Dyke Suite Research Articles

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.

Mapping temporal and spatial patterns of zircon U-Pb disturbance: A Yilgarn Craton case study

The zircon U-Pb system is a robust geochronometer, however disturbance to this system can be widely diagnosed on U-Pb concordia diagrams where it is typically interpreted as a consequence of radiogenic-Pb loss. In many cases, removal of radiogenic-Pb is not complete, and Discordia regressions on concordia diagrams may then potentially track both the original age of crystallization of the rock, and the timing of isotopic disturbance. However, U-Pb disturbance, resulting in normal discordance, can also be driven by U gain. In this contribution, we expand on a novel approach to determine the most pervasive time of U-Pb disturbance in a sample. We introduce the Concordant-Discordant Comparison (CDC) test which evaluates the similarity between a sample's concordant age structure and a modelled age structure, from discordant analyses, over a wide range of potential disturbance times. The closest similarity in concordant and discordant age populations as derived from a specified time of elemental mobility, is interpreted as the best estimate for the time of U-Pb disturbance. The CDC test is appropriate for magmatic as well as detrital samples, and yields dates that can be spatially interpolated to produce regional maps that define domains of U-Pb disturbance.We highlight the application of the CDC test using a case study from the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia. U-Pb SIMS analysis of 95 zircon samples from across the Eastern Goldfields Superterrane reveals two dominant times of U-Pb disturbance – recent and at ~600Ma. The ~600Ma time of U-Pb disturbance shows a statistically significant spatial relationship to a buried set of undated dykes (the Cosmo Newbery Dyke Suite), which are apparent in geophysical images. Thermal modelling indicates dyke temperatures alone are insufficient to cause widespread Pb loss in host rock zircon. We therefore suggest dyke emplacement caused U-Pb disturbance through reactive fluids leaching radiogenic-Pb. Additionally, we use the difference between the measured and calculated zircon Th/U ratio to distinguish between Pb loss and elemental gain processes, as the calculated Th/U ratio derived from 208Pb/206Pb will only match measured Th/U where no later addition or loss of Th or U has occurred. CDC modelling of discordant U-Pb zircon analyses may provide a means to recognise the distal footprint of otherwise difficult to date tectonothermal events and extract useful information from often discarded analyses.

Paleomagnetism and U–Pb geochronology of the Black Range dykes, Pilbara Craton, Western Australia: a Neoarchean crossing of the polar circle

ABSTRACTWe report a new paleomagnetic pole for the Black Range Dolerite Suite of dykes, Pilbara craton, Western Australia. We replicate previous paleomagnetic results from the Black Range Dyke itself, but find that its magnetic remanence direction lies at the margin of a distribution of nine dyke mean directions. We also report two new minimum ID-TIMS 207Pb/206Pb baddeleyite ages from the swarm, one from the Black Range Dyke itself (>2769 ± 1 Ma) and another from a parallel dyke whose remanence direction lies near the centre of the dataset (>2764 ± 3 Ma). Both ages are slightly younger than a previous combined SHRIMP 207Pb/206Pb baddeleyite weighted mean date from the same swarm, with slight discordance interpreted as being caused by thin metamorphic zircon overgrowths. The updated Black Range suite mean remanence direction (D = 031.5°, I = 78.7°, k = 40, α95 = 8.3°) corresponds to a paleomagnetic pole calculated from the mean of nine virtual geomagnetic poles at 03.8°S, 130.4°E, K = 13 and A95 = 15.0°. The pole's reliability is bolstered by a positive inverse baked-contact test on a younger Round Hummock dyke, a tentatively positive phreatomagmatic conglomerate test, and dissimilarity to all younger paleomagnetic poles from the Pilbara region and contiguous portions of Australia. The Black Range pole is distinct from that of the Mt Roe Basalt (or so-called ‘Package 1’ of the Fortescue Group), which had previously been correlated with the Black Range dykes based on regional stratigraphy and imprecise SHRIMP U–Pb ages. We suggest that the Mt Roe Basalt is penecontemporaneous to the Black Range dykes, but with a slight age difference resolvable by paleomagnetic directions through a time of rapid drift of the Pilbara craton across the Neoarchean polar circle.

