Abstract

A model of the cooling history of tin-bearing granitic magma forming the Schellerhau granites (Eastern Erzgebirge, Germany) is shown on the basis of quartz textures. Similar grain size, similar grain habit and correlatable growth textures of phenocrysts in different granite varieties give proof of a common crystallisation history before the melts of the Schellerhau granite varieties were intruded. Four nucleation events occurred during crystallisation in different crustal levels between about 20 and 1 km depth. The parental melt of the Schellerhau granites is interpreted to have contained <2.5 wt.% H2O originally. The water content of the melt during the subvolcanic intrusion stage amounted to more than 5 wt.% and characterises highly evolved residual melts that enable the formation of tin deposits. This paper contributes to a better understanding of the development and behaviour of fractionated tin-bearing granitic melts, and links quartz cathodoluminescence (CL) with microanalytical studies.

7.2 Introduction

MacLellan and Trembath (1991) developed a quantitative model for evaluating the cooling history of a granitic magma on the basis of quartz textures. The relative chemical and structural stability of quartz is responsible for the conservation of quartz generations of different size, habit and structural state in granites and rhyolites. In contrast the chemical composition of feldspar changes during cooling because it is in equilibrium with melt.

Therefore quartz morphology provides reliable information about the cooling history of granitic melts.

In this paper we report about the application of cathodoluminescence (CL) microscopy to

the growth textures of quartz. However, it is not the aim of this paper to provide a detailed explanation of the complex causes of luminescence in quartz. The CL of quartz results from substitutional and interstitial incorporation of trace elements and from different types of intrinsic and extrinsic defect centres in the quartz lattice (e.g. Ramseyer et al., 1988).

Consequently, the luminescence behaviour is a complex function of the concentration of trace elements which may act as CL activators, like Al, Fe and Ti, or as quenchers. The CL is related to the conditions of mineral crystallisation, and alteration and of natural γ- and α -radiation. The analysis of the luminescence spectra makes it possible to quantify the emission properties of single grains. The scanning electron microscope cathodoluminescence (SEM-CL) facilitates high resolution of intragranular growth textures of magmatic quartz. Zoning within quartz is normally invisible by conventional optical microscopy and rarely documented by quantitative analysis. We also determined trace element concentrations in quartz using the electron microprobe. In addition, the detailed analysis of quartz texture and size distribution was carried out. The intragranular growth textures, the quartz framework and the grain size distribution yield important information on the nature and evolution of the melt from which the crystal grew. Plots of maxima of grain size distributions for a single mineral indicate bursts of nucleation which are caused by a high degree of melt undercooling (Dowty, 1989).

In the following is given a short introduction in the general interpretation of growth textures, based on the present level of knowledge. A general compilation of observable growth and resorption textures is illustrated in Fig. 7.1.

The different primary growth textures represent disturbances of growth and diffusion rates during crystallisation. Growth and diffusion rates depend on melt composition, crystal transport in the melt (e.g., convection) and ascent velocity of the melt and related pressure and temperature changes. These criteria regulate type and quantity of defects, few of which are luminescent active and incorporated into the quartz lattice. Wavelength and amplitudes of individual zones are directly related to the physical and chemical melt properties. The

Fig. 7.1 Synoptical scheme of primary growth textures in magmatic quartz phenocrysts with a diameter of 1 to 5 mm contrasted by SEM-CL. 1 - Nucleation of hexagonal β-quartz or rhombohedral α-quartz; 2 - Resorped surface; 3 - Skeletal growth; 4 - Step zoning (50 - 1000 µm); 5 - Oscillation zoning (2 - 20 µm); 6 - Growth impediment; 7 - Inclusion (entrapment) of melt or a bubble; 8 - Growth impediment caused by adjacent phenocrysts; 9 - Rim of anhedral quartz of the final magmatic crystallisation.

diffusion rate in the melt controls the compositional variation and width of the zoning. The higher the diffusion rates, the less compositional differences occur in the quartz.

