A NEW INTERPRETATION FOR THE GARNET ZONING IN … · 2016. 10. 27. · EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 12, No. 1 (June 2008): 7-30 A NEW INTERPRETATION FOR

EARTH SCIENCES

RESEARCH JOURNAL

Earth Sci. Res. J. Vol. 12, No. 1 (June 2008): 7-30

A NEW INTERPRETATION FOR THE GARNET ZONING IN

METAPELITIC ROCKS OF THE SILGARÁ FORMATION,

SOUTHWESTERN SANTANDER MASSIF, COLOMBIA

Carlos Alberto Ríos Reyes1, Oscar Mauricio Castellanos Alarcón2 and Akira Takasu3

1 Escuela de Geología, Universidad Industrial de Santander, A.A 678, Bucaramanga, Colombia.2 Programa de Geología, Universidad de Pamplona, Colombia.

3 Geosciences Department, Shimane University, Japan.

Corresponding author: Carlos Alberto Ríos Reyes, School of Applied Sciences, University ofWolverhampton, Wulfruma Street, Wolverhampton WV1 1SB, UK

Tel. +44 (0)1902 322679; Fax. +44 (0)1902 322714; E-mail: [email protected]

Abstract

A Barrovian sequence of the Silgará Formation at the southwestern Santander Massif, Colombian Andes, con-tains zoned garnets in which major and trace element zoning correlates with distribution of mineral inclusions,which may indicate that garnet growth rate varied through time and affected both composition and texture ofgarnets, although different garnet producing reactions have also played an important role in the chemical zoningof garent. However, a local metasomatism process associated to the action of late magmatic fluids associated tothe emplacement of the Pescadero Pluton (external forcing mechanism) would be also considered. In particular,Ca, Mn and Y zoning patterns in some garnets correspond with inclusion-rich vs. inclusion-free zones, althoughthe distribution of inclusions does not correlate with chemical zoning (i.e., the same inclusions are found inCa-rich and Ca-poor zones of the garnet). There is a similar lack of correlation with accessory phases (apatite,monazite, xenotime, ilmenite or rutile). In a garnet from the garnet-staurolite zone, a high Mn core containsabundant and randomly oriented apatite, monazite and ilmenite inclusions, while a euhedral low Ca mantle zoneis inclusion-free and the high Ca / low Mn rim zone contains apatite, monazite and ilmenite aligned parallel tothe margins of the garnet. Inclusions in garnet can also represent mineral phases were not completely consumedduring garnet growth. Association of garnet zoning trends and patterns with inclusion distribution may help dif-ferentiate between processes that identically affect major-element zoning but that produced variable textures inthe garnet.

7

Manuscript received September 7, 2007.Accepted for publication January 30, 2008.

Key words: Colombia; garnet; zoning; Santander Massif; Silgará Formation.

Resumen

Una secuencia Barroviana de la Formación de Silgará en la región suroccidental del Macizo de Santander, An-des colombianos, contiene granates zonados en los cuales la zonación de elementos mayores y trazas secorrelaciona con la distribución de las inclusiones minerales, lo cual puede indicar que la tasa de crecimiento delgranate varió con el tiempo y afectó la composición y la textura de los granates, aunque diferentes reaccionesque producen granate han jugado también un papel importante en la zonación química del granate. Sin embargo,un proceso local de metasomatismo generado por la acción tardia de fluidos magmáticos asociados alemplazamiento del Plutón de Pescadero (mecanismo de fuerza externa) es también aquí considerado. En partic-ular, los patrones de zonación de Ca, Mn e Y en algunos granates corresponden con zonas ricas en inclusionesvs. zonas sin inclusiones, aunque la distribución de inclusiones no se correlaciona con la zonación química (esdecir, las mismas inclusiones se encuentran en las zonas ricas y pobres en Ca del granate). Hay una carencia sim-ilar de correlación con las fases accesorias (apatito, monacita, xenotima, ilmenita o rutilo). En un granate de lazona del granate-estaurolita, un núcleo alto en Mn contiene abundantes inclusiones aleatoriamente orientadas deapatito, monacita e ilmenita, mientras que una zona euhedral baja en Ca carece de inclusiones y la zona de borderica de alto Ca / bajo Mn contiene inclusiones de apatito, monacita, e ilmenita orientadas paralelo a los bordesdel granate. Inclusiones en granate pueden también representar fases minerales que no fueron completamenteconsumidas durante el crecimiento del granate. La asociación de los patrones de zonación del granate con ladistribución de las inclusiones puede ayudar a distinguir entre los procesos que afectan idénticamente lazonación de elementos mayores pero produjo texturas variables en el granate.

Palabras claves: Colombia; granate; zonación; Macizo de Santander; Formación Silgará.

Introduction

Garnet is one of the most studied minerals in relationwith chemical variations in metamorphic rocks sinceit preserves a record of its growth history expressedby the chemical zoning and inclusions of other min-erals. The broad range in mineral composition pro-motes the growth of garnet in rocks of very differentchemical compositions and over a wide spectrum ofmetamorphic conditions. Furthermore, its refractorycharacter allows the preservation of chemical andtextural zoning that is important when making inter-pretations about the metamorphic history of the hostrock. Of particular interest in recent years have beenthe trace element zoning in garnet as an importantsource of information for petrologic processes(Hickmott & Shimizu, 1990; Schwandt et al., 1996;Chernoff & Carlson, 1999; Yang & Rivers, 2001;Skelton et al., 2002) and for relating the growth of

geochronologically important accessory phases (e.g.,monazite) to metamorphic reaction histories (Pyle &Spear, 1999; Gibson et al., 2004). Major and trace el-ement zoning in garnet have been typically explainedby (1) elemental fractionation during mineral growth(e.g., Hollister 1966; Cygan and Lasaga 1982), (2)by intracrystalline diffusion (e.g., Anderson andBuckley 1973), (3) limitations at the mineral-matrixinterface (e.g., Carlson 1989), (4) interaction with ametasomatic fluid (e.g., Hickmott et al. 1987; Youngand Rumble 1993; Chamberlain and Conrad 1993;Erambert and Austrheim 1993; Jamtveit et al. 1993;Jamtveit and Hervig 1994), (5) the breakdown orgrowth of trace element-rich minerals (Hickmott &Shimizu, 1990; Hickmott and Spear 1992) or (6)changes in the garnet-mineral matrix partition coeffi-cients because of changes in pressure and tempera-ture conditions, garnet composition or the number ofmineral phases in the assemblage or their proportions

8

CARLOS ALBERTO RÍOS REYES, ÓSCAR MAURICIO CASTELLANOS ALARCÓN AND AKIRA TAKASU

(Yang and Rivers, 2002). The majority of these pro-cesses occur during crystal growth. Diffusionalreequilibration is the only primary postgrowth pro-cess that leads to zoning in minerals. In an extremecase, compositional zoning can arise in garnet afterits crystallization with a homogeneous compositionthat is in desequilibrium with the matrix. Because achemical gradient exists between the garnet and thesurrounding matrix, often biotite-rich, volume diffu-sion acts to reequilibrate the garnet composition withthe matrix (e.g., Tracy et al., 1976). Diffusion occursas long as the temperature remains sufficiently high(e.g., Lasaga 1983). Trace elements are extremelysensitive to changes in accessory mineral assemblageand/or fluid composition and many trace-elementdiffusivities in garnet must be very much slower thandiffusivities for major elements (Mg, Mn, Fe) in gar-net, but probably on the same order as the diffusivityof Ca in garnet (Pyle & Spear, 1999). Trace elementdistributions in garnet must, however, be interpretedwith caution. Previous studies have documentedtrends in Ca, Mn and Y zoning in garnet (e.g.,Chernoff & Carlson, 1997; Pyle & Spear, 1999;Yang & Rivers, 2002) and discussed the possibilitythat low- or high- annuli within garnet are related tolocal disequilibrium in some elements, but not all.For example, patterns that deviate from normalgrowth zoning in garnet (e.g., euhedral bands con-centric about the garnet core, patches or spiral tocurving patterns) might form by garnet overgrowthof phases enriched or depleted in particular major ortrace elements (e.g., Mn; Yang & Rivers, 2001;Hirsch et al., 2003), by resorption of garnet duringgarnet-consuming reactions that produce staurolite(Menard & Spear, 1993) or by growth of garnet aftera matrix phase has been depleted (e.g., epidote,chloritoid; Whitney & Ghent, 1993). The resultinggarnet zoning patterns will be a function of the localchemical heterogeneities, the temperature of meta-morphism during and after garnet growth and thegrowth rate of garnet, factors that will affect both thezoning and the distribution of mineral inclusions ingarnet. Chernoff & Carlson (1997, 1999) consideredthe possibility of independent, local scales of equi-librium for different elements. They demonstrated

that many trace elements, as well as Ca, reflect dis-equilibrium at thin-section scale and thereforethermobarometric estimates that involve grossularcontents may be in error. Garnet zoning may also beaffected by fluid flow (e.g., Stowell et al. 1996;Skelton et al., 2002) and deformation and these pro-cesses may be coupled, as deformation will changegrain size and adjust grain boundaries, affecting ratesand pathways for diffusion. Garnets are frequentlyzoned in the major elements Fe, Mg, Mn and Ca. Athigh temperature, major element growth zoning maybe significantly modified by intracrystalline diffu-sion while trace element zoning may be less suscepti-ble to diffusion (e.g., Hickmott & Spear, 1992;Lanzirotti, 1995; Chernoff & Carlson, 1999). In thispaper, we report zoning patterns for garnets of themetapelitic Silgará Formation of the southwesternSantander Massif, Colombian Andes. We documentmajor and trace element zoning in garnets and evalu-ate the processes that control and influence zoning

during prograde metamorphism.

