Early Cretaceous Mafic Dikes Research Articles

First identification of Early Cretaceous mafic dikes in the Baingoin area, central Tibet: Implications for crust-mantle interactions and magmatic flare-up

Mafic dikes are generally emplaced in extensional tectonic settings and provide key information regarding deep mantle processes and sources. The Bangong-Nujiang suture zone was formed by the collision of the Qiangtang and Lhasa terranes and experienced intense magmatism during the Early Cretaceous. However, the deep mantle processes and mechanisms involved in this magmatic flare-up (ca. 115 Ma) in the collisional belt remain controversial because of the lack of evidence for coeval mafic magmatism. Here, we present detailed petrological, geochronological, geochemical, and Sr-Nd-Hf-O isotope data for the newly discovered hypersthene-bearing mafic dikes in the Baingoin area in the middle-eastern parts of the Bangong-Nujiang suture zone, central Tibet. Secondary ion mass spectroscopy (SIMS) zircon U-Pb dating showed that the mafic dikes were emplaced during 120−115 Ma. These mafic rocks are characterized by variable MgO contents (2.7−5.2 wt%) and Mg# values (38.5−52.8), slight enrichment in light rare earth elements (REEs; [La/Yb]N = 7.5−8.1), relatively flat heavy REE patterns ([Gd/Yb]N = 1.75−1.84), and negative Eu, Ta, Nb, and Ti anomalies. The dikes also have relatively low initial 87Sr/86Sr ratios (0.7060−0.7062) and negative εNd(t) (−2.2 to −1.6) and positive εHf(t) (+2.5 to +3.6) values, and variable zircon εHf(t) (−2.2 to +7.2) and slightly elevated zircon δ18O (5.6‰−7.0‰) values. These geochemical characteristics indicate that the mafic dikes were derived from an enriched lithospheric mantle source. However, compared with coeval magmatic rocks, the mafic dikes have relatively high εNd(t) and εHf(t) values, indicating that they contain a depleted mantle component. The mafic dikes contain clinopyroxene and orthopyroxene (i.e., hypersthene), indicative of derivation from a high-temperature magma source. Clinopyroxene-melt thermobarometry yielded a temperature range of 1167−1213 °C, further supporting the involvement of a high-temperature asthenospheric component. Therefore, we suggest that the parental magmas of the Nakoulai mafic dikes were probably generated by the interaction between the asthenospheric mantle and overlying metasomatized lithospheric mantle. Combined with data from nearby Cretaceous magmatic rocks and sedimentary rocks, we suggest that the mafic dikes were generated in a postcollisional setting caused by upwelling of asthenospheric mantle owing to slab breakoff beneath the Bangong-Nujiang suture zone. Slab breakoff played a key role in the crust-mantle interactions and the onset of the magmatic flare-up in the middle-eastern parts of the Bangong-Nujiang suture zone.

Unraveling the tectonic setting and crystallization history of the Equatorial Atlantic Magmatic Province

In NE Brazil, Early Cretaceous mafic dikes and sills intrude the Precambrian Borborema Province and the Paleozoic Parnaíba basin, where they form the Equatorial Atlantic Magmatic Province. The Rio Ceará-Mirim dike swarm (RCM) is a giant arcuate structure ca. 1100 km in length composed of sets of dikes ranging from E-W to NE-SW. There are two additional giant swarms with WNW and NE-SW orientations spanning 380 km and 355 km, respectively, in the NW and SE regions of the Borborema Province. We investigated the tectonic setting and crystallization history of the mafic magmatism through orientation, dimensions (960 dikes), and 32 representative samples of plagioclase crystal size distributions (CSD) to shed light on the connection between the dikes and sills. Plagioclase compositions in EQUAMP intrusions indicate normal crystallization trends with the magma becoming more evolved towards the final stages of crystallization. CSD data suggests that residence times are similar for dikes and sills (≤ 1.5 yrs), although some dikes display higher residence times (3.6–1.8 yrs) in agreement with a longer crystallization history. These results suggest a deeper crystallization setting for the dikes in agreement with a plumbing system in that dikes penetrate the “stiffer” basement units to feed upward the sills accommodated between sedimentary layers with contrasted rheologies. The regional distribution of the mafic magmatism resulted from a bulk NW-trending extensional deformation, where the brittle (re)activation and control of pre-existing structures allowed the nucleation of the arcuate giant dike system and other satellite swarms. We also examine connections between tholeiitic igneous events in NE South America and tectonomagmatic episodes affecting the Pangea and West Gondwana landscapes in the Mesozoic Era.

