Crystallization and differentiation in abyssal tholeiites and gabbros from mid-oceanic ridges
Abstract
Tholeiitic gabbros from the Mid-Atlantic Ridge near 24°N were found to show remarkable differentiation, producing high-iron, high-titanium gabbros and aplite in later stages. Crystallization of olivine and plagioclase from abyssal tholeiite magma approximately follows cotectic relation in the system olivine-plagioclase-pyroxene.
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Cited by (111)
Gerf Neoproterozoic ophiolitic rocks in the Southern Eastern Desert of Egypt represent the largest ophiolite nappe in the Arabian-Nubian Shield and have been preserved as part of the N-S striking Allaqi-Heiani suture zone. Landsat-8 OLI/TIRS, ASTER and Sentinel-1B data successfully discriminate the Gerf ophiolitic section and the structural framework of the study area. This study has applied spectral transform approaches, consisting of principal component analysis (PCA), band ratio (BR) and minimum noise fraction (MNF) for lithological and structural mapping. NW-SE and N-S structural trends are dominant and control the distribution of talc‑carbonates and ophicarbonates. The mantle section comprises dominantly harzburgites with subordinate dunites and is rarely cut by dike-like bodies of clinopyroxenites; these peridotites have been partially to completely converted to serpentinites and related rocks. The primary Gerf peridotites are low in TiO2 (0.01 wt%), Al2O3 (0.50 wt%), and CaO (0.44 wt%) content on average, but are rich in Ni (up to 2758 ppm) and Cr (2906 ppm) relative to primitive mantle, suggesting their highly refractory residual nature after high degrees of partial melting, similar to forearc peridotites. This is confirmed by chemistry of their relic primary minerals, namely olivine (Fo: 91–93.7; NiO: 0.28–0.43 wt%), chromian spinel (Cr# 0.75 on average), orthopyroxene (Mg#: 0.92–0.93) and clinopyroxene (Mg#: 0.89–0.90). Gerf serpentinized peridotites also show ranges of oxygen fugacity (Δlog ƒO2, FMQ + 0.2 – FMQ + 1.4) and equilibrium temperature (770–900 °C), consistent with those of forearc peridotites. Bulk-rock analyses of ultramafic rocks and in-situ analyses of their pyroxenes reveal enrichment of fluid mobile elements (FME: e.g., B, Cs, Pb, Sr) relative to high-field strength elements (e.g., Nb, Zr, Ti, Ta), indicating intense metasomatism of Gerf peridotites by slab-derived fluids. The Gerf peridotites have been subjected to intense CO2 input from the subducted slab to form carbonate-rich rocks beneath the arc-forearc region. Carbonates (mainly magnesite) replace serpentine minerals and their formation synchronizes with the transition from lizardite to antigorite at temperatures, ~250° to ~350 °C. The CO2-rich fluids increase the Au content because of alteration and break down of Au-bearing sulphides, and this process formed Au mineralization in carbonate-rich rocks. The maximum amount of CO2 expulsion from the subducted slab increases with increasing mantle depth, and structural trends as ophicarbonates are abundant in thick parts of the ophiolite sequence in the Gerf area that was highly dissected by NW-SE, N-S and E-W striking faults. This confirms that structures control on distribution of the ophiocarbonate rocks in Gerf ophiolite. Calculated parental melts in equilibrium with Gerf peridotite spinels have boninitic affinities, suggesting their generation during the forearc stage, but parental melts of Gerf clinopyroxenite veins resemble N-MORB-like melts, indicating melt metasomatism of sub-arc mantle by impregnated mafic melts during early subduction initiation. Clinopyroxenite veins crystallized from MOR-like melts as a result of interaction between these melts and depleted peridotites in the sub-arc mantle.