Geological setting

The Santander Massif lies within the Eastern Cordil-lera of the Colombian Andes, where it divides intothe northeast-trending Perijá in Colombia and theeast-northeast-trending Mérida Andes in Venezuela(Figure 1). The metamorphic history of this massif isimportant for interpretation of the geologic and tec-tonic evolution of the northwestern continental mar-gin of South America. The Santander Massif isunderlain by deformed and metamorphosed rocksthat have been tectonostratigraphically divided intothree metamorphic units (Bucaramanga Gneiss Com-plex, Silgará Formation and Orthogneiss), which areintruded by several igneous bodies, most of them ofTriassic-Jurassic age and some of Paleozoic age(e.g., Goldsmith et al., 1971; Boinet et al., 1985;Dörr et al., 1995; Ordoñez, 2003). The intrusivesform part of the Santander Plutonic Complex and areinterpreted as calc-alkaline crustal bodies emplacedafter peak-metamorphism. Sedimentary rocks rang-ing in age from Devonian to Tertiary flank the corerocks. The garnet-bearing samples investigated in the

9

A NEW INTERPRETATION FOR THE GARNET ZONING IN METAPELITIC ROCKS OF THE SILGARÁ FORMATION,


present study belong to the metapelitic sequence ofthe Caledonian Silgará Formation that representswhat was originally a very thick pile of volcano-sedi-mentary rocks. Well-exposed sections of this meta-morphic unit crop out at the southwestern region ofthe Santander Massif (Figure 1), which is long estab-lished as classic area for the study of rock metamor-phism and deformation caused by continentalcollision during the Caledonian orogeny. Accordingto Ríos et al. (2003), this region experienced crustal

thickenning during the initial stages of collision, fol-lowed by a slow uplift and erosion period, resultingin a decompression accompanied by heating. TheSilgará Formation structures and metamorphic fea-tures here are complex and early workers had greatdifficulty in making sense of the geology of the area.The metamorphic rocks of the Silgará Formationgenerally strike NW-SE and dip to the southwest. TheNW-trending Bucaramanga Fault represents a verymajor break in the crust in the northeastern part of the

10


Figure 1. Lower left side, map of the Colombian Andes, showing the location of the Santander Massif in the EasternCordillera. Upper left side, simplified geological map of the Santander Massif, showing the distribution of the SilgaráFormation metamorphic rocks in grey (modified after Goldsmith et al., 1971) and the location of the study area in black.Right side, generalized geological map of the study area (modified after Ward et al., 1973), showing locations of garnetsamples investigated (black stars). Metamorphic zones modified after Ríos et al. (2003). BF: Bucaramanga Fault; LSAF. LosSantos - Aratoca Fault.

study area and separates the Precambrian Buca-ramanga Gneiss Complex from the Triassic-JurassicPescadero and Mogotes batholiths. This fault is prob-ably strike slip with a protracted history of displace-ment (Goldsmith et al., 1971). In the southwest, nearAratoca, the Silgará Formation is cut by theNW-SE-trending Los Santos - Aratoca Fault and isunconformably overlain by Mesozoic sedimentaryrocks (Figure 1). It is also unconformably overlain bya Mesozoic sequence on the northwest and the south-west. The Triassic-Jurassic Pescadero and Mogotesbatholiths intruded the Silgará Formation in thenortheastern and eastern part of the area afterpeak-metamorphism (Goldsmith et al., 1971). TheSilgará Formation is dominated by metapelitic rockswith minor intercalations of metabasites. The lithol-ogy of the metapelitic sequence changes in composi-tion northeastward, from quartz-rich pelitic schists inthe southwest to feldspar-rich semipelitic schists inthe northeast. A millimeter-scale compositionalbanding consists of alternating granoblastic quartz-plagioclase-rich domains with minor K-feldspar andmuscovite-biotite-rich domains. Subordinated rocktypes of the Silgará Formation are well-foliated am-phibole-bearing schists, orthoamphibolites andcalc-silicate rocks. Microstructural evidence showsthat the Silgará Formation underwent prograde meta-morphism during at least three deformation phases,including extensive retrograde metamorphism dur-ing the last stage. The dominant schistosity or slatycleavage in the metapelitic rocks may have devel-oped at the same time as early folds and thrusts. Themetapelites display a well-developed schistosity, al-though well-preserved sedimentary features are lo-cally preserved in low-grade rocks. In general, laterstructures such as flat-lying crenulations, small chev-ron folds and kink bands overprint the main foliation.Retrograde textures include partial replacement ofgarnet by chlorite and/or muscovite or chlorite andbiotite along cracks; heavily corroded staurolite crys-tals surrounded by muscovite; biotite and calcic-am-phibole replaced by chlorite (although in some casesamphibole is replaced by biotite); and feldspar partlyreplaced by sericite. A progressive sequence of meta-morphic zones has developed in pelitic rocks of the

Silgará Formation. The regional metamorphic gradeincreases with structural depth from the biotite zoneto the staurolite-kyanite zone. Garnet-bearing pelitesare widespread throughout the region. Barrovian re-gional metamorphism occurred under low- tohigh-temperature and medium-pressure conditions.The distinction between the staurolite-kyanite andlower sillimanite (fibrolite) zones is not well definedbecause fibrolitic sillimanite is present in traceamounts in staurolite-kyanite bearing samples. Theoccurrence of fibrolite in sample PCM-473 lackingother Al2SiO5 phases near the contact with thePescadero Pluton is interpreted as formed in the wan-ing stages of a thermal event. Therefore, thefibrolitization process should not be always consid-ered as a polymorphic reaction. Ríos et al. (2003) esti-mated the temperatures and pressures of equilibrationof mineral assemblages from metamorphic rocks ofthe southwestern Santander Massif. P-T conditionswere 500-520°C and 4.4-5.5 kbar in the garnet zoneand 590-700°C and 5.5-7.5 kbar in the gar-net-staurolite zone.

Analytical methods

X-ray maps were collected and analyses were carriedout using the JEOL 8800 electron probemicroanalyzer at the Research Center for Coastal La-goon Environments at the Shimane University (Ja-pan) and the JEOL 8900 electron probe micro-analyzer at the Department of Geology and Geophys-ics at the University of Minnesota (USA). Accessorymineral phases were identified using back-scatteredelectron imaging (BEI) and energy-dispersive spec-troscopy (EDS). The analytical conditions were asfollows: accelerating voltage of 15 kV and beam cur-rent of 20-25 nA (for quantitative analyses) and25-75 nA (with dwell times of 40-55 msec/pixel, formajor element maps) or higher beam current (100nA) and longer dwell times (up to 90 msec/pixel) fortrace element maps. Qualitative X-ray maps were ob-tained first to delineate geochemically significantzones and to guide quantitative spot analysis (in ra-dial traverses and spacing decreasing near the rim).Data adquisition and reduction were carried out us-

11



ing the ZAF correction procedures. A combination ofnatural and synthetic minerals were used as stan-dards.