Möwe Bay Dykes, Northwestern Namibia: Geochemical and geochronological evidence for different mantle source regions during the Cretaceous opening of the South Atlantic

Dyke emplacement in the course of Paraná-Etendeka volcanism in northwestern Namibia has been considered as a short-lived event related to a specific magma source at approximately 135–130Ma. New geochemical, whole rock Sr-Nd-Pb isotope data and 40Ar/39Ar ages reveal that at least three geochemically and isotopically different tholeiitic dyke generations, with intrusion ages of 135.2±0.7, 124.1±0.8Ma and 113.0±0.5Ma, can be distinguished in the Möwe Bay area, Skeleton Coast of northwestern Namibia. Distinct mantle source components were identified in the petrogenesis of the various dyke generations. Magma composition of the two older dyke suites, both of which were emplaced in a tectonic setting dominated by E-W extension, evolved from a within-plate to an enriched mid-ocean ridge basalt type. Contamination by continental crust and/or lithospheric mantel is suggested by a high 238U/204Pb-value (=μ2) of 9.97, which is typical of the Kalahari Craton. The third dyke generation, which intruded in a tectonic regime dominated by SE-NW extension, corresponds to high-Ti ocean island-type basalt with unradiogenic isotopic compositions derived from a mantle source with a μ2-value of 9.35.The formation of the older dykes is attributed to the presence of a thermal anomaly in the upper sublithospheric mantle. This anomaly was most likely caused by the peripheral part of a mantle plume that impinged at the base of the lithosphere and caused erosion of the subcontinental mantle lithosphere and melting above the plume head. The initial break-up of SW Gondwana and the formation of early oceanic crust were most likely due to passive rather than active rifting. Continued plume upwelling facilitated by progressive thinning of SW-Gondwana crust led to the formation of the younger, c. 113Ma old dykes, which are chemically and isotopically identical to coeval rocks from the northeastern portion of the Walvis Ridge and thus are interpreted as onshore expressions of the Tristan-Gough plume head at that time. The difference in the dominant extension directions of the older and the younger dyke generations can be explained by rotation induced by the Aptian/Albian opening of the Equatorial Central Atlantic, accompanied by a substantial increase in the South Atlantic spreading rate.

Primary Magmas in Continental Arcs and their Differentiated Products: Petrology of a Post-plutonic Dyke Suite in the Tertiary Adamello Batholith (Alps)

Determining the primary compositions of arc magmas is fundamental in retracing the chemical differentiation processes responsible for the formation of juvenile arc crust and the thermal structure of the mantle wedge. We have investigated a series of post-plutonic dykes that intruded the gab- broic to tonalitic southern part of the Tertiary Adamello Batholith in the Alps. The dyke rocks range in composition from primary, hydrous high-Mg basalts to basalts, basaltic andesites, andesites and dacites. Field relationships and high-precision U–Pb dating of titanite and zircon show that the dyke suite ranges in age from 41.67 ± 0.06 Ma for the high-Mg basalt to 38.62 ± 0.12 Ma for the youngest dacitic dykes, closely associated with plutonic activity from 42.5 to 39.0 Ma. Andesites and dacites have primitive 87Sr/86Sri (0.7032–0.7038) and 143Nd/144Ndi (eNdCHUR +3.5–3.2) isotopic signatures strongly limiting the extent of crustal assimilation, whereas some of the high-Mg basalts have se- lectively assimilated pelitic metasedimentary rocks as shown by high Cs/Rb, Rb/Sr and Rb/Zr ratios, and isotopically more enriched compositions (87Sr/86Sri 0.7039–0.7046; eNdCHUR +1.6–0.0). Primitive high-Mg basaltic dykes that escaped assimilation processes are primary mantle partial melts that were extracted from their source at pressures of 2.7 ± 0.2 GPa and temperatures of 1390 ± 30°C, conditions corresponding to the spinel–garnet transition in mantle peridotite. Major elem- ent modelling constrains the degree of melting to 20 6 2% leaving a harzburgite residue, consistent with the trace element chemistry of the high-Mg basalts, which have moderate [Gd/Yb]N ratios of 1–1.2. Differentiated basaltic andesites and dacites follow experimentally constrained liquid lines of descent for fractional crystallization at mid- to deep crustal levels. The trace element chemistry of amphiboles from basaltic andesite and andesite dykes reveals the coexistence of amphibole with primitive melts, indicating elevated pressures and H2O contents in their parental magmas. Thermobarometric constraints for amphibole phenocrysts result in pressures from 0.65 to 0.78 GPa and temperatures ranging from 930 to > 1000°C. The absence of any significant Eu-anomaly in the rare earth element patterns in these amphiboles indicates the late appearance of plagioclase in the crystallization sequence. The crystallization of amphibole drives the differentiated magmas to slightly peraluminous, corundum-normative compositions that are common for tonalites building the major part of the Adamello Batholith. Fractionation models at mid- to lower crustal conditions result in the cumulative crystallization of 17% olivine, 2% Cr-rich spinel, 18% clinopyroxene, 41% amphibole, 4% plagioclase and 0.1% magnetite to obtain an andesitic composition from a primary, hydrous high-Mg continental arc basalt. Cumulates formed during fractional crystallization atmid- to deep crustal levels are dunites and wehrlites followed by hornblendites and hornblende- gabbros. The trace element signatures of basaltic andesites and dacites display low Rb/Zr and Rb/ Sr, and are consistent with fractionation-dominated processes within the crust in an active contin- ental margin. Significant crustal assimilation is not required to obtain the trace element signatures of the evolved andesitic magmas.

