Petrography, Mineral Composition and Geochemistry of' Volcanic and Subvolcanic Rocks of CRP-3, Victoria 1,ancl Basin, Antarctica
Abstract - T h e petrography. mineralogy a n d g e o c h c m i s ~ r y of volcanic and s u b v o l c a n i c r o c k s in C R P - 3 c o r e have been e x a m i n e d in dctail in o r d e r to characterise and to compare them with volcanic and subvolcanic rocks cropping on1 in tlie Victoria Land area. and to define the clask provenance or to establish possible volcanic activity coeval with deposition.
C l a s t s w i t h s i z e s r a n g i n g f r o m g r a n u l e t o b o u l d e r s h o w geoclicmical and mineralogical features comparable with those of Fcrrar Supergroup rocks. They display a subalkaline affinity and compositions ranging from basalts to dacitc.
Three different petrographic groups with distinct textural and grain siLc features
( s u b o p h i t i c . intergranular-intersertal. and g l a s s y - h y a l o p i l i t i c ) arc recognised a n d arc related to the ei~~placeinent/cooling mechanism.
I n the sand to silt fraction, the few glass shards that have been recognised are strongly altered: however chemical analyses show they have subalkalic magmatic affinity. Mineral compositions of the abundant free cli~iopyroxene grains found in the core. are less affected by alteration proccsscs. and indicate an origin from subalkaline magmas. This excludes the presence, during the deposition of CRP-3 rocks of alkaline volcanic activity comparable with the McMurdo Volcanic Group.
Strong alteration of the magmatic body intruded the Beacon sandstones obliterates the original mineral assemblage. Geochemical investigations confirm that intrusion is part of the Fen'ar Large I g n e o ~ ~ s Province.
INTRODUCTION
Mackay Glacier drains the East Antarctic ice sheet crossing the Transantarctic Mountains at about 76O. In this sector of the ~ransantarctic Mountains (TAM) the principal units of the Cambro-Ordovician crystalline basement crop out (Granite Harbour Igneous Complex and Koettlitz Group) (Allibone, 1992: Allibone et al.:
1993): as well as the overlying Devonian-Triassic q u a r t z o s e s e d i m e n t a r y s u c c e s s i o n ( B e a c o n Supergroup). This sedimentary sequence is intruded and overlain by Jurassic sills and volcanics rocks that f o r m part of t h e F e r r a r L a r g e Igneous P r o v i n c e (FLIP) (Kyle, 1998). The uplift. more than 5 km, of t h e T r a n s a n t a r c t i c M o u n t a i n s began in t h e l a t e Cretaceous, filling the Ross Sea basins with detrital sediments resulting from denudation (Fitzgerald, 1999). Cenozoic alkalic McMurdo Volcanic Group nagm mat ism began in this sector of the TAM in the Oligocene, as evidenced by volcanic glasses and tephra found in the upper 300 metres cored in CRP-2 (Arn~ienti et al.. this volume; McIntosh, 2001) T h e tephra are the only expression of volcanisin in the vicinity of C a p e Roberts as surface exposures are lacking. Marine magnetic anomalies. west of the CRP drill sites ( B o z z o et al., 1997), may represent the roots of volcanic centres that produced the tephra.
CRP-3 cored about 800 meters of Cenozoic strata, in which igneous materials form an important fraction of the sediments, and about 200 meters of Devonian sandstone intruded by a small magmatic body (Cape Roberts Science Team, 2000).
In this p a p e r w e discuss t h e petrography.
mineralogy and geochemistry of volcanic and subvolcanics rocks in CRP-3 in order to characterise and then to compare them with volcanic successions cropping out in the Victoria Land area and finally to d e f i n e their p r o v e n a n c e or to e s t a b l i s h possible syndepositional volcanic activity. We investigate as well, compositional and textural characteristics of the magmatic body found at the base of the core, in order to c o m p a r e it with s i m i l a r b o d i e s c r o p p i n g o u t onland. Further recognition among the volcanic clasts.
of distinct lithologies whose stratigraphical positions are well constrained in the onland succession gives information on the uplift and erosion style of the mountain range and on the evolution of the basins.
STRATIGRAPHY OF VOLCANIC AND SUBVOLCANIC ROCKS IN VICTORIA LAND
Volcanic and subvolcanic rocks in Victoria Land were formed in two distinct episodes and contrasting emplacement styles. The older episode. Jurassic in
"Corresponding author (pompilio@ct.ingv.it)
g e , includes s i l l s (Fen'ar D o l e r i t e ) . lava f l o w s ( K i r k p a t r i c k B a s a l t s ) , and p y r o c l a s t i c d e p o s i t s (Mawson and Exposure Hill Formations, Kirkpatrick Basalt pyroclasts) (Elliot, 2000: Elliot et al., 1995).
These rocks belong to the Ferrar Supergroup which is now referred 10 as the Fcrrar Large Igneo~is province (FLIP) (Kyle, 1998). Young volcanics rocks (Eocene to present) formed volcanoes in the McMiircio area and in nonhcrn Victoria Land (Kyle. 1990) aiul arc known as the Me Murdo Volcanic Group (McMVG).
Sills represent the I I I I X ~ widespread expression of Jurassic niagmatism, in thc Transa~itarctic Mountain, and intrude Beacon Supergroup and rarely the Granite H a r b o u r intrusive c o m p l e x . T h e s i l l s r a n g e in thickness from a Sew tens to over 300 metcrs (Elliot et a!., 1995). They commonly show regular columnar joi~iiing and exhibit sharp intrusive co~itacts with the host rocks. Rafts of tilted and rotated Beacon strata occur within the sills. Altcmtion of Beacon sandstones a n d Ordovician granitoids caused by IiydrotIien~irtI circulation clue to sill intrusion has been documented in the Taylor Glacier region (Craw & Findlay, 1984), and phreatic eruption related to very shallow inu'usioii lias been described as well (Grapes et al.. 1973).