Chemical zoning and textural relationsin garnet

A detailed petrographic study of six thin sections wascarried out on selected specimens which are the sameas those studied by Ríos (1999) and corresponding toa series of pelitic rocks of the Silgará Formation com-posed of quartz + plagioclase + muscovite + biotite +garnet ± staurolite and accessory fibrolitic sillimaniteand rare crystals of andalusite and kyanite (one sam-ple contained the three polymorphs). In the garnetzone, garnets are commonly idioblastic, whereas inthe garnet-staurolite and staurolite-kyanite zonesthey tend to be rounded and embayed, displaying re-action rims. As follows we describe petrographic fea-tures of the major and accessory minerals in theanalyzed samples as illustrated in Figure 2. The min-eral assemblages of the analyzed samples are given inTable 1. Mineral abbreviations are after Kretz(1983). All the examined samples contain garnetporphyroblasts that preserve growth zoning to vary-ing degrees. Garnet is almandine-rich and exhibitszoning in which Mn decreases from core to rim asMg and Fe increase. The zoning of Fe, Mg and Mnzoning is strongly correlated with respect to the de-gree of local equilibrium in contrast with Ca zoningthat does not mimic Fe, Mg and Mn zoning exactly(e.g., Chernoff & Carlson, 1997; Spear & Daniel,1998). According to Hirsch et al. (2003), it reflects acombination of changing availability of these ele-ments to the surface of the growing garnet and chang-ing partitioning of elements among the mineralphases in the rock. Garnet shows abrupt variations inCa distribution from core to rim, developing in somecases a complex cyclic zoning with euhedral low- orhigh-Ca annuli. Trace element zoning in garnet willdepend on the presence or absence of saturingphases, such as xenotime for Y, xenotime, apatite ormonazite for P and ilmenite, titatine and rutile for Ti.Analyzed garnets do not show zoning in Y, Sc, P, Cror Ti, except garnet zone sample PCM-441, which ex-

hibits Y zoning. Yttrium is potentially useful formonitoring reactions involving Ca-rich mineralssuch as epidote and plagioclase because this elementsubstitutes for Ca in mineral structures and Y discon-tinuities in garnet zoning may correlate with inflec-tions in Ca zoning (Hickmott & Spear, 1992). In thefollowing sections, we describe the main features ofchemical zoning in garnet from the Silgará Forma-tion pelites. Table 2 shows representative chemicalcompositions of analyzed garnets. Analytical pointsare indicated by white dots and numbers in the X-raymaps, with numbers keyed to the table.

PCM-441 Garnet. Pelitic schist from the gar-net-zone, which typically contain a mineral assem-blage of muscovite + quartz + plagioclase + garnet ±biotite, with minor K-feldspar, tourmaline, apatite,zircon, epidote, calcite and Fe-Ti oxides. Numer-ous, very fine-grained (0.03-0.42 mm in diameter),euhedral grains of garnet, with a hexagonal or pen-tagonal outline, occur in this sample (Figure 2a).Garnet grew after an early foliation and contain in-clusion trails of ilmenite and graphite concordant tothe main foliation (crenulation cleavage), althoughit may have inclusion-free rims. It is replaced alongtheir margins and fractures by chlorite, biotite andFe-oxides. The most striking characteristic of thisgarnet is the high Mn concentration, which fromcore to rim varies from 54 to 25 mol% spessartine(Figure 3a). Fe and Mg increase from core to rimwith a small decrease in Fe/Fe+Mg. Garnet has alow-Ca core (7 mol%) with an inflection midway(14 mol%) between core and rim, decreasing to-wards the rim (8 mol%), as shown in Figure 4a.Trace element maps for garnet in sample PCM-441show that yttrium is the only one that displays zon-ing. With the possible exception of Yb, whichshows a slight enrichment in the garnet core, theother trace elements (Sc, P, Cr, Ti and Yb) are in lowabundance and homogeneous across the garnet.Garnet is characterized by a euhedral outline,high-Y core (Figure 4b), which is larger than thefaint Yb-rich region. The Y-rich core region corre-sponds exactly to the low Ca core region. RelativelyY-enriched regions of the garnet also occur discon-tinuously along the rim and in the Ca-poor core of

12


13



Figure 2. Plane-polarized light photomicrographs of garnet-bearing metamorphic rocks of the Silgará Formation. (a)Numerous very fine-grained garnet crystals; sample PCM-441 (garnet zone). (b) Garnet with inclusion-rich (quartz andilmenite) core and inclusion-free rim regions; sample PCM-420 (garnet-staurolite zone). (c) Garnet porphyroblast displayingtwo different inclusion patterns involving ilmenite; sample PCM-361 (garnet-staurolite zone). (d) Inclusion trails of quartzand ilmenite preserved in garnet porphyroblasts; sample PCM-516 (garnet-staurolite zone). (e) Fibrolitic sillimaniteoccurring at core of muscovite adjacent to garnet; sample PCM-473 (staurolite-kyanite zone). (f) Garnet-bearingcalc-silicate rock occurring as numerous fine- to medium-grained crystals associated with calcic amphibole; samplePCM-514 (garnet-staurolite zone).

14


Tab

le1.

Min

eral

asse

mbl

ages

ofth

ean

alyz

edsa

mpl

esof

the

Sil

gará

For

mat

ion

peli

tic

rock

sfr

omth

eso

uthw

este

rnS

anta

nder

Mas

sif

Met.

Zo

ne

Sam

ple

Ch

lB

tM

sQ

zP

lK

fsG

rtSt

An

dK

ySil

Ca-A

Zr

Ap

Mgn

Ilm

Ru

tEp

Mn

zX

en

Tu

rC

al

Garn

et

PC

M-4

41

rX

XX

Xm

Xm

mm

mm

r

Garn

et-St

auro

lite

PC

M-3

61

rX

X,r

X,i

XX

Xi

ii

ii

i

Garn

et-St

auro

lite

PC

M-5

16

rX

XX

,iX

XX

II

Garn

et-St

auro

lite

PC

M-5

14

rX

,iX

,iX

,iX

,iX

X,i

ii

ii

r

Garn

et-St

auro

lite

PC

M-4

20

rX

X,i

X,i

XX

ii

Sta

uro

lite-

Kyanite

PcM

-47

3r

X,i

X,r

,iX

,iX

,iX

Xm

,ii

mm

Qtz

,Bt,

Pl,

and

retr

ogra

deC

hlar

eco

mm

onin

alls

ampl

es.X

-pre

sent

,bla

nk-a

bsen

t,m

-min

orph

ase,

i-in

clus

ions

inga

rnet

,r-r

etro

grad

em

iner

als.

Min

eral

abbr

evia

tion

saf

ter

Kre

tz(1

983)

15



Figure 3. Zoning profiles rim-core-rim of almandine, pyrope, spessartine and grossular components and Fe/(Fe+Mg) ratioin garnet from samples (a) PCM-441 (garnet zone), (b) PCM-514 (garnet-staurolite zone), (c) PCM-361 (garnet-staurolitezone), (d) PCM-516 (garnet-staurolite zone), (e) PCM-420 (garnet-staurolite zone) and (b) PCM-473 (staurolite-kyanitezone). Note the different scales for Xsps, Xprp and Xgrs on the left and for Xalm and Fe/(Fe+Mg) on the right.

an adjacent small garnet. X-ray maps for Ca distri-bution in garnet and its vicinity indicate that thereare no Ca-rich accessory phases included in garnetand only one small apatite crystal in the matrix just

beyond the field of view in Figure 4.

PCM-361 Garnet. Pelitic schist from the gar-net-staurolite-zone characterized by the peak meta-morphic assemblage of quartz + plagioclase +muscovite + biotite + garnet + staurolite. Garnetporphyroblasts (1.5-2.6 mm in diameter) areanhedral to subhedral with an elliptical outline andcontain numerous inclusions of ilmenite that definedifferent pattern of distribution (Figure 2c). At thecore region, the inclusion patterns can be concordantor discordant to the main foliation of the rock orramdonly oriented, whereas at the rim region, theyfollow the rim of the garnet, defining an approxi-mately circular pattern. Garnet porphyroblasts areawrapped by a penetrative foliation (crenulationcleavege). Garnet is strongly zoned in Mn (Figures3c, 5b), which decreases from 21 mol% in the core to1-2 mol% in the rim. Fe and Mg increase from core torim, with a small decrease in Fe/Fe+Mg. Ca de-

creases from core (5 mol%) towards the rim, with aslight discontinuity in the zoning midway betweencore and rim, reaching a minimum (1-2 mol%) repre-sented by an euhedral low-Ca annulus (Figures 5a,6a); then increases towards the rim (6 mol%). Thelow-Ca annulus coincides with the euhedral zone ofhigh-Mn (6-9 mol%) and is truncated in the bottompart due to partial resorption of garnet. Analyzed gar-nets all exhibit complex Ca zoning, but there is no ap-parent zoning in analyzed trace elements (Y, Yb, P,Ti, Cr). Garnet is texturally zoned with respect to thedistribution and shape orientation of ilmenite inclu-sions and textural zoning can be related to chemicalzoning (Figures 2c, 5, 6a). In the Mn-rich garnet coreregion, ilmenite inclusions are randomly oriented. In-clusions are absent in the low-Ca annulus. In thehigh-Ca / low-Mn near-rim zone, ilmenite inclusionsare abundant and are aligned parallel to the garnetrim, creating a circular pattern, which is also apparentin matrix ilmenite adjacent to garnet (Figure 2c).Ilmenite occurs as inclusions in garnet that have beenpartially pseudomorphed by rutile (Figure 7) or as amatrix accessory mineral. Other inclusions in garnetare quartz, apatite, monazite, zircon and rutile, which

