Dyke Intrusion Research Articles

The ground deformation of the south-eastern flank of Mount Etna monitored by GNSS and SAR interferometry from 2016 to 2019

SUMMARY The south-eastern sector of the Mount Etna, Italy, is characterized by numerous active faults, in particular the Belpasso–Ognina lineament, the Tremestieri–San Gregorio–Acitrezza fault, the Trecastagni fault and the Fiandaca–Nizzeti fault including the Timpe Fault System. Their activity is the result of both volcanism and tectonics. Here, we analyse the ground deformation occurred from 2016 to 2019 across those active faults by using the GNSS data acquired at 22 permanent stations and 35 campaign points observed by the Etna Observatory (INGV) and by the University of Catania. We also use the time-series of line of sight displacement of permanent scatterers SENTINEL-1 A-DInSAR obtained by using the P-SBAS tool of the ESA GEP-TEP (Geohazards Thematic Exploitation Platform) service. We discriminate the contributions of the regional tectonic strain, the inflations, the deflations of the volcano and the gravitational sliding in order to analyse the deformation along the faults of the south-eastern flank of Etna. The shallow and destructive Mw = 4.9 earthquake of 2018 December 26 occurred within the studied area two days after a dyke intrusion, that propagated beneath the centre of the volcano accompanied by a short eruption. Both GNSS and InSAR time-series document well those events and allow to investigate the post-seismic sliding across the faults of south-eastern flank. We analyse the slow slip events (SSE) that are observed in the GNSS and InSAR time-series in the vicinity of the Acitrezza fault. We quantify and discuss the tectonic origin of the Belpasso–Ognina lineament that we interpreted as a tear fault.

Formation of magmatic segments within the Aluto-Gedemsa area, Main Ethiopian Rift

The formation of magmatic segments in the Main Ethiopian Rift (MER) is key to the argument that the rift is transitioning from a continental to an oceanic spreading center. In this study, we used gravity data from Global Gravity Model plus2013 to interpret the formation and nature of magmatic segmentation within the Aluto- Gedemsa area, central MER. Our gravity data show that the Aluto and Gedemsa magmatic segments represent two distinct features and no linkage between the two occurs. This observation is in contradiction to surface tectonic mappings, which argue that Aluto and Gedemsa volcanoes belong to a single magmatic segment. Our results suggest that magmatic localization beneath the two segments may not be controlled by the same magmatic process: The Gedemsa magmatic segment (G-MS) is associated with an elongated NW trending gravity maxima, which suggests that its formation could be linked to and controlled by transversal structures in the rift. On the other hand, circular positive gravity maxima characterize the Aluto magmatic segment (AL-MS), suggesting that dyke intrusion controls the formation of the segment. Our results, together with existing data, support the occurrence of bilateral off-rift magmatic extension zones, in addition to the axial magmatic segments (AL-MS and G-MS). These results suggest that the central MER is still in a relatively immature stage of continental rifting where strain is accommodated by border faults, axial magmatic segments, and off-rift magmatic zones.