Early Cretaceous mafic dikes in the northern Qinling Orogenic Belt, central China: Implications for lithosphere delamination

Mafic dikes preserve a considerable amount of geological information about crustal evolution. Yanshanian granitoids and granitites are widespread in the southern margin of the North China Craton (NCC) and northern Qinling Orogenic Belt (NQOB), respectively. However, further study is required to resolve the many contrasting hypotheses relating to the mechanisms of petrogenesis and origins. The present study therefore reports on geochemical, geochronological, and Sr-Nd-Pb isotopic data for a suite of mafic dikes in the Mokou area of the NQOB, central China. The Mokou dikes are composed of diabase and lamprophyre intruded into the Kuanping Group (Pt3). The zircon U-Pb ages of two diabases and one lamprophyre were 132 ± 3 Ma and 132 ± 1 Ma, and 132 ± 1 Ma, respectively. The ages of a total of 126 zircon grains from three rock samples were assigned to nine age groups: ~2.2 Ga, ~946 Ma, ~519 Ma, ~440 Ma, ~413 Ma, ~385 Ma, ~160 Ma, ~145 Ma, and ~132 Ma. This complex zircon age spectrum corresponds to regional magmatic or metamorphic events from the Paleoproterozoic to the Early Cretaceous. We determined that the Mokou dikes are dominantly alkaline rocks that belong to the shoshonite and high-K calc-alkaline series. The Mokou dike samples were enriched in large ion lithophile elements (e.g., Rb and Ba) and presented pronounced negative anomalies in the high-field-strength elements (e.g., Nb, Ta, Zr, and Hf). Both diabase and lamprophyre in the Mokou area were enriched in light rare earth elements (11.9 < (La/Yb)N < 30.8) with no obvious Eu anomalies (δEu = 0.92–1.03). They had high initial 87Sr/86Sr ratios (0.70625–0.71433), negative ε Nd (t) values (−12.9 to −8.3), and slightly variable initial ratios of 206Pb/204Pb (17.539–18.185), 207Pb/204Pb (15.518–15.693), and 208Pb/204Pb (38.090–38.996). Our results indicate that the Mokou dikes originated from an enriched lithospheric mantle and a contaminated asthenosphere. Continental crust materials in the magmas/fluid reservoirs were involved during the rapid intrusion of the Mokou mafic magma, as opposed to the assimilation of wall rocks in the ascent conduit. We conclude that the Mokou dikes are products of lithosphere delamination, which occurred ca. 132 Ma in the NQOB, thus suggesting that lithospheric thinning of the NCC also affected the NQOB.

Water Contents of Early Cretaceous Mafic Dikes in the Jiaodong Peninsula, Eastern North China Craton: Insights into an Enriched Lithospheric Mantle Source Metasomatized by Paleo–Pacific Plate Subduction–Related Fluids

The Early Cretaceous lithospheric mantle beneath the Jiaodong Peninsula has been recognized as ancient and enriched mantle with widespread mafic dike swarms. However, the enriching agent and the process of the lithospheric mantle remain under debate. In this study, we recovered the H2O contents of the primitive magmas of Early Cretaceous mafic dikes in the Jiaodong Peninsula by using the H2O contents of clinopyroxene (Cpx) phenocrysts and H2O partition coefficients DH2Ocpx/melt. We also present whole-rock geochemical data and in situ major- and trace-element contents for Cpx phenocrysts from these mafic dikes. These dikes are characterized by low SiO2 contents, high MgO contents, enrichment of large-ion lithophile elements and light rare earth elements, depletion of high-field-strength elements, high (87Sr/86Sr)i ratios, and low εNd(t) values. These geochemical features imply an origin of enriched subcontinental lithospheric mantle. The calculated H2O contents of the primitive magmas range from 1.38 to 4.45 wt%, with associated H2O/Ce ratios varying from 237 to 322, suggesting that their mantle source was metasomatized by oceanic slab subduction–related fluids. In addition, covariations between H2O/Ce and incompatible elements indicate the addition of a dehydrated marine sediment component in the mantle source of the Jiaodong mafic dikes. In combination with the H2O contents of Early Cretaceous basaltic rocks in the southeastern North China Craton, the lithospheric mantle was metasomatized through fluids related to paleo–Pacific plate subduction.