16


Figure 4. X-ray compositional maps in garnet from sample PCM-441 (garnet zone). (a) Ca map, showing a high-Ca band atmantle region and the relationship between the low-Ca at core and the high-Y annulus. Numbered spots are keyed toanalyses in Table 2. (b) Y map, displaying a high-Y annulus at core characterized by sharp euhedral and straight outlines. Inthe X-ray maps, the elemental distributions are indicated by white color for the highest and black color for the lowestconcentration.



Tab

le2.

Rep

rese

ntat

ive

com

posi

tion

sof

met

amor

phic

garn

etfr

omth

eS

ilga

ráF

orm

atio

n,so

uthw

este

rnS

anta

nder

Mas

sif.

Majo

r

ele

men

ts

PC

M-

441

PC

M-3

1P

CM

-

516

PC

M-

514

PC

M-

420

PC

M-4

73

rim

core

rim

rim

core

rim

rim

rim

core

rim

core

rim

rim

core

rim

wt%

12

34

51

23

45

67

11

23

45

67

81

23

41

23

4

SiO

236,3

936,7

636,1

736,9

636,8

237,7

337,4

536,7

636,7

936,6

736,6

937,1

037,6

136,8

837,3

136,6

237,4

436,7

637,3

336,8

537,6

636,6

336,5

37,2

636,7

637,3

837,4

537,3

037,6

7

TiO

20,0

20,0

60,0

80,0

50,0

50,0

00,0

00,0

90,1

20,0

00,0

10,0

50,0

20,0

40,0

20,1

20,1

60,4

60,0

50,0

30,0

30,1

30,0

20,0

00,0

00,0

80,0

00,0

00,0

4

Al 2

O3

20,6

420,3

220,0

320,5

920,6

920,6

120,4

920,5

320,5

520,5

220,6

320,4

119,9

120,2

820,1

920,4

619,8

119,1

720,4

620,3

120,7

019,7

920,2

420,6

320,2

520,1

020,4

620,5

020,2

3

FeO

*25,4

217,9

516,6

122,1

326,2

638,8

637,0

733,8

432,1

034,6

534,9

437,7

527,9

223,3

923,4

622,6

423,8

022,5

522,9

221,4

523,8

524,6

625,7

232,0

931,2

234,8

334,1

734,9

134,1

5

MnO

12,2

820,3

422,9

214,5

211,2

01,9

12,7

86,5

19,2

15,5

84,9

90,7

59,2

78,7

28,8

58,8

59,1

49,2

49,7

910,1

48,4

213,8

911,6

3,4

34,6

83,3

92,5

73,7

33,8

MgO

2,2

61,1

20,9

81,7

22,4

72,2

92,7

62,0

41,6

82,2

12,2

22,7

73,5

12,8

62,7

62,4

02,6

52,3

62,5

92,4

32,8

11,5

71,7

22,7

72,4

71,7

22,8

51,7

82,0

3

Ca

O2,9

54,2

22,5

25,0

22,7

61,2

10,6

31,6

01,6

11,2

70,9

71,9

32,0

96,3

95,9

87,9

16,5

67,5

26,2

57,3

15,9

83,5

23,6

02,9

72,7

21,8

82,1

61,8

52,0

0

Na

2O

0,0

00,0

10,0

70,0

30,0

10,0

40,0

60,0

40,0

60,0

50,0

30,0

40,0

40,0

30,0

60,0

30,0

30,0

40,0

30,0

00,0

20,0

10,0

40,0

00,0

10,0

20,0

10,0

50,0

5

K2O

0,0

60,0

40,0

50,0

40,1

70,0

70,0

50,0

50,0

60,0

30,0

50,0

40,0

80,0

80,0

40,0

60,0

50,0

40,0

50,0

30,0

40,0

40,0

50,0

50,0

50,0

40,0

40,0

10,0

8

Cr 2

O3

0,0

00,0

00,0

00,0

00,0

00,0

40,0

10,0

00,0

30,0

00,0

00,0

70,0

00,0

10,0

00,0

00,0

10,0

20,0

00,0

20,0

00,0

10,0

00,0

10,0

20,0

00,0

10,0

20

To

tal

100,0

1100,8

299,4

3101,0

7100,4

3102,2

5101,2

8101,4

6102,2

0100,9

8100,8

2100,9

0100,4

598,6

898,6

7100,0

999,6

598,1

699,4

798,5

799,5

1100,2

599,4

999,2

198,1

899,4

299,7

3100,1

9100,0

4

Cati

on

sb

ase

do

n12

ato

ms

of

oxy

gen

Si

2,9

61

2,9

82

2,9

92,9

74

2,9

78

3,0

12

3,0

11

2,9

70

2,9

63

2,9

74

2,9

94

2,9

92

3,0

27

2,9

96

3,0

33,0

08

3,0

20

3,0

13,0

13

3,0

20

2,9

91

2,9

89

3,0

20

3,0

21

3,0

52

3,0

28

3,0

26

3,0

52

Ti

0,0

01

0,0

04

0,0

10,0

03

0,0

03

0,0

00

0,0

00

0,0

05

0,0

07

0,0

00

0,0

01

0,0

03

0,0

01

0,0

02

00,0

07

0,0

10

0,0

30

00,0

02

0,0

08

0,0

01

0,0

00

0,0

00

0,0

05

0,0

00

0,0

00

0,0

03

Al

1,9

79

1,9

42

1,9

51,9

52

1,9

72

1,9

38

1,9

41

1,9

55

1,9

51,9

61

1,9

67

1,9

39

1,8

88

1,9

41

1,9

30

1,9

28

1,8

81,8

51,9

41,9

51,9

56

1,9

05

1,9

53

1,9

71

1,9

61

1,9

34

1,9

50

1,9

65

1,9

32

Fe

3+

0,0

96

0,0

87

0,0

70,0

93

0,0

66

0,0

38

0,0

38

0,0

94

0,1

09

0,0

91

0,0

42

0,0

72

0,0

56

0,0

62

0,0

17

0,0

41

0,0

58

0,0

60,0

40,0

60,0

00

0,0

97

0,0

67

0,0

00

0,0

00

0,0

00

0,0

00

0,0

00

0,0

00

Fe

2+

1,6

33

1,1

31,0

81,3

96

1,7

10

2,5

22

2,4

54

2,1

93

2,0

53

2,2

59

2,3

22

2,4

74

1,8

23

1,5

26

1,5

71,4

73

1,5

51,4

82

1,5

10

1,4

1,5

99

1,5

87

1,6

94

2,1

75

2,1

46

2,3

78

2,3

10

2,3

68

2,3

14

Mn

0,8

46

1,3

97

1,6

0,0

90

0,7

67

0,1

29

0,1

89

0,4

45

0,6

28

0,3

83

0,3

42

0,0

51

0,6

32

0,6

00

0,6

08

0,5

99

0,6

20,6

40,6

70,7

0,5

72

0,9

61

0,8

04

0,2

35

0,3

26

0,2

34

0,1

76

0,2

56

0,2

61

Mg

0,2

75

0,1

35

0,1

20,2

06

0,2

98

0,2

73

0,3

31

0,2

46

0,2

02

0,2

67

0,2

68

0,3

33

0,4

21

0,3

46

0,3

30,2

86

0,3

20,2

90,3

10,2

90,3

36

0,1

91

0,2

10

0,3

35

0,3

03

0,2

09

0,3

44

0,2

15

0,2

46

Ca

0,2

57

0,3

67

0,2

20,4

32

0,2

39

0,1

03

0,0

54

0,1

38

0,1

39

0,1

10

0,0

84

0,1

67

0,1

80

0,5

56

0,5

20

0,6

78

0,5

70,6

60

0,5

40

0,6

40,5

14

0,3

08

0,3

16

0,2

58

0,2

39

0,1

64

0,1

87

0,1

61

0,1

73

Na

0,0

01

0,0

03

0,0

30,0

11

0,0

02

0,0

11

0,0

18

0,0

12

0,0

18

0,0

16

0,0

09

0,0

13

0,0

13

0,0

10

0,0

20

0,0

10,0

10

0,0

13

0,0

10

0,0

00

0,0

06

0,0

03

0,0

13

0,0

00

0,0

03

0,0

06

0,0

02

0,0

15

0,0

15

K0,0

12

0,0

09

0,0

10

0,0

09

0,0

36

0,0

14

0,0

10

0,0

11

0,0

12

0,0

06

0,0

11

0,0

07

0,0

17

0,0

17

0,0

10,0

13

0,0

10,0

10,0

10,0

10,0

09

0,0

09

0,0

11

0,0

11

0,0

11

0,0

08

0,0

09

0,0

03

0,0

18

Cr

0,0

00

0,0

00

0,0

00

0,0

00

0,0

00

0,0

02

0,0

00

0,0

00

0,0

02

0,0

00

0,0

00

0,0

04

0,0

00

0,0

01

0,0

00

0,0

00

00

0,0

00

00,0

00

0,0

01

0,0

00

0,0

01

0,0

01

0,0

00

0,0

01

0,0

01

0,0

00

To

tal

8,0

61

8,0

56

8,0

78,0

67

8,0

71

8,0

43

8,0

46

8,0

70

8,0

84

8,0

67

8,0

41

8,0

54

8,0

58

8,0

58

8,0

48,0

43

8,0

58,0

53

8,0

38

8,0

48,0

15

8,0

60

8,0

58

8,0

05

8,0

12

7,9

91

8,0

07

8,0

10

8,0

12

XM

g0,1

40,1

10,1

00,1

30,1

50,1

00,1

