H.P. Zeck

An erupted migmatite from Cerro del Hoyazo, SE Spain

The almandine-bearing biotite-cordierite-labradorite dacite of the Cerro del Hoyazo is part of the Neogene volcanic range in SE Spain, extending roughly from Cabo de Gata to Cartagena.About 1 vol. % of the lava consists of rock inclusions, measuring over 1 cm, made up of almandine-biotite-sillimanite gneiss, quartz-cordierite gneiss and spinel-cordierite rock. On the ground of their abundance, chemical composition, mineral content and structure, the first and the second type are interpreted as restite inclusions and the third type as recrystallized restite. These restites and the dacitic magma were derived syngenetically from a (semi-) pelitic rock sequence by means of anatexis: the (semi-)pelitic rocks separated into a granitoid melt and Al-rich restites. Euhedral almandine crystals found in the glass base of the dacite have a pre-magmatic origin, and may be compared directly to those in the restites.Another type of inclusion is represented by basic igneous rocks of varying grain size, comprising mainly basaltoid rocks and quartz-rich gabbros. These inclusions commonly bear some restite fragments of the kind mentioned above, and therefore are interpreted as representing basic magma of deeper origin that has absorbed some anatectic material. In part, the composite basic melt thus formed crystallized under plutonic conditions and fragments of the resulting quartz-rich gabbro were incorporated in a later stage in the dacitic melt. Another portion of the composite basic magma was incorporated in the dacitic melt (probably shortly before the eruption of the dacitic magma) as magma blebs (Ø ≅1–20 cm) which subsequently crystallized in part, some of them showing a comparatively fine-grained border zone.Two possible hypotheses are suggested for the time relation between the anatexis of the (semi-) pelitic complex and the appearance of the basic magma: (a) the anatexis was of a regional nature, and was in progress when the basic magma entered the stage (and assumedly triggered the eruption of the granitoid magma); (b) the regional anatexis took place considerably earlier and the basic magma intruded an essentially solid migmatite complex, which was then melted down (contact anatexis) and subsequently erupted.

A 1,220±60 M.Y. Rb-Sr isochron age representing a Taylor-convection caused recrystallization event in a granitic rock suite

In the Tisselskog area (N. Dalsland), in the eastern, external part of the Grenvillian belt in N. Europe, two formations of granitic rocks form the basement for the basal sediments of the Grenvillian, Dalslandian Group. Basement and cover went through a low grade regional metamorphism at 1,030±40 m.y. (2σ) ago. The granitic rocks are strongly recrystallized, but have preserved their granitic texture. Most magmatic crystals are pseudomorphed by a suite of metamorphic crystals of quartz, albite, chlorite, white mica, epidote, titanite, hematite, pyrite and carbonate. It is remarkable that these rocks with a perfect magmatic appearance have lost so much of their magmatic mineralogy. The older of the two intrusions, the Ballsjön Granodiorite Formation, yields a 7 point, 1,220±60 m. y. Rb-Sr isochron (initial Sr isotope ratio=0.7048±0.0004, λ87Rb=l.42×10−11·y−1). This age is interpreted as representing the pervasive recrystallization in the rocks, which was induced by a hydrothermal convective circulation system — a Taylor convection — set up by the intrusion itself or by the younger Tisselskog Leucogranite Formation which intrudes it. In dating a phase of pervasive hydrothermal resetting by whole rock Rb-Sr dating one should try to strike a balance between maximal Rb/Sr variation and maximal similarity in the starting mineralogy in the rock samples, the latter aiming at geochemical coherence during the hydrothermal alteration.