Volcanic and volcaniclastic material form the P r e b b l e Formation in the central T r a n s a n t a r c t i c Mountains, Mawson Formation in S o u t h Victoria Land (SVL) and Exposure Hill Formation in northern Victoria Land (NVI.) (Elliot. 2000). They overlie
a
silicic luffaceous sequence (13anso11 Forination) of probable Early Jurassic age (Elliot, 1996) and Triassic volcanichistic rocks of Lashiy, Fremouw and Falla Pormations which conti~in clasis coming from both craton and volcanic arc. In SVI,., close to the CRP-3 sitc, pyrociastic (tuff-brcccia and lapilli-breccia of the M a w s o n f o r m a t i o n ) a n d v o l c a n i c l a s t i c r o c k s (Carapace Sandstone) are more than 400 in of thick.Kirkpatrick Basalis form ;I sequence of lava flows variable from 1 to more than 100 metres thick and with a total thickness in NVL of more than 700 in (Elliot e t al.. 1986). Lavas a r e i n t e r b e d d e d with volcaniclastic detritus, and locally pillow or pillow- breccia form the base of the lava secluc~~ce (Roland &
Worner, 1996). Among tlie Kirkpatrick Basalts two compositional types can be distinguished o n the basis of m a j o r and trace elenients c o n c e n t r a t i o n s . T h e Mt.Fa/.io chemical type (MFCT) forms the ma,jority tlie succession. whereas Scarab Peak chemical type (SPCT) forms only the uppermost flows or series of flows. SI^T flows are more evolved than MFCT and shows marked iron and titanium enrichme~it (Elliot et al., 1995).
C e n o ~ o i c magmatic rocks include lavas, teplira ( L e M a s u r i e r & Thornson. 1 9 9 0 ) and i n t r u s i o n s (Tonarini et al., 1997). T h e extrusive rocks fc)rm the youngest stratigraphic unit. except for glacial deposits.
in a large part of Victoria Land but intrusion cllso o c c u r witiiin t h e Victoria L a n d basin s e d i m e n t s (Boy.y.o et al., 1997).
PETROGRAPHY
VOLCANIC AND SUBVOLCANIC CLASTS
Detailed petrographic descriptions of the igneous cletritiis in CRP-3 arc inclucicd i n the Initial Report (Cape Roberts Science Team, 2000). A summary of the petrograpliical (laid is given below. Igneous clasis with s i z e s r a n g i n g from g r a n u l e to b o u l d e r a r e medium- to fine-grained black to vey rocks. They are gencrully massive. although amygdaloidal fragi'ncms with vesicles filled with secondary minerals are also common.
All the sampled clasis s h a r e the same mineral a s s e m b l a g e : major m i n e r a l s a r e pkigioclasc and augite; pigeonitc, K-felclspar, orthopyroxene and
quart^ are also found in inost of the samples, llmei~ite and mapnetitc are ubiquitous accessory minerals.
Altefiition is significant and mainly affects pyroxenes and the grouni.imass. C l a s t s i n which the original mineral assemblage is touilly replaced by secondary minerals a r c u n c o m m o n . 'Textural and "rain s i z e features observed in thin sections endbled us to divide a tlie s a m p l e d c i a s t s into t h r e e g r o u p s . T h e s e textural differences have been related (Cape Roberts Science Team, 2000) to the cooling, rate associated with clislinct empl;icement mechcinisms,
G r o u p I c o n s i s t s of m e d i u m to fine-grained l i o l o c r y s t a l l i ~ ~ c r o c k s , with s u b o p h i t i c texture.
Plagioclasc is the most a b u n d a n t phase. forming subhedral crystals (maximum length=2 m m ) ; dcilteric scricite alteration commonly replaces crystals. Augitc is si.~bordin;ite a n d f o r m s s i ~ b l i e d r ; ~ l and ti~ihedral crysti~ls (maximum lcngtl1=4 mm); in n-iosl samples i t is largely transformed to smectites. Pigconite is minor a s well and is associated witli augite as subl~edral crysttils, Both rhombohedral and cubic Fe-Ti oxides crystals arc present in most samples. Less commonly.
acicular ilmenite crystals with leucoxene coatinpare p r e s e n t . In most s a m p l e s . f i n e - g r a i n e d qu;irtz- felclspatliic intergrowths a r e visible in interstices between plagiioclase and pyroxene crystals. In fine- grained clasts plagioclase is euhedral, pyroxenes lend to be acicuhir a n d ;i very fine-grained c r y p t o - crystalline matrix is quite often strongly altered to scricite and clay minenils.
Group 11 is made-up of medium- to fine- grained hypocrystalline rocks with textures varying from intergranular to intersertal, Plagioclase is the most abundant phase and f o r m s a complex network of acicular crystals with pyroxene. Augitc crystals are rarely preserved, normally being altered to smectite.
Relicts o f the origirizil clinopyroxene are preserved only in the cores o f few 1zu-g~ prismcUic crystiils.
Pigeonitc is rare. and orthopyroxene occurs as single prismatic euheclral crystals. Anhedral quartz is present as discontinuous segregations. The interstitial material ranges from very fine-gmirincd cryptocrystalline matrix to very rare piilagonitised brown glass that encloses
tiny plagioclase and pyroxene aggregates. Group I1 c l , cists . . itte . . generally non-vcsici-ik~r, although s o m e irregularly s h a p e d vesicles filled by s e c o n d a r y minerals (carbonate. chalcedony, cristobalitc, and /-eolitcs) have been observed.
Group 111 includes aphyric and weakly porphyritic c l , tists. . . g e n e r a l l y hypocrystalline bin nirely holohy;iIi~ie. These rocks .show a low vesiculiiri~y, although i n s o m e specimens amygdales filled with secondary minerals form as much as 30% of the rock.