Concentration gradients develop in the melt when the growth rate exceeds the diffusion rate, and in the solid when the reaction rate of crystals with the liquid is lower than the growth rate (Sibley et al., 1976).

The development of growth zoning during magmatic crystallisation is described by a number of models (e.g. Sibley et al., 1976; Anderson, 1984) that have been derived from the zonal structure of plagioclase. We apply these models to the crystallisation of magmatic quartz having a very similar zonal growth as plagioclase.

Major discontinuities in the zoning (50 - 1000 µm width) being non-periodic and showing a significant change of the luminescence colours result in physicochemical changes of external

4

5 7

9

6 2 1 8

3

composition (Bottinga et al., 1966; Allègre et al., 1981). Bottinga et al. (1966) defined those zones as compositional zoning, and Allègre et al. (1981) called them stepped zoning.

The fine oscillatory zoning (2 - 20 µm width) within the step zones is explained by a self-organised (“intrinsic” acc. to Shore and Fowler, 1996) diffusion controlled mechanism on the crystal-melt boundary layer and exists in a number of recent models (Sibley et al., 1976;

Haase et al., 1980; Allègre et al., 1981; Loomis, 1982; Simakin, 1984; Pearce, 1993).

Oscillatory growth happens very slowly at low degrees of undercooling and oversaturation. It may only take place when the crystallising system is not disturbed on the solid-liquid interface, thus the melt should not convect (Allègre et al., 1981). Investigations on natural plagioclases by Greenwood and McTaggart (1957) and Wiebe (1968) confirm that oscillation zones of crystals grown from the same melt cannot be correlated due to the self-organising character of the process. To develop oscillation zones physical or chemical changes in the bulk magma are not required.

Discussions about the distinction of resorption structures and growth impediments in quartz phenocrysts have been controversial (Kozlowski, 1981; Harris and Anderson, 1984).

Laemmlein (1930) first recognised and described lobate depressions at the crystal surface as growth embayments. The fact that these are growth impediments may be recognised because the zonation adapts to the shape of the embayments. In contrast, resorption surfaces cut pre-existing zones. Growth impediments are caused by immiscible liquids, vapor bubbles, molten sulfide or fluid-rich melt droplets which stick on the crystal surface, hinder the crystal growth and result in lobate depressions and entrapments (Kozlowski, 1981; Donaldson and Henderson, 1988; Lowenstern, 1995). The resorption (melting) of quartz surfaces is due to SiO2-undersaturation of the melt which may be caused by increase in temperature, isothermal depressurisation or magma mixing. Crystals may undergo a rounding due to thermal weakening, chemical interaction and mechanical abrasion even during transport in the melt.

The occurrence of resorption which results in strong rounding of the quartz crystals is in accordance with the rapid ascent of granitic melts by dyke formation as found by Holtz and Johannes (1994) and Johannes and Holtz (1996).

The presence of skeletal growth indicates supercooling and a related decrease of the ratio of diffusion rate/growth rate. In such cases compositional gradients on the crystal-melt interface develop, planar crystal/melt interfaces become unstable and the growth results in skeletal morphology (Kirkpatrick, 1981; Fowler, 1990).

As object of studies we have chosen the Schellerhau Granite Complex (SGC) in the Eastern Erzgebirge/Germany. It has been studied by Pälchen and Ossenkopf (1967), Helbig and Beyer (1970), Seim et al. (1982), Just et al. (1987), and Schilka and Baumann (1996) so that an extensive state of knowledge based on mapping, availability of drill cores and resulting chemical and petrographical data exists. The pluton was studied by the authors across a vertical sampling profile of about 1000 m using the relief differences of up to 300 m caused by erosion and dislocations, and available drill cores of depths of up to 1000 m (drilling Niederpöbel 1/58, Helbig and Beyer, 1970). On the basis of the collected data, we present a model of the quartz crystallisation history of the Schellerhau granite magma.