Fission-track constraints on timing of Alpine nappe emplacement and rates of cooling and exhumation, Torrox area, Betic Cordilleras, S. Spain

Abstract Fission-track analyses were performed on apatite and zircon from 8 samples of an exhumed gneiss body belonging to the Alpine Alpujarride nappe complex of the Internal Zone of the Betics. The eight FT apatite ages range from 13.0 to 20.4 Ma, with a central age for six of them of 16.6 ± 0.8 Ma (1σ); a single zircon FT age determination yielded a central age of 17.2 ± 2.5 Ma. Apatite fission-track length distributions are narrow with mean lengths between 14 and 14.7 μm. Such distributions are indicative for very high cooling rates, confirming the results of a previous thermochronological study using RbSr, KAr and ArAr techniques; cooling rates of 250–350°C/m.y. from above 600°C to below ∼350°C at 20-18 Ma were indicated by the tight clustering pattern of the mineral ages. This age bracket may represent the best current estimate for the termination of the Alpujarride nappe emplacement in this region and the very fast large-scale tectonic unroofing. Subsequent cooling may be characterised by lower rates of ∼150°C/m.y. in the period 19-17 Ma and down to ∼5°C/m.y. in the period 17-14 Ma as indicated by best-fit thermal history modelling based on spontaneous apatite fission-track length distributions. This late-stage cooling process may (in part) be related to restoring the isotherm pattern disturbed by the rapid uplift. Further cooling at rates of ∼2°C/m.y. is suggested for the period 14-5 Ma and this may be related to late-stage exhumation of the metamorphic rocks. The data indicate that the period of very fast cooling and coeval extensional tectonics in this area ended at ∼18–19 Ma.

Prehnite-pumpellyite facies metamorphism in Precambrian basement rocks of S Sweden

Very small amounts of prehnite and pumpellyite have been found, often as lenses in biotite crystals, in otherwise quite ordinary quartzo-feldspathic gneisses and granitoid rocks forming common components of the Precambrian basement of S Sweden. It is concluded that both minerals belong to the same metamorphic paragenesis, which also includes quartz, K-feldspar, albite, epidote, colourless mica and chlorite. The presence of this paragenesis, in however slight amounts, signifies that the rocks bear the imprint of a phase of metamorphism belonging to the prehnite-pumpellyite facies.

Metasomatic control of K/Rb ratios in amphibolites

Abstract Amphibolite facies metamorphism of gabbroic dykes, together with their quartzo-feldspathic, granulite facies country rock (Archean basement, west Greenland), transformed the gabbroic rocks into amphibolites with remarkably high K/Rb values (up to 2300, average 727), reflecting correspondingly high K/Rb values in the country rock. A similar dyke transformation in an amphibolite facies complex (Precambrian basement, southwest Sweden) produced amphibolites with more common K/Rb values (average 375) similar to those found in the country rock. As the protolith chemistry and the P-T conditions of origin for the two amphibolite occurrences are very similar, it is concluded that the difference in K/Rb distribution is caused by the difference in country rocks. The K and Rb values in the two amphibolite occurrences lie between protolith and country-rock values, suggesting an exchange of K and Rb between basic rock and quartzo-feldspathic country rock, probably through the hydrothermal fluids which assisted in the amphibolite formation.

Oxygen-isotope systematics of a strongly recrystallized granitic rock complex, Grenvillian Belt, SW Sweden

