Evangelos Moulas

The problem of depth in geology: When pressure does not translate into depth

We review published evidence that rocks can develop, sustain and record significant pressure deviations from lithostatic values. Spectroscopic studies at room pressure and temperature (P-T) reveal that in situ pressure variations in minerals can reach GPa levels. Rise of confined pressure leads to higher amplitude of these variations documented by the preservation of α-quartz incipiently amorphized under pressure (IAUP quartz), which requires over 12 GPa pressure variations at the grain scale. Formation of coesite in rock-deformation experiments at lower than expected confined pressures confirmed the presence of GPa-level pressure variations at elevated temperatures and pressures within deforming and reacting multi-mineral and polycrystalline rock samples. Whiteschists containing garnet porphyroblasts formed during prograde metamorphism that host quartz inclusions in their cores and coesite inclusions in their rims imply preservation of large differences in pressure at elevated pressure and temperature. Formation and preservation of coherent cryptoperthite exsolution lamellae in natural alkali feldspar provides direct evidence for grain-scale, GPa-level stress variations at 680°C at geologic time scales from peak to ambient P-T conditions. Similarly, but in a more indirect way, the universally accepted’ pressure-vessel’ model to explain preservation of coesite, diamond and other ultra-high-pressure indicators requires GPa-level pressure differences between the inclusion and the host during decompression at temperatures sufficiently high for these minerals to transform into their lower pressure polymorphs even at laboratory time scales. A variety of mechanisms can explain the formation and preservation of pressure variations at various length scales. These mechanisms may double the pressure value compared to the lithostatic in compressional settings, and pressures up to two times the lithostatic value were estimated under special mechanical conditions. We conclude, based on these considerations, that geodynamic scenarios involving very deep subduction processes with subsequent very rapid exhumation from a great depth must be viewed with due caution when one seeks to explain the presence of microscopic ultrahigh-pressure mineralogical indicators in rocks. Non-lithostatic interpretation of high-pressure indicators may potentially resolve long-lasting geological conundrums.

P-T estimates and timing of the sapphirine-bearing metamorphic overprint in kyanite eclogites from Central Rhodope, northern Greece

Sapphirine-bearing symplectites that replace kyanite in eclogites from the Greek Rhodope Massif have previously been attributed to a high-pressure granulite-facies metamorphic event that overprinted the eclogitic peak metamorphic assemblage. The eclogitic mineralogy consisted of garnet, omphacitic pyroxene, rutile and kyanite and is largely replaced by low-pressure minerals. Omphacite was initially replaced by symplectites of diopside and plagioclase that were subsequently replaced by symplectites of amphibole and plagioclase. Garnet reacted during decompression to form a corona of plagioclase, amphibole and magnetite. Rutile was partly transformed to ilmenite and kyanite decomposed to produce a high-variance mineral assemblage of symplectitic spinel, sapphirine, plagioclase and corundum. The presence of quartz and corundum in the kyanite eclogites is evidence for the absence of bulk equilibrium and obviates a conventional analysis of phase equilibria based on the bulk-rock composition. To circumvent this difficulty we systematically explored the pressure-temperature-composition (P-T-X) space of a thermodynamic model for the symplectites in order to establish the pressure-temperature (P-T) conditions at which the symplectites were formed after kyanite. This analysis combined with conventional thermometry indicates that the symplectites were formed at amphibolite-facies conditions. The resulting upper-pressure limit (∼0.7 GPa) of the sapphirine-producing metamorphic overprint is roughly half the former estimate for the lower pressure limit of the symplectite forming metamorphic event. Temperature was constrained (T ∼ 720°C) using garnet-amphibole mineral thermometry. The P-T conditions inferred here are consistent with thermobarometry from other lithologies in the Rhodope Massif, which show no evidence of granulite-facies metamorphism. Regional geological arguments and ion-probe (SHRIMP) zircon dating place the post-eclogite-facies metamorphic evolution in Eocene times.

From Mesoproterozoic magmatism to collisional Cretaceous anatexis: Tectonomagmatic history of the Pelagonian Zone, Greece

The magmatic history of the Pelagonian Zone, in northern Greece, is constrained with secondary ion mass spectrometer (SIMS) U-Pb dating on zircons of various granitoids whose structural positions were defined with respect to the regional main foliation. Ages pertain to four groups: (i) Mesoproterozoic (circa 1430u2009Ma) crystallization of granites inferred from inherited magmatic zircon cores that have been partially molten during the (ii) Neoproterozoic at circa 685u2009Ma (metamorphic zircon rims) and subsequently intruded by a Neoproterozoic leucogranite (circa 600u2009Ma). (iii) Late- or post-Variscan calc-alkaline granitoids (315–301u2009Ma) were in turn intruded by a subvolcanic dike at about 280u2009Ma. In the Early Permian the eNd(t) in magmas decreased from −7.3 to −1.3, hinting to mantle-derived melts produced during extension. Rifting is further heralded by two acidic and one mafic dike containing Lower-Middle Triassic zircons (246–242u2009Ma). (iv) Early Cretaceous anatectic melts at 117u2009±u20098u2009Ma formed during regional metamorphism. This age is the first report of in situ anatexis in the Pelagonian Zone. Cretaceous anatexis developed during the Mesozoic collision of Pelagonia with the Eurasian margin. Major- and trace-element geochemistry of amphibolites further attests for the complex pre-Alpine tectonic history with Neoproterozoic calc-alkaline and back-arc geochemical signature and Triassic alkali-magmatism.

