Igneous and metamorphic petrology of lavas and dykes of the Macquarie Island ophiolite complex

IGNEOUS AND METAMORPHIC PETROLOGY OF

LAVAS

AND DYKES OF THE MACQUARIE

ISLAND

O P H I O L I T E COMPLEX.

J

Brendon

J.

G r i f f i n

Submitted

i n f u l f i l m e n t o f

the requirements

for the degree

of

Doctor o f

Philosophy.

University of Tasmania

This t h e s i s c o n t a i n s n o m a t e r i a l which has been

accepted f o r t h e award o f any o t h e r d e g r e e o r diploma i n any

u n i v e r s i t y , and t o t h e b e s t of my knowledge and b e l i e f , c o n t a i n s no copy o r p a r a p h r a s e of m a t e r i a l p r e v i o u s l y p u b l i s h e d o r w r i t t e n by a n o t h e r p e r s o n , except where due r e f e r e n c e i s made i n t h e t e x t o f t h i s t h e s i s .

B . J. G r i f f i n

CONTENTS

List of Figures List of Tables List

of

Plates Abstract

Chapter 1 INTRODUCTION 1.1 Introduction

1 . 2 Acknowledgements

Chapter 2 LOGISTICS AND TECHNICAL DEVELOPMENTS

2.1 Location and tectonic setting of Macquarie Island 2.2 Access and field conditions

2.3' Electron microprobe development and calibration Chapter 3 GEOLOGY OF MACQUARIE ISLAND

3.1 Introduction

3.2 General geology

3.3 Structural geology

3.4 Age,

Chapter 4 PETROGRAPHY AND PRIMARY PHASE MINERALOGY

4.1 General petrography of the lavas

and

dykes

4.2 Phenocrysts distribution and mineral chemistry 4.2.1 Plagioclase feldspar

4.2.2 Clinopyroxene

4.2.3 Olivine 4.2.4 Spinel

4 . 3 Phenocryst crystallization order 4.4 Origin of the phenocrysts

4.5 Petrography and mineralogy of young ocean-floor basalts

4 . 6 Comparison with other ophiolite complexes

4.7 Summary

v

viii

X

xi i

Chapter 5 METAMORPHISM OF THE LAVAS AND

DYKES

5.1 Introduction 5.1

5.2 Sample .collections and mineral identification

techniques 5 . I

5.3 Metamorphism: definitions and terminology 5.2

5.4 Metamorphic assemblages in the pillow lnvas 2nd

dyke swarm zones of the Macquarie I s l a n d c o m ~ ~ l c x 5 . 4

5.4.1 Ocean-f loor weathering 5 . 5

5 -4.2 Zeolite facies alterat-ion 5.10

5.4.3 Lower greenschist facics alteration 5 . 3 1

5.4.4 Upper greenschist to amphibolite facics

Contents cont

.

5.5

Sulphide mineralization and associated alteration

5 . 5 . 1

Introduction

5 . 5 . 2

Discrete sulphides amongst lavas

and dykes

5.5.3

Vein mineralization

5 . 5 . 4

Stockwork mineralization

5 - 5 . 5

Sulphide mineralization

-

a summary

5.6

Oxygen and carbon isotope study of the

metamorphism

5.7

Conditions and processes of metamorphism

5.7.1 Specific conditions

of

metamorphism

5..

7

- 2

Regional nature

of

the metamorphism and

volcanic stratigraphy

5 . 8 A summary

of ocean-floor metamorphism exposed at

Macquarie Island

5.9

Fluid convection and sulphide mineralization

Chapter 6

MAJOR

AND

TRACE ELEMENT

GEOCHEMISTRY

6.1

~ntrodbction

6.2

Geochemical effects of the alteration and

metamorphism

6 . 2 . 1 H20 abundances and F ~ ~ + / F c ~ + ratios

6 . 2 . 2

Comparison of petrography and CIPW

normative and major element chemistry

6 . 2 . 3

Core-rim variations in pillow

l a v a s

and variations across a massive

flo\v

6 . 2 . 4

Inter-element correlations of

major

and

trace elements

6 . 3

Igneous geochemistry

Petrogenesis of the lavas and dykes:

implications from their geochemistry

Major element and compatible trace

element variations

Major element composition modelling and

constraints

on

petrogenesis

Major element petrogenesis

Incompatible

and

hygromagmatophilc

trace element variations

and

character-

ization of the volcanic racks

Implications of incompatible

and

hygromagmatophile clement

rstjos

Relationships of the

incompatible

and

hygromagmatophi

le element

ahundanccs to

the petrologic and normative

. chaructcr .

of the samples

Refinement of partial melting

estimates

C o n t e n t s c o n t .

iii

page Chapter 7 EVOLUTION OF hlACQUARIE ISLAND-TYPE OCEANIC CRUST

7 . 1 I n t r o d u c t i o n

7 . 2 Metamorphic p r o c e s s e s i n t h e o c e a n i c c r u s t 7 . 3 I m p l i c a t i o n s

and

f u r t h e r s t u d i e s

7 . 4 Igneous p r o c e s s e s a t o c e a n i c r i d g e s p r e a d i n g c e n t r e s

7 . 5 D e r i v a t i o n o f p r i m i t i v e magmas and n a t u r e of t h e mantle h e t e r o g e n e i t y

7.6 N e a r - s u r f a c e p r o c e s s e s

a t

a Macquarie I s l a n d - type mid-oceanic r i d g e

REFERENCES

Appendix 1 Major element analyses and 'CIPW norm

c a l c u l a t i o n s f o r Macquarie 1 s l a n d 1 a v a s and

dykes

CIPW normative compositions of Macquarie I s l a n d l a v a s and d y k e s '

Primary p h a s e microprobe a n a l y s e s

Microprobe ' a n a l y s e s of secondary p h a s e s i n Macquarie I s l a n d lavas and dykes

XRD r e s u l t s .on secondary m i n e r a l separates A5 .1

XRF and

wet chemical a n a l y s i s t e c h n i q u e s A 6 . 1

Sample c a t a l o g u e A 7 . J

Oxygen i s o t o p e geochemistry of t h e Macquarie i n I s l a n d O p h i o l i t e , by J . D . Cocker, B.J. G r i f f i n G pocket K . Muehlenbachs.

The o u t l y i n g i s l a n d s of Macquaric l s l i ~ ~ ~ d , h y I I

D . J . Lugg, G.W. J o h n s t o n e G R . . ) . ( ; r i f f i n .

Energy d i s p c r s i v c a n a l y s i s system c a 1 i llr'nt' i ; ~ i l r l I I

o p e r a t i o n w i t h 'I'AS-SUEUS, an cldvai~ccd i 1ht.cr;rc t i.vc

d a t a - r e d u c t i o n package, by 6 . J . ( ; r i f f i 11.

The p e t r o l o g y of t h e Macquaric I s l a n d o p h i o l i t c I I

a s s o c i a t i o n : M i d - T e r t i a r y o c e a n i c c r u s t o f t h o S o u t h e r n Ocean, by B . J . G r i f f i n G It. V:~rnc,

The Macquaric I s l a n d o p h i o l i t c a s s o c i a t i o n : Mid- I I T e r t i a r y o c e a n i c c r u s t o f t h c Southern Occ:~n,

by B . J . G r i f f i n and R . Varne.

C o n t e n t s c o n r

.

Appendix 8 cant. The Macquarie I s l a n d o p h i o l i t e a s s o c i a t i o n : i n

b l i d - T c r t i a r y o c e a n i c c r u s t o f t h e S o u t h e r n _pocket

Ocean, by B . J . G r i f f i n

4

R . Vawe.

Oxygen and c a r b o n i s o t o p e g e o c h e m i s t r y I I

Contents cont. .

