dykes from the Underberg region of southern KwaZulu-Natal (South Africa) were intruded at 178 Ma, coincident in age with the major Okavango Dyke Swarm of Botswana, and also coincident with minor Karoo-related intrusions of the northern and central Lebombo. The dykes are all low-Ti-Zr tholeiites, they trend NW-SE and are presumed to continue into the Karoo central area of the Lesotho Highlands. In many respects, the Underberg dykes are similar to the majority of the low-Ti-Zr volcanic and subvolcanic intrusions of the Karoo; however, their 87Sr/86Sr and Nd isotope ratios are either ‘Ferrar-like’ (87Sr/86Sr 0·710; Nd < -3) or transitional between Karoo low-Ti-Zr and Ferrar low-Ti magmas. A potential Ferrar source for at least some of the Underberg dykes is supported by anisotropy of magnetic susceptibility analyses of the dyke suite, which demonstrate absolute flow direction from the SE to the NW, consistent with Gondwana reconstructions. The role of crustal contamination and combined fractional crystallization is also demonstrated to have played a key role in the petrogenesis of the Underberg dykes, involving a local upper crust contaminant. However, the composition of the ‘Ferrar-like’ dykes cannot be easily explained by AFC processes, but they do demonstrate that melting of a lithospheric mantle source enriched to a small degree by subduction-derived fluid was also important.

Geochemical and isotopic (Nd–O) evidence bearing on the origin of late- to post-orogenic high-K granitoid rocks in the Western Superior Province: implications for late Archean tectonomagmatic processes

The granitoid rock dominated central Wabigoon subprovince of the Superior Province records low-K trondhjemite–tonalite–granodiorite (TTG) type magmatic episodes at <2.83–2.74 and 2.722–2.709 Ga, and high-K mafic to felsic plutonism at 2.690–2.685 Ga. High-K units consist of granite to granodiorite dykes and sills, a K-feldspar megacrystic granodiorite suite of sanukitoid affinity and a suite of mafic dykes and intrusions. Initial ϵ Nd values (−3.1 to +3.3) indicate variable input to all units from light REE-enriched older crustal materials. The δ 18O (VSMOW) range of felsic compositions (+7.1 to +8.9%) overlaps closely that of average upper Superior Province crust. The granite/granodiorite units probably received melt components derived from both older tonalitic crust and isotopically juvenile supracrustal material. The thermal flux for partial melting was provided by mafic components of the coeval megacrystic granodiorite suite. This latter suite likely formed by extensive crustal assimilation and fractionation of enriched-mantle-derived high-Mg dioritic magmas in a post-collisional setting, possibly resulting from slab breakoff or broader scale lithospheric delamination. A genetic link is inferred between mafic magmatism and the late- to post-tectonic high-K granitoid magmatism that typically represents the last stabilization event within Superior subprovinces. That crustal recycling processes played a major role in the petrogenesis of central Wabigoon high-K granitoid suites is consistent with other evidence that supports repeated and substantial continental recycling within this subprovince as far back as the Mesoarchean.

Modelling anatexis in intra-cratonic rift basins: an example from the Neoproterozoic rocks of the Scottish Highlands