A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes

The George Fisher and Hilton Zn-Pb-Ag deposits are located approximately 20 km north of Mount Isa. Although previous workers have noted the existence of a dolerite dyke at the Hilton Zn-Pb-Ag deposit (Valenta, 1994b; Valenta, 1994a), a dolerite dyke at the George Fisher Zn-Pb-Ag deposit has not been previously documented. Progressive underground expansion at the George Fisher mine has resulted in the discovery of a dolerite dyke in the southernmost portion of the deposit. The temporal and petrological relationship between the dolerite dykes and the adjacent Zn-Pb-Ag mineralisation at the George Fisher and Hilton Zn-Pb-Ag deposits has not been considered by previous studies, and has significant implications for the currently accepted syn-diagenetic metallogenic model. In assay data, the dolerite dykes correspond with a large spike in TiO2 and V values. In drill core, the dolerite dykes occur as a light grey to grey-brown coloured rock that is commonly overprinted along its margins by Zn-Pb-Ag mineralisation. In-situ U-Pb geochronology performed on igneous apatite produce a lower intercept age of 1611 ± 21 Ma (MSWD = 0.93) and 1619 ± 22 Ma (MSWD = 0.53) for the dolerite dykes at the George Fisher and Hilton deposits, respectively. This age is consistent with dyke intrusion during the earliest phases of deformation during the (1620–1500 Ma) Isan Orogeny. The dolerite dykes have experienced multiple stages of post-emplacement hydrothermal alteration/veining, which have paragenetic equivalents in the adjacent Zn-Pb-Ag orebodies. The hydrothermal stages include: (1) Quartz ± albite ± K-feldspar alteration/veining; (2) Dolomite alteration/veining; (3) Biotite-chlorite-sulphide alteration; (4) Paragenetically late calcite, dolomite and/or sulphide veining. Monazite from a quartz ± albite ± K-feldspar vein in the Hilton dyke produces a lower intercept age of 1513 ± 16 Ma (MSWD = 1.1), constraining the maximum age of alteration within the dolerite dykes. To assess the timing of alteration in the adjacent George Fisher Zn-Pb-Ag deposit, in-situ Lu-Hf geochronology was performed on pre-mineralisation calcite from a section of stratabound Zn-Pb-Ag mineralisation, and a paragenetically late cross-cutting sphalerite-calcite vein. Calcite from the pre-mineralisation alteration assemblage produces a Lu-Hf age of 1501 ± 32 Ma (MSWD = 1.04), which is interpreted to constrain the maximum age of stratabound Zn-Pb-Ag mineralisation. Calcite from the late cross-cutting vein produced a Lu-Hf age of 1289 ± 26 Ma (MSWD = 1.00), which is interpreted to constrain the age of post-mineralisation faulting throughout the deposit. The evidence presented in this study indicates that the dolerite dykes intruded during the early Isan Orogeny at ca. 1620 Ma, and experienced subsequent hydrothermal alteration during D3 of the Isan Orogeny coeval with Zn-Pb-Ag mineralisation.

Quantifying Dyke-Induced Graben and Dyke Structure Using 3D Seismic Reflection Data and The Role of Interpretation Bias

During dyke intrusion, tensile stresses concentrated within the overlying rock may lead to the formation of normal faults. These faults typically form graben-bounding pairs that are sub-parallel to, and dip toward, the upper tip of their underlying dyke. Many studies use geometric properties extracted from the surface expression of such dyke-induced faults to estimate the geometry of subsurface dykes. These methods assume dyke-induced faults are planar and nucleate at the surface. However, recent seismic reflection-based investigations of the 3D structure of dyke-induced faults confirm they can be non-planar and have complex growth histories. Here, we use 3D seismic reflection surveys from offshore NW Australia to: (1) examine how the surface expression of dyke-induced faults relates to subsurface dyke geometry and depth; and (2) test whether subjective bias may influence the quantitative analyses of dyke-induced faults using seismic reflection data. We show displacement and dip vary across dyke-induced faults, supporting previous suggestions that faults nucleate between dyke upper tips and the free surface. We also find that prediction of dyke upper tip depths using graben width and area of loss methods are sensitive to fault dip variations and interpretation biases, but often still produce similar results to measured dyke depths. Both measured and predicted dyke depths vary by several hundred metres along-strike, which we relate to the preservation of dyke heads, segmentation, and/or magma density changes. Overall, we show reflection seismology provides a better understanding of the 3D structure of dyke-induced faults and their relationship to the geometry and emplacement dynamics of their causal dykes.

Deep long period seismicity preceding and during the 2021 Fagradalsfjall eruption, Iceland

We use a dense seismic network on the Reykjanes Peninsula, Iceland, to image a group of earthquakes at 10–12 km depth, 2 km north-east of 2021 Fagradalsfjall eruption site. These deep earthquakes have a lower frequency content compared to earthquakes located in the upper, brittle crust and are similar to deep long period (DLP) seismicity observed at other volcanoes in Iceland and around the world. We observed several swarms of DLP earthquakes between the start of the study period (June 2020) and the initiation of the 3-week-long dyke intrusion that preceded the eruption in March 2021. During the eruption, DLP earthquake swarms returned 1 km SW of their original location during periods when the discharge rate or fountaining style of the eruption changed. The DLP seismicity is therefore likely to be linked to the magma plumbing system beneath Fagradalsfjall. However, the DLP seismicity occurred ~ 5 km shallower than where petrological modelling places the near-Moho magma storage region in which the Fagradalsfjall lava was stored. We suggest that the DLP seismicity was triggered by the exsolution of CO2-rich fluids or the movement of magma at a barrier to the transport of melt in the lower crust. Increased flux through the magma plumbing system during the eruption likely adds to the complexity of the melt migration process, thus causing further DLP seismicity, despite a contemporaneous magma channel to the surface.