List of Figures

Figure 2.1 Locality map

Generalized geologic map of Macquarie 1,sland Geological map of the northern part of Macquarie Island

Structural measurements on lavas

Spatial distribution of alkaline and tholeiitic lava variants

Size distribution of plagioclase phenoc~ysts between.tholeiitic and alkaline lava variants and for the lavas as a group

Plagioclase compositions in sample 211

Plagioclase compositions in the lavas

and

dykes Microprobe analyses of cores of clinopyroxene phenocrysts plotted in the Ca-Mg-Fe compositional triangle

Cr203-Mg/Mg+Fe variations in clinopyroxene

phenocrysts .

ovar ria ti on

of

Crz03 in clinopyroxene phenocrysts

from

lavas

Compositional variations of groundmass clinopyroxenes

Ti

vs Mg/Mg+Fe for tholeiitic lava pyroxenes and alkaline lava pyroxenes

Cr vs Mg/Mg+Fe as for Figure 4.9

Compositions of spinel phenocrysts

Distribution of secondary assemblages in Plncquaric

Island lavas 5. l l

Mass balance for zeolites analyzed u s i n g t h c TI1[)

microprobe 5.13

Charge balance of zeolites analyzcd using t.hc 'r1'11

microprobe. 5 . I4

Mass balance

for

zeolites analyzcd with thc I:t)AX

system 5.15

Charge balance for zeolites analyzed with t h c L:.I)AX

system 5 , I 0

Compositional fields f o r zeolitcs on a N:i:2(:;1: f N i 7 1 ( ; ) ! ; . I 8

Zeolite analyses from Macquaric 1 s l a n d l a v i i s 1,lot i.o(l

on a Na-2Ca-(Si-16) cation plot ! ; , I ! )

Phillipsite analyses from Macquaric Is1:lnd l : ~ v a s

and modern ocean-f loor b a s a l ts 5 . 2 7

FeO vs MgO plot for "chlori tcs" from Mncc(11:i r,i c

Contents c o n t .

Figure 5.10 FeO vs MgO p l o t f o r c h l o r i t e s from i n d i v i d u a l l a v a s t h a t have undergone lower g r c e n s c l l i s t , f a c i e 5 metamorphism

Sulphide occurrences

Oxygen i s o t o p e composition f o r whole r o c k s of t h e Macquarie I s l a n d o p h i o l i t e vs H 2 0 c o n t e n t

Oxygen and carbon i s o t o p i c composition f o r c a l c i t e i n t h e Macquarie I s l a n d o p h i o l i t e

Secondary phase d i s t r i b u t i o n i n Macquarie I s l a n d u n i t s

Temperature ranges o f z e o l i t e s i n a c t i v e hydro- thermal a r e a s

A c t i v i t y diagram a t c o n s t a n t t e m p e r a t u r e , p r e s s u r e and a c t i v i t y o f Hz0 shpwing

phase

r e l a t i o n s f o r a l b i t e , analcime and h e u l a n d i t e

S i m p l i f i e d metamorphic d i s t r i b u t i o n and s t r u c t u r e o f l a v a s and dykes

Schematic o f o c e a n - f l o o r metamorphism 5.89

llistograms of

LOI

f o r l a v a s and d y k e s from

d i f f e r e n t metamorphic grades 6 . 4 Histograms o f

~ e ~ + / ~ e ~

f o r

l a v a s and dykes o f

d i f f e r e n t metamorphic grades 6 . 7 R e l a t i v e p r o p o r t i o n s o f normative 0 1 , h y , d i , ne

and Q i n Macquarie I s l a n d b a s a l t s 6 . 9

Normalized REE abundances 6 . 1 3

N i and C r c o n t e n t s o f Macquarie I s l a n d b a s a l t s and

d o l e r i t e s 6 . 1 5

Geochemical p a t t e r n s i n MORB and o t h e r mafic lavas 6.20 AFM diagram f o r Macquarie I s l a n d b a s a l t s , d o l e r i t e s ,

l a y e r e d gabbros and w e h r l i t e s , and h a r z b u r g i t e s 6 . 2 3 P l o t o f major elements a g a i n s t Mg/Mg+Fc f o r

l a v a s and dykes G . 7 5

T i and Z r c o n t e n t s o f Macquaric

Island

h a s l i l t s

and d o l e r i t e s 6 . 3 7

T i ,

~ r '

and Y c o n t e n t s of Macquaric Island b a s a l e s

and d o l e r i t e s 0 . 3 n

T i , Z r and S r c o n t e n t s o f Macquarj e I s l a r ~ d b i ~ s ; ~ l t.s

and d o l e r i t e s ( I

.

:f!J

Zr and Nb c o n t e n t s o f Macquarie Island b a s a l t s nncl

d o l e r i t e s 0 . 4 0

Z r and Y c o n t e n t s of Macquarie I s l a n d b a s n l t s a n d

d o l e r i t e s 0 4 0 '

6.14 Masuda-Coryell REE p l o t o f Macquaric I s l a n d

Contents c o n t .

S r , Rb, K 2 0 and P 2 0 5 a g a i n s t Nb f o r t h e l a v a s

and dykes

La,

Zr, Ti02 and Y a g a i n s t

Nb

f o r t h e l a v a s and

dykes,

Geochemical p a t t e r n s normalized t o

MORB

abundances f o r "enriched", " d e p l e t e d u and

MORB-

e q u i v a l e n t l i q u i d s from Macquarie I s l a n d

. ... - - - -

Ne- and hy-normative composition c o n t e n t a g a i n s t Nb abundance.

.

A schematic s e c t i o n through Macquarie I s l a n d - t y p e o c e a n i c l i t h o s p h e r e . The s e c t i o n i s based on t h e t r a v e r s e from North Head t o Eagle P o i n t .

Dynamic

n a t u r e and e v o l u t i o n o f t h e o c e a n i c crust

sequence o f Macquarie I s l a n d

A schematic model of n e a r - s u r f a c e magma

vii

i

Contents cont.

. .

List of Tables

Table 3.1

Structural measurements off lavas and sediments

at the same locality

3 . 2

Angular relationships between lavas and intruding

dykes

4.1

Phenocryst assemblages and abundances in lavas

4.2

Phenocryst assemblages

and abundances in alkaline

and tholeiitic lavas

4 . 3

Representative microprobe analyses of plagioclase

phenocrysts and groundmass crystals

4 . 4 A

Representative microprobe analyses of pyroxene

phenocrysts

4.43

Representative microprobe abalyses of goundmass

pyroxenes and amphiboles

4 . 5

Representative microprobe analyses of olivine

4 . 6

Representative microprobe analyses of spinels

5 . 1

Representative microprobe analyses of smectites

Zeolite structures

and optical properties in

Macquarie Island rocks

Representative microprobe analyses of Group I

zeolites

Repre'sentative microprobe analyses of Group 5

zeolites

~e~resentative

microprobe analyses

of

gyro1

i

t c

and ?hi1

lebrandite

Phillipsite analyses from literature

Representative microprobe analyses of mixed-layer

silicates

Microprobe analyses of Ti-rich mixed-layer

silicate

Representative microprobe analyses of albite

and

K-feldspar

Representative microprobc analyses of

prchnitc

Representative microprobe anal

yscs o f cpi tlor.cs

Representative microprobc

analyscs of

sphcno

:111(1

recalculated sphene and epidote

a n a l y s c s

Representative mlcroprobe analyses of

scconcI;~ry

amphiboles

and

talc

in the

dyke swarm

snmylcs

Relict clinopyroxene and rim amphibole

microprobe analyses

Secondary and primary plagioclase feldspar

compositions from altered dyke swarm

samples

+

~ c ' + / ~ e ~ +

ratios,

H20

and

C02

data for

o c c a n - f l u o r

[image:11.612.78.540.58.782.2]

Contents cont

.

Table 6 . 1

COz

a n d H 2 0 a n a l y s e s of l a v a s and dykes

6 . 2 Rim-core p i l l o w l a v a and edge-core massive flow a n a l y s e s

6 . 3 Pearson c o r r e l a t i o n c o e f f i c i e n t

matrix

f o r major and t r a c e elements i n Macquarie I s l a n d l a v a s and dykes

6 . 4 C o r r e l a t i o n c o e f f i c i e n t s of Zr with o t h e r e l e m e n t s / oxides f o r r e c a l c u l a t e d a n a l y s e s

6 . 5 Macquarie I s l a n d l a v a s a n d d y k e s : m a j o r e l e m e n t d a t a summary

6.6 P o s s i b l e primary l i q u i d s

6 . 7 A REE abundances i n Macquarie l s l a n d l a v a s and dykes

6.7B L i t e r a t u r e REE analyses of Macquarie I s l a n d l a v a s

and BCR s t a n d a r d a n a l y s i s

6 . 8 Mixing c a l c u l a t i o n s : Macquarie I s l a n d 1 a v a s . a n d dykes

6 . 9 Macquarie I s l a n d l a v a s and dykes: t r a c e element

d a t a swmnary

6.10 S e l e c t e d major and t r a c e element r a t i o s

i n

t h e REE d a t a s u b s e t .