The Tachakoucht–Iriri–Tourtit arc complex (Moroccan Anti-Atlas): Neoproterozoic records of polyphased subduction-accretion dynamics during the Pan-African orogeny

We report new mapping, tectonic, metamorphic and U–Pb zircon dating data on the polyphased Tachakoucht–Iriri and Tourtit arc-related units within the Moroccan Pan-African belt (Sirwa window, Anti-Atlas). The studied area contains four different sub-units, from south to north: (1) the Tachakoucht gneisses intruded to its northern part by (2) Iriri intrusions. To the north, the Tachakoucht–Iriri massif is thrusted by (3) the south-verging 760Ma Khzama ophiolitic sequence intruded by (4) the Tourtit meta-granitic complex. The Tachakoucht gneiss represents former andesitic to dacitic porphyritic rocks crystallized around 740–720Ma in an intra-oceanic arc setting (IOAS). Subsequently, it has been buried and metamorphosed to 700°C, 8kbar in response to early accretion of the arc onto the West African Craton (WAC). This tectono-metamorphic event also led to the dismembering and stacking of back-arc ophiolite onto the arc unit. Subsequently, the Iriri intrusions, a suite of hydrous mafic dykes (hornblende gabbro and fine-grained basalt) and ultramafic (hornblendite) plutons showing subduction zone affinities, intruded the Tachakoucht gneiss under P–T conditions of 750–800°C and 2–5kbar. Emplacement of Iriri intrusions led locally to pronounced partial melting of the Tachakoucht gneiss and to the production of leucogranitic melts. These melts crop out into the Iriri–Tachakoucht gneiss contacts as leucogneissic bands (former leucosomes, dated at 651±5Ma) but also intruded the Khzama ophiolite to form the Tourtit granite (dated at 651±3Ma). These ages (651–641Ma) also constrain the timing of Iriri intrusion emplacement. The entire complex has been overprinted by a second deformation event under greenschist to amphibolite facies conditions marked by transposition of primary structures and a development of mylonitic shear zones. These results and those published on the Bou Azzer window show that two phases of subduction-related magmatism occurred in the Anti-Atlas belt and that they were separated by an early accretion of the intra-oceanic arc system (IOAS) onto the West African craton passive margin. Our interpretations also validate thermo-mechanical models predicting an intense perturbation of subduction dynamics during arc-continent collision (i.e. composite subductions, polarity reversal) which can expand the production of typical hydrous arc magma and induces a late magmatic phase after partial or total accretion of the IOAS.

Early to late Neoproterozoic magmatism and magma mixing–mingling in Sri Lanka: Implications for convergent margin processes during Gondwana assembly