20,1

00,0

90,1

10,1

00,1

20,1

90,1

80,1

70,1

60,1

70,1

60,1

70,1

70,1

70,1

10,1

10,1

30,1

20,0

80,1

30,0

80,1

Xalm

0,5

60,3

90,3

70,4

80,5

80,8

40,8

10,7

30,6

90,7

60,7

70,8

20,6

00,5

10,5

20,4

90,5

20,4

90,5

0,4

70,5

30,5

40,5

70,7

20,7

10,8

00,7

70,7

90,7

7

Xsp

s0,2

70,4

50,5

20,3

20,2

50,0

40,0

60,1

40,2

00,1

20,1

10,0

20,2

00,1

90,2

0,1

90,2

00,2

00,2

20,2

30,1

90,3

10,2

60,0

80,1

10,0

80,0

60,0

90,0

9

Xp

rp0,0

90,0

40,0

40,0

70,1

00,0

90,1

10,0

80,0

60,0

90,0

90,1

10,1

40,1

10,1

10,0

90,1

00,0

90,1

00,1

0,1

10,0

60,0

70,1

10,1

00,0

70,1

10,0

70,0

8

Xgrs

0,0

80,1

20,0

70,1

40,0

80,0

30,0

20,0

40,0

40,0

40,0

30,0

50,0

60,1

80,1

70,2

20,1

80,2

10,1

80,2

10,1

70,1

00,1

00,0

90,0

80,0

50,0

60,0

50,0

6

XM

G=

Mg/

(Fe+

Mg)

*T

otal

Fe

asF

eO+

Fe 2

O3

also occur as matrix phases. Apatite is most abundantin the Mn-rich core region (also outlined by thelow-Ca annulus) and is rare in the outer parts of thegarnet except very close to the rim, where a few largegrains are located (Figure 5, below). Monazite in-creases in abundance from garnet core to rim to ma-trix, but is entirely lacking from the low-Ca zone.Xenotime occurs only as a matrix phase.

PCM-516 Garnet. Pelitic schist from the garnet-staurolite-zone, which contains a mineral assemblageof quartz + plagioclase + K-feldspar + muscovite +

biotite + garnet. Garnet porphyroblasts are subhedral,0.3-2.5 mm in diameter, pseudopentagonal andpoiquiloblastic (Figure 2d). They contain inclusiontrails of quartz and platy ilmenite that preserve evi-dence of an earlier fabric occurring at a high angle tothe main fabric, which corresponds to a crenulationcleavage. An inclusion-free rim separates the corefrom the external fabric. Garnet is associated withsymmetrical and asymmetrical pressure shadows.The fabric outside of garnet porphyroblasts generallydisplays evidence of recrystallization and grain-size

18


Figure 5. Above, X-ray compositional maps in garnet from sample PCM-361 (garnet-staurolite zone). (a) Occurrence of alow-Ca annulus characterized by sharp pentagonal and straight outlines. (b) Relationship between the euhedral zone ofhigh-Mn region and the low-Ca annulus. In the X-ray maps, the elemental distributions are indicated by white color for thehighest and black color for the lowest concentration. (c) Sketch of same garnet showing that the low-Ca annulus lacks ofinclusions. Accessory mineral phases are indicated by black dots (apatite), x marks (xenotime), stars (monazite) and dashes(ilmenite). Courtesy of Donna Whitney.

coarsening, respect to the inclusions within garnet. Asecond stage of garnet growth is indicated by the in-clusion-free rim that separates the core from the dis-cordant external fabric. Garnet shows decreasing Mnfrom the core (34 mol%) to the rim (16 mol%). Feand Mg increase from core to rim, with a small de-crease in Fe/Fe+Mg. This garnet is characterized by asmall reversal in zoning near the rim. Fe, Mg and Mndistributions are strongly correlated with each other,whereas Ca distribution is not (Figures 3d, 6b). Cadecreases outwards and reaches a minimum atmid-region (4 mole %), then increases towards therim. Inclusions in garnet are ilmenite and quartz inthe core region. Irregular patches of calcite occurwithin garnet and in the matrix and may be texturallylate, but we note that the distribution of calcite in gar-net is confined within the core region, which is bor-dered by the low-Ca ring. Calcite does not occur inthe grossular-rich outer core/rim of the garnet, al-though the outer core/rim region is highly fractured.Trace elements were not analyzed.

PCM-514 Garnet. Calc-silicate rock from thegarnet-staurolite-zone characterized by a mineral as-semblage of quartz + plagioclase + K-feldspar + gar-net + calcic amphibole. A very interesting texture isobserved in this sample, with clusters of numerousfine- to medium-grained crystals of garnet, which are0.1-3.3 mm in diameter, anhedral to subhedral andpseudohexagonal and rounded shape, containingabundant quartz inclusions throughout its core (Fig-ure 2f). Other inclusions are epidote, plagioclase, bi-otite and ilmenite. Each crystal of garnet has majorelement zoning (Figure 3c), but no detectable traceelement zoning in the analyzed elements. Thegrossular component displays a complex oscillatoryzoning, fluctuating between 17 and 24 mol%, oppo-site to the trend in zoning in almandine (Figure 6c).From core to rim, spessartine decreases andFe/(Fe+Mg) increases. Analyzed garnets have atleast two high-Ca annuli. The variable grossular con-tent may be related to reactions involving other calcicphases in this rock calcic amphibole, plagioclase,epidote). The garnet core, defined as the regionwithin the low-Ca ring, contains abundant mineralinclusions. Quartz is most common, but epidote,

plagioclase, K-feldspar, calcic amphibole, biotite,magnetite and ilmenite also occur. Patchy calcite ispresent in the garnet and appears to be related to frac-tures. Calcite occurs both within and beyond thelow-grossular ring in the outer core. Figure 8a showsthat Na is antipathetic with Ca, which suggest that theanorthite content in plagioclase varies with thegrossular content in garnet and therefore it is neces-sary a source of Ca in addition to the garnet andplagioclase. On the other hand, Ti show a positivecorrelation with Ca (Figure 8b), which could reflectthe consumption of a Ti-rich mineral phase such as il-menite or rutile during garnet growth. Similar oscil-latory zoning has been reported by other authors(e.g., Jamtveit et al., 1995; Holten et al., 1997;Ivanova et al., 1998; Jamtveit, 1999; Pollok et al.2001), probablye due to fluctuations in supersa-turation, which might have originated from external(boiling, fluid mixing, temperature, and pressurefluctuations) or internal factors, resulting fromself-organization in the interplay of fluid convectionand crystal growth in the near vicinity of the growingcrystals (Jamtveit et al. 1995; Holten et al. 1997;Pollok et al. 2001).