I'henocrysts iissemblage include Iiypidiornorphic plagiocliise and iliigite. More than 90% of the rock comprises a quench matrix consisting o f acicular crystals o f plagioclasc m d feathery inicrolites o f clinopyroxene and riirely. brown piilagonitic glass. In the more vesiculatecl, scoriticeous clasis, portions with c l e a r brown g l a s s without microlitcs becoiiie predominant.
GI..ASS SHARDS
The C1W3 Initial Report (Cape Roberts Science Team. 2000) recorded the presence of glass shards in the sand to silt fractions of the CRP-3 core. T h i n sections f r o m selected core intervals. where glass shards a p p e a r e d to be more abundant, have been investigated by optical a n d s c a n n i n g e l e c t r o n microscopy (SEMI. Glass shards i n thin section are difficult to identify due to the very sinall size ( c 2 0 0 inid-on) and the flake-like sliiipe. Backscauered SEM imiiges show that they are very finely crystallised (Fig. lit) and indicate they are not red! glass shards as occur in volccinic ash. but rather altered fr~igincnts of'
f i glassy or hyalopilitic rocks (Fig. l 1) & c).
INTRUSIVli BODY
Petrographical fctit~ircs of the inirusive body, its variability with depth ancl the relationship to the host- rocks have been described in detail in the preliminary report. A synthesis is rcported here. Tile rock forming t h e i n t r u s i o n is mainly massive and is s t r o n g l y altered. It is a I~ypocrystalline, apliyric rock. with a few (<S%) large (?-mm) phenocrysts whose outlines alone arc only recognizable. Phenocrysts are t t i b ~ ~ l a r scricitised plagioclase and prismatic (augite ? ) to e q u a n t (orllio ?) p y r o x e n e s totally r e p l a c e d b y hematite. c h l o r i t e , c l a y minerals and c a r b o n a t e . S a m p l e s f r o m t h e upper p a n (at 9 0 2 . 1 3 , 9 0 3 . 3 0 metres below sea floor (mbsf)) tend 10 preserve the original t e x t u r e better. a l t h o u g h the o r i g i n a l mineralogy can be detected only with the scanning electron microscope (Fig. Id).
T h e g r o u n d m a s s has a texture v a r y i n g froni interserial to intergnmular and is composed mainly of altered subhedral plagiocliise (now sericite). Between the pliigioclase laths, reddish heiriiitite grains a r e abundant and probably arc replaced mafic minerals.
Inlerstitii~l areas with acicular feldspars (albite or K-
feldspar), smcctites. serpentine and ilmenite needles arc also found and have probably repiacecl original glass (Fig. Id),
In samples from the lower portion of [lie intrusion (depth >906 nibsf), the tcxture, as well as the original ininemlogy. is totiilly destroyed: a n aniistomising web of carbonates. sinectites. probable serpentine, and lic~rieitite has completely rephiced most of the rock (Pig. 1c).
MINERAL CHEMISTRY
Chemical analyses of phases were carried out with both wiivelcngtli dispersive ( C a m e c n - C a m e b a x m i c r o p r o b e a t C N R - C S E S M R R o m e at 2 0 kv of acceleration tension, 30 inA of probe current, spot si7.c variable from 5 p m in minerals 10 20
!~m
in g l a s s e s ) and e n e r g y d i s p e r s i v e s y s t e m s (SKM Cambridge S360 iiiici Link Kxl EDS microanalysis at INGV-Catania with 15 k v , 0 . 5 m A , s p o t s i z e 5 - 20 {im) using natural minerals as standards and ZAP corrections.T h e composition of minerals ( p h e n o c r y s t s o r çiicrolites fo~iiid in volcanic clasis and as crystal Si'agnicnts are reported in tables 1 and 2 and in figure 2.
Pkigioclasc composition range from An8,, to more anorthitic compositions are found in phenocrysts belonging to G r o u p 1 elasts (Tab. i and Fig. 22). The highest anorthite contents are found in phenocryst cores and rims and ~ ~ ~ i c r o p l i e n o c r y s t s clre syslcmatically more albitic. A larger cornposilional range in plagioclasc is reported for basal dolerite boulder in CIROS-1 (Grapes et i l l . , 1 9 8 9 ) and in volcanic detritus o f CR1'-l and 2 (Arinienti et al..
1998; Armienti et al. ibis volume). In particular in C R P - 1 and 2 c o r e s a n a b u n d a n c e of a n o n h o s e c r y s t a l s a t t r i b ~ i t a b l c to M c M i w d o Cetiozoic magmatism was observed (An'nienti e t al.. 1998).
Conversely tile c o m p o s i t i o n a l range observed in feldspars of CRP-3 volcanic detritus, lies whitin that mcasi.irecI in Perrar dolcriies a n d Kirkpatrick basalts (both MFCT and SPCT types) reported in Elliot et al.
( 1 9 9 5 ) and Hornig ( 1 9 9 3 ) ( F i g . 221). Most of the analysed pyroxenes are augites with ¥ distinct iron- enrichment trend (Tab. 2 & Pig. 2b). A small subset of d a t a e x h i b i t s s i ~ E ~ c c a l i c aiigite ;ind p i g e o n i t e composition which is also observed petrograpl~ically.
This compositional range is typical of subalkaline volcanic rocks of the Fcn'ar Supergroup (Elliot et al..
1995; Hornig, 1993). In the Ca+Na vs Ti discriiiiinam d i a g r a m ( L e t e r r i e r et a l . , 1 9 8 2 ) ( F i g . 3 ) , C R P - 3 clinopyroxencs fall in tlie subalkali field together with subalkaline pyroxenes that characterise ciiamictite 01 t h e lower portion of C R P - 2 . S o d i c p y r o x e n e s observed amorig the clasts and crystal fragnients in C R P - 1 and 2 ( A r m i c n t i et a l . , 1 9 9 8 ) a r e totally l a c k i n g i n C R P - 3 s u g g e s t i n g t h a t t h e whole
s c d i ~ ~ i e n t a r y pile was deposited before onset of the Cenoxoic MeMVG ~n~dgiiiatism in the area,
In the intrusive b o d y drillecl at the b o n o m of CRP-3 (901.09-919.05 ~nibsl'). thc original ~inagin;itic panigenesis is totiilly replaced by demerit: minenils.
