Hassina Mouri

Sm-Nd garnet and U-Pb monazite dating of high-grade metamorphism and crustal melting in the West Uusimaa area, southern Finland

Abstract The 100 km wide late Svecofennian granite-migmatite zone in southern Finland contains the Sulkava, the Turku and the West Uusimaa low-pressure, high-temperature granulite areas. In the West Uusimaa area the peak metamorphic conditions are estimated at T = 750-800°C and P = 4-5 kbars. Detailed isotopic dating of different parts of migmatites (mesosomes and leucosomes) as well garnet-orthopyroxene gneisses was undertaken by conventional analysis of U-Pb on monazite and Sm-Nd on garnet. U-Pb monazite ages show that the West Uusimaa area underwent a granulite facies metamorphism at peak conditions between 1832±2 Ma and 1816±2 Ma. The area was then cooled down to 700-600°C at 1.81-1.79 Ga according to Sm-Nd garnet-whole rock data. These results together with previous data show that all the three granulite areas in southern Finland share a coeval thermal event probably stemming from common or similar heat sources.

A magmatic age and four successive metamorphic events recorded in zircons from a single meta-anorthosite sample in the Central Zone of the Limpopo Belt, South Africa

Abstract: Ion microprobe U–Th–Pb analyses combined with cathodoluminescence imaging of zircons from a meta-anorthosite from the Central Zone of the Limpopo Belt record five distinct events. The most concordant ages of cores of zircon yielded a weighted mean 207Pb/206Pb age of 3344 ± 3.6 Ma, representing the protolith age. The unzoned overgrowths yielded four discrete groups with weighted mean 207Pb/206Pb ages of 3150 ± 15 Ma, 2614 ± 4 Ma, 2488 ± 22 Ma and 2009 ± 28 Ma, representing four possible metamorphic events in the Central Zone. This is the first time that a succession of several metamorphic events has been demonstrated in a single rock-type from the Limpopo Belt. Supplementary material: Analytical methods and data are available at http://www.geolsoc.org.uk/SUP18368.

First occurrence of the rare 'corundum+quartz' assemblage in the high-grade zone from the Namaqualand Metamorphic Complex, South Africa : evidence for higher P, T metamorphism?

Abstract We report in this paper the first occurrence of the rare assemblage corundum + quartz in the high-grade metamorphic zone in the core of the Namaqualand Metamorphic Complex (NMC) (South Africa). The magnetite-bearing rocks hosting the corundum-quartz assemblage are very rare, and occur discontinuous bands a few cm wide, associated with diorite, tonalite and anorthosite within quartz- feldspar ± sillimanite ± biotite ± spinel supracrustal rocks extensively intruded by a foliated granite gneiss. The magnetite-bearing rocks are characterized by the presence of abundant crystals of magnetite together with ilmenite, spinel and large (~1 cm in diameter) euhedral crystals of corundum both displaying a sharp contact with magnetite and quartz. However, in some micro-domains, spinel and quartz, as well as corundum and quartz, are separated by a thin corona of sillimanite with or without magnetite. These textural features suggest that the following reactions have taken place: quartz + corundum → sillimanite, and quartz + spinel + O2 → sillimanite + magnetite. These reactions are interpreted as occurring at higher, as yet unknown, P-T conditions than previously estimated for the NMC. Whether corundum and quartz were once in equilibrium is worth investigating, as it could constitute a geothermobarometric assemblage for ultra-high T granulites.

Origin and evolution of FeAl-granulite in the thermal aureole of the Chilka Lake anorthosite, Eastern Ghats Province, India