118–115 Ma magmatism in the Tethyan Himalaya igneous province: Constraints on Early Cretaceous rifting of the northern margin of Greater India

Understanding the dynamics of Large Igneous Provinces (LIPs) is critical to deciphering processes associated with rupturing continental lithosphere. Microcontinental calving, the rifting of microcontinents from mature continental rifted margins, is particularly poorly understood. Here we present new insights into these processes from geochronological and geochemical analyses of igneous rocks from the Tethyan Himalaya. Early Cretaceous mafic dikes are widely exposed in the eastern and western Tethyan Himalaya, but no such rocks have been reported from the central Tethyan Himalaya. Here we present an analysis of petrological, geochronological, geochemical, and Sr–Nd–Hf–Os isotopic data for bimodal magmatic rocks from the center-east Tethyan Himalaya. Zircon U–Pb dating yields six weighted-mean concordant 206Pb/238U ages of 118±1.2 to 115±1.3 Ma. Mafic rocks display MORB-like compositions with flat to depleted LREE trends, and positive εNd(t) (+2.76 to +5.39) and εHf(t) (+8.0 to +11.9) values. The negative Nb anomalies and relatively high 187Os/188Os ratios (0.15–0.19) of these rocks are related to variable degrees (up to 10%) of crustal contamination. Geochemical characteristics indicate that mafic rocks were generated by variable degrees (2–20%) of partial melting of spinel lherzolites in shallow depleted mantle. Felsic rocks are enriched in Th and LREE, with negative Nb anomalies and decoupling of Nd (εNd(t) = −13.39 to −12.78) and Hf (εHf(t) = −4.8 to −2.0), suggesting that they were derived mainly from garnet-bearing lower continental crust. The geochemical characteristics of the bimodal magmatic associations are comparable to those of associations that form in a continental rift setting. Results indicate that Early Cretaceous magmatism occurred across the whole Tethyan Himalaya, named here as the “Tethyan Himalaya igneous province”. Separation of the Tethyan Himalaya from the Indian craton may have occurred during ongoing Early Cretaceous extension related to the Kerguelen mantle plume during the nascent stages of a global plate-reorganization event. If this is the case, our findings provide clues to the nature of the Tethyan Himalaya, challenging traditional view of the India–Asia single-stage collision model.

Petrogenesis and tectonic implications of ∼130 Ma diabase dikes in the western Tethyan Himalaya (western Tibet)

Early Cretaceous mafic dikes are widely exposed in the eastern Tethyan Himalaya. However, coeval mafic rocks are limited in the western Tethyan Himalaya. We report the major and trace elements and zircon U–Pb ages of the newly discovered diabase dikes in the Zhongba area in the western Tethyan Himalaya. The ICP-MS zircon U–Pb analyses from one representative diabase dike yields a crystallization age of 129.8±1.3Ma (MSWD=6.7). These mafic dikes are characterized by high concentrations of TiO2 (2.81–3.27%), Fe2O3T (10.79–12.14%), and MgO (5.11–6.71%). The chondrite-normalized diagram indicates the enrichment of LREE and the depletion of HREE. The primitive mantle-normalized trace element patterns do not display negative anomalies of Nb, Ta, and Ti. These characteristics imply an OIB affinity. Trace element modelling (Sm/Yb vs. Sm) suggests that the dikes originated from an enriched mantle source region, composed primarily of garnet and spinel (spinel<garnet), with a low degree of partial melting (4–8%). Comparing with coeval OIB-affinity mafic rocks from the areas of Gyangze, Kangma, Cona, and Laguila, we propose that the Cretaceous mafic rocks were generated in an extensional setting along a passive continental margin and were derived from similar magma sources with some crustal contamination in the case of the Zhongba intrusives. Because of the similar geochemical characteristics, ages and tectonic settings, of the Zhongba diabase rocks and other volcanic rocks from the Kerguelen LIP, we suggest that the Zhongba diabase rocks are likely also part of the Kerguelen LIP.

Rift brittle deformation of SE-Brazilian continental margin: Kinematic analysis of onshore structures relative to the transfer and accommodation zones of southern Campos Basin

High-resolution drone-supported mapping and traditional field work were used to refine the hierarchy and kinematics of rift-related faults in the basement rocks and Early Cretaceous mafic dikes onshore of the Campos Basin, SE-Brazil. Two sets of structures were identified. The most significant fault set is NE-SW oriented with predominantly normal displacement. At mesoscale, this fault set is arranged in a rhombic pattern, interpreted here as a breached relay ramp system. The rhombic pattern is a penetrative fabric from the thin-section to regional scale. The second-order set of structures is an E-W/ESE-WNW system of normal faults with sinistral component. These E-W structures are oriented parallel with regional intrabasinal transfer zones associated with the earliest stages of Campos Basin's rift system. The crosscutting relationship between the two fault sets and tholeiitic dikes implies that the NE-SW fault set is the older feature, but remained active until the final stages of rifting in this region as the second-order fault set is older than the tholeiitic dikes. Paleostresses estimated from fault slip inversion method indicated that extension was originally NW-SE, with formation of the E-W transfer, followed by ESE-WNW oblique opening associated with a relay ramp system and related accommodation zones.