Characterization of an Active Fault through a Multiparametric Investigation: The Trecastagni Fault and Its Relationship with the Dynamics of Mt. Etna Volcano (Sicily, Italy)

The Trecastagni Fault (TF) is an important tectonic structure in the middle-lower southern flank of Mt. Etna volcano. It is characterised by evident morphological slopes with normal dip-slip ruptures that directly affect roads and buildings. The TF plays a key role in the complex framework of the volcano dynamics since it represents part of the southern boundary of the unstable sector. Seismic surveys have been performed on three different areas of the fault to gain insights into the seismic stratigraphic structure of the subsoil. We considered the seismic activity of a sector of the territory affecting the surface evidence of the Trecastagni Fault in the period between 1980 and 2021 in order to highlight the main seismic release and define the space–time distribution of seismicity. Most of the seismicity is located in the north-western portion, while the central and southern sectors are characterised by low seismic activity. The strongest earthquakes occur mainly within the first 5 km of depth in the form of swarms and/or isolated shocks. Ground deformation techniques (levelling, In-SAR and two continuous extensometers) evidence a continuous aseismic slip of the TF that is interrupted by short accelerations accompanied by shallow seismicity. The Trecastagni Fault dynamics are strictly linked to magma pressurisation and intrusive episodes of Mt. Etna that induce additional stress and promote its slip along the fault plane. Multidisciplinary data analysed in this work, evidenced the dual behaviour of the fault, from aseismic creep to stick-slip, and the relation with magmatic activity, also suggesting the time delay in the response of the fault after the intense stress induced by dyke intrusion.

Magmatic inflation, miniature dyke intrusion and prolonged torrential rainfall led to the emergence of the 2018 aseismic fissure in Kenya

Magmatic inflation, miniature dyke intrusion and prolonged torrential rainfall led to the emergence of the 2018 aseismic fissure in Kenya

Precursor-free eruption triggered by edifice rupture at Nyiragongo volcano

Classical mechanisms of volcanic eruptions mostly involve pressure buildup and magma ascent towards the surface1. Such processes produce geophysical and geochemical signals that may be detected and interpreted as eruption precursors1–3. On 22 May 2021, Mount Nyiragongo (Democratic Republic of the Congo), an open-vent volcano with a persistent lava lake perched within its summit crater, shook up this interpretation by producing an approximately six-hour-long flank eruption without apparent precursors, followed—rather than preceded—by lateral magma motion into the crust. Here we show that this reversed sequence was most likely initiated by a rupture of the edifice, producing deadly lava flows and triggering a voluminous 25-km-long dyke intrusion. The dyke propagated southwards at very shallow depth (less than 500 m) underneath the cities of Goma (Democratic Republic of the Congo) and Gisenyi (Rwanda), as well as Lake Kivu. This volcanic crisis raises new questions about the mechanisms controlling such eruptions and the possibility of facing substantially more hazardous events, such as effusions within densely urbanized areas, phreato-magmatism or a limnic eruption from the gas-rich Lake Kivu. It also more generally highlights the challenges faced with open-vent volcanoes for monitoring, early detection and risk management when a significant volume of magma is stored close to the surface.

Mineralogy of the Imalia Au-Sn-bearing polymetallic sulfide deposit, Mahakoshal belt, Central India

The Au-Sn-bearing, polymetallic sulfide deposit at Imalia in the western part of Mahakoshal belt, Central India is hosted by recrystallized dolostone and subsidiary phyllitic dolostone. These host rocks are transected by shallow level intrusion of quartz porphyry dykes. There are two major NS trending ore zones (∼1.5 km cumulative length) and a subsidiary-one, confined to fractures and shears in the dolostones or along its contact with the intrusive dykes. Polyphase mineralization include: irregularly disseminated pyrite crystals of diagenetic/metamorphic origin, patchy to stringery Pb-Zn sulfide ores showing pervasive metamorphic fabric, dominant massive vein type pyrite-arsenopyrite ores with significant amounts of invisible gold and tin. Other ore minerals include cassiterite, molybdenite, wolframite and roquesite in a magnetite-rich oxidic halo, and electrum, enargite/luzonite, wittichenite, ourayite, eclarite, aikinite and idaite in the sulfidic veins. Some tentatively identified ore minerals in these veins include calaverite, proustite, famatinite, sakuraiite, and tenorite. Pyrite contains structurally located invisible gold as high as 17.6 ppm whereas arsenopyrite contains a maximum of 20–24 ppm Au. Pyrite also contains unusually high Sn (upto 2673 ppm). The mineralogical assemblage of rare phases and their paragenesis in the Imalia vein type ores reflect the high sulfur (log fS2 = −9 to −13 bar) and oxygen (logfO2 = −32 bar) fugacities at a temperature range of 350 °C to 250 °C (according to thermometric calculations) during their emplacement.