S o m e p s e u c l o ~ i ~ o r p l i s with the original s h a p e s o f pyroxenes cimi Ic1dsp;irs arc present but the original composition h;is been connpletciy erased. Wicicspre;i~l patches of siclcrite chai'clcterise lhe groundniass of the intrnsivc body (l-'ig. ie). showing tliiit fluids 1pI;iycd an i n i ~ o n a n i role in tlnc alteri~iion of the ii'itrusion.
At) e Of
138 A lid
GEOCHEMISTRY
VOLCANIC AND SUBVO1,CANIC CLASTS AND GLASS SHARDS
B u l k c o i i i p o s i t i o n s of l a r g e r ~ l a s t s w h i c h a r c r e p r e s e n u ~ t i v e of the three cliffereiit pen'ographical groups are reported in table 3, Microdiiiilyscs of gliiss sliiirds and intergranular "lasses i n clasts arc also included in table 4.
The large loss of ignition (1-01) v i n e s measured in the bulk rock samples, the low total for glasses and tlic occurrence of a small amount of corundum i n the CIPW norm. indicate that rocks and shards have been subjected to a s i g n i f i c a n t and variable degrees o f alteration.
Chemical alteration was checked using ternary d i a g r a m s p r o j ~ o s e d by N e s b i t t & Young ( 1 9 8 9 ) (Fig. 4). In these diagrams. bulk compositions of the ku-gcst analysed clasts and iiicsoslasis g l a s s e s ' p i o l very close to those of basalts-gabbros and show only
1 l i m i t e d s c a t t e r w h i c h is p r o h i ~ b l y r c l i ~ t c d to differentiation processes. I n the same diagrams most of the glass shards plot along a cliiTerentiation trend whereas only a few "lass coiiipositioiis point toward kaolinile (or kanciite) and illite end-members denoting iniponant alteration processes.
O n the basis of this diagram, altered glasses were excluded and the less altered compositions plotted in t h e total a l k a l i - s i l i c a ( T A S ) d i a g r a m i n o r d e r to
. .' '
dssify a n d t o c o m p a r e them with v o l c a n i c successions cropping out on land (Fig. 5 ) .
L a g e r d u s t s range iin composition from basaltic
163.3 185.5 Ill-f free ~
57.52 54.25 25.35
(1.40 0.31 7.60 6.44 0.65 98.27 X620 1.361 0.0 l5 HOOT 0.37 1 0.569 0.038 37.9 58.2
370.8 free
p.
51.61 28.84 0.86 0.29 13.13 3.69 0.27 98.09 2,379 1.567 0.033 0.003 0,649 11.330 0.016 65.2 33.2
. 1.6
aiicicsite to dacite. In partictilar rocks b c l o n g i ~ i g to grroup I are basalts. whereas group I 1 and groin; Ill are respectively, basaltic andesites anci ciaciles, Group l ami group I1 rocks show compositions comparable with those reported by (Elliot cl al., 1995) for Ferrar S u p e r g r o u p r o c k s . G r o u p 111 r o c k s s h o w 21 m o r e
474 M . I'oinpilio cl ill . .
i d . 2 - Compositions of selected pyroxenc crystals in CRP-3 core.
Depth (mbsf) 5.93 5.93 5.93 5.93 47.74 727.61
GroupIType
-. . I I I I I -- 11
,SiOi 49.89 49.80 47.64 51.70 53.30 50,54
1'iOi 0.83 0.73 0.71 0.00 0.53 0.38
A1,0, 1.26 1.03 0.72 1.10 l .89 l .40
1 'c0 22.16 24.10 33.65 27.08 1 1,89 17.18
M 1 1 0 0.47 0.43 0.61 0.57 0.39 0.38
M g 0 10.69 8.55 8.29 16.19 16.61 13.19
(.;iO 15.00 15.72 7.59 4.67 16.53 15.73
Tot 100.3 100.36 99.21 101.3 101.14 98.8
l-'i)i.iniiici recalculated on (lie basis o f 4 cation.\ (6 0 )
Si 1.939 1.959 1.941 1.965 1.962 1.950
*l l\: 0.058 0.041 0.035 0.035 0.038 0.050
I.,, W 0.004 0.000 0.024 0.000 0.000 0.000
AI "l 0.000 0.007 0.000 0.014 0.044 0.013
3+ \;l 0.013 0.000 0.015 0.021 0.000 0.015
'Fi 0.024 0.022 0.022 0.000 0.015 0.011
M g 0.619 0.502 0.504 0.917 0.912 0.759
0.704 0.793 1.107 0.839 0.366 0.540
M n 0.015 0.014 0.021 0.018 0.012 0.012
Ca 0.624 0.663 0.331 0.190 0.652 0.650
WO 31.61 33.61 16.75 9.57 33.58 32.91
En 31.35 25.44 25.46 46.18 46.95 38.40
Fs 37.05 40.95 57.79 44.25 19.48 28.68
727.61 5.79 I 1 free 50.77 49.87 0.5 1 0.55 l .S4 l .09 14.92 19.78 0.47 13.62 8.16 16.84 18.77 98.2 98.69
5.79 free
Formula calculated following IMA rules (Morimoto et al. 1988). Fe3+ calculated assuming charge balance
evolved composition that is rare in the FLIP. With the only exception of one sample described by Brotzu et al., (1988). rocks which are classified as dacites based on geochemistry, are all from evolved regions of sills or interstitial areas in thick lava flows (Elliot et al., 1995).
