Muscovite-bearing Granites Research Articles

Differentiation of strongly peraluminous, perphosphorus granites: The pedrobernardo pluton, central Spain

The Pedrobernardo pluton (295 ± 2 Ma; initial 87 Sr/ 86 Sr = 0.7122) is a layered, sheet-like granite body with distinct vertical zoning: biotite ± muscovite-bearing granites in the lower zone, muscovite ± biotite leucogranites in the middle zone, and muscovite-bearing aplites and pegmatites in the upper zone. The magma was anatectically generated, intruded almost free of restite, and evolved as a closed system by crystallization in situ. Geochemical modelling indicates that the lower and middle zones followed an equilibrium crystallization path, with no separation between solids and melt, and the upper zone followed a fractional crystallization path. Magmatic fractionation produced leucocratic differentiates with high P concentrations and a P/Ca ratio higher than in apatite. The excess P is located in plagioclase and K-feldspar. Monazite (+apatite) fractionation in the lower and middle zones produced extreme depletions in Th, U, Y, and the REEs, and chondrite-normalized REE patterns with low LREEs/HREEs and a slight discontinuity between Nd and Sm. About 90% of the Th, U, Y, and REEs reside in accessory minerals, especially in monazite. Biotite and muscovite have very low REE contents with small positive or no Eu anomalies. K-feldspar and plagioclase in both the lower and upper zones have moderate REE contents with positive Eu anomalies. Feldspars from the upper zone, in contrast, have negative Eu anomalies and have very low REE contents. Compared to available data from literature, D Sr, Ba, Th, U, Y, REEs bio/ mineral are lower by one to three orders of magnitude, and D REEs plag/ Kfsp are also lower by one order of magnitude. These huge discrepancies are attributed not only to contamination with REE-rich inclusions in mineral concentrates used in previous studies, but to real differences in geochemical behavior of trace elements in peraluminous systems, with respect to subaluminous systems. Biotite/melt and, probably, K-feldspar/melt distribution coefficients for Th, U, Y, and the REEs in peraluminous magmas must be much lower than previously considered.

Magnetic susceptibilities of Westland-Nelson plutonic rocks: Discrimination of Paleozoic and Mesozoic granitoid suites

Abstract A portable magnetic susceptibility meter allows rapid in situ estimation of the magnetite content of granitoid rocks, and hence provides a guide to their I- or S-type character. Except for some muscovite-bearing granites above about 71% SiO2 and some orthogneisses of the Charleston Metamorpbic Group, the susceptibility measurements clearly discriminate between Westland-Nelson granitoids of known Mesozoic (magnetite-bearing, I-type) and Paleozoic (magnetite-free, S-type) age. Although some Mesozoic granites lack magnetite, no magnetite-bearing Paleozoic granites have been recognised. Precise susceptibility measurements of a suite of S-type biotite granites show that their very weak susceptibilities are proportional to modal biotite content. Ilmenite and pyrrhotite in these S-type granites have no measureable affect on susceptibility measurements. Deformation locally reduces susceptibility on an outcrop scale, whereas contact metamorphism/hydrothermal alteration may increase or decrease susceptibility. Aeromagnetic anomalies in the Westland-Nelson region of New Zealand are associated more generally with magnetite-bearing I-type granitic rocks than with lamprophyre dikes (and hypothetical magnetic source bodies), and it is considered that several anomalies are due to the granitic rocks, one variety of which is apparently associated with lamprophyre swarms.

Cretaceous crustal structure and metamorphism in the hinterland of the Sevier thrust belt, western U.S. Cordillera

Combined structural and geochronologic studies to the west of the Sevier belt at lat 39{degree}-40{degree}N indicate that peak metamorphism of the upper 15 km of crust occurred during the Late Cretaceous (70-90 Ma), overlapping in time with the intrusion of muscovite-bearing granites and pegmatites. Metamorphism was accompanied by penetrative deformation (top-to-the-east layer-parallel shear); both were localized at the currently exposed structural levels, resulting in impressive vertical and lateral strain gradients and T{sub max} gradients of > 50 C/km. Inception of thrust faulting in the Sevier belt may have predated peak metamorphism and intrusion of granites in the hinterland by 10-15 m.y., but it continued during and ended simultaneously with the end of metamorphism at about 70 Ma. The rise of muscovite-bearing granites and hot fluids, generated by metamorphism and crustal anatexis at greater depth, provides and explanation for the observed localized metamorphism and deformation at relatively high structural levels.

Garnet, tourmaline and muscovite-bearing leucogranites, gneisses and migmatites of the Higher Himalayas from Zanskar, Kulu, Lahoul and Kashmir

Summary Approximately 30–50% of the map area of the Central Crystalline Complex of the Higher Himalayas in Zanskar, Kulu, Lahoul and Kishtwar-Kashmir (NW India) is composed of Miocene leucogranites intruding metamorphic basement rocks. Ultrametamorphism at deeper crustal levels resulted in in situ anatexis, migmitization and lit-par-lit intrusions or stromatic layering within high-grade metasedimentary rocks. Garnet, tourmaline and muscovite-bearing granites locally contain biotite, apatite or kyanite. Leucogranites at higher crustal levels are associated with a widespread network of anastomosing dykes and sills with high tourmaline content. Rare-earth-element (REE) abundances are relatively low. Garnets are compositionally variable and are usually unzoned. Extremely high 87 Sr/ 86 Sr initial ratios of leucogranites (0.755–0.769) indicate a crustal anatectic origin while the wide range of Sr initial ratios can be explained by melting of a variety of metasedimentary rocks of the subducting Indian Shield. Radiometric ages of Himalayan leucogranites (28–12 Ma) follow closely in time a thermal peak of metamorphism (40–25 Ma). Inversion of metamorphic isograds during large-scale crustal subduction along several major thrust zones in the Higher Himalayas followed immediately after generation of the leucogranites. The leucogranites are interpreted as anatectic melts of high-grade metamorphic crystalline basement, generated at depth by residual (Barrovian) heat with a large amount of volatile and fluid flux. Frictional heating along the N-dipping Main Central Thrust is not considered a major source of heat for generation of the leucogranites.

Lithium Distribution in Canadian Granitoid Rocks

Analytical data on the lithium content of granitoid rocks mainly from the Cordilleran and Appalachian regions of Canada are presented and compared with published data for various areas of the Canadian Precambrian Shield. The comparison suggests that the average lithium content of granitic rocks in whole geological regions and provinces in which lithium deposits are numerous is not appreciably higher, if at all, than in those where lithium deposits are rare or absent. It is appreciably higher in some granitic rocks, especially muscovite-bearing granite, in limited areas in which lithium deposits are numerous, but is high also in muscovitic and greisenized granite in some areas where lithium deposits are scarce or absent.

Occurrence of zinc in granitic biotites

37 granitic plutons in Great Britain and the United States were sampled and biotite separates prepared. These biotites were analysed for zinc and iron and a modal (point-count) analysis was conducted on the granitic samples. The results of these analyses were examined for differences that would characterise mineralised and non-mineralised granites, and for differences between biotites coexisting with muscovite and those coexisting with hornblende. The possibility of differentiating between mineralised and non-mineralised granites on the basis of the zinc content of biotites is rejected, but significant differences in composition are found between biotites from muscovite-biotite-granites and those from hornblende-biotite-granites. Muscovite-bearing granites have low biotite contents, these biotites having high iron and low zinc concentrations; hornblende-bearing granites have high biotite contents but these biotites, in contrast, have comparitively low iron and high zinc concentrations.