Process and Mechanisms of Deep Fluid Effects on Hydrocarbon Generation and Pore Space in Shale: A Case Study from the Mesoproterozoic Xiamaling Formation in the Zhaojiashan Section

AbstractHeat carried by deep fluid might greatly affect hydrocarbon generation and pore space in shale. Dyke intrusion carrying high levels of heat may be a means by which to explore the influence of deep fluid on shale reservoirs. This study evaluates hydrocarbon generation and analyzed the evolution of shale storage space in the third member of the Xiamaling Formation in the Zhaojiashan section, Hebei Province, based on experimental data such as TOC, SEM, VRo, low‐temperature N2 adsorption and high‐pressure mercury injection. The results show that the dyke intrusion reduced the shale TOC content drastically—by up to 77%—and also induced instantaneous hydrocarbon generation over a range about 1.4 times the thickness of the intrusion. Furthermore, the dyke intrusion might transform organic pores in surrounding shales into inorganic pores. There were two shale porosity peaks: one appeared when VRo = 2.0%, caused by the increase of organic pores as thermal maturity increased, the other occurred when the VRo value was between 3% and 4%, caused by the increase of inorganic mineral pores. It can be concluded that dyke intrusion can be an effective tool with which to study how deep fluid affects instantaneous hydrocarbon generation and pore space in shale.

Temporal-spatial characterization of mining-induced seismicity in the vicinity of a dyke – A case study

Major faults and dykes are common geological discontinuities in underground mines and can often contribute to significant failures such as coal bursts during mining. The failure mechanisms of faults have been widely studied, whereas the seismic characteristics associated with dykes have been rarely investigated for mine dynamic hazard risk management. This research aimed to characterize the seismic behavior near an intrusive dyke that intersected with the roadways in an underground longwall coal mine in Australia. The temporal-spatial variations of seismic multi-parameters around the stiff dyke were analyzed, including clustered seismicity, event locations, Es/Ep ratio, b-value and correlation with the released seismic energy. Four stages were defined based on the evolutions of seismic characteristics. The rock fractures initiated near the dyke when the longwall face was 400 m from the dyke. The seismicity started to increase when the longwall was <100 m from the dyke and decreased drastically after the longwall mined 50 m past the dyke. The seismic events were concentrated at the inbye side of the dyke and propagated to the outbye side of the dyke as the longwall mined through the dyke. The spatial distribution of Es/Ep ratios and temporal variations of the b-value indicated that higher magnitude seismic events dominated by shear failures occurred in the hanging wall of the dyke, suggesting more sudden seismic energy release and escalating seismic risks around the roadways at the inbye side of the dyke. Meanwhile, when the longwall was mining through the dyke, even though the seismicity was intensive, seismic energy was more frequently and gently released so that stress was not significantly accumulated to induce any major failures at the outbye side of the dyke. The results demonstrated that the geological features of the dyke and the longwall mining progress had a controlling effect on the seismic characteristics in the time-space domain. The comprehensive analysis of seismic multi-parameters reliably revealed the rock mass response near a major dyke and can provide guidance for assessing and managing the risks of dynamic mining hazards around dykes.

22 years of satellite imagery reveal a major destabilization structure at Piton de la Fournaise

Volcanic activity can induce flank failure, sometimes generating large earthquakes and tsunamis. However, the failure structures have never been fully characterized and the failure mechanism is still debated. Magmatic activity is a possible trigger, either through fault slip, which might be induced by dyke intrusions, or through sill intrusions, which might be undergoing coeval normal displacements and slip. At the Piton de la Fournaise volcano, satellite imagery combined with inverse modeling highlights the pathways of 57 magmatic intrusions that took place between 1998 and 2020. We show that a major arcuate dyke intrusion zone is connected at depth to a sill intrusion zone, which becomes a fault zone towards the sea, forming a spoon-shaped structure. Some sills are affected by coeval normal displacement and seaward slip. Overall, the structure is characterized by a continuum of displacement from no slip, to sheared sills and finally pure slip. Repeated intrusions into this spoon-shaped structure could trigger catastrophic collapses.