The Sri Lankan fragment of Gondwana preserves the records of Neoproterozoic tectonothermal events associated with the final assembly of the supercontinent. Here we investigate a suite of magmatic rocks from the Wanni, Kadugannawa and Highland Complexes through geological, petrological, geochemical and zircon U–Pb and Lu–Hf isotopic techniques. The hornblende biotite gneiss, charnockites, metagabbro and metadiorites investigated in this study show geochemical features consistent with calc-alkaline affinity and subduction-related signature including LILE enrichment relative to HFSE coupled with distinct Nb–Ta depletion and weak negative Zr–Hf anomalies. The felsic suite falls in the volcanic arc granites (VAGs) field and the mafic suite shows island arc basalt affinity in tectonic discrimination plots, suggesting that the protoliths of the rocks were derived from arc-related magmas in a convergent margin setting. LA-ICPMS zircon U–Pb analyses show crystallization of charnockite and dioritic mafic magmatic enclave from the Highland Complex during ca. 565 and 576Ma corresponding to bimodal magmatism. The diorite also contains metamorphic zircons of ca. 525Ma. Hornblende–biotite gneiss from the Kadugannawa Complex shows protolith emplacement age at 973–980Ma, followed by new zircon growth during repeated thermal events through late Neoproterozoic. The dioritic enclaves in these rocks are much younger, and form part of a deformed and metamorphosed dyke suite with emplacement ages of 559Ma, broadly coeval with the bimodal magmatism in the Highland Complex at that time. The youngest group of zircons in this rock shows ages of 508Ma, corresponding to the latest thermal event. A charnockite from this locality shows oldest group of zircons at 962Ma, corresponding to emplacement age similar to that of the magmatic protolith of the hornblende biotite gneiss. This rock also shows zircon growth during repeated thermal events at 832Ma, 780Ma, 721Ma and 661–605Ma. The lower intercept age of 543Ma marks the timing of collisional metamorphism. Charnockite from the Wanni Complex shows emplacement age at 1000Ma, followed by thermal event at 570Ma, the latter correlating with the bimodal magmatic event in the Highland Complex. The dioritic enclave within this charnockite shows an age of ca. 980Ma, suggesting intrusion of mafic magma into the felsic magma chamber. Zircons in the diorite also record multiple zircon events during 950 to 750Ma. Zircons in the Highland Complex charnockite possess negative εHf(t) values in the range −6.7 to −12.6 with TDMC of 2039–2306Ma suggesting magma derivation through melting of Paleoproterozoic source. In contrast, the εHf(t) range of −11.1 to 1.6 suggests a mixed source of both of older crustal and juvenile material. The εHf(t) values of −4.5 to 4.5 and TDMC of 1546–1962Ma for the hornblende biotite gneiss also shows magma derivation from mixed sources that included Paleoproterozoic components. The younger dioritic intrusive, however, has a more juvenile magma source as indicated by the mean εHf(t) value of 1.3. The associated charnockite shows a tight positive cluster of εHf(t) from 0.6 to 5.1, suggesting juvenile input. Charnockite from the Wanni Complex shows clearly positive εHf(t) values of up to 13.1, and TDMC in the range 937–1458Ma suggesting much younger and depleted mantle source. The diorite enclave also has positive εHf(t) values with an average value of 8.5 and TDMC in the range of 709–1443Ma clearly suggesting younger juvenile sources. The early and late Neoproterozoic bimodal suites are correlated to convergent margin magmatism associated with the assembly of Sri Lanka within the Gondwana supercontinent.

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.

Petrogenesis of Gunbarrel magmatic rocks: Homogeneous continental tholeiites associated with extension and rifting of Neoproterozoic Laurentia

The ca. 780Ma Gunbarrel Igneous Event of northwest Laurentia consists of spatially discrete suites of sills, dykes and lavas distributed over a vast area extending from Wyoming in the south to the Wopmay Orogen and the Mackenzie Mountains of Northwest Canada. Thick (≤100m) sills and rare dykes in Wopmay orogen and thinner (≤30m) sills, dykes and rare lavas in the Mackenzie Mountains are moderately evolved, augite+oligoclase–labradorite+ilmenite–magnetite gabbros and amygdaloidal basalts. Systematic petrochemical differences between units reveals that each is likely derived from subtly distinct parental magmas collectively exhibiting mutually consistent element variations. The dataset is remarkably homogenous, in particular, the incompatible trace elements and the Sm–Nd isotopes. All rocks preserve petrochemical evidence of an enriched MORB-like mantle source, but a small lithospheric component in the primary magmas resulted in elevated LILE, minor negative HFSE anomalies and sub-depleted mantle but supra-bulk earth ɛNd values. The lithospheric component was slightly older, modestly fractionated, Sr-depleted, garnet-free (pyroxenitic?) lower crust or, similar material that was previously recycled into the lithospheric mantle. Mineral chemical data for plagioclase and clinopyroxene in chill margin samples from a Hottah sheet in Wopmay orogen, indicates rapid and repeated turbulent mixing of geochemically and thermally similar magmas. These were staged from large, lowermost crust(?) magma chambers centred over an asthenospheric thermochemical anomaly thought to lie to the west of present-day North America. These magmas were then rapidly emplaced across western Laurentia.

Application of portable X-ray fluorescence analysis to characterize dolerite dykes at the Plutonic Gold Mine, Western Australia

The amphibolite-facies, Au-mineralized mafic rocks at the Plutonic Gold Mine are intruded by a suite of dolerite dykes of unknown age. The zones between these intrusive units often host significant Au mineralization. It is unclear whether this enrichment in Au mineralization is a function of the intrusion of the dolerites themselves or the influence of pre-existing structures (e.g. faults or shears). Geochemical characterization of the different microcrystalline dolerite units is important to the understanding of the structural architecture of the deposit and to the possible relationship of the dolerites to Au mineralization. The collection of a large geochemical dataset ( n = 497) from the dolerite dykes from across the deposit using portable X-ray fluorescence technology allows us to break them into four distinct geochemical groupings. Thus we can define their geometries with greater confidence than was possible using lithology alone. Traverses across individual dolerite dykes indicate that the chill margins are the most geochemically homogenous and most likely to represent the chemistry of the source magma. Plots of Ti v. Zr combined with principal component analysis (PCA) define four geochemically distinct suites of dolerites. By applying this understanding to dolerites in a small area of the deposit, a new interpretation was generated whereby significant amounts of rock that were previously modelled as being dolerite were reclassified as potential host-rock, thus increasing the potential for Au in this area.