6.11 La/Sm and La/Yb r a t i o s of Macquarie I s l a n d samples

grouped by grade of a l t e r a t i o n and metamorphism

6.12 Trace element ' abundances i n end member .components

d e f i n e d by

Nb

v a r i a n c e i n t h e Macquarie I s l a n d l a v a s and dykes

6.13 MORB normalized v a l u e s o f hygromagmatophile

elements i n t h e "enriched", tfdepleted" a n d

"MORB-

e q u i v a l e n t " components .

6 . 1 4 E f f e c t s of i n c r e a s i n g K 2 0 and P205 on

CTPW

[image:12.612.77.534.81.581.2]

Contents cont.

List of Plates

-

Plate 4.1

Quenched olivine crystallites in unaltered

basaltic glass.

4.2

Subvariolitic texture in a chilled

lava

pillow

r i m .

4.3

Xntersertal texture in an alkaline basalt.

4'. 4

Discrete kaersutitic amphibole in an alkali

01 ivine basalt.

4.5 .

Hydrous segregation zones in an alkaline basalt.

Subophitic texture in a tholeiitic basalt flow.

Corroded plagioclase phenocrysts.

A

rounded and embayed augite phenocryst.

Altered olivine glomerocryst.

Spinel phenocrysts included in

a

large plagioclase

phenocryst

.

A

zoned augite'phenocryst including plagioclase.

Replacement,of

olivine

by smectites and calcite.

Natrolite-thomsonite-mesolite

intergrowths filling

an amygdule rimmed by smectites.

Laumontite filling

a

vein rimmed by albite.

Euhedral calcite rhombs rimmed by brown smectites

in a zeolite filled amygdule.

A c t i n o l i t e - s p h e n e - c h l o r i t e

assemblage replacing

mesostasis.

Brown-olive green Ti-rich chlorite

as n sccondnry

m a t r i x t o

relict titanomagnetitc.

Subradiating epidote and chloritc filling

an

amygdule in an altered tholeiitic

l a v a .

Uralitized doleritic dyke

with

primary twinning

preserved in uralitized clinopyroxcne.

Relict clinopyroxene with

marginal

alterat.jon

t.o

actinolite amphibole in a partially uralitized

dyke.

Randomly oriented actinolitc

f

ibrcs

r e p 1 ac i r~g

Contents cont.

Plate 5.11

Secondary sphene mantling titanomagnetite in a

uralitized

dyke

swarm sample.

5.12 Trernolite-talc-magnetite

amygdule assemblage in

a

dyke swarm sample.

5.13

Massive Fe-rich amphibole in a lower amphibolite

facies altered dyke swarm sample.

5 . 1 4

Palimpsest structure in quartz in

a

quartz-

sulphide vein.

ABSTRACT

Macquarie I s l a n d

i s

an emergent p a r t of t h e Macquarie Ridge, which

runs s o u t h from New Zealand t o j o i n the I n d i a n - P a c i f i c r i d g e system, and

marks t h e boundary between t h e I n d i a n - A u s t r a l i a n and t h e P a c i f i c p l a t e s .

Most o f Macquarie I s l a n d i s composed o f fault-bounded b l o c k s o f

v o l c a n i c r o c k s t h a t a r e commonly b a s a l t i c p i l l o w l a v a s w i t h r a r e r massive

flows and minor sediments. The sediments r a n g e from

GZobCgeriw

oozes and

r e d s i l t s t o n e s through c o a r s e r l i t h i c wackes t o agglomerates. D o l e r i t e

dyke swarms, gabbroic m a s s e s ' i n c l u d i n g a l a y e r e d complex, and s e r p e n t i n i z e d p e r i d o t i t e s a l s o o c c u r i n t h e n o r t h e r n p a r t o f t h e i s l a n d , where an

o c e a n i c l i t h o s p h e r e s e c t i o n has been r e c o g n i z e d .

The b a s a l r s and d o l e r i t e s a r e usual l y p o r p h y r i t i c , c a r r y i n g

p l a g i o c l a s e (ha 7-Ane o ) a s t h e dominant phenocryst phase w i t h l e s s abundant o l i v i n e (FOB 9 -Fos 5)

,

chrome s p i n e l and r a r e l y c l inopyroxene

{Cat, S M ~ S Fes -Ca3 a ~ g s 0 ~ e l z )

.

~ o r m a t i v e l y t h e r o c k s range from n k - t o Q-

b e a r i n g , with most f a l l i n g n e a r t h e c r i t i c a l p l a n e o f normative s i l i c a

u n d e r s a t u r a t i o n . Dykes t e n d t o b e more i r o n - r i c h t h a n lnvns, and -inclttdc t h e more tli-poor r o c k s . Tile rocks a l s o r a i g c i n composition from t y p i c a l

o c e a n - f l o o r b a s a l t s through v a r i e t i e s r e l a t i v e l y e n r i c h e d i n some

incompatible t r a c e e l e m e n t s , p a r t i c u l a r l y Nb ( 2 0 - 7 3 ppm) and t h e l i g h t

r a r e e a r t h e l e m e n t s , t h a t o t h e r w i s e r e t a i n o c e a n - f l o o r b a s a l t phenocryst

assemblages, major element compositions and T i , N i , C r and Z r c o n t e n t s .

These l a t t e r v a r i e t i e s c l o s e l y resemble t h e o c e a n - f l o o r b a s n l t s from t h c "abnormal" r i d g e segments n e a r 4 5 % and 3 6 O N (FAMOUS) on t h e M i t l -

Mixing c a l c l l l a t i o n s u s i n g phenocryst and rock compositions s u g g e s t t h a t

much

o f t h e compositional v a r i a t i o n i n t h e v o l c a n i c s c o u l d have a r i s e n by low-pressure c r y s t a l f r a c t i o n a t i o n . However, low-pressure

f r a c t i o n a t i o n p r o c e s s e s cannot a l o n e account f o r a l l o f t h e v a r i a t i o n s i n

c o n c e n t r a t i o n i n t h e incompatible e l e m e n t s : some may aZso a r i s e from

d i f f e r e n t degrees of p a r t i a l m e l t i n g of a p o s s i b l y inhomogeneous s o u r c e .

Four grades o f a l t e r a t i o n 'and metamorphism have been d i s t i n g u i s h e d

i n t h e l a v a s and dykes. The lowest grade o f a l t e r a t i o n produced s m e c t i t e -

c a r b o n a t e dominated assemblages, p r i n c i p a l l y a f f e c t i n g o l i v i n e and g l a s s .

Lavas t h a t have s u f f e r e d t h i s o c e a n - f l o o r weathering a l t e r a t i o n have been

shown

t o

r e t a i n t h e magnetic p r o p e r t i e s o f t y p i c a l o c e a n i c c r u s t , and

were probably a t t h e t o p of t h e p i l e , i n t h e uppermost 200 m. Underlying

t h e s e i s a complex zone o f z e o l i t e a l t e r a t i o n d e f i n e d by t h e development o f Ca and Na z e o l i t e s , p r i n c i p a l l y n a t r o l i t e , thomsonite, a n a l c i t e , w a i r a k i t e and a t t h e bottom o f t h e zone, l a u m o n t i t c . The degree o f a l t e r a t i o n

i s

v a r i a b l e : f r e s h g l a s s h a s been found w i t h i n a fcw.nietres o f i n t e n s e l y z e o l i t i z e d lavas. Beneath t h e s e z e o l i t e f n c i e s asscmblagcs a r e a l b i t e - c h l o r i t e - e p i d o t e - s p h e n e assemblages o f t h e lower g r e c n s c h i s t

f a c i e s , p r e s e n t a t t h e b a s e o f t h e l a v a p i l e where up t o h a l f of t1lc o u t c r o p i s composed o f b a s a l t i c dykes. In c o n t r a s t , t h e dykes o f t h e dyke swarms have . s u f f e r e d a d i s t i n c t i v e a c t i n o l i t e aml~hibole a . l t e ~ * a t i o n and

replacement o f t h e primary

mafic

m i n e r a l s t h a t h a s l e f t t h e p l a g i o c l a s e l i t t l e a l t e r e d . This " u r a l i t i z a t i o n ' ' r e f l e c t s t h e a t t a i n m e n t o f

c o n d i t i o n s o f t h e upper g r e e n s c h i s t f a c i c s grade of metamorphism, i ~ n d s p o r a d i c a l l y a t t h e base of t h e dyke swarm u n i t , whcrc v e i n s o f IlornbIondc a r e p r e s e n t , t h e lower a m p h i b o l i t e f a c i c s grade o f metrrn~orphisnl.