PCM-420 Garnet. Quartz-feldespatic pelitic rockfrom the garnet-staurolite-zone that contains a min-eral assemblage of quartz + plagioclase + K-feldspar+ biotite + garnet. Garnet occurs as large (1-2 mm indiameter) subhedral and subrounded porphyroblastswith cores densely packed with very fine inclusionsof quartz and minor ilmenite with no preferred orien-tation that give the host crystal a spongy appearanceand inclusion-free rims or as small (0.5-1 mm in di-ameter) subhedral and subrounded grains with inclu-sion-free cores (Figure 2b). The fabric outside ofgarnet porphyroblasts shows recrystallization andgrain-size coarsening, respect to the inclusionswithin garnet. Small crystals of magnetite occurs ad-jacent to garnet. Garnet exhibits growth zoning: fromcore to near rim, there is an increase in Mg (from 3 to12 mol%) and a decrease in Mn (from 43 to 6 mol%;Figure 6d) and in Ca (from 18 to 8 mol%), with asmall decrease in the Fe/(Fe+Mg) ratio. Concentra-tions of Mn, Mg and Fe in the cores of the small gar-nets correspond with near-rim compositions in the

19



20


larger crystals, but Ca composition is not similarlysystematic. The concentration of Ca in the cores ofsmall garnets does not correspond to near-rim com-positions in the larger crystals. No discontinuity inthe compositional zoning of Mn occurs at the bound-ary between the inclusion-rich core and the inclu-sion-free rim. Within the high-Mn region, which also

corresponds to an inclusion-rich region in the garnet,zoning is not concentric about the core (Figure 6d).

PCM-473 Garnet. Pelitic schist from the

staurolite-kyanite-zone. It is characterized a mineral

assemblage of quartz + plagioclase + muscovite + bi-

otite + garnet + staurolite. Garnet in this sample oc-

Figure 6. X-ray compositional maps in pelitic garnets from the garnet-staurolite zone. (a) and (b) illustrate low-Ca annuliwith sharp pentagonal outlines in garnet from samples PCM-361 and PCM-516, respectively. In (a), at the bottom of thegarnet, the low-Ca annulus is truncated against biotite; white areas correspond to calcite (garnet core region and matrix) andCa-rich plagioclase (matrix). (c) Cyclic Ca zoning in garnet from sample PCM-514 with low- and high-Ca annuli with sharphexagonal outlines. (d) Mn distribution in garnet from sample PCM-420, showing a small reversal zoning at rim. Numberedspots are keyed to analyses in Table 2. In the X-ray maps, the elemental distributions are indicated by white color for thehighest and black color for the lowest concentration.

curs as large porphyroblasts, which are up to 5 mm in

diameter, anhedral (generally elongated) and contain

a small number of inclusions of quartz, biotite, mus-

covite, plagioclase, ilmenite and magnetite. It has

been highly cracked and partially replaced by quartz,

muscovite and sillimanite (Figure 2e). Fibrolite com-

monly occurs within muscovite that is associated to

garnet or as minute crystals sometimes penetrating

quartz grains, which develop embayment in garnet,

within which the polymorph of Al2SiO5 also occurs.

The staurolite usually occurs as relicts in muscovite.

Rutile occurs only as a matrix phase. Garnet exhibits re-

verse zoning, with a decrease in Mg (from 11 to 6

mol%), Figure 9a, and increase in Mn (from 7 to 10

mol%), Figure 9b, from core to rim and a small increase

in the Fe/(Fe+Mg) ratio. Composition is more homoge-

nous in the interior of the crystal and Ca content in-

creases slightly within the outer core (5-7 mol%),

where it reaches a maximum, then decreases to 5 mol%

at rim, developing a low-Ca annulus (Figure 9c).

Reaction history

A detailed discussion of the reaction history amongmajor mineral phases in metapelitic rocks of the

Silgará Formation at the southwestern SantanderMassif has been presented by Ríos (1999) and Ríos etal. (2003). Therefore, only metamorphic reactions ingarnet-bearing pelites will be considered here. Thetypical Barrovian progression of metamorphic reac-tions in these rocks involve low-grade garnet growth,intermediate-grade garnet consumption duringstaurolite producing and high-grade resumption ofgarnet growth during kyanite producing at expensesof staurolite, which is similar to what reported Kohn& Malloy (2004).

The first appearance of garnet was the result of aFe-Mg-Mn continuous reaction. The characteristicgarnet of the garnet zone is almandine-rich and prob-ably grew by two continuous reactions:

(1) Chl + Ms = Grt + Bt + Qtz + H2O

(2) Chl + Ms + Qtz = Grt + Bt + H2O

which can explain the formation of almandineand pyrope in garnet, but the high Ca amount ob-served in the garnet core or rim suggests that aCa-rich phase also participated in the garnet-formingreaction. The source of Mn is inferred to be Mn-richepidote, probably enclosed in clinozoisite or othermatrix minerals. The garnet isograd reaction is

21



Figure 7. Back-scattered electron images of garnet in sample PCM-361 (garnet-staurolite zone), which contains ilmeniteinclusions. (a)-(b) Unreacted crystal of ilmenite and adjacent pseudomorph of rutile after ilmenite.

strongly dependent on bulk rock composition andspecially the MnO and CaO contents. In rocks of highamount of those components, garnet may appear attemperatures below 450°C whereas in rocks of lowamount of them, garnet may not appear to well above500°C (Spear, 1993). Respect to the cyclic zoningobserved in calc-silicate rocks (e.g., samplePCM-514, Figure 6c), we consider that the high- andlow-Ca annuli in garnet may have grown, respec-tively, by the following reactions

(3) Chl + Ms + Ep = Grt + Bt + Pl + Ca-Amp ++ Bt + H2O,

(4) Chl + Pl + Ms + Qtz = Grt + Bt + Ca-Amp ++ H2O

However, the behavior of Ca-rich fluids shouldbe evaluated further by considering the trace elementconcentrations in garnet. According to Pollok et al.(2001), oscillatory zoning in garnet can be explainedthrough two different sets of processes: local growthdynamics vs external forcing mechanisms.

At the staurolite isograd, the first appearance ofstaurolite can be related to two reactions. Texturalevidence shows that staurolite may be produced atthe expense of garnet by the discontinuous reaction

(5) Grt + Chl + Ms = St + Bt + Qtz + H2O

In garnet-free assemblages, staurolite may beproduced by the continuous reaction

(6) Chl + Ms = St + Bt + Qtz + H2O

which can be deduced from metamorphic tex-tures, considering that staurolite grew mainly inphyllosilicate-rich layers (Graebner and Schenk,1999). When chlorite is completely consumed, asstaurolite becomes less abundant it may be pre-served as inclusions or armoured relics in musco-vite and garnet growth can be explained by thereaction

(7) St + Bt + Qtz = Grt + Ms + H2O

Staurolite may dissapears with further growth ofgarnet at the kyanite isograd as a result of the discon-tinuous reaction

(8) St + Ms + Qtz = Ky + Grt + Bt + H2O

If garnet is not involved, the staurolite break-down is produced by the continuous reaction

(9) St + Ms + Qtz = Al2SiO5 + Bt + H2O

which reflects lower temperatures than the dis-continuous reaction (6) and P-T conditions nearlyindependient of the MnO and CaO contents of therock. However, if garnet breaks down as sillimanite

22


Figure 8. Zoning profiles rim-core-rim in garnet from sample PCM-514 (garnet-staurolite zone), showing the correlation

between the weight percent of CaO and the (a) Na2O and (b) TiO2 contents. Lower axis indicates distance (mm). Left axisindicates concentration (wt%).

is produced, so the initial growth of sillimanite maybe considered to result from two separate continuousreactions operating simultaneously

(10) St + Ms + Qtz = Sil + Bt + H2O

(11) Grt + Ms = Bt + Sil + Qtz

Chemical zoning and sillimanite inclusions inmuscovite and quartz may indicate decrease in pres-sure after the formation of sillimanite by reaction (9)and during staurolite-consuming reactions (increas-ing temperature).

There is textural evidence to support the firstappearance of kyanite in pelitic rocks of the SilgaráFormation by the polymorphic reaction andalusite

� kyanite. On the other hand, a net reaction such

as andalusite � sillimanite must not have occurredat some point during metamorphism, because wedon’t really know if the andalusite reacted directlyto produce the sillimanite or if the andalusite wasfirst dissolved in a fluid phase, with the fluid phasecarrying the dissolved components to a new loca-tion where the fluid then precipitated thesillimanite. However, the presence of sillimanite,kyanite and andalusite in the area or in some sam-ples seems to suggest a more complex P-T meta-morphic evolution.