AbstractIn the eastern, external part of the Grenvillian Belt in SW Sweden, five formations of granitic rocks were found in the basement of the Dalslandian Supracrustal Group. The granitic rocks have been strongly recrystallized but have preserved most of their granitic texture in the process. Most magmatic crystals have been pseudomorphed by metamorphic minerals: quartz, albite, chlorite, biotite, white mica, epidote, titanite, hematite, pyrite and carbonate. Two of the formations have subsequently been affected by a cataclastic deformation and at present consist of mylogneisses. δ18O whole-rock values for the granitic rocks vary from +3.0‰ to +11.1‰. Quartz-apatite, quartz-zircon and quartz-titanite pairs show 18O/16O fractionations corresponding to equilibrium temperatures of 550–700° C, which are believed to reflect in the main continued closed-system isotopic exchange at high temperatures following solidification. In contrast highly positive 18O/16O fractionations for quartz-K-feldspar, quartz-biotite, quartz-chlorite and quartz-sericite pairs in some granitic samples indicate that these rocks have exchanged oxygen with heated, meteoric, H2O dominated fluids. Other granitic samples, however, show virtually undisturbed magmatic 18O/16O fractionation values for the same mineral pairs, even though these rocks are equally altered.It is concluded that all granitic rock units recrystallized under greenschist facies conditions during the infiltration of fluids under the influence of hydrothermal convection systems set up by the intrusion of the granitic plutons. The fluids probably had a range of δ18O values from ca. -14‰ to ca. +10‰, indicating the mixing of distinct fluid reservoirs, one of meteoric origin and the others of magmatic and/or metamorphic origin. The temperature of alteration is estimated at 450–500° C.Estimation of pre-alteration δ18O whole rock values for the five granitic units suggests that three units should be assigned a dominantly S-type origin, where as the other two units may partly or wholly have an I-type origin.

Statistical evaluation of metasomatic effects in meta-igneous rock series

Abstract Comparison of the chemical composition of a series of tholeiitic, gabbroic rocks with that of its metamorphic derivatives, a series of amphibolites, reveals that the differences are so small that they can only be evaluated by special statistical analysis. Application of current statistical tests directly to the raw percentage and ppm values may yield confusing results, because the metasomatic changes are compounded with the within-series magmatic differences. Instead a method is proposed which is based on a comparison of the individual data points of these two populations with the regression line in an X MgO  Tio 2 MgO diagram modelling the magmatic evolution. Application of the rank-sum test and the t-test to the differences thus found provides a statistical assessment of the metasomatic effect of the metamorphism.

Volume effect of a gabbro-amphibolite transition

Abstract Specific gravities of a series of gabbroic rocks and their metamorphic equivalents are compared with their major-element compositions using the Gresens equation. Care was taken to assure a close parent-daughter rock match by careful field control and by applying composition-volume screening. The study indicates that correct matching of protoliths with their metamorphic derivatives is the limiting factor deciding the accuracy of volume effect determinations. The metamorphic transition is consistent with a constant-SiO2,-TiO2,-Al2O3,-FeO★,-MgO model. A small but well-defined volume increase of 2% is indicated.

Pre-Dalslandian deformation and recrystallization in the basement of the Dalslandian supracrustals, Grenvillian (Sveconorwegian) Belt, south-west Sweden

Abstract Reinvestigation of the tectonic and microstructural relations in the Dalslandian supracrustals and in the underlying granitic to granodioritic, mylo-gneiss basement provides evidence for a pre-Dalslandian deformation and recrystallization in the basement. This result is in agreement with earlier suggestions by van Overeem (1948; PhD thesis, Leiden University, Holland). It is demonstrated that the metamorphic and structural development in the basement is different from that in the Dalslandian supracrustals.

Oxygen isotope systematics indicating large-scale circulation of fluids in granitic rocks from southwest Sweden

Abstract In the Precambrian basement of SW Sweden two formations of granitic rocks have been strongly recrystallized but have preserved their granitic texture. Most magmatic crystals have been replaced by green-schist-facies mineral assemblages. δ 18 O whole-rock values for the granitic rocks vary from +1.7‰ to +8.2‰ (relative to SMOW) and extend below the normal range for granitic rocks (+6 to +10‰). Quartz-K-feldspar pairs show 18 O 16 O fractionations of +5.4‰ and +7.8‰, indicating disturbance of magmatic equilibrium fractionations. It is concluded that the O-isotope redistribution took place during the pervasive recrystallization of the rocks, under the influence of a large-scale flux of water (partly) of meteoric origin, probably in a convective hydrothermal system set up by the intrusion of the granites themselves. Estimation of pre-alteration δ 18 O whole-rock values for one of the granitic units suggests a partly S-type origin.