I t i s argued t h a t t h e observed p r o g r e s s i o n o f sccondary :1sscn~l1 I : ~ j i c s

a r o s e from hydrothermal a l t e r a t i o n under varying tcmperaturc :111tl I)rcs:;llt*e

conditions, which a f f e c t e d t h e o c e a n i c c r u s t s c c t i o n t o t h c I);lsc 01: t.lic

x i v t h i s corresponds t o a depth o f approximately t h r e e k i l o m e t r e s . Prelim-

i n a r y oxygen and carbon i s o t o p e s t u d i e s show t h a t s e a w a t e r was t h e

i n i t i a l f l u i d and t h a t t h e f l u i d became s u b s t a n t i a l l y modified with depth

through r o c k ' i n t e r a c t i o n . F l u i d movement was c o n c e n t r a t e d a l o n g f r a c t u r e

systems i n t h e l a v a s .

The major e f f e c t s o f geochemical a l t e r a t i o n o f t h e massive r o c k s ,

+

away from t h e f r a c t u r e zon'es, are i n c r e a s e s i n t h e W 2 0 c o n t e n t and

~ e " / ~ e ~ + r a t i o . The magnitude o f t h e changes

i s

dependent on s t r a t i -

g r a p h i c p o s i t i o n : both d e c r e a s e with d e p t h . Gypsum d e p o s i t s , c o n t a i n i n g minor amounts o f s u l p h i d e s , a r e p r e s e n t i n t h e t o p o f t h e dyke swarm

u n i t . Pyrite-dominated sulphide-quartz-carbonate assemblages a r e p r e s e n t

i n v e i n s i n t h e base o f t h e l a v a p i l e and upper p a r t o f t h e dyke swarm

u n i t . A l a r g k stockwork d e p o s i t , with t h e same mineralogy, i s a l s o

p r e s e n t i n t h e b a s e o f t h e l a v a s e c t i o n . I t i s suggested t h a t t h e gypsum-

r i c h d e p o s i t s r e f l e c t temperature-induced s u l p h a t o s a t u r a t i o n o f t h e descending f l u i d whereas t h e s u l p h i d e - b e a r i n g vein assemblages have been

C h a p t e r 1

I NTRODUCTION

1.1 INTRODUCTION

'Ihe Macquarie Ridge i s a narrow a r c u a t e system o f submarine r i d g e s t h a t r u n s . s o u t h from New l e a l a n d t o j o i n t h e I n d i a n - P a c i f i c r i d g e

system, and marks t h e boundary' between t h e I n d i a n - A u s t r a l i a n and t h e P a c i f i c p l a c e s . The c r e s t o f t h e Macquarie Ridge i s a t w a t e r d e p t h s o f l e s s t h a n 200 m i n s e v e r a l p l a c e s , and n e a r 54"s 15g0E h a s emerged t o form Macquarie I s l a n d ( C u l l e n , 1969; Hayes

G

Talwani, 1 9 7 2 ) .

L i n e a r e a s t - w e s t t r e n d i n g marine magnetic anomalies s o u t h o f A u s t r a l i a have been t r a c e d a c r o s s the ridge t o t h e Macquarie Trough e a s t o f Macquarie I s l a n d . Geophysical d a t a show t h a t t h e i s l a n d r e p r e s e n t s ' o c e a n i c c r u s t produced d u r i n g T e r t i a r y s e a - f l o o r s p r e a d i n g a t t h e I n d i a n - P a c i f i c r i d g e system, which was l a t e r u p l i f t e d d u r i n g marginal i n t e r a c t i o n s between t h e I n d i a n - A u s t r a l i a n plate moving n o r t h and t h e P a c i f i c p l a t e moving west (Williamson, 1974, 1979). Magnetic p r o p e r t i e s of pillow l a v a s from t h e i s l a n d correspond well w i t h t h o s e of ocean f l o o r b a s a l t s ( B u t l e r e t a Z . , 1976) and t h e i s l a n d i s r e c o g n i z e d t o have t h e h i g h e s t p o t e n t i a l o f known o p h i o l i t e s f o r m o d e l l i n g t h e magnetic l a y e r o f t h e o c e a n i c l i t h o s p h e r e (Levi et n l . , 1 9 7 8 ) .

Macquarie I s l a n d t h e r e f o r e p r o v i d e s t h e o p p o r t u n i t y t o cxmnirlc o c e a n i c c r u s t , u s i n g t h e same methodology and a t thc! s;imc s c : ~ l c :IS l u r ~ c l - based s t u d i e s o f o p h i o l i t e complexes. Thc i s l a n d i s ttlc connecting 1 i11k between t h e o p h i o l i t e s o f c o n t i n e n t a l environments, and t.hc in ::.itu

o c e a n i c c r u s t s t u d i e d by d r e d g e , d r i l l and s u b m e r s i b l e .

P r e v i o u s g e o l o g i c a l s t u d i e s (Mawson, 1943; Varne E Hubenacli , 1 9 7 2 , 1973) i n d i c a t e , w i t h i n t h e p i l l o w l a v a l a y e r , a r a n g e from ~inmctamorptrased, b u t o x i d i z e d assemblages, through z e o l i t e f a c i e s grade n s s c ~ n b l : ~ j ; e s , t o

o c e a n i c p i l l o w l a v a l a y e r . The d o l e r i t e dyke s w a n s have been meta-

morphosed under c o n d i t i o n s o f upper g r e e n s c h i s t and v a r y i n g grades o f

a m p h i b o l i t e f a c i e s metamorphism. Dykes i n t r u d i n g t h e p i l l o w l a v a

l a y e r a r e n o t e d t o c o n t a i n metamorphic assemblages s i m i l a r t o t h e

e n c l o s i n g l a v a s . The p l u t o n i c rocks p r e s e n t have been v a r i a b l y s e r p e n -

t i n i z e d , p r e h n i t i z e d and p a r t i a l l y o r completely metamorphosed. S i m i l a r

grades and v a r i a t i o n s i n metamorphism have been r e p o r t e d from modern

o c e a n - f l o o r r o c k s (Cann, 1971) and o p h i o l i t e s found i n c o n t i n e n t s

(Smewing, 1975; Coleman, 1977; Coish, 1 9 7 7 ) .

T h i s t h e s i s p r e s e n t s an i n v e s t i g a t i o n o f t h e s t y l e and n a t u r e of

t h e ocean f l o o r metamorphism i n t h e p i l l o w l a v a and dyke

swarm

u n i t s o f

t h e o c e a n i c l i t h o s p h e r e exposed on Macquarie I s l a n d . A summary o f

background s t u d i e s , l o g i s t i c s and i n s t r u m e n t development a s s o c i a t e d

w i t h , t h i s p r o j e c t i s given i n t h e f o l l o w i n g c h a p t e r . The f i e l d r e l a t i o n -

s h i p s and macroscopic f e a t u r e s of the lavas and dykes a r e d e s c r i b e d i n Chapter 3 . The primary phase mineralogy and p e t r o l o g i c a l n a t u r e o f the

l a v a s and dykes a r e p r e s e n t e d i n Chapter 4 a s a background t o the i n t e g r a t e d p e t r o g r a p h i c , e l e c t r o n m i c r o - a n a l y t i c and i s o t o p i c s t u d y o f

t h e metamorpl~ism of t h e s e rocks, given i n Chapter 5 . E f f e c t s o f t h e

a l t e r a t i o n and metamorphism on t h e primary geochemistry o f selected

l a v a s and dykes a r e d i s c u s s e d i n Chapter 6 . The geochemical c h a r a c t e r - i z a t i o n o f o c e a n i c c r u s t , based on t h e Macqrlarie I s l a n d d a t a , i s a l s o

c o n s i d e r e d . i n Chapter 6 ; t o g e t h e r with a p r e l i m i n a r y djsci~ssiori o f t11c

p e t r o g e n e s i s o f t h e samples. The major c o n c l u s i o n s of' t h i s tticsis : ~ r u

1 . 2 ACKNOWLEDGEMENTS

I must f i r s t thank my s u p e r v i s o r , D r R. Varne, f o r h i s a s s i s t a n c e

i n a11 a s p e c t s o f t h i s s t u d y and w i t h o t h e r problems t h a t i n e v i t a b l y

appear

during

Ph.D. s t u d i e s . P r o f e s s o r D.H. Green i s a l s o g r a t e f u l l y acknowledged for h i s h e l p and s u p p o r t .