Formation and preservation of fresh lawsonite: Geothermobarometry of the North Makran Blueschists, southeast Iran

A low-grade metamorphic “Coloured Melange” in North Makran (SE Iran) contains lenses and a large klippe of low temperature, lawsonite-bearing blueschists formed during the Cretaceous closure of the Tethys Ocean. The largest blueschist outcrop is a >1000 m thick coherent unit with metagabbros overlain by interlayered metabasalts and metavolcanoclastic rocks. Blueschist metamorphism is only incipient in coarse grained rocks whereas finer grained, foliated samples show thorough metamorphic recrystallization. The low variance blueschist peak assemblage is glaucophane, lawsonite, titanite, jadeite ± phengitic mica. Investigated phase-diagram sections of three blueschists with different protoliths yield peak conditions of ~300-380 °C at 9-14 kbar. Magnesio-hornblende and rutile cores indicate early amphibolite-facies metamorphism at >460 °C and 2-4 kbar. Later conditions at slightly higher pressures of 6-9 kbar at 350-450 °C are recorded by barroisite, omphacite and rutile assemblages before entering into the blueschist facies and finally following a retrograde path through the pumpellyite-actinolite facies across the lawsonite stability field. Assuming that metamorphic pressure is lithostatic pressure, the corresponding counterclockwise P-T path is explained by burial along a warm geothermal gradient (~15 °C/km) in a young subduction system, followed by exhumation along a cold gradient (~8 °C/km); a specific setting that allows preservation of fresh undecomposed lawsonite in glaucophane-bearing rocks. n nThis article is protected by copyright. All rights reserved.

Metamorphic conditions and structural evolution of the Kesebir-Kardamos dome: Rhodope metamorphic complex (Greece-Bulgaria)

The synmetamorphic nappe system of the Rhodope Metamorphic Complex has been deformed into dome-and-basin structures attributed to syn- to post-convergent exhumation. We document the deformation style and present new thermobarometric and geochronological constraints for the Kesebir–Kardamos dome in southern Bulgaria and northern Greece. The dome consists of a migmatitic core overlain by high-grade thrust sheets. Kinematic indicators indicate a continuum from ductile to brittle conditions during exhumation. Thermodynamic modeling applied to the high-grade, intermediate thrust sheets yielded peak conditions of 1.2xa0GPa and ca 730u2009°C. New U–Pb SHRIMP-II dating of zircons from rocks of the same unit revealed Late Jurassic–Early Cretaceous (145xa0Ma) as the time of metamorphic crystallization; some zircon rims yielded Eocene ages (53 and 44xa0Ma) interpreted as having been thermally reset owing to coeval granitoid magmatism. The high-grade rocks were covered by Lutetian–Priabonian marine sediments after exhumation. Slumps suggest that sedimentation took place in a tectonically active environment. Our new structural, petrological and geochronological results suggest that the major shear zone in the core of the Kesebir-Kardamos dome is equivalent to the Late Jurassic–Early Cretaceous Nestos Shear Zone. Post-Jurassic metamorphic ages recorded in the Rhodope most likely represent crustal rather than deep subduction geodynamic processes.

Tiny timekeepers witnessing high-rate exhumation processes

Tectonic forces and surface erosion lead to the exhumation of rocks from the Earth’s interior. Those rocks can be characterized by many variables including peak pressure and temperature, composition and exhumation duration. Among them, the duration of exhumation in different geological settings can vary by more than ten orders of magnitude (from hours to billion years). Constraining the duration is critical and often challenging in geological studies particularly for rapid magma ascent. Here, we show that the time information can be reconstructed using a simple combination of laser Raman spectroscopic data from mineral inclusions with mechanical solutions for viscous relaxation of the host. The application of our model to several representative geological settings yields best results for short events such as kimberlite magma ascent (less than ~4,500u2009hours) and a decompression lasting up to ~17 million years for high-pressure metamorphic rocks. This is the first precise time information obtained from direct microstructural observations applying a purely mechanical perspective. We show an unprecedented geological value of tiny mineral inclusions as timekeepers that contributes to a better understanding on the large-scale tectonic history and thus has significant implications for a new generation of geodynamic models.

Relation between mean stress, thermodynamic, and lithostatic pressure

Handling Editor: Doug Robinson Abstract Pressure is one of the most important parameters to be quantified in geological problems. However, in metamorphic systems the pressure is usually calculated with two different approaches. One pressure calculation is based on petrological phase equilibria and this pressure is often termed thermodynamic pressure. The other calculation is based on continuum mechanics, which provides a mean stress that is commonly used to estimate the thermodynamic pressure. Both thermodynamic pressure calculations can be justified by the accuracy and applicability of the results. Here, we consider systems with low‐differential stress (<1 kbar) and no irreversible volumetric deformation, and refer to them as conventional systems. We investigate the relationship between mean stress and thermodynamic pressure. We discuss the meaning of thermodynamic pressure and its calculation for irreversible processes such as viscous deformation and heat conduction, which exhibit entropy production. Moreover, it is demonstrated that the mean stress for incompressible viscous deformation is essentially equal to the mean stress for the corresponding viscous deformation with elastic compressibility, if the characteristic time of deformation is five times longer than the Maxwell viscoelastic relaxation time that is equal to the ratio of shear viscosity to bulk modulus. For typical lithospheric rocks, this Maxwell time is smaller than c. 10,000 years. Therefore, numerical simulations of long‐term (>10 kyr) geodynamic processes, employing incompressible deformation, provide mean stress values that are close to the mean‐stress value associated with elastic compressibility. Finally, we show that for conventional systems the mean stress is essentially equal to the thermodynamic pressure. However, mean stress and, hence, thermodynamic pressure can be significantly different from the lithostatic pressure.