Volcano-tectonic interactions at the southern margin of the Okataina Volcanic Centre, Taupō Volcanic Zone, New Zealand

The c. 15 km-long Ngapouri-Rotomahana Fault (NRF) is a major splay of the Paeroa Fault at the eastern margin of the modern Taupō Rift, the active tectonic structure embedded within the Taupō Volcanic Zone of North Island, New Zealand. The NRF and Paeroa Fault extend to the southern margin of the Okataina Volcanic Centre (OVC) and lie southwest of the Tarawera vent lineation, which is the source of approximately half of the eruptions of the OVC in the past 25 cal. ka BP. Here, we explore volcano-tectonic relationships between the OVC and the NRF and Paeroa Fault. Collective evidence used in our analysis includes: volcanic processes interpreted as occurring during the historic 1886 Tarawera (basalt) and the prehistoric 1314 ± 12 CE Kaharoa (basalt triggered rhyolite) eruptions, both on the Tarawera vent lineation; exposures in five trenches excavated across the NRF and seven trenches across the Paeroa Fault; data on a series of explosion craters formed to the southwest of the volcano associated with the ~1314 CE Kaharoa eruption and the Rotoma rhyolite (~9.4 cal. ka BP) eruption from the OVC; and mafic dykes that primed several of the OVC eruptions. Data from the twelve trenches on the two faults reveal eight surface fault ruptures since 15.6 cal. ka BP, with most closely coinciding with volcanic eruptions, providing a first-order indication of probable causality. Three principal modes of interaction are identified. Firstly, large displacement events on the Paeroa fault, arguably immediately prior to the Mamaku and Rotoma rhyolite eruptions (~7.9 and ~9.4. cal. ka BP, respectively) and on the NRF immediately prior to the ~1314 CE Kaharoa eruption are candidates for earthquake static or dynamic stress triggers for those explosive eruptive events. Secondly, basalt dyke intrusion was also involved in the initiation of the Kaharoa eruption, so the spatial and temporal relationships between dyke intrusion, smaller displacement fault ruptures and initiation of the Kaharoa eruption appear closely connected. Thirdly, faulting events that are interpreted as co- or post-eruption may be the result of stress triggers associated with magma chamber deflation.

Geological and geochemical characteristics of the Wulong gold deposit, Liaodong Peninsula: Implications for gold mineralization

As a typical hydrothermal vein-type gold deposit in the eastern Liaodong Peninsula, northeast of North China Craton, the Wulong gold deposit is associated with the Jurassic gneissic two-mica granites with gold reserves over 80 t. In this paper, the gold precipitation and enrichment mechanism of the Wulong deposit is investigated by studying the morphological structure and geochemical characteristics of pyrite. Pyrites are divided into four types based on their microstructural characteristics: Py1 is represented by disseminated pyrite, which was developed in granites; Py2a is dominated by porous pyrite, which developed in the early quartz-pyrite vein; Py2b commonly wraps Py2a, distinguished from Py2a by its “smooth” surface, and developed in the quartz-polymetallic sulfide stage; Py3 was observed in quartz-calcite-fluorite stage. X-Ray Fluorescence Spectrometer (XRF) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) analysis revealed the enrichment of trace elements in pyrite at different stages: 1) In an overall view, the strong positive correlation among chalcophile elements was detected; 2) The total amount of trace elements in Py1 and Py3 was significantly lower than that in Py2a and Py2b; 3) In Py2a, the content of chalcophile elements represented by Ag, Te, Bi and Pb is higher than that of other types of pyrite, and gold mainly coexists with those elements in the form of nanoscale micro-inclusions (Au0); 4) The lattice gold (Au+) was positively correlated with As in Py2b. In contrast to the extremely low invisible gold content (Py1: 0.94 ppm; Py2a: 2.23 ppm; Py2b: 4.45 ppm; Py3: 0.28 ppm) in pyrite, there is a large amount of visible gold in the deposit in the form of scattered natural gold and electrum particles (silver content 4.38–36.01%). In addition, these visible gold is often associated with mineral aggregates consisting of galena, native bismuth, joséite-B, etc. Combined with the research results of previous studies, we proposed that visible gold is the main source of gold in the Wulong gold deposit. It is concluded that the occurrence of a large amount of visible gold in this deposit is closely related to the thinning and destruction event of the North China Craton during the Mesozoic. Different from the deposition characteristic of gold in gold-rich pyrite, in the high arsenic (>2000 ppm) area of pyrite, only very low gold content less than 5 ppm was detected. Meanwhile, the complex dyke intrusion and cutting phenomena observed in the mining area indicate the multi-stage tectonic activities in the metallogenic stage. Therefore, a reasonable explanation is that the pressure of the ore-forming fluid decreased sharply during tectonic activity, which promoted the separation of fluid phases and resulted in the massive precipitation of natural gold and chalcophile elements. This paper is of great significance to the gold prospecting in the Liaodong Peninsula, and also provides an important reference for the comparative study of metallogenic mechanisms in the Liaodong Peninsula and Jiaodong Peninsula.