Many people h e l p e d with this p r o j e c t through t h e i r w i l l i n g n e s s t o l i s t e n and a d v i s e and I would p a r t i c u l a r l y l i k e t o thank

Dr

R . Berry,

DT C . F . B u r r e t t , D r A . Bush, D r C . J . E a s t o e , D r

D.J.

E l l i s , R . J . F o r d , D r J . Foden, S . H a r l e y , G . A . J e n n e r , D r

M.

Solomon, D r J . C . van Floort,

and D r 3 . L . Walshe.

Technical problems I encountered were g r e a t l y e a s e d by h e l p from W .

Doran,

R . Lincolme, Dr A . McKee, D r C . kuckolds and

N.G.

Ware.

Ms

J . Pongratz i s thanked f o r t y p i n g and s u p e r v i s i n g t h e p u b l i c a t i o n o f t h i s t h e s i s ;

The p r o j e c t was funded by r e s e a r c h g r a n t s from t h c L l n i v e r s i t ~ . o f

Tasmania and t h e A u s t r a l i a n Research Grants Committee. The blacquarie

I s l a n d Advisory Committee, t h e National Parks and W i l d l i f e S e r v i c e o f

Tasmania and t h e A n t a r c t i c D i v i s i o n o f t h e A u s t r a l i a n Department o f

Science and t h e Environment generously provided t r a n s p o r t t o and

l o g i s t i c s u p p o r t on Macquarie I s l a n d . I was s u p p o r t e d i n p a r t by

r e s e a r c h s c h o l a r s h i p s from t h e Department o f Education o f thc

Commonwealth o f A u s t r a l i a a n d , t h e Esso O i l Company P t y I,t.il, t o whan~ 1

Chapter 2

LOGISTICS

AND TECHNICAL

DEVELOPMENTS

2 . 1 LOCATION AND TECTONIC SETTING OF MACQUARIE ISLAND

Sub-Antarctic Macquarie I s l a n d (54'30 '5; 1 5 9 " ~ ) l i e s i n t h e South P a c i f i c Ocean, 1280 km s o u t h e a s t of Hobart, Tasmania, and 1440 km n o r t h o f t h e A n t a r c t i c c o n t i n e n t (Figure 2 . 1 ) . I t i s a s u b a e r i a l exposure of t h e Macquarie Ridge. The i s l a n d i s about 37 km l o n g by 4 . 8 km wide and c o n s i s t s o f

a

h i l l y p l a t e a u a t about 300 m a l t i t u d e ; t h e h i g h e s t p o i n t i s 433 m.

The Macquarie Ridge i s a narrow, a r c u a t e r i d g e t h a t runs s o u t h from New Zealand t o j o i n t h e I n d i a n - P a c i f i c r i d g e system. It h a s a

rugged and complicated topography and i n some p l a c e s has a double c r e s t s e p a r a t e d by a deep and narTow d e p r e s s i o n (Summerhayes, 1974). The r i d g e i s l o c a l l y topped by a flat p l a t f o r m about 160 rn below sea. l e v e l t o mark where it may have been exposed i n t h e P l i o c e n e . A s u b s i d i a r y r i d g e occurs e a s t o f t h e main ridge i n t h e Macquarie I s l a n d ' r e g i o n , . s e p a r t e d from

it by a narrow b u t deep t r e n c h , t h e Macquarie Trench. T h i s t r e n c h c o n t a i n s undeformed sediments [Hayes

F

Talwani, 1972)

.

The r i d g e i s s e i s m i c a l l y active; and i s g e n e r a l l y considered t o

mark t h e boundary between t h e I n d i a n - A u s t r a l i a n and t h e , P a c i f i c p l a t e s . ~ a r t h ~ u a k ' e s a r e shallow, and f i r s t motion a n a l y s e s imply t h a t t h e r i d g e i s a s s o c i a t e d with normal, t h r u s t and s t r i k e - s l i p f a u l t i n g . P l a t e

t e c t o n i c a n a l y s e s s u g g e s t t h a t the predominant motion along t h e r i d g e i s r i g h t - l a t e r a l s t r i k e - s l i p (Hayes

E

Talwani, 1972) , Nevertheless, d i f f e r e n t p a r t s of t h e r i d g e a r e probably a l s o governed by e x t e n s i o n a l and compress- i o n a l regimes a s a consequence of t h e c l o s e n e s s of t h e r i d g e . t o the

P a c i f i c - I n d i a n p o l e of r e l a t i v e motion. Marginal i n t e r a c t i o n s between t h e p l a t e s a s s o c i a t e d with r a p i d , r e c e n t motions o f t h i s pole (LC Pichon

[image:22.612.72.532.57.628.2]

complex s t r u c t u r a l development o f t h e r i d g e , and i n c i d e n t a l l y t o t h e exposure o f p a r t o f

it

t o form Macquarie I s l a n d .

Local 1 y

,

r a d i o m e t r i c

'

c

ages o f Aptenodytes

patagonica

( b l i l l e r ) i n u p l i f t e d t e r r a c e s s u g g e s t t h e a c t u a l emergence o f Macquarie I s l a n d

i n t h e e a r l y P l e i s t o c e n e , w i t h an average u p l i f t age o f t h e i s l a n d between 1 . 5 m and 4 . 5

rn

p e r 1000 y e a r s (CoLhoun & Goede, 1 9 7 3 ) .

2 . 2 ACCESS AND FIELD CONDITIONS

Macquarie I s l a n d i s a Tasmanian S t a t e Reserve and i s a l s o classified as an A n t a r c t i c b a s e . Its s t a t u s as a r e s e r v e r e q u i r e s t h a t pcrmiss.ion t o work on and sample t h e i s l a n d be o b t a i n e d from t h e N a t i o n a l Parks and W i l d l i f e S e r v i c e of Tasmania. I t s c l a s s i f i c a t i o n a s an A n t a r c t i c b a s e means t h a t access i s g e n e r a l l y l i m i t e d l o p e r s o n n e l o f e x p e d i t i o n s o f t h e A n t a r c t i c D i v i s i o n of t h e A u s t r a l i a n Department o f S c i e n c e and t h e Environment. Access i s normally o n l y p o s s i b l e by s e a and t h e t h r e e f i e l d t r i p s I made d u r i n g t h i s s t u d y i n v o l v e d 4 - 6 day voyages (each way) on t h e M.V. NeZZa Dan, o r M.V. Tka2a Dan. T l ~ e f i e l d t r i p s were f o r t h e . four-month 1975/76 summer s e a s o n , and two f i v e - d a y v i s i t s d u r i n g t h e 1876 and 1979 S p r i n g R e l i e f e x p e d i t i o n s . These e n a b l e d t h e completion o f mapping and sampling p r o j e c t s i n i t i a t e d by e a r l i e r workers (Varne e t aZ., 1969; Varne E Rubenach, 1972, 1973).

The o u t c r o p on Macquarie I s l a n d v a r i e s . I t i s g e n e r a l l y good a l o n g t h e c o a s t a l t h o u g h t h e r a p i d emergence o f thc i s l a n d Ilas l c f t :;omc l a r g e f o s s i l beach d e p o s i t s where t h e r e c o g n i t i o n o f i n s i t u a:ltcrial c a n be d i f f i c u l t . On t h e p l a t e a u t h e r e i s l e s s o u t c r o p and wcathcring has a f f e c t e d t h e r o c k s , t h e metamorphosed m a t c r i a l b c i n g p a r t i c u l a r l y s u s c e p t i b l e .

immediate v i c i n i t y o f t h e b a s e camp,

i s

on f o o t . Small f i e l d h u t s a r e

spaced a t approximately four-hour walking i n t e r v a l s down t h e e a s t c o a s t

o f t h e i s l a n d , w i t h o n l y one h u t on t h e west c o a s t a t Bauer Bay.

