Granitoid textures using CL-petrography: examples from the Illapel Intrusive suite, Chile Michael D. Higgins

1 and Diego Morata

2*

1Sciences Appliqués, Université du Québec à Chicoutimi, 555 blvd de l'Université, Chicoutimi, Québec, G7H 2B1, Canada.

2Departamento de Geología y Centro de Excelencia en Geotermia de los Andes (CEGA-FONDAP). Facultad de Ciencias

Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile *Contact email: dmorata@cec.uchile.cl Abstract. Cathodoluminiscence (CL) petrography permits identification and characterization of feldspar textures in plutonic rocks. This technique has been applied to plutonic rocks from the Lower Cretaceous Illapel Plutonic Complex (Coastal Range of central Chile). Magmatic and subsolidus textures can be observed using cold cathode CL-microscope. This inexpensive, powerful and easily to apply technique reveals some critical aspects related with feldspars texture and chemical composition that are not evident using classical polarised microscopy or some more expensive scanning electron techniques. Keywords: Cathodoluminiscence, petrography, plutonism,

petrogenesis

1 Introduction

The petrogenesis of igneous rocks is recorded by the

development of its texture (microstructure). Texture is

taken here as the complete geometric description of the

crystals in the rock, together with their internal structural,

chemical and isotopic variations. The geometric aspects of

texture have been quantified in studies of many different

types of plutonic and volcanic rocks, except granitoids

(Higgins 2006). Part of the reason for this is the

complexity of the textures of many granitoids, as revealed

by heterogeneities within crystals.

Information on the textural variation within crystals can be

studied using point analyses or by imaging techniques. In

this paper we will discuss cathodoluminescence (CL:

electron-excited luminescence) imaging, an inexpensive,

powerful and well-known technique (Pagel et al., 2000)

that has been rarely applied to the study of granitoids (e.g.

Slaby and Götze, 2004; Catlos et al., 2010; Dalby et al.,

2010). CL easily reveals the presence of different feldspar

type and textures because subtle variations in feldspar

chemistry may result in different CL colours and intensities

(see Dalby et al., 2010 and references therein).

Specifically, vivid red CL is observed in almost pure albite

(Ab>95), contrasting with the blue CL colours that

characterized K-feldspar. Consequently, chemical

heterogeneities in minerals revealed by CL have been used

to identify magmatic evolution (e.g., 6áDE\� DQG� *|W]H��

2004), or in relation with subsolidus processes (e.g., Dalby

et al., 2010 and references therein).

In this contribution we show that CL-petrography can add

much information to simple optical examination of thin

sections, and guide further investigations by more

expensive and spatially restricted methods.

2 Methododology

The bombardment of materials by electrons produces a

wide range of secondary particles and radiations, including

light, which is the phenomenon of CL (e.g. Pagel et al.,

2000). Electrons lose their energy fast in minerals; hence

the CL light is produced close to the surface. CL light has a

wide range of wavelengths, but usually only visible light is

used for geological purposes, especially in the microscope-

based system used in this study.

CL images can also be made using a dedicated instrument

attached to an optical microscope. A wide beam of

electrons from a cold-cathode source is directed towards a

polished thin section, which is in a vacuum chamber

attached to the microscope stage. An area about 3 mm in

diameter is obliquely illuminated with electrons. Light

from the section escapes though a window into the

objective of the microscope. The system is fast, as no

scanning is required, and can detect both colour and

intensity of the CL. Each image can cover a much larger

area, but the resolution is lower than in SEM based

systems as it limited by the window and the sample-

objective distance. This type of CL system is useful if large

areas are to be imaged: it is quite fast and feasible to image

a significant fraction of a thin section. As granites are

generally quite coarse-grained, this was the CL method

used in this study.

3 Granitoid samples from the Illapel Plutonic Suite

Studied samples are from the Illapel Plutonic Complex, a

Cretaceous N-S elongated body that intrudes Upper

Jurassic to Lower Cretaceous volcanic and volcaniclastic

rocks (Parada HW�Dl., 1999; Rivano HW�DO., 1985; Morata HW�

DO�, 2010). This plutonic complex ranges from medium-

grained gabbro to trondhjemites, with hornblende and

biotite ± clinopyroxene bearing tonalites and granodiorites

as the most abundant lithologies. Three samples have been

chosen to illustrate the applications of the CL technique.

395

ILL-08-74 is a fresh, relatively fine-grained granodiorite.

The XL image (figure 1A) shows that the minerals are

fresh, with relatively unequilibrated textures. Orthoclase

forms large plates, some of which are not evident as they

are in extinction. In the CL image (figure 1B) plagioclase

is green and is the dominant mineral (42%). The texture

shows that it was the first major mineral to form. The

interior of the grains are bright green and very fresh. Some

zoning is revealed at the edges with darker green colours,

which correspond to clear rims in the XL image. Quartz

has is very deep blue in CL. No zoning could be seen in

this sample or others in this study. The texture shows that

quartz was the second mineral to crystallise. Orthoclase is

pale blue in CL. It is unzoned and clearly crystallised after

plagioclase and quartz. Its distribution is quite

heterogeneous, and appears to have filled in residual

porosity in the crystal framework. Mafic and oxide

minerals are black in CL.