Thermal impact of dykes on ignimbrite and implications for fluid flow channelisation in a caldera

Ignimbrites within calderas host intrusions with hazardous and/or economically significant hydrothermal systems. The Hvítserkur ignimbrite at Breiðuvík caldera, north-eastern Iceland, is intruded by basaltic dykes. Our data show that the ignimbrite immediately adjacent to the dyke is hard, dark-coloured, recrystallised quartz, plagioclase, and alkali feldspar with a low permeability and porosity and frequent macrofractures. At 1-2 m from the dyke, the ignimbrite is hard, dominantly glassy with pervasive perlitic microfractures, has high permeability, but low porosity and frequent macrofractures. A narrow zone of pervasive unlithified clay exists 2 m from the dyke. Beyond this, the ignimbrite is soft and zeolite-rich, has low permeability, high porosity and fewer macrofractures. The dyke intrusion promoted a narrow zone of welding, fracturing and perlitisation in the ignimbrite resulting in high permeability and focussed alteration. Our study shows how intrusions and their thermal aureoles create vertical pathways for, and horizontal barriers to, geothermal fluid flow.

Thermal impact of dykes on ignimbrite and implications for fluid flow compartmentalisation in calderas

Ignimbrites within calderas host intrusions with hazardous and/or economically significant hydrothermal systems. The Hvítserkur ignimbrite at Breiðuvík caldera, north-eastern Iceland, is intruded by basaltic dykes. Our data show that the ignimbrite immediately adjacent to the dyke is hard, dark-coloured, recrystallised quartz, plagioclase, and alkali feldspar with a low permeability and porosity and frequent macrofractures. At 1-2 m from the dyke, the ignimbrite is hard, dominantly glassy with pervasive perlitic microfractures, has high permeability, but low porosity and frequent macrofractures. A narrow zone of pervasive unlithified clay exists 2 m from the dyke. Beyond this, the ignimbrite is soft and zeolite-rich, has low permeability, high porosity and fewer macrofractures. The dyke intrusion promoted a narrow zone of welding, fracturing and perlitisation in the ignimbrite resulting in high permeability and focussed alteration. Our study shows how intrusions and their thermal aureoles create vertical pathways for, and horizontal barriers to, geothermal fluid flow.

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.

Geology and genesis of auriferous porphyritic monzogranite and its correlation with the Qiyugou porphyry-breccia system in East Qinling, Central China