The Neoproterozoic metasediments of northwestern Scotland were deformed during the 470 Ma Grampian orogeny. Their pre-Ordovician history has proved difficult to elucidate, due to conflicting evidence. While the stratigraphic record indicates deposition in intracontinental rift basins associated with the break-up of Rodinia, isotopic dates in the range 870–780 Ma from granite gneiss, early pegmatites and metamorphic garnets have been attributed to a Neoproterozoic ‘Knoydartian’ orogeny. Stratigraphic evidence for this orogeny is lacking, and it is not represented elsewhere on the Laurentian margin. An alternative interpretation is that much of the Knoydartian history can be related to extensional, not collisional processes. Specifically, it has been proposed that the 870 Ma West Highland granite gneiss that is intruded into the Moine rocks of northwestern Scotland is not the product of synorogenic anatexis but represents a suite of granite sheets that were generated during extensional rifting and were subsequently deformed and gneissified during the Grampian orogeny. This contribution presents numerical models of extension-related anatexis to test this hypothesis.We first develop a methodology to estimate stretch values and the duration of extension and thermal subsidence for the Moine rift basins. A thermal model is then constructed for these basins using transient finite element techniques. This model shows that lithospheric extension sufficient to produce major rift basins, even if they are filled with feldspathic sediment with Neoproterozoic heat production characteristics, will not lead to crustal anatexis. However, a regional suite of mafic dykes in the more easterly (Loch Eil) Moine suggests that stretching led to decompression melting of the mantle. We model the effect of advecting heat into the extending lithosphere by the introduction of a modest volume of basaltic magma, and show that substantial granitic melt can be generated in the basement beneath the basin. The amount of anatexis varies with the locus of basalt intrusion. Some 30% more granite is generated by dykes emplaced along basin-bounding faults than by either dykes emplaced beneath the centre of the basin, or by underplating sills. The spatial distributions of the West Highland gneiss and of the mafic suite are compatible with this finding. There is clear field evidence that the protolith of the West Highland gneiss consisted of a suite of pre-tectonic granite sheets, and our modelling demonstrates that they could have been generated during the later stages of extensional rifting and Moine sedimentation.

The youngest Neoproterozoic mafic dyke suite in the Arabian Shield: mildly alkaline dolerites from South Jordan – their geochemistry and petrogenesis

The Arabian–Nubian Shield evolved through a sequence of tectonomagmatic cycles, which took place during Neoproterozoic time (1000–540 Ma). Dyke emplacement constitutes one of the conspicuous features of the Arabian–Nubian Shield, with mafic dykes being the most abundant. The investigated dykes represent the youngest Neoproterozoic mafic dykes and have been dated in Jordan at 545 ± 13 Ma. Geochemically the studied dykes are mildly alkaline, are enriched in large ion lithophile elements (LILE) and high field strength cations (HFSC), show moderate enrichment of REE, and lack Nb anomaly. These features are consistent with a predominantly extensional continental tectonic setting. Crystallization temperatures of the suite fall between 1050 and 800 °C to as low as 650 °C as deduced from pyroxene thermometry. The investigated dykes were derived from a metasomatized lithospheric mantle by 5 % modal batch partial melting of phlogopite-bearing spinel lherzolite, according to geochemical modelling. The intra-suite geochemical features are explicable by 64 % fractional crystallization of olivine, pyroxene, plagioclase and titanomagnetite and possibly other accessories like apatite at a later stage. The cumulate produced from this fractionation of the investigated dyke suite contributed to the formation of the mafic lower crust of the Arabian–Nubian Shield. Elemental ratios and petrographic evidence indicate possible minor crustal contamination of the suite. The youngest mafic dykes show striking geochemical similarities to the same generation of dolerite dykes in the adjacent countries, to transitional young basalt suites of the Main East African Rift, and to Quaternary Jordanian basalts. The youngest mafic dyke suite, the rhyolites of the Aheimir suite, and St Katherina rhyolites of Sinai represent the last igneous activity in the Arabian–Nubian Shield before the onset of the Cambrian at about 545 Ma ago.

Cyclic continental mafic tuff and flood basalt volcanism in the Late Archaean Nullagine and Mount Jope Supersequences in the eastern Pilbara, Western Australia