ILL-08-104 is a fresh granite. In XL the rock does not

appear to be very different from ILL-08-74 (figure 2A),

but CL shows a different texture (figure 2B). Plagioclase

appears to be the earliest phase, but is less abundant than in

ILL-08-74 and does not appear to form a framework.

Zoning is very evident in both XL and CL. Quartz is a

major phase and appears as independent crystals.

Orthoclase is homogeneously distributed in the section and

is unaltered. In commonly appears to separate plagioclase

and quartz grains, almost as if the grains were physically

disaggregated and the orthoclase crystallised in the space.

Mafic minerals are black in CL, but contain abundant

small grains of apatite, which are very bright green. This

association was found in most other samples.

Sample ILL-08-81 is an unfoliated leucogranite with no

mafic minerals visible in the field. In figure 3A quartz is

clearly visible as a major component and it is clear that the

feldspars are altered. The CL image (figure 3B) reveals

some of the details of the feldspars and the alteration

processes related with subsolidus fluid-rock interaction.

Some plagioclase relicts are still present (green), but if it

was an abundant mineral it is now completely altered.

Orthoclase has a range of CL colours. Relatively unaltered

parts are blue; pink shows the presence albitisation; pale

areas are a mixture of different types of alteration. It is

clear that there are strong variations in the nature of the

alteration over a scale of a few millimetres.

4 Conclusions

Microscope based cathodoluminescence colour images

reveal many aspects of the texture and alteration of

granitoids that are not evident or even visible in plane-

polarised or cross-polarised light images. Feldspar CL

colours permit their easy identification: red for pure albite,

green tones from the albite-anorthite solid solutions and

light blue for K-feldspar. If the equipment is available then

it is a relatively cheaper way to characterise granitoids

petrographically in more detail. Textures indicative of

magmatic evolution or even subsolidus, fluid-rock

interactions can be then easily identified and characterized.

Acknowledgements

This work has been supported by the Chilean National

Science Foundation (FONDECYT) Project 1080468 to

DM and a “Discovery” grant from NRSER (Canada) to

MDH. This research is a contribution to the FONDAP-

CONICYT Project #15090013.

References Catlos, E.J.; Baker, C.; Sorensen, S.S.; Çemen, I.; Hançer, M. 2010.

Geochemistry, geochronology, and cathodoluminiscence

imaginery of the Salihi and Turgutlu granites (central Mederes

Massif, western Turkey): implications for Aegean tectonics.

Tectonophysics 488: 110-130.

Dalby, K.N.; Anderson, A.J.; Mariano, A.N.; Gordon, R.A.;

Mayanovic, R.A.; Wirth, R. 2010. An investigation of

cathodoluminiscence in albite from the A-type Georgeville

granite, Nova Scotia. Lithos 114: 86-94.

Higgins, M.D, 2006. Quantitative Textural Measurements in Igneous

and Metamorphic Petrology. Cambridge University Press,

Cambridge, UK.

Morata, D.; Varas, M.I.; Higgins, M.D.; Valencia, V.; Verhoort, J.D.

2010, Episodic emplacement of the Illapel Plutonic Complex

(Coastal Cordillera, central Chile): Sr and Nd isotopic, and

zircon UPb geochronological constraints. In VII SSAGI South

American Symposium on Isotope Geology,Brasília

Pagel, M.; Barbin, V.; Blanc, P.; Ohnenstatter, D. 2000.

Cathodoluminescence in Geosciences. Springer-Verlag, Berlin.

Parada, M.A.; Nystrom, J.O.; Levi, B. 1999. Multiple sources for the

Coastal Batholith of central Chile (31-34 degrees S):

geochemical and Sr-Nd isotopic evidence and tectonic

implications. Lithos 46(3):505-521.

Rivano, S.; Sepúlveda, P.; Hervé, M.; Puig, A. 1985. Geocronología

K- Ar de las rocas intrusivas entre los 31°-32°S, latitud sur,

Chile. Revista Geológica de Chile 17:205-214.

Slaby, E.; Götze, J. 2004. Feldspar crystallization under magma-

mixing conditions shown by cathodoluminiscence and

geochemical modelling – a case study from the Karkonosze

pluton (SW Poland). Mineralogical Magazine 68(4). 561-577.

396

Figure 1. ILL-08-74 – field of view 8 mm. A) Linear cross-polarised light. B) Cathodoluminescence (CL) image.

Figure 2. ILL-08-104 – field of view 5 mm. A) Linear cross-polarised light B) CL image.

Figure 3. ILL-08-81 – field of view 5 mm. A) Linear cross-polarised light B) CL image.

397