T h e glasses a r e rhyolitic in composition and resemble pyroclastic rocks in the Hanson Formation (redefined Upper Falla formation; Elliot, 1996). Using Zr, Ti, Nb and Y elements, that are considered to be i m m o b i l e during the weathering and alteration
processes, bulk clast compositions are subalkalic and range in composition from basalts to andesite (Fig. 6).
In particular, with the exception of a single Group l sample, the analysed clasts samples fall outside the fields for MFCT and SPCT rocks described by (Elliol e t al., 1995; Roland & Worner, 1996) and dolcrite clasts of CRP-l (Kyle, 1998). In addition on a Zr vs T i 0 7 diagrams (Fig. 7) C R P - 3 volcanic clasis of group I and one of group I1 fall along the trend of MFCT and dolerite clasts of CRP-1, whereas group I11 clasts and one group I1 clast fall outside.
F i e . 4 - A-CN-K and A-CNK-FM A1203
diagrams (molar proportion) for glasses (shards and clast mesostasis),
bulk clasts and intrusive body of AI203
CRP-3 core. Heavy curve with arrow represents a differentiation trend from gabbro to granite. dashed lines describe weathering trends (Nesbitt
& Young, 1989).
CaO*+Na20
Petrography. Mineral Composition am1 (Icochemistry of Volcanic and Suhvolcanic Rocks 475 7i//1. 3 - Major and trace clement hulk-rock composition of selected ('RI' 3 clasts.
.- -----p
1)cptIi (mbsf) 33.60-66 344.6 1-64
I'ctiogi aphic gi oup I 1
469.24-28
. . . - . . Ill .-
63.60 0.83 12.50 3.00 5.22 0. I I 3.86 3.3 I 2.17
l .49 0. I I 3.8 1 100.0 1 54.16 30.45 1.63
8 189 2 1 125
7 1 32 307 12 43 62 230 34
627.1 1-14 Ill 63.70 0.9 1 12.78
3.55 4.1 l 0.1 1 3.40 4.38 2.3 1
1.51 0.1 1 3.12 99.99 53.01 28.39
10 207 19 124 69 39 342 17 45 6 1 235
34 Analyses carried out at DST-Pisa on powder pellets. using a Philips PW1480 XRF spectroineter. with a full matrix correction of Franzini et al. (1975) and Leoni & Saitta (1976). Mg0 and Na,0 were determined by AAS. Fe0 by titraton. L01 gravimetrically.
In conclusion. no analysed clast is comparable to S P C T rocks. but t a k i n g i n t o a c c o u n t the scatter caused by alteration andlor possible analytical bias m o s t of the analysed clasts can b e related to the MFCT rocks.
INTRUSIVE BODY
Eight samples representing the central portion of t h e igneous i n t r u s i o n ( s e e t a b l e 5 f o r depth of sampling and analyses) and larger clasts enclosed in the external fractured zones were analysed for major a n d trace elements by X R F at Victoria University, Wellington and P i s a University. Any systematic difference is detectable between samples analysed from two laboratories, and thus inter-laboratory bias can be considered not-significant.
The chemical compositions of these rocks confirm t h e occurrence of extensive alteration, as clearly shown in petrographical observations. In all samples,
major element concentrations appear to be strongly modified by alteration processes as suggested by large loss of ignition values (LOI) and normative corundum in the CIPW norm. In the ternary diagrams of Nesbitt
& Young (1989) (Fig. 5). samples from the intrusive body plot along weathering lines mainly controlled by the alteration of plagioclase and formation of smectite and in s o m e cases illite. In both diagrams, t h e s e i n t r u s i v e rocks display a w i d e s c a t t e r p r o b a b l y reflecting varying degrees of alteration. In figure 8 L 0 1 a n d ratios between m o b i l e ( F e , M g ) and i m m o b i l e (Ti. Nb) elements during alteration are plotted against the depth. Though petrographical observations indicate more pronounced alteration in t h e l o w e r portion of the i g n e o u s body (below 906.00 mbsf) no clear correlation is evident between the above compositional parameters and the depth. On the other hand, peaks with high LOI, F e O / T i O , , . MgOITiO- at 909 and 915 mbsf confirm that some portions of the intrusion suffered significant changes
A MesoslastS
0 Group I Group I1
Dacite
Fig. 5 - Total Alkali -Silica (TAS) classification diagram for selected. less altered volcanic rocks in CRP-3 core. Fields for Mouni l-'c~/lo Chemical Type (MFCT) and Scarab Peak Chemical Type (SPCT) in Ferrar Dolerite and Kirkpatrick basalt rocks (Elliot el al.. 1005:
Fleming et al.. 1992. 1995: Hornig. 1993). volcanics rocks in CRP-2 (Armienti et al.. 2001) and rocks from the Hanson Formation (Flliot
& Larsen. 1993) are reported for comparisons.
Tab. 4 - Average composition of glass shards and glassy inesostasis in CRP-3 core
depth 5.79 47.74 185.5 185.5 370.8 375.3 735.4
group I1 mesostasis
# ans 2 1 4 2 1 1
SiOl 59.90 56.37 75.60 76.41 77.96 76.01
TiO? 1.42 1.02 0.00 0.79 0.00 0.16
A1207 25.17 15.21 13.93 12.33 12.34 14.87
FeOtot 4.36 14.21 0.61 2.49 1.11 2.20
MnO 0.00 0.00 0.00 0.00 0.00 0.01
MgO 1.90 4.93 0.07 0.00 0.00 3.17
CaO 0.31 2.20 2.36 3.14 3.35 0.05
Na10 1.63 1.47 3.05 3.08 2.93 0.98
KiO 4.39 4.25 4.34 1.69 1.56 2.54
P@, 0.00 0.00 0.00 0.00 0.64 0.00
Cl 0.93 0.35 0.03 0.07 0.12 0.00
Range of totals 72-73 79 79-96 94-97 94 72 94-95
in original analyses
Mg# 50.6 44.0 22.8 0.0 0.0 78.8 63.2
Q norm 29.1 12.2 35.8 44.9 49.1 54.8 32.9
C norm 17.3 4.2 0.0 0.0 1.3 10.4 0.0
Analyses are normalised to 100.