2 . 3

ELECTRON

MICROPROBE DEVELOPMENT AND CALIBRATION

An i n t e g r a l p a r t of t h i s r e s e a r c h p r o j e c t

was

t h e development of

s o f t w a r e and hardware n e c e s s a r y f o r c a l i b r a t i o n and r o u t i n e o p e r a t i o n

o f an energy d i s p e r s i v e m i c r o - a n a l y t i c a l system f o r a c c u r a t e

q u a n t i t a t i v e a n a l y s e s of g e o l o g i c a l m a t e r i a l s . The system i s an

EDAX EDS system a t t a c h e d t o a J E O L JXA-50A s c a n n i n g e l e c t r o n microprobe

i n the Central Science Laboratory, U n i v e r s i t y of Tasmania. The s o f t w a r c

and o p e r a t i n g procedures developed for t h i s system a r e d e s c r i b e d i n

d e t a i l by G r i f f i n (1979), Appendix 8 of t h i s t h e s i s .

The i n i t i a l development o f t h i s system b y myself r e q u i r e d one y e a r , and t h u s comprises a s i g n i f i c a n t p r o p o r t i o n o f t h e s t u d y . This was e s s e n t i a l t o the p r o j e c t a s t h e f i n e - g r a i n e d n a t u r e o f both primary

and secondary f e a t u r e s o f t h e samples excluded normal i d e n t i f i c a t i o n

t e c h n i q u e s . Subsequent c a l i b r a t i o n and u p d a t i n g o f c o r r e c t i o n r o u t i n e s

werc performed independently o f t h i s s t u d y , a s a c o n s u l t a n t . The s y s t e m

currently p r o v i d e s up t o 2 0 a n a l y s e s p c r h o u r of norrnal s i 1 icclte

Chapter 3

GEOLOGY OF MACQUARI E

IS

LAND

3 . 1 INTRODUCTION

The g e n e r a l geology o f Macquarie I s l a n d h a s been d e s c r i b e d by

Mawson (1943) and Varne

E

Rubenach (1972). The r e s u l t s of f i e l d w o r k

a s s o c i a t e d with t h i s t h e s i s g e n e r a l l y c o r r o b o r a t e d t h e work by Varne

e t

aZ.

(1969) and Varne

E

Rubenach (1972), i n p a r t i c u l a r s u p p o r t i n g t h e i r

r e c o g n i t i o n of t h e ''Older Basic Groupff (Mawson, 1943) a s dyke swarm

units, and t h e r e f o r e t h i s c h a p t e r i s o n l y a s h o r t summary o f t h c s e e a r l i e r s t u d i e s , m a i n l y drawn from G r i f f i n

E

Varne (1980) (Appendix 7 ) .

However i t has proved p o s s i b l e t o d i s t i n g u i s h s t r a t i g r a p h i c r e l a t i o n s h i p s

between d i f f e r e n t v o l c a n i c b l o c k s , and t h i s a s p e c t o f t h e f i e l d w o r k i s

d e s c r i b e d i n Chapter 5 . These f i e l d r e l a t i o n s h i p s form t h e b a s i s f o r

t h e r e c o g n i t i o n o f s e v e r a l p a r t i a l s e c t i o n s through the v o l c a n i c sequence,

t h a t eluded t h e p r e v i o u s ' w o r k e r s , based on t h e s p a t i a l d i s t r i b u t i o n o f t h c

v a r i o u s secondary assemblages and new s t r u c t u r a l i n f o r m a t i o n . I

3 . 2 GENERAL GEOLOGY

Fault-bounded b l o c k s o f v o l c a n i c s and dyke swarms colnpose about.

80% o f t h e o u t c r o p o f Macquarie I s l a n d ( F i g u r e 3 . 1 ) . An i n t r u s i v e

complex of dyke swarms, massive and l a y e r e d gabbros, and scrpcnt:inizod p e r i d o t i t e s l o c a t e d a t t h e n o r t h e r n end o f t h e i s l a n d ( F i g u r c 3 . 2 ' ~

I

c o n s t i t u t e t h e remainder.

I

The v o l c a n i c b l o c k s a r e mainly p i l l o w l a v n s , t o g e t h c r with v ; ~ r y i n g

p r o p o r t i o n s o f r a r e massive l a v a f l o w s , b a s a l t i c dykes a n d v a r i o u s sediments. These sediments i n c l u d e c a l c a r e o u s

CZobigerina

oozc :tr~cj

F i g u r e 3 . 1 , G e r i e r a l i z e d g e o l o g i c map o f blacquarie I s l a n d w i t h s u p e r f i c i a l d e p o s i t s o m i t t e d . S e r p e n t i n i z e d p e r i d o t i t & and g a b b r o masses a r e marked by c r o s s e s ; e x t r u s i v e . v o l c a n i c r o c k s and a s s o c i a t e d s e d i m e n t s a r e marked b y ' v e e s ; dyke swarms a r e b l a n k . S t r i k e s on l a v a s a r e shown w i t h a s i n g l e t i c k , and strikes on dykes a r e ' s h o w n w i t h a d o u b l e t i c k . F a u l t e d c o n t a c t s a r e drawn as heavy l i n e s , and g r a d a t i o n a l o r u n c e r t a i n c o n t a c t s a r e drawn a s d o t t e d l i n e s . (From Varne

[image:26.612.117.429.47.681.2]

F i g u r e 3 . 2 G e o l o g i c a l map o f t h e n o r t h e r n p a r t o f F l a c q u a r i e I s l a n d , w i t h s u p e r f i c i a l d e p o s i t s o m i t t e d (from V a r n e

E

Rubenach, 1 9 7 2 ) . S e r p e n t i n i z e d p e r i d o t i t e b o d i e s a r e marked b y heavy d i a g o n a l c r o s s - h a t c h i n ' g ; l a y e r e d g a b b r o complcx i s marked by c r o s s e s ; o t h e r g a b b r o masses a r e marked by l i g h t c r o s s - h a t c h i n g ;

[image:27.612.148.508.37.509.2]

;II~J i r1*cgul:lr*, thin,

lcnsoid

: i c c t i m i i l ; ~ t i o ~ ~ s int.crsti t i n 1 t o t h c p i 1 low

l a v n s , (11)

to

8 CIII i n

thickness.

It is

usually rccryst;lll

i z c d a n d / o r

silicified. Poorly preserved coccoliths in this ooze have hccn intcr-

preted to yield an Early, or perhaps Cliddle, Miocene age (Quilty

e t

GZ.,

1973).

The volcaniclastic sediments range from siltstones to volcanic

brcccias.

All are reddish-brown, due to a high proportion of iron

oxides in the matrix'cement.

The

finer-grained sediments occur

in

small lenses, up

to

2

m thick, intercalated with

the

lava flows. Their

lrlteral

e x t e n t

is difficult to estimate.

One

lens is exposed for about

60 rn

at Mawson Point, on the northwestern coast of Macquarie Island

(Figurc 3.1).

These,lenses

commonly contain graded beds, from siltstone

to

lithicwackes,

and rarely include truncated, convoluted laminae.

The

volcanic breccia deposits are lenses ranging from

2-3 m up

to 15

m

in

thickness, again well exposed

at ~ a w s o n Point. These

are unstratified

rocks with angular clasts varying up to 15 cm (in maximum scction) from

n

silt-sized matrix and with

a

highly variable degree of packing.

I

liyaloclastite breccias occur interstitially to lava pillows and

as

I

small

l e n s e s . I

Varnc

c t

aZ.

(1969)

have noted that the association of pillow

lava with interstitial

GZobigerina

ooze, hyaloclastite and lithiwacke

is

characteristic of ocean-floor deposition and probably occurred at

water depths between 2000 m

and

4000

m.

The

range of sediment types,

including small turbidite flows, reflects

a variety of depositonal

environments, from low to

high energy. The volcanic breccias closely

resemble talus-slope and rubble deposits observed at various places on

the Mid-Atlantic Ridge.

The

dolerite dyke swarms

are

a notable geological feature, and

occur as fault-bounded blocks almost entirely composed of series

of

s h e e t e d dyke complex. S c r e e n s o f massive gabbros, s e r p e n t i n i z e d

p e r i d o t i t e s and v o l c a n i c s a r e p r e s e n t . Although m u l t i p l e i n t r u s i o n s ,

evidenced by s e r i e s o f s i m i l a r l y f a c i n g c h i l l e d e d g e s , make e s t i m a t e s

o f t h i c k n e s s d i f f i c u l t , t h e average dyke t h i c k n e s s i s about 1

m,

w i t h r a r e examples r a n g i n g t o 3 m e t r e s .

Rocks o f t h e l a y e r e d gabbro complex around Half-Moon Bay

(Figure 3 . 2 ) i n c l u d e w e h r l i t e , p l a g i o c l a s e w e h r l i t e , t r o c t o l i t e , o l i v i n e gabbor and g a b b r o .