The evolution of Eastern Tornquist-Paleoasian Ocean and subsequent continental collisions: A case study from the Western Tatra Mountains, Central Western Carpathians (Poland)
2017, Gondwana ResearchThe crystalline basement of the Tatra Mountains in the Central Western Carpathians, forms part of the European Variscides and contains fragments of Gondwanan provenance. Metabasite rocks of MORB affinity in the Tatra Mountains are represented by two suites of amphibolites present in two metamorphic units (the Ornak and Goryczkowa Units) intercalated with metapelitic rocks. They are interpreted as relics of ocean crust, with zircon δ18OVSMOW values of 4.97–6.96‰. Zircon REE patterns suggest oxidizing to strongly oxidizing conditions in the parent mantle-derived basaltic magma. LA-MC-ICP-MS U-Pb dating of magmatic zircon cores yields a crystallization age of c. 560 Ma, with inherited components at c. 600 Ma, corresponding to the Pannotia break-up event and to the formation of the Eastern Tornquist–Paleoasian Ocean.
However, the zircon rims of both suites yield evidence for two different geological histories. Zircon rims from the Ornak amphibolites record two overgrowth phases. The older rims, dated at 387 ± 8 Ma are interpreted as the result of an early stage of Variscan uplift while the younger rims dated at 342 ± 9 Ma are attributed to late Variscan collisional processes. They are characterized by high δ18OVSMOW values of 7.34–9.54‰ and are associated with migmatization related to the closure of the Rheic Ocean.
Zircon rims from the Goryczkowa amphibolites yield evidence of metamorphism at 512 ± 5 Ma, subsequent Caledonian metamorphism at 447 ± 14 Ma, followed by two stages of Variscan metamorphism at 372 ± 12 Ma and 339 ± 7 Ma, the latter marking the final closure of the Rheic Ocean during late-Variscan collision.
The presented data are the first direct dating of ocean crust formation in the eastern prolongation of the Tornquist Ocean, which formed a probable link to the Paleoasian Ocean.
The Lower Oceanic Crust
2013, Treatise on Geochemistry: Second EditionThe lower oceanic crust forms when melts that are generated within the mantle beneath mid-ocean ridges cool and crystallize at depth below the seafloor. The compositions of mid-ocean ridge basalts (MORBs), such as their low Mg# (Mg/(Mg + Fe2 +)), show that they have been significantly modified after separation from the mantle by partial crystallization within the lower oceanic crust.
There are currently only a handful of locations in the modern ocean basins where significant sampling of the lower oceanic crust has occurred. Trace element and isotopic data from these oceanic plutonic rocks suggest that the parental melts were relatively homogeneous. In turn, this suggests either that melt extraction from the mantle efficiently mixes melts generated under different conditions or that crustal level homogenization is very efficient. Several lines of evidence, including disequilibrium between the crystals and host basalt in many MORBs, suggest that magma mixing is an important process within the lower oceanic crust. The plutonic rocks record evidence for extensive melt–rock reaction within crystal mush zones that fractionates incompatible trace elements from one another more than predicted by fractional crystallization. The bulk composition of the lower oceanic crust and, hence, of Moho-crossing melts is poorly constrained due to inadequate sampling. However, there is evidence for depletion of incompatible elements within the lower crust compared to the overlying upper crust in all areas studied, with the extent of depletion proportional to the element's partition coefficient.
Spreading rate plays an important role in controlling lower crustal processes through its impact on the thermal structure near the ridge axis. At fast-spreading ridges, the flux of magma (and hence specific and latent heat) into the crust is large; however, geophysical surveys show that there is only a small melt sill at the base of the sheeted dike complex; this sill is underlain by a larger mush zone. The lack of a large molten region requires efficient hydrothermal heat extraction from the lower crust. Several lines of evidence suggest that much of the latent heat of crystallization is extracted from the lower crust through the roof of the small sill, with crystals subsiding down and out from this body to form the lower crust. Residual interstitial melt is largely compacted out of the crystal mush zone. At slow-spreading ridges, the magma flux into the crust can be an order of magnitude lower than at fast-spreading ridges and steady-state magma chambers do not form. Instead, lower crustal magma chambers are transient features in which crystallization leads to a wide range of cumulate compositions forming during the final stages of solidification.
Several characteristics of oceanic gabbros and MORB differentiation trends at slow-spreading ridges have been interpreted as requiring crystallization at elevated pressure. However, there are alternative explanations for all of the data, and it is unlikely that substantial amounts (>10%) of crystallization occurs at high pressure (>0.3 GPa). Despite the limited heat supply at slow-spreading ridges, extensive crystallization at >0.3 GPa would require efficient hydrothermal heat extraction from this depth for which there is little evidence.
