Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizário ultramafic amphibolite, Encantadas Complex, southernmost Brazil

The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizário ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at 1989 ± 21 Ma. This ultramafic rock was re-metamorphosed at 702±21 Ma during a greenschist facies eventM2; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257±12 Ma) and Camboriú Orogeny (~ 1989 Ma) of the Trans-Amazonian Cycle, followed by an orogenic event (702±21 Ma) of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma) corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton.

The Role of Tholeiitic Magmatism in the English Lake District: Evidence from Dykes in Eskdale

Mafic dykes occur in close association with, and both cut and are cut by, the Eskdale granite in the south-western Lake District. The dykes range compositionally from magnesian basalt to andesite and are divided into two groups: (1) high-Fe-Ti rocks of tholeiitic affinity forming most of the dykes and (2) a lower-Fe-Ti group, comparable in composition to the lavas of the Borrowdale Volcanic Group. The dykes extend the range of tholeiitic magmatism in the Lakes into late Ordovician, and possibly Silurian times, and indicate that published plate tectonic models partly based on the distribution of magma types are perhaps over-simplified. The Eskdale dykes form one end of a spectrum of Lake District compositions from tholeiitic to calc-alkaline. All the magma types may have shared a common mantle source, their final composition reflecting residence times in the crust or LIL-enriched mantle.

Geochemistry of a continental tholeiite suite: late Palaeozoic quartz dolerite dykes of Scotland

The late Carboniferous quartz dolerite suite of Scotland consists mainly of quartz tholeiites, with subordinate olivine tholeiites and tholeiitic andesites. The low pressure evolution of the magmas was controlled by fractionation of olivine–plagioclase–pyroxene–oxides assemblages from more magnesian compositions and plagioclase–pyroxene–oxides–apatite removal from intermediate compositions. A higher pressure stage dominated by olivine fractionation is suggested by the presence of olivine nodules in a magnesian basalt dyke from Fife.The suite is of high-Fe-Ti type, closely comparable to certain basalts erupted in areas of active lithospheric spreading or “hot spots”, such as Iceland and Hawaii. The ppO2 can be inferred to have been rather higher in the Scottish rocks than in comparative suites, promoting earlier separation of Fe-Ti oxides, with the consequent effects on trace element distribution.Apart from varying degrees of fractionation, chemical variations in the dykes are of three types: rather minor variations along individual dykes, variations across certain thicker dykes, and minor and trace element variations reflecting chemical heterogeneities in the mantle sources. The dykes and sills are inferred to have been fed from a plexus of small, partly independent, magma reservoirs.New trace element data on tholeiitic lavas from the Oslo Rift confirm recently revived suggestions that the Scottish and northern English quartz dolerites are part of a larger province extending into Scandinavia.

Experimental Petrology and Petrogenesis of Apollo 12 Basalts

Experimental studies at 1 bar and up to 30 kbar establish the crystallization sequences for basalts 12021, 12065, 12022, 12009 and 12040. Olivine is the liquidus phase at low pressures, with minor chromium spinel and pigeonitic clinopyroxene joining the olivine at lower temperatures or accompanying the olivine in the less magnesian basalts (12021, 12065). At higher pressures, subcalcic clinopyroxene becomes the liquidus phase except in the most magnesian basalt (12040) where orthopyroxene joins the olivine and becomes the liquidus phase at pressures of 25 kbar. Integration of experimental studies with observed mineralogy of natural rocks shows conclusively that the basalt compositions studied do not lie on a plagioclase + pyroxene + spinel ± olivine cotectic nor have these rocks been derived by accumulation of olivine or pyroxene into such a low temperature cotectic liquid. The Apollo 12 basalts provide clear evidence for the genesis of olivine-rich basalts in the lunar interior. The nature of the source rock is deduced to be pyroxenite or olivine-bearing pyroxenite in which orthopyroxene is probably the major phase with lesser sub-calcic or pigeonitic clinopyroxene. The 100 Mg/(Mg + Fe) ratio of the source region in the deep lunar interior is 75–80.