Discussion

In this study we reveal a number of important observa-tions regarding the major and trace element zoningand its correlation with accesory mineral phases ingarnet-bearing pelites and associated lithologies of theSilgará Formation at the southwestern Santander Mas-sif. Fe, Mg and Mn zoning is strongly correlated withrespect to the degree of local equilibrium in contrastwith Ca zoning that does not mimic zoning of those el-ements exactly (e.g., Chernoff & Carlson, 1997; Spear& Daniel, 1998). This strong correlation supports thehypothesis that these elements achieved a close ap-proach to local equilibrium during garnet growth. Ifvariations of these elements were produced only by lo-cal variations in bulk composition, then mass balanceand phase equilibrium considerations would predict

antithetic zoning of Fe, Mg and XFe with respect toMn. The chemical zoning in the analyzed garnets maybe originated by growth zoning, as a result of chang-ing external conditions (pressure, temperature or bulkrock chemistry) and by diffusion zoning also due tochanging external conditions, but with no growth orconsumption of garnet. According to Spear (1993), achange in chemical zoning character from growth zon-ing to diffusion zoning by progressive homogeniza-tion is attributed to diffusion with increasingmetamorphic grade. However, a local metasomatic

23



Figure 9. X-ray compositional maps showing (a) Mg, (b) Mn and (c) Ca distribution in garnet from sample PCM-473(staurolite-kyanite zone). White arrows indicate embayments. Numbered spots are keyed to analyses in Table 2. In the X-raymaps, the elemental distributions are indicated by white color for the highest and black color for the lowest concentration.

event has also played a very important role in control-ling metamorphic reactions.

Major element zoning. The analyzed garnets usu-ally show a normal zoning with Mn content decreas-ing from core to rim, suggesting a progrademetamorphism. A high Mn content correlates with ahigh Fe/(Fe+Mg), which suggests that areas of highMn content formed at lower temperature. Therefore,Mn zoning can be used as a time line in these rocks(Spear and Daniel, 1998). Crystal size distribution(CSD) analysis of garnets is consistent with continuousnucleation and growth of garnet throughout the garnetcrystallization episode (Ríos et al., 2003), which indi-cates that large garnets nucleated early and the small-est garnets nucleated last. In contrast to theobservations of Chernoff & Carlson (1997) for garnetsin the Picuris Range, New Mexico, high-Mn,low-Mg+Fe cores or low-Mn, high-Mg+Fe cores donot correlate with garnet size in the Silgará Formation.However, the strong correlation of Mn and Fe, regard-less of garnet crystal size and spatial distribution, sug-gests a homogenous distribution of these divalentcations in the intergranular medium during garnetgrowth. Ca zoning can exhibit different trends than theFe-Mn-Mg and is generally unrelated to zoning pat-terns in Fe, Mn and Mg, although in some cases Caand Mn zoning patterns are spatially related (e.g., sam-ple PCM-361, Figure 5a, 5b). It is well known thatchanges in the reactant mineral phases and product as-semblages occur at different times during the reactionhistory of garnet crystals of different size and, there-fore, they can not be the result of any event affectingthe entire rock, such as a change in pressure, tempera-ture or fluid composition but they reflect kinetic fac-tors that cause Ca to fail achieving chemicalequilibrium during garnet growth. Although garnetoutlines are sometimes slightly rounded (e.g.,PCM361, Figure 6a; PCM-516, Figure 6b), it is possibleto observe that their rims were basically parallel to theeuhedral low-Ca annuli and that the majority of garnetconsumption occurred at the corners or intersectionsof growth crystal faces of garnet. A likely explanationfor the difference in appearance of the annuli is that aparticular thin section plane did not necessarily passthrough the centre of the garnet and in some cases a

high-annuli at core could be missed. Garnet showsabrupt variations in Ca distribution from core to rim,probably due to the consumption of Ca-enriched min-eral phases in garnet-producing reactions, developingzoning patterns with euhedral low-Ca annuli (Figures6a, 6b) parallel to the garnet outlines, which is typicalof growth zoning and suggest that very little diffusiontook place after growth (Chernoff & Carlson, 1999).Cal-silicate rocks contain a variety of Ca-rich phases(e.g., epidote, plagioclase, Ca amphibole and apatite)in addition to garnet. If the consumption or productionof these phases and the changes in how Ca and otherelements are locally partitioned between garnet andother minerals, accounts for garnet zoning, thereshould be trends in zoning in other Ca-rich phases.Matrix plagioclase typically displays normal zoning,with an increase in XAn spatially associated with a de-crease in Xgrs of garnet rims (Ríos et al., 2003).Low-Ca annuli within garnets are unlikely to havebeen caused by resorption of garnet because the bandsare euhedral. A garnet resorption would be expected toproduce an irregular, embayed garnet margin and anovergrowth on such irregular boundary should pro-duce a subhedral to anhedral annulus, which has notbeen observed in this study. A sharp decrease ofgrossular content from core to the mid-region has beeninterpreted by Menard & Spear (1993) as produced byresorption of garnet during productions of staurolite orloss of epidote from the assemblage, which is unlikelybecause the garnet core is euhedral. A change in min-eral assemblage may account for some of the zoningtrends observed, but to explain the complex oscillatoryzoning observed in calc-silicate rocks (samplePCM-514, Figure 6c), this would require appearanceand disappearance of Ca-rich phases or some otherprocess that control fluctuations in the availability ofelements. The Ca-rich bands in garnet may correlatewith the breakdown of epidote as well as with the pres-ence of a Ca-rich fluid. Garnet growth continued withdecreasing grossular content after each of theseevents, showing that Ca was fractionated into garnet,plagioclase and/or calcic amphibole. Ti and Ca con-tents in garnet from sample PCM-514 are positivelycorrelated, which may reflect consumption of ilmeniteor rutile to liberate Ti during growth of Ca- rich garnet

24


or metasomatic addition of Ti. There is not doubt that acomplex oscillatory zoning as described here has acontroversial origin and we agree with Stowell et al.(1996) in the sense that it is difficult to explain it by acyclic addition and loss of mineral phase(s) from thechemical system. Instead, it could reflect episodicmetasomatism by influx of Ca-rich fluids during gar-net growth near of adjacent to hydrothermal veins ofquartz-epidote. Therefore, the results obtained by Ríos& Takasu (1999) for Ca concentrations would not beinterpreted as event markers or as recording complexbarometric histories, when in fact they reflect only lo-cal effects that can not have occurred simultaneouslythroughout the rock.

Trace element zoning. Analysis of the trace ele-ment zoning in garnet was done as a useful supple-ment to major-element zoning to improve thepetrological interpretation. According to Pyle & Spear(1999), trace elements zoning in garnet growing in aclose system depends on the presence or absence of“saturing” mineral phases such as xenotime for Y,xenotime, apatite or monazite for P and ilmenite, titan-ite or rutile for Ti. However, the accessory phases thatlikely reflect interactions with major phases could bexenotime and monazite, as a consequence of reactioncoupling with garnet and the high compatibility of Yin all three phases. In our case, we have documentedthe presence of these accessory mineral phases, butunfortunately they did not show trace element zoning,except in sample PCM-441 (Figure 4b), where Y zon-ing shows a remarkably euhedral core that is not reallyparallel to crystal faces of garnet, representing agrowth zone that probably was not modified by diffu-sion. High-Y annuli in garnet have been reported byPyle & Spear (1999) in staurolite-bearing samples,which may form by garnet overgrowth of proximalmatrix enrichment in Y due to garnet consumptionduring discontinuous staurolite-forming reactions.Therefore, the high-Y annulus observed in the garnetstudied from the garnet zone can not be explained by aresorption-regrowth process, which also will producea chemical zoning characterized by an asymmetric an-nulus with steeper slopes and embayed and irregularshapes. However, the growth history of this garnet canbe represented by a period of garnet growth at an ap-