Petrography, Mineral Composition and Geochemistry of Volcanic and Snbvolcanic Rocks 4 7 7 Tub. 5 - Miijor and trace element of the magmatic intrusive body.
. .
Intei veil
SlO, TlO, A1,0, Fe,O, F e 0 M n O M g O c a o N a , 0 K,O P,O, L 0 1 T o t M g # Q no1 m C noun N b Zl Y S1 R b
ce
B a La Nl Cr
v
CO As c u Ga Pb s c Th U
Z n nd 4 2 6 6 nd 5 8 4
9 8
140 137
3 8 18
6 7 7 8
88 7 9
4 3 32
246 225
17 13
47 43
77 80
196 178
4 0 3 3
nd nd
nd nd
nd nd
nd nd
nd nd
nd nd
lld lld
l1 d nd
*Analysts: R. Grapes. J.E. Patterson (Victoria Univ. Analytical Facility - Wellington. NZ). Remaining analyses carried out at DST Pisa on powder pellets, using an Philips PW1480 XRF spectrorneter, with a full matrix correction of Franzini et al. (1975) and Leoni and Saitta (1976). MgO and Na20 were determined by AAS. F e 0 by titraton. L 0 1 gravimetrically.
i n composition d u e to extensive precipitation of carbonates (mostly siderite) from circulating fluids.
O n the basis of the a b o v e o b s e r v a t i o n i s concluded that major element concentrations do not represent the original compositions of the igneous body and cannot be used to infer magmatic affinity.
n o r correspondence w i t h v o l c a n i c s u c c e s s i o n s cropping out onland.
M i n o r and trace e l e m e n t s show an a n a l o g o u s variability that can be ascribed to re-mobilisation due to alteration. However, some elements (Ti, P, Zr, Nb, a n d Y ) that a r e k n o w n to b e l e s s affected by alteration processes can still be used for conlparisons
with exposed igneous rocks and to infer magmatic affinities. In f i g u r e 6 t h e s a m p l e s of the C R P - 3 intrusions a r e p l o t t e d in t h e classification- discrimination d i a g r a m of Floyd & Whinchester ( 1 9 7 8 ) which is b a s e d o n ratios of i m m o b i l e elements. Even with a significant scatter exists, the samples fall inside the subalkaline field. astride the divide between basaltic and andesitic compositions.
These results rule out the association with the alkaline C e n o z o i c M c M V G a n d s u g g e s t a r a t h e r c l o s e correspondence with the Jurassic Ferrar Supergroup.
Samples of the CRP-3 intrusion have Ti-Zr (Fig. 7) and Zr-Nb ratios (Cape Roberts Science Team, 2000) consistent with a Fen'ar origin.
Fig. 6 - N b I Y a n d ZrITiO, classification d i a g r a m ( F l o y d &
Winchester. 1978) for CRP-3 volcanics clasts and intrusive rocks.
Fields for rocks belonging to the Mount Fazio Chemical Type ( M F C T ) and Scarab Pcak Chemical Type (SPCT) of the Ferrar Supergroup (Elliot et al.. 1995: Fleming et al.. 1992, 1995: Hornig.
1993) and dolerite clasts in CRP-I (Kyle. 1998) are reported for comparison.
CONCLUSIVE REMARKS
Mineral and glass chemistry, bulk clast chen~istry and textures of igneous detritus in the CRP-3 confirm they are Ferrar Supergroup material. Even strongly altered clasts show an unequivocal subalkaline affinity typical of Jurassic rocks b e l o n g i n g to the F e r r a r
I'hc large textural diSferences within clasts rcl'lwt the cooling rate associated with different empli~cen~cilt mechanisms. T h u s holocrystallinc c l a s l s with suhopliitic textures typical of intrusive ~i~i~j:~iniilic bodies would represent the erosion of Ferrar sills or would belong to t h e inner portions o f thick Kirkpatrick lava flows; intersertal to iiltergi.:iii~iliir textures. with a variable abundance of interslilial glass, could represent the external part of these sills, but more probably they a r e eroded f r a g m e n t s of Kirkpatrick lava f l o w s . Glass-rich, vesiciiliited volcanic rocks and quench crystals indicate ;I high cooling rate, such as that associated with chilling of magma o n c o n t a c t w i t h water or c o u n t r y rocks.
Alternatively they may represent the external portions of Kirkpatrick lava flows or alternatively, could lie
L s ic or pillow-lava fragments derived from the p y r o c l x ~ ~ ' volca~iiclastic deposits that are intercalated i n the Kirkpatrick succession. Bulk chemistry o f Group I and G r o u p I1 c l a s t s roughly m a t c h e s lie compositional range of the MFCT Ferrar Dolerite and Kirkpatrick B a s a l t . G r o u p I11 rocks, a l t l i o i ~ g h subalkaline, show an evolved composition that wiis not o b s e r v a b l e in any of t h e volcanics c r o p p i n g o n l a n d . Finally, r a r e g l a s s s h a r d s s h o w a good correspondence with the Hanson unit, they would therefore derive from the erosion of these volcanics which being probably early Jurassic in age, underlie the Ferrar Supergroup succession. This m a g i i a t i c affinity excludes these glasses from being products of C e n o z o i c a l k a l i n e activity coeval w i t h their deposition, as hypothesised in the initial report. and indirectly supports the onset of Cenozoic activity in
-, - -
, - Hanson and Prebble Fmts. -,-S'
- - - . . -.. -.-- --- -- -,--
-- .-.-- * < - c
- - - - . ..--S-.---
0 50 100 150 200 250 300
zr ( P P ~ I )
Fig. 7 - Zr vs TiO, plot for CRP-3 volcanics clasts and intrusive rocks. Data sources for MFCT. SCPT CRP-l dolerite clasts and Prebble and Hanson Formations are the same as figures 5 and 6.