The Qiyugou deposit in the Qinling Orogenic Belt includes breccia pipe hosted gold mineralization and newly discovered porphyry gold mineralization, whereas the genetic relationship between porphyry stocks, stockwork-disseminated mineralization and breccia pipe-hosted mineralization remains obscure. Here we present the results from an integrated field investigation, petrography, whole-rock geochemistry, zircon U-Pb geochronology, trace elements and Hf-O isotopes on a suite of porphyritic monzogranite from the newly discovered auriferous ‘191 pluton’, to elucidate the petrogenesis, magma source, timing of magmatic-hydrothermal event and the possible genetic link with the Qiyugou porphyry-breccia system. The porphyritic monzogranite shows varying degree of hydrothermal alterations including potassic alteration, phyllic alteration, silicification, sericitization and chloritization, resulting in the high loss on ignition values (LOI = 2.88–6.64 wt%) and linear relationships between LOI and large ion lithophile elements (LILEs) which suggest element mobility in fluids. The fluid immobile high field strength elements (HFSEs) and zircon trace elements indicate that the porphyritic monzogranite is typical I-type granite. Zircon U-Pb dating of the porphyritic monzogranites yield weighted mean ages of 136.4 ± 1.1 Ma, 134.6 ± 1.8 Ma and 135.2 ± 1.1 Ma, representing the emplacement age of the auriferous ‘191 pluton’. Zircon Hf-O isotopes show εHf(t) values from −25.6 to −20.0 with crustal model ages ranging from 2807 to 2458 Ma, and δ18O values from 4.75‰ to 5.66‰, indicating that the parental magma involved mixing between the partially melted Taihua Group lithologies and mantle-derived component. The available geochronological data constrain the timing and duration of magmatic-hydrothermal activities in the Qiyugou deposit, including pre-mineralization 158.7–150.1 Ma granitic magmatism, syn-mineralization 136.4–128.6 Ma porphyry magmatism and Au mineralization, and post-ore 128.9–124.7 Ma granitic dyke intrusion. The duration of the magmatic-hydrothermal activity in the Qiyugou deposit ranges between 136.4 Ma and 128.6 Ma, when the breccia pipes and granitoid plutons formed through multiple pulses of magma emplacement and fluid exsolution from a fertile magma chamber. The 136.4–131.3 Ma porphyry magmatism and gold mineralization in the ‘191 pluton’ represent an early stage magmatic-hydrothermal activity in the Qiyugou porphyry-breccia system. We predict that the deeper levels of the breccia pipes could be potential target for porphyry gold ore bodies in the Qiyugou deposit.

Remagnetization Age and Mechanism of Cretaceous Sediments in Relation to Dyke Intrusion, Hainan Island: Tectonic Implications for South China and the Red River Fault

AbstractHainan Island lies near the Red River Fault, a prominent tectonic feature produced by the India‐Asia collision. There, we carried out a geochronologic and paleomagnetic study on Cretaceous rocks in order to better understand the kinematic history of the region. U‐Pb zircon dating of tuff intercalated in red bed sedimentary rocks yielded a concordant age of 106.6 ± 0.3 Ma; a mafic dyke intruding the red beds yielded a concordant age of 104.6 ± 0.7 Ma. Stepwise demagnetization experiments on 448 sedimentary rock samples and 191 dyke samples isolate solely normal polarities. Paleomagnetic directions of the dykes cluster in two distinct populations in geographic coordinates, indicating that dyke intrusion occurred in two pulses of limited duration (secular variation was not averaged) after tilting of the sediments. Baking of the sediments from the dykes only occurred near the contacts. Together with published data, the mean directions of 104 sites most tightly group at 58.3 ± 3.2% unfolding, indicative of a synfolding remagnetization, which can be constrained to have occurred within a 2 Myr period between sedimentation and dyke intrusion. We suggest that warm (50–100°C) fluid interaction during basin development led to new mineral growth spawning chemical remagnetization. The corresponding paleomagnetic pole at 81.5°N, 145.2°E (A95 = 2.4°) is indistinguishable from the coeval Eurasian reference pole, suggesting the South China Block has remained fixed to Eurasia since 105 Ma. A contour map of paleomagnetic rotations from 115 studies in the region shows that the Red River Fault roughly demarcates rotation magnitudes/signs, suggestive of a major tectonic boundary.

Emplacement of Carbonatite and Associate Rocks of Siriwasan Carbonatite Complex, Gujarat: Evident by Field Relationship and Petrology

The Siriwasan carbonatite complex is situated in the western part of Deccan Basalt Province and belongs to Chhota Udepur carbonatite-alkaline sub-province. The area consist a variety of rock types including sedimentary rocks of Bagh Group (sandstone, limestone and shale), Deccan basalts, carbonatized sandstone, pyroclastic rocks, carbonatite breccia, carbonatites (Alvikite and Sovite), tinguaite, trachyte and dolerite. The detailed field studies and petrography of the carbonatites and associated rocks of the area concluded that there should be at least two episodes of carbonatitic magmatism of different ages. The older episode was extrusive coupled with intrusive dykes and the later event was intrusive only. The tinguaite is older than Deccan basalts while the trachyte and dolerite are younger rock units.