Age and paleomagnetism of the 1210 Ma Gnowangerup–Fraser dyke swarm, Western Australia, and implications for late Mesoproterozoic paleogeography

Dolerite dykes of the Gnowangerup–Fraser Dyke Suite are subparallel to the southern and southeastern margins of the Yilgarn Craton. We collected oriented samples for paleomagnetic study from 19 dykes along the Phillips and Fitzgerald Rivers and north of Ravensthorpe. Alternating-field (AF) demagnetization revealed a stable two-polarity remanence in 14 dykes, and the primary nature of the magnetic directions is supported by a positive baked-contact test and by rock-magnetic evidence. U–Pb zircon and baddeleyite ages for two dykes confirm that the Gnowangerup–Fraser dykes are part of the 1210Ma Marnda Moorn Large Igneous Province. The mean paleomagnetic pole, at 55.8°N, 323.9°E, A95=6.5°, is almost identical to the previously reported VGP of the 1212Ma Fraser dyke, also supported by a positive baked-contact test. The combined robust paleopole places the West Australian Craton in a near-polar position at 1210Ma. Comparison with coeval Laurentian paleopoles indicates that Laurentia and Australia were widely separated at that time. We present a paleomagnetically permissible drift model for these two continents between 1210 and 1070Ma. One dyke yields a stable remanence with a VGP similar to the paleopole for the 755Ma Mundine Well dykes indicating that this dyke may have been emplaced during the same event at c. 755Ma. Differences in lengths and shapes of late Mesoproterozoic Apparent Polar Wander Paths of several continents suggests that a large supercontinent did not exist between about 1300 and 1050Ma. This may have been a transitional time between the final breakup of Nuna and the assembly of Rodinia.

Late Variscan calc-alkaline lamprophyres in the Krupka ore district, Eastern Krušné hory/Erzgebirge: their relationship to Sn-W mineralization

*Corr esponding author Variscan lamprophyres occur in the greisen tin-, tungsten- and molybdenum-ore district of Krupka in the Eastern Krusne hory/Erzgebirge (KHE). They belong to a bimodal dyke suite of aplites, felsic porphyries, microgranites and mafic dykes associated with late Variscan tin-bearing granites and include minettes, kersantites and spessartites, while vogesite reported earlier has not been confirmed. One altered mafic dyke is interpreted as original microdiorite. All lamprophyres are basic to intermediate rocks (47.3–56.9 wt. % SiO 2 ) with shoshonitic to ultrapotassic composition (3.1–7.5 wt. % K 2 O). The high concentrations of MgO (4.7–11.4 wt. %), molar Mg/(Mg + Fe) ratios (0.56–0.74) and abundances of compatible elements (350–800 ppm Cr, 130–360 ppm Ni) indicate that lamprophyres represent primary mantle melts that underwent no or little fractionation or contamination, and high LREE/HREE ratios point to magma formation in the stability field of garnet peridotite. In addition, high contents of potassium and LILE (50–370 ppm Li, 150–920 ppm Rb, 750–3100 ppm Ba) indicate metasomatic enrichment of the upper mantle prior to partial melting. The LILE–HFSE–REE patterns indicate involvement of slab components (subducted siliciclastic and carbonate sediments). Strong enrichment in U (6–29 ppm) and Th (17–75 ppm) is another characteristic feature of lamprophyres from the Eastern KHE and elsewhere in central Europe, and it is consistent with the metasomatic transport via oxidized saline fluids from the slab to the mantle wedge. The lamprophyres in the Krupka district were variably greisenized in the vicinity of granite greisens and Sn–W hydrothermal veins and their original minerals were replaced by an assemblage of lithian phlogopite, topaz, fluorite, apatite and titanium-bearing phases. During alteration, they were strongly depleted in Na 2 O, CaO, Sr and Ba, moderately depleted in REE, and enriched in Li, Rb, Cs, Sn and F. By contrast, Al and Zr behaved as immobile elements and their abundances indicate overall mass loss of 10–18 % during greisenization consistent with the increase of porosity, which facilitated the hydrothermal dissolution–precipitation reactions. The spatial association of greisens and lamprophyres suggests that the greisenizing fluids migrated along similar geological structures, which were previously accessible to the mantle-derived media (melts and/or fluids). On a local scale, the lamprophyre dykes intersected by greisen veins provided geochemical or lithological barrier, which favoured the cassiterite deposition. The timing of lamprophyre dykes also indicates that the mantle metasomatism beneath the KHE area occurred before the late Variscan granitic magmas were generated.

