Graphitized diamonds from a peridotite massif in Morocco and implications for anomalous diamond occurrences

Pearson, D.G.; Davies, G.R.; Nixon, P.H.; Milledge, H. J.

doi:10.1038/338060a0

Cited by 122 publications

(66 citation statements)

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“…Ronda and Beni Bousera peridotites, Loomis 1975;Pearson et al 1989). For the Ronda and Beni Bousera peridotites, graphite pseudomorphs after diamond indicate depths >125 km (Pearson et al 1989;Tabit et al 1990). The initial stages of exhumation are attributed to Mesozoic rifting (Vissers et al 1995).…”

Section: Continental Collision Zonesmentioning

confidence: 98%

Exhumation processes

et al. 1999

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Deep-seated metamorphic rocks are commonly found in the interior of many divergent and convergent orogens. Plate tectonics can account for high-pressure metamorphism by subduction and crustal thickening, but the return of these metamorphosed crustal rocks back to the surface is a more complicated problem. In particular, we seek to know how various processes, such as normal faulting, ductile thinning, and erosion, contribute to the exhumation of metamorphic rocks, and what evidence can be used to distinguish between these different exhumation processes.In this paper, we provide a selective overview of the issues associated with the exhumation problem. We start with a discussion of the terms exhumation, denudation and erosion, and follow with a summary of relevant tectonic parameters. Then, we review the characteristics of exhumation in different tectonic settings. For instance, continental rifts, such as the severely extended Basin-and-Range province, appear to exhume only middle and upper crustal rocks, whereas continental collision zones expose rocks from 125 km and greater. Mantle rocks are locally exhumed in oceanic rifts and transform zones, probably due to the relatively thin crust associated with oceanic lithosphere.Another topic is the use of P-T-t data to distinguish between different exhumation processes. We conclude that this approach is generally not very diagnostic since erosion and normal faulting show the same range of exhumation rates, reaching maximum rates of >5-10 km Ma -1 for both processes. In contrast, ductile thinning appears to operate at significantly slower rates. The pattern of cooling ages can be used to distinguish between different exhumation processes. Normal faulting generally shows an asymmetric distribution of cooling ages, with an abrupt discontinuity at the causative fault, whereas erosional exhumation is typically characterized by a smoothly varying cooling-age pattern with few to no structural breaks. Last, we consider the challenging problem of ultrahigh-pressure crustal rocks, which indicate metamorphism at depths greater than 100-125 kin. Understanding the exhumation of these rocks requires that we first know where and how they were formed. One explanation is that metamorphism occurred within a thickened crustal root, but it does seem unlikely that the crust, including an eclogitized mafic lower crust, could get much thicker than c. 110 km while maintaining a reasonable Moho depth (<70 km, assuming that the seismically defined Moho would be observed to lie above the eclogitized lower crust). Diamondbearing crustal rocks cannot be explained by this scheme. The alternative is to accrete the upper 10-40 km of lithospheric mantle into the orogenic root. This scenario will provide sufficient pressures for both coesite-and diamond-bearing eclogite-facies metamorphism, while maintaining a reasonable Moho depth (<70 km) and reasonable mean topography (<3 kin). We speculate that the detachment and foundering of the mantle root may contribute to the exhumation of any crust...

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Section: Continental Collision Zonesmentioning

confidence: 98%

Exhumation processes

et al. 1999

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“…It is probable that the aluminous mafic rocks experienced higher-pressure conditions than high-pressure granulite to eclogite facies expected from petrographic characteristics because graphitized diamond-bearing garnet pyroxenites indicating ultra-high-pressure conditions were reported from both the Ronda and Beni Bousera massifs (Slodkevich, 1983;Pearson et al, 1989;Davies et al, 1993). Note that these diamond-bearing rocks have distinct mineralogical and geochemical signatures from the aluminous mafic rocks (Morishita et al, 2001).…”

Section: Sample Descriptionsmentioning

confidence: 97%

“…Graphitized-diamond-bearing garnet pyroxenites were reported from both massifs (Slodkevich, 1983;Pearson et al, 1989;Davies et al, 1993). The diamond-bearing garnet pyroxenites have distinct differences from the Type II mafic rocks, that is the aluminous mafic rocks (Morishita et al, 2001).…”