Petrography, Mineral Composition ;ind (icochcmistry ol' Volcanic i i n t l Snhvolcanic Rocks 47 9
Fig. 8 - Loss of Ignition. FeO-TiO.,. Mg-TiO.,. Nb-TiO, ratios vs depth for rocks, belonging to the intrusive body.
this area as being recorded in CRP-2 at 280 mbsf and dated at 24.22 Ka (McIntosh, 2001).
A s in CRP-1, no clasts having compositional or petrographical characters attributable to the SPCT has been found. SPCT rocks cap the Jurassic lavas, and may represent a good marker in the reconstruction of the erosion history of the area. In contrast to CRP-1, clasts in CRP-3, that could have been derived from t h e Kirkpatrick B a s a l t , a r e very c o m m o n . T h e increase in subalkaline basalt clasts mainly derived f r o m Ferrar Supergroup rocks indicate that a large part of the erosion of the Jurassic units must have taken place in Eocene-Oligocene time (Cape Roberts Science Team, 2000).
Finally, though heavily affected by alteration, the intrusive body s h o w s a c l e a r subalkaline affinity, which rules out a Cainozoic age and indicates that it b e l o n g s to the J u r a s s i c F e r r a r S u p e r g r o u p . T h i s intrusive body can be considered as one of the many
intrusive bodies emplaced into Beacon rocks a t various stratigraphic levels. T h e g e o m e t r i c a l
~~eI:itionships between the intrusion and the country rock a n d thc petrographical characteristics of t h e f o r m e r can be used to make inferences on t h e e m p l a c e m e n t a n d c o o l i n g s t y l e s . T h e i n f e r r e d presence of glass points to relatively fast cooling o f t h e margin o f the magmatic body. whereas t h e occurrence of breccia i n a clay-rich zone indicates an i m p o r t a n t e p i s o d e of brittle deformation a n d hydrotlicrmal alteration at a lower temperature that accompaniecl or followed the cmplacement. T h e s e processes caused modification of the outer part of the i n t r u s i o n mechanical and c h e m i c a l . T h e l a c k o f vesiculation excludes emplacement at very shallow depths, (e.g. neck or plug) but the presence of a large quantity of fluids suggests that intrusion occurred in wet permeable sediments. Fluids from secondary boiling of the magma, coupled with the generation of steam in the surrounding sediments, caused fracturing and brecciation of both the chilled margin of t h e intrusive body and surrounding sediment strata. T h e l a c k o f significant t h e r m a l m e t a m o r p h i c e f f e c t s suggests a small size intrusion occurred in a short time span Although infrequent this emplacement style has already been recognised in Taylor Glacier and in Allan Hills (Craw & Findlay, 1984; Grapes et al.,
1973).
ACKNOWLEDGEMENTS - We are grateful to D.Elliot and P.Kyle for reviews and improvements of the manuscripts.
We thank also R. Grapes and J.E. Patterson for providing fast analyses of the intrusion. A special thank to S . Sandroni. J . Smellie a n d F. Talarico for their friendly support. This work has been performed with the financial support of the Italian Programme! Nazionale di Ricerche i n Antartide (PNRA).
REFERENCES
A l l i b o n e A . H . 1 9 9 2 . L o w - P r e s s u r e High-Temperature Metamorphism of Koettlitz Group schists. Taylor Valley and Upper Ferrar Glacier Area, South Victoria Land. Antarctica.
New Zealand Journal of Geology and Geophysics, 35. 115-127.
Allibone A.H. Cox S.C.. Graham I.J.. Smillie R.W.. Johnstonc R.D., Ellery S.G.. & Palmer K.. 1993. Granitoids of the Dry Valleys area Southern Victoria Land. Antarctica - Plutons, field.
relationships, and isotopic dating. New Zealand Journal of Geology and Geophysics. 36. 281-297.
A r n ~ i e n t i P,. Messiga B . & Vannucci R., 1998. Sand provenance from Major and Trace element Analyses of Bulk rock and sand grains. Terra Antartica. 5, 589-599.
Armienti P,. Pompilio M,. & Tamponi M,. 2001. Sand provenance from major and trace element analyses of bulk rock and sand grains from CRP-212A. Victoria Land Basin. Antarctica. This volume.
Bozzo E.. Damaske D.. Caneva G.. Chiappini M,. Ferraccioli F..
G a m b e t t a M . & Meloni A., 1 9 9 7 . A High Resolution Aeromagnetic Survey over Proposed Drill Sites Off Shore of Cape Roberts in the Southwestern Ross Sea (Antarctica). In:
Ricci C.A. (ed.). The Antarctic Region: Geological Evolution and Processes. Terra Antartica Publication, Siena. 1129-1 134.
Brotzu P., Capaldi G.. Civetta L.. Melluso L. & Orsi G.. 1088, Jurassic Ferrar Dolerites and Kirkpatrick Basal! in norllicrn Victoria Land (Antarctica): Stratigraphy, Geochronology, and Petrology. Memorie della Societd Geologica 1tali(111(1. 13. 97-
116.
Cape Roberts Science Team. 2000. Studies form the Cape Robcrts Project. Ross Sea. Antarctica Initial Report o n Cl<!'-3. P r r a Antartica. 7. 1-209.
Craw D. & Findlay R. H.. 1984. Hydrothermal alteration of Lower Orclovician granitoids and Devonian Beacon Sandstone at Taylor Glacier. M c M u r d o Sound. Antarctica. N e i r Zealand Journal o f Geology and Geophysics. 27. 465-475.
illicit D.H.. 2000. Stratigraphy of Jurassic Pyroclastic rocks in the Transarctic Mountains. Journal of African Earth Sciences. 31.
77-89.
Hlliot D.H.. 1996. The Hanson Formation: a new stratigraphic unit in Transantarctic Mountains. Antarctica. Antarctic Science. 8.