The

rocks a r e m i n e r a l o g i c a l l y s i m p l e . They mainly "

c o n s i s t of o l i v i n e , p l a g i o c l a s e and clinopyroxene. S p i n e l

i s

a r a r e

b u t widespread a c c e s s o r y m i n e r a l , and orthopyroxene o c c u r s v e r y r a r e l y

a s t h i n rims around o l i v i n e o r a s t i n y r e l i c t g r a i n s w i t h i n clinopyroxene.

Cumulate phenocryst phases a r e o l i v i n e , p l a g i o c l a s e and' s p i n e l . Clino-

pyroxene i s a l w a y s postcumulus; p l a g i o c l a s e

i s

a l s o postcumulus i n some o l i v i n e - r i c h r o c k s .

The massive gabbros exposed on t h e east c o a s t , and t o t h e n o r t h e a s t o f t h e l a y e r e d gabbro complex ( F i g u r e 3 . 2 ) p o s s e s s complicated i n t r u s i v e c o n t a c t s , c o n t a i n i n c l u s i o n s o f o t h e r gabbros, a r c i n p l a c e s deformed,

!

and a r e :ilso c u t by d y k e s . T h e i r b u l k chetnical c o m l ~ o s i t i o n s

are

s i m i l a r t o t h o s e of some o f t h e l a v a s and dykes ( G r i f f i n

G

Varne, 1950) :3nd t h e y

range i n t e x t u r e from g a b b r o i c t o d o l e r i t i c . A zone o f massive gabbros

t h a t have undergone high-temperature s u b - s o l i d u s r e c r y s t a l l i z a t i o n o c c u r s

around I s l a n d Lake and n o r t h o f Eagle P o i n t (Figure 3 . 2 ) . T h i s zone

s e p a r a t e s t h e w e h r l i t e s o f t h e l a y e r e d gabbro compl c x from n h:irzl,~r 1.1: i t-i:

' h e complcx s t ~ u c t u r o of

Macquorie

I s i a n d i . ~ a .resu.lt o f t h e wi.dc- s p r e a d f a u l t i n g , probably on a l l s c a l e s , and t i l t i n g o f t h e f a u l t - '

bounded b l o c k s . Dips measured on sediments r a n g e from n e a r - h o r i z o n t a l

t o 45' and f o r p i l l o w e d l a v a u n i t s t o 80' ( F i g u r e 3 . 3 ) . These l a t t e r

measurements a r e l e s s r e l i a b l e b u t a r e s u p p o r t e d by good agreement where

o u t c r o p s a l l o w measurements from both l a v a s and sediments (Table 3 . 1 ) .

Two s t r i k e d i r e c t i o n s a r e common, about 150' and 230'. I t h a s been argued

t h a t t h e t i l t i n g o f t h e v o l c a n i c rocks o c c u r r e d i n two s t a g e s : t h e f i r s t

stage involved t i l t i n g around axes t h a t were n e a r - h o r i z o n t a l and p a r a l l e l

to dyke-bedding p l a n e i n t e r s e c t i o n s and t h e r e f o r e o n l y caused v a r i a t i o n s

i n d i p ; t h e second was

a l a t e r

r o t a t i o n about v e r t i c a l axes o f t h e t i l t e d r o c k s t h a t caused t h e v a r i a t i o n s i n s t r i k e (Varne E Rubenach, 1972)

.

This i n t e r p r e t a t i o n i s s u p p o r t e d by palaeomagnetic s t u d i e s (Williamson, I

1979)

.

Angular r e l a t i o n s h i p s between l a v a s and dykes measured a t 14 . localities range from 46' t o 86' (Table 3 . 2 )

.

The mean a n g l e i s 72'

0

and a t 11 o f t h e 14 l o c a l i t i e s t h e a n g l e i s g r e a t e r t h a n 60

,

s u p p o r t i n g a model where t h e l a v a s have been i n t r u d e d by n e a r - v e r t i c a l

o r

v e r t i c a l dykes. The d e v i a t i o n s o f t h e a n g u l a r r e l a t i o n s h i p s from 90' may i n p a r t r e s u l t from n o n - h o r i z o n t a l a t t i t u d e s o f t h e l a v a s a t t h e t i m e o f

i n t r u s i o n .

The

dykes seem t o have been i n t r u d e d o r i g i n a l l y s t r i k i n g e a s t - s o u t h e a s t . T h i s o r i e n t a t i o n corresponds v e r y w e l l w i t h t h e i n f e r r e d

o r i e n t a t i o n o f t h e s p r e a d i n g a x i s of t h e I n d i a n - P a c i f i c r i d g e d u r i n g

Anomaly 7 time, as p r e s e r v e d b y . t h e p r e s e n t o r i e n t a t i o n o f t h e l i n e a r

marine magnetic anomalies

i n

t h e r e g i o n (Weissel & Hayes, 1 9 7 2 ; Williamson:

Table

3 . 1

- . . . . -. . . .

STRUCTURAL MEASUREMENTS FtiOIl PILLOW LAirAS

AND

SEDIMENTS

AT THE

SAME

LOCALITY

1

Locality Lava Sediment

[image:32.612.95.449.78.569.2]

Table

3.2

ANGULAR

RELATIONSHIPS

BETWEEN

LAVAS

AND INTRUDING

DYKES

'

Locality

Angle

( O ) *

Bishop

I s . ' 8 2

Average = 7 2 O

Range = 86"-46"

I

I

*

True

angles were obtained

by standard

stereographic

[image:33.612.152.497.42.559.2]

R o t a t i o n s o f t h e c r u s t a l b l o c k s o f t h e ' i s l a n d t h e r e f o r e

a p p a r e n t l y o c c u r r e d f i r s t a b o u t h o r i z o n t a l a x e s p a r a l l e l t o t h e s p r e a d i n g a x i s , probably a s an i n t e g r a l p a r t of t h e s p r e a d i n g process ( B a l l a r d 6

Van Andel, 1 9 7 7 ) . L a t e r r o t a t i o n s a b o u t v e r t i c a l axes c o u l d have been I caused by r e c e n t s t r i k e - s l i p movements a l o n g t h e Macquarie Ridge.

3 . 4 _- AGE

I

C o c c o l i t h s p r e s e n t i n t h e GZobigerina o o z e s , i n t e r s t i t i a l t o t h e p i l l o w l a v a s , i n d i c a t e a n age o f E a r l y , o r p e r h a p s Middle, Miocene

( Q u i l t y

e t

aZ., 1 9 7 3 ) . T h i s i s a range o f about 1 1 - 2 2 Ma i n a b s o l u t e

terms. The c o r r e l a t i o n of marine magnetic Anomaly 7 a c r o s s Maequal-ie I s l a n d (Williamson, 1974) s u g g e s t s an age o f a b o u t 27 Ma u s i n g t h e H e i r t z l e r

e t

aZ. (1968) m a g n e t o s t r a t i g r a p h i c time s c a l e , o r 25.5 Ma

us ing t h e more r e c e n t t i m e s c a l e o f L a Brecque et at. (1977) . Thus the

g e n e r a l age a p p e a r s w e l l c o n s t r a i n e d , b u t a s p e c i f i c age a e t e r m i ~ l a t i o n

I

Chapter 4

1'ETROC;RAI'IIY AN11 I'll1 MARY PIIAS [I MlNERA1.OC;Y -

4 . 1 GENERAL FETROGRAPlIY

OF

THE LAVAS

AND

DYKES

The l a v a s range from g l a s s y t o medium g r a i n e d c r y s t a l l i n e l a v a s , and

from a p h y r i c t o s t r o n g l y p o r p h y r i t i c v a r i e t i e s . P l a g i o c l a s e i s t h e

most abundant phenocryst phase followed by o l i v i n e , s p i n e l and r a r e l y

c l i n o p y r o x e n e . Dykes i n t r u d i n g t h e l a v a s show similar p e t r o g r a p h i c

v a r i a t i o n s but a r e coar.ser g r a i n e d .

Dyke swarm rocks r a n g e

from

f i n e t o c o a r s e g r a i n e d , a g a i n with both a p h y r i c and s t r o n g l y p o r p h y r i t i c v a r i a n t s . P l a g i o c l a s e

i s

more

abundant i n t h e s e r o c k s t h a n i t i s i n t h e l a v a s and a s s o c i a t e d dykes,

as h a s been noted by Varne E Rubenach (1972).