Genesis of the 1.21 Ga Marnda Moorn large igneous province by plume–lithosphere interaction

The 1.21Ga Marnda Moorn large igneous province (LIP) of the Yilgarn Craton is important for understanding the final breakup of the Nuna (Columbia) supercontinent. However, its petrogenesis is poorly understood owing to the lack of geochemical data. We conducted geochemical analyses of the Gnowangerup–Fraser Dyke Suite, a major part of the Marnda Moorn LIP, and report the first geochemical and Nd isotope data for this LIP. Results of a complementary paleomagnetic study of these dykes will be published elsewhere. Most of the studied dykes consist of predominately tholeiitic and OIB-like dolerite (Group 1) and one arc-like and more felsic dyke (Group 2). Group 1 samples have incompatible trace element compositions similar to those of tholeiitic Hawaiian plume-induced OIB and typical asthenospheric mantle-derived Nd isotopes with ɛNd(t) varying from +3.7 to +7.5, produced mainly within the spinel stability field (<75km depth). Their source region most likely contains recycled oceanic crust. Samples from the Group 2 dyke are characterized by extremely unradiogenic Nd isotopes with ɛNd(t) of about −12, strong depletion of Nb–Ta–Zr–Hf–Ti, chondritic Nb/Ta ratios (20–18), oversaturated silica, and strong deficiencies in CaO, FeOt, TiO2, and Ni. This implies that the dyke was produced by partial melting of enriched sub-continental lithospheric mantle. The coexistence of OIB- and arc-like end-members but mainly Hawaiian OIB-like tholeiitic mafic dykes, interpreted large-scale asthenosphere upwelling in a very short time, and the large volume of mafic magma, favour a plume origin for the Marnda Moorn LIP. The geochemical and emplacement characteristics are attributed to relief of the lithosphere–asthenosphere boundary across the Yilgarn craton and a complex interplay between the plume, heated lithosphere, normal asthenosphere, and recycled components. We propose a two-stage melting model to explain the geochemical composition and emplacement of the Marnda Moorn LIP. Our plume-lithosphere interaction model is consistent with the occurrence of synchronous ultrahigh-temperature events in the Musgrave Province of central Australia.

Mapping the Dyke Swarms of the Neoproterozoic Basement in Southwestern Jordan Using Remote Sensing and GIS Techniques

The current study documents for the first time the existence of four suites of dykes intruded the Neoproterozoic crustal rocks in southwestern Jordan. The oldest suite strike E-W followed by suites strike NE-SW, N-S and NW-SE which is the youngest one. Remote sensing and GIS techniques were used for mapping the dykes observed at the satellite image of the study area. Landsat Thematic Mapper color composite scenes allowed mapping of most the dyke swarms in the study area. Digital image processing technique was performed to map the dyke swarms in the study area. The emplacement of the dykes is consistent with extensional continental tectonic setting during the Precambrian time and before the deposition of the Cambrian sediments. The study gives an evidence for an anticlockwise rotation of the extensional stress regimes that caused these suites of dykes. Remote sensing and GIS technique used here could thus guide the interpretation and provide exact and accurate dyke mapping tools.

Multiple 3.8–3.1 Ga tectono-magmatic events in a newly discovered area of ancient rocks (the Shengousi Complex), Anshan, North China Craton