Section: Geological Backgroundmentioning

confidence: 99%

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Constraints on the evolutionary history of aluminous mafic rocks in the Ronda peridotite massif (Spain) from trace-element compositions of clinopyroxene and garnet

et al. 2009

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Multiple trace-element compositions of clinopyroxene and garnet in aluminous mafic layers (±corundum and/or sapphirine) alternating with peridotites from the Ronda massif, Spain, were determined by an ArF 193 nm excimer laser system coupled with a quadrupole ICP-MS. The studied aluminous mafic rock is divided into several sublayers based on the modal mineral variations. Trace-element compositions of minerals vary between sublayers. Primitive mantle-normalized trace-element distribution patterns have more significant positive Eu anomaly for both garnet and clinopyroxene in the corundum-bearing sublayers than for those in the other sublayers whereas corundum-bearing sublayers contain more abundant secondary plagioclase than corundum-free sublayers. These geochemical signatures of the corundum-bearing sublayers intercalated with corundum-free sublayers were acquired as plagioclase-rich portions of gabbroic layered sequence originated at the lower crustal to the uppermost mantle sections rather than selective accumulation of metamorphic plagioclase due to deformation during the late stage of exhumation of the massif. Partition coefficients of trace elements between clinopyroxene and garnet show systematic variations with respect to major element composition, indicating local attainment of chemical equilibrium. The trace element compositions of clinopyroxene and garnet were equilibrated with secondary metamorphic plagioclase, i.e., at granulite-facies conditions, because plagioclase is required for the geochemical balance in Sr and Eu contents of the whole-rock trace element compositions. Positive Eu anomaly of clinopyroxene (±garnet) is also found in mantle-derived aluminous eclogitic xenoliths and eclogitic mineral inclusions in diamond captured in kimberlites, and is thus a useful indicator to explore recycled crustal components in the mantle.

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“…Based on these age constraints, Blichert-Toft et al (1999) estimated a fast decompression rate of the rising peridotite, on the order of 10 cm year −1 , consistent with a nearly adiabatic ascent. Moreover, the clinopyroxene-garnet assemblage locally contains graphitized diamonds (Pearson et al 1989;Pearson and Nixon 1996;Ruiz Cruz et al 2011), thus indicating high temperatures in the range of 1200-1350°C and crystallization pressures of at least 20 Kb (Kornprobst et al 1990), within the diamond stability field. It is noteworthy that the graphitized diamonds may represent 10 to as much as 15 vol% of the host rocks and reach up to 20 mm in size (Pearson et al 1989;Pearson and Nixon 1996;Ruiz Cruz et al 2011).…”

Section: Introductionmentioning

confidence: 98%

Geologic and Metallogenic Framework of North Africa

Bouabdellah

Slack

2016

Mineral Deposits of North Africa

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North Africa is noteworthy for having a great diversity of geology and mineral resources. Geologically, the region contains numerous terranes that contain rocks ranging in age from Archean to Quaternary, including diverse igneous, sedimentary, and metamorphic lithologies. Major geological features were built during the main phases of Proterozoic and subsequent Phanerozoic orogenies, the break-up of the supercontinents Rodinia, Gondwana, and Pangea, and during the opening and closing of major ocean basins such as the Iapatus, Atlantic, Pacific, and Tethys. Three major tectono-stratigraphic domains are recognized (1) the Precambrian province that includes the African-Nubian Shield (ANS) to the east and the West African Craton (WAC) to the west, and the intervening Tuareg Shield and "Saharan Metacraton"; (2) the Variscan (Paleozoic) fold belt; and (3) the Atlas-Alpine (Mesozoic-Cenozoic) system. Mineral deposits of North Africa formed in a variety of geologic settings at different time periods from Archean to Quaternary. Mineral commodities including gold, silver, cobalt, nickel, chromium, arsenic, copper, lead, zinc, iron, and many other elements. Major deposit types present in the region are (1) orthomagmatic Cr-Ni-platinum group elements (PGE); (2) rare-metal granites and related rare-element pegmatites; (3) volcanic-hosted massive sulphide (VHMS); (4) sedimentaryexhalative (SEDEX); (5) orogenic and intrusion-related gold; (6) iron oxide-copper-gold (IOCG); (7) banded iron formation (BIF); (8) Mississippi Valley-type (MVT) lead-zinc; (9) sediment-hosted stratiform copper;Electronic supplementary material The online version of this chapter

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Graphitized diamonds from a peridotite massif in Morocco and implications for anomalous diamond occurrences

Cited by 122 publications

References 8 publications

Exhumation processes

Exhumation processes

Constraints on the evolutionary history of aluminous mafic rocks in the Ronda peridotite massif (Spain) from trace-element compositions of clinopyroxene and garnet

Geologic and Metallogenic Framework of North Africa

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