Gabbro Discovery in Discovery Deep: First Plutonic Rock Samples From the Red Sea Rift Axis

Plutonic rocks such as gabbros provide information on magmatic and tectonic processes which occur beneath a mid-ocean rift axis as well as on the formation of the oceanic crust. Igneous rocks, reported from the Red Sea Rift valley, have been limited to extrusive basalts so far. The only deeper crustal rocks found in the Red Sea area are from the rift flanks and are interpreted as late-stage continental rift magmatism. Here, we present the geochemistry of the first recovered gabbro fragments from the axis of the Red Sea Rift, sampled from a crater structure within the brine-filled Discovery Deep at the axis of the Red Sea Rift. Petrology and geochemistry show characteristics of a typical mid-ocean ridge gabbro formed at shallow crystallization depth. Clinopyroxene core mineral data fall within two groups, thus pointing to a multiphased magmatic history, including different magma batches and a joint late-stage fractional crystallization. Geobarometry, based on clinopyroxene cores, suggests lower crystallization pressures than similar geobarometric data reported for gabbroic samples from Zabargad (8–9 kbar) and Brother’s Islands (2.5–3.5 kbar) at the rift flanks. However, based on the evolved whole rock composition, its multiphase history, the thickness of the crust, the current location of the samples, and the uncertainties in the barometer, geobarometric estimates for the samples are likely overestimated. Instead, we propose that these rock fragments originate from the upper part of a fully developed oceanic crust in the central Red Sea Rift. High-resolution bathymetry and sparker seismic data reveal that the Discovery Deep is characterized by a significant normal fault and a strong reflector near the rift axis, which we interpret as a potential sill intrusion in an approximate depth of 400 m. Based on the lack of progressive alteration and the sampling location within a sediment-free crater structure, we interpret that the emplacement of the gabbros has to be geologically recent. We interpret the gabbro either as a xenolith transported by the eruptive volcanism that formed the crater, potentially related to the sill intrusion visible at depth, or as intrusive gabbro, which was uplifted and deposited in a talus fan by the adjacent normal fault, exposed by the formation of the volcanic crater.

Sill intrusion into pyroxenitic mush and the development of the Lower-Upper Critical Zone boundary of the Bushveld Complex: Implications for the origin of stratiform anorthosites and chromitites in layered intrusions

Layered mafic-ultramafic intrusions are the fossilized remnants of magmatic plumbing systems and provide excellent natural laboratories to investigate the processes of magma differentiation and solidification. The Rustenburg Layered Suite is the plutonic mafic-ultramafic part of the Bushveld Complex of South Africa and it has traditionally been assumed to have formed from an upwardly-aggrading (and in-sequence) crystal pile in a melt-dominated chamber. In this study, we present field and petrological observations, complemented with detailed plagioclase mineral chemistry (molar An, LREE and strontium isotopes) for the first stratiform anorthosite layer (MG3F anorthosite) at the Lower-Upper Critical Zone boundary (LCZ-UCZ) in the eastern limb of the Bushveld Complex. We use these data to test the overarching paradigm of a melt-dominated chamber for the magmatic evolution of the Rustenburg Layered Suite. The MG3F anorthosite is immediately overlain by the MG3 chromitite and both are surrounded by pyroxenite. A distinctive ‘egg-box’ structure, consisting of round pyroxenite blocks mantled by chromitite, marks the LCZ-UCZ boundary, and represents an erosional disconformity at the base of the MG3F anorthosite. The MG3F anorthosite is laterally continuous for 100s km in the eastern limb. In the northern-central sector of the eastern limb, the 1.5 m thick MG3F anorthosite is characterized by non-cotectic proportions of foliated plagioclase and chromite chains that lie parallel to the foliation. The MG3F anorthosite is divisible into two sub-layers on the basis of (i) a compositional break in plagioclase molar An, LREE and strontium isotope composition and, (ii) a peak in chromite mode (up to 12 vol%). In the lower half of the layer plagioclase LREE concentrations increase upward, molar An shows a marginal decrease upward and strontium isotopes are relatively homogeneous (87Sr/86Sr2.06Ga 0.7056-0.7057). In the upper half of the layer, plagioclase LREE concentrations decrease upward, molar An shows a marginal increase upward and strontium isotopes show strong inter- and intra-grain variability (87Sr/86Sr2.06Ga 0.7053-0.7064). Strontium isotopes in interstitial plagioclase in the immediate footwall and hangingwall pyroxenites show similar 87Sr/86Sr2.06Ga values to the MG3F anorthosite and decrease with distance from the MG3F anorthosite. In the southern sector of the eastern limb, the 4 m thick MG3F anorthosite exhibits identical stratigraphic compositional trends in terms of molar An in plagioclase. We infer that the MG3F anorthosite formed by two successive sill-like injections of magma into a resident viscoplastic pyroxenitic crystal mush. An initial pulse of plagioclase-saturated melt underwent in situ fractional crystallization, manifested as upwardly decreasing molar An and upwardly increasing LREE in plagioclase in the lower half of the MG3F anorthosite. Sill intrusion caused deformation of the viscoplastic pyroxenite mush and vortices of superheated liquid generated by frictional viscous heating caused disaggregation of the footwall pyroxenitic mush. Disaggregated blocks of pyroxenitic mush reacted with the superheated liquid (a hybrid chromite-saturated melt) to produce chromite-rich rims at the base of the MG3F anorthosite (egg-box structure). A second sill-like injection of magma then entered the chamber that halted in situ crystallization. This sill was a plagioclase slurry that contained isotopically distinct plagioclase laths compared to those present in the previous sill. The upward increase in molar An of plagioclase, and decreasing LREE, may be explained by the slurry becoming more primitive in melt composition with time. The second sill also caused mush disaggregation and renewed the production of a hybrid chromite-saturated melt. Chromite crystals were then mobilized and injected as slurries at the interface between the sill and resident mush towards the back of the flow, culminating in the development of the MG3 chromitite. Our model for the development of the Lower-Upper Critical Zone boundary questions the existence of a melt-dominated chamber and it has implications for the origin of stratiform anorthosites (and chromitites) in crustal magma chambers.