A l l o f t h e samples examined have been a l t e r e d o r metamorphosed t o some e x t e n t b u t t h e primary igneous t e x t u r e s have n o t been

s i g n i f i c a n t l y d i s r u p t e d . T h i s

i s

a g e n e r a l f e a t u r e o f t h e a l t e r a t i o n o f mafic r o c k s from o p h i o l i t e s (Coleman, 1977) and from t h e ocean f l o o r

(Cann, 1971). The m i n e r a l o g i c a l changes imposed on t h e s e r o c k s d u r i n g

t h e v a r i o u s mctamorphic e v e n t s a r e d i s c u s s e d i n d e t a i l i n Chapter 5 .

Cameron e t

aZ.

(1979) have recognized t h r e e t y p e s of b a s a l t s from

Macquarie I s l a n d i n t h e A u s t r a l i a n N a t i o n a l U n i v e r s i t y c o l l e c t i o n .

Type-] b a s a l t s a r e p l a g i o c l a s e - p h y r i c (Ansz-70) w i t h s u b - v a r i o l i t i c

t e x t u r e s and c o n t a i n e u h e d r a l o l i v i n e p h e n o c r y s t s (Fogo-88) and, r a r e l y ,

c l i n o p y r o x e n e . The p l a g i o c l a s e p h e n o c r y s t s c o n t a i n g l a s s y i n c l u s i o n s

and sometimes t i n y e u h e d r a l s p i n e l s and a r e n o t s t r o n g l y zoned.

Q p e - 2 b a s a l t s c o n t a i n microphenocrysts o f a l t e r e d o l i v i n e , e l o n g a t e

l a t h s o f p l a g i o c l a s e f e l d s p a r and dark a r e a s o f m e s o s t a s i s which are

in altered olivine or as rnegacrysts up to 0.6 mm.

a p e - 3

basalts

contain primar). kaersutitc

as discrete groundnrass crystals

or as

rims

on colourless ai~gite

and microphenocrysts of plagioclasc, clinopyroxene

and chrome spinel.

Type-I

basalts dominate the

A.N.U.

collection'and,

with type-2

basalts, are noted to

be

texturally and mineralogically similar to

basalts generated at mid-ocean ridges or in marginal basins. The

amphibole-bearing

type-3 basalt is suggested to have no ocean-floor

equivalent and possibly to represent the type of vulcanism .that

eventually forms seamounts (Cameron

et

aZ.,

1980) although this seems

unlikely from the field relationships.

The study reported here

is

the first made on a comprehensive

collection from Macquarie Island and involved the petrographic

exam-

ination of

198

lava samples and

41

dyke samples. Three major petro-

graphic

variations are present

in the

samp1e;'in the degree

of

crystallinity, porphyritic nature and, in the coarser variants,

groundmass mineralogy.

..-. . - .. --

The majority of the samples is porphyritic, and relative proportions

and assemblages o f ' p h e n ~ c r ~ s t s

are given

in Table 4

-1,

based on the

initial sample set (129 lava samples: Appendix

9).

The major feature

shown by Table 4.1

(1)

is the predominance of plagioclase amongst the

phenocryst phases.

It

is present.in 91% of the phyric lavas, followed

,

by olivine

( 4 7 % ) ,

clinopyroxene (27.5%) and spinel

(26.5%) ).

The

various phenocryst assemblages present are listed in Table 4.1

( 3 ,

together with their relative frequencies

of occurrence.

The lavas vary in degree

of

crystallinity from glassy pillow-

selvedge material and spalled glass fragments

in

interstitial hyalo-

clastite breccias

( e . g . , as

commonly exposed in the North Head and

Pyramid Peak areas) to medium grained holocrystalline rocks in the

Table 4.1

PHENOCRYST ASSEMBLAGES

AND ABUNDWCES IN

LAVAS

(129 samples)

No. 0.

'0

Aphyric samples Porphyritic samples

PORPHYRITIC SAMPLES

7

. -

1 . I n d i v i d u a l p h a s e o c c u r r e n c e s

plagioclase is contained by 93* 91.2

olivine 11 48* 47.1

spinel I f 27* 26.5

2.. Phenocryst assemblage occurrences

plag 37 36.3

0 1 3 2 -9

s

P

1 1

.o

plag + cpx 10 9.8

p l a g + 01 13

12.7

plag + sP 5 4.9

cpx + s p 01 + s p

plag + 01 +

cpx

plag + 01 +

sp

plag + 01 + cpx + sp

[image:37.612.104.455.129.663.2]

blawson P o i n t . A complete v a r i a t i o n e x i s t s between t h e s e two extremes.

Flany o f t h e g l o s s e s c o n t a i n quenched c r y s t a l l i t e s , commonly of o l i v i n e .

These c r y s t a l l i t e s may c o n t a i n two-phase i h c l u s i o n s o f g l a s s and.

vapour ( P l a t e 4 . 1 ) . More slowly-cooled samples c o n t a i n g r a n u l a r

plumes o f clinopyroxene with t h e development

of

s u b - v a r i o l i t i c t e x t u r e s ( P l a t e 4 . 2 )

.

The groundmass mineralogy o f t h e h o l o c r y s t a l l i n e l a v a s and dykes

e x h i b i t s a wide v a r i a t i o n . A t one. extreme t h e l a v a s have s u b o p h i t i c

t o ophi t i c t e x t u r e s ( P l a t e 4 - 6 1 with equant subhedral p l a g i o c l a s e p a r t i a l l y o r completely e n c l o s e d by g r a n u l a r t o s u b h e d r a l a u g i t e t o g e t h e r with t i t a n o m a g n e t i t e and g l a s s . I n c o a r s e r g r a i n c d examples

t h e a u g i t i c clinopyroxenes may have t h i n p u r p l i s h t i t a n a u g i t e

rims

and i n r a r e l a t e - s t a g e s e g r e g a t i o n v e i n s k a e r s u t i t i c arrrphibolc, t o g e t h e r

w i t h a p a t i t e rods and s k e l e t a l i l m e n i t e , may a l s o be p r e s e n t a s a l a t e

growth.on t h e c l i n o p y r o x e n e , b u t n o t a s d i s c r e t e g r a i n s . O l i v i n e

i s

n o t p r e s e n t a s

a

groundmass p h a s e . Such r o c k s a r e n o t t r u e t h o l e i i t e s

i n t h a t t h e y l a c k p i g e o n i t e o r orthopyroxene

as a

groundmass phase ( J o p l i n , 1 5 7 1 ) . As Carmichael etaZ.(1974) have remarked, a c e a n - f l o o r

b a s a l t s have a l k a l i n e b a s a l t mineralogy b u t t h o l c i i t i c c h e m i s t r y . Ilowever, a s n o t e d by Cameron

e.t

aZ. (1980), p e t r o g r a p h i c a l l y t h e s e

r o c k s a r e i d e n t i c a l t o modern o c e a n - f l o o r b a s a l t s o r "abyssal t h o 1 e i . i . t ~ ~ "

(Cann,

1971). I n the f o l l o w i n g t c x t t h e s e v a r j a n t s a r e conscclucnt l y r e f e r r e d t o a s t h e t h o l c i i t i c v a r i a n t s .

-- - . - . . . .-. --... ~ . .

A t t h e o t h e r e x t r e m e . t h e l a v a s a r e c h a r a c t e r i s e d by p u r p l i s h t i t a n a u g i t e s g e n e r a l l y developed as sheaves o f t a b u l a r , e l o n g a t e

I ,

c r y s t a l s o r i e n t e d p e r p e n d i c u l a r t o randomly-oriented, c l o n g a t c laths of: p l a g i o c l a s e , ' forming _{an i n t e r s e r t a l t e x t u r e (Plate 4.3)}

.

_{'Ihc ~ ) y r a x c n c s} a r e uniform i n c o l o u r , and t h i s d i s t i n g u i s h a b l e from t h c Latc stilgc

Plate 4.1 Quenched olivine crystallite in glass containing

symmetric glass and glass + vapour inclusions

(sample 46). Scale: 1 cm = 10 microns.

Plate 4.2 Subvariolitic texture in chilled lava pillow rim

P l a t e 4 . 3 Intersertal

texture in

alkaline

basalt

(sample 38391).

Scale:

1 cm = 800 microns

Plate 4.4 Reddish-brown kaersutitic amphibole as discrete crystals

and rims on zoned titaniferous augites

in

an alkali