proximately constant activity of Y (xenotime stable),followed by garnet at much lower Y activity (xeno-time absent). Lanzirotti (1995) describes differentmechanisms for annular rings of high-Y in garnet,such as open system-fluid infiltration, garnet resorp-tion and renewed growth (unlikely argument for Yzoning described above), changes in garnet growthrates and comsumption of Y-enriched mineral phases.Therefore, the production of an annulus without gar-net resorption requieres that the accessory-phase-outreaction be essentially discontinuous or the resultwould be a gradual, rather than abrupt, increase in gar-net major or trace element content (Pyle & Spear,1999). Hickmott & Spear (1992) concluded that con-centrations of Y, Zr and REE in garnet should varywith Ca concentrations because these elementssusbtitute into epidote-group minerals. Therefore, re-actions involving consumption of these phases shouldsupply both Ca and these trace elements for garnetgrowth. Unfortunately, garnet from sample PCM-514does not show trace-element zoning in Y and thereforeis difficult to correlate it with Ca in garnet. The addi-tion of calcium would control chemical reactions pro-ducing garnet (Xgrs increases), plagioclase (XAn

increases), calcic amphibole and epidote, whereas re-actions in absence of metasomatism would returnXgrs and XAn to previous levels. Xenotime is presentonly as a matrix accessory phase in sample PCM-361(Figure 5c), which can be interpreted as a stable min-eral phase throughout the entire growth history of gar-net. At this respect, Pyle & Spear (1999) consider thatthe scarcity of matrix xenotime at grades above thegarnet-zone in garnet-bearing pelites implies that thegrowth of garnet is accompanied by the consumptionof xenotime. Monazite is a common accessory mineralin samples from the higher metamorphic zones (gar-net-staurolite and staurolite-kyanite), being virtuallyabsent at garnet-grade. Monazite inclusions occur ingarnet (e.g., PCM-361, Figure 5c), suggesting thatthey grew before or during growing of garnet. Garnetgrowth should be previous to or synchronous with ma-trix monazite growth. Its growth may be related to theprograde consumption of allanite as been reported indifferent studies, assuming P is derived from apatite(e.g., Catlos et al., 2001). Although it is probable that a

25



precursor LREE mineral was present, garnet,plagioclase and phyllosilicates host sufficient P andLREEs and the reaction between them could stabilizemonazite (Kohn & Malloy, 2004). Where analyses arenot disrupted by the presence of inclusions, Ti concen-tration decreases from core to rim. The increasing con-centration towards the rim in garnet may be a result ofchanges in the Ti-bearing minor phase assemblagefrom ilmenite-rutile in the garnet core to rutile in thegarnet rim. Accesory ilmenite, titanite and rutile coex-ist with the silicates. Inclusions of ilmenite (by far themost prominent Ti-bearing phase) in garnet have beenpartly replaced by rutile as observed in Figure 7 andshow from core to rim an increase in MnO and a de-crease in FeO. Apatite is abundant in all rocksthroughout the garnet outboard and in the matrix. Ac-cessory phases such as zircon, monazite, titanite,epidote and allanite may contain significant Y, butthese should be of limited modal extent in the samplesexamined with respect to garnet to explain why thereis not apparent trace element zoning in garnet (exceptfor Y in sample PCM-441).

Diffusion and resorption. Garnet usually showsa normal zoning with Mn content decreasing fromcore to rim, although a minimum Mn content near rim(e.g., sample PCM-420, Figure 6d) sometimes is ob-served, and this type of zoning is characterized as re-versal zoning, which in many cases reflectspost-peak resorption and reequilibration during cool-ing by elemental diffusion during retrograde meta-morphism. Whitney and Ghent (1993) consider thatno significant increase in Mn at garnet rims suggestsno major late metamorphic resorption of garnet.However, a resorption process cannot explain a re-versal zoning in euhedral garnet. Possible explana-tions involve growth or post-crystallization mo-dification. In the case of the former, asymmetric zon-ing can be explained by overgrowth of Mn-richphase(s) (top half of crystal in Figure 6d) as well as alower Mn phase by garnet at low temperature (so thatdiffusion was limited and Mn was confined to the vi-cinity of the overgrown phase(s); e.g., Hirsch et al.,2003). The core, represented by a low Mn concentra-tion, is not situated in the geometrical center of thegrain, implying either asymmetrical growth or that a

significant amount of resorption has taken place,which is compatible with the corroded appearance ofthe rim. Mn is not concentric about individual partsof the garnet, but rather is zoned in irregular,amoeba-like shapes, a pattern that reflects fastgrowth along grain boundary surfaces and slowerdissolution and replacement of quartz inclusions(Spear & Daniel, 1999). However, we consider thatthis garnet where is in contact with progressive shearzones displays a tectonic dissolution, as revealed bythe chemical zoning, which is abruptly truncatedagainst the main metamorphic foliation of the rock.Chemical zoning in garnet from sample PCM-473can be affected by a post-growth thermal history.Garnet zoning is reversal in this sample and is bestexplained by diffusion during partial resorption offirst stage garnet, with a second stage of growth nearthe outer rim of garnet, marking the onset of a newprograde garnet producing reaction. Resorption iscompatible with the corroded appearance of the rimand also with the traditional interpretation of thelow-Mg (Figure 9a) and high-Mn (Figure 9b) rim.Figure 9c illustrates partial preservation of a low-Caannulus at the outermost rim, which is truncated bydissolution and resorption of garnet, developingembayments At rim, the chemical zoning are patchyand the Mn or Mg (samples PCM-361, PCM-420,PCM-514 or PCM-516) and Y (sample PCM-441) dis-tributions are characterized by a small reversal zon-ing, indicating some garnet resorption and backdiffusion after maximum temperature was achievedas a consequence of reaction zones of chlorite or bio-tite around garnet that could contain inclusions of ac-cessory mineral phases such as xenotime, monazite,epidote, zircon or titanite that can incorporate signifi-cant amounts of Y as proposed in differet studies(e.g., Wopenka et al., 1996; Heinrich et al., 1997;Finger et al., 1998).

Conclusions

We propose that our zoning and textural observationsare most consistent with a continuous garnet growthprocess, involving various Ca-rich phases, and thatgarnet growth may have varied in terms of different

26


garnet producing reactions during prograde meta-morphism. The occurrence of inclusion-rich and in-clusion-free zones in garnet has been explained by achange in the rate of garnet growth (Yang and Rivers,2001), i.e., garnet growth in the inclusion-rich zonesmust have been sufficiently rapid to allow entrap-ment of abundant inclusions. Therefore, we can in-terpret inclusion-poor rims and inclusion-rich coresas representing different growth rates, which is sup-ported here by the occurrence of garnet showinglow-Ca annuli lack of inclusions (e.g., PCM-361 andPCM-516) and high-Mn inclusion-rich core (e.g.,PCM-420). In sample PCM-361, for example, thehigh Ca / low Mn rim zone contains apatite, monaziteand ilmenite aligned parallel to the margins of thegarnet, whereas the euhedral low-Ca annulus withinthe garnet corresponds to a change in mineral inclu-sion abundance, but does not correspond to a changein the mineral inclusion assemblage itself (Figures 5,6a). The low- and/or high-Ca annuli may be used astime markers during garnet growth history and trun-cation of annuli is not only an evidence of tectonicdissolution in progressive shear zones as proposed byRíos et al. (2003) but also an excellent indication ofsubsequent garnet resorption. Accessory-phase as-semblages may consider an early garnet growth at theexpense of chlorite or biotite, with resultant xenotimeconsumption and production of monazite. Most im-portantly, this study shows that monazite crystalliza-tion is not an isolated event, but occurs throughoutthe metamorphic history of the rock. We have shownthat valuable petrogenetic information can be ob-tained from major and trace-element distribution inpelitic and associated rocks of the Silgará Formationand that a complete understanding of reaction historymay be achieved if accessory phases are considered.However, our interpretation could be tested by run-ning Gibbs method simulations, utilising internallyconsistent thermodynamic datasets and relevant ac-tivity-composition models.

Acknowledgements

This research forms part of the M.Sc. program car-ried out by C. Ríos at Shimane University. We are

very grateful to the Instituto Colombiano para elDesarrollo de la Ciencia y la Tecnología “FranciscoJosé de Caldas” (COLCIENCIAS), as well as theUniversidad Industrial de Santander, entities thatthrough Grant No. 1102-05-083-95 financially sup-ported fieldwork (Research Project: “Metamorphismand associated metallogeny of the Santander Massif,Eastern Cordillera, Colombian Andes”). We are in-debted to the Research Center for Coastal LagoonEnvironments at Shimane University for allowing usthe use of the electron microprobe analyzer for acqui-sition and assistance of major element data. We alsothank Dr. D. Whitney from the Department of Geo-logical and Geophysics at University of Minnesotafor her assistance with the adquisition of major andtrace element data, substantial contributions and helpin manuscript preparation.

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