Near Anshan city in the North China Craton, the oldest rocks in Asia (⩾3300Ma) have been thoroughly documented at two localities (Baijiafen and Dongshan). In this paper we report the full geological context for more ancient rocks from a third Anshan locality – the polyphase migmatite Shengousi Complex. SHRIMP U–Pb zircon dating indicates a protracted tectono-magmatic history for the Shengousi Complex: The oldest recognised component is banded trondhjemitic gneiss (3773±6Ma), which is veined by strongly deformed granitic pegmatite. These occur with a second generation of trondhjemitic rocks (3454±8 and 3448±9Ma). The next generation of plutonic rocks is a composite suite of iron-enriched mafic dykes (3332±6 and 3331±8Ma) with broadly coeval felsic veins (3311±4Ma). Finally there was intrusion of monzogranite (3129±6Ma). Strong deformation has generally brought these 3773–3129Ma plutonic phases into close concordance to form banded rocks. However, locally cross-cutting relationships are preserved in small lower strain domains, that give a relative chronology agreeing with the absolute zircon U–Pb chronology. All of this complex history is recorded in a single<50m wide outcrop.Some of the<3600Ma rocks of the Shengousi Complex contain 3780–3730 and 3660–3600Ma inherited zircon xenocrysts. The stronger prominence of 3660–3600Ma components distinguishes the Shengousi Complex from the Baijiafen and Dongshan complexes, where components of this age are rarer. One possibility is that at 3660–3600Ma, the Shengousi Complex was at a deeper crustal level than the Baijiafen and Dongshan complexes, and underwent migmatisation at that time.The ∼3450Ma igneous phases discovered in the Shengousi Complex are new ages for igneous rocks in the Anshan area. The Precambrian geology of the Anshan area is thus marked by polyphase Eoarchaean orthogneisses, the newly recognised ∼3450Ma phases, widespread 3360–3300 magmatic activity, found in association with Mesoarchaean plutonic rocks and the Neoarchaean Anshan Group and Palaeoproterozoic Liaohe Group metasedimentary rocks. This shows that the geology of the Anshan area has a strong similarity with the Narryer Gneiss Complex of Western Australia, which contains the polyphase Eoarchaean Meeberrie gneisses, the 3490–3440Ma Eurada gneiss association, ∼3300Ma granites and migmatisation, and is intercalated with younger Precambrian metasedimentary rocks of different ages containing Hadean detrital zircons. No other ancient gneiss complexes in the world show such a close match in their history with the Anshan area rocks. This means that the Anshan area is prospective for locating new occurrences of Hadean crustal components intercalated with younger sedimentary rocks.

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.

GEOCHEMISTRY OF THE EARLY CRETACEOUS KOEGEL FONTEIN ANOROGENIC IGNEOUS COMPLEX, SOUTH AFRICA

The Koegel Fontein Igneous Complex in the Western Cape Province of South Africa is a high-level granite-syenite intrusive suite with minor plugs and dykes of felsic and mafic composition. Age dating and field relations show that the felsic rocks intruded as two sequential series, beginning with syenites and the Kerskloof suite of felsic dykes (andesite-trachyandesite, trachyte-rhyolite) at about 144 Ma, followed by the Rietpoort granite pluton and quartz-porphyry dykes at about 134 Ma. The intrusion of the mafic dykes and plugs took place between the two felsic series. The early Cretaceous age of the Koegel Fontein associates its formation with Gondwana breakup and establishment of the volcanic rifted margin in southwest Africa. This paper reports results from the first geochemical study of the Koegel Fontein Complex. Two groups of mafic units are distinguished based on mineralogy, field characteristics and chemical composition. The tholeiitic group comprises basalt and basaltic andesites, the alkaline group ranges from basanite-tephrite to phono-tephrite and basaltic trachy-andesite. Distinctions of incompatible element concentrations (K, P, Ti) and ratios (Y/Nb, Zr/Nb) indicate the presence of two separate magma series. Regional dolerite dykes (Gariep dykes and others) associated with contemporary magmatism along the continental margin occur in and near the complex. These regional dolerites are much more homogeneous compositionally than the mafic dykes of Koegel Fontein. Compositionally, they closely resemble the less-differentiated tholeiite dykes. The felsic units at Koegel Fontein show geochemical and isotopic evidence for two magma series, which correspond to the two intrusive episodes suggested by geochronology and field relations. The first series comprises the volumetrically minor intrusions of syenites, the Kerskloof dykes and the Rooivleitjie granite, with high values (ɛNd(t) from −0.4 to −1.8) indicating a strong mantle input. Similarities of trace element signatures suggest an affinity with the alkaline mafic units. The second series of felsic magmas makes up the bulk of the Koegel Fontein complex, with the large Rietpoort granite pluton and quartz-porphyry dykes (ɛNd(t) = −4.8 to −6.9). These rocks may be related to the tholeiitic mafic series but their isotopic composition indicates a large crustal component. The granitic rocks at Koegel Fontein have geochemical characteristics regarded as typical for anorogenic (A-type) granites, including high contents of Y, Nb, U and Th. In this and in many other features, the Koegel Fontein complex resembles the contemporary intrusive complexes in the Damaraland of northwest Namibia. Like the Damaraland granites, the Koegel Fontein magmas are of mixed origin, and their isotopic composition reflect the proportion of mantle vs. crustal source contributions.