The Chilka Lake anorthosite (792 ± 2 Ma) in the northeastern part of the Eastern Ghats Province provides an excellent example of deep-crustal diapiric emplacement and ultra-high temperature (UHT) contact metamorphism. Within a narrow contact zone, supracrustal granulates were heated to temperatures in excess of 1000°C, causing renewed partial anatexis and melt segregation under dynamic conditions, with hybridization of the marginal leuconorite. The thermal history of the innermost aureole is well manifested by reaction textures in a FeAl-granulite occurring at the immediate contact SE of Balugaon. The tightly folded rock shows a finely banded structure. Typically, spinel+ilmenite layers are separated from quartz+garnet 1 ± perthite layers by successive coronitic layers of sillimanite and garnet 2 . The sillimanite fabric indicates synkinematic development of the corona. The reaction texture, mineral and bulk chemical characteristics, and geothermobarometric modelling suggest the following thermal evolution: (1) near-isobaric heating of a banded metapelitic granulite (khondalite: grt 1 +sil+qtz+kfsp) to c . 1100°C (7–8 kbar). Coeval with partial melting and extraction of the felsic melt, the divariant reaction (FeMg)grt+2sil → 3(FeMg)spl+6qtz is crossed at c . 1050°C, leading to the formation of a restitic FeAl-granulite made up of alternating spl+ilm and qtz+grt 1 layers. The segregated felsic melt causes hybridization of the adjacent leuconorite. (2) Subsequent near-isobaric cooling of the FeAl-granulite (spl+qtz+ilm+grt 1 ) from c . 1100°C to the stable geotherm. Reversal of the divariant reaction [3(FeMg)spl+6qtz → (FeMg)grt+2sil] produces the multi-layered coronitic texture. During this stage, coronal garnet forms also in the closely associated ferrodiorite via the reaction opx+pl → grt+qtz. The P-T data indicate that the Chilka Lake anorthosite intruded thickened crust at a depth of c . 28 km.

Geochemical characterization and petrogenesis of mafic granulites from the Central Indian Tectonic Zone (CITZ)

Abstract A mafic magmatic sequence of the Bhandara–Balaghat Granulite (BBG) Belt is represented by gabbroic rocks containing orthopyroxene (Opx)–clinopyroxene (Cpx)–plagioclase (Pl)–hornblende±quartz±garnet and showing tholeiitic affinity. These rocks are divided into two groups: (I) garnet-bearing; and (II) garnet-free. The garnet-bearing group is characterized by nearly flat REE patterns. In the multi-element plots, Sr, Zr and Ti show negative anomalies, indicating plagioclase, Ti-magnetite and apatite fractionation. The garnet-free rocks are geochemically subdivided into two subgroups: IIa and IIb. Subgroup IIa is marked by flat REE patterns; the LREE shows 20–30 times chondrite abundances and small positive Eu anomalies. Multi-element patterns show negative anomalies of Nb, P and Ti. Subgroup IIb is characterized by slightly enriched patterns; the LREE shows 10–60 times chondrite abundances. The REE patterns for the Subgroup IIb show moderately to highly fractionated LREE with flat HREE. Multi-element plots show negative anomalies in Nb, Ti and Zr. The Nd–Ce relationship suggests that mafic granulites of the BBGs are derived from higher degrees (Group I, c. 15–30%; Subgroup IIa, c. 20–40%; and Subgroup IIb, c. 18–35%) of partial melting of variably enriched mantle sources, followed by the evolution of the parental melt by fractional crystallization of Opx–Cpx–Pl. The geochemical signatures also suggest that the magma was further modified by crustal contamination during the course of its evolution. The Nd (TDM) model ages, which vary from 3.2 to 1.6 Ga, suggest a long-term evolution of the mafic granulites, possibly starting with overprinting of the isotope composition of their mantle source by crustal isotope signatures as a consequence of crustal recycling; evolving by emplacement and crystallization of the protolith at 2.7 Ga, as well as through later tectonotermal events up to granulite-facies metamorphism and exhumation of the BBG Belt during the collision of the Archaean Bundelkhand and Bastar cratons, and the formation of the Central Indian Tectonic Zone (CITZ) at 1.5 Ga.

The emerging field of medical geology in brief: some examples

Emerging medical problems present medical practitioners with many difficult challenges. Emergent disciplines may offer the medical community new opportunities to address a range of these diseases. One such emerging discipline is medical geology, a science that is dealing with the influence of natural environmental factors on the geographical distribution of health in humans and animals. It involves the study of the processes and causes of diseases and also the use research findings to present solutions to health problems.

Correction to: Natural occurrence of potentially harmful fluoride contamination in groundwater: an example from Nakuru County, Kenyan Rift Valley

Author would like to make corrections in the original publication.