A late Archaean (ca. 2775–2715 Ma) continental flood basalt and extensional basin succession in the eastern Pilbara of Western Australia has been divided into 11 unconformity-bounded packages of rock, and four mafic dyke suites have been correlated chemically with subaerial flood basalts in the succession. The stratigraphic divisions follow unconformity-based mapping, whole-rock geochemical, palaeomagnetic, volcanological, sedimentological, petrographic and airborne geophysical studies. The 6 km thick lower Chichester Range Megasequence (Fortescue Group minus the Jeerinah Formation) in the Nullagine Synclinorium comprises lower and upper subaerial flood basalt units separated by a thick succession of terrestrial clastic sedimentary and felsic volcanic rocks. A repeated pattern of mafic volcanism is apparent in the upper part of the succession comprising six tuff to basalt cycles, each representing a flood basalt event. Subaerial flood basalt volcanism in each cycle was preceded by mafic hydrovolcanism resulting in a mafic tuff horizon beneath each subaerial flood basalt pile. Mafic tuff horizons were deposited and resedimented in mostly terrestrial settings. Initial pyroclastic volcanism is attributed to interaction between rising mafic magma batches and shallow ground water or surface water. Overlying flood basalt piles represent relatively rapid outpourings of mafic lava that complete the volcanic cycle. Most geological time in this part of the succession is represented either by boundaries between tuff/basalt cycles or by the resedimented tuff horizons. This study supports a rift-related geotectonic model for the succession as a whole, though it is shown to be more complex than previously proposed.

Mantle plumes and Antarctica-New Zealand rifting: evidence from mid-Cretaceous mafic dykes

Ocean floor magnetic anomalies show that New Zealand was the last continental fragment to separate from Antarctica during Gondwana break-up, drifting from Marie Byrd Land, West Antarctica, about 84 Ma ago. Prior to continental drift, a voluminous suite of mafic dykes (dated by Ar–Ar laser stepped heating at 107 ± 5 Ma) and anorogenic silicic rocks, including syenites and peralkaline granitoids (95–102 Ma), were emplaced in Marie Byrd Land during a rifting event. The mafic dyke suite includes both high- and low-Ti basalts. Trace element and Sr and Nd isotope compositions of the mafic dykes may be modelled by mixing between tholeiitic OIB (asthenosphere-derived) and alkaline high- to low-Ti alkaline magmas (lithospheric mantle derived). Pb isotopes indicate that the OIB component had a HIMU composition. We suggest that the rift-related magmatism was generated in the vicinity of a mantle plume. The plume helped to control the position of continental separation within the very wide region of continental extension that developed when the Pacific–Phoenix spreading ridge approached the subduction zone. Separation of New Zealand from Antarctica occurred when the Pacific–Phoenix spreading centre propagated into the Antarctic continent. Sea floor spreading in the region of the mantle plume may have caused an outburst of volcanism along the spreading ridge generating an oceanic plateau, now represented by the 10–15 km thick Hikurangi Plateau situated alongside the Chatham Rise, New Zealand. The plateau consists of tholeiitic OIB-MORB basalt, regarded as Cretaceous in age, and similar in composition to the putative tholeiitic end-member in the Marie Byrd Land dykes. The mantle plume is proposed to now underlie the western Ross Sea, centred beneath Mount Erebus, where it was largely responsible for the very voluminous, intraplate, alkaline McMurdo Volcanic Group. A second mantle plume beneath Marie Byrd Land formed the Cenozoic alkaline volcanic province.

The structural evolution of the Rauer Group, East Antarctica: mafic dykes as passive markers in a composite Proterozoic terrain

Archaean gneisses in the Rauer Group of islands, East Antarctica, record a prolonged history of high-grade deformational episodes, many of which predate that identified in mid-Proterozoic gneisses. Eleven generations of mafic dykes, belonging to discrete chemical suites, have been used as relative time markers to constrain this deformational history. Based on the timing of intrusion with respect to structures, dykes in the Rauer Group have been correlated with largely undeformed and dated dyke suites in the adjacent Vestfold Hills. This has allowed absolute ages to be inferred for the early- to mid-Proterozoic mafic dyke suites in the Rauer Group, and a correlation of the interspersed structural events. Most structures in the Rauer Group, however, developed in response to high-grade progressive deformation at approximately 1000 Ma. During this deformational episode, strains were repeatedly partitioned into sub-vertical, noncoaxial, high-strain zones recording NW-directed sinistral transpression, that separated zones of lower strain dominated by coaxial folding with axes parallel to the shear direction. Three additional mafic dyke suites intruded during this deformation which was followed by three stages of brittle-ductile deformation and a final suite of lamprophyre dykes. Due to the numerous intrusive time markers, the Rauer Group serves as an excellent illustration of how complicated gneiss terrains may be.