Whin Sill contact metamorphism in the Cow Green reservoir boreholes, Northern England: evidence for an Upper Teesdale source for the Whin Sill magma

Contact metamorphism of Carboniferous rocks by the Whin Sill in Upper Teesdale is documented utilizing boreholes drilled in the 1960s ground investigation for the Cow Green reservoir. Hedenbergite, prehnite and datolite are recorded for the first time, and the first analyses for seven contact minerals are presented. Pure limestones are recrystallized into saccharoidal marbles with average calcite grain sizes increasing from <0.1 mm at >20 m from the contact, up to 0.5 mm within 5 m from the contact. Grossular is the most abundant mineral, and along with epidote is recorded over 20 m from the sill contact; all other minerals are restricted to < c. 10 m from the contact. This substantial contact metamorphism around Cow Green is unique in comparison with other boreholes across the Alston Block that have penetrated even thicker leaves of the sill, and for which no contact metamorphic mineral development has been reported. The currently favoured model has the magma sourced from dykes on the southern and northern borders of the Alston Block, but the evidence in support is circumstantial. The contrast in the metamorphic development is definitive evidence that the heat flow linked to the Whin Sill intrusion in the Upper Teesdale area lasted much longer than other areas across the Block, giving strong support to the magma being sourced in this area, rather than from the dykes.Supplementary material: X-ray fluorescence whole rock Sr (ppm) analyses of limestone beds in boreholes 17, 18, 21, 22, 35, 39, 40 and 41 in Table S1, and Electron MicroProbe Analysis of garnet, feldspar, epidote, idocrase, prehnite, pyroxene and chlorite in Table S2 are available at: https://doi.org/10.6084/m9.figshare.c.5077640

Magma-driven accommodation structures formed during sill emplacement at shallow crustal depths: The Maiden Creek sill, Henry Mountains, Utah

In areas of exceptional exposure, upper-crustal intrusions and their immediate wall rocks commonly preserve direct evidence of the emplacement, magma flow pathways, and strains associated with the intrusion process. Such excellent exposure is displayed by the Paleogene Maiden Creek intrusion—a small satellite body related to the Mount Hillers intrusive complex, Henry Mountains, Utah. An intermediate plagioclase-hornblende porphyritic magma was intruded into the Entrada Sandstone Formation at an estimated depth of ∼3 km. The southern part of the intrusion is overlain by the newly identified Maiden Creek shear zone (MCSZ): a subhorizontal, top-to-the-WNW detachment formed at the contact with the overlying sandstone country rocks. From observations of both syn-emplacement deformation and the exposed intrusion geometries, it is proposed that the southern Maiden Creek intrusion comprises westerly derived, inclined sill sheets. Host-rock sandstones were sandwiched (∼E–W constriction) between these intrusive bodies beneath the MCSZ. It is proposed that the MCSZ is a syn-emplacement magma-driven accommodation structure, with a shear sense antithetic to the magma flow direction, which played a critical role in accommodating the westerly derived sill intrusion. Our results show that inelastic syn-emplacement deformation structures, such as the MCSZ, are very important in the accommodation of magma in the subsurface. Such small structures are unlikely to be imaged by seismic-reflection surveys, highlighting the importance of detailed field studies in our understanding of intrusion geometry and emplacement mechanisms.