Petrogenesis of Early Cretaceous mafic dikes in southeastern Jiaolai basin, Jiaodong Peninsula, China

ABSTRACTMafic dikes of mainly Early Cretaceous age (130–110 Ma) are widely developed on the Jiaodong Peninsula, China. Previous studies of the dikes, which have focused mainly on occurrences in the Jiaobei uplift and in the Sulu orogenic belt, have thoroughly examined their petrogenesis and geodynamic setting. This study identified four previously unknown mafic dikes (dolerite and lamprophyre) in southeastern Jiaolai basin (near Haiyang city), Jiaodong Peninsula. Detailed geochemical and geochronological analyses were conducted to determine the petrogenesis of the dikes and to infer their geodynamic setting. Zircon U–Pb dating by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) indicates that the dikes were emplaced at ~126 Ma. The dikes are characterized by low SiO2 contents (44.3–52.3 wt.%), high contents of MgO (7.28–10.13 wt.%), Cr (267–652 ppm), and Ni (93–335 ppm), and high Mg# values (63–73); they are enriched in large ion lithophile elements (LILEs; Ba, K, and Sr), depleted in high field strength elements (HFSEs; Nb, Ta, P, and Ti), and are characterized by high (87Sr/86Sr)i isotope ratios (0.707226–0.708222), low εNd(t) values (−12.3 to −13.6), and zircon εHf(t) values (−15.6 to −23.6). These features suggest that the dikes were derived from enriched subcontinent lithospheric mantle (SCLM). The wide range of Rb/Sr (0.04–0.18) and Ba/Rb (5–34) ratios, and the low and limited range of Dy/Yb (1.93–2.52) and K/Yb (5.71–11.99) ratios of the dikes indicate that the magmas originated from a low degree of partial melting of an amphibole- and phlogopite-bearing lherzolite in the spinel–garnet transition zone. The parental magma might have experienced fractionation of olivine and clinopyroxene during its ascent without significant crustal contamination. Evident depletion of Nb–Ta and Zr–Hf, low and limited range of Th/Yb ratios, elevated Ba/La ratios, constant chondritic Zr/Hf ratios, and a large range of Hf/Sm ratios further indicate that the mantle sources of the dikes were altered by carbonate-related metasomatism from subducted slab-derived fluids, which were most likely related to subduction of the Palaeo-Pacific plate during the Mesozoic. The mafic dikes in the southeastern Jiaolai basin resemble the arc-like mafic dikes in the Jiaobei terrain and the Sulu orogenic belt, and possibly indicate lithospheric thinning induced by slab rollback of the Palaeo-Pacific plate.

Contribution of subducted Pacific slab to Late Cretaceous mafic magmatism in Qingdao region, China: A petrological record

Abstract The occurrence of the Pishikou mafic dike in the Qingdao region, China provides important constraints on the origin of Late Cretaceous (86–78 Ma) mafic magmatism on the eastern North China craton. The Pishikou mafic dike is distributed in the Cretaceous Laoshan granitoid body, Qingdao region and contains peridotitic and granulitic xenoliths, xenocrysts, and megacrysts. Rocks from the Pishikou mafic dike are basanites and have low SiO2 (&lt; 42 wt%) and Al2O3 (12.5 wt%) contents, and high MgO (&gt; 8 wt%), total alkalis (Na2O + K2O &gt; 4.8 wt%, Na2O/K2O &gt; 1), TiO2 (&gt; 2.5 wt%), CaO (&gt; 9 wt%) and P2O5 (&gt; 1 wt%). In trace element abundances, they are highly enriched in large ion lithophile elements (LILEs) and light rare‐earth elements (LREEs) (ΣREE = 339–403 ppm, (La/Yb)N = 39–42) without high field strength element (HFSE) depletion. These rocks have radiogenic Sr and Pb, and less radiogenic Nd isotopic compositions [(87Sr/86Sr)i &gt; 0.7059, εNd ≈ 2.7–3.8 (206Pb/204Pb)i ≈ 18.0 ± 0.1]. The diagnostic elemental ratios, such as Nb/La, Nb/U, and Nb/Th, are compatible with those of mid‐oceanic ridge basalts (MORBs) and oceanic island basalts (OIBs). Therefore, the Pishikou mafic dike has a geochemical feature completely different from those of the Early Cretaceous mafic dikes from the Qingdao region, but similar to those of back‐arc basalts from the Japan Sea. This geochemical feature suggests that the Pishikou mafic dike was derived from an asthenosphere source, but contaminated by materials from the subducted Pacific slab. The discovery of this mafic dike thus provides a petrological evidence for the contribution of subducted Pacific slab to the Late Cretaceous magmatism in the Qingdao region of the eastern North China craton.