High Fe-Ti mafic magmatism and tectonic setting of the Paleoproterozoic Broken Hill Block, NSW, Australia
2007, Precambrian ResearchWe present petrographic, geochemical (major, traces and rare earth elements (REE)) and isotopic (Sm–Nd and Rb–Sr) data from ca. 1685 Ma mafic rocks of the Willyama Supergroup in the Broken Hill Block of western NSW, Australia. The mafic rocks occur throughout the lower Willyama Supergroup stratigraphy and are interpreted here as shallowly emplaced sills that were metamorphosed to upper amphibolite and granulite facies during the Olarian Orogeny (ca. 1600–1580 Ma). Our data indicate that the metabasites originated as a result of variable degrees of partial melting of a depleted mantle source, but slightly enriched with incompatible elements compared to present day N-MORB. This was followed by simple crystal fractionation or by an assimilation-fractional crystallisation (AFC) process involving only small degrees of crustal assimilation (rate of assimilation to rate of crystal fractionation, r = 0.05–0.2). Crystal fractionation proceeded along a tholeiitic trend of extreme primary iron and titanium enrichment, leading to melts with up to 25 wt% of total iron as Fe2O3 and 4.2 wt% of TiO2. Volumetrically minor intermediate rocks evolved from this fractionation, but the bulk of the contemporary felsic magmatic rocks (Rasp Ridge and Hores/Potosi Gneisses) are not linked by fractional crystallisation to the mafic melt that produced the meta-igneous amphibolites and are products of the anatexis of crustal material from the Willyama sedimentary pile. Based on the occurrence of bimodal magmatism, a depleted mantle source, partial melting modelling and minimal crustal contamination of the mafic rocks, we infer that the Broken Hill Block (ca. 1685 Ma) was the extensional axis and depositional centre of an advanced stage intra-cratonic rift with relatively thin crust and lithosphere. Data from the neighbouring Olary Domain, in contrast, imply smaller degrees of partial melting, higher degrees of crustal contamination and the presence of a subcontinental lithospheric mantle source, which suggests relatively thicker lithosphere and places the Olary Domain on the rift margin. Active faulting during the rift stage, coupled with submarine sedimentation and an anomalous geothermal gradient driven by lithospheric thinning, provide an ideal theoretical environment for the formation of the Broken Hill Pb–Zn–Ag orebody.
The Lower Oceanic Crust
2007, Treatise on GeochemistryOrigin of eclogitic metagabbro mass in the Sambagawa belt: Geological and geochemical constraints
2006, LithosEclogite-bearing metagabbro masses in the high-P/T Sambagawa metamorphic belt in SW Japan have long been regarded as tectonic blocks derived from the mantle wedge or hanging-wall side of the former Sambagawa subduction zone. The origin of one of the metagabbro masses (the Seba metagabbro) is reexamined by: (i) geological (chemico-structural) study focusing on the relationship between this body and the surrounding metasedimentary schists; and (ii) comparative major- and trace-element bulk-chemistry with other gabbros/metagabbros from the world. The geological studies reveal the presence of a marginal shear zone that: (i) is located between the enveloping pelitic schist and the central metagabbro; (ii) has a bulk chemical composition intermediate between the two; and (iii) shows up to 10 cm-scale prominent mafic–felsic banding. Based on the difficulty of forming such marginal shear zones by a tectonic block hypotheses, and key field observations in neighboring regions indicating formation of the marginal shear zone as sediments, the Seba metagabbro is concluded to have originated on the ocean floor as an olistolith. Comparative bulk-chemical studies show that Sr content of the Seba metagabbro is too high for gabbros derived from fracture zones associated with a spreading ridge. A comparison with other eclogitic metagabbro masses in the Sanbagawa belt reveals numerous similarities with the Seba metagabbro. This suggests all the metagabbro masses can be regarded as members of an ‘olistostrome complex’ that subducted and underwent the eclogite-facies metamorphism. The eclogitic complex includes the Western Iratsu mass that is considered to be an ancient seamount. This suggests that the metagabbro masses represent fragments of the ancient Western Iratsu seamount.
Lamont-Doherty Geological Observatory Contribution No. 1423.