Magnesian dyke suites of the 2.7 Ga Kambalda Sequence, Western Australia: Evidence for coeval melting of plume asthenosphere and metasomatised lithospheric mantle

Three premetamorphic magnesian dyke suites are prominent in the Woolyeneer basalt Formation of the ∼2.7 Ga Kambalda Sequence, Kalgoorlie Terrane, Yilgarn craton, Western Australia: the Butterfly (Mg# 76–70; Ni 640–370 ppm), the Norseman (Mg# 72–66, Ni 1105–1030 ppm), and the Crown (Mg# 73–72, Ni 1240–1170 ppm) Dyke suites. Butterfly Dykes have flat HREE at 6–8 times chondrite, mildly depleted LREE, and Nb/Th > 8; compositionally they resemble high-Mg basalts associated with komatiites and tholeiitic basalts in many Archean greenstone terranes erupted from mantle plumes. Crustally contaminated counterparts have the conjunction of greater (La/Sm) N with Nb/Th < 8. Norseman and Crown suites have composite compositional characteristics: (1) high MgO contents, low Al 2O 3 and Al 2O 3/TiO 2 ratios, enriched LREE, but fractionated HREE; and (2) positive Nb anomalies, fractionated REE, deep negative P anomalies, and downturn from Pr to Th on primitive mantle normalised diagrams. The former characteristics are of norites, abundant in Neoarchean to Proterozoic intrusions, and interpreted to be melts of low-Al metasomatised continental lithospheric mantle (CLM), whereas the latter characteristics are typical of high-μ ocean island basalts (HIMU-OIB) where the concensus is of ocean crust processed through a subduction zone and recycled into the mantle plume source of HIMU-OIB. Epsilon Nd values of +2.4 to +3.6 are in keeping with a depleted mantle source(s). Accordingly these dyke suites are consistent with melting of depleted continental lithospheric mantle metasomatised by low degree melts from HIMU-OIB. On the Th/Yb versus Nb/Yb discrimination diagram, Butterfly Dykes plot near depleted mantle, and Noresmam and Crown suites within the MORB-OIB array towards the HIMU endmember; consequently, the MORB-OIB array was established in the Archean mantle. Taken with published trace element and Sm–Nd isotope data, the most straightforward geodynamic-magmatic scenario is that the Butterfly Dykes are intrusive equivalents of high-Mg flows in the Kambalda Sequence, erupted from a mantle plume melting at the base of the lithosphere, whereas Norseman and Crown Dyke suites represent melts of metasomatised mantle lithosphere induced by some combination of rifting and thermal input from the plume.

THE STABLE AND RADIOGENIC ISOTOPIC ATTRIBUTES OF PRECIOUS-METAL-BEARING POLYMETALLIC VEINS FROM THE COBALT EMBAYMENT, NORTHERN ONTARIO, CANADA: GENETIC AND EXPLORATION IMPLICATIONS

The Cobalt Embayment is a large (~30,000 km 2 ) domain of Paleoproterozoic (2.45–2.22 Ga) siliciclastic sedimentary rocks (the Huronian Supergroup) that unconformably overlies Archean basement rocks of the Abitibi Greenstone Belt. The sedimentary rocks were affected by subgreenschist-facies metamorphism and regionally distributed metasomatism related to waning stages of the Penokean Orogeny ( ca . 1.9–1.7 Ga). Precious-metal-bearing polymetallic calcite–quartz vein systems occur throughout the embayment and share many common features, including their morphological characteristics, ore and gangue mineralogy, paragenetic sequence, and spatial association with regional faults rooted in the Archean basement. The polymetallic veins are typically hosted in Nipissing Diabase, a regionally distributed suite of mafic sills and dykes, which intruded the Huronian sedimentary rocks at ca. 2.2 Ga. Integrated carbon, oxygen, and strontium isotopic data for calcite, the major mineral in the gangue, support a common hydrothermal origin for the regionally distributed systems of polymetallic veins, involving mixing of meteoric-water-dominated fluids with basin brines. Local variations in δ 13 C likely reflect the availability of, and degree of hydrothermal fluid interaction with, organic matter in different regions of the embayment. Significant intradeposit variation in δ 34 S values of vein sulfides (pyrite, chalcopyrite, galena) suggests that localized sources of sulfur existed in the Huronian and Archean rocks, with positive values (>0‰) in several of the mineralized vein systems likely reflecting the introduction of sulfur remobilized from the basement as a result of the conversion of pyrite to pyrrhotite in the underlying Archean rocks. Data and observations suggest that metals were transported in oxidized fluids, and precipitation occurred as a result of oxidation–reduction reactions along reactivated faults, which facilitated fluid mixing. Together with Pb isotope data for the vein systems, the isotopic data presented support a model for ore genesis in which the systems of precious-metal-bearing polymetallic calcite–quartz veins formed, and were reactivated, in response to two major episodes of hydrothermal fluid activity that occurred throughout the Cobalt Embayment, at ca. 2.2 and ca. 1.7 Ga. The proposed genetic model identifies the polymetallic veins as one manifestation of hydrothermal fluid activity during these basin-wide events and also recognizes the regional unconformity as a potential site for undiscovered, unconformity-associated hydrothermal polymetallic mineralization.