Geological and Geochemical Characteristics of Early Cretaceous Mafic Dikes in Northern Jiangxi Province, SE China and Their Geodynamic Implications

Abstract The development of Early Cretaceous mafic dikes in northern and southern Jiangxi allows an understanding of the geodynamic setting and characteristics of the mantle in southeast China in the Cretaceous. Geological and geochemical characteristics for the mafic dikes from the Wushan copper deposit and No. 640 uranium deposit are given in order to constrain the nature of source mantle, genesis and tectonic implications. According to the mineral composition, the mafic dikes in northern Jiangxi can be divided into spessartite and olive odinite types, which belong to slightly potassium-rich calc-alkaline lamprophyre characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), large depletion in high strength field elements (HSFE) and with negative Nb, Ta and Ti anomalies, as well as 87Sr/86Sr ratios varying from 0.7055 to 0.7095 and 143Nd/144Nd ratios varying from 0.5119 to 0.5122. All features indicate that the magma responsible for the mafic dikes was derived mainly from metasomatic lithosphere mantle related to dehydration and/or upper crust melting during subduction. Differences in geochemical characteristics between the mafic dikes in northern Jiangxi and the Dajishan area, southern Jiangxi were also studied and they are attributed to differences in regional lithospheric mantle components and/or magma emplacement depth. Combining geological and geochemical characteristics with regional geological history, we argue that southeast China was dominated by an extensional tectonic setting in the Early Cretaceous, and the nature of the mantle source area was related to enrichment induced by asthenosphere upwelling and infiltration of upper crust-derived fluids responding to Pacific Plate subduction.

Middle Jurassic to Early Cretaceous Igneous Rocks Along Eastern North American Continental Margin

Late Middle Jurassic and Early Cretaceous mafic dikes, sills, flows, and local volcaniclastic sediments are intercalated within continental shelf sediments from the Baltimore Canyon Trough northward to the Grand Banks of Newfoundland. The igneous rocks on the eastern North American margin are mainly alkali basalts of intraplate affinity. The late Middle Jurassic igneous activity was of short duration, at about 140 Ma, and was restricted to Georges Bank where it led to construction of several volcanic cones. The main period of igneous activity was concentrated at about 120 Ma in the Aptian/Barremian. The activity consists of dike swarms in Baltimore Canyon, occasional dikes on the Scotian Shelf, and the growth of stratovolcanoes on the Scotian Shelf and Grand Banks. Younge dikes (~95 Ma) also are present on the Grand Banks. The major phases of igneous activity are synchronous with major regional tectonic events, namely the initiation of rifting in the Labrador-Greenland region (140 Ma), separation of continental plates between Iberia and the Grand Banks (115 Ma), and separation of Labrador and Greenland continental plates (83-92 Ma). Most of the igneous activity occurred along old fracture zones that appear to have been reactivated and/or along fractures or fracture zones in the continental crust, resulting from changes in stress patterns caused by plate motion. The co-occurrence of Middle Jurassic and Early Cretaceous volcanic activity on the continental slope off Georges Bank is evidence that a hot-spot process alone cannot explain the linear character and age trends within the New England Seamounts. Propagation of a fracture zone into oceanic crust more satisfactorily accounts for the age and distribution of the volcanism. With regard to oil exploration on the continental margin, care must be taken to properly identify igneous and volcaniclastic rocks on mechanical logs, drill cuttings, and cores. Reflection seismic profiles can be used to map the areal extent of sills, flows, and low-angle dikes, which commonly show distinctive seismic responses. However, steeply dipping dikes generally produce little, if any, seismic response. Isotopic-age determinations of igneous rocks, combined with biostratigraphic-age determinations of adjacent strata, are invaluable for stratigraphic correlation, establishing chronology of seismic sequences, and analysis of basin sedimentation and tectonic history.