Search results for "petrology"

The geochemical riddle of “low-salinity gypsum” deposits

2022

International audience; Gypsum makes up about one fifth of giant salt deposits formed by evaporation of seawater throughout Earth’s history. Although thermodynamic calculations and precipitation experiments predict that gypsum precipitates when the salinity of evaporating seawater attains about 110 g kg-1, gypsum deposits of the Mediterranean Salt Giant often bear the geochemical signature of precipitation from less saline water masses. Addressing this geochemical riddle is important because marine gypsum deposition and continental gypsum erosion affect the global carbon cycle. We investigated gypsum deposits formed in the marginal basins of the Mediterranean Sea during the Messinian Salini…

New zircon ages and regional significance for the evolution of the Pan-African orogen in Madagascar

1999

New 207 Pb/ 206 Pb single zircon evaporation ages for granulites, gneisses and granites in southern and central Madagascar record a widespread Pan-African metamorphic and magmatic event in the period c. 650–556 Ma, but also earlier ages in the range 1890–1710 Ma, inherited from protolith material and reflecting heterogeneous crustal sources. South of the Ranotsara shear zone, metasedimentary gneisses and granulites contain an early population of detrital zircons with ages in the range 1890–1740 Ma; a detrital grain with an age of 899 ± 2 Ma suggests that some sedimentary protoliths were deposited later than c. 900 Ma. Metamorphic zircons have a mean age of 564.2 ± 0.9 Ma. North of the Ranot…

Mafic dykes of Salvador (State of Bahia, Brazil): geological and petrological characteristics

1989

Mesozoic mafic dyke swarm from Rio Ceará-Mirim (northeast Brazil)

1989

The origin of fibrous veins: constraints from geochemistry

2002

Metamorphic petrology and zircon geochronology of high-grade rocks from the central Mozambique Belt of Tanzania: crustal recycling of Archean and Pal…

2003

New data on the metamorphic petrology and zircon geochronology of high-grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan-African event. The metamorphic rocks are characterized by a clockwise P–T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite-bearing metapelites. The meta-granitoid rocks are of calc-alkaline composition, range in age fr…

Carbon isotope geochemistry and geochronological constraints of the Neoproterozoic Sirohi Group from northwest India

2012

a b s t r a c t The early Neoproterozoic carbonates of the Sirohi Group, northwestern India, were studied for geochronology as well as for oxygen and carbon isotopes across three different sections to understand their relationship with contemporaneous global events. New single zircon Pb-Pb evaporation data con- strain the age of the Sirohi Group between ca. 920 and ca. 820 Ma, though actual span for deposition of the carbonates may be little shorter taking into allowance the time for peneplanation. Narrow range in variation of 18O values is generally observed in the carbonate which is consistent with generally well-preserved original compositions. Two sections (SA and SS) out of the three t…

Occurrence and Genesis of Palygorskite and Associated Clay Minerals in a Pleistocene Calcrete Complex, Sde Boqer, Negev Desert, Israel

1996

AbstractPalygorskite and associated clay minerals have been studied in a Pleistocene calcrete complex from the Negev desert (Sde Boqer, Israel). This complex is divided into five main parts: the chalky and marly bedrock overlain by its weathered product, a brecciated calcrete hardpan; the laminar crust; loess pockets trapped in the calcrete; and the overlying soft surficial soil. The distribution of clay minerals is directly dependent on the position of the sample in the calcrete complex. Smectite is inherited from the bedrock in the calcrete with an aeolian enrichment in loess pockets and upper soft soil. The source of kaolinite is mainly detrital, related to desert dust. Illite is partly …

Der meteorit von Breitscheid—III petrographische untersuchung

1959

Abstract The meteorite of Breitscheid is a grey-veined bronzite-olivine-chondrite containing nickel-iron and troilite. It has a remarkably high volume of pores of about 10 per cent. The density of the solid is 3·7, of the stone as a whole 3.3. The chondrules take about 20 per cent of the volume of the stone. The ore minerals are: nickel-iron (kamacite and taenite), troilite, chromite, ilmenite and besides, in the melting crust, magnetite. Especially ilmenite—until recently doubtful as a constituent of meteorites—could be proved in a convincing manner. The silicate constituents are: olivine, bronzite, clinopyroxene, plagioclase (?), glass and a few minerals which could not be identified. The…

Subduction-related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India: Constraints from petrol…

2015

Abstract Major and trace elements as well as the first oxygen isotopes are reported on eclogite xenoliths from the Mesoproterozoic KL2 and P3 kimberlite pipes of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India. Garnets in kyanite-bearing samples are rich in grossular, whereas they are predominantly pyrope-almandines in the bimineralic (kyanite-free) samples. The kyanite eclogite from the P3 pipe is more Mg-rich than those from KL2 pipe. Equilibration temperatures indicate derivation from 4.5 to 5.3 GPa and 1060 to 1220 °C for the KL2 samples and 3.6 GPa, 918 °C for the P3 sample. Garnet rare earth element patterns show two characteristic types, one with relatively lo…