389-394.
i o t D.H.. Fleming T.H.. Haban M.A.. & Siders M . A . , 1995.
Petrology and mineralogy of the Kirkpatrick basalt and Ferrar dolerite. Mesa range region. North Victoria Land, Antarctica.
Contribiitiofis to Antarctic Research /V - Antcircfic research series. 67. 103-141.
HIliot D.H.. Haban M.A.. Siders M.A.. 1986. The Exposure Hill F o r m a t i o n . M e s a R a n g e . In S t u m p , E . ( E d . ) . G e o l o g i c a l Investigations in Northern Victoria Land. American Geophysical Union. Washington DC. Antarctic research series. 46. 267-284.
Flerning T.H., Elliot D.H.. Jones L.M.. Bowman J.R.. & Siders M.A.. 1992. Chemical and isotopic variations in an iron-rich lava flow from the Kirkpatrick Basalt. north Victoria Land, A n t a r c t i c a : i m p l i c a t i o n s f o r low t e m p e r a t u r e a l t e r a t i o n . Contributions to Miiieralogy and P e t ~ d o g y . 111. 440-457.
Fleming T.H.. Poland K.A. & Elliot D.H.. 1995. Isotopic and c h e m i c a l c o n s t r a i n t s o n t h e c r u s t a l evolution and s o u r c e signature of Ferrar magmas. north Victoria Land. Antarctica.
Contributions to Mineraloe\ and Petrology, 121. 217-236.
Fitzgerald P,. 1999. Cretaceous - Cenozoic Tectonic Evolution of the Antarctic Plate. T e r n Antartica Reports. 3. 109-130.
Franzini M . & Leoni, L.. 1972. A full matrix correction in X-ray f l u o r e s c e n c e a n a l y s i s o f rock s a m p l e s : A t t i dellcl Societd T o s c m di Science N a f w a l i . 79. 7-22.
F l o y d P.A. & W i n c h e s t e r J . A . . 1 9 7 8 . I d e n t i f i c a t i o n a n d discrimination of altered and methanlorphosed volcanic rocks using immobile elements. Chemical Geology. 21. 291-306.
Gamble J.. Barrett P.J. & Adams C.J.. 1986. Basaltic clasts from
Unit 8 . In: Barrett P.S. ( c d . ) . Aiiliti'i'lic C e i w z i i i r / i i \ / o n ' . /W.s'STS-I drilll~ole. IISIR ISiill(~ti~~. 237,
Grapes R.. Gamble S. & Palmcr P,. 1989. Basal dolerilc h o ~ ~ k l i - i ~ . In: Barrett P J . (cd.). Antarctic Cciio:oic / ~ i . ~ o ~ : ~ , f r o m ('lR0,Y I drillhole: McMiirdo Sound. / I S M R~illriin. 245. 169- 17.1, Grapes R.H.. Reid D.L. & McPhcrson S. G.. 1973. Shallow ilolrii~r
intrusion and phreatic eruption in the Allan Hills i - r g i o ~ ~ . iVi'ir Zealand Journal of Geolov\ anil Gc~o/)l~ysic.s. 17. 56:3-570.
Hornig I.. 1993. High-Ti and Low-Ti tholeitcs in the Junissic I'vrr;~r Group. Antarctica. Geologi.scl~(~.\ Jcihrhiich. E47. 3 3 5 . W ) . Kyle P.R.. 1981. Mineralogy and geocheiiiistry of a hiisiinitr 1 0
phonolite sequence at Hut Point Peninsula. Antarcticti, h:isr(l nil
core from Dry Valley Drilling Pro.jcct Drillholes l , 2 :III{I 1.
Journal of Petrology. 22. 45 1-500.
Kyle P.R., 1998. Ferrar dolci-itc clasls from CRP-1 drillcore. ' I ' m i i
Antarfica. 5. 61 1-612.
Kyle P.R.. 1 9 9 0 , M c M u r d o Volcanic G r o u p - W e s t e r n Ross Enlbayment. Introduction. In: Volcanoes of the Antarctic I'lnlc
& Southern O c e a n s , pp. 19-134. Antarctic Research Scrii"..
American Geophysical Union. Washington.
LeMasurier W.E.. 1990. Late Cenozoic volcanism on t h e Anliirdii' Plate: an overview. I n : Volcanoes of the A n t a r c t i c I'liiic A S o u t h e r n O c e a n s , p p . 1 - 1 7 . Antarctic R e s e a r c h Scrii."..
Washington.
Leoni L . & Saitta M , . 1976. X-ray fluorescence a n a l y s i s 01' 20 trace elements in rocks and mineral standards: Rendicoini ilcllii Societd Italiaiza di Mi~ieralogia e Petrologia. 32. 497-5 10.
Leterrier J . . Maury R.C.. Thonon P,. Girard D. & Miirchnl h4..
1982. Clinopyroxene composition as a method of icicn~il'iciiti~~ii of the magmatic affinities of palaeo-volcanic s e r i e s . Liirlli Planetan- Science Letters, 59. 139-154.
Mclntosh W.C.. 2001. ^ArlZyAr Chronology of Teplira and Volcanic Clasts in CRP-2A. Ross Sea. Antarctica. Terra Antci~.iicii. 7, 621-630.
Morimoto. N.. 1988. Nomenclature of pyroxenes. Mineriilo,qx tiiliil Petrology. 39, 55-76.
Nesbitt H. W. & Young G. M , . 1982. Early Protemzoic climates and plate motions inferred from major element chemistry of lutites. Nature. 299. 715-717.
Nesbitt H. W. & Young G. M,, 1989. Formation and diagcnesis of a weathering profiles. Journal of Geology. 97. 129-147.
Roland N. W. & Worner G.. 1996. Kirkpatrick flows and associiilcil pyroclastics: new occurrences. definition. and a s p e c t s ol' :I Jurassic Transantarctic rift. Geologiscl~es J a h h